Abstract
Abstract
The idea of using single-molecules as components in electronic devices is fas-
cinating. For this idea to come into fruition, a number of technical and theo-
retical challenges must be overcome. In this PhD thesis, the electron-phonon
interaction is studied for a special class of molecules, which is characterised
by destructive quantum interference. The molecules are cross-conjugated,
which means that the two parts of the molecules are conjugated to a third
part, but not to each other. This gives rise to an anti-resonance in the trans-
mission.
In the low bias and low temperature regime, the electrons can tunnel in-
elastically from the left to the right electrode. This is the process behind
inelastic electron tunnelling spectroscopy (IETS), which is a single-molecule
spectroscopic method, where the vibrational ngerprint of a molecule is di-
rectly observed by the tunnelling current This process has been studied in
detail for ordinary conjugated or saturated molecules. Selection rules does
not exist in IETS, but some modes are favoured over others, and this is the
bases for the propensity rules in IETS that has been rationalised.
In this thesis, we study IETS for cross-conjugated molecules. We nd that
the vibrational modes that would be expected to dominate, following the
propensity, rules are very weak. Instead, other modes are found to be the
dominant ones. We study this phenomenon for a number of cross-conjugated
molecules, and link these ndings to the anti-resonance in the transmission.
We then go on to study current induced heating and cooling, and nd that
there is a basis for using quantum interference to design molecules that can
be cooling by the tunnelling current. The basic idea is to align the incoming
and the outgoing transmission channels such that absorption of a phonon is
favoured over emission of a phonon. The incoming and outgoing channels
are usually very alike, but by separating them using quantum interference it
is possible to tune the system to observe a cooling eect. The basis is illus-
trated in a simple tight-binding model, and the subsequent cooling process
is then observed using general atomistic transport software.
The idea of using single-molecules as components in electronic devices is fas-
cinating. For this idea to come into fruition, a number of technical and theo-
retical challenges must be overcome. In this PhD thesis, the electron-phonon
interaction is studied for a special class of molecules, which is characterised
by destructive quantum interference. The molecules are cross-conjugated,
which means that the two parts of the molecules are conjugated to a third
part, but not to each other. This gives rise to an anti-resonance in the trans-
mission.
In the low bias and low temperature regime, the electrons can tunnel in-
elastically from the left to the right electrode. This is the process behind
inelastic electron tunnelling spectroscopy (IETS), which is a single-molecule
spectroscopic method, where the vibrational ngerprint of a molecule is di-
rectly observed by the tunnelling current This process has been studied in
detail for ordinary conjugated or saturated molecules. Selection rules does
not exist in IETS, but some modes are favoured over others, and this is the
bases for the propensity rules in IETS that has been rationalised.
In this thesis, we study IETS for cross-conjugated molecules. We nd that
the vibrational modes that would be expected to dominate, following the
propensity, rules are very weak. Instead, other modes are found to be the
dominant ones. We study this phenomenon for a number of cross-conjugated
molecules, and link these ndings to the anti-resonance in the transmission.
We then go on to study current induced heating and cooling, and nd that
there is a basis for using quantum interference to design molecules that can
be cooling by the tunnelling current. The basic idea is to align the incoming
and the outgoing transmission channels such that absorption of a phonon is
favoured over emission of a phonon. The incoming and outgoing channels
are usually very alike, but by separating them using quantum interference it
is possible to tune the system to observe a cooling eect. The basis is illus-
trated in a simple tight-binding model, and the subsequent cooling process
is then observed using general atomistic transport software.
| Original language | English |
|---|
| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
|---|---|
| Number of pages | 100 |
| Publication status | Published - 2014 |