Self-trapped excitons at the quartz(0001) surface

We have studied self-trapped excitons in alpha-quartz using density functional theory (DFT), both in the crystal and at the (0001) surface. The excitons are triplet excited states that distort the crystal locally. They have a long lifetime, of the order of a millisecond, and become thermally equilibrated. We have calculated the drop in the exciton energy as it approaches the surface from the interior of the crystal. In the subsurface layer of the -OH terminated (0001) surface, the energy has dropped by 0.7 eV. Another 0.4 eV drop occurs as the exciton enters the surface layer, where it breaks off an OH radical. The drop in energy can be understood from the greater ease of structural distortion at the surface. These calculations illustrate that excitons formed in the bulk could migrate out to the surface and form chemically active surface species. Molecules adsorbed at the surface could also serve as traps for the excitons and could, in principle, be induced to undergo structural or chemical transitions.