二維碳材料的能帶結構與電性傳輸機制的探討

Abstract

本研究涵蓋了探討導電特性最好的石墨烯到近乎絕緣體的氧化石墨烯，利用電子束微影技術完成電子元件以及直流電訊號的分析，從電性上探討石墨烯與金屬電極接面的電性傳輸。在5 K到300 K的溫度區間，比較實驗數據與幾種可能的傳輸機制理論，並確認熱擾致誘發穿隧理論(Fluctuaion Induced Tunneling Conduction)可以解釋接面電阻的傳輸行為，這隱含有一絕緣體存在石墨烯與金屬電極之中，透過擬合參數的計算，估計出等效能障的寬度以及穿隧面積的大小與接面電阻的關係，並由傳輸模型提供一個降低接面電阻的方法。利用穿隧結(Tunneling junction)結構探討還原氧化石墨烯不同厚度造成的能帶結構差異，比較單層還原氧化石墨烯大範圍的穿隧能譜與完美石墨烯的差異，並利用外加電場調控費米能階與狄拉克點的能量差，估計考慮無序結構的單層還原氧化石墨烯的費米速度。進一步同時探討單層還原氧化石墨烯的還原程度對電荷傳輸以及能態密度的影響，隨著殘留氧官能基的比率升高，其傳輸機制逐漸偏離二維變程跳躍傳輸，此時穿隧能譜亦同時觀察到還原氧化石墨烯由沒有能隙的半金屬態轉變為具有能隙的絕緣態。利用Tight-binding理論計算石墨烯的能態密度與能帶結構由低氧鍵結比率到高氧鍵結比率的變化，比較理論計算與實驗的結果，提出在微觀尺度碳原子與氧原子的鍵結模型，並解釋還原氧化石墨烯從半金屬態到絕緣態其能帶結構的演進。

Simple two-probe devices on mechanically exfoliated graphene flakes are fabricated and the temperature behavior of resistance is measured from room temperature down to liquid helium temperature for the study of electron transport in the interface. Comparing experimental data with several different transport theories, it is confirmed that the model of fluctuation-induced tunneling conduction describes precisely the electron transport and indicates the existence of a thin insulating layer in the metal-graphene interface. Through the interface probing by electron transport measurements, the way to reduce the contact resistance is suggested. More than that, Differential conductance, revealing density of states (DOS) of reduced graphene oxides (rGO) sheets in a wide voltage range. A difference in DOS of rGO sheets with different thickness is identified. For the single-layer rGO, the DOS shows a whole band with band edges in line with theoretical predictions, and gating DOS is used to estimate electron’s Fermi velocity. Disorder effects on both electronic transport property and DOS of single-layer rGO were investigated by the simultaneous approach of two different measurements: one is the characterization of electrical transport property from two ohmic-contact probed devices; the other is the characterization of DOS from the differential conductance (dI/dV) measurements on tunneling junction devices. Here we try to propose an atomic structure of rGO to describe suitably the experimental discoveries of the electron transport variation, phase transition, the band gap opening and the band-tail disorder effects. The gradual transition from graphene to graphene oxides is experimentally determined in condensed material for the first time.