Amardeep Manikrao Jagtap

Bertille Martinez

Nicolas Goubet

Audrey Chu

Clément Livache

Charlie Gréboval

Julien Ramade

Dylan Amelot

Paul Trousset

Amaury Triboulin

Sandrine Ithurria

Mathieu G Silly

Benoit Dubertret

Emmanuel Lhuillier

Design of Unipolar Barrier for Nanocrystal Based Short Wave Infrared Photodiode

Keywords: nanocrystals, infrared photodetection, short wave infrared, photodiode

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INTRODUCTION

Colloidal nanocrystals are promising materials for the design of low-cost optoelectronic devices. Beyond their bright luminescence properties, which are used in displays and light emitting diodes (LEDs), their use for infrared detection have generated a lot of interest over the recent years [START_REF] Kershaw | Narrow Bandgap Colloidal Metal Chalcogenide Quantum Dots: Synthetic Methods, Heterostructures, Assemblies, Electronic and Infrared Optical Properties[END_REF] especially for two applications: solar cells 2 (ie at wavelengths below 1 µm) and thermal imaging (for wavelengths above 3 µm). [START_REF] Keuleyan | Mid-Infrared HgTe Colloidal Quantum Dot Photodetectors[END_REF] The intermediate range of wavelengths from 1 to 3 µm is called short wave infrared range (SWIR) and finds applications such as imaging of biological tissue sections, [START_REF] Thimsen | Shortwave-Infrared (SWIR) Emitters for Biological Imaging: A Review of Challenges and Opportunities[END_REF][START_REF] Bruns | Next-Generation in Vivo Optical Imaging with Short-Wave Infrared Quantum Dots[END_REF] telecommunications, night-glow assisted night vision [START_REF] Hansen | Overview of SWIR Detectors, Cameras, and Applications[END_REF] and active imaging. [START_REF] Demiguel | Theoretical Analysis and Comparison of SWIR Active Imaging Detectors[END_REF] InGaAs is the leading technology in this range of wavelengths. Performances reached by InGaAs detectors are undoubtedly impressive with high quantum efficiency around 80 % and low dark current densities (≈10 -9 A•cm -2 ). However, InGaAs technology suffers from two main issues, which are its high fabrication cost and the lack of wavelength tunability. In this sense, nanocrystals may offer an interesting low-cost platform especially if they can reach wavelengths where InGaAs is not effective.

Here we typically target the extended SWIR for detection up to 2.5 µm (4000 cm -1 or 0.5 eV). Mercury chalcogenides are certainly the most mature material to achieve absorption and photoconduction in this range of wavelengths. There is nevertheless a limited number of reports based on mercury chalcogenides photodiodes, which are generally focused either on shorter [START_REF] Chen | Photocurrent Enhancement of HgTe Quantum Dot Photodiodes by Plasmonic Gold Nanorod Structures[END_REF][START_REF] Nam | Near-Infrared-Sensitive Bulk Heterojunction Solar Cells Using Nanostructured Hybrid Composites of HgTe Quantum Dots and a Low-Bandgap Polymer[END_REF] or longer wavelengths. 10- 12 Our group has recently reported photodetection at this wavelength (2.5 µm) using HgTe nanocrystals. [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF] In addition to report an encapsulation strategy to obtain stable in air operability, we have also pointed that photodiodes can be an interesting way to enhance the device detectivity, compared to photoconductive devices. [START_REF] Guyot-Sionnest | Background Limited Mid-Infrared Photodetection with Photovoltaic HgTe Colloidal Quantum Dots[END_REF] The reported level of performances (quantum efficiency below 1% and D*≈2•10 7 Jones at room temperature) was however too limited. The proposed diode was relying on a Schottky junction with the following structure: ITO/TiO2/HgTe/Au, a scheme of its energetic profile is shown in Figure 1c. This structure was nevertheless suffering from two major flaws: (i) energetically unoptimized contact design, which limited the extraction of photocharged carriers and (ii) a too high dark current, as shown in Figure 1c.

To further enhance the performances of this device, not only we improve the charge extraction by optimizing the contact design, but we propose to introduce an unipolar barrier to reduce the electronic dark current. While the concept of unipolar barrier has been extensively investigated for infrared photodetection based on III-V epitaxially grown semiconductors, [START_REF] Martyniuk | Barrier Infrared Detectors[END_REF][START_REF] Savich | Dark Current Filtering in Unipolar Barrier Infrared Detectors[END_REF][START_REF] Savich | Benefits and Limitations of Unipolar Barriers in Infrared Photodetectors[END_REF] it has only been marginally applied to colloidal nanocrystal based solar cells to funnel the carriers in a graded band gap structure, [START_REF] Kramer | Solar Cells Using Quantum Funnels[END_REF] or in p-n junctions. [START_REF] Chuang | Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering[END_REF] Here, we integrate this unipolar barrier to reduce the dark current of our device and increase the detectivity, see Figure 1b andd. We report HgTe nanocrystal-based photodiodes with detectivity reaching 3•10 8 Jones for 2.5 µm detection at room temperature. This is an order of magnitude larger than the value reported for previous diodes at the same wavelength.

DISCUSSION

Two families of nanocrystal materials are potentially interesting to achieve absorption in the SWIR: lead and mercury chalcogenides. Lead chalcogenide nanocrystals have been extensively used to harvest the near infrared 2 (from 800 nm to 1 µm) part of the solar spectrum in quantum dots based solar cells. However, in the extended SWIR (>2.5 µm), they are difficult to integrate [START_REF] Pietryga | Pushing the Band Gap Envelope: Mid-Infrared Emitting Colloidal PbSe Quantum Dots[END_REF][START_REF] Konstantatos | Ultrasensitive Solution-Cast Quantum Dot Photodetectors[END_REF] into photodiodes with a vertical geometry. Indeed, large nanoparticles are required to reduce the confinement energy and achieve band edge energy around 0.5 eV, and such nanoparticles tend to have a poor colloidal stability. This leads to CQD films with a low quality (ie with pinholes) and results in electrical shortcuts. To build a detector based on CQDs in the extended SWIR, narrower band gap materials are required and mercury chalcogenides compounds offer the most mature alternative. [START_REF] Green | Synthetic Routes to Mercury Chalcogenide Quantum Dots[END_REF][START_REF] Lhuillier | Recent Progresses in Mid Infrared Nanocrystal Based Optoelectronics[END_REF][START_REF] Huo | MoS2-HgTe Quantum Dot Hybrid Photodetectors beyond 2 µm[END_REF][START_REF] Cryer | 300 Nm Spectral Resolution in the Mid-Infrared with Robust, High Responsivity Flexible Colloidal Quantum Dot Devices at Room Temperature[END_REF] In the following we will focus on HgTe as active material to address the extended SWIR range. HgTe CQDs have been synthetized using the procedure developed by Keuleyan et al. [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF] Nanoparticles have a tetrapodic shape with a size around 8 nm, see transmission electron microscopy (TEM) image in Figure 1a, and they present an optical band edge at 4000 cm -1 (0.5 eV or 2.5 µm), see Figure 1a. It was previously demonstrated that the material is an ambipolar conductor (i.e. it conducts both holes and electrons), see supporting information figure S1 and S3-5. Its work function is equal to 4.7 ± 0.1 eV and it presents an Urbach energy, which describes the gap trap distribution, of 35 meV. [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF][START_REF] Martinez | Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals[END_REF] Figure 1 a. Infrared spectrum of HgTe nanocrystals with a band-edge energy at 4000 cm -1. The background is a TEM image of HgTe nanocrystals. b. Scheme of electrically active photodiode. c. Energy band profile of the first generation of short wave infrared photodiode with a structure ITO/TiO2/HgTe (ambipolar -4000 cm -1 )/Au. d. Energy band profile of the second generation of short wave infrared photodiode with a structure FTO/TiO2/HgTe (ambipolar -4000 cm -1 )/HgTe (p type -6000 cm - 1 )/MoO3/Au. The relative position of the band is determined assuming a 4.7 eV work function for ITO, [START_REF] Park | Work Function of Indium Tin Oxide Transparent Conductor Measured by Photoelectron Spectroscopy[END_REF] 4.4 eV work function for FTO, [START_REF] Qiao | Water-Soluble Polythiophene∕nanocrystalline TiO2 Solar Cells[END_REF] 5.1 eV for gold and 3.2 eV band gap for anatase TiO2 [START_REF] Ola | Review of Material Design and Reactor Engineering on TiO2 Photocatalysis for CO2 Reduction[END_REF] with HOMO and LUMO respectively at 7.3 eV and 4.1 eV. Band alignment for MoO3 (figure S1) and values for HgTe (figure S6) are the one determined in the supporting information.

Jagtap et al [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF] proposed a first generation of HgTe based photodiode operating in the extended SWIR based on a ITO/TiO2/HgTe/Au structure, see Figure 1c. While oxides layers are processed in air, all the following steps of the device fabrication are conducted in air-free conditions to avoid the substantial increase of dark conductance observed when ligand-exchanged HgTe CQD films are exposed to air. [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF][START_REF] Chen | Fast, Air-Stable Infrared Photodetectors Based on Spray-Deposited Aqueous HgTe Quantum Dots[END_REF][START_REF] Lhuillier | Mid-Infrared HgTe/As2S3 Field Effect Transistors and Photodetectors[END_REF] On top of the device, a thick Poly(methyl-methacrylate) (PMMA; water repellant) and Poly (vinyl alcohol) (PVA; O2 repellant) are deposited at low temperature (room temperature and annealed at 50 ˚C) to encapsulate and obtain air-stable performances. [START_REF] Jagtap | Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances[END_REF] The I-V curve rectifying behavior is the result of the TiO2 layer, which is used as electron extractor and hole blocking layer (see Figure 1c). The responsivity of this structure (a few µA.W -1 around 0 V) is actually very low, which is the signature of a poor photocharge extraction. Optimizing the electron and hole extraction is required to increase device performances. In the first section of this paper we discuss how both hole and electron extractions can be improved by designing better contacts.

We first screen the effect of adding a hole extraction layer between the HgTe CQD layer and the gold contact. We chose to deposit a MoO3 layer (a scheme of the device is given in Figure 1b). The effect of this layer on the device responsivity is shown in Figure 2a. In presence of this layer, we observe an increase of the responsivity up to a factor two. The optimal thickness is in the range from 10 to 20 nm, see the inset of Figure 2a.

Similarly, we screen different strategies to prepare the electron extraction layer. Two transparent conductive oxides have been tested (indium tin oxide: ITO and fluorine doped tin oxide: FTO) and two electron transport layers have been tested (ZnO and TiO2), see Figure 2b. Out of these four combinations, the combination of FTO and TiO2 is the one leading to the highest responsivity with a factor two improvement compared to the former generation, see the inset of Figure 2b. This improvement comes from the better band alignment between FTO and the top of the conduction band of TiO2, see Figure 1c-d.

