Sheng-Hsiung Yang

Nano Approach Investigation of the Conduction Mechanism in Polyaniline Nanofibers

A nanotechnological approach is applied to measurements of the electric field dependence of resistance under a high electric field while in low voltage. With this technique, the conduction mechanism on a mesoscopic scale is explored in a single, nonagglomerated nanofiber. Polyaniline nanofibers are prepared by vigorous mixing of aniline and oxidation agent ammonium persulfate in acid solution. They exhibit a uniform nanoscale morphology rather than agglomeration as that produced via conventional chemical oxidation. The as-synthesized polyaniline nanofibers are doped (dedoped) with a HCl acid (NH(3) base), and their temperature behaviors of resistances follow an exponential function with an exponent of T(-1/2). To measure the conduction mechanism in a single nanofiber, the dielectrophoresis technique is implemented to position nanofibers on top of two electrodes with a nanogap of 100-600 nm, patterned by electron-beam lithography. After the devices are irradiated by electron beam to reduce contact resistances, their temperature behaviors and electric field dependences are unveiled. The experimental results agree well with the theoretical model of charging energy limited tunneling. Other theoretical models such as Efros-Shklovskii and Motts one-dimensional hopping conduction are excluded after comparisons and arguments. Through fitting, the size of the conductive grain, separation distance between two grains, and charging energy per grain in a single polyaniline nanofiber are estimated to be about 4.9 nm, 2.8 nm, and 78 meV, respectively. The nanotechnological approach, where the nanogap and the dielectrophoresis technique are used for single nanofiber device fabrication, is applied for determination of mesoscopic charge transport in a polyaniline conducting polymer.

Liquid crystal alignment on zinc oxide nanowire arrays for LCDs applications

The zinc oxide (ZnO) nanowire arrays on the indium tin oxide (ITO) glass substrates were fabricated by using the two-step hydrothermal method. A high transmittance ~92% of ZnO nanowire arrays on ITO substrate in the visible region was obtained. It was observed that the liquid crystal (LC) directors were aligned vertically to the (ZnO) nanowire arrays. The properties of ZnO nanowire arrays as vertical liquid crystal (LC) alignment layers and their applications for hybrid-aligned nematic LC modes were investigated in this work.

Tunable Surface Wettability of ZnO Nanoparticle Arrays for Controlling the Alignment of Liquid Crystals

The control of the liquid crystal (LC) alignment is very important for both academic research and practical applications. LC molecules aligned on the ZnO nanoparticle arrays (ZnO NPAs) are demonstrated and the pretilt angles of LCs can be controlled by using ZnO NPAs with different surface wettability. The wettability of ZnO NPAs fabricated by the solution-based hydrothermal method can be controlled by changing the annealing temperature of the as-prepared ZnO NPAs. The measurements of the energy-dispersive spectra and photoluminescence have shown that the chemical properties of ZnO NPAs have been changed with the annealing temperature. Our results show that the pretilt angle of LCs can be tuned continuously from ∼0 to ∼90° as the contact angle of water on ZnO NPAs changes from 33 to 108°.

Solution-processable phosphorescent to organic light-emitting diodes based on chromophoric amphiphile/silica nanocomposite

We report the synthesis of a tris-cyclometalated iridium complex which emits sky-blue light and its potential use in phosphorescent light-emitting devices. The hybrid meso-structured nanocomposites by sol-gel co-assembly with tetraethyl ortho-silicate and corresponding molecular interactions within mesopores were also demonstrated. Electroluminescent devices were fabricated using carbazole-based monomers and iridium complex as the active layer, acting as a host/guest system through a co-assembled sol-gel process. Devices based on this nanocomposite showed improved luminescent efficiencies several times higher than that of similar chromophores elaborated in the literature. A triple-layer electroluminescence device with the configuration of ITO/PEDOT/ Ir(F2OC11ppy)(3):CA-C11:PBD nanocomposite/TPBI/Ca/Al showed a maximum brightness of 1389 cd m(-2) at 12 V and a maximum efficiency of 3.29 cd A(-1).

Improved efficiency of perovskite solar cells based on Ni-doped ZnO nanorod arrays and Li salt-doped P3HT layer for charge collection

We demonstrate regular-type perovskite solar cells based on vertically grown Ni-doped ZnO nanorod arrays and doped P3HT for solar energy harvesting. PCBM was introduced between Ni-doped ZnO nanorod arrays and the perovskite layer to improve electron extraction, while P3HT doped with bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) was used as hole transporting layer. Three types of perovskite materials, including MAPbI3, (MA)x(FA)1-xPbI3, and (MA)y(GA)1-yPbI3, were used as light harvesting layers to exploit conversion efficiency of photovoltaic devices. The optimized perovskite solar cell with the configuration of ITO/Ni-doped ZnO nanorods/PCBM/(MA)y(GA)1-yPbI3/P3HT + Li-TFSI/Au revealed an open-circuit voltage of 0.83 V, a short-circuit current density of 23.73 mA/cm2, a fill factor of 70%, and a power conversion efficiency of 13.79%.

