Measuring the surface dynamics of glassy polymers

The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk alpha relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk alpha relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.     

Dislocations and Flux Pinning in YBa2Cu3O7-delta

Bulk YBa(2)Cu(3)O(7-delta) superconductors, under certain processing conditions such as melt texturing, exhibit a very high dislocation density of 10(9) to 10(10) per square centimeter. In addition, the density of low-angle grain boundaries in such samples can be significantly increased (to less than 700-nanometer spacing) through a dispersion of submicrometer-sized Y(2)BaCuO(5) inclusions. These defect densities are comparable to those in high critical current thin films as revealed through scanning tunneling microscopy, and yet the critical current densities in the bulk materials (at 77 kelvin and a field of 1 tesla for example) remain at a 10(4) amperes per square centimeter level, about two orders of magnitude lower than in thin films. The results imply that these defect density levels are not significant enough to explain the difference in flux pinning strength between the thin film and bulk materials. The observation of spiral-like growth of the superconductor phase in bulk Y-Ba-Cu-O is also reported.     

Molecular confinement accelerates deformation of entangled polymers during squeeze flow

The squeezing of polymers in narrow gaps is important for the dynamics of nanostructure fabrication by nanoimprint embossing and the operation of polymer boundary lubricants. We measured stress versus strain behavior while squeezing entangled polystyrene films to large strains. In confined conditions where films were prepared to a thickness less than the size of the bulk macromolecule, resistance to deformation was markedly reduced for both solid-glass forging and liquid-melt molding. For melt flow, we further observed a complete inversion of conventional polymer viscosity scaling with molecular weight. Our results show that squeeze flow is accelerated at small scales by an unexpected influence of film thickness in polymer materials.     

Epitaxial BiFeO3 multiferroic thin film heterostructures

Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.     

Viscoelastic dynamics of confined polymer melts

The frequency-dependent shear response of an ultrathin polymer melt (polyphenylmethylsiloxane) confined between adsorbing surfaces (parallel plates of mica) is described. The sinusoidal deformations were sufficiently small to give linear response, implying that measurement did not perturb the film structure. A remarkable transition was observed with decreasing thickness. When the film thickness was less than five to six times the unperturbed radius of gyration, there emerged a strong rubber-like elasticity that was not characteristic of the bulk samples. This result indicates enhanced entanglement interactions in thin polymer films and offers a mechanism to explain the slow mobility of polymers at surfaces.     

The life and death of "Bare" viscous bubbles

Air bubbles collect and explode at the surface of many viscous liquids, as observed with polymer foams, in glass furnaces, and during volcanic eruptions. The liquid film separating the bubble from bulk air can have a long lifetime (if it is viscous) even if it is not protected by a surfactant. These "bare" films display unusual dynamic behaviors in drainage and rupture. Two different model systems were studied: a polymer melt (silicone oil) and a molten (borosilicate) glass of comparable viscosity. Although the two systems differ greatly in their relaxation time, they are described by the same set of laws, which can be understood from a relatively simple hydrodynamic model.     

Growth of uniformly oriented silica MFI and BEA zeolite films on substrates

Applications of zeolite films benefit from alignment of the integrated channels, but methods for film growth have nearly always introduced orientational randomization in the direction normal to the substrate. We now report facile methods to grow silicalite-1 films and pure silica beta zeolite films on substrates with straight or sinusoidal channels positioned uniformly upright at a thickness of up to 8 micrometers. Precise gel compositions and processing temperatures are critical to promote secondary growth on pre-formed oriented crystal monolayers while suppressing self-crystallization in the bulk medium. Preliminary results highlight the potential of these uniformly oriented films in the nonlinear optical response and separation of xylene isomers.     

Photoinduced electron transfer from a conducting polymer to buckminsterfullerene

Evidence for photoinduced electron transfer from the excited state of a conducting polymer onto buckminsterfullerene, C(60), is reported. After photo-excitation of the conjugated polymer with light of energy greater than the pi-pi* gap, an electron transfer to the C(60) molecule is initiated. Photoinduced optical absorption studies demonstrate a different excitation spectrum for the composite as compared to the separate components, consistent with photo-excited charge transfer. A photoinduced electron spin resonance signal exhibits signatures of both the conducting polymer cation and the C(60) anion. Because the photoluminescence in the conducting polymer is quenched by interaction with C(60), the data imply that charge transfer from the excited state occurs on a picosecond time scale. The charge-separated state in composite films is metastable at low temperatures.     

