Strong Light Emission Research Articles

Application of Ultrathin Layer in White Organic Electroluminescent Devices

Based on 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) acting as dopant and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) acting as hole-blocking layer, the EL spectra of two kinds of organic light-emitting diodes (OLEDs) ITO/NPB/BCP/Alq3∶DCJTB/Alq3/Al (structure A) and ITO/NPB/BCP/Alq3/Alq3∶DCJTB/Alq3/Al (structure B) were investigated. Our experimental data showed that the relative emission intensity of green light was low in the EL spectra of devices with structure A, and it was hard to enhance the relative emission intensity of green light by only increasing the thickness of Alq3 layer. On the other hand, the thin layer of Alq3 inserted between BCP layer and dopant layer (Alq3∶DCJTB) had a tremendous impact on the relative emission intensity of green light for devices with structure B, and a strong green light emission could be obtained by inserting an ultrathin layer of Alq3. Further, by optimizing the device structure, a stable white OLED ITO/NPB(50 nm)/BCP(3 nm)/Alq3(3 nm)/Alq3∶DCJTB(1%(w))(5 nm)/Alq3(7 nm)/Al was obtained with ultrathin organic layers of BCP, Alq3 and Alq3∶DCJTB. The CIE of the device almost kept at (0.33, 0.37) with increasing the applied voltages (14-18 V), and its luminance reached 11521 cd·m-2 at the current density of 432 mA·cm-2.

Stress-induced band gap tuning in ⟨112⟩ silicon nanowires

We show via density functional calculations that the electronic band structures of ⟨112⟩-oriented hydrogenated silicon nanowires (SiNWs) could be significantly altered by axial stresses. In particular, an axial compression could cause an indirect-to-direct band gap transition in ⟨112⟩ SiNWs. As direct energy band may induce strong light-emission properties of Si, the possibility of indirect-to-direct band transition via axial stress has fundamental implications in exploiting SiNWs for optoelectronic applications.

Continuous production of fine zinc oxide nanorods by hydrothermal synthesis in supercritical water

Continuous production of highly crystalline ZnO nanorods by supercritical hydrothermal synthesis was reported in this article. Zinc nitrate aqueous solution was pressurized to 30 MPa at room temperature and rapidly heated to 673 K by mixing with supercritical water and then fed into a tubular reactor. Residence time is about 10 s. Production of ZnO nanorod particles with uniform particle size distribution showed a strong ultraviolet light emission at room temperature. This article also reported in-situ surface modification of ZnO nanorods with organic reagents by the supercritical hydrothermal synthesis.

Synthesis and photoluminescence, field emission properties of stalactite-like ZnS-ZnO composite nanostructures

Wurtzite stalactite-like quasi-one-dimensional ZnS nanoarrays with ZnO protuberances were synthesized through a thermal evaporation route. The structure and morphology of the samples are studied and the growth mechanism is discussed. X-ray diffraction (XRD) results show both the ZnS stem and the ZnO protuberances have wurtzite structure and show preferred [001] oriented growth. The photoluminescence and field emission properties have also been investigated. Room temperature photoluminescence result shows it has a strong green light emission, which has potential application for green light emitter. Experimental results also show that the stalactite arrays have a good field emission property, with turn-on field of 11.4 V/mu m, and threshold field of 16 V/mu m. The ZnO protuberances on the ZnS stem might enhance the field emission notably.

Synthesis of GaN nanorods using Tantalum catalyst by magnetron sputtering

Single crystalline wurzite GaN nanorods are successfully synthesized on the tantalum catalyzed Si substrate by RF magnetron sputtering. The products are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and photoluminescence (PL). The results show that the nanorods have a hexagonal wurtzite structure with diameters ranging from 80 to 200 nm and lengths typically up to 10 μm. The PL spectrum exhibits a strong UV light emission at 364 nm. The growth mechanism of the crystalline GaN nanorods is discussed briefly.

Simple Synthesis of a Variety of Nano-structures Using Silicide Alloys with Ga Droplets

A variety of nano-structures, such as nanofibers, nanotubes, nanocapsules, nanoribbons and nanorods, were synthesized using silicide alloys with Ga droplets. It was found that the growth morphology and the structural property of the nanostructures significantly depended on the silicides seed materials. In addition, the amorphous SiOx nanofibers show strong ultraviolet and/or visible light emissions, and the PL spectra of the nanofibers depended on the seed materials. For some of the nanofiber syntheses, the formation of nanoflakes or nanoribbons of β-Ga2O3 occurs, and a variety of growth morphologies for the β-Ga2O3 nanostructures was obtained. The series of morphological and structural images are shown for a variety of nanostructures. The obtained SiOx nanofibers are expected to be materials for use in cheap, abundant and safe luminescent devices for visible light applications.

