Intrinsic Emission Band Research Articles

The specificity of photoluminescence n-CdS/p-CdTe in semiconductor heterostructures

The low-temperature (4.2 K) near-band-edge photoluminescence spectrum of a thin fine-grained (h, dcr≤1μm) polycrystalline CdTe layer in an n-CdS/p-CdTe film heterostructure subjected to frontal excitation by an Ar+ laser with an intensity of ~44 W/cm2 consists of a dominant intrinsic (e-h) emission band with a half-width ΔA=10-12 meV and a blue shift ΔEr » 25 meV of the red edge with respect to Eg, its LO+nLA phonon replica (ΔB»40 meV) with a weak doublet structure, and a wide (ΔD»100 meV) surface-interface luminescence band peaking at a frequency ħω»1.49 eV. Rear-side illumination of the photoresistive CdS layer in the intrinsic absorption range with an intensity Lill ≈ 5.102 lx almost completely destroys the e-hband and all related luminescence lines, which are replaced with an asymmetric polariton emission doublet having an exciton resonance frequency ħω»1.59 eV(Δex» 25 meV) and a wide line of shallow donor–acceptor pairs (ΔDAP»40 meV) at a frequency of ħω»1.54 eV, whose maximum intensity is almost two orders of magnitude lower than that of the A line in the absence of illumination.

Novel red-emitting phosphor La2Ti6O15:Eu3+ for fabricating warm white LED-devices

Eu3+-doped La2Ti6O15 nanoparticles were prepared by the sol-gel process. The phase-formations, crystal structure, photoluminescence properties, and lighting application were reported. La2Ti6O15 has band energy of 3.6 eV, which can be narrowed with the increase of Eu3+-doping in the lattices. A broad intrinsic emission band (400–750 nm) peaked at 515 nm was observed at 300 K Eu3+-doped La2Ti6O15 phosphors present bright red-luminescence from 5D0→7F2 transitions (615 nm). The luminescence has an excellent thermal stability performance with an activation energy of ΔE = 0.33 eV. Importantly, it fits well with the commercial LED-chip because of its efficient excitation in blue (465 nm) and near-UV region (395 nm). Red-LED device packaged with the optimal phosphor was achieved with luminous efficiency of 43.75 lm/W. It was suggested that La2Ti6O15:Eu3+ could be a potential candidate for fabricating high-power warm white-LEDs.

Luminescence enhancement of La4Ti9O24:Eu3+ red-emitting phosphors via cation-substitution of M (Ga, Nb, Ta, Zr, Al) ions

Eu3+- and M (Ga3+, Nb5+, Ta5+, Zr3+, Al3+)-co-doped La4Ti9O24 phosphors were prepared by the solid-state reaction method. The structural phase formation, luminescence properties, and possible LED-lighting application were investigated. The undoped La4Ti9O24 shows an indirect transition with a band-energy of 2.88 ​eV La4Ti9O24 has a broad intrinsic emission band (350–700 ​nm) peaked at 530 ​nm (2.34 ​eV) at 300K. Eu3+-doped La4Ti9O24 presents red luminescence from 5D0→7FJ (J ​= ​0, 1, 2, 3, 4) transitions, which was obviously enhanced with the cation-substitution of M (Ga3+, Nb5+, Ta5+, Zr3+, Al3+) on Ti4+ sites in the lattices. The typical thermal quenching of La3.72Eu0.28Ti8.55M0.45O24 (M ​= ​Al) phosphor is ΔE ​= ​0.342 ​eV. Importantly, the optical excitation of the samples fit well with the commercial LED-chip because of its efficient absorption in near-UV region (395 ​nm) and blue (465 ​nm) wavelength region. The red-LED device packaging with the optimal phosphor was achieved, which has a luminous efficiency of 42 lm/W with a working current of 200 ​mA and a testing voltage of 3.2 ​V. The experiments suggest that the phosphor could be potential for solid-state LED-lighting and display.

