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Radioluminescence, thermoluminescence and dosimetric properties of ZnO ceramics

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Larisa Grigorjeva, Aleksejs Zolotarjovs, Sergej Yu Sokovnin, Donats Millers, Krisjanis Smits, Vladislav G. Il`ves

Two types of ZnO ceramics were fabricated and characterized by XRD, SEM methods. The radioluminescence spectra were measured within the 300–550 K range. The defect luminescence band peaking at ~2.35 eV is the dominant one in radioluminescence spectra in both of the fabricated ceramics. The thermostimulated luminescence (TSL) glow-curves were measured after X-ray irradiation at 300 K. It was concluded that the complex overlapping peak within the 320–450 K temperature range consists of two components (~360–375 K and 400–420 K). The ratio of component intensities differs in both ceramics. The positions of high temperature TSL components (480–520 K) also differ in both samples; therefore not only sintering conditions but also the properties of the initial powder are very important for characteristics of TSL. A linear dependence of peak intensity on irradiation dose was observed up to ~3 kGy for ceramic 1 and up to 9 kGy for ceramic 2.

cer_int_2017_img

Ceramics International

DOI: 10.1016/j.ceramint.2017.02.016

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Effect of Mg2+ ions co-doping on luminescence and defects formation processes in Gd3(Ga,Al)5O12:Ce single crystals

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V. Babin , P. Bohacek , L. Grigorjeva , M. Kucera , M. Nikl , S. Zazubovich , A. Zolotarjovs
Photo- and radioluminescence and thermally stimulated luminescence characteristics of Ce3+ – doped
and Ce3+, Mg2+ co-doped Gd3(Ga,Al)5O12 (GAGG) single crystals of similar composition are investigated in the 9-500 K temperature range. The Ce3+ – related luminescence spectra and the photoluminescence decay kinetics in these crystals are found to be similar. Under photoexcitation in the Ce3+ – and Gd3+ – related absorption bands, no prominent rise of the photoluminescence intensity in time is observed neither in GAGG:Ce,Mg nor in GAGG:Ce crystals. The afterglow is strongly reduced in GAGG:Ce,Mg as compared to GAGG:Ce, and the afterglow decay kinetics is much faster. Co-doping with Mg2+ results in a drastic decrease of the thermally stimulated luminescence (TSL) intensity in the whole investigated temperature range and in the appearance of a new complex Mg2+ – related TSL glow curve peak around 285 K. After irradiation in the Ce3+ – related 3.6 eV absorption band, the TSL intensity in GAGG:Ce,Mg is found to be comparable with that in the GAGG:Ce epitaxial film of similar composition. The Mg2+ – induced changes in the concentration, origin and structure of the crystal lattice defects and their influence on the scintillation characteristics of GAGG:Ce,Mg are discussed.

2017 gagg paper

Optical Materials 66 (2017) 48-58

DOI: 10.1016/j.optmat.2017.01.039

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Colorimetric gas detection by the varying thickness of a thin film of ultrasmall PTSA-coated TiO2 nanoparticles on a Si substrate

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Urmas Joost, Andris Šutka, Meeri Visnapuu, Aile Tamm, Meeri Lembinen, Mikk Antsov, Kathriin Utt, Krisjanis Smits, Ergo Nõmmiste and Vambola Kisand

Colorimetric gas sensing is demonstrated by thin films based on ultrasmall TiO2 nanoparticles (NPs) on Si substrates. The NPs are bound into the film by p-toluenesulfonic acid (PTSA) smits 2017 imageand the film is
made to absorb volatile organic compounds (VOCs). Since the color of the sensing element depends on the interference of reflected light from the surface of the film and from the film/silicon substrate interface, colorimetric detection is possible by the varying thickness of the NP-based film. Indeed, VOC absorption causes significant swelling of the film. Thus, the optical path length is increased, interference wavelengths are shifted and the refractive index of the film is decreased. This causes a change of color of the sensor element visible by the naked eye. The color response is rapid and changes reversibly within seconds of exposure. The sensing element is extremely simple and cheap, and can be fabricated by common coating processes.

 

Beilstein Journal of Nanotechnology

Beilstein J. Nanotechnol. 2017, 8, 229–236.
DOI:10.3762/bjnano.8.25

The paper is open access. Download it from journal page here or by pressing the buttons below:

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Tunneling luminescence in long lasting afterglow of SrAl2O4:Eu,Dy

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Virginija Liepina, Donats Millers, Krisjanis Smits

A long lasting (afterglowing) luminescent material SrAl2O4: Eu2+, Dy3+ was prepared by high temperature solid-state reaction method. The processes responsible for long lasting luminescence were studied by means of luminescence spectra, thermally stimulated luminescence and afterglow kinetics.Two processes are found to contribute in excited Eu2+ creation – the thermally released electrons recombination and electron tunneling from trap to Eu3+. The possible scheme of long lasting luminescence mechanism is proposed.

virginija raksts image.

Journal of Luminescence

http://dx.doi.org/10.1016/j.jlumin.2017.01.011

DOI: 10.1016/j.jlumin.2017.01.011

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Looking back. UPCON 2016: Bright future for upconversion

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A great review of the may-2016 conference that took place in Wroclaw, Poland!

Check out the view from Dayong Jin published in Nature Photonics here.

upcon_image

Krisjanis Smits and Aleksejs Zolotarjovs had a really productive and interesting visit. We hope to meet again soon with all the participants of the conference.

European Cooperation in Science and Technology (COST) Action CM1403

Mechanical Reinforcement of Electrospun Poly(Vinyl Alcohol) by a-FeOOH Nanowires

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Anna Sutka, Martin Jarvekulg, Andris Sutka, Ivo Heinmaa, Uno Maeorg, Krisjanis Smits, Martin Timusk

We report the mechanical performance of α-FeOOH nanowire reinforced poly(vinyl alcohol) (PVA) composite nanofiber mat, fabricated using straightforward aqueous processing methods. Goethite (α-FeOOH) nanocrystals have a high elastic modulus and –OH rich surface, ensuring strong interactions with hydrophilic polymers and effective reinforcement. Needle-less electrospinning resulted in alignment of the nanowires along fibre axis, as confirmed by transmittance electron microscopy studies. Produced composite PVA nanofibers containing 10 wt% goethite nanoparticles exhibited an outstanding fivefold increase in Young’s modulus and 2.5-fold improvement of tensile strength compared to mats of neat PVA. The addition of α-FeOOH had a significant influence on glass transition temperature indicating formation of interphase regions around nanowire inclusions. Observed properties are explained by nanowire grafting in the precursor solution, extensive interactions between the adsorbed PVA chains and the matrix and percolation of interphase regions at 10 wt% α-FeOOH.

Published in Polymer Composites, John Wiley and Sons Ltd.

DOI: 10.1002/pc.24231

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Unexpected Epitaxial Growth of a Few WS2 Layers on 1100 Facets of ZnO Nanowires

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Boris Polyakov, Alexei Kuzmin, Krisjanis Smits, Janis Zideluns, Edgars Butanovs,
Jelena Butikova, Sergei Vlassov, Sergei Piskunov, and Yuri F. Zhukovskii

untitled-1

Core-shell nanowires is an interesting and perspective class of radially heterostructured nanomaterials where epitaxial growth of the shell can be realized even at signi ficant core-shell lattice mismatch. In this study epitaxial hexagonally-shaped shell consisting of WS2 nanolayers was grown on f1100g facets of prismatic wurtzite-structured
[0001]-oriented ZnO nanowires for the first time. A synthesis was performed by annealing in a sulfur atmosphere of ZnO/WO3 core-shell structures, produced by reactive
DC magnetron sputtering of amorphous a-WO3 layer on top of ZnO nanowire array.
The morphology and phase composition of synthesized ZnO/WS2 core-shell nanowires were con firmed by scanning and transmission electron microscopy (SEM and TEM),micro-Raman and photoluminescence spectroscopy. Epitaxial growth of WS2(0001) layer(s) on f1100g facets of ZnO nanowire is unexpected due to incompatibility of their symmetry and structure parameters. To relax the interfacial incoherence, we propose a model of ZnO/WS2 interface containing WS2 bridging groups inside and use first-principles simulations to support its feasibility.

