**4.1. SHG instrumentation**

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

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

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.

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

this period, the second-harmonic signals were recorded.

0 2 4 6 8 10

Number of bilayers

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

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

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 shows absorbance at 445 nm (due to the azodye sidechain of PS-119) as a function of number of bilayers for

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

, thickness 4 mm) were prepared from aqueous solutions of PAH and Poly

(area 10 x 30 mm2

44 Advanced Electromagnetic Waves

conditions [35].

[35].

0.0

0.2

0.4

Absorbance at 445 nm / arb. u.

0.6

0.8

 pH 3.5 pH 7.0 pH 10.0

**4.1. SHG instrumentation**

interpretation of experimental results.

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

MΩ∙cm) and dried by nitrogen-flow right before use.

Due to electronic resonance at 532 nm, PS-119 polyelectrolyte is strongly active in secondharmonic generation if excited by a 1064 nm laser beam [see Eq. (8)], while PAH is optically inactive. Therefore, using this pump wavelength we are probing only one polyelectrolyte (PS-119), while the other is used only to assemble the film. This facilitates the interpretation of experimental results.

contamination and stored for a period of 48 hours at room temperature (~ 23°C) and air humidity around 40%. Only after 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 matching condition along the surface plane leads to *kSHG*sin*αSHG* =2*kIR*sin*αIR*, which gives αSHG = αIR in the reflection direction (air side). Polarizers are used to set the polarization combination.

**Figure 8.** Layout for SHG measurements, consisting of a mode-locked Nd+3:YAG (1064 nm) laser pumping the sample, which is positioned at a rotation/translation stage, and a detection system based on a photomultiplier tube (PMT).

For studying the effect of heating, we used a temperature-controlled sample cell and the SinPout polarization combination. Figure 9 shows the sample cell for SHG measurements as a function of temperature, which is aligned by a mirror mount positioned on top of a computerized xy translation and rotation stage. A commercial temperature controller was used to vary the temperature with ~ 0.34°C/min heating rate from room temperature (around 25°C) to 190°C. Figure 9 also shows in more detail the heating cell used in this experiment, with a sample inside. on top of a computerized xy translation and rotation stage. A commercial temperature controller was used to vary the temperature with ~ 0.34C/min heating rate from room temperature (around 25C) to 190C. Figure 9 also shows in more detail the heating cell

**Figure 9: Experimental sample cell used to probe SHG signal as a function of temperature. It allows measurements either in air or vacuum, and for alignment purposes it is positioned on a mirror mount attached to the rotation/translation stage. Figure 9.** Experimental sample cell used to probe SHG signal as a function of temperature. It allows measurements either in air or vacuum, and for alignment purposes it is positioned on a mirror mount attached to the rotation/transla‐ tion stage.

### 4.2. Molecular ordering as a function of number of layers **4.2. Molecular ordering as a function of number of layers**

used in this experiment, with a sample inside.

Measurements of SHG as a function of azimuthal angle of rotation Ω were performed for the SP (S pump and P SHG signal, or SinPout) and SS (S pump and S SHG signal, or SinSout) polarization combinations for LbL films of PAH/PS-119, varying the number of monolayers and the pH of the adsorption/rinsing solutions. For samples with isotropic ordering, the signal in the SP polarization combination should be intense, while for the SS polarization combination the SHG signal is only allowed if the sample is anisotropic. However, if there are orientational domains much smaller than the area of the pump beam in the sample, but larger than the wavelength of the beam, there should be an Measurements of SHG as a function of azimuthal angle of rotation Ω were performed for the SP (S pump and P SHG signal, or SinPout) and SS (S pump and S SHG signal, or SinSout) polari‐ zation combinations for LbL films of PAH/PS-119, varying the number of monolayers and the pH of the adsorption/rinsing solutions. For samples with isotropic ordering, the signal in the SP polarization combination should be intense, while for the SS polarization combination the SHG signal is only allowed if the sample is anisotropic. However, if there are orientational domains much smaller than the area of the pump beam in the sample, but larger than the wavelength of the beam, there should be an intense and isotropic SHG signal for both SP and SS polarization, confirming the microscopic anisotropy of the sample. Therefore, the absence of signal in the SS polarization is indicative of an isotropic molecular arrangement at the scale of the pump beam wavelength.

intense and isotropic SHG signal for both SP and SS polarization, confirming the microscopic anisotropy of the sample. Therefore, the absence of signal in the SS polarization is indicative of an isotropic molecular arrangement at the scale of the pump beam wavelength. As a control measurement, an SHG rotational scan was obtained for a sample of z-cut quartz crystal and also for a gold surface prepared by thermal evaporation on glass. As can be seen in Figure 10, the measurement for quartz presents six directions where the signal is maximum, reflecting the C3v symmetry of this quartz crystal surface. For the gold surface, it is isotropic and the electronic resonance at 532 nm yields a high second-order susceptibility [22] charac‐

