Thermopower in Transition from Tunneling to Hopping

• Published in: Nano letters Vol. 22; no. 18; pp. 7682 - 7689
• Main Authors: Park, Sohyun, Jo, Jeong Woo, Jang, Jiung, Ohto, Tatsuhiko, Tada, Hirokazu, Yoon, Hyo Jae
• Format: Journal Article
• Language: English
• Published: American Chemical Society 28-09-2022
• Subjects: Seebeck coefficient; molecular junction; tunneling; hopping; thermopower; transition
• ISSN: 1530-6984; 1530-6992
• DOI: 10.1021/acs.nanolett.2c03083

The Seebeck effect of a molecular junction in a hopping regime or tunneling-to-hopping transition remains uncertain. This paper describes the Seebeck effect in molecular epitaxy films (OPI n where n = 1–9) based on imine condensation between an aryl amine and aldehyde and investigates how the Seebeck coefficient (S, μV/K) varies at the crossover region. The S value of OPI n linearly increased with increasing the molecular length (d, nm), ranging from 7.2 to 38.0 μV/K. The increasing rate changed from 0.99 to 0.38 μV·K–1 Å–1 at d = 3.4 nm (OPI4). Combined experimental and theoretical studies indicated that such a change stems from a tunneling-to-hopping transition, and the small but detectable length-dependence of thermopower in the long molecules originates from the gradual reduction of the tunneling contribution to the broadening of molecular orbital energy level, rather than its relative position to the Fermi level. Our work helps to bridge the gap between bulk and nanoscale thermoelectric systems.