Opto-electronic properties of Zn(1-x)VxO: Green emission enhancement due to V4+ state

Abstract

Vanadium incorporation in ZnO modifies the lattice structure. The valence state of V plays an important role, controlling the oxygen content and thereby dimensions of the lattice. Both V4+ and V5+ are more electropositive than Zn2+ and reduce oxygen vacancies, resulting in lattice expansion. However, the sizes of both V4+ and V5+ are smaller than Zn2+, thereby resulting in the lattice contraction. The internal competition of increasing oxygen content and reducing effective crystal radius decides the lattice expansion and contraction. This affects the lattice strain and changes electronic levels, which modify absorption and emission processes in between the valence and conduction bands. A strong green emission band not due to oxygen vacancy but due to defects contributed by vanadium is also dependent on the oxidation state of vanadium. Bandgap also increases with the increase in the V4+ content.

Vanadium substitution: A simple and economic way to improve UV sensing in ZnO

“…As a result oxygen vacancies, V0 /•• + 5/4Oo X ; where, Oo X is an oxygen ion in its site. Oxygen content was found to increase in the substituted samples, as revealed from XANES studies [15]. Hence, if surface oxygen defects would have been the mechanism then with reduced oxygen vacancies, sensitivity should have reduced with increased vanadium incorporation.…”

“…Fraction of V 4+or5+ . XANES analysis provided an estimation of ratio of V 4+ and V 5+ content [15]. Both V 4+ and V 5+ contents show increasing trends similar to sensitivity as shown in inset of figure 3 (a).…”

“…It has been discussed in general, that adsorption and desorption of oxygen molecules from the surface of a material, is affected by UV radiation [13]. In dark condition, oxygen molecules capture free electrons found to increase in the substituted samples, as revealed from XANES studies [15]. Hence, if surface oxygen defects would have been the mechanism then with reduced oxygen vacancies, sensitivity should have reduced with increased vanadium incorporation.…”

“…To be noted that the trend of increment in sensitivity is strikingly similar to that of V 4+ ions. Hence vanadium (V 4+ or V 5+ ) incorporation must be creating defect states in between the valence and conduction bands of ZnO [15][16][17], which trap free electrons from the lattice thereby reducing the dark current. While V 5+ state is empty, V 4+ state has one available electron.…”

Influence of V2O5 and B2O3 addition on the sintering behaviour and physical properties of ZnO ceramics

“…This results in the lattice distortion. The internal competition of increasing oxygen content and reducing effective crystal radius determines the direction of lattice expansion and contraction [27].…”