Laboratoire de Physique et d’Étude des Matériaux

Strong electronic correlations are often associated with the proximity of a Mott-insulating state. In recent years however, it has become increasingly clear that the Hund’s rule coupling (intra-atomic exchange) is responsible for strong correlations in multiorbital metallic materials that are not close to a Mott insulator. Hund’s coupling has two effects: It influences the energetics of the Mott gap and strongly suppresses the coherence scale for the formation of a Fermi liquid. A global picture has emerged recently, which emphasizes the importance of the average occupancy of the shell as a control parameter. The most dramatic effects occur away from half-filling or single occupancy. We review the theoretical understanding and physical properties of these Hund’s metals, together with the relevance of this concept to transition-metal oxides (TMOs) of the 3d, and especially 4d, series (such as ruthenates), as well as to the iron-based superconductors (iron pnictides and chalcogenides).

Graphene and graphene-like two-dimensional (2D) materials have attracted much attention due to its extraordinary electronic and optical properties, which accommodate a large potential in optoelectronic applications such as photodetection. However, although much progress has been made, many challenges exist in fundamental and practical aspects hindering graphene and graphene-like 2D materials from photodetector and other photonic and optoelectronic applications. Here, we review the recent progress in photodetection based on graphene and graphene-like 2D materials and start with the summary of some most important physical mechanisms, including photoelectric, photo-thermoelectric, and photo-bolometric regimes. Then methodology-level discussions are given from viewpoints of state-of-the-art designs in device geometry and materials. It is worth emphasizing that emerging photodetection and photodetectors based on graphene-like 2D materials such as metal chalcogenide nanosheets are reviewed systematically. Finally, we conclude this review in a brief discussion with remaining challenges in photodetection of two-dimensional photonics and optoelectronics (2D POE) and note that complete understandings of 2D materials and 2D POE may inspire solar energy conversion and other new applications.

Semiconductor quantum dots (QDs) could significantly impact the performance of biomedical near-infrared (NIR) imaging by providing fluorescent probes that are brighter and more photostable than conventional organic dyes. However, the toxicity of the components of NIR emitting II-VI and IV-VI QDs that have been made so far (Cd, Hg, Te, Pb, etc.) has remained a major obstacle to the clinical use of QDs. Here, we present the synthesis of CuInS(2)/ZnS core/shell QDs emitting in the NIR ( approximately 800 nm) with good quantum yield and stability even after transfer into water. We demonstrate the potential of these QDs by imaging two regional lymph nodes (LNs) in vivo in mice. We then compare the inflammatory response of the axillary LN induced by different doses of CuInS(2)/ZnS and CdTeSe/CdZnS QDs and show a clear difference in acute local toxicity, the onset of inflammation only occurring at a 10 times more concentrated dose for CuInS(2)/ZnS QDs than for their Cd-containing counterparts.

In this paper, we review recent progress on colloidal growth of 2D nanocrystals. We identify the four main sources of anisotropy which lead to the formation of plate- and sheet-like colloidal nanomaterials. Defect-induced anisotropy is a growth method which relies on the presence of topological defects at the nanoscale to induce 2D shapes objects. Such a method is particularly important in the growth of metallic nano-objects. Another way to induce anisotropy is based on ligand engineering. The availability of some nanocrystal facets can be tuned by selectively covering the surface with ligands of tunable thickness. Cadmium chalcogenides nanoplatelets (NPLs) strongly rely on this method which offers atomic control in the thinner direction, down to a few monolayers. Two-dimensional objects can also be obtained by post or in situ self-assembly of nanocrystals. This growth method differs from the previous ones in the sense that the elementary objects are not molecular precursors and is a common method for lead chalcogenide compounds. Finally, anisotropy may simply rely on the lattice anisotropy itself as it is common for rod-like nanocrystals. Colloidally grown transition metal dichalcogenides (TMDC) in particular result from such process. We also present hybrid syntheses which combine several of the previously described methods and other paths, such as cation exchange, which expand the range of available materials. Finally, we discuss in which sense 2D objects differ from 0D nanocrystals and review some of their applications in optoelectronics, including lasing and photodetection, and biology.

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 (<2 nm) nanocrystal CdSe seeds. The nanoplatelet thickness is fixed by the seed dimension and remains constant during the platelet formation. The nanoplatelet lateral dimensions can be tuned using additional precursor injection. Absorption and fluorescence analysis of the CdSe nanoplatelets as they continuously extend laterally confirms a continuous transition from 3D to 1D confined nanoparticles. The formation of the CdSe platelets is found to be similar for different platelet thicknesses that we control with a precision of one CdSe monolayer.

