Hydrothermal growth of ZnO nanostructures

Finally, we have addressed some relevant findings on the importance of having well-defined synthetic strategies developed for the generation of MNPs, with a focus on particle formation mechanism and recent modifications made on the preparation of monodisperse samples of relatively large quantities not only with similar physical features, but also with similar crystallochemical characteristics. Then, different methodologies for the functionalization of the prepared MNPs together with the characterization techniques are explained. Theorical views on the magnetism of nanoparticles are considered.

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Magnetic nanoparticles: preparation, physical properties, and applications in biomedicine

A wide variety of cation-exchangeable layered transition metal oxides and their relatively rare counterparts, anion-exchangeable layered hydroxides, have been exfoliated into individual host layers, i.e., nanosheets. Exfoliation is generally achieved via a high degree of swelling, typically driven either by intercalation of bulky organic ions (quaternary ammonium cations, propylammonium cations, etc.) for the layered oxides or by solvation with organic solvents (formamide, butanol, etc.) for the hydroxides. Ultimate two-dimensional (2D) anisotropy for the nanosheets, with thickness of around one nanometer versus lateral size ranging from submicrometer to several tens of micrometers, allows them to serve either as an ideal quantum system for fundamental study or as a basic building block for functional assembly. The charge-bearing inorganic macromolecule-like nanosheets can be assembled or organized through various solution-based processing techniques (e.g., flocculation, electrostatic sequential deposition, or the Langmuir-Blodgett method) to produce a range of nanocomposites, multilayer nanofilms, and core-shell nanoarchitectures, which have great potential for electronic, magnetic, optical, photochemical, and catalytic applications.

Nanosheets of oxides and hydroxides: Ultimate 2D charge-bearing functional crystallites.

Biomedical nanomagnetics is a multidisciplinary area of research in science, engineering and medicine with broad applications in imaging, diagnostics and therapy. Recent developments offer exciting possibilities in personalized medicine provided a truly integrated approach, combining chemistry, materials science, physics, engineering, biology and medicine, is implemented. Emphasizing this perspective, here we address important issues for the rapid development of the field, i.e., magnetic behavior at the nanoscale with emphasis on the relaxation dynamics, synthesis and surface functionalization of nanoparticles and core-shell structures, biocompatibility and toxicity studies, biological constraints and opportunities, and in vivo and in vitro applications. Specifically, we discuss targeted drug delivery and triggered release, novel contrast agents for magnetic resonance imaging, cancer therapy using magnetic fluid hyperthermia, in vitro diagnostics and the emerging magnetic particle imaging technique, that is quantitative and sensitive enough to compete with established imaging methods. In addition, the physics of self-assembly, which is fundamental to both biology and the future development of nanoscience, is illustrated with magnetic nanoparticles. It is shown that various competing energies associated with self-assembly converge on the nanometer length scale and different assemblies can be tailored by varying particle size and size distribution. Throughout this paper, while we discuss our recent research in the broad context of the multidisciplinary literature, we hope to bridge the gap between related work in physics/chemistry/engineering and biology/medicine and, at the same time, present the essential concepts in the individual disciplines. This approach is essential as biomedical nanomagnetics moves into the next phase of innovative translational research with emphasis on development of quantitative in vivo imaging, targeted and triggered drug release, and image guided therapy including validation of delivery and therapy response.

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Biomedical Nanomagnetics: A Spin Through Possibilities in Imaging, Diagnostics, and Therapy

