Aleksejs Zolotarjovs has successfully defended his Doctoral thesis in physics

We congratulate Aleksejs Zolotarjovs for successfully defending his doctoral thesis on “OPTICAL PROPERTIES OF PLASMA ELECTROLYTIC OXIDATION COATINGS ON ALUMINIUM ALLOY SURFACE” in physics.


Plasma Electrolytic Oxidation (PEO) is a process for producing thick (tens of microns), hard and chemically stable oxide coatings on metal surfaces. For decades, the process is under investigation by both scientific and industrial communities due to the low operational cost and ease of production. Due to the flexibility of the process, in the last couple of years, novel possibilities of PEO are quickly emerging: biocompatibility, antibacterial, photovoltaic, photocatalysis and other properties of the coatings are intensively studied. This work explores the different aspects of luminescence properties of PEO coatings which was not studied prior. Both process modification possibilities and practical applications are studied, with the focus on correlation between synthesis parameters and optical properties of the coatings. Three main topics are covered in this thesis:

1) Doping of the coating oxide structure with additives that alter the luminescence output

2) Production of coatings with dosimeter properties and evaluation of their performance

3) Synthesis of complex oxide structure of doped strontium aluminate thus creating persistent phosphorescent coatings

In combination with inherited PEO properties like outstanding hardness, great adhesion to the surface, chemical stability and quick production, the added luminescence properties can make this coating technology promising for practical applications in various fields of science and technology.


Madara Leimane and Ernests Einbergs defend their Master’s thesis outstandingly and excellently!

Congratulations to Madara Leimane for outstandingly defending her thesis on “Sol-gel synthesis of monolithic silicon dioxide” in chemistry as well as to Ernests Einbergs for excellently defending his thesis on “Usability of mecanoluminiscent materials in optoelectronic sensors and the development of a method for studying their properties”

Growth and characterization of PbI2-decorated ZnO nanowires for photodetection applications

In this study, we demonstrated for the first time the growth of ZnO nanowires (NWs) decorated with highly crystalline few-layer PbI2 and fabricated two-terminal single-nanowire photodetector devices to investigate the photoelectric properties of the hybrid nanostructures. We developed a novel two-step growth process for uniform crystalline PbI2 nanosheets via reactive magnetron deposition of a lead oxide film followed by subsequent iodination to PbI2 on a ZnO NW substrate, and we compared as-grown hybrid nanostructures with ones prepared via thermal evaporation method. ZnO–PbI2 NWs were characterized by scanning and transmission electron microscopy, X-ray diffraction analysis and photoluminescence measurements. By fabricating two-terminal single-nanowire photodetectors of the as-grown ZnO–PbI2 nanostructures, we showed that they exhibit reduced dark current and decreased photoresponse time in comparison to pure ZnO NWs and have responsivity up to 0.6 A/W. Ab initio calculations of the electronic structure of both PbI2 nanosheets and ZnO NWs have been performed, and show potential for photoelectrocatalytic hydrogen production. The obtained results show the benefits of combining layered van der Waals materials with semiconducting NWs to create novel nanostructures with enhanced properties for applications in optoelectronics or X-ray detectors.

DOI: 10.1016/j.jallcom.2020.154095

The boron effect on low temperature luminescence of SrAl2O4:Eu, Dy

Temperature dependence of the afterglow of persistent luminescence material SrAl2O4:Eu,Dy is a major problem for outdoor low temperature applications. Therefore this publication deals with tailoring the material for better outdoor use by exploring the second mechanism, that is involved in the afterglow – charge tunnelling from the trapping center to the luminescence center. Structure, morphology, emission and thermally stimulated luminescence properties have been measured for SrAl2O4:Eu,Dy samples with different added boron percentage. The results indicate a change in morphology of the samples with increasing boron concentration, as well as a change in afterglow times. The low temperature luminescence intensity and afterglow time dependence of boron addition turns out to be different from the room temperature luminescence intensity and afterglow time dependence from boron concentration. Boron addition in necessary amount plays a key role to creating trapping centers in the material that are located spatially close to the luminescence center thus making the material afterglow possible even in low temperatures.

DOI: 10.1016/j.ceramint.2020.01.208

Luminescence of natural α-quartz crystal with aluminum, alkali and noble ions impurities

Photoluminescence and thermally stimulated luminescence of synthetic and natural (morion and smoky) α-quartz crystals doped with aluminum and alkali ions were studied. The samples were examined both untreated and subjected to substitution of alkali ions for copper or silver ions. The photoluminescence spectrum of the untreated crystals is characterized with the main blue band around 400 nm (~3.1 eV). The corresponding luminescence center is based on a defect containing aluminum and alkali as compensators in natural and synthetic quartz crystals. Photoluminescence is subjected to thermal quenching and can be detected at high temperatures above 700 K, however the main intensity decay takes place at 200 K. The thermal quenching activation energy is 0.15 ± 0.05 eV and the frequency factor is 3·107 s−1. In the samples with silver ions the main luminescence band is located at ~260 nm (~4.7 eV) with a time constant of ~37 μs at 80 K, and in the samples with copper ions the PL band is at ~ 360 nm (~3.4 eV) with a time constant ~ 50 μs at 80 K. The initial luminescence of crystals is greatly reduced after introduction of noble ions. The luminescence of noble ions quenches at 700 K without drop in intensity at 200 K. For luminescence associated with silver the energy of thermal quenching is 0.7 ± 0.1 eV with a frequency coefficient of 1 · 101 3 s−1, and for the luminescence related to copper, these parameters are 0.55 ± 0.1 eV and 1 · 101 2 s−1. The differences in intra-center luminescence properties of the same defect containing alkali ions or noble ions are based on differences in electronic transitions. In the case of alkali ions the charge transfer transitions between oxygen and alkali ions. In the case of noble ions absorption – luminescence corresponds to intra ion transitions. Radiation properties are related to trapping of an electron on one valence ion. Created atom moves out of aluminum containing defect. The hole remains on aluminum-oxygen defect. Thermally stimulated luminescence is related to release of atom, it diffusion to aluminum defect with the hole on oxygen and following radiative recombination. Optically stimulated luminescence is explained by the similar process of optical release of excited atom and movement to aluminum defect and recombination of electron with hole.

DOI: 10.1016/j.jlumin.2019.116602

On low-temperature luminescence quenching in Gd3(Ga,Al)5O12:Ce crystals

Temperature dependences of the photoluminescence and X-ray excited luminescence intensity and thermally stimulated luminescence glow curves are measured in the 4.2–300 K temperature range for the undoped and Ce3+ – doped Gd3(Ga,Al)5O12 crystals. The conclusion is made that no low-temperature quenching of the Ce3+ – related photoluminescence takes place. In both the undoped and the Ce3+ – doped crystals, temperature dependences of the X-ray excited recombination luminescence intensity correlate with the position and shape of thermally stimulated luminescence glow curve peaks of the hole origin. Low-temperature quenching of the X-ray excited luminescence in these crystals is explained by the fact that at low temperatures, free holes are trapped at oxygen ions while electrons are trapped at various intrinsic defects. In Ce3+ – doped Gd3(Ga,Al)5O12 crystals, thermally stimulated release of the trapped holes and electrons and their subsequent recombination at Ce3+ ions result in the enhancement of the Ce3+ – related electron recombination luminescence with the increasing temperature in the 10–180 K range.

DOI: 10.1016/j.optmat.2019.109252


Spectral and kinetic characteristics of pyroelectric luminescence in LiGaO2

Pyroelectric luminescence was observed in noncentrosymmetrical crystal LiGaO2 with the direct band gap around 6 eV. For the first time spectral and kinetic characteristics of pyroelectric luminescence were obtained. The temporal structure of the PEL signal was determined as a sequence of pulses with duration not longer than several nanoseconds. This allowed proposing of the luminescence mechanism: in vacuum conditions in LiGaO2 crystal pyroelectric luminescence occurs inside the sample due to radiative recombination of electrons with the positively charged intrinsic luminescence centres.

DOI: 10.1016/j.optmat.2019.05.014

Photoconductivity & photoelectron emission of LiGaO2 crystal excited in intrinsic absorption range

The photoelectric response spectra were studied in a LiGaO2 crystal. The obtained spectra were interpreted as the release and diffusion of charge carriers, as well as external photoemission of electrons. The charge carrier release starts from excitation with a photon with energy greater than the optical gap of the LiGaO2 crystal (∼6 eV). The efficiency of generation of charge carriers exponentially increases up to 8 eV with saturation beginning from 9 eV. The nature of the photoelectric response is attributed to the Dember effect corresponding to the higher mobility of electrons compared to holes. The low yield of the internal photoelectron in the range of the intrinsic absorption band at 6 eV is associated with its exciton nature. Above 9.5–10 eV, an additional response was revealed that was interpreted as external photoelectron emission with a threshold of about Eth = 10.1 ± 0.2 eV. The relative emission yield of electrons increases as (hν – Eth)3, which corresponds to indirect transitions between the valence and conduction bands. The emission of photoelectrons for the direct band-to-band transitions increases linearly with the photon energy and the corresponding threshold is defined as 12.7 ± 0.2 eV.

