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  • 1. Alfredsson, Maria
    et al.
    Ojamae, Lars
    Hermansson, Kersti
    Uppsala universitet, Strukturkemi.
    A comparison of Hartree-Fock, MP2, and DFT results for the HCN dimer and crystal1996Ingår i: INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Vol. 60, nr 3, s. 767-777Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A number of hydrogen-bond related quantities-geometries, interaction energies, dipole moments, dipole moment derivatives, and harmonic vibrational frequencies-were calculated at the Hartree-Fock, MP2, and different DFT levels for the HCN dimer and the pe

  • 2.
    Andres Cisneros, Gerardo
    et al.
    Wayne State University, MI 48202 USA.
    Thor Wikfeldt, Kjartan
    University of Iceland, Iceland; Stockholm University, Sweden.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Lu, Jibao
    University of Utah, USA.
    Xu, Yao
    Ruhr University of Bochum, Germany.
    Torabifard, Hedieh
    Wayne State University, USA.
    Bartok, Albert P.
    University of Cambridge, England.
    Csanyi, Gabor
    University of Cambridge, England.
    Molinero, Valeria
    University of Utah, USA.
    Paesani, Francesco
    University of Calif San Diego, USA.
    Modeling Molecular Interactions in Water: From Pairwise to Many Body Potential Energy Functions2016Ingår i: Chemical Reviews, ISSN 0009-2665, E-ISSN 1520-6890, Vol. 116, nr 13, s. 7501-7528Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Almost 50 years have passed from the first computer simulations of water, and a large number of molecular models have been proposed since then to elucidate the unique behavior of water across different phases. In this article, we review the recent progress in the development of analytical potential energy functions that aim at correctly representing many-body effects. Starting from the many-body expansion of the interaction energy, specific focus is on different classes of potential energy functions built upon a hierarchy of approximations and on their ability to accurately reproduce reference data obtained from state-of-the-art electronic structure calculations and experimental measurements. We show that most recent potential energy functions, which include explicit short-range representations of two-body and three-body effects along with a physically correct description of many-body effects at all distances, predict the properties of water from the gas to the condensed phase with unprecedented accuracy, thus opening the door to the long-sought "universal model" capable of describing the behavior of water under different conditions and in different environments.

  • 3.
    Atakan, Aylin
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Erdtman, Edvin
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Mäkie, Peter
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Odén, Magnus
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Nanostrukturerade material. Linköpings universitet, Tekniska fakulteten.
    Time evolution of the CO2 hydrogenation to fuels over Cu-Zr-SBA-15 catalysts2018Ingår i: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 362, s. 55-64Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Time evolution of catalytic CO2 hydrogenation to methanol and dimethyl ether (DME) has been investigated in a high-temperature high-pressure reaction chamber where products accumulate over time. The employed catalysts are based on a nano-assembly composed of Cu nanoparticles infiltrated into a Zr doped SiOx mesoporous framework (SBA-15): Cu-Zr-SBA-15. The CO2 conversion was recorded as a function of time by gas chromatography-mass spectrometry (GC-MS) and the molecular activity on the catalyst’s surface was examined by diffuse reflectance in-situ Fourier transform infrared spectroscopy (DRIFTS). The experimental results showed that after 14 days a CO2 conversion of 25% to methanol and DME was reached when a DME selective catalyst was used which was also illustrated by thermodynamic equilibrium calculations. With higher Zr content in the catalyst, greater selectivity for methanol and a total 9.5% conversion to methanol and DME was observed, yielding also CO as an additional product. The time evolution profiles indicated that DME is formed directly from methoxy groups in this reaction system. Both DME and methanol selective systems show the thermodynamically highest possible conversion.

  • 4.
    Bounechada, Djamela
    et al.
    Chalmers, Dept Chem & Biol Engn, SE-41296 Gothenburg, Sweden Chalmers, Competence Ctr Catalysis, SE-41296 Gothenburg, Sweden .
    Darmastuti, Zhafira
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Skoglundh, Magnus
    Chalmers, Dept Chem & Biol Engn, SE-41296 Gothenburg, Sweden Chalmers, Competence Ctr Catalysis, SE-41296 Gothenburg, Sweden.
    Carlsson, Per-Anders
    Chalmers, Dept Chem & Biol Engn, SE-41296 Gothenburg, Sweden Chalmers, Competence Ctr Catalysis, SE-41296 Gothenburg, Sweden.
    Vibrational Study of SOx Adsorption on Pt/SiO22014Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, nr 51, s. 29713-29723Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The formation of ad-SOx species on Pt/SiO2 upon exposure to SO2 in concentrations ranging from 10 to 50 ppm at between 200 and 400 degrees C has been studied by in situ diffuse reflectance infrared Fourier transformed spectroscopy. In parallel, first-principles calculations have been carried out to consolidate the experimental interpretations. It was found that sulfate species form on the silica surface with a concomitant removal/rearrangement of silanol groups. Formation of ad-SOx species occurs only after SO2 oxidation to SO3 on the platinum surface. Thus, SO2 oxidation to SO3 is the first step in the SOx adsorption process, followed by spillover of SO3 to the oxide, and finally, the formation of sulfate species on the hydroxyl positions on the oxide. The sulfate formation is influenced by both temperature and SO2 concentration. Furthermore, exposure to hydrogen is shown to be sufficiently efficient as to remove ad-SOx species from the silica surface.

  • 5.
    Bounechada, Djamela
    et al.
    Chalmers Institute of Technology, Gothenburg.
    Darmastuti, Zhafira
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Ojamae, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Skoglundh, Magnus
    Competence Centre for Catalysis / Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Carlsson, P-A
    Competence Centre for Catalysis / Department of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Vibrational analysis of SO2 on Pt / SiO2 systemManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    In situ diffuse reflectance infrared Fourier transformed spectroscopy was used to study the interactions of SOx species with Pt/SiO2 between 200 and 400°C, and for SO2 concentrations between 10 and 50 ppm, which represents a concentration range where MISFET sensors exhibit good responses. In parallel, first-principles calculations have been carried out to support the experimental interpretations. It was found that sulfate species were formed on the silica surface, accompanied with removal/rearrangement of silanol groups upon exposure to SO2. Both experimental and theoretical calculations also suggest that the surface species were only formed after SO2 oxidation to SO3 on the metal surface. These evidences support the idea of SO2 oxidation to SO3 as the first step in the process of sulfate formation, followed by spillover of SO3 to the oxide, and finally the formation of sulfate species on the hydroxyl positions on the oxide. The results also indicate that the sulfate formation on silica depends both on the temperature and the SO2 concentration. Furthermore, hydrogen exposure was shown to be efficient for sulfur removal from the silica surface.

  • 6.
    Brena, B.
    et al.
    Department of Physics, Uppsala University, Box 518, SE-751 20 Uppsala, Sweden.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Surface effects and quantum confinement in nanosized GaN clusters: Theoretical predictions2008Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 112, nr 35, s. 13516-13523Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The structure and the electronic properties of stoichiometric (GaN) n clusters (with 6 = n = 48) were investigated by means of quantum-chemical hybrid density functional theory (DFT) using the B3LYP functional. Particular emphasis was put on the investigation of the evolution of the physical properties of the clusters as a function of their size. Two types of model clusters were studied. Cage-type structures were found to be the most stable for smaller cluster sizes, whereas for larger sizes conformations cut out from the GaN wurtzite crystal were favorable. The study of the electronic structure shows that the energy gap of the clusters tends to become larger as the dimensions of the clusters increase. The vertical electronic absorption energies were calculated by means of time-dependent (TD) DFT. For such small clusters, probably due to the predominant amount of surface atoms, well-defined quantum confinement effects, as commonly observed in crystalline quantum dots, are not apparent. © 2008 American Chemical Society.