Synthesis of hierarchical heterostructure of Si 3N 4/Si

Three-dimensional Si-based hierarchical heterostructures with the assistance of catalyst Fe have been synthesized by CVD process. It is found that the major stem wires are single-crystalline Si, and the side branches are single-crystalline Si 3N 4 nanowires. A plausible two-stage growth mechanism has been proposed for the formation of these uniquely stacked structures. Firstly, Si stems are formed via VS mechanism. Secondly, the catalyst Fe deposited onto the surfaces of the stem and then serves as the nucleate and growth sites for the formation of Si 3N 4 nanowires. The photoluminescence (PL) spectrum of the product shows a strong blue light emission centered at 413 nm with a shoulder at 440 nm under excitation at 360 nm.

Quantized Growth of CdTe Quantum Dots; Observation of Magic-Sized CdTe Quantum Dots

This work presents experimental observation of the quantized growth of CdTe quantum dots (QD) in the presence of hexadecylamine (HDA), hexylphosphonic acid (HPA), and trioctylphosphine oxide (TOPO) above 200 °C. The crystal growth of CdTe QDs is monitored by in situ UV−vis absorption spectroscopy. The high-temperature absorption spectra indicate the evolution of multiple peaks corresponding to various sizes of QDs. Analysis of the growth kinetics suggests quantized growth of the CdTe QDs in the coordinating solvent mixture. The high-resolution transmission electron microscopy (HRTEM) images and electron diffraction pattern show that most of the QDs have the zinc blende crystal structure. The HRTEM images indicate nanotwinning and stacking faults in larger CdTe QDs. Domain sizes in the HRTEM images correlate well with the smallest observed magic-sized CdTe QDs, in agreement with the proposed aggregation growth mechanism under the experimental conditions. The smallest observed zinc blende CdTe QDs with the diameter of 1.9 ± 0.3 nm are isolated by quenching the reaction mixture during the initial phase of the QD synthesis. The experimental observation suggests that the surprising stability of the magic-sized CdTe QDs is the result of the surface stabilization of the QDs of the HDA and/or HPA. As previously suggested, the aggregation is driven by dipole−dipole interaction between CdTe nanoparticles. The results show that the aggregation of quantum dots could be very important at the early stage of the growth. The magic-sized QDs can be dissolved in either methanol or toluene, which suggests heterogeneity of their surface chemistry. The QDs dissolved in the methanol phase exhibit relatively strong white light emission from 400 to 650 nm with an emission quantum yield of approximately 4%. The QDs dissolved in the toluene phase exhibit very weak emission.

Electronic excitations induced by the impact of coinage metal ions and clusters on a rare gas matrix: Neutralization and luminescence

Low-energy collisions of atomic ions and cluster ions with rare gas covered metal substrates can lead to strong light emission which is element and size specific. Instead of a direct energy transfer into the electronic system of the cluster the observed emission originates from excitons trapped in the rare gas layer which carries the excitation energy. The nature of the processes involved in the neutralization and excitation of the neutral clusters electronic system are discussed. Strong analogies to thermoluminescence are found.

Near ultraviolet light emitting diode composed of n-GaN∕ZnO coaxial nanorod heterostructures on a p-GaN layer

The authors report on the fabrication and characteristics of near ultraviolet nanorod light emitting diodes (LEDs) composed of n-GaN∕ZnO nanorod heterostructures on p-GaN substrates. The nanorod LEDs consist of the vertically aligned n-GaN∕ZnO coaxial nanorod arrays grown on a p-GaN substrate. The LEDs demonstrated strong near ultraviolet emission at room temperature. The nanorod LEDs were turned on a forward-bias voltage of 5V, and exhibited a large light emitting area. From electroluminescent spectra, dominant emission peaks were observed at 2.96 and 3.24eV for an applied current of 2mA. The origins of the strong and large area light emission are also discussed in terms of enhanced carrier injection from n-GaN nanostructures to p-GaN substrates.

Synthesis and photoluminescence of single-crystalline GaN nanowires and nanorods

Single-crystalline GaN nanowires and nanorods have been successfully grown on Si(1 1 1) substrates by magnetron sputtering through ammoniating the Ga 2O 3/Nb films at 900 °C in a quartz tube. The GaN nanowires and nanorods have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), field-emission transmission electron microscope (FETEM), and photoluminescence. The results show that the diameters of the nanowires are about 50 nm while the diameters of the nanorods are within 100–200 nm. Photoluminescence of the GaN nanostructure materials revealed only a strong and broad UV light emission peak at 369 nm. Finally, the growth mechanism of GaN nanostructure materials is also briefly discussed.

