2021 г.

Автори: B. Stamboliyska, S. Tapanov, E. Velcheva, D. Yancheva, M. Rogozherov, Z. Glavcheva, G. Lalev, M. Dimitrov

Резюме: The purpose of the present study is to deepen our knowledge about the painting materials and technology characteristics of iconography during the Bulgarian National Revival. Our efforts have focused on studying the murals in the catholicon (main church) “The Nativity of the Virgin” of the Rila Monastery, which is one of Bulgarian leading historical monuments. A series of micro samples from the altar mural paintings were analyzed by a combination of attenuated total reflectance Fourier transform infrared (ATR-FTIR), micro-Raman spectroscopy, scanning electron microscopy energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and Optical microscopy (OM). The data analysis showed that the painter’s palette is made up of nine mineral pigments: smalt, chrome yellow, emerald green, vermilion, red lead, green earths, yellow ochre, sienna and calcite. The impression of rich colorful palette was achieved by using various pigment mixtures. The presence of egg binder in many paint samples suggested that the murals were executed by the traditional egg tempera technique, with the only exception of the smalt paint of the background where carbohydrate glue is present as binder.

Списание: Spectrochimica Acta Part A, 247 (2021) 119087

Автори: S. Todorova, J.L. Blin, A. Naydenov, B. Lebeau, D. Karashanova, H. Kolev, P. Gaudin, R. Velinova, L. Vidal, L. Michelin, L. Josien, D. Filkova, I. Ivanova, A. Dotzeva, K. Tenchev

Резюме: The hierarchical macro-mesoporous silica (MMS) was used for a first time as a support for catalysts for oxidation reactions. The macro-mesoporous silica was synthesized by the emulsions templating mechanism and modified separately or simultaneously using cobalt and manganese oxides. The obtained materials were characterized by different physicochemical methods and tested in the oxidation of CO and n-hexane combustion reactions. The modification of the MMS materials does not change significantly the mesopores characteristics; however, its pores are partially blocked by the oxides. For Co-MM sample agglomerates consisting of Co3O4 with average size of 100−150 nm and small spherical aggregates, encapsulated in the mesopores are formed. The amorphous manganese oxide preferentially fills up the mesopores in Mn-MM sample. Mixed oxide Co-Mn phases situated in the mesoporous network are formed in the bi-component Co-Mn samples. No significant change is observed either in the texture, or in the structural features of the catalysts after reaction.

The highest catalytic activity for Co-MM sample in CO and n-hexane oxidation is related to the predomination of Co3+ species on the surface of Co3O4 and the more accessible oxide particles located outside the mesopores. The encapsulation of mixed Co-Mn oxides particles in the pores of the macro-mesoporous silica is responsible for a lower catalytic activity in comparison with that of the mono-component cobalt sample.

Списание: Spectrochimica Acta Part A, 247 (2021) 119087 Catalysis Today, 2020

Автори: V. Petkov, S. Simeonova, M. Kandeva, R. Valov

Резюме: The objective of this study was to deposit directly chromium with diamond nanoparticles (ND) on aluminum alloys and investigate the coating surface. The chromium coatings on aluminum alloys were obtained by electrochemical deposition. The coatings were doped with ND. The diamond nanoparticles were obtained by detonation synthesis. Chromium coatings were deposited on aluminum alloys with a silicon content of 7 % and 10 %. The ND concentration in the electrolyte was 25 g/l. The surface analysis was performed by means ofAtomic force microscopy. The surface of the coating of chromium with ND on Al10Si is twice more even than that on Al7Si. The microstructure and microhardness were examined with a metallographic microscope and a microhardness tester. The microhardness of the coated samples is 9163 MPa compared to 893 MPa of uncoated aluminum samples. The thickness of the chromium coatings doped with diamond nanoparticles is between 45 – 55 μm. The coatings are dense, continuous and uniform with good adhesion to the substrate material.

Списание: Списание Archives of Foundry Engineering, 20(4):115-120 2020

Автори: Yulian Zagranyarski, Monika Mutovska, Petia Petrova, Reni Tomova, Petar Ivanov, Stanimir Stoyanov

Резюме: A synthetic approach for preparation of a new class of 1,8-naphthalimide derivatives with fused benzo and naphtho-dioxin systems is reported. The proposed reaction conditions for the first step allow selective bromination of 1,8-naphthalic anhydride to the corresponding 3,4,6-tribromo derivative, which has the potential to be versatile building block in the chemistry of naphthalimides. The dioxin-annulated products were isolated in very good yields and their applicability for further modifications by metal-catalysed coupling reactions was proven. The synthesized fluorescent dyes showed media dependant emission and their photophysical and electroluminescent properties were evaluated with respect to their application in OLED. TGA measurements showed that all compounds are stable up to above 400 °C. The best OLED test device has turn-on voltage of 8 V; maximum luminescent intensity of 3031 cd/m2, and current efficiency of 6.9 cd/A.

Списание: Dyes and Pigments, Volume 184, 2021

2020 г.

Автори: K. Lazarova, Bozhilova, S.; Ivanova, S.; Christova, D.; Babeva, T.

Резюме: In this paper, the possibility to design flexible humidity sensors by spin-coating of moisture-sensitive polymer on three types of substrates—poly(ethylene terephthalate) (PET), polylactide (PLA) and composite polysiloxane is investigated. The optical properties, surface morphology and roughness of the substrates covered with polymer are studied by transmittance measurements and surface profiling, respectively. Thin polymer films of amphiphilic copolymerobtained by partial acetalization of poly(vinyl alcohol) are used as humidity sensitive media. The sensing properties are probed through transmittance measurements at different levels of relative humidity (RH). The influence of substrate type is studied by comparing the hysteresis of flexible sensors with those that are deposited on glass substrates.

