Xinzheng Lan

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.

High-performance CdS:P nanoribbon field-effect transistors constructed with high-κ dielectric and top-gate geometry

High-performance field-effect transistors (FETs) based on single phosphorus-doped n-type CdS nanoribbon with high-κ HfO2 dielectric and top-gate geometry were constructed. In contrast to the nano-FETs that were fabricated on SiO2/Si substrate with back-gate device configuration, the top-gate FETs exhibit a substantial improvement in performances, i.e., work voltage was reduced to a small value of within ±5 V, the subthreshold swing was reduced to 200 mV/dec and the Ion/Ioff ratio was increased by about six orders of magnitude. The top-gate CdS:P nano-FET shows high sensitivity upon light irradiation, revealing that the top-gate FETs are promising candidates for nanoelectronic and optoelectronic applications.

Construction of high-quality CdS:Ga nanoribbon/silicon heterojunctions and their nano-optoelectronic applications

Silicon based optoelectronic integration is restricted by its poor optoelectronic properties arising from the indirect band structure. Here, by combining silicon with another promising optoelectronic material, the CdS nanoribbon (NR), devices with heterojunction structure were constructed. The CdS NRs were also doped with gallium to improve their n-type conductivity. A host of nano-optoelectronic devices, including light emitting diodes, photovoltaic devices, and photodetectors, were successfully constructed on the basis of the CdS:Ga NR/Si heterojunctions. They all exhibited excellent device performances as regards high stability, high efficiency, and fast response speed. It is expected that the CdS NR/Si heterojunctions will have great potential for future applications of Si based optoelectronic integration.

High quantum-yield CdSexS1−x/ZnS core/shell quantum dots for warm white light-emitting diodes with good color rendering

Composition-controllable ternary CdSe(x)S(1-x) quantum dots (QDs) with multiple emission colors were obtained via a hot-injection-like method at a relatively low injection temperature (230 ° C) in octadecene. Then highly fluorescent CdSe(x)S(1-x)/ZnS core/shell (CS) QDs were synthesized by a facile single-molecular precursor approach. The fluorescent quantum yield of the resulting green (λ(em) = 523 nm), yellow (λ(em) = 565 nm) and red (λ(em) = 621 nm) emission of CS QDs in toluene reached up to 85%, 55% and 39%, respectively. Moreover, a QDs white light-emitting diode (QDs-WLED) was fabricated by hybridizing green-, yellow- and red-emitting CdSe(x)S(1-x)/ZnS CS QDs/epoxy composites on a blue InGaN chip. The resulting four-band RYGB QDs-WLED showed good performance with CIE-1931 coordinates of (0.4137, 0.3955), an R(a) of 81, and a T(c) of 3360 K at 30 mA, which indicated the combination of multiple-color QDs with high fluorescence QYs in LEDs as a promising approach to obtain warm WLEDs with good color rendering.

Synthesis of high quality and stability CdS quantum dots with overlapped nucleation-growth process in large scale

A low-cost, green, and reproducibly non-injection one-pot synthesis of high-quality CdS quantum dots (QDs) is reported. The synthesis was performed in the open air by mixing precursors cadmium stearate and S powder into a new solvent N-oleoylmorpholine. An overlapped nucleation-growth stage followed by a dominated growth stage was observed. The resulting QDs exhibited well-resolved absorption fine substructure and a dominant band-edge emission with a narrow size distribution (the full width at half maximum (fwhm) was only 22-24nm). The maximum photoluminescence (PL) quantum yield (QY) was as high as 46.5%. Highly monodispersed CdS QDs with tunable sizes and similar PL fwhm and QYs could also be obtained from the CdS QDs in a large-scale synthesis. The high-resolution transmission electron microscopy (HRTEM) images and powder X-ray diffraction (XRD) pattern suggested that the as-prepared QDs with high crystallinity had a cubic structure. A significant PL improvement and a continuous QY increase for the CdS QDs were observed during a long storage time in air and in a glovebox under room temperature. A slow surface reconstruction was proposed to be the cause for the PL enhancement of CdS QDs.

High-performance photodetectors and enhanced field-emission of CdS nanowire arrays on CdSe single-crystalline sheets

Vertically aligned nanowire arrays (NWAs) of semiconductor materials, combined with the merits of the large surface-to-volume ratio and low reflectance induced by light scattering and trapping, have attracted growing interests in the fabrication of high-performance optoelectronic nano-devices due to their exceptional geometrical structure and device architecture. However, an inexpensive synthesis of II–VI group semiconductor NWAs, e.g. CdS, CdSe NWAs, especially their heterostructures, is still a great challenge, and the devices (photodetector, field emitter, etc.) based on these NWA heterostructures have therefore been rarely studied. Here, we report a new method of synthesizing high-quality vertical CdS NWAs which heteroepitaxially grow on CdSe single-crystalline sheets (SCSs). The CdS NWA/CdSe SCS heterostructures as a whole are designed and fabricated into novel photodetectors via E-beam lithography. The obtained photodetectors exhibit excellent performance with high photosensitivity, responsivity, and external quantum efficiency, alongside fast response speed, and wide range response spectrum. Additionally, field-emission data of these vertically tapered CdS NWAs on CdSe SCS show enhanced properties with low turn-on field, high enhancement factor, and good stability. The results indicate that the synthesized CdS NWA/CdSe SCS heterostructure is a good candidate for broadband (ultraviolet-visible) photodetectors and field-emitters.

