Photoluminescence Spectra Of Thin Films Research Articles

Distinguishing between Triplet-Pair State and Excimer Emission in Singlet Fission Chromophores Using Mixed Thin Films

In the photoluminescence spectra of thin films made of singlet fission (SF) materials emission features that are red-shifted from the free exciton emission are of particular interest. They can be fingerprints of the correlated triplet-pair state and as such offer insights into the mechanisms of the multistep SF process. However, excimer formation or trap-state population can also cause such features and a clear disentanglement of the various contributions can be challenging. Here, we use blends of anthradithiophene (ADT) and weakly interacting organic semiconductors to control the polarizability of the molecular environment and, thus, to distinguish between excimer emission and emission from the correlated triplet-pair state. Using time-resolved photoluminescence spectroscopy measurements, we clarify the relation between excimer formation and SF in ADT and find that excimer formation constitutes a parallel relaxation channel for the exciton and neither mediates nor hinders SF.

Structural, optical and photoluminescence properties of Ga2O3 thin films deposited by vacuum thermal evaporation

In this study, Gallium oxide (Ga2O3) thin films were deposited by vacuum thermal evaporation technique. The effects of annealing temperature on the structural, optical and photoluminescence (PL) properties of Ga2O3 thin films were investigated in detail. The Ga2O3 thin films annealed at lower temperatures were amorphous, while those annealed at above 800 °C were nanocrystalline. The annealing temperature increasing from 400 to 1100 °C results in the band-gap increase from 4.70 to 5.13 eV due to the reduction in density of defect states. Under the excitation at 250 nm, the PL spectra of thin films consisted of a UV emission (300 nm) and two blue emissions (400 nm and 438 nm). The UV and blue luminescence intensities increased as the annealing temperature was increased from 400 to 1100 °C. The blue luminescence is suggested to originate from the recombination of trapped electrons in the donor with trapped holes in the acceptor, where the origin of the donor should be an double ionized oxygen vacancy (VO++) and the acceptor should be a gallium vacancy (VGa) or gallium-oxygen vacancy pair (VGa-VO++).

Improvement of structural, optical and electrical properties of iron doped indium oxide thin films by high gamma radiations for photocatalysis applications

Iron doped indium oxide (In2O3:Fe) thin films have been deposited on glass substrates by spray pyrolysis technique and irradiated by four high gamma doses 1, 5, 10 and 100 kGy. We have investigated the influence of applied gamma radiations on structural, optical, photoluminescence and electrical properties of In2O3:Fe thin films using X-ray diffraction, Raman spectroscopy, spectrophotometer, Photoluminescence spectrometer and Hall Effect measurements. Structural analysis revealed that preferred orientation (400) plan depicted at 2θ = 35.5° was kept after exposure to gamma radiations with a noticeable improvement of crystallinity for all doses. Average transmission values in the transparent domain [600–2500] nm were increased from 79% to a maximum value of 88% for irradiated thin layer by 10 kGy. Band gap energy was also increased from 3.32 eV to 3.5 eV after irradiation by 10 kGy. Different optical parameters such as refractive index n(λ), extinction coefficient K(λ), lattice dielectric constant (εL), high frequency dielectric constant (ε∞), plasma frequency (ωP) and the ratio of carrier concentration to electron effective mass (Nm*) were determined and analyzed. Single-oscillator Wemple-Didomenico model was applied to calculate the dispersion parameters E0 and Ed after gamma irradiation. Photoluminescence spectra of thin films show an overall decrease in their peak intensities after γ-irradiation. Hall Effect measurements of In2O3:Fe thin films revealed that electrical resistivity have been decreased from 5.07 * 10−2 Ω cm to a minimum value of 1.75 * 10−2 Ω cm at 10 kGy. All these good experimental results can lead to a potential use of gamma irradiated iron doped indium oxide thin film for optoelectronic devices. Photocatalytic activity of γ-irradiated In2O3:Fe thin films to decompose methylene blue were studied under sun light. Optimum irradiated thin film shows an enhancement of photocatalytic activity under sun light with a percentage of dye decolorization equals to 87%.

Crystallographic and Optical Characteristics of Thin Films of Cu2ZnSn(S x Se1–x )4 Solid Solutions

Results of x-ray diffraction and optical studies of thin films of Cu2ZnSn(SxSe1–x)4 solid solutions were presented. Unit-cell constants a and c of Cu2ZnSn(SxSe1–x)4 were determined as functions of composition. It was shown that a and c decreased linearly with increasing S concentration. Optical transmission and photoluminescence spectra of thin films of Cu2ZnSn(SxSe1–x) solid solutions were measured. Absorption coefficients and band-gap energies were determined as functions of the solid-solution composition.

