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Self-Assembly and Characterization of Fullerene Monolayers on Si(100) Surfaces Wenju Feng and Barry Miller* Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078 Received August 6, 1998. In Final Form: December 14, 1998 Self-assembly of C60 molecules on planar n- or p-type Si(100) has been accomplished by direct tethering onto the Si surface without using any intermediate functional hydrocarbon chain. Photoresponse has been observed on these monolayer modified electrodes in both aqueous and nonaqueous media. The p-type Si(100) surface with a bound C60 monolayer is also capable of mediating redox reactions. These C60 monolayer modified Si electrodes are very stable in both acidic aqueous and polar nonaqueous solvents. Characterization by fast atom bombardment mass spectroscopy (FAB-MS) suggests that simultaneous hydrosilylation and hydrogenation reactions account for the nature of the monolayer formation.
Introduction Covalently bonded organic monolayers on solid substrates are of interest for controllable studies of electron transfer and, generally, for potential applications in organic circuits and devices.1-4 Since fullerene thin films have exhibited interesting electrical and optical properties and C60 has displayed rich chemistry,5-10 self-assembled monolayers (SAM) of fullerene have been tethered on substrates such as Au, In-Sn oxide (ITO), and SiO2/Si by using thiol or silane hydrocarbon chains.11-15 As charge transfer rate decays exponentially with distance, a shorter chain is desirable to expedite electron exchange. In fact, it has been shown that hydrocarbons 10 carbon atoms in length are virtually too long to transfer electrons through the interface effectively.16-19 However, if monolayers have (1) Dulcey, C. S.; Georger, J. H., Jr.; Krauthamer, V.; Stenger, D. A.; Fare, T. L.; Calvert, J. M. Science 1991, 252, 551. (2) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Chem. Mater. 1995, 7, 526. (3) Kumar, A.; Abbott, N. L.; Kim, E.; Biebuyck, H. A.; Whitesides, G. M. Acc. Chem. Res. 1995, 28, 219. (4) Dressick, W.; Calvert, J. M. Jpn. J. Appl. Phys. 1993, 32, 5829. (5) Electronic Properties of Fullerenes; Kuzmany, H., Fink, J., Mehring, M., Roth, S., Eds.; Springer-Verlag: Heidelberg, Germany, 1993. (6) Haddon, R. C.; Hebard, A. F.; Rosseinsky, M. J.; Murphy, D. W.; Duclos, S. J.; Lyons, K. B.; Miller, B.; Rosamilia, J. M.; Fleming, R. M.; Kortan, A. R.; Glarum, S. H.; Makhija, A. V.; Muller, A. J.; Eick, R. H.; Zahurak, S. M.; Tycko, R.; Dabbagh, G.; Thiel, F. A. Nature 1991, 350, 320. (7) Miller, B.; Rosamilia, J. M. J. Chem. Soc., Faraday Trans. 1993, 89, 273. (8) Jehoulet, C.; Obeng, Y. S.; Kim, Y. T.; Zhou, F.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 4237. (9) Miller, B.; Rosamilia, J. M.; Dabbagh, G.; Tycko, R.; Haddon, R. C.; Muller, A. J.; Wilson, W.; Murphy, D. W.; Hebard, A. F. J. Am. Chem. Soc. 1991, 113, 6291. (10) The Chemistry of Fullerene. Advanced Series in Fullerene; Talor, R., Ed.; World Scientific: Singapore, 1995; Vol. 4. (11) Shi, X.; Caldwell, W. B.; Chen, K.; Mirkin, C. A. J. Am. Chem. Soc. 1994, 116, 11598. (12) Chen, K.; Caldwell, W. B.; Mirkin, C. A. J. Am. Chem. Soc. 1993, 115, 1193. (13) Chupa, J. A.; Xu, S.; Fischetti, R. F.; Strongin, R. M.; McCauley, J. P., Jr.; Smith, A. B., III; Blasie, J. K. J. Am. Chem. Soc. 1993, 115, 4383. (14) Tsukruk, V. V.; Lander, L. M.; Brittain, W. J. Langmuir 1994, 10, 996. (15) Caldwell, W. B.; Chen, K.; Mirkin, C. A.; Babinec, S. J. Langmuir 1993, 9, 1945. (16) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (17) Finklea, H. O.; Avery, S.; Lynch, M.; Furtsch, T. Langmuir 1987, 3, 409.
