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Tsvetanka Babeva   Dr.  Institute, Department or Faculty Head 
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Tsvetanka Babeva published an article in March 2019.
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Top co-authors
Svetlana Mintova

51 shared publications

State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corp. (CNPC)China University of Petroleum (East China) Qingdao 266555 P.R. China

Hussein Awala

21 shared publications

Laboratoire Catalyse et Spectrochimie, Normandie University, ENSICAEN, UNICAEN, CNRS, 14050 Caen, France;(S.K.);(M.Z.);(H.A.);(V.R.)

Marina Vasileva

5 shared publications

Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Katerina Lazarova

4 shared publications

Institute of Optical Materials and Technologies, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

Sébastien Thomas

4 shared publications

Laboratoire Catalyse & Spectrochimie, Université de Caen, 6, boulevard du Maréchal Juin, 14050 Caen Cedex, France; s:(H.A.);(S.T.);(S.M.)

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Distribution of Articles published per year 
(2014 - 2019)
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Publications
Article 0 Reads 0 Citations Aluminum-doped zinc oxide thin films deposited by electrospray method Georgi Marinov, Konstantin Lovchinov, Violeta Madjarova, Vel... Published: 01 March 2019
Optical Materials, doi: 10.1016/j.optmat.2019.01.055
DOI See at publisher website
PROCEEDINGS-ARTICLE 0 Reads 0 Citations Organic framework engineering in mesoporous Nb2O5 thin films used as an active medium for organic vapors sensing Rosen Y. Georgiev, Darinka Christova, Biliana Georgieva, Tsv... Published: 05 June 2018
Advances in Optical Thin Films VI, doi: 10.1117/12.2322527
DOI See at publisher website
Article 2 Reads 1 Citation Thin films from hydrophilic poly(N,N-dimethyl acrylamide) copolymers as optical indicators for humidity K Lazarova, L Todorova, D Christova, M Vasileva, R Georgiev,... Published: 01 January 2017
Journal of Physics: Conference Series, doi: 10.1088/1742-6596/794/1/012022
DOI See at publisher website
Article 0 Reads 0 Citations NOVEL METHODS FOR THE ASSESSMENT OF CRACK PROPAGATION IN ENDODONTICALLY TREATED TEETH Еkaterina Karteva, Neshka Manchorova-Veleva, Vesela Stefanov... Published: 20 September 2016
Journal of IMAB - Annual Proceeding (Scientific Papers), doi: 10.5272/jimab.2016223.1308
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Article 0 Reads 4 Citations Optical Properties of Sol-Gel Nb 2 O 5 Films with Tunable Porosity for Sensing Applications Rosen Georgiev, Biliana Georgieva, Marina Vasileva, Petar Iv... Published: 01 January 2015
Advances in Condensed Matter Physics, doi: 10.1155/2015/403196
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Thin Nb2O5 films with tunable porosity are deposited by the sol-gel and evaporation induced self-assembly methods using organic template Pluronic PE6100 with different molar fractions with respect to NbCl5 used as a precursor for synthesis of Nb sol. Surface morphology and structure of the films are studied by Transmission Electron Microscopy and Selected Area Electron Diffraction. The optical characterization of the films is carried out through reflectance spectra measurements of the films deposited on silicon substrates and theoretical modeling in order to obtain refractive index, extinction coefficient, and thickness of the films. The overall porosity of the films and the amount of adsorbed acetone vapors in the pores are quantified by means of Bruggeman effective medium approximation using already determined optical constants. The sensing properties of the samples are studied by measuring both the reflectance spectra and room-temperature photoluminescence spectra prior to and after exposure to acetone vapors and liquid, respectively. The potential of using the studied mesoporous Nb2O5 films for chemooptical sensing is demonstrated and discussed.1. IntroductionIn recent years, Nb2O5 emerges as a multifunctional material due to its interesting properties such as electrochromic behavior [1], photoelectric and photocatalytic activity [2], excellent chemical stability, and corrosion resistance in both acidic and alkaline media [3]. Besides, many applications in photonics demand high refractive index materials with good optical quality and negligible scattering. Thus, various innovative applications have been developed and thin Nb2O5 films have found applications in photonics for improving the optical performance of different devices such as optical filters [4], waveguide-based optical circuits [5], and transparent conductive electrodes [6]. A number of novel applications of