Adsorption of C60 Buckminster Fullerenes on an 11-Amino-1

Dec 18, 2007 - Buckminster fullerene C60 was used as a model to understand the attachment chemistry of large ... The chemical state and topography of ...
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Langmuir 2008, 24, 810-820

Adsorption of C60 Buckminster Fullerenes on an 11-Amino-1-undecene-Covered Si(111) Substrate Xiaochun Zhang and Andrew V. Teplyakov* Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed August 26, 2007. In Final Form: October 17, 2007 Buckminster fullerene C60 was used as a model to understand the attachment chemistry of large molecules on amine-terminated self-assembled monolayers (SAM) on semiconductor substrates. This type of interface serves as a prototype for future devices in such fields as solar energy conversion, biosensing, catalysis, and molecular electronics. Fullerene C60 was attached to 11-amino-1-undecene self-assembled monolayers on a Si(111) surface. The chemical state and topography of the C60-modified surface were characterized by surface analytical spectroscopic and microscopic methods and by computational investigation. X-ray photoelectron spectroscopy revealed that the secondary amine group is formed between the C60 and the 11-amino-1-undecene SAM on the surface. The appearance of the π-π* C 1s shakeup peak confirmed the presence of C60 on the surface. Infrared spectroscopic studies verified several characteristic features of the C60 skeleton vibration and the 11-amino-1-undecene vibrational signature. The atomic force microscopy investigation suggested that the fullerene molecules produce surface features with an apparent height of ∼2 nm and an average width of ∼20 nm. A parallel study was performed on a Au(111) surface for comparison with the results obtained on the silicon substrate. The reaction between fullerene molecules and ∼1% 11-amino-1undecene diluted in decene SAM on the Si(111) surface accordingly yielded dilute and uniformly distributed C60 molecules on the surface, which indicated that the amine groups were the reactive sites.

1. Introduction Over the last 10 years, there has been an array of advances in designing novel interfaces for future devices needed for solar energy conversion, biosensing, catalysis, nanostructure fabrication, and microelectronics. An increasing number of studies on coupling various highly functional molecules to solid substrates have been explored in recent years because of the important fundamental role of the interfaces created in all of the abovementioned systems. Interestingly, it has long been recognized that the immobilization of large molecules on solid substrates lays the groundwork for most practical applications. However, the important role of the substrates and, even more importantly, the specific interaction between the molecules and the substrates have attracted significantly less attention. Traditionally, most of the work that involves attaching large, polyfunctional molecules to solid supports was performed on glass1-6 or gold surfaces,7-17 which are very amenable to detailed investigations. Only recently * Corresponding author. E-mail: [email protected]. Tel: (302) 8311969. Fax: (302) 831-6335. (1) Waddell, T. G.; Leyden, D. E.; DeBello, M. T. J. Am. Chem. Soc. 1981, 103, 5303-5307. (2) Chrisey, L. A.; Lee, G. U.; O’Ferrall, C. E. Nucleic Acids Res. 1996, 24, 3031-3039. (3) Guschin, D.; Yershov, G.; Zaslavsky, A.; Gemmell, A.; Shick, V.; Proundnikov, D.; Arenkov, P.; Mirzabekov, A. Anal. Biochem. 1997, 250, 203211. (4) Lee, P. H.; Sawan, S. P.; Modrusan, A.; Arnold, L. J.; Jr.; Reynolds, M. A. Bioconjugate Chem. 2002, 13, 97-103. (5) Lipshutz, R. J.; Fodor, S. P. A.; Gingeras, T. R.; Lockhart, D. J. Nat. Genet. 1999, 21, 20-24 (supplement). (6) Jung, G. Y.; Stephanopoulos, G. Science 2004, 304, 428-431. (7) Hashimoto, K.; Ito, K.; Ishimori, Y. Anal. Chem. 1994, 66, 3830-3833. (8) Frutos, A. G.; Brockman, J. M.; Corn, R. M. Langmuir 2000, 16, 21912197. (9) Lee, H. J.; Goodrich, T. T.; Corn, R. M. Anal. Chem. 2001, 73, 55255531. (10) Lu, M.; Hall, J. G.; Shortreed, M. R.; Wang, L.; Berggren, W. T.; Stevens, P. W.; Keiso, D. M.; Lyamichev, V.; Neri, B.; Skinner, J. L.; Smith, L. M. J. Am. Chem. Soc. 2002, 124, 7924-7931. (11) Boon, E. M.; Ceres, D. M.; Drummond, T. G.; Hill, M. G.; Barton, J. K. Nat. Biotechnol. 2000, 18, 1096-1100. (12) Georgiadis, R.; Peterlinz, K. P.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166-3173.

has it been realized that in the majority of practical applications the interactions of this type of molecule with semiconductors will likely play a very important role because existing knowledge of the electronic properties of semiconductor materials allows for the controlled modification and manipulation of potential interfaces. Understanding the chemistry and characteristics of these interface systems will be the key for the future realization of many nanoscale applications. Despite the obvious need to understand the interaction of large molecules with semiconductor substrates, only a limited number of investigations have truly addressed these interactions on the molecular scale. Because silicon single-crystalline substrates are at the heart of the modern semiconductor industry, the possibility of designing devices starting with this substrate seems to be the most practical, as the technology and machinery needed to make such a jump is closely related to the existing processing. Thus, for the purpose of this discussion, we will mainly focus on ways to interface large polyfunctional molecules with silicon single crystals. Hamers and Smith have investigated a potential method for attaching specific oligonucleotides to silicon surfaces.18-20 Cha and co-workers have studied the immobilization of oligonucleotides on brush-coated Si surfaces.21 Diamond surfaces as (13) Zhou, D.; Bruckbauer, A.; Ying, L.; Abell, C.; Klenerman, D. Nano Lett. 2003, 3, 1517-1520. (14) Liu, D.; Bruckbauer, A.; Abell, C.; Balasubramanian, S.; Kang, D.-J.; Klenerman, D.; Zhou, D. J. Am. Chem. Soc. 2006, 128, 2067-2071. (15) Frutos, A. G.; Liu, Q.; Thiel, A. J.; Sanner, A. M. W.; Condon, A. E.; Smith, L. M.; Corn, R. M. Nucleic Acids Res. 1997, 25, 4748-4757. (16) Frutos, A. G.; Smith, L. M.; Corn, R. M. J. Am. Chem. Soc. 1998, 120, 10277-10282. (17) Liu, Q.; Wang, L.; Frutos, A. G.; Condon, A. E.; Cron, R. M.; Smith, L. M. Nature 2000, 402, 175-179. (18) Strother, T.; Hamers, R. J.; Smith, L. M. Nucleic Acids Res. 2000, 28, 3535-3541. (19) Strother, T.; Cai, W.; Zhao, X.; Hamers, R. J.; Smith, L. M. J. Am. Chem. Soc. 2000, 122, 1205-1209. (20) Zhang, L.; Strother, T.; Cai, W.; Cao, X.; Smith, L. M.; Hamers, R. J. Langmuir 2002, 18, 788-796. (21) Cha, T.-W.; Boiadjiev, V.; Lozano, J.; Yang, H.; Zhu, X.-Y. Anal. Biochem. 2002, 311, 27-32.

