Junling Qu

Prachi Rastogi

Charlie Gréboval

Clément Livache

Marion Dufour

Audrey Chu

Sang-Soo Chee

Julien Ramade

Zhen Xiang

Sandrine Xu

Ithurria

Xu

Sandrine Ithurria

Emmanuel Lhuillier

A nanoplatelet-based light emitting diode and its use for all-nanocrystal LiFi-like communication

Keywords: nanoplatelets, light emitting diode, efficiency droop, electronic transport, nanocrystalbased communication

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Colloidal nanocrystals have been incorporated as green and red source into liquid crystal display (LCD), however, electrically pumped light emitting diodes (LEDs) application of them has been challenged. [START_REF] Wood | Colloidal Quantum Dot Light-Emitting Devices[END_REF] Even though there may remain several concerns (lifetime and light extraction) to reach industrial integration, a key open question is related to the choice of the material. Among possible candidates, CdSe two-dimensional (2D) nanoplatelets (NPLs) [START_REF] Lhuillier | Two-Dimensional Colloidal Metal Chalcogenides Semiconductors: Synthesis, Spectroscopy, and Applications[END_REF] appear as one of the most promising because they combine (i) the narrowest photoluminescence (PL) signal, [START_REF] Tessier | Spectroscopy of Single CdSe Nanoplatelets[END_REF] (ii) the high maturity of II-VI nanocrystal colloidal syntheses, (iii) spectral tunability based on quantum confinement [START_REF] Christodoulou | Chloride-Induced Thickness Control in CdSe Nanoplatelets[END_REF][START_REF] Diroll | Shape-Selective Optical Transformations of CdSe Nanoplatelets Driven by Halide Ion Ligand Exchange[END_REF][START_REF] Dufour | Halide Ligands To Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness[END_REF][START_REF] Cho | Direct Synthesis of Six-Monolayer (1.9 Nm) Thick Zinc-Blende CdSe Nanoplatelets Emitting at 585 Nm[END_REF] or doping [START_REF] Khan | Tunable and Efficient Red to Near-Infrared Photoluminescence by Synergistic Exploitation of Core and Surface Silver Doping of CdSe Nanoplatelets[END_REF][START_REF] Sharma | Understanding the Journey of Dopant Copper Ions in Atomically Flat Colloidal Nanocrystals of CdSe Nanoplatelets Using Partial Cation Exchange Reactions[END_REF][START_REF] Dufour | Doping as a Strategy to Tune Color of 2D Colloidal Nanoplatelets[END_REF][START_REF] Liu | Light-Emitting Diodes with Cu-Doped Colloidal Quantum Wells: From Ultrapure Green, Tunable Dual-Emission to White Light[END_REF] with (iv) anisotropic emission diagram [START_REF] Feng | Consequence of Shape Elongation on Emission Asymmetry for Colloidal CdSe/CdS Nanoplatelets[END_REF] , which means a macroscopic directive emission can be obtained from a well-aligned NPLs assembly [START_REF] Jana | Stacking and Colloidal Stability of CdSe Nanoplatelets[END_REF][START_REF] Yoon | Origin of Shape-Dependent Fluorescence Polarization from CdSe Nanoplatelets[END_REF] , thus improving the light extraction efficiency of LED devices by overcoming total internal reflection.

NPLs are 2D-shaped nanocrystals with confinement occurring only along the thickness. The specific growth mechanism of these nanocrystals leads to a lack of roughness along this direction and consequently the PL linewidth is not limited by inhomogeneous broadening as for spherical nanoparticles. Beyond their narrow luminescence spectrum, NPLs have also attracted interest for the design of transistors, [START_REF] Lhuillier | Investigating the N-and p-Type Electrolytic Charging of Colloidal Nanoplatelets[END_REF] light detectors [START_REF] Lhuillier | Electrolyte-Gated Colloidal Nanoplatelets-Based Phototransistor and Its Use for Bicolor Detection[END_REF] and low threshold lasers. [START_REF] She | Blue Amplified Spontaneous Emission and Lasing Using Colloidal CdSe Nanoplatelets[END_REF][START_REF] Grim | Continuous-Wave Biexciton Lasing at Room Temperature Using Solution-Processed Quantum Wells[END_REF][START_REF] Rossinelli | Compositional Grading for Efficient and Narrowband Emission in CdSe-Based Core/Shell Nanoplatelets[END_REF][START_REF] Olutas | Lateral Size-Dependent Spontaneous and Stimulated Emission Properties in Colloidal CdSe Nanoplatelets[END_REF] While the narrow PL achieved by NPLs was enough to motivate the design of LED from core-only objects, [START_REF] Dufour | Halide Ligands To Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness[END_REF][START_REF] Fan | Colloidal CdSe1-XSx Nanoplatelets with Narrow and Continuously-Tunable Electroluminescence[END_REF] the integration of NPLs into LED has strongly benefit from the development of core/crown [START_REF] Liu | Nanocrystal Light-Emitting Diodes Based on Type II Nanoplatelets[END_REF] and core/shell structures. For long, only low temperature methods, [START_REF] Mahler | Core/Shell Colloidal Semiconductor Nanoplatelets[END_REF] such as colloidal atomic layer deposition, [START_REF] Ithurria | Colloidal Atomic Layer Deposition (c-ALD) Using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media[END_REF] has been used for the growth of core/shell NPLs. This approach enhances the PL efficiency by passivating surface dangling bonds and preserves the narrow linewidth of the NPLs, but the PL quantum efficiency remains generally limited below 50% with a large batch-to-batch fluctuation. In spite of these limitations, first core/shell NPL-based LEDs [START_REF] Xiao | Emergence of Nanoplatelet Light-Emitting Diodes[END_REF][START_REF] Vitukhnovsky | Electroluminescence from Colloidal Semiconductor CdSe Nanoplatelets in Hybrid Organic-Inorganic Light Emitting Diode[END_REF] have been reported by Chen et al. [START_REF] Chen | Quasi-2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence[END_REF] and improved latter by Giovanella et al. [START_REF] Giovanella | Efficient Solution-Processed Nanoplatelet-Based Light-Emitting Diodes with High Operational Stability in Air[END_REF] More recently, high temperature growth of shells on NPLs which preserve the 2D shape of the particles have been developed, [START_REF] Cruguel | Electronic Structure of CdSe-ZnS 2D Nanoplatelets[END_REF][START_REF] Feng | Probing the Fluorescence Dipoles of Single Cubic CdSe/CdS Nanoplatelets with Vertical or Horizontal Orientations[END_REF][START_REF] Polovitsyn | Synthesis of Air-Stable CdSe/ZnS Core-Shell Nanoplatelets with Tunable Emission Wavelength[END_REF][START_REF] Altintas | Giant Alloyed Hot Injection Shells Enable Ultralow Optical Gain Threshold in Colloidal Quantum Wells[END_REF][START_REF] Rossinelli | High-Temperature Growth of Thick-Shell CdSe/CdS Core/Shell Nanoplatelets[END_REF][START_REF] Kelestemur | Colloidal CdSe Quantum Wells with Graded Shell Composition for Low-Threshold Amplified Spontaneous Emission and Highly Efficient Electroluminescence[END_REF][START_REF] Altintas | Highly Stable, Near-Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero-Nanoplatelets Enabled by ZnS-Shell Hot-Injection Growth[END_REF] leading to high PL efficiencies (90% and more) in the red. The improvement in the PL efficiency has significantly enhanced the electroluminescence (EL) efficiency [START_REF] Kelestemur | Colloidal CdSe Quantum Wells with Graded Shell Composition for Low-Threshold Amplified Spontaneous Emission and Highly Efficient Electroluminescence[END_REF][START_REF] Liu | Record High External Quantum Efficiency of 19.2% Achieved in Light-Emitting Diodes of Colloidal Quantum Wells Enabled by Hot-Injection Shell Growth[END_REF] , which has brought the NPL based LEDs to a new level of maturity and inspired the effort for their integration to more complex systems.

In this paper, we choose the strategy of integrating high temperature grown core/shell NPLs into LEDs. The shell alloy ratio is optimized to match the band alignment of the diode structure and favor the charge balance in the device. The obtained LED based on NPLs achieves 5.15% of external quantum efficiency (EQE), sub-bandgap turn-on voltage and shows an extrapolated half-lifetime of 3160 hours (time after which the emitted power drops by half for 100 Cd.m -2 luminance). We then investigate transport in this LED at low temperature and show that the current droop occurs at higher current when temperature is decreased. We use this observation to suggest one path of improvement for this diode structure. Finally, we couple this LED to a NC-based solar cell used as a broad-band detector. We demonstrate that this couple of all NC-based devices can be used as a LiFi-like communication setup.

DISCUSSION

We start by growing core/shell NPLs made of a CdSe core and an alloyed shell of CdS and ZnS. Current NC-based LED structures rely on organic hole transport layers (HTLs) while electron transport layers (ETLs) are typically based on oxides (ZnO or TiO2). A successful structure for CdSebased nanocrystal LED has been proposed by Dai et al [START_REF] Dai | Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots[END_REF] which includes the following stack of layers: ITO/PEDOT:PSS/Poly-TPD/PVK/NCs/ZnO/Ag, where PEDOT:PSS, Poly-TPD and PVK are respectively poly (3,4-ethylenedioxythiophene) polystyrene sulfonate, Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine and Poly(9-vinyl) carbazole. The HTLs have been identified as limiting blocks for the LED structure because of (i) the acidity of PEDOT:PSS which can etch the transparent conductive electrode, (ii) limited stability of the organic layer and (iii) lower carrier mobility of the HTLs with respect to ETL. To minimize the limitation coming from the HTL, we design the NPL in order to minimize the HTL/NPL energy barrier.

