Strong Light Emission Research Articles

Influence of Supersaturation and Spontaneous Catalyst Formation on the Growth of PbS Wires: Toward a Unified Understanding of Growth Modes

High quality stoichiometric lead sulfide (PbS) wires were synthesized by a simple chemical vapor deposition (CVD) process using pure PbS powder as the material source. Growth mechanisms were systematically investigated under various growth conditions, with three modes of growth identified: direct vapor-liquid-solid (VLS) wire growth nucleating from the substrate surface, bulk PbS crystallites by vapor-solid (VS) deposition, and subsequent VLS growth nucleating on top of the bulk deposition through spontaneously formed catalyst particles. Furthermore, we found that these growth modes can be organized in terms of different levels of supersaturation, with VS bulk deposition dominating at high supersaturation and VLS wire growth on the substrate dominating at low supersaturation. At intermediate supersaturation, the bulk VS deposition can form larger crystallites with domains of similarly oriented wires extending from the flat facets. Both predeposited catalysts and spontaneously formed Pb particles were observed as nucleation catalysts, and their interplay leads to various interesting growth scenarios such as reversely tapered growth with increasing diameter. The VLS growth mechanism was confirmed by the presence of Pb-rich caps revealed in an elaborate cross-sectional transmission electron microscopy (TEM) experiment after focused ion beam milling in a modified lift-out procedure. Temperature-dependent photoluminescence (PL) of PbS wires was performed in the mid-infrared wavelength range for the first time, demonstrating strong light emission from band edge, blue-shifted with increasing temperature. The high optical quality of PbS wires may lead to important applications in mid-infrared photonics. The substrate growth temperature as low as 400 °C allows for silicon-compatible processing for integrated optoelectronics applications.

Fabrication and optical properties of needle-like ZnO array by a simple hydrothermal process

A simple low temperature hydrothermal process was employed to fabricate the needle-like ZnO array in the diluted butyl amine. The microstructure, morphology and the photoluminescence property of the as-prepared products were characterized by x-ray diffraction (XRD), transmission electron microscope (TEM), field emission environment scanning electron microscope (SEM) and photoluminescence spectrum (PL). The results show that the needle-like ZnO crystals are hexagonal wurtzite monocrystal with slender figure and smooth surface. A possible growth mechanism of the needle-like ZnO array related to the diluted butyl amine is proposed. The PL spectrum of the needle-like ZnO array shows a strong blue light emission at 437 nm and a relatively low ultraviolet emission at 377 nm.

Effect of [OH–] Linkages on Luminescent Properties of ZnO Nanoparticles

Optical properties of ZnO nanoparticles prepared from a simple chemical method using sodium zincate bath show strong white light emission. X-ray absorption fine structure studies reveal a completely different local environment around Zn in these ZnO nanoparticles. The observed luminescence properties and local structural changes have been explained on the basis of a linkage between Zn and OH– ions in the surface layers of ZnO nanoparticles.

One-step growth of Si 3N 4 stem–branch featured nanostructures: Morphology control by VS and VLS mode

We report here one-step synthesis of Si 3N 4 nanodendrites by selectively applying a vapor–solid (VS) and vapor–liquid–solid (VLS) strategy via direct current arc discharge method. The resultant nanodendrites were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy and X-ray powder diffraction. The spine-shaped nanodendrites were generated by a noncatalytic growth following a VS mode. The uniform secondary nanowire branches were epitaxial grown from two side surfaces of the nanowire stems. The pine-shaped nanodendrites were obtained through a catalytic growth in a VLS process. These branch nanowires were unsystematically grown from the nanocone-like stems. The photoluminescence spectra of the nanodendrites show a strong white light emission around 400–750 nm, suggesting their potential applications in light and electron emission devices.

Metal/graphene sheets as p-type transparent conducting electrodes in GaN light emitting diodes

We demonstrate the use of graphene based transparent sheets as a p-type current spreading layer in GaN light emitting diodes (LEDs). Very thin Ni/Au was inserted between graphene and p-type GaN to reduce contact resistance, which reduced contact resistance from ∼5.5 to ∼0.6Ω/cm2, with no critical optical loss. As a result, LEDs with metal-graphene provided current spreading and injection into the p-type GaN layer, enabling three times enhanced electroluminescent intensity compared with those with graphene alone. We confirmed very strong blue light emission in a large area of the metal-graphene layer by analyzing image brightness.

