Photoluminescence Phenomena Research Articles

Abnormal photoluminescence phenomena in (Tb3+, Eu3+) codoped Ga2O3 phosphor

The structural and optical properties of (Tb3+, Eu3+)-codoped β-Ga2O3 crystals were examined using X-ray diffraction measurement, photoluminescence (PL) analysis, PL excitation (PLE) spectroscopy, and PL decay measurement. The (Tb3+, Eu3+)-codoped samples were synthesized by metal-organic decomposition (MOD). The PL and PLE measurements showed that the (Tb3+, Eu3+)-codoped samples exhibit clearly different luminescence properties when excited at wavelengths below or above λex ∼ 350 nm, which corresponds to the boundary in wavelength between the 4f6 → charge transfer state and 4f6 → 4f6 transitions to occur in Eu3+. At λex > 350 nm, efficient energy transfer occurred from Tb3+ to Eu3+, resulting in the decreased Tb3+ and enhanced Eu3+ emissions. At λex < 350 nm, not only the Tb3+ but also the Eu3+ emissions decreased with increasing Tb3+/Eu3+ concentration. Furthermore, the PL intensity degradation in the seconds time scale was observed by exciting light at λex < 350 nm. Such unusual PL phenomena at λex < 350 nm seem to be due to an interaction between Tb3+ and Eu3+ with producing nonradiative relaxation channels in the Tb3+/Eu3+ emission pathway. The temperature dependence of the PL intensity was also measured at T = 20–450 K in increments of 10 K and analyzed using newly developed models.

Synthesis and characterization of Li2TiSiO5 obtained by melting and solid-state reaction

In this work, the synthesis of the Li2TiSiO5 photoluminescent compound, obtained from a stoichiometric mixture of Li2CO3, SiO2 and TiO2 nanoparticles, was investigated by two different routes named route I (melting and quenching) and route II (solid-state reaction). In the route I, the raw materials were melted (1550 °C/2 h) and cooled into water. Subsequently, the solidified product was milled (30 min) in a high-energy mill. In the route II, the raw materials were directly milled (10 min). The obtained powders, from both routes, were heat treated at different temperatures (900–1000 °C/60 min) and then subjected to crystallographic (XRD) and thermal (DSC) analyses in order to investigate the resulting structure and the associated physical–chemical reactions. The results showed that it is possible to obtain the Li2TiSiO5 compound from both synthesis routes but, apparently, the route I seems to be more appropriated to obtain the Li2TiSiO5 compound and to demonstrate the photoluminescence phenomena with and even without thermal treatment.

Core/shell quantum dots in ferroelectric liquid crystals matrix: effect of spontaneous polarisation coupling with dopant

ABSTRACTIn order to fabricate efficient and superior performance liquid crystal (LC) devices, the physical parameters of the LC mesogens can be duly altered by incorporating non-mesogenic materials like quantum dots (QDs), graphene and polymers. In the present work, the effect of adding core/shell QDs in two ferroelectric liquid crystals (FLCs), along with the change in their physical properties, has been investigated. A small concentration of QDs is dispersed into the two FLCs and temperature variations of vital parameters like spontaneous polarisation (Ps), rotational viscosity, response time, relative permittivity and relaxation strength have been measured for both the FLC materials. The contrast ratio, UV–near visible absorbance as well as photoluminescence (PL) of both the mesogens have also been determined and compared. A faster electro-optical response and the induced phenomenon of PL with a temperature-dependent low-frequency relaxation mode have been observed in Felix 17/100 after the addition of QDs. The present study also provides valuable information about the interaction between QDs and the two FLC systems depending upon polarisation–field (P–E) coupling. The same dopant can interact with FLCs in dissimilar fashion if the intrinsic properties of both the FLCs are different thereby producing different modifications in their respective physical parameters.

