Seebeck coefficient of thermoelectric molecular junctions: First-principles calculations

A first-principles approach is presented for the thermoelectricity in molecular junctions formed by a single molecule contact. The study investigates the Seebeck coefficient considering the source-drain electrodes with distinct temperatures and chemical potentials in a three-terminal geometry junction. We compare the Seebeck coefficient in the amino-substituted and unsubstituted butanethiol junctions and observe interesting thermoelectric properties in the amino-substituted junction. Due to the novel states around the Fermi levels introduced by the amino substitution, the Seebeck coefficient could be easily modulated by using gate voltages and biases. When the temperature in one of the electrodes is fixed, the Seebeck coefficient varies significantly with the temperature in the other electrode and such dependence could be modulated by varying the gate voltages. As the biases increase, richer features in the Seebeck coefficient are observed, which are closely related to the transmission functions in the vicinity of the left and right Fermi levels.

Figure 1

(Color online) The Seebeck coefficient $S$ in a three-terminal geometry with $V_{\text{SD}}=0.01\text{V}$: (a) $S$ versus $V_G$ where $T_L=T_R=T$ for the amino-substituted [black (thick) lines] and unsubstituted [red (thin) lines] butanethiol for $T=50\text{K}$ (solid line), $T=100\text{K}$ (dashed line), and $T=150\text{K}$ (dotted line). The inset shows the schematic of the three-terminal junction. The gate field is applied in a direction perpendicular to the direction of charge transport. (b) The density of states and the transmission function (inset): the left panels for the amino-substituted butanethiol junction at $V_G=-3.65$, $-2.60$, and 1.72 V; the right panels are for the unsubstituted butanethiol junction at $V_G=-1.76$, $-0.58$, and 2.92 V. (c) $S$ versus $T_L$ for an amino-substituted butanethiol junction for $V_G=-3.38$, $-2.60$, and 1.72 V, where $T_R=0\text{K}$.

Figure 2

(Color online) (a) The Seebeck coefficient $S$ as a function of source-drain biases in a two-terminal geometry for amino-substituted (main graph) and unsubstituted (inset in the upper right corner) butanethiol for $T=50\text{K}$ (solid lines), $T=100\text{K}$ (dashed lines), and $T=150\text{K}$ (dotted lines). (b) The density of states and the transmission function (inset) for various source-drain biases ($V_{\text{SD}}=-0.9$, $-0.5$, $-0.3$, 0.2, and 0.8 V) in the amino-substituted butanethiol junction.