DOI: 10.1016/j.optmat.2019.05.005

Hybrid Tribo-Piezo-Electric Nanogenerator with Unprecedented Performance Based on Ferroelectric Composite Contacting Layers

It was recently reported that more efficient triboelectric nanogenerator (TENG)-like devices can be prepared using inversely polarized ferroelectric films made of same material as the contacting layers. In the present work, a clear correlation between the piezoelectric response of inversely polarized ferroelectric PVDF/BaTiO3 nanocomposite films and the performance of the TENG-like device based on these films is demonstrated. This observation is explained by magnified electrostatic induction that is driven by piezoelectric charges and ferroelectric properties of these films. A double capacitor model is proposed that effectively portrays the interactions between ferroelectric layers during contact-separation and subsequent charge redistributions in the external circuit. The new understanding has allowed the result of 3-fold higher open circuit voltages (2.7 kV from 5 cm2) as compared to that of a state of the art TENG. Furthermore, findings uncover the potential for vast improvement in the field of nanogenerators for mechanical energy harvesting as a significantly better piezoelectric performance of flexible nanogenerators has been reported elsewhere.

DOI: 10.1021/acsaem.9b00836

X-ray induced defects in advanced lithium orthosilicate pebbles with additions of lithium metatitanate

Advanced lithium orthosilicate (Li 4 SiO 4 ) pebbles with additions of lithium metatitanate (Li 2 TiO 3 ) as a secondary phase have attracted international attention as an alternative solid-state candidate for the tritium breeding in future nuclear fusion reactors. In this research, the generation of radiation-induced defects in the Li 4 SiO 4 pebbles with various contents of Li 2 TiO 3 was analysed in-situ by X-ray induced luminescence technique. After irradiation with X-rays, the accumulated radiation-induced defects in the Li 4 SiO 4 pebbles were studied by electron spin resonance, thermally stimulated luminescence and absorption spectrometry. On the basis of the obtained results, it is concluded that the generation mechanism and the structure of primary radiation-induced defects (except Ti 3+ centres) in the advanced Li 4 SiO 4 pebbles with additions of Li 2 TiO 3 under exposure to X-rays is similar to the single-phase ceramics. In addition, it is expected that the additions of Li 2 TiO 3 can increase the probability for the recombination processes of primary radiation-induced defects in the advanced Li 4 SiO 4 pebbles during irradiation and thereby reduce the formation of thermally stable radiolysis products, such as colloidal lithium particles.

DOI: 10.1016/j.fusengdes.2019.03.096

Identification of Active Sites for Oxygen Reduction Reaction on Nitrogen- and Sulfur-Codoped Carbon Catalysts

Nitrogen- and sulfur-codoped carbon catalysts were prepared as electrocatalytic materials for the oxygen reduction reaction (ORR). Herein, we propose a novel and effective one-pot synthetic approach to prepare a NS-doped carbon catalyst by using the mixture of graphene oxide and multi-walled carbon nanotubes as a carbon support. Successful NS-doping of carbon and formation of the catalytically active sites were confirmed by X-ray photoelectron spectroscopy and with energy dispersion spectroscopy. The ORR activity of NS-codoped carbon was investigated by using a rotating disc electrode method. The NS-doped carbon shows superior ORR performance in alkaline media, and the electrocatalytic mechanism for the reduction of oxygen was well explained by density functional theory calculations of graphene sheets.

DOI: 10.1021/acs.jpcc.9b00117

Thermostimulated luminescence of plasma electrolytic oxidation coatings on 6082 aluminium surface

For decades, plasma electrolytic oxidation (PEO) coatings are actively studied and applied to protect the surface of various valve metals from chemical or mechanical damage. However, over the last couple of years intense research is being done to explore additional possibilities of the PEO coatings apart from their classical application. One of the possible additional uses is thermostimulated luminescence dosimetry that is already widely applied for environmental and health monitoring. This research proposes a method to produce a novel functional coating on aluminium surface exhibiting intense thermostimulated luminescence signal that could be used for dosimetry of ionizing radiation. The result was achieved using plasma electrolytic oxidation with modified electrolyte to introduce carbon ions into the oxide thus inducing defects in the crystalline structure of the coating. Al6082 aluminium alloy was used as a substrate, KOH and ethanol mixture as an electrolyte. A bipolar pulsed regime was used for 15 min. The obtained coating combines the desired luminescence properties with a good mechanical stability due to the relatively hard cubic phase of the coating. Scalability of the technology and low production cost makes the coatings prospective for various practical applications.

DOI: 10.1016/j.radmeas.2019.02.020


Sub-nanosecond excitonic luminescence in ZnO:In nanocrystals

The effects of indium concentration influence on the morphology, luminescence spectra and luminescence decay kinetics of ZnO:In nanocrystals prepared by the solar physical vapour deposition method are investigated. While undoped ZnO nanocrystals exhibit tetrapod-like morphology, with increasing indium concentration the tetrapods are transformed into particles whose average size decreases with increasing indium concentration. The results of time-resolved luminescence studies of undoped and indium doped ZnO nanocrystals showed that by increasing indium concentration the decay time falls and luminescence intensity decreases.

DOI: 10.1016/j.radmeas.2019.02.016

Nanoengineered Gd 3 Al 2 Ga 3 O 12 Scintillation Materials with Disordered Garnet Structure for Novel Detectors of Ionizing Radiation

The search for engineering approaches to improve the scintillation properties of Gd 3 Al 2 Ga 3 O 12 crystals and enable their production technology is of current interest. This crystal, while doped with Ce, is considered a good multi-purpose scintillation material for detecting gamma-quanta and neutrons. It is observed that co-doping with Mg affected intrinsic defects in the crystal structure that create shallow electronic traps. Other point structure defects, which are based on local variations of the crystal stoichiometry, are significantly diminished by use of a co-precipitated raw material. Nano-structuring of the raw material enables production of a homogeneous precursor mixture for growing a crystal with minimal evaporation of Ga from the melt. The demonstrated nano-engineering approach increased the light yield from the crystal by approximately 20%, enabling its applications in well logging.

DOI: 10.1002/crat.201800172

Electronic processes in doped ZnO nanopowders

ZnO nanocrystals, undoped and doped with Iridium or Indium were prepared by solar irradiation in Heliotron reactor in PROMES CNRS facilities, France. The comparative analysis of the excitonic spectra of ZnO single crystal and ZnO nanocrystals (NCs) doped with In and Ir was performed. It is shown that the excitonic processes in Ir doped nanocrystals coincide well with electronic processes in undoped NC and single crystal; however, the electronic processes in In-doped nanocrystals are significantly different from those in single crystal. The radioluminescence spectra of ZnO:In was analysed and additional luminescence band at ∼3.18 eV was detected due to In-doping. The luminescence decay time depends on In concentration in nanocrystals and is significantly less in ZnO:In compared with undoped nanocrystals. The fast scintillation of ZnO:In makes this material promising for application.

DOI: 10.1088/1757-899X/503/1/012017

Contribution of molecular structure to self-assembling and biological properties of bifunctional lipid-like 4-(N-alkylpyridinium)-1,4-Dihydropyridines

The design of nanoparticle delivery materials possessing biological activities is an attractive strategy for the development of various therapies. In this study, 11 cationic amphiphilic 4-(N-alkylpyridinium)-1,4-dihydropyridine (1,4-DHP) derivatives differing in alkyl chain length and propargyl moiety/ties number and position were selected for the study of their self-assembling properties, evaluation of their cytotoxicity in vitro and toxicity on microorganisms, and the characterisation of their interaction with phospholipids. These lipid-like 1,4-DHPs have been earlier proposed as promising nanocarriers for DNA delivery. We have revealed that the mean diameter of freshly prepared nanoparticles varied from 58 to 513 nm, depending upon the 4-(N-alkylpyridinium)-1,4-DHP structure. Additionally, we have confirmed that only nanoparticles formed by 4-(N-dodecylpyridinium)-1,4-DHP derivatives 3 and 6, and by 4-(N-hexadecylpyridinium)-1,4-DHP derivatives 10 and 11 were stable after two weeks of storage. The nanoparticles of these compounds were found to be homogenous in size distribution, ranging from 124 to 221 nm. The polydispersity index (PDI) values of 1,4-DHPs samples 3, 6, 10, and 11 were in the range of 0.10 to 0.37. We also demonstrated that the nanoparticles formed by 4-(N-dodecylpyridinium)-1,4-DHP derivatives 3, 6, and 9, and 4-(N-hexadecylpyridinium)-1,4-DHP derivatives 10 and 11 had zeta-potentials from +26.07 mV (compound 6) to +62.80 mV (compound 11), indicating a strongly positive surface charge and confirming the relative electrostatic stability of these nanoparticle solutions. Transmission electron microscopy (TEM) images of nanoaggregates formed by 1,4-DHPs 3 and 11 confirmed liposome-like structures with diameters around 70 to 170 nm. The critical aggregation concentration (CAC) value interval for 4-(N-alkylpyridinium)-1,4-DHP was from 7.6 µM (compound 11) to 43.3 µM (compound 6). The tested 4-(N-alkylpyridinium)-1,4-DHP derivatives were able to quench the fluorescence of the binary 1,6-diphenyl-1,3,5-hexatriene (DPH)—1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) system, demonstrating hydrophobic interactions of 1,4-DHPs with phospholipids. Thus, 4-(N-dodecylpyridinium)-1,4-DHP derivative 3 quenched the fluorescence of the DPH–DPPC system more efficiently than the other 4-(N-alkylpyridinium)-1,4-DHP derivatives. Likewise the compound 3, also 4-(N-dodecylpyridinium)-1,4-DHP derivative 9 interacted with the phospholipids. Moreover, we have established that increasing the length of the alkyl chain at the quaternised nitrogen of the 4-(N-alkylpyridinium)-1,4-DHP molecule or the introduction of propargyl moieties in the 1,4-DHP molecule significantly influences the cytotoxicity on HT-1080 (human fibrosarcoma) and MH-22A (mouse hepatocarcinoma) cell lines, as well as the estimated basal cytotoxicity. Additionally, it was demonstrated that the toxicity of the 4-(N-alkylpyridinium)-1,4-DHP derivatives on the Gram-positive and Gram-negative bacteria species and eukaryotic microorganism depended on the presence of the alkyl chain length at the N-alkyl pyridinium moiety, as well as the number of propargyl groups. These lipid-like compounds may be proposed for the further development of drug formulations to be used in cancer treatment.

