Erwan Bossavit

Les « quantum dots » (QD) colloïdaux sont des cristaux de semiconducteurs avec une taille de l'ordre de quelques nanomètres, qui sont synthétisés en solution et qui ont une luminescence accordable avec leur taille. Ainsi, dans les dernières années, les QD ont émergé comme une alternative aux semiconducteurs traditionnels, dont la croissance est très couteuse en énergie. Bien que dans le visible le potentiel des QD ait été pleinement réalisé, atteignant une distribution commerciale, des efforts supplémentaires sont encore nécessaires dans l'infrarouge. En particulier, l'efficacité de luminescence des QD infrarouges chute drastiquement lors de leur intégration des diodes électroluminescentes (LED). Cela est dû à une combinaison d'un mauvais équilibre de l'injection des charges dans le matériau actif, d'une densité accrue de chemin de relaxation non-radiatifs dont les QD intégrés, et du piégeage de la lumière générée à l'intérieur du dispositif. Durant mon projet de thèse, j'ai cherché à résoudre ces points critiques en obtenant le contrôle sur l'environnement électronique, chimique, et optique des QD au sein des LED. Pour cela, j'ai eu recours à différentes méthodes. Tout d'abord, j'ai ajusté la structure de bande électronique des QD en modifiant par le dépôt d'atomes de métaux alcalins leur travail de sortie, que j'ai suivi par mesures XPS sur la ligne TEMPO du synchrotron SOLEIL. J'ai ensuite régulé la densité de pièges à la surface des QD dans les LED par une combinaison d'un traitement aux ions halogénures, qui a permis de multiplier par un facteur 3 l'efficacité des LED et a mené à une EQE record de 2.3 %, ainsi que du refroidissement des dispositifs, qui a étendu leur durée de vie de deux ordres de grandeur. J'ai également induit une anisotropie dans l'émission des QD initialement isotropes par le couplage à deux types de structures photoniques, les réseaux plasmoniques et les microcavités diélectriques, ce qui a le potentiel d'améliorer l'extraction de la lumière dans les LED. Enfin, j'ai transféré le couplage au microcavité diélectrique à un système de photodétection, menant à des pics ultrafins de photocourant et largement accordable dans la gamme télécom.

Mercury telluride (HgTe) nanocrystals are cornerstone materials for infrared optoelectronics, yet all previously reported forms of HgTe have crystallized in the zinc blende phase. Here, we develop a comprehensive cation exchange route to access metastable wurtzite (WZ) HgTe in both spherical and nanorod morphologies. Structural and spectroscopic characterizations show that WZ HgTe NCs retain the strong confinement tunability of their optical properties while introducing non-cubic lattice and distinct electronic topology. Ab initio modeling reveals that bulk WZ HgTe is a Dirac semimetal, whereas quantum confinement opens a direct gap that enables bright short-wave infrared emission. High-pressure studies demonstrate an irreversible WZ-to-zinc blende phase transition, consistent with its metastable nature, while the WZ phase remains stable at cryogenic temperatures. Electrically driven light-emitting diodes based on WZ HgTe nanorods exhibit superior electroluminescence beyond 2 μm, establishing a platform bridging topological semimetals and confined infrared emitters.

Colloidal nanomaterials are now a cost‐effective strategy for the design of infrared imagers. Their increased maturity brings them to the point where the evaluation of their potential for space and astronomy applications becomes relevant. The first step is to test their robustness against irradiation, since this process is responsible for the performance degradation of most imagers. In this study, we explore the potential of HgTe nanocrystals for short‐wave infrared sensing under X‐ray and He ⁺ ion irradiation. To first assess the degradation of the material itself, we test the evolution of a single‐pixel photoconductive device upon irradiation and observe that only marginal degradation occurs for doses relevant to space applications, and degradation only occurs at a very high threshold, making this colloidal semiconductor quite promising to operate in harsh radiation environments. Complementary spectroscopic studies reveal very different degradation mechanisms upon X‐ray (leading to oxidation) and He ⁺ ion (sintering particles). We tested the potential of this material in a focal plane array and observed degradation, though not preventing imaging, in a range of doses where no effect was observed for the simple photoconductive device. This suggests that the CMOS readout circuit might be the bottleneck regarding operation under radiation.

