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

1106 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
tradense memories, organic electronics, new classes
<strong>of</strong> biosensors, and electronic devices based on quantum
effects.
1.2. Surfaces and Interfaces in Nanoscience
One distinguishing characteristic <strong>of</strong> nanometerscale
structures is that, unlike macroscopic materials,
they typically have a high percentage <strong>of</strong> their constituent
atoms at a surface. The volume <strong>of</strong> an object
(V ∝ l 3 , where l is the characteristic length) decreases
more quickly than its surface area (S ∝ l 2 ) as the size
diminishes: S/V ∝ l -1 , where l has atomic or molecular
dimensions. This scaling behavior leads, in the
most extreme case, to structures where nearly every
atom in the structure is interfacial. In some sense,
nanostructures are “all surface”. 16
We believe that surfaces represent a fourth state
<strong>of</strong> matter-they are where the gradients in properties
are greatest. (In bulk phases <strong>of</strong> matter-gas, liquid,
solid-the gradients are usually zero.) Atoms or
molecules at the surface <strong>of</strong> a material experience a
different environment from those in the bulk and
thus have different free energies, electronic states,
reactivities, mobilities, and structures. 17,18 The structure
and chemical composition within macroscopic
objects determines many physical properties, e.g.,
thermal and electrical conductivity, hardness, and
plasticity. In contrast, the physical properties <strong>of</strong>
nanostructures depend to a much greater extent on
their surface and interfacial environment than do
bulk materials.
1.3. SAMs and Organic Surfaces
Bare surfaces <strong>of</strong> metals and metal oxides tend to
adsorb adventitious organic materials readily because
these adsorbates lower the free energy <strong>of</strong> the
interface between the metal or metal oxide and the
ambient environment. 18 These adsorbates also alter
interfacial properties and can have a significant
influence on the stability <strong>of</strong> nanostructures <strong>of</strong> metals
and metal oxides; the organic material can act as a
physical or electrostatic barrier against aggregation,
decrease the reactivity <strong>of</strong> the surface atoms, or act
as an electrically insulating film. Surfaces coated
with adventitious materials are, however, not welldefined:
they do not present specific chemical functionalities
and do not have reproducible physical
properties.(e.g., conductivity, wettability, or corrosion
resistance).
<strong>Self</strong>-assembled monolayers (SAMs) provide a convenient,
flexible, and simple system with which to
tailor the interfacial properties <strong>of</strong> metals, metal
oxides, and semiconductors. SAMs are organic assemblies
formed by the adsorption <strong>of</strong> molecular
constituents from solution or the gas phase onto the
surface <strong>of</strong> solids or in regular arrays on the surface
<strong>of</strong> liquids (in the case <strong>of</strong> mercury and probably other
liquid metals and alloys); the adsorbates organize
spontaneously (and sometimes epitaxially) into crystalline
(or semicrystalline) structures. The molecules
or ligands that form SAMs have a chemical functionality,
or “headgroup”, with a specific affinity for a
substrate; in many cases, the headgroup also has a
high affinity for the surface and displaces adsorbed
adventitious organic materials from the surface.
There are a number <strong>of</strong> headgroups that bind to
specific metals, metal oxides, and semiconductors
(Table 1). The most extensively studied class <strong>of</strong> SAMs
is derived from the adsorption <strong>of</strong> alkanethiols on
gold, 19-27 silver, 26,28,29 copper, 26 palladium, 30,31 platinum,
32 and mercury. 33 The high affinity <strong>of</strong> thiols for
the surfaces <strong>of</strong> noble and coinage metals makes it
possible to generate well-defined organic surfaces
with useful and highly alterable chemical functionalities
displayed at the exposed interface. 23,34
1.4. SAMs as Components <strong>of</strong> Nanoscience and
Nanotechnology
SAMs are themselves nanostructures with a number
<strong>of</strong> useful properties (Figure 1). For example, the
thickness <strong>of</strong> a SAM is typically 1-3 nm; they are the
most elementary form <strong>of</strong> a nanometer-scale organic
thin-film material. The composition <strong>of</strong> the molecular
components <strong>of</strong> the SAM determines the atomic composition
<strong>of</strong> the SAM perpendicular to the surface; this
characteristic makes it possible to use organic synthesis
to tailor organic and organometallic structures
at the surface with positional control approaching
∼0.1 nm. SAMs can be fabricated into patterns
having 10-100-nm-scale dimensions in the plane <strong>of</strong>
a surface by patterning using microcontact printing
(µCP), 130,131 scanning probes, 132-134 and beams <strong>of</strong>
photons, 135-138 electrons, 139 or atoms. 140,141 Phaseseparated
regions in SAMs comprising two or more
constituent molecules can have ∼100-nm 2 -scale dimensions.
142
SAMs are well-suited for studies in nanoscience
and technology because (1) they are easy to prepare,
that is, they do not require ultrahigh vacuum (UHV)
or other specialized equipment (e.g., Langmuir-
Blodgett (LB) troughs) in their preparation, (2) they
form on objects <strong>of</strong> all sizes and are critical components
for stabilizing and adding function to preformed,
nanometer-scale objectssfor example, thin
films, nanowires, colloids, and other nanostructures,
(3) they can couple the external environment to the
electronic (current-voltage responses, electrochemistry)
and optical (local refractive index, surface
plasmon frequency) properties <strong>of</strong> metallic structures,
and (4) they link molecular-level structures to macroscopic
interfacial phenomena, such as wetting,
adhesion, and friction.
1.5. Scope and Organization <strong>of</strong> the Review
This review focuses on the preparation, formation,
structure, and applications <strong>of</strong> SAMs formed from
alkanethiols (and derivatives <strong>of</strong> alkanethiols) on gold,
silver, copper, palladium, platinum, mercury, and
alloys <strong>of</strong> these metals. It emphasizes advances made
in this area over the past 5 years (1999-2004). It
does not cover organic assemblies formed by Langmuir-Blodgett
techniques, 143 from alkylsiloxanes
and alkylsilanes, 144 or from surfactants adsorbed on
polar surfaces. 145 The objectives <strong>of</strong> this review are as
follows: (1) to review the structure and mechanism
<strong>of</strong> formation <strong>of</strong> SAMs formed by adsorption <strong>of</strong> n-