Size-dependent surface luminescence in ZnO nanowires

Abstract

Nanometer sized whiskers ͑nanowires͒ offer a vehicle for the study of size-dependent phenomena. While quantum-size effects are commonly expected and easily predicted, size reduction also causes more atoms to be closer to the surface. Here we show that intensity relations of below-band-gap and band-edge luminescence in ZnO nanowires depend on the wire radius. Assuming a surface layer wherein the surface-recombination probability is 1 ͑surface-recombination approximation͒, we explain this size effect in terms of bulk-related to surface-related material-volume ratio that varies almost linearly with the radius. This relation supports a surface-recombination origin for the deep-level luminescence we observe. The weight of this surfaceluminescence increases as the wire radius decreases at the expense of the band-edge emission. Using this model, we obtain a radius of 30 nm, below which in our wires surface-recombination prevails. More generally, our results suggest that in quantum-size nanowires, surface-recombination may entirely quench band-to-band recombination, presenting an efficient sink for charge carriers that unless deactivated may be detrimental for electronic devices.

Magnetic Properties of ZnO Nanoparticles

“…Recently, Shalish et al [23] demonstrated that this luminescence arise form the surface of the ZnO while Norberg and Gamelin established it is directly correlated with the surface hydroxide concentration [24]. Thus, the study of this emission is a suitable tool to investigate the modification in the surface dependence of the PL visible emission with the type of capping molecule is in agreement with the surface origin [23,24], as the molecules can alter only the electronic structure close to the surface.…”

Broadband ZnO Single-Nanowire Light-Emitting Diode

“…The weak luminescence centered at ∼380 nm has been attributed to excitonic recombination. 5,6,9,11,14,18 The broad sub-bandgap emission in the PL and EL spectra is believed to be due to defects and surface states. 5,6,11,18 The spectral characteristics are reproducible across different devices, and all three devices exhibit an enhancement of the short wavelength emission with increasing bias voltage.…”

Multicolor Luminescence from Semiconductor Nanocrystal Composites Tunable in an Electric Field

“…The visible luminescence of ZnO nanocrystals located around 510-520 nm (''green'' emission) is affected by the surface (Matsumoto et al, 2002;Norberg & Gamelin, 2005;Ramakrishna & Ghosh, 2003;Shalish et al 2004;VanDijken et al, 2000a;Van Dijken et al, 2001). A direct relationship between the size and the excitonic emission intensity in ZnO crystals in the 40-2500 nm size range was reported (Matsumoto et al, 2002;Shalish et al 2004;Van Dijken et al, 2001). Moreover the ''green'' emission intensity was reported to display a direct correlation with surface hydroxide concentrations (Liu et al, 1992;Norberg & Gamelin, 2005;Van Dijken et al, 2000b).…”

“…Improvement of the (NBE/VB) intensity ratio indicates that dangling bonds and defect states in the surface layer of ZnO nanocrystals are significantly passivated by dye. The visible luminescence of ZnO nanocrystals located around 510-520 nm (''green'' emission) is affected by the surface (Matsumoto et al, 2002;Norberg & Gamelin, 2005;Ramakrishna & Ghosh, 2003;Shalish et al 2004;VanDijken et al, 2000a;Van Dijken et al, 2001). A direct relationship between the size and the excitonic emission intensity in ZnO crystals in the 40-2500 nm size range was reported (Matsumoto et al, 2002;Shalish et al 2004;Van Dijken et al, 2001).…”