DOI: 10.3390/pharmaceutics11030115

Production of Phosphorescent Coatings on 6082 Aluminum Using Sr0.95Eu0.02Dy0.03Al2O4-δ Powder and Plasma Electrolytic Oxidation

In this study, a new approach for producing phosphorescent aluminum coatings was studied. Using the plasma electrolytic oxidation (PEO) process, a porous oxide coating was produced on the Al6082 aluminum alloy substrate. Afterwards, activated strontium aluminate (SrAl2O4: Eu2+, Dy3+) powder was filled into the cavities and pores of the PEO coating, which resulted in a surface that exhibits long-lasting luminescence. The structural and optical properties were studied using XRD, SEM, and photoluminescence measurements. It was found that the treatment time affects the morphology of the coating, which influences the amount of strontium aluminate powder that can be incorporated into the coating and the resulting afterglow intensity.

DOI: 10.3390/coatings9120865

The search for defects in undoped SrAl2O4 material

SrAl2O4:Eu,Dy is a very efficient long afterglow phosphor with wide range of possible applications. The luminescence properties and the possible luminescence mechanism of this material have been studied extensively, but there is almost no information available about the undoped material. Therefore, this article deals with the luminescence and thermally stimulated luminescence of an undoped SrAl2O4, revealing the possible defects that might be involved in the creation of the long afterglow in doped material. We conclude that undoped material exhibits some luminescence under X-ray irradiation in low temperature; close to room temperatures luminescence is almost fully thermally quenched in comparison to low temperatures. We can observe F and F2 center luminescence as well as trace metal luminescence in the emission spectrum. TSL glow curve yields the peaks that are close to those observed in material with Eu and Dy doping; therefore these peaks are clearly related to intrinsic defects. The peak at around 400 K, that is shifting with rare earth doping, might be due to dopant interaction with intrinsic defects.

DOI: 10.1016/j.optmat.2018.06.004

Characterization of electrospun poly(lactide) composites containing multiwalled carbon nanotubes

The main aim of this work was to obtain conductive polymer-based materials by incorporation of different amounts of multiwalled carbon nanotubes (MWCNTs) into poly(lactide)(PLA) using the electrospinning technique. Fiber-based nonwovens with 0.2, 0.5, 1, and 3 wt% of MWCNTs were characterized regarding conductivity, morphology, thermal, and mechanical properties. It was confirmed that an increase of the MWCNTs content does not influence the increase of the material conductivity, since the conductivity was 170 ohm sq−1 for all composites. Scanning electron microscopy and transmission electron microscopy analyses revealed that smooth and beadless fibers were obtained, but also average diameters of composite nanofibers decreased with the increase of the MWCNTs content. Differential scanning calorimetry analysis showed that the presence of MWCNTs in the PLA matrix had a significant influence on the crystallization behavior of PLA nanofibers, because the decrease in crystallization temperature (Tc) was detected. Also, the incorporation of MWCNTs into PLA fibers affected the melting process, enabling the generation of α′ form, while had no influence on ordered α crystal. The enthalpy of composite degradation decreased, because MWCNTs are well-known for good heat conductivity, and with that the second step of degradation slowed down, as it was confirmed by thermogravimetric analysis. The addition of MWCNTs improved mechanical properties of composite fibers and caused the increase of both elasticity and tensile strengths of nanofibers. © The Author(s) 2019.

DOI: 10.1177/0892705719857780

In Memoriam

In August 2019, Larisa Grigorjeva, a habilitated Doctor of Physics and a long time employee at the Solid State institute of Physics at the University of Latvia, has passed away. The results of her scientific research have gained international attention and her significant contribution to Latvian science has been highly appreciated by the Latvian Academy of Sciences, awarding her the Edgars Siliņš Name Award and the title of Emeritus Scientist of Latvia. Larisa Grigorieva has organized and led a number of scientific projects, including international ones, in her 15-years of being the head of the Solid State Radiation Laboratory. It has been the basis for the development of the current UL ISSP Optical Materials Laboratory. Successfully combining research with teaching, under the leadership of Larisa Grigorieva many Bachelor, Master, and Doctorate degrees have been successfully obtained. LU CFI has lost an active researcher, Larisa Grigorjeva, and colleagues are saddened by her departure and will keep her in her memory.

Recent progress in understanding the persistent luminescence in SrAl2O4:Eu,Dy

Virginija Vitola, Donats Millers, Ivita Bite, Krisjanis Smits & Agnese SpustakaEver since the discovery of SrAl2O4:Eu,Dy persistent afterglow material,that can intensively glow up to 20h, the mechanism of long-lasting luminescence has been a popular area of research. The research is focused on discovering the mechanism of persistent luminescence in order to prolong the duration and intensity of afterglow in a controlled way. Although most researchers agree on the  general things, there are still many unclarities and ambiguities to discuss upon.This review paper briefly sketches in the highlights of past research on the luminescence mechanism in SrAl2O4:Eu,Dy, mainly focusing on the research conducted in the past decade dedicated to clearing these ambiguities.This paper provides an overview of the latest persistent luminescence mechanisms offered by researchers.

Keywords: Persistent luminescence, long afterglow, luminescence mechanism, luminescence mechanism; strontium aluminate

Published in Materials Science and Technology


Luminescence of localized states in oxidized and fluorinated silica glass


Photoluminescence of nominally pure oxidized and fluorinated silica glasses, which have good optical transparency in the optical gap, has been investigated under excitation by excimer lasers. The data are compared with the data for chlorinated silica glass, as well as glass containing OH. The excitation of localized states provides the release and capture of charge carriers, despite the low absorption coefficient. The holes are self-trapped. The recombination in the thermally activated or tunnel process creates luminescence of oxygen-deficient centers. It is concluded that oxidation and fluorination do not change the concentration of localized states, but they change the electronic transitions of the localized states to higher energies. In type III glass with a high OH content, luminescence is suppressed by the presence of hydrogen and appears after irradiation with 7.9 eV photons of the F2 laser. This observation suggests the existence of localized states even in these glasses.

• Photoluminescence of oxidized and fluorinated pure silica glasses was studied.
• optical absorption below 8.1 eV is 1–3 cm-1.
• two bands UV and blue luminescence of ODC was observed.
• UV band decay is slower than 4.5 ns and that of blue band is faster than 10 ms.
• recombination process of luminescence excitation.

Pure silica glass, Localized states, Self-trapped hole, Recombination luminescence, Oxygen deficient centers, Excimer lasers, Time resolved photoluminescence

Published in Journal of Non-Crystalline Solids

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Short update on ERDF project Phosphorescent coatings prepared by plasma electrolytic oxidation

We are happy to announce that the project ERDF Project Nr. “Phosphorescent coatings prepared by plasma electrolytic oxidation (2017-2019)” was successfully evaluated and the green light for the last stage was given. However, although the project is nearing its completion, great amount of work is still required. With finalization of measurements and synthesis, the publications are in active development.

Defending of the doctoral thesis “Electronic excitations and processes in long lasting luminescence material SrAl2O4” – Virgīnija Vītola

Persistent luminescent material SrAl2O4: Eu2+, Dy3+ is widely used and studied for energy efficient applications. A study of SrAl2O4:Eu,Dy; SrAl2O4:Dy,  and undoped SrAl2O4 material was conducted. Improvements for the mechanism of persistent luminescence have been suggested, that involve tunnelling of electron from trapping center to Eu3+, and the proof for stable Dy4+ existence in strontium aluminate has been presented. Undoped material luminescence has been observed, consisting of trace impurity and intrinsic defects luminescence.The results of this work give new knowledge and improve the existing concepts of persistent luminescence in Eu activated strontium aluminates. They also open some possibilities for wider applications of persistent phosphors.

Photoelectric response of localized states in silica glass


The photoelectric response of pure silica glasses excited by excimer lasers has been studied. The samples were made under various conditions. Photoelectric polarization of samples due to the Dember effect has been registered. The signal was recorded under the conditions of a space charge limited current. The space charge resulting from the capture of electrons and holes creates a static electric field that prevents diffusion of released charge carriers. The current registration in the external circuit stops, despite the continuation of photoexcitation. This effect was used as a fact of measuring the photocurrent in the sample volume instead of the parasitic current that is not associated with the sample. The screen has been chosen to prevent the influence of a spurious signal. It has been found that charge carriers are released when excited in the spectral absorption range of localized states of silica. Based on the Dember effect, the sign of the photoelectric response shows the type of charge carriers – an electron or a hole is mobile. Thus, a sample containing aluminum without alkali ions gives a negative signal, which indicates the diffusion of electrons at 290 K, since aluminum is an effective hole trap. An oxygen-deficient sample at 290 K provides a positive signal indicating the diffusion of holes, because the center of oxygen deficiency is an effective electron trap. This sample at 100 K provides a negative signal due to the effective self-trapping of holes.

