M Denoual
email: mdenoual@ensicaen.fr

V Menon

T Sato

O De Sagazan

A W Coleman

H Fujita

Liquid cell with temperature control for in-situ TEM chemical studies

Keywords: liquid cell, TEM, temperature control, capacitively coupled heat feedback PACS numbers:

Liquid cells are hermetically sealed capsules that make it possible to image through liquids at the nanoscale using Transmission Electron Microscopes (TEM). Liquid cells can be passive vessels or integrate electrodes up to now for electrodeposition experiments. Temperature is a key parameter for chemical kinetics of processes. This paper describes a temperature controlled liquid cell for in-situ TEM studies. The liquid cell integrates a resistive element for both heating and sensing temperature. A capacitively coupled heat feedback technique is implemented to control the temperature of the liquid cell from room temperature to 100°C with accuracy 0.1°C. In-situ TEM experiment illustrate the heating capability of such temperature controlled liquid cell while observing at the nanoscale.

Introduction

The physical and chemical properties of matter are intimately related to its structure. Highly driven by nanotechnologies, the study of natural, biological or synthetic material at the nanoscale is a current topic common to a variety of fields including biology, chemistry, medicine and materials science. In their conventional usage, tools such as X-ray diffractometer, Transmission Electron Microscopes (TEM) and Atomic Force Microscopes, enable a static observation of matter at the nanoscale. Among those tools, TEM exhibits the most versatility as an extension of optical microscope at a lower scale and has become standard analytical tool. Yet, the conventional usage does not authorize the study of liquid samples and consequently the access to fundamental process dynamics.

With the development of microfabrication technologies, a dedicated usage for liquid samples, proposed in the 1930's [START_REF] Marton | La microscopie électronique des objets biologiques[END_REF][START_REF] Abrams | A closed cell for electron microscopy[END_REF], has become possible with a new class of devices referred to as liquid A c c e p t e d M a n u s c r i p t cells. Those devices enrich TEM with the capability to achieve real-time observation of nanometerscale dynamic phenomena within fluid media, such observation of liquid samples provides unique insights into life and materials science. Commercial solutions exist based on dedicated perfused holders with silicon microfabricated lids and O-rings assembled at the tip. Those perfused holders usually integrate 3 or 4 electrodes for liquid biasing or heating. The interest in these commercial tools is particularly driven by electrochemical studies [START_REF] He | Operando liquid cell electron microscopy of discharge and charge kinetics in lithium-oxygen batteries[END_REF] in the field of batteries. Alternative solution to the micro-lid type liquid cell associated to a perfused holder is the stand-alone liquid cell type.

Stand-alone liquid cells are capsules or vessels containing the liquid sample and sealed hermetically to prevent evaporation in the high vacuum environment of the TEM. The observation with the electron beam of the TEM is achieved through membranes thin enough to reduce electron scattering and enable high resolution [START_REF] De Jonge | Electron Microscopy of specimens in liquid[END_REF][START_REF] Ross | Opportunities and challenges in liquid cell electron microscopy[END_REF]. The thin membranes are usually made out of silicon nitride, even if some development work is being conducted using graphene sheets [START_REF] Daulton | In Situ environmental cell-transmission electron microscopy study of microbial reduction of chromium (VI) using electron energy loss spectroscopy[END_REF][START_REF] Krueger | Drop casted self assembling graphene oxide membranes for scanning electron microscopy on wet and dense gaseous samples[END_REF]. The most common process consists in the assembly of two silicon chips with silicon nitride membranes (20 to 200 nm thick) aligned forming the observation hole. The two chips are stacked one over the other with an intermediate spacer material forming the space wherein liquid is sandwiched and the assembly is finally hermetically sealed [START_REF] De Jonge | Electron Microscopy of specimens in liquid[END_REF][START_REF] Ross | Opportunities and challenges in liquid cell electron microscopy[END_REF]. The resulting liquid cell allows for the observation of liquid samples even in conventional, non-specialized TEM setups with regular TEM holders. Compared to micro-lid type liquid cell with perfused holder, two-chip liquid cells operate with smaller volumes of liquid and the studied samples can be unloaded from the TEM holder in a non-destructive way.

Observation of liquid samples at the nanoscale has been a breakthrough of the 2000's for the fields of life and material sciences. Nowadays much effort is being expended on recreating the natural or artificial environment of the sample, biological or other, to be able to in-situ and operando study process dynamics, and to interact with the sample. For this purpose, liquids cells integrating electrodes for electrochemical interaction [START_REF] Grogan | The Nanoaquarium: A Platform for In Situ Transmission Electron Microscopy in Liquid Media[END_REF][START_REF] Zeng | In Situ Study of Lithiation and Delithiation of MoS2 Nanosheets Using Electrochemical Liquid Cell Transmission Electron Microscopy[END_REF][START_REF] Egawa | In-situ real time monitoring of nanoscale gold electroplating using micro-electromechanical systems liquid cell operating in transmission electron microscopy[END_REF] have been proposed. This work focuses on the achievement of temperature control of the liquid sample in a liquid cell. Some previous work has investigated the temperature effect in similar devices but dedicated to gas study using a micro-machined heater with estimated uncertainty of 10% on temperature [START_REF] Creemer | Atomic-scale electron microscopy at ambient pressure[END_REF]. This work proposes a specific design approach dedicated to cells containing liquids associated with a previously proposed heat-feedback technique [START_REF] Denoual | Bolometer with heat feedback[END_REF] enabling accurate temperature control at the location of the sample observation. The design can be adapted to various holders as long as they have electrical contacts.

