Van der Waals'force-controlled nano Raman scattering spectroscopy

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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.