**3. Layer-by-layer thin films**

G. Decher and co-workers [28, 29] were the first to propose this simple and efficient method of physical deposition. Specifically, layer-by-layer (LbL) deposition is a fast and practical deposition process based on the electrostatic interaction between polyelectrolytes and opposite charges on solid substrates, such as glass, silica, or mica [29–33]. It can be used to fabricate thin films from a few nanometers to hundreds of nanometers. In most cases the mechanism responsible for adsorption is mainly the electrostatic interaction, but secondary interactions such as hydrophobicity, Van der Waals, or H-bonding are also relevant [7].

Some advantages of electrostatic LbL with respect to other thin film fabrication techniques, for example, Langmuir–Blodgett (LB) depositions, are the use of water-soluble molecules involving a large variety of materials, its independence of size and topology of substrate [31, 32], and the applicability to almost any hydrophobic or hydrophilic solid, such as glass, quartz, mica, and gold [33].

In the LbL self-assembly process, spontaneous sequential adsorption of oppositely charged polyelectrolytes (polyions) is carried out from most often dilute aqueous solutions on charged surfaces. Figure 5 illustrates the experimental procedures for adsorbing LbL films. Typically, a charged substrate (frequently glass or quartz) is immersed in an oppositely charged polyion solution. Electrostatic attraction occurs between the charged surface and the oppositely charged molecules in solution. It is expected that adsorption occurs until overall charge neutrality or charge reversal is reached at the substrate surface, implying that the adsorption process is self-limited. After washing the substrate with an aqueous solution (usually of the same pH as the adsorption solution) in order to remove excess adsorbed material and to ensure that only one strongly adsorbed monolayer remains, the substrate is dried by N2 flow. We have shown that this drying step promotes the inhomogeneity of the film in the micrometer scale [34], making it unfit for certain applications. The next step is immersing the substrate with the first adsorbed layer in an oppositely charged polyion solution. The oppositely charged polyelectrolytes will complex at the film/solution interface, leading to adsorption of the second layer and overall charge reversal again. Now the signal of net surface charge (substrate plus adsorbed film) is restored to that of the original substrate. Other rinsing and drying steps complete the fabrications of the first bilayer. The whole procedure can be repeated as many times as necessary, with the same or a different pair of materials, which may also include nanoparticles, dendrimers, enzymes, etc. Therefore, in addition to allowing precise control of film thickness, the LbL method allows making films with their compositions controlled at the nanometer scale up to several hundreds of nanometers in thickness, simply by properly choosing the materials used for fabricating each layer.

Since there are no restrictions in the selection of polyelectrolytes, there are many materials that may be employed in the manufacture of LbL films. Thus, some of the most used are the PEI (poly(ethylene imine)), PAH (poly(allylamine chloride)), and PDAC (poly(dimethyldially‐ lammonium chloride)) as polycations; and PVS (poly(vinyl sulfonic acid)), PSS (poly(sodium styrene sulfonate)), Ma-co-DR13 (a side-chain-substituted azobenzene copolymer derived from azodye Disperse Red 13), PS-119 (Poly(vinylamine) backbone azo chromophore), and Probing the Molecular Ordering in Azopolymer Thin Films by Second-Order Nonlinear Optics http://dx.doi.org/10.5772/61180 43

**3. Layer-by-layer thin films**

42 Advanced Electromagnetic Waves

mica, and gold [33].

G. Decher and co-workers [28, 29] were the first to propose this simple and efficient method of physical deposition. Specifically, layer-by-layer (LbL) deposition is a fast and practical deposition process based on the electrostatic interaction between polyelectrolytes and opposite charges on solid substrates, such as glass, silica, or mica [29–33]. It can be used to fabricate thin films from a few nanometers to hundreds of nanometers. In most cases the mechanism responsible for adsorption is mainly the electrostatic interaction, but secondary interactions

Some advantages of electrostatic LbL with respect to other thin film fabrication techniques, for example, Langmuir–Blodgett (LB) depositions, are the use of water-soluble molecules involving a large variety of materials, its independence of size and topology of substrate [31, 32], and the applicability to almost any hydrophobic or hydrophilic solid, such as glass, quartz,

In the LbL self-assembly process, spontaneous sequential adsorption of oppositely charged polyelectrolytes (polyions) is carried out from most often dilute aqueous solutions on charged surfaces. Figure 5 illustrates the experimental procedures for adsorbing LbL films. Typically, a charged substrate (frequently glass or quartz) is immersed in an oppositely charged polyion solution. Electrostatic attraction occurs between the charged surface and the oppositely charged molecules in solution. It is expected that adsorption occurs until overall charge neutrality or charge reversal is reached at the substrate surface, implying that the adsorption process is self-limited. After washing the substrate with an aqueous solution (usually of the same pH as the adsorption solution) in order to remove excess adsorbed material and to ensure that only one strongly adsorbed monolayer remains, the substrate is dried by N2 flow. We have shown that this drying step promotes the inhomogeneity of the film in the micrometer scale [34], making it unfit for certain applications. The next step is immersing the substrate with the first adsorbed layer in an oppositely charged polyion solution. The oppositely charged polyelectrolytes will complex at the film/solution interface, leading to adsorption of the second layer and overall charge reversal again. Now the signal of net surface charge (substrate plus adsorbed film) is restored to that of the original substrate. Other rinsing and drying steps complete the fabrications of the first bilayer. The whole procedure can be repeated as many times as necessary, with the same or a different pair of materials, which may also include nanoparticles, dendrimers, enzymes, etc. Therefore, in addition to allowing precise control of film thickness, the LbL method allows making films with their compositions controlled at the nanometer scale up to several hundreds of nanometers in thickness, simply by properly

