Ya Ping Chiu

Origin of field-dependent optical transmission of magnetic fluid films

The physical mechanism of the optical transmission of magnetic fluid films under perpendicular magnetic fields is investigated in this work. Under perpendicular magnetic fields, originally dispersed magnetic particles agglomerate to form magnetic columns. The liquid phase is transparent, whereas the columns are opaque. Hence, the liquid phase dominates the optical transmission of the magnetic fluid film. When the field strength is raised, more columns are formed, and the area of the liquid phase is reduced. This leads to the decrease in the optical transmission of the film under higher field strength. The variation in the concentration of the liquid phase under various field strengths also contributes to the transmission of the film. By taking account of the variations of the effective concentration and the area ratio of the liquid phase in the magnetic fluid film under magnetic fields, the resultant magnetic field dependence of the optical transmission was calculated and found to be consistent with the experimental results. This provides evidence for the origin of the field-dependent optical transmission of the magnetic fluid film under external fields.

Atomic-scale evolution of local electronic structure across multiferroic domain walls

In complex, correlated oxides, heterointerfaces have emerged as key focal points of current condensed matter science. [ 1–3 ] For ferroic oxides, in order to minimize the total energy, domain walls emerge as natural interfaces. Multiferroic materials show a wealth of controllable multiple ferroic order through stress, optical excitation, electric, or magnetic fi elds in the same phase, which in turn suggest potential applications in the realization of oxide-based electronic devices, such as spintronics, information storage devices, or communications. [ 4–7 ] According to the detailed classifi cation given by Mermin in ferroic systems, [ 8 ]

Local Conduction at the BiFeO3‐CoFe2O4 Tubular Oxide Interface

In strongly correlated oxides, heterointerfaces, manipulating the interaction, frustration, and discontinuity of lattice, charge, orbital, and spin degrees of freedom, generate new possibilities for next generation devices. In this study, existing oxide heterostructures are examined and local conduction at the BiFeO(3)-CoFe(2)O(4) vertical interface is found. In such hetero-nanostructures the interface cannot only be the medium for the coupling between phases, but also a new state of the matter. This study demonstrates a novel concept on for oxide interface design and opens an alternative pathway for the exploration of diverse functionalities in complex oxide interfaces.

Ferroelectric Control of the Conduction at the LaAlO3/SrTiO3 Heterointerface

Modulation of band bending at a complex oxide heterointerface by a ferroelectric layer is demonstrated. The as-grown polarization (Pup ) leads to charge depletion and consequently low conduction. Switching the polarization direction (Pdown ) results in charge accumulation and enhances the conduction at the interface. The metal-insulator transition at a conducting polar/nonpolar oxide heterointerface can be controlled by ferroelectric doping.

Atomic-Scale Interfacial Band Mapping across Vertically Phased-Separated Polymer/Fullerene Hybrid Solar Cells

Using cross-sectional scanning tunneling microscope (XSTM) with samples cleaved in situ in an ultrahigh vacuum chamber, this study demonstrates the direct visualization of high-resolution interfacial band mapping images across the film thickness in an optimized bulk heterojunction polymer solar cell consisting of nanoscale phase segregated blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We were able to achieve the direct observation of the interfacial band alignments at the donor (P3HT)-acceptor (PCBM) interfaces and at the interfaces between the photoactive P3HT:PCBM blends and the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) anode modification layer with an atomic-scale spatial resolution. The unique advantage of using XSTM to characterize polymer/fullerene bulk heterojunction solar cells allows us to explore simultaneously the quantitative link between the vertical morphologies and their corresponding local electronic properties. This provides an atomic insight of interfacial band alignments between the two opposite electrodes, which will be crucial for improving the efficiencies of the charge generation, transport, and collection and the corresponding device performance of polymer solar cells.