Figure 2 a. Responsivity for a photodiode with the following structure ITO/ZnO/HgTe (ambipolar -4000 cm-1)/MoO3/Au as a function of the applied bias over the diode for different thicknesses of the MoO3 layer. The inset plot is the relative evolution of the responsivity as a function of the MoO3 thickness for a 50 mV bias. b. Responsivity for a photodiode with the following structure of transparent conductive layer/n-type conduction layer/HgTe (ambipolar -4000 cm -1 )/MoO3/Au as a function of the applied bias over the diode for different couples of transparent conductive layer (ITO and FTO) and n-type extraction layer (ZnO and TiO2). The thickness of the MoO3 layer is 10 nm. The inset plot is the relative evolution of the responsivity for the different considered couples of transparent conductive layer and n-type extraction layer, under 50 mV bias.

In the following, we aim to design an unipolar barrier, [START_REF] Martyniuk | Barrier Infrared Detectors[END_REF][START_REF] Savich | Dark Current Filtering in Unipolar Barrier Infrared Detectors[END_REF][START_REF] Savich | Benefits and Limitations of Unipolar Barriers in Infrared Photodetectors[END_REF] whose role will be to prevent the electron injection from the gold contact, while still letting the photogenerated holes being collected, see Figure 1d. There are three main criteria for the design of such unipolar barrier. (i) First, this unipolar barrier needs to present a wider band gap than the absorbing layer (ie above 0.5 eV here). (ii) Secondly, the band alignment needs to be suited to let the photohole flows (ie no valence band offset). In other words, the unipolar barrier valence band needs to be aligned with the absorbing layer valence band while the conduction band needs to be offset, by an amount >> kBT where kB is Boltzmann constant and T is the operating temperature. Last, (iii) the barrier must have an unipolar behavior (here p-type is desirable) from a transport point of view.

In the following, we demonstrate that another layer of HgTe with a wider band gap can be used as unipolar barrier. To reach this goal, a first challenge is to identify a strategy to control the majority carrier in a layer of HgTe CQDs.

We have tested the effect of changing the mercury precursor during the synthesis. The procedure developed by Keuleyan is based on HgCl2. [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF] It was recently demonstrated that other halides may have a huge impact on reaction kinetics and final shape of particles. [START_REF] Goubet | Terahertz HgTe Nanocrystals: Beyond Confinement[END_REF] It was also demonstrated that for the same ligands (S 2-), mercury chalcogenides might be p-type when the (111) facet is exposed [START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] or might even become n-type when the (100) facet is exposed [START_REF] Livache | Charge Dynamics and Optolectronic Properties in HgTe Colloidal Quantum Wells[END_REF] . As a result, we may have speculated that the small change of shape may result in different doping magnitude. However, no such effect has been observed, see figure S3. A second hypothesis to tune the majority carrier is a change in the reaction stoichiometry. It is known that n-type character of II-VI semiconductor CQDs results from their cationrich surface. [START_REF] Jeong | Air-Stable n-Doped Colloidal HgS Quantum Dots[END_REF] As the reaction conditions are changed from a 1:1 Hg/Te ratio to a 50% excess in mercury or tellurium, no significant change in the Fermi level position or carrier mobility has been observed, see Figure S4.

On the other hand, we determine that the ligand used to quench the reaction is playing a critical role on the nature of the majority carrier, see Figure 3a. Compared to the Keuleyan's procedure [START_REF] Keuleyan | Synthesis of Colloidal HgTe Quantum Dots for Narrow Mid-IR Emission and Detection[END_REF] for which the synthesis is quenched by adding dodecanethiol and TOP, we now use oleic acid. By doing so, the infrared spectrum presents very limited changes, see Figure 3a and3c. The change of capping ligands nevertheless comes at the price of a much lower colloidal stability. Indeed, as opposed to thiols, acids are hard bases from Pearson's theory and thus poorly bond to Hg 2+ , which is a soft acid. Then for both initial capping ligands, we conduct a solid-state ligand exchange toward ethanedithiol. Transport properties are conducted in an ion gel quasi solid electrolyte gated transistor configuration. [START_REF] Kang | High Carrier Densities Achieved at Low Voltages in Ambipolar PbSe Nanocrystal Thin-Film Transistors[END_REF][START_REF] Bisri | Low Driving Voltage and High Mobility Ambipolar Field-Effect Transistors with PbS Colloidal Nanocrystals[END_REF][START_REF] Lhuillier | Investigating the N-and p-Type Electrolytic Charging of Colloidal Nanoplatelets[END_REF] This field effect transistor configuration allows (i) air operability of the device, (ii) low operating biases and (iii) gating of thick films. HgTe quenched with dodecanethiol leads to an ambipolar behavior, see Figure 3b, while the oleic acid quenched materials are only p-type, see Figure 3d. We speculate that this difference of behavior is the result of the weak bonding of oleic acid to the CQD surface. As a result, oleic acid ligands are easier to strip from the CQD surface than the DDT molecules. Thus, we have identified a path to control the majority carrier of HgTe CQDs at the synthetic level by tuning the ligand used to quench the reaction. In addition of the surface chemistry effect, we have observed a huge change of transport behavior with the CQD size. Transistor measurements, see Figure 4a-c and figure S5, reveal a switch from p-type to ambipolar and finally to n-type as the CQD size increases. In particular, materials with band gap larger than 5500 cm -1 (0.68 eV) present only p-type behavior, see Figure 4d. These results are consistent with the reconstructed electronic spectrum of this material probed by photoemission, see Figure 4e andS6. The Fermi level is in the bottom part of the band gap for the widest band gap materials (7000 cm -1 ≈0.87 eV and 6000 cm -1 ≈0.75 eV band edge energy); in the upper part of the band gap, but still close to the middle for intermediate band gap (4000 cm -1 ≈0.5 eV and 2700 cm -1 ≈ 0.33 eV band edge energy); while for the smallest band gap, the Fermi level is found to be within the conduction band. This degenerate doping is confirmed by the optical spectrum, see figure S5, on which an intraband feature is appearing.

Figure 4 a, b and c are respectively transfer curves for electrolyte gated thin films of HgTe CQDs with band edge at 7000 cm -1 , 4000 cm -1 and 700 cm -1 . All transistor curves are acquired at room temperature under a 400 mV bias. d. Ratio of the electron and hole mobilities (µe-/µh+) for thin films of HgTe with different band edges. When material presents only p-type conduction, we set the ratio µe-/µh+ to 10 -2 . e. Band alignment of HgTe nanocrystals with different values of band edge energy. In particular, we highlight the weak valence band offset between 4000 cm -1 and 6000 cm -1 band edge energy materials.

To build an unipolar barrier, the band alignment is certainly the most critical aspect. The combination of infrared spectroscopy (to determine the band edge energy) and photoemission spectroscopy (to locate the valence band with respect to the Fermi level) has been used to determine the relative position of HgTe band gap as a function of the nanocrystal size, see Figure 4e. As opposed to what has been measured with electrochemistry, [START_REF] Chen | Reversible Electrochemistry of Mercury Chalcogenide Colloidal Quantum Dot Films[END_REF] we observe almost no valence band offset for the smallest size of CQDs. As a result, using a layer of HgTe CQDs with a 6000 cm -1 (≈0.75 eV) band edge energy appears as a suitable path for the design of an unipolar barrier. 90 % of the increase of band gap, with respect to the absorbing material band gap, is a conduction band offset, while the valence band offset is typically of the order of kBT at room temperature. Moreover, the material has a clear unipolar behavior, conducting only holes. In the next step, we integrate a thin layer of p-type HgTe with a 6000 cm -1 band gap between the absorbing layer and the hole extraction layer made of MoO3.

Thanks to the unipolar barrier based on HgTe CQDs with a 6000 cm -1 band edge, we observe a clear increase of the photoresponse, especially in the low bias range, see Figure 5a. The internal quantum efficiency, (ηint %) = ΔIPh/(eφ), where ΔI Ph is the photocurrent at 0 V bias, e is the electronic charge and Φ is the photon flux (photon/seconds), is enhanced by ≈ 26-fold as compared to the photovoltaic device without p-type HgTe layer.

The noise of the device has been measured, see figure S10 and is clearly limited by 1/f contribution. 40- 42 The addition of the p-type barrier clearly impacts the detectivity, which is increased by an order of magnitude reaching value of 3•10 8 Jones at room temperature, see the inset of Figure 5a. To further motivate this study, we compare the progress in term of responsivity and detectivity between the first and second generations of diodes, see Figure 5b andc. We notice that the fast time response of the first generation has been preserved and show that the 3 dB cut-off frequency is above 10 kHz (actually measurement is setup-limited in this case), see Figure 5d. We provide in table S1 a comparison of the current device performances with other detectors reported in the SWIR range and based on colloidal nanocrystals. The performance of the diode is typically the highest reported for a HgTe CQD based photovoltaic device in the extended SWIR regime. By the time we submit our paper we became aware of even higher performance (D* of 8•10 9 Jones at room temperature) obtained in a photoconductive configuration by the group of Halpert's group, [START_REF] Cryer | 300 Nm Spectral Resolution in the Mid-Infrared with Robust, High Responsivity Flexible Colloidal Quantum Dot Devices at Room Temperature[END_REF] thanks to inorganic capping ligands.

Figure 5 a. Responsivity as a function of the applied bias for different thicknesses of the HgTe 6000 cm -

1 p-type layer used as electron blocking layer for a FTO/TiO2/HgTe (ambipolar -4000 cm -1 )/HgTe (p type -6000 cm -1 )/MoO3/Au photodiode. The inset reports the detectivity value of this diode, at 0 V and room temperature, for different thicknesses of the HgTe 6000 cm -1 p-type layer used as electron blocking layer. b. Responsivity as a function of the applied bias for ITO/TiO2/HgTe (ambipolar -4000 cm -1 )/Au and for FTO/TiO2/HgTe (ambipolar -4000 cm -1 )/HgTe (p-type -6000 cm -1 )/MoO3/Au photodiode. c. Detectivity as a function of the applied bias for ITO/TiO2/HgTe (ambipolar -4000 cm -1 )/Au and for FTO/TiO2/HgTe (ambipolar -4000 cm -1 )/HgTe (p-type -6000 cm -1 )/MoO3/Au photodiode. d Bode diagram for the device of the second generation giving the frequency dependence of the photocurrent. The inset is a time trace of the photocurrent as the light source (here a 1.55 µm laser diode) is turned on and off.

