Lower Photoluminescence Intensity Research Articles

Reduced Graphene Oxide/InGaZn Mixed Oxide Nanocomposite Photocatalysts for Hydrogen Production

A series of reduced graphene oxide and indium-gallium-zinc mixed oxide (RGO/IGZ) nanocomposites were successfully synthesised by a simple one-step hydrothermal method. The as-synthesised nanocomposites were characterised by crystallographic, microscopic, and spectroscopic methods to explore the robust photocatalytic activity of the prepared materials. XRD patterns confirmed the formation of highly pure, single-phase, hexagonal In2 Ga2 ZnO7 with no impurity-related peaks. All the photocatalysts absorbed visible light as observed from the diffuse reflectance UV/Vis spectra. The electron-hole recombination is effectively minimised by the formation of an RGO/metal oxide nanocomposite, which was successfully derived from a photoluminescence (PL) study and photoelectrochemical measurements. The decoration of IGZ nanocrystals onto reduced graphene sheets leads to significant quenching of its luminescent intensity, dramatically improved photocurrent generation (33 times more than neat IGZ) and significantly enhanced photostability. The high photocatalytic activity for H2 production is explained by the strong interaction between the IGZ nanocrystals with RGO sheets, low PL intensity, high photocurrent and large surface area.

Trench-Shaped Defects on AlGaInN Quantum Wells Grown under Different Growth Pressures

We obtained high-quality AlGaInN/GaN quantum wells (QWs) with a 385 nm emission by adding Al sources to GaInN QWs grown at a low growth pressure of 150 mbar. When AlGaInN QWs were grown at a relatively high growth pressure of 400 mbar, a considerable amount of peculiar trench-shaped defects were observed on the surface of the AlGaInN QWs. We found that the trench defects acted as nonradiative centers, leading to low photoluminescence (PL) intensities. Our experiments reveal that a low growth pressure is effective in suppressing the trench defect formation, resulting in high-quality AlGaInN QWs.

Effect of starting properties and annealing on photocatalytic activity of ZnO nanoparticles

Eight commercial ZnO samples were investigated, as-received and after annealing in the flow of oxygen or forming gas. We found that ZnO nanoparticles with higher photoluminescence (PL) intensity (fewer nonradiative defects) exhibited a high photocatalytic activity. Samples exhibiting high PL intensity presented lower concentrations of surface adsorbates, which is consistent with lower native defect density in those samples. It was observed that annealing did not result in an improvement in photocatalytic activity of the samples with low defect density (high PL intensity). For the samples with low PL intensity, the same annealing condition could result in an improvement or worsening of photocatalytic activity, depending on the starting properties of the samples. Variable effects of annealing on samples with different properties indicate that the effect of a post-growth treatment cannot be generalized for all ZnO samples. The improvements in the photocatalytic activity were observed for a cationic dye methylene blue, while for an anionic dye acid red 27 annealing resulted in the changes in the amount of adsorbed dye, but no improvement in the photocatalytic activity for dye degradation.

CNTs–TiO2/Al2O3 composite membrane with a photocatalytic function: Fabrication and energetic performance in water treatment

Integrating a microfiltration with photocatalysis is a rising method for improving the anti-fouling capability of membrane by degradating pollutants blocked the pores of membrane via photocatalysis. However, the quick recombination of photogenerated charges in the photocatalytic layer limited the performance of this photocatalytic membrane. To inhibit the recombination of photogenerated charges, a CNTs–TiO2/Al2O3 composite membrane was designed and fabricated. Compared with TiO2/Al2O3 membranes (the typical photocatalytic membrane), the CNTs–TiO2/Al2O3 composite membrane displayed lower photoluminescence intensity and higher photocurrent density, which indicated the higher separation efficiency of its photogenerated charges. To get a good combination of membrane flux and rejection, the CNTs content and the thickness of CNTs–TiO2 layer was optimized. Under the best prepared parameters, the Polyethylene glycol (PEG) rejection and permeate flux was 70% and 980Lm−2h−1, respectively. The optimized CNTs–TiO2/Al2O3 composite membrane under UV light irradiation exhibited 3 times higher of the stable permeate flux than filtration alone, and the humic acid removal rate of composite membrane was 10% higher than TiO2/Al2O3 membranes. This work could provide an alternative way to improve rejection and photocatalytic efficiency for conventional photocatalytic membranes and facilitate their practical application in water treatment.

