Recombination Luminescence Research Articles

Recombination luminescence in Li2B4O7 doped with manganese and copper

Non-irradiated and irradiated ceramics of Li2B4O7:Mn, Li2B4O7:Mn,Be, and Li2B4O7:Mn,Cu were studied using electron paramagnetic resonance and luminescence spectroscopy. The emission of Mn2+ centres is observed in the thermoluminescence spectra of the low-temperature and dosimetric high-temperature glow peaks in irradiated samples. Arguments are given in favour of hole mobility being responsible for the dosimetric thermoluminescence peak at 490 K in Li2B4O7:Mn. The co-doping of Li2B4O7:Mn with Cu+ is shown to increase the sensitivity of the material to ionizing radiation.

Intra-center and recombination luminescence of bismuth defects in fused and unfused amorphous silica fabricated by SPCVD

Photoluminescence (PL) of bismuth doped silicon dioxide excited by UV excimer lasers (ArF — 193nm, KrF — 248nm) and a green light laser diode (532nm) is studied in a wide spectral band at temperatures ranging from 12 to 750K. Two types of samples are investigated: unfused, 100μm in thickness amorphous layer immediately deposited on the inner surface of silica substrate tube, and the same material after profusion resulted from tube collapsing to a rod by external heating. PL bands centered at 620–650nm, 820nm and 1400nm wavelengths are observed in both fused and unfused samples. Under excitation by the green laser diode decay time constants for 650nm (orange) and 1400nm (NIR) PL bands measured at room temperature amount 3μs and 600μs respectively. These rather long decay times point to partly forbidden intra-center electron transitions. PL intensities of the orange and NIR bands are not temperature dependent within 12–450K range. At higher temperatures the orange band manifests an intra-center thermal quenching, activation energy and frequency factor being 0.42±0.04eV and 5·109s−1 respectively, while intensity of the NIR band weakly depends on temperature up to 700K. Electron-hole recombination excitation mechanism is found to contribute to the orange, but not to the NIR PL band under UV laser pumping. The specific feature of orange PL excited by recombination is the magnitude of decay time constant being about milliseconds at temperature of 12K and decreasing with the temperature increase. Localized state ionization and two-photon absorption of intense UV light excite electron-hole pairs in silica host. Some bismuth defects serve as traps for the electrons, while holes transmute into self-trapped state thus generating self-trapped holes (STHs). Thermally activated escape of the STHs followed by their subsequent recombination with trapped electrons forms the mechanism to transfer the excitation to particular bismuth defects responsible for orange PL. At the same time no signature of the impact of such recombination process on the excitation of NIR PL is observed. This permits one to conclude that the nature of bismuth defects responsible for these two PL bands varies, and these two types of defects are available in both fused and infused silicon dioxides.

高激发功率密度与真空条件下的反斯托克斯发光过程

通常上转换机制大体可归纳为三类：1) 基态与激发态的步进吸收过程；2) 掺杂离子间的能量传递型上转换；3) 光子雪崩型上转换，这些跃迁过程都发生在带间掺杂稀土离子的4fN~4fN组态之间。在高激发功率密度与真空条件下，稀土氧化物发生了一类特殊的上转换过程，在激发发射机制上不同于上述已知的上转换形式。光子数拟合和光电导测量结果表明这是一种带隙激发过程，激发过程始于稀土离子基态，经多光子过程到达导带；电子由导带的退激发过程导致了起源于电荷迁移态、带间能级辐射跃迁、激子复合等多种发光过程，加上光致黑体辐射形成了具有多种发光起源的混合反斯托克斯发射，在光谱上表现为宽带连续强发射。因此发光机制是掺杂离子(激发态过程)与基质(带隙激发)体系的整体协同激发发射过程，分立发光与复合发光两类不同的发光机制中的各种形式，都参与了这种特殊的上转换激发发射过程。激发功率密度、激活离子掺杂浓度、样品所处环境的真空度以及基质带隙宽度，是实现这种近红外激发下的反斯托克斯发射的关键实验条件。在功率密度为50 W/cm2的1 W 980 nm近红外激发下，能量转化率可达10%以上，亮度可达100,000 cd/m2，流明效率达到15 lm/W，色度可通过对真空度、激发功率密度及掺杂浓度等实验参数来连续调配。这类特殊的反斯托克斯过程不仅在机制分析和新掺杂体系尝试两方面有大量的理论和实验工作值得深入挖掘拓展，在应用方面还有开发特种需求的高亮度白光点光源，非接触型气压传感器的潜在价值。 Usually, the mechanisms of upconversion are mainly based on excitated state absorption, energy transfer upconversion and photon avalanche, where 4fN transitions are included in bandgap of host. Here, we present a special upconversion of lanthanide oxide under high power density excitation in vacuum that differs from three known mecha-nisms. Upconversion intensity-power fitting and photon conductance measuring demanstrate it is a bandgap excitation process from rare earth ion ground state to conduction band through a multi-photon process. The following deexcitation process from conduction band leads to various luminescence processes that arise from charge transfer band, radiative energy level transitions, exciton recombination and so on, together with thermal blackbody emission, forming a strong continue anti-stokes emission. Therefore this process is a collective interaction of the whole system for doping ion and substrate, and has luminescence mechanisms for both discrete luminescence centre and recombination of electrons and holes. Excitation power density, concentration of activated ion, pressure and bandgap of substrate are key experiment conditions to fulfill this anti-Stokes emission. Under 1 W excitation power with 50 W/cm2 power density of 980 nm NIR, it can achieve more than 10% energy transfer efficiency, 100,000 cd/m2 illumination, and 15 lm/W luminous effi-ciency and its chrominance can be continually tuned by pressure, power density and doping concentration. This kind of special anti-Stokes is not only worth investigating its mechanism and trying various systems in both theory and experi-ment, but also has potential applications in high luminosity point white light source, non-contact pressure meter and so on.

