22 Search Results

Inorganic nanocomposites synthesized by combination of colloidal nanocrystals (NCs) and inorganic clusters have recently emerged as new materials with novel and unique functionalities. Much of the demonstrated promise of nanocomposites derives from the unique interactions between NC and matrix components—this generates new material properties, which direct unique transport behavior in the overall solid or nanocomposite—be it mass, charge, or heat. While measured empirically, it has remained largely impossible to take an a priori look at material properties and use those as a guideline to design desired transport behavior. Fundamentally, this is because the structural and electronic changes manifest at thosemore » interfaces have remained hidden from examination. We provide experimental evidence that transport behavior in nanocrystal-in-matrix (NIM) composites is dictated primarily by interfacial charge transfer associated with electronic and structural reconstructions as the composite forms. Our approach building continuous composite superlattices serves as a starting point for systematic probing of the nanointerface of NIM composites via ultrathin films. A combination of field effect transistor device characterization and photoemission spectroscopy reveals the systematic dependence of the polarity of charge transfer on the selection of matrix materials in NIM composites. We use this insight to combine, by design, different components to tune the carrier type in NIM composites.« less

Quaternary alloys are essential for the development of high-performance optoelectronic devices. However, immiscibility of the constituent elements can make these materials vulnerable to phase segregation, which degrades the optical and electrical properties of the solid. High-efficiency III–V photovoltaic cells are particularly sensitive to this degradation. InAlAsSb lattice matched to InP is a promising candidate material for high-bandgap subcells of a multijunction photovoltaic device. However, previous studies of this material have identified characteristic signatures of compositional variation, including anomalous low-energy photoluminescence. In this paper, atomic-scale clustering is observed in InAlAsSb via quantitative scanning transmission electron microscopy. Finally, image quantification of atomicmore » column intensity ratios enables the comparison with simulated images, confirming the presence of nonrandom compositional variation in this multispecies alloy.« less

The radiation-sensitive nature of halide perovskites has hindered structural studies at the atomic scale. We overcome this obstacle by applying low dose-rate in-line holography, which combines aberration-corrected high-resolution transmission electron microscopy with exit-wave reconstruction. This technique successfully yields the genuine atomic structure of ultrathin two-dimensional CsPbBr₃ halide perovskites, and a quantitative structure determination was achieved atom column by atom column using the phase information of the reconstructed exit-wave function without causing electron beam-induced sample alterations. An extraordinarily high image quality enables an unambiguous structural analysis of coexisting high-temperature and low-temperature phases of CsPbBr₃ in single particles. On a broader level,more » our approach offers unprecedented opportunities to better understand halide perovskites at the atomic level as well as other radiation-sensitive materials.« less

Low-temperature synthesis of crystalline silicon and silicon-containing nanowires remains a challenge in synthetic chemistry due to the lack of sufficiently reactive Si precursors. We report that colloidal Si nanowires can be grown using tris(trimethylsilyl)silane or trisilane as the Si precursor by a Ga-mediated solution-liquid-solid (SLS) approach at temperatures of about 200 °C, which is more than 200 °C lower than that reported in the previous literature. We further demonstrate that the new Si chemistry can be adopted to incorporate Si atoms into III-V semiconductor lattices, which holds promise to produce a new Si-containing alloy semiconductor nanowire. This development represents anmore » important step toward low-temperature fabrication of Si nanowire-based devices for broad applications.« less

Artificial photosystems are advanced by the development of conformal catalytic materials that promote desired chemical transformations, while also maintaining stability and minimizing parasitic light absorption for integration on surfaces of semiconductor light absorbers. We demonstrate that multifunctional, nanoscale catalysts that enable high-performance photoelectrochemical energy conversion can be engineered by plasma-enhanced atomic layer deposition. The collective properties of tailored Co ₃ O ₄ /Co(OH) ₂ thin films simultaneously provide high activity for water splitting, permit efficient interfacial charge transport from semiconductor substrates, and enhance durability of chemically sensitive interfaces. Furthermore, these films comprise compact and continuous nanocrystalline Co ₃ O ₄more » spinel that is impervious to phase transformation and impermeable to ions, thereby providing effective protection of the underlying substrate. Moreover, a secondary phase of structurally disordered and chemically labile Co(OH) ₂ is introduced to ensure a high concentration of catalytically active sites. Application of this coating to photovoltaic p ⁺ n-Si junctions yields best reported performance characteristics for crystalline Si photoanodes.« less

