Auger-free luminescence due to interatomic p–d transition and self-trapped exciton luminescence in Rb2ZnCl4 crystals

It is reported that Auger-free (AF) luminescence appears with two bands at 4.5 and 6.3 eV in Rb₂ZnCl₄. This luminescence originates from a radiative transition of the Cl 3p valence electrons into the Zn 3d outermost-core holes. The present work is the first observation of AF luminescence due to interatomic p–d transitions in halide crystals. The appearance of two AF luminescence bands suggests the existence of two types of AF transitions following core hole creation. A largely Stokes-shifted luminescence band is also found to appear at 1.9 eV. This band has an excitation threshold at the fundamental absorption edge, and is ascribed to the radiative decay of a self-trapped exciton.     

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr³⁺ centers in submicron and nanostructured Al₂O₃ crystals of different phase compositions at temperatures of 300 and 7.5 K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of ²E, ⁴T₂, and ⁴T₁ excited states of Cr³⁺ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al₂O₃ nanoscale crystals is formed by intracenter transitions (2.5–5.5 eV region), by charge transfer band (6.9 eV) and by effective formation of impurity-bound excitons (9.0 eV region). Such impurity-bound excitons correspond to O₂p→Al₃s electron transition in surroundings of an impurity Cr³⁺ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100 nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al₂O₃ crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al₂O₃ at an excitation energy above 19 eV (more than 2E_g).     

Defect related luminescence in Hg0.2Cd0.8Te nano- and micro-crystals grown by the solvothermal method

The photoluminescence (PL) properties of nano- and micro-crystalline Hg_1?xCd_xTe (x≈0.8) grown by the solvothermal method have been studied over the temperature range 10–300 K. The emission spectra of the samples excited with 514.5 nm Ar⁺ laser consist of five prominent bands around 0.56, 0.60, 0.69, 0.78 and 0.92 eV. The entire PL band in this NIR region is attributed to the luminescence from defect centers. The features like temperature independent peak energy and quite sensitive PL intensity, which has a maximum around 50 K is illustrated by the configuration coordinate model. After 50 K, the luminescence shows a thermal quenching behavior that is usually exhibited by amorphous semiconductors, indicating that the defects are related to the compositional disorder.     

Preparation and luminescence properties of Eu2+-doped BaSi2O5 glass-ceramics

We report on the preparation of Eu²⁺-doped BaSi₂O₅ glass-ceramics by crystallizing an Eu³⁺-doped barium-silicate glass at temperatures in the range from 750 to 1100 °C. Single phase BaSi₂O₅ glass ceramics can be obtained by thermal annealing at temperatures of about 950 °C. The luminescence intensity of Eu²⁺ increases dramatically if monoclinic BaSi₂O₅ is formed. Monoclinic Eu²⁺:BaSi₂O₅ shows efficient, broad band luminescence between 450 and 550 nm by excitation in the near UV. Annealing at temperatures >1000 °C leads to orthorhombic BaSi₂O₅ with much smaller Eu²⁺ luminescence. Static and time-resolved luminescence measurements indicate that Eu²⁺ ions are incorporated into the BaSi₂O₅ crystallites while Eu³⁺ ions remain in the amorphous phase.     

Temperature dependence of the PbWO4:F,Eu luminescence

Luminescence of the PbWO₄:F,Eu single crystal was investigated in the temperature region of 10–300 K. Besides two well known “blue” (2.80 eV) and “green” (2.45 eV) luminescence bands an additional band at 2.25 eV was observed in the whole temperature region and was assigned to the WO₃F defect centres. Europium dopant evinced as a narrow weak luminescence band at 2.02 eV only at 300 K. Temperature dependence of the excitation spectra was simulated assuming existence of the two defect absorption bands located near the fundamental absorption edge.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

Electronic excitations and luminescence of SrAlF5 crystals doped with Ce3+ ions

This paper reports the results of a time-resolved photoluminescence and energy transfer processes study in Ce³⁺ doped SrAlF₅ single crystals. Several Ce³⁺ centers emitting near 4 eV due to 5d-4f transitions of Ce³⁺ ions substituting for Sr²⁺ in non-equivalent lattice sites were identified. The lifetime of these transitions is in the range of 25–35 ns under intra-center excitation in the energy region of 4–7 eV at T = 10 K. An effective energy transfer from lattice defects to dopant ions was revealed in the – 7–11 eV energy range. Both direct and indirect excitation channels are efficient at room temperature. Excitons bound to dopants are revealed at T = 10 K under excitation in the fundamental absorption region above 11 eV, as well as radiative decay of self-trapped excitons resulting in luminescence near 3 eV.     

