Structure and dynamics of C60 molecules on Au(111)

Abstract

Earlier studies of 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 (2Ý3 × 2Ý3)R30°p hase. We have identified the adsorption geometries of the two states that image as dim and bright in STM. These consist of a C 60 molecule with a hexagon side down in a vacancy (hex-vac) and a 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 C 60 molecules, but there is a rearrangement of the substrate near the C 60 molecules. The two types of molecules differ in height, by about 0.7Å, 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 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)-(2Ý3 × 2Ý3)R30°-C 60 . In both systems, the formation of the commensurate 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.

Bicomponent Supramolecular Architectures at the Vacuum–Solid Interface

“…12, 244 In contrast, when the bonding energy is of the same order of magnitude as the diffusion energy, the adsorbate behavior on the surface is more sensitive to temperature effects, and an equilibrium exists between a 2D gas phase on the surface and the gas phase itself. Although surface diffusion of atoms and of small molecules, such as carbon monoxide, is well understood (and often follows a simple single particle hopping model), the problem is significantly more complex in the case of larger molecules 245 because they span several surface adsorption sites, 246 deform during diffusion, 247 interact with surface defects, 248 can induce local surface reconstruction (see below), 249 can diffuse only in specific directions, 250 and can be immobilized or highly mobile depending on the orientation with respect to the surface atomic lattice, 251 among other effects.…”

Observing single-atom diffusion at a molecule-metal interface

“…For an in-depth understanding, it is advantageous to use a scanning tunneling microscope (STM) for imaging because it provides information about the local environment of each individual molecule, which is important since the molecular properties might differ locallydue to defects, step edges, or different areas of a surface reconstruction [4,5]. For instance, individual C 60 molecules were found to change between dark and bright appearances in STM images, caused by the diffusion of surface [6] or bulk [7] vacancies, charge transfer between molecule and surface [8], or adsorption on small metal islands [9].Adatoms are known to diffuse on close-packed metal surfaces if the thermal energy is sufficient for their detachment from step edges [10]. When molecules are adsorbed on such a surface, the adatoms can interact with and even be trapped by molecules [11][12][13][14][15], leading to characteristic metallic nanostructures that reflect the molecular shape [16][17][18].…”

“…For an in-depth understanding, it is advantageous to use a scanning tunneling microscope (STM) for imaging because it provides information about the local environment of each individual molecule, which is important since the molecular properties might differ locallydue to defects, step edges, or different areas of a surface reconstruction [4,5]. For instance, individual C 60 molecules were found to change between dark and bright appearances in STM images, caused by the diffusion of surface [6] or bulk [7] vacancies, charge transfer between molecule and surface [8], or adsorption on small metal islands [9].…”

C60-induced Devil's Staircase transformation on a Pb/Si(111) wetting layer

“…C 60 can eject different number of substrate atoms and generate vacancies of different sizes, i.e., a single vacancy on Al, 17,18 Pt, 19,20 Ag, 21,22 and Au [23][24][25] (111) surfaces while a seven-atom nanopit on Cu(111) 26,27 . Here using density functional theory 28,29 are linear combinations generated from these two phases for 1.20ML < θ < 1.33ML, as θ is increased gradually.…”

“…DFT-LDA calculations have been shown to describe well the C 60 /metal interfaces, to explain successfully the puzzle in work function change on noble metal surfaces after C 60 adsorption, 32,33 and to predict the energetics of the surface reconstructions induced by C 60 , 20,21 and even the different orientations of C 60 at the single vacancy on Ag(111) and Au(111) . 25 To estimate the van der Waals (vdW) interaction in the system 38,47 , DF1-optPBE has been used to check key results. We use a 400 eV kinetic energy cut-off, 12 Å vacuum layer, and ( …”