Списание: Engineering Proceedings MDPI, 42, 16, 2020

Автори: D. Gerginova, S. Simova, M. Popova, M. Stefova, J. P. Stanoeva, V. Bankova

Резюме: Bulgaria and North Macedonia have a long history of the production and use of honey; however, there is an obvious lack of systematic and in-depth research on honey from both countries. The oak honeydew honey is of particular interest, as it is highly valued by consumers because of its health benefits. The aim of this study was to characterize honeydew and floral honeys from Bulgaria and North Macedonia based on their NMR profiles. The 1D and 2D 1H and 13C-NMR spectra were measured of 16 North Macedonian and 22 Bulgarian honey samples. A total of 25 individual substances were identified, including quinovose, which was found for the first time in honey. Chemometric methods (PCA—principal component analysis, PLS-DA—partial least squares discriminant analysis, ANOVA—analysis of variance) were used to detect similarities and differences between samples, as well as to determine their botanical and geographical origin. Semiquantitative data on individual sugars and some other constituents were obtained, which allowed for the reliable classification of honey samples by botanical and geographical origin, based on chemometric approaches. The results enabled us to distinguish oak honeydew honey from other honey types, and to determine the country of origin. NMR was a rapid and convenient method, avoiding the need for other more time-consuming analytical techniques.

Списание: Molecules 25, 4687-4698 (2020)

Автори: Maya G. Shopska, Iskra Z. Shtereva, Hristo G. Kolev, Krasimir K. Tenchev, Silviya Z. Todorova, Georgi B. Kadinov

Резюме: The paper is focused on evaluation of active centres and adsorbed species type impact on 10%Co+0.5%Pd)/Al2O3 catalyst system performance aiming selectivity optimization. Application of different sets of precursor pretreatment and reductions resulted in catalysts exhibiting high CO conversion or high methane selectivity. Sample of high selectivity was prepared by pretreatment in hydrogen and the performance was determined by lower amount of strongly adsorbed CO, strongly adsorbed carbonate species, higher amount of reduced metal and bimetallic particles. A more active system was formed by pretreatment in air leading to larger amount of unreduced metal and CO-bridged species on the surface, stable coverage of hydroxyl groups on the support, and medium-strength sites for adsorption of carbonates. Ratios of adsorbed H/СО and strongly/weakly СО species appeared as important criteria for catalyst efficiency together with supported metal state, amount of unreduced ions, bimetallic particle formation, and alumina’s ability to adsorb CO and CO2.

Списание: Croatica Chemica Acta, Vol. 93 No. 2, 2020, https://hrcak.srce.hr/247328

Автори: R. Georgiev, Georgieva, B., Lazarova, K., Vasileva, М., Babeva, T.

Резюме: Present study aims at development of thin flms of sol–gel Ta2O5 with tunable refractive index that could be used as an active medium for volatile organic compound (VOCs) optical detection. In order to tune the refractive index, mesoporosity is introduced through soft templating approach where series of co-polymers (Pluronics) are utilized as structuredirecting agents. Thin flms are prepared by spin-coating method using tantalum sol synthesized from TaCl5 as a precursor and ethanol as a solvent. The temperature annealing at 320 °C fnalizes the tantalum oxide lattice formation and decomposes the template being used. The morphology and elemental composition of the flms are studied by transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy in STEM, respectively, and the amorphous status of the flms is confrmed by selected area electron diffraction. Non-linear curve ftting method and non-destructive UV–Vis refectance measurements are used for determination of optical parameters of the flms. The free volume fraction in the flms is estimated through modeling of refractive index by Bruggeman efective medium approximation. The flms are exposed to acetone vapors as a commonly studied VOC. The reaction of the flm due to vapors’ exposure is recorded as a change in the refection spectra of the flm and the adsorbed acetone quantity is estimated. The infuence of template composition on optical and sensing properties of Ta2O5 flms is revealed and discussed.

Списание: Opt Quant Electron 52, 437 (2020)

Автори: D. Tsyganov, N. Bundaleska, J. Henriques, E. Felizardo, A. Dias, M. Abrashev, J. Kissovski, A. M. Botelho do Rego, A. M. Ferraria, and E. Tatarova

Резюме: An experimental and theoretical investigation on microwave plasma-based synthesis of free-standing N-graphene, i.e., nitrogen-doped graphene, was further extended using ethanol and nitrogen gas as precursors. The in situ assembly of N-graphene is a single-step method, based on the introduction of N-containing precursor together with carbon precursor in the reactive microwave plasma environment at atmospheric pressure conditions. A previously developed theoretical model was updated to account for the new reactor geometry and the nitrogen precursor employed. The theoretical predictions of the model are in good agreement with all experimental data and assist in deeper understanding of the complicated physical and chemical process in microwave plasma. Optical Emission Spectroscopy was used to detect the emission of plasma-generated “building units” and to determine the gas temperature. The outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman, and X-ray photoelectron spectroscopies.

Списание: Materials, MDPI, 2020, 13(18), 4213 (IF 3.057)

Автори: Petar Todorov, Stela Georgieva, Petia Peneva, Rusi Rusew, Boris Shivachevc and Anton Georgiev

Резюме: Herein, the synthesis and characterization of four novel 5,50 -diphenylhydantoin Schiff bases containing different aromatic species are presented. Their structure–property relationship was studied by X-ray, optical and electrochemical methods as well as DFT calculations in terms of their E/Z photoisomerization and enol/keto phototaumerization. The big challenge in photoinduced motion is achieving control and stability over the two isomers. Solvent-driven bidirectional photoswitching behavior was studied in nonpolar 1,4-dioxane and polar aprotic DMF. T-type photochromism in 1,4-DOX and opposite behavior in DMF as P-type switches (bistable system) were observed. The obtained results lead to a conclusion that by variation of the solvent environment a direct control over the bidirectional switching behaviour from T-type to P-type can be achieved.

