Structural, optical, and magnetic properties of (Co, Cu)-codoped ZnO films with different Co concentrations

Abstract

Room temperature ferromagnetic and ultraviolet optical properties of Co-doped ZnO nanocluster films Zn 0.99-x Co x Cu 0.01 O films with different Co concentrations from 0% to 20% were fabricated by a sol-gel method. Moderate Co doping is found to improve the surface uniformity and crystal quality of the films, and causes a redshift of the band edge of Zn(Co,Cu) films. X-ray photoelectron spectroscopy reveals that the introduction of Co ions causes the valence state of Cu to change from þ2 to þ1; while at Co concentrations lower than 10%, the Co exists in the þ2 valence state. Strong blue emission at $420 and 440 nm are observed, decreasing with increasing Co concentration, but becoming strong again as the concentration is increased to 20%. Enhanced room-temperature ferromagnetism is observed for the (Co, Cu)-codoped ZnO films at Co concentrations lower than 10%. These interesting magnetic properties are explained based on charge transfer, together with the defect-related model for ferromagnetism. V C 2014 AIP Publishing LLC.

Effect of sol pH on microstructures, optical and magnetic properties of (Co,Fe)-codoped ZnO films synthesized by sol–gel method

“…As is well known, the redshift of a band edge could be due to the spin-exchange interaction between the sp band and localized spins of transition-metal ions [32]. That is, the exchange interaction between the transition-metal ions and the band electrons, the so-called sp-d interaction, causes negative and positive corrections, respectively, to the conduction and valence band energy, and hence decreases the E g of the films [4].…”

“…few ZnO particles are generated with a large amount of Zn (OH) 4 2-from a sol of low pH value, according to formula (3), accompanied by weak thin film luminescent intensity. One can see that the intensity of the UV peak increases gradually with increasing pH value.…”

“…In recent years, it has been established that ferromagnetic properties in doped ZnO systems can be enhanced through co-doping with ions of two metals [16]. As is well known, cobalt's electronic shell structure is similar to that of Zn 2+ [4], just as iron's is. Based on this, ZnO systems codoped with Fe and Co have attracted much attention to their microstructures, optical and ferromagnetic properties.…”

“…Zinc oxide (ZnO) has emerged as a promising material for applications such as light-emitting diodes, gas sensors, UV detectors, cosmetics and biomaterials due to its large bandgap of 3.37 eV and exciton binding energy of 60 meV at room temperature [1][2][3][4][5][6]. In particular, transition metal (TM)-doped ZnO films are promising candidates for a variety of practical applications [7][8][9].…”

Ferromagnetic mechanism of (Co, Cu)-codoped ZnO films with different Co concentrations investigated by X-ray photoelectron spectroscopy

“…Also, Lin [14] synthesized ZnCoO that exhibited obvious ferromagnetic ordering above room temperature if prepared with a particular amount of Cu doping by a standard solid-state reaction method. Recently, we reported that (Co, Cu)-doped ZnO shows a red-shift compared to the case when either Co or Cu atoms alone are used to dope ZnO [4,15].…”