Lun Dai

Single-Crystalline CdS Nanobelts for Excellent Field-Emitters and Ultrahigh Quantum-Efficiency Photodetectors

The single crystalline CdS nanobelts were synthesized by an improved vapor-liquid-solid (VLS) process. Field emission measurements show that the nanostructures have a low turn-on field of 3.7 V&m-1 at a current density of 10 μA cm-1, a low threshold field of 9.3 V μm cm-1 and a high enhancement factor of 1298. When assembled into nanoscale visible light photodetectors, the CdS nanobelts showed good sensitivity and wavelength selectivity. The results imply that the present CdS nanobelts are excellent candidates for applications in high-performance field emitters and photodetectors.

Self-powered, ultrafast, visible-blind UV detection and optical logical operation based on ZnO/GaN nanoscale p-n junctions.

Ultrafast-response (20 μs) UV detectors, which are visible-blind and self-powered, in devices where an n-type ZnO nanowire partially lies on a p-type GaN film, are demonstrated. Moreover, a CdSe-nanowire red-light detector powered by a nanoscale ZnO/GaN photovoltaic cell is also demonstrated, which extends the device function to a selective multiwavelength photodetector and shows the function of an optical logical AND gate.

Single-Nanowire Single-Mode Laser

We demonstrate single-mode laser emission in single nanowires. By folding a 200 nm diameter CdSe nanowire to form loop mirrors, single-mode laser emission around 738 nm wavelength is obtained with line width of 0.12 nm and low threshold. The mode selection is realized by the vernier effect of coupled cavities in the folded nanowire. In addition, the loop structure makes it possible to tune the nanowire cavity, opening an opportunity to realize a tunable single-mode nanowire laser.

High-Performance Single CdS Nanowire (Nanobelt) Schottky Junction Solar Cells with Au/Graphene Schottky Electrodes

High-performance single CdS nanowire (NW) as well as nanobelt (NB) Schottky junction solar cells were fabricated. Au (5 nm)/graphene combined layers were used as the Schottky contact electrodes to the NWs (NBs). Typical as-fabricated NW solar cell shows excellent photovoltaic behavior with an open circuit voltage of ∼0.15 V, a short circuit current of ∼275.0 pA, and an energy conversion efficiency of up to ∼1.65%. The physical mechanism of the combined Schottky electrode was discussed. We attribute the prominent capability of the devices to the high-performance Schottky combined electrode, which has the merits of low series resistance, high transparency, and good Schottky contact to the CdS NW (NB). Besides, a promising site-controllable patterned graphene transfer method, which has the advantages of economizing graphene material and free from additional etching process, was demonstrated in this work. Our results suggest that semiconductor NWs (NBs) are promising materials for novel solar cells, which have potential application in integrated nano-optoelectronic systems.

Self-powered high performance photodetectors based on CdSe nanobelt/graphene Schottky junctions

Self-powered photodetectors based on CdSe nanobelt (NB)/graphene Schottky junctions are fabricated and investigated. Typically such Schottky junctions exhibit good rectifying behavior without light illumination. The on/off ratio is more than 1 × 105 when the voltage changes from −1 to 1 V. Under zero bias, typically such photodetectors show high photosensitivity (∼3.5 × 105), which is defined as (Iphoto − Idark)/Idark, to above-band-gap irradiation. Under 1000 Hz light switching frequency, the response and recovery times of such photodetector are typically 82 and 179 μs, respectively, and the photoconductive gain is 28, greater than unity. The high photosensitivity and gain, as well as fast response speed, guarantee the feasibility of such self-powered photodetectors.

Single ZnO Nanowire/p‐type GaN Heterojunctions for Photovoltaic Devices and UV Light‐Emitting Diodes

We fabricate heterojunctions consisting of a single n-type ZnO nanowire and a p-type GaN film. The photovoltaic effect of heterojunctions exhibits open-circuit voltages ranging from 2 to 2.7 V, and a maximum output power reaching 80 nW. Light-emitting diodes with UV electroluminescence based on the heterojunctions are demonstrated.

