Showing papers by "Ho Won Jang published in 2010"

Ferroelastic switching for nanoscale non-volatile magnetoelectric devices

Abstract: Multiferroics, where (anti-) ferromagnetic, ferroelectric and ferroelastic order parameters coexist, enable manipulation of magnetic ordering by an electric field through switching of the electric polarization. It has been shown that realization of magnetoelectric coupling in a single-phase multiferroic such as BiFeO(3) requires ferroelastic (71 degrees, 109 degrees) rather than ferroelectric (180 degrees) domain switching. However, the control of such ferroelastic switching in a single-phase system has been a significant challenge as elastic interactions tend to destabilize small switched volumes, resulting in subsequent ferroelastic back-switching at zero electric field, and thus the disappearance of non-volatile information storage. Guided by our phase-field simulations, here we report an approach to stabilize ferroelastic switching by eliminating the stress-induced instability responsible for back-switching using isolated monodomain BiFeO(3) islands. This work demonstrates a critical step to control and use non-volatile magnetoelectric coupling at the nanoscale. Beyond magnetoelectric coupling, it provides a framework for exploring a route to control multiple order parameters coupled to ferroelastic order in other low-symmetry materials.

One-dimensional oxide nanostructures as gas-sensing materials: review and issues.

Abstract: In this article, we review gas sensor application of one-dimensional (1D) metal-oxide nanostructures with major emphases on the types of device structure and issues for realizing practical sensors. One of the most important steps in fabricating 1D-nanostructure devices is manipulation and making electrical contacts of the nanostructures. Gas sensors based on individual 1D nanostructure, which were usually fabricated using electron-beam lithography, have been a platform technology for fundamental research. Recently, gas sensors with practical applicability were proposed, which were fabricated with an array of 1D nanostructures using scalable micro-fabrication tools. In the second part of the paper, some critical issues are pointed out including long-term stability, gas selectivity, and room-temperature operation of 1D-nanostructure-based metal-oxide gas sensors.

Ferroelectricity in strain-free SrTiO3 thin films.

Abstract: Biaxial strain is known to induce ferroelectricity in thin films of nominally nonferroelectric materials such as SrTiO3. By a direct comparison of the strained and strain-free SrTiO3 films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that all SrTiO3 films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longer-range correlation of preexisting nanopolar regions, likely originating from minute amounts of unintentional Sr deficiency in nominally stoichiometric samples. These findings highlight the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostructures, and interfaces.

Template engineering of Co-doped BaFe2As2 single-crystal thin films.

Abstract: Understanding new superconductors requires high-quality epitaxial thin films to explore intrinsic electromagnetic properties and evaluate device applications. So far, superconducting properties of ferropnictide thin films seem compromised by imperfect epitaxial growth and poor connectivity of the superconducting phase. Here we report new template engineering using single-crystal intermediate layers of (001) SrTiO(3) and BaTiO(3) grown on various perovskite substrates that enables genuine epitaxial films of Co-doped BaFe(2)As(2) with a high transition temperature (T(c,rho=0) of 21.5 K, where rho=resistivity), a small transition width (DeltaT(c)=1.3 K), a superior critical current density J(c) of 4.5 MA cm(-2) (4.2 K) and strong c-axis flux pinning. Implementing SrTiO(3) or BaTiO(3) templates to match the alkaline-earth layer in the Ba-122 with the alkaline-earth/oxygen layer in the templates opens new avenues for epitaxial growth of ferropnictides on multifunctional single-crystal substrates. Beyond superconductors, it provides a framework for growing heteroepitaxial intermetallic compounds on various substrates by matching interfacial layers between templates and thin-film overlayers.

