Michel Nasilowski

Benoit Mahler

Emmanuel Lhuillier

Sandrine Ithurria

Benoit Dubertret
email: benoit.dubertret@espci.fr

Two-Dimensional Colloidal Nanocrystals

Keywords: 2D, beyond graphene, nanoplates, colloidal growth, anisotropy, TOC:

published or not. The documents may come

Table of contents 1. Introduction

Nanocrystals are particles with at least one dimension comprised between 1 and 100 nm.

For the last decades, they have been of growing interest as they exhibit unique properties due to their small size. They are often described as artificial atoms, as their sparse density states can be modified tuning their shape, size and composition. [START_REF] Yin | Colloidal Nanocrystal Synthesis and the Organic-inorganic Interface[END_REF] Indeed, electron confinement in those nanostructures provides the nanocrystals with properties that are much different from their bulk counterparts and can be used to enhance electric, optical or magnetic properties. [START_REF] Xia | Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?[END_REF] A typical example of the influence of size on the crystal properties are Quantum Dots (QDs) which are nanocrystals made of semiconducting materials. The band-edge energy between the conduction and the valence band depends directly on their size, which gives their fluorescence wavelength a strong size-dependence. The shape is another parameter of key importance to control the properties of such nanoparticles as the motion of electrons (and therefore holes and plasmons) can be confined in given directions. For example, not only the catalytic activity of metals is enhanced when their size is reduced, [START_REF] Valden | Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties[END_REF][START_REF] Zecchina | Selective Catalysis and Nanoscience: An Inseparable Pair[END_REF] but the selectivity of the catalyzed reactions is also strongly dependent on the exposed facets, and therefore on the shape of the nanocrystals (NCs). [START_REF] Shi | The Effects of Gas Adsorption on Particle Shapes in Supported Platinum Catalysts[END_REF][START_REF] Narayanan | Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability[END_REF] Besides, the surface plasmon resonance (SPR) in Au NCs is mostly dependent on their shape: [START_REF] Kelly | The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment[END_REF] changing the geometry of the nanocrystal changes the energy of the plasmon peak more than the mere increase of the size of the spherical particles.

Finally, the size, but also the shape of the nanoparticles (NPs) plays an important role in the cellular uptake. [START_REF] Chithrani | Determining the Size and Shape Dependence of Gold Nanoparticle Uptake into Mammalian Cells[END_REF][START_REF] Chithrani | Elucidating the Mechanism of Cellular Uptake and Removal of Protein-Coated Gold Nanoparticles of Different Sizes and Shapes[END_REF] The need to control the shape of the NPs is therefore strong and the growing interest in shape-controlled syntheses of NCs is confirmed by numerous contributions for different applications such as optics (photodetectors, [START_REF] Lhuillier | Nanoplatelets Bridging a Nanotrench: A New Architecture for Photodetectors with Increased Sensitivity[END_REF][START_REF] Deng | Solution Synthesis of Ultrathin Single-Crystalline SNS Nanoribbons for Photodetectors via Phase Transition and Surface Processing[END_REF] lasers, [START_REF] Grim | Continuous-Wave Biexciton Lasing at Room Temperature Using Solution-Processed Quantum Wells[END_REF] fluorescence [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF][START_REF] Joo | Low-Temperature Solution-Phase Synthesis of Quantum Well Structured CdSe Nanoribbons[END_REF][START_REF] Chen | Quasi-2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence[END_REF] ), electronics, [START_REF] Takami | Continuous Hydrothermal Synthesis of Nickel Oxide Nanoplates and Their Use as Nanoinks for P-Type Channel Material in a Bottom-Gate Field-Effect Transistor[END_REF][START_REF] Cha | Two-Dimensional Chalcogenide Nanoplates as Tunable Metamaterials via Chemical Intercalation[END_REF][START_REF] Wang | Gate-Controlled Surface Conduction in Na-Doped Bi 2 Te 3 Topological Insulator Nanoplates[END_REF] catalysis, [START_REF] Somorjai | Colloid Science of Metal Nanoparticle Catalysts in 2D and 3D Structures. Challenges of Nucleation, Growth, Composition, Particle Shape, Size Control and Their Influence on Activity and Selectivity[END_REF] biological sensing [START_REF] Gao | Highly Stable Silver Nanoplates for Surface Plasmon Resonance Biosensing[END_REF] and imaging. [START_REF] Homan | Silver Nanoplate Contrast Agents for in Vivo Molecular Photoacoustic Imaging[END_REF][START_REF] Li | Topological Insulator Bismuth Selenide as a Theranostic Platform for Simultaneous Cancer Imaging and Therapy[END_REF] The top-down (chemical or mechanical exfoliation [START_REF] Li | Preparation and Applications of Mechanically Exfoliated Single-Layer and Multilayer MoS 2 and WSe 2 Nanosheets[END_REF][START_REF] Eda | Photoluminescence from Chemically Exfoliated MoS 2[END_REF][START_REF] Chen | Two-Dimensional Graphene Analogues for Biomedical Applications[END_REF][START_REF] Novoselov | Two-Dimensional Atomic Crystals[END_REF] by ion intercalation, ion exchange, sonication [START_REF] Nicolosi | Liquid Exfoliation of Layered Materials[END_REF] or by the "Scotchtape" method [START_REF] Jariwala | Emerging Device Applications for Semiconducting Two-Dimensional Transition Metal Dichalcogenides[END_REF] ) as well as the bottom-up approaches have so far been explored to synthesize 2D nanostructures with different techniques used such as physical deposition methods (vapor deposition [START_REF] Shi | Recent Advances in Controlled Synthesis of Two-Dimensional Transition Metal Dichalcogenides via Vapour Deposition Techniques[END_REF] , molecular beam epitaxy (MBE) [START_REF] Mata | A Review of MBE Grown 0D, 1D and 2D Quantum Structures in a Nanowire[END_REF] ), lithography, [START_REF] Haynes | Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics[END_REF][START_REF] Tabor | Dependence of the Threshold Energy of Femtosecond Laser Ejection of Gold Nanoprisms from Quartz Substrates on the Nanoparticle Environment[END_REF] or wet chemical methods. The latter present the advantage of being relatively cheap, easy to implement and scalable, but the precise control of the size and shape of the synthesized NPs is still challenging. However, in the last decades, numerous groups have improved the wet chemical growth techniques and have intensively studied the growth of anisotropic structures. This review will focus on the solvothermal syntheses protocols and mechanisms used to obtain colloidal metal and semiconductor nanocrystals with a two-dimensional geometry.

The synthesis of nanoparticles using wet chemical methods requires, in general, the same three components: precursors which provide the atoms for the growth of the NP (by thermal decomposition or conversion to more reactive species); ligands which ensure colloidal stability and which have a key role to precisely control the size and shape of the obtained nano-objects ; and the solvent which provides a homogeneous reaction medium in which the precursors are soluble and the final NCs are stable (it is to be noted that the solvent can also act as a source of precursors or ligands [START_REF] Acharya | Shape-Dependent Confinement in Ultrasmall Zero-, One-, and Two-Dimensional PbS Nanostructures[END_REF] ). All these components, depending on their reactivity or their affinity to specific crystal facets provide good tools to control not only the size but also the shape of NCs. Before entering into the details of shape control, we will briefly remind the LaMer theory and introduce the two important steps for the bottom-up colloidal synthesis of NCs: nucleation and growth. After those steps, Ostwald ripening can occur; [START_REF] Ostwald | Über Die Vermeintliche Isomerie Des Roten Und Gelben Quecksilberoxyds Und Die Oberflächenspannung Fester Körper[END_REF] it is generally avoided to preserve a good size monodispersity of the particles.

NCs are produced following a general synthesis scheme: precursors, which provide atoms to build the NCs, decompose or rearrange into monomers and form nuclei during the nucleation step. These nuclei can further grow to form bigger crystals either by continuous deposition of monomers on their surface, by attachment of several nuclei together or by Ostwald ripening. The nucleation-growth theory has been introduced by La Mer in the 50s, adapted from a theory developed by Becker and Döring in the 20s. [START_REF] Mer | Nucleation in Phase Transitions[END_REF][START_REF] Becker | Kinetische Behandlung Der Keimbildung in Übersättigten Dämpfen[END_REF] Homogeneous nucleation occurs spontaneously and randomly, without the need for a nucleation site. It requires however a solution that is supersaturated in precursors.

Heterogeneous nucleation, on the other hand, occurs at a nucleation site, which is a solid phase in contact with the liquid (or vapor) phase containing the precursors. In La Mer's theory, the so-called nucleation, which is the spontaneous formation of nuclei in the solution, is a homogeneous process, while the growth can be considered as a heterogeneous one, as precursors deposit on a nucleation site in the solution (the already formed nuclei).

The basic idea behind the theory is that a thermodynamic system tends to minimize its Gibbs free energy or increase its enthropy. [START_REF] Polte | Fundamental Growth Principles of Colloidal Metal Nanoparticles -a New Perspective[END_REF] The Gibbs free energy of a spherical cluster can be expressed as follows:

∆𝐺 = - 4 3 𝜋𝑟 3 |∆𝐺 𝑉 | + 4𝜋 2 𝛾 Equation 1
where 𝑟 is the radius, |∆𝐺 𝑉 | is the difference is the Gibbs free energy per unit volume, and 𝛾 is the surface energy per unit area, i.e. the energy needed to create a surface of unit area. The Gibbs free energy consists of two terms. The first one, negative, expresses a decrease of the Gibbs free energy, and thus a favorable event. In this case, it corresponds to the bonding of a monomer to the cluster. The second term, positive, expresses the unfavorable increase of the energy surface after the bonding of a monomer. The change in the Gibbs free energy due to the formation of a bond between a cluster and a monomer therefore corresponds to a competition between the decrease of the volume energy and the increase of the surface one.

Below a certain critical size, corresponding to the critical radius 𝑟 𝑐 = 2𝛾 |∆𝐺 𝑉 | ⁄ (calculated by canceling the derivative of Equation 1), the positive surface energy increase is higher, thus pushing the system towards the dissolution: the growth is unfavorable. Above 𝑟 𝑐 , the negative volume energy decrease is higher, thus growth is favored. Figure 1 shows the evolution of the Gibbs free energy of a cluster. The energy barrier ∆𝐺 𝑐 corresponds to ∆𝐺 at 𝑟 𝑐 . Once this energy barrier is overcome and the critical radius reached, stable clusters form in solution: the nuclei. These nuclei can further grow through heterogeneous nucleation.

Indeed, the surface energy necessary for nucleation is lower at phase boundaries, which become so-called nucleation sites. Therefore, once the nuclei are formed, subsequent nucleation occurs preferentially on the present nuclei. However, homogeneous nucleation is still possible, and can occur in the same time as heterogeneous nucleation. The concept of burst nucleation was introduced by Victor La Mer and relies on the separation of nucleation and growth of the nanoparticles. In order to achieve efficient separation, the idea is to produce all the nuclei at the same time, and let the growth happen only later. Figure 2 describes the evolution of the concentration of monomers versus time, showing the mechanism of the burst nucleation: in phase I, either because of a swift injection of precursors or because a critical temperature is reached, the precursors convert into monomers whose concentration increases. The supersaturation is reached at a concentration 𝐶 𝑆 , however the energy barrier to start nucleation has still to be overcome. In phase II, the saturation keeps increasing to reach a concentration 𝐶 𝑚𝑖𝑛 at which the activation energy is overcome:

homogeneous nucleation can occur. This nucleation is very fast, hence the name "burst nucleation". Due to the nucleation, the concentration of monomers drops, decreasing the saturation and therefore ending the nucleation step. In phase III, nucleation has stopped and the remaining monomers will only attach to the existing nuclei to grow the NPs. The NP will grow in such a way to reduce its surface energy, which is the limiting parameter, already present in the nucleation step. Historically, in 1874, [START_REF] Gibbs | On the Equilibrium of Heterogeneous Substances[END_REF] Gibbs suggested that the shape of a crystal is dictated by the minimization of the total surface energy. Therefore, the shape of crystals (and nanocrystals, as it has been shown that the equilibrium shapes of the macroscopic crystals and nanocrystals are the same when they have the same chemical composition 39,[START_REF] Barmparis | Nanoparticle Shapes by Using Wulff Constructions and First-Principles Calculations[END_REF] ) is therefore not dictated by the minimum total surface (which is a sphere), but depends strongly on crystalline facets. The final equilibrium shape will be dominated by the facets with the lowest surface energies. In 1901, Wulff [START_REF] Wulff | Zur Frage Der Geschwindigkeit Des Wachsthums Und Der Auflösung Der Krystallflächen[END_REF] assumed that the surface free energy of a crystal depends on the facet's crystallographic orientation: the shape of the NP is then determined by the crystallographic nature of the expressed facets with low surface energy. In isotropic phases, this turns out to be a sphere, but for NPs which express different facets, the shape may be more complex than the mere spherical shape. An easy construction method that allows to determine the equilibrium shape of NCs has been developed by Wulff and is called the Wulff construction. [START_REF] Wulff | On the Question of Speed of Growth and Dissolution of Crystal Surfaces[END_REF] It requires knowing the energy needed to create a surface of unit area in the directions normal to the atomic planes (hkl) (which is the direction of the vector [hkl]) 𝛾 ℎ𝑘𝑙 . This energy is roughly proportional to the number of broken bonds on a surface: due to smaller distances between atoms on facets with high coordination such as close-packed facets, those facets have less dangling bonds per area unit, lowering the surface energy compared to facets with more dangling bonds. When this energy is known, after choosing a set of axes, one can plot the plane normal to [hkl] at a distance 𝑐 • 𝛾 ℎ𝑘𝑙 (where 𝑐 is a constant) of the origin of the axes. The same operation is repeated for all the (hkl) planes, and the geometrical shape lying inside all the plotted lines corresponds to the shape with the minimal total free energy. [START_REF] Barmparis | Nanoparticle Shapes by Using Wulff Constructions and First-Principles Calculations[END_REF][START_REF] Viswanath | Mechanistic Aspects of Shape Selection and Symmetry Breaking during Nanostructure Growth by Wet Chemical Methods[END_REF] Figure 3 presents an example of Wulff construction for an orthorhombic structure, with 𝛾 100 = 𝛾 110 = 1/2𝛾 010 , knowing that all other (hkl) planes have much higher surface energies. The equilibrium shape in this conditions is a rod-like prism, that is indeed found experimentally for some orthorhombic structures such as aragonite CaCO3. [START_REF] De Leeuw | Surface Structure and Morphology of Calcium Carbonate Polymorphs Calcite, Aragonite, and Vaterite: An Atomistic Approach[END_REF][START_REF] Jones | Morphology and Growth of Aragonite Crystals in Hot-Spring Travertines at Lake Bogoria, Kenya Rift Valley[END_REF] For a face-centered cubic (fcc) structure, the surface energies can be estimated as follows: 𝛾 100 = 4(𝜀 𝑎 2 ⁄ ), 𝛾 110 = 4.24(𝜀 𝑎 2 ⁄ ) and 𝛾 111 = 3.36(𝜀 𝑎 2 ⁄ ) where 𝜀 is the bond strength and 𝑎 is the lattice constant. [START_REF] Xia | Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?[END_REF] Thus, the surface energies can be ordered 𝛾 111 < 𝛾 100 < 𝛾 110 , showing that the NC should adopt a shape to favor (111) facets. However, not only the surface energy is important, the surface area also needs to be taken into account, and the NC will tend to minimize it too, resulting in truncated octahedrons enclosed by (111) and (100) facets. [START_REF] Viswanath | Predicting the Growth of Two-Dimensional Nanostructures[END_REF][START_REF] Kang | Synthesis and Characterization of Nanometer-Size Fe 3 O 4 and γ-Fe 2 O 3 Particles[END_REF][START_REF] Zhang | A Systematic Study of the Synthesis of Silver Nanoplates: Is Citrate a "Magic" Reagent?[END_REF][START_REF] Elechiguerra | The Role of Twinning in Shape Evolution of Anisotropic Noble Metal Nanostructures[END_REF] These first examples show that the equilibrium shape of a NC cannot be determined only by the minimization of the surface area as it would always yield spherical-shaped NCs.

Due to symmetry reasons and to asymmetric bond strength, some materials prefer growing in one or two dimensions. Their equilibrium shape is therefore nanorods or nanoplates. Table 1 gives examples of preferred thermodynamic shapes for some chalcogenide NCs. There are however in the literature numerous examples of nanocrystals whose shape is not their thermodynamic equilibrium shape. For example, FeS2 tend to grow in three dimensions, but two-dimensional FeS2 nanoplates have been reported. [START_REF] Kirkeminde | Synthesis and Optoelectronic Properties of Two-Dimensional FeS 2 Nanoplates[END_REF][START_REF] Li | Electrical and Magnetic Properties of FeS 2 and CuFeS 2 Nanoplates[END_REF] Thus, certain synthesis parameters force the NCs to grow in a non-energetically favored shape and yield two-dimensional structures. What drives this anisotropic growth?

The syntheses of NPs seldom happen in conditions close to the equilibrium. They rather deviate from these conditions and follow driving-forces that favor one shape over the other. Several mechanisms can explain the anisotropic growth of NCs.

During the nucleation of NCs, not all the nuclei are the same. Some of them might have a specific shape, others might present defects. The initial seed is therefore important in determining the final shape of the NCs as different nuclei will grow differently. Defect induced anisotropy, such as twin plane in fcc metals, has been widely studied and is one of the major driving force for two-dimensional growth, and can yield metal nanoprisms or nanoplates (Figure 4). [START_REF] Alloyeau | Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy[END_REF][START_REF] Lukman | Facile Synthesis, Stabilization, and Anti-Bacterial Performance of Discrete Ag Nanoparticles Using Medicago Sativa Seed Exudates[END_REF][START_REF] Zeng | Successive Deposition of Silver on Silver Nanoplates: Lateral versus Vertical Growth[END_REF][START_REF] Cutler | Colloidal Gold and Silver Triangular Nanoprisms[END_REF] This growth mechanism will be developed in part 2. Because some nanoparticles have shapes that do not correspond to their thermodynamic equilibrium shapes, kinetic effects have to be taken into account. Indeed, depending on the growth rate of the nanoparticles, or on their ligands, the nanoparticles can be blocked in a local energy minimum. The nanoparticle nucleation takes place on time scales ranging from a fraction of a millisecond to a few milliseconds. The reorganization of atoms may occur on longer time ranges, not giving enough time for the structure to find the absolute energy minimum, thus being blocked in a metastable configuration that can give the NP its final shape. [START_REF] Baletto | Structural Properties of Nanoclusters: Energetic, Thermodynamic, and Kinetic Effects[END_REF] When the growth rate of nanoparticles is high, the fast addition of precursors does not favor any facets, exhibiting high-energy facets such as {221} facets in fcc metals. [START_REF] Personick | Making Sense of the Mayhem behind Shape Control in the Synthesis of Gold Nanoparticles[END_REF][START_REF] Langille | Defining Rules for the Shape Evolution of Gold Nanoparticles[END_REF] On the other hand, when the growth rate is slowed down (lower temperature, slower reduction rate…), the growth is faster on high-energy planes, therefore more energetically-favored facets are expressed, such as low-indexes {111} planes in fcc metals (Figure 5). 60 This growth sequence can be dramatically changed because of the influence of ligands.

Indeed, they can cap specific facets, slowing the growth of the passivated facets compared to the other ones. Because the growth in some crystallographic directions is hindered, rod-or plate-like symmetries can be obtained. This effect is called chemical poisoning.

