Quantitative Characterization of Obliquely Deposited Substrates of Gold by Atomic Force Microscopy:  Influence of Substrate Topography on Anchoring of Liquid Crystals

Abstract

We report the use of atomic force microscopy (AFM) to characterize quantitatively the structural anisotropy within ultrathin (thickness of ∼10 nm) obliquely deposited films of gold and thereby calculate the influence of this anisotropy on the orientations of liquid crystals (LCs) supported on these surfaces. Whereas visual inspection of AFM images (real space or reciprocal space) reveals no obvious structural anisotropy within these gold films, a quantitative analysis of the AFM profiles does show a subtle level of anisotropy on wavelengths comparable to the lateral dimensions of the gold grains (∼30 nm). Our analysis reveals the root-mean-square (rms) slope of the surface topography to be ∼1°greater in a direction parallel to the direction of deposition of the gold as compared to the perpendicular direction. We also demonstrate the rms curvature of the grains of gold to be greatest in a direction parallel to deposition. Because the amplitude of the surface roughness (∼2 nm) is small compared to its wavelength (∼30 nm), the influence of the surface roughness on the orientations of supported LCs can be described through an elastic mechanism of anchoring. By combining the multimode Berreman-de Gennes model for the elastic free energy density of a nematic LC with AFM profiles of the topography of obliquely deposited gold films, we calculate the azimuthal anchoring energy of the supported LC to be ∼0.015 mJ/m 2 , a value that is consistent with estimates of anchoring energies obtained by fabrication of twisted nematic LC cells. The results reported in this paper provide a route to the characterization of surfaces with designed levels of anisotropy suitable for control of the anchoring of LCs. This capability will, we believe, find application in studies aimed at exploring the use of LCs for amplification and transduction of events of molecular recognition (e.g., antigenantibody) at surfaces.

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