Si nanosheets are a promising candidate material for nanoscale devices but a thorough understanding of their surface properties is still lacking. Here we find, using first-principles molecular-dynamics calculations, a new surface structure specific to a double-layer Si nanosheet. The new surface structure, which can be labeled as Si(1 1 1)-2 x 2 in the same fashion as the bulk Si surface, is formed during surface reconstruction at finite temperature. The reconstructed nanosheet surface consists of highly distorted tetrahedra as on the Si(1 1 1)-2 x 1 surface, but its electronic band structure is markedly different from that of Si(1 1 1)-2 x 1. The vibrational properties of the reconstructed Si nanosheet are also found to reflect the distortion in the tetrahedral order. (c) 2011 Elsevier B.V. All rights reserved.