Electrical characteristics of single molecule fullerene based devices
File(s)
Author(s)
Thong, Aaron
Type
Thesis
Abstract
This thesis investigates the design of a unimolecular donor-acceptor
system (4TPA-C60) for the purpose of developing biomimetic Turin IETS
sensors. The gas phase 4TPA-C60 molecule is calculated to have a localised
double-well electronic structure, which is similar to that of a
nanowire resonant tunnelling device. 4TPA-C60 decorated gold surfaces
are prepared from scratch, and characterised at each step. STM imaging
confirms successful grafting of the molecular species to the gold surface.
Scanning tunneling spectroscopy performed on these molecular
double-barrier tunnel junctions show a slightly asymmetric I(V ) profile,
similar to predictions from ab initio calculations. DFT calculations
also reveal that the behaviour of the device is strongly dependent on
the supramolecular couplings at both metal/molecule interfaces. A new
phenomenon is identified, where pinning of the LUMO to the HOMO
states maintains the resonant transmission channel and prevents crossing
of the frontier molecular orbitals at higher biases. The HOMO-LUMO
pinning effect is determined to arise from charge accumulation on the
C60 cage, due to smaller coupling at the C60/metal interface. By preventing
crossing of the states, HOMO-LUMO pinning delays the onset
of the NDR feature, resulting in a wider current peak and lower resolution
of the sensor. Based on the charge transport mechanism, several
alternative systems are proposed in which HOMO-LUMO pinning
can be minimized. An endohedral fullerene derivative, 4TPA-F@C60, is
found to be the most promising candidate, displaying much narrower
NDR peaks. These findings not only help to improve future designs of
molecular Turin IETS sensors, but also contribute significantly to our understanding
in the broader field of molecular devices in general.
system (4TPA-C60) for the purpose of developing biomimetic Turin IETS
sensors. The gas phase 4TPA-C60 molecule is calculated to have a localised
double-well electronic structure, which is similar to that of a
nanowire resonant tunnelling device. 4TPA-C60 decorated gold surfaces
are prepared from scratch, and characterised at each step. STM imaging
confirms successful grafting of the molecular species to the gold surface.
Scanning tunneling spectroscopy performed on these molecular
double-barrier tunnel junctions show a slightly asymmetric I(V ) profile,
similar to predictions from ab initio calculations. DFT calculations
also reveal that the behaviour of the device is strongly dependent on
the supramolecular couplings at both metal/molecule interfaces. A new
phenomenon is identified, where pinning of the LUMO to the HOMO
states maintains the resonant transmission channel and prevents crossing
of the frontier molecular orbitals at higher biases. The HOMO-LUMO
pinning effect is determined to arise from charge accumulation on the
C60 cage, due to smaller coupling at the C60/metal interface. By preventing
crossing of the states, HOMO-LUMO pinning delays the onset
of the NDR feature, resulting in a wider current peak and lower resolution
of the sensor. Based on the charge transport mechanism, several
alternative systems are proposed in which HOMO-LUMO pinning
can be minimized. An endohedral fullerene derivative, 4TPA-F@C60, is
found to be the most promising candidate, displaying much narrower
NDR peaks. These findings not only help to improve future designs of
molecular Turin IETS sensors, but also contribute significantly to our understanding
in the broader field of molecular devices in general.
Version
Open Access
Date Issued
2017-04
Date Awarded
2017-07
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Horsfield, Andrew
Shaffer, Milo
Sponsor
Singapore. Agency for Science, Technology and Research
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)