The structure of the interface between water and self-assembled organic monolayers having different roughness and polarity is investigated by molecular dynamics computer simulations. The electronic absorption spectrum of an adsorbed chromophore at these different interfaces is computed and correlated with the structure of the interface. Several different electronic transitions characterized by different changes in the electric dipole moment are considered. We find that the effective polarity of the interface is greater than that of bulk water in contrast to the situation at several liquid/liquid interfaces, but in agreement with recent experiments at the liquid/solid interface. We show that this higher effective polarity is due to a contribution from the polar groups at the surface that is larger than the loss due to the decreased interaction with water.

High-pressure optical absorption spectroscopic measurements of both erbium-doped and undoped Si nanoparticles have been carried out in a diamond anvil cell up to pressures of 180 kbar. The emphasis here is with respect to (a) the effect of particle size on the pressure dependence of the band gap as well as (b) indirect examination of the structural impact of the erbium dopant on the pressure-induced phase transition(s). It is found that in terms of electronic structure these Er-doped Si nanocrystals act very much like indirect gap silicon, with an observed band gap pressure dependence of -1.4 × 10 -6 eV/bar. Measurements of the optical spectra in terms of integrated area as a function of pressure of these doped nanoparticles reveal that the first-order phase transition must lie above 180 kbar, substantially elevated from the bulk value of 120 kbar. Thus, doped nanocrystals of this dimension maintain a significant elevation in the phase transition pressure (known in homogeneous Si nanocrystals relative to bulk crystalline Si), but the Er dopant does not introduce the type of structural defects that would lower the energy barrier to such a transformation.