Published in Journal of Non-Crystalline Solids

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    Photoluminescence and Electron Spin Resonance of Silicon Dioxide Crystal with Rutile Structure (Stishovite)

    Anatoly Trukhin, Andris Antuzevics

    An electron spin resonance (ESR) and photoluminescence signal is observed in the as grown single crystal of stishovite indicating the presence of defects in the non‐irradiated sample. The photoluminescence of the as received stishovite single crystals exhibits two main bands – a blue at 3 eV and an UV at 4.75 eV. Luminescence is excited in the range of optical transparency of stishovite (below 8.75 eV) and, therefore, is ascribed to defects. A wide range of decay kinetics under a pulsed excitation is observed. For the blue band besides the exponential decay with a time constant of about 18 μs an additional ms component is revealed. For the UV band besides the fast component with a time constant of 1–3 ns a component with a decay in tens μs is obtained. The main components (18 μs and 1–3 ns) possess a typical intra‐center transition intensity thermal quenching. The effect of the additional slow component is related to the presence of OH groups and/or carbon molecular defects modifying the luminescence center. The additional slow components exhibit wave‐like thermal dependences. Photo‐thermally stimulated creation–destruction of the complex comprising host defect and interstitial modifiers explains the slow luminescence wave‐like thermal dependences.

    Published in Physica Status Solidi Applications and Materials Science

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    Creation of glass-characteristic point defects in crystalline SiO2 by 2.5 MeV electrons and by fast neutrons

    Linards Skuja, Nadège Ollier, Koichi Kajihara, Krisjanis Smits

    Point defects in crystalline SiO2, created by 2.5 MeV electron irradiation at dose below the amorphizationthreshold or by fast neutrons, were compared by luminescence spectroscopy. Oxygen dangling bonds (“non-bridging oxygen hole centers”, NBOHCs), peculiar to amorphous state of SiO2, were detected for the first time in electron-irradiated non-amorphized α-quartz crystal. Their presence may signal the formation of nucleation centers in crystal structure as the first step to radiation-induced amorphization. Compared to crystal, irradiated by 1019 cm−2 fast neutrons, their concentration was over 100 times lower, and their inhomogeneous broadening was at least 2.5 times smaller. Divalent silicons (“silicon oxygen deficiency centers”, SiODC(II)), inherent to oxygen-deficient or irradiated SiO2 glass, were detected in neutron-irradiated (1019 n/cm2) α-quartz but were not found after the electron irradiation. Radiation-induced interstitial O2 molecules, characteristic to irradiated glassy SiO2 and other oxide glasses, are found in α-quartz only after neutron irradiation. The oxygen atoms, displaced by the 2.5 MeV e irradiation of α-quartz for fluences up to 1019 e/cm2 evidently stays entirely in the peroxy linkage (Si-O-O-Si bond) form.

    Published in Journal of Non-Crystalline Solids

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    Reversible Photodoping of TiO2 Nanoparticles for Photochromic Applications

    Urmas Joost, Andris Šutka, Marek Oja, Krisjanis Smits, Nicola Döbelin, Ardi Loot, Martin Jarveku,  Mika Hirsimaki, Mika Valden, and Ergo Nõmmiste

    Observations on the strong photochromic effect of crystalline
    TiO2 quantum dots (mean size ≈ 4 nm) are presented. The synthesized
    quantum dots consist of irregularly shaped anatase TiO2 nanoparticles (NPs)
    and are dispersed in butanol (8% by mass). Obtained NPs exhibit a dramatic
    photoresponse to UV light, enabling effective transmittance modulation in a
    broad wavelength range extending from the visible to near-infrared region, and
    even the thermal black body radiation regime beyond 10 μm. The exceptional
    photoresponse is attributed to hole-scavenging by butanol, TiO2 self-reduction,
    injection of electrons to the conduction band, and consequent localized surface
    plasmon resonances in NPs. The observed optical effect is reversible, and the
    initial high transmittance state can be restored simply by exposing the NPs to
    air. The applied NP synthesis route is economic and can be easily scaled for
    applications such as smart window technologies

    Published in Chemistry of Materials

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    Magnetron sputtering fabrication of α-Al2O3:Cr powders and their thermoluminescence properties

    Larisa Grigorjeva, Aleksejs Zolotarjovs, Donats Millers, Krisjanis Smits, Peter Krug, Johannes Stollenwerk, Alan Osman, Thomas Tenostendarp

    For many years doped α-Al2O3 has attracted interest as a dosimeter for personal, environment monitoring and food control. The alumina single crystal growth is a difficult process; however, materials in form of powders, ceramics and coating are possible to obtain. In this study for the first time Cr doped α-Al2O3powders were prepared by DC reactive magnetron sputtering followed by milling and oxidation. The morphology and phase analysis was performed; content of residual impurities was determined and thermostimulated glow curves were measured after different x-ray irradiation times (radiation doses). The prepared powder shows the dosimetry properties up to 20 kGy dose.

    Published in Radiation Measurements

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      F. Muktepavela*, J. Maniks, L. Grigorjeva, R. Zabels, P. Rodnyi, E. Gorokhova

      Transparent ZnO ceramics are of interest for use as material for highefficiency fast scintillators. Doping ZnO ceramics in order to improve complex of their properties is a promising direction. In the present research, the role of indium in the ZnO nanopowders surface interactions and in the change of microstructures and photoluminescence (PL) characteristics of sintered ceramics is considered. Undoped and 0.13 wt% In doped ZnO ceramics are obtained by hot pressing sintering. It has been found that indium leads to the transition of initially faceted ZnO particles to rounded, contributing to good sintering with formation of diffusion active grain boundaries (GBs). Unlike ZnO ceramics, ZnO:In ceramics microstructure is characterised by the transcrystalline mode of fracture, faceted GBs with places of zig-zag forms and predominant distribution of In at the GBs. Such indium induced modifications of GBs promote removal of point defects and reduce PL parameter α = Idef/Iexc in comparison with the undoped ceramics. Results characterise ZnO:In ceramics with improved GBs properties as a prospective material for scintillators.


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      A comparative study of photocatalytically active nanocrystalline tetragonal zyrcon-type and monoclinic scheelite-type bismuth vanadate

      Slobodan D.Dolić, Dragana J.Jovanović, Krisjanis Smits, Biljana Babić, Milena Marinović-Cincović, Slavica Porobić, Miroslav D.Dramićanin

      Monoclinic scheelite-type BiVO4 is currently considered as one of the most promising non-titania photocatalysts, wheras tetragonal zircon-type BiVO4 is still poorly understood. Herein, a new and simple synthetic approach was applied and nanostructured single-phase zircon-type BiVO4 was successfully prepared by a controllable ethylene-glycol colloidal route. In addition, nanostructured monoclinic scheelite-type BiVO4 powders were also fabricated through annealing of the as-prepared samples. A comparative study of the two BiVO4 polymorphs was conducted and it turned out that the novel synthetic approach had a significant impact on porosity and photocatalytic performance of zircon-structured BiVO4. All the prepared materials, as-prepared and annealed, were mesoporous, while measured values of specific surface area of some zircon-structured samples (∼34 m2/g) were ∼7 times higher than those reported thus far for this phase. Interestingly, for the first time, zircon-type BiVO4, previously considered to be a poor photocatalyst, exhibited a better overall performance in degradation of methyl orange compared to monoclinic scheelite-type BiVO4. Hence, it could be expected that the here-presented synthesis and observations will both arouse interest in scarcely studied tetragonal zircon-type BiVO4 and facilitate as well as speed up further research of its properties.

      Published in Ceramics International

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      Solvothermal synthesis derived Co-Ga codoped ZnO diluted magnetic degenerated semiconductor nanocrystals

      Andris Šutka, Tanel Käämbre, Urmas Joost, Kuno Kooser, Mati Kook, Roberto Felix Duarte, Vambola Kisand, Mikhael Maiorov, Nicola Döbelin, Krisjanis Smits

      Here we are reporting solvothermal synthesis derived diluted magnetic and plasmonic Co-Ga co-doped ZnOnanocrystals with high magnetization values (from 1.02 to 4.88 emu/g) at room temperature. Co-Ga co-doped ZnO nanocrystals show up to 2 fold increase in saturation magnetization compared to Co doped ZnO nanocrystals at the same Co concentration, with the observed room temperature magnetization higher than previously reported values for multifunctional magnetic and plasmonic nanocrystals, and the effect of Ga suggesting some role of the correspondingly introduced itinerant charge. While at the lowest Ga content the nanoparticles appear homogeneously doped, we note that already a moderate Ga content of several percent triggers a fraction of Co to segregate in metallic form in the bulk of the nanoparticles. However, the amount of segregated Co is not sufficient to account for the total effect, whereas a dominating contribution to the observed magnetism has to be related to itinerant charge mediated exchange interactions.

