Photoluminescence resulting from semiconductor-metal solid solution observed in one-dimensional semiconductor nanostructures

A narrow band photoluminescence (PL) emission peak resulting from CdS-Au solid solution was observed when growing one-dimensional nanostructures of CdS via the vapor-liquid-solid mechanism by using Au as the catalyst. This emission peak was located at 680 nm, a wavelength longer than the near band edge emission of CdS at 520 nm, and was shown not to be caused by the usual trap states of CdS which lead to a broad band emission. Here, the one-dimensional nanostructures of CdS were grown in a simple, low-temperature (360 degrees C) metal-organic chemical vapor deposition process with a single source precursor of CdS. Straight nanowires of diameter 50-70 nm and wormlike nanorods of diameter 100-200 nm were obtained. Both the upper and lower portions of the nanorods/nanowires possessed single crystallinity as judged from the corresponding high-resolution transmission electron microscopy images and selected area electron diffraction data. This work demonstrates the feasibility of adjusting PL emission peaks of optoelectronic semiconductors through alloying with metals.     

Synthesis and thermal behavior of side-chain liquid crystalline polymethacrylates containing tolane-based mesogenic side groups

The synthesis and characterization of nine polymethacrylates containing 4-alkoxy-4′-trifluoromethyltolane, 4-alkoxy-4′-cyanotolane, and 4-alkoxy-4′-nitrotolane side groups were described in this study. The phase behavior of the prepared monomers and polymers was characterized by differential scanning calorimetry, optical polarizing microscopy, and x-ray diffraction. All of the obtained monomers exhibit no mesophase, while most of the synthesized polymers reveal enantiotropic mesomorphism. The polymethacrylate containing 4-propanyloxy-4′-nitrotolane side groups was the only one which shows no mesomorphic behavior. Both the spacer length and the nature of terminal groups have profound influence on the phase transition temperatures and thermal stability of the mesophase. The polymers with longer spacers tend to form a more ordered mesophase with a wider temperature range. Among three polymers with the same spacer length, the polymer with a trifluoromethyl terminal end group is inclined to form a more ordered mesophase than the other two polymers. No side chain crystallization occurred for all obtained polymers. © 1994 John Wiley & Sons, Inc.     

Molecular orientation of evaporated pentacene films on gold: alignment effect of self-assembled monolayer

Pentacene films deposited on self-assembled monolayers (SAMs) bearing different terminal functional groups have been studied by reflection-absorption IR, grazing angle XRD, NEXAFS, AFM, and SEM analyses. A film with pentacene molecules nearly perpendicularly oriented was observed on Au surfaces covered with an SAM of alkanethiol derivative of X-(CH2)(n)-SH, with X = -CH(3), -COOH, -OH, -CN, -NH(2), C(60), or an aromatic thiol p-terphenylmethanethiol. On the other hand, a film with the pentacene molecular plane nearly parallel to the substrate surface was found on bare Au surface. A similar molecular orientation was found in thinner ( approximately 5 nm) and thicker (100 nm) deposited films. Films deposited on different surfaces exhibit distinct morphologies: with apparently smaller and rod-shaped grains on clean bare Au surface but larger and islandlike crystals on SAM-modified surfaces. X-ray photoemission electron microscopy (X-PEEM) was used to analyze the orientation of pentacene molecules deposited on a SAM-patterned Au surface. With the micro-NEXAFS spectra and PEEM image analysis, the microarea-selective orientation control on Au was characterized. The ability to control the packing orientation in organic molecular crystals is of great interest in fabricating organic field effect transistors because of the anisotropic nature of charge transport in organic semiconducting materials.     

Electrophoresis of a concentrated spherical dispersion at arbitrary electrical potentials

The electrophoretic behavior of a concentrated spherical dispersion is investigated theoretically. The present analysis extends those in the literature in that both the surface potential of a particle and the strength of the applied electric field are arbitrary and both the effects of double-layer polarization and the overlapping between neighboring double layers are taken into account. Results based on these conditions are highly desirable since they cover essentially all the possible experimental conditions in practice. We show that, for a fixed surface potential and strength of applied electric field, the higher the concentration of particle, the smaller the mobility. Counterions are found to accumulate at the downstream side of a particle. Double-layer polarization is inappreciable if either it is thick or the concentration of the particle is high.     

Preparation and evaluation of a functionalized polymer-coated silica based sorbent for metal ion separation

A silica based sorbent with an anion complexone polymer coating, [24]ane-N₆ macrocycle, was prepared. The chelation properties of this material were investigated by elemental analysis, infrared spectra and Voige’s method. The polymer-coated silica column (25– 40 μm, 100 × 4.6 mm i.d.) was employed for trace metal analyses. Oxalic acid, malonic acid, succinic acid, citric acid, phthalic acid and acetic acid were used as mobile phases. Their retention characteristics were elucidated. Oxalic acid was found to be the most effective eluent. With a mobile phase consisting of oxalic acid (25 mM) and sodium nitrate (25 mM) at pH 4.2, the separation of copper(II), cadmium(II), cobalt(II) and zinc(II) in sea water could be achieved. The identification of metal ions was performed at 510 nm using 4(2-pyridylazo)resorcinol (1 × 10^–4 M) as post column reagent. The limits of detection were 5 × 10^–7 M, 1 × 10^–5 M, 3 × 10^–5 M and 2 × 10^–6 M for copper(II), cadmium(II), cobalt(II) and zinc(II) based on three times the standard deviation of the response for the lowest concentration (n = 5) in the chromatogram with a sample volume of 50 μL. For evaluation of data reliability, oyster tissue (NIST SRM 1566 a) was studied with the proposed system. Received: 9 February 1998 / Revised: 15 May 1998 / Accepted: 16 June 1998     