CONCLUSION

We develop a strategy to design an unipolar barrier to filter the electron dark current while letting the photo-holes flow in HgTe nanocrystals based photodiode. To do so, we develop a method to control the majority carrier in HgTe CQD films based on the choice of the quenching ligand during the synthesis. We also demonstrate that size strongly drives the nature of the majority carriers from holes for small particles to electrons for large HgTe CQDs. Combination of transport and photoemission measurements is used to demonstrate that a layer of HgTe CQDs with a band edge energy at 6000 cm - 1 actually fits with criteria for the design of unipolar barrier with almost no valence band offset but a >200 meV barrier in the conduction band. The addition of this layer is used to enhance the responsivity and detectivity of the device. The latter now reaches 3•10 8 Jones at room temperature with 3 dB cutoff frequency above 10 kHz.

METHODS

Nanocrystal synthesis 1 M TOP:Te precursor: 2.54 g of Te powder is mixed in 20 mL of TOP in a three neck flask. The flask is kept under vacuum at room temperature for 5 min and then the temperature is raised to 100 °C. Furthermore, degassing of flask is conducted for the next 20 min. The atmosphere is switched to Ar and the temperature is raised to 275 °C. The solution is stirred until a clear orange coloration is obtained. The flask is cooled down to room temperature and the color switches to yellow. Finally, this solution is transferred to an Ar filled glove box for storage.

HgTe CQD synthesis with band edge at 4000 cm -1 : In a 100 mL three-neck flask, 513 mg of HgCl2 and 60 mL of oleylamine are degased at 110 °C for 1 h. Under Ar at 80 °C, a solution containing 1.9 mL of 1 M TOP:Te and 10 mL of oleylamine is quickly injected after warming it up with heat gun. After 3 min, the reaction is quickly quenched by adding a mixture of toluene (9 mL) and dodecanethiol (1 mL), the flask is then cooled down using air flux. The nanocrystals are precipitated in ethanol and redispersed in toluene. This washing is repeated one more time. Finally, the HgTe CQDs dispersed in toluene are filtered through a 0.2 µm PTFE filter. The obtained solution is used for further characterizations and fabrication of devices. Synthesis of HgTe nanocrystals with other sizes are discussed in the supporting information.

ITO patterning: ITO substrates are cut into 15 × 15 mm size and thoroughly cleaned by sonication in acetone for 5 min, rinsed with acetone and isopropanol, then dried completely with dry N2 flow. AZ 5214E photoresist is spin-coated for 30 s and subsequently the substrates are baked at 110 °C for 90 s. At next stage, standard photolithography is performed using mask aligner for exposing the substrates to UV light for 5 s through a lithography mask (1 mm width). Photoresist is developed using AZ 726 developer for 40 s and immediately rinsed with de-ionized water. Thus, exposed ITO surface is completely etched out with 25 % HCl (in water) for 12 min and substrates are dipped immediately in de-ionized water. Then, lift-off is conducted in an acetone bath and patterned ITO substrates are cleaned with acetone and isopropanol. Finally, substrates are dried with dry N2 flow.

FTO patterning: FTO substrates are cut into 15 × 15 mm size and thoroughly cleaned by sonication in acetone bath for 5 min, rinsed with acetone and isopropyl alcohol and finally dried completely with dry N2 flow. Photoresist (AZ 5214E) is spin-coated for 30 s and subsequently the substrates are baked at 110 °C for 90 s. At next stage, photolithography is performed using mask aligner for exposing the substrates to UV light for 5 s through a lithography mask (1 mm width). AZ 726 developer is used for developing the pattern in 40 s and immediately rinsed with de-ionized water. Afterwards, Zn power is sprinkled on developed substrates and excess Zn powder is removed from substrates. This process is repeated twice. Thus, exposed FTO surface is completely etched out with 2 M HCl (2 mL of 37 % HCl in 10 mL of DI water) for 15 min and cotton knob is used to remove any remaining Zn powder from substrate surface. At next stage, substrates are dipped immediately in de-ionized water. Then, lift-off is carried out in an acetone bath and patterned FTO substrates are thoroughly cleaned with acetone and isopropyl alcohol. Finally, substrates are dried with dry N2 flow. TiO2 and ZnO film preparation: 200 µL of anatase TiO2 nanoparticle solution is spin-coated on above patterned ITO and FTO substrates at 5000 rpm for 30 s. The TiO2 film is annealed at 200 °C for 15 min and its thickness is measured to be 65 nm with Dektak profilometer. ZnO nanocrystal film is obtained by spin coating ZnO nanocrystals at 3000 rpm for 45 s and annealing the film at 250 °C for 30 min. This process is repeated one more time to build a 200 nm thick layer HgTe nanocrystal film preparation and EDT ligand exchange in air free conditions: Film preparation and ligand exchange processes are carried out inside a N2 filled glovebox. In a typical procedure, 80 µL of concentrated HgTe nanocrystals (25 mg/mL) from toluene is spin coated at 2000 rpm for 30 s on above fabricated Glass/ITO/TiO2 substrates. After complete evaporation of solvent, ligand exchange is carried out by dipping the film in 1 -2 wt % EDT solution in ethanol for 90 s and rinsing it in pure ethanol for 30 s. Afterwards, a quick annealing step at 50 °C, for 1 min is carried out. This procedure is repeated for 8 -9 times to get thicker (180 -200 nm) and pin-hole free HgTe nanocrystal film. Same procedure is followed for ITO/ZnO, FTO/TiO2 and FTO/ZnO. MoO3/Au electrode deposition in air free conditions: Fabricated Glass/ITO/TiO2/HgTe substrates are transferred from glove box to the thermal evaporator chamber under N2 environment. If MoO3 is included in the device, it is thermally evaporated through a shadow mask (1 mm width) from MoO3 powder at a rate of ≈ 1 -2 Å•s -1 . Then 80 nm of Au is thermally evaporated at a rate of 2 -3 Å•s -1 through the same shadow mask. The latter is aligned to get a pixel of 1 × 1 mm 2 area.

Protective layer of PMMA/PVA: Above fabricated devices are transferred from thermal evaporator to glovebox under N2 environment. PMMA (5 wt % in CHCl3) solution is spin-coated on substrates at 2000 rpm for 60 s, then a quick annealing step at 50 °C for 1 min is performed. In following steps, PVA (centrifuged solution at 10 wt % in water) is spin coated at 4000 rpm for 60 s. After this step, substrates are annealed for 1 min at 50 °C. Finally, device is kept in vacuum overnight for the complete drying of encapsulation layers. Thus, obtained thicknesses for these encapsulation films are found to be 1.3 µm and 0.5 µm for PMMA and PVA, respectively.

Responsivity measurements:

The sample is biased using a DC bias from Zurich instrument MFLI lockin amplifier. The light from a blackbody source at 927 °C is used as broadband infrared source. A Ge filter is used to cut the light at wavelength below 1.8 µm. This light is then chopped using a mechanical chopper. This chopper is used as reference frequency for the lock-in. Photocurrent signal is finally acquired on the lock-in while the chopping frequency is tuned. Figure 5d shows the frequency dependence of the photosignal showing a 3 dB cut-off frequency above 10k Hz. In this case the signal device is illuminated by a laser at 1.55 µm, which input is electrically modulated with a signal generator.

Noise measurements:

The sample is biased using the input of a Femto DLPCA 200 amplifier. The current through the sample is then amplified using the same current amplifier. The current spectral density is then acquired using a low frequency spectrum analyzer SR 780 from Stanford Research system.

ACKNOWLEDEGMENTS

EL thanks the support ERC starting grant blackQD (grant n° 756225). We acknowledge the use of cleanroom 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, AJ and JR thank Nexdot for post doc funding.

ASSOCIATED CONTENTS

Supporting information

The Supporting Information is available free of charge on the ACS Publications website at DOI: Nanocrystals synthesis for various size, material characterization, transport measurement; effect of halide, stoichiometry and size on doping, noise measurement and comparison with state of the art device in the SWIR range

Figure 3 a

 3 Figure 3 a. Infrared spectrum for dodecanethiol quenched HgTe CQDs with band edge at 4000 cm -1 . b. Transfer curve (drain current as a function of gate bias) for dodecanethiol quenched HgTe CQDs with band edge at 4000 cm -1 . c Infrared spectrum for oleic acid quenched HgTe CQDs with band edge at 4000 cm -1 .d. Transfer curve (drain current as a function of gate bias) for oleic acid quenched HgTe CQDswith band edge at 4000 cm -1 . Note that for both transistor measurements, the long ligands have been exchanged for ethanedithiol. All transistor curves are acquired at room temperature under a 400 mV bias.


