35

Chapter 2 - The incorporation of dye molecules in ionically self-assembled

monolayer thin films containing cationic polyelectrolyte,

Poly(diallyldimethylammonium chloride), PDDA, and synthetic hectorite Laponite

RD

2.1 Abstract

Multilayer growth of ionically self-assembled monolayer thin films containing a

cationic polyelectrolyte poly(diallyldimethylammonium chloride), PDDA, a synthetic

hectorite Laponite RD and a polymeric dye PCBS was studied with three different film

characterization techniques: atomic force microscopy, ellipsometry, and UV/Vis

spectroscopy. Layer-by-layer growth of the thin films was monitored with these

techniques for up to 25 quadlayers comprising of two layers of the polycation and one

layer each of the polyanionic dye and laponite with the sequence of polycation- laponite-

polycation-dye. An average thickness of 1.8 nm every quadlayer was found for multilayer

films of PDDA, laponite and PCBS deposited at pH conditions of 7, 10, and 7,

respectively. AFM images of the terminal laponite layer show flat tile- like deposition of

laponite platelets that are about 25-35 nm in diameter. This flat deposition of the platelets

was seen even after 15 quadlayers of the deposited films.

2.2 Introduction

Ultrathin organic films have gained interest in many areas such as integrated

optics [1], sensors, friction reducing coatings or surface orientation layers, and

microelectronics [2] for a variety of reasons. One of the reasons comes from the central

principle of modern technology: miniaturization is good and thin films can significantly

contribute to size reduction of devices. The high surface-to-mass ratio of thin films

makes them a suitable prospect for sensor applications. Due to a variety of assembly

techniques such as physisorption, chemisorption and ionic assembly, numerous organic

and inorganic materials can be integrated into thin films.

The fabrication of organic/inorganic, nanostructured materials with tailored

functionalities, particularly optoelectronic functionalities, is an area of intense interest in

36

current materials research. Modern telecommunications - signal processing, data storage,

transmission – have created the need for high performance optical devices such as lasers,

electro-optic (EO) modulators, image processors, optical memories and harmonic

generators. Bandwidth at a bargain is the primary driver of the fiber-optics industry and

thus improved electro-optic modulators are needed [3]. EO modulators are often

fabricated in the form of thin film devices and thus fabrication techniques for thin,

nanostructured films will be reviewed. These techniques include simple blending of

polymers and inorganic nanoparticles, Langmuir-Blodgett (LB) monolayers, covalent

assemblies, and ionically self-assembled monolayers.

Recent interest in the intercalation of organic polymers into layered clays by

simple blending is due to the unique physical and mechanical properties arising from the

synergistic interactions of the individual components that make up these novel polymer-

ceramic nanocomposites. For example, including as little as 2 % w/w of mica-type

layered silicates dispersed in polyimide causes a 60 % decrease in the permeability of

water, reduces the thermal expansion coefficient by 25 % and enhances the in-plane

storage modulus, while maintaining the dielectric characteristics of the bulk polymer [4].

Langmuir-Blodgett (LB) deposition involves the deposition of preformed

monolayers from a gas- liquid interface to a solid planar substrate [5-8]. Layered films

made by the LB method have been studied for several decades [5-8]. In a recent study,

ordered multilayers composed of CdS, ZnS, PbSe and silver nanoparticles sandwiched

between amphiphile bilayers were constructed using the Langmuir-Blodgett technique

[9]. Due to the crystalline nature of these inorganic particles, the resulting composites

have enhanced structural properties as compared to purely organic composites. Kotov et

al [10] constructed Langmuir-Blodgett monolayers with hectorite (a natural clay that

resembles the platelet- like structure of laponite, but is an order of magnitude larger than

laponite) [11] plates in various organic assemblies. There are several disadvantages of the

LB film technique. One of them is that it can only be used for molecules that water-

insoluble and have surfactant- like properties. Another aspect that has to be controlled

accurately for this type of deposition is the surface pressure, which has to be held

37

constant. Any change in the pressure can change the packing of the molecules and hence

alter the orientation of the chromophores. Excess pressure can break the monolayers. The

resultant films formed are held together by Van der Waals forces and hence have poor

thermal and mechanical stability [7-8]. LB films containing nonlinear optically active

chromophores typically experience a decay of chromophore orientation over time. In

some cases, X and Z type films, both of which can re-orient and form the

thermodynamically stable Y-films [12]. This rearrangement can be extremely

unfavorable for nonlinear optical (NLO) applications, as both X and Z type films are

noncentrosymmetric in nature and hence are NLO active, whereas Y type films are

inherently structurally symmetric and hence are NLO inactive. Due to these

disadvantages of LB films, this method is not very feasible for commercial optoelectronic

device applications. Ionic self-assembly techniques described below offset many of these

disadvantages of LB films as described below.

Covalent self-assembly of molecular adsorbates onto solid substrates involves

covalent bonds between the monolayers and is used to overcome the instability problems

associated with the LB films [13]. In this method, a treated surface is brought in contact

with a material that can covalently bind to the surface. This deposited monolayer is then

chemically treated to enable the deposition of the next layer. Crosslinking of these films

can also be done to further improve its properties [14]. In a study conducted by Yoon et

al [15], they showed that the thermal stability of the NLO-activity of an epoxy polymer

was increased from room temperature to about 100 °C, by crosslinking it with a sol-gel

technique. Covalent self-assembly requires careful control of reaction conditions and, in

some cases, can be a time consuming process and is limited in the choice of materials.

This is because certain reactions may require deposition conditions of high temperatures

and pressures and thus can be expensive. For example in the above described study of

Yoon et al. [15], the crosslinking alone takes 5 hours at an elevated temperature of 80 °C.

Yang et al., have reported that the immersion times could vary from 4 hours to several

days [16]. It is also limited by the extent of reaction for each of the assembly processes.

Thus although this method boasts of high optical constants, has several limitations that

has prevented this method to become very commercially popular.

38

Another technique used for applications of polymer films in nonlinear optical

applications is the guest-host technique (poled polymers), in which the polymer films are

deposited by mechanical methods such as spin-casting on a substrate [17-19]. Here an

NLO-active chromophore is doped into an optically inactive polymer and then aligned

with an electric field. Lastly we talk about ionic self-assembly [20], in which polymers

are adsorbed onto charged substrates in a self- limiting manner and the monolayers are

stably held together by ionic interactions and possibly hydrogen bonding interactions.

