Julijana Velevska - Academia.edu (original) (raw)

Papers by Julijana Velevska

Prussian Blue (PB) thin films were prepared by chemical bath deposition onto fluorine doped tin o... more Prussian Blue (PB) thin films were prepared by chemical bath deposition onto fluorine doped tin oxide (FTO) coated glass substrates. Elelctrochromic behavior of the films was examined in an electrochromic device (ECD) consisted of home-built glass cell with a 1 mol/dm3 KCl as an electrolyte, PB film as working, and FTO as counter electrodes. These films showed electrochromism changing color from deep blue in as deposited state, to green in oxidized state, and back to blue and transparent (colorless) in reduced state. Visible transmittance spectra of the PB thin films in bleached and colored states were recorded in-situ. Absorption coefficients spectra were calculated using the transmittance spectra. The output integral of the spectral intensity and the integral of the spectral modulation were calculated by taking the solar irradiance spectrum AM 1.5 for a normal illumination on a PB-based ECD, and the absorption coefficients spectra of the PB films in their bleached and colored states.

9TH INTERNATIONAL PHYSICS CONFERENCE OF THE BALKAN PHYSICAL UNION (BPU-9), 2016

Prussian blue (PB) is an anodically coloring material which finds its application in the wide are... more Prussian blue (PB) is an anodically coloring material which finds its application in the wide area of electrochromic devices, due to its four-color polyelectrochromicity. Here we use electrochemical impedance spectroscopy as a technique to determine the electrical behavior of an electrochromic Prussian blue device under as deposited (0V), green (2V) and bleached (-2V) states. The values of real and imaginary part of the impedance for green and bleached state, confirm the higher rate of charge transport during the oxidation process and lower rate during reduction process. The experimental data of impedances were used to propose equivalent circuit and to determine the parameters of the device system applying nonlinear least square (NLLS) method.

International Journal of Sciences: Basic and Applied Research, Feb 15, 2017

Nickel oxide (NiO x) thin films were prepared by three different deposition techniques: electroch... more Nickel oxide (NiO x) thin films were prepared by three different deposition techniques: electrochemical (EC) deposition, low vacuum evaporation (LVE), and chemical bath deposition (CBD). Those films were deposited onto fluorine doped tin oxide (FTO) coated glass substrates. Electrochromic behavior of the films was examined in an electrochromic device (ECD) constructed by using NiO x films as working electrodes, together with the FTO coated glass as a counter electrode in alkaline environment (0.1 M NaOH aqueous solution). The films exhibited anodic electrochromism, changing color from transparent to dark brown. Visible transmittance spectra of the nickel oxide thin films in bleached and colored states were recorded in-situ. Absorption coefficients spectra were calculated using the transmittance spectra. The output integral of the spectral intensity and the integral of the spectral modulation were calculated by taking the solar irradiance spectrum AM 1.5 for a normal illumination on a nickel oxide-based ECD, and the absorption coefficients spectra of the nickel oxide films in their bleached and colored states.

Materials Research Bulletin, Dec 1, 2011

Radiation & Application, 2017

In this work, electrochromic Prussian blue (PB) and tungsten oxide (WO3) thin films have been pre... more In this work, electrochromic Prussian blue (PB) and tungsten oxide (WO3) thin films have been prepared by the chemical bath deposition method. The films have been deposited onto fluorine doped tin oxide (FTO) coated glass substrates. Electrochromic behavior of each film was studied by cyclic voltammetry. An electrochromic test device (ECTD) was constructed by using WO3 as a working electrode, together with a PB film as an opposite (counter) electrode, and aqueous solution of 1 mol/dm 3 KCl, slightly acidified with 2 drops of conc. HCl in 100 ml, as an electrolyte. The optical transmission spectra of the bleached and colored states were recorded in the visible part of the spectrum. The contrast ratio was calculated from these spectra. The coloration efficiency and the time response of the ECTD were also examined.