Comprehensive Numeric Study of Gallium Nitride Light-Emitting Diodes Adopting Surface-Plasmon-Mediated Light Emission Technique

The coupling of quantum well (QW) spontaneous emissions to surface plasmons (SPs) has been a promising technique to increase emission rate of LEDs. We carried out numeric investigations to explore the electromagnetic nature of these SP modes. It has been shown that the SP resonance frequency on a flat silver/GaN interface, and hence the corresponding emission enhancement factor can be easily tuned by altering the thickness of silver film and the separation between QW and metal. By using coupled SPPs, partial energy transfer across silver film can be achieved, where strong directional light emission can be re-emitted. We also utilized semiconductor simulation technique to investigate the internal operations of our proposed LED. We found that the internal inefficiency of the device might be attributed to the current crowding effect, poor carrier injection, as well as bad overlap of electron and hole wave functions inside the well. The combination of electromagnetic and semiconductor simulation techniques has been presented as a powerful tool in theoretical analysis of SP-mediated emission LED.

One-Dimensional Cd(II) and Co(II) Coordination Polymers with Dinuclear Secondary Building Units Constructed by 1,1′-Biphenyl-3,3′-Dicarboxylic Acid: Synthesis, Structures, Luminescent and Magnetic Properties

Bilayered diffusion and hydrothermal synthetic methods using pyridine (py) and 2,6-dimethylpyrazine (dmpz) as weak bases and small terminal ligands generated two Cd(II) and Co(II) 1D coordination polymers with dinuclear metal clusters as secondary building units (SBUs) from an organic bridging ligand, 1,1′-biphenyl-3,3′-dicarboxylic acid (H2 L). They are {[Cd2(L)2(py)4] (py)(CH3OH)} n (1) and [Co2(L)2(dmpz)4(H2O)] n (2). Compounds 1 crystallizes in the monoclinic space group P2(1)/c, while 2 crystallizes in the triclinic space group P-1. Especially, compound 2 possesses the 1D chain structure that contains two linked left-handed and right-handed helices. Fluorescent measurements for 1 display strong blue light emissions with the maximum emission at 396 nm with the excitation of 320 nm. Magnetization study shows the existence of weak antiferromagnetic coupling for 1 with the negative −E 2/k value of −1.3 K.

Low temperature preparation and characterization of CdTiO 3 nanoplates

An alternative low cost method was proposed for the low temperature (600 °C) preparation of CdTiO 3 nanoplates, using low-melting-point Cd(NO 3) 2 · 4H 2O and high-reactive-activity TiO 2 nanocrystals as the reactants. The structure, composition, specific surface area, thermal stability and optical properties of the as-synthesized products were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, field emission scanning electron microscopy, Brunauer–Emmett–Teller surface area analysis, UV–vis absorption and room temperature photoluminescence spectra. The as-synthesized CdTiO 3 nanoplates were thermally stable at temperatures ≤ 900 °C, and their phase changed from ilmenite to perovskite when heated in air at 1000 °C for 2 h. Besides, they exhibited a strong and broad visible light emission in the range of about 470–750 nm upon laser excitation at 325 nm, enabling their use as a luminescent material.

Strong white light emission from a processed porous silicon and its photoluminescence mechanism

We have prepared various porous silicon (PS) structures with different surface conditions (any combination of oxidation, carbonization as well as thermal annealing) to increase the intensity of photoluminescence (PL) spectrum in the visible range. Strong white light (similar to day-light) emission was achieved by carrying out thermal annealing at 1100 °C after surface modification with 1-decene of anodic oxidized PS structures. Temperature-dependent PL measurements were first performed by gradually increasing the sample temperature from 10 to 300 K inside a cryostat. Then, we analyzed the measured spectrum of all prepared samples. After the analysis, we note that throughout entire measured spectrum, only two main peaks corresponding to blue and green-orange emission lines (which can be interpreted by quantum size effect and/or configuration coordinate model) were seem to be predominant for all temperature range. To further reveal and analysis these peaks, finally, measured data were inputted into the formula of activation energy of thermal excitation. We found that activation energies of blue and green-orange lines were approximately 49.3 and 44.6 meV, respectively.

