Steric spacing of molecular linkers on passivated Si(111) photoelectrodes

Surfaces with high photoelectrochemical and electronic quality can be prepared by tethering small molecules to single-crystalline Si(111) surfaces using a two-step halogenation/alkylation method (by Lewis and co-workers).1-7 We report here that the surface coverage of custom-synthesized, phenyl-based molecular linkers can be controlled by varying the steric size of R-groups (R=CH3, C6H11, 2-ethylhexyl) at the periphery of the linker. Additionally, the linkers possess a para triflate group (-O2SCF3) that serves as a convenient analytical marker and as a point of covalent attachment for a redox active label. Quantitative X-ray photoelectron spectroscopy (XPS) measurements revealed that the surface coverage systematically varies according to the steric size of the linker: CH3 (6.7±0.8%), CyHex (2.9±1.2%), EtHex (2.1±0.9%). The stability of the photoelectrochemical cyclic voltammetry (PEC-CV) behavior was dependent on an additional methylation step (with CH3MgCl) to passivate residual Si(111)-Cl bonds. Subsequently, the triflate functional group was utilized to perform Pd-catalyzed Heck coupling of vinylferrocene to the surface-attached linkers. Ferrocene surface coverages measured from cyclic voltammetry on the ferrocene-functionalized surfaces Si(111)-8a/CH3-Fc (R=CH3) and Si(111)-8c/CH3-Fc (R=2-EtHex) are consistent with the corresponding Fe 2p XPS coverages and suggest a ∼1:1 conversion of surface triflate groups to vinyl-Fc sites. The surface defect densities of the linker/CH3 modified surfaces are dependent on the coverage and composition of the organic layer. Surface recombination velocity (SRV) measurements indicated that n-Si(111)-8a/CH3 and the ferrocene coupled n-Si(111)-8a/CH3-Fc exhibited relatively high surface carrier lifetimes (4.51 and 3.88 μs, respectively) and correspondingly low S values (3880 and 4510 cm s(-1)). Thus, the multistep, linker/Fc functionalized surfaces exhibit analogously low trap state densities as compared to the fully passivated n-Si(111)-CH3 surface.