J. Phys. Chem. C

DOI: 10.1021/acs.jpcc.6b06139

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Luminescent PEO Coatings on Aluminum

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Aleksejs Zolotarjovs, Krisjanis Smits, Anete Krumina, DonatsMillers, and Larisa Grigorjeva

Results show the possibilities of pore filling approach to modify alumina coatings with various materialspeo-raksta-bilde in order to enhance coating optical (or other) properties and develop new functional materials; as well as demonstrate novel alumina phase transition detection approach. Luminescent PEO coatings were
produced on aluminum surface using pore-filling method. Three stage process was developed to modify alumina coating in order to enhance its luminescent properties. Eu3+ recharging to Eu2+ followed by significant (up to 10 times) total luminescence intensity increase was observed, Eu ion presence evaluated in final coating by measuring fast decay kinetics. Structure of obtained coatings was analyzed using XRD and FTIR spectroscopy indicating presence of η-alumina phase.

ECS Journal of Solid State Science and Technology, 5 (9) R150-R153 (2016)

DOI: 10.1149/2.0401609jss

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Complex tribomechanical characterization of ZnO nanowires: nanomanipulations supported by FEM simulations

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Sergei Vlassov, Boris Polyakov, Sven Oras, Mikk Vahtrus,
Mikk Antsov, Andris Šutka Krisjanis Smits, Leonid M Dorogin and
Rünno Lõhmus

Abstract
In the present work, we demonstrate a novel approach to nanotribological measurements based
on the bending manipulation of hexagonal ZnO nanowires (NWs) in an adjustable halfsuspended
configuration inside a scanning electron microscope. A pick-and-place manipulation
technique was used to control the length of the adhered part of each suspended NW. Static and
kinetic friction were found by a ‘self-sensing’ approach based on the strain profile of the
elastically bent NW during manipulation and its Young’s modulus, which was separately
measured in a three-point bending test with an atomic force microscope. The calculation of static
friction from the most bent state was completely reconsidered and a novel more realistic crackbased
model was proposed. It was demonstrated that, in contrast to assumptions made in
previously published models, interfacial stresses in statically bent NW are highly localized and
interfacial strength is comparable to the bending strength of NW measured in respective bending
tests.

Nanotechnology 27 (2016) 335701 (10pp)

doi:10.1088/0957-4484/27/33/335701

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Europium(III)-doped A2Hf2O7 (A ¼ Y, Gd, Lu) nanoparticles: Influence of annealing temperature, europium(III) concentration and host cation on the luminescent properties

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Jelena Papan , Dragana J. Jovanovic , Katarina Vukovic , Krisjanis Smits ,
Vesna ÐorCevic , Miroslav Dramicanin 

The detailed analyses of structure and luminescence of europium(III)-doped A2Hf2O7 (A ¼ Y, Gd, Lu)
nanoparticles is presented. Samples were prepared by time and cost effective combustion method that
utilize polyethylene glycol both as a chelating agent and as a fuel, with different europium(III) concentrations
(from 1 to 12 at.%), annealed at temperatures ranging from 800 to 1400 C, and with alternating
A3þ cation in the A2Hf2O7 host. Then, structural variations between materials were analysed by Xray
diffraction and structural refinement, while the changes in the luminescence were assessed from the
Judd-Ofelt analyses of emission spectra. Nanoparticles prepared at the lowest temperature (800 C) had
the smallest particle size of ~6 nm and showed the highest quantum efficiency when doped with 1 and
2 at.% of europium(III). Radiative transition rate and quantum efficiency of emission showed
Lu2Hf2O7 > Gd2Hf2O7 > Y2Hf2O7 trend.

Optical Materials xxx (2016) 1e9

doi:10.1016/j.optmat.2016.04.007

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Effect of Point Defects on Luminescence Characteristics of ZnO Ceramics

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P. A. Rodnyi, K. A. Chernenko, A. Zolotarjovs, L. Grigorjeva,
E. I. Gorokhova, and I. D. Venevtsev

Abstract—Photo- and thermally stimulated luminescence of ZnO ceramics are produced by uniaxial hot pressing. The luminescence spectra of ceramics contain a wide band with a maximum at 500 nm, for which oxygen vacancies VO are responsible, and a narrow band with a maximum at 385 nm, which is of exciton nature. It follows from luminescence excitation spectra that the exciton energy is transferred to luminescence centers in ZnO. An analysis of the thermally stimulated luminescence curves allowed detection of a set of discrete levels of point defects with activation energies of 25, 45, 510, 590 meV, and defects with continuous energy distributions in the range of 50–100 meV. The parameters of some of the detected defects are characteristic of a lithium impurity and hydrogen centers. The photoluminescence kinetics are studied in a wide temperature range.
DOI: 10.1134/S1063783416100309

ISSN 1063-7834,

Physics of the Solid State, 2016, Vol. 58, No. 10, pp. 2055–2061.

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Ag sensitized TiO2 and NiFe2O4 three-component nanoheterostructures: synthesis, electronic structure and strongly enhanced visible light photocatalytic activity

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A. Šutka, T.
Käämbre, R. Pärna, N. Doebelin, M. Vanags, K. Smits and V. Kisand

This study reports on the synthesis and characterisation of two- and three-component visible light active photocatalytic
nanoparticle heterostructures, based on TiO2 and NiFe2O4 and sensitized with Ag. We observe that Ag content as small as
1 at.% in the TiO2/NiFe2O4 heterostructure increases by more than an order of magnitude the rate constant for the visible
light photocatalytic process. We rationalise this in terms of the measured structure and electronic structure data of the
binary and ternary combinations of the component materials and focus on details, which show that an optimised
deposition sequence is vital for attaining the high values of photocatalytic efficiency, because the charge transfer across
the interfaces appears to be sensitive to where the Ag is loaded in the heterostructure. The overall higher visible light
photocatalytic activity of the TiO2/Ag/NiFe2O4 heterostructure was observed and is attributed to enhanced charge carrier
separation efficiency and migration via vectorial electron transfer.

RSC Advances., 2016,

DOI: 10.1039/C6RA00728G

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Rare earth doped glass–ceramics containing NaLaF4 nanocrystals

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E. Elsts , G. Krieke, U. Rogulis, K. Smits, A. Zolotarjovs, J. Jansons, A. Sarakovskis, K. Kundzins

Oxyfluoride glasses 16Na2O–9NaF–5LaF3–7Al2O3–63SiO2 (mol%) activated with 3% terbium, dysprosium,
praseodymium and neodymium fluorides have been prepared and studied by differential thermal analysis,
cathodoluminescence, X-ray induced luminescence, X-ray diffraction, scanning electron microscopy
and energy dispersive X-ray spectroscopy. We found out that the presence of crystalline phase enhances
the X-ray induced luminescence intensity. X-ray induced luminescence is the most intense for the sample
activated with terbium and treated at 700 C, whereas the praseodymium and neodymium activated
samples have the fastest decay times.

Optical Materials xxx (2016) xxx–xxx

doi: 10.1016/j.optmat.2016.01.005

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Longitudinal spin-relaxation in nitrogen-vacancy centers in electron irradiated diamond

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A. Jarmola, A. Berzins, J. Smits, K. Smits, J. Prikulis, F. Gahbauer, R. Ferber, D. Erts, M. Auzinsh, and D.
Budker

We present systematic measurements of longitudinal relaxation rates (1=T1) of spin polarization in
the ground state of the nitrogen-vacancy (NV–) color center in synthetic diamond as a function
of NV– concentration and magnetic field B. NV– centers were created by irradiating a Type 1b
single-crystal diamond along the [100] axis with 200 keV electrons from a transmission electron
microscope with varying doses to achieve spots of different NV– center concentrations. Values of
(1=T1) were measured for each spot as a function of B

Applied Physics Letters 107, 242403 (2015);

doi: 10.1063/1.4937489

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Luminescence of polymorphous SiO2

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A.N. Trukhin, K. Smits, J. Jansons, A. Kuzmin

The luminescence of self-trapped exciton (STE) was found and systematically studied in tetrahedron
structured silica crystals (a-quartz, coesite, cristobalite) and glass. In octahedron structured stishovite
only host material defect luminescence was observed. It strongly resembles luminescence of oxygen
deficient silica glass and g or neutron irradiated a-quartz. The energetic yield of STE luminescence for aquartz
and coesite is about 20% of absorbed energy and about 5(7)% for cristobalite. Two types of STE
were found in a-quartz. Two overlapping bands of STEs are located at 2.5e2.7 eV. The model of STE is
proposed as SieO bond rupture, relaxation of created non-bridging oxygen (NBO) with foundation of a
bond with bridging oxygen (BO) on opposite side of c or x,y channel. The strength of this bond is
responsible for thermal stability of STE. Similar model of STE was ascribed for coesite and cristobalite
with difference related to different structure. STE of Silica glass is strongly affected by disordered
structure.