Crystal growth, optical, luminescence and scintillation characterization of Li2Zn2(MoO4)3 crystal

A Li2Zn2(MoO4)3 (LZMO) single crystal was grown by the flux method under the condition of a low temperature-gradient. The obtained crystal has a single-phase confirmed by a powder X-ray diffraction study. Luminescence properties of the crystal have been studied from room to cryogenic (300–6 K) temperatures under excitation by a 280 nm light-emitting diode (LED). At 6 K, the grown crystal exhibits an intrinsic emission band with emission wavelength ranges from 400 to 900 nm, and the maximum peak at ~612 nm. From 6 to 280 K, the luminescence decay time of the crystal was studied using the 280 nm LED. The decay times for the entire temperature range were fitted with two exponential decay functions. The temperature-dependent (300–10 K) scintillation light yield was studied under the excitation of a 90Sr (beta) radioactive source. We compared scintillation and luminescence light yields of the LZMO with a reference CaMoO4 crystal. Thermoluminescence study of the crystal was carried out from 9 to 300 K and different kinematic parameters such as activation energy and frequency factor have been calculated. From this study, it shows that the LZMO crystal has potential as detector material in experiments searching for neutrinoless double-beta decay of 100Mo at cryogenic temperatures.

Scintillation Properties of Tetrafluoroaluminate Crystal

In this article, single crystals of a new nonhygroscopic scintillating material of tetrafluoroaluminate (TlAlF4) were grown using the Bridgman–Stockbarger technique. The diffraction pattern obtained from powder X-ray diffraction was found to be in good agreement with the reference data. The X-ray-induced luminescence measurements of the grown crystal sample exhibit a very strong intrinsic emission band in the wavelength range from 300 to 450 nm, with a peak maximum at 390 nm. Scintillation properties, such as pulse height spectra and decay time of the grown crystals, were investigated using $\gamma $ -rays from the 137Cs radiation source. The scintillation light yield was found to be ~11 800 ph/MeV. At room temperature, the measured decay time profile was fitted with three exponential functions; the result showed a fast component of 194 ns (13.1%), a medium component of 992 ns (84.6%), and a slow component of 3.5 $\mu \text{s}$ (2.3%). Trap centers were studied using the results obtained from thermally stimulated luminescence. The temperature dependence of the scintillation light yield and decay time was also studied. The results showed that TlAlF4 is a promising scintillator crystal, and thus, a potential candidate for X-ray and $\gamma $ -ray detection.

Optical and mechanical properties of Li2Mg2(MoO4)3 crystal grown by Czochralski method

A Li2Mg2(MoO4)3 crystal was grown by self-flux Czochralski method in a platinum crucible. The polycrystalline compound was synthesised using standard solid-state reaction method. The purity of the synthesised compound was analysed through powder X-ray diffraction method. Single crystal growth was carried out using a Czochralski chamber in argon atmosphere and the pulling rate was maintained 0.5 mm/h with a rotation rate of 8−10 rpm. The temperature dependent luminescence and decay time measurements were carried out in a temperature range of 10–300 K. The grown crystal sample exhibits an intrinsic emission band in a wavelength range from 320 to 800 nm with the maximum peak at 590 nm. At low temperatures, a significant increase in emission intensity is observed as well as the red shift in the peak position. The luminescence decay time was fitted with three decay components. Thermally stimulated luminescence glow curve was recorded to study the trap centers and found a trap center at 25 K with 3 meV trap depth. Mechanical behaviour of the grown crystal was analysed by using Vicker’s micro hardness testing. The results suggest that Li2Mg2(MoO4)3 crystal can be used for luminescence applications.

Refractive index mediated plasmon hybridization in an array of aluminium nanoparticles.

The arrangement of plasmonic nanoparticles in a non-symmetrical environment can feature far-field and/or near-field interactions depending on the distance between the objects. In this work, we study the hybridization of three intrinsic plasmonic modes (dipolar, quadrupolar and hexapolar modes) sustained by one elliptical aluminium nanocylinder, as well as behavior of the hybridized modes when the nanoparticles are organized in arrays or when the refractive index of the surrounding medium is changed. The position and the intensity of these hybridized modes were shown to be affected by the near-field and far-field interactions between the nanoparticles. In this work, two hybridized modes were tuned in the UV spectral range to spectrally coincide with the intrinsic interband excitation and emission bands of ZnO nanocrystals. The refractive index of the ZnO nanocrystal layer influences the positions of the plasmonic modes and increases the role of the superstrate medium, which in turn results in the appearance of two separate modes in the small spectral region. Hence, the enhancement of ZnO nanocrystal photoluminescence benefits from the simultaneous excitation and emission enhancements.