The location of Co and Ni promoter atoms in industrial-style hydrotreating catalysts is examined by combining aberration-corrected scanning transmission electron microscopy and electron energy loss spectrum imaging. The observations unambiguously demonstrate that both Co and Ni promoter atoms occupy sites at all low-indexed edge terminations of hexagonally shaped multi-layer MoS₂ nanocrystals. In contrast, similar observations for single-layer MoS₂ nanocrystals show that Co-promoter atoms preferentially attach at the (-1 0 0) S-edge termination and are absent at the (1 0 0) Mo-edge termination. The apparent discrepancy between single- and multi-layer MoS₂ nanocrystals can be explained by the 2H-MoS₂ crystal structure, formore » which successive MoS₂ layers alternatingly expose Mo- and S-edge terminations in any of the low-indexed directions. Thus, the multi-layer Co-Mo-S and Ni-Mo-S nanocrystals, formed in the present type of industrial-style hydrotreating catalyst, are consistently described as a superposition of single-layer Co-Mo-S and Ni-Mo-S structures, and in turn, provide promoted edge sites with different steric accessibility for the organic compounds in mineral oil distillates.« less

Making connections between performance and structure in bimetallic catalysts requires knowledge of how the two elements are spatially associated. Elemental maps obtained by analytical TEM methods are an invaluable tool for identifying the location of different elements, but for many samples, visual inspection of elemental maps is insufficient for assessing the degree of element spatial correlation. This is particularly true for beam-sensitive materials where short mapping acquisition times lead to images with high noise and low color depth. In these situations, statistical analysis of elemental maps can be used to identify spatial correlations among the elements in a sample. Inmore » this work, the relationship between catalyst performance and bimetallic spatial association was explored using Mn-promoted Co-based Fischer-Tropsch synthesis catalysts prepared by different pretreatment methods. Mn was used as a catalyst additive to suppress methane formation. Catalysts that underwent calcination before reduction produced more methane and fewer long-chain hydrocarbons than catalysts that were directly reduced. The extent to which Co and Mn were spatially associated was assessed using correlation metrics, colocation plots, and histograms generated using data from STEM-EDS maps. Although both catalysts yielded visually similar elemental maps, the results of statistical analysis suggested that the calcined catalyst exhibited greater spatial segregation between the Co and Mn. These findings support the hypothesis that having Mn in close proximity to the Co is essential for the manifestation of Mn promotion effects in Co-based FTS catalysts.« less

Ce-rich mixed metal oxides comprise a recently discovered class of ­electrocatalysts for the oxygen evolution reaction (OER). In particular, at current densities below 10 mA cm^-2, Ni_0.3Fe_0.07Co_0.2Ce_0.43O_x exhibits ­superior activity compared to the corresponding transition metal oxides, despite the relative inactivity of ceria. To elucidate the enhanced activity and underlying catalytic mechanism, detailed structural characterization of this quinary oxide electrocatalyst is reported in this paper. Transmission electron microscopy imaging of cross-section films as-prepared and after electrochemical testing reveals a stable two-phase nanostructure composed of 3–5 nm diameter crystallites of fluorite CeO₂ intimately mixed with 3–5 nm crystallites of transition metalmore » oxides alloyed in the rock salt NiO structure. Dosing experiments demonstrate that an electron flux greater than ≈1000 e Å^-2 s^-1 causes the inherently crystalline material to become amorphous. A very low dose rate of 130 e Å^-2 s^-1 is employed for atomic resolution imaging using inline holography techniques to reveal a nanostructure in which the transition metal oxide nanocrystals form atomically sharp boundaries with the ceria nanocrystals, and these results are corroborated with extensive synchrotron X-ray absorption spectroscopy measurements. Finally, ceria is a well-studied cocatalyst for other heterogeneous and electrochemical reactions, and our discovery introduces biphasic cocatalysis as a design concept for improved OER electrocatalysts.« less