Polyhydrogenated Graphene: Excited State Dynamics in Photo- and Electroactive Two-Dimensional Domains

Understanding the phenomenon of intense photoluminescence in carbon materials such as hydrogenated graphene, graphene nanoribbons, and so forth is at the forefront of investigations. In this study, six different types of hydrogenated graphene (phG) produced from different starting materials were fully characterized in terms of structure and optical spectroscopy. Comprehensive photoluminescence lifetime analyses of phGs were conducted by combining time-correlated single-photon counting with steady-state fluorescence spectroscopy and femtosecond transient absorption spectroscopy. The conclusion drawn from these assays is that graphene islands with diameters in the range from 1.1 to 1.75 nm reveal band gap photoluminescence between 450 and 800 nm. As a complement, phGs were implemented in hybrids with water-soluble electron accepting perylenediimides (PDIs). By virtue of mutual π-stacking and charge transfer interactions with graphene islands, PDIs assisted in stabilizing aqueous dispersion of phG. Implicit in these ground state interactions is the formation of 300 ps lived charge separated states once photoexcited.

Spontaneous crystalization and aggregation of DCNP pyrazoline-based organic dye as a way to tailor random lasers

The 3-(1,1-dicyanoethenyl)-1-phenyl-4,5-dihydro-1H-pyrazole (DCNP) compound is showing versatile optical features including nonlinear optical effects, photoluminescence and stimulated emission resulting from its molecular design as well as polar alignment in crystal lattice. We present detailed studies carried out for the DCNP compound on a photoluminescence phenomenon, showing that its emission properties are strongly different for molecular and aggregated forms. Experiments have proved that stimulated emission can occur only from the crystaline form of DCNP and the precipitation of it from a solution leads to the constitution of gain and different, random feedback mechanisms depending on aggregate size. The immobilization of DCNP crystals and the controlled crystalization in a viscous biopolymeric matrix have been utilized to generate random micro-cavities, that support weak light localization and coherent random laser emission.

Photoluminescence and nonlinear optical phenomena in plasmonic random media—A review of recent works

Photoluminescence properties and nonlinear optical response of metal–dielectric nanocomposites (MDNCs)—germanate glasses, bio-cellulose membranes and colloids containing either silver (Ag) or gold (Au) nanoparticles (NPs)—are reviewed. The phenomena discussed are: i. the photoluminescence enhancement observed from rare-earth doped PbO–GeO2 glass containing Ag NPs; ii. optical amplification at 1530nm in RIB waveguides made with PbO–GeO2 thin films covered with Au NPs; iii. Random Laser emission from a bio-cellulose membrane infiltrated with Rhodamine 6G and containing Ag NPs; iv. the nonlinearity management of high-order processes in colloids containing Ag NPs suspended in acetone. In all discussed cases the influence of the metallic NPs is clearly demonstrated and a procedure to control the nonlinear propagation of light beams in heterogeneous media is presented.

Optical and thermal properties of azo derivatives of salicylic acid thin films

N-acryloyl-4-aminosalicylic acid (4-AMSA), monomer (HL) and 5-(4′-alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HLn) are synthesized and characterized with various physico-chemical techniques. Thin films of 5-(4′-alkyl phenylazo)-N-acryloyl-4-aminosalicylic acid (HLn) are prepared by spin coating technique. The X-ray diffraction (XRD) patterns of 4-aminosalicylic acid (4-ASA) and its derivatives are investigated in powder and thin film forms. Thermal properties of the compounds are investigated by thermogravemetric analysis (TGA). The optical energy gap and the type of optical transition are investigated in the wavelength range (200–2500nm) for 4-ASA, HL and HLn. The values of fundamental energy gap (Eg) are in the range 3.60–3.69eV for all compounds and the type of optical transition is found to be indirect allowed. The onset energy gap Eg∗ appeared only for azodye compounds is found to be in the range 0.95–1.55eV depending on the substituent function groups. The refractive index, n, shows a normal dispersion in the wavelength range 650–2500nm, while shows anomalous dispersion in the wavelength rang 200–650nm. The dispersion parameters ε∞, εL, Ed, Eo and N/m∗ are calculated. The photoluminescence phenomena (PL) appear for thin films of 4-ASA and its derivatives show three main emission transitions.

