Self assembled nanostructures

Self Assembly of
complex structures

A self assembled monolayer (SAM) is an organized layer of amphiphilic
molecules in which one end of the molecule, the “head group” shows a
special affinity for a substrate. SAMs also consist of a tail with a
functional group at the terminal end as seen in Figure 1.

Characteristics of SAM
The word SAM generally denotes a monomolecular thick film of organic
compounds on flat (i.e., two-dimensional) metal or semiconductor surfaces.
SAM formation provides one of the easiest ways to obtain ordered monolayers
through strong chemisorption between the head group of a desired compound
and the metal surface leading to the preparation of thermodynamically stable
monolayers 1–3 as compared to LB (Langmuir–Blodgett) and other techniques.
Long-chain alkane thiols (containing more than six to seven methylene units)
form more well-ordered defect-free monolayers than short-chain alkane thiols,
disulphides or sulphides.
Aromatic (pi systems like benzene, naphthalene or diphenylene systems) or/and
hydrogen-bonded molecules with multiple contacts, containing functional groups
like thiols, amines, sulphides, selenides etc. provide improved stability

Schematic diagram of an ideal, single-crystalline SAM of
alkanethiolates supported on a gold surface

SAMs are created by the chemisorption of hydrophilic “head groups” onto a substrate
from either the vapor or liquid phase[
followed by a slow two-dimensional organization
of hydrophobic “ tail groups”. Initially, adsorbate molecules form either a disordered
mass of molecules or form a “lying down phase”]
, and over a period of hours,
begin to form crystalline or semicrystalline structures on the substrate surface. The
hydrophilic “head groups” assemble together on the substrate, while the hydrophobic
groups assemble far from the substrate. Areas of close-packed molecules nucleate
and grow until the surface of the substrate is covered in a single monolayer.

• Adsorbate molecules adsorb readily because they lower the
surface energy of the substrate and are stable due to the
strong chemisorption of the “head groups.” These bonds
create monolayers that are more stable than the physisorbed
bonds of Langmuir-Blodgett films. Thiol-metal bonds, for
example, are on the order of 100 kJ/mol, making the bond
stable in a wide variety of temperature, solvents, and
potentials. The monolayer packs tightly due to
van der Waals interactions, thereby reducing its own free
energy. The adsorption can be described by the
Langmuir adsorption isotherm if lateral interactions are
neglected. If they cannot be neglected, the adsorption is
better described by the Frumkin isotherm].

Types of SAMs
• Selecting the type of head group depends on the
application of the SAM. Typically, head groups are
connected to an alkyl chain in which the terminal end can
be functionalized (i.e. adding –OH, -NH3, or –COOH groups)
to vary the interfacial properties. An appropriate substrate
is chosen to react with the head group. Substrates can be
planar surfaces, such as silicon and metals, or curved
surfaces, such as nanoparticles. Thiols and disulfides are
the most commonly used on noble metal substrates.
Currently, gold is the standard for these head groups. Gold
is an inert and biocompatible material that is easy to
acquire.

Preparation of SAMs
• Metal substrates for use in SAMs can be produced
through physical vapor deposition techniques,
electrodeposition or electroless deposition.
Alkanethiol SAMs produced by adsorption from
solution are made by immersing a substrate into a
dilute solution of alkane thiol in ethanol for 12 to 72
hours at room temperature and dried with nitrogen.
SAMs can also be adsorbed from the vapor phase.
For example, chlorosilane SAMs (which can also be
adsorbed from the liquid phase), are often created in
a reaction chamber by silanization in which silane
vapor flows over the substrate to form the
monolayer.

Advantages of alkanethiol SAMs on gold are
summarized below:
• Gold is a relatively inert metal, resisting oxidation.
• Gold has a strong specific interaction with sulfur that allows
the formation of monolayers in the presence of other
functional groups.or example, the homolytic bond strength of
methanethiolates on Au(111) is ~ 45 kcal/mol.
• Long-chain alkanethiols form densely packed, crystalline or
semi-crystalline monolayers on gold due to the van der Waals
forces between the carbon chain.
• The macroscopic surface properties can be dramatically
altered by changing the terminal head group. Such surface
properties include wettability, blood interactions, protein/cell
adhesion and charge distribution.
• Preparation of alkanethiol SAMs is a simple process not
requiring elaborate and expensive equipment or extensive
experience to be performed successfully.

