The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Monodisperse samples of silver nanocubes were synthesized in large quantities by reducing silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). These cubes were single crystals and were characterized by a slightly truncated shape bounded by {100}, {110}, and {111} facets. The presence of PVP and its molar ratio (in terms of repeating unit) relative to silver nitrate both played important roles in determining the geometric shape and size of the product. The silver cubes could serve as sacrificial templates to generate single-crystalline nanoboxes of gold: hollow polyhedra bounded by six {100} and eight {111} facets. Controlling the size, shape, and structure of metal nanoparticles is technologically important because of the strong correlation between these parameters and optical, electrical, and catalytic properties.

http://science.sciencemag.org/content/sci/298/5601/2176.full.pdf

Shape-Controlled Synthesis of Gold and Silver Nanoparticles

This review focuses on the synthesis, protection, functionalization, and application of magnetic nanoparticles, as well as the magnetic properties of nanostructured systems. Substantial progress in the size and shape control of magnetic nanoparticles has been made by developing methods such as co-precipitation, thermal decomposition and/or reduction, micelle synthesis, and hydrothermal synthesis. A major challenge still is protection against corrosion, and therefore suitable protection strategies will be emphasized, for example, surfactant/polymer coating, silica coating and carbon coating of magnetic nanoparticles or embedding them in a matrix/support. Properly protected magnetic nanoparticles can be used as building blocks for the fabrication of various functional systems, and their application in catalysis and biotechnology will be briefly reviewed. Finally, some future trends and perspectives in these research areas will be outlined.

Magnetic nanoparticles: Synthesis, protection, functionalization, and application

1. Introduction 20642. Synthesis of Magnetic Nanoparticles 20662.1. Classical Synthesis by Coprecipitation 20662.2. Reactions in Constrained Environments 20682.3. Hydrothermal and High-TemperatureReactions20692.4. Sol-Gel Reactions 20702.5. Polyol Methods 20712.6. Flow Injection Syntheses 20712.7. Electrochemical Methods 20712.8. Aerosol/Vapor Methods 20712.9. Sonolysis 20723. Stabilization of Magnetic Particles 20723.1. Monomeric Stabilizers 20723.1.1. Carboxylates 20733.1.2. Phosphates 20733.2. Inorganic Materials 20733.2.1. Silica 20733.2.2. Gold 20743.3. Polymer Stabilizers 20743.3.1. Dextran 20743.3.2. Polyethylene Glycol (PEG) 20753.3.3. Polyvinyl Alcohol (PVA) 20753.3.4. Alginate 20753.3.5. Chitosan 20753.3.6. Other Polymers 20753.4. Other Strategies for Stabilization 20764. Methods of Vectorization of the Particles 20765. Structural and Physicochemical Characterization 20785.1. Size, Polydispersity, Shape, and SurfaceCharacterization20795.2. Structure of Ferro- or FerrimagneticNanoparticles20805.2.1. Ferro- and Ferrimagnetic Nanoparticles 20805.3. Use of Nanoparticles as Contrast Agents forMRI20825.3.1. High Anisotropy Model 20845.3.2. Small Crystal and Low Anisotropy EnergyLimit20855.3.3. Practical Interests of Magnetic NuclearRelaxation for the Characterization ofSuperparamagnetic Colloid20855.3.4. Relaxation of Agglomerated Systems 20856. Applications 20866.1. MRI: Cellular Labeling, Molecular Imaging(Inﬂammation, Apoptose, etc.)20866.2.

Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications

Nanocrystals are fundamental to modern science and technology. Mastery over the shape of a nanocrystal enables control of its properties and enhancement of its usefulness for a given application. Our aim is to present a comprehensive review of current research activities that center on the shape-controlled synthesis of metal nanocrystals. We begin with a brief introduction to nucleation and growth within the context of metal nanocrystal synthesis, followed by a discussion of the possible shapes that a metal nanocrystal might take under different conditions. We then focus on a variety of experimental parameters that have been explored to manipulate the nucleation and growth of metal nanocrystals in solution-phase syntheses in an effort to generate specific shapes. We then elaborate on these approaches by selecting examples in which there is already reasonable understanding for the observed shape control or at least the protocols have proven to be reproducible and controllable. Finally, we highlight a number of applications that have been enabled and/or enhanced by the shape-controlled synthesis of metal nanocrystals. We conclude this article with personal perspectives on the directions toward which future research in this field might take.

Shape‐Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics?