less than eight carbons, the hydrocarbon chains cannot stand straight above the surface but are tilted or otherwise contorted.20 With such surfaces, further functionalization is difficult and good quality films cannot be obtained. We report here a method to self-assemble the fullerene molecule on Si surfaces by the shortest possible bond and its favorable consequences. Generally, to modify a Si surface with two-dimensional ordered monolayer structures, an organosilane compound is necessary to premodify the Si surface, so that further derivatization can be carried out. Recently, direct functionalization of Si surface by utilizing H-terminated Si surfaces has been exploited.21-24 This direct functionalization ought to facilitate the charge transfer between the substrate and the attaching functional group as the distance between them is shortened compared to the indirect way of using organosilanes. We and others have found that thermally driven chemical reactions occurred between C60 monolayer molecules and the H-terminated Si(100) substrate surfaces.24-26 We are able to covalently bond fullerene molecules directly on H-terminated Si(100) surfaces based on homogeneous hydrosilylation. A homogeneous hydrosilylation reaction under heat is shown in eq 1.
R3Si-H + CH2dCHR′ 9 8 R3Si-CH2-CH2R′ (1) ∆ Chidsey and co-workers21 have previously published reactions which amount to heterogeneous hydrosilylation; i.e., an alkene reacts with H-Si surface through an addition reaction. Our work exploits a similar target where the “alkene” is a double bond on the C60 cage, as shown in Scheme 1. The surface Si atoms on Si(100) are dihydride (18) Chidsey, C. E. D. Science 1991, 251, 919. (19) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (20) Ohtake, T.; Mino, N.; Ogawa, K. Langmuir 1992, 8, 2081. (21) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (22) Cleland, G.; Horrocks, B. R.; Houlton, A. J. Chem. Soc., Faraday Trans. 1995, 91, 4001. (23) Sailor, M. J.; Lee, E. J. Adv. Mater. 1997, 9, 783. (24) Feng, W.; Miller, B. Recent Advances in the Chemistry and Physics of Fullerenes and Related Materials; Kadish, K. M., Ruoff, R. S., Eds.; The Electrochemical Society, Inc.: Pennington, NJ; PV97-42, 1997; Vol. 5, pp 33-43. (25) Feng, W.; Miller, B. Electrochem. Solid State Lett. 1998, 1 (4), 172. (26) Schmidt, J.; Hunt, M. R. C.; Miao, P.; Palmer, R. E. Phys. Rev. B, 1997, 56, 9918.
10.1021/la980999s CCC: $18.00 © 1999 American Chemical Society Published on Web 03/31/1999
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Scheme 1. Sketch of the Possible Reaction between the CdC Bond on C60 and the Si-H Bond on H-terminated Si Surface where Filled Circles Represent the Surface Si Atoms and the Open Circles Represent the Bulk Si Atoms
or trihydride after NH4F etching.27,28 Since trihydride hydrogen atoms are observed to desorb at ∼77-277 °C by Yates et al.,29 it is possible that the trihydrides are active when the C60 molecule approaches the surface. For simplicity, in Scheme 1, only trihydrides are sketched. Presumably, one of the 6-6 ring carbon-carbon double bonds will open and an addition reaction will take place between the carbon-carbon double bond and one of the Si-H bond on the H-terminated Si surface. Here we report the syntheses of the covalently bonded C60 monolayers on Si(100) and their interesting redox and photoelectrochemical behavior. Experimental Section Materials. The C60 used was kindly donated by J. R. Degenfelder. The purity of C60 was checked by mass spectroscopy and chromatography and found to be 99.9%. Other chemicals are reagent grade or as follows: o-dichlorobenzene (H2O < 0.005%); benzene (99.9%); tetra n-butylammonium fluoroborate (Bu4NBF4) (99%); toluene (99.9%); acetone, acetonitrile, hexane (99.9%). Acetonitrile was distilled from CaH2 when used as electrolytic solvent for cyclic voltammetry. Czochralski silicon (100) wafers of n-type (P doped), 3.4-4.6 Ω cm, and p-type (B doped), 8 Ω cm, were used as substrates. Preparation of C60 Monolayer on Si (100). The Si(100) wafers were cleaned in a series of organic solvents ultrasonically (acetone, toluene, acetone again, and ethanol, each for 20 s) and then rinsed thoroughly with deionized water. They were then exposed to a 1:1 H2SO4/30% H2O2 mixture for 15 min, etched in 40% NH4F for 5∼10 min, and finally rinsed with acetonitrile or hexane. The Si surface thus treated is known to be H-terminated.27,28 We also checked the H-termination by galvanostatic potentialtime transients.25 The above pretreated