Hybrid density-functional theory calculations of electronic and optical properties of mercaptocarboxylic acids on ZnO $(10{overline 1}0)$ surfaces

In this work we investigate the electronic properties of mercaptocarboxylic acids with several carbon chain lengths adsorbed on ZnO-(10-10) surfaces via density functional theory calculations using semi-local and hybrid exchange-correlation functionals. Amongst the investigated structures, we identify the monodentate adsorption mode to be stable. Moreover, this mode introduces optically active states in the ZnO gap, is further confirmed by the calculation of the dielectric function at PBE0 and TD-PBE0 levels. One interesting finding is that adsorption mode and the dielectric properties of the hybrid system are both rather insensitive to the chain length, since the acceptor molecular state is very localized on the sulphur atom. This indicates that even small molecules can be used to stabilize ZnO surface and to enhance its functionality for opto-electronic applications.