Photoluminescence Emission Band Research Articles

Exploring the Emissive States of Heteroatom-Doped Graphene Quantum Dots

The photoluminescence (PL) emission states of heteroatom-doped graphene quantum dots (GQDs) remain unknown, particularly the assignment of the low-energy excitation band (more than 330 nm). To address these issues, this work synthesized three different types of GQDs: undoped GQDs (UGQDs), nitrogen-doped GQDs (NGQDs), and boron-doped GQDs (BGQDs), with similar sizes, chemical compositions (types and compositions of surface functional groups), and defects using a constant potential electrolysis method. The PL emissive states in these GQDs and the effects of the dopant heteroatom on the PL were revealed based on the combination of spectroscopic methods and theoretical calculations. The results indicated that the GQDs exhibit multiemissive centers for the PL emission mechanism. An excitation-independent PL emission band (band I) results from a high-energy transition originating from the quantum confinement of the carbon core (carbon π–π* transitions in sp2 domain), and an excitation-dependent PL emission band...

Down-conversion photoluminescence of ZrO2:Er3+ coatings formed by plasma electrolytic oxidation

Plasma electrolytic oxidation of zirconium in alkaline solution containing Er2O3 powder was used for preparation of ZrO2:Er3+ coatings. Photoluminescence (PL) emission spectra of ZrO2:Er3+ excited by ultraviolet irradiation are composed of broad PL band associated with ZrO2 host and sharp bands corresponding to f–f transitions of Er3+. The strongest green PL emission band of Er3+ in the range from 540 nm to 580 nm is assigned to 4S3/2 → 4I15/2 transition. The PL excitation spectra of ZrO2:Er3+ characterize broad band from 250 nm to 350 nm associated with charge transfer state of Er3+ and the series of peaks in the range from 350 nm to 530 nm which are associated with 4f transitions of the Er3+ from ground state 4I15/2 to higher levels. Obtained results allowed the identification of down-conversion PL mechanism.

Structural variation and near infrared luminescence in Mn5+-doped M2SiO4 (M = Ba, Sr, ca) phosphors by cation substitution

Alkaline earth orthosilicates M2SiO4 (M = Ba, Sr and Ca) phosphors doped with Mn5+ ions have been prepared by the conventional solid-state reaction method. The crystal structures were verified by X-ray diffraction. By varying the host composition from Ba to Ca, the position shifting of photoluminescence emission bands corresponding to the 1E → 3A2 transition are induced by the increasing angular distortion and covalence. The nonradiative energy transfer becomes a dominant role in the photoluminescence process with substituting Ba2+ sites for smaller cations. The structural evolution and relevant luminescence mechanisms in (Ba1−xCax)2SiO4 phosphors also have been investigated in detail. It reveals that the chemical composition modification and structural variation can be an efficient approach to optimize the photoluminescence behaviors.

Thermoluminescence of the persistent-luminescence phosphor, BaAl2O4; A stuffed tridymite

BaAl2O4 is a stuffed tridymite used as a long-lasting phosphor. The thermoluminescence of BaAl2O4 prepared by solution-combustion is reported. Analysis of the sample using X-ray diffraction shows that it formed as a single phase compound with a hexagonal structure following annealing at 1200 °C. A broad photoluminescence emission band between 300 and 650 nm was detected due to excitation at 248 nm. The phosphor showed a natural TL peak at 102 °C for measurement at 1 oCs−1 and, when beta irradiated to 100 Gy, two broad peaks at 123 and 318 °C also for heating at 1 oCs−1. The analysis of the main glow peak at 123 °C suggests that it is a combination of several collocated peaks, that is, peaks embedded within each other. We resolved four such components labelled peaks 1 to 4. The thermoluminescence decreases with heating rate in a way consistent with thermal quenching whose activation energy was determined as ∼0.65eV using peak 3. Interestingly, this value of the activation energy for thermal quenching for BaAl2O4, a stuffed derivative of silica, is similar to literature values for quartz (a silica), suggesting that the recombination centre in the two cases may be similar.

