College Park, Maryland June 6 - 10 , 2004
TP47: Reflectometry Studies of Multi-Layered Non-Linear Optical Films
E. Watkins, J. Majewski, J. Robinson, H. Wang, P. Chiarelli, J. Casson, F. Trouw (Los Alamos National Laboratory)
Neutron and x-ray reflectivity measurements were used to characterize the structure of non-linear optical (NLO) films containing S-azo-C18 molecules. Langmuir-Blodgett (LB) multilayers were prepared by the sequential transfer of S-azo-C18 and stearic acid molecules on octadecyltrichlorosilane (OTS) coated quartz and silicon substrates. To provide good contrast for neutron measurements, the stearic acid molecules were deuterated. One and two bilayer samples were prepared for both possible layer orderings. Neutron measurements were performed on the SPEAR reflectometer at the Los Alamos Neutron Science Center (LANSCE) and complementary x-ray reflectivity measurements were used to pin down certain aspects of the molecular architecture.
For the single bilayer samples, the Kiessig fringe spacing observed (dQ ~ 0.07 Å-1) corresponds to a length scale of approximately 90 A and matches the predicted thickness of the bilayer. Competing length scales make it difficult to identify the complementary fringe spacing for the two-bilayer samples. Box model fitting of the data sets show the following: (1) well packed layers of OTS are present, (2) the density of the stearic acid layers are low indicating either patchy layers or intermixing of molecules with adjacent layers, (3) the thickness of the layer corresponding to the S-azo molecules is greater than predicted, and (4) intermixing and disorder is apparent within a single bilayer and the degree of disorder increases with additional layers. Despite second harmonic generation (SHG) measurements that indicate a linear increase in the SHG signal with increasing bilayer number, reflectivity measurements show that the films are disordered on the molecular scale.
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