Si wafer was reacted with C60 to form the C60 monolayer on Si surface following either solvent casting or vapor deposition of C60. Solvent Casting. Immediately after NH4F etching and rinsing, 10 µL of 1.0 × 10-4 to 0.01 M C60/benzene or C60/ODCB solution was typically cast onto ∼1 cm2 of a Si(100) substrate. This amount of C60 on the Si(100) substrate corresponds to about 5-500 monolayers of coverage based on the full monolayer density of C60, 1.9 × 10-10 mol/cm2.12,30 The cast Si is then heated in an oven under Ar flowing at 180 °C to 210 °C for 1.5 h, followed by rinsing in benzene to remove unbound fullerene. Vapor Deposition. Right after NH4F etching and rinsing, the Si substrate is put in a quartz tube in an oven with Ar flowing through the quartz tube. The Si substrate in the quartz tube is positioned where the temperature is 190-230 °C, downstream of sublimed C60, provided from a C60 source located in the quartz (27) Chabal, Y. J.; Higashi, G. S.; Raghavachari, K. Burrows, V. A. J. Vac. Sci. Technol. A 1989, 7, 2104. (28) Bjorkman, C. H.; Fukuda, M.; Yamazaki, T.; Miyazaki, S.; Hirose, M. Jpn. J. Appl. Phys. 1995, 34, 722. (29) Cheng, C. C.; Yates, J. T., Jr. Phys. Rev. B 1991, 43, 4041. (30) A calculated value assuming a hexagonal tight packing model for C60 with molecular diameter 10 Å (van der Waals diameter).
tube and held at 360 °C to 400 °C. After 1 h deposition under an Ar flow, the samples were rinsed in benzene to remove unbound fullerene. Electrochemical Experiment. A three-compartment cell was used when the electrolyte is based on an organic solvent, and a one-compartment cell was used when the electrolyte is aqueous. Illumination was either at 632.8 nm from a He-Ne laser or broadband from a halogen lamp. A light expander was used to expand the laser beam over the entire electrode surface. The intensity of light before entering the cell was read from a Metrologic Radiometer and was not corrected for losses due to the cell glass window and the thin layer electrolyte (∼2 mm) between the cell window and the front surface of the electrode. Ar bubbling of the cell electrolyte was maintained. A saturated calomel electrode was used as reference and looped Pt wire as counter electrode. A Model 175 universal programmer driving the Model 173 potentiostat, or the Pine RDE3 potentiostat were used for cyclic voltammetry. Potential in all cases shown here was scanned from positive of C60 reduction to negative, then return. FAB-MS. The mass spectra were measured by a modified VG AutoSpec tandem mass spectrometer,31 a trisector E1BE2 instrument. The electrode was attached to the probe and introduced into the ion source. The operating pressure in the source is about 10-6 mbar. The species on the electrode surface were bombarded and ionized by a Cs+ ion gun operated at 20 kV. The ions exited from the source were accelerated by 8000 V. Only those ions with 8000 V kinetic energy can pass through E1, and the m/z of ions was measured by scanning the magnetic field of the magnetic sector. Each spectrum is the average of about 20 scans.
Results and Discussion Cyclic Voltammetry. The cyclic voltammetry of C60 monolayer modified n-Si(100) is different from that of C60 in solution on a bare Si(100) electrode. In Figure 1, we see three reversible reduction-reoxidation waves of solution C60 on a Pt electrode and three, less reversible appearing, reduction-reoxidation waves on bare n-Si(100). For n-Si(100) modified with a C60 monolayer, however, the first redox step appears to be both more reversible and shifted positive in potential compared to the solution C60 on n-Si. The origin of this may lie in fast electron transfer between the Si substrate and the attached C60 molecules and/or a thermodynamically more favorable reaction for C60 tethered directly on the Si surface. However, the nature of the difference in cyclic voltammetry between the bonded C60 monolayer and solution C60 is not clear at this time and needs further investigation. The electrochemistry of the bonded C60 monolayer on n-Si shows the expected n-type semiconductor behavior, i.e., the oxidation of the reduced C60 in the light takes (31) Polce, M. J.; Cordero, M. M.; Wesdemiotis, C.; Bott, P. A. Int. J. Mass. Spectrum. 1992, 113, 35.
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Figure 3. C60 monolayer/n-Si (100) in 1 M HClO4. He-Ne Laser, 632.8 nm at 2.51 mW/cm2. Scan rates (mV/s): (‚‚‚) 100; (---) 200; (s) 500. In the inset, Ipa is the anodic peak current. Area of the electrode: 0.84 cm2.
