Laser ablation and the production of polymer films

The formation of high-quality thin films of polytetrafluoroethylene (PTFE) is important in many applications ranging from material reinforcement to molecular electronics. Laser ablation, a technique widely used to deposit a variety of inorganic materials, can also be used as a simple and highly versatile method for forming thin polymer films. The data presented show that PTFE films can be produced on various supports by the evaporation of a solid PTFE target with a pulsed ultraviolet laser. The composition of the ablation plume suggests that PTFE ablation and subsequent film formation occur by way of a laser-induced pyrolitic decomposition with subsequent repolymerization. The polymer films produced by this method are composed of amorphous and highly crystalline regions, the latter being predominantly in a chain-folded configuration with the molecular axis aligned parallel to the substrate surface.     

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.     

Synthesis of reverse osmosis membranes by plasma polymerization of allylamine

The polymerization of allylamine in a radio-frequency electrodeless plasma to form thin polymer films on microporous filter media provides very effective dry composite membranes for reverse osmosis. Salt and urea rejections as high as 98 percent and 46 percent, respectively, have been achieved from a solution containing 10 grams of sodium chloride per liter and 10 grams of urea per liter. The plasma conditions and polymerization technique are discussed.     

Polymer gate dielectric surface viscoelasticity modulates pentacene transistor performance

Nanoscopically confined polymer films are known to exhibit substantially depressed glass transition temperatures (Lg's) as compared to the corresponding bulk materials. We report here that pentacene thin films grown on polymer gate dielectrics at temperatures well below their bulk Tg's exhibit distinctive and abrupt morphological and microstructural transitions and thin-film transistor (TFT) performance discontinuities at well-defined growth temperatures. The changes reflect the higher chain mobility of the dielectric in its rubbery state and are independent of dielectric film thickness. Optimization of organic TFT performance must recognize this fundamental buried interface viscoelasticity effect, which is detectable in the current-voltage response.     

Microstructure of Polymers by Tritium Autoradiography

Polystyrene, polypropylene, and polyethylene, containing tritiumlabeled additives and crystallized in thin films, were examined with autoradiographic stripping film. In the structures formed by isotactic polystyrene, tritiated atactic polymer concentrated in specific patterns in the outer regions. Polypropylene spherulites showed marked differences in the distribution of the tritiated additive, dilauryl thiodipropionate, whenever their optical properties also differed. Autoradiographs of polyethylene spherulites containing low molecular weight tritiated polyethylene exhibited concentric ring patterns similar to those observed on viewing the polymer film in a polarizing microscope.     

In situ interfacial mass detection with piezoelectric transducers

The converse piezoelectric effect, in which an electric field applied across a piezoelectric material induces a stress in that material, has spurred many recent developments in mass measurement techniques. These methods commonly rely on the changes in the vibrational resonant frequency of piezoelectric quartz oscillators that result from changes in mass on the surface of the oscillator. The dependence of frequency on mass has been exploited extensively for mass measurements in vacuum or gas phase, for example, thickness monitors for thin-film preparation and sensors for chemical agents. Advances in piezoelectric methodology in the last decade now allow dynamic measurements of minute mass changes (< 10(-9) grams per square centimeter) at surfaces, thin films, and electrode interfaces in liquid media as well. Mass measurements associated with a diverse collection of interfacial processes can be readily performed, including chemical and biological sensors, reactions catalyzed by enzymes immobilized on surfaces, electron transfer at and ion exchange in thin polymer films, and doping reactions of conducting polymers.     

退火对掺铟氧化锌薄膜结构及光学性能的影响

采用溶胶-凝胶法在石英衬底上制备掺铟氧化锌薄膜,研究不同退火温度对薄膜结构及发光性能的影响.结果表明,掺铟氧化锌薄膜仍为六角纤锌矿结构的ZnO相,在大于450 nm的波段薄膜样品的透射率都较高;随着退火温度的升高,透射率先增后减,600℃时达到最大;薄膜样品的光学带隙都小于纯ZnO的理论值(3.37 eV),且随退火温度的升高呈先减后增趋势;样品的结晶度与发光强度随着退火温度的升高而增强.     

