| 1. |
- Gonzalez-Borrero, P. P., et al.
(författare)
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Optical band-gap determination of nanostructured WO3 film
- 2010
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 96:6
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Tidskriftsartikel (refereegranskat)abstract
- The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green's function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data.
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| 2. |
- Arvizu, Miguel A, et al.
(författare)
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Electrochromism in sputter-deposited W-Ti oxide films : Durability enhancement due to Ti
- 2014
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 125, s. 184-189
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of W-Ti oxide were prepared by reactive DC magnetron sputtering and were characterized by Rutherford bathcattering spectrometry, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The electrochromic properties were studied by cyclic voltammetry in an electrolyte of lithium perchlorate in propylene carbonate and by optical transmittance measurements. The addition of Ti significantly promoted the amorphous nature of the films and stabilized their electrochemical cycling performance and dynamic range for electrochromism. (C) 2014 Elsevier B.V. All rights reserved.
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| 3. |
- Green, Sara, et al.
(författare)
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Structure and composition of sputter-deposited nickel-tungsten oxide films
- 2011
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 519:7, s. 2062-2066
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Tidskriftsartikel (refereegranskat)abstract
- Films of mixed nickel-tungsten oxide, denoted NixW1-x oxide, were prepared by reactive DC magnetron co-sputtering from metallic targets and were characterized by Rutherford backscattering spectrometry. X-ray photoelectron spectroscopy, X-ray diffractometry and Raman spectroscopy. A consistent picture of the structure and composition emerged, and at x<0.50 the films comprised a mixture of amorphous WO3 and nanosized NiWO4, at x = 0.50 the nanosized NiWO4 phase was dominating, and at x>0.50 the films contained nanosized NiO and NiWO4.
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| 4. |
- Niklasson, Gunnar A., 1953-, et al.
(författare)
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Determination of scattering and absorption coefficients from experimental data : Application to sunscreen lotions
- 2013
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Ingår i: Optics & Photonics in Sweden 2013.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Materials with pronounced light scattering are of large interest in a variety of scientific and technical disciplines. Their optical properties can to a good approximation be described by two-flux (Kubelka-Munk) theory [1]. This theory considers one light beam propagating in the forward direction and one beam propagating in the backward direction in the material. The optical properties of the material are described by phenomenological scattering (S) and absorption (K) coefficients. Additional parameters are the internal reflection coefficients for diffuse light at the interfaces and the reflection coefficient for incident collimated light at the front interface. The large amount of parameters makes it difficult to invert experimental measurements of transmittance (T) and reflectance (R) in order to obtain the parameters S and K characterizing the material [2,3]. We propose a method to partially overcome this problem in special cases by invoking physically realistic approximations for the reflection coefficients. We consider viscous emulsions or polymeric materials positioned between two transparent substrates separated by a spacer. Many organic and polymeric materials have refractive indices sufficiently close to those of glass and quartz substrates so that reflections between the material and the glass can be neglected. The reflection coefficient of the substrate for collimated light is known from its refractive index. Measurements of R and T in a region of weak scattering and absorption allow us to put quite stringent limits on the internal reflection coefficients for diffuse light. In this way good approximative values for S and K can be obtained.We apply this method to spectrophotometric measurements on two commercial sunscreen lotions. A drop of lotion was applied on a quartz substrate and a second quartz plate was put on top. The quartz plates were separated by aluminium foil of thickness 11 mm, except in the area covered by the lotion. Measurements of total R and T were taken by a Perkin-Elmer lambda-900 spectrophotometer equipped with an integrating sphere, in the wavelength range 250-800 nm. The samples were completely diffuse scattering and measurements in a low scattering region indicated a preferential forward scattering. The absorption coefficient was found to be low in the visible region and increased very much in the ultraviolet (UV). The scattering coefficient increased towards shorter wavelengths in the visible but exhibited differing behaviours in the UV. A sunscreen containing TiO2 particles exhibited a very low UV scattering together with higher scattering in the visible range, see fig. 1. Fig. 1. Scattering (S) and absorption (K) coefficients for a sunscreen containing a mix of chemical pigments and TiO[1] P. Kubelka, J. Opt. Soc. Am. 38 (1948) 448.[2] R. Levinson, P. Berdahl and H. Akbari, Solar Energy Mater. Solar Cells, 89 (2005) 319.[3] D. Barrios, R. Vergaz, J.M. Sanchez-Pena, C.G. Granqvist and G.A. Niklasson, Solar Energy Mater. Solar Cells, 111 (2013) 115.
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| 5. |
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| 6. |
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| 7. |
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| 8. |
- Barrios, David, et al.
