Formation and Structure of Self-Assembled Monolayers
Abraham Ulman
Department of Chemical Engineering, Chemistry and Materials Science, and the Herman F. Mark Polymer Research Institute, Polytechnic University,
Six MetroTech Center, Brooklyn, New York 11201
Received November 3, 1995 (Revised Manuscript Received April 18, 1996)
Contents
I. Introduction 1533
II. Self-Assembled Monolayers 1534
1. Monolayers of Fatty Acids 1534
2. Monolayers of Organosilicon Derivatives 1535
3. Organosulfur Adsorbates on Metal and
Semiconductor Surfaces
1539
4. Alkyl Monolayers on Silicon 1543
5. Multilayers of Diphosphates 1544
III. Competing Interactions in the Formation of
Self-Assembled Monolayers
1545
1. Why are Alkanethiolates on Au(111) and
Ag(111) Different?
1545
2. The Interlocking of Molecular Parts 1546
3. Alkanethiolates on Au(100): A Different
Symmetry
1546
5. Specific Intermolecular Interactions in SAMS 1548
6. Surface Engineering Using SAMs 1549
IV. Conclusions 1551
V. References 1552
I. Introduction
The field of self-assembled monolayers (SAMs) has
witnessed tremendous growth in synthetic sophisti-
cation and depth of characterization over the past 15
years.
1
However, it is interesting to comment on the
modest beginning and on important milestones. The
field really began much earlier than is now recog-
nized. In 1946 Zisman published the preparation of
a monomolecular layer by adsorption (self-assembly)
of a surfactant onto a clean metal surface.
2
At that
time, the potential of self-assembly was not recog-
nized, and this publication initiated only a limited
level of interest. Early work initiated in Kuhn’s
laboratory at Go¨ttingen, applying many years of
experience in using chlorosilane derivative to hydro-
phobize glass, was followed by the more recent
discovery, when Nuzzo and Allara showed that SAMs
of alkanethiolates on gold can be prepared by adsorp-
tion of di-n-alkyl disulfides from dilute solutions.
3
Getting away from the moisture-sensitive alkyl trichlo-
rosilanes, as well as working with crystalline gold
surfaces, were two important reasons for the success
of these SAMs. Many self-assembly systems have
since been investigated, but monolayers of alkane-
thiolates on gold are probably the most studied SAMs
to date.
The formation of monolayers by self-assembly of
surfactant molecules at surfaces is one example of
the general phenomena of self-assembly. In nature,
self-assembly results in supermolecular hierarchical
organizations of interlocking components that pro-
vides very complex systems.
4
SAMs offer unique
opportunities to increase fundamental understanding
of self-organization, structure-property relation-
ships, and interfacial phenomena. The ability to
tailor both head and tail groups of the constituent
molecules makes SAMs excellent systems for a more
fundamental understanding of phenomena affected
by competing intermolecular, molecular-substrates
and molecule-solvent interactions like ordering and
growth, wetting, adhesion, lubrication, and corrosion.
That SAMs are well-defined and accessible makes
them good model systems for studies of physical
chemistry and statistical physics in two dimensions,
and the crossover to three dimensions.
SAMs provide the needed design flexibility, both
at the individual molecular and at the material
levels, and offer a vehicle for investigation of specific
interactions at interfaces, and of the effect of increas-
ing molecular complexity on the structure and stabil-
ity of two-dimensional assemblies. These studies
may eventually produce the design capabilities needed
for assemblies of three-dimensional structures.
5
How-
ever, this will require studies of more complex
systems and the combination of what has been
learned from SAMs with macromolecular science.
The exponential growth in SAM research is a
demonstration of the changes chemistry as a disci-
Abraham Ulman was born in Haifa, Israel, in 1946. He studied chemistry
in the Bar-Ilan University in Ramat-Gan, Israel, and received his B.Sc. in
1969. He received his M.Sc. in phosphorus chemistry from Bar-Ilan
University in 1971. After a brief period in industry, he moved to the
Weizmann Institute in Rehovot, Israel, and received his Ph.D. in 1978 for
work on heterosubstituted porphyrins. He then spent two years at
Northwestern University in Evanston, IL, where his main interest was one-
dimensional organic conductors. In 1985 he joined the Corporate
Research Laboratories of Eastman Kodak Company, in Rochester, NY,
where his research interests were molecular design of materials for
nonlinear optics and self-assembled monolayers. In 1994 he moved to
Polytechnic University where he is the Alstadt-Lord-Mark Professor of
Chemistry. His interests encompass self-assembled monolayers, surface
engineering, polymers at interface, and surfaces phenomena.
