Excitation Of Luminescence Centers Research Articles

Luminescent Properties of Activated Nanocomposites Based on Core–Shell Nanoparticles

Formulas for the rates of spontaneous radiative decay of excitation of luminescent centers located in subwavelength spherical core–shell nanoparticles and in the nanocomposite matrix are derived. Effective medium models and local field problems concerning the optical properties of activated nanocomposites based on core–shell nanoparticles are discussed. A detailed analysis of two modern approaches to the problem of spontaneous transitions in luminescent centers located in homogeneous spherical nanoparticles is made.

Energy transport and scintillation of cerium-doped elpasolite Cs2LiYCl6: Hybrid density functional calculations

Elpasolites are a large family of halides which have recently attracted considerable interest for their potential applications in room-temperature radiation detection. Cs2LiYCl6 is one of the most widely studied elpasolite scintillators. In this paper, we will show hybrid density functional calculations on electronic structure, energetics of small electron and hole polarons and self-trapped excitons, and the excitation of luminescence centers (Ce impurities) in Cs2LiYCl6. The results provide important understanding in energy transport and scintillation mechanisms in Cs2LiYCl6 and rare-earth elpasolites in general.

Electron-excited luminescence of SiO2 layers on silicon

An analysis of cathodoluminescence and electroluminescence spectra of Si-SiO2 structures suggests a conclusion concerning the processes involved in excitation of the luminescence centers generated in the UV spectral region and their localization. The electroluminescence observed in this region of the spectrum is generated in excitation of luminescence centers localized in the immediate vicinity of the Si-SiO2 phase boundary. In the case of cathodoluminescence, the observed emission bands at ∼4.3 and ∼2.7 eV appear in excitation of the luminescence of silylene centers at the Si-SiO2 phase boundary.

Electroluminescence mechanisms in SiO x N y (Si) nanocomposite films

With a view to creating the Si LED, the mechanisms of electroluminescence (EL) in SiOxNy(Si) nanocomposite films with Si nanocrystals embedded in the SiOxNy matrix are studied experimentally and theoretically. The most important results are obtained from a Au/SiOxNySi)/p-Si structure having a semitransparent electrode, the oxynitride film containing Si nanocrystals with a mean size of 3–5 nm and a concentration of ∼1018 cm−3; the measurements are made on a reverse-biased structure (substrate potential negative). Room-temperature EL is observed in the visible and IR ranges; the respective peaks are located at wavelengths of 600–700 and about 1200 nm. The study examines current-voltage characteristics of the structure and the dependence of integrated EL intensity on current, voltage, film thickness, the type of substrate conductivity, etc. The following conclusions are drawn from the experimental and theoretical results: The IR branch is mainly associated with carrier heating, avalanche ionization, and formation of light-emitting microplasmas near the substrate-film interface. The visible branch is linked to (i) hot-electron injection from the substrate into the film and (ii) impact excitation of luminescent centers at nanocrystal-matrix interfaces.

Electroluminescence kinetics of film structures based on manganese-doped zinc sulfide

A theoretical and experimental study is made of the growth and decay constants of the brightness as functions of the rise time and amplitude of the pulse from electroluminescent structures based on manganese-doped zinc sulfide and deposited on smooth and rough substrates, and as functions of the rise time and amplitude of the ramped (linearly rising) exciting voltage. The curves obtained are used to determine a variety of parameters and characteristics of the electroluminescence process: the lifetime of the excited luminescence centers, the excitation and relaxation probabilities of luminescence centers per unit time, and the cross section of impact excitation of luminescence centers and their dependences on the rise time and amplitude of the linearly rising exciting voltage. Explanations are given for the fact that the indicated characteristics show different behavior for structures on smooth and rough substrates.

Luminescence excitation assisted by phonons in porous silicon

The high-energy tail of the luminescence spectrum of porous Si excited with low photon energies has been observed to extend beyond the excitation energy. The temperature dependence of the luminescence intensity strongly depends on the excitation energy. These phenomena are explained in terms of phonon-assisted excitation of luminescence centers with strong phonon coupling.

