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  • 151.
    Ivanov, Sergey
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Karpov Inst Phys Chem, Ctr Mat Sci, Vorontsovo Pole 10, Moscow Russia.
    Andersson, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Cedervall, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Lewin, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Bazuev, G. V.
    Russian Acad Sci, Inst Solid State Chem, Ural Branch, Ekaterinburg 620990, Russia.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Temperature-dependent structural and magnetic properties of R2MMnO6 double perovskites (R=Dy, Gd; M=Ni, Co)2018In: Journal of materials science. Materials in electronics, ISSN 0957-4522, E-ISSN 1573-482X, Vol. 29, no 21, p. 18581-18592Article in journal (Refereed)
    Abstract [en]

    The structural and magnetic properties of the Dy2CoMnO6, Dy2NiMnO6 and Gd2CoMnO6 double perovskites are investigated using X-ray powder diffraction and squid magnetometry. The materials adopt an orthorhombic structure (space ground Pnma) with disordered Co(Ni)/Mn cations, and exhibit ferrimagnetic transitions near T(C)85, 95, and 115K respectively. T-C was found to monotonously depend on the orthorhombic distortion (a-c)/(a+c) of the compounds. The crystal structure of the compounds was investigated as a function of temperature (16-1100K range), evidencing changes in the BO6 octahedron near T-C. The magnetic entropy changes are estimated for comparison of the magnetocaloric properties to those from literature.

  • 152.
    Ivanov, Sergey
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. Karpov Inst Phys Chem, Ctr Mat Sci, Vorontsovo Pole 10, Moscow 105064, Russia.
    Beran, Premysl
    CAS, Inst Nucl Phys, Rez, Czech Republic.
    Bush, Alexandr A.
    Moscow Technol Univ, Moscow 119454, Russia.
    Sarkar, Tapati
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Shafeie, Samrand
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Wang, Duo
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sanyal, Biplab
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Eriksson, Olle
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Sahlberg, Martin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Kvashnin, Yaroslav
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
    Tellgren, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Structural Chemistry.
    Nordblad, Per
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Mathieu, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Cation ordering, ferrimagnetism and ferroelectric relaxor behavior in Pb(Fe1-xScx)(2/3)W1/3O3 solid solutions2019In: European Physical Journal B: Condensed Matter Physics, ISSN 1434-6028, E-ISSN 1434-6036, Vol. 92, no 8, article id 163Article in journal (Refereed)
    Abstract [en]

    Ceramic samples of the multiferroic perovskite Pb(Fe1-xScx)(2/3)W1/3O3 with 0 <= x <= 0.4 have been synthesized using a conventional solid-state reaction method, and investigated experimentally and theoretically using first-principle calculations. Rietveld analyses of joint synchrotron X-ray and neutron diffraction patterns show the formation of a pure crystalline phase with cubic (Fm3(_)m) structure with partial ordering in the B-sites. The replacement of Fe by Sc leads to the increase of the cation order between the B and B '' sites. As the non-magnetic Sc3+ ions replace the magnetic Fe3+ cations, the antiferromagnetic state of PbFe2/3W1/3O3 is turned into a ferrimagnetic state reflecting the different magnitude of the magnetic moments on the B ' and B '' sites. The materials remain ferroelectric relaxors with increasing Sc content. Results from experiments on annealed and quenched samples show that the cooling rate after high temperature annealing controls the degree of cationic order in Pb(Fe1-xScx)(2/3)W1/3O3 and possibly also in the undoped PbFe2/3W1/3O3.

  • 153.
    Jacobsson, Jesper
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    ZnO Quantum Dots: Size Dependent Optical, Vibrational and Photoelectrochemical Properties2012Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis is devoted to quantum dots of ZnO in the size regime 2.5-10 nm.

    The focus is directed towards their size dependent properties with special

    emphasis on the optical, vibrational and photocatalytic properties. The particles

    were synthesized by hydrolysis in alkaline zinc acetate solution.

    Analysis and characterisation were performed on both growing particles in

    solution as well as on thin particle films. The main characterisation techniques

    were UV-vis, florescence, XRD, Raman, electrochemical- and photoelectrochemical

    methods.

    Based on a large set of XRD and UV-vis measurements an empirical relation

    between the band gap and the particle size were developed. This enabled

    size resolved measurements of the visible fluorescence on growing particles

    in solution. We report evidence of mobile trap states responsible for the fluorescence

    and discuss a possible surface dependent mechanism for this.

    The quantum confinement of the phonon modes were investigated with

    Raman spectroscopy and molecular dynamic simulations. We report on a

    size depended suppression of the phonon modes and the contribution from

    surface effects.

    The absolute position of the conduction band edge was determined as a

    function of particle size with different photoelectrochemical methods that we

    describe in detail. We demonstrate that most of the size dependent shift in

    the band gap occurs by a change in the position of the conduction band. We

    also show that the parabolic band approximation is valid in a region of

    slightly less than 0.1 eV from the conduction band edge, and this even under

    an external electric field. An interesting electroabsorption phenomenon

    where the absorption locally increases under an applied potential are described

    and explained in terms of the quantum confined Stark effect and the

    Franz-Keldysh effect.

    The produced films of the particles show antireflective properties on conducting

    glass substrates, which could be interesting from a technological

    perspective. The films are demonstrated to show photocatalytic activity with

    respect to degradation of organic dyes and for solar water splitting.

    List of papers
    1. Antireflective coatings of ZnO quantum dots and their photocatalytic activity
    Open this publication in new window or tab >>Antireflective coatings of ZnO quantum dots and their photocatalytic activity
    2012 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 2, no 27, p. 10298-10305Article in journal (Refereed) Published
    Abstract [en]

    Thin films of ZnO quantum dots of different sizes have been deposited on conducting glass substrates. The films are transparent and work as antireflective coatings in the visible region. The negative absorption reaches down to -0.25 which represent a 77% increase in the transmitted light. Over a large part of the visible spectrum the increased transmittance is over 25%, and we demonstrate this to be a thin film effect. Under simulated solar illumination these films show a relatively high photocatalytic activity towards decomposition of methylene blue. The rate of photodecomposition depends on particle size and the smallest particles, which are less than 4 nm in diameter, show the highest quantum efficiency. We find the overall efficiency to be in the same order of magnitude to what's reported for commercial photocatalytic products like Degussa P25 and Pilkinton Active™, and maybe even somewhat better. We also demonstrate an increased hydrophilicity for the films under UV radiation. The photocatalytic oxidation of water into oxygen as a function of applied bias was measured in a three electrode system. The overall efficiency is small due to the high band gap but the internal quantum efficiency reaches over 10%.

