ITO coated quartz fibers for heat radiative applications

Radiation heat transfer through fibrous materials is very strong at high temperatures (up to 1000 °C). Indium tin oxide (ITO) thin films were sol-gel deposited onto the surfaces of fibers to reduce the radiation heat transfer as radiation reflective coatings. SEM, XRD and FT-IR techniques were used to characterize the microstructure and performance of films. Results show that ITO thin film is uniformly deposited on fibers with a thickness of about 200 nm and can be used to apply a radiative reflective coating. Moreover, the efficiency of radiation reflective properties of films is improved as the annealing temperature increases. Results prove that ITO film is an excellent candidate to reduce the radiation heat transfer as radiation reflective coatings on fibrous materials.     

Geometric optics of arrays of reflective surfaces

We present an analysis of the geometric optics of spherically curved arrays of reflective surfaces. In particular we consider optical devices in which reflective surfaces are arranged on a spherical interface and every ray reflects once from a reflector. The orientation of the reflective surfaces is not necessarily related in any way to the orientation of the interface. The analysis can be applied to any radiation that may specularly reflect from the reflectors. This may be reflection from stacks of mirrors or diffraction from the atomic planes. The principles are applied to x-ray optical systems such as capillary arrays and curved crystals. The calculations are used to find optimum configurations of reflective arrays for applications such as x-ray condensers and telescopes, to find the tolerances to which reflective arrays must be constructed, and to find the conditions in which primary aberrations are eliminated.     

Cover Picture: High‐Performance Organic Single‐Crystal Transistors on Flexible Substrates (Adv. Mater. 17/2006)

The cover shows a comparison of thin and thick rubrene single crystals where the flexibility of the thin rubrene crystals is clearly illustrated. On p. 2320, Yang, Bao, and co‐workers report that high performance flexible transistors on plastic substrates fabricated by using these rubrene “thin‐film” single‐crystals demonstrate mobility as high as 4.6 cm² Vs^–1 and ON/OFF ratios of approximately 10⁶.     

取样光纤布拉格光栅特性的研究

用传输矩阵法从理论上计算了取样光纤布拉格光栅的反射谱特性。这种方法将光栅视为多层均匀薄膜的叠加,利用每一层的传输矩阵相乘获得了光栅的反射谱特性。研究表明,随着光栅长度的增加和采样率、折射率调制深度的减少,反射峰的均匀性得到了改善,旁瓣的反射率变小,带宽明显变窄,而反射峰间隔保持不变。反射峰的间隔由光栅周期决定,与采样率无关,而某些文献则要求采样率小于10%。这与频谱分析所得结论相吻合。     

Reabsorption trapping of luminescence in laser crystals: enhancement of energy storage and upconversion

It is shown that reabsorption of luminescence in laser crystals can enhance energy storage, energy transfer, and upconversion in solid-state laser media. These effects, experimentally observed in Yb-doped and Er-doped crystals, can potentially decrease the threshold for compact cw pumped lasers. The influence of parasitic laser modes and the amplification of spontaneous emission on population inversion in reflective coated laser elements is discussed.     

Morphology oftrans-1, 4-polyisoprene crystallized in thin films

The development of melt-grown crystalline morphology in thin films of trans-1,4-polyisoprene (TPI) was studied by optical and electron microscopy. The crystalline morphologies observed are explained in terms of thin chainfolded lamellar crystals which would grow (if unrestricted) to a diamond shape. The electron diffraction patterns obtained can be indexed using the unit cells proposed by Fisher [3]. The preferred growth faces for the low melting form crystals (LMF) are the (120) planes and probably the (110) planes for the high melting form crystals (HMF). LMF crystals are exclusive to LMF spherulites and HMF crystals are exclusive to HMF spherulites. At large supercoolings both LMF and HMF spherulites nucleate as bundles of lamellar crystals and grow by extensive, twisting, branching and spawning. LMF spherulites grown at small supercoolings develop as hedrites/axialites, or as splayed groups of large crystals differing in orientation with respect to the electron beam. The frequency of twisting, branching and spawning being minimal at these supercoolings. Row nucleated structures are observed in strained films. They consist of long wavy backbone crystals (5 to 25 nm thick) lying in the direction of strain, overgrown by lamellar crystals (5 to 10 nm thick) oriented at right angles to the strain direction. At some temperatures both LMF and HMF crystals nucleate in the same row. The morphologies observed in thin films are compared with and discussed in terms of the observations made of bulk-grown and solution-grown crystals.     

