Guogang Qin

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.

A simple and scalable graphene patterning method and its application in CdSe nanobelt/graphene Schottky junction solar cells.

We have developed a simple and scalable graphene patterning method using electron-beam or ultraviolet lithography followed by a lift-off process. This method, with the merits of: high pattern resolution and high alignment accuracy, being free from additional etching or harsh processes, being universal to arbitrary substrates, and being compatible to Si microelectronic technology, can easily be applied to diverse graphene-based devices, especially in array-based applications, where large-scale graphene patterns are desired. We have applied this method to fabricate CdSe nanobelt (NB)/graphene Schottky junction solar cells, which have potential applications in integrated nano-optoelectronic systems. A typical as-fabricated solar cell shows excellent photovoltaic behavior, with an open-circuit voltage of ∼0.51 V, a short-circuit current density of ∼5.75 mA cm(-2), and an energy conversion efficiency of ∼1.25%. We attribute the high performance of the cell to the as-patterned high-performance graphene, which can form an ideal Schottky contact with CdSe NB. Our results suggest that both the developed graphene patterning method and the as-fabricated CdSe NB/graphene Schottky junction solar cells have reachable application prospects.

Light coupling and modulation in coupled nanowire ring-Fabry-Pérot cavity.

CdS nanowire (NW) ring cavities were fabricated and studied for the first time. The rings with radii from 2.1 to 5.9 microm were fabricated by a nanoprobe system installed in a scanning electron microscope. Radius dependent whispering gallery modes (WGMs) were observed. A straight CdS NW with Fabry-Pérot (F-P) cavity structure was fabricated and placed by the side of a NW ring cavity to form a coupled ring-F-P cavity. When the NW ring was excited by a focused laser, a bright green light spot was observed at the output end of the straight NW, indicating that the latter had served as an effect waveguide to couple the light out from the ring cavity. The corresponding light spectrum showed that the WGMs had been modulated. We confirmed that the NW F-P cavity had served as a modulator. Such a coupled cavity has potential application in a nanophotonic system.

Multicolor graphene nanoribbon/semiconductor nanowire heterojunction light-emitting diodes

We report novel graphene nanoribbon (GNR)/semiconductor nanowire (SNW) heterojunction light-emitting diodes (LEDs) for the first time. In the device, the GNR and SNW have face-to-face contact with each other, which has the merits of a larger active region and smaller series resistance, and may benefit high-efficiency electroluminescence and even electrically driven lasers in the future. ZnO, CdS, and CdSe NWs were employed in our case. At forward biases, the GNR/SNW heterjunction LEDs could emit light with wavelengths varying from ultraviolet (380 nm) to green (513 nm) to red (705 nm), which were determined by the band-gaps of the involved SNWs. The mechanism of light emitting for the GNR/SNW heterojunction LEDs was discussed. Our work opens new routes to developing diverse graphene-based nano-optoelectronic devices, which are basic components in integrated optoelectronic system. Besides, the novel graphene/SNW hybrid devices, by taking advantage of both graphene and SNW, will be promising candidates for use in applications such as high-sensitivity sensor and transparent flexible devices in the future.

Two-dimensional CdS nanosheet-based TFT and LED nanodevices

Semiconductor nanosheets have several unique applications in electronic and optoelectronic nanodevices. We have successfully synthesized single-crystalline n-type CdS nanosheets via a chemical vapor deposition (CVD) method in a Cd-enriched ambient. The as-synthesized nanosheets are typically 40-100 nm thick, 10-300 µm wide, and up to several millimeters long. Using the nanosheets, we fabricated for the first time (to our knowledge), nano thin-film transistors (nano-TFTs) based on individual CdS nanosheets. A typical unit of such nanosheet TFTs has a high on-off ratio (∼1.7 ×10(9)) and peak transconductance (∼14.1µS), which to our knowledge are the best values reported so far for semiconductor nano-TFTs. In addition, we fabricated n-CdS nanosheet/p(+)-Si heterojunction light emitting diodes (LEDs) with a top electrode structure. This structure, where the n-type electrode is directly above the junction, has the advantage of a large active region and injection current favorable for high-efficiency electroluminescence (EL) and lasing. Room-temperature spectra of the LEDs consist of only an intense CdS band-edge emission peak (∼507.7 nm) with a full width at half-maximum of about 14 nm.

