利用具空間解析度的電子能量損失能譜技術來研究奈米材料的表面激發現象

Abstract

本論文藉由掃描穿透式電子顯微鏡加載電子能量損失能譜儀來從事奈米材料的體積與表面激發的研究，可以在奈米尺度下更進一步的觀察微結構與電子結構。 此聚集的電子束有如0.2奈米的探針，具有非常高的空間分辨率，結合電子能量損失能譜儀可以完全在原子級空間解析下研究電子激發，在低掠射角下探索單一個別奈米材料的表面激發，其空間解析與較高的激發能量範圍是近場光學的量測所無法比擬的。本論文分為三個部分，分別在三種不同的奈米材料上研究電子激發，並藉由理論模擬計算探討與驗證實驗結果。第一部分著重於奈米金顆粒包埋在氧化矽奈米線的表面電漿子強度分佈，利用電子束優越的高空間分辨率研究其光學反應。在包埋金顆粒的氧化矽奈米線中，實驗發現金顆粒的表面電漿子具有異向性的強度分佈，在奈米線的橫向方向呈現較高的強度分佈。基於此現象，對於這些具有光增強導電性的金顆粒-氧化矽奈米線，推測電荷載體的產生傾向於奈米線表面附近，表明由衰減蛻變的表面電漿子發揮了重要作用，並且藉由偏振相關之光電導率的量測結果肯定這個推論。第二和第三部分描述半導體奈米結構之電子激發的研究工作，包含了三角形的氮化鎵奈米線和長方形的氧化鋅奈米線，涵蓋可見光與紫外光能譜範圍。經由高空間解析的能量損失能譜的仔細量測，詳細探討主要的體電漿子和表面電漿子外，亦包含較少被討論的表面波導模式、表面激子激發模式與及結合表面波導和表面激子激發模式。實驗與理論推導結果一致地呈現出，表面激子激發模式在材料表面具有較遠的分佈範圍，其局部的光學電場分佈隨著離表面距離漸遠，強度逐漸衰減的程度比表面電漿子較為緩慢，意味著其電磁波能量大都侷限在表面。此外，我們還利用二維能譜成像技術，即能量過濾透射電子顯微鏡，呈現出特定能量的電子激發在實空間的強度分佈圖。

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.