18

polarization is indicative of an isotropic molecular arrangement at the scale of the pump Probing the Molecular Ordering in Azopolymer Thin Films by Second-Order Nonlinear Optics http://dx.doi.org/10.5772/61180 47

on top of a computerized xy translation and rotation stage. A commercial temperature

controller was used to vary the temperature with ~ 0.34C/min heating rate from room

temperature (around 25C) to 190C. Figure 9 also shows in more detail the heating cell

**Figure 9: Experimental sample cell used to probe SHG signal as a function of temperature. It allows measurements either in air or vacuum, and for alignment purposes it is positioned on a mirror mount** 

Measurements of SHG as a function of azimuthal angle of rotation Ω were

performed for the SP (S pump and P SHG signal, or SinPout) and SS (S pump and S SHG

signal, or SinSout) polarization combinations for LbL films of PAH/PS-119, varying the number of monolayers and the pH of the adsorption/rinsing solutions. For samples with

isotropic ordering, the signal in the SP polarization combination should be intense, while for the SS polarization combination the SHG signal is only allowed if the sample is

anisotropic. However, if there are orientational domains much smaller than the area of the pump beam in the sample, but larger than the wavelength of the beam, there should be an

intense and isotropic SHG signal for both SP and SS polarization, confirming the

microscopic anisotropy of the sample. Therefore, the absence of signal in the SS

used in this experiment, with a sample inside.

4.2. Molecular ordering as a function of number of layers

**attached to the rotation/translation stage.** 

beam wavelength.

For studying the effect of heating, we used a temperature-controlled sample cell and the SinPout polarization combination. Figure 9 shows the sample cell for SHG measurements as a function of temperature, which is aligned by a mirror mount positioned on top of a computerized xy translation and rotation stage. A commercial temperature controller was used to vary the temperature with ~ 0.34°C/min heating rate from room temperature (around 25°C) to 190°C. Figure 9 also shows in more detail the heating cell used in this experiment, with a sample

on top of a computerized xy translation and rotation stage. A commercial temperature controller was used to vary the temperature with ~ 0.34C/min heating rate from room temperature (around 25C) to 190C. Figure 9 also shows in more detail the heating cell

**Figure 9: Experimental sample cell used to probe SHG signal as a function of temperature. It allows measurements either in air or vacuum, and for alignment purposes it is positioned on a mirror mount** 

**Figure 9.** Experimental sample cell used to probe SHG signal as a function of temperature. It allows measurements either in air or vacuum, and for alignment purposes it is positioned on a mirror mount attached to the rotation/transla‐

Measurements of SHG as a function of azimuthal angle of rotation Ω were

performed for the SP (S pump and P SHG signal, or SinPout) and SS (S pump and S SHG signal, or SinSout) polarization combinations for LbL films of PAH/PS-119, varying the number of monolayers and the pH of the adsorption/rinsing solutions. For samples with isotropic ordering, the signal in the SP polarization combination should be intense, while for the SS polarization combination the SHG signal is only allowed if the sample is anisotropic. However, if there are orientational domains much smaller than the area of the pump beam in the sample, but larger than the wavelength of the beam, there should be an intense and isotropic SHG signal for both SP and SS polarization, confirming the microscopic anisotropy of the sample. Therefore, the absence of signal in the SS polarization is indicative of an isotropic molecular arrangement at the scale of the pump

As a control measurement, an SHG rotational scan was obtained for a sample of z-cut quartz crystal and also for a gold surface prepared by thermal evaporation on glass. As can be seen in Figure 10, the measurement for quartz presents six directions where the signal is maximum, reflecting the C3v symmetry of this quartz crystal surface. For the gold surface, it is isotropic and the electronic resonance at 532 nm yields a high second-order susceptibility [22] charac‐

Measurements of SHG as a function of azimuthal angle of rotation Ω were performed for the SP (S pump and P SHG signal, or SinPout) and SS (S pump and S SHG signal, or SinSout) polari‐ zation combinations for LbL films of PAH/PS-119, varying the number of monolayers and the pH of the adsorption/rinsing solutions. For samples with isotropic ordering, the signal in the SP polarization combination should be intense, while for the SS polarization combination the SHG signal is only allowed if the sample is anisotropic. However, if there are orientational domains much smaller than the area of the pump beam in the sample, but larger than the wavelength of the beam, there should be an intense and isotropic SHG signal for both SP and SS polarization, confirming the microscopic anisotropy of the sample. Therefore, the absence of signal in the SS polarization is indicative of an isotropic molecular arrangement at the scale

18

inside.

46 Advanced Electromagnetic Waves

tion stage.

used in this experiment, with a sample inside.

**attached to the rotation/translation stage.** 

beam wavelength.

of the pump beam wavelength.

