We report band gap formation in graphene by using bonding interactions with Si impurities in the form of Si cluster (Si-n). We demonstrate that neither the distortion in graphene nor the periodicity of the adsorption can lead to band gap opening. The calculated band gap for Si-2 is 0.83 eV at Dirac points and the effect of this Si-C interaction is maintained even when the cluster size is increased. However, there is a strong dependence of the size of the band gap on the size of the Si-n. Analysis of the trend points to the change in the dispersion of the gap states due to the change in the Si-C bond. (C) 2011 Elsevier B.V. All rights reserved.