Bertille Martinez

Rémi Plamont

Charlie Gréboval

Prachi Rastogi

Yoann Prado

Junling Qu

Audrey Chu

Clément Livache

Hervé Xiangzhen Zhen Xu

Charlie Greboval

Hervé Cruguel

Sandrine Ithurria

Mathieu G Silly

Nicolas Goubet

Emmanuel Lhuillier

Azobenzene as Light-Activable Carrier Density Switches in Nanocrystals

come

INTRODUCTION

Nanocrystals (NCs) appear as an important building block for solution-processed optoelectronic devices [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] such as light emitting diodes, [START_REF] Wood | Colloidal Quantum Dot Light-Emitting Devices[END_REF] solar cells [START_REF] Semonin | Peak External Photocurrent Quantum Efficiency Exceeding 100% via MEG in a Quantum Dot Solar Cell[END_REF] and infrared (IR) sensors. [START_REF] Mcdonald | Solution-Processed PbS Quantum Dot Infrared Photodetectors and Photovoltaics[END_REF][START_REF] Keuleyan | Mid-Infrared HgTe Colloidal Quantum Dot Photodetectors[END_REF] In this quest, the doping control becomes of utmost interest: it is critical for the design of p-n junctions, [START_REF] Chuang | Improved Performance and Stability in Quantum Dot Solar Cells through Band Alignment Engineering[END_REF] to reduce lasing threshold [START_REF] Christodoulou | Single-Exciton Gain and Stimulated Emission Across the Infrared Optical Telecom Band from Robust Heavily-Doped PbS Colloidal Quantum Dots[END_REF] or to induce intraband absorption in the mid infrared. [START_REF] Jagtap | Emergence of Intraband Transitions in Colloidal Nanocrystals[END_REF][START_REF] Wang | Electrochromic Nanocrystal Quantum Dots[END_REF] Doping of NCs can be obtained from various methods: [START_REF] Schimpf | Electronic Doping and Redox-Potential Tuning in Colloidal Semiconductor Nanocrystals[END_REF][START_REF] Erwin | Doping Semiconductor Nanocrystals[END_REF][START_REF] Rinehart | Photochemical Electronic Doping of Colloidal CdSe Nanocrystals[END_REF] introduction of extrinsic impurities [START_REF] Nag | A Study of Mn2+ Doping in CdS Nanocrystals[END_REF][START_REF] Knowles | Luminescent Colloidal Semiconductor Nanocrystals Containing Copper: Synthesis, Photophysics, and Applications[END_REF][START_REF] Sahu | Electronic Impurity Doping in CdSe Nanocrystals[END_REF] within the NCs or within the array of nanocrystals, non-stoichiometry of the material (Cu2-xS, [START_REF] Luther | Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots[END_REF] (Bi;Sb)2Te3, HgSe [START_REF] Deng | 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] ), metal functionalization, [START_REF] Mahler | Metallic Functionalization of CdSe 2D Nanoplatelets and Its Impact on Electronic Transport[END_REF][START_REF] Lee | Au-PbS Core-Shell Nanocrystals: Plasmonic Absorption Enhancement and Electrical Doping via Intra-Particle Charge Transfer[END_REF] functionalization by redox molecules [START_REF] Koh | Heavily Doped n-Type PbSe and PbS Nanocrystals Using Ground-State Charge Transfer from Cobaltocene[END_REF][START_REF] Martinez | Polyoxometalate as Control Agent for the Doping in HgSe Self-Doped Nanocrystals[END_REF] or surface dipole functionalization. [START_REF] Miller | Revisiting the Valence and Conduction Band Size Dependence of PbS Quantum Dot Thin Films[END_REF][START_REF] Kroupa | Tuning Colloidal Quantum Dot Band Edge Positions through Solution-Phase Surface Chemistry Modification[END_REF][START_REF] Robin | Surface Control of Doping in Self-Doped Nanocrystals[END_REF][START_REF] Brown | Energy Level Modification in Lead Sulfide Quantum Dot Thin Films through Ligand Exchange[END_REF][START_REF] Martinez | HgSe Self-Doped Nanocrystals as a Platform to Investigate the Effects of Vanishing Confinement[END_REF] In this latter case, the dipole moment associated with the grafted ligand on the nanocrystal surface induces a band shift (ΔE) according to the following equation ΔE = 𝜎𝜇 𝜀 0 𝜀 𝑟 with 𝜎, µ, 𝜀 0 and 𝜀 𝑟 being respectively the dipole surface density, the dipole magnitude, the vacuum permittivity and the dielectric constant.

Once under thin film form, all these techniques suffer from a lack of tunability. Alternatively, tuning carrier density can be achieved thanks to gating in a field effect transistor [START_REF] Gréboval | Field-Effect Transistor and Photo-Transistor of Narrow-Band-Gap Nanocrystal Arrays Using Ionic Glasses[END_REF][START_REF] Talapin | PbSe Nanocrystal Solids for N-and p-Channel Thin Film Field-Effect Transistors[END_REF] configuration or with an electrochemical cell. [START_REF] Wang | Electrochromic Nanocrystal Quantum Dots[END_REF] Optical pumping can also be used to fill the conduction band and so saturate the interband absorption. In this case the carrier density tuning is either short-lived when pulsed excitation is used, or requires high power continuous wave (CW) excitation, which may damage the nanocrystal array due to sample heating.

Here, we explore an alternative approach for the control of carrier density with light, which is reversible and compatible with low excitation power(< 100 mW.cm -2 ). To do so, we functionalize NCs with azobenzenes (AZBs), see Figure 1. [START_REF] Peng | Light Controlled Protein Release from a Supramolecular Hydrogel[END_REF][START_REF] Zhao | Reversible Trapping and Reaction Acceleration within Dynamically Self-Assembling Nanoflasks[END_REF] The reversible nitrogen-nitrogen double-bond configuration can switch from "cis" to "trans" and is triggered respectively by UV (cis to trans) and visible (trans to cis) irradiations, which makes AZBs promising molecular switches or tools to induce self-assembly by external stimuli. [START_REF] Credi | Artificial Molecular Motors Powered by Light[END_REF] During conformation change, the dipole associated to the molecule is dramatically affected and changes by ≈ 3 Debye, [START_REF] Hartley | The Dipole Moments of Cis-and Trans-Azobenzenes and of Some Related Compounds[END_REF] see Figure 2c and e. Thus, by using AZBs as a surfacecapping molecules for nanocrystals, we propose a method where the nanocrystals surface dipole is light reconfigurable. Compared to conventional optical excitation, this method can be operated under much lower excitation power and so reduces the material degradation. We bring evidence that it is possible to control the response of infrared nanomaterials while shining light (with low-power excitation) in the UV.

METHODS

Synthesis of thiol functionalized azobenzenes

Materials for thiol functionalized azobenzenes: All reactions are carried under N2. THF was purified and dried over Braun solvent purification system (MB-SPS-800), other solvents (CH2Cl2, heptane, methanol, ethyl acetate) were purchased dry from Sigma-Aldrich and used without further purification. The chemical reagents were purchased from Synthon, abcr, Sigma-Aldrich and used without further purifications. (1.58 g, 8 mmol, 1 eq.), 3-bromoprop-1ene (0.7 mL, 0.97 g, 8 mmol, 1 eq.) and potassium t-butoxide (1 g, 10.4 mmol, 1.3 eq.) were dissolved in 20 mL of dry THF. The reaction mixture was stirred at reflux for 16 hours after what it was cool down to room temperature. The solvent was evaporated under reduced pressure and the residue dissolved in 100 mL CH2Cl2. The organic phase was washed with distilled water (3 x 60 mL). The organic phase was dried over MgSO4 and the solvent was evaporated under reduced pressure. The product was purified by flash chromatography on silica with ethyl acetate / heptane (1:9) as eluent. The product was collected as orange solid, 1.525 g, yield 80%. Compound 2: 3-(4-phenyldiazenyl)phenoxy)propyl-ethanethioate : In a dried three neck round bottom flask, compound 1 (1 g, 4.2 mmol, 1 eq.) and AIBN (1.45 g, 8.8 mmol, 2.1 eq.) were dissolved in 20 mL of dry toluene. Thioacetic acid (0.63 mL, 0.67 g, 8.8 mmol, 2.1 eq.) was added dropwise to the mixture under stirring. The reaction mixture was stirred at reflux for 16 hours. The reaction was then cool down to room temperature and diluted with 60 mL of toluene. The organic phase was washed with saturated NaHCO3 solution in water (60 mL) and distilled water (2 x 60 mL). The organic phase was dried over MgSO4 and the solvent was evaporated under reduced pressure. The product was purified by flash chromatography on silica with ethyl acetate / heptane (1:9) as eluent. The product was collected as orange solid, 1 g, yield 76%. 1 H NMR (400 MHz, CDCl3) δ = 2.12 (m, 2H, C-CH2-C), 2.36 (s, 3H, -COCH3), 3.09 (t, 2H, CH2-S), 4.10 (t, 2H, O-CH2), 7.01 (d, 2H, Ar-H), 7.50 (m, 3H, Ar-H), 7.88 ppm (m, 4H, Ar-H).

Compound 3: 3-(4-(phenyldiazenyl)phenoxy)propane-1-thiol :

Compound 2 (0.5g, 1.6 mmol, 1 eq.) was dissolved in 40 mL of degassed MeOH under N2. 2 mL of HCl (37%) (14 eq.) were added dropwise and the reaction mixture was refluxed for 3 h. The reaction was then cool down to room temperature and the excess HCl was neutralized with a NaOH solution in water (1M). 100 mL of CH2Cl2 were added to the reaction mixture and the organic phase was washed with distilled water (2 x 100 mL). The organic phase was collected and dried over MgSO4 and the solvent was evaporated under reduced pressure. The product was purified by flash chromatography on silica with ethyl acetate / heptane (1:15) as eluent. The product was collected as orange solid, 0.260 g, yield 60%.

Nanocrystal syntheses

Chemicals: For nanocrystals, the following list of product have been used: mercury acetate (Hg(OAc)2, Sigma-Aldrich), mercury chloride (HgCl2, Sigma-Aldrich, 99%), tellurium powder (Te, Sigma-Aldrich, 99.99%), selenium powder (Se, Strem Chemicals), oleic acid (OA, Sigma-Aldrich, 90%), oleylamine (OLA, Sigma-Aldrich, 80-90%), trioctylphosphine (TOP, Sigma-Aldrich, 97%), n-hexane (VWR, 99%), ethanol absolute anhydrous (EtOH, Carlo Erba, 99.5%), methanol (MeOH, Carlo Erba, 99.8%), toluene (VWR, 99.3%), chloroform (Carlo Erba), lithium perchlorate (LiClO4, Sigma-Aldrich, 98%), polyethylene glycol (PEG 6k, Mw = 6.10 3 g.mol -1 , Fluka).

HgSe CQDs with azobenzene grafting: in a 50 mL three neck flask, 0.5 g of Hg(OAc)2, 10 mL of OA and 25 mL of OLA were degassed at 85 °C for 1 h. Under argon atmosphere at 90 °C, 1.6 mL of a 1 M solution of Se in TOP was quickly injected into the flask. After 4 min, the reaction was quenched by adding quickly a mix of 1 mL of CHCl3 and 45 mg of azobenzenes, and cooled down to room temperature using ice bath. The obtained dark solution was then opened to air, precipitated by addition of ethanol and methanol and centrifuged. The supernatant was discarded and the pellet was redispersed in chloroform. The cleaning procedure was repeated two more times. The average nanoparticle diameter in this fraction was 4.3 ± 0.7 nm, according to the measure by transmission electron microscopy (TEM), see Figure S2. The intraband absorption peak was measured at 3080 cm -1 .

HgTe CQDs with azobenzene grafting: 513 mg of HgCl2 was added to 60 mL of OLA in a 100 mL round flask. The solution was placed under vacuum and heated to 110 °C for 1 h. Then, the temperature was decreased to 80 °C and the solution was placed under Ar atmosphere. 1.9 mL of TOP:Te (1 M) with 10 mL of OLA were added to the mercury solution. The solution color immediately turns to dark brown and the reaction was stopped after 3 min. A solution made of 35 mg of azobenzenes and 10 mL of chloroform was quickly added to quench the reaction. The nanocrystals were then precipitated with ethanol. After centrifugation, the nanocrystals were redispersed in chloroform. The washing step was repeated one more time. The interband peak was measured to be at 4860 cm -1 .

Material characterization

Analytical thin layer chromatography (TLC) was carried out on Merck silica gel60 F254. Products were revealed by ultraviolet light (254 or 366 nm) and stained with dyeing reagents solutions as potassium permanganate solution. Flash chromatography was performed on Combiflash® Companion or with Merck silica gel 60 (230-400 mesh). 1 H and 13 C NMR spectra were recorded at ambient temperature on Bruker Ascend TM 400 spectrometer operating at 400 MHz 1 H. [START_REF] Rinehart | Photochemical Electronic Doping of Colloidal CdSe Nanocrystals[END_REF] C nucleus was observed with 1 H decoupling. Solvent residual signals were used as internal standard. Chemical shifts (δ) in ppm. The peaks patterns are indicated as the following format multiplicity (s: singlet; d: doublet; t: triplet; q: quartet; sept: septuplet; m: multiplet; dd: doublet of doublet; dt: doublet of triplet; dm: doublet of multiplet, etc.). The prefix br. indicates a broadened signal.

