Modification of electronic properties of top-gated graphene devices by ultrathin yttrium-oxide dielectric layers.

We report the structure characterization and electronic property modification of single layer graphene (SLG) field-effect transistor (FET) devices top-gated using ultrathin Y(2)O(3) as dielectric layers. Based on the Boltzmann transport theory within variant screening, Coulomb scattering is confirmed quantitatively to be dominant in Y(2)O(3)-covered SLG and a very few short-range impurities have been introduced by Y(2)O(3). Both DC transport and AC capacitance measurements carried out at cryogenic temperatures demonstrate that the broadening of Landau levels is mainly due to the additional charged impurities and inhomogeneity of carriers induced by Y(2)O(3) layers.

Asymmetric ZnO panel-like hierarchical architectures with highly interconnected pathways for free-electron transport and photovoltaic improvements.

Through a rapid and template-free precipitation approach, we synthesized an asymmetric panel-like ZnO hierarchical architecture (PHA) for photoanodes of dye-sensitized solar cells (DSCs). The two sides of the PHA are constructed differently using densely interconnected, mono-crystalline and ultrathin ZnO nanosheets. By mixing these PHAs with ZnO nanoparticles (NPs), we developed an effective and feasible strategy to improve the electrical transport and photovoltaic performance of the composite photoanodes of DSCs. The highly crystallized and interconnected ZnO nanosheets largely minimized the total grain boundaries within the composite photoanodes and thus served as direct pathways for the transport and effective collection of free electrons. Through low-temperature (200 °C) annealing, these novel composite photoanodes achieved high conversion efficiencies of up to 5.59% for ZnO-based quasi-solid DSCs.

ZnO hierarchical structures for efficient quasi-solid dye-sensitized solar cells.

We report a direct precipitation method for mass production of ZnO microflowers (MFs) containing hierarchical structures. The ZnO MFs are constructed by interlaced single crystalline and porous nanosheets which are ideal photoanode material for dye-sensitized solar cells (DSCs) because the MFs can largely improve the energy harvesting performance and the efficiency of DSCs. Compared with other forms of nano-sized structures, the novel hierarchical structures show obvious advantages in DSC application because of their large surface area for dye-loading, good light scattering efficiency and excellent electrical transport property. The quasi-solid state DSCs fabricated with the MF hierarchical structures exhibited an efficiency of 4.12%, much higher than that of ZnO nanoparticle-based DSCs, indicating a great potential for the development of highly-efficient quasi-solid DSCs.

Vertically aligned ZnO/amorphous-Si core-shell heterostructured nanowire arrays.

We report the synthesis of vertically aligned ZnO/a-Si core-shell nanowire arrays (ZnO nanowires coated with amorphous silicon) through chemical vapor deposition. The core-shell heterostructured nanowires possessed uniform morphology and the thickness of the amorphous silicon shells could be controlled easily by tuning the deposition duration and temperature. The core-shell heterostructured nanowires exhibited enhanced antireflection and absorption performance as well as tunable PL properties. Because the individual ZnO/a-Si nanowires showed p-type characteristics and the ZnO cores were n-type semiconductors, the core-shell nanowires formed p-n junctions naturally.

Nanostructural transformation and formation of heterojunctions from Si nanowires.

Si nanowires coated with Ni showed interesting structural transformation behaviors as observed by in situ transmission electron microscopy. Owing to the presence of the native oxide on Si nanowire surfaces, the Ni thin shells initially segregated into nanosized droplets on the oxide surfaces. The native oxide shells protected the Si cores from reacting with Ni at temperatures below 1350 °C. Ni started the reaction with Si nanowires preferentially at the defects or bending regions of the nanowires. Because the reaction temperature was sufficiently high, the structural transformation was extremely fast and completed within 0.1 s. The resulting nanowires were single crystalline NiSi(2), the most desirable Ni silicide structure for potential applications. Nanowire junctions of NiSi(2)/Si and nanowire-nanotube junctions of NiSi(2)/SiC have been obtained upon further annealing.

High-quality ZnO nanowire arrays directly fabricated from photoresists.

We report a simple and effective method for fabricating and patterning high-quality ZnO nanowire arrays using carbonized photoresists to control the nucleation site, density, and growth direction of the nanowires. The ZnO nanowires fabricated using this method show excellent alignment, crystal quality, and optical properties that are independent of the substrates. The carbonized photoresists provide perfect nucleation sites for the growth of aligned ZnO nanowires and they also perfectly connect to the nanowires to form ideal electrodes that can be used in many applications of ZnO nanomaterials.