The effect of crystal size, pressure, temperature, and their coupling on the bandgap (E G ) of ZnO crystals have been investigated based on the Hamiltonian perturbation, using the extended BOLS correlation theory. The functional dependence of the E G on the identities (order, nature, length, energy) of the representative bond for a specimen and the response of the bonding identities to the applied stimuli have been established. Theoretical reproduction of the measurements confirms that the E G expansion originates from the bond contraction/compression and bond strength gain due to (i) Goldschmidt-Pauling's rule of bond contraction induced by undercoordination, (ii) low-temperature enhanced stability, and (iii) mechanical energy storage. It is found that the multiple-field coupling effect dominates in the surface skin up to three atomic layers. The presented approach provides a guideline for harnessing the photoluminescence, photoabsorption, and exciton emission from ZnO and other semiconductors as well.

Although the size-and shape-induced blueshift in the photoluminescence and photoabsorption of nanostructured ZnO has been extensively investigated, the underlying mechanism remains yet unclear. Here we show that theoretical reproduction of the observed trends clarifies that the blueshift originates from the Hamiltonian perturbation due to the broken-bond-induced local strain and quantum trapping and electron-phonon coupling in the surface skin up to two atomic layers in depth while bonds in the core interior retain their bulk nature. The extent of the blue shift depends on the tunable fraction of undercoordinated atoms in the surface skin. Therefore, the quantum confinement effect is indeed more "superficial" than first thought ͓H. Winn, OE Mag. 8, 10 ͑2005͔͒.