Heteroatom engineering for enhancing the thermoelectric power factor of molecular junctions†

Abstract

Engineering the power factor (PF) of molecular junctions is one of the most attractive research topics in the field of thermoelectronics for applications in thermal management and high-performance thermoelectric energy conversion at the nanoscale. Here, we modified the chemical structure of self-assembled monolayers (SAMs) formed by the widely investigated alkanethiolate (HS–C_n) through heteroatom substitutions, including terminal iodine (I) atom substitution and replacing backbone methylene units (–CH₂–) with oxygen (O) atoms, to obtain iodo-substituted oligo(ethylene glycol) thiolates (HS–(C₂O)_m–C₂–I). The electrical conductivity (σ) and Seebeck coefficient (S) of the SAMs with HS–(C₂O)_m–C₂–I can be enhanced simultaneously compared to those of the length-matched SAMs of HS–C_n, resulting in the PF of HS–(C₂O)₄–C₂–I being over five orders of magnitude higher than that of HS–C₁₄, which was attributed to the resonant states contributed from the substituted HS–(C₂O)_m–C₂–I near the Fermi energy level. Our findings highlight the significance of chemically engineering the organic molecules to dramatically boost the PF of molecular junctions for further applications of highly efficient nanoscale thermoelectric devices.