The highest occupied molecular orbital (HOMO) of the PVK is located 5.8±0.1 eV below the vacuum level. To determine the energy of the maximum of the valence band in the core/shell NPL, we use X-ray photoemission (XPS) and determine the work function (Figure 1c) and the relative position of the valence band with respect to the Fermi energy (Figure 1d). For ZnS shell, we found the valence band located 5.7±0.1 eV below the vacuum level, which is quasi resonant with the PVK within the accuracy of the photoemission.

To further optimize this band alignment and the balance of the charge injection, we tune the confinement energy. In NPLs changing the core level leads to discrete values for the band edge energy and this approach is not a continuous path to tune the band edge energy. Here we rather choose to tune the shell content thanks to alloying. The introduction of a small amount of CdS in the ZnS shell favor the electron delocalization over the whole structure, reducing the band edge energy. This allows a fine continuous tuning of the exact energy of the valence and conduction band. We observe that a 5% content of CdS within the CdZnS shell minimize the LED turns on voltage while maximizing its quantum efficiency, see figure S1 andtable S1. nanoplatelets. e. Electronic spectrum of the core/shell nanoplatelets. f. Energy alignement for the LED. The energies for the different layers except the NPL are taken from ref. [START_REF] Chen | Quasi-2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence[END_REF][START_REF] Dai | Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots[END_REF] We thus choose to focus on CdSe/Cd0.05Zn0.95S core/shell nanoplatelets in the following. [START_REF] Feng | Probing the Fluorescence Dipoles of Single Cubic CdSe/CdS Nanoplatelets with Vertical or Horizontal Orientations[END_REF][START_REF] Heuclin | Core-Shell Nanoplatelets Film and Display Device Using the Same[END_REF] The absorption and PL of the material are given in Figure 1a. The PL peaks at 620 nm (≈2 eV) and presents a full width at half maximum (FWHM) of 27 nm. The particles are of a parallelipedic shape with a lateral extension of 10x40 nm 2 as shown on Figure 1b. Photoemission spectra unveil a work function of 4.4 eV and a Fermi level deep in the band gap located 1.3 eV above the valance band. This suggests a n-type nature for this material, consistent with previous transport measurements for CdSe core based NPLs. [START_REF] Lhuillier | Investigating the N-and p-Type Electrolytic Charging of Colloidal Nanoplatelets[END_REF] We thus propose an effective electronic spectrum for the heterostructure as given in Figure 1e. The chosen alloy composition provides a quasi-resonant band alignment with the HTL, while reducing the band conduction band offset with ZnO compared to pure ZnS shell, see Figure 1f.

We build the all solution-processed ITO/PEDOT:PSS/Poly-TPD/PVK/NCs/ZnO/Ag diode with the optimized NCs layer, see a scheme of the device in Figure 2a. We also add an encapsulation layer to avoid any exposure of the device to oxygen and moisture during operation. In the following, the device is stored and operated in room condition (i.e. air exposed). The LED presents an EL spectrum which matches the band gap of CdSe/Cd0.05Zn0.95S core/shell nanoplatelets, with an peak wavelength at 622 nm, see Figure 2b. The FWHM of the EL is 28 nm. The CIE coordinate in the chromaticity diagram is shown in Figure 2c. Note that the emission of the NPL based LED follows a Lambertian angular dependence, see Figure S2. The I-V curve, luminance and EQE of the device are then shown in Figure 3a andb. The diode presents a sub-bandgap threshold with a turn on voltage as low as 1.63 V (about 0.4 V lower than the band gap), which is the lowest reported so far. [START_REF] Luo | Origin of Subthreshold Turn-On in Quantum-Dot Light-Emitting Diodes[END_REF] The maximum EQE has been determined to be 5.15%, partly limited by the PL efficiency of around 35%. The luminance can reach 35 100 cd.m - 2 under 7 V operation, see Figure S3. The lifetime of the diode with an initial luminance of 73 cd.m - 2 and 1000 cd.m -2 have been monitored, under constant current. A reasonable extrapolation allows to estimate that 50% of the extracted power will remain after 11 000 h (at 73 cd.m -2 ), see Figure 3c. Under higher luminance of 1000 cd.m -2 , we found the half lifetime to be 100 h, see figure S4. Assuming that the L 1.5 T50 is constant [START_REF] Dai | Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots[END_REF] , with L the initial luminance and T50 the duration when the LED drops to half of its efficiency, we can determine the half lifetime under 100 cd.m -2 luminance to be at least 3160 h.

It is worth pointing that Kelestemur et al [START_REF] Kelestemur | Colloidal CdSe Quantum Wells with Graded Shell Composition for Low-Threshold Amplified Spontaneous Emission and Highly Efficient Electroluminescence[END_REF] recently reported a similar LED based on core/shell NPLs grown at high temperature and for which the HTL is made of PEDOT:PSS and PVK. Our LED achieve better EQE than the CdSe/CdS NPL-based LED but lower than the one based on core/shell/shell (CdSe/CdS/CdZnS) NPL. The latter material leads to EQE as high as 10%, but the turn-on voltage (2.3 V) is higher. It indicates a less optimized band alignment for charge injection. More comparisons of the LED performance with previously reported LED based on NPLs are given in Table 1. The LED shows interesting performance such as a narrow EL signal, low turn-on operation and long lifetime, however, under high driving current, we observe an efficiency droop. Efficiency droop is a general feature for LEDs and nanocrystal-LEDs where the EQE decreases with the higher current. Currently, the most common strategy to prevent the efficiency droop is to design the nanocrystal band profile to reduce the Auger effect. [START_REF] Lim | Droop-Free Colloidal Quantum Dot Light-Emitting Diodes[END_REF] Graded interfaces has been proposed as a possible route to reach this goal. [START_REF] Cragg | Suppression of Auger Processes in Confined Structures[END_REF] We then investigate the thermal properties of the LED, because low temperature transport is a powerful probe to reveal transport mechanisms and possible ways to improve LED structure. Upon cooling the EL signal shows a blue shift which follows the shift of the PL signal. Thus, we can attribute the EL shift to the change of band gap of NPLs with temperature. Temperature-dependent transport reveals two regimes of bias in the I-V curve. In the first region at low bias, typically below the turn-on voltage of the LED, the transport is almost temperature independent, see Figure 4a and S2. At higher bias, the current depends more strongly on the temperature and we particularly observe a shift of the voltage (Vt) where the device enters a strongly conductive state. This shift is also responsible for a similar shift of the turn-on voltage with temperature in the EQE curve as a function of bias, see Figure 4c. We may have naively attributed this shift of the transport and EQE to the change of band gap with temperature. However, the shift of PL and EL with temperature are much smaller than the one observed in transport, see Figure S5 and S6. Indeed, this shift with temperature in the I-V curve is also observed in the diode built without NPLs, see Figure S7. As a result, we believe that the NPLs are not related to the shift. This is consistent with the situation where in NC-LEDs the nanoparticles are mostly a recombination center and transport is entirely driven by the ETL and HTLs. Complementarily with the shift of the turn-on voltage depending on the temperature, we observe that the EQE droop is shifted toward higher current density at lower Figure 4d and its inset.

The fact that the appearing of the droop is strongly temperature-dependent suggests two origins for the droops. The first path is the temperature-dependent Auger recombination within the NPL [START_REF] Pelton | Hot-Carrier Relaxation in CdSe/CdS Core/Shell Nanoplatelets[END_REF][START_REF] Li | Area-and Thickness-Dependent Biexciton Auger Recombination in Colloidal CdSe Nanoplatelets: Breaking the "Universal Volume Scaling Law[END_REF] .

Temperature dependence of the Auger process remains so far uninvestigated in NPL. However, NPLs thanks to their large volume [START_REF] Klimov | Quantization of Multiparticle Auger Rates in Semiconductor Quantum Dots[END_REF] should present a reduced auger recombination with respect to other kinds of nanocrystals. In addition, NPL with thin shell, as it is here, are supposed to be the one with the lowest Auger rate [START_REF] Pelton | Nonmonotonic Dependence of Auger Recombination Rate on Shell Thickness for CdSe/CdS Core/Shell Nanoplatelets[END_REF] . Thus, it is likely that the delay of droop is assisted by Auger effect occurring in the transport layers. Such speculation is supported by the fact that the shift of the Vt, which follows shift of the droops with temperature, is also observed in the NPL free device, see Figure S7. Nanocrystals-based LEDs and detectors have for the moment been used independently. The maturity of these devices has reached a high level, which motivates their integration into more complex systems. Here, we explore the possibility to design an all-nanocrystal-based communication setup which operates in LiFi-like operation. In such setup, a light source, here the NPL-based LED, is used not only for lightning but also to send information. For this application, we have updated the LED device and expanded it to a larger active area, typically 7x8 mm 2 to generate more photons, see Figure 5a. The thicknesses of the ZnO and PEDOT:PSS layers have also been increased to prevent electrical shorts in the device which are more likely to occur as the device size is increased. However, the device structure maintained and so is the low turn-on voltage around 1.7 V. The electrical and optical characteristics of the diode are given in Figure S8 and S9.

