Probing Charge Transport and Thermoelectricity in Molecular Junctions

Abstract

The study of charge transport and thermoelectricity in molecular junctions is of fundamental interest for understanding charge transport mechanisms and provides knowledge critical for the development of nanotechnologies including electronics, energy conversion and thermal management. In spite of a large amount of theoretical and experimental work into charge transport and thermoelectric properties of various molecular junctions, several important questions remain unsolved. Quantum phenomena dominate transport in molecular junctions, therefore, a natural question to ask is whether it is possible to tune the thermoelectric properties of molecular junctions via quantum interferences. To answer this question, I present work where we investigated charge and thermoelectric properties in molecular junctions based on two oligo (phenylene ethynylene) (OPE) derivatives where quantum interference effects are expected to arise. Our experiments reveal that meta-OPE3 junctions, which are expected to exhibit destructive interference effects, yield a higher thermopower (with ~20 μV/K) compared with para-OPE3 (with ~10 μV/K). Results from both single-molecule junction and monolayer experiments correspond well with each other and agree well with computational predictions made by our collaborators. Our results show that quantum interference effects can indeed be employed to enhance the thermoelectric properties of molecular junctions. Along with enhancing thermoelectric properties of molecular junctions via quantum interference, past theoretical work has proposed another strategy to tune the thermoelectric properties in molecular junctions by varying the metal centres incorporated in porphyrins. The tunability of thermoelectric properties in these junctions, however, have not been experimentally explored. To explore the feasibility of tuning the thermoelectric properties in these junctions, we conducted measurements in Au-porphyrin-Au, Au-(Cu-porphyrin)-Au and Au-(Zn-porphyrin)-Au junctions. To achieve better thermoelectric performance, we replaced the thiol end groups that are typically employed in a series of metallo-porphyrins with triisopropylsilyl end groups, which enable a direct C-Au bond resulting in an increase of the electrical conductance. In fact, our single molecule experiments show nearly two orders of magnitude increase in the electrical conductance of junctions compared to previous work that employed thiol end groups. The thermoelectric experiments reveal tunable thermopower through molecules with different metal centres. Overall, among the molecules studied in our work, we find that Au-(Zn-porphyrin)-Au junctions exhibit the best thermoelectric performance. In addition to the energy conversion of heat-to-electricity as discussed above, thermoelectric effects are also expected to enable cooling in molecular junctions through Peltier effects and in principle such Peltier cooling in molecular junctions may be applied to fabricate refrigerators at molecular scale. However, experimental observation of Peltier cooling in molecular junctions has not been possible. Here, I discuss the experimental observation of Peltier cooling in molecular junctions. The molecular junctions studied here are Au-(biphenyl-4,4'-dithiol)-Au, Au-(terphenyl-4,4''-dithiol)-Au and Au-(4,4'-bipyridine)-Au, of which the charge transport and thermoelectric properties are widely studied in the field. Our results unambiguously show cooling in molecular junctions under small bias voltage and reveal the relationship between heating or cooling and charge transport mechanisms in studied molecular junctions. Our experimental results are supported by computational results from our collaborators.