PbWO4 Single Crystals Research Articles

Revealing defect centers in PbWO4 single crystals using thermally stimulated current measurements

The trap centers have a significant impact on the electronic properties of lead tungstate (PbWO4), suggesting their crucial role in optoelectronic applications. In the present study, we investigated and revealed the presence of shallow trap centers in PbWO4 crystals through the utilization of the thermally stimulated current (TSC) method. TSC experiments were performed in the 10–280 K range by applying a constant heating rate. The TSC spectrum showed the presence of a total of four peaks, two of which were overlapped. As a result of analyzing the TSC spectrum using the curve fit method, the activation energies of revealed centers were found as 0.03, 0.11, 0.16, and 0.35 eV. The trapping centers were associated with hole centers according to the comparison of TSC peak intensities recorded by illuminating the opposite polarity contacts. Our findings not only contribute to the fundamental understanding of the charge transport mechanisms in PbWO4 crystals but also hold great promise for enhancing their optoelectronic device performance. The identification and characterization of these shallow trap centers provide valuable insights for optimizing the design and fabrication of future optoelectronic devices based on PbWO4.

Investigation of linear and nonlinear optical properties of PbWO4 single crystal

In this manuscript, PbWO4 single crystals having great importance for device applications were studied in terms of linear and nonlinear optical properties. For this purpose, transmission and reflection experiments were performed in the wavelength range of 350–1000 nm. X-ray diffraction analysis presented crystalline structure as scheelite type tetragonal. Spectral dependencies of absorption coefficient, skin depth, refractive index, dielectric function were reported. Optical band gap of the crystal was estimated as 3.25 eV from the Tauc and derivative spectral methods. Urbach, critical point, single oscillator and dispersion energies, static refractive index and dielectric constant were revealed for PbWO4 single crystal. First- and third-order nonlinear susceptibilities, and nonlinear refractive index were also computed for the studied crystal.

Spectroscopic ellipsometry characterization of PbWO4 single crystals

Optical characterization of PbWO4 single crystals grown by Czochralski method was achieved in virtue of spectroscopic ellipsometry experiments carried out in the energy region of 1.0–5.6 eV at room temperature. Tetragonal scheelite structure with lattice parameters of a = b = 5.4619 Å and c = 12.0490 Å was determined for the bulk crystal utilizing from XRD analysis. Analyses of the ellipsometry data presented the photon energy dependencies of complex dielectric function of the crystal. The real part of the dielectric function exhibited increasing behavior with energy in the below 4.1 eV and then decreased immediately. Zero frequency refractive index and dielectric constant were determined to be 2.02 and 4.08, respectively, using Wemple and DiDomenico oscillator model. High frequency dielectric constant was calculated as 4.30 by Spitzer-Fan model. Optical band gap of PbWO4 was found to be 3.24 eV from the dielectric relaxation time spectrum. Moreover, existence of two critical points with energies of 3.70 and 4.58 eV was revealed from the analyses of extinction coefficient and second derivative of the dielectric function. These levels were considered to be due to creation of cation exciton (Pb2+ 6s2 − Pb2+ 6s6p) and transitions in the [WO4]2− group.

Effect of temperature on band gap of PbWO4 single crystals grown by Czochralski method

The structural and optical properties of PbWO4 single crystals grown by Czochralski method and investigated by x-ray diffraction (XRD) and transmission experiments. XRD pattern presented well-defined and intensive peaks related with tetragonal scheelite structure. Transmission experiments were accomplished for the first time at various temperatures between 10 and 300 K on PbWO4 single crystals to reveal variation of band gap with temperature. Derivative spectroscopy method presented the experimentally observed band gap energy as increasing from 3.20 to 3.35 eV when the temperature was decreased to 10 K from room temperature. The revealed energy was associated with transition taking place between delocalized and trap levels. Temperature-band gap energy plot was analyzed by Varshni and Bose–Einstein models. The fitting processes under the light of these models revealed optical characteristics of absolute zero experimentally observed band gap, variation rate of gap energy with temperature and Debye temperature of PbWO4 single crystal.

