Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes

This work was funded by the European Regional Development Fund (ERDF) project no. 1.1.1.1/19/A/138. A.S. and K. S. acknowledge the funding from the European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under grant agreement No. 739508, project CAMART2.Nitrogen-doped multiwalled carbon nanotubes (N-MWCNTs) are known as a perspective material for a variety of applications in nanoelectronic devices, sensors, catalysts for carbon dioxide reduction, and flexible thermoelectrics. However, up to date most of the reports on the properties of N-MWCNTs are focused on a narrow niche of research, for example, a study of low-temperature magnetoresistance or room-temperature thermoelectrical properties. In this work, N-MWCNTs were synthesized using benzene:pyridine precursor in different ratios, and both magnetoresistance and thermoelectrical properties of the synthesized N-MWCNTs were systematically investigated in the temperature range 2-300 K and compared with the properties of undoped MWCNTs. Unexpected switching of the magnetoresistance of the N-MWCNTs at low temperatures from negative to positive values was observed, and the processes underlying this effect are discussed. The study of the thermoelectrical properties revealed n-type conductance in the N-MWCNTs, which was attributed to the impact of nitrogen defects incorporated in the MWCNT structure. Performed for the first-time investigations of the thermal stability of the Seebeck coefficient of N-MWCNTs in air revealed that the Seebeck coefficient retains its negative values and even increases after annealing of the N-MWCNTs in air at 500 °C. These findings illustrate the high potential of the presented in this work N-MWCNTs for applications in different devices in a wide range of temperatures. --//-- Jana Andzane, Mikhail V. Katkov, Krisjanis Buks, Anatolijs Sarakovskis, Krisjanis Smits, Donats Erts, Synthesis, magnetoresistance, and thermoelectrical properties of environmentally stable n-type nitrogen-doped multiwalled carbon nanotubes, Carbon Trends, Volume 13, 2023,100302, ISSN 2667-0569, https://doi.org/10.1016/j.cartre.2023.100302. (https://www.sciencedirect.com/science/article/pii/S2667056923000573). Published under the CC BY-NC-ND licence.ERDF project no. 1.1.1.1/19/A/138. A.S; European Union's Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-Teaming Phase2 under grant agreement No. 739508, project CAMART2

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

The publication is prepared with the financial support of National Research Programme IMIS2.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 Eu 2+ creation – the thermally released electrons recombination and electron tunneling from trap to Eu3+. The possible scheme of long lasting luminescence mechanism is proposed.IMIS2; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

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.This research project was supported financially by ERDF Project No: Nr.1.1.1.1/16/A/182; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

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

The support from M -ERANET project “MyND” is acknowledged. K.K. was partially supported by the Collaborative Research Project of Laboratory for Materials and Structures, Tokyo Institute of Technology . Visiting researcher support from Ecole Polytechnique, Palaiseau is appreciated. Mr. Olivier Cavani is thanked for the expert help with irradiations.Point defects in crystalline SiO2, created by 2.5 MeV electron irradiation at dose below the amorphization threshold 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.M -ERANET; Tokyo Institute of Technology; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Niobium enhanced europium ion luminescence in hafnia nanocrystals

This work was supported by the UL ISSP grant for Scientific Research Projects for Students and Young Researchers SJZ/2016/15 .In this work we demonstrate a method where by adding Nb ions, Ln3+ ion luminescence intensity in HfO2 is increased for up to 15 times (in a sample containing 5 mol%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.ISSP UL SJZ/2016/15; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Thermostimulated luminescence of plasma electrolytic oxidation coatings on 6082 aluminium surface

Financial support provided by Scientific Research Project for Students and Young Researchers Nr. SJZ/2016/12 realized at the Institute of Solid State Physics, University of Latvia is greatly acknowledged. Authors are grateful to R. Ignatans for measurements and analysis of XRD spectra and ElGoo Tech ltd. for providing PEO equipment.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.ISSP UL Scientific Research Project for Students and Young Researchers Nr. SJZ/2016/12; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

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

This work was conducted with the funding of Scientific Research Project for Students and Young Researchers realised at the Institute of Solid State Physics, University of Latvia [SJZ/2018/2].Ever since the discovery of SrAl2O4:Eu,Dy persistent afterglow material, that can intensively glow up to 20 h, 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.Latvijas Universitate SJZ/2018/2; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

The search for defects in undoped SrAl2O4 material

This research project was supported financially by ERDF Project No: Nr.1.1.1.1/16/A/182 .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.European Regional Development Fund Nr.1.1.1.1/16/A/182; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

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

This research project was supported financially by ERDF Project No 1.1.1.1/16/A/182.ERDF Project No 1.1.1.1/16/A/182; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Luminescence properties of chlorine molecules in glassy SiO2 and optical fibre waveguides

The support from Latvian Research Program IMIS 2, project “Photonics and materials for photonics” is acknowledged. K.K. was partially supported by the Collaborative Research Project of Materials and Structures Laboratory, Tokyo Institute of Technology. The publication costs of this article were covered by the Estonian Academy of Sciences and the University of Tartu.Glassy SiO2 is the basic material for optical fibre waveguides and manufacturing-induced Cl impurities reduce their transparency in UV spectral range. This work reports in-depth study/spectroscopic parameters of the near-infrared (1.23 eV) low-temperature photo-luminescence (PL) of interstitial Cl2 molecules in SiO2. The zero-phonon line position was estimated at 2.075 eV on the basis of anharmonicity of Cl2 PL vibronic data. The vibronic sub-bands are broadened by coupling to phonons and by an additional contribution from the glassy disorder. The Huang‒Rhys factor is ≈13. The PL decay time is between 1 and 10 ms in the temperature range 100 K‒13 K and can be reproduced by 3 exponents. Cl2 PL retains relatively high quantum yield and its characteristic structured shape, when the temperature is increased from 13 K to the liquid nitrogen temperature. This allows using it conveniently as a high-sensitivity diagnostic tool for detecting Cl2 impurities in optical fibre waveguides. Time-resolved measurements of optical fibre waveguides indicate that the lower detection limit is below 1010 Cl2/cm3.Tokyo Institute of Technology, MSD K.K.,Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART²