Stimulated emission from distyrylbenzene derivative crystals grown by vapor

Narrowed spectra at 452 nm from a thin platelike crystal of distyrylbenzene derivative, 2,5-diphenyl-1,4-distyrylbenzene with two trans double bonds (trans-DPDSB) grown by vapor deposition, are observed. The trans-DPDSB crystal is irradiated by the third harmonic (355 nm) of a Nd:YAG laser. The FWHM of the narrowed spectra can reach 6 nm for the crystal when the pumping energy is 400 mu J/pulse. The threshold value for an optically pumped laser is approximately 350 mu J/pulse.

Tethered diblock copolymer chains on platelets prepared by semicrystalline ABC triblock copolymers in toluene with trace amounts of water

Lamellar platelets of triblock copolymers grown in dilute toluene solution with trace amounts of water can be used as templates for tethered diblock copolymer chain preparation and analysis. Polystyrene-bpoly(2-vinylpyridine)-b-poly(ethylene oxide) (PS-b-P2VP-b-PEO) with two different block fractions were used as model templates to generate tethered P2VP-b-PS chains on the platelet basal surfaces. In toluene solution the aggregation states of PS-b-P2VP-b-PEO were sensitive to the water content in the solution. For toluene with trace amount of water, spherical micelles were formed in the early stage and large square platelets would gradually grow from these spherical micelles. The hydrogen bonding between water and EO units was responsible for the formation of micelles and subsequent square platelets in the solution. Tethered P2VP-b-PS chains on basal surface of PEO platelets could be regarded as diblock copolymer brushes and the density (or: 0.086-0.36) and height (d: 3.5-14.3 nm) of these tethered chains could be easily modulated by changing the crystallization condition and/ or the molecular weight of each block. The tethered P2VP-b-PS chains were responsive to different solvent vapor.

Self-assembly morphology effects on the crystallization of semicrystalline block copolymer thin film

Self-assembly morphology effects on the crystalline behavior of asymmetric semicrystalline block copolymer polystyrene-block-poly(L-lactic acid) thin film were investigated. Firstly, a series of distinctive self-assembly aggregates, from spherical to ellipsoid and rhombic lamellar micelles (two different kinds of rhombic micelles, defined as rhomb 1 and rhomb 2) was prepared by means of promoting the solvent selectivity. Then, the effects of these self-assembly aggregates on crystallization at the early stage of film evolution were investigated by in situ hot stage atomic force microscopy. Heterogeneous nucleation initiated from the spherical micelles and dendrites with flat on crystals appeared with increasing temperature. At high temperature, protruding structures were observed due to the thickening of the flat-on crystals and finally more thermodynamically stable crystallization formed. Annealing the rhombic lamellar micelles resulted in different phenomena. Turtle-shell-like crystalline structure initiated from the periphery of the rhombic micelle 1 and spread over the whole film surface in the presence of mostly noncrystalline domain interior. Erosion and small hole appeared at the surface of the rhombic lamellar micelle 2; no crystallization like that in rhomb 1 occurred. It indicated that the chain-folding degree was different in these two micelles, which resulted in different annealing behaviors.

Colloidal photonic crystals with a graded lattice-constant distribution

In this work, a gradient polystyrene colloidal photonic crystal was fabricated by annealing in a graded temperature field. The lattice constant of the gradient crystal gradually varied along the temperature-gradient direction. The positional bandgap wavelength as well as the attenuation of the bandgap wavelength could be tuned dependent on the position of the gradient colloidal crystal along the gradient direction because of the lattice-constant variation.