UV and visible spectroscopy: Spectra have been acquired using a JASCO V-730 spectrometer.

Infrared spectroscopy: FTIR (Fourier Transform Infrared Spectroscopy) spectra are acquired using a Fischer Nicolet iS50 in ATR configuration. The spectra are averaged over 32 acquisitions and have a 4 cm -1 resolution.

TEM:

For TEM imaging, the solution of nanocrystals is drop-cast on a copper TEM grid and dried overnight under vacuum to remove remaining solvent. The TEM images are then obtained using a JEOL 2010 microscope operated at 200 kV.

Transport measurements

Electrodes fabrication: The surface of a Si/SiO2 wafer (400 nm oxide layer) was cleaned by sonication in acetone. The wafer was rinsed with isopropanol and finally cleaned using an O2 plasma. AZ 5214E resist was spin-coated and baked at 110 °C for 90 s. The substrate was exposed under UV through a pattern mask for 2 s. The film was further baked at 125 °C for 2 min to invert the resist. Then a 40 s flood exposure was performed. The resist was developed using a bath of AZ 326 for 32 s, before being rinsed in pure water. A 5 nm chromium layer and 80 nm gold layer were deposed by using a thermal evaporator. The lift-off was performed by dipping the film in acetone for 1 h. The electrodes were finally rinsed using isopropanol and dried by an air flow. The electrodes were 2.5 mm long and spaced by 20 µm. These electrodes were used for DC measurements (IV curves and transistor measurements).

Film preparation: Films were prepared using spin-coating deposition onto electrodes. The solutions were diluted to 20 mg/mL and spin-coated onto substrates at a speed of 600 rpm, with an acceleration of 1000 rpm/s for 20 s. After drying, the film was dipped in an ethanol solution to remove any remaining free azobenzene ligand. This is a crucial step because free azobenzene can percolate and rotate into the film and so their dipole can be orientated in any direction, leading to less efficient surface gating.

Dependency of dark current with dipole:

The current of a film of HgX (X = Te, Se) nanocrystals grafted with AZB was measured with a Keithley 2634b, with an applied voltage of 0.5 to 1 V. Light (from LED flashlights in the UV at 365 nm (15 mW/cm²) and in the blue at 460 nm (100 mW/cm²) was shone on the film for 30 s to change azobenzene configuration and so, the surface dipole. Dark current after illumination was measured once the current value is stabilized.

Transistor measurements using electrolyte gating: For electrolyte gating, we first mix, in a glove box, 0.5 g of LiClO4, with 2.3 g of PEG (MW = 6 kg.mol -1 ). The vial is heated at 170 °C on a hot plate for 2 h until the solution gets clear. To use the electrolyte, the solution is warmed around 100 °C and brushed on the top of the HgTe film. The sample is then connected to a Keithley 2634b which sets the drain bias (VDS = 200-500 mV), controls the gate bias (VGS) with a step of 1 mV and measures the associated currents IDS and IGS.

Figure 2 a. TEM picture of HgTe NCs with AZB grafted at the surface. b. Absorption spectra of AZB grafted HgTe NCs after UV (λ=365 nm) and blue illumination (λ=460 nm). c. Scheme of HgTe NCs grafted with AZB in trans-configuration. d. Electronic spectrum of the HgTe NCs with pristine surface. e. Scheme of HgTe NCs with AZB in cis-configuration. f. Electronic spectrum of the HgTe NCs with cis AZB grafted.

DISCUSSION

As the band shift induced by surface dipole is not depending on the material band gap, we choose to work with narrow band gap materials which will present a larger band gap renormalization upon the change of AZB configuration. In other words, the effect of band shift in those nanocrystals should be a significant fraction of their bandgap energy. We use NCs of HgTe and HgSe according to established procedures given respectively in ref 35 and 19. These two materials have attracted a significant interest for the design of IR detectors [START_REF] Lhuillier | Recent Progresses in Mid Infrared Nanocrystal Based Optoelectronics[END_REF][START_REF] Cryer | 300 Nm Spectral Resolution in the Mid-Infrared with Robust, High Responsivity Flexible Colloidal Quantum Dot Devices at Room Temperature[END_REF][START_REF] Chen | Mercury Telluride Quantum Dot Based Phototransistor Enabling High-Sensitivity Room-Temperature Photodetection at 2000 Nm[END_REF][START_REF] Tang | Dual-Band Infrared Imaging Using Stacked Colloidal Quantum Dot Photodiodes[END_REF][START_REF] Livache | A Colloidal Quantum Dot Infrared Photodetector and Its Use for Intraband Detection[END_REF] with tunable absorption from short-wave infrared and up to THz. [START_REF] Goubet | Terahertz HgTe Nanocrystals: Beyond Confinement[END_REF] In such arrays of NCs, infrared detection is possible but the performance (dark current, spectral and time response) dramatically depends on the doping magnitude. It is thus of utmost interest to have an external knob to tune the carrier density.

Both materials have a zinc blende structure, according to their diffraction pattern (figure S1). HgTe presents a tetrapodic shape, see Figure 2a with 8-nm-long arm, while HgSe is more spherical (Figure S2) with a size around 4.7 nm. Using a combination of photoemission [START_REF] Sarma | X-Ray Photoelectron Spectroscopy: A Unique Tool To Determine the Internal Heterostructure of Nanoparticles[END_REF] and infrared spectroscopy (see figure S3 for procedure details) it is possible to determine the electronic spectrum of each material in absolute energy scale (i.e. vs vacuum). These two materials have been chosen because they present p (HgTe) and n (HgSe) nature. HgTe is quasi intrinsic, with a Fermi level in the lower part of the band gap, leading to small p-type nature [START_REF] Chu | HgTe Nanocrystals for SWIR Detection and Their Integration up to the Focal Plane Array[END_REF] , see Figure 2d, while HgSe is degenerately ndoped as discussed later in the text. [START_REF] Deng | 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] The goal of this paper is to demonstrate that the surface dipole resulting from the AZB can be tuned using light. To graft the AZBs on the mercury chalcogenides NCs, we take advantage of the strong affinity of surface mercury cations for sulfur. The AZB is first functionalized with a thiol, according to the procedure given in Figure 1 and Figure S4-6. The thiol-functionalized AZB is then used to quench the nanocrystal growth. The confirmation of the grafting is obtained by UV-visible (Figure 2b and S6) and infrared spectroscopies (Figure S7). In particular, we observe a strong blueshift of the UV absorption spectrum of the AZB grafted NCs after UV illumination (λ=365 nm; P<<W.cm -2 ) which can be associated to the cis conformation of the AZB. After illumination in the visible (λ=460 nm; P<<W.cm -2 ), this band redshifts, which is consistent with the switching to the trans conformation of the AZB. We expect that the lighttriggered switching of the ligand conformation increases the surface dipole which will result in a band shifting. As a result, the relative position of these bands with respect to the Fermi level is tuned leading to a change in the nanocrystal carrier density.

We start our study with p-type mercury telluride nanocrystals. Their transport properties are first probed in an electrolyte-gated field effect transistor (FET) configuration. [START_REF] Martinez | Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals[END_REF] A scheme of the device can be seen in Figure 3a. Ion gel gating approach has been quite successful to achieve gating on mercury chalcogenides nanocrystals. [START_REF] Izquierdo | Strongly Confined HgTe 2D Nanoplatelets as Narrow Near-Infrared Emitters[END_REF] In addition of air operability, this approach leads to large gate capacitance which can tune the carrier density up to several carriers per nanocrystal. [START_REF] Lhuillier | Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz[END_REF] HgTe nanocrystals present an ambipolar behavior, with a higher mobility for the holes than for electrons (Figure 3b), which is expected with the electronic spectrum proposed in Figure 2d. The mobility of the carrier is estimated to be 50 times smaller than for conventionally used short dithiol (ethanedithiol), see figure S8. Another striking difference between EDT and AZB capping is the fact than AZB present stronger hole conduction compared to electron conduction, while the opposite behavior is observed for EDT. This suggests than AZB favor the p-doping of the HgTe. [START_REF] Martinez | Probing Charge Carrier Dynamics to Unveil the Role of Surface Ligands in HgTe Narrow Band Gap Nanocrystals[END_REF] We then use transport measurement as a probe of the change of carrier density with AZB conformation. We follow charge transport as a function of time after pulses of UV and visible light to switch the conformation of the AZB, see Figure 3c. During the illumination, we observe a large increase of the current. This current modulation is due to photocharge generation within the HgTe semiconductor. Then as the sample returns to dark condition, we first see a transient current. The latter is not due to transient photocurrent since the response of HgTe is fast [START_REF] Livache | Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals[END_REF] (>MHz and see figure S9). This regime actually reflects that AZB conformation thermally comes back to trans. Finally, the dark current stabilizes at a value higher than before UV exposure. This modulation of the current is the signature of the change of AZB dipole on the NC surface. After a pulse of visible light, the current in the dark recovers its initial value. The AZB change of conformation is fairly reversible over the first cycles, as shown in Figure 3d and S10. For sake of comparison, we have also checked that the sample without AZB grafted, does not lead to a persistent modulation of the dark current after the illumination, see Figure S9. Assuming a value of µ = 2.5x10 -5 cm 2 V -1 s -1 (see Supplementary information) for the carrier mobility, we can estimate the change of carrier density ∆𝑛 within the nanocrystal ∆𝑛 = ∆𝑗 𝑒µ𝐹 , with ∆𝑗 the change of current density, e the elementary charge and F the electric field. The initial hole density in this p-type material is 1.8x10 -4 thermally activated holes per nanocrystal and it increases after the light activation of the surface dipole to 2.6x10 -4 . Because the hole carrier density is far below 1 per nanocrystal, we can assume that only the 1Sh state is filled. We can thus estimate the relative position of this level with respect to the Fermi level (EF) 𝑝 = 2𝑓 𝐹𝐷 (1𝑆 ℎ -𝐸 𝐹 ) with fFD the Fermi Dirac distribution. Thus, the observed change of current is the result of a 7 meV shift of the valence band toward the Fermi level. Knowing this shift, we can determine the surface density of AZB on the HgTe. We found typically ≈3 AZBs per nanocrystal. This rather low value can be explained by the tetrapodic shape of the HgTe nanoparticle and the bulkiness of the AZB, which conformation change also limits the grafting density. To further enhance the grafting density of AZB on the nanocrystals surface, we then switch to HgSe NCs which spherical shape is more favorable. [START_REF] Grigel | The Surface Chemistry of Colloidal HgSe Nanocrystals, toward Stoichiometric Quantum Dots by Design[END_REF][START_REF] Nag | Inorganic Surface Ligands for Colloidal Nanomaterials[END_REF] The other striking interest to use HgSe is to have a doping level very different from HgTe. HgSe is indeed an example of degenerately n-doped material. The size of the NC has been chosen to have a full filled 1Se state, see the electronic spectrum in Figure 4a and the transistor transfer curve in Figure 4b which confirm that the material only presents a n-type character. In p-doped material (i.e. HgTe), the dipole-related shift of the band was increasing the hole density. Because the shift direction is the same for HgSe, it should induce a drop in the electron carrier density.

