Search

International audience; As the field of nanocrystal-based optoelectronics matures, more advanced techniques must be developed in order to reveal the electronic structure of nanocrystals, particularly with devicerelevant conditions. So far, most of the efforts have been focused on optical spectroscopy, and electrochemistry where an absolute energy reference is required. Device optimization requires probing not only the pristine material but also the material in its actual environment (i.e., surrounded by a transport layer and an electrode, in the presence of an applied electric field). Here, we explored the use of photoemission microscopy as a strategy for operando investigation of NC-based devices. We demonstrate that the method can be applied to a variety of materials and device geometries. Finally, we show that it provides a direct access to the metal-semiconductor interface band bending as well as the distance over which the gate effect propagates in field-effect transistors.

International audience; After the use of nanocrystals as light down-converters, infrared sensing appears to be one of the first market applications where they can be used while being both electrically and optically active. Over recent years, tremendous progress has been achieved, leading to an apparent rise in the technological readiness level (TRL). So far, the efforts have been focused on PbS nanocrystals for operation in the near-infrared. Here, we focus on HgTe since its narrower band gap offers more flexibility to explore the extended shortwave and mid-wave infrared. We report a photoconductive strategy fort the design of short wave infrared focal plane array with enhanced image quality. An important aspect often swept under the rug at an early stage is the material stability. It appears that HgTe remains mostly unaffected by oxidation under air operation. The evaporation of Hg, a potentially dramatic aging process, only occurs at temperatures far beyond the focal plane array's standard working temperature. The main bottleneck appears to be the particle sintering resulting from joule heating of focal plane array. This suggests that a cooling system is required, with a first role of preventing the material from sintering even before targeting dark current reduction.

International audience; Integration of photonic structures in nanocrystal-based photodetectors has been demonstrated to improve device performances. Furthermore, bias-dependent photoresponse can be observed in such devices as a result of the interplay between hopping transport and inhomogeneous electromagnetic field. Here, we investigate the main physical concepts leading to a voltage-dependent photoresponse. We first bring evidence of bias-dependent carrier mobilities in a nanocrystal array over a wide range of temperatures. Then, we realize an infrared sensing device using HgTe nanocrystals, where the electrodes also play the role of a grating, inducing a spatially inhomogeneous absorption. The obtained device exhibits a significant bias-dependent photoresponse while possessing a competitive detection performance in the extended shortwave and mid-wave infrared, with detectivity reaching 7x10 10 Jones at 80 K and a fast response time of around 70 ns. This work provides the foundation for further advancements in nanocrystal-basedactive photonics devices.

International audience; Visible nanocrystal-based light-emitting diodes (LEDs) are about to become commercially available. However, their infrared counterparts suffer from two key limitations. First, III–V semiconductor technologies are strong competitors. Second, their potential for operation beyond 1.7 µm remains unexplored. The range from 1.5 to 4 µm corresponds to a technological gap in which the efficiency of interband quantum-well-based devices vanishes and quantum cascade lasers are not efficient enough. Powerful infrared LEDs in this range are needed for applications such as active imaging, organic molecule sensing and airfield lighting. Here we report the design of a HgTe nanocrystal-based LED with luminescence between 2 and 2.3 µm. With an external quantum efficiency of 0.3% and radiance up to 3 W Sr−1 m−2, these HgTe LEDs already present a competitive performance for emission above 2 µm.

International audience; While the integration of nanocrystals as an active medium for optoelectronic devices progresses, light management strategies become required. Over recent years, several photonic structures (plasmons, cavities, mirrors, etc.) have been coupled to nanocrystal films to shape the absorption spectrum, tune the directionality, and so on. Here, we explore a photonic equivalent of the acoustic Helmholtz resonator and propose a design that can easily be fabricated. This geometry combines a strong electromagnetic field magnification and a narrow channel width compatible with efficient charge conduction in spite of hopping conduction. At 80 K, the device reaches a responsivity above 1 A•W-1 and detectivity above 10 11 Jones (3 µm cutoff) while offering a significantly faster time-response than vertical geometry diodes.

International audience; Nanocrystals (NCs) are one of the few nanotechnologies to have attained mass market applications with their use as light sources for displays. This success relies on Cd-and In-based wide bandgap materials. NCs are likely to be employed in more applications as they provide a versatile platform for optoelectronics, specifically, infrared optoelectronics. The existing material technologies in this range of wavelengths are generally not cost effective, which limits the spread of technologies beyond a few niche domains, such as defense and astronomy. Among the potential candidates to address the infrared window, mercury chalcogenide (HgX) NCs exhibit the highest potential in terms of performance. In this review, we discuss how material developments have facilitated device enhancements. Because such NCs are primarily used because of their infrared optical features, we first review the strategies for the associated colloidal growth and electronic structure. The review is organized considering three main device-related applications: light emission, electronic transport and infrared photodetection.

