High density vertical zinc oxide nanowire arrays were fabricated using highly ordered channels in anodic alumina membranes via chemical vapor deposition assisted by electrochemical deposition methods. Using conductive atomic force microscopy, the electrical transport and photoconduction of individual vertical nanowires were investigated. A negative photoconductivity was observed as a result of electron trapping in the alumina membrane. In contrast, positive photoconductivity was observed using a thermally annealed anodic alumina membrane as the nanowire growth template. These studies provide a pathway for constructing highly integrated nanoscale electronic and optoelectronic circuits, such as logic circuits, light emitting diodes, solar cells, and ultrahigh resolution imaging sensors.
Power conversion efficiency of gallium arsenide (GaAs) nanowire (NW) solar cells is severely limited by enhanced charge recombination (CR) at sidewall surfaces, but its atomistic mechanisms are not well understood. In addition, GaAs NWs usually contain a high density of twin defects that form a twin superlattice, but its effects on CR dynamics are largely unknown. Here, quantum molecular dynamics (QMD) simulations reveal the existence of an intrinsic type-II heterostructure at the (110) GaAs surface. Nonadiabatic quantum molecular dynamics (NAQMD) simulations show that the resulting staggered band alignment causes a photoexcited electron in the bulk to rapidly transfer to the surface. We have found orders-of-magnitude enhancement of the CR rate at the surface compared with the bulk value. Furthermore, QMD and NAQMD simulations show unique surface electronic states at alternating (111)A and (111)B sidewall surfaces of a twinned [111]-oriented GaAs NW, which act as effective CR centers. The calculated large surface recombination velocity quantitatively explains recent experimental observations and provides microscopic understanding of the underlying CR processes. V C 2015 AIP Publishing LLC. [http://dx.
As a first step towards mechanistic studies of fuel cell electrodes with both well-defined and functionally representative structural features, two-dimensional anti-dot metal films with tunable features are prepared. The fabrication employs a facile, sacrificial templating method, known as polymer sphere lithography, and the resulting metal films are fully connected, yet fully porous. Using initial bead sizes in the range of 500 nm to 3.2 mm and oxygen plasma etching to remove from ¼ to 3 ⁄4 of the original bead diameter, computed triple phase boundary densities in the porous films of 2,000 to 43,500 cm cm À2 are achieved. Image analysis shows the computed (theoretical) and experimental structural features to be in good agreement, demonstrating sufficient perfection in the films for electrochemical studies. Furthermore, thermal stability under hydrogen of thermally evaporated Ni films is excellent, with negligible change in triple phase boundary length as required for quantitative electrochemical measurements. Ultimately, these two-dimensional metallic networks may also serve as the platform for future fabrication of three-dimensional electrodes with truly optimized structural features.
Semiconductor nanowires (NWs) often contain a high density of twin defects that form a twin superlattice, but its effects on electronic properties are largely unknown. Here, nonadiabatic quantum molecular dynamics simulation shows unique surface electronic states at alternating (111)A and (111)B sidewall surfaces of a twinned [111]-oriented GaAs NW, which act as effective charge-recombination centers. The calculated large surface recombination velocity quantitatively explains recent experimental observations and provides microscopic understanding of the underlying surface-recombination processes.
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