Nicolas Goubet

Maxime Thomas

Charlie Gréboval

Audrey Chu

Junling Qu

Prachi Rastogi

Sang-Soo Chee

Mayank Goyal

Yimin Zhang

Zhen Xiang

Xu

Gregory Cabailh

Sandrine Ithurria

Emmanuel Lhuillier

Near to Long-Wave Infrared Mercury Chalcogenide Nanocrystals from Liquid Mercury

HgTe nanocrystals are currently the most promising colloidal material for infrared detection, combining broadly tunable infrared absorption and photoconductive properties. Current synthesis leads to a limited amount of material and relies on a highly toxic water-soluble form of Hg. Here, we explore the possibility of using Hg thiolate as Hg source and demonstrate that the latter can be formed in situ from liquid Hg. The developed protocol allows large masses (7 g) and highly concentrated (100 g/L) synthesis, which is a step forward for the transfer of this material towards industry. The transport properties of the material have also been investigated and we observe a transition from p to n-type with size. We observe that the threshold of the p to n switch depends on the growth method which enables for a given size of nanocrystal the formation of p-n junction. This work has great potential to design infrared sensor with optimized charge dissociation.

INTRODUCTION

The ease for tuning the spectrum of colloidal quantum dots (CQDs) from UV to the THz range [START_REF] Goubet | Terahertz HgTe Nanocrystals: Beyond Confinement[END_REF] is certainly one of their most striking properties. While in the visible range their emitting properties have been used for applications such as displays and light emitting diodes [START_REF] Shirasaki | Emergence of Colloidal Quantum-Dot Light-Emitting Technologies[END_REF] , in the infrared range [START_REF] Lu | Infrared Quantum Dots: Progress, Challenges, and Opportunities[END_REF] , it is rather their absorption and photoconductive features which are valuable to design solar cells [START_REF] Lu | High-Efficiency PbS Quantum-Dot Solar Cells with Greatly Simplified Fabrication Processing via "Solvent-Curing[END_REF][START_REF]Colloidal Quantum Dot Optoelectronics and Photovoltaics[END_REF][START_REF] Ramade | Potential of Colloidal Quantum Dot Based Solar Cell for Near-Infrared Active Detection[END_REF] and infrared sensors. [START_REF] Tang | Dual-Band Infrared Imaging Using Stacked Colloidal Quantum Dot Photodiodes[END_REF][START_REF] Jagtap | Design of a Unipolar Barrier for a Short-Wave Infrared Photodiode[END_REF][START_REF] Tang | Scalable Fabrication of Infrared Detectors with Multispectral Photoresponse Based on Patterned Colloidal Quantum Dot Films[END_REF] In the near infrared, PbS and PbSe [START_REF] Pietryga | Pushing the Band Gap Envelope: Mid-Infrared Emitting Colloidal PbSe Quantum Dots[END_REF] CQDs have reached a high level of maturity with polydispersity down to 3%. This corresponds to a size fluctuation of a single monolayer. [START_REF] Weidman | Air-Stable PbS Nanocrystals via Precursor Stoichiometry Control[END_REF] When longer infrared wavelengths (λ>2 µm) are concerned, HgTe has led to the best photoconductive properties [START_REF] Chen | Mercury Telluride Quantum Dot Based Phototransistor Enabling High-Sensitivity Room-Temperature Photodetection at 2000 Nm[END_REF][START_REF] Rauch | Near-Infrared Imaging with Quantum-Dot-Sensitized Organic Photodiodes[END_REF][START_REF] Keuleyan | Mid-Infrared HgTe Colloidal Quantum Dot Photodetectors[END_REF][START_REF] Ackerman | Fast and Sensitive Colloidal Quantum Dot Mid-Wave Infrared Photodetectors[END_REF] and promising light emission [START_REF] Abdelazim | Room Temperature Synthesis of HgTe Quantum Dots in an Aprotic Solvent Realizing High Photoluminescence Quantum Yields in the Infrared[END_REF][START_REF] Sergeev | Tailoring Spontaneous Infrared Emission of HgTe Quantum Dots with Laser-Printed Plasmonic Arrays[END_REF] in the mid-infrared or lasing [START_REF] Geiregat | Continuous-Wave Infrared Optical Gain and Amplified Spontaneous Emission at Ultralow Threshold by Colloidal HgTe Quantum Dots[END_REF] in the short-wave infrared.

When it comes to synthesizing nanocrystals (NCs) absorbing above 3 µm, which is critical for thermal imaging applications, the number of available synthesis methodologies becomes limited. [START_REF] Kershaw | Narrow Bandgap Colloidal Metal Chalcogenide Quantum Dots: Synthetic Methods, Heterostructures, Assemblies, Electronic and Infrared Optical Properties[END_REF][START_REF] Green | Synthetic Routes to Mercury Chalcogenide Quantum Dots[END_REF][START_REF] Kovalenko | Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap Energies: From Telecommunications to Molecular Vibrations[END_REF] Kovalenko et al [START_REF] Kovalenko | Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap Energies: From Telecommunications to Molecular Vibrations[END_REF] proposed a synthesis based on the in situ formation of HgTe in aqueous medium to overcome the difficulty of handling a gaseous precursor. However, the material was limited to wavelengths below 3.5 µm. Later, Keuleyan et al [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF][START_REF] Keuleyan | Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 Μm[END_REF] developed an organic approach using HgCl2 dissolved in oleylamine that reacts with Te complexed with trioctylphosphine (TOP:Te). The same group then improved the size distribution and shape control by replacing the TOP:Te by bis(trimethylsilyl)telluride, which does not go in the direction of reduced toxicity or easier handling. [START_REF] Shen | Synthesis of Nonaggregating HgTe Colloidal Quantum Dots and the Emergence of Air-Stable n-Doping[END_REF] Indeed, most of the previous literature on HgTe synthesis has focused on the use of mercury salts, where Hg is in the Hg +II state, which is extremely soluble in water. In addition, the material quality and, in particular the size distribution is strongly improved by diluting the reaction medium. [START_REF] Keuleyan | Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 Μm[END_REF] As a result, current syntheses are conducted in diluted conditions and this is a key limitation to scale-up the HgTe NCs synthesis. It was recently demonstrated that the workforce is the main cost relative to nanocrystal synthesis. [START_REF] Jean | Synthesis Cost Dictates the Commercial Viability of Lead Sulfide and Perovskite Quantum Dot Photovoltaics[END_REF] In this sense, dilute processes require more syntheses to obtain the same amount of material and further increase workforce exposure to heavy metals. Developing new path which allow concentrated synthesis of HgTe while preserving the spectral tunability in the infrared appears of utmost interest.

In this paper, we develop a new synthetic method for the synthesis of mercury chalcogenide CQDs based on liquid mercury as Hg source. Liquid Hg does not appear as a common mercury source for colloidal synthesis, whereas it has been widely used in the case of liquid phase epitaxy as employed for the growth of bulk HgCdTe and HgTe based heterostructures. In addition, to reduce the toxicity of the mercury precursor, this procedure allows to achieve extremely concentrated syntheses (up to 400 mM [Hg]). The procedure is used to produce up to 7 g of material from a single synthesis carried out in conventional laboratory conditions (i.e. from a 100 mL total volume). In addition, the procedure leads to broadly tunable absorption from 1000 to 10000 cm -1 . Last, we demonstrate that the doping magnitude of the material can be controlled by the choice of the growth procedure, enabling the formation of p-n junction made of only one nanocrystal size.

EXPERIMENTAL SECTION

Nanocrystal synthesis

Chemicals : Te powder 30 mesh (Alfa Aesar, 99.99%), Se powder (Sigma-Aldrich, 99,99%), sulfur powder (Sigma Aldrich, 99,99%), bis(trimethylsilyl) sulfide (Sigma-Aldrich), (NH4)2S (Sigma Aldrich, 20 wt. % in H2O), mercury acetate (Sigma-Aldrich, ≥98%), mercury chloride (Sigma-Aldrich, ≥99,5%), mercury bromide (Strem Chemicals), mercury iodide (Strem Chemicals, 99%), iodine (Sigma-Aldrich, 99.99%), Trioctylphosphine (TOP, Cytek, 90%), dodecanethiol (DDT, Sigma Aldrich), ethanedithiol (EDT, Sigma-Aldrich, ≥98%), oleylamine (Acros, 80-90%), ethanol (VWR, ≥99.8%), methanol (VWR, ≥99.8%), toluene (VWR, ≥99.8%), lithium perchlorate (Sigma-Aldrich, 98%), polyethylene glycol (MW = 6 kg mol -1 ), the water used was purified with a Milipore system (18.2 M).