Generation of nitrogen-vacancy color center in nanodiamonds by high temperature annealing

We make use of inherent vacancies and nitrogen substitutions in nanodiamonds to generate nitrogen-vacancy (NV) centers by high temperature annealing. After 800 °C annealing, low temperature photoluminescence intensity of nanodiamonds shows more than eight times improvement compared with that of unannealed samples. Confocal microscope images of well dispersed nanodiamonds indicate a dramatically increased proportion of nanodiamonds containing NV centers after annealing. Optically detected magnetic resonance spectrum of single NV center demonstrates that the resultant nanodiamonds are suitable for further application as magnetic field sensor. The annealing-oxidation method could be an attractive option for NV center generation in nanodiamonds.

Enhanced hydrogen production over CdSe QD/ZTP composite under visible light irradiation without using co-catalyst

A series of CdSe quantum dot (QD)/zirconium titanium phosphate (ZTP) was synthesized by solvothermal method using ethylene diamine by varying Cd to Se ratio from 1:1 to 1:4 and examined as robust catalysts for hydrogen evolution under visible light irradiation without using any co-catalyst. Extensively, the structural, optical, morphological, elemental and photoresponse spectra measurement of the composite system was studied. The catalytic activity of the materials was correlated with photoluminescence spectra, band gap energy and the photosensitization effect of CdSe quantum dot. Though neat CdSe quantum dot and zirconium titanium phosphate (ZTP) exhibited photocatalytic hydrogen evolution, the composite material showed remarkable high activity. Among these, 1CdSe quantum dot/zirconium titanium phosphate (ZTP) composite showed the highest hydrogen production (905.4μmol) within 3h which is consistent with low photoluminescence (PL) intensity, wide band gap energy and the photosensitization effect of CdSe quantum dot.

Seeding Layer Influence on the Low Temperature Photoluminescence Intensity of 3C-SiC Grown on 6H-SiC by Sublimation Epitaxy

We report on n-type 3C-SiC samples grown by sublimation epitaxy. We focus on the low temperature photoluminescence intensity and show that the presence of a first conversion layer, grown at low temperature, is not only beneficial to improve the homogeneity of the polytype conversion but, also, to the LTPL signal intensity. From the use of a simple model, we show that this comes from a reduced density of non-radiative recombination centers.

Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots

Photoluminescence blinking--random switching between states of high (ON) and low (OFF) emissivities--is a universal property of molecular emitters found in dyes, polymers, biological molecules and artificial nanostructures such as nanocrystal quantum dots, carbon nanotubes and nanowires. For the past 15 years, colloidal nanocrystals have been used as a model system to study this phenomenon. The occurrence of OFF periods in nanocrystal emission has been commonly attributed to the presence of an additional charge, which leads to photoluminescence quenching by non-radiative recombination (the Auger mechanism). However, this 'charging' model was recently challenged in several reports. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging, with the goal of clarifying the role of charging in blinking. We find that two distinct types of blinking are possible: conventional (A-type) blinking due to charging and discharging of the nanocrystal core, in which lower photoluminescence intensities correlate with shorter photoluminescence lifetimes; and a second sort (B-type), in which large changes in the emission intensity are not accompanied by significant changes in emission dynamics. We attribute B-type blinking to charge fluctuations in the electron-accepting surface sites. When unoccupied, these sites intercept 'hot' electrons before they relax into emitting core states. Both blinking mechanisms can be electrochemically controlled and completely suppressed by application of an appropriate potential.