Factors that Influence Monolayer Order
• The final order and quality of the assembled monolayer are dependent
on several factors,
1. The cleanliness and purity of the original gold surface
2. Nature of the surface
3. The nature, purity and concentration of the alkanethiol and assembly
solution
4. The nature and purity of the solvent used
4. The length and composition of the spacer chain
5. The type of head group (size and properties)
6. The amount of time the monolayer is allowed to assemble
7. The crystallinity of the substrate also play a crucial role in determining the
compactness

Cleanliness of the Gold
Due to the high relative surface energy of gold, “clean” gold can
only be created in a vacuum environment. Once a gold
surface is exposed to the air it will immediately be coated
with a layer of hydrocarbon. Nevertheless, the presence of
adventitious hydrocarbons is not as problematic as other
surface contaminants such as oils and common polymer
contaminants such as poly(dimethyl siloxane). Due to the
strong driving force of the sulfur-gold interaction, stabilization
of the van der Waals interactions and the lack of a bulky head
group, standard methyl terminated alkanethiols with long
(~10 carbons) carbon chains are able to remove adventitious
contamination. For other thiols with bulky, flexible, or
charged head groups, the cleanliness of the gold is more
important. These types of head groups can reduce the driving
force for assembly, making it more difficult to remove
contamination from the gold surface. In either case, it is
recommended that freshly prepared gold be used for
monolayer formation, and if necessary the gold surfaces be
stored under inert gas.

Purity of the Alkanethiol
All alkanethiols do not have the same affinity for gold.
Some thiols will out compete others for surface sites
resulting in a surface composition that differs greatly
from the bulk solution concentration. For example,
alkanethiols with longer alkane chains will generally
assemble faster from a mixture than alkanethiols
with shorter alkane chains. This means that in a
competitive adsorption of a long and short chain
alkanethiol, at a given solution composition, it would
be expected that the resulting surface would be
enriched in the longer chain alkanethiol. Alkanethiols
with small head groups tend to out-compete
alkanethiols with bulkier head groups.

The Length of the Spacer Chain
A longer alkanethiol with a small head group is recommended.
Type of End group
To form a monolayer, the alkanethiols must pack into a crystalline-type lattice in specific
arrangements on the gold surface. As with any system, optimal packing requires
uniform, geometrically defined components. Straight chain alkanethiols with small end
can assemble together in a close packed arrangement with few
defects, while alkanethiols with bulky head groups typically form less ordered
monolayers. This disorder is largely driven by steric hindrance that interferes with the
molecular packing.

If an ordered monolayer is required for a given application that
also requires an alkanethiol with a bulky end group, it is often
possible to use a mixed monolayer system.
Monolayer Formation Kinetics
• The process of self-assembly is fast. For most alkanethiols, a
monolayer is formed after just a few minutes. Once the
monolayer is formed, the layer still goes through changes as
more alkanethiols pack into the layer and the molecules
rearrange to their optimal configuration. This annealing of the
monolayer can take hours to days and will lead to a
technically superior monolayer. The amount of time
required to obtain a given level of order within a monolayer
will depend on the initial solution concentration, the
temperature, and the characteristics of the alkanethiol being
used. Assembly from dilute solutions requires longer times to
reach a well ordered state than assembly from more
concentrated solutions. For most monolayers, assembly for 1
to 2 days will result in an equilibrium state, where the
majority of the molecules are arranged in their final, optimal
configuration.

