Static Time-of-Flight Secondary Ion Mass Spectrometry of Monolayers

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was performed ...... Hersam, M. C.; Guisinger, N. P.; Lyding, J. W. Nanotechnology 2000, 11,...
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Static Time-of-Flight Secondary Ion Mass Spectrometry of Monolayers on Scribed Silicon Derived from 1-Alkenes, 1-Alkynes, and 1-Haloalkanes Yit-Yian Lua,† Travis L. Niederhauser,† Reija Matheson,† Cara Bristol,† Ian A. Mowat,‡ Matthew C. Asplund,† and Matthew R. Linford*,† Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602, and Charles Evans & Associates, 810 Kifer Road, Sunnyvale, California 94086-5203 Received October 4, 2001. In Final Form: April 10, 2002 Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was performed on monolayers prepared by scribing silicon under a homologous series of 1-alkenes, 1-alkynes, and 1-haloalkanes: CH2dCH(CH2)nCH3 (n ) 2, 5, 9), HCtC(CH2)nCH3 (n ) 2, 5, 9), Cl(CH2)nCH3 (n ) 4, 7, 9), Br(CH2)nCH3 (n ) 4, 7, 11), I(CH2)nCH3 (n ) 0, 1, 2, 4, 7, 11), and I13CH3. Numerous SiCxHy+ and CxHy+ fragments and adduct ions were observed. The results support a proposed binding model that 1-haloalkanes bind to the silicon surface through one C-Si bond and that 1-alkenes and 1-alkynes generally bind through two C-Si bonds. For instance, silicon surfaces scribed under 1-haloalkanes show less carbon by X-ray photoelectron spectroscopy (XPS) than silicon scribed under 1-alkenes and 1-alkynes with the same number of carbon atoms, but they show more intense SiCxHy+ fragments by ToF-SIMS. Above a certain chain length, the relative intensities of the fragment and adduct ions for a homologous series generally increase with increasing alkyl chain length, which is in agreement with carbon surface coverages measured by XPS and the proposed binding models. Anomalously strong SiCH3+ and SiC2H5+ fragments observed in silicon scribed under CH3I and CH3CH2I suggest formation of methyl- and ethyl-terminated silicon, respectively. An isotopic study of silicon scribed under 13CH3I and CH3I provides additional evidence for formation of methyl-terminated silicon and suggests sputter-induced decomposition of the near-surface region by ToF-SIMS. Ab initio calculations of a few SiCxHy+ type fragments are shown to verify assignments of structure. We also note an alternative explanation for some of the results based on the density of alkyl chains on the surfaces.

Introduction We have recently reported1,2 a new method of modifying and patterning silicon, which consists of (a) cleaning a silicon wafer to remove adventitious contaminants from its surface, leaving its thin native oxide layer (10-15 Å thick), (b) wetting the dry surface of the clean silicon with a reactive organic molecule, or “scribing liquid”, (c) mechanically scribing the silicon with a diamond-tipped instrument while it is wet with the scribing liquid, and (d) cleaning the scribed surface to remove excess scribing liquid and silicon particles that are produced by scribing. The scribing chemically activates the surface, which reacts with the scribing liquid to produce alkyl monolayers on silicon. Experimental evidence1,2 suggests that scribing silicon under 1-alkenes, 1-alkynes, or 1-haloalkanes leads to covalent attachment of alkyl chains to the Si surface through silicon-carbon bonds. It was proposed1 that 1-alkenes and 1-alkynes bind to scribed silicon through two3 (and sometimes one) bonds and that 1-haloalkanes bind to scribed silicon through one bond2 as shown in Figure 1. These proposals were based on earlier studies of the chemisorption of organic * To whom correspondence may be addressed. E-mail: [email protected]. † Brigham Young University. ‡ Charles Evans & Associates. (1) Niederhauser, T. L.; Jiang, G.; Lua, Y.-Y.; Dorff, M. J.; Woolley, A. T.; Asplund, M. C.; Berges, D. A.; Linford, M. R. Langmuir 2001, 19, 5889-5900. (2) Niederhauser, T. L.; Lua, Y.-Y.; Sun, Y.; Jiang, G.; Strossman, G. S.; Pianetta, P.; Linford, M. R. Chem. Mater. 2002, 14, 27-29. (3) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Zgierski, M. Z.; Wolkow, R. A. J. Am. Chem. Soc. 1999, 121, 4532-4533.

