Probing the Visible Luminescence Mechanism in ZnO Nanoparticles by Band Edge Modulation

Abstract

The visible luminescence mechanism of ZnO is an important but controversial problem. In this paper, we report the structural and optical properties of Mg-doped ZnO nanoparticles (approximately 3-4 nm) synthesized via the sol-gel method. X-ray diffraction (XRD) analysis and absorption spectra observations revealed that Mg ions replace Zn ions in the lattice. In the room temperature photoluminescence (PL) spectra, three emission bands, ultraviolet (UV), blue, and green, were observed. With increasing concentration of Mg, the near band edge (NBE) emission band progressively shifted to the higher energy side. The green emission in the visible emission band, however, presented an inconspicuous shift. The reason is probably that the bottom of the conduction band in ZnO is determined by the Zn 4s state, and the top of the valence band is determined by the O 2p state. Mg ions in ZnO substitute for zinc ions and enter a slightly distorted tetrahedral site, which causes the bottom of the ZnO conduction band to be shifted to higher energy and leaves the top of the valence band unchanged. This combined with the fact that the deep level position is insensitive to the shift of the band edge led us to conclude that the green emission originates from electronic transition between the deep defect level and the top of the valence band (or very shallow acceptor level).

Mg-Induced Shift of Band Edge in ZnO:Mg Probed by the Visible Luminescence

“…In contrast the green band has only small blue-shift. The phenomenon was also observed by X. L. Chen et al [14]. The 0.04 eV shift of the absorption edge in studied region indicates that the absorption edges may be dominated by the transition between defects and one band.…”

“…A blue shift of absorption edge was clearly observed with Mg doped into ZnO. Although Mg does lead to decease of the crystalline particle size, as mentioned above, the crystallites studied here are still much larger than those of quantum dots studied in the literature, which are about 4 nm [14,15]. These indicate that the blue shift is due to the influence of Mg 2+ ions rather than the decrease of particle size.…”

Synthesis of Quantum-Sized ZnO Nanoparticles in Different Medium and Their Application to NO<SUB>2</SUB> Sensors

“…Until now many methods have been developed to synthesize quantumsized zinc oxide nanoparticles including vapor phase growth, vapor-liquid-solid processes, metal organic chemical vapor deposition, electrophoretic deposition, sol-gel process and homogeneous precipitation. [7][8][9][10][11] However, it is * Authors to whom correspondence should be addressed. still a great challenge to fabricate quantum-sized ZnO with controlled-size by wet chemical methods.…”