Showing papers by "Michael H. Huang published in 2018"
TL;DR: Recognizing that facet-dependent behaviors are intrinsic semiconductor properties, researchers should pay attention to their influence in the explanation of the measured photocatalytic properties, and consider ways to enhance photoc atalytic efficiency or design electrical components utilizing the facet effects.
Abstract: Recent studies on the electrical conductivity and photocatalytic activity properties of semiconductor nanocrystals such as Cu2O, Ag2O, TiO2, PbS, and Ag3PO4 exposing well-defined surfaces have revealed strong facet effects. For example, the electrical conductivity of Cu2O crystals can vary from highly conductive to nonconductive, and they can be highly photocatalytically active or inactive depending on the exposed faces. The crystal surfaces can even tune their light absorption wavelengths. Our understanding is that the emergence of these unusual phenomena can be explained in terms of the presence of an ultrathin surface layer having different band structures and degrees of band bending for different surfaces, which affects charge transport and photons into and out of the crystals. This review uses primarily results from our research on this frontier area of semiconductor properties to illustrate the existence of semiconductor facet effects. A simple adjustment to normal semiconductor band diagram allows ...
90 citations
TL;DR: In this paper, a modified and general band diagram for Cu2O crystals has been constructed incorporating their optical size and facet effects with surface band bending, and a more complete understanding of the properties of these nanocubes can be found.
Abstract: By making Cu2O nanocubes, octahedra, and rhombic dodecahedra with tunable sizes and recording their light absorption and emission spectra, their absorption and emission bands shift steadily to longer wavelengths with increasing particle sizes from 10 nm to beyond 250 nm. Emission intensities are highest for the smallest nanocubes. Photoluminescence band shifts exceed 130 nm over this size range. For particles having the same volume, rhombic dodecahedra absorb light of shortest wavelength, while cubes show most red-shifted absorption with their band gaps differing by 0.17 eV (or 51.5 nm). They show obviously different colors. The presence of optical size and facet effects in semiconductors means that their emission wavelengths are tunable through facet control and use of nanocrystals much larger than quantum dots. A modified and general band diagram for Cu2O crystals has been constructed incorporating their optical size and facet effects with surface band bending. In addition, a more complete understanding...
48 citations
TL;DR: The calculation results suggest the observation of facet-dependent electrical-conductivity properties of germanium materials; when making transistors from germium, the facet effects with shrinking dimensions approaching 3 nm may also need to be considered.
Abstract: To find out if germanium possesses facet-dependent electrical-conductivity properties, surface-state density functional theory (DFT) calculations were performed on one to six layers of germanium (100), (110), (111), and (211) planes. Tunable Ge(100) and Ge(110) planes always present the same semiconducting band structure with a band gap of 0.67 eV expected of bulk germanium. In contrast, one, two, four, and five layers of Ge(111) and Ge(211) plane models show metal-like band structures with continuous density of states (DOS) throughout the entire band. For three and six layers of Ge(111) and Ge(211) plane models, the normal semiconducting band structure was obtained. The plane layers with metal-like band structures also show Ge-Ge bond-length deviations and bond distortions, as well as significantly different 4s and 4p frontier-orbital electron counts and relative percentages integrated over the valence and conduction bands from those of the semiconducting state. These differences should contribute to strikingly dissimilar band structures. The calculation results suggest the observation of facet-dependent electrical-conductivity properties of germanium materials; when making transistors from germanium, the facet effects with shrinking dimensions approaching 3 nm may also need to be considered.
42 citations
TL;DR: In this article, ammonium borane was used to pseudomorphically convert polyhedral Cu2O cubes, octahedra, and rhombic dodecahedra to Cu particles.
Abstract: Cu2O cubes, octahedra, and rhombic dodecahedra can be pseudomorphically converted to Cu crystals of the corresponding morphologies through the addition of ammonia borane. Nitroarene can be completely reduced during the compositional transformation with four equivalents of ammonia borane at 30 °C in 25 min. All the obtained polyhedral Cu crystals can give 100% nitroaniline conversion to p-phenylenediamine exclusively, but commercial Cu2O powder shows a comparatively lower 4-bromonitrobenzene conversion and yields a mixture of products. Use of sodium borohydride as a reducing agent resulted in the formation of deformed Cu particles and a low nitroaniline conversion percentage. Cu2O cubes cannot be converted to Cu particles with the addition of hydrazine, and nitroaniline conversion did not occur. Nitro group reduction is successful with high yields for diverse nitroarene molecules giving only a single product starting from a solution of the nitroarene compound, Cu2O cubes and ammonia borane.