(författare)
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Toward a quantitative model for suspended particle devices : Optical scattering and absorption coefficients
- 2013
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 111, s. 115-122
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Tidskriftsartikel (refereegranskat)abstract
- Suspended particle devices (SPDs) allow rapid voltage-controlled modulation of their optical transmittance and are of interest for solar-energy-related and other applications. We investigated the spectral total and diffuse transmittance of an SPD, including its angular dependence. The optical modulation was large for visible light but almost nil in the infrared, and the devices had noticeable haze. A theoretical two-flux model was formulated and provided a quantitative description of the absorption and scattering coefficients and thereby of the detailed optical performance. This analysis gives a benchmark for assessing improvements of the SPD technology as well as for comparing it with alternative technologies for optical modulation.
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| 9. |
- Bayrak Pehlivan, I., et al.
(författare)
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A polymer electrolyte with high luminous transmittance and low solar throughput : Polyethyleneimine-lithium bis(trifluoromethylsulfonyl) imide with In2O3:Sn nanocrystals
- 2012
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 100:24, s. 241902-
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Tidskriftsartikel (refereegranskat)abstract
- Chemically prepared similar to 13-nm-diameter nanocrystals of In2O3:Sn were included in a polyethyleneiminelithium bis(trifluoromethylsulfonyl) imide electrolyte and yielded high haze-free luminous transmittance and strong near-infrared absorption without deteriorated ionic conductivity. The optical properties could be reconciled with effective medium theory, representing the In2O3:Sn as a free electron plasma with tin ions screened according to the random phase approximation corrected for electron exchange. This type of polymer electrolyte is of large interest for opto-ionic devices such as laminated electrochromic smart windows.
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| 10. |
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| 11. |
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| 12. |
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| 13. |
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| 14. |
- Bayrak Pehlivan, İlknur
(författare)
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Functionalization of polymer electrolytes for electrochromic windows
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Saving energy in buildings is of great importance because about 30 to 40 % of the energy in the world is used in buildings. An electrochromic window (ECW), which makes it possible to regulate the inflow of visible light and solar energy into buildings, is a promising technology providing a reduction in energy consumption in buildings along with indoor comfort. A polymer electrolyte is positioned at the center of multi-layer structure of an ECW and plays a significant role in the working of the ECW.In this study, polyethyleneimine: lithium (bis(trifluoromethane)sulfonimide (PEI:LiTFSI)-based polymer electrolytes were characterized by using dielectric/impedance spectroscopy, differential scanning calorimetry, viscosity recording, optical spectroscopy, and electrochromic measurements.In the first part of the study, PEI:LiTFSI electrolytes were characterized at various salt concentrations and temperatures. Temperature dependence of viscosity and ionic conductivity of the electrolytes followed Arrhenius behavior. The viscosity was modeled by the Bingham plastic equation. Molar conductivity, glass transition temperature, viscosity, Walden product, and iso-viscosity conductivity analysis showed effects of segmental flexibility, ion pairs, and mobility on the conductivity. A connection between ionic conductivity and ion-pair relaxation was seen by means of (i) the Barton-Nakajima-Namikawa relation, (ii) activation energies of the bulk relaxation, and ionic conduction and (iii) comparing two equivalent circuit models, containing different types of Havriliak-Negami elements, for the bulk response.In the second part, nanocomposite PEI:LiTFSI electrolytes with SiO2, In2O3, and In2O3:Sn (ITO) were examined. Adding SiO2 to the PEI:LiTFSI enhanced the ionic conductivity by an order of magnitude without any degradation of the optical properties. The effect of segmental flexibility and free ion concentration on the conduction in the presence of SiO2 is discussed. The PEI:LiTFSI:ITO electrolytes had high haze-free luminous transmittance and strong near-infrared absorption without diminished ionic conductivity. Ionic conductivity and optical clarity did not deteriorate for the PEI:LiTFSI:In2O3 and the PEI:LiTFSI:SiO2:ITO electrolytes.Finally, propylene carbonate (PC) and ethylene carbonate (EC) were added to PEI:LiTFSI in order to perform electrochromic measurements. ITO and SiO2 were added to the PEI:LiTFSI:PC:EC and to a proprietary electrolyte. The nanocomposite electrolytes were tested for ECWs with the configuration of the ECWs being plastic/ITO/WO3/polymer electrolyte/NiO (or IrO2)/ITO/plastic. It was seen that adding nanoparticles to polymer electrolytes can improve the coloring/bleaching dynamics of the ECWs.From this study, we show that nanocomposite polymer electrolytes can add new functionalities as well as enhancement in ECW applications.
