An X-ray reflectometry study of Cd- and Mn-substituted tricosanoate Langmuir-Blodgett films

The structures of Langmuir-Blodgett films of 8 and 14 bilayers of Cd²⁺- and Mn²⁺-substituted tricosanoate have been measured by X-ray reflectometry. The specular reflectivity profiles contain up to 21 orders of Bragg peak, which indicates that there is an almost perfect stacking of the bilayers normal to the surface of the films. These profiles also exhibit a characteristic variation in intensity between the even and odd orders of Bragg peak for all four samples. The lineshape of the off-specular scattering at the positions of the Bragg peaks was found to be Lorentzian. The half-width at half-maximum (HFHM) of these peaks varies according to the square of the longitudinal scattering vector q_z, which confirms the presence of self-affine rough interfaces in these samples. The magnitude of the interface roughness is found to be smallest in the films with the greatest number of layers. This is possibly associated with structural defects, caused by an incomplete transfer on to the substrate of the first few layers of the LB film, which can propagate through subsequent layers causing a phase shift of the bilayer structure whilst still preserving the overall bilayer period.