It is worth noting that the degenerate doping makes that, under the low excitation power used here, the photocurrent signal is negligible compared to the dark current. As a result, the effect of the light on HgSe sample is mostly the result of the dark current modulation. This contrasts with HgTe where photocurrent modulation was large due to a lower carrier density. As we follow the current in the sample as a function of time, we indeed observe a significant decrease of the current in the steady state after UV illumination (Figure 4c). After illumination by blue visible light (return to the trans configuration), the current increases again, even if the reversibility is not as good. With the same procedure as before, we estimate the change of carrier density from 1.94 electrons per dot down to 1.72 electrons in the AZB cis configuration. This corresponds to a 40 meV shift for the relative energy of the 1Se state with respect to the Fermi level and to approximately 8 AZBs per HgSe NC.

CONCLUSION

In conclusion, we have demonstrated the remote control of the carrier density thanks to mechanized nanocrystals. The emergence of the functionality at the macroscopic level comes from the grafting of thiol functionalized Azobenzene at the surface of HgTe/HgSe nanoparticles. This molecular machine has been selected for its dipole moment shift resulting from the reversible conformation switches upon irradiation. The AZB conformation switches towards the cis conformation and leads to an increase of the hole density for p-type materials and a reduction of the electron density for n-type materials. This method allows the tuning of carrier density using low irradiance (< 100 mW.cm -2 ) which does not damage the sample through heating. Further work will have to focus on the increase of the AZB surface density of the NC surface and on boosting reversibility.

Figure 1 :

 1 Figure 1: Synthetic route to thiol functionalized azobenzene.

1 H

 1 NMR (400 MHz, CDCl3) δ = 4.62 (t, 2H, O-CH2), 5.39 (m, 2H, =CH2), 6.08 (m, 1H, =CH-), 7.04 (d, 2H, Ar-H), 7.50 (m, 3H, Ar-H), 7.91 ppm (m, 4H, Ar-H).

1 H

 1 NMR (400 MHz, CDCl3) 𝛿 = 1.42 (t, 1H, -SH), 2.12 (m, 2H, C-CH2-C), 2.76 (m, 2H, S-CH2), 4.16 (t, 2H, O-CH2), 7.02 (d, 2H, Ar-H), 7.50 (m, 3H, Ar-H), 7.92 ppm (m, 4H, Ar-H), see FigureS4

  .

13 C

 13 NMR (100 MHz, CDCl3) 𝛿 = 21.30 (CH2-S), 33.34 (C-CH2-C), 66.06 (CH2-O), 114.81 (CAr), 122.68 (CAr), 124.87 (CAr), 129.14 (CAr), 130.49 (CAr), 147.15 (CAr-N), 152.85 (CAr-N), 161.41 ppm (CAr-O), see Figure S5.

Figure 3 a

 3 Figure 3 a. Scheme of an electrolyte gated field effect transistor (FET) which channel is made of a thin film of HgTe. b. Transfer curve (drain-source current as a function of gate bias) for the FET based on AZB functionalized HgTe channel c. Drain source current of an HgTe capped with azobenzene film as a function of time. UV (λ=365 nm) or blue light (λ=460 nm) is shone on the film during the intervals highlighted with purple and blue rectangles respectively. ΔIUV is the change of current as the AZB switches from trans to cis, while ΔIvis is the change of current as the AZB switches from cis to trans d. Current modulation (as shown on Figure 3c) induced by UV and visible pulse of light as a function of pulse event.

Figure 4 a

 4 Figure 4 a. Electronic spectrum of the HgSe with pristine surface. b. Transfer curve (current as a function of gate bias) for the FET based on HgSe channel. c. Drain source current of an HgTe capped with azobenzene film as a function of time. UV (λ=365 nm) or blue light (λ=460 nm) is shone on the film during the intervals highlighted with purple and blue bars respectively.

ACKNOWLEDEGMENTS

EL thanks the support ERC starting grant blackQD (grant n° 756225), while SI thanks the support of ERC starting grant Ne2Dem. 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-Nano, Copin, Graskop, Frontal. JQ thanks Chinese Scholar council for PhD grant.


Dylan Amelot

Prachi Rastogi

Bertille Martinez

Charlie Gréboval

Clément Livache

Francesco Andrea Bresciani

Junling Qu

Audrey Chu

Mayank Goyal

Sang-Soo Chee

Charlie Greboval

Nicolas Casaretto

Zhen Xiang

Xu

Christophe Méthivier

Hervé Cruguel

Abdelkarim Ouerghi

Angshuman Nag

Mathieu G Silly

Nadine Witkovski

Emmanuel Lhuillier

Revealing the Band Structure of FAPI Quantum Dot Film and Its Interfaces with Electron and Hole Transport Layer Using Time Resolved Photoemission

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

INTRODUCTION

Lead halide perovskite has emerged as a promising candidate in the field of nanocrystals. [START_REF] Kovalenko | Properties and Potential Optoelectronic Applications of Lead Halide Perovskite Nanocrystals[END_REF][START_REF] Shamsi | Metal Halide Perovskite Nanocrystals: Synthesis, Post-Synthesis Modifications, and Their Optical Properties[END_REF] It becomes possible to grow bright nanocrystals [START_REF] Swarnkar | Colloidal CsPbBr3 Perovskite Nanocrystals: Luminescence beyond Traditional Quantum Dots[END_REF] without using core/shell structure, making traditional II-VI heterostructures such as CdSe/ZnS almost obsolete. [START_REF] Protesescu | Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut[END_REF] While for perovskite thin film the solar cell has been the main targeted application [START_REF] Hodes | Perovskite-Based Solar Cells[END_REF] , under nanocrystal form, this is for lightning that the lead halide perovskite appears as the most promising [START_REF] Yan | LEDs Using Halide Perovskite Nanocrystal Emitters[END_REF] material. There, the defect tolerance [START_REF] Steirer | Defect Tolerance in Methylammonium Lead Triiodide Perovskite[END_REF] is used as a path to avoid surface induced trap states.

It is only later that perovskite nanocrystals have also been considered as a viable material for solar cell design. Since their synthesis was well developed for two years, CsPbI3 nanocrystals already have shown superior performances than those of PbS quantum dot solar cell. [START_REF] Sanehira | Enhanced Mobility CsPbI3 Quantum Dot Arrays for Record-Efficiency, High-Voltage Photovoltaic Cells[END_REF][START_REF] Mir | Strategy to Overcome Recombination Limited Photocurrent Generation in CsPbX3 Nanocrystal Arrays[END_REF][START_REF] Swarnkar | Quantum Dot-Induced Phase Stabilization of α-CsPbI3 Perovskite for High-Efficiency Photovoltaics[END_REF] In spite of these tremendous improvements, perovskites nanocrystals still suffer from two main limitations which are (i) their stability (to air, light, moisture,…) and (ii) their unoptimized band gap energy. CsPbI3 is certainly the most integrated material for the design of lead halide perovskite nanocrystal based solar cells, [START_REF] Ling | 1% CsPbI3 Perovskite Quantum Dot Solar Cells via Cesium Cation Passivation[END_REF] however, its black phase (also called α phase) has a band gap around 700 nm, which renders unabsorbed a significant part of the solar spectrum. Secondly, this phase is easily transformed to the yellow phase (δ phase) which has an even larger band gap. This latter issue has been partly solved by the introduction of a better-suited ligand exchange procedure. However, the band gap remains still 300 meV too large compared with the optimal band gap of a single junction solar cell.

Narrower band gap materials will be of utmost interest. Currently, Cs/FASnI3 is the narrowest band gap material reported (≈1000 nm) for halide perovskite [START_REF] Jellicoe | Synthesis and Optical Properties of Lead-Free Cesium Tin Halide Perovskite Nanocrystals[END_REF][START_REF] Fang | Long-Lived Hot-Carrier Light Emission and Large Blue Shift in Formamidinium Tin Triiodide Perovskites[END_REF] , but the stability issue of tin based material appears to be even more dramatic than for lead based perovskite. Formamidinium (CH5N2 + ) lead iodide [START_REF] Fang | Exciton Recombination in Formamidinium Lead Triiodide: Nanocrystals versus Thin Films[END_REF] (FAPI) is a hybrid material consisting organic and inorganic ions. It is currently one of the best trade-off to combine a reasonably small band gap (i.e. reasonably matching ideal gap to absorb solar light) with stability. [START_REF] Zhao | High Efficiency Perovskite Quantum Dot Solar Cells with Charge Separating Heterostructure[END_REF] The fine electronic structure of FAPI nanocrystals [START_REF] Fu | Unraveling Exciton-Phonon Coupling in Individual FAPbI3 Nanocrystals Emitting near-Infrared Single Photons[END_REF] at the few meV scale, has been controversial during the past years because the exact nature of the ground state (triplet vs singlet) is unclear. [START_REF] Ramade | Fine Structure of Excitons and Electron-Hole Exchange Energy in Polymorphic CsPbBr3 Single Nanocrystals[END_REF][START_REF] Canneson | Negatively Charged and Dark Excitons in CsPbBr3 Perovskite Nanocrystals Revealed by High Magnetic Fields[END_REF][START_REF] Sercel | Exciton Fine Structure in Perovskite Nanocrystals[END_REF][START_REF] Becker | Bright Triplet Excitons in Caesium Lead Halide Perovskites[END_REF][START_REF] Tamarat | The Ground Exciton State of Formamidinium Lead Bromide Perovskite Nanocrystals Is a Singlet Dark State[END_REF] On the other hand, at the eV scale, the electronic structure which really drives the solar cell performances needs to be better revealed. In particular, the nature of the majority carrier (electron vs hole) still remains an open question. This makes that the design of solar cell based on FAPI mostly follows empirical approaches.

In addition, the performance of devices based on lead halide perovskite is strongly driven by the design of interfaces. [START_REF] Sanehira | Influence of Electrode Interfaces on the Stability of Perovskite Solar Cells: Reduced Degradation Using MoOx/Al for Hole Collection[END_REF][START_REF] Schulz | Halide Perovskites: Is It All about the Interfaces?[END_REF][START_REF] Hellmann | The Difference in Electronic Structure of MAPI and MASI Perovskites and Its Effect on the Interface Alignment to the HTMs Spiro-MeOTAD and CuI[END_REF] . Here we combine photoemission and (photo)transport measurement to demonstrate that the Fermi level of this material is located deeply within the band gap and that trap plays a limited role on charge recombination.

