Bright blue emission from Te-doped ZnS nanowires

Optical properties of Te-doped ZnS (ZnS:Te) nanowires (NWs) synthesized by a thermal chemical vapor deposition method were investigated by cathodeluminescence and photoluminescence (PL) measurements. ZnS:Te NWs exhibit the blue emission with the maximum peak at ∼440 nm at room temperature. We calculated Te-induced states on the valence band and conduction band in ZnS bulk crystal compared with PL peaks of ZnS:Te NWs. Temperature-dependent PL indicated that the activation energy of electron confined in ZnS:Te NWs is 85 meV. Blue light-emitting dot matrix displays were also fabricated using ZnS:Te NWs. This result suggested that ZnS:Te NWs could be applied as a blue-color-emitter on display devices.     

Aligned growth of ZnO nanowires by NAPLD and their optical characterizations

Not only vertically aligned ZnO nanowires but also horizontally aligned ZnO nanowires have been successfully grown on the annealed (0 0 0 1) c-cut and (1 1 2 0) a-cut sapphire substrates, respectively using catalyst-free nanoparticle-assisted pulsed-laser ablation deposition (NAPLD). The as-synthesized ZnO nanowires exhibit an ultraviolet emission at around 390 nm and the absent green emission under room temperature. The single ZnO nanowire was collected in the electrode gap by dielectrophoresis (DEP). Under the optical pumping, the single ZnO nanowire exhibited UV emission at around 390 nm with several sharp peaks whose energy spacings are almost constant, which greatly differs from the broad UV emission of the film with many nanowires, suggesting ZnO nanowires as candidates for laser media. The single ZnO nanowire showed polarized photoluminescence (PL). The as-synthesized ZnO nanowires could find many interesting applications in short-wavelength light-emitting diode (LED), laser diode and gas sensor.     

Photoluminescence spectroscopy study on tris(8-hydroxyquinoline) aluminum film

In situ photoluminescence spectroscopy (PL) measurements of tris(8-hydroxyquinoline) aluminum (Alq₃) film were carried out. Upon deposition of Alq₃ on the glass substrate, the PL intensity changes dramatically, while the peak position of Alq₃ emission shows a sharp red-shift from 524 nm at the initial deposition of Alq₃, and tends to a saturation value of 536 nm for the film thickness range from 2 to 500 nm. This red-shift is associated with the change from the 2D to 3D exciton state with increasing Alq₃ film thickness. Temperature dependent PL spectra of Alq₃ films showed, besides the changes in the PL intensity, clearly a blue-shift of Alq₃ emission about 9 nm for the film annealing up to 150 °C, while no any shift of Alq₃ emission was observed for the film annealing below 130 °C. Both changes in PL intensity, and especially in the peak position of Alq₃ emission were attributed to crystallization (thermal) effect of Alq₃ film upon annealing.     

Mechanism of intense blue photoluminescence in silica wires

The photoluminescence (PL) properties of our silica wires were investigated with PL, PL excitation and PL decay. A high brightness photoluminescence band at 2.8 eV with a shoulder around 3.0 eV was observed in our silica wires. Two PL excitation bands for the 2.8 eV emission were observed at 4.77 and 3.37 eV. The 3.37 eV excitation band is reported for the first time. The characteristic of the blue PL in our silica wires was different from that of the well-known 2.7 eV PL in bulk silica material, suggesting a negation of previous attribution of blue emission in silica nanowires. The mechanism of the PL was also discussed.     

Size-dependent photoluminescent properties of uncapped CdS particles prepared by acoustic wave and microwave method

CdS particles with crystallite size of 5-12 nm have been prepared via acoustic wave stimulated (sonochemical) route and microwave initiated combustion method. X-ray line broadening and transmission electron microscopy (TEM) suggest that sonochemical powders are more amorphous (5-10 nm) compared to microwave-synthesized sulphides (10-15 nm). The photoluminescent (PL) properties of powders with size <10 nm show a clearly blue shifted, resolved emission with full-width at half-maxima (FWHM) ∼100 nm, while powders with size >15 nm show dominant blue to green narrow emission with FWHM ∼60 nm. The mechanistic details of the synthetic route appear to affect the morphology and consequently the PL properties to a significant extent.     