As a sensor, we use a PbS nanocrystal-based solar cell which have been proved to be an efficient broad band absorber over the visible range. We fabricate solar cell structure proposed by Chuang et al [START_REF] Chuang | Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering[END_REF] and the detailed fabrication can be found in the supporting information and ref [START_REF] Ramade | Potential of Colloidal Quantum Dot Based Solar Cells for Near-Infrared Active Detection[END_REF] . The PbS solar cell characteristics are given in Figure S10. Briefly, the structure relies on PbS nanocrystals with a band gap at 920 nm. The p-n junction is made of a thick layer of iodide-capped PbS and a ptype layer of ethanedithiol capped PbS nanocrystals. The dark current of such diode operated at 0 V can be as low as 10 nA.cm -2 and the diode present a strong current modulation under illumination. Interestingly, the time response of the diode has been found to be 10 ns for the rise time and 1 µs for the decay time. This is too slow to establish a high speed (GHz or THz) telecommunication channel but can be used to transfer information. To operate this system, we apply a constant driving voltage to the LED, which is compatible with lightning application. On the top of this signal we setup an AC modulated (@10kHz here) signal to generate signal modulation. The light is then collected by the solar cell as described in Figure 5b. The current in the photodiode is then filtered to keep the AC part only. We first investigate how the distance between the source and the detector affects the signal acquisition, see Figure 5c. In the current configuration, where signal collection is not optimized and conservative bias are applied on the LED, photocurrent to dark current ratio above 1 can be preserved up to 1.5 m. The drop of the signal with distance follows a decay slower than 1/R 2 (ie the optical power decay), with R the source to detector distance. We attribute this behavior to the improvement in the quantum efficiency of the solar cell when the incident power flux is reduced. We also consider the impact of the luminance and the signal modulation amplitude of the LED, see Figure 5d. For luminance compatible with lightning (5000 cd.m -2 ), we can conserve photocurrent to dark current ratio above 1 for signal modulation as low as 25% which is not eye detectable. Transmission of binary signal, beyond a simple plane wave, has also been tested with a 1 m distance, see Figure S11 and the obtained signal matches the LED input.

CONCLUSION

We build a LED based on core/shell NPLs grown at high temperature. The obtained diode presents a maximal EQE of 5.15% with a record low turn-on voltage of 1.63 V and extremely long lifetime (3160 h at 100 cd.m -2 ). Under large current injection an efficiency droop appears, where the current threshold of droop appears to be strongly temperature dependent. Lower temperatures shift the droop toward larger current density. Such strong temperature dependence on the droop threshold suggest that future efforts need to be focused on the transport layers. We have also successfully demonstrated the potential of the LED into a LiFi-like communication setup and show that communication can be obtained once the LED is coupled to a solar cell as a detector. This paves the way for future developments of all nanocrystal-based communication setup.

METHODS

Chemicals for NPL growth : Octadecene (ODE, Alfa Aeser, 90%), cadmium acetate dihydrate (Cd(Ac)2, Sigma, 98%), cadmium oxide (CdO, Strem, 99.99%), zinc oxide (ZnO, Sigma, 99,99%), myristic acid (Fluka, 98%), oleic acid (OA, Alfa Aesar, 90%), selenium powder (Strem Chemicals 99.99%), zinc acetate dihydrate (Aldrich, 98%), octanethiol (Fluka, 97%), Trioctylamine (TOA, sigma aldrich, 98%), hexane (VWR, 99%), ethanol (EtOH, VWR, < 99%), toluene (Carlo Erba, < 99%), methanol (VWR, < 99%).

Precursors:

Cadmium myristate Cd(Myr)2: In a 50 mL three neck flask, 2.56 g of CdO and 11 g of myristic acid were mixed and degassed at 90 °C for an hour. Then, under Ar flux, the temperature was set to be 200 °C. When the solution turns colorless (in approximately 40 minutes), the reaction was stopped and the temperature decreased. At 60 °C, 30 mL of methanol were added to the mixture. The cadmium myristate was washed 4 times with methanol and dried overnight at 50 °C under vacuum.

Cadmium oleate in ODE (0.1M): CdO (6.42 g, 50 mmol) was added to 100 mL of OA in a 250 mL three-neck flask and heated at 160 °C. When the mixture became colorless, the flask is cooled down to 70 °C and degassed under vacuum for 1 h. Then 400 mL of ODE were added to obtain the final concentration. The solution was transferred into a bottle and stored for several months. The precursor was heated prior to use.

Zinc oleate in ODE (0.1 M): In a 250 mL flask, 50 mmol of ZnO (4.074 g) and 100 mL of OA are introduced. The mixture was heated to 180 °C and stayed at that temperature until there was a clear solution. The reaction was cooled down immediately to 120 °C and degassed at this temperature to remove the water. Then the heating mantle was removed, and the mixture were allowed to cool down naturally. To get 0.1 M zinc oleate, 16 mL of ODE was added to 4 mL of 0.5 M zinc oleate.

The precursor was heated prior to use.

CdxZn1-x oleate (0.1 M) can be obtained by mixing cadmium oleate (0.1 M) and zinc oleate (0.1 M) described above according to the desired ratio.

CdSe core nanoplatelets synthesis: A mixture of 340 mg of Cd(Myr)2 (0.6 mmol), 24 mg of Se powder (0.3 mmol) and 30 mL of ODE were degassed in a three neck flask for 30 min under vacuum. Then, under Ar flux, the temperature was set to 240 °C and as it reached 180 °C, 80 mg (0.3 mmol) of Cd(OAc)2•2H2O solid was rapidly added to the flask. 12 min after the addition of Cd(OAc)2, the reaction was quenched by injecting 2 mL of OA . Then the mixture was cooled down to room temperature. Hexane and ethanol were added to the reaction mixture before it was precipitated by centrifugation. A second cleaning was conducted with hexane/ethanol and the final CdSe core nanoplatelets were redispersed in 12 mL of ODE.

CdSe/CdxZn1-xS core/shell nanoplatelets synthesis: The recipe was taken from literature with modifications. [START_REF] Feng | Probing the Fluorescence Dipoles of Single Cubic CdSe/CdS Nanoplatelets with Vertical or Horizontal Orientations[END_REF] 15 mL of TOA was added to a 100 mL three-neck flask and degas at 100 °C for 30 min. Then the atmosphere was switched to Ar and the temperature was set to 300 °C. Meanwhile, 7 mL of 0.12 M octanethiol and 7 mL of 0.1 M of CdxZn1-x oleate (x=0.05 or 0.1) were put into two syringes and set on a pump. When the temperature stabilized at 300 °C, the syringe pump was started and pumped at a constant rate of 4.7 mL/h. After several drops of pumping, 3 mL of NPL core in ODE were swiftly injected to the flask. Then the mixture was baked for another 90 min at 300 °C after all the injection were finished. The flask was then cooled down with air flux and at the temperature of 100 °C, 2 mL of OA was injected. 15 mL of hexane and 10 mL of ethanol were added to precipitate the NPLs by centrifugation. After a second purification with ethanol and hexane, the NPLs were finally dispersed in 5 mL of toluene.

Material characterization

Absorption and photoluminescence: JASCO V-730 spectrometer was used for UV-Visible spectroscopy. Photoluminescence (PL) spectra and PL quantum yield in a solution sample were obtained by an Edinburgh Instrument spectrometer.

Transmission electron microscopy image:

A JEOL 2010 transmission electron microscope operated at 200 kV was used to observe the nanocrystals. The grids were prepared by drop casting a diluted solution of NPLs dispersed in toluene and degassed overnight under secondary vacuum to remove left over of solvent and volatile organics.

X-ray photoemission measurements (XPS):

For ultraviolet photoemission spectroscopy, the films of nanocrystals were spin-casted on a gold coated Si substrate (gold layer is 80 nm thick). The film was ligand exchanged by dipping the film into a 1% in volume solution of ethanedithiol in ethanol.

The samples were introduced in the preparation chamber and degassed until the vacuum reached below 10 -9 mbar. Then samples were introduced to the analysis chamber. Photo energy in the 60 to 1000 eV range were used. The signal was acquired by a detector SCIENTA SES2002 analyzer equipped with a delay line detector developed by ELETTRA. A gold substrate was used to measure the Fermi energy. The exact value of the energy of the beamline was determined by measuring the second harmonic of Cd core level peaks. The measurement of the low binding part of the photoemission spectrum was used to determine the value of VB-EF. Then the energy of the conduction band was determined from the valence band offset by the absorption gap. Finally, the work function was determined by measuring the cut-off of secondary electrons.

LED fabrication

Materials for LED: PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate, Al 4083, M121, Ossila), Poly-TPD (Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine, Ossila), PVK (Poly(9-vinyl) carbazole, average Mn 25,000-50,000, Aldrich), Chlorobenzene 99.8%, Sigma-Aldrich), m-Xylene (anhydrous, ≥99%, Sigma-Aldrich), Zinc acetate dihydrate (97+%, Alfa Aesar), Tetramethylammonium hydroxide pentahydrate (TMAOH, 98%, Alfa Aesar), Dimethyl Sulfoxide (DMSO, ≥99.9%, Sigma-Aldrich), ethyl acetate (VMR), ethanol absolute anhydrous (VMR), acetone (VWR), and toluene (Carlo Erba, 99.3%). All the materials were used as received.

Synthesis of ZnO nanoparticles:

The procedure was taken from ref [START_REF] Dai | Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots[END_REF] . In flask A, 3 mmol of Zinc acetate were dissolved in 30 mL of DMSO by vigorous stirring. At the same time, 5.5 mmol of TMAOH were dissolved with 10 mL of ethanol in flask B. Then the content of the two flasks were mixed and stirred for 24 hours under ambient conditions. The reaction mixture turned whitish during the first few seconds and became clear soon after. ZnO particles were precipitated by ethyl acetate and redispersed in ethanol. 160 µL of 2-ethanolamine were added to stabilize the nanoparticles before they were precipitated and redispersed with ethyl acetate and ethanol respectively again. Finally, the ZnO nanoparticles in ethanol is filtered using a 0.22 µm PTFE filter.

ITO substrate patterning: ITO substrates (10 Ω/sq) were cut into 15 mm × 15 mm pieces and cleaned by sonication in acetone for 5 min. After sonication, the substrates were rinsed with acetone and isopropanol before dried completely with N2 flow. The substrates were further cleaned with O2 plasma for 5 min to remove organic residuals on the surface. After cleaning, TI-Prime and AZ 5214E photoresist were sequentially spin-coated on the surface of ITO substrates at the rate of 4000 rpm for 30 s and baked at 110 °C for 120 s and 90 s, respectively. In the next stage, a mask aligner was used to expose the substrates to UV light for 20 s through a lithography mask (1 mm width). Photoresist was then developed using AZ 726 developer for 20 s before rinsed with deionized water and dried with N2 flux. After another 5-minute plasma cleaning, the substrates were etched in a 25% HCl (in water) bath for 10 min at 40 °C before they were dipped immediately in deionized water. Finally, the lift-off was conducted in an acetone bath. Before using, the patterned ITO substrates were cleaned with acetone and isopropanol first and put under plasma for 10 min.

