Do Thermoelectric Materials in Nanojunctions Display Material Property or Junction Property?

The miniaturization of thermoelectric nanojunctions raises a fundamental question: do the thermoelectric quantities of the bridging materials in nanojunctions remain to display material properties or show junction properties? In order to answer this question, we investigate the Seebeck coefficient $S$ and the thermoelectric figure of merit $ZT$ especially in relation to the length characteristics of the junctions from the first-principles approaches. For $S$, the metallic atomic chains reveal strong length characteristics related to strong hybridization in the electronic structures between the atoms and electrodes, while the insulating molecular wires display strong material properties due to the cancelation of exponential scalings in the DOSs. For $ZT$, the atomic wires remain to show strong junction properties. However, the length chrematistics of the insulation molecular wires depend on a characteristic temperature $T_{0}= \sqrt{\beta/\gamma(l)}$ around 10K. When $T \ll T_{0}$, where the electron transport dominates the thermal current, the molecular junctions remain to show material properties. When $T \gg T_{0}$, where the phonon transport dominates the thermal current, the molecular junctions display junction properties.