Ping-Yen Hsieh

High Mobility of Graphene-Based Flexible Transparent Field Effect Transistors Doped with TiO2 and Nitrogen-Doped TiO2

Graphene with carbon atoms bonded in a honeycomb lattice can be tailored by doping various species to alter the electrical properties of the graphene for fabricating p-type or n-type field-effect transistors (FETs). In this study, large-area and single-layer graphene was grown on electropolished Cu foil using the thermal chemical vapor deposition method; the graphene was then transferred onto a poly(ethylene terephthalate) (PET) substrate to produce flexible, transparent FETs. TiO2 and nitrogen-doped TiO2 (N-TiO2) nanoparticles were doped on the graphene to alter its electrical properties, thereby enhancing the carrier mobility and enabling the transistors to sense UV and visible light optically. The results indicated that the electron mobility of the graphene was 1900 cm(2)/(V·s). Dopings of TiO2 and N-doped TiO2 (1.4 at. % N) lead to n-type doping effects demonstrating extremely high carrier mobilities of 53000 and 31000 cm(2)/(V·s), respectively. Through UV and visible light irradiation, TiO2 and N-TiO2 generated electrons and holes; the generated electrons transferred to graphene channels, causing the FETs to exhibit n-type electric behavior. In addition, the Dirac points of the graphene recovered to their original state within 5 min, confirming that the graphene-based FETs were photosensitive to UV and visible light. In a bending state with a radius of curvature greater than 2.0 cm, the carrier mobilities of the FETs did not substantially change, demonstrating the application possibility of the fabricated graphene-based FETs in photosensors.

High Stability Electron Field Emitters Synthesized via the Combination of Carbon Nanotubes and N2-Plasma Grown Ultrananocrystalline Diamond Films

An electron field emitter with superior electron field emission (EFE) properties and improved lifetime stability is being demonstrated via the combination of carbon nanotubes and the CH4/N2 plasma grown ultrananocrystalline diamond (N-UNCD) films. The resistance of the carbon nanotubes to plasma ion bombardment is improved by the formation of carbon nanocones on the side walls of the carbon nanotubes, thus forming strengthened carbon nanotubes (s-CNTs). The N-UNCD films can thus be grown on s-CNTs, forming N-UNCD/s-CNTs carbon nanocomposite materials. The N-UNCD/s-CNTs films possess good conductivity of σ = 237 S/cm and marvelous EFE properties, such as low turn-on field of (E0) = 3.58 V/μm with large EFE current density of (J(e)) = 1.86 mA/cm(2) at an applied field of 6.0 V/μm. Moreover, the EFE emitters can be operated under 0.19 mA/cm(2) for more than 350 min without showing any sign of degradation. Such a superior EFE property along with high robustness characteristic of these combination of materials are not attainable with neither N-UNCD films nor s-CNTs films alone. Transmission electron microscopic investigations indicated that the N-UNCD films contain needle-like diamond grains encased in a few layers of nanographitic phase, which enhanced markedly the transport of electrons in the N-UNCD films. Moreover, the needle-like diamond grains were nucleated from the s-CNTs without the necessity of forming the interlayer that facilitate the transport of electrons crossing the diamond-to-Si interface. Both these factors contributed to the enhanced EFE behavior of the N-UNCD/s-CNTs films.

A high carrier-mobility crystalline silicon film directly grown on polyimide using SiCl4/H2 microwave plasma for flexible thin film transistors

An approach for the direct synthesis of crystalline Si films with high carrier mobility on a flexible polyimide (PI) substrate is reported. Microwave plasma enhanced chemical vapor deposition was applied using H2-diluted silicon tetrachloride (SiCl4) as a precursor. Dense crystalline Si films with a columnar structure were directly obtained on an unheated substrate without the extra recrystallization technique and the transfer process under an optimal H2 flow rate of 100 sccm. The films show a crystalline volume fraction of >98% and a high growth rate of 4.1 nm s−1. Detailed TEM studies show that crystalline Si films can be directly grown on a substrate without an amorphous incubation layer even at the initial stage. Moreover, a thin layer of a single-crystalline (111) grain exists on the topmost surface. Such a microstructure feature is responsible for the measured high Hall carrier mobility of 170 cm2 V−1 s−1. The adhesion between the Si film and the PI substrate achieves the highest rank, 5B. Mechanical static bending tests reveal the critical radius of curvature (Rc) at 8.0 mm. The decrease in electrical conductivity is only 23% after dynamic bending tests at Rc for 1000 cycles. Top-gate flexible Si thin film transistors employing an optimized 100 nm crystalline Si channel layer exhibit a field-effect carrier mobility as high as 106 cm2 V−1 s−1, a threshold voltage of 2.5 V, and an on/off current ratio of 1.2 × 106. No obvious deterioration of transfer characteristics was observed when the devices were subjected to bending at Rc. These results thus represent important steps toward a low-cost approach to high-performance flexible crystalline Si film based electronics.