Списание: New J. Chem., Royal Society of Chemistry, 2020, 44, 15081

Автори: N. Burdzhiev, A. Ahmedova, B. Borrisov, R. Graf

Резюме: Tautomerization of 2-phenylimidazolecarbaldehydes has not been studied in detail so far, although this process is a well-known phenomenon for imidazole derivatives. That is why we focus our study on a series of 2-phenylimidazolecarbaldehydes and their parent alcohols that were synthesized and studied by detailed 1H and 13C NMR in solution and in the solid state. The apparent problem is that the fast tautomerization impedes the full structural description of the compounds by conventional 13C NMR measurements. Indeed, the 13C NMR spectra in solution exhibit poor resolution, and in most cases, signals from the imidazole ring are not detectable. To avoid this problem, we used 13C CP-MAS NMR as an alternative spectroscopic method for unambiguous spectroscopic characterization of the studied series of 2-phenylimidazoles. The data were analyzed in combination with quantum chemical DFT-GIAO methods by considering the tautomerization process and the intermolecular interactions. The DFT (B3LYP/6-31G(d,p)) calculations allowed to identify and suggest the preferred tautomer in the gas phase and in DMSO solvent, which for alcohols are (2-phenyl-1H-imidazol-4-yl)methanol and its analogs, and for the aldehydes are the 2-phenyl-1H-imidazole-5-carbaldehydes. The gas-phase calculated energy differences between the two possible tautomeric forms are in the range 0.645–1.415 kcal/mol for the alcohols and 2.510–3.059 kcal/mol for the aldehydes. In the DMSO solvent, however, for all compounds, the calculated energy differences go below 1.20 kcal/mol. These data suggest that both tautomeric forms of the studied 2-phenylimidazoles can be present in solution at room temperature. Our data from detailed 2D NMR measurements in the solid state (1H-13C HETCOR and 1H-1H double-quantum coherence MAS NMR) suggested that also in the solid state both tautomers coexist in different crystalline domains. This fact does not obscure the 13C CP-MAS NMR spectra of the studied 2-phenyl substituted imidazoles and suggests this spectroscopic method as a powerful tool for a complete structural description of tautomeric systems with aromatic conjugation.

Списание: Molecules, MDPI, 2020, 25(17), 3770

Investigation of photophysical, electrochemical and electroluminescent properties of Iridium(III)bis[2 phenylbenzo[d] thiazolato N,C2′ ] quinolin 8 olate for white organic light emitting diodes application

Автори: P. Ivanov, P. Petrova, R. Tomova

Резюме: The complex Iridium(III)bis[2-phenylbenzo[d]thiazolato-N,C2 ′]-quinolin-8-olate ((bt)2Irq) was synthesized and identifed by 1 H NMR and IR spectroscopy. Its photophysical, electrochemical and electroluminescent properties for White Organic Light-Emitting Diodes (WOLED) application were investigated. It was established that (bt)2Irq irradiates yellow-orange light in CH2Cl2 (DCM) solution and solid flm with maximum peaks, respectively, at 578 and 666 nm and shoulders at 628 and 720 nm. The complex was used as stand-alone emitter and a dopant in the matrixes of hole transporting layer (HTL) or electroluminescent layer (EL) in the range from 0 to 16.5 w%. The base OLED structure was Anode/HTL/EL/ETL/Cathode, where HTL is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) included in a matrix of poly(N-vinylcarbazole) (PVK), EL is Aluminium (III) bis(2-methyl-8-quninolinato)-4-phenylphenolate (BAlq) and ETL is an electron transporting layer of Tris-(8-hydroxyquinoline) aluminium (Alq3) or Zinc (II) bis(2-(hydroxyphenyl) benzothiazole (Zn(bt)2). It is found that the electroluminescent spectra of both types OLEDs were the sum of the greenish-blue and yellow-orange emissions of BAlq and Ir complex and CIE coordinates of the emitted light remain within the white range over wide dopant concentrations. The best current efciency and CIE coordinates close to the ideal white (0.3300; 0.3300) demonstrate devices with 2 wt% doped HTL (6 Cd/A, (0.2735; 0.3613)) and 16.5 w% doped EL (5.1 Cd/A, (0.2721; 0.3284)). It was found that both types of doped devices operate through a charge trapping mechanism.

Списание: Journal of Materials Science: Materials in Electronics 31 (2020) 15707–15717

Автори: N. Bundaleska, A. Dias, N. Bundaleski, E. Felizardo, J. Henriques, D. Tsyganov, M. Abrashev, E. Valcheva, J. Kissovski, A. M. Ferraria, A. M. Botelho do Rego, A. Almeida, J. Zavašnik, U. Cvelbar, O. M. N. D. Teodoro, Th. Strunskus, and E. Tatarova

Резюме: The ability to change the secondary electron emission properties of nitrogen-doped graphene (N-graphene) has been demonstrated. To this end, a novel microwave plasma-enabled scalable route for continuous and controllable fabrication of free-standing N-graphene sheets was developed. High-quality N-graphene with prescribed structural qualities was produced at a rate of 0.5 mg/min by tailoring the high energy density plasma environment. Up to 8% of nitrogen doping levels were achieved while keeping the oxygen content at residual amounts (~1%). The synthesis is accomplished via a single step, at atmospheric conditions, using ethanol/methane and ammonia/methylamine as carbon and nitrogen precursors. The type and level of doping is afected by the position where the N-precursor is injected in the plasma environment and by the type of precursors used. Importantly, N atoms incorporated predominantly in pyridinic/pyrrolic functional groups alter the performance of the collective electronic oscillations, i.e. plasmons, of graphene. For the frst time it has been demonstrated that the synergistic efect between the electronic structure changes and the reduction of graphene π-plasmons caused by N doping, along with the peculiar “crumpled” morphology, leads to sub-unitary (< 1) secondary electron yields. N-graphene can be considered as a prospective low secondary electron emission and plasmonic material.

Списание: Nature – Scientific Reports 10, 13013 (2020);

Автори: Dancho Yordanov, Vera Deneva, Anton Georgiev, Aurelien Crochet, Katharina M. Fromm, Liudmil Antonov

Резюме: The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV–Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (KT in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol−1 of 4 and 0.63 kcal mol−1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.

Списание: Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy,237,118416, 2020

Автори: Koseva, P. Tzvetkov, P. Ivanov, P. Petrova, R. Tomova, A. Yordanova, V. Nikolov

Резюме: Powder samples of Ca2GeO4 doped with 0.5, 1, 2 and 3 at.% Tb3+ were prepared via solid-state synthesis technique. The obtaining of pure phase at all dopant concentrations was proved through XRD analyses. In the emission and excitation spectra of the obtained powders, the characteristic peaks of Tb3+ ion were observed. The Tb3+ excitation spectrum in the range from 300 to 500 nm shows characteristic transitions of Tb3+, attributed to the f–f transitions. The strongest peak is located at 379 nm corresponding to the 7F6 → 5D3 transition. The main emission peak of Tb3+ is 5D4 → 7F5 transitions at 545 nm, corresponding to green color. Other transitions are located at 416 (5D3 → 7F5), 437 (5D3 → 7F4), 458 (5D3 → 7F3), 488 (5D4 → 7F6), 588 (5D4 → 7F4), 621 (5D4 → 7F3), 651 (5D4 → 7F2) and 675 (5D4 → 7F1) nm. The optimum emission is observed for 2 at.% Tb3+ ion concentration. At this concentration dominate also (5D4 → 7F4) and (5D4 → 7F3) transitions. CIE coordinates of the samples show different emission colors depending on the active ion concentration. The obtained results confirm that as-prepared terbium-doped materials could be used like green, yellow and reddish phosphors.

