Characterization of Asymmetric Polydiacetylene Ultrathin Films
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of 5 mm/min and 2.5 mm/min respectively. The films were dried for 18 mins in between two dipping cycles. UV/Vis absorption studies were carried out using a Perkin-Elmer Lambda 9 spectrophotometer. Vibration spectra were recorded using a Perkin-Elmer FT-IR spectrometer 1760 with a room-temperature TGS detector. Thickness of the LB multilayers was measured using an AutoELR-III Automatic Ellipsometer (Rudolph Research) with a He-Ne laser (632.8 nm). Second-harmonic (SH) generation measurements were performed using a Q-switched NdYAG laser (1064 nm, pulse width 10 ns). Third order nonlinear optical effects were measured using the technique of degenerate four wave mixing (DFWM). A Nd-YAG laser of 1064 nm was frequency-doubled with the second harmonic generator crystal to provide 17 ps full width at half maximum (FWHM) single pulses. These pulses were utilized to perform a DFWM experiment. Transmission electron microscopy (TEM) studies were carried out using a Philips EM 400 electron microscopy. LB films were deposited on a 200 mesh copper grid supported with a thin
carbnon film. AFM images of the LB film surface were obtained using a atomic force microscope (Park Scientific Instruments, CA) with a Si 3 N4 tip. The images recorded were reproducible and obtained with an applied force in 10 nN range under well-calibrated conditions. RESULTS AND DISCUSSION Figure 1 shows the surface pressure-area isotherm and chemical structure of the asymmetric polydiacetylene, poly- 1-{[(butoxycarbonylmethyl)amidoyloxo]-4-pyrimidyl I -diacetylene (PBPOD), used for this work. The isotherm indicates that the monolayer is quite stable upto the pressure of 50 mN/m. Extrapolation of the curve to zero pressure at the low compressibility region from the isotherm gives a molecular area of approximately 50 A2 per repeat unit. Interestingly, the multilayer is formed by Z-type deposition. As shown in Figure 1, the chemical structure of P-BPOD consists of two hydrophilic parts; viz. the pyrimidyl ring, and the urethaneester linkage with only four methylene groups. This unusual amphiphilic structure seems to contribute to the Z-type multilayer assembly on hydrophobic substrates. Figure 2 shows the comparison of the electronic absorptions for the solution, cast films, and LB multilayers. Red-shifted excitonic bands in LB films (583 nm as compared to 573 nm in cast films) is explained by enhanced effective conjugation length due to the backbone orientation. The absorption intensity increase at 583 nm with the number of layers in the inset of Figure 2 indicates reproducible multilayer formation. The polarized electronic absorption spectroscopy of multilayers were carried out at normal incidence. The absorption (A//) polarized parallel to the dipping direction is significantly larger than that (A 1 ) polarized perpendicular to the dipping direction (Figure 3). This implies that the backbone is preferentially oriented along the dipping direction. The polarized absorption spectra at 45 degree incident angle (data not shown) exhibits much larger intensity i
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