Efficient silicon quantum dots light emitting diodes with an inverted device structure

We use silicon quantum dots (SiQDs) with an average diameter of 2.6 ± 0.5 nm as the light emitting material and fabricate inverted structure light emitting diodes (SiQD-LEDs) with bottom cathodes. ZnO nanoparticles with high electron mobility, a deep valence band edge, and robust features to resist dissolving by the SiQD solvent were used as the electron transport layer. 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) with high hole transport mobility and a high lowest unoccupied molecular orbital level was used as the hole transport layer. Poly(ethylene imine) (PEI) modified indium-tin oxide (ITO) was used as the low work function (∼3.1 eV) cathode and MoO3/Al as the high work function anode. Electroluminescence of the SiQD-LEDs is mainly from the SiQDs with a peak located at ∼700 nm. The maximum external quantum efficiencies of the SiQD-LEDs are 2.7%.

Selected-control hydrothermal synthesis and photoresponse properties of Bi2S3 micro/nanocrystals

Orthorhombic phase Bi2S3 micro/nanocrystals with various morphologies have been successfully synthesized via a facile one-step hydrothermal approach without any templates or surfactants. The formation mechanisms of the Bi2S3 micro/nanocrystals have been proposed on the basis of the experimental results. The photoresponse properties using Bi2S3 nanorods as a representative system show a significantly enhanced conductivity and a steady photoresponse with a fast response time of 177.1 ms, indicating its potential application in high-speed and high-sensitivity photodetectors and photoelectronic switches.

High-performance CdSe nanobelt based MESFETs and their application in photodetection

High performance metal–semiconductor field-effect transistors (MESFETs) based on single n-CdSe nanobelts (NBs) have been fabricated and applied as photodetectors. Au is used as the gate metal, which formed a good Schottky contact with the CdSe NB with a rectification ratio of about 2 × 108. The CdSe NB MESFETs exhibit a near-zero threshold voltage (−0.55 V), low subthreshold swing (60.4 mV per dec), no clearly observed current hysteresis, and the highest on/off current ratio (5 × 108) reported so far for NW/NB MESFETs. We have also investigated the photoresponse properties of these MESFETs. Typical CdSe NB MESFET based photodetectors have high current responsivity (∼1.4 × 103 A W−1), high gain (∼2.7 × 103), and fast photoresponse speed (the rise time and the decay time are about 35 and 60 μs, respectively.) under a gate voltage of −1 V. All these results show that the CdSe NB based MESFETs can be promising candidates for both electronic and opto-electronic nanodevices.

Novel optoelectronic devices based on single semiconductor nanowires (nanobelts)

Semiconductor nanowires (NWs) or nanobelts (NBs) have attracted more and more attention due to their potential application in novel optoelectronic devices. In this review, we present our recent work on novel NB photodetectors, where a three-terminal metal–semiconductor field-effect transistor (MESFET) device structure was exploited. In contrast to the common two-terminal NB (NW) photodetectors, the MESFET-based photodetector can make a balance among overall performance parameters, which is desired for practical device applications. We also present our recent work on graphene nanoribbon/semiconductor NW (SNW) heterojunction light-emitting diodes (LEDs). Herein, by taking advantage of both graphene and SNWs, we have fabricated, for the first time, the graphene-based nano-LEDs. This achievement opens a new avenue for developing graphene-based nano-electroluminescence devices. Moreover, the novel graphene/SNW hybrid devices can also find use in other applications, such as high-sensitivity sensor and transparent flexible devices in the future.

Fully Transparent Quantum Dot Light-Emitting Diode with a Laminated Top Graphene Anode

A new method to employ graphene as top electrode was introduced, and based on that, fully transparent quantum dot light-emitting diodes (T-QLEDs) were successfully fabricated through a lamination process. We adopted the widely used wet transfer method to transfer bilayer graphene (BG) on polydimethylsiloxane/polyethylene terephthalate (PDMS/PET) substrate. The sheet resistance of graphene reduced to ∼540 Ω/□ through transferring BG for 3 times on the PDMS/PET. The T-QLED has an inverted device structure of glass/indium tin oxide (ITO)/ZnO nanoparticles/(CdSSe/ZnS quantum dots (QDs))/1,1-bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC)/MoO3/graphene/PDMS/PET. The graphene anode on PDMS/PET substrate can be directly laminated on the MoO3/TAPC/(CdSSe/ZnS QDs)/ZnO nanoparticles/ITO/glass, which relied on the van der Waals interaction between the graphene/PDMS and the MoO3. The transmittance of the T-QLED is 79.4% at its main electroluminescence peak wavelength of 622 nm.