Jack C. Chang

Controlled synthesis of oriented single-crystal ZnO nanotube arrays on transparent conductive substrates

Large-scale arrays of highly oriented single-crystal ZnO nanotubes (ZNTs) are successfully fabricated on transparent conductive substrates by a simple method from an aqueous solution at a low temperature (typically 85°C). The tubular morphology of the ZnO nanostructures is formed by a defect-selective chemical etching of the electrodeposited ZnO nanorods. The size of the ZNT arrays is determined by that of ZnO nanorod arrays which can be readily controlled by tuning several electrodeposition parameters. The present method can be employed to prepare ZNT arrays on flexible, conductive substrates, as well as on patterned conductive substrates.

Polyhedral Organic Microcrystals: From Cubes to Rhombic Dodecahedra

Research Grants Council of Hong Kong SAR, China [CityU101608]; City University of Hong Kong [7002275]; National Basic Research Program of China [2006CB933000, 2007CB936000]; National Natural Science Foundation of China [50825304, 50903059]

One‐Step Self‐Assembly, Alignment, and Patterning of Organic Semiconductor Nanowires by Controlled Evaporation of Confined Microfluids

Organic semiconductors, which have unique electronic and optical properties that differ from those of their inorganic counterparts, have attracted intense attention for potential applications in optoelectronic devices such as organic lightemitting diodes (OLEDs), 2] organic field-effect transistors (OFETs), organic solar cells (OSCs), and gas sensors. 11] Numerous reports have indicated that organic semiconductor molecules predominantly aggregate and selfassemble into one-dimensional (1D) nanowires or nanorods along the direction of p–p stacking or other directional intermolecular interactions. Owing to excellent performance in carrier transport, such one-dimensional nanostructures may serve as attractive building blocks in future organic electronic applications. However, to fabricate practical devices on a large scale, a major challenge is to design a method to deposit and align a large number of such nanowires in a desired position. In most cases, nanostructures selfassembled directly from solution tend to be distributed in a macroscopically random fashion on the substrate. Disordered alignment of organic semiconductors may significantly increase the overall cost due to material consumption and also result in poor performance of electronic devices. Therefore, a facile deposition and patterning method for organic semiconductor molecules is highly desirable. To date, several strategies for alignment of 1D nanowires have been investigated, including the Langmuir–Blodgett technique, electric or magnetic field assisted alignment, dip coating, electrostatic alignment, and so on. However, these methods usually require an external facility and are limited in producing large-area ordered patterns. In recent years, evaporation-induced self-assembly (EISA) has been reported to prepare well-ordered 2D patterns. The EISA method depends on the simple fact that a drop of colloidal solution always leaves a ringlike deposit at the perimeter. During the evaporation process, the loss of solvent mainly occurs at the contact line, and an outward capillary flow carries the solvent and dispersed solute from the interior to the contact line. Therefore, the key parameter to achieve well-ordered 2D patterns is an efficient method to control the contact line. Recently, Lin et al. reported a simple method for controlling droplet evaporation in a confined geometry, which leaves behind well-organized gradient concentric ring patterns. With a spherical lens on the substrate, the contact line is well controlled and hence gradient concentric rings are obtained. However, the asprepared patterns are usually amorphous and no specific nanostructures are formed because of the hard-to-crystallize materials used in evaporation process, such as polymer and inorganic quantum dots (QDs). On the other hand, organic semiconductor molecules can easily self-assemble into 1D nanostructures by evaporation. 44] We have developed a facile method to prepare aligned organic nanowires on a solid substrate or liquid/liquid interface based on the EISA method. 45] With the aid of solvent evaporation, selfassembly of molecules and alignment of as-obtained nanostructures can be combined to produce a large-area ordered pattern of organic nanowires or films. However, the method wastes a lot of solvent, and the contact line is not easy to control. We have now integrated the EISA method with the concentric ring patterns of Lin et al. , so that simultaneous self-assembly, alignment, and patterning of organic semiconductor nanowires can be achieved in one step. Here we demonstrate such a facile approach to fabricate large-scale concentric arrays of nanowires by solvent evaporation in a confined geometry. N,N’-Dimethylquinacridone (DMQA) was selected as a nonvolatile solute in this experiment. It is an industrially important red organic dye with intense fluorescence, which is widely used in photovoltaic and other organic electroluminescent devices. It was synthesized according to the reported procedure and was purified twice by vacuum sublimation. Concentric ring patterns of DMQA nanowires were prepared from chloroform solutions of DMQA with concentrations of 0.2, 0.1, and 0.05 mmolL . The confined [*] Z. L. Wang, R. R. Bao, X. M. Ou, Prof. J. C. Chang, Prof. X. H. Zhang Nano-organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion and Optoelectronic Materials Technical Institute of Physics and Chemistry Chinese Academy of Sciences, Beijing (China) E-mail: xhzhang@mail.ipc.ac.cn