Radiation Measurements 90 (2016) 6e13

doi:10.1016/j.radmeas.2015.12.002

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Multicolor upconversion luminescence of GdVO4:Ln3þ/Yb3þ (Ln3þ ¼ Ho3þ, Er3þ, Tm3þ, Ho3þ/Er3þ/Tm3þ) nanorods

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Tamara V. Gavrilovic , Dragana J. Jovanovic,  Krisjanis Smits, Miroslav D. Dramicanin 

Lanthanide-doped GdVO4 nanorods that exhibit upconversion emission under 982 nm excitation have
been prepared by a facile room-temperature chemical co-precipitation method followed by a subsequent
annealing at temperatures of 600 C, 800 C and 1000 C. Multicolor upconversion emission, including
white, was achieved by tuning the concentrations of dopant lanthanide ions (Ho3þ, Er3þ, Tm3þ and Yb3þ)
in GdVO4. It is found that four GdVO4 samples emit light with the white chromaticity coordinates of
(0.326, 0.339), (0.346, 0.343), (0.323, 0.327) and (0.342, 0.340) respectively, under a single-wavelength
NIR excitation. These coordinates are very close to the standard equal energy white light coordinates
(0.333, 0.333) according to the 1931 CIE diagram. By varying dopant lanthanide concentrations in
nanorods it is possible to produce upconversion emission with colors between red (0.504, 0.369), green
(0.282, 0.577) and blue (0.142, 0.125) coordinates.

Dyes and Pigments 126 (2016) 1e7

doi:10.1016/j.dyepig.2015.11.005

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Gas sensitive luminescence of ZnO coatings obtained by plazmaelectrolytic oxidation

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L. Grigorjeva, D. Millers, K. Smits, A. Zolotarjovs

The ZnO coatings on Zn substrate were obtained using plasma electrolytic oxidation method. The XRDand SEM methods were used for structural and morphological characterization of obtained coatings. Theluminescence of ZnO coatings were studied and compared with luminescence characteristics of ZnO sin-gle crystal. It is shown that luminescence intensity in ZnO defect band depends on oxygen concentrationin ambient atmosphere. The effect is of interest for oxygen sensing based on ZnO coating luminescence

Sensors and Actuators A 234 (2015) 290–293

doi:10.1016/j.sna.2015.09.018

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Raman, electron microscopy and electrical transport studies of x-ray amorphous Zn-Ir-O thin lms deposited by reactive DC magnetron sputtering

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M. Zubkins, R. Kalendarev, J. Gabrusenoks, K. Smits, K. Kundzins,
K. Vilnis, A. Azens, J. Purans

Abstract. Zn-Ir-O thin lms on glass and Ti substrates were deposited by reactive DC
magnetron sputtering at room temperature. Structural and electrical properties were
investigated as a function of iridium concentration in the lms. Raman spectrum of Zn-Ir-
O (61.5 at.% Ir) resembles the spectrum of rutile IrO2, without any distinct features of wurtzite
ZnO structure. SEM images indicated that morphology of the lms surface improves with
the iridium content. EDX spectroscopy and cross-section SEM images revealed that the lms
growing process is homogeneous. Crystallites with approximately 2 – 5 nm size were discovered
in the TEM images. Thermally activated conductivity related to the variable range hopping
changes to the non-thermally activated before iridium concentration reaches the 45 at.%.

IOP Conference Series: Materials Science and Engineering 77 (2015) 012035

doi:10.1088/1757-899X/77/1/012035

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Luminescence of Er/Yb and Tm/Yb doped FAp nanoparticles and ceramics

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L. Grigorjeva, K. Smits, D.Millers, Dz.Jankoviča

Abstract. The nanoparticles of hydroxiapatite and fluorapatite doped with
Er/Yb and Tm/Yb were synthesized and characterized by FTIR, XRD, SEM and
TEM methods. The results of up-conversion luminescence studies were
presented for the samples as prepared, annealed at 500°C and at 900-1000 °C.
At annealing above 800°C the ceramic state was formed. It is shown that
fluorapatite host is more appropriate than hydroxiapatite host for rare ions
luminescence and up-conversion processes. The post preparing annealing of
nanarticles significantly enhanced the luminescence intensity. The Tm/Yb
doped fluorapatite shows intense up-conversion luminescence in 790-800 nm
spectral region and is potentially useful for biomedical applications.

IOP Conference Series: Materials Science and Engineering 77 (2015) 012036

doi:10.1088/1757-899X/77/1/012036

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UV and yellow luminescence in phosphorus doped crystalline and glassy silicon dioxide

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A.N.Trukhin , K.Smits , J.Jansons , D.Berzins , G.Chikvaidze , D.L.Griscom 

Luminescence of phosphorus doped crystalline α-quartz and phosphosilicate glass with content
3P2O5  7SiO2 was studied.Water and OH groups are found by IR spectra in these materials.The spectrum of lumines cence contains many bands in the range 1.5–5.5 eV. The luminescence bands in UV range at 4.5–5 eV are similar in those materials. Decay duration in exponential approximation manifests a time constant about 37 ns. Also a component in ms range was detected. PL band of ms component is shifted to low energy withrespect to that of 37 ns component. This shift is about 0.6 eV. It is explained as singlet–triplet splitting of excited state. Below 14 K increase of luminescence kinetics duration in ms range was observed and it was ascribed to zero magnetic field splitting of triplet excited state of the center.
Yellow–red luminescence was induced by irradiation in phosphorus doped crystalline α-quartz,
phosphosilicate glasses.The yellow luminescence contains two bands a t600 and 740 nm. Their decay is similar under 193 nm laser and maybe fitted with the first order fractal kinetics or stretched exponent.
Thermally stimulated luminescence contains only band at 600 nm. The 248 nm laser excites luminescence at 740 nm according to intracenter process with decay time constant about 4ms at 9 K.
Both type of luminescence UV and yellow were ascribed to different defects containing phosphorus.
P-doped α-quartz sample heated a t550 °C become opalescent. IR spectra related to water and OH
groups are changed. Photoluminescence intensity of all three bands,UV (250nm), yellow (600nm) and
red (740nm) strongly diminished and disappeared after heating to 660 °C. Radiation induced red
luminescence of non-bridging oxygen luminescence center (NBO) appeared in crystal after heat treatment. We had observed a crystalline version of this center (Skujaetal., Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms. 2012; 286: pp.159–168).
Effect of heat treatment explained as sedimentation of phosphorus in some state. Keeping of treated
sample at 450–500 °C leads to partial revival of ability to create yellow luminescence center under irradiation.

Journal of Luminescence 166(2015)346–355

doi:10.1016/j.jlumin.2015.05.045

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Luminescence of Eu ion in alumina prepared by plasma electrolyticoxidation

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Krisjanis Smitsa, Donats Millers, Aleksejs Zolotarjovs,Reinis Drunka, Martins Vanks

Eu ion luminescence in aluminium oxide nanocrystals and layers prepared by plasma electrolytic oxida-tion (PEO) are investigated in this study. The Eu ion in PEO coatings has intense luminescence allowingsuch material to be used for preparation of various phosphor materials. In this study, Eu ion doped coatingswere prepared with two methods: anodization and pulsed bipolar plasma electrolytic oxidation. Also,for comparative studies, alumina nanocrystals with the same amount of Eu ions were prepared using SolGel and molten salts methods.Obtained Eu-doped coatings were studied using luminescence methods. Typical Eu ion luminescencebands were observed, however intensity and spectral distribution differs drastically depending on prepa-ration method and parameters used, therefore the Eu ion luminescence could be used as coating qualityluminescent probe. Additionally, the possibility to incorporate the Eu ions in trivalent or divalent stateexhibiting bright red and blue luminescence accordingly was demonstrated by using different oxidationparameters. Moreover, comparison measurements between powder samples and coatings were made.

