Photoluminescence Spectra of GaS0.75Se0.25 Layered Single Crystals

Photoluminescence (PL) spectra of GaS_0.75Se_0.25 layered single crystals have been studied in the wavelength region of 500‐850 nm and in the temperature range of 10‐200 K. Two PL bands centered at 527 ( 2.353 eV, A‐band) and 658 nm (1.884 eV, B‐band) were observed at T = 10 K. Variations of both bands have been studied as a function of excitation laser intensity in the range from 8 × 10^‐3 to 10.7 W cm^‐2. These bands are attributed to recombination of charge carriers through donor‐acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.043 and 0.064 eV below the bottom of conduction band to acceptor levels located 0.088 and 0.536 eV above the top of the valence band are suggested to be responsible for the observed A‐ and B‐bands in the PL spectra, respectively.     

Temperature and excitation intensity tuned photoluminescence in Tl4GaIn3S8 layered single crystals

Photoluminescence spectra of Tl₄GaIn₃S₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 500–780 nm and in the temperature range of 26–130 K with extrinsic excitation source (λ_exc = 532 nm), and at T = 26 K with intrinsic excitation source (λ_exc = 406 nm). Three emission bands A, B and C centered at 514 nm (2.41 eV), 588 nm (2.11 eV) and 686 nm (1.81 eV), respectively, were observed for extrinsic excitation process. Variations in emission spectra have been studied as a function of excitation laser intensity in the 0.9‐183.0 mW cm^–2 range for extrinsic excitation at T = 26 and 50 K. Radiative transitions from the donor levels located at 0.03 and 0.01 eV below the bottom of the conduction band to the acceptor levels located at 0.81 and 0.19 eV above the top of the valence band were proposed to be responsible for the observed A‐ and C‐bands. The anomalous temperature dependence of the B‐band peak energy was explained by configurational coordinate model. From X‐ray powder diffraction and energy dispersive spectroscopic analysis, the monoclinic unit cell parameters and compositional parameters of Tl₄GaIn₃S₈ crystals were determined, respectively. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Excitation intensity and temperature‐dependent photoluminescence and optical absorption in Tl4Ga3InSe8 layered crystals

Photoluminescence (PL) spectra of Tl₄Ga₃InSe₈ layered crystals grown by Bridgman method have been studied in the wavelength region of 600‐750 nm and in the temperature range of 17‐68 K. A broad PL band centered at 652 nm (1.90 eV) was observed at T = 17 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.13 to 55.73 mW cm^‐2 range. Radiative transitions from donor level located at 0.19 eV below the bottom of conduction band to shallow acceptor level located at 0.03 eV above the top of the valence band were suggested to be responsible for the observed PL band. From X‐ray powder diffraction and optical absorption study, the parameters of monoclinic unit cell and the energy of indirect band gap were determined, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of photoluminescence from ordered GaInP2 epitaxial layers

The temperature behavior of the integrated intensity of photoluminescence (PL) emission from ordered GaInP₂ epitaxial layer was measured at temperatures of 10 ‐ 300 K. Within this temperature range the PL emission is dominated by band‐to‐band radiative recombination. The PL intensity temperature dependence has two regions: at low temperatures it quenches rapidly as the temperature increases, and above 100 K it reduces slowly. This temperature behavior is compared with that of disordered GaInP₂ layer. The specter of the PL emission of the disordered layer has two peaks, which are identified as due to donor‐accepter (D‐A) and band‐to‐band recombination. The PL intensity quenching of these spectral bands is very different: With increasing temperature, the D‐A peak intensity remains almost unchanged at low temperatures and then decreases at a higher rate. The intensity of the band‐to‐band recombination peak decays gradually, having a higher rate at low temperatures than at higher temperatures. Comparing these temperature dependencies of these PL peaks of ordered and disordered alloys and the temperature behavior of their full width at half maximum (FWHM), we conclude that the different morphology of these alloys causes their different temperature behavior. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Flux Bridgman growth and the influence of annealing temperatures on PL properties of ZnO single crystals