Surfactant-assisted alignment of ZnO nanocrystals to superstructures.

Self-organization of ZnO nanoparticles into various superstructures (sheet, platelet, ring) has been achieved with the assistance of micelles formed by surfactant cetyltrimethylammonium bromide (CTAB) under one-pot condition. The CTAB-modified zinc hydroxy double salt (Zn-HDS) mesocrystals act as intermediates to form ZnO hexagonal superstructures at temperatures as low as 50 degrees C. The decomposition temperature of Zn-HDS mesocrystals is much lower than that of the corresponding bulk crystals because the organic additive CTAB effectively decreases the degree of crystallinity. Taking advantage of temperature-induced phase transformation of micelles, two-stage self-organization can form ZnO platelets and ring mesocrystals, that is, ZnO ellipsoidal superstructures formed through vertical attachment on (0001) facets of basic units can further assemble to form ZnO platelets and rings through vertical attachment on (0001) facets of ZnO ellipsoidal superstructures. The structural transformation of micelles as shape templates can offer a new route for self-assembly of nonspherical colloids into three-dimensional photonic crystals. ZnO sheet, ring, and platelet mesocrystals with a high population of polar Zn-(0001) plane are expected to have high photocatalytic activity.

ZnO nanowire-based all-optical switch with Reset-Set flip-flop function

An all-optical switch with Reset-Set (RS) flip-flop function has been developed by attaching a derivative of spiropyran on the surface of zinc oxide (ZnO) Nanowire. Using UV/visible irradiation and the fluorescence of spiropyran-modified ZnO nanowire as inputs—set/reset and output, RS flip-flop function can be performed on a single ZnO nanowire or a nanowire array. The configuration of the current all-optical switch represents a potential for developing small-sized all-optical devices, which could be further exploited at higher level of integration.

High-efficiency endothermic energy transfer in polymeric light-emitting devices based on cyclometalated Ir complexes

We report polymeric light-emitting diodes (PLEDs) made from pinene-substituted iridium(III) phosphorescent dopants: tris(5-(4-difluoro phenyl)-10,10-dimethyl-4-aza-tricycloundeca-2,4,6-triene) iridium (III) [Ir(F-pppy)3] and tris(5-(2,4-difluorophenyl)-10,10-dimethyl-4-aza-tricycloundeca-2,4,6-triene) iridium (III) [Ir(F2-pppy)3]. The pinene substitution introduces steric hindrance to molecular structure of the dopant that reduces triplet-triplet annihilation between dopants and consequently enhances device performance. Via endothermic energy transfer from poly(vinylcarbazole) to Ir(F-pppy)3 and Ir(F2-pppy)3, a peak electroluminescent efficiency of 32.8cd∕A or 12.3cd∕A at 12wt% Ir(F-pppy)3 or 15wt% Ir(F2-pppy)3 doped and solution-processed PLEDs have been obtained. These values represent significant improvement in performance over previously reported endothermic energy-transfer based electrophosphorescent devices.