Figure 1. Proposed binding of (a) 1-alkynes, (b) 1-alkenes, and (c) 1-haloalkanes to scribed silicon (illustrated for the Si(100) surface).

molecules, such as acetylene,4-8 ethylene,7,9-12 propylene,13,14 other alkenes,5,15-18 CH3I,19 CH3Cl,20 and CH3(4) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. Chem. Phys. Lett. 1986, 30, 170-174. (5) Hamers, R. J.; Wang, Y. Chem. Rev. 1996, 96, 1261-1290. (6) Nishijima, M.; Yoshinobu, J.; Tsuda, H.; Onchi, M. Surf. Sci. 1987, 192, 383-397. (7) Cheng, C. C.; Wallace, R. M.; Taylor, P. A.; Choyke, W. J.; Yates, J. T., Jr. J. Appl. Phys. 1990, 67, 3693-3699. (8) Taylor, P. A.; Wallace, R. M.; Cheng, C. C.; Weinberg, W. H.; Dresser, M. J.; Choyke, W. J.; Yates, J. T., Jr. J. Am. Chem. Soc. 1992, 114, 6754-6760. (9) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. J. Chem. Phys. 1987, 87, 7332-7340. (10) Cheng, C. C.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1990, 231, 289-296. (11) Mayne, A. J.; Avery, A. R.; Knall, J.; Jones, T. S.; Briggs, G. A. D.; Weinberg, W. H. Surf. Sci. 1993, 284, 247-256. (12) Liu, H.; Hamers, R. J. J. Am. Chem. Soc. 1997, 119, 7593-7594. (13) Bozack, M. J.; Taylor, P. A.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1986, 177, L933-L937.

10.1021/la0115132 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/16/2002

Monolayers on Scribed Silicon

CH2Br,21,22 on unpassivated Si(100) and Si(111). Structures a and b of Figure 1 show the predicted binding for 1-alkynes and 1-alkenes on a bare Si(100) type surface. On Si(111)-(7 × 7), acetylene most likely forms “two new σ bonds (a “di-σ” configuration), in a manner qualitatively similar to its behavior on Si(001).”5 For our rough surfaces we no doubt expose multiple crystal faces of silicon. With regard to the cyclic structures shown in structures a and b of Figures 1, Inagaki and co-workers23 showed that cyclotetrasilene, which contains one double bond, is less strained than the analogous cyclic saturated compound by 3.8 kcal/mol. Also, according to their calculations cyclobutene is only slightly less stable than cyclobutane (by 6.1 kcal/mol). These results raise the possibility that the four-membered ring in Figure 1a with a double bond might be no less stable or perhaps even slightly more stable than the saturated ring in Figure 1b. We again emphasize that the structures in Figure 1 are, at present, only hypothetical, but we believe that they are the most reasonable first guesses for our new materials because 1-alkenes, 1-alkynes, and 1-haloalkanes have been found to bind to unpassivated silicon in the manner shown in Figure 1. We also note the possibility that 1-alkenes and 1-alkynes may bind to silicon through two carbon-silicon bonds to form five- or six-membered rings and the likelihood that the real monolayers contain alkyl chains bonded in a variety of different ways. Furthermore, we recognize that while the data shown in this work can be viewed as consistent with the presence of ring structures on silicon scribed under 1-alkenes and 1-alkynes, they do not prove their existence. To our knowledge these are the first wet chemical preparations of monolayers on silicon that do not employ hydrogen-terminated silicon as a starting material.24-38 (14) Bozack, M. J.; Choyke, W. J.; Muehlhoff, L.; Yates, J. T., Jr. Surf. Sci. 1986, 176, 547-566. (15) Hamers, R. J.; Hovis, J. S.; Lee, S.; Liu, H.; Shan, J. J. Phys. Chem. B 1997, 101, 1489-1492. (16) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D. M.; Wolkow, R. A. J. Am. Chem. Soc. 2000, 122, 3548-3549. (17) Schwartz, M. P.; Ellison, M. D.; Coulter, S. K.; Hovis, J. S.; Hamers, R. J. J. Am. Chem. Soc. 2000, 122, 8529-8538. (18) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N., Jr. Acc. Chem. Res. 2000, 33, 617-624. (19) Gutleben, H.; Lucas, S. R.; Cheng, C. C.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1991, 257, 146-156. (20) Bronikowski, M. J.; Hamers, R. J. J. Vac. Sci. Technol., A 1995, 13, 777-781. (21) Keeling, L. A.; Chen, L.; Greenlief, C. M.; Mahajan, A.; Bonser, D. Chem. Phys. Lett. 1994, 217, 136-141. (22) Brown, K. A.; Ho, W. Surf. Sci. 1995, 338, 111-116. (23) Naruse, Y.; Ma, J.; Inagaki, S. Tetrahedron Lett. 2001, 42, 65536556. (24) Buriak, J. M. Chem. Commun. 1999, 1051-1060. (25) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudho¨lter, E. J. R. Adv. Mater. 2000, 12, 1457-1460. (26) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631-12632. (27) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (28) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. (29) Bateman, J. E.; Eagling, R. D.; Worrall, D. R.; Horrocks, B. R.; Houlton, A. Angew. Chem., Int. Ed. Engl. 1998, 37, 2683-2685. (30) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48-51. (31) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1999, 15, 8288-8291. (32) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831-3835. (33) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11491-11502. (34) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688-5695.