Phase-separated composite films for liquid crystal displays

A method of preparing liquid crystal devices by phase separation of liquid crystal from its solution in a prepolymer, which results in adjacent layers of liquid crystal and polymer, is described. Liquid crystals in these phase-separated composite films exhibit electro-optical properties not observed in devices prepared by conventional methods, polymer dispersion, or polymer-stabilization methods. Devices incorporating ferroelectric liquid crystals have gray scale and switch 100 times faster at low fields than conventional surface-stabilized devices. This method makes it possible to prepare devices with liquid crystal film thickness comparable to optical wavelengths.     

Synthesis of two-dimensional polymers

A synthetic pathway is described to construct "in bulk" two-dimensional (2D) polymers shaped as molecular sheets. A chiral oligomeric precursor is used that contains two reactive sites, a polymerizable group at one terminus and a reactive stereogenic center near the middle of the molecule. The bulk reaction yields bilayer 2D polymers of molecular weight in the order of millions and a monodisperse thickness of 50.2 angstroms. The 2D molecular objects form through molecular recognition by the oligomers, which self-organize into layers that place the reactive groups within specific planes. The oligomers become catenated by two different stitching reactions involving the reactive sites. At room temperature, stacks of these molecular objects can organize as single crystals and at higher temperatures melt into smectic liquid crystals. Nonlinear optical experiments reveal that solid films containing the 2D polymers form structures that are thermally and temporally more stable than those containing analogous 1D polymers. This observation suggests that the transformation of common polymers from a 1D to a 2D architecture may produce generations of organic materials with improved properties.     

Thin Films of n-Si/Poly-(CH3)3Si-Cyclooctatetraene: Conducting-Polymer Solar Cells and Layered Structures

The optical and electronic properties of thin films of the solution-processible polymer poly-(CH(3))(3)Si-cyclooctatetraene are presented. This conjugated polymer is based on a polyacetylene backbone with (CH(3))(3)Si side groups. Thin transparent films have been cast onto n-doped silicon (n-Si) substrates and doped with iodine to form surfacebarrier solar cells. The devices produce photovoltages that are at the theoretical limit and that are much greater than can be obtained from n-Si contacts with conventional metals. Two methods for forming layered polymeric materials, one involving the spincoating of preformed polymers and the other comprising the sequential polymerization of different monomers, are also described. An organic polymer analog of a metal/insulator/metal capacitor has been constructed with the latter method.     

A simple kinetic model of polymer adsorption and desorption

A model of the desorption and adsorption of a polymer layer at a planar surface indicates a transition from exponential kinetics at high temperatures to nonexponential kinetics (stretched exponential with index one-half) at lower temperatures where these processes are diffusion-limited. Measurements of polystyrene desorption through polyisoprene overlayers show this predicted transition. Corroborative results are obtained for polystyrene desorption through polymethylmethacrylate overlayers. This identification of two distinct kinetic regimes suggests a unifying perspective from which to analyze polymer and biopolymer mobility at surfaces.     

Quantum electronic stability of atomically uniform films

We have studied the structural stability of thin silver films with thicknesses of N = 1 to 15 monolayers, deposited on an Fe(100) substrate. Photoemission spectroscopy results show that films of N = 1, 2, and 5 monolayer thicknesses are structurally stable for temperatures above 800 kelvin, whereas films of other thicknesses are unstable and bifurcate into a film with N +/- 1 monolayer thicknesses at temperatures around 400 kelvin. The results are in agreement with theoretical predictions that consider the electronic energy of the quantum well associated with a particular film thickness as a significant contribution to the film stability.     

露天矿区排土场复垦地土壤容重差异、GPR特征识别与反演

为揭示复垦地不同剖面土壤容重的总体特征和深度差异,以中煤平朔安太堡露天矿南排土场为研究对象,基于探地雷达（GPR）图像特征分析土壤容重差异,构建介电常数与土壤容重之间的关系。分析方法采用环刀采样称重法、探地雷达探测法、方差分析法、拟合分析法等。结果表明：矿区排土场平台土壤容重大于边坡,边坡坡度越大,土壤容重越小。在土壤含水率较小时,各个剖面0~30 cm处有较多大振幅信号,土壤容重较小,探地雷达信号图上大振幅信号的多少与土壤容重大小成反比。土壤容重随介电常数的增大而呈线性增加趋势,且二者拟合后的R²值达0.813 9,相伴概率小于0.01。研究表明,矿区排土场不同位置及同一剖面不同深度土壤容重大小差异明显,通过探地雷达可对土壤容重差异进行分析。     

Mercury-Based Cuprate High-Transition Temperature Grain-Boundary Junctions and SQUIDs Operating Above 110 Kelvin

