Moonkyu Park

Vertical ZnO nanowires/graphene hybrids for transparent and flexible field emission

We present a transparent and flexible optoelectronic material composed of vertically aligned ZnO NWs grown on reduced graphene/PDMS substrates. Large-area reduced graphene films were prepared on PDMS substrates by chemical exfoliation from natural graphitevia oxidative aqueous dispersion and subsequent thermal reduction. ZnO NWs were hydrothermally grown on the reduced graphene film substrate and maintained their structural uniformity even in highly deformed states. The electrical contact between semiconducting ZnO NWs and the metallic graphene film was straightforwardly measured by electric force microscopy (EFM). It shows a typical metal–semiconductor ohmic contact without a contact barrier. Owing to the mechanical flexibility, transparency, and low contact barrier, the ZnO NWs/graphene hybrids show excellent field emission properties. Low turn-on field values of 2.0 V μm−1, 2.4 V μm−1, and 2.8 V μm−1 were measured for convex, flat, and concave deformations, respectively. Such variation of field emission properties were attributed to the modification of ZnO NWs emitter density upon mechanical deformation.

Biomineralized N-Doped CNT/TiO2 Core/Shell Nanowires for Visible Light Photocatalysis

We report an efficient and environmentally benign biomimetic mineralization of TiO(2) at the graphitic carbon surface, which successfully created an ideal TiO(2)/carbon hybrid structure without any harsh surface treatment or interfacial adhesive layer. The N-doped sites at carbon nanotubes (CNTs) successfully nucleated the high-yield biomimetic deposition of a uniformly thick TiO(2) nanoshell in neutral pH aqueous media at ambient pressure and temperature and generated N-doped CNT (NCNT)/TiO(2) core/shell nanowires. Unlike previously known organic biomineralization templates, such as proteins or peptides, the electroconductive and high-temperature-stable NCNT backbone enabled high-temperature thermal treatment and corresponding crystal structure transformation of TiO(2) nanoshells into the anatase or rutile phase for optimized material properties. The direct contact of the NCNT surface and TiO(2) nanoshell without any adhesive interlayer introduced a new carbon energy level in the TiO(2) band gap and thereby effectively lowered the band gap energy. Consequently, the created core/shell nanowires showed a greatly enhanced visible light photocatalysis. Other interesting synergistic properties such as stimuli-responsive wettabilites were also demonstrated.

Self-Assembly-Induced Formation of High-Density Silicon Oxide Memristor Nanostructures on Graphene and Metal Electrodes

We report the direct formation of ordered memristor nanostructures on metal and graphene electrodes by a block copolymer self-assembly process. Optimized surface functionalization provides stacking structures of Si-containing block copolymer thin films to generate uniform memristor device structures. Both the silicon oxide film and nanodot memristors, which were formed by the plasma oxidation of the self-assembled block copolymer thin films, presented unipolar switching behaviors with appropriate set and reset voltages for resistive memory applications. This approach offers a very convenient pathway to fabricate ultrahigh-density resistive memory devices without relying on high-cost lithography and pattern-transfer processes.

Three-dimensional ferroelectric domain imaging of epitaxial BiFeO3 thin films using angle-resolved piezoresponse force microscopy

Here we introduce angle-resolved piezoresponse force microscopy (AR-PFM), whereby the sample is rotated by 30° increments around the surface normal vector and the in-plane PFM phase signals are collected at each angle. We obtained the AR-PFM images of BaTiO3 single crystal and cube-on-cube epitaxial (001) BiFeO3 (BFO) thin film on SrRuO3/SrTiO3 substrate, and confirmed that the AR-PFM provides more unambiguous information on the in-plane polarization directions than the conventional PFM method. Moreover, we found eight additional in-plane polarization variants in epitaxial BFO thin films, which are formed to mitigate highly unstable charged domain boundaries.