10.1021/la702631g CCC: $40.75 © 2008 American Chemical Society Published on Web 12/18/2007

Adsorption of C60 on a Si(111) Substrate

substrates for immobilizing various biomolecules have been studied as well.22-24 The biggest challenge of these studies was to control the stability and functionality of the surface while modifying the interfaces. One of the prototypical model systems that can be used to understand the attachment of macromolecules to semiconductor surfaces is fullerenes. For many years, buckyballs have attracted substantial attention from the scientific and engineering communities because of their unique structure and electronic properties and their great thermal and chemical stability. One of the most important motivations for using the C60 molecule as the model in our work is that it is easy to detect and characterize using both microscopic and spectroscopic techniques. The abundance of studies on attaching fullerene C60 to different substrates and nanostructures focused on applying different techniques,25 including spin coating,26 solution evaporation,27,28 thermal evaporation,29,30 the Langmuir-Blodgett technique,31,32 interfacial hydrogen bonding,33 electrochemical deposition,34,35 and selfassembled monolayers (SAMs)36-43 to reproducibly build welldefined, stable interface. One of the most successful approaches to forming highly ordered, well-defined structures in a reproducible manner is the use of self-assembled monolayers composed of functionalized molecules as a substrate for attaching fullerenes. Compared to other venues, the functionalized self-assembled monolayers form a more-ordered, better-defined interface, which is a legitimate platform for further modification.37-44 Gold substrates and alkane-thiol SAMs are often a preferred chemical system used as C60 support materials, and the aggregation of C60 on such substrates has been explored.42 The C60/SAM system has been studied with a variety of different microscopic methods, including transmission electron microscopy (TEM),39 atomic force microscopy (AFM),43 scanning tunneling microscopy (STM),42 and water contact angle measurements38 or spectroscopic surface (22) Rao, T. N.; Yagi, I.; Miwa, T.; Tryk, D. A.; Fujishima, A. Anal. Chem. 1999, 71, 2506-2511. (23) Mathieu, H. J. Surf. Interface Anal. 2001, 32, 3-9. (24) Ushizawa, K.; Sato, Y.; Mitsumori, T.; Machinami, T.; Ueda, T.; Ando, T. Chem. Phys. Lett. 2002, 351, 105-108. (25) Bonifazi, D.; Enger, O.; Diederich, F. Chem. Soc. ReV. 2006, 36, 390414. (26) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.; Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128, 6328-6329. (27) Chlistunoff, J.; Cliffel, D.; Bard, A. J. Thin Solid Films 1995, 257, 166184. (28) Jehoulet, C.; Bard, A. J.; Wudl, F. J. Am. Chem. Soc. 1991, 113, 54565457. (29) Moriarty, P.; Upward, M. D.; Ma, Y.-R.; Dunn, A. W.; Beton, P. H.; Teehan, D.; Woolf, D. A. Surf. Sci. 1998, 405, 21-26. (30) Yoshida, Y.; Tanigaki, N.; Yase, K. Thin Solid Films 1996, 281-282, 80-83. (31) Mirkin, C. A.; Caldwell, W. B. Tetrahedron 1996, 52, 5113-5130. (32) Wang, S.; Leblanc, R. M.; Arias, F.; Echegoyen, L. Langmuir 1997, 13, 1672-1676. (33) Arias, F.; Godinez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086-6087. (34) Koh, W.; Dubois, D.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1993, 97, 6871-6879. (35) Balch, A. L.; Costa, D. A.; Winkler, K. J. Am. Chem. Soc. 1998, 120, 9614-9620. (36) Cecchet, F.; Rapino, S.; Margotti, M.; Da, Ros, T.; Prato, M.; Paolucci, F.; Rudolf, P. Carbon 2006, 44, 3014-3021. (37) Hoang, V. T.; Rogers, L. M.; D’Souza, F. Electrochem. Commun. 2002, 4, 50-53. (38) Song, F.; Zhang, S.; Bonifazi, D.; Enger, O.; Diederich, F.; Echegoyen, L. Langmuir 2005, 21, 9246-9250. (39) Fujihara, H.; Nakai, H. Langmuir 2001, 17, 6393-6395. (40) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. 1999, 38, 2403-2405. (41) Sahoo, R. R.; Patnaik, A. J. Colloid Interface Sci. 2003, 268, 43-49. (42) Patnaik, A.; Okudaira, K. K.; Kera, S.; Setoyama, H.; Mase, K.; Ueno, N. J. Chem. Phys 2005, 122, 154703. (43) Tsukruk, V. V.; Everson, M. P.; Lander, L. M.; Brittain, W. J. Langmuir 1996, 12, 3905-3911. (44) Yoshimoto, S.; Tsutsumi, E.; Narita, R.; Murata, Y.; Murata, M.; Fujiwara, K.; Komatsu, K.; Ito, O.; Itaya, K. J. Am. Chem. Soc. 2007, 129, 4366-4376.