    Keywords
    Anti reflective coatings, Applied bias, Conducting glass, Degussa P25, Different sizes, Film effects, Internal quantum efficiency, Large parts, Methylene Blue, Overall efficiency, Photo-catalytic, Photo-decomposition, Photocatalytic activities, Photocatalytic oxidations, Solar illumination, Three electrode-system, Transmitted light, Visible region, Visible spectra, ZnO quantum dots, Aromatic compounds, Coatings, Photocatalysis, Quantum efficiency, Semiconductor quantum dots, Substrates, Thin films, Ultraviolet radiation, Zinc oxide
    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-184913 (URN)10.1039/c2ra21566g (DOI)000312138400028 ()
    Available from: 2012-11-19 Created: 2012-11-15 Last updated: 2019-02-18Bibliographically approved
    2. Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots: Size and Band Gap Correlation and Evidence of Mobile Trap States
    Open this publication in new window or tab >>Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots: Size and Band Gap Correlation and Evidence of Mobile Trap States
    2011 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 50, no 19, p. 9578-9586Article in journal (Refereed) Published
    Abstract [en]

    ZnO nanoparticles constitute a convenient model system for fundamental studies with many possible technical applications in, for example, sensors and the field of catalysis and optoelectronics. A large set of ZnO quantum dots in the size range 2.5-7 nm have been synthesized and analyzed in detail. Time resolved in situ UV-vis absorption measurements were used to monitor the growth of these particles in solution by correlating the optical band gap to particle size given from X-ray diffraction (XRD) measurements. The particles formed were isotropic in shape, but small initial deviations gave indications of a transition from thermodynamic to kinetically controlled growth for particles around 4 nm in diameter. On the basis of this, the behavior and mechanisms for the particle growth are discussed. The fluorescence dependence on particle size was investigated by combining fluorescence and UV-vis measurements on growing particles. This revealed that the positions of the fluorescence trap states are mobile toward the conduction- and valence band. A broadening of the trap states was also found, and a surface dependent mechanism of the trap state shift and broadening is proposed.

    National Category
    Chemical Sciences Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-161945 (URN)10.1021/ic201327n (DOI)000295115000047 ()
    Available from: 2011-11-23 Created: 2011-11-21 Last updated: 2019-02-18Bibliographically approved
    3. Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
    Open this publication in new window or tab >>Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
    2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 29, p. 15692-15701Article in journal (Refereed) Published
    Abstract [en]

    The absolute position of the conduction and the valence band edges of ZnO quantum dots (Qdots) has been determined as a function of particle size with potential dependent absorption spectroscopy. The absolute position of the band edges are vital for which catalytic reactions that can occur at the surface. They are also crucial parameters for charge injection and extraction in nanoparticular solar cells and other optoelectronic devices based on nanoparticles. The position of the conduction band edge was determined by potentiostatic population of the conduction band states and monitoring the resulting increase in the optical band gap. This was performed for ZnO particles in the quantum confined region with diameters ranging between 4 and 9 nm. The particles were deposited into thin films giving an ensemble of particles for which the analysis could be performed. The relevant equations were derived and their validity in terms of applied potential and kinetic considerations was quantified. We find that essentially all of the quantum size effect of increased band gap is occurring by a shift of the conduction band edge. The extent of the validity of the parabolic approximation, which is one of the assumptions in the analysis, is investigated, both experimentally and with density functional theory calculations of bulk ZnO Here, we find that the parabolic approximation only is valid in an energy range of slightly less than 0.1 eV from the conduction band edge but in that regime constitutes an excellent approximation. We also demonstrate that the validity of the parabolic approximation follows a rising Fermi level into the conduction band energy levels.

    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-179918 (URN)10.1021/jp302220w (DOI)000306725200061 ()
    Available from: 2012-08-27 Created: 2012-08-27 Last updated: 2019-02-18Bibliographically approved
    4. Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
    Open this publication in new window or tab >>Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
    2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 12, p. 6893-6901Article in journal (Refereed) Published
    Abstract [en]

    The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E-2 Raman active optical phonon at 436 cm(-1) is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260-340 attributed to surface vibrations. The E-1-LO mode at 585 cm(-1) is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 angstrom) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode. 

    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-129300 (URN)10.1021/jp300985k (DOI)000302051100015 ()
    Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2019-02-18Bibliographically approved
  • 154.
    Jacobsson, Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Antireflective coatings of ZnO quantum dots and their photocatalytic activity2012In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 2, no 27, p. 10298-10305Article in journal (Refereed)
    Abstract [en]

    Thin films of ZnO quantum dots of different sizes have been deposited on conducting glass substrates. The films are transparent and work as antireflective coatings in the visible region. The negative absorption reaches down to -0.25 which represent a 77% increase in the transmitted light. Over a large part of the visible spectrum the increased transmittance is over 25%, and we demonstrate this to be a thin film effect. Under simulated solar illumination these films show a relatively high photocatalytic activity towards decomposition of methylene blue. The rate of photodecomposition depends on particle size and the smallest particles, which are less than 4 nm in diameter, show the highest quantum efficiency. We find the overall efficiency to be in the same order of magnitude to what's reported for commercial photocatalytic products like Degussa P25 and Pilkinton Active™, and maybe even somewhat better. We also demonstrate an increased hydrophilicity for the films under UV radiation. The photocatalytic oxidation of water into oxygen as a function of applied bias was measured in a three electrode system. The overall efficiency is small due to the high band gap but the internal quantum efficiency reaches over 10%.

  • 155.
    Jacobsson, Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Quantum Confined Stark Effects in ZnO Quantum Dots Investigated with Photoelectrochemical Methods2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 22, p. 12061-12072Article in journal (Refereed)
    Abstract [en]

    The optical absorption behavior of ZnO quantum dots has been investigated as a function of particle size in the quantum confined regime, between 4 and 9 nm in diameter, by using photoelectrochemical methods. Thin films of quantum dots, with 18 different sizes, were prepared on conducting substrates where the Fermi level could be controlled potentiostatically simultaneously as absorption measurements were performed. While raising the Fermi level into the conduction band, the dominant effect is a decrease in absorption as a consequence of increased electron population in the conduction band. This is a potentiostatic analogue to the Burstein-Moss shift for degenerate semiconductors. For applied potentials in an interval of 0.2 eV below the conduction band edge, the absorption does, however, increases instead of decreases. This absorption increase was found to be caused by a transition into states located within the band gap, which are introduced as a consequence of the applied potential. The magnitude of this effect is for the smallest particles (4 nm) approximately 9% compared to the magnitude of the Burstein-Moss bleaching. The effect decreases with increased particle size and essentially disappears for particles approaching 9 nm. The phenomenon is analyzed in terms of the Stark effect where the consequence of the applied potential is a buildup of an electric field within the particles, breaking the symmetry and splitting the energy levels in the conduction band. The gradual disappearance of the effect for the growing particles gives the extent of the quantum confinement effects of this phenomenon. The size-dependent absorption probability is analyzed and gives important information concerning the nature of both the perturbed states above the conduction band edge and the formation of the subband edge states.

  • 156.
    Jacobsson, Jesper T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Fjällström, Viktor
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Sahlberg, Martin Häggblad
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency2013In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 6, no 12, p. 3676-3683Article in journal (Refereed)
    Abstract [en]

    Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuInxGa1-xSe2, are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results.

  • 157.
    Jacobsson, Jesper T.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Platzer-Björkman, Charlotte
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    CuInxGa1-xSe2 as an efficient photocathode for solar hydrogen generation2013In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 38, no 35, p. 15027-15035Article in journal (Refereed)
    Abstract [en]

    Utilizing the energy in the sun to efficiently split water into hydrogen and oxygen can have a huge beneficial impact on a future post-carbon energy system. There is still, however, some way to go before this concept will be fully competitive. At the heart of the problem is finding and designing materials that can drive the photoreaction in an efficient and stable way. In this work we demonstrate how CIGS (CuInxGa1-xSe2), can be used for photo reduction of water into hydrogen. CIGS, which is a proven good solar cell material, does not in itself have the appropriate energetics to drive the reaction to any larger extent. Here we show that by utilizing a solid state pn-junction for charge separation and a catalyst deposited on the surface, the efficiency is significantly improved and photocurrents of 6 mA/cm(2) are demonstrated for the reduction reaction in the configuration of a photo-electrochemical cell. The stability of CIGS in water under illumination turns out to be a problem. In our present set-up, we demonstrate that separation between the charge carrier generation, which takes place in the solar cell, from the catalysis, which takes place in the electrolyte leads to improved stability, while keeping the essential functions of the processes. By incorporating appropriate charge separation layers and optimizing the catalytic conditions at the surface of the electrodes, photocurrents in excess of 20 mA/cm2 are reached for the reduction half reaction, demonstrating how essentially the full potential of GIGS as an efficient absorber material can be utilized in photocatalytic reduction of water into hydrogen.