  • 7.
    Buchholt, Kristina
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Ieva, E.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi.
    Käll, Per-Olov
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Torsi, L
    Universita degli Studi di Bari, Italy.
    Lutic, D.
    Växjö universitet.
    Strand, M
    Växjö universitet.
    Sanati, M.
    Växjö universitet.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    FET devices with gold nanoparticle gate material as nitrogen oxide gas sensors2006Ingår i: Proceedings from E-MRS 2006, Nice France, May 29- June 1, 2006, 2006, s. 87-92Konferensbidrag (Refereegranskat)
    Abstract [en]

       

  • 8.
    Buttera, Sydney C.
    et al.
    Carleton University, Canada.
    Ronnby, Karl
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten.
    Pedersen, Henrik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Barry, Sean T.
    Carleton University, Canada.
    Thermal study of an indium trisguanidinate as a possible indium nitride precursor2018Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films, ISSN 0734-2101, E-ISSN 1520-8559, Vol. 36, nr 1, artikel-id 01A101Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Tris-N,N,-dimethyl-N,N -diisopropylguanidinatoindium(III) has been investigated both as a chemical vapor deposition precursor and an atomic layer deposition precursor. Although deposition was satisfactory in both cases, each report showed some anomalies in the thermal stability of this compound, warrenting further investigation, which is reported herein. The compound was found to decompose to produce diisopropylcarbodiimide both by computational modeling and solution phase nuclear magnetic resonance characterization. The decomposition was shown to have an onset at approximately 120 degrees C and had a constant rate of decomposition from 150 to 180 degrees C. The ultimate decomposition product was suspected to be bisdimethylamidoN, N,-dimethyl-N,N -diisopropylguanidinato-indium(III), which appeared to be an intractable, nonvolatile polymer. Published by the AVS.

  • 9. Cavalleri, M
    et al.
    Näslund, LÅ
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska fakulteten.
    Edwards, DC
    Wernet, Ph
    Ogasawara, H
    Myneni, S
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Odelius, M
    Nilsson, A
    Pettersson, LGM
    The local structure of protonated water from x-ray absorption and density functional theory2006Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 124, s. 194508-194508Artikel i tidskrift (Refereegranskat)
    Abstract [en]

      

  • 10.
    Ciobanu, C V
    et al.
    Ohio State Univ, Dept Phys, Columbus, OH 43210 USA.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Shavitt, I
    Ohio State Univ, Dept Phys, Columbus, OH 43210 USA.
    Singer, S J
    Ohio State Univ, Dept Phys, Columbus, OH 43210 USA.
    Structure and vibrational spectra of H(+)(H(2)O)(8): Is the excess proton in a symmetrical hydrogen bond?2000Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 113, nr 13, s. 5321-5330, artikel-id PII [S0021-9606(00)30735-8]Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The energetics, structure, and vibrational spectra of a wide variety of H + (H 2 O) 8 structures are calculated using density functional theory and second-order Møller–Plesset ab initio methods. In these isomers of H + (H 2 O) 8 the local environment of the excess proton sometimes resembles a symmetric H 5 O + 2 structure and sometimes H 3 O + , but many structures are intermediate between these two limits. We introduce a quantitative measure of the degree to which the excess proton resembles H 5 O + 2 or H 3 O + . Other bond lengths and, perhaps most useful, the position of certain vibrational bands track this measure of the symmetry in the local structure surrounding the excess proton. The general trend is for the most compact structures to have the lowest energy. However, adding zero-point energy counteracts this trend, making prediction of the most stable isomer impossible at this time. At elevated temperatures corresponding to recent experiments and atmospheric conditions (150–200 K), calculated Gibbs free energies clearly favor the least compact structures, in agreement with recent thermal simulations [Singer, McDonald, and Ojamäe, J. Chem. Phys. 112, 710 (2000)]. © 2000 American Institute of Physics.

  • 11.
    Danielsson, Örjan
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Li, Xun
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Pedersen, Henrik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Forsberg, Urban
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska fakulteten.
    A model for carbon incorporation from trimethyl gallium in chemical vapor deposition of gallium nitride2016Ingår i: Journal of Materials Chemistry, ISSN 0959-9428, E-ISSN 1364-5501, Vol. 4, nr 4, s. 863-871Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Gallium nitride (GaN) semiconductor material can become semi-insulating when doping with carbon. Semi-insulating buffer layers are utilized to prevent leakage currents in GaN high power devices. Carbon is inherently present during chemical vapor deposition (CVD) of GaN from the use of trimethyl gallium (TMGa) as precursor. TMGa decomposes in the gas phase, releasing its methyl groups, which could act as carbon source for doping. It is previously known that the carbon doping levels can be controlled by tuning the CVD process parameters, such as temperature, pressure and precursor flow rates. However, the mechanism for carbon incorporation from TMGa is not yet understood. In this paper, a model for predicting carbon incorporation from TMGa in GaN layers grown by CVD is proposed. The model is based on ab initio quantum chemical calculations of molecular adsorption and reaction energies. Using Computational Fluid Dynamics, with a chemical kinetic model for decomposition of the precursors and reactions in the gas phase, to calculate gas phase compositions at realistic process conditions, together with the proposed model, we obtain good correlations with measurements, for both carbon doping concentrations and growth rates, when varying the inlet NH3/TMGa ratio. When varying temperature (800 – 1050°C), the model overpredicts carbon doping concentrations at the lower temperatures, but predicts growth rates well, and the agreement with measured carbon doping concentrations is good above 1000°C.

  • 12.
    Danielsson, Örjan
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Sukkaew, Pitsiri
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Kordina, Olof
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Shortcomings of CVD modeling of SiC today2013Ingår i: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 132, nr 11, s. 1398-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The active, epitaxial layers of silicon carbide (SiC) devices are grown by chemical vapor deposition (CVD), at temperatures above 1,600 °C, using silane and light hydrocarbons as precursors, diluted in hydrogen. A better understanding of the epitaxial growth process of SiC by CVD is crucial to improve CVD tools and optimize growth conditions. Through computational fluid dynamic (CFD) simulations, the process may be studied in great detail, giving insight to both flow characteristics, temperature gradients and distributions, and gas mixture composition and species concentrations throughout the whole CVD reactor. In this paper, some of the important parts where improvements are very much needed for accurate CFD simulations of the SiC CVD process to be accomplished are pointed out. First, the thermochemical properties of 30 species that are thought to be part of the gas-phase chemistry in the SiC CVD process are calculated by means of quantum-chemical computations based on ab initio theory and density functional theory. It is shown that completely different results are obtained in the CFD simulations, depending on which data are used for some molecules, and that this may lead to erroneous conclusions of the importance of certain species. Second, three different models for the gas-phase chemistry are compared, using three different hydrocarbon precursors. It is shown that the predicted gas-phase composition varies largely, depending on which model is used. Third, the surface reactions leading to the actual deposition are discussed. We suggest that hydrocarbon molecules in fact have a much higher surface reactivity with the SiC surface than previously accepted values.

  • 13.
    Darmastuti, Zhafira
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Larsson, Mikael
    Alstom Power, Sweden.
    Lindqvist, Niclas
    Alstom Power, Sweden.
    SiC based field effect transistor for H2S detection2011Ingår i: Proc. IEEE Sensors 2011, Limerick, Ireland, October 28-31, IEEE , 2011, s. 770-773Konferensbidrag (Refereegranskat)
    Abstract [en]

    Experimental characterization and quantum chemical calculations were performed to evaluate the performance of a SiC based Field Effect Transistors with Pt and Ir gates as H2S sensors. The sensors were tested against various concentrations of H2S gas at the operating temperature between 150 and 350 °C. It was observed that Ir was very sensitive and selective to H2S at 350 °C. This phenomenon was studied further by comparing the reaction energy when H2S is exposed to Pt and Ir with density functional theory (DFT) calculations.

