Hydrosilylation has been a key reaction in preparing monolayers on silicon surfaces. This process involves the reaction of a terminally unsaturated reactant with the Si surface. Over the past 20 years, several advances have been accomplished to obtain better (i.e. denser and more stable) monolayers with various reactants (alkenes, alkynes, dienes, etc.) under different reaction conditions (e.g. thermal initiation, ultraviolet light, etc.).^1,2 Such a higher density is advantageous for the structural ordering, stability and a wide range of applications. The procedure used in our lab (as well as labs around the world) involves wet-chemical techniques for the surface modifications. As the name implies, the reactants with these techniques must be available as liquids under the reaction conditions. Due to this constraint, only monolayers of relatively long chain lengths have been made, because shorter chains evaporate under thermal conditions (or are even a gas). In the current project, we have prepared and characterized a new class of monolayers with (short) chain lengths that were previously inaccessible.

H-Si(111) surfaces were modified with gaseous alkynes in a pressure-resistant PARR reactor. This novel method in silicon-carbon chemistry allows the chemisorption of compounds that were previously unusable in surface modification due to its volatility. Si–C-bonded monolayers derived from propyne, 1-butyne and 5–functionalized-pent-1-ynes (-COOH, -Cl, & -NH₂) were prepared and characterized using various surface-sensitive techniques. Si(111)–propenyl and butenyl silicon-monolayers display a higher packing density (up to 75%) than any wet-chemically prepared alkyne-derived monolayer. Furthermore, a combination of experimental and theoretical data shows that propyne chemisorption happens in a temperature-dependent manner, not observed for any other alkyne studied up to now: through addition of the second carbon (-iso) at temperatures below 90 ° C, and of the terminal carbon (-lin) above 90 ° C. Finally, 5-chloro-1-pentyne and 4-Pentynoic acid were shown to bind at high surface densities and (near-)exclusively via the terminal carbon of the triple bond. These end groups allow for further functionalization of the monolayer.

(1) Rijksen, B.; Pujari, S. P.; Scheres, L.; van Rijn, C. J. M.; Baio, J. E.; Weidner, T.; Zuilhof, H. Langmuir2012, 28, 6577-6588.