  • 158.
    Jacobsson, Jesper T
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Viarbitskaya, Sviatlana
    Mukhtar, Emad
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots2014In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 27, p. 13849-13857Article in journal (Refereed)
    Abstract [en]

    The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indicates a stabilization of the excitons in the smallest particles. The lifetime of the UV fluorescence is in the picosecond time scale and interestingly, it is discontinuous with respect to particle size. For the smallest particles, the exciton emission life time reaches 30 ps, which is three times longer than that for the largest particles. This demonstrates a transition between two different mechanisms for the UV-fluorescence. We suggest that this is an effect of surface trapping and stabilization of the excitons occurring in the smallest particles but not in the larger ones. We also discuss the time scale limit for slowed hot carrier dynamics in ensembles of quantum confined ZnO particles.

  • 159.
    Jacobsson, Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Viktor, Fjällström
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Marika, Edoff
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Tomas, Edvinsson
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Sustainable Solar Hydrogen Production: From Photo-Electrochemical Cells to PV-Electrolysis and Back Again2014In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706Article in journal (Refereed)
    Abstract [en]

    Sustainable hydrogen production could, in principle, be accomplished along several different routes, where some of the most promising approaches involve utilization of solar energy. Photoelectrochemical cells (PEC-cells) and PV-electrolyzers for solar hydrogen production are here analyzed and compared. The analysis is performed by theoretically designing a number of intermediate devices, successively going from PEC-cells to PV-electrolyzers. The main physical processes: absorption, charge carrier separation, charge carrier transport, and catalysis are analyzed in the different devices. This demonstrates how the two concepts are related, and how one could easily be transformed and converted into the other. The awareness of the close relationship between PEC-cells and PV-electrolyzers is not as widely recognized as it should be. Traditionally, these two approaches have often been considered as fundamentally different, and are far too seldom analyzed in the same context. We argue that the different device designs for solar hydrogen production are best seen as essentially equivalent approaches, and as topological variations of the same basic theme, and can in many cases be unified under the acronym photo driven catalytic (PDC) devices. We further argue that much is to gain by acknowledging the similarities between PEC water splitting and PV-electrolysis, and that one concept alone should not be considered without also considering the other. The analysis and discussion presented could potentially lead to an increased fruitful crossbreeding of the accumulated knowledge in the respective sub-discipline, and aid in realizing solar hydrogen production as a sustainable and economically compatible energy alternative.

  • 160.
    Jacobsson, T. Jesper
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Highly Efficient CIGS Based Devices for Solar Hydrogen Production and Size Dependent Properties of ZnO Quantum Dots2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Materials and device concepts for renewable solar hydrogen production, and size dependent properties of ZnO quantum dots are the two main themes of this thesis.

    ZnO particles with diameters less than 10 nm, which are small enough for electronic quantum confinement, were synthesized by hydrolysis in alkaline zinc acetate solutions. Properties investigated include: the band gap - particle size relation, phonon quantum confinement, visible and UV-fluorescence as well as photocatalytic performance. In order to determine the absolute energetic position of the band edges and the position of trap levels involved in the visible fluorescence, methods based on combining linear sweep voltammetry and optical measurements were developed.

    The large band gap of ZnO prevents absorption of visible light, and in order to construct devices capable of utilizing a larger part of the solar spectrum, other materials were also investigated, like hematite , Fe2O3, and CIGS, CuIn1-xGaxSe2.

    The optical properties of hematite were investigated as a function of film thickness on films deposited by ALD. For films thinner than 20 nm, a blue shift was observed for both the absorption maximum, the indirect band gap as well as for the direct transitions. The probability for the indirect transition decreased substantially for thinner films due to a suppressed photon/phonon coupling. These effects decrease the visible absorption for films thin enough for effective charge transport in photocatalytic applications.

    CIGS was demonstrated to be a highly interesting material for solar hydrogen production. CIGS based photocathodes demonstrated high photocurrents for the hydrogen evolution half reaction. The electrode stability was problematic, but was solved by introducing a modular approach based on spatial separation of the basic functionalities in the device. To construct devices capable of driving the full reaction, the possibility to use cells interconnected in series as an alternative to tandem devices were investigated. A stable, monolithic device based on three CIGS cells interconnected in series, reaching beyond 10 % STH-efficiency, was finally demonstrated. With experimental support from the CIGS-devices, the entire process of solar hydrogen production was reviewed with respect to the underlying physical processes, with special focus on the similarities and differences between various device concepts.

    List of papers
    1. Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots: Size and Band Gap Correlation and Evidence of Mobile Trap States
    Open this publication in new window or tab >>Absorption and Fluorescence Spectroscopy of Growing ZnO Quantum Dots: Size and Band Gap Correlation and Evidence of Mobile Trap States
    2011 (English)In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 50, no 19, p. 9578-9586Article in journal (Refereed) Published
    Abstract [en]

    ZnO nanoparticles constitute a convenient model system for fundamental studies with many possible technical applications in, for example, sensors and the field of catalysis and optoelectronics. A large set of ZnO quantum dots in the size range 2.5-7 nm have been synthesized and analyzed in detail. Time resolved in situ UV-vis absorption measurements were used to monitor the growth of these particles in solution by correlating the optical band gap to particle size given from X-ray diffraction (XRD) measurements. The particles formed were isotropic in shape, but small initial deviations gave indications of a transition from thermodynamic to kinetically controlled growth for particles around 4 nm in diameter. On the basis of this, the behavior and mechanisms for the particle growth are discussed. The fluorescence dependence on particle size was investigated by combining fluorescence and UV-vis measurements on growing particles. This revealed that the positions of the fluorescence trap states are mobile toward the conduction- and valence band. A broadening of the trap states was also found, and a surface dependent mechanism of the trap state shift and broadening is proposed.

    National Category
    Chemical Sciences Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-161945 (URN)10.1021/ic201327n (DOI)000295115000047 ()
    Available from: 2011-11-23 Created: 2011-11-21 Last updated: 2019-02-18Bibliographically approved
    2. Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
    Open this publication in new window or tab >>Investigation of Vibrational Modes and Phonon Density of States in ZnO Quantum Dots
    2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 12, p. 6893-6901Article in journal (Refereed) Published
    Abstract [en]

    The ability to understand the phonon behavior in small metal oxide nanostructures and their surfaces is of great importance for thermal and microelectronic applications in successively smaller devices. Here the development of phonons in successively larger ZnO wurtzite quantum dots (QDs) is investigated. Raman spectroscopic measurements for particles from 3 to 11 nm reveal that the E-2 Raman active optical phonon at 436 cm(-1) is the first mode to be developed with a systematic increase with particle size. We also find a broad phonon band at 260-340 attributed to surface vibrations. The E-1-LO mode at 585 cm(-1) is the next to be developed while still being strongly suppressed in the confined particles. Other modes found in bulk ZnO are not developed for particles below 11 nm. Results from density functional theory showed an excellent agreement with the experimental molecular vibrations in the zinc acetate precursor and phonon modes in bulk ZnO. To elucidate the vibration behavior and phonon development in the ZnO QDs under nonzero temperature conditions and incorporating surface reconstruction, we performed reactive force field calculations. We show that the experimentally developed phonon modes in the QDs are the ones expected from dynamic theory. In particular, we show that the surface phonon modes in the very outermost surface (5 angstrom) can explain the observed broad phonon band and give the precise relation between the intensity of the surface and bulk phonons as the particle size increases. Calculations with temperatures between 50K and 1000K also show distinction of temperature effects in the material and that the phonon peaks are not generally shifted when the system is heated and quantum confined but instead reveal a dependence on the symmetry of the phonon mode. 

