Tieying Yang

Self-assembly and magnetic properties of cobalt nanoparticles

Two- and three-dimensional superlattices of passivated cobalt nanoparticles were formed by a self-assembly technique. The size and stabilization of the cobalt nanoparticles are controlled by using the combination of oleic acid and triphenylphosphine. The cobalt nanoparticles are stable for at least 90 days without oxidation at room temperature under ambient conditions. The magnetic properties of the cobalt nanoparticles in different forms are compared, which provides helpful information on the magnetostatic interaction of the nanoparticles.

Suppression of Structural Phase Transition in VO2 by Epitaxial Strain in Vicinity of Metal-insulator Transition

Mechanism of metal-insulator transition (MIT) in strained VO2 thin films is very complicated and incompletely understood despite three scenarios with potential explanations including electronic correlation (Mott mechanism), structural transformation (Peierls theory) and collaborative Mott-Peierls transition. Herein, we have decoupled coactions of structural and electronic phase transitions across the MIT by implementing epitaxial strain on 13-nm-thick (001)-VO2 films in comparison to thicker films. The structural evolution during MIT characterized by temperature-dependent synchrotron radiation high-resolution X-ray diffraction reciprocal space mapping and Raman spectroscopy suggested that the structural phase transition in the temperature range of vicinity of the MIT is suppressed by epitaxial strain. Furthermore, temperature-dependent Ultraviolet Photoelectron Spectroscopy (UPS) revealed the changes in electron occupancy near the Fermi energy EF of V 3d orbital, implying that the electronic transition triggers the MIT in the strained films. Thus the MIT in the bi-axially strained VO2 thin films should be only driven by electronic transition without assistance of structural phase transition. Density functional theoretical calculations further confirmed that the tetragonal phase across the MIT can be both in insulating and metallic states in the strained (001)-VO2/TiO2 thin films. This work offers a better understanding of the mechanism of MIT in the strained VO2 films.

Structure, Optical Absorption, and Performance of Organic Solar Cells Improved by Gold Nanoparticles in Buffer Layers

11-Mercaptoundecanoic acid (MUA)-stabilized gold nanoparticles (AuNPs) embedded in copper phthalocyanine (CuPc) were used as a buffer layer between a poly(3-hexyl-thiophene) (P3HT)/[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk heterojunction and anodic indium-tin oxide (ITO) substrate. As systematic synchrotron-based grazing incidence X-ray diffraction (GIXRD) experiments demonstrated that the AuNPs present in the buffer layer can improve the microstructure of the active layer with a better lamella packing of P3HT from the surface to the interior, UV-visible absorption spectrum measurements revealed enhanced optical absorption due to the localized surface plasma resonance (LSPR) generated by the AuNPs. The device of ITO/poly(3,4-ethylenedioxythiophene):polystyrenesulfonate/CuPc:MUA-stabilized AuNPs/P3HT:PCBM/LiF/Al was found with over 24% enhancement of power conversion efficiency (PCE) in comparison with reference devices without AuNPs. This remarkable improvement in PCE should be partially attributed to LSPR generated by the AuNPs and partially to improved crystallization as well as preferred orientation order of P3HT due to the presence of the AuNPs, which would promote more applications of metal NPs in the organic photovoltaic devices and other organic multilayer devices.

Surface double-layer structure in (110) oriented BiFeO3 thin film

Surface double-layer structure different from the interior was found in BiFeO3 thin film grown on SrRuO3 covered SrTiO3 (110) substrate by pulsed laser deposition. It was shown that BiFeO3 film exhibits epitaxial phase with single domain. X-ray reflectivity and X-ray photoelectron spectroscopy results revealed a skin layer of less than 1 nm with a reduced electron density and different surface state. Grazing incidence x-ray diffraction convinced a surface multi-domain structure of several nm beneath the surface skin layer. The double-layer near surface structure would be originated from the large depolarization field produced by the single-domain structure with strain.

Direct Growth of Graphene on Silicon by Metal-Free Chemical Vapor Deposition

AbstractThe metal-free synthesis of graphene on single-crystal silicon substrates, the most common commercial semiconductor, is of paramount significance for many technological applications. In this work, we report the growth of graphene directly on an upside-down placed, single-crystal silicon substrate using metal-free, ambient-pressure chemical vapor deposition. By controlling the growth temperature, in-plane propagation, edge-propagation, and core-propagation, the process of graphene growth on silicon can be identified. This process produces atomically flat monolayer or bilayer graphene domains, concave bilayer graphene domains, and bulging few-layer graphene domains. This work would be a significant step toward the synthesis of large-area and layer-controlled, high-quality graphene on single-crystal silicon substrates.

High-Tc SQUID magnetometer with grain boundary junctions and its application in non-destructive evaluation

We propose a simple, systematic, and efficient method to enlarge the gap of a photonic crystal. In this method, we add a small fraction of a third component into the existing photonic crystal. The dielectric property of the third component as well as its insertion position in a unit cell are chosen according to the held-energy distribution of two Bloch states at the band edges. The method is very general. It can be applied to ally microstructure and is independent of the dimensionality of the original photonic crystal. Thus, it: opens up a way to engineer photonic band gaps. Here, we demonstrate the validity of this method explicitly in two dimensions for both s and p waves. We show that, for certain microstructures, absolute gaps can be significantly enlarged. The method is also demonstrated in microwave experiments.

Electric field effect of YBa2Cu3O7 film modulated by Pb(Zr0.53Ti0.47)O3 gate layer

The electric field effect of YBa2Cu3O7/Pb(Zr0.53Ti0.47)O-3 (YBCO/PZT) integrated film has been investigated. At 100 K, the resistance of the 12 nm YBCO channel increases 7% which was induced by a spontaneous polarization with +1 V external voltage applied across a 200-nm-thick PZT layer. It was not found that the critical temperature of the YBCO layer is significantly changed by the ferroelectric field within the experimental error. The results are consistent with the theory analysis. The sample was examined by TEM with a cross-sectional specimen

High-Tc SQUID magnetometer and planar gradiometer and their applications in non-destructive evaluation

We have carried out experimental investigation of eddy current non-destructive evaluation using a high-T-c SQUID magnetometer. By mapping out field distributions with the SQUID magnetometer, flaws 10 mm below the surface in conducting aluminum structures can be clearly identified. We have also designed and fabricated a first-order planar gradiometer on a large area LaAlO3 substrate with a baseline of 16 mm.

Infrared detector fabricated by YBCO Josephson junctions in series

Abstract The infrared detectors were fabricated by YBCO bicrystal and step-edge junctions in series. The detectors exhibited an high responsivity of 780 V/W at 77K and low NEP of 9.0 × 10 −13 W/√Hz. The optical response of the detectors below T c is mainly of nonequilibrium origin.

Sensitive dc-SQUIDs and magnetometers using step-edge junctions

Abstract Superconducting quantum interference devices (SQUIDS) using YBa 2 Cu 3 O 7 step-edge grain boundary junctions on LaAlO 3 substrates were fabricated. The I–V and V -φ characteristics of these step-junction dc-SQUIDs were studied. At 77K, a square washer configuration with inductance L s =150 pH yields flux noise of about 5.08×10 −5 φ 0 √Hz, corresponding to a magnetic field and energy resolutions were 270 ft/√Hz and 3.68×10 −29 J/Hz respectively above 100 Hz.