Doctor of Philosophy
Institute for Superconducting and Electronic Materials
Zhu, Chengbo B., Study of ballistic transport in molecular junctions based on H2O@C60, B40 and phenalenyl-based molecule, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4477
Molecular electronics has been an attractive area for the past two decades. New concepts, with no classical analogues, have been inspired by nanoscale devices. As electronic devices are scaled down to nanometer dimensions, their operation depends on the detailed atomic structure. Recently, more and more attention has been paid to the physical properties of metal–molecule–metal junctions that go beyond electronic transport characterizations.
In this thesis, a short history and the latest progress on molecular electronics are introduced. Then, we briefly describe the theory and simulation methods.
First, the transport properties of H2O@C60-based nanostructures sandwiched between electrodes have been calculated. We find that, unlike the single endohedral fullerene molecule in electrostatic field, such nanostructures can no longer act as a Faraday cage under voltage bias. The screening effect disappears completely. In addition, the disappearance of the screening effect is water-position-independent. Nevertheless, the conductance of the junction is water-position-dependent. When the encapsulated water molecule moves towards the centre of the C60, the conductance of the molecular junction decreases, and vice versa. For this highly symmetric dipolar molecule, with the same contact geometry, its transport properties can be manipulated by controlling the encapsulated water molecule.
Secondly, the conductance of two H2O@C60 molecules in series order is reported, as well as how the number of encapsulated water molecules influences the transport properties of the junction. Encapsulating an H2O molecule in one of the C60 cages increases the conductance of the dimer. Negative differential resistance is found in the dimer systems, and its peak-to-valley current ratio depends on the number of encapsulated H2O molecules. The conductance of the C60 dimer and the H2O@C60 dimer is two orders of magnitude smaller than that of the C60 monomer. Furthermore, we demonstrate that the conductance of the molecular junctions based on the H2O@C60 dimer can be tuned by moving the encapsulated H2O molecules. The conductance is H2O-position dependent. Our findings indicate that the H2O@C60 can be used as a building block in C60-based molecular electronic devices and sensors.
Thirdly, the transport properties and thermopower of individual B40 molecules are calculated. Our study suggests that B40 is a highly conductive molecule compared with C60. The conductance of Au-B40-Au junctions can be as high as several times that of Au-C60-Au junctions with similar contact geometries. As a rule of thumb, in single-molecule junctions based on π-conjugated molecules or C60 fullerene, the number of conduction channels usually equals the number of C atoms in contact with the electrode. However, the number of conduction channels in a B40- molecule junction is less than the number of B atoms in direct contact with the electrode, due to the unique electronic structure of B40. Moreover, we have found that the thermopower of B40 with gold electrodes is dramatically smaller than that of the Au-C60-Au junction and is negative, except for one configuration, due to the fact that the lowest unoccupied molecular orbital dominates the charge transport. There is reason to believe that chemical modification and functionalization of the B40 are possible. This may lead to finding molecules with higher conductance after doping. The B40 fullerene is a new platform for highly conductive single-molecule junctions for future molecular circuits.
Fourthly, we propose a way of connecting phenalenyl-based molecules to gold electrodes with their spin-polarized state preserved. As a result, spin-polarized transmission is found in the phenalenyl (C13H9) molecular junction. Remarkably, the peak positions for both spins are found to differ by more than 0.5 eV. The spinpolarized transmission is suppressed or enhanced by replacing two carbon atoms of phenalenyl with boron or nitrogen atoms. The current of the nitrogen-doped junction is spin-balanced at low bias but spin-polarized at finite bias. This leads to a device that can generate spin-polarized current at the desired bias by doping. By B-doping, the spin-polarized transmission is enhanced, as the orbitals in one spin channel resonate with the electrons on the electrodes, indicating its potential application in making spin filter devices.