    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-129300 (URN)10.1021/jp300985k (DOI)000302051100015 ()
    Available from: 2010-08-10 Created: 2010-08-10 Last updated: 2019-02-18Bibliographically approved
    3. Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
    Open this publication in new window or tab >>Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots
    2012 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 29, p. 15692-15701Article in journal (Refereed) Published
    Abstract [en]

    The absolute position of the conduction and the valence band edges of ZnO quantum dots (Qdots) has been determined as a function of particle size with potential dependent absorption spectroscopy. The absolute position of the band edges are vital for which catalytic reactions that can occur at the surface. They are also crucial parameters for charge injection and extraction in nanoparticular solar cells and other optoelectronic devices based on nanoparticles. The position of the conduction band edge was determined by potentiostatic population of the conduction band states and monitoring the resulting increase in the optical band gap. This was performed for ZnO particles in the quantum confined region with diameters ranging between 4 and 9 nm. The particles were deposited into thin films giving an ensemble of particles for which the analysis could be performed. The relevant equations were derived and their validity in terms of applied potential and kinetic considerations was quantified. We find that essentially all of the quantum size effect of increased band gap is occurring by a shift of the conduction band edge. The extent of the validity of the parabolic approximation, which is one of the assumptions in the analysis, is investigated, both experimentally and with density functional theory calculations of bulk ZnO Here, we find that the parabolic approximation only is valid in an energy range of slightly less than 0.1 eV from the conduction band edge but in that regime constitutes an excellent approximation. We also demonstrate that the validity of the parabolic approximation follows a rising Fermi level into the conduction band energy levels.

    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-179918 (URN)10.1021/jp302220w (DOI)000306725200061 ()
    Available from: 2012-08-27 Created: 2012-08-27 Last updated: 2019-02-18Bibliographically approved
    4. Antireflective coatings of ZnO quantum dots and their photocatalytic activity
    Open this publication in new window or tab >>Antireflective coatings of ZnO quantum dots and their photocatalytic activity
    2012 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 2, no 27, p. 10298-10305Article in journal (Refereed) Published
    Abstract [en]

    Thin films of ZnO quantum dots of different sizes have been deposited on conducting glass substrates. The films are transparent and work as antireflective coatings in the visible region. The negative absorption reaches down to -0.25 which represent a 77% increase in the transmitted light. Over a large part of the visible spectrum the increased transmittance is over 25%, and we demonstrate this to be a thin film effect. Under simulated solar illumination these films show a relatively high photocatalytic activity towards decomposition of methylene blue. The rate of photodecomposition depends on particle size and the smallest particles, which are less than 4 nm in diameter, show the highest quantum efficiency. We find the overall efficiency to be in the same order of magnitude to what's reported for commercial photocatalytic products like Degussa P25 and Pilkinton Active™, and maybe even somewhat better. We also demonstrate an increased hydrophilicity for the films under UV radiation. The photocatalytic oxidation of water into oxygen as a function of applied bias was measured in a three electrode system. The overall efficiency is small due to the high band gap but the internal quantum efficiency reaches over 10%.

    Keywords
    Anti reflective coatings, Applied bias, Conducting glass, Degussa P25, Different sizes, Film effects, Internal quantum efficiency, Large parts, Methylene Blue, Overall efficiency, Photo-catalytic, Photo-decomposition, Photocatalytic activities, Photocatalytic oxidations, Solar illumination, Three electrode-system, Transmitted light, Visible region, Visible spectra, ZnO quantum dots, Aromatic compounds, Coatings, Photocatalysis, Quantum efficiency, Semiconductor quantum dots, Substrates, Thin films, Ultraviolet radiation, Zinc oxide
    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-184913 (URN)10.1039/c2ra21566g (DOI)000312138400028 ()
    Available from: 2012-11-19 Created: 2012-11-15 Last updated: 2019-02-18Bibliographically approved
    5. A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots
    Open this publication in new window or tab >>A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots
    2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 10, p. 5497-5504Article in journal (Refereed) Published
    Abstract [en]

    We here devise an electrochemical method for determining the absolute energetic position of trap levels involved in fluorescence. The method utilizes potentiostatic control of the Fermi level in the material, and thereby also the electronic population of the energy states involved in the fluorescence. The method is especially useful for nanoparticle semiconductor electrodes. Here we exemplify the method by determining the position of the trap levels involved in the green fluorescence in thin films of ZnO quantum dots. The exact mechanism and the absolute positions of these states have been debated in the literature. Here we show that the visible fluorescence is caused by a transition from energy levels slightly below the conduction band edge to a deep trap within the band gap. We further pinpoint the location of the upper trap level to be at 0.35 +/- 0.03 eV below the conduction band edge. Particles between 5 and 8 nm in diameter have been analyzed, which is in the quantum confined region of ZnO. We also show that the position of the upper trap level shifts with the size of the quantum dots in the same way as the conduction band.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-198385 (URN)10.1021/jp31190St (DOI)000316308400068 ()
    Available from: 2013-04-15 Created: 2013-04-15 Last updated: 2019-02-18Bibliographically approved
    6. A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots
    Open this publication in new window or tab >>A size dependent discontinuous decay rate for the exciton emission in ZnO quantum dots
    2014 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 16, no 27, p. 13849-13857Article in journal (Refereed) Published
    Abstract [en]

    The time resolved UV-fluorescence in ZnO quantum dots has been investigated using femtosecond laser spectroscopy. The measurements were performed as a function of particle size for particles between 3 and 7 nm in diameter, which are in the quantum confined regime. A red shift in the fluorescence maximum is seen while increasing the particle size, which correlates with the shift in band gap due to quantum confinement. The energy difference between the UV-fluorescence and the band gap does, however, increase for the smaller particles. For 3.7 nm particles the fluorescence energy is 100 meV smaller than the band gap energy, whereas it is only 20 meV smaller for the largest particles. This indicates a stabilization of the excitons in the smallest particles. The lifetime of the UV fluorescence is in the picosecond time scale and interestingly, it is discontinuous with respect to particle size. For the smallest particles, the exciton emission life time reaches 30 ps, which is three times longer than that for the largest particles. This demonstrates a transition between two different mechanisms for the UV-fluorescence. We suggest that this is an effect of surface trapping and stabilization of the excitons occurring in the smallest particles but not in the larger ones. We also discuss the time scale limit for slowed hot carrier dynamics in ensembles of quantum confined ZnO particles.

