Self-Assembled Monolayers of Thiolates on Metals as - Whitesides ...

1142 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
number <strong>of</strong> different junctions confirm that the current
through devices containing SAMs <strong>of</strong> these
molecules depends exponentially on the thickness <strong>of</strong>
the SAM. These results suggest the primary mechanism
<strong>of</strong> conductance through these molecules is
electron tunneling. 4,560,595,608-610
Challenges for Molecular Electronics. Studying
the rates <strong>of</strong> electron transfer and mechanisms for
charge transport in molecules confined in solid-state
junctions presents several challenges that are different
from those familiar from studies <strong>of</strong> electrontransfer
processes in solution or at electrode-solution
interfaces. Some challenges relate to the ambiguity
<strong>of</strong> the structure, orientation, and identity <strong>of</strong> the
species present in the junction after assembly or
fabrication; others relate to uncertainties concerning
the interfaces between the molecules and the electrical
contacts. 611 Still others are connected to the
relatively harsh processing conditions used in fabricating
or testing the devices (thermal evaporation,
intense electric fields) and potential for structural
rearrangements <strong>of</strong> these interfaces (for example,
formation <strong>of</strong> metal filaments bridging two electrodes
612 ); the latter is especially important for junctions
involving gold. The development <strong>of</strong> unambiguous
and predictive models that correlate the structure
<strong>of</strong> individual molecules to their electronic properties
will require research that addresses the factors
described below.
Structure <strong>of</strong> “Complex” SAMs. The majority <strong>of</strong>
the types <strong>of</strong> molecules that are predicted to have
interesting electronic characteristics (insulating, conducting,
rectifying) are not simple alkanethiols; they
contain various organic functional groups, interlocking
rings, branching structures, and organometallic
redox sites. 613 The orientation and arrangement <strong>of</strong>
molecules formed from thiols on gold, silver, palladium,
and platinum are only known (to varying
degrees) for n-alkanethiols, some aromatic compounds
(biphenyls, phenylene ethynylenes), 614 and
related compounds with minor structural variations
(different end groups) (Table 2). SAMs <strong>of</strong> other types
<strong>of</strong> molecules simply are not characterized.
Nature <strong>of</strong> the Interfaces between SAMs and
Their Electrical Contacts. How the number and
type <strong>of</strong> interfaces (van der Waals, chemical bonding,
vacuum, solvent) affect the conductance observed for
molecules in the junctions is unclear. At least one
interface usually involves a metal-thiolate bond
(when a SAM is used as the organic component). The
influence <strong>of</strong> this interface on the electrical behavior
<strong>of</strong> the system is, however, poorly understood, 576
especially for different types <strong>of</strong> metals with substantially
different surface chemistries, e.g., palladium
and silver (section 3.1).
In most junctions there is even less knowledge
regarding the nature <strong>of</strong> the electrical contact between
the SAM and the second electrode attached to the
SAM and its influence on electron transport. It is
evident that evaporation <strong>of</strong> a metal electrode onto the
SAM can have a number <strong>of</strong> detrimental outcomes
including formation <strong>of</strong> metallic filaments and extensive
chemical reactions with the SAM. 612,615 Other
contacts, such as scanning probes and mercury drops,
may be less damaging than evaporated contacts, but
these junctions introduce additional interfaces (SAM/
vacuum/electrode or SAM/solvent/electrode) that also
increase the complexity <strong>of</strong> the system. Rogers and
co-workers reported a procedure for forming s<strong>of</strong>tlaminate
contacts that might serve to eliminate some
<strong>of</strong> the most serious problems encountered with metal
contacts evaporated on top <strong>of</strong> organic thin films. 616
Mechanisms <strong>of</strong> Electron Transport in Metal-
Molecule(s)-Metal Junctions. The basic mechanism
for electron transport in these types <strong>of</strong> systems
is generally thought to involve electron tunneling-a
process where the molecule mediates the electron
transfer but the electron does not reside on the
molecule for any significant period <strong>of</strong> time-or, perhaps
in some cases, resonant tunneling. 559,617 The
subtle details <strong>of</strong> the electron tunneling process remain
controversial, however, especially in molecules
that are more complicated than linear alkane chains.
Also, the degree to which other conduction mechanisms,
such as thermionic emission (transport above
the potential barrier <strong>of</strong> the junction) or electron
hopping (transport where the electron does localize
to specific sites in the molecule during transport),
contribute to the total current remains an open
question. Temperature-dependent current measurements
are a common method for analyzing solid-state
semiconductor devices but rarely are used to analyze
molecule-based devices; these measurements may
provide some mechanistic details <strong>of</strong> how the electrons
are moving through the molecule in the junction.
4,608,618
Perturbations to the Electronic and Vibrational
States <strong>of</strong> Molecules. The effect <strong>of</strong> the intense
electric fields applied in these measurements on the
energy levels within the molecules is not known.
Inelastic tunneling spectroscopy (IETS) is one method
to determine the vibrational states excited in the
molecule during tunneling events and can confirm
the presence <strong>of</strong> the molecule in the junction after it
is formed. 609,619 Unlike IR and Raman spectroscopy,
the selection rules for excitation <strong>of</strong> different modes
are not well understood.
Interpretation <strong>of</strong> I-V Curves for Two-Terminal
Devices. Experimentally observed I-V data are
difficult, if not impossible, to interpret correctly from
two-terminal junctions because small variations in
the electrostatic environment near the junction and
in the electronic and physical couplings between the
molecules and the electrical contacts affect the measured
response. The addition <strong>of</strong> a third electrode (a
gate) to metal-molecule(s)-metal junctions provides
a means to vary the electrostatic environment <strong>of</strong> the
molecule in a controlled manner and eliminates some
<strong>of</strong> the variability common in two-terminal junctions.
605,620
Defects in Junctions. The role <strong>of</strong> defects (both
intrinsic and extrinsic) on the measured electrical
responses has been recognized as a problem (Figure
17). There are, however, no real tools for characterizing
their role in the electron-transfer processes and
no good techniques for reproducing the defects in
experimental systems. Some defects, such as filaments
formed by evaporation, can be eliminated by