Synthesis of silicon quantum dots in zinc silicate matrix by low-temperature process: Optical, structural and electrical characterization

Silicon nano-crystallites dispersed in an insulating matrix of zinc silicates have been obtained by a two-step process, consisting of sputtering deposition of several alternated amorphous silicon and zinc oxide thin layers, followed by a chemical reaction carried out under vacuum at comparatively low temperature (from 450°C to 560°C) between stratified reagents. This low cost, low temperature and scalable process has to be considered promising in quantum dot fabrication technology. Several characterization techniques have been employed and large evidence has been found for the presence of silicon and zinc oxide nano-crystallites showing quantum confinement effect and related band gap enlargement. Photoluminescence phenomenon has been observed in the range 550–850nm and a consistent explanation has been given in terms of exciton radiative recombination on trap states at the interface between Si dot and silicate matrix. Electrical measurements have given evidence for an expected material anisotropy where large difference between lateral and perpendicular resistivity values has been found. A technological strategy for transport property improvement has been prospected.

Europium incorporated barium titanate thin films for optical applications

BaTiO3:Eu (BT:Eu) thin films were deposited onto quartz substrates by RF magnetron sputtering. The effect on structural, morphological, optical and photoluminescence (PL) properties in the films with different Eu concentrations (0–5 wt%) were investigated. The X-ray diffraction (XRD) pattern of the undoped BT thin film revealed a tetragonal (T) phase with orientations along (101) plane. From XRD pattern, the crystallinity of the films increased with increase in Eu concentration. The SEM images revealed that the films exhibited tetragonal shape, crack free and good adherence to the substrate. Atomic force microscopy studies showed an increase of grain growth with doping concentration. The rms roughness value increased with increase in Eu concentration and the film surface revealed positive skewness and high value of kurtosis which make them suitable for tribological applications. X-ray photoelectron spectroscopy revealed the presence of barium, titanium, europium and oxygen in BT:Eu film. An average transmittance of >80 % (in visible region) was observed for all the films. Optical band gap of Eu doped BT films decreased from 3.86 to 3.53 eV. Such films with optical properties such as high transparency, decrease in band gap and high refractive index are suitable for optoelectronic applications. PL properties showed a sharp line at 625 nm and a broad line at 552 nm due to europium (Eu3+) transitions. PL phenomena were observed, owing to the electronic structure of Eu3+ ions as well as BT nanocrystallites in the films. The sharp and intense red luminescence is useful for photoelectric devices and optical communications.

Tracing photon transmission in dye-doped DNA-CTMA optical nanofibers

We experimentally demonstrate the novel phenomena of photoluminescence (PL) and fluorescence resonance energy transfer (FRET) assisted three-color PL separating in DNA optical nanofibers consisting of the stretched and connected DNA-cetyltrimethyl ammonium wires. The PL experiments are performed to comparatively trace photon transmission between single dye-doped DNA-CTMA optical nanofiber and PMMA optical nanofiber. A cascade FRET including DNA minor groove binder and DNA intercalators is used to further trace photon transmission inside DNA-CTMA wire. These experimental results will help to intrigue the new applications of DNA-CTMA as molecular waveguide in optobioelectronics area.

Development of accurate two-dimensional dose-imaging detectors using atomic-scale color centers in Ag-activated phosphate glass and LiF thin films

Novel disk-type X-ray two-dimensional (2D) imaging detectors have been developed by utilising atomic-scale defects as minimum luminescent units such as radiation-induced Ag-related species in Ag-activated phosphate glass and F-aggregated centres in lithium fluoride (LiF) thin films. Such luminescent detectors are based on the radiophotoluminescence (RPL) and photoluminescence (PL) phenomena, respectively. Accurate accumulated 2D dose distributions with a high spatial resolution of micron order over large areas and a wide dynamic range covering 8 orders of magnitude were rapidly reconstructed and were successfully demonstrated for the first time by combining the Ag-doped glass with LiF thin films. These detectors should be suitable for X- and gamma-ray imaging in radiation diagnostics and clinical radiotherapy.

