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Electroluminescence of molecules in a scanning tunneling microscope: Role of tunneling electrons and surface plasmons
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0002-3915-300X
KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.ORCID iD: 0000-0003-0007-0394
2011 (English)In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 20, 205419- p.Article in journal (Refereed) Published
Abstract [en]

We study electroluminescence from molecules confined in a scanning tunneling microscope based on a recently proposed density-matrix approach. The molecule is treated by a two-state model with each state consisting of a set of vibrational energy levels. The interband transition probabilities are described by Franck-Condon factors as calculated from a harmonic model. The role played by the tunneling electron as well as by the surface plasmon resonance is investigated. The dependence of the electron-tunneling-induced electroluminescence on the temperature, the bias voltage, the coupling strength between the molecule and the electrodes, and the radiative decay rate of the excited state has been systematically studied. It is found that under high temperature, due to the thermally assisted electron tunneling, photon emissions can still be detected when the bias voltage is less than the excitation energy of the molecule. We also find that when the molecule is asymmetrically coupled to the electrodes, electroluminescence does not strictly follow the Franck-Condon distribution. Our simulations also show that the increase of the radiative decay rate of the excited state cannot lead to the hot luminescence from higher vibrational levels. The involvement of the surface plasmon can drastically alter the spectral profiles, resulting in hot luminescence from molecules, when the surface plasmon in a scanning tunneling microscope is of high strength and ultrashort duration. The influence of the strength and the duration of the plasmon pulse on the electroluminescence spectra has been discussed.

Place, publisher, year, edition, pages
2011. Vol. 84, no 20, 205419- p.
Keyword [en]
National Category
Engineering and Technology
URN: urn:nbn:se:kth:diva-52548DOI: 10.1103/PhysRevB.84.205419ISI: 000297104700019ScopusID: 2-s2.0-82755177359OAI: diva2:467679
Swedish Research Council
QC 20111219Available from: 2011-12-19 Created: 2011-12-19 Last updated: 2013-05-20Bibliographically approved
In thesis
1. Electron-vibration coupling and its effects on optical and electronic properties of single molecules
Open this publication in new window or tab >>Electron-vibration coupling and its effects on optical and electronic properties of single molecules
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The thesis is devoted to theoretical investigations of electron-vibration coupling and its effects on optical and electronic properties of single molecules, especially for molecules confined between metallic electrodes.

A density-matrix approach has been developed to describe the photon emission of single molecules confined in the scanning tunneling microscope (STM). With this new method electronic excitations induced by both the tunneling electron and the localized surface plasmon (LSP) can be treated on an equal footing. Model calculations for porphyrin derivatives have successfully reproduced and explained the experimentally observed unusual variation of the photon emission spectra. The method has also been extended to study the STM induced fluorescence and phosphorescence of C60 molecules in combination with the first principles calculations. In particularly, the non-Condon vibronic couplings have been exclusively included in the calculations. The experimental spectra have been nicely reproduced by our calculations, which also enable us to identify the unique spectral fingerprint and origin of the measured spectra. The observed rich spectral features have been finally correctly assigned.

The electron transport properties of molecular junctions with bipyridine isomers have been studied in the sequential tunneling (SET) regime by assuming that the molecules are weakly coupled to metallic electrodes. It is shown that the strong electron-vibration coupling in the 2, 2’-bipyridine molecule and the 4,4’-bipyridine molecule can lead to observable Franck-Condon blockade. Taking advantage of such novel effect, a gate-controlled conductance switch with ideal on-off ratio has been proposed for a molecular junction with the 4, 4’-bipyridine molecule.

The effect of the electron-vibration coupling on one-photon and two-photon absorption spectra of green fluorescent protein (GFP) has been systematically examined. The hydroxybenzylidene-2, 3-dimethylimidazolinone molecule in the deprotonated anion state (HBDI) is used to model the fluorescence chromophore of the GFP. Both Condon and non-Condon vibronic couplings have been considered in the calculations. The calculated spectra are in good agreement with the available experimental spectra. It confirms the notion that the observed blue-shift of the two-photon absorption spectrum with respect to its one-photon absorption counterpart is caused by the non-Condon vibronic coupling.

All the calculations are carried out with our own software package, DynaVib. It is capable of modeling a variety of vibrational-resolved spectroscopies, such as absorption, emission, and resonant Raman scattering (RRS) spectra. In our package, the Duschinsky rotation and non-Condon effect have been fully taken into account. Both time-independent and time-dependent approaches have been implemented, allowing to simulate the spectra of very large molecules.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xiv, 66 p.
Trita-BIO-Report, ISSN 1654-2312 ; 2013:10
National Category
Theoretical Chemistry
urn:nbn:se:kth:diva-122180 (URN)978-91-7501-773-0 (ISBN)
Public defence
2013-06-07, FA32, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 14:00 (English)

QC 20130520

Available from: 2013-05-20 Created: 2013-05-14 Last updated: 2013-05-20Bibliographically approved

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