  • 14.
    Darmastuti, Zhafira
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Bhattacharyya, P.
    Dept. of Electronics and Telecommunication Engineering, Bengal Engineering and Science University, India.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Kanungo, Jayita
    IC Design & Fabrication Centre, Dept. of Electronics & Telecommunications Engineering, Jadavpur University, Kolkata, India.
    Basu, Sukumar
    IC Design & Fabrication Centre, Dept. of Electronics & Telecommunications Engineering, Jadavpur University, Kolkata, India.
    Käll, Per-Olov
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Oorganisk Kemi.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    SiC-FET methanol sensors for process control and leakage detection2013Ingår i: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 187, nr SI, s. 553-562Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two types of SiC based field effect transistor sensors, with Pt or Ir gate, were tested to detect methanol in the concentration range of 0–1600 ppm for both process control and leak detection applications. The methanol response was investigated both with and without oxygen, since the process control might be considered as oxygen free application, while the sensor is operated in air during leak detection. Pt sensors offered very fast response with appreciably high response magnitude at 200 °C, while Ir sensors showed both higher response and response time up to 300 °C, but this decreased considerably at 350 °C. Cross sensitivity effect in presence of oxygen, hydrogen, propene and water vapor was also investigated. The presence of oxygen improved the response of both sensors, which is favorable for the leak detection application. Hydrogen had a large influence on the methanol response of both sensors, propene had a negligible influence, while water vapor changed direction of the methanol response for the Pt sensor. The detection mechanism and different sensing behavior of Pt and Ir gate sensors were discussed in the light of model reaction mechanisms derived from hybrid density-functional theory quantum-chemical calculations.

  • 15.
    Darmastuti, Zhafira
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Bhattacharyya, Partha
    Bengal Engineering and Science University, India.
    Basu, Sukumar
    Jadavpur University, India.
    Käll, Per-Olov
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    SiC - FET Sensors for methanol leakage detection2012Ingår i: Proceeding of the 14th International Meeting on Chemical Sensors (IMCS 2012), 2012, s. 1579-1582Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

    Pt and Ir SiC based Field Effect Transistor sensors were tested to detect low concentration of methanol (<200 ppm) for both process control and leak detection applications. Pt sensors gave good and very fast response at 200°C, while Ir sensors gave larger but much slower response. The presence of oxygen improved the response of the sensor which was favorable for the leak detection application. The influence of hydrogen and propene to the sensor response was also studied. Beside the experimental work, the detection mechanism and different sensing behavior of Pt and Ir were studied by quantum chemical calculations.

  • 16.
    Darmastuti, Zhafira
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Bounechada, Djamela
    Competence Centre for Catalysis / Dept. of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Carlsson, P-A
    Competence Centre for Catalysis / Dept. of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Lindqvist, N.
    Alstom Power AB, Växjö, Sweden.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Skoglundh, Magnus
    Competence Centre for Catalysis / Dept. of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Carlsson, P-A
    Competence Centre for Catalysis /Dept. of Chemical and Biological Engineering, Chalmers University of Technology, Göteborg, Sweden.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Ojamae, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Detection mechanism studies of SO2 on Pt / SiO2 systemManuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Experiment was performed with Pt-gate SiC-FET sensors to study the detection mechanism of the sensors. The sensing measurement showed that oxygen influenced the response quite strongly. The sensor response became larger in the presence of oxygen. Experiment with mass spectroscopy indicated the formation of SO3 during the sensing measurement. Further experiment with DRIFT spectroscopy showed the formation of sulfate species on the oxide surface, accompanied by the disappearance of the silanol groups. An explanatory model was built based on quantum-chemical calculations. The results strengthened the experimental results by showing that it was more energetically favorable for SO2 to oxidize into SO3 before being adsorbed on the oxide surface. It was also observed that the overall adsorption reaction was exothermic, the activation energy for the SO2 oxidation was 48,75 kJ/mol, and the rate limiting step was the desorption of SO3 from the Pt surface.

  • 17.
    Darmastuti, Zhafira
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska högskolan.
    SiC-FET as SO2 Sensors - Detection Mechanism Studies2014Ingår i: Proc IMCS 2014, Buenos Aires, Argentine, March 17-19, MPS-T3-4, 2014Konferensbidrag (Refereegranskat)
  • 18.
    D'Ercole, A
    et al.
    Unita INFM Torino, I-10125 Turin, Italy.
    Giamello, E
    Unita INFM Torino, I-10125 Turin, Italy.
    Pisani, C
    Unita INFM Torino, I-10125 Turin, Italy.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Embedded-cluster study of hydrogen interaction with an oxygen vacancy at the magnesium oxide surface1999Ingår i: JOURNAL OF PHYSICAL CHEMISTRY B, ISSN 1089-5647, Vol. 103, nr 19, s. 3872-3876Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An embedded-cluster Hartree−Fock approximation is adopted for simulating the formation of Fs(H) color centers at the (001) surface of magnesium oxide. This process is assumed to take place in two steps at an isolated surface anion vacancy:  first, a hydrogen molecule is adsorbed dissociatively at the defect; second, following UV irradiation, a neutral hydrogen atom is removed and an electron remains trapped at the vacancy with a hydroxyl group nearby. According to the present calculations, the activation energy for the dissociation is appreciable (about 25 kcal/mol) and the products (a proton bound to a low-coordinated oxygen and a hydride ion above the vacancy) are considerably less stable than the reactants. The excitation of the adsorbed species owing to the UV irradiation is simulated by considering a singlet−triplet transition of the hydride−vacancy complex, which then dissociates into an H atom and a trapped lone electron. The electronic structure and the EPR parameters of the resulting paramagnetic state are explored. The theoretical results agree in many respects with the experimental data as concerns one of the forms of heterolitically dissociated hydrogen which are found at the defective MgO surface. However, from the viewpoint of the energetics, this model is untenable because that species is known to form irreversibly at room temperature with low activation energy.

  • 19.
    Edvardsson, Sverker
    et al.
    Mittuniversitetet, Institutionen för informationsteknologi och medier.
    Ojamäe, L
    Thomas, John O
    A study of vibrational modes in Na(+)beta-Alumina by molecular-dynamics simulation1994Ingår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 6, nr 7, s. 1319-1332Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The vibrational properties of crystalline Na+ beta-alumina (Na1.22Al11O17.11) have been studied using the molecular dynamics simulation technique. The vibrational density of states was calculated from the velocity autocorrelation function, and the infrared spectrum from the dipole-dipole autocorrelation function. Knowledge of the vibrations in different crystallographic directions for the different atomic species facilitates the assignment of spectral peaks. The sodium in-plane vibrations are 59, 88 and 112 cm-1, and the out-of-plane vibrations are at 146 cm-1. The stoichiometric compound is also studied, and in this case the sodium in-plane vibrations are at 80 cm-1 and the out-of-plane vibrations at 140 cm-1. The density of states is used to calculate thermodynamic properties: heat capacity, entropy and internal and free energy. The values obtained at 300 K are C(upsilon) = 410 J K-1 mol-1, S(upsilon) = 300 J K-1 mol-1, U = 370 kJ mol-1 and F = 280 kJ mol 1. The heat capacity and entropy values are in good agreement with experiment, and thus strongly support the empirical force field used in the simulation

  • 20.
    Erdtman, Edvin
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska fakulteten.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad sensorvetenskap. Linköpings universitet, Tekniska fakulteten.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Simulations of the thermodynamics and kinetics of NH3 at the RuO2 (110) surface2017Ingår i: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, Vol. 656, s. 9s. 77-85Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ruthenium(IV)oxide (RuO2) is a material used for various purposes. It acts as a catalytic agent in several reactions, for example oxidation of carbon monoxide. Furthermore, it is used as gate material in gas sensors. In this work theoretical and computational studies were made on adsorbed molecules on RuO2 (110) surface, in order to follow the chemistry on the molecular level. Density functional theory calculations of the reactions on the surface have been performed. The calculated reaction and activation energies have been used as input for thermodynamic and kinetics calculations. A surface phase diagram was calculated, presenting the equilibrium composition of the surface at different temperature and gas compositions. The kinetics results are in line with the experimental studies of gas sensors, where water has been produced on the surface, and hydrogen is found at the surface which is responsible for the sensor response.

