Fig. 4: The geometric and electronic characterizations of C₂₀.

a _I–a _IV STM image, AFM image, AFM simulation, and Laplace-filtered AFM image of a C₂₀. STM set points: I = 5 pA, V = 0.3 V. Reference set point of Δz: I = 5 pA, V = 0.3 V. b Calculated frontier orbitals of C₂₀ and superposition of orbital densities of the nearly energetically degenerated orbitals. Calculations were conducted at the ωB97XD/def2-TZVP level. (c) Scanning tunneling spectroscopy (STS) of C₂₀ conducted on NaCl. The differential conductance (dI/dV) signal shows two peaks that can be attributed to the PIR and NIR states. Inset: STM (I = 5 pA, V = 0.3 V, 0 to 1.6 Å) and AFM (Δz = −0.6 Å, reference set point: I = 5 pA, V = 0.3 V, −3.4 Hz to −0.2 Hz) images of a C₂₀. Scale bar: 0.5 nm. STM images (d _I–f _I) and Laplace-filtered STM images (d _II–f _II) of C₂₀ obtained with a CO-tip. (d _I–d _II) NIR, constant current mode; (e _I–e _II) PIR, constant height mode (Δz = 0 Å, reference set point: I = 5 pA, V = −2.25 V); (f _I and f _II) NIR, constant height mode (Δz = 0 Å, reference set point: I = 5 pA, V = 1.55 V). The dI/dV spectrum sweeping from zero to negative bias (blue line) was conducted on the triple bond of C₂₀ (blue cross in Fig. 4c), and the dI/dV spectrum sweeping from zero to positive bias (red line) was conducted on the single bond of C₂₀ (red cross in Fig. 4c).