CsPbBr3 microarrays with tunable periodicity, optoelectronic and field emission properties using self-assembled polystyrene template and co-evaporation method.

The booming growth of all inorganic cesium lead halide perovskites in optoelectronic applications has prompted extensive research interest in the fabrication of ordered nanostructures or microarrays for enhanced device performances. However, the high cost and complexity of commercial lithographic approaches impede the facile fabrication of perovskite microarrays. Herein, CsPbBr3 microarrays with tunable periodicities have been fabricated using a self-assembled polystyrene nanosphere template and a co-evaporation method. The periodicity of CsPbBr3 microarrays is precisely manipulated by simply modifying the size of polystyrene nanospheres. These microarrays are beneficial for light harvesting, leading to better light absorption ability and prolonged photoinduced carrier lifetime. The longest average carrier lifetime of 58.3 ns is obtained for CsPbBr3 microarrays with a periodicity of 1.0 µm. More importantly, the periodic structures of CsPbBr3 microarrays result in a tunable density of emitter tips in field emission devices. Compared to compact CsPbBr3 films, a 68.2% decrease of the turn-on field is observed for CsPbBr3 microarrays when the periodicity is 150 nm. The higher density of emitter tips leads to larger local field enhancement, and hence the largest field enhancement factor of 3346.6. Finally, a good emission current stability for CsPbBr3 microarray-based field emission devices has been demonstrated.

Exciton localization and optical properties improvement in nanocrystal-embedded ZnO core-shell nanowires.

We present a comparative investigation of the morphological, structural, and optical properties of vertically aligned ZnO nanowires (NWs) before and after high energy argon ion (Ar(+)) milling. It is found that the outer regions of the as-grown sample change from crystalline to amorphous, and ZnO core-shell NWs with ZnO nanocrystals embedded are formed after Ar(+) milling. Optical properties of the ZnO NWs have been investigated systematically through power and temperature dependent photoluminescence measurements, and the phenomenon of exciton localization as well as the relevant favorable photoluminescence characteristics is elucidated. Interestingly, under high density optical pumping at room temperature, coherent random lasing action is observed, which is ascribed to exciton localization and strong scattering. Our results on the unique optical properties of localized exciton in ZnO core-shell nanostructures shed light on developing stable and high-efficiency excitonic optoelectronic devices such as light-emitting diodes and lasers.

Broadband Photodetection of Centimeter-Scale T-Phase Gallium Telluride Grown by Molecular Beam Epitaxy.

Two-dimensional (2D) semiconductors have recently attracted considerable attention due to their promising applications in future integrated electronic and optoelectronic devices. Large-scale synthesis of high-quality 2D semiconductors is an increasingly essential requirement for practical applications, such as sensing, imaging, and communications. In this work, homogeneous 2D GaTe films on a centimeter scale are epitaxially grown on fluorphlogopite mica substrates by molecular beam epitaxy (MBE). The epitaxial GaTe thin films showed an atomically 2D layered lattice structure with a T phase, which has not been discovered in the GaTe geometric isomer. Furthermore, semiconducting behavior and high mobility above room temperature were found in T-GaTe epitaxial films, which are essential for application in semiconducting devices. The T-GaTe-based photodetectors demonstrated respectable photodetection performance with a responsivity of 13 mA/W and a fast response speed. By introducing monolayer graphene as the substrate, we successfully realized high-quality GaTe/graphene heterostructures. The performance has been significantly improved, such as the responsivity was enhanced more than 20 times. These results highlight a feasible scheme for exploring the crystal phase of 2D GaTe and realizing the controlled growth of GaTe films on large substrates, which could promote the development of broadband, high-performance, and large-scale photodetection applications.

Transition-metal salt-containing silica nanocapsules elaborated via salt-induced interfacial deposition in inverse miniemulsions as precursor to functional hollow silica particles.

Aqueous core-silica shell nanocapsules were successfully prepared using liquid droplets containing transition-metal salt as templates in inverse miniemulsions. The formation of the silica shell was attributed to the interfacial deposition of silica species induced by the presence of the transition-metal salt. In addition to the control of the particle morphology, the incorporated transition-metal salts could be used to derivatize the particles and confer additional functionalities to the hollow silica particles. To demonstrate the derivatization, the magnetic hollow silica particles were prepared by converting iron salts to magnetic iron oxides by heat treatment. The particle morphology, size, and size distribution were characterized by transmission electron microscopy and scanning electron microscopy. The results show that the particle properties strongly depend on the type and the amount of salts, the amount of tetraethoxysilane (TEOS), the pH of the droplets, and the ratios of 2-hydroxyethyl methacrylate to aqueous HCl solution. The specific surface area and pore properties were characterized by N2 sorption measurements. The pore properties and specific surface area could be tuned by varying the amount of salt. Levels of elements and of iron oxides in the magnetic hollow particles were measured by energy-dispersive X-ray spectroscopy. Iron was distributed homogenously with silicon and oxygen in the sample. The magnetization measured by a magnetic property measurement system confirmed the successful conversion of the iron salts to magnetic iron oxides.

Impact of Graphene Work Function on the Electronic Structures at the Interface between Graphene and Organic Molecules.

The impact of graphene work function (WF) on the electronic structure at the graphene/organic interface has been investigated. WF manipulation of graphene is realized using self-assembled monolayers (SAMs) with different end groups. With this method, the upper surface of the functionalized graphene remains intact, and thus precludes changes of molecular orientation and packing structures of subsequently deposited active materials. The WF of NH2-SAM functionalized graphene is ~3.90 eV. On the other hand, the WF of graphene increases to ~5.38 eV on F-SAM. By tuning the WF of graphene, an upward band bending is found at the ZnPc/graphene interface on F-SAM. At the interface between C60 and NH2-SAM modified graphene, a downward band bending is observed.