International audience; Mercury telluride (HgTe) nanocrystals are among of the most versatile infrared (IR) materials with the absorption of first optical absorption which can be tuned from visible to the THz range. Therefore, they have been extensively considered as near IR emitters and as absorbers for low-cost IR detectors. However, the electroluminescence of HgTe remains poorly investigated in spite of its ability to go toward longer wavelengths compared to traditional lead sulfide (PbS). Here, we demonstrate a light emitting diode (LED) based on an indium tin oxide (ITO)/zinc oxide (ZnO)/ZnO-HgTe/PbS/gold stacked structure, where the emitting layer consists of a ZnO/HgTe bulk heterojunction which drives the charge balance in the system. This LED has low turn-on voltage, long lifetime, and high brightness. Finally, we conduct short wavelength infrared (SWIR) active imaging, where illumination is obtained from a HgTe NC-based LED, and demonstrate moisture detection.

Despite broad applications in imaging, energy conversion, and telecommunications, few nanoscale moieties emit light efficiently in the shortwave infrared (SWIR, 1000-2000 nm or 1.24-0.62 eV). We report quantum-confined mercury chalcogenide (HgX, where X = Se or Te) nanoplatelets (NPLs) can be induced to emit bright (QY30%) and tunable (900-1500+ nm) infrared emission from attached quantum dot (QD) "defect" states. We demonstrate near unity energy transfer from NPL to these QDs, which completely quench NPL emission and emit with a high QY through the SWIR. This QD defect emission is kinetically tunable, enabling controlled midgap emission from NPLs. Spectrally resolved photoluminescence demonstrates energy-dependent lifetimes, with radiative rates 10-20 times faster than those of their PbX analogues in the same spectral window. Coupled with their high quantum yield, midgap emission HgX dots on HgX NPLs provide a potential platform for novel optoelectronics in the SWIR.

Nanocrystals (NCs) are now established building blocks for optoelectronics and their use as down converters for large gamut displays has been their first mass market. NC integration relies on a combination of green and red NCs into a blend, which rises post-growth formulation issues. A careful engineering of the NCs may enable dual emissions from a single NC population which violates Kasha’s rule, which stipulates that emission should occur at the band edge. Thus, in addition to an attentive control of band alignment to obtain green and red signals, non-radiative decay paths also have to be carefully slowed down to enable emission away from the ground state. Here, we demonstrate that core/crown/crown 2D nanoplatelets (NPLs), made of CdSe/CdTe/CdSe, can combine a large volume and a type-II band alignment enabling simultaneously red and narrow green emissions. Moreover, we demonstrate that the ratio of the two emissions can be tuned by the incident power, which results in a saturation of the red emission due to non-radiative Auger recombination that affects this emission much stronger than the green one. Finally, we also show that dual-color, power tunable, emission can be obtained through an electrical excitation.

Optical antennas have become ubiquitous tools to enhance the spontaneous emission of atoms, molecules and quantum dots. In this presentation, we report a series of experimental results investigating the emission of light by ensembles of interacting emitters coupled to resonators. First, we report the observation of a strong plasmon−exciton coupling regime in a system consisting of a layer of nanoplatelets on top of a gold planar surface. Reflectometry measurements and mode analysis lead to the non-ambiguous derivation of a Rabi splitting between two polaritonic branches. Secondly, we investigate the polarized and directional emission of light by a patterned layer of nanoplatelets optically pumped. Models based on the paradigm of the Purcell effect mediated radiation fail to fully explain spectral and spatial features observed in such experiments, such as the emergence of spatial coherence or the suppression of quenching. We discuss and highlight the differences between emission by a single emitter and by a thermalized assembly of quantum emitters to show that a statistical framework is required to understand their interactions with optical antennas. Based on these considerations, we introduce a model of light emission by thermalized ensembles of emitters, and find good agreement between our model and experimental data.

