Ordered fragmentation of oxide thin films at submicron scale.

Crack formation is typically undesirable as it represents mechanical failure that compromises strength and integrity. Recently, there have also been numerous attempts to control crack formation in materials with the aim to prevent or isolate crack propagation. In this work, we utilize fragmentation, at submicron and nanometre scales, to create ordered metal oxide film coatings. We introduce a simple method to create modified films using electroplating on a prepatterned substrate. The modified films undergo preferential fragmentation at locations defined by the initial structures on the substrate, yielding ordered structures. In thicker films, some randomness in the characteristic sizes of the fragments is introduced due to competition between crack propagation and crack creation. The method presented allows patterning of metal oxide films over relatively large areas by controlling the fragmentation process. We demonstrate use of the method to fabricate high-performance electrochromic structures, yielding good coloration contrast and high coloration efficiency.

Diameter dependence of the void formation in the oxidation of nickel nanowires.

In this work, we show how the vacancy diffusion length scale must be considered, in the context of the diameter of a nanowire, when utilizing the Kirkendall phenomenon in the fabrication of metal oxide nanotubes starting from metal nanowires. We find that the diameter of the nanowire relative to the diffusion length scale of the vacancy will affect greatly the type of voids that can be generated. By using a larger diameter nickel nanowire, we show that segmented heterojunction void formation can be avoided and that the resulting structure will serve as a precursory 'template' for subsequent oxidation processes at high temperatures. In doing so, we can prevent the formation of bamboo-like structures and obtain uniform nickel oxide nanotubes through direct oxidation that has proven to be difficult previously. The result from this work is also significant as the interplay of vacancy diffusion length and nanostructure dimension is important in the oxidation of other types of metal nanostructures, especially when void formation and the Kirkendall effect are involved.

Investigation of bipolar resistive switching and the time-dependent SET process in silver sulfide/silver thin films and nanowire array structures.

We report on the bipolar resistive switching (RS) behaviour observed from silver sulfide/silver (Ag2S/Ag) nanowire array and thin film structures fabricated under similar conditions. By examining the RS parameters measured using electrical probing with a similar tungsten probe on both types of structures, we conclude that the larger SET voltage and lower ON-state conductance in the thin film structures are the result of the longer conductive filamentary paths formed during the SET process. In addition, we found that the metal filament can be established at a constant voltage bias which is much lower than the measured SET voltage for a sweeping voltage bias, as long as the constant bias/stress voltage is applied for a sufficiently long duration. This time dependency in the SET process is possibly related to the migration and reduction of silver ions to form silver atoms at the filamentary formation site. Experimental results also show that an applied voltage bias, either with increasing magnitude or duration, will increase the ON-state conductance. This is explained by the increased cross-sectional area of the conductive filamentary path. From the comparative investigation of the RS parameter values obtained from the two different structures, it is concluded that nanostructuring of the Ag2S/Ag heterostructure from a two-dimensional thin film to a one-dimensional nanowire structure results in an improvement in the SET process parameters.

Heterostructures of germanium nanowires and germanium-silicon oxide nanotubes and growth mechanisms.

We report on a method to fabricate one-dimensional heterostructures of germanium nanowires (GeNWs) and germanium-silicon oxide nanotubes (GeSiO(x)NTs). The synthesis of the wire-tube heterostructures is carried out using a simple furnace set-up with germanium tetraiodide and germanium powders as growth precursors, gold-dotted silicon wafers as substrates and by controlling the temperature ramp rate/sequence of the growth precursors. Two types of wire-tube heterostructures resulting from distinct growth mechanisms are obtained. The type-1 heterostructure consists of a GeNW, grown via a gold-catalyzed vapour-liquid-solid process, at the lower end and a GeSiO(x)NT at the upper end. In contrast, the type-2 heterostructure is made up of a solid wire at the upper end and a hollow tube at the lower end. The solid wire portion of the type-2 heterostructure is formed through an oxide-assisted growth process.

Comparison of the synthesis of Ge nanocrystals in hafnium aluminum oxide and silicon oxide matrices.

Growth of germanium (Ge) nanocrystals in silicon (Si) oxide and hafnium aluminum oxide (HfAlO) is examined. In Si oxide, nanocrystals were able to form at annealing temperatures of 800 degrees C to 1000 degrees C. Nanocrystals formed at 800 degrees C were round and approximately 8 nm in diameter, at 900 degrees C they become facetted and at 1000 degrees C they become spherical again. In HfAlO, at 800 degrees C nanocrystals formed are relatively smaller (approximately 3 nm in diameter) and lower in density. While at 900 degrees C and 1000 degrees C, nanocrystals did not form due to out-diffusion of Ge. Different nanocrystal formation characteristics in the matrices are attributed to differences in their crystallization temperatures.