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«ANNUAL REPORT Riga 2012 Annual Report 2011, Institute of Solid State Physics, University of Latvia. Editor: A.Krumins. Composed matter: A.Muratova. ...»

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3. J. Jansons. Fiziķu centieni 20. Gs. 50. – 60. Gados atgriezt fundamentālo zinātni Latvijas Universitātē. – Apvienotais pasaules latviešu zinātnieku III un Letonikas IV kongress „Zinātne, sabiedrība un nacionālā identitāte”, sekcija „Tehniskās zinātnes” tēžu krājums, RTU Izdevniecība, 2011, 135. Lpp Doctoral thesis E. Elsts “Spectroscopic Studies of Scintillator Materials: CsI:Tl, CdWO4:Mo and Tb activated oxyfluorides”

Bachelor thesis:

1. J. Bergmane „Rekristalizēta amorfa silicija plāno kārtiņu elektrofizikālās īpašības”, vad. Guntis Mārciņš.

2. A. Romanova „Rekristalizēta amorfa silicija plāno kārtiņu struktūru veidošanās likumsakarības”, vad. Guntis Mārciņš

3. I. Brice „Aktivitēti oksifluorīdi redzamās gaismas luminoforos” vad. Uldis Rogulis.

–  –  –

The electronic properties of advanced materials for scintillators, light transformers, gas sensors, radiation detectors, photocatalysis, persistent phosphors, as well as high-performance optical glass for NIR to VUV spectral range are studied by means of spectroscopic methods including time-resolved spectroscopy.

Scientific Visits Abroad

1. Dr.habil.phys.L.Grigorjeva, Estonia (4 days)

2. Dr.habil.phys. D.Millers, Poland (3 days)

3. Dr.K.Smits, France (14 days)

4. Dr.habil.phys. D.Millers, France (14 days)

5. Dr.Habil.phys. L.Grigorjeva, France (14 days)

6. Dr.Habil.phys. L.Grigorjeva, Germany (7 days)

7. Dr.Habil.phys. L.Grigorjeva, France (6 days)

8. Dr.K.Smits., The Nitherlands (5days)

9. Dr.habil. L.Skuja, Japan (59 days)

10. Dr.habil. L.Skuja, China ( 7 days)

Cooperation

Latvia SIA “Baltic Scientific Instruments (Dr.V.Gostilo, M.Shorohov)

SIA RITEC

Riga Technical University, Institute of Inorganic Chemistry (Dr.habil.sc.ing. J.Grabis, Dr.

Dz.Jankoviča) Riga Technical University, Institute of Silicate Materials ( Prof.A.Medvids, Prof.M.Knite)) Institute of Atomic Physics and Spectroscopy, University of Latvia (Prof. J.Spigulis, Dr.

A.Skudra) Estonia Institute of Physics, Tartu (Dr.S.Zazubovich) Russia GOI, St.Peterburg (Dr.L.Maksimov) Burjatia State University, (Dr.A.V.Nomoev) Kotel’nikov Institute of Radio-engineering and Electronics of RAS, Russia (Prof.,Dr.

Konstantin Golant) Poland Institute of High Pressure Physics, PAN, Warszawa, Poland (Prof.W.Lojkowski,) Institute of Low Temperatures and Structure Researchs, PAS Wroclaw (Prof.W.Strek) France CNRS Processes, Material and Solar Energy Laboratory, (PROMES), Odeillo (Dr.C.Monty) Université Jean Monnet Of Saint-Etienne (France) (Prof. Y Ouerdane).

Japan Tokyo Institute of Technology (Prof. H.Hosono, M.Hirano) Tokyo Metropolitan University (Prof. K. Kajihara) Israel Prof. A.Gedanken, Bar-Ilan University, Ramat Gan.

–  –  –

Absorption spectroscopy. FTIR absorption spectroscopy: EQUINOX 55 (10000-400 cm-1 and 22000-7000 cm-1 spectral regions) was developed for dispersed materials.

Transient absorption under electron beam excitation.

Luminescence spectroscopy. Luminescence excitation by the following sources is available :

a pulsed electron beam accelerator (10 ns, 270 keV, 1012 electrons/pulse), X rays, YAG:Nd laser (266 nm, 532 nm), nitrogen laser (337 nm), excimer lasers (248, 193 and 157 nm), deuterium and xenon lamps. Luminescence detection is performed using photomultipliers/monochromators and cooled CCD camera coupled with spectrograph. Timeresolved luminescence is detected by digital oscilloscopes, multichannel photon counters or time-correlated single-photon counting.

Vacuum ultraviolet spectroscopy: McPherson 234/302 200 mm monochromator with D2 lamp with MgF2-window serving as light source (120-250 nm). Excimer F2 pulsed laser (157 nm).

