Search

Nanoshell formation has been studied experimentally in Ag/Au and Ag/Pd systems in a hemispherical geometry at different temperatures. The void formation in these systems is the result ofpureKirkendall-porosity formation, because it is caused mainly by the inequality of the intrinsic atomic fluxes and other effects (e.g. stresses), inevitably present during nanoshell formations in solid state reactions (oxides, sulphides), can be less important or can be neglected. The kinetics of the process was followed by Transmission Electron Microscopy. Both the growth and shrinkage regimes of the process were observed at the same temperature and even the temperature dependence of the characteristic time (tcr) describing the crossover of the two different regimes was observed. We succeeded to show that tcrshifts to smaller values with increasing temperature. This confirms the theoretical results:the growth and the shrinkage regimes are controlled by the faster as well as the slower diffusion coefficients (DAas well as DB), respectively. It is also illustrated that, confirming recent theoretical predictions, the pore radius linearly depends on the initial particle radius and the slope of this straight line increases with the average composition of the faster component.

It is shown that both the growth and shrinkage of hollow shells in Ag/Pd hemispherical core-shell nanostructures take place at the same temperature. The crossover time, t cr, between these regimes is shifted to lower values with increasing temperature. This result confirms that the growth and shrinkage regimes are controlled by the faster and slower diffusion coefficients (D Ag and D Pd), respectively. The pore radius, confirming recent theoretical predictions, linearly depends on the initial particle radius, and the slope of this straight line increases with the average composition of the faster component.

Interdiffusion in Zn/InSb system has been investigated under high (0.59×106 G) and low (1 G) gravity conditions at 673, 593 and 573K, respectively. Samples annealed at 0.59×106 G, 673K for 60 hours, indicated the formation of a periodic reaction layer structure. Since such structures can be formed in solid state reactions of ternary systems [, the effect of high gravitational field and high hydrostatic pressure (approximately 3kbar) in the formation of periodic patterns was investigated. Systematic investigations at ambient pressure and low gravitational field were carried out at 593 and 573 K in sandwich geometry. SEM and EDX analysis had shown that there are different phases between the initial components. Starting from the Zn side of the specimen there is a very thin single-phase with high (about 90%) In content. Next to it is a thin two-phase layer, containing mainly 50-50% InSb and some elongated Zn particles and then there is a thick phase with the composition of Zn5In2Sb4 which is followed by a similar two-phase mixture (InSb+Zn) similar to the Zn side of the sample. Although the diffusion zone is not a well developed periodic structure, but every layer (clearly distinguishable form the others and was either a single-or multiphase layer) grows with the time.

Solid state reactions between amorphous Si and crystalline Co have been investigated by 4W electrical resistance and TEM. Multilayered (with 10 periods of 5nm a-Si/5nm Co and 10 nma- Si/10nm Co layers) as well as tri-layered samples (20nm a-Si/3nmCoSi/6nm Co) were produced by magnetron sputtering and isothermally heat treated at different temperatures between 473 and 523K. From the time evolution of the normalized resistance the kinetics of the process were determined by fitting a power law, tk, and k was between 0.8 and 1. Possibility of the interface reaction control and/or the effect of the diffusion asymmetry (which was recently published for the non-parabolic interface shifts on the nanoscale) will be discussed.

Laser-matter coupling results specific structural changes in amorphous chalcogenide semiconductor layers which originate from electron-hole excitations, defect creation or modification and subsequent atomic motions. These changes can be influenced by plasmon fields. Plasmon enhanced photo-darkening and bleaching, optical recording in thin As x Se 1 − x films have been demonstrated in this paper, specifically in As 20 Se 80 and As 2 Se 3 compositions which revealed the best effects of stimulated expansion or optical darkening respectively due to the He–Ne laser (λ = 633 nm) illumination. Gold nanoparticles deposited on the silica glass substrate and covered by an amorphous chalcogenide film satisfy the conditions of efficient surface plasmon resonance in this spectral region. These experimental results support the importance of localized electric fields in photo-structural transformations of chalcogenide glasses as well as suggest better approaches for improving the performance of these optical recording media.

Interdiffusion has been studied in Ag/Au hemispherical core-shell structures on sapphire substrate. In isothermal heat treatments first a relatively fast growth of nanovoids was observed, which was followed by a slower shrinkage process. The void formation is interpreted by pure Kirkendall-porosity formation since Ag-50%Au solid solution has been formed in the shell. In contrary, in all previous publications on hollow nanoshell formation a chemical reaction took place and the shell consisted of the reaction product (i.e., of sulphide or oxide). Furthermore, in these cases the shrinkage was observed at temperatures higher than the formation temperature.