C O M M U N I C A T I O N S
lone-pair orbital on the ligand also affect the step lengths. Larger
more diffused orbitals give bonds with weaker force constants; thus,
the SMe ligand gives significantly longer steps than the NH2, even
though the two bond strengths are roughly equal.
We have shown that the donor-acceptor bonds between these
ligands and gold result in selective bonding that provides an
unambiguous conductance signature for single-molecule junctions.
The σ-donation from the lone pair to the metal is strongest for the
phosphines, followed by the amines, and weakest for the sulfides,
while π-back-donation from the metal to the ligand is known to be
more significant in phosphines than in sulfides than in amines.10
The trend in the calculated bond strength reflects the balance of
these processes. We have successfully explained trends in the
conductance through amine-gold-linked molecules as mediated by
Au s-N lone-pair σ-states.3 We hypothesize that the increased
availability of ligand d states in sulfides and phosphines leads to a
π-channel for electron transfer through the SMe- and PMe2-linked
junctions. This additional channel accounts for the trends seen in
the contact resistance. In particular, the strongest role for π-back-
donation in the phosphines corresponds to the increased conductivity
seen for phosphines compared with methyl sulfides and amines.
In summary, we have shown that the link group is a rich area
for study of molecular conductance. We find that trialkyl phosphines
have extremely sharp conductance histograms and have the lowest
contact resistance. These results augur well for the preparation of
new types of linkages that go beyond donor-acceptor bonds to
form electrically transparent contacts.
Figure 3. (A) Step-length histograms showing the distribution of lengths
that molecular junctions of butane with PMe2 (blue), SMe (red), and NH2
(green) link groups can be extended. Step-length distributions are determined
by an automated algorithm. Calculated junction formation energy for two
L-Au bonds (B) and applied force (C) versus elongation of model junctions
of the three molecules shown in A. (D) Illustration for butane with PMe2
links shown with 1.4 Å elongation. Gold clusters represent the underco-
ordinated Au contact atom and its environment. The junction is elongated
by stepwise displacing the four highlighted Au atoms on each side, allowing
full relaxation of all other atoms.
What is the origin of these large differences in the step length
distributions for different end groups? To help answer this question,
we have performed density functional theory (DFT)-based calcula-
tions8 for the n-butyl molecules bound to Au clusters (see
Supporting Information). We find that a donor-acceptor bond is
formed through the delocalization of the lone pair on the phosphorus
in PMe2 and the sulfur in SMe to an undercoordinated gold atom
as we had previously shown for the amine link group.3 The
unambiguous peak seen in our data for all three ligands indicates
that this donor-acceptor bond is both selective and well-defined.
We find that the Au-NH2R and Au-SMeR bonds have similar
strengths around 0.6 eV, while the Au-PMe2R bond is significantly
stronger (∼1.2 eV), suggesting that the binding energies alone do
not explain the step-length distributions.
Acknowledgment. This work was supported primarily by the
Nanoscale Science and Engineering Initiative of the National
Science Foundation (NSF) under NSF award number CHE-0641523
and by the New York State Office of Science, Technology, and
Academic Research (NYSTAR). This work was supported in part
by the U.S. Department of Energy, Office of Basic Energy Sciences,
under contract number DE-AC02-98CH10886. M.L.S. thanks the
MRSEC Program of the NSF under award number DMR-0213574.
In a computational experiment, the junctions were elongated
stepwise, allowing the Au contact atoms and the molecule to relax
fully at each step. The resulting junction energy curves and their
derivatives that measure the force applied to the junction at each
step are shown in Figure 3B and C. In each case, at the point of
maximum force, the donor-acceptor bond is elongated and each
contact Au atom begins to relax back toward its parent Au4 cluster.
This implies that the junctions break at a ligand-Au bond. The
calculated maximum force is less than or comparable to the
measured breaking force in Au point contacts (1.5 nN).9 The
elongation from the energy minimum to the force maximum
systematically increases from NH2 to SMe to PMe2 (represented
by horizontal arrows in Figure 3C), consistent with trends in orbital
size and the measured step-length histogram trends. Interestingly,
the force curves all show a flattening at smaller elongation that
corresponds to the point at which the contact Au atom shifts laterally
(arrows in Figure 3D). The force and energy required to shear the
undercoordinated Au atom from a hollow to a bridge site are
small: this is a soft degree of freedom in the junction. The donor-
acceptor bonds discussed here are thus strong enough to move the
contact Au atoms; this surface-atom mobility is one source of the
long steps seen here. Stronger ligand-metal bonds lead to more
metal-surface distortion; therefore, the junctions using phosphine
bridges give the longest steps. However, the size and shape of the
Supporting Information Available: Synthesis procedures, data
analysis, and theoretical methods. This material is available free of
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