Ionic layer-by-layer self-assembly is a novel method for the fabrication of thin

films. The first description of multilayer assemblies by spontaneous adsorption of

alternating layers of positively and negatively charged colloids on charged substrates was

suggested by Iler [21]. In this method a charged substrate is alternately dipped into

oppositely charged polyelectrolytes to formed controlled multilayers. This pioneering

work of Iler was further developed more recently by Decher et al whose work focused

initially on films made by the layer-by-layer adsorption of linear polyions [20,22-24].

The method has distinct advantages over the other thin film fabrication processes such as

LB films [25]. Firstly the preparative procedure is simple and elaborate apparatus is not

required. Secondly, this process is based on the adsorption of soluble or colloidal

components onto solid substrates and thus a wide variety of polyions can be used with a

wide range of hydrophilic or hydrophobic constituents. The water-solubility of polyions

means that film processing does not require organic solvents. Thirdly, the simplicity of

this process makes it suitable to use any kind of charged substrate, thus expanding the

scope of the possible applications. The monolayers formed are physically robust even on

rough surfaces [17]. Further a variety of polymeric materials from biopolymers in the

form of proteins [26-31] to inorganic materials such as clays [32-33] can also be used

with this technique. Finally, an extremely important advantage these films have is that

they deposit within just a few minutes as compared to the time-consuming and high

temperature conditions associated with some of the alternative methods mentioned above.

McAloney et al [34] suggest that in general, initial polymer adsorption occurs in less that

10 seconds. In our present study, the maximum deposition time was not more than 5

39

minutes for all polymeric systems. Thus within a matter of hours, a multilayer film can be

prepared with relative ease. For similar number of layers, other techniques can take as

much as a few days.

The first ISAM films containing a clay were made by Kleinfeld and Ferguson

[35] where they used laponite (0.2% w/w aqueous solution), a synthetic clay and PDDA

(5% w/w aqueous solution) obtaining a bilayer thickness of ~ 3 nm. Kleinfeld and

Ferguson suggested that these nanocomposites offer a potentially powerful strategy for

use in various applications due to the control they provide over structure and thickness.

For example, they demonstrated that ionically self-assembled thin films of PDDA (20%

w/w aqueous solution) and Laponite RD (0.5%w/w aqueous solution) could be used as

humidity sensors due to the water absorption properties of laponite. They showed that

this system has a rapid and reversible absorption of ambient moisture that can be

monitored with ellipsometric and gravimetric techniques [35]. As another example, gold

substrates were coated with an ω-mercaptoalkylammonium compound to promote

adsorption of a silicate layer, thus providing the possibility for preparation of electrodes

with controllable barrier layers or electrical insulators.

Since the initial work by Kleinfeld and Ferguson, a variety of clays have been

used to make ISAM films with the majority of the work done with montmorillonite and

laponite. Lvov et al [26,33] have made films of montmorillonite with several polycations

such as PDDA and poly(ethylenimine) (PEI) with bilayer thicknesses of ~3.6 and 3.3 nm

for PDDA and PEI respectively. Lvov made ISAM films involving proteins and anionic

montmorillonite. He speculated that montmorillonite would potentially form better

structures (flat tile- like montmorillonite deposition with low surface roughness) with

proteins compared to ISAM films made with proteins and organic, linear chain

polyanions.

Multilayer films with laponite have been made by several groups such as van

Duffel and coworkers [36-37] Kleinfeld and Ferguson [35,38-40] and Nicolai et al [41]

where they have shown multilayer growth of laponite containing films with several

polycations. Van Duffel and coworkers have demonstrated layer-by- layer growth of

40

ISAM films that contain a monomeric dye. In our work we have extended this three-

component system to use a polymeric dye.

This chapter is the first part of a two-part study involving the use of laponite clay

platelets in ISAM films fabricated for nonlinear optical (NLO) applications. Recently

there has been considerable interest in the second-order NLO properties of organic films.

Lenahan et al [42] and Heflin et al [43-45] have used commercially available polymeric

dyes such as a linear polydye Poly S-119, in ISAM films that exhibit second harmonic

generation (SHG). In a part of this study we will concentrate on one of these specific

areas, which is the use of materials exhibiting second harmonic generation or frequency

doubling that are used in electro-optic (EO) modulators. This has been a subject of much

interest and study since Franken et al [46] first observed frequency doubling in quartz.

Frequency changing occurs due to the materials ability to change its refractive

index and thus altering the frequency of the light passing through it [47]. This

phenomenon of frequency altering by a medium when an electric field is applied is called

the Pockels effect [48]. An example of this is the altering of the commercial infrared laser

(wavelength of 1064 nm), to green light (wavelength of 532 nm), when passed through

one of these second-order nonlinear media. This transformation is useful because it

quadruples the amount of information the laser can write on an optical disc [49].

Materials that exhibit this frequency doubling undergo a polarization when subjected to

an electric field, which is given by

P(t) = χ(1) E(t) + χ(2) [E(t)]2 + χ(3) [E(t)]3 + …….. (1)

where:

E(t) is the electric field at time t, either from incident light, or applied externally

χ(1) is the linear dielectric susceptibility

χ(2) , χ(3) are the higher order nonlinear susceptibilities.

When the higher order terms are zero, the material is said to be linear. A nonlinear

optical (NLO) material has non-zero higher order susceptibilities. The specific classes of

nonlinear materials that are discussed in this chapter are second-order nonlinear materials

41

or those with a non-zero χ(2). Materials possessing a nonzero χ(2) are noncentrosymmetric.

For those such materials comprised of NLO-active chromophores, noncentrosymmetry

requires preferential alignment so that χ(2) is given by

χ(2) = NFβ〈cos3θ〉 (2)

where N is the density of NLO active chromophores in the film, F is the local field

correction factor, β is the hyperpolarizability of the chromophore, and 〈cos3θ〉 is the

average of cosine3θ, where θ is the angle of orientation of the chromophore relative to

the normal of the film

For successful NLO applications of organic, multilayered films exhibiting SHG, it

is essential that chromophore deposition be uniform from one layer to the next and that

chromophore alignment be maximized. For polymeric ISAM films, chromophore

alignment is complicated by the interpenetration of the constituent monolayers. Several

studies have confirmed that two-component multilayers are not stratified into well-

defined layers but are dispersed and interpenetrating [50-52]. In work by Rubner et al

[53-55], it was shown that the degree of interpenetration of polymeric ISAM films

increased as the number of layers increased. That is, the effect of interpenetration of

polymeric layers was more prominent further away from the impenetrable glass substrate.

In NLO work done by Figura [56] with polymeric ISAM films exhibiting SHG, it was

seen that the SHG signal was maximum at layers close to the substrate and diminished

with layers further away from the substrate with a film of PAH/PS-119.

