Center for Nanochemistry, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
ACS Nano. 2011 Jul 26;5(7):5338-44. doi: 10.1021/nn103237x. Epub 2011 Jun 22.
We studied the modulation of Raman scattering intensities of molecules on graphene by tuning the graphene Fermi level with electrical field effect (EFE). A series of metal phthalocyanine (M-Pc) molecules (M = Mn, Fe, Co, Ni, Cu, Zn), which have different molecular energy levels, were used as probe molecules. The Raman intensities of all these M-Pc molecules become weaker when the graphene Fermi level is up-shifted by applying a positive gate voltage, while they become stronger when the graphene Fermi level is down-shifted by applying a negative gate voltage. However, this Raman intensity modulation only occurs when applying the gate voltage with a fast sweep rate, while it is nearly absent when applying the gate voltage with a slow sweep rate, which is likely due to the arising of the hysteresis effect in the graphene EFE. In addition, the Raman modulation ability for M-Pc molecules with smaller energy gaps is larger than that with larger energy gaps due to the difference in the energy alignment between graphene and these M-Pc molecules. Furthermore, this modulation shows the greatest one on single-layer graphene and mainly comes from the first layer of molecules which are in direct contact with graphene. The Raman modulation of molecules in GERS with the EFE suggests that the Raman enhancement for GERS occurs through a chemical enhancement mechanism.
我们通过电场所致的石墨烯费米能级（EFE）调节来研究分子在石墨烯上的拉曼散射强度的调制。一系列具有不同分子能级的金属酞菁（M-Pc）分子（M = Mn、Fe、Co、Ni、Cu、Zn）被用作探针分子。当施加正栅极电压使石墨烯费米能级上移时，所有这些 M-Pc 分子的拉曼强度都会变弱，而当施加负栅极电压使石墨烯费米能级下移时，它们的拉曼强度会变强。然而，这种拉曼强度调制仅在施加快速扫描速率的栅极电压时发生，而在施加缓慢扫描速率的栅极电压时几乎不存在，这可能是由于石墨烯 EFE 中出现滞后效应所致。此外，由于石墨烯和这些 M-Pc 分子之间的能量对准的差异，具有较小能隙的 M-Pc 分子的拉曼调制能力大于具有较大能隙的 M-Pc 分子的拉曼调制能力。此外，这种调制在单层石墨烯上表现出最大的调制效果，主要来自与石墨烯直接接触的第一层分子。EFE 对 GERS 中分子的拉曼调制表明，GERS 的拉曼增强是通过化学增强机制发生的。
