Showing papers by "Michael H. Huang published in 2015"
TL;DR: When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components.
Abstract: It is interesting to examine facet-dependent electrical properties of single Cu2O crystals, because such study greatly advances our understanding of various facet effects exhibited by semiconductors. We show a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is nonconductive. The conductivity differences are ascribed to the presence of a thin surface layer having different degrees of band bending. When electrical connection was made on two different facets of a rhombicuboctahedron, a diode-like response was obtained, demonstrating the potential of using single polyhedral nanocrystals as functional electronic components. Density of state (DOS) plots for three layers of Cu2O (111), (100), and (110) planes show respective metallic, semimetal, and semiconducting band structures. By examining DOS plots for varying number of planes, the surface layer thicknesses responsible for the facet-dependent electrical properties of Cu2O crystals have been determined to be below 1.5 nm for these facets.
192 citations
TL;DR: This work again shows that the observed facet-dependent optical phenomenon in metal-Cu2O core-shell nanocrystal nanocrystals is derived from the Cu2O shells.
Abstract: Pd–Cu2O core–shell nanocubes and truncated octahedra with six average sizes for each particle shape have been synthesized from 29 nm Pd nanocubes. The nanocubes have average edge lengths of 64–124 nm, while the truncated octahedra are 107–183 nm in the opposite tip distance. The core and shell composition and lattice orientation have been determined, showing the formation of single-crystalline Cu2O shells. The surface plasmon resonance (SPR) band from the Pd nanocrystal cores is barely visible. However, the Cu2O shells display facet-dependent optical properties. The Cu2O absorption band for smaller Pd–Cu2O cubes is consistently more red-shifted than somewhat larger Pd–Cu2O truncated octahedra. This work again shows that the observed facet-dependent optical phenomenon in metal–Cu2O core–shell nanocrystals is derived from the Cu2O shells. The use of 40 nm Pd cubes as cores gave uniform and size-tunable Pd–Cu2O nanocubes and truncated octahedra that display the Pd SPR band. The Pd SPR band is consistently located at 650 nm for Pd–Cu2O truncated octahedra, and 670 nm for the cubes despite large variation in the shell thickness. Both the Cu2O absorption and the Pd plasmonic band exhibit facet-dependent optical properties.
50 citations
TL;DR: Au-Cu2 O core-shell nanocubes, octahedra, and rhombic dodecahedra display facet-dependent optical properties and clearly show a transition from the shell thickness-independent gold surface plasmon resonance band for oct ahedra with smaller gold cores to progressive red-shifts of the band with increasing shell thickness in octahedral gold cores.
Abstract: Au-Cu2 O core-shell nanocubes, octahedra, and rhombic dodecahedra display facet-dependent optical properties. Furthermore, different-sized Au-Cu2 O octahedra with 58, 65, 68, and 73 nm octahedral gold cores clearly show a transition from the shell thickness-independent gold surface plasmon resonance band for octahedra with smaller gold cores to progressive red-shifts of the band with increasing shell thickness in octahedra with larger gold cores.
49 citations
TL;DR: In this article, a review provides examples and advances in fabricating polyhedral supercrystals on a substrate during the process of solvent evaporation and through diffusion transport of surfactant to generate free-standing super-crystals.
49 citations
TL;DR: The use of Au cores not only allows the successful formation of Au-Cu2O core-shell nanocrystals with tunable sizes, but also enables the observation of facet-dependent optical properties in these crystals through the Au localized surface plasmon resonance (LSPR) absorption band.
Abstract: The ability to prepare Au-Cu2O core-shell nanocrystals with precise control over particle size and shape has led to the discovery of facet-dependent optical properties in cuprous oxide crystals. The use of Au cores not only allows the successful formation of Au-Cu2O core-shell nanocrystals with tunable sizes, but also enables the observation of facet-dependent optical properties in these crystals through the Au localized surface plasmon resonance (LSPR) absorption band. By tuning the Cu2O shell morphology from rhombic dodecahedral to octahedral and cubic structures, and thus the exposed facets, the Au LSPR band position can be widely tuned. Such facet-dependent optical effects are not observed in bimetallic Au-Ag and Au-Pd core-shell nanocrystals with the same precisely tuned particle sizes and shapes. It is believed that similar facet-dependent optical properties could be observed in other ionic solids and other metal-metal oxide systems. The unusually large degree of plasmonic band tuning covering from the visible to the near-infrared region in this type of nanostructure should be quite useful for a range of plasmonic applications.
48 citations
TL;DR: Observation of cross-sectional samples indicates shape preservation of interior nanocrystals within a supercrystal, and the generality of the surfactant diffusion approach to form supercrystals with diverse composition is shown.
Abstract: Au–Pd core–shell nanocrystals with cubic, truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures have been employed to form micrometer-sized polyhedral supercrystals by both the droplet evaporation method and novel surfactant diffusion methods. Observation of cross-sectional samples indicates shape preservation of interior nanocrystals within a supercrystal. Low-angle X-ray diffraction techniques and electron microscopy have been used to confirm the presence of surfactant between contacting nanocrystals. By diluting the nanocrystal concentration or increasing the solution temperature, supercrystal size can be tuned gradually to well below 1 μm using the surfactant diffusion method. Rectangular supercrystal microbars were obtained by increasing the amounts of cubic nanocrystals and surfactant used. Au–Ag core–shell cubes and PbS cubes with sizes of 30–40 nm have also been fabricated into supercrystals, showing the generality of the surfactant diffusion approach to form supercrystal...