Our hypothesis is that if we introduce a layer in these films that mimics the effect

of a solid substrate, we may be able to optimize the chromophore orientation and hence

maximize SHG. This chapter deals with achieving the layer-by- layer growth of

multilayer films of the three-component system consisting of a polycation (PAH or

PDDA), laponite clay, and the polymeric NLO-active dye PCBS. Our goal was to define

the deposition conditions and materials requirements for layer-by-layer deposition of the

organic and inorganic constituents with a focus on finding conditions needed for tiling of

the laponite clay.

42

2.3 Experimental

Materials

The materials used for this work include two polycations, an NLO-active

polymeric dye, and Laponite RD clay. Deionized water was obtained from a NanoPure II

system with a resistivity of 18 Ω-cm. Reagent grade NaOH, HCl, H2O2, NH4OH, and

phosphoric acid were used.

The polycations used in this work were poly(diallyldimethylammonium chloride)

or PDDA, [Sigma-Aldrich; 100,000 < Mw < 200,000 g/mole; “low molecular weight”

received as a 20 wt% solution in water; repeat unit molecular weight of 161 g/mole] and

poly(allylamine hydrochloride) or PAH [Polysciences; Mw ~ 70,000 g/mole; repeat unit

molecular weight of 92 g/mole]. The structures of the polycations are shown in Figure 1.

Poly{1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium

salt} or PCBS [Sigma-Aldrich; MW ~ 100,000 g/mole; repeat unit molecular weight of

371 g/mole]. The structure of PCBS is shown in Figure 2. PCBS and the two polycations

were used as received.

The clay used for this study was Laponite RD [Southern Clay Products, Texas], a

synthetic hectorite-type clay with a density of 2570 Kg/m3, a surface area of 900 m2/g

[57], and a mean chemical composition: SiO 2 66.2 %, MgO 30.2 %, Na2O 2.9 %, and

Li2O 0.7 % [41]. When properly dispersed, Laponite has a reported high degree of

monodispersity, which should facilitate flatter deposition. The idealized chemical

formula for Laponite RD is given by [(Si8(Mg5.34Li0.66)O20(OH)4]Na0.66. The cation

exchange capacity (CEC) of laponite platelets is 0.95 meq/g. The primary platelets

dimensions are 0.97 nm thick and a diameter ~ 25-35 nm. Dispersion conditions are

discussed in more detail in the Results section below. Laponite platelets, in the dry form,

usually form aggregates in which clay sheets are separated by exchangeable cations and

one or two water layers [37]. Each layer comprises three sheets, two outer tetrahedral

silica sheets and a central octahedral magnesium sheet and has a thickness of 0.97 nm.

Some of the magnesium in the central sheet is replaced by lithium leading to a net

negative charge of the layer, which is balanced by sodium ions located between adjacent

layers in a stack [58]. The polyvalent nature of laponite leads to its capability in ISAM

films to “heal” packing defects formed in the preceding adsorption cycle [40].

43

Another reason for choosing Laponite was the reported smoothness of ISAM

films made with it as compared to those made with other clays [36]. There are a variety

of natural and synthetic clays that could be used. Suspensions of natural colloidal clay

platelets exhibit a wide variety of structural and mechanical properties and are frequently

used as thickeners, fillers and antisettling agents [59]. Montmorillonite is one such

natural clay which deposits as single sheets or thin platelets composed of 2-3 sheets in

which the negative charge is balanced by inter and intra- lamellar sodium cations [60].

These platelets are about 200 nm in size. The disadvantage that natural clays have when

compared with their synthetic counterparts is that they are usually very polydisperse in

both shape and size. Synthetic clays comparatively have a much higher degree of

monodispersity when in solution. Van duffel et al [36] compared ISAM film formation

using laponite and a natural hectorite as the anionic species and PDDA as the polycation.

Films formed by natural clays were found to be significantly rougher than films made

with synthetic clays. This was believed to be due to either the overlapping of natural clay

particles more with each other as compared to the synthetic clays when adsorbed on

positively charged surfaces or possibly a lesser extent of adsorbtion as compared to

synthetic clays. Thompson et al have done a pH study on laponite that indicate that it is

best (in terms of adsorption) for the pH of laponite to be around 10 to avoid degradation

or dissolution of the laponite particles [61]. Even a high pH (>11.0) is not favorable,

since at this pH some chemical breakdown could occur and also the ionic strength at this

pH is too high for full dispersion.

Solution and Suspension Preparation

Stock solutions of polymers and laponite were prepared by mixing in a beaker

with about 80% of the final volume of water and the solutions were stirred for about 1

hour. The remaining amounts of water were then added and the solutions were typically

stirred overnight. The pH was then adjusted to the desired value with NaOH or HCl. In

the case of the laponite RD suspension, phosphoric acid was used to adjust the pH to 6.

The solution was left to stir until the pH remained unchanged and then was further

adjusted if required.

44

Slide Cleaning

Glass microscope slides obtained from Fischer Scientific (Premium grade) were used as

substrates for the films. The slides were prepared for dipping using the RCA cleaning

method given below . In the RCA cleaning method, glass was cleaned using two

solutions – a base and an acid solution. The base solution was composed of 92.6 ml H2O,

10.3 ml H2O2 (30% w/w), 17.2 ml NH4OH mixed in a volumetric flask - the mixture was

swirled for 30 sec after the addition of each component. The acid solution was composed

of 96.0 ml H2O, 9.0 ml H2O2, 15.0 ml HCl (1.18M) mixed in a volumetric flask - the

mixture was swirled for 30 sec after the addition of each component.

Glass slides were put in a glass-slide holder, base solution was added, the holder

was covered with a lid and placed in a dish filled with DI water. This assembly was then

placed on a heating plate. After the temperature reached 70 °C, it was left for 20 min and

then the slides were removed. The slides were then rinsed with the rising procedure

mentioned above and placed in a clean glass-slide holder, after which the acid solution

added. After 20 min, they were removed and rinsed again. These were then put in another

glass-slide holder and placed uncovered in the oven at 130 °C, for at least an hour. Then

they were removed from the oven and allowed to cool. The slides treated in this manner

are negatively charged for deposition of the first monolayer of the polycation, which was

positively charged.

Film Formation Procedure

Dipping conditions.

For both polycations, the concentration used in the dipping experiments was 0.01

mole repeat unit/liter and the dipping time was 5 minutes. The polycations were

deposited at pH 7 and 10 although most of the work was done at a pH 7. The

concentration of the PCBS was 0.01 mole repeat unit/liter and was deposited at pH 7 in

all cases with a dipping time of 5 minutes. No salts were added to the polymer solutions –

only acid or base were added to adjust the pH.

