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Cadmium chalcogenide nanoplatelets present unique optical properties resulting from their atomically precise thickness control. They have emerged as a new promising class of nanomaterial for optoelectronic applications. The goal of this thesis is to promote new optical properties through the design and surface chemistry of nanoplatelets. Firstly, CdSe/CdSe1-xTex core/crown heterostructures were synthesized and showed bicolor emission at the scale of a single nanoplatelet. This bicolor emission results from a competition between the localization of the electron in the CdSe core driven by the conduction band offset and the direct exciton recombination in the crown favored by the high exciton binding energy. Silver doped nanoplatelets were synthesized in the second part. This doping, performed through partial cation exchange, creates a silver-related emissive state in the band gap. The amount of silver incorporated in the nanoplatelet allows a progressive control of the emission color from green to red thanks to the coexistence of the band edge and the silver related emission. The silver state is strongly bound and is located 340 meV above the bulk valence band. Finally, the modification of the surface chemistry with bromide and oleylamine reduces the surface stresses and improves the surface passivation. This leads to a shift, beyond the discrete confinement, of the emission wavelength and increases the quantum yield up to 75 %.; Les nanoplaquettes de chalcogénures de cadmium présentent des propriétés optiques uniques résultant d’une épaisseur contrôlée à l’échelle de la monocouche atomique. Elles apparaissent comme une nouvelle classe de nanomatériaux à fort potentiel applicatif. L’objectif de cette thèse est, par le design de la particule inorganique et de la chimie de surface, de faire émerger de nouvelles propriétés optiques. Dans un premier temps des hétérostructures cœur/couronne de CdSe/CdSe1-xTex ont été synthétisées et ont montré une bi-émission à l’échelle de la nanoplaquette unique. Cette bi-émission émerge d’une compétition entre la localisation de l’électron dans le cœur de CdSe induite par le décalage entre les bandes de conduction et d’une recombinaison directe dans la couronne favorisée par la grande énergie de liaison de l’exciton dans les nanoplaquettes. Des nanoplaquettes de CdSe dopées à l’argent ont été synthétisées dans un second temps. Le dopage, fait par échange cationique partiel, créé des pièges émissifs dans la largeur de bande interdite. La quantité d’argent incorporée permet un contrôle progressif de la couleur d’émission du vert au rouge grâce à la coexistence de la recombinaison de bord de bande et de l’émission liée à l’argent. Ces niveaux d’argent sont fortement liés à CdSe et se situent 340 meV au-dessus de la bande de valence du CdSe massif. Enfin, la modification de chimie de surface par un mélange de bromures et d’oleylamine diminue les contraintes de surface et améliore la passivation de surface. Ceci induit un décalage, au-delà du confinement discret, de la longueur d’onde d’émission et une augmentation du rendement quantique jusqu’à 75%.