    National Category
    Inorganic Chemistry
    Research subject
    Chemistry with specialization in Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-221255 (URN)10.1039/C4CP00254G (DOI)000338116700036 ()
    Available from: 2014-03-27 Created: 2014-03-27 Last updated: 2019-02-18Bibliographically approved
    7. Quantum Confined Stark Effects in ZnO Quantum Dots Investigated with Photoelectrochemical Methods
    Open this publication in new window or tab >>Quantum Confined Stark Effects in ZnO Quantum Dots Investigated with Photoelectrochemical Methods
    2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 22, p. 12061-12072Article in journal (Refereed) Published
    Abstract [en]

    The optical absorption behavior of ZnO quantum dots has been investigated as a function of particle size in the quantum confined regime, between 4 and 9 nm in diameter, by using photoelectrochemical methods. Thin films of quantum dots, with 18 different sizes, were prepared on conducting substrates where the Fermi level could be controlled potentiostatically simultaneously as absorption measurements were performed. While raising the Fermi level into the conduction band, the dominant effect is a decrease in absorption as a consequence of increased electron population in the conduction band. This is a potentiostatic analogue to the Burstein-Moss shift for degenerate semiconductors. For applied potentials in an interval of 0.2 eV below the conduction band edge, the absorption does, however, increases instead of decreases. This absorption increase was found to be caused by a transition into states located within the band gap, which are introduced as a consequence of the applied potential. The magnitude of this effect is for the smallest particles (4 nm) approximately 9% compared to the magnitude of the Burstein-Moss bleaching. The effect decreases with increased particle size and essentially disappears for particles approaching 9 nm. The phenomenon is analyzed in terms of the Stark effect where the consequence of the applied potential is a buildup of an electric field within the particles, breaking the symmetry and splitting the energy levels in the conduction band. The gradual disappearance of the effect for the growing particles gives the extent of the quantum confinement effects of this phenomenon. The size-dependent absorption probability is analyzed and gives important information concerning the nature of both the perturbed states above the conduction band edge and the formation of the subband edge states.

    National Category
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-221257 (URN)10.1021/jp503098q (DOI)000337013400064 ()
    Available from: 2014-03-27 Created: 2014-03-27 Last updated: 2019-02-18Bibliographically approved
    8. Optical quantum confinement in low dimensional hematite
    Open this publication in new window or tab >>Optical quantum confinement in low dimensional hematite
    2014 (English)In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 10, p. 3352-3363Article in journal (Refereed) Published
    Abstract [en]

    Hematite is considered to be a promising material for various applications, including for example photoelectrochemical cells for solar hydrogen production. Due to limitations in the charge transport properties hematite needs to be in the form of low-dimensional particles or thin films in several of these applications. This may however affect the optical properties, introducing additional complications for efficient design of photo-active devices. In this paper the optical absorption is analyzed in detail as a function of film thickness for 35 thin films of hematite ranging between 2 and 70 nm. Hematite was deposited by atomic layer deposition on FTO-substrates using Fe(CO)(5) and O-2 as precursors. It was found that for film thicknesses below 20 nm the optical properties are severely affected as a consequence of quantum confinement. One of the more marked effects is a blue shift of up to 0.3 eV for thinner films of both the indirect and direct transitions, as well as a 0.2 eV shift of the absorption maximum. The data show a difference in quantum confinement for the indirect and the direct transitions, where the probability for the indirect transition decreases markedly and essentially disappears for the thinnest films. Raman measurements showed no peak shift or change in relative intensity for vibrations for the thinnest films indicating that the decrease in indirect transition probability could not be assigned to depression of any specific phonon but instead seems to be a consequence of isotropic phonon confinement. The onset of the indirect transition is found at 1.75 eV for the thickest films and shifted to 2.0 eV for the thinner films. Two direct transitions are found at 2.15 eV and 2.45 eV, which are blue shifted 0.3 and 0.45 eV respectively, when decreasing the film thickness from 20 to 4 nm. Low dimensional hematite, with dimensions small enough for efficient charge transport, thus has a substantially lower absorption in the visible region than expected from bulk values. This knowledge of the intrinsic optical behavior of low dimensional hematite will be of importance in the design of efficient photo-active devices.

    National Category
    Natural Sciences
    Identifiers
    urn:nbn:se:uu:diva-221059 (URN)10.1039/c3ta14846g (DOI)000331249900012 ()
    Available from: 2014-03-26 Created: 2014-03-25 Last updated: 2019-02-18Bibliographically approved
    9. CuInxGa1-xSe2 as an efficient photocathode for solar hydrogen generation
    Open this publication in new window or tab >>CuInxGa1-xSe2 as an efficient photocathode for solar hydrogen generation
    2013 (English)In: International journal of hydrogen energy, ISSN 0360-3199, E-ISSN 1879-3487, Vol. 38, no 35, p. 15027-15035Article in journal (Refereed) Published
    Abstract [en]

    Utilizing the energy in the sun to efficiently split water into hydrogen and oxygen can have a huge beneficial impact on a future post-carbon energy system. There is still, however, some way to go before this concept will be fully competitive. At the heart of the problem is finding and designing materials that can drive the photoreaction in an efficient and stable way. In this work we demonstrate how CIGS (CuInxGa1-xSe2), can be used for photo reduction of water into hydrogen. CIGS, which is a proven good solar cell material, does not in itself have the appropriate energetics to drive the reaction to any larger extent. Here we show that by utilizing a solid state pn-junction for charge separation and a catalyst deposited on the surface, the efficiency is significantly improved and photocurrents of 6 mA/cm(2) are demonstrated for the reduction reaction in the configuration of a photo-electrochemical cell. The stability of CIGS in water under illumination turns out to be a problem. In our present set-up, we demonstrate that separation between the charge carrier generation, which takes place in the solar cell, from the catalysis, which takes place in the electrolyte leads to improved stability, while keeping the essential functions of the processes. By incorporating appropriate charge separation layers and optimizing the catalytic conditions at the surface of the electrodes, photocurrents in excess of 20 mA/cm2 are reached for the reduction half reaction, demonstrating how essentially the full potential of GIGS as an efficient absorber material can be utilized in photocatalytic reduction of water into hydrogen.

    Keywords
    Solar water splitting, Hydrogen production, CIGS, CuInGaSe2, PEC, Water electrolysis
    National Category
    Natural Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Electronics
    Identifiers
    urn:nbn:se:uu:diva-214041 (URN)10.1016/j.ijhydene.2013.09.094 (DOI)000328006500010 ()
    Available from: 2014-01-07 Created: 2014-01-07 Last updated: 2019-02-18Bibliographically approved
    10. A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency
    Open this publication in new window or tab >>A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency
    Show others...
    2013 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706, Vol. 6, no 12, p. 3676-3683Article in journal (Refereed) Published
    Abstract [en]

    Efficient production of hydrogen from solar energy is anticipated to be an important component in a future sustainable post-carbon energy system. Here we demonstrate that series interconnected absorbers in a PV-electrolysis configuration based on the compound semiconductor CIGS, CuInxGa1-xSe2, are a highly interesting concept for solar water splitting applications. The band gap energy of CIGS can be adjusted to a value close to optimum for efficient absorption of the solar spectrum, but is too low to drive overall water splitting. Therefore we connect three cells in series, into a monolithic device, which provides sufficient driving force for the full reaction. Integrated with a catalyst this forms a stable PV/photo-electrochemical device, which when immersed in water reaches over 10% solar-to-hydrogen efficiency for unassisted water splitting. The results show that series interconnected device concepts, which enable use of a substantial part of the solar spectrum, provide a simple route towards highly efficient water splitting and could be used also for other solar absorbers with similar electro-optical properties. We discuss how the efficiency could be increased for this particular device, as well as the general applicability of the concepts used in this work. We also briefly discuss advantages and disadvantages of photo-electrochemical cells in relation to PV-electrolysis with respect to our results.

