Direct observation of room-temperature out-of-plane ferroelectricity and tunneling electroresistance at the two-dimensional limit.

Out-of-plane ferroelectricity with a high transition temperature in nanometer-scale films is required to miniaturize electronic devices. Direct visualization of stable ferroelectric polarization and its switching behavior in atomically thick films is critical for achieving this goal. Here, ferroelectric order at room temperature in the two-dimensional limit is demonstrated in tetragonal BiFeO3 ultrathin films. Using aberration-corrected scanning transmission electron microscopy, we directly observed robust out-of-plane spontaneous polarization in one-unit-cell-thick BiFeO3 films. High-resolution piezoresponse force microscopy measurements show that the polarization is stable and switchable, whereas a tunneling electroresistance effect of up to 370% is achieved in BiFeO3 films. Based on first-principles calculations and Kelvin probe force microscopy measurements, we explain the mechanism of polarization stabilization by the ionic displacements in oxide electrode and the surface charges. Our results indicate that critical thickness for ferroelectricity in the BiFeO3 film is virtually absent, making it a promising candidate for high-density nonvolatile memories.

Columnar structured FePt films epitaxially grown on large lattice mismatched intermediate layer.

The microstructure and magnetic properties of the FePt films grown on large mismatched ZrN (15.7%) intermediate layer were investigated. With using ZrN intermediate layer, FePt 10 nm films exhibited (001) texture except for some weaker FePt (110) texture. Good epitaxial relationships of FePt (001) <100>//ZrN (001) <100>//TiN (001) <100> among FePt and ZrN/TiN were revealed from the transmission electron microscopy (TEM) results. As compared with TiN intermediate layer, although FePt-SiO2-C films grown on ZrN/TiN intermediate layer showed isotropic magnetic properties, the large interfacial energy and lattice mismatch between FePt and ZrN would lead to form columnar structural FePt films with smaller grain size and improved isolation. By doping ZrN into the TiN layer, solid solution of ZrTiN was formed and the lattice constant is increased comparing with TiN and decreased comparing with ZrN. Moreover, FePt-SiO2-C films grown on TiN 2 nm-20 vol.% ZrN/TiN 3 nm intermediate layer showed an improved perpendicular magnetic anisotropy. Simultaneously, columnar structure with smaller grain size retained.

Nanogranular TiN-ZrO2 intermediate layer induced improvement of isolation and grain size of FePt thin films.

The effects of TiN-ZrO2 intermediate layer on the microstructures and magnetic properties of FePt films were investigated. The TiN-ZrO2 intermediate layer was granular consisting of grains of solid solution of Ti(Zr)ON segregated by amorphous ZrO2. By doping ZrO2 into TiN intermediate layer, the FePt grains became better isolated from each other and the FePt grain size was reduced. For 20 vol. % ZrO2 doping into TiN, the grain size decreased dramatically from 11. 2 nm to 6. 4 nm, and good perpendicular anisotropy was achieved simultaneously. For the FePt 4nm-SiO2 35 vol. % -C 20 vol. % films grown on top of the TiN-ZrO2 20 vol. % intermediate layer, well isolated FePt (001) granular films with coercivity higher than 18. 1 kOe and an average size as small as 6. 4 nm were achieved.

Note: application of a pixel-array area detector to simultaneous single crystal X-ray diffraction and X-ray absorption spectroscopy measurements.

X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) are two main x-ray techniques in synchrotron radiation facilities. In this Note, we present an experimental setup capable of performing simultaneous XRD and XAS measurements by the application of a pixel-array area detector. For XRD, the momentum transfer in specular diffraction was measured by scanning the X-ray energy with fixed incoming and outgoing x-ray angles. By selecting a small fixed region of the detector to collect the XRD signal, the rest of the area was available for collecting the x-ray fluorescence for XAS measurements. The simultaneous measurement of XRD and X-ray absorption near edge structure for Pr0.67Sr0.33MnO3 film was demonstrated as a proof of principle for future time-resolved pump-probe measurements. A static sample makes it easy to maintain an accurate overlap of the X-ray spot and laser pump beam.

Nanocrystalline nickel ferrite particles synthesized by non-hydrolytic sol-gel method and their composite with biodegradable polymer.

