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)_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.