Figure 1. Comparison between C60 monolayer and C60 in solution. All CVs in 0.5 M Bu4NBF4/ODCB. Key: (a) Pt in 1 mM C60 solution, 100 mV/s; (b) bare n-Si(100) in 1.5 mM C60 solution, 100 mV/s; (c) C60 monolayer on n-Si(100), 1 V/s. In parts b and c, a He-Ne laser was used: 632.8 nm; 2.0 mW/cm2 illumination.
Figure 2. C60 monolayer/n-Si(100) in 1 M HClO4: (---) in the dark; (-) under He-Ne laser, 632.8 nm. Scan rate, 200 mV/s. Light intensities: 1, 1.0; 2, 1.3; 3, 1.7; and 4, 2.5 mW/cm2.
place at more negative potential than in the dark, Figure 2. The photovoltages observed in 1 M HClO4 and in 0.5 M Bu4NBF4/ACN at light intensity 2.5 mW/cm2 of He-Ne light at 632.8 nm are 0.12 V and 0.22 V, respectively. The CVs change little with different light intensity in this range indicating no photogenerated carrier limitations under these experimental conditions. The photocurrent is proportional to scan rate (Figure 3 and inset), which is consistent with the behavior of surface confined species. For C60 monolayer modified p-Si(100), we see a photocathodic current which occurs at more positive potentials than in the dark, Figure 4. Similar to the behavior for n-Si(100), these currents are proportional to scan rate and are therefore due to surface-confined species. The C60 moieties on Si surfaces are very stable. No decrease in current density was observed upon continuous cycling (for instance, several tens of cycles) of the above CVs and upon many days of exposure to1M HClO4 solution. Rapid decays (several cycles) in current density were observed in physically deposited C60 films on Pt electrodes by Bard et al.8 or on unmodified Si from our own test
Figure 4. C60 monolayer/p-Si(100) in 1M HClO4. Light source: halogen lamp, 0.70 mW/cm2. Key: (1) (---) in the dark and (s) under light, 150 mV/s; (2) current vs scan rate under light. Aera of the electrode: 0.91 cm2.
experiments. The stability of the C60-modified electrodes indicates that there is chemical bonding between the C60 molecules and the substrate Si. It is very likely that a chemical reaction as depicted in Scheme 1 occurred in the monolayer preparation process, and the bound C60 protects the underlying Si against oxidation. The chemical bonding character is also reflected in the FAB-MS results (vide infra). On the basis of the lattice constant of Si (100), a ) 5.42 Å, and the van der Waals diameter of C60, 10 Å, we estimated a full monolayer coverage of C60 on Si(100) as 1.4 × 10-10 mol/cm2 when the C60 molecules are bonded to Si atoms.32 This is smaller than 1.9 × 10-10 mol/cm2, the monolayer coverage for “close packed” C60 molecules on any substrate in which there is no chemical interaction necessary to the C60 molecules.12,29 For our C60 monolayer modified n-Si(100) and p-Si(100) electrodes, the coverages of C60 on the surfaces reach about 10% of the estimated one full monolayer coverage (1.4 × 10-10 mol/cm2), based
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Figure 5. Mediation of solution Fe(CN)63- by C60 monolayer/ p-Si(100). He-Ne laser, 632.8 nm at 2.5 mW/cm2. Scan rate: 500 mV/s. Key: (s) 1 M HClO4 ; (---), 2 mM K3Fe(CN)6/1 M HClO4; (‚‚‚) 5 mM K3Fe(CN)6/1 M HClO4. Electrode area: 0.91 cm2.