Spin injection/detection using an organic-based magnetic semiconductor

Abstract: T he new paradigm of electronics, ‘spintronics’, promises to extend the functionality of information storage and processing in conventional electronics 1 . The principal spintronics device, the ‘spin valve’, consists of two magnetic layers decoupled by a spin-transporting spacer, which allows parallel (on) and antiparallel (off) alignment of the magnetizations (spins) of the two magnetic layers. The device resistance then depends on the spin alignment controlled by the external magnetic field. In pursuit of semiconductor spintronics 2 , there have been intensive efforts devoted to develop roomtemperature magnetic semiconductors 3 and also to incorporate both inorganic semiconductors 4 and carbon-based materials 5‐11 as the spin-transporting channels. Molecule/organic-based magnets, which allow chemical tuning of electronic and magnetic properties, are a promising new class of magnetic materials for future spintronic applications 12,13 . Here, we report the realization of an organic-based magnet as an electron spin polarizer in the standard spintronics device geometry. A thin non-magnetic organic semiconductor layer and an epitaxial ferromagnetic oxide film were employed to form a hybrid magnetic tunnel junction. The results demonstrate the spin-polarizing nature of the organic-based magnetic semiconductor, vanadium(TCNE: tetracyanoethylene)x (x 2; Tc 400 K), and its function as a spin injector/detector in hybrid magnetic multilayer devices. Molecule/organic-based magnets are relatively new materials created by chemical synthesis targeting magnetic properties at the macroscopic and/or molecular level. These materials, ranging from solely organic to organic/inorganic hybrid complexes, exhibit scientific richness in both physics and chemistry. The flexibility of organic chemical methodology, which can adjust molecular units within these systems, introduces tunability of tailor-made magnetic and electronic properties 12 . The presence of large molecular units generally leads to a low spin density, which makes these materials difficult to use in practical applications of conventional magnets. However, strong onsite Coulomb interaction together with ‘weak’ intermolecular overlapping within these materials can lead to highly spinpolarized bands with a relatively narrow bandwidth 13 . This anticipation of high spin polarization in metal-(TCNE) magnets has been broadly supported by both experimental studies 1416 and theoretical calculations 1618 indicating that these materials are promising candidates for future spintronics applications. An intriguing challenge arises concerning whether these materials can be incorporated into the platform of conventional magnetic multilayer devices.

Ferroelectricity in Strain-Free SrTiO$_{3}$ Thin Films

Abstract: Biaxial strain is known to induce ferroelectricity in thin films of nominally nonferroelectric materials such as SrTiO3. By a direct comparison of the strained and strain-free SrTiO3 films using dielectric, ferroelectric, Raman, nonlinear optical and nanoscale piezoelectric property measurements, we conclude that all SrTiO3 films and bulk crystals are relaxor ferroelectrics, and the role of strain is to stabilize longerrange correlation of preexisting nanopolar regions, likely originating from minute amounts of unintentional Sr deficiency in nominally stoichiometric samples. These findings highlight the sensitive role of stoichiometry when exploring strain and epitaxy-induced electronic phenomena in oxide films, heterostructures, and interfaces.

Strong vortex pinning in Co-doped BaFe2As2 single crystal thin films

Abstract: We report the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to single crystals and a maximum pinning force Fp(0.6Tc)>5 GN/m3 at H/Hirr∼0.5 indicative of strong high-field vortex pinning. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H∥ab plane, inverting the expectation of the Hc2 anisotropy. High resolution transmission electron microscopy reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.

Phase-transition temperatures of strained single-crystal SrRuO3 thin films.

Abstract: SrRuO3 (SRO) is widely used as an electrode in oxide electronic device applications due to its excellent material properties such as metallic conductivity, chemical stability, good lattice match with multifunctional oxides, and atomically smooth and welldefined surfaces. Especially in the fabrication of epitaxial thin-film heterostructures, the crystal symmetry and domain structure of the overlayer thin film are strongly dependent on those of the bottom electrode. Thus, it is critical to investigate the crystal symmetry and domain structure of the bottom electrode at the growth temperature and during cooling of the epitaxial heterostructures to room temperature if the bottom electrode undergoes structural transitions. In ABO3 perovskite materials, the ideal cubic symmetry can be distorted by several mechanisms such as distortions of the octahedra, cation displacementswithin the octahedra, and tilting of the octahedra. The first two mechanisms are driven by electronic instabilities of the octahedral metal ion as exemplified by the Jahn-Teller distortion in KCuF3 or the ferroelectric displacement of titanium in BaTiO3. [7,8] The third and most common mechanism, octahedral tilting, can be realized by tilting essentially rigid BO6 octahedra while maintaining their corner-sharing connectivity. This type of distortion is typically observedwhen theA cation is too small for the cubic BO3 corner-sharing octahedral network. At room temperature bulk SRO exhibits orthorhombic symmetry (Pbnm). Figure 1 shows the sequence of phase transitions in unstrained bulk SRO from orthorhombic to tetragonal and then cubic symmetry with increasing temperature. According to the Glazer notation, octahedral tilting in orthorhombic SRO is described by a a cþ, implying that RuO6 octahedra are rotated in opposite directions by equivalent magnitude along [100] and [010] and in the same direction about [001]. Tetragonal SRO is a one-tilt system, where RuO6 octahedra are rotated only about the [001] direction (aac ). The tetragonal phase of SRO is stable within the very narrow temperature range from 547 to 677 8C and, finally, high-symmetry cubic perovskite (Pm3m) becomes stable above 677 8C. Enormous strains exist in thin films when one material is deposited onto a substrate due to differences in crystal symmetry, lattice parameters, and thermal expansion coefficients between the film and the underlying substrate. As a result, the properties of thin films can be differ widely from the intrinsic properties of the unstrained bulk counterparts. For example, recent experiments have shown strain-induced ferroelectricity in SrTiO3 (STO) films at room temperature [15] and huge changes in the ferroelectric transition temperature in both BaTiO3. [16]