Effect of Fe3+ doping on structural,optical and electrical properties δ-Bi2O3 thin films

Fe³⁺ doped δ-Bi₂O₃ thin films were prepared by sol–gel method on quartz glass substrate at room temperature and annealed at 800 °C. The thin films were then characterized for structural, surface morphological, optical and electrical properties by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), optical absorption measurements and d.c. two-probe, respectively. The XRD analyses revealed the formation δ-Bi₂O₃ followed by a mixture of Bi₂₅FeO₄₀ and Bi₂Fe₄O₉. SEM images showed reduction in grain sizes after doping and the optical studies showed a direct band gap which reduced from 2.39 eV for pure δ-Bi₂O₃ to 1.9 eV for 10% Fe³⁺ doped δ-Bi₂O₃ thin film. The electrical conductivity measurement showed the films are semiconductors.     

Photoluminescence of AgGaS2 and CuGaS2 doped with rare-earth impurities

Photoluminescence (PL) related to rare-earth (RE) impurities (Ho, Er and Eu) in AgGaS₂ and CuGaS₂ crystals has been studied. In Ho-doped AgGaS₂ and CuGaS₂, two series of PL lines are observed in 1.86–1.92 eV region and 2.24 eV region, and they are assigned to ⁵F₃–⁵I₇ and ⁵S₂–⁵I₈ transitions of the Ho³⁺ ion, respectively. Similarly, in Er-doped AgGaS₂ and CuGaS₂, Er³⁺-related two PL series are observed: 1.83–1.88 eV region (⁴F_9/2–⁴I_15/2) and 2.22–2.26 eV region (⁴S_3/2–⁴I_15/2). For both Ho and Er impurities, the profile of the PL spectrum in AgGaS₂ is complex, and PL exhibited large number of lines compared with that in CuGaS₂. The differences in PL spectra between this two compounds are related to the crystal field at the cation site and the local atomic arrangement of the RE impurities. This work also refers to the PL band at 2.28 eV observed for the Eu-doped AgGaS₂ crystal.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Synthesis,characterization and studies of radiative properties on Eu3+-doped ZnAl2O4

Eu³⁺-doped ZnAl₂O₄ phosphors were successfully synthesized in air atmosphere at 900 °C. The phosphors were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), thermally stimulated luminescence (TSL) and photoluminescence (PL) techniques. The average particle size of the system as determined from SEM was found to be 100–150 nm (for samples annealed at 900 °C). PL spectra of the doped phosphors showed emission peaks corresponding to Eu³⁺ ions. Lifetime studies revealed Eu³⁺ ions to be in two different sites. The asymmetric ratio (I₆₁₆/I₅₉₂) was observed to be about 3.75. This suggested that Eu³⁺ ion entered the host mainly substituting Al³⁺ site distorting the local environment and is partly located on surface of the phosphors. A prominent glow peak at 430 K was observed in the TSL of γ-irradiated Eu³⁺-doped ZnAl₂O₄ phosphors. Trap parameters for this peak have been determined and the probable mechanism for the glow peak is proposed. CIE chromaticity coordinates for the system were evaluated. It was observed that, the system could be employed as a potential red emitting phosphor. Commercial utility of the phosphor was investigated by comparing it with commercial red phosphor. The PL intensity of the as prepared phosphors was 63% of that of the commercial phosphor. Apart from this, various radiative properties such as the Judd–Ofelt intensity parameters, spontaneous emission probabilities, luminescence branching ratios, radiative lifetimes and quantum efficiency were evaluated for the system.     

Magneto-optical properties of α-Fe2O3@ZnO nanocomposites prepared by the high energy ball-milling technique

