Yuzhan Wang

High-coercivity Co-ferrite thin films on (100)-SiO2 substrate

Co-ferrite films were deposited on SiO2 single-crystal substrates. The as-deposited films were amorphous. The crystallization required an annealing at 700 °C or higher. Magnetic properties were found to be strongly dependent on annealing temperature, annealing duration, and film thickness. A small film thickness can restrict the formation of large particles. A coercivity as high as 9.3 kOe was achieved in the 50 nm film after annealing at 900 °C for 15 min deposited on (100)-SiO2 substrate. The high coercivity was associated with a nanostructure, lattice strain, and larger Raman shift with a relatively sharp peak.

Quasi-free-standing epitaxial graphene on SiC (0001) by fluorine intercalation from a molecular source.

We demonstrated a novel method to obtain charge neutral quasi-free-standing graphene on SiC (0001) from the buffer layer using fluorine from a molecular source, fluorinated fullerene (C(60)F(48)). The intercalated product is stable under ambient conditions and resistant to elevated temperatures of up to 1200 °C. Scanning tunneling microscopy and spectroscopy measurements are performed for the first time on such quasi-free-standing graphene to elucidate changes in the electronic and structural properties of both the graphene and interfacial layer. Novel structures due to a highly localized perturbation caused by the presence of adsorbed fluorine were produced in the intercalation process and investigated. Photoemission spectroscopy is used to confirm these electronic and structural changes.

Charged defects and their effects on electrical behavior in Bi1-xLaxFeO3 thin films

Ferroelectric and dielectric characteristics of Bi1−xLaxFeO3 thin films deposited on SrRuO3 as bottom electrode have been investigated. In accordance with the Rayleigh model, it is in principle established that La doping in BiFeO3 effectively reduces the concentration of charged defects and dielectric loss, although there is a slight deviation at the high level of La doping (x=0.2). This departure is attributed to the reversible bending movement of pinned 180° domain walls, which contributes to the dielectric permittivity nonlinearly without inducing loss. In addition, the competition between domain wall pinning and depinning is determined to be the dominant fatigue mechanism, as shown by the enhanced fatigue endurance at the high La-doping level, test frequency, and electrical field.

Charge transfer dynamics of 3,4,9,10-perylene-tetracarboxylic-dianhydride molecules on Au(111) probed by resonant photoemission spectroscopy

Charge transfer dynamics across the lying-down 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) organic semiconductor molecules on Au(111) interface has been investigated using the core-hole clock implementation of resonant photoemission spectroscopy. It is found that the charge transfer time scale at the PTCDA∕Au(111) interface is much larger than the C 1s core-hole lifetime of 6 fs, indicating weak electronic coupling between PTCDA and the gold substrate due to the absence of chemical reaction and∕or bonding.

Effects of heat treatment and magnetoannealing on nanocrystalline Co-ferrite powders

This work consists of three parts: the effects of heat treatment (slow cooling and quenching), magnetoannealing, and postannealing of samples with induced anisotropy. It has been found that noncomplete inverse spinel structure was the result after annealing at higher temperature and quenching. Our Mossbauer spectroscopy study confirmed noncomplete inverse structure after quenching, while inverse spinel structure was formed after slow cooling. The kinetics of the formation of induced anisotropy during magnetoannealing has been investigated in this study. Reduction of crystalline magnetic anisotropy was observed, as coercivity decreased after magnetoannealing. The change of remanence ratio and coercivity followed the expected equations for ion diffusion. A relative large anisotropy in magnetization was evident. A postannealing resulted in the conversion into the initial isotropic stage. The process could be well described using the equations of ion diffusion.

Temperature-dependent electrical behavior of La0.7Sr0.3MnO3-buffered Bi0.9La0.1FeO3 thin films

Temperature-dependent ferroelectric and dielectric behaviors of La0.7Sr0.3MnO3-buffered Bi0.9La0.1FeO3 thin films were investigated. It was observed that the coercive voltage remarkably increases with decreasing temperature, and for a fixed driving voltage the area of hysteresis loop demonstrates a maximum at a certain temperature, revealing the competition between the coercivity and driving voltage. The dielectric constant versus ac electric field evolves from the nonlinear behavior at room temperature, which is ascribed to the reversible bending movement of domain walls, to the linear Rayleigh law at 90 K, indicating the increased density of pinning centers and suppressed domain wall motion. The fatigue resistance is deteriorated at the low temperature as a result of the enhanced domain pinning.