The Laponite RD was deposited at a concentration of 0.2 % w/w in DI water

where the unadjusted pH was approximately 10. This condition was determined by

dynamic light scattering experiments, described below, to give individual platelets with

45

equivalent spherical hydrodynamic diameters of 25-35 nm. The dipping time for the

laponite suspensions was 5 minutes in all cases.

Dipping was done in standard dipping jars with about 40 ml of the solutions. First

the charged glass slide was dipped into the polycation solution (PDDA or PAH) and was

then rinsed thoroughly with DI water. The rinsed slide was then dipped into the

polyanionic PCBS solution.

Rinsing between the dipping steps was done with DI water. Each slide, after

removal from the dipping jar, was first agitated in a 40 ml beaker containing DI water for

about 5 sec to remove loosely bound polymer and then 500 ml of DI water was poured

over it for further rinsing. This procedure was repeated twice.

Drying of slides.

Slides were dried in a stream of nitrogen gas filtered through a 5 micron filter.

Drying was done usually every 5 bilayers/quadlayers, and at the end of the final

bilayer/quadlayer deposition. It was also done after the deposition of every laponite layer.

Deposition sequence.

A polycation layer was first deposited on a cleaned, negatively charged glass

slide. This was followed by a layer of Laponite clay and was then followed by another

layer of the polycation and then the PCBS dye. This constitutes a quadlayer and is

illustrated in Figure 3. This sequence was then repeated to deposit more quadlayers.

Dynamic Light Scattering

Dynamic Light Scattering (DLS) measurements were performed with a DynaPro-

801 TC from Protein Solutions Inc with a laser operating at 836.4 nm. Prior to

conducting the light scattering experiments, the sample chamber was flushed with 1-2 ml

of DI water using a 0.02 µm Whatman Anotop syringe filter. Experiments were

performed at 25°C using a 0.1 µm Whatman Anotop syringe filter for the Laponite RD.

The laponite concentration was 0.2 % w/w. Size distribution analyses were conducted

using two algorithms in the Dynamics software [62]: Regularization and Dynals. Both

methods report at least three peaks (the Dynals method can report more than three),

46

giving the relative scattering percentages for each and reporting a value for the

hydrodynamic radius, given by the following Stokes-Einstein equation:

RH = kbT/6pηDT (2)

where kb is Boltzmann’s constant, T is the temperature, η is the solvent viscosity, Rh is

the hydrodynamic radius and DT is the translation diffusion coefficient. The light

scattering was performed at a fixed angle of 90o.

UV-Visible Spectrophotometry

Absorbance measurements were done on U-2000 Spectrophotometer made by

Hitachi Instruments Inc. The characteristic absorbance peak for PDDA is 275.5 nm while

the characteristic absorbance peak for PCBS is 359 nm. Layer-by- layer growth of the

films was characterized by measuring the absorbance at 359 nm of films after the

deposition of every 5 quadlayers. The slope of the plots and their standard deviations

were calculated by linear least squares regression[63].

Ellipsometry

Thickness measurements were made with the VB-200 VASE Ellipsometer made

by J.A.Woollam Company. The data were analyzed by the WVASE32 software, version

3.361. The instrument was calibrated, prior to the experiments with a silicon wafer. After

the calibration, each ISAM film sample was mounted on the vacuum mount and exposed

to monochromatic light at different wavelengths. The range of wavelengths used for our

measurements was 200-1000 nm. The VASE measures the change in polarization of light

reflected off a sample as a function of optical wavelength and incident angle. The beam

averages the lateral properties of the sample within a 3 mm by 10 mm spot size. Film

thickness was then estimated by the WVASE software by modeling the experimental

spectra using the Lorentz oscillator model [64]. The thickness data were measured after

the deposition of every 5 quadlayers and were plotted as a function of the number of

quadlayers.

47

Atomic Force Microscopy

Surface roughness and laponite deposition were conducted using the Digital

Instruments atomic force microscope Dimension 3000, which uses Nanoscope IIIa SPM

controller. For our experiments, we used the TappingMode™ AFM. This operates by

scanning a silicon tip attached to the end of an oscillating cantilever across the sample

surface. The tip taps lightly on the sample surface during scanning, contacting the surface

at the bottom of its swing. By maintaining a constant oscillation amplitude, a constant tip-

sample interaction is maintained during imaging. The distance the tip travels with the

help of the cantilever, within the specified surface range, to keep the tip-sample

interaction is measured by a detector, which translates it into a surface image with the

help of the operating software.

2.4 Results

Dispersion of Laponite RD in Aqueous Suspension

In order to obtain flat, tiled, relatively impenetrable Laponite surfaces suitable for

SHG film characterization, it was necessary to deposit the clay platelets from well-

dispersed suspensions at conditions of ionic strength and pH at which Laponite is

chemically stable. Thus, dynamic light scattering was used to determine suspension

conditions needed to disperse the Laponite platelets. Laponite RD at a concentration of

0.2 % w/w in DI water has an unadjusted pH of ~ 10. At these conditions, the average

size of the laponite platelets obtained from DLS experiments was 35 nm. Both the Dynals

and Regularization algorithms of the DLS software showed a single peak indicating that

the polydispersity was close to zero. After the pH was adjusted to 6 with phosphoric acid,

with the Laponite concentration still at 0.2 % w/w, the solution could not be injected into

the DLS chamber with a 0.1 µm filter indicating significant aggregation. For this reason,

no ISAM films were made with Laponite at pH = 6.

Characterization of Film Deposition

The absorbance at 359 nm and thicknesses of the films were measured after the

deposition of every 5 quadlayers. Absorbance data for films made with PDDA deposited

48

at pH = 7 and 10 are shown in Figure 4 whereas the film thicknesses as determined by

ellipsometry are shown in Figure 5. The slopes of the absorbance/quadlayer plot and the

thickness plot and their standard deviations of the slopes determined by linear least

squares regression are summarized in Table 1. The relative standard deviations of the

absorbance/quadlayer slopes were 4-8% while those of the thickness per quadlayer were

5.6-7%. The absorbance/quadlayer slope increased by about 70% as the pH of deposition

of the PDDA increased from 7 to 10 whereas the thickness per quadlayer increased by

about 55% over that same pH range.

Similar film deposition experiments conducted with PAH as the polycation

yielded visually cloudy and laterally inhomogeneous films. This cloudiness was visible at

quadlayer numbers as low as two when the PAH was deposited at pH = 10. When the

PAH was deposited at pH = 7, the cloudiness and film inhomogenieties appeared after the

deposition of six quadlayers. No experiments with PAH were done with more than six

quadlayers and no further characterization of the films made with PAH was done.