Hollow Few-Layer Graphene-Based Structures from Parafilm Waste for Flexible Transparent Supercapacitors and Oil Spill Cleanup

We report a versatile strategy to exploit parafilm waste as a carbon precursor for fabrication of freestanding, hollow few-layer graphene fiber mesh (HFGM) structures without use of any gaseous carriers/promoters via an annealing route. The freestanding HFGMs possess good mechanical flexibility, tailorable transparency, and high electrical conductivity, consequently qualifying them as promising electrochemical electrodes. Because of the hollow spaces, electrolyte ions can easily access into and contact with interior surfaces of the graphene fibers, accordingly increasing electrode/electrolyte interfacial area. As expected, solid-state supercapacitors based on the HFGMs exhibit a considerable enhancement in specific capacitance (20-30 fold) as compared to those employing chemical vapor deposition compact graphene films. Moreover, the parafilm waste is found to be beneficial for one-step fabrication of nanocarbon/few-layer graphene composite meshes with superior electrochemical performance, outstanding superhydrophobic property, good self-cleaning ability, and great promise for oil spill cleanup.

High-performance flexible electron field emitters fabricated from doped crystalline Si pillar films on polymer substrates

We report a new approach for the synthesis of various crystalline Si nanostructures on a polyimide (PI) substrate via microwave plasma enhanced chemical vapor deposition (MWPECVD) using SiCl4/H2 as precursors, and study the effects of conducting type (i.e., intrinsic, n-type, and p-type) on the electron field emission (EFE) properties of the Si nanostructures. H2 plasma treated B-doped crystalline Si pillars (H2: p-Si pillars) with a diameter of 50 nm and a sharp tip radius of 16 nm on a Mo-coated PI substrate reveals the best EFE performance with a low turn-on field of 5.85 V μm−1, high current density of 1.37 mA cm−2@10 V μm−1, and an extremely high field enhancement factor of 1281.13. This superior EFE performance is achieved because of its geometric features and high conductivity across the emitters. In addition, a flexible crystalline Si film-based field emission prototype device using the H2: p-Si pillar sample as the cathode is constructed. No obvious deterioration on EFE characteristics is observed when the device is subjected to bending at a radius of curvature (R) of 10 mm. According to the lifetime test, we achieve a half-life time over 10 h when a repeating FE-on/off test of 9 times at an R of 10 mm is performed, indicating high flexibility and good stability. These results thus demonstrate important steps toward a low-cost approach for creating high-performance and flexible field emission displays.

Flexible Solar Cells Using Doped Crystalline Si Film Prepared by Self-Biased Sputtering Solid Doping Source in SiCl4/H2 Microwave Plasma.

We developed an innovative approach of self-biased sputtering solid doping source process to synthesize doped crystalline Si film on flexible polyimide (PI) substrate via microwave-plasma-enhanced chemical vapor deposition (MWPECVD) using SiCl4/H2 mixture. In this process, P dopants or B dopants were introduced by sputtering the solid doping target through charged-ion bombardment in situ during high-density microwave plasma deposition. A strong correlation between the number of solid doping targets and the characteristics of doped Si films was investigated in detail. The results show that both P- and B-doped crystalline Si films possessed a dense columnar structure, and the crystallinity of these structures decreased with increasing the number of solid doping targets. The films also exhibited a high growth rate (>4.0 nm/s). Under optimal conditions, the maximum conductivity and corresponding carrier concentration were, respectively, 9.48 S/cm and 1.2 × 10(20) cm(-3) for P-doped Si film and 7.83 S/cm and 1.5 × 10(20) cm(-3) for B-doped Si film. Such high values indicate that the incorporation of dopant with high doping efficiency (around 40%) into the Si films was achieved regardless of solid doping sources used. Furthermore, a flexible crystalline Si film solar cell with substrate configuration was fabricated by using the structure of PI/Mo film/n-type Si film/i-type Si film/p-type Si film/ITO film/Al grid film. The best solar cell performance was obtained with an open-circuit voltage of 0.54 V, short-circuit current density of 19.18 mA/cm(2), fill factor of 0.65, and high energy conversion of 6.75%. According to the results of bending tests, the critical radius of curvature (RC) was 12.4 mm, and the loss of efficiency was less than 1% after the cyclic bending test for 100 cycles at RC, indicating superior flexibility and bending durability. These results represent important steps toward a low-cost approach to high-performance flexible crystalline Si film-based photovoltaic devices.

