Download self-assembled monolayers lecture

CHE499 : A Nanotechnology Course in Chemical &
Materials Engineering
Spring 2006
Self-Assembled
Monolayers
By Drs. Lloyd Lee, Winny Dong 5GD6ER

Self-Assembled Monolayers
(SAMs)

History
• Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481.

Introduction
• Self-assembled monolayers (SAMs) can be prepared using
• alkylsiloxane monolayers,
• fatty acids on oxidic materials and
• alkanethiolate monolayers.
O O
• The principle is simple: A molecule which is essentially an
alkane chain, typically with 10-20 methylene units, is given
a head group with a strong preferential adsorption to the
substrate used. Thiol (S-H) head groups and Au(111)
substrates have been shown to work excellently. The thiol
molecules adsorb readily from solution onto the gold,
creating a dense monolayer with the tail group pointing
outwards from the surface. By using thiol molecules with
different tail groups, the resulting chemical surface
functionality can be varied within wide limits. Alternatively,
it is also possible to chemically functionalize the tail groups
by performing reactions after assembly of the SAM.
Book: Ultrathin Organic Films by A. Ulman, 2000.
H 5 C 2 O
O
H2N
Si
O
Si
OH HO
OH
Si
H 2 N
Si
O
Si
O
O
H 2 N
Si
O
Si
O
O
H 2 N
H 2 N
H 2 N
HO
HO
Si OH
Si O
Si O
O OH
Si Si
O

Applications:
• Applications are molecular recognition, SAMs as
model substrates and biomembrane mimetics in
studies of biomolecules at surfaces, selective binding
of enzymes to surfaces, chemical force microscopy,
metallization of organic materials, corrosion
protection, molecular crystal growth, alignment of
liquid crystals, pH-sensing devices, patterned
surfaces on the µm scale, electrically conducting
molecular wires and photoresists.

Manufacture
• The preferred crystal face for alkanethiolate
SAM preparation on gold substrates is the
(111) direction, which can be obtained either
by using single crystal substrates or by
evaporation of thin Au films on flat supports,
typically glass or silicon. A schematic outline
of the SAM preparation procedure on such
gold substrates is given in the Figure,
together with a schematic of a mixed SAM
(see below). Several different solvents are
usable at the low thiol concentrations
(typically 1-2 mM). The most commonly
used solvent is ethanol. Even though a selfassembled
monolayer forms very rapidly on
the substrate, it is necessary to use
adsorption times of 15 h or more to obtain
well-ordered, defect-free SAMs. Multilayers
do not form, and adsorption times of two to
three days are optimal in forming highestquality
monolayers.
The substrate, Au on Si, is
immersed in an ethanol solution of
the desired thiol(s). Initial adsorption
is fast (seconds); then an
organization phase follows which
should be allowed to continue for
>15 h for best results. A schematic
of a fully assembled SAM is shown
to the right.

Tail Groups: Functionalization
• As mentioned above, the tail
group that provides the
functionality of the SAM can be
widely varied. CH3-terminated
SAMs are commercially
available; other functional
groups can be synthesized by
any well-equipped chemical
laboratory, providing almost
infinite possibilities of variation.
In addition, chemical
modification of the tail group is
entirely possible after formation
of the SAM, expanding the
available range of functionalities
even further.

SAM on AU (111) Surface
A schematic model of the (sqrt(3)×sqrt(3))R30° overlayer structure formed
by alkanethiolate SAMs on Au(111).

