Electric-field-induced changes in absorption and emission spectra of CdS nanoparticles doped in a polymer film

Electric-field-induced changes in absorption and emission spectra of colloidal CdS nanoparticles ranging in size from 1.0 to 5.0 nm in diameter have been measured by using electric field modulation spectroscopy. The analysis of the electroabsorption spectra indicates that the dipole moment in the first exciton state becomes larger with increasing particle size. The presence of the large dipole moment following photoexcitation into the first exciton band suggests that the CdS nanoparticles have large CT character in the first exciton state. The quantum yields both of the exciton emission and of the trap emission are markedly reduced by application of an electric field. On the basis of the direct measurements of the field-induced change in emission decay profile, it is suggested that the field-induced de-enhancement of these emission yields results from the field-induced decreases both in lifetime and in initial population of each emitting state. It is also found that the emission intensity of CdS nanoparticles increases under the UV light irradiation in air and decreases in a vacuum condition and that fluorescence lifetime in the former case is longer than that in the latter. This enhancement and de-enhancement process in emission intensity is almost reversible at least in several cycles.     

A novel ultrasound-assisted approach to the synthesis of CdSe and CdS nanoparticles

Hexagonal CdSe and hexagonal CdS nanoparticles have been prepared using Cd(Ac)₂ and less hazardous elemental Se or S as precursors, respectively, with the aid of ultrasound irradiation under an atmosphere of H₂/Ar (5/95, V/V). The products consist of 7-10 nm nanocrystallites which aggregated in the form of polydispersive nanoclusters with sizes in the range 30-40 nm in the case of CdSe, and near monodispersive nanoclusters with a mean size of about 40 nm in the case of CdS. X-ray diffraction, high-resolution TEM and SAED patterns (selected area electron diffraction patterns) show that the as-prepared particles are well crystallized. X-ray photoelectron spectroscopy (XPS) measurements further confirm the formation of CdSe and CdS. Diffuse reflection spectra indicate that both the CdSe and the CdS nanocryslallites are direct band-gap semiconductors with band-gap values of about 1.83 and 2.62 eV, respectively. Control experiments demonstrate that the hydrogen is the reducing agent, and the extreme high temperature induced by the collapse of the bubble accelerates the reduction of elemental Se or S by hydrogen. An ultrasound assisted in situ reduction/combination mechanism is proposed.     

A new approach to DSC calibration in wax/polymer blending

DSC analysis of wax/polymer blends is carried out between 270 and 420 K. Calibration for melting point and enthalpy is normally carried out using indium (melting point 430 K), which is unsatisfactory for these materials. IUPAC organic standards covering this range tend to sublime and their onset temperatures are variable. Pure alkanes have similar thermal characteristics to wax/polymer blends and some have been well characterised by adiabatic calorimetry. They are being investigated as alternative secondary calibration standards to give more accurate thermal characterisation of wax/polymer blends. Also,n-triacontane can be used to check DSC resolution.     

Controlled nucleation and growth of CdS nanoparticles in a polymer matrix

In-situ synchrotron X-ray diffraction (XRD) was used to monitor the thermal decomposition (thermolysis) of Cd thiolates precursors embedded in a polymer matrix and the nucleation of CdS nanoparticles. A thiolate precursor/polymer solid foil was heated to 300 degrees C in the X-ray diffraction setup of beamline W1.1 at Hasylab, and the diffraction curves were each recorded at 10 degrees C. At temperatures above 240 degrees C, the precursor decomposition is complete and CdS nanoparticles grow within the polymer matrix forming a nanocomposite with interesting optical properties. The nanoparticle structural properties (size and crystal structure) depend on the annealing temperature. Transmission electron microscopy (TEM) and photoluminescence (PL) analyses were used to characterize the nanoparticles. A possible mechanism driving the structural transformation of the precursor is inferred from the diffraction features arising at the different temperatures.     

A controlled approach to iron oxide nanoparticles functionalization for magnetic polymer brushes

In this article, we report a detailed study of surface modification of magnetite nanoparticles by means of three different grafting agents, functional for the preparation of magnetic polymer brushes. 3-Aminopropyltriethoxysilane (APTES), 3-chloropropyltriethoxysilane (CPTES), and 2-(4-chlorosulfonylphenyl)ethyltrichlorosilane (CTCS) were chosen as grafting models through which a wide range of polymer brushes can be obtained. By means of accurate thermogravimetric analysis a good control over the amount of immobilized molecules is achieved, and optimal operating conditions for each grafting agent are consequently determined. Graft densities ranging from approximately 4 to 7 molecules per nm(2) are obtained, depending on the conditions used. In addition, the surface-initiated atom transfer radical polymerization (ATRP) of methyl methacrylate (MMA) carried out with CTCS-coated nanoparticles is presented as an example of polymer brushes, leading to a well-defined and dense polymeric coating of around 0.6 PMMA chains per nm(2).     

