Size-dependent characterization of embedded Ge nanocrystals: Structural and thermal properties

Abstract

A combination of conventional and synchrotron-based techniques has been used to characterize the sizedependent structural and thermal properties of Ge nanocrystals ͑NCs͒ embedded in a silica ͑a-SiO 2 ͒ matrix. Ge NC size distributions with four different diameters ranging from 4.0 to 9.0 nm were produced by ion implantation and thermal annealing as characterized with small-angle x-ray scattering and transmission electron microscopy. The NCs were well represented by the superposition of bulklike crystalline and amorphous environments, suggesting the formation of an amorphous layer separating the crystalline NC core and the a-SiO 2 matrix. The amorphous fraction was quantified with x-ray-absorption near-edge spectroscopy and increased as the NC diameter decreased, consistent with the increase in surface-to-volume ratio. The structural parameters of the first three nearest-neighbor shells were determined with extended x-ray-absorption finestructure ͑EXAFS͒ spectroscopy and evolved linearly with inverse NC diameter. Specifically, increases in total disorder, interatomic distance, and the asymmetry in the distribution of distances were observed as the NC size decreased, demonstrating that finite-size effects govern the structural properties of embedded Ge NCs. Temperature-dependent EXAFS measurements in the range of 15-300 K were employed to probe the mean vibrational frequency and the variation of the interatomic distance distribution ͑mean value, variance, and asymmetry͒ with temperature for all NC distributions. A clear trend of increased stiffness ͑higher vibrational frequency͒ and decreased thermal expansion with decreasing NC size was evident, confirming the close relationship between the variation of structural and thermal/vibrational properties with size for embedded Ge NCs. The increase in surface-to-volume ratio and the presence of an amorphous Ge layer separating the matrix and crystalline NC core are identified as the main factors responsible for the observed behavior, with the surrounding a-SiO 2 matrix also contributing to a lesser extent. Such results are compared to previous reports and discussed in terms of the influence of the surface-to-volume ratio in objects of nanometer dimensions.

Ion Beam Formation and Modification of Cobalt Nanoparticles

“…Figure 6 shows that the NPs exhibit less thermal expansion compared to the bulk Co standards and the effect is more pronounced for NPs of smaller sizes. Similar results have been observed for Pt [58] and Ge [45] NPs embedded in SiO 2 , where both studies demonstrated that the thermal expansion of the interatomic distances was lowest for small NPs and increased with increasing NP size. It was suggested that the SiO 2 matrix restricts the thermal expansion of the embedded NPs.…”

“…The prominent features of the bulk HCP Co and c-Co NP spectra in Figure 10 result from multiple scattering resonances of the 1s photoelectron [20]. The a-Co NP spectrum (and those of disordered or amorphous materials in general) exhibits less fine structure due to the loss of such multiple scattering contributions [45,65]. This spectrum is also similar to that reported previously for thin-film amorphous Co [66].…”

“…XANES spectra for two bulk standards plus c-and a-Co NPs are shown in Figure 10. The sensitivity of the XANES technique to valence state, crystallographic phase, multiple scattering effects (and more) makes for an effective means of "fingerprinting" the crystallographic phase of a material under investigation [45,64]. The prominent features of the bulk HCP Co and c-Co NP spectra in Figure 10 result from multiple scattering resonances of the 1s photoelectron [20].…”

“…The observed structural changes with decreasing NP size relative to the bulk standard are as follows: (a) the decrease in peak height (or lower coordination) stems from the under-coordinated surface atoms; (b) the increasing FWHM (or increasing disorder) results from the relaxation and reconstruction of the under-coordinated surface atoms; (c) surface-curvature-induced capillary pressure yields a BL contraction; and finally (d) the asymmetric bondlength distribution (or negative C 3 ) results from the superposition of the different BLs from the core and surface/interface (where the latter are shorter). For semiconductor NPs embedded in SiO 2 , simulations [44] and experiments [45] have demonstrated that the interfacial atom environment is highly distorted to accommodate material and bonding differences across the interface. Similar trends have been reported for other embedded elemental metal NPs [46][47][48][49].…”

On EXAFS Debye-Waller factor and recent advances

“…The EXAFS DW factor is nowadays a significant parameter of EXAFS analyses, which can be exploited to gain original information in a number of different cases, such as local mechanisms of negative thermal expansion (Bridges et al, 2014),), pressure-induced phase transitions (Principi et al, 2004;Popescu et al, 2011), structure and dynamics of nanoparticles (Ikemoto & Miyanaga, 2007;Comaschi et al, 2008;Araujo et al, 2008) and thermally induced atomic strains (Ruffoni et al, 2007). This paper is intended to be an updated version of the previous paper (Dalba & Fornasini, 1997).…”