CdSe/CdTe core-crown type-II nanoplatelet heterostructures are two-dimensional semiconductors that have attracted interest for use in light-emitting technologies due to their ease of fabrication, outstanding emission yields, and tunable properties. Despite this, the exciton dynamics of these complex materials, and in particular how they are influenced by phonons, is not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and temperature-dependent structural measurements to investigate CdSe/CdTe nanoplatelets with a thickness of four monolayers. We show that charge-transfer (CT) excitons across the CdSe/CdTe interface are formed on two distinct time scales: initially from an ultrafast (∼70 fs) electron transfer and then on longer time scales (∼5 ps) from the diffusion of domain excitons to the interface. We find that the CT excitons are influenced by an interfacial phonon mode at ∼120 cm-1, which localizes them to the interface. Using low-temperature PL spectroscopy we reveal that this same phonon mode is the dominant mechanism in broadening the CT PL. On cooling to 4 K, the total PL quantum yield reaches close to unity, with an ∼85% contribution from CT emission and the remainder from an emissive sub-band-gap state. At room temperature, incomplete diffusion of domain excitons to the interface and scattering between CT excitons and phonons limit the PL quantum yield to ∼50%. Our results provide a detailed picture of the nature of exciton-phonon interactions at the interfaces of 2D heterostructures and explain both the broad shape of the CT PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that to maximize the PL quantum yield both improved engineering of the interfacial crystal structure and diffusion of domain excitons to the interface, e.g., by altering the relative core/crown size, are required.

Two dimensional ultra thin CdSe nanoplatelets have attracted a large interest due to their optical properties but their formation mechanism is not well understood. Several different mechanisms have been proposed: confined growth in a surfactant mesophase acting as a template, anisotropic ripening of small seeds into 2D nanoplatelets or continuous anisotropic growth of a limited number of nuclei. However, quantitative in situ data that could validate or disprove these formation scenarios are lacking. We use synchrotron-based small-angle and wide-angle X-ray scattering to probe the formation mechanism of CdSe nanoplatelets synthesized using a heating-up method. We prove the absence of a molecular mesophase in the reactive medium at the onset of nanoplatelet formation ruling out a templating effect. We also show that our data are inconsistent with the anisotropic ripening of small seeds whereas the evolution of the SAXS patterns during the reaction is consistent with the continuous lateral growth of nanoplatelets fed by reactive monomers. Finally, we show that when the final temperature of the synthesis is lowered, nanoplatelets with larger lateral dimensions form. We reveal that they bend in solution during their growth to yield nanoscrolls.

Two dimensional ultra thin CdSe nanoplatelets have attracted a large interest due to their optical properties but their formation mechanism is not yet well understood. Several different mechanisms and models have been proposed but quantitative in situ data that could validate or disprove them are lacking. We use synchrotron-based small-angle and wide-angle X-ray scattering to probe in situ the formation mechanism of CdSe nanoplatelets synthesized using a heating-up method. We prove the absence of a molecular mesophases in the reactive medium at the onset of nanoplatelet formation ruling out a templating effect. A q-2 regime is observed from the start of the reaction which extends towards smaller wave vectors with time, consistent with the continuous lateral growth of nanoplatelets from a limited number of seeds fed by reactive monomers. A ripening mechanism where small cluster fuse to yield nanoplatelets can also be ruled out by our data. When the final temperature is lowered, larger nanoplatelets are obtained and the SAXS patterns exhibit marked oscillations due to their rolling into curved nanoscrolls. Our experiments thus show that nanoplatelet curvature appears during their synthesis.

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.

International audience; Germanane is a two-dimensional, strongly confined form of germanium. It presents an interesting combination of (i) ease of integration with CMOS technology, (ii) low toxicity, and (iii) electronic confinement which transforms the indirect bandgap of the bulk material into a direct bandgap featuring photoluminescence. However, the optoelectronic properties of this material remain far less investigated than its structural properties. Here, we investigate the photoluminescence and transport properties of arrays of methyl-terminated germanane flakes. The photoluminescence appears to have two contributions, one from the band edge and the other from trap states. The dynamics of the exciton appear to be in the range of 1–100 ns. Conduction in this material appears to be p-type, while the photoconduction time response can be made as short as 100 μs.

International audience; Two-dimensional colloidal nanoplatelets (NPLs), owing to the atomic-level control of their confined direction (i.e., no inhomogeneous broadening), have demonstrated improved photoluminescence (PL) line widths for cadmium chalcogenide-based nanocrystals. Here we use cation exchange to synthesize mercury chalcogenide NPLs. Appropriate control of reaction kinetics enables the 2D morphology of the NPLs to be maintained during the cation exchange. HgTe and HgSe NPLs have significantly improved optical features compared to existing materials with similar band gaps. The PL line width of HgTe NPLs (40 nm full width at half-maximum, centered at 880 nm) is a factor of 2 smaller than typical PbS nanocrystals (NCs) emitting at the same wavelength. The PL has a lifetime of 50 ns, almost 2 orders of magnitude shorter than small PbS colloidal quantum dots (CQDs), and a quantum yield of ∼10%, almost 2 orders of magnitude shorter than small PbS colloidal quantum dots (CQDs). These materials are promising for a large variety of applications spanning from telecommunications to the design of colloidal topological insulators.

