Half-flat vs. atomically flat: Alkyl monolayers on morphologically controlled Si(100) and Si(111) have very similar structure, density, and chemical stability.
Chemists have long preferred the Si(111) surface for chemical functionalization, as a simple aqueous etch can be used to produce ideal, atomically flat H/Si(111) surfaces for subsequent reactions. In contrast, industry-standard etches produce rough H/Si(100) surfaces terminated by nanohillocks. The recent discovery of an aqueous etch that produces morphologically controlled H/Si(100) surfaces with a near atomically flat or "half-flat" morphology challenges the assumption that Si(111) is an inherently preferable starting point for chemical functionalization. This study shows that alkyl functionalization of morphologically controlled, "half-flat" Si(100) surfaces by terminal alkenes produces dense, tightly packed monolayers that are essentially identical to those on atomically flat Si(111). The striking similarity between the infrared spectra on these two surfaces - in terms of absolute absorbance, line shape and position, and polarization dependence - strongly suggests that alkyl monolayers on morphologically controlled Si(111) and Si(100) have essentially identical structures. The principle difference between the two surfaces is the amount of residual H at the Si/organic interface, a difference that is dictated by the structure of the Si(100) surface. Alkyl monolayers on morphologically controlled Si(111) and Si(100) surfaces were shown to be equally resistant to harsh oxidants. As a result, there appears to be no chemical reason to prefer one surface over the other, at least for functionalization with terminal alkenes.
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
William J I
WJ
DeBenedetti
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
Thomas L
TL
Li
Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853, USA.
Melissa A
MA
Hines