以甲基偶氮於銅(111、110、100)表面成長石墨烯之掃描顯微研究

Abstract

根據文獻我們知道甲基是(CH3·)石墨烯生長過程中一樣重要的中間產物，在實驗中我們嘗試直接用甲基(CH3·)在銅單晶上生長石墨烯並與甲烷做比較，我們所用的甲基是由甲基偶氮分子裂解而來，當甲基偶氮分子在接近1000度高溫裂解時會產生許多小分子，包含甲基、氮氣、NCH3與其他少量的分子，我們發現如果用甲基當作前驅物，石墨烯在600~700度的低溫下就可以在(111)、(110)、(100)這幾個常見晶面上長出很好的品質，我們發現這些石墨烯的形狀與其生長的銅基質結構有非常大的相關性，並且藉由這些NCH3分子，我們發現生長出來的石墨烯具有~0.05%的氮參雜。另外，不同與甲烷我們發現以甲基為前驅物時，石墨烯的生長速率並不會隨著銅的晶面不同而改變，這可能是因為甲基與甲烷在不同晶面上脫氫效率不同的影響，所以當我們直接用甲基為前驅物時，石墨烯的生長速度不會隨著晶面不同而有所改變。最後，我們在用甲基生長石墨烯的實驗中觀察到，石墨烯的生長伴隨著「銅基質的移除」，這些石墨烯在生長的過程中能夠將其下的銅移除，導致看起來石墨烯是陷於兩旁的銅之下，我們認為這是因為銅原子在生長過程中很可能會與甲基鍵結形成類似CH3-M或CH3-MH的結構(M為金屬原子，例如:銅)，進而造成銅原子被帶離金屬表面。

Methyl group (CH3·) is an important intermediate specie in the growth of graphene when using hydrocarbon as precursors. In this work, we demonstrate a one-shot synthesis method of high-quality nitrogen-doped graphene on copper substrates by using methyl from pyrolysis of azomethane, and discuss an observation about the growth of graphene. Pyrolysis of azomethane gas at ~1000oC generates mainly methyl, methylnitrene, nitrogen and a few other minor species. We discovered that, with methyl, graphene can readily grow at a low temperature of 600~700°C with a high probability, on all major Cu orientations. Though the islands’ shape and nucleation density of graphene are affected by the crystal orientations, graphene has a uniform growing rate on Cu(111), Cu(110) and Cu(100). Nitrogen doping density with N/C ratio of ~ 0.05% is observed. The dopants are generated due to the incorporation of methylnitrene during growth. Though having a low doping density, evidence shows that the synthesis of highly doped graphene can be achieved. Furthermore, when using methyl group as precursor, a “copper etching effect” is observed. Graphene islands are discovered to be capable to “dig a hole” on copper surface. We surmise that this is due to the reaction between copper atoms and methyl which forms intermediate species such as CHx-Cu. Thus, copper atoms at surface can be carried away in this structure.