Dielectrophoretic placement of quasi-zero-, one-, and two-dimensional nanomaterials into nanogap for electrical characterizations

DEP is one of promising techniques for positioning nanomaterials into the desirable location for nanoelectronic applications. In contrast, the lithography technique is commonly used to make ultra‐thin conducting wires and narrow gaps but, due to the limit of patterning resolution, it is not feasible to make electrical contacts on ultra‐small nanomaterials for a bottom‐up device fabrication. Thus, integrating the lithography and dielectrophoresis, a real bottom‐up fabrication can be achieved. In this work, the device with the nanogap in between two nanofinger‐electrodes is made using electron‐beam lithography from top down and the ultra‐small nanomaterials, such as colloidal PbSe quantum dots, polyaniline nanofibers, and reduced‐graphene‐oxide flakes, are placed in the nanogap by DEP from bottom up. The threshold electric field for the DEP placement of PbSe nanocrystals was roughly estimated to be about 8.3 × 104 V/cm under our experimental configuration. After the DEP process, several procedures for reducing contact resistances are attempted and measurements of intrinsic electron transport in versatile nanomaterials are performed. It is experimentally confirmed that electron transport in both PbSe nanocrystal arrays and polyaniline nanofibers agrees well with Prof. Ping Shengs model of granular metallic conduction. In addition, electron transport in reduced‐graphene‐oxide flakes follows Motts 2D variable‐range‐hopping model. This study illustrates an integration of the electron‐beam lithography and the DEP techniques for a precise manipulation of nanomaterials into electronic circuits for characterization of intrinsic properties.

UV-treated ZnO films for liquid crystal alignment

The surface wettability of ZnO films prepared by a sol–gel process is altered from hydrophobicity to hydrophilicity via ultraviolet (UV) light irradiation. The results indicate that hydrophilicity of the ZnO films is enhanced along with the increase of UV irradiation energy. The measurements of X-ray photoelectron spectroscopy and photoluminescence show that the amount of oxygen vacancies in ZnO films is increased with the UV irradiation. The results of UV-vis absorption spectroscopy and X-ray diffraction also show that the crystalline quality of the ZnO films is slightly changed after UV treatment. The increase in oxygen vacancies means that water molecules can easily coordinate into the oxygen vacancy sites, leading to the increase of surface wettability. It is observed that liquid crystal (LC) molecules can be aligned on the UV-treated ZnO films, and the orientation of LCs on the UV-treated ZnO films can be tuned from homeotropic to homogeneous alignment by controlling the surface wettability of ZnO films. Our results show that the pretilt angle of LCs on ZnO films depends on their surface wettability, and it can be successfully adjusted over a wide range from 89.5° to 0.5° as the contact angle of water on ZnO films changes from 97° to 60°.

Hybrid materials based on polymer nanocomposites for environmental applications

Abstract Environmental pollution is one of the most important issues that our society has to face in the coming years for its survival. To protect the media in which we live, several strategies have been developed aiming at elimination of pollutants and then at exploitation of clean and sustainable energies. Furthermore, new technologies enable the efficient storage and savings of produced energy. In this chapter, we provide an overview of material strategies employing polymer-based composites in environment protection. We examine the synthesis process of materials and the formation of composites. Their applications in devices used in photocatalysts for degradation of pollutants, solar cells for energy harvesting, batteries for energy storage, and light-emitting diodes for economic lighting are described and discussed. Despite multiple issues remaining to be addressed, the progress in hybrid material research is promising for obtaining the needed energy for industrial and domestic activities in an environmentally acceptable way.

Liquid crystal alignment on ZnO nanostructure films

The study of liquid crystal (LC) alignment is important for fundamental researches and industrial applications. The tunable pretilt angles of liquid crystal (LC) molecules aligned on the inorganic zinc oxide (ZnO) nanostructure films with controllable surface wettability are demonstrated in this work. The ZnO nanostructure films are deposited on the ITO- glass substrates by the two-steps hydrothermal process, and their wettability can be modified by annealing. Our experimental results show that the pretilt angles of LCs on ZnO nanostructure films can be successfully adjusted over a wide range from ~90° to ~0° as the surface energy on the ZnO nanostructure films changes from ~30 to ~70 mJ/m. Finally we have applied this technique to fabricate a no-bias optically-compensated bend (OCB) LCD with ZnO nanostructure films annealed at 235 °C.

Synthesis of fluorene-based polyelectrolytes tethering different counterions for single-component white light-emitting electrochemical cells

A series of polyfluorene (PF) electrolytes bearing Br−, BF4 −, or PF6 − counterions were synthesized and characterized. 2,1,3-benzoselenadiazole moieties were incorporated into polymer main chains to produce single-component white lightemitting polymers. The thermal stability of Br-containing ionic PF was decreased because of the Hofmann elimination occurred at higher temperature. By replacing Br− with BF4 − or PF6 − counterions, the thermal stability of polymers was significantly improved. The emission intensity around 550 nm was decreased for ionic polyelectrolytes. The optimized spin-coated light-emitting electrochemical cell (LEC) with the configuration of ITO/PEDOT/polymer/Ag showed a maximum luminescence efficiency of 1.56 lm/W at a low operation bias of 3 V. The single-component LEC device exhibited pure white light emission with CIE’1931 coordinates approaching (0.33, 0.33) and high color rendering index (CRI < 85), referring to its potential use in solid-state-lighting application.