Local structure of Ge quantum dots determined by combined numerical analysis of EXAFS and XANES data

Abstract

The sensitivity of X-ray absorption near-edge structure (XANES) to the local symmetry has been investigated in small (∼4 nm) matrix-free Ge quantum dots. The FDMNES package was used to calculate the theoretical XANES spectra that were compared with the experimental data of as-prepared and annealed nanoparticles. It was found that XANES data for an as-prepared sample can only be adequately described if the second coordination shell of the diamond-type structural model is included in the FDMNES calculations. This is in contrast to the extended X-ray absorption fine-structure data that show only the first-shell signal. These results suggest that, despite the high degree of disorder and a large surface-to-volume ratio, as-prepared small Ge quantum dots retain the diamond-type symmetry beyond the first shell. Furthermore, we utilized this sensitivity of XANES to the local symmetry to study annealed Ge quantum dots and found evidence for significant structural distortion which we attribute to the existence of surface disorder in the annealed oxygen-free Ge quantum dots.

Structural Characterization of Oleylamine- and Dodecanethiol-Capped Ge1–xSnx Alloy Nanocrystals

“…Small σ 2 values (σ 2 ≤ 0.010 Å 2 ) are observed for the first NN peaks for both the Ge and Sn edge data, providing support for a fairly well-ordered structure. Observation of the Ge–Ge and Sn–Ge second NN peaks also indicates the presence of long-range order. , The observed values of σ 2 for the Ge–Ge first NN peaks, 0.003–0.004 Å 2 , are similar to but slightly smaller than those of comparably sized reference cubic Ge NCs (∼0.0026–0.0029 Å 2 ) measured at 10 K . This indicates that alloying with Sn introduces more disorder than is observed in similarly sized Ge NCs.…”

“…Observation of the Ge−Ge and Sn−Ge second NN peaks also indicates the presence of long-range order. 54,55 The observed values of σ 2 for the Ge− Ge first NN peaks, 0.003−0.004 Å 2 , are similar to but slightly smaller than those of comparably sized reference cubic Ge NCs (∼0.0026−0.0029 Å 2 ) measured at 10 K. 56 This indicates that alloying with Sn introduces more disorder than is observed in similarly sized Ge NCs. The σ 2 values for the Ge−Ge second NN peaks are quite large, again indicating significant disorder beyond the first neighbor shell, but are similar to those of reference cubic Ge.…”

Investigation of the photophysical properties of energy-relevant inorganic nanocrystals

“…in the presence of suitable surfactants [oleylamine, octadecene (ODE), trioctylphosphine (TOP)] is widely used to make monodisperse Ge nanocrystals. [56][57][58][59][60][61][62][63][64][65] Heating a solution of GeBr2 or GeI2 with a surfactant has also been shown to generate Ge nanocrystals. [66][67][68] Co-reduction of GeI2 and GeI4 is another common strategy for generating Ge nanocrystals in the ~2-20 nm size regime,…”

Germanium–Tin/Cadmium Sulfide Core/Shell Nanocrystals with Enhanced Near-Infrared Photoluminescence

“…in the presence of suitable surfactants [oleylamine, octadecene (ODE), trioctylphosphine (TOP)] is widely used to make monodisperse Ge nanocrystals. [13][14][15][16][17][18][19][20][21][22] Heating a solution of GeBr2 or GeI2 with a surfactant has also been shown to generate Ge nanocrystals. [23][24][25] Co-reduction of GeI2 and GeI4 is another common strategy for generating Ge nanocrystals in the ~2-20 nm size regime, where the precursor ratio controls the particle size.…”