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resonance peak height for d-CoPc decreased
by about a factor of 4 (Fig. 2A), but the height
of the d z 2 OMT resonance peak for the intact
CoPc varied by only È15%.
The peak position, the line shape, and the
temperature-dependent peak intensity all suggest
that the resonance near E F
for d-CoPc
molecules likely arises through the Kondo effect.
The good fit of the peak at different temperatures
in the Fano model (22), which has
been successfully applied to surface Kondo
systems to describe the quantum interference
between a localized magnetic impurity and a
continuum (23, 24), further supports the notion
of the Kondo effect (Fig. 2, C to E). The Fano
model here can be described by the relation
dI
dV
ðq þ ẽÞ2
º RðẽÞ º
1 þ ẽ
,whereR is the transi-
2
tion rate, ẽ 0 eV j a
G=2
represents the energy
parameter as a function of the resonance energy
and width, q is the interference parameter that
controls the resonance shape, e is the elementary
charge, a is the energy shift of the
resonance center with respect to E F
,andG 0
2k B
T K
is the width of the resonance, where k B
is the Boltzmann constant and T K
the Kondo
temperature.
At 5 K, fitting all the dI/dV spectra for different
tips and d-CoPc molecules to the Fano
model gives the average values a 0 j4 T 3
meV, G 0 49 T 5 meV, and q 0 j9 T 4 (Fig.
2C). The temperature-dependent resonance
width also shows a
q
good
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
fit to an approximate
formula EG 0 2 ðpk B TÞ 2 þ 2ðk B T K Þ 2 ,
where T is the measurement temperature^ developed
from Fermi liquid theory (25) and
gives a T K
of È208 K (Fig. 2D). The T K
value obtained here is much higher than any
previously reported temperature for magnetic
atoms (23, 24, 26–29) or clusters (30)
on surfaces. For comparison, we also studied
CuPc molecules, which have a nonmagnetic
ion center, in contrast with CoPc. The
CuPc molecules adsorbed on a Au(111) surface
can also be dehydrogenated by the same
method. The central part of an STM image
of the CuPc molecule is a hole (Fig. 2B),
but it is a protrusion in a fully dehydrogenated
CuPc (d-CuPc), and there is no noticeable
resonance appearing near E F
in the
dI/dV spectra.
In order to understand qualitatively our
experimental observations, we carried out
Fig. 2. Kondo resonance of d-CoPc at different temperatures. (A) TypicaldI/dV spectra measured at the
centers of a CoPc molecule at 5 K (black line), showing a d z 2 OTM resonance, and a d-CoPc molecule at
5, 90, and 150 K (colored lines), showing strong resonance near E F
. Spectra from bare Au(111) (gray line)
is shown for comparison. (B) Topographic three-dimensional view of CuPc and d-CuPc, together with
the corresponding dI/dV spectra measured at their centers. All spectra in (A) and (B) were taken with
the same set point of V 0 600 mV and I 0 0.4 nA. (C) A fit (red line) to the resonance at 5 K in (A)
according to the Fano model, with parameters of width È 44 meV, q Èj6, and a Èj5 meV.
Black symbols indicate experimental results. (D) The resonance width against measured
temperature. Error bars represent standard deviations. (E) The temperature-dependent height of
the Kondo resonance peak, which decreases approximately logarithmically from 20 to 150 K and
becomes nearly saturated at lower temperatures.
first-principles studies on the structural and
electronic properties of CoPc and d-CoPc molecules
adsorbed on Au(111) (31). We used a
slab model for the adsorption system, consisting
of three atomic layers with 56 Au atoms
each for the Au substrate and a vacuum seven
atomic layers thick (Fig. 3, A and B). The
distance between the molecule and the gold
substrate is È3.0 ). The interaction between
the molecule and substrate clearly changes
the electronic structure and magnetic property
of the CoPc molecule. In a free CoPc
molecule, the Co atom has unpaired d electrons
and the magnetic moment of the Co
atom is 1.09 Bohr magnetons (m B
). In the CoPc
adsorption system, the magnet moment is completely
quenched by the molecule-substrate interaction.
The spin-polarized partial density
of states (PDOS) of the Co atom in the CoPc
adsorption system (Fig. 3C), and in a free
CoPc molecule (Fig. 3D), revealed that the
spin-down states were filled more than the
spin-up states for the free CoPc molecule.
However, the filling difference disappeared for
the CoPc adsorbed on Au(111). The theoretical
STM image of a CoPc molecule on Au(111)
simulated with the Tersoff-Hamann formula
(32) (Fig. 3E) reproduces the main feature of
the experimental image (Fig. 1D).
Dehydrogenation induces a marked change
of the molecular structure (Fig. 4, A and B),
so that the d-CoPc molecule on Au(111) is
no longer planar. The smallest separation
between the end C atoms of the benzene
ring and the gold substrate is È1.9 ), leading
to a much stronger binding to the gold
substrate. The central Co atom in the d-
CoPc molecule shifts upward remarkably (the
d Co-Au
distance is È3.8 ) for d-CoPc but
3.0 ) for CoPc).
More importantly, the magnetic moment
is recovered for the d-CoPc adsorption system.
The spin-polarized PDOS of the Co atom
in the d-CoPc adsorption system (Fig. 4C) near
E F
has an empty minority spin peak that comes
from the magnetic quantum number m 0 0(d z 2 )
states. This peak is consistent with our experimental
spectra measured at different temperatures,
in which an observable peak appears
near 135 meV (fig. S3). The magnetic moment
of the d-CoPc molecule is now 1.03 m B
,very
close to the value of a free CoPc molecule.
The simulated STM image with a large bright
spot for the d-CoPc adsorption system (Fig.
4D) agrees quite well with the observed image
(Fig. 1H).
To understand the high Kondo temperature
in the d-CoPc/Au(111) system, we compared
its PDOS with that of a single Co adatom on
an Au(111) surface (33) (Fig. 4E). The average
spin splitting of the d-CoPc/Au(111) system
is smaller than that of Co/Au(111). The
on-site Coulomb repulsion U is proportional to
this splitting, so the U of the d-CoPc/Au(111)
system is smaller than that of the Co/Au(111)
www.sciencemag.org SCIENCE VOL 309 2 SEPTEMBER 2005 1543