Micro-/macroscopic and density functional studies of the interactions between molybdenum trioxide and C₆₀ molecule

We have investigated the interactions between C₆₀ and (MoO₃)_n using scanning tunneling microscopy with spectroscopy (STM/STS) and ex situ ultraviolet-visible-near-infrared (UV-vis-NIR) spectroscopy in combination with density functional theory (DFT) calculations. The formation of (MoO₃)_n chemically bound to C₆₀ is energetically favorable due to ΔG < 0 for n = 1, 2, 4, 6, 8, and 9, and they well reproduced the histogram of the height of (MoO₃)_n on the C₆₀ (111) terrace obtained by a STM height-profile. STS results demonstrated the upward energy shift of both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of C₆₀ in the vicinity of (MoO₃)_n (n = 6 or 9), which is consistent with the previous results of the co-deposited C₆₀/MoO₃ film obtained using photoemission and inverse photoemission spectroscopy [Wang and Gao, Appl. Phys. Lett. 105, 111601 (2014), Yang et al., J. Phys.: Condens. Matter 28, 185502 (2016), and Li et al., J. Phys. Chem. C 118, 4869 (2014)]. Theoretical calculations of (MoO₃)_n (n = 1, 2, 4, 6, 8, and 9) chemically bound to C₆₀ indicated that 0.01-0.32 holes are injected into C₆₀ by (MoO₃)_n nanoclusters, and UV-vis-NIR and DFT results found that the hole doping to C₆₀ is caused via the electron transfer from the HOMO of C₆₀ to the LUMO of (MoO₃)_n. Furthermore, it is noted that the C₆₀-(MoO₃)_n interactions exhibit a high heat resistance up to 250 °C by examining the UV-vis-NIR spectra of a co-deposited C₆₀/MoO₃ (6:4) film before and after thermal annealing. The present findings provide useful information for the practical use of P-type C₆₀-based thermoelectric devices.