Amorphous Hydrogenated Silicon Carbide (a‐SiC:H) Coatings Produced by Remote Hydrogen Microwave Plasma CVD from Bis(dimethylsilyl)ethane ‐ a Novel Single‐Source Precursor

The effect of substrate temperature (TS) on the growth rate, chemical structure, surface morphology, density, refractive index and photoluminescence (PL) of a‐SiC:H films is reported. The increase in TS from 30°C to 400°C causes the elimination of organic moieties from the film and the formation of Si‐carbidic network structure. The films produced at high TS are morphologically homogeneous, dense, and hard materials of high refractive index and extremely low PL intensity.

Deposition of ZnO multilayer on LiNbO3 single crystals by DC-magnetron sputtering

Zinc oxide (ZnO) thin films were deposited on LiNbO3 (LN) single crystals with 200nm thicknesses by three different ways, where coating of zinc (Zn) film was followed by thermal oxidation for four, two, and one steps with 50, 100, and 200nm thicknesses repeatedly. Sample, which was produced at 4-step of deposition and oxidation of Zn layer, showed high transmittance and low structural defect due to a lower photoluminescence intensity and Urbach energy. Average grain size in X-ray diffraction (XRD), scanning electron microscopy (SEM) micrograph, and atomic force microscopy (AFM) images for multilayer of ZnO was lower than monolayer of ZnO thin films. Applying multilayer coating technique leads to decrease of surface roughness and scattering on light on surface and fabrication of LiNbO3 waveguides with lower optical loss.

On the mechanism of enhanced photocatalytic activity of composite TiO2/carbon nanofilms

In the search for the origin of the enhanced photocatalytic activity of composite TiO2–carbon systems, we have fabricated and analyzed well-defined model samples consisting of anatase and graphitic carbon with and without modifying the interface between them by a thin SiO2 space layer. The films with a TiO2/C interface show noticeably lower photoluminescence intensity and shorter carrier life times compared to single TiO2 films with the same thickness and composition. The stronger non-radiative recombination is mainly assigned to charge carrier leakage (transfer) at the interface between TiO2 nanocrystallites and the carbon film.

Confocal spectroscopy of InGaN LED structures

Photoluminescence of InGaN structures for green light-emitting diodes (LEDs) with multiple quantum wells as an active medium was studied with spatial and spectral resolution using confocal microscopy. Bright spots of ∼200 nm diameter were observed. Emission from these bright areas was up to 8 times more intense than from the rest of the sample surface and the band peak position in these areas was blueshifted with respect to the band position in the background surface of lower photoluminescence intensity. The data on emission properties in bright and dark areas and the dependence of these properties on the excitation power density were interpreted by assuming inhomogeneous distribution of defects acting as nonradiative recombination centres.

Optimization of InGaAsN(Sb)/GaAs quantum dots for optical emission at 1.55 μm with low optical degradation

Low optical degradation in GaInAsN(Sb)/GaAs quantum dots (QDs) p–i–n structures emitting up to 1.55μm is presented in this paper. We obtain emission at different energies by means of varying N content from 1 to 4%. The samples show a low photoluminescence (PL) intensity degradation of only 1 order of magnitude when they are compared with pure InGaAs QD structures, even for an emission wavelength as large as 1.55μm. The optimization studies of these structures for emission at 1.55μm are reported in this work. High surface density and homogeneity in the QD layers are achieved for 50% In content by rapid decrease in the growth temperature after the formation of the nanostructures. Besides, the effect of N and Sb incorporation in the redshift and PL intensity of the samples is studied by post-growth rapid thermal annealing treatments. As a general conclusion, we observe that the addition of Sb to QD with low N mole fraction is more efficient to reach 1.55μm and high PL intensity than using high N incorporation in the QD. Also, the growth temperature is determined to be an important parameter to obtain good emission characteristics. Finally, we report room temperature PL emission of InGaAsN(Sb)/GaAs at 1.4μm.