Study of defect-dipoles in an epitaxial ferroelectric thin film

Abstract: We have analyzed the defect contributions to the in-plane polarization switching of epitaxial (001) BiFeO3 thin films on (110) TbScO3 substrates. Interdigitated electrodes were patterned with respect to ferroelectric stripe domains in the BiFeO3 film. Polarization measurements exhibited a clear double hysteresis caused by the presence of a static defect field (∼40 kV/cm); the field resulted from ordered defect-dipoles initially aligned to the spontaneous polarization. By monitoring the defect field, both realignment and disassociation of the defect-dipoles were demonstrated. These results establish the arrangement of defect-dipoles in epitaxial ferroelectric thin films, guiding technologies and opening an avenue for defect related studies.

Highly sensitive CO sensors based on cross-linked TiO2 hollow hemispheres

Abstract: We report the fabrication of a highly sensitive gas sensor based on a network of nanostructured TiO 2 hollow hemispheres (NTHH). O 2 plasma etching induces cross-linking of hexagonal-close-packed polystyrene array, which is adopted as a template substrate for room-temperature deposition of TiO 2 thin film. High-temperature calcination effectively removes polystyrene template beads and promotes crystallization of TiO 2 , finally producing cross-linked NTHH via nanobridges. The gas-sensing capability of a NTHH-based sensor is demonstrated using 1–500 ppm CO. Our sensor exhibits a very high response of 4220% change in resistance when exposed to 500 ppm CO at 250 °C, whereas a gas sensor based on a plain TiO 2 film shows a 195% change. The high sensitivity of the NTHH-based sensor is attributed to the enhanced gas sensing performance of the narrow nanobridges between hollow hemispheres.

Ferroelectricity in nonstoichiometric SrTiO3 films studied by ultraviolet Raman spectroscopy

Abstract: Homoepitaxial Sr1+xTiO3+δ films with −0.2≤x≤0.25 grown by reactive molecular-beam epitaxy on SrTiO3 (001) substrates have been studied by ultraviolet Raman spectroscopy. Nonstoichiometry for strontium-deficient compositions leads to the appearance of strong first-order Raman scattering at low temperatures, which decreases with increasing temperature and disappears at about 350 K. This indicates the appearance of a spontaneous polarization with a paraelectric-to-ferroelectric transition temperature above room temperature. Strontium-rich samples also show a strong first-order Raman signal, but the peaks are significantly broader and exhibit a less pronounced temperature dependence, indicating a stronger contribution of the disorder-activated mechanism in Raman scattering.

Metallicity in LaTiO 3 thin films induced by lattice deformation

Abstract: Orthorhombic bulk ${\text{LaTiO}}_{3}$ is a correlated Mott insulator whose delicate antiferromagnetic insulating ground state can be easily destroyed by cation vacancies, excess oxygen, or alkaline-earth dopants, resulting in metallicity. Here we show that metallic conduction can also be realized in epitaxial ${\text{LaTiO}}_{3}$ thin films on (001) ${\text{SrTiO}}_{3}$ substrates, while films grown on (110) ${\text{DyScO}}_{3}$ and ${\text{GdScO}}_{3}$ substrates are insulating. These results illustrate the sensitivity of electrical transport to lattice effects and demonstrate the tunability of correlated oxide thin-film properties via substrate-induced deformation.

