# Graphene as infrared and electron transparent electrode applied to the design of narrow band gap nanocrystal-based photodiode

Dario Mastrippolito, Albin Colle, Clement Gureghian, Tommaso Gemo, Adrien Khalili, Mariarosa Cavallo, Erwan Bossavit, Huichen Zhang, Yanjun Ma, Yoann Prado, et al.

► To cite this version:

Dario Mastrippolito, Albin Colle, Clement Gureghian, Tommaso Gemo, Adrien Khalili, et al. Graphene as infrared and electron transparent electrode applied to the design of narrow band gap nanocrystal-based photodiode. Advanced Optical Materials, 2025, 13 (21), pp.2500708. 10.1002/adom.202500708. hal-05487783

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https://hal.science/hal-05487783v1

Submitted on 1 Feb 2026

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Graphene as infrared and electron transparent electrode applied to the design
of narrow band gap nanocrystal-based photodiode

Dario Mastrippolito<fnref n="1" />,<fnref n="2" />, Albin Colle<fnref n="1" />, Clement Gureghian<fnref n="1" />, Tommaso Gemo<fnref n="1" />,<fnref n="3" />, Adrien Khalili<fnref n="1" />,
Mariarosa Cavallo<fnref n="1" />, Erwan Bossavit<fnref n="1" />,<fnref n="2" />, Huichen Zhang<fnref n="1" />, Yanjun Ma<fnref n="1" />, Yoann Prado<fnref n="1" />, James K.
Utterback<fnref n="1" />, Gregory Vincent<fnref n="4" />, Nicolas Péré-Laperne<fnref n="3" />, Ding Pei<fnref n="2" />, Pavel Dudin<fnref n="2" />, José Avila<fnref n="2" />,
Debora Pierucci<fnref n="1" />, Emmanuel Lhuillier<fnref n="1" />*

**Abstract:** Colloidal nanocrystals (NCs) are a promising platform for infrared optoelectronics. Efforts focus on designing NCs that absorb in the short- and mid-wave infrared and integrating them into diode stacks. A major challenge is coupling these sensors to read-out integrated circuits (ROICs) for infrared imaging, which requires infrared-transparent top electrodes. Conventional materials like tin-doped indium oxide lose transparency at longer wavelengths, limiting their effectiveness. Metallic grids have emerged as an alternative but struggle to maintain a uniform potential, as shown by nanobeam X-ray photoemission microscopy. To address this, graphene is explored as a transparent electrode. A novel diode stack is proposed to maintain a backside mirror, accommodate HgTe NCs' chemical constraints, and incorporate electrodes that efficiently extract both electrons and holes. Unlike conventional designs limited to near-zero bias, this stack operates optimally under CMOS ROIC conditions. Additionally, its transparent electrode allows photoelectron emission from within the diode, enabling in-situ electric field analysis. This capability could rationalize the optimization process of photodiode design.

**Keywords:** Nanocrystals, short wave infrared, HgTe, graphene, transparent conductive electrode, photodiode.

*To whom correspondence should be sent: el@insp.upmc.fr

<footnote n="1">¹ Sorbonne Université, CNRS, Institut des NanoSciences de Paris, 4 place Jussieu, 75005 Paris,
France.</footnote>

<footnote n="2">² Synchrotron SOLEIL, L'Orme des Merisiers, Départementale 128, 91190 Saint-Aubin, France.</footnote>

<footnote n="3">³ LYNRED, Actipole - CS 10021, 364 route de Valence, 38113 Veurey-Voroize, France</footnote>

<footnote n="4">⁴ DOTA, ONERA, Université Paris Saclay, 6 Chemin de la Vauve aux Granges, 91120 Palaiseau,
France.</footnote>
INTRODUCTION

Quantum confinement confers size-tunable optical properties to semiconductor nanocrystals (NCs). For this reason, these NCs have been massively investigated and have been successfully integrated into commercially available devices for visible wavelength emission. This is, nevertheless, not the only use for colloidal semiconductors. The broad library of available materials enables to reach a large range of band gaps down to the narrow ones that are relevant to infrared optoelectronics.¹,² In this spectral range, the production cost of state-of-the-art devices remains prohibitive for many applications, which tends to limit the potential of infrared optoelectronics to a few niche applications. To create cost disruption, new technologies have to offer reduced costs for the material growth and easier device integration, while keeping similar performances compared to epitaxially grown thin films. Colloidal-based optoelectronics have the potential to address all these points. However, when it comes to NCs for infrared sensing, most of the efforts have been focused on single-pixel devices acting as proof of concept, and more efforts are required to demonstrate their potential in full imagers. While there have been a few examples of such cameras,³⁻⁶ more has to be done to make them compatible with longer detection wavelengths, increase their operating temperature, and ensure their long-term operation.