Characterization:
• Among the most frequently used techniques are infrared spectroscopy,
ellipsometry, studies of wetting by different liquids, x-ray photoelectron
spectroscopy, electrochemistry, and scanning probe measurements. It has
been clearly shown that SAMs with an alkane chain length of 12 or more
methylene units form well-ordered and dense monolayers on Au(111)
surfaces. The thiols are believed to attach primarily to the threefold hollow
sites of the gold surface, losing the proton in the process and forming a
(sqrt(3)×sqrt(3))R30° overlayer structure (shown in Figure ). The distance
between pinning sites in this geometry is 5.0 Å, resulting in an available
area for each molecule of 21.4 Å2. Since the van der Waals diameter of
the alkane chain is somewhat too small (4.6 Å) for the chain to completely
occupy that area, the chains will tilt, forming an angle of approximately 30°
with the surface normal. Depending on chain length and chain-terminating
group, various superlattice structures are superimposed on the
(sqrt(3)×sqrt(3))R30° overlayer structure. The most commonly seen
superlattice is the c(4×2) reconstruction, where the four alkanethiolate
molecules of a unit cell display slightly different orientations when
compared with each other.

• The Au-thiolate bond is strong - homolytic bond strength 44 kcal/mol
- and contributes to the stability of the SAMs together with the van
der Waals forces between adjacent methylene groups, which
amount to 1.4-1.8 kcal/mol. The latter forces add up to significant
strength for alkyl chains of 10-20 methylenes and play an important
role in aligning the alkyl chains parallel to each other in a nearly alltrans
configuration. At low temperatures, typically 100 K, the order is
nearly perfect, but even at room temperature there are only few
gauche defects, concentrated to the outermost alkyl units.
One convenient method of checking a SAM for well-ordered and
dense structure is infrared reflection-absorption spectroscopy
(IRAS). The CH stretching vibrations of the alkyl chain are very
sensitive to packing density and to the presence of gauche defects,
which makes them ideally suited as probes to determine SAM
quality. In particular, the antisymmetric CH2 stretching vibration (d-)
at ~2918 cm-1 is a useful indicator; its position varies from 2916 or
2917 cm-1 for SAMs of exceptional quality or cooled below room
temperature, via 2918 cm-1 which is the normal value for a highquality
SAM, to ~2926 cm-1 which is indicative of a heavily
disordered, "spaghetti-like" SAM. A typical IRAS spectrum of the CH
stretching region of a hexadecanethiolate (HS(CH2)15CH3 ) SAM is
shown in the following Figure...

IRAS spectrum of a hexadecanethiolate SAM in the CH stretching
region. The most prominent vibrations are indicated. d+ and d- are
the symmetric and antisymmetric CH2 stretches; r+ and r- are the
symmetric and antisymmetric CH3 stretches, respectively. At the
measurement temperature (82 K), the ra- and rb-components of the r-
peak are resolved.

• Thickness measurements using ellipsometry yield SAM
thicknesses that are in good agreement with the 30° chain tilt
mentioned above. For example, reported ellipsometric
thicknesses of hexadecanethiolate SAMs lie in the 21±1 Å
range, to compare with the 21.2 Å that result if a fully extended
hexadecanethiol molecule of 24.5 Å length is tilted 30°.
• Contact angle measurements further confirm that alkanethiolate
SAMs are very dense and that the contacting liquid only
interacts with the topmost chemical groups. Reported advancing
contact angles with water range from 111° to 115° for
hexadecanethiolate SAMs. At the other end of the wettability
scale, there are hydrophilic monolayers, e.g., SAMs of 16-
mercaptohexadecanol (HS(CH2)16OH), that display water
contact angles of

Lithography

U Bielefeld

In Si(100), the anisotropic etch characteristic of a KOH was exploited for the
fabrication of 35 nm wide and 30 nm deep grooves. The grating pattern was
written in Octadecylthrichlorosilane (OTS) adsorbed onto hydroxilized Si(100).

