Article pubs.acs.org/Langmuir
Multifunctional Silicon Surfaces: Reaction of Dichlorocarbene Generated from Seyferth Reagent with Hydrogen-Terminated Silicon (111) Surfaces Wenjun Liu,†,‡ Ian D. Sharp,*,†,§ and T. Don Tilley*,†,∥,⊥ †
Joint Center for Artificial Photosynthesis, ‡Materials Sciences Division, §Physical Biosciences Division, and ∥Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ⊥ Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Insertion of dichlorocarbene (:CCl2), generated by decomposition of the Seyferth reagent PhHgCCl2Br, into the Si−H bond of a tertiary silane to form a Si−CCl2H group is an efficient homogeneous, molecular transformation. A heterogeneous version of this reaction, between PhHgCCl2Br and a silicon (111) surface terminated by tertiary Si−H bonds, was studied using a combination of surface-sensitive infrared and X-ray photoelectron spectroscopies. The insertion of dichlorocarbene into surface Si−H bonds parallels the corresponding reaction of silanes in solution, to produce surface-bound dichloromethyl groups (Si−CCl2H) covering ∼25% of the silicon surface sites. A significant fraction of the remaining Si−H bonds on the surface was converted to Si−Cl/Br groups during the same reaction, with PhHgCCl2Br serving as a halogen atom source. The presence of two distinct environments for the chlorine atoms (Si−CCl2H and Si−Cl) and one type of bromine atom (Si−Br) was confirmed by Cl 2p, Br 3d, and C 1s X-ray photoelectron spectroscopy. The formation of reactive, halogen-terminated atop silicon sites was also verified by reaction with sodium azide or the Grignard reagent (CH3MgBr), to produce Si−N3 or Si−Me functionalities, respectively. Thus, reaction of a hydrogen-terminated silicon (111) surface with PhHgCCl2Br provides a facile route to multifunctional surfaces possessing both stable silicon−carbon and labile silicon−halogen sites, in a single pot synthesis. The reactive silicon−halogen groups can be utilized for subsequent transformations and, potentially, the construction of more complex organic−silicon hybrid systems.
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INTRODUCTION Wet-chemical functionalization of silicon surfaces to give a monolayer of organic molecules represents an effective and practical tool for control of chemical, physical, and electronic properties of the underlying silicon.1−3 The resulting organic− silicon hybrid structures find applications in electronics,3−5 chemical/bio sensing,6,7 and photoelectrochemical energy conversion.8−10 Among the different functionalizations for silicon surfaces, those supported by strong and nonpolar Si−C linkages provide better chemical stabilization of the silicon against oxidation and electronic passivation of deleterious surface states,11 as compared to those resulting from either weak Si−X (X = Si or I) or strong but polar Si−X (X = H, N, F, or Cl) bonds.12,13 Starting from a freshly etched, hydrogenterminated silicon (111) surface (H−Si(111)), surface Si−C bonds are usually introduced either via a hydrosilylation reaction with unsaturated hydrocarbons (alkenes or alkynes) (Scheme 1a)14−24 or by a two-step chlorination/alkylation sequence proceeding through a chlorine-terminated silicon (111) surface (Cl−Si(111)) as an intermediate state (Scheme 1b).25−32 Here, we describe an alternative strategy to create Si−C linkages on silicon (111) surfaces. This approach is inspired by © 2013 American Chemical Society
the chemistry of tertiary silanes, considered to be discrete molecular analogues of sites on the extended H−Si(111) surface, and relies on a well-established reactivity mode for the Si−H bond that has not previously been exploited for wetchemical modification of H−Si(111) surfaces. As shown in Scheme 1c, dichlorocarbene (:CCl2), generated in situ by thermal decomposition of the Seyferth reagent PhHgCCl2Br, inserts into the Si−H bonds of tertiary silanes, for example, triethylsilane and triphenylsilane.33−36 We envisioned a similar reaction on the H−Si(111) surface, which is coated by an array of tertiary Si−H bonds. Indeed, we found that this insertion reaction also occurs on the H−Si(111) surface, resulting in a coverage of ∼25% of the atop silicon sites with dichloromethyl functionalities (Si−CCl2H). Interestingly, a significant fraction of the remaining Si−H groups was converted to Si−Cl/Br groups during the course of the reaction, presumably through a radical-initiated halogenation. Our findings highlight similarities and differences between homogeneous, molecular transformations in solution and heterogeneous reactions on surfaces. Received: October 1, 2013 Revised: December 10, 2013 Published: December 12, 2013 172
dx.doi.org/10.1021/la403789a | Langmuir 2014, 30, 172−178
Langmuir
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with Milli-Q water, the samples were etched in an ammonium fluoride solution (40% in water; Transene Co., Inc.; deoxygenated by sparging with N2 for 30 min) for 15 min, rinsed with Milli-Q water, and dried under a constant flow of nitrogen. The resulting H−Si(111) surfaces were immediately brought into a N2-filled drybox for subsequent surface functionalization. Functionalization of H−Si(111) Surfaces. In a drybox, H− Si(111) surfaces were placed in a solution of PhHgCCl2Br in chlorobenzene (20 mM) in a sealed vial. This mixture was heated to 80 °C with slow stirring for 4 h. Afterward, the samples were removed from the drybox and sonicated in methanol for 5 min. This procedure resulted in mixed surface terminations composed of both dichloromethyl and halogen (Br and Cl) groups (CCl2H/Cl/Br−Si(111)). Secondary Functionalization of CCl2H/Cl/Br−Si(111) Surfaces. In a drybox, CCl2H/Cl/Br−Si(111) surfaces were immersed either in a saturated solution of sodium azide in hexamethylphosphoramide (HMPA) for 4 h or in a solution of methylmagnesium bromide in 2-methyltetrahydrofuran (∼3.2 M; Sigma-Aldrich) at 70 °C overnight, to produce CCl2H/N3−Si(111) or CMe2H/Me−Si(111) surfaces, respectively. Fourier-Transform Infrared Spectroscopy (FT-IR). IR spectra were collected using a Bruker VERTEX 70 spectrometer with a liquid nitrogen-cooled MCT detector. Spectra of monolayers on silicon surfaces were measured in the attenuated-total-reflectance (ATR) mode, using a VariGATR accessory (Harrick Scientific Products, Inc.). The functionalized, polished side of a wafer was pressed against a hemispherical Ge crystal, which was set to an IR light incident angle of 62.5° with respect to the surface normal. ATR-IR spectra were collected under a constant flow of nitrogen in the optical path, and with a resolution of 4 cm−1. All spectra reported here were presented as an average of 1240 consecutive scans and referenced to the spectrum of the bare Ge crystal in air as the background. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed using a Kratos Axis Ultra DLD system. Measurements were carried out at a base pressure of
