Yuehua Peng

Broad spectral response photodetector based on individual tin-doped CdS nanowire

High purity and tin-doped 1D CdS micro/nano-structures were synthesized by a convenient thermal evaporation method. SEM, EDS, XRD and TEM were used to examine the morphology, composition, phase structure and crystallinity of as-prepared samples. Raman spectrum was used to confirm tin doped into CdS effectively. The effect of impurity on the photoresponse properties of photodetectors made from these as-prepared pure and tin-doped CdS micro/nano-structures under excitation of light with different wavelength was investigated. Various photoconductive parameters such as responsivity, external quantum efficiency, response time and stability were analyzed to evaluate the advantage of doped nanowires and the feasibility for photodetector application. Comparison with pure CdS nanobelt, the tin-doped CdS nanowires response to broader spectral range while keep the excellect photoconductive parameters. Both trapped state induced by tin impurity and optical whispering gallery mode microcavity effect in the doped CdS nanowires contribute to the broader spectral response. The micro-photoluminescence was used to confirm the whispering gallery mode effect and deep trapped state in the doped CdS nanowires.

Memristive properties of hexagonal WO3 nanowires induced by oxygen vacancy migration

Tungsten trioxide (WO3) is always oxygen-deficient or non-stoichiometric under atmospheric conditions. Positively charged oxygen vacancies prefer to drift as well as electrons when the electric field is strong enough, which will alter the distribution of oxygen vacancies and then endow WO3 with memristive properties. In Au/WO3 nanowire/Au sandwich structures with two ohmic contacts, the axial distribution of oxygen vacancies and then the electrical transport properties can be more easily modulated by bias voltage. The threshold electric field for oxygen vacancy drifting in single-crystal hexagonal WO3 nanowire is about 106 V/m, one order of magnitude less than that in its granular film. At elevated temperatures, the oxygen vacancy drifts and then the memristive effect can be enhanced remarkably. When the two metallic contacts are asymmetric, the WO3 nanowire devices even demonstrate good rectifying characteristic at elevated temperatures. Based on the drift of oxygen vacancies, nanoelectronic devices such as memristor, rectifier, and two-terminal resistive random access memory can be fabricated on individual WO3 nanowires.

Multi-layered MoS2 phototransistors as high performance photovoltaic cells and self-powered photodetectors

An optoelectronic diode based on a p–n junction is one of the most fundamental device building blocks with extensive applications. Compared with graphene, layered transition-metal dichalcogenides demonstrate promising applications in novel valley-electronics and opto-electronics. Here we reported the fabrication and optoelectronic properties of a single multilayer MoS2 sheet. Our results indicate that the thin MoS2 shows a linear transport property while thick MoS2 shows diode characteristics with well-defined current rectification behavior. We assign that the rectification behavior is due to the formation of a p–n junction in the single multilayer MoS2 piece. The intrinsic defects in MoS2 can change the conduction polarity, such as: sulfur vacancies contribute to the n-type behavior while sulfur interstitials and molybdenum vacancies contribute to the p-type conduction. The variation of intrinsic defects and stoichiometry is obvious over the micrometer range in thick MoS2. The fabricated MoS2 transistors were assessed under bias and gate voltage modulation when exposed to red, green and UV light under vacuum. The multilayer MoS2 shows dominant p-type behavior under dark conditions while its shows dominant n-type conduction under light illumination. In addition, this MoS2 phototransistor shows an evident photovoltaic effect. The open-circuit voltage (Voc) and short-circuit current (Isc) are observed to be −0.48 V and 494 nA under red illumination. These results demonstrate the potential application of a single multilayer MoS2 sheet in optoelectronics, such as light-emitting diodes (LEDs), field-effect photovoltaic cells and photodetectors.

Enhanced memristive performance of individual hexagonal tungsten trioxide nanowires by water adsorption based on Grotthuss mechanism

The electrical transport properties of Au/WO3 nanowire/Au sandwich structures were studied under different levels of relative humidity at room temperature. Experimental results indicate that the memristive performance of the structures is enhanced remarkably when the relative humidity is above a critical level, which might be attributed to H+ drifting based on Grotthuss mechanism. H+ (produced by oxidizing water molecules with holes) drifting results in not only changes in the barrier heights but also an increase in electrical current. The Au/WO3 nanowire/Au sandwich structures, whose memristive performance can be modulated by gas molecule adsorption, might be a good candidate for exploiting next generation memory devices.

Reconfigurable resistive switching devices based on individual tungsten trioxide nanowires

In the two-terminal Au/WO3 nanowire/Au electronic device with two Schottky barriers, drifting of oxygen vacancies under strong electric field induced by the bias voltage applied at short distance will result in the effective width of the reverse biased Schottky barrier decreasing, and then result in the memristive effect or resistive switching phenomenon. By unidirectional bias voltage sweeping, the Au/WO3 Schottky contact can be turned gradually and reversibly into Ohmic contact, and then the two-terminal Au/WO3 nanowire/Au resistive switching device can be reconfigured gradually and reversibly from non-rectifying state to either a forward or reverse rectifying state.

