Jiannong Wang

Highly monodisperse polymer-capped ZnO nanoparticles: Preparation and optical properties

We report the preparation of highly monodisperse ZnO nanoparticles using poly(vinyl pyrrolidone) (PVP) as the capping molecules. The surface-modified ZnO nanoparticles were found to be remarkably stable. The optical absorption shows distinct excitonic features. Markedly enhanced near-band-edge ultraviolet photoluminescence and significantly reduced defect-related green emission were also observed. We attribute this observation to the nearly perfect surface passivation of the ZnO nanoparticles by the PVP molecules. The third-order nonlinear optical response of these PVP-capped ZnO nanoparticles in a dilute solution was found to be significantly larger (by at least two orders of magnitude) than that of the bulk ZnO.

High-Performance Hole-Extraction Layer of Sol-Gel-Processed NiO Nanocrystals for Inverted Planar Perovskite Solar Cells**

Hybrid organic/inorganic perovskite solar cells have been rapidly evolving with spectacular successes in both nanostructured and thin-film versions. Herein, we report the use of a simple sol-gel-processed NiO nanocrystal (NC) layer as the hole-transport layer in an inverted perovskite solar cell. The thin NiO NC film with a faceted and corrugated surface enabled the formation of a continuous and compact layer of well-crystallized CH3 NH3 PbI3 in a two-step solution process. The hole-extraction and -transport capabilities of this film interfaced with the CH3 NH3 PbI3 film were higher than those of organic PEDOT:PSS layers. The cell with a NiO NC film with a thickness of 30-40 nm exhibited the best performance, as a thinner layer led to a higher leakage current, whereas a thicker layer resulted in a higher series resistance. With the NiO film, we observed a cell efficiency of 9.11 %, which is by far the highest reported for planar perovskite solar cells based on an inorganic hole-extracting layer.

Impurity effect on weak antilocalization in the topological insulator Bi2Te3.

We study the weak antilocalization (WAL) effect in topological insulator Bi(2)Te(3) thin films at low temperatures. The two-dimensional WAL effect associated with surface carriers is revealed in the tilted magnetic field dependence of magnetoconductance. Our data demonstrate that the observed WAL is robust against deposition of nonmagnetic Au impurities on the surface of the thin films, but it is quenched by the deposition of magnetic Fe impurities which destroy the π Berry phase of the topological surface states. The magnetoconductance data of a 5 nm Bi(2)Te(3) film suggests that a crossover from symplectic to unitary classes is observed with the deposition of Fe impurities.

High‐Efficiency All‐Polymer Solar Cells Based on a Pair of Crystalline Low‐Bandgap Polymers

All-polymer solar cells based on a pair of crystalline low-bandgap polymers (NT and N2200) are demonstrated to achieve a high short-circuit current density of 11.5 mA cm-2 and a power conversion efficiency of up to 5.0% under the standard AM1.5G spectrum with one sun intensity. The high performance of these NT:N2200-based cells can be attributed to the low optical bandgaps of the polymers and the reasonably high and balanced electron and hole mobilities of the NT:N2200 blends due to the crystalline nature of the two polymers.

Field emission from crystalline copper sulphide nanowire arrays

Straight crystalline copper sulphide (Cu2S) nanowire arrays have been grown by using a simple gas–solid reaction at room temperature. These were demonstrated to exhibit semiconductor properties. Field emission was observed at a field of ∼6 MV/m, and its current-field characteristics deviate from Fowler–Nordheim theory, i.e., showing a nonlinear Fowler–Nordheim plot. The uniform emission from the whole arrays was observed using transparent anode technique, and their variation with applied field was recorded. The emission from individual nanowires was also studied using a field emission microscope, and was found to consist of a number of spatially resolved diffuse spots. Finally, stable emission current at different levels and over time was recorded. These findings indicate that semiconductor nanowires as cold cathode have a potential future, worthy of further comprehensive investigation. The technical importance of using semiconductor nanowires as cold cathode emitter is given.

Mono-sized single-wall carbon nanotubes formed in channels of AlPO4-5 single crystal

An alternative approach to the synthesis of mono-sized and parallel-aligned single-wall carbon nanotubes (SWCNs) is reported. The SWCNs are formed in 0.73 nm sized channels of microporous aluminophosphate crystallites by pyrolysis of tripropylamine molecules in the channels. They are characterized through transmission electron microscopy, polarized Raman scattering, and electrical transport measurements. Our results would open a door to further detailed studies on the intrinsic properties of carbon nanotubes now in progress.

Cost-efficient clamping solar cells using candle soot for hole extraction from ambipolar perovskites

Ambient-unstable hole transporters and expensive and complicated noble metal electrode deposition are incompatible with the large scale and low-cost production of perovskite solar cells and thus would hamper their commercialization. Herein we report a new modality of perovskite solar cells that do away with the use of conventional hole transporters by directly clamping a selective hole extraction electrode made of candle soot and a deliberately engineered perovskite photoanode. The key soot/perovskite interface, which promotes hole extraction and electron blocking by forming a Schottky junction, was established seamlessly by pre-wetting and reaction embedding the carbon particles. Femtosecond time-resolved photoluminescence revealed a high hole extraction rate at 1.92 ns−1. We have now achieved 11.02% efficiency, making an important step towards roll-to-roll production of perovskite solar cells.

The van der Waals epitaxy of Bi2Se3 on the vicinal Si(111) surface: an approach for preparing high-quality thin films of a topological insulator

The epitaxial growth of thin films of the topological insulator Bi2Se3 on nominally flat and vicinal Si(111) substrates was studied. In order to achieve a planar growth front and better quality epifilms, a two-step growth method was adopted for the van der Waals epitaxy of Bi2Se3 to proceed. By using vicinal Si(111) substrate surfaces, the in-plane growth rate anisotropy of Bi2Se3 was exploited in order to achieve single crystalline Bi2Se3 epifilms, in which threading defects and twins are effectively suppressed. The optimization of the growth parameters has resulted in the vicinal Bi2Se3 films showing a carrier mobility of ~2000 cm2 V−1 s−1 and a background doping of ~3×1018 cm−3 of the as-grown layers. Such samples not only show a relatively high magnetoresistance but also show a linear dependence on the magnetic field.

High‐Performance Graphene‐Based Hole Conductor‐Free Perovskite Solar Cells: Schottky Junction Enhanced Hole Extraction and Electron Blocking

Multilayered graphene and single-layered graphene are assembled onto perovskite films in the form of Schottky junctions and ohmic contacts, respectively, for the production of a graphene-based hole transporting material-free perovskite solar cell. Multilayered graphene extracts charge selectively and efficiently, delivering a higher efficiency of 11.5% than single-layered graphene (6.7%).

Enhanced ultraviolet emission and optical properties in polyvinyl pyrrolidone surface modified ZnO quantum dots

Optical properties of ZnO quantum dots (QDs) capped with polyvinyl pyrrolidone (PVP) molecules have been investigated. It is demonstrated that surface modification by PVP can dramatically change the emission spectra of the ZnO QDs. At the optimized condition with a PVP/Zn2+ ratio of 3:5, the photoluminescence (PL) spectrum of ZnO QDs shows a strong ultraviolet (UV) emission while the low energy green emission is fully quenched. This is a result of the surface passivation of the ZnO QDs by the PVP molecules. The origin of the green emission is attributed to the surface states associated with oxygen vacancies. Temperature and excitation power dependent PL studies suggest that the UV emission is associated with localized states.