Thermoelectric properties of oligoglycine molecular wires

Abstract

We have investigated the electrical and thermoelectrical properties of glycine chains with and without cysteine terminal groups. The electrical conductance of (Gly) n , (Gly) n Cys and Cys(Gly) n Cys 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) n Cys and Cys(Gly) n Cys respectively, while β = 0.92 Å -1 for (Gly) n . We also find that replacing the peptide bond with a methylene group (-CH 2 -) increases the conductance of (Gly) 3 Cys. Furthermore, we find the (Gly) 1 Cys and Cys(Gly) 1 Cys 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) 1 Cys at room temperature, which increases further with increasing temperature, suggesting that peptide-based SAM junctions are promising candidates for thermoelectric energy harvesting.