We have recently synthesized atomically flat semiconductor colloidal nanoplatelets with quasi 2D geometry. Here, we show that core/shell nanoplatelets can be obtained with a 2D geometry that is conserved. The epitaxial growth of the shell semiconductor is performed at room temperature. We report the detailed synthesis of CdSe/CdS and CdSe/CdZnS structures with different shell thicknesses. The shell growth is characterized both spectroscopically and structurally. In particular, the core/shell structure appears very clearly on high-resolution, high-angle annular dark-field transmission electron microscope images, thanks to the difference of atomic density between the core and the shell. When the nanoplatelets stand on their edge, we can precisely count the number of atomic planes forming the core and the shell. This provides a direct measurement, with atomic precision, of the core nanoplatelets thickness. The constraints exerted by the shell growth on the core is analyzed using global phase analysis. The core/shell nanoplatelets we obtained have narrow emission spectra with full-width at half-maximum close to 20 nm, and quantum yield that can reach 60%.

We collect and resolve spectrally and temporally the photoluminescence of single CdSe nanoplatelets. The emission intensity of single nanoplatelets at room temperature shows ON and OFF periods with a usual blinking statistics, while at 20 K, their emission intensity can be extremely stable in time. At room temperature, the emission spectra of single nanoplatelets are similar to ensemble measurements with a full width at half-maximum of 40 meV. At 20 K, we obtain a resolution-limited spectral line width (<0.4 meV). The fluorescence lifetime of single nanoplatelets decreases when the temperature decreases to reach 200 ps at 20 K. This lifetime shortening is concomitant with an increase of the nanoplatelets' emission intensity.

The introduction of a mobile and polarized organic moiety as a cation in 3D lead-iodide perovskites brings fascinating optoelectronic properties to these materials. The extent and the time scales of the orientational mobility of the organic cation and the molecular mechanism behind its motion remain unclear, with different experimental and computational approaches providing very different qualitative and quantitative description of the molecular dynamics. Here we use ultrafast 2D vibrational spectroscopy of methylammonium (MA) lead iodide to directly resolve the rotation of the organic cations within the MAPbI3 lattice. Our results reveal two characteristic time constants of motion. Using ab initio molecular dynamics simulations, we identify these as a fast (∼300 fs) "wobbling-in-a-cone" motion around the crystal axis and a relatively slow (∼3 ps) jump-like reorientation of the molecular dipole with respect to the iodide lattice. The observed dynamics are essential for understanding the electronic properties of perovskite materials.

We show that electron- and hole-doped BaFe(2)As(2) are strongly influenced by a Mott insulator that would be realized for half-filled conduction bands. Experiments show that weakly and strongly correlated conduction electrons coexist in much of the phase diagram, a differentiation which increases with hole doping. This selective Mottness is caused by the Hund's coupling effect of decoupling the charge excitations in different orbitals. Each orbital then behaves as a single-band doped Mott insulator, where the correlation degree mainly depends on how doped is each orbital from half filling. Our scenario reconciles contrasting evidences on the electronic correlation strength, implies a strong asymmetry between hole and electron doping, and establishes a deep connection with the cuprates.

We show that the main mechanism for the dc voltage or dc current induced insulator-metal transition in vanadium dioxide VO(2) is due to local Joule heating and not a purely electronic effect. This "tour de force" experiment was accomplished by using the fluorescence spectra of rare-earth doped micron sized particles as local temperature sensors. As the insulator-metal transition is induced by a dc voltage or dc current, the local temperature reaches the transition temperature indicating that Joule heating plays a predominant role. This has critical implications for the understanding of the dc voltage or dc current induced insulator-metal transition and has a direct impact on applications which use dc voltage or dc current to externally drive the transition.

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 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.

The first functional light‐emitting diodes (LEDs) based on quasi 2D colloidal core/shell CdSe/CdZnS nanoplatelets (NPLs). The solution‐processed hybrid devices are optimized with respect to their electroluminescent characteristics, first, by improving charge injection through exchanging the as‐synthesized NPL long‐chain ligands to shorter ones such as 3‐mercaptopropionic acid, and second, by comparing different hole‐transporting layers. NPL‐LEDs exhibit a maximum luminance of 4499 cd m ‐2 and external quantum efficiencies of 0.63%. In particular, over different applied voltages, systematically narrow electroluminescence of full width at half maximum (FWHM) in the range of 25–30 nm is observed to be independent from the choice of device configuration and NPL ligands. As spectrally narrow electroluminescence is highly attractive in terms of color purity in the context of LED applications, these results emphasize the unique potential of this new class of colloidal core/shell nanoplatelet in achieving bright and functional LEDs of superior color purity.

We experimentally demonstrate the control of the spontaneous emission rate and the radiation pattern of colloidal quantum dots deterministically positioned in a plasmonic patch antenna. The antenna consists of a thin gold microdisk separated from a planar gold layer by a few tens of nanometers thick dielectric layer. The emitters are shown to radiate through the entire patch antenna in a highly directional and vertical radiation pattern. Strong acceleration of spontaneous emission is observed, depending on the antenna geometry. Considering the double dipole structure of the emitters, this corresponds to a Purcell factor up to 80 for dipoles perpendicular to the disk.