4.2. Molecular ordering as a function of number of layers

**4.2. Molecular ordering as a function of number of layers**

As described in Section 2.4, the results were adjusted to Equations (28) – (33) to determine **Figure 10: SHG measurements as a function of sample orientation () for (a) z-cut -quartz and (b) Figure 10.** SHG measurements as a function of sample orientation (Ω) for (a) z-cut α-quartz and (b) thermally evapo‐ rated gold film, with the SinPout polarization combination.

teristic of this metal. Thus, the SHG signal from gold is independent of orientation and is fairly strong. 1 through 6, and the best fit lines are shown in Figure 11. Using their definitions **thermally evaporated gold film, with the SinPout polarization combination.** 

As an example of determining the orientational distribution function for layer-by-layer films, we consider the results displayed in Figure 11, where we can see the SHG signal as a function of azimuthal angle Ω for a 10 bilayer film of PAH/Ma-co-DR13 [36]. As described in Section 2.4, the results were adjusted to Equations (28) – (33) to determine χ1 through χ6, and the best fit lines are shown in Figure 11. Using their definitions (Equations (22) – (27)) and the orien‐ tation distribution function given by Equation (34), the six equations were solved to find the orientation parameters in F(θ, φ). For these films, the following values were determined: θ0 = 37.41±0.24, σ = 12.85±0.53, d1 = 0.009±0.001, d2 = 0.014±0.002, d3 = -0.001±0.003, and Ω<sup>0</sup> = -11.87±4.78, which fully characterize the orientational distribution of azo-groups in the sample. (Equations (22) – (27)) and the orientation distribution function given by Equation (34), the six equations were solved to find the orientation parameters in F(,). For these films, the following values were determined: θ0 = 37.410.24, = 12.850.53, d1 = 0.0090.001, d2 = 0.0140.002, d3 = -0.0010.003, and 0 = -11.874.78, which fully characterize the orientational distribution of azo-groups in the sample.

**Figure 11: Azimuthal dependence of SHG signal for azopolimer (PAH/Ma-co-DR13)10 layer-by-layer films. (From Reference [36]) Figure 11.** Azimuthal dependence of SHG signal for azopolimer (PAH/Ma-co-DR13)10 layer-by-layer films. (From Ref‐ erence [36])

films can also be investigated by SHG. Figure 12(a) shows SHG measurements as a

function of azimuthal angle for a one bilayer film of (PAH/PS-119) obtained at three

different points on each sample, for two samples fabricated with solutions of different pH

values and dried by slow evaporation. They show that this fabrication procedure leads to

isotropic and homogeneous LbL films, in agreement with the results already found for

films of PAH/PSS [34]. The opposite was observed for self-assembled films of PAH/Ma-

co-DR13, shown in Figure 12(b), which were fabricated with dry nitrogen flow drying.

Indeed BAM (Brewster Angle Microscopy) measurements revealed the presence of

orientational domains and inhomogeneity [37]. However, we have found that thicker films

tend to became globally (not locally) more isotropic, since the SHG signals show smaller

variations as a function of rotation, but the SS polarization combination does not vanish.

The marked effect of drying on the molecular arrangement of azopolymer LbL

19

18

each layer.

sample.

The marked effect of drying on the molecular arrangement of azopolymer LbL films can also be investigated by SHG. Figure 12(a) shows SHG measurements as a function of azimuthal angle Ω for a one bilayer film of (PAH/PS-119) obtained at three different points on each sample, for two samples fabricated with solutions of different pH values and dried by slow evaporation. They show that this fabrication procedure leads to isotropic and homogeneous LbL films, in agreement with the results already found for films of PAH/PSS [34]. The opposite was observed for self-assembled films of PAH/Ma-co-DR13, shown in Figure 12(b), which were fabricated with dry nitrogen flow drying. Indeed BAM (Brewster Angle Microscopy) meas‐ urements revealed the presence of orientational domains and inhomogeneity [36]. However, we have found that thicker films tend to became globally (not locally) more isotropic, since the SHG signals show smaller variations as a function of rotation, but the SS polarization combi‐ nation does not vanish. The marked effect of drying on the molecular arrangement of azopolymer LbL films can also be investigated by SHG. Figure 12(a) shows SHG measurements as a function of azimuthal angle Ω for a one bilayer film of (PAH/PS-119) obtained at three different points on each sample, for two samples fabricated with solutions of different pH values and dried by slow evaporation. They show that this fabrication procedure leads to isotropic and homogeneous LbL films, in agreement with the results already found for films of PAH/PSS [34]. The opposite was observed for self-assembled films of PAH/Ma-co-DR13, shown in Figure 12(b), which were fabricated with dry nitrogen flow drying. Indeed BAM (Brewster Angle Microscopy) measurements revealed the presence of orientational domains and inhomogeneity [37]. However, we have found that thicker films tend to became globally (not locally) more isotropic, since the SHG signals show smaller variations as a function of rotation, but the SS polarization combination does not vanish.