International Journal of Sciences: Basic and Applied Research, Mar 18, 2016

The manifestation of electrochromic phenomena makes Prussian blue (PB) thin films very attractive... more The manifestation of electrochromic phenomena makes Prussian blue (PB) thin films very attractive, especially because they exhibit four-color polyelectrochromicity. In this work PB thin films were prepared by simple and low cost shemical deposition method. The films were deposited onto fluorine doped tin oxide (FTO) coated glass substrates. An electrochromic test device (ECTD) was constructed by using these films as working electrodes, and FTO coated glass as an opposite electrode in aqueous solution of 1 mol/dm 3 KCl as supporting electrolyte. The electrochemical properties of the films were characterized by cyclic voltammetry. The obtained films exhibited electrochromism, changing color from deep blue in as deposited state into green, and back to blue and colorless. Visible transmittance spectra of PB films were studied in-situ in as deposited, colored and bleached states. From those spectra, contrast ratio (CR) and the optical band gaps E g were evaluated. The dependence of the optical density (OD) on charge density was examined and the coloration efficiency (CE) was calculated to be 149.8 cm 2 C-1. The time response during the film switching between transparent and blue states was also examined.

Journal of Electrochemical Science and Engineering, Mar 22, 2017

Tungsten oxide (WO3) thin films were prepared by a simple, economical, chemical bath deposition m... more Tungsten oxide (WO3) thin films were prepared by a simple, economical, chemical bath deposition method onto fluorine doped tin oxide (FTO) coated glass substrates. The electrochemical properties of the films were characterized by cyclic voltammetry. The obtained films exhibited electrochromism, changing color from initially colorless to deep blue, and back to colorless. Visible transmittance spectra of (WO3) films were recorded insitu in their both, bleached and colored states. From those spectra, absorption coefficient () and the optical energy gaps were evaluated. The dependence of the optical density on the charge density was examined and the coloration efficiency () was calculated to be 22.11cm 2 C-1. The response times of the coloring and bleaching to an abrupt potential change from-2.5 V to +2.5 V and reverse, were found to be 9.3 and 1.2 s respectively. The maximum light intensity modulation ability of the films, when the AM1.5 spectrum is taken as an input, was calculated to be about 50 %.

Silpakorn University Science and Technology Journal, 2012

The electrical characteristics of MOS capacitors with Ta 2 O 5 as oxide and with different gate e... more The electrical characteristics of MOS capacitors with Ta 2 O 5 as oxide and with different gate electrodes (Al, Au, W, TiN) are investigated using high frequency capacitance-voltage (C-V) and current-voltage (I-V) measurements. The influence of deposition techniques of gate electrode (reactive sputtering and evaporation) and the type of electrode material (different work functions) are observed. Charged trapping properties were studied by measuring the gate voltage shift due to trapped charge generation in order to investigate the response to constant current stress (CCS) under various current/time conditions at room temperature. The results showed that the presence of pre-existing electron traps leads the changes of time dependent voltage during gate injected CCS in the initial stage, followed by the slow positive charge build-up, same for all the structures and characteristic for this oxide. Gate-induced defects due to ther rate of reaction of gate electrode with the oxide are responsible for different behavior of the structures with different gate electrodes, observed in the initial stage of CCS. The Au-gated devices appear to be the most susceptible to the constant current stress degradation. They also have highest values of capacitance and lowest leakage currents.

Silpakorn University Science and Technology Journal, 2011

Macedonian Journal of Chemistry and Chemical Engineering, Dec 20, 2012

International Journal of Sciences: Basic and Applied Research, Jul 18, 2018

In this work a simple chemical method for depositing cobalt hexacyanoferrate (CoHCF) films has be... more In this work a simple chemical method for depositing cobalt hexacyanoferrate (CoHCF) films has been developed. The films have been prepared by successive immersion of the fluorine doped glass substrates (FTO) into an acidic aqueous solution of CoCl 2 and K 4 [Fe(CN) 6 ]. The characterization of the films with X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) showed that the films have crystalline structure. The electrochemical properties of the films were characterized by cyclic voltammetry. Obtained films exhibited electrochromism, changing colour reversibly between transparent and brown. Visible transmittance spectra of CoHCF films in their bleached and coloured states were recorded insitu. Those spectra were used to estimate the optical band gaps. The dependence of the optical density on charge density was examined and used to calculate the colouration efficiency. The response times of the colouring and bleaching to an abrupt potential change from-2 V to +2 V and reverse were also examined. The maximum light intensity modulation ability of the films and saved energy, when the AM 1.5 spectrum is taken as an input, were calculated to be 55% and 243.56 Wm-2 , respectively, which makes this films suitable for application in electrochromic devices.