Influence of a Surface Finishing Method on Light Collection Behaviour of PWO Scintillator Crystals

In the field of scintillators, high scintillation and light production performance require high-quality crystals. Although the composition and structure of crystals are fundamental in this direction, their ultimate optical performance is strongly dependent on the surface finishing treatment. This paper compares two surface finishing methods in terms of the final structural condition of the surface and the relative light yield performances. The first polishing method is the conventional “Mechanical Diamond Polishing” (MDP) technique. The second polishing technique is a method applied in the electronics industry which is envisaged for finishing the surface treatment of scintillator crystals. This method, named “Chemical Mechanical Polishing” (CMP), is efficient in terms of the cost and material removal rate and is expected to produce low perturbed surface layers, with a possible improvement of the internal reflectivity and, in turn, the light collection efficiency. The two methods have been applied to a lead tungstate PbWO4 (PWO) single crystal due to the wide diffusion of this material in high energy physics (CERN, PANDA project) and diagnostic medical applications. The light yield (LY) values of both the MDP and CMP treated crystals were measured by using the facilities at CERN while their surface structure was investigated by Scanning Electron Microscopy (SEM) and Grazing Incidence X-ray Diffraction (GID). We present here the corresponding optical results and their relationship with the processing conditions and subsurface structure.

Application of two-photon absorption in PWO scintillator for fast timing of interaction with ionizing radiation

This work was aimed at searching for fast phenomena in scintillators in sub-10-ps range, a benchmark timing for the time response of radiation detectors in particle colliders. The pump-and-probe optical absorption technique with a tunable-wavelength parametric oscillator as the pump and a continuous-spectrum source as the probe beam was used to study lead tungstate PbWO4 (PWO) single crystals. It is shown that the rise time of the probe pulse absorption induced by the pump pulse is shorter than the pump pulse width of 200fs. The approximately linear dependence of the probe absorption on the pump pulse energy density evidences that the induced absorption is caused by two-photon absorption involving one probe and one pump photon. We demonstrate that the intensity of the induced absorption at certain wavelengths is influenced by gamma irradiation, provided that an appropriate light polarization is selected. The application of the irradiation-sensitive nonlinearity for fast timing in radiation detectors is discussed.

Magnetic resonance studies on 207Pb nuclei and paramagnetic impurities in PbWO4:Eu, PbWO4:Gd, and pure PbWO4 single crystals

The electron paramagnetic resonance (EPR) of Eu2+ and Gd3+ ions in PbWO4 crystals doped with paramagnetic impurities was investigated using an X-band spectrometer. It was found that during the growth of these crystals, Gd3+ ions substitute for the Pb2+ ions more effectively than Eu2+ ions. In addition, the relaxation rate 1/T1 of the 207Pb nuclei in pure and doped PbWO4 single crystals was studied using a nuclear magnetic resonance (NMR) spectrometer over a temperature range of 180–400K. The relaxation mechanisms of the pure and doped crystals were Raman processes, resulting in relaxation rates that were proportional to the square of temperature.

Luminescence of PbWO4 single crystals doped with fluorine

The influence of fluorine doping on the luminescent properties of lead tungstate, PbWO4, a scintillating material used in high-energy physics, was studied. Two series of crystals that were grown by the Czochralski method in two different laboratories were investigated. It was shown that the luminescent properties of the fluorine-doped samples were similar although those of the undoped PbWO4 samples from the different series differed significantly. It was concluded that this was associated with the formation of WO3F complexes in the fluorine-doped PbWO4 crystals.

Bridgman crystal growth and spectral properties of Er doped PbWO4 as stimulated Raman crystals

Several Er doped (0.2at%, 0.5at%, and 1.0at%) PbWO4 single crystals were grown by Bridgman method with the size of Φ20×100mm. The absorption spectra of Er:PWO samples were measured to investigate the spectral properties of Er3+ ions according to Judd-Ofelt theory. The phenomenological intensity parameters Ωλ were fitted to calculate the spectral parameters of several radiative transitions of Er3+ ions, and the possibility of 4I13/2→4I15/2 transition at about 1.5μm of Er3+ ions in PWO crystal was also discussed. The energy transfer from PWO matrix to Er3+ ions was investigated based on the photoluminescence spectra, and the frequency shifting effect of Er3+ ions on PWO crystal was discussed according to the spontaneous Raman spectra. The 1.5μm fluorescence spectra of different Er:PWO samples were measured under the excitation at 980nm.