Applied Surface Science 337 (2015) 166–171

doi:10.1016/j.apsusc.2015.02.085

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Thermally stimulated luminescence of undoped and Ce3+-doped Gd2SiO5 and (Lu,Gd)2SiO5 single crystals

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V. Bondar, L. Grigorjeva, T. Kärner, O. Sidletskiy,
K. Smits, S. Zazubovich, A. Zolotarjovs

Thermally stimulated luminescence (TSL) characteristics (TSL glow curves and TSL
spectra) are investigated in the 4-520 K temperature range for the X-ray irradiated at 4 K, 8
K, or 80 K single crystals of gadolinium and lutetium-gadolinium oxyorthosilicates. The
nominally undoped Gd2SiO5 and (Lu,Gd)2SiO5 crystals, containing traces of Ce3+, Tb3+, and
Eu3+ ions, and Ce3+-doped Gd2SiO5 and (Lu,Gd)2SiO5 crystals are studied. For the first time,
the TSL glow curves of these materials are measured separately for the electron (intrinsic,
Ce3+- or Tb3+-related) and hole (Eu3+-related) recombination luminescence, and the TSL glow
curve peaks, arising from thermal decay of various electron and hole centers, are identified.
The origin of the traps related to the TSL peaks is discussed, and thermal stability parameters
of the electron and hole traps are calculated.

Journal of Luminescence

DOI:10.1016/j.jlumin.2014.11.034

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Studies of radiation defects in cerium, europium and terbium activated oxyfluoride glasses and glass ceramics

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E. Elsts , U. Rogulis, K. Bulindzs, K. Smits, A. Zolotarjovs, L. Trinkler, K. Kundzins

Terbium, cerium and europium activated oxyfluoride glasses and glass ceramics have been studied by
thermally stimulated luminescence (TSL) and optical absorption techniques after the X-ray irradiation.
A creation of colour centres in oxyfluoride glass matrix and TSL peaks depending on the activator type
were observed. LaF3 and rare earth activators were analysed by SEM–EDS.

Optical Materials 41 (2015) 90–93

DOI: 10.1016/j.optmat.2014.10.042

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Luminescence of Phosphorus Containing Oxide Materials: Crystalline SiO2-P and 3P2O5·7SiO2; CaO·P2O5; SrO·P2O5 glasses

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A. N.Trukhin, K. Smits, J. Jansons, D. Berzins, G. Chikvaidze,
D.L.Griscom

ABSTRACT. Luminescence of phosphate glasses such as CaO·P2O5 and SrO·P2O5 is compared with that of phosphorus doped
crystalline -quartz and phosphosilicate glass with content 3P2O5·7SiO2. Water & OH groups are found by IR spectra in these
materials. The spectrum of luminescence contains many bands in the range 1.5 – 5.5 eV. The luminescence bands in UV range at
4.5- 5 eV are similar in those materials. Decay duration in exponential approximation manifests a time constant about 37 ns. Also
a component in μs range was detected. PL band of μs component is shifted to low energy with respect to that of ~37 ns
component. This shift is about 0.6 eV. It is explained as singlet-triplet splitting of excited state. Below 14 K increase of
luminescence kinetics duration in μs range was observed and it was ascribed to zero magnetic field splitting of triplet excited
state of the center. Yellow-red luminescence was induced by irradiation in phosphorus doped crystalline -quartz,
phosphosilicate glasses. The yellowl uminescence contains two bands at 600 and 740 nm. Their decay is similar under 193 nm
laser and may be fitted with the first order fractal kinetics or stretched exponent. Thermally stimulated luminescence contains
only band at 600 nm. The 248 nm laser excites luminescence at 740 nm according to intra center process with decay time
constant about 4 ms at 9 K. Both type of luminescence UV and yellow were ascribed to different defects containing phosphorus.
P-doped α-quartz sample heated to 550 co become opalescent. Ir spectra related to water & OH groups are changed.
Photoluminescence intensity of all three bands, UV (250 nm), yellow (600 nm) and red (740 nm) strongly diminished and
disappeared after heating to 660 Co. Radiation induced red luminescence of non-bridging oxygen luminescence center (NBO)
appeared in crystal after heat treatment. We had observed a crystalline version of this center (l. Skuja et al, Nuclear Instruments
and Methods in Physics Research B 286,159–168 (2012)). Effect of heat treatment explained as sedimentation of phosphorus in
some state. Keeping of treated sample at 450-500 Co leads to partial revival of ability to create yellow luminescence center
under irradiation.

AIP Conf. Proc. 1624, 154-166 (2014);

doi: 10.1063/1.4900472

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Characteristic and sinterability of alumina-zirconia-yttria nanoparticles prepared by different chemical methods

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GRABIS Janis, JANKOVICA Dzidra, STEINS Ints, SMITS Krisjanis,and SIPOLA Inta

Abstract. The characteristics and sinterability of the Al2O3-ZrO2(Y2O3) nanoparticles produced by
simple and effective microwave and molten salts methods and processed by using spark plasma
sintering were studied and compared. The crystalline powders with the specific surface area in the
range of 72–108 m2/g and crystallite size of 5–13 nm were obtained by calcination of samples
prepared by both methods at 800 oC. The content of t-ZrO2 phase depends on concentration of
Al2O3, Y2O3 and on calcination temperature but the impact of the preparation method is
insignificant. The phase transition of tetragonal ZrO2 to monoclinic for the samples without Y2O3
started at 1000 oC though it was incomplete in the case of high content of Al2O3. The bulk materials
with relative density of 86.1–98.7% were fabricated by the spark plasma sintering method at 1500–
1600 oC depending on the content of Al2O3 and Y2O3.

Advances in Science and Technology Vol. 87 (2014) pp 30-35

doi:10.4028/www.scientific.net/AST.87.30

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Luminescence properties of zirconia nanocrystals prepared by solar physical vapor deposition

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Krisjanis Smits , Larisa Grigorjeva , Donats Millers , Karlis Kundzins , Reinis Ignatans , Janis Grabis , Claude Monty

Zirconia nanocrystals have attracted considerable interest as biolabels, which can be used as probes for
medical imaging and biosensor applications. However, zirconia particle agglomeration forms amajor limitation
to its use for biolabeling. In this backdrop, for the first time, well-separated zirconia nanocrystals
were obtained in a Heliotron reactor (PROMES CNRS, France) via the solar physical vapor deposition
(SPVD) method. As the raw material target for solar evaporation, zirconia nanopowders obtained via
the sol–gel process were used. The luminescence and upconversion luminescence properties of the Sol
Gel nanopowders were compared with those of the SPVD nanocrystals. Erbium was chosen as the luminescence
center with ytterbium as the sensitizer, and along with these two dopants, niobium was also
used. Niobium acts as a charge compensator to compensate for depletion in the charge due to the
introduction of trivalent erbium and ytterbium at tetravalent zirconium sites. Consequently, the
oxygen-vacancy concentration is reduced, and this results in a significant increase in the upconversion
luminescence.
The SPVD-prepared samples showed less agglomeration and a fine crystal structure as well as high
luminescence, and thus, such samples can be of great interest for biolabeling applications.