Synthesis and luminescence characterization of Pr3+, Gd3+ co‐doped SrF2 transparent ceramics

AbstractPr3+, Gd3+ co‐doped SrF2 transparent ceramic, as the potential material for visible luminescent applications, was prepared by hot‐pressing of precursor nanopowders. The microstructure, phase compositions, and in‐line transmittance, as well as the photoluminescence properties were investigated systematically. Highly optical quality Pr,Gd:SrF2 transparent ceramic with nearly pore‐free microstructure was obtained at 800°C for 1.5 hours. The average in‐line transmittance of the x at.% Pr, 6 at.% Gd:SrF2 (x = 0.2, 0.5, 1.0, 2.0) transparent ceramics reached to 87.3 % in the infrared region. The photoluminescence spectra presented intense visible light emissions under the excitation of 444 nm, the main intrinsic emission bands located at 483 and 605 nm, which were attributed to the transitions of Pr3+: 3P0 → 3H4 and 1D2 → 3H4, respectively. With the co‐doping of Gd3+ ions, the emission intensity of the Pr:SrF2 transparent ceramic was greatly enhanced. All the emission bands of x at.% Pr, 6 at.% Gd:SrF2 transparent ceramics exhibited the highest luminescence intensity with the 1.0 at.% Pr3+ doping concentrations, whereas the lifetimes decreased dramatically with the Pr3+ doping contents increasing from 0.2 to 2.0 at.% due to its intense concentration quenching effect. The 1 at.% Pr, 6 at.% Gd:SrF2 transparent ceramic is a promising material for visible luminescent device applications.

Luminescence characteristics of magnesium aluminate spinel crystals of different stoichiometry

Magnesium aluminate spinel single crystals with different stoichiometry, MgAl2O4 (1:1 spinel) and MgO 2.5Al2O3 (1:2.5) were investigated using different optical methods (cathode-, photo- and thermally stimulated luminescence (TSL), optical absorption, “creation spectra” of TSL peaks and phosphorescence by VUV radiation). Low-temperature charge carrier traps and the position of intrinsic UV emission bands depend on the degree of stoichiometry. Antisite defects (ADs), Mg2+ or Al3+ located in a “wrong” cation site (Mg|A1 or Al|Mg) are the main as-grown structural defects, which serve also as efficient traps for electrons and holes as well as seeds for bound excitons. AD concentration is especially high in 1:2.5 spinel. There are several manifestations of ADs (electronic excitations near ADs) in the spectral region of 7–7.5 eV, slightly below the energy gap.

The nature of luminescence of KI and KI-Na crystals at low temperature deformation after natural decrease in the symmetry of the lattice

The X-ray luminescence spectra at low temperature (90K) uniaxial deformation were researched for KI and KI-Na crystals, that were stored for more than 10 years. The following regularities were observed: firstly, the intensities of the intrinsic emission bands at 3.3 eV (π) and 4.1 eV ( σ ) become equal in com- parison with freshly grown crystals; and secondly, with an increase in the degree of low-temperature uniaxial deformation, a gradual shift of the emission spectra occurs in two directions: the luminescence maximum at 3.3 eV shifts towards short wavelengths, the final position is fixed at 3.9 eV, which prac- tically merges with σ-luminescence; the radiation maximum at 3.0 eV (Ex-radiation) is shifted toward long wavelengths, the final position which is fixed at 2.8 eV. Thus, low temperature uniaxial deformation leads to the separation of the emission spectra at 3,3 eV(π) → 3,9 eV and 3.0 eV ( Ex ) → 2,8 eV, which are interpreted by the separation of weak → on and weak → strong exciton configurations , respectively.

Ratiometric fluorescence detection of mercuric ions by sole intrinsic dual-emitting gold nanoclusters

Ratiometric fluorescence sensing based on intrinsic dual-emitting nanomaterials is very intriguing for their simplicity and efficacy. However, intrinsic dual-emitting gold nanocluster-based fluorescence ratiometry remains unexplored. In this work, a ratiometric fluorescence assay for detection of mercuric ions (Hg2+) has been developed using sole intrinsic dual-emitting gold nanoclusters (DE-AuNCs). The DE-AuNCs, simply synthesized by reducing chloroauric acid with an appropriate amount of glutathione, had two distinct intrinsic emission bands peaked at 610 nm and 810 nm respectively. When exposed to Hg2+, the 610 nm emission of the DE-AuNCs was remarkably quenched serving as a reporter signal, while the 810 nm emission almost kept unchanged serving as a reference signal, and thus giving a ratiometric signal. This ratiometric method showed high sensitivity and selectivity for sensing Hg2+ with low LOD down to 0.19 ppb which is quite lower than the WHO drinking-water guideline for inorganic mercury, and was successfully applied to the detection of Hg2+ in environmental water samples.

Pressure-induced emission band separation of the hybridized local and charge transfer excited state in a TPE-based crystal.