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| 15. |
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| 16. |
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| 17. |
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| 18. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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Ion conduction of branched polyethyleneimine-lithium bis(trifluoromethylsulfonyl) imide electrolytes
- 2011
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Ingår i: Electrochimica Acta. - : Elsevier BV. - 0013-4686 .- 1873-3859. ; 57, s. 201-206
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Tidskriftsartikel (refereegranskat)abstract
- Ionic conductivity of polymer electrolytes containing branched poly (ethylene imine) (BPEI) and lithium bis(trifluoromethyl sulfonyl)imide (LiTFSI) was measured between temperatures of 20 and 70◦C and molar ratios of 20:1 and 400:1. The electrolytes were characterized by impedance spectroscopy, differential scanning calorimetry, and viscosity measurements. At room temperature, the maximum conductivity was 2×10−6 S/cm at a molar ratio of 50:1. The molar conductivity of the electrolytes displayed first a minimum and then a maximum upon increasing salt concentration. A proportionality of molar conductivity to segmental mobility was seen from glass transition temperature and viscosity measurements. Analysis of the Walden product and isoviscosity conductivity showed that the percentage of ions bound in ion pairs increased at low concentrations below 0.1 mol/kg. The average dipole moment decreased with salt concentration. The temperature dependence of the ionic conductivity showed an Arrhenius behavior.
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| 19. |
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| 20. |
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| 21. |
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| 22. |
- Bayrak Pehlivan, Ilknur, et al.
(författare)
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[PEI-SiO2]:[LiTFSI] nanocomposite polymer electrolytes : Ion conduction and optical properties
- 2012
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 98, s. 465-471
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Tidskriftsartikel (refereegranskat)abstract
- Ion conductivity and optical properties were investigated for polymer electrolytes based on poly (ethyleneimine) and lithium bis(trifluoromethylsulfonyl)imide and also containing up to 9 wt.% of 7-nm-diameter SiO2 nanoparticles. The [N]:[Li] molar ratio was kept constant at 50:1. Impedance measurements were performed in the frequency range 10(-2)-10(7) Hz and between the temperatures 20 and 70 degrees C with an applied ac voltage of 1 V. Spectrophotometric data of total and diffuse transmittance were taken between the wavelengths 300 and 2500 nm. The bulk impedance was fitted to a conductive Havriliak-Negami circuit model. The ion conductivity increased monotonically for increasing SiO2 contents: specifically its room temperature value went from 8.5 x 10(-7) S/cm without nanoparticles to 3.8 x 10(-5) S/cm for 8 wt.% of SiO2 while the diffuse transmittance remained at similar to 1% so that optical clarity prevailed.
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| 23. |
- Cindemir, Umut, et al.
(författare)
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Porous Nickel Oxide Sensor for Formaldehyde Detection
- 2014
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Ingår i: European Materials Society (E-MRS) Spring Meeting, Lille, France, May 26-30, 2014..
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Formaldehyde is a volatile organic compound, which is a harmful indoor pollutant, causing sick building syndrome (SBS) and is released from household and building materials. Since higher concentrations of formaldehyde are considered to be carcinogenic, monitoring them indoors is of great importance. Advanced gas deposition has here been used to fabricate highly porous nickel oxide (NiO) thin films for formaldehyde sensing. The films were deposited on Al2O3 substrates with prefabricated comb-structured electrodes, and a resistive heater at the opposite face. The morphology of the films was investigated with scanning electron microscopy, and the porosity was determined by nitrogen adsorption isotherms with the Brunauer-Emmett-Teller method. The particle size was found to be less than 10 nm, as determined by x-ray diffraction. X-ray photoelectron spectroscopy of the NiO films was also done. Gas sensing measurements were done using a total gas flow rate of 200 ml/min. Resistivity values of sensors were recorded with formaldehyde diluted in synthetic air. Sensor resistances were recorded at 50 ppm, 25ppm, 10ppm and 5 ppm formaldehyde concentration. NiO films showed promising formaldehyde gas sensing properties implying lower levels of detection limit.
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| 24. |
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| 25. |
- Furlani, Maurizio, 1958, et al.
(författare)
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Concentration dependence of ionic relaxation in lithium doped polymer electrolytes
- 2010
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Ingår i: Journal of Non-Crystalline Solids. - : Elsevier BV. - 0022-3093 .- 1873-4812. ; 356:11-17, s. 710-714
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Tidskriftsartikel (refereegranskat)abstract
- A detailed impedance spectroscopy study at ambient temperature was carried out on polymer electrolytes based on low molecular weight poly(ethylene oxide) 400, poly(propylene oxide) 400 and a random copolymer of molecular weight 600, to which were added LiN(CF3SO2)(2) (LiTFSI) salt. The ionic conductivity exhibits a maximum at intermediate salt concentrations and is significantly higher for poly(ethylene oxide) and the copolymer. A dielectric relaxation was found in a frequency region above the one, where the ion conductivity dominates the dielectric response, and below the region of the relaxations of the polymer host. The relaxation strength scales with ion concentration, as appropriate for an ion pair relaxation in systems above the glass transition. The frequency of this relaxation, multiplied by the relaxation strength, has been found to be proportional to the ion conductivity, and the relaxation has therefore been assigned to short-range ionic motion in the polymer. It exhibits characteristics similar to conductivity relaxations in inorganic solid ion conductors, and is considered to be due to the same species that give rise to the ion conductivity. (C) 2010 Elsevier B.V. All rights reserved.
