Xiaoli Lu

Magnetoelectric Coupling in Ordered Arrays of Multilayered Heteroepitaxial BaTiO3/CoFe2O4 Nanodots

Fully epitaxial BaTiO(3)/CoFe(2)O(4) ferroelectric/ferromagnetic multilayered nanodot arrays, a new type of magnetoelectric (ME) nanocomposite with both horizontal and vertical orderings, were fabricated via a stencil-derived direct epitaxy technique. By reducing the clamping effect, ferroelectric domain modification and distinct magnetization change proportional to different interfacial area around the BaTiO(3) phase transition temperatures were found, which may pave the way to quantitative introducing of ME coupling at nanoscale and build high density multistate memory devices.

Yellow luminescence of polar and nonpolar GaN nanowires on r-plane sapphire by metal organic chemical vapor deposition.

We have grown horizontal oriented, high growth rate, well-aligned polar (0001) single crystalline GaN nanowires and high-density and highly aligned GaN nonpolar (11-20) nanowires on r-plane substrates by metal organic chemical vapor deposition. It can be found that the polar nanowires showed a strong yellow luminescence (YL) intensity compared with the nonpolar nanowires. The different trends of the incorporation of carbon in the polar, nonpolar, and semipolar GaN associated with the atom bonding structure were discussed and proved by energy-dispersive X-ray spectroscopy, suggesting that C-involved defects are the origin responsible for the YL in GaN nanowires.

Hydrothermal synthesis of single-crystalline La0.5Ca0.5MnO3 nanowires at low temperature

Single-crystalline La0.5Ca0.5MnO3 nanowires have been successfully prepared by a hydrothermal method at low temperature. X-Ray diffraction (XRD) results show that the La0.5Ca0.5MnO3 nanowires have an orthorhombic perovskite structure without any other impurity phase. Transmission electron microscopy (TEM) images demonstrate that the as-synthesized samples are made up of a large quantity of nanowires with lengths ranging from several to several tens of micrometers and uniform diameter (∼80 nm). High-resolution transmission electron microscopy (HRTEM) images reveal that the nanowires with very clean surfaces grow along [100]. The result of magnetic measurements indicates that the nanowires have an enhanced Tc due to the shrinkage of the unit cell volume.

Nucleation-Induced Self-Assembly of Multiferroic BiFeO3-CoFe2O4 Nanocomposites

Large areas of perfectly ordered magnetic CoFe2O4 nanopillars embedded in a ferroelectric BiFeO3 matrix were successfully fabricated via a novel nucleation-induced self-assembly process. The nucleation centers of the magnetic pillars are induced before the growth of the composite structure using anodic aluminum oxide (AAO) and lithography-defined gold membranes as hard mask. High structural quality and good functional properties were obtained. Magneto-capacitance data revealed extremely low losses and magneto-electric coupling of about 0.9 μC/cmOe. The present fabrication process might be relevant for inducing ordering in systems based on phase separation, as the nucleation and growth is a rather general feature of these systems.

Homoepitaxial branching: an unusual polymorph of zinc oxide derived from seeded solution growth

The development of hydrothermal synthesis has greatly promoted bottom-up nanoscience for the rational growth of diverse zinc oxide (ZnO) nanostructures. In comparison with normal ZnO nanowires, ZnO nanostructures with a larger surface area, for instance, branched nanowires, are more attractive in the application fields of catalysis, sensing, dye-sensitized solar cells etc. So far the ZnO branched nanowires achieved by either one-step or multistep growth always present a boundary-governed nonepitaxial branch/stem interface. In this report, seeded growth of single-crystalline ZnO hexabranched nanostructures was achieved by selecting polyethylene glycol (PEG) as capping agent based on a low-temperature, laterally epitaxial solution growth strategy. We investigated the generality of this PEG-assisted growth process using different ZnO seed layers including continuous film, patterned dots, and vertically aligned nanowire arrays. It was revealed that PEG is a distinctive c-direction inhibitor responsible for the lateral growth and subsequent branching of ZnO due to its nonionic and nonacidic feature and weak reactivity in the solution system. All the obtained branched nanostructures are of single crystallinity in nature, which is methodologically determined by the homoepitaxial growth mode. This PEG-assisted process is versatile for diameter tuning and branch formation of ZnO nanowires by secondary growth. Our proof-of-concept experiments demonstrated that the ZnO hexabranched nanostructures presented superior photocatalytic efficiency for dye degradation relative to the normal ZnO nanowires.