TOP:Te precursor (1 M) : 2.54 g of Te powder are mixed in 20 mL of TOP in a threeneck flask. The flask is kept under vacuum at room temperature for 1 hour. The atmosphere is then switched to Ar and the temperature is raised to 275 °C. The solution is stirred until a clear orange solution is obtained. The flask is then cooled down and the color switches to yellow. At room temperature, the flask is degassed again for 10 min. Finally, this solution is transferred to a glove box for storage. Mercury thiolate precursor: 3.18 g of mercury acetate are dissolved in 150 mL of water. The mercury solution is then added to a solution of 5 mL of dodecanethiol in 250 mL of ethanol. Mercury dodecylthiolate is precipitated immediately in white powder. After decanting, the solid product is filtered and washed several times with ethanol (150 mL). The white powder is dried and recrystallized in hot toluene.

Synthesis of HgTe nanocrystal from Hg thiolate: 0.6 g of mercury dodecylthiolate is dissolved in 4 mL of oleylamine and degassed at 100 °C for 10 min. Under Ar atmosphere, 1 mL of TOP:Te (1M) is injected to the thiolate solution at the desired temperature. The solution immediately turns from colorless to black-brown color. After 1 min, 1 mL of dodecanethiol is injected to quench the reaction. Nanocrystals are precipitated with 10 mL of methanol. After precipitation, the nanocrystals are redispersed with 2 mL of chloroform and few drops of dodecanethiol. The previous two steps are repeated twice with 5 mL of methanol. Finally, nanocrystals are redispersed in chloroform.

Synthesis from liquid mercury: 0.2 g of liquid mercury is sonicated in 2 mL of dodecanethiol during 15 min. The solution turn from colorless to a metallic grey color. 2 mL of oleylamine and 26 mg of iodine are added to the mercury solution and degassed at room temperature for 15 min. Slight discoloration is observed during the degassing step. Under argon atmosphere, 1 mL of TOP:Te (1 M) is injected to the mercury solution at the desired temperature. The solution color turns from grey to a black-brown solution. After 1 min, 1 mL of dodecanethiol is injected to quench the reaction. Nanocrystals are precipitated with 10 mL of methanol. After precipitation, the nanocrystals are redispersed with 2 mL of chloroform and few drops of dodecanethiol. The precipitation step is repeated with 5 mL of methanol. Finally, nanocrystals are redispersed in chloroform.

Large scale synthesis from liquid mercury

In a 250 mL three-neck flask, 8 g of liquid mercury is sonicated in 30 mL of dodecanethiol for 8 hours. After adding 1.04 g of iodine and 30 mL of oleylamine, the mercury solution is degassed at room temperature for 30 min. The atmosphere is flushed with Ar and placed again under vacuum. At 90 °C, 40 mL of TOP:Te (1 M) is injected under vacuum to increase the injection speed [START_REF] Yarema | Upscaling Colloidal Nanocrystal Hot-Injection Syntheses via Reactor Underpressure[END_REF] and the reaction is conducted for 3 min. The reaction is quenched by adding 20 mL of dodecanethiol. The temperature decreases with ice bath to room temperature. Nanocrystals are precipitated with 40 mL of methanol. After precipitation, the nanocrystals are redispersed with 20 mL of chloroform and few drops of dodecanethiol. The precipitation step is repeated with 10 mL of methanol. Finally, nanocrystals are redispersed in chloroform. In summary, only 100 mL of reaction medium and 50 mL of cleaning solvent are necessary to obtain ≈7 g of short wave infrared active nanomaterials.

Material characterization

Optical spectroscopy: Absorption spectra are acquired using a Shimadzu 3600 spectrometer for the UV visible part, while a Thermo Fischer IS50 Fourrier transform infrared spectrometer in ATR configuration for the near infrared and a Bruker Vertex 70 for the mid infrared are used.

Microscopy:

For transmission electron microscopy, we used a JEOL 2010. The grids were prepared by a drop-cast of diluted solution of nanocrystals dispersed in hexane and degassed overnight under secondary vacuum.

Diffraction: X-ray diffraction pattern is obtained by drop-casting the solution of nanocrystals on a Si wafer. The diffractometer is a Philips X'Pert, based on the emission of the Cu Kα line operated at 40 kV and 40 mA current.

Elemental analysis using X-ray dispersive spectroscopy (EDX):

For EDX analysis the particles are spread on a conductive carbon tape. The EDX analysis is performed by a FEI Magellan scanning electron microscope operated at 15 kV and 1.6 nA. The X-ray analysis is made with an Oxford probe.

Photoemission spectroscopy: 80 nm-thick gold is evaporated on a Si wafer and used as conductive substrate. The film of nanoparticles is deposited, and the pristine ligands are exchanged with ethanedithiol. The film is then introduced to the preparation chamber and then degassed until a vacuum in the 10 -10 mbar range is reached. The sample is finally loaded into the analysis chamber. During the XPS measurements, the photoelectrons are detected at normal emission from the polarization vector 𝐸 ⃗ . A photon energy of 1486.7 eV (Al Kα) was used. For the photoelectrons analysis we used 50 eV pass energy. To calibrate the system, we shift the data to set the energy of the C 1s core level equal to 284.8 eV. This shift is applied to all other spectra to get the actual kinetic energy value. Then all spectra are plotted as a function of binding energy following the formula BE = 1486.7 -cKE, where cKE is the corrected kinetic energy.

Transport measurements

Electrode fabrication: The surface of a Si/SiO2 wafer (400 nm oxide layer) is cleaned by sonication in acetone. The wafer is rinsed with isopropanol and finally cleaned using an O2 plasma. AZ 5214E resist is spin-coated and baked at 110 °C for 90 s. The substrate is exposed under UV through a pattern mask for 2 s. The film is further baked at 125 °C for 2 min to invert the resist. Then a 40 s flood exposure is performed. The resist is developed using a bath of AZ 726 for 32 s, before being rinsed in pure water. We then deposit a 5 nm chromium layer and 80 nm gold layer using a thermal evaporator. The lift-off is performed by dipping the film in acetone for 1 hour. The electrodes are finally rinsed using isopropanol and dried by an air flow. The electrodes are 2.5 mm long and spaced by 20 µm.

Electrolyte preparation:

For electrolyte gating, we first mixed 0.5 g of LiClO4 with 2.3 g of PEG (MW = 6 kg.mol -1 ) in a glove box. The solution is heated at 170 °C on a hot plate for 2 hours until the solution gets clear. To use the electrolyte, the solution is warmed around 80 °C and brushed on the top of the HgTe nanoparticle film.

Thin film deposition and ligand exchange: Ligand exchange is performed on solidstate film. The HgTe QDs film is prepared by drop-cast of a colloidal solution in chloroform on 20 µm spaced electrodes. The film is dipped in a 1-2 vol% EDT/ethanol solution for 1 min. Ethanol is used to rinse the film after each HgTe QD deposition and EDT ligand exchange cycle. A drop of viscous electrolyte is deposited on the nanocrystal film. After PEG solidification, the device is dried overnight under vacuum.

Transistor characterization:

The sample is connected to two Keithley 2400. The first, sets the drain bias (50 mV < VDS < 500 mV) and measures the associated drain current. The second, tunes the gate bias (VGS) between -2 and +2 V with a step of 1 mV and measures the associated gate current. All measurements are conducted under room conditions (temperature and pressure). A scheme of the device is given in Figure S18.