Tuning the interfacial hole injection barrier between p-type organic materials and Co using a MoO3 buffer layer

We demonstrate that the interfacial hole injection barrier Δh between p-type organic materials (i.e., CuPc and pentacene) and Co substrate can be tuned by the insertion of a MoO3 buffer layer. Using ultraviolet photoemission spectroscopy, it was found that the introduction of MoO3 buffer layer effectively reduces the hole injection barrier from 0.8 eV to 0.4 eV for the CuPc/Co interface, and from 1.0 eV to 0.4 eV for the pentacene/Co interface, respectively. In addition, by varying the thickness of the buffer, the tuning effect of Δh is shown to be independent of the thickness of MoO3 interlayer at both CuPc/Co and pentacene/Co interfaces. This Fermi level pinning effect can be explained by the integer charge-transfer model. Therefore, the MoO3 buffer layer has the potential to be applied in p-type organic spin valve devices to improve the device performance via reducing the interfacial hole injection barrier.

A synchrotron-based photoemission study of the MoO3/Co interface

The electronic structures at the MoO(3)∕Co interface were investigated using synchrotron-based ultraviolet and x-ray photoelectron spectroscopy. It was found that interfacial chemical reactions lead to the reduction of Mo oxidation states and the formation of Co-O bonds. These interfacial chemical reactions also induce a large interface dipole, which significantly increases the work function of the cobalt substrate. In addition, two interface states located at 1.0 and 2.0 eV below the Fermi level are identified. These two states overlap at film thickness of between 2-4 nm, which suggests the MoO(3) intermediate layer may facilitate ohmic charge transport.

Negative differential resistance based on electron injection/extraction in conducting organic films

This work reports a mechanism of negative differential resistance (NDR) observed for perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) films. The NDR is based on electron injection and extraction at the metal/PTCDA interface, and is governed by the joint effect of electronic and ionic components. Consequently, the NDR behavior exhibits a monotonous dependence on the voltage scan rate, and the number of NDR peaks is also sensitive to the work function of metal electrodes. The results provide further understanding on the diverse manifestation of NDR, and would be useful in organic electronic applications.

A percolating membrane with superior polarization and power retention for rechargeable energy storage.

Polarization is associated with the property of a material to form electric dipoles in the direction of an external field E. In this case, the separated positive and negative charges remain in the material, and thus create an internal field. The amount of polarizable charges determines the value of polarization P in the unit of C·cm−2. When the external field is removed, the material can maintain its polarized charges for certain time, and this ability is defined to be the polarization or charge retention. Basically, there are two types of polarizable materials – dielectrics and ferroelectrics. Dielectrics are electrically insulating, and their polarization is based on the slight shift of charges under a field.[1–6] Ferroelectric polarization is related to field-induced phase transitions which involve the switching of ionic domain walls.[7–10] Dielectrics are often used as insulators and frequency resonators,[1–6] while ferroelectrics are applicable to switching memory devices.[7–10] Due to their small polarization P in the order of μC·cm−2, neither dielectrics nor ferroelectrics are suitable for electric energy storage. This is because the amount of polarizable charges is so small that the corresponding current is only μA·cm−2, which is too low a current compared to that of common energy-storage devices such as batteries and supercapacitors.[11–15] This work reports the discovery of a third type of polarizable material – the percolating organic membrane. Compared to dielectrics and ferroelectrics, this new class of polarizable membrane exhibits a few extraordinary properties. First, it has the largest polarization known so far, with P (up to 1.0 C·cm−2) being 105–106 times that of dielectrics and ferroelectrics. Second, the membrane exhibits good polarization retention, and is able to retain 50% of the polarized charges for 24 hours. In addition, the membrane polarization follows an anti-intuition mechanism in which percolation effect is at work, making thicker membranes much more polarizable and ionic-conducting than thinner ones. The large polarization and long charge retention renders the membrane a promising material in energy storage applications. In the following sections, we present the above unique features of the membrane, and demonstrate its immediate applicability in storing the electric energy generated by a Si solar panel.