In particular, a key challenge in the transition from single-pixel devices to focal plane arrays (FPAs) relates to sample illumination, which can no longer occur through the substrate due to the presence of the read-out integrated circuit (ROIC). Therefore, top illumination becomes a requirement, and so do transparent conductive electrodes. A few groups avoided this issue by designing planar devices with both electrodes on the bottom part.⁷⁻⁹ Such an approach has the benefit of simplifying the fabrication process with fewer layers to be deposited. However, the transport length is increased, which is generally detrimental for a material in which charge conduction operates by hopping conduction. This is why most of the current NC-based FPAs still rely on photodiode structures.¹⁰⁻¹³ For organic electronics and NC-based optoelectronics, the electrodes of the photodiodes often rely on conductive transparent oxides; see a scheme of such a device in **Figure 1a**. The most popular among them is tin-doped indium oxide (ITO). In the visible, ITO combines a high transparency (>80 %) and a high conductivity. However, plasmonic resonance in the short-wave infrared range reduces ITO's transparency in this range,¹⁴ as pointed out in **Figure 1b**, and makes it opaque in the mid-infrared.¹⁵,¹⁶ To mitigate this effect, a simple strategy consists of reducing the thickness of the electrode to limit its absorption, at the obvious price of an increased contact resistance. This problem is quite dramatic for infrared devices where the narrow band gap nature of the active layer leads to a reduced resistance, masking the contact resistance as a significant fraction of the total resistance.¹⁷

<figure><img src="image_2.png" /></figure>
**Figure 1 Design of top electrode.** a. Schematic of a diode stack made of a bottom mirror connected to a nanocrystal-based stack and an ITO layer used as partially transparent electrode. b. Transmission through ITO-coated glass, with various thicknesses. The dip at 4.26 µm is due to CO₂ absorption, as measurement is performed in air. The grey dashed line corresponds to the cut-off wavelength of an InGaAs camera (cut off 1.7 µm). The inset images are images of the glass and ITO-coated glass obtained with an InGaAs camera. The image highlights the lack of IR transparency for thick ITO. c. Schematic of a diode stack made of a bottom mirror connected to a nanocrystal-based stack and a metallic grid used as partially transparent electrode.

Therefore, alternative methods must be explored. Dang *et al* have proposed to replace the transparent conductive oxide with a metallic grid.<sup>15,16</sup> For standard metallic grids, their transparency is given by the metal filling factor. However, Dang *et al* proposed to take advantage of the resonances occurring when grids reach the nanometric scale to design electrodes with transparency significantly exceeding the metal's coverage fraction. Moreover, by combining three resonances, they achieved a broadband (from 2 to 5 µm) absorption of around 80 %. While this strategy looks very appealing for designing the light-matter coupling, its electrostatic properties clearly raise questions.

In this work, we start by assessing the electrical operation of metallic grids, using operando X-ray photoemission microscopy as a local probe of the device's potential landscape, and we find some clear limitations in using such grids as electrodes. Therefore, we then explore the use of graphene as an infrared and electron-transparent top electrode.<sup>18-22</sup> We use graphene electrodes for the design of short-wave infrared (SWIR) vertical diodes based on HgTe NCs.<sup>1,23</sup> HgTe is specifically chosen because it is currently the only platform that, in addition to the short-wave range, also covers long- and mid-infrared regions, where the design of transparent electrodes becomes even more critical. Beyond optical transparency, we also demonstrate that the introduction of the graphene electrode improves the compatibility of the diode stack with the usual operation of ROICs, and allows for in-situ probing of the electric field inside the diode stack.

## RESULTS AND DISCUSSION

As pointed out by **Figure 1b**, transparent conductive oxides such as ITO or FTO (fluoride-doped tin oxide), and other doped semiconductors, present a plasmonic absorption in the infrared range, reducing their potential as transparent electrodes. In the quest for alternative transparent electrodes, metallic grids appear promising, as the introduction of optical resonances can push the transparency of the electrode close to unity, thanks to exalted transmission and lead to a large overall absorption in the diode stack. In addition, a metallic grid relies on classical lithography processes and, consequently, its fabrication is more straightforward. Nevertheless, their actual use as electrodes, to efficiently apply electric fields and collect charges, must be verified.

To understand the challenge raised by the use of metallic grids, it must be remembered that in a NC film, charge conduction occurs through a hopping mechanism, which tends to limit the charge diffusion length. As a result, photodiodes rely on a thin stack whose exact thickness is a trade-off between the efficacy of the charge collection (maximized below 100 nm) and the absorption depth (at around 1 µm). Therefore, the resulting dimensions of the devices are highly anisotropic, with a thickness of a few hundred nm and a lateral size of several µm (pixel pitch on SWIR cameras typically ranges from 5 to 15 µm). As a result of this asymmetry, when a grid is used as the electrode, the absorbing material located in-between the metallic stripes is physically much closer to the bottom electrode than it is to the grid. Consequently, the potential of this same material is much closer to
the bottom electrode's potential than to the grid's potential. To actually quantify this problem, we fabricated a model system, where a layer of NCs (HgTe to match the absorbing material of the later proposed diode, 350 nm thick) is coupled to a metallic bottom electrode and a top metallic grid. The schematic of the device can be found in **Figure 1c**, and the various geometries of the tested grids are shown in Figure S1.