Optical Materials 37 (2014) 251–256

http://dx.doi.org/10.1016/j.optmat.2014.06.003

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The role of Nb in intensity increase of Er ion upconversion luminescence in zirconia

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K. Smits, A. Sarakovskis, L. Grigorjeva, D. Millers, and J. Grabis

It is found that Nb co-doping increases the luminescence and upconversion luminescence intensity
in rare earth doped zirconia. Er and Yb-doped nanocrystalline samples with or without Nb
co-doping were prepared by sol-gel method and thermally annealed to check for the impact of
phase transition on luminescence properties. Phase composition and grain sizes were examined by
X-ray diffraction; the morphology was checked by scanning- and high-resolution transmission
electron microscopes. Both steady-state and time-resolved luminescence were studied. Comparison
of samples with different oxygen vacancy concentrations and different Nb concentrations
confirmed the known assumption that oxygen vacancies are the main agents for tetragonal or cubic
phase stabilization. The oxygen vacancies quench the upconversion luminescence; however, they
also prevent agglomeration of rare-earth ions and/or displacement of rare-earth ions to grain
surfaces. It is found that co-doping with Nb ions significantly (>20 times) increases upconversion
luminescence intensity. Hence, ZrO2:Er:Yb:Nb nanocrystals may show promise for upconversion
applications

Journal of Applied Physics 115, 213520 (2014)

DOI:10.1063/1.4882262

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The role of Nb in intensity increase of Er ion upconversion luminescence in zirconia

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K. Smits, A. Sarakovskis, L. Grigorjeva, D. Millers, J. Grabis

It is found that Nb co-doping increases the luminescence and upconversion luminescence intensity
in rare earth doped zirconia. Er and Yb-doped nanocrystalline samples with or without Nb
co-doping were prepared by sol-gel method and thermally annealed to check for the impact of
phase transition on luminescence properties. Phase composition and grain sizes were examined by
X-ray diffraction; the morphology was checked by scanning- and high-resolution transmission
electron microscopes. Both steady-state and time-resolved luminescence were studied. Comparison
of samples with different oxygen vacancy concentrations and different Nb concentrations
confirmed the known assumption that oxygen vacancies are the main agents for tetragonal or cubic
phase stabilization. The oxygen vacancies quench the upconversion luminescence; however, they
also prevent agglomeration of rare-earth ions and/or displacement of rare-earth ions to grain
surfaces. It is found that co-doping with Nb ions significantly (>20 times) increases upconversion
luminescence intensity. Hence, ZrO2:Er:Yb:Nb nanocrystals may show promise for upconversion
applications.

VC 2014 AIP Publishing LLC.

doi: 10.1063/1.4882262

http://dx.doi.org/10.1063/1.4882262

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Luminescence of rutile structured crystalline silicon dioxide (stishovite)

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A.N.Trukhin , K.Smits, G.Chikvaidze, T.I.Dyuzheva, L.M.Lityagina 

Luminescence spectrum of synthetic mono-crystalline stishovite comprises as low blue band at 400 nm
(3.1 eV) and a fast UV band at 260 nm (4.7 eV), as well as some bands in near-infrared range of spectra.The NIR luminescence of stishovite crystal, excited with lasers 532 nm, 248 nm and 193 nm as well as x-ray, possesses several sharp lines. Azerophonon line is situated at 787 nm (1.57 eV) and grows with cooling. An anti-Stokes line is located at 771 nm (1.68 eV). This line disappears with cooling. In a powder sample of stishovite created by shockwaves generated by the impact of a 50m-diameter meteorite in Arizona 50,000 years ago, the PL broad blue band is situated at 425 nm (2.9eV), the UV band at 260 nm (4.7eV), and the sharp lines, seen only under 193 nm laser, at 689 nm (1.789 eV), 694 nm (1.785 eV) and 706 nm (1.754 eV).
We ascribe the fast UV luminescence to singlet–singlet transitions and the slow blue band to triplet–
singlet transitions of the same intrinsic defect of stishovite in both types of samples. The blue band in
stishovite crystal exhibits delayed luminescence of recombination nature, whereas the blue band of Arizona’s powder sample does not exhibit such effect. This difference is explained by different surroundings of luminescence center in those samples. NIR luminescence of mono-crystalline stishovite is ascribed to carbon impurity penetrated in the sample from graphite heater. NIR luminescence of powder from Arizona has not yet found an explanation.

Solid State Communications 189(2014)10–14

DOI: 10.1016/j.ssc.2014.03.010

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TSL and fractional glow study of Ge-doped α-quartz.

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A. Zolatarjovs, A.N. Trukhin, K. Smits, D. Millers

Abstract. Crystalline α-quartz doped with 0.1wt% and 0.9wt% germanium was studied using
TSL and FGT equipment. Sample was chosen because previously it is known that Ge in quartz
is efficient trap for electrons, therefore it could be used for detection of hypothetic self-trapped
hole in α-quartz. However previous investigations of ODMR and TSL shows that in α-quartz
the hole is still mobile and trapping occurs only on defect states. The activation energies for
both TSL peaks are found by fractional glow and Hoogenstraaten method. The TSL
distribution changes depending on Ge concentration and also on irradiation type. The TSL
peaks below 70K in quartz doped with Ge could belong to hole trapped on Ge.

IOP Conference Series: Materials Science and Engineering 49 (2013) 012056

doi:10.1088/1757-899X/49/1/012056

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Photocatalytic Properties of TiO2 and ZnO Nanopowders

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L. Grigorjeva, J. Rikveilis, J. Grabis, Dz. Jankovica,
C. Monty, D. Millers, K. Smits

Photocatalytic activity of TiO2 and ZnO nanopowders is studied
depending on the morphology, grain sizes and method of synthesizing.
Photocatalysis of the prepared powders was evaluated by degradation of the
methylene blue aqueous solution. Absorbance spectra (190–100 nm) were
measured during exposure of the solution to UV light. The relationships
between the photocatalytic activity and the particle size, crystal polymorph
phases and grain morphology were analyzed. The photocatalytic activity of
prepared TiO2 nanopowders has been found to depend of the anatase-to-rutile
phase ratio. Comparison is given for the photocatalytic activity of ZnO
nanopowders prepared by sol-gel and solar physical vapour deposition
(SPVD) methods

Latvian Journal of Physics and Technical Sciences 2013, N 4

DOI: 10.2478/lpts-2013-0025

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Characterization of hydroxyapatite by time-resolved luminescence and FTIR spectroscopy

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L.Grigorjeva, D.Millers, K.Smits, Dz.Jankovica, L.Pukina

Abstract. Time-resolved luminescence and FTIR absorption spectra of undoped and Eu
and Ce doped hydroxyapatite nanocrystalline powders prepared by sol-gel method were
studied. The luminescence band at 350-400 nm was detected and two decay times (11 ns
and 38 ns) was determinated for Ce doped samples. The luminescence spectra and decay
kinetics were analized for Eu doped nanopowders. The Eu3+ ion was incorporated in
different Ca sites. The process of energy transfer to Eu3+ excited state (5D0) was detected
from luminescence decay kinetics.

IOP Conference Series: Materials Science and Engineering 49 (2013) 012005

doi:10.1088/1757-899X/49/1/012005

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Ultraviolet luminescence of ScPO4, AlPO4 and GaPO4 crystals

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Anatoly N Trukhin, Krishjanis Shmits, Janis L Jansons and
Lynn A Boatner

Abstract
The luminescence of self-trapped excitons (STEs) was previously observed and described for
the case of tetragonal-symmetry ScPO4 single crystals. The subject band in this material is
situated in the UV spectral range of 210 nm or 5.8 eV. In the present work, we are both
expanding this earlier luminescence study and seeking to identify similar luminescence
phenomena in other orthophosphate crystals, i.e., AlPO4 and GaPO4. These efforts have
proven to be successful—in spite of the structural differences between these materials and
ScPO4. Specifically we have found that for AlPO4 and GaPO4, in addition to an -quartz-like
STE, there is a UV luminescence band that is similar in position and decay properties to that
of ScPO4 crystals. Potentially this represents an STE in AlPO4 and GaPO4 crystals that is
analogous to the STE of ScPO4 and other orthophosphates. The decay kinetics of the UV
luminescence of ScPO4 was studied over a wide temperature range from 8 to 300 K, and they
exhibited some unusual decay characteristics when subjected to pulses from an F2 excimer
laser (157 nm). These features could be ascribed to a triplet state of the STE that is split in a
zero magnetic field. A fast decay of the STE was detected as well, and therefore, we conclude
that, in addition to the slow luminescence corresponding to a transition from the triplet state,
there are singlet–singlet transitions of the STE. Time-resolved spectra of the slow and fast
decay exhibit a small shift (0.15 eV) indicating that the singlet–triplet splitting is small and
the corresponding wavefunction of the STE is widely distributed over the atoms of the ScPO4
crystal where the STE is created.