on the integration of the experimental cyclic voltammograms. Mediation Reactions of Solution Species by C60 Monolayer on Si (100). Redox reactions of solution species can be mediated through surface-confined species. Specifically, as shown by Wrighton et at.,33,34 a surfaceconfined film of ferrocene was first oxidized to ferrocenium by photogenerated holes from n-Si, then Fe(CN)64- (the solution species) was subsequently oxidized by ferrocenium. We observed similar mediation phenomena from C60 monolayer modified p-Si(100), Figure 5. In 1 M HClO4, the C60 monolayer showed rather symmetric reduction and oxidation peaks, while with addition of Fe(CN)63- to the solution, the cathodic current increased and the oxidation current was attenuated. This results when C60 is reduced by a photogenerated electron to C60-, C60reduces Fe(CN)63- to Fe(CN)64- and then is itself oxidized to C60, leaving less C60- to be oxidized at positive scan. A similar mediated reaction was also observed in Fe(ClO4)3 solution. These solution species are not reduced at the oxide-passivated, non-C60-covered, silicon surface. Such mediation reactions are interesting in that they not only further confirm that we have C60 films on the electrode, but, more importantly, that such C60 monolayer modified Si(100) surfaces may have catalytic applications MS Chacterization and Possible Monolayer Formation Mechanism. Fast atom bombardment mass spectrometry (FAB-MS) has been used to characterize the C60 film on modified Si(100) surfaces. To check the position of C60, we evaporated C60 onto an oxidized Si(100) surface (3:1 H2SO4 /30% H2O2 at 100 °C for 1 h). The highest FAB-MS peak is 720 ( 2 au in the spectrum, which is due to intact C60, Figure 6a. Figure 6b is the spectrum of the C60 monolayer modified H-terminated n-Si(100) in which the highest peak is ∼733 au, corresponding to C60Hx (the x is 13 here). A similar spectrum is observed for C60 monolayer modified H-terminated p-Si(100), Figure 6c (in this case, the x is 17). The number of hydrogen atoms on the C60 ball, x, is actually temperature dependent. The (32) In our case, the estimation of the monolayer coverage of C60 on Si(100) is based on the belief of chemical bonding between C60 molecules and Si atoms. Each C60 molecule (10 Å in diameter) is bonded to a Si atom centered in an area of (2 × 5.42)2 ) 117.5 Å2 in which there are 12 more Si atoms. The closest C60 neighbor occupies the next 117.5 Å2 area. Because of the chemical bonding situation, the packing of the C60 molecules are loose compared to that of a “closed packed” situation, as seen in refs 12 and 29. (33) Bolts, J. M.; Bocarsly, A. B.; Palazzotto, M. C.; Walton, E. G.; Lewis, N. S.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101, 1378. (34) Lewis, N. S.; Bocarsly, A. B.; Wrighton, M. S. J. Phys. Chem. 1980, 84, 2033.
Figure 6. FAB-MS spectra of C60 films on Si(100): (a) C60 film on SiO2/n-Si(100); (b) C60 monolayer/n-Si(100); (c) C60 monolayer/p-Si(100).
higher the deposition temperature, the larger is x (we observed x from 13 to 43 in the film preparation temperature range 190-230 °C). This dependence was also observed with C60 monolayer modified porous Si surfaces. 25 Both the MS peak positions and the temperature dependence indicate that in addition to the hydrosilylation reaction as sketched in Scheme 1, hydrogenation may also happen simultaneously in the monolayer formation process. This is consistent with the trihydride hydrogen desorption at ∼77-277 °C observed by Yates et al.29 A schematic representation of the monolayer formation process is shown in Figure 7, in which the desorbed hydrogen atoms add to some CdC bonds on the C60 cage in addition to the addition reaction of one Si-H bond to a CdC bond. After the C60 molecules react with the Si
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Figure 7. Scheme for the C60 monolayer/Si(100) formation. The filled circles represent the surface Si atoms, and the open circles represent the bulk Si atoms.
surface, the Si atoms on the surface will have dangling bonds left by the loss of hydrogen atoms. These Si atoms may undergo reconstruction;35,36 i.e., the Si atoms next to each other may bind to form dimers. Conclusions A new type of fullerene monolayer modified n- or p-type Si(100) has been made which has the shortest distance possible from the monolayer to the substrate. FAB-MS showed the composition of the C60 monolayer on Si(100) is C60Hx where x is 13 to 43. Simultaneous hydrosilylation and hydrogenation reactions are believed to be responsible for the monolayer formation. Such C60 monolayers on Si(35) Schlier, R. E.; Farnsworth, H. E. J. Chem. Phys. 1959, 30, 917. (36) Haneman, D. Rep. Prog. Phys. 1987, 50, 1045.
(100) surfaces exhibit photoeffects and mediation reactions of solution species which correlate well to the expected properties of n- or p-type substrates coupled to a surfaceattached reversible and stable redox moiety. Since, very recently, a large enhancement of these photocurrent and photovoltage effects has been observed on C60 monolayer modified porous Si surfaces,25 it is encouraging to speculate that these C60 monolayer modified Si surfaces may find applications in photosensors and photovoltaics.
Acknowledgment. The authors are grateful to Professor C. Wesdemiotis and J. Wu of the University of Akron for FAB-MS analyses. LA980999S