33 citations
TL;DR: Cu2O nanocrystals can be pseudomorphically converted into Cu crystals through ammonia borane reduction, releasing hydrogen for stereoselective semihydrogenation of diphenylacetylene.
Abstract: Cu2O cubes, octahedra, and rhombic dodecahedra can be pseudomorphically converted into Cu crystals of corresponding shapes through reduction by ammonia borane in ethanol at 50 °C or below within 3 min, demonstrating the feasibility of making challenging polyhedral metal particles from metal oxide crystals. Hydrogen gas is also produced from ammonia borane in the process. The obtained Cu crystals have a slightly nanoporous interior. Addition of diphenylacetylene in the formation of Cu rhombic dodecahedra leads to complete stereoselective production of sterically hindered (Z)-stilbene. Semihydrogenation of other alkynes also gives pure (Z)-alkenes. Cu cubes and octahedra also showed considerable (Z)-stilbene selectivity along with minor formation of (E)-stilbene and bibenzyl as compared to CuCl2 and commercial Cu2O particles. Mechanistic studies reveal that the low binding affinity of alkenes on the rhombic dodecahedra surfaces leads to high product selectivity. These Cu crystals act as a green and low-cost catalyst for the synthesis of high-purity (Z)-alkenes.
30 citations
TL;DR: Wurtzite CdS nanoparticles have been lightly deposited on Cu2O cubes, octahedra, and rhombic dodecahedra to examine facet effects on the interfacial charge transfer in a photocatalytic reaction as mentioned in this paper.
Abstract: Wurtzite CdS nanoparticles have been lightly deposited on Cu2O cubes, octahedra, and rhombic dodecahedra to examine facet effects on the interfacial charge transfer in a photocatalytic reaction. Instead of an expected photocatalytic activity enhancement on the basis of a favorable band alignment at the heterojunction, CdS-decorated Cu2O octahedra and rhombic dodecahedra show drastically reduced photocatalytic activities. Further increasing the CdS deposition amount leads to complete suppression of photocatalytic activity. Cu2O cubes remain inactive even after CdS deposition. Transmission electron microscopy analysis reveals epitaxial growth of the (101) planes of CdS on the (110) planes of a Cu2O rhombic dodecahedron, whereas the (110) planes of CdS align parallel to the (111) planes of a Cu2O octahedron. Because facet-dependent photocatalytic activity can be understood from different degrees of band bending at the crystal surfaces, significantly upward bending for the CdS-contacting planes can explain th...
29 citations
TL;DR: In this article, the authors showed that changing the reaction cell potential enables particle shape evolution by adjusting the metal precursor concentration has not been demonstrated before, although various parameters in the Nernst equation has been used to achieve this.
Abstract: Past work on the synthesis of gold nanocrystals has revealed that changing the reaction cell potential enables particle shape evolution. Although various parameters in the Nernst equation has been used to achieve this, adjusting the metal precursor concentration has not been demonstrated before. Using the reported synthetic conditions with cetyltrimethylammonium chloride (CTAC) surfactants to grow gold nanocubes, rhombic dodecahedra, and octahedra as the starting points, fixing the ascorbic acid volume, and tuning the volume of HAuCl4 solution introduced allow the formation of the same series of crystal shape evolution from rhombic dodecahedral to trisoctahedral and cubic structures in the presence of a tiny amount of NaBr. In another series, by gradually increasing the HAuCl4 solution volume but fixing potassium iodide and ascorbic acid volumes, gold nanocrystals with tunable morphologies from octahedra to corner-truncated octahedra, edge- and corner-truncated octahedra, and rhombic dodecahedra can be sy...
21 citations
TL;DR: This work demonstrates that germanium wafers also possess facet-dependent electrical conductivity responses that can be utilized in the fabrication of novel fin field-effect transistors (finFET).
Abstract: Electrical conductivity properties of Ge {100}, {110}, {111}, and {211} facets have been measured by breaking Ge (100) and (111) wafers to expose {110} and {211} surfaces and contacting the different facets with tungsten probes. Ge {111} and {211} faces are far more conductive than the already conductive Ge {100} and {110} faces, matching with recent density functional theory (DFT) predictions. Asymmetric I-V curves resembling those of p-n junctions have been collected for the {110}/{111} and {110}/{211} facet combinations. The current-rectifying effects stem from different degrees of surface band bending for the highly and less conductive faces and the direction of current flow. This work demonstrates that germanium wafers also possess facet-dependent electrical conductivity responses that can be utilized in the fabrication of novel fin field-effect transistors (finFET).
17 citations