Photoluminescence characteristics of strontium titanate thin films prepared by spray pyrolysis

Strontium titanate, SrTiO3 thin films were successfully prepared by spray pyrolysis. The strontium titanate, STO precursor solid thin film were fabricated from an aqueous solution of Sr(NO3)2/[(CH3)2CHO]4Ti/HNO3 = 19:1:20 (molar ratio) at room temperature on a substrate. The as-deposited STO thin film with those annealed at different temperature were prepared. Photoluminescence spectra of thin films were obtained using Photoluminescence Spectrophotometer and optical properties were carried out by using Shimadzu Uv-Vis Spectrophotometer. In conclusion, the band gap energy and the bandwidth of thin films have been analysed. ________________________________________GRAPHICAL ABSTRACT

Influence of thickness on optical and structural properties of BiFeO3 thin films: PLD grown

We report the influence of thickness on optical and structural properties of BFO thin films grown by PLD method in this paper. In the present work, highly oriented single phase BFO thin films of ∼200nm and 600nm thicknesses were deposited on ITO coated glass substrates. The XRD results reveal the growth of BFO thin films along (010) plane with rhombohedral perovskite structure. The compositional investigation of thin films was carried out by energy dispersive X-ray spectroscopy. The optical properties of thin films have been recorded using UV-vis and photoluminescence spectroscopy. The values of optical band gap of BFO thin films were found to be 2.60eV and 2.36eV for 200nm and 600nm thickness respectively. Photoluminescence spectra of thin films reveal that the near band edge emission centered over the 484nm.

Relationship between structure and optoelectrical properties of organic–inorganic hybrid materials containing fullerene derivatives

Novel lead iodide-based layered perovskite compounds, which contain fullerene derivatives, N-methyl-2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (AmPF) and N-( n-dodecyl)-2-(4-ammoniumphenyl)-fulleropyrrolidine iodide (C12AmPF), in their organic layers were fabricated as thin solid films by spin-coating. The XRD profiles showed that (AmPF)PbI 4 molecules were arranged in a closer-packing form, compared with (C12AmPF)PbI 4. The photoluminescence spectra of thin films suggested the presence of energy transfer between C 60 moiety and lead(II) iodide layers, which led to the disappearance of fluorescence peak at 517 nm and the appearance of a new fluorescence peak at 780 nm. (AmPF)PbI 4 and (C12AmPF)PbI 4 films exhibited photoconductivity when ultraviolet light was irradiated, and the photocurrent values with applying 1.0 V bias voltage were 1.77 μA and 1.48 × 10 −2 μA, respectively.

Optical properties of Cu(In, Ga)Te2 thin films

Structural and optical properties of polycrystalline Cu(In, Ga)Te2 thin films were analysed. They were deposited onto glass substrate by the flash evaporation technique and annealed at 450 °C in an argon atmosphere. Their optical properties in the near-infrared and visible range, 500–3000 nm, were studied to determine the gap. To identify the energy levels associated with various defect structures, photoluminescence (PL) measurements were carried out. The behaviour of the PL spectrum as a function of temperature and excitation intensity in these samples is reported. The PL spectrum of thin films after annealing in an argon atmosphere at 450 °C showed two peaks ranging from 0.90 to 1.15 eV. The first one, localized at 1.00 eV, was attributed to donor–acceptor pair transition while the second one at 1.05 eV was ascribed to a free-to-bound (F–B) transition.

Photoluminescence spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532-nm laser radiation and gamma-rays

We have investigated temporal behavior of the photoluminescence (PL) spectra of thin films containing CdSe/ZnS quantum dots irradiated by 532 nm laser radiation and gamma-rays. Under ∼100 W/cm 2 laser radiation, the PL intensity ( I PL) increases with irradiation time upto about 500 s and thereafter declines linearly. The wavelength of the PL emission ( λ peak) exhibits a blue-shift with exposure time. Upon simultaneous irradiation by 100 W/cm 2 532-nm laser, as well as 0.57 and 1.06 MeV gamma-rays, the temporal behaviors of both I PL and λ peak are significantly different; I PL increases to a saturation level, and the magnitude of the blue-shift in λ peak is reduced. We discuss possible mechanisms underlying these results.

