Shouke Yan

Nanowire crystals of a rigid rod conjugated polymer.

In this paper, we show that well-defined, highly crystalline nanowires of a rigid rod conjugated polymer, a poly(para-phenylene ethynylene)s derivative with thioacetate end groups (TA-PPE), can be obtained by self-assembling from a dilute solution. Structural analyses demonstrate the nanowires with an orthorhombic crystal unit cell wherein the lattice parameters are a approximately = 13.63 A, b approximately = 7.62 A, and c approximately = 5.12 A; in the nanowires the backbones of TA-PPE chains are parallel to the nanowire long axis with their side chains standing on the substrate. The transport properties of the nanowires examined by organic field-effect transistors (OFETs) suggest the highest charge carrier mobility approaches 0.1 cm(2)/(V s) with an average value at approximately 10(-2) cm(2)/(V s), which is 3-4 orders higher than that of thin film transistors made by the same polymer, indicating the high performance of the one-dimensional polymer nanowire crystals. These results are particular intriguing and valuable for both examining the intrinsic properties of PPEs polymer semiconductors and advancing their potential applications in electronic devices.

Solution-Processed, High-Performance Nanoribbon Transistors Based on Dithioperylene

Solution-processed, high-performance 1D single-crystalline nanoribbon transistors fabricated from dithioperylene are described. The integration of two sulfur atoms into the perylene skeleton induces a compressed highly ordered packing mode directed by S···S interactions. The mobilities of up to 2.13 cm(2) V(-1) s(-1) for a dithioperylene individual nanoribbon make it particularly attractive for electronic applications.

Low-temperature magnetotransport behaviors of heavily Mn-doped (Ga,Mn)As films with high ferromagnetic transition temperature

We report the low-temperature magnetotransport behaviors of (Ga,Mn)As films with the nominal Mn concentration x larger than 10%. The ferromagnetic transition temperature T-C can be enhanced to 191 K after postgrowth annealing (Ga,Mn)As with x=20%. The temperature T-m, corresponding to the resistivity minimum in the curve of resistivity versus temperature at temperature below T-C, depends on Mn concentration, annealing condition, and magnetic field. Moreover, we find that the variable-range hopping may be the main conductive mechanism when temperature is lower than T-m.

Synthesis of graphene/Ni–Al layered double hydroxide nanowires and their application as an electrode material for supercapacitors

Graphene oxide (GO) as a template for the morphology controllable synthesis of inorganic particles has recently attracted enormous attention. In this study, we report the synthesis of graphene/Ni–Al layered double hydroxide (LDH) nanowires by using GO and adjusting the loading of urea in hydrothermal reactions. Transmission electron microscopy observation demonstrates that the graphene/Ni–Al LDH nanowires are composed of nano-sized Ni–Al LDH sheets. This finding demonstrates a new assembly form of LDH materials. Symmetric supercapacitors have been prepared by using the graphene/Ni–Al LDH nanowires as an electrode material. Compared to other Ni–Al LDH materials, the graphene/Ni–Al LDH nanowires exhibit much better performance as an electrode material for supercapacitors, due to the preferential conductive behaviour of the nanowire structures and the conductive networks formed by the nanowires in the corresponding electrodes.

Morphologies and deformation behavior of poly(vinylidene fluoride)/poly(butylene succinate) blends with variety of blend ratios and under different preparation conditions

The morphological features and mechanical properties of PVDF/PBS blends with a variety of blend ratios and under different preparation conditions have been studied by optical and atomic force microscopy, as well as by tensile tests. It was found that, at high PVDF crystallization temperature, a small amount of PBS in the 70/30 PVDF/PBS blends has been expelled into the PVDF spherulite margin areas and interspherulitic regions due to the high diffusion ability of PBS and the slower crystal growth rate of the PVDF at high temperature. Nevertheless, the PBS affects the crystallization of PVDF significantly, which has been revealed by the increase in birefringence of PVDF spherulites of both α and γ types and the increase in band period of the α PVDF spherulites. With increasing PBS content, the increase in birefringence of PVDF spherulites and the increase in band period of the α PVDF spherulites get more evident, reflecting a more efficient influence of PBS on the crystallization of PVDF. In the PBS-rich blends, e.g., in the 40/60 and 30/70 PVDF/PBS blends, the PVDF forms isolated spherulites with large non-crystallizing PBS melt regions, which results in an interspherulitic phase separation with bigger interspherulitic PVDF areas. There is, however, PBS dispersed within the PVDF spherulites. During the crystallization of PBS at low temperature, it was found that the PBS in the interspherulitic and interaspherulitic regions growth in different manner with different growth rates. At lower PVDF crystallization temperatures, the PVDF crystallizes first and fills all the volume in regardless its content in the blends. This leads to the PBS being distributed in the interlamellar or interfibrillar regions of PVDF spherulites only. However, the growth of PBS in banded and non-banded PVDF matrix is different, reflecting the influence of pre-existing PVDF crystals on the crystallization of PBS. Tensile test shows that the deformation behavior depends remarkably on the blend ratio and the crystallization temperature of the PVDF. Such phenomena has been correlated and explained in view of the inner morphological change.

Ordering rigid rod conjugated polymer molecules for high performance photoswitchers.