Influence of Mo/Sb Co-Doping on Scintillation Properties of PbWO<sub>4 </sub>Crystal

Lead tungstate, PbWO4 single crystals co-doped with Mo6+ and Sb3+ ions were grown using the modified Bridgman method. The X-ray powder diffraction, optical transmission, X-ray excited luminescence, photoluminescence, light output and decay time have been investigated. Compared to non-doped PbWO4, the co-doped PbWO4:(Mo,Sb) crystals exhibit improved transmittance in the short wavelength region. Luminescence and light output measurements demonstrated that Mo6+ and Sb3+ co-doping could enhance the luminescence of PbWO4 and increase the light output to about 7.0% of Bi4Ge3O12 crystal. Doped Mo6+ and Sb3+ ions in PbWO4 were tentatively considered to occupy W and Pb sublattice sites mostly. The second excitation peak at 385 nm, which is the second effective excitation for the enhanced blue-green emission in as-grown PbWO4:(Mo,Sb) crystal, should be related to [MoO4]2- group and oxygen vacancy.

Erratum: “Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals” [J. Chem. Phys. 131, 034505 (2009)

Erratum: “Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals” [J. Chem. Phys. 131, 034505 (2009)

Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals

A multifrequency electron paramagnetic resonance (EPR) investigation of Nd(3+) impurities in PbWO(4) single-crystals at the conventional microwave frequency (MF) 9.43 GHz, and at the 95, 190, and 285 GHz high frequencies was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective electron spin of one-half and to a tetragonal symmetry. For the magnetic field along the tetragonal axis, the g(parallel)-factor and the hyperfine constant A(parallel) of the lowest doublet of the ground multiplet decreases with frequency increase. For the magnetic field perpendicular to the tetragonal axis, the g(perpendicular)-factor exhibits a small azimuthal angular dependence that increases with increasing the frequency due to the S(4) site symmetry. The azimuthal angular dependence allows to clearly distinguish between different local axial symmetries. These properties are interpreted as high field/frequency (HF) effects associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. We show that from the combined analysis of the multifrequency MF- and HF-EPR spectra and of the optical data, an accurate description of the ground multiplet of the Kramers rare earth ions in solid matrices can be derived.

Spectral properties and thermoluminescence of codoped PbWO4:(Mo,Y) and PbWO4:(F,Y) crystals

AbstractLead tungstate, PbWO4 single crystals codoped with Mo6+/F– and Y3+ ions were grown using the modified Bridgman method. Optical transmission, X‐ray excited luminescence, photoluminescence, ultrashort pulsed X‐ray excited fluorescent lifetime and thermoluminescence have been investigated. Compared to pure PbWO4, the codoped PbWO4:(Mo,Y) and PbWO4:(F,Y) crystals exhibit improved transmittance in the short‐wavelength region. Luminescence and light‐yield measurements demonstrated that Mo6+/F– and Y3+ codoping could enhance the luminescence of PbWO4 and reduce slow decay components. Doped Mo6+ and F– ions in PbWO4 were tentatively considered to occupy W and O sites, while Y3+ ions codoped in PbWO4:Mo/F mostly occupy Pb sublattice sites. The second excitation peak at 335 nm, which is the second effective excitation for the enhanced blue‐green emission in as‐grown PbWO4:(Mo,Y) and PbWO4:(F,Y) crystals, should be related to MoO42– groups and O vacancies (VO). Thermoluminescence glow curve measurement between RT and 400 °C provides complementary information about trapping states and the effect of Y3+ ion codoping resulting in the reduction of stable and temporary hole centers. Further work is needed to explain the doping and energy‐transfer mechanism. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Luminescence and scintillation characteristics of heavily Pr3+-doped PbWO4 single crystals

PbWO 4 single crystals with praseodymium doping in a wide concentration range (0.1–5 at % in the melt) were grown by Czochralski method. Absorption, luminescence, and scintillation characteristics were measured at room temperature. Similar to other trivalent dopants, heavy Pr3+ doping on one hand suppresses intrinsic host scintillation. On the other hand though, in the scintillation decay it introduces components with characteristic decay times within 500–2000 ns based on Pr3+ 4f-4f emission lines in the green-red part of the spectra. We discuss the suitability of such material for dual scintillation/Cherenkov light detector.

Luminescence and Scintillation Properties of Doubly-Doped PbWO4 Single Crystals with Mo6+/PbF2, Mo6+/Gd3+ and PbF2/Y3+

Luminescence and Scintillation Properties of Doubly-Doped PbWO4 Single Crystals with Mo6+/PbF2, Mo6+/Gd3+ and PbF2/Y3+

The optical and electrical properties in Nd3+ doped lead tungstate single crystals

The influence of Nd3+ doping with different concentration on the luminescence properties of PbWO4 single crystals were investigated by means of the thermo-luminescence, X-ray excited luminescence and photoluminescence spectra. Nd3+ doping has almost the same doping effects as that of La3+ doping in PbWO4. The doping mechanism of Nd3+ was briefly discussed. Dielectric loss spectroscopy and conductibility were measured when the PbWO4:Nd3+ single crystals were sequentially annealed in air atmosphere from 100 °C to 1000 °C. The heavily Nd3+-doped PbWO4 present a larger conductivity at the high temperature with low activation energy.

