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Optical properties of nanocrystalline WO3 and WO3-x thin films prepared by DC magnetron sputtering
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Fysikalisk kemi.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Kemiska sektionen, Institutionen för kemi - Ångström, Fysikalisk kemi.
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Fasta tillståndets fysik.ORCID-id: 0000-0002-8279-5163
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Fasta tillståndets fysik.
2014 (Engelska)Ingår i: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, nr 21, s. 213510-Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

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.

Ort, förlag, år, upplaga, sidor
American Institute of Physics (AIP), 2014. Vol. 115, nr 21, s. 213510-
Nationell ämneskategori
Fysik Teknik och teknologier Den kondenserade materiens fysik
Forskningsämne
Teknisk fysik med inriktning mot fasta tillståndets fysik
Identifikatorer
URN: urn:nbn:se:uu:diva-228539DOI: 10.1063/1.4880162ISI: 000337161600016OAI: oai:DiVA.org:uu-228539DiVA, id: diva2:734474
Tillgänglig från: 2014-07-17 Skapad: 2014-07-16 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
Ingår i avhandling
1. Nanocrystalline Tungsten Trioxide Thin Films: Structural, Optical and Electronic Characterization
Öppna denna publikation i ny flik eller fönster >>Nanocrystalline Tungsten Trioxide Thin Films: Structural, Optical and Electronic Characterization
2014 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

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.

Ort, förlag, år, upplaga, sidor
Uppsala: Acta Universitatis Upsaliensis, 2014. s. xii+129
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1103
Nyckelord
tungsten trioxide, nanocrystallin structure, thin films, optical properties, electronic properties
Nationell ämneskategori
Teknik och teknologier
Forskningsämne
Teknisk fysik med inriktning mot fasta tillståndets fysik
Identifikatorer
urn:nbn:se:uu:diva-211855 (URN)978-91-554-8824-6 (ISBN)
Disputation
2014-01-30, Polhemsalen, Ångdtrömlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (Engelska)
Opponent
Handledare
Tillgänglig från: 2014-01-08 Skapad: 2013-12-02 Senast uppdaterad: 2014-11-17

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