Magnetic–fluorescent nanocomposites (NCs) with 10 wt% of α-Fe₂O₃ in ZnO have been prepared by the high energy ball-milling. The crystallite sizes of α-Fe₂O₃ and ZnO in the NCs are found to vary from 65 nm to 20 nm and 47 nm to 15 nm respectively as milling time is increased from 2 to 30 h. XRD analysis confirms presence of α-Fe₂O₃ and ZnO in pure form in all the NCs. UV–vis study of the NCs shows a continuous blue-shift of the absorption peak and a steady increase of band gap of ZnO with increasing milling duration that are assigned to decreasing particle size of ZnO in the NCs. Photoluminescence (PL) spectra of the NCs reveal three weak emission bands in the visible region at 421, 445 and 485 nm along with the strong near band edge emission at 391 nm. These weak emission bands are attributed to different defect – related energy levels e.g. Zn-vacancy, Zn interstitial and oxygen vacancy. Dc and ac magnetization measurements show presence of weakly interacting superparamagnetic (SPM) α-Fe₂O₃ particles in the NCs. ⁵⁷Fe-Mössbauer study confirms presence of SPM hematite in the sample milled for 30 h. Positron annihilation lifetime measurements indicate presence of cation vacancies in ZnO nanostructures confirming results of PL studies.     

Energy transfer induced Eu3+ photoluminescence enhancement in tellurite glass

In this work, structural, thermal and optical properties of Eu³⁺ doped TeO₂–La₂O₃–TiO₂ glass were investigated. The differential scanning calorimetry (DSC) measurements reveal an important stability factor ΔT=143.52 K, which indicates the good thermal and mechanical stabilities of tellurite glass. From the absorption spectrum, the optical band gap was found to be direct with E_g=3.23 eV. The temperature dependences of photoluminescence (PL) properties of Eu-doped and Eu–Tb codoped tellurite glass are investigated. As the temperature increases from 7 to 300 K, both the PL intensity and the PL lifetime relative to the ⁵D₂→⁷F₀ are nearly constant below 230 K and then an enhancement takes place. This anomalous feature is attributed to the thermally activated carrier transfer process from charged intrinsic defects states to Eu³⁺ energy levels.By co-doping tellurite glasses with Eu and Tb, a strong Eu³⁺ PL enhancement is shown due to excitation transfer from Tb³⁺ and intrinsic defects to Eu ions.     

Luminescence enhancement by the reduction–oxidation synthesis in monoclinic RE2O3 (RE=Eu,Gd) phosphors containing Eu3+ activator

A novel synthesis was developed for enhanced luminescence in sesquioxide phosphors containing Eu³⁺ activator. It consisted of two annealing steps: reduction under vacuum with gaseous H₂ at 10 Torr and 1300 °C and re-oxidation at 300–1500 °C in air. The integrated luminescence intensity of the monoclinic Eu₂O₃ phosphor was enhanced ca. 21 times by this method compared with conventional processing. The photoluminescence (PL) intensity was maximized at re-oxidation temperatures of 500–1100 °C. The PL characteristics of monoclinic Eu₂O₃ and Gd₂O₃:0.06Eu samples were compared with a commercial cubic Y₂O₃:Eu phosphor. The evolution of physical characteristics during the two-step annealing was studied by Raman spectroscopy, XPS, XRD, PL decay analysis, and SEM. PL decay lifetime increased proportionally to the PL intensity over the range 0.5–100 μs. Additional vibrational modes appeared at 490, 497, and 512 cm^?1 after the two-step annealing. The increase in PL intensity was ascribed to the formation of excess oxygen vacancies and their redistribution during annealing. Resonance crossovers between the charge transfer state and the emitting ⁵D_J states are discussed in relation to reported luminescence saturation mechanisms for oxysulfides Ln₂O₂S:Eu³⁺ (Ln=Y, La).     

Time-resolved spectroscopy in LiCaAlF6 doped with Cr3+ as a function of pressure and temperature: Excited-state crossover and phase-transition effects

This work investigates phase transition (PT) and excited-state-crossover (ESCO) effects on the photoluminescence (PL) properties of LiCaAlF₆: Cr³⁺. The structural requirements for changing the Cr³⁺ PL behavior from a broad-band emission at 1.59 eV (781 nm) at ambient conditions, to ruby-like narrow-line emission at 1.87 eV (663 nm) are analyzed in the 0–35 GPa range. We report a PL study on LiCaAlF₆: Cr³⁺ by means of time-resolved emission as a function of pressure and temperature. In particular we focus on the PL variations occurring around the pressure-induced trigonal-to-monoclinic first-order PT in LiCaAlF₆ at 7 GPa.     

On the origin of blue light emission from Ge-nanocrystals containing a-SiOxfilms

We have studied the photoluminescence of a-Si_xGe_yO_{1 − x − y}films with average Ge-nanocrystal sizes ranging from over 100 nm down to 2 nm. No systematic peak shift of the luminescence bands at 3.0 eV and 2.0 eV with the diameter of the nanocrystals is observed. Comparision with a simplified confinement model shows that quantum size effects cannot explain the blue luminescence. We propose the Ge²₀defect as a likely source for this band, based on considerations about the crystallization process.     