Raman and luminescence spectroscopy: Andor Shamrock303i spectrometer with Newton DU971N electron multiplying cooled CCD, NIR to UV spectral range. Hamamtsu mini spectrometer C10082CAH VIS –UV spectral range.

Energy-dispersive X-ray fluorescence microanalysis (EDAX Eagle III spectrometer, Rhodium X-ray source with microcapillary focusing lens, detected elements from Na to U, spatial resolution ~50 μm).

–  –  –

ZnWO4 powders with grain size in range 20 nm-10 μm have been synthesized by a simple combustion method and subsequent calcinations. The photocatalytic activities of powders were tested by degradation of methylene blue solution under UV light. The luminescence spectra and luminescence decay kinetics were studied and luminescence decay time dependence on powder average grain size was obtained. The correlation between self-trapped exciton luminescence decay time and photocatalytic activity of ZnWO4 powders was shown.

A model explaining the excitonic luminescence decay time correlation with photocatalytic activity was proposed.

–  –  –

Transparent Ce and Ce/Pr doped YAG ceramics were prepared under high pressures (up to 8 GPa) and relative low temperature (450oC). Grain size of the ceramics is less than 50 nm.

However unknown defects or disorder strains on grain boundaries caused the additional absorption in these ceramics. The luminescence intensity, spectra and the decay time dependence on pressure applied during ceramic preparation were studied. Concentration of some intrinsic point defect was reduced under the high pressure applied for sintering process.





It is shown that formation time of the excited state of Ce luminescence depends on the pressure applied during ceramic sintering.

UP-CONVERSION LUMINESCENCE IN ZrO2 NANOCRYSTALS

K.Smits, A.Sarakovskis, D.Millers, Dz.Jankovia, L.Grigorjeva A set of undoped and Er/Yb doped ZrO2 samples with different dopant concentrations were prepared and studied. The dopant concentration impacts to up-conversion luminescence intensity and luminescence temperature dependence. There are correlation between Er/Yb concentration and tetragonal or even structure stabilization as well as intrinsic defect concentration. The role of intrinsic defects to up-conversion luminescence characteristics was shown.

–  –  –

The time-resolved luminescence of undoped as well as europium doped (0.1 mol% - 20 mol%) ZrO2 nanocrystals obtained by sol-gel synthesis were studied. The electronic excitations in zirconia are mobile, so with the increasing of activator concentration the intrinsic defects related luminescence decreases and intensity of activator luminescence increases until it reaches saturation. It is known that the Eu acts as phase stabilizer and as well Eu3+ is used as luminescence probe. The mechanisms of excited state creation and the possible models of luminescence centers are studied.

–  –  –

Exchange between oxygen molecules embedded in amorphous SiO2 (interstitial O2 ) and oxygen atoms in the a-SiO2 network is found to be remarkably slow at 500 °C. Thermal loading of 18O2 at this temperature yields a-SiO2 containing 18O-labeled interstitial O2 whose O fraction is as high as ~90%. The 18O fraction of interstitial O2 in this sample is quickly decreased by thermal annealing at or above 700 °C because of the oxygen exchange accompanied by the release of 16O from the a-SiO2 network. This finding indicates that the oxygen exchange starts at much lower temperatures than indicated by previous works, based on monitoring of the isotopic composition of oxygen atoms in the aSiO2 network.

–  –  –

The properties of electron paramagnetic resonance (EPR) signal of oxygen dangling bonds in amorphous SiO2 ("non-bridging oxygen hole centers", NBOHC) in excimer laser-irradiated amorphous SiO2 were studied in the temperature range 20K to 295K.

NBOHCs strongly affect optical and chemical properties of amorphous SiO2 -based (nano) structures and their surfaces. The behaviour of their EPR signal is complicated due to a nearly degenerate electronic ground state. It was found that EPR signal has a nonCurie (~1/T) T-dependence down to 40K, indicating that EPR-based concentration estimates routinely obtained at T=77K underestimate the center concentrations at least by a factor of 1.7. The estimates of NBOHC concentration, based on EPR, are typically ~10 times lower than those derived from optical spectroscopy, evidently due to incomplete accounting for temperature, microwave saturation and due to degenerate ground state coupled to disorder effects. The EPR signal of NBOHCs shows a strong microwave saturation at T40K which allows for a high-sensitivity detection by 2nd-harmonic EPR registration techniques. Using it, the low intensity lowfield wing of the EPR signal was shown to extend to g values as large as g=2.4.