Electronic-vibrational spectra of neodymium activated crystals

17. H. Sobue, T. Uryu, K. Matsuzaki, and J. Tabata, "Stereoregularity of polymethacrylo- nitrile," J. Polym. Sci., l, No. 8, Part B, 409-411 (1963). 18. H. Herma, V. Groebe, and R. Schmolke, "Characterization of copolymers. 2. Composition and sequential length distribution of acrylonitrile--methacrylonitrile copolymers," Faserforsch. Textiltechnik, 117, No. 2, 56-60 (1966). ELECTRONIC-VIBRATIONAL SPECTRA OF NEODYMIUM ACTIVATED CRYSTALS V. F. Zolin, V. M. Markushev, V. I. Tsaryuk, and Ch. M. Briskina UDC 535.34:541.65 Electron vibrational (EV) spectra of neodymium (Nd ~+) compounds [I, 2] are interesting from different points of view. They could be useful in connection with investigations of the structure of luminescence centers [3-5]. The presence of EV bands can restrict the applica- bility of selective laser excitation spectroscopy in studying luminescence centers in dis- ordered media (for example, glasses) [6-8]. EV bands can make a considerable contribution to the overlap integrals of the luminescence spectra of donors and the absorption spectra of ac- ceptors, whose magnitude determines the probability of transferring exGitation energy in the luminescence quenching and sensitization processes, as well as with migration and cross- relaxation [9, 10]. Finally, the probabilities of multiphonon transitions, determining the rate of relaxation of excitation of luminescence centers in crystals and glasses activated by lanthanides, depends on the magnitude of EV interaction and the phonon density of states, which can be estimated from the EV spectra [11-13]. The EV spectra of europium and terbium compounds were studied in [4, 5, 14]. The inter- pretation of these spectra was facilitated by the relative simplicity of the Stark structure of the energy levels of the Eu 3+ ion and the similarity of the spectral properties of Eu ~+ and Tb s+. In this paper we present the results of investigations of EV s~ectra of analogous neodymium compounds, for which the EV interaction is stronger than for Eu + and Tb 3+. The use of laser excitation permitted observing easily the EV bands in the luminescence spectra of neo- dymium. The magnitudes of the EV interaction for the Nd s+ ion were estimated from the ratios of the integral intensities of the single-phonon and phononless bands (Io~/Ioo) [12]. Technique and Objects of Investigation. We investigated the spectra of materials used in laser technology: spectra of neodymium pentaphosphate NdPsO~, neodymium tetraphosphates and tetraphosphates of alkali metals MeNdP~O~ (Me = Na, K, Rh, Cs), binary molybdates with composition Na,La~-mNdm(MoO~)~ (m = l, 0.9, 0.25) and BaNda-mGdm(MoO~)~ (m = O, 1,8), binary tungstates Me~Lao.,Ndo.~(WO~)~ (Me = Na, Cs), calciumtungstates CaWOd, and garnets Y3AI2(A104)3 (YAG), Gd,Gaa(GaO~), (GGG), activated with 1% neodymium. Absorption spectra in the region 24,100-14,200 cm -~ were recorded photographically using an ISP-51 spectrograph with a long focal length UF-90 chamber and photoelectrically usin~ a DFS-12 spectrometer and F~U-79. The luminescence spectra in the region 9600-ii,700 cm , excited with a Rhodamine 6G tunable dye laser, were recorded by an MDR-23 monochromator using F~U-62 and a PAR 160 integrator. It is convenient to observe the single-phonon EV satellites, related to internal vibra- tions of ligands, in the absorption spectra of neodymium between bands of the electronic transitions ~I,/2--~G,/2 and ~I,/2--~G,/a, ~G5/2 and in the luminescence spectra between bands of the electronic transitions ~F,/a--~I,/~ and ~F~/a--~I~/2. EV satellites, generated by the transitions dIg/2--dGT/2~ 2G9/2, 2K~,/, and ~I,/a--~F,/,, ~S,/,, are also clearly evident. To separate the EV satellites and to identify them, we used the following: temperature dependence of the line intensities, comparison of EV spectra of different compounds with one another, as well as comparison with IR spectra and Raman scattering spectra, In a number of cases, it is useful to compare with EV spectra corresponding =o europium and terbium com- pounds. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 38, No. 4, pp. 614-620, April, 1983. Original article submitted February i, 1982. 448 0021-9037/83/3804-0448507.50 9 1983 Plenum Publishing Corporation

Cathodoluminescence of thorium in the presence of O 2, CO and gas mixtures of CO-O 2 and CO-H 2

The cathodoluminescence (2–6 keV incident electrons) observed from thorium (111) and (533) crystal faces was recorded and analyzed for surfaces produced under various conditions. The blue luminescence observed in the presence of a partial oxygen pressure ∼ 133 μPa (∼10 -6 Torr) was found to consist of a broad asymmetric major band that peaked around 468 nm on which weak bands or lines were superimposed at approximately 433, 489, 502, and 534 nm. The emission was almost totally extinguished in the presence of a partial CO pressure ∼ 133 μPa (∼10 -6 Torr). The thorium-oxygen cathodoluminescence (CL) is interpreted as arising from the formation of ThO 2 and the excitation of luminescence centers by the incident electron beam and their subsequent decay. The major luminescence at 468 nm arises from F centers in ThO 2. The weak bands at 433 and 534 nm may arise from surface F + and F centers designated as F + s and F s. The former may also be due to an OH luminescence center. The two longer wavelength lines (489, 502 nm) superimposed on the broad major band at approximately 468 nm are interpreted as arising from Pr 3+ impurities in the thorium lattice that gave rise to fluorescence emission. The line at 468 nm also may be due in part to the fluorescence of ThO. The cathodoluminescence spectra observed in the presence of CO, and (CO+O 2) and (CO+H 2) gas mixtures were consistent with an interpretation that O 2 in the gas phase was required in order to obtain ThO 2 on and below the surface to produce significant luminescence. Auger spectroscopy showed that exposure to CO left approximately as much oxygen on the surface as in the case of O 2 but did not produce appreciable cathodoluminescence.

Electroluminescence of CdF 2:Ho 3+/Au diodes

We studied light emitting Au/CdF 2:Ln 3+ diodes. Our results on I(V), B(V) or B(I) curves show that two different mechanisms are responsible for light emission; at low voltages tunnel injection followed by impact excitation has been found while at higher voltages (≳12V) impact ionisation of the matrix and excitation of luminescent centers by (e-,h) pairs are more likely.