      Published in Journal of Alloys and Compounds

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      Phase stability and oxygen-sensitive photoluminescence ofZrO2:Eu,Nb nanopowders

      Valter Kiisk,*, Laurits Puust, Hugo Mandar, Peeter Ritslaid, Mihkel Rahn,Ivita Bite, Dzidra Jankovica, Ilmo Sildos, Raivo Jaaniso

      We studied structure and oxygen-sensitive photoluminescence (PL) of ZrO2:Eu,Nb nanocrystalline powders synthesized via a sol-gel route and heat-treated up to 1200 °C. The material containing only 2 at% Eu3+ was predominantly monoclinic, whereas 8 at% of Eu3+ stabilized tetragonal phase. Comparable amount of niobium co-doping effectively suppressed the formation of tetragonal phase.

      PL of Eu3+ ions was observed under direct excitation at 395 nm. PL decay kinetics showed that the luminescence was partially quenched, depending on doping concentrations and ambient atmosphere. At 300 °C, the PL intensity of all samples systematically responded (with up to 70% change) to changing oxygen content in the O2/N2 mixture at atmospheric pressure. At low doping levels, the dominant factor controlling the PL intensity was an energy transferfrom excited PL centers to randomly distributed defects in the ZrO2 lattice. We argue that the charge transfer between the defects and adsorbed oxygen molecules alters the ability of the defects to quench Eu3+ luminescence. At high doping levels, another type of sensor response was observed, where some Eu3+emitters are effectively switched on or off by the change of ambient gas. A remarkable feature of the studied material is a reversing of the sensor response with the variation of the Nb concentration.

      Published in Materials Chemistry and Physics

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      Up-conversion luminescence of GdVO4:Nd3+/Er3+ and GdVO4:Nd3+/Ho3+ phosphors under 808 nm excitation

      Dragana J.Jovanović, Tamara V.Gavrilović, Slobodan D.Dolić, Milena Marinović-Cincovi, KrisjanisSmits, Miroslav D.Dramićanin

      In recent years, there exists a tendency in research of up-conversion materials to shift excitation from 980 nm to shorter wavelengths. Here, in order to produce up-conversion luminescence emission of GdVO4-based materials under 808 nm excitation, polycrystalline powders of GdVO4:Er3+/Nd3+ and GdVO4:Ho3+/Nd3+ were successfully prepared by a high-temperature solid-state reaction technique. The prepared powders were highly crystalline with a single-phase zircon-type GdVO4 structure and consisted of micrometer-sized irregular spherical particles (2–6 μm in diameter). In all studied samples, visible up-conversion luminescence was successfully achieved under 808 nm illumination. Near-infraredpumping produced emission bands in the green, yellow-orange and green regions of the visible spectrum. The bands in the green and red regions of GdVO4: Er3+/Nd3+ as well as GdVO4:Ho3+/Nd3+ were, respectively, characteristic of Er3+ and Ho3+ ions. The dominant band originating from the 4G7/2 → 4I11/2transition in Nd3+ ions was observed around 597 nm in all samples.

      Published in Optical Materials

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      Mechanical reinforcement of electrospun poly(vinyl alcohol) by α‐FeOOH nanowires

      Anna Šutka Martin Järvekülg Andris Šutka Ivo Heinmaa Uno Mäeorg 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. POLYM. COMPOS., 39:2461–2468, 2018. © 2016 Society of Plastics Engineers

      Published in Polymer Composites

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      Excitation Transfer Engineering in Ce-Doped 2 Oxide Crystalline Scintillators by Codoping with Alkali3 Earth Ions

      Etiennette Auffray, Ramunas Augulis, Andrei Fedorov, Georgy Dosovitskiy, Larisa Grigorjeva, Vidmantas Gulbinas, Merry Koschan, Marco Lucchini, Charles Melcher, Saulius Nargelas, Gintautas Tamulaitis, Augustas Vaitkevicius, Aleksejs Zolotarjovs, and Mikhail Korzhik*

      Time-resolved spectroscopic study of the photoluminescence response to femtosecond pulse excitation and free carrier absorption at different wave10 lengths, thermally stimulated luminescence measurements and investigation of differential absorption are applied to amend the available data on excitation transfer in GAGG:Ce scintillators, and an electronic energy-level diagram in this single crystal is suggested to explain the influence of codoping with divalent Mg on luminescence kinetics and light yield. The conclusions are generalized by comparison of the influence of aliovalent doping in garnets (GAGG:Ce) and oxyorthosilicates (LSO:Ce and YSO:Ce). In both cases, the codoping facilitates the energy transfer to radiative Ce3þ centers, while the light yield is increased in the LYSO:Ce system but reduced in GAGG:Ce.

      Published in Physica Status Solidi

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      Aqueous synthesis of Z-scheme photocatalyst powders and thin-film photoanodes from earth abundant elements

      A.Šutka, M.Vanags, U.Joost, K.Šmits, J.Ruža, J.Ločs, J.Kleperis, T.Juhna

      Solid-state narrow band gap semiconductor heterostructures with a Z-scheme charge-transfer mechanism are the most promising photocatalytic systems for water splitting and environmental remediation under visible light. Herein, we construct all-solid Z-scheme photocatalytic systems from earth abundant elements (Ca and Fe) using an aqueous synthesis procedure. A novel Z-scheme two-component Fe2O3/Ca2Fe2O5 heterostructure is obtained in a straightforward manner by soaking various iron-containing nanoparticles (amorphous and crystalline) with Ca(NO3)2 and performing short (20 min) thermal treatments at 820 °C. The obtained powder materials show high photocatalytic performances for methylene blue dye degradation under visible light (45 mW/cm2), exhibiting a rate constant up to 0.015 min−1. The heterostructure exhibits a five-fold higher activity compared to that of pristine hematite. The experiments show that amorphous iron-containing substrate nanoparticles trigger the Fe2O3/Ca2Fe2O5 heterostructure formation. We extended our study to produce Fe2O3/Ca2Fe2O5nanoheterostructure photoanodes via the electrochemical deposition of amorphous iron-containing sediment were used. The visible-light (15 mW/cm2) photocurrent increases from 183 μA/cm2to 306 μA/cm2 after coupling hematite and Ca2Fe2O5. Notably, the powders and photoanodes exhibit distinct charge-transfer mechanisms evidenced by the different stabilities of the heterostructures under different working conditions.

      Published in Journal of Environmental Chemical Engineering

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      Particle size effects on the structure and emission of Eu3+:LaPO4 and EuPO4 phosphors

      Tamara Gavrilovića, Jovana Perišaa, Jelena Papana, Katarina Vukovića, Krisjanis Smits,Dragana J. Jovanovića, Miroslav D. Dramićanina

      This paper provides the detailed study of (nano)particle’s size effect on structural and luminescent properties of LaPO4:Eu3+ synthesized by four different methods: high temperature solid-state, co-precipitation, reverse micelle and colloidal. These methods delivered monoclinic monazite-phase submicron particles (> 100 nm), 4 × 20 nm nanorods and 5 nm spheres (depending on the annealing temperature), 2 × 15 nm nanorods, and ultrasmall spheres (2 nm), respectively. The analysis of emission intensity dependence on Eu3+ concentration showed that quenching concentration increases with a decrease of the particle size. The critical distance for energy transfer between Eu3+ ions is found to be 18.2 Å, and the dipole-dipole interaction is the dominant mechanism responsible for the concentration quenching of emission. With the increase in Eu3+ concentration, the unit-cell parameter slightly increases to accommodate larger Eu3+ ions at sites of smaller La3+ ions. Photoluminescent emission spectra presented four characteristic bands in the red spectral region: at 592 nm (5 D0→7 F1), at 612 nm ( 5 D0→7 F2), at 652 nm (5 D0→7 F3) and at 684 nm (5 D0→7 F4), while in small colloidal nanoparticles additional emission bands from host defects appear at shorter wavelengths. Intensities of f-f electronic transitions change with particles size due to small changes in symmetry around europium sites, while emission bandwidths increase with the reduction of particle size due to increased structural disorder. Judd-Ofelt analysis showed that internal quantum yield of Eu3+ emission is strongly influenced by particle’s morphology.

      Published in Journal of Luminescence

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      Synthesis, structure and spectroscopic properties of luminescent GdVO 4 :Dy 3+ and DyVO 4 particles

      Dragana J. Jovanovic, Andrea Chiappini, Lidia Zur, Tamara V. Gavrilovic, Thi Ngoc Lam Tran, Alessandro Chiasera, Anna Lukowiak, Krisjanis Smits, Miroslav D. Dramicanin, Maurizio Ferrari

      In this work, we focused on the syntheses, structure and spectroscopic properties of GdVO4:Dy3þ and DyVO4 (nano)particles of different sizes and shapes (spherical nanoparticles of 2 nm, 4 nm, and 20 nm in size, nanorods with a few nanometers in diameter and up to 10e20 nm in length and microparticles of 1 e8 mm) obtained by four synthetic methods. The size effect on the structure, Raman active modes, and photoluminescence emission intensities was analyzed by X-ray diffraction, Raman and photoluminescence spectroscopy, scanning and transmission electron microscopy, and diffuse reflection spectroscopy. All X-ray diffraction patterns clearly indicated presence of a single tetragonal zircon-type phase; absence of impurity phases indicate that the dopant Dy3þ ions were successfully and uniformly incorporated into the GdVO4 host lattice due to the equal valence and similar ionic radii. Micro-Raman measurements support the XRD measurements and showed Raman-active modes of the REVO4 systems (RE ¼ Gd, Dy). The difference between the two hosts in the diffuse reflectance spectra was observed and it could be attributed to more effective Gd3þ ions on the charge transfer bands and different polarization (compared to bulk material) in smaller nanoparticles. Photoluminescence spectroscopy showed several bands in the visible and near-infrared regions which can be exclusively attributed to the fef transitions of Dy3þ ions.

