Decay kinetics of the green emission in tungstates and molybdates

Luminescence characteristics of a number of undoped and variously doped PbWO₄ crystals were studied at 0.4–400 K by the time-resolved spectroscopy and compared with those of ZnWO₄,CdWO₄ and PbMoO₄ crystals. Two types of green emission centres are detected in PbWO₄ crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4 eV emission observed under excitation around 3.90–3.95 eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5 eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb⁺-WO₃ centres at T>180 K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated.     

闪烁晶体的发光研究进展

概述了近年来闪烁体发光研究的进展,主要介绍用于未来高能物理实验的新型闪烁体发光机理研究,选取我们在研BaF₂,BaF₂:RE,CeF₃以及PbWO₄中的一些新进展。重点谈及三点:(1)在BaF₂的“价带^→芯带”跃迁发光研究基础上进行稀土(Gd³⁺-Eu³⁺)掺杂时观察到的量子剪裁以及对多光子发光的新思考;(2)CeF₃晶体发光的级联能量传递中,Ce³⁺(290nm发射带)与缺陷发光中心(340nm发射带)间能量传递及其传递效率的温度依赖;(3)PbWO₄晶体的发光中心研究中,提出以“WO₄^-2+O_i”绿光中心替代“WO³+F”中心观点的依据。同时也简介了医用闪烁体的最新进展。     

PbWO4闪烁晶体的发光动力学模型

在对PbWO₄闪烁晶体的光谱特性、发光衰减及其温度依赖以及热释光的研究基础上,并结合理论计算,提出了PbWO₄晶体发光的动力学模型,给出了PbWO₄晶体的基本能带结构及激子发光中心能态、陷阱能级在能隙中的位置。用此模型可以完整说明PbWO₄的发光过程,特别是导致室温下发光效率低的原因。最后还对其主发射成分蓝、绿发光中心的起源作了简要讨论。     

Electron paramagnetic resonance study of copper impurity charge-states in PbWO4 scintillator

The study of two types of Cu²⁺ centres observed in nominally pure PbWO₄ crystals grown by Bridgman and Czochralski methods was carried out by electron paramagnetic resonance (EPR). One of centres, called Cu²⁺(I), arises after oxygen compensation, while the second one, called Cu²⁺(II) requires, in addition, infrared illumination at low temperatures, being thermally stable only up to 22–23 K. The EPR spectra of both Cu²⁺ centres are described by rhombic symmetry g-tensors with the Z-principal axis lying close to, or in the (ab) plane of the crystal. It is proposed that in both centres the Cu²⁺ ions substitute for Pb²⁺ ions. The additional reduction of the local crystal-field symmetry is connected with a CuWO₄ wolframite-type lattice distortion for the Cu(I) and a Jahn–Teller distortion of the regular PbWO₄ lattice for the Cu(II). It was also found that either IR irradiation or thermal heating activate the transfer of an electron between two closely spaced Cu²⁺ centres.     

Recombination luminescence in lead tungstate scintillating crystals

Room temperature radioluminescence and photoluminescence decay kinetics measurements of Ba-doped PbWO₄ crystals were compared with those of undoped and Mo-doped samples. Photoluminescence decay measurements focus on the coexistence of the immediate (fast) decay having a decay time of a few nanoseconds with slower delayed recombination decay processes. The radioluminescence emission peaking at 500 nm in Ba-doped crystals is similar to that observed in Mo-doped samples. However, photoluminescence of the Ba-doped crystals shows much faster decay kinetics with respect to that of PbWO₄:Mo. Wavelength-resolved thermally stimulated luminescence data (10–300 K) provides complementary information about trapping states and is correlated to photoluminescence decay kinetics.     

Emission properties of Ta-centres in KCl

Electrolytic colouration at about 600°C and 350 V/cm, for KCl crystal containing Pb²⁺ and for KCl:Pb²⁺ crystals co-doped with Li⁺, Na⁺ and Rb⁺ has been undertaken. Several absorption bands were observed in both doped and co-doped crystals in the visible-UV region. Excitation into these bands gives rise to the same 0.86 eV emission band except for the Li⁺-co-doped crystal which gives rise to a 0.80 eV emission band. These absorption bands are due to the same T_a-centre related to Pb⁻. The observed infrared emission intensity of the crystal with F-centres is higher than of without F-centres.     

