Energy stabilization of the $s$-symmetry superatom molecular orbital by endohedral doping of C${}_{82}$ fullerene with a lanthanum atom

Energy stabilization of the superatom molecular orbitals (SAMOs) in fullerenes is investigated with the goal of involving their nearly free-electron bands in practical charge transport applications. Combining low-temperature scanning tunneling microscopy-based spectroscopic methods and density functional theory calculations on an endohedral metallofullerene La@C${}_{82}$, we confirm that the $s-$SAMO of C${}_{82}$ fullerene is stabilized by as much as 2 eV with respect to that of C${}_{60}$ by endohedral doping with the La atom. On the copper metal substrate, the $s-$SAMO energy is further lowered to just 1 eV above the Fermi level, making the applications of $s-$SAMO state in transport more plausible. We conclude that in an endohedral metallofullerene, the $s-$SAMO state is stabilized through the hybridization with the $s$-symmetry valence state of the metal atom and the stabilization energy correlates with the ionization potential of the free atom.

Figure 1

The anticorrelation between the $s$-SAMO energies calculated by DFT of $M$@C${}_{60}$ molecules with different metal atoms, $M$, placed at the center of fullerene vs the first ionization potential of $M$. The data points for La, Dy, and Lu represent the energies from spectroscopic STM measurements (filled triangles), which we attribute to the $s$-SAMO.[42, 65] For these measurements, the metal atoms are presumed to be at their equilibrium positions within $M$@C${}_{82}$ molecules. For an estimate of the destabilization of $s$-SAMO due to the displacement of EA from the cage center, the calculated equilibrium value of $s$-SAMO for Li@C${}_{60}$ is also given (filled triangle).

Figure 2

(a) STM topographic images of La@C${}_{82}$ molecules on the C${}_{60}$/Cu(111) substrate. Isolated La@C${}_{82}$ molecules are located at interfaces between differently oriented C${}_{60}$ domains or interfaces of domains on different terraces. The image size is 50 × 50 nm. The inset (3.8 × 3.8 nm) shows an enlarged image of a single La@C${}_{82}$ molecule. (b) Combined $dI$/$dV$ and d(lnz)/d(lnV) spectra obtained for a single isolated La@C${}_{82}$ molecule. Below 3 eV, the peaks in $dI$/$dV$ spectra coincide with those of d(lnz)/d(lnV). (c) and (d) STM topographic image (50 × 50 nm) as well as combined $dI$/$dV$ and d(lnz)/d(lnV) spectra of La@C${}_{82}$ molecules adsorbed on the Cu(111) step edge. Single La@C${}_{82}$ molecules and assemblies of several molecules are located at step edges on the lower terrace. The inset (12 × 12 nm) shows a single La@C${}_{82}$ molecule, a trimer, and a quadrimer. The $dI$/$dV$ and d(lnz)/d(lnV) spectra in (b) and (d) are combined using an arbitrary vertical scale.

Figure 3

Comparisons between the calculated total DOS of La@C${}_{82}$ molecule (black), the projected DOS on La 4$f$ and 5$d$ (red dotted) and the projected DOS of SAMOs (green dash-dotted) with the experimental $dI$/$dV$ and d(lnz)/d(lnV) spectra (light and dark blue) obtained on isolated La@C${}_{82}$ molecules adsorbed on C${}_{60}$/Cu(111). The calculated $s$-SAMO is ∼2.09 eV above the $E_f$. There is good agreement in peak positions between the experimental and theoretical data.

Figure 4

The calculated total, local, and nonlocal DOS of isolated La@C${}_{82}$ (a) and C${}_{82}$ (b). The $s$- and $p$-SAMOs belong to the nonlocal DOS. The strong influence of La${}^{3+}$ on the $s-$SAMO results in an s-p SAMO splitting of ∼2.3 eV in the case of La@C${}_{82}$, whereas it is only ∼0.6 eV for the empty C${}_{82}$. Compared with $s-$SAMO, the $p-$SAMOs are less affected by the presence of La${}^{3+}$ inside the cage.

Figure 5

The energy difference between the $s-$ and $p-$SAMOs of isolated La@C${}_{82}$ and C${}_{60}$ molecules on a C${}_{60}$ buffer layer. For the La@C${}_{82}$ molecule, the energy difference is 2.86 eV, whereas for the isolated C${}_{60}$, it is 0.56 eV.

Figure 6

The Cu(111) substrate effect on the SAMOs. (a) d(lnz)/d(lnV) spectra of C${}_{60}$ on Cu(111) and C${}_{60}$/Cu(111) surfaces. (b) d(lnz)/d(lnV) spectra of La@C${}_{82}$ on Cu(111) and C${}_{60}$/Cu(111) surfaces. In both cases, the energy levels shift down in the presence of the Cu(111) substrate.