Temperature controlled liquid cell

Design

The device, as with conventional liquid cells, comprises two silicon chips with thin electron transparent silicon nitride membranes forming the observation chamber. The bottom chip with the A c c e p t e d M a n u s c r i p t resistive element for temperature control and the top chip with the liquid inlet and outlet are assembled and sealed to form an hermetic vessel embedding the liquid medium to be observed; general schematics of the device are illustrated in Figure 1.

The resistive element is integrated inside the liquid chamber in order to control the temperature at the very location of the observation. The center of the observation window is kept without resistive element to remain transparent to the electron beam (e-beam). The central area clear of metal is about 20 µm in diameter. For temperature control, since the resistive element is in direct contact with the liquid medium, the operating voltage has to be kept below the hydrolysis threshold of water, i.e.

V (water-based liquid medium solutions represent the vast majority of the solutions used/of interest).

As a rule of thumb, the higher the thermal insulation is, the less the power required for heating is. The thermal insulation is mainly related to the diameter of the self-sustained membrane and to its thickness. Practical process fabrication considerations enabling high good-die yield fix a membrane radius of 200 µm (r2). The resistive element for temperature control at the center of the membrane is designed to be 40 µm in radius (r1). Considering a worst case scenario with the thickest (200 nm) nitride layer, a rough evaluation of the thermal conductance Gth using the following formula, A design margin considering a maximum total thermal conductance of 100 µW/K is applied to encompass process discrepancies, silicon nitride thermal conductivity variations, liquid medium loading and other thermal exchange phenomena. Experimentally for 70 nm and 180 nm thick membrane devices, average thermal conductivities of 7-15 µW/K and 30 µW/K were extracted from measures respectively for liquid cells unfilled with liquid and liquid cells filled with liquid.

𝐺 𝑡ℎ = 2𝜋 •  • 𝑡 ln (𝑟 2 𝑟 1 ⁄ ) ⁄ [ 15
The required heating power to ensure temperature variations of up to 100°C can then be derived and yields a maximum Joule power of 10 mW. This required power and the voltage limitation to prevent hydrolysis fix the maximum value for the resistance of the resistive element to about 100 .

Taking into account the resistivity of thin metallic layer deposited, the length and thickness of the resistive element designed to achieve resistance values below 100 , in practice 100 nm thick 6 µm wide metal tracks. A 4-wire resistive element is designed to suppress the effect of contact resistance and conductive paths. The measured resistance then corresponds only to the central circular resistance.

A c c e p t e d M a n u s c r i p t The devices are fabricated using clean-room micro-fabrication technologies as in [START_REF] Egawa | In-situ real time monitoring of nanoscale gold electroplating using micro-electromechanical systems liquid cell operating in transmission electron microscopy[END_REF]. A c c e p t e d M a n u s c r i p t

Temperature control

The role of the temperature control is to enable the setting of a defined temperature at the location of the observation window independently of the device used and of the liquid filling the liquid cell.

A single resistive element is used for both temperature sensing through resistance variation measurement and heating with Joule power. The possibility to produce heat at the very same location of the temperature measurement with a single element, contrary to systems with separated heater and thermometer [START_REF] Denoual | On-chip fluidic actuation for TEM liquid cells[END_REF] favors a compact and simpler design and reduces the number of required pads.

The temperature control of this single resistive element is obtained with a heat feedback technique previously applied to infrared detectors [START_REF] Denoual | Bolometer with heat feedback[END_REF] and gas sensors [START_REF] Denoual | Thermal resonant zeolite-based gas sensor[END_REF]; the capacitively coupled heat feedback technique. A detailed description of the digital version of the capacitively coupled electrical substitution technique can be found in [START_REF] Denoual | Digital implementation of the capacitively coupled electrical substitution for resistive bolometers[END_REF]. The main principles and characteristics are summarized hereafter.

In this technique, the electrical bias point for temperature measurement and the Joule power signal for heating are separated in different frequency domains. The electrical bias point and consequently the temperature measure is at low frequency while the Joule power feedback signal is at high frequencies (several MegaHertz). This configuration allows independent configuration for the temperature measurement resolution and for the heating range. Figure 3 schematically illustrates the temperature control configuration. The resistive element is 4-wire. Two wires are used for the electrical biasing and coupled heat feedback and the other two wires are connected to an instrumentation amplifier for voltage measurement across the resistive element. The biasing resistor and the bias voltage at the output of a digital to analog converter are chosen to limit the elevation of temperature due to biasing power below 0.1°C. For example, in practice, Vbias is 1 V and Rbias 10 kOhm producing a maximum biasing Joule power of 1 µW in the resistive element. The instrumentation amplifier has a voltage gain of 1000 to take advantage of the full-scale voltage range of the component supplied by a symmetrical 12 V power supply for a measurement range between room temperature and 100°C. The amplified and filtered signal is digitized for subsequent digital control processing with a Proportional Integral (PI) controller. A digital version, with a PWM (Pulsed Width Modulation) modulated output, of the capacitively coupled heat feedback technique is implemented to benefit from its intrinsic linearity that eases the PI controlled design [START_REF] Denoual | Digital implementation of the capacitively coupled electrical substitution for resistive bolometers[END_REF]. The feedback PWM modulated signal is transposed at higher frequency with a carrier Vo at 3 MHz. The temperature setting for the desired working point is given to the controlled and the closed-loop system ensures that the desired temperature is reached. For the first practical implementation, a NI6251 acquisition board is used for the digital-to-analog, analog-todigital and PWM functionalities. The PI controller is programmed in a Labview interface associated to the NI6215 acquisition board.