DISCUSSION

Current syntheses of mid-infrared HgTe NCs particularly suffer from the low solubility of HgCl2 in amines which are commonly used as coordinating solvent. Thus organosoluble precursors of mercury are of utmost interest. Here we take advantage of the strong affinity of Hg for sulfur to prepare mercury thiolate as the Hg source. [START_REF] Busupalli | Ultrathin Sheets of Metal or Metal Sulfide from Molecularly Thin Sheets of Metal Thiolates in Solution[END_REF] The latter is prepared by the reaction of Hg 2+ salt with dodecanethiol (DDT). DDT has been previously introduced in HgTe syntheses, however, the purpose was to slow down the reaction to grow small NCs. [START_REF] Geiregat | Continuous-Wave Infrared Optical Gain and Amplified Spontaneous Emission at Ultralow Threshold by Colloidal HgTe Quantum Dots[END_REF][START_REF] Kim | Bandgap Engineered Monodisperse and Stable Mercury Telluride Quantum Dots and Their Application for Near-Infrared Photodetection[END_REF] The obtained mercury dodecylthiolate precursor is a white powder, see Figure 1a, with a lamellar crystalline structure according to X-ray diffraction and scanning electron microscopy (SEM), see Figure 1a andS1. The reaction to synthesize HgTe nanocrystals (Figure 1b) is conducted using trioctylphosphine telluride (TOP:Te) as Te source in presence of oleylamine. The amine is critical to obtain non-aggregated NCs with a sharp band edge transition, see Figure S2.

Figure 1c displays the infrared spectrum of the obtained nanoparticles with various sizes obtained for different temperatures of synthesis. There are three main contributions in the spectra of these nanoparticles. (i) At large wavenumber (ie high energy) there is a broad band absorption relative to interband transitions (ie transition from valance band to conduction band). By tuning the reaction temperature from 50 to 180 °C the interband-edge energy can be tuned from 10000 cm -1 to 2000 cm -1 (equivalent to the range between 1.2 eV and 250 meV), see Figure S3, S14 and Figure 1c. (ii) At low wavenumber below 1000 cm -1 (ie low energy part of the spectrum) there is a peak relative to intraband transition. [START_REF] Shen | Synthesis of Nonaggregating HgTe Colloidal Quantum Dots and the Emergence of Air-Stable n-Doping[END_REF] This peak is only observed for the largest nanoparticles. This absorption is the result of self-doping, [START_REF] Jeong | Air-Stable n-Doped Colloidal HgS Quantum Dots[END_REF][START_REF] Robin | Surface Control of Doping in Self-Doped Nanocrystals[END_REF] which is a very specific doping mechanism of narrow band gap nanocrystal. [START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF] Because of their narrow band gap nature and because of their small non-stoichiometry (energy dispersive X-ray spectroscopy reveals an Hg excess, see table S1) the Fermi level can be above the conduction band leading to degenerate doping. In HgTe, this process only occurs when the band gap is narrow enough, 1,24 as a result only the largest nanoparticles present this behavior. (iii) Last, the spectrum presents some sharp lines resulting from organic and this is mostly associated with the capping ligands. Here dodecanethiol is the main grafted molecule and we observe clear signatures for C-H bond stretch (2924 cm -1 ), C-H bond bending (1466 cm -1 ), C-S bond at 725 cm -1 . CO2 contribution at 2340 cm -1 may also appear due to the fact that measurements are conducted in air.

Because HgTe is a semimetal (i.e. zero band gap semiconductor) and has a large Bohr radius [START_REF] Izquierdo | Strongly Confined HgTe 2D Nanoplatelets as Narrow Near-Infrared Emitters[END_REF][START_REF] Rinnerbauer | Effect of Quantum Confinement on Higher Transitions in HgTe Nanocrystals[END_REF] (≈40 nm), we can directly attribute the observed spectral tunability to quantum confinement: NCs size is tuned from 3 to 25 nm with the synthesis, see Figure 1b and Figure S4. Obtained NCs have a zinc blende lattice (Figure S5) and are cationrich according to energy dispersive X-ray spectroscopy, see Table S1. While mercury thiolate appears as a promising route, we aim to use this strategy to reduce the toxicity of the protocol. Commonly used Hg 2+ salts are strongly water (and then blood) soluble, which is not the case of liquid mercury. [START_REF] Daeneke | Liquid Metals: Fundamentals and Applications in Chemistry[END_REF] In the following, we propose a synthetic procedure where the mercury thiolate is prepared in situ from liquid mercury. We sonicate liquid Hg in DDT for several hours [START_REF] Pokroy | Sonication-Assisted Synthesis of Large, High-Quality Mercury Thiolate Single Crystals Directly from Liquid Mercury[END_REF] and then obtain a mixture of mercury thiolate crystals and colloidal liquid Hg, [START_REF] Ramesh | Mercury Nanodrops and Nanocrystals[END_REF] see Figure 2a andb and Figure S7.

If the reaction toward HgTe NCs is conducted from this mixture, we collect a significant amount of colloidally unstable material. The reaction leads to the formation of a HgTe nanorod array, see Figure S8 and S9. The addition of iodine [START_REF] Hudson | Conduction Band Fine Structure in Colloidal HgTe Quantum Dots[END_REF] in the reaction medium favours the formation of the stable fraction, see Figure S10. In this case, iodine is expected to behave as an oxidizing agent for Hg 0 toward Hg 2+ . [START_REF] Hudson | Conduction Band Fine Structure in Colloidal HgTe Quantum Dots[END_REF] This method allows a broad tuning of the band-edge energy from 8000 to 1000 cm -1 for interband transitions and up to 500 cm -1 for intraband transitions, see Figure 2d and S11. The obtained NCs have a non-spherical shape, see Figure 2c andS12.

There are two important details that remain to be revealed about this procedure, including the crystallinity and the actual redox state of Hg. We confirm that obtained nanoparticles are crystalline with a zinc-blende atomic lattice (Figure 2f). It is worth mentioning that EDX also confirmed that the formed NCs are cation-rich. Using photoemission spectroscopy, see Figure 2e, S6 and S13, we observe a single contribution for the Hg 4f state. The binding energy of the Hg 4f7/2 is determined to be 100.3 eV which is consistent with the +II redox state of Hg. Within the photoemission resolution, we can thus exclude the presence of metallic Hg in the final material.

Hg as precursor. (d) Absorbance spectra of HgTe QDs synthesized from 50 °C to 200 °C using liquid Hg as precursor. * is here to highlight the intraband contribution of the absorption spectrum. (e) X-ray photoemission signal, relative to the Hg 4f state, obtained from a film made of HgTe NCs and for which the Hg precursor is liquid Hg. (f) X-ray diffraction pattern obtained from a film made of HgTe NCs and for which the Hg precursor is liquid Hg. The diffractogram is consistent with the zinc blende phase of HgTe.

A second key advantage of this approach based on liquid mercury is the possibility to conduct highly concentrated syntheses leading to several grams of NCs per experiment. [START_REF] Williamson | Prodigious Effects of Concentration Intensification on Nanoparticle Synthesis: A High-Quality, Scalable Approach[END_REF] A significant effort has been conducted to achieve gram scale synthesis for PbS [START_REF] Yarema | Upscaling Colloidal Nanocrystal Hot-Injection Syntheses via Reactor Underpressure[END_REF][START_REF] Cademartiri | Solventless, and Diffusion-Controlled Route to Highly Monodisperse PbS Nanocrystals[END_REF][START_REF] Bera | Solvent-Less Solid State Synthesis of Dispersible Metal and Semiconducting Metal Sulfide Nanocrystals[END_REF][START_REF] Hendricks | A Tunable Library of Substituted Thiourea Precursors to Metal Sulfide Nanocrystals[END_REF] , but this effort is still lacking for HgTe which has been identified as a major limitation toward industrial transfer of mid-IR NCs. [START_REF] Livache | Road Map for Nanocrystal Based Infrared Photodetectors[END_REF] Here, we have been able to develop a procedure which allows the synthesis of 7 g of HgTe NCs out of 100 mL of reaction medium and which used only 50 mL of solvent for two steps of cleaning, see Figure 3a, b and Figure S15. The obtained material, observed by TEM presents similar shape and absorption features as the material synthesized with a reduced quantity of precursors, see Figure 3c. The final concentration of NCs in the reaction medium is around 100 g.L -1 , which is by far the largest value reported for HgTe, see Figure 3d.