On-line induced absorption measurement on PbWO4, YAlO3:Ce and CsI scintillating crystals

On-line measured radiation-induced absorption spectra and their kinetics are presented for the doped PbWO₄, YAlO₃:Ce and CsI crystals. In the doubly doped PbWO₄:Mo,Y crystals the saturation level of the induced absorption increased with Mo concentration up to 2750 ppm. With a higher concentration it again gradually decreased. Yttrium co-doping around 100 ppm improved considerably the radiation hardness of Mo-doped PbWO₄. Characteristic recovery time of these crystals was about 30 min. Positive influence of Zr⁺⁴ co-doping on characteristics of a set of YAlO₃:Ce samples resulted in decrease in the induced absorption intensity. The presence of very slow recovery processes was in good agreement with thermoluminescence characteristics above room temperature reported earlier. An absorption band round 420 nm appeared in as-grown CsI crystals doped with Tl, and was related to the oxygen contamination.     

Luminescence study of pure and Fe- or Mo-doped ZnWO4 crystals

ZnWO₄, ZnWO₄:Fe and ZnWO₄:Mo crystals were investigated by the methods of time-resolved spectroscopy in the temperature range of 4.2–300 K. It is shown that the Mo and Fe impurities significantly reduce the light yield of ZnWO₄. The main 2.5 eV emission of ZnWO₄ and the 1.77 eV emission band of ZnWO₄:Mo are shown to originate from the triplet excited state of the WO₆ and MoO₆ complex, respectively. In ZnWO₄:Fe,Mo the MoO₆ emission band is shifted to lower energies due to the perturbing influence of the iron impurity. No perturbing effect of Fe or Mo ions was observed for the main emission of ZnWO₄:Fe and ZnWO₄:Mo. The creation spectrum of self-trapped holes was measured for ZnWO₄, ZnWO₄:Fe and ZnWO₄:Mo crystals in the energy region of 4–30 eV.     

Luminescence of [Be] centre in MgO:Be

Doping of MgO single crystals with Be results in the formation of numerous Be-containing paramagnetic centres, easily detectable by EPR, and creates an intensive luminescence band at 6.2 eV, observable at T<200 K, and gives rise to new thermoluminescence peaks at 147 and 190 K. A paramagnetic centre with a rhombic symmetry that decays at 160 K was identified as a [Be]⁺ (i.e. O⁻–Be²⁺) centre—a hole trapped by O²⁻ lattice ion near a Be²⁺ perturbing defect. The luminescence excitation and isochronal annealing studies led to the conclusion that the 6.2 eV luminescence arises at the radiative decay of electron excitations near Be²⁺. These excitations can be created at the recombination of electrons with the holes localised in the [Be]⁺ centres, at the recombination of holes with the electrons trapped in the Be¹⁺ centre or at a direct excitation of oxygen near the Be²⁺ ion.     

The role of alkali ions in the 190 K TSL peak in quartz

The role of alkali ions in the creation of the thermally stimulated luminescence (TSL) peak at 190 K in quartz has been investigated by wavelength resolved TSL and thermally stimulated current (TSC) measurements performed on synthetic crystals, both as grown and hydrogen swept, characterised by alkali content of about 1–3 and 0.1 ppm respectively. The 190 K peak has been efficiently produced in as-grown crystals by a double irradiation procedure consisting of a first x-ray irradiation at 90 K followed by heating in the 170–300 K temperature range and a second irradiation at 90 K; this effect has not been observed in the hydrogen swept crystal. Moreover, the study of the spectral composition of the emitted light has shown the existence of two emission bands, one peaking at 450 nm (T<150 K) and the second one evidenced for T>150 K and peaking at around 380 nm.

In as grown samples, TSC peaks at 205, 260 and at around 350 K (composite structure) have been detected: their intensities are much stronger in the as grown crystal and with the electric field oriented along the z-axis indicating that they have an ionic character. By taking into account the mechanism of formation of the [SiO₄/M⁺]⁰ (M⁺=Li⁺, Na⁺) traps (previously found to be responsible for the 190 K TSL peak), the 205 K TSC peak can be attributed to the radiation induced dissociation of alkalis from [AlO₄/M⁺]⁰ defect centres and subsequent migration near to Si sites; on the other hand, the 260 K TSC peak can be related to the subsequent disintegration of [SiO₄/M⁺]⁰ defects involving the migration of alkalis to different ionic traps.