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| 26. |
- Granqvist, Claes-Göran, et al.
(författare)
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Advances in chromogenic materials and devices
- 2010
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 518:11, s. 3046-3053
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Tidskriftsartikel (refereegranskat)abstract
- Chromogenic materials allow the transmittance of visible light and solar energy to be varied under the action of an external stimulus This paper first discusses buildings related energy savings that can be accomplished by chromogenic technologies, and their beneficial effects on comfort issues We then summarize recent work on thermochromic VO2-based thin films with particular attention to multi-layers of VO2 and TiO2 and to new VO2 Mg films for which the doping gives significantly lowered absorption of visible light The final part covers electrochromic materials and devices with foci on coloration efficiency and on durability issues for foil-type constructions based on films of WO3 and NiO.
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| 27. |
- Granqvist, Claes-Göran, et al.
(författare)
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Chromogenics for Sustainable Energy : Some Advances in Thermochromics and Electrochromics
- 2010
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Ingår i: Advances in Science and Technology. - 1662-0356. ; 75, s. 55-64
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Tidskriftsartikel (refereegranskat)abstract
- Chromogenic materials are able to change their optical properties in response to external stimuli such as temperature (in thermochromic materials) and electrical charge insertion (in electrochromic materials). Below we review some recent advances for these types of materials. Specifically we first discuss the limitations of thermochromic VO2 films for energy efficient fenestration and show from calculations that nanocomposites containing VO2 can have superior properties and display high luminous transmittance and large temperature-dependent solar transmittance modulation. Even better results may be found for nanoparticles of VO2:Mg. In the second part of the paper we survey some recent progress for electrochromic devices and show that W oxide films have increased coloration efficiency when some Ni oxide is added. We also present initial results for flexible electrochromic foils produced by roll-to-roll coating and continuous lamination.
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| 28. |
- Granqvist, Claes-Göran, 1946-, et al.
(författare)
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Electrochromics and thermochromics for energy efficient fenestration : Functionalities based on nanoparticles of In2O3:Sn and VO2
- 2014
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 559, s. 2-8
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Tidskriftsartikel (refereegranskat)abstract
- Windows incorporating electrochromic (EC) and thermochromic (TC) materials are of great interest for today's and tomorrow's buildings and can create energy efficiency jointly with indoor comfort. This paper summarizes several recent studies and shows that nanoparticles of transparent conducting oxides-specifically In2O3: Sn (ITO) and thermochromic VO2-can lead to desirable functionalities. We consider three examples: (i) the use of ITO nanoparticles in conventional polaronic EC devices in order to suppress near-infrared solar transmittance, (ii) performance limits for plasmonic EC devices based on ITO nanoparticles, and (iii) ITO-VO2-based nanocomposites combining low thermal emittance with TC properties. We also consider Mg doping of VO2 to enhance the luminous transmittance and Al2O3/VO2 double layers with improved durability. Both experimental and theoretical results are reported. (C) 2013 Elsevier B. V. All rights reserved.
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| 29. |
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| 30. |
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| 31. |
- Green, Sara, et al.
(författare)
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Electrochromism in Nickel-Tungsten Oxides.
- 2012
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Ingår i: IME-10. Tenth International meeting on Electrochromism, Holland, MI USA, August 12-16, 2012.. ; , s. 17-
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Konferensbidrag (refereegranskat)
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| 32. |
- Green, Sara V., et al.
(författare)
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Electrochromic properties of nickel oxide based thin films sputter deposited in the presence of water vapor
- 2012
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 520:10, s. 3839-3842
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Tidskriftsartikel (refereegranskat)abstract
- Electrochromic nickel oxide based thin films were prepared by reactive RF magnetron sputtering from metallic nickel in the presence of Ar, O-2 and H2O. The water vapor led to enhanced optical modulation and charge capacity. At a wavelength of 550 nm the bleached state transmittance was 0.73 and the transmittance for the colored state was 0.28 and 0.15 for water partial pressures of p(H2O)<10(-3) Pa and p(H2O) similar to 7 x 10(-2) Pa, respectively. The charge densities were 14 and 25 mC/cm(2) for p(H2O)<10(-3) Pa and P-H2O similar to 7 x 10(-2) Pa, respectively. The coloration efficiency was decreased with increased water partial pressure, from about 0.07 to 0.06 cm(2)/mC. Preliminary results show that the H2O promotes an amorphous structure and makes the films increasingly hydrous.
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| 33. |
- Green, Sara V, et al.
(författare)
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Electrochromism in sputter deposited nickel-containing tungsten oxide films
- 2012
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 99, s. 339-344
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of NixW1-x oxide were prepared by reactive DC magnetron co-sputtering and were investigated by optical and electrochemical measurements. Electrochromism was found only for 0<x<0.3 but not for 0.3<x<0.6, though films with x~0.4 could still sustain reversible charge insertion. The coloration efficiency was largest for 0.10<x<0.15. The charge capacity of the NixW1-x oxide films decreased upon increasing the value of x as a consequence of a decreasing ion diffusion coefficient leading to slower kinetics.