Control of Wall Thickness and Extraordinarily High Hydrothermal Stability of Nanoporous MCM‐41 Silica

Wall thickness of siliceous MCM‐41 could be controlled systematically up to 36.1 Å. A reasonable model explaining formation of thicker MCM‐41 walls, not enlarging pore channel is proposed on the basis of TGA and ¹³C MAS NMR data of samples. Thermal restructuring process under mild basic condition favors the silica redeposition on silica wall and building up thicker wall. Most mesostructure of calcined MCM‐41 with thicker wall was retained even after hydrothermal treatment in boiling water for 14 days. To our best knowledge, the excellent hydrothermal stability of the MCM‐41 silica reported herein has not been described before and facilitates practical applications of mesoporous molecular sieves in future.     

Electrophoresis of a charge-regulated spheroid along the axis of an uncharged cylindrical pore

Boundary effects can have a profound influence on the electrophoretic behavior of a charged entity, in particular, when the entity is nonspherical and its surface conditions are dependent upon the nearby environment. In this study, the electrophoresis of a spheroid along the axis of an uncharged cylindrical pore is analyzed for the case where the electrical potential is low and the applied electric field is weak. We consider the case where the surface of a particle contains dissociable acidic and basic functional groups, which simulate biological colloids and entities covered by an artificial membrane. This leads to a mixed-type boundary value problem, which extends the conventional constant-surface-potential and constant-surface-charge-density models to a more general case. The effects of the particle aspect ratio, the relative magnitudes of particle and pore, the thickness of the double layer surrounding a particle, and the pH of the liquid phase on the electrophoretic mobility of a particle are investigated. Several interesting results are observed; for example, if the volume of a particle is fixed, its mobility may have a local maximum as the relative magnitudes of its two axes vary.     

Photophoresis of an aerosol sphere normal to a plane wall

A combined analytical-numerical study is presented for the quasisteady photophoretic motion of a spherical aerosol particle of arbitrary thermal conductivity and surface properties exposed to a radiative flux perpendicular to a large plane wall. The Knudsen number is assumed to be so small that the fluid flow is described by a continuum model with a temperature jump, a thermal slip, and a frictional slip at the surface of the radiation-absorbing particle. In the limit of small Peclet and Reynolds numbers, the appropriate equations of conservation of energy and momentum for the system are solved using a boundary collocation method and numerical results for the photophoretic velocity of the particle are obtained for various cases. The presence of the neighboring wall causes two basic effects on the particle velocity: first, the local temperature gradient on the particle surface is enhanced or reduced by the wall, thereby speeding up or slowing down the particle; second, the wall increases viscous retardation of the moving particle. The net effect of the wall can decrease or increase the particle velocity, depending upon the relative conductivity and surface properties of the particle as well as the relative particle-wall separation distance. In general, the boundary effect of a plane wall on the photophoresis of an aerosol particle can be quite significant in some situations. In most aerosol systems, the boundary effect on photophoresis is weaker than that on the motion driven by a gravitational field.     

Study on aqueous polymerizations of vinyl monomers initiated by metal oxidant–chelating agent redox initiators

In this study, a series of chelating type reductants containing redox pairs were tested as the initiator for aqueous polymerizations. The redox pairs consist of Ce(IV) or several first-row transition metals coupled with chelating agents of amino acids, dibasic acids, or diamine. The initial rates and limiting conversions of acrylamide polymerization initiated by those redox pairs were determined. The reductive reactivity of the chelating agents with Ce(IV) and the oxidative half-wave potential of Ce(III)-chelating agent/Ce(IV)-chelating agent were measured to evaluate the feasibility of these redox pairs as initiators. After the evaluation, the redox pairs other than Ce(IV)-amino acid type chelating agent were precluded to be promising initiators for aqueous polymerizations. Those Ce(IV)-amino acid type chelating agent redox pairs which could form at least two five- or six-membered rings were found to be potential initiators. The Ce(IV)-NTA pair was the most promising one. The mechanism of initiation of the redox pairs was proposed and further confirmed by the ¹³C- and ¹H-NMR spectra of NTA-terminated polyacrylamide. The complex formation constants (K) and disproportionation constants (k_d) of the Ce(IV)-amino acid type chelating agent redox initiators for acrylamide polymerization were evaluated. The factors governing the parameters of chelated complexes and the performance of polymerizations were discussed. These redox pairs were also used as the initiators for aqueous polymerizations of acrylic acid and acrylonitrile. © 1993 John Wiley & Sons, Inc.