Low Temperature Synthesis of Lithium-Doped Nanocrystalline Diamond Films with Enhanced Field Electron Emission Properties

Low temperature (350 °C) grown conductive nanocrystalline diamond (NCD) films were realized by lithium diffusion from Cr-coated lithium niobate substrates (Cr/LNO). The NCD/Cr/LNO films showed a low resistivity of 0.01 Ω·cm and excellent field electron emission characteristics, viz. a low turn-on field of 2.3 V/µm, a high-current density of 11.0 mA/cm2 (at 4.9 V/m), a large field enhancement factor of 1670, and a life-time stability of 445 min (at 3.0 mA/cm2). The low temperature deposition process combined with the excellent electrical characteristics offers a new prospective for applications based on temperature sensitive materials.

Nitrogen-Incorporated Ultrananocrystalline Diamond Electrodes for Dopamine Determination

In this paper, nitrogen incorporated ultrananocrystalline diamond (NUNCD) films were fabricated for use as electrodes to detect dopamine. The NUNCD electrodes achieved high sensitivity, great selectivity, and excellent detection limits for dopamine sensing. The NUNCD electrode, fabricated as a potential sensitive biosensor for dopamine without any catalyst or mediators, demonstrated good activity for the direct detection of dopamine by simply putting the bare NUNCD electrode into a dopamine solution. Furthermore, the marked selectivity of the NUNCD electrode is very favorable for the determination of dopamine (DA) concentration (0.32 μM) in the presence of ascorbic acid (AA) and uric acid (UA). Considering dopamine detection in real biological fluid samples, the NUNCD electrode performed excellently with a detection limit of 0.39 μM and a high recovery ranging from 90-120%, revealing that NUNCD electrodes have promising use in the sensing of dopamine.

Superhydrophobic graphene-based sponge as a novel sorbent for crude oil removal under various environmental conditions

Mechanical recovery of oils using oil sorbents is one of the most important approaches to manage marine oil spills. However, the properties of the oils spilled into sea are influenced by external environmental conditions. In this study, we present a graphene-based (GB) sponge as a novel sorbent for crude oil removal and compare its performance with that of a commercial sorbent sheet under various environmental parameters. The GB sponge with excellent superhydrophobic and superoleophilic characteristics is demonstrated to be an efficient sorbent for crude oils, with high sorption capacity (up to 85-95 times its weight) and good reusability. The crude-oil-sorption capacity of our GB sponge is remarkably higher (about 4-5 times) than that of the commercial sheet and most other previously reported sponge sorbents. Moreover, several challenging environmental conditions were examined for their effects on the sorption performance, including the weathering time of oils, seawater temperature, and turbulence (wave effect). The results show that the viscosity of the oil increased with increasing weathering time or decreasing temperature; therefore, the sorption rate seemed to decrease with longer weathering times and lower temperatures. Turbulence can facilitate inner sorption and promote higher oil sorption. Our results indicate that the extent of the effects of weather and other environmental factors on crude oil should be considered in the assessment of the effective adsorption capacity and efficiency of sorbents. The present work also highlights the widespread potential applications of our GB sponge in marine spilled-oil cleanup and hydrophobic solvent removal.

The enhancement of the electron field emission behavior of diamond/CNTs materials via the plasma post-treatment process for the applications in triode-type vacuum field emission transistor

Plasma post-treatment (ppt) process for enhancing the electron field emission (EFE) properties of diamond/CNTs films for the applications in triode vacuum field emission (VFE) transistors is being reported. The EFE properties of UNCD/CNTs films were markedly improved by modifying the granular structure by ppt-process. The factor which resulted in enhanced EFE properties is due to the formation of nano-graphitic layers, while the nano-sized diamond grains coalesced. The triode VFE transistors performance was also significantly enhanced due to the utilization of DGC/CNTs (or HiD/CNTs) films as cathode that renders the triode VFE transistors possessing great potential for applications in vacuum microelectronics.