A c c e p t e d M a n u s c r i p t

Discussion

Equipping the liquid cell with temperature control capabilities will enable providing information at the nanoscale on real-time temperature effects on process dynamics. In life science, being able to study at the nanoscale biological specimen at 37°C is fundamental to have access to observation closer to real conditions. For material sciences, cycling temperature can provide a fast way to optimize liquid phase synthesis or to determine the effect of temperature on the growth or structuration of nanostructures A c c e p t e d M a n u s c r i p t that ultimately impact their physical and chemical properties. The liquid cell with temperature control presented here answers such type of requirements with temperature control over the range room temperature up to 100°C. Future work will involve the usage of those temperature controlled liquid cells for in-situ TEM chemical experiments. Partial integration of the conditioning electronics and control will be investigated to miniaturize the experimental set-up and possibility to take advantage of faster temperature cycles, indeed such small liquid volumes in the observation window can be heated up with response time of the order of milliseconds or sub-milliseconds [START_REF] Arata | Miniaturized thermocontrol devices enable analysis of biomolecular behavior on their timescales, second to millisecond[END_REF].

Temperature control adds to electrical biasing [START_REF] Egawa | In-situ real time monitoring of nanoscale gold electroplating using micro-electromechanical systems liquid cell operating in transmission electron microscopy[END_REF] and liquid actuation [START_REF] Denoual | On-chip fluidic actuation for TEM liquid cells[END_REF] previously developed to propose a set of on-chip functionalities for in-situ TEM studies.

Conclusion

In this paper, the design of liquid cell with temperature control is presented. Capacitively coupled heat feedback implementation ensures the temperature control of the liquid from room temperature to 100°C. The ability to control the temperature of a liquid sample while observing it in a TEM is a new capability that will enable providing information at the nanoscale on real-time temperature effects on process dynamics.

  ], yields to values for a single membrane in the range [7.8-33 µW/K] depending on silicon nitride thermal conductivity (Si3N4  [10-43 W/m.K]).

Figure 1 :

 1 Figure 1: Schematics of the liquid cell, a. perspective view of the liquid cell, b. top view of the sole bottom chip, c. side view of the liquid cell with top and bottom chips stacked and aligned, d. zoomed view of the 4-wire resistive element. The liquid cell consists of two chips hermetically sealed. The top and bottom chips include free-standing thin (50-200 nm) silicon nitride membranes that aligned together form the observation chamber across which the electron beam of the TEM passes interacting with the studied specimen.

Figure 2 (

 2 Figure 2 (a) shows a top chip and a bottom stacked and aligned. The resistive element at the center of the observation chamber is visible through the transparent silicon nitride membranes. Before loading into the holder, the liquid cell is filled dropping the liquid sample over the inlet while the outlet allows gas to evacuate, then the liquid cell is hermetically sealed with UV-hardening epoxy and loaded into the sample holder as a static liquid cell. Figure 2 (b) shows an assembled liquid cell loaded into a sample holder with electrical contact. The holder is a custom designed holder with inserted wires enabling 5 electrical contacts. The presence of electrical contacts is the only requirement to be able to use this sealed liquid cell technology with integrated temperature control. The particular holder used here has no ability to flow liquid.

Figure 2 :

 2 Figure 2: Pictures of realised devices. (a) Bottom and top chips stacked and aligned. (b) Liquid cell device loaded into a (S)TEM holder with electrical contacts.

Figure 3 :

 3 Figure 3: Temperature control scheme involving capacitively coupled electrical substitution.

Figure 4

 4 Figure 4 shows an example of regular temperature cycling obtained with the temperature control implementation and a liquid cell. The set temperature cycle is a progressive ramp with 5°C steps of 30 s duration up to 85°C. Here, the response is damped to avoid overshoot and the response time to settle the temperature is set to 5 s with the conditioning electronics and PI controller. To visualize the temperature evolution the resistive element is covered with thermochromic ink (Thermochromic screen ink REF 47°C SFXC)[START_REF] Ueno | Thermochromic ink as internal probes of thermal behavior in micro-fluidic systems[END_REF]. The red color of this thermochromic ink turns transparent at temperature higher than 47°C. Picture insets for each temperature level are displayed next to the temperature axis. Two larger pictures illustrate the ink color state over the resistive element at 25°C and 85°C. From the temperature curve, a standard deviation of 0.11°C is calculated and noise analysis with a long time constant temperature setting yields to a 0.01°C 2 /Hz white noise level in the working frequency range. The inset pictures with the red thermochromic ink allow visualizing the progressive temperature evolution around the resistive element and rather homogeneous behavior at the center of the membrane.