Many factors influence the chemical poisoning effect. First, because the passivation of some facets is the primary driving force for anisotropic growth, the ligands will play a crucial role in determining which facets will grow. Their affinity to the atomic species at the surface of the nanoparticle will help a better or worse passivation. For CdSe nanoparticles, for example, a strong ligand such as trioctylphosphine (TOP), or trioctylphosphine oxide (TOPO),

shows such strong affinity to surface cadmium ions that it reduces the energy difference between the different facets, annihilating the chemical poisoning effect: the growth rate of all the facets becomes very similar, leading to isotropic, spherical CdSe NPs. [START_REF] Son | Colloidal Synthesis of Ultrathin Two-Dimensional Semiconductor Nanocrystals[END_REF] The nature of the ligand is of course very important: it can be inorganic [START_REF] Personick | Shape Control of Gold Nanoparticles by Silver Underpotential Deposition[END_REF] or organic, [START_REF] Son | Colloidal Synthesis of Ultrathin Two-Dimensional Semiconductor Nanocrystals[END_REF][START_REF] Son | Large-Scale Soft Colloidal Template Synthesis of 1.4 Nm Thick CdSe Nanosheets[END_REF][START_REF] Li | Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects[END_REF] and a small change in its nature can yield totally different results. [START_REF] Yu | Formation and Stability of Size-, Shape-, and Structure-Controlled CdTe Nanocrystals: Ligand Effects on Monomers and Nanocrystals[END_REF] Controlling the chemical poisoning induced by ligands can lead to different shapes using the same material and crystal structure. For example, wurtzite CdS or CdSe/CdS nanocrystals can be synthetized with rod [START_REF] Peng | Shape Control of CdSe Nanocrystals[END_REF] , tetrapod [START_REF] Milliron | Colloidal Nanocrystal Heterostructures with Linear and Branched Topology[END_REF] or plate like shapes [START_REF] Cheng | Shape Control of Monodisperse CdS Nanocrystals: Hexagon and Pyramid[END_REF][START_REF] Cassette | Colloidal CdSe/CdS Dot-in-Plate Nanocrystals with 2D-Polarized Emission[END_REF] depending on the synthesis conditions and the use of phosphonic acid ligands which hinder the nanocrystals growth perpendicular to the wurtzite c axis, leading to the formation of rod-shaped nanocrystals.Another important characteristic of ligands is their propensity to form close-packed layers: it is called the soft-templating effect, developed in part 3. While electrostatic forces bind the ligands to the surface of the NPs (eg. negatively charged acids with positively charged ions), the organic chains of the ligands, which ensure the colloidal stability, can form a stable assembly that decrease the energy of the facets and therefore hinder the growth in this direction, yielding nanoplatelets (see Figure 6), or increase the steric repulsion between NPs through their bulky and rigid chains, decreasing the probability of random attachment between several NPs (see part on Oriented Attachment). [START_REF] Son | Large-Scale Soft Colloidal Template Synthesis of 1.4 Nm Thick CdSe Nanosheets[END_REF][START_REF] Li | Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects[END_REF][START_REF] Chen | Controlled Growth of Cubic Cadmium Sulfide Nanoparticles Using Patterned Self-Assembled Monolayers as a Template[END_REF][START_REF] Pileni | The Role of Soft Colloidal Templates in Controlling the Size and Shape of Inorganic Nanocrystals[END_REF] Another important factor in the ligand-mediated shape control is the temperature of the growth reaction. Not only can the temperature play a role in the kinetic control of the NP growth by lowering the differences in the energies of the facets, but it also acts on the formation of the ligand assemblies. Indeed, low temperature growth will enhance the subtle energy differences between the facets, and subsequently the corresponding growth speeds differences. Besides, the close-packed layers of surfactants can form under a certain temperature, above which they decompose and cannot play their role as a soft template anymore. [START_REF] Li | Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects[END_REF][START_REF] Barman | Contrasting Melting Behavior of Zinc Stearate and Zinc Oleate[END_REF][START_REF] Bakshi | Aqueous-Phase Room-Temperature Synthesis of Gold Nanoribbons: Soft Template Effect of a Gemini Surfactant[END_REF] Once the nuclei are present in solution, they can grow through the addition of precursors or self-assemble in solution to form new, larger crystals. This mechanism is developed in part 4. In 1998, R. L. Penn and J. F. Banfield described on TiO2 a mechanism for the anisotropic growth of NCs: oriented attachment (OA). In this mechanism, two adjacent NPs reorganize so that they share a common crystallographic orientation. The decrease of the surface energy (through the decrease of the number of unsatisfied surface bonds) is the driving force for the attachment of these NPs, to form a new crystal. [START_REF] Penn | Imperfect Oriented Attachment: Dislocation Generation in Defect-Free Nanocrystals[END_REF] This mechanism requires the NPs to be free to move, as in solution, which explains while this mechanism is found to occur in wet chemical syntheses. However, oriented attachment can also occur between NPs of different crystalline structure, the only condition is for the two "daughter"

NCs to have facets with similar sizes. During the OA mechanism, several steps are involved, presented in Figure 7. First, the NCs need to diffuse and be in close vicinity to each other.

Then, because the diffusion can be assimilated to a random walk process driven by Brownian motion, the orientation of the NCs is not necessarily coherent and OA cannot happen. Thus, the rotation of the NPs is needed for the NC to align with its neighbor and decrease the graingrain boundary energy. Finally, the desorption of ligands at the interface and the attachment of NCs yield a new crystal. Kinetic studies seem to show that the limiting step is the diffusion of NCs. [START_REF] Zhang | A Multistep Oriented Attachment Kinetics: Coarsening of ZnS Nanoparticle in Concentrated NaOH[END_REF] Figure 8 shows in situ transmission electron microscopy images of the OA mechanism for iron oxide nanoparticles. The attachment of NCs does not always occur with the exact same crystalline orientation: two NCs can attach with slightly different orientations, but that still decrease the overall energy. Such misorientations can lead to the incorporation of defects (such as twin defects, Figure 9) in the new crystal. This allows to combine different mechanisms for anisotropic growth. [START_REF] Penn | Imperfect Oriented Attachment: Dislocation Generation in Defect-Free Nanocrystals[END_REF][START_REF] Penn | Oriented Attachment and Growth, Twinning, Polytypism, and Formation of Metastable Phases: Insights from Nanocrystalline TiO2[END_REF] Adapted with permission from ref. 76. Copyright (1998) Mineralogical Society Of America.

Another class of material consists of naturally layered materials such as graphene or transition metal dichalcogenide structures. Here, the atoms are convalently bond to each other within one layer. The layers, however, are maintained together through weak van der Waals interactions only (Figure 10). In this case, the shape is directly governed by the Wulff construction, the challenge is thus to control the thickness of the nanosheets down to the monolayer. These materials and their colloidal syntheses are developed in part 5.

Defect induced anisotropy

Symmetry breaking during nucleation

Since bulk silver and gold have an fcc structure, the formation of anisotropic shapes is thermodynamically not favored. However, several works have reported anisotropic Au or Ag nanoplates or nanoprisms. [START_REF] Zeng | Successive Deposition of Silver on Silver Nanoplates: Lateral versus Vertical Growth[END_REF][START_REF] Xu | Facile Fabrication of Single Crystal Gold Nanoplates with Micrometer Lateral Size[END_REF] Therefore, symmetry breaking needs to occur during the formation of metallic nanoparticles. Given the nucleation-growth mechanism, the size, shape and crystalline structure of the initial nuclei play an important role in determining the shape of the final NC. Indeed, if the synthesis is performed under thermodynamic control only, the nuclei formed will obey the same rule and tend to minimize their surface free energy.

However, if, right after nucleation, they deviate from the thermodynamic equilibrium shape, it means there is a driving force that stabilizes other shapes. One of the reasons for such a deviation is that the synthesis enters kinetic control, instead of thermodynamic control.

Indeed, in practice, the synthesis is performed far from the equilibrium conditions, as several parameters such as a large surface-to-volume ratio, a high proportion of atoms at the edge or low difference in energy between different structures, strongly influence the formation of nuclei in solution. [START_REF] Mottet | Modeling Free and Supported Metallic Nanoclusters: Structure and Dynamics[END_REF][START_REF] Swaminarayan | Polycrystalline Surface Properties from Spherical Crystallites: Ag, Au, Cu[END_REF] These factors lead to the formation of defects and to polycrystalline nuclei then to symmetry breaking that will start the anisotropic growth.

Twin defects

The most common kinds of defects in face-centered cubic (fcc) structures are twin defects and stacking faults. [START_REF] Elechiguerra | The Role of Twinning in Shape Evolution of Anisotropic Noble Metal Nanostructures[END_REF] Twin defects are of particular interest in the shape control of NCs. In fcc structures, the atomic planes are organized in three different manners, and stacked according to the "abcabcabc"… sequence. If, after an "a" plane, the atoms deposit as if they were on a "c" plane (instead of "b"), and then follow the regular stacking, their sequence will be the following: "abcacbacb". The highlighted "a" plane is a mirror plane, or twin plane (Figure 11).

The reason that such defects are very common in fcc structures in the (111) direction is that the energy difference between the perfect crystal and the twinned one is very low (the coordination number of atoms in each layer is the same for the perfect crystal and for the twinned one). [START_REF] Elechiguerra | The Role of Twinning in Shape Evolution of Anisotropic Noble Metal Nanostructures[END_REF][START_REF] Personick | Making Sense of the Mayhem behind Shape Control in the Synthesis of Gold Nanoparticles[END_REF] The local symmetry around the twin defect is hexagonal close-packed (hcp);

for example, between the fcc and hcp arrangement of atoms in copper along the (111) direction, the difference in binding energy is only 0.01 eV per atom. Adatoms that attach to a surface can therefore easily switch from and fcc to an hcp structure, forming a twin plane. [START_REF] Sau | Nonspherical Noble Metal Nanoparticles: Colloid-Chemical Synthesis and Morphology Control[END_REF][START_REF] Sansoz | An Atomistic Perspective on Twinning Phenomena in Nano-Enhanced Fcc Metals[END_REF] As twin defects are good ways for the crystals to release surface stress, multiply twinned particles can also be formed. Figure 12: Schematic diagram of the formation of Au nanoplates. r1 corresponds to the reduction rate of Au 3+ that dictates the period of nucleation and the amount of Au 3+ in the subsequent growth stage. r2 is the rate of spherical nucleation, leading to spherical nanoparticles (with growth rate r4). r3 is the rate of twinned-particle nucleation, followed by rapid deposition of Au atoms on the sides of the nanoparticule, leading to nanoplates (with growth rate r5). Attachment of atom cluster is also possible via rate r6 (discussed below). Reprinted with permission from ref. 87. Copyright (2012) American Chemical Society.

Anisotropic growth due to twin defects

Singly-twinned nanoparticles

When a single twin plane is formed, the resulting nucleus, at the early stage of growth, is a hexagonal plate due to the six-fold symmetry of fcc structures (Figure 13). Each side of the hexagon is either a concave side or a convex side because of the presence of a twin plane.

When atoms diffuse towards the nucleus and adsorb to grow the NC, they can either adsorb on a concave or on a convex face. On the convex side, each atomic site has only three closest neighbors, and the stabilization energy is therefore quite low. On the other hand, on the concave side, newly adsorbed atoms are stabilized by four neighbors from several facets, increasing the stabilization energy. Atoms will then preferentially adsorb on the concave sides, favoring the growth of the NC along these sides rather than the convex sides. This can be seen on Figure 13 where a singly-twinned nucleus is represented. The growth will be favored on the facets that present a concave-type surface while the convex-type surface does not favor adsorption of new atoms, and will remain unchanged. This can also be explained by the Gibbs-Thomson formula which states that the chemical potential is inversely proportional to the curvature:

∆𝜇 = 𝛾𝛺(1 𝑅 1 ⁄ + 1 𝑅 2 ⁄ )
where ∆𝜇 is the change in the chemical potential, 𝛾 the interfacial free energy, 𝛺 the atomic volume and 𝑅 1 and 𝑅 2 are the radii curvature. [START_REF] Xia | Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?[END_REF] For a convex surface where the curvature is positive, the chemical potential change is positive, and the adsorption of atoms is favored on a flat surface rather that a convex surface. On the other hand, for a concave surface where the curvature is negative, the change in the chemical is negative, and the adsorption of atoms on the concave side is favored.

Multiply-twinned nanoparticles

In the case of multiply-twinned nuclei, several cases can be observed. First, if a second twin plane is parallel to the first one, all the side of the nucleus will exhibit a concave and a convex structure, equalizing the stabilization energy for adatoms, leading to a homogenous growth along the six directions (Figure 14). The out-of-plane growth is however still not favored as atoms prefer to adsorb on concave surfaces rather than flat surfaces. One can further imagine the effect of extra twin planes parallel to the first one: triply-twinned nanoparticles will grow in the six directions, but with different growth rates as some sides exhibit only one concave-type surface while the others exhibit two. Those nanoparticles will yield non-equilateral hexagons as shown in Figure 15. Reprinted with permission from ref. 89. Copyright (2011), American Institute of Physics.

Second, because twin planes can also form in directions that are not parallel to each other, different morphologies can be obtained. Indeed, in fcc metals for example, {111} twin planes can form at an angle of 70.53°, while 2𝜋 5 ⁄ = 72°. This explains why the five-fold symmetry is often present in fcc structures. Thus, icosahedra and decahedra are often formed. [START_REF] Baletto | Microscopic Mechanisms of the Growth of Metastable Silver Icosahedra[END_REF] The remaining gap of 7.35° between two {111} facets is filled by the stretching of the bonds between atoms, which leave their low energy configuration but allow a more energy-favored arrangement of atoms at the surface (Figure 16). [START_REF] José-Yacaman | Defect Structure in Nanoalloys[END_REF] This stretching, however, is favored for small size of NPs. Calculations show that multiply-twinned Ag seeds are the more energetically favored. [START_REF] Baletto | Reentrant Morphology Transition in the Growth of Free Silver Nanoclusters[END_REF] An increase in the size of the NP increases the stretching of the bonds, decreasing the stability of the structure that was stuck in a local minimum of energy. [START_REF] José-Yacaman | Defect Structure in Nanoalloys[END_REF] The stability of the structure therefore depends on their size [START_REF] Baletto | Microscopic Mechanisms of the Growth of Metastable Silver Icosahedra[END_REF][START_REF] Baletto | Reentrant Morphology Transition in the Growth of Free Silver Nanoclusters[END_REF] but also on a complex interplay between the minimization of the surface energy (shape) and the minimization of the bulk energy (local atomic orientation). [START_REF] Reinhard | Size-Dependent Icosahedral-to-Fcc Structure Change Confirmed in Unsupported Nanometer-Sized Copper Clusters[END_REF] At small sizes, icosahedra are favored, and as the NP grows, decahedra become predominant at medium sizes, and finally thermodynamically favored Wulff polyhedra at large sizes. [START_REF] Xia | Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?[END_REF] Simulations performed in 2002 show that for Ag nanoparticles of small sizes (below 170 atoms), the icosahedral shape is dominant, then the icosahedral shape becomes predominant, and above 600 atoms, fcc structures compete with decahedra (Figure 17). squares are decahedra and triangles are fcc structures. Reprinted from ref. 94, Copyright (2002), with permission from Elsevier.

Inducement of twin defects and oxidative etching

The mere role of ligand templating cannot explain the anisotropic growth of NPs. The nature and shape of the initial seeds definitely play an important role, as seen previously. An interesting aspect is the selection of the initial seeds: why are the anisotropic seeds, less thermodynamically stable, selected to grow over spherical seeds? What induces the twin defects described above, and what is the mechanism that removes the other shapes of NPs to leave in solution up to 90% of anisotropic NCs? [START_REF] Chen | High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching[END_REF][START_REF] Lee | High-Yield, Size-Controlled Synthesis of Silver Nanoplates and Their Applications as Methanol-Tolerant Electrocatalysts in Oxygen Reduction Reaction[END_REF] Chen et al. [START_REF] Chen | High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching[END_REF] have developed a seedless growth of gold nanoplates that yields 90% of monodisperse Au seeds without any purification step. They used hexadecyltrimethylammonium chloride (CTAC) as the surfactant, and iodide ions that have already been observed to direct the anisotropic growth. [START_REF] Millstone | Iodide Ions Control Seed-Mediated Growth of Anisotropic Gold Nanoparticles[END_REF] In a mixture of CTAC, potassium iodide and sodium tetrachloroaurate solution, ascorbic acid is added and starts the reduction of Au 3+ ions to Au + . NaOH is used to regulate the pH. The obtained nanoplates have a thickness of ≈15 nm and their edge length can be tuned from ≈40 nm to ≈120 nm by changing the concentration of precursors. When the reduction of gold ions is sped up by using HAuCl4, no gold nanoplates can be obtained, showing the importance of the kinetics in obtaining anisotropic NPs (see 2.5). As without iodide no nanoplates can be obtained, it seems to play an important role in the formation of anisotropic structures. Millstone et al. suggested that iodide preferentially binds to {111} facets of Au nanoplates, favoring the anisotropic growth. [START_REF] Millstone | Iodide Ions Control Seed-Mediated Growth of Anisotropic Gold Nanoparticles[END_REF] However, iodide can also play the role of selective etchant to remove less stable NCs, leaving in solution only planar structures. Indeed, iodide can oxidize metallic gold nanoparticles following the mechanism: 96 Reprinted with permission from ref. 96. Copyright (2002) American Chemical Society.

4I -+ O2 + 2H2O = 2I2 + 4OH - I2 + I -= I3 - 2Au + I3 -+ I -= 2AuI2 -
Thus, iodide ions can have a dual-function as capping agent that directs growth and as etchant that selects planar structures.

A similar etching mechanism has been observed by Zhang et al. during the synthesis of silver nanoplates. [START_REF] Zhang | A Systematic Study of the Synthesis of Silver Nanoplates: Is Citrate a "Magic" Reagent?[END_REF] When AgNO3 is reduced with NaBH4 in the presence of trisodium citrate and poly(vinylpyrrolidone) (PVP), [START_REF] Métraux | Rapid Thermal Synthesis of Silver Nanoprisms with Chemically Tailorable Thickness[END_REF] spherical Ag NPs are obtained. However, after the addition of H2O2, the extinction spectra showed a shoulder at 500 nm corresponding to the formation of anisotropic structures. After the optimization of the nature and ratio of reagents, Ag NPs can be obtained with a yield of 100% (with succinate and citramalate replacing the citrate).

The proposed mechanism for such a high yield is that upon injection of NaBH4 silver nanoparticles are produced and stabilized by the citrate and borohydride ions. However, due to the presence of H2O2 that acts as a etchant, a dynamic equilibrium in solution is formed between the reduction of metallic ions by NaBH4 and the oxidation of metallic nanoparticles by H2O2 (following the equations: Ag 0 + H2O2 = Ag + + OH -+ OH • and Ag 0 + OH • = Ag + + OH -). 100

Therefore, the growth of the Ag NPs is hindered by the etchant. Over time, the NaBH4 is consumed and the stabilization of the NPs weakens, allowing for their growth. Due to the presence of the etchant, the resulting NPs exhibit lots of defects, including twin planes that in turn favor planar growth. H2O2 thus removes less stable NPs from solution and introduces twin defects in the remaining ones.

Although the proposed mechanism differs from the previous one, Yu et al. also found that H2O2 plays a crucial role in the formation of metallic nanoplates. 101 The equilibrium between the reduction by NaBH4 and the etching by H2O2 is still an important step. However, in basic conditions, where it exists under the form of HO2 -, H2O2 acts not as an oxidant, but as a reducing agent: Ag + + HO2 -= Ag 0 + HO2 • . The formed radical can dissociate into O2 that further oxidizes Ag NPs to form Ag + ions that can be used for the growth of NPs.

This oxidative etching can be detrimental to anisotropic NPs. Indeed, single crystals are more resistant to oxidative etching that twinned NPs that present at their surface some less stable features. This is somehow contradictory to the previous mechanisms, but it is in fact complementary. The oxidative etching needs to be controlled in such a way as to introduce some twin defects in the NCs but not dissolve them completely. It is possible to selectively eliminate singly-or multiply-twinned NPs from solution by using more or less reactive agents. [102][103][104] Getting rid of the oxidative species might be a solution to control the etching, either by bubbling to remove the oxygen or by using proper ligands that stabilize the surface efficiently. [105][106][107] The mechanism of anisotropic growth is therefore complicated, and strongly depends and the species in solution, in the environment (the oxygen present in air for instance) as well as on the speed of the reaction, the shape of the initial seeds and the ligand templating effect.