Journal of Physics: Condensed Matter 25 (2013) 385502 (6pp)

doi:10.1088/0953-8984/25/38/385502

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Cathodoluminescence of oxyfluoride glass-ceramics

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U. Rogulis, E. Elsts , J. Jansons , A. Sarakovskis , G. Doke , A. Stunda , K. Smits

Tb, Ce, Eu activated oxyfluoride glass-ceramics with the composition SiO2 $ Al2O3 $ Li2O $ LaF3 have been
studied by cathodoluminescence (CL). We compared CL intensities and decay times of the Tb, Ce, Eu
activated glass-ceramic samples and observed that the Tb activated sample has the most intense
luminescence, but the Ce activated sample has the shortest decay times. Induced optical absorption and
thermostimulated luminescence have been observed after X-ray irradiation of samples.

Radiation Measurements xxx (2013) 1-4

DOI: 10.1016/j.radmeas.2012.12.020

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The electron beam induced short lived absorption in PLZT optical ceramics

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D. Millers , V. Dimza, L. Grigorjeva, M. Antonova, K. Smits, M. Livins

The short lived absorption induced by a pulsed electron beam in the led–lanthanum–zirconate–titanate
(PLZT) optical ceramics was studied. The measured absorption spectrum covers 1.1–2.9 eV energy range
and consists of several strongly overlapping peaks. The rise of the absorption is delayed with respect to
the excitation pulse, due to charge carrier migration before trapping at centers responsible for the
absorption. The formation rate of absorption centers and decay rate of absorption depend on the photon
energy. The kinetics of short lived absorption varies over spectrum and can be approximated with a
stretched exponent. The stretched exponent parameters indicate that the local disorder of a matrix is
similar around all absorption centers.

Optical Materials xxx (2013) xxx–xxx

DOI 10.1016/j.optmat.2012.12.029

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Up-conversion luminescence dependence on structure in zirconia nanocrystals

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Krisjanis Smits, Dzidra Jankovica , Anatolijs Sarakovskis, Donats Millers

The zirconia samples containing two different concentrations of Er and Yb dopants were prepared using
the Sol–Gel method and up-conversion luminescence was studied using the time-resolved techniques.
The up-conversion luminescence depends on the oxygen content in surrounding gasses during annealing
as well as on the annealing temperature. These dependencies indicate that ZrO2 intrinsic defects annealing
and generation, phase transition as well as dopant redistribution take place. The possible role of these
processes on up-conversion luminescence is discussed. The results of experiments confirmed that the
annealing temperature has a crucial influence on up-conversion luminescence for samples containing
small concentrations of Er and Yb; whereas for samples containing large concentrations of Er and Yb,
the primary change of up-conversion luminescence is due to the grain size growth during annealing.
The optimal annealing temperature depends upon the Er and Yb ion concentration. It is crucial to obtain
up-conversion zirconia material with high quantum efficiency.

Optical Materials 35 (2013) 462–466

DOI: 10.1016/j.optmat.2012.09.038

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Nanocomposites for Novel Sensing Systems

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A. Sternberg, I. Muzikante, R. Dobulans, D. Millers, L. Grigorjeva, K. Smits, M. Knite, and G. Sakale

Abstract The work performed under the National Research Program of Latvia
and presented here is about advanced, chemical stimuli responsive materials.
Sensor materials are produced on the basis of metallophthalocyanines or zirconia
nanocrystals with the purpose of detecting small size molecular compounds like
NH3, O2 and O3. Polymer/nanostructured carbon composite is produced to detect
large size molecular compounds like organic vapour. Basic gaseous compound
sensing principles are discussed within this paper.

Physics and Biophysics

DOI 10.1007/978-94-007-4119-5_12

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Zirconia nanocrystals as submicron level biological label

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K Smits, J Liepins, M Gavare, A Patmalnieks, A Gruduls and D Jankovica

Abstract. Inorganic nanocrystals are of increasing interest for their usage in biology and pharmacology research. Our interest was to justify ZrO2 nanocrystal usage as submicron level biological label in baker’s yeast Saccharomyces cerevisia culture. For the first time (to our knowledge) images with sub micro up-conversion luminescent particles in biologic media were made. A set of undoped as well as Er and Yb doped ZrO2 samples at different concentrations were prepared by sol-gel method. The up-conversion luminescence for free standing and for nanocrystals with baker’s yeast cells was studied and the differences in up-conversion luminescence spectra were analyzed. In vivo toxic effects of ZrO2 nanocrystals were tested by co-cultivation with baker’s yeast.

IOP Conference Series: Materials Science and Engineering 38 (2012) 012050

doi:10.1088/1757-899X/38/1/012050

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The luminescent properties of persistent strontium aluminate phosphor prepared by solar induced solid state synthesis

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V Liepina, K Smits, D Millers, L Grigorjeva, C Monty

Abstract. A novel method – solar induced solid state synthesis – for the synthesis of aluminate phosphor doped with Eu and Dy and the study of its luminescent properties are presented in this article. Two kinds of samples of persistent strontium aluminate phosphor have been prepared – ones via this new method and ones via conventional high temperature solid state reaction. Both kinds of samples were made using the same raw materials. The samples were tested for their chemical composition using X-Ray diffraction method. The luminescence properties and phase composition of the obtained samples have been compared. The behaviour of afterglow, thermally stimulated luminescence, excitation and emission spectra are presented.

IOP Conference Series: Materials Science and Engineering 38 (2012) 012045

doi:10.1088/1757-899X/38/1/012045

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The time-resolved luminescence characteristics of Ce and Ce/Pr doped YAG ceramics obtained by high pressure technique

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L. Grigorjeva , D. Millers , K. Smits , A. Sarakovskis , W. Lojkowski , A. Swiderska-Sroda , W. Strek ,
P. Gluchowski

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

Optical Materials 34 (2012) 986–989

doi:10.1016/j.optmat.2011.05.023

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Defect Luminescence of YAG Nanopowders and Crystals

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L. Grigorjeva, D. Jankoviča, K. Smits, D. Millers, S. Zazubovich

Undoped and rare-earth-ion-doped Y3Al5O12 (YAG) nanopowders are prepared by the sol-gel low-temperature combustion method. The luminescence characteristics of the YAG, YAG:Ce, YAG:Pr, and YAG:Ce/Pr nanopowders are compared with those of the single crystals. The luminescence band peaking around 3.1 eV is complex and excited at about 3.6 eV, 3.9 eV and 4.3 eV. The 3.1 eV emission was peculiar to all the samples studied. The Stokes shift of this band is ~0.5 eV. The decay time of the ~3.1 eV emission at 80 K is ~14 ns and the slower decay (afterglow) components are practically absent. The 3.1 eV luminescence was suggested to arise from different intrinsic lattice defects.

Latvian Journal of Physics and Technical Sciences 2012, N 4

DOI: 10.2478/v10047-012-0022-4

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

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A.N. Trukhin, K. Smits , A. Sharakosky , G. Chikvaidze , T.I. Dyuzheva , L.M. Lityagina

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; F2, 157 nm) were used for photoluminescence
(PL) excitation. Two PL bands were observed. One, in UV range with the maximum at 4.770.1 eV with
FWHM equal to 0.9570.1 eV, mainly is seen under ArF laser. Another, in blue range with the maximum
at 370.2 eV with FWHM equal to 0.870.2 eV, is seen under all three lasers. The UV band main fast
component of decay is with time constant t¼1.270.1 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 1771 ms 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 F2 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 ms. For blue band recombination
luminescence decay is lasting to ms range of time with power law decay t1.
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 0.0370.01 eV for the case of X-ray
luminescence and it is 0.1570.05 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 a-quartz crystal presumes similar nature
of centers in those materials.

