Abstract
Semiconductor nanocrystals have the potential for a range of applications in optoelectronicsand nonlinear optics. As the surface-to-volume ratio increases, surface emissionprocesses become more important. Using infrared (IR) and photoluminescence (PL)spectroscopy, we have developed a unified model for the acceptor and intragap surfacestates of ZnO nanocrystals. A PL peak was observed at 2.97 eV, in agreementwith an acceptor level previously observed in the IR (Teklemichael et al 2011Appl. Phys. Lett. 98 232112). The temperature dependence of the IR absorptionpeaks, which correspond to a hole binding energy of 0.46 eV, showed an ionizationactivation energy of only 0.08 eV. This activation energy is attributed to thermalexcitation of the hole to surface states 0.38 eV above the valence band maximum.Therefore, while the acceptor is deep with respect to the bulk valence band, itis shallow with respect to surface states. A strong red PL emission centered at1.84 eV, with an excitation onset of 3.0 eV, is attributed to surface recombination.
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