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Quantum chemical studies of deposition and catalytic surface reactions
Linköping University, Department of Physics, Chemistry and Biology, Chemistry. Linköping University, Faculty of Science & Engineering.
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Quantum chemical calculations have been used to model chemical reactions in epitaxial growth of silicon carbide by chemical vapor deposition (CVD) processes and to study heterogeneous catalytic reactions for methanol synthesis. CVD is a common method to produce high-quality materials and e.g. thin films in the semiconductor industry, and one of the many usages of methanol is as a promising future renewable and sustainable energy carrier. To optimize the chemical processes it is essential to understand the reaction mechanisms. A comprehensive theoretical model for the process is therefore desired in order to be able to explore various variables that are difficult to investigate in situ. In this thesis reaction paths and reaction energies are computed using quantum chemical calculations. The quantum-chemical results can subsequently be used as input for thermodynamic, kinetic and computational fluid dynamics modelling in order to obtain data directly comparable with the experimental observations.

For the CVD process, the effect of halogen addition to the gas mixture is studied by modelling the adsorption and diffusion of SiH2, SiCl2 and SiBr2 on the (0001̅) 4H-SiC surface. SiH2 was found to bind strongest to the surface and SiBr2 binds slightly stronger than the SiCl2 molecule. The diffusion barrier is shown to be lower for SiH2 than for SiBr2 and SiCl2 which have similar barriers. SiBr2 and SiCl2 are found to have similar physisorption energies and bind stronger than the SiH2 molecule. Gibbs free-energy calculations also indicate that the SiC surface is not fully hydrogen terminated at CVD conditions since missing-neighboring pair of surface hydrogens is found to be common. Calculations for the (0001) surface show that SiCl, SiCl2, SiHCl, SiH, and SiH2 likely adsorb on a methylene site, but the processes are thermodynamically less favorable than their reverse reactions. However, the adsorbed products may be stabilized by subsequent surface reactions to form a larger structure. The formation of these larger structures is found to be fast enough to compete with the desorption processes. Also the Gibbs free energies for adsorption of Si atoms, SiX, SiX2, and SiHX where X is F or Br are presented. Adsorption of Si atoms is shown to be the most thermodynamically favorable reaction followed by SiX, SiHX, and SiX2, X being a halide. The results in this study suggest that the major Si contributors in the SiC–CVD process are Si atoms, SiX and SiH.

Methanol can be synthesized from gaseous carbon dioxide and hydrogen using solid metal-metal oxide mixtures acting as heterogeneous catalysts. Since a large surface area of the catalyst enhances the speed of the heterogeneous reaction, the use of nanoparticles (NP) is expected to be advantageous due to the NPs’ large area to surface ratio. The plasma-induced creation of copper NPs is investigated. One important element during particle growth is the charging process where the variation of the work function (W) with particle size is a key quantity, and the variation becomes increasingly pronounced at smaller NP sizes. The work functions are computed for a set of NP charge numbers, sizes and shapes, using copper as a case study. A derived analytical expression for W is shown to give quite accurate estimates provided that the diameter of the NP is larger than about a nanometer and that the NP has relaxed to close to a spherical shape. For smaller sizes W deviates from the approximative expression, and also depends on the charge number. Some consequences of these results for NP charging process are outlined.

Key reaction steps in the methanol synthesis reaction mechanism using a Cu/ZrO2 nanoparticle catalyst is investigated. Two different reaction paths for conversion of CO2 to CO is studied. The two paths result in the same complete reaction 2 CO2 → 2 CO + O2 where ZrO2 (s) acts as a catalyst. The highest activation energies are significantly lower compared to that of the gas phase reaction. The presence of oxygen vacancies at the surface appear to be decisive for the catalytic process to be effective. Studies of the reaction kinetics show that when oxygen vacancies are present on the ZrO2 surface, carbon monoxide is produced within a microsecond. The IR spectra of CO2 and H2 interacting with ZrO2 and Cu under conditions that correspond to the catalyzed CH3OH production process is also studied experimentally and compared to results from the theoretical computations. Surface structures and gas-phase molecules are identified through the spectral lines by matching them to specific vibrational modes from the literature and from the new computational results. Several surface structures are verified and can be used to pin point surface structures in the reaction path. This gives important information that help decipher how the reaction mechanism of the CO2 conversion and ultimately may aid to improve the methanol synthesis process.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2018. , p. 65
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1925
National Category
Theoretical Chemistry Nano Technology Physical Chemistry
Identifiers
URN: urn:nbn:se:liu:diva-148757DOI: 10.3384/diss.diva-148757ISBN: 9789176853337 (print)OAI: oai:DiVA.org:liu-148757DiVA, id: diva2:1220575
Public defence
2018-08-30, Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2018-06-19 Created: 2018-06-19 Last updated: 2018-06-19Bibliographically approved
List of papers
1. Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations
Open this publication in new window or tab >>Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations
2013 (English)In: Theoretical Chemistry accounts, ISSN 1432-881X, E-ISSN 1432-2234, Vol. 132, no 12Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
Springer Verlag (Germany), 2013
Keywords
Quantum-chemical calculations, Density functional theory (DFT), B3LYP, Chemical vapour deposition (CVD), Silicon carbide (SiC), SiCl2, SiH2, Surface reactions, Adsorption, Reaction path, Activation energy, Diffusion, Hydrogen termination
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-100478 (URN)10.1007/s00214-013-1403-3 (DOI)000325724400001 ()
Available from: 2013-11-08 Created: 2013-11-08 Last updated: 2018-06-19Bibliographically approved
2. Brominated chemistry for chemical vapor deposition of electronic grade SiC
Open this publication in new window or tab >>Brominated chemistry for chemical vapor deposition of electronic grade SiC
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2015 (English)In: Chemistry of Materials, ISSN 0897-4756, E-ISSN 1520-5002, Vol. 27, no 3, p. 793-801Article in journal (Refereed) Published
Abstract [en]

