First-principles study of polar molecule adsorption on hydrogenated diamond (001)

Density functional theory calculations reveal that adsorption of small polar molecules such as H$_{2}$O, HF, and HCl on hydrogenated diamond (001) surfaces can result in unexpected dihydrogen bonding. This implies that in a C-H bond, H may be more electronegative than C, thus deviating from the widely-accepted Pauling's electronegativity scale, C(2.55) \textit{vs} H (2.20). Detailed analysis of the self-consistent charge densities confirms that electrons are indeed accumulated more at the H site than at the C site with respect to the free atoms. It further explains why dihydrogen bond can form for H$_{2}$O and HF on the surfaces, but not for NH$_{3}$. The true physical origin for the well-known reduction of the work function due to surface hydrogenation is now attributed to the replacement of loosely bonded surface C $\pi $ electrons by more tightly bonded C-H $\sigma $ electrons. We also propose that the favorable formation of the dihydrogen bonds may contribute to the observed p-type conductivity of diamond surfaces in acidic conditions.