Photosensitization of TiO2 nanoparticulate thin film electrodes by CdS nanoparticles

Surface photovoltage spectra (SPS) measurements of TiO₂ show that a large surface state density is present on the TiO₂ nanoparticles and these surface states can be efficiently decreased by sensitization using CdS nanoparticles as well as by suitable heat treatment. The photoelectrochemical behavior of the bare TiO₂ thin film indicates that the mechanism of photoelectron transport is controlled by the trapping/detrapping properties of surface states within the thin films. The slow photocurrent response upon the illumination can be explained by the trap saturation effect. For a TiO₂ nanoparticulate thin film sensitized using CdS nanoparticles, the slow photocurrent response disappears and the steady-state photocurrent increases drastically, which suggests that photosensitization can decrease the effect of surface states on photocurrent response. Electronic Publication     

Rutile TiO2 nanowires on anatase TiO2 nanofibers: a branched heterostructured photocatalysts via interface-assisted fabrication approach

A water-dichloromethane interface-assisted hydrothermal method was employed to grow rutile TiO(2) nanowires (NWs) on electrospun anatase TiO(2) nanofibers (NFs), using highly reactive TiCl(4) as precursor. The water-dichloromethane interface inhibited the formation of rutile NWs in water phase, but promoted the selective radial growth of densely packed rutile NWs on anatase NFs to form a branched heterojunction. The density and length of rutile NWs could be readily controlled by varying reaction parameters. A formation mechanism for the branched heterojunction was proposed which involved (1) the entrapment of rutile precursor nanoparticles at water-dichloromethane interface, (2) the growth of rutile NWs on anatase NFs via Ostwald ripening through the scavengering of interface-entrapped rutile nanoparticles. The heterojunction formed at anatase NF and rutile NW enhanced the charge separation of both under ultraviolet excitation, as evidenced by photoluminescence and surface photovoltage spectra. The branched TiO(2) heterostructures showed higher photocatalytic activity in degradation of rodamine B dye solution than anatase NFs, and the mixture of anatase NFs, and P25 powders, which was discussed in terms of the synergistic effect of enhanced charge separation by anatase-rutile heterojunction, high activity of rutile NWs, and increased specific area of branched heterostructures.     

Controlled strain on a double-templated textured polymer film: a new approach to patterned surfaces with bravais lattices and chains

A double template-assisted fabrication method for making surface patterns with tunable lattice geometries on a polymer surface is reported. This technique is based on a locally nonuniform strain produced in a double-templated polymer film that has a strong modulation in thickness. It can produce all 2D primitive Bravais lattices as well as chains on the surface of a polymer. The lattice parameters are controllable with nanoprecision by varying the direction and amount of the applied strain.     

A facile and green synthetic approach toward fabrication of starch-stabilized magnetite nanoparticles

A facile and green synthetic approach for fabrication of starch-stabilized magnetite nanoparticles was implemented at moderate temperature. This synthesis involved the use of iron salts, potato starch,sodium hydroxide and deionized water as iron precursors, stabilizer, reducing agent and solvent respectively. The nanoparticles(NPs) were characterized by UV-vis, PXRD, HR-TEM, FESEM, EDX, VSM and FT-IR spectroscopy. The ultrasonic assisted co-precipitation technique provides well formation of highly distributed starch/Fe_3O_4-NPs. Based on UV–vis analysis, the sample showed the characteristic of surface plasmon resonance in the presence of Fe_3O_4-NPs. The PXRD pattern depicted the characteristic of the cubic lattice structure of Fe_3O_4-NPs. HR-TEM analysis showed the good dispersion of NPs with a mean diameter and standard deviation of 10.68 4.207 nm. The d spacing measured from the lattice images were found to be around 0.30 nm and 0.52 nm attributed to the Fe3O4 and starch, respectively. FESEM analysis confirmed the formation of spherical starch/Fe_3O_4-NPs with the emission of elements of C, O and Fe by EDX analysis. The magnetic properties illustrated by VSM analysis indicated that the as synthesized sample has a saturation magnetization and coercivity of 5.30 emu/g and 22.898 G respectively.Additionally, the FTIR analysis confirmed the binding of starch with Fe_3O_4-NPs. This method was cost effective, facile and eco-friendly alternative for preparation of NPs.     