Resolution function normalisation and secondary extinction in neutron triple-axis spectrometry

The theory of resolution correction in triple-axis spectrometry is developed from first principles. It is demonstrated that for ideally imperfect thin crystals the formulation coincides with that introduced initially by Cooper and Nathans and subsequently considered by Dorner. The predicted energy variation of peak Bragg reflectivities of monochromator and analyser crystals in Bragg case scattering is such as to confirm experimental data. In the Laue case to obtain results compatible with experiment one has to invoke theories of secondary extinction. In an attempt to accommodate these observations a new finite threshold model of secondary extinction is proposed which interpolates thin crystals formulas and conventional secondary extinction formulas obtained in the zero threshold limit.     

Some proposals for the application of liquid crystals to photoelastic instrumentation

Liquid crystal displays (L.C.D.'s) are well known for their application to electronic watches, calculators, games and digital thermometers, the technology involved being derived from electro-optic effects observed in liquid crystals since about 1968. This paper describes proposals for a totally different application of one particular class of liquid crystal, as an optical element in a photoelastic transmission or reflective polariscope.
The proposals are currently under development* and considerable work remains before this particular application of liquid crystals is perfected. Early results are promising and if successful they could well produce a revolution in the design of optical equipment with particular attraction to automatic systems incorporating microcomputer control.     

Deep-ultraviolet Raman microspectroscopy: characterization of wide-gap semiconductors

We have developed a high-throughput deep-ultraviolet (DUV) Raman microspectrometer with excitation from a continuous wave (cw) laser operated at 244 nm that enables us to characterize thin surface layers of wide-gap semiconductors. This spectrometer system consists of a filter spectrometer for the rejection of stray light and a high-dispersion spectrograph combined with a liquid nitrogen cooled charge-coupled device (CCD) detector and extends the low-frequency limit of the observable spectral range down to 170 cm(-1). In the microscope we use a Cassegrain reflective objective for the collection of the scattered light and an off-axis mirror for introduction of the excitation laser light. DUV Raman spectroscopy has been applied for studying wide-gap semiconductors including SiC and AlGaN epitaxial films and shallow implanted layers of these materials. Raman spectra of various crystals have also been measured for examining the performance of this system. Resonance enhancement of Raman bands has been observed for several semiconductors, and the results are discussed.     

Directed Self‐Assembly of Liquid‐Crystalline Molecular Building Blocks for Sub‐5 nm Nanopatterning

The thin‐film directed self‐assembly of molecular building blocks into oriented nanostructure arrays enables next‐generation lithography at the sub‐5 nm scale. Currently, the fabrication of inorganic arrays from molecular building blocks is restricted by the limited long‐range order and orientation of the materials, as well as suitable methodologies for creating lithographic templates at sub‐5 nm dimensions. In recent years, higher‐order liquid crystals have emerged as functional thin films for organic electronics, nanoporous membranes, and templated synthesis, which provide opportunities for their use as lithographic templates. By choosing examples from these fields, recent progress toward the design of molecular building blocks is highlighted, with an emphasis on liquid crystals, to access sub‐5 nm features, their directed self‐assembly into oriented thin films, and, importantly, the fabrication of inorganic arrays. Finally, future challenges regarding sub‐5 nm patterning with liquid crystals are discussed.     

