Bence Parditka

Silicene on Ag(111) : domains and local defects of the observed superstructures

On Ag(111) substrate, the growth of a silicene layer (the equivalent of graphene for silicon) gives rise to four main superstructures due to the epitaxy between the silicene layer (honeycomb structure) and the Ag(111) substrate. Depending on both the substrate temperature and the deposition rate, the following superstructures were observed: (4×4), (× )R13.9°, (2× 2)R30° and (× )R19.1°. Each one corresponds to a specific rotation of the silicene layer with respect to the silver substrate. In this paper we show and discuss for each superstructure all the expected equivalent domains due to the symmetry properties of both the silicene honeycomb structure and the trigonal silver surface. Finally, we show, from STM images of the (2× 2)R30° superstructure, the role played by periodic local defects on the average direction of the superstructures.

Nanoporous Gold Nanoparticles and Au/Al2O3 Hybrid Nanoparticles with Large Tunability of Plasmonic Properties

Nanoporous gold nanoparticles (NPG-NPs) with controlled particle size and pore size are fabricated via a combination of solid-state dewetting and a subsequent dealloying process. Because of the combined effects of size and porosity, the NPG-NPs exhibit greater plasmonic tunability and significantly higher local field enhancement as compared to solid NPs. The effects of the nanoscale porosity and pore size on the optical extinction are investigated for the NPG-NPs with different particle sizes experimentally and theoretically. The influences of both porosity and pore size on the plasmonic properties are very complicated and clearly different for small particles with dominated dipole mode and large particles with dominated quadrupole mode. Au/Al2O3 hybrid porous NPs with controlled porosity and composition ratio are fabricated through plasma-enhanced atomic layer deposition of Al2O3 into the porous structure. In the Au/Al2O3 hybrid porous NPs, both Au and Al2O3 components are bicontinuously percolated over the entire structure. A further red shift of the plasmon peak is observed in the hybrid NPs due to the change of the environmental refractive index. The high tunability of the plasmonic resonances in the NPG-NPs and the hybrid porous NPs can be very useful for many applications in sensing biological and organic molecules.

Effect of Diffusion Induced Driving Forces on Interdiffusion - Stress Development/Relaxation and Kinetics of Diffusion Processes

General description of the interplay between the Kirkendall shift (as a special way of relaxation) and diffusion induced driving forces in diffusion intermixing of binary systems is given. It is shown that, if the Kirkendall shift is negligible, a steady state Nernts-Planck regime is established with diffusion coefficient close to the slower diffusivity, independently of the type of the diffusion induced field and also independently whether this is a single field or a combination of different fields (e.g. stress field and extra chemical potential of non-equilibrium vacancies). Deviations from parabolic kinetics are expected only before or after this steady state stage. Using the results of our previous paper, on development and relaxation of diffusion induced stresses, it is illustrated that the setting of time of the Nernst-Planck regime is very short: intermixing on the scale of few tenths of nanometer is enough to reach it. It is also illustrated that this stage is realized even in the case of asymmetric interdiffusion (in one side of the diffusion zone the diffusion is orders of magnitude higher than in the other), when the stress distribution has a more complex form (having a sharp peak at the interface). Surprisingly the steady state is longer than it would be expected from the relaxation time of Newtonian flow: This is so because the composition profile is not static but changes fast in the timescale of the stress relaxation, and thus the stress re-develops continuously.

Photocatalytic hollow TiO 2 and ZnO nanospheres prepared by atomic layer deposition

Carbon nanospheres (CNSs) were prepared by hydrothermal synthesis, and coated with TiO2 and ZnO nanofilms by atomic layer deposition. Subsequently, through burning out the carbon core templates hollow metal oxide nanospheres were obtained. The substrates, the carbon-metal oxide composites and the hollow nanospheres were characterized with TG/DTA-MS, FTIR, Raman, XRD, SEM-EDX, TEM-SAED and their photocatalytic activity was also investigated. The results indicate that CNSs are not beneficial for photocatalysis, but the crystalline hollow metal oxide nanospheres have considerable photocatalytic activity.

Core-shell carbon nanosphere-TiO2 composite and hollow TiO2 nanospheres prepared by atomic layer deposition

Core-shell carbon-TiO2 composite and hollow TiO2 nanospheres were prepared using carbon nanospheres as hard-templates, coating them with TiO2 using atomic layer deposition, and subsequent burning out of the carbon cores. The bare carbon, the composite carbon-TiO2 and the hollow TiO2 nanospheres were characterized with TG/DTA-MS, FTIR, XRD and SEM-EDX.

Electron irradiation induced amorphous SiO2 formation at metal oxide/Si interface at room temperature; Electron beam writing on interfaces

Al2O3 (5 nm)/Si (bulk) sample was subjected to irradiation of 5 keV electrons at room temperature, in a vacuum chamber (pressure 1 × 10−9 mbar) and formation of amorphous SiO2 around the interface was observed. The oxygen for the silicon dioxide growth was provided by the electron bombardment induced bond breaking in Al2O3 and the subsequent production of neutral and/or charged oxygen. The amorphous SiO2 rich layer has grown into the Al2O3 layer showing that oxygen as well as silicon transport occurred during irradiation at room temperature. We propose that both transports are mediated by local electric field and charged and/or uncharged defects created by the electron irradiation. The direct modification of metal oxide/silicon interface by electron-beam irradiation is a promising method of accomplishing direct write electron-beam lithography at buried interfaces.

Phase Growth in Amorphous Si-Cu and Si-Co Systems: Combination of SNMS, XPS, XRD, and APT Techniques

Abstract. It is shown, by the combination of SNMS, (Secondary Neutral Mass Spectrometry), XRD, XPS and APT (Atom Probe Technique) that the growth of the Cu3Si crystalline layer at 408 K between the amorphous Si and nanocrystalline Cu thin films follows a linear law and the shifts of the Cu3Si/Cu and Cu3Si/a-Si interfaces approximately equally contributed to the growth of this phase. It is also illustrated that the Si atoms diffuse fast into the grain boundaries of the nanocrystalline Cu, leading to Si segregation. Both the SNMS and APT results indicate that even during the deposition of Cu on the amorphous Si an intermixed region is formed at the interface. This region easily transforms into a homogeneous Cu3Si crystalline reaction layer subsequently which further grows following apparently an interface controlled linear kinetics. Similar experiments performed in Co/a-Si system to study the formation and growth kinetics of the intermetallic phase. However, interestingly, homogenous formation of the new phase at the Co/a-Si interface was not always observed.

Interfacial reaction and phase growth for various metal/amorphous silicon system

The practical importance of the reactions between a semiconductor and a metal system cannot be overstated. Fundamental physics itself offers also a wide variety of interesting phenomena. Unlike the “clean” metal-metal reactions nucleation or interface control is observed for some metal-silicon reactions.