It has been previously discussed by Livache et al, [START_REF] Livache | Road Map for Nanocrystal Based Infrared Photodetectors[END_REF] that the introduction of a 1 cm 2 large infrared sensor in every car sold in Europe (20 millions/year) will require the synthesis of 50 kg of HgTe nanocrystals. Such amount also includes 90% of waste (from spin coating) at the fabrication steps. Previous HgTe nanocrystal methods were incompatible with mass scale production leading to such amounts. Here we have been able to obtain 7 g from less than 100 mL of reaction medium. If we assume that an operator works around 200 days/year, a simple benchtop setup can already lead to 1.4 kg/year. In SME companies, the use of large glass reactor is now well established. The amount of 50 kg can potentially be fully produced from a single 30 L reactor. [START_REF] Green | A Simple Metalorganic Route to Organically Passivated Mercury Telluride Nanocrystals[END_REF][START_REF] Li | Room Temperature Synthesis of HgTe Nanocrystals[END_REF][START_REF] Piepenbrock | A Low-Temperature Synthesis for Organically Soluble HgTe Nanocrystals Exhibiting Near-Infrared Photoluminescence and Quantum Confinement[END_REF][START_REF] Kovalenko | Colloidal HgTe Nanocrystals with Widely Tunable Narrow Band Gap Energies: From Telecommunications to Molecular Vibrations[END_REF][START_REF] Kim | Bandgap Engineered Monodisperse and Stable Mercury Telluride Quantum Dots and Their Application for Near-Infrared Photodetection[END_REF][START_REF] Im | Efficient HgTe Colloidal Quantum Dot-Sensitized near-Infrared Photovoltaic Cells[END_REF][START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF][START_REF] Keuleyan | Photoluminescence of Mid-Infrared HgTe Colloidal Quantum Dots[END_REF][START_REF] Keuleyan | Mercury Telluride Colloidal Quantum Dots: Electronic Structure, Size-Dependent Spectra, and Photocurrent Detection up to 12 Μm[END_REF][START_REF] Nengjie | MoS2-HgTe Quantum Dot Hybrid Photodetectors beyond 2 Μm[END_REF][START_REF] Shen | Synthesis of Nonaggregating HgTe Colloidal Quantum Dots and the Emergence of Air-Stable n-Doping[END_REF][START_REF] Hudson | Conduction Band Fine Structure in Colloidal HgTe Quantum Dots[END_REF] .

(d) Comparison of mass yield from this work to previous studies assuming a total conversion of the limiting reagent to a HgTe material from references

The developed strategy based on Hg thiolate and in situ formed Hg thiolate is not limited to HgTe and can be extended to other chalcogenides (i.e. HgS and HgSe), see Figure 4b andc. As for previously reported syntheses, the spectrum of HgS [START_REF] Jeong | Air-Stable n-Doped Colloidal HgS Quantum Dots[END_REF] and HgSe [START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF][START_REF] Deng | Colloidal Quantum Dots Intraband Photodetectors[END_REF] presents an intraband feature in the mid-infrared, see Figure 4a, S16 and S17. It is worth noting that the chalcogenide precursors cannot be TOP:S and TOP:Se and we rather use (NH4)2S and Se in oleylamine and DDT respectively for the synthesis of HgS and HgSe. To demonstrate the potential of this new material for optoelectronic applications, we probed their transport properties in a field effect transistor. [START_REF] Lhuillier | Mid-Infrared HgTe/As2S3 Field Effect Transistors and Photodetectors[END_REF] We used an ion gel electrolyte configuration, see Figure S18 for a scheme of the setup. [START_REF] Lhuillier | Investigating the N-and p-Type Electrolytic Charging of Colloidal Nanoplatelets[END_REF] The large capacitance of the electrolyte allows to broadly tune the relative position of the Fermi level with respect to the band over several hundred of meV, which is similar to the band gap value of this material. As a result, it is possible to achieve both hole and electron injection in this material. Consequently, all material appears ambipolar, we nevertheless observe a clear change in the ratio of hole vs electron conduction with the growth method. It was previously demonstrated [START_REF] Jagtap | Design of a Unipolar Barrier for a Short-Wave Infrared Photodiode[END_REF] that HgTe NCs show a transition from p-type to n-type as size (and confinement) gets increased (respectively reduced). We have measured the transfer curves of HgTe NCs based transistors with various sizes for the three types of synthesis based on: (i) HgCl2, (ii) Hg thiolate and (iii) liquid Hg, see Figure S19-S21. They do present, as expected, a transition of the conduction from p to n-type as the NC size is increased, see Figure 5d. Note that such transition is also consistent with the appearing of the intraband contribution in the infrared spectrum of the largest nanoparticle which indicate a stronger doping magnitude.

However, the threshold of this transition varies from synthesis to synthesis. For a given size, the synthesis based on liquid Hg tends to be more n-type, while the one based on thiolate is more p-type, see Figure 5a-c. Because we observe from photoemission no trace of Hg 0 , we can exclude that the change of doping is the result of a change of the Hg redox state. Alternatively, we can speculate that each growth condition affects the facet which are the most present on the nanocrystal surface. This leads to small modification of the surface Hg excess and finally tune the doping.

In the last part of the paper we discuss the use of this material (HgTe prepared from liquid Hg) for infrared sensing. We synthetize particle with a band gap at 4900 cm -1 (600 meV) corresponding to an absorption the extended short-wave infrared, see Figure 6a. Once the material is processed under thin and the ligand exchange for ethanedithiol (EDT) the band edge energy remains almost unchanged but the feature gets broader. Transport in HgTe nanoparticles connected to gold electrodes appears to be ohmic, see Figure 6b. More striking is the temperature dependence of the material, which presents a large decay of the film conductance upon cooling, see the inset of Figure 6b. An Arrhenius fit of the current leads to an activation energy of 226 meV. This is quite close to half the value of the optical band gap (600/2=300 meV) which suggests a quasi-intrinsic nature for the semiconductor. It is also worth pointing that this value is also much larger than the one obtained for HgTe nanoparticles resulting from the mercury halide precursor. In the latter case, the activation energy is 150 meV for a band gap of 720 meV 52 (6000 cm -1 ) or 114 meV for a band gap of 500 meV [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF] (4000 cm -1 ). Last the time response of the sample has been tested under illumination by a 1.55 µm laser diode, see Figure 6c. Turn on time has been estimated to be 21 µs. For the decay time, we observe two components a fast one around 3µs and a longer one at 153 µs. All characteristic time remains compatible with fast imaging.

CONCLUSIONS

We propose a new synthetic strategy for the synthesis of mercury chalcogenides NCs. The method relies on the use of Hg thiolate as the Hg source. We demonstrate that the Hg thiolate can be prepared in situ from liquid mercury. This approach can be used to obtain highly concentrated NCs solutions (≈100 g.L -1 ). Significant amount (7 g) of infrared NCs can be obtained from extremely reduced reaction volume. Compared to conventional HgTe synthesis, the method reduces exposition of workforce to hydrophilic mercury species and drastically decreases the solvent consumption. The procedure can be extended to HgS and HgSe. The transport properties of the obtained HgTe have been tested in a transistor configuration and we observe a transition from p to n-type with size. The size threshold is strongly dependent on the way the NCs are grown. This is promising for the design of p-n photodiodes with enhanced photocharges dissociation. Another promising direction will relate to the use of amalgam. Many noble metal can be mixed in liquid Hg, [START_REF] Mertens | Au@Hg Nanoalloy Formation Through Direct Amalgamation: Structural, Spectroscopic, and Computational Evidence for Slow Nanoscale Diffusion[END_REF] this approach may be use to further control the doping of the nanoparticle though the introduction of impurities with redox state different from +II. Last, we have tested the potential of this material for IR sensing and demonstrate similar performances to previously reported synthesis.

ASSOCIATED CONTENT

Figure 1 (

 1 Figure 1 (a) Optical and scanning electron microscopy SEM pictures of mercury dodecylthiolate (Hg(DDT2)) and its use to grow HgTe NCs. (b) TEM pictures of HgTe QDs synthesized with Hg(DDT2) at 150 °C. (c) Absorbance spectra of HgTe QDs synthesized from 50 °C (blue) to 200 °C (orange) using Hg(DDT2) as Hg precursor. * is here to highlight the intraband contribution of the absorption spectrum.

Figure 2 (

 2 Figure 2 (a) Image of a liquid Hg drop. (b) Optical and SEM pictures of the solution of mercury dodecylthiolate prepared from liquid Hg. (c) TEM picture of HgTe QDs synthesized using liquid

Figure 3 (

 3 Figure 3 (a) Result of a 100 mL synthesis made from liquid Hg and leading to a highly concentrated solution of HgTe NCs. (b) Dried powder of 6.9 g HgTe QDs obtained with 100 mL of synthesis solvent. (c) Absorption spectrum of the HgTe NCs obtained after the synthesis described in part (a) and (b). The picture in the back is a TEM image of the obtained HgTe NCs. (d) Comparison of mass yield from this work to previous studies assuming a total conversion of the limiting reagent to a HgTe material from references43-45,21,28,46,22,47,23,48,24,37 .

