Recent advances in single-molecule thermoelectricity has isolated and identified different families of high-performance molecules. However, to realize the commercial potential of these molecules and convert them into real-world thin-film energy-harvesting devices, fundamental issues surrounding parallel-aligned junctions within these devices need to be addressed.
A team of Researchers from the UK and Spain have studied a junction composed of two parallel C60 molecules sandwiched between two graphene monolayers, in an attempt to boost the electrical and thermoelectric performance against current single-junction mechanisms.
Molecular devices composed of single or multiple molecules which are bridged by at least two electrodes have gathered a lot of attention from both a theoretical and experimental point of view. Such devices have been found to possess a plethora of properties which facilitate excellent tunability and transport mechanisms, including negative differential resistance (NDR), electrical switching and thermoelectric power generation.
Common thermoelectric materials of the inorganic variety, i.e. Pb, Bi, Co, Sb are toxic and expensive due to finite sources across the globe. So, to circumnavigate the resource shortage, Researchers turned to using single organic molecules, which has worked with great effect so far. But, to prove their commercial value, issues surrounding junctions being placed in parallel need to be resolved.
As a step in the right direction to solving this scientific conundrum, the Researchers believed that a controlled scalability may hold the key. The Researchers have taken to using density functional theory (DFT) calculations to help determine how parallel junctions can be addressed.
The Researchers assembled a four-terminal device at the edges of two graphene sheets which sandwiched two C60 molecules. The terminals were set up as semi-infinite crystalline leads to eliminate edge effects on the graphene sheets. The Researchers used a code called SIESTA to obtain the optimized geometry and density approximation, and used a transport code named GOLLUM to compute the electrical and thermoelectric properties of the devices from the mean-field Hamiltonian and overlap matrices. The Researchers chose a double-z plus polarization (DZP) basis set for their calculations.
The Researchers investigated the properties of the C60 molecules by placing them parallel to each other and sandwiched between the two graphene sheet electrodes. Unlike in classical conductors, the Researchers found that increasing the number of parallel junctions from one to two caused the electrical conductivity to increase by at least a factor of two.
The Researchers also found that the Seebeck coefficient, i.e. the thermoelectric power or thermoelectric sensitivity, is sensitive to the number of molecules sandwiched between the electrodes. In classical conductors, the sensitivity would not change. The Seebeck coefficient sensitivity was also shown to not be proportional to the increase of parallel molecules.
The non-classical behavior exhibited by the junctions was found to arise from inter-junction quantum interference, mediated by the electrodes and caused an enhanced response within the device. This was demonstrated under an environment with no direct inter-molecular coupling, and instead, was a product of indirect inter-molecular interactions. These indirect interactions could be used to boost the electrical and thermoelectric performance of a device if they can be controlled, and as such represent an interesting find.
In addition, the electrical conductance between the two sandwiching graphene electrodes did not follow Ohm’s law and was a result of the conductance being twice as large as the conductance of a single C60 molecule.
The findings from these computations are significant because they show that single-molecule thermoelectric properties will not translate into thin-film materials formed in classical self-assembly processes. However, it does show that utilizing the quantum interface can yield a thermoelectric performance which can exceed classical expectations.
The Researchers also analyzed a tight binding model which featured parallel electron transport through two sites. The model predicted that the dimer’s thermoelectric properties will oscillate with the dimer separation. This was found to occur up to a phase coherence length, which was also found to decrease with increasing temperature.
Whilst this is only a theoretical study, if scaled correctly, the electrical and thermoelectric performance boost displayed in the computations could realize these single molecule thermoelectric devices.
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“Thermoelectricity in vertical graphene-C60-graphene architectures”- Wu Q., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-10938-2