The observation of ferromagnetism in magnetic ion doped II–VI diluted magnetic semiconductors (DMSs) and oxides, and later in (Ga,Mn)As materials has inspired a great deal of research interest in a field dubbed “spintronics” of late, which could pave the way to exploit spin in addition to charge in semiconductor devices. The main challenge for practical application of the DMS materials is the attainment of a Curie temperature at or preferably above room temperature to be compatible with junction temperatures. Among the studies of transition-metal doped conventional III–V and II–VI semiconductors, transition-metal-doped ZnO and GaN became the most extensively studied topical materials since the prediction by Dietl et al., based on mean field theory, as promising candidates to realize a diluted magnetic material with Curie temperature above room temperature. The underlying assumptions, however, such as transition metal concentrations in excess of 5% and hole concentrations of about 1020 cm−3, have not gotten as much attention. The particular predictions are predicated on the assumption that hole mediated exchange interaction is responsible for magnetic ordering. Among the additional advantages of ZnO-and GaN-based DMSs are that they can be readily incorporated in the existing semiconductor heterostructure systems, where a number of optical and electronic devices have been realized, thus allowing the exploration of the underlying physics and applications based on previously unavailable combinations of quantum structures and magnetism in semiconductors. This review focuses primarily on the recent progress in the theoretical and experimental studies of ZnO- and GaN-based DMSs. One of the desirable outcomes is to obtain carrier mediated magnetism, so that the magnetic properties can be manipulated by charge control, for example through external electrical voltage. We shall first describe the basic theories forwarded for the mechanisms producing ferromagnetic behavior in DMS materials, and then review the theoretical results dealing with ZnO and GaN. The rest of the review is devoted to the structural, optical, and magnetic properties of ZnO- and GaN-based DMS materials reported in the literature. A critical review of the question concerning the origin of ferromagnetism in diluted magnetic semiconductors is given. In a similar vein, limitations and problems for identifying novel ferromagnetic DMS are briefly discussed, followed by challenges and a few examples of potential devices.

/pdf/ferromagnetism-of-zno-and-gan-a-review-4bjmm699d4.pdf

Ferromagnetism of ZnO and GaN: A Review

Using complementary experiments we show that the room temperature ferromagnetism observed in anatase $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ films is not carrier mediated, but coexists with the dielectric state. TEM and x-ray absorption spectroscopy reveal a solid solution of Co in anatase, where Co is not metallic but in the $+2$ state substituting for Ti. Measurements at 300 K yield a ${M}_{\mathrm{S}}$ of $1.1\text{ }\text{ }{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{Co}$ atom, while all films are highly insulating. The evidence of intrinsic ferromagnetism in the dielectric ground state of $\mathrm{Co}\ensuremath{\mathbin:}{\mathrm{TiO}}_{2}$ leads to new considerations for the origin of ferromagnetism in transition metal doped oxides.

Intrinsic Ferromagnetism in Insulating Cobalt Doped Anatase TiO2

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

$\mathrm{Co}$-doped $\mathrm{ZnO}({\mathrm{Co}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O})$ is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial ${\mathrm{Co}}_{x}{\mathrm{Zn}}_{1\ensuremath{-}x}\mathrm{O}$ films on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}(012)$ substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of $\mathrm{Co}(x\ensuremath{\leqslant}0.35)$ can be uniformly incorporated into the film without phase segregation. $\mathrm{Co}$ is found to be in the $+2$ oxidation state, independent of $x$, by both surface-sensitive core-level $x$-ray photoemission and bulk-sensitive optical absorption spectroscopies. This material can be grown $n$-type by the deliberate incorporation of oxygen vacancies, but not by inclusion of $\ensuremath{\sim}1$ at. % $\mathrm{Al}$. Semiconducting films remain ferromagnetic up to $350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per $\mathrm{Co}$ basis was very small $(l0.1\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}∕\mathrm{Co})$, even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the $\mathrm{Co}(\mathrm{II})$ ions behave as magnetically isolated $S=3∕2$ ions.

http://depts.washington.edu/kkgroup/publications/PDF/2004_Tuan_CoZnO_epitaxialg.pdf

Epitaxial growth and properties of cobalt-doped ZnO on α − Al 2 O 3 single-crystal substrates

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

We present a systematic approach to fabricate a variety of magnetic nanoparticles with desirable structure and controlled magnetic properties based on our studies of the process kinetics. The morphology of binary alloy particles is dependent on their bulk thermodynamics—for immiscible heterogeneous systems (Co–Au) core-shell structures are obtained while miscible systems (Fe–Pt) lead to alloy nanoparticles. The annealing effects on FePt nanoparticles show that the coercivity and magnetic anisotropy increase dramatically after annealing at temperatures above 650°C. Studies of Co–Au core-shell structure show that the core is magnetic, but the Au shell does not significantly affect its magnetic properties.

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

A. B. Pakhomov

Papers

Intrinsic Ferromagnetism in Insulating Cobalt Doped Anatase TiO2

Nanomagnetism and spin electronics: materials, microstructure and novel properties

Epitaxial growth and properties of cobalt-doped ZnO on α − Al 2 O 3 single-crystal substrates

Ferromagnetic Cr-doped ZnO for spin electronics via magnetron sputtering

A general approach to synthesis of nanoparticles with controlled morphologies and magnetic properties