Nanoscale Chemistry in One-dimension Peidong Yang, University of California, Berkeley, NSF-CAREER DMR Platonic Gold Nanocrystals Known to the ancient Greeks, there are five Platonic solids that can be constructed by selecting a regular convex polygon and having the same number of them meet at each corner: tetrahedron, octahedron, hexahedron (cube), icosahedron, dodecahedron. The beauty in their symmetry and their apparent simplicity continue to inspire generations of mathematicians and scientists. Recently, the concept of shape control has started to revitalize the centuries-old metal colloidal synthesis. The preparation of nanoparticles of highly symmetric Platonic shapes with a unified method, however, has yet to be demonstrated, and is by itself a scientific curiosity and great challenge that requires exquisite crystal growth control. We have developed a systematic shape-evolution of gold nanocrystals with sizes of nm in a modified polyol process. By adding surface-regulating polymer and foreign ions, we can readily access the distinct shapes of tetrahedron, cube, octahedron, and icosahedron with high yield and good uniformity. These nanocrystals have the perfect symmetry for 2- and 3-dimensional packing and therefore could enable the rational tuning of their optical, electrical, and catalytic properties. Angew. Chem. Int. Ed. 43, 3673, 2004.

Single Crystalline GaN Nanotubes The growth of single crystalline semiconductor nanotubes would be advantageous in potential nanoscale electronics, optoelectronics, and biochemical sensing applications. We’ve developed an “epitaxial casting” approach for the synthesis of single crystalline technologically important GaN nanotubes with inner diameters of nm and wall thicknesses of 5-50 nm. Hexagonal ZnO nanowires were used as templates for the epitaxial overgrowth of thin GaN layers in a chemical vapor deposition system. The ZnO nanowire templates were subsequently removed by simple thermal reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the substrates. This is the first example of single crystalline GaN nanotubes and the novel templating process should be applicable to many other semiconductor systems. Nanoscale Chemistry in One-dimension Peidong Yang, University of California, Berkeley, NSF-CAREER DMR Nature, 422, 599, C&EN cover story, Aug. 2005

Crystallographic Alignment of High Density Gallium Nitride Nanowire Arrays Crystallographic Alignment of High Density Gallium Nitride Nanowire Arrays Single-crystalline, one-dimensional semiconductor nanostructures are considered to be one of the critical building blocks for nanoscale optoelectronics. Elucidation of the vapor-liquid-solid (VLS) growth mechanism has already enabled precise control over nanowire position and size, yet to date no reports have demonstrated the ability to choose from different crystallographic growth direction of a nanowire array. Control over the nanowire growth direction is extremely desirable in that anisotropic parameters such as thermal and electrical conductivity, index of refraction, piezoelectric polarization, and band gap may be used to tune the physical properties of nanowires made from a given material. We have demonstrated the use of metal- organic chemical vapor deposition and appropriate substrate selection to control the crystallographic growth directions of high-density arrays of gallium nitride nanowires with distinct geometric and physical properties. Epitaxial growth of wurtzite gallium nitride on (100)  - LiAlO2 and (111) MgO single crystal substrates resulted in the selective growth of nanowires in the orthogonal [1-10] and [001] directions, exhibiting triangular and hexagonal cross sections and drastically different optical emission. The MOCVD process is entirely compatible with the current GaN thin film technology, which would lead to easy scale-up and device integration. Nanoscale Chemistry in One-dimension Peidong Yang, University of California, Berkeley, NSF-CAREER DMR Nature Materials, 3, 528, 2004.

Nanoribbon Waveguides for Subwavelength Photonics Integration While the electrical integration of chemically synthesized nanowires has been achieved using lithography, optical integration, which promises high speeds and greater device versatility, remains unexplored. We have investigated the properties and functions of individual crystalline oxide nanoribbons that act as subwavelength optical waveguides and assess their applicability as nanoscale photonic elements. The length, flexibility and strength of these structures enable their manipulation on surfaces, including the optical linking of nanoribbon waveguides and other nanowire elements to form networks and device components. We demonstrate the assembly of ribbon waveguides with nanowire light sources and detectors as a first step toward building nanowire photonic circuitry. Nanoscale Chemistry in One-dimension Peidong Yang, University of California, Berkeley, NSF-CAREER DMR Science, 305, 1269, Proc. Nat. Acad. Sci., 102, 7800, J. Phys. Chem. B. (Feature Article), 109, 15191, 2005.