Some reasons of emission variation in InAs quantum dot-in-a-well structures

Photoluminescence (PL) and X ray diffraction have been studied in InAs quantum dots (QDs) embedded in symmetric In0.15Ga1-0.15As/GaAs quantum wells (dot-in-a-well, DWELL) with QDs grown at different temperatures. The density of QDs decreases from 1.1×1011 down to 1.3×1010 cm-2 with increasing the QD growth temperatures from 470 to 535°C. The QD density decreasing in DWELLs is accompanied by the non monotonous variation of QD parameters. The PL intensity increases and the PL peak shifts to low energy in structures with QDs grown at 490 and 510°C. On the contrary the structures with QDs grown at 525 and 535°C are characterized by lower PL intensities and PL peak positions shifted to higher energy.The method of X-ray diffraction has been applied with the aim to study the variation of elastic strain in DWELL structures with QDs grown at different temperatures. It was shown that the minimum of elastic strain corresponds to DWELL with QDs grown at 490-525 °C. For lower (470 °C) and higher (535 °C) QD growth temperatures the level of compressive strain increased in DWELLs. The reasons of strain variation are discussed as well.

Optical and electrical quality improvements of undoped InAs∕GaSb superlattices

The performance and operating temperature of infrared (IR) detectors are largely limited by thermal generation and noise processes in the active region of the device. Particularly, excess background charge carriers enhance dark currents and depress the detector figures of merit. Therefore, reducing the overall defects and background carriers in the undoped region of p-i-n diodes is an important issue for developing high-operating temperature IR detectors. In this article, the authors discuss how several postgrowth annealing conditions and interface shutter sequences are optimized to reduce the density of nonradiative defect trap centers and background carriers and studied their relevance to the photoluminescence (PL) emission qualities of typical mid-IR InAs∕GaSb superlattices (SLs). Among the several in situ postgrowth annealing temperatures investigated, the SLs annealed at 450°C had the highest carrier density and the lowest PL intensity, while the SLs annealed at 475°C had the lowest carrier density and the highest PL intensity. In situ annealing did not change the carrier type, but slightly reduced the carrier densities as compared to the as-grown SL sample of 1.8×1011cm−2. While in situ annealing improves the surface morphological quality and the PL intensity, ex situ annealing creates many surface pits and significantly reduces the PL intensity. The carrier density was sensitive to the variation in interface composition control. With a minor variation in the interface shutter sequence, the carrier density dramatically increased from ∼2×1011to5×1012cm−2, and the corresponding mobility decreased from 6600to26cm−2∕Vs, indicating a degradation of the interfacial quality. All of the SL samples investigated for this study were p-type.

Synthesis of ZnO:N thin films by reactive dc magnetron sputtering

Structural, electrical, and optical properties of nitrogen doped zinc oxide (ZnO:N) thin films deposited by direct current magnetron sputtering are analysed in this work. Nitrogen doping allows creation of p-type semiconducting ZnO thin films for their most important application as light emitting diodes. The scanning electron microscope images of sample crosssection show dense structure with columnar growth features. The X-ray diffraction patterns indicate semi-amorphous structure, explaining very low photoluminescence intensity. Optical transmittance measurements reveal significant changes in optical properties upon different amount of nitrogen doping. The resistance measurements in 30–90 °C temperature range of ZnO:N thin films demonstrate similar to semiconductor behaviour and show resistance change up to 50%. Keywords: reactive magnetron sputtering, nitrogen doped zinc oxide, p-type ZnO thin films, LED

Self-selective recovery of photoluminescence in amphiphilic polymer encapsulated PbS quantum dots

Self-selected recovery of the photoluminescence (PL) of amphiphilic polymer encapsulated PbS quantum dots (QDs) was observed in water for the first time and possible mechanisms were proposed based on investigations by means of transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction and fluorescence spectroscopy. Water-soluble PbS QDs were synthesized by transferring monodispersed QDs capped with hydrophobic ligands of oleylamine from an organic solvent into water via amphiphilic polymers poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). The water transfer process leads to a double size distribution (5.6 ± 0.9 nm and 2.7 ± 0.4 nm), attributed to ligand etching together with Ostwald ripening, as well as the fast decay of PL. The automatic recovery of the PL in PbS QDs stored in water in the dark for 3 months was only observed for the subset of smaller QDs and is largely due to the removal of surface defects with aging, as evidenced by the decreased percentage of unpassivated surface atoms from XPS studies. In contrast, the PL of the subset of larger QDs in the same sample does not self-recover in water and can only be slightly recovered by transferring them into environments with less external quenches. The results strongly suggest that it is the surface defect in the larger QDs themselves, introduced during Ostwald ripening, that is primarily responsible for their non-emitting status or rather low PL intensity under different conditions. The increase of unpassivated Pb atoms in larger PbS QDs after the 3 month aging has been confirmed by XPS, which explains their non-recovery behavior in water. The PL-recovered QD sample in water is very stable and shows comparable photostability to the initial QDs dispersed in an organic phase.