International audience; We explore the gold functionalization of 2D CdSe nanoplatelets (NPL) as a possible way to tune their electronic and transport properties. We demonstrate that the size and location of the gold tip can be controlled using light and temperature. The Au tip–CdSe NPL hybrid presents a large rise of the conductance compared to the pristine semiconductor (i.e., without gold functionalization). The role of the semiconductor in this transport remains unclear and needs to be better understood. We hypothesize four mechanisms: (i) a reduction of the band gap energy due to the formation of a gold–selenium compound, (ii) a charge transfer between the metal and the semiconductor leading to an increase in carrier concentration, (iii) a change in the inter-nanoparticle tunnel barrier height, and (iv) a simple percolation process between the metallic grain. X-ray photoelectron spectroscopy (XPS) shows that the CdSe NPL are unaffected by oxidation and that gold is in the metallic state Au0. We consequently exclude the formation of a narrow band gap Au2Se phase as the possible mechanism leading to the observed rise of conductance. Moreover, Kelvin probe force microscopy and XPS give evidence for an increase in work function upon gold tipping, which can be interpreted in terms of a shift of the Fermi level toward the valence band maximum. As hole conduction in CdSe NPLs is very unlikely to occur, we rather favor the hypothesis that the strong increase in conduction is largely driven by percolation between the metallic tips as the main mechanism responsible for transport in this hybrid system.

Colloidal nanocrystals are promising materials for achieving low cost optoelectronic devices. In this paper, we focus on the transport and photo transport properties of 2D nanoplatelet thin films and their use for photodetection. We present evidence that improved performances relies on good trap passivation as well as overcoming the inherent large exciton binding energy of the 2D NPL. This can be achieved using a phototransistor configuration with transport at the single particle scale (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Colloidal 2D nanoplatelets (NPLs) are a class of nanoparticles that offer the possibility of forming two types of heterostructures, by growing either in the confined direction or perpendicular to the confined direction, called core/crown NPLs. Here, we demonstrate that bicolor emission can be obtained from 2D NPLs with a core/crown geometry. To date, for CdSe/CdTe NPLs with type-II band alignment, only charge transfer emission has been observed due to the very fast (

International audience; Among the colloidal quantum dots, 2D nanoplatelets present exceptionally narrow optical features. Rationalizing the design of heterostructures of these objects is of utmost interest; however, very little work has been focused on the investigation of their electronic properties. This work is organized into two main parts. In the first part, we use 1D solving of the Schrödinger equation to extract the effective masses for nanoplatelets (NPLs) of CdSe, CdS, and CdTe and the valence band offset for NPL core/shell of CdSe/CdS. In the second part, using the determined parameters, we quantize how the spectra of the CdSe/CdS heterostructure get affected by (i) the application of an electric field and (ii) by the presence of a dull interface. We also propose design strategies to make the heterostructure even more robust.

CONSPECTUS: Semiconductors are at the basis of electronics. Up to now, most devices that contain semiconductors use materials obtained from a top down approach with semiconductors grown by molecular beam epitaxy or chemical vapor deposition. Colloidal semiconductor nanoparticles have been synthesized for more than 30 years now, and their synthesis is becoming mature enough that these nanoparticles have started to be incorporated into devices. An important development that recently took place in the field of colloidal quantum dots is the synthesis of two-dimensional (2D) semiconductor nanoplatelets that appear as free-standing nanosheets. These 2D colloidal systems are the newborn in the family of shaped-controlled nanoparticles that started with spheres, was extended with rods and wires, continued with tetrapods, and now ends with platelets. From a physical point of view, these objects bring 1D-confined particles into the colloidal family. It is a notable addition, since these platelets can have a thickness that is controlled with atomic precision, so that no inhomogeneous broadening is observed. Because they have two large free interfaces, mirror charges play an important role, and the binding energy of the exciton is extremely large. These two effects almost perfectly compensate each other, it results in particles with unique spectroscopic properties such as fast fluorescent lifetimes and extreme color purity (narrow full width at half-maximum of their emission spectra). These nanoplatelets with extremely large confinement but very simple and well-defined chemistry are model systems to check and further develop, notably with the incorporation in the models of the organic/inorganic interface, various theoretical approaches used for colloidal particles. From a chemical point of view, these colloidal particles are a model system to study the role of ligands since they have precisely defined facets. In addition, the synthesis of these highly anisotropic objects triggered new research to understand at a mechanistic level how this strong anisotropy could be generated. Luckily, some of the chemical know-how built with the spherical and rod-shaped particles is being transferred, with some adaptation, to 2D systems, so that 2D core/shell and core/crown heterostructures have recently been introduced. These objects are very interesting because they suggest that multiple quantum wells could be grown in solution. From the application point of view, 2D colloidal nanoplatelets offer interesting perspectives when color purity, charge conductivity, or field tunable absorption are required. In this Account, we review the chemical synthesis, the physical properties, and the applications of colloidal semiconductor nanoplatelets with an emphasis on the zinc-blende nanoplatelets that were developed more specifically in our group.

