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 surface2. Nature of the surface3. The nature, purity and concentration of the alkanethiol and assembly

solution4. The nature and purity of the solvent used4. The length and composition of the spacer chain5. The type of head group (size and properties)6. The amount of time the monolayer is allowed to assemble7. 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.

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.Therefore in a mixture of a straight chain alkanethiol and an alkanethiol with a bulky headgroup, it would be expected that the straight chain alkanethiol would dominate the surface.

The Length of the Spacer Chain

A longer alkanethiol with a small head group is recommended.

Type of End groupTo form a monolayer, the alkanethiols must pack into a crystalline-type lattice in specificarrangements 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 fewdefects, 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 spectra2. The ratio S/Au, calculated on the basis of XPS results considered to compare the amounts of adsorbed thiols1. 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 10 μg/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 onSAM covered surfaces

Micrographs of microcrystalline zirconia on (a) a bare gold substratecycled 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 PURPOSEIf there is any ether group attach to the alkyl chain it can be used to trap the various metal ions from the samplee.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 thephotoactive 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 componentleads to the preparation of a sensor device. If the monolayer is immobilizedon 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 monolayerformingcomponent in sensor preparation. In the example shown in Fig. 4.41,a self-assembled monolayer of calixarene was immobilized on an electrodeof a quartz crystal microbalance. The quartz crystal microbalance is a masssensingdevice capable of nanogram-level precision (see Chap. 5). When themonolayer-coveredmicrobalance was exposed to guest gas, the guest bonding caused a change in the resonant frequency. In this case, tetrachloroethylenegave 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 theirassociation 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

Langmuir-Blodgett Assembly

• 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 thebulk 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 throughtechniques 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 plateg= 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 TroughTo 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 achieved by successive dipping through alternating monolayers.

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 verycommon 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. Suchasymmetric LB films are attractive materials for nonlinear optics. However,sometimes the monolayer unit folds over during the transfer process, changingthe X or Z film into a symmetric Y film. Therefore, special techniques arerequired 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, andsubsequently 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.

•Application to glass and make antireflecting but at the same time allowing 99% of visible light to pass through.