Structure of and Light Emission in Matrix-Free Germanium Quantum Dots.

Abstract

The connection between light emission and structure of Germanium nanoparticles (3-10 nm) prepared by top-down (etching) and bottom-up (sol-gel and colloidal synthesis) has been investigated using Raman spectroscopy, TEM, x-ray absorption spectroscopy (XAS), x-ray di raction (XRD), and photoluminescence (PL). It was found that TEM, Raman spectroscopy, PL, and XRD techniques all result in di ering values for the nanoparticle size which don't all agree in the limit of experimental error. Several structural models have been proposed and tested by high pressure Raman measurements. It was found that a Raman peak corresponding to diamond-type Ge structure is observed well above the transition pressure of both amorphous ( six GPa) and crystalline ( 11 GPa) Ge. The pressure dependence of the Raman signal peak position was observed to follow an unexpected non-linear shift with a corresponding increase in peak width (FWHM). Possible structural origins of these trends have been investigated by adapting the widely used phonon con nement model to high pressure conditions and comparing experimental data with the model behaviour under assumptions of constant, and size-dependent bulk modulus. Considered collectively with the ambient structural data, the results of the analysis of the high pressure behaviour point to the phenomenon of gradual surface induced amorphisation under pressure in matrix-free Ge nanoparticles. The best structural model to describe this is a core-shell with the small crystalline core and a disordered surface layer. The local structure of samples was investigated using XAS, while opticallydetected XAS, using x-ray excited optical luminescence (XEOL), was used to link structure with optical emission. The emission was found to depend on surface termination; in oxygen terminated nanoparticles the oxide rich regions are responsi- 4 ble for light emission, while in their hydrogen terminated counterparts' pure Ge regions contribute to the luminescence. Furthermore, with the aid of molecular dynamics simulations it was shown that in hydrogen-terminated samples, optical emission is due to a topologically disordered (amorphous) region close to the surface of the nanoparticles. We demonstrated that OD-XAS can potentially provide subnanoparticle resolution due to its sensitivity to the light emitting sites in a sample. We further investigated the microscopic origins of such sensitivity and identi ed possible limitations. This work clearly demonstrates that a combination of methods sensitive to short-range and long-range structure are required for comprehensive characterisation of nanoscale systems.