Tip enhanced Raman spectra of AB-stacked bilayer graphene (BLG) and twisted bilayer graphene (TwBLG) nanofragments have been studied by using density functional theory. Different from a normal Raman spectrum of BLG, a unique Raman band Gr+ is observed in its tip enhanced Raman spectrum and assigned as a split of the G band. We attribute this split to the nonuniformity distribution of the charge on carbon atoms. Compared with BLG, the Gr+ band intensity of TwBLG is dramatically enhanced at a small twist angle and decreases with the angle increasing. Interlayer Coulomb interaction represented in the Gr+ mode of TwBLG matches well with the atom vibration strength distribution of the Gr+ band at different twist angles, and thereby the properties of the Gr+ band can be tuned by the twisted angle. The results may help to further understand the Raman spectra of TwBLG and provide deep insights for exploring vibrational modes of two-dimensional nanomaterials.

Chemical interaction between the tips and molecules is one of the main contributing mechanisms to tip-enhanced Raman spectroscopy (TERS). In this work, we calculate the TERS spectra of the biphenylene (BP) dimer at 13 nonequivalent tip sites by means of density functional theory and explore the influence of the TERS tip on vibrational mode characters and Raman intensity. The Raman intensity of the vibrational mode involving the antisymmetric stretching of tetra-rings is found to be specifically enhanced. We attribute this specific enhancement to the electronic sensitive atom vibrational character of the mode and infer that the vibrational strength of atoms can be tuned by the TERS tip. The results provide an intuitive interpretation on the effects of tip-induced electronic redistributions on specific vibrational modes in TERS and indicate the possibility to further improve the TERS resolution.

The length dependence of the Raman spectra and vibrational properties of biphenylene strips are explored using density functional theory.

The atomic-scale mechanism of plasmon-mediated H2 dissociation on gold nanoclusters is investigated using time-dependent density functional theory. The position relationship between the nanocluster and H2 has a strong influence on the reaction rate. When the hydrogen molecule is located in the interstitial center of the plasmonic dimer, the hot spot here has a great field enhancement, which can promote dissociation effectively. The change in the molecular position results in symmetry breaking, and the molecular dissociation is inhibited. For the asymmetric structure, direct charge transfer from the gold cluster to the antibonding state of the hydrogen molecule by plasmon decay makes a prominent contribution to the reaction. The results provide deep insights into the influence of structural symmetry on plasmon-assisted photocatalysis in the quantum regime.