Then, we used X-ray nanobeam photoemission microscopy as a tool to unveil the *operando* changes of the potential landscape.<sup>24–29</sup> The experiment is conducted on the Antares beamline of the SOLEIL synchrotron.<sup>30,31</sup> The monochromatized soft X-ray source (hν = 95 eV) is first focused through a Fresnel zone plate to deliver a 700 nm beam spot at the sample level, setting the experiment's spatial resolution. The sample is then scanned, and for each point, a photoemission spectrum is collected. A schematic of the experiment is proposed in **Figure 2a**. Here, we track the Te 4d core level state, which has a binding energy of 40.2 eV when no bias is applied. Since the whole photoemission spectrum shifts rigidly when bias is applied, any shift in this core level directly corresponds to a shift in the valence band. Therefore, this method allows us to measure how the bands shift under an applied bias. Tracking a core level instead of the valence band simplifies the data fitting process because core level peaks have a well-defined shape.
<figure><img src="image_3.png" /><figcaption>**Figure 2** Nanobeam X-ray photoemission microscopy used to unveil the potential generated by a metallic grid used as top electrode. a. Schematic of the nanobeam photoemission microscopy experiment used for operating probing of the potential generated in the semiconductor by a metallic grid used as top electrode. OA stands for order sorting aperture. b. Map of the Te 4d<sub>5/2</sub> core level component intensity used as a chemical map. c. Te 4d core level probed in the opening of the grid under various potentials of the bottom electrode. d. Profile (same color code as in part c) of the Te 4d binding energy along the grid, highlighting a large shift of the potential as we shift from the top of the metal to the opening between electrodes.</figcaption></figure>

**Figure 2b** displays the intensity map of the Te 4d<sub>5/2</sub> core level component for a grating electrode. This map provides a chemical mapping showing higher intensity in the opening regions and strong reduction where the NCs are covered by the metal electrode. It should be noted that the intensity is reduced in this case, but is non-zero due to the ability of HgTe to form an amalgam with metal,³² and thus diffuse into the electrode area.

For a photon energy of 95 eV, the inelastic mean free path (IMFP) of Te 4d core-level photoelectrons (with a kinetic energy of KE≈50 eV) is extremely short (less than 1 nm), resulting in a very shallow
electron escape depth.³³ Consequently, only the potential coming from the very top layer of the film is probed. Here, the experiment is conducted as the potential of the top grid is grounded. However, upon bias application on the bottom electrode, we observed a shift of the Te 4d spectrum, as shown in **Figure 2c-d**. The profile collected along the y-axis of **Figure 2b**, clearly shows the inability of the grid to maintain its potential all along the top part of the active film, which instead acquires the potential of the underlying bottom electrode.

<figure><img src="image_4.png" /><figcaption>**Figure 3 Electrostatic simulation of the potential.** a. Simulated potential for a grid with a 5 µm period. The top electrode is grounded, while the bottom electrode is set to a 0.5 V potential. b. Zoom on the graph of part a in the edge of the electrode. c. Potential profile at set x coordinates for a position corresponding to the middle of the top electrode (green), edge of the electrode (gold), and away from the electrode (purple).</figcaption></figure>

To further quantify this problem, we have conducted an electrostatic simulation (**Figure 3**). In this case, the grid is made of a 5 µm strip with a 10 µm period (50 % filling factor), see **Figure 3a**. The field corresponding to the color change contrast is clearly located below the strip, while away from the strip, the potential of the film matches the one from the bottom electrode, see the potential profile in **Figure 3c**. A zoom on the edge of the metallic stripe (**Figure 3b**) shows that the field decays extremely rapidly as we move away from the electrode, typically on a length scale that is similar to the film thickness. The profile of potential determined by this combination of simulations and photoemission measurements indicates that a significant part of the device with the metallic grid is electrically ineffective. This conclusion matches Dang et al.'s, where the authors recommend the use of a high-filling factor grid.¹⁵ This is why in the second part of the paper, we explore an alternative design of the transparent electrode based on graphene.

The pioneering work of Konstantatos et al demonstrated the coupling of graphene to NCs as a synergetic approach,³⁴ where graphene brings its capacity for conduction, while the NCs offer their light emission and absorption capabilities. The geometry of the first device was focused on the graphene channel, while the NCs acted as light sensitizers, which found some important limitations,
such as a large dark current due to the absence of band gap in graphene and a slow response, which comes as the drawback of the very large photocurrent gain.

This is why later devices rather chose to use graphene as the (transparent) electrode either for light-emitting diodes³⁵,³⁶ or for photodetectors.²⁶,³⁷⁻⁴¹ In most of these devices, the graphene is directly functionalized to the substrate, while the design of the diode onto a ROIC rather pushes for the graphene to be the top electrode. The electrode being on top requires the precise positioning of the graphene flake with respect to the underneath diode stack while maintaining the latter's properties after the deposition process.

While we chose to maintain established electron and hole transport layers, respectively SnO₂ and Hg-treated Ag₂Te, replacing the top metallic electrode with a graphene layer calls for further updates to the diode stack. Metallic electrodes also act as mirrors, favoring the formation of a photonic cavity, which boosts the absorption magnitude, which raises the incentive to keep a metal as the bottom electrode. However, in our diode stack, see **Figure 4a**, the bottom electrode acts as the electron extractor, which is particularly challenging to design with HgTe NCs. The electron-extracting electrode should typically be made of a low-work-function metal, but these suffer from a high tendency toward oxidation. Additionally, noble metals such as silver (but clearly not limited to it) form amalgams with HgTe NCs.³² The formation of amalgams of Ag₂Te from the silver electrode is unfavorable for the carrier extraction and induces a loss of reflectivity from the electrode. This is why we chose to design the bottom electrode from a combination of a gold layer, acting as back backside mirror, and an ITO layer, which combines a reduced work function with respect to gold and a strong chemical compatibility with HgTe NCs.