Nanoscale piezoresponse studies of ferroelectric domains in epitaxial BiFeO3 nanostructures

We report the dependence of the ferroelectric domain configuration and switching behavior on the shape (square versus round) of epitaxial BiFeO3 (BFO) nanostructures. We fabricated (001) oriented BFO(120 nm)/SrRuO3(SRO,125 nm) film layers on (001) SrTiO3 single crystals by rf magnetron sputter deposition, and patterned them to square (500×500 nm2) and round (502 nm in diameter) shaped nanostructures by focused ion-beam lithography. The surface morphology and the crystalline structure of the nanostructures were characterized by scanning electron microscopy and x-ray diffraction, respectively, while the domain configuration was investigated using piezoelectric force microscopy. We found that the square-shaped nanostructures exhibit a single variant domain configuration aligned along the [1¯11¯] direction, whereas the round-shaped nanostructures exhibit seven variants of domain configuration along the [1¯11¯], [11¯1¯], [111¯], [111], [1¯11], [11¯1], and [1¯1¯1] directions. Moreover, local d33 piezoelectric c...

Local surface potential distribution in oriented ferroelectric thin films

Using Kelvin force microscopy, the authors have investigated the potential distribution on ferroelectric films. The local distribution of potential was observed on downward, prepoled areas. The polarity of the potential corresponds to the screen charge. It was found that the electrical properties of the grain boundary affect the potential distribution. Most of the grain boundaries show a lower potential than the area inside the grain. The authors identified certain regions at the grain boundary with a very low potential. Such potential pits may act as efficient screen charge draining paths and may lead to important perturbations on the device level.

Effect of local surface potential distribution on its relaxation in polycrystalline ferroelectric films.

We have studied the effect of local surface potential distribution on its relaxation in the polycrystalline ferroelectric thin films. A lower surface potential region, i.e., potential pit, is generated near a grain boundary. The deep potential pit has a faster relaxation than the area far away from the potential pit due to the acceleration of the screen charge draining near the grain boundary and the domains formed by applying higher voltage have a faster relaxation due to the larger gradient of screen charge distribution. In addition, the surface potential and its relaxation depend on the sign of applying voltage. The result shows that the surface potential distribution may influence significantly to the reliability of bit signal on the memory devices.

The piezoresponse force microscopy investigation of self-polarization alignment in poly(vinylidene fluoride-co-trifluoroethylene) ultrathin films

We report the self-polarization alignment without external poling in spin-coated poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), thin films on transparent and flexible substrates. Piezoresponse force microscopy (PFM) allows the quantitative analysis of preferentially aligned polarization in the ferroelectric thin films. We found that as-received P(VDF-TrFE) thin films on transparent poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) electrodes showed stronger self-polarization alignment than those on indium-tin oxide (ITO) electrodes. The relative ratios for the aligned polarization per unit volume on PEDOT:PSS and ITO electrodes were 18.6% and 4%, respectively.

Piezoresponse force microscopy studies of PbTiO3 thin films grown via layer-by-layer gas phase reaction

We fabricated 20 nm thick PbTiO3 films via reactive magnetron sputtering and studied the domain switching phenomena and retention properties using piezoresponse force microscopy. We found that multistep deposited PbTiO3 thin films showed 29% smaller rms roughness (2.5 versus 3.5 nm), 28% smaller coercive voltage (1.68 versus 2.32 V), 100% higher d33 value, and improved retention characteristic (89% versus 52% of remained poled domain area in 1280 min after poling) than single-step deposited PbTiO3 thin films. We attribute the improvement to the more complete chemical reaction between PbO and TiO2 during the film growth.

Spin-coated ultrathin poly(vinylidene fluoride-co-trifluoroethylene) films for flexible and transparent electronics

We demonstrate spin-cast ultrathin poly(vinylidene fluoride-co-trifluoroethylene), P(VDF-TrFE), films on flexible and transparent substrates. Importantly, the PEDOT:PSS electrode allowed for the superior piezoresponse value (17.14 ± 2.37 pm V−1) and the low coercive voltage (1.76 ± 0.79 V) of the 20 nm thick P(VDF-TrFE) films. The use of PEDOT:PSS allows for the coherent interface between PEDOT:PSS and P(VDF-TrFE), the strong adhesion at the interface and no interfacial interaction when compared to that of indium–tin oxide (ITO).