Defect engineering in ZnO nanocones for visible photoconductivity and nonlinear absorption

Abstract

Nanostructured ZnO is a promising material for optoelectronic and nonlinear optical applications because of the flexibility of band gap engineering by means of various defect states present in it. Employing the time-correlated single photon counting photoluminescence technique, the correlation between defect levels and optoelectronic and nonlinear optical properties of ZnO is explored in this work. By a facile solution method, ZnO nanocones with a dominating preferential orientation along energetically less favorable, oxygen terminated (10̄11) facets were synthesized using a passivating capping agent. Photoluminescence spectra demonstrate that the as-grown samples have both oxygen and zinc vacancies, and after calcination in air oxygen vacancies vanish, but zinc vacancies are enhanced. Photoconductivity of the samples reduces significantly upon calcination, confirming the reduction in oxygen vacancies. However, the samples exhibit a significant enhancement in the nonlinear optical absorption coefficient upon calcination, indicating that the effective two-photon absorption causing the nonlinear optical behaviour originates from zinc vacancies. These results illustrate the vast possibilities of band gap engineering in intrinsic ZnO for future optoelectronic applications.

Controlled Defects of Zinc Oxide Nanorods for Efficient Visible Light Photocatalytic Degradation of Phenol

“…When ZnO nanorods with lower surface defects were used, only about 20% degradation of phenol was observed after 300 min (Figure 4b), showing a degradation rate constant of 1 × 10 −3 min −1 . In this regard, it has been reported earlier that the presence of surface defects in ZnO nanostructures increases the rate of electron-hole pair generation through increased sub-bandgap absorption, enhancing the photocatalytic activity of the material in the visible region [41,42]. Additionally, the increased surface defect density in the 350 °C annealed ZnO nanorods increases the number of active sites near the surface of the nanorods by acting as trap sites for the photo-generated electrons, allowing the photoactive charges to interact easily with the phenol molecules [41].…”

Reduced graphene oxide–ZnO self-assembled films: tailoring the visible light photoconductivity by the intrinsic defect states in ZnO

“…44,45 The quenching in visible emission in rGO-ZnO may be due to the passivation of defects in ZnO during the formation of rGO-ZnO. In ZnO, green emission is due to the capture of holes by V O to generate V O , followed by a transition between V O to the valence band.…”

“…44 In rGO-ZnO, the green emission corresponding to oxygen vacancies is passivated during the reduction of graphene oxide. A dispersion of samples in PAM, spin coated on FTO glass (with active area of 0.25 cm 2 ), was used as the working electrode.…”

Effect of cobalt doping on structural, thermo and photoluminescent properties of ZnO nanopowders

“…The broad blue band emission peaks are assigned to surface defects such as oxygen vacancies (V o ) and Zinc interstitials (Zn i ). The green emission origin is generally assigned to the radiative recombination of photo generated holes in the valence band with electrons in singly occupied oxygen vacancies [20,22]. The PL emission intensity is found to decrease with the increase in cobalt concentration as shown in the inset of Fig.…”