Списание Journal of Optics 49 (2020) 403–407,

Автори V. Lyutov and V. Tsakova

Резюме The redox behavior of polyaniline (PANI) doped with polysulfonic acids is studied in slightly acidic and neutral solutions (from pH 4 to pH 7). The dopants used are polystyrene sulfonic acid (PSSA), poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) and poly-(4,4’-(2,2′-disulfonic acid)-diphenylene-tere-phthalamide) (t-PASA). Thin PANI layers are used in order to identify clearly the redox peak structure. Based on deconvolution of the reductive currents the pH dependence of the constituent peaks is studied and pathways of the reductive transitions are suggested. With increasing pH a loss of electroactivity is observed for all three dopants but to a different extent: the largest for PAMPSA- and the smallest PSSA-doped PANIs. In the same time a stabilization of the conductive emeraldine salt state is observed at pH 7 for all PANIs with largest extent of stabilization in the PANI/PAMPSA case. The electrocatalytic properties of the polysulfonic acids-doped PANIs are comparatively studied for ascorbic acid (AA) and dopamine (DA) oxidation at pH 7. It is found that all three types of PANI are good electrocatalysts for both reactions with advantage of the PAMPSA-doped layers showing higher absolute oxidative currents for both reactions. The results concerning DA oxidation demonstrate the possibility to involve polysulfonic acid-doped PANIs in electrocatalytic reactions in neutral solutions occurring at high positive potentials beyond the peak potential of the PANI oxidative transition

Списание: Journal of Solid State Electrochemistry (2020) 24:3113-3123

Автори: T. Todorova, P. Shestakova, T. Petrova, M. Popova, H. Lazarova, Y. Kalvachev

Резюме: The post-synthesis treatment of AlZSM-5 and GaZSM-5 zeolites by etching with buffer solution of ammonium fluoride and 0.25 M HF acid was carried out. The treatment is applied in order to obtain secondary pores in crystals and to provide easier access to the catalytically active zeolite centers. Hydrothermal synthesis of these zeolites was performed from systems containing tetrapropylammonium bromide as a template. The reaction parameters of synthesis conditions for both zeolites were optimized in order to obtain pure crystalline phases. The zeolites obtained were characterized by X-ray diffraction, IR spectroscopy, scanning electron microscopy, physical adsorption–desorption of nitrogen and solid-state NMR spectroscopy. It has been found that the result materials remain with high crystallinity while new mesoporous are created in their structure. The volume due to the presence of mesopores increases by up to 67% from the total volume, which is a drastic increase compared with the parent solids. The investigations with FTIR spectroscopy of low-temperature CO adsorption and solid-state NMR spectroscopy show that the treatment is slightly selective toward dissolution of heteroatom (aluminum or gallium). This phenomenon is observed for the first time for gallium-substituted zeolites. In order to study the influence of the metal atom in zeolite structure and the efficacy of the acid attack on the catalytic activity, the samples obtained were tested in reaction of m-xylene and toluene transformation.

Списание:  J. Mater. Sci. 55, 13799-13814 (2020)

Автори: St. Todorova, V. Rangelova, L. Mihaylov, T. Spassov

Резюме: MmNi5 hydrogen storage alloy with an average particle size of 120 μm was subjected to hydriding treatment at 50oC, under 40 atm. pure hydrogen atmosphere. As a result, a large density of particle cracks was formed, which led to an increase in the particle surface area. The enlarged particles surface, in turn, led to substantially improved hydrogen sorption kinetics compared to the as-received (untreated) alloy. It was also found that hydrogen-induced decrepitation of the MmNi5 powder results in enhancement of the hydrogen diffusivity mainly due to shortening of its diffusion distances.

Списание: International Journal of ELECTROCHEMICAL SCIENCE

Int. J. Electrochem. Sci., 15 (2020) 4900 – 4907, doi: 10.20964/2020.06.29

Автори: Elena Stoykova, Dimana Nazarova, Lian Nedelchev, Kwan-Jung Oh, Joongki Park

Резюме: Analysis of dynamic speckle patterns enables visualization of faster or slower changes for 3D objects. Compression of data in this type of metrology by coarse quantization at non – uniform illumination is studied by simulation and experiment.

Списание: Imaging and Applied Optics Congress 2020, Optical Society of America (OSA), paper HTh5H.5 

Автори: L. Nedelchev, B. Blagoeva, G. Mateev, D. Nazarova, and E. Stoykova

Резюме: Characteristics of polarization holographic gratings (PHG) in non-covered azopolymer thin films and azopolymer films covered with glass slide are compared. An improvement of the polarization properties of the PHG in the covered films is established.

Списание: Imaging and Applied Optics Congress, Technical Digest, Optical Society of America, 2020,

Автори: E. Haladjova, M6. Smolíček, I. Ugrinova, D. Momekova, P. Shestakova, Z. Kroneková, J. Kronek, S. Rangelov

Резюме: Poly(2-methyl-2-oxazoline)-polyethylenimine (PMeOx-co-PEI) copolymers differing by degree of polymerization (DP = 50 and 200) and PEI content (from 37 to 99 mol%) were synthesized by living cationic ring-opening polymerization of 2-methyl-2-oxazoline, followed by partial hydrolysis. Upon mixing with DNA in a wide range of N/P ratios, they formed well-defined polyplex particles of small size (typically below 100 nm) and narrow size distribution. The polyplexes demonstrated good colloidal stability and very low in vitro cytotoxicity. The copolymers exhibited buffering capacity of over 50% relative to that of the reference PEI implying effective endo-lysosomal escape of the polyplexes. Increased cellular internalization of both PCR fragments and plasmid DNA, attributable to the strongly positive ζ potential and small size of the polyplexes, was observed. In spite of these favorable prerequisites, the transfection efficiency was low (below 20% relative to the control PEI) and was attributed to retarded swelling of the polyplex particles, endo-lysosomal rupture, and DNA release.