Cadmium chalcogenide nanoplatelets present unique optical properties resulting from their atomically precise thickness control. They have emerged as a new promising class of nanomaterial for optoelectronic applications. The goal of this thesis is to promote new optical properties through the design and surface chemistry of nanoplatelets. Firstly, CdSe/CdSe1-xTex core/crown heterostructures were synthesized and showed bicolor emission at the scale of a single nanoplatelet. This bicolor emission results from a competition between the localization of the electron in the CdSe core driven by the conduction band offset and the direct exciton recombination in the crown favored by the high exciton binding energy. Silver doped nanoplatelets were synthesized in the second part. This doping, performed through partial cation exchange, creates a silver-related emissive state in the band gap. The amount of silver incorporated in the nanoplatelet allows a progressive control of the emission color from green to red thanks to the coexistence of the band edge and the silver related emission. The silver state is strongly bound and is located 340 meV above the bulk valence band. Finally, the modification of the surface chemistry with bromide and oleylamine reduces the surface stresses and improves the surface passivation. This leads to a shift, beyond the discrete confinement, of the emission wavelength and increases the quantum yield up to 75 %., Les nanoplaquettes de chalcogénures de cadmium présentent des propriétés optiques uniques résultant d’une épaisseur contrôlée à l’échelle de la monocouche atomique. Elles apparaissent comme une nouvelle classe de nanomatériaux à fort potentiel applicatif. L’objectif de cette thèse est, par le design de la particule inorganique et de la chimie de surface, de faire émerger de nouvelles propriétés optiques. Dans un premier temps des hétérostructures cœur/couronne de CdSe/CdSe1-xTex ont été synthétisées et ont montré une bi-émission à l’échelle de la nanoplaquette unique. Cette bi-émission émerge d’une compétition entre la localisation de l’électron dans le cœur de CdSe induite par le décalage entre les bandes de conduction et d’une recombinaison directe dans la couronne favorisée par la grande énergie de liaison de l’exciton dans les nanoplaquettes. Des nanoplaquettes de CdSe dopées à l’argent ont été synthétisées dans un second temps. Le dopage, fait par échange cationique partiel, créé des pièges émissifs dans la largeur de bande interdite. La quantité d’argent incorporée permet un contrôle progressif de la couleur d’émission du vert au rouge grâce à la coexistence de la recombinaison de bord de bande et de l’émission liée à l’argent. Ces niveaux d’argent sont fortement liés à CdSe et se situent 340 meV au-dessus de la bande de valence du CdSe massif. Enfin, la modification de chimie de surface par un mélange de bromures et d’oleylamine diminue les contraintes de surface et améliore la passivation de surface. Ceci induit un décalage, au-delà du confinement discret, de la longueur d’onde d’émission et une augmentation du rendement quantique jusqu’à 75%.

The gating of nanocrystal films is currently driven by two approaches: either the use of a dielectric such as SiO2 or the use of electrolyte. SiO2 allows fast bias sweeping over a broad range of temperatures but requires a large operating bias. Electrolytes, thanks to large capacitances, lead to the significant reduction of operating bias but are limited to slow and quasi-room-temperature operation. None of these operating conditions are optimal for narrow-band-gap nanocrystal-based phototransistors, for which the necessary large-capacitance gate has to be combined with low-temperature operation. Here, we explore the use of a LaF3 ionic glass as a high-capacitance gating alternative. We demonstrate for the first time the use of such ionic glasses to gate thin films made of HgTe and PbS nanocrystals. This gating strategy allows operation in the 180 to 300 K range of temperatures with capacitance as high as 1 μF·cm-2. We unveil the unique property of ionic glass gate to enable the unprecedented tunability of both magnitude and dynamics of the photocurrent thanks to high charge-doping capability within an operating temperature window relevant for infrared photodetection. We demonstrate that by carefully choosing the operating gate bias, the signal-to-noise ratio can be improved by a factor of 100 and the time response accelerated by a factor of 6. Moreover, the good transparency of LaF3 substrate allows back-side illumination in the infrared range, which is highly valuable for the design of phototransistors.

International audience; Zinc blende II-VI semiconductor nanoplatelets (NPLs) are defined at the atomic scale along the thickness of the nanoparticle and are initially capped with carboxylates on the top and bottom [001] facets. These ligands are exchanged on CdSe NPLs with halides that act as X-L-type ligands. These CdSe NPLs are costabilized by amines to provide colloidal stability in nonpolar solvents. The hydrogen from the amine can participate in a hydrogen bond with the lone pair electrons of surface halides. After ligand exchange, the optical features are redshifted. Thus, ligand tuning is another way, in addition to confinement, to tune the optical features of NPLs. The improved surface passivation leads to an increase in the fluorescence quantum efficiency of up to 70% in the case of bromide. However, for chloride and iodide, the surface coverage is incomplete, and thus, the fluorescence quantum efficiency is lower. This ligand exchange is associated with a decrease in stress that leads to unfolding of the NPLs, which is particularly noticeable for iodide-capped NPLs.