Photoluminescent Green Emission Band Induced by the Systematic Change of -CH3, -OCH3 and Naphthyl Groups in Chiral Imines

Herein, we report the morphological, optical, and structural modifications induced by the change of different functional groups in the para-position of the benzene ring in a series of chiral imines. These organic compounds were examined using scanning electron microscopy (SEM), optical absorption, X-ray diffraction, and photoluminescence (PL) techniques. SEM images showed drastic morphological changes, and the absorbance results showed significant changes in the bands located in the ∼200–400 nm range, associated with π → π*, δ → δ*, and n → π* transitions. An optical behavior similar to that of semiconductors (in the UV region), with two transitions in the ∼3.3–4.3 eV range was observed for the compounds. The results obtained by PL spectra exhibited changes in intensity, with gradually shifting increases in the green band emission. However, it is more intense with the crystals of the imine bearing the -OCH3 functional group, considering that the aforementioned green band is associated with the different m...

Excitation wavelength dependent photoluminescence emission behavior, UV induced photoluminescence enhancement and optical gap tuning of Zn0.45Cd0.55S nanoparticles for optoelectronic applications

In the present study, we investigate the excitation wavelength (λex) dependent photoluminescence (PL) behavior in Zn0.45Cd0.55S nanoparticles. The deconvoluted PL emission bands for nanopowders and nanocolloids reveal noticeable spectral blue shift with decreasing λex accompanied by intensity enhancement. This unusual behavior is explained in terms of selective particle size distribution in nanostructures, advancing of fast ionization process at short λex; and solvation process in polar solvent. In addition, we attributed the UV-induced PL intensity enhancement and blue shift of the optical gap to the reduction in particle size by photo-corrosion process associated with the improvement in the quantum size effect; surface modification due to cross-linkage improvement of capping molecules at NPs surface; the creation of new radiative centers and the formation of photo-passivation layers from ZnSO4 and CdSO4, and photo-enhanced oxygen adsorption on Zn0.45Cd0.55S nanoparticles surface.

Effect of topological structure on photoluminescence of PbSe quantum dot‐doped borosilicate glasses

AbstractBorosilicate glasses doped with PbSe quantum dots (QDs) were prepared by a conventional melt‐quenching process followed by heat treatment, which exhibit good thermal, chemical, and mechanical stabilities, and are amenable to fiber‐drawing. A broad near infrared (NIR) photoluminescence (PL) emission (1070‐1330 nm) band with large full‐width at half‐maximum (FWHM) values (189‐266 nm) and notable Stokes shift (100‐210 nm) was observed, which depended on the B2O3 concentration. The PL lifetime was about 1.42‐2.44 μs, and it showed a clear decrease with increasing the QDs size. The planar [BO3] triangle units forming the two‐dimensional (2D) glass network structure clearly increased with increasing B2O3 concentration, which could accelerate the movement of Pb2+ and Se2− ions and facilitate the growth of PbSe QDs. The tunable broadband NIR PL emission of the PbSe QD‐doped borosilicate glass may find potential application in ultra‐wideband fiber amplifiers.

Self-cleaning and spectral attributes of erbium doped sodium-zinc-tellurite glass: Role of titania nanoparticles