Списание: Journal of Applied Polymer Science. 137 (45), 49400 (2020) 

Автори: K. Lazarova, Bozhilova, S., Christova, D., Babeva

Резюме: The optical and humidity-sensing properties are studied of hydrophobically modified PVA, namely poly(vinyl alcohol-co-vinyl acetal)s of varied copolymer composition. Copolymers are synthesized by reacting PVA with acetaldehyde in aqueous solution, thus introducing cyclic acetal functionalities in the polymer chain. Thin polymer films are deposited by spin-coating and their optical properties are studied as a function of the copolymer composition, i.e., degree of acetalization. Reflectance measurements at different relative humidities in the range 5 – 95 % RH are conducted in order to probe the sensing behavior. The comparison with neat PVA thin films confirms the influence of the copolymer’s acetal fraction on the optical and sensing properties and degree of hysteresis. The feasibility of applying poly(vinyl alcohol-co-vinyl acetal) thin films for optical sensing of humidity is demonstrated and discussed.

Списание: Journal of Physics: Conference Series, 1492, 2020.

Автори: Dimitre Dimitrov, Che-Liang Tsai, Vera Marinova, Stefan Petrov, Dimitrina Petrova, Blagovest Napoleonov, Blagoy Blagoev, Velichka Strijkova, Ken Yuh Hsu and Shiuan Huei Lin

Резюме: The integration of high uniformity, conformal and compact transparent conductive layers into next generation indium tin oxide (ITO)-free optoelectronics, including wearable and bendable structures, is a huge challenge. In this study, we demonstrate the transparent and conductive functionality of aluminum-doped zinc oxide (AZO) thin films deposited on glass as well as on polyethylene terephthalate (PET) flexible substrates by using an atomic layer deposition (ALD) technique. AZO thin films possess high optical transmittance at visible and near-infrared spectral range and electrical properties competitive to commercial ITO layers. AZO layers deposited on flexible PET substrates demonstrate stable sheet resistance over 1000 bending cycles. Based on the performed optical and electrical characterizations, several applications of ALD AZO as transparent conductive layers are shown—AZO/glass-supported liquid crystal (LC) display and AZO/PETbased flexible polymer-dispersed liquid crystal (PDLC) devices.

Списание: Coatings 10, 539; (2020)  

Автори: M. V. Abrashev,  V. G. Ivanov,  B. S. Stefanov,  N. D. Todorov, J. Rosell, and  V. Skumryev

Резюме: Synthetic powder, ore samples, and mineral single crystals of goethite (αα-FeOOH) were investigated with polarized Raman spectroscopy at temperatures from 293 K to 473 K. The symmetry of the vibrational modes, observed in different scattering configurations, was determined unequivocally. The assignment of the Raman-active modes to definite atomic vibrations is supported by two types of lattice-dynamical calculations: empirical shell model and ab initio density functional theory. The temperature dependencies of the line shape parameters of some Raman-active vibrations at temperatures near to the antiferromagnetic–paramagnetic phase transition infers for a significant spin–lattice coupling in this compound. The most informative in this aspect is the B3gB3g phonon at 387cm−1387cm−1, which overlays a broad scattering background and displays a pronounced asymmetric Fano-line shape. The asymmetry increases in the paramagnetic state above the Néel temperature (TN=393KTN=393K), indicating a strong interaction of this mode with the underlying excitation continuum. The origin of the excitation background is discussed in light of our experimental results and the existing data for the magnetic structure and transport properties of αα-FeOOH. By using the molecular-orbital dimer approach, we calculate the spin–phonon coupling constants for the B3gB3g mode as J′=−0.2eV/ÅJ′=−0.2eV/Å and J″=+0.18eV/Å2J″=+0.18eV/Å2, respectively. Thus, we rationalize that, most probably, the scattering background stems from magnetic excitations, and the asymmetric shape of the B3gB3g band is a result of a linear spin–phonon coupling of this mode with the Fe–O1–Fe spin dimers. Another mechanism, a phonon interaction with thermally activated charge carriers above TNTN, is also considered.

Списание: Journal of Applied Physics 127, 205108 (2020);

Автори: Tanya Tsoncheva, Ivanka Spassova, Gloria Issa, Radostina Ivanova, Daniela Kovacheva, Daniela Paneva, Daniela Karashanova, Nikolay Velinov, Boiko Tsyntsarski, Biliana Georgieva, Momtchil Dimitrov, Nartzislav Petrov

Резюме: This work is focused on the preparation of Ni0.5M0.5Fe2O4 mixed ferrites (M=Zn or Cu), supported on nanoporous carbons materials. The carbon supports are obtained from various waste residues, such as peach stones from the canning industry and by-products from the low rank coals pyrolysis. X-ray difraction (XRD), nitrogen physisorption, high-resolution transmission electron microscopy (HRTEM), Moessbauer spectroscopy and temperature programmed reduction (TPR) analyses as well as mesoporous silica with tridimensional structure type KIT-6 silica based reference samples are used for detail characterization of the obtained materials. It was established that the loaded on the carbon supports phase is a complex mixture of fnely dispersed ferrite, substituted magnetite, metal (Cu, Fe, FeNi alloy) and ZnO particles. Their dispersion and composition depend on the texture characteristics of the carbon support, which could be easily controlled by the waste precursor used. The existence of mesoporosity in the carbon host matrix provokes the formation of more fnely dispersed and easily reducible spinel particles, which ensures higher initial catalytic activity, but fast deactivation of the catalysts. The formation of activated carbon mesoporosity is facilitated by the presence of cellulose and hemicellulose in the biomass or the addition of furfural to the coal tar pitch precursor. The Ni0.5Zn0.5Fe2O4 modifcations demonstrate higher potential as catalysts for hydrogen production via methanol decomposition.