• There is evidence that SAM formation occurs in two steps, an
initial fast step of adsorption and a second slower step of
monolayer organization. Many of the SAM properties, such as
thickness, are determined in the first few minutes. However, it
may take hours for defects to be eliminated via annealing and for
final SAM properties to be determined. The exact kinetics of SAM
formation depends on the adsorbate, solvent and substrate
properties. In general, however, the kinetics are dependent on
both preparations conditions and material properties of the
solvent, adsorbate and substrate. Specifically, kinetics for
adsorption from a liquid solution are dependent on:
• Temperature – room temperature preparation improves kinetics
and reduces defects.
• Concentration of adsorbate in the solution – low concentrations
require longer immersion times and often create highly crystalline
domains.

Patterning of SAMs
1. Locally attract
• This first strategy involves locally depositing self-assembled monolayers on the
surface only where the nanostructure will later be located. This strategy is
advantageous because it involves high throughput methods that generally involve
less steps than the other two strategies. The major techniques that use this
strategy are
• micro-contact printing
is analogous to printing ink with a rubber stamp. The SAM molecules are inked onto an pre-
shaped elastomeric stamp with a solvent and transferred to the substrate surface by
stamping. The SAM solution is applied to the entire stamp but only areas that make
contact with the surface allow transfer of the SAMs. The transfer of the SAMs is a
complex diffusion process that depends on the type of molecule, concentration,
duration of contact, and pressure applied.
• Dip-pen nanolithography
– Dip-pen nanolithography is a process that uses an atomic force microscope to transfer
molecules on the tip to a substrate. Initially the tip is dipped into a reservoir with an ink.
The ink on the tip evaporates and leaves the desired molecules attached to the tip.
When the tip is brought into contact with the surface a water meniscus forms between
the tip and the surface resulting in the diffusion of molecules from the tip to the
surface. These tips can have radii in the tens of nanometers, and thus SAM molecules
can be very precisely deposited onto a specific location of the surface.
•

2. Locally remove
• The locally remove strategy begins with covering the entire surface with a SAM.
Then individual SAM molecules are removed from locations where the deposition
of nanostructures is not desired. The end result is the same as in the locally attract
strategy, the difference being in the way this is achieved. The major techniques
that use this strategy are:
• Scanning tunneling microscope
– The scanning tunneling microscope can remove SAM molecules in many different ways.
The first is to remove them mechanically by dragging the tip across the substrate
surface. This is not the most desired technique as these tips are expensive and dragging
them causes a lot of wear and reduction of the tip quality. The second way is to degrade
or desorb the SAM molecules by shooting them with an electron beam.
• Atomic force microscope
– The most common use of this technique is to remove the SAM molecules in a process
called shaving, where the atomic force microscope tip is dragged along the surface
mechanically removing the molecules.
• Ultraviolet irradiation
– In this process, UV light is projected onto the surface with a SAM through a pattern of
apperatures in a chromium film. This leads to photo oxidation of the SAM molecules.
These can then be washed away in a polar solvent. This process has 100nm resolutions
and requires exposure time of 15-20 minutes

3. Modify tail groups
• The final strategy focuses not on the deposition or removal of SAMS, but
the modification of terminal groups. In the first case the terminal group
can be modified to remove functionality so that SAM molecule will be
inert. In the same regards the terminal group can be modified to add
functionality so it can accept different materials or have different
properties than the original SAM terminal group. The major techniques
that use this strategy are:
• Focused electron beam and ultraviolet irradiation
– Exposure to electron beams and UV light changes the terminal group
chemistry. Some of the changes that can occur include the cleavage of bonds,
the forming of double carbon bonds, cross-linking of adjacent molecules,
fragmentation of molecules, and confromational disorder.
• Atomic force microscope
– A conductive AFM tip can create an electrochemical reaction that can change
the terminal group.

Quality of SAM
• Purity of the adsorbate – impurities can affect the
final physical properties of the SAM
• Dirt or contamination on the substrate –
imperfections can cause defects in the SAM
• The final structure of the SAM is also dependent on
the chain length and the structure of both the
adsorbate and the substrate. Steric hindrance and
metal substrate properties, for example, can affect
the packing density of the film, while chain length
affects SAM thickness].