PACS: 78.60.K–61.72.J–42.70.Ce     

铥、铽及铕离子掺杂的BaAl2O4膜的阴极射线发光特性

利用喷雾热解法合成了Tm、Tb及Eu离子掺杂的铝酸钡(BaAl₂O₄)发光膜,研究了合成条件对其结构和发光特性的影响.在退火温度达到700℃时,生成了BaAl₂O₄膜.Tm³⁺和Tb³⁺掺杂的BaAl₂O₄膜分别发蓝光和绿光,而Eu³⁺掺杂的BaAl₂O₄膜的发光既有Eu²⁺的特征发射——宽的蓝光发射带,也有Eu³⁺的特征发射——窄的红光发射峰.BaAl₂O₄:Tm³⁺的发射主峰位于462nm,在电压为5kV和电流密度为57μA/cm²的条件下,其阴极射线发光(CL)亮度可达25cd/m²,效率可达0.11lm/W.BaAl₂O₄:Tb³⁺的发射主峰位于549nm,在相同的条件下,其阴极射线发光亮度可达120cd/m²,效率可达0.55lm/W.BaAl₂O₄:Eu³⁺的发射主峰位于616nm,其阴极射线发光亮度为50cd/m²,效率为0.23lm/W.BaAl₂O₄:Eu²⁺发蓝光,峰值位于452nm,其阴极射线发光亮度为640cd/m²,效率为2.93lm/W.     

Effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition

The effect of high-temperature annealing on AlN thin film grown by metalorganic chemical vapor deposition was investigated using atomic force microscopy, Raman spectroscopy, and deep ultra-violet photoluminescence(PL) with the excitation wavelength as short as ～ 177 nm. Annealing experiments were carried out in either N2 or vacuum atmosphere with the annealing temperature ranging from 1200℃ to 1600℃. It is found that surface roughness reduced and compressive strain increased with the annealing temperature increasing in both annealing atmospheres. As to optical properties,a band-edge emission peak at 6.036 eV and a very broad emission band peaking at about 4.7 eV were observed in the photoluminescence spectrum of the as-grown sample. After annealing, the intensity of the band-edge emission peak varied with the annealing temperature and atmosphere. It is also found that a much stronger emission band ranging from 2.5 eV to 4.2 eV is superimposed on the original spectra by annealing in either N2 or vacuum atmosphere. We attribute these deep-level emission peaks to the VAL–ONcomplex in the AlN material.     

EPR characterization of Mn impurity ions in PbWO4 single crystals

The electron paramagnetic resonance (EPR) properties of the Mn²⁺ ions in PbWO₄ single crystals grown by the Czochralski method have been investigated in the X-band microwave frequency, at T=20 K. The angular dependence of the EPR line positions obtained by rotating the magnetic field in the main crystallographic planes shows that the local symmetry at the Mn²⁺ impurity ions is tetragonal, strongly suggesting that the Mn²⁺ ions substitute for the Pb²⁺ lattice cations, without charge compensation. The resulting spin Hamiltonian parameters compare well with the corresponding values for the Mn²⁺ ions in other isomorphous tungstates. The observed strong angular variation of the EPR linewidth has been quantitatively described considering a random distribution of lattice strains.     

Co,Zn共掺铌酸锂电子结构和吸收光谱的第一性原理研究

利用基于密度泛函的第一性原理的计算方法,研究了Co单掺及Co和Zn共掺LiNbO_3晶体的电子结构和吸收光谱.研究显示,各掺杂体系铌酸锂晶体的带隙均较纯铌酸锂晶体变窄. Co:LiNbO_3晶体禁带宽度为3.32 eV; Co:Zn:LiNbO_3晶体, Zn的浓度低于阈值或达到阈值时,禁带宽度分别为2.87或2.75 eV. Co:LiNbO_3晶体在可见-近红外光波段2.40, 1.58, 1.10 eV处形成吸收峰,这些峰归结于Co 3d分裂轨道的跃迁;加入抗光折变离子Zn~(2+),在1.58, 1.10 eV处的吸收峰增强,可以认为Zn~(2+)与Co~(2+)之间存在电荷转移,使e_g轨道电子减少,但并不影响t_(2g)轨道电子.结果表明,晶体中的Co离子在不同共掺离子下可充当深能级中心(2.40 eV),或可充当浅能级中心(1.58 eV),两种情况下,掺入近阈值的Zn离子均有助于实现优化存储.     