Homogeneous Si0.5Ge0.5 bulk crystal growth as substrates for strained Ge thin films by the traveling liquidus-zone method

Compositionally uniform Si_0.5Ge_0.5 bulk crystals were grown by the traveling liquidus-zone method which we developed for alloy crystal growth. Grown crystals were characterized as substrates for compressive-strained Ge thin films for high mobility p-channels of complementary metal oxide semiconductor transistors. Compositional uniformity was excellent and crystallinity was also excellent for 10 mm diameter crystals. However, crystallinity was degraded for 30 mm diameter crystals although compositional uniformity was excellent. Transmission electron microscope (TEM) observation showed high dislocation density at the interface between a Si seed and a grown crystal due to lattice mismatch. However, the dislocation density decreased as crystal growth proceeded. High quality 30 mm diameter crystals will be grown when the single crystal length is extended judging from TEM results. In this paper, we report on the growth and characterization of Si_0.5Ge_0.5 crystals as substrates for strained Ge thin films.     

Applications of multidirectional asymmetrical microlens-array light-control films on reflective liquid-crystal displays for image quality enhancement

The multidirectional asymmetrical microlens-array light-control film (MAMA-LCF) is developed for enhancing the image brightness and contrast ratio of various reflective liquid-crystal displays. By use of index-matching material, the interface reflection is greatly reduced. Through optimized designs, the surface-scattering effect is also suppressed; thus the contrast ratio is much enhanced. From experimental results, the MAMA-LCF leads to a approximately 1.5 x gain in brightness over the MgO standard white and a 15:1 contrast ratio for the reflective color super-twist nematic liquid-crystal display, 2.8 x MgO and a 23:1 contrast ratio for the polymer-dispersed liquid-crystal, and 2.8 x MgO and a 13:1 contrast ratio for the cholesteric liquid-crystal display. Potential applications of this low-cost plastic thin film for reflective liquid-crystal displays are foreseeable.     

The effects of oxygen partial pressure on the acoustic velocity in zirconia films studied by picosecond ultrasonics

We have performed a picosecond ultrasonic study of the effects of oxygen partial pressure on the acoustic velocity in zirconium oxide films made by radio frequency reactive sputtering. To derive more accurate and reliable values of sound velocity, various reflective layers were used to enhance the echo signals of acoustic pulses photoexcited by an ultrafast laser. It is found that the acoustic responses of the samples with a W reflective layer are much stronger than those with a Si or SiO₂ reflective layer. In addition, the W reflective layer generates extra photoacoustic waves, which can be utilized to improve the accuracy of velocity measurement. The thin film velocities we measured were 10∼24% less than the bulk value, and exhibited a strong dependence on the growth conditions and the microstructure of films.     

Extended-chain polydiacetylene crystals

The results of a preliminary study of the structure and morphology of a 100% crystalline polydiacetylene (pTS) have been presented. The crystals of the polymer have been prepared by solution crystallization of the monomer and subsequent thermal polymerization. They have been obtained in various morphological forms which include thin films, thin lamellae and fibrillar sections cleaved from macroscopic crystals. These various morphologies all have the polymer molecules in a chain-extended conformation and the polymer molecules lie in the plane of the thin crystals or sections. These morphologies are compared with the morphologies of conventional chain-folded semicrystalline polymers and they offer a unique opportunity to study new aspects of the structure and deformation of polymer crystals.     

Carbon nanotube dispersion in nematic liquid crystals: An overview

The aim of present review article is to present a comprehensive and current overview of scientific advancement in liquid crystal and carbon nanotube suspension. Particular attention has been paid to the recent developments and fundamental understanding of carbon nanotube dispersion in nematic liquid crystals. The dispersion and interaction of carbon nanotube in liquid crystal matrices and more elaborately the effect of dispersion on the properties of liquid crystalline materials has been extensively discussed. Recent progress has shown that even a very minute concentration of carbon nanotube in liquid crystals can have a reflective impact on the electrical and optical properties of liquid crystals. Liquid crystals provide a distinctive environment for controlling the alignment of carbon nanotubes whereas carbon nanotubes are important for the enhancement and fine-tuning of liquid crystalline properties. Potential applications of liquid crystal and carbon nanotube suspension are briefly discussed. Conclusion and future perspectives of this rapidly emerging field is provided at the end.     

