Quantitative characterization of obliquely deposited substrates of gold byatomic force microscopy: Influence of substrate topography on anchoring ofliquid crystals

We report the use of atomic force microscopy (AFM) to characterize quantita tively the structural anisotropy within ultrathin (thickness of similar to 10 nm) obliquely deposited films of gold and thereby calculate the influenc e 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 gol d films, a quantitative analysis of the AFM profiles does show a subtle lev el of anisotropy on wavelengths comparable to the lateral dimensions of the gold grains (similar to 30 nm). Our analysis reveals the root-mean-square (rms) slope of the surface topography to be similar to 1 degrees greater in a direction parallel to the direction of deposition of the gold as compare d to the perpendicular direction. We also demonstrate the rms curvature of the grains of gold to be greatest in a direction parallel to deposition. Be cause the amplitude of the surface roughness (similar to 2 nm) is small com pared to its wavelength (similar to 30 nm), the influence of the surface ro ughness on the orientations of supported LCs can be described through an el astic mechanism of anchoring. By combining the multimode Berreman-de Gennes model for the elastic free energy density of a nematic LC with AFM profile s of the topography of obliquely deposited gold films, we calculate the azi muthal anchoring energy of the supported LC to be similar to 0.015 mJ/m(2), a value that is consistent with estimates of anchoring energies obtained b y fabrication of twisted nematic LC cells. The results reported in this pap er provide a route to the characterization of surfaces with designed levels of anisotropy suitable for control of the anchoring of LCs. This capabilit y will, we believe, find application in studies aimed at exploring the use of LCs for amplification and transduction of events of molecular recognitio n (e.g., antigen-antibody) at surfaces.