No abstract

Caesium iodide is one of the more extensively studied scintillators. Here we present X-ray luminescence spectra, scintillation light output and decay curves as function of temperature, from room temperature down to below 10 K. Features of the observed intrinsic luminescence are explained in terms of radiative recombination of on-and off-centre STE. A model permitting interpretation of the dynamics of luminescence changes in CsI with temperature is suggested. This model includes adiabatic potential energy surfaces (APES) associated with singlet and triplet states of self-trapped excitons (STE) and explains the variation of the luminescence spectra with temperature as a result of re-distribution in the population between on-and off-centre STE. The temperature dependence of the scintillation light yield is discussed in the framework of the Onsager mechanism.

The importance of luminescent tungstates and molybdates in several technological applications motivated the study of the structural, luminescence and scintillation properties of the MgWO4–MgMoO4 system. X-ray diffraction studies allowed the identification of three main types of structures in the pseudo-binary MgWO4–MgMoO4 system (sanmartinite β-MgMoO4, cuprosheelite α-MgMoO4, and wolframite MgWO4) and the refinement of the parameters of the crystal lattice. It is found that the single-phase solid solution MgMo1−xWxO4 with a β-MgMoO4 structure is created only at x<0.10, while for a higher tungsten content a mixture of different phases is formed. The x-ray luminescence spectra of a series of samples of the MgWO4–MgMoO4 system are measured at T = 8 K. The principal emission bands are assigned to the main structural phases as follows: β-MgMoO4, 520 nm; α-MgMoO4, 590 nm; MgWO4 (wolframite), 480 nm. The phase composition of the sample determines the actual shape of the observed spectra. Possible relations between the crystal structure and luminescence properties of different phases are discussed in terms of a configuration coordinate model. Of all the compounds under test, MgWO4 is found to have the best scintillation response for particle excitation (0.90 ± 0.15 that of ZnWO4 at T = 295 K). Further, the light yield also remains high with decreasing temperature, which makes this material potentially useful for cryogenic applications.

The variation of luminescence and excitation spectra of titanium doped Al 2 O 3 for the concentration of Ti ranging from 10 to 1000 ppm was investigated using synchrotron radiation. In the lightly doped Al 2 O 3 -Ti (<100 ppm) samples we identified several emission bands. These are the emission of the excitons localized at Ti (290 nm), the emission due to F þ centers (325 nm), the band around 420 nm traditionally attributed to F center emission, and the luminescence of Ti 3þ ions at 720 nm. The emphasis in this study is on the clarification of the nature of the blue emission band in the samples with high concentration of Ti (!100 ppm), where the luminescence and excitation spectra of the blue emission exhibit noticeable variability. This is explained by a model of the luminescence process of Ti 4þ -F centers that includes the photoionization of Ti 3þ , the subsequent capture of electrons at F þ -centers, formation of excited F-centers and, finally, the emission of a blue photon. The quenching of the blue emission with increasing Ti concentration is interpreted in terms of competition between oxygen vacancies and Ti 4þ centers in the capture of the electron.

The data concerning the light‐emitting diodes based on ZnO homojunction with ITO/n ‐ZnO/p ‐ZnO+photoresist/Au/Ti and ITO/n ‐ZnO/p ‐ZnO+pho‐toresist+PEDOT:PSS/Au/Ti structures are reported. The p ‐ZnO nanowires were grown by electrochemical deposition method. It is suggested that the p ‐type conductivity first of all is connected with the zinc vacancies VZn. The clear rectifying behavior was observed on the current‐voltage curve of the ZnO p ‐n homojunction. The broad electroluminescence (EL) emission bands in the blue, green and red regions lead to observation of the white light by a naked eye under forward bias. The intrinsic defects such as Zni, VZn, Vo and Oi would be responsible for occurrence of these bands. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)