Nb2O5 films rely on the ability to deposit high quality films using relatively simple and inexpensive techniques such as sol-gel and spin or dip coating. The sol-gel method attracts considerable scientific attention because of its versatility, low cost, and low temperature processing [7, 8]. Besides, it allows control of the microstructure of the coating and produces durable and chemically stable films. Moreover, the versatility of the sol-gel process to prepare porous film has an additional advantage to be exploited.Mesoporous materials (pore diameter in the range of 2–50 nm) have attracted the scientific interest due to their remarkable properties and potential applications in many advanced areas such as drug delivery systems, sensors, catalysis, photovoltaic cells, and fuel cells [9]. One attractive application of the mesoporous films can be realized when they are incorporated in one-dimensional stacks (Bragg stacks) as high refractive index building blocks [10, 11]. In this case, an optical sensing could be realized on the basis of refractive index change due to the condensation of vapors in the pores [12–14].Among several methods of porous films fabrication, the evaporation-induced self-assembly (EISA) process is the most advantageous because it offers good reproducibility and the reactions are finely controlled without any special techniques simply by changing sizes of the used templates and mole ratios of template to inorganic species [15–17]. Block copolymers are very attractive materials as soft-templating agents because they tend to self-assemble into micelles whose morphology and size depend on block composition and solution parameters [18, 19].Recently, we have shown that it is possible to control and optimize the optical and sensing properties of mesoporous Nb2O5 by changing the porosity degree and free volume fraction inside the films through varying the molar ratio of organic template (PE6800, BASF) and NbCl5 used for preparing the Nb sol [20]. An improvement of sensing properties of mesoporous films is demonstrated as compared to the dense films (films prepared without template). However, despite the high volume fraction of free space inside the films (around 50%), only small amount of acetone is adsorbed (1.5%). Bad interconnectivity of the pores and their encapsulation inside the film are discussed as probable reasons for the weak response. In order to improve the sensing properties of thin Nb2O5 films, we intend to use Pluronic PE6100 as an organic template. As compared to PE6800, this template has four times lower molecular weight and 8 times lower percentage of polyethylene glycol (PEG) in the molecule. Moreover, in contrast to PE6800, PE6100 is insoluble in hydrochloric acid. Considering that Nb sol used for the preparation of the films consists of some amount of hydrochloric acid left as a side product from the synthesis, we expected PE6100 to lead to stronger porosity.In the present paper, we study thin Nb2O5 films with tuned porosity prepared by the sol-gel and evaporation-induced methods using Pluronic PE6100 as an organic template. In order to tailor the porosity, different molar fractions of PE6100 with respect to NbCl5 are used. Optical properties and thickness of the films are calculated from measured reflectance spectra through nonlinear curve fitting. Bruggeman effective medium approximation is applied for estimation of the overall porosity of the films and the amount of adsorbed acetone vapors in the pores. Reflectance spectra and room-temperature photoluminescence spectra collected prior to and after exposure to acetone are used for proving the sensing abilities of the films. The potential of using the studied mesoporous Nb2O5 films for chemooptical sensing is demonstrated and discussed.2. Materials and MethodsThin niobia (Nb2O5) films were prepared by spin-coating (3000 rpm, 30 s) of Nb sol and subsequent high temperature annealing at 320°C for 30 min at 5°C/min acceleration rate [8]. The Nb sol was synthesized by a sonocatalytic method using 0.400 g NbCl5 (99%, Aldrich) as a precursor, 8.3 mL ethanol (98%, Sigma-Aldrich), and 0.17 mL distilled water [8].The synthesis of mesoporous films was carried out through an evaporation-induced self-assembly method using organic template Pluronic PE6100 (BASF). PE6100 is a block copolymer in which the central polypropylene glycol group (PPG) is flanked by two polyethylene glycol groups (PEG). The molar mass of PPG and its percentage in the molecule are 1750 g/mol and 90%, respectively. The mass ratio of template to NbCl5 for different samples is given in Table 1. After deposition, the template is eliminated by annealing at 320°C for 30 min at 5°C/min acceleration rate.Table 1: Refractive