Yanchun Han

Sol–gel synthesis and photoluminescent properties of LaPO4:A (A = Eu3+, Ce3+, Tb3+) nanocrystalline thin films

Rare-earth ion (Eu3+, Tb3+, Ce3+)-doped LaPO4 nanocrystalline thin films and their patterning were fabricated by a Pechini sol–gel process combined with soft lithography on silicon and silica glass substrates. X-Ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), atomic force microscopy (AFM), scanning electron microcopy (SEM), optical microscopy, absorption and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicate that the films begin to crystallize at 700 °C and the crystallinity increases with increasing annealing temperature. The morphology of the thin film depends on the annealing temperature and the number of coating layers. The 1000 °C annealed single layer film is transparent to the naked eye, uniform and crack-free with a thickness of about 200 nm and an average grain size of 100 nm. Patterned thin films with different strip widths (5–50 µm) were obtained by micromolding in capillaries (soft lithography). The doped rare earth ions show their characteristic emission in the nanocrystalline LaPO4 films, i.e., Eu3+5D0–7FJ (J = 1, 2, 3, 4), Tb3+5D3,4–7FJ (J = 6, 5, 4, 3, 2) and Ce3+ 5d–4f transition emissions, respectively. Both the lifetimes and the PL intensities of Eu3+ and Tb3+ increase with increasing annealing temperature, and the optimum concentrations for them were determined to be 5 mol% and 16 mol% of La3+ in LaPO4 thin films, respectively. An energy transfer phenomenon from Ce3+ to Tb3+ has been observed in LaPO4 nanocrystalline thin films, and the energy transfer efficiency depends on the doping concentration of Tb3+ if the concentration of Ce3+ is fixed.

Preparation, patterning and luminescent properties of nanocrystalline Gd2O3 : A (A = Eu3+, Dy3+, Sm3+, Er3+) phosphor films via Pechini sol-gel soft lithography

Nanocrystalline Gd2O3:A (A = Eu3+, Dy3+, Sm3+, Er3+) phosphor films and their patterning were fabricated by a Pechini sol-gel process combined with a soft lithography. X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical microscopy, UV/vis transmission and photoluminescence (PL) spectra as well as lifetimes were used to characterize the resulting films. The results of XRD indicated that the films began to crystallize at 500 degreesC and that the crystallinity increased with the elevation of annealing temperatures. Uniform and crack free non-patterned phosphor films were obtained by optimizing the composition of the coating sol, which mainly consisted of grains with an average size of 70 nm and a thickness of 550 nm. Using micro-molding in capillaries technique, we obtained homogeneous and defects-free patterned gel and crystalline phosphor films with different stripe widths (5, 10, 20 and 50 mum). Significant shrinkage (50%) was observed in the patterned films during the heat treatment process. The doped rare earth ions (A) showed their characteristic emission in crystalline Gd2O3 phosphor films due to an efficient energy transfer from Gd2O3 host to them. Both the lifetimes and PL intensity of the rare earth ions increased with increasing the annealing temperature from 500 to 900 degreesC, and the optimum concentrations for Eu3+, Dy3+, sm(3+), Er3+ were determined to be 5, 0.25, 1 and 1.5 mol% of Gd3+ in Gd2O3 films, respectively.

A New Method to Improve Poly(3-hexyl thiophene) (P3HT) Crystalline Behavior: Decreasing Chains Entanglement To Promote Order−Disorder Transformation in Solution

We promoted order-disorder transformation of poly(3-hexylthiophene) (P3HT) in solution by ultrasonic oscillation which substantially improved crystallinity in its pure film. P3HT with low molecular weight (M(w)) dispersed very well in p-xylene solvent and few aggregates generated in the solution. For P3HT with high M(w), the results suggested the coexistence of two phases: disordered coils in solution and ordered microcrystals in suspension. Upon ultrasonic oscillating, more ordered precursors generated in solution due to increased self-assembly from disordered to ordered configuration, which resulting from decreased extent of chains entanglement existing in high molecular system, and red shift at absorption maximum and growing intensity of the pi-pi* absorption at ca. 604 nm were observed in solution. The films prepared from the oscillated solution then showed increased degree of crystallinity, pi-pi interactions and homogeneously distributed nanofibrils, which should be attributed to the ordered precursors constructed in solution. Furthermore, the best crystallinity of the film was obtained at the oscillating time of 4 min, showing the equilibrium state between the increased content of crystalline molecules and the shortened crystalline length. This simple method paves the way for the decreasing chains entanglement during crystallizing process of conjugated polymer in solution, and it enriches the ways to improve crystalline order in thin films comprising crystallizable polymers.

Morphologies in solvent-annealed thin films of symmetric diblock copolymer

We have systematically studied the thin film morphologies of symmetric poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer after annealing to solvents with varying selectivity. Upon neutral solvent vapor annealing, terraced morphology is observed without any lateral structures on the surfaces. When using PS-selective solvent annealing, the film exhibits macroscopically flat with a disordered micellar structure. While PMMA-selective solvent annealing leads to the dewetting of the film with fractal-like holes, with highly ordered nanoscale depressions in the region of undewetted films. In addition, when decreasing the swelling degree of the film in the case of PMMA-selective solvent annealing, hills and valleys are observed with the coexistence of highly ordered nanoscale spheres and stripes on the surface, in contrast to the case of higher swelling degree. The differences are explained qualitatively on the basis of polymer-solvent interaction parameters of the different components.