Synthesis and Characterization of Triphenylethylene Derivatives with Aggregation-Induced Emission Characteristics

New aggregation-induced emission (AIE) compounds derived from triphenylethylene were synthesized. The thermal, photophysical, electrochemical and aggregation-induced emissive properties were investigated. All the compounds had strong blue light emission capability and good thermal stability. Their maximum fluorescence emission wavelengths were between 443 to 461nm in solid states, while their glass transition temperatures ranged from 86 to 129°C. The decomposition temperatures of the synthesized compounds were in the range of 432-534°C. The synthesized compounds possessed aggregation-induced emission properties, namely exhibited enhanced fluorescence emission in aggregated states. The highest occupied molecular orbital (HOMO) energy levels estimated from the oxidation potentials were between 5.61 and 5.66eV and the lowest unoccupied molecular orbital/highest occupied molecular orbital (LUMO/HOMO) energy gap values were found to be in the range of 3.18-3.22eV. The compounds 4-(4-(2,2-bis(4-(naphthalen-1-yl)phenyl)vinyl)phenyl) dibenzothiophene [(BN)(2)Bt] and 4-(4-(2,2-di(biphenyl-4-yl)vinyl)phenyl) dibenzothiophene [(BB)(2)Bt] exhibited vibronic fine-structure photoluminescence spectra when the water fraction was less than 70%.

Photoluminescent properties of Dy 3+ in MgO-Ga 2O 3-SiO 2 nano-glass-ceramic prepared by sol–gel method

The MgO-Ga 2O 3-SiO 2 (MGS) glass and glass-ceramic (GC) containing MgGa 2O 4 nanocrystals and Dy 3+ ions were prepared by a simple sol–gel method. MgGa 2O 4 nanocrystals in MgO-Ga 2O 3-SiO 2 GC formed when the heating temperature reached 800 °C. The energy transfer from MgGa 2O 4 nanocrystals to Dy 3+ was observed. It is important that the strong white light emission is observed corresponding to the excitation band gap of MgGa 2O 4 nanocrystals. As exciting Dy 3+ ions, only weak yellow and blue emissions were observed.

Multi-zone light emission in a one-dimensional ZnO waveguide with hybrid structures

We observed multi-zone light emission in a one-dimensional waveguide based on an individual pearl-like ZnO nanowire with hybrid structures, which was obtained through an electrical breakdown process. E2 (high) mode in Raman spectra revealed a blueshift while a redshift of UV near band edge emission was observed by focusing laser on the polycrystalline parts at room temperature. Strong light emission was observed from the polycrystalline parts except the end in the pearl-like ZnO nanowires as compared with columnar ones, which is associated with the light propagation in the waveguide determined by the different dielectric constants between single crystal and polycrystal.

Molecular Beam Epitaxial Growth, Fabrication, and Characterization of High Efficiency InGaN/GaN Dot-in-a-Wire White Light Emitting Diodes on Si(111)

We report on the molecular beam epitaxial growth, fabrication, and characterization of InGaN/GaN dot-in-a-wire white light emitting diodes (LEDs) grown on Si(111). By varying the In compositions in the dot layers, we have demonstrated strong white light emission. The dot-in-a-wire LEDs exhibit a relatively high internal quantum efficiency of ~ 36.7% and virtually zero efficiency droop for current densities up to ~ 200 A/cm2 at room temperature, which are attributed to the superior 3-dimensional carrier confinement provided by the dots and to the use of nearly defect and strain-free GaN nanowires.

