Benchmarking break-junction techniques: electric and thermoelectric characterization of naphthalenophanes

Break-junction techniques provide the possibility to study electric and thermoelectric properties of
single-molecule junctions in great detail. These techniques rely on the same principle of controllably
breaking metallic contacts in order to create single-molecule junctions, whilst keeping track of the junc-
tion’s conductance. Here, we compare results from mechanically controllable break junction (MCBJ) and
scanning tunneling microscope (STM) methods, while characterizing conductance properties of the same
novel mechanosensitive para- and meta-connected naphtalenophane compounds. In addition, thermo-
power measurements are carried out for both compounds using the STM break junction (STM-BJ) tech-
nique. For the conductance experiments, the same data processing using a clustering analysis is per-
formed. We obtain to a large extent similar results for both methods, although values of conductance and
stretching lengths for the STM-BJ technique are slightly larger in comparison with the MCBJ. STM-BJ
thermopower experiments show similar Seebeck coefficients for both compounds. An increase in the
Seebeck coefficient is revealed, whilst the conductance decreases, after which it saturates at around
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10 μV K${}^{-1}$ . This phenomenon is studied theoretically using a tight binding model. It shows that changes of
molecule-electrode electronic couplings combined with shifts of the resonance energies explain the cor-
related behavior of conductance and Seebeck coefficient.