The results of Shubnikov–de Haas (SdH) oscillation measurements on p-CoSb3 single crystals are presented. The values of the cyclotron effective mass were determined. It was found that the effective mass increases from 0.11 m 0 to 0.15 m 0 with increasing hole concentration, indicating a non-parabolicity of the valence band of CoSb3. Using the two-band Kane model a band-edge effective mass of mn/m0=0.050±0.002 and an energy gap Eg=35±2 meV were estimated.

The valence band parameters of CoSb3 determined by Shubnikov-de Haas effect

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Publisher’s Note: “Simple models for dynamic hysteresis loop calculations of magnetic single-domain nanoparticles: Application to magnetic hyperthermia optimization” [J. Appl. Phys. 109, 083921 (2011)]

The temperature dependence of the conductivity, σ(T), of FeSi at T ≈ 100–170 K follows the Arrhenius law with the value of the band gap Eg ≈ 50 ± 5 meV. Between Tν ≈ 15–25 K and Tν∗ ≈ 3–4 K it agrees with the Mott-type varriable-range hopping. No tendency of saturation in σ(T) is observed down to 1.4 K. The relative magnetoresistance, Δϱ/ϱ, is negative at T ⩽ 4.2 K and H < 1 T with the amplitude value ≈0.2%. In higher fields, up to 35 T, it is positive and demonstrates a complex magnetic field and temperature dependence suggesting both positive and negative contributions. At T ⩾ 77 K, Δϱ/ϱ is negligible. The positive component of Δϱ/ϱ agrees with that of the Osaka model which takes into account both correlation and spin-flip effects. The negative contribution is attributed to the Zeeman splitting of the mobility threshold predicted by Kamimura and Kurobe. The temperature dependence of the hopping frequency with the pre-exponent ν0 ≈ 2.8 × 1012s−1, the value of the internal field, HI ≈ 60 kOe, and the maximum value of the correlation energy, Um ≈ 17 K, are obtained.

Conductivity and magnetoresistance of FeSi in the Anderson-localized regime

We measure the magnetization of fine cobalt particles by SQUID and pulsed magnetic fields up to 35 T. These measurements have been made on two samples (C1, C2) with nonagglomerated particles. The analysis of the magnetic meaurements evidences very narrow log-normal size distribution centered around 1.5 nm (≅150 atoms) and 1.9 nm (≅310 atoms) for C1 and C2, respectively. Magnetization at 4.2 K seems to saturate in fields up to 5 T leading to an enhanced mean magnetic moment per atom compared to bulk value (1.72μB). However, magnetization measurements up to 35 T do not permit to reach saturation, and show a continuous increase of μCo reaching 2.1±0.1μB (C1) and 1.9±0.1μB (C2). The effective magnetic anisotropies are found to be larger than those of bulk materials and decrease with increasing particle size. These features are associated with the large influence of the surface atoms.

Magnetization measurements on fine cobalt particles

Ferromagnetic (FM) nanostructures embedded in semiconductors are attracting interest because their physical properties could be used in new devices such as memories, sensors or more generally involving “spintronics”. In this work, we will present experimental results on the influence of the thermal annealing on the structural and magnetic properties of nanosized MnAs ferromagnets buried in GaAs. These nanocrystals have been obtained by Mn+As co-implantation at a dose of 2×10 16 ions cm −2 for each species, performed at room temperature into GaAs wafers followed by thermal annealing at 750°C. Increasing the duration of the annealing process (tanneal=15,60, and 120 s ) leads to a reduction of the Mn atomic densities. High-resolution transmission electron microscopy and diffraction analysis exhibit unambiguously a population of MnAs precipitates located at the projected range. Their mean diameters are ranged from 8 to 10 nm as tanneal is longer. The nanocrystals display the expected ferromagnetic behavior with Curie temperatures in the range of 360 K . The extracted spontaneous magnetizations from the FM response are found slightly smaller than expected for the bulk phase, probably due to a small fraction of diluted Mn. The influence of the nanoparticle size histogram and concentration on the superparamagnetic behavior is briefly discussed.

Marc Respaud

Papers

The valence band parameters of CoSb3 determined by Shubnikov-de Haas effect

Publisher’s Note: “Simple models for dynamic hysteresis loop calculations of magnetic single-domain nanoparticles: Application to magnetic hyperthermia optimization” [J. Appl. Phys. 109, 083921 (2011)]

Conductivity and magnetoresistance of FeSi in the Anderson-localized regime

Magnetization measurements on fine cobalt particles

Synthesis of MnAs ferromagnets by Mn+ ion implantation