index (), extinction coefficient (), thickness (), volume fractions of air () and acetone () in %, absolute reflectance change , and refractive index change after exposure to acetone vapors as a function of Pluronic PE6100/NbCl5 ratio.The surface morphology and structure of the films were studied by Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction (SAED) using HRTEM JEOL JEM 2100 (Japan) microscope.The optical properties (refractive index and extinction coefficient ) along with the thickness of the films were determined from reflectance spectra of the films measured at normal light incidence by UV-VIS-NIR spectrophotometer Cary 05E (Varian, Australia) using nonlinear curve fitting method [8]. The experimental errors for , , and are 0.005, 0.003, and 2 nm, respectively.Photoluminescence spectra of the films were measured at excitation wavelength of 325 nm in the range of 340–640 nm using Fluorolog-3 spectrofluorometer (Horiba/Jobin-Yvon, France).The vapor sensing measurements of the films were implemented in a Cary 05E spectrophotometer equipped with a homemade bubbler system for generation of vapors from liquids with controlled concentrations [11]. Reflectance spectra of the samples were measured before and after exposure to the vapors.3. Results and DiscussionsFigure 1 shows typical TEM images of the dense samples deposited without organic template (Figure 1(a)) and porous one obtained with addition of PE6100 in ratio of PE6100 : NbCl5 = 0.07 : 1 (Figure 1(b)). The comparison of morphology shows that the template leads to the formation of pores in the mesosize range arranged nonperiodically. The SAED diffraction (shown as insets) had proven that all samples were amorphous. This result is consistent with our previous studies of sol-gel Nb2O5 films that revealed amorphous structure for film annealed at 320°C and crystalline one when films are annealed at 450°C [21].Figure 1: TEM images of dense Nb2O5 film (a) and porous Nb2O5 film (b) prepared with the ratio of PE6100 : NbCl5 equal to 0.07 : 1. SAED diffraction patterns are shown as insets.Figure 2(a) presents reflectance spectra of the films in the spectral range from 320 to 900 nm. The comparison between dense film (s1) and porous ones (s2–s5) shows stepwise red shift of the minima with increasing of the Pluronic concentration.Figure 2: Reflectance spectra (a) and refractive index dispersion curves (b) of the studied Nb2O5 films.The measured spectra were further used for calculation of refractive index , extinction coefficient , and thickness of the films. Figure 2(b) presents the dispersion curves of refractive index for the films with different amount of PE6100. All curves exhibit normal dispersion ( decreases with increasing of wavelength of light) that is expected for spectral region of slight absorption. With increasing the amount of PE from 0 to 0.07 (Table 1), a substantial decrease in from 2.198 to 1.615 measured at 600 nm is observed. It is interesting to note that further increase of PE from 0.07 to 0.14 does not lead to additional significant change of . The next decrease in is observed for value of PE of 0.23: changes from 1.668 to 1.507. No further reduction of for s5 is noticed although PE increases twice as compared to s4.The obtained dependencies for are consistent with the data calculate
Article 4 Reads 10 Citations Vapor Responsive One-Dimensional Photonic Crystals from Zeolite Nanoparticles and Metal Oxide Films for Optical Sensing Katerina Lazarova, Hussein Awala, Sébastien Thomas, Marina V... Published: 09 July 2014
Sensors, doi: 10.3390/s140712207
DOI See at publisher website PubMed View at PubMed ABS Show/hide abstract
The preparation of responsive multilayered structures with quarter-wave design based on layer-by-layer deposition of sol-gel derived Nb2O5 films and spin-coated MEL type zeolite is demonstrated. The refractive indices (n) and thicknesses (d) of the layers are determined using non-linear curve fitting of the measured reflectance spectra. Besides, the surface and cross-sectional features of the multilayered structures are characterized by scanning electron microscopy (SEM). The quasi-omnidirectional photonic band for the multilayered structures is predicted theoretically, and confirmed experimentally by reflectance measurements at oblique incidence with polarized light. The sensing properties of the multilayered structures toward acetone are studied by measuring transmittance spectra prior and after vapor exposure. Furthermore, the potential of the one-dimensional photonic crystals based on the multilayered structure consisting of Nb2O5 and MEL type zeolite as a chemical sensor with optical read-out is discussed.
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