Kinetic control and selective passivation of nanoplate growth

As mentioned previously, the kinetics of the reaction also plays an important role in the shape determination of the NPs. Indeed, a change in the reduction rate of the metallic ions changes the shape of the metallic NPs. By speeding the reduction of Au 3+ ions with HAuCl4, nanospheres are obtained rather than nanoplates. [START_REF] Chen | High-Yield Seedless Synthesis of Triangular Gold Nanoplates through Oxidative Etching[END_REF] Similarly, iodide ions have been proposed

to bind to {111} facets, stabilizing the nanoparticles, and therefore decreasing the growth rate along the corresponding direction. 108 Binding strength of halide ions is the following: Cl -<Br -<I - , decreasing the available metallic surface that acts as a catalyst for the reduction of metal ions. 109 Besides, the reduction rate of Au + ions is strongly affected by the presence of halide ions in the growth solution, and the reduction rate of gold ions will be decreased with the use of a larger halide ion. [START_REF] Langille | Defining Rules for the Shape Evolution of Gold Nanoparticles[END_REF]110 The binding of halide ions to the surface of the NPs and their influence on the reduction rate of gold ions are critical in the kinetics of Au NPs growth. For example, the use of a high concentration of iodide ions will yield nanoplates, while the use of bromide yields single crystalline cubes (Figure 19 A-C). [START_REF] Langille | Defining Rules for the Shape Evolution of Gold Nanoparticles[END_REF] This is due to the strongly reduced reduction rate of gold ions with iodides: the growth of planar structures requires twin defects to be present in the NPs, defects that appear at low growth rate under kinetic control. Iodide, slowing down the reduction of gold ions, and limiting the availability of the NP surface, allows therefore the formation of twin defects.

This shows the importance of kinetic control in the growth of planar NPs. Slower reduction rate generally leads to more thermodynamically favored shapes with lower-energy facets while, with faster reduction rates, kinetically-controlled growth yields nanoparticles with a variety of shapes, with higher-energy facets. The limit between thermodynamic growth and kinetic growth can be determined by a critical growth speed: under the speed of deposition, the NPs are driven to grow into their thermodynamically favored shape (plates, given they present twin defects), while speeding the deposition rate forms more complex shapes, energetically unfavorable. [START_REF] Alloyeau | Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy[END_REF] The reducing agent used (ascorbic acid, NaBH4…), the environmental conditions (pH, temperature…), as well as the use of selective adsorbents on the metallic NPs can dramatically change the growth kinetics. Ascorbic acid is a milder reducing agent than NaBH4, and its reducing strength can be tuned through the change of the pH in the growth solution. [START_REF] Personick | Making Sense of the Mayhem behind Shape Control in the Synthesis of Gold Nanoparticles[END_REF]111 The selective passivation of a given facet also affects the growth. For example, the deposition of Ag atoms on the surface of gold NPs passivates and stabilizes these surfaces, slowing down their growth. This deposition depends on the amount of Ag + ions in solution, on the reducing agents used, and on the available surface of the Au NPs. Ag will deposit preferentially on the surface with a high coordination number, thus with as many Au atoms as possible. [START_REF] Personick | Shape Control of Gold Nanoparticles by Silver Underpotential Deposition[END_REF] (see Figure 19 D-G). The growth of planar nanoparticles is therefore not only controlled by the speed of the reaction, but also by a precise control of the stabilization ligands used for the synthesis (see part 3) and the species present in the growth solution. In the absence of Ag + ions, halide ions slow down the reduction of metal, slowing down the particle growth, while in the presence of Ag + ions, halide ions, that also bind to the surface of the NPs, destabilize the Ag layer, speeding up the growth of the NPs. [START_REF] Personick | Making Sense of the Mayhem behind Shape Control in the Synthesis of Gold Nanoparticles[END_REF][START_REF] Langille | Defining Rules for the Shape Evolution of Gold Nanoparticles[END_REF] Furthermore, the growth of planar structures requires the presence of twin planes, which form during the nucleation step: the shape of the original seed is another critical parameter for the determination of the shape of the final NP. 112 Selective passivation of facets during growth also allowed to synthesize even more anisotropic structures, such as Freestanding ultrathin metallic nanosheets (FUMNSs) which are only a few atomic-layers-thick. 113 For example, CO molecules strongly adsorbs on the {111} planes of Pd nanosheets, favoring growth in the perpendicular directions. 114

Ligand engineering

Ligand templating

Since the early 2000s, it has been shown that in a synthesis the right choice of ligands could tune the shape of nanocrystals, first leading to spherical, then to rods and to multipods and finally to 2D materials [START_REF] Yin | Colloidal Nanocrystal Synthesis and the Organic-inorganic Interface[END_REF]115,116 . In particular ligands as amines can dissolve metallic salts at room temperature leading to lamellar phases which could serve as template for the growth of nanocrystals. In addition, anisotropic crystal structures are more convenient to give rise to anisotropic shapes nanocrystals. In, 2006 Hyeon and co-workers have shown the synthesis of wurtzite CdSe nanoribbons [START_REF] Joo | Low-Temperature Solution-Phase Synthesis of Quantum Well Structured CdSe Nanoribbons[END_REF] then described as a templated-growth synthesis [START_REF] Son | Large-Scale Soft Colloidal Template Synthesis of 1.4 Nm Thick CdSe Nanosheets[END_REF] . These 2D nanoribbons or nanosheets are equivalent to quantum belts 117 and quantum platelets synthesized by Buhro and co-workers. 118 The syntheses of these materials are done below 100°C and consist of the reaction of a metallic salt dissolved in a primary amine (for example octylamine) or a mixture of a primary and a binary amine with a chalcogenide precursor such as octylammonium selenocarbamate or selenouera for selenium or trioctylphosphine tellurium (TOPTe) for tellurium. This is a soft template process. Indeed, the dissolution of a cadmium salt, either CdCl2 or Cd(Ac)2 (Ac = acetate), in an amine produces a lamellar aminebilayer mesophase confirmed by low angle X-Ray diffraction. These 2D nanoparticles exhibit a wurtzite structure with the thickness in the [11-20] direction, the length in the Concerning the understanding of the growth mechanism, CdSe remains the most attractive semiconductor and the reader can refer to the following reviews. [START_REF] Son | Colloidal Synthesis of Ultrathin Two-Dimensional Semiconductor Nanocrystals[END_REF][125][126][127] The cadmium precursor dissolved in amines forms anisotropically ordered layers which will serve as templates for the nucleation and growth of the wurtzite nanoplates once the precursors of selenium is introduced. Both Hyeon 128 and Buhro's groups 118 have spectroscopically observed at the early stage of the synthesis the formation of magic sized clusters with a majority of (CdSe)13 and (CdSe)34. The (CdSe)34 magic sized clusters are less thermodynamically stable than the (CdSe)13, thus after few hours or less, only the (CdSe)13 clusters are observed. These clusters are locked into stripes stabilized by the amines, which stack in the thickness and the width directions. If they are unbundled before the crystallization, these stripes will appear as sheets stacked along their width (Figure 20 d). The amines are weakly bound to the clusters, which favors the assembly of clusters up to the formation of nanoplatelets. Thus, these (CdSe)13 are then converted to 2D nanocrystals, either with an annealing step at higher temperature which stay below 100°C or with a longer time of reaction (up to 48h). After crystallization the quantum belts unbundle in their width direction but are stacked in the thickness direction. The addition of a long primary amine, such as oleylamine, and sonication allows the separation of the quantum belts. The as-obtained nanoparticles have a welldefined thickness of few monolayers. The top and bottom surfaces of these platelets are nonpolar with an equal number of cadmium and chalcogenide atoms and are stabilized by uncharged amine. An increase of the reaction temperature thermodynamically favors the synthesis of thicker nanoparticles.

More recently, Buhro and co-workers 124 have also shown that instead of using a pure primary amine, a mixture of a primary and secondary amine to dissolve the cadmium precursor allows the formation of CdSe quantum platelets with pure (CdSe)34 magic sized cluster as intermediate of the reaction at room temperature. The (CdSe)34 clusters can also convert to the more thermodynamically stable (CdSe)13 by lowering the temperature to 0°C, but then the annealing temperature should be higher to crystallize them in quantum platelets.

These precisely defined clusters can be achieved with different materials leading either to quantum platelets 120 or to quantum nanowires such as ZnTe. 129 These quantum wires exhibit similar optical properties as quantum platelets with well defined absorption peaks and unusually thin transitions. Wurtzite quantum platelets and zinc-blende nanoplatelets exhibit the same optical features which will be treated in section 3.2. As original objects, we can cite the heavily doped Mn:CdSe nanoribbons resulting from the crystallization of doped (CdSe)13 magic-sized cluster which can include up to 2 Mn atoms per cluster, meaning the clusters have for composition (Cd13-nMnnSe13) with n=0, 1 or 2. 128 All these nanoplates present a wurtzite structure but more recently it has been shown that PbS nanosheets which present a rock-salt cubic structure could also be synthesized by this soft template process. 120

The case of zinc-blende cadmium chalcogenides nanoplatelets

Cadmium chalcogenides nanoplatelets are one of the most successful syntheses among 2D colloidal systems. 130 These 2D nanoparticles 127,131,132 have been named nanoribbons, nanobelts, 118 nanosheets, nanoplatelets [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF] (NPLs) by the different groups. 133 What makes them appealing is their anisotropic growth controlled at the atomic scale. In these NPLs, the quantum confinement only occurs in one direction, since the lateral extension is typically larger than the Bohr radius (7nm for CdSe). Moreover, there is no roughness along the confined direction. This latter property provides the NPLs with exceptionally narrow optical features. The absence of roughness explains that there is no inhomogeneous broadening. As a result, the spectrum of a single NPL overlaps with the spectrum of the ensemble measurement, while the full width at half maximum can be as low as 7 nm.

CdSe NPLs have been investigated first. [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF] Their first reported synthesis begins with a mixture made of long-chain cadmium carboxylate (myristate or oleate), selenium powder and octadecene used as non-coordinating solvent. After the degassing step, the temperature is raised. The selenium powder typically dissolves around 150°C and, quickly after that, the solution turns yellow to pale orange. At this point a second metallic precursor is introduced under a solid form and capped with short carboxylic acid (acetate typically). Generally, cadmium acetate is used but the synthesis actually works with other cations. The time of the introduction of this short carboxylic chain is critical. It has to occur when some small nanocrystals are already formed, but before the formation of large QD.

The obtained nanoplates have a parallepipedic shape. The stoichiometry of the precursor also controls the shape of the lateral extension and the shape can be tuned from square (see Figure 21 a) to rectangle. Thanks to the perfect control of their thickness, they tend to stack under pile and can even self-assemble 134,135 into µm-scale needles. 136 The lateral extension also strongly depends on the thickness of the NPL. Thinner NPLs tend to present a larger lateral extension, up to the point where they are able to roll on themselves 132,137 (see Figure 21 b).

In spite of huge progress in the synthesis of the 2D NPLs, they are generally synthetized with spherical 0D QD as by-products. As a result, a selective precipitation is generally conducted. The general procedure takes advantage of the large lateral extension of the NPLs which tends to limit their colloidal stability. Addition of a limited amount of non-solvent (ethanol or acetone) in the crude mixture after the synthesis generally allows precipitating the NPLs while keeping the QD in solution.

To reduce the amount of solvent used during this cleaning procedure, electrophoretic sorting of the NPLs is also possible. 138 By applying a "large" electric field (400V over 1cm) on the solution resulting from the synthesis in presence of acetone, the NPLs get deposited on the positive electrode with a selectivity (defined as the ratio of the deposition rates) of 400 compared to QD. The most striking property of the NPLs is their exceptionally narrow optical feature (Figure 21 d), with full width at half-maximum (FWHM) as low as 7nm for a photoluminescence (PL) signal around 500nm. Thanks to their perfectly controlled thickness, no inhomogeneous broadening is observed and the PL signal of a single NPL overlaps with the signal obtained from an ensemble. 139 The homogeneous broadening is typically below 2kbT. For core only structures, we obtain PL efficiency of 20%. [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF] A general trend is that nanoplates with a smaller lateral extension tends to have a larger PL efficiency. We believe that such behavior result from the larger probability for the large object to include defect acting as a non-emissive recombination center. The NPLs present almost no Stoke-shift between the energy of the absorption and the energy of the emission (see Figure 21 d). As the thickness rises, the optical features get redshifted as expected from quantum confinement. [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF] However, due to the perfect control of the thickness with an integer number of layer, the optical features can no longer be controlled continuously as for spherical objects, see Figure 21 e. In the case of CdSe NPLs, different populations can be obtained with a first excitonic feature at 390, 460, 510 and 550 nm. Possibly, if the lateral extension is very small, some lateral confinement occurs and the peaks can be slightly blue-shifted. In these CdSe 2D quantum wells, the optical features have been attributed to the heavy hole-electron and light hole-electron transition. 140 The dynamics of the exciton is also strongly impacted by the 2D aspect of the nanoparticles.

Indeed, the typical PL decay time of CdSe NPLs is of a few ns at room temperature, which is one order of magnitude faster than for QD at the same wavelength and with similar quantum yield. At low temperature, the exciton lifetime get even shorter and can be as low as 150 ps. 139 From an experimental perspective, the absorption cross section and the extinction coefficient 141 have been recently determined and are of utmost interest for the determination of the NPLs concentration. Surprisingly the scale of the absorption seems to have a dependence with the surface which is superlinear.

Quickly after the synthesis of the CdSe NPLs, the procedure has been extended to other chalcogenides. 140,142 CdS nanoplates come with a large lateral extension. Compared to CdSe, their optical absorption is broader. The broadening results from the low spin-orbit coupling in CdS which tends to limit the splitting of heavy-hole and light-hole bands. As a result, the two transitions, which appear splitted in CdSe, are here only splitted by an energy smaller than their homogeneous broadening. Finally, the PL efficiency of pure CdS NPLs is low. CdTe nanoplates are the one with the narrowest excitonic features, down to 7 nm, while a value of 12 nm is more typical for CdSe NPLs. CdTe NPL are obtained using TOPTe as precursor and cadmium propionate instead of the acetate. Moreover, the CdTe NPLs are synthetized with the largest lateral extension up to 1µm (see Figure 21 c).

Thickness tunability

In this section, we will discuss the thickness tunability of zinc blende cadmium chalcogenides nanoplatelets. In this cubic structure, all the three directions x, y and z should be equivalent. Nevertheless, during the synthesis, the introduction of an acetate salt "breaks" the isotropic growth of nanocrystals and leads to nanoplatelets of 2, 3, 4 or 5 monolayers. The thickness of the nanoplatelets is in the [001] direction and a monolayer is defined as a succession of a plan of cadmium and a plan of chalcogenide in the z direction. The top and bottom surfaces are terminated by cadmium planes and are passivated by the oleate ligands (see Figure 6). When the nanoplatelets stack, the space between two nanoplatelets is of few nm 132,134 which corresponds to two interpenetrated oleate ligands.

Although the optical properties of the CdSe zinc-blende nanoplatelets and the wurtzite quantum platelets are similar, the growth conditions are different. While for the wurtzite nanoparticles magic-sized clusters are intermediate of the reaction (discussed in section 3.1), such clusters have not been highlighted as intermediate for the zinc-blende nanoplatelets. 130 If the growth of zinc-blende nanoplatelets occurs via the formation of some magic-sized clusters, they react much faster than they are produced and it remains difficult to isolate them.

Thus, the growth of zinc-blende nanoplatelets can be seen as a two-step process: (1) nucleation of nanocrystals and ( 2) growth of nanocrystals up to some sizes which are seeds for nanoplatelets, followed by the lateral extension of these seeds leading to nanoplatelets initiated by an acetate or a propionate salt 130 (Cd(prop)2) (see Figure 22 a). During the growth of the nanoplatelets, the exciton absorption spectra continuously red shifts as the nanoplatelets grow, expressing the weakening of the quantum confinement from 0D nanocrystals to 2D nanomaterials. It should be pointed out, that in 2014 Asaula and coworkers 143 have synthesized CdSe nanoplatelets with pure cadmium octanoate in the presence of selenourea without using a mixture of a short and a long carboxylate chain.

Nevertheless, the crystal structure of the synthesized nanoplates remains unclear. In the following discussion we will mostly focus on the syntheses with mixture of two carboxylate chains. Unfortunately, the thickest nanoplatelets cannot be obtained the same way, except for CdS as it would be explained in the next paragraph. 144 In the following discussion, we will mostly focus on CdSe nanoplatelets, a model material in the II-VI semiconductors. In order to synthesize 4MLs and 5MLs nanoplatelets, it is necessary to overpass a limit size of nanocrystals, the seeds of nanoplatelets, to grow thickest nanoplatelets at the expense of thinnest. A simple way to describe the synthesis of thicker-than-3MLs zinc-blende (ZB) nanoplatelets is a homogeneous growth of "spherical" quantum dots which can reach critical sizes -the thickness of the nanoplatelets -followed by their lateral extension initiated by the introduction of a short aliphatic carboxylate chain salt such as cadmium acetate or cadmium propionate. Thus, for 2MLs, the seeds size would be 7.6 Å, and for 3MLs (resp. 4MLs and 5MLs)

it would be 10.64 Å (resp 13.7 Å and 16.74 Å). Figure 23 shows the ideal crystalline seeds for the 3MLs, 4MLs and 5MLs nanoplatelets exhibiting both {100} and {111} facets. During these syntheses, the nucleation rate should be low enough to keep in solution enough precursors for the lateral extension. Then, one would expect that with bigger nanocrystals seeds, it would be possible to reach thicker-than-5MLs nanoplatelets. Nevertheless, these kinetics considerations do not take into account the crystal structure and the stability of the facets.

And, during the growth, a crystal minimizes its surface energy which also depends on the ligands and their ability to stabilize facets. Regarding the packing density and the number of unsaturated atoms of the {111} and the {100} facets, the surface energy of the {100} facet is smaller than the {111} facets, thus the growth of atoms on these latest is favored. 145 On the other hand, the calculated values of the adsorption energy of oleic acid on both plans show that oleic acid would stabilize {100} plans less than {111} plans. 145 Thus, depending on the temperature of reaction and the ligands, it is in some points possible to tune the shape of the nanocrystals. Here, during the synthesis of nanoplatelets, a short carboxylate aliphatic chain (less than three carbons) with a low boiling point is introduced. The acetate or propionate salt will lower the influence of the facet stabilization energy by the ligands, due to a fast kinetic of adsorption and desorption with short chain ligands. This will favor the growth of nanocrystals with low-index crystallographic planes such as the {001} facets. The nanoplatelets are indeed exhibiting the top and bottom facets as cadmium rich {001} facets stabilized by oleate ligands, not acetate ligands which are too small to stabilize NPLs in non-polar solvent. Nevertheless, it does not explain the breaking in the isotropic growth which, instead of giving cubic nanocrystals, leads to nanoplatelets. For the four existing populations of nanoplatelets (2 to 5MLs), the seeds diameter would be inferior to 1.7 nm with a number of atoms inferior to 200 with more than 30% of them on the surface (see Figure 23). For the two thickest populations of nanoplatelets, the synthesis requires a first step of growth without the acetate (resp.

propionate) salt, where spherical zinc-blende nanocrystals are obtained with mostly cadmium rich surfaces, since cadmium is in excess and the only ligands are oleate which can mostly bind cations. Once the acetate salt is introduced at high temperature (superior to 190°C), equilibrium of exchange may happen on the different facets between acetate and oleate.

Presumably, regarding the oleate steric hindrance compared to acetate, acetate capping nanocrystals would mostly exchange with oleate on two opposite facets. With a simultaneous precursor reaction, it can lead to the growth of nanoplatelets. Given this fact, it could explain the difficulty to synthesize nanoplatelets thicker than 5MLs. If the growth conditions imply first spherical nanocrystals, they will typically exhibit both {100} and {111} facets and the larger the nanocrystals, the larger the surface of these facets. By supposing that the seeds of 6MLs nanoplatelets are 2 nm nanocrystals (6.5 monolayers x lattice parameter of 0.304nm = 1.976nm), the probability to exhibit the {100} facets in all the six directions is higher than for smaller nanocrystals. And despite the introduction of the short ligands, two facets perpendicular to each other may be stabilized by the oleate thus blocking the anisotropic growth. This limitation may be overpass with a better control of the syntheses.

It should be pointed out that Peng and co-workers 144 have successfully synthesized pure CdS nanodisks with similar optical properties as nanoplatelets 140 of four pure populations. In these syntheses, all the precursors including cadmium acetate dihydrate, sulfur and a fatty acid, are conjointly introduced in octadecene in a three-neck flask and are heated to various temperatures under inert atmosphere. In addition to the annealing temperature and reaction duration, the choice of the fatty acid is a key factor to control the thickness of the nanodisks.