Film Surface Characterization

AFM was used to characterize the surfaces of the clean glass slides and terminal

laponite layers on the slides. Height images are shown in Figure 6 while the

corresponding phase images are shown in Figure 7. In Figure 6, roughness calculations

were performed on the height images on both the entire image as well as on smaller areas

indicated by the boxes. The average roughness - specified as “Img. Ra” in the figures - of

the cleaned glass slide was 0.19 nm while that of the terminal Laponite layer deposited on

only one PDDA layer (pH 7) on the glass was about 0.97 nm. After 15 quadlayers were

deposited, the roughness of the terminal Laponite layer was about 1.5 nm. The domain

sizes in the phase images of the terminal laponite layers in Figures 7(b) and 7(c) vary

somewhat but are consistent over much of the micrographs with the lateral dimensions of

Laponite platelets measured as ~35 nm.

49

2.5 Discussion

Dispersion of Laponite RD in Aqueous Suspension

Both dynamic light scattering (DLS) and atomic force microscopy (AFM) show

these platelets to be about 35 nm when the suspension pH = 10. Kleinfeld et al. found this

platelet size to be in the range of about 25-35 nm [35] and Van Duffel et al obtained sizes

of about 30 nm for these platelets [37]. These differences could be due to various reasons

such as dispersion times, solution concentrations as well as difference in dispersion

methods of the separate groups. The aggregation of Laponite at pH = 6 and the stability at

pH = 10 are consistent with earlier reports on the dispersion of Laponite RD in water by

Tawari et al [65] and Nicolai et al [66]. Tawari et al. showed that Laponite platelets in

aqueous suspensions have negative charges on their basal planes and edges that are

amphoteric and that are protonated for pH < 11. For pH ~ 10 under dilute conditions (0.2

% w/w), the basal planes are quite negatively charged while the edges are only slightly

positively charged and thus the platelets are well dispersed. For pH ~ 6, the edges are

more protonated and aggregation occurs due to face-edge attraction.

Effect of Polycation pH on Layer-by-Layer Growth

The linear plots in Figures 4 and 5 and their relative standard deviations of the

slopes shown in Table 1 are consistent with layer-by- layer deposition of the film

components. The ~70% increase in the absorbance/quadlayer slope and the

corresponding ~55% increase in quadlayer thickness as the PDDA pH increased from 7

to 10 are consistent with the deposited PDDA layer becoming thicker with more loops

and tails at the higher pH, thus providing more binding sites for the PCBS. At pH= 10,

the PDDA is less soluble in solution due to the higher ionic strength, than at pH = 7.

Thus, the PDDA adsorbs at pH = 10 with thicker loops and tails whereas, at pH = 7, it

adsorbs more in flat trains. The thicker films at pH = 10 can bind more of the polyanionic

PCBS. The quadlayer thicknesses of 1.8 nm at a PDDA pH= 7 and 2.8 nm at a pH = 10

are consistent with there being, on the average, a Laponite layer one platelet thick,

deposited in a flat, tile- like manner per quadlayer with the remainder comprised of PDDA

and PCBS.

50

One of the essential conditions for the regular layer-by- layer growth in this

system is that the laponite platelets deposit in a flat, tile- like manner, which is seen from

the AFM images in Figures 6 and 7 that indicate roughnesses of 0.97-1.49 nm. The AFM

images in Figures 6(b-c) and 7(b-c) show platelets with sizes that are consistent with the

dynamic light scattering results of hydrodynamic diameter ~ 35 nm which is further

evidence of largely flat, tile-like deposition. By contrast, if the platelets had deposited

with a more random orientation of their basal planes, this would have likely hindered

layer-by- layer deposition of the PDDA and PCBS layers.

In the work done by Lvov et al [33], with montmorillonite platelets and Kleinfeld

and Ferguson’s [35] study with laponite, they noticed that during the dipping procedure,

platelets mostly anchored to the surface and further relaxation and layer formation

occurred during the intermediate drying step.

In comparison to the present work, Kleinfeld and Ferguson found the Laponite

platelet size in PDDA films to be about 25 nm by AFM [35] whereas Van Duffel et al

obtained sizes of about 30 nm for Laponite in ISAM films made also with PDDA [36].

Given the differences in experimental techniques, i.e. differences in dispersion times and

mixing for the laponite and PDDA concentrations, and the uncertainties inherent in

interpreting AFM images, these studies are in reasonable agreement with the present

work on the finding that the Laponite deposits principally in a flat, tiling manner and that

the PDDA deposits in a layer-by- layer manner as well. The principal finding in the

present work is that the PCBS component is also incorporated into the

PDDA/Laponite/PCBS films in such a manner that all three components exhibit layer-by-

layer growth.

Effect of Polycation Type on Film Formation

The inhomogeneous ISAM films of Laponite, PCBS, and the polycation PAH

were likely due to PAH being a weaker polycation than PDDA. The secondary amine in

PAH has a pKa ~ 8.7 and thus its degree of ionization and solubility is more pH-

dependent than those of PDDA. At the higher pH of 10, PAH is only slightly charged and

hence becomes less soluble, forming aggregates in solution. We hypothesize that the

PAH deposits on the Laponite at least partly as aggregates which, in turn, leads to

51

subsequent Laponite deposition in the form of aggregates that scatter light, thus yielding

cloudy films. For the deposition of PAH at pH = 7, there is something more complicated

that could be possibly occurring. Laponite has a high cation exchange capacity

(0.95meq/g) [39], meaning that it pulls protons from solution and releases corresponding

amounts of cations. This is the reason why it has a natural unadjusted pH of about 10.

Thus, when the laponite film is immersed in a PAH solution, even though the pH of the

PAH is 7, the release of the cations could possibly increase the local pH of the solution

near the laponite layers which would then again result in the deposition of PAH in

aggregates. The possibility of the increase in local pH for PAH was indicated by the pH

measurements of the PAH solution after deposition, showing an increase from 7 to ~ 8.5.

This increase was not seen with the PDDA solution. Thus for such nanocomposites to be

constructed with PAH as the polycation, it would be more advisable to use a clay that has

a lower ion exchange capacity than laponite. However, a problem that could arise with a

clay with a lower ion exchange capacity is the possibility of inadequate clay deposition.