• SAMs have been characterized using a large
number of surface analytical tools.
• Infrared spectroscopy (FTIR)
• Ellipsometry
• Contact angle measurement (studies of
wetting by different liquids)
• X-ray photoelectron spectroscopy (XPS)
• Electrochemistry
• 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.
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 high-quality 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 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 used (82 K), the
ra- and rb-components of the r- peak are resolved.

Ellipsometry measures the change in amplitude and phase of light upon reflection.
Using these values, the thickness and refractive index of a film can be calculated. This
procedure can be used to determine the thickness of a monolayer before and after
reactions to detect adsorption of molecules onto the SAM. 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 <10°. These two extremes are only possible to achieve
if the SAM surfaces are uniform and expose only the chain-terminating group at the
interface. Mixed SAMs of CH3- and OH-terminated thiols can be tailor-made with any
wettability (in terms of contact angle) between these limiting values.

The characteristics of mixed two-component SAMs depend strongly upon the
precise chemical identity of the components and upon their proportion in the
preparation solution, as already stated above. Apart from the composition of the
SAMs, the issue of island formation is very important for mixed monolayers. In
mixed CH3/CO2CH3 SAMs, scanning tunnelling microscopy has revealed island
formation on the 20-50 Å scale. For mixed SAMs of hexadecanethiol and 16-
mercaptohexadecanol,well-mixed monolayers, although mixing at a true
molecular level has neither been contradicted nor confirmed at the present
stage. Undoubtedly though, macroscopic phase segregation into single
component domains does not occur.

XPS study
1. An intense S 2p peak on the surface XPS spectra
2. The ratio S/Au, calculated on the basis of XPS results considered to compare
the amounts of adsorbed thiols
1. The thickness of the adsorbed thiol layer estimated from the decrease of gold
signal intensity when compared to a bare gold surface
2. On spectrum, the contribution of two S 2p doublets; the first doublet, at lower
binding energy (162 and 163.3 eV for S 2p3/2 and 2p1/2 respectively)
corresponds to the sulfur bound to gold (Sb), the second, at higher energy (163.6
and 164.9 eV) can be attributed to the sulfur not involved in a covalent bond with
gold, but remaining in SH bonds, that we call “free sulfur”.
The Sb/Sf ratio considered as a signature of whether thiols are grafted forming
bridges, or physically adsorbed. Indeed. If thiol molecules form bridges onto the
gold surface, Sb/Sf ratio is expected to be higher than 1. Eventually,a Sb/Sf ratio
lower than 1 indicates the presence of physically adsorbed thiols, not involved in
covalent bonds with gold, and probably forming multilayers on surfaces.

SAMs have been employed as cross-linkers primarily to aid functionalization of
SERS substrates.. As shown in Figure the SERS-active nanoparticles which were
derived are first immobilized with carboxylic acid-terminated SAMs. Molecules of
anti-human insulin (IgG) were then bound to the SAM via the commonly used EDC
chemistry to develop SERS-based immuno-nanosensors. The immuno-
nanosensors were then used to detect insulin in cell lysates at a concentration of
10μg/mL

Areas of application for SAMs
•Systems (nanoelectromechanical systems (NEMS)
•Microelectromechanical MEMS)
• Household goods.
•Studying membrane properties of cells and organelles
and cell attachment on surfaces.
•To control electron transfer in electrochemistry.
•To protect metals from harsh chemicals and etchants.
reduce sticking of NEMS and MEMS components in humid
environments.
• To create a hydrophobic monolayer on car windshields to
keep them clear of rain
•Molecular recognition

SELF CLEANING AND MOISTURE
REPELLENT APPLICATIONS
• By self assembly, we can get a such type of
monolayer in which the hydrophobicity of
chain can be used to apply the self cleaning
action (water removal)of various surfaces.
This technique can get a potential application
in self cleaning of car glasses during raining.
• Moreover, by this property of monolayer, we
can protect the MEMS- NEMS devices from
moisture.

