Effect of bismuth, cobalt and bismuth-cobalt codoped zinc oxide nanowires on stable room-temperature ferromagnetism for spintronics applications

Abstract

Room-temperature ferromagnetism (RTFM) in wide and direct bandgap diluted magnetic semiconductor zinc oxide (ZnO) is attributed to the intrinsic defects and p-orbital porbital (p-p) coupling interaction. As a result of oxidation, the ferromagnetism induced by defects is unstable. Besides, present literature gives an unclear picture of the origin of RTFM. This work aims to (1) synthesis the pristine ZnO nanowires (NWs) using simple solution process and (2) control the amount of dopants (Bi, Co and Bi-Co) to achieve the high crystallinity of pristine and doped ZnO NWs to obtain stable RTFM. The experimental findings were compared with density functional theory (DFT) studies to affirm our findings. Using a simple solution process also enables us to provide a room to control the amount of dopant and even helps to achieve comparable crystallinity compared to physical deposition method. Solution process is also very helpful in achieving high yield of the prepared samples. Three set of samples were prepared Bidoped ZnO NWs (Bi = 1, 3 and 5%), Co-doped ZnO NWs (Co = 2.5, 4, 10 and 15%), and Bi-Co codoped ZnO NWs (Bi=1%, Co = 2.5, 5, 10 and 15%). The morphology and the crystalline quality of the samples were observed by field emission electron microscopy (FE-SEM) and transmission electron microscopy (TEM), respectively. By FESEM images, all the dopants shown that the grown ZnO NWs have preferred orientation along the c-axis in (001) direction due to the anisotropic crystal nature of ZnO. TEM analysis further confirmed the crystal growth direction along the (001) direction. The X-ray diffraction (XRD) findings showed that the NWs have a wurtzite crystal structure and have a high intensity at (002) peak, indicating that most of the reflection comes from the hexagonal face of the NWs. X-ray photoelectron spectroscopy (XPS) confirmed the presence of all expected elements. At higher doping, the oxide phase appears and shows that the OH- groups exist on the surface of ZnO NWs. UV-Vis analysis showed decreased optical bandgap from 3. 5 eV to 3.08 eV upon doping that is probably due to the sp-d exchange interaction between localized d-electrons band electrons of dopants. The magnetic studies showed that the pure ZnO NWs showed a diamagnetic response at room-temperature (RT) and ferromagnetic response at lowtemperature (4K) that may be related to defects related species. On the other hand, all the Bi, Co, and Bi-Co codoped samples showed a ferromagnetic (FM) response at RT and 4K. Each dopant shown FM at certain doping concentration. For Bi-doped ZnO, the Bi:ZnO-1% found to have highest saturation magnetization (Ms) of 0.87 and 0.94 emu/cm3 at RT and 4K, respectively. Doping of Bi will introduce acceptor levels due to its p-orbital, and hence free holes in the crystal. The free carriers in the localized orbitals are responsible for the itinerant magnetism. For the Co-doped ZnO NWs, the highest Ms was observed for Co:ZnO-10%. It was calculated to be 0.34 and 0.50 emu/cm3 at RT and 4K, respectively. The highest Ms value among Bi, Co and Bi-Co samples was observed when Bi and Co were codoped with Bi-1, Co-10% -ZnO concentration. The Ms values were 1.67 and 1.28 emu/cm3 at RT and 4K, respectively. Both codoped Bi and Co in their 3+ valence states probably play their prominent role in the enhancement of FM related to defect formation. Our DFT studies were in line with the experimental results. Overall, obtained results demonstrated that all the doped and codoped (Bi = 1, Co = 2.5, 5, 10%) samples could be used to tune both the optical and magnetic properties of ZnO NWs, hence paving the pathway for future spintronics and spin-polarized optoelectronics applications.