Mercury telluride (HgTe) nanocrystals (NCs) are promising materials for infrared photodetection due to their size-tunable band edges and compatibility with colloidal synthesis. While optical studies have established the temperature dependence of their relative bandgaps, the evolution of their absolute electronic spectrum with cooling remains poorly understood, despite its critical role in the device operation. In this work, we investigate in situ the temperature-induced absolute energy shift of HgTe NCs' electronic states with varying band edge energies, using X-ray photoemission spectroscopy. We observe a systematic and reversible rigid shift of core levels toward lower binding energies upon cooling, reaching approximately 80 meV per 100 K, four times larger than the corresponding optical band edge shift (≈20 meV). This behavior is consistent across different NC sizes, making the impact more dramatic for the narrower bandgap NCs. Such absolute energy shifts can significantly alter carrier densities and interfacial band alignments, potentially creating Schottky barriers and reducing extraction efficiency in photodiode architectures. Our findings highlight the necessity of accounting for temperature-induced absolute energy shifts in the design of next-generation HgTe NC-based infrared detectors.

Van der Waals heterostructures offer a playground for optoelectronics with the promise of coupling a broad range of materials while lifting the constraints of epitaxy. Optimization of such devices also requires careful control of the spatial localization of the in-plane and out-of-plane electric field. Therefore, operando tools that give direct access to the local energy and electric field landscape are necessary. Here, we demonstrate that nanobeam X-ray photoemission imaging is an effective method to map the electric field in a 2D flake-based multielectrode transistor and a WS₂/MoSe₂ heterostructure, while the electrical biases are applied. We discuss how the shape and geometry of the flake, the electrical contacts, and the overlapping of the flakes impact the field distribution. In the region where flakes overlap, the in-plane electric field distribution aligns in the two materials, whereas the out-of-plane field distribution resulting from the gating charge injection deviates from a homogeneous distribution.

After the success of semiconductor nanocrystals as light sources for displays in the visible range, the infrared range now offers a complementary playground. Applications requiring chemical contrast in images and applications to LIDAR technology incentivize the development of devices with narrow spectral responses. However, the solutions that rely on introducing notch filters still suffer from imperfect transmission at the wavelength of interest in a spectral range where their detection is already difficult. Here, we explore the integration of a short-wave infrared detector directly into a dielectric microcavity. Our approach simultaneously achieves ultranarrow absorption lines below 30 cm^-1 at a telecom wavelength, together with a broadband, continuous, postfabrication spectral tunability over 1200 cm^-1. By taking advantage of the field magnification inside the cavity, we demonstrate that the spectral shaping properties can be obtained while maintaining performances on par with an uncoupled device, stressing the benefit of this method compared to filter-only approaches.

HgTe nanocrystals (NCs) are now an established material for colloidal optoelectronics. In the mid‐ and far‐infrared range, there is to date no viable colloidal alternative material that can emit or detect light with such a high efficiency. Advancements have been rapid, primarily concentrating on the direct application of the narrow bandgap material spectrum in devices. However, certain material properties, such as carrier density, have been suboptimally addressed, despite their critical importance to device design. Therefore, this review aggregates most of the recent developments related to the determination and control of the carrier density with HgTe NCs. The manuscript is organized along three scales: the intrinsic properties that mostly relate to the inorganic core of the NCs, the nanoscale functionalization that relates to the surface modification of the NCs, and the device scale mostly driven by gating methods. The review provides a systematic discussion on strategies and technologies to control the carrier density and their use for optoelectronics. Finally, a discussion is done on the directions to be developed to push even further the efficiency of devices based on this material.

Colloidal nanocrystals (NCs) are a promising platform for infrared optoelectronics. Current efforts focus on designing NCs that absorb in the short‐ and mid‐wave infrared and integrating them into diode stacks. A major challenge is to 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 read‐out‐integrated‐circuit (ROIC) conditions. Additionally, its transparent electrode allows photoelectron emission from within the diode, enabling in situ electric field analysis. This capability enables to rationalize the optimization process of photodiode design.

As the integration of transition metal dichalcogenides (TMDC) becomes more advanced for optoelectronics, it is increasingly relevant to develop tools that can correlate the structural properties of the materials with their electrical output. To do so, the determination of the electronic structure must go beyond the hypothesis that the properties of the pristine material remain unaffected after the device integration, which generates changes in the dielectric environment, including electric fields that are likely to renormalize the electronic spectrum. Here, we demonstrate that nanobeam photoemission spectroscopy is a well-suited tool to unveil the device energy landscape under operando conditions. Both the gate vertical field and the drain in-plane vectorial electric field can be determined with a sub-μm resolution. We provide a correlative description of a field-effect transistor to connect its bias-modified energy landscape with the transistor electrical output. The method appears highly suited to unveil how the actual geometry of the flake (thickness, edge effect, presence of structural defects, etc.) is driving the current flow within the device. Lastly, the method appears fully compatible with traditional device fabrication, therefore making it relevant for systematic rational optimization of TMDC-based electronic devices.