Tunable blue photoluminescence from methoxy substituted diphenyl quinoline

Blue-emitting phenylated quinoline conjugated derivatives in liquid and solid phases have generated considerable interest owing to their high photoluminescence (PL) quantum yields. A blue light-emitting material, methoxy (OCH 3) substituted diphenyl quinoline (2,4′-methoxy phenyl, 4-phenyl quinoline) has been synthesized and characterized by FT-IR, UV–vis absorption and photoluminescence measurements. Two absorption peaks at 270 and 330 nm due to conjugated side and π–π* transition of the conjugated main chains, respectively, are reported. The synthesized polymeric compound demonstrates a bright emission in blue region at 434 nm in the solid. It also shows hypsochromic shift in THF and bathochromic shift in formic acid. The PL spectra of thin films of the OMe-DPQ embedded in PMMA matrix were investigated. The OMe-DPQ exhibits a concentration tunable photoluminescence. A narrow full width at half-maximum (FWHM) value of PL spectra can be obtained, may be useful for developing OLED or solar cell devices, or in chemical sensors.

The Effect of Ar ∕ O2 Sputtering Gas on the Phosphorus-Doped p -Type ZnO Thin Films

The influence of gas on phosphorus-doped ZnO thin films deposited by radio frequency magnetron sputtering was studied. As the partial pressure of increased in the sputtering gas, the structure of changed from nanorods to a smooth thin film. The electron concentration of the thin film was also decreased due to the reduction in oxygen vacancies that act as donors. Photoluminescence spectra of thin films also showed a reduction in the intensity of the deep level emission peaks, due to the reduction in native defects related to the oxygen vacancies as the partial pressure was increased. The thin films grown under an gas ratio of showed -type characteristics after a rapid thermal annealing (RTA) activation process. The -type showed a resistivity of , a mobility of , with a hole concentration of .

Photoluminescence spectra of a conjugated polymer: from films and solutions to single molecules

The purpose of this work is to address the issue of applicability of single-molecule spectroscopy (SMS) results for conjugated polymers to "bulk" samples, e.g. conjugated polymer films. Also, some apparent inconsistencies in the literature on SMS regarding the photoluminescence spectral position of conjugated polymers are discussed. We present a series of photoluminescence spectra of thin films of the conjugated polymer poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene) (MEH-PPV) with a wide range of varying thickness. The thickness was varied from approximately 20 nm to the value corresponding to well-separated single molecules (SMS sample). The thickness variation resulted in a strong ( approximately 2000 cm(-1)) blue-shift and broadening of the spectrum. The result was reproduced on isolated molecules embedded into a PMMA matrix. This effect cannot be explained by a decrease in energy transfer "freedom" alone. We performed a comprehensive comparison of presented and elsewhere published spectra of MEH-PPV polymer and oligomers in different samples: films, solutions, isolated-molecule coatings and standard SMS samples. The comparison allows that the main reason behind the blue shift is conformational disorder, which is largely dependent on the sample. We also discuss some experimental aspects of SMS, such as representativeness of detected molecules, spectral sensitivity of a setup and temperature. Together with differences in sample preparation method, these issues can explain the existing inconsistencies in the literature.

Cationic Bis-cyclometalated Iridium(III) Diimine Complexes and Their Use in Efficient Blue, Green, and Red Electroluminescent Devices

A series of cationic Ir(III) complexes with the general formula (C/N)2Ir(N/N)(+)PF6- featuring bis-cyclometalated 1-phenylpyrazolyl-N,C2' (C/N) and neutral diimine (N/N, e.g., 2,2'-bipyridyl) ligands were synthesized and their electrochemical, photophysical, and electroluminescent properties studied. Density functional theory calculations indicate that the highest occupied molecular orbital of the compounds is comprised of a mixture of Ir d and phenylpyrazolyl-based orbitals, while the lowest unoccupied molecular orbital has predominantly diimine character. The oxidation and reduction potentials of the complexes can be independently varied by systematic modification of either the C/N or N/N ligands with donor or acceptor substituents. The electrochemical redox gaps (E(ox)-E(red)) were adjusted to span a range between 2.39 and 3.08 V. All of the compounds have intense absorption bands in the UV region assigned to 1(pi-pi*) transitions and weaker charge-transfer (CT) transitions that extend to the visible region. The complexes display intense luminescence both in fluid solution and as neat solids at 298 K that is assigned to emission from a triplet metal-ligand-to-ligand CT (3MLLCT) excited state. The energy of the 3MLLCT state varies in nearly direct proportion to the size of the electrochemical redox gap, which leads to emission colors that vary from red to blue. Three of the (C/N)2Ir(N/N)(+)PF6- complexes were used as active materials in single-layer light-emitting electrochemical cells (LECs). Single-layer electroluminescent devices were fabricated by spin-coating the Ir complexes onto an ITO-PEDOT/PSS substrate followed by deposition of aluminum contacts onto the organic film. Devices were prepared that give blue, green, and red electroluminescence spectra (lambda(max) = 492, 542, and 635 nm, respectively), which are nearly identical with the photoluminescence spectra of thin films of the same materials. The single-layer LECs give peak external quantum efficiencies of 4.7, 6.9, and 7.4% for the blue, green, and red emissive devices, respectively.

