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Theory of vibrationally assisted tunneling for hydroxyl monomer flipping on Cu(110)
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.ORCID iD: 0000-0003-2659-4161
Toyama Univ, Japan.
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering.
2014 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 90, no 16, Article ID: 165413- p.Article in journal (Refereed) Published
Abstract [en]

To describe vibrationally mediated configuration changes of adsorbates on surfaces we have developed a theory to calculate both reaction rates and pathways. The method uses the T-matrix to describe excitations of vibrational states by the electrons of the substrate, adsorbate, and tunneling electrons from a scanning tunneling probe. In addition to reaction rates, the theory also provides the reaction pathways by going beyond the harmonic approximation and using the full potential energy surface of the adsorbate which contains local minima corresponding to the adsorbates different configurations. To describe the theory, we reproduce the experimental results in [T. Kumagai et al., Phys. Rev. B 79, 035423 (2009)], where the hydrogen/deuterium atom of an adsorbed hydroxyl (OH/OD) exhibits back and forth flipping between two equivalent configurations on a Cu(110) surface at T = 6 K. We estimate the potential energy surface and the reaction barrier, similar to 160 meV, from DFT calculations. The calculated flipping processes arise from (i) at low bias, tunneling of the hydrogen through the barrier, (ii) intermediate bias, tunneling electrons excite the vibrations increasing the reaction rate although over the barrier processes are rare, and (iii) higher bias, overtone excitations increase the reaction rate further.

Place, publisher, year, edition, pages
2014. Vol. 90, no 16, Article ID: 165413- p.
National Category
Condensed Matter Physics
Research subject
Physics, Condensed Matter Physics
Identifiers
URN: urn:nbn:se:lnu:diva-38309DOI: 10.1103/PhysRevB.90.165413ISI: 000343771900005OAI: oai:DiVA.org:lnu-38309DiVA: diva2:765616
Available from: 2014-11-24 Created: 2014-11-24 Last updated: 2015-09-28Bibliographically approved
In thesis
1. Modeling of non-equilibrium scanning probe microscopy
Open this publication in new window or tab >>Modeling of non-equilibrium scanning probe microscopy
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

The work in this thesis is basically divided into two related but separate investigations.

The first part treats simple chemical reactions of adsorbate molecules on metallic surfaces, induced by means of a scanning tunneling probe (STM). The investigation serves as a parameter free extension to existing theories. The theoretical framework is based on a combination of density functional theory (DFT) and non-equilibrium Green's functions (NEGF). Tunneling electrons that pass the adsorbate molecule are assumed to heat up the molecule, and excite vibrations that directly correspond to the reaction coordinate. The theory is demonstrated for an OD molecule adsorbed on a bridge site on a Cu(110) surface, and critically compared to the corresponding experimental results. Both reaction rates and pathways are deduced, opening up the understanding of energy transfer between different configurational geometries, and suggests a deeper insight, and ultimately a higher control of the behaviour of adsorbate molecules on surfaces.

The second part describes a method to calculate STM images in the low bias regime in order to overcome the limitations of localized orbital DFT in the weak coupling limit, i.e., for large vacuum gaps between a tip and the adsorbate molecule. The theory is based on Bardeen's approach to tunneling, where the orbitals computed by DFT are used together with the single-particle Green's function formalism, to accurately describe the orbitals far away from the surface/tip. In particular, the theory successfully reproduces the experimentally well-observed characteristic dip in the tunneling current for a carbon monoxide (CO) molecule adsorbed on a Cu(111) surface. Constant height/current STM images provide direct comparisons to experiments, and from the developed method further insights into elastic tunneling are gained.

Place, publisher, year, edition, pages
Växjö: Linneaus Univesity, 2015. 80 p.
Keyword
scanning tunneling microscopy, molecular dynamics, density functional theory, non-equilibrium Green's functions
National Category
Condensed Matter Physics
Research subject
Physics, Condensed Matter Physics
Identifiers
urn:nbn:se:lnu:diva-46448 (URN)978-91-87925-73-3 (ISBN)
Presentation
2015-09-17, Ny227, Kalmar Nyckel, Kalmar, 10:00 (English)
Opponent
Supervisors
Available from: 2015-09-28 Created: 2015-09-23 Last updated: 2015-09-28Bibliographically approved

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