<figure><img src="image_5.png" /><figcaption>**Figure 4** Diode stack with graphene as top electrode. a. Schematic of the diode stack, in particular, the bottom electrode is made of gold/ITO combination, while the top electrode is graphene. b. IV curve of the diode stack depicted in part a, at room temperature and 200 K. c. Photocurrent spectra acquired at various temperatures, highlighting a redshift as the sample is cooled down. d. Simulated absorption map within the diode stack, assuming 200 nm of HgTe. e. Simulated absorption spectra of the diode stack (i.e., total including loss in contact) and occurring within the NCs (i.e., the effective one leading to photocurrent generation).</figcaption></figure>

The IV curve of this diode, acquired in dark conditions, is rectifying, as shown in **Figure 4b**, and this behavior is amplified upon cooling. We also notice a modest redshift of the band edge upon cooling (**Figure 4c**), which agrees with previous observations on this material.⁴²,⁴³ Thanks to the design of
the electrode, the maximum absorption is located in the HgTe layer, see **Figure 4d**. For a 200 nm thick HgTe film, the absorption at the band edge reaches around 20 %. Such a thickness remains too low to generate a vertical Fabry Perot cavity, which will require a film thickness of the order of $\lambda/2n$, with $\lambda$ the band edge wavelength and $n$ the refractive index ($\approx 2.3$),$^{44,45}$ that corresponds to a film thickness of the order of $\approx 400$ nm, see also Figure S2. Nevertheless, we choose to operate a thin device to also be compatible with later *operando* photoemission tests, whereas thicker devices are more likely to be subject to photo-charging effects that will induce parasitic shifts.

Under illumination, the diode operation of the stack is further confirmed by the presence of an open-circuit voltage, see **Figure 5a**. The responsivity (**Figure 5b** and S3) of the diode appears to be modest at 0 V (sub mA.W⁻¹ regime), but considering the typical work function values of graphene (4.5 eV),$^{46}$ ITO (4.7 eV),$^{47}$ and gold (5.1 eV),$^{48}$ we attribute this to the lower work function difference of the graphene/ITO combination compared to the gold/ITO combination. However, the responsivity massively rises under bias, reaching 1 A.W⁻¹ under -0.6 V. A conventional diode stack (Figure S4) with bottom illumination through ITO also experiences an increase of responsivity with bias application, but the change in responsivity is typically within an order of magnitude, see Figure S4b. As a result, the best detectivity is obtained at quite a low driving bias (below -0.2 V) for the conventional stack. In comparison, the graphene-based stack shows a two-order-of-magnitude increase of detectivity for values above -0.2 V, setting the operating detection regime of the stack, and the detectivity reaches a maximum at -0.5 V. This change of driving condition needs to be put in the context of coupling such diodes to a ROIC. Very low driving voltages are incompatible with the CMOS transistor operation included in the pixel of the ROIC, and a voltage closer to 1 V is a preferable operating condition to be compatible with existing ROIC technologies. This requirement is quite critical to avoid the large cost associated with new ROIC development.

<figure><img src="image_6.png" /><figcaption>**Figure 5** SWIR detection performance from the diode stack with graphene as top electrode.
a. IV curves in the dark and under illumination by a 1.55 µm laser diode with various powers at room temperature. b. Responsivity and detectivity, at 100 Hz, as a function of the applied voltage measured at 200 K. Illumination is ensured by a blackbody radiation at 980°C.</figcaption></figure>

The repeatability of the fabrication process has also been tested, see Figures S5-S7, both for a given graphene provider, while testing multiple diodes on the same chip, and also comparing two sample providers. Overall, the repeatability is quite good (within a factor of 2 for the dark current) on
a given chip. Response-wise, ACS Materials graphene appears twice better (Figure S 7), and we attribute this difference to the higher work function of the latter (4.5 eV compared to 4.3 eV for Graphenea). The graphene being used on the hole extraction side, a larger work function appears more favorable.

Beyond the targeted transparency in the infrared window, single-layer graphene presents the advantage of being transparent to electrons photoemitted from the active material underneath. Such a property has already been used to increase the coupling of channel to gate in a FET geometry.<sup>37,38</sup> Therefore, it becomes possible to conduct X-ray photoemission to study the diode operation *in-situ*. If the measurement is conducted at 95 eV (surface-sensitive condition), the escape depth is very short, and only the HgTe layer is probed in the very vicinity of the graphene. In such a case, no shift is observed under bias application, see **Figure 6a**. In other words, the graphene is efficient in setting the potential of the device's top layer. Conversely, if we choose a photon energy leading to a longer photoelectron escape depth (700 eV as opposed to 95 eV, i.e., KE (Te 4d) ≈ 655 eV), we can probe the buried HgTe. Under such conditions, we can observe a clear shift of the Te 4d core level as a bias is applied. The (reversible) shift reaches 0.22 eV, see **Figure 6b**. This value is only a fraction of the applied bias and actually reflects the electric field distribution at the depth where the photoelectrons are probed. Conducting such an experiment with a continuously tunable hard X-ray source will be of utmost interest because it will give access to the full distribution of potential vs thickness.