Figure 4 (

 4 Figure 4 (a) Absorbance spectra of HgSe (red) and HgS (blue) QDs synthesized with mercury dodecylthiolate. * is here to highlight the intraband contribution of the absorption spectrum. TEM pictures of (b) HgSe and (c) HgS QDs. Data relative to synthesis with liquid Hg are reported in S16 and S17.

Figure 5

 5 Figure 5 Transfer curves of HgTe QDs with excitonic feature around 4000 cm -1 synthesized with liquid mercury (a), mercury halogenide (b) and mercury dodecylthiolate (c) with drainsource current (black) and gate-source current (blue). (d) Mobility ratio of electrons and holes for HgTe QDs synthesized using the three different mercury precursors and with different confinement energies (ie taken equal to the interband transition energies).

Figure 6a .

 6a Figure 6a. absorption and photocurrent spectra of HgTe nanocrystals obtained from liquid Hg synthesis. b. I-V curve from a thin film of HgTe nanocrystals obtained from liquid Hg synthesis after EDT ligand exchange. The inset provide the temperature dependence of the current for thin film of HgTe nanocrystals obtained from liquid Hg synthesis. The red curve is an arrhenius'fit of the data revealing an activation energy of 226 meV. c. Current as a function of time for a thin film of HgTe nanocrystals obtained from liquid Hg synthesis under illumination by a 1.55 µm laser turning on and off. The laser in on state corresponds to the grey part of the graph.

ACKNOWLEDGMENT

EL acknowledges the financial support of the European Research Council (ERC) starting grant (blackQD -n°756225). We thank Agence Nationale de la Recherche for funding through grant IPER-nano2, Copin, Graskop and Frontal. This work was 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. We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre". This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K. JQ thanks Chinese Scholar Council for PhD. funding. AC thanks Agence Innovation Defense for PhD funding.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Supporting information include chemical and details about material characterization, as well as methods for precursors and nanocrystal synthesis. Additional data relative to electronic transport are also provided (PDF).

AUTHOR INFORMATION

Corresponding Author nicolas.goubet@sorbonne-universite.fr and el@insp.upmc.fr


Charlie Gréboval

Eva Izquierdo

Clément Livache

Bertille Martinez

Marion Dufour

Nicolas Goubet

Nicolas Moghaddam

Junling Qu

Audrey Chu

Julien Ramade

Hervé Aubin

Hervé Cruguel

Mathieu Silly

Emmanuel Lhuillier

Sandrine Ithurria

Impact of dimensionality and confinement on the electronic properties of mercury chalcogenide nanocrystals

Keywords: nanoplatelets, HgTe, confinement, dimensionality, carrier dynamics, infrared

We demonstrate the growth of 2D nanoplatelets (NPLs) made of a HgTe/CdS heterostructure, with an optical absorption reaching the shortwave infrared range. The material is an interesting platform to investigate the effect of dimensionality (0D vs 2D) and confinement on the electronic spectrum and carrier dynamics in colloidal materials. We bring consistent evidence for the p-type nature of this material from transport and photoemission measurements. The majority carrier dynamics probed using pump-probe photoemission is found to be mostly dependent on the presence of a confinement barrier at the surface rather than on the material dimensionality. The minority carrier, on the other hand, is strongly affected by the material shape showing longer lived minority carrier in 2D NPLs compared to their 0D equivalent with similar band gap. Finally, we test the potential of this material for photodetection in the short-wave infrared range (SWIR) and show that fast photoresponse and detectivity reaching 10 9 Jones at room temperature can be achieved.

INTRODUCTION

Among colloidal quantum dots (CQDs), 2D nanoplatelets 1,2 (NPLs) of II-VI semiconductors present unique optical properties with an exceptionally narrow photoluminescence (PL) signal. 3 In the visible range, this makes CdSe NPLs highly promising for the design of wide gamut display. This property results from their specific growth mechanism [4][5][6] which induces no roughness along the only confined direction. However, this growth mechanism was up to recently limited to Cadmium chalcogenides with optical features in the visible range of wavelengths.

Imaging in the short-wave infrared range of wavelengths (between 1 and 3 µm, SWIR) is useful in many applications such as active imaging, bio-imaging, telecom or LIDAR detection. To achieve absorption in the SWIR, narrow band gap materials 7,8 have to be used. PbS nanosheets 9,10 have been synthetized 11,12 and show tunable absorption 13 almost up to the bulk band gap value. 14 They also present a large lateral extension [15][16][17] (up to several µm) which is of great interest for the design of single-particle devices such as field effect transistors 18,19 (FETs) and photodetectors. 9 On the other hand, PbS nanosheets remain quite strongly aggregated and their optical features are broad due to scattering and lack of atomic scale control of the thickness. In this sense, 2D colloidal materials with well-defined IR optical features were lacking up to the recent synthesis of HgX NPLs by Izquierdo et al. 20 These NPLS were obtained from a cation exchange on cadmium chalcogenides NPLs.

Mercury chalcogenides, especially HgTe and HgSe, are interesting candidates for SWIR detection because they are semimetals under bulk form. Consequently, band edge energy of HgTe or HgSe nanocrystals is only due to confinement and can theoretically be as low as wanted 21 . The HgTe NPLs obtained by cation exchange show narrow absorption and PL in the near infrared (IR) range (around 900 nm). The cation exchange is nevertheless self-limited, meaning that only alloys 22,23 have been obtained in the case of quantum dots and only surface exchange over the first two monolayers is achieved for NPLs 24 . In practice attempts to conduct the process on NPLs thicker than 3 monolayers have led to CdX-HgX core-shell heterostructures. 24 The exchange is limited to the two external planes of the NPLs. These 2D heterostructures based on HgX were not able to reach the SWIR range.

To keep the advantages of a 2D geometry while pushing the band edge energy in the SWIR range, one has to find a way to decrease the confinement along the thickness of HgTe nanoplatelets. In this paper, we describe the growth of a CdS shell on HgTe nanoplatelets to obtain tunable optical features in the SWIR range. Beyond the possibility to push the absorption toward redder wavelengths, the promise of such heterostructures is also to reduce the exciton binding energy. In 2D NPLs, the binding energy 25,26 is larger than their 0D counterpart (HgTe 2D nanoplatelets are certainly the most confined materials with a confinement energy around 1.5 eV), which prevents the effective splitting of the electron-hole pair. 27 Building a heterostructure has previously been demonstrated as a viable way to enhance charge dissociation. 28 This is why in the following, we investigate the electronic properties of the HgTe/CdS heterostructure. More specifically, we focus on the spectrum of this material in absolute energy scale which is critical for the future integration of the material into devices. We then compare the carrier relaxation dynamics of majority and minority carriers of HgTe NPLs (2D material), HgTe/CdS NPLs (2D material + confinement) and HgTe CQDs (0D material with same band gap as the HgTe/CdS NPLs) to get the influence of dimensionality and confinement on transport. To do so, we use a combination of wide bandwidth transient photocurrent measurements, pump-probe photoemission and transport measurements conducted in a FET configuration. Finally, we investigate the photoconductive properties of the HgTe/CdS NPLs and discuss how the observed changes in carrier dynamics impact their detection performances.

DISCUSSION

This paper is focused on the optoelectronic properties of HgTe/CdS NPLs because they combine a 2D aspect with a reduced confinement. The growth of HgTe/CdS NPLs relies on three steps, see Figure 1a: (i) growth of CdTe NPLs using the procedure described by Pedetti et al, 29 (ii) cation exchange to replace cadmium atoms by mercury ones, according to the procedure of Izquierdo et al. 20 And (iii) the CdS shell growth by colloidal atomic layer deposition (c-ALD) 30 .

