Dynamic Tilting of Ferroelectric Domain Walls Caused by Optically Induced Electronic Screening.

Optical excitation perturbs the balance of phenomena selecting the tilt orientation of domain walls within ferroelectric thin films. The high carrier density induced in a low-strain BaTiO_{3} thin film by an above-band-gap ultrafast optical pulse changes the tilt angle that 90° a/c domain walls form with respect to the substrate-film interface. The dynamics of the changes are apparent in time-resolved synchrotron x-ray scattering studies of the domain diffuse scattering. Tilting occurs at 298 K, a temperature at which the a/b and a/c domain phases coexist but is absent at 343 K in the better ordered single-phase a/c regime. Phase coexistence at 298 K leads to increased domain-wall charge density, and thus a larger screening effect than in the single-phase regime. The screening mechanism points to new directions for the manipulation of nanoscale ferroelectricity.

Mesoscopic Elastic Distortions in GaAs Quantum Dot Heterostructures.

Quantum devices formed in high-electron-mobility semiconductor heterostructures provide a route through which quantum mechanical effects can be exploited on length scales accessible to lithography and integrated electronics. The electrostatic definition of quantum dots in semiconductor heterostructure devices intrinsically involves the lithographic fabrication of intricate patterns of metallic electrodes. The formation of metal/semiconductor interfaces, growth processes associated with polycrystalline metallic layers, and differential thermal expansion produce elastic distortion in the active areas of quantum devices. Understanding and controlling these distortions present a significant challenge in quantum device development. We report synchrotron X-ray nanodiffraction measurements combined with dynamical X-ray diffraction modeling that reveal lattice tilts with a depth-averaged value up to 0.04° and strain on the order of 10-4 in the two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. Elastic distortions in GaAs/AlGaAs heterostructures modify the potential energy landscape in the 2DEG due to the generation of a deformation potential and an electric field through the piezoelectric effect. The stress induced by metal electrodes directly impacts the ability to control the positions of the potential minima where quantum dots form and the coupling between neighboring quantum dots.

Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer.

Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved X-ray reflectivity studies show that the reconfiguration of a densely packed rhenium-azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

Photoinduced Domain Pattern Transformation in Ferroelectric-Dielectric Superlattices.

The nanodomain pattern in ferroelectric-dielectric superlattices transforms to a uniform polarization state under above-band-gap optical excitation. X-ray scattering reveals a disappearance of domain diffuse scattering and an expansion of the lattice. The reappearance of the domain pattern occurs over a period of seconds at room temperature, suggesting a transformation mechanism in which charge carriers in long-lived trap states screen the depolarization field. A Landau-Ginzburg-Devonshire model predicts changes in lattice parameter and a critical carrier concentration for the transformation.

Thermal Fluctuations of Ferroelectric Nanodomains in a Ferroelectric-Dielectric PbTiO_{3}/SrTiO_{3} Superlattice.

Ferroelectric-dielectric superlattices consisting of alternating layers of ferroelectric PbTiO_{3} and dielectric SrTiO_{3} exhibit a disordered striped nanodomain pattern, with characteristic length scales of 6 nm for the domain periodicity and 30 nm for the in-plane coherence of the domain pattern. Spatial disorder in the domain pattern gives rise to coherent hard x-ray scattering patterns exhibiting intensity speckles. We show here using variable-temperature Bragg-geometry x-ray photon correlation spectroscopy that x-ray scattering patterns from the disordered domains exhibit a continuous temporal decorrelation due to spontaneous domain fluctuations. The temporal decorrelation can be described using a compressed exponential function, consistent with what has been observed in other systems with arrested dynamics. The fluctuation speeds up at higher temperatures and the thermal activation energy estimated from the Arrhenius model is 0.35±0.21 eV. The magnitude of the energy barrier implies that the complicated energy landscape of the domain structures is induced by pinning mechanisms and domain patterns fluctuate via the generation and annihilation of topological defects similar to soft materials such as block copolymers.

Nanolithography on graphene by using scanning tunneling microscopy in a methanol environment.

Since it was discovered in 2004, graphene has attracted enormous attention as an emerging material for future devices, but it has been found that conventional lithographic processes based on polymer resist degrade its intrinsic performance. Recently, our group studied a resist-free scanning tunneling microscopy-based lithography in various atmospheres by injecting volatile liquids into a chamber. In this study, multilayer graphene was scanned and etched by controlling bias voltage under methanol pressure. We focused on improving patterning results in terms of depth and line width, while the previous study was performed to find an optimum gas environment for patterning on a graphite surface. Specifically, we report patterning outputs depending on conditions of voltage, current, and pressure. The optimum conditions for methanol environment etching were a gas pressure in the range of 41-50 torr, a -4 V tip bias, and a 2 nA tunneling current.

Hydrogeochemical Evaluation of Groundwater Quality Parameters for Ogallala Aquifer in the Southern High Plains Region, USA.

The purposes of this study are to analyze the groundwater quality of the Ogallala Aquifer and evaluate the hydrological characteristics in the southern High Plains region of the Permian Basin, Texas. Levels of chloride, fluoride, nitrate, selenium, pH, and total dissolved solids (TDS) were analyzed for the period 1990-2016. Data concerning a total of 133 wells were collected from the Texas Water Development Board (TWDB), which is an open database provided by the US government. The average levels of contaminants were compared to their respective Maximum Contaminant Levels (MCL) stipulated by the Environmental Protection Agency (EPA). The study area experienced high concentrations of most parameters including chloride, fluoride, nitrate, selenium, and TDS, within the contaminants' respective MCLs. Borden and Dawson counties experienced the highest overall amounts of groundwater pollutants. Possible sources of each contaminant are discussed, with oil and gas activities, agricultural practices, and other human actions impacting the conditions. This research provides important information about groundwater quality of the Ogallala Aquifer and contributes to understanding the response to development in the Permian Basin, Texas.