We herein report a newly synthesized simple molecule, named TPE[double bond, length as m-dash]C4, with twisted D-A structure. TPE[double bond, length as m-dash]C4 showed two intrinsic emission bands ascribed to the locally excited (LE) state and the intramolecular charge transfer (ICT) state, respectively. In the crystal state, the LE emission band is usually observed. However, by applying hydrostatic pressure to the powder sample and the single crystal sample of TPE[double bond, length as m-dash]C4, dual-fluorescence (445 nm and 532 nm) was emerged under high pressure, owing to the pressure-induced emission band separation of the hybridized local and charge transfer excited state (HLCT). It is found that the emission of TPE[double bond, length as m-dash]C4 is generally determined by the ratio of the LE state to the ICT state. The ICT emission band is much more sensitive to the external pressure than the LE emission band. The HLCT state leads to a sample with different responsiveness to grinding and hydrostatic pressure. This study is of significance in the molecular design of such D-A type molecules and in the control of photoluminescence features by molecular structure. Such results are expected to pave a new way to further understand the relationship between the D-A molecular structure and stimuli-responsive properties.

Influence of substrate on structural, morphological and optical properties of TiO2 thin films deposited by reaction magnetron sputtering

Titanium dioxide (TiO2) films have been prepared by DC reaction magnetron sputtering technique on different substrates (glass, SiO2, platinum electrode-Pt, Silicon-Si). X-ray diffraction (XRD) patterns showed that all TiO2 films were grown along the preferred orientation of (110) plane. Samples on Si and Pt substrates are almost monophasic rutile, however, samples on glass and SiO2 substrates accompanied by a weak anatase structure. Atomic force microscopy (AFM) images revealed uniform grain distribution except for films on Pt substrates. Photoluminescence (PL) spectra showed obvious intrinsic emission band, but films on glass was accompanied by a distinct defect luminescence region. Raman spectroscopy suggested that all samples moved to high wavenumbers and films on glass moved obviously.

On the low-temperature photoluminescence and photovoltaic properties of fine-grained CdTe films

In the low-temperature (T = 4.2 K) photoluminescence spectrum of a fine-grained (the grain size is smaller than 1 μm) obliquely sputtered CdTe thin film, the dominant intrinsic emission band caused by potential barriers at grain boundaries and the edge doublet band representing the LO-phonon replica of the dominant band are observed. Doping of the film with an In impurity quenches the doublet band and subsequent heat treatment activates the intrinsic emission band. The full-width at half maximum and the short-wavelength shift of the red edge of the latter correlate with a maximum anomalously high photovoltage of 2 × 102–103 V generated by the film.

Optical temperature sensing based on the luminescence from YAG:Pr transparent ceramics

The YAG:Pr transparent ceramic was fabricated using a conventional solid-state reactive method to explore its possible application in optical thermometry. Photoluminescence and temperature-dependent luminescence were elaborately investigated under 452 nm excitation. The ceramic showed two intrinsic emission bands at 488 and 594 nm, which were attributed to characteristic Pr3+: 3P0 → 3H4 and 3P1 → 3H6 transitions, respectively. Down-conversion emissions from the two thermally coupled excited states of Pr3+ were recorded in the temperature range of 293–593 K. The Boltzmann distribution theory was adopted to interpret the temperature-dependent luminescence of Pr3+. The temperature sensitivity exhibited an increasing trend with the increase of temperature, typically, 0.0025 K−1 at 593 K. The results indicated that the present ceramic was a promising candidate for optical temperature sensor.

Luminescent, optical and electronic properties of La3Ta0.5Ga5.5O14 single crystals grown in different atmospheres

Luminescent, optical and electronic properties of La3Ta0.5Ga5.5O14 single crystals grown in different atmospheres are presented. The absorption bands at 255, 290, 350 and 480nm were detected; the intensity of bands increases with the concentration of oxygen in the growth atmosphere. It is shown that the shift of the fundamental absorption edge with the temperature obeys Urbach rule. The corresponding fitting allowed to estimate the slope coefficient σ=0.35, which implies self-trapping of excitons in La3Ta0.5Ga5.5O14. Calculations of the band structure, partial densities of states and reflectivity spectra were performed. The bandgap of La3Ta0.5Ga5.5O14 was determined as Eg=5.6eV. The luminescence properties under UV, VUV and X-ray excitation were studied. Intrinsic emission band at 440–450nm is attributed to the excitons self-trapped at TaO6 molecular complexes. Extrinsic emission bands at 410, 440 and 550nm are attributed to the emission of excitons trapped by antisite defects, F-centers and oxygen deficient oxyanionic complexes.

