Showing papers by "Christopher B. Murray published in 2001"

Colloidal synthesis of nanocrystals and nanocrystal superlattices

Abstract: This paper provides an overview of the synthetic techniques used to prepare colloidal nanocrystals (NCs) of controlled composition, size, shape, and internal structure and the methods for manipulat...

Monodisperse 3 d Transition-Metal (Co,Ni,Fe) Nanoparticles and Their Assembly intoNanoparticle Superlattices

Abstract: Magnetic colloids, or ferrofluids, have been studied to probe the fundamental size-dependent properties of magnetic particles and have been harnessed in a variety of applications. The magnetorheological properties of magnetic colloids have been exploited in high-performance bearings and seals. The deposition of magnetic dispersions on platters and tapes marked the earliest embodiments of magnet information storage. Magnetic particles enhance contrast in magnetic resonance imaging and promise future diagnostic and drug delivery applications. The need to explore the scaling limits of magnetic storage technology has motivated the preparation of size-tunable monodisperse magnetic nanoparticles with controlled internal structures. The study of these nanoparticles is critical to efforts to separate the role of defects from intrinsic, finite size effects.

Synthesis of Monodisperse Nanoparticles of Barium Titanate: Toward a Generalized Strategy of Oxide Nanoparticle Synthesis

Abstract: The physical properties of materials as they progress from the bulk to the nanoscale regime (1 00 nm) continue to be of immense interest and increasing importance for future technological applications. Nanocrystals display properties generally found to be scientifically different from the bulk material or the atomic or molecular species from which they can be derived. Examples of this phenomenon are manifold. 1-3 The study of semiconductor nanocrystal quantum dots is a well-established field, yielding rich, useful ,and application-oriented research. 4 A coherent field of study is currently emerging for the systematic examination of nanocrystal oxides with the aim of producing nanoparticles with narrow size distributions and size tunability in the nanoscale regime. Among the important characteristics of nanoparticle systems are facile manipulation and reversible assembly which allow for the possibility of incorporation of nanoparticles into electric, electronic, or optical devices. Such “bottom up” or “self-assembly” approaches are the benchmark of nanotechnology. Ferroelectric materials have been under investigation due to the prospect that the stable polarization states could be used to encode the 1 and 0 of the Boolean algebra that form the basis of memory and logic circuitry (FRAM). The family of complex ferroelectric oxides such as BaTiO 3, Pb(Zr,Ti)O3, and (Ba,Sr)TiO3 has far reaching applications in the electronics industry for transducers, actuators, and high-k dielectrics. 5 Ferroelectricity is the phenomenon designated to crystals in possession of a spontaneous polarization and hysteresis effects with respect to the dielectric displacement in the presence of an applied electric field. The precise nature of ferroelectricity at the nanoscale, such as critical size dependent suppression in particles and thin films, 6

Compositionally controlled FePt nanoparticle materials

Abstract: High temperature solution phase decomposition of Fe(CO)/sub 5/ and reduction of Pt(acac)/sub 2/ in the presence of stabilizers, oleic acid and oleyl amine, are employed to produce 4 nm diameter FePt nanoparticles. The Fe and Pt composition of the nanoparticle materials can be tuned by adjusting the molar ratio of Fe(CO)/sub 5/ to Pt(acac)/sub 2/, and the compositions ranging from Fe/sub 30/Pt/sub 70/ to Fe/sub 80/Pt/sub 20/ are obtained. The nanoparticle materials are easily dispersed into alkane solvent, facilitating their self-organization into nanoparticle superlattices. As synthesized FePt nanoparticles possess a disordered fcc structure and show superparamagnetic behavior. Thermal annealing induces a change of internal particle structure and thus of the magnetic properties of the particles. Composition dependent structure analysis shows that an annealed FePt nanoparticle assembly with a composition around Fe/sub 55/Pt/sub 45/ will lead to the highly ordered fct phase. This Fe/sub 55/Pt/sub 45/ nanoparticle assembly yields high coercivity, and will be a candidate for future ultra-high density magnetic recording media applications.

Competing interactions in dispersions of superparamagnetic nanoparticles

Abstract: We study the magnetic properties of dispersions of uniformly sized, chemically synthesized cobalt nanoparticles (NP's), observing a crossover from a blocked state to a superparamagnetic one with increasing temperature. By analyzing magnetization data, we determine the distributions of NP volumes and anisotropies, and establish that variations in the shapes of the magnetic cores of the NP's generate the anisotropy governing the crossover. We characterize the frustrated low-temperature state produced by the competition between dipolar interactions and anisotropy at sufficiently high NP density through remanent magnetization measurements, and explain the results through analysis of a simple model.