–  –  –

Creation of point defects by ArF (6.4 eV) and F2 laser (7.9 eV) irradiation in synthetic “wet” silica glass thermally loaded with interstitial O2 molecules was studied by optical absorption, electron paramagnetic resonance and infrared absorption. The presence of excess oxygen caused a significant increase of laser-induced ultraviolet (UV) absorption, which was 4 times (7.9 eV-irradiation) and 20 times stronger (ArF irradiation) as compared to O2 -free samples. The spectral shape of photoinduced absorption nearly completely coincided with the spectral shape of oxygen dangling bonds (NBOHC) in 3 to 6.5 eV regions. The contribution of Si dangling bonds (E' centers) was less than few % and was not dependent on oxygen content. Peroxy radical defects were not detected. The photoinduced NBOHCs thermally decayed at 400...500 C. However, a subsequent brief 7.9 eV irradiation restored their concentration up to 70%. This sensitization can be in part attributed to generation of interstitial Cl 2 and HCl. These data show that oxygen stoichiometry is an important factor for maximizing the laser toughness of wet silica.

–  –  –

Point defects strongly influence optical properties of synthetic amorphous silica (synthetic aSiO2) used in excimer laser photolithography and their properties are intensively studied.

Decomposition of an Si-O-Si bond into a pair of oxygen vacancy and interstitial oxygen species is an intrinsic defect process in a-SiO2. It is similar to the creation of vacancyinterstitial pairs in crystalline materials and is regarded as “Frenkel defect process” in an amorphous material. Oxygens are also known to be emitted from a-SiO2 surfaces under irradiation with vacuum-ultraviolet (VUV) light or electron beam. However, the anion part of the Frenkel pair in a-SiO2, interstitial oxygen atom, lacks reliable spectroscopic signatures. Therefore, Frenkel process has been studied much less than another intrinsic defect process in a-SiO2, a simple cleavage of an SiO bond, yielding a pair of silicon and oxygen dangling bonds. Interstitial oxygen molecule (O2), a common form of the interstitial oxygen species in a-SiO2, exhibits characteristic infrared photoluminescence (PL) at 1272 nm. This PL band allows interstitial O2 to be detected selectively with a high sensitivity, and is useful in studying Frenkel defect processes in both a-SiO2 and crystalline SiO2. The Frenkel process is dominant over the formation of the dangling bond pairs in highpurity synthetic a-SiO2. Both these processes are influenced by the degree of the structural disorder of a-SiO2 characterized by distribution of Si-O-Si angles. Fluorine doping promotes the structural relaxation and is useful in decreasing the concentration of “strained” Si-O-Si bonds, which have Si-O-Si bond angles widely different from the relaxed angle and are vulnerable to radiation. Moderate fluorine doping is effective in improving both UV-VUV transparency and radiation hardness, whereas heavy fluorine doping tends to enhance defect processes involving the Frenkel mechanism and to degrade the radiation hardness.

EXCHANGE BETWEEN INTERSTITIAL OXYGEN MOLECULES AND

NETWORK OXYGEN ATOMS IN AMORPHOUS SiO2 STUDIED BY 18O ISOTOPE

LABELING AND INFRARED

PHOTOLUMINESCENCE SPECTROSCOPY

K.Kajihara, T.Miura, H.Kamioka, M.Hirano, L.Skuja, H.Hosono Amorphous SiO2 (a-SiO2) thermally annealed in an oxygen atmosphere incorporates oxygen molecules (O2) in interstitial voids. When the thermal annealing is performed in 18O2 gas, interstitial 18O2 as well as interstitial 16O18O and 16O2 are formed due to the oxygen exchange with the a-SiO2 network. The infrared photoluminescence band of interstitial O2 was utilized to quantitatively analyze the oxygen exchange, taking into account the influences of common network modifiers in synthetic a-SiO2 (SiOH, SiF, and SiCl groups). The presence of network modifiers does not significantly change the average rate of 18O transfer from interstitial O2 to the a-SiO2 network and its activation energy, suggesting that the network modifiers themselves do not serve as preferential oxygen exchange sites. When the concentration of SiOH groups is low, the oxygen exchange rate is distributed, indicating that only a small part of the network oxygen atoms participates in the oxygen exchange. However, the distribution of the oxygen exchange rate is distinctly narrow in the sample with high SiOH concentration. It is attributed to the redistribution of the network 18O atoms and the modification of the a-SiO2 network topology caused by reactions with mobile interstitial water molecules, which are transiently formed by dehydroxylation of paired SiOH groups. The activation energy for the average oxygen exchange rate is larger than that of the permeation of interstitial O2 in a-SiO2. Furthermore, the average exchangefree diffusion length of interstitial O2 below 900C (≳1 μm) is far larger than the scale of the interstitial voids in a-SiO2 (≲1 nm). These observations confirm that the oxygen exchange is not necessarily involved in the permeation of interstitial O2.

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