The application of nanocrystals as heterogeneous catalysts and plasmonic nanoparticles requires fine control of their shape and chemical composition. A promising idea to achieve synergistic effects is to combine two distinct chemical and/or physical functionalities in bi-metallic core-shell nanocrystals. Although techniques for the synthesis of single-component nanocrystals with spherical or anisotropic shape are well established, new methods are sought to tailor multicomponent nanocrystals. Here, we probe etching in a controlled redox environment as a synthesis technique for multi-component nanocrystals. Our Monte Carlo computer simulations demonstrate the appearance of characteristic non-equilibrium intermediate microstructures that are further thermodynamically tested and analyzed with molecular dynamics. Convex platelet, concave polyhedron, pod, cage and strutted-cage shapes are obtained at room temperature with fully coherent structure exposing crystallographic facets and chemical elements along distinct particle crystallographic directions. We observe that structural and dynamic properties are markedly modified compared to the untreated compact nanocrystal.
The electronic properties of hydrogenated, spherical, Si/Ge and Ge/Si core-shell nanocrystals with a diameter ranging from 1.8 to 4.0 nm are studied within Density Functional Theory. Effects induced by quantum confinement and strain on the near-band-edge states localization, as well as the band-offset properties between Si and Ge regions, are investigated in detail. On the one hand, we prove that Si(core)/Ge(shell) nanocrystals always show a type II band-offset alignment, with the HOMO mainly localized on the Ge shell region and the LUMO mainly localized on the Si core region. On the other han…
The instability of metal halide perovskites upon exposure to moisture or heat strongly hampers their applications in optoelectronic devices. Here, we report the large-yield synthesis of highly water-resistant total-inorganic green luminescent CsPbBr3/CsPb2Br5 core/shell heteronanocrystals (HNCs) by developing an in situ phase transition approach. It is implemented via water-driven phase transition of the original monoclinic CsPbBr3 nanocrystal and the resultant tetragonal CsPb2Br5 nanoshell has small lattice mismatch with the CsPbBr3 core, which ensures formation of an epitaxial interface for …
We report the synthesis and optical characterization of fully inorganic gradientshell CdSe/CdZnS nanocrystals (NCs) with high luminescence quantum yield (QY, 50 percent), which were obtained by replacing native oleic-acid (OA) ligands with halide ions (Br and Cl). Absorption, photoluminescence excitation (PLE) and photoluminescence (PL) spectra in solution were unaffected by the ligand-exchange procedure. The halide-capped NCs were stable in solution for several weeks without modification of their PL spectra; once deposited as unprotected thin films and exposed to air, however, they did show s…
Outstanding optoelectronic properties and a facile synthesis render halide perovskite nanocrystals (NCs) a promising material for nanostructure-based devices. However, the commercialization is hindered mainly by the lack of NC stability under ambient conditions and inefficient charge carrier injection. Here, we investigate solutions to both problems, employing methylammonium lead bromide (MAPbBr_3) NCs encapsulated in diblock copolymer core-shell micelles of tunable size. We confirm that the shell does not prohibit energy transfer, as FRET efficiencies between these NCs and 2D CsPbBr_3 nanopla…
Unique nanorod precipitates with a core-shell structure are found to nucleate from the grain boundaries of a bulk nanocrystalline Al-Ni-Y alloy fabricated via powder consolidation, contributing significantly to stabilization and strengthening. The local structure, chemistry, and evolution of these features during annealing is reported here. In the as-consolidated state, the nanorods can be either structurally ordered or disordered, yet a consistent chemical patterning is found where the core is primarily Al plus C while the shell is enriched with Y. As annealing time increases, more nanorods t…
We show that the decomposition of caesium lead halide perovskite nanocrystals under continuous X-ray illumination depends on the surface ligand. For oleic acid/oleylamine, we observe a fast decay accompanied by the formation of elemental lead and halogen. Upon surface functionalization with a metal porphyrin derivate, the decay is markedly slower and involves the disproportionation of lead to Pb0 and Pb3+. In both cases, the decomposition is preceded by a contraction of the atomic lattice, which appears to initiate the decay. We find that the metal porphyrin derivative induces a strong surface…
Engineering the structure of core-shell colloidal semiconductor nanoparticles (CSNPs) is attractive due to the potential to enhance photo-induced charge transfer (PICT) and induce favourable optical and electronic properties. Nonetheless, the sensitivity of telluride CSNPs to high temperatures makes it challenging to precisely modulate their surface crystallinity. Herein, we have developed an efficient strategy for synthesising telluride CSNPs with thin amorphous shells using aqueous cation exchange (ACE). By changing the synthesis temperature in the range 40 to 110C, the crystallinity of the …