    National Category
    Natural Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Electronics
    Identifiers
    urn:nbn:se:uu:diva-213462 (URN)10.1039/c3ee42519c (DOI)000327250300028 ()
    Available from: 2014-01-02 Created: 2013-12-23 Last updated: 2019-02-18Bibliographically approved
    11. Sustainable Solar Hydrogen Production: From Photo-Electrochemical Cells to PV-Electrolysis and Back Again
    Open this publication in new window or tab >>Sustainable Solar Hydrogen Production: From Photo-Electrochemical Cells to PV-Electrolysis and Back Again
    2014 (English)In: Energy & Environmental Science, ISSN 1754-5692, E-ISSN 1754-5706Article in journal (Refereed) Published
    Abstract [en]

    Sustainable hydrogen production could, in principle, be accomplished along several different routes, where some of the most promising approaches involve utilization of solar energy. Photoelectrochemical cells (PEC-cells) and PV-electrolyzers for solar hydrogen production are here analyzed and compared. The analysis is performed by theoretically designing a number of intermediate devices, successively going from PEC-cells to PV-electrolyzers. The main physical processes: absorption, charge carrier separation, charge carrier transport, and catalysis are analyzed in the different devices. This demonstrates how the two concepts are related, and how one could easily be transformed and converted into the other. The awareness of the close relationship between PEC-cells and PV-electrolyzers is not as widely recognized as it should be. Traditionally, these two approaches have often been considered as fundamentally different, and are far too seldom analyzed in the same context. We argue that the different device designs for solar hydrogen production are best seen as essentially equivalent approaches, and as topological variations of the same basic theme, and can in many cases be unified under the acronym photo driven catalytic (PDC) devices. We further argue that much is to gain by acknowledging the similarities between PEC water splitting and PV-electrolysis, and that one concept alone should not be considered without also considering the other. The analysis and discussion presented could potentially lead to an increased fruitful crossbreeding of the accumulated knowledge in the respective sub-discipline, and aid in realizing solar hydrogen production as a sustainable and economically compatible energy alternative.

    National Category
    Inorganic Chemistry Engineering and Technology
    Research subject
    Engineering Science with specialization in Electronics
    Identifiers
    urn:nbn:se:uu:diva-221258 (URN)10.1039/C4EE00754A (DOI)000337977600001 ()
    Available from: 2014-03-27 Created: 2014-03-27 Last updated: 2019-02-18Bibliographically approved
  • 161.
    Jacobsson, T. Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots2013In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 10, p. 5497-5504Article in journal (Refereed)
    Abstract [en]

    We here devise an electrochemical method for determining the absolute energetic position of trap levels involved in fluorescence. The method utilizes potentiostatic control of the Fermi level in the material, and thereby also the electronic population of the energy states involved in the fluorescence. The method is especially useful for nanoparticle semiconductor electrodes. Here we exemplify the method by determining the position of the trap levels involved in the green fluorescence in thin films of ZnO quantum dots. The exact mechanism and the absolute positions of these states have been debated in the literature. Here we show that the visible fluorescence is caused by a transition from energy levels slightly below the conduction band edge to a deep trap within the band gap. We further pinpoint the location of the upper trap level to be at 0.35 +/- 0.03 eV below the conduction band edge. Particles between 5 and 8 nm in diameter have been analyzed, which is in the quantum confined region of ZnO. We also show that the position of the upper trap level shifts with the size of the quantum dots in the same way as the conduction band.

  • 162.
    Jacobsson, T. Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots2012In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 116, no 29, p. 15692-15701Article in journal (Refereed)
    Abstract [en]

    The absolute position of the conduction and the valence band edges of ZnO quantum dots (Qdots) has been determined as a function of particle size with potential dependent absorption spectroscopy. The absolute position of the band edges are vital for which catalytic reactions that can occur at the surface. They are also crucial parameters for charge injection and extraction in nanoparticular solar cells and other optoelectronic devices based on nanoparticles. The position of the conduction band edge was determined by potentiostatic population of the conduction band states and monitoring the resulting increase in the optical band gap. This was performed for ZnO particles in the quantum confined region with diameters ranging between 4 and 9 nm. The particles were deposited into thin films giving an ensemble of particles for which the analysis could be performed. The relevant equations were derived and their validity in terms of applied potential and kinetic considerations was quantified. We find that essentially all of the quantum size effect of increased band gap is occurring by a shift of the conduction band edge. The extent of the validity of the parabolic approximation, which is one of the assumptions in the analysis, is investigated, both experimentally and with density functional theory calculations of bulk ZnO Here, we find that the parabolic approximation only is valid in an energy range of slightly less than 0.1 eV from the conduction band edge but in that regime constitutes an excellent approximation. We also demonstrate that the validity of the parabolic approximation follows a rising Fermi level into the conduction band energy levels.

  • 163.
    Jacobsson, T. Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Fjällstrom, Viktor
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    A theoretical analysis of optical absorption limits and performance of tandem devices and series interconnected architectures for solar hydrogen production2015In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 138, p. 86-95Article in journal (Refereed)
    Abstract [en]

    Photo-driven catalytic (PDC) water splitting, using either photoelectrochemical cells (PEC-cells), PV-electrolyzers, or some hybrid system in-between, has attracted a lot of attention. In single-cell device architectures for solar hydrogen production, based on single band gap photoabsorbers, there is a fundamental efficiency problem originating from the energy distribution of the solar spectrum and the thermodynamic and kinetic requirements for splitting water. The minimum band gap for a single-junction device in order to withhold unbiased overall water splitting is considered to be at least 2.0 eV. This is far from the 1.35 eV which is the optimal band gap of a semiconductor for maximum power conversion of light in the solar spectrum. This discrepancy has been termed as the solar spectrum mismatch problem (the SSM-problem). The standard solution to this problem is to construct tandem devices, whereas an alternative is to interconnect several one band gap cells in series, side by side. Both approaches enable the use of low energy photons in the solar spectrum while still providing a sufficiently high photopotential for driving the full reaction, without seriously compromising with the area efficiency. In this paper, the tandem and serial architectures for handling the SSM-problem are analyzed and compared. The analysis is focused towards differences in the limits of optical absorption, the optimal number of optical. absorbers, and their corresponding band gaps. Taking losses due to charge carrier separation and catalysis into account, the maximum STH-efficiency for a series interconnected solar splitting device was found to be 24.6%, compared to 32.0% for an optimum tandem device at 1 Sun (air mass 1.5, 1000 W m(-2)). This can be compared with the maximum efficiency of 18.0% for an ideal single band gap photoabsorber in single junction device. The analysis shows that the maximum STH efficiency limits for series interconnected architectures for unassisted solar water splitting are not particularly far behind the more commonly studied tandem devices. They could then be an interesting alternative given the simplicity and versatility of series interconnected device architectures. The analysis also compares how tandem devices and series interconnected devices can differ in terms of charge carrier separation, charge carrier transport, catalysis, overall efficiency, device architecture, and expected cost.

