Extended Data Fig. 6: Thermopower data of Si2-Si222-Si2 and Si8.

a, Chemical structures of Si8 and Si2-Si222-Si2. b, c, Two-dimensional histograms of thermoelectric current measured while Si2-Si222-Si2 junctions are held at ΔT = 0 K and 37 K, respectively. d, Histogram of the measured thermoelectric current for Si8, which has the same number of Si atoms across the molecule as Si2-Si222-Si2, with ΔT = 0 K and 27 K. Inset: Histogram of thermopower determined from the thermoelectric current for Si8 junctions. After subtracting the thermopower of Au of 2 μV K⁻¹ (to account for the thermoelectric current between the hot substrate and the cold set-up), the average thermopower for Si8 is 35 ± 17 μV K⁻¹, smaller than that of Si2-Si222-Si2. e, Transmission curves for Si8 junction along with Si2-Si222-Si2 junction showing different slopes at the Fermi level. f, Thermopower calculated as the slope of the transmission as a function of energy. Theory underestimates the thermopower of both Si2-Si222-Si2 and Si8 by approximately an order of magnitude. Furthermore, the energy alignment between the antiresonance and the Fermi energy is not exact because of inherent errors of DFT^31,42, which results in the opposite sign of the thermopower at the Fermi energy compared to the experimental value.