Electrochemical nucleation of gold nanoparticles in a polymer film at a liquid-liquid interface

Simultaneous nucleation of gold nanoparticles and polymerization of tyramine has been carried out at an immiscible electrolyte interface. By transferring the gold ion of tetraoctylammoniumtetracloroaurate (TOAAuCl(4)) from the organic to the aqueous phase, a fast homogeneous electron transfer from the tyramine monomer reduces the gold ion. Electropolymerization then proceeds, and gold nanoparticles form. The newly formed nanoparticles act as nucleation sites for the deposition of the oligomers/polymer (and possibly vice versa). This results in gold nanoparticles stabilized in a polytyramine matrix. The size of the nanoparticles is controlled by the concentration of oligomers/polymer in solution. The polymer nanoparticle composite film was analyzed with TEM, XPS, and AFM.     

A simple CdS nanoparticles cascading approach for boosting N3 dye/ZnO nanoplates DSSCs overall performance

Enhanced photosensitization in presence of CdS nanoparticles is achieved in electrochemically deposited ZnO nanoplates and N3 loaded dye-sensitized solar cells. Chemically embedded CdS nanoparticles act as a sandwiching layer between ZnO nanoplates and dye molecules by overcoming current limiting serious Zn²⁺/dye insulating complex formation and CdS photo-corrosion issues. The X-ray diffraction and the scanning electron microscopy confirm the ZnO with vertically aligned nanoplates, perpendicular to the substrate surface. Amorphous CdS is monitored using electron dispersive X-ray analysis. The low and high resolution transmission electron microscope images confirm the presence of CdS nanoparticles over ZnO nanoplates which later is supported by an increase in optical absorbance and shift in band edge. About 400% increase in solar conversion efficiency with this cascade arrangement is achieved when compared with without CdS which could be fascinating while designing solid state solar cells in presence of suitable p-type layer.     

Direct synthesis of CdS nanoparticles via simple one-pot calcination of new two-dimensional Cd(II) coordination polymer

Assembly of quinoxaline with Cd(II) in the presence of SCN^? anion produces a new coordination polymer [Cd(Quinoxaline)(SCN)₂(CH₃CN)] _n [Cd(Quinoxaline)₂(SCN)₂] _n (1) and is characterized by IR, ¹H NMR and ¹³C NMR spectroscopy. The single-crystal X-ray data of compound 1 show that this coordination polymer grows in two-dimensional supramolecular structure. In this polymer, the organic ligand chelate through its one nitrogen atom to cadmium(II) atom and thiocyanate anion acts as a bridge to two cadmium(II) atom in the chains. Also, nanoscale of CdS has been synthesized by calcination at 400, 500 and 600 °C under air atmosphere and characterized by scanning electron microscopy, X-ray powder diffraction and energy-dispersive X-ray. The thermal stability of compound 1 was studied by thermal gravimetric and differential thermal analyses.     

A novel layer-by-layer approach for the fabrication of conducting polymer/RNA multilayer films for controlled release

Poly(anilineboronic acid) (PABA)/ribonucleic acid (RNA) multilayer films were prepared under neutral condition using a layer-by-layer deposition of PABA and RNA. RNA was used both as a polyelectrolyte for multilayer formation as well as dopant for PABA. Photoelastic modulated infrared reflection absorption spectroscopy measurements suggest that PABA interacts covalently with RNA through the formation of a boronate ester, a boron-nitrogen dative bond, as well as electrostatic interactions of anionic phosphates with cationic amines. The deposition procedure was monitored with UV-vis absorption spectroscopy, showing a linear dependence of absorbance with the number of PABA/RNA bilayers deposited. The multilayer films were further characterized using X-ray photoelectron spectroscopy and ellipsometry, which yielded a PABA/RNA bilayer thickness of approximately 10 nm. The PABA/RNA multilayer films are redox-active at neutral pH, consistent with the formation of a self-doped polymer. Electrochemical control of PABA under these conditions allows potential-induced controlled release of RNA from a multilayer at neutral pH, suggesting that this may serve as a novel method for controlled release of RNA under physiological conditions.     

