Lin-Bao Luo

Incorporation of Graphenes in Nanostructured TiO(2) Films via Molecular Grafting for Dye-Sensitized Solar Cell Application

This paper presents a systematic investigation on the incorporation of chemical exfoliation graphene sheets (GS) in TiO(2) nanoparticle films via a molecular grafting method for dye-sensitized solar cells (DSSCs). By controlling the oxidation time in the chemical exfoliation process, both high conductivity of reduced GS and good attachment of TiO(2) nanoparticles on the GS were achieved. Uniform GS/TiO(2) composite films with large areas on conductive glass were prepared by electrophoretic deposition, and the incorporation of GS significantly improved the conductivity of the TiO(2) nanoparticle film by more than 2 orders of magnitude. Moreover, the power conversion efficiency for DSSC based on GS/TiO(2) composite films is more than 5 times higher than that based on TiO(2) alone, indicating that the incorporation of GS is an efficient means for enhancing the photovoltaic (PV) performance. The better PV performance of GS/TiO(2) DSSC is also attributed to the better dye loading of GS/TiO(2) film than that of TiO(2) film. The effect of GS content on the PV performances was also investigated. It was found that the power conversion efficiency increased first and then decreased with the increasing of GS concentration due to the decrease in the transmittance at high GS content. Further improvements can be expected by fully optimizing fabrication conditions and device configuration, such as increasing dye loading via thicker films. The present synthetic strategy is expected to lead to a family of composites with designed properties.

p-Type ZnO nanowire arrays.

Well-aligned ZnO nanowire (NW) arrays with durable and reproducible p-type conductivity were synthesized on alpha-sapphire substrates by using N2O as a dopant source via vapor-liquid-solid growth. The nitrogen-doped ZnO NWs are single-crystalline and grown predominantly along the [110] direction, in contrast to the [001] direction of undoped ZnO NWs. Electrical transport measurements reveal that the nondoped ZnO NWs exhibit n-type conductivity, whereas the nitrogen-doped ZnO NWs show compensated highly resistive n-type and finally p-type conductivity upon increasing N2O ratio in the reaction atmosphere. The electrical properties of p-type ZnO NWs are stable and reproducible with a hole concentration of (1-2) x 10(18) cm(-3) and a field-effect mobility of 10-17 cm2 V(-2) s(-1). Surface adsorptions have a significant effect on the transport properties of NWs. Temperature-dependent PL spectra of N-doped ZnO NWs show acceptor-bound-exciton emission, which corroborates the p-type conductivity. The realization of p-type ZnO NWs with durable and controlled transport properties is important for fabrication of nanoscale electronic and optoelectronic devices.

Monolayer Graphene Film on ZnO Nanorod Array for High‐Performance Schottky Junction Ultraviolet Photodetectors

A new Schottky junction ultraviolet photodetector (UVPD) is fabricated by coating a free-standing ZnO nanorod (ZnONR) array with a layer of transparent monolayer graphene (MLG) film. The single-crystalline [0001]-oriented ZnONR array has a length of about 8-11 μm, and a diameter of 100∼600 nm. Finite element method (FEM) simulation results show that this novel nanostructure array/MLG heterojunction can trap UV photons effectively within the ZnONRs. By studying the I-V characteristics in the temperature range of 80-300 K, the barrier heights of the MLG film/ZnONR array Schottky barrier are estimated at different temperatures. Interestingly, the heterojunction diode with typical rectifying characteristics exhibits a high sensitivity to UV light illumination and a quick response of millisecond rise time/fall times with excellent reproducibility, whereas it is weakly sensitive to visible light irradiation. It is also observed that this UV photodetector (PD) is capable of monitoring a fast switching light with a frequency as high as 2250 Hz. The generality of the above results suggest that this MLG film/ZnONR array Schottky junction UVPD will have potential application in future optoelectronic devices.

Core-shell heterojunction of silicon nanowire arrays and carbon quantum dots for photovoltaic devices and self-driven photodetectors.

Silicon nanostructure-based solar cells have lately intrigued intensive interest because of their promising potential in next-generation solar energy conversion devices. Herein, we report a silicon nanowire (SiNW) array/carbon quantum dot (CQD) core-shell heterojunction photovoltaic device by directly coating Ag-assisted chemical-etched SiNW arrays with CQDs. The heterojunction with a barrier height of 0.75 eV exhibited excellent rectifying behavior with a rectification ratio of 10(3) at ±0.8 V in the dark and power conversion efficiency (PCE) as high as 9.10% under AM 1.5G irradiation. It is believed that such a high PCE comes from the improved optical absorption as well as the optimized carrier transfer and collection capability. Furthermore, the heterojunction could function as a high-performance self-driven visible light photodetector operating in a wide switching wavelength with good stability, high sensitivity, and fast response speed. It is expected that the present SiNW array/CQD core-shell heterojunction device could find potential applications in future high-performance optoelectronic devices.

