Haisheng Song

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.

Enhanced photocatalytic performance of TiO2-ZnO hybrid nanostructures

We studied the photocatalytic properties of rational designed TiO2-ZnO hybrid nanostructures, which were fabricated by the site-specific deposition of amorphous TiO2 on the tips of ZnO nanorods. Compared with the pure components of ZnO nanorods and amorphous TiO2 nanoparticles, these TiO2-ZnO hybrid nanostructures demonstrated a higher catalytic activity. The strong green emission quenching observed from photoluminescence of TiO2-ZnO hybrid nanostructures implied an enhanced charge transfer/separation process resulting from the novel type II heterostructures with fine interfaces. The catalytic performance of annealing products with different TiO2 phase varied with the annealing temperatures. This is attributed to the combinational changes in Eg of the TiO2 phase, the specific surface area and the quantity of surface hydroxyl groups.

Flexible and Semitransparent Organolead Triiodide Perovskite Network Photodetector Arrays with High Stability.

Organolead triiodide perovskite (CH3NH3PbI3) as a light-sensitive material has attracted extensive attention in optoelectronics. The reported perovskite photodetectors (PDs) mainly focus on the individual, which limits their spatial imaging applications. Uniform perovskite networks combining transparency and device performance were synthesized on poly(ethylene terephthalate) (PET) by controlling perovskite crystallization. Photodetector arrays based on above network were fabricated to demonstrate the potential for image mapping. The trade-off between the PD performance and transparency was systematically investigated and the optimal device was obtained from 30 wt % precursor concentration. The switching ratio, normalized detectivity, and equivalent dark current derived shot noise as the critical parameters of PD arrays reached 300, 1.02 × 10(12) Jones, and 4.73 × 10(-15)A Hz(-1/2), respectively. Furthermore, the PD arrays could clearly detect spatial light intensity distribution, thus demonstrating its preliminary imaging function. The perovskite network PD arrays fabricated on PET substrates could also conduct superior flexibility under wide angle and large number of bending. For the common problem of perovskite optoelectronics in stability, the perovskite networks sheathed with hydrophobic polymers greatly enhanced the device stability due to the improved interface contacts, surface passivation, and moisture isolation. Taking into consideration transparency, flexibility, imaging and stability, the present PD arrays were expected to be widely applied in visualized portable optoelectronic system.

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.

Sulfurization induced surface constitution and its correlation to the performance of solution-processed Cu2ZnSn(S,Se)4 solar cells.

To obtain high photovoltaic performances for the emerging copper zinc tin sulfide/selenide (CZTSSe) thin film solar cells, much effort has deservedly been placed on CZTSSe phase purification and CZTSSe grain size enhancement. Another highly crucial but less explored factor for device performance is the elemental constitution of CZTSSe surface, which is at the heart of p-n junction where major photogenerated carriers generate and separate. In this work we demonstrate that, despite the well-built phase and large grained films are observed by common phases and morphology characterization (XRD, Raman and SEM), prominent device efficiency variations from short circuited to 6.4% are obtained. Insight study highlights that the surface (0–250 nm) compositions variation results in different bulk defect depths and doping densities in the depletion zone. We propose that suitable sulfurization (at ~10 kPa sulfur pressure) drives optimization of surface constitution by managing the Cu, Zn and Sn diffusion and surface reaction. Therefore, our study reveals that the balance of elemental diffusion and interface reactions is the key to tuning the surface quality CZTSSe film and thus the performance of as resulted devices.

Tunable p-type conductivity and transport properties of AlN nanowires via Mg doping.

Arrays of well-aligned AlN nanowires (NWs) with tunable p-type conductivity were synthesized on Si(111) substrates using bis(cyclopentadienyl)magnesium (Cp(2)Mg) vapor as a doping source by chemical vapor deposition. The Mg-doped AlN NWs are single-crystalline and grow along the [001] direction. Gate-voltage-dependent transport measurements on field-effect transistors constructed from individual NWs revealed the transition from n-type conductivity in the undoped AlN NWs to p-type conductivity in the Mg-doped NWs. By adjusting the doping gas flow rate (0-10 sccm), the conductivity of AlN NWs can be tuned over 7 orders of magnitude from (3.8-8.5) × 10(-6) Ω(-1) cm(-1) for the undoped sample to 15.6-24.4 Ω(-1) cm(-1) for the Mg-doped AlN NWs. Hole concentration as high as 4.7 × 10(19) cm(-3) was achieved for the heaviest doping. In addition, the maximum hole mobility (∼6.4 cm(2)/V s) in p-type AlN NWs is much higher than that of Mg-doped AlN films (∼1.0 cm(2)/V s). (2) The realization of p-type AlN NWs with tunable electrical transport properties may open great potential in developing practical nanodevices such as deep-UV light-emitting diodes and photodetectors.