As an example of determining the orientational distribution function for layer-by-layer films, we consider the results displayed in Figure 11, where we can see the SHG signal as a function of azimuthal angle Ω for a 10 bilayer film of PAH/Ma-co-DR13 [36]. As described in Section 2.4, the results were adjusted to Equations (28) – (33) to determine <sup>1</sup> through 6, and the best fit lines are shown in Figure 11. Using their definitions (Equations (22) – (27)) and the orientation distribution function given by Equation (34), the six equations were solved to find the orientation parameters in F(, ). For these films, the following values were determined: θ0 = 37.41±0.24, σ = 12.85±0.53, d1 = 0.009±0.001, d2 = 0.014±0.002, d3 = -0.001±0.003, and Ω0 = -11.87±4.78, which fully characterize the orientational distribution of azo-groups in the

Figure 11. Azimuthal dependence of SHG signal for azopolimer (PAH/Ma-co-DR13)10 layer-by-layer films. (From Reference [36])

Figure 12. (a) SHG measurements in three different spots for two different (PAH/PS-119)1 films fabricated with spontaneous drying (pH **Figure 12.** a) SHG measurements in three different spots for two different (PAH/PS-119)1 films fabricated with sponta‐ neous drying (pH 7.0 and pH 10.0 solutions). (b) SHG measurements in two different spots of the same (PAH/Ma-co-DR13)1 film fabricated with nitrogen flow drying.

7.0 and pH 10.0 solutions). (b) SHG measurements in two different spots of the same (PAH/Ma-co-DR13)1 film fabricated with nitrogen flow drying. SHG measurements as a function of the azimuthal angle Ω for films of PAH/PS-119 of different thicknesses prepared at pH 7 showed that the films are always isotropic in the sample plane, since they have strong signal with SP polarization combination that is independent of the sample orientation. Furthermore, the SS polarization signal is practically zero SHG measurements as a function of the azimuthal angle Ω for films of PAH/PS-119 of different thicknesses prepared at pH 7 showed that the films are always isotropic in the sample plane, since they have strong signal with SP polarization combination that is independent of the sample orientation. Furthermore, the SS polarization signal is practically zero whatever the number of layers, indicating that the samples are also microscopically isotropic. However, we

whatever the number of layers, indicating that the samples are also microscopically isotropic. However, we note that the SP signal undergoes a change with the number of layers, which is related to the relative orientation of the azo-groups in

The marked effect of drying on the molecular arrangement of azopolymer LbL films can also be investigated by SHG. Figure 12(a) shows SHG measurements as a function of azimuthal angle Ω for a one bilayer film of (PAH/PS-119) note that the SP signal undergoes a change with the number of layers, which is related to the relative orientation of the azo-groups in each layer.

The marked effect of drying on the molecular arrangement of azopolymer LbL films can also be investigated by SHG. Figure 12(a) shows SHG measurements as a function of azimuthal angle Ω for a one bilayer film of (PAH/PS-119) obtained at three different points on each sample, for two samples fabricated with solutions of different pH values and dried by slow evaporation. They show that this fabrication procedure leads to isotropic and homogeneous LbL films, in agreement with the results already found for films of PAH/PSS [34]. The opposite was observed for self-assembled films of PAH/Ma-co-DR13, shown in Figure 12(b), which were fabricated with dry nitrogen flow drying. Indeed BAM (Brewster Angle Microscopy) meas‐ urements revealed the presence of orientational domains and inhomogeneity [36]. However, we have found that thicker films tend to became globally (not locally) more isotropic, since the SHG signals show smaller variations as a function of rotation, but the SS polarization combi‐

As an example of determining the orientational distribution function for layer-by-layer films, we consider the results displayed in Figure 11, where we can see the SHG signal as a function of azimuthal angle Ω for a 10 bilayer film of PAH/Ma-co-DR13 [36]. As described in Section 2.4, the results were adjusted to Equations (28) – (33) to determine <sup>1</sup> through 6, and the best fit lines are shown in Figure 11. Using their definitions (Equations (22) – (27)) and the orientation distribution function given by Equation (34), the six equations were solved to find the orientation parameters in F(, ). For these films, the following values were determined: θ0 = 37.41±0.24, σ = 12.85±0.53, d1 = 0.009±0.001, d2 = 0.014±0.002, d3 = -0.001±0.003, and Ω0 = -11.87±4.78, which fully characterize the orientational distribution of azo-groups in the

Figure 11. Azimuthal dependence of SHG signal for azopolimer (PAH/Ma-co-DR13)10 layer-by-layer films. (From Reference [36])

show smaller variations as a function of rotation, but the SS polarization combination does not vanish.

150

210

0

SHG intensity / arb. u.

0

 MS SP Spot 1

 SP 1 SP 2 SP 3 SS 1 SS 2 SS 3

330

30

0

330

30

60

90

120

(PAH/PS-119)1

60

300

300

90

270

270

120

240

(PAH/Ma-co-DR13)1

240

DR13)1 film fabricated with nitrogen flow drying.

