以化學氣相沉積法合成大面積高品質單晶石墨烯

Abstract

石墨烯(graphene)自從被發現以來即被受到高度的重視與研究，其高化學穩定性、高載子遷移率、高導熱及導電性、獨特的光學性質等，有潛力取代目前以矽為主的電晶體，成為新一代的二維材料，因此合成大面積且高品質的石墨烯也成為近年來熱門的課題。 本研究利用化學氣相沉積法(chemical vapor deposition, CVD)在銅箔上合成大面積且高品質的單晶石墨烯，為了擴大石墨烯的面積，其中關鍵的要素在於降低石墨烯的成核密度(nucleation density)，因此我們嘗試了各種方法，包含銅箔的前處理、合成時的裝置設計及各參數的調整，致力於得到更大面積的石墨烯。 在銅箔前處理部分，因裂解的碳原子沿著銅箔的表面成長，故銅箔表面的形貌會影響到石墨烯品質，因此我們先利用電解拋光(electropolishing)的技術製備表面平坦的銅箔；接著，我們將銅箔氧化形成一層氧化銅，在高溫下裂解的碳原子會與銅箔表面的氧氣反應形成二氧化碳及一氧化碳並離開表面，因此降低了碳原子在銅箔表面的成核機率。進行合成時，我們採用狹縫狀的腔體設計，並改變狹縫的長度，來改善現有的化學氣相沉積系統，不僅降低了成核密度也提升了石墨烯的品質，目前成功合成出單一核種達700 μm的單晶高品質石墨烯。另外也使用光學顯微鏡、電子顯微鏡、拉曼光譜及聚焦電子束繞射，進一步鑑定所合成為高品質的石墨烯。

Graphene has been highly paid attention to since its discovery because of the highly chemical stability, high carrier mobility, thermal conduction, and the unique optical properties. It is the candidate of the new 2D material and has the potential to replace the silicon transistor. As a result, synthesizing large-area and high-quality graphene has been the popular research recently. In our research, we synthesize large-area and high-quality single-crystalline graphene on copper foil by chemical vapor deposition. In order to enlarge the graphene domain size, the key point is to lower its nucleation density. We try several methods, including the copper pretreatment, device design and parameter adjustment during the graphene growth. In the part of copper pretreatment, we do electropolishing and oxidation. Because the active carbon species grow along the copper surface and the morphology of copper affects the quality of graphene, we do electropolishing to flatten the copper surface to lower the possibility of the graphene nucleation. On the other hand, we oxidize copper to form cuprous oxide(Cu2O) and cupric oxide(CuO2) on the surface. During the high temperature, carbon atoms will act with oxide and become carbon oxide(CO) and carbon dioxide(CO2) which leave the surface. This also helps to reduce the nucleation density. During synthesis, we design a special CVD reactor to allow the graphene to grow within a confined reaction space, which also reduces the nucleation density. Finally, we use optical microscope, electron microscope, Raman spectrometer and selected-area electron pattern(SAED) to identify the quality of graphene. From now on, we have successfully synthesized single-crystalline graphene with 700 μm in diagonal length.