Optical and photoluminescence properties of nanostructured ZrO2:Tb thin films

Tb doped ZrO2 thin films were deposited on quartz substrates by sol–gel dip coating technique. X-ray diffraction patterns showed the formation of ZrO2:Tb thin films with a tetragonal structure. The morphological difference has been observed in the nanostructured ZrO2:Tb thin film with a terbium concentration of 5mol% compared to other concentrations. An average transmittance of >80% (in UV–Vis region) was observed for all samples. Optical band gap was found to vary as a function of doping concentration. ZrO2:Tb thin film with a terbium concentration of 5mol% displayed a refractive index supremacy which can be directly employed in extending the range of tunability of the refractive index. Moreover, these films also demonstrated tuning of band gap values due to the incorporation of Tb into the solid networks of ZrO2 and modified microstructure. Photoluminescence (PL) properties showed several sharp lines due to terbium (Tb3+) transitions. PL phenomena were observed, owing to the electronic structure of terbium (Tb3+) ions as well as zirconia nanocrystallites in the films. Noticeable influences of terbium to terbium (Tb↔Tb) cross relaxation, which favors the green lines over the blue lines, are observed. An enhancement of luminescence intensity is found due to change in the crystalline environment and reduction in OH amount with temperature. The intense green PL emission in ZrO2:Tb thin films makes them suitable for generation of solid state light emitting diodes.

Photoluminescence of colloids of pristine MgAl layered double hydroxides

We present an unexpected photoluminescence (PL) phenomenon for colloids of pristine MgAl layered double hydroxides (LDHs). Moreover, the PL spectra displayed little dependency on LDH platelet size, excitation wavelength or colloid concentration. The possible mechanism was discussed mainly in terms of specific surface areas, numerous surface defects and excited electron–hole recombination.

Photoluminescence phenomena prevailing in c-axis oriented intrinsic ZnO thin films prepared by RF magnetron sputtering

The substantial amount of –OH groups attached to the Zn lattice has been correlated to the dominant c-axis orientation of the hexagonal ZnO crystals with wurtzite structure, which demonstrates two preferred first-order Raman peaks and also exhibits a distinct UV luminescence band due to the typical exciton emission or near-band-edge emission.

UV/O3 Generated Graphene Nanomesh: Formation Mechanism, Properties, and FET Studies

The bandgap engineering of graphene is a challenging task for its potential application. Forming unique structures such as nanoribbons or nanomeshes is an effective way to open up a bandgap in graphene. In this work, a graphene nanomesh (GNM) was prepared through UV-mediated oxidation of a graphene oxide (GO) film at atmosphere. Atomic force microscopy (AFM) was used to track the evolution of the surface morphology of GO during the irradiation. It was observed that a nanoporous network structure was progressively produced in the basal plane, which can be attributed to the fact that highly reactive oxygen species preferentially attack sp3 carbon-rich regions of the GO. In particular, the as-prepared GNM shows interesting semiconducting characteristics and photoluminescence (PL) phenomenon, which make it become a promising candidate for the use of electronics, optoelectronics, and biomedical engineering. Finally, the field-effect transistors (FETs) were fabricated using the as-prepared GNM as the active cha...

A comparative study of optical and radiative characteristics of X-ray-induced luminescent defects in Ag-doped glass and LiF thin films and their applications in 2-D imaging

We report novel disk-type X-ray two-dimensional (2-D) imaging detectors utilising Ag-doped phosphate glass and lithium fluoride (LiF) thin films based on the radiophotoluminescence (RPL) and photoluminescence (PL) phenomena, respectively. The accumulated X-ray doses written in the form of atomic-scale Ag-related luminescent centres in Ag-doped glass and F-aggregated centres in LiF thin films were rapidly reconstructed as a dose distribution using a homemade readout system. The 2-D images reconstructed from the RPL and PL detectors are compared with that from the optically stimulated luminescence (OSL) detector. In addition, the optical and dosimetric characteristics of LiF thin films are investigated and evaluated. The possibilities of dose distributions with a high spatial resolution on the order of microns over large areas, a wide dynamic range covering 11 orders of magnitude and a non-destructive readout are successfully demonstrated by combining the Ag-doped glass with LiF thin films.