Targeted drug delivery has been one of the most important biomedical applications for magnetic particles. Such applications require magnetic particles to have functionalized surfaces/surface coatings that facilitate their incorporation into a polymer matrix to produce a polymer composite. In this paper, nanocrystalline nickel ferrite particles with an oleic acid surface coating were synthesized using a non-hydrolytic sol-gel method and incorporated into a biodegradable polymer matrix, poly(D,L-lactide) PLA prepared using a double emulsion method. As-synthesized nickel ferrite particles had a multi-crystalline structure with chemically adsorbed oleic acid on their surface. After forming the PLA composite, nickel ferrite particles were encapsulated in PLA microspheres. At low nickel ferrite concentrations, composites showed very similar surface charges to that of PLA. The composites were magnetically responsive and increasing the nickel ferrite concentration was found to increase magnetization of the composite.

Interlayer exchange coupling effect of L1(0) CoPt based exchange coupled composite media.

In this work, effects of exchange coupling of soft magnetic layer on switching field and magnetization reversal behaviour of CoPt-SiO2(soft)/CoPt-SiO2(hard) exchange coupled media were investigated. With increasing the thickness of the soft layer, both the coercivity and magnetization squareness of composite media decreased. Soft layer thickness 4 nm and below was more effective to significantly reduce the switching field than that above 4 nm. More incoherent switching behavior was observed with increasing soft layer thickness.

Synthesis of LiYF4, BaYF5, and NaLaF4 optical nanocrystals.

This paper reports the synthesis of high quality LiYF4, BaYF5, and NaLaF4 nanocrystals by high-temperature co-decomposition of precursors in organic solvents. Their bulk counterparts have long been used as efficient luminescent hosts for various applications including lasers, upconversion fluorescence, and quantum cutters. The particles were characterized using TEM, XRD, dynamic light scattering (DLS), and fluorescence spectrometry. Trifluoroacetic acid (CF3COOH) and the reaction temperature were crucial for the formation of NaLaF4 and LiYF4 nanoparticles. NaLaF4 was not formed without using CF3COOH, only LaF3 and NaF mixture was formed. NaLaF4 nanoparticles were obtained only when CF3COOH was added in the reaction solution and the temperature was > or =330 degrees C. For the synthesis of LiYF4,, in the absence of CF3COOH in the reaction, a mixture of YOF and LiYF4 nanoparticles was formed. Pure LiYF4 particles were obtained only until CF3COOH was added in the reaction at 340 degrees C or above. The nanoparticles were easily dispersed in organic solvents include hexane, toluene, and chloroform and formed transparent colloidal solutions. The ease of doping of these as-synthesized host nanoparticles for designed optical properties was assessed. The LiYF4, BaYF5, and NaLaF4 nanoparticles, co-doped with 20% Ytterbium (Yb) and 2% Erbium (Er), showed bright upconversion fluorescence upon 980 nm NIR excitation, confirming the high quality of as-synthesized nanoparticles. These nanoparticles are potential candidates for nano-optical devices, thin films, telecommunication, and bio-probes.

Synthesis of monodisperse iron oxide and iron/iron oxide core/shell nanoparticles via iron-oleylamine complex.

Monodisperse magnetic nanoparticles are of great scientific and technical interests. This paper reports a single-step synthesis of monodisperse magnetite nanoparticles with particle size of 8 nm. Iron/maghaemite core/shell nanoparticles with particle size of 11 nm were obtained by reducing the concentration of oleylamine. TEM and in-situ FTIR results suggested that iron-oleylamine intermediate was generated in-situ and decomposed at higher temperature. Oleylamine was also found on the surface of nanoparticles, indicating its role as capping agent which provided steric protection of as-synthesized nanoparticles from agglomeration. Both magnetite and iron/maghaemite core/shell nanoparticles were superparamagnetic at room temperature with a blocking temperature at 80 K and 67 K, respectively.

The effects of substrate position on electroless polyol deposited nanostructured FeNi films.

The nanostructured FeNi thin films were deposited on a polycrystalline Cu substrate by reducing constituent metal salts in refluxing ethylene glycol. The effect of substrate position was investigated. During deposition, the substrate was subject to one of the following processes: (a) complete immersion in solution, (b) repeated immersion followed by suspension above solution (denoted as quenching), and (c) suspension above the solution. Compared to the conventional polyol synthesis of FeNi where Fe concentration could not exceed 30 at%, the quenching process dramatically increased Fe at% to above 40%. Complete suspension of substrate above the solution resulted in Fe-rich films where Fe at% >90%. The microhardness, adhesion, and magnetic properties of deposited films showed a strong dependence on the long-range and short-range order of the film, which, in turn, depended on the substrate position. Quenched films with ordered local Ni environment and higher crystallinity had the highest Vickers hardness, best adhesion to substrate, and largest saturation magnetization compared to those deposited on substrates placed in other positions. The oxidation of Fe occurring in the vapor deposition significantly affected the film properties.

The effects of particle size and surface coating on the cytotoxicity of nickel ferrite.