Monolayer graphene film/silicon nanowire array Schottky junction solar cells

Schottky junction solar cells were constructed by combining the monolayer graphene (MLG) films and the Si nanowire (SiNW) arrays. Pronounced photovoltaic characteristics were investigated for devices with both p-MLG/n-SiNWs and n-MLG/p-SiNWs structures. Due to the balance between light absorption and surface carrier recombination, devices made of SiNW arrays with a medium length showed better performance and could be further improved by enhancing the MLG conductivity via appropriate surface treatment or doping. Eventually, a photoconversion efficiency up to 2.15% is obtained by the means of filling the interspace of SiNW array with graphene suspension.

High-Quality Graphenes via a Facile Quenching Method for Field-Effect Transistors

Single- and few-layer graphene sheets with sizes up to 0.1 mm were fabricated by simply quenching hot graphite in an ammonium hydrogen carbonate aqueous solution. The identity and thickness of graphene sheets were characterized with transmission electron microscopy, atomic force microscopy, and Raman spectroscopy. In addition to its simplicity and scalability, the present synthesis can produce graphene sheets with excellent qualities in terms of sizes, purity, and crystal quality. The as-produced graphene sheets can be easily transferred to solid substrates for further processing. Field-effect transistors based on individual graphenes were fabricated and shown to have high ambipolar carrier mobilities.

Graphene-β-Ga2 O3 Heterojunction for Highly Sensitive Deep UV Photodetector Application.

A deep UV light photodetector is assembled by coating multilayer graphene on beta-gallium oxide (β-Ga2 O3 ) wafer. Optoelectronic analysis reveals that the heterojunction device is virtually blind to light illumination with wavelength longer than 280 nm, but is highly sensitive to 254 nm light with very good stability and reproducibility.

Device structure-dependent field-effect and photoresponse performances of p-type ZnTe:Sb nanoribbons

Sb-doped ZnTe nanoribbons (NRs) with enhanced p-type conductivity were successfully synthesized by a simple thermal co-evaporation method. Nanodevices, including nano-field-effect transistors (FETs) and nano-photodetectors (nanoPDs), were constructed based on the ZnTe:Sb NRs and their structure dependent device performances were systemically investigated. It is found that the transport properties of the ZnTe nanostructures as well as the device structures play a critical role in determining the device performances. In contrast to the nano-metal-oxide-semiconductor FETs (nanoMOSFETs) with back-gate structure, the top-gate nano-metal-insulator-semiconductor FETs (nanoMISFETs) show much enhanced performances in all aspects. On the other hand, owing to the appropriate p-type doping, nano-photodetectors (nanoPDs) based on the ZnTe:Sb NRs exhibit excellent device performances, such as high responsivity and photoconductive gain, fast response speed, large detectivity and so on. Moreover, the response time could be effectively shortened by using nano-heterojunction photodetectors (nanoHPDs). It is expected that knowledge gained from this work could be readily extended to nanodevices based on other nanostructures.

Self-powered and fast-speed photodetectors based on CdS:Ga nanoribbon/Au Schottky diodes

Self-powered photodetectors based on CdS:Ga nanoribbons (NR)/Au Schottky barrier diodes (SBDs) were fabricated. The as-fabricated SBDs exhibit an excellent rectification characteristic with a rectification ratio up to 106 within ±1 V in the dark and a distinctive photovoltaic (PV) behavior under light illumination. Photoconductive analysis reveals that the SBDs were highly sensitive to light illumination with very good stability, reproducibility and fast response speeds at zero bias voltage. The corresponding rise/fall times of 95/290 μs represent the best values obtained for CdS based nano-photodetectors. It is expected that such self-powered high performance SBD photodetectors will have great potential applications in optoelectronic devices in the future.

Monolayer hexagonal arsenene with tunable electronic structures and magnetic properties via impurity doping

Monolayer hexagonal arsenene (hAs), a typical two-dimensional semiconducting material with a wide band gap and high stability, has attracted increasing research interest due to its potential applications in optoelectronics. Using first-principles calculations, we have investigated the electronic and magnetic properties of x-substituted hAs (x = B, C, N, O, Ga, Ge, Se, and monovacancy) and x-adsorbed hAs (x = As). Our results show that the B-, N-, and Ga-substituted hAs have spin-unpolarized semiconducting characters like pristine hAs, and indirect–direct band gap transitions are induced in the B- and N-substituted systems. In contrast, the O-, Se-, and monovacancy-substituted hAs are metallic, and the C- and Ge-substituted hAs show spin-polarized semiconducting characters with band gaps of 1.1 and 1.3 eV for the spin-up channels and 1.0 and 0.7 eV for the spin-down channels, respectively. For the As-adsorbed hAs, the Fermi level crosses the spin-up states, yielding metallic behavior, while the spin-down channel retains semiconducting character. Detailed analysis of electronic structures for the C-substituted, Ge-substituted, and As-adsorbed hAs shows that strong hybridizations between the doping atoms and As atoms lead to energy splitting near the Fermi level and consequently induce magnetic moments. By selective doping, hAs can be transformed from a spin-nonpolarized semiconductor to a spin-polarized semiconductor, to a half-metal, or even to a metal, which indicates that the doped hAs will have promising potential in future electronics, spintronics, and optoelectronics.