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.
Authors
Little, William RobertCollections
- Theses [3822]