To further reveal the nature of the electronic states of FAPI nanocrystal thin film in interaction with the surrounding charge transport layers, we use a non-contact surface photovoltage measurement. This measurement is based on pump-probe photoemission to reveal in situ band alignment and charge transfer at the interface between FAPI layer and electron/hole transport layers. We reveal that hole is the majority carrier in FAPI nanocrystals and we present its band profile and interfaces under dark condition as well as under illumination.

METHODS

Chemicals: PbI2 (Alfa Aesar, 98.5%), formamidinium acetate (Fa(OAC)2, Alfa Aesar, 99%), oleylamine (OLA, Sigma-Aldrich, 80-90%), oleic acid (OA, Sigma-Aldrich), n-hexane (VWR, 99%), n-heptane (Merck, >99%), ethanol absolute anhydrous (EtOH, Carlo Erba, 99.5%), methanol (MeOH, Carlo Erba, 99.8%), toluene (VWR, 99.3%), chloroform (Carlo Erba), octadecene (ODE, Acros Organics, 90%) lead acetate (Pb(OAc)2.2Pb(OH), Alfa Aesar), ethyl acetate (Et(OAc)2, Carlo Erba), and titanium dioxide nanoparticles (TiO2, SOLARONIX Ti Nanoxide BL/SC) Preparation of stock solution of formamidinium oleate: In a 50 mL three neck flask, 392 mg of formamidinium acetate with 18 mL of ODE and 12 mL of OA are mixed. The flask is degassed under vacuum at room temperature. Once vacuum below 1 mbar is reached, the flask is heated at 100 °C. After the salt is fully dissolved, the flask is cooled down and further degassed for 15 min.

FAPI nanocrystal synthesis:

In a 100 mL three-neck flask, 240 mg of PbI2 are stirred in 18 mL of ODE. The flask is degassed at 110 °C under vacuum. After degassing for 20 min, 2 mL of OLA are added. Once vacuum has recovered, 1 mL of OA is added. At this point the lead salt is fully dissolved and the solution turns clear yellow. The atmosphere is switched to Ar and the temperature set at 80 °C, 20 mL of the formamidinium oleate are quickly injected. The solution turns dark red with a bright red luminescence. After 15 s the heating mantle is removed and a water bath is used to cool the flask. The solution is centrifuged at 6000 rpm. The supernatant is discarded and then the formed pellet is redispersed in hexane. Ethyl acetate is added to precipitate the particle a second time. After centrifugation the pellet is dispersed in fresh hexane.

Ligand exchange on FAPI film: Pb(OAc)2 ligand exchange was performed on FAPI nanocrystals. A thin layer (around 20-30 nm) of the nanocrystal solution is deposited on a substrate by spin coating. Meanwhile a saturated solution of Pb(OAc)2 in ethyl acetate is prepared. The solution is centrifuged and only the supernatant is saved. Then the film of FAPI nanocrystal is dipped in the previous solution for 30 s, and then rinsed in pure ethyl acetate for 30 s and subsequently annealed at 100 °C for 2 min. To build thicker films of nanocrystals, this operation can be repeated several times.

MoO3 deposition:

MoOx layer was deposited by thermal evaporation from MoO3 powder. The samples were fixed on evaporation sample holder and introduced in thermal evaporator. Once a vacuum level of 5.10 -6 mbar is reached, MoOx is evaporated on the samples at a rate of 2-3 Å•s -1 monitored with in situ quartz crystal microbalances.

TiOx deposition: we use a commercial TiO2 nanocrystal solution. First, this solution was diluted by a factor of 30 with absolute ethanol. Then, the diluted solution was deposited on samples by spin coating, using the following parameters: 5000 rpm (speed)/1000 rpm s -1 (acceleration)/30 s (duration). The thickness of such a layer is determined to be less than 15 nm with profilometry.

Infrared absorption: for infrared spectroscopy, we use a Fischer Nicolet iS50 FTIR in either Attenuated Total Reflection (ATR) or in transmission configuration. The spectra are averaged over 32 acquisitions and have a 4 cm -1 resolution.

UV-visible absorption:

for UV-visible spectroscopy, we use a JASCO V-730 spectrometer.

X-ray photoemission measurements (XPS):

For photoemission spectroscopy, we use the Tempo beamline of synchrotron Soleil. Some additional core level analysis made on laboratory photoemission instrument are also provided in the supporting information. In this case an Al source is used. Films of nanocrystals are spincasted on a gold coated Si substrate (gold layer is 80 nm thick). The ligand of the nanocrytals are exchanged using the previously described procedures. [START_REF] Mir | Strategy to Overcome Recombination Limited Photocurrent Generation in CsPbX3 Nanocrystal Arrays[END_REF][START_REF] Swarnkar | Quantum Dot-Induced Phase Stabilization of α-CsPbI3 Perovskite for High-Efficiency Photovoltaics[END_REF] Samples are introduced in the preparation chamber and degassed until vacuum below 10 -9 mbar is reached. Then samples are introduced in the analysis chamber. Photons in 60 eV to 1000 eV energy range are used. The signal is acquired onto a Scienta Ses2002 electron analyzer equipped with a delay line detector developed by Elettra [START_REF] Bergeard | Time-Resolved Photoelectron Spectroscopy Using Synchrotron Radiation Time Structure[END_REF] . Acquisition is done at constant pass energy within the detector. Core levels are acquired with a 100 eV pass energy, while time resolved measurements are conducted with a 200 eV pass energy. A gold substrate is used to calibrate the Fermi energy. The absolute value of the incoming photon energy is determined by measuring the second harmonic of Pb core level peaks. Low binding energy part of the photoemission spectrum is used to determine the value of the valence band maximum with respect to the Fermi level, VB-EF. Then the energy of the conduction band is determined from the valence band offset of the absorption gap. Finally, the work function is determined by measuring the cut-off of secondary electrons.

DISCUSSION

We start by synthetizing FAPI nanocrystals (NCs) with previously reported method. [START_REF] Protesescu | Monodisperse Formamidinium Lead Bromide Nanocrystals with Bright and Stable Green Photoluminescence[END_REF] The obtained NCs have a cubic shape (see transmission electron microscopy image in Figure 1b) and cubic lattice according to Xray diffraction (Figure S1). [START_REF] Protesescu | Dismantling the "Red Wall" of Colloidal Perovskites: Highly Luminescent Formamidinium and Formamidinium-Cesium Lead Iodide Nanocrystals[END_REF] The band edge energy, determined from the photoluminescence spectrum is found to be 1.65 eV, see Figure 1a. The absorption spectrum is barely structured, which suggests a lack of quantum confinement in this material. The latter observation is consistent with the size of the FAPI nanoparticle (>10 nm) which is larger than the material Bohr radius (5.5 nm). 28 We then have investigated the transport properties of thin film of FAPI NCs. To render them conductive, we use the same procedure as the one developed for CsPbI3, [START_REF] Sanehira | Enhanced Mobility CsPbI3 Quantum Dot Arrays for Record-Efficiency, High-Voltage Photovoltaic Cells[END_REF] where the initial long capping ligands (oleylamine) are stripped by dipping the film in ethyl acetate. Identical procedure is also used to prepare the film for photoemission to avoid film charging. Under illumination by a light source with photon energy larger than the band gap, we observe an increase of the conductance by two orders of magnitude, see Figure 1c. The power dependence of the current with the incident light can be reasonably fitted by a power law, of which exponent is close to ½, see Figure 1d. Such dependence of the photocurrent on the incident photon flux suggests that photo-carrier generation is limited by bimolecular process (i.e. band to band recombination) rather than by trapping (monomolecular process). [START_REF] Willis | Controlling Nanogap Quantum Dot Photoconductivity through Optoelectronic Trap Manipulation[END_REF] This is consistent with previous results for transport in lead halide perovskite NC film. [START_REF] Mir | Strategy to Overcome Recombination Limited Photocurrent Generation in CsPbX3 Nanocrystal Arrays[END_REF] The temperature dependence on the current is given in Figure 1e. The activation energy around room temperature can be obtained from an Arrhenius fit of the current, a value of 295 meV has been extracted from our measurements. In such array of NC, the transport is driven by hopping and the activation energy is mostly the result of the thermal activation of the carrier density. From this activation energy, we can conclude that the Fermi level is at least 300 meV away from any band which suggests that the Fermi level is deeply located within the band gap. The absolute energy levels of the FAPI sample are then revealed using photoemission made at Tempo beamline of Synchrotron Soleil. The sample presents contributions of C, N, Pb and I from the FAPI and a limited O contamination, see the survey spectrum in Figure 2a. The analysis of core levels is shown in Figure 2b-e. The I 4d state appears with a single contribution at a binding energy of 49.36 eV (the full width at half maximum (FWHM) is 0.79 eV, while the spin-orbit coupling (SO) is 1.7 eV). The Pb 4f state reveals two contributions appearing at 136.91 eV and 138.44 eV (FWHM = 0.73 eV, SO = 4.86 eV), which are attributed to Pb 0 and Pb 2+ , respectively. The C 1s contribution appears to have at least three contributions (BE = 284.46 eV; 285.27 eV and 286.35 eV, FWHM = 1.13 eV) which are mixtures with carbon resulting from air exposure and the one from formamidinium and ligands. The N 1s state is made of two components appearing at binding energy of 398.51 eV and 400.72 eV (FWHM = 1.74 eV). These two contributions are associated with the nitrogen from the oleylamine [START_REF] Dufour | Halide Ligands To Release Strain in Cadmium Chalcogenide Nanoplatelets and Achieve High Brightness[END_REF] used as a capping ligand and the one from the formamidinium. [START_REF] Shallcross | Determining Band-Edge Energies and Morphology-Dependent Stability of Formamidinium Lead Perovskite Films Using Spectroelectrochemistry and Photoelectron Spectroscopy[END_REF] The analysis of the cut-off of the secondary electron reveals a work function of 4.05 eV, see Figure S3. Note that it is very close to the value found for CsPbI3 which is 4.1 ± 0.05 eV. [START_REF] Mir | Strategy to Overcome Recombination Limited Photocurrent Generation in CsPbX3 Nanocrystal Arrays[END_REF][START_REF] Zhang | Bright Perovskite Nanocrystal Films for Efficient Light-Emitting Devices[END_REF] The vicinity of these two values relates to the fact that in lead halide perovskite material, the states close to the Fermi level mostly result from lead orbitals, which give rise to similar values between these two compounds. [START_REF] Lee | Comprehensive Understanding and Controlling the Defect Structures: An Effective Approach for Organic-Inorganic Hybrid Perovskite-Based Solar-Cell Application[END_REF] The analysis of the valence band is generally used to determine the exact position of the Fermi level within the band gap. The Fermi level in the lower half of the band gap is associated with a p-type nature (i.e. hole as majority carrier), while a Fermi level in the upper part of the band gap is associated with a n-type semiconductor with electron as majority carrier. As expected, from the transport measurement, the photoemission signal reveals a Fermi level very close to the middle of the gap, see Figure 2f and S4. However, depending on the photon energy used to probe the valence band, the Fermi level can be determined just below or above the middle of the gap. In other words, the limited accuracy of the measurement and method to extract valence band maximum (≈50 to 100 meV), prevents a clear determination of the majority carrier. In addition, the exact determination of the valence energy level becomes even more difficult once the FAPI material is coupled to metal oxide charge transport layers. Indeed, the metal oxide being defective, oxygen vacancies leads to low binding energy states (see Figure S5) which hide the semiconductor valence level. [START_REF] Greiner | Transition Metal Oxide Work Functions: The Influence of Cation Oxidation State and Oxygen Vacancies[END_REF][START_REF] Greiner | Thin-Film Metal Oxides in Organic Semiconductor Devices: Their Electronic Structures, Work Functions and Interfaces[END_REF] Thus, alternative methods to standard photoemission are required to obtain a clear determination of the energy level profile.

