Sandrine Ithurria

Colloidal nanoplatelets with two-dimensional electronic structure

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.

Quasi 2D colloidal CdSe platelets with thicknesses controlled at the atomic level.

Well documented procedures to grow zero-dimensional systems, dots, and one-dimensional systems, wires and tubes, as colloidal particles in solution have been reported. In contrast, there are no methods of preparation that yield optically active two-dimensional soluble particles. Yet, ultrathin films (quantum wells) of II−VI and III−V semiconductors epitaxially grown on substrates by molecular beam epitaxy for example have proven extremely useful for both fundamental studies and a wealth of applications in optoelectronics. We show that II−VI cadmium selenide platelets, with thicknesses tuned at the atomic level, can be synthesized in solution. We describe the method for the preparation of these new colloidal nanocrystals and characterize them structurally and optically. We identified three platelets populations with emission maximums at 462, 513, and 550 nm with corresponding thicknesses estimated at 1.9, 2.2, and 2.5 nm, respectively. Despite the fact that the platelet aspect ratio within a population can...

Colloidal Atomic Layer Deposition (c-ALD) using Self-Limiting Reactions at Nanocrystal Surface Coupled to Phase Transfer between Polar and Nonpolar Media

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.

Low Voltage, Hysteresis Free, and High Mobility Transistors from All-Inorganic Colloidal Nanocrystals

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.

Two-Dimensional Colloidal Metal Chalcogenides Semiconductors: Synthesis, Spectroscopy, and Applications

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.

Two-Dimensional Colloidal Nanocrystals

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.

Efficient exciton concentrators built from colloidal core/crown CdSe/CdS semiconductor nanoplatelets.

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.

Type-II CdSe/CdTe Core/Crown Semiconductor Nanoplatelets

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 excitons 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.

Monitoring Morphological Changes in 2D Monolayer Semiconductors Using Atom-Thick Plasmonic Nanocavities

Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.

Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz

Infrared thermal imaging devices rely on narrow band gap semiconductors grown by physical methods such as molecular beam epitaxy and chemical vapor deposition. These technologies are expensive, and infrared detectors remain limited to defense and scientific applications. Colloidal quantum dots (QDs) offer a low cost alternative to infrared detector by combining inexpensive synthesis and an ease of processing, but their performances are so far limited, in terms of both wavelength and sensitivity. Herein we propose a new generation of colloidal QD-based photodetectors, which demonstrate detectivity improved by 2 orders of magnitude, and optical absorption that can be continuously tuned between 3 and 20 μm. These photodetectors are based on the novel synthesis of n-doped HgSe colloidal QDs whose size can be tuned continuously between 5 and 40 nm, and on their assembly into solid nanocrystal films with mobilities that can reach up to 100 cm(2) V(-1) s(-1). These devices can be operated at room temperature with the same level of performance as the previous generation of devices when operated at liquid nitrogen temperature. HgSe QDs can be synthesized in large scale (>10 g per batch), and we show that HgSe films can be processed to form a large scale array of pixels. Taken together, these results pave the way for the development of the next generation mid- and far-infrared low-cost detectors and camera.