Ba2SiO4:Ce3+,Mn2+荧光粉的制备与光谱性质

用高温固相反应法合成了Ba₂SiO₄:xCe³⁺,yMn²⁺(x=0~0.2, y=0~0.15)荧光粉,研究了荧光粉的晶体结构和发光性质。在紫外光激发下,Ba₂SiO₄:xCe³⁺的发射光谱为位于384 nm附近的宽带。Ba₂SiO₄:Mn²⁺样品的发射光谱位于376 nm的宽带较强,红光发射极弱。在Ce³⁺和Mn²⁺共掺的Ba₂SiO₄:xCe³⁺,yMn²⁺样品中,位于606 nm附近的红光发射较强,来源于Mn²⁺的⁴T₁(⁴G)-⁶A₁(⁶S)跃迁。这说明Ce³⁺离子将部分能量传递给了Mn²⁺离子,有效地敏化了Mn²⁺离子的发光。当Ce³⁺的摩尔分数为0.2、Mn²⁺的摩尔分数为0.075时,Ba₂SiO₄:xCe³⁺,yMn²⁺荧光粉位于606 nm的Mn²⁺的发射峰最强。     

Spectroscopic properties of ZnWO4 single crystal doped with Fe and Li impurities

The influence of Fe and Li co-doping on optical properties of ZnWO₄ crystals have been investigated by optical spectroscopy and electron paramagnetic resonance (EPR). The iron ions were added to the ZnWO₄ composition in the form of Fe⁰, FeO or Fe₂O₃. EPR has been used for determination of the Fe³⁺ concentration. The luminescence and excitation spectra obtained at different temperatures are presented. Under excitation 325 nm, two bands were observed: blue-green emission peaked at 490 nm and red peaked at 650 nm. The blue- green emission is related to Fe³⁺ and their shape depends on the doping method. The red emission is not related to the Fe³⁺.     

Scintillation properties of Yb-doped garnet crystals

Luminescence properties of heavily Yb-doped Lu₃Al₅O₁₂ (Yb:LuAG) and Lu₃Ga₅O₁₂ (Yb:LGG) single crystals grown by micro-pulling-down method were characterized. Charge transfer luminescence of Yb³⁺ was observed in both crystals. Photoluminescence spectra, temperature dependence of emission intensity and decay kinetics of these crystals were studied. Mean decay time of about 24 ns at 90 K (Yb5%:LuAG) and 21 ns at 110 K (Yb5%:LuGG) were observed. Strong thermal quenching at room temperature was measured for both Yb:LuAG and Yb:LGG.     

Impurity levels of iron-group ions in TiO2(II)

Photocurrent measurements on oxidized, single-crystal TiO₂ undoped and doped with V, Cr, Mn or Fe were made both before and after exposure to white light from a tungsten lamp; they revealed a band of states centered at 2.0 eV below the conduction-band edge E_c that appears to be the band of surface states active in the photoelectrolysis of water. They also revealed a band of bulk states extending from near the valence-band edge E_v to about 1.9 eV below E_c. The ESR signals from V⁴⁺, Cr³⁺ and Fe³⁺ ions in oxidized samples and from Mn³⁺ ions in reduced samples were measured before and after exposure to white and monochromatic light; changes in the ESR signal intensities allow placement of the V⁴⁺: d¹ level about 2.1 eV below E_c, the Cr³⁺: d³ level about 2.7 eV belo E_c, the Fe³⁺ level at or just below the valence band edge E_v which is 3.0 eV below E_c, the Fe²⁺ level approximately at E_c, the Mn³⁺ level about 1.9 eV below E_c. Comparison of experiments on oxidized and reduced samples suggests that the bulk states are associated with cation vacancies, and measurements with polarized light gave anisotropies that are qualitatively interpretable with a cation-vacancy model for the oxidized samples.     

Luminescence of defect centers in yttrium–aluminum garnet crystals

Spectral–kinetic properties of the 2.38 eV emission in neutron-irradiated and as-received yttrium aluminum garnet (Y₃Al₅O₁₂) crystals are investigated. This luminescence band as well as its lifetime and the temperature dependence are analyzed. It is suggested that the 2.38 eV luminescence band is due to the strongly allowed transition of the F⁺-center in garnet crystals. Possible reasons that may explain the observed temperature dependence of this emission intensity are discussed.     

2018年5期 电子期刊

研究了不同Mn/Pb量比的Mn掺杂CsPbCl₃（Mn：CsPbCl₃）钙钛矿量子点的发光性质。Mn/Pb的量比增加引起的Mn²⁺发光峰的红移,被认为是来源于高浓度Mn²⁺掺杂下的Mn²⁺-Mn²⁺对。进一步研究了Mn：CsPbCl₃量子点的发光效率与Mn/Pb的量比之间的关系,发现随着量比达到5：1时,其发光效率明显下降。这种发光效率下降是由于Mn掺杂浓度引起的发光猝灭。Mn：CsPbCl₃量子点的变温发光光谱证实,随着温度的升高,Mn离子发光峰蓝移,线宽加宽,但其发光强度明显增加。