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analytical techniques such as cyclic voltammetry (CV),37-39 attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR),41 X-ray photoelectron spectroscopy (XPS),41 and near-edge X-ray absorption fine structure measurements (NEXAFS).42 However, only a very limited number of studies attempted to combine microscopic and spectroscopic methods. In the paper by Tsukruk et al., C60 was chemically adsorbed onto an azide-terminated alkylsilane SAM on a silicon substrate.43 In that work, fullerene molecules were covalently anchored onto a silicon substrate through the SAM and the properties of the interface such as surface morphology and friction forces were examined. We focus on a different chemistry to attach fullerene molecules to silicon substrates using amino-terminated SAMs. It has been established that the fullerene molecules react readily with the amine group41,42 and thus can be used as a reliable probe of surface reactivity. At the same time, amine-terminated SAMs can also be used in a variety of practical surface-modification schemes. In the present study, the buckminster fullerenes C60 were used as models for understanding the attachment of macromolecules to semiconductor substrates. We combined two chemistriess the reaction between an alkene and a hydrogen-terminated Si surface to form a SAM45,46 and the reactivity of an amine group with fullerene molecules. Previous studies by Sahoo and Patnaik et al.41,42,47 set the benchmark of the chemical reaction between fullerene C60 molecules and SAM with an amine functional group. The microscopic and spectroscopic characteristics reported in these papers are used as a reference in our work. Fullerenes (C60) were attached to an 11-amino-1-undecene SAM on a Si(111) surface. The chemical state and topography of the C60-modified surface were characterized by surface analytical spectroscopic and microscopic methods. The surface chemistry was characterized by XPS and FTIR, whereas the surface topography was examined by AFM. Parallel experiments were carried out on the Au surface, and the results were compared with those on the Si surface. Selected properties of C60 attached to an amine-octanethiol SAM on a Au surface model were investigated previously by computational methods. For example, the typical lowest unoccupied molecular orbitals of this model were calculated using density functional theory (DFT).41,42,47 In our work, we predict a number of experimental spectroscopic characteristics of C60 attached to 11-amine-1-undecene SAM on Si(111) investigated by DFT. With the optimized model structure, the vibrational frequencies and the core-level energy were investigated. As shown below, the predicted observables are consistent with the experimental results. 2. Experimental Section 2.1. Materials. All chemicals were reagent grade or better and used as received: 11-chloro-1-undecene (Aldrich, 97%), trifluoroacetic acid (TFA) (Aldrich, 99%), potassium phthalimide (Fluka, g99.0%), hydrazine monohydrate (Fluka, g98.0%), di-tert-butyl dicarbonate (Sigma-Aldrich, 99%), buckminsterfullerene C60 (Fluka, g98%), cysteamine hydrochloride (Aldrich, g98%), 1-decene (Acros, 95%), dimethyl formamide (DMF) (Fisher, 99.8%), methylene chloride (Fisher, 99.9%), petroleum ether (Fisher, Certified ACS), ethyl ether (Fisher, Laboratory Grade), methanol (Fisher, 99.9%), ethanol (Pharmco, 99.5%), ammonium hydroxide (Fisher, 14.8 N), and toluene (Fisher, 99.9%). The silicon substrates were (45) Sieval, A. B.; Linke, R.; Meijer, G.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 2001, 17, 7554-7559. (46) Sieval, A. B.; Demirel, A. L.; Nissink, W. M.; Linford, M. R.; van der Maas, J. H.; de Jen, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759-1768. (47) Sahoo, R. R.; Patnaik, A. Appl. Surf. Sci. 2004, 245, 26-38.

812 Langmuir, Vol. 24, No. 3, 2008 double-polished Si(111) wafers (5-10 Ω cm, 440-450 µm) (Micro Fabrication). Commercial Au(111) substrates deposited on mica were obtained from Molecular Imaging. The deionized water (18 mΩ resistance) used to rinse the surfaces came from a first-generation Milli-Q water system (Millipore). 2.2. Synthesis of t-Butyloxycarbonyl (t-BOC)-Protected 11Amino-1-undecene. t-BOC-protected 11-amino-1-undecene was prepared by standard organic synthesis methods.18,45 Potassium phthalimide (12.8 g) was added to a solution of 10.0 g of 1-chloro10-undecene in 25 mL of dry DMF. The resulting suspension was stirred at 90 °C for 24 h. The obtained reaction mixture was cooled to room temperature, and 75 mL of water was added. The aqueous layer was extracted once with 75 mL and subsequently twice with 30 mL of ether. The combined organic layers were washed with 25 mL of a 0.2 M NaOH solution and with 25 mL of brine and dried over anhydrous MgSO4. Evaporation of the solvent yielded the crude product as a yellow solid. Recrystallization from 50 mL of distilled methanol after standing at 18-20 °C in a closed flask for 18 h yielded N-(ω-undecylenyl)-phthalimide as white needles. Hydrazine (2.5 g) was added to a solution of 10.0 g of N-(ω-undecylenyl)phthalimide in 100 mL of ethanol. The resulting mixture was heated to reflux for 3 h. The solution was then cooled to room temperature and acidified to pH 1 to 2 with 1 M HCl. The white suspension was filtered through glass fibers, and the residue was washed twice with 20 mL of 1 M HCl. The combined filtrates were made alkaline (pH 10 to 11) by the addition of NaOH (tablets) and concentrated to a volume of 100 mL by evaporation under reduced pressure. The resulting turbid aqueous layer was extracted four times with 50 mL of ether. The combined organic layers were washed once with 20 mL of a 0.2 M NaOH solution and once with 20 mL of brine, to which a few milliliters of the 0.2 M NaOH solution has been added. The organic layer was dried over NaOH (solid) for 1 to 2 h. Evaporation of the solvent yielded crude 1-amino-10-undecene as a yellow oil. About 5 g of 1-amino-10-undecene was dissolved in 60 mL of chloroform that was added to a solution of 3 g of NaHCO3 in 50 mL of water. Sodium chloride (6.45 g) was added along with 7.18 g of di-tert-butyl dicarbonate dissolved in a few milliliters of chloroform. This mixture was refluxed for 90 min and extracted twice with 50 mL of ether. The collected organic extracts were dried over magnesium sulfate and filtered, and the ether was removed by rotary evaporation. The t-BOC-protected product was purified by vacuum distillation. NMR confirmed the identity of each step’s product: N-(ω-undecylenyl)-phthalimide, 1-amino-10-undecene, and t-butyloxycarbonyl (t-BOC)-protected 11-amino-1-undecene. 2.3. Monolayer Preparation. The preparation steps are shown in Scheme 1. A 5 mL quantity of a solution of the t-BOC-protected 1-amino-10-undecene was placed into a 25 mL flask fitted with a reflux condenser and kept under flowing N2. The solution was deoxygenated with dry N2 for at least 1 h. The hydrogen-terminated Si(111) surface was prepared using the etching procedure described in detail in the Supporting Information section. The flask was immersed in an oil bath, and the solution was refluxed for 2 h while maintaining a slow N2 flow. The sample was then removed from the solution and cleaned in petroleum ether (40-60 °C), methanol, and dichloromethane. Afterward, treating the surface with 25% TFA in methylene chloride for 1 h was followed by a 5 min rinse in 10% NH4OH to remove the t-BOC protecting group and to form the primary amine-terminated surface. The surface was then rinsed with water and dried with N2. 2.4. Preparation of the Fullerene-Modified Surface. The C60modified surface was prepared using a similar method to that used previously on a Au surface.41,42 As outlined in Scheme 1, the freshly prepared amine-terminated Si surface was placed in a 1 mM (or 0.01 mM) toluene solution of C60 and allowed to reflux for 2 days under N2 flow. Then the surface was rinsed with toluene for 5 min to remove physisorbed C60 and dried with N2 before characterization. For comparison, similar chemistry was carried out on the Au surface as shown in Scheme 2. The Au surface was annealed at 400 °C for 2 h at 10-6 Torr pressure. Then the Au surface was soaked in a 1 mM toluene solution of cysteamine for 24 h at room temperature. After that, the surface was rinsed with toluene, ethanol, and water