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| 34. |
- Hu, Shuanglin, et al.
(författare)
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Optical properties of Mg-doped VO2 : Absorption measurements and hybrid functional calculations
- 2012
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 101:20, s. 201902-
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Tidskriftsartikel (refereegranskat)abstract
- Mg-doped VO2 thin films with thermochromic properties were made by reactive DC magnetron co-sputtering onto heated substrates, and spectral absorption was recorded at room temperature in the 0.5 < <(h)over bar>omega < 3.5 eV energy range. Clear evidence was found for a widening of the main band gap from 1.67 to 2.32 eV as the Mg/(V + Mg) atomic ratio went from zero to 0.19, thereby significantly lowering the luminous absorption. This technologically important effect could be reconciled with spin-polarized density functional theory calculations using the Heyd-Scuseria-Ernzerhof [Heyd et al., J. Chem. Phys. 118, 8207 (2003); ibid. 124, 219906 (2006)] hybrid functional. Specifically, the calculated luminous absorptance decreased when the Mg/(V + Mg) ratio was increased from 0.125 to 0.250.
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| 35. |
- Ji, Yu-Xia, et al.
(författare)
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Durability of thermochromic VO2 thin films under heating and humidity : Effect of Al oxide top coatings
- 2014
-
Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 562, s. 568-573
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Tidskriftsartikel (refereegranskat)abstract
- An explorative study was performed on sputter-deposited thermochromic (TC) VO2 films exposed to heat treatment under dry and humid conditions. The ambient conditions were harsh and 80-nm-thick VO2 films were rapidly converted to non-TC V2O5. It was found that a 30-nm-thick sputter-deposited Al oxide top coating provided good protection and delayed the oxidation for more than one day upon heating in dry air at 300 degrees C and that protection occurred for several days at 95% relative humidity and 60 degrees C. The thickness of the Al oxide was important and, expectedly, increased thickness yielded enhanced protection. Our results are important for TC fenestration as well as for other technical applications.
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| 36. |
- Ji, Yu-Xia, et al.
(författare)
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Durability of VO 2 -based thin films at elevated temperature : Towards thermochromic fenestration
- 2014
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Ingår i: INERA Workshop. - : IOP Publishing. ; , s. UNSP 012005-
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Konferensbidrag (refereegranskat)abstract
- An explorative study was performed on sputter-deposited thermochromic VO 2 films with top coatings of Al oxide and Al nitride. The films were exposed to dry air at a high temperature. Bare 80-nm-thick VO 2 films rapidly converted to non-thermochromic V 2 O 5 under the chosen conditions. Al oxide top coatings protected the underlying VO 2 films and, expectedly, increased film thickness yielded improved protection. Specifically, it was found that a 30-nm-thick sputter-deposited Al oxide top coating delayed the oxidation by more than one day upon heating at 300°C. The results demonstrate the importance of protective layers in thermochromic windows for practical application.
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| 37. |
- Johansson, Malin B., et al.
(författare)
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Electronic and optical properties of nanocrystalline WO3 thin films studied by optical spectroscopy and density functional calculations
- 2013
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 25:20, s. 205502-
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Tidskriftsartikel (refereegranskat)abstract
- The optical and electronic properties of nanocrystalline WO3 thin films prepared by reactive dc magnetron sputtering at different total pressures (P-tot) were studied by optical spectroscopy and density functional theory (DFT) calculations. Monoclinic films prepared at low P-tot show absorption in the near infrared due to polarons, which is attributed to a strained film structure. Analysis of the optical data yields band-gap energies E-g approximate to 3.1 eV, which increase with increasing P-tot by 0.1 eV, and correlate with the structural modifications of the films. The electronic structures of triclinic delta-WO3, and monoclinic gamma- and epsilon-WO3 were calculated using the Green function with screened Coulomb interaction (GW approach), and the local density approximation. The delta-WO3 and gamma-WO3 phases are found to have very similar electronic properties, with weak dispersion of the valence and conduction bands, consistent with a direct band-gap. Analysis of the joint density of states shows that the optical absorption around the band edge is composed of contributions from forbidden transitions (>3 eV) and allowed transitions (>3.8 eV). The calculations show that E-g in epsilon-WO3 is higher than in the delta-WO3 and gamma-WO3 phases, which provides an explanation for the P-tot dependence of the optical data.
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| 38. |
- Johansson, Malin B., et al.