Figure 4 :

 4 Figure 4: Temperature ramps with 5°C steps every 30 s up to 85°C. Inset pictures of the heater/temperature sensing resistive element covered with thermochromic ink for each temperature level are depicted next to the temperature axis.

Figure 5

 5 Figure 5 illustrates an in-situ STEM heating experiment. PMMA (PolyMethyl MethAcrylate) polymer fragments are continuously observed while changing the temperature setting of the liquid cell.Figure

Figure 9 (

 9 a) shows the fragments shape for temperature below 92°C. Figure9(b) illustrates the fragments after A c c e p t e d M a n u s c r i p t temperature setting over 92°C. The fragments have densified and present in rounded shapes. The temperature step lasts 15 minutes long but the shape evolution is observable right after the temperature increase (more precisely the shape evolution is observable after the re-focusing time of about 10 s required after the temperature increase). Considering the glass transition temperature of PMMA polymer ranges from 85°C to 165°C, the observed densification and rounding could be related to such a phase transition phenomenon. The pictures are taken in Z-contrast mode, also called dark-field mode.In this mode, the image is formed by scattered electrons collected by an annular detector and the contrast is directly related to the atomic number of the atoms being observed. Some small air bubbles, dark spots in the image, can appear during temperature experiments. Some are visible at the left-top side of Figure9(b).

Figure 5 :

 5 Figure 5: In Situ TEM thermal experiments, images in Z-contrast mode. (a) Polymer fragments for temperature below 92°C. (b) The same polymer fragments after 92°C has been exceeded. Magnification 90k, scale bar 300 nm. The plain arrow in each figure indicates a polymer fragment. The dashed arrow points small bubbles that appeared during the heating.

Acknowledgments

Some micro-fabrication process steps were done in the Takeda clean room at the University of Tokyo, partially financed by the MEXT Nanotechnology Platform.


Edwin A Baquero

Héloïse Virieux

Robert A Swain

Angélique Gillet

Arnaud Cros-Gagneux

Yannick Coppel

Bruno Chaudret

Céline Nayral

Fabien Delpech

Synthesis of Oxide-Free InP Quantum Dots: Surface Control and H 2 -Assisted Growth

Control over particle surfaces and interfaces is a principal requirement to fully take advantage of semiconducting Quantum Dot (QDs) properties. In the case of indium phosphide, the sensitivity of the material to water renders it challenging to synthesize oxide-free particles. We demonstrate the ability to achieve complete control over the surface by developing synthetic strategies based on a novel reactive indium precursor, tris(N,N'diisopropylacetamidinato) indium(III). This complex permits the synthesis of InP QDs at temperatures as low as 150°C with no inherent surface oxidation. At higher temperatures (230°C), the concomitant use of an H 2 atmosphere yields oxide-free InP QDs. The prevention of such an amorphous oxide layer provides a clean surface suitable for subsequent growth. An extensive solid-state 2 H NMR spectroscopy study demonstrates an unprecedented H 2 bond dissociation at the QD surface yielding QDs-H species and in so doing protecting the surface from oxidation.

