Formation of nanoscale fine-structured silicon by pulsed laser ablation in hydrogen background gas

Abstract

The formation of nanoscale fine structures during pulsed laser ablation of a silicon target in a hydrogen atmosphere has been studied by analyzing the deposited silicon fine structures prepared under different conditions. Transmission electron microscopy, scanning electron microscopy ͑SEM͒, Raman scattering, and infrared absorption studies on the deposited samples indicate that silicon nanocrystallites are produced when the background gas pressure is higher than a critical value. The deposited substance is found to show hierarchical structure having surface hydrogenated silicon nanocrystallites as the primary structure and aggregates of the nanocrystallites as the secondary structure. The secondary structure depends on the hydrogen background gas pressure, while the size of the primary nanocrystallites is 4 -5 nm independent of the pressure. These results suggest that the fine structure is formed in two steps; the silicon nanocrystallites having a stable surface are initially formed and they are subsequently aggregated to form the secondary structure. Analysis of surface free energy suggests that the stability is acquired by termination of the surface by creation of Siu H bonds. We carried out fractal analysis of the SEM image of the deposits and found that the secondary structure shows good self-similar structure when deposited at higher background gas pressure. The fractal dimension of aggregated secondary structure varies from 1.7 to more than 2.0 with decreasing background gas pressure. Comparison of these values with reported results for the fractal growth simulation indicates that the region at which aggregation of the nanocrystallites takes place changes from in the plume to on the substrate with decreasing background gas pressure. Effects of the hydrogen background gas on the nanocrystallization process and spatial distribution of formed nanocrystallites in the plume are discussed. The formation of surface stabilized Si nanocrystallites and their spatial confinement by background gas in the first and second steps determine the hierarchical structure of deposited substance.

Grain size tuning of nanostructured Cu2O films through vapour phase supersaturation control and their characterization for practical applications

“…Although the fractal dimension does not directly correspond to the porosity, aggregated materials which have smaller fractal dimensions are reported 29,30 to have higher porosity and hence a decrease in the fractal dimension may be considered to be qualitatively corresponding to an increase in the porosity. 31 In the present study, the lower values of the fractal dimension of the nanostructured Cu 2 O films deposited at lower supersaturations indicate larger porosity in agreement with the larger measured values of porosity of these films (Table I).…”

Aluminum Nano-polycrystalline Substance with Ferromagnetics and Application to High-Frequency Core Inductor

“…Producing any kind of nanoparticle with a diameter close to 2 nm in liquids is easily possible. It has been proven in various papers in which scanning transmission electron microscope (STEM) analyses were performed, and the structures and sizes have already been observed [12][13][14]. The size of the produced nanoparticles is not changed by materials.…”

“…The size of the produced nanoparticles is not changed by materials. Additionally, the proof of the mean size, i.e., the size of the produced Si nanoparticles needed for them to emit violet light, should be lower than 2 nm, and the bandgap energy enhanced by the size effect must be larger than 2 eV [12][13][14].…”

“…SQUID measurements of the magnetic hysteresis revealed that the Al nano-polycrystalline structure was ferromagnetic. Using this method, sintered Al nano-polycrystalline structures from Al oxide powder can be easily fabricated at low cost in a short time [12][13][14][15][16][17]. These nano-metals with small poly-crystalline structures are novel materials, and their magnetic and mechanical properties, aside from some of the electrical properties, have not yet been clarified.…”

“…Bulk Al is normally known to be non-magnetic. However, authors fabricated Al nanoparticles with a mean diameter of less than 20 nm from Al oxide (alumina) particles [12][13][14][15][16], and obtained Al nano-polycrystalline structures using the Al nanoparticles [15]. SQUID measurements of the magnetic hysteresis revealed that the Al nano-polycrystalline structure was ferromagnetic.…”