<figure><img src="image_7.png" /><figcaption>**Figure 6** Graphene as electron-transparent electrode for operando investigation. a. (resp b.) Te 4d core level probed by XPS at 95 eV (resp 700 eV) on the diode stack under 0 V and under +0.25 V applied. At 700 eV, the spectra display a 0.22 eV shift.</figcaption></figure>

**Table 1** Comparison of the various performances obtained for the IR NC-based photodiode for three different approaches of transparent electrode design.
<table><thead><tr><th>Transparent electrode strategy</th><th>Transparent oxide (ITO)</th><th>Metallic grid</th><th>Graphene</th></tr></thead><tbody><tr><td>Transparency</td><td>Limit in the SWIR (80 %)<br>Incompatible with MWIR (&lt;50%)</td><td>From a filling factor limited (no resonance) to high with exalted transmission</td><td>High (&gt;95 %)<br>SWIR and MWIR compatible</td></tr><tr><td>Absorption</td><td>10-30 %<br>Higher if the thickness enables a Fabry-Perot cavity</td><td>Up to 80 %<sup>16</sup></td><td>20 %</td></tr><tr><td>Fabrication</td><td>Sputtering may be problematic as the top electrode</td><td>Trivial if non-resonant e-beam step for resonance (more challenging as top)</td><td>Transfer step</td></tr><tr><td>Electrical charge extraction/contact resistance</td><td>Homogeneous<br>R=10-100 Ω (depends on thickness)</td><td>Highly inhomogeneous<br>R=few Ω</td><td>Homogeneous<br>R=10-30 Ω (depends on thickness)</td></tr></tbody></table>

To summarize, we bring quantitative evidence that the current strategy for the design of transparent infrared electrodes used for NCs-based devices faces major limitations. On one hand, ITO is clearly not transparent enough. On the other hand, in spite of fully satisfactory optical properties, metallic grids act as poor electrodes to set the potential of a photodiode's top layer, as evidenced by nanobeam X-ray photoemission coupled with simulations. For these reasons, graphene was then explored for the design of the top electrode coupled to a short-wave photodiode. Though the diode performances do not match the current state of the art,⁴⁹⁻⁵¹ due to weak absorption and limited operating temperature, the diode enables a larger bias operation, which increases its compatibility with a CMOS ROIC. In **Table 1**, we summarize the benefits and drawbacks of each strategy regarding transparency, contact resistance, and challenges regarding fabrication. It is worth pointing out that this table is not complete since other strategies are also explored, such as metallic wires⁵²,⁵³ into polymer matrix, or thin polymeric conductive layer.⁵⁴⁻⁵⁶ It is also worth stressing that strategies should not be opposed to each other, since a promising future direction will be the combination of graphene and metallic grids as a strategy to combine both light resonances with homogeneous electrostatic properties.

In this work, we have been focused on HgTe as the infrared active material, but the concept is not limited and can be implemented for other IR active NCs such as PbS and III-V semiconductors. This may nevertheless require switching the graphene transfer process since the current wet transferring process may damage materials that are more prone to oxidation than HgTe.

Furthermore, we demonstrate that the benefit of the graphene electrode is not limited to the infrared sensor niche design but can be applied to many *operando* photoemission studies since it is also
transparent to electrons. We bring a proof of concept that the potential drop can be probed in *situ*.
This has a great potential for not only the optimization of photodiode stacks, but also in many other
fields where the application of an electric field is used as a knob to tune material properties: tracking
ion migration in perovskite, inducing superconductivity thanks to a gate...

METHODS

**Chemicals.** Mercury chloride (HgCl₂, Strem Chemicals, 99%), tellurium powder (Te, Sigma-Aldrich, 99.99%), trioctylphosphine (TOP, Alfa aesar, 90%), oleylamine (Thermo scientific, 80-90%), dodecanethiol (DDT, Sigma-Aldrich, 98%), 2-mercaptoethanol (MPOH, Thermo scientific, 98%), N,N-dimethylformamide (DMF, VWR rectapur), ethanol absolute anhydrous (VWR), methanol (VWR normapur), chloroform (VWR technical), chlorobenzene (VWR rectapur).
SnO₂ nanoparticles (15% dispersion in water, Alfa Aesar), ammonium chloride (Alfa Aesar, 98 %) silver nitrate (Alfa Aesar, 99.9%), 1,2-ethanedithiol (EDT, Fluka, 98%), graphene (Graphenea, Easy Transfer, now discontinued or from ACS Materials, Trivial Transfer).

All chemicals are used as received, except oleylamine, which is centrifuged before use. **Mercury and salts are toxic, handle them with great care.**

**1 M TOP:Te precursor.** In a three-neck flask, 6.35 g of Te powder is mixed in 50 mL of TOP. The flask is kept under vacuum at room temperature for 5 minutes and then heated up to 100 °C and kept under vacuum for an additional 45 minutes. The atmosphere is changed to Ar and the temperature is raised to 275 °C. The solution is stirred until a clear orange color is obtained. The flask is then cooled down, and the color switches to yellow. The flask is degassed again for 10 minutes at room temperature. Finally, this solution is transferred to a nitrogen-filled glovebox for storage.