Poly(methacrylic acid)-mediated morphosynthesis of PbWO4 micro-crystals

PbWO4 crystals with various morphologies were fabricated via a facile poly(methacrylic acid)-mediated hydrothermal route. Novel microsized PbWO4 single crystals with a needle-like shape as well as other morphologies, such as a fishbone, dendrite, sphere, spindle, ellipsoid, rod, and dumbbell with two dandelion-like heads, could be produced. The presence of PMAA, [Pb2+]/[WO42-] molar ratio (R), and aging temperature played key roles in the formation of the PbWO4 needle-like structures. Between temperatures of 60 to 150 °C, the length and photoluminescence intensities of the PbWO4 micro needles significantly increased with aging temperature, while the diameter did not change remarkably. Time-dependent experiments revealed that the formation of PbWO4 microneedles involved an unusual growth process, involving nucleation, oriented assembly and controlled mesoscale restructuring of nanoparticle building blocks.

Yellow-to-violet, blue, and green frequency upconversions in Nd3+-doped PbWO4 single crystal

Upconversion emission properties of Nd3+ ions doped in PbWO4 crystal are investigated by pulsed laser excitation into the G5∕24+G2(1)7∕2 levels at temperatures from 10to295K. The main upconversion emission consists of strong blue emission bands in the wavelength region of 435–447nm corresponding to the P1∕22→I9∕24 transitions. Some UV bands located in the wavelength region of 356–397nm corresponding to the D3∕24→I9∕24, I11∕24, and I13∕24 transitions and weak green emission at 525.2nm corresponding to the G7∕24→I9∕24 transition are observed. Several upconversion blue emissions from the P1∕22 level and the absoprtion lines due to the I9∕24→P1∕22 transition indicate multisite structure of the Nd3+ ions in the PbWO4 lattices. We investigate the excitation spectra of the upconversion emissions and their pump power dependence and lifetimes from which the probable upconversion mechanisms are proposed and analyzed. The dominant mechanisms for the upconversion emission originating from the D3∕24 level (UV emission) and the P1∕22 level (blue emission) are proven to be the sequential two-photon excited-state absorption, while the energy transfer upconversion process is responsible for the green upconversion emission from the G7∕24 level of Nd3+.

Stress dependence of Raman vibrational bands of PbWO4 single crystals

AbstractA microscopic piezo‐spectroscopic (PS) investigation of the stress dependence of the Raman bands of lead tungstate was conducted, which used a ball‐on‐ring flexure method, aimed at adding controlled biaxial stresses on thin‐plate samples. Spectral shifts varying with applied loads and in‐depth positions were observed for a ‐ and c ‐plane PbWO4 single crystals, confirmed by polarized Raman experiments. Taking account of the laser probe function, theoretical convolutions were performed to determine precisely the PS coefficients, which were determined as Π 11 = Π 22 = –1.29 ± 0.20 cm–1/GPa and Π 33 = –0.87 ± 0.25 cm–1/ GPa for the 902 cm–1 A g species. Accordingly, the residual stresses generated by mechanical treatments were analysed based on the derived coefficients. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

EPR and optical studies of Cr-doped PbWO4 single crystals

Abstract Electron paramagnetic resonance (EPR) studies were performed on Cr ions in PbWO 4 crystals using an X-band spectrometer. With the help of Fourier-transform infrared (FTIR) and thermoluminescence (TSL) data, it was proposed that Cr 3+ impurities enter PbWO 4 lattice in the Pb-site and result in large relaxation of crystal lattice, which was illustrated by the significant distortion of WO 4 tetrahedron and the rapid decrease of blue luminescence band. Further, for heavily doped samples some new EPR signals, which did not appear in the lightly doped sample, were detected. The new EPR lines were ascribed to the exchange-coupled effect of Cr 3+ -ion pairs. Luminescence spectra provide support to the idea about defect clusters. The presence of Cr 3+ -ion pairs or Cr 3+ -ion associations in heavily doped PWO is the reason for the luminescence quench of PWO host and Cr 3+ ion.