Hydrothermal growth of ZnO nanostructures

This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films

Zinc oxide (ZnO), with its excellent luminescent properties and the ease of growth of its nanostructures, holds promise for the development of photonic devices. The recent advances in growth of ZnO nanorods are discussed. Results from both low temperature and high temperature growth approaches are presented. The techniques which are presented include metal?organic chemical vapour deposition (MOCVD), vapour phase epitaxy (VPE), pulse laser deposition (PLD), vapour?liquid?solid (VLS), aqueous chemical growth (ACG) and finally the electrodeposition technique as an example of a selective growth approach. Results from structural as well as optical properties of a variety of ZnO nanorods are shown and analysed using different techniques, including high resolution transmission electron microscopy (HR-TEM), scanning electron microscopy (SEM), photoluminescence (PL) and cathodoluminescence (CL), for both room temperature and for low temperature performance. These results indicate that the grown ZnO nanorods possess reproducible and interesting optical properties. Results on obtaining p-type doping in ZnO micro-?and nanorods are also demonstrated using PLD. Three independent indications were found for p-type conducting, phosphorus-doped ZnO nanorods: first, acceptor-related CL peaks, second, opposite transfer characteristics of back-gate field effect transistors using undoped and phosphorus doped wire channels, and finally, rectifying I?V characteristics of ZnO:P nanowire/ZnO:Ga p?n junctions. Then light emitting diodes (LEDs) based on n-ZnO nanorods combined with different technologies (hybrid technologies) are suggested and the recent electrical, as well as electro-optical, characteristics of these LEDs are shown and discussed. The hybrid LEDs reviewed and discussed here are mainly presented for two groups: those based on n-ZnO nanorods and p-type crystalline substrates, and those based on n-ZnO nanorods and p-type amorphous substrates. Promising electroluminescence characteristics aimed at the development of white LEDs are demonstrated. Although some of the presented LEDs show visible emission for applied biases in excess of 10 V, optimized structures are expected to provide the same emission at much lower voltage. Finally, lasing from ZnO nanorods is briefly reviewed. An example of a recent whispering gallery mode (WGM) lasing from ZnO is demonstrated as a way to enhance the stimulated emission from small size structures.

https://iopscience.iop.org/article/10.1088/0957-4484/20/33/332001/pdf

Zinc oxide nanorod based photonic devices: recent progress in growth, light emitting diodes and lasers.

We present an overview of the critical issues of current interest in magnetic and magnetoelectronic materials. A broad demonstration of the successful blend of materials synthesis, microstructural evolution and control, new physics and novel applications that is central to research in this field is presented. The critical role of size, especially at the nanometer length scale, dimensionality, as it pertains to thin films and interfaces, and the overall microstructure, in determining a range of fundamental properties and potential applications, is emphasized. Even though the article is broad in scope, examples are drawn mainly from our recent work in magnetic nanoparticles and core-shell structures, exchange, proximity and interface effects in thin film heterostructures, and dilute magnetic dielectrics, a new class of materials for spin electronics applications.

http://depts.washington.edu/kkgroup/publications/PDF/2006_Krish_Nanomag_JMSreview.pdf

Nanomagnetism and spin electronics: materials, microstructure and novel properties

Magnetic oxide semiconductors, oxide semiconductors doped with transition metal elements, are one of the candidates for a high Curie temperature ferromagnetic semiconductor that is important to realize semiconductor spintronics at room temperature. We review in this paper recent progress of research on various magnetic oxide semiconductors. The magnetization, magneto-optical effect and magneto-transport such as the anomalous Hall effect are examined from the viewpoint of feasibility to evaluate the ferromagnetism. The ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.