(författare)
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Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering
- 2014
-
Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 115:21, s. 213510-
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of tungsten trioxide thin films prepared by DC magnetron sputtering, withdifferent oxygen vacancy (Vo) concentration, have been studied by spectrophotometry andphotoluminescence (PL) emission spectroscopy. Absorption and PL spectra show that the filmsexhibit similar band gap energies, Eg 2.9 eV. The absorption spectra of the films show twopronounced absorption bands in the near-infrared region. One peak (P1) is located atapproximately 0.7 eV, independent of Vo concentration. A second peak (P2) shifts from 0.96 eV to1.16 eV with decreasing Vo concentration. Peak P1 is assigned to polaron absorption due totransitions between tungsten sites (W5þ!W6þ), or an optical transition from a neutral vacancystate to the conduction band, Vo0!W6þ. The origin of peak P2 is more uncertain but may involveþ1 and þ2 charged vacancy sites. The PL spectra show several emission bands in the range 2.07 to3.10 eV in the more sub-stoichiometric and 2.40 to 3.02 eV in the less sub-stoichiometric films.The low energy emission bands agree well with calculated optical transition energies of oxygenvacancy sites, with dominant contribution from neutral and singly charged vacancies in the lesssub-stoichiometric films, and additional contributions from doubly charged vacancy sites in themore sub-stoichiometric films.
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| 39. |
- Johansson, Malin B, et al.
(författare)
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Structural and optical properties of visible active photocatalytic WO3 thin films prepared by reactive dc magnetron sputtering
- 2012
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Ingår i: Journal of Materials Research. - : Springer Science and Business Media LLC. - 0884-2914 .- 2044-5326. ; 27:24, s. 3130-3140
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Tidskriftsartikel (refereegranskat)abstract
- Nanostructured tungsten trioxide films were prepared by reactive dc magnetron sputteringat different working pressures P-tot = 1-4 Pa. The films were characterized by scanning electron microscopy, x-ray diffraction, Rutherford backscattering spectroscopy, Raman spectroscopy, and ultraviolet-visible spectrophotometry. The films were found to exhibit predominantly monoclinic structures and have similar band gap, E-g approximate to 2.8 eV, with a pronounced Urbach tail extending down to 2.5 eV. At low P-tot, strained film structures formed, which were slightly reduced and showed polaron absorption in the near-infrared region. The photodegradation rate of stearic acid was found to correlate with the stoichiometry and polaron absorption. This is explained by a recombination mechanism, whereby photoexcited electron-hole pairs recombine with polaron states in the band gap. The quantum yield decreased by 50% for photon energies close to E-g due to photoexcitations to band gap states lying below the O-2 affinity level.
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| 40. |
- Johansson, Malin, 1972-
(författare)
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Nanocrystalline Tungsten Trioxide Thin Films : Structural, Optical and Electronic Characterization
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns experimental studies of nanocrystalline tungsten trioxide thin films. Functional properties of WO3 have interesting applications in research areas connected to energy efficiency and green nanotechnology. The studies in this thesis are focused on characterization of fundamental electronic and optical properties in the semiconducting transition metal oxide WO3. The thesis includes also applied studies of photocatalytic and photoelectrochemical properties of the material. All nanocrystalline WO3 thin films were prepared using DC magnetron sputtering. It was found that structures like hexagonal and triclinic phase with different properties can be produced with sputtering technique. Thin film deposition has been performed using different process parameters with emphasis on sputter pressure and films that mainly consist of monoclinic γ-phase, with small contributions of ε-phase. Changes in the pressure are shown to affect the number of oxygen vacancies in the WO3 thin film, with close to stoichiometric WO3 formed at high pressures (30 mTorr), and slightly sub-stochiometric WO3-x, x = 0.005 at lower pressures (10 mTorr). Both stoichiometric and sub-stoichiometric thin films have been characterized by several structural, optical and electronic techniques. The electronic structure and especially band gap states have been explored and optical properties of WO3 and WO3-x have been studied in detail. The band gap has been determined to be in the range 2.7-2.9 eV. Absorption due to polaron absorption (W5+ -W6+), oxygen vacancy sites (Vo -W6+), and due to differently charged oxygen vacancy states in the band gap have been determined by spectrophotometry and photoluminescence spectroscopy, in good agreement with resonant inelastic x-ray spectroscopy and theoretical calculations. The density of electronic states in the band gap was determined from cyclic voltammetry measurements, which correlate with O vacancy concentration as compared with near infrared absorption. By combining different experimental methods a thorough characterization of the band gap states have been possible and this opens up the opportunity to tailor the WO3 functionalities. WO3 has been shown to be visible active photocatalyst, and a promising electrode material as inferred from photo-oxidation and water splitting measurements, respectively. Links between device performance in photoelectrochemical experiments, charge transport and the electronic structure have been elucidated.
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| 41. |
- Kaden, Heike, et al.