The synthetized core NPLs present three monolayers in thickness (ie there are 3 plans of Te surrounded by 4 plans of cations). This corresponds to a thickness of 1.1 nm, while the lateral extension is typically of 100 nm x 200 nm. The shape of the particles is preserved during these steps as confirmed by electron microscopy, see Figure 1b andc. The initial CdTe NPL spectrum presents a very narrow absorption feature at 500 nm, see Figure 1e. After the cation exchange the absorption edge is strongly redshifted and appears at 870 nm. The growth of the CdS shell is conducted using the room temperature c-ALD procedure. 30 The method relies on the successive exposition of the NPLs to cation and anion precursors. The most conventional method is to phase-transfer the particles from a non-polar solvent to a polar solvent (N-Methyl Formamide typically) and conducts the growth in this polar phase. However, this approach has been unsuccessful (ie no shift of the excitonic feature, nor evidence of Cd deposition in energy dispersive X -ray spectroscopy). We thus choose to conduct the c-ALD while keeping the material into the non-polar phase, see experimental section for details. The 2D shape of the material is preserved, but the particles are enrolled due to the lattice mismatch (≈10%) between HgTe and CdS, see Figure 1d, S1 and S2 form TEM imaging. While the diffraction pattern, see figure S3 and table S1, is almost unaffected by the cation exchange step, because CdTe and HgTe have very close lattice parameter, we observe a strong modification of the peak relative to HgTe after the growth of the CdS shell. In particular, we can notice a contraction of the lattice parameter of 5%. After the growth of the CdS shell, the absorption is again strongly redshifted and appears at 1200 nm with 1 layer of CdS, see Figure 1e. As a thicker (3ML) CdS shell is grown, the absorption is further redshifted and the band edge reaches 1600 nm. This shift is the combination of reduced confinement and strain 31 . This is the first time that HgTe based 2D NPLs reach absorption in the SWIR range. In the following, we will describe a HgTe NPL with n layers of CdS shell as HgTe/nCdS. We will more specifically focus on HgTe/3CdS since it has a similar band gap energy as InGaAs semiconductor which is the current leading technology for photodetection in the SWIR range. Beyond the optical shift, the growth of the CdS shell can be confirmed by energy dispersive X-ray spectroscopy. Table 1 and Table 2 confirm that the CdS material is incorporated in the heterostructure in a very similar amount as the one expected for infinite layers. In addition, photoemission measurements, conducted on the Tempo beamline of Soleil synchrotron, further confirm the presence of Cd and S in the final material, see Figure 2a. A detailed analysis of the core levels is given in Figure S5. The electronic spectrum of the material is reconstructed using a combination of photoemission and infrared spectroscopy. Photoemission is used to locate valence band and vacuum level with respect to the Fermi level which reveals the doping and work function of the material, see Figure 2b. IR spectroscopy is used to determine both the band edge energy and the conduction band energy. The work function of the material is found to be 4.7±0.1 eV, see Figure 2c, which is consistent with previous observation on HgTe nanocrystals [START_REF] Jagtap | ACS Photonics[END_REF] .

The energy profile of the HgTe/3 CdS NPLs is compared with the one obtained for pure HgTe NPLs (with a larger confinement) and with HgTe CQDs (typical size 6 nm) with similar band gap in order to understand the effect of material dimensionality, see Figure 2c andd. Because CdS has a wide band gap (2.4 eV), it forms with HgTe a type I heterostructure [START_REF] Nethercot | [END_REF] . Nevertheless the (quantum and dielectric) confinement is so strong in the HgTe NPLs that the wavefunction partly delocalized in the CdS shell, leading to the observed reduction of band edge energy. This is consistent with the band edge shift observed on HgTe NPLs as the surface chemistry is modified 20 . In the following, we will study how this barrier not only affects the spectrum but also the carrier dynamics. Small size HgTe CQDs can be grown to have a similar band gap as the HgTe/3CdS NPLs, in this case the energy profile is found to be very similar, see Figure 2c. Those two materials will be compared to understand the effect of the CdS confinement. The relative position of the Fermi level with respect to the middle of the band gap is used to determine the nature of doping in the material. In all cases, the Fermi level is found to be closer to the valence band suggesting a p-type character for the material, see Figure 2c. Again this is consistent with previous observation on HgTe where a switch from p-type to n-type has been observed with the reduction of confinement energy. 21,26 The HgTe NPLs and QDs being strongly confined (> 700 meV of confinement), we expect them to be p-type. The nature of the majority carrier is also confirmed by transport measurements. In this case, a thin film of NPLs or QDs is prepared and crosslinked using ethanedithiol as coupling ligand to increase the inter-nanoparticle electronic coupling. A mixture of LiClO4 in polyethylene glycol is used as ion-gel electrolyte 34 , see Figure S7. This electrolyte gate combines several advantages such as a large capacitance, air operability, low bias operability and possibility to gate thick films thanks to the diffusion of ions within the nanocrystal film. The transfer curves are given in Figure 3a-c and present a current modulation (on/off ratio) above two orders of magnitude. HgTe NPLs with and without shells as well as HgTe CQDs exhibit a clear increase of the conductance under hole injection (ie negative gate bias), while the electron conduction remains limited.

The temperature dependency of the transport has been investigated, see Figure 3d-f. Close to room temperature, the I-T curve is well fitted by an Arrhenius law. The extracted activation energy, which quantifies the impact of temperature on the device dark current, is typically 250 meV for the HgTe NPLs. HgTe/3CdS and the HgTe CQDs present a lower activation energy of 140-150 meV. This value is quite close to the measured energy spacing between the valence band and the Fermi level and thus reflects the thermal activation of holes up to the valence band. We finally use a third strategy to investigate the nature of the majority carriers in this confined form of HgTe. To do so, we have used transient photovoltage measurements conducted in a pump-probe configuration. [35][36][37] A scheme of the setup is given in Figure 4a. A short (80 fs) pulse of light (λ = 800 nm) is generated from a Ti:Sapphire laser and shone on the sample. After this pump excitation, a 600 eV photon source from the synchrotron is used to probe every 10 ns the photoemission signal from the Hg 4f state. The binding energy is then plotted as a function of time and the sign of the observed energy shift is a signature of the majority carrier nature. More details about the principle of this measurement can be found in the supporting information, see figure S6.

In all cases, we have observed a negative shift (ie a decrease of the binding energy) which is consistent with a p-type semiconductor. The decay time of the binding energy is related to the majority carrier diffusion time. [START_REF] Jagtap | ACS Photonics[END_REF] The hole lifetime is found to be 230-240 ns for HgTe NPLs without CdS barrier and for HgTe QDs, see Figure 4b. It thus poorly depends on band gap value nor dimensionality. On the other hand, the introduction of the CdS shell considerably increases the hole lifetime up to ≈ 1 µs. CdS presents a type I band alignment with the HgTe semimetal. CdS forms a barrier in addition to the existing one resulting from the ligand, which slows down the hopping transfer from one nanoparticle to another. As a result, we can conclude that the dynamics of majority carriers is driven by the confining barrier. We also investigated the dynamics of the minority carriers (ie the electrons). Here we used a wide dynamic setup 38 which combines both broad temporal bandwidth (from ns to 10 ms) and large signal dynamics (4 orders of magnitude typically). A scheme of this transient photocurrent experiment is given as an inset of Figure 5a. Briefly, a short (0.9 ns) UV (λ = 355 nm) pulse is shone on a thin film of nanoparticles deposited on biased interdigitated electrodes. The wide dynamic of the setup reveals a long living photoresponse, see Figure 5a. As discussed previously by Martinez et al, 21 the temporal response can be split in two parts. At early time scale, there is a fast decay corresponding to carrier recombination, this decay time generally matches well with the PL decay time. 39 In the case of HgTe CQDs, we observe a fast decay of 0.5 ns, see Figure 5b. This measurement is actually limited by the pulse length which is consistent with previously reported PL decay 40 measured on similar size of HgTe CQDs for which a bi-exponential behavior was observed with decay time of 50 ps and 2 ns. On the other hand, the decay time measured for the HgTe and the HgTe/3CdS NPLs is longer than the setup resolution and has been determined to be 1.8 ns. This result is quite promising for the integration of such NPLs for photodetection since this decay time will drive the magnitude of the photoresponse.

