Electronic structure of germanium nanocrystals core and (001)-(1 x 1) oxidised surface

The electronic structure of (001)-(1 × 1) oxidised surface and inner core of germanium nanocrystals (1.5 - 2.3 nm in diameter) is studied separately using the self-consistent Hartree-Fock method coupled with the large unit cell (LUC) method. The separate evaluation of surface and core electronic structure leads to the exact discrimination between nanocrystal surface and core properties. The results show that the lattice constant and the energy gap of the core part decrease as nanocrystals increase in size in agreement with previous studies. The small differences between the core and the surface lattice constant reflects good adherence of the oxide layer. The nanocrystal oxidised surface is characterised by a smaller gap and a wider valence band than the core. This result is owing to the splitting of the highly degenerate core states as they approach the surface in addition to the newly added levels of oxygen surface atoms. The present two gap results resolved the ambiguity in different gap and photoluminescence results of Ge nanocrystals. The surface layers have oscillatory negative and positive charges that diminish as they reach the inner fourth germanium layer. LUC method predicts nearly direct transitions in small germanium nanocrystals that explain their suitability for light-emitting applications.