探討不同缺陷程度的熱還原氧化石墨烯之能態密度變化

本實驗採用Hummers法製備的氧化石墨烯(GO)，單層厚度約為1.1 nm，於高真空環境下進行熱退火還原處理400 ℃、24小時，以去除附著在表面上的碳氧官能基團，形成熱還原氧化石墨烯(rGO)薄膜。利用電子束微影及熱蒸鍍技術於同一樣品製作兩點量測rGO元件及金屬－氧化層－rGO之穿隧結元件。量測兩點式rGO元件室溫及變溫環境下的電流對電壓關係並討論其電性傳輸機制，同時也用穿隧結結構於室溫真空環境中量測穿隧電流對電壓關係，以求得rGO樣品的能態密度。　　量測小偏壓範圍的電流對電壓線性關係，可得到室溫電阻率分別位於10^5、10^7及10^9 Ω三種數量級附近。分別將三類樣品置於變溫環境下量測其電阻率對溫度關係，發現電阻率隨著溫度下降而逐漸上升，呈現半導體行為，且這三類樣品都可用二維變程式跳躍傳輸理論解釋其電性傳輸機制，但無法從擬合參數推論影響樣品電阻率的原因為何。為了將樣品能態密度與電阻率作連結，利用穿隧結結構的量測，發現電阻率為10^5 Ω的rGO於沒有外加偏壓的能態密度圖上沒有能隙的產生，且隨著外加閘極偏壓，其能態密度最小值，迪拉克點(dirac point)所對應到的電壓值會有所改變。而在電阻率為10^7及10^9 Ω的rGO的樣品上，發現其能態密度有1.02和2.91 eV的能隙存在。推論是由於rGO樣品的殘留碳氧官能基的多寡造成於費米能階處之能態密度有所差異，而缺陷越多會導致能態密度下降進而產生能隙，使得電阻率上升。藉由穿隧結結構和rGO的變溫量測，將rGO能態密度、電阻率及電性傳輸機制連結。

關鍵字

氧化石墨烯；能態密度

並列摘要

In this study, transport properties and density of states of reduced-graphene oxide (r-GO) have been investigated simultaneously. Graphene oxide (GO) sheets are synthesized by the modified Hummers’ method. Prior to device fabrication, the GO sheets are subjected to thermal annealing at 400 ℃ for 24 h to remove oxygen functional groups and to restore GO sheets back to higher conduction of r-GO sheets. Then, by using electron-beam lithography and thermal evaporation techniques, we make two Ohmic-contact electrodes and one tunnel junction on the same r-GO device. The separation distance between current leads is maintained at 3 μm in all r-GO devices. The r-GO devices reveal a variation in room-temperature (RT) resistivity from 10^5 to 10^9 Ω. All electrical measurements are carried out under a 760-Torr helium gas and at temperature ranging from 300 to 80 K. We discovered that the electron transport in r-GO devices can be well-described by two-dimensional Mott’s variable range hopping. On the other hand, the tunneling-junction measurement and dI/dV spectrum are carried out for a study of the density of states. For those devices having a RT resistivity of ~10^5Ω, they reveal a minimum conductance at zero voltage under zero gate voltage. Then, by changing the gate voltage, the minimum conductance in dI/dV spectra makes a shift which is attributed to electrons tunneling to the Dirac point of the r-GO sheets. As for other devices having RT resistivities of 10^7 Ω and 10^9 Ω, we observed bandgaps of 1.02 and 2.91 eV in dI/dV spectra. It is argued that the more the residues of oxygen functional groups in r-GO samples are, the larger the bandgap is and the higher the RT resistivity is. Through the simultaneous measurements of electron transport and density of states on r-GO sheets, we can find a relation between the density of state and the resistivity in rGO sheets.

並列關鍵字

reduce graphene oxide ； density of states

參考文獻

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探討不同缺陷程度的熱還原氧化石墨烯之能態密度變化

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