  • 21.
    Flemström, A.
    et al.
    Department of Inorganic, Structural and Physical Chemistry, Arrhenius Laboratory, Stockholm University, S-10691, Stockholm, Sweden.
    Hirsch, T. K.
    Department of Inorganic, Structural and Physical Chemistry, Arrhenius Laboratory, Stockholm University, S-10691, Stockholm, Sweden.
    Sehlstedt, L.
    Department of Inorganic, Structural and Physical Chemistry, Arrhenius Laboratory, Stockholm University, S-10691, Stockholm, Sweden.
    Lidin, S.
    Department of Inorganic, Structural and Physical Chemistry, Arrhenius Laboratory, Stockholm University, S-10691, Stockholm, Sweden.
    Ojamäe, L.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Effects from hydrogen bonds on water structure in (H3O)2[Mo6Cl8X6yH2O X=Cl (y=7), Br (y=6), or I (y=6)2002Ingår i: Solid State Sciences, ISSN 1293-2558, E-ISSN 1873-3085, Vol. 4, nr 8, s. 1017-1022Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The compounds (a) (H3O)2[Mo6Cl8Cl6]·7H2O, (b) (H3O)2[Mo6Cl8Br6]·6H2O and (c) (H3O)2[Mo6Cl8I6]·6H2O were synthesized from MoCl2 and the corresponding halide acid. The structures were determined by X-ray diffraction and refined in the monoclinic space groups, (a) C2/c and for (b) and (c) P21/a. The cell parameters were for (a), a=17.3607(2), b=9.1351(7), c=18.6300(2) Å and β=98.13(1)°, (b) a=17.4295(2), b=9.3803(10), c=9.3769(12) Å and β=101.04(1)° and (c) a=18.0083(10), b=9.7612(10), c=9.8139(12) Å and β=100.20(2)°. The positions of the hydrogen atoms were determined by theoretical energy optimization. The structures are compared with respect to the effect of hydrogen bonding on the water structure.

  • 22.
    Gunnarsson, Rickard
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Plasma och beläggningsfysik. Linköpings universitet, Tekniska fakulteten.
    Brenning, Nils
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Plasma och beläggningsfysik. Linköpings universitet, Tekniska fakulteten. KTH Royal Inst Technol, Sweden.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Kalered, Emil
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Raadu, Michael Allan
    KTH Royal Inst Technol, Sweden.
    Helmersson, Ulf
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Plasma och beläggningsfysik. Linköpings universitet, Tekniska fakulteten.
    Nucleation of titanium nanoparticles in an oxygen-starved environment. II: theory2018Ingår i: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, nr 45, artikel-id 455202Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The nucleation and growth of pure titanium nanoparticles in a low-pressure sputter plasma has been believed to be essentially impossible. The addition of impurities, such as oxygen or water, facilitates this and allows the growth of nanoparticles. However, it seems that this route requires such high oxygen densities that metallic nanoparticles in the hexagonal alpha Ti-phase cannot be synthesized. Here we present a model which explains results for the nucleation and growth of titanium nanoparticles in the absent of reactive impurities. In these experiments, a high partial pressure of helium gas was added which increased the cooling rate of the process gas in the region where nucleation occurred. This is important for two reasons. First, a reduced gas temperature enhances Ti-2 dimer formation mainly because a lower gas temperature gives a higher gas density, which reduces the dilution of the Ti vapor through diffusion. The same effect can be achieved by increasing the gas pressure. Second, a reduced gas temperature has a more than exponential effect in lowering the rate of atom evaporation from the nanoparticles during their growth from a dimer to size where they are thermodynamically stable, r*. We show that this early stage evaporation is not possible to model as a thermodynamical equilibrium. Instead, the single-event nature of the evaporation process has to be considered. This leads, counter intuitively, to an evaporation probability from nanoparticles that is exactly zero below a critical nanoparticle temperature that is size-dependent. Together, the mechanisms described above explain two experimentally found limits for nucleation in an oxygen-free environment. First, there is a lower limit to the pressure for dimer formation. Second, there is an upper limit to the gas temperature above which evaporation makes the further growth to stable nuclei impossible.

  • 23.
    HERMANSSON, KERSTI
    et al.
    Uppsala universitet, Kemiska institutionen.
    OJAMAE, LARS
    ON THE ROLE OF ELECTRIC-FIELDS FOR PROTON-TRANSFER IN WATER1995Ingår i: Solid State Ionics, ISSN 0167-2738, E-ISSN 1872-7689, Vol. 77, s. 34-42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The influence of uniform and non-uniform electric fields on one-dimensional proton transfer curves for (H2O)(2), H5O2+ and H3O2- has been examined using quantum-mechanical ab initio calculations. Both liquid-state and solid-state environments are discussed. For the charged complexes the transfer barrier is removed or greatly reduced by a field as small as 0.005 a.u. (2.5 X 10(7) V/cm). Local field fluctuations of this size are easily produced in condensed aqueous systems at room temperature. For the asymmetric single-well potential of an (H2O)(2) complex, a field ten times larger is needed to move the minimum from one side to the other across the O ... O bond. Such local fields can be achieved in ionic aqueous systems. The energy barrier for proton transfer in ice Ih has been computed using a periodic Hartree-Fock approach; the barrier for a fully concerted proton transfer is similar to 60 kJ/mol.

  • 24.
    Hermansson, Kersti
    et al.
    Uppsala universitet, Kemiska institutionen.
    Ojamäe, Lars
    From Molecule to Cluster to Bulk - Water OH Vibrations in Different Surroundings1992Ingår i: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 42, nr 5, s. 1251-1270Artikel, forskningsöversikt (Refereegranskat)
    Abstract [en]

    Vibrational spectra for the O-H stretching motion of HDO molecules in different surroundings have been calculated by quantum mechanical ab initio methods and compared with experimental spectra. The free water molecule, water chains, and ion-water clusters are discussed. Solvent effects on OH vibrations in liquid water have been calculated as well as "in-crystal" OH frequencies in some ice and ionic crystalline hydrate structures. The importance of nonadditivity effects, electron correlation (at the MP2 level), and long-range interactions for the total frequency downshift is demonstrated. It is shown that the inclusion of these effects, in conjunction with a variational quantum mechanical treatment of the anharmonic vibrational stretching motion (force constants up to the fourth order), yields vibrational frequencies in quantitative agreement with experiment for a wide range of aqueous systems.

  • 25.
    Hirsch, T.K.
    et al.
    Department of Physical Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    An investigation of H-atom positions in sulfuric acid crystal structures2004Ingår i: Acta Crystallographica Section B: Structural Science, ISSN 0108-7681, E-ISSN 1600-5740, Vol. 60, nr 2, s. 179-183Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hydrogen conformations in crystalline H2SO4· 8H2O and H2SO4·6.5H2O have been studied using a system developed by Hirsch [(2003), Z. Anorg. Allg. Chem. 629, 666-672]. New H-atom coordinates, as estimated from DFT calculations, are given for these structures. © 2004 International Union of Crystallography Printed in Great Britain - all rights reserved.

  • 26.
    Hirsch, T.K.
    et al.
    Physical Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Quantum-chemical and force-field investigations of ice Ih: Computation of proton-ordered structures and prediction of their lattice energies2004Ingår i: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 108, nr 40, s. 15856-15864Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The different possible proton-ordered structures of ice Ih for an orthorombic unit cell with 8 water molecules were derived. The number of unique structures was found to be 16. The crystallographic coordinates of these are reported. The energetics of the different polymorphs were investigated by quantum-mechanical density-functional theory calculations and for comparison by molecular-mechanics analytical potential models. The polymorphs were found to be close in energy, i.e., within approximately 0.25 kcal/mol H2O, on the basis of the quantum-chemical DFT methods. At 277 K, the different energy levels are about evenly populated, but at a lower temperature, a transition to an ordered form is expected. This form was found to agree with the ice phase XI. The difference in lattice energies among the polymorphs was rationalized in terms of structural characteristics. The most important parameters to determine the lattice energies were found to be the distributions of water dimer H-bonded pair conformations, in an intricate manner.