We investigate the potential use of colloidal nanoplates of Sb2Te3 by conducting transport on single particle with in mind their potential use as 3D topological insulator material. We develop a synthetic procedure for the growth of plates with large lateral extension and probe their infrared optical and transport properties. These two properties are used as probe for the determination of the bulk carrier density and agree on a value in the 2–3 × 1019 cm−3 range. Such value is compatible with the metallic side of the Mott criterion which is also confirmed by the weak thermal dependence of the conductance. By investigating the transport at the single particle level we demonstrate that the hole mobility in this system is around 40 cm2V−1s−1. For the bulk material mixing n-type Bi2Te3 with the p-type Sb2Te3 has been a successful way to control the carrier density. Here we apply this approach to the case of colloidally obtained nanoplates by growing a core-shell heterostructure of Sb2Te3/Bi2Te3 and demonstrates a reduction of the carrier density by a factor 2.5.

In this article we study the ultrafast dynamics of excitons and charge carriers photogenerated in two-dimensional in-plane heterostructures, namely, CdSe-CdTe nanoplatelets. We combine transient absorption and two-dimensional electronic spectroscopy to study charge transfer and delocalization from a few tens of femtoseconds to several nanoseconds. In contrast with spherical nanocrystals, the relative alignment of the electron and hole states of CdSe and CdTe in thin 2D nanoplatelets does not lead to a type-II heterostructure. Following the excitation in CdSe or CdTe materials, the electron preferentially delocalises instantaneously over the whole heterostructure. In addition, depending on the crown material (CdTe versus CdTeSe), the hole transfers either to trap states or to the crown, within a few hundreds of femtoseconds. We conclude that the photoluminescence band, at lower energy than the CdSe and CdTe first exciton transition, does not result from the recombination of the charge carriers at the charge transfer state but involves localised hole states.

International audience; We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material,. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping of the HgTe nanoplatelets by S 2-ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 µs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100 ns to 500 ns range, which suggests that the current performances are rather limited by geometrical factors.

Quasi-two dimensional CdSe nanoplatelets with a well-controlled thickness exhibit several advantageous properties for optical and opto-electronic application, such as in quantum dot sensitized solar cells. Due to the quantum confinement effects arising from their thickness of typically a few nanometers, the excitonic and charge carrier properties of these nanoobjects can be easily tuned by varying the number of monolayers they are composed of and the passivating ligands adsorbed on their surface. We have performed a density functional theory (DFT) investigation of the geometrical and electronic properties of non-stoichiometric CdSe zinc blende nanoplatelets with different thicknesses in the (100) direction, stabilized by various organic (HCOO−) and inorganic (SH− and OH−) ligands. The relaxation parameters and adsorption energies of the studied ligands on the polar zinc blende (100) surface have been calculated, along with the band gaps, band structures, density of states and a detailed Mulliken charge analysis of these hybrid nanostructures. The latter revealed a major electron transfer from the SH− ligand towards the surface of the nanocrystals, in line with what is observed from the orbital-projected density of states. CdSe zinc blende nanoplatelets of various thicknesses, stabilized by fatty acids, SH− and OH− ligands have also been synthesized, and their band gaps have been measured by absorption spectroscopy. A good agreement is found between the experimental and calculated values, especially for the evolution of the band gaps with the thickness of the nanoplatelets. Taken all together, the established theoretical model and computational approach can potentially serve as a powerful tool to provide a qualitative and quantitative description of the geometrical and electronic properties of quasi-two dimensional nonstoichiometric polar inorganic semiconductor materials, at low computational cost.