1

2

**Figure 12.** a) SHG measurements in three different spots for two different (PAH/PS-119)1 films fabricated with sponta‐ neous drying (pH 7.0 and pH 10.0 solutions). (b) SHG measurements in two different spots of the same (PAH/Ma-co-

SHG measurements as a function of the azimuthal angle Ω for films of PAH/PS-119 of different thicknesses prepared at pH 7 showed that the films are always isotropic in the sample plane, since they have strong signal with SP polarization combination that is independent of the sample orientation. Furthermore, the SS polarization signal is practically zero whatever the number of layers, indicating that the samples are also microscopically isotropic. However, we

180

0

0

330

30

60

90

120

(PAH/PS-119)1 pH 10

> MS SP Spot 2

 SP 1 SP 2 SP 3 SS 1 SS 2 SS 3

330

30

60

300

300

90

270

270

whatever the number of layers, indicating that the samples are also microscopically isotropic. However, we note that the SP signal undergoes a change with the number of layers, which is related to the relative orientation of the azo-groups in

120

240

(PAH/Ma-co-DR13)1

240

1

150

210

2

SHG intensity / arb. u.

180

nation does not vanish.

48 Advanced Electromagnetic Waves

(a) pH 7

SHG intensidade / arb. u.

180

150

210

(b)

flow drying.

each layer.

SHG intensity/ arb. u.

180

150

210

sample.

obtained at three different points on each sample, for two samples fabricated with solutions of different pH values and dried by slow evaporation. They show that this fabrication procedure leads to isotropic and homogeneous LbL films, in agreement with the results already found for films of PAH/PSS [34]. The opposite was observed for self-assembled films of PAH/Ma-co-DR13, shown in Figure 12(b), which were fabricated with dry nitrogen flow drying. Indeed BAM (Brewster Angle Microscopy) measurements revealed the presence of orientational domains and inhomogeneity [37]. However, we have found that thicker films tend to became globally (not locally) more isotropic, since the SHG signals Similarly to the study of PAH/PSS films [24], for PAH/PS-119 films fabricated at pH 7.0 there is an alternation of the SP intensities as the last layer of the film is PAH or PS-119. Since the samples are always isotropic, Figure 13 shows the average intensities of the azimuthal SHG measurements with SP polarization as a function of the number of layers for the three pH values studied. Some groups have reported a linear increase of χ(2) with the number of bilayers, especially above 10 bilayers [37–41], implying that average chromosphere orientations in each bilayer is the same (e.g., all pointing up, on average, in every bilayer). As noted in Figure 13, the square root of the SHG signal (which is related to the effective value of χ(2)) does not grow linearly with the number of bilayers for these PAH/PS-119 films. For pH 10.0, the signal rapidly decreases with the number of layers, up to 20 layers, but remains approximately constant for pH 7. In the case of pH 3.5, there is a slight increase of signal with thickness up to 10 bilayers, but the signal is significantly reduced for thicker films, around 30 bilayers (not shown). Moreover, we always noted alternating SHG intensity after adsorption of each polyelectrolyte (integer vs. half-integer number of bilayers), at least for the first few bilayers. Specifically, for the films of PAH/PS-119 at pH 3.5 the authors of references [38] and [39] report a linear growth of χ(2) for films up to 100 bilayers. However, we observe that for the same pH value the signal initially grows with the number of bilayers, but also alternating as the last layer is PAH or PS-119, and decreases from ~ 10 bilayers, in disagreement with references [37–41]. Even for other films fabricated at other pH values, the increase was not linear. This behavior was reproduced in another set of samples manufactured with other solutions. Similar effect was observed by Lvov and co-workers for PDDA/PAZO LbL films where they reported an increase of χ(2) up to 5 bilayers, but a reduction for thicker films [15]. At the moment we have no explanation for this discrepancy between our experimental results and those of references [37– 41]. However, we have reported the changes in molecular conformation including the molecular ordering after adsorption of the subsequent layer [24], and therefore a linear increasing of χ(2) would be quite surprising, because that would mean that each and every PS-119 layer has an identical average orientation, and in the same direction. This would be especially unexpected considering effects such as interpenetration of layers and increasing film roughness with the number of layers, as has been observed for films of POMA/PVS [43] and PAH/Ma-co-DR13 [42].

> In summary, comparing the intensities in Figure 13, we see that the signal initially grows with the number of layers for low pH (3.5), and it only decreases for high pH (10.0) and remains nearly unchanged for almost neutral pH (7.0). When the number of bilayers is high, we observed that the SHG signal always decreases considerably.