Thus, as for other materials the two thinnest populations of nanodisks could be synthesized in mild conditions with various fatty acids of moderated hydrocarbon chain length from C14 (myristic acid) to C18 (oleic acid). On the other hand, the only way to get the pure thickest population seems to be the use of a fatty acid with an unusually long hydrocarbon chain: the melissic acid (C30). This observation is in accordance with the fact that thick CdSe nanoplatelets are better stabilized by longer chains. [START_REF] Li | Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects[END_REF] Thus, if the synthesized nanodisks are too thin, they will tend to grow in the thickness to reach a better stabilization with the ligands present in the reaction mixture.

Colloidal 2D heterostructures

NPLs combine very narrow optical features with fast PL decay, which makes them appealing candidates for next generation quantum emitters, but also for more applied perspectives such as their use as display phosphors or for lasing. In this sense boosting further their luminescence yield is of utmost interest. For spherical objects, the well-established strategies is the in situ growth of an heteostructure. 146 With type-I and quasi-type-II band alignment (see Figure 24 a and b), the exciton wavefunction is more confined within the core and has less opportunity to visit the surface defects. As a result, the PL efficiency can be boosted. Alternatively type-II band alignment (see Figure 24 c), provides the exciton with a mixed material character and in particular allows shifting of the optical features below the band gap of each material. In this section we discuss the specific growth method of the 2D colloidal heterostructures. The latter can be obtained either thanks to isotropic growth leading to core/shell structures or only in their plane to achieve core/crown objects. With the growth of the shell, a significant redshift is observed resulting from the decrease of the confinement direction, i.e. the thickness. Current core/shell structures based on cadmium chalcogenides present a red excitonic feature (630-670nm). Their PL efficiency can be as high as 80%, 148 however this method suffer from a limited reproducibility. As the shell is grown, the linewidth of the PL emission rises but remains between 19 and 25 nm

(depending on the nature of the shell and the thickness) which is still below the value obtained for spherical QDs (see Figure 25 a and b).

The previous approach only offers a limited tunability on the thickness shell. Another low temperature approach has been developed to offer as for SILAR 149 (Successive Ionic Layer Adsorption and Reaction) the possibility to grow the shell layer by layer. The procedure has been named colloidal ALD. 150 Similarly to the atomic layer deposition, the process is selflimited. In this case, the growth is conducted in polar medium, which allows the direct use of S 2-precursor as sulfide source. The growth relies first on the phase transfer of the NPLs from a non-polar phase where the NPLs are suspended with oleate ligands to a polar phase (dimethylformamide as solvent) where the new ligands are typically S 2-or SH -. The ligands react with the nanocrystal surface which comes with a change of color resulting from the partial extension of the carrier wavefunction in the newly formed sulfide layer. After the washing of the excess ligands, some cadmium precursor is added in the polar phase; again, the color of the reaction medium redshifts. The reaction can be continued by successively introducing the Cd and S precursors and washing the unreacted precursors between each step. Compared to the previous method, this procedure allows the fine tuning of the shell thickness. Moreover, this method allows tuning the surface from cation-rich to anion-rich and thus to adjust the surface gating. In particular, this method has been used to tune the nature of the majority carriers (electron or hole) in thin-film transistors based on narrow band-gap transistors. 151,152 This method has also been extended to other chalcogenides such as Se 153 and Te, however selenide and telluride precursors are not available under a commercial and stable form. Thus a preliminary reduction step of the chalcogenides by a strong reducing agent (sodium borohydride) is necessary. Moreover the growth of the shell needs to be conducted air free.

These low temperature growths of the shell have led to improved performances in terms of PL efficiency. However there is still a need to develop a high temperature shell growth to obtain more reliable syntheses and a lower density of defects, leading to even higher PL yields.

Core-crown

While the growth of a shell is isotropic and occurs over all the facets of the NPLs, the 2D shape allows another growth mode occurring only in the plane direction. [START_REF] Cassette | Colloidal CdSe/CdS Dot-in-Plate Nanocrystals with 2D-Polarized Emission[END_REF]156 In this case we rather obtain core -crown objects 154 (see Figure 24 e).

The reaction is conducted in a medium very similar to the one used to grow the core and which has to favor the 2D growth rather than secondary nucleation. In a typical reaction, a mixture of cadmium oleate and acetate is first prepared. Then the core NPLs are added and this step is followed by the quick addition of the S precursor (sulfur in octadecene, or S-ODE).

The core NPLs are used as nucleation sites and a surrounding CdS layer is formed. The lateral extension of the crown is controlled via the amount of introduced precursor and can be tuned from a few lattice parameters up to 100 nm. Core/crown CdSe/CdS NPLs present a fairly high PL efficiency (60%), while maintaining their narrow emission linewidth (less than 15nm).

The CdS crown is actively absorbing and the absorption spectrum presents a typical increase around 500 nm due to the CdS (see Figure 25 c). However the PL signal remains driven by the CdSe core and no PL coming from CdS is observed (see Figure 25 c). This actually results from the fast transfer of the hole to the CdSe due to more favorable confinement, which is followed by the transfer of the electron due to the large binding energy of the exciton in the NPLs. As a result, the CdSe core behaves as the emissive recombination center.

The core/crown heterostructure has been extended to type-II band alignment. 155,157 In this case, the crown is made of CdTe. Te under the form of TOP-Te is used instead of the S-ODE. Thanks to the band alignment between CdSe and CdTe the PL signal can be pushed further than the bulk band gap of each material (800nm) (see Figure 25 d). Due to the reduced wavefunction overlap, the electron being localized within the CdSe and the hole within the CdTe, the recombination of the electron hole pair become less likely to happen and long lifetime are observed (200-300ns).

Alloying and doping:

Alloying

Because of the perfect control of the thickness in the cadmium chalcogenides nanoplatelets, there is no inhomogeneous broadening of the optical features. However this comes at the price of the loss of continuous tunability of the optical properties (see Figure 21 e). To recover this tunability of the optical features, alloying of the CdSe NPLs with S 158 and Te have been proposed.

Alloying with sulphide is used to obtain tunability between the blue and the green (see

Doping

The control of the optical and electrical properties of the NPLs also comes through their doping. So far no work has studied the electrical doping of the NPLs. On the other hand, magnetic doping by Mn 2+ has been reported using wurtzite CdSe nanoribbons. 128 The introduction of the cation is somehow tricky since acetate precursor will not lead to Mn incorporation but the acetate will rather be used as ligand. It was proposed to introduce the manganese ions under MnCl2 form and their incorporation has been pushed up to 10% where it saturates. The manganese ions are indeed located in the bulk of the structure rather than on the surface as evidenced by the manganese phosphorescence (see Figure 26 c).

3.6. Hybrid system

SiO2 encapsulation

The encapsulation of NPLs into an inorganic matrix raised some interest to ensure their surface passivation. Moreover, such protective coating is highly desirable in their use as phosphor for displays (see the discussion in section 7.2). One successful method achieved encapsulation of NPLs in a silica coating. 162 The synthesis allows a conformal growth of a thin (3-20nm) silica shell around the NPLs. To do so, tetraethylorthosilicate is used as the silicon precursor and the reaction is slowly conducted in a basic medium. Such functionalization may also be used as a spacer while trying to couple the NPLs to plasmonic structures. 163

Metal functionalization

The metal functionalization of the NPLs is of utmost interest to tune their transport 165,166 and photocatalytic properties. 167 It has been successfully reported with Au 117,164 , Pd, 164 Pt 164 and Ni. 167 The general method is close to the one developed to functionalized nanorods. 168,169 It typically relies on the metal salt reduction in presence of amine used as metal ions reducer and some ligands (didecyldimethylammonium bromide). The reaction occurs at temperatures as low as room temperature. The tips are typically located on the edges of the NPLs (see Figure 27 c-e), and come under the form of small metallic bids. Their size typically ranges from 2 to 10 nm.

Self-assembly

While the previously described methods to induce anisotropy rely on molecular precursors, self-assembly differs from these methods in the sense that the elementary brick is far larger than the atomic lattice. In this section, we describe the growth of large structures obtained by assembly of colloidal nanocrystals. [170][171][172] Here we voluntarily discard the "usual" self-assembly of nanocrystals which has been discussed in several reviews [173][174][175] to focus on the self-assembly occurring during the synthesis and leading to an epitaxial connection between the nanocrystals. We in particular focus on lead chalcogenides (IV-VI compounds)

which are typically obtained with this growth process. 176

The case of lead chalcogenides self-assembly

Leads chalcogenides compounds are known to be able to form a broad range of nanocrystal shapes from 0D spherical objects to nanocubes, 177 wires 178 and stars 179 (see Figure 28 a-d). The different shapes are obtained by tuning the reaction conditions (temperature, coordination of the solvent, presence of coligand), but they actually all result from the assembly of small nanocrystals. While all shapes and dimensionalities are accessible with this material, 2D sheets have been one of the last to be obtained 180 (see Figure 28 e andf).

The synthesis of 2D sheets of PbS relies on two critical parameters 175 : (i) first the reaction is conducted far from stoichiometry. There is a lead excess compared to sulfide 181,182 The obtained sheets have a tunable thickness 180 from 2 to 50 nm 185 typically. Their lateral extension is generally quite large 184 from 200 nm up to 10 µm (see Figure 28 f). However, their surface is not as well controlled as for cadmium chalcogenides nanoplates obtained with the ligand templating method and the optical features are even broader than for quantum dots of the same size and composition. The PbS nanosheets also present PL signal typically in the near IR 186 and telecom wavelengths. 182 The PL efficiency is around 6 % and can be raised to 11 % by growing a CdS 187 shell. The latter is obtained through a cation exchange process while the dots are heated in a solution of cadmium oleate at moderate temperature (60 to 100°C). PbS nanosheets have also attracted a large interest because of their large extension which make them good candidates for single nanocrystal electronics and for their use as transistors 184 and for photovoltaics. 188,189

Colloidal structure with 2-x dimensionality

Among the 2D class of colloidally prepared lead chalcogenides nanomaterial, some special networks need to be mentioned. By self-assembling small lead chalcogenide nanocrystals, square and honeycomb lattices can be formed. 190 They have been proposed almost simultaneously by the Hanrath's 191 and the Vanmaekelbergh's groups. 190 Since they are full of holes, their dimensionality 192 lies between 1 and 2. What makes such structure appealing is that for the first time it is possible to obtain a colloidal-nanocrystal-based self-assembly structure combing both order and high degree of coupling between nanoparticles. So far there was really two types of structures. On the one hand, it was possible to make self-assembly structures with high coherence length (up to mm), but due to the presence of long capping ligands, nanocrystals were not electronically coupled between each other. On the other hand, nanocrystals have been used under thin film form for transport, but in this case the main concern regards the local coupling rather than the long distance order. To achieve large coupling, ligand exchange toward short ligands is performed, 193,194 but it degrades the film quality by shrinking the volume of the film and forming cracks. This new type of assembly, combining order and coupling is appealing to conduct material engineering at the nanoscale and formed artificial crystals.

The most successful path to build such mesocrystal is to assemble the nanocrystals at a liquid-air interface. Among the large zoology of shape (Figure 28) that can be obtained by selfassembling lead chalcogenide nanocrystals, this method inherently induces anisotropy towards a sheet form. In a typical preparation, a PbSe nanocrystal solution dispersed in toluene with a sub µM concentration is deposited on the top of ethylene glycol. 190 The two liquids are not miscible and due to the lower boiling point of toluene, the latter quickly evaporate. It leaves nanocrystals on the top of glycol. At first the QDs entropically get order on a hexagonal phase, 195 where the ligands remain present. To observe the necking of the QDs, the bottom phase needs to have some affinity with the QD capping ligands 191 and it is generally admitted that air free conditions 190,191 are necessary to induce this assembly of the nanocrystals and their assembly into square and honeycomb lattice. The removing of the ligands from the nanocrystal surface will induce the fusion of their facets. 196 If the (100) facets merge, the square lattice is obtained (see Figure 29 c), while the honeycomb lattice results from the fusion of the ( 110) facets (see Figure 29 a and b). The latter phase is only obtained while starting from small nanocrystals (<5nm) and remains difficult to obtain. In this case, the nanocrystals are not all in the same plane and the structure is silicene like. This material has also attracted a huge theoretical effort for the modeling of its band structure. [198][199][200] The latter is intermediate between individual QD in which the states are not dispersive and the quantum well. Indeed the mesocrystal keeps presenting some confinement and the excitonic structure is only little affected 191 (see Figure 29 d). Among the theoretical predictions regarding these structures, there are miniband states, Dirac cones (for the honeycomb lattice) and topological gap, 198 ( see Figure 29 f). This material can thus be seen as an artificial graphene, with several advantages such as the presence of a band gap or heavier mass with respect to carbon leading to spin orbit coupling. Miniband state is also an appealing prediction to achieve in nanocrystal solid band-like transport [201][202][203] and their associated large Finally this platform of materials is not limited to lead chalcogenides since cation exchange can be conducted towards other cations while preserving the overall mesocrystal structure.

Demonstration toward cadmium chalcogenides has already been successfully obtained. 204 Achieving cation exchange towards mercury is also highly desirable from an electronic structure point of view since the final object will benefit from the inverted band structure of the HgTe 205 compound and can possibly generate some topological non trivial states 198,200 .

Colloidal synthesis of chalcogenides with a layered crystalline structure

Chalcogenides materials possessing lamellar crystal structures have been the subject of numerous studies over the past few years. Such materials, including the transition metal dichalcogenides (TMDCs) exhibit a broad range of properties, from metals to insulators, semiconductors, superconductors, semimetals… rendering them particularly attractive for numerous applications: bio-labeling [START_REF] Chen | Two-Dimensional Graphene Analogues for Biomedical Applications[END_REF] , photothermal therapy 206 , transistors 207 , lasing 208 , optoelectronics [209][210][211] , thermoelectrics, electro-and photocatalysis 212,213 , photovoltaics 214 , or spintronics. Furthermore, similarly to graphene, the electronic properties of these materials can undergo drastic changes when studied at the monolayer scale. For example, the indirect band-gap semiconductors MoS2 and WS2 become direct when reduced down to the monolayer.

Liquid phase exfoliation

The initial explorations of the electro-optical properties of monolayers have been made

possible by the use of the now well known "scotch tape method" (initially used to prepare graphene sheets [START_REF] Novoselov | Two-Dimensional Atomic Crystals[END_REF] ) to physically exfoliate monolayers from microscopic crystals. The obtained layers are generally quite large and of good crystalline quality. This method has been used on numerous lamellar materials due to its ease of use and the quality of the obtained layers, but this approach is inapplicable for the preparation of large quantities of monolayers. Alternative liquid phase exfoliation strategies have then been developed to achieve larger scale production of colloidaly stable nanosheets. Liquid exfoliation of layered materials is discussed in great details in the eponymous review [START_REF] Nicolosi | Liquid Exfoliation of Layered Materials[END_REF] and will only be briefly described here.

We can distinguish two different main approaches: mechanical exfoliation and exfoliation driven by intercalation. Coleman and coworkers recently demonstrated that mechanical exfoliation of layered materials powders dispersed in a solvent is possible when the solvent surface energy is similar to that of the exfoliated sheets. 215,216 In this case, the reaggregation is avoided, leading to stable dispersions of nanosheets in liquids. This strategy has been successfully applied to a broad range of materials (graphene, BN, MoS2, WS2, MoSe2, MoTe2, Bi2Se3, TaS2, SnS2, GaS…). [START_REF] Nicolosi | Liquid Exfoliation of Layered Materials[END_REF]215,217,218 One can distinguish two main ways to provide the necessary energy to exfoliate the powders. The first demonstrated one has been to sonicate powders in a suitable solvent such as N-methyl-pyrrolidone (NMP) 216,219 . More recently, the same group used a shear exfoliation strategy to prepare the nanosheets. 220 This technique is very promising for large-scale production, as the synthesis has been demonstrated using very simple apparatus such as kitchen blenders. 221 Diverse solvents are suitable for these mechanical exfoliation strategies in liquids and depend on the material to exfoliate 215 . For example, MoS2 is most easily exfoliated using NMP or N-Cyclohexyl-2-pyrrolidone (CHP). 215 Such mechanical exfoliation in liquid phase can even take place in pure water if heated enough to ensure the colloidal stabilization of the nanosheets. 222 The main drawback of mechanical exfoliation in liquid is generally the lack of dimension control of the exfoliated nanosheets: the dispersion does not contain monolayers only but a mixture of mono-and multilayers. Long exfoliation times can even lead to the formation of TMDCs "quantum dots" as exemplified for MoS2, MoSe2 WS2, WSe2, ReS2, TaS2 and NbSe2. 223 A widely used alternative to mechanical exfoliation is exfoliation through intercalation.

Lamellar materials can act as hosts for a variety of small molecules (generally ions), which can penetrate into the space between layers. The interlayer space expands, destabilizing the structure and renders the exfoliation through sonication easy. In the case of TMDCs, intercalation is often associated with simultaneous reduction of the layers especially when the intercalation involves alkali metals such as lithium 224 . Such reduction can strongly modify the resulting monolayer properties. For example, in the case of MoS2 and WS2, a phase transition occurs during lithium exfoliation, induced by the charging of the layers. The resulting monolayers exhibit a 1T crystal structure (octahedral coordination for the metal) instead of the usual 2H structure (where the chalcogen atoms surrounding the metal adopt a triangular prismatic geometry). This charge and crystal structure change profoundly affects the electronic properties of the resulting monolayers, 2H-MS2 are direct band gap semiconductors whereas 1T-MS2 exhibit a metallic character. The widespread use of these exfoliation methods is hindered by the dangerousness of alkaline metals containing intercalants such as nbutyllithium. Alternative intercalants have been explored to increase the exfoliation yields and render these protocols safer and more cost effective. 225 It is also possible to perform electrochemical lithium intercalation to increase the yield and quality of the obtained nanosheets. 226 The exfoliation by intercalation technique is a method of choice to obtain a high yield of monolayers, at the expense of a crystal structure change induced by the intercalation itself. This structure change is reversible by gentle heating (between 200 and 300°C for example in the case of MoS2). 227 Nanosheets obtained through liquid phase exfoliation are generally colloidaly stable. In the case of mechanical liquid exfoliation, the stabilization is insured by interactions with the solvent itself, the concentration of stable dispersions is therefore limited and pretty low (a few hundreds of mg/L). On the other hand, for exfoliation through intercalation, the stability is generally induced by the surface charges generated by the intercalation process. One can then obtain highly concentrated dispersions of nanosheets in polar solvent such as water. 222 Neither of these liquid phase exfoliation methods allows for a complete morphological control of the obtained nanosheets. The mechanical exfoliation produces a mixture of monoand multilayers nanosheets, whereas intercalation driven exfoliation induces crystal structure changes. Furthermore, none of these methods allow for a good control of the nanosheets lateral extension. These drawbacks have motivated the development of direct colloidal syntheses for the production of monolayered metal chalcogenides.

An alternative strategy combining advantages of both approaches has been recently developed as tandem molecular intercalation (TMI). 228 This strategy, tested on different TMDCs (TiS2, ZrS2 and NbS2 obtained via colloidal synthesis and commercial powders of WSe2, MoS2 and TiS2) consists of the intercalation of a mixture of short and long chains Lewis bases such as propyl-, butyl-and hexyl-amine or sodium ethoxide and sodium hexanolate. This exfoliation does not induce any crystal structure change of the monolayers but is more efficient for rather small (100nm) crystallites.

We will present in the following some successful strategies to directly synthesize colloidal stable lamellar metal chalcogenides nanosheets. A first part focuses on TMDCs, with emphasis on TiS2 as a prototype of IVB and VB dichalcogenides, then on molybdenum and tungsten dichalcogenides. After the presentation of some transformations, manipulations and applications of TMDCs colloidal nanosheets, other lamellar chalcogenides systems will be discussed, such as IV-VI and V-VI chalcogenides.