**HgTe NCs synthesis:** The synthesis is based on the process proposed by Keuleyan et al.⁵⁷ In a 100 mL three-neck flask, 543 mg of HgCl₂ and 50 mL of oleylamine are degassed under vacuum at 110 °C for 30 minutes. Then, the atmosphere is switched to N₂, and the temperature is stabilized at 110 °C. Then, a mixture of 2 mL of TOP:Te (1 M) and 8 mL of oleylamine are quickly injected. After 3 minutes, 10 mL of a mixture of 10% in volume of DDT in toluene is injected, and a water bath is used to rapidly decrease the temperature. The content of the flask is split over three centrifuge tubes. The solution is precipitated first with MeOH, and the resulting pellet is redispersed in one centrifuge tube with chloroform. A second precipitation is performed with absolute EtOH, and the formed pellet is redispersed in 6 mL of chlorobenzene.

**Ag₂Te CQD synthesis:** The synthesis is based on the process proposed by Ackerman et al.⁵⁸ In a 50 mL three-neck flask, 170 mg of AgNO₃ is mixed together with 25 mL of oleylamine and 2.5 mL of oleic acid. The flask is then degassed under vacuum at room temperature for 15 min and then at 70 °C for 10 min. The atmosphere is replaced with nitrogen and 2.5 mL of TOP is added to the solution. Then, the temperature of the flask is raised to 160 °C. After 30 min, the solution becomes orange. At this step, 0.5 mL of TOP:Te (1M) is injected, and the reaction is quenched after 10 min with a water bath. The crude solution was stored in a freezer at -20 °C.
**Grid electrodes:** A Si/SiO₂ substrate is cleaned by sonication in an acetone bath and rinsing in isopropanol. A final cleaning is made using O₂ plasma. A standard UV lithography process is carried out to define the bottom electrode. First, an adhesion promoter (TI PRIME) is spin-coated at 4000 rpm for 30 seconds and baked at 110 °C for 1 minute. A photoresist (AZ5214E) is then spin-coated and baked under the same conditions. The resist is exposed to UV light through a mask for 1.5 seconds, followed by a reversal bake at 110 °C for 2 minutes. A flood exposure of 40 seconds is performed, and the resist is developed in AZ726MIF for 20 seconds. Metal deposition is carried out via thermal evaporation, with 10 nm of Ti followed by 40 nm of Au. The lift-off process is then performed in an acetone bath for 20 minutes. To form the active layer, the HgTe QD ink is spin-coated onto the substrate, producing a 350 nm-thick film. For the top electrode (grid), a second UV lithography step is performed on top of the previously deposited HgTe QD film. This time, no adhesion promoter is used, and the reversal bake of the photoresist is conducted at 115 °C for 2 minutes. A 50 nm-thick Au layer is deposited via thermal evaporation, followed by a lift-off process in acetone for 20 minutes.

## Diode Fabrication

**Back electrode:** A glass slide substrate (1.1 mm thickness) is cleaned via sonication in acetone. The glass is rinsed with acetone, then isopropanol, and dried with a N₂ gun. A further cleaning is made using an O₂ plasma. An adhesion promoter (TI PRIME) is spin-coated onto the glass substrate and baked at 110 °C for 60 s. Photoresist (AZ5214E) is spin-coated and baked at 110 °C for 60 s. The substrate is exposed to UV through a pattern mask for 1.5 s. The film is then baked at 110 °C for 2 min to invert the resist. Then, a 40 s UV flood exposure is performed. The resist is developed using a bath of AZ726MIF for 19 s, rinsed in pure water, and finally dried with N₂. After cleaning with O₂ plasma, 10 nm of Cr and 40 nm of Au are deposited using thermal evaporation. Then, 30-40 nm of ITO is deposited via sputtering. The final lift-off is performed by dipping the film in acetone for 30 min. The patterned substrate is rinsed using isopropanol and dried with an N₂ flow.

**SnO₂ deposition:** For light-doped SnO₂ (SnO₂-L), 200 µL of commercial colloidal SnO₂ solution is diluted with 1 mL of 24 mM NH₄Cl solution in water. For heavy-doped SnO₂ (SnO₂-H), 200 µL of commercial SnO₂ solution is diluted with 1 mL of 48 mM NH₄Cl solution in water. The SnO₂-L solution is deposited onto the patterned substrate via spin-coating. The SnO₂-H solution is deposited onto the SnO₂-L layer via spin-coating. The film is annealed on a hot plate at 70 °C for 1 h.

**HgTe ink:** 1.75 mL of the HgTe solution in toluene (1.27 OD at 400 nm after a 1000x dilution) is mixed with 1.05 mL of exchange solution (30 mg HgCl₂, 18 mL DMF, 2 mL MPOH) and 0.7 mL of DMF. The solution is stirred throughout vortexing and sonication for 3 min to help the ligand exchange. Then, the solution is precipitated by adding toluene and centrifuged at 6000 rpm for 4 min. The supernatant is discarded, and the QDs are dried using a N₂ gun. Finally, the NCs are redispersed in 0.25 mL of DMF, and centrifuged for 1 min at 6000 rpm. 30 µL of the ink is deposited by spin coating on the SnO₂ substrate, pre-cleaned by O₂ plasma. The spin-coating is performed in two steps: a first spreading step at 1400 rpm (acceleration 500 rpm.s⁻¹) for 120 s, and a second drying step at 2000 rpm (1000 rpm.s⁻¹) for 60 s. The obtained film thickness ranges around 220-250 nm.