(författare)
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Detection of moisture differences of a swellable and a non-swellable clay in the low and mid frequency range
- 2011
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Ingår i: 6th Conference on Innovative Moisture Measurement in Research and Practise, Karlsruhe, Oct. 2011. ; , s. 10pp-
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Konferensbidrag (refereegranskat)abstract
- The sensitivity of dielectric spectroscopy to display changes in moisture content was studied for one swellable (bentonite) and one non-swellable clay (illite) in the low (1.0E-04…1.0E+06 Hz) and mid (2.0E+08…1.1E+09 Hz) frequency range. Therefore, both air dry materials were stored until equilibration at 11% and 93% relative humidity (r.h.) and their volumetric water content (Wvol) was measured with static and dynamic heating as well as with dielectric spectroscopy. Transformation of complex permittivity to Wvol was accomplished using the calibration functions after Topp and Roth. Moisture distinction could be displayed for both materials in the lower frequency range, whereas the changes in water content could only be described for the illite within the mid frequency range. Estimation of volumetric water content by transformation of real relative permittivity with Topp-function, resulted in underestimation of Wvol, especially for samples stored at high relative humidity (93% r.h.). Determination of Wvol with the calibration function after Roth was successful for illite at 11% r.h., but failed at higher relative humidity and for the bentonite in both hydration states. Additionally, complex dielectric data were clearly influenced by other processes beside bound water polarization.
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| 42. |
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| 43. |
- Kaden, Heike, et al.
(författare)
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Low-frequency dielectric properties of three bentonites at different adsorbed water states
- 2013
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Ingår i: Journal of Colloid and Interface Science. - : Elsevier BV. - 0021-9797 .- 1095-7103. ; 411, s. 16-26
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Tidskriftsartikel (refereegranskat)abstract
- Three bentonites of varying smectite content were investigated by dielectric spectroscopy in the frequency range 10(-4) to 10(6)Hz after storage at well-defined humidities. The identification of relaxation processes from complex permittivity measurements was difficult, since conductivity effects were superimposed on the underlying relaxations. Relaxation peaks revealed by the dissipation factor indicated the occurrence of interfacial processes between 10(2) and 10(6) Hz. The intensity of the polarization of the electrochemical double-layer at the clay-water interface was promoted by increasing water content and was shifted to higher frequencies the higher the water content in the bentonites. Below ∼1Hz, electrode polarization (EP) was shown to be a participating process with capacitance values ranging from 0.6(*)10(-3) to 7.3(*)10(-3)F due to the accumulated charges. An equivalent circuit model was introduced that successfully described the low-frequency dielectric behavior of bentonites at low moisture levels. An included series R-CPE connection was used to describe the double-layer relaxation. At water contents up to 17%, the bulk resistivity was mainly influenced by smectite content and cation exchange capacity, whereas at water contents of ⩾19%, interlayer occupation and hydration state became more important.
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| 44. |
- Laaksonen, Katri, et al.
(författare)
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Nanoparticles of TiO2 and VO2 in dielectric media : Conditions for low optical scattering, and comparison between effective medium and four-flux theories
- 2014
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Ingår i: Solar Energy Materials and Solar Cells. - : Saunders Elsevier. - 0927-0248 .- 1879-3398. ; 130:SI, s. 132-137
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Tidskriftsartikel (refereegranskat)abstract
- Spectral transmittance and reflectance in the 300 to 2500 nm solar-optical wavelength range were calculated for nanoparticles of titanium dioxide and vanadium dioxide with radii between 5 and 100 nm embedded in transparent dielectric media. Both of the materials are of large importance in green nanotechnologies: thus TiO2 is a photocatalyst that can be applied as a porous film or a nanoparticle composite on indoor or outdoor surfaces for environmental remediation, and VO2 is a thermochromic material with applications to energy-efficient fenestration. The optical properties, including scattering, of the nanoparticle composites were computed from the Maxwell–Garnett effective-medium theory as well as from a four-flux radiative transfer model. Predictions from these theories approach one another in the limit of small particles and in the absence of optical interference. Effects of light scattering can be modeled only by the four-flux theory, though. We found that nanoparticle radii should be less than ~20 nm in order to avoid pronounced light scattering.
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| 45. |
- Lansåker, Pia C., et al.
(författare)
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Au-Based Transparent Conductors for Window Applications : Effect of Substrate Material
- 2010
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Ingår i: Advances in Science and Technology. - 1662-0356. ; 75, s. 25-30
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Tidskriftsartikel (refereegranskat)abstract
- Thin films of Au were made by sputter deposition onto glass substrates with and without transparent and electrically conducting layers of SnO2:In. The Au films were up to ~11 nm in thickness and covered the range for thin film growth from discrete islands, via large scale coalescence and formation of a meandering conducting network, to the formation of a more or less “holey” film. Scanning electron microscopy and atomic force microscopy showed that the SnO2:In films were considerably rougher than the glass itself. This roughness influenced the Au film formation so that large scale coalescence set in at a somewhat larger thickness for films on SnO2:In than on glass. Measurements of spectral optical transmittance and electrical resistance could be reconciled with impeded Au film formation on the SnO2:In layer, leading to pronounced “plateaus” in the near infrared optical properties for Au films on SnO2:In and an accompanying change from such two-layer films having a lower resistance than the single gold film at thicknesses below large scale coalescence to the opposite behavior for larger film thicknesses.