  • 27.
    Huang, C
    et al.
    Stanford Synchrotron Radiat Lightsource.
    Wikfeldt, K T
    Stockholm University.
    Tokushima, T
    RIKEN SPring 8, Mikazuki, Hyogo, Japan .
    Nordlund, D
    Stanford Synchrotron Radiat Lightsource.
    Harada, Y
    RIKEN SPring 8, Mikazuki, Hyogo Japan .
    Bergmann, U
    Stanford Synchrotron Radiat Lightsource.
    Niebuhr, M
    Stanford Synchrotron Radiat Lightsource.
    Weiss, T M
    Stanford Synchrotron Radiat Lightsource.
    Horikawa, Y
    RIKEN SPring 8, Mikazuki, Hyogo Japan .
    Leetmaa, M
    Stockholm University.
    Ljungberg, M P
    Stockholm University.
    Takahashi, O
    Hiroshima University.
    Lenz, Annika
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Lyubartsev, A P
    Stockholm University.
    Shin, S
    RIKEN SPring 8, Mikazuki, Hyogo Japan .
    Pettersson, L G M
    Stockholm University.
    Nilsson, A
    Stanford Synchrotron Radiat Lightsource.
    Letter: Reply to Soper et al.: Fluctuations in water around a bimodal distribution of local hydrogen-bonded structural motifs2010Ingår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 107, nr 12, s. E45-E45Artikel i tidskrift (Övrigt vetenskapligt)
    Abstract [en]

    n/a

  • 28.
    Huang, C
    et al.
    Stanford Synchrotron Radiat Lightsource.
    Wikfeldt, K T
    AlbaNova.
    Tokushima, T
    RIKEN.
    Nordlund, D
    Stanford Synchrotron Radiat Lightsource.
    Harada, Y
    RIKEN.
    Bergmann, U
    Stanford Synchrotron Radiat Lightsource.
    Niebuhr, M
    Stanford Synchrotron Radiat Lightsource.
    Weiss, T M
    Stanford Synchrotron Radiat Lightsource.
    Horikawa, Y
    RIKEN.
    Leetmaa, M
    AlbaNova.
    Ljungberg, M P
    AlbaNova.
    Takahashi, O
    Hiroshima University.
    Lenz, Annika
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Lyubartsev, A P
    Stockholm University.
    Shin, S
    RIKEN.
    Pettersson, L G M
    AlbaNova.
    Nilsson, A
    Stanford Synchrotron Radiat Lightsource.
    The inhomogeneous structure of water at ambient conditions2009Ingår i: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, ISSN 0027-8424, Vol. 106, nr 36, s. 15214-15218Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of approximate to 1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (non-directional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.

  • 29.
    Huang, X.
    et al.
    Cherry L. Emerson Ctr. of Sci. Comp., Department of Chemistry, Emory University, Atlanta, GA 30322, United States.
    Cho, H.M.
    Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, OH 43210, United States.
    Carter, S.
    Cherry L. Emerson Ctr. of Sci. Comp., Department of Chemistry, Emory University, Atlanta, GA 30322, United States, Department of Chemistry, University of Reading, Reading, United Kingdom.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Bowman, J.M.
    Cherry L. Emerson Ctr. of Sci. Comp., Department of Chemistry, Emory University, Atlanta, GA 30322, United States.
    Singer, S.J.
    Department of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, OH 43210, United States.
    Full dimensional quantum calculations of vibrational energies of H5O2+2003Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 107, nr 37, s. 7142-7151Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The full dimensional (15 degrees-of-freedom) quantum calculations of vibrational energies of H5O2+ are reported using the global potential energy surface (OSS) of Ojamäe et al. (J. Chem. Phys. 1998, 109, 5547). One set of calculations uses the diffusion Monte Carlo (DMC) method with a highly flexible initial trial wave function. This method is limited to the ground vibrational state, but produces what we believe is a highly accurate, benchmark energy and wave function for that state. The DMC wave function is analyzed to identify coordinates that are strongly correlated in zero-point fluctuations. A simple harmonic model is developed to elucidate the energetic consequences of these correlations. The other set of calculations is based on the code MULTIMODE, which does configuration interaction (CI) calculations using a basis determined from a vibrational self-consistent field (VSCF) Hamiltonian, but which uses a representation of the potential with mode coupling limited to a maximum of four modes. Good agreement is obtained between the DMC and the CI MULTIMODE energies for the ground vibrational state. When less sophisticated theoretical treatments are applied, either variational Monte Carlo or vibrational self-consistent field, fairly large errors are found. Vibrationally excited-state energies obtained with MULTIMODE are also reported.

  • 30.
    Kalered, Emil
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Brenning, N.
    KTH Royal Institute Technology, Sweden.
    Pilch, Iris
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tunnfilmsfysik. Linköpings universitet, Tekniska fakulteten.
    Caillault, L.
    University of Paris 11, France.
    Minea, T.
    University of Paris 11, France.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    On the work function and the charging of small (r <= 5 nm) nanoparticles in plasmas2017Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, nr 1, artikel-id 013702Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The growth of nanoparticles (NPs) in plasmas is an attractive technique where improved theoretical understanding is needed for quantitative modeling. The variation of the work function W with size for small NPs, r(NP) amp;lt;= 5 nm, is a key quantity for modeling of three NP charging processes that become increasingly important at a smaller size: electron field emission, thermionic electron emission, and electron impact detachment. Here we report the theoretical values of the work function in this size range. Density functional theory is used to calculate the work functions for a set of NP charge numbers, sizes, and shapes, using copper for a case study. An analytical approximation is shown to give quite accurate work functions provided that r(NP) amp;gt; 0.4 nm, i.e., consisting of about amp;gt; 20 atoms, and provided also that the NPs have relaxed close to spherical shape. For smaller sizes, W deviates from the approximation, and also depends on the charge number. Some consequences of these results for nanoparticle charging are outlined. In particular, a decrease in W for NP radius below about 1 nm has fundamental consequences for their charge in a plasma environment, and thereby on the important processes of NP nucleation, early growth, and agglomeration. Published by AIP Publishing.

  • 31.
    Kalered, Emil
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Pedersen, Henrik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Janzén, Erik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska högskolan.
    Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations2013Ingår i: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 132, nr 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The effect chlorine addition to the gas mixture has on the surface chemistry in the chemical vapour deposition (CVD) process for silicon carbide (SiC) epitaxial layers is studied by quantum-chemical calculations of the adsorption and diffusion of SiH2 and SiCl2 on the (000-1) 4H–SiC surface. SiH2 was found to bind more strongly to the surface than SiCl2 by approximately 100 kJ mol−1 and to have a 50 kJ mol−1 lower energy barrier for diffusion on the fully hydrogen-terminated surface. On a bare SiC surface, without hydrogen termination, the SiCl2 molecule has a somewhat lower energy barrier for diffusion. SiCl2 is found to require a higher activation energy for desorption once chemisorbed, compared to the SiH2 molecule. Gibbs free energy calculations also indicate that the SiC surface may not be fully hydrogen terminated at CVD conditions since missing neighbouring pair of surface hydrogens is found to be a likely type of defect on a hydrogen-terminated SiC surface.

  • 32.
    Kanungo, Jayita
    et al.
    IC Design & Fabrication Centre, Dept. of Electronics & Telecommunications Engineering, Jadavpur University, Kolkata, India.
    Andersson, Mike
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Darmastuti, Zhafira
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Basu, Sukumar
    IC Design & Fabrication Centre, Dept. of Electronics & Telecommunications Engineering, Jadavpur University, Kolkata, India.
    Käll, Per-Olov
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Lloyd Spetz, Anita
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
    Development of SiC-FET methanol sensor2011Ingår i: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 160, nr 1, s. 72-78Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A silicon carbide based field effect transistor (SiC-FET) structure was used for methanol sensing. Due to the chemical stability and wide band gap of SiC, these sensors are suitable for applications over a wide temperature range. Two different catalytic metals, Pt and Ir, were tested as gate contacts for detection of methanol. The sensing properties of both Ir gate and Pt gate SiC-FET sensors were investigated in the concentration range 0.3–5% of methanol in air and in the temperature range 150–350 °C. It was observed that compared to the Ir gate sensor, the Pt gate sensor showed higher sensitivity, faster response and recovery to methanol vapour at comparatively lower temperature, with an optimum around 200 °C. Quantum-chemical calculations were used to investigate the MeOH adsorption and to rationalize the observed non-Langmuir behavior of the response functions. The methanol sensing mechanism of the SiC-FET is discussed.