Intrinsic and defect related luminescence in double oxide films of Al–Hf–O system under soft X-ray and VUV excitation

Low temperature time-resolved luminescence spectra in the region of 2.5–9.5eV under soft X-ray excitation as well as time-resolved luminescence excitation spectra in the UV–VUV region (3.7–12eV) of solid solutions AlxHfyO1−x−y thin films were investigated. The values of x and Al/Hf ratio were determined from X-ray photoelectron srectroscopy data. Hafnia films and films mixed with alumina were grown in a flow-type chemical vapor deposition reactor with argon as a carrier gas. In addition, pure alumina films were prepared by the atomic layer deposition method. A strong emission band with the peak position at 4.4eV and with the decay time in the μs-range was revealed for pure hafnia films. The emission peak at 7.74eV with short nanosecond decay kinetics was observed in the luminescence spectra for pure alumina films. These emission bands were ascribed to the radiative decay of self-trapped excitons (an intrinsic luminescence) in pure HfO2 and Al2O3 films, respectively. Along with intrinsic host emission, defect related luminescence bands with a larger Stokes shift were observed. In the emission spectra of the solid solution films (x=4; 17; 20at%) the intrinsic emission bands are quenched and only the luminescence of defects (an anion vacancies) was observed. Based on transformation of the luminescence spectra and ns-luminescence decay kinetics, as well as changes in the time-resolved luminescence and luminescence excitation spectra, the relaxation processes in the films of solid solution are discussed.

Photoluminescence of monoclinic Li3AlF6 crystals under vacuum ultraviolet and soft X-ray excitations

Using Bridgman technique we have grown monoclinic β-LiAF crystals suitable for optical studies, performed XRD-identification and Rietveld refinement of the crystal structure and carried out a photoluminescence study upon vacuum ultraviolet (VUV) and extreme ultraviolet (XUV)-excitations, using the low-temperature (T=7.2K) time-resolved VUV-spectroscopy technique. The intrinsic PL emission band at 340–350nm has been identified as due to radiative recombination of self-trapped excitons. The electronic structure parameters were determined: bandgap Eg≈12.5eV, energy threshold for creation of unrelaxed excitons 11.8eV<En<12.5eV. The PL emission bands at 320–325 and 450nm were attributed to luminescence caused by lattice defects. We have discovered an efficient excitation of PL emission bands in the energy range of interband transitions (Eex>13.5eV), as well as in the energy range of core transitions at 130eV. We have revealed UV–VUV PL emission bands at 170 and 208nm due to defects. A reasonable assumptions about the origin of the UV–VUV bands were discussed.

Optical properties of nanocrystalline magnesium aluminate spinel synthesized from industrial wastes

In this work, magnesium aluminate spinel (MA) nanoparticles were synthesized via co-precipitation method using industrial wastes of magnesium and aluminum. The obtained powders were calcined at a temperature range of 650–1500 °C. The calcined powders were characterized by XRD, FT-IR, DTA, FE-SEM and HR-TEM. XRD data illustrated that the spinel MA with a crystallite size of ∼3.8 nm was formed at 650 °C. Optical properties of the MA spinel revealed that the optical reflectance is highly dependent on calcination temperature. Photoluminescence analysis revealed intrinsic emission bands with high intensity centered at 390, 370 and 485 nm, when the samples are excited by a 350 nm light from a Xe lamp at room temperature. Results suggest that co-precipitation method of aluminum and magnesium wastes at low temperatures is a promising way for the production of highly pure and active nanocrystalline MA spinel with expected unique properties.

Characterization of Functional Layers of CdTe Crystals Subjected to Different Surface Treatments

The surface functional layers of commercial detector-grade CdTe crystals are characterized after different surface treatments such as chemical polishing etching in bromine-containing solutions and laser annealing with nanosecond pulses of the second harmonic of a YAG:Nd laser. The effects of etching and laser irradiation with energy density near the melting threshold of CdTe on the optical properties of the CdTe crystal surface are studied using ellipsometry and photoluminescence (PL). The samples after storage in air are also investigated. The intrinsic emission band at 1.58 eV and broad dopant band peaked at 1.45 eV are observed in the PL spectra excited by a semiconductor laser with wavelength of 405 nm at 80 K. The low energy band is associated with carrier recombination at structural defects in the surface layer. The redistribution of the PL band intensities after chemical polishing is found and it is attributed to transformation of the point defect structure and defect formation in the surface region of CdTe. The laser treatment essentially changes the surface state and structure of the surface layer that can be used for effective modification of characteristics of the functional layers of CdTe crystals particularly band bending before formation of electrical contacts.