Molecules of a rigid rod conjugated polymer, a derivative of poly(para-phenylene ethynylene)s with thioacetyl end groups (TA-PPE), were well aligned by drop-casting the polymer solution onto the friction-transferred poly(tetrafluoroethylene) substrates. TA-PPE molecules were found to be exactly oriented with their conjugated backbones along the PTFE sliding direction. Photoresponse characteristics based on the uniaxially ordered film were significantly improved compared to those of devices with the disordered film. For example, the switch on/off ratio of the photoswitchers with aligned molecules was as high as 330-400, while that of devices without alignment was only 8-12. It was due to the efficient carrier transport along the highly aligned polymer films, in which the molecules of TA-PPE oriented along the carrier transport direction of the devices.

Carbazole-based polysiloxane hosts for highly efficient solution-processed blue electrophosphorescent devices

The efficient host materials containing carbazole moieties linked to the backbone of polysiloxane through a phenyl bridge have been synthesized and characterized. They exhibit good film forming ability, high thermal decomposition temperatures and suitable glass transition temperatures, so as to form stable amorphous states. Moreover, the silicon–oxygen linkage disrupts their conjugation and results in a sufficiently high triplet energy level (3.0 eV). Iridium bis(4,6-difluorophenyl)pyridinato-N,C2 picolinate (FIrpic)-based devices using them as hosts show good overall performance with low efficiency roll-off. The device using PCzMSi as the host demonstrates the best performance with a maximum current efficiency of 22.8 cd A−1, a maximum power efficiency of 9.4 lm W−1 and a maximum external quantum efficiency of 11.9% at a practical luminance of 1165 cd m−2. Even at a brightness of 5000 cd m−2 level, the external quantum efficiency (EQE) still remains as high as 10%, suggesting a gentle roll-off of device efficiency at high current density. In addition, typically, the host PCzMSi film displays good mechanical performance by a nanoindentation technique to meet the practical application. These results demonstrate that the design of polysiloxane-based host materials is a promising approach to realize high performance solution-processed blue phosphorescent polymer light emitting diodes (PhPLEDs).

A versatile hybrid polyphenylsilane host for highly efficient solution-processed blue and deep blue electrophosphorescence

A universal hybrid polymeric host (PCzSiPh) for blue and deep blue phosphors has been designed and synthesized by incorporating electron-donating carbazole as pendants on a polytetraphenylsilane main chain. The polymer PCzSiPh (4) has a wide bandgap and high triplet energy (ET) because of the tetrahedral geometry of the silicon atom in the tetraphenylsilane backbone. The distinct physical properties of good solubility, combined with high thermal and morphological stability give amorphous and homogenous PCzSiPh films by solution processing. As a result, using PCzSiPh as host with the guest iridium complex TMP-FIrpic gives blue phosphorescent organic light-emitting diodes (PhOLEDs) with overall performance which far exceeds that of a control device with poly(vinylcarbazole) (PVK) host. Notably, FIrpic-based devices exhibit a maximum external quantum efficiency (EQE) of 14.3% (29.3 cd A−1, 10.4 lm W−1) which are comparable to state-of-the-art literature data using polymer hosts for a blue dopant emitter. Moreover, the versatility of PCzSiPh extends to deep blue PhOLEDs using FIr6 and FCNIrpic as dopants, with high efficiencies of 11.3 cd A−1 and 8.6 cd A−1, respectively.

Arylsilanes and siloxanes as optoelectronic materials for organic light-emitting diodes (OLEDs)

Organic light emitting diodes (OLEDs) are currently receiving much attention for applications in new generation full-colour flat-panel and flexible displays and as sources for low energy solid-state lighting. Arylsilanes and siloxanes have been extensively studied as components of OLEDs, mainly focusing on optimizing the physical and electronic properties of the light-emitting layer and other functional layers within the OLED architecture. Arylsilanes and siloxanes display the advantages of good solubility in common organic solvents and excellent resistance to thermal, chemical and irradiation degradations. In this review, we summarize the recent advances in the utilization of arylsilanes and siloxanes as fluorophore emitters, hosts for phosphor emitters, hole and exciton blocking materials, and as electron and hole transporting materials. Finally, perspectives and challenges related to arylsilanes and siloxanes for OLED applications are proposed based on the reported progress and our own opinions.

Synthesis of Dibenzothiophene‐Containing Ladder Polysilsesquioxane as a Blue Phosphorescent Host Material

A ladder polysilsesquioxanes with side chain of dibenzothiophene groups (BS-LPSQ) was successfully synthesized. The ladder structure of BS-LPSQ was characterized by MALDI-TOF MS, XRD, and (1)H NMR spectroscopy. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), atomic force microscopy (AFM), and spectroscopic analyses revealed that the BS-LPSQ has good film-forming ability, high thermal and morphological stability, and good miscibility to the dopant iridium bis(4,6-difluorophenyl)pyridinato-N,C(2)-picolinate (FIrpic), high triplet energy, and a wide bandgap. In addition, compared with the ringed polysiloxane BS-PSQ phosphorescent host material reported previously, the ladder structure of BS-LPSQ has not only a higher thermal resistance, but also could prevent molecular aggregation and effectively avoid quenching of fluorescence. Thus, the BS-LPSQ may be used as a better host for the blue-light-emitting iridium complex FIrpic. The performance of the electrophosphorescent device, based on the ladder BS-LPSQ as the active layer, is superior to that of ringed BS-PSQ and any other polyhedral oligomeric silsesquioxane (POSS)-based or polymer host materials.