Free-standing two-dimensional nanostructures, which are referenced as nanoplates, nanomembranes, or nanosheets, are at the center of intense research and development. These structures can be metallic, dielectric, or semiconductor and have a controlled and a uniform thickness that is small compared to their lateral dimensions. Some of these structures are grown in solution and emerge as a novel class of colloidal material with physical properties close to the quantum wells and a chemistry similar to that developed for the colloidal quantum dots. They can be easily manipulated, and their surface chemistry can be tuned with methods similar to those developed for the colloidal nanocrystals. Here, we review the syntheses in solution, the properties, and the first applications of these flat colloidal semiconductor nanostructures.

We show that colloidal nanoplatelets can self-assemble to form a 1D superlattice. When self-assembled, an additional emission line appears in the photoluminescence spectrum at low temperatures. This emission line is a collective effect, greatly enhanced when the NPLs are self-assembled. It is attributed to the longitudinal optical (LO) phonon replica of the band-edge exciton, and its presence in self-assembled nanoplatelets is explained using a model based on an efficient photons reabsorption between neighboring nanoplatelets. The presence of phonon replica at low temperatures in ensemble measurements suggests the possibility to design a laser, based on self-assembled nanoplatelets.

We report the continuous lateral extension of cadmium selenide nanoplatelets into nanosheets using continuous injection of precursors at high temperature. We show that we can obtain CdSe nanosheets with lateral dimensions up to 700 nm and a well-defined thickness that can be tuned with atomic precision. When the nanosheets’ lateral size increases, they roll on themselves to form nanoscrolls that can unroll upon surface modification. The final geometry of the nanosheets can be tuned to different morphologies using precursors that favor the growth of specific crystal facets. We provide a detailed study of the CdSe nanosheets growth and its optimization for three different thicknesses.

International audience; Self-doped nanocrystals raise great interest for infrared (IR) optoelectronics because their optical properties span from near to far IR. However, their integration for photodetection requires a fine understanding of the origin of their doping and also a way to control the magnitude of the doping. In this paper, we demonstrate that a fine control of the doping level between 0.1 and 2 electrons per dot is obtained through ligand exchange. The latter affects not only the interparticle coupling but also their optical properties because of the band-shift resulting from the presence of surface dipoles. We demonstrate that self-doping is a bulk process and that surface dipoles can control its magnitude. We additionally propose a model to quantify the dipole involved with each ligand. We eventually use the ligand design rule previously evidenced to build a near-infrared photodetector on a soft and transparent substrate. The latter significantly improves the performance compared to previously reported colloidal quantum dot-based photodetectors on plastic substrates operated in the telecom wavelength range.

High-mobility solution-processed all-inorganic solid state nanocrystal (NC) transistors with low operation voltage and near-zero hysteresis are demonstrated using high-capacitance ZrO(x) and hydroxyl-free Cytop gate dielectric materials. The use of inorganic capping ligands (In(2)Se(4)(2-) and S(2-)) allowed us to achieve high electron mobility in the arrays of solution-processed CdSe nanocrystals. We also studied the hysteresis behavior and switching speed of NC-based field effect devices. Collectively, these analyses helped to understand the charge transport and trapping mechanisms in all-inorganic NCs arrays. Finally, we have examined the rapid thermal annealing as an approach toward high-performance solution-processed NCs-based devices and demonstrated transistor operation with mobility above 30 cm(2)/(V s) without compromising low operation voltage and hysteresis.

The syntheses of strongly anisotropic nanocrystals with one dimension much smaller than the two others, such as nanoplatelets, are still greatly underdeveloped. Here, we demonstrate the formation of atomically flat quasi-two-dimensional colloidal CdSe, CdS and CdTe nanoplatelets with well-defined thicknesses ranging from 4 to 11 monolayers. These nanoplatelets have the electronic properties of two-dimensional quantum wells formed by molecular beam epitaxy, and their thickness-dependent absorption and emission spectra are described very well within an eight-band Pidgeon-Brown model. They present an extremely narrow emission spectrum with full-width at half-maximum less than 40 meV at room temperature. The radiative fluorescent lifetime measured in CdSe nanoplatelets decreases with temperature, reaching 1 ns at 6 K, two orders of magnitude less than for spherical CdSe nanoparticles. This makes the nanoplatelets the fastest colloidal fluorescent emitters and strongly suggests that they show a giant oscillator strength transition.

