探索少數層石墨烯在熱電效應下的長程庫倫交互作用

Investigation of Long-Range Coulomb Interactions on Thermoelectric Effect in Few-Layer Graphene

黃奕凱(Yi-Kai Huang)

指導教授 ： 簡紋濱

國立交通大學/理學院/電子物理系所/碩士(2019年)

摘要

本實驗將天然石墨原塊以機械剝離法製備少數層石墨烯於二氧化矽/矽基板上，透過電子束微影與熱蒸鍍鍍膜等技術製作成少數層石墨烯元件。藉由加熱電極施加熱源使材料產生溫度梯度，並量測熱電勢進而觀察其熱電效應。本研究將探討少數層石墨烯在熱電效應下由長距離電子-電子庫倫交互作用力所引起的非局部效應。
實驗中觀察到熱電效應下的非局部訊號存在於石墨烯狄拉克點附近，隨著載子濃度增加而被屏蔽消失。此外於存在熱電非局部效應的樣品，也可於相同環境下的迪拉克點附近觀測到其電性非局部效應之電導，因此推測熱電的非局部效應也與長距離電子-電子庫倫交互作用力有關。並且所觀察到之電性與熱電的非局部效應皆會發生於一至參層的石墨烯樣品中。
熱電非局部效應的載子傳輸特徵為在狄拉克點附近出現非局部電洞與非局部電子訊號，且此現象的發生常隨著環境溫度下降而愈加明顯，另外此熱電非局部電壓訊號在部分元件中，會受額外載子摻雜影響，使得僅觀察到的訊號往電子或電洞表現偏移。此外在雙層石墨烯中，我們發現熱電非局部效應的載子濃度表現範圍與雜質濃度關係為正相關，故適當的無序性可以使非局部效應較不易受屏蔽效應的影響而消失。

關鍵字

少數層石墨烯 ； 熱電效應 ； 長距離電子-電子庫倫交互作用 ； 非局部效應

並列摘要

In this experiment, few-layer graphene flakes were exfoliated from nature graphite and transferred on SiO_2 covered Si substrates. The graphene devices were fabricated by standard electron beam lithography and thermal evaporation. By studying the thermoelectric effect, a temperature gradient was generated by a microscale heater and the heat generate voltage across the graphene flake was measured to evaluate the thermoelectric power (TEP). In this study, we focus on the long-range electron-electron Coulomb interactions in the TEP of few-layer graphene flakes.
The non-local TEP effect was observed, exists near the Dirac point of the graphene and vanishes at high carrier concentration because of the screening effect. In addition, at the same condition near the Dirac point, a non-local electrical conductance is observed simultaneously corresponding to the non-local TEP effect. It is argued that the non-local TEP effect is attributed to the same long-range electron-electron Coulomb interaction that results in the non-local electrical conductance. These two non-local effects can be observed on single, bilayer, and tri-layer graphene flakes.
The non-local TEP thermal voltages comes probably from the fluctuation of electron and hole charges near the Dirac point. The electron and hole charge fluctuation is much more obvious at low temperatures thus enhancing the two non-local effects. In some devices, the non-local TEP voltages shift due to other electron or hole doping effects from the surrounding environment. Moreover, in bilayer graphene flakes, we observed that the non-local TEP effect is strongly dependent on the disorder (impurity carrier concentrations) of the system because the disorder can reduce the screening effect from the high carrier concentration.

並列關鍵字

Few-layer graphene ； Thermoelectric effect ； Long-range electron-electron Coulomb interactions ； Non-local effect

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