Scale Dependence of the Orientation and Conformation Distribution Analysis of a Molecular Monolayer Using Sum Frequency Generation Imaging Microscopy

A systematic approach of experimentally measuring the average orientation and distribution of octadecanethiol (ODT) monolayer on gold was performed by using sum frequency generation imaging microscopy (SFGIM). By constructing maps from region of interests (ROIs) of different length scales, measuring from 5 to 1000 μm, two-dimensional domains could be distinguished to contain less-oriented monolayers. By generating the tilt angle distributions of the scaled ROIs, SFGIM determines the experimental orientation, θ, and distribution width, σ 0, as dictated by the SFGIM resolution limit. Using the same statistical methods, the heterogeneities of ODT monolayers on Au and Cu were compared. The quantitative analysis of these orientation parameters is essential for future work on characterizing monolayer integrity on more complex surfaces such as reactive metals and alloys. Composite maps that merge orientation and conformational information were demonstrated as potential tools for spectromicroscopic measurement of the reactivity of metal substrates to surface processes such as corrosion and wettability.