A study of mechanical forces induced by low-energy (10-100 eV) electron irradiation of alkanethiol self-assembled monolayers on Au(111)
Electron irradiation is widely used in various applications such as fabricating masks for nano-imprint lithography, manufacturing cross linked polymers and synthesizing novel chemicals. Low-energy secondary electrons scattered from the incident high-energy source in lithography and other techniques may introduce unwanted damage to the mask. Although a great deal of research has been conducted on irradiation induced chemistry, mechanical effects are still poorly understood. Studying the mechanical consequences of low-energy electron irradiation is important for understanding the fundamental chemistry in irradiation induced processes, as well as for improving irradiation related techniques. Alkanethiol (AT) self-assembled monolayers (SAMs), which can spontaneously assemble on metals (i.e. Ag, Au), were selected for this research due to their uniformity and sensitivity to modification using electron beams. C-H bond rupture occurs during irradiation and leads to the formation of radical sites and C-C, C=C, and S-S bond formation within the monolayer. Formations of these bonds between chains can physically distort the organic layer, which will induce mechanical force changes on the gold substrate. In order to detect the stress change, AT SAMs were deposited on gold coated microcantilevers, which are silicon nitrate wafers of 0.6 micrometer thickness. Changes in the bending of these cantilevers were determined using a HeNe laser beam and a position sensitive detector (PSD). By using this cantilever-laser system, we were able to measure stress change with a resolution of 10-3 N/m. These studies have shown that the net stress change is always tensile; suggesting that cross-linking between chains is the dominant electron-initiated process.