At the nanoscale, the role of the particle surface is predominant for the chemical (solubility, colloidal stability, catalytic activi-ty…) and physical (optical, magnetic, transport…) properties. Understanding the surface properties of nanoparticles is thus key to controlling their features. The surfaces of several semiconductor nanocrystals (Quantum Dots, QDs) have been investigated by many, leading to a substantial amount of knowledge and control. 1- 4 This has opened the door to great achievements in many scientific disciplines, as exemplified by the use of these materials as bio-labels, lasers, light-emitting diodes (LEDs), and solar cells. [START_REF] Hendricks | A Tunable Library of Substituted Thiourea Precursors to Metal Sulfide Nanocrystals[END_REF][START_REF] Kovalenko | of Nanoscience with Nanocrystals[END_REF][START_REF] Boles | The Surface Science of Nanocrystals[END_REF][START_REF] Hens | A Solution NMR Toolbox for Characterizing the Surface Chemistry of Colloidal Nanocrystals[END_REF][START_REF] Talapin | Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications[END_REF] However, most such advancements have been achieved for toxic Cd-and Pb-based systems, while others, such as III-V semiconductors (in particular InP), [START_REF] Gary | Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates[END_REF][START_REF] Tamang | Chemistry of InP Nanocrystal Syntheses[END_REF] have not been as thoroughly studied. Indeed, the more covalent nature of the In-P solid and its high sensitivity toward oxidation account for the complexity of controlling both the InP surface chemistry and, in the case of core/shell systems, the shelling interface. [START_REF] Cros-Gagneux | Surface Chemistry of InP Quantum Dots: A Comprehensive Study[END_REF][START_REF] Virieux | InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description[END_REF] We and others have demonstrated the sensitivity of these materials to the water present in solution either as a reactant impurity or as a by-product generated under the high-temperature conditions of currently reported syntheses. [START_REF] Cros-Gagneux | Surface Chemistry of InP Quantum Dots: A Comprehensive Study[END_REF][START_REF] Virieux | InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description[END_REF][START_REF] Baquero | Identifying Short Surface Ligands on Metal Phosphide Quantum Dots[END_REF][START_REF] Xie | Effect of Trace Water on the Growth of Indium Phsophide Quantum Dots[END_REF][START_REF] Ramasamy | Beneficial Effects of Water in the Colloidal Synthesis of InP/ZnS Core-Shell Quantum Dots for Optoelectronic Applications[END_REF] This results in the formation of oxide and hydroxide species, which dramatically and irreversibly alter the surface. It is important to note that the presence of water in the InP synthesis medium is not an exception but rather the general case. Most procedures are predicated on the use of indium carboxylate precursors and/or other oxygen-containing ligands at high tempera-tures (>188°C). Carboxylic acids, one such stabilizing agent, produce water as a co-product of decarboxylative coupling in this temperature range, [START_REF] Cros-Gagneux | Surface Chemistry of InP Quantum Dots: A Comprehensive Study[END_REF][START_REF] Virieux | InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description[END_REF] leading to unexpected and unwanted effects, [START_REF] Cros-Gagneux | Surface Chemistry of InP Quantum Dots: A Comprehensive Study[END_REF][START_REF] Virieux | InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description[END_REF][START_REF] Xie | Effect of Trace Water on the Growth of Indium Phsophide Quantum Dots[END_REF][START_REF] Ramasamy | Beneficial Effects of Water in the Colloidal Synthesis of InP/ZnS Core-Shell Quantum Dots for Optoelectronic Applications[END_REF] most notably the inhibition of further particle growth. This phenomenon serves as an immense obstacle to the accurate study of QD growth mechanisms, as it renders impossible the independent isolation and investigation of other parameters. The true effect of acid concentration on particle growth, for example, cannot be satisfactorily described while the concentration of water varies simultaneously. [START_REF] Gary | Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates[END_REF][START_REF] Tamang | Chemistry of InP Nanocrystal Syntheses[END_REF] Moreover, the pivotal (and sometimes detrimental) effect of water has been demonstrated even in the case of materials considered to be largely resistant. For instance, in both PbS QDs and CdSe nanoplatelet systems, water and hydroxide ligands alter the surface energy of specific planes. [START_REF] Zherebetskyy | Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid[END_REF][START_REF] Bertrand | Shape Control of Zincblende CdSe Nanoplatelets[END_REF] Likewise, water is suspected to engender deterioration of the optical and electrical properties of PbSe and PbS QDs-based films. [START_REF] Malgras | The Effect of Surface Passivation on the Structure of Sulphur-Rich PbS Colloidal Quantum Dots for Photovoltaic Application[END_REF][START_REF] Luther | Optical, and Electrical Properties of Self-Assembled Films of PbSe Nanocrystals Treated with 1,2-Ethanedithiol[END_REF] Herein, we address the issue of InP QD surface chemistry control, and in particular, the prevention of surface oxidation/hydroxylation, even when in the presence of O-containing ligands and water. We describe two alternative approaches based on a novel indium precursor: tris(N,N'-diisopropylacetamidinato) indium(III). Interestingly, while indium amidinate complexes have been known for a long time, [START_REF]The Group 13 Metals Aluminium, Gallium, Indium and Thallium: Chemical Patterns and Peculiarities[END_REF] they have only very recently been used as reagents for CVD and ALD growth of indium-based materials. [START_REF] Gebhard | New Amidinate Complexes of Indium(III): Promising CVD Precursors for Transparent and Conductive In2O3 Thin Films[END_REF][START_REF] Mccarthy | Oxygen-Free Atomic Layer Deposition of Indium Sulfide[END_REF] The use of this particular indium amidinate complex in our colloidal synthesis produces particles completely devoid of oxides, allowing them to grow to larger diameters. The mechanism of protection from oxidation at 150°C (under argon) and at 230°C (under dihydrogen) are described. In the latter case, an unprecedented H 2 bond dissociation at the QD surface is demonstrated by 2 H Magic Angle Spinning (MAS) NMR.

