在扶手型帶狀石墨烯中新穎態與其量子傳輸特性之研究

A study on novel states and quantum transport in armchair graphene nanoribbons

邱志宣(Chi-Hsuan Chiu)

指導教授 ： 朱仲夏

國立交通大學/理學院/電子物理系所/博士(2015年)

https://doi.org/10.6842/NCTU.2015.00282

摘要

此論文意在探索與了解在扶手型帶狀石墨烯中各種新穎態與其量子傳輸特性。此工作分為兩部分:一為由邊緣電位引起的扶手型石墨烯邊緣態，另一為由吸附原分子引起的節點型量子束縛態。

我們發現由邊緣電位引發起的贋自旋翻轉是形成邊緣態的關鍵過程。在扶手型石墨烯開放邊界和扶手型帶狀石墨烯這兩種邊緣組態中，均能有邊緣態形成。在扶手型石墨烯開放邊界上，即使沒有破壞次晶格對稱，邊緣電位被證實能開啟贋自旋翻轉(能谷內)的散射。此機制使開放邊界產生兩個反射波。此雙波特徵讓邊緣態的形成變為可能。此邊緣態的物理來源與半導體中的Tamm態不同。而在無能隙的帶狀扶手型石墨烯中，施加邊緣電位造成能隙開啟。邊緣態與能隙開啟均在電導性質中顯現出與眾不同的特徵。

我們也發現一個在金屬性扶手型帶狀石墨烯節點上的吸附原分子能產生一個束縛態。在量子傳輸上，此單獨一個節點型吸附原分子束縛態是無法顯現出任何的反射。然而一個在縱向距離上出現的非節點吸附原分子(或是散射點)能將其顯現在電導中，表現出Fano共振現象。節點型吸附原分子相關的本質加上Fano結構能大幅提升了能量上的解析度，這兩方面招致了利用傳輸性質來偵測吸附原分子種類的運用。因著Fano共振態有局域性質，在輕微的混亂如邊緣空缺之下，Fano頂點被證實是很穩固的。我們以推廣型的H#westeur061#ckel模型用來描述吸附原分子，並以氟原子和氫氧根做為案例。

關鍵字

奈米帶狀石墨烯 ； 扶手型開放邊界 ； 邊界效應 ； 節點物理 ； 扶手型邊緣態 ； 節點型吸附原分子束縛態

並列摘要

This thesis aims to explore and to understand various novel states and their quantum transport in armchair graphene nanoribbons (AGNR). This work consists of two main parts: AGNR edge states induced by edge potentials and metallic AGNR nodal-bound states induced by adsorbates.

Pseudospin flipping is found to be the key process leading to the formation of edge states due to edge-potentials. Both edge configurations, namely an armchair graphene open boundary and armchair graphene nanoribbon, are found to support our edge-state formation finding. At an armchair graphene open boundary, an edge-potential U0 is shown to turn on pseudospin-flipped (intravalley) scattering even though U0 does not post an apparent breaking of the AB site (basis atoms) symmetry. This opens up two-wave features emanating (or reflecting) from the boundary. The same two-wave feature makes possible the formation of the edge state. The physical origin of the edge state is different from that for the Tamm states in semiconductors. For an armchair graphene nanoribbon with gapless energy spectrum, applying U0 to both edges opens up an energy gap. Both edge-states and energy gap opening exhibit their distinct features in the nanoribbon
conductance.

An adsorbed atom on a nodal site of the gapless subband in a metallic AGNR is found to induce a bound state. The nodal-adsorbate bound state alone does not cause reflection in the AGNR quantum transport in the one propagating-channel regime. Yet its manifestation, as a Fano resonance in G, is brought forth by the presence of a non-nodal adsorbate (or a scatterer) located on a longitudinally-separated site. Both the adsorbate-
dependent nature and the significantly enhanced energy resolution of the Fano structure in G invite adsorbate recognition. The Fano peak is shown to be robust against weak disorder such as from edge-vacancies, due to the localized nature of the Fano resonant state. Adsorbates F and OH, described within an extended H#westeur005#uckel model, are considered as examples.

並列關鍵字

graphene nanoribbon ； armchair open boundary ； boundary effects ； nodal physics ； armchair edge state ； nodal adsorbate bound state

參考文獻

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