Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore.
ACS Nano. 2011 Mar 22;5(3):1600-5. doi: 10.1021/nn200010m. Epub 2011 Feb 23.
Though graphene has been intensively studied by Raman spectroscopy, in this letter, we report a study of the second-order overtone and combination Raman modes in a mostly unexplored frequency range of 1690-2150 cm(-1) in nonsuspended commensurate (AB-stacked), incommensurate (folded) and suspended graphene layers. On the basis of the double resonance theory, four dominant modes in this range have been assigned to (i) the second order out-of-plane transverse mode (2oTO or M band), (ii) the combinational modes of in-plane transverse acoustic mode and longitudinal optical mode (iTA+LO), (iii) in-plane transverse optical mode and longitudinal acoustic mode (iTO+LA), and (iv) longitudinal optical mode and longitudinal acoustic mode (LO+LA). Differing from AB-stacked bilayer graphene or few layer graphene, single layer graphene shows the disappearance of the M band. Systematic analysis reveals that interlayer interaction is essential for the presence (or absence) of the M band, whereas the substrate has no effect on the presence (or absence) of the M band. Dispersive behaviors of these "new" Raman modes in graphene have been probed by laser excitation energy-dependent Raman spectroscopy. It is found that the appearance of the M band strictly depends on the AB stacking, which could be used as a fingerprint for AB-stacked bilayer graphene. This work expands upon the unique and powerful abilities of Raman spectroscopy to study graphene and provides another effective way to probe phonon dispersion, electron-phonon coupling, and to exploit the electronic band structure of graphene layers.
尽管拉曼光谱学对石墨烯进行了深入研究，但在这封信件中，我们报告了在一个未被广泛探索的频率范围内（1690-2150cm(-1))，对非悬浮共形（AB 堆叠）、非共形（折叠）和悬浮石墨烯层中的二次泛频和组合拉曼模式的研究。基于双共振理论，我们将这个范围内的四个主要模式分配给（i）面外横向模式的二次泛频（2oTO 或 M 带），（ii）面内横向声子和纵向光学声子的组合模式（iTA+LO），（iii）面内横向光学声子和纵向声学声子（iTO+LA），以及（iv）纵向光学声子和纵向声学声子（LO+LA）。与 AB 堆叠双层石墨烯或少层石墨烯不同，单层石墨烯中 M 带消失。系统分析表明，层间相互作用对于 M 带的存在（或不存在）是必不可少的，而衬底对 M 带的存在（或不存在）没有影响。通过激光激发能量相关的拉曼光谱技术，我们研究了这些“新”拉曼模式在石墨烯中的色散行为。结果表明，M 带的出现严格取决于 AB 堆叠，这可以作为 AB 堆叠双层石墨烯的指纹。这项工作扩展了拉曼光谱学在研究石墨烯方面的独特而强大的能力，并提供了另一种有效的方法来探测声子色散、电子-声子耦合以及利用石墨烯层的电子能带结构。