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Static time-of-flight secondary ion mass spectrometry (ToF-SIMS), secondary ion mass spectrometry (SIMS), and other ion beam techniques have been effectively used to study and characterize monolayers24,39-41 on silicon, oxidized silicon, and gold. Schenkel, Chidsey and coworkers42 performed ToF-SIMS on alkyl monolayers on silicon using highly charged ions as a primary ion source. Buriak and co-workers33 used depth-profiled SIMS to study porous silicon that was derivatized with alkyl monolayers. Chang and co-workers used static SIMS to study the adsorption of tert-butylacetylacetate (or tert-butylacetoacetate: CH3COCH2CO2C(CH3)3) on Si(100).43 Linford and co-workers have reported SiCxHy+ and CxHy+ type ions in ToF-SIMS spectra of monolayers of 1-alkenes and 1-alkynes on scribed silicon,1 and SiCl+, SiBr+, and SiI+ fragments in ToF-SIMS spectra of silicon that is scribed under 1-chloro-, 1-bromo-, and 1-iodoalkanes.2 SIMS has also been used to study the structure and formation of silane monolayers on SiO2,44 including the relationship between photoresist lift-off and surface coverage45 of trimethylsiloxy-terminated silicon oxide,46 surface patterning of 7-12 µm lines47 of trimethylsiloxy-terminated silicon oxide,48 the kinetics of surface bromomethyldimethylsilylation,49 the thermal stability and degradation mechanisms of trialkylsilane-modified silica powder,50 HPLC stationary phases,51,52 (3-aminopropyl)triethoxysilane-coated surfaces,53-56 and silane deposition on ultrathin silicon dioxide films.57 In contrast to many surface characterization techniques that are best used with or that require planar surfaces (35) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2000, 16, 10359-10368. (36) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allonge, P. Langmuir 2000, 16, 7429-7434. (37) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S. J. Am. Chem. Soc. 1996, 118, 7225-7226. (38) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.-Y.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056-1058. (39) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (40) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151-256. (41) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (42) Schenkel, T.; Schneider, M.; Hattass, M.; Newman, M. W.; Barnes, A. V.; Hamza, A. V.; Schneider, D. H.; Cicero, R. L.; Chidsey, C. E. D. J. Vac. Sci. Technol., B 1998, 16, 3298-3300. (43) Chang, C.-C.; Huang, I.-J.; Lung, C.-H.; Hwang, H.-Y.; Teng, L.-Y. J. Phys. Chem. B 2001, 105, 994-1002. (44) Spool, A. M. IBM J. Res. Dev. 1994, 38, 391-411. (45) Ponjee´, J. J.; Marriott, V. B.; Michielsen, M. C. B. A.; Touwslager, F. J.; Van Velzen, P. N. T.; van der Wel, H. J. Vac. Sci. Technol., B 1990, 8, 463-466. (46) Niehuis, E.; Van Velzen, P. N. T.; Lub, J.; Heller, T.; Benninghoven, A. Surf. Interface Anal. 1989, 14, 135-142. (47) Schwieters, J.; Cramer, H.-G.; Heller, T.; Ju¨rgens, U.; Niehuis, E.; Zehnpfenning, J.; Benninghoven, A. J. Vac. Sci. Technol., A 1991, 9, 2864-2871. (48) Van Velzen, P. N. T. Surface Characterization at (Sub)monomolecular Level with Time-of-Flight SIMS. In Proceedings SIMS VI; Benninghoven, A., Huber, A. M., Werner, H. W., Eds.; Wiley: Chichester, 1988; pp 1013-1020. (49) Van Velzen, P. N. T.; Ponjee´, J. J.; Benninghoven, A. Appl. Surf. Sci. 1989, 37, 147-159. (50) Severin, J. W.; van der Wel, H.; Camps, I. G. J.; Baken, J. M. E.; Vankan, J. M. J. Surf. Interface Anal. 1992, 19, 133-138. (51) Brown, V. A.; Barrett, D. A.; Shaw, P. N.; Davies, M. C.; Ritchie, H. J.; Ross, P.; Paul, A. J.; Watts, J. F. Surf. Interface Anal. 1994, 21, 263-273. (52) Simko, S. J.; Miller, M. L.; Linton, R. W. Anal. Chem. 1985, 57, 2448-2451. (53) Wang, D.; Jones, F. R.; Denison, P. J. Mater. Sci. 1992, 27, 3648. (54) Van Ooij, W. J.; Sabata, A. Characterization of films of organofunctional silanes by TOFSIMS and XPS. Part I. Films of N-[2(vinylbenzylamino)-ethyl]-3-aminopropyltrimethoxysilane on zinc and γ-aminopropyltriethyoxysilane on steel substrates. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; Koninklijke Wo¨hrmann B.V.: Utrecht, 1992; pp 323-343.