Transparent ZnO crystals were obtained by the flux Bridgman method from high temperature solution of 22 mol% ZnO‐78 mol% PbF₂ system. The influence of annealing temperatures on the photoluminescence (PL) of ZnO crystal was investigated. An ultraviolet emission peak at about 379 nm was observed in PL spectra and the peak position has a weak blueshift for annealed samples. A green band centered at 523 nm appeared in the annealed samples and its intensity enhanced with the increase of annealing temperatures, while the intensity of the ultraviolet peak decreased considerably. However, the ultraviolet emission peak became the strongest after annealing at 1000 °C. This phenomenon was considered to be associated with oxygen vacancy and F^‐ impurities induced by the PbF₂ flux. The results show that high temperature annealing in air seems helpful for improving the PL properties of ZnO crystal. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Luminescence of GaN single crystals prepared by heating a Ga melt in Na–N2 atmosphere

Colorless transparent prismatic crystals (0.5‐2.0 mm long) and hopper crystals (1.0‐2.5 mm long) of GaN were prepared by heating a Ga melt at 800°C in Na vapor under N₂ pressures of 7.0 MPa for 300 h. The photoluminescence (PL) spectrum of a prismatic crystal at 4 K showed the emission peaks of neutral donor‐bound exciton (D⁰‐X) and free exciton (X_A) at 3.472 eV and 3.478 eV, respectively, in the near band edge region. The full‐width at half‐maximum (FWHM) of (D⁰‐X) peak was 1.9 meV. The emission peaks of a donor–acceptor pair transition (D⁰‐A⁰) and its phonon replicas were observed in a lower energy range (2.9‐3.3 eV). The emission peaks of the D⁰‐A⁰ and phonon replicas were also observed in the cathodoluminescence (CL) spectrum at 20 K. The (D⁰‐X) PL peak of a hopper crystal at 4 K was at 3.474 eV (2.1 meV higher), having a FWHM of 6.1 meV which was over 3 times larger than that of the prismatic crystal. A strong broad band with a maximum intensity around 1.96 eV was observed for the hopper crystals in the CL spectrum at room temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation,structure and photoluminescence properties of SiO2–coated ZnS nanowires

It is essential to passivate one‐dimensional (1D) nanostructures with insulating materials to avoid crosstalking as well as to protect them from contamination and oxidation. The structure and influence of thermal annealing on the photoluminescence properties of ZnS‐core/SiO₂‐shell nanowires synthesized by the thermal evaporation of ZnS powders followed by the sputter deposition of SiO₂ were investigated. Transmission electron microscopy and X‐ray diffraction analyses revealed that the cores and shells of the core‐shell nanowires were single crystal zinc blende‐type ZnO and amorphous SiO₂, respectively. Photoluminescence (PL) measurement showed that the core‐shell nanowires had a green emission band centered at around 525 nm with a shoulder at around 385 nm. The PL emission of the core‐shell nanowires was enhanced in intensity by annealing in an oxidative atmosphere and further enhanced by subsequently annealing in a reducing atmosphere. Also the origin of the enhancement of the green emission by annealing is discussed based on the energy‐dispersive X‐ray spectroscopy analysis results. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stoichiometric single crystal growth of AgGaS2 by iodine transport method and characterization

High quality single crystals of ternary AgGaS₂ (AGS) semiconductor with chalcopyrite structure have been grown by chemical vapor transport (CVT) technique using iodine as a transporting agent at different growth zone temperatures. The powder X‐ray diffraction and single crystal X‐ray diffraction studies indicate that the as‐grown AGS crystals belong to the tetragonal (chalcopyrite) system with (112) plane as the dominant peak. The full width at half maximum (FWHM) of the X‐ray rocking curve for the as‐grown AGS single crystal is 5 arcsec. The energy dispersive X‐ray analysis (EDAX) and optical transmission spectra of as‐grown AGS single crystals grown at different conditions show the almost same composition and band gap (2.65 eV). Photoluminescence (PL) spectra of as‐grown AGS single crystals show prominent band edge emission at 2.61 eV. The resistivity of the as‐grown AGS single crystal has been measured. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Properties of Sm‐doped CaNb2O6 thin films deposited by radio‐frequency magnetron sputtering