Списание: Waste and Biomass Valorization, 2020

Автори: Katerina Lazarova, Silviya Boycheva, Marina Vasileva, Denitza Zgureva and Tsvetanka Babeva

Резюме: In this study, solid waste from coal combustion in thermal power plants (TPPs) was used for the synthesis of zeolite Na-X samples. They were prepared by the long-term alkaline atmospheric conversion of coal ash collected from the electrostatic precipitators in the TPP “AES Galabovo”. When used in the form of thin films/layers, the optical detection of volatile organic compounds (VOCs) is possible due to a change in their reflectance spectra and color. In order to improve the sensing properties of synthesized zeolites, they were wet milled for 60 s and both milled and unmilled zeolites were used as dopants for the niobium oxide matrix in the form of thin films deposited by the spin-coating method on a silicon substrate. The surface morphology and structure of both zeolite powders were studied by scanning electron microscopy, while their size was determined by dynamic light scattering (DLS) spectra. Optical constants (refractive index, n, and extinction coefficient, k) and the thickness of the films were calculated from reflectance measurements. The change in the reflection coefficient ∆R of the films was determined from measured reflectance spectra prior to and after exposure to probe acetone molecules. An increase in the reaction of the films with milled zeolites to acetone, compared to the samples with unmilled zeolites, is demonstrated.

Списание: Materials Proceedings 2020, 2, 3.

Автори: Silviya Boycheva, Denitza Zgureva, Katerina Lazarova, Tsvetanka Babeva, Cyril Popov, Hristina Lazarova and Margarita Popova

Резюме: Fly ash (FA) from lignite coal combusted in different Thermal Power Plants (TPPs) was used for the synthesis of zeolites (FAZs) of the Na-X type by alkaline activation via three laboratory procedures. FAZs were characterized with respect to their morphology, phase composition and surface properties, which predetermine their suitability for applications as catalysts and adsorbents. FAZs were subsequently modified with metal oxides (CuO) to improve their catalytic properties. The catalytic activity of non-modified and CuO-modified FAZs in the total oxidation of volatile organic compounds was investigated. FAZs were studied for their potential to retain CO2, as their favorable surface characteristics and the presence of iron oxides make them suitable for carbon capture technologies. Thin films of FAZs were deposited by in situ crystallization, and investigated for their morphology and optical sensitivity when exposed to pollutants in the gas phase, e.g., acetone. This study contributes to the development of novel technological solutions for the smart and valuable utilization of FA in the context of the circular economy and green energy production.

Списание: Materials 2020, 13(9), 2014 

Автори: Anton Georgiev, Dancho Yordanov, Deyan Dimov, Ivailo Zhivkov, Dimana Nazarova, Martin Weiter

Резюме: Herein, we report the synthesis and E→Z photoswitching behavior of two 4-substituted azomethine phthalimides containing anthracenyl and 4-(dimethylamino)phenyl moieties (EAMP1 and EAMP2). These compounds represent newly synthesized and unstudied photoswitches with dual fluorescence properties as E-isomers and at photostationary state (PSS) depending on the solvent polarity. Steady-state fluorescence measurements were performed in various solvents and the results show strong sensitivity on the environmental polarity. The kinetics of E→Z photoswitching to PSS was studied in AcCN by visible light activation at 410 nm (EAMP1) and long wavelength UV-light activation at 350 nm (EAMP2). The quantitative and qualitative performance of the switching behavior was evaluated by the degree of photoisomerization (R) and the rate constant (k). It was found for EAMP1 R = 6.95 %, k = 8.87 × 10−4 s −1 and EAMP2 R = 88.72 %, k = 4.00 × 10−4 s −1 , respectively. The reason for the lower photoconversion of EAMP1 compared to the EAMP2 was analyzed through optimization of the molecular geometry of E- and Z-isomers in the ground state (S0) and first excited state (S1) by DFT/ TD-DFT calculations with B3LYP/6-31+G(d,p) level of theory using IEFPCM in AcCN. It was found that Eisomers in the S0 have nonplanar conformation, while the Z-isomer of EAMP1 prefers twisted conformation and the Z-isomer of EAMP2 T-shaped conformation is energetically advantageous compared to the twisted one. The reason is the weak H…..π noncovalent interaction (NCI) between 4-(dimethylamino)phenyl moiety and phthalimide ring. Moreover, the Z-isomer of EAMP2 is unusual stable up to 600 min at room temperature in dark compared to the EAMP1, which undergoes full Z→E relaxation for less than 60 min at the same conditions. The Z→E relaxation of EAMP2 is achieved for 90 min at 60 °C. The fluorescence E→Z switching behavior was studied by emission measurements in AcCN and 1,4-DOX as E-isomers and at PSS in room and liquid nitrogen (77 K) temperatures. In the polar and nonpolar solvents, red-shifted emissions with increased fluorescence quantum (Φfl) yields have been observed at PSS compared to the E-isomers. The molecular rotor behavior was studied in the binary mixture of glycerol:ethanol and the results show a sensitivity of the emission bands depending on the environmental viscosity. Time-resolved fluorescence decay measurements were performed in AcCN and 1,4-DOX as E-isomers and at PSS to estimate the mechanism of fundamental fluorescence bands. We found that dyes at PSS have longer lifetime (τ) compared to the E-isomers, especially in less polar 1,4-DOX.

Списание: Journal of Photochemistry and Photobiology A: Chemistry, Volume 393, 2020 

Автори: Blagoeva, B., Nedelchev, L., Mateev, G., Stoykova, E., Nazarova, D.

Резюме: We present a study of the diffraction efficiencies of polarization holographic gratings recorded in thin films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]). Two series of layers have been prepared using two different solvents – distilled water and methanol. The gratings are inscribed by two plain waves with orthogonal circular polarizations (left and right circular) from a He-Cd gas laser (442 nm) at recording angle 20°, corresponding to grating period 1.3 μm. Higher diffraction efficiency is obtained for the thin film samples spin-coated from the methanol solution for thicknesses below 600 nm. Diffraction efficiency higher than 27% was achieved, as well as surface relief height more than 500 nm.

Списание: Photosensitive Materials and their Applications, Proceedings, Volume 11367; 113671G (2020),

Автори: Mariela Tsanova-Stamatova, Neshka Manchorova-Veleva, Tsvetanka Babeva

Резюме: 3D profilometry is a highly technological method successfully used in dentistry for evaluating materials’ roughness. The aim of our in vitro study was to estimate the effect of mechanical and chemical agents on surface roughness and microscopic structure of zirconia polycrystals by means of 3D profilometry. Seven groups of yttria-stabilized KATANA™Zirconia (Kuraray Noritake Dental Inc.) samples were prepared: one control group (no pretreatment); four groups sandblasted with 50 µm and 250 µm Al2O3 particles at a pressure of 2 and 4 bars and two groups treated with HF (hydrofluoric acid) of 10% and 40% concentration. Each specimen was with diameter and thickness of 11.8 mm and 5.0 mm, respectively. 3D optical profilometry was used to study the surface topography and roughness of the samples. By comparison with control group it was demonstrated that the most suitable treatment that leads to the highest surface roughness values is sand blasting with 50 µm Al2O3 particles at a pressure of 4 bars. This method can be suggested as an effective method for surface pretreatment of zirconia.