Chemical synthesis of monodisperse and magnetic alloy nanocrystal containing thin films

Abstract: A method and structure for forming magnetic alloy nanoparticles includes forming a metal salt solution with a reducing agent and stabilizing ligands, introducing an organometallic compound into the metal salt solution to form a mixture, heating the mixture to a temperature between 260° and 300° C., and adding a flocculent to cause the magnetic alloy nanoparticles to precipitate out of the mixture without permanent agglomeration. The deposition of the alkane dispersion of FePt alloy particles, followed by the annealing results in the formation of a shiny FePt nanocrystalline thin film with coercivity ranging from 500 Oe to 6500 Oe.

Crystalline, Shape, and Surface Anisotropy in Two Crystal Morphologies of Superparamagnetic Cobalt Nanoparticles by Ferromagnetic Resonance

Abstract: Ferromagnetic resonance (FMR) techniques are used to investigate superparamagnetic cobalt nanoparticles (NP's) with different crystalline structures and sizes ranging from 4 to 9 nm in diameter. Magnetic contributions from NC shape, crystallographic structure, defects, and surface structure are discussed. An independent-superparamagnetic-grain model is employed to simulate the FMR measurements. The results from both single crystalline and polycrystalline cobalt NP's reveal that a particle's effective anisotropy, and thus its magnetic properties, are extremely sensitive to internal structure as well as overall particle shape. Finally, surface chemical properties were found to yield unique FMR signatures for NP's at low temperatures.

MOKE spectra and ultrahigh density data storage perspective of FePt nanomagnet arrays

Abstract: Superlattices of self-assembled, monodisperse FePt nanomagnet arrays have been studied using magneto-optical (MOKE) spectroscopy in the photon energy range 0.8-5.3 eV. The nanomagnets are chemically synthesized and subsequently deposited on SiO/sub 2/ substrates for structural, magnetic and optical characterization. Large room temperature coercivities up to 9000 Oe are obtained after annealing to temperatures up to 580/spl deg/C. They are attributed to the transformation from the chemically disordered fcc phase to the chemically ordered L1/sub 0/ fct phase of FePt. The chemical ordering process is accompanied by changes in the electronic structure of the materials, which leads to characteristic MOKE spectral changes. In particular, the occurrence of a strong MOKE peak at 2 eV photon energy is observed. Polar and transverse (out-of-plane and in-plane) Kerr hysteresis studies indicate 3D random distribution of the magnetic easy axes in these superlattices. These nanomagnet assemblies with the control on magnet spacing and spatial order are prospective candidates for future ultrahigh density magnetic recording media with potential areal densities beyond Tbit/in/sup 2/.

Self-Assembled Magnetic Nanoparticle Arrays

Abstract: Advances in magnetic recording technology have resulted primarily from proportional scaling of each individual storage bit [1]. To control the signal to noise ratio and other important recording parameters, it is essential to maintain a large number of independent ferromagnetic grains per bit. These trends have driven the development of new magnetic thin film media with smaller grains, high coercivity, low magnetization and minimal magnetic exchange coupling between neighboring grains [2–6]. Today, the grain size of Co-alloy based advanced recording media has been reduced to dimensions close to the onset of superparamagnetic behavior. At such dimensions the magnetization of individual grains tends to fluctuate randomly with ambient thermal energy, resulting in irreversible thermal decay of stored information [7] (Chaps. 5 and 6).

Synthesis and Properties of Lead Selenide Nanocrystal Solids

Abstract: We present results of our investigation of the synthesis, structural properties and electrical transport properties of lead selenide (PbSe) nanoparticle-derived solids. Stable colloidal solutions containing crystalline PbSe particles with sizes on the order of 5–10 nm were synthesized using an organometallic lyothermal growth method in high-temperature organic solvents (100∼200 ◦ C). The nanoparticle powders have been characterized by X-ray scattering (WAXS/SAXS), electron microscopy and optical absorption. Thin films were formed by controlled precipitation of the nanoparticles from solution onto insulating substrates. Electrical resistance (R) and Seebeck coefficient (S) for conductive PbSe films from different annealing conditions were studied and compared. We were able to obtain conductive PbSe films from colloids by low temperature annealing which did not disturb the nanoparticle self-assembly.

Nanometer-sized Bismuth Crystallites Synthesized from a High-temperature Reducing System

Abstract: Nanometer-sized bismuth has successfully been prepared using a high-temperature organic reducing system by presence of proper capping/stabilizing agents. Self-assembly of bismuth was, at the first time, obtained using size-selected nanoparticles (15nm in size). Various synthetic conditions, which may significantly affect the formation of self-assembled nanocrystalline bismuth, have been optimized and discussed in this paper. The as-prepared nanocrystallites exist in a single rhombohedral phase with high crystallinity, and oxidation problem has been efficiently overcome within limited period by employing this method.