  • 33.
    Knight, C
    et al.
    Ohio State University.
    Singer, SJ
    Ohio State University.
    Kuo, J-L
    Nanyang Technological University.
    Hirsch, TK
    Stockholm University.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Klein, ML
    University of Pennsylvania.
    Hydrogen bond topology and the ice VII/VIII and Ih/XI proton ordering phase transitions2006Ingår i: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics: Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics, ISSN 1063-651X, E-ISSN 1095-3787, Vol. 73, s. 056113-056113Artikel i tidskrift (Refereegranskat)
    Abstract [en]

      Ice Ih, ordinary ice at atmospheric pressure, is a proton-disordered crystal that when cooled under special conditions is believed to transform to ferroelectric proton-ordered ice XI, but this transformation is still subject to controversy. Ice VII, also proton disordered throughout its region of stability, transforms to proton-ordered ice VIII upon cooling. In contrast to the ice Ih/XI transition, the VII/VIII transition and the crystal structure of ice VIII are well characterized. In order to shed some light on the ice Ih proton ordering transition, we present the results of periodic electronic density functional theory calculations and statistical simulations. We are able to describe the small energy differences among the innumerable H-bond configurations possible in a large simulation cell by using an analytic theory to extrapolate from electronic DFT calculations on small unit cells to cells large enough to approximate the thermodynamic limit. We first validate our methods by comparing our predictions to the well-characterized ice VII/VIII proton ordering transition, finding agreement with respect to both the transition temperature and structure of the low-temperature phase. For ice Ih, our results indicate that a proton-ordered phase is attainable at low temperatures, the structure of which is in agreement with the experimentally proposed ferroelectric Cmc21 structure. The predicted transition temperature of 98  K is in qualitative agreement with the observed transition at 72  K on KOH-doped ice samples.

  • 34. KNUTS, SÖREN
    et al.
    OJAMAE, LARS
    HERMANSSON, KERSTI
    Uppsala universitet, Kemiska institutionen.
    AN AB-INITIO STUDY OF THE OH STRETCHING FREQUENCIES IN ICE-II, ICE-VIII, AND ICE-IX1993Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 99, nr 4, s. 2917-2928Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ab initio studies of the uncoupled, anharmonic OH and OD stretching frequency shifts in the three proton-ordered ice phases known, ice II, ice VIII, and ice IX, are presented. The ice structures are simulated by (H2O)5 supermolecules surrounded by point charges representing the correct crystal potentials. The calculations include electron correlation at the MP2 (DZP) level. For the eight different OH (OD) vibrators studied, the crystal environment leads to a downshift of the anharmonic OD frequency in the range 195-265 cm-1, in good agreement with experimental values (222-281 cm-1) when corrections are made for the limited supermolecular size (approximately - 45 cm-1), and, for ice VIII, also for the effects of the nonhydrogen bonded network (approximately + 75 cm- 1). Also the agreement between absolute experimental and theoretical OD frequencies is good when errors due to basis set limitation (approximately - 75 cm-1 ) are taken into account. The calculations suggest a reassignment of two of the experimental OD bands in ice II and all three experimental OD bands in ice IX. Calculations for charge-embedded (H2O)9 and (H2O)13 ice clusters show that at least a nonamer is needed to avoid boundary effects from the size of the supermolecule. Theoretical correlation curves between H-bond parameters-R (O ... 0), nu(OH), r(e)(OH), and infrared absorption intensity-are presented for the three ice phases and are compared to liquid water computations.

  • 35.
    Kula, Mathias
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    A theoretical study of the electronic structure of GaN nanorods2012Ingår i: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 112, nr 7, s. 1796-1802Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a theoretical study of the evolution of the electronic structure of wurtzite GaN nanorods for different lengths (2.415.4 nm) in the [0001] direction and different diameters (0.972.25 nm). This study includes both a hybrid density functional theory study and a comparison with the k.p empirical band structure method. From the quantum chemical calculations, surface effects are found to be important. When these have been compensated for the electronic structure properties as a function of rod length or diameter approximately follow the trend expected from the quantum confinement effect. The k.p method predicts a similar behavior although deviations are apparent for smaller sizes.

  • 36.
    Kuo, J. L.
    et al.
    Univ Penn, Dept Chem, Ctr Mol Modeling, Philadelphia, PA 19104 USA.
    Klein, M. L.
    Univ Penn, Dept Chem, Ctr Mol Modeling, Philadelphia, PA 19104 USA.
    Singer, S. J.
    Ohio State Univ, Dept Chem, Columbus, OH 43210 USA.
    Ojamäe, L.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Ice lh-Ice XI phase transition: A quantum mechanical study2004Konferensbidrag (Övrigt vetenskapligt)
  • 37.
    Kuo, Jer-Lai
    et al.
    Department of Chemistry, Ohio State University, Columbus, Ohio 43214.
    Coe, James V.
    Department of Chemistry, Ohio State University, Columbus, Ohio 43214.
    Singer, Sherwin J.
    Department of Chemistry, Ohio State University, Columbus, Ohio 43214.
    Band, Yehuda B.
    Departments of Chemistry and Physics, Ben-Gurion University of the Negev, Beer-Sheva, Israel.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    On the use of graph invariants for efficiently generating hydrogen bond topologies and predicting physical properties of water clusters and ice2001Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 114, nr 6, s. 2527-2540Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Water clusters and some phases of ice are characterized by many isomers with similar oxygen positions, but which differ in direction of hydrogen bonds. A relationship between physical properties, like energy or magnitude of the dipole moment, and hydrogen bond arrangements has long been conjectured. The topology of the hydrogen bond network can be summarized by oriented graphs. Since scalar physical properties like the energy are invariant to symmetry operations, graphical invariants are the proper features of the hydrogen bond network which can be used to discover the correlation with physical properties. We demonstrate how graph invariants are generated and illustrate some of their formal properties. It is shown that invariants can be used to change the enumeration of symmetry-distinct hydrogen bond topologies, nominally a task whose computational cost scales like N2, where N is the number of configurations, into an N ln N process. The utility of graph invariants is confirmed by considering two water clusters, the (H2O)6 cage and (H2O)20 dodecahedron, which, respectively, possess 27 and 30 026 symmetry-distinct hydrogen bond topologies associated with roughly the same oxygen atom arrangements. Physical properties of these clusters are successfully fit to a handful of graph invariants. Using a small number of isomers as a training set, the energy of other isomers of the (H2O)20 dodecahedron can even be estimated well enough to locate phase transitions. Some preliminary results for unit cells of ice-Ih are given to illustrate the application of our results to periodic systems.

     

  • 38.
    Kuo, J.-L.
    et al.
    Department of Chemistry, Ohio State University, Columbus, OH 43210, United States.
    Ciobanu, C.V.
    Department of Physics, Ohio State University, Columbus, OH 43210, United States.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Shavitt, I.
    Department of Chemistry, University of Illinois, Urbana, IL 61801, United States.
    Singer, S.J.
    Department of Chemistry, Ohio State University, Columbus, OH 43210, United States.
    Short H-bonds and spontaneous self-dissociation in (H2O)20: Effects of H-bond topology2003Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 118, nr 8, s. 3583-3588Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The affects of H-bond topology and spontaneous self-dissociation in (H2O)20 was discussed. The enthalpy of dissociation of water to H+ and OH- in bulk water was found to be 13.5 kcal/mol. The surface of ice was characterized by dangling hydrogens and variable H-bond topology.