Figure 12. (a) SHG measurements in three different spots for two different (PAH/PS-119)1 films fabricated with spontaneous drying (pH 7.0 and pH 10.0 solutions). (b) SHG measurements in two different spots of the same (PAH/Ma-co-DR13)1 film fabricated with nitrogen SHG measurements as a function of the azimuthal angle Ω for films of PAH/PS-119 of different thicknesses prepared at pH 7 showed that the films are always isotropic in the sample plane, since they have strong signal with SP polarization combination that is independent of the sample orientation. Furthermore, the SS polarization signal is practically zero From the absorbance measurements in Figure 7, the amount of PS-119 per bilayer is constant in each sample for all three pH values. Therefore, we may conclude that the azopolymer chains do not remain with the same degree of ordering as the film grows, otherwise we should have observed a linear increase of χ(2) with thickness. Specifically, since there is a decrease in signal with increasing number of layers, it is necessary that the adsorption of the last layers is affected by the average ordering of the previous layers, pointing on average in the opposite direction

and 10.0.

Similarly to the study of PAH/PSS films [24], for PAH/PS-119 films fabricated at pH 7.0 there is an alternation of the SP intensities as the last layer of the film is PAH or PS-119. Since the samples are always isotropic, Figure 13 shows the average intensities of the azimuthal SHG measurements with SP polarization as a function of the number of layers for the three pH values studied. Some groups have reported a linear increase of χ(2) with the number of bilayers, especially above 10 bilayers [37–41], implying that average chromosphere orientations in each bilayer is the same (e.g., all pointing up, on average, in every bilayer). As noted in Figure 13, the square root of the SHG signal (which is related to the effective value of χ(2)) does not grow linearly with the number of bilayers for these PAH/PS-119 films. For pH 10.0, the signal rapidly decreases with the number of layers, up to 20 layers, but remains approximately constant for pH 7. In the case of pH 3.5, there is a slight increase of signal with thickness up to 10 bilayers, but the signal is significantly reduced for thicker films, around 30 bilayers (not shown). Moreover, we always noted alternating SHG intensity after adsorption of each polyelectrolyte (integer vs. half-integer number of bilayers), at least for the first few bilayers. Specifically, for the films of PAH/PS-119 at pH 3.5 the authors of references [38] and [39] report a linear growth of χ(2) for films up to 100 bilayers. However, we observe that for the same pH value the signal initially grows with the number of bilayers, but also alternating as the last layer is PAH or PS-119, and decreases from ~ 10 bilayers, in disagreement with references [37–41]. Even for other films fabricated at other pH values, the increase was not linear. This behavior was reproduced in another set of samples manufactured with other solutions. Similar effect was observed by Lvov and co-workers for PDDA/PAZO LbL films where they reported an increase of χ(2) up to 5 bilayers, but a reduction for thicker films [15]. At the moment we have no explanation for this discrepancy between our experimental results and those of references [37–41]. However, we have reported the changes in molecular conformation including the molecular ordering after adsorption of the subsequent layer [24], and therefore a linear increasing of χ(2) would be quite surprising, because that would mean that

unexpected considering effects such as interpenetration of layers and increasing film roughness with the number of

layers, as has been observed for films of POMA/PVS [43] and PAH/Ma-co-DR13 [42].

Figure 13. SHG signal and square root of SHG signal as a function of number of bilayers for PAH/PS-119 films fabricated at pH 3.5, 7.0, **Figure 13.** SHG signal and square root of SHG signal as a function of number of bilayers for PAH/PS-119 films fabricat‐ ed at pH 3.5, 7.0, and 10.0.

of the first ones. If the first few layers were ordered and the following ones had an isotropic arrangement, the SHG signal should initially grow and then saturate at a constant value for thick films. In summary, comparing the intensities in Figure 13, we see that the signal initially grows with the number of layers for low pH (3.5), and it only decreases for high pH (10.0) and remains nearly unchanged for almost neutral pH (7.0). When the number of bilayers is high, we observed that the SHG signal always decreases considerably.

At pH 3.5, PAH has an ionization degree of about 100%, while the glass is hardly ionized (only about 9% of the surface Si-OH groups, according to references [43, 44]) and the first layer of PAH would be expected to be very thin due to the high charge density in the chains and low substrate charge. However, due to a high ionic strength, the electrostatic shield makes the PAH chain a little more coiled. Therefore, these films at pH 3.5 are slightly thicker and rougher due to the reduction of electrostatic interactions by an increased ionic strength. This means that the SHG signal initially increases with thickness, but the net ordering of each additional bilayer is reduced for thick films, leading to a saturation (and eventual reduction) of SHG signal, as shown in Figure 13. For pH 10, the glass is highly charged, but the PAH is only about 30% ionized, thus forming more folded layers than at pH 3.5. Therefore, the adsorbed amount is large but there are few sites in the PAH layer available for complexation with the PS-119, thus decreasing the drive for orientational ordering of PS-119 and leading to films that rapidly become disordered with increasing thickness, reducing the SHG signal. For pH 7, both the substrate and PAH are quite charged, with a low ionic strength in the solutions, favoring electrostatic interactions and allowing the film growth with a relative stability of the SHG signal.

