Orientation of individual ${\mathrm{C}}_{60}$ molecules adsorbed on Cu(111): Low-temperature scanning tunneling microscopy and density functional calculations

Density functional theory (DFT) and low-temperature scanning tunneling microscopy (STM) have been combined to examine the bonding of individual ${\mathrm{C}}_{60}$ molecules on Cu(111). Energy-resolved differential-conductance maps have been measured for individual ${\mathrm{C}}_{60}$ molecules adsorbed on a Cu(111) surface by means of low-temperature STM, which are compared to and complemented by theoretically computed spectral images. It has been found that ${\mathrm{C}}_{60}$ chemisorbs with a six-membered ring parallel to the surface at two different Cu(111) binding sites that constitute two exclusive hexagonal sublattices. On each sublattice, ${\mathrm{C}}_{60}$ is bonded in one particular rotational conformer, i.e., ${\mathrm{C}}_{60}$ molecules bind to the Cu(111) surface in two different azimuthal orientations differing by 60° depending on which sublattice the binding site belongs to. The binding conformation of ${\mathrm{C}}_{60}$ and its orientation with regard to the copper surface can be deduced by this joint experimental-theoretical approach. Six possible pairs of ${\mathrm{C}}_{60}$ configurations on three different Cu surface binding sites have been identified that fulfil the requirements of the two sublattices and are consistent with all experimental and theoretical data. Theory proposes that two of these configuration pairs are most likely. We have found that DFT does not get the binding energy between rotational conformers in the correct order. We also report two different ${\mathrm{C}}_{60}$ monolayers on Cu(111): one with alternating orientations of neighboring molecules at low temperature and the other with $(4\times 4)$ structure after annealing above room temperature.

Figure 1

(Color) Model structure for ${\mathrm{C}}_{60}$ bonded to a Cu(111) surface in top and side views. The copper clusters are composed of 55 atoms. The sum of the Mulliken charges per layer of ${\mathrm{C}}_{60}$ when bonded to copper are given next to the side view representation.

Figure 2

Individual ${\mathrm{C}}_{60}$ molecules on Cu(111). The image is obtained with a high-pass (Laplace) filter enhancing the intramolecular resolution. The ${\mathrm{C}}_{60}$ molecules show a threefold rotational symmetry and two different orientations with respect to the copper substrate. The inset shows the topography image for a single molecule displayed without high-pass filtering (tunneling parameters $I=3.8\times {10}^{-11}\mathrm{A}$ and $V=1.7\mathrm{V}$).

Figure 3

(Color) All binding configurations of ${\mathrm{C}}_{60}$ on the on-top, hcp-hollow, and fcc-hollow sites of the Cu(111) surface. The structure of the bottom of ${\mathrm{C}}_{60}$ (black lines) with regard to Cu(111) (orange balls, Cu atoms; dark balls, top surface Cu atoms; and light balls, second layer Cu atoms) is shown. The structure of the top of ${\mathrm{C}}_{60}$ relates to the bottom structure by 60° rotation.

Figure 4

$dI∕dV$ images of ${\mathrm{C}}_{60}$ acquired at constant current mode for a set of voltages (V) are shown in the middle two rows, and the simulated counterparts with Kohn–Sham orbital energies (eV) are in the top and bottom row. Fermi level is at $0\mathrm{V}$ (eV). The calculated images were obtained for the on-top site, but very similar images have been computed for the hcp-hollow site.

Figure 5

(Color) Local $dI∕dV$ spectroscopy at the center (blue) and at two off-center (dotted red and green lines) positions of a ${\mathrm{C}}_{60}$ molecule. The two off-center positions were $5\mathrm{\AA }$ away from the center in the direction of the pentagons and hexagons of the upper part of the cage, respectively. For easy reference, these positions are marked in the $dI∕dV$ image of the inset. The vertical tip position corresponds to feedback parameters of $I=65\mathrm{pA}$ at $V=+2155\mathrm{mV}$.

Figure 6

(Color) A single ${\mathrm{C}}_{60}$ molecule has been manipulated using the STM tip, approaching ${\mathrm{C}}_{60}$ in a succession of manipulations from all sides and angles, and for each movement, the relative position and orientation of the cage has been recorded. The position has been determined with respect to another ${\mathrm{C}}_{60}$ serving as a marker (located at $x=0$ and $y=0$). The blue and red triangles show the resulting positions and orientations. The corners of the triangles point in the directions of the five-membered rings in the upper half of the ${\mathrm{C}}_{60}$ cage. The black dots map out the majority (preferred) hexagonal sublattice as a guide for the eye.