Exploring seebeck-coefficient fluctuations in endohedral-fullerene, single-molecule junctions

• Published in: Nanoscale horizons Vol. 7; no. 6; pp. 616 - 625
• Main Authors: Ismael, Ali K, Rincón-García, Laura, Evangeli, Charalambos, Dallas, Panagiotis, Alotaibi, Turki, Al-Jobory, Alaa A, Rubio-Bollinger, Gabino, Porfyrakis, Kyriakos, Agraït, Nicolás, Lambert, Colin J
• Format: Journal Article
• Language: English
• Published: England Royal Society of Chemistry 31.05.2022
• Subjects: Buckminsterfullerene; Electrical junctions; Extreme values; Fullerenes; Histograms; Inhomogeneity; Metallofullerenes; Seebeck effect; Standard deviation; Thermoelectric materials
• ISSN: 2055-6756; 2055-6764; 2055-6764
• DOI: 10.1039/d1nh00527h

For the purpose of creating single-molecule junctions, which can convert a temperature difference Δ T into a voltage Δ V via the Seebeck effect, it is of interest to screen molecules for their potential to deliver high values of the Seebeck coefficient S = −Δ V /Δ T . Here we demonstrate that insight into molecular-scale thermoelectricity can be obtained by examining the widths and extreme values of Seebeck histograms. Using a combination of experimental scanning-tunnelling-microscopy-based transport measurements and density-functional-theory-based transport calculations, we study the electrical conductance and Seebeck coefficient of three endohedral metallofullerenes (EMFs) Sc 3 N@C 80 , Sc 3 C 2 @C 80 , and Er 3 N@C 80 , which based on their structures, are selected to exhibit different degrees of charge inhomogeneity and geometrical disorder within a junction. We demonstrate that standard deviations in the Seebeck coefficient σ S of EMF-based junctions are correlated with the geometric standard deviation σ and the charge inhomogeneity σ q . We benchmark these molecules against C 60 and demonstrate that both σ q , σ S are the largest for Sc 3 C 2 @C 80 , both are the smallest for C 60 and for the other EMFs, they follow the order Sc 3 C 2 @C 80 > Sc 3 N@C 80 > Er 3 N@C 80 > C 60 . A large value of σ S is a sign that a molecule can exhibit a wide range of Seebeck coefficients, which means that if orientations corresponding to high values can be selected and controlled, then the molecule has the potential to exhibit high-performance thermoelectricity. For the EMFs studied here, large values of σ S are associated with distributions of Seebeck coefficients containing both positive and negative signs, which reveals that all these EMFs are bi-thermoelectric materials. STM measurements and DFT calculations to study 3 bi-thermoelectric endohedral metallofullerenes, benchmarked against C 60 , show that the fluctuations in S are correlated with the charge inhomogeneity and the geometrical disorder within the junction.