Chlorinated chemical vapor deposition (CVD) chemistry for growth of homoepitaxial layers of silicon carbide (SiC) has paved the way for very thick epitaxial layers in short deposition time as well as novel crystal growth processes for SiC. Here, we explore the possibility to also use a brominated chemistry for SiC CVD by using HBr as additive to the standard SiC CVD precursors. We find that brominated chemistry leads to the same high material quality and control of material properties during deposition as chlorinated chemistry and that the growth rate is on average 10 % higher for a brominated chemistry compared to chlorinated chemistry. Brominated and chlorinated SiC CVD also show very similar gas phase chemistries in thermochemical modelling. This study thus argues that brominated chemistry is a strong alternative for SiC CVD since the deposition rate can be increased with preserved material quality. The thermochemical modelling also suggest that the currently used chemical mechanism for halogenated SiC CVD might need to be revised.

National Category
Chemical Sciences Physical Sciences
Identifiers
urn:nbn:se:liu:diva-111075 (URN)10.1021/acs.chemmater.5b00074 (DOI)000349934500016 ()
Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2018-06-19Bibliographically approved
3. Growth Mechanism of SiC Chemical Vapor Deposition: Adsorption and Surface Reactions of Active Si Species
Open this publication in new window or tab >>Growth Mechanism of SiC Chemical Vapor Deposition: Adsorption and Surface Reactions of Active Si Species
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2018 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 122, no 1, p. 648-661Article in journal (Refereed) Published
Abstract [en]

Silicon carbide is a wide bandgap semiconductor ideally suitable for high temperature and high power applications. An active SiC layer is usually fabricated using halide-assisted chemical vapor deposition (CVD). In this work, we use quantum chemical density functional theory (B3LYP and M06-2X) and transition state theory to study adsorptions of active Si species in the CVD process on both the Si face and the C face of 4H-SiC. We show that adsorptions of SiCl, SiCl2, SiHCl, SiH, and SiH2 on the Si face likely occur on a methylene site, CH2(ads), but the processes are thermodynamically less favorable than their reverse or desorptions. Nevertheless, the adsorbed products become stabilized with the help of subsequent surface reactions to form a larger cluster. These cluster formation reactions happen with rates that are fast enough to compete with the desorption processes. On the C face, the adsorptions likely occur on a surface site terminated by a dangling bond, *(ads), and produce the products which are thermodynamically stable. Lastly, we present the Gibbs free energies of adsorptions of Si atoms, SiX, SiX2, and SiHX, for X being F and Br. Adsorptions of Si atoms are shown to be the most thermodynamically favorable among all the species in the study. Among the halide-containing species, the Gibbs free energies (ARG) from smallest to largest are observed in the adsorptions of SiX, SiHX, and SiX2, for X being the halides. The results in this study suggest that the major Si contributors in the SiC CVD process are Si atoms, SiX (for X being the halide) and SiH.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC, 2018
National Category
Inorganic Chemistry
Identifiers
urn:nbn:se:liu:diva-144885 (URN)10.1021/acs.jpcc.7b10751 (DOI)000422814200069 ()
Note

Funding Agencies|Swedish Foundation for Strategic Research from Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swedish Research Council (VR) [2016-05137_4]

Available from: 2018-02-09 Created: 2018-02-09 Last updated: 2018-06-19
4. On the work function and the charging of small (r <= 5 nm) nanoparticles in plasmas
Open this publication in new window or tab >>On the work function and the charging of small (r <= 5 nm) nanoparticles in plasmas
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2017 (English)In: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 24, no 1, article id 013702Article in journal (Refereed) Published
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.

Place, publisher, year, edition, pages
AMER INST PHYSICS, 2017
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:liu:diva-136218 (URN)10.1063/1.4973443 (DOI)000395395100092 ()
Note

Funding Agencies|Knut and Alice Wallenberg foundation (KAW) [2014.0276]; Swedish Research Council via the Linkoping Linneaus Environment LiLi-NFM [2008-6572]

Available from: 2017-03-31 Created: 2017-03-31 Last updated: 2018-06-19

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