1533Chem. Rev. 1996, 96, 1533−1554
S0009-2665(95)00235-4 CCC: $25.00 1996 American Chemical Society

pline has been going through. Chemistry has been
moving away from traditional disciplines and into
interdisciplinary areas, and chemists are engaged in
research at the interface of chemistry with physics,
biology, and engineering. The fabrication and ma-
nipulations of molecular assemblies, molecular rec-
ognition, biomineralization, hierarchical structure
and function, and computational chemistry to eluci-
date structure-function relationships have become
central themes in modern chemistry. These impor-
tant changes can find their origin partly in the areas
of Langmuir-Blodgett and self-assembled monolay-
ers, which continue to serve as major techniques for
the fabrication of supramolecular structures.
The interest in the general area of self-assembly,
and specifically in SAMs, stems partially from their
perceived relevance to science and technology. In
contrast to ultrathin films made by, for example,
molecular beam epitaxy (MBE), and chemical vapor
deposition (CVD), SAMs are highly ordered and
oriented and can incorporate a wide range of groups
both in the alkyl chain and at the chain termina.
Therefore, a variety of surfaces with specific inter-
actions can be produced with fine chemical control.
6
Due to their dense and stable structure, SAMs have
potential applications in corrosion prevention, wear
protection, and more. In addition, the biomimetic
and biocompatible nature of SAMs makes their
applications in chemical and biochemical sensing
promising. Their high molecular order parameter in
SAMs makes them ideal as components in electro-
optic devices Recent work on nanopatterning of SAMs
suggests that these systems may have applications
in patterning of GaAs and in the preparation of
sensor arrays.
7
While the majority of papers in recent years deal
with thiols on gold, this by no means is the only
system to consider. Silanes on hydroxylated surfaces
are important systems for many technological ap-
plications, and efforts continue to achieve better
reproducibility in monolayer preparation. SAMs of
fatty acid derivatives are an important link between
the Langmuir-Blodgett and the self-assembly tech-
niques and, as such, continue to be studied. In this
review, we discuss structural factors in the formation
of SAMs. We describe different SAMs, their unique
features and provide examples of various systems.
We then attempt to provide a general picture of self-
assembly on surfaces, as it emerges from a consid-
eration of the interplay of different forces that control
this process.
II. Self-Assembled Monolayers
SAMs are ordered molecular assemblies formed by
the adsorption of an active surfactant on a solid
surface (Figure 1). This simple process makes SAMs
inherently manufacturable and thus technologically
attractive for building superlattices and for surface
engineering. The order in these two-dimensional
systems is produced by a spontaneous chemical
synthesis at the interface, as the system approaches
equilibrium. Although the area is not limited to long-
chain molecules,
8
SAMs of functionalized long-chain
hydrocarbons are most frequently used as building
blocks of supermolecular structures.
1. Monolayers of Fatty Acids
Spontaneous adsorption of long-chain n-alkanoic
acids (C
n
H
2n+1
COOH) has been studied in the past
few years. This is an acid-base reaction, and the
driving force is the formation of a surface salt
between the carboxylate anion and a surface metal
cation. Allara and Nuzzo
9,10
and Ogawa et al.
11
studied the adsorption of n-alkanoic acids on alumi-
num oxide. Schlotter et al. studied the spontaneous
adsorption of such acids on silver.
12
Huang and Tao
studied SAMs of long-chain diacetylene amphiphiles.
13
In this rigid, rodlike systems, the diacetylene pi-sys-
tem breaks the cylindrical symmetry of the all-trans-
alkyl chain. Thus, if one views the two-dimensional
packing of fatty acids as a close-packed assembly of
ordered all-trans-alkyl chains, the diacetylene units
introduce a stratum of defects into this assembly.
14
As a result, the highest contact angle was recorded
on a monolayer where the diacetylene group is
connected to the carboxylic head group, i.e., the
stratum of defects is as far as possible from the air-
monolayer interface, so that high ordering and close-
Figure 1. Self-assembled monolayers are formed by simply immersing a substrate into a solution of the surface-active
material. The driving force for the spontaneous formation of the 2D assembly includes chemical bond formation of molecules
with the surface and intermolecular interactions.
1534 Chemical Reviews, 1996, Vol. 96, No. 4 Ulman