Another interesting feature is the alternation of the SHG signal for films of a few bilayers (data not shown). For example, for the film of 3 layers (or 1.5 bilayers), (PAH/PS-119)/PAH, typically the signal is canceled out or significantly reduced, except for pH 7 where the alternating signals are all large. Here, we will consider only the films formed at pH 3.5 and 10, which show a different behavior from that observed in our previous report [24]. Compared to the first bilayer, PAH/PS-119, which presents considerable SHG signal, the (PAH/PS-119)/PAH film has a new ordering of PS-119 azo-groups, with random or symmetric configuration in the z-direction of film growth, which yields a vanishing SHG signal. This because in the 1 bilayer film, the negatively charged azo-groups of PS-119 are oriented on average toward the cationic PAH layer. In the three-layer film, the signal is greatly reduced, since the third PAH layer modifies the orientation of the previously adsorbed PS-119 layer, thereby reducing the SHG signal. In particular, for pH 3.5 the signal is almost completely canceled, indicating an almost perfectly symmetrical configuration of the azopolymer active groups. This is reasonable because the two PAH layers of the PAH/PS-119/PAH film are highly and equally charged, exerting nearly the same influence on the central PS-119 layer.

To investigate in more detail the orientation of azo-groups of PS-119 in very thin films, in the anomalous region of ordering as a function of thickness for pH 3.5 films (see Figure 13), we performed a direct measurement of the phase of χ(2) with the SP polarization combination, using as a reference a thin film of zinc sulfide (ZnS), as described in Section 2.3. Figure 14(a) shows that the phase of the SHG signal from the sample, which is related to the average direction of orientation of the azopolymer is always the same for films formed at pH 3.5 with an integer number of bilayers where the last layer is the

Figure 14(a) shows that the phase of the SHG signal from the sample, which is related to the average direction of orientation of the azopolymer is always the same for films formed at pH 3.5 with an integer number of bilayers where the last layer is the azopolymer, covering a highly charged cationic layer of PAH. For the first bilayer, it is expected that the preferred arrange‐ ment of the azo-groups of PS-119 is toward the highly charged layer of PAH, that is, toward the substrate side. Thus, this behavior is preserved for all films whose last layer is PS-119. Since the signal for the film with 3 layers, PAH/PS-119/PAH, is null, we can conclude that occurs a rearrangement of the chromophores in direction to both layers of PAH, such as observed for PAH/PSS films [24]. Therefore, the results in Figure 14(a) confirm our assumptions about the orientation of the azopolymer groups. azopolymer, covering a highly charged cationic layer of PAH. For the first bilayer, it is expected that the preferred arrangement of the azo-groups of PS-119 is toward the highly charged layer of PAH, that is, toward the substrate side. Thus, this behavior is preserved for all films whose last layer is PS-119. Since the signal for the film with 3 layers, PAH/PS-119/PAH, is null, we can conclude that occurs a rearrangement of the chromophores in direction to both layers of PAH, such as observed for PAH/PSS films [24]. Therefore, the results in Figure 14(a) confirm our assumptions about the orientation of the azopolymer groups.

of the first ones. If the first few layers were ordered and the following ones had an isotropic arrangement, the SHG signal should initially grow and then saturate at a constant value for

**Figure 13.** SHG signal and square root of SHG signal as a function of number of bilayers for PAH/PS-119 films fabricat‐

the number of bilayers is high, we observed that the SHG signal always decreases considerably.

0 5 10 15 20 25 30 35 40

Number of bilayers

Figure 13. SHG signal and square root of SHG signal as a function of number of bilayers for PAH/PS-119 films fabricated at pH 3.5, 7.0,

In summary, comparing the intensities in Figure 13, we see that the signal initially grows with the number of layers for low pH (3.5), and it only decreases for high pH (10.0) and remains nearly unchanged for almost neutral pH (7.0). When

0 2 4 6 8 10 12 14 16 18 20 22

0.0 0.2 0.4 0.6 0.8 1.0

pH 3.5

pH 10.0

pH 7.0

0.0 0.5 1.0 1.5 2.0 (SHG intensity)1/2 / arb. u.

0.0 0.1 0.2 0.3 0.4

0 1 2 3 4 5 6 7 8 9 10

layers, as has been observed for films of POMA/PVS [43] and PAH/Ma-co-DR13 [42].