      Published in Optical Materials

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      Towards metal chalcogenide nanowire-based colour-sensitive photodetectors

      Edgars Butanovs*, Jelena Butikova, Aleksejs Zolotarjovs, Boris Polyakov

      In recent years, nanowires have been shown to exhibit high photosensitivities, and, therefore are of
      interest in a variety of optoelectronic applications, for example, colour-sensitive photodetectors. In this
      study, we fabricated two-terminal PbS, In2S3, CdS and ZnSe single-nanowire photoresistor devices and
      tested applicability of these materials under the same conditions for colour-sensitive (405 nm, 532 nm
      and 660 nm) light detection. Nanowires were grown via atmospheric pressure chemical vapour transport
      method, their structure and morphology were characterized by scanning and transmission electron
      microscopy (SEM and TEM), X-ray diffraction (XRD), and optical properties were investigated with
      photoluminescence (PL) measurements. Single-nanowire photoresistors were fabricated via in situ
      nanomanipulations inside SEM, using focused ion beam (FIB) cutting and electron-beam-assisted platinum welding; their current-voltage characteristics and photoresponse values were measured. Applicability of the tested nanowire materials for colour-sensitive light detection is discussed.

      Published in Optical Materials

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      Effect of Ga content on luminescence and defects formation processes in Gd3(Ga,Al)5O12:Ce single crystals

      L. Grigorjeva, K. Kamada, M. Nikl, A. Yoshikawa, S. Zazubovich, A. Zolotarjovs

      Luminescence characteristics of Ce3+ – doped Gd3GaxAl5-xO12 single crystals with different Ga contents (x = 1, 2, 3, 4, 5) are studied in the 9-500 K temperature range. The spectra of the afterglow, photoluminescence, radioluminescence, and thermally stimulated luminescence (TSL) of each crystal coincide. The increase of the Ga content results in the high-energy shift of the spectra while the radioluminescence intensity at 9 K remains practically constant up to x = 4. No Ce3+ emission is observed in case of x =5. The total TSL intensity drastically increases, reaches the maximum value around x =2e3, and then decreases due to the thermal quenching of the Ce3+ emission. The TSL glow curve maxima are gradually shifting to lower temperatures, and the dependence of the maxima positions and the corresponding trap depths on the Ga content is close to linear. However, the activation energy of the TSL peaks creation under irradiation of the crystals in the 4f – 5d1 absorption band of Ce3+ decreases drastically with the increasing Ga content (especially in the range of x = 1-2), and this dependence is found to be strongly nonlinear. Possible reasons of the nonlinearity are discussed.

      Published in Optical Materials

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      2018. gada sasniegums Latvijas zinātnē

      Par vienu no nozīmīgākajiem zinātnes sasniegumiem Latvijā 2018. gadā, Latvijas Zinātņu akadēmija nosaukusi mūsu  ERAF projekta “Fosforiscējoša pārklājuma iegūšana plazmas elektrolītiskajā oksidācijas procesa” Nr. tapušo darbu “Jauna energoefektīva metode ilgi spīdošo pārklājumu iegūšana uz metāliem”.

      Publikācija par jaunu metodi fosforiscējošo pārklājumu izveidei uz alumīnija virsmas

      Mēs ar lepnumu paziņojam, ka oktobrī Materials & Design tika publicēta jaunā publikācija “Jauna metode fosforiscējošo pārklājumu izveidei uz alumīnija virsmas.” Šo publikāciju finansēja ERAF projekts “Fosforiscējoša pārklājuma iegūšana plazmas elektrolītiskajā oksidācijas procesā” Nr. .

      Jauna metode fosforiscējošo pārklājumu izveidei uz alumīnija virsmas

      Jauna metode fosforiscējošo pārklājumu izveidei uz alumīnija virsmas

      Autori: Ivita Bite, Guna Krieke, Aleksejs Zolotarjovs, Katrina Laganovska, Virginija Liepina, Krisjanis Smits, Krisjanis Auzins,Larisa Grigorjeva, Donats Millers un Linards Skuja

      Šis pētījums piedāvā jaunu pieeju fosforescējošu pārklājumu radīšanai uz metāla virsmām. Stroncija alumināti ir vispopulārākie modernie luminiscējoši materiāli, kuri izrāda ilgu pēcspīdēšanu istabas temperatūrā un plašu spektrālo sadalījumu redzamajā diapazonā. Tomēr, neraugoties uz lielo pētījumu skaitu, šādu materiālu sintēzes metodes joprojām ir relatīvi neefektīvas un videi nedraudzīgas. Ilgas pēcspīdēšanas pārklājumi, kas satur SrAl2O4: EU2 +, Dy3 + ir tikuši sagatavoti ar plazmas elektrolītiskās oksidācijas metodi uz komerciālā alumīnija sakausējuma Al6082 virsmas. Šajā procesā esošo elektrisko izlāžu rezultātā stroncija alumināts veidojas līdzīgi kā cietvielu reakcijas metodēs. Rentgenstaru difrakcijas analīze apstiprina, ka pārklājumā ir monoklīniskā SrAl2O4 fāze. Iegūtā pārklājuma optiskās īpašības tika analizētas ar luminiscences metodēm, ko klasiski izmanto luminoforu pētījumos. Pārklājuma veiktspēju salīdzināja ar komerciāli pieejamu stroncija alumināta pulveri. Piedāvātā pārklājuma sintēzes metode var būt noderīga, lai izstrādātu energoefektīvu un ilgstošu automobiļu un sabiedrības drošības infrastruktūru.

      Atslēgvārdi: fosforiscējošais pārklājums, strioncija alumināts, pēcspīdēšana, luminofors, elektrolītiskā oksidācija

      Žurnāls: Materials & Design

       Lejupielādēt PDF

      Novel method of phosphorescent strontium aluminate coating preparation on aluminum

      Author list: Ivita Bite, Guna Krieke, Aleksejs Zolotarjovs, Katrina Laganovska, Virginija Liepina, Krisjanis Smits, Krisjanis Auzins,Larisa Grigorjeva, Donats Millers and Linards Skuja

      This study presents a novel approach to produce phosphorescent coatings on metal surfaces. Strontium aluminates are the most popular modern phosphorescent materials exhibiting long afterglow at room temperature and a broad spectral distribution of luminescence in the visible range. However, despite a large amount of research done, methods for synthesis of such materials remain relatively energy inefficient and environmentally unfriendly. A long-afterglow luminescent coating containing SrAl2O4:Eu2+, Dy3+ is prepared by the plasma electrolytic oxidation on the surface of commercial aluminum alloy Al6082. During the electrical discharges in this process, the strontium aluminate is formed in a similar way to the solid-state reaction method. X-ray powder diffraction analysis confirms that the monoclinic SrAl2O4 phase is present in the coating. Optical properties of the obtained coating were analyzed with luminescence methods classically used for studies of luminophores. The performance of the coating was compared with commercially available strontium aluminate powder. The proposed method of coating synthesis may be of value for the development of energy-efficient and long-lasting automotive and public safety infrastructure.

      Keywords: phosphorescent coating, strontium aluminate, long afterglow, persistent luminophore, electrolytic oxidation

      Published in Materials & Design

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      Niobium enhanced Europium ion luminescence in Hafnia nanocrystals

      K. Laganovska*, I. Bite, A. Zolotarjovs, K. Smits

      In this work we demonstrate a method where by adding Nb ions, Ln3+ ion lumi
      nescence intensity in HfO2 is increased for up to 15 times (in a sample containing
      5mol%Eu). The effect is described as niobium acting as a charge compensator and neutralizing the charge resulting from Ln3+ ion insertion in Hf4+ site and
      hence reducing the number of defects present. This is the second system where
      such an effect was observed, so it is expected that other metal oxides would show the same effect. The optical properties of HfO2:Eu3+ and HfO2:Eu3+, Nb5+,
      synthesized using the sol-gel method and annealed at various temperatures are
      studied. A conclusion that the structure of hafnia does not affect luminescence
      intensity directly and a larger role is played by factors such as defect presence
      and the size of the particles is drawn based on XRD and TSL measurements.
      Time-resolved luminescence measurements were also carried out and significant
      changes depending on dopant concentration and annealing temperatures were
      observed. Judd Ofelt theory was used to determine quantum efficiency and the local symmetry of Eu3+ ion sites.
      Keywords: Hafnia, Luminescence, Eu3+, Charge compensation, Judd Ofelt

      Published in Journal of Luminescence

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      The search for defects in undoped SrAl2O4 material

      Virginija Vitola, Donats Millers, Krisjanis Smits, Ivita Bite, Aleksejs Zolotarjovs

      SrAl2O4:Eu,Dy is a very efficient long afterglow phosphor with wide range of possible applications. The luminescence properties and the possible luminescence mechanism of this material have been studied extensively, but there is almost no information available about the undoped material. Therefore, this article deals with the luminescence and thermally stimulated luminescence of an undoped SrAl2O4, revealing the possible defects that might be involved in the creation of the long afterglow in doped material. We conclude that undoped material exhibits some luminescence under X-ray irradiation in low temperature; close to room temperatures luminescence is almost fully thermally quenched in comparison to low temperatures. We can observe F and F2 center luminescence as wellas trace metal luminescence inthe emission spectrum.TSL glow curveyields the peaksthat are close to those observed in material with Eu and Dy doping; therefore these peaks are clearly related to intrinsic defects. The peak at around 400K, that is shifting with rare earth doping, might be due to dopant interaction with intrinsic defects.