Zhang and TeplyakoV Scheme 1. Preparation Steps of the Amino Undecene SAM and Fullerene Attachment

Scheme 2. Preparation Steps of Fullerene-Attached Cysteamine on the Au Surface

and dried with N2. The cysteamine-covered Au surface was then immediately placed in a 1 mM (or 0.01 mM) toluene solution of C60 for 2 days under reflux and N2 flow conditions. Then the surface was rinsed with toluene and dried with N2. 2.5. Characterization. 2.5.1. Atomic Force Microscopy. Tapping mode AFM imaging was performed on a Multimode SPM (Vecco) with a NanoScope IIIA controller (CA) in air at room temperature. BS-Tap 300Al tips (budget sensors) with a force constant of 40 N/m and a resonance frequency of 300 kHz were used. The AFM images were collected at 256 pixels × 256 pixels per image and were analyzed using the Nanoscope software (version 6) with first-order flattening. 2.5.2. X-ray Photoelectron Spectroscopy. XPS measurements were carried out on a VG ESCALAB 220i-XL electron spectrometer (VG Scientific Ltd., U.K.) with monochromatic Al KR X-rays (hν ) 1486.7 eV) at 10-10 Torr. The operating conditions for the X-ray

Adsorption of C60 on a Si(111) Substrate source were a 400 µm nominal X-ray spot size (fwhm) with operation at 15 kV, 8.9 mA, and 124 W for both survey and high-resolution spectra. A 2-µm-thick aluminum window was used to isolate the X-ray chamber from the sample analysis chamber to prevent highenergy electrons from impinging on the sample. Survey spectra were collected from 0 to 1200 eV in binding energy, with a pass energy of 100 eV, a dwell time of 100 ms per data point, and an energy resolution of ∼1.0 eV. High-resolution spectra were collected at a pass energy of 20 eV, an energy resolution of ∼0.1 eV, a dwell time of 100 ms/data point, and 20-70 scans depending on the various elements. The C 1s peak, centered at 284.6 eV, did not require charge shifting and therefore was used to calibrate the spectra. The data analysis was performed using Casa software. 2.5.3. FTIR. Single transmission Fourier transform infrared measurements were performed on a Nicolet Magna-IR 560 spectrometer utilizing a liquid-nitrogen-cooled external MCT-A detector. The sample was positioned at a 60° angle with respect to the incoming infrared beam. FTIR spectra of the samples were collected in the range of 4000-650 cm-1 with 256 scans per spectrum at 8 cm-1 resolution. The native-oxide-covered Si(111) wafer was used as the background. 2.6. Computational Details. Computational investigation was used to verify several experimental results in this system. Density functional calculations were performed using the B3LYP method,48,49 as implemented in the Gaussian 03 suite of programs.50 The Si(111) surface was modeled by a Si10H15 cluster that represents one silicon surface atom, three subsurface silicon atoms, three third-layer silicon atoms, and three forth-layer silicon atoms. Other than the surface silicon atom, all silicon atoms were terminated by hydrogen to maintain their hybridization. The cluster structure was fully relaxed. The long-chain molecule and attached C60 were not geometrically constrained. Energy optimization and vibrational frequency calculations were both performed using the LANL2DZ basis set. The calculated vibrational spectra were scaled by a factor of 0.944877 for comparison with the experimental results. This scaling factor was obtained by a comparison of the predicted and experimental frequencies of the major vibrational species, for instance, C-H stretching and scissoring and C-N stretching, for the amineterminated SAM surface. This scaling factor is consistent with the value that was used by our group for different molecules using the B3LYP/LANL2DZ approach.51-54 The final predicted spectra, shown as solid bars in Figures 8 and 9, are presented with only the calculated frequencies whose intensities are more than 1% of the highestintensity absorption band. This simplification allows us to focus on the main features in the predicted spectra. The only exception to this approach is the expected vibrational signature of the asymmetric and symmetric NH2 stretching modes. They are shown in the Figures despite the fact that their predicted intensities are low. N 1s core-level energies in the models are predicted using Koopmans’ theorem, and the binding energy of an electron is assumed (48) Becke, A. D. J. Chem. Phys 1993, 98, 5648. (49) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785. (50) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (51) Pirolli, L.; Teplyakov, A. V. Surf. Sci. 2006, 600, 3313-3320. (52) Pirolli, L.; Teplyakov, A. V. J. Am. Chem. Soc. 2006, 110, 4708-4716. (53) Bocharov, S.; Dmitrenko, O.; Me´ndez De Leo, L. P.; Teplyakov, A. V. J. Am. Chem. Soc. 2006, 128, 9300-9301. (54) Rodrı´guez-Reyes, J. C. F.; Teplyakov, A. V. J. Phys. Chem. C 2007, 111, 4800-4808.