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| 46. |
- Lansåker, Pia C, et al.
(författare)
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Characterization of gold nanoparticle films : Rutherford backscattering spectroscopy, scanning electron microscopy with image analysis, and atomic force microscopy
- 2014
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Ingår i: AIP Advances. - : AIP Publishing. - 2158-3226. ; 4:10, s. 107101-
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Tidskriftsartikel (refereegranskat)abstract
- Gold nanoparticle films are of interest in several branches of science and technology, and accurate sample characterization is needed but technically demanding. We prepared such films by DC magnetron sputtering and recorded their mass thickness by Rutherford backscattering spectroscopy. The geometric thickness d(g)-from the substrate to the tops of the nanoparticles-was obtained by scanning electron microscopy (SEM) combined with image analysis as well as by atomic force microscopy (AFM). The various techniques yielded an internally consistent characterization of the films. In particular, very similar results for d(g) were obtained by SEM with image analysis and by AFM.
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| 47. |
- Lansåker, Pia C, et al.
(författare)
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Thin gold films on SnO2:In : Temperature-dependent effects on the optical properties
- 2012
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Ingår i: Thin Solid Films. - : Elsevier BV. - 0040-6090 .- 1879-2731. ; 520:9, s. 3688-3691
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Tidskriftsartikel (refereegranskat)abstract
- Gold films with thicknesses of 5 +/- 0.5 nm were sputter deposited onto SnO2:In-coated glass kept at different temperatures up to 140 degrees C, and similar films, deposited onto substrates at 25 degrees C, were annealing post treated at the same temperatures. Nanostructures and optical properties were recorded by scanning electron microscopy and spectrophotometry in the 0.3 to 2.5 mu m wavelength range, respectively. Annealing had a minor influence on the optical transmittance despite significant changes in the scale of the nanostructure, whereas deposition onto substrates heated to 140 degrees C yielded granular films with strong plasmon absorption of luminous radiation. These results are of considerable interest for optical devices with gold films prepared at elevated temperature or operating at such temperature.
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| 48. |
- Lansåker, Pia C., et al.
(författare)
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Thin sputter deposited gold films on In2O3:Sn, SnO2:In, TiO2 and glass : Optical, electrical and structural effects
- 2013
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Ingår i: Solar Energy Materials and Solar Cells. - : Elsevier BV. - 0927-0248 .- 1879-3398. ; 117:SI, s. 462-470
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Tidskriftsartikel (refereegranskat)abstract
- Thin gold films are promising transparent conductors with many actual and potential uses in "green" technologies, transparent electronics, etc. These applications require different substrate materials, and hence it is important to understand the role of the substrate on Au thin film growth. Such effects have been studied in this work wherein Au films ranging from island structures, via large scale coalescence into meandering metal networks, to thin homogenous layers were deposited by DC magnetron sputtering onto glass substrates and In2O3:Sn (ITO), SnO2:In and TiO2 base layers backed by glass. Optical, electrical and structural properties were recorded for films deposited onto unheated substrates. We found distinct and characteristic differences in Au growth on the various backings. Thus ITO and SnO2:In base layers yielded gold films with island features remaining to larger thicknesses than for deposition directly onto glass, and the sheet resistance was lower for gold deposition onto SnO2:In and ITO only when the gold films were less than similar to 5 nm in thickness. Our results highlight the complexity of substrates' influence on thin film formation.
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| 49. |
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| 50. |
- Lansåker, Pia, et al.
(författare)
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Spectral density analysis of thin gold films : Thickness and structure dependence of the optical properties
- 2013
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Ingår i: Proceedings. - Stockholm : The Electromagnetics Academy. - 0351-6067. ; , s. 443-447, s. 443-447
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Recension (övrigt vetenskapligt/konstnärligt)abstract
- In this paper we study the feasibility of representing the optical properties of ultrathin gold films by effective medium theories. Gold films with mass thicknesses in the range of 1.4 to 9.2 nm were deposited by DC magnetron sputtering onto non-heated glass substrates. Optical measurements in the range 0.25 to 2 µm were carried out by spectroscopic ellipsometry, and the effective complex dielectric function of each film was determined. The gold films were modelled as a mixture of gold and air, and a general effective medium description using the spectral density function (SDF) was used to describe their optical properties. Numerical inversion of the experimental dielectric function gave a broad and rather featureless SDF with a few superimposed peak structures, both for island structures and percolating films. The broad background is qualitatively similar to the predictions of the Bruggeman [Ann. Phys. (Leipzig), 5th series, 24 (1935) 636-679] model.
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