Elucidating Si-Si dimmer vibration from the size-dependent Raman shift of nanosolid Si

It has been surprising that with the solid size reduction, the transverse optical (TO) Raman mode shifts to lower frequency and new low-frequency Raman (LFR) acoustic modes are generated and shift to higher frequency upon nanosolid Si formation. Understanding the mechanism behind the TO red shift and the LFR creation and blue shift has long been a challenge. On the basis of the BOLS correlation [Sun et al. J. Phys. Chem. B 2002, 106, 10701], here we show that the TO red shift arises from the cohesive bond weakening of the lower coordinated atoms near the surface region of the nanograin, and the LFR arises from intergrain interaction. Strikingly, simulating the TO peak shift confirms the assumption, originated by Shi and Jiang, that the magnitude of surface atomic vibration is always higher than the bulk value and remains constant at particle size greater than 1 nm. Practice also provides a way for elucidating information of an isolated Si-Si dimer vibration.