Steric Spacing of Molecular Linkers on Passivated Si(111) Photoelectrodes

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• 作者：Feng Li ; Victoria M. Basile ; Ryan T. Pekarek ; Michael J. Rose
• 刊名：ACS Applied Materials & Interfaces
• 出版年：2014
• 出版时间：November 26, 2014
• 年：2014
• 卷：6
• 期：22
• 页码：20557-20568
• 全文大小：554K
• ISSN：1944-8252

文摘

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鈭? 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 (鈭扥₂SCF₃) 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 卤 0.8%), CyHex (2.9 卤 1.2%), EtHex (2.1 卤 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)鈥揅l 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)鈥?b>8a/CH₃鈥揊c (R = CH₃) and Si(111)鈥?b>8c/CH₃鈥揊c (R = 2-EtHex) are consistent with the corresponding Fe 2p XPS coverages and suggest a 鈭?: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)鈥?b>8a/CH₃ and the ferrocene coupled n-Si(111)鈥?b>8a/CH₃鈥揊c exhibited relatively high surface carrier lifetimes (4.51 and 3.88 渭s, respectively) and correspondingly low S values (3880 and 4510 cm s^鈥?). Thus, the multistep, linker/Fc functionalized surfaces exhibit analogously low trap state densities as compared to the fully passivated n-Si(111)鈥揅H₃ surface.

Keywords:

Si(111); surface functionalization; passivation; steric spacing; photoelectrochemistry; covalent attachment