探討還原氧化石墨烯的熱電特性

Abstract

本實驗使用石墨烯以及還原氧化石墨烯經由電子束微影、熱蒸鍍鍍膜等技術製作元件。使用兩點量測法觀察石墨烯以及還原氧化石墨烯的電性傳輸特性，並分別利用四點量測以及鎖頻放大技術觀察樣品兩端溫度差以及電壓差去分析熱電特性。石墨烯以及還原氧化石墨烯在施加背閘極偏壓後，可以觀察到兩者的場效應都有雙極性(bipolar)的特性。從變溫量測電阻的結果，顯示還原氧化石墨烯的電性傳輸符合Mott二維的變程跳躍傳輸理論。在熱電量測中可以從正負號去判斷傳輸的載子為電子或電洞，並且在場效應觀察到石墨烯以及還原氧化石墨烯的熱電功率(thermoelectric power)在狄拉克點有正負號的變化，顯示出調控背向閘極偏壓足以使傳輸載子從電洞轉變成電子。最後利用還原氧化石墨烯的電導率去估算其表面上殘留氧的覆蓋率，結果顯示還原氧化石墨烯的熱電功率會隨著氧覆蓋率增加而上升。當氧覆蓋率增加時，因為空洞缺陷及無序性的產生使能隙打開，電子跟電洞可以被分離，電性相反載子的補償作用減少，使還原氧化石墨烯的熱電功率增加。

Graphene and reduced graphene oxide device are fabricated using standard electron-beam lithography and thermal evaporation. The device resistance is measured in the temperature range from 200 to 300 K. The result shows that electron transport in reduced graphene oxide can be described by Mott’s two-dimensional variable range hopping. By applying back gate voltage, both graphene and reduced graphene oxide show ambipolar behavior in field effect responses. The sign of thermoelectric power (TEP) of graphene and reduced graphene oxide changes across the charge neutrality point since the majority carriers are changed from electrons to holes. The oxygen coverage of reduced graphene oxide can be estimated from its conductivity. The TEP of reduced graphene oxide increases with increasing oxide coverage.