Control of Second-Order Nonlinear Optical Susceptibility in Ionically Self-Assembled Films by pH and Ionic Strength
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Relative Value
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Thickness per Bilayer
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Absorbance per Bilayer
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Immersion Time (seconds)
Figure 1 . Relative film thickness and absorbance for thirty-bilayer Poly S-119/PAH ISAM films. each solution. In less than one minute, the absorbance and thickness each reach values greater than 80% of the maximum value. Films immersed for thirty minutes in each solution produced values roughly 10% less than those immersed for 300 seconds. Second harmonic generation (SHG) measurements were made using the 1200 nm output from a broadband BBO optical parametric oscillator pumped by the 355 nm third harmonic of a Q-switched Nd:YAG laser. The beam was weakly focused into the sample to a spot size of a few hundred microns. The film was polished off of one side of the substrate to eliminate interference effects, and the film was rotated 45 away from normal incidence about the vertical axis with incident p-polarized light. As described in Ref. 3, the SHG intensity exhibited the expectde quadratic dependece on the number of bilayers up to 100 bilayers for Poly S-119/PAH ISAM films. The quadratic growth of the SHG intensity is particularly important because it illustrates that the degree of polar orientation is maintained for each successive bilayer. If the degree of orientation was decreased for the latter deposited layers, the SHG intensity would have a subquadratic dependence on the number of bilayers, as has been observed in some reports.4,6 EFFECTS OF pH AND SALT CONCENTRATION It has been found that the thickness of ISAM layers can be dramatically varied by different pH and salt concentrations in the immersion solutions.7 As an example, Figure 2 illustrates the dependence of the peak absorbance (480 nm) as a function of the number of bilayers for pH values of 1.5, 2.5, and 3.5. In each case, the excellent linear dependence demonstrates the excellent homogeneity of ISAM film formation from layer to layer. However, the larger slope for decreased pH values indicates that more chromophore is incorporated into each layer. This is confirmed by direct measurements
of thickness. In all our studies, thickness and absorbance have yielded excellent linear dependence for a given set of polymeric components. The conformation of a polyelectrolyte in solution is determined by the Debye length κ -1.8 The Debye length represents the effective electrostatic interaction range between charges on the polymer and is given by κ −1=
kT 2cs z 2e 2
where cs is the total ionic strength (concentration of small ions) and z is the counterion valency. In solutions with added NaCl, the ionic strength is the total concentration of Na+ and H+ ions. As the Debye length is reduced, the repulsion of neighboring ions on the polymer is reduced due to electrostatic screening by the counterions. This, in turn, allows more curvature in the polymer backbone, or correspondingly, a reduction in the persistence length. When the electrostatic screening is low, the polyelectrolyte adopts an extended conformation
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