Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan.
Nano Lett. 2012 Jan 11;12(1):414-9. doi: 10.1021/nl203733r. Epub 2011 Dec 15.
Surface contamination by polymer residues has long been a critical problem in probing graphene's intrinsic properties and in using graphene for unique applications in surface chemistry, biotechnology, and ultrahigh speed electronics. Poly(methyl methacrylate) (PMMA) is a macromolecule commonly used for graphene transfer and device processing, leaving a thin layer of residue to be empirically cleaned by annealing. Here we report on a systematic study of PMMA decomposition on graphene and of its impact on graphene's intrinsic properties using transmission electron microscopy (TEM) in combination with Raman spectroscopy. TEM images revealed that the physisorbed PMMA proceeds in two steps of weight loss in annealing and cannot be removed entirely at a graphene susceptible temperature before breaking. Raman analysis shows a remarkable blue-shift of the 2D mode after annealing, implying an anneal-induced band structure modulation in graphene with defects. Calculations using density functional theory show that local rehybridization of carbons from sp(2) to sp(3) on graphene defects may occur in the random scission of polymer chains and account for the blue-shift of the Raman 2D mode.
聚合物残留物的表面污染长期以来一直是探测石墨烯固有特性的一个关键问题，也是在表面化学、生物技术和超高速度电子学中利用石墨烯独特应用的一个关键问题。聚甲基丙烯酸甲酯（PMMA）是一种常用于石墨烯转移和器件加工的高分子，会留下一层薄薄的残留物，需要通过退火来进行经验性清洁。在这里，我们使用透射电子显微镜（TEM）结合拉曼光谱，对 PMMA 在石墨烯上的分解及其对石墨烯固有特性的影响进行了系统研究。TEM 图像显示，物理吸附的 PMMA 在退火过程中经历了两步失重，并且在石墨烯易受破坏的温度之前不能完全去除。拉曼分析表明，退火后 2D 模式发生了显著的蓝移，这意味着退火导致了石墨烯中具有缺陷的能带结构调制。使用密度泛函理论的计算表明，在聚合物链的随机断裂过程中，石墨烯缺陷上的碳原子可能会发生从 sp(2)到 sp(3)的局部重杂化，这可以解释拉曼 2D 模式的蓝移。
