Stacked Ge nanocrystals with ultrathin SiO2 separation layers.

The aim of this work is the tailored growth of Ge nanocrystals (NCs) in (GeO(x)/SiO(2)) multilayers (ML) for photovoltaic applications. For this purpose the fabrication of regularly stacked Ge NCs separated by ultrathin SiO(2) layers is essential to enable charge carrier transport by direct tunnelling. In this paper we report on the fabrication of (GeO(x)/SiO(2))(50) multilayer stacks via reactive dc magnetron sputtering and Ge NCs formation after subsequent annealing. It is shown that magnetron sputtering allows us to deposit very regular ML stacks with a total thickness of about 300 nm, characterized by ultrathin (down to 1 nm) and very smooth (roughness ∼ 0.6 nm) SiO(2) separation layers. A main challenge is to keep these properties for a thermal budget necessary to form Ge NCs. For this reason, the temperature dependence of phase separation. Ge crystallization and ML morphology was investigated by Rutherford backscattering, x-ray scattering, Raman spectroscopy and electron microscopy. The formation of size confined Ge NCs of about 5 nm after annealing of only 550 °C is confirmed. This low thermal budget ensures the suppression of GeO emanation and multilayer stability. Spectroscopic ellipsometry was applied to determine the optical Ge NC bandgap to (1.65 ± 0.5) eV.

Reversible shape changes of Pd nanoparticles on MgO(100).

We studied the interaction of oxygen with MgO(100) supported Pd nanoparticles at 10(-5) mbar oxygen pressure and a sample temperature of 570 K. We employed high-resolution X-ray reciprocal space mapping, which allows us to resolve the average particle shape from the quantitative analysis of intensity diffraction rods running perpendicular to corresponding facet surfaces. We identified the oxygen induced formation of nanosized (112) facets which is reversible in a CO atmosphere. Our results give direct evidence for the microscopic evolution of the nanoparticle shape under reactant exposure, which is essential for an atomistic understanding of catalytic reactions on nanoparticles.

Influence of hydrogen on thermally induced phase separation in GeO/SiO2 multilayers.

The influence of the annealing atmosphere on the temperature induced phase separation of Ge oxide in GeO(x)/SiO(2) multilayers (x≈1), leading to size controlled growth of Ge nanocrystals, is explored by means of x-ray absorption spectroscopy at the Ge K-edge. Ge sub-oxides contained in the as-deposited multilayers diminish with increasing annealing temperature, showing complete phase separation at approximately 450 °C using inert N(2) ambient. The use of reducing H(2) in the annealing atmosphere influences the phase separation even at an early stage of the disproportionation. In particular, the temperature regime where the phase separation occurs is lowered by at least 50 °C. At temperatures above 400 °C the sublayer composition, and thus the density of the Ge nanocrystals, can be altered by making use of the reduction of GeO(2) by H(2).