Transferable GaN layers grown on ZnO-coated graphene layers for optoelectronic devices

We fabricated transferable gallium nitride (GaN) thin films and light-emitting diodes (LEDs) using graphene-layered sheets. Heteroepitaxial nitride thin films were grown on graphene layers by using high-density, vertically aligned zinc oxide nanowalls as an intermediate layer. The nitride thin films on graphene layers show excellent optical characteristics at room temperature, such as stimulated emission. As one of the examples for device applications, LEDs that emit strong electroluminescence emission under room illumination were fabricated. Furthermore, the layered structure of a graphene substrate made it possible to easily transfer GaN thin films and GaN-based LEDs onto foreign substrates such as glass, metal, or plastic.     

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.     

Two-dimensional electronic excitations in self-assembled conjugated polymer nanocrystals

Several spectroscopic methods were applied to study the characteristic properties of the electronic excitations in thin films of regioregular and regiorandom polythiophene polymers. In the regioregular polymers, which form two-dimensional lamellar structures, increased interchain coupling strongly influences the traditional one-dimensional electronic properties of the polymer chains. The photogenerated charge excitations (polarons) show two-dimensional delocalization that results in a relatively small polaronic energy, multiple absorption bands in the gap where the lowest energy band becomes dominant, and associated infrared active vibrations with reverse absorption bands caused by electron-vibration interferences. The relatively weak absorption bands of the delocalized polaron in the visible and near-infrared spectral ranges may help to achieve laser action in nanocrystalline polymer devices using current injection.     

可见光响应氮杂化TiO2-N薄膜对水源水体的杀菌作用

以尿素为氮源,采用Sol-gel法及浸渍-提拉法制备氮杂化TiO2-N薄膜,并运用正交试验法优化TiO2-N薄膜的最佳制备条件;利用AFM分析技术对TiO2-N薄膜表面形貌进行表征;在太阳光光催化反应器中,探讨TiO2-N薄膜对饮用水供水水源水体中大肠菌群的杀菌作用。结果表明,热处理条件是影响TiO2-N薄膜制备的关键因素,各个影响因素的主次顺序为热处理温度〉停留温度〉热处理时间,在最佳条件下制备的TiO2-N薄膜,在太阳光作用下光解30min时,对饮用水供水水源水体中大肠菌群的杀菌率达到57.7%以上;Ag的协同掺杂（Ag-TiO2-N）或在TiO2-N薄膜表面的沉积（Ag/TiO2-N）,显著提高了TiO2-N薄膜的杀菌作用,对大肠菌群的杀菌率可达90%以上。     

均质和非均质保鲜膜的MA机理与数学模型研究

围绕聚烯烃基材料聚合物薄膜的透气、透湿性能,重点研究了适于果蔬MA保鲜的均质膜和非均质膜的透气、透湿规律和机理,建立了相关数学模型。     

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．     

Mussel-inspired surface chemistry for multifunctional coatings

We report a method to form multifunctional polymer coatings through simple dip-coating of objects in an aqueous solution of dopamine. Inspired by the composition of adhesive proteins in mussels, we used dopamine self-polymerization to form thin, surface-adherent polydopamine films onto a wide range of inorganic and organic materials, including noble metals, oxides, polymers, semiconductors, and ceramics. Secondary reactions can be used to create a variety of ad-layers, including self-assembled monolayers through deposition of long-chain molecular building blocks, metal films by electroless metallization, and bioinert and bioactive surfaces via grafting of macromolecules.     

