Two Different Length-Dependent Regimes in Thermoelectric Large-Area Junctions of n-Alkanethiolates

Molecular thermoelectrics is relatively unexplored compared with its analogous research field, molecular electronics. This is surprising considering that the two research fields share an identical energy landscape across molecular junctions and similar quantum-chemical mechanisms. This paper describes the length dependence of thermopower in self-assembled monolayers comprising structurally simple wide band gap molecules, n-alkanethiolates (SC_n; n = 2, 4, 6, 8, 10, 12, 14, 16, 18) chemisorbed on gold. Thermovoltage measurements at zero bias have enabled the determination of the Seebeck coefficient of n-alkanethiolates for the first time. A plot of the Seebeck coefficient versus the length of the n-alkane chain reveals the presence of two different length-dependent regimes. The rate of the decrease of the Seebeck coefficient as the molecular length increases changes at SC₁₀ from -0.54 to -0.10 μV(K·n_C)^-1. The theoretically proposed presence of metal-induced gap states (MIGS) in the short but not in the long n-alkanethiolates accounts for the two observed length-dependent regimes. Owing to the length dependence of the transmission function coefficient of MIGS in short n-alkanethiolates, the Seebeck coefficient decreases linearly as the length increases. The nearly zero rate of decrease in the long n-alkanethiolates mirrors the insignificant MIGS in the long n-alkanethiolates.