| Summary: | 博士 === 國立清華大學 === 化學系 === 104 === In Chapter 1, we examined the facet-dependent electrical conductivity properties of single Cu2O crystals. We showed that a Cu2O octahedron is highly conductive, a cube is moderately conductive, and a rhombic dodecahedron is non-conductive. 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.
In Chapter 2, we have synthesized Au‒Cu2O core‒shell nanocubes, octahedra, and rhombic dodecahedra with three different sizes for each particle shape by using 35 nm octahedral gold cores to investigate their facet-dependent optical properties. Structural characterization of the Au‒Cu2O rhombic dodecahedra has been extensively performed. The surface plasmon resonance (SPR) absorption band arising from the gold cores is centered at 694, 721, and 741 nm for the Au‒Cu2O rhombic dodecahedra, octahedra, and nanocubes, respectively. Remarkably, tuning the oxide shell surface can produce a band position difference as large as 47 nm. The SPR band position is fixed despite large changes in the shell thickness. Cu2O shells also exhibit facet-dependent optical properties. A mixed sample of edge- and corner-truncated octahedra and truncated rhombic dodecahedra having significant {110} facets gives a gold SPR band at 710 nm, showing the band position is highly linked to the proportions of different Cu2O surfaces exposed. In addition, different-sized Au‒Cu2O core‒shell octahedra with 58, 65, 68, and 73 nm octahedral gold cores were prepared to clearly show the transition from the shell thickness-independent gold SPR peak for octahedra with smaller gold cores to a progressive red-shift of the band with increasing shell thickness in octahedra with larger gold cores.
In Chapter 3, we have synthesized Au@Ag‒Cu2O core‒shell rhombic dodecahedra, truncated octahedra and cuboctahedra with different sizes for each shape by using 42 nm cubic Au‒Ag cores for facet-dependent optical property examination. Truncated octahedra and cuboctahedra with tunable sizes were also prepared using 38 and 50 nm cubic Au‒Ag cores. The magnitudes of Au‒Ag SPR band red-shift at 260‒280 nm are much larger than those seen in Au‒Cu2O nanocrystals. By tuning the Cu2O shell shape from rhombic dodecahedral to cuboctahedral structures, the SPR absorption band separation can reach 55 nm. Facet-dependent optical absorption effect from the Cu2O shells can also be identified.
This thesis enhances our understanding of how crystal facets can strongly affect the electrical conductivity and optical absorption of Cu2O nanocrystals. In other studies, we have also shown that photocatalysis and organocatalysis, and more recently heat transmission, are also facet-dependent for Cu2O nanocrystals. As more demonstrations of facet-dependent effects are reported for Cu2O and other semiconductor materials, people would gradually realize and accept that many, if not all, semiconductor materials really possess various facet-dependent properties.
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