LED Fabrication: PEDOT:PSS solution (filtered through 0.45 µm filter) was spin-coated on a patterned ITO glass electrode at 4000 rpm for 60 s and annealed at 140 °C for 10 min in air. Inside a Nitrogen-filled glovebox, Poly-TPD (8 mg.mL -1 in chlorobenzene), PVK (1.5 mg.mL -1 in m-xylene), NPLs and ZnO nanoparticles were successively spin-coated at 2000 rpm for 45 s on the PEDOT: PSS-coated substrate. After the deposition of Poly-TPD, the sample was annealed at 110 °C for 20 min, and for PVK the annealing was at 170 °C for 30 min. Finally, 80 nm of Ag was deposited on top of the ZnO using a shadow mask by thermal evaporation. The thickness of NPL and ZnO layer were 18 nm and 80 nm respectively, as obtained by profilometry. The devices were encapsulated inside the glove box with a piece of glass by Araldite crystal glue. The size of the pixel was 1 mm 2 which was the overlap of ITO and Ag electrode.

LED characterization:

The EQE of the device was determined according to the method from Ref [START_REF] Chen | Efficient Near-Infrared Organic Light-Emitting Diodes Based on Multimetallic Assemblies of Lanthanides and Iridium Complexes[END_REF] , with 𝑉(𝜆) the function of photonic eye sensitivity. For the characterization, we collected current-voltage-luminance characteristics with a Keithley K2634B sourcemeter unit and a PM100A powermeter coupled with the S120C Si detector from Thorlabs. Knowing that the working diameter of detector area is 9.5 mm and assuming the distance between detector and device to be 6.5 mm, the geometry related value of

Operation of the communication setup:

To probe the ability of the LED for all-nanocrystal-based communication we used a MFLI lock-in amplifier to drive the LED and measure the detected signal from the PbS solar cell. The signal output of the MFLI was used to send a periodic signal composed of an offset voltage (Voffset) over which is added a sinusoidal signal at 10 kHz with an amplitude (zero-to-peak) of Vpk. On the detection side, the solar cell is operated at 0 V bias and current is amplified through a Femto DLPCA-100 transimpedance amplifier (TIA). The output voltage of the TIA was then sent to the signal input of the lock-in. After demodulation the photocurrent at the reference frequency is measured. The amplitude of the modulation is defined as 𝑉 pk /𝑉 offset .

Figure 1 a

 1 Figure 1 a. Absorption and photoluminescence spectra of CdSe/Cd0.05Zn0.95S core/shell nanoplatelets. b. Transmission electron microscopy image of CdSe/Cd0.05Zn0.95S core/shell nanoplatelets. c. Photoemission spectrum relative to the secondary electron cut-off for the core/shell nanoplatelets. d. Photoemission spectrum relative to the valence band for the core/shell nanoplatelets. e. Electronic spectrum of the core/shell nanoplatelets. f. Energy alignement for the LED. The energies for the different layers except the NPL are taken from ref.[START_REF] Chen | Quasi-2D Colloidal Semiconductor Nanoplatelets for Narrow Electroluminescence[END_REF][START_REF] Dai | Solution-Processed, High-Performance Light-Emitting Diodes Based on Quantum Dots[END_REF]

Figure 2 a

 2 Figure 2 a. Scheme of the LED made of glass/ITO/hole transport layers (PEDOT :PSS/PolyTPD/PVK)/ CdSe/Cd0.05Zn0.95S core/shell nanoplatelets/ZnO/Ag. b. Electroluminescence spectra of the LED which emitting layer is made of CdSe/Cd0.05Zn0.95S core/shell nanoplatelets at room temperature and under various applied bias. c. Chromaticity diagram and the color associated with the spectrum of the LED based on CdSe/Cd0.05Zn0.95S core/shell nanoplatelets.

Figure 3 .

 3 Figure 3 .a Current density and luminance as a function of the applied bias for the LED based on CdSe/Cd0.05Zn0.95S core/shell nanoplatelets. b. External quantum efficiency (EQE) and luminance as a function of the driving current. c. Normalized light power extracted from the LED as a function of time. The LED is driven under a constant current of 14 µA here which correspond to a initial luminance of 73 cd.m -2 . The experimental data are in black, while the red(solid) curve is an extrapolation. The extracted power drops by half after ≈11 000 h.

Figure 4 a

 4 Figure 4 a. Current as a function of bias under various temperatures of the LED based on CdSe/Cd0.05Zn0.95S core/shell nanoplatelets. b. Electroluminescence spectra obtained at different temperatures under 5 V bias. A clear red-shift is observed as the temperature increases. c. Uncalibrated EQE as a function of the applied bias for various temperatures . d. Uncalibrated EQE as a function of the driving current under different temperatures.

Figure 5 a

 5 Figure 5 a. Picture of the large LED based on NPLs under operation. b. Scheme of the developed communication setup. The LED is driven with a constant bias on the top of which a modulated signal is applied. The signal is then collected on a PbS nanocrystal based solar cell used as broad band detector. The AC component of the signal is then amplified using a lock-in setup. c. Photocurrent and photocurrent to dark current ratio as a function of the LED to detector distance. 1 /R 2 decay law is also plotted. Signal is modulated at 10 kHz here. d. Photocurrent to dark current ratio as a function of the luminance of the LED and as a function of the signal modulation. 100% modulation means that the LED is turned on and off, while for lower modulation the LED is always emitting some light.

1 𝑙 2 1 . 2 ,

 1212 . Considering the Lambertian emission of LED device, the flux leaving the device directly can be described as 𝐹 𝑒𝑥𝑡 = = 𝜋𝐿 0 , with 𝐿 0 the flux per solid angle of light leaving the device in the forward direction. Since the solid angle from the photodetector to the light source is Ω = 𝑆 with 𝑆 1 the area of the detector and 𝑙 the distance between the light source and detector, then 𝐿 0 The number of photons emitted per second to the forward direction then can be calculated by 𝑁 𝑃 = , with 𝜆 the wavelength of electroluminescence, ℎ the Plank's constant and 𝑐 the speed of light. The number of electrons injected per second can be obtained by 𝑁 𝑃 = 𝐼 𝑒 , with 𝐼 the current flow of the device. Thus, the EQE can be calculated as 𝐸𝑄𝐸 = 𝑁 𝑝 𝑁 𝑒 = 𝜋𝑃 ⅆ𝑒𝑡 𝑙 2 𝜆𝑒 𝑠 1 ℎ𝑐𝐼 . The irradiance of the device is 𝑅 = with 𝑆 2 the area of the pixel. The luminance of the device 𝐿 = 683.𝑉(𝜆).𝐹 𝑒𝑥𝑡 𝜋.𝑆 2

Table 1 :

 1 Comparison of different figure of merit for NPL-based LEDs

	Refer ence	Material (NPL)	Wavelength (nm)	PLQY (%)	FWHM (nm)	EQEmax (%)	Lmax (nt)	VON (V)	Pixel size (mm 2 )	t1/2 at 100 nt (h)	J90 (mA.cm -2 )
		CdSe/Cd0.15Zn0.85S	648	86		5.66	16010	2.4	4
	36	CdSe/Cd0.25Zn0.75S	650	≈100	26	19.32	23490	2.4	4	12.8
		CdSe/ZnS	618	85		4.69	18730	2.8	4
	34	CdSe/CdS/CdZnS	634-648	75-89	21	9.92	46000	2.3	16	560	276
		CdSe/CdS	650	48	21	1.8	10650	2.3	16		372
	28	CdSe/CdZnS	658	40±5		8.39	1540	4.05	5.4
	22	CdSe/CdSeTe Core/crown	599	85	>40	3.57	34520	1.9	1
	40	CdSe/CdS	556	60	14	5	33000 2.25	4
	27	CdSe/CdZnS	646	30	26	0.63	4499	4.7	4
	21	CdSe/CdSSe	520	35	12.5		100	2.1
	This	CdSe/Cd0.05Zn0.95S	620	35	29	5.15	35100 1.63	1	3160	295
	work	CdSe/Cd0.1Zn0.9S	640	65	23			≈1.7	56

ACKNOWLEDEGMENTS

The project is supported by ERC starting grant blackQD (grant n° 756225) and Ne2Dem (grant n° 853049). 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, Copin, Frontal, Graskop. JQ thanks Chinese Scholarship Council for PhD grant while AC thanks Agence innovation defense.

Additional information

Supplementary information is available for this paper at. It includes informations relative to the LED characterization (angular dependence operation under high electric field, low temperature operation), Operation of the all nanocrystal-based communication setup (large scale LED fabrication, solar cell fabrication and characterization, transmission of binary signal). A video illustrating the operation of the LEDs under pulsed mode is also provided.


Junling Qu

Nicolas Goubet

Clément Livache

Bertille Martinez

Dylan Amelot

Charlie Gréboval

Audrey Chu

Julien Ramade

Hervé Cruguel

Sandrine Ithurria

Mathieu G Silly

Emmanuel Lhuillier

Intraband Mid-Infrared Transitions in Ag2Se Nanocrystals: Potential and Limitations for Hg-Free Low-Cost Photodetection

Infrared photodetection based on colloidal nanoparticles is a promising path toward low cost devices. However, mid-infrared absorption relies on interband transitions in heavy metal-based materials, which is a major flaw for the development toward mass market. In the quest of mercury-free infrared active colloidal materials, we here investigate Ag2Se nanoparticles presenting intraband transition between 3 and 15 µm. With photoemission and infrared spectroscopy, we are able to propose an electronic spectrum of the material in absolute energy scale. We also investigate the origin of doping and demonstrate that it results from a cation excess under Ag + form. We demonstrate photoconduction into this material under resonant excitation of the intraband transition. However, performances are currently quite weak with (i) a slow photoresponse (several seconds), and (ii) some electrochemical instabilities at room temperature.