This is because, the cation exchange ability of the clay is the driving force for it being

able to deposit for film formation. That would explain the results obtained by Durand-

Piana and coworkers [67], where they find that polycations with quaternary ammonium

groups, like PDDA, form the most stable complexes with clays. These types of

polycations have charge densities that are relatively insensitive to changing pH

conditions. Glinel and associates [68] confirmed, in a study comparing complexation of

clay platelets of laponite with polycations of various structures, that best conditions in

terms of film growth are obtained with PDDA.

2.6 Conclusions

In this first part of a two-part study, we have shown that the polyanionic dye

PCBS can be incorporated into ionically self-assembled monolayer films with the

polycation PDDA and the synthetic clay Laponite RD. Layer-by- layer growth of these

films was demonstrated by both UV/Vis spectrophotometry and by ellipsometry when the

PDDA was deposited at pH = 7 and 10. Film thickness and the amount of PCBS dye

deposited increased by roughly the same proportion – 55 – 70% - as the pH at which the

52

PDDA was deposited increased from 7 to 10. This increase is consistent with greater

electrostatic screening at pH = 10 leading to thicker and more loopy deposited layers. The

thickness per quadlayer and AFM images of terminal Laponite layers are consistent with

Laponite deposition in a tile-like fashion. We believe that this flat deposition would limit

the degree of interpenetration of these ISAM films due to the formation of an

impenetrable surface. In the next part, we will study the effect of the incorporation of the

clay particles on the second-order NLO behavior of these ISAM films. We plan to test the

hypothesis that the Laponite platelets can mimic the effect of the substrate and hence

enhance the resultant SHG signal.

2.7 Acknowledgements

• This work was supported by grant # ECS-9907747 from the National Science

Foundation.

• Stephen McCartney in the Materials Research Institute at Virginia Tech., for help

with the AFM imaging.

• Brian Dickerson in Materials Engineering at Virginia Tech., for help with the

ellipsometric measurements.

2.8 References

[1] “Polar Orientation of Dyes in Robust Multilayers by Zirconium Phosphate-

Phosphonate Interlayers”, H. E. Katz, G. Scheller, T. M. Putvinski, M. L. Schilling,

W. L. Wilson, C. E. D. Chidsey, Science, 254, 1485-1487, 1991.

[2] “Metal-Insulator-Semiconductor and Metal-Insulator-Metal Devices Derived from

Zirconium Phosphonate Thin Films”, L. J. Kepley, D. D. Sackett, C. M. Bell, T. E.

Mallouk, Thin Solid Films 207, 132-136, 1992.

[3] Kristen Lewotsky, Spie’s Oemagazine, 20-23, January 2001.

[4] “Synthesis and properties of two-dimensional nanostructures by direct intercalation

of polymer melts in layered silicates”, R. Vaia, H. Ishii, E. Giannelis, Chem. Mater,

5, 1694-1696, 1993.

53

[5] “The constitution and fundamental properties of solids and liquids. II. Liquids”, I.

Langmuir, J. Am. Chem. Soc., 39, 1848-1906, 1917.

[6] “Films Built by Depositing Successive Monomolecular Layers on a Solid Surface”,

K. Blodgett, J. Am. Chem. Soc., 57, 1007-1022, 1935.

[7] A. Ulman, “An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to

Self-Assembly”, Academic Press, Boston, MA, 1991.

[8] G. L. Gaines Jr., “Insoluble Monolayers at Liquid-Gas Interfaces”, Interscience,

New York, 1966.

[9] “Nanochemistry: Synthesis in Diminishing Dimensions”, G. Ozin, Adv. Mater., 4,

612-649, 1993.

[10] “Spreading of Clay Organocomplexes on Aqueous Solutions: Construction of

Langmuir-Blodgett Clay Organocomplex Multilayer Films”, N. Kotov, F. Meldrum,

J. Fendler, E. Tombacz, I. Dekany, Langmuir, 10, 3797-3804, 1994

[11] Technical Directory for Laponite, Laporte Industries Ltd., UK.

[12] A. Ulman, “Characterization of Organic Thin Films”, Butterworth-Heinemann,

Boston, 1995.

[13] “Adsorption of ordered zirconium phosphonate multilayer films on silicon and gold

surfaces”, H. Lee, L. Kepley, H. Hong, S. Akhter, T. Mallouk, J. Phys. Chem., 92,

2597-2601, 1988.

[14] “A new approach to construction of artificial monolayer assemblies”, L. Netzer, J.

Sagiv, J. Am. Chem. Soc., 105, 674-676, 1983.

[15] “New Second-order nonlinear optical (NLO) epoxy polymer treated by sol-gel

processing”, C. Yoon, H. Shim, Macromol. Chem. Phys., 199, 2433-2437, 1998.

[16] “A Molecular Architectural Approach to Second-Order Nonlinear Optical

Materials”, X. Yang, D. McBranch, B. Swanson, D. Li, Mat. Res. Soc. Symp. Proc.,

392, 27-32, 1995.

[17] “Second-order nonlinearity in poled polymer systems”, D.M. Burland, R.D. Miller,

C.A. Walsh, Chem. Rev., 94, 3-29, 1994.

[18] “Second-order non- linear optical polymers”, C. Samyn, T. Verbiest, A. Persoons,

Macromol. Rapid Commun., 21, 1-15, 2000.

[19] P. Pantellis, J. Hill, L. A. Hornak, “Polymers for Lightwave and Integrated Optics”,

54

ed., Marcel Dekker, Inc., New York, 1992.

[20] “Buildup of ultrathin multilayer films by a self-assembly process:III. Consecutively

alternating adsorption of anionic and cationic polyelectrolytes on charged surfaces”,

G. Decher, J. D. Hong, J. Schmitt, Thin Solid Films 210/211, 831-835, 1992.

[21] “Multilayers of colloidal particles”, R. K. Iler, J. Colloid Interface Sci., 21, 569-594,

1966.

[22] “Buildup of Ultrathin Multilayer Films by a Self-assembly Process: II. Consecutive

Adsorption of Anionic and Cationic Bipolar Amphiphiles and Polyelectrolytes on

Charged Surfaces”, G. Decher, J. D. Hong, Ber. Bunsen-Ges. Phys. Chem., 95,

1430-1434, 1991.

[23] “Assembly, structural characterization, and thermal behavior of layer-by- layer

deposited ultrathin films of poly(vinyl sulfate) and poly(allylamine)”, Y. Lvov, G.

Decher, H. Mohwald, Langmuir, 9, 481-486, 1993.

[24] “Neutron reflectivity analysis of self-assembled film super lattices with alternate

layers of deuterated and hydrogenated polystyrenesulfonate and polyallylamine”, D.

