Assembly and transport properties of zero and one dimensional structures

Two novel assembly processes of zero dimensional structures are presented. First, two-dimensional (2-D), submonolayer annular rings of organically functionalized Ag nanocrystals are formed on solid substrates after controlled evaporation of Ag nanocrystal solution and three-dimensional (3-D) cylindrical ring structures are fabricated based on the template of two-dimensional (2-D) Ag nanocrystal rings. Second, the 2-D structure map (similar to a phase diagram) of Ag nanocrystal superstructures was elucidated. These superstructures were formed at the air-water interface. Two different nanocrystal building blocks, such as Ag nanocrystals with larger metal core/short-chain surfactants and Ag nanocrystals with smaller metal core/long-chain surfactants were used for this work. Three different superstructural motifs, each depending on the composition of the building blocks, are presented with the results of the corresponding assembly structures.; Fabrication and transport properties of one-dimensional (1-D) structures are presented. Chemical vapor deposition of SiH₄, coupled with Au- and Zn-nucleated vapor-liquid-solid (VLS) growth, was used to synthesize high aspect ratio, 20 ± 5 nm diameter single crystal Si nanowires (SiNWs), characterized by a nanowire growth directions of ⟨111¯⟩ and ⟨211⟩. The resistivity of as-prepared Au- and Zn-SiNW devices is greater than 10 6 Ω·cm. The resistance of these devices can be lowered significantly via thermal annealing in flowing Ar/H₂. The contacts of the devices are characterized by Schottky barriers, and temperature dependent measurements indicate that tunneling is the dominant mechanism for carrier injection. After annealing the nanowire devices in the temperature range 750–800°C, the resistivities of annealed Si-NW's are ∼30 Ω·cm and 10 mΩ·cm for Zn-SiNWs and Au-SiNWs, respectively, and the contacts were effectively Ohmic. This decreased resistance of the nanowire devices was attributed to doping of the nanowires by the Au or Zn catalyst used for wire nucleation and growth. The nanowire resistivities reported for the most highly doped wires are substantially lower than would be expected based on the solid solubility of either Au or Zn in Si. The role that surface states play in electron transport through these nanowire devices is unknown, but are likely responsible for the observed low resistivities.