Solvent-induced microphase separation in diblock copolymer thin films with reversibly switchable morphology

We have studied the surface morphology of symmetric poly(styrene)-block-poly(methyl methacrylate) diblock copolymer thin films after solvent vapor treatment selective for poly(methyl methacrylate). Highly ordered nanoscale depressions or striped morphologies are obtained by varying the solvent annealing time. The resulting nanostructured films turn out to be sensitive to the surrounding medium, that is, their morphologies and surface properties can be reversibly switchable upon exposure to different block-selective solvents.

Oriented Poly(3-hexylthiophene) Nanofibril with the π−π Stacking Growth Direction by Solvent Directional Evaporation

In this article, the uniaxial alignment of poly(3-hexylthiophene) (P3HT) nanofibrils with a π-π stacking growth direction in which P3HT chains adopt a flat-on conformation was obtained by solvent directional evaporation using a glass cover slide and a poly(dimethylsiloxane) (PDMS) sheet to press the P3HT film in a carbon disulfide (CS(2)) atmosphere. By controlling the CS(2) vapor pressure during the film-forming process, we got a well-oriented P3HT film whose order parameter reached as high as 0.97. The orientation of the film was induced by the crystallization nucleation of P3HT and the directional evaporation of the solvent. Under a CS(2) vapor atmosphere, P3HT crystals preferred to adopt the form II modification, which started by nucleation. Owing to the solvent directional evaporation from the center to the margin, P3HT at the center of the sample would precipitate first to induce nucleation. Then the peripheral P3HT would directly diffuse, precipitate, and then adhere to the nucleus to form the uniaxial alignment of P3HT nanofibrils along the direction of solvent evaporation. Furthermore, in the P3HT nanofibrils, the π-π stacking direction of P3HT lamellae was parallel to the crystal growth direction, which would provide an effective path for charge transport.

Patterning thin polymer films by surface-directed dewetting and pattern transfer

The pattern evolution processes of thin polystyrene (PS) film on chemically patterned substrates during dewetting have been investigated experimentally. The substrates have patterns of self-assembly monolayers produced by microcontact printing with octadecyltrichlorosilane. Optical microscopy and atomic force microscopy images reveal that ordered micrometer scale pattern can be created by surface direct dewetting. Various pattern sizes and pattern complexities can be achieved by controlling the experimental parameters. The dewetting pattern has been transferred to form PDMS stamp for soft lithography.

Constructing the nanointerpenetrating structure of PCDTBT:PC70BM bulk heterojunction solar cells induced by aggregation of PC70BM via mixed-solvent vapor annealing

In this paper, mixed-solvent vapor annealing (M-SVA) was proposed to improve polymer crystallinity and inhibit fullerene intercalation into polymer side chains in a poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT):[6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) blend system. Experiments showed the optimal volume ratio of a mixed solvent, i.e., for tetrahydrofuran (THF) and carbon disulfide (CS2) vapor it is 1 : 1, and the appropriate annealing time is 5 min. After the M-SVA process, out-of-plane grazing incidence X-ray diffraction (GIXD) and spectroscopic ellipsometry (SE) data show that PCDTBT crystals with larger size and higher crystallinity were formed due to enhanced π–π interaction of polymer chains; the aggregation of PC70BM was promoted as well, which inhibited the intercalation behavior, resulting in a continuous electron path and improved phase purity. The morphology transition improved the carrier mobility, in particular electron mobility. Also, the recombination of a charge transfer (CT) state was reduced and the lifetime of mobile carriers was increased, which could be verified by transient-absorption (TA) spectra. As a result, the power conversion efficiency (PCE) of the BHJ reached 6.60%, an increase of more than 40% compared with the reference device (PCE = 4.54%), even though the thickness of the active layer is up to 220 nm and the PC70BM content is as low as 71.4 wt% by weight.

Sol–gel fabrication, patterning and photoluminescent properties of LaPO4:Ce3+, Tb3+ nanocrystalline thin films

Abstract Ce 3+ and/or Tb 3+ -doped LaPO 4 nanocrystalline thin films and their patterning were fabricated by a sol–gel process combined with soft lithography on silicon and quartz glass substrates. The results of XRD indicated that the films began to crystallize at 700 °C. The 1000 °C annealed single layer films are transparent by eyes, uniform and crack-free with a thickness of about 200 nm and an average grain size of 100 nm. Patterned thin film with different band widths (5–50 μm) were obtained by micro-molding in capillaries technique. The luminescence and energy transfer properties of Ce 3+ and Tb 3+ were studied in LaPO 4 films.

Detection of explosives with porous xerogel film from conjugated carbazole-based dendrimers

The carbazole-based conjugated dendrimer H2-BCz with tert-butyl surface groups shows selective gelation behavior in a good solvent and anti-solvent mixture. The sol–gel conversion of H2-BCz can be observed on heating and cooling. The H2-BCz xerogel films show three-dimensional porous networks with the nanofibers (layered structures with a period of 1.38 nm) entangling and intersecting with each other. The π–π interactions between the aryl units may be the driving force for the formation of one-dimensional H2-BCz fibers during the gelation process. Due to the electron-donating property of the H2-BCz molecule and the porosity of the fibrous film, the xerogel films can be used as efficient fluorescence quenching based chemsensors for the detection of nitroaromatic explosives, such as 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT). The quenching efficiency was as high as 77% and 91% when exposed to the vapor of TNT and DNT, respectively.