Journal of Luminescence 131 (2011) 2273–2278

doi:10.1016/j.jlumin.2011.05.062

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Intrinsic defect related luminescence in ZrO2

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K. Smits , L. Grigorjeva , D. Millers, A. Sarakovskis, J. Grabis, W. Lojkowski

The studies of ZrO2 and yttrium stabilized ZrO2 nanocrystals luminescence as well as yttrium stabilized
single crystal luminescence and induced absorption showed that the intrinsic defects are responsible
for luminescence at room temperature. These defects form a quasi-continuum of states in ZrO2 band
gap and are the origin of the luminescence spectrum dependence on the excitation energy.
Luminescence centers are oxygen vacancies related but not the vacancies themselves. At room
temperature, in ZrO2, deep traps for electrons and holes exist. The oxygen vacancies are proposed to
be the traps for electrons.

Journal of Luminescence 131 (2011) 2058–2062

doi:10.1016/j.jlumin.2011.05.018

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Luminescence of silicon Dioxide – silica glass, α -quartz and stishovite

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Anatoly N. Trukhin , Krishjanis Smits, Georg Chikvaidze, Tatiana I. Dyuzheva, Ludmila M. Lityagina

This paper compares the luminescence of different modifications of silicon dioxide – silica glass, -quartz
crystal and dense octahedron structured stishovite crystal. Under x-ray irradiation of pure silica glass and
pure -quartz crystal, only the luminescence of self-trapped exciton (STE) is detected, excitable only in
the range of intrinsic absorption. No STE luminescence was detected in stishovite since, even though its
luminescence is excitable below the optical gap, it could not be ascribed to a self-trapped exciton. Under
ArF laser excitation of pure -quartz crystal, luminescence of a self-trapped exciton was detected under
two-photon excitation. In silica glass and stishovite mono crystal, we spectrally detected mutually similar
luminescences under single-photon excitation of ArF laser. In silica glass, the luminescence of an oxygen
deficient center is presented by the so-called twofold coordinated silicon center (L.N. Skuja et al., Solid
State Commun. 50, 1069 (1984)). This center is modified with an unknown surrounding or localized
states of silica glass (A.N. Trukhin et al., J. Non-Cryst. Solids 248, 40 (1999)). In stishovite, that same
luminescence was ascribed to some defect existing after crystal growth. For -quartz crystal, similar to
silica and stishovite, luminescence could be obtained only by irradiation with a lattice damaging source
such as a dense electron beam at a temperature below 80 K, as well as by neutron or -irradiation at
290 K.
In spite of a similarity in the luminescence of these three materials (silica glass, stishovite mono crystal and
irradiated -quartz crystal), there are differences that can be explained by the specific characteristics of
these materials. In particular, the nature of luminescence excited in the transparency range of stishovite is
ascribed to a defect existing in the crystal after-growth. A similarity between stishovite luminescence and
that of oxygen-deficient silica glass and radiation induced luminescence of -quartz crystal presumes a
similar nature of the centers in those materials.

Central European Journal of Physics • 9(4) • 2011 • 1106-1113

DOI: 10.2478/s11534-011-0016-5

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Europium doped zirconia luminescence

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Krisjanis Smits , Larisa Grigorjeva a, Donats Millers , Anatolijs Sarakovskis , Agnieszka Opalinska ,
Janusz D. Fidelus , Witold Lojkowski 

The luminescence properties and crystalline structure of ZrO2:Eu nanocrystals doped with different concentrations
of Eu were studied. Luminescence from the Eu2+ state was not observed even if the electrons
and holes were created up to 1019 cm3; thus it was suggested the Eu3+ was not an efficient trap for
electrons possibly due to Eu3+ negative charge relative to the crystalline lattice. The mutual interaction
between Eu3+ ions was not strong up to 5 at.% concentration. The stabilization of ZrO2 tetragonal as well
as cubic structure by Eu3+ is possible.

Optical Materials 32 (2010) 827–831

doi:10.1016/j.optmat.2010.03.002

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The luminescence of ZnO ceramics

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L. Grigorjeva , D.Millers , K.Smits , J.Grabis , J.Fidelus , W. qojkowski , T.Chudoba , K.Bienkowski 

The luminescence properties of ZnO ceramics with grains 100-5000 nm sintered by different techniques from nanopowders were studied. The luminescence decay times were compared with that obtained for ZnO singlecrystal. The temperature dependence of non-exponential decay of defect luminescence (2.0-2.6 eV) was measured in wide time, intensity and temperature range.The luminescence decay kinetic at T=20 K shows the decay close to I(t) w t1 dependence. At temperature region 50-250 K the decay kinetics is more complicated since the TSL was observed in this temperature region. It is shown that the luminescence properties of NP and ceramics strongly depend on defect distribution on grains surface
and the volume/surface ratio determine the luminescence decay in ZnO nanostructures and ceramics.

Radiation Measurements

doi:10.1016/j.radmeas.2010.03.012

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Time-resolved cathodoluminescence and photoluminescence of nanoscale oxides

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L. Grigorjeva, D. Millers, A. Kalinko, V. Pankratov, K. Smits

Abstract
The nanostructured oxide materials such as ZnO, ZrO2, and Y3Al5O12 (YAG) are perspective materials for transparent scintillating and/or
laser ceramics. The luminescence properties of single crystals, nanopowders and ceramic were compared. Nominally pure and rare-earth doped
nanopowders and ceramics have been studied by means of time-resolved luminescence spectroscopy.
The fast blue luminescence band was studied in ZnO ceramics sintering from different raw materials.
The luminescence centres of ZrO2:Y were compared in a single crystal, ceramic and nanopowder.
It is shown that ceramic sintering parameters have a strong influence on time-resolved luminescence characteristics in cerium-doped YAG.

Journal of The European Ceramic Society

doi:10.1016/j.jeurceramsoc.2008.03.037

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Advanced nanocrystalline ZrO2 for optical oxygen sensors

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Janusz D. Fidelus and Witold Łojkowski, Donats Millers, Krisjanis Smits and Larisa Grigorjeva

Abstract It was shown that ZrO2 nanopowders and
nanoceramics can be used as an optical oxygen sensor,
where the luminescence signal is proportional to the
partial oxygen pressure in gases. The nanopowders were
obtained in a hydrothermal microwave driven process
followed by annealing at 750oC. Nanoceramics were
obtained by sintering at pressures up to 6 GPa and at
250oC so that grain growth did not occur. Luminescence
of both materials depends linearly on the oxygen content
in nitrogen-oxygen mixtures for 2.1% – 25 vol% oxygen
content. For luminescence excitation using a laser beam,
the luminescence intensity decreases as oxygen pressure
increases. For excitation with an electron beam, the
opposite effect is observed – the lower the oxygen
pressure, the lower the luminescence signal. The
experimental results are explained in terms of
luminescence centers being distorted lattice sites close to
vacancies.

Proceedings of IEEE Sensors 2009

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Excitonic luminescence in ZnO nanopowders and ceramics

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L. Grigorjeva , D. Millers , K. Smits , V. Pankratov , W. Łojkowski, J. Fidelus , T. Chudoba , K. Bienkowski , C. Monty 

Fast photoluminescence spectra in the spectral region of 3.1–3.45 eV in ZnO and ZnO:Al ceramics were
studied at 14 and 300 K. Ceramics with grains smaller than 100 nm were sintered from nanopowders
by high pressure (8 GPa) and low temperature (350 C). Ceramics with grain sizes 1–5 lm were sintered
at 1400 C. It is shown that excitonic luminescence spectra depend on the ceramics grain size, post preparing
annealing and doping. The excitonic luminescence decay time was faster than 2 ns and the afterglow
at 30 ns was 0.05%.