Wavefunction engineering using intraband transition is the most versatile strategy for the design of infrared devices. To date, this strategy is nevertheless limited to epitaxially grown semiconductors, which lead to prohibitive costs for many applications. Meanwhile, colloidal nanocrystals have gained a high level of maturity from a material perspective and now achieve a broad spectral tunability. Here, we demonstrate that the energy landscape of quantum well and quantum dot infrared photodetectors can be mimicked from a mixture of mercury selenide and mercury telluride nanocrystals. This metamaterial combines intraband absorption with enhanced transport properties (i.e. low dark current, fast time response and large thermal activation energy). We also integrate this material into a photodiode with the highest infrared detection performances reported for an intraband-based nanocrystal device. This work demonstrates that the concept of wavefunction engineering at the device scale can now be applied for the design of complex colloidal nanocrystal-based devices., The field of wavefunction engineering using intraband transition to design infrared devices has been limited to epitaxially grown semiconductors. Here the authors demonstrate that a device with similar energy landscape can be obtained from a mixture of colloidal quantum dots made of HgTe and HgSe.

The use of intraband transition is an interesting alternative path for the design of optically active complex colloidal materials in the mid-infrared range. However, so far, the performance obtained for photodetection based on intraband transition remains much smaller than the one relying on interband transition in narrow-band-gap materials operating at the same wavelength. New strategies have to be developed to make intraband materials more effective. Here, we propose growing a heterostructure of HgSe/HgTe as a means of achieving enhanced intraband-based photoconduction. We first tackle the synthetic challenge of growing a heterostructure on soft (Hg-based) material. The electronic spectrum of the grown heterostructure is then investigated using a combination of numerical simulation, infrared spectroscopy, transport measurement, and photoemission. We report a type-II band alignment with reduced doping compared with a core-only object and boosted hole conduction. Finally, we probe the photoconductive properties of the heterostructure while resonantly exciting the intraband transition by using a high-power-density quantum cascade laser. Compared to the previous generation of material based on core-only HgSe, the heterostructures have a lower dark current, stronger temperature dependence, faster photoresponse (with a time response below 50 μs), and detectivity increased by a factor of 30.

International audience; Among semiconductor nanocrystals (NCs), 2D nanoplatelets (NPLs) are a special class of nanomaterials with well controlled optical features. So far most of the efforts have been focused on wide band gap materials such as cadmium chalcogenide semiconductors. However, optical absorption can be pushed toward the Infra-Red (IR) range using narrow band gap materials such as mercury chalcogenides. Here we demonstrate the feasibility of a core/shell structure made of a CdSe core with two HgSe external wells. We demonstrate that the optical spectrum of the heterostructure is set by the HgSe wells and this, despite the quasi type II band alignment which makes the band edge energy independent of the inner core thickness. On the other hand, these core/shell NPLs behave, from a transport point of view, as a wide band gap material. We demonstrate that the introduction of a wide band gap CdSe core makes the material less conductive and with a larger photoresponse. Hence the heterostructure presents an effective electric band gap wider than the optical band gap. This strategy will be of utmost interest to design infrared effective colloidal materials for which the reduction of the carrier density and the associated dark current is a critical property.

International audience; Colloidal quantum dots (QDs) are now being used as the new generation of phosphor for displays. White light is obtained by combining the blue emission of a diode with green and red population of QDs. Having bicolor emission within a single nanocrystal population can be even more convenient. Here, we demonstrate that bicolor emission can be obtained from 2D nanoplatelets (NPLs) with a core/crown geometry. In CdSe/CdTe NPLs with type II band alignment, only the charge transfer emission was observed so far due to the very fast (

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

International audience; HgTe nanocrystals are extremely interesting materials to obtain a highly tunable absorption spectrum in the infrared range. Here, we discuss the two extreme cases of strongly confined and barely confined HgTe nanocrystals. We discuss the synthesis and optoelectronic properties of HgTe 2D nanoplatelets where the confinement energy can be as large as 1.5 eV. This material presents enhanced (mostly narrower) light emitting properties compared to spherical nanocrystals emitting at the same wavelength. Moreover, absorption spectra, majority carriers and time response can be tuned by carefully choosing the surface chemistry and applying a well-chosen gate bias. HgTe can also be used to explore the effect of vanishing confinement and to obtain quasi bulk properties with tunable absorption in the THz, up to 150 µm.