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Figure 2. The ratio of areas of the uncorrected C 1s XPS peak to the Si 2p XPS peak for 1-alkenes and 1-alkynes (squares: upper half black (1-alkynes), lower half black (1-alkenes)) and 1-haloalkanes (circles: upper half black (1-iodoalkanes), right side black (1-bromoalkanes), lower half black (1-chloroalkanes)).

(optical ellipsometry, X-ray reflectivity, atomic force microscopy (AFM) and wetting), SIMS effectively characterizes rough, scribed silicon. Here we report a ToFSIMS study of monolayers prepared by scribing silicon under a homologous series of 1-alkenes, 1-alkynes, and 1-haloalkanes: CH2dCH(CH2)nCH3 (n ) 2, 5, 9), HCt C(CH2)nCH3 (n ) 2, 5, 9), Cl(CH2)nCH3 (n ) 4, 7, 9), Br(CH2)nCH3 (n ) 4, 7, 11), I(CH2)nCH3 (n ) 0, 1, 2, 4, 7, 11), and I13CH3. In the spectra of our surfaces we observe numerous SiCxHy+ and CxHy+ fragment ions and adduct ions (combinations of the molecular adsorbate with one or more atoms from the substrate).58 Similar ions have previously been observed with SIMS studies of ultrathin organic films: Brown and co-workers51 saw [Si(CH3)2CxH2x+1]+ (x ) 1, 2, 3, 4, 8, 12, 18), C2H3+, C3H5+, and C4H7+ in spectra of dimethylalkylsilyl chains on silica, and Linton and co-workers52 identified CH3+, C3H5+, C4H5+, C4H7+, (CH3)2SiH+, C6H13+, and C7H15+ in octadecyldimethylchlorosilane-derivatized silicas. In a static SIMS study of monolayers of thiols on silver, Leggett and co-workers59 noted that in the positive ion spectra of octanethiol, dodecanethiol, and octadecanethiol “the low mass region (m/z < 100) is dominated by hydrocarbon fragments, presumably by fragmentation of the alkyl chain.” Wrighton and co-workers similarly observed CxHy+ ions with x ) 2-6 for monolayers of thiols on gold.60 With regard to the SiCxHy+ and CxHy+ fragments from our surfaces, we have observed the following: (1) Above a certain alkyl chain length of the scribing liquid, ion intensities by ToF-SIMS correlate with coverages measured by X-ray photoelectron spectroscopy (see Figures 2-6), i.e., relative fragment intensity scales as the amount of carbon on the surface, as would be expected if monolayers are produced by scribing silicon under reactive compounds.1,2 (2) Anomalously strong SiCH3+ and SiC2H5+ (55) Wang, D.; Jones, F. R.; Denison, P. TOF SIMS and XPS study of the interaction of hydrolyzed γ-aminopropyltriethoxysilane with E-glass surfaces. