Superatom orbitals of ${\text{Sc}}_3{\text{N@C}}_{80}$ and their intermolecular hybridization on $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ surface

We investigate the electronic structure of an endohedral fullerene, ${\text{Sc}}_3{\text{N@C}}_{80}$, chemisorbed on $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ surface by scanning tunneling microscopy and density-functional theory. Scanning tunneling microscopy and spectroscopy identify a series of delocalized atomlike superatom molecular orbitals (SAMOs) in the ${\text{Sc}}_3{\text{N@C}}_{80}$ and its aggregates. By contrast to ${\text{C}}_{60}$, the encapsulated ${\text{Sc}}_3\text{N}$ cluster in ${\text{Sc}}_3{\text{N@C}}_{80}$ distorts the nearly-spherical central potential of the carbon cage, imparting an asymmetric spatial distribution to the SAMOs. When ${\text{Sc}}_3{\text{N@C}}_{80}$ molecules form dimers and trimers, however, the strong intermolecular hybridization results in highly symmetric hybridized SAMOs with clear bonding and antibonding characteristics. The electronic-structure calculations on ${\text{Sc}}_3{\text{N@C}}_{80}$ and its aggregates confirm the existence of SAMOs and reproduce their hybridization as observed in the experiment.

Figure 1

(Color online) (a) Molecular model of ${\text{Sc}}_3{\text{N@C}}_{80}$. (b) Schematic model of the partially oxidized $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ surface. (c) High-resolution STM topographic image of the partially oxidized $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ surface showing both the 0.76 nm Cu troughs and the $\left(2\times 1\right)\text{-O}$ domains. (d) STM image of ${\text{Sc}}_3{\text{N@C}}_{80}$ adsorbed on $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ surface. The tunneling conditions are $V_{bias}=0.5\text{V}$ and $I_{setpoint}=400\text{pA}$ for (a), and $V_{bias}=0.79\text{V}$ and $I_{setpoint}=150\text{pA}$ for (d). (e) $z\text{−}V$ spectra for ${\text{Sc}}_3{\text{N@C}}_{80}$ and $\text{Cu}\left(110\right)\text{−}\left(2\times 1\right)\text{-O}$ domain measured with initial tunneling conditions of $V_{bias}=1.0\text{V}$ and $I_{setpoint}=200\text{pA}$. (f) Numerical derivative of (e). The curves are displaced with respect to each other for clarity.

Figure 2

(Color online) [(a)–(d)] $dI/dV$ mapping showing the LDOS of SAMOs of an individual ${\text{Sc}}_3{\text{N@C}}_{80}$ molecule at different energies. [(e)–(h)] $dI/dV$ mapping of the SAMOs of another ${\text{Sc}}_3{\text{N@C}}_{80}$ molecule showing different spatial distributions. The measurement bias voltages are indicated in the figures; the tunneling current $I_{setpoint}$ is 200 pA.

Figure 3

(Color online) (a) $dz/dV$ spectra measured at the center and an end of a ${\text{Sc}}_3{\text{N@C}}_{80}$ dimer. Top inset: STM topography of a dimer adsorbed on a 0.76 nm Cu trough; the red circle and blue square indicate the positions where the $dz/dV$ spectra were recorded. Bottom inset: $dI/dV$ image of the dimer recorded at 3.4 V showing the LDOS of the Cu trough. Imaging dimensions for both insets are $5.0\times 5.0{\text{nm}}^2$. [(b) and (c)] $dI/dV$ mapping of the SAMOs in two different ${\text{Sc}}_3{\text{N@C}}_{80}$ dimers. SAMOs show typical bonding and antibonding characteristics originating from the strong hybridization between molecules. In (b) and (c) the dimers are adsorbed on a Cu trough and $\left(2\times 1\right)\text{-O}$ domain, respectively. At 3.6 V, the dimer adsorbed on the $\left(2\times 1\right)\text{-O}$ domain shows distinct LDOS at each end [black arrows in (c)], which is caused by scattering of surface state electrons of the substrate at adsorbed molecules. The measurement bias voltages are indicated in the figures; the tunneling current $I_{setpoint}$ is 200 pA, and the imaging size is $4.2\times 4.2{\text{nm}}^2$.

Figure 4

(Color online) (a) $dz/dV$ spectra measured at the center and the end of a linear ${\text{Sc}}_3{\text{N@C}}_{80}$ trimer adsorbed on a Cu trough. Inset: STM topography of a linear trimer; the red circle and blue square indicate the positions where the $dz/dV$ spectra were recorded. [(b)–(g)] $dI/dV$ mapping of the trimer at different energies revealing the intermolecular hybridization of the SAMOs. The measurement bias voltages are indicated within the figures; the tunneling current $I_{setpoint}$ is 200 pA.

Figure 5

(Color online) (a) $dz/dV$ spectra measured at different positions of a triangular ${\text{Sc}}_3{\text{N@C}}_{80}$ trimer. Inset: STM topography of a trimer, where red circle, green triangle, and blue square indicate the positions where the $dz/dV$ spectra were recorded. [(b)–(f)] $dI/dV$ mapping of the triangular ${\text{Sc}}_3{\text{N@C}}_{80}$ trimer at different energies. The measurement bias voltages are indicated in the figures; the tunneling current $I_{setpoint}$ is 200 pA.

Figure 6

(Color online) (a) Calculated DOS of an isolated ${\text{Sc}}_3{\text{N@C}}_{80}$ molecule analyzed into components that are localized on the carbon cage and ${\text{Sc}}_3\text{N}$ cluster, and delocalized from the atomic centers. The energy of LUMO is set to zero. (b) Calculated $s$- and $p\text{-SAMO}$ wave functions for an isolated ${\text{Sc}}_3{\text{N@C}}_{80}$ molecule. The energy is relative to LUMO. The inset shows the top view of the molecular structure.

Figure 7

(Color online) The calculated SAMO probability densities obtained by adding the distributions of two ${\text{Sc}}_3{\text{N@C}}_{80}$ molecules with different ${\text{Sc}}_3\text{N}$ cluster azimuths, as shown in the molecular structures. (a) monomer; (b) dimer; and (c) trimer.