Graphite patterning in a controlled gas environment.

Although a number of methods using scanning probe lithography to pattern graphene have already been introduced, the fabrication of real devices still faces limitations. We report graphite patterning using scanning probe lithography with control of the gas environment. Patterning processes using scanning probe lithography of graphite or graphene are normally performed in air because water molecules forming the meniscus between the tip and the sample mediate the etching reaction. This water meniscus, however, may prevent uniform patterning due to its strong surface tension or large contact angle on surfaces. To investigate this side effect of water, our experiment was performed in a chamber where the gas environment was controlled with methyl alcohol, oxygen or isopropanol gases. We found that methyl alcohol facilitates graphite etching, and a line width as narrow as 3 nm was achieved as methyl alcohol also contains an oxygen atom which gives rise to the required oxidation. Due to its low surface tension and highly adsorptive behavior, methyl alcohol has advantages for a narrow line width and high speed etching conditions.

Domain alignment within ferroelectric/dielectric PbTiO3/SrTiO3 superlattice nanostructures.

The ferroelectric domain pattern within lithographically defined PbTiO3/SrTiO3 ferroelectric/dielectric heteroepitaxial superlattice nanostructures is strongly influenced by the edges of the structures. Synchrotron X-ray nanobeam diffraction reveals that the spontaneously formed 180° ferroelectric stripe domains exhibited by such superlattices adopt a configuration in rectangular nanostructures in which domain walls are aligned with long patterned edges. The angular distribution of X-ray diffuse scattering intensity from nanodomains indicates that domains are aligned within an angular range of approximately 20° with respect to the edges. Computational studies based on a time-dependent Landau-Ginzburg-Devonshire model show that the preferred direction of the alignment results from lowering of the bulk and electrostrictive contributions to the free energy of the system due to the release of the lateral mechanical constraint. This unexpected alignment appears to be intrinsic and not a result of distortions or defects caused by the patterning process. Our work demonstrates how nanostructuring and patterning of heteroepitaxial superlattices allow for pathways to create and control ferroelectric structures that may appear counterintuitive.

Effect of Corynebacterium glutamicum on Livestock Material Burial Treatment.

In recent years, foot-and-mouth disease has occurred in all parts of the world. The animals with the disease are buried in the ground; therefore, their concentration could affect ground or groundwater. Moreover, the complete degradation of carcasses is not a certainty, and their disposal is important to prevent humans, livestock, and the environment from being affected with the disease. The treatment of Corynebacterium glutamicum is a feasible method to reduce the risk of carcass decomposition affecting humans or the environment. Therefore, this study aimed to investigate the effect of C. glutamicum on the soil environment with a carcass. The composition of amino acids in the soil treated with C. glutamicum was generally higher than those in the untreated soil. Moreover, the plant root in the soil samples treated with C. glutamicum had 84.0% amino acids relative to the standard value and was similar to that of the control. The results of this study suggest the possibility to reduce the toxicity of a grave land containing animals with this disease.

Spatially confined low-power optically pumped ultrafast synchrotron x-ray nanodiffraction.

The combination of ultrafast optical excitation and time-resolved synchrotron x-ray nanodiffraction provides unique insight into the photoinduced dynamics of materials, with the spatial resolution required to probe individual nanostructures or small volumes within heterogeneous materials. Optically excited x-ray nanobeam experiments are challenging because the high total optical power required for experimentally relevant optical fluences leads to mechanical instability due to heating. For a given fluence, tightly focusing the optical excitation reduces the average optical power by more than three orders of magnitude and thus ensures sufficient thermal stability for x-ray nanobeam studies. Delivering optical pulses via a scannable fiber-coupled optical objective provides a well-defined excitation geometry during rotation and translation of the sample and allows the selective excitation of isolated areas within the sample. Experimental studies of the photoinduced lattice dynamics of a 35 nm BiFeO3 thin film on a SrTiO3 substrate demonstrate the potential to excite and probe nanoscale volumes.

Patterning of photosensitive polyimide LB film and its application in the fabrication of biomolecular microphotodiode array.

Ultra thin film of photosensitive polyimide having benzene and sulfonyloxyimide moieties in the main chain was prepared using a Langmuir-Blodgett (LB) technique, and then micro array pattern of the polyimide LB film on a gold substrate was obtained by deep UV lithographic technique. In order to array cytochrome c molecules along the micro-patterned gold substrate, the well-characterized monolayer of cytochrome c was immobilized with a mixed monolayer of 11-mercaptoundecanoic acid (11-MUDA) and decanethiol. The redox activity and electron transfer between cytochrome c molecular center and gold electrode interface for the self-assembled cytochrome c monolayer were investigated by cyclic voltammetry measurement. Biomolecular photodiode consisting of cytochrome c and green fluorescent protein (GFP) onto the patterned gold substrate was fabricated by self-assembly process. The integration and morphology of cytochrome c and GFP were studied from the measurements of atomic force microscopy (AFM) and fluorescence emission. Especially, current-voltage characteristics of the protein multilayers were investigated by scanning tunneling microscopy (STM) and its application in biomolecular photodiode was also examined.

Adsorption characteristics of As(V) on iron-coated zeolite.

The sorption characteristics of arsenic(As(V)) on iron-coated zeolite (ICZ) were investigated through batch and column studies. As(V) was completely removed within 30 min in a concentration of 2mg/l, with a 100g/l dose of ICZ. Optimum dose of ICZ was 33.3g/l at a concentration of 20.12 mg/l and the effect of solution pH was negligible at a pH range of 3.0