Specific features of the radiative relaxation of excited states of polymethine dyes of different ionicity in films of photoconductive and nonphotoconductive polymers

The spectral and luminescent properties of film composites based on photoconductive poly-N-epoxipropylcarbazole and nonphotoconductive polyvinylbutyral with admixtures of cationic and anionic polymethine dyes, as well as the effect of an external magnetic field on these properties, are studied. It is found that the magnetic field affects the intensity and kinetics of the delayed fluorescence and recombination luminescence of the cationic dye in photoconductive films. This is explained by specific features of photogeneration of charge pairs, namely, by the participation of the singlet and triplet excited states of dye molecules in this process, as well as by the singlet-triplet conversion in dye molecules and photogenerated charge pairs.

Continuous wave OSL in bulk AlN single crystals

AbstractThe kinetics of recombination luminescence of β‐irradiated AlN single crystals has been studied with continuous wave optically stimulated luminescence (CW‐OSL) method. It is shown that the OSL process is characterized by two exponential components with a decay time τ = 32 and 212 s. Photoionization cross‐sections σ = 4.8·10‐19 and 7.3·10‐20 cm2 have been evaluated, assuming each component is related to electron traps based on VN‐ and/or ON‐centers at various charged states. It is established that dose dependences of the OSL response of the bulk crystals under study are linear with taking into correct accounting for the radiation induced afterglow and hence can be used for quantitative estimates in solid state dosimetry. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Cathodoluminescence and Spatial Variation in Mobility-Lifetime <formula formulatype="inline"><tex Notation="TeX">$(\mu \tau)$</tex></formula> Product in Bulk Doped Thallium Bromide

Nonuniformity in charge transport properties is a limiting factor in energy resolution of radiation detectors. In this paper, we investigate variations in the low temperature ambipolar diffusion length and the mobility-lifetime (μτ) product in bulk doped TlBr using cathodoluminescence (CL) and transport imaging. One TlBr crystal was doped with sodium (Na), aluminum (Al), and silver (Ag). A second TlBr crystal was doped with copper (Cu), iron (Fe), and zinc (Zn). We report the first low temperature high resolution CL spectroscopy and mapping in bulk doped TlBr, showing spatial variation in recombination luminescence on a scale of ~10 μm. Transport imaging is applied to quantify these variations in TlBr at 5 K. Ambipolar diffusion lengths and μτ products, dominated by the transport of holes, are mapped across a 40 μm segment of TlBr at a resolution of 2 μm. Ambipolar diffusion lengths are found to vary between 4.6 μm and 11.2 μm, on a spatial scale comparable to the variation observed in the CL map.