SAM mediated solid state synthesis
• ω-functionalised SAMs as organic templates for controlled nucleation
and growth . Due to the structural as well as functional similarities of SAM
to protein surfaces, the preparation strategy is sometimes known as
biomimetic synthesis as the monolayers play an important role by
providing the suitable functionalities necessary to initiate the growth of
the inorganic layers.
• SAM of 1,4-benzenedimethenethiol (BDMT) to link Zr4+ from an aqueous
solution of appropriate zirconium salt to subsequently form
microcrystalline, monoclinic zirconia at room temperature.
• First a quasi two-dimensional SAM surface is formed with different
terminal functional groups as a general synthetic strategy before
attaching Zr species from aqueous ZrOCl2 solution.

Potential cycling of this Zr attached samples as working electrodes in 1M
aqueous KCl solution between –1×1 to 0×7 V vs saturated calomel electrode
(SCE) gives microcrystalline zirconia.
When the SAM surface is terminated with CH3 (hydrophobic surface),
amorphous ZrO2 is formed while SH and NH2 functional groups cause
crystalline ZrO2 different morphological features.
General synthetic strategy for formation of microcrystalline ZrO2 on
SAM covered surfaces

Micrographs of microcrystalline zirconia on (a) a bare gold substrate
cycled for longer time, (b) a DDT SAM surface, (c) a 1,5-pentane dithiol
SAM surface, and (d) a amino-thiophenol SAM (amine terminated)
surface.

FOR PURIFICATION PURPOSE
If there is any ether group attach to the
alkyl chain it can be used to trap the
various metal ions from the sample
e.g. various transition and alkaline
metal ions

Azobenzene derivatives grafted on gold exhibit reversible photoswitching,
probably facilitated by the use of a short alkyl spacer partially preserving the
photoactive azobenzene unit from the influence of the surface . This result
highlighted the importance of the chemical structure of the spacer used to
connect the photoactive switching units to the considered conductive surface.

The application of a host-type molecule as a monolayer-forming component
leads to the preparation of a sensor device. If the monolayer is immobilized
on an electrode, guest binding can be detected as an electric signal. A self-
assembledmonolayer of α-cyclodextrin was immobilized on a gold electrode.
Because the α-cyclodextrincan accommodate hydroquinone, the binding behavior
of the hydroquinone to the α-cyclodextrin monolayer was detected by a redox
signal from the hydroquinone. Using the hydroquinone as a competitive guest, the
binding of another guest molecule can be analyzed from the decrease of the
hydroquinone response. For example, methyl red is capable of binding with α-
cyclodextrin, and its binding behavior was quantitatively determined by analyzing
the suppression of the hydroquinone current. Systematic analyses revealed that
the p-isomer binds more strongly than the o-isomer (regioselectivity) and the R-
isomer binds more strongly than the S-isomer (stereoselectivity).

Instead of cyclodextrin, calixarene has also been used as the monolayerforming
component in sensor preparation. In the example shown in Fig. 4.41,
a self-assembled monolayer of calixarene was immobilized on an electrode
of a quartz crystal microbalance. The quartz crystal microbalance is a masssensing
device capable of nanogram-level precision (see Chap. 5). When the
monolayer-coveredmicrobalance was exposed to guest gas, the guest bonding
caused a change in the resonant frequency. In this case, tetrachloroethylene
gave a large response.

By grafting light-triggered switches on gold nanoparticles it is possible to control this
interparticle separation within the network. To do this, a connecting unit containing
an azobenzene functional group was grafted onto the surface of different
nanoparticles by two thiol groups positioned at the extremities of the linker. Gold
nanoparticles thus form a network connected by light-sensitive linkers. The total
length of these linkers shrinks from 3 nm in the trans form to 2 nm in the cis form.
SPR spectroscopy that the interparticle spacing is controlled by the reversible cis !
trans photoisomerization of the azobenzene moiety of the linker.