Highly transparent and durable self-cleaning materials became demanding for diverse applications. To attain such goals, glass system with composition (69−x)TeO2-20ZnO-10Na2O-1Er2O3-(x)TiO2, where x=0.0, 0.1, 0.2, 0.3 and 0.4mol% were synthesized using the conventional melt quenching method and characterized. For the first time, the influence of embedded TiO2 (Titania) nanoparticles (TNPs) concentration variation on the self-cleaning and spectral properties was examined to establish their correlation. TEM micrograph revealed the nucleation of spherical TNPs (average size ≈14nm) inside the amorphous matrix. Reduction in the optical band gap energy (from 3.08–3.03eV) and water contact angle (from 68o to 43o) both were evidenced with the increase in TNPs contents, wherein the later one (reduced contact angle or enhanced wettability) was attributed to the enhanced hydrophilicity of the glass samples. Conversely, a slight increase in the methylene blue (MB) degradation rate with the increase of TNPs contents up to 0.2mol% indeed indicated an improved photocatalytic activity of the synthesized glass. The absorption spectra exhibited ten significant bands of Er3+ ions in the wavelength range of 407 to 1532nm. The emergent three prominent photoluminescence (PL) emission bands of the glass sample (with 0.2mol% of TNPs) positioned at 535nm, 555nm and 670nm were enhanced by a factor of 6.77, 4.56 and 2.00, respectively. Surface plasmon resonance (SPR) band of the embedded TNPs inside the glass was detected around 581nm. Furthermore, Raman band of the glass (containing 0.4mol% of TNPs) centred at 845cm−1 displayed an intensity enhancement by a factor of 3.58 times, which was ascribed to the TNPs localized SPR field mediated effect. It is established that the measured enhanced hydrophilicity (self-cleanliness) and improved spectral features of the present glass composition was steered by the TNPs surface plasmon assisted effects.

Blue photoluminescence of sponge-like highly porous alumina synthesized in hydrofluoric acid based electrolytes

For the first time nanostructures of anodized aluminum oxide (AAO) were synthesized using hydrofluoric acid based electrolytes under potentiostatic mode with varied oxidation conditions. As-grown oxide layers were amorphous and had sponge-like disordered structure with ramified pores system of 50 to 300 nm diameters. All samples under daylight demonstrated blue emission with power up to 10 μW that was seen by naked eye. It was shown that integral intensity of photoluminescence (PL) emission band in 350 – 650 range increases up to 7 – 60 times depending on synthesis details. Observed PL spectra were approximated by superposition of two components with Emax = 2.74 and 2.44 eV, FWHM = 0.63 and 0.53 eV. It was substantiated that studied emission have intrinsic origin and can be attributed to F22+ - and F2-centers.

Photoluminescence and reflectivity studies of high energy light ions irradiated polymethyl methacrylate films

The self-standing films of non-conducting polymethyl methacrylate (PMMA) were irradiated in vacuum using high energy light ions (HELIs) of 50 MeV Lithium (Li+3) and 80 MeV Carbon (C+5) at various ion dose to induce the optical changes in the films. Upon HELI irradiation, films exhibit a significant enhancement in optical reflectivity at the highest dose. Interestingly, the photoluminescence (PL) emission band with green light at (514.5 nm) shows a noticeable increase in the intensity with increasing ion dose for both ions. However, the rate of increase in PL intensity is different for both HELI and can be correlated with the linear energy transfer by these ions in the films. Origin of PL is attributed to the formation of carbon cluster and hydrogenated amorphous carbon in the polymer films. HAC clusters act as PL active centres with optical reflectivity. Most of the harmful radiation like UV are absorbed by the material and is becoming opaque after irradiation and this PL active material are useful in fabrication of optoelectronic devices, UV-filter, back-lit components in liquid crystal display systems, micro-components for integrate optical circuits, diffractive elements, advanced materials and are also applicable to the post irradiation laser treatment by means of ion irradiation.

Induced magnetoelectric coupling and photoluminescence response in solution-processed CoFe2O4/Pb0.6Sr0.4TiO3 multiferroic composite film

ABSTRACTCoFe2O4/(Pb0.6Sr0.4)TiO3 bi-layered composite film exhibits ferromagnetic and ferroelectric responses (Ms ∼ 162 emu/cm3, Hc ∼ 1.8 kOe, 2Pr ∼ 8.6 µC/cm2 and 2Ec ∼ 634 kV/cm) at room temperature. The temperature dependent dielectric anomaly near magnetic phase transition temperature suggests magnetoelectric coupling effect in the composite film. The maximum MD effect can reach as high as ∼49% at 3 kOe applied DC magnetic field. The composite film exhibits magnetoelectric voltage co-efficient αME ∼ 67 mVcm−1Oe−1. The photoluminescence emission bands of the composite film have a significant blue shift as compared to those reported for CFO and PST nanostructures.

