Evidence of room-temperature ferromagnetism in vertically aligned Bi–Co co-doped ZnO nanowires

Abstract

We are the first to report Bi–Co co-doped zinc oxide nanowires (ZnO NWs) to investigate theoretically and experimentally the alignment of relative spin and the location of Bi3+ and Co2+ as the primary cause of magnetism. In the present study, we carefully performed theoretical and experimental studies of ZnO NWs prepared by the hydrothermal method and obtained exciting outcomes. The x-ray diffraction findings show that the NWs have a wurtzite crystal structure and high intensity at the (002) peak, indicating that most of the reflection comes from the hexagonal surface of the NWs. This means that the extended NWs are mainly aligned along the c-axis. The field-emission scanning electron microscope results show that, due to its anisotropic crystal existence, the grown NWs have a preferred path along the c-axis in the (001) direction. X-ray photoelectron spectroscopy studies established the presence of both Bi and Co in all the Bi–Co co-doped samples. They also showed that OH− groups exist on the surface of ZnO NWs, and increasing Co content tends to increase the O-deficient region in the ZnO matrix. UV–vis analysis showed a decreased optical band gap upon doping, which is probably due to the sp-d exchange interaction between localized d-electron bands of dopants. Optical studies also suggested that the doping induced an increase in electron concentration. The Raman results indicate that the ZnO NWs contain crystallization with few defects after Bi–Co-doping. From the magnetic studies, it was observed that the ferromagnetism probably originates from the defect-rich regions, i.e. NW surface and Bi and Co sit at ZnO’s substitutional site. In contrast, the core of the NWs remains under the other magnetic state. Our density functional theory studies are in line with the experimental results.

Free carrier-mediated ferromagnetism in nonmagnetic ion (Bi–Li) codoped ZnO nanowires

“…The complete details on the whole development process and growth directions of doped and codoped ZnO NWs has been discussed elsewhere. 22,23,25 In the Li-doped samples, the preferred growth direction is 002 as can be seen in the XRD spectra but with Bi-1 wt% codoping in Li:ZnO NWs, the 002 is no more the preferred orientation and other planes in the super cell are also observed, and even with the Bi-1, Li-9 wt% ZnO NWs sample, the preferred orientation direction is 100 planes. This is also evident from the FESEM images shown in Fig.…”

“…Previous works of ours have described in detail the development process of ZnO nanoparticles and nanowires, as well as the influence of the dopant on the shape. 22,23,25 In undoped, tetrahedrally-coordinated ZnO, the c -axis has the lowest surface energy density (9.9 eV nm −2 ). As a result, the free energy of the system is minimised if ZnO grows in elongated structures along the c -axis.…”

“…29 Symmetrical characteristics in the Zn 2p 3/2 and 2p 1/2 spectra disprove the existence of several Zn components. 23 Fig. 5(d) shows the Li 1s spectra.…”

“…22 Due to its low melting point, Bi is an ideal catalyst for low-temperature production of ZnO NWs. 23 Itinerant ferromagnetism, as opposed to ferromagnetism caused by defects, makes Bi atoms a possible heavy-ion non-magnetic contender. Since the ion cores of Bi atoms are associated with high spin–orbit energy, they aid in stabilising the FM.…”

Magneto-Transport and Enhanced Spin-Polarized Photo Response in Solution-Processed Vertically Aligned Zn0.9Ni0.1O Nanowires

“…ZnO-based DMS systems have drawn tremendous attention because of their pronounced optical properties at short wavelengths and robust magnetic properties at room temperature, making them suitable for applications in electronics and spintronics [7,8]. For device applications, the Curie temperature up to or at least at room temperature is highly desired [9][10][11] to realize technological applications [12,13].…”