Langmuir–Blodgett film
• A Langmuir–Blodgett film contains one or more monolayers of an
organic material, deposited from the surface of a liquid onto a solid
by immersing (or emersing) the solid substrate into (or from) the
liquid. A monolayer is adsorbed homogeneously with each
immersion or emersion step, thus films with very accurate thickness
can be formed. This thickness is accurate because the thickness of
each monolayer is known and can therefore be added to find the
total thickness of a Langmuir-Blodgett Film. The monolayers are
assembled vertically and are usually composed of amphiphilic
molecules with a hydrophilic head and a hydrophobic tail (example:
fatty acids). Langmuir–Blodgett films are named after Irving Langmuir
and Katharine B. Blodgett.

Mechanism
• LB films are formed when amphiphilic molecules like
surfactants interact with air at an air-water interface.
Surfactants (or Surface acting agents) are molecules with
hydrophobic 'tails' and hydrophilic 'heads'. When surfactant
concentration is less than critical micellar concentration (CMC
), the surfactant molecules arrange themselves as shown in
Figure. This tendency can be explained by surface-energy
considerations. Since the tails are hydrophobic, their
exposure to air is favoured over that to water. Similarly, since
the heads are hydrophilic, the head-water interaction is more
favourable than air-water interaction. The overall effect is
reduction in the surface energy (or equivalently, surface
tension of water).

This amphiphilic nature of surfactants is responsible for their
association behaviour in solution (micelles, bilayers, vesicles, etc.) and their
accumulation at interfaces (air/water or oil/water). The hydrophobic part usually
consists of hydrocarbon or fluorocarbon chains, while the hydrophilic part consists
of a polar group (-OH, -COOH, -NH3 +, -PO4 -(CH2)2NH3 + etc.)
.

For very small concentrations, far less than
critical micellar concentration (CMC), the surfactant
molecules execute a random motion on the water-air
interface. This motion can be thought to be similar to the
motion of ideal gas molecules enclosed in a container. The
corresponding thermodynamic variables for the surfactant
system are, surface pressure (Π), surface area (A) and
number of surfactant molecules (N). This system behaves
similarly to a gas in a container. The density of surfactant
molecules as well as the surface pressure increase upon
reducing the surface area A ('compression' of the 'gas').
Further compression of the surfactant molecules on the
surface shows behavior similar to phase transitions. The
‘gas’ gets compressed into ‘liquid’ and ultimately into a
perfectly closed packed array of the surfactant molecules on
the surface corresponding to a ‘solid’ state. Instruments like
Langmuir-Blodgett trough can be used to quantify such
phenomena.

A schematic of a Langmuir Blodgett trough
1.Amphiphile monolayer 2. Liquid subphase 3. LB Trough 4. Solid substrate 5. Dipping
mechanism 6. Wilhelmy Plate 7. Electrobalance 8. monolayer Barrier 9. Barrier
Mechanism 10. Vibration reduction system 11. Clean room enclosure closure

• Trough : Teflon (polytetrafluoroethylene). Teflon is hydrophobic and
chemically inert, making it a highly suitable material and the most
commonly used for troughs today. Occasionally metal or glass troughs
coated with a thin layer of Teflon are used.
• Barrier :Most commonly used systems are comprised of movable Teflon
barrier blocks that slide parallel to the walls of the trough and are in
contact with the top of the fluid. Another version with a variable perimeter
working zone is the circular trough in which the monolayer is located
between two radial barriers.
• Balance :An important property of the system is its surface pressure (the
surface tension of the pure subphase minus the surface tension of the
subphase with amphiphiles floating on surface) which varies with the
molecular area. The surface pressure – molecular area isotherm is one of
the important indicators of monolayer properties. Additionally, it is
important to maintain constant surface pressure during deposition in order
to obtain uniform LB films. Measurement of surface pressure can be done
by means of a Wilhelmy plate or Langmuir balance.