Plasmon resonance modulated photoluminescence and Raman spectroscopy of diindenoperylene organic semiconductor thin film

In a comparison between a bare diindenoperylene (DIP) film and a DIP film spin-coated with a layer of gold nanoparticles, we have investigated the influence of plasmon resonances in the gold particles on spectroscopic properties of the molecular film. Under off-resonant excitation with a laser at 633 nm, the bare DIP film showed only weak photoluminescence (PL) and Raman signals, but after spin-coating gold nanoparticles on such a DIP film, we found an enhancement of both the PL and Raman signals by a factor of about 3, whereas no enhancement could be observed when the same sample was excited with laser light of 488 nm. This difference reveals that at 633 nm, plasmon resonances in the gold nanoparticles are excited, leading in turn to an enhancement of PL and Raman signals of the weakly absorbing DIP film via coupling between plasmons in the gold particles and exciton-polaritons in the molecular film. For the laser at 488 nm, due to a much larger absorption coefficient of DIP, excitons in the molecular film are directly excited, out-weighing the influence of an off-resonant coupling to the plasmon resonances in the gold particles occurring at much lower energy.     

Infrared luminescence from spark-processed silicon

The infrared (IR) photoluminescence (PL) emission of spark-processed silicon (sp-Si) was investigated. A broad and strong room temperature PL peak in the 945 nm (1.31 eV) spectral range was observed when sp-Si was excited with an argon laser. This peak is different from the PL commonly reported for anodically etched porous silicon and other silicon-based materials. The PL intensity increases substantially after annealing sp-Si between 350 and 500 °C in air after which it decreases again. The PL wavelength is observed to peak at 1010 nm by annealing sp-Si near 450 °C. It was further found that the most efficient PL occurs for a Si/O ratio of 0.3, for a small spark gap of about 1 mm, and for spark-processing times in the 15-60 min range.A model for the IR PL is proposed which mirrors that for visible PL. Specifically, it is proposed that the electrons which have been pumped by the laser from the ground state into a broad quasi-absorption band (or closely spaced absorption lines between 1.7 and 2.3 eV) revert back to lower IR levels at 1.31 eV by a non-radiative transition from where they revert radiatively to the ground state by emitting the observed 945 nm light.     

Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride

Dandelion-like gallium nitride (GaN) microstructures were successfully synthesized via Ni catalyst assisted chemical vapor deposition method at 1200 °C under NH₃ atmosphere by pre-treating precursors with aqueous ammonia. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). X-ray diffraction analysis revealed that as-synthesized dandelion-like GaN was pure and has hexagonal wurtzite structure. SEM results showed that the size of the dandelion-like GaN structure was in the range of 30-60 μm. Dandelion-like GaN microstructures exhibited reasonable field emission properties with the turn-on field of 9.65 V μm⁻¹ (0.01 mA cm⁻²) and threshold field of 11.35 V μm⁻¹ (1 mA cm⁻²) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer. Photoluminescence (PL) measurements of dandelion-like GaN showed a strong near-band-edge emission at 370.2 nm (3.35 eV) with blue band emission at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. The room-temperature photoluminescence properties showed that it has also potential application in light-emitting devices. The tentative growth mechanism for the growth of dandelion-like GaN was also described.     

On the origin of light emission from porous anodic alumina formed in sulfuric acid

We have investigated the photoluminescence (PL) property of porous anodic alumina membranes (PAAMs) formed on bulk Al foils in 0.3 M sulfuric acid. Different from that from the PAAMs formed in oxalic acid, the obtained PL spectra show two emission bands which have different origins. One centered at ∼465 nm (α-band) weakens its intensity in the PAAM annealed in O₂ and is thus attributed to optical transition in oxygen vacancies. The other in the blue (β-band) redshifts with increasing excitation wavelength. On the basis of spectral examinations and analyses, we ascribe the β-band to radiative recombination of carriers in the isolated hydroxyl groups at the surface of the pore wall, whereas the photogeneration of carriers takes place in oxygen vacancies in the pore wall. This work improves the understanding of the light-emitting property of the PAAMs formed in sulfuric and oxalic acid.     

Fabrication of complex nanostructures on ZnO crystals by the interference of three femtosecond laser beams

By adjusting the laser polarization combinations, fluences and pulse numbers, we fabricated several types of two-dimensional (2D) complex nanostructures on the surface of c-cut ZnO single crystal by the interference of three femtosecond laser beams with central wavelength of 800 nm, pulse duration of 50 fs and pulse repetition frequency of 1 kHz. The hexagonal 2D nanostructures with a period of 600 nm are very regular and uniform, in which nanoparticles, nanorings and nanoripples with sizes of 200 nm are embedded. Excited by 800 nm femtosecond laser pulses, the photoluminescence (PL) micrographs reveal that the 2D nanostructures can emit purer and brighter blue light compared with the plane surface. These nanostructures have potential applications in blue light-emitting diodes (LEDs), high density optical storage and other optoelectronic devices.     

