Thermopower in Transition from Tunneling to Hopping

Area of Science:

• Materials Science
• Condensed Matter Physics
• Nanotechnology

Background:

• The Seebeck effect in molecular junctions is not fully understood, especially concerning the transition between tunneling and hopping transport regimes.
• Molecular epitaxy films offer a platform to study fundamental thermoelectric properties at the nanoscale.
• Understanding length-dependent thermoelectricity is crucial for designing efficient molecular electronic devices.

Purpose of the Study:

• To investigate the Seebeck effect in organic molecular epitaxy films (OPI_n) with varying molecular lengths.
• To determine how the Seebeck coefficient changes across the tunneling-to-hopping transition region.
• To elucidate the underlying mechanisms governing thermopower in molecular systems.

Main Methods:

• Fabrication of OPI_n films (n=1-9) using imine condensation.
• Measurement of the Seebeck coefficient (S) as a function of molecular length (d).
• Theoretical calculations and experimental data analysis to understand transport mechanisms.

Main Results:

• The Seebeck coefficient (S) increased linearly with molecular length (d) from 7.2 to 38.0 μV/K.
• A change in the rate of increase was observed at d = 3.4 nm (OPI₄), indicating a transition.
• Experimental and theoretical findings attribute this transition to a shift from tunneling to hopping transport.

Conclusions:

• The study reveals a tunneling-to-hopping transition in OPI_n films, influencing their thermoelectric properties.
• The length-dependence of thermopower in longer molecules is due to reduced tunneling contribution affecting molecular orbital broadening.
• This research bridges the understanding of thermoelectricity between bulk materials and nanoscale molecular systems.