  We investigate the impact of water, a polar solvent, on the optical
absorption of prototypical silicon clusters with oxygen passivation. We
approach this complex problem by assessing the contributions of three factors:
chemical reactivity; thermal equilibration and dielectric screening. We find
that the silanone (Si=O) functional group is not chemically stable in the
presence of water and exclude this as a source of significant red shift in
absorption in aqueous environments. We perform first principles molecular
dynamics simulations of the solvation of an oxygenated silicon cluster with
explicit water molecules at 300 K. We find a systematic 0.7 eV red shift in the
absorption gap of this cluster, which we attribute to thermal strain of the
molecular structure. Surprisingly, we find no observable screening impact of
the solvent, in contrast with consistent blue shifts observed for similarly
sized organic molecules in polar solvents. The predicted red shift is expected
to be significantly smaller for larger Si quantum dots produced experimentally,
guaranteeing that their vacuum optical properties are preserved even in aqueous
environments.