Surface Tension
The molecules in a liquid have a certain degree of attraction to each
other. The degree of this attraction, also called cohesion, is dependent on
the properties of the substance. The interactions of a molecule in the
bulk of a liquid are balanced by an equally attractive force in all directions.
The molecules on the surface of a liquid experience an imbalance of
forces i.e. a molecule at the air/water interface has a larger attraction
towards the liquid phase than towards the air or gas phase. Therefore,
there will be a net attractive force towards the bulk and the air/water
interface will spontaneously minimize its area and contract.
The net effect of this situation is the presence of free energy at the
surface. The excess energy is called surface free energy and can be
quantified as a measurement of energy/area. It is also possible to
describe this situation as having a line tension or surface tension which is
quantified as a force/length measurement. Surface tension can also be
said to be a measurement of the cohesive energy present at an interface.
The common units for surface tension are dynes/cm or mN/m. These
units are equivalent. Solids may also have a surface free energy at their
interfaces but direct measurement of its value is not possible through
techniques used for liquids.

The Wilhelmy method consists of a plate partially immersed in the liquid connected
to an electronic linear-displacement sensor, or electrobalance. The plate can be
made of platinum or filter paper which has been presoaked in the liquid to maintain
constant mass. The plate detects the downward force exerted by the liquid meniscus
which wets the plate. The surface tension can then be calculated by the following
equation:
m= mass of the plate
g= gravitational accelaration
tp = thickness of the plate, wp = width of the plate

The weight of the plate can be determined beforehand and set to zero on the
electrobalance, while the effect of buoyancy can be removed by extrapolating
the force back to the zero depth of immersion. Then the remaining
component force is only the wetting force. Assuming that perfect wetting of
the plate occurs (θ = 0, cos(θ) = 1), then the surface tension can be
calculated.
The surface pressure is then the change in surface tension due to the
addition of the monolayer.
Π = γ ─ γ 0
Where, Π = surface pressure ,γ = Surface tension of the subphase
with momolayer, γ 0 = Surface tension of the subphase

Preparations of Langmuir-Blodgett Trough
To eliminate contamination from the air, the LB trough can be enclosed in a
clean room. The trough set-up may also be mounted on a vibration isolation
table, to further stabilize the monolayer. The exact calibration of the
electrobalance is also very important for force measurements. Even small
contaminations can have substantial effects on results. If an aqueous subphase
is used, the water must be purified to remove organics and deionized to a
resistivity not less than 1.8 GΩ-m. Impurities as small as 1ppm can radically
change the behavior of a monolayer.
The trough and barriers be thoroughly cleaned by a solvent such as ethanol to
remove any residual organics. The liquid subphase is added to a height such that
the meniscus just touches the barriers. The amphiphilic molecules dissolved in
solvent are slowly dropped onto the liquid surface using a microsyringe, with care
being taken to spread it uniformly across the surface. Some time must be taken
to allow for evaporation of the solvent, and the spreading of the amphiphile. The
Wilhelmy plate to be used must be absolutely clean. The Wilhelmy plate is then
mounted on the electrobalance such that it is immersed perpendicular to the
surface of the liquid and a uniform meniscus is achieved.
The transfer of a monolayer to a substrate is a delicate process dependent on
many factors, such as, the direction and speed of the substrate, the surface
pressure, composition, temperature, and pH of the subphase. Multilayers can be

A Wilhelmy plate
It is a thin plate that is used to measure equilibrium surface or interfacial
tension at an air-liquid or liquid-liquid interface. In this method, the plate is
oriented perpendicular to the interface, and the force exerted on it is
measured. The Wilhelmy plate consists of a thin plate usually on the order of a
few centimeters square. The plate is often made from glass or platinum which
may be roughened to ensure complete wetting. The plate is cleaned
thoroughly and attached to a scale or balance via a thin metal wire. The force
on the plate due to wetting is measured via a tensiometer or microbalance and
used to calculate the surface tension (σ) using the Wilhelmy equation:
σ = F /(2 l cosθ)
l = the wetted length of the Wilhelmy
plate, θ is the contact angle between
the liquid phase and the plate. In practice
to complete wetting (θ = 0) is assumed.

The Wilhelmy plate measurements
give pressure – area isotherms that
show phase transition-like behaviour
of the LB films. In the gaseous phase,
there is minimal pressure increase for
a decrease in area. This continues
until the first transition occurs and
there is a proportional increase in
pressure with decreasing area. Moving
into the solid region is accompanied by
another sharp transition to a more
severe area dependent pressure. This
trend continues up to a point where
the molecules are relatively close
packed and have very little room to
move. Applying an increasing pressure
at this point causes the monolayer to
become unstable and destroy the
monolayer.