Списание: Comptes rendus de l’Academie bulgare des Sciences, 73, (11), No 11, 1510-1516, 2020, DOI: 10.7546/CRABS.2020.11.04

Автори: D. Tsyganov, N. Bundaleska, A. Dias, J. Henriques, E. Felizardo, M. Abrashev, J. Kissovski, A. M. Botelho do Rego, A. M. Ferraria, and E. Tatarova

Резюме: Free-standing N-graphene was synthesized using a microwave plasma-based method at atmospheric pressure conditions through a single step and in a controllable manner. Using ethanol and ammonia as precursors, N-graphene with low relative amount of bonded oxygen and low level of saturated sp3 carbon bonds was produced. Adjusting the injection position of the nitrogen precursor in the plasma medium leads to selectivity in terms of doping level, nitrogen configuration and production yield. A previously developed theoretical model, based on plasma thermodynamics and chemical kinetics, was further updated to account for the presence of nitrogen precursor. The important role of HCN attachment to the graphene sheets as the main process of N-graphene formation is elucidated. The model predictions were validated by experimental results. Optical Emission Spectroscopy was used to detect the emission of plasma generated ‘‘building units’’ and to determine the gas temperature. The plasma outlet gas was analyzed by Fourier-Transform Infrared Spectroscopy to detect the generated gaseous by-products. The synthesized N-graphene was characterized by Scanning Electron Microscopy, Raman and X-ray photoelectron spectroscopies.
Списание: PCCP, Royal Society of Chemistry, 2020, 22, 4772, (IF 3.567).

Автори: R. Georgiev, K. Lazarova, M. Vasileva, T. Babeva

Резюме: Vapor responsive all Nb2O5 Bragg stacks were prepared using spin-coating of alternating dense and porous flms with quarter-wavelength thickness. The required porosity in the flms was achieved by soft-templating method using micellar solution of Pluronic copolymer as an organic template. The surface morphology and structure of the flms were studied by transmission electron microscopy and selected area electron difraction, respectively, while optical characterization was performed by combination of ellipsometric and refectance measurements. Vapor sensing response of the flms was tested by refectance measurements of the samples prior to and after exposure to acetone vapors selected as probe molecules. Incorporation of “defect” layer in the multilayered structures was frstly simulated theoretically and then realized experimentally aiming at improvement of the optical response to volatile organic compounds. Furthermore, the potential of all niobia stacks consisting of alternating dense and porous sol–gel Nb2O5 flms as chemical sensors with optical read-out is demonstrated and discussed.

Списание: Optical and Quantum Electronics, 52:114, 2020

Автори: Valentina Lyubomirova, Veronika Mihaylova, Rumyana Djingova

Резюме: The accurate determination of macroelements (Na, K, Ca, Mg and Si) in potable waters with inductively-coupled plasma mass spectrometry (ICP-MS) is difficult due to their high concentrations. In the present study, optimisation of cell-based ICP-MS for application of a bandpass parameter (RPa) for signal reduction was performed to extend their linear ranges. Individual values of the RPa for each isotope have been selected. A method for the determination of macroelements without dilution using optimised RPa values has been developed and applied for potable water analysis. The accuracy was evaluated by analyses of a surface water reference material.

Списание: Spectroscopy Europe, 32 (5), 2020

2019 г.

Автори: Katerina Lazarova, Silvia Bozhilova, Siika Ivanova, Darinka Christova and Tsvetanka Babeva

Резюме: Hydrophobically modified poly(vinyl alcohol)s of varied copolymer composition were tested as active media for optical sensing of humidity. Copolymer thin films were deposited on silicon substrate using water-methanol solution in a volume ratio of 20:80 and concentration of 1 wt%. Films were subjected to low (60 °C) and moderate (180 °C) temperature annealing in order to study the temperature influence on optical and humidity sensing properties. Refractive index, extinction coefficient along with thickness of the films were determined by non-linear minimization of the goal function comprising measured and calculated reflectance spectra at normal light incidence. The humidity sensing ability of the films was studied through reflectance measurements at different humidity levels in the range 5–95 %RH. The influence of temperature annealing on optical and sensing properties was demonstrated and discussed.

Списание: Proceedings 2019 (MDPI), 42, 16; doi:10.3390/ecsa-6-06555

Автори N.M. Ferreira, A.R. Sarabando, S. Atanasova-Vladimirova, R. Kukeva, R. Stoyanova, B.S. Ranguelov, F.M. Costa

Резюме Mg-Al-Si-O glassy systems have a great importance in a wide range of industrial applications, specifically as an electrolyte for molten oxide electrolysis processes in steelmaking. Understanding how the iron oxidation state of the raw material (Fe2+/Fe3+) and its corresponding amount influence this glassy system’s properties will be the aim of the current work. Iron oxides (as Fe2O3 or Fe3O4) were used to dope Mg-Al-Si-O system obtaining amorphous materials through an unconventional method: Laser Floating Zone (LFZ). Above 8% mol of Fe formation of magnetic phases or iron clusters, were observed in the glass matrix. Samples with Fe2O3 showed a higher crystal concentration, when compared with Fe3O4. The electron paramagnetic resonance measurements show a strong dependence on the iron source (Fe3O4 or Fe2O3). In addition, the magnetization decreases linearly with iron content, independently of iron oxidation state, except for samples with a higher concentration of Fe2O3(15% mol), due to sample crystallization. Moreover, with Fe3O4 as raw material there is an improvement (~250 times) of the electrical conductivity when compared with Fe2O3. The results show that the presence of Fe2+ on the glass influences the electrical conductivity, which could have impact in the efficiency of molten oxide electrolysis process.