Positive Potential Operation of a Cathodic Electrogenerated Chemiluminescence Immunosensor Based on Luminol and Graphene for Cancer Biomarker Detection

In this work, we report a cathodic electrogenerated chemiluminescence (ECL) of luminol at a positive potential (ca. 0.05 V vs Ag/AgCl) with a strong light emission on the graphene-modified glass carbon electrode. The resulted graphene-modified electrode offers an excellent platform for high-performance biosensing applications. On the basis of the cathodic ECL signal of luminol on the graphene-modified electrode, an ECL sandwich immunosensor for sensitive detection of cancer biomarkers at low potential was developed with a multiple signal amplification strategy from functionalized graphene and gold nanorods multilabeled with glucose oxidase (GOx) and secondary antibody (Ab(2)). The functionalized graphene improved the electron transfer on the electrode interface and was employed to attach the primary antibody (Ab(1)) due to it large surface area. The gold nanorods were not only used as carriers of secondary antibody (Ab(2)) and GOx but also catalyzed the ECL reaction of luminol, which further amplified the ECL signal of luminol in the presence of glucose and oxygen. The as-proposed low-potential ECL immunosensor exhibited high sensitivity and specificity on the detection of prostate protein antigen (PSA), a biomarker of prostate cancer that was used as a model. A linear relationship between ECL signals and the concentrations of PSA was obtained in the range from 10 pg mL(-1) to 8 ng mL(-1). The detection limit of PSA was 8 pg mL(-1) (signal-to-noise ratio of 3). Moreover, the as-proposed low-potential ECL immunosensor exhibited excellent stability and reproducibility. The graphene-based ECL immunosensor accurately detected PSA concentration in 10 human serum samples from patients demonstrated by excellent correlations with standard chemiluminescence immunoassay. The results suggest that the as-proposed graphene ECL immunosensor will be promising in the point-of-care diagnostics application of clinical screening of cancer biomarkers.

Photocatalytic activities of wet oxidation synthesized ZnO and ZnO–TiO2 thick porous films

Highly porous zinc oxide (ZnO) film was pro- duced by using reactive magnetron sputtering zinc target followed by wet oxidation. Titanium dioxide (TiO2) was mixed to the porous films by using either TiO2 target magnetron sputter deposition or sol-spin method. The film thickness could reach 50 lm with uniform porosity. On the sputtering prepared ZnO-TiO2 film surface, fine nanorods with small anatase TiO2 nano-clusters on the tips were observed by SEM and TEM, and the titanium (Ti) com- position was determined by XPS as 0.37%. The sol-spin treatment could increase the Ti composition to 4.9%, with reduced pore size compared to the untreated ZnO porous film. Photoluminescence measurements showed that the Ti containing porous film has strong ultraviolet-visible light emission. In the photo-catalysis testing, ZnO and ZnO- TiO2 have similar photo-catalysis activity under 365 nm UV irradiation, but under visible light, the photocatalysis activities of ZnO-TiO2 films were twice higher than that of ZnO porous film, implying promising applications of this porous oxide composite for industrial and dairy farm wastewater treatment.

Synthesis, Structure, Aggregation‐Induced Emission, Self‐Assembly, and Electron Mobility of 2,5‐Bis(triphenylsilylethynyl)‐3,4‐diphenylsiloles

2,5-Bis(triphenylsilylethynyl)-3,4-diphenylsiloles with different 1,1-substituents [XYSi(CPh)(2) (C-C≡C-SiPh(3))(2)] (Ph=phenyl) were synthesized in high yields by the Sonogashira coupling of 2,5-dibromo-3,4-diphenylsiloles with triphenylsilylacetylene, and two of these were characterized crystallographically. Crystal structures and theoretical calculations showed that the new silole molecules had higher conjugation than 2,5-diarylsiloles. They possessed low HOMO and LUMO energy levels due to the electron-withdrawing effect of the triphenylsilylethynyl groups. Cyclic voltammetry analysis revealed low electron affinities, which were comparable to those of perfluoroarylsiloles. B3LYP/6-31* calculations demonstrated that the new siloles possessed large reorganization energies for electron and hole transfers and high electron mobilities. A mobility of up to 1.2×10(-5) cm(2) V(-1) s(-1) was obtained by the transient electroluminescence method, which was about fivefold higher than that of tris(8-hydroxyquinolinato)aluminum, a widely used electron-transport material, under the same conditions. All of the silole molecules possessed high thermal stability. Although, their solutions were weakly emissive, their nanoparticle suspensions and thin films emitted intense blue-green light upon photoexcitation, demonstrating a novel feature of aggregation-induced emission (AIE). Polarized emissions were observed in the silole crystals. The addition of solvents, which did not dissolve the silole molecules, into silole-containing solutions caused self-assembly of the molecules, which produced macroscopic fibrils with strong light emissions.