Strong visible and near infrared photoluminescence from ZnO nanorods/nanowires grown on single layer graphene studied using sub-band gap excitation

Fabrication and optoelectronic applications of graphene based hybrid 2D-1D semiconductor nanostructures have gained tremendous research interest in recent times. Herein, we present a systematic study on the origin and evolution of strong broad band visible and near infrared (NIR) photoluminescence (PL) from vertical ZnO nanorods (NRs) and nanowires (NWs) grown on single layer graphene using both above band gap and sub-band gap optical excitations. High resolution field emission scanning electron microscopy and X-ray diffraction studies are carried out to reveal the morphology and crystalline quality of as-grown and annealed ZnO NRs/NWs on graphene. Room temperature PL studies reveal that besides the UV and visible PL bands, a new near-infrared (NIR) PL emission band appears in the range between 815 nm and 886 nm (1.40–1.52 eV). X-ray photoelectron spectroscopy studies revealed excess oxygen content and unreacted metallic Zn in the as-grown ZnO nanostructures, owing to the low temperature growth by a physical vapor deposition method. Post-growth annealing at 700 °C in the Ar gas ambient results in the enhanced intensity of both visible and NIR PL bands. On the other hand, subsequent high vacuum annealing at 700 °C results in a drastic reduction in the visible PL band and complete suppression of the NIR PL band. PL decay dynamics of green emission in Ar annealed samples show tri-exponential decay on the nanosecond timescale including a very slow decay component (time constant ∼604.5 ns). Based on these results, the NIR PL band comprising two peaks centered at ∼820 nm and ∼860 nm is tentatively assigned to neutral and negatively charged oxygen interstitial (Oi) defects in ZnO, detected experimentally for the first time. The evidence for oxygen induced trap states on the ZnO NW surface is further substantiated by the slow photocurrent response of graphene-ZnO NRs/NWs. These results are important for tunable light emission, photodetection, and other cutting edge applications of graphene-ZnO based 2D-1D hybrid nanostructures.

Identification of trapping and recombination levels, structure, morphology, photoluminescence and optical absorption behavior of alloyed ZnxCd1−xS quantum dots

Cubic ZnxCd1−xS nanoparticles (NPs) synthesized by chemical precipitation method with average crystallite size of 2.9 ± 0.2 nm were characterized using X-ray diffraction (XRD), X-ray fluorescence (XRF), high-resolution transmission electron microscope (HRTEM), UV–vis absorption, Fourier transform infrared red (FTIR), photoluminescence (PL) emission and Raman spectroscopy. XRD analysis demonstrated systematic shift of diffraction peaks to higher diffraction angles accompanied by a decrease in the lattice parameters with increasing Zn content (x); this confirmed the formation and homogeneity of ZnxCd1−xS nanoalloys. In addition, the dependence of lattice parameters and Raman shift on x showed linear behavior and good agreement with Vegard's law. TEM images of ZnxCd1−xS NPs revealed nearly spherical shape NPs, relatively narrow particle size distribution and standard deviation in the range 1.8–3.4%.; as well as HRTEM images showed well resolved diffraction crystalline planes of zinc-blende cubic type structure. Analysis of optical absorption spectra showed blue shift of both the direct optical band gap from 3.25 to 4.05 eV and excitonic absorption shoulder from 2.56 to 3.88 eV with increasing x, confirming the homogeneity of ZnxCd1−xS alloyed semiconductor NPs. At excitation wavelength 325 nm, the deconvoluted structural defects related PL emission bands are broadened and revealed stronger PL intensity than that at 370 nm. Furthermore, increasing x resulted in PL enhancement accompanied by blue shift of green emission band centered at 515 nm–445 nm. To explain composition dependent PL emission process; trapping and recombination localized levels in ZnxCd1−xS alloyed NPs were identified quantitatively and an energy band diagram was suggested.

