Ordering and dynamical properties of superbright C${}_{60}$ molecules on Ag(111)

C${}_{60}$ monolayers grown on Ag(111) at room temperature form incommensurate lattices that convert into a commensurate (2$\sqrt{3}$ × 2$\sqrt{3}$)R30° phase upon annealing. The C${}_{60}$ molecules in the commensurate phase have been observed to exist in three different states on Ag(111), namely bright, dim, and superbright (SB). All three species are in dynamical equilibrium at 280 < $T$ < 440 K. The bright and dim species were the subject of an earlier low-energy electron diffraction study that determined their geometries on the surface and the dynamics of the switching between those two states. The study presented here takes a closer look at the SB molecules, which appear and disappear at temperature-dependent rates with a measured activation barrier of 1.5 eV. The SB molecules in the commensurate phase comprise about 0.5% of the molecules and have a spatially random distribution. The evidence suggests that the formation of the three different states of C${}_{60}$ on Ag(111) is a result of stress imposed by the substrate as the C${}_{60}$ adopts the commensurate spacing that is slightly smaller than its natural spacing. In the incommensurate phases, there is no bright-dim contrast, but SB C${}_{60}$ molecules form and organize into ordered arrays that appear to correspond to the moiré patterns that are produced by the mutually incommensurate lattices. This suggests that the substrate responds to the nonuniform forces imposed by the C${}_{60}$ molecules by producing raised islands of Ag atoms at the vertices of the moiré structure. A similar island structure may account for the SB molecules in the commensurate phase.

Figure 1

(a) STM image (125  ×  142 nm${}^2$, +2V, 0.06 nA) at 0.2 ML coverage of C${}_{60}$ on Ag (111), taken at room temperature, showing that it grows from step edges in both directions, and that different islands have different characteristics. (b) Magnified view of the rectangular zone shown in (a).

Figure 2

STM image (47 × 80 nm${}^2$, +2 V, 0.1 nA), taken at room temperature, of 1 ML coverage of C${}_{60}$ after postannealing the sample to 423 K. Some regions (O) show ordered SB, others (D) show disordered SB and bright-dim relief. Others (E) show no bright-dim contrast or SB.

Figure 3

Left: (a) STM image, taken at room temperature after annealing to 423 K, at negative bias (34 × 38 nm${}^2$, −1 V, 0.3 nA) and with two different height profiles showing bright-dim contrast (I) and SB height (II). (b) Same as (a) but for positive bias (34 × 38 nm${}^2$, 1 V, 0.3 nA). Zero height corresponds to the mean position of the layer.

Figure 4

STM image (69 × 100 nm${}^2$, +2 V, 0.1 nA) showing a region of the surface having a majority of R30° disordered (D) domains and a minority of R30° domains (E) with no SB or bright-dim contrast. The surface has been annealed to 688 K after C${}_{60}$ deposition, the image was taken at room temperature.

Figure 5

(a) Consecutive 80 × 80 nm${}^2$ STM images (−2.5 V, 0.06 nA) taken 273 s apart, showing the activity of the SB molecules at $T$ = 407 K. The circles show the locations of appearance and disappearance of SB. (b) Appearance and disappearance rates, per molecule, of the SB as a function of temperature. The slope corresponds to an energy barrier of 1.5 ± 0.1 eV. (c) Two consecutive STM images (+2 V, 0.1 nA, 18 × 22 nm${}^2$) taken at 300 K, showing that in this instance, an SB is replaced by a bright. Note that at 300 K, the kinetics of the SB conversion is very slow and only one event occurs in the images shown, but the bright-dim flipping, which has an activation energy of 0.84 eV, can be observed [7].

Figure 6

Sections of three of the STM images in Fig. 5, taken at 407 K, showing two immobile vacancies in the C${}_{60}$ lattice, and two SB molecules in each frame.

Figure 7

A sequence of consecutive 57.2 × 57.2 nm${}^2$ images, 96 s apart, from the commensurate (2$\sqrt{3}$ × 2$\sqrt{3}$)R30° phase taken at a sample temperature of 332 K as In is dosed onto the surface. The In has apparently already migrated to the step edge in scan 1 and has begun to diffuse across the lower terrace, but not the upper terrace. As it progresses, the dim and SB molecules disappear.

Figure 8

Comparison of STM image with calculated moiré images using the structural parameters measured from the STM images. (a) STM image taken at $T$ = 50 K (−2 V and 0.21 nA) for an incommensurate structure of C${}_{60}$ on Ag(111) that shows the clear moiré formation. The domain to the right has a different lattice orientation and also shows a moiré effect but not as strongly for these parameters. (b) Section of the STM image taken at RT, from Fig. 2. The moiré is not as obvious, but the SBs have a similar arrangement as in (a). (c) A moiré generated by graphically overlapping gray-scale C${}_{60}$ molecules with gray-scale Ag atoms based on the structure in (b), and applying smoothing. (d) The same moiré with a threshold applied, so that only the brightest parts of the moiré are visible.

Figure 9

Schematic drawing of possible configurations of additional Ag atoms beneath a SB molecule. Gray atoms are the top layer of the Ag substrate, darker gray denotes the locations of the atoms or vacancies that are occupied by C${}_{60}$ molecules. The blue (darker gray) shades of the Ag atoms beneath the central C${}_{60}$ refer to the possible additional Ag atoms beneath the SB; different shading denotes the presence of 3, 6, or 12 extra atoms. Note that an annular island (e.g., remove the darkest blue atoms) is also a possible scenario.