Abstract
The field of molecular electronics have been shown to span a huge
range of properties. In an effort to extract the parameters of the
system that governs these properties, a number of methods that decomposes
the current have been developed. These methods function
not just as tools for data extraction, but also serves as the foundation
upon which to gain insights into the physics that governs the molecular
properties. As such, the understanding of the applicability and
the development of new methods to decompose the current may be
a goal in it self.
In this thesis we will explore some of these methods, and use the
insights from this study to develop new methods. First, we will compare
two methods that decompose the current into the transmission
from a single conducting level of the molecular device, by extracting
level position and broadening. In general we see that the method
that relies on I/V fitting produces better results than the method
that is based on just the measurement of the Seebeck coefficient and
the zero-bias conductance. We then combine the two methods to develop
a new method to obtain two-level information. However, while
this new two-level method achieves better agreement with the actual
transmission of the molecule, cooperative quantum effects between all
of the levels of the molecule means that oligo-level information may
not be experimentally available. We do, however, see that when the
two single-level methods produce parameters that are similar, there
is an improved likelihood that these parameters are representative for
the molecule.
We will continue by exploring the current through multi-path
molecules. We note that for most of the molecules of this study, the
current exhibit coherence effects between linkers to the same electrode.
To explore how this coherence manifests itself, we develop a
method that decomposes the current into interatomic transmission
that can be mapped onto the molecule, and then splits this even further
into the channels of the molecules. For some of the molecules we see that the coherence is an inherent property. This is seen as the
molecules having only a single channel with the transmission entering
though both connections to one lead, going through the molecule,
and exiting through both connections to the other. For the other
molecules, the transmission splits into two channels; one channel with
the transmission entering from each connector to one electrode, and
exiting through a specific connection to the other, based on which
was used to enter. For such molecules the coherence manifests itself
as interplay between the two channels.
In the last part of the thesis, we will develop a method that allows
for the mapping of interatomic transmission in the presence of
electron-phonon couplings that allow for inelastic effects. This is developed
as a perturbative series and we will show explicit equations
for terms up to second order in the electron-phonon interaction. Here,
especially the first order correction proves to be interesting, as this
term has no non-interatomic counterpart. Using this method we are
able to track the spatial location of the vibrations from the interatomic
transmission, and we will give an interpretation on how to
understand the different corrections in terms of previously existing
methods to allow this method to draw upon already existing knowledge.
range of properties. In an effort to extract the parameters of the
system that governs these properties, a number of methods that decomposes
the current have been developed. These methods function
not just as tools for data extraction, but also serves as the foundation
upon which to gain insights into the physics that governs the molecular
properties. As such, the understanding of the applicability and
the development of new methods to decompose the current may be
a goal in it self.
In this thesis we will explore some of these methods, and use the
insights from this study to develop new methods. First, we will compare
two methods that decompose the current into the transmission
from a single conducting level of the molecular device, by extracting
level position and broadening. In general we see that the method
that relies on I/V fitting produces better results than the method
that is based on just the measurement of the Seebeck coefficient and
the zero-bias conductance. We then combine the two methods to develop
a new method to obtain two-level information. However, while
this new two-level method achieves better agreement with the actual
transmission of the molecule, cooperative quantum effects between all
of the levels of the molecule means that oligo-level information may
not be experimentally available. We do, however, see that when the
two single-level methods produce parameters that are similar, there
is an improved likelihood that these parameters are representative for
the molecule.
We will continue by exploring the current through multi-path
molecules. We note that for most of the molecules of this study, the
current exhibit coherence effects between linkers to the same electrode.
To explore how this coherence manifests itself, we develop a
method that decomposes the current into interatomic transmission
that can be mapped onto the molecule, and then splits this even further
into the channels of the molecules. For some of the molecules we see that the coherence is an inherent property. This is seen as the
molecules having only a single channel with the transmission entering
though both connections to one lead, going through the molecule,
and exiting through both connections to the other. For the other
molecules, the transmission splits into two channels; one channel with
the transmission entering from each connector to one electrode, and
exiting through a specific connection to the other, based on which
was used to enter. For such molecules the coherence manifests itself
as interplay between the two channels.
In the last part of the thesis, we will develop a method that allows
for the mapping of interatomic transmission in the presence of
electron-phonon couplings that allow for inelastic effects. This is developed
as a perturbative series and we will show explicit equations
for terms up to second order in the electron-phonon interaction. Here,
especially the first order correction proves to be interesting, as this
term has no non-interatomic counterpart. Using this method we are
able to track the spatial location of the vibrations from the interatomic
transmission, and we will give an interpretation on how to
understand the different corrections in terms of previously existing
methods to allow this method to draw upon already existing knowledge.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 148 |
| Publication status | Published - 2016 |