The surface and bulklike phonon modes of Si(100)c(4 X 2) nanometer-thin film were investigated using density functional calculations with a 4 x 4 supercell consisting of 10 layers. The surface phonon modes were dispersed over a wide range with two collective peaks around 120 and 190 cm(-1), including interference from both surface phonons. The distribution of the bulklike phonon modes revealed similar peaks to the bulk phonons, and the two collective peaks were newly confirmed on the lower frequency side. These characteristic peaks provide large spectral variation in the phonons of nano-thin films. (c) 2006 Elsevier B.V. All rights reserved.