DNA-Self
Self-assembled
monolayers

DNA SAMs

Biotinylated SAMs (NIST)
NIST

Mercury Removal:
SAMs in Mesopores
(Pacific NN Lab)

Mesoporous Silica for Mercury Removal
• Xiangdong Feng, while a researcher for the Pacific Northwest
National Laboratory, developed a process where molecules that
can grab mercury out of the water are placed inside
mesoporous silica. This spongelike rock has a surface area
thousands of times larger than its size allowing it to grab the
mercury quickly and efficiently. Eventually, Feng says, it should
be possible to reduce the presence of mercury to a few parts per
trillion, compared with a few parts per billion for current
techniques, and do it more quickly as well. In addition, Feng
says his invention can remove just about any pollutant or heavy
metal from contaminated water. "People say I should quit my
day job and start mining for gold."

Mercury Removal: SAM in
Mesopores (Pacific NN Lab)
• Under this task, a proprietary new technology, Self-Assembled Monolayers on
Mesoporous Supports (SAMMS), for RCRA metal ion removal from aqueous
wastewater and mercury removal from organic wastes such as vacuum pump oils is
being developed at Pacific Northwest National Laboratory (PNNL).
• The six key features of the SAMMS technology are 1) large surface area (>900 m2/g)
of the mesoporous oxides (SiO2, ZrO2, TiO2) ensures high capacity for metal
loading (more than 1 g Hg/g SAMMS); 2) molecular recognition of the interfacial
functional groups ensures the high affinity and selectivity for heavy metals without
interference from other abundant cations (such as calcium and iron) in wastewater;
3) suitability for removal of mercury from both aqueous wastes and organic wastes;
4) the Hg-laden SAMMS not only pass TCLP tests, but also have good long-term
durability as a waste form because the covalent binding between mercury and
SAMMS has good resistance to ion exchange, oxidation, and hydrolysis; 5) the
uniform and small pore size (2 to 40 nm) of the mesoporous silica prevents bacteria
(>2000 nm) from solubilizing the bound mercury; and 6) SAMMS can also be used
for RCRA metal removal from gaseous mercury waste, sludge, sediment, and soil.
Resource Conservation and Recovery Act (RCRA)

Hg-removal by SAM in Silica
(SiO2, ZrO2, TiO2)

• Molecular self-assembly is a unique phenomenon in which functional
molecules aggregate on an active surface, resulting in an organized
assembly having both order and orientation.9-11 In this approach,
bifunctional molecules containing a hydrophilic head group and a
hydrophobic tail group adsorb onto a substrate or an interface as
closely packed monolayers. The driving forces for the self-assembly
are the inter- and intra-molecular interactions between the functional
molecules (such as van der Waals forces). The tail group and the head
group can be chemically modified to contain certain functional groups
to promote covalent bonding between the functional organic molecules
and the substrate on one end, and molecular bonding between the
organic molecules and the metals on the other.9 By populating the
outer interface with specific functional groups, an effective means for
scavenging heavy metals is made available. The metal-loading
capability is determined by the available surface area of the underlying
inorganic support. A high surface area support allows for high RCRA
metal loading.

High-Surface
Mesoporous Supports
• The unique mesoporous oxide supports provide high surface area
(>900 m2/g), thereby enhancing the metal loading capacity. They also
provide an extremely narrow pore size distribution, which can be
specifically tailored from 15 Å to 400 Å, thereby minimizing
biodegradation from microbes and bacteria. Mesoporous structures can
be disposed of as stable waste forms.
The porous supporting materials used in this research (SiO2, ZrO2,
TiO2) are synthesized through a co-assembly process using oxide
precursors and surfactant molecules.12-15 The material synthesis is
accomplished by mixing surfactants and oxide precursors in a solvent
and reacting the solution under mild hydrothermal conditions. The
surfactant molecules form ordered liquid crystalline structures, such as
hexagonally ordered rod-like micelles, and the oxide materials
precipitate on the micellar surfaces to replicate the organic templates
formed by the rod-like micelles. Subsequent calcination to 500°C
removes the surfactant templates and leave a high surface area oxide
skeleton. The pore size of the mesoporous materials is then
determined by the rod-like micelles, which are extremely uniform. Using
different chain length surfactants produces mesoporous materials with
different pore sizes.