A molecular dynamics study on vibration spectra of a-SiO2 surface

The charge equilibration (QEq) method was used in the molecular dynamics study of the vibration spectra on the a-SiO2 surface. The vibrational density of states (VDOS) of a silica glass surface and partial contribution of Si and 0 atoms were compared with those obtained from the fixed-charge (FQ) model. The VDOS in each of the samples has two groups of vibrational bands, i.e., wide-lower-frequency band (WB) between 0-25 THz and narrow-higher-frequency band (NB) between 25 and 40 THz, which is similar to the bulk VDOS obtained experimentally. The QEq surface exhibits a peak and two shoulders while two peaks are found for the FQ surface. At the surface, an excess peak (at 1-2 THz) was observed by substracting the Debye VDOS from the calculated VDOS, which is regarded as the experimentally observed the boson peak (BP). The position of the BP is shorter than that in the case of the bulk. In the FQ surface, the VDOS consists of narrower bands, while in the QEq sample, wider bands exist due to charge variation. (C) 2003 Elsevier Science B.V. All rights reserved.