Study of the Effect of Glow Discharges Near aM = 3 Bow Shock

This paper aims at investigating the effect of discharges generated near steady bow shocks and their possible use for localized heat deposition leading to drag reduction. We report the observation of steady discharges generated in front of a blunted model in a M = 3 supersonic flow. The test model is designed as a pair of coaxial electrodes, and set up in the ONERA R1Ch M = 3 blowdown wind tunnel. A high voltage power supply is used to generate negative discharges. A corona regime, a glow regime, and a filamentary arc regime are observed. The negative corona regime consists of Trichel pulses dissipating less than 200 mW of electrical power. The glow discharge absorbs powers up to 0.5 kW. It displays a strong light emission in the vicinity of the shock front, followed by a darker region downstream of the shock. The drag coefficient is measured and shows no measurable change when a glow discharge is switched on. To explain this and further investigate the effect of localized heat deposition at the shock front, modified Rankine-Hugoniot jump relations are computed, taking into account a volumetric heat source. This allows one to compute a fair estimate of the drag coefficient and shows that drag reduction by localized heating in the shock front is possible. However, it also shows that in our experiment, the plasma thermal power is too small to appreciably reduce the drag, possibly because of the role of the electron impact excitation of N 2 vibrations, whose fairly long relaxation time could shift downstream the effective gas heating. More generally, the model shows that power-efficient plasma-induced drag reduction requires high plasma heating efficiency.

Towards UV imaging sensors based on single-crystal diamond chips for spectroscopic applications

The recent improvements achieved in the Homoepitaxial Chemical Vapour Deposition technique have led to the production of high-quality detector-grade single-crystal diamonds. Diamond-based detectors have shown excellent performances in UV and X-ray detection, paving the way for applications of diamond technology to the fields of space astronomy and high-energy photon detection in harsh environments or against strong visible light emission. These applications are possible due to diamond's unique properties such as its chemical inertness and visible blindness, respectively. Actually, the development of linear array detectors represents the main issue for a full exploitation of diamond detectors. Linear arrays are a first step to study bi-dimensional sensors. Such devices allow one to face the problems related to pixel miniaturisation and of signal read-out from many channels. Immediate applications would be in spectroscopy, where such arrays are preferred. This paper reports on the development of imaging detectors made by our groups, starting from the material growth and characterisation, through the design, fabrication and packaging of 2× n pixel arrays, to their electro-optical characterisation in terms of UV sensitivity, uniformity of the response and to the development of an electronic circuit suitable to read-out very low photocurrent signals. The detector and its electronic read-out were then tested using a 2×5 pixel array based on a single-crystal diamond. The results will be discussed in the framework of the development of an imager device for X–UV astronomy applications in space missions.

Fabrication, properties and sintering of ZnO nanopowder

Pure ZnO nanopowder was unintentionally prepared by a solvothermal synthesis method in dimethylformamide (DMF) solvent, using zinc chloride, potassium hydroxide, and potassium borohydride as starting materials. The size of the powder synthesized at 180 °C for 48 h is about 35 nm. The nanopowder exhibits much better absorbance in ultraviolet band than commercial ZnO powder and shows a strong violet light emission excited by a 325-nm line. The nanopowder can be pressurelessly sintered to high density (>98% of theoretical density) at 950 °C in atmosphere.

Single-step fabrication of Fresnel microlens array on sapphire substrate of flip-chip gallium nitride light emitting diode by focused ion beam

A simple one-step focused ion beam milling technique was used to fabricate a Fresnel microlens array on the backside of sapphire substrate of a gallium nitride blue light emitting diode. The optical output power from the flip-chip gallium nitride blue light emitting diode is enhanced to about 1.68 times at the injection current of 20mA. The spatial light distribution from the backside of flip-chip gallium nitride blue light emitting diode with a Fresnel microlens array shows a uniform and stronger light emission.

