S. G. Zhang

Improved electroluminescence from n-ZnO/AlN/p-GaN heterojunction light-emitting diodes

n-ZnO/p-GaN heterojunction light-emitting diodes with and without a sandwiched AlN layer were fabricated. The electroluminescence (EL) spectrum acquired from the n-ZnO/p-GaN displays broad emission at 650 nm originating from ZnO and weak emission at 440 nm from GaN, whereas the n-ZnO/AlN/p-GaN exhibits strong violet emission at 405 nm from ZnO without GaN emission. The EL intensity is greatly enhanced by inserting a thin AlN intermediate layer and it can be attributed to the suppressed formation of the GaOx interfacial layer and confinement effect rendered by the AlN potential barrier layer.

Localized surface plasmon-enhanced electroluminescence from ZnO-based heterojunction light-emitting diodes

We demonstrate the surface plasmon (SP) enhanced n-ZnO/AlN/p-GaN light-emitting diodes (LEDs) by inserting the Ag nanoparticles (NPs) between the ZnO and AlN layers. The ultraviolet/violet near band edge emission of the device is significantly enhanced while the green defect-related emission is modestly suppressed compared to the LEDs without Ag NPs. The red-shift of electroluminescence (EL) peak and the reduced photoluminescence decay lifetime of ZnO suggest that the improved EL performance of the device with Ag NPs is attributed to the resonant coupling between excitons in ZnO and localized SPs in Ag NPs.

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15 +/- 0.15 eV and conduction band offset is -0.90 +/- 0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.

Ultraviolet electroluminescence from ordered ZnO nanorod array/p-GaN light emitting diodes

The highly ordered and aligned ZnO nanorod arrays were grown on p-GaN substrates via a facile hydrothermal process assisted by the inverted self-assembled monolayer template, from which the ZnO nanorod/p-GaN heterojunction light emitting diodes (LEDs) were fabricated. The ZnO nanorod-based LEDs exhibit a stronger ultraviolet emission of 390 nm than the ZnO film-based counterpart, which is attributed to the low density of interfacial defects, the improved light extraction efficiency, and carrier injection efficiency through the nano-sized junctions. Furthermore, the LED with the 300 nm ZnO nanorods has a better electroluminescence performance compared with the device with the 500 nm nanorods.

Controllable Growth of Highly Ordered ZnO Nanorod Arrays via Inverted Self-Assembled Monolayer Template

This article presents a facile and effective approach to the controllable growth of highly ordered and vertically aligned ZnO nanorod arrays on the GaN substrate via a hydrothermal route by using the TiO(2) ring template deriving from the polystyrene microsphere self-assembled monolayer. The size of TiO(2) ring template can be flexibly tuned from 50 to 400 nm for the 500 nm polystyrene microspheres by varying the time of reactive ion etching and the concentration of TiO(2) sol. As a result, the diameter of the individual ZnO nanorods can be potentially tuned over a wide range. The combination of several characterization techniques has demonstrated that the ordered ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays with high crystalline quality.

Raman peak enhancement and shift of few-layer graphene induced by plasmonic coupling with silver nanoparticles

Few-layer graphene was transferred directly on top of Ag nanoparticles, and the coupling between graphene and localized surface plasmons (LSPs) of Ag nanoparticles was investigated. We found that the surface enhanced Raman spectroscopy of graphene was increased approximately 7-fold by near-fields of plasmonic Ag nanoparticles and the enhancement factor of graphene G peak increased with the particle size. Meanwhile, the LSP resonances of Ag nanoparticles exhibit a 10 nm redshift and a 13 nm broadening by the presence of graphene, which can be attributed to the coupling between the Ag LSPs and the graphene.

Ordered ZnO nanorods-based heterojunction light-emitting diodes with graphene current spreading layer

Ordered ZnO nanorods-based heterojunction light-emitting diodes (LEDs) have been fabricated by adopting few-layer graphene as a current spreading layer. The strong emission at low currents infers the high interfacial quality between GaN and ordered ZnO nanorods, and the current spreading effect resulting from graphene. The improved electroluminescence performance was achieved compared to the ZnO nanorods-based LED with a conventional indium-tin-oxide electrode, which can be attributed to the stable, reliable, and low resistance ohmic-contacts between graphene and ZnO nanorods, as well as the high transmittance of graphene. These results demonstrate feasibility of using graphene as electrodes for high-efficiency ZnO nanorods-based LEDs.

Plasmon enhanced polymer solar cells by spin-coating Au nanoparticles on indium-tin-oxide substrate

To enhance light absorption in polymer solar cells, the Au nanoparticles (NPs) with different sizes were incorporated to the devices by spin-coating Au colloid solution on the indium-tin-oxide substrates prior to deposition of buffer layer. It has been found that the power conversion efficiency of bulk heterojunction cells can be increased from 3.50% to 4.07% after incorporating the 60 nm Au NPs, corresponding to an improvement of 16%. The improved device performance is ascribed to the localized surface plasmon excitation of the Au NPs. The method we report herein is a kind of simple and quick solution process.

Optimization of electroluminescence from n-ZnO/AlN/p-GaN light-emitting diodes by tailoring Ag localized surface plasmon

The localized surface plasmon (LSP)-enhanced n-ZnO/AlN/p-GaN light-emitting diodes (LEDs) were fabricated by inserting Ag nanoparticles (NPs) into the ZnO/AlN interface. To investigate the effects of morphology of Ag NPs on the electroluminescence (EL) of device, the Ag NPs with various sizes were prepared by annealing Ag thin films with different deposition times. It is found that the insertion of Ag NPs with suitable size and surface coverage is favorable for the effective resonant coupling between excitons in ZnO and LSP of Ag NPs, and thereby significantly improve the EL performance of the device. For the n-ZnO/AlN/p-GaN LED with 10 nm Ag NPs, a maximum EL enhancement factor of 3.7 was observed at 420 nm at an injection current of 10 mA. For the device with the smaller Ag NPs, only the weaker enhancement is observed due to the smaller scattering cross section. On the other hand, in the case of the larger Ag NPs, the energy mismatch between the LSP of Ag NPs and the near band-edge emission of ZnO, as w...

Efficiency enhancement of polymer solar cells by localized surface plasmon of Au nanoparticles

We demonstrate the improvement of power conversion efficiency (PCE) in bulk heterojunction polymer solar cells based on blended poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester by introducing 40 nm Au nanoparticles (NPs) with various concentrations. The Au NPs were deposited on indium-tin-oxide (ITO) substrates by spin-coating from colloidal solution prior to deposition of poly (3,4-ethylene dioxythiophene:poly (styrene sulfonate) (PEDOT:PSS) buffer layer. It has been found that both short-circuit current density and PCE increase after incorporating Au NPs between ITO and PEDOT:PSS layer, and a suitable area density of Au NPs is required to achieve a maximum enhancement of device efficiency. The PCE of solar cells has been increased from 3.50% to 3.81% with 0.9 wt. % Au NPs. The PCE enhancement is attributed to the localized surface plasmon excitation of Au NPs. The method employed herein is a kind of simple and convenient solution process, and it has great potential in future practical a...