Since there are no restrictions in the selection of polyelectrolytes, there are many materials that may be employed in the manufacture of LbL films. Thus, some of the most used are the PEI (poly(ethylene imine)), PAH (poly(allylamine chloride)), and PDAC (poly(dimethyldially‐ lammonium chloride)) as polycations; and PVS (poly(vinyl sulfonic acid)), PSS (poly(sodium styrene sulfonate)), Ma-co-DR13 (a side-chain-substituted azobenzene copolymer derived from azodye Disperse Red 13), PS-119 (Poly(vinylamine) backbone azo chromophore), and

such as hydrophobicity, Van der Waals, or H-bonding are also relevant [7].

choosing the materials used for fabricating each layer.

**Figure 5.** Self-assembly process. Part A: adsorption, rinsing, and drying of the first layer polyelectrolyte (polycation). Part B: adsorption, rinsing, and drying of the second layer (polyanion). Repetition of this process determines the de‐ sired number of bilayers.

PAA (poly(acrylic acid)) as polyanions. Figure 6 displays the structural formulas of some of these polyelectrolytes.

**Figure 6.** Structural formula for some polyelectrolytes used in LbL assembly.

In this chapter, we discuss the molecular orientation of self-assembled LbL films fabricated with polyelectrolytes containing the azo-group. PAH (Mw = 15000) and Poly S-119 (Mw = unknown) were obtained from Aldrich and used as received. LbL films on BK7 glass substrates (area 10 x 30 mm2 , thickness 4 mm) were prepared from aqueous solutions of PAH and Poly S-119 with 1.0 mg/ml concentration and pH 3.5, 7.0, and 10.0. For a given choice of pH, both polyelectrolyte solutions and the rinsing solution had the same pH value, which was adjusted by addition of HCl (from Qhemis, 37%, analytical grade) and NaOH (from Aldrich, electronic grade, purity 99.99%). Substrates were cleaned by piranha solutions (H2SO4/H2O2 at 3:1 proportions by volume) for 20 min, extensively rinsed with Milli-Q water (resistivity 18.3 MΩ∙cm) and dried by nitrogen-flow right before use.

The LbL films were prepared by alternate adsorption of cationic (PAH) and anionic (Poly S-119) polyelectrolytes on the BK7 glass substrates, as described in literature [29, 33]. In this work, we used just one final drying process: drying by slow water evaporation, that is, the films were prepared without any drying after adsorption or rinsing stages. In order to dry the samples after the self-assembly is complete, the substrates were loosely covered by a Petri dish to avoid contamination and stored for a period of 48 hours at room temperature (~ 23°C) and air humidity around 40%. Only after this period, the second-harmonic signals were recorded. contamination and stored for a period of 48 hours at room temperature (~ 23°C) and air humidity around 40%. Only after this period, the second-harmonic signals were recorded.

Figure 7 shows absorbance at 445 nm (due to the azodye sidechain of PS-119) as a function of number of bilayers for films fabricated with three different pH values. As we can see, the film content of PS-119 increases linearly with the number of bilayers, demonstrating that the same amount of azopolymer is adsorbed at each bilayer. Figure 7 also shows that the adsorbed amount per layer is larger for pH 10, suggesting the formation of thicker films under such conditions [35]. Figure 7 shows absorbance at 445 nm (due to the azodye sidechain of PS-119) as a function of number of bilayers for films fabricated with three different pH values. As we can see, the film content of PS-119 increases linearly with the number of bilayers, demonstrating that the same amount of azopolymer is adsorbed at each bilayer. Figure 7 also shows that the adsorbed amount per layer is larger for pH 10, suggesting the formation of thicker films under such conditions [35].

**4. Second-harmonic generation from LbL films**

Due to electronic resonance at 532 nm, PS-119 polyelectrolyte is strongly active in second-harmonic generation if excited by a 1064 nm laser beam [see Eq. (8)], while PAH is optically inactive. Therefore, using this pump wavelength we are

Our SHG instrumentation is shown in Figure 8 for SHG measurements as a function of azimuthal angle Ω, where we can see a double-functional rotation/translation stage, allowing rotation of sample around the *z* axis (azimuthal angle Ω), and its translation on the horizontal plane *xy*. A pulsed Nd3+:YAG laser is used to excite the samples. The repetition rate, pulse duration, and the pump energy of the IR beam at 1064 nm were 20 Hz, 30 ps, and 2.0 mJ, respectively. The area of beam on the sample surface was approximately 2 mm2, and the angles of incidence/reflection were 60°, since the phase

2*k* sin

, which gives αSHG = αIR in the

probing only one polyelectrolyte (PS-119), while the other is used only to assemble the film. This facilitates the

matching condition along the surface plane leads to *SHG SHG IR IR k* sin

reflection direction (air side). Polarizers are used to set the polarization combination.

Figure 7. UV-vis absorbance at 445 nm for PAH/PS-119 LbL films fabricated at different pH values. **Figure 7.** UV-vis absorbance at 445 nm for PAH/PS-119 LbL films fabricated at different pH values.

interpretation of experimental results.

**4.1. SHG instrumentation**