The general consensus suggests that the elevated temperatures necessary to standard InP syntheses are a direct consequence of the high stability of the indium precursors. [START_REF] Tamang | Chemistry of InP Nanocrystal Syntheses[END_REF] Hoping to prepare InP QDs under softer conditions, a more reactive indium precursor featuring In-N bonds (186 kJ/mol) instead of In-O bonds (320 kJ/mol for the common precursor indium acetate) was selected. We successfully prepared InP QDs under argon at temperatures as low as 150°C by adding the tris(trimethylsilylphosphine (P(TMS) 3 ) to a solution containing the indium precursor as well as hexadecylamine (HDA) and palmitic acid (PA) in a 1:3 ratio, both of which play only the role of ligands (1 in Scheme 1). Transmission electron microscopy (TEM) images of 1 (Figure S1a) indicate the presence of monodisperse spherical InP QDs (2.0 ± 0.3 nm). X-ray diffraction (XRD) analysis reveals that the particles have a zinc blende structure (Figure S1b), and the UV-Vis spectrum shows an absorption peak at 530 nm (Figure S1c). The replacement of indium carboxylate with indium amidinate thus allows us to significantly decrease the required temperature for the preparation of high-quality InP QDs. In all previously published syntheses, temperatures higher than 160°C are required. [START_REF] Gary | Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates[END_REF] Interestingly, both the 31 P Hahn-echo and the 1 H-31 P Cross-Polarization (CP) Magic Angle Spinning (MAS) solid state NMR spectra of this sample (Figure S5) show only one resonance (located at δ -200 ppm). This high-field signal is assigned to nanoparticulate metal phosphides. [START_REF] Cros-Gagneux | Surface Chemistry of InP Quantum Dots: A Comprehensive Study[END_REF][START_REF] Virieux | InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description[END_REF] No other signals are present in the spectrum, meaning that there are no surface oxides (typically found at chemical shifts between 50 and 0 ppm). [START_REF] Cros-Gagneux | Surface Chemistry of InP Quantum Dots: A Comprehensive Study[END_REF][START_REF] Virieux | InP/ZnS Nanocrystals: Coupling NMR and XPS for Fine Surface and Interface Description[END_REF] Prior to this study, whenever surfaces have been precisely and rigorously characterized by XPS or 31 P NMR, surface oxidation has been ubiquitously implicated on particles stabilized by oxygencontaining ligands. [START_REF] Tomaselli | NMR Study of InP Quantum Dots: Surface Structure and Size Effects[END_REF][START_REF] Lucey | Monodispersed InP Quantum Dots Prepared by Colloidal Chemistry in a Noncoordinating Solvent[END_REF][START_REF] Ryu | Step-Wise Synthesis of InP/ZnS Core-Shell Quantum Dots and the Role of Zinc Acetate[END_REF][START_REF] Li | Surface Activation of Colloidal Indium Phosphide Nanocrystals[END_REF] Evidence of coordination of PA and HDA ligands to the QD surface is provided by the 1 H-13 C (CP) MAS spectrum (top in Figure S7), the carboxylate resonance appears at δ 182 ppm and the alpha-CH 2 group of HDA at δ 42 ppm. Two additional broad peaks at 162 and 172 ppm are assigned to the resonances of the amidinate sp 2 carbon and to the acyl group of the amide, respectively. The intensity of these resonances are significantly decreased in the [START_REF] Zherebetskyy | Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid[END_REF] C Direct Polarization (DP) spectrum (bottom in Figure S7), an effect of long relaxation times which are due to the ligand rigidity at the QD surface. In contrast, the signal intensity of the terminal methyl group (δ 15 ppm) is enhanced due to its increased mobility. Firstly, this demonstrates the complexity of the surface of these QDs, which possess a coordination sphere consisting of at least four types of molecules. Secondly and more importantly, we also note the formation of amide, which results from a reaction between acid and amine, and is accompanied by in-situ water formation. [START_REF] Wu | Synthesis of Colloidal Uranium-Dioxide Nanocrystals[END_REF][START_REF] Protiere | Amine-Induced Growth of an In2O3 Shell on Colloidal InP Nanocrystals[END_REF] However, despite the presence of water, no oxidation occurs at the InP QD surface (Figure S5), suggesting that at temperatures as low as 150°C, water cannot effectively diffuse through the layer of the coordination sphere. In order to grow larger particles, the reaction was performed at 230°C (2 in Scheme 1). By TEM we observe spherical QDs with a diameter of 4.1 ± 0.4 nm (Figure S2a), indicating that the temperature increase favors growth. This growth is also apparent in the UV-Vis spectrum (Figure S2c), which shows a red-shifted absorption peak with respect to that of 1 (570 vs 530 nm). Increasing the temperature also results in some modifications to the QD coordination sphere, as can be seen in the MAS [START_REF] Zherebetskyy | Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid[END_REF] C spectra (Figure S9). The characteristic amidinate peak at 162 ppm is no longer present, while a significant increase in the intensity of the resonance of amide relative to that of the carboxylate is observed in the 1 H- [START_REF] Zherebetskyy | Hydroxylation of the Surface of PbS Nanocrystals Passivated with Oleic Acid[END_REF] C CP MAS spectrum (top in Figure S9). This suggests that conversion of carboxylate proceeds further into amide at 230°C than at 150°C. In contrast to our synthesis at 150°C, and as has been observed in previous InP syntheses, [START_REF] Baquero | Identifying Short Surface Ligands on Metal Phosphide Quantum Dots[END_REF] the oxidation of the surface occurs concurrently with the production of amide and water: hence, in addition to the broad and high-field resonance of metal phosphides at -185 ppm, we observe two more phosphorus environments (identified as InPO 4 and InPO 3 ) 8-9 as evidenced by the resonances at δ 2 and 28 ppm in the 31 P Hahn-echo MAS NMR spectrum (Figure 1a). This sequence also allows quantification (the relative ratio is 85:7:8 respectively) that 15% of the P atoms of the QDs are oxidized. In an effort to avoid surface oxidation, the same procedure was carried out at 230°C under an H 2 atmosphere (1.8 bar) (3 in Scheme 1). In the field of III-V semiconductor technology, the use of hydrogen plasmas was identified long ago as a practical method for passivating defects and removing surface carbon contaminations and native oxides leaving a clean and wellordered surface for epitaxial growth. [START_REF] Bruno | On the Use of H2 Plasma for the Cleaning and Passivation of InP Substrates[END_REF] In contrast, hydrogenation via an H 2 atmosphere was identified as being inefficient. [START_REF] Seager | Semiconductors and Semimetals; Hydrogenation methods[END_REF] In the matter at hand, however, dihydrogen treatment proves to be a highly valuable upgrade. TEM images show QDs of 4.5 ± 0.5 nm (Figure S3a) indicating that the reductive atmosphere favors the growth of the QDs. This observation is confirmed in the UV-Vis spectrum as the absorption peak appears at 610 nm (Figure S3c), significantly red-shifted to that of sample 2 (570 nm). As expected, XRD measurements indicate that the particles are zinc blende structure (Figure S3b). The H 2 atmosphere does not inhibit the condensation reaction between acid and amine, as the C=O amide resonance is again visible at δ 171.8 ppm in the 1 H-13 C CP-MAS NMR spectrum (Figure S12). However, the 1 H-31 P CP-MAS and the 31 P MAS NMR spectra show only one resonance at δ -190 ppm (Figures 1b andS10) demonstrating that under these conditions, surface oxidation is prevented. While these uncoated QDs 2 and 3 do not display any photoluminescence, shelling with ZnS using a previously reported procedure [START_REF] Tessier | Economic and Size-Tunable Synthesis of InP/ZnE (E = S, Se) Colloidal Quantum Dots[END_REF] (See Supporting Information for details) yields core/shell QDs emitting at 610 and 680 nm (Figures S20 andS22) with 13% and 33% quantum yields, both values taken respectively.

