Figure 1

(a)–(d) $dI/dV$ over Cu(111) for various ${\mathrm{C}}_{60}$ tips (1.0 V, 0.1 nA). The orientation of ${\mathrm{C}}_{60}$ at each tip apex is determined by constant-current images over a Au atom (sample bias: $-2.0\mathrm{V}$, current: 0.1 nA, size: $12\times 12{\AA }^2$). Each orientation is flanked by a Hückel simulation of the $\mathrm{LUMO}+1$ orbital, and for better visibility, by the same orbital cut along the $xy$ plane (the $\mathrm{LUMO}+1$ orbital is localized on the pentagons forming the ${\mathrm{C}}_{60}$ skeleton). Panel (a) present also a $dI/dV$ spectrum over Cu(111) acquired with a copper-coated $W$ tip (dashed line, feedback loop opened at $-3.0\mathrm{V}$ and 5 nA).Reuse & Permissions

Figure 2

$dI/dV$ acquired above Cu(111) with two different tips (1.0 V, 1.0 nA), having (a) same corner orientation for ${\mathrm{C}}_{60}$ [as in Fig. 1a], (b) same $6:6$ orientation for ${\mathrm{C}}_{60}$ [as in Fig. 1c].Reuse & Permissions

Figure 3

(a) $dI/dV$ spectra for various barrier thicknesses. The spectra are normalized by the conductance ($G=I/V$) at which the feedback loop was opened, respectively, from top to bottom: 0.01, 0.1, 1 and 10 nS. (b) NDC intensity versus tip-sample distance (left vertical axis) and corresponding changes of $G$ (right vertical axis). From the tunneling regime, we extract an apparent-barrier height of $\Phi =5\mathrm{eV}$. The distance is estimated by taking $z=0$ for the contact conductance ($G$ vs $z$ curve acquired at 1 V), the hatched lines highlighting the contact regime. The NDC intensity corresponds to the minimum of the $dI/dV$ curves in (a). The lines correspond to the simulation detailed in the text: a realistic DOS (dashed line) and a stepped DOS without $d$-bulk contribution (dash-dotted line).Reuse & Permissions

Figure 4

(a) DOS of a low-symmetry ${\mathrm{C}}_{60}$ tip (upper half-plane, thick line) and Cu(111) surface (lower half-plane, dash-dotted line: $V=0\mathrm{V}$, thick line: $V=-2.35\mathrm{V}$) used in the WKB simulation ($m=0$ and $m\ne 0$ components shown with dashed and dotted lines, respectively). Copper DOS: step function of unit amplitude centered at $-0.45\mathrm{eV}$ in ${\rho }_s^{m=0}$ and a constant background set to 0.5 in ${\rho }_s^{m\ne 0}$ with low-energy upturn. ${\mathrm{C}}_{60}$ DOS: three Lorentzian peaks mimicking the HOMO ($-2.0\mathrm{eV}$), LUMO (0.4 eV) and $\mathrm{LUMO}+1$ (1.9 eV) with widths 0.5 eV, 1.0 eV, and 0.5 eV, respectively, and amplitudes proportional to the free-molecule degeneracies (thick line). The occupied states are indicated by the shaded blue areas for the sample voltage $V=-2.35\mathrm{V}$ where the Shockley surface-state edge matches the $\mathrm{LUMO}+1$ resonance. (b) Simulated $dI/dV$ for (a) via Eq. (1). (c) Energy-resolved current for $V=-2.35\mathrm{V}$ (dark orange [dark gray]) and $V=-2.40\mathrm{V}$ (light orange [light gray]) for $z=6\AA$. (d)–(f) Similar to (a)–(c) but for a high-symmetry ${\mathrm{C}}_{60}$ orientation where the $m=0$ component is suppressed for HOMO and $\mathrm{LUMO}+1$.Reuse & Permissions