This research focuses on the use of the Langmuir monolayers to fabricate ultrathin polar multilayers for the applications of pyroelectricity and second-order nonlinear optics. Nitrobiphenyl was used as the chromophore to provide polarity for pyroelectricity and nonlinear optical response. Two approaches were adopted here: alternating layers and polymeric layers.
First, two nitrobiphenyl containing compounds (VBPhNO2 and NO2BPhOH) were examined. The monolayer formation of the above compounds was facilitated by mixing with materials bearing similar structures (VBPhOH and CH3BPhOH). Transfer behavior of the monolayers was examined by vertical and horizontal deposition. All the monolayers showed Z-type vertical deposition. X-ray diffraction results show that the multilayer structure is Y-type, irrespective of the deposition type.
An untraditional alternating-layer method was developed, using only one polar unit- nitrobiphenyl. X-ray diffraction and pyroelectric measurements suggest that a polar film was constructed.
In the second approach, side chain liquid crystalline polymers were used. Nitrobiphenylether was used as the active chromophore and attached to polymethylsiloxane through methylene units (-(CH2)n-, n = 4 or 11). Methoxyethoxymethoxy was incorporated to e nhance the spreadability and monolayer formation of the polymers.
Horizontal deposition was used to produce transparent polymeric multilayers. X-ray diffraction, second harmonic measurements and pyroelectric studies all support the existence of a non-centrosymmetric structure for the polymeric multilayers. Second harmonic measurements and pyroelectric studies also show the good stability of Cop11 multilayers.
The molecular susceptibility of nitrobiphenyl was measured to be 9.8 ± 1.0 × 10-30 esu. The linear relationship between the square root of the second harmonic intensity (pp) and the number of layers for Cop11 suggests the good quality of the films. An unusual pyroelectric response was observed for Cop11 multilayers. From FTIR-RA studies, the aromatic rings of the molecules appeared to stay oriented in liquid crystalline phases