Списание Ceramics International, 45 (17) Part А, 21379-21384, doi.org/10.1016/j.ceramint.2019.07.125

Автори Miroslav V. Abrashev, Petko Chernev, Paul Kubella, Mohammad Reza Mohammadi, ChiaraPasquini, Holger Dau and Ivelina Zaharieva,

Резюме Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure–activity relations, we investigate the influence of moderate annealing (100–300 °C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure–reactivity relations. Oxide structures featuring both di-μ-oxo bridged Mn ions and (close to) linear mono-μ-oxo Mn3+–O–Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like ζ-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 °C) and this is the first report where it was stable under ambient conditions.

Списание J. Mater. Chem. A, 2019, 7, 17022

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Автори L. Nedelchev, D. Ivanov, B. Blagoeva, D. Nazarova

Резюме Azopolymers are highly efficient materials, able to register the polarization state of light. They have broad range of applications from diffractive optical elements with unique polarization properties to biophysics. We present a study of birefringence kinetics, induced with five different pump lasers with wavelengths 355, 442, 491, 514 and 532 nm in thin films of widely used amorphous azopolymer PAZO – poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfon amido]-1,2-ethanediyl, sodium salt]. During the real-time measurement, the value of the photoinduced birefringence (Δn) is determined at 635 nm, using a DPSS laser. When saturation is reached, the spectrum of the birefringence is measured. Our results indicate that the investigated azopolymer can be used for applications in a wide wavelength range.

Списание, Journal of Photochemistry & Photobiology A: Chemistry, 376, 1–6, doi.org/10.1016/j.jphotochem.2019.02.009

Автори G. Mateev, L. Nedelchev, A. Georgiev and D. Nazarova

Резюме We present a study of the photoinduced birefringence in nanocomposite films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfon amido]-1,2-ethanediyl, sodium salt]) doped with TiO2 nanoparticles (NP) with different concentrations before and after thermal annealing. The NP represent nanopowder with primary particle size 21 nm. The concentration of the NP was varied from 0% (non-doped azopolymer film) to 5 wt%. The thermal process, applied to the nanocomposite films, includes 1 h heating at 200°C. Previous studies of PAZO show that the polymer is stable up to 270°C. We study the dependence of the maximal birefringence induced with He-Cd laser (λ= 442 nm) on the concentration of the TiO2 NP in the azopolymer thin films as well as thermal effect on the absorbance spectra of the thin films. As indicated by our results, the birefringence is higher for the thermally annealed samples. An increase of the photoinduced birefringence is also observed for the nanocomposite layers with 1% NP for the non-annealed films, and with 2% NP for the annealed films.

Списание Open Materials Science, 4, 1–5 doi.org/10.1515/oms-2019-0001

Автори K. Lazarova, D. Christova , R. Georgiev, B. Georgieva and T. Babeva

Резюме Thin films with nanometer thicknesses in the range 100–400 nm are prepared from double hydrophilic copolymers of complex branched structures containing poly(N,N-dimethyl acrylamide) and poly(ethylene oxide) blocks and are used as humidity sensitive media. Instead of using glass or opaque wafer for substrates, polymer thin films are deposited on Bragg stacks and thin (30 nm) sputtered Au–Pd films thus bringing color for the colorless polymer/glass system and enabling transmittance measurements for humidity sensing. All samples are characterized by transmittance measurements at different humidity levels in the range from 5% to 90% relative humidity. Additionally, the humidity induced color change is studied by calculating the color coordinates at different relative humidity using measured spectra of transmittance or reflectance. A special attention is paid to the selection of wavelength(s) of measurements and discriminating between different humidity levels when sensing is performed by measuring transmittance at fixed wavelengths. The influence of initial film thickness, sensor architecture, and measuring configuration on sensitivity is studied. The potential and advantages of using top covered Bragg stacks and polymer/metal thin film structures as humidity sensors with simple optical read-outs are demonstrated and discussed.

Списание Nanomaterials, 9, 875, doi:10.3390/nano9060875

Автори B. Blagoeva, G. Mateev, D. Nazarova, L. Nedelchev

Резюме In this work we study the dependence of the photoinduced birefringence on the initial temperature of the layer, as well as the possibility of recording and termal erasure in thin azopolymer films. To determine the temperature dependence, the sample is placed on a hot stage, which can maintain or vary the temperature of the sample with a high precision. Birefringence is induced with a linearly polarized light from a laser at 442 nm, and its value is determined by measuring the Stokes parameters of a probe laser beam after the sample. The birefringence is erasured by placing the sample on a hot stage.

Списание Journal of Physics and Technology 3(1), 3-7

Автори D. Ivanov, V. Strijkova, L. Nedelchev, D. Nazarova and E. Borisova

Резюме This study presents optical and chemical methods to visualize healthy and malignant sections of histological samples, by using polarized light imaging and staining with different contrast agents. This approach complements the diagnostic conclusions made by the physicians and improves qualitatively healthy versus tumor tissue differentiation, which may be in practical use for pathologistsin their diagnostic conclusions.

Списание Journal of Physics and Technology, 3(1) 14-17

Автори G. Mateev, D. Nazarova and L. Nedelchev

Резюме In this article, we investigate the photoinduced birefringence in commercially available azo-polymer PAZO doped with TiO2 spherical nanoparticles (21 nm size). The films are prepared by spin-coating water solution of the polymer mixed with the nanoparticles with different concentrations from 0 to 10 wt %. To induce the birefringence we use He-Cd laser with wavelength 442 nm. For measurement of the effect, probe laser with wavelength 635 nm is used. The experimental setup includes also polarimeter, which measures the Stokes parameters in real time. We observe increase of the photo-induced birefringence in azopolymer films doped with TiO2 nanoparticles. The maximal value of n is achieved for the sample with 1 wt. % concentration.

Списание Journal of Physics and Technology, 3(1) 18-21

Автори S. Kozhukharov, V. Zhelev and S. Tchaoushev

Книга Advances in Materials Science Research Vol. 37, Ed. By: M. Wythers, NOVA Sci. Publ. pp.101-166; ISBN: 978-1-53615-038-4

Автори Stephan V. Kozhukharov and Christian A. Girginov

Книга Phenomena and Theories in Corrosion Science. Methods of Prevention, ed. By A. Gergely NOVA Sci. Publ. pp. 3 – 150; ISBN 978-153-615253-1