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2008, 112, 19168–19172 Published on Web 11/14/2008
Chemical Passivation of Silicon Nanowires with C1-C6 Alkyl Chains through Covalent Si-C Bonds Muhammad Y. Bashouti,† Thomas Stelzner,‡ Andreas Berger,‡,§ Silke Christiansen,‡,§ and Hossam Haick*,† The Department of Chemical Engineering and Russell Berrie Nanotechnology Institute, Technion-Israel Institute of Technology, Haifa 32000, Israel, Institute of Photonic Technology e.V., Albert-Einstein-Str. 9, 07745 Jena, Germany, and Max-Planck-Institute of Mikrostructure Physics, Weinberg 2, 06120 Halle, Germany ReceiVed: August 31, 2008; ReVised Manuscript ReceiVed: October 16, 2008
We report on the functionalization of Si NWs with C1-C6 alkyl chains using a versatile two step chlorination/ alkylation process. We show that Si NWs terminated with C1-C6 molecules, through Si-C bonds, connect alkyl molecules to 50-100% of the Si atop sites and provide surface stability that depends on the chain length and molecular coverage, according to the following order: C1-Si NW > C2-Si NW > (C3-C6)-Si NW. Our results indicate that the oxidation resistance of (C1-C2)-Si NWs is significantly higher than equivalent 2D Si(100) surfaces, whereas (C3-C6)-Si NWs are comparable to 2D (C3-C6)-Si(100). These discrepancies can be explained as follows: the lower the molecular coverage, the higher the probability for interaction between oxidizing agents (O2 or H2O) and molecule-free sites. Our results are of practical importance when reduced amounts of oxide are required, e.g., for radial epitaxy on NWs to realize vertical P-N junctions for solar cells or for radial Si/Ge superlattices for application in optoelectronics. 1. Introduction Silicon nanowires (Si NWs) offer unique opportunities as building blocks for nanoelectronics1 and (bio)sensors.2-4 However, for many of the specific device settings, the presence of native oxide (SiOx), which instantaneously forms on the Si NW surfaces when exposed to air, is undesirable.5-7 This is because the quality of the oxide that forms on the surface is poor, thereby inducing a high number of unwanted, uncontrolled interface states in the band gap of Si.7,8 These effects become more significant as Si NWs become smaller in diameter, because the properties of the semiconductor surface begin to dominate the properties of the device as a whole.6,9,10 Freshly prepared H-terminated Si NWs have low surface recombination velocities but are not stable in air for more than a few hours.11,12 Recently, we have reported on Si NWs that were chemically modified by CH3 functionalities, through a covalent Si-C bond, and exhibited a tremendously enhanced stability (more than 40 days) against oxidation.5 The resulting CH3-Si NWs allowed formation of air-stable Si NW field effect transistors (FETs) having on-off ratios in excess of 105 over a relatively small gate voltage swing ((2 V).5 However, to achieve electronic devices and (bio)sensors with improved control over their properties, there is a need for a wide variety of other functionalities that allow control at molecular levels as well as locating certain functionalities and/or sorption properties.13 * To whom correspondence should be addressed. E-mail: hhossam@ technion.ac.il. Fax: +972-4-8295672. † Technion-Israel Institute of Technology. ‡ Institute of Photonic Technology e.V. § Max-Planck-Institute of Mikrostructure Physics.