The two InP samples 2 (oxidized QDs) and 3 (non-oxidized QDs) also provide a unique opportunity to investigate the influence of the oxide layer on otherwise similar QD samples. As such, growing experiments were performed on freshly prepared InP samples 2 and 3 by adding (in the same relative stoichiometry) all starting materials several times (one-third of the initial amount used for the preparation of InP QDs in each addition). The reaction was monitored by TEM (Figures S17 andS18) and UV-Vis spectroscopy (Figure 2). Both techniques clearly demonstrate successful particle growth of sample 3: in the UV-Vis spectrum (Figure 2b), the absorption peak redshifts with each successive addition (for an overall change from 600 to 680 nm), and TEM images indicate a final size of 6.7 nm, 50% larger than the initial 4.5 nm particles. The size distribution of the particles is un-changed, but the morphology, spherical during the initial additions, becomes tetrahedral after the final addition (Figure S17). On the other hand, the growth is blocked for sample 2 (Figure 2a) after the 3 rd addition with only a minimal overall size change (the absorption peak shifts from 570 to 600 nm and the particle diameter from 3.9 to 4.4 nm, Figure S18). This slight increase in size is possible because there remain some non-oxidized surface sites: By the 31 P MAS NMR spectrum, 15% of the P atoms are oxidized (for 3.9 nm QDs, ca. 28% of the total number of P atoms are expected to be surface atoms). [START_REF] Peng | An Essay on Synthetic Chemistry of Colloidal Nanocrystals[END_REF] The substantial difference between samples 2 and 3 over the course of this growing experiment proves that surface oxides significantly hinder particle growth. To elucidate the mechanism of surface protection against oxidation, InP QDs were synthesized under the same conditions as that of sample 3, with D 2 (1.8 bar) used in place of H 2 (sample 4, see Supporting Information for details). [START_REF] Kovalenko | of Nanoscience with Nanocrystals[END_REF] H NMR spectroscopy has previously been identified as a useful tool to study the interaction of hydrogen with metal nanoparticles. [START_REF] Truflandier | Where Does Hydrogen Absorb on Ru Nanoparticles? A Powerful Joint 2H MAS-NMR/DFT Approach[END_REF][START_REF] Pery | Direct NMR Evidence for the Presence of Mobile Surface Hydrides on Ruthenium Nanoparticles[END_REF] The lower gyromagnetic ratio of 2 H compared to 1 H results in decreased dipolar coupling and thus 2 H NMR spectra affords a narrower linewidth. As expected, 4 shows similar physical properties (shape, size, absorption peak and crystalline structure, Figure S4) and chemical composition (unoxidized surface, Figure S13 and amide formation, Figure S14) to its analogous particles synthesized under H 2 (3). The experimental and simulated 2 H solid-echo MAS NMR spectra of 4 are shown in Figure 3. A near-perfect fitting (Figure 3a) shows four signals, three of them presenting a significant quadrupolar coupling constant (C Q ): (i) δ iso = 8.8(± 0.2) ppm (C Q = 170(± 5) kHz), (ii) δ iso = 3.1(± 0.3) ppm (C Q = 145(± 15) kHz) and (iii) δ iso = 2.0(± 0.3) ppm (C Q = 95(± 15) kHz). The fourth contribution shows a Lorentzian shape without quadrupolar coupling at δ iso = 1.1(± 0.1) ppm. The three components with quadrupolar coupling also appear in the 2 H-1 H CP-MAS experiment (Figure S15), indicating that they are in close proximity to [START_REF] Hendricks | A Tunable Library of Substituted Thiourea Precursors to Metal Sulfide Nanocrystals[END_REF] H nuclei and that their motion is restricted. The signal at ~8.8 ppm is attributed to deuterated amide in the N atom (e.g. -NDCO-), whereas the 3.1 and 2.1 signals are assigned to P-D species (respectively In 3 PD and In 2 PD 2 ). C Q values provide valuable information about the mobility of the deuteron; smaller values correspond to increased mobility. [START_REF] Pery | Direct NMR Evidence for the Presence of Mobile Surface Hydrides on Ruthenium Nanoparticles[END_REF] The amide resonances display the highest C Q values as the consequence of tight coordination and steric hindrance. Considering the two types of P-D species, the smaller C Q is attributed to the deuteron of In 2 PD 2 consistent with the less-hindered environment relative to that of In 3 PD. The last component at 1.1 ppm in 2 H solid-echo MAS NMR spectrum is not observed in the 2 H-1 H CP-MAS spectrum (Figure S15) due to its large mobility (which averages the C Q to zero). This resonance can be assigned to the ND 2 of residual mobile amine molecules. The deuterium is thus incorporated both at the surface of the QDs and in the coordinated ligands. A probable mechanism to account for these observations is the activation of gaseous deuterium onto the InP surface followed by H/D exchange in both the amide and the amine. We note that deuterium incorporation requires the presence of the InP QDs, as no signal could be observed in the 2 H solid-echo MAS spectrum of a mixture of PA/HDA alone in the same conditions (21h at 230°C, 1.8 bar D 2 , Figure S16). These findings clearly support that H 2 (D 2 ) molecules are activated (dissociated) at the QDs surface yielding QD-H(D) bonds. While the formation of P-D bonds is unambiguously evidenced, the signature of In-D bonds is not apparent. This is not surprising, as extensive broadening is anticipated on account of both the wide distribution of chemical environments and (more importantly) the large quadrupole moment of indium nuclei (I=9/2). [START_REF] Hibbs | Crystal and Molecular Structure of the First Indium Trihydride Complex [InH3{CN(Pr i )C2Me2N(Pr i )}[END_REF] This formation of QD-H surface species explains why InP QDs are not oxidized in the presence of H 2 despite the waterforming condensation reaction simultaneously occurring during the QDs synthesis. In conclusion, drawing on a profound understanding of InP QD surface chemistry, we have designed two novel approaches to overcome the ubiquitous surface alteration by O-containing ligands and by-products. We first demonstrate that the high energy indium precursor tris(N,N'-diisopropylacetamidinato) indium(III) is reactive at temperatures low enough (150°C) to prevent the oxidation of the resulting particles, despite the in-situ water formed as a by-product. Alternatively, at higher temperatures (230°C), when such an oxidation cannot be avoided under argon, we demonstrate the benefits of using an H 2 atmosphere. We show that H 2 dissociation occurs at the QD surface, ultimately protecting the particles from oxidation and leaving a clean surface suitable for subsequent growth. This opens the door for accurate study of the fundamental chemistry of these systems (growth mechanism, influence of the synthesis parameters…) but also of the physics of InP QDs, which is undoubtedly affected by the untilnow unavoidable defects caused by amorphous surface oxides. Moreover, our approach could be extended to various other semiconductor materials, further opening new perspectives -even for highly-characterized systems such as cadmium and lead chalcogenides.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website. Experimental section, TEM, UV-Vis, XRD and solid state NMR data of InP QDs 1-4, and the ZnScoated versions 5-6.