Competition of lamellar orientation in thin films of a symmetric poly(styrene)-b-poly(L-lactide)diblock copolymer in melt state

We have studied the lamellar orientation in thin films of a model diblock copolymer, symmetric poly(styrene)-b-poly(L-lactide) (PS-PLLA), in the melt state on supported silicon wafer surface. In this system, while the PLLA block prefers to wet the polymer/substrate interface, the polymer/air as well as polymer/polymer interface is neutral for both blocks due to the similar surface energies of PS and PLLA in melt state. Our results demonstrate that the interplay of the interfaces during phase separation results in a series of structures before approaching the equilibrium state. Lamellar orientation of thin films with different initial film thicknesses at different annealing stages has been investigated using atomic force microscopy (AFM), transmission electronic microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). It is found that in the early stage (annealing time t < 10 min), the polymer/substrate interface dominates the structure evolution, leading to a parallel lamellar structure with holes or islands formed depending on the initial film thickness. Later on, the neutral air interface becomes important and leads to a transition of lamellar orientation from parallel to perpendicular. It is interesting to see that for films with thickness h > 2L, where L is the bulk lamellar period, the lamellar orientation transition can occur independently in different parallel lamellar domains due to the neutrality of polymer/polymer interface.

Conducting probe atomic force microscopy investigation of anisotropic charge transport in solution cast PBD single crystals induced by an external field

2-(4-Biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxdiazole (PBD) is a good electron-transporting material and can form single crystals from solution. In this work, solution cast PBD single crystals with different crystallographic axes (b, c) perpendicular to the Au/S substrates in large area are achieved by controlling the rate of solvent evaporation in the presence and absence of external electrostatic field, respectively. The orientation of these single crystals on Au/S substrate was characterized by transmission electron microscopy (TEM) and atomic force microscopy (AFM). Conducting probe atomic force microscopy (CP-AFM) was used to measure the charge transport characteristics of PBD single crystals grown on Au/S substrates. Transport was measured perpendicular to the substrate between the CP-AFM tip and the Au/S substrate. The electron mobility of 3 x 10(-3) cm(2)/(V s) for PBD single crystal along crystallographic b-axis is determined. And the electron mobility of PBD single crystal along the c-axis is about 2 orders of magnitude higher than that along the b-axis due to the anisotropic charge transport at the low voltage region.

Reversibly strain-tunable elastomeric photonic crystals

Reversibly strain-tunable polymeric photonic crystals made of thermoplastic polyester elastomer (TPEE) were fabricated by using the self-assembled silica opals as templates. The stop band of the polymeric photonic crystal locates at the near infrared (IR) regime in its transmission spectrum, and exhibits a blue shift with the increase of the incident angle. Because of the elasticity of the TPEE, the stop band of the TPEE photonic crystal can also be reversibly tuned at ambient temperature through to and fro uniaxially or biaxially stretching and recovering by changing the lattice spacing and the symmetry of the crystal along (1 1 1) plane.

Vaterite-type YBO3 : Eu3+ crystals: hydrothermal synthesis, morphology and photoluminescence properties

Vaterite-type YBO3:Eu3+ crystals with interesting flower and hedgehog fungus-like structures composed of nanosheets were obtained by controlled crystallization of Y2O3 and Eu2O3 in H3BO3 solutions under acidic hydrothermal (HT) conditions. Nanosheets of uniform thicknesses were formed by preferential crystal growth along the (100) crystallographic plane and specific three-dimensional structures were further developed through a homocentric growth mechanism. Optical emission measurements showed that the HT-grown nanosheet crystals exhibited a higher ratio of the emitted red-to-orange light ratio than crystals grown from solid-state reactions. The photoluminescence intensity and emission lifetimes were also studied as a function of the Eu3+ dopant concentration and the HT synthesis temperature. The effect of some additives: a chelating ligand, a surfactant and a polymer, on the YBO3:Eu3+ crystals morphology was also investigated.

Grafting polymerization of L-lactide on the surface of hydroxyapatite nano-crystals

A novel method of grafting ring-opening polymerization of L-lactide (LLA) onto the surface of hydroxyapatite nano-particles (n-HAP) was developed. PLLA was directly connected onto the HAP surface through a chemical linkage. The PLLA-g-HAP particles could be stably dispersed in organic solvent such as chloroform for several weeks. The n-HAP particles still retained the original dimension and shape after the grafting of PLLA. Compared with the P-31 MAS-NMR spectrum of pure HAP powders, there appeared a downfield displacement of 1.2 ppm in the spectrum of PLLA-g-HAP. Fourier transformation infrared (FT-IR) spectra further confirmed the existence of PLLA on the surface of PLLA-g-HAP. The amount of grafted polymer determined by thermal gravimetric analysis (TGA) was about 6% in weight. The tensile strength and elongation at break of the PLLA/PLLA-g-HAP composite containing 8 wt% of PLLA-g-HAP were 55 MPa and about 10-13%, respectively, while those of the PLLA/n-HAP composites were 40 MPa and 3-5%, respectively.