INTRODUCTION

Thanks to the ease of color tunability from UV to THz range, [START_REF] Goubet | Terahertz HgTe Nanocrystals: Beyond Confinement[END_REF] colloidal quantum dots (CQDs) have created extensive interest in applications such as displays and optoelectronics. [START_REF] Konstantatos | Colloidal Quantum Dot Optoelectronics and Photovoltaics[END_REF][START_REF] Talapin | Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications[END_REF] Their solution processability makes CQD-based infrared optoelectronics of utmost interest for the design of a new generation of low cost devices. In particular, CQDbased mid-infrared (IR) photodetection [START_REF] Lhuillier | Recent Progresses in Mid Infrared Nanocrystal Based Optoelectronics[END_REF] has reached a high level of maturity with the demonstration of mid-IR photoconduction, [START_REF] Keuleyan | Mid-infrared HgTe Colloidal Quantum Dot Photodetectors[END_REF] mid-IR focal plane array, [START_REF] Buurma | MWIR Imaging with Low Cost Colloidal Quantum Dot Films[END_REF] single particle photodetection, [START_REF] Wang | Transport in a Single Self-Doped Nanocrystal[END_REF] multi-color photodetection, [START_REF] Lhuillier | Electrolyte-Gated Colloidal Nanoplatelets-Based Phototransistor and its Use for Bicolor Detection[END_REF][START_REF] Tang | Scalable Fabrication of Infrared Detectors with Multispectral Photoresponse Based on Patterned Colloidal Quantum Dot Films[END_REF] plasmon assisted photodetection, [START_REF] Chen | Photocurrent Enhancement of HgTe Quantum Dot Photodiodes by Plasmonic Gold Nanorod Structures[END_REF][START_REF] Yifat | Mid-IR Colloidal Quantum Dot Detectors Enhanced by Optical Nano-Antennas[END_REF] gas sensing [START_REF] Chen | Mercury Telluride Quantum Dot based Phototransistor Enabling High Sensitivity Room-Temperature Photodetection at 2000 nm[END_REF] and fire detection. [START_REF] Lacovo | High Responsivity Fire Detectors Based on Pbs Colloidal Quantum Dot Photoconductors[END_REF] All these demonstrated applications currently remain led by mercury chalcogenides compounds (HgS, 14 , 15 HgSe, 16 , 17 HgTe [START_REF] Seong | Photocurrent Characteristics of Solution-Processed HgTe Nanoparticle Thin Films Under the Illumination of 1.3 µm Wavelength Light[END_REF] ) which present both interband [START_REF] Kershaw | Narrow Bandgap Colloidal Metal Chalcogenide Quantum dots: Synthetic Methods, Heterostructures, Assemblies, Electronic and Infrared Optical Properties[END_REF] and intraband [START_REF] Zhiyou | Colloidal Quantum Dots Intraband Photodetectors[END_REF][START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF][START_REF] Jagtap | Emergence of Intraband Transitions in Colloidal Nanocrystals[END_REF] transitions in the mid-IR range. However, the presence of mercury is often seen as a fundamental flaw of this technology.

Narrow energy transitions in the mid-IR typically require materials with a significant number of bands, which generally comes with a large atomic number and is thus heavy metal-free incompatible. The demand for non-toxic infrared active CQDs is more likely to result from doped semiconductors presenting intraband or plasmonic transitions. Several candidates might be considered such as extrinsically doped oxides, as Si nanocrystals and selfdoped chalcogenides such as Ag2Se. The latter material is of utmost interest in the sense that its surface chemistry is closer to those of the HgX compounds. Consequently the surface chemistry toolbox transfer to Ag2Se materials seems more straightforward than to oxides or group IV compounds. Interest for Ag2Se compounds was initially driven by near-IR photoluminescence [START_REF] Gu | Ultrasmall Near-Infrared Ag2Se Quantum Dots with Tunable Fluorescence for In Vivo Imaging[END_REF][START_REF] Langevin | Size-Dependent Extinction Coefficients and Transition Energies of Near-Infrared β-Ag2Se Colloidal Quantum Dots[END_REF] and thermoelectric applications. [START_REF] Yang | Bendable Thermoelectric Generators Composed of p-and n-type Silver Chalcogenide Nanoparticle Thin Films[END_REF] Sahu et al [START_REF] Sahu | Quantum Confinement in Silver Selenide Semiconductor Nanocrystals[END_REF][START_REF] Sahu | Facile Synthesis of Silver Chalcogenide (Ag2E; E=Se, S, Te) Semiconductor Nanocrystals[END_REF] were the first to report mid-IR tunable transitions in Ag2Se compounds; however, several questions are still unanswered, in particular the potential of this material for photoconduction and the exact nature of the mid-IR absorption feature. Recently, Park et al [START_REF] Park | Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se Nanocrystals[END_REF] answered to the latter question using spectroelectrochemistry and brought evidence of its intraband character. Here we expand the investigation on the Ag2Se CQDs: we report about (i) the electronic structure of this material, (ii) the origin of doping in this material and finally (iii) the potential and the limitations of Ag2Se CQDs for mid-IR photoconduction.

METHODS

Chemicals:

Tri-n-octylphosphine oxide (TOPO, Alfa Aescar, 95%), Oleylamine (OLA, Acros, 80-90%), Trioctylphosphine (TOP, Cytek, 90%), Selenium powder (Strem Chemicals, 99.99%), Silver Chloride (AgCl, Alfa Aesar, 99.9%), Dodecanethiol (DDT, Sigma-Aldrich, 98%), 1,2,ethanedithiol (EDT, Fluka, 98.0%), Butanol (VMR, GPR RECTAPUR), Chloroform (CHCl3, Carlo Erba), Ethanol absolute anhydrous (Carlo Erba, 99.9%), Lithium perchlorate (LiClO4, Sigma-Aldrich, 98%) and Polyethylene glycol (PEG, Mw=6 kg/mol) were used.

Ag2Se nanocrystal synthesis: 7.8 g of TOPO and 6.6 mL of OLA were put in a 50 mL three-neck flask. The mixture was degassed at 120 °C for 1 hour before the atmosphere was switched to argon. Then, 6 mL of selenium solution in TOP (1 M) was injected at 120 °C followed by heating the mixture to 140 °C, 180 °C or 250 °C. 4 mL of AgCl solution in TOP (1 M) was quickly injected at 140 °C and allowed to grow for 1 minute to obtain small-sized nanocrystals, injected at 180 °C and grew for 20 minutes to obtain large-sized ones, or injected at 250 °C for 20 minutes to obtain extra-large Ag2Se nanocrystals. The growth reactions were quenched by 0.5 mL of DDT. After that, the flask was cooled down with air flux and 5 mL of butanol was injected at 60 °C to avoid the solidification of the reaction products. The nanocrystals were then precipitated with ethanol and redispersed with CHCl3 twice. To obtain clean small-sized Ag2Se, a 0.22 µm filter was also used after two washes.

FTIR:

For infrared spectroscopy, we use a Vertex 70 from Bruker in the ATR configuration. The solution of nanocrystals is drop-casted on the diamond cell. Once the solvent is dried, the spectrum is acquired with a 4 cm -1 resolution and averaged 32 times.

TEM:

For TEM pictures, a drop of diluted CQD solution is drop-casted onto a copper grid covered with an amorphous carbon film. A JEOL 2010F is used at 200 kV for the acquisition of pictures.

X-ray diffraction: X-ray diffraction is obtained by drop-casting the Ag2Se CQD solution on a Si wafer. The diffractometer is a Philips X'Pert based on the emission of the Cu Kα line at λ=0.154 nm, operated at 40 kV and 40 mA.

EDX analysis: Energy dispersive X-ray (EDX) analysis is conducted by depositing a solution of nanocrystals onto Si wafer. The sample is then introduced into a FEI Magellan scanning electron microscope. The current of the beam is adjusted to 1.6 nA and the operating bias is set as 15 kV. The EDX analysis is obtained from an Oxford EDX probe.

RBS analysis:

Rutherford Backscattering Spectrometry has been used as a complementary analysis to EDX to determine the Ag/Se ratio. To do so, the SAFIR source at INSP was used with He + as ion accelerated under 1.8 MeV. The beam size is 1.5 mm 2 and the collection angle is 165°. The He + beam current is operated at 50 nA. The energy calibration is done using a Bi-implanted Si wafer with a 6.10 15 cm -2 dose. From the backscattered yield, the Ag/Se ratio is determined by 𝑅 = Ultraviolet and X-ray photoemission measurements: For photoemission, the Ag2Se nanocrystal solution has been drop-casted on an 80 nm gold coated Si/SiO2 wafer. The film is then ligand exchanged using ethanedithiol as capping ligands. The ligand exchange is obtained by dipping the film into a solution of EDT (1%wt) in ethanol.

The valence band and the cut off of the secondary electrons have been obtained while exciting the film using a He lamp operated at 21.2 eV on the Tempo beamline of synchrotron Soleil, while for the core level the synchrotron beam operated at 600 eV has been used as X-ray photon source. The photoelectrons were detected at normal emission and at 46° from the polarization vector 𝐸 ⃗ . The Fermi edge of a bulk gold substrate is used as energy reference (ie its binding energy is taken equal to 0 eV). All other spectra are shifted accordingly.

Si/SiO2/Au electrodes:

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 h. The electrodes are finally rinsed using isopropanol and dried by an air flow. The electrodes are 2.5 mm long and spaced by 10 µm. These electrodes are used for photoconductive device and electrolyte gated transistor measurements.

Electrolyte gating:

For electrolyte gating we first mix in a glove box 0.5 g of LiClO4 with 2.3 g of PEG. The vial is heated at 170 °C on a hot plate for 2 h until the solution becomes clear. To use the electrolyte, the solution is warmed around 100 °C and brushed on the top of the Ag2Se CQD film.

