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.

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.

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.