  • 164.
    Jacobsson, T. Jesper
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Fjällstrom, Viktor
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edoff, Marika
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    CIGS based devices for solar hydrogen production spanning from PEC-cells to PV-electrolyzers: a comparison of efficiency, stability and device topology2015In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 134, p. 185-93Article in journal (Refereed)
  • 165.
    Jacobsson, T. Jesper
    et al.
    Ecole Polytech Fed Lausanne, Inst Chem Sci & Engn, Lab Photomol Sci, CH-1015 Lausanne, Switzerland..
    Schwan, L. Josef
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
    Ottosson, Mikael
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Hagfeldt, Anders
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry. Ecole Polytech Fed Lausanne, Inst Chem Sci & Engn, Lab Photomol Sci, CH-1015 Lausanne, Switzerland..
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Determination of Thermal Expansion Coefficients and Locating the Temperature-Induced Phase Transition in Methylammonium Lead Perovskites Using X-ray Diffraction2015In: Inorganic Chemistry, ISSN 0020-1669, E-ISSN 1520-510X, Vol. 54, no 22, p. 10678-10685Article in journal (Refereed)
    Abstract [en]

    Lead halogen perovskites, and particularly methylammonium lead iodine, CH3NH3PbI3, have recently attracted considerable interest as alternative solar cell materials, and record solar cell efficiencies have now surpassed 20%. Concerns have, however, been raised about the thermal stability of methylammonium lead iodine, and a phase transformation from a tetragonal to a cubic phase has been reported at elevated temperature. Here, this phase transition has been investigated in detail using temperature-dependent X-ray diffraction measurements. The phase transformation is pinpointed to 54 degrees C, which is well within the normal operating range of a typical solar cell. The cell parameters were extracted as a function of the temperature, from which the thermal expansion coefficient was calculated. The latter was found to be rather high (alpha(v) = 1.57 X 10(-4) K-1) for both the tetragonal and cubic phases. This is 6 times higher than the thermal expansion coefficient for soda lime glass and CIGS and 11 times larger than that of CdTe. This could potentially be of importance for the mechanical stability of perovskite solar cells in the temperature cycling experienced under normal day night operation. The experimental knowledge of the thermal expansion coefficients and precise determination of the cell parameters can potentially also be valuable while conducting density functional theory simulations on these systems in order to deliver more accurate band structure calculations.

  • 166. Janssens, Stoffel D.
    et al.
    Drijkoningen, Sien
    Saitner, Marc
    Boyen, Hans-Gerd
    Wagner, Patrick
    Larsson, Karin
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Haenen, Ken
    Evidence for phase separation of ethanol-water mixtures at the hydrogen terminated nanocrystalline diamond surface2012In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 137, no 4, p. 044702-Article in journal (Refereed)
    Abstract [en]

    Interactions between ethanol-water mixtures and a hydrophobic hydrogen terminated nanocrystalline diamond surface, are investigated by sessile drop contact angle measurements. The surface free energy of the hydrophobic surface, obtained with pure liquids, differs strongly from values obtained by ethanol-water mixtures. Here, a model which explains this difference is presented. The model suggests that, due to a higher affinity of ethanol for the hydrophobic surface, when compared to water, a phase separation occurs when a mixture of both liquids is in contact with the H-terminated diamond surface. These results are supported by a computational study giving insight in the affinity and related interaction at the liquid-solid interface.

  • 167.
    Jansson, Ulf
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Lewin, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Carbon-containing multi-component thin films2019In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 688, article id 137411Article, review/survey (Refereed)
    Abstract [en]

    High entropy alloys (HEAs) have been a hot research area for many years. They are solid solutions of at least five elements in approximately equimolar compositions. The HEAs are assumed to be stabilized by a high entropy of mixing favouring a solid solution phase instead of a mixture of intermetallic phases. The importance of entropy of mixing and the true nature of HEAs are debated but the concept has contributed to an interesting development of new alloys. They idea of stabilizing solid solutions with many elements have recently been expanded to nitrides, borides, oxides and carbides. Furthermore, a growing number of thin film studies of these compounds are now published. In this paper we summarise recent results from studies of carbon-containing multi-component thin films based on the HEA concept. We will summarise some general observations connected to "high-entropy" materials. We also describe some general trends in metal-carbon interactions for transition metals and discuss how they should influence the formation of multi-component carbides. A summary of results on bulk multi-component carbide materials is also presented. We review published studies of carbon-containing multi-component thin films mainly deposited with magnetron-sputtering. The crystal structure, microstructure and properties of these films are described. Finally, we highlight some interesting topics for future research.

  • 168.
    Jansson, Ulf
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Lewin, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Sputter deposition of transition-metal carbide films - A critical review from a chemical perspective2013In: Thin Solid Films, ISSN 0040-6090, E-ISSN 1879-2731, Vol. 536, p. 1-24Article, review/survey (Refereed)
    Abstract [en]

    Thin films based on transition-metal carbides exhibitmany interesting physical and chemical properties making them attractive for a variety of applications. The most widely used method to produce metal carbide films with specific properties at reduced deposition temperatures is sputter deposition. A large number of papers in this field have been published during the last decades, showing that large variations in structure and properties can be obtained. This review will summarise the literature on sputter-deposited carbide films based on chemical aspects of the various elements in the films. By considering the chemical affinities (primarily towards carbon) and structural preferences of different elements, it is possible to understand trends in structure of binary transition-metal carbides and the ternary materials based on these carbides. These trends in chemical affinity and structure will also directly affect the growth process during sputter deposition. A fundamental chemical perspective of the transition-metal carbides and their alloying elements is essential to obtain control of the material structure (from the atomic level), and thereby its properties and performance. This review covers a wide range of materials: binary transition-metal carbides and their nanocomposites with amorphous carbon; the effect of alloying carbide-based materials with a third element (mainly elements from groups 3 through 14); as well as the amorphous binary and ternary materials from these elements deposited under specific conditions or at certain compositional ranges. Furthermore, the review will also emphasise important aspects regarding materials characterisation which may affect the interpretation of data such as beam-induced crystallisation and sputter-damage during surface analysis. 

  • 169.
    Ji, Yu-Xia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Niklasson, Gunnar A.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Granqvist, Claes Göran
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Boman, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Thermochromic VO2 films by thermal oxidation of vanadium in SO22016In: Solar Energy Materials and Solar Cells, ISSN 0927-0248, E-ISSN 1879-3398, Vol. 144, p. 713-716Article in journal (Refereed)
    Abstract [en]

    Thermochromic films of VO2 were prepared by a two-step procedure: Sputtering was first used to deposit metallic vanadium, and such layers were subsequently oxidized in SO2 at a temperature in the 600-650 degrees C range. X-ray diffraction, Raman spectroscopy, measurements of temperature-dependent electrical resistance, and spectrophotometric transmittance data at different temperatures were employed to demonstrate that the films consisted of polycrystalline VO2 with good thermochromism, especially when oxidized at the highest temperature. Oxidation in SO2 is able to produce VO2 without the stringent process control that can be an obstacle for making VO2 by oxidation in O-2.