At long time scale, the photocurrent follows a power law, see Figure 5c, which is the signature of multi-trapping transport. 41,42 In this regime, charges which have been trapped at early times are thermally released at the band edge. The deeper the trap, the longer it takes to be released. The exponent of the power law fit (b≈0.38, on Figure 5c) directly relates to the Urbach energy (EU) which describes the in-gap distribution of traps, through the relation

U b E T k b + - = 1
with kbT the thermal energy. While the value of the Urbach energy has been found to be strongly dependent on the nanocrystal surface chemistry, 21 it appears that particle shape and presence of barrier barely affects the trap distribution. We measured a value of 44±3 meV for all materials, see Figure 5c. To conclude, we have investigated the potential of these HgTe/3CdS NPLs for detection in the SWIR. First, we have measured the photocurrent spectrum and compared it with the absorption of the nanoparticles before ligand exchange. As often, with thiol ligand exchange, the optical feature gets broader. 43 More surprisingly, we have observed a blue shift of the transition, see Figure 6a. This suggests that the NPL surface is partly etched as ligand exchange is conducted and that thiols strip some of the surface cadmium. The material presents a clear photoresponse under excitation by a laser source at 1.55 µm, see Figure 6b, with a strong increase of the conductance under illumination. The responsivity has been determined and is typically in the few 100s µA/W under illumination in the mW range, see Figure 6c. 44,45 The device also presents a fast time response, see Figure 6d andS3, with a rise and fall time below 10 µs which actually is limited by the laser rise time in this experiment. The noise (see Figure S9) has been determined to be 1/f limited, see the inset of Figure 6e. This leads to device detectivity reaching 10 9 Jones at room temperature and under low flux, see Figure 6e. Performance-wise the material is not reaching state of the art device made of PbS and operated at the same wavelength. 46 This difference results from the much larger dielectric constant of PbS 47 (εPbS = 20-40) compared to HgTe 48 (εHgTe = 13): as a result, charges behave as quasi-free carriers in PbS while the exciton remains strongly bound in HgTe. In addition, the 2D aspect of the NPLs further enhanced the exciton binding energy. 26 It was nevertheless proposed that such difficulty can be overcome by integrating the NPL into nanotrench device 27 where the high exciton binding energy can be balanced by the application of a high electric field.

CONCLUSION

We demonstrate the possibility to grow heterostructure from Hg based 2D nanoplatelets with optical absorption in the short-wave infrared. This material presents a far better-defined optical feature than the one previously for 2D materials based on lead chalcogenides. The material is determined to be p-type from transport and photoemission measurements. We demonstrate that the majority carrier lifetime in confined HgTe nanocrystals is poorly depending on dimensionality but depends on confinement and is considerably extended in the case of HgTe/CdS compared to HgTe quantum dots at the same band gap. On the other hand, the minority carrier lifetime is found to be longer in 2D NPLs compared to 0D HgTe CQDs and is poorly dependent on the presence of the shell. Finally, we demonstrate that HgTe/3CdS can be used for the design of fast (< 10 µs response time) short wave infrared photodetector operated at room temperature. Integration of this material into nm scale electrode is proposed as a path to enhance the charge dissociation and reach higher detectivity.

Experimental details

Chemicals. Te powder (Sigma-Aldrich, 99.99 %), Cadmium oxide (CdO, Strem Chemicals, 99.99 %), Mercury acetate (Hg(Ac)2, Sigma-Aldrich, > 99.0 %), Oleylamine (OAm, Acros, 80-90 %), Trioctylphosphine (TOP, Cytec, 90 %), Propionic acid (Aldrich, 99 %), Cadmium bromide tetrahydrate (CdBr2,(H2O)4, Aldrich, 98%), Ammonium sulfide solution ((NH4)2S, Fluka), Oleic acid (Aldrich, 90 %), Trioctylamine (Alfa Aesar, 95%), Octadecene (ODE, Aldrich, 90 %), Ethanol (VWR, > 99.9 %), n-Hexane (VWR, 98 %), Acetone (VWR, 99 %), Methanol (Carlo Erba, > 99,9 %).

Precursors

Cadmium propionate. Cadmium propionate (Cd(prop)2) is obtained by mixing 1.036 g of CdO powder with 10 mL of propionic acid under argon flow for 1 h at 70 °C. The flask is then open and heated at 140 °C until the volume gets reduced by a factor two. The white solution is precipitated with acetone. The solution is centrifuged and the supernatant discarded. The solid is dried under vacuum at room temperature for 24 h.

TOPTe (1 M). In a three-neck flask, 2.54 g of Te powder are mixed with 20 mL of trioctylphosphine and degassed under vacuum at room temperature until the pressure gets reduced below 1 mbar. Then the flask is heated under argon at 275 °C until the black Te powder gets fully dissolved. The orange solution is then cooled and turns yellow. Once the temperature is below 40 °C, the solution is further degassed under vacuum for 10 min and finally stored in an air free glove box.

Sulfide solution. Ammonium sulfide solution (100 µL, 0.3 mmol), oleylamine (125 µL, 0.4 mmol) and 2 mL of hexane are introduced in a flask. 200 µL of methanol are added to remove water. A biphasic mixture is obtained. To remove the excess of polysulfide, 75 µL of TOP (0.2 mmol) are introduced leading to the formation of TOPS.

Nanocrystals synthesis.

Synthesis of 3 MLs CdTe NPLs. In a three-neck flask, Cd(prop)2 (390 mg, 1.5 mmol), oleic acid (240 µL, 0,75 mmol) and 30 mL of octadecene are degassed under vacuum at 90 °C for 1 hour. Under argon flow, TOPTe at 1 M (300 µL) diluted in 1.5 mL of octadecene is swiftly injected at 210 °C. After 30 minutes of reaction, the mixture is cooled down and 1.5 mL of oleic acid is injected. The resulting nanoplatelets are precipitated twice with an excess of ethanol and are redispersed in 30 mL of hexane.

Cation exchange for synthesis of 3 MLs HgTe NPLs. In a flask, 110 mL of hexane and 2 mL of CdTe nanoplatelets are introduced. At room temperature, 5 mL of a freshly prepared solution at 0.01 M of Hg(Ac)2 in trioctylamine are introduced (the ratio Cd:Hg is 1:2). The cation exchange is followed by absorption spectroscopy. Once the first excitonic peak has reach 824 nm, 1 mL of oleic acid are injected to stabilize the HgTe NPLs. The excess of precursor is removed by centrifugation (9000 rpm, 5 min). The nanoplatelets are then resuspended in hexane for further experiments.

Shell growth by c-ALD. 1 mL of the 3 MLs HgTe NPL solution is diluted in 1 mL of hexane. 100 µL of the sulfide solution is introduced in the NPLs solution. After a quick stirring, NPLs are precipitated by centrifugation (10 000 rpm, 30 seconds). The nanoplatelets are redispersed in 2 mL of hexane. The NPLs are stable in apolar phase due to the oleylammonium on their surface. To grow the half layer of Cd, 200 µL of a solution of CdBr2,4H2O in ethanol at 0.1 M are added. The solution is vigorously stirred until the NPLs are aggregated. To obtain a stable colloidal suspension, 20 µL of oleylamine are then injected. Finally, the NPLs are precipitated and suspended in 2 mL of hexane. In this way, a monolayer of CdS is grown on both sides of the NPLs (HgTe/1CdS). The same process is reproduced for each additional layer of CdS.

HgTe CQDs synthesis with band edge at 6000 cm -1 : 513 mg of HgCl2 was added to 60 mL of oleylamine in a 100 mL round flask. The solution was placed under vacuum and heated to 110 °C for 1 h. Then, the temperature is decreased to 60 °C and solution placed to Ar atmosphere. 1.9 mL of TOP:Te (1 M) with 10 mL of oleylamine are added to the mercury solution. The solution color gradually turns to dark brown and the reaction is made during 3 min. A solution made of 1 mL of dodecanethiol and 9 mL of toluene is quickly added to quench the reaction. The nanocrystals are then precipitated with ethanol. After centrifugation, the nanocrystals are redispersed in chloroform. The washing step is repeated one more time. The solution is redispersed in chloroform and filtered with a 0.2 µm filter. Two additional washing steps are applied with final redispersion in chloroform.

Material characterization.

For UV-Vis spectroscopy, diluted solutions of nanocrystals in hexane are prepared. The absorption spectra are acquired using a Shimadzu UV 3600 spectrometer. For electron microscopy imaging, a diluted solution of nanocrystal is dropcasted on a copper TEM grid. The grid is degassed overnight under secondary vacuum. TEM observations were performed with a JEOL 2010. For film thickness determination, we used a Dektak 150 Veeco profilometer.

