Studies of surface excitations in nano-materials by spatially resolved electron energy loss spectroscopy
Date Issued
2010
Date
2010
Author(s)
Wu, Chien-Ting
Abstract
The bulk and surface excitations of nano-materials are investigated by electron energy-loss spectroscopy (EELS) in conjunction with scanning transmission electron microscopy (STEM) in order to further study the microstructure and electronic structures of matters in nano-scale. STEM-EELS work was carried out in a FEI (Tecnai F20) STEM capable of forming an electron probe as small as 0.2 nm. With the 0.2 nm electron probe in STEM-EELS, a very high spatial resolution unmatched by optical near-field experiments, the electronic excitations of an individual nano-object can be thoroughly explored by brining the atomic-scale probe to nanomaterials in grazing incidence. In contrast to volume excitations which are easily observed in extended bulky objects, modern nano-materials thus provide new opportunities for studying surface excitations. This dissertation is divided into three sections consisting of the studies of electronic excitations on three different nano-materials. First section focuses on the intensity distribution of Au surface plasmons (SPs) excited by fast electrons with excellent spatial resolution in Au-silica peapod nanowires (NWs) to understand the optical response of the hybrid NWs. In fact, the SP excitations of gold-silica nanowire were found to be anisotropic with stronger SP intensities observed along the transverse direction of the nanowire. This indicates that the charge carriers generated near the surface of the nanowires by the decay of SP resonance plays a significant role to the enhanced photo-conductivity. This conclusion is reaffirmed by the polarization dependent photo-conductivity measurement. The second and third sections describe the investigations of electronic excitations in the individual NWs (the triangular GaN NWs and the rectangular ZnO NWs in particular) in the visible/UV spectral regime. Detailed EELS characterizations of the individual single GaN or ZnO NW show that, in addition to the predominant excitations of bulk plasmons (BPs) and SPs, other rare types of surface excitations such as surface guided modes, surface exciton polaritons (SEPs) and SEPs-like modes are also present. The SEPs occurring near inter band transitions appear in a spectral regime beyond conventional optical accessible ranges, thus less studied previously, and the guided modes bear certain resemblances to the optical guiding waves recently uncovered in individual-NW spontaneous emissions. The SEPs-like modes are associated with larger real and raising imaginary parts of dielectric function above the band gap in which the surface guided mode becomes weaker by way of absorption, and therefore the SEPs component would become competitive in intensity. Hence, the SEPs become predominant than surface guided mode in surface excitation above band gap. We also note that the wave fields of these SEP excitations which decay much more slowly than SPs are predominantly distributed outside the nanowire surface.
We have also employed the 2-D spectral imaging technique, i.e., energy-filtered transmission electron microscopy (EFTEM), to map out the intensity distribution of excitation in real space, and the line-profile in 2-D mapping is in good agreement with the results from 1-D linear mapping STEM-EELS.
Subjects
scanning transmission electron microscopy
Electron energy-loss spectroscopy
Energy-filtered transmission electron microscopy
Gold
Nanowire
Silicon compounds
surface plasmons
Bulk plasmons
photo-conductivity
polarization dependent
GaN
ZnO
Surface guided modes
Surface exciton polaritons
Type
thesis
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