Temperature induced size selective synthesis of hybrid Cds/pepsin nanocrystals and their photoluminescence investigation

There is growing interest in materials chemistry for taking advantage of the physical and chemical properties of biomolecules in the development of next generation nanoscale materials for opto-electronic applications. A biomimetic approach to materials synthesis offers the possibility of controlling size, shape, crystal structure, orientation, and organization. The great progress has been made in the control that can be exerted over optical materials synthesis using biomolecules (protein, nucleic acid)/mineral interfaces as templates for directed synthesis. We have synthesized the CdS nanocrystals using pepsin by biomimetic technique at four different set temperatures. X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) results showed that we are able to tune the size and distribution profile just by tuning the reaction (Rx) temperature and goes towards excitonic Bhor radius (2.5 nm) at low temperature (4 °C). The narrow absorption peak at 260 nm from Cd²⁺-pepsin complex dominates and indicates the size dispersion of the modified CdS nanoparticles are fairly monodisperse. Effective mass approximation (EMA) shows large blue-shift (~1 eV) in the band gap for the cubic phase from bulk hexagonal CdS. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra are dominated by a strong and narrow band-edge emission tunable in the blue region indicating a narrow size distribution. The reduction in PL efficiency is observed when the Rx temperature increases however no change in PLE spectra and temporal profiles of the band-edge PL is observed. At 4 °C, high emission efficiency with shift of PL spectrum in the violet region is observed for 1.7 nm size CdS quantum dots (QDs). Presence of pepsin has slowed the PL decay which is of the order of 100 μs.     

Electroplex emission at PVK/Bphen interface for application in white organic light-emitting diodes

White organic light-emitting diode (WOLED) with a structure of ITO/poly(N-vinylcarbazole) (PVK)/4,7-diphenyl-1, 10-phenanthroline (Bphen)/tris(8-hydroxyquinoline)aluminum (Alq₃)/LiF/Al has been fabricated via the thermal evaporation technique. The electroluminescence (EL) spectrum of the as-fabricated WOLED covers from 380 to 700 nm of the visible light region with a wide blue emission from PVK and an interesting new red emission. The red emission at 613 nm in EL spectra of the WOLED was attributed to electroplex emission at PVK/Bphen interface since it was not observed in photoluminescence spectra. The WOLED showed a Commission International De l'Eclairage coordinate of (0.31, 0.32), which is very close to the standard white coordinate (0.33, 0.33).     

Two peaks observed in the electroluminescence spectra of Alq3-based OLEDs

Tris(8-hydroxyquinolato) aluminum (Alq₃)-based organic light-emitting diodes were fabricated with or without using a hole transport layer (HTL). As a conventional device, the ITO/Alq₃/Mg-Ag device yielded a green-light emission with a single peak at 525 nm in the electroluminescence (EL) spectrum. In contrast, two sub-peaks were observed in the EL spectrum of some ITO/HTL/Alq₃/Mg-Ag devices. This difference was tentatively explained by comparing EL with the photoluminescence (PL) spectrum reported in the literature.     

Ex situ synthesis and optical properties of ZnO-PbS nanocomposites

Zinc oxide (ZnO) and lead sulphide (PbS) nanoparticles separately synthesized by a precipitation method were combined by an ex situ route to prepare ZnO-PbS nanocomposites with different molar ratios of ZnO and PbS. The structure and morphology of the ZnO, PbS and ZnO-PbS samples were analyzed with X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). A UV-vis spectrophotometer was used to collect the absorption and 325 nm He-Cd and 488 nm Ar lasers were used to collect the photoluminescence data from the samples. ZnO nanoparticles showed a broad and stable emission peak at ∼570 nm, while a strongly quantum confined emission from PbS nanoparticles was detected at ∼1344-1486 nm. The ZnO-PbS nanocomposites exhibited dual emission in the visible and near-infrared (NIR) regions that is associated with defects and recombination of excitonic centres in the ZnO and PbS nanoparticles, respectively. The PL intensity of the visible emission from the ZnO-PbS nanocomposite was shown to increase when the ZnO to PbS molar ratio was 5:1 and the emission was almost quenched at molar ratios of 1:1 and 1:5. For different molar ratios of ZnO to PbS, the PL intensity of the NIR emission from the ZnO-PbS nanocomposites was more intense than that of PbS nanoparticles.     

Visible luminescence of Al2O3 nanoparticles embedded in silica glass host matrix

This paper deals with the sol-gel elaboration and defects photoluminescence (PL) examination of Al₂O₃ nanocrystallites (size ∼30 nm) confined in glass based on silica aerogel. Aluminium oxide aerogels were synthesized using esterification reaction for hydrolysis of the precursor and supercritical conditions of ethyl alcohol for drying. The obtained nanopowder was incorporated in SiO₂ host matrix. After heating under natural atmosphere at 1150 °C for 2 h, the composite Al₂O₃/SiO₂ (AS) exhibited a strong PL bands at 400-600 and 700-900 nm in 78-300 K temperature range. PL excitation (PLE) measurements show different origins of the emission. It was suggested that OH-related radiative centres and non-bridging oxygen hole centres (NBOHCs) were responsible for the bands at 400-600 and 700-900 nm, respectively.     

