Vibrational properties of Ge nanocrystals determined by EXAFS

Abstract

The vibrational properties of Ge nanocrystals (NCs) produced by ion implantation in SiO 2 followed by thermal annealing were determined from temperature dependent Extended X-Ray Absorption Fine Structure (EXAFS) spectroscopy measurements. Using a correlated anharmonic Einstein model and thermodynamic perturbation theory it was possible to extract information about thermal and static disorder, thermal expansion and anharmonicity effects for the Ge NCs. Comparison with results for bulk crystalline and amorphous Ge indicates that the Ge NCs bonds are stiffer than those of both bulk phases of Ge. Also, the values of the anharmonic linear thermal expansion and the thermal expansion coefficient obtained for the Ge NCs were considerably smaller those for bulk crystalline Ge. Similar trends are reported in the literature for other semiconductor NC systems. They suggest that the increased surface to volume ratio of nanocrystals and the presence of the surrounding SiO 2 matrix might be responsible for the different vibrational properties of the nanocrystal phase.

In situ X-ray absorption spectroscopy of germanium evaporated thin film electrodes

“…Table 1). Although crystalline Ge is made of 4 atoms at 2.45 Å and 12 atoms at 4 Å, the FT of the room temperature EXAFS spectrum only consists of a dominant backscattering from the atoms located at 2.45 Å and a weak backscattering corresponding to the 12 atoms located at 4 Å [18]. Thus, it is not so surprising that the three shells of Ge atoms positioned at distances above 4.5 Å are not detected for Li 15 Ge 4 .…”

Thermodynamic properties of Pt nanoparticles: Size, shape, support, and adsorbate effects

“…Enhanced thermal stability and superheating was detected for faceted NP shapes with good crystallinity and coherent or semi-coherent (nearly epitaxial) interfaces around the embedded NPs, or when NPs were coated by a high melting-point matrix. 16,33,34 On the other hand, reduced T m values were found for NPs randomly embedded (e.g. without epitaxial relationships) in similar matrixes.…”

“…For larger NPs (> 200 atoms), monotonic size-dependent trends in T m were observed. 11 For a given material system, the specific thermal behavior was found to be drastically affected by environmental influences such as the presence of a support, an encapsulating matrix, the internal defect density within a NP, the structural and chemical nature of the NP/support interface, and the presence of ligands or surface adsorbates [12][13][14][15][16][17] .…”

“…TEM, X-ray diffraction, differential scanning calorimetry, EXAFS, etc.) for several nanoscale material systems, including supported or embedded Pt 5,6,28,29 , Ag 17 , and Ge NPs 16,30,31 , as well as unsupported nanocrystalline agglomerates such as Ag 32 , Au 32 , ZnS 12 , and thiol-capped CdS 13 and CdTe 14 nanocrystals. A comparison between the different references is challenging due to the distinct sample preparation and characterization conditions, and in some cases, the lack of detail on the specific structure of the systems investigated.…”

Amorphization of Ge and Si nanocrystals embedded in amorphousSiO2by ion irradiation

“…We analyzed the temperature-dependent data for the 9.0 nm Ge NCs irradiated with Si to a fluence of 2 ϫ 10 14 cm −2 according to the multiple data set fitting procedure described in Ref. 19.…”