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Theoretical Modeling of Intra- and Inter-molecular Charge Transport
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on theoretical study of charge transportproperties in molecular systems. The understanding of the transportprocess and mechanism in molecular systems is essential forthe design of new functional molecular materials and molecularelectronic devices. The molecular junctions and organic molecularcrystals have been used as the model systems to highlight the usefulnessof theoretical modelling. A molecular junction is a system that consists ofone or several molecules sandwiched between two electrodes.The charge transport in molecular junctions is a very complex processthat is affected by the interaction between molecules and electrodes,the surroundings, as well as electron-electron (e-e) andelectron-phonon (e-p) couplings. When the molecule-electrode couplingis strong, the transport process can be very quick. If the e-p couplingis weak, the inelastic tunneling has only negligible contributions to thetotal current and the elastic electron tunneling plays the dominant role.Furthermore, the hopping process becomes dominant in the case of strong e-pcoupling, for which the geometric relaxation of the molecule needsto be considered. In this thesis, we have examined these three kinds oftransport processes separately.

The first studied system is a molecular junction consisting of aromaticallycoupled bimolecules. Its elastic electron tunneling property is simulatedusing Green's functional theory at density functional theory level.The dependence of the conductance of bimolecular junctions on the vertical distances,horizontal distances and the tilt angles has been systematically studied. Theinelastic electron tunneling spectra (IETS) of molecular junctions have beencalculated for several systems that were experimentally measured with conflictingresults and controversial assignments. Our calculations provide the reliableassignments for the experimental spectra and revealed unprecedented detailsabout the molecular conformations within the junctions under different conditions.It demonstrates that a combined theoretical and experimental IETS study is capableof accurately determining the structure of a single molecule inside the junction.The hopping process is a dominant charge transfer process in organic molecularcrystals. We have studied the charge transport ability of four kinds of n-typeorganic semiconductor materials to find out the related structure-to-propertyrelationship. It is done by adopting the quantum charge transfer rate equationcombined with the random walk approach.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , x, 68 p.
Series
Trita-BIO-Report, ISSN 1654-2312 ; 2012:15
Keyword [en]
charge transport, molecular junction, organic molecular materials, Green's function, first-principles simulation
National Category
Physical Chemistry
Research subject
SRA - Transport
Identifiers
URN: urn:nbn:se:kth:diva-94103ISBN: 978-91-7501-371-8 (print)OAI: oai:DiVA.org:kth-94103DiVA: diva2:525339
Public defence
2012-06-05, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
TrenOp, Transport Research Environment with Novel Perspectives
Note
QC 20120515Available from: 2012-05-15 Created: 2012-05-07 Last updated: 2012-05-15Bibliographically approved
List of papers
1. Formation and electronic transport properties of bimolecular junctions based on aromatic coupling
Open this publication in new window or tab >>Formation and electronic transport properties of bimolecular junctions based on aromatic coupling
2010 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 22, no 32, 325102- p.Article in journal (Refereed) Published
Abstract [en]

A systematic first-principles study on conductance-voltage characteristics of bi-(quasi) oligo(phenylene ethynylene)-monothiol molecular junctions reported by Wu et al (2008 Nat. Nanotechnol. 3 569) is presented. The so-called ortho-and para-conformations of the bimolecular junction are considered. Our calculation indicates that the bimolecular junction prefers to take the ortho-conformation because of its lower energy. The simulation supports the experimental findings that aromatic coupling between two molecules is strong enough to induce the formation of molecular junctions. By comparing with experimental results, structure parameters for a probable bimolecular junction are determined. The underlying mechanism for formation of the bimolecular junction and its electron transport is discussed.

Keyword
SINGLE-MOLECULE JUNCTIONS, CONDUCTANCE, WIRES, TRANSISTOR
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-29417 (URN)10.1088/0953-8984/22/32/325102 (DOI)000280542900013 ()2-s2.0-77956927461 (Scopus ID)
Note
QC 20110216Available from: 2011-02-16 Created: 2011-02-02 Last updated: 2017-12-11Bibliographically approved
2. Inelastic Electron Tunneling Spectroscopy of Gold-Benzenedithiol-Gold Junctions: Accurate Determination of Molecular Conformation
Open this publication in new window or tab >>Inelastic Electron Tunneling Spectroscopy of Gold-Benzenedithiol-Gold Junctions: Accurate Determination of Molecular Conformation
2011 (English)In: ACS NANO, ISSN 1936-0851, Vol. 5, no 3, 2257-2263 p.Article in journal (Refereed) Published
Abstract [en]