Electron excitations in LiB3O5 crystals with defects: Low-temperature time-resolved luminescence VUV spectroscopy

The dynamics of electron excitations and luminescence of LiB₃O₅ (LBO) single crystals was studied using low-temperature luminescence vacuum ultraviolet spectroscopy with a subnanosecond time resolution under photoexcitation with synchrotron radiation. The kinetics of the photoluminescence (PL) decay, the time-resolved PL emission spectra, and the time-resolved PL excitation spectra of LBO were measured at 7 and 290 K, respectively. The PL emission bands peaking at 2.7 eV and 3.3 eV were attributed to the radiative transitions of electronic excitations connected with lattice defects of LBO. The intrinsic PL emission bands at 3.6 and 4.2 eV were associated with the radiative annihilation of two kinds of self-trapped electron excitations in LBO. The processes responsible for the formation of localized electron excitations in LBO were discussed and compared with those taking place in wide-gap oxides.     

Disorder driven optical processes in nanocrystalline ZnO

A systematic study on the modification of optical properties in mechanically milled ZnO powder has been reported here. The average grain size of the powder becomes ～20 nm within 4 h of milling. Fluctuations of average grain size have been noticed at the initial stage of milling (within 15 min). Changes in grain morphology with milling have also been noticed in scanning electron micrographs of the samples. Room temperature optical absorption data shows a systematic red shift of absorption band edge (～3.25 eV). The band tail parameter (extracted from the optical absorption just below the band edge) follows a simple exponential relation with the inverse of the average grain size. Significant increase of the band tail parameter has been noticed at low grain size regime. It has been analyzed that high values of band tail parameter is a representative of V_Zn–V_O type divacancy clusters. Room temperature photoluminescence spectra show decrease (except for 120 min milling) of band edge emission intensity with increase of milling time. Subsequent decrease of sub-band edge emission is, however, less prominent. The variation of PL intensity ratio (intensity at band edge peak with that at 2.3 eV) follows simple exponential decrease with the increase of band tail parameter. This indeed shows that band edge emission in ZnO is related with the overall disorder in the system, not grain size induced only.     

Fabrication and luminescent properties of Al2O3:Cr3 + microspheres via a microwave solvothermal route followed by heat treatment

AlOOH:Cr^3 + powders were synthesized via a microwave solvothermal route at 433 K for 30 min and were used as the precursor and template for the preparation of γ-Al₂O₃:Cr^3 + by thermal transformation at 773 K for 2 h in air. The obtained γ-Al₂O₃ based powders were microspheres with an average diameter about 1.9 μm. Photoluminescence (PL) spectra showed that the Al₂O₃:Cr^3 + particles presented a symmetric broad R band at 696 nm without appreciable splitting when excited at 462 nm. It is shown that the 0.04 mol% of doping concentration of Cr^3 + ions in γ-Al₂O₃:Cr^3 + is optimum. According to Dexter's theory, the critical distance between Cr^3 + ions for energy transfer was determined to be 47.54 Å. Based on the corresponding PL spectrum, full width at half maximum (FWHM) of Al₂O₃:Cr^3 + (0.04 mol%) was calculated to be 3.35 nm.     

Synthesis and luminescence of La2BaZnO5 phosphors

A modified synthesis of La₂BaZnO₅ phosphors activated with rare earths Eu³⁺, Tb³⁺, Pr³⁺ and Sm³⁺, and ns² ion Bi³⁺ is reported. RE₂BaZnO₅ compounds are conventionally prepared by two step solid state reaction. In the first step, carbonates or similar precursors are intimately mixed and heated at 900 °C to decompose the precursors to oxides. To eliminate the unwanted phases like BaRE₂O₄, the resulting powders are reheated at 1100 °C for long time. We prepared La₂BaZnO₅ phosphors activated with various activators by replacing the first step by combustion synthesis. Results on photoluminescence are presented. PL results on Eu³⁺ and Tb³⁺ are in good agreement with the literature reports. PL emission from Sm³⁺, Pr³⁺ and Bi³⁺ had not been reported earlier. Excitation spectrum of Eu³⁺ is dominated by a charge transfer band around 318 nm, while for the other rare earths a band at 240 nm is always present. This is attributed to the host absorption.