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; Koninklijke Wo¨hrmann B.V.: Utrecht, 1992; pp 345-364. (56) Eldridge, B. N.; Buchwalter, L. P.; Chess, C. A.; Goldberg, M. J.; Goldblatt, R. D.; Novak, F. P. A time-of-flight static secondary ion mass spectrometry and X-ray photoelectron spectroscopy study of 3-aminopropyltrihydroxysilane on water plasma treated chromium and silicon surfaces. In Silanes and Other Coupling Agents; Mittal, K. L., Ed.; Koninklijke Wo¨hrmann B.V.: Utrecht, 1992; pp 305-321. (57) Knotter, D. M. Appl. Surf. Sci. 1996, 99, 99-110. (58) Karolewski, M. Secondary Ion Mass Spectrometry Tutorial. http://kickme.to/kalypso Version 1.0, 2-3. 2000. Ref Type: Electronic Citation. (59) Hutt, D. A.; Cooper, E.; Leggett, G. J. J. Phys. Chem. B 1998, 102, 174-184. (60) Frisbie, C. D.; Martin, J. R.; Duff, R. R., Jr.; Wrighton, M. S. J. Am. Chem. Soc. 1992, 114, 7142-7145.

Figure 3. Relative intensities of signals corresponding to SiCH3+ as a function of the number of carbon atoms in (a) 1-alkene and 1-alkyne scribing liquids and (b) 1-haloalkane scribing liquids.

Figure 4. Relative intensities of signals corresponding to SiC2H5+ as a function of the number of carbon atoms in (a) 1-alkene and 1-alkyne scribing liquids and (b) 1-haloalkane scribing liquids.

fragments from silicon scribed under CH3I and CH3CH2I suggest formation of methyl- and ethyl-terminated silicon, respectively. (3) Silicon scribed under 1-haloalkanes has less carbon, as shown by XPS, than silicon scribed under 1-alkenes and 1-alkynes with the same number of carbon atoms, but it shows more intense SiCxHy+ fragments in ToF-SIMS spectra (this supports the binding model that has been proposed earlier that 1-haloalkanes are only bonded through one C-Si bond but that 1-alkenes and 1-alkynes are bonded to two and sometimes one carbon atom, although another possible explanation for these results is that SiCxHy+ cations are more easily produced from monolayers with less densely packed alkyl chains).1,2 (4) An isotopic study of silicon scribed under 13CH3I and CH3I provides additional evidence for formation of methylterminated silicon and suggests sputter-induced decomposition of the near-surface region by ToF-SIMS. Finally, we present ab initio calculations, with different levels of sophistication, of a few SiCxHy+ type fragments to verify

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particles,64 (d) electronic desorption of hydrogen from hydrogen-terminated silicon with an STM tip and its reaction with reactive molecules,30,65 and (e) surface patterning of lipid bilayers to form corrals on glass with lines scratched on glass at basic pH.66 Experimental Section

Figure 5. Relative intensities of signals corresponding to SiC3H7+ as a function of the number of carbon atoms in (a) 1-alkene and 1-alkyne scribing liquids and (b) 1-haloalkane scribing liquids.

