Department of Chemistry, Columbia University, New York, New York, 10027 United States.
ACS Nano. 2010 Nov 23;4(11):7005-13. doi: 10.1021/nn102227u. Epub 2010 Oct 14.
Strong Raman scattering is observed from iodine anions adsorbed at ca. 3% coverage on single layer graphene. In addition, the Raman signal from just one bromine intercalation layer inside three and four layer thick graphenes is observed. We analyze and model the intramolecular electronic, charge-transfer, and multiple reflection electromagnetic mechanisms responsible for this unusual sensitivity. Graphene is an excellent Raman substrate for adsorbed species showing intramolecular electronic resonance, because graphene efficiently quenches interfering excited-state luminescence. The Raman sensitivity for adsorbed and intercalated molecular species is highest for single layer graphene and decreases with increasing thickness. These phenomena are compared with surface enhanced Raman spectroscopy field enhancement and "chemical" Raman processes in aggregated Ag particles and on flat, highly reflective metal surfaces. The Raman spectra of adsorbed bromine layers are not observed, despite significant charge transfer to graphene. Charge transfer from adsorbed bromine is about one-half of charge transfer from intercalated bromine. We attribute the large Raman signal for both adsorbed iodine and intercalated bromine species to intramolecular electronic resonance enhancement. The signal evolution with varying graphene thickness is explained by multiple reflection electromagnetic calculations.
在单层石墨烯上约 3%的覆盖度下吸附的碘阴离子表现出强烈的拉曼散射。此外，还观察到了三层和四层厚石墨烯中仅一个溴插层层内的拉曼信号。我们分析并建立了分子内电子、电荷转移和多次反射电磁机制模型，这些机制解释了这种不寻常的敏感性。对于表现出分子内电子共振的吸附和插层分子物种，石墨烯是一种极好的拉曼基底，因为石墨烯能有效地猝灭干扰的激发态发光。对于单层石墨烯，吸附和插层分子物种的拉曼灵敏度最高，随着厚度的增加而降低。这些现象与聚集的 Ag 颗粒上的表面增强拉曼光谱场增强和“化学”拉曼过程以及在平坦、高反射金属表面上的现象进行了比较。尽管溴向石墨烯转移了大量电荷，但没有观察到吸附溴层的拉曼光谱。吸附溴的电荷转移约为插层溴的一半。我们将吸附碘和插层溴物种的大拉曼信号归因于分子内电子共振增强。随着石墨烯厚度的变化，信号的演化可以通过多次反射电磁计算来解释。