Korneev, Y. Lvov, G. Decher, J. Schmitt, S. Yarodaikin, Physica B, 213/214,

954-956, 1995.

[25] “Assembling Alternate Dye-Polyion Molecular Films by Electrostatic Layer-by-

Layer Adsorption”, K. Ariga, Y. Lvov, T. Kunitake, J. of Am. Chem. Soc., 119,

2224-2231, 1997.

[26] “Assembly of Multicomponent Protein Films by means of Electrostatic Layer-by-

Layer Adsorption”, Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, J. of Am. Chem.

Soc., 117, 6117-6123, 1995.

[27] “Layer-by- layer assembly of alternate protein/polyion ultrathin films with electrolyte

attraction as driving force”, Y. Lvov, K. Ariga, T. Kunitake, Chem. Lett.,

2323-2326, 1994.

[28] “Sequential actions of glucose oxidase and peroxides in molecular films assembled

layer-by- layer alternate adsorption”, M. Onda, Y. Lvov, K. Ariga, T. Kunitake,

Biotechnol. Bioeng., 51, 163-167, 1996.

[29] “Layer-by- layer Architectures of Concanavalian A by means of Electrostatic and

Biospecific Interactions”, Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, J. Chem.

55

Soc., Chem. Commun. 2313-2314, 1995.

[30] “Successive Deposition of Alternate Layers of Polyelectrolytes and a Charged

Virus”, Y. Lvov, H. Haas, G. Decher, H. Mohwald, A. Mikhailov, B.

Mtchedlishvily, E. Morgunova, B. Valinshtein, Langmuir, 10, 4232-4236, 1994.

[31] “A new kind of immobilized enzyme multilayer based on cationic and anionic

interaction”, W. Kong, X. Zhang, M. Gao, H. Zhou, W. Li, J. Shen, J. Macromol.

Rapid Commun., 15, 405-409, 1994.

[32] “Layer-by- layer Self-Assembly of Polyelectrolyte-Semiconductor Nanoparticle

Composite Films”, N. Kotov, I. Dekany, J. Fendler, J. Phys. Chem., 99, 13065-

13069, 1995.

[33] “Formation of Ultrathin Multilayer and Hydrated Gel from Montmorillonite and

Linear Polycations”, Y. Lvov, K. Ariga, I. Ichinose, T. Kunitake, Langmuir, 12,

3038-3044, 1996.

[34] “In Situ Investigations of Polyelectrolyte Film Formation by Second Harmonic

Generation”, R.A. McAloney, M.C. Goh, J. of Phys. Chem. B, 103, 10729-10732,

1999.

[35] “Stepwise Formation of Multilayered Nanostructural Films from Macromolecular

Precursors”, E. R. Kleinfeld, G. S. Ferguson, Science, 265, 370-373, 1994.

[36] “Multilayered Clay Films: Atomic Force Microscopy Study and Modelling”, B. van

Duffel, R. A. Schoonheydt, Langmuir, 15, 7520-7529, 1999.

[37] “Fuzzy Assembly and Second Harmonic Generation of Clay/Polymer/Dye

Monolayer Films”, B. van Duffel, T. Verbiest, S. V. Elshocht, A. Persoons, F. C.

De Schryver, R. A. Schoonheydt, Langmuir, 17, 1243-1249, 2001.

[38] “Rapid, Reversible Sorption of Water from the Vapor by a Multilayered Composite

Film: A Nanostructured Humidity Sensor”, E. R. Kleinfeld, G. S. Ferguson, Chem.

Mater., 7, 2327-2331, 1995.

[39] “Mosaic Tiling in Molecular Dimensions”, E. R. Kleinfeld, G. S. Ferguson, Adv.

Mater., 7, 414-416, 1995.

[40] “Healing of Defects in the Stepwise Formation of Polymer/Silicate Multilayer

Films”, E. Kleinfeld, G. Ferguson, Chem. Mater., 8, 1575-1578, 1996.

[41] “Structure of gels and aggregates of disk- like colloids”, T. Nicolai, S. Cocard, Eur.

56

Phys. J. E 5, 221-227, 2001.

[42] “Novel Polymer Dyes for Nonlinear Optical Applications Using Ionic Self-

Assembled Monolayer Technology”, K. Lenahan, Y. Wang, Y. Liu, Adv. Mater.,

10, 853-855, 1998.

[43] “Noncentrosymmetric Ionically Self-Assembled Thin Films for Second Order

Nonlinear Optics”, J. R. Heflin, Y. Liu, C. Figura, D. Marciu, R. Claus, Organic

Thin Films for Photonics Applications, v 14, 78-80, 1997.

[44] “Second Order Nonlinear Optical Thin Films Fabricated from Ionically Self-

Assembled Monolayers”, J. R. Heflin, Y. Liu, C. Figura, D. Marciu, R. Claus, Proc.

SPIE, 3147, 10-19, 1997.

[45] “Thickness Dependence of Second-Harmonic Generation in Thin Films Fabricated

from Ionically Self-assembled Monolayers”, J. R. Heflin, C. Figura, Y. Liu, D.

Marciu, R. Claus, Appl. Phys. Lett., 74, 495-497, 1999.

[46] “Generation of Optical Harmonics”, P. A. Franken, A. E. Hill, C. W. Peters and G.

Weinreich, Phys. Rev. Lett., 7, 118-119, 1961.

[47] “Electro-optic Effects and Optically Nonlinear materials”, W. Du,

www.phys.vt.edu/~graupner/5555/Du/sld001.htm, 1999.

[48] R. W. Boyd, “Nonlinear Optics”, Academic Press, Rochester, New York, 1992.

[49] “Devices Based on Electro-optic Polymers Begin To Enter Marketplace”, Ron

Dagani, Chemical & Engineering News, March 4,22-27, 1996.

[50] “Fuzzy Nanoassemblies Toward Layered Polymeric Multicomposites”, G. Decher,

Science, 277, 1232-1237, 1997.

[51] “Detailed Structure of Molecularity Thin Polyelectrolyte Multilayer Films on Solid

Substrates as revealed by Neutron Reflectometry”, M. Losche, J. Schmitt, G.

Decher, W. G. Bouwman, K. Kjaer, Macromolecules, 31, 8893-8906, 1998.

[52] “Internal structure of layer-by- layer adsorbed polyelectrolyte films: a neutron and

x- ray reflective study”, J. Schmitt, T. Grunewald, G. Decher, P. S. Pershan, K.

Kjaer, M. Losche, Macromolecules, 26, 7058-7063, 1993.