Langmuir, Vol. 24, No. 3, 2008 813 to be equal to its orbital energy. In this work, the N 1s core-level energy shift was predicted and compared with the XPS experimental results. The absolute values of binding energies were calibrated by comparing experimental XPS results for p-nitroaniline (p-NH2C6H4-NO2)55 with the computational results for the same molecule at the B3LYP/LANL2DZ level of theory. The reason for choosing p-nitroaniline was the fact that the two N atoms in this molecule have binding energies that cover a wide range that can effectively be used as a “ruler” for comparison with models containing N atoms. As a result, the added correction factor to the predicted core-level energies was found to be 8.5 eV. All of the predicted core-level energies from calculations in this article were corrected by this factor and then were compared to the experimental results.

3. Results and Discussion 3.1. AFM. Figure 1 shows the AFM images of Si (A-D) and Au (E-H) after different surface-modification chemistry steps. The bottom part of the Figure gives the corresponding line profiles. At the starting point, both hydrogen-terminated Si(111) (image A) and clean Au(111) (image E) show a roughness (rms) of substantially less than 0.5 nm. After the formation of the 11amino-1-undecene SAM, the Si surface became slightly rougher, with a roughness (rms) of ∼0.5 nm (image B). Similar behavior is observed for the cysteamine SAM-modified Au surface (image F). A small difference between the two might arise as a result of the much shorter carbon chain of cysteamine (2 carbons) as compared to 11-amino-1-undecene (11 carbons). It has been reported previously that the longer-chain hydrocarbons form more-ordered SAMs on the Si substrates.46,56 Some effect may also be caused by oxidation processes and other chemistries affecting unprotected amino groups under ambient conditions. After being modified with C60 molecules, the surface topography changed significantly. As shown in image C, the Si substrate is covered with clusters with a width of up to 30 nm and a height of up to 10 nm. Modified with the same concentration of C60, the Au surface also exhibits C60 clusters observed by AFM (image G). The width of the clusters remains ∼30 nm. The approximate height of the clusters is ∼5 nm, which is slightly lower than on silicon. Notice that compared to the Si surface there are some smaller clusters observed on the Au surface. To see the effect of fullerene concentration on the size of the clusters on these surfaces, a less concentrated C60 reagent was used. In Figure 1, images D and H indicate that the average cluster sizes decreased when a 0.01 mM C60 solution was used. The clearly observed clusters on both surfaces have a height of ∼2 nm and a width of ∼20 nm. This seems to suggest that the C60 molecules still tend to aggregate while chemically attaching to the surface. To further understand the role of the reactive sites on the surface, we set up an experiment to probe the reactivity of amine head groups among unreactive methyl head groups on the surface. As a diluent, we chose decene molecule with a double bond and a methyl head group, which should be nonreactive with respect to fullerenes. The reaction between the double bond in the longchain alkene molecules and the hydrogen-terminated Si(111) surface plays a key role in the surface chemistry in this case. As shown in Scheme 3, t-BOC 11-amino-1-undecene was diluted in decene with a ratio of 1:99. The mixture of alkenes then reacted with the hydrogen-terminated Si(111) surface and formed a mixed monolayer. The amine groups were then deprotected on the surface. This surface was then reacted with an ∼0.01 mM fullerene toluene solution according to the same procedure as (55) Agren, H.; Roos, B. O.; Bagus, P. S.; Gelius, U.; Malmquist, P. A.; Svensson, S.; Maripuu, R.; Siegbahn, K. J. Chem. Phys. 1982, 77, 3893-3901. (56) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

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Figure 1. Top AFM images: (A) H-terminated Si(111); (B) amino-undecene SAM on Si(111); (C) ∼1 mM C60 on SAM-modified Si(111); (D) ∼0.01 mM C60 on SAM-modified Si(111); (E) Au(111); (F) cysteamine SAM on Au(111); (G) ∼1 mM C60 on SAM-modified Au(111); and (H) ∼0.01 mM C60 on SAM-modified Au(111). Bottom: Corresponding section analysis of the images.

used before. Figure 2 shows the AFM images of this surface. Image (a) revealed that the surface was covered by evenly distributed clusters of fullerene molecules. Compared to the result of the fullerene solution of the same concentration reacting with the 11-amino-1-undecene SAM, the concentration of features in image (a) decreased accordingly, and the distribution was more uniform. Image (b) shows the scan of the square area in image (a). The sizes of the individual features on the surface were similar to each other, with an apparent height of ∼2 nm and a width of ∼15 nm. The 11-amino-1-undecene molecules functioned as probes on the surface regardless of the concentration of the fullerene molecules that are available in the reaction. These results suggest that the amine groups are indeed the reactive sites. The control experiment was performed to confirm the absence of covalent binding between a SAM prepared from decene and the fullerene solution of the same concentration as on a mixed SAM on Si(111). There were no fullerene molecules attached to the surface according to the AFM results as shown in Figure 3. It was previously reported by Zeng et al.57 that alkanethiol chains annealed on the Au(111) surface formed 1D facial layers that enabled the fullerene molecules subsequently dosed onto this substrate to bind to the exposed S-containing sites. However, the configuration and packing of decene SAMs on Si(111) in our work makes this type of interaction between fullerenes and the underlying substrate impossible, and the vibrational studies presented below confirm this assessment. By themselves, these results do not prove the presence of individual C60 molecules on the surface. The size of the features observed (on average 2 nm × 15 nm) is substantially larger than

