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Modeling of non-equilibrium scanning probe microscopy
Linnaeus University, Faculty of Technology, Department of Physics and Electrical Engineering. (Condensed Matter Physics)ORCID iD: 0000-0003-2659-4161
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 [en]
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: urn:nbn:se:lnu:diva-46448ISBN: 978-91-87925-73-3 (print)OAI: oai:DiVA.org:lnu-46448DiVA: diva2:856187
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
List of papers
1. Theory of vibrationally assisted tunneling for hydroxyl monomer flipping on Cu(110)
Open this publication in new window or tab >>Theory of vibrationally assisted tunneling for hydroxyl monomer flipping on Cu(110)
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.

National Category
Condensed Matter Physics
Research subject
Physics, Condensed Matter Physics
Identifiers
urn:nbn:se:lnu:diva-38309 (URN)10.1103/PhysRevB.90.165413 (DOI)000343771900005 ()2-s2.0-84908052258 (Scopus ID)
Available from: 2014-11-24 Created: 2014-11-24 Last updated: 2017-12-05Bibliographically approved
2. Scanning tunneling microscopy current from localized basis orbital density functional theory
Open this publication in new window or tab >>Scanning tunneling microscopy current from localized basis orbital density functional theory
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 11, 115434Article in journal (Refereed) Published
Abstract [en]

We present a method capable of calculating elastic scanning tunneling microscopy (STM) currents from localized atomic orbital density functional theory (DFT). To overcome the poor accuracy of the localized orbital description of the wave functions far away from the atoms, we propagate the wave functions, using the total DFT potential. From the propagated wave functions, the Bardeen's perturbative approach provides the tunneling current. To illustrate the method we investigate carbon monoxide adsorbed on a Cu(111) surface and recover the depression/protrusion observed experimentally with normal/CO-functionalized STM tips. The theory furthermore allows us to discuss the significance of s- and p-wave tips.

National Category
Atom and Molecular Physics and Optics
Identifiers
urn:nbn:se:lnu:diva-46504 (URN)10.1103/PhysRevB.93.115434 (DOI)000372715600004 ()2-s2.0-84962071169 (Scopus ID)
Available from: 2015-09-28 Created: 2015-09-28 Last updated: 2017-12-01Bibliographically approved
3. Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory
Open this publication in new window or tab >>Influence of atomic tip structure on the intensity of inelastic tunneling spectroscopy data analyzed by combined scanning tunneling spectroscopy, force microscopy, and density functional theory
Show others...
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 93, no 16, 165415Article in journal (Refereed) Published
Abstract [en]

Achieving a high intensity in inelastic scanning tunneling spectroscopy (IETS) is important for precise measurements. The intensity of the IETS signal can vary by up to a factor of 3 for various tips without an apparent reason accessible by scanning tunneling microscopy (STM) alone. Here, we show that combining STM and IETS with atomic force microscopy enables carbon monoxide front-atom identification, revealing that high IETS intensities for CO/Cu(111) are obtained for single-atom tips, while the intensity drops sharply for multiatom tips. Adsorption of the CO molecule on a Cu adatom [CO/Cu/Cu(111)] such that the molecule is elevated over the substrate strongly diminishes the tip dependence of IETS intensity, showing that an elevated position channels most of the tunneling current through the CO molecule even for multiatom tips, while a large fraction of the tunneling current bypasses the CO molecule in the case of CO/Cu(111).

National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Physics, Electrotechnology
Identifiers
urn:nbn:se:lnu:diva-46508 (URN)10.1103/PhysRevB.93.165415 (DOI)000373878100002 ()2-s2.0-84963756980 (Scopus ID)
Available from: 2015-09-28 Created: 2015-09-28 Last updated: 2017-12-01Bibliographically approved

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