Electrolyte gated field effect transistor measurements:

To measure the transfer curve of the electrolyte gated transistor, the transistor is connected to a home-made probe station connected to a Keithley 2634B used as a sourcemeter. All measurements are conducted in air at room temperature and ambient pressure.

Photoconduction measurements: For photoconduction measurements with interband excitation at room temperature, we use a near infrared (1.55 µm) laser diode. The sample is connected to a Zurich instrument MFLI lock-in, which also biased the sample. The laser is mechanically chopped using a wheel and this signal is used as reference by the lock-in. For low temperature measurements, the sample is introduced in a closed-cycle helium cryostat and cooled down. A quantum cascade laser operating at 4.4 µm is used to resonantly excite the intraband transition. The sample is biased using the source of Femto DLCPA-200 current amplifier, which is also used to magnify the current. The signal is acquired on a Tektronik TDS-5034 oscilloscope.

DISCUSSION

We first synthetize a series of Ag2Se CQDs using the method proposed by Norris' group. [START_REF] Sahu | Quantum Confinement in Silver Selenide Semiconductor Nanocrystals[END_REF][START_REF] Sahu | Facile Synthesis of Silver Chalcogenide (Ag2E; E=Se, S, Te) Semiconductor Nanocrystals[END_REF] The quenching of the reaction has been updated by introducing some DDT at the end of the reaction to enhance colloidal stability and reduce the material aggregation. The infrared spectra for three sizes of Ag2Se CQDs are given in Figure 1a. They are composed of an interband feature at short wavelength and a strongly absorbing peak in the mid-IR. Tuning the size of the nanoparticles (see Figure 1c andd and Figure S1) from 5 nm to 28 nm allows to tune the energy of the intraband feature from 2100 cm -1 (260 meV or 4.8 µm) to 650 cm -1 (80 meV or 15.4 µm). The synthesis leads to low size polydispersity, between 5 and 10 %. The lattice structure is particularly well resolved in the case of the large Ag2Se CQDs, see Figure 1d, where the X-ray diffraction confirms a tetragonal structure (Figure 1b). In the following we will focus on the Ag2Se small size CQDs because they present the best optical features to target photodetection in the 3-5 µm range. The first open question is the electronic structure of the material in absolute energy scale, which is a critical step for the integration of this material into device with optimized ohmic contacts. To answer this question, we have conducted ultraviolet photoemission on Tempo beamline of Soleil Synchrotron. As shown in Figure 2a, a zoom on the cut-off of the secondary electrons reveals that the work function is equal to 4.3 eV. The low binding part of the spectrum (Figure 2b) is used to locate the valence band with respect to the Fermi energy and we deduce a value of Vb-Ef of 0.68 eV. Park et al [START_REF] Park | Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se Nanocrystals[END_REF] , have deduced from spectroelectrochemistry the value of the interband gap to be 0.6 eV for this size of Ag2Se CQD, which basically brings the 1Se state of the conduction band below the Fermi level and confirms the degenerately n-doped nature of the Ag2Se CQDs. From the energy of the intraband feature deduced from the IR spectrum, the 1Pe state is 260 meV above the 1Se state. A reconstructed electronic spectrum is proposed in Figure 2c. In this sense, the obtained spectrum is quite close to the one previously observed for HgSe QDs with same sizes. [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF]

Figure 2a. X-ray photoemission signal around the cut off of the secondary electrons for a film made of small Ag2Se CQDs. The determined work function is 4.3 eV. b. X-ray photoemission signal associated to the valence band for a film made of small Ag2Se CQDs. c. Electronic spectrum in absolute energy scale from a film made of small Ag2Se CQDs.

The origin of doping needs to be further investigated. Park et al [START_REF] Park | Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se Nanocrystals[END_REF] have reported a metal excess in these particles using Energy Dispersive X-ray spectroscopy (EDX). The actual stoichiometry of their compound is Ag2.3Se. In our case EDX and Rutherford back scattering spectroscopy (RBS) gives consistent results of a silver excess and a final stoichiometry of Ag2.6Se, see Figure S2 andS3. We attribute the fact to have even more silver than Park et al to the difference of synthetic process to grow the Ag2Se CQDs. In II-VI semiconductor based CQDs, it is well known that surface tends to be metal rich leading to a n-type character.

The presence of metal silver is also further revealed by high-resolution transmission electron microscopy, see Figure 34 andS5. We indeed observe the presence of Janus nanoparticles, see Figure S4 with some darker spot on the QD image. A careful analysis of the diffraction pattern (Figure S4b-d) reveals that the main part of the particle is made of tetragonal Ag2Se, while the dark spot is made of pure metallic silver. In addition, see figure S5, we also notice the presence of nanoparticles with a 5-fold symmetry which is characteristic of noble metal nanoparticles in facecentered cubic (fcc) lattice. [START_REF] Goubet | Size and Nanocrystallinity Controlled Gold Nanocrystals: Synthesis, Electronic and Mechanical Properties[END_REF][START_REF] Ino | Multiply Twinned Particles at Earlier Stages of Gold Film Formation on Alkalihalide Crystals[END_REF][START_REF] Goris | Measuring Lattice Strain in Three Dimensions Through Electron Microscopy[END_REF] Thus, a naive explanation of doping may have been an energy transfer from the metallic Ag 0 phase to the narrow band gap semiconductor phase. Indeed Ag 0 , thanks to its low work function is commonly used as electron injector for light emitting diode. However, this hypothesis is ruled out by X ray photoemission, which is the most suited tool to discriminate between two oxidation states of a chemical species, see Figure 3. The overview signal (Figure 3a) reveals the presence of gold from the substrate and oxygen due to air exposure of the sample while other peaks (Ag, Se for the semiconductor and S, C from the ligands) actually relate to the sample itself. The signal from the Ag 3d peak is shown in Figure 3b and without ambiguity can be fitted by a single Voigt contribution. The binding energy of the Ag 3d5/2 state is measured to be 368.06 eV, while the spin-orbit coupling is 6 eV [START_REF] Wagner | Hanbook of X-ray Photoelectron Spectroscopy[END_REF] and the full width at half maximum (FWHM) is 0.65 eV. This value of the binding energy is typically 200 meV below the value usually reported for Ag 0 . 34 Moreover, no loss feature is observed in this spectrum which could have been a signature of metallic silver. Thus, we can confidently state that XPS only reveals Ag + and that the small Ag 0 island observed in TEM are just non-statistically representative of the sample. We can conclude that doping in Ag2Se is somehow quite similar to most II-VI compounds for which the cation surface excess leads to a ntype character. [START_REF] Cruguel | Electronic Structure of CdSe-ZnS 2D Nanoplatelets[END_REF][START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] For the sake of completeness, we also have analyzed the signal relative to the Se 3d state, see Figure 3c. In this case, two components are observed, the most electron-rich peak is centered at a binding energy of 53.7 eV while the second peak presents a binding energy of 54.3 eV. Both contributions are fitted with a FWHM of 0.71 eV and spin orbit coupling of 0.86 eV. [START_REF] Wagner | Hanbook of X-ray Photoelectron Spectroscopy[END_REF] Figure 3a. X-ray photoemission overview of a film of Ag2Se small nanoparticles deposited on a gold substrate. The photon energy is set to be 600 eV. b. X-ray photoemission signal relative to the Ag 3d state from a film of Ag2Se small nanoparticles. c. X-ray photoemission signal relative to the Se 3d state from a film of Ag2Se small nanoparticles.

In the following, we investigate the conduction and photoconduction properties of thin films made of Ag2Se. At first, we integrate the small Ag2Se CQDs into an electrolyte gated transistor [START_REF] Lhuillier | Investigating the n-and p-Type Electrolytic Charging of Colloidal Nanoplatelets[END_REF] (a scheme of the device is proposed in Figure 4a). The latter is very convenient to tune the carrier density of heavily doped CQDs. To make the film of Ag2Se CQDs conductive, we performed a solid-state ligand exchange toward EDT capping and check that this procedure preserves the intraband feature (see Figure S6). The transfer curve (Figure 4b) reveals a clear n-type behavior with an increase of conductance upon electron injection (ie toward positive gate bias), which is fully consistent with the electronic spectrum proposed in Figure 2c. The on-off ratio of the transistor is typically around 50. In the following we have used two light sources to excite the small Ag2Se CQDs: (i) a near infrared laser diode source at 1.55 µm to pump the interband transition and (ii) a quantum cascade laser source at 4.4 µm to resonantly excite the intraband transition. At room temperature and under near-IR illumination, the material presents a clear photoconductive behavior, see Figure 4c-e. The photocurrent presents a sub-linear dependence with the incident photon flux, see Figure 4c. The time response, see Figure 4d-e and S7, is for near -infrared excitation reasonably fast with a 3 dB cutoff frequency around 20 kHz. This behavior is obtained while the material is operated in the ohmic regime of the IV curve (ie below 200 mV). On the other hand, when the bias range is increased, the I-V curve starts to strongly deviate from linear regime and a clear electrochemical signature is observed, see Figure 4f. We speculate that the electrochemical behavior observed results from the limited stability of the Ag + oxidation state. This electrochemical instability of the thin film of Ag2Se CQDs is a severe limitation. We nevertheless observe that at low temperature, the material recovers its ohmic behavior, see figure S8, with no hysteresis. To finish, we explore the photoconductive behavior of the material at low temperature while we resonantly excite the intraband transition using quantum cascade laser as mid-IR source. A scheme of the experimental setup is given in Figure 5a. As the sample is cooled, the temperature dependence of the current follows a monotonic behavior, which can be fitted by an Arrhenius behavior. The activation energy in the 300 to 200 K range is 164 meV, see Figure 5b. This value is within the accuracy of the photoemission experiment (50 meV) consistent with the energy spacing from the Fermi level to the 1Pe state (180 meV), which is the first empty state available for transport. To obtain a clear photocurrent signal while exciting the sample resonantly with the intraband absorption, very low temperature has to be reached (<200K), see Figure 5c-e and S9. The responsivity has been estimated to be 8 µA.W -1 , which is orders of magnitude lower than what has been reported for HgSe CQDs. [START_REF] Zhiyou | Colloidal Quantum Dots Intraband Photodetectors[END_REF][START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF] On the other hand, similarly to HgSe, the time response is extremely slow (several tens of s), see Figure 5d-e. This strongly contrasts with performance in HgTe CQDs where time response as short as 10 ns has been reported at the same wavelength. [START_REF] Martinez | Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals[END_REF][START_REF] Livache | Band-Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals[END_REF]

