Supplementary material
Estimation of Cu atom density at the C60/Cu(111) interface by in-situ monitoring of C60
growth at 400 K
The number of missing Cu atoms, N, in a (4 × 4) unit cell can be obtained from a
mass-flow analysis by in-situ STM monitoring of C60 growth. This number is of critical
importance because it allows us to readily rule out implausible models. We discovered
that, in the initial stage, Cu atoms were ejected by C60 adsorbed at the upper step edges.
These extracted adatoms were subsequently captured through intercalation by C60
adsorbed at the lower step edges. Consequently, the (4 × 4) C60 region grows from both
sides of a step but with identical height[1] as opposed to the bi-directional step flow C60
growth reported previously[2]. This “adatom recycling” behavior permits a rather
accurate estimate of the Cu adatom density at the reconstructed interface, which helps to
exclude implausible models. In Fig. S1, we prepared many monolayer-deep vacancy
islands on a large terrace of width > 1 m . We used ~500 eV neon ions to briefly sputter
(~ 10 - 60 s total time) the Cu surface with subsequent brief annealing at T~400 K.
Because of the large terrace width, the Cu mass flow is captured solely by nearby
vacancies and the total Cu mass is thus fixed. By analyzing the vacancy boundary
propagation during C60 growth, we established a simple relation between the grown C60
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areas interior (Si) and exterior (Se) to the original vacancy boundary (Vb). That is,
S
e
 N   Si  (16  N ) , where N is the number of Cu atoms removed in each
unreconstructed (4 × 4) unit cell. From a series of ~ 20 images, we calculated N from
image pairs and obtained N=7.1±0.7.
Figure S1
At 400 K the interface already fully reconstructs. Identical C60 areas grow
from both sides of the vacancy boundary (Vb), through transporting ejected Cu adatoms
from the C60 region exterior to the vacancies (Se) to the C60 region interior to the
vacancies (Si). a, C60 coverage at 0.05 ML. b, C60 coverage at 0.30 ML. Image size: 500
nm × 500 nm. From a series of ~ 20 images like those presented here, we calculated N
using the formula
S
e
 N   Si  (16  N ) , from image pairs. In the upper right inset, the
horizontal axis (DATA) indexes the time difference (number of image frames) of the
calculated image pairs. We obtained N=7.1±0.7.
Structure of a stacked monolayer C60 island
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Figure S2
A monolayer C60 island can show stacked layers because of coexisting
reconstructed and unreconstructed interfaces. There are three distinct regions A, B, C. A
line profile passing region C-A-B is shown. Image size: 50 nm × 50 nm. Image taken at
77 K. The C60 island was prepared at room temperature.
Figure S2 illustrates a stacked monolayer C60 island and the STM topographic height
line profile of three regions; (A) C60 region with reconstructed interface, (B) C60 region
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with unreconstructed interface, and (C) C60 region with reconstructed interface plus one
additional Cu layer underneath. The topographic height in area B is ~ 0.4 Å slightly
lower than that in area C, because a C60 in a 7-atom vacancy hole cannot "sink" by a full
monolayer step height. The STS spectra at regions A and C are identical, but are different
from the spectra at region B.
Ab-initio calculation details
We performed ab initio band structure calculations using the highly accurate
full-potential projected augmented wave method, as implemented in the VASP package,
within the local density approximation with generalized gradient correction (GGA). The
unreconstructed and reconstructed systems are modeled by a (4 × 4) C60 monolayer at hcp
or fcc sites on a Cu(111) slab (thickness ranging from 8 to 30 Cu layers). The periodic
slabs are separated by 10 Å vacuum slabs. The very thick Cu slab (e.g., 30 layers) is
needed to extract the C60 band dispersion and obtain convergence of the r-fcc and r-hcp
model energy difference. The calculations were performed on a 41 k-point mesh over the
irreducible Brillouin zone, using 450,000 plane waves with a cut-off energy of 400 eV
and with full lattice relaxation. The LUMO-derived and HOMO bands were extracted by
analyzing the carbon-projected electron density. The calculated bands, depicted as the
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white circles and white dashed guide line in Fig. 1b, are aligned to EF and scaled in such
a way to match the measured and calculated HOMO band energy.
Comparison of best-fit LEED model and the best-energy model
Our best-fit LEED is calculated with Barbieri/Van Hove SATLEED package(see
http://www.ap.cityu.edu.hk/personal-website/Van-Hove_files/leed/leedpack.html).The
calculated coordinates perpendicular to the surface differ from the best-energy model by
~0.05 Å to 0.1 Å for C60 and ~0.13 Å to 0.2 Å for Cu, consistent with the decreasing
LEED sensitivity with depth caused by electron damping; parallel to the surface, we
found differences of ~0.05 Å to 0.2 Å, mainly because LEED is less sensitive to parallel
coordinates. We note that the best-energy model is obtained for temperature T=0 K and
the best-fit LEED model is for T=300 K.
Energy difference between models of reconstructed and unreconstructed C60/Cu(111)
interfaces
The total energy in the reconstructed and unreconstructed cases should be compared
with care since they are of different system sizes. The adsorption energies are ~3.0
eV/cell and ~0.8 eV/cell, respectively. The energy cost for substrate reconstruction,
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estimated as the surface energy difference (~2.0 eV/cell) in both cases, is subtracted from
the ~3.0 eV/cell value. Therefore, the reconstructed model is favored by ~0.2 eV/cell
over the unreconstructed case.
[1] C. H. Lin et al., Journal of Nanoscience and Nanotechnology 8, 602 (2008).
[2] J. C. Dunphy et al., Surface Science 383, L765 (1997).
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