Similarly to the study of PAH/PSS films [24], for PAH/PS-119 films fabricated at pH 7.0 there is an alternation of the SP intensities as the last layer of the film is PAH or PS-119. Since the samples are always isotropic, Figure 13 shows the average intensities of the azimuthal SHG measurements with SP polarization as a function of the number of layers for the three pH values studied. Some groups have reported a linear increase of χ(2) with the number of bilayers, especially above 10 bilayers [37–41], implying that average chromosphere orientations in each bilayer is the same (e.g., all pointing up, on average, in every bilayer). As noted in Figure 13, the square root of the SHG signal (which is related to the effective value of χ(2)) does not grow linearly with the number of bilayers for these PAH/PS-119 films. For pH 10.0, the signal rapidly decreases with the number of layers, up to 20 layers, but remains approximately constant for pH 7. In the case of pH 3.5, there is a slight increase of signal with thickness up to 10 bilayers, but the signal is significantly reduced for thicker films, around 30 bilayers (not shown). Moreover, we always noted alternating SHG intensity after adsorption of each polyelectrolyte (integer vs. half-integer number of bilayers), at least for the first few bilayers. Specifically, for the films of PAH/PS-119 at pH 3.5 the authors of references [38] and [39] report a linear growth of χ(2) for films up to 100 bilayers. However, we observe that for the same pH value the signal initially grows with the number of bilayers, but also alternating as the last layer is PAH or PS-119, and decreases from ~ 10 bilayers, in disagreement with references [37–41]. Even for other films fabricated at other pH values, the increase was not linear. This behavior was reproduced in another set of samples manufactured with other solutions. Similar effect was observed by Lvov and co-workers for PDDA/PAZO LbL films where they reported an increase of χ(2) up to 5 bilayers, but a reduction for thicker films [15]. At the moment we have no explanation for this discrepancy between our experimental results and those of references [37–41]. However, we have reported the changes in molecular conformation including the molecular ordering after adsorption of the subsequent layer [24], and therefore a linear increasing of χ(2) would be quite surprising, because that would mean that each and every PS-119 layer has an identical average orientation, and in the same direction. This would be especially unexpected considering effects such as interpenetration of layers and increasing film roughness with the number of

At pH 3.5, PAH has an ionization degree of about 100%, while the glass is hardly ionized (only about 9% of the surface Si-OH groups, according to references [43, 44]) and the first layer of PAH would be expected to be very thin due to the high charge density in the chains and low substrate charge. However, due to a high ionic strength, the electrostatic shield makes the PAH chain a little more coiled. Therefore, these films at pH 3.5 are slightly thicker and rougher due to the reduction of electrostatic interactions by an increased ionic strength. This means that the SHG signal initially increases with thickness, but the net ordering of each additional bilayer is reduced for thick films, leading to a saturation (and eventual reduction) of SHG signal, as shown in Figure 13. For pH 10, the glass is highly charged, but the PAH is only about 30% ionized, thus forming more folded layers than at pH 3.5. Therefore, the adsorbed amount is large but there are few sites in the PAH layer available for complexation with the PS-119, thus decreasing the drive for orientational ordering of PS-119 and leading to films that rapidly become disordered with increasing thickness, reducing the SHG signal. For pH 7, both the substrate and PAH are quite charged, with a low ionic strength in the solutions, favoring electrostatic interactions and allowing the film growth with a relative stability of the SHG

thick films.

and 10.0.

ed at pH 3.5, 7.0, and 10.0.

0.0 0.2 0.4 0.6 0.8 1.0

0.0 0.5 1.0 1.5 2.0 2.5

SHG intensity / arb. u.

50 Advanced Electromagnetic Waves

0.00 0.05 0.10 0.15

signal.

**Figure 14: SHG interference pattern for PAH/PS-119 films fabricated at pH 3.5 with various numbers of bilayers. is the angle of the silica compensator plate. Figure 14.** SHG interference pattern for PAH/PS-119 films fabricated at pH 3.5 with various numbers of bilayers. θ is the angle of the silica compensator plate.

measurement, while difficult to be performed, confirms that the reordering of the azogroups also occurs, as shown in Figure 14(b). When the film is finished with a layer of PAH, PS-119 chromophores undergo reorientation and then acquire a small average ordering in the direction opposite to the substrate, causing a reversal of the phase of the SHG signal. Therefore, films with an even number of layers have the same phase, with azogroups pointing toward the substrate, while the 2.5 layer film had the opposite net

4.3. Effect of temperature on the ordering of azopolymer films

orientation.

For films with 2 or 2.5 bilayers, whose signals are less intense, the phase

Now, we shall discuss the effect of heating on the ordering of azopolymer films.

Figure 15 shows results for PAH/PS-119 films fabricated from solutions at pH 3.5, which

24

For films with 2 or 2.5 bilayers, whose signals are less intense, the phase measurement, while difficult to be performed, confirms that the reordering of the azo-groups also occurs, as shown in Figure 14(b). When the film is finished with a layer of PAH, PS-119 chromophores undergo reorientation and then acquire a small average ordering in the direction opposite to the substrate, causing a reversal of the phase of the SHG signal. Therefore, films with an even number of layers have the same phase, with azo-groups pointing toward the substrate, while the 2.5 layer film had the opposite net orientation. opposite to the substrate, causing a reversal of the phase of the SHG signal. Therefore, films with an even number of layers have the same phase, with azo-groups pointing toward the substrate, while the 2.5 layer film had the opposite net