The understanding of depletion layers is of major importance to control the optical and electronic properties of metal oxide (MO) nanocrystals (NCs). Here, we show that depletion layer engineering is the main mechanism of photodoping of MO NCs. We show that the introduction of different electronic interfaces induces a double-bending of the electronic bands and a distinct carrier density profile. We found that the light-induced depletion layer modulation and bending of the bands close to the surface of the nanocrystal is the main mechanism responsible for the storage of extra electrons after ph…
Full Quantum and Mixed Quantum--Classical Dynamics of Hot Exciton Cooling in Semiconductor Nanocrystals
University of California, Berkeley（加州大学伯克利分校）；University of Freiburg（弗赖堡大学）；Universität Heidelberg（海德堡大学）；The Hebrew University of Jerusalem（耶路撒冷希伯来大学）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）
Chemical transformations, such as ion exchange, are commonly employed to modify nanocrystal compositions. Yet the mechanisms of these transformations, which often operate far from equilibrium and entail mixing diverse chemical species, remain poorly understood. Here, we explore an idealized model for ion exchange in which a chemical potential drives compositional defects to accumulate at a crystal's surface. These impurities subsequently diffuse inward. We find that the nature of interactions between sites in a compositionally impure crystal strongly impacts exchange trajectories. In particula…
Recent progress achieved in metal-assisted chemical etching (MACE) has enabled the production of high-quality micropillar arrays for various optoelectronic applications. Si micropillars produced by MACE often show a porous Si/SiOx shell on crystalline pillar cores introduced by local electrochemical reactions. In this paper, we report the distinct optoelectronic characteristics of the porous Si/SiOx shell correlated to their chemical compositions. Local photoluminescent (PL) images obtained with an immersion oil objective lens in confocal microscopy show a red emission peak (about 650 nm) alon…
Semiconducting, core-shell nanocrystals (NCs) are promising building blocks for the construction of higher dimensional artificial nanostructures using oriented attachment. However, the assembly and epitaxial attachment steps critical to this construction introduce disorder and defects which inhibit the observation of desirable emergent electronic phenomena. Consequently, understanding defect formation and remediation in these systems as a function of dimensionality is a crucial step to perfecting their synthesis. In this work, we use in situ high resolution transmission electron microscopy to …
A complex InAs/CdSe/ZnSe Core/Shell1/Shell2 (CSS) structure is synthesized, where the intermediate CdSe buffer layer decreases strain between the InAs core and the ZnSe outer shell. This structure leads to significantly improved fluorescence quantum yield as compared to previously prepared core/shell structures and enables growth of much thicker shells. The shell growth is done using a layer-by-layer method in which the shell cation and anion precursors are added sequentially allowing for excellent control and a good size distribution is maintained throughout the entire growth process. The CSS…
Ligand-Controlled Phonon Dynamics in CsPbBr3 Nanocrystals Revealed by Machine-Learned Interatomic Potentials
Georgia Institute of Technology（佐治亚理工学院）；Queens College, City University of New York（纽约市立大学皇后学院）；The Graduate Center, City University of New York（纽约市立大学研究生中心）
Amorphous Co-rich microwires with excellent soft magnetic and mechanical properties produced by melt-extraction technique are emerging as a multifunctional material for a variety of applications ranging from ultrasensitive magnetic field sensors to structural health self-monitoring composites. There is a pressing need for enhancing these properties to make the microwires practical for integration into new technologies. Conventional heat treatments at temperature below crystallization may improve the magnetic softness of an as-quenched amorphous wire, but usually deteriorate the good mechanical…
We report the interactions of cationic latex particles synthesized by RAFT/MADIX-mediated emulsion polymerization with anionic cellulose nanocrystals (CNCs) and cotton fabrics. Latexes in the size range 200-300 nm with poly(butyl acrylate) or poly(2-ethylhexyl acrylate) hydropho-bic cores and hydrophilic shell are synthesized. We show that the latex/CNC interaction is me-diated by electrostatics, the interaction being the strongest with the most charged particles. The adsorption process is efficient and does not require any functionalization step for either cellulose or latex. A major result i…
Optical and X-ray Photo-emission Spectroscopies of Core/Shell Colloidal CdSe/CdS Quantum Dots: Modeling and Experimental Determination of Band Alignment