Relative measurements of second-order susceptibility with reflective second-harmonic generation

There is a strong demand for a simple and reliable technique for second-order susceptibility measurements of thin films. Since the Maker fringe technique is limited to transparent substrates we propose an experimental protocol based on reflective second-harmonic generation (SHG). The proposed protocol is based on relative measurements of Z-cut quartz. An analytical expression of the reflective SHG intensity dependence of the polarizer, analyzer, and sample azimuth is presented. An error analysis is also presented. Thin organic film of the side-chain polymer poly(Disperse Red 1 Methacrylate-Co-Methyl-Methacrylate) is investigated. Results for different wavelengths are reported.     

Organic light-emitting diode microcavities from transparent conducting metal oxide photonic crystals

We report herein on the integration of novel transparent and conducting one-dimensional photonic crystals that consist of periodically alternating layers of spin-coated antimony-doped tin oxide nanoparticles and sputtered tin-doped indium oxide into organic light emitting diode (OLED) microcavities. The large refractive index contrast between the layers due the porosity of the nanoparticle layer led to facile fabrication of dielectric mirrors with intense and broadband reflectivity from structures consisting of only five bilayers. Because our photonic crystals are easily amenable to large scale OLED fabrication and simultaneously selectively reflective as well as electronically conductive, such materials are ideally suited for integration into OLED microcavities. In such a device, the photonic crystal, which represents a direct drop-in replacement for typical ITO anodes, is capable of serving two necessary functions: (i) as one partially reflecting mirror of the optical microcavity; and (ii) as the anode of the diode.     

Fringe field switching of a twisted nematic liquid crystal device for a single-cell-gap transflective display

We propose an optical configuration of a twisted nematic liquid crystal device driven by a fringe field for a single-cell-gap transflective display. The dark state of the reflective part is realized by a nematic liquid crystal layer with a twist angle of 63.6 degrees and retardation of 194 nm, while a quarter-wave plate is inserted for the dark state of the transmissive part. Wavelength dispersion of the liquid crystal layer is suppressed by introducing a half-wave plate. Different directions of electric fields rotate liquid crystals to 15 degrees for the bright state of the reflective part, but to -30 degrees for that of the transmissive part. With the proposed configuration, we can realize a high-brightness single-gamma transflective display in a single-cell-gap structure without any in-cell retardation layers.     

Mica, a potential two-dimensional-crystal gate insulator for organic field-effect transistors

2D mica crystals (with thickness < 100 nm) obtained by mechanical exfoliation are incorporated for the first time into the design of organic thin film field-effect transistor arrays and organic single crystal transistors as a gate insulator. The size of mica crystals could be up to the dimensions of an A4 piece of paper. All devices so fabricated exhibited high mobility and low operating voltage, indicating the high quality of the crystals and the great potential of mica crystals as a flexible, low-cost, transparent insulator for organic electronics.     

Simulation of growth dynamics in atomic layer deposition. Part III. Polycrystalline films from tetragonal crystallites

Materials with one- and two-dimensional crystal structures often form crystals with the shape of needles and platelets, respectively. This should be expected to have great influence on the properties of thin films grown from such materials, and the effect on texture and topography is shown in this paper by a series of simulations. The topic of this paper is the effect of different aspect ratios of crystals with tetragonal symmetry on the resulting thin films. However the major results should also apply to crystals of other types of symmetry having aspect ratios deviating from one. The growth dynamics in atomic layer deposition of polycrystalline thin films have been simulated for randomly oriented and randomly positioned crystallites. Crystalline seed objects adapting shapes of tetragonal boxes with aspect ratios from 0.1 (platelets) to 10 (needles) has been used as examples for growth of films from tetragonal crystals. The dependence of roughness, surface crystal density, and texture on the film thickness is shown. Topography and cross sections of simulated films with different aspect ratios are discussed. Non-linear growth regimes are consistently found in the initial stage of the film formation. A conversion from type-2 to type-1 substrate inhibited growth is observed as the aspect ratio deviates significantly from one.