Secondary ion mass spectrometry for characterizing photopatterned, self-assembled monolayers on gold

Abstract

Secondary ion mass spectrometry (SIMS) can be used to determine the lateral distribution of molecular photoproducts in a photopatterned, self-assembled monolayer (SAM) on Au. The photosensitive SAMs, Au-I, are made by immersing evaporated Au films into a solution of a disulfide with a pendant aryl azide. Near-UV irradiation (λ≳320 nm) of these SAMs through a Cr-on-glass mask and a thin film of N-(2,2,2-trifluoroethyl)-N-(2-ferrocenylethyl) amine, II, results in the photoattachment of II to the Au surface where irradiated. Cyclic voltammetry establishes the presence of 2.8×10−10 mol/cm2, or about one monolayer, of ferrocene centers on the irradiated substrate. Positive SIMS survey spectra of the photosensitive Au-I SAMs show several peaks assigned to the aryl azide functionality. Survey spectra of Au-I substrates irradiated in the presence of II show peaks assigned to ferrocene fragments. Importantly, the high lateral resolution imaging capability of SIMS is used to map the lateral distribution of molecular and elemental species in a SAM photopatterned with II. The lateral resolution demonstrated was 10 μm. The chemical maps for C12H12Fe+ (m/z 212) and F− (m/z 19) reproduce the pattern of the mask. An important manifestation of the exceptional sensitivity of SIMS is the speed of image acquisition. Maps are generally acquired in less than 30 s with total ion doses of ∼1013 ions/cm2. A damage study was made showing counts for ions with m/z 27, 56, 197, and 212 as a function of primary ion dose. The half-life for the C12H12Fe+ (m/z 212) fragment was approximately 8.5 min at 45 nA/cm2 primary beam current density. This is ≳15 times longer than it takes to acquire a well-resolved image of m/z 212 distribution in a patterned SAM.