2015

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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