Sm‐doped CaNb₂O₆ (CaNb₂O₆:Sm) phosphor thin films were prepared by radio‐frequency magnetron sputtering on sapphire substrates. The thin films were grown at several growth temperatures and subsequently annealed at 800 °C in air. The crystallinity, surface morphology, optical transmittance, and photoluminescence of the thin films were investigated by X‐ray diffraction, scanning electron microscopy, ultraviolet‐visible spectrophotometry, and fluorescence spectrophotometry, respectively. All of the thin films showed a main red emission radiated by the transition from the ⁴G_5/2 excited state to the ⁶H_9/2 ground state of the Sm³⁺ ions and several weak bands under ultraviolet excitation with a 279 nm wavelength. The optimum growth temperature for depositing the high‐quality CaNb₂O₆:Sm thin films, which was determined from the luminescence intensity, was found to be 400 °C, where the thin film exhibited an orthorhombic structure with a thickness of 370 nm, an average grain size of 220 nm, a band gap energy of 3.99 eV, and an average optical transmittance of 85.9%. These results indicate that the growth temperature plays an important role in controlling the emission intensity and optical band gap energy of CaNb₂O₆:Sm thin films.     

Photoluminescence and radiation damage in W,Mg, Ca doped Bi4Ge3O12 crystals

Spectral properties and radiation damage in W, Mg and Ca doped Bi₄Ge₃O₁₂ (W:BGO, Mg:BGO, Ca:BGO) crystals before and after thermal treatment and gamma‐ray (γ‐) or ultraviolet (UV) radiation were studied. The absorption and the photoluminescence (PL) spectra of doped BGO crystals in visible region were measured. Before γ or UV radiation, the emission intensity of W:BGO crystal is stronger than that of pure BGO at about 500 nm. After γ radiation (10k Gy), the emission intensities of doped BGO are all weaker than pure BGO under identically condition. However, the emission intensity of W:BGO is stronger than that of pure BGO after UV exposure (10k Gy). Thermal treated (annealed in N₂ at 550 °C for 6 h) W:BGO has shown a much stronger emission intensity than others. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis by organic molten salt method and characterization of chalcopyrite AgInS2 nanorods

In this paper, chalcopyrite AgInS₂ nanorods were synthesized for the first time by a one‐step, ambient pressure, environment friendly organic molten salt (OMS) method at 200 °C. The as‐synthesized products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), respectively. The XRD results reveal that the as‐synthesized products at 120–160 °C under ambient pressure contain AgIn₅S₈ which will decrease with the increase of growth temperature. A sample containing only the chalcopyrite AgInS₂ phase is successfully obtained at 200 °C. Furthermore, the elemental compositions are found to become increasingly stoichiometric with increasing temperature. UV‐Vis and photoluminescence (PL) spectra are utilized to investigate the optical properties of AgInS₂ nanorods. By testing on UV‐Vis spectra, it is concluded that the limiting wavelength of the AgInS₂ nanorods is 661 nm and the band gap is 1.88 eV. A broad red emission band peak centered at about 1.874 eV (662 nm) is clearly observed at room temperature, and the intensity of the emission increases with excitation wavelength. In addition, the photoluminescence quantum yield (PLQY) of the nanocrystals at the excitation wavelength of 250 nm was determined to be 13.2%. A possible growth mechanism of AgInS₂ nanorods was discussed. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence study of annealing effects on CuI crystals grown by evaporation method

Cuprous iodide (CuI) is the ultrafastest inorganic scintillation crystal at present. But the low intensity of its ultrafast component luminescence limits the wide application of CuI at room temperature. In this paper, the photoluminescence (PL) characteristics of different quality CuI crystals before and after annealing in various conditions have been investigated in terms of peak position and peak intensity. The origin of different emission band peaked around 426 nm, 680 nm, 718 nm and 820 nm is discussed and the excitation spectra of two mainly emission bands is obtained. Meanwhile, the relative peak intensity of the ultrafast luminescence component to slow lumiescence component of CuI crystals has been studied with respect to the defect concentration of I vacancies. Especially, the method of improving the intensity of ultrafast compentent luminescence of CuI crystals is concluded. These results can provide an important reference for optimizing the luminescence performance of CuI crystals.     