Photoluminescence of gold, copper and niobium as a function of temperature

A specially constructed instrument for measuring the low intensity photoluminescence emission spectra of metals is described. It uses low luminescence optical components and dedicated sample mounting techniques. Room temperature measurements agree closely with literature spectra for high-purity gold and are found to be sensitive to 100 ppm impurities. Detailed spectra are presented, which are weakly temperature dependent, for gold, copper and unpolished niobium between room temperature and 100 K. We conclude that this work provides accurate luminescence data for Au from 300 K down to 100 K. Although the (variable temperature) luminescence data for Cu are consistent both with the room temperature experimental data in the literature and theory, we conclude the role of surface adsorbates and/or oxides cannot be ruled out. Theory suggests that Nb has a factor ∼50 lower luminescence intensity than Au and Cu because the real part of the refractive index is a factor ∼5 higher and the density of states ∼2 eV below the Fermi energy is a factor of ∼4 lower than Au and Cu. Measurements are presented for unpolished Nb, but given the lack of signal detection for polished Nb and that theory predicts very weak signals, we conclude that the luminescence signals from pure Nb still remain below the sensitivity of our instrument.

Optical transitions and multiphonon Raman scattering of Cu doped ZnO and MgZnO ceramics

Cu doped ZnO and MgZnO ceramics were created via a process of cold pressing and annealing, and their optical properties and phonon dynamics were studied. It was found that the ceramics exhibit infrared absorption peak energies at 5783 and 5822 cm−1, indicative of intraband transitions in a substitutional Cu ion of oxidation state +2. The UV photoluminescence (PL) intensity of the ceramics was found to weaken significantly relative to an undoped sample. The low PL intensity is discussed in terms of the CuxZn1−xO alloy system and the indirect bandgap of the CuO end member, as well as in terms of the nonradiative Cu centers. Due to the weak PL, up to ten LO multiphonons were observed in the Raman spectra, pointing to a strong polaron coupling. The resonance behavior of the highest intensity mode was found to exhibit outgoing resonance characteristics.

Terbium photoluminescence in yttrium aluminum garnet xerogels

Based on a colloidal solution containing terbium, yttrium, and aluminum metal ions, a powder was synthesized and films of terbium-doped yttrium aluminum garnet Tb0.15Y2.85Al5O12 were grown on single-crystal silicon and porous anodic alumina. Annealing of the sample in a temperature range from 200–1100°C results in an increase in the photoluminescence intensity in the wavelength range from 480–640 nm, which is caused by Tb3+ ion intra-atomic transitions 5D4→7Fj (j = 3, 4, 5, 6). Annealing at 900°C and higher temperatures gives rise to low-intensity photoluminescence bands in the region of 667 and 681 nm, which correspond to transitions 5D4→7F0, 5D4→7F1, and room-temperature Stark term splitting, which suggests the existence of a crystalline environment of Tb3+ ions. The FWHM of spectral lines in the region of 543 nm decreases from ∼10 to ∼(2–3) nm as the xerogel annealing temperature is increased from 700 to 900°C and higher. Three bands with maxima at 280, 330, and 376 nm, which correspond to Tb3+ ion transitions 7F6→5I8, 5L6, 5G6, 5D3, are observed in the photoluminescence excitation spectra of the studied structures for the emission wavelength at 543 nm. X-ray diffraction detected the formation of a crystalline phase for a terbium-doped yttrium aluminum garnet powder after annealing at 1100°C.