Strong Electron–Phonon Coupling and Self-Trapped Excitons in the Defect Halide Perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb)

The optical and electronic properties of Bridgman grown single crystals of the wide-bandgap semiconducting defect halide perovskites A3M2I9 (A = Cs, Rb; M = Bi, Sb) have been investigated. Intense Raman scattering was observed at room temperature for each compound, indicating high polarizability and strong electron–phonon coupling. Both low-temperature and room-temperature photoluminescence (PL) were measured for each compound. Cs3Sb2I9 and Rb3Sb2I9 have broad PL emission bands between 1.75 and 2.05 eV with peaks at 1.96 and 1.92 eV, respectively. The Cs3Bi2I9 PL spectra showed broad emission consisting of several overlapping bands in the 1.65–2.2 eV range. Evidence of strong electron–phonon coupling comparable to that of the alkali halides was observed in phonon broadening of the PL emission. Effective phonon energies obtained from temperature-dependent PL measurements were in agreement with the Raman peak energies. A model is proposed whereby electron–phonon interactions in Cs3Sb2I9, Rb3Sb2I9, and Cs3Bi...

Optical and gas-sensing properties, and electronic structure of the mixed-phase CaCu3Ti4O12/CaTiO3 composites

Combined experimental and theoretical investigations were conducted on the electronic structure, as well as the optical and gas-sensing properties of mixed-phase CaCu3Ti4O12/CaTiO3 (CCTO/CTO) composites, which were synthesized by a solid-state reaction using various milling durations. Our results revealed that these CCTO/CTO composites have a broad photoluminescence (PL) emission band located at 450nm, which is strongly influenced by the milling process duration. Scanning electron microscopy images confirmed that the use of longer milling durations favored the formation of pores and increased the active surface area of the CCTO/CTO thick films. The thick films were prepared using the screen-printing technique, and hence, could be structured for gas-sensing applications. Our theoretical findings may further elucidate the electronic structure associated with the interfacial band alignment (type I-straddling gap) of the CCTO/CTO composite systems.

Strain Engineered Band Structure and Optical Properties of Confined GaAs Quantum Dots

Understanding the physics that correlate strain and physical properties of quantum dots (QDs) is crucial for technology applications. In this paper, GaAs QDs confined in Al2O3 matrix are synthesized using the pulsed laser deposition method and rapid thermal annealing technique. It is revealed that the confined GaAs QDs experience compressive strain during the growth process. The strain can be used to improve and tailor the optical properties of confined GaAs QDs by engineering the bandgap and thus the photoluminescence emission band to a distinct wavelength. These findings presented here can engineer the properties of GaAs QDs for potential application in optoelectronic and photonic devices.

Luminescence and Raman Detection of Molecular Cl2 and ClClO Molecules in Amorphous SiO2 Matrix

Chlorine is a common undesirable impurity in synthetic SiO2 glass for ultraviolet optics and optical fibers. It is usually incorporated into glass as bound Si–Cl groups or interstitial Cl2 molecules. We report a high-sensitivity detection of Cl2 in amorphous SiO2 (a-SiO2) by photoluminescence (PL) and also by Raman spectroscopy. The Cl2 PL emission band at 1.22 eV (1016 nm) appears at T < 160 K and shows a characteristic vibronic progression with separations ≈(520–540) cm–1 and an average lifetime of ≈5 ms at 13 K. Its excitation spectrum coincides with the shape of the 3.78 eV (328 nm) optical absorption band of Cl2 in a-SiO2, corresponding to the X → A 1Πu transition to repulsive excited state. Direct X → a singlet-to-triplet excitation was also observed at 2.33 eV (532 nm). Cl2 PL may serve as a sensitive and selective tool for monitoring Cl impurities and their reactions in a-SiO2. A Raman band of Cl2 is found at 546 cm–1. Cl2 photodissociation at energies up to 4.66 eV (266 nm) was not detected, poin...