Tunable formation of ordered wrinkles in metal films with controlled thickness gradients deposited on soft elastic substrates.

Controlled wrinkled surface is useful for a wide range of applications, including flexible electronics, smart adhesion, wettability, stamping, sensoring, coating, and measuring. In this work, thickness-gradient-guided spontaneous formation of ordered wrinkling patterns in metal films deposited on soft elastic substrates is revealed by atomic force microscopy, theoretic analysis, and simulation. It is observed that in the thicker film region, broad cracks form, and the film surface remains flat. In the thinner film region, the cracks attenuate along the direction of the thickness decrease, and various wrinkle patterns including branched stripes, herringbones, and labyrinths can coexist. The interplay between the residual compression and the thickness gradient leading to the formation of such wrinkling patterns is discussed based on a nonlinear wrinkling model. The simulated wrinkling patterns as well as the variation trends of the wrinkle wavelength and amplitude along the gradient direction are in good agreement with the experimental observations. The report in this work could promote better understanding and fabrication of such ordered wrinkling patterns by tunable thickness gradient.

Superparamagnetic Fe3O4/Poly(N-isopropyl acrylamide) Nanocomposites Synthesized in Inverse Miniemulsions: Magnetic and Particle Properties.

In the present study, superparamagnetic Fe3O4/poly(N-isopropyl acrylamide) nanocomposites were synthesized by one-step inverse miniemulsion copolymerization of N-isopropyl acrylamide and N,N'-methylene diacrylamide. The loading of Fe3O4 nanoparticles in the nanocomposites was 27 wt%, and the saturation moment of the nanocomposites was 12.4 emu x g(-1). Fe3O4 nanoparticles were prepared through a coprecipitation method. The amount of stabilizer (poly(acrylic acid)) significantly influenced the size and size distribution of the Fe3O4 nanoparticles, and, therefore, their magnetic properties. Superparamagnetism of the Fe3O4 nanoparticles was preserved in the nanocomposites. The effects of synthetic parameters on the particle properties, namely surfactant loading, concentration of ferrofluid, type of lipophobe and initiator, and amount of cross-linker were investigated. Nanocomposites of Fe3O4/poly(N-isopropyl acrylamide) displayed a guava-like morphology, which they could retain after being redispersed in polar solvents.

Enhanced crystallization rate of poly(L-lactic acid) (PLLA) by polyoxymethylene (POM) fragment crystals in the PLLA/POM blends with a small amount of POM.

Phase diagrams and glass transition behaviors of poly(L-lactic acid)/polyoxymethylene (PLLA/POM) blends have been investigated in our previous work (Macromolecules 2013, 46, 5806-5814). In this work, the crystallization behaviors and physical properties of the PLLA/POM blends with the PLLA as the major component have been systematically studied. POM was crystallized into the fragment crystals that were finely dispersed in the PLLA matrix when cooling down from the melt of the blends. It was found that the POM fragment crystals accelerated the crystallization process of PLLA matrix and increased the final crystallinity of PLLA significantly in the blends. At the same time, the PLLA spherulites nucleated by POM fragment crystals were much smaller than those obtained from neat PLLA. It was further found that the crystallization rate of PLLA was quite dependent upon the POM loadings and the highest crystallization rate was observed at POM loadings of 7 wt %. It is considered that the POM fragment crystals take the nuclei role to initiate the crystallization of PLLA at low POM loadings, while a high content of POM in the blends leads to the large POM spherulites that cannot nucleate PLLA crystallization effectively. The obtained PLLA/POM blends at low POM loadings with small PLLA spherulites exhibited excellent optical transmittance and good mechanical performance.

Manipulating the charge transfer at CuPc/graphene interface by O2 plasma treatments.

The manipulation of charge transfer at CuPc/graphene interface has been demonstrated by treating pristine graphene with O2 plasma. As revealed by in situ ultraviolet photoelectron spectroscopy measurements, a much stronger interfacial charge transfer occurs when the pristine graphene is exposed to O2 plasma prior to the growth of CuPc films, which is attributed to the increased work function of graphene after O2 plasma treatment. Moreover, the highest occupied molecular orbital leading edge of CuPc locates at ∼0.80 eV below substrate Fermi level on O2 plasma treated graphene, whereas it locates at ∼1.10 eV on pristine graphene. Our findings provide detailed information regarding the electronic structure at CuPc/graphene and CuPc/O2 plasma treated graphene interfaces. The increased work function in combination with the relatively smaller energy offset between the highest occupied molecular orbital of CuPc and Fermi level of O2 plasma treated graphene facilitates the extraction of holes at the interface, and hence paves the way for improving the performance of graphene-based organic photovoltaic cells.

Fabrication of nanogel core-silica shell and hollow silica nanoparticles via an interfacial sol-gel process triggered by transition-metal salt in inverse systems.

Nanogel (hydrophilic polymer nanoparticles) core-silica shell nanoparticles were successfully fabricated via hydrolysis and condensation reaction of tetraethoxysilane (TEOS). Transition-metal tetrafluoroborate-containing nanogels were used as templates for fabrication in inverse systems (cyclohexane as continuous phase). Magnetic, hollow silica particles were subsequently formed by removing the polymer core and converting iron salts to iron oxides via heat treatment. We propose that the formation of the core-shell morphology is induced by the promoted precipitation of silica species at the surface of nanogels due to the interaction between silica species and transition-metal tetrafluoroborate. The influence of the synthesis parameters (type and amount of salts, pH of the nanogels, and amount of TEOS) on the particle morphology was systematically investigated. The pore properties and specific surface area of the hollow silica particles could be modified by the varying the amount of salt.