      Published in Optical Materials


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      High resolution luminescence spectroscopy and thermoluminescence of different size LaPO4:Eu3+ nanoparticles

      Tamara Gavrilovića,  Katrīna Laganovska, Aleksejs Zolotarjovs, Krisjanis Smits, Dragana J. Jovanović, Miroslav D. Dramićanin

      Nanoparticles (5nm) and nanorods (2nm×15nm and 4nm×20nm) of monoclinic monazite LaPO4:Eu3+ were prepared by reverse micelle and co-precipitation techniques. Effects of the particle size and surface defects on the intensity of luminescence and the emission spectrum shapes were analyzed by high resolution spectroscopy under laser (266nm) and X-rays excitation. All synthesized LaPO4:Eu3+ samples showed similar spectral features with characteristic Eu3+ ions emission bands: 5D0→7F0 centered at 578.4nm, magnetic-dipole transition 5D0→7F1 at 588–595nm, electric-dipole transition 5D0→7F2 at 611.5–620.5nm, 5D0→7F3 at (648–652nm) and 5D0→7F4 at (684–702.5nm), with the most dominant electric-dipole 5D0→7F2 transition. Additionally, the thermally stimulated luminescence was studied for the most dominant peak at 611.5nm. It was shown that the Eu3+ doping creates traps in all samples. Two prominent and well resolved glow peaks at 58.7K and 172.3K were detected for 5nm nanoparticles, while low-intensity glow-peaks at 212.1K and 212.2K were observed in the X-rays irradiated nanorods. Displayed glows could be attributed to free and bound electrons and holes or to the recombination of electrons of ionized oxygen vacancies with photogenerated holes. To obtain information about the processes and specific defect type it is necessary to carry out additional analysis for all synthesized samples. The glow curves were analyzed and trap parameters were estimated and discussed throughout the paper.

      Published in Optical Materials

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      ERDF Project Nr. KC-PI -2017/90 Mini spectrometer for food analyses (2018)

      The proposed project aims to create a mini spectrometer for food analysis, which would be affordable, small and multifunctional. Our goal is to offer the parties interested a way to constantly control the quality of their products without buying serious lab equipment or hiring additional staff. The first prototype created will be customized specifically for the beer industry – measuring color, turbidity, bitterness, number of live yeast cells and other properties. The measurement results and other information will be displayed in a mobile application which would be connected to the phone via bluetooth. We aim for the application to be as simple, user friendly and straightforward as possible, so that the users could use it from the get-go. Later on we plan on adapting the device to other industries as well, such as dairy and wine industries, and also individual uses to check for lactose or preservative presence.


      Projekta mēķis ir izstrādāt uz spektrofotometrijas balstītu iekārtu pārtikas nozares kvalitātes kontrolei. Lai gan sākotnēji plānots izstrādāt ierīci tieši alus nozarei – krāsas, rūgtuma, duļķainības, rauga veselības u.c. noteikšanai, vēlāk tā tiks papildināta un pielāgota arī citām nozarēm. Ierīces priekšrocības ir tās zemās izmaksas, multifunkcionalitāte, pielāgojamība un mazais izmērs.

      Projekta pirmās kārtas maksimālās kopējās izmaksas ir 25 000 euro ar ERAF atbalsta intensitāti 90%.
      1. kārtas laikā notiek Tirgus izpēte, Tehniski ekonomiskā priekšizpētes un Komercializācijas stratēģijas izstrāde.

      ERAF projekts Nr. – Fosforiscējoša pārklājuma iegūšana plazmas elektrolītiskajā oksidācijas procesā – jaunumi

      Pēdējo pāris mēnešu laikā panākts ievērojams uzlabojums procesa elektrisko parametru kontrolē,ko nodrošināja barošanas bloka veiktā modernizācija. PEO iekārtai ir vairāki uzlabojumi ieskaitot augstāku spriegumu limitu (līdz 1000 V), kā arī ar ievērojami vieglāk lietojamu programmatūru un automatizētiem vadības algoritmiem, kas ļauj nepārtraukti mainīt jebkuru vēlamo parametru procesa laikā. Šīs iespējas ļauj padziļināti izpētīt augsta sprieguma ietekmi uz pārklājuma veidošanas procesu un RE jonu ieviešanu tajā. Turklāt tika uzlabota barošanas avota stabilitāte, nodrošinot ilgāku nepārtrauktu apstrādes laiku pie lielām slodzēm. Tas ir svarīgi dažādiem oksīda fāzes veidošanās procesiem, jo pārkristalizācija notiek tikai ilgākos apstrādes laikos.
      Tika sagatavots paraugu komplekts un turpināta parametru optimizācija. Galvenais mērķis ir nemainīgs: uzlabot ilgspīdošu PEO pārklājumu ar kvalitāti, kas sakrīt vai pārsniedz līdzīgu materiālu kvalitāti.

      X-ray excited luminescence of SrAl2O4:Eu,Dy at low temperatures

      V. Liepina, D. Millers * , K. Smits, A. Zolotarjovs, I. Bite

      SrAl2O4:Eu,Dy is a very efficient long afterglow phosphor with wide range of possible applications. In this study we carried out the study of X-ray excited luminescence of SrAl2O4:Dy and SrAl2O4:Eu,Dy samples, including the measurements of afterglow at low temperatures within extended time scale. We observed both Eu and Dy luminescence peaks in the afterglow and TSL measurements. In recent articles the tunneling of electron from trap levels to excited Eu luminescence center was discussed, and in this research we conclude that under X-ray irradiation Eu2+ and Dy3+ serve as hole traps; electron tunneling is present in both SrAl2O4:Eu, Dy and SrAl2O4: Dy, and luminescence afterglow at 10 K arises from decay of excited Eu2+ and Dy3+ centers created via electron tunneling from host trap to Eu3+ and Dy4+ ions.

      Journal of Physics and Chemistry of Solids

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      Jaunumi ERAF Post Doc project No. Jauni nanoizmēru oksīdu materiāli ar upconversion luminiscenci

      Mēs esam priecīgi paziņot, ka nesen projektā ir panākts ievērojams progress, un ir tikušas izstrādātas un optimizētas četras dažādas uz fosfātiem un vanadānu balstītas fosforu sagatavošanas metodes: augstas temperatūras, cietvielu, līdz-izgulsnēšanās, reversās micellas un koloidālās ķīmijas.
      Tika iegūti paraugi ar dažādām morfoloģijām un izmēriem ar pietiekamu luminiscences intensitāti. Iekļaujot tos silīcija dioksīda viļņvados, daži no tiem varētu atrast potenciālu pielietojumu 1,3 μm fotoluminiscences uzlabošanā.
      Darbs, kurā pētītas luminiscējošās lantāna nanodaļiņas, tika prezentēts starptautiskajā konferencē ‘’Spectral shaping for biomedical and energy applications’’, SHIFT 2017, 13.-17. Novembris, Tenerife, Spānija.
      Publicēšanai tika pieņemts raksts ar detalizētu pētījumu par nanodaļiņu lieluma ietekmi uz dopēta lantāna un europija fosfāta strukturālajām un optiskajām īpašībām. Raksts ir atrodams tiešsaistē. Lepojamies arī ziņot, ka tika iesniegts kopīgs manuskripts no mūsu komandas un pētniekiem no Itālijas, Vjetnamas, Polijas un Serbijas, kuri pētīja fosforus, kuru pamatā ir vanadāta matrica, un pieņemts publicēšanai.
      Mēs turpināsim savu darbu meklējot materiālus ar uzlabotām īpašībām un plašākiem praktiskajiem pielietojumiem.

      ERAF Post Doc project No. Jauni nanoizmēru oksīdu materiāli ar upconversion luminiscenci

      Galvenā darbības joma:Šis starpdisciplinārais projekts aptver plaši pētītu problēmu risinājumus luminiscējošas augšup-pārveidojošo nanomateriālu jomā. Piemēram, virsmas stabilizācija, modifikācija, (bio) funkcionalizācija un materiālu toksiskums, kā rezultātā tiks izstrādāti un sagatavoti daudzfunkcionāli nanomateriāli ar uzlabotām raksturīpašībām.

      Mūsdienās viena no vissarežģītākajām materiālu zinātnes problēmām ir sintētisko metožu, kas nodrošina augstas fāzes tīrības, kristalitātes, lieluma, morfoloģijas un sastāva vienveidīgumu, optimizācija un attīstība. Lai iegūtu šāda veida neorganiskas augšup-pārveidojošās luminiscējošās nanodaļiņas, tiks izmantotas vairākas atšķirīgas sintēzes metodes: koloidālās ķīmijas metodes, inversās micellas tehnika, līdz-izgulsnēšanās, mikroviļņu hidrotermālais, sol-gel, kūstošā sāls un cietvielu.