Colloidal synthesis of transition metal dichalcogenides

Column IVB and VB chalcogenides

The first attempts to produce size-controlled colloidal TMDCs nanostructures where driven by the will to study the quantum confinement effects in these materials. 229 Due to small excitonic Bohr radius, confinement effects arise for very small lateral sizes and cluster syntheses were developed. Such clusters were mainly produced by reactions in inverse micelles. In a typical MoS2 cluster synthesis, 230,231 MoCl4 is dissolved in a degassed inverse micelle solution containing octane as a solvent, hexanol as a cosurfactant and trididecylmethylammonium chloride as a surfactant. Clusters are immediately formed after H2S injection, and control over the cluster size is possible by changing the nature of the surfactant. Colloidally stable cluster dispersions are monodisperse in size and exhibit strong confinement effects as evidenced by absorption spectra displaying well resolved excitonic features, strongly blue-shifted compared to the bulk spectrum. STM measurements of the clusters indicate that their thickness corresponds to one S-Mo-S trilayer only. Similar synthesis has been reported in details for WS2 232 , MoSe2 and WSe2 233 , all of them reported as single trilayers, with sizes ranging from 2.5 to 8 nm diameter. Furthermore, some evidence has been given of an aggregative growth mechanism for MoS2 clusters, unstable 4nm clusters spontaneously evolve to form 8nm clusters over time. 234 TiS2 clusters have also been synthesized using the same inverse micelle approach, the reported sizes range between 2 and 10 nm. 235 Subsequently, different hydrothermal synthetic strategies have been described, giving generally access to TMDC nanosheets but without colloidal stability and control over size, shape or aggregation of the obtained nanostructures. [236][237][238] The nanosheets often selfassemble during synthesis to give flower like morphologies. 239 In the following we will mainly focus on organic-phase colloidal syntheses of TMDC nanosheets, which gives access to stable dispersions of nanostructures. Direct colloidal synthesis of larger TiS2 monolayers has been first reported in 2008, and consists this time in a heating-up process. 240 TiCl4 is dissolved in dried oleylamine by sonication. A sulfur/oleylamine precursor is prepared by dissolving sulfur powder into oleylamine at 110°C. Both precursor solutions are then mixed at room temperature, heatedup to 215°C, and reacted for 12 hours, leading to 50nm diameter TiS2 monolayer nanodiscs (Figure 31-1). Changing the sulfur stoichiometry and the reaction time allows for a lateral size control from 18 to 50nm diameter. The obtained monolayers have been reported to be unstable, and undergo rapid oxidation when exposed to air.

Syntheses carried out using the same precursors (sulfur and titanium tetrachloride) but using 1-octadecene as the solvent 243 lead to the formation of flower-like or flake-like nanostructures, consisting of numerous randomly oriented aggregated nanosheets.

Subsequent reported syntheses are mainly based on the use of oleylamine both as solvent and stabilizing ligand. The morphology of the obtained nanosheets depends on the experimental conditions: nature of the sulfur precursor, type of synthesis (mainly heating-up process or hot injection protocol), working temperature and annealing time. Obtaining welldefined morphologies and preventing aggregation in solution during growth has been the focus of subsequent studies. For example, a simple protocol modification reported by Kuno et al. 241 allowed them to synthesize much larger TiS2 nanosheets, with average lateral dimensions of 500 nm × 500 nm and thicknesses between 4 and 6 nm (Figure 31-2). The synthesis consisted in dissolving a slight molar excess of elemental sulfur in oleylamine, followed by the introduction of TiCl4 at room temperature and heating up the mixture at 300°C for 3 h. By changing the sulfur precursor to carbon disulfide (CS2) and using a hot-injection protocol at 300°C (directly injecting CS2 into a mixture of titanium(IV) chloride and oleylamine), Cheon group reported the formation of well-defined TiS2 colloidal nanodiscs 242 .

These nanocrystals are about 100nm diameter and a few monolayers in thickness (4 to 6 nm, Figure 31-3). The same group used a simillar colloidal synthesis strategy to explore the preparation of colloidal nanodiscs made of diverse transition metal sulfides and selenides from the IVB and VB columns: TiS2, ZrS2, HfS2, VS2, NbS2, TaS2 and TiSe2, ZrSe3, HfSe3, VSe2, NbSe2, and TaSe2. 245,246 Sulfides where systematically prepared using a hot injection protocol with carbon disulfide as the sulfur precursor whereas selenides are synthetized via a heating up process using elemental selenium as the selenide precursor. They also demonstrate that the use of elemental sulfur dissolved in oleylamine is a poor sulfide precursor for the formation of high quality nanodiscs as its reaction pathway generates reactive radicals degrading the crystal structure of the already formed nanodiscs.

Cheon and coworkers suggest still another sulfur precursor change and a modification of protocol from fast hot-injection to slow introduction or generation of the sulfide precursor. 244 The strategy here is to suppress the nucleation burst, which leads to multilayers nuclei and small nanodiscs. The so-called "diluted chalcogen continuous influx (DCCI)" has been demonstrated efficient for the synthesis of group IVB TMDCs monolayers (ZrS2, TiS2 and HfS2, Figure 32). In a typical synthesis, a mixture of metal chloride, oleylamine and dodecanethiol (DDT) is heated between 230°C and 245°C for prolonged amount of time (about 10 hours). At these temperatures, the DDT slowly decomposes to release hydrogen disulfide (H2S) during the whole synthesis time. The absence of a strong increase in sulfide precursors at the beginning of the synthesis prevents the fast nucleation of multilayers. At the same time, the small number of generated nuclei allows for the formation of large monolayer nanosheets, between 500nm and 1μm.

Column VIB chalcogenides, molybdenum and tungsten compounds.

We can retrace the same evolution in synthesis strategies for molybdenum and tungsten dichalcogenides. A first successful approach by Cheon and coworkers has been to sufidize WO3 nanorods 247 . The solvent is here hexadecylamine and the sulfidation is carried out by fast injection of CS2 at 330°C and annealing for 1 hour. This first synthesis produces bundles of WS2 nanosheets where the lateral dimensions are controlled by the initial length of the WO3 nanorods used as the tungsten source. A simpler one-pot strategy has also been reported for MoS2 and WS2, based on high-temperature decomposition of single source precursors in oleylamine at 360°C. 248 The authors used (NH4)2MoS4 and (NH4)2WS4 as precursors for MoS2

and WS2 nanosheets respectively. This synthetic scheme does not produce well-defined freestanding monolayers and the reaction products look slightly aggregated. Synthesis of colloidaly-stable WSe2 nanosheets was reported by Brutchey and coworkers for the preparation of conductive thin-films. The synthesis was carried out by the injection of di-tertbutyl diselenide (tBu2Se2) into a solution of WCl4 in dodecylamine at 150 °C under nitrogen.

The solution was then heated to 225 °C and held at this temperature for 6 h prior to quenching.

In this case, the generated products are diverse in shape. TEM images show the presence of nanosheets, onion structures and aligned plates. 249 et al. in 2015. 251 This time, instead of playing with the precursors introduction to control the nucleation and growth events -then the production of monolayers-they explored the influence of the solvent (acting also as a ligand) on the morphologies of nanocrystalline WSe2.

They systematically changed the solvent in a one-pot reaction where W(CO)6 is reacted with diphenyl diselenide (Ph2Se2) in oleylamine, oleyl alcohol, or oleic acid (ligands having the same number of carbon atoms, but different terminal functional groups) at 330 °C under an Ar atmosphere for 12 h. Nanostructures obtained in oleylamine are small (5nm) multilayer ( 4 Even if the exact formation mechanism to obtain colloidal TMDC monolayers is still unclear, it is possible to perceive some general guidelines. The protocols developed so far seek first to control the nucleation rate to avoid the formation of multilayers. This is achieved either by the slow injection of the precursors into the reaction mixture or the use of slowly decomposing precursors inducing a controlled release of active species over time. A second important parameter is the nature of the solvent, always coordinating the formed nanosheets to prevent aggregation during synthesis. It is then possible to favor the lateral growth through the use of suitable ligands selectively binding to the basal planes while keeping the edges available for the growth reaction. The colloidal synthesis of lamellar metal chalcogenide nanosheets is still in its infancy, numerous structures and materials have been exfoliated but not directly synthetized among the about sixty layered TMDC identified by Yoffe. 253

Transformations and applications

Colloidal TMDCs nanodiscs and nanosheets can act as building blocks to develop more complex architectures or directly find diverse technological applications. These first experiments demonstrate the particular reactivity of the edges compared to the basal planes in the case of two-dimensional lamellar dichalcogenides.

Such 2D-heterostructure synthesis via exchange of one of the elements of the dichalcogenide nanodisk can also be carried out with modification of the anion, in this case replacement of the sulfur by oxygen. 257 Depending on the oxygen source and the experimental conditions, the sulfur can be fully replaced by oxygen with conservation of the initial nanostructure morphology, leading to the formation of pure TiO2 nanodisks (Figure 34 j). In this case, the reaction is induced by bubbling oxygen gas into a hot oleylamine mixture (140°C)

containing the TiS2 nanodisks. The working temperature generates the reactive 1 O2 species.

On the other hand, when TiS2 nanodisks (5mg) are mixed with water (0.3mL) in toluene (3mL) at room temperature for time ranging from 12 to 60 hours, TiO2 gradually appears on the edges, ultimately forming pure TiO2 toroidal nanocrystals and completely consuming the initial TiS2 nanodisks(Figure 34 k).

Reactivity of TMDC nanosheets towards metal salts can be also used to generate numerous metal/TMDC nanosheets heterostructures. A simple route consists in mixing nanosheets, metal salts and a reducing agent, as demonstrated for the formation of MoS2 or MoSe2 nanosheets decorated with Au, Ag or Pt nanoparticles. 258 The reduction of the metal salt can be induced using chemical, microwave or thermal routes for example. 259 The noble metal decoration of TMDCs nanosheets has been demonstrated down to the monolayer.

Chemically exfoliated MoS2 can be decorated with Pd, Pt or Ag nanostructures using different reducing agents and surfactants depending on the metal deposited. 260 Gold nanoparticles can also be grown on chemically exfoliated monolayers without the use of any reducing agents, simply by mixing a water dispersion of MoS2 or WS2 monolayers with HAuCl4 solution in water. 261 This spontaneous gold reduction can be induced by the negatively charged nature of the chemically exfoliated monolayers. Alternatively, the TMDC nanosheets themselves can act as the reducing agents, allowing spontaneous gold reduction at the surface of the layers, even for the mechanically exfoliated 2H-MoS2 nanosheets. 262 A mechanism involving the oxidation of the molybdenum or tungsten atoms from +IV to +VI has been proposed in the case of WS2 nanotubes and fullerene-like MoS2. 263 Metal deposition on TMDCs monolayers can be controlled by a careful choice of the reducing agents, capping molecules and nature of both the nanosheet and the metal itself to induce an epitaxial growth of the metallic structure on top of the monolayer. 260 More complex phenomena have been recently observed on these TMDC/metal systems, leading to the formation of gold nanowires on MoS2 surfaces 264 , or to the synthesis of TMDC hybrid nanobelts (MoS2, TaS2, TiS2, WSe2 or TaSe2) decorated with Pt or PtAg alloy nanoparticles. 265 Dedicated reviews discuss in more details the synthesis of TMDC/metal heterostructures 266 and the formation of composites including nanosheets. 267 268 It is worth mentioning an alternative strategy to prepare TMDC-containing heterostructures where the two components are synthetized at the same time. Using a "onepot" synthesis, Zhang and coworkers successfully synthetized MoS2/CdS and WS2/CdS nanocrystals containing one TMDC monolayer on the (0001) surface of the wurtzite CdS nanocrystals. 269 The protocol consists in the injection of (NH4)2WS4 oleylamine precursor solution into a hot mixture of ODE, oleylamine, oleic acid and cadmium oxide. (NH4)2WS4

serves both as the tungsten and sulfur precursor.

Manipulations

The development of colloidal TMDCs nanosheets applications implies to master their manipulation: assembly, orientation, organization and thin film deposition for example. Some techniques have then been developed to specifically deal with colloidal nanosheets. Using solution-synthetized colloidal TiS2 nanodiscs as a model system, Cheon and coworkers reported two interesting phenomena. After synthesis, dispersions of TIS2 nanodiscs have a tendency to vertically stack in solution -forming aggregates-due to strong interactions between the basal planes. Under pulsed photoexcitation at high enough power to transiently modify the charge carrier distribution of the nanodiscs, the stacks disaggregate, forming stable dispersions of individual particles if the solvent is polar enough to stabilize them (Figure 35 a).

These light-induced dispersions are stable over time and the vertical stacks can be created again via a decrease of the solvent polarity. 270 The same TiS2 nanodiscs can also be vertically oriented on a surface using electric fields (Figure 35 b). Contrarily to other anisotropic nanocrystals such as nanorods, applying a continuous electric field does not result in orientation control of the nanodiscs in solution. The problem is circumvented by applying a DC square-wave electric field, the orientation occurs during the changes in intensity. 271 The charged nature of lithium-exfoliated nanosheets also gives access to electrostatic assembly strategies. For example, a layer-by-layer assembly strategy has been demonstrated using negatively charged MoS2 nanosheets and a polyelectrolyte (in this case poly(allylamine hydrochloride), PAH), allowing for the formation of MoS2/PAH multilayers on a substrate (Figure 35 d). After annealing and 2H crystal structure restoration, the multilayers MoS2/polyelectrolyte exhibit characteristic monolayer fluorescence, highlighting the successful electronic decoupling of the sheets by the polymer spacing. 273 Self-assembly can also be used to create complex structures in solution. It is for example possible to synthesize chiral nanofibers from exfoliated TMDC nanosheets in solution (Figure 35 c). 272 The structuration is induced in this case by strong stirring of a nanosheets solution (1500rpm) and stabilized by the presence of a non-ionic surfactant such as Pluronic P123, Triton X-100 or Tween 20.

Finally, stable dispersions of nanosheets can also directly be used as ink to prepare functional electronic devices. 274 Different ways of deposition have been explored, such as drop-casting, inkjet printing or vacuum filtration, all of them leading to working devices. 274

Functionalization

TMDCs have the unique property to form stable colloidal suspensions after exfoliation even without the use of any stabilizing agents such as surface ligands. 215 Being able to functionalize them with organic molecules can further increase their stability and add new functionalities to the nanosheets. The approach is different than for colloidal II-VI quantum dots, where the synthesis is carried out in presence of organic ligands. They remain at the surface of the nanocrystals, where they can be exchanged in a subsequent step. 275 In the case of TMDCs nanosheets, 276 as ligands interactions are weak on the basal planes of the sheets, the functionalization takes place only on defects and edges, or is mediated by a chemical transformation of the sheet itself to render it more reactive (under the charged 1T structure for example). The following ligand-functionalization techniques have been developed mainly for molybdenum and tungsten disulfides but could be applied on other TMDCs nanosheets as well. They often involve first a chemical exfoliation step using lithium intercalation, inducing charging and structure change of the exfoliated monolayers. A first successful ligand conjugation protocol has been developed by Dravid group, using chemically exfoliated MoS2 monolayers (Figure 35a-d). 277 Different water-soluble pegylated thiols (dissolved in water or methanol depending their structure) are simply shaken and/or sonicated with the purified exfoliated MoS2 monolayers for 24 hours. After purification by dialysis or centrifugation, the conjugated nanosheets exhibit ζ-potential change consistent with the ligand charge. The presence of ligands anchored on the nanosheets is also evidenced by FT-IR spectra showing characteristic ligands vibrations. Finally, strong differences in electrochemical hydrogen evolution reaction (HER) efficiency suggest that the ligands are bound on the edges sites, efficiently passivating them.

A second approach, developed by Voiry et al. is based on covalent functionalization. 279 The authors exploit the electron rich nature of the 1T phase (MoS2, WS2 or MoSe2) to make it react with various electrophiles such as iodomethane, 2-iodoacetamide or aryl diazonium salts (in this case, 4-bromobenzenediazonium tetrafluoroborate, see Figure 35 e). A typical procedure consists of mixing an aqueous solution of purified lithium exfoliated TMDC nanosheets with 2-iodoacetamide in tenfold excess and let it react for 5 days at room temperature. The successful functionalization is characterized by XPS, FTIR and C-NM, highlighting the addition of amide or methyl moieties directly on the chalcogen layer.

Surprisingly, the functionalized monolayers keep a 1T structure but evolve from metallic to semiconducting, exhibiting a strong photoluminescence (Figure 35 f). The covalent functionalization of the 1T phase can at the same time stabilize the structure and tune the electronic properties of the monolayers. 280 Alternatively, it is possible to directly functionalize 2H-MoS2 nanosheets through metal salts coordination. 281 This strategy, initially reported for inorganic MoS2 fullerenes functionalization 282 is based on the use of a transition metal cation with high affinity for sulfur and octahedral coordination, coupled with a strong chelating ligand. The metal cation strongly binds to the TMDC surface and act as an anchor for organic chelating ligands. For example, copper (II) acetate can be adsorbed on MoS2 surfaces with a degree of functionalization as high as 50%. This adsorption is reversible and does not induce structural changes in the nanosheets structure.

Among the different exposed functionalization strategies, thiol conjugation is playing a particularly important role in the development of biomedical applications using nanosheets. [START_REF] Chen | Two-Dimensional Graphene Analogues for Biomedical Applications[END_REF] Employing dithiols as binding group (such as lipoic acid-PEG molecules)

increases the affinity of the ligand for the nanosheet surface. 278 Thanks to their broad light absorption in the visible and near-infrared regions of the spectrum, TMDC nanosheets have been first used as photothermal therapy agents. 283,284 Further studies have also demonstrated their potential as drugs delivery agents, 285,286 using their large surface area to load active molecules. Using direct synthesis pathways, metal-doped WS2 have been prepared to combine magnetic resonance imaging (contrast from the dopant) with X-ray imaging (high Zelements forming the nanosheet). 287 Furthermore, single TMDC nanosheets exhibit different affinities for single and double-strand DNAs and efficiently quench the fluorescence of organic molecules, making them promising platforms for sensing of DNA and biomolecules. [288][289][290] Biomedical applications of TMDC nanosheets are described in details in recent reviews. [START_REF] Chen | Two-Dimensional Graphene Analogues for Biomedical Applications[END_REF]291 The large surface area of TMDC nanosheets render them particularly attractive for diverse energy related applications. In catalysis, molybdenum and tungsten dichalcogenides have been demonstrated to be highly efficient electrocatalysts for hydrogen evolution reaction.

Active sites being located on the edges, 292 it is beneficial to work with small dimensions or disordered nanostructures. 293 Alternatively, the activity of the catalyst can be increased by phase engineering, the metallic 1T phase being more active in the case of MoS2 213 and WS2 212 nanosheets for hydrogen evolution. Different reviews deal in details with the use of TMDC nanosheets for hydrogen evolution reactions 294 or more generally for light-harvesting applications. 295 The large surface-on-volume ratio of colloidal nanosheets makes then also particularly promising for the development new energy storage materials. Colloidal WS2 247 and TiS2 296 nanosheets, obtained through direct colloidal syntheses have been studied as electrode materials for electrochemical storage through lithium intercalation. Both materials behave differently than their bulk counterparts with a much higher storage capacity. Similarly, chemically exfoliated 1T-MoS2 monolayers exhibit high volumetric capacitance of ≈700 F cm -3 , 20 times higher than the 2H crystal structure modification. 297 These promising results are generating a growing interest for the use of TMDC nanosheets in energy storing devices. 298,299

IV-VI lamellar chalcogenides

IV-VI semiconductors, such as tin or germanium sulfides and selenides can also crystallize into lamellar systems. Depending on their stoichiometry, they either adopt a Typical TMDC structure for SnS2 and SnSe2, crystallizing into a CdI2-type structure, whereas the monosulfides and selenides (GeS, GeSe, SnS and SnSe) crystallize into an orthorhombic structure as described in Figure 10.

SnS2 single layers can be prepared through chemical exfoliation in liquid formamide. 300 Hexagonal nanoplates (lateral dimensions of 150nm and mean thickness of 16nm) have been synthetized similarly to other TMDC colloidal nanosheets, employing thermal decomposition of tin(IV) diethyldithiocarbamate Sn(S2CNEt2)4 in oleylamine at elevated temperature (280°C). 301 Such SnS2 nanoplates can be also synthesized through hydrothermal 302 or solvothermal 303 protocols, with equally good results. The colloidal synthesis of tin diselenide (SnSe2) nanosheets has been comparatively less explored, protocols yielding phase pure nanosheets have been developed, mainly through solvothermal reactions. For example, reaction of SnCl2 and SeO2 in benzyl alcohol and PVP (polyvinylpyrrolidone) in an autoclave at 180°C for 16h yields hexagonal nanosheets of 1 micron diameter which are only 6 nm thick. 304 One of the problems of the successful development of such nanosheets is the possible existence of mixed phases SnSe-SnSe2. 305 Both germanium and tin monochalcogenides nanosheets have been prepared using solvothermal approaches. Colloidal nanosheets of GeS and GeSe have been synthesized using a "one-pot" protocol, where a mixture of GeI4, hexamethyldisilazane, oleylamine, oleic acid, and dodecanethiol (for GeS) or trioctylphosphine selenide (TOP-Se, for GeSe) are heated to 320 °C for 24 h. 308 The obtained nanosheets have lateral dimensions up to a few microns with thicknesses ranging from 3 to 100nm (about 5nm for GeS and more than 20nm for GeSe).

