Thermoelectric properties of oligoglycine molecular wires

We have investigated the electrical and thermoelectrical properties of glycine chains with and without cysteine terminal groups. The electrical conductance of (Gly)n, (Gly)nCys and Cys(Gly)nCys molecules (where Gly, Cys represent glycine and cysteine and n = 1-3) was found to decay exponentially with length l as e-βl. Our results show that connecting the molecules to gold electrodes via the sulphur atom of the cysteine moiety leads to higher β factors of 1.57 Å-1 and 1.22 Å-1 for (Gly)nCys and Cys(Gly)nCys respectively, while β = 0.92 Å-1 for (Gly)n. We also find that replacing the peptide bond with a methylene group (-CH2-) increases the conductance of (Gly)3Cys. Furthermore, we find the (Gly)1Cys and Cys(Gly)1Cys systems show good thermoelectrical performance, because of their high Seebeck coefficients (∼0.2 mV K-1) induced by the sulphur of the cysteine(s). With the contributions of both electrons and phonons taken into consideration, a high figure of merit ZT = 0.8 is obtained for (Gly)1Cys at room temperature, which increases further with increasing temperature, suggesting that peptide-based SAM junctions are promising candidates for thermoelectric energy harvesting.