[53] “Controlling Bilayer Composition and Surface Wettability of Sequentially Adsorbed

Multilayers of Weak Polyelectrolytes”, D. Yoo, S. Shiratori, M. Rubner,

Macromolecules, 31, 4309-4318, 1998.

57

[54] “Forster Energy Transfer Studies of Polyelectrolyte Heterostructures Containing

Conjugated Polymers: A Means To Estimate Layer Interpenetration”, J. Baur, M.

Rubner, J. Reynolds, S. Kim, Langmuir, 15, 6460-6469, 1999.

[55] “pH-Dependent Thickness Behavior of Sequentially Adsorbed Layers of Weak

Polyelectrolytes”, S. Shiratori, M. Rubner, Macromolecules, 33, 4213-4219, 2000.

[56] C. Figura, “Second Order Nonlinear Optics in Ionically Self-Assembled Thin

Films”, Ph.D. dissertation, Department of Physics, Virginia Tech, 1999.

[57] Technical Directory for Laponite, Laporte Industries Ltd., UK.

[58] “Unusual Thixotropic Properties of Aqueous Dispersions of Laponite RD”, N.

Willenbacher, J. Colloid Interface Sci., 182, 501-510, 1996.

[59] “Sol-Gel Transitions in Aqueous Suspensions of Synthetic Takovites. The Role of

Hydration Properties and Anisotropy”, L. J. Michot, J. Ghanbaja, V. Tirtaatmadja, P.

J. Scales, Langmuir, 17, 2100-2105, 2001.

[60] “Mechanism of and Defect Formation in the Self-Assembly of Polymeric Polycation

– Montmorrilonite Ultrathin Films”, N. A. Kotov, T. Haraszti, L. Turi, G. Zavala, R.

E. Geer, I. Dekany, J. H. Fendler, J. Am. Chem. Soc., 119, 6821-6832, 1997.

[61] “Nature of laponite and its aqueous dispersions”, D. W. Thompson, J. T.

Butterworth, J. Colloid Interface Sci., 151, No. 1, 236-243, 1992.

[62] See www.protein-solutions.com for description of particle size distribution analysis.

[63] P. R. Bevington, “Data Reduction and Error Analysis for the Physical Sciences”,

McGraw Hill, N.Y. 1969.

[64] R. M. Azzam and N. M. Bashara, "Ellipsometry and Polarized Light", Elsevier,

N.Y. 1987

[65] “Electrical Double-Layer Effects on the Brownian Diffusivity and Aggregation Rate

of Laponite Clay Particles”, S. Tawari, D.L. Koch, C. Cohen, J. Coll. & Interf. Sci.,

240, 54-66, 2001.

[66] “Light Scattering Study of Dispersion of Laponite”, T. Nicolai, S. Cocard,

Langmuir, 16, 8189-8193, 2000.

[67] “Flocculation and Adsorption Properties of Cationic Polyelectrolytes toward

Na-Montmorillonite Dilute Suspensions”, G. Durand-Piana, F. Lafuma,

R. Audebert, J. Colloid Interface Sci., 119, 474-480, 1987.

58

[68] “Ordered Polyelectrolyte “Multilayers”. 3. Complexing with Clay Platelets with

Polycations of Varying Structure”, K. Glinel, A. Laschewsky, A. M. Jonas,

Macromolecules, 34, 5267-5274, 2001.

59

2.9 Tables & Figures

Table I – Absorbance and Thickness per quadlayer

PDDA

pH

Absorbance*/Quadlayer σ absorbance Thickness/Quadlayer

(nm)

σ thickness

(nm)

7 0.00186 0.00015 1.8 0.1

10 0.0032 0.00014 2.8 0.2

*measured at 359 nm

(Note: Standard deviation (σ) was calculated using least squares regression [63]).

60

(a) (b)

Figure 1. Polycation structures: (a) PDDA (b) PAH

61

(

CO2- Na

+

N

SO2

NH

N

) n

OH

Figure 2. Structure of PCBS

62

Substrate

Figure 3. Quadlayer deposition sequence: Polycation, Dye, Clay

63

0

0.05

0.1

0.15

0.2

0.25

0 10 20 30 40 50 60

Quadlayers Deposited

Ab

sorb

ance

at

359

nm

PDDA pH = 10

PDDA pH = 7

Figure 4: Absorbance at 359 nm versus quadlayer number for films made with PDDA

under the different pH conditions of 7 and 10. Note: The quadlayer numbers shown in the

chart are double that of the deposition cycles, since it considers the film deposited on

both sides of the glass substrate.

64

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30

Quadlayers Deposited

Th

ickn

ess

in n

m

PDDA pH = 10

PDDA pH = 7

Figure 5: Thickness Vs Quadlayer number for films made with PDDA under the different

pH conditions of 7 and 10. The quadlayer numbers shown in the chart are double that of

the deposition cycles.

65

Figure 6 (a)

Roughness Analysis

66

Roughness Analysis

Figure 6 (b)

67

Figu re 6 (c)

Figure 6: Height images obtained by AFM of: (a) Plain Glass slide with an average

roughness (Img. Ra) of 0.49 nm over the entire image area and 0.26 nm over the area

covered in the box (indicated by Mean roughness, Ra). (b) Terminal laponite layer

deposited after a PDDA layer (pH = 7.0) on cleaned glass (laponite layer of the first

quadlayer), with an average roughness (Img. Ra) of 0.97 nm over the entire image area

and 0.93 nm over the area in the box. (c) Terminal laponite layer deposited after the

deposition of 15 quadlayers (PDDA pH = 7.0 & PCBS = 7.0), with an average roughness

(Img. Ra) of 1.48 nm and 1.49 nm over the area in the box.

Note: In image and box statistics, the Z range and Rmax give the difference between the

highest and lowest points on the image, relative to the central plane. Img. Ra and Mean

Roughness Analysis

68

roughness (Ra) represent the average roughness in the image and box statistics

respectively, and Img. Rms and Rms represent the RMS value of the roughness in the

image and box statistics respectively. The surface area differential for both the image and

the box, represents the percentage of the 3-D area of the image to the 2-D area produced

by the projection of the surface onto the threshold plane.

69

Figure 7 (a)

Figure 7 (b)

70

Figu re 7 (c)

Figure 7: Phase images obtained by AFM of: (a) Plain glass slide. (b) Terminal laponite

layer deposited after a PDDA layer (pH = 7.0) on cleaned glass (laponite layer of the first

quadlayer) (c) Terminal laponite layer deposited after the deposition of 15 quadlayers

(PDDA pH = 7.0 & PCBS = 7.0). (Note: The Z-range for each of the phase images, from

lightest to darkest is 10 degrees).