Solar Energy Materials and Solar Cells, Jun 1, 2002

Nickel oxide (NiO x) thin films were prepared by a low vacuum evaporation (LVE) onto fluorine dop... more Nickel oxide (NiO x) thin films were prepared by a low vacuum evaporation (LVE) onto fluorine doped tin oxide (FTO) coated glass substrates. Electrochromic test device (ECTD) was constructed by using these films as working electrodes, together with the FTO/glass as an opposite (counter) electrode in alkaline environment (0.1 M NaOH aqueous solution). Those films exhibited anodic electrochromism, changing color from transparent white-yellowish to dark brown. The coloration and bleaching times, defined as time to reach 2 3 of the final change in transmittance, were found to be 1.7 and 4.2 s, respectively. The electrochromic characteristics were recorded by using cyclic voltammetry. The optical transmission spectra of the bleached and colored states were recorded in the visible part of the spectrum. The Fourier-transform infrared (FTIR) reflection spectra of the sample in the bleached and colored states were measured at near normal incidence angle. The dependence of the transmission of the sample at wavelength l ¼ 670 nm on the electric charge transported to the anode were also examined. The coloration efficiency was calculated to be 32.4 cm 2 /C from the slope of the graphical presentation of the optical density as a function of the charge density, during the charge extraction. The spontaneous ex situ change of coloration with time and stability of the sample in the bleached and colored states were also examined.

Solar Energy Materials and Solar Cells, Feb 1, 2002

Nickel oxide (NiOx) thin films were prepared by the chemical deposition method (solution growth) ... more Nickel oxide (NiOx) thin films were prepared by the chemical deposition method (solution growth) on two kinds of substrates: (1) glass and (2) glass/SnO2:F. Films were thermally treated at 200°C for 10min in atmosphere. The texture, microstructure and composition were examined by optical microscopy, X-ray diffraction patterns (XRD) and X-ray photoelectron spectroscopy (XPS) analysis of the surface layer. The films

Physics and Chemistry of Liquids, Mar 1, 2013

Dielectric and electric behaviour of calcite and dolomite aqueous solutions were investigated usi... more Dielectric and electric behaviour of calcite and dolomite aqueous solutions were investigated using dielectric spectroscopy and electric modulus formalism. The electrode polarisation influence was observed in the low-frequency region. The influence of including grain-size (5 µm, 10 µm and 20 µm) concentration (5 × 10–10 g L) and temperature (295–313 K) to dielectric constant, electric modulus and conductivity was established. The Cole–Cole plot showed only one suppressed semicircle

Thin Solid Films, Apr 1, 2007

Thin cuprous oxide films were prepared by a low cost, chemical deposition (electroless) method on... more Thin cuprous oxide films were prepared by a low cost, chemical deposition (electroless) method onto glass substrates pre-coated with fluorine doped tin oxide. The X-ray diffraction pattern confirmed the Cu 2 O composition of the films. Visible transmittance spectra of the cuprous oxide films were studied for the as-prepared, colored and bleached films. The cyclic voltammetry study showed that those films exhibited cathode coloring electrochromism, i.e. the films showed change of color from yellowish to black upon application of an electric field. The transmittance across the films for laser light of 670 nm was found to change due to the voltage change for about 50%. The coloration memory of those films was also studied during 6 h, ex-situ. The coloration efficiency at 670 nm was calculated to be 37 cm 2 /C.

Physica B: Condensed Matter, 2011

Single phase perovskite Ba(Sr1/3Nb2/3)O3 ceramics was prepared using the columbite precursor meth... more Single phase perovskite Ba(Sr1/3Nb2/3)O3 ceramics was prepared using the columbite precursor method. X-ray diffraction (XRD) technique is used to verify the single phase formation. It stabilizes in hexagonal phase with lattice constants a=12.1243Å and c=15.3747Å. Impedance analysis shows distributed relaxation time. Single semicircular arc observed in complex impedance plot confirms that only semiconducting grains are contributing to the polarization. The

Thin Solid Films, 2007

Solar Energy Materials and Solar Cells, 2002