Scheme 1 .

 1 Scheme 1. Synthesis of InP QDs based on the use of a more reactive In precursor. PA = Palmitic Acid; HDA = Hexadecylamine

Figure 1 .

 1 Figure 1. 31 P Hahn-echo MAS NMR spectrum of (a) InP QDs 2 (spinning rate 16 kHz), (b) InP QDs 3 (spinning rate 18 kHz)

Figure 2 .

 2 Figure 2. UV-Vis spectra of growth InP QDs from a) 2 and b) 3 (each addition corresponds to one-third amount of the initial amount of precursors and ligands used for the synthesis of either 2 or 3)

Figure 3 .

 3 Figure 3. (a) Experimental (blue) and simulated (red) 2 H solidecho MAS spectrum of InP QDs 4 (spinning rate 8 kHz). (b, c, d) simulated spectra of the signal at 8.8, 3.1 and 2.0 ppm (e) Expanded view of the +10/-10 kHz area with full intensity and the simulated lorentzian signal at 1.1 ppm (in black). The simulated spectrum was slightly shifted to facilitate comparison.

ACKNOWLEDGMENT

This work was supported by the Université Paul Sabatier, the Région Midi-Pyrénées, the CNRS, the Institut National des Sciences Appliquées of Toulouse, the European Commission for ERC Grant No. 694159 and the Agence Nationale pour la Recherche (Project ANR-13-IS10-0004-01). E. A. B. is grateful to Marie Curie Actions and Campus France for a PRESTIGE post-doc fellowship (FP7/2007-2013) under REA grant agreement no. PCOFUND-GA-2013-609102.

AUTHOR INFORMATION Corresponding Author

*E-mail cnayral@insa-toulouse.fr (C.N.) and fdelpech@insatoulouse.fr (F.D.)

Notes

The authors declare no competing financial interests.