Publisher's Note: Radiative exciton recombination and defect luminescence observed in single silicon nanocrystals [Phys. Rev. B86, 125302 (2012)

Publisher's Note: Radiative exciton recombination and defect luminescence observed in single silicon nanocrystals [Phys. Rev. B<b>86</b>, 125302 (2012)

Registration of radical anions of Al, Ga, In tris-8-oxyquinolinates by magnetosensitive and spectrally resolved recombination luminescence in benzene solutions

Formation and recombination of transient radical anions of Al, Ga, and In tris-8-oxyquinolinates (Alq3, Gaq3, and Inq3), the key species in certain organic electroluminescing devices, was studied in X-irradiated benzene solution. The spectra of recombination luminescence were obtained and effective yields of recombination luminescence referred to Alq3 were estimated as 0.3 (Gaq3) and 0.15 (Inq3). Magnetic field effects in recombination luminescence involving the radical anions of the three oxyquinolinates were obtained, from which hyperfine coupling constants at the central ions were estimated as 0.45 (IAl=5/2), 0.8 (IGa=3/2), and 0.6 (IIn=9/2)mT for radical anions of Alq3, Gaq3, and Inq3, respectively.

Radiative exciton recombination and defect luminescence observed in single silicon nanocrystals

The role of quantum confinement (QC) and surface-related defect centers (DCs) on the photoluminescence (PL) of individual silicon nanocrystals (Si NCs) is investigated using confocal microscopy with radially and azimuthally polarized laser beams. It is shown that the multiple-peak PL spectra of single Si NCs revealing SiO${}_{2}$ phonon side bands are associated with a linear transition dipole moment (TDM) and a short lifetime of 4 ns, indicating that the PL originates from defect centers. In a new study applied to free-standing single Si NCs obtained by Si cluster beam deposition, we could discriminate between PL centers with one- and more-dimensional TDM. Nanoparticles revealing a three-dimensional TDM are characterized by a significantly weaker and structureless Gaussian PL band at larger wavelengths and by $\ensuremath{\mu}$s lifetimes. These PL features are attributed to the radiative recombination of quantum-confined excitons generated in the Si cores. Within the same sample, we found coexistence of Si NCs exhibiting QC PL and others showing DC PL.

Recombination luminescence of CaSO4:Tb3+ and CaSO4:Gd3+phosphors

Abstract A comparative study of the excitation of luminescence by VUV radiation as well as of thermally and photostimulated luminescence has been carried out for CaSO4:Tb3+ and CaSO4:Gd3+ phosphors, where Na+ or F− ions are used for charge compensation. The distinction in hole processes for the phosphors with Na+ or F− compensators is determined by the differing thermal stability of the holes localized at/near Tb3+Na+ and Gd3+Na+ (up to 100–160 K) or at/near Tb3+F− V Ca and Gd3+F− V Ca centers involving also a cation vacancy (up to 400–550 K). Tunnel luminescence in the pairs of localized electrons and holes nearby Tb3+ or Gd3+ has been detected. The mechanisms of electron-hole, hole-electron and tunnel recombination luminescence as well as a subsequent released energy transfer to RE3+ ions are considered.

Features of the ionization mechanism of radical recombination luminescence excitation

A comparison of the temperature-time cycles of radical recombination luminescence (RRL) in atomic hydrogen, based on ZnS and ZnCdS at different partial pressures, is presented. The obtained results allow us to explain the temperature dependences of chemisorption of the charged form of N+ and the optical transition rate upon RRL.

SU-E-T-62: Modeling Influence Factors of Al2O3:C Optically Stimulated Luminescence Detectors (OSLDs) Exposed to Radiotherapy Beams

Purpose: The optically stimulated luminescence(OSL) technique is becoming increasingly popular in medical dosimetry. This is partially because of the dosimetric properties of Al2O3:C, the most common commercially available OSLdetector (OSLD). Currently, there is no reference dosimetry protocol based on measurements using Al2O3:C OSLDs in radiotherapy beams. Influence factors, if not properly accountedfor, may introduce large systematic uncertainties in the dose measurements.In this work we develop a phenomenological model based on band structuretheory that can be used to investigated the role of different influence factors on the OSL response of Al2O3:C OSLDs exposed to photon beams. Methods: To describe the charge transportation processes that lead to the OSL response of Al2O3:C OSLDs, we used a band diagram model consisting of three electron traps (shallow, main and deep traps) and two luminescence recombination centers. Based on this model we established a system of differential equations that represent the transport of charge carriers (electrons and holes) during irradiation, relaxation and stimulation of the material. Then we solved this system of differential equations todetermine the role of factors that may influence the OSL response including: irradiation temperature, time elapsed since irradiation (fading), non‐linearityof the dose response, accumulated dose and resetting or bleaching method. Results: The OSL signal: 1) depends on irradiation temperature in the temperature range relevant to medical dosimetry (15 to 40°C); 2) fades as a function of time elapsed since irradiation. The fading is dose dependent; 3) has a non‐linear dose‐response, which depends on the detectedOSL emission; and 4) changes as a function of accumulated dose and resetting method. Conclusion: The proposed band diagram model may be used tounderstand the role of influence factors on Al2O3:C OSLDs, which willhelp in the development of reference dosimetry protocols based on measurements using Al2O3:C OSLDs. Natural Sciences and Engineering Research Council of Canada