Direct observation of ferroelectric polarization-modulated band bending at oxide interfaces

This study presents a direct visualization of the influences of ferroelectric polarization on the electronic properties of the Schottky contact at the Nb-SrTiO3/BiFeO3 hetero-interface using scanning tunneling microscopy and spectroscopy (STM/S). The evolution of the local density of states across the Nb-SrTiO3/BiFeO3 interface reveals the interfacial band alignment and the characteristic quantities of the metal/ferroelectric contact. The unique combination of STM and STS in this study delivers an approach to obtain critical information on the interfacial electronic configurations of ferroelectric oxide interfaces and also their variation with ferroelectric polarization switching.

An electrically switchable surface free energy on a liquid crystal and polymer composite film

An electrically switchable surface free energy on a liquid crystal and polymer composite film (LCPCF) resulting from the orientations of liquid crystal molecules is investigated. By modification of Cassie’s model and the measurement based on the Chibowski’s film pressure model (E. Chibowski, Adv. Colloid Interface Sci. 103, 149 (2003)), the surface free energy of LCPCF is electrically switchable from 36×10−3J/m2 to 51×10−3J/m2 while the average tilt angle of LC molecules changes from 0° to 32° with the applied pulsed voltage. The switchable surface free energy of LCPCF can help us to design biosensors and photonics devices, such as electro-optical switches, blood sensors, and sperm testers.

Parallel p–n Junctions across Nanowires by One-Step Ex Situ Doping

The bottom-up synthesis of nanoscale building blocks is a versatile approach for the formation of a vast array of materials with controlled structures and compositions. This approach is one of the main driving forces for the immense progress in materials science and nanotechnology witnessed over the past few decades. Despite the overwhelming advances in the bottom-up synthesis of nanoscale building blocks and the fine control of accessible compositions and structures, certain aspects are still lacking. In particular, the transformation of symmetric nanostructures to asymmetric nanostructures by highly controlled processes while preserving the modified structural orientation still poses a significant challenge. We present a one-step ex situ doping process for the transformation of undoped silicon nanowires (i-Si NWs) to p-type/n-type (p-n) parallel p-n junction configuration across NWs. The vertical p-n junctions were measured by scanning tunneling microscopy (STM) in concert with scanning tunneling spectroscopy (STS), termed STM/S, to obtain the spatial electronic properties of the junction formed across the NWs. Additionally, the parallel p-n junction configuration was characterized by off-axis electron holography in a transmission electron microscope to provide an independent verification of junction formation. The doping process was simulated to elucidate the doping mechanisms involved in the one-step p-i-n junction formation.

Sunlight-activated graphene-heterostructure transparent cathodes: enabling high-performance n-graphene/p-Si Schottky junction photovoltaics

Compared to widely-reported graphene-based anodes, the task of obtaining a stable graphene-based cathode is generally more difficult to achieve because n-type graphene devices have very limited thermal and chemical stabilities, and are usually sensitive to the influence of the ambient environment. This work developed a novel “sunlight-activated” graphene-heterostructure transparent electrode in which photogenerated charges from a light-absorbing material are transferred to graphene, resulting in the modulation of electrical properties of the graphene transparent electrode caused by a strong light–matter interaction at graphene-heterostructure interfaces. A photoactive graphene/TiOx-heterostructure transparent cathode was used to fabricate an n-graphene/p-Si Schottky junction solar cell, achieving a record-high power conversion efficiency (>10%). The photoactive graphene-heterostructure transparent electrode, which exhibits excellent tunable electrical properties under sunlight illumination, has great potential for use in the future development of graphene-based photovoltaics and optoelectronics.

Precisely Controlled Ultrastrong Photoinduced Doping at Graphene-Heterostructures Assisted by Trap-State-Mediated Charge Transfer.

Ultrastrong and precisely controllable n-type photoinduced doping at a graphene/TiOx heterostructure as a result of trap-state-mediated charge transfer is demonstrated, which is much higher than any other reported photodoping techniques. Based on the strong light-matter interactions at the graphene/TiOx heterostructure, precisely controlled photoinduced bandgap opening of a bilayer graphene device is demonstrated.