The quantity and the quality of the deposited monolayer on a solid support is
measured by the transfer ratio, t.r. This is defined as the ratio between the decrease
in monolayer area during a deposition stroke, Al, and the area of the substrate, As.
For ideal transfer the t.r. is equal to 1. Depending on the behaviour of the molecule
the solid substrate can be dipped through the film until the desired thickness of the
film is achieved. Different kind of LB multilayers can be produced and/or obtained by
successive deposition of monolayers on the same substrate . The most common one
is the Y-type multilayer, which is produced when the monolayer deposits to the solid
substrate in both up and down directions. When the monolayer deposits only in the
up or down direction the multilayer structure is called either Z-type or X-type.
Intermediate structures are sometimes observed for some LB multilayers and they
are often referred to be XY-type multilayers. The production of so called alternating
layers which consist of two different kind of amphiphiles is also possible by using
highly sophisticated instruments. In such an instrument there is a trough with two
separate compartments both possessing a floating monolayer of a different
amphiphile. These monolayers can then be alternatingly deposited on one solid
substrate.

Isotherms of a fatty acid with a single hydrocarbon chain (left) and a phospholipid
with two hydrocarbon chains (right) are illustrated in Figure. Following the definitions
above one can see that the fatty acid has three distinct regions gas (G), liquid (L1)
and solid (S), while the phospholipid has an additional almost horizontal transition
phase (L2-L1) between the two different liquid phases. This is very
common for phospholipids and the position of this horizontal transition phase is very
temperature dependent. As the temperature is increased the surface pressure value
at which the horizontal transition phase occurs will increase and vice versa.

Modified methods of LB transfer

The mode of monolayer transfer achieved depends on the polarity of the hydrophilic
head of the amphiphile and the surface pressure. If monolayer transfer occurs on both
the down stroke and the up stroke, head-to-head and tail-to-tail orientations of the
monolayers are achieved . This transfer mode is called Y-type, and the LB film obtained
is called Y film. When the monolayer is only transferred during the down stroke of the
solid support, the transfer mode is X-type. The opposite type of monolayer transfer,
transfer only during the up stroke, is called Z-type. In the X film and Z film, the monolayer
orients in a particular direction to form an asymmetric assembly. Such
asymmetric LB films are attractive materials for nonlinear optics. However,
sometimes the monolayer unit folds over during the transfer process, changing
the X or Z film into a symmetric Y film. Therefore, special techniques are
required to ensure asymmetrically assembled LB films.

Application
•LB films can be used as passive layers in MIS (metal-insulator-semiconductor)
which have more open structure than silicon oxide, and they allow gases to
penetrate to the interface more easily and have obvious effects. A ferroelectric
crystal can be electrically switched between two stable polarization states, and
therefore is particularly attractive for use in nonvolatile random-access memories.
Polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene (TrFE)
are particularly attractive ferroelectrics for nonvolatile memory applications due to
their chemical stability, ease of fabrication in thin film form, large polarization,
and excellent switching characteristics. The substrate was a highly polished n-
type silicon wafer and had a 100 nm thick silicon oxide grown on top. Ohmic
contact was made to the back side of the silicon with a vacuum-evaporated
aluminum film annealed at 450 °C for 10 min. A 100 monolayer (ML) ferroelectric
film of the P(VDF-TrFE 70:30) copolymer was fabricated on top of the oxide layer
by horizontal LB deposition from a water subphase at a temperature of 25 °C and
a surface pressure of 5 mN/m, topped with a 90 nm thick Al gate electrode, and
subsequently annealed at 130 °C for 1 h. The device exhibited clear capacitance
hysteresis as the gate voltage was cycled between 625 V, with a capacitance
dynamic range of 8:1 and threshold voltage shift of 2.8 V.

Self assembled nanostructures

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