Probing the Local Coordination Environment for Transition Metal Dopants in Zinc Oxide Nanowires

Abstract

It is hypothesized that a highly ordered, relatively defect-free dilute magnetic semiconductor system should act as a weak ferromagnet. Transition-metal-doped ZnO nanowires, being single crystalline, single domain, and single phase, are used here as a model system for probing the local dopant coordination environments using X-ray absorption spectroscopy and diffraction. Our X-ray spectroscopic data clearly show that the dopant resides in a uniform environment, and that the doping does not induce a large degree of disorder in the nanowires. This homogeneous nature of the doping inside the oxide matrix correlates well with observed weakly ferromagnetic behavior of the nanowires.Dilute magnetic semiconductors are of great current research interest owing to their novel magnetic properties. These materials are envisioned to have potential use in so-called "spintronic" devices, in which one seeks to control both the charge and spin degrees of freedom of the electron; a few such prototypical devices are already in existence. 1,2 Despite improvements in device fabrication, the fundamental questions regarding the origins of the magnetic behavior in dilute magnetic semiconducting (DMS) systems remain unsatisfactorily answered. It is known that much of the controversy has stemmed from the fact that clustering or phase separation of the magnetic dopant ions can result in magnetic data that is misleading or unreliable. [3][4][5] Even in systems that appear to be quite pure, reported magnetic behavior can vary widely, ranging from room-temperature ferromagnetism 6 to no magnetic ordering at all. 7,8 This variance in magnetism is attributed to the multitude of synthetic methods used for the production of DMS materials, which can result in differences in dopant environment, structural disorder, or carrier concentration. Therefore, to extract the most accurate information from any magnetic data on a DMS material, it is vital to perform a rigorous structural characterization.A combination of X-ray spectro-microscopy and X-ray diffraction (XRD) techniques can provide excellent insight into the structure of a DMS system, down to the local environment of the magnetic cation. Although these types of experiments have been conducted previously on bulk and thin film systems of DMS materials, 9-12 very few studies on one-dimensional nanostructures of transition-metal (TM)-doped semiconductors have been reported. In this Letter, we report results of a detailed structural analysis on TM-doped (Co and Mn) ZnO nanowires grown by a novel solutionbased synthesis, and we correlate these findings with the magnetic properties of the nanowires. These transition-metaldoped ZnO nanowires, being single crystalline, single domain, and single phase, serVe as an excellent model system for probing the local dopant coordination enVironments and their correlation with magnetic properties. XRD, microextended X-ray absorption fine structure spectroscopy (µ-EXAFS), as well as high-resolution imaging combined with near-edge X-ray absorption fine struct...

One-dimensional ZnO nanostructures: Solution growth and functional properties

“…Reproduced with permission did not induce a large degree of disorder in the nanowires. This homogeneous and uniform doping of the transition metal ions correlated with the weak ferromagnetic behavior of the nanowires [261]. Electron transport measurements on single Co-doped ZnO nanowires showed that the electron mobility could be as high as 75 cm 2 /(V·s) [262].…”

“…In addition, the peak centered at 680 nm in the photoluminescence spectrum corresponds to the transition from 4 [258]. The local dopant coordination environments were studied by X-ray diffraction and absorption spectroscopy [261]. The results showed that the transition metal doped ZnO nanowires were single crystal, single domain, and single phase in nature.…”

X-ray absorption fine structure spectroscopy in nanomaterials

“…Especially, Co-doped ZnO nanowires (NWs) have attracted considerable attention. Yuhas and colleagues [77] synthesized single crystalline, single domain, and single phase Co:ZnO NWs with uniform diameters of 35 ± 5 nm and lengths ranging from 2 to 6 μm using a solution-based synthetic route. The Co concentrations, ranging from 5.35 to 15.50 at.%, are far beyond the equilibrium solubility of Co in bulk ZnO.…”

Crossed Ga2O3/SnO2 Multiwire Architecture: A Local Structure Study with Nanometer Resolution

“…Nevertheless, most of the local up-to-date structural analyses on nanowires have reported average interatomic distances over a macroscopic volume. 17,18 If nanofocusing optics is added to an X-ray absorption experiment, the challenge of study single nanowires can be overcome using X-ray nanobeams.…”