CONCLUSION

We have explored the potential of Ag2Se CQDs for mid-IR low-cost photoconduction as a heavy metal free colloidal material. Using photoemission and transport measurements, we have confirmed the degenerately n-type doping of the material and are able to propose an electronic spectrum in absolute energy scale where the work function is estimated to be 4.3 eV and the Fermi level is lying in the conduction band between the 1Se and 1Pe state. A cation excess is observed from EDX and RBS analysis, in agreement with recent report on the same material [START_REF] Park | Mid-Infrared Intraband Transition of Metal Excess Colloidal Ag2Se Nanocrystals[END_REF] and the X-ray photoemission demonstrates that the excess metal is under Ag + form. Thus doping is the result of material nonstoichiometry and the narrow band gap phase which makes possible the reduction of the particle by the environment as proposed for mercury chalcogenides compounds. [START_REF] Robin | Surface Control of Doping in Self-Doped Nanocrystals[END_REF] Material can be made photoconductive using conventional ligand exchange procedure, however we have observed a very limited range of bias stability (200 mV) likely due to oxidation and reduction of the Ag + . This major failure needs to be solved, possibly by the addition of a shell, to really consider this material for room temperature IR photodetection. Under mid-IR excitation resonant with the intraband absorption, the photoconduction is determined to be very weak and slow compared to performances reported for Hg containing CQDs.

  AAg denotes the area of the peak resulting from backscattering at Ag atoms, ASe equals the area of the Se peak, and ZAg and ZSe are the atomic number of Ag and Se, respectively.

Figure 1a .

 1a Figure 1a. Infrared spectra of films made of Ag2Se CQDs with three different sizes. b. X-ray diffractogram of Ag2Se CQDs with three different sizes. c. TEM image of small Ag2Se CQDs. d. High resolution TEM image of extra large Ag2Se CQDs.

Figure 4a .

 4a Figure 4a. Scheme of an electrolyte gated transistor for which the channel is made of a small Ag2Se CQD film. b Transfer curve (drain current as a function of applied gate bias) for a film made of small Ag2Se CQDs. c. Photocurrent as a function of the incident light power from a near-IR (λ=1.55 µm) laser diode. d. Time trace of the photocurrent while the near-IR laser is turned on and off. The laser power is set at 25 mW. e. Bode diagram (photocurrent as a function of signal frequency) for a film made of small Ag2SeCQDs under illumination by the near-IR laser diode. f. IV curve of a film made of small Ag2Se CQDs obtained when a larger range of biases is explored.

Figure 5 a

 5 Figure 5 a. Scheme of the setup used for the illumination of the sample by a mid-IR source (ie a quantum cascade laser operated at 4.4 µm with a 10 mW power, Reprinted with permission from ref 38 (2018) American Chemical Society. b. Dark current under 1V as a function of the temperature for a film made of small Ag2Se CQDs. c. IV curve for a film made of small Ag2Se CQDs under dark condition and under illumination by the quantum cascade laser. d. Current as function of time while the mid-IR source is turned on and off. e. Bode diagram (photocurrent as a function of signal frequency) for a film made of small Ag2Se CQDs.

SUPPORTING INFORMATION

Supporting information includes additional structural (TEM), electronic (XPS, RBS, EDX) and transport measurements.

ACKNOWLEDEGMENTS

EL thanks the support ERC starting grant blackQD (grant number 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 Nanodose and H2DH. NG and AJ thank Nexdot for post doc funding.


Ulrich Nguétchuissi Noumbé

Charlie Gréboval

Clément Livache

Brulé Thibault

Bernard Brulé

Abdelkarim Doudin

Emmanuel Ouerghi

Jean-Francois Lhuillier

Dayen

Thibault Brule

Bernard Doudin

Abdelkarim Ouerghi

Emmanuel Lhuillier

Jean-Francois Dayen
email: jean-francois.dayen@ipcms.unistra.fr

Ionic Glass-Gated 2D Material-Based Phototransistor: MoSe 2 over LaF 3 as Case Study

Keywords: (photodetector, phototransistor, two dimensional material, ionic glass, field effect transistor)

Introduction

Because they combine a band gap with new optical features 1 resulting from their low dimensionality Transition Metal DiChalcogenides (TMDC) are generating a lot of interests to revisit the 2D physics of semiconductor. [2][3][4] The control of carrier density becomes of utmost importance to design building block such as pn junction 5,6 , field effect transistor (FET) 7 , light emitting diode 8 and photodetector 3,9,10 . Due to chalcogenides vacancy TMDC are not behaving as intrinsic semiconductor, MoSe2 is n type while WSe2 is p type for example. Post synthesis control of this doping is also possible thanks to hydrogenation, 11 deposition of small electron donor and acceptor molecule. 12,13 All these methods however still suffer from a lack of post fabrication tunability, and may require alternative device fabrication process such as resist free patterning 14,15 . Gate control of the carrier density remains the most conventional method to tune the carrier density at the device level. 16 Two main strategies have been proposed to gate 2D material which are conventional dielectric (typically the silica layer on the top of a silicon wafer) and electrolyte (ionic liquid, ion gel…). In the first approach, dielectric breakdown limits the maximum accumulated carrier density within the 10 12 cm -2 range. The second method is of utmost interest when larger sheet density (>10 13 cm -2 ) are targeted. [17][18][19] Use of electrolyte has been exploited in TMDC to develop ambipolar FET 20 , to induce superconductivity 21 or for the design of pn junction. [22][23][24] An important drawback of electrolyte gate technology, is that the electrolyte covers the gated material, hindering direct top optical excitation since it can absorb or scatter the light, and preventing its use for optoelectronics devices and optical studies. Some alternative to dielectric and electrolyte have been proposed and rely on ferroelectric polymer 25 and more recently on the use of ionic glasses. The Shukla's group 22 has proposed to use borosilicate and soda-lime glasses as ionic glass media. By applying a large electric field on such glass slide at high temperature (400 K), ions within the glass substrate can accumulate at the sample surface leading to a gate effect. While the method can be used to reach high carrier density above 10 14 cm -2 , the gate effect can only be obtained far above room temperature. The Cui's group 28 later proposed to change these glass for LaF3 and demonstrated gate effect under dark condition on MoS2, while preserving the gate tunability down to 180K circumventing the need for large electric field and high temperature imposed by borosilicate and soda-lime glasses. However, there is no report to date on the use of ionic glass for photodetection. Here, we explore for the first time the possibility to control the photoresponse of a 2D material over an extended doping range with ion glass gating, using MoSe2 flakes over LaF3 ionic glass as archetypal system. Our MoSe2/Ionic Glass phototransistor demonstrate high and fast photoresponsivity respect to the state of the art.

Moreover, we demonstrate that ionic glass gating can be used as a knob to tune the nature of the carrier recombination processes, while annihilating completely the traps contribution in electron injection regime. This new functionality enables to modulate electrostatically the magnitude of the photoresponse over more than two orders of magnitude. Surprisingly, this change is achieved while preserving the photoresponse time, i.e, while amplifying the gain bandwidth product.

Results and discussion

We first start by preparing a MoSe2 flake using exfoliation method and transfer it onto the LaF3 substrate. Few layers MoSe2 flake was mechanically exfoliated from bulk crystal on Polydimethylsiloxane (PDMS) using adhesive tapes, and then transferred onto LaF3 substrate by dry transfer method 29 . The optical transparency of the LaF3 substrate allows for simple identification of the flakes by optical contrast. The lack of PL signal suggests that the flake is made of multilayers MoSe2. 30 This hypothesis is further confirmed by the Raman map and spectrum, see while it is expected to be at 240 cm -1 for the monolayer MoSe2, confirming the multilayer aspect of our sample. Finally using Atomic Force Microscopy, we determine the thickness of the sample to be 2.5±0.5 nm, which correspond to 4-5 monolayers of MoSe2.

The flake is then connected to gold electrodes by electron beam lithography using a standard bilayer PMMA (Polymethyl methacrylate) resist and an additional conductive Protective Coating resist. A scheme of the phototransistor device as well as scanning electron microscopy image are given in Figure 1.a-b. Finally, the sample is inserted into a cryostat for temperature dependent phototransport study.

As gate bias is applied, F -vacancy moves toward or away from the surface which induce electrostatic gating effect 28 . The MoSe2 flake presents a n-type characteristic which is consistent with previous MoSe2 based FET using SiO2 as dielectric, 31,32 see Figure 2a-b. It is important to note that field effect is preserved even for operation below room temperature.

Actually we observe a maximum of the current modulation for a device temperature around Because the LaF3 substrate is quite thick (>1 mm), its capacitance is weak. At lower frequency, ions get enough time to reach the surface and can form the equivalent of a ionic double layer, and a significant rise of the capacitance is observed which now reach 1 µF.cm -2 , a value quite similar to those reported for electrolytes 17 . Such large capacitance allows low gate bias operability, typically around a few volts, since carrier concentration as large as 5x10 13 cm -2 are achieved under 8 V operation.