Magnificent CdS three-dimensional nanostructure arrays: the synthesis of a novel nanostructure family for nanotechnology

Magnificent CdS three-dimensional nanostructure arrays, composed of caky plating made up of Cd micro-platelet arrays which are surrounded by an outer layer of well-aligned CdS nanowire/pillar arrays, were successfully prepared through a combination of electroplating and subsequent solvothermal reaction on Cd-coated copper substrates. The nucleation and growth behavior of CdS nanowire/pillars were qualitatively analyzed in terms of kinetics. The results demonstrated that the microstructure of the products shows great dependence on the synthetic conditions, including reactant concentration, growth temperature and time, and the morphological characteristics of the substrate. The coalescence growth of adjacent CdS nanowire/pillars during the growth stage was firstly demonstrated. In addition, the formation mechanism of caky Cd plating was well discussed and also a hydrogen bubbles-assisted growth mechanism based on electrochemistry was proposed to explain the unique plating. The relationship between the microstructure of the products and the synthetic conditions is beneficial for modifying the shape of CdS nanostructures. The as-prepared CdS three-dimensional nanostructure arrays are advantageous for their large surface area, highly ordered structure and conductive growth substrates, and thus may have great potential in many advanced material areas, especially in the field of solar cells.

Construction of crossed heterojunctions from p-ZnTe and n-CdSe nanoribbons and their photoresponse properties

Sb-doped p-type ZnTe nanoribbons (NRs) and Ga-doped n-type CdSe NRs were synthesized via a co-thermal evaporation method in a horizontal tube furnace, respectively. Crossbar heterojunction diode (HD) devices were constructed from p-ZnTe:Sb NRs and n-CdSe NRs by a convenient route. The p-ZnTe/n-CdSe NR HD device exhibits a significant rectification characteristic with a rectification ratio up to 103 within ±5 V and a low turn-on voltage of 2.6 V. Photoresponse analysis reveals that such HD devices were highly sensitive to light illumination with excellent stability, reproducibility and fast response speeds of 37/118 μs at reverse bias voltage. It is expected that such HD devices will have great potential applications in electronic and optoelectronic devices in the future.

Mechanism and growth of flexible ZnO nanostructure arrays in a facile controlled way

Nanostructure arrays-based flexible devices have revolutionary impacts on the application of traditional semiconductor devices. Here, a one-step method to synthesize flexible ZnO nanostructure arrays on Zn-plated flexible substrate in Zn(NO3)2/NH3 ċ H2O solution system at 70-90°C was developed. We found out that the decomposition of Zn(OH)2 precipitations, formed in lower NH3 ċ H2O concentration, in the bulk solution facilitates the formation of flower-like structure. In higher temperature, 90°C, ZnO nanoplate arrays were synthesized by the hydrolysis of zinc hydroxide. Highly dense ZnO nanoparticale layer formed by the reaction of NH3 ċ H2O with Zn plating layer in the initial self-seed process could improve the vertical alignment of the nanowires arrays. The diameter of ZnO nanowire arrays, from 200 nm to 60 nm, could be effectively controlled by changing the stability of Zn(NH3)42+ complex ions by varying the ratio of Zn(NO3)2/NH3 ċ H2O which further influence the release rate of Zn2+ ions. This is also conformed by different amounts of the Zn vacancy as determined by different UV emissions of the PL spectra in the range of 380-403 nm.

Ultralow-voltage and high gain photoconductor based on ZnS:Ga nanoribbons for the detection of low-intensity ultraviolet light

A low-intensity ultraviolet photodetector (PD), with a gain as high as ∼2.4 × 106, has been successfully constructed based on gallium (Ga) doped zinc sulfide (ZnS) nanoribbons (NRs). The device exhibits excellent photoconductive properties upon a bias voltage as low as ∼0.01 V in terms of high sensitivity to UV light with an intensity of 1 μW cm−2 (corresponding to an incident power of 10−14 W), relatively fast response times of ∼3.2 ms, and an extremely high detectivity of ∼1.3 × 1019 cm Hz1/2 W−1. The high gain and fast response time are attributed to the excellent ohmic contact obtained by using a high quality ITO electrode and having a carrier mobility as high as 130 cm2 V−1 s−1, which was confirmed from the back-gate field effect transistors. These results show that the single-crystalline n-type ZnS:Ga NRs will have potential applications in future high-performance low-intensity ultraviolet photodetectors.