Zhiyuan Zuo

Surfactantless photochemical growth of Ag nanostructures on GaN epitaxial films with controlled morphologies and their application for SERS

Ag nanostructures with controlled morphologies have been synthesized on GaN epitaxial films through a convenient, fast, and surfactantless photochemical method. On n-type GaN, uniform spheroidic Ag nanoparticles (NPs) of high density were obtained after photochemical reaction for a short time (30 s to 5 min) in AgNO3 aqueous solution. Reducing AgNO3 concentration yielded branched Ag nanostructures. The Ag NPs on the GaN substrates exhibit a Fabry–Perot-type surface plasmon resonance (SPR). On p-type GaN, Ag nanocrystals (NCs) with well-defined shapes ranging from cubes, truncated cubes, cuboctahedra, truncated octahedra, and octahedra were obtained through varying the concentration of AgNO3 solution. We propose that the formation of polyhedral Ag NCs is induced by the separation of the nucleation and growth processes by the GaN surface. Both surface states and carrier concentration are considered to play substantial roles during the growth process. This surfactantless photochemical method is general and enables the facile synthesis of Ag nanostructures at room temperature, which may be extended for the synthesis of other noble metal nanostructures, e.g.Au, Pt, and Pd, on GaN or other wide bandgap semiconductors, e.g.SiC. The obtained Ag/GaN hybrid systems show potential application as solid substrates for surface enhanced Raman scattering (SERS) analysis.

Enhanced Optoelectronic Performance on the (110) Lattice Plane of an MAPbBr3 Single Crystal

Hybrid organic-inorganic lead halide perovskites have attracted significant attention due to their impressive optoelectronic properties. MAPbX3 (MA= CH3NH3+, X= Cl, Br or I), the most popular member of this family, has been recognized as an important next-generation optoelectronic materials contender, and remarkable progress has been achieved in both thin films and single crystals. However, the lack of optimizations in energy harvest, transportation, carrier extraction, and process compatibility is hindering their future development. In this study, a triangle prism MAPbBr3 single crystal exposing (100) and (110) crystallographic planes was successfully synthesized, and the optoelectronic performances of these two lattice planes were systematically explored by employing a planar metal-semiconductor-metal (MSM) device. Compared to the device fabricated on the (100) plane, a 153.33% enhancement of responsivity was achieved under 10 μW irradiation and 10 V bias on the (110) plane. Finally, possible mechanism for such an enhancement was discussed based on the different defect migration behaviors of (100) and (110) planes.

Ultra bright AlGaInP light emitting diodes with pyramidally patterned GaP spreading layer

A mild photo-assisted chemical method is developed to fabricate pyramidally patterned GaP:Mg spreading layer to improve the light extraction efficiency of 625 nm AlGaInP red light emitting diodes. The pyramid patterns fabricated by using hydrofluoric acid and H2O2 mixed solution in combination with 532 nm laser radiation, are experimentally confirmed to be very efficient for light extraction. Angle resolved electroluminescent tests reveal that the patterned chips show an average 66.2% enhancement of the light emission intensity over the original chips. The output power measured on encapsulated samples exhibits an enhancement of 41.3%.

Metal-assisted electroless fabrication of nanoporous p-GaN for increasing the light extraction efficiency of light emitting diodes

We report metal-assisted electroless fabrication of nanoporous p-GaN to improve the light extraction efficiency of GaN-based light emitting diodes (LEDs). Although it has long been believed that p-GaN cannot be etched at room temperature, in this study we find that Ag nanocrystals (NCs) on the p-GaN surface enable effective etching of p-GaN in a mixture of HF and K2S2O8 under ultraviolet (UV) irradiation. It is further shown that the roughened GaN/air interface enables strong scattering of photons emitted from the multiple quantum wells (MQWs). The light output power measurements indicate that the nanoporous LEDs obtained after 10 min etching show a 32.7% enhancement in light-output relative to the conventional LEDs at an injection current of 20 mA without significant increase of the operating voltage. In contrast, the samples etched for 20 min show performance degradation when compared with those etched for 10 min, this is attributed to the current crowding effect and increased surface recombination rate.

Resonant actuation of microcantilever by pulse wave of one-nth the resonant frequency

We achieve the resonant actuation of a microcantilever by using a pulse wave of one-nth the microcantilever resonant frequency. Our studies on the relationship between pulse duration and microcantilever resonant amplitude reveal that the most effective actuation can be acquired when the pulse duration is odd multiples of the half period of microcantilever resonant oscillation. This actuation method is generally applicable and may find applications in ultrahigh frequency actuation of microcantilever or similar resonant systems for high sensitivity detection.

Enhanced photoluminescence of gallium phosphide by surface plasmon resonances of metallic nanoparticles

We report here enhanced photoluminescence (PL) from GaP, an indirect band gap semiconductor, by metallic Au and Ag nanoparticles. The metallic Au and Ag nanoparticles were sputtering-coated onto GaP followed by heat treatment. After optimization of the surface coverage ratios, we find 5.6-fold and 14.5-fold PL enhancement for the Au and Ag coated samples, respectively. Time resolved photoluminescence spectroscopy indicates that Ag nanoparticles can effectively accelerate the PL decay time. Our results indicate that the PL enhancement comes from enhanced photon scattering and localized field enhancement of metallic Au and Ag nanoparticles.

Plasmonic GaInP solar cells with improved energy conversion efficiency

We report here a new method to improve the performance of GaInP solar cells by fabricating Ag nanoparticles (Ag NPs) atop the devices through a simple wet-chemical method. The size and density of Ag NPs can be easily optimized by changing the concentration of AgNO3 solution and/or the reaction time. The Ag NPs support surface plasmonic resonances (SPRs) and thus greatly increase light absorbance and photocurrent responses of the solar cells by the scattering and re-reflection of incident light. We find that, for a Ag NPs density of ∼3.37 × 1010 cm−2, the short-circuit current density (Jsc) increases by 19.5% from 14.9 to 17.8 mA cm−2, and the power conversion efficiency (PCE) increases by 20.4% from 15.2% to 18.3%.

Periodic indentation patterns fabricated on AlGaInP light emitting diodes and their effects on light extraction

We report a novel method for fabricating periodic indentation patterns on AlGaInP light emitting diodes (LEDs) that can effectively improve the light extraction efficiency. The patterns consist of hemispherical pits created by directly imprinting the top GaP window layer of AlGaInP LEDs with patterned sapphire (PS). The angle resolved electroluminescence tests reveal that the patterned chips show an average light emission enhancement of 155% over the original planar chips.

111) Twinned BaTiO3 microcrystallites

We report here the controlled synthesis of (111) twinned BaTiO3 microcrystallites, which are widely known for their (110) ferroelectric twins. This (111) twinned structure was obtained through a composite hydroxide mediated process by using amorphous TiO2 powders. A twin assisted re-entrant growth model is proposed to account for the microstructural aspects determined. It is considered that the (111) twins originate from the Ti–O octahedra of the amorphous TiO2 powders. The role of oriented aggregation is also discussed.