To extract the clear band profile of the FAPI nanocrystals film, we conducted pump-probe photoemission measurement, of which working principle is explained in Figure 3a. A pulsed laser (640 nm) is used to generate an exciton in the semiconductor because of the photon energy being larger than the band gap of FAPI. Then, the signal from a core level (Pb 4f) using photon at 600 eV are probed. The photoemission spectrum is then collected every 10 ns. Each spectrum is fitted with two gaussians and we then follow the shift of the maximum of each gaussian as a function of time. This approach leads to higher resolution compared with static photoemission.

The signal obtained from pristine FAPI nanocrystals film is given in Figure 3b. Under illumination, we observe a decrease of 25 meV in the binding energy of the Pb 4f state. Such a decrease of the binding energy is the signature of a downward band bending as expected for a p-type semiconductor [START_REF] Spencer | Time-Resolved Surface Photovoltage Measurements at n-Type Photovoltaic Surfaces: Si(111) and ZnO(101 ̅ 0)[END_REF][START_REF] Bergeard | Time-Resolved Photoelectron Spectroscopy Using Synchrotron Radiation Time Structure[END_REF][START_REF] Spencer | Chemically-Specific Time-Resolved Surface Photovoltage Spectroscopy: Carrier Dynamics at the Interface of Quantum Dots Attached to a Metal Oxide[END_REF][START_REF] Livache | Charge Dynamics and Optolectronic Properties in HgTe Colloidal Quantum Wells[END_REF] . The turn-on time, see Figure 3c, is associated with the displacement of the minority carrier toward the surface, which tends to reduce the surface band bending. The duration of the electron displacement to reach a new equilibrium condition of the interface band alignment lasts for 150 µs. Once the light is turned off, the hole then moves from the bulk and recombines with the electron accumulated at the surface under illumination. As the number of electrons at the interface will be reduced, the quasi-flat band will bend downward and finally restore the ptype interface, see Figure 3d. The decay time of the binding energy is found to be 1.4 ms and relates to the transport duration of the majority carrier to the surface. It is worth to compare the obtained majority lifetime with the one determined for PbS quantum dot which was up to recently one of the most studied material for quantum dot-based solar cell. For PbS NCs, the turn-on and-off time have been determined to be around 63 and 210 µs, respectively. [START_REF] Spencer | Charge Dynamics at Heterojunctions for PbS/ZnO Colloidal Quantum Dot Solar Cells Probed with Time-Resolved Surface Photovoltage Spectroscopy[END_REF] The longer living time obtained in FAPI suggests that there is less recombination path for carrier in FAPI than in PbS. This result is consistent with a defect tolerant character of electronic structure as expected for lead halide perovskite material.

Figure 3 a. Scheme of the pump-probe photoemission experiment. A laser diode at 640 nm is used to excite the FAPI nanocrystals film, while the probe is the synchrotron beam at 600 eV. Photoemission of the Pb 4f state is then followed as a function of time. b. Energy of the Pb 4f state of FAPI nanocrystals film as a function of time, as the laser light at 640 nm is turned on and off. c. Scheme of the band bending occuring in a p-type semiconductor film under illumination and d. after the illumination is turned-off.

We then investigate the interfaces of FAPI with electron and hole transport layers, see Figure S6 for XPS overview spectra. We have chosen TiOx and MoOx electron and hole transport layers, respectively. A conventional diode structure [START_REF] Swarnkar | Quantum Dot-Induced Phase Stabilization of α-CsPbI3 Perovskite for High-Efficiency Photovoltaics[END_REF][START_REF] Hellmann | The Difference in Electronic Structure of MAPI and MASI Perovskites and Its Effect on the Interface Alignment to the HTMs Spiro-MeOTAD and CuI[END_REF][START_REF] Akkerman | Strongly Emissive Perovskite Nanocrystal Inks for High-Voltage Solar Cells[END_REF] for lead halide perovskite generally includes an additional layer of spiro-OMeTAD as hole transport layer in the vicinity of the perovskite. However, because of its continuous character, the presence of such layer would have prevented the extraction of the photoelectrons. We consequently remove this layer and explore the direct effect of hole extraction layer on MoOx. The thin layer of TiOx is obtained by spin coating, being formed by commercial TiOx precursor for the FAPI film, while the MoOx layer is thermally evaporated. The atomic force microscopy represents that the FAPI film remains fairly smooth, see Figure S2. The analysis of the photoemission peak for Ti 2p and Mo 3d, are shown in Figure S7 and S8. It reveals that both materials are slightly non stoichiometric. Ti shows two oxidation states +III and +IV state, whereas Mo exhibits three contributions from three oxidation states consisting of +IV, +V and +VI. In general, perfect stoichiometry for oxides will have required a high temperature (T > 300 °C) annealing step under oxygen, which would damage irreversibly the FAPI nanocrystals.

Regarding the FAPI in contact with the electron transport layer (i.e. TiOx), the time-resolved photoemission reveals a slight downward band bending (Figure 4c) at the interface evidenced by the increased Pb 4f binding energy of about 50 meV, see Figure 4a, facilitating electron extraction. Standard photoemission, Figure S7, reveals that Ti get oxidized in the presence of FAPI nanocrystals as shown by the disappearance of the +III contribution in Ti 2p components. This effect can be interpreted as an electron transfer from the TiOx to the FAPI layer. Reverse effect is observed when FAPI is in contact with the hole transport layer (MoOx). Under illumination, time-resolved photoemission presents an inverted shift (Figure 4b), which suggests a slight upward band bending at the interface between FAPI and MoOx, see Figure 4d. Moreover, the analysis of the Mo 3d in Figure S8, clearly shows a change in the relative weight of the different contribution of the redox state of Mo. The +V state is more favored than the + VI state, which is a signature of the MoOx reduction suggesting an electron transfer from the FAPI to the MoOx layer as already observed in the case of bulk methylammonium lead iodide. [START_REF] Schulz | High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid Organic-Inorganic Perovskite Solar Cells[END_REF] It is worth pointing that the recovery time of the dark band bending condition is typically four times longer at the MoOx interface than for the TiOx interface (380 µs for MoOx and 1.7 ms for TiOx). We can also notice that the magnitude of the light induced shift is around 50 meV for both interfaces, this is much smaller than the shift induced by the interface. From our experimental findings, we propose a full description of FAPI NCs in contact with hole and electron transport layer, in the dark and under illumination. In Figure 4c andd, FAPI NCs adopt a n-type character at the TiOx interface whereas a p-type character is given by the MoOx interface.

CONCLUSION

To summarize, FAPI appears an interesting platform for the design of NC based solar cell because of its higher stability compared to CsPbI3 and its slightly narrower band gap. Conductive film of the material can be obtained using similar method as developed for the black phase of CsPbI3 (i.e. by stripping ligand in ethyl acetate). In addition to photoconduction, transport measurement reveals a limited role of the trap in the charge recombination and the Fermi level deep in the band gap. The latter prediction is confirmed using Xray photoemission spectroscopy, which reveals the Fermi level is closely located to the middle of the band gap. Pump-probe time resolved photoemission reveals a p-type nature for the surface of pristine FAPI nanocrystals film. Once in presence with TiOx used as electron transport layer, the downward band bending of FAPI NCs is increased and we observe signature from an electron transfer from the TiOx to the FAPI. The reverse effect (upward band bending) is observed at the interface of FAPI NCs with MoOx used as hole transport layer. We finally propose a full description of the energy landscape of FAPI nanocrystals film and its interface with both electron and hole transport layers.

SUPPORTING INFORMATION

Additional informations include X-ray diffraction, AFM and additional photoemission measurements.

ACKNOWLEDEGMENTS

EL thanks the support ERC starting grant blackQD (grant n° 756225). We acknowledge the use of cleanroom 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-Nano, Copin, Frontal, Graskop. JQ thanks Chinese Scholar council for PhD grant while AC thanks Agence innovation defense. This work has been partially funded by the CNRS Energy unit (Cellule Energie) through the project P-NANOSOL.

Figure 1 a

 1 Figure 1 a. Absorption and photoluminescence spectra of a FAPI nanocrystals film. b. Transmission electron microscopy image of FAPI nanocrystals. The inset is a higher magnification image. c. I-V curves of a FAPI nanocrystals film under dark condition and under illumination (λ = 405 nm and P = 3 mW), at room temperature. d. Current as a function of the incident power (λ = 405 nm) for a FAPI nanocrystals film. e. Current as a function of temperature for a FAPI nanocrystal film in the dark.

Figure 2 a

 2 Figure 2 a. Overview of the photoemission spectrum for a FAPI nanocrystals film. The photon energy is set at 600 eV. Photoemission spectra relative to the core level of N 1s (b.), C 1s(c.), Pb 4f (d.) and I 4d (e.) states. f. Energy profile of a FAPI nanocrystals film with downward band bending at the surface as revelad by pump-probe time resolved photoemission.

Figure 4

 4 Figure 4 Binding Energy of the Pb 4f state as a function of time, during light on and off, for a FAPI nanocrystals film with a. TiO2 b. MoO3 deposited on it. In part a and b the red line is obtained by averaging the experimental points. c and d are respectively the energy profiles of a FAPI nanocrystals film and its interfaces with TiOx and MoOx. For interfaces, black full curves are band alignement in the dark, while dotted grey line are obtained under illumination.