Self-organized discotic liquid crystals for high-efficiency organic photovoltaics

Self-organization of liquid crystalline and crystalline-conjugated materials has been used to create, directly from solution, thin films with structures optimized for use in photodiodes. The discotic liquid crystal hexa-peri-hexabenzocoronene was used in combination with a perylene dye to produce thin films with vertically segregated perylene and hexabenzocoronene, with large interfacial surface area. When incorporated into diode structures, these films show photovoltaic response with external quantum efficiencies of more than 34 percent near 490 nanometers. These efficiencies result from efficient photoinduced charge transfer between the hexabenzocoronene and perylene, as well as from effective transport of charges through vertically segregated perylene and hexabenzocoronene pi systems. This development demonstrates that complex structures can be engineered from novel materials by means of simple solution-processing steps and may enable inexpensive, high-performance, thin-film photovoltaic technology.     

Using selective withdrawal to coat microparticles

We report a method that uses the process of selective withdrawal of one fluid through a second immiscible fluid to coat small particles with polymer films. Fluid is withdrawn through a tube with its orifice slightly above a water-oil interface. Upon increasing the flow rate, there is a transition from a state where only oil is withdrawn to a state where the water, containing the particles to be coated and appropriate prepolymer reagents, is entrained in a thin spout along with the oil. The entrained particles eventually cause the spout interface to break, producing a thin coat of controllable thickness around each particle, which can be subsequently polymerized using chemical reagents, light, or heat. This method allows flexibility in the chemical composition and thickness of the conformal coatings.     

Ferroelectric Bi3.25La0.75Ti3O12 films of uniform a-axis orientation on silicon substrates

The use of bismuth-layered perovskite films for planar-type nonvolatile ferroelectric random-access memories requires films with spontaneous polarization normal to the plane of growth. Epitaxially twinned a axis-oriented La-substituted Bi4Ti3O12 (BLT) thin films whose spontaneous polarization is entirely along the film normal were grown by pulsed laser deposition on yttria-stabilized zirconia-buffered Si(100) substrates using SrRuO3 as bottom electrodes. Even though the (118) orientation competes with the (100) orientation, epitaxial films with almost pure (100) orientation were grown using very thin, strained SrRuO3 electrode layers and kinetic growth conditions, including high growth rates and high oxygen background pressures to facilitate oxygen incorporation into the growing film. Films with the a-axis orientation and having their polarization entirely along the direction normal to the film plane can achieve a remanent polarization of 32 microcoulombs per square centimeter.     

退火对非晶态碲镉汞薄膜稳定态光电导的影响

利用射频磁控溅射方法,在宝石衬底上制备了非晶态碲镉汞(a-HgCdTe)薄膜。对原生a-HgCdTe薄膜进行了不同退火时间和不同退火温度的热退火,在80～300K温度范围内,分别测量了原生和退火处理后的a-HgCdTe薄膜样品的稳定态光电导,研究了退火时间和退火温度对非晶态HgCdTe薄膜的稳定态光电导和激活能的影响。结果表明,原生和退火a-HgCdTe薄膜的稳定态光电导具有热激活特性;随着退火时间增加或退火温度升高,a-HgCdTe薄膜的晶化程度提高,导致光电导增大,光电导激活能降低。利用非晶-多晶转变机制讨论了实验结果。     

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.     

Chain pullout and mobility effects in friction and lubrication

The interfacial shear stress that occurs when a network of a polymer that is highly mobile at the segment level (an elastomer) is slid over a smooth surface of an immobile (glassy) polymer has been measured. The glassy material is covered by a thin layer of end-attached chains of the mobile material. The experiment was designed so that there were no free chains at the interface; the slip occurred between network chains on the one side and rigid material plus end-attached mobile chains on the other side. Two main results were obtained. (i) The interfacial shear stress is strongly affected by the segment mobility of the materials on both sides of the slip plane, and considerably lower stress is observed when the materials on both sides of the interface are highly mobile. (ii) Very thin layers of tethered chains can increase the interfacial friction. Both results are relevant to the understanding of a number of practical situations that range from the operation of thin layers of lubricants, such as those found in magnetic storage devices, to the problem of wall slip and melt fracture in polymer processing.