Wearable and sensitive heart-rate detectors based on PbS quantum dot and multiwalled carbon nanotube blend film

Wearable and sensitive photodetectors (PDs) have been demonstrated based on a blend film of PbS quantum dots (QDs) and QDs modified multiwalled carbon nanotubes (MWCNTs). Owing to the synergetic effect from high light sensitivity of PbS QDs and excellent conductive and mechanical properties of MWCNTs, the blend PDs show high sensitivity and flexibility performance: device responsivity and detectivity reach 583 mA/W and 3.25 × 1012 Jones, respectively, and could stand large number (at least 10 000 cycles) and wide angle (up to 80°) bending. Furthermore, the wearable and sensitive PDs have been applied to measure the heart rate in both red and near infrared (NIR) ranges. The presented PDs are expected to work as sensor candidates in integrated electronic skin.

Self-assembly and horizontal orientation growth of VO2 nanowires.

Single-crystalline vanadium dioxide (VO2) nanostructures have attracted an intense research interest recently because of their unique single-domain metal-insulator phase transition property. Synthesis of these nanostructures in the past was limited in density, alignment, or single-crystallinity. The assembly of VO2 nanowires (NWs) is desirable for a “bottom-up” approach to the engineering of intricate structures using nanoscale building blocks. Here, we report the successful synthesis of horizontally aligned VO2 NWs with a dense growth mode in the [1-100]quartz direction of a polished x-cut quartz surface using a simple vapor transport method. Our strategy of controlled growth of VO2 NWs promisingly paves the way for designing novel metal-insulator transition devices based on VO2 NWs.

Investigation of the Interaction between Perovskite Films with Moisture via in Situ Electrical Resistance Measurement.

Organometal halide perovskites have recently emerged as outstanding semiconductors for solid-state optoelectronic devices. Their sensitivity to moisture is one of the biggest barriers to commercialization. In order to identify the effect of moisture in the degradation process, here we combined the in situ electrical resistance measurement with time-resolved X-ray diffraction analysis to investigate the interaction of CH3NH3PbI(3-x)Cl(x) perovskite films with moisture. Upon short-time exposure, the resistance of the perovskite films decreased and it could be fully recovered, which were ascribed to a mere chemisorption of water molecules, followed by the reversible hydration into CH3NH3PbI(3-x)Cl(x)·H2O. Upon long-time exposure, however, the resistance became irreversible due to the decomposition into PbI2. The results demonstrated the formation of monohydrated intermediate phase when the perovskites interacted with moisture. The role of moisture in accelerating the thermal degradation at 85 °C was also demonstrated. Furthermore, our study suggested that the perovskite films with fewer defects may be more inherently resistant to moisture.

High Quantum Yield Blue Emission from Lead-Free Inorganic Antimony Halide Perovskite Colloidal Quantum Dots

Colloidal quantum dots (QDs) of lead halide perovskite have recently received great attention owing to their remarkable performances in optoelectronic applications. However, their wide applications are hindered from toxic lead element, which is not environment- and consumer-friendly. Herein, we utilized heterovalent substitution of divalent lead (Pb2+) with trivalent antimony (Sb3+) to synthesize stable and brightly luminescent Cs3Sb2Br9 QDs. The lead-free, full-inorganic QDs were fabricated by a modified ligand-assisted reprecipitation strategy. A photoluminescence quantum yield (PLQY) was determined to be 46% at 410 nm, which was superior to that of other reported halide perovskite QDs. The PL enhancement mechanism was unraveled by surface composition derived quantum-well band structure and their large exciton binding energy. The Br-rich surface and the observed 530 meV exciton binding energy were proposed to guarantee the efficient radiative recombination. In addition, we can also tune the inorganic perovskite QD (Cs3Sb2X9) emission wavelength from 370 to 560 nm via anion exchange reactions. The developed full-inorganic lead-free Sb-perovskite QDs with high PLQY and stable emission promise great potential for efficient emission candidates.