Wavelength Tunable CdSe Nanowire Lasers Based on the Absorption‐Emission‐Absorption Process

Wavelength tunability of lasers is one of the most important parameters for practical applications such as optical communication, environmental monitoring, and spectroscopy analysis. [ 14–18 ] Recently, tunable semiconductor NW and nanoribbon lasers have been realized, using tunable bandgap nanostructures as the composition tunable gain media. [ 19 –26 ] In this approach, alloyed semiconductor NWs with different bandgaps are necessary components for wavelength tuning; a careful composition control is required in order to obtain high crystal quality stoichiometric NWs. [ 27–29 ] Alternatively, the peak wavelength of NW lasers can also be tuned by changing the geometry of the cavity structures, but the tunable range is limited to about 10 nm. [ 7 , 13 ]

Semiconductor nanowire lasers

Semiconductor nanowires (or other wire-like nanostructures, including nanoribbons and nanobelts) synthesized by bottom-up chemical growth show single-crystalline structures, excellent geometric uniformities, subwavelength transverse dimensions, and relatively high refractive indices, making these one-dimensional structures ideal optical nanowaveguides with tight optical confinement and low scattering loss. When properly pumped by optical or electrical means, lasing oscillation can be readily established inside these high-gain active nanowires with feedback from endface reflection or near-field coupling effects, making it possible to realize nanowire lasers with miniature sizes and high flexibilities. Also, the wide-range material availability bestows the semiconductor nanowire with lasing wavelength selectable within a wide spectral range from ultraviolet (UV) to near infrared (IR). As nanoscale coherent light sources, in recent years, nanowire lasers have been attracting intensive attention for both fundamental research and technological applications ranging from optical sensing, signal processing, and on-chip communications to quantum optics. Here, we present a review of the status and perspectives of semiconductor nanowire lasers, with a particular emphasis on their optical characteristics categorized in two groups: (1)xa0waveguiding related properties in Sectionxa03, which includes waveguide modes, near-field coupling, endface reflection, substrate-induced effects, and nanowire microcavities, and (2)xa0optically pumped semiconductor nanowire lasers in Sectionxa04, starting from principles and basic types of UV, visible, and near-IR nanowire lasers relying on Fabry–Perot cavities, to advanced configurations including wavelength-tunable, single-mode operated, fiber-coupled, and metal-incorporated nanowire lasing structures for more possibilities. In addition, the material aspects of semiconductor nanowires, including nanowire synthesis and electrically driven nanowire lasers, are briefly reviewed in Sectionsxa02 and 5, respectively. Finally, in Sectionxa06 we present a brief summary of semiconductor nanowire lasers regarding their current challenges and future opportunities.

Fast-speed and high-gain photodetectors of individual single crystalline Zn3P2 nanowires

We report high-performance photodetectors of individual single crystalline Zn3P2 nanowires. A facile catalyst-free physical evaporation process was developed to synthesize high-quality single crystalline Zn3P2 nanowires (NWs) with high yield. A typical individual Zn3P2 NW photodetector exhibited fast speeds (the response and recovery times were less than 25 and 26 μs, respectively) and high gain (∼470), which is an excellent result towards getting a balance in the speed and gain trade-off of the semiconductor nanostructure-based photodetectors. In addition, the device had the advantages of high photosensitivity (Iphoto − Idark/Idark ≈ 100) and small device size (constructed on an individual nanowire). All these merits substantiate that the Zn3P2 nanowires can serve as excellent photoconductive materials, and the Zn3P2 NW photodetectors are very promising for practical applications.

Graphene-based Schottky junction solar cells

The Schottky junction, with merits of material universality, low cost and easy fabrication, is an alternative structure for solar cells. Compared to traditional indium-tin-oxide (ITO) based Schottky junction solar cells, graphene-based ones have merits of low cost, performance stability, and are applicable to flexible devices. In this highlight, we survey the recent research on graphene-based Schottky junction solar cells, including graphene-on-silicon Schottky junction solar cells and graphene/single NW (NB) Schottky junction solar cells. The working principle of them is discussed. These works demonstrate that graphene-based Schottky junction structures are promising candidates for developing diverse novel high-efficient and low-cost photovoltaic devices. The perspective and challenge of them are also discussed and anticipated.