**Ag₂Te ligand exchange:** 800 µL of the unfrozen crude solution is precipitated with MeOH by centrifugation (6000 rpm, 60 s). The formed pellet is dispersed in a mixture of 200 µL of DDT and 600 µL of chlorobenzene. Then, the NCs solution is precipitated again with MeOH by centrifugation.
The pellet is dispersed using 800 µL of chlorobenzene. The NCs solution is precipitated again with MeOH by centrifugation. The pellet is then dispersed in 1.6 mL of a mixture of hexane/octane (9:1 volume ratio) and stored in a dark vial to avoid photodegradation. On top of the HgTe film, 50 µL of Ag₂Te NCs solution is spin-coated at 1500 rpm (500 rpm·s⁻¹) for 30 s. 50 µL of HgCl₂ (10 mM in MeOH) solution is dropped onto the HgTe film and spin-dried after 15 s. Then, the film is rinsed with IPA. This procedure is repeated one more time. Finally, an ethanedithiol ligand exchange is performed by dipping the film in 1% ethanedithiol in acetonitrile solution for 30 s and rinsing with pure acetonitrile. The overall Ag₂Te film thickness is around 50 nm.

**Graphene top electrode:** Graphene electrodes are realized starting from commercial CVD-grown graphene on polymer film (PMMA sacrificial layer/graphene/polymer film from Graphenea "Easy transfer" product now discontinued, or ACS Materials "Trivial Transfer" product). A sheet of PMMA/graphene film (0.5x0.5 cm²) is released from the bottom polymer support in deionized water. The free PMMA/graphene stack is fished and transferred directly on top of the Ag₂Te film and then dried in air for 30 min to remove any water residues. To improve the adhesion of graphene to the Ag₂Te film, the vertical stack is annealed at 45 °C for 60 min and stored in a primary vacuum for 24 hours. Finally, the sacrificial PMMA layer is removed by dipping the vertical stack in hot acetone (40 °C, 60 min), isopropanol (60 min), and dried by a gentle N₂ flow. The mean overlapping region between the top graphene electrode and the back electrode is around 1 mm², which defines the pixel area.

**Infrared spectroscopy:** A Thermo Fisher Nicolet iS50 in attenuated total reflection (ATR) mode is used. The spectra are averaged over 32 acquisitions and have a 4 cm⁻¹ resolution.

**Photoemission microscopy** is conducted on the ANTARES beamline of Synchrotron SOLEIL. The synchrotron beam is focused using a Fresnel zone plate, see **Figure 2a**. The latter is made to operate at low energy (below 100 eV), to obtain optimal focusing. In this case, the beam width at the sample position can be down to around 700 nm. The sample is then raster-scanned and the photoelectrons are collected using an MBS A-1 hemispherical electron analyzer. The sample holder is moved with step sizes ranging from 500 to 2000 nm and, for each point, a series of photoemission spectra is collected. These spectra are acquired with a 200 eV pass energy, providing a 15.8 eV window with a 17 meV energy step, and are then averaged. Here, we focus on the Te 4d core level doublet with a SO splitting of 1.44 eV and a binding energy for the Te 4d<sub>5/2</sub> of around 40 eV. Data acquisition lasts from a few minutes to an hour, depending on the targeted map size. We used linearly horizontal polarized photons with a photon energy of 95 eV and all measurements are conducted at 250 K, with the combined energy resolution better than 90 meV (for pass energy 200 eV). For all maps, the angular dispersion of the analyzer (±15°) is neglected by integrating the spectra over all angles. The samples are mounted on the *operando* sample holder of the ANTARES beamline, which is equipped with electrical contacts. These electrical contacts are made through gold wires bonded to the electrical pads of the sample holder, while silver paste is applied to the electrical pads of the sample. The sample is connected to a Keithley 2450 to apply bias.

**Work function measurements:** Graphene work function values were obtained by measuring at room temperature the secondary electron cutoff by X-ray photoemission spectroscopy of graphene sheets transferred to SiO₂/Si substrate (graphene grounded to the sample holder). A PHI GENESIS setup is used, which is equipped with a monochromatic Al Kα X-ray source (hv=1486.6 eV) with a spot size of 100 µm in normal emission configuration with a pass energy (PE) of 6.5 eV and an applied bias of -10 V. The zero-binding energy reference, corresponding to the Fermi level, was
calibrated using the leading edge of a clean Ag foil. The position of the secondary electron cutoff was determined by extrapolating the leading edge of the spectra with respect to the zero baseline.