Nanoscale rectification at the LaAlO3/SrTiO3 interface

Abstract: Control over electron transport at scales that are comparable to the Fermi wavelength or mean-free path can lead to a variety of electronic devices. Here we report electrical rectification in nanowires formed by nanoscale control of the metal-insulator transition at the interface between LaAlO3 and SrTiO3. Controlled in-plane asymmetry in the confinement potential produces electrical rectification in the nanowire, analogous to what occurs naturally for Schottky diodes or by design in structures with engineered structural inversion asymmetry. Nanostructures produced in this manner may be useful in developing a variety of nanoelectronic, electro-optic, and spintronic devices.

The role of reflective p-contacts in the enhancement of light extraction in nanotextured vertical InGaN light-emitting diodes

Abstract: We report effective methods for improving light extraction efficiency for n-side-up vertical InGaN light-emitting diodes (LEDs). For the LEDs with high reflectance Ag-based p-contacts, nanotexturing of the n-GaN surface using a combination of photonic crystals and photochemical etching drastically enhances the efficiency of extraction from the top surface. In contrast, the LEDs with low reflectance Au-based p-contacts show significantly less improvement through the nanotexturing. These experimental results indicate the critical role of high reflectance p-contacts as well as surface texturing in improving the light extraction efficiency of the vertical LEDs for solid-state lighting.

Size effects in the CO sensing properties of nanostructured TiO2 thin films fabricated by colloidal templating

Abstract: This study investigates the CO sensing properties of nanostructured TiO2 thin film gas sensors fabricated with colloidal templates using different sizes of polymer spheres. Compared to plain films, the nanostructured films show enhanced gas sensing in the form of greater sensitivity and a faster response. More interestingly, the use of colloidal templates with smaller spheres (300 nm in diameter) leads to close-packed nanostructured TiO2 thin films with very large-scale uniformity and a more pronounced improvement in CO sensing compared to the use of larger spheres (1 μm in diameter). This result suggests that an understanding of the sphere size effects on the gas sensing properties of nanostructured TiO2 thin films created by colloidal templating is important in the development of these films for actual applications.

Anisotropic relaxation and crystallographic tilt in BiFeO3 on miscut SrTiO3 (001)

Abstract: Epitaxial BiFeO3 thin films on miscut (001) SrTiO3 substrates relax via mechanisms leading to an average rotation of the crystallographic axes of the BiFeO3 layer with respect to the substrate. The angle of the rotation reaches a maximum in the plane defined by the surface normal of the film and the direction of the surface miscut. X-ray microdiffraction images show that each BiFeO3 mosaic block is rotated by a slightly different angle and contains multiple polarization domains. These effects lead to a complicated overall symmetry in BiFeO3 thin films. This relaxation mechanism can be extended to other complex oxides.

Mechanism of the sensitivity enhancement in TiO2 hollow-hemisphere gas sensors

Abstract: We investigate the mechanism of the sensitivity enhancement in TiO2 hollow-hemisphere gas sensors. Using monolayer close-packed polystyrene microspheres as a sacrificial template, a TiO2 thin film based on a network of ordered hollow hemispheres is formed by room-temperature sputtering deposition and subsequent calcination at 550°C. A thin film gas sensor based on the TiO2 hollow hemispheres exhibits a 225% change in its resistance when exposed to 50 ppm CO at 250°C, whereas a gas sensor based on a flat TiO2 film shows an 85% change. Numerical analysis reveals that the enhancement of the gas sensitivity in the hollow-hemisphere gas sensor is simply the result of an increase in the effective surface area for the adsorption of gas molecules.

Kelvin probe force microscopy for conducting nanobits of NiO thin films.

