Morphological evolution of silver nanoparticles and its effect on metal-induced chemical etching of silicon.

In this report, we have demonstrated the morphological evolution of the silver nanoparticles (AgNPs) by controlling the growth conditions and its effect on morphology of silicon (Si) during metal-induced electroless etching (MICE). Self-organized AgNPs with peculiarly shape were synthesized by an electroless plating method in a conventional aqueous hydrofluoric acid (HF) and silver nitrate (AgNO3) solution. AgNP nuclei were densely created on Si wafer surface, and they had a strong tendency to merge and form continuous metal films with increasing AgNO3 concentrations. Also, we have demonstrated that the fabrication of aligned Si nanowire (SiNW) arrays in large area of p-Si (111) substrates by MICE in a mixture of HF and hydrogen peroxide (H2O2) solution. We have found that the morphology of the initial AgNPs and oxidant concentration (H2O2) greatly influence on the shape of the SiNW etching profile. The morphological results showed that AgNP shapes were closely related to the etching direction of SiNWs, that is, the spherical AgNPs preferred to move vertical to the Si substrate, whereas non-spherical AgNPs changed their movement to the [100] directions. In addition, as the etching activity was increased at higher H2O2 concentrations, AgNPs had a tendency to move from the original [111] direction to the energetically preferred [100] direction.

Ultralong ZnO nanowire arrays synthesized by hydrothermal method using repetitive refresh.

Long ZnO nanowire arrays (> or = 10 microm) were fabricated using the hydrothermal method and the refresh process of the reactant solution. The diameter of the synthesized nanowires was controlled by varying the solution concentration of the seed layers, without reducing their length. The maximum temperature in this process was 95 degrees C and the repeated refresh process at 95 degrees C provided the driving force for the growth of ultralong nanowires by exchanging the reactants. Interestingly, the diameter of the refreshed ZnO nanowires strongly depended on the solution temperature during refresh. The exchange of the reactant solution at the same temperature as the synthesis temperature induced the synthesis of ultralong nanowires and the length of the resultant nanowires can be controlled by varying the repetition number. The illumination of the ultraviolet light induced considerably enhanced current flow in the ultralong nanowires from mid 10(-10) to 10(-7) A at 5 V.

Over 95% of large-scale length uniformity in template-assisted electrodeposited nanowires by subzero-temperature electrodeposition.

In this work, we report highly uniform growth of template-assisted electrodeposited copper nanowires on a large area by lowering the deposition temperature down to subzero centigrade. Even with highly disordered commercial porous anodic aluminum oxide template and conventional potentiostatic electrodeposition, length uniformity over 95% can be achieved when the deposition temperature is lowered down to -2.4°C. Decreased diffusion coefficient and ion concentration gradient due to the lowered deposition temperature effectively reduces ion diffusion rate, thereby favors uniform nanowire growth. Moreover, by varying the deposition temperature, we show that also the pore nucleation and the crystallinity can be controlled.

Controlled growth of heteroepitaxial zinc oxide nanostructures on gallium nitride.

ZnO epitaxial layers were grown on GaN underlying films by metalorganic chemical vapor deposition at various temperatures. An increase in growth temperature led to morphological changes from a smooth film with hexagonal-shaped surface pits to honeycomb-like nanostructures with deep hollow, and additionally resulted in a decrease in dislocation density in the interfacial layers. The reduced dislocation density at the higher growth temperature was attributed to an increase in the size of the critical nucleus and the low nucleation density at the initial stage. The shifts in the peak positions in the X-ray diffraction and photoluminescence were also observed in the samples grown at different temperatures, and were caused by the variation of residual strains after the complete coalescence of the nuclei.

Ga-doped ZnO nanorod arrays grown by thermal evaporation and their electrical behavior.

Vertically well-aligned Ga-doped ZnO nanorods with different Ga content were grown by thermal evaporation on a ZnO template. The Ga-doped ZnO nanorods synthesized with 50 wt% Ga with respect to the Zn content showed minimum compressive stress relative to the ZnO template, which led to a rapid growth rate along the c-axis due to the rapid release of stored strain energy. A further increase in the Ga content improved the conductivity of the nanorods due to the substitutional incorporation of Ga atoms in the Zn sites based on a decrease in lattice spacing. A p-n diode structure with Ga-doped ZnO nanorods as an n-type layer displayed a distinct white light luminescence from the side view of the device, showing weak ultraviolet and various deep-level emissions.

Synthesis and characterization of ZnO/MgZnO heterostructure nanorods by simple two-step evaporation.

ZnO-core/MgZnO-shell heterostructure nanorods with high aspect ratio were synthesized using a two-step thermal evaporation procedure, in which the core and the shell layers were formed separately at different temperatures. Microstructural characterization revealed a position dependence of the crystal structure and composition in the shell layer. The shell layer in the upper region consisted of MgO with quantum dot-like structure having cubic phases embedded in an amorphous oxide layer, while a Mg(0.35)Zn(0.65)O shell layer with a self-assembled superlattice structure of triple periodicity was formed in the middle region.