Enhancement of anti-stokes sensitized luminescence in AgCl(I) crystals in the presence of silver nanoparticles

We have observed a significant increase in the intensity with anti-Stokes excitation of recombination luminescence in AgCl(I) microcrystals sensitized by methylene blue molecules in the presence of silver nanoparticles.

Recombination luminescence and trap levels in undoped and Al-doped ZnO thin films on quartz and GaSe (0 0 0 1) substrates

Photoluminescence spectra of ZnO and ZnO:Al (1.00, 2.00 and 5.00at.%) films on GaSe (0001) lamellas and amorphous quartz substrates, obtained by annealing, at 700K, of undoped and Al-doped metal films, are investigated. For all samples, the nonequilibrium charge carriers recombine by radiative band-to-band transitions with energy of 3.27eV, via recombination levels created by the monoionized oxygen atoms, forming the impurity band laying in the region 2.00−2.70eV. Al doping induces an additional recombination level at 1.13eV above the top of the valence band of ZnO films on GaSe substrates. As a result of thermal diffusion of Zn and Al into the GaSe interface layer from ZnO:Al/GaSe heterojunction, electron trap levels located at 0.22eV and 0.26eV below the conduction band edge of GaSe, as well as a deep recombination level, responsible for the luminescent emission in the region 1.10−1.40eV, are created.

Multipeak-structured photoluminescence mechanisms of as-prepared and oxidized Si nanoporous pillar arrays

Silicon dominates the electronic industry, but its poor optical properties mean that it is not preferred for photonic applications. Visible photoluminescence (PL) was observed from porous Si at room temperature in 1990, but the origin of these light emissions is still not fully understood. This paper reports that an Si nanocrystal, silicon nanoporous pillar array (Si-NPA) with strong visible PL has been prepared on a Si wafer substrate by the hydrothermal etching method. After annealing in O2 atmosphere, the hydride coverage of the Si pillar internal surface is replaced by an oxide layer, which comprises of a great quantity of Si nanocrystal (nc-Si) particles and each of them are encapsulated by an Si oxide layer. Meanwhile a transition from efficient triple-peak PL bands from blue to red before annealing to strong double-peak blue PL bands after annealing is observed. Comparison of the structural, absorption and luminescence characteristics of the as-prepared and oxidized samples provides evidence for two competitive transition processes, the band-to-band recombination of the quantum confinement effect of nc-Si and the radiative recombination of excitons from the luminescent centres located at the surface of nc-Si units or in the Si oxide layers that cover the nc-Si units because of the different oxidation degrees. The sizes of nc-Si and the quality of the Si oxide surface are two major factors affecting two competitive processes. The smaller the size of nc-Si is and the stronger the oxidation degree of Si oxide layer is, the more beneficial for the luminescent centre recombination process to surpass the quantum confinement process is. The clarification on the origin of the photons may be important for the Si nanoporous pillar array to control both the PL band positions and the relative intensities according to future device requirements and further fabrication of optoelectronic nanodevices.

Synchrotron and laser excitation of luminescence in PbWO4:Tb crystals at different temperatures

The effect of temperature on the spectral luminescence characteristics of PbWO4:Tb3+ crystals with synchrotron and laser excitation is studied. If PbWO4:Tb3+ is excited by synchrotron radiation with λ = 88 nm at 300 K, a faint recombination luminescence of the impurity terbium is observed against the matrix luminescence. When the temperature is reduced to 8 K, the luminescence intensity of PbWO4:Tb3+ increases by roughly an order of magnitude and the characteristic luminescence of the unactivated crystal is observed. Excitation of PbWO4:Tb3+ by a nitrogen laser at 300 K leads to the appearance of emission from Tb3+ ions. At 90 K, a faint matrix luminescence is observed in addition to the activator emission. The formation of the luminescence excitation spectra for wavelengths of 60–320 nm is analyzed and the nature of the emission bands is discussed.

