Structure and dynamics of ${\text{C}}_{60}$ molecules on Au(111)

Earlier studies of ${\text{C}}_{60}$ adsorption on Au(111) reported many interesting and complex features. We have performed coordinated low-energy electron diffraction, scanning tunneling microscopy (STM), and density functional theory studies to elucidate some of the details of the monolayer commensurate (2√3 × 2√3)R30° phase. We have identified the adsorption geometries of the two states that image as dim and bright in STM. These consist of a ${\text{C}}_{60}$ molecule with a hexagon side down in a vacancy (hex-vac) and a ${\text{C}}_{60}$ molecule with a carbon-carbon 6:6 bond down on a top site (6:6-top), respectively. We have studied the detailed geometries of these states and find that there is little distortion of the ${\text{C}}_{60}$ molecules, but there is a rearrangement of the substrate near the ${\text{C}}_{60}$ molecules. The two types of molecules differ in height, by about 0.7 Å, which accounts for most of the difference in their contrast in the STM images. The monolayer displays dynamical behavior, in which the molecules flip from bright to dim, and vice versa. We interpret this flipping as the result of the diffusion of vacancies in the surface layers of the substrate. Our measurements of the dynamics of this flipping from one state to the other indicate that the activation energy is 0.66 ± 0.03 eV for flips that involve nearest-neighbor ${\text{C}}_{60}$ molecules, and 0.93 ± 0.03 for more distant flips. Based on calculated activation energies for vacancies diffusing in Au, we interpret these to be a result of surface vacancy diffusion and bulk vacancy diffusion. These results are compared to the similar system of Ag(111)-(2√3 × 2√3)R30°-${\text{C}}_{60}$. In both systems, the formation of the commensurate ${\text{C}}_{60}$ monolayer produces a large number of vacancies in the top substrate layer that are highly mobile, effectively melting the interfacial metal layer at temperatures well below their normal melting temperatures.

Figure 1

STM images recorded at sample voltages between 2 and 2.2 V of ${\text{C}}_{60}$ islands on Au(111). (a),( b) $6.3\times 3.6{\mathrm{nm}}^2$ images of two different domains of a (2√3 × 2√3)R30° superlattice. (c) $6.3\times 4.7{\mathrm{nm}}^2$ image of the (7 × 7)R0° structure. (d) $11.2\times 11.2{\mathrm{nm}}^2$ image of a (√589x√589)R14.5° structure. Yellow rhombuses indicate the unit cells of the superstructures.

Figure 2

Constant-current images recorded at Vs = 2 V. (a) 34 × 17 nm${}^2$ overview. (b),(c) 3.7 × 3.7 nm${}^2$ subimages of (a) from the areas indicated by white squares showing (√589x√589)R14.5° and (2√3 × 2√3)R30° β-type superstructures. (d) Cross-sectional profile of (b) and (c) measured along the indicated lines. Short arrows highlight the molecules that were used to determine height differences.

Figure 3

LEED patterns from the Au(111)-(2√3 × 2√3)R30°-${\text{C}}_{60}$ structure at 80 K for electron energies of (a) 82, (b) 119, and (c) 275 eV. Representative first-order spots from the substrate and the ${\text{C}}_{60}$ structure are indicated by gray annuli. These patterns are imaged on a flat plane and therefore the patterns appear distorted relative to a perfect hexagonal symmetry.

Figure 4

Representative LEED spectra from the best-fit mixture of 80% hex-par vac and 20% 6:6 C-C top for ${\text{C}}_{60}$ on Au(111). The full data set is provided in Ref [37].

Figure 5

(4√3 × 2√3)R30° surface supercell used in the DFT calculation for mixed configurations of ${\text{C}}_{60}$ on Ag(111) and Au(111). Top (bottom) panel is for top (side) view. The left (right) ${\text{C}}_{60}$ sits on a vacancy (top) site with a hexagon (short C-C bond) facing down to the surface. Δ$z$ labels the height difference between the two ${\text{C}}_{60}$ molecules on different sites. This is the starting/reference orientation of the system for the energy-orientation plot in Fig. 6. The orientation change of the right ${\text{C}}_{60}$ involves the anticlockwise rotation along the $z$ axis, and three tilt angles for each rotation.

Figure 6

Adsorption energy per ${\text{C}}_{60}$ vs tilt angle of the right ${\text{C}}_{60}$ at different rotational angles (see Fig. 5) on (a) Ag(111) and (b) Au(111). The energy scale is relative to the hex-vac + hex-top configuration (dotted horizontal line).

Figure 7

Simulated STM image of the mixed configurations at +1 V for the right ${\text{C}}_{60}$ (a) at 120° with tilt angle of 4.6° on Ag(111) and (b) at 90° on Au(111) with tilt angle of −11.5° (see Fig. 5).

Figure 8

(a) 40 × 40 nm${}^2$ STM image of Au(111)- (2√3 × 2√3)R30°-${\text{C}}_{60}$ at T = 272 K and tunneling parameters I = 0.09 nA, V = +2.2 V. Inset: Difference between the image shown and the previous one, 69 s earlier. (b) 40 × 40 nm${}^2$ STM image of same surface at T = 298 K and I = 0.06 nA, V = +2.3 V. The inset showing the difference of two successive frames indicates a higher rapid flipping rate at higher temperature. (c) Difference between two successive STM images (40 × 40 nm${}^2$) of Au(111)- (2√3 × 2√3)R30°-${\text{C}}_{60}$ at T = 280 K recorded 81 s apart. Examples of NN, NNN, and uncorrelated flips are outlined. (d) Temperature dependence of the fraction of NN, NNN, and uncorrelated flips on Au(111)- (2√3 × 2√3)R30°-${\text{C}}_{60}$. (e) Plot of the natural logarithm of the bright to dim or dim to bright flipping rate (events/nm${}^2$ s) as a function of inverse temperature for NN flips and other flips. The slopes of the graph indicate activation energies of 0.66 ± 0.03 eV and 0.93 ± 0.03 eV, respectively.