Figure 6. Relative intensities of signals corresponding to SiC4H9+ as a function of the number of carbon atoms in (a) 1-alkene and 1-alkyne scribing liquids and (b) 1-haloalkane scribing liquids.

that our assignments of structure are appropriate and to better understand the appropriate level of theory for modeling these ions, similar to the calculations by Fa¨ngstro¨m et al., who also studied Si-containing radical cations.61 Finally, we note that this work is closely related to earlier examples of chemical activation of materials through mechanical means. These include (a) removal of regions of alkanethiol monolayers on gold with micromachining techniques, i.e., a scalpel or carbon fiber, and exposure of the bare gold to a solution of a new thiol,62 (b) removal (or nanoshaving) of thiols from gold with an AFM tip in a thiol-containing solution,63 (c) grinding silicon in the presence of 1-alkenes to create functionalized silicon (61) Fa¨ngstro¨m, T.; Lunell, S.; Engels, B.; Eriksson, L. A.; Shiotani, M.; Komaguchi, K. J. Chem. Phys. 1997, 107, 297-306. (62) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380-1382. (63) Xu, S.; Liu, G. Langmuir 1997, 13, 127-129.

General Comments. Sample preparation and cleaning have been previously reported.1,2 CH3I was obtained from Fisher (99.8%) and 13CH3I (99 atom % 13C), 1-chlorodecane (98%), 1-bromododecane (97%), and 1-iodododecane (98%) were obtained from Aldrich. All chemicals were used as received. Unless otherwise specified, in SiCxHy+ and CxHy+ fragments, Si, C, and H denote 28Si, 12C, and 1H, respectively, and x and y are integers. Peak areas were measured with instrument software and normalized to the areas of the 28Si+ and 28Si- peaks for the positive and negative scans, respectively, which were consistently some of the largest features in the spectra. (In some of the plots shown in the Supporting Information, peak heights were used instead of peak areas. However, while the standard deviations in the data were lower if peak areas were used, plots obtained from peak heights showed the same trends as those from peak areas.) Static Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Static ToF-SIMS (Cameca/ION-TOF TOF-SIMS IV) was performed with a monoisotopic 25 keV 69Ga+ primary ion source in “bunched mode” to achieve a mass resolution of ∼10000 (m/∆m). The primary ion (target) current was typically 3 pA, with a pulse width of 20 ns before bunching, and the raster area of the beam was 500 × 500 µm2. For the experiments that compared Si scribed under CH3I to Si scribed under 13CH3I, data were obtained with a Physical Electronics PHI TRIFT II time-of-flight secondary ion mass spectrometer, using a 69Ga liquid metal ion gun (LMIG) primary ion source. The instrument was operated in an ion microprobe mode in which the bunched, pulsed primary ion beam was rastered across the sample’s surface. The analytical area was 60 × 60 µm2, the acquisition time was 240 s per spectrum, and no charge compensation was employed. The primary ion beam potential was 12 kV for positive ions and 18 kV for negative ions. The primary ion current (dc) was 2 nA, no masses were blanked, and the energy filter and contrast diaphragm were both used to obtain enhanced mass resolution. The mass resolution achievable using these conditions was 6500 (m/∆m at m/z 41) in positive ion mode and 5600 (m/∆m at m/z 60) in negative ion mode. These mass resolutions were somewhat below the optimal mass resolution typically obtained on a polished wafer surface because of surface roughness. The resolution was still sufficient to distinguish SiO2 from 13CH3SiO in negative ion mode. X-ray Photoelectron Spectroscopy (XPS). XPS of 1-chlorodecane-, 1-bromododecane-, and 1-iodododecane-modified silicon was performed with an SSX-100 X-ray photoelectron spectrometer with a monochromatic Al KR source and a hemispherical analyzer. The analytical chamber had a typical base pressure during data acquisition of