From Molecules to Surfaces: Radical-Based Mechanisms of Si–S and Si–Se Bond Formation on Silicon

Abstract

The derivatization of silicon surfaces can have profound effects on the underlying electronic properties of the semiconductor. In this work, we investigate the radical surface chemistry of silicon with a range of organochalcogenide reagents (comprising S and Se) on a hydride-terminated silicon surface, to cleanly and efficiently produce surface Si-S and Si-Se bonds, at ambient temperature. Using a diazonium-based radical initiator, which induces formation of surface silicon radicals, a group of organochalcogenides were screened for reactivity at room temperature, including di-n-butyl disulfide, diphenyl disulfide, diphenyl diselenide, di-n-butyl sulfide, diphenyl selenide, diphenyl sulfide, 1-octadecanethiol, t-butyl disulfide, and t-butylthiol, which comprises the disulfide, diselenide, thiol, and thioether functionalities. The surface reactions were monitored by transmission mode Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ionization mass spectrometry. Calculation of Si-Hx consumption, a semiquantitative measure of yield of production of surface-bound Si-E bonds (E = S, Se), was carried out via FTIR spectroscopy. Control experiments, sans the BBD diazonium radical initiator, were all negative for any evident incorporation, as determined by FTIR spectroscopy. The functional groups that did react with surface silicon radicals included the dialkyl/diphenyl disulfides, diphenyl diselenide, and 1-octadecanethiol, but not t-butylthiol, diphenyl sulfide/selenide, and di-n-butyl sulfide. Through a comparison with the rich body of literature regarding molecular radicals, and in particular, silyl radicals, reaction mechanisms were proposed for each. Armed with an understanding of the reaction mechanisms, much of the known chemistry within the extensive body of radical-based reactivity has the potential to be harnessed on silicon and could be extended to a range of technologically relevant semiconductor surfaces, such as germanium, carbon, and others.

A Mechanistic Study of the Oxidative Reaction of Hydrogen-Terminated Si(111) Surfaces with Liquid Methanol

“…[1][2] The reactivity of hydrogen-terminated Si(111) (H-Si(111)) surfaces toward organic nucleophiles, including alkenes, [3][4] alkynes, [5][6] amines, [7][8][9][10][11] thiols and disulfides, [12][13] Grignards, [14][15] and alcohols, [16][17][18][19][20][21][22][23][24][25] has been widely exploited to impart desirable functionality to the Si interface. These surface reactions have been used to control the interface between Si and metals, [26][27][28][29][30][31] metal oxides, [32][33][34][35] polymers, [36][37][38][39][40][41] and redox assemblies.…”

Expanding the Repertoire of Molecular Linkages to Silicon: Si–S, Si–Se, and Si–Te Bonds

“…66−68 Assuming the central role of the Si· radical species, the formation of the Si−E bond could occur through two possible pathways. As is well-established in the molecular silane radical literature, 37,56 dialkyl and diaryl disulfides and diselenides can add to the silyl radical through an S H 2 mechanism (Scheme 2a), producing a sulfuranyl or seleranyl intermediate that then collapses to form the Si−ER final product, releasing an equivalent of ·ER (E = S and Se). The equilibria between silanes and thiols under radical conditions have been extensively studied, and although there are few examples of direct coupling of a silane radical, Si·, and a chalcogenide radical, ·ER, to produce Si−ER bonds, there is precedent.…”

“…The RSSR molecules all result in sulfur incorporation on the silicon surfaces ( Figures 3a,c,e,g, and S20), the energy of which corresponds to the known binding energy of Si−S−Rg r o u p s . 31,37 The corresponding silicon Si 2p spectra reveal little oxidation, which would appear as conspicuous higher energy features above 102 eV.…”

“…37,54 The disulfide molecule, bis(4-chlorophenyl) disulfide, which has a chlorine tag, was also reacted with the porous silicon, and the distinctive Cl 2p doublet was observed at 200.6 and 202.2 eV, in addition to the sulfur feature ( Figure S20). XPS data for the chalcogenide features of the starting materials (molecules REER, E = S, Se, and Te), as well as reported values for Si− ER when known, are shown in Figure 3 and Tables S1 and S2. XPS analyses were complemented by depth profiling with ToF-SIMS, which provides information regarding elemental content, uniformity of functionalization throughout the layer, and comparative information regarding substitution levels.…”

“…36 Our group showed how porous silicon could be functionalized through a room-temperature radical-initiated process using dialkyl and diaryl disulfides and diselenides, but the chemistry did not proceed with sterically hindered disulfides (i.e., with t-butyl substituents) or ditellurides (the latter because of very low solubility). 37 Finally, to the best of our knowledge, there are no reports of SAMs attached to silicon through Si−Te bonds. To summarize, the chemistry of silicon-chalcogenide-based interfacing of molecules on silicon surfaces is still in very early stages when compared to the richness of the literature pertaining to silicon−carbon and silicon−oxygen bond formation.…”