探討熱還原之氧化石墨烯電性研究

Abstract

本實驗採用以Hummers法製備的氧化石墨烯(GO)溶液，薄膜單層厚度約為1 nm左右。於高真空環境下進行熱還原處理400℃、24小時，以脫除氧化石墨烯上的含氧官能基，形成還原氧化石墨烯(r-GO)薄膜。固定元件製程參數，利用電子束與微影與熱蒸鍍技術，製成背閘極(back-gate)元件。分別量測室溫環境下(300 K)與變溫過程(300~120 K)的電流-電壓關係圖特性，並利用外加閘極偏壓對元件電流的關係，探討其電性傳輸機制與載子遷移率的變化。 實驗量測於室溫環境及零偏壓下，其電流與電壓關係曲線皆為線性關係，呈現歐姆接觸行為，電阻值介於510∼810 Ω之間約有三個數量級的變化，遷移率值的分布大約為10^(-3)~10^(-1) cm^2/V*s，二維系統的電洞載子濃度約為10^12~10^14 cm^(-2)。若於變溫環境中，隨著溫度下降可發現樣品的電阻值逐漸上升，其電性特性呈現一半導體行為。當室溫電阻值越大者，隨著溫度下降其電阻上升的變化率將會更明顯。由於薄膜表面的缺陷、皺摺帶狀條紋及破碎程度不一，造成電子於無序結構中的侷域態間跳躍，我們可以利用二維變程式跳躍傳輸，定性地解釋量測結果，進一步發現 T0 的變化，可歸因於還原氧化石墨烯薄膜上的能態密度改變。

In this study, transport properties of thermally reduced-graphene oxide (r-GO) nanodevices have been investigated. Graphene oxide sheets are made via a modified Hummers’ method and are stocked in de-ionized water. Graphene oxide sheets are then deposited on SiO2 (300 nm)/Si substrates with pre-patterned alignment marks and micrometer scale electrodes. The Si substrate is taken as a back gate electrode. Prior to electron beam lithography process, as-deposited sheets are subjected to annealing at 400℃ for 24 h in order to remove oxygen functional groups and to restore themselves to high conducting graphene. Afterwards, the electron beam lithography is employed to fabricate current leads in conjunction with those r-GO sheets. The separation distance between two current leads is purposely maintained at 1 μm in all r-GO nanodevices for analysis. All electrical measurements are carried out under a 760-Torr helium gas environment at temperature ranging from 300 to 100 K. The r-GO nanodevices reveal the room-temperature (RT) resistances and the mobilities, varying from 10^5 to 10^8 Ω and 10^(-3) to 10^(-1) cm^2/ V*s, respectively. Moreover, electron transport in r-GO nanodevices can be well-described by two-dimensional Mott’s-variable range hopping. According to fitting parameters of the Mott model, we found that the large variation of RT resistance is ascribed to the change of number of localized state at Fermi level in r-GO, rather than that of the localization length. Our investigation provides a pathway to understand the fundamental conduction mechanism in thermally reduced graphene oxide and to engineer for electronic and optical applications in the near future.