Electrochemiluminescent detection of labile radical intermediates of electrochemical reactions

The article considers the basic principles of electrochemiluminescent spin traps diagnostic method which consists of efficient trapping of intermediate labile radical products (free radicals of complex organic molecules) by electrogenerated radical ions of electrochemiluminescers in different liquid-phase systems including bioobjects. The method permits qualitative and quantitative identification of such particles by measurement of electrolysis radical recombination luminescence (electrogenerated chemiluminescence) intensity which is emitted by electrolyzed solutions of organic electrochemiluminescers. The unique possibilities of the method are illustrated on examples of experimental study of systems with sodium tetraphenylborate and chloride ion.

Recombination luminescence of poly(arylene phthalide) films induced by visible light

Long-term afterglow emission (AG) of poly(arylene phthalide) films (PAP) irradiated with visible light has been discovered. Recombination nature of the AG was experimentally proven as in the case of UV irradiation. Features of the observed AG spectra were analyzed in comparison with the photo-, electro-, and thermoluminescence spectra of the PAP films in relation to the commonness of the AG generation mechanism, which is associated with the involvement of the two-photon reactions and triplet states in the photoprocesses in PAP.

Luminescence of dense, octahedral structured crystalline silicon dioxide (stishovite)

It is obtained that, as grown, non-irradiated stishovite single crystals possess a luminescence center. Three excimer pulsed lasers (KrF, 248 nm; ArF, 193 nm; F 2, 157 nm) were used for photoluminescence (PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.7±0.1 eV with FWHM equal to 0.95±0.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum at 3±0.2 eV with FWHM equal to 0.8±0.2 eV, is seen under all three lasers. The UV band main fast component of decay is with time constant τ=1.2±0.1 ns for the range of temperatures 16–150 K. The blue band decay possesses fast and slow components. The fast component of the blue band decay is about 1.2 ns. The slow component of the blue band well corresponds to exponent with time constant equal to 17±1 μs within the temperature range 16–200 K. deviations from exponential decay were observed as well and explained by influence of nearest interstitial OH groups on the luminescence center. The UV band was not detected for F 2 laser excitation. For the case of KrF laser only a structure less tail up to 4.6 eV was detected. Both the UV and the blue bands were also found in recombination process with two components having characteristic time about 1 and 60 μs. For blue band recombination luminescence decay is lasting to ms range of time with power law decay ∼ t −1. For the case of X-ray excitation the luminescence intensity exhibits strong drop down above 100 K. such an effect does not take place in the case of photoexcitation with lasers. The activation energies for both cases are different as well. Average value of that is 0.03±0.01 eV for the case of X-ray luminescence and it is 0.15±0.05 eV for the case of PL. So, the processes of thermal quenching are different for these kinds of excitation and, probably, are related to interaction of the luminescence center with OH groups. Stishovite crystal irradiated with pulses of electron beam (270 kV, 200 A, 10 ns) demonstrates a decrease of luminescence intensity excited with X-ray. So, irradiation with electron beam shows on destruction of luminescent defects. The nature of luminescence excited in the transparency range of stishovite is ascribed to a defect existing in the crystal after growth. Similarity of the stishovite luminescence with that of oxygen deficient silica glass and induced by radiation luminescence of α-quartz crystal presumes similar nature of centers in those materials.

Luminescence centers in Y3Al5O12:La single crystals

The nature of luminescence centers created by La3+ isoelectronic impurities (IIs) in Y3Al5O12 (YAG) crystals has been considered. The La3+ dopant in YAG crystals gives rise to intensive UV luminescence in the complex band peaked at 4.28 eV at 300 K with a decay time of the main component of 575 ns. It has been shown that the complex UV emission band in YAG:La crystals in the 10–300 K range is a superposition of the luminescence of excitons localized near La dopants (LE(La) centers), the recombination luminescence of La3+ ions in the positions of Y3+ cations (LaY centers) and the low-intensive recombination luminescence of YAl antisite defects in the bands peaked at 4.65–4.62, 4.29–4.25 and 3.86–3.84 eV, respectively. In the visible range the emission spectra of YAG:La crystals also consist of the luminescence of F+ and F-centers in the bands peaked at 3.1 and 2.69 eV with a decay time of the main components of 2.3 ns and in the μs-ms range, respectively. It has been shown that the luminescence of F+ and F-centers in YAG:La crystals can be excited by emission of LE(LaY) and LaY centers.