Luminescence of crystals of divalent tungstates

Crystals of divalent tungstates are characterized by two main luminescence spectral ranges: a short-wavelength (blue) luminescence band in the range 390–420 nm and a group (often two groups) of longer wavelength (green) bands in the range 480–520 nm. For crystals of calcium, strontium, barium, cadmium, magnesium, zinc, and lead tungstates, it is shown that the wavelength corresponding to the maximum of the blue luminescence band (λ_max) correlates with the melting temperature (T_m) of these compounds. The position of the blue luminescence band is the same (in the range 510–530 nm) for crystals with different divalent cations. Annealing in vacuum and electron irradiation decrease the intensity of both blue and green luminescence bands but do not change the ratio of their maximum intensities. This circumstance suggests that vacancies serve as luminescence quenchers to a greater extent rather than facilitate the formation of emission centers responsible for a particular luminescence band.     

Photoluminescence in doped and annealed GeSe2 glass

Photodoping of GeSe₂ glass with Ag, Zn, In and Sn shifted the centre of the photoluminescence (PL) band towards lower energies and gave rise to a decrease in its half width, but no new band appeared. The excitation spectrum at 77 K and the temperature dependence of the PL intensity of Ag-doped samples were measured in the temperature range (77–280) K. An activation energy of (253 ± 10) meV for non radiative transitions was foundat T > 210 K which is smaller than that measured in undoped GeSe₂ glass. Thermal annealing shifted the peak of the PL band towards higher energies but had no effect on its half width.     

Effects of passivation and postannealing on the photoluminescence properties of MgO/SiO2 core‐shell nanorods

MgO nanorods were grown by the thermal evaporation of Mg₃N₂ powders on the Si (100) substrate coated with a gold thin film. The MgO nanorods grown on the Si (100) substrate were a few tens of nanometers in diameter and up to a few hundreds of micrometers in length. MgO/SiO₂ core‐shell nanorods were also fabricated by the sputter‐deposition of SiO₂onto the MgO nanorods. Transmission electron microscopy (TEM) and X–ray diffraction (XRD) analysis results indicated that the cores and shells of the annealed core‐shell nanorods were a face‐centered cubic‐type single crystal MgO and amorphous SiO₂, respectively. The photoluminescence (PL) spectroscopy analysis results showed that SiO₂ coating slightly decreased the PL emission intensity of the MgO nanorods. The PL emission of the MgO/SiO₂ core‐shell nanorods was, however, found to be considerably enhanced by thermal annealing and strongly depends on the annealing atmosphere. The PL emission of the MgO/SiO₂ core‐shell nanorods was substantially enhanced in intensity by annealing in a reducing atmosphere, whereas it was slightly enhanced by annealing in an oxidative atmosphere. The origin of the PL enhancement by annealing in a reducing atmosphere is discussed with the aid of energy‐dispersive X‐ray spectroscopy analyses. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of Flux on thermoluminescence in Flux-grown BaFCl crystals

BaFCl crystals have been grown using BaF₂ and BaCl₂ by flux technique. Glow curves, optical absorption, and TL emission spectra of x/r — irradiated crystals are studied. The results have been compared with those BaFCl crystals grown from NaF flux so as to study the effects of flux on these properties. It is found that crystals grown from BaF₂ flux are relatively purer. An additional TL glow peak at 460 K, an optical absorption band at 775 nm and TL emission band at 485 nm have been obtained in the presently grown crystals. The additional glow peak, optical absorption band have been attributed to F(¯F) aggregate centers, whereas the 485 nm TL emission band to impurity centers.     