K, see

Using the C(f) curve it is now possible to estimate the field effect electronic mobility using

the expression µ 𝐹𝐸 = 𝐿 𝑤𝐶 𝛴 𝑉 𝐷𝑆 𝜕𝐼 𝐷𝑆 𝜕𝑉 𝐺𝑆
, where L is the electrode spacing (2.6 µm), W is the electrode length (9.3 µm), CΣ the surface capacitance taken equal to 0.1 µF.cm -2 at 200K (see inset of Figure 2d). We found electron mobility to be µ 𝐹𝐸 ≈ 15 cm 2 •V -1 •s -1 at 200 K, see Figure 2d. This value is very similar to the one reported for MoSe2 multilayer gated with silica. 32 While for TMDC the concept of dark current control using LaF3 has already been reported 28 , we here extend this concept and demonstrate for the first time that the photoresponse of 2D semiconducting material can also be tuned thanks to the ionic glass gate (see Supporting Information and Figure S2 for technical details). Under illumination above the band gap (by a 405 nm blue laser), we observe a photoconductive signal, see Figure 3a.

The magnitude of the signal can be strongly tuned using the gate bias, see Figure 3b.

Typically, the responsivity of the film reaches 50 mA/W at 0 V of gate bias and is enhanced by a factor 4 under strong positive gate bias now exceeding 0.2 A/W. It is worth pointing that the increase of the responsivity is not as fast as the one of the dark current. As a result the gate bias which maximizes the signal to noise ratio is obtained under hole injection, see Figure 3d.

This regime corresponds to the one which minimizes the dark current. Rise-and fall-times (respectively tON and tOFF in Figure 3.c) extracted from the photocurrent characteristics under negative gate bias are close to 200 µs and 600 µs respectively. This is two orders of magnitude faster than time responses previously reported on usual exfoliated and CVD MoSe2 10,33-37 , 50 times faster than encapsulated MoSe2 devices 38 , and more than one decade shorter than the fastest MoSe2 phototransistor reported so far 39 . Another very striking feature comes from the very weak gate dependence of the time response with gate bias, see Figure 3c. The on and off time change by less than 30 % from the hole injection to the electron injection regimes. This is remarkable since the gain bandwidth product (which can be seen as the ratio of responsivity divided by the characteristic time constant) is actually increasing with the applied gate. This result contrast with what is observed for phototransistor based on nanocrystal film, 40,41 where the change of responsivity is made at quasi constant gain bandwidth product. In other words, a higher photoresponse comes at the prize of a slower response, on the contrary of our device.

We have then used the power dependence of the photoresponse to reveal the recombination mechanism. While the incident power is increased, the photocurrent rises, see Figure 4a-b. However this increase presents a sub-linear dependence and appears to be gate tunable, see Figure 4b. Under electron injection (ie positive gate bias) the current as a function of incident power follows a power law with an exponent of 0.42, which is quite close to the value of ½ expected for bimolecular process [START_REF] Rose | Concepts in Photoconductivity and Allied Problems[END_REF][START_REF] Willis | [END_REF] (ie recombination occurs through electron hole recombination). On the other hand, under hole injection we observe an increase of the exponent value which now reached 0.67. This value is typically intermediate between the value of ½ expected for bimolecular process and the value of 1 obtained for monomolecular process (ie for trapping). By applying a negative gate bias the Fermi level is brought more deeply in the band gap and the recombination centers get unfilled and can act as trap states, which explain the increase of the power law exponent, see the associated mechanism in the inset of Figure 4b.

Conclusion

We have explored the potential of LaF3 ionic glass to build TMDC based phototransistor with low operating bias and preserving gate tunability down to 180 K. Optimal operating temperature is around 200K and limits the leakage through the substrate while preserving a strong ionic mobility, with ION/IOFF ratio exceeding 5 decades. Not only the dark conductance can be tuned but also the photoresponse which demonstrates time responses down to 200 µs, up to two decades faster than state of the art MoSe2 phototransistors reported so far. Careful analysis of photocurrent properties reveals that the relative contribution of traps on the carrier recombination can be dramatically suppressed thanks to the very high accumulated carrier concentration achieved by ion glass gating. In other words, the ion glass gate can be used as knob to switch on and off monomolecular recombination processes. Remarkably, this novel functionality allows for modulating the device photoresponse by more than two orders of magnitude, while at the same time increasing its gain bandwidth product, a unique property never observed on any other nanomaterials such as nanocrystals.

This study shows that ionic glass gate are complementary method to the usual electrolyte gate approaches, as they enable optical experiments in high doping regime that are not possible when the 2D material is buried into the electrolyte gate. The performances of our ionic glass photodetector demonstrate the potential of this method to explore novel photoconduction processes and alternative architectures of devices. Our approach is generic, and can be easily extended to any photoconducting nanomaterials, including organic and molecular systems [44][45][46] , nanocrystals 47,48 , and hybrid materials [49][50][51][52][53][54] .

Experimental section

Device fabrication : Few layer MoSe2 was mechanically exfoliated from bulk crystal (2H phase from graphene supermarket) on PDMS (Polydimethylsiloxane) using adhesive tapes, and then transferred onto LaF3 substrate by dry transfer method 29 . By optical contrast, we identified the flake and, we used Ebeam lithography with a Zeiss Supra 40 scanning electron microscope coupled to a Raith lithography system to pattern electrodes. We used a standard bilayer PMMA (Polymethyl methacrylate) resist and a conductive Protective Coating resist (e-spacer) to write on the insulator surface. The PMMA bilayer consists of AR-PMMA 669.04 and AR-PMMA 679.02, that were spin-coated at 4000 revolutions per minute (rpm) during 1 min and annealed at 180 °C during 90 seconds (220nm / 4000rpm and 70nm / 4000rpm respectively). The e-spacer AR-PC 5090.02 was then spincoated at the same speed (4000 rpm) but annealed at 90 °C during 90 seconds (90nm /4000rpm). After the development in 1:3 MIBK-IPA at 25 °C, metal deposition Ti(3nm)/Au(47nm) was performed by electron beam evaporator in high vacuum chamber, followed by lift-off in acetone, rinsing with isopropanol and nitrogen jet drying.

Scanning electron microscopy : SEM is made using a FEI Magellan microscope. Images are acquired with the e-beam accelerated with a 3 kV bias and the current from the electronic gun set at 6 pA.

Raman and photoluminescence spectra :The micro-Raman and photoluminescence (PL) measurements were conducted using commercial confocal HORIBA LabRAM HR Evolution micro-Raman microscope with an objective focused 532 nm and 633 nm lasers in an ambient environment at room temperature. The excitation laser was focused onto the samples with a spot diameter of ≈0.5 µm and incident power of ≈0.1 mW. Measurements were performed on the same microscope with a 100× objective and a CCD detector (detection range from 1.2 to 6.2 eV).

Photocurrent and electrical measurements :

Figure 1 .

 1 c and Figure 1.d. The A1g peak of MoSe2 here appears at 242 cm -1 ,

Figure 2c .

 2c There the ION/IOFF current modulation exceeds 5 orders of magnitude (Ion/Ioff=2x10 5 for VDS=0.5 V), see Figure 2b-c. Above 200 K, the On-off ratio is limited by a large off current due to the relatively large leakage through the LaF3. At temperature below 180K, the ionic mobility of the F -vacancy gets strongly reduced and the ON current gets lower, see Figure 2c. A subthreshold slope, defined as S=(d(logIDS)/dVGS) -1 of S≈240 mV/decade can be extracted at VDS=1V. To have a deeper insight into the ionic glass gating mechanisms, the capacitance of LaF3 as a function of frequency (C(f)) has been determined using an impedance meter, see Inset of

Figure 2d (

 2d Figure 2d (see also Supporting Information and Figure S1 for technical details). At high

  Photocurrent and electrical measurements :The sample is connected to a Keithley K2634 B

Figure 1 .

 1 Figure 1. a) Scheme of the photodetector device and electrical connections. A flake of MoSe2

Figure 2 .

 2 Figure 2. a) Transfer curve of an MoSe2 flake under 1 V drain-source bias at 200 K in linear

Figure 3 .

 3 Figure 3. a) Photocurrent of the device illuminated under 12 nW received power from a 405

Figure 4 .

 4 Figure 4. a) Photocurrent at 100 Hz and 180 K for different incident power (from 11 nW to

Acknowledgments

JFD thanks financial support of the Agence Nationale de la Recherche (under Labex NIE 11-LABX-0058_NIE within the Investissement d'Avenir program ANR-10-IDEX-0002-02, and under H2DH ANR-15-CE24-0016). EL thanks the support ERC starting grant blackQD (grant n° 756225). We acknowledge the use of clean-room facilities from the StNano platform and the "Centrale de Proximité Paris-Centre". We acknowledge Etienne Lorchat and Stéphane Berciaud for technical help with 2D material transfert. This work has been supported by the

((will be filled in by the editorial staff))

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author. TOC text : We report for the first time 2D material based photodetector using ionic glass as electrostatic gating method, choosing MoSe2 over LaF3 ionic glass as archetypal system. We unveil the wider possibilities offered by this architecture, and provide a careful analysis of its unique optoelectronic properties.

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2018.

Supporting Information

Ionic glass gated 2D material based phototransistor : MoSe2 over LaF3 as case study.

Ulrich Nguétchuissi Noumbé, Charlie Gréboval, Clément Livache, Thibault Brule, Bernard Doudin, Abdelkarim Ouerghi, Emmanuel Lhuillier*, Jean-Francois Dayen*.

To whom correspondence should be sent: el@insp.upmc.fr , jean-francois.dayen@ipcms.unistra.fr

AFM measurements

The thickness of the sample was revealed using Atomic Force Microscoscopy. It is found to be 2.5±0.5 nm, which correspond to 4-5 monolayers of MoSe2.

Impedance measurements

To measure the capacitance of the sample, 80 nm of Au is evaporated on both sides of the LaF3 substrate. It is then connected to a MFLI lock-in amplifier that applies an AC bias of 300 mV and measures the current, as scheme of the setup is shown in Figure S1.

Photocurrent measurement