Transport of charge and energy in metal -molecule -metal junctions

This dissertation presents the experimental study of the electrical conductance (G) and Seebeck coefficient (S) of metal-molecule-metal junctions. The dependence of the electrical conductance of metal-molecule-metal junctions on the molecule's structure is elucidated. The existence of a measurable Seebeck coefficient in metal-molecule-metal junctions is shown for the first time and the sign of the measured Seebeck coefficient is used to determine whether the charge transport in metal-molecule-metal junctions is dominated by positive (p-type) or negative (n-type) charge carriers.;The electrical conductance of a series of thiol (-SH), amine (-NH 2) terminated aliphatic and aromatic molecules was measured using a modified scanning tunneling microscope break junction technique. A new method called the last-step analysis (LSA) was introduced to analyze data obtained in these measurements. This analysis in contrast to previous work does not require any data pre-selection, making the results less subjective and more reproducible. We first studied the electrical conductance of aliphatic molecular junctions. It was found that Au-hexanedithiol-Au, Au-octanedithiol-Au and Au-decanedithiol-Au junctions have an electrical conductance of 3.6 X 10 -4 Go, 4.4 X 10-5 Go and 5.7 X 10-6 Go, respectively, where Go is the fundamental quantum of electrical conductance. The electrical conductance decreases exponentially with the length of the alkane chains suggesting that the mechanism for electrical transport through these molecular junctions is quantum mechanical tunneling. On varying the end groups of aliphatic molecules 0rom thiols to amines we found that the electrical conductance was almost identical, suggesting that the end group had no significant effect on the electrical conductance in this case. We also measured the electrical conductance of aromatic molecular junctions: Au-1,4-benzenediamine-Au, Au-4,4'-dibenzenediamine-Au and Au-4.4''-tribenzenediamine-Au and they were found to have an electrical conductance of 1.05 X 10 -2 Go, 1.41 X 10-3 Go and 2.05 X 10-4 Go, respectively. Although, the electrical conductance decreases exponentially with the lengths of the molecules even in the case of aromatic molecules, the length dependence of the electrical conductance was much weaker than that of aliphatic molecular junctions. From the data it can be seen that for a given length aromatic molecular junctions have a larger conductance than aliphatic molecular junctions.;The Seebeck coefficient (S) of molecular junctions was measured using a modified break junction technique by trapping molecules between two gold electrodes with a temperature difference across them. The junction Seebeck coefficient of Au-1,4-benzenedithiol-Au, Au-4,4'-dibenzenedithiol-Au and Au-4,4"-tribenzenedithiol-Au was measured at room temperature to be (+8.7 +/- 2.1) microvolts per Kelvin (muV/K), (+12.9 +/- 2.2) muV/K, and (+14.2 +/- 3.2) muV/K, respectively. The positive sign unambiguously indicates p-type (hole) conduction through these heterojunctions, and the Au Fermi level position for AuBDT-Au junctions was identified at 1.2 eV above the highest occupied molecular orbital (HOMO) level of BDT. Our study provides the first experimental answer to the question of whether charge transport through molecular junctions is dominated by p-type or n-type charge carriers. In perspective, the ability to study thermoelectricity in molecular junctions as demonstrated here allows us to address some of the fundamental transport problems in molecular electronics.