Optical Materials 2009

doi:10.1016/j.optmat.2008.10.052

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Time-resolved absorption and luminescence following electron-hole pair creation in ZnO

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R. Andrew Wall, Kyle C. Lipke, K. B. Ucer, R. T. Williams, D. Millers, K. Smits, and L. Grigorjeva

We report transient absorption induced by electron-hole excitation
in undoped ZnO. A laser pump/continuum probe
method covers 2–300 ps, and an electron pulse with lamp
transmission covers 8–300 ns. The broad absorption spectrum
increases monotonically with wavelength from 900 to 1600
nm. Following a reasonable hypothesis that the free-carrierlike
induced infrared absorption is proportional to the total
number of free carriers, excitons, and shallow-trapped carriers
in the sample, these data allow setting an upper limit on
the quantum efficiency of a specified lifetime component of
luminescence. For the undoped commercial ZnO studied in
this report, the quantum efficiency of room temperature excitonic
luminescence is less than 5%. This means that there is
significant room for improvement in applications aiming to
use room-temperature excitonic blue luminescence of ZnO
for fast scintillators and light sources. Direct observation that
a large majority of excitations are tied up for more than 100
nanoseconds in shallow traps confirms the premise for studies
undertaken at Oak Ridge National Laboratory to improve the
excitonic luminescence yield and decay rate of ZnO by donor
doping. The preliminary results presented here on undoped
ZnO suggest that induced absorption measurements should be
a useful diagnostic of quantum efficiency while studying such
dopant effects.

Physica Status Solidi (C) Current Topics in Solid State Physics

DOI 10.1002/pssc.200879896

phys. stat. sol. (c) 6, No. 1, 323–326 (2009)

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Radiative Decay of Electronic Excitations in ZrO Nanocrystals and Macroscopic Single Crystals

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Krisjanis Smits, Larisa Grigorjeva, Donats Millers, Janusz D. Fidelus, and Witold Lojkowski

Abstract—The time-resolved luminescence was studied for
ZrO:Y single crystal and nanocrystals. The similar recombination
centres were found in both—single crystal and nanocrystals.
Luminescence decay is within 200 ns in nanocrystals, whereas it
extends to the microseconds in single crystal. It was shown that
the defects responsible for transient absorption were not involved
directly in radiative recombination process.

IEEE Transactions on Nuclear Science, VOL. 55, NO. 3, JUNE 2008

Digital Object Identifier 10.1109/TNS.2008.924077

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Luminescence Properties of ZnO Nanocrystals and Ceramics

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Larisa Grigorjeva, Donats Millers, Janis Grabis, Claude Monty, Aleksandr Kalinko, Krishjanis Smits,
Vladimir Pankratov, and Witold Łojkowski

Abstract—The luminescence excitation spectra, luminescence
spectra and the nanosecond-scale decay kinetics were studied.
The ZnO and ZnO:Al nanopowders were prepared by vaporization-
condensation in a solar furnace using different raw powders:
commercial, hydrothermal and those obtained by plasma synthesis.
Exciton-phonon as well as exciton-exciton interaction
processes in nanopowders, a bulk crystal and ZnO ceramics were
studied and compared. The fast decay and low afterglow intensity
of ZnO nanopowders and ceramics support these materials for
scintillators.

IEEE Transactions on Nuclear Science, VOL. 55, NO. 3, JUNE 2008

Digital Object Identifier 10.1109/TNS.2008.921931

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Transient and stable color centers in neutron irradiated MgO

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Vera Skvortsova, Nina Mironova-Ulmane, Larisa Grigorjeva,
Donats Millers, Krisjanis Smits

Abstract
The transient absorption and luminescence induced by the pulsed electron beam have been investigated in the MgO single crystal
containing transition metal ion (Cr, Mn, Fe) impurities and preliminary irradiated by the fast neutrons. It is supposed that the different
behavior of the absorption spectra of the MgO samples preliminary irradiated by the different fast neutron fluence is connected with the
destruction of the hole centers and with the creation of interstitial protons and the formation of the microphase Mg(OH)2. We assume
that the luminescence band at 3.2 eV is connected with F+ color centers.

Nuclear Instruments and Methods in Physics Research B 266 (2008) 2941–2944

doi:10.1016/j.nimb.2008.03.143

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Zirconia Based Nanomaterials for Oxygen Sensors – Generation, Characterisation and Optical Properties

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Janusz D. Fidelus, Witold Lojkowski, Donats Millers,
Larisa Grigorjeva, Krishjanis Smits and Robert R. Piticescu

Abstract: Microwave driven hydrothermal synthesis and hydrothermal synthesis were used to
obtain ZrO2 nanopowders. Their production with varying phase composition, the characterisation
and selected optical properties concerning their potential use as luminescence oxygen sensors are
reported. It was found that the powders obtained by the microwave driven hydrothermal method
and annealed at 750 0C in air show experiment repeatability within an accuracy of 6 %.

Solid State Phenomena Vol. 128 (2007) pp. 141-150

© (2007) Trans Tech Publications, Switzerland

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Comparison of ZrO2:Y Nanocrystals and Macroscopic Single Crystal Luminescence

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Krisjanis Smits, Donats Millers, Larisa Grigorjeva, Janusz D. Fidelus, Witold Lojkowski

Abstract. The luminescence spectra of a tetragonally structured ZrO2:Y single crystal and
nanocrystals were compared. It was found that the number of luminescence centers contributed
to the spectra. The excitation of luminescence within the band gap region led to different
luminescence spectra for the single crystal and nanocrystal samples, whereas recombinative
luminescence spectra were the same for both samples. The origin of this difference is that in
the nanocrystals, even under excitation within the band gap, charge carriers were created.
Zirconium- oxygen complexes distorted by intrinsic defects were proposed to be the
luminescence centres responsible for the wide luminescence band observed.

Journal of Physics: Conference Series 93 (2007) 012035

doi:10.1088/1742-6596/93/1/012035

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The Luminescence Properties of ZnO nanopowders

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Aleksandr Kalinko, Janusz D. Fidelus, Larisa Grigorjeva, Donats Millers, Claude J. Monty, Adam Presz and Krisjanis Smits

Abstract. Pure and Al3+ doped ZnO nanopowders were studied by means of time-resolved
luminescence spectroscopy. The powders were synthesized by hydrothermal and plasma
methods. These powders were used as a raw material for vaporization-condensation process
inside the Solar reactor. The commercially available ZnO nanopowder was studied for a
comparison. Exciton to defect band luminescence intensity ratio was estimated in different
types of ZnO nanopowders. It was found that nanopowders with whiskers morphology show
superlinear luminescence intensity depending on excitation density. The observed effect
depends on the average nanoparticle size and on the powder morphology.

IOP Publishing
Journal of Physics: Conference Series 93 (2007) 012044

doi:10.1088/1742-6596/93/1/012044

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The Luminescence Properties of ZnO:Al Nanopowders Obtained by Sol-gel, Plasma and Vaporization-condensation Methods

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Larisa Grigorjeva, Donats Millers, Krishjanis Smits, Claude Monty, Jules Kouam and Lassad El Mir

Abstract: ZnO nanocrystals were synthesized and characterized by XRD and SEM methods. The
luminescence spectra and decay kinetics were studied under pulsed laser excitation (266 nm, 8 ns).
ZnO and ZnO:Al powders were prepared by sol-gel and plasma chemical synthesis. These powders
were used as a raw material for the SPVD (Solar Physical Vapour Deposition) process. In this way,
the vaporisation-condensation phenomenon (VC) led to the formation of ZnO whiskers and
nanopowders. The luminescence properties of the VC nanopowders were studied and compared to
those displayed by the raw material. The Al dopant, present in the raw powders as a solid solution
and ZnAl2O4 precipitates, was only present as Al ions in the nanopowders after the SPVD. The blue
luminescence intensity increased considerably after SPVD. The whiskers type microstructures
showed nonlinear blue luminescence dependent on the excitation pulse density.

Solid State Phenomena 2007 Continue reading “The Luminescence Properties of ZnO:Al Nanopowders Obtained by Sol-gel, Plasma and Vaporization-condensation Methods” →

Luminescence of oxygen related defects in zirconia nanocrystals

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K. Smits, L. Grigorjeva, W. Łojkowski, and J. D. Fidelus

The luminescence of undoped tetragonal structure ZrO2 nanocrystals was studied. The luminescence intensity
depends on oxygen content in gases mixture in which the nanocrystals were annealed. The distorted
Zr-O bond is suggested to be the recombination center for band carriers. The oxygen deficient defect
is proposed to be responsible for photoluminescence.

Physica Status Solidi (C) Current Topics in Solid State Physics 4, No. 3, 770– 773 (2007)

DOI 10.1002/pssc.200673850

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