Nanoscale Patterning

SURFACE SCIENCE

What's more useful: a quick and easy nanoscale fabrication method that yields lots of loosely assembled products, or a painstaking procedure for making robust structures one molecule at a time?

This question highlights a trade-off commonly encountered in nanoscale research. But now, scientists can choose a third alternative.

University of Toronto chemists have shown that weakly bound self-assembled layers of molecules can be secured to solid surfaces via strong chemical bonds by irradiating the molecules with ultraviolet light. The study advances the understanding of surface reaction mechanisms and may lead to simple, yet precise, procedures for patterning surfaces with nanometer-sized features.

By exposing a cold, pristine silicon crystal to small quantities of methyl bromide, chemistry professor and Nobel Laureate John C. Polanyi and coworkers Sergey Dobrin, Xuekun Lu, Fedor Y. Naumkin, and Jody S. Y. Yang decorate the crystal surface with well-ordered patterns of molecules. Scanning tunneling microscopy images show that, on the particular silicon crystal face examined by the group, the molecules assemble into arrays of neat circles consisting of 12 molecules per circle.

As with other surfaces and adsorbed species, the patterns form readily at low temperatures (50 K in this study). But because such "physisorbed" assemblies are held in place loosely through weak physical interactions, they lose their high degree of order at warmer temperatures.

To take advantage of self-assembly yet form stable structures, the Toronto researchers irradiated the molecules with UV light. The group reports that the radiation causes a reaction in which the C–Br bond in the adsorbed molecules is cleaved and a Si–Br bond to the crystal is formed with retention of the self-assembled pattern [Surf. Sci., published online Nov. 4, http://dx.doi.org/10.1016/j.susc.2004.09.048]. The process, which can also be carried out with an electron beam, results in an array of circles (12 bromine atoms per circle) in which each atom is bonded at the exact location where the parent methyl bromide molecule had been bonded before irradiation. The group notes that the bromine circles are stable at 200 °C.

On the basis of experimental and computational work, Polanyi and coworkers propose that the UV light liberates electrons from the surface, resulting in charge transfer to the adsorbed molecules. The process dissociates the molecules, causing methyl radicals to be ejected and Br² ions to recoil toward the surface, where they are neutralized and captured in a chemically bound state.

Richard M. Osgood Jr., professor of electrical engineering and applied physics at Columbia University, remarks that the study suggests "powerful possibilities" for patterning surfaces with "unexpectedly high spatial resolution." Osgood, whose comments are scheduled to appear in the same issue of Surface Science, adds that the sturdy surface structures may be used in subsequent patterning steps as chemical resists and passivation layers.