Tunable Light Emission from GaN-Based Photonic Crystal with Ultraviolet AlN/AlGaN Distributed Bragg Reflector

In this paper, the characteristics of GaN-based two-dimensional (2D) photonic crystal bandedge coupling operation with an ultraviolet AlN/AlGaN distributed Bragg reflector (DBR) have been investigated and analyzed. The 25-pair AlN/Al0.2GaN0.8 DBR shows a high reflectivity of 85% at 375 nm with the stop-band width of 15 nm. A strong light emission was observed from GaN photonic crystals within high reflectivity region of DBR. The emission wavelength can be also manipulated by varying the radius to lattice constant ratio. The photonic crystal bandedge mode was also characterized with three-dimensional plane-wave expansion (PWE) and finite-difference time-domain (FDTD) simulation.

Low temperature synthesis and optical properties of CaTiO 3 nanoparticles from Ca(NO 3) 2·4H 2O and TiO 2 nanocrystals

Choosing low-melting-point Ca(NO 3) 2·4H 2O and high-reactive-activity TiO 2 nanocrystals as the raw materials, a simple and cost-effective route was developed for the synthesis of CaTiO 3 nanoparticles at 600 °C, which is much lower than that (about 1350 °C) used in the conventional solid state reaction methods. X-ray diffraction, energy dispersive X-ray spectroscopy and field emission scanning electron microscopy revealed the formation of orthorhombic phase CaTiO 3 nanoparticles with oxygen-deficiency at the surface. UV–vis absorption spectrum of the as-obtained CaTiO 3 nanoparticles displayed an absorption peak centered at around 325 nm (3.8 eV), together with a tail at lower energy side. Room temperature photoluminescence spectrum of the as-obtained CaTiO 3 nanoparticles upon laser excitation at 325 nm demonstrated a strong and broad visible light emission ranging from about 527 to 568 nm, which may be originated from the surface states and defect levels.

Gallium and indium co-doping of epitaxial zinc oxide thin films grown in water at 90 °C

Zinc oxide (ZnO) thin films were intentionally co-doped with group III elements (gallium) in order to investigate and understand the effects of co-doping on the morphological, electrical and optical properties of gallium-doped ZnO (GZO) films. The co-doped films were grown on MgAl2O4 spinel substrates using a low-temperature solution-phase method known as hydrothermal synthesis. Gallium with indium co-doped ZnO (GIZO) films displayed a dramatic improvement in surface morphology as compared with the Ga-doped ZnO (GZO) films due to the compensation effect of gallium and indium doping which reduced the lattice strain. The 0.0033M gallium with 3.3 × 10−4M indium co-doped film exhibited an electron concentration of 3.14 × 1020 cm−3 and resistivity of 7.4 × 10−4 Ω cm which were both enhancements of 1.5 times over the GZO film. These films were comparable to the films fabricated by more expensive and complicated vapour-phase methods. The figure of merit for this film was determined to be 1.63 × 10−2 sq/Ω which was very close to the indium tin oxide conducting films currently used commercially. Finally, the GIZO film was hydrothermally grown on a p-GaN film to form an n-ZnO/p-GaN heterojunction light-emitting diode (LED). This LED showed diode I–V characteristics and exhibited strong cool-white light emission which signified the prospect of using GIZO as an effective and low-cost n-type layer in LEDs.

One-step, solid-state reaction to ZnO nanoparticles in the presence of ionic liquid

ZnO nanoparticles have been synthesized in ionic liquids (1-butyl-3-methylimidazolium chloride, [BMIM]Cl) by one-step solid-state reaction at room temperature for short time. The structure and morphology of the products were characterized by means of X-ray powder diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM), whose results indicated that the sample was spherical in morphology with diameters of 15–20 nm. The room temperature photoluminescence spectra (PL) showed strong and broad green light emission. In addition, [BMIM]Cl could be collected and reused for subsequent reactions. This method provides a facile, one-step and low-cost route for the synthesis of nanostructures of metal oxides with potential photo-catalysis.