10.1021/jp8077437 CCC: $40.75
In this letter, we report on a versatile two step chlorination/ alkylation process14,15 for functionalizating Si NWs with C1-C6 alkyl chains, while preserving the Si NWs’ original length and diameter. We show that Si NWs terminated with C1-C6 molecules, through Si-C bonds, connect alkyl molecules to 50-100% of the Si atop sites and provide surface stability that depends on the chain length and molecular coverage. The alkylation process of Si NWs, the oxidation resistance of the modified structures and the differences from 2D surfaces are discussed. 2. Experimental Section 2.1. Synthesis of Si NWs. Si NWs were prepared by the vapor-liquid-solid (VLS) method using chemical vapor deposition (CVD) with silane on Si(111) substrates (see Figure 1). For that purpose, the substrates were etched in a diluted HF solution to remove the native oxide and subsequently a 2 nm thick Au film was sputtered on the substrate and the sample was transferred into the CVD chamber. The substrates were annealed at ∼580 °C and at a pressure of ∼5 × 10-7 mbar for 10 min. The temperature was then reduced to ∼520 °C and a mixture of 5-10 sccm Ar and 5 sccm SiH4 was introduced for 20 min at a pressure of 0.5-2 mbar for growth of nanowires. 2.2. Alkylation of Si NWs through Si-C bond. The Si NWs were terminated with alkyl chains using a two-step chlorination/alkylation route (see Figure 2). Before any chemical treatment, each sample was cleaned by N2 flow. After being cleaned, H-terminated Si NWs were prepared by etching the amorphous SiOx coating by exposing the Si NWs to buffered HF solution (pH ) 5) for 60 s and then NH4F for 30 s. The samples were then removed and rinsed in water for C2-Si NW (68% coverage) > (C3-C6)-Si NW (49-56% coverage). The results show further that the oxidation resistance of C1-Si NWs (100% coverage) > 2D C1-Si(100) (100% coverage), C2-Si NWs (∼68% coverage) > 2D C2-Si(100) (60% coverage), (C3-C6)-Si NWs (49-56% coverage) ≈ 2D
19172 J. Phys. Chem. C, Vol. 112, No. 49, 2008 (C3-C6)-Si(100) (30% coverage). The higher the coverage beyond ∼50% Si atop sites, the higher the oxidation resistance of Si NWs and the higher the difference between the oxidation resistance between Si NWs and 2D Si(100). For molecularly modified surfaces that fall into this category, a Si-C bond on Si NWs, which is stronger than that of 2D Si(100), seems to play a role in determining the oxidation resistance of the Si surfaces, although further studies are required to fully understand this role. Below 50% coverage of Si atop sites, the oxidation resistance of Si NWs and 2D Si(100) terminated with C3-C6 alkyl chains seems to be independent of the coverage of the Si atop sites. These trends could change, however, at longer exposure times (>336 h) to ambient conditions, especially after a growth of (stable) native oxide in the molecule-free sites (or pinholes). In this case, the determining factor for oxidation resistance would be the stability of the molecular islands. Our results are of practical importance, for example, when reduced amounts of oxide are required for radial epitaxy on NWs to realize vertical P-N junctions for solar cells or for radial Si/ Ge superlattices for application in optoelectronics. A comprehensive study that targets the kinetic mechanism of the alkylation process of Si NW surfaces, the oxidation resistance of the modified structures, and the differences compared to 2D surfaces are underway and will be published elsewhere. Acknowledgment. H.H. and M.Y.B. acknowledge the Marie Curie Excellence Grant of the FP6, the U.S.-Israel Binational Science Foundation, and the Russell Berrie Nanotechnology Institute for financial support, and Mr. Ossama Assad (Technion) for fruitful discussions. M.Y.B. thanks the Zeff Fellowship for partial support. H.H. holds the Horev Chair for the Leaders in Science and Technology. S.H.C. acknowledges financial support by the Deutsche Forschungsge-meinschaft (DFG) under contract number CH159/1. References and Notes (1) Thelander, C.; Agarwal, P.; Brongersma, S.; Eymery, J.; Feiner, L. F.; Forchel, A.; Scheffler, M.; Riess, W.; Ohlsson, B. J.; Go¨sele, U.; Samuelson, L. Mater. Today 2006, 9, 28–35. (2) Patolsky, F.; Lieber, C. M. Mater. Today 2005, 8, 20–28. (3) Law, M.; Kind, H.; Messer, B.; Kim, F.; Yang, P. Angw. Chem., Inter. Ed. 2002, 41, 2405–2408. (4) Assad, O.; Haick, H. In IEEE ISIE; Cambridge, U.K., 2008. (5) Haick, H.; Hurley, P. T.; Hochbaum, A. I.; Yang, P.; Lewis, N. S. J. Am. Chem. Soc. 2006, 128, 8990–8991. (6) Kamins, T. Electrochem. Soc. Interf. 2005, 14, 46–49.
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