Similarly, SnSe nanosheets have been synthesized via a "one-pot" procedure, slowly heating a mixture of SnCl2, oleylamine, TOP-Se, and HMDS to 240 °C and holding for 30 min. 306 In this case, square-like nanosheets are obtained (see Figure 36 d) with lateral dimensions of about 400nm and thicknesses ranging from 35 to less than 10 nm, depending on the amount of Sn and Se precursors used. This protocol allowed the authors to study the formation mechanism of these SnSe nanosheets: the formation occurs via coalescence of individual nanoparticle building blocks to yield a single-crystal nanosheet (Figure 36 a-d). If enough precursors are available, thickness growth takes place through coalescence of nanoparticles at the surface of the existing nanosheet. The same material can be colloidally synthesized with a controlled thickness down to the monolayer, still using a "one-pot" heating-up reaction. 307 In a typical procedure, a mixture of SnCl4•5H2O, SeO2, oleylamine, and 1,10-phenanthroline is degassed, heated up to 260 °C and annealed for 30 min under N2 atmosphere. AFM measurements of the obtained nanosheets confirm their monolayer thickness of about 1nm (Figure 36 e-f)). It is worth noting that, without phenanthroline, 3D SnSe nanoflowers are obtained using the same procedure. In this case, the phenanthroline acts as a strong complexing agent, suppressing vertical growth and aggregation of thicker nanosheets arising during the synthesis of the nanoflowers. The formation mechanism investigation reveals again that an oriented attachment process leads to the nanosheets formation. The same group also reported the colloidal synthesis of SnS and SnS2 nanosheets using tin diethyldithiocarbamate phenanthroline complexes. 309 Similarly to tin selenide system, tin sulfide nanocrystals and nanosheets can be either obtained under the SnS or SnS2 stoechiometry, depending on the experimental conditions used. 310 Alternatively, SnS have been recently exfoliated using a liquid phase exfoliation protocol with N-methyl-2-pyrrolidone as a solvent. 311

Tetradymite colloidal compounds

Bismuth and antimony chalcogenides have attracted huge interest in the field of thermoelectric 312,313 materials because of their heavy mass. More recently, they have appeared as a model system for 3D topological insulators. 314 In this phase of the matter, the bulk material is expected to be an insulator, while surface conducting states are formed.

Bi2Te3, Sb2Te3 and Bi2Se3, are narrow band-gap semiconductors, with band gap value ranging from 0.2eV to 0.3eV. They have a rhombohedral structure with a lattice parameter of roughly 1 nm (quintuplet layer). As a result, they tend to grow under a layer form, with covalent bonds within the plane and where the quintuplets are stacked by weak Van der Waals interaction, as evidenced by TEM picture on Figure 37 b. Due to their lattice geometry, they form hexagonal plates in solution with a tunable lateral extension ranging from 100 nm to a few µm (see Figure 37 a). Their colloidal synthesis 316 has been mostly investigated using hydrothermal path in water [317][318][319][320] or polar solvents 315,321,322 as reaction media. More recently, a synthesis conducted in organic non-polar phase has been proposed 323 . The growth relies on the preparation of Bi and antimony oleate. The latter is mixed with a non-coordinating solvent (octadecene) and the reaction with TOP-Te occurs above 120°C. The obtained nanoplates have a thickness between 3 and 50 nm while the lateral size can be tuned with time and temperature. As for Cd based nanoplates, the synthesis has been extended to heterostructures in a core/crown 315 (Figure 37 c) and core/shell 323 (Figure 37 d-e) geometry.

The obtained plates tend to present some antistructural defects [324][325][326] where in the case of Sb2Te3, Te atoms replace some of the cations which tends to result in a p-type doping. 327

Bi2Se3 is generally n doped because of similar defects. Among the open questions regarding these materials is the control of their doping, which is a critical question for the observation of the topological surface states. In this sense synthesis leading to a better control of the stoichiometry of the compound is highly desirable.

Alternative growths and hybrid materials

While most of the methods to grow 2D anisotropic colloidal nanoparticle can be ranked among the four previously described mechanisms, we have identified several other alternative processes which can possibly expand the range of available 2D materials. Here we discuss the light induced anisotropic growth, the cation exchange and the growth of 2D perovskites.

Light induced methods

Although wet chemical methods have mostly been studied in the recent past, other synthesis methods exist that present some advantages. In the case of metal nanoparticles for example, direct chemical or thermal synthesis do not offer the same level of control as photochemical synthesis where the reaction of NP growth can be stopped only by removing the irradiation light. In such a synthesis, the excitation light (of a wavelength usually chosen at the plasmon resonance energy of the NP) is absorbed by NPs, and creates an electron-hole pair where the electron reduces the metallic precursors for further growth, while the hole oxidizes the ligands on the surface on NPs. 328,329 This method has been used to grow silver nanoprisms by Maillard et al. 329 The ligand used in this synthesis was citrate. Although the final size of the NPs can be controlled by the wavelength of the irradiation light (the NP grows until it stops absorbing light at this particular wavelength), the latter does not seem to play a role in the control of the thickness of the NPs, 330,331 suggesting that the final anisotropy of the NPs is not due to the irradiation, but to some intrinsic properties of the initial seeds or ligand templating, as described previously. [332][333][334] It has been observed that the illumination of citrate-capped Ag spherical seeds (<10 nm), in the presence of bis(psulfonatophenyl)phenylphosphane dihydrate dipotassium salt (BSPP), although a similar result was obtained with poly(vinylpyrrolidone) (PVP), 335 formed 100 nm-edge-long nanoprisms. 336 The mechanism behind this conversion from spherical NPs to nanoprisms can be explained by the selective aggregation of the seeds into small clusters, some of them with an anisotropic, two-dimensional shape. The latter are probably further stabilized by citrate ions whose binding to {111} facets is favored compared to other facets where ligands may be destabilized due to plasmon excitation. 337,338 The subsequent dissolution of the clusters allows for the growth of the nanoprisms at the expense of the clusters (see Figure 38). Light-induced methods are not limited to photochemical redox reactions. Indeed, light can also be used as a driving force towards oriented aggregation. Using a fluorescence tube, Ji et al. 337 have seen that a longer excitation wavelength produces larger Ag nanoprisms. By changing the wavelength from 450 to 750 nm, the edge of the nanoprisms increased from ~40

to 120 nm. 337 The authors explained this by an oriented attachment mechanism (see Figure 39) which could be suppressed using a secondary excitation beam with a properly chosen wavelength (see Figure 40). By choosing a secondary excitation wavelength for the quadrupole plasmon excitation, the interparticle fusion was stopped due to a reorganization of charges at the particle surface. The dipole plasmon excitation, however, which is also present when a fluorescence tube was used, favors aggregation. Finally, many other parameters have to be taken into account, such as pH 339 or temperature, that could also change the final shape of the nanoparticles in the light-mediated synthesis. 328 for six different-sized nanoprisms (1-6 edge length: 38±7 nm, 50±7 nm, 62±9 nm, 72±8 nm, 95±11nm and 120±14 nm) prepared by varying the primary excitation wavelength (central wavelength at 450, 490, 520, 550, 650 and 750 nm, respectively; width, 40 nm) coupled with a secondary wavelength (340 nm; width, 10 nm). (c) The edge lengths as a function of the primary excitation wavelength. (d-f) TEM images of Ag nanoprisms with average edge lengths of 38±7nm (d), 72±8nm (e) and 120±14nm (f). Scale bar applies to panels d-f. Reprinted by permission from Macmillan Publishers Ltd: Nature from ref. 337 , copyright (2003).

Cation exchange

In spite of huge progress over the past few years, some materials remain difficult to obtain under a 2D form. In this perspective cation exchange 340 is an interesting path to expand the range of available materials. The method was adapted to nanocrystals by Son et al. 341 It consists in introducing some small cations such as Ag + or Cu + , in the presence of nanocrystals.

Thanks to their small cationic radius, the ions can penetrate the atomic lattice of the nanocrystal. If the reaction is slow enough, the anion lattice remain unaffected while the initial cations get removed and replaced by the new cations. Since the Ag + and Cu + ions are strongly sensitive to oxygen, the reaction needs to be conducted in a glove box. Starting from a cadmium chalcogenides NPLs, such process leads to the formation of Ag2X or Cu2X, 342 as shown in Figure 41 c. The optical properties of the NPLs are strongly affected and switch from an excitonic feature to a plasmon resonance. A second cation exchange step can then be conducted back to the initial cation or towards another cation. The second cation exchange is not as easy: the new cation has to be introduced in a large excess and the solution has to be heated. Nevertheless it is possible to obtain with this process NPLs of ZnS and PbS 343 (see

2D perovskites

The rediscovery and fast development of the halide perovskite materials has been initially

induced by their successful use as sensitizers for DSSC solar cells. 344 After a few years of development only, the perovskite solar cells reach record efficiencies of about 20%. 345 Meanwhile, other potentialities of halide perovskites have been explored. Thanks to their high absorption cross-section, 346 near unity fluorescence quantum efficiency 347,348 and exceptional charge diffusion length, 349 halide perovskites, under bulk, thin films or nanoparticles shapes are a potential game changer in diverse applications such as light-harvesting, 350 photodetection 351 or light-emitting devices. 352,353 Halide perovskites are of general formula ABX3, where X is a halogen anion, A is generally lead or tin and B is an organic ammonium or a cesium cation. They can adopt diverse crystal structures depending on their composition and generally undergo several phase changes depending on the temperature. 354,355 While methylammonium lead iodide has been extensively studied over the past few years for photovoltaic applications, other compositions are scrutinized as well for different applications. In particular, if lamellar hybrid perovskites systems have been known for quite a long time, 356,357 the colloidal synthesis of two dimensional hybrid perovskites has only been achieved very recently. Lamellar hybrid perovskites can be obtained when B is an organic ammonium with a bigger carbon chain such as butylamine for example. Furthermore, by varying the ratio metylammonium/longammonium cations, it is possible to prepare different lamellar perovskites with a perfectly defined number of PbX6 -octahedra planes sandwiched by long ammonium ligands, of general formula (CH3(CH2)3NH3)2(CH3NH3)n-1PbnX3n+1 (n =1, 2, 3, and 4) for example 358 and behaving as multiple quantum wells. 359 We can either see their colloidal synthesis as similar to the preparation of colloidal nanosheets from materials with a layered crystal structure, or as a soft templating system. On can also point out the similarities of such syntheses with other hybrid lamellar semiconductor preparation schemes like the II-VI semiconductors diamines hybrid materials previously reported. These hybrids can be of diverse compositions: ZnTe 360 , ZnSe 361 , MnSe 361 , CdSe 362 , and the thickness of the inorganic layer can also be perfectly controlled through the reaction temperature 363 . Modifications of these syntheses led to the preparation of colloidal CdSe nanosheets for example. [START_REF] Son | Large-Scale Soft Colloidal Template Synthesis of 1.4 Nm Thick CdSe Nanosheets[END_REF] Different strategies have been reported to prepare and study these ultrathin hybrid perovskite crystals. As for other lamellar systems, a first way to obtain them is to mechanically exfoliate bulk lamellar crystals. For example, Baumberg and coworkers report the micromechanical exfoliation of (C6H9C2H4NH3)2PbI4 crystals and identify mono-and few-layers areas with an interlayer spacing of 1.6nm. Differences of behavior from the thinnest regions compared to the bulk are reportedly due to strain relaxation and ligands reorganization. 364 Similarly, Heinz and colleagues exfoliated (C4H9NH3)2PbI4 crystals but observed strong differences between the bulk and the nanolayers. Due to the change in dielectric environment for example, the exciton binding energy increases by 1/3, up to 490 meV. 359 Direct growth of hybrid perovskites on a substrate is also possible, simply by dropping a diluted solution of precursors on a hot substrate and let the solvent evaporate. Squared nanosheets of (C4H9NH3)2PbBr4 are obtained, with lateral dimensions of a few micrometers and thicknesses down to the monolayer. The observed band gap shift towards higher energies compared to the bulk is attributed to structural relaxations. 365 These methods reported above do not lead to colloidal nanosheets in solution and use lamellar hybrid perovskite systems to obtain monolayers. Direct colloidal syntheses, inspired by the existing ones forming isotropic nanocrystals, 366 have been recently reported for ammonium as well as cesium lead halide perovskites nanoplatelets.

A first approach uses a modification of the colloidal CH3NH3PbBr3 perovskite synthesis reported by Schmidt et al.,367 consisting in the reaction of PbBr2 with a mixture of CH3NH3Br and longer ammonium bromide in the presence of oleic acid and octadecene. Using octylammonium bromide and carefully purifying the resulting nanostructures, Tisdale group isolated 2D perovskite nanoplatelets of monolayer thickness. 368 It is also possible to tune the thickness of the perovskite nanoplatelets by varying the octylammonium / methylammonium bromide ratio. In this case, nanoplatelets of 1 to 5 PbBr6 octahedra planes are formed, allowing the authors to study the quantum confinement effects in these perovskite quantum wells. 369

A two-step procedure has also been reported, involving the synthesis of PbI2 nanocrystals which are subsequently infiltrated with ammonium ions, leading to colloidal nanocrystals of lamellar perovskite. 370

Cesium lead halide perovskites have also been synthetized under the shape of nanoplatelets, with a good control of the thickness of the plates. A first report by Alivisatos and coworkers describes the formation of nanoplates with thicknesses ranging from 1 to 5 unit cells, 371 using a modification of the colloidal synthesis of CsPbBr3 nanocubes by Protesescu et al. 348 These nanostructures are strongly luminescent (up to 84% QY) and their composition can be subsequently tuned using a cation exchange procedure, to prepare CsPbI3 and CsPbCl3 nanoplates. The same materials under the shape of nanoplatelets have also been reported by the group of Manna, 372 using this time a room temperature synthesis conducted in air, where a mixture of Cs oleate, oleylamine, oleic acid, PbBr2 and HBr is destabilized by injection of acetone, leading within seconds to the formation of nanoplatelets. In this case the thickness is simply controlled by the amount of HBr initially present in the mixture.

Such colloidal 2D nanostructures with extremely high quantum yield and narrow photoluminescence can readily find applications for the development of light-emitting diodes 373 or laser gain media. 347

Applications and perspectives

In the last section of this review we discuss some applications of the 2D colloidal nanocrystals. At first we discuss in which way the physical properties of 2D objects are affected by their dimensionality. We then review some of their use for optoelectronic, chemical and biological applications.

Before developing the different applications of 2D colloidal nanocrystals, it is needed to point out that the syntheses rarely yield only one population of NCs, and several sizes as well as several shapes can coexist in the reaction mixture. In order to get rid of the unwanted shapes of NCs, sorting of NPs has to be performed. Several methods exist to select one size or shape over the others. The first one is centrifugation. 130,374 Two-dimensional platelets of CdSe can be separated from spherical QDs by mere centrifugation due to their quite large difference in lateral size. This separation is possible thanks to the shape-dependent drag in the solution which gives different sedimentation velocities according to the ratio between effective mass and friction factor. 374,375 Size-selective precipitation can also be performed by adding a nonsolvent that will destabilize the largest NPs which can then be easily separated by centrifugation. 376 These methods are often limited by the small differences in the sedimentation coefficients of the different shapes of NPs, and by the necessary repeated centrifugation steps that make them time consuming. Capillary electrophoresis as well as size exclusion chromatography have also been used to separated different shapes of Au and Ag

NPs. The aggregation of NPs in the capillary and the chromatography column was avoided by adding a surfactant that also increased the resolution. The overall yield and throughput of these methods remain limiting for larger scale applications. 377,378 The addition of surfactants can be used in a different way to separate different shapes: indeed, the formation of micelles, above the critical micellar concentration, has been used to perform depletion-induced shape sorting. 379 When colloids (NPs) are separated by a distance smaller that the size of another species in solution (micelles), the formation of a concentration gradient produces an attractive osmotic pressure that pushes towards the aggregation and precipitation of NPs having the largest contact area. By playing on the concentration of the micelles, the attractive potentials between colloids can be discriminated for colloids of different shapes, and shape-selective precipitation can be performed. Finally, electrophoretic separation of NPs has been recently presented. 138 Directly after the synthesis, depending on the surface charge of the NPs, nanoplatelets can be easily separated from the spherical particles present in solution, without any major loss in their optical properties. 2014) American Chemical Society.

Why is 2D special ?

In addition to the challenge of their synthesis, 2D colloidal materials raise great promises in terms of new properties with respect to 0D nano-objects. In this section we discuss some of the specific physical properties resulting from their dimension.

From an electronic transport point of view, 2D materials are of utmost interest to achieve improved conduction. Transport in nanocrystals array occurs through a hopping process. At each step, the charge carriers have to face a tunneling barrier resulting from the presence of ligands at the nanoparticle surface. Short ligand passivation 194,380,381 allows the improvement of carrier mobility, nevertheless reducing the number of hopping steps may also be an alternative method to rise the overall conductivity. Thanks to their large lateral extension, 2D nanomaterials are perfect candidates to reduce the number of hopping steps by a ratio of their lateral size over their confined direction. In addition, the wavefunction overlap of two plates is expected to be proportional to their overlapping area. 2D objects will consequently present larger neighbor-to-neighbor coupling than the punctual contact arising from two spheres. However careful experimental evidence of this dimensionality effect still needs to be obtained.

The electronic structure of 2D colloidal objects differs from 0D objects in the sense that quantum confinement is not prevailing in all directions and plane-wave-like states possibly exist within the plane direction. From an optical perspective, 2D objects present a fairly different density of states than the sparse one of 0D nanoparticles. It raises the possibility to inject more carriers and excitons within the same nanocrystal. Compared to bulk object they also differ by their lower screening of the charge. Indeed, colloidal nanocrystals are surrounded by a low dielectric index medium (solvent, ligand…), contrary to bulk or even epitaxially grown quantum wells. As a result, there is strong dielectric contrast between the semiconductor and its environment, which results in dielectric confinement. Due to this dielectric contrast, the mirror-charge effect is enhanced and the charges are pushed away from the surface. This leads to a positive value for the self-energy which should impact the predicted energy spectrum, however the latter is almost perfectly balanced by the binding energy (negative correction to the energy spectrum) of the exciton. For QDs, the binding energy is typically of a few meV for lead chalcogenides QDs (due to their very large dielectric constant) and of few tens of meV for cadmium chalcogenides. In the case of 2D nanoplates, this value is typically ten times larger. On PbS nanosheets, 382 it was recently estimated to be around 50meV while for CdSe, its value can be as large as 200-300 meV. 383,384 Due to this cancelation, the effective mass model solved for an infinite well leads to surprisingly good prediction of the energy spectrum of the colloidal nanosheets. [START_REF] Ithurria | Quasi 2D Colloidal CdSe Platelets with Thicknesses Controlled at the Atomic Level[END_REF] Nevertheless the large dielectric contrast and large binding energy of the exciton keeps impacting optical and transport properties of the NPLs as discussed in the following part of this section.

Application for light emission

NPLs, with their extremely narrow PL linewidth are obvious candidates as next generation phosphor for display. Over the last three years, colloidal QDs have started to Lasing is also an obvious application of NPLs, because it also takes advantage of the short PL lifetime (few ns at room temperature) of the NPLs. Colloidal quantum dots have started being used as gain medium 389 to build random laser in the beginning of the 2000's. In spite of huge progress, the threshold irradiance to reach the stimulated regime remains high 390 (several mJ.cm -2 ). NPLs have really been a breakthrough in this sense, since the threshold level has been lowered 386 by three decades (see Figure 43 a andb). With NPLs, the threshold 387 can be below 10 µJ.cm -2 with gain around 600 cm -1 . Insertion of the NPLs in an optical cavity has led to lasing signal [START_REF] Grim | Continuous-Wave Biexciton Lasing at Room Temperature Using Solution-Processed Quantum Wells[END_REF]386,391 (see Figure 43 c). The lasing linewidth is typically 2 nm. 386 This performance also stands while the device is pumped in a multiphoton regime. 388

Two 386 and three 388 photon pumping have been demonstrated. In a two-photon pumped (λ≈800nm) configuration, the stimulated emission threshold remains low (1-10 mJ.cm -2 ). The exact origin of this improvement of the lasing performance remains mostly ununderstood, however we can speculate that the reduced Auger and larger density of states of the NPLs are key elements.