The gold benzenedithiol gold junction is the classic prototype of molecular electronics. However, even with the similar experimental setup, it has been difficult to reproduce the measured results because of the lack of basic information about the molecular confirmation inside the junction. We have performed systematic first principles study on the inelastic electron tunneling spectroscopy of this classic junction. By comparing the calculated spectra with four different experimental results, the most possible conformations of the molecule under different experimental conditions have been successfully determined. The relationship between the contact configuration, and the resulted spectra is revealed. It demonstrates, again that one should always combine the theoretical and experimental inelastic electron tunneling spectra to determine the molecular conformation in a junction. Our simulations have also suggested that in terms of the reproducibility and stability, the electromigrated nanogap technique is much better than the mechanically controllable break junction technique.

Keyword
inelastic electron tunneling, molecular junctions, molecular conformations, density functional theory, vibrations
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-32041 (URN)10.1021/nn103522k (DOI)000288570600085 ()2-s2.0-79952931567 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20110404Available from: 2011-04-04 Created: 2011-04-04 Last updated: 2012-05-15Bibliographically approved
3. Assignments of Inelastic Electron Tunneling Spectra of Semifluorinated Alkanethiol Molecular Junctions
Open this publication in new window or tab >>Assignments of Inelastic Electron Tunneling Spectra of Semifluorinated Alkanethiol Molecular Junctions
2011 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 115, no 41, 20301-20306 p.Article in journal (Refereed) Published
Abstract [en]

The peculiar experimental inelastic electron tunneling spectra of a series of hexadecanethiol molecular junctions have finally been reproduced by first-principles simulations. It is found that the debated spectral profile around 0.38 eV indeed originated from the C-H stretching vibration associated with at least two terminal methylene groups close to the sulfur atom. The intensity of this spectral peak becomes dominant, as observed in the experiments when the molecule is titled -40 degrees relative to the normal of the electrode surface, which is due to the opening of a new tunneling pathway bypassing the end sulfur atom. The dependence of this strong vibrational feature on the titled angle of the molecule is predicted with the help of the concept of effective coupling energy. The degree of the fluorination on the inelastic electron tunneling spectrum of hexadecanethiol molecules has also been discussed in detail.

Keyword
spectroscopy, transport, simulations, pathways, rules
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-46844 (URN)10.1021/jp206085u (DOI)000295700800033 ()2-s2.0-80054769181 (Scopus ID)
Note
QC 20111107Available from: 2011-11-07 Created: 2011-11-07 Last updated: 2017-12-08Bibliographically approved
4. Determination of the Configuration of a Single Molecule Junction by Inelastic Electron Tunneling Spectroscopy
Open this publication in new window or tab >>Determination of the Configuration of a Single Molecule Junction by Inelastic Electron Tunneling Spectroscopy
Show others...
2010 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 114, no 11, 5199-5202 p.Article in journal (Refereed) Published
Abstract [en]

First-principles calculations for inelastic electron tunneling spectroscopy (IETS) of a single 1,3-propanedithiol molecule covalently bound to gold electrodes are presented. Inelastic electron tunneling spectra of the single molecule junction with different contact geometries and molecular orientations at the interface are simulated. It is demonstrated that the delicate variation in the configuration of the single molecule junction caused by separating the two electrodes call result in significant changes in the inelastic electron tunneling spectral profile of the junction. The two Most probable configurations of the molecular junction formed in the experiment (Nano Lett. 2008, 8, 1673) are theoretically identified, and the experimental IET spectra are correctly assigned.

Keyword
transport
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-19318 (URN)10.1021/jp101428d (DOI)000275708600061 ()2-s2.0-77949796797 (Scopus ID)
Note
QC 20110114Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. Charge Transport inPerylene Diimides Based n-type Organic Semiconductors
Open this publication in new window or tab >>Charge Transport inPerylene Diimides Based n-type Organic Semiconductors
(English)Article in journal (Other academic) Submitted
Identifiers
urn:nbn:se:kth:diva-95168 (URN)
Note
QS 2012Available from: 2012-05-15 Created: 2012-05-15 Last updated: 2012-05-15Bibliographically approved

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