the size of an individual molecule. Previous STM studies implied that the size of individual C60 molecules adsorbed on Si substrates is expected to be less than 1 nm.58,59 Patnaik et al. used STM investigation to report that the C60 molecules attached to the 11-amino-1-undecene thiol SAM on the Au substrate aggregated into clusters with an apparent width of ∼5 nm and a height of ∼0.4 nm.42 In addition, compared to that of STM, the AFM’s lower lateral resolution in the studies presented here limited our ability to quantify the actual width of the observed features. We believe that the size of the features observed in our work is consistent with the results in previous investigations. It is also important to mention that it is very difficult to obtain atomicresolution AFM images of molecules that are smaller than 50 nm at room temperature, especially if they are adsorbed on top of a “soft” layer for reasons such as thermal motion, the radius of the tips (typically 20-10 nm), and the tip-broadening effect during scanning. The mechanism of C60 aggregation on the aminoterminated monolayers on the Si(111) surface is not completely understood; however, plausible explanations of fullerene aggregation have been explored.60 3.2. XPS. The chemical characteristics of the 11-amino-1undecene SAM on the Si(111) surface and the C60-modified SAM were investigated by high-resolution XPS. C 1s and N 1s (57) Zeng, C.; Wang, B.; Li, B.; Wang, H.; Hou, J. G. Appl. Phys. Lett. 2001, 79, 1685-1687. (58) Chen, D.; Sarid, D. Surf. Sci. 1995, 329, 206-218. (59) Yao, X.; Ruskell, T. G.; Workman, R. K.; Sarid, D.; Chen, D. Surf. Sci. 1996, 366, L743-L749. (60) Alargova, R. G.; Deguchi, S.; Tsujii, K. J. Am. Chem. Soc. 2001, 123, 10460-10467.

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Scheme 3. Preparation Steps of Fullerene Reacted with Diluted Amine Sites on the Si(111) Surface

XPS spectra are presented in Figure 4. In the C 1s XPS spectrum of the 11-amino-1-undecene SAM (a), the peak at 286.6 eV is assigned to the C-N bond in the 11-amino-1-undecene SAM,20,41 and the C-O bond could originate from the residual t-BOC 11-amino-1-undecene molecules or from partial oxidation.18,20,41 The peak at 288.8 eV is due to oxidized C species in the form of CdO on the surface.18,20 Compared to spectrum (a), the intensity of the C 1s peak at 284.6 eV in spectrum (c) increased by 87% because of the C60 attachment to the surface. In spectrum (c), the additional C 1s shakeup peak at 290.1 eV is attributed to the π-π* electron transition, indicating that the C60 molecules are attached to the surface.41 Comparing the N 1s spectra of the 11-amino-1-undecene SAM (b) and the C60 attached surfaces (d), the total N 1s peak intensity decreased by 20%, which might be due to oxidation and damage occurring during the C60 reaction with the surface or, more likely, to the screening effect of the buckyball molecules. In spectrum (b), the main N 1s XPS peak is at 400.0 eV, which is assigned to primary amine group -NH2 in the 11-amino-1-undecene SAM.18,20 In spectrum (d), the N 1s XPS has two major components. The peak at 400.2 eV is attributed by primary amine group -NH2. The peak at 397.9 eV is assigned to secondary amino group -NH-, which binds directly to the C60 buckyball.41 The peak area of -NH2 is about twice the peak area corresponding to -NH-. It can be roughly estimated that less than about one-third of the primary amine groups react with C60 on the surface. However, the exact quantification is hindered by ex situ experiments in our case and is also limited by the possible screening effect of the C60 buckyball molecules. Parallel experiments were performed on the Au surface as described in the Experimental Section. Figure 5 shows the C 1s and N 1s high-resolution XPS spectra of the cysteamine SAM on the Au surface (a, b) and the C60-modified Au surface (c, d). Similar to the chemical composition of the Si surface, the C 1s XPS of cysteamine SAM has three components. The peak at

286.1 eV is assigned to the C-N bond in the cysteamine monolayer, and the peak at 287.2 eV is due to the oxidized carbon in the system. Compared to spectrum (a), the C 1s 284.6 eV peak intensity increased 39% in spectrum (c), which suggests the attachment of the C60 molecules to the Au surface. In spectrum (c), the carbon π-π* shakeup peak was observed at 290.4 eV, which was very close to the shakeup peak of C60 on the Si surface (290.1 eV). The N 1s XPS spectra of the cysteamine-covered Au surface (b) have one peak at 399.5 eV, which is assigned to the primary amine on the surface. However, N 1s of the C60-treated surface has two components. The peak at 399.7 eV is assigned to the primary amine group (-NH2) left on the surface, and the peak at 397.3 eV corresponds to the secondary amine (-NH-) that reacts with the C60 buckyball. However, the portion of the secondary amine in the mixture of species is only 10%, which could be due to the differences between the Si and Au surfaces and the different characteristics of monolayers formed. The 11amino-1-undecene molecules that reacted with the H-Si(111) surface have 11 carbons in each molecule, but the cysteamine molecules that react with the Au(111) surface consist of only 2 carbons. Because of the short length of the cysteamine molecules, it is expected that the density of the cysteamine SAM would be lower and the order of the cysteamine SAM would be worse than that of the 11-amino-1-undecene SAM on the Si(111) surface. All of these factors account for the different surface chemical environments for Si and Au, which caused the difference in the N 1s XPS results. 3.3. FTIR. The characteristic features on the surface and the order of the SAM were studied by FTIR. It has been reported previously that the vibrational frequencies of the CH2 stretching region of long alkyl chain molecules are very sensitive to the

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Figure 2. AFM images: (a) Fullerene molecules absorbed on Si(111) covered by 1% amino undecene in decene SAM. (b) Enlarged scan of the square area in part (a). Below is the corresponding section analysis of the images.