Photoluminescence of Colloids of Pristine ZnAl Layered Double Hydroxides

Colloids of pristine ZnAl layered double hydroxides (LDHs) were found to display an unexpected photoluminescence (PL) phenomenon disparate from the corresponding LDH solids. First, ZnAl LDHs with nitrate in the interlayer were prepared by a hexamethylenetetramine hydrolysis method. Their colloids were obtained by subsequent delamination in formamide at room temperature. Interestingly, an unexpected PL phenomenon was observed for the colloids, different from that of the relevant solid powder. The Zn/Al ratio and concentration did not change the Stokes shift. However, they were found to be the major factors in the emission intensity. Numerous surface defect sites and interaction of LDH with formamide were believed to be the main reasons.

The phenomenon of induced photoluminescence in ferroelectric mesophase

Most of ferroelectric liquid crystal (FLC) or ferroelectric mesophase show no photoluminescence (PL). But when, Cu doped ZnO (Cu-ZnO) nanoparticles (NPs) are dispersed in FLC, it shows an induced photoluminescence. This induced phenomenon of PL strongly depends on the concentration of dopant NPs. The Cu-ZnO-FLC composite system has shown two distinct emission peaks near 400nm and 650nm that correspond to the violet and red emission respectively. The characteristic features of both the emission peaks suggest their different source of origins. In the present paper, the aspects related to induced PL and its applications have been discussed.

Photoluminescence measurements of LiF TL detectors

Abstract The properties of standard lithium fluoride (LiF) thermoluminescent (TL) detectors, which are routinely used in radiation protection systems, were investigated under light stimulation. The luminescence of different types of LiF detectors, which were irradiated with gamma rays of energy up to 300 Gy and alpha particles with a fluence up to 5*109 cm−2, were stimulated by a blue light and were heated up to temperature of 240 °C or 400 °C, depending on the type of detectors. The irradiated LiF detectors during the blue-light (460 nm) stimulation emit green photoluminescence (PL) with a wavelength of 530 nm. The LiF detectors showed a PL effect of much higher efficiency after they were irradiated with alpha particles than after they were irradiated with gamma rays. However, in contrast to PL, the TL readout showed a significantly lower efficiency of LiF detectors after alpha particle irradiation. These effects result from the different trap mechanisms that are responsible for TL and PL phenomena. The temperature stability of the traps responsible for the PL effect for both types of LiF detectors was studied.

Novel planar and star-shaped molecules: Synthesis, electrochemical and photophysical properties

Three novel planar and star-shaped molecules containing thiophene-functionalized group and acetylenic spacers, 4-((5″-iodo-[2,2′:5′,2″-terthiophen]-5-yl)ethynyl)aniline (I3TEA), 4,4′-([2,2′:5′,2″-terthiophene]-5,5″-diylbis(ethyne-2,1-diyl))dianiline (3TDDA), 1,3,5-tris((5-((trimethylsilyl)ethynyl)thiophen-2-yl)ethynyl)benzene (TETEB) were synthesized through Sonogashira coupling reaction. Their photophysical and electrochemical properties were explored by UV–vis, photoluminescence (PL) emission and DFT calculation. Three compounds possess unusual phenomena of PL, with the concentration of gradually decreased, the intensity of PL firstly increased and then decreased, this may be due to the aggregate-enhanced emission (AEE) effect and aggregation-caused quenching (ACQ) effect. Their spectroscopic data demonstrate that D–π–A–π–D structured organic dye 3TDDA based on NH2 as donor units and 3T as acceptor units with acetylenic spacers π-conjugated chain between them exhibits good luminescent properties. Dye I3TEA with donor(D)–acceptor(A) structure shows good charge transfer because of amino functional group and iodine atom. Octupolar organic π-conjugated star-shaped molecule TETEB possesses excellent luminescent properties because of its unique structure applied in the optoelectronic field. The redox curves and results of DFT calculations suggest that three compounds have low ionization potential and low Eoxd. All the good properties demonstrate three compounds could be promising candidates for photovoltaic materials.