G. Z. Xing

Correlated d 0 ferromagnetism and photoluminescence in undoped ZnO nanowires

We report the correlated d(0) ferromagnetism and photoluminescence in undoped single-crystalline ZnO nanowires synthesized by using a vapor transport method. We systematically tune the oxygen deficiency in the ZnO nanowires from 4% to 20% by adjusting the growth conditions, i.e., selecting different catalyst (Au or Ag) and varying the growth temperature. Our study suggests that oxygen vacancies induce characteristic photoluminescence and significantly boost the room-temperature ferromagnetism. Such undoped ZnO nanowires with tunable magnetic and optical properties are promising to find applications in multifunctional spintronic and photonic nanodevices.

Electrode dependence of resistive switching in Mn-doped ZnO: Filamentary versus interfacial mechanisms

We carry out a comparative study on resistive switching in Mn-doped ZnO thin films; samples grown on Pt and Si show unipolar and bipolar switching behaviors, respectively. Fittings of the current-voltage curves and area dependence of the device resistance reveal the filamentary conduction in Pt/Mn:ZnO/Pt. On the other hand, the interfacial effect dominates in Pt/Mn:ZnO/Si, and its low resistance state exponentially relaxes toward the high resistance state in contrast to the good data retention in Pt/Mn:ZnO/Pt. Our results suggest that selecting electrodes dictates the resistive switching mechanism presumably by affecting the migration dynamics of oxygen vacancies.

Defect-induced magnetism in undoped wide band gap oxides: Zinc vacancies in ZnO as an example

To shed light on the mechanism responsible for the weak ferromagnetism in undoped wide band gap oxides, we carry out a comparative study on ZnO thin films prepared using both sol-gel and molecular beam epitaxy (MBE) methods. Compared with the MBE samples, the sol-gel derived samples show much stronger room temperature ferromagnetism with a magnetic signal persisting up to ∼740 K, and this ferromagnetic order coexists with a high density of defects in the form of zinc vacancies. The donor-acceptor pairs associated with the zinc vacancies also cause a characteristic orange-red photoluminescence in the sol-gel films. Furthermore, the strong correlation between the ferromagnetism and the zinc vacancies is confirmed by our first-principles density functional theory calculations, and electronic band alteration as a result of defect engineering is proposed to play the critical role in stabilizing the long-range ferromagnetism.

Photoluminescence characteristics of high quality ZnO nanowires and its enhancement by polymer covering

We investigated the photoluminescence (PL) properties of ZnO nanowires with and without covering with polymethyl methacrylate (PMMA). Low temperature PL spectra of as-grown ZnO nanowires are dominated by near band edge (NBE) emission due to donor bound excitons and free-to-bound recombination (FB). FB emission persists till 300 K and together with free exciton emission governs the lineshape of the PL spectra. After covering with PMMA, the integral intensity of NBE emission increases about three times, indicating significantly improved excitonic emission efficiency. A model based on surface states and energy bands theory was proposed to interpret this emission enhancement.

Robust Room-Temperature Ferromagnetism with Giant Anisotropy in Nd-Doped ZnO Nanowire Arrays

As an important class of spintronic material, ferromagnetic oxide semiconductors are characterized with both charge and spin degrees of freedom, but they often show weak magnetism and small coercivity, which limit their applications. In this work, we synthesized Nd-doped ZnO nanowire arrays which exhibit stable room temperature ferromagnetism with a large saturation magnetic moment of 4.1 μ(B)/Nd as well as a high coercivity of 780 Oe, indicating giant magnetic anisotropy. First-principles calculations reveal that the remarkable magnetic properties in Nd-doped ZnO nanowires can be ascribed to the intricate interplay between the spin moments and the Nd-derived orbital moments. Our complementary experimental and theoretical results suggest that these magnetic oxide nanowires obtained by the bottom-up synthesis are promising as nanoscale building blocks in spintronic devices.

P-type electrical, photoconductive, and anomalous ferromagnetic properties of Cu2O nanowires

Cu2O nanowires are synthesized by reduction of CuO nanowires with hydrogen gas. Strong green photoluminescence dominated by band-edge emission is observed. Field effect transistors fabricated from individual Cu2O nanowires present high on-off ratio (>106) and high mobility (>95 cm2/V s). Furthermore, the device demonstrates a fast photoelectric response to blue illumination in air at room temperature. In addition, anomalous ferromagnetism appears in Cu2O nanowires, which may originate from the defects in Cu2O nanowires. This work shows the application potentials of the Cu2O nanowires, especially in an electrical and photonic device.

Tuning ferromagnetism in MgxZn1−xO thin films by band gap and defect engineering

We investigate the room temperature ferromagnetism in band gap tunable MgxZn1−xO (x≤0.22) alloy thin films and find that ferromagnetism is significantly enhanced in p-type MgxZn1−xO (x≥0.17) compared with the n-type counterparts (x≤0.15). Temperature-dependent photoluminescence measurements reveal the correlation between the p-type behavior, enhanced ferromagnetism, and zinc vacancies. First-principle calculations demonstrate that the formation energy of zinc vacancies decreases with the increasing Mg content and the zinc vacancies in MgxZn1−xO alloys stabilize the ferromagnetic coupling. Our results suggest a viable route to tune the magnetic properties of oxides through band gap and defect engineering.

Ferroelectric Transistors with Nanowire Channel: Toward Nonvolatile Memory Applications

We report the fabrication and characterization of ZnO nanowire memory devices using a ferroelectric Pb(Zr(0.3)Ti(0.7))O(3) (PZT) film as the gate dielectric and the charge storage medium. With a comparison to nanowire transistors based on SiO(2) gate oxide, the devices were evaluated in terms of their electric transport, retention, and endurance performance. Memory effects are observed as characterized by an eminent counterclockwise loop in I-V(g) curves, which is attributed to the switchable remnant polarization of PZT. The single-nanowire device exhibits a high (up to 10(3)) on/off ratio at zero gate voltage. Our results give a proof-of-principle demonstration of the memory application based on a combination of nanowires (as channels) and ferroelectric films (as gate oxide).

Characteristics of ultraviolet photoluminescence from high quality tin oxide nanowires

We investigate the optical properties of ultraviolet range emission from high quality tin oxide nanowires prepared by vapor-liquid-solid growth technique. Temperature dependent photoluminescence (PL) measurement is performed between 10 and 300 K. At low temperatures, the PL originates from radiative recombination of excitons bound to neutral donors, donor-acceptor pair transition and their associated longitudinal optical (LO) phonon replicas. The LO-phonon replicas up to third order with Huang–Rhys factor of 0.34 are observed. Evolution of the peaks and the origin of PL thermal quenching at high temperatures are discussed in detail.

Emergent ferromagnetism in ZnO/Al2O3 core-shell nanowires : towards oxide spinterfaces.

We report that room-temperature ferromagnetism emerges at the interface formed between ZnO nanowire core and Al2O3 shell although both constituents show mainly diamagnetism. The interface-based ferromagnetism can be further enhanced by annealing the ZnO/Al2O3 core-shell nanowires and activating the formation of ZnAl2O4 phase as a result of interfacial solid-state reaction. High-temperature measurements indicate that the magnetic order is thermally stable up to 750 K. Transmission electron microscopy studies reveal the annealing-induced jagged interfaces, and the extensive structural defects appear to be relevant to the emergent magnetism. Our study suggests that tailoring the spinterfaces in nanostructure-harnessed wide-band-gap oxides is an effective route towards engineered nanoscale architecture with enhanced magnetic properties.