  • 170.
    Jogi, Indrek
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. Univ Tartu, Inst Phys, EE-51014 Tartu, Estonia..
    Jacobsson, T. Jesper
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Fondell, Mattis
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Watjen, Timo
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Carlsson, Jan-Otto
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Boman, Mats
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Edvinsson, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics. Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics.
    Phase Formation Behavior in Ultrathin Iron Oxide2015In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 31, no 45, p. 12372-12381Article in journal (Refereed)
    Abstract [en]

    Nanostructured iron oxides, and especially hematite, are interesting for a wide range of applications ranging from gas sensors to renewable solar hydrogen production. A promising method for deposition of low-dimensional films is atomic layer deposition (ALD). Although a potent technique, ALD of ultrathin films is critically sensitive to the substrate and temperature conditions where initial formation of islands and crystallites influences the properties of the films. In this work, deposition at the border of the ALD window forming a hybrid ALD/pulsed CVD (pCVD) deposition is utilized to obtain a deposition less sensitive to the substrate. A thorough analysis of iron oxide phases formation on two different substrates, Si(100) and SiO2, was performed. Films between 3 and SO rim were deposited and analyzed with diffraction techniques, high-resolution Raman spectroscopy, and optical spectroscopy. Below 10 nm nominal film thickness, island formation and phase dependent particle crystallization impose constraints for deposition of phase pure iron oxides on non-lattice-matching substrates. Films between 10 and 20 nm thickness on SiO2 could effectively be recrystallized into hematite whereas for the corresponding films on Si(100), no recrystallization occurred. For films thicker than 20 nm, phase pure hematite can be formed directly with ALD/pCVD with very low influence of the substrate on either Si or SiO2. For more lattice matched substrates such as SnO2:F, Raman spectroscopy indicated formation of the hematite phase already for films with 3 rim nominal thickness and clearly for 6 nm films. Analysis of the optical properties corroborated the analysis and showed a quantum confined blue-shift of the absorption edge for the thinnest films.

  • 171.
    Johansson, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Correlation between process parameters and milling efficiency2012Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    An experimental ball-milling study was performed to compare the deagglomeration behavior and the evolution of the particle size distribution with increasing milling time of two relatively coarse WC powders used for the production of cemented carbide cutting tools. The WC-powders were found to have distinctly different particle size distributions and particle morphologies prior to milling.

    Lab-scale WC samples were made using a range of different process parameters and milling times. These were then analysed by means of microscopy, laser light scattering, gas adsorption BET analysis and X-ray powder diffraction, XRD, to attain particle size distribution, specific surface area and a mean crystal size, respectively.

    The results suggested a linear relation between log(particle size) and log(milling time) between 10 and 80 hours milling. The viscosity was shown to have a minor effect on the milling efficiency. Both the number of collisions of milling balls per unit time as well as the kinetic energy of the milling ball affected the size reduction; more collisions or higher energy resulted in a higher milling efficiency.

    The evaluation of the effect of the process parameters on milling efficiency was facilitated by the use of simple scaling factors. For example, all milling curves for samples with different WC amounts coincided when rescaling the milling time using a scaling factor based on the weight of the WC and milling balls. The same scaling factor could be used with success for rescaling the results from different trials obtained with laser light scattering, gas adsorption and XRD.

    The results of this work are useful for future work on modeling of the milling process which should lead to more accurate predictions of the outcome of milling unit operations.

  • 172.
    Johansson, Fredrik
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Ahlberg, Patrik
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Jansson, Ulf
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.
    Zhang, Shi-Li
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Lindblad, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Molecular and Condensed Matter Physics.
    Nyberg, Tomas
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Electronics.
    Minimizing sputter-induced damage during deposition of WS2 onto graphene2017In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 110, no 9, article id 091601Article in journal (Refereed)
    Abstract [en]

    We demonstrate the sputter-deposition of WS2 onto a single-layer graphene film leaving the latter disorder-free. The sputtering process normally causes defects to the graphene lattice and adversely affects its properties. Sputtering of WS2 yields significant amounts of energetic particles, specifically negative S ions, and reflected neutral Ar, and it is therefore used as a model system in this work. The disorder-free sputtering is achieved by increasing the sputteringpressure of Ar thereby shifting the kinetic energy distribution towards lower energies for the impinging particle flux at the substrate. Raman spectroscopy is used to assess the amount of damage to the graphene film. Monte Carlo simulations of the sputteringprocess show that W is completely thermalized already at relatively low sputtering pressure, whereas Ar and S need a comparably higher pressure to thermalize so as to keep the graphene film intact. Apart from becoming completely amorphous at 2.3 mTorr, the graphene filmremains essentially disorder-free when the pressure is increased to 60 mTorr. The approach used here is generally applicable and readily extendable to sputter-deposition of other material combinations onto sensitive substrates. Moreover, it can be used without changing the geometry of an existing sputtering setup.

  • 173.
    Johansson, Kristina
    Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry. Uppsala University.
    Multicomponent Nitride Thin Films by Reactive Magnetron Sputtering2018Licentiate thesis, comprehensive summary (Other academic)
    List of papers
    1. Influence of oxygen content on structure and material properties of reactively sputtered Al-Ge-O-N thin films
    Open this publication in new window or tab >>Influence of oxygen content on structure and material properties of reactively sputtered Al-Ge-O-N thin films
    2018 (English)In: Journal of Alloys and Compounds, ISSN 0925-8388, E-ISSN 1873-4669, Vol. 738, p. 515-527Article in journal (Refereed) Published
    Abstract [en]

    Ternary Al-Ge-N and quaternary Al-Ge-O-N coatings were deposited by reactive dc magnetron cosputtering of Al and Ge targets in an Ar/N-2 or Ar/N-2/O-2 atmosphere at a substrate temperature of 250 degrees C. The structure and material properties of the coatings were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), nanoindentation, UV-vis spectroscopy and optical profilometry. In agreement with literature, the ternary Al-Ge-N coatings were found to be nanocomposite materials with nanocrystalline (Al1-xGex) N-y solid solution phase in a Ge3N4-z amorphous matrix. The Al-Ge-O-N coatings consisted of a nanocrystalline wurzite-type (Al1-xGex)( N1-yOy) solid solution phase for low oxygen concentrations with a possible co-existence of an amorphous Ge-N matrix phase. For higher O contents, the coatings became X-ray amorphous. The mechanical properties of the Al-Ge-O-N films were improved for low oxygen content, as compared to the ternary Al-Ge-N samples, showing an increase in hardness up to 29 GPa and Young's modulus to 320 GPa. The oxygen addition also resulted in an additional design parameter of the optical properties compared to the ternary Al-Ge-N films. The optical absorption edge was thus tuneable towards both shorter and longer wavelength by changing the O and Ge content respectively, and ranged from 302 to 373 nm, corresponding to an optical bandgap (E-04) between 4.1 and 3.3 eV. After annealing of the Al-Ge-O-N coatings in ultra-high vacuum at 500 degrees C, indications of increased thermal stability for the coating with high oxygen content were observed. For the annealed Al-Ge-O-N films the mechanical properties were improved upon heat treatment, while the optical properties were only slightly changed. These results suggests that coatings of the Al-Ge-O-N system could be suitable as protective optical coatings at elevated temperatures.

    Place, publisher, year, edition, pages
    ELSEVIER SCIENCE SA, 2018
    Keywords
    Reactive sputtering, Al-Ge-O-N, Optical properties, Hard coatings, Thermal stability
    National Category
    Inorganic Chemistry
    Identifiers
    urn:nbn:se:uu:diva-341556 (URN)10.1016/j.jallcom.2017.12.185 (DOI)000419214800062 ()
    Funder
    Swedish Research Council, C0514401Swedish Foundation for Strategic Research , RIF14-0053
    Available from: 2018-02-13 Created: 2018-02-13 Last updated: 2019-09-08Bibliographically approved
    2. Multicomponent Hf-Nb-Ti-V-Zr nitride coatings by reactive sputter deposition
    Open this publication in new window or tab >>Multicomponent Hf-Nb-Ti-V-Zr nitride coatings by reactive sputter deposition
    <