Figure 3. AFM images: control reaction between fullerene molecules and the decene SAM on Si(111). On the right is the section analysis of the image.

conformational order of the SAM on the surface.46,56,61 The asymmetrical CH2 stretching frequency νa and the symmetrical CH2 stretching frequency νs would both shift to lower wave(61) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150.

numbers when the degree of the order of the thin film increased. More specifically, for a well-ordered system (crystalline state), νa(CH2) is expected to be observed between 2920 and 2918 cm-1, and νs(CH2) is expected to be observed between 2851 and 2850 cm-1. For a disordered system (liquid state), νa(CH2) is

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Figure 4. High-resolution XPS of the Si surfaces: (a) C 1s for the amino-undecene SAM on Si(111); (b) N 1s for the amino-undecene SAM on Si(111); (c) C 1s for the C60-modified SAM on Si(111); and (d) N 1s for the C60-modified SAM on Si(111).

Figure 5. High-resolution XPS of the Au surfaces: (a) C 1s for the cysteamine SAM on Au(111); (b) N 1s for the cysteamine SAM on Au(111); (c) C 1s for the C60-modified SAM on Au(111); and (d) N 1s for the C60-modified SAM on Au(111).

expected to be between 2928 and 2925 cm-1, and νs(CH2) is expected to be between 2856 and 2855 cm-1.45,46,62 Figure 6 (62) Perring, M.; Dutta, S.; Arafat, S.; Mitchell, M.; Kenis, P. J. A.; Bowden, N. B. Langmuir 2005, 21, 10537-10544.

shows the FTIR spectrum of the SAM of 11-amino-1-undecene on Si(111) surface. The two C-H stretching modes, asymmetrical and symmetrical at 2921 and 2851 cm-1, respectively, reveal that the 11-amino-1-undecene SAM formed in our studies is

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Zhang and TeplyakoV Scheme 4. Optimized Structure of the Models Based on the Si10H15 Cluster Representing the Si(111) Surfacea

Figure 6. Infrared spectrum of the SAM of 11-amino-1-undecene on the Si(111) surface. The inset is the C-H stretching region.

a (a) 11-Amino-1-undecene SAM and (b) Fullerene attached to this SAM on Si(111) calculated at the B3LYP/LANL2DZ level of theory.

Figure 7. Infrared spectrum of the fullerene C60 attached to the SAM of 11-amino-1-undecene on the Si(111) surface. The inset is the C-H stretching region.

highly ordered on the Si(111) surface. The broad band around 1700 cm-1 is due to physically adsorbed water,63 whose intensity decreased after being left in the IR purging system for 12 h. The region between 1620 and 1580 cm-1 is assigned to -NH2 amine deformation modes.64,65 The peak at 1450 cm-1 is attributed to the C-H scissoring vibration from the methylene group that is attached to the amino group.41 The peak around 1379 cm-1 is likely from the C-N stretching mode,65 and the band from 1345 to 1260 cm-1 is assigned to the CH2 twisting and wagging modes.65 The broad band around 3500-3000 cm-1 corresponds to the -NH2 stretch. All these assignments are confirmed below by computational investigations. Figure 7 presents the IR spectrum of the C60 fullerene attached to the SAM on the Si(111) surface. The presence of two C-H stretching modes, asymmetrical and symmetrical at 2920 and 2850 cm-1, respectively, revealed that the SAM is still highly ordered on the Si(111) substrate after the reaction with fullerene molecules. The band from 3500 to 3220 cm-1 is attributed to the N-H stretching modes.65 The broad peak at approximately 1700 cm-1 is assigned to physically adsorbed water.63 The region from 1577 to 1490 cm-1 is likely due to amine deformation modes.64 The peak at 1459 cm-1 is assigned to C-H scissoring (63) Kulkarni, S. A.; Mirji, S. A.; Mandale, A. B.; Vijayamohanan, K. P. Thin Solid Films 2006, 496, 420-425. (64) Hooper, A. E.; Werho, D.; Hopson, T.; Palmer, O. Surf. Interface Anal. 2001, 2001, 809-814. (65) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569.

vibrations, and the peak at 1395 cm-1 is from C-N stretching.65 The peaks from 1370 to 1250 cm-1 are due to CH2 twisting and wagging modes.65 The covalently attached fullerene molecules showed several vibrational features at 1428, 1394, and 1180 cm-1 due to the C60 skeleton ring vibrations.41,66,67 We also investigated the process of monolayer formation on the hydrogen-terminated Si(111) surface. The Si(111)-H bond, with a vibrational frequency of 2083.7 cm-1, was studied before and after 11-amino-1-undecene SAM formation. It has been previously established that approximately half of the silicon hydride species react when a monolayer is formed on the H-Si(111) surface.68-70 Theoretically, about a half of the silicon monohydride species initially covering the substrate should remain on the surface after the formation of the monolayers. In our case, it was observed that the intensity of the νSi-H signal was reduced by approximately half after the SAM modification, which is consistent with the theoretical prediction. However, this result should not be used as a reliable quantitative reference because in several previously published investigations essentially the entire Si-H stretch absorption signature has been observed to disappear after the SAM has formed.62,71 The detailed results are included in the Supporting Information section of this article. 3.4. Computational Results. A density functional theory (DFT) computational investigation was used to verify several experimental results. The B3LYP/LANL2DZ level of theory was used to optimize the cluster models, calculate the vibrational frequencies, and predict the XPS core-level shift. The models used in these computational calculations are presented in Scheme 4 and represent a single 11-amino-1-undecene molecule attached to a Si10H15 cluster representing the Si(111) substrate and a single (66) Fabian, J. Phys. ReV. B 1996, 53, 13864-13870. (67) Bowmart, P.; Kurmoo, M.; Green, M. A.; Pratt, F. L.; Hayes, W.; Day, P.; Kikuchi, K. J. Phys.: Condens. Matter 1993, 5, 2739-2748. (68) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145-3155. (69) Zhang, L.; Wesley, K.; Jiang, S. Langmuir 2001, 17, 6275-6281. (70) Yuan, S.; Zhang, Y.; Li, Y.; Xu, G. Colloids Surf., A 2004, 242, 129-135. (71) Miramond, C.; Vuillaume, D. J. Appl. Phys. 2004, 96, 1529-1536.