International audience; We discuss the transport properties of CsPbBrxI3x perovskite nanocrystal arrays as a modelensemble system of caesium lead halide-based perovskite nanocrystal arrays. While this material isvery promising for the design of light emitting diodes, laser, and solar cells, very little work hasbeen devoted to the basic understanding of their (photo)conductive properties in an ensemblesystem. By combining DC and time-resolved photocurrent measurements, we demonstrate fastphotodetection with time response below 2 ns. The photocurrent generation in perovskitenanocrystal-based arrays is limited by fast bimolecular recombination of the material, which limitsthe lifetime of the photogenerated electron-hole pairs. We propose to use nanotrench electrodes asa strategy to ensure that the device size fits within the obtained diffusion length of the material inorder to boost the transport efficiency and thus observe an enhancement of the photoresponse by afactor of 1000.

Quantum wells (QWs) are thin semiconductor layers than confine electrons and holes in one dimension. They are widely used for optoelectronic devices, particularly semiconductor lasers, but have so far been produced using expensive epitaxial crystal-growth techniques. This has motivated research into the use of colloidal semiconductor nanocrystals, which can be synthesized chemically at low cost, and can be processed in the solution phase. However, initial demonstrations of optical gain from colloidal nanocrystals involved high thresholds. Recently, colloidal synthesis methods have been developed for the production of thin, atomically flat semiconductor nanocrystals, known as nanoplatelets (NPLs). We investigated relaxation of high-energy carriers in colloidal CdSe NPLs, and found that the relaxation is characteristic of a QW system. Carrier cooling and relaxation on time scales from picoseconds to hundreds of picoseconds are dominated by Auger-type exciton-exciton interactions. The picosecond-scale cooling of hot carriers is much faster than the exciton recombination rate, as required for use of these NPLs as optical gain and lasing materials. We therefore investigated amplified spontaneous emission using close-packed films of NPLs. We observed thresholds that were more than 4 times lower than the best reported value for colloidal nanocrystals. Moreover, gain in these films is 4 times higher than gain reported for other colloidal nanocrystals, and saturates at pump fluences more than two orders of magnitude above the ASE threshold. We attribute this exceptional performance to large optical cross-sections, relatively slow Auger recombination rates, and narrow ensemble emission linewidths.

Enhanced plasmonic fields are a promising way to increase the efficiency of photocatalytic water splitting. The availability of atomically thin materials opens up completely new opportunities. We report photocatalytic water splitting on ultrathin CdSe nanoplatelets placed in plasmonic nanogaps formed by a flat gold surface and a gold nanoparticle. The extreme field intensity created in these gaps increases the electron–hole pair production in the CdSe nanoplatelets and enhances the plasmon-mediated interfacial electron transfer. Compared to individual nanoparticles commonly used to enhance photocatalytic processes, gap-plasmons produce several orders of magnitude higher field enhancement, strongly localized inside the semiconductor sheet thus utilizing the entire photocatalyst efficiently.

We have synthesized atomically flat CdSe/CdTe core/crown nanoplatelets (NPLs) with thicknesses of 3, 4, and 5 monolayers with fine control of the crown lateral dimensions. In these type-II NPLs, the charges separate spatially, and the electron wave function is localized in the CdSe core while the hole wave function is confined in the CdTe crown. The exciton’s recombination occurs across the heterointerface, and as a result of their spatially indirect band gap, an important emission red shift up to the near-infrared region (730 nm) is observed with long fluorescence lifetimes that range from 30 to 860 ns, depending on the type of interface between the core and the crown. These type-II NPLs have a high quantum yield of 50% that can be further improved to 70% with a gradient interface. We have characterized these novel CdSe/CdTe core/crown NPLs using UV–vis, emission, and excitation spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy.

International audience; The optical and optoelectronic properties of colloidal quantum dots strongly depend on the passivation of their surface. Surface states are however difficult to quantify using optical spectroscopy and techniques based on back gated field effect transistors are limited in the range of carrier density that can be probed, usually significantly below one charge carrier per particle. Here we show that electrolyte gating can be used to quantitatively analyze the increase of defects in a population of nanoparticles with increasing surface irregularities. We illustrate this method using CdSe nanoplatelets that are grown in their thickness using low temperature layer-by-layer method. Spectroscopic analysis of the samples confirm that the nanoplatelet thickness is controlled, on average, with atomic precision, but structural analysis with transmission electron microscopy shows that the number of surface defects increases with the nanoplatelet thickness. The amount of charge defects is probed quantitatively using electrolyte-gated field effect transistor (EFET). We observe that the threshold voltage of the EFET increases with the NPL thickness, in agreement with the structural analysis. All samples displayed n-type conduction with strong current modulation (subthreshold swing slope of 100 mV/decade and on/off ratio close to 107). We also point out that an efficient electrolyte gating of the film requires a fine control of the nanoparticle film morphology.