Barycenter of energy of lanthanide 4f(N)-(1)5d configuration in inorganic crystals

A semiempirical method for the evaluation of the barycenter of energy of 4f(N-1)5d configurations is presented. The environmental factors affecting the barycenter are given to be the bond volume polarization, fractional covalence of the chemical bond between the central ion and the nearest anion, and presented charge of the nearest anion in the chemical bonds. The barycenter energies of 4f(N-1)5d configurations of Eu2+ and Ce3+ are calculated in various crystals, and the results are in good agreement with the experimental values. A relationship is found between the barycenter of energy of the 4f(N-1)5d configuration on Eu2+ method offers the advantage of applicability to a broad class of luminescence materials and initiates a link between macroscopic properties and microscopic structure.

Solvent-induced crystallization of spherical micelles formed by copolymer blends

We have followed the morphological evolution and crystallization process of spherical micelles formed by the mixture of polystyrene-b-poly(acrylic acid) (PS-b-PAA) and polystyrene-b-poly(2-vinylpyridine)b-poly(ethylene oxide) (PS-b-P2VP-b-PEO) (the core of the spherical micelles was made of P2VP and PAA blocks through hydrogen bonding in neutral solvent N,N-dimethylformamide, DMF) via DMF vapor treatment. Different phenomena, such as rupture of the film, formation of cylinder aggregates and regular square lamellae, were observed when the micelle film was treated in DMF for different times. At the early stage of annealing in DMF vapor, the micelle film became unstable and ruptured. Cylinder aggregates, within which the PEO blocks achieved the association and primary chain folding, formed as the mesophases before the nucleation of the PEO single crystals at this stage. Further treatment in DMF vapor resulted in the nucleation of the PEO blocks at the corners of quasi-square lamellae. Then a quite regular "sandwich" lamellar structure, constructed by a PEO single-crystal layer covered by two tethered layers of other amorphous blocks on the top and bottom crystal basal surfaces, formed when the film of micelles was annealed in DMF vapor for sufficient times.

Optical intensity gradient by colloidal photonic crystals with a graded thickness distribution

Gradient colloidal crystals with a thickness gradient were prepared by the vertical deposition technique with vertically graded concentration suspensions. The thickness of the gradient colloidal crystal gradually changes at different positions along the specific gradient direction of the crystal. The thickness gradient was determined by the concentration gradient, depending on the initial colloidal concentration and the settling time. The optical transmission intensity at the dip wavelength can be tuned by changing the thickness of the colloidal crystals. The gradient colloidal crystals lead to a gradient of optical intensity at the dip in transmission light. The gradient of optical intensity at the dip increases as the thickness gradient of the colloidal crystal increases.

Large-scale synthesis of Berlin green Fe[Fe(CN)(6)] microcubic crystals

Berlin green FeFe(CN)(6) microcubic crystals have been successfully prepared by a simple hydrothermal process between K-3[Fe(CN)(6)] with Na2S2O3 aqueous solution, free of any surfactant or template. The experimental results clearly show that the molar ratio of K-3[Fe(CN)(6)] to Na2S2O3 and their concentrations are the dominant processing factors in controlling the size, morphology, and composition of the resulting products.

Tunable photonic crystals by mixed liquids

Tunable photonic crystals were prepared by infiltrating mixed liquids into the voids of silica opals. The stop bands of the tunable photonic crystals were continuously tuned by changing the relative refraction index contrast n(mix)/ns(pheres) of the spheres and the mixed liquids by varying the volume fraction of the component of the mixed liquids. The peak wavelengths of the stop band have a red shift as the refractive index of the mixed liquids increases. The transmittance at the stop band decreases with the increase of the refractive index contrast n(mix)/n(spheres). The larger the diameter of silica opals is, the wider the tunable range of the stop band is. The position and transmittance of the peak wavelength obtained by the photonic band calculations agreed very well with the experimental data.