Abstract To use water as the source of electrons for proton or CO 2 reduction within electrocatalytic devices, catalysts are required for facilitating the proton‐coupled multi‐electron oxygen evolution reaction (OER, 2 H 2 O→O 2 +4 H + +4 e − ). These catalysts, ideally based on cheap and earth abundant metals, have to display high activity at low overpotential and good stability and selectivity. While numerous examples of Co, Mn, and Ni catalysts were recently reported for water oxidation, only few examples were reported using copper, despite promising efficiencies. A rationally designed nanostructured copper/copper oxide electrocatalyst for OER is presented. This material derives from conductive copper foam passivated by a copper oxide layer and further nanostructured by electrodeposition of CuO nanoparticles. The generated electrodes are highly efficient for catalyzing selective water oxidation to dioxygen with an overpotential of 290 mV at 10 mA cm −2 in 1 m NaOH solution.

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.

Free standing two-dimensional materials appear as a novel class of structures. Recently, the first colloidal two-dimensional heterostructures have been synthesized. These core/shell nanoplatelets are the first step toward colloidal quantum wells. Here, we study in detail the spectroscopic properties of this novel generation of colloidal nanoparticles. We show that core/shell CdSe/CdZnS nanoplatelets with 80% quantum yield can be obtained. The emission time trace of single core/shell nanoplatelets exhibits reduced blinking compared to core nanoplatelets with a two level emission time trace. At cryogenic temperatures, these nanoplatelets have a quantum yield close to 100% and a stable emission time trace. A solution of core/shell nanoplatelets has emission spectra with a full width half-maximum close to 20 nm, a value much lower than corresponding spherical or rod-shaped heterostructures. Using single particle spectroscopy, we show that the broadening of the emission spectra upon the shell deposition is not due to dispersity between particles but is related to an intrinsic increased exciton-phonon coupling in the shell. We also demonstrate that optical spectroscopy is a relevant tool to investigate the presence of traps induced by shell deposition. The spectroscopic properties of the core/shell nanoplatelets presented here strongly suggest that this new generation of objects will be an interesting alternative to spherical or rod-shaped nanocrystals.

High-performance perovskite photodetectors based on solution-processed all-inorganic CsPbBr₃ thin films were fabricated with a high photoresponsivity and on/off photocurrent ratio.

Just like insulators can present topological phases characterized by Dirac edge states, superconductors can exhibit topological phases characterized by Majorana edge states. In particular, one-dimensional topological superconductors are predicted to host zero-energy Majorana fermions at their extremities. By contrast, two-dimensional superconductors have a one-dimensional boundary which would naturally lead to propagating Majorana edge states characterized by a Dirac-like dispersion. In this paper we present evidences of one-dimensional dispersive in-gap edge states surrounding a two-dimensional topological superconducting domain consisting of a monolayer of Pb covering magnetic Co-Si islands grown on Si(111). We interpret the measured dispersive in-gap states as a spatial topological transition with a gap closure. Our method could in principle be generalized to a large variety of heterostructures combining a Rashba superconductor with a magnetic layer in order to be used as a platform for engineering topological quantum phases.

The Nernst effect is the transverse electric field produced by a longitudinal thermal gradient in the presence of a magnetic field. At the beginning of this century, Nernst experiments on cuprates were analyzed assuming that: (i) the contribution of quasi-particles to the Nernst signal is negligible; and (ii) Gaussian superconducting fluctuations cannot produce a Nernst signal well above the critical temperature. Both these assumptions were contradicted by subsequent experiments. This paper reviews experiments documenting multiple sources of a Nernst signal, which, according to the Bridgman relation, measures the flow of transverse entropy caused by a longitudinal particle flow. Along the lines of Landauer's approach to transport phenomena, the magnitude of the transverse magneto-thermoelectric response is linked to the quantum of thermoelectric conductance and a number of material-dependent length scales: the mean free path, the Fermi wavelength, the de Broglie thermal wavelength and the superconducting coherence length. Extremely mobile quasi-particles in dilute metals generate a widely-documented Nernst signal. Fluctuating Cooper pairs in the normal state of superconductors have been found to produce a detectable Nernst signal with an amplitude conforming to the Gaussian theory, first conceived by Ussishkin, Sondhi and Huse. In addition to these microscopic sources, mobile Abrikosov vortices, mesoscopic objects simultaneously carrying entropy and magnetic flux, can produce a sizeable Nernst response. Finally, in metals subject to a magnetic field strong enough to truncate the Fermi surface to a few Landau tubes, each exiting tube generates a peak in the Nernst response. The survey of these well-established sources of the Nernst signal is a helpful guide to identify the origin of the Nernst signal in other controversial cases.