Effect of Er3+ ions on the phase formation and properties of In2O3nanostructures crystallized upon microwave heating

Regular sized nanostructures of indium oxide (In2O3) were homogeneously grown using a facile route, i.e. a microwave-hydrothermal method combined with rapid thermal treatment in a microwave oven. The presence of Er3+ doping plays an important role in controlling the formation of cubic (bcc) and rhombohedral (rh) In2O3 phases. The samples presented broad photoluminescent emission bands in the green-orange region, which were attributed to the recombination of electrons at oxygen vacancies. The photocatalytic activities of pure bcc-In2O3 and a bcc-rh-In2O3 mixture towards the UVA degradation of methylene blue (MB) were also evaluated. The results showed that Er+3 doped In2O3 exhibited the highest photocatalytic activity with a photonic efficiency three times higher than the pure oxide. The improved performance was attributed to the higher surface area, the greater concentration of electron traps due the presence of the dopant and the possible formation of heterojunctions between the cubic and rhombohedral phases.

Influence of Pr doping on the thermal, structural and optical properties of novel SLS-ZnO glasses for red phosphor

A novel environmental friendly strategy towards red phosphors in optoelectronic applications employing Pr6O11 doped SLS-ZnO with chemical composition x(Pr6O11)·100-x(SLS·ZnO) where x=0, 1, 2, 3, 4 and 5wt% via melt-quenching technique was successfully synthesized. The X-ray Diffraction (XRD) patterns of all these glasses show broad and diffused humps, which confirm the amorphous structure of samples. The Differential Thermal Calorimetry (DSC) indicated that the value of glass transition is higher from 625°C to 637°C with increasingly of Pr6O11 content. Fourier Transform Infrared Spectra (FTIR) spectra display a decreasing trend towards a smaller wavenumber with the increase of Pr content is due to the formation of non-bridging oxygen (NBO) in SLS-ZnO host matrix. The absorption spectra had revealed the most intense absorption band at ∼444nm, which was assigned as excitation wavelength to determine the photoluminescence (PL) emission intensity of the glass. The indirect band gap values varies from ∼2.44eV to ∼3.02eV as a function of Pr6O11 concentration. The PL emission bands at ∼530 (blue), ∼556 (green), ∼613 (red) and ∼650 (red)nm increases from 0wt% to 4wt% and slightly decreases as Pr6O11 increases with a maximum at 5wt%. Therefore, the SLS-ZnO doped with Pr6O11 as a good potential as red phosphors in an optoelectronic application in accordance with the highest red emission intensity at ∼613nm and ∼650nm.

Color Change Effect in an Organic–Inorganic Hybrid Material Based on a Porphyrin Diacid

Porphyrinic materials show a range of interesting and useful optical and electrical properties. The less well-known subclass of porphyrin diacids has been used in this work to construct an ionic hybrid organic–inorganic material in combination with a halogenidometalate anion. The resulting compound, [H6TPyP][BiCl6]2 (1) (TPyP = tetra(4-pyridyl)porphyrin), has been obtained via a facile solution-based synthesis in single crystalline form. The material exhibits a broad photoluminescence emission band between 650 and 850 nm at room temperature. Single crystals of [H6TPyP][BiCl6]2 show a photocurrent in the fA and a much higher dark current in the nA range. They also display an unexpected reversible color change upon wetting with different liquids. This phenomenon has been investigated with optical spectroscopy, SEM, XPS, and NEXAFS techniques, showing that a surface-based structural coloration effect is the source of the color change. This stands in contrast to other materials where structural coloration typ...