Application for photodetection

Thanks to the fine control of the band edge energy, NPLs appear as interesting candidates for photodetection with sharp control of the cut-off wavelength. The phototransport in nanoplatelets arrays have been first reported by Pedetti et al. 142 With CdTe NPLs, the current under illumination is two to three times larger than under dark condition.

With CdSe or CdSe/CdS NPLs this modulation is even larger 392 and can be as high as 10 4 .

However the overall photoresponse of the film remains low (tens of µA.W -1 ) which corresponds to weak internal efficiency (<<1%).

To improve the performance, it was proposed to switch from a photoconductive device to a phototransistor, to take advantage of the non-linear dependence of the current with the gate bias. Moreover, gating of the film allows a fine control of the carrier density and is able to tune the filling of the trap states. Electrolyte gating of cadmium chalcogenides NPLs has been proposed. 153 The method relies on the use of ion gel electrolyte (LiClO4 in polyethylene glycol: PEG). This method combines several advantages compared to conventional gating based on liquid electrolyte. First, the size of the device can be shrinked since no bulky and leaky electrochemical cell are necessary. Secondly, the device can be operated in air. Finally, since PEG is a solvent for LiClO4, no solvent needs to be added and this prevents the exposition of the NPLs to an annealing step necessary to remove the electrolyte solvent. This gating allows a large tuning of the conductance 393 with on/off ratio above 10 8 , while keeping the voltage range low (<3V).

This gating method can be used not only to tune the dark conductivity but also the photoconductivity of the system. Indeed, electrolyte gating allows a large tuning of the Fermi level of the system which can be used to control the filling of the trap states. In non-gated semiconductors, a photogenerated electron has a limited lifetime because of the possible trapping of the photocarriers by shallow traps. Once the system is gated, the Fermi level can be pushed toward the band presenting trap states, and fill these traps. As a result, the photogenerated carriers can no longer be trapped and their lifetime in the band is extended, leading to gain effect. This method has been applied to NPL films (see the inset of Figure 44 a for a scheme of the device). The idea is to prepare the system close to the current turn on point in the transistor subthreshold regime. Here the current dependence with the carrier density is exponential. Thus the photogenerated carrier density leads to an enhanced current modulation as shown in Figure 44 a, where the photocurrent rises by more than two decades thanks to the gate. The gate bias cannot be pushed too far otherwise (i) dark current also rises and (ii) the absorption of the carriers gets bleached once the Fermi level is pushed into one of the band. Another interesting use of this electrolyte gating of NPL film is the development of bicolor detectors. 392 By combining in parallel a film of CdSe/CdS NPLs with some HgTe nanocrystals, 394,395 one can use the gate as a switch to selectively obtain photoresponse in the visible and near infrared 396 using mercury chalcogenide 381,397,398 nanocrystals or in the visible only when the photoresponse of the NPLs prevails.

In spite of the progress resulting from the filling of the trap states, the overall responsivity of the gated NPL film remains low (10 mA.W -1 ). However, the NPLs have so far been treated as conventional (i.e. 0D) nanocrystals and the device geometry does not consider their 2D nature. As stated earlier, 2D nanosheets present a large binding energy (50-250 meV >> kbT). The latter needs to be overcome to achieve an efficient electron-hole dissociation. As a result, a fairly large electric field have to be applied over each NPL. To do so, an interesting way is to shrink the device size. Indeed, such modification of the device not only leads to larger electric fields but also to a lower sensitivity of the device to the presence of defects within the film. Indeed, to ensure a strong local coupling, ligand exchanges are generally conducted, however they lead to a decrease of the volume of the film due to the shortening of the ligands and this tends to form cracks at the µm scale. These cracks make the percolation path of the carrier up to the electrodes even more complex. Smaller devices are thus expected to lead to larger conductivity. 399 The size of the device can be reduced down to a single NPL. To do so nanofabrication is necessary. Nanogap and nanotrench electrodes have been obtained through a variety of methods such as e-beam lithography, 400 electromigration 401,402 or shadowing 403 methods. Nanocrystal based photodetectors using nm-spaced electrodes have been proposed by Prins et al 404 and Willis et al 405 , however the device keeps operating in the hopping regime since the nanocrystal are not larger that the electrode-spacing. On the other hand, NPLs, thanks to their large lateral extension can be more easily connected to the electrodes and this releases the constrain on the electrodes fabrication which "only" have to be spaced by tens of nm (see Figure 44 b). Such geometry of device has led to the best photodetection performance for colloidal based cadmium chalcogenides nanocrystals with extremely large photoresponse 10 (kA.W -1 ). The device is able to conciliate large gain (3000 electrons are generated per absorbed photon) with high bandwidth (>kHz). This performance corresponds to an improvement 10 by a factor 10 7 compared to the same material connected with µm spaced electrodes operated in the hopping regime (see Figure 44 b). The only limitation to this approach is the decrease of the optical area which tends to limit the photon absorption. Finally, other alternative paths have been tested to overcome the hopping transport limitation. In this sense, a hybrid system with graphene has been proposed. In such a device, QDs are used for their optical absorption and graphene for its high mobility. The device relies on the selective transfer of one carrier of the electron-hole pair towards the graphene. Indeed, if both carriers were transferred to graphene they would quickly recombine without leading to a current. The selectivity of the transfer of the carrier results from the higher proximity of one band of the semiconductor with the Dirac cones of the graphene. The charge remaining in the semiconductor is used to photogate the graphene. 406 The living time of this carrier in the semiconductor is generally longer than the transit time of the other carrier within graphene, thanks to its high mobility, which results in gain. With PbS quantum dots 407,408 exceptional device performances have been reported with record responsivity (10 7 A.W -1 ), even if the bandwidth is generally limited. Lead sulphide QDs strongly differ from the NPLs in the sense that the photocarriers almost behave as free charges due to the high dielectric constant of the material, while excitons within the NPLs are more strongly bound. As a result, the charge dissociation needs to be assisted at the nanocrystal scale. Robin et al 409 demonstrated that a possible way to split the electron-hole pair is to use heterostructured NPLs. They monitor the charge transfer using an electrolytic transistor configuration, as shown in Figure 44 c. They evidence that quasi-type-II and type-II band alignment allow a fine control of the nanocrystal-to-graphene charge transfer. Heterostructure NPLs present a lower binding energy due to their lower overlap of the electron-hole wavefunction. This property is further used to control the sign and intensity of the transfer rate.

Applications for biology and chemistry

Metal nanoparticles are widely used in applications for chemical or biological sensing.

These properties are mainly based on their strong surface plasmon resonance (SPR) which depends on the size and the shape of the NP. 410,411 For Au nanoplates for example, the spectral range of their SPR extends from the visible to the infrared window, covering the biological optical window for tissue. [START_REF] Cutler | Colloidal Gold and Silver Triangular Nanoprisms[END_REF] By coupling the optical properties of Au nanoplates to a matched laser excitation, gold nanoplates show an efficient photothermal conversion for thermal damage on HeLa cells. 412 The strong SPR can be used for chemical sensing through Surface Enhanced Raman Spectroscopy (SERS) which is strongly dependent of the scattering properties of the metal NPs.

Non-spherical metallic particles are of particular interest in this field as calculations have

shown that the electromagnetic field is increased at sharp vertices such as tips of nanoplates, increasing the sensitivity of the SERS method. 413 Also used for their catalytic properties, Au NPs show a facet-dependent selectivity; the exposed facet is itself dependent on the shape of the NPs. [START_REF] Narayanan | Catalysis with Transition Metal Nanoparticles in Colloidal Solution: Nanoparticle Shape Dependence and Stability[END_REF] Moreover, the catalytic activity changes on different regions of gold nanoplates, increasing form flat facets to edges to corners. The specific activity of reduction of resazurin to resorufin by NH2OH is ~8% higher at the edges than on the flat facets, and ~80% higher on the corners than of the edges of the Au NPs. 414 This again shows the dependence on the shape of the nanoparticle, and the advantages of a 2D structure over a spherical, 3D structure. Semiconductor nanoplatelets, due to their exceptional optical properties presented previously (see part 3), are excellent candidates for biological probes. Their molar extinction coefficient is three orders of magnitude higher than the one of green fluorescent protein (GFP), and their narrow emission band could help perform multicolor imaging with limited bleed-through in the other imaging channels. However, their synthesis is performed in organic solvents and their stabilization in aqueous media is even more challenging that for 0D QDs as NPLs exhibit more surface atoms: the ligand exchange step to make them water-soluble strongly affects the surface atoms reducing their photoluminescence efficiency. A recent work by Sung Jun Lim and co-workers 415 shows that encapsulation of NPLs in lipoproteins makes them efficient probes for in cellulo imaging (see Figure 48). Not only that, but when compared to 0D QDs, only the NPLs showed efficient uptake in live human epidermoid carcinoma cells.

The authors explain this uptake selectivity not by the surface chemistry (as other compounds with the same chemistry did not show any uptake), but by the large surface area of NPLs and their low curvature, facilitating multivalent interaction with the cell membrane.

Conclusion

We discuss the growth of 2D colloidal nanocrystals. We have identified four main mechanisms which lead to the growth of such sheet-shaped objects which are:

 Defect induced anisotropy which is a very common mechanism to grow 2D metallic nanoparticles;

 Ligand engineering which includes the formation of lamellar phases but also ligandtunable accessibility to the nanocrystal surface. This process leads to extremely high control of the thickness down to the atomic scale;

 Self-assembly of pre-formed nanocrystals into a 2D mesostructured;

 Anisotropic growth resulting from an anisotropic atomic lattice; this process is particularly common to grow transition metal dichalcogenides material.

In addition to these four main processes, alternative mechanisms relying on light activated growth and cation exchanges also extend the tool-box of chemists to grow 2D colloidal objects with a broader range of available materials.

In the final section, of the review we discuss in what ways the physical properties of 2D object differ from their 0D analog. In particular, the mirror charges and the large exciton binding energy strongly affect their optoelectronic properties. Their short PL lifetime have attracted a lot of interest for their use as light emitter for down conversion and lasing.

Moreover, they are also very promising for photodetection as long as the device geometry addresses their2D aspect by taking advantage of their anisotropy and by addressing how the exciton can be splitted. Finally, the use of 2D colloidal objects for chemistry and biology offers new possibilities for nanoparticle functionalization or for stronger selectivity.

Figure 1 :

 1 Figure 1: Evolution of the Gibbs free energy of a cluster versus its size (black curve). The evolution of the two terms in the Gibbs free energy equation is also shown; the volume (or bulk) energy (blue curve) and the surface energy (in red). The critical radius 𝒓 𝒄 and the activation energy ∆𝑮 𝒄 are also presented on the figure. Adapted from ref. 37 -Published by The Royal Society of Chemistry.

Figure 2 :

 2 Figure 2: Evolution of the monomer concentration vs time according to La Mer's theory of burst nucleation. Adapted from ref. 37 -Published by The Royal Society of Chemistry.

Figure 3 :

 3 Figure 3: Example of Wulff construction for an orthorhombic structure. The equilibrium shape is shown in grey and is comprised in the lines corresponding to the low energy facets. Adapted from ref. 40 -Belstein J. Nanotechnol.

Figure 4 :

 4 Figure 4: a. and b. Field Emission Scanning Electron Microscopy images of Au nanoplates with edge lengths of varying size. Adapted with permission from Ah et al. (2005) (ref. 57). Copyright (2005) American Chemical Society. c. Transmission Electron Microscopy (TEM) images of Ag nanoplates, with, in the inset, stacked Ag nanoplates, standing vertically on their edges. Adapted with permission from ref 48. Copyright (2011) American Chemical Society.

Figure 5 :

 5 Figure 5: Shape evolution of a nanoparticle during growth. Rapid addition to high-energy facets y (relative to x-edges) results in the elongation of the x, low-energy edges. From ref. 2. Copyright 2009 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

Figure 6 :

 6 Figure 6: A and B. 2D CdSe nanoparticles of different lateral size. C. Scheme of the CdSe twodimensional nanoparticle. The ligands are fatty acids that attach with their negatively charged acid function to the positively charged Cd ions at the surface of the NP. Their organic backbone form a fatty-acid layer that acts as a soft template hindering the growth in the direction of the passivated planes. Scale bar is 50 nm. Adapted with permission from ref. 64. Copyright (2011) American Chemical Society.

Figure 7 :

 7 Figure 7: Scheme of oriented attachment mechanism. After the diffusion of NCs, the rotation of NCs is necessary for proper crystallographic orientation, and then attachment and selfrecrystallization occur. Reproduced from Ref.77 with permission from The Royal Society of Chemistry.

Figure 8 :

 8 Figure 8: Sequence of in situ TEM images showing the details of the oriented attachment method for iron oxide nanoparticles. (A) and (D to H) Sequence showing attachment at a lattice-matched interface. (A) shows the arrangement of particles before attachment. The asymmetric particle in front of the smaller spherical particle is not involved in the attachment process. (D) to (G) show formation of the interface. Two edge dislocations denoted in (E) to (G) by red dashed lines translate to the right, leaving a defect-free structure in (H). (B) and (I to K)

Figure 9 :

 9 Figure 9: Schemes of NCs during and after oriented attachment. a) primary particles during the oriented attachment mechanism that yields b) a new crystal that contains twin defects. Adapted with permission from ref.76. Copyright (1998) Mineralogical Society Of America.

Figure 10 :

 10 Figure 10: Layered structures of metal chalcogenides. (a) Single nanocrystal is composed of multiple layers stacked along the c-axis by weak van der Waals force. Reproduced from Ref. 79 with permission from The Royal Society of Chemistry. (b) Single layer of metal chalcogenides exits as various polymorphs. Each layer may have tri-, tetra-or penta-atomic layers of metal and chalcogen. Reproduced from ref. 80 with permission of Taylor and Francis. (c) Rare GeS type of 2D layered metal chalcogenides. Reproduced from p.91 from ref. 81 with permission of Springer.

Figure 11 :

 11 Figure 11: Model of a twin plane in a face-centered cubic (fcc) metal: (A) fcc lattice; (B) fcc lattice with the repeating « abcabcabc » stacking pattern annotated; (C) annotated fcc lattice from (B) oriented to the [110] zone axis so that the {111} repeating layers are more easily visible; (D) annotated fcc lattice which contains a twin plane, as viewed down the [110] zone axis. Here, the insertion of a « c » layer in place of a « b » layer (« abcacba ») in the repeating pattern has resulted in a structure which is mirrored around the central « a » layer (marked with a horizontal line). Reprinted with permission from ref.59. Copyright (2013) American Chemical Society.

Figure 13 :

 13 Figure 13: Growth model for a singly-twinned nanoparticle. The A-type concave facets grow more rapidly than B-type convex facets, yielding triangular prisms with exposed B-type facets. From ref.88. Copyright 2005 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

Figure 14 :

 14 Figure 14: Growth model for a doubly-twinned nanoparticle. All six sides now exhibit A-type concave facets (dashed arrows) which causes rapid growth in all the six dimensions (solid arrows). From ref.88. Copyright 2005 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.

Figure 15 :

 15 Figure 15: Growth model for triply-twinned nanoparticles yielding non-equilateral hexagons.Reprinted with permission from ref.89. Copyright (2011), American Institute of Physics.

94

 94

Figure 16 :

 16 Figure 16: a. Sketch of penta-twinned decahedral nanoparticle. The tetrahedra that make the decahedron share a common axis, and present an angle of 70.52° between two {111} facets, leaving a theoretical gap of 1.48° between two tetrahedral. From ref.85. Copyright 2010 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc. b. TEM image of a gold decahedron nanoparticle. Reprinted with permission from ref.95. Copyright (2008) American Chemical Society.

Figure 17 :

 17 Figure 17: Dependence of shape with size of Ag clusters. a, b, c and d: typical structures obtained for Ag clusters. a. Icosahedron at 147 atoms, b. icosahedron at 309 atoms, c. decahedron at 561 atoms and d. fcc polyhedron at 923 atoms. e. Percentage of different shapes obtained for increasing number of atoms N in the cluster. Circles are icosahedra,

Figure 18 :

 18 Figure 18: Growth of Au nanoplates through oxidative etching of Au spherical nanoparticles.Reprinted with permission from ref.96. Copyright (2002) American Chemical Society.

Figure 19 :

 19 Figure 19: (A-C) Scanning Electron Microscope (SEM) images of gold nanoparticles synthesized in the presence of (A) 0, (B) 10, and (C) 75 µM of NaI. Adapted with permission from ref.60. Copyright (2012) American Chemical Society. (D-G) SEM images of gold nanoparticles synthesized in the presence of an increasing concentration of Ag + ions. Higher concentrations of Ag + lead to the stabilization of facets with an increasing number of exposed surface atoms. Reprinted with permission from ref.59. Copyright (2013) American Chemical Society.

  [0001] direction and the width in the [1-100] direction. In addition to CdSe semiconductor, various semiconductors such as CdS, CdTe, 119 zinc chalcogenide, 120,121 CuS 122 or more recently PbS have been synthesized by this path. 120

Figure 20 :

 20 Figure 20: a) Scheme of the three steps synthesis in wurtzite 2D nanocrystals starting with dissolution of a salt in amines, then the introduction of the chalcogenide precursor, finally leading to 2D nanocrystals. b) Low angle X-ray diffraction of the lamellar structure containing

Figure 21 :

 21 Figure 21: TEM Images of CdSe NPLs with small (a) and large lateral extension (b). The latter rolled on themselves. c. TEM image of CdTe NPLs. d Absorption and photoluminescence spectra of CdSe NPLs with a first excitonic feature at 510nm. There is almost no Stoke shift between the maximum of the absorption and the photoluminescence (PL) signal. The PL linewidth is 10nm. e. Absorption spectrum of CdSe NPLs with different thicknesses and presenting a first optical absorption feature at 390nm, 460nm, 510nm and 550nm.

Figure 22 :

 22 Figure 22: a) Scheme of the two-step growth of cadmium chalcogenide nanoplatelets, b) absorption spectra of CdTe nanoplatelets at different times during a reaction first at 180°C and then at 210°C (dashed line) with Cd(prop)2 as a short chain carboxylate. c) Absorbance at 350nm as a function of time for the CdTe nanoplatelets growth. Adapted with permission from ref.142. Copyright (2013) American Chemical Society.

Figure 23 :

 23 Figure 23: Scheme of CdE (E= S, Se or Te) nanocrystals, "seeds" of the 3, 4 and 5MLs nanoplatelets. The different facets are represented in red for the direction <100> and in green and yellow for the direction <111>. Number of atoms in the nanocrystals and on their surfaces.

Figure 24 :

 24 Figure 24 : Scheme of a heterostructure band strcture with a type-I (a), quasi-type-II (b) and type-II (c) band aligment. d TEM image of a core/shell (CdSe/CdS) NPL. The inset is a scheme of the core/shell heterostrcture e. TEM image of a core/crown (CdSe/CdS) NPL. The inset is a scheme of the core/crown heterostructure.

Figure 25 :

 25 Figure 25 : Absorption (a) and photoluminescence (b) spectra of CdSe core and CdSe/CdS core/shell. Adapted with permission from ref.148. Copyright (2013) American Chemical Society. c. Photoluminescence excitation spectrum of CdSe/CdS core/crown NPLs with different lateral extension of the CdS crown. Adapted with permission from ref.154. Copyright (2014) American Chemical Society. d. Photoluminescence (red line), photoluminescence excitation (dashed line) and absorption spectra (blue line) of CdSe-CdTe core crown NPLs. Adapted with permission from ref.155. Copyright (2014) American Chemical Society. The PL signal is strongly redshifted compared to the absorption of both CdSe (absorption at 510nm) and CdTe (absorption at 560nm) confirming the type-II band alignment.

Figure 26 a

 26 Figure 26 a). It is simply obtained by mixing sulphide powder with the selenium precursor.159,160 Alternatively selenium disulphide can also replace the mixture of Se and S in a 1:2 stoechiometry.