Luminescence of molecules adsorbed on surfaces of wide gap materials

We report on the photoluminescence spectra of thin films of chain-like π-conjugated molecules on well-defined surfaces of wide gap inorganic materials. The aim is to study the energy transfer processes across the organic/substrate interface which limit the luminescence of molecules in close contact to substrate surfaces. We discuss quaterthiophene (4 T) adsorbed on the c(2×2)-ZnSe(0 0 1) surface and tetracene (Tc) on the surface of a thin epitaxial Al 2O 3 film on Ni 3Al(1 1 1). For thin films vapor-deposited at low substrate temperatures, we observe no luminescence signal for both systems, which indicates the presence of fast luminescence quenching processes at the organic/inorganic interface. After annealing, luminescence spectra corresponding to those of the bulk crystals are obtained. This can be explained by the formation of 3D-crystallites, which effectively separate most molecules from the organic/inorganic interface.

The Optical Properties of ZnO Nanoparticles Capped with Polyvinyl Butyral

ZnO nanoparticles capped with polyvinyl butyral (PVB) have been synthesized by the sol-gel process. Photoluminescence (PL) spectra show a remarkable decrease in visible emission intensity after ZnO nanoparticles are capped with PVB, which indicates that dangling bonds and defect states at the surface of ZnO nanoparticles are markedly passivated. As a result, the process of surface-trapped hole tunneling back into the particles to form V**O recombination center is blocked. The PL spectra of thin films show a strong ultraviolet (UV) emission with very weak visible emission. The typical intensity ratio of the UV emission at 3.45 eV to the visible emission at about 2.41 eV is 43.3, which shows an obvious improvement in luminescence properties by the surface passivation with PVB. Low-temperature PL spectra of ZnO powder at 93.8 K are dominated by free exciton, bound exciton and the LO-phonon replica of the bound exciton.

Study of photoluminescence spectra of Si-rich SiN x films

The silicon-rich silicon nitride thin films were prepared by low pressure chemical vapor deposition (LPCVD) technique. Greenish-blue visible photoluminescence (PL) were observed at room temperature, with six peaks at 2.97, 2.77, 2.55, 2.32, 2.10 and 1.90 eV, respectively. After the rapid thermal annealing (RTA) process, the position of PL spectrum of thin film has no obvious shift, but relative intensity of some peaks has changed. We found that the abundance of Si and some oxygen in the films, which introduce great amount of dangling bond defects, are the dominating factors leading to the strong room-temperature visible PL emission. A simple model of gap state was built to explain the PL mechanism of silicon nitride thin films.

Photoluminescence of C60 Thin Films at Low Temperatures

Photoluminescence spectra of thin films (free and on (001) surface of NaCl wafer) of fullerite C60 are studied at 5 K. Film structures were analyzed by standard transmission high energy electron diffraction (THEED) technique. Influence of film structure and substrate on luminescence spectra was observed. It is assumed that the decay of Frenkel excitons on grains boundaries in nanodispersive films competes with energy transport to X-traps and the intensity of excited X-traps photoluminescence strongly decrease. It is supposed that this effect is revealed when a singlet exciton diffusion length is comparable with grains sizes.

Charge Compensation of Er3+ Ions in Ca1—xErxF2+x Thin Films at Low Rare-Earth Substitution Rates

The Er 3+ ions in Ca 1-x Er x F 2+x (x ≤ 0.01) thin layers grown by sublimation on Si substrates were selectively excited using a laser-induced up-conversion technique under focused excitation at liquid nitrogen temperature. By comparison with the results previously published on bulk crystals, two (Er 3+ , F i-) centers of tetragonal and trigonal symmetry and one (Er 3+ , O 2- ) center of trigonal symmetry were identified in these films. These centers exhibit noticeable green luminescence when the laser wavenumber is tuned on the excited-state transition 4 I 13/2 → 4 F 5/2 of each one of them. The emitting 4 S 3/2 level is populated through a two-step absorption process. A comparison of photoluminescence spectra of thin films and bulk crystals testifies to a lower density of aggregated centers in the films.