Photoemission measurements. 5 nm of Cr and 80 nm of Au were deposited by thermal evaporation on silicon wafers to get conductive substrates. On these substrates, a drop of the HgTe-xCdS solution was deposited at room temperature and the toluene evaporated quickly. A ligand exchange with EDT was performed (see procedure in the solid-state ligand exchange section below) and the film was then introduced in the preparation chamber, degassed at room temperature until a vacuum of 10 -10 mbar was reached, and finally transferred in the analysis chamber. For XPS measurements, a photon energy around 600 eV was used and the photoelectrons were detected at normal emission from the polarization vector, using a 50 eV pass energy. The experimental value of photon energy was measured on a clean Au substrate: the first and second orders of Au 4f7/2 core level were acquired and the difference of measured kinetic energies between the two orders to actual photon energy hνexp. Finally, to recalibrate the system, the Fermi level kinetic energy of the same clean gold substrate was acquired and shifted to be at hνexp (corresponding to a binding energy of 0 eV). This shift was then applied to all the measured kinetic energies before calculating the binding energies using the formula BE = hνexp -cKE where cKE is the corrected kinetic energy.

Device fabrication Electrodes fabrication.

A glass substrate is first rinsed then sonicated in acetone for 5 min. The wafer is then rinsed with acetone and isopropanol and finally O2 plasma cleaned for 5 min. Then AZ 5214E resist is spin coated on the substrate and baked at 110 °C for 90 s. The resist is exposed through a shadow mask to UV illumination for 2 s, with black paper below the substrate. The resist is then baked again at 125 °C for 2 min and re-exposed to UV (without the mask) for 40 s. The resist is then developed using AZ726. The film is then rinsed with water and finally dried. The remaining organic layer is removed by plasma cleaning. 5 nm of chromium and 80 nm of gold are evaporated and the remaining resist is removed by immersing the film in acetone for one hour. The electrodes are then rinsed with isopropanol and finally dried. The interdigitated electrodes include 25 digits, each 2.5 mm long with a 20 μm gap between them. The optical area of the device is 0.037cm 2 .

Solid state ligand exchange. Solutions at 1% in mass of EDT in ethanol are prepared. A drop of HgTe NPL solution in hexane:octane (9:1 volume ratio) is drop-casted onto the electrodes and left to dry. The resulting thin film is dipped in the ligand solution and rinsed with acetone. Once the film is dried, a second layer of HgTe NPLs is deposited and the ligand exchange step is repeated. The deposition process is performed five to six times in total.

Electrolyte gating.

For electrolyte gating we first mixed in a glove box 0.5 g of LiClO4 with 2.3 g of PEG (MW = 6 kg.mol -

). The vial is heated at 170 °C on a hot plate for 2 h until the solution appears clear. The electrolyte solution is warmed to around 100 °C and brushed on the top of the HgTe NPL film.

SUPPORTING INFORMATIONS

Supporting information content analysis of core level spectrum obtained from photoemission and detail about transport and phototransport characterization setup.

STATEMENT OF CONTRIBUTION

EL and SI designed the project. EI, MD, and SI grow the material and perform material characterization. CG, CL, BM, NG, JQ, AC, JR, HC, MS and EL conduct the synchrotron experiment. CG, HA, EI and CL conduct the transport measurements. CG and EL write the manuscript with inputs and feedback from all authors

Figure 1 a

 1 Figure 1 a. Scheme of the growth mechanism used to achieve HgTe/CdS core-shell NPLs. b. TEM image of CdTe NPLs. c. TEM image of HgTe NPLs. d. TEM image of HgTe/3CdS NPLs. e. Absorption spectrum of CdTe, HgTe and HgTe/CdS (1 layer of CdS) NPLs. f. Absorption spectrum of HgTe/CdS NPLs with 1, 2 and 3 layers of CdS in the shell.

Figure 2 a

 2 Figure 2 a. X-ray photoemission overview spectrum from HgTe/3CdS NPLs. Photon energy is set at 600 eV. Red lines relate to core level peaks, while grey lines refer to Auger features. b. Signal relative to the valence band in the X-ray photoemission spectrum from HgTe/3CdS NPLs. c. Electronic spectrum in absolute energy scale for HgTe NPLs, HgTe/3CdS NPLs and HgTe QDs (6nm). The Fermi level has been set equal to zero (pink). Valence band position is extracted from the low binding energy part of the photoemission spectrum, while the conduction band energy is determined with respect to the valence band using the absorption band gap. Finally the vacuum level is determined from the cut-off of the secondary electrons. The grey levels in the gap correspond to trap states which distribution width can be determined using transient photocurrent measurements and was measured to be around 44 meV. A more detailed description of these measurements is provided further in the article, especially onFigure 5. d. Scheme of the valence band and conduction band energy profile for the Hgte NPLs, the HgTe 3CdS NPLs and the HgTe QDs.

Figure 3

 3 Figure 3 Transfer curves (drain and gate currents as a function of applied gate bias) for thin films of HgTe NPLs (a), HgTe/3CdS NPLs (b) and HgTe QDs (c). The applied drain-source bias is 500 mV for HgTe and 100 mV for HgTe/CdS. Current as a function of temperature for thin films of HgTe NPLs (d), HgTe/3CdS NPLs (e) and HgTe QDs (f). For the I-T curve, the measurement is conducted in a two contact geometry (ie without gate)

Figure 4 a

 4 Figure 4 a. Scheme of the pump probe experiment used to measure the majority carrier lifetime. Typically, an 800 nm pulsed laser is used to excite the system. Then, every 10 ns, a photoemission spectrum of the Hg 4f state is acquired. The peak position is then fitted and plotted as a function of time. b. Shift of the binding energy relative to the Hg 4f state as a function of time after illumination by an 80 fs long pulse of 800 nm light for thin film of HgTe NPLs, HgTe/3CdS NPLs and HgTe QDs.

Figure 5 a

 5 Figure 5 a. Photocurrent as a function of time after illumination by a 0.8 ns long pulse of 355 nm light. The inset is a scheme of the setup used for measurement. b. Zoom on the graph of part a. at early timescale. The scattered lines are exponential fit. The associated decay time are also provided. c. Zoom on the graph of part a. at long timescale.

Figure 6a .

 6a Figure 6a. Photocurrent and absorption spectra for a thin film of HgTe/3CdS NPLs. The applied bias is 1.5V. b. IV curve for a thin film of HgTe/3CdS NPLs under various excitation powers ranging from dark condition to 25 mW from a 1.55 µm laser diode source. c. Responsivity as a function of light excitation power (λ=1.55 µm) for a thin film of HgTe/3CdS. The applied bias is 1.5V, measurement is conducted at room temperature. d. Photocurrent as a function of time while the sample is illuminated by a pulse of 1.55 µm light for different temperatures. The applied bias is 1.5V. e. Detectivity as a function of light excitation power (λ=1.55 µm) for a thin film of HgTe/3CdS. The applied bias is 1.5V, measurement is conducted at room temperature. The inset is a measure of the noise current spectral density, under 1.5V.

Table 1

 1

	Atomic ratios of S, Cd, Te and Hg in HgTe/1CdS measured by energy dispersive X-ray spectroscopy (EDS) and
	expected from the number of layers in the NPL.
	HgTe/1CdS	S (%)	Cd (%)	Te (%)	Hg (%)
	Determined by EDS 16±2	16±2	29±2	39±2
	Predicted	18	18	27	36
	stoichiometry

Table 2 Atomic

 2

	HgTe/3CdS	S (%)	Cd (%)	Te (%)	Hg (%)
	Determined by EDS 30.5±2	30.5±2	18±2	21±2
	Predicted	32	32	16	21
	stoichiometry

ratios of S, Cd, Te and Hg in HgTe/3CdS measured by energy dispersive X-ray spectroscopy (EDS) and expected from the number of layers in the NPL.

ACKNOWLEDEGMENTS

EL thanks the support ERC starting grant blackQD (grant n°756225). We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre". This work has been supported by the Region Ile-de-France in the framework of DIM Nano-K (grant dopQD). This work was 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 and also by the grant IPER-nano2. NG and JR thank Nexdot for post doc funding and JQ thanks the Chinese scholar council for PhD grant.