Abstract: We demonstrated the writing and reading of conducting nanobits on a NiO thin film deposited on Pt/TiO2/SiO2/Si substrates for a resistive random access memory (RRAM) application using conducting atomic force microscopy (CAFM) and Kelvin probe force microscopy (KFM). A Pt/NiO/Pt RRAM capacitor showed a typical unipolar switching behavior with bistable resistances. Conducting nanobits with diameters of 22 nm written with a bias of 3.0 V on the NiO thin film were observed with CAFM. The conducting nanobits observed by KFM exhibited negative potentials relative to the insulating regions when there was no bias at the Pt bottom electrode because image charges were induced by charges formed at the end of the KFM tip by the reference AC bias. Enhancement of the KFM signals for conducting nanobits was achieved using specific biases at the Pt bottom electrode, which provided clear KFM images for conducting nanobits.

Longitudinal and transverse magnetization components in thin films: A resonant magnetic reflectivity investigation using circularly polarized soft x-rays

Abstract: An in-plane vectorial analysis of the magnetization of thin magnetic films is presented. Longitudinal soft x-ray resonant magnetic reflectivity curves display characteristic nodes where the longitudinal scattering component is suppressed by x-ray interference. The transverse magnetic component can be effectively retrieved at these nodal points, despite the use of circular polarization and longitudinal scattering geometry. Using a single geometric configuration, transverse and longitudinal magnetic hysteresis loops can be clearly separated. Calculations based on a Stoner–Wohlfarth model satisfactorily describe both loops. Therefore, this method presents a viable alternative to standard vectorial analysis techniques, with the additional benefit of element specificity.

Erratum: Spin injection/detection using an organic-based magnetic semiconductor

Abstract: Nature Materials 9, 638–642 (2010); published online: 18 July 2010; corrected after print: 23 July 2010. In the original version of this article published online and in print, the received date should have been 29 December 2009, not 2010. This has been corrected in the PDF and HTML versions of this Letter.

Analytical Determination of Cyanide in Maesil (Prunus mume) Extracts

Abstract: Picrate, enzyme-picrate and instrumental analysis methods using IC (Ion Chromatography) and HPLC (High Performance Liquid Chromatography) were compared for their effectiveness in determining cyanide in extracts of Maesil, which is classified as a harmful substance. First, the picrate method showed the shortest analysis time (about 5 hr). The color of picrate paper changed at 0.01 mg/. However, it was difficult to detect cyanide from amygdalin of glucosides. Second, we performed a qualitative analysis for total cyanide (free cyanide and cyanide from amygdalin) by the enzyme-picrate method using -glucosidase and a quantitative analysis by spectrophotometry. Finally, analysis of cyanide by IC and HPLC required the longest determining time (about 17 hr) as well as pretreatment for each free cyanide and amygdalin. These results suggest that enzyme-picrate is the most effective analysis method for the detection of cyanide in Maesil extracts.

Strong vortex pinning in Co-doped BaFe2As2 single crystal thin films

Abstract: We report measurements of the field and angular dependences of Jc of truly epitaxial Co-doped BaFe2As2 thin films grown on SrTiO3/(La,Sr)(Al,Ta)O3 with different SrTiO3 template thicknesses. The films show Jc comparable to Jc of single crystals and a maximum pinning force Fp(0.6Tc) > 5 GN/m3 at H/Hirr ~ 0.5 indicative of strong vortex pinning effective up to high fields. Due to the strong correlated c-axis pinning, Jc for field along the c-axis exceeds Jc for H//ab plane, inverting the expectation of the Hc2 anisotropy. HRTEM reveals that the strong vortex pinning is due to a high density of nanosize columnar defects.

Formation of Nanoporous TiO 2 Thin Films on Si by Anodic Oxidation

Abstract: Nanoporous titanium dioxide () is very attractive material for various applications due to the high surface to volume ratio. In this study, we have fabricated nanoporous thin films on Si by anodic oxidation. 500-nm-thick titanium (Ti) films were deposited on Si by using electron beam evaporation. Nanoporous structures in the Ti films were obtained by anodic oxidization using ethylene glycol electrolytes containing 0.3 wt% and 2 vol% under an applied bias of 5 V. The diameter of nanopores in the Ti films linearly increased with anodization time and the whole Ti layer could become nanoporous after anodizing for 3 hours, resulting in vertically aligned nanotubes with the length of 200~300 nm and the diameter of 50~80 nm. Upon annealing at in air, the anodized Ti films were fully crystallized to of rutile and anatase phases. We believe that our method to fabricate nanoporous films on Si is promising for applications to thin-film gas sensors and thin-film photovoltaics.