Thermal and EPR investigations of thallium gallium disulphide single crystal

In this research, the results of the differential scanning calorimetry (DSC) and electron paramagnetic resonance (EPR) investigations of TlGaS₂ single crystal are presented. Specific heat capacity (C_p) anomalies of layered TlGaS₂ have been obtained by using a new DSC technique for such crystals. Remarkable heat capacity anomalies have been revealed at the temperatures of 137.7 K, 174.5 K and 238.5 K. It is found that the anomalies appear at maximum with a small deviation (by 3‐4%) from the regular values, and C_p discontinuity amounted to approximately 5%. Additionally, EPR spectra of Fe doped TlGaS₂ single crystals have been recorded at various temperatures down to 6 K for different orientations of the applied magnetic field. Transformations of present EPR spectra are not sufficient for the confirmation of structural phase transitions, in contrast to the cases in iso structural TlInS₂ and TlGaSe₂ compounds. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Enhancement of the emission from TeO2 nanorods by encapsulation with ZnO

TeO₂‐core/ZnO‐shell nanorods were synthesized by a two–step process comprising thermal evaporation of Te powders and atomic layer deposition of ZnO. Scanning electron microscopy images exhibit that the core‐shell nanorods are 50 ‐ 150 nm in diameter and up to a few tens of micrometers in length, respectively. Transmission electron microscopy and X‐ray diffraction analysis revealed that the cores and shells of the core‐shell nanorods were polycrystalline simple tetragonal TeO₂ and amorphous ZnO with ZnO nanocrystallites locally, respectively. Photoluminescence measurement revealed that the TeO₂ nanorods had a weak broad violet band at approximately 430 nm. The emission band was shifted to a yellowish green region (∼540 nm) by encapsulation of the nanorods with a ZnO thin film and the yellowish green emission from the TeO₂‐core/ZnO‐shell nanorods was enhanced significantly in intensity by increasing the shell layer thickness. The highest emission was obtained for 125 ALD cycles (ZnO coating layer thickness: ∼15 nm) and its intensity was much higher than that of the emission from the uncapsulated TeO₂ nanorods. The origin of the enhancement of the emission by the encapsulation is discussed in detail. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural and optical response of 2‐Mercaptoethonal capped CdS nanocrystals to Fe3+ ions

Cadmium sulfide (CdS) semiconductor nanocrystals (NCs) doped with Fe³⁺ have been synthesized via a solution‐based method utilizing dopant concentrations of (0–5%) and employing 2‐mercaptoehonal as a capping agent. X‐ray diffraction (XRD) results showed that the undoped CdS NCs are in mixed phase of cubic and hexagonal, where as the doped CdS NCs are in hexagonal phase. The crystallite size was increased from ∼1.2 nm to ∼2 nm. Diffuse reflectance spectroscopy studies (DRS) reveals that the band gap energy was decreased with Fe doping and it lies in the range of 2.58 ‐ 2.88 eV. Photoluminescence (PL) spectra of undoped CdS NCs show a strong green emission peak centered at 530 nm and a weak red emission shoulder positioned at 580 nm. After doping all the luminescence intensity was highly quenched and the green emission peak was shifted to orange region (580 nm), but the position of weak red emission shoulder was unaltered with doping. FTIR studies revealed that the NCs were sterically stabilized by 2‐mercaptoethanol. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies on the growth and optical characterization of Tm3+‐doped BaY2F8 single crystals

The BaY₂F₈ crystals doped with different concentrations of Tm³⁺ ions were prepared by the temperature gradient technique (TGT). X‐ray powder diffraction was applied to analyze the phase. The cracking phenomenon along (010) and (100) planes of the crystals grown by temperature gradient technique was studied on the basis of the structure of BaY₂F₈ crystals. The absorption spectra were measured and investigated in the ultraviolet‐visible and near‐infrared ranges at room temperature. Several characteristic absorption bands of Tm³⁺‐doped BaY₂F₈ crystal were observed. The emission and excitation spectra were obtained and investigated at room temperature and 12 K, showing the characteristic emission peaks of Tm³⁺ ions. The temperature dependence of Photoluminescence curve was also investigated in the range of 12–296 K. The luminescence intensity of emission bands decreased with increasing temperature, while the effective bandwidth increased. The up‐conversion spectrum excited at 650 nm was recorded and up‐conversion mechanism was analyzed in detail. The result showed the purple, green and yellow emissions corresponding to ³P₁→³F₃, ¹D₂→³H₅ and ³P₀→¹G₄ transitions, respectively.