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Figure 8. Comparison of the experimental infrared spectrum recorded for the 11-amino-1-undecene SAM on the Si(111) surface (solid lines) and the computational results (bars) for model (a) in Scheme 4.

Figure 9. Comparison of the experimental infrared spectrum of C60 attached to the 11-amino-1-undecene SAM on the Si(111) surface (solid lines) and the computational results (bars) for model (b) in Scheme 4. Longer bars are used to identify the C60 skeletal vibrations.

fullerene C60 molecule attached via an amine linker. Despite this simplification, these models can yield valuable information, especially on N 1s core-level energies and vibrational frequencies. The atomic coordinates, the calculated vibrational frequencies, and the predicted core-level energies of N atoms in both models are included in the Supporting Information section of this article. Figure 8 compares the experimental IR spectrum (solid line) of the representative spectral regions with the predicted infrared absorption spectrum for model (a) in Scheme 4. A correction factor of 0.944877 was applied to the predicted frequencies to account for systematic errors. The calculated frequency spectrum confirms the assignments offered earlier. The peak at approximately 1700 cm-1 in the experimental results due to physically adsorbed water does not correlate with any calculated absorption features because water is not considered in the model system. The experimental IR results of C60 attached to the 11-amino1-undecene SAM on the Si(111) surface (solid line) and the computationally predicted spectrum for model (b) are compared in Figure 9. Here, the calculated vibrational frequencies originating from the C60 entity in the model are labeled with longer bars in Figure 9 for easier identification. Similarly to the analysis presented above for model (a), the same correction factor was used for the predicted spectrum. The physically adsorbed water peak around 1700 cm-1 does not correlate with any predicted absorption bands because the computational model does not include water molecules. Notice that the stretching mode observed experimentally in the 3200-3500 cm-1 spectral region correlates only with the predicted N-H stretching vibration at 3291 cm-1. This difference may arise from several effects, including the

presence of water and possible hydrogen bonding or other factors affecting the structure of the SAM or the systematic errors unaccounted for by the scaling factor that was calculated for a SAM without fullerenes. In addition to the prediction of vibrational spectra, the predicted core-level energy shifts were also compared with the experimental XPS spectra. The N 1s core-level energies of models (a) and (b) in Scheme 4 were calibrated for the comparison with the N 1s binding energies as described in section 2.6. As summarized in Figure 10, the experimental binding energy of primary amine group -NH2 of 11-amino-1-undecene on Si(111) is observed at 400.2 eV, and secondary amine group -NH- formed during the C60 attachment is observed at 397.2 eV. Thus, the XPS feature corresponding to the primary amine is higher in energy than that corresponding to the secondary amine by 2.0 eV. The theoretical prediction for these features is 398.73 eV for the primary amine group in model (a) of Scheme 4 (solid bar in Figure 10) and 397.53 eV for the secondary amine in model (b) (dashed line in Figure 10). The difference follows the same trend as the experimental results: the primary amine has a higher binding energy than the secondary amine. The calculated difference is 1.2 eV, which is consistent with the experiment.

4. Conclusions In this work, fullerene C60 molecules were covalently attached to an amine-terminated SAM on a Si(111) substrate. The analytical spectroscopic and microscopic methods and computational investigation were combined so that we could understand the chemistry of this attachment, and parallels between this chemistry

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Figure 10. Comparison of the N 1s XPS spectrum of C60 attached to the 11-amino-1-undecene SAM on Si(111) and the N atom corelevel energy in the -NH2 group predicted for model (a) (vertical solid bar) and the -NH- group in model (b) (vertical dashed bar). The exact positions of the predicted core-level energies are calibrated as described in section 2.6.

and similar reactions on a Au substrate covered with a SAM formed by thiol chemistry were drawn. The AFM images revealed the change in the topography during each step of the attachment. The average size of the features on the C60-modified surface suggests that the aggregation of fullerenes is possible either in the reactant solution or on the surface. The Si(111) covered with a SAM prepared by the dilution of 11-amino-1-undecene with decene reacts with C60 exclusively via amino groups. An AFM investigation suggests that the density of the features corresponding to fullerenes on the surface of this diluted SAM decreased substantially and that the feature size indicates the presence of small fullerene clusters or possibly single fullerene molecules. These findings verified the important role of the amine group as a reactive site on the surface. The N 1s XPS suggested

Zhang and TeplyakoV

the formation of a secondary amine group in the reaction of an amine-terminated SAM with a C60 buckyball, whereas C 1s XPS verified the presence of the π-π* C 1s shakeup peak after the attachment. The characteristic IR frequencies confirmed the properties of the surfaces before and after their reactions with the buckyball. The two C-H stretching modes, asymmetrical at 2921-2920 cm-1 and symmetrical at 2851-2850 cm-1, revealed that the alkyl chains in the SAMs are highly ordered before and after this modification. In addition, a computational investigation was used to confirm the experimental results. Despite the size limitations of the model cluster systems used, the predicted vibrational frequencies are in agreement with the experimental IR spectrum. The N 1s core-level energies of the models successfully predicted the trend of the binding-energy difference between primary amine before the C60 attachment and secondary amine formation during this reaction. Acknowledgment. This work was partially supported by the National Science Foundation (CHE-0415979). We acknowledge the Surface Analysis Facility (University of Delaware) and the research group of Professor Thomas P. Beebe, Jr. for their help with XPS measurements. We also acknowledge the Bio-imaging Center (Delaware Biotechnology Institute) and Dr. Liz Adam for AFM support. GridChem, Computational Chemistry Grid (CGG) (www.gridchem.org) is acknowledged for computational resources and services. We thank Mr. Patrick McMahon (Department of Chemistry and Biochemistry, University of Delaware) for his help with vibrational frequency calculations for one of the computational models. Supporting Information Available: Detailed procedure for the preparation of the hydrogen-terminated Si(111) surface, computational models, atomic coordinates, predicted vibrational frequencies, and corelevel energies. This material is available free of charge via the Internet at http://pubs.acs.org. LA702631G