Unexpected Oxidation Behavior of Cu Nanoparticles Embedded in Porous Alumina Films Produced by Molecular Layer Deposition

This work reports an unexpected oxidation behavior of Cu nanoparticles embedded in porous Al(2)O(3) confinements that are produced by annealing alucone (an organic-inorganic hybrid material) deposited by molecular layer deposition. An oxidation of such encapsulated Cu nanoparticles by annealing in air produces Cu oxide nanoparticles attached to the outer surface of the hollow Al(2)O(3) nanostructures, which is in strong contrast to bare or compact, nonporous Al(2)O(3)-coated Cu nanoparticles, which result in hollow oxide nanospheres or do not undergo oxidation, respectively. The conversion from encapsulated Cu to supported oxide nanoparticles is explained by a concerted pore-assisted diffusion and oxidation mechanism. The micropores in the films, having diameters of several angstroms, permit a selective out-diffusion of Cu atoms and prevent the inward diffusion of oxygen. The subsequent oxidation occurs at the pore entrances, which work as multiple nucleation sites for the formation of oxide nanoparticles with a small size and good dispersion.

Role of measurement voltage on hysteresis loop shape in Piezoresponse Force Microscopy

The dependence of field-on and field-off hysteresis loop shape in Piezoresponse Force Microscopy (PFM) on driving voltage, Vac, is explored. A nontrivial dependence of hysteresis loop parameters on measurement conditions is observed. The strategies to distinguish between paraelectric and ferroelectric states with small coercive bias and separate reversible hysteretic and non-hysteretic behaviors are suggested. Generally, measurement of loop evolution with Vac is a necessary step to establish the veracity of PFM hysteresis measurements.

Styrene hydrogenation performance of Pt nanoparticles with controlled size prepared by atomic layer deposition

Highly dispersed Pt sub-nanoparticles supported on carbon nanotubes (CNTs) with well-controlled size have been prepared by atomic layer deposition. Their particle size distribution was characterized by TEM. The obtained Pt sub-nanoparticles exhibit unusual catalytic performance for styrene hydrogenation. It is revealed that the turnover frequency (TOF) of the Pt/CNTs catalysts for this reaction correlated well with the Pt particle size. The highest TOF was obtained with the Pt/CNTs catalyst having an average Pt particle size around 0.5–0.7 nm.

A Compact Variable-Temperature Broadband Series-Resistor Calibration

We present a broadband on-wafer calibration from 45 MHz to 40 GHz for variable temperature measurements, which requires three standards: a thru, reflect, and series resistor. At room temperature, the maximum error of this technique, compared to a benchmark nine-standard multiline thru-reflect-line (TRL) method, is comparable to the repeatability of the benchmark calibration. The series-resistor standard is modeled as a lumped-element -network, which is described by four frequency-independent parameters. We show that the model is stable over three weeks, and compare the calibration to the multiline TRL method as a function of time. The approach is then demonstrated at variable temperature, where the model parameters are extracted at 300 K and at variable temperatures down to 20 K, in order to determine their temperature dependence. The resulting technique, valid over the temperature range from 300 to 20 K, reduced the total footprint of the calibration standards by a factor of 17 and the measurement time by a factor of 3.

Flexible Quasi-Two-Dimensional CoFe2O4 Epitaxial Thin Films for Continuous Strain Tuning of Magnetic Properties

Epitaxial thin films of CoFe2O4 (CFO) have successfully been transferred from a SrTiO3 substrate onto a flexible polyimide substrate. By bending the flexible polyimide, different levels of uniaxial strain are continuously introduced into the CFO epitaxial thin films. Unlike traditional epitaxial strain induced by substrates, the strain from bending will not suffer from critical thickness limitation, crystalline quality variation, and substrate clamping, and more importantly, it provides a more intrinsic and reliable way to study strain-controlled behaviors in functional oxide systems. It is found that both the saturation magnetization and coercivity of the transferred films can be changed over the bending status and show a high accord with the movement of the curvature bending radius of the polyimide substrate. This reveals that the mechanical strain plays a critical role in tuning the magnetic properties of CFO thin films parallel and perpendicular to the film plane direction.