      Pētījuma projekta laikā tiks izveidotas, pārklātas un modificētas dažādas luminiscējošas augšup-pārveidojošās nanomateriālu grupas. To vidū ir: metālu oksīdi (TiO2, ZnO, ZrO2, HfO2), alumināti (SrAlO4), silikāti (SrSiO4), fosfāti (APO4, A = La, Y, Gd, Lu), vanadāti (piemēram, GdVO4), fluorīdi (NaYF4 , LaF4) un oksisulfīdi (piemēram, Gd2O2S). Turklāt piedāvātās struktūras var tikt izmantotas kā cietvielu lāzeru pastiprinātāji, pretviltošanas etiķetes, kā arī kontrastviela multimodālajai attēlveidošanai, vēža noteikšanai, fotodinamiskā terapija un zāļu piegāde

      ERAF projekts Nr. – Fosforiscējoša pārklājuma iegūšana plazmas elektrolītiskajā oksidācijas procesā – jaunumi

      PEO Projekta izpildē sasniegts nozīmīgs atskaites punkts – pirmo reizi ir iegūti luminiscējoši pārklājumi ar pietiekamu fosforiscences intensitāti, lai to būtu iespējams saskatīt ar neapbruņotu aci. Turpinām darbs pie intensitātes palielināšanas un luminiscences pēcspīdēšanas parametru variēšanas. Sasniegtie rezultāti tiks apkopoti zinātniskajā rakstā.

      Tāpat ir iesniegta publikācija, kurā pētītas stroncija alumināta pulvera optiskās īpašības.

      Sākot ar septembri Boriss Poļakovs un Krišjānis Auziņš ir pievienojušies projekta komandai un aktīvi strādā, lai veicinātu turpmāku projekta veiksmīgu norisi.

      Rezultātu labākai atkārtojamībai, ir veikti uzlabojumi PEO reaktora kamerā (skat. Fotoattēlu), ļaujot pētīt pārklājumus detalizētāk. Tehniskie uzlabojumi ir veikti arī pašos alumīnija paraugos – sagatavēs. Jaunā tipa standartizētos paraugus ir iespējams sadalīt mazākās daļās, pilnībā izslēdzot parauga virsmas nejaušu bojāšanu dalīšanas procesā.

      Doped zirconia phase and luminescence dependence on the nature of charge compensation

      Krisjanis Smits, Dags Olsteins, Aleksejs Zolotarjovs, Katrina Laganovska, Donats Millers, Reinis Ignatans & Janis Grabis

      Zirconia is a relatively new material with many promising practical applications in medical imaging, biolabeling, sensors, and other fields. In this study we have investigated lanthanide and niobium doped zirconia by luminescence and XRD methods. It was proven that charge compensation in different zirconia phases determines the incorporation of intrinsic defects and activators. Thus, the structure of zirconia does not affect the Er luminescence directly; however, it strongly affects the defect distribution around lanthanide ions and the way in which activator ions are incorporated in the lattice. Our results demonstrate the correlation between the crystalline phase of zirconia and charge compensation, as well as the contribution of different nanocrystal grain sizes. In addition, our experimental results verify the theoretical studies of metastable (tetragonal, cubic) phase stabilization determined using only oxygen vacancies. Moreover, it was found that adding niobium drastically increases activator luminescence intensity, which makes Ln3+doped zirconia even more attractive for various practical applications. Although this study was based on the luminescence of the Er ion, the phase stabilization, charge compensation, and luminescence properties described in our results are expected to be similar for other lanthanide elements. Our results suggest that the luminescence intensity of other oxide matrices where lanthanides incorporate in place of tetravalent cations could be increased by addition of Nb ions.

      Scientific Reports 7, Article number: 44453

      DOI: 10.1038/srep44453

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      ERAF projekts -Fosforiscējoša pārklājuma iegūšana plazmas elektrolītiskajā oksidācijas procesā-.

      Līguma numurs

      Projekta ilgums: 27.02.2017 -31.10.2019

      Projekta zinātniskais vadītājs: Dr.phys. Krisjanis Smits


      Projekta mērķis ir ne-saimnieciskajā rūpnieciskajā pētījumā iegūt alumīniju saturošu fosforiscējošu pārklājumu plazmas elektrolītiskās oksidācijas (PEO) procesā.

      Projekta gaitā paredzēts PEO procesā iegūt leģētus ar Eu joniem aktivētus kompleksos oksīdus, izveidot aktivatoru nanokristālu pārklājumu poru aizpildīšanai un, veicot gala oksidāciju, PEO procesā iegūt luminiscentu pārklājumu. Paralēli veicamajām darbībām, tām tiks pielāgota SIA “ElGoo tech” īpašumā esošā PEO aparatūra un tās parametri, kā rezultātā iegūsim validētu nepatentētu tehnoloģiju. Tiks veikta iegūto materiālu īpašību (luminiscences, struktūras un morfoloģijas) izpēte. Plānots iepirkums, kura nolikumā, vērtēšanā un tehniskajā specifikācijā tiks piemēroti Zaļā publiskā iepirkuma kritēriji un principi.

      Ar saimniecisko darbību nesaistīto projektu plānots īstenot kopā ar sadarbības partneri – komersantu SIA “ElGoo tech” no 2017.gada 1.februāra līdz 2019.gada 31.oktobrim, ar kuru kopā ir izstrādāts šis projekts un noslēgts sadarbības līgums. Plānotās projekta izmaksas ir 539 844.50 EUR.

      Plānots iegūt 120 gab. fosforiscējoša pārklājuma paraugus, vienu šī procesa validētas nepatentētas tehnoloģijas aprakstu, 6 zinātniskos rakstus, piesaistīto privāto finansējumu un 1 komersantu, kurš sadarbojas ar pētniecības organizāciju. Pētījumā tiks nodarbināti vismaz 11 darbinieki, 4, kuriem tiks izveidotas jaunas darba vietas.

      Projektu plānots īstenot inovatīvo materiālu un tehnoloģijas nozarē izmantojot starpdisciplinaritāti – dabas zinātnes (1.3 fizika (Physical sciences) un 1.4 ķīmija (Chemical sciences)) un inženierzinātnes un tehnoloģija (2.5 materiālu zinātne (Materials engineering) un 2.10 nanotehnoliģijas (Nano-technology)).

      Tiks pētīta iespēja izveidot luminiscentus pārklājumus, izmantojot plazmas elektroķīmisko oksidāciju (PEO). Patreiz PEO procesā iegūtie pārklājumi tiek aktīvi izmantoti, jo tiem piemīt labas aizsargājošās (liela cietība, laba nodilumizturība, termiskā aizsardzība, pretkorozijas uc.) īpašības. Metālu un dažu tā sakausējumu oksīdiem, ir labas optiskās īpašības, taču, piemēram, luminiscence PEO procesā iegūtiem pārklājumiem iepriekš nav pētīta. Pirmie pētījumi tika veikti SIA “ElGoo tech” sadarbojoties ar LU CFI, kuros iegūti pirmie labi luminiscējošie pārklājumi. Šie rezultāti un literatūrā atrodamie cita veida pētījumi parāda, ka ar PEO metodi iespējams izveidot arī fosforiscentos pārklājums, kuriem būtu ievērojams pielietojumu potenciāls, piemēram, SrAl2O4:Eu:Dy, kuram piemīt vienas no lielākajām kvantu efektivitātēm un ilgākā pēcspīdēšana.

      Tādēļ šī pētījumu projekta ietvaros plānots iegūt alumīniju saturošus pārklājumus ar ilgu pēcspīdēšanu (fosforiscenci). Iegūstamie pārklājumi būs caurspīdīgi tuvajā UV apgabalā, tādejādi gala pārklājumam būs vairākas reizes lielāka efektivitāte salīdzinot ar pārdošanā esošajām fosforiscentajām krāsām. vienlaicīgi fosforiscējošais pārklājums kalpos kā metālu atikorozijas pārklājums un šāda pārklājuma noturība, ilgmūžība būs daudz lielāka salīdzinot ar krāsām.

      Projekta sasniegtajiem rezultātiem pēc projekta beigām tiks nodrošināta:

      1. Finansiālā ilgtspēja – no valsts un privātā sektora budžeta tiks finansētas 4 jaunizveidotās darba vietas un finansēta projekta rezultātu publiskā pieejamība, SIA „ElGoo tech” no savas saimnieciskās darbības peļņas uzturēs projektā modificēto PEO aparatūru un budžetā paredzēs realizēt eksperimentālo pētījumu;
      2. Institucionālā ilgtspēja – 4 projektā iesaistītie zinātniskie darbinieki tiks nodarbināti jaunizveidotajās darba vietās, tā saglabājot pētījumā iegūto praktisko pieredzi un zinātību un izmantojot to tālākajā darbā, savukārt projekta īstenotāji (LU CFI un SIA „ElGoo tech”) apņemas pētījuma rezultātus izmantot savā turpmākajā zinātniskajā un saimnieciskajā darbībā;
      3. Politiskā ilgtspēja – ir nodrošināta, jo projekta mērķi atbilst RIS3 noteiktajiem tautsaimniecības transformācijas virzieniem un izaugsmes 1., 2. un 3.prioritātei. Projektu raksturojošie atslēgas vārdi – plazmas elektroķīmiskā oksidācija, pārklājumi, fosforiscence, luminiscence, oksīdi, nanokristāli.


      Oficiālā lapa par projektu ir atrodama šeit.