  • 39.
    Käll, Per-Olov
    et al.
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Ojamäe, Lars
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi.
    Pedersen, Henrik
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Materiefysik.
    Söderlind, Fredrik
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi.
    Petoral, Rodrigo Jr
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Sensorvetenskap och Molekylfysik.
    Uvdal, Kajsa
    Linköpings universitet, Tekniska högskolan. Linköpings universitet, Institutionen för fysik, kemi och biologi, Sensorvetenskap och Molekylfysik.
    Zhang, H.
    Zouc, X.
    Synthesis, Characterisation and Molecular Functionalisation of Gd2O3 Nanocrystals2004Ingår i: NAN:-8,2004, 2004Konferensbidrag (Övrigt vetenskapligt)
    Abstract [en]

      

  • 40.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Kariis, Hans
    Swedish Defence Research Agency.
    Pohl, Anna
    Swedish Defence Research Agency.
    Persson, Petter
    Lund University.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    The electronic structure and reflectivity of PEDOT:PSS from density functional theory2011Ingår i: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 384, nr 03-jan, s. 44-51Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The geometric and electronic structure of condensed phase organic conducting polymer PEDOT:PSS blends has been investigated by periodic density functional theory (DFT) calculations with a generalized-gradient approximation (GGA) functional, and a plane wave basis set. The influence of the degree of doping of the PEDOT polymer on structural and optical parameters such as the reflectivity, absorbance, conductivity, dielectric function, refractive index and the energy-loss function is studied. A flip from the benzoid to the quinoid structure is observed in the calculations when the neutral PEDOT is doped by negatively charged PSS. Also the optical properties are affected by the doping. In particular, the reflectivity was found to be very sensitive to the degree of doping, where higher doping implies higher reflectivity. The reflectivity is highly anisotropic, with the dominant contribution stemming from the direction parallel to the PEDOT polymer chain.

  • 41.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Karlsson, Maria
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Quantum-chemical investigations of phenol and larger aromatic molecules at the TiO2 anatase (101) surface2008Ingår i: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 117, s. 012020-(8 pp)Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Adsorption of aromatic molecules at the (101) surface of titanium dioxide anatase is studied by quantum-chemical B3LYP computations, where both cluster and periodic calculations were performed and compared. For phenol different adsorption modes at a TiO2 cluster were mapped out and the energetically most favourable conformation was used for investigation of the electronic structure, for periodic calculations, and as a mould for the adsorption modes of phenylmethanol, phenylethanol, naphthalen-2-ol, phenanthren-2-ol, pyren-2-ol and perylen-2-ol. The alcohols form a H-bond to a surface O and a O(molecule)-Ti bond. For the larger aromatic molecules their increasingly higher HOMO levels decrease the effective bad gap of the system. Inclusion of spacer groups as in phenylmethanol and phenylethanol results in higher adsorption energies and larger band gaps. The LUMOs for the adsorbates help visualize the electronic coupling to the surface. Comparison of the cluster with the periodic model indicates that the former describes the electronic coupling in a similar manner as the latter, although the former lacks in the description of the anatase substrate.

  • 42.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    A theoretical study of water clusters: the relation between hydrogen-bond topology and interaction energy from quantum-chemical computations for clusters with up to 22 molecules2005Ingår i: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 7, s. 1905-1911Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Quantum-chemical calculations of a variety of water clusters with eight, ten and twelve molecules were performed, as well as for selected clusters with up to 22 water molecules. Geometry optimizations were carried out at the B3LYP/cc-pVDZ level and single-point energies were calculated at the B3LYP/aug-cc-pVDZ level for selected clusters. The electronic energies were studied with respect to the geometry of the oxygen arrangement and six different characteristics of the hydrogen-bond arrangement in the cluster. Especially the effect of the placement of the non-hydrogen bonding hydrogens on the interaction energy was studied. Models for the interaction energy with respect to different characteristics of the hydrogen-bond arrangement were derived through least-square fits. The results from the study of the clusters with eight, ten and twelve molecules are used to predict possible low-energy structures for various shapes of clusters with up to 22 molecules.

  • 43.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    A theoretical study of water equilibria: The cluster distribution versus temperature and pressure for (H2O)n, n=1–60, and ice2009Ingår i: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, nr 13, s. 134302-134302-13Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The size distribution of water clusters at equilibrium is studied using quantum-chemical calculations in combination with statistical thermodynamics. The necessary energetic data is obtained by quantum-chemical B3LYP computations and through extrapolations from the B3LYP results for the larger clusters. Clusters with up to 60 molecules are included in the equilibrium computations. Populations of different cluster sizes are calculated using both an ideal gas model with noninteracting clusters and a model where a correction for the interaction energy is included analogous to the van der Waals law. In standard vapor the majority of the water molecules are monomers. For the ideal gas model at 1 atm large clusters [56-mer (0–120 K) and 28-mer (100–260 K)] dominate at low temperatures and separate to smaller clusters [21–22-mer (170–280 K) and 4–6-mer (270–320 K) and to monomers (300–350 K)] when the temperature is increased. At lower pressure the transition from clusters to monomers lies at lower temperatures and fewer cluster sizes are formed. The computed size distribution exhibits enhanced peaks for the clusters consisting of 21 and 28 water molecules; these sizes are for protonated water clusters often referred to as magic numbers. If cluster-cluster interactions are included in the model the transition from clusters to monomers is sharper (i.e., occurs over a smaller temperature interval) than when the ideal-gas model is used. Clusters with 20–22 molecules dominate in the liquid region. When a large icelike cluster is included it will dominate for temperatures up to 325 K for the noninteracting clusters model. Thermodynamic properties (Cp, H) were calculated with in general good agreement with experimental values for the solid and gas phase. A formula for the number of H-bond topologies in a given cluster structure is derived. For the 20-mer it is shown that the number of topologies contributes to making the population of dodecahedron-shaped cluster larger than that of a lower-energy fused prism cluster at high temperatures.

  • 44.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Computational studies of the stability of the (H2O)100 nanodrop2010Ingår i: Journal of Molecular Structure: THEOCHEM, ISSN 0166-1280, Vol. 944, nr 1-3, s. 163-167Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The stability of the (H2O)100 nanodrop, experimentally known from a polyoxomolybdatecrystal structure (Müller et al. Inorg. Chem. Commun., 2003, 6, 52) and other structuresinferred from clathrate structures, are studied by quantum-chemical B3LYP computations.The free energies are compared to the trends for smaller clusters with 15-30 molecules. Forthe small clusters both cage-based structures and denser structures with a larger number of Hbondsobtained by an evolutionary algorithm (Bandow and Hartke, J. Phys. Chem. A, 2006,110, 5809) are used. The dense structures are most often found to be lower in electronicenergy. The cage-based structures, to which the structure of the experimentally found(H2O)100 cluster can be categorized, become more stable when Gibbs free energy is calculatedat 298 K. Additional cage-based clusters in the 35-81 molecular range were constructed forcomparison. The experimental cluster with 100 molecules (C2h/Ci-symmetry for oxygens/allatoms) and the constructed cluster with 42 molecules are found to be lower in energy than aplausible overall trend. The (H2O)42 cluster has an extraordinary high symmetry (S6), evenwhen the hydrogens are considered. The (H2O)100 cluster is the only of the studied clusters forwhich ΔG is negative at 298 K.

  • 45.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    On the stability of dense versus cage-shaped water clusters: quantum-chemical investigations of zero-point energies, free energies, basis-set effects and IR spectra of (H2O)12 and (H2O)202006Ingår i: Chemical Physics Letters, ISSN 0009-2614, E-ISSN 1873-4448, Vol. 418, s. 361-367Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The energetics of water clusters with 12 and 20 molecules are studied by quantum-chemical computations using the B3LYP, MP2, MP4 and CCSD methods. The effect of electron-correlation method, basis set, zero-point energy, thermal energy and Gibbs free energy on the relative stability of fused clusters (structures consisting of cubic- or prismatic-shaped subparts) versus cage-shaped clusters (more open structures with only three-coordinated molecules) are investigated. The O–H stretching IR vibrational spectra are studied. The contribution of zero-point and Gibbs free energy will diminish the energy difference between fused- and cage-shaped clusters, but the fused structures are still slightly more favorable.