The superconducting transport characteristics of HgBa(2) CaCu(2)O(6+delta) (Hg-1212) films and grain-boundary junctions grown on (100)-oriented SrTiO(3) bicrystal substrates have been investigated. The films exhibit a zero-resistance temperature of approximately 120 kelvin and sustain large critical current densities, with values as high as 10(6) amperes per square centimeter at around 100 kelvin. On the other hand, the grain boundaries behave as weak links, with substantially lower critical currents, as is observed for other cuprate superconductors. A reduction of three orders of magnitude in critical current was observed for transport across a 36.8 degrees grain boundary. The current-voltage characteristics of bridges across such a grain boundary show weak-link behavior qualitatively resembling that of a resistively shunted junction. Single-level direct-current superconducting quantum interference devices (SQUIDs) have been fabricated with such bicrystal junctions. These SQUIDs show clear periodic voltage modulations when subjected to applied magnetic fields. The SQUIDs operate at temperatures as high as 111.8 kelvin, which makes them attractive for operation in portable sensors and devices that utilize nonconventional cooling methods.     

Epitaxial cubic gadolinium oxide as a dielectric for gallium arsenide passivation

Epitaxial growth of single-crystal gadolinium oxide dielectric thin films on gallium arsenide is reported. The gadolinium oxide film has a cubic structure isomorphic to manganese oxide and is (110)-oriented in single domain on the (100) gallium arsenide surface. The gadolinium oxide film has a dielectric constant of approximately 10, with low leakage current densities of about 10(-9) to 10(-10) amperes per square centimeter at zero bias. Typical breakdown field is 4 megavolts per centimeter for an oxide film 185 angstroms thick and 10 megavolts per centimeter for an oxide 45 angstroms thick. Both accumulation and inversion layers were observed in the gadolinium oxide-gallium arsenide metal oxide semiconductor diodes, using capacitance-voltage measurements. The ability to grow thin single-crystal oxide films on gallium arsenide with a low interfacial density of states has great potential impact on the electronic industry of compound semiconductors.     

硫化温度和时间对SnxSy薄膜结构和成分的影响

用热蒸发技术在玻璃基片上沉积一层sn薄膜,在真空条件下,将其在150～300℃下硫化30．60min．对在不同温度和时间下硫化的薄膜进行结构、成分和表面形貌分析,结果表明：在不同温度和不同时间下硫化,所得到的薄膜在物相结构、成分和表面形貌上都存在差异．当硫化温度为240℃、硫化时间为45min时,所制得的薄膜为正交结构的SnS多晶薄膜,其均匀性、致密性以及对基片的附着力都较好,具有（111）方向优先生长,薄膜粒径在200～800nm,且晶格常数与标样的数值吻合很好．     

Giant piezoelectricity on Si for hyperactive MEMS

Microelectromechanical systems (MEMS) incorporating active piezoelectric layers offer integrated actuation, sensing, and transduction. The broad implementation of such active MEMS has long been constrained by the inability to integrate materials with giant piezoelectric response, such as Pb(Mg(1/3)Nb(2/3))O(3)-PbTiO(3) (PMN-PT). We synthesized high-quality PMN-PT epitaxial thin films on vicinal (001) Si wafers with the use of an epitaxial (001) SrTiO(3) template layer with superior piezoelectric coefficients (e(31,f) = -27 ± 3 coulombs per square meter) and figures of merit for piezoelectric energy-harvesting systems. We have incorporated these heterostructures into microcantilevers that are actuated with extremely low drive voltage due to thin-film piezoelectric properties that rival bulk PMN-PT single crystals. These epitaxial heterostructures exhibit very large electromechanical coupling for ultrasound medical imaging, microfluidic control, mechanical sensing, and energy harvesting.     

Stable ordering in Langmuir-Blodgett films

Defects in the layering of Langmuir-Blodgett (LB) films can be eliminated by depositing from the appropriate monolayer phase at the air-water interface. LB films deposited from the hexagonal phase of cadmium arachidate (CdA2) at pH 7 spontaneously transform into the bulk soap structure, a centrosymmetric bilayer with an orthorhombic herringbone packing. A large wavelength folding mechanism accelerates the conversion between the two structures, leading to a disruption of the desired layering. At pH > 8.5, though it is more difficult to draw LB films, almost perfect layering is obtained due to the inability to convert from the as-deposited structure to the equilibrium one.     

Total alignment of calcite at acidic polydiacetylene films: cooperativity at the organic-inorganic interface

Biological matrices can direct the absolute alignment of inorganic crystals such as calcite. Cooperative effects at an organic-inorganic interface resulted in similar co-alignment of calcite at polymeric Langmuir-Schaefer films of 10,12-pentacosadiynoic acid (p-PDA). The films nucleated calcite at the (012) face, and the crystals were co-aligned with respect to the polymer's conjugated backbone. At the same time, the p-PDA alkyl side chains reorganized to optimize the stereochemical fit to the calcite structure, as visualized by changes in the optical spectrum of the polymer. These results indicate the kinds of interactions that may occur in biological systems where large arrays of crystals are co-aligned.