Y. L. Zhu

Large-scale synthesis of single crystalline gallium nitride nanowires

Large-scale synthesis of single crystalline GaN nanowires in anodic alumina membrane was achieved through a gas reaction of Ga2O vapor with a constant flowing ammonia atmosphere at 1273 K. X-ray diffraction, Raman backscattering spectroscopy, scanning electron microscopy, and transmission electron microscopy indicated that those GaN nanowires with hexagonal wurtzite structure were about 14 nm in diameter and up to several hundreds of micrometers in length. The growth mechanism of the single crystalline GaN nanowires is discussed.

Ordered nanostructure of single-crystalline GaN nanowires in a honeycomb structure of anodic alumina

Ordered nanostructure of single-crystalline GaN nanowires in a honeycomb structure of anodic alumina was synthesized through a gas reaction of Ga2O vapor with a constant ammonia atmosphere at 1273 K in the presence of nano-sized metallic indium catalysis. Atomic force microscopy, x-ray diffraction, Raman backscattering spectroscopy, scanning electron microscopy, and transmission electron microscopy indicate that the ordered nanostructure consists of single-crystalline hexagonal wurtzite GaN nanowires in the uniform pores of anodic alumina about 20 nm in diameter and 40-50 mu m in length. The growth mechanism of the ordered nanostructure is discussed. The photoluminescence spectrum of this nanostructure is also reported.

Microstructural characterization of bulk Ti3AlC2 ceramics

The microstructural characteristics of the bulk Ti3AlC2 ceramic, synthesized by a solid–liquid reaction and simultaneous in-situ hot pressing, have been studied by electron diffraction and high-resolution transmission electron microscopy. It was found that the as-synthesized ceramic is fully dense and free from amorphous phase at the grain boundaries. The ceramic is predominantly a single phase of hexagonal Ti3AlC2 with a minor volume of tetragonal Al3Ti and cubic TiC. The Al3Ti grains are crystallographically independent of the Ti3AlC2, whereas TiC is intergrown with Ti3AlC2, forming thin platelets and maintaining a close orientational relationship with the Ti3AlC2 matrix.

Angular dependent magnetoresistance with twofold and fourfold symmetries in A-type antiferromagnetic Nd0.45Sr0.55MnO3 thin film

The angular dependent magnetoresistance (AMR) of Nd0.45Sr0.55MnO3 thin film epitaxially grown on SrTiO3 (001) is examined at different temperatures and magnetic fields. Twofold and fourfold symmetric AMR and a transition between them are observed under two different measurement modes and are found to be dependent on temperature and/or strength of a magnetic field. In comparison with AMR occurring in other systems, we believe that the twofold/fourfold symmetric AMR observed here corresponds to different spin-canted states induced by the magnetic field at certain temperatures below the Neel temperature.

On the benefit of aberration-corrected HAADF-STEM for strain determination and its application to tailoring ferroelectric domain patterns.

Revealing strains on the unit-cell level is essential for understanding the particular performance of materials. Large-scale strain variations with a unit-cell resolution are important for studying ferroelectric materials since the spontaneous polarizations of such materials are strongly coupled with strains. Aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy (AC-HAADF-STEM) is not so sensitive to the sample thickness and therefore thickness gradients. Consequently it is extremely useful for large-scale strain determination, which can be readily extracted by geometrical phase analysis (GPA). Such a combination has various advantages: it is straightforward, accurate on the unit-cell scale, relatively insensitive to crystal orientation and therefore helpful for large-scale. We take a tetragonal ferroelectric PbTiO3 film as an example in which large-scale strains are determined. Furthermore, based on the specific relationship between lattice rotation and spontaneous polarization (Ps) at 180° domain-walls, the Ps directions are identified, which makes the investigation of ferroelectric domain structures accurate and straightforward. This method is proposed to be suitable for investigating strain-related phenomena in other ferroelectric materials.

Magnetic anisotropy and metal-insulator transition in SrRuO3 thin films at different growth temperatures

Magnetic and transport properties of SrRuO3 film grown on SrTiO3 at different substrate temperatures have been investigated. Metallic behavior over the temperature range from 5 to 300 K is observed in the film grown at 750 °C. With a decrease in the growth temperature, a metal-insulator transition occurs for films grown at 700 and 650 °C, with transition temperatures of 15 K and 250 K, respectively, and a complete insulator behavior shows up in the film grown at 600 °C. Correspondingly, out-of–plane (OOP) magnetic anisotropy is gradually weakened, leading to complete magnetic isotropy in the film grown at 600 °C. The OOP lattice constant increases from 0.395 nm, for the film grown at 750 °C, up to 0.403 nm for the film grown at 600 °C. The correlation between the magnetic properties, transport properties, and the lattice constants indicates that the magnetic anisotropy and the metal-insulator transition (or insulator behavior) are caused mainly by strain in the SRO films, with correspondingly larger strai...

Oriented domains in a thin film of La0.8Sr0.2MnO3 prepared by laser molecular-beam epitaxy

Abstract A thin film of La0 8Sr0.2MnO3, prepared by computer-controlled laser molecular-beam epitaxy, on a single-crystal SrTiO3 substrate, has been characterized by transmission electron microscopy. Electron microdiffraction and high-resolution imaging reveal that the as-received thin film with a thickness of 200 nm is epitaxially grown on the SrTiO3(001) substrate. The microstructures in the whole film are clarified in terms of the oriented microdomains. The crystallographic relationships of these domains are discussed on the basis of an orthorhombic unit cell. Theoretical calculations based on a geometrical model that was recently proposed and applied to a number of epitaxial systems have been carried out to rationalize the present observations.

Growth orientation of one-dimensional silicon nanowires prepared by thermal evaporation

One-dimensional silicon nanowires have been grown by thermal evaporation and their growth orientations determined by transmission electron microscopy studies. The nanowires, which are often highly curved in morphology and heavily twinned in microstructure, are crystallographically separated into several sections, each with a characteristic crystallographic orientation along the wire axis. Straight nanowires, or straight sections in a curved nanowire, are found to have non-unique crystallographic orientations when {111} twinning occurs.

Synthesis of orderly nanostructure of crystalline GaN nanoparticles on anodic porous alumina membrane

Synthesis of an orderly nanostructure of crystalline GaN nanoparticles on anodic porous alumina membrane through a gas reaction of Ga2O vapor with a constant ammonia atmosphere at 900°C was achieved. The investigation using atomic force microscopy, x-ray diffraction, transmission electron microscopy and high resolution electron microscopy indicated that the orderly nanostructure consisted of polycrystalline GaN nanoparticles with a hexagonal wurtzite structure and about 10–20 nm in diameter. The growth mechanism of the orderly nanostructure of the GaN nanoparticles was discussed. The photoluminescence spectrum of the orderly nanostructure was also reported.

Single [101]-oriented growth of La0.9Sr0.1MnO3 films on vicinal SrTiO3(001) substrates

Thin films of orthorhombic La0.9Sr0.1MnO3, have been grown by computer-controlled laser molecular-beam epitaxy on SrTiO3(001) substrate and vicinal SrTiO3(001) substrates. Electron diffractions and high-resolution imaging reveal that the as-received thin films with thickness of 300 nm are epitaxially grown on the substrates. The microstructures in the film grown on SrTiO3(001) substrate are clarified in terms of the oriented microdomains, while the films on vicinal SrTiO3(001) substrates are predominated by a single [101]-oriented growth, which provides a useful routine for a design of preferred physical properties. Based on the minimization of surface energy, the mechanism of such a single domain formation is proposed.