46 citations
TL;DR: The concentrations of KBr and KI in the solution are so low that spectral shifts were not detected upon their addition to the solution and the Pd nanocrystals can readily be used for various applications after simple removal of surfactant.
Abstract: Palladium octahedra, truncated octahedra, cuboctahedra, truncated cubes, and nanocubes with sizes of tens of nanometers have been synthesized in an aqueous mixture of H2PdCl4 solution, cetyltrimethylammonium chloride (CTAC) surfactant, KBr solution, dilute KI solution, and ascorbic acid solution at 35 °C for 30 min. By tuning the amount of dilute KBr solution introduced, particle shape control can be achieved. Adjusting the volumes of the Pd precursor and KBr solutions added, smaller and larger Pd nanocrystals were obtained with excellent shape control. Extensive structural and optical characterization of these nanocrystals has been performed. Two absorption bands in the ultraviolet region can be discerned for these Pd nanocrystals. Concave Pd cubes can also be prepared. Pd cubes were found to grow at a faster rate than that for the formation of octahedra. The concentrations of KBr and KI in the solution are so low that spectral shifts were not detected upon their addition to the solution. The Pd nanocrys...
43 citations
TL;DR: In this article, four types of gold nanoparticles with distinctly different shapes and great uniformity were designed and synthesized, including cubes, rhombic dodecahedra (RD), edge-and corner-truncated octahedras (ECTO), and triangular plates, to systematically probe their influences on photovoltaics.
Abstract: In the work, mechanisms behind various 3D nanocrystals enhanced performance of bulk heterojunction solar cells were studied comprehensively. Four types of gold nanoparticles (NPs) with distinctly different shapes and great uniformity were designed and synthesized, including cubes, rhombic dodecahedra (RD), edge- and corner-truncated octahedra (ECTO), and triangular plates, to systematically probe their influences on photovoltaics. RD and triangular plates show a higher growth rate, while slower growth favors cubes and ECTO formation by controlling the reduction agent and capping ion amount. NPs with increasing corners and proper size of cross-section induce stronger near-field coupling and far-field scattering in P3HT:PC61BM-based active layers. Both finite-difference time-domain simulation and UV–visible absorption spectra firmly support that RD exhibit the strongest localized surface plasmon resonance and optical scattering. With optimized conditions, a high power conversion efficiency exceeding 4% was ...
38 citations
TL;DR: Aqueous phase synthesis of small Au-Ag core-shell nanocubes, cuboctahedra, and octahedr is achieved through the deposition of Ag shells on small octahedral Au cores, which show efficient photothermal activity and can assemble into supercrystals.
Abstract: Aqueous phase synthesis of small Au-Ag core-shell nanocubes, cuboctahedra, and octahedra is achieved through the deposition of Ag shells on small octahedral Au cores. These nanocrystals show efficient photothermal activity and can assemble into supercrystals.
25 citations
TL;DR: Examination of the reagent introduction sequence indicates that early or late addition of nitric acid to tune the initial solution pH can strongly influence crystal growth rate and result in the formation of PbS crystals with different sizes and shapes.
Abstract: A one-pot synthesis for the growth of PbS nanocubes and octahedra has been developed by heating an aqueous mixture of cetyltrimethylammonium bromide (CTAB), thioacetamide (TAA), lead acetate, and nitric acid at 90 °C for 3 h. The method allows for direct large-scale production of small and uniform PbS nanocubes. The PbS cubes and octahedra exhibit different surface properties, as evidenced by zeta potential measurements and stability tests in methyl orange and methylene blue solutions. Examination of the reagent introduction sequence indicates that early or late addition of nitric acid to tune the initial solution pH can strongly influence crystal growth rate and result in the formation of PbS crystals with different sizes and shapes. Remarkably, the use of pre-formed PbS nanocubes for further crystal growth under low TAA concentration can transform the cubes into large octahedra through a series of particle shape evolution.
24 citations
TL;DR: Pd nanocubes, cuboctahedra, and octahedras with good size control were used to catalyze CN bond formation in the Tsuji-Trost allylic amination reaction of aniline as mentioned in this paper.
Abstract: Pd nanocubes, cuboctahedra, and octahedra with good size control were used to catalyze CN bond formation in the Tsuji–Trost allylic amination reaction of aniline. Nanocubes gave either monoallylaniline or diallylaniline depending on the amount of allyl bromide used, but the octahedra and cuboctahedra only gave mixtures of monoallylaniline and diallylaniline under the same reaction conditions. The Pd nanocubes were stable over multiple cycles of the reaction. The cubes and octahedra were demonstrated to catalyze the amination reaction by using a wide variety of substituted anilines, but the cubes were the best catalyst with consistently the highest efficiency, product yield, and product selectivity. This work demonstrates that the use of metal nanocrystals with proper facet control is important for catalyzing coupling reactions with product selectivity.
TL;DR: Pd nanoparticles with cubic, cuboctahedral, and octahedral shapes were used to catalyze the Tsuji-Trost allylic amination reaction of aniline derivatives as discussed by the authors.
Abstract: Pd nanoparticles with cubic, cuboctahedral, and octahedral shapes are used to catalyze the Tsuji—Trost allylic amination reaction of aniline derivatives.
TL;DR: In this paper, complete regioselectivity for formation of 1,4-triazoles was obtained using gold rhombic dodecahedra, which was obtained by using a gold-rhombic lattice.
Abstract: Complete regioselectivity for formation of 1,4-triazoles is obtained using gold rhombic dodecahedra.