The safety and toxicity of nanoparticles are of growing concern despite their significant scientific interests and promising potentials in many applications. The properties of nanoparticles depend not only on the size but also the structure, microstructure and surface coating. These in turn are controlled by the synthesis and processing conditions. The dependence of cytotoxicity on particle size and on the presence of oleic acid as surfactant on nickel ferrite particles were investigated in vitro using the Neuro-2A cell line as a model. For nickel ferrite particles without oleic acid prepared by ball milling, cytotoxicity was independent of particle size within the given mass concentrations and surface areas accessible to the cells. For nickel ferrite particles coated with oleic acid prepared by the polyol method, the cytotoxicity significantly increased when one or two layers of oleic acid were deposited. Large particles (150+/-50 nm diameter) showed a higher cytotoxicity than smaller particles (10+/-3 nm diameter).

FePt-Ag nanocomposite thin films with longitudinal magnetic anisotropy.

A well-controlled method to fabricate FePt thin films with the (200) texture and longitudinal magnetic anisotropy for high-density magnetic recording media is reported. FePt-Ag nanocomposite thin films with L1(0) ordered FePt grains embedded in an Ag matrix were deposited on the Cr90Ru10/glass by co-sputtering from Ag and FePt targets. The Ag doping suppressed the (001) texture but improved the L1(0) FePt (200) texture. The magnetic easy axis of FePt-Ag thin films changed from perpendicular to longitudinal in direction. In-plane coercivity of the films varied from 0.8 kOe to 6.5 kOe, depending on Ag contents in the films and under-layer thickness. The change from the (001) to (200) texture could be due to the competition of grain-boundary energy and epitaxial-strain energy.

Imaging cells and tissues with refractive index radiology.

Can individual cells, including live cells, be imaged using hard x rays? Common wisdom until now required sophisticated staining techniques for this task. We show instead that individual cells and cell details can be detected in culture solution and tissues with no staining and no other contrast-enhancing preparation. The sample examined can be much thicker than for many other microscopy techniques without sacrificing the capability to resolve cells. The key factor in our approach is the use of a coherent synchrotron source and of contrast mechanisms based on the refractive index. The first successful tests were conducted on a variety of cell systems including skin and internal leaf cells, mouse neurons, rabbit fibroblast cells, and human tumor cells.

Structural effects of Ti underlayer on CoCrPt magnetic films.

The effects of long-range and short-range orders of Ti underlayer thickness on the magnetic properties of sputtered Co72 Cr21 Pt7 films were investigated using synchrotron X-ray scattering and X-ray absorption near-edge structure spectroscopy. The results were consistent with that of magnetic measurements and X-ray photoelectron spectroscopy. For thin Ti underlayers (10 nm), the oxidation of Ti and significant mixing of other elements within this underlayer did not promote texture development, further resulting in poor texturing of magnetic films and undesirable magnetic properties. Increased crystallinity and texture of metallic Ti in thicker underlayers enhanced the magnetic peak alignment and its properties.

Surface-modified diamond nanoparticles as antigen delivery vehicles.

Recognition of antigens by immunocompetent cells involves interactions that are specific to the chemical sequence and conformation of the epitope (antigenic determinant). Adjuvants that are currently used to enhance immunity to antigens tend to either alter the antigen conformation through surface adsorption or shield potentially critical determinants, e.g., functional groups. It is demonstrated here that surface-modified diamond nanoparticles (5-300 nm) provide conformational stabilization, as well as a high degree of surface exposure to protein antigens. By enhancing the availability and activity of the antigen in vivo, a strong, specific immune response can be elicited. Results are demonstrated for mussel adhesive protein (MAP), a substance for which conventional adjuvants have proven only marginally successful in evoking an immune response. Surface-modified diamond nanoparticles as antigen delivery vehicles are a novel example of the exciting marriage of materials science, chemistry, and biology.

Self-arrangement of molybdenum particles into cubes.

An unusual distribution of particle sizes has been observed following the formation of molybdenum particles by argon ion sputtering. Many of the molybdenum particles produced by sputtering at the threshold pressure for particle formation in the vapor appear to be single crystalline cubes. There are two prominent peaks in the edge length distribution of the cubes, one centered at 4.8 nanometers with a halfwidth of approximately 1.3 nanometers and the other at 17.5 nanometers. The peak for the larger cubes is approximately square and has a total width of 7.0 nanometers. Evidence is presented that the larger cubes are formed by a 3 by 3 by 3 self-arrangement of the smaller cubes, which contain approximately 7000 atoms. Self-arrangement in inorganic structures is normally only observed when the building blocks are atoms, molecules, or clusters of less than 100 atoms.