Electrical characterization of H2S adsorption on hexagonal WO3 nanowire at room temperature

We have characterized the electrical transport properties of Au/WO3 nanowire/Au devices in ambient air and gaseous H2S to investigate the adsorption kinetics of H2S molecules on the surface of WO3 nanowire at room temperature. The WO3 nanowire devices exhibit increasing linear conductance and electrical hysteresis in H2S. Furthermore, the contact type between Au electrode and WO3 nanowire can be converted from original ohmic/Schottky to Schottky/ohmic after being exposed to H2S. These results suggest that adsorbed H2S molecules are oxidized by holes to form hydrogen ions and S atoms, which will result in formation of hydrogen tungsten bronze and desorption of previously chemically adsorbed H2O molecules. Adsorbed H2S molecules can also oxidize previously adsorbed and ionized oxygen, which will release the electrons from the ionized oxygen and then weaken upward band bending at the surface of WO3 nanowire.

Modulating memristive performance of hexagonal WO3 nanowire by water-oxidized hydrogen ion implantation.

In a two-terminal Au/hexagonal WO3 nanowire/Au device, ions drifting or carriers self-trapping under external electrical field will modulate the Schottky barriers between the nanowire and electrodes, and then result in memristive effect. When there are water molecules adsorbed on the surface of WO3 nanowire, hydrogen ions will generate near the positively-charged electrode and transport in the condensed water film, which will enhance the memristive performance characterized by analogic resistive switching remarkably. When the bias voltage is swept repeatedly under high relative humidity level, hydrogen ions will accumulate on the surface and then implant into the lattice of the WO3 nanowire, which leads to a transition from semiconducting WO3 nanowire to metallic HxWO3 nanowire. This insulator-metal transition can be realized more easily after enough electron-hole pairs being excited by laser illumination. The concentration of hydrogen ions in HxWO3 nanowire will decrease when the device is exposed to oxygen atmosphere or the bias voltage is swept in atmosphere with low relative humidity. By modulating the concentration of hydrogen ions, conductive hydrogen tungsten bronze filament might form or rupture near electrodes when the polarity of applied voltage changes, which will endow the device with memristive performance characterized by digital resistive switching.

High-performance photodetectors based on bandgap engineered novel layer GaSe0.5Te0.5 nanoflakes

Layered two-dimensional (2D) gallium monochalcogenide (GaX, X = S, Se, Te) semiconductor crystals hold great promise for potential electronics and photonics application. In this paper, we reported the optoelectronic properties of 2D bandgap engineered GaSe0.5Te0.5 nanoflakes. The GaSe0.5Te0.5 nanoflakes were synthesized by chemical vapor deposition (CVD) and characterized by XRD, SEM, TEM, XPS, Raman and PL spectra, which demonstrate the high crystal quality of as-prepared nanoflakes. The photodetector based on single GaSe0.5Te0.5 nanoflake shows fast response time, high reversibility and stability both in air and vacuum. The photo-responsivity is up to 22 A W−1 under illumination of 532 nm light. More interesting, the GaSe0.5Te0.5 nanoflake photodetector demonstrate extended light response range, as compared with pure GaSe. The photo-responsivity is 13 A W−1 for 650 nm red light. The present results suggest strongly that the bandgap engineered 2D GaSe0.5Te0.5 nanoflakes hold extensive applications in next-generation photodetection and photosensing nanodevices.

Large-scale synthesis and electrical transport properties of single-crystalline SmB6 nanowires

Topological Kondo insulator samarium hexaboride (SmB6) nanowires, with diameters of 60–150 nm and lengths up to 1–5 μm, were successfully synthesized in large scale by chemical vapor deposition using BCl3 and SmCl3 as precursors at 1070 °C. Transmission electron microscopy observation and selected area electron diffraction analysis indicate that SmB6 nanowires are single-crystalline and grow in a preferred direction of [1 0 0]. It also indicates that the growth of SmB6 nanowires might be governed by a vapor–solid mechanism. Conventional four-terminal resistance measurements show that the resistance of an SmB6 nanowire increases with decreasing temperature, but saturates at temperatures less than 10 K, which might be attributed to a true topological insulator with a metallic surface and fully insulating bulk states. Resistance measurements also indicate that the contribution of surface states to conductance in the SmB6 nanowire is enhanced remarkably; therefore the high-quality single-crystalline SmB6 nanowires with large surface-to-bulk ratio might be the best candidate for investigating the topological properties of this material.

Surface polarons and optical micro-cavity modulated broad range multi-mode emission of Te-doped CdS nanowires

The optical confinement and strong carrier coupling within a semiconductor nanostructure cavity are crucial for the modulation of emission properties. Fundamental understanding of the light-matter interaction in a low dimensional system is important. In this paper, we synthesized high-quality hexagonal Te-doped CdS nanowires by two-step chemical vapor deposition and investigated systematically the doping concentration, temperature, excitation power, excitation wavelength dependent Raman, photoluminescence and carrier lifetime decay. Scanning electron microscopy, energy dispersive x-ray spectrometry and x-ray diffraction confirmed Te-doping in the as-prepared samples. The strong surface optical (SO) phonon mode is observed in the micro-Raman spectra of an individual Te-CdS nanowire, which is unsuitable in large-sized structures. In situ micro-photoluminescence (μ-PL) characterization shows dominant confined defect state emission with whispering gallery mode (WGM) characteristics. The emission peak position shifts under increased excitation power, demonstrating the inelastic scattering by bound carriers. In addition, the short wavelength emission modes are dominant at a low temperature (80 K) while the long wavelength emission modes are dominant at a high temperature (300 K) due to different recombination processes contributing to the WGM resonant bands, which was also confirmed by the time-resolved PL measurement. All these results reflect strong coupling between the surface evanescent-wave in the WGM cavity and the SO phonon/polaron, which will facilitate the rational tailoring of surface/interface relevant properties for nanophotonic device applications.