Molecular quantum well at the ${\mathrm{C}}_{60}∕\mathrm{Au}\left(111\right)$ interface

A molecular quantum well is observed in epitaxial thin films of ${\mathrm{C}}_{60}$ on Au(111). This quantum well is characterized by nearly free-electron-like dispersion in the surface plane and splitting of the electronic band with increasing number of ${\mathrm{C}}_{60}$ overlayers. The experimental quantum well structure can be well described by tight binding theory with an intermolecular charge transfer integral of $\beta =-33\pm 3\mathrm{meV}$, corresponding to a full bandwidth of $400\pm 40\mathrm{meV}$. Quantum mechanical calculation based on time-dependent density functional theory provides excellent agreement with this $\beta$ value. The molecular quantum well is populated by photoinduced electron transfer from the Au substrate and has a transient lifetime of $4\mathrm{fs}<t<20\mathrm{fs}$.

Figure 1

Left: Schematic illustration of the ${\mathrm{C}}_{60}$ $(2\sqrt{3}\times 2\sqrt{3})R30°$ superlattice on the Au(111) surface. The dashed arrow indicates the $k_{\Vert }$ direction for dispersion measurements, and solid line is the $11\overline{2}$ direction. Right: LEED image of the 1 ML ${\mathrm{C}}_{60}∕\mathrm{Au}\left(111\right)$ surface. The solid line is the $11\overline{2}$ direction, which is $8\pm 1°$ from the horizontal direction for dispersion measurement.

Figure 2

2PPE spectra taken at a photon energy of $4.593\mathrm{eV}$ for ${\mathrm{C}}_{60}$ covered Au(111) surfaces at various molecular film thicknesses ($\theta =1–20$ ML). IR1 & IR2: $n=1$ and 2 image resonances; L2: the molecular resonance. The $x$ axis is energy of the nominally unoccupied resonances referenced to the Fermi level. The gray bar on each spectrum represents the relative position of peak center, as expected from the image potential.

Figure 3

(a) Parallel dispersions of the L2 molecular resonance on 1 ML (solid circles) and 2 ML (open circles) ${\mathrm{C}}_{60}$ covered Au(111). The solid and dashed curved are fits to the tight binding approximation, as detailed in the text. (b) Parallel dispersions of the $n=1$ image resonance on 1 ML (solid circles) and 2 ML (open circles) ${\mathrm{C}}_{60}$ covered Au(111). The solid and dashed curves are fits to the tight binding approximation, as detailed in the text.

Figure 4

The energy required, in eV, to promote an electron from the gold $7s^1$ orbital into the $\mathrm{LUMO}+2$ (solid symbols) and $\mathrm{LUMO}+3$ (open symbols) orbitals of ${\mathrm{C}}_{60}$ (triangles) and ${\left({\mathrm{C}}_{60}\right)}_2$ (circles). Results from three levels of theory [B97-1, B3LYP, and B3LYP(ac)] are shown.