TY - BOOK
T1 - Electron and Phonon Transport in Molecular Junctions
AU - Li, Qian
PY - 2015
Y1 - 2015
N2 - Molecular electronics provide the possibility to investigate electron and phonon transport at the smallest imaginable scale, where quantum effects can be investigated and exploited directly in the design. In this thesis, we study both electron transport and phonon transport in molecular junctions. The system we are interested in here are π-stacked molecules connected with two semi-infinite leads. π-stacked aromatic rings, connected via π-π electronic coupling, provides a rather soft mechanical bridge while maintaining high electronic conductivity. We investigate electron transport and the thermoelectric response of five representative π-stacked systems. We find that the transmission and power factor are both enhanced by increasing the conjugation length or adding substituent groups. The local transmission shows that several extra paths are added by cyano groups, which increases the total transmission at the Fermi energy. We propose and analyze a way of using π stacking to design molecular junctions to control heat transport. We develop a simple model system to identify optimal parameter regimes and then use density functional theory (DFT) to extract model parameters for a number of specific π-stacked systems. It is found that the proposed mechanism can indeed signicantly reduce the phonon conductance but also increase it depending on the specific
molecules and stackings used. Furthermore, we study how a molecule's conformation influence phonon transport by mechanically compressing an alkane chain in the junction with graphene leads, using DFT combined with scattering matrix approach. We find that the thermal conductance of the compressed junction drops by half in comparison to the extended junction, making it possible to turn on and o the heat current. The reduced thermal conductance in the compressed junction mainly stems from a large suppression of the transmission coefficients of the longitudinal and the in-plane transverse channels of the leads. Finally, we return and investigate phonon transport through π-stacked molecules connected to graphene leads including all modes of the system
using DFT method. It is found that the thermal conductance of π-stacked
systems can be reduced by 95%, compared with that in a single-molecule
junction. Phonon transmission of π-stacked systems is reduced dramatically
in the whole frequency range and the left transmission mainly remains below
5 THz.
AB - Molecular electronics provide the possibility to investigate electron and phonon transport at the smallest imaginable scale, where quantum effects can be investigated and exploited directly in the design. In this thesis, we study both electron transport and phonon transport in molecular junctions. The system we are interested in here are π-stacked molecules connected with two semi-infinite leads. π-stacked aromatic rings, connected via π-π electronic coupling, provides a rather soft mechanical bridge while maintaining high electronic conductivity. We investigate electron transport and the thermoelectric response of five representative π-stacked systems. We find that the transmission and power factor are both enhanced by increasing the conjugation length or adding substituent groups. The local transmission shows that several extra paths are added by cyano groups, which increases the total transmission at the Fermi energy. We propose and analyze a way of using π stacking to design molecular junctions to control heat transport. We develop a simple model system to identify optimal parameter regimes and then use density functional theory (DFT) to extract model parameters for a number of specific π-stacked systems. It is found that the proposed mechanism can indeed signicantly reduce the phonon conductance but also increase it depending on the specific
molecules and stackings used. Furthermore, we study how a molecule's conformation influence phonon transport by mechanically compressing an alkane chain in the junction with graphene leads, using DFT combined with scattering matrix approach. We find that the thermal conductance of the compressed junction drops by half in comparison to the extended junction, making it possible to turn on and o the heat current. The reduced thermal conductance in the compressed junction mainly stems from a large suppression of the transmission coefficients of the longitudinal and the in-plane transverse channels of the leads. Finally, we return and investigate phonon transport through π-stacked molecules connected to graphene leads including all modes of the system
using DFT method. It is found that the thermal conductance of π-stacked
systems can be reduced by 95%, compared with that in a single-molecule
junction. Phonon transmission of π-stacked systems is reduced dramatically
in the whole frequency range and the left transmission mainly remains below
5 THz.
UR - https://rex.kb.dk/primo-explore/fulldisplay?docid=KGL01009161671&context=L&vid=NUI&search_scope=KGL&tab=default_tab&lang=da_DK
M3 - Ph.D. thesis
BT - Electron and Phonon Transport in Molecular Junctions
PB - Department of Chemistry, Faculty of Science, University of Copenhagen
ER -