**Electrostatic simulation:** Electrostatic calculations were carried out using COMSOL AC/DC Module with Electric Currents physics. Electric potentials were defined at the interfaces between the electrodes and the semiconductor material. Other interfaces with this material are defined as Electric Insulation. The conductivity of electrodes is assumed to be $45.2 \times 10^6$ S/m and that of the semiconductor is supposed to be $10^5$ lower. However, the latter parameter would only influence currents, which are not represented in this study. A free triangular mesh is used, with 185 664 elements.

**Electromagnetic simulation:** The graphene-topped photodiode has been simulated using the COMSOL Multiphysics Software. The simulations used Electromagnetic Waves module in Frequency Domain for a two-dimensional model. From top to bottom, the stacking is composed of the following: an input port backed with a 1 µm-thick PML. A 5 µm-thick air layer is used to avoid any non-physical effect of evanescent waves forming at the interface with the diode modeled as vacuum. The graphene layer for which the optical effect is assumed to be null and, therefore, not taken into account in the model. An $Ag_2Te$ layer of 50 nm thickness and a 200 nm thick (unless specified) layer of $HgTe$, both modeled with the same refractive index based on measurements conducted on mercury telluride films. A $SnO_2$ layer of 40 nm and 30 nm of ITO whose refractive index models are based on measurements. A gold layer of 40 nm, for which the index is based on a Drude model, serves as a back mirror. A 5 µm-thick layer of $SiO_2$ modeled with a refractive index of 1.44 serves as the substrate before an output port backed by a 1 µm PML. The period was arbitrarily taken as 1 µm.

The top port was set as input by enabling wave excitation at this port, while the excitation of the bottom port was kept disabled. The top port input power was set so that the electric field value is unity. Subsequent power dissipation in the structure was computed using the local electric losses (emw.Qe) normalized by the input power. The meshing of the structure was performed based on extremely fine automatic meshing calibrated for semiconductor physics and then adjusted manually to be two orders of magnitude tighter inside the diode stacking. This resulted in a meshing with about 53 000 elements, of which about 36 000 were within the diode and the gold mirror. Dissipation within the diode and within the whole structure has been computed for angles of incidence spanning from 0 to 60 degrees. Each simulation was run using a transverse electric (TE) and a transverse magnetic (TM) excitation. Absorption under unpolarized light was computed as the mean of absorptions under TE and TM excitation.

## Optoelectronic characterization

**I-V Curves:** Current-voltage characteristics of the photodiode are measured while the sample is mounted on the cold finger of a closed-cycle cryostat. The setup was regulated in temperature thanks to a LakeShore Model 335 temperature controller. After the setpoint temperature is reached and thermalization is completed, the current flowing through the photodiode is measured for different bias voltages using a Keithley 2634B SourceMeter. The bias step is set to 5 mV, and each point is integrated over 10 measurements.
**Blackbody Response:** For responsivity measurements, the same cryostat setup is used with a double-coated ZnSe window, enabling illumination of the detector. A Black Point BB-4A Calibration Source from Omega Engineering set to 980°C is placed 41 cm away from the photodiode. The radiation emitted from the blackbody is first filtered through a FEL1000 longpass filter from Thorlabs and then chopped at 100 Hz using a 2-slot chopper blade on a Thorlabs MC2000 Optical Chopper System. It then passes through the two cryostat windows before reaching the device. The photocurrent is then measured using synchronous detection through a MFLI Lock-In Amplifier made by Zurich Instruments. Total power in the spectral region of interest is calculated by integrating the theoretical blackbody radiation, considering the transmission of the filter and both coated ZnSe windows, between the passing wavelength of the long pass filter (cut-off at 1 µm) and the NCs band edge.

**Noise measurements:** Noise measurements are conducted while the device is mounted in the cryostat cold finger. The current from the photodiode is fed into a Femto DLPCA-200 transimpedance amplifier (TIA), also used as the bias source. The output from the TIA is fed into a spectrum analyzer (Stanford Research Systems, SR780).

## DATA AVAILABILITY

The data that support the findings of this study are available from the corresponding author upon request.

## ACKNOWLEDGMENTS

The authors thank Bruno Domenichini for suggesting the seminal concept of the study and Stephane Lorc y for his help in sample preparation at ANTARES. The project is supported by ERC grant AQDtive (grant n°101086358) and from French National Research Agency (ANR) through the grants Bright (ANR-21-CE24-0012-02), MixDferro (ANR-21-CE09-0029), Quicktera (ANR-22-CE09-0018), Operatwist (ANR-22-CE09-0037-01), E-map (ANR-23-CE50-0025), DIRAC (ANR-24-ASM1-0001), camIR (ANR-24-CE42-2757), Piquant (ANR-24-CE09-0786) and as part of the "Investissements d'Avenir" program (Labex NanoSaclay, reference: ANR-10-LABX-0035). We acknowledge the use of clean-room facilities from the "Centrale de Proximité Paris-Centre" and support from Renatech+ for micro and nanofabrication. This project has also received financial support from the CNRS through the MITI interdisciplinary programs (project WITHIN) and Région Ile de France through the Sesame project INSIDE.

## COMPETING INTEREST

The authors declare no competing interests.

## Supporting Information

Supporting Information includes (i) Optical image of the electrodes used for X-ray photoemission microscopy, (ii) Optical simulation of the graphene diode, (iii) Temperature dependence for the graphene-based diode stack, (iv) Bias dependence for the conventional diode stack.
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