A new numerical approach to dense polymer brushes and surface instabilities

We present a numerical self-consistent field (SCF) method which describes freely jointed chains of spherical monomers applied to densely grafted polymer brushes. We discuss both the Flory-Huggins model and the Carnahan-Starling equation of state and show the latter being preferable within our model at polymer volume fractions above 10%. We compare the results of our numerical method with data from molecular dynamics (MD) simulations [G.-L. He, H. Merlitz, J.-U. Sommer, and C.-X. Wu, Macromolecules 40, 6721 (2007)] and analytical SCF calculations [P. M. Biesheuvel, W. M. de Vos, and V. M. Amoskov, Macromolecules 41, 6254 (2008)] and obtain close agreement between the density profiles up to high grafting densities. In contrast to prior numerical and analytical studies of densely grafted polymer brushes our method provides detailed information about chain configurations including fluctuation, depletion, and packing effects. Using our model we could study the recently discovered instability of densely grafted polymer brushes with respect to slight variations of individual chain lengths, driven by fluctuation effects [H. Merlitz, G.-L. He, C.-X. Wu, and J.-U. Sommer, Macromolecules 41, 5070 (2008)]. The obtained results are in very close agreement with corresponding MD simulations.     

Anisotropic island growth: a new approach to thin film electrocrystallization

The electrochemical deposition of metals onto foreign substrates usually occurs through Volmer-Weber island growth, and hence the structure and properties of thin films are critically dependent on the mechanism of nucleation and growth. For example, high nucleus densities are essential for achieving island coalescence at small thickness. Here we demonstrate a new approach to controlling thin film microstructure through the control of island geometry. By promoting anisotropic island growth, film coalescence can be achieved at smaller thickness and with lower island densities.     

A new approach to control the dispersion of platinum nanoparticles in a metal oxide coating

The present work shows the feasibility of preparing transparent titania coatings being doped with platinum nanoparticles by sol–gel processing. The used platinum nanoparticles are modified by two different functional thiol ligands, mercaptoethanol and mercaptopropionic acid. The functional ligands are used to create a nanoparticle network and they can also promote anchorage of titanium alkoxides as sol–gel precursors, ensuring a regular distribution of the metal nanoparticles within the coating as well as a good stability to the film.     

A novel approach for Apocynum venetum/cotton blended fabrics modification by cationic polymer nanoparticles

Apocynum venetum/cotton blended fabrics have been subjected to treat with cationic polymer nanoparticles followed by dyeing with Acid Red B,and then studied for their dyeing performance and morphology.The investigation on the effect of modification factors on the blended fabrics indicated that the 0.5 g/L nanoparticles concentration,60 min treating time,60 °C treating temperature and pH 6-8 are the optimum modification process to improve the dyeability of acid dye.In addition,the SEM images show that nanoparticles can be adsorbed on the surface of modified A.venetum or cotton fibers,and the two different fibers could have the same adsorption ability to Acid Red B.     

Fluorescent nanoparticles based on a microporous organic polymer network: fabrication and efficient energy transfer to surface-bound dyes

Well-defined nanoparticles composed of a tetraphenylmethane-based microporous polymer network with an average particle diameter of 30-60 nm were fabricated by a miniemulsion polymerization technique. Strong green emission was observed and efficient excitation energy transfer from nanoparticles to surface-bound dye molecules was explored.     

A new inorganic-organic polymer for the passivation of thin film capacitors

Inorganic-organic polymers with barrier properties against water vapor, excellent electrical data (3, 2, R _D>10¹⁶ cm, E _D up to 400 V/µm) and good adhesion to various substrate materials have been developed. Tailored modifications of these materials provide an excellent protective coating for thin film capacitors. Several mm thick, expensive, encapsulations could be replaced by thin coatings (up to 10 µm). The polymer coating allows the use of thin film chip capacitors in surface-mount technology. As a measure for the efficiency of the coating, the capacitance decrease under controlled humidity has been used. The influence of the material composition, the type of catalyst during sol-gel processing and the curing conditions have been studied. Adhesion and water vapor permeation properties of the polymers and rheological properties of the coating solutions have been investigated. A protective coating is developed, which increases the withstandness of capacitors against humid conditions (90°C, 100% rel. humidity) by a factor of about 30 (compared to uncoated capacitors) and shows no crack formation during thermal cycling.