https://iopscience.iop.org/article/10.1088/0268-1242/20/4/012/pdf

Magnetic oxide semiconductors

Through nonequilibrium rf magnetron cosputtering of ZnO and Cr metal on a-plane Al2O3 we find ferromagnetic ordering with a room-temperature saturation moment of 1.4μB per Cr ion at a doping concentration of ∼9.5 at. % after UHV postanneal. No secondary phases are detected in the films via x-ray diffraction (XRD). Increased Cr doping causes disorder within the films resulting in decreased overall, and per Cr, moment. The Curie temperature exceeds 365 K, the maximum temperature reached in our experiment. All films are dielectric with a resistivity higher than 106Ωcm at room temperature. The lack of carriers indicates that the ferromagnetic mechanism is not carrier mediated.

http://depts.washington.edu/kkgroup/publications/PDF/2005_Roberts_CrZnO_sputtering.pdf

Ferromagnetic Cr-doped ZnO for spin electronics via magnetron sputtering

Multilayers of Co/ZnO, with varying nominal thickness of metal (2–10 A) and semiconductor (2–20 A), were prepared by ion-beam sputtering. Magnetic, transport, and magnetotransport measurements were carried out over a temperature range of 2.5 to 300 K. Upon decreasing Co thickness and increasing ZnO thickness in the multilayer stack, the properties of the samples undergo a crossover from those of granular metallic Co/semiconductor multilayers to a dilute magnetic semiconductor superlattice. We interpret ferromagnetism in the latter case as due to ordering in the Co-rich layers, mediated by carriers from lightly doped, high carrier concentration layers.

Transition from granular to dilute magnetic semiconducting multilayers in ion-beam-deposited ZnO/Co

We explore the effects of codoping with hydrogen on magnetism, conductivity, and spin polarization of carriers in Cr-doped ZnO. Zn0.99Cr0.01O:H films sputter deposited on sapphire show a correlation between magnetization and conductivity when H is introduced. In the first method, dielectric and weakly magnetic films grown in pure Ar are subsequently annealed at 400°C in a 5% H2 95% Ar 1atm flowing tube furnace. These films show increases in conductivity and saturation and remnant magnetization postanneal. In the second method, conducting ferromagnetic films are grown in the H∕Ar mixture. They are magnetic as grown but show a small decrease in saturation and remnant magnetization and conductivity post-H∕Ar anneal. Ferromagnetic CrO2 with TC=390K or antiferromagnetic phases are not detected in hydrogenated films. We studied spin polarization of carriers using anomalous Hall effect; however, initial experiments show no such signs, hence spin polarization is not yet confirmed. Hydrogen doped in dielectric Cr:...

Effect of hydrogen codoping on magnetic ordering and conductivity in Cr:ZnO thin films

Measurements of the spontaneous magnetic moment and conductance of oxygen-saturated ZnO:Cr thin films and tunnel junctions show an exponential drop with increasing thickness. All films are in the insulating state. The type and concentration of dominant point defects in the bulk do not provide conditions for magnetic ordering, while surfaces play an important role in magnetism. We suggest that in film thicknesses below a characteristic length (∼30nm), surface states or enhanced concentration of point defects near the surface (oxygen vacancies or incorporated hydrogen) lead to surface magnetism. Ballistic tunneling is observed in junctions <100nm, with penetration depth of ∼10nm.

Surface scaling of magnetism in Cr:ZnO dilute magnetic dielectric thin films

Methods of producing graphene, reaction chambers for forming graphene, and graphene sheets formed from the methods are described herein. A method may include adding at least one metal catalyst in a reaction chamber, adding at least one hydrocarbon gas in the reaction chamber, allowing the at least one metal catalyst and the at least one hydrocarbon gas to contact one another to produce a product, and dehydrogenating the product to produce the graphene.

Bradley Kirk Roberts

Papers

Ferromagnetic Cr-doped ZnO for spin electronics via magnetron sputtering

Transition from granular to dilute magnetic semiconducting multilayers in ion-beam-deposited ZnO/Co

Effect of hydrogen codoping on magnetic ordering and conductivity in Cr:ZnO thin films

Surface scaling of magnetism in Cr:ZnO dilute magnetic dielectric thin films

Method of producing graphene from a hydrocarbon gas and liquid metal catalysts