Tris(4‐cyanophenyl)amine: Simple Synthesis via Self‐assembly; Strong Fluorescence in Solution, Nano/microcrystals, and Solid

AbstractThe threefold symmetric molecule, tris(4‐cyanophenyl)amine (TCPA) is synthesized from 4‐fluorocyanobenzene by treatment with potassium carbonate in dimethylsulfoxide; the occurrence of the reaction without any amine reagent suggests the involvement of a novel self‐assembly process. A reprecipitation strategy provides stable colloids containing highly monodisperse nano/microcrystals with well‐defined cubic morphology and sizes tunable from 250 to 500 nm. The solution, colloid, and solid states of TCPA exhibit strong blue fluorescence; the colloid shows dual emission with an unusually small Stokes shift. Computational investigations are carried out on the molecule and supramolecular assemblies derived from the crystal structure. Coupled with detailed spectroscopic studies, they show that the emission in the colloidal and solid states can be attributed to energy levels resulting from the intermolecular interactions within different aggregation motifs in the condensed phase and energy cascades between them. The computations also reveal the presence of cooperative interactions in the molecular crystal contributing to its high thermal stability. The strong light emission exhibited by TCPA, concurrently in the solution, nano/microcrystal, and solid states establishes it as a novel molecular material of potential practical utility; it has led to the exploration of the underlying mechanism that describes the phenomena observed in the different physical states.

Formation and luminescent properties of face-centered-cubic Si nanocrystals in silica matrix by magnetron sputtering with substrate bias

Si nanocrystals with face-centered-cubic (fcc) structure were fabricated using common sputtering technique, but applying bias on the substrate, the thermodynamic calculation indicates that the bombardment of Ar+ ions arising from the bias should be responsible for the crystallization and phase transition of Si nanocrystals. The silica films containing fcc-Si nanocrystals give strong blue and ultraviolet light emissions, which suggest that the band structure of Si material could be modified by changing the crystal structure.

Surfactant-assisted self-assembly growth of single-crystalline ZnO microflowers at low temperature

Uniform single-crystalline ZnO microflower with well-defined multilayer spiny petals and hexangular prism pistil have been successfully prepared through the surfactant-assisted self-assembly route at 60°C. For the as-prepared product, three emitting bands were observed, of which included a very strong and broad green light emission at around 580nm, and two shoulders emission at 404nm and 439nm, respectively. ESR experiments confirm that there existed a great fraction of oxygen vacancies in the ZnO microflowers. The possible self-assembly growth mechanism has also been proposed.

Effect of NH 3 flow rate on growth, structure and luminescence of amorphous silicon nitride films by electron cyclotron resonance plasma

In this paper, hydrogenated amorphous silicon nitride (a-SiN x :H) films have been deposited using an electron cyclotron resonance chemical vapor deposition system. The effect of NH 3 flow rate R on the deposition rate, structure and luminescence were studied using various techniques such as optical emission spectroscopy, Fourier Transform Infrared absorption (FTIR), X-ray photoelectron spectroscopy (XPS) and fluoro-spectroscopy, respectively. Optical emission behavior of SiH 4 + NH 3 plasma shows that atomic Si radical concentration determines the film deposition rate. Structural transition of a-SiN x film from Si-rich one to near-stoichiometric/N-rich one with R was revealed by FTIR and the two phase separation of a-Si and a-Si 3N 4 was also convinced in Si-rich SiN x films by XPS. Either photo- or electroluminescence for all the SiN x films with R > 3 sccm shows a strong light emission in visible light wavelength range. As R < 6 sccm, recombination of electrons and holes in a-Si quantum dots is the main mechanism of photo/electroluminescence for Si-rich SiN x films, however, for photoluminescence, gap states' luminescence is also in competition; as R > 6 sccm, light emission of the SiN x film originates from defect states in its band gap.

Cathodoluminescent and nonlinear optical properties of undoped and erbium doped nanostructured ZnO films deposited by spray pyrolysis

We have deposited zinc oxide (ZnO) and erbium doped zinc oxide (ZnO:Er) thin films on heated glass substrates using spray pyrolysis technique. The effect of erbium dopant on structural, morphological, luminescent and nonlinear optical properties was studied. The deposited films have been analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM), ex situ compositional analysis (ESCA), profilometry, cathodoluminescence (CL) and third harmonic generation (THG) measurements. All films were polycrystalline, having a preferential growth orientation along the ZnO (002) plane, with a corresponding average crystallite size of less than 41nm. Addition of erbium can effectively control the film surface morphology and its cathodoluminescent properties. The films containing low erbium concentration show a uniform surface covered with hexagonal shaped grains and a strong UV light emission intensity as well as TH response. In contrast, when the erbium doping ratio exceeds 3%, a porous surface with columnar textural growth becomes more pronounced, and a substantial reduction of the cathodoluminescent and TH response. A strong TH signal was obtained for the film with good crystalline quality at the concentration of 2%. Third order nonlinear optical susceptibility (χ〈3〉) values of the studied materials were in the remarkable range of 10−12esu.