Length-Dependent Thermopower of Highly Conducting Au–C Bonded Single Molecule Junctions

Abstract

We report the simultaneous measurement of conductance and thermopower of highly conducting single-molecule junctions using a scanning tunneling microscope-based break-junction setup. We start with molecular backbones (alkanes and oligophenyls) terminated with trimethyltin end groups that cleave off in situ to create junctions where terminal carbons are covalently bonded to the Au electrodes. We apply a thermal gradient across these junctions and measure their conductance and thermopower. Because of the electronic properties of the highly conducting Au-C links, the thermoelectric properties and power factor are very high. Our results show that the molecular thermopower increases nonlinearly with the molecular length while conductance decreases exponentially with increasing molecular length. Density functional theory calculations show that a gateway state representing the Au-C covalent bond plays a key role in the conductance. With this as input, we analyze a series of simplified models and show that a tight-binding model that explicitly includes the gateway states and the molecular backbone states accurately captures the experimentally measured conductance and thermopower trends.

Thermopower of molecular junctions: Tunneling to hopping crossover in DNA

“…This expression predicts the following properties-which were verified in experiments 7,8,10 : In the deep tunneling regime, the thermopower increases linearly with temperature and with the number of molecular sites N . As well, LUMO conduction, with ǫ B situated above the Fermi energy, shows a negative thermopower.…”

“…As well, in this deep tunneling regime S should scale linearly with the leads' temperature. These predictions were verified experimentally by several groups 2,8,10 .…”

“…Specifically, measurements of the thermopower of single organic molecules reveal whether electrons or holes are the primary charge carriers 3 . The thermopower can further reveal relative orbital levelalignment [4][5][6] , and pinpoint transport mechanisms, in support of electrical conductance measurements [7][8][9][10][11][12] .…”

Effects of vibrational anharmonicity on molecular electronic conduction and thermoelectric efficiency

“…(13), it can be shown that the spectral functions take a Lorentzian lineshape centered about ǫ d,a ,…”

“…Beyond electrical conductance, the Seebeck coefficient, which measures the voltage that develops when a small temperature difference is applied, under the condition that the net charge current vanishes, hands over information about the structure and energetics of molecular junctions. It reveals, e.g., the nature of molecular orbitals hybridizing with the metal electrodes, and whether the conductance is HOMO or LUMO dominated [7][8][9][10][11][12][13][14] .…”