International audience; CdSe nanoplatelets show perfectly quantized thicknesses of few monolayers. They present a situation of extreme, yet well de ned quantum con nement. Due to large dielectric contrast between the semiconductor and its ligand environment, interaction between carriers and their dielectric images strongly renormalize bare single particle states. We discuss the electronic properties of this original system in an advanced tight-binding model, and show that Coulomb interactions, including self-energy corrections and enhanced electron-hole interaction, lead to exciton binding energies up to several hundred meVs.

We present the synthesis and the optical properties of a new type of two-dimensional heterostructure: core/crown CdSe/CdS nanoplatelets. They consist of CdSe nanoplatelets that are extended laterally with CdS. Both the CdSe core and the CdS crown dimensions can be controlled. Their thickness is controlled at the monolayer level. These novel nanoplatelet-based heterostructures have spectroscopic properties that can be similar to nanoplatelets or closer to quantum dots, depending on the CdSe core lateral size.

It has recently become possible to chemically synthesize atomically flat semiconductor nanoplatelets with monolayer-precision control over the platelet thickness. It has been suggested that these platelets are quantum wells; that is, carriers in these platelets are confined in one dimension but are free to move in the other two dimensions. Here, we report time-resolved photoluminescence and transient-absorption measurements of carrier relaxation that confirm the quantum-well nature of these nanomaterials. Excitation of the nanoplatelets by an intense laser pulse results in the formation of a high-temperature carrier population that cools back down to ambient temperature on the time scale of several picoseconds. The rapid carrier cooling indicates that the platelets are well-suited for optoelectronic applications such as lasers and modulators.

Known manipulations of semiconductor thermal transport properties rely upon higher-order material organization. Here, using time-resolved optical signatures of phonon transport, we demonstrate a "bottom-up" means of controlling thermal outflow in matrix-embedded semiconductor nanocrystals. Growth of an electronically noninteracting ZnS shell on a CdSe core modifies thermalization times by an amount proportional to the overall particle radius. Using this approach, we obtain changes in effective thermal conductivity of up to 5× for a nearly constant energy gap.

Atomic layer deposition (ALD) is widely used for gas-phase deposition of high-quality dielectric, semiconducting, or metallic films on various substrates. In this contribution we propose the concept of colloidal ALD (c-ALD) for synthesis of colloidal nanostructures. During the c-ALD process, either nanoparticles or molecular precursors are sequentially transferred between polar and nonpolar phases to prevent accumulation of unreacted precursors and byproducts in the reaction mixture. We show that binding of inorganic ligands (e.g., S(2-)) to the nanocrystal surface can be used as a half-reaction in c-ALD process. The utility of this approach has been demonstrated by growing CdS layers on colloidal CdSe nanocrystals, nanoplatelets, and CdS nanorods. The CdS/CdSe/CdS nanoplatelets represent a new example of colloidal nanoheterostructures with mixed confinement regimes for electrons and holes. In these materials holes are confined to a thin (∼1.8 nm) two-dimensional CdSe quantum well, while the electron confinement can be gradually relaxed in all three dimensions by growing epitaxial CdS layers on both sides of the quantum well. The relaxation of the electron confinement energy caused a shift of the emission band from 510 to 665 nm with unusually small inhomogeneous broadening of the emission spectra.

We report heat dissipation times in semiconductor nanocrystals of CdSe. Specifically, a previously unresolved, subnanosecond decay component in the low-temperature photoluminescence decay dynamics exhibits longer decay lifetimes (tens to hundreds of picoseconds) for larger nanocrystals as well as a size-independent, ~25-meV spectral shift. We attribute the fast relaxation to transient phonon-mediated relaxation arising from nonequilibrium acoustic phonons. Following acoustic phonon dissipation, the dark exciton state recombines more slowly via LO-phonon assistance resulting in the observed spectral shift. The measured relaxation time scales agree with classical calculations of thermal diffusion, indicating that interfacial thermal conductivity does not limit thermal transport in these semiconductor nanocrystal dispersions.

We study the formation of colloidal CdSe nanoplatelets using both tansmission electron microscopy (TEM) and spectroscopic analysis. We show that the platelets form by continuous lateral extension of small (