Structural, optical and electrical properties of ZnO/Zn2GeO4 porous-like thin film and wires

Zinc oxide/zinc germanium oxide (ZnO/Zn₂GeO₄) porous-like thin film and wires has been fabricated by simple thermal evaporation method at temperature about 1120 °C for 2.5 h. The structural and optical properties of the porous-like-thin film and wires have been investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and photoluminescence (PL) spectroscopy. Metal semiconductor metal (MSM) photodetector structure was used to evaluate the electrical characteristics by using current-voltage (I-V) measurements. Room temperature photoluminescence spectrum of the sample shows one prominent ultraviolet peak at 378 nm and a shoulder at 370 nm. In addition, broad visible blue emission peak at wavelength 480 nm and green emission peak at 500 nm are also observed. Strong photoelectric properties of the MSM in the UV demonstrated that the porous-like-thin film and wires contribute to its photosensitivity and therefore making ZnO/Zn₂GeO₄ wires potential photodetector in the shorter wavelength applications.     

Variation of spark-processing parameters on the photoluminescence properties of spark-processed silicon

Electrical and physical parameters, which influence the photoluminescence (PL) properties of spark-processed silicon (sp-Si), were systematically varied in order to obtain optimal PL emission. Among these parameters are the average spark current, the pulse width of the spark events, the frequency of the pulses, the processing time, the electrode diameter, the distance between the electrodes, the spark-processing environment, and the gas ambient pressure. It was found that for optimal PL emission the processing current needs to be between 20 and 40 mA, and the pulse frequency of the sparks between 10 and 15 kHz. Further, the N₂/O₂ ratio of the processing environment needs to be about 7:3 and the ambient gas pressure and the processing time as large as feasible. The conditions that are favorable for green PL are a small pulse width, a small counter electrode diameter, a small gap between electrodes, a relatively large nitrogen concentration in the processing chamber, and a comparatively large spark frequency. In the opposite cases, a UV/blue PL is predominantly observed. The results are discussed in terms of various thermal effects on the resulting molecules or defects, which are believed to be important for the PL emission.     

The growth and optical properties of ZnO nanowires at the junctions of nanowalls

Highly pure ZnO nanowires at the junctions of nanowalls have been successfully prepared via a simple vapor phase route at relative low temperature in the absence of catalyst. SEM, TEM, Raman scattering and PL were employed to characterize as-grown samples. The nanowires have average diameter of 10 nm with [001] growth direction. Raman scattering shifted from higher frequency to lower frequency and the enhancement of PL intensity of the green emission to the UV may be attributed to smaller nanowires and thinner nanowalls. This nanostructure may have important applications in batteries, light-emitting and energy conversion devices and other fields demanding high surface area materials.     

Concentration dependence of photoluminescence properties in poly[(2-methoxy,5-octoxy)1,4-phenylenevinylene] thin films

Photoluminescence and selective excitation photoluminescence measurements at room temperature have been performed on poly[(2-methoxy,5-octoxy) 1,4-phenylenevinylene] (MO-PPV) thin films, which are prepared from MO-PPV chloroform solutions of different concentrations. The position of the S0→S1 absorption peak shows red-shift and broadened relative to that in an MO-PPV solution form due to the solidification effect, while no relative shifts between the absorption spectra of these film samples are observed. A long wavelength emission component near 630 nm has been identified as S2→S0 vibronic transition through the Gaussian decomposition method and confirmed by below-gap PL and selective-excitation PL experiments. This second vibronic component cannot be observed in the spectra of thick films. The PL efficiency of MO-PPV thin film is also investigated through comparison with that of an MEH-PPV thin film and explained by the side substituent effect.     

Laser on porous silicon after oxidation by irradiation and annealing

Stimulated emission has been observed from oxide structure of silicon when optically excited by 514 nm laser. The photoluminescence (PL) pulse has a Lorentzian shape with a full width at half maximum (FWHM) of 0.5-0.6 nm. The twin peaks at 694 nm and 692 nm are dominated by stimulated emission which can be demonstrated by its threshold behavior and transition from sub-threshold to linear evolution in light emission. The gain coefficient from the evolution of the peak-emission intensity as a function of the optically pumped sample length has been measured. The oxide structure was fabricated by laser irradiation and annealing treatment on silicon. A model for explaining the stimulated emission has been proposed in which the trap states of the interface between oxide of silicon and porous nanocrystal play an important role.