  • 46.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Structures of the I-, II- and H-Methane Clathrates and the Ice−Methane Clathrate Phase Transition from Quantum-Chemical Modeling with Force-Field Thermal Corrections2011Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 115, nr 23, s. 6169-6176Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Methane hydrates with the three clathrate structures I, II and H are studied by quantumchemicalmethods. The periodic B3LYP computations are combined with force-field methodsfor the thermal energy corrections. The pressure dependencies for the crystal structures, latticeenergies and guest molecule interactions are derived. Quantum-chemical geometryoptimizations predict too small cell volumes compared to experimental data, but includingzero-point energy and thermal energy the cell volume increases and the correct densities areobtained. Phase diagram for the three structures are investigated, and phase transitions werefound at 5 GPa for the MH-I–MH-II transition and at 10 GPa for the MH-II–MH-H transition.

  • 47.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Theoretical IR spectra for water clusters (H2O)n (n = 6-22, 28, 30) and identification of spectral contributions from different H-Bond conformations in gaseous and liquid water2006Ingår i: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 110, nr 50, s. 13388-13393Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The vibrational IR spectra in the O-H stretching region are computed for water clusters containing 6-22, 28, and 30 molecules using quantum-chemical calculations (B3LYP and an augmented basis set). For the cluster with 20 molecules, several different structures were studied. The vibrational spectrum was partitioned into contributions from different molecules according to their coordination properties. The frequency shifts depend on the number of donated/accepted H-bonds primarily of the two molecules participating in the H-bond, but also of the surrounding molecules H-bonding to these molecules. The frequencies of H-bonds between two molecules of the same coordination type are spread over a broad interval. The most downshifted hydrogen-bond vibrations are those donated by a single-donor 3-coordinated molecule where the H-bond is accepted by a single-acceptor molecule. The H-bonded neighbors influence the downshift, and their contribution can be rationalized in the same way as for the central dimer. Single donors/acceptors cause larger downshifts than 4-coordinated molecules, and the least downshift is obtained for double donors/acceptors. This result is at variance with the conception that experimental liquid water spectra may be divided into components for which larger downshifts imply higher H-bond coordination. A mean spectral contribution for each coordination type for the donor molecule was derived and fitted to the experimental liquid water IR spectrum, which enabled an estimation of the distribution of H-bond types and average number of H-bonds (3.0 ± 0.2) in the liquid. © 2006 American Chemical Society.

  • 48.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi. Linköpings universitet, Tekniska högskolan.
    Pohl, Anna
    Swedish Defence Research Agency.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Persson, Petter
    Lund University.
    Computational study of the catalytic effect of platinum on the decomposition of DNT2012Ingår i: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 112, nr 7, s. 1852-1858Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The catalytic decomposition of dinitrotoluene (DNT; 3-4-DNT), a by-product of the explosive trinitrotoluene (trotyl), on a platinum surface is investigated computationally. Reaction paths have been computed for a DNT molecule interacting with a Pt-cluster under varying temperatures using quantum-chemical density functional theory. Two possible initiation steps where DNT split either into nitro-tolyl and NO2, or in nitro-tolyl-oxidanyl and NO, are considered. The energy barrier for the catalytic process is found to decrease significantly for the Pt catalyzed reaction compared with the uncatalyzed reaction.

  • 49.
    Lenz, Annika
    et al.
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Selegård, Linnea
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Molekylär ytfysik och nanovetenskap. Linköpings universitet, Tekniska fakulteten.
    Söderlind, Fredrik
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Oorganisk Kemi. Linköpings universitet, Tekniska fakulteten.
    Larsson, Arvid
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Holtz, Per-Olof
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Halvledarmaterial. Linköpings universitet, Tekniska högskolan.
    Uvdal, Kajsa
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Molekylär ytfysik och nanovetenskap. Linköpings universitet, Tekniska fakulteten.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Fysikalisk Kemi. Linköpings universitet, Tekniska högskolan.
    Käll, Per-Olov
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Oorganisk Kemi. Linköpings universitet, Tekniska högskolan.
    ZnO Nanoparticles Functionalized with Organic Acids: An Experimental and Quantum-Chemical Study2009Ingår i: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, nr 40, s. 17332-17341Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Electrochemical synthesis and physical characterization of ZnO nanoparticles functionalized with four different organic acids, three aromatic (benzoic, nicotinic, and trans-cinnamic acid) and one nonaromatic (formic acid), are reported. The functionalized nanoparticles have been characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV−vis, and photoluminescence spectroscopy. The adsorption of the organic acids at ZnO nanoparticles was further analyzed and interpreted using quantum-chemical density-functional theory computations. Successful functionalization of the nanoparticles was confirmed experimentally by the measured splitting of the carboxylic group stretching vibrations as well as by the N(1s) and C(1s) peaks from XPS. From a comparison between computed and experimental IR spectra, a bridging mode adsorption geometry was inferred. PL spectra exhibited a remarkably stronger near band edge emission for nanoparticles functionalized with formic acid as compared to the larger aromatic acids. From the quantum-chemical computations, this was interpreted to be due to the absence of aromatic adsorbate or surface states in the band gap of ZnO, caused by the formation of a complete monolayer of HCOOH. In the UV−vis spectra, strong charge-transfer transitions were observed.

  • 50.
    Liu, Yuan
    et al.
    Beijing Univ Chem Technol, Peoples R China; Univ Nebraska, NE 68588 USA.
    Huang, Yingying
    Chinese Acad Sci, Peoples R China; Dalian Univ Technol, Peoples R China.
    Zhu, Chongqin
    Univ Nebraska, NE 68588 USA; Univ Penn, PA 19104 USA; Univ Penn, PA 19104 USA.
    Li, Hui
    Beijing Univ Chem Technol, Peoples R China.
    Zhao, Jijun
    Dalian Univ Technol, Peoples R China.
    Wang, Lu
    Univ Sci and Technol China, Peoples R China.
    Ojamäe, Lars
    Linköpings universitet, Institutionen för fysik, kemi och biologi, Kemi. Linköpings universitet, Tekniska fakulteten.
    Francisco, Joseph S.
    Univ Nebraska, NE 68588 USA; Univ Penn, PA 19104 USA; Univ Penn, PA 19104 USA.
    Zeng, Xiao Cheng
    Beijing Univ Chem Technol, Peoples R China; Univ Nebraska, NE 68588 USA.
    An ultralow-density porous ice with the largest internal cavity identified in the water phase diagram2019Ingår i: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 116, nr 26, s. 12684-12691Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The recent back-to-back findings of low-density porous ice XVI and XVII have rekindled the century-old field of the solid-state physics and chemistry of water. Experimentally, both ice XVI and XVII crystals can be produced by extracting guest atoms or molecules enclosed in the cavities of preformed ice clathrate hydrates. Herein, we examine more than 200 hypothetical low-density porous ices whose structures were generated according to a database of zeolite structures. Hitherto unreported porous EMT ice, named according to zeolite nomenclature, is identified to have an extremely low density of 0.5 g/cm(3) and the largest internal cavity (7.88 angstrom in average radius). The EMT ice can be viewed as dumbbell-shaped motifs in a hexagonal close-packed structure. Our first-principles computations and molecular dynamics simulations confirm that the EMT ice is stable under negative pressures and exhibits higher thermal stability than other ultralow-density ices. If all cavities are fully occupied by hydrogen molecules, the EMT ice hydrate can easily outperform the record hydrogen storage capacity of 5.3 wt % achieved with sII hydrogen hydrate. Most importantly, in the reconstructed temperature-pressure (T-P) phase diagram of water, the EMT ice is located at deeply negative pressure regions below ice XVI and at higher temperature regions next to FAU. Last, the phonon spectra of empty-sII, FAU, EMT, and other zeolite-like ice structures are computed by using the dispersion corrected vdW-DF2 functional. Compared with those of ice XI (0.93 g/cm(3)), both the bending and stretching vibrational modes of the EMT ice are blue-shifted due to their weaker hydrogen bonds.

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