Neutron and X-Ray Reflectometry Studies of Rough Interfaces
in a Langmuir-Blodgett Film
H. Kepa, L. J. Kleinwaks,
N. F. Berk, C. F. Majkrzak, V. I. Troitsky, R. Antolini, and L. A. Feigin,
Proc. Int. Conf. on Neutron Scattering,
Toronto (1997). To be published in Physica B.
Neutron and x-ray reflectometry are used to study interlayer
roughness and islanding in a 20-bilayer barium stearate
Langmuir-Blodgett film with alternating hyrogenated and deuterated
bilayers. The interlayer roughness is highly conformal, and analysis of
the diffuse x-ray peak widths suggest it is approximately self-affine
with a roughness exponent approximately 0.8. This exponent
describes the film in all directions, even though steps on the Si
substrate interrupt the correlation of the film across the steps. The
neutron and x-ray specular reflectivities have modulated Kiessig
fringes, indicating the presence of islands on the top of the film.
Odd Bragg peaks in the neutron specular reflectivity are broadended,
implying long-range disorder in the H-D bilayer structure. These
data suggest that the islands arose from incomplete coverage during
the film preparation.
This work was presented as an invited
paper to ICNS by Henryk Kepa, a guest researcher at NIST from Oregon
State University. The study is a collaboration between scientists at
NIST and the from the Center for Materials and Medical Biophysics,
Povo, Italy (Antonlini), Technobiochip, Marciana, Italy (Berzina, Troitsky),and the
Institute of Crystallography, Moscow, Russia (Feigin).
Logan J. Kleinwaks contributed to this
study while a senior at Thomas Jefferson High School for
Science and Technology in Alexandria, VA.
Values of the self-affine roughness
exponent have been published which range from 0.2 to 0.8 (the value
found here). Such a span, if reliable, is likely the result of several
distinct physical process at work during L-B film fabrication. Values
near 1 are consistent with models of random crumpling, as during
drying; e.g., Tzschichholz,
Hansen, and Roux.