Figure 26 :

 26 Figure 26: a. Photoluminescence spectrum of CdSSe NPLs with different ratios of S and Se, adapted with permission from ref.159. Copyright (2015) American Chemical Society. b. Photoluminescence spectrum of CdSeTe NPLs, with different ratios of Te and Se. 1 corresponds to the Pure CdSe phase while 0 corresponds to the CdTe phase. The data evidences a bowing effect. Adapted from ref. 161. c. Absorption and photoluminescence spectrum of Mn doped CdSe ribbons. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials from ref. 128, copyright (2010).

Figure 27 :

 27 Figure 27 : TEM images of CdSe nanoplates before (a) and after (b) their coating by a thin SiO2 layer. Adapted from ref.162. Copyright (2014) American Chemical Society. c, d and e are respectively TEM images of CdSe nanoplates functionalized with gold, palladium and platinum tips. Adapted with permission from ref.164. Copyright (2015) American Chemical Society.

  by a factor 20 to 1000. Such Pb:S ratio aimed to promote the nucleation step rather than the growth. It leads to the formation of small nanocrystals (3nm and less). The second critical point (ii) is the introduction of chloroalcane (trichloroethylene). Its exact role remains unclear, but halides are also pointed as a source of anisotropy for metal nanoparticles, 183 but have to be introduced in the early time of the reaction. Due to the low boiling point of this chloroalcane, it cannot be introduced until the degassing steps are finished. The chlorine is certainly used to prevent the growth along certain facets and promote the attachment along the[110] direction180,181 .

Figure 28 :

 28 Figure 28 : TEM images of PbSe QD (a), cubes (b), wires (c) and stars (d). e is an SEM image of PbS sheet, adapted from ref 181 with permission of The Royal Society of Chemistry . f. is a TEM image of a PbS sheet. Reprinted with permission from ref.184. Copyright (2012), American Institute of Physics.

Figure 29 :

 29 Figure 29 : a TEM image of a PbSe-nanocrystal-based honeycomb lattice. The nanocrystals appear in white. b is a zoom on the inter-dot interface. Adapted with permission from ref. 190 Copyright (2012) American Chemical Society. c. TEM image of a PbSe-nanocrystal-based percolative network with a square geometry. Adapted from ref 197. d. Near-infrared absorbance of the PbSe nanocrystals before and during their assembly. The spectra broaden, but the confinement is preserved. e. Plot of the inter-dot distance and film conductivity as a function of the assembly duration. Adapted with permission from ref.191. Copyright (2013) American Chemical Society. f. Calculated band structure for an artificial honeycomb lattice made of HgTe. Adapted from ref.198.

  carrier mobility. Even if further experimental evidences are necessary to confirm the existence of the predicted states, improved transport properties compared with unfused nanocrystals have already been obtained. Baumgardner et al. reported a rise of the conductance by four decades (see Figure 29 e), during the self-assembly process, while Evers et al. have measured transport mobility 197 close to 10cm 2 V -1 s -1 .

Figure 30 :

 30 Figure 30: 1-(a) TEM image showing top view of 50-nm TiS2 nanodisks, (b) structural diagram, (c) EDS spectrum, and d) PXRD pattern. From ref.240. Copyright 2008 by John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc. 2-(a) Low-and (b) high-resolution TEM images of TiS2 NSs. (c) SAED pattern of TiS2 NSs stacked atop each other. Adapted with permission from ref.241. Copyright (2012) American Chemical Society. 3-TEM images of TiS2 discs: (a, b) Low-resolution and magnified images of TiS2 disc nanocrystals (c) Side view of TiS2 discs stacked together. (d) HRTEM image and (inset) FFT pattern of TiS2. Adapted with permission from ref.242. Copyright (2011) American Chemical Society.

Figure 31 :

 31 Figure 31: Single-layer TiS2, ZrS2 or HfS2 nanosheets (a) Single-layer TiS2 nanosheets formed by using DDT. (b) Low-magnification TEM image; (c) pseudocolor image for a selected area; (d) SAED pattern of a single-layer TiS2 nanosheet. (e) TEM image of the single-layer ZrS2 nanosheets; (f) pseudocolor image of a small area. (g) Chemical reaction for HfS2. (h) TEM image; (i) pseudocolor image for a selected area; and (j) SAED pattern. Adapted with permission from ref.244. Copyright (2014) American Chemical Society.

Figure 32 :

 32 Figure 32: Colloidal tungsten and molybdenum dichalcogenides monolayers. 1-(a) Lowmagnification TEM image of 1T-WS2 nanosheets. (b) HAADF-HRSTEM image of a 1T-WS2 monolayer, showing the zigzag pattern of the tungsten atoms characteristic of the distorted 1T structure as well as numerous voids and tungsten vacancies. Adapted with permission from ref.250. Copyright (2014) American Chemical Society. 2-Single-layer WSe2 nanosheets. (a) Low-magnification TEM image and (b) pseudocolor image of the selected area in the box of panel a. (c) High-resolution TEM image, of a single-layer WSe2 nanosheet. Adapted with permission from ref.251. Copyright (2015) American Chemical Society. 3-Single-layer MoSe2 nanosheets. (a) Low-magnification TEM image and (b) pseudocolor image of the selected area in the box of panel a. (c) High-resolution TEM image, of a single-layer MoSe2 nanosheet. Adapted with permission from ref.251. Copyright (2015) American Chemical Society.

- 8 layers

 8 ) nanosheets. The products obtained in oleyl alcohol are thinner(2-3 monolayers) and larger (20nm). Finally, when oleic acid is used as solvent and ligand, the nanosheets obtained are monolayers with large lateral sizes of 200-400nm (Figure33-2 and 33-3). The proposed explanation is based on DFT calculations showing that primary amines strongly bound to the edges of the nanostructures, thus favoring the multilayer growth. On the other side, oleic acid is a weak ligand favoring lateral growth and the formation of monolayers. A close protocol, using dibenzyl diselenide and Bis(acetylacetonato)dioxomolybdenum(VI) (MoO2(acac)2) as precursors with oleylamine as the solvent, yields MoSe2 hierarchical nanostructures after only 20 minutes of reaction at 240°C.252

Figure 33 :

 33 Figure 33: a) Large-area image of an intermediate TiS2_Cu2S structure obtained with CuCl2/TiS2 = 1. (b) High-magnification image at the interface between TiS2 and Cu2S showing orientation match between of Cu2S and TiS2 crystal structures. The FFT pattern (inset) shows green and red dots corresponding to TiS2 and Cu2S, respectively. (c) Side-view TEM image and EELS elemental analysis (d, e) of an intermediate TiS2_Cu2S structure. Ti is shown in green and Cu in red. (f) Schematic rotation of the intermediate structure by 90°, showing both planar and side-view images. TEM images of toroidal-shaped nanostructures synthesized by reactions of TiS2 discs with Ag, Mn, and Cd cations: (g) TiS2_Ag2S; (h) TiS2_MnS; (i) TiS2_CdS. The insets show highermagnification images. Adapted with permission from ref.242. Copyright (2011) American Chemical Society. j) Reaction of TiS2 upon 1O2 as chemical stimulus: TEM images of TiS2 nanocrystals treated with 1O2 for (i) 0 h, (ii) 6 h and (iii) 24 h. Dark contrast dots appear in (ii), EELS analysis of nanocrystals collected at 6 h (Ti, blue; S, yellow; O, red) shows sulfur containing dots (TiS2) surrounded by oxygen (TiO2). k) Disc to toroid TiO2 morphological transformation driven by H2O. TEM images at each transformation stage from TiS2 nanodiscs to toroid TiO2 nanocrystals by reacting TiS2 with H2O for (i) 0 h, (ii) 12 h, (iii) 36 h and (iv) 60 h. Adapted with permission from ref.257. Copyright (2013) American Chemical Society.Using a model system based on colloidal TiS2 nanodisks, Cheon and coworkers have explored diverse cation or anion exchange reactions, leading to different heterostructures. In a first paper, 242 they reported the reaction of TiS2 nanosheets with copper (II) cations. The cations react preferentially at the edges of the nanodisks, leading to the formation of TiS2_Cu2S heterostructures and then to pure Cu2S toroidal nanocrystals if the reaction is complete (Figure34a-f). Such metallic cation reaction on TiS2 edges is broadly applicable and allows the formation of TiS2/Ag2S, TiS2/MnS and TiS2/CdS heterostructures (Figure34g, h, i).

Figure 34 :

 34 Figure 34: a) TEM image of a TiS2 sample before and after 30 min of photoexcitation under pulsed laser irradiation. The scale bar is 200 nm. Reprinted with permission from ref.270. Copyright (2014) American Chemical Society. b) AFM images of TiS2 nanodiscs deposited on a microscope coverslip in the presence (top) and the absence (bottom) of a 2 kHz square wave Ez field. Reprinted with permission from ref.271. Copyright (2015) American Chemical Society. c) SEM images of chiral MoS2 nanofibers. Reprinted with permission from ref.272. Copyright (2015) American Chemical Society. d) A representative cross-sectional TEM image of 2 MoS2/polymer multilayer with the corresponding line scan profile representing the thickness of a single MoS2 nanosheet and single polymeric trilayer (MTL) of the MoS2/polymer. Reprinted with permission from ref.273. Copyright (2014) American Chemical Society.

Figure 35 :

 35 Figure 35: a) Structural model illustrating thiol conjugation of chemically exfoliated MoS2 sheets. Reprinted with permission from ref.277. Copyright (2013) American Chemical Society. b) Photos of MoS2 and MoS2-PEG in water, PBS and cell medium after 3,000 rpm centrifugation. c) AFM images of MoS2 before and after PEGylation. d) AFM-measured diameter and thickness distributions of MoS2 and MoS2-PEG. From ref.278 Copyright 2014 by John Wiley & Sons, Inc.Reprinted by permission of John Wiley & Sons, Inc. e) Schematic of covalent functionalization:The 2H phase of TMDs is converted to the 1T phase via lithiation using butyllithium (BuLi), the nanosheets are then functionalized using 2-iodoacetamide or iodomethane (R-I) solution. f) Photoluminescence spectra obtained from single-layer MoS2 grown by CVD (2H phase), from the metallic 1T phase and from the functionalized 1T phase. A strong photoluminescence peak at ≈1.8 eV is observed for the 2H phase, consistent with single-layer 2H MoS2, whereas photoluminescence is quenched after conversion to the metallic 1T phase. The functionalized 1T phase exhibits strong photoluminescence, characterized by two bands at ≈1.9 eV and ≈1.6 eV. Inset: typical CVD-grown monolayer triangular nanosheet on which the photoluminescence spectra were measured. Scale bar, 25 μm. Reprinted by permission from Macmillan Publishers Ltd: Nature Chemistry from ref.279, copyright (2014).

Figure 36 :

 36 Figure 36: (a-d) TEM images of aliquots taken during the formation of the SnSe nanosheets highlighting the different growth stages, from initial nucleation, agglomeration, and coalescence of nanoparticle seeds into two-dimensional dendritic nanostructures then lateral growth via nanoparticle attachment resulting in a uniform square-like nanosheet morphology. Scale bars are 100 nm. Reprinted with permission from ref.306. Copyright (2011) American Chemical Society. (e, f) TEM and AFM images of SnSe nanosheets with their height data, showing the successful synthesis of uniform single layer SnSe nanosheets. Reprinted with permission from ref.307. Copyright (2013) American Chemical Society.

Figure 37 a

 37 Figure 37 a. TEM image of Sb2Te3 nanoplates. b. High resolution TEM image of Bi2Te3 nanoplates. c. TEM image of Bi2Te3-Sb2Te3 core/crown structure, adapted with permission from ref.315. Copyright (2015) American Chemical Society. d. TEM image of Bi2Te3-Sb2Te3 core/shell structure. e and f are EDX maps on for Bi and Sb respectively on the same area as the part seen in d.

Figure 38 :

 38 Figure 38: At the early stages of the synthesis, nanoplates and spherical particules are formed. The clusters dissolve and redeposit on the nanotriangles that act as seeds for further growth. From ref.336. Reprinted with permission from AAAS.

Figure 39 :

 39 Figure 39 : Light-induced nanoparticle aggregation, where 4 smaller nanoplates attach in an oriented manner to form a larger one. Reprinted by permission from Macmillan Publishers Ltd: Nature from ref.337, copyright (2003).

Figure 40 :

 40 Figure 40: (a) Schematic diagram of dual-beam excitation. (b) The optical spectra (normalized)for six different-sized nanoprisms(1-6 edge length: 38±7 nm, 50±7 nm, 62±9 nm, 72±8 nm, 95±11nm and 120±14 nm) prepared by varying the primary excitation wavelength(central wavelength at 450, 490, 520, 550, 650 and 750 nm, respectively; width, 40 nm) coupled with a secondary wavelength (340 nm; width, 10 nm). (c) The edge lengths as a function of the primary excitation wavelength. (d-f) TEM images of Ag nanoprisms with average edge lengths of 38±7nm (d), 72±8nm (e) and 120±14nm (f). Scale bar applies to panels d-f. Reprinted by permission from MacmillanPublishers Ltd: Nature from ref.337 , copyright (2003).

Figure 41 a

 41 Figure 41 a). Bouet et al. 343 evidenced that the cation exchange occurs gradually along the thickness direction (see Figure 41 b), which is consistent with the coexistence of the CdSe and Cu2Se phase in the XRD diffractogram, as shown in Figure 41 c. The same strategy has been used by Kuno group to prepare ultrathin Cu2-xSe nanosheets (1.6 nm thick) exhibiting quantum confinement as well as a strong surface plasmon resonance in the infrared, induced by the copper vacancies. 342

Figure 41 :

 41 Figure 41 : a. Absorption spectra of CdS NPLs, of CdS NPLs after their back exchange from Cu2S, of ZnS NPLs after their exchange from Cu2S and of PbS NPL after their exchange from Cu2S. b. Cu content for different position on a partially (left) and completely (right) exchanged CdSe/CdS NPLs. The top part is a TEM image where the rounds indicate the position of the EDX

Figure 42 :

 42 Figure 42: A. Schematic drawing of a centrifuge tube after precipitation of Au nanoparticles depending on their size and aspect ratio. B. Corresponding solutions of Au NPs precipitated at the bottom or on the side wall of the centrifuge tube in A. Reproduced from ref.374. Copyright (2009) National Academy of Sciences, USA. C. Scheme of depletion-induced size-and shapesorting. Reprinted with permission from ref.379. Copyright (2010) American Chemical Society. D. Scheme of electrophoretic separation of spherical and two-dimensional NPs of semiconductor in a non-polar solvent after the synthesis. Adapted with permission from ref.138. Copyright (2014) American Chemical Society.

  replace rare earth based phosphor to produce white light for displays.385 Indeed, the PL signal coming from lanthanide based compound is broad(80-100nm) which presents two main drawbacks: (i) a limited gamut (ie how many colors can a display reproduce) which is especially limiting in the green. And (ii) near infrared emission which corresponds to light emission in an eye-insensitive range of wavelengths. The latter can alternatively be seen as loss of plug efficiency. Colloidal QDs present narrower PL signal (30-40nm FWHM for core/shell nanocrystals) and are now used for gamut enhanced display. Compared to OLEDs, they are cheaper because they are compatible with LCD technology while producing similarly high quality of color. NPLs with FWHM down to 20 nm can become the next generation of nanocrystal-based phosphors for display as long as they are able to sustain the operating conditions (high flux: 100 mW.cm -2 and high temperature: 100°C) requested for such applications. This will request the development of encapsulation methods suited for NPL surface chemistry.

Figure 43 :

 43 Figure 43: a. Amplified stimulated emission spectra of CdSe NPLs for different intensities of the pump, Adapted with permission from reference 386 . Copyright (2014) American Chemical Society. b. Integrated intensity of the luminescence as a function of the pump intensity for CdSe-CdS core shell NPLs, Adapted with permission from reference 387 . Copyright (2014) American Chemical Society. c. PL and lasing of CdSe NPLs under different pump powers. Reprinted with permission from Macmillan Publishers Ltd: Natutre Nanotechnology from ref.12, copyright (2014). d. Two photon (λ≈800nm) pumped cadmium chalcogenides nanoplates stimulated emission intensity as a function of the pump power, adapted from reference 388 .

Figure 44 :

 44 Figure 44 : a. Photoresponsivity, of a CdSe-CdS NPL film gated with an ion-gel electrolyte, as a function of the applied gate bias. The inset is a scheme of the device. Adapted with permission from reference 392 . Copyright (2014) American Chemical Society b. Photoresponsivity of a CdSe/CdS-nanoplates film bridging a nanotrench as a function of the incident light power. The inset is a SEM image of the device. The data associated with the same film connected to 20 µm spaced electrodes are also plotted for comparison. Adapted with permission from reference 10. Copyright (2015) American Chemical Society. c. Current as a function of the applied gate bias for an epitaxially grown graphene layer functionnalized with CdSe/CdS nanoplates under different level of incident light power. The inset is a scheme of the device.

Figure 45 :

 45 Figure 45: Precisely tuning the plasmon mode of GTNPs to the bio-window of tissue for the demonstration of photothermal conversion. (A) In pink, the extinction spectra of Au nanoplates with a plasmon resonance at 808 nm, in black the extinction spectra of Au nanoplates with a plasmon resonance at 976 nm, and in red, the emission spectrum of an NIR laser used in these experiments with a central wavelength at 808 nm. In green, extinction spectra of fat and water in this region as well as their relatively low extinction region (the white area); (B) photo of a agarose-fat channel as a simulated tissue. (C and D) The infrared images of the channel loaded with the Au nanoplates suspensions with plasmon modes at (C) 976 nm and (D) 808nm and then irradiated by the 808 nm NIR laser for 4 min. The temperatures at the center of the chip were up to 30 and 43°C for (C) and (D), respectively. Reproduced from Ref. 412 with permission from The Royal Society of Chemistry.

Figure 46 :

 46 Figure 46: Electromagnetic field enhancement around Ag nanoparticles with different shapes: (a) Ag nanospheres (radius 19 nm); (b) Ag nanodisks (size=40 nm, thickness=9 nm) ; (c) and (d) Ag triangular nanoprisms (size=60 nm, thickness= 12 nm). The number in green indicates the maximal enhancement of the electromagnetic field at the wavelength indicated in red. Adapted from ref. 413 . With permission from Springer Science+Business Media.

Figure 47 :

 47 Figure 47: Spatially resolved activity quantitation on single silica-coated Au nanoplates. (A) Locations of 2325 product molecules overlaid on top of the SEM image of a silica-coated Au nanoplate. Each dot is the location of one product molecule. The locations are color coded according to their respective regions on the nanoplate: flat facet (red), edges (blue), and corners (green). The solid black line outlines the outer contour of the silica shell. The dashed black line outlines the perimeter of the Au nanoplate core. (B) Same as (A) but for a different silica-coated Au nanoplate with 1579 products detected. (C) Specific activities of the different regions of the nanoplates from (A, top) and (B, bottom). Adapted with permission from ref. 414 . Copyright (2013) American Chemical Society.

Figure 48 :

 48 Figure 48: Fluorescence confocal microscopy images of A431 human cells after 24h of exposure to (a) NPLs or (b) QDs. The top boxes correspond to the 3D z-stack along the green line, while the side boxes correspond to the 3D z-stack along the red line. Adapted with permission from ref.415. Copyright (2016) American Chemical Society.

Table 1 :

 1 Thermodynamic preferences of chalcogenide nanocrystals. Reproduced from Ref. 50 with permission from The Royal Society of Chemistry.

  analysis. Adapted with permission from343 . Copyright (2014) American Chemical Society. c. XRD diffractogram of CdSe, CdSe/Cu2Se and Cu2Se NPLs obtained at different step of the Cu + ligand exchange. Adapted with permission from 342 . Copyright (2014) American Chemical Society.

Nanostructures with Enhanced Thermoelectric Performance. ACS Appl. Mater. Interfaces 2015, 7 (26), 14263-14271.

Acknowledgments .................................................................................................................... 86 References ................................................................................................................................ 87 Acknowledgments BD and SI thanks Agence Nationale de la recherche (ANR) for funding through the grant SNAP H2DH and Nanodose. This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K. This work was also supported by French state funds managed by the ANR within the Investissements d'Avenir programme under reference ANR-11-IDEX-0004-02, and more specifically within the framework of the Cluster of Excellence MATISSE led by Sorbonne Universités