Weichang Zhou

Continuous Alloy-Composition Spatial Grading and Superbroad Wavelength-Tunable Nanowire Lasers on a Single Chip

By controlling local substrate temperature in a chemical vapor deposition system, we have successfully achieved spatial composition grading covering the complete composition range of ternary alloy CdSSe nanowires on a single substrate of 1.2 cm in length. Spatial photoluminescence scan along the substrate length shows peak wavelength changes continuously from approximately 500 to approximately 700 nm. Furthermore, we show that under strong optical pumping, every spot along the substrate length displays lasing behavior. Thus our nanowire chip provides a spatially continuously tunable laser with a superbroad wavelength tuning range, unmatched by any other available semiconductor-based technology.

Luminescence and local photonic confinement of single ZnSe:Mn nanostructure and the shape dependent lasing behavior.

One-dimensional Mn-ZnSe nanostructures with high crystallite quality were synthesized by the CVD method. Transmission electron microscopy was used to study the defect state, crystal lattice and growth direction of as-prepared nanostructures. Raman spectra under varied excitation wavelengths confirmed the dopant modes of Mn(II) and the inhomogeneity. The micro-photoluminescence (PL) spectra of individual nanostructures under CW laser excitation with different powers showed the dominant trapped state emission with periodic multi-peaks. The selected peak mapping indicated that there were many integrated Fabry-Perot cavities and whispering gallery mode cavities within the nanowires/nanoneedles and nanobelts, respectively, which can be accounted for by inhomogeneous optical phases in the Mn-ZnSe nanostructure. The phase may be introduced by both Mn doping and structural relaxation. The micro-PL spectra under nanosecond pulse laser excitation produce low threshold lasing lines near the band edge of Mn-ZnSe nanostructures. The lasing occurs due to the dominant interaction between bound excitons at high density, evidenced by its appearance close to the LO phonon replica. The belts show much stronger lasing emission due to larger 2D coherent space than the wires due to the inhomogeneity induced by the doping process. The different optical behavior with changing excitation pulses may find applications in future photonic devices of II-VI nanostructures.

One-step synthesis of low-dimensional CdSe nanostructures and optical waveguide of CdSe nanowires

Low-dimension (1D) semiconductor CdSe nanostructures, including nanosheets, nanoribbons, nanowires and branchlike, have been successfully synthesized in bulk quantities by a simple and low-cost process based on thermal evaporation of CdSe powders onto a silicon substrate. The temperature of the substrates and the concentration of CdSe vapour are the critical parameters for the formation of different morphologies of CdSe nanostructure by scanning electron microscopy and transmission electron microscopy characterization. The growth of nanowires is controlled by the conventional vapour–liquid–solid (VLS) mechanism, while the nanobelts comply with a combination of VLS and vapour–solid (VS) processes; the formation of sheets primarily follows the nanobelt formation with more VS growth. The photoluminescence properties of CdSe nanowires were studied by confocal microscopy; a clear optical waveguide was observed and discussed for the first time.

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.

Ordered CdS micro/nanostructures on CdSe nanostructures

Composite structures of aligned and orientation-ordered quasi-one-dimensional CdS micro/nanostructures on CdSe substrates of different shaped nanostructures have been synthesized by using two-step thermal evaporation processes. The CdSe substrate crystalline orientations and local temperatures play their roles in the CdS nanostructure growth step, which is in some contrast with the vapor-liquid-solid (VLS) growth mechanism. Micro-photoluminescence measurements show strong luminescence responses on the six-fold symmetrical CdSe/CdS nanostructure. Controllable growth on various shaped substrates may find applications in obtaining many other aligned orientation-ordered hetero-nano/microstructure materials.

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.

The effect of dopant and optical micro-cavity on the photoluminescence of Mn-doped ZnSe nanobelts

Pure and Mn-doped ZnSe nanobelts were synthesized by a convenient thermal evaporation method. Scanning electron microscopy, X-ray powder diffraction, energy dispersive X-ray spectroscopy and corresponding element mapping, and transmission electron microscope were used to examine the morphology, phase structure, crystallinity, composition, and growth direction of as-prepared nanobelts. Raman spectra were used to confirm the effective doping of Mn2+ into ZnSe nanobelts. Micro-photoluminescence (PL) spectra were used to investigate the emission property of as-prepared samples. A dominant trapped-state emission band is observed in single ZnSeMn nanobelt. However, we cannot observe the transition emission of Mn ion in this ZnSeMn nanobelt, which confirm that Mn powder act as poor dopant. There are weak near-bandgap emission and strong 4T1 → 6A1 transition emission of Mn2+ in single ZnSeMnCl2 and ZnSeMn(CH3COO)2 nanobelt. More interesting, the 4T1 → 6A1 transition emission in ZnSeMn(CH3COO)2 nanobelt split into multi-bands. PL mapping of individual splitted sub-bands were carried out to explore the origin of multi-bands. These doped nanobelts with novel multi-bands emission can find application in frequency convertor and wavelength-tunable light emission 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.