Van der Waals'force-controlled nano Raman scattering spectroscopy

我们发现碳纳米管在受到货车德瓦尔斯力作用时，其特定的拉曼谱带发生了位移。利用原子力显微镜的银悬臂梁针尖测量了单根碳纳米管束在外加应力作用下的拉曼光谱，并原位观察了碳纳米管束的拉曼谱带的变化。当应力从0 nN到2.4nN连续施加时，G带的下峰（~ 1600 cm ~（-1））红移了18 cm ~（-1）。G带反映了碳纳米管的电学性质，具有两个相邻的峰。另一方面，在G带的较高峰中没有观察到峰位移。这一现象源于两个峰对应于不同振动方向的振动模式，并且这两个振动模式相对于彼此具有不同的剪切应力。此外，我们观察到径向呼吸模式（RBM）的峰移约200 cm-1至300 cm-1，最多为5 cm-1。RBM取决于CNT的直径。这种峰位移可以归因于这样的事实，即通过对CNT施加单轴应力，CNT的横截面变形为椭圆形状。我们还观察到当施加应力时，所有拉曼带的强度都增强。这表明，由于碳纳米管沿着长轴方向的形变，使得碳纳米管的带隙能量接近共振能（2.33eV），从而增加了共振拉曼效应的贡献。计算表明，如果金属簇中至少含有四个Ag原子，则可以很好地再现由络合物形成机制引起的实验观察到的带移。

We found spectral shift of specific Raman band of carbon nanotubes (CNTs) when van der Waals's force is applied onto the CNTs. We measured Raman spectrum of single CNT bundle while applying stress on it with silver coated cantilever tip of atomic force microscope, and observed spectral change of Raman band of CNT in situ. When we apply stress from 0nN to 2.4nN successively, lower peak of G-band (〜1600cm-1) was red-shifted by 18cm-1. G-band reflects electric property of CNT and has two adjacent peaks. On the other hand, no peak shift was observed in higher peak of G-band. This phenomenon originates from the fact that two peaks are corresponding to the vibrational modes of different vibrational direction, and these two vibrational modes have different shearing stresses with respect to each other. Furthermore, we observed peak shift of radial breathing mode (RBM) around 200cm-1〜300cm-1 at most 5cm-1. RBM depends on the diameter of CNT. This peak shift can be attributed to the fact that the cross-section of CNT is deformed into ellipse shape by applied uniaxial stress on the CNT. We also observed intensity of all Raman band are enhanced when applying stress. This suggests that the band gap energy of CNT comes near to the resonance energy(2.33eV) because of the deformation of CNT along the long axis of CNT, thus contribution of resonant Raman effect is increased.We also performed vibrational calculation of molecule-metal cluster including a number of metal atom based on density functional theory. The calculation showed the experimentally observed band shift by complex formation mechanism is well reproduced if the metal cluster contains at least four Ag atoms.