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Si-C(N) σ Linkages and N f Si Dative Bonding at Pyridine/Si(111)-7 × 7 Feng Tao, Yee Hing Lai, and Guo Qin Xu* Department of Chemistry, National University of Singapore, 10 Kent Ridge, Singapore 119260 Received October 10, 2003 We experimentally demonstrated that pyridine/Si(111)-7×7 can act as an electron donor/acceptor pair as a result of the charge transfer from the electron-rich N atom of pyridine to the electron-deficient adatom of the Si surface, evidenced by the upshift of 1.8 eV (state A) for the N(1s) core level upon the formation of a datively bonded complex compared to physisorbed molecules. Another state (B) whose N(1s) binding energy downshifts by 1.2 eV was assigned to an adduct through Si-C and Si-N covalent linkages, formed via a [4 + 2]-like addition mechanism on Si(111)-7 × 7. Binding molecules through the formation of the dative bond resulted from significant electron transfer opens a new approach for the creation of Si-based molecular architectures and modification of semiconductor interfacial properties with unsaturated organic molecules.
Organic modification and functionalization of silicon surfaces have attracted much attention recently due to its potential applications in Si-based microelectronics and sensing technologies.1 To achieve fine control in incorporating desired organic functionalities into existing device technologies, the growing efforts have been dedicated to the mechanistic understanding of chemical attachment of unsaturated organic molecules on silicon surfaces at molecular level.2,3 Si(111)-7 × 7, one of the most important semiconductor surfaces, can be clearly described with the so-called dimeradatom-stacking (DAS) faulted model.4 The spatial arrangement of surface adatoms and rest atoms in a unit cell is schematically presented in Figure 1. Nineteen dangling bonds in every unit cell are located at 12 adatoms, 6 rest atoms and 1 corner hole. Our systematic studies 5-11 clearly demonstrated that the adjacent adatom-rest atom pair can be alternatively viewed as a highly reactive “di-radical” for covalent attachment of unsaturated organic molecules through [4 + 2]- or [2 + 2]-like addition strategies. On the other hand, the charge transfer occurs preferentially from adatoms to rest atoms, resulting in completely occupied dangling bonds at the rest atoms and corner hole with a charge of ∼-1 and partially occupied dangling bonds at the adatoms (+7/12). Therefore, the electron-deficient adatoms and electron-rich rest atoms * Author to whom correspondence should be addressed. Tel.: (65) 6874 3595. Fax: (65) 6779 1691. E-mail:
[email protected]. (1) Yates, J. T., Jr. Science 1998, 279, 335-336. (2) Wolkow, R. A. Annu. Rev. Phys. Chem. 1999, 50, 413-441. (3) Lu, X.; Xu, X.; Wang, N. Q.; Zhang, Q.; Lin, M. C. J. Phys. Chem. B 2001, 105, 10069-10075. (4) Takayanagi, K.; Tanishiro, T.; Tanahashi, S.; Takahashi, M. J. Vac. Sci. Technol., A 1985, 3, 1502-1506. (5) Cao, Y.; Yong, K. S.; Wang, Z. Q.; Chin, W. S.; Lai, Y. H.; Deng, J. F.; Xu, G. Q. J. Am. Chem. Soc. 2000, 122, 1812-1813. (6) Cao, Y.; Wang, Z. H.; Deng, J. F.; Xu, G. Q. Angew. Chem. Int. Ed. 2000, 39, 2740-2744. (7) Tao, F.; Wang, Z. H.; Chen, X. F.; Xu, G. Q. J. Am. Chem. Soc. 2002, 124, 7170-7180. (8) Tao, F.; Sim, W. S.; Xu, G. Q.; Qiao, M. H. J. Am. Chem. Soc. 2001, 123, 9397-9403. (9) Tao, F.; Wang, Z. H.; Lai, Y. H.; Xu, G. Q. J. Am. Chem. Soc. 2003, 125, 6687-6696. (10) Tao, F.; Chen, X. F.; Wang, Z. H.; Xu, G. Q. J. Phys. Chem. B 2002, 106, 3890-3895. (11) Qiao, Q. H.; Cao, Y.; Deng, J. F.; Xu, G. Q. Chem. Phys. Lett. 2000, 325, 508-512.
Figure 1. Spatial arrangement of surface electron-deficient adatoms and electron-rich rest atoms in a (7 × 7) unit cell (a) and the di-radical reactive site of an adjacent adatom-rest atom pair (b).
of Si(111)-7 × 7 (Figure 1) may possibly act as electron acceptors and electron donors, respectively. The formation of a dative bond between organic molecules and semiconductor surfaces was previously found for trimethylamine on Si(100) and Si(111)-7 × 712-14 and aliphatic amines on Ge(100).15 In addition, the possible existence of a dative bond between acetone and Si(100) was also theoretically suggested.16 Recently, a similar binding mechanism in pyridine adsorption on Si(100)17and Ge(100)18 was experimentally demonstrated. As schematically shown in Figure 1, Si(111)-7 × 7 has a distinctly different structure from Si(100). For Si(111)-7 (12) Cao, X.; Hamers, R. J. J. Am. Chem. Soc. 2001, 123, 1098810996. (13) Cao, X.; Hamers, R. J. J. Phys. Chem. B 2002, 106, 1840-1842. (14) Wang, G. T.; Mui, C.; Tannaci, J. F.; Filler, M. A. Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2003, 107, 2982-2986. (15) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2001, 105, 3295-3299. (16) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2001, 105, 12559-12565. (17) Tao, F.; Qiao, M. H.; Wang, Z. H.; Xu, G. Q. J. Phys. Chem. B 2003, 107, 6384-6390. (18) Cho, Y. E.; Maeng, J. Y.; Kim, S.; Hong, S. Y. J. Am. Chem. Soc. 2003, 125, 7514-7515.
10.1021/la035900i CCC: $27.50 © 2004 American Chemical Society Published on Web 12/12/2003
Si-C(N) σ Linkages and N f Si Dative Bonding
Figure 2. (Left panel) N(1s) BE for pyridine adsorbed on Si(111)-7 × 7 as a function of exposure at 110 K. (Right panel) N(1s) BE for pyridine adsorbed on Si(111)-7 × 7 as a function of temperature (110-300 K).
× 7, the largely unequal distributions of electron density at the adatom and rest atom make them possibly act as an electron acceptor and donor to form a dative bond with other molecules/atoms. On the other hand, the nitrogen atom of pyridine has a lone pair of electrons that are not in conjugation with the aromatic ring, forming a higher electron density at the nitrogen atom. It may possibly function as an electron donor to form a dative bond with the electron-deficient adatom of Si(111)-7 × 7. In this study, therefore, we explore the possibility of forming a dative bond at the pyridine/Si(111)-7 × 7 interface. In addition, the aromatic ring may react with the adjacent adatom-rest atom pair through [4 + 2]- or [2 + 2]-like addition pathways to form a di-σ-bonded surface adduct. The electronic structures and vibrational signatures of pyridine on Si(111)-7 × 7 were investigated by using X-ray photoelectron spectroscopy (XPS) and vibrational highresolution electron energy loss spectroscopy (HREELS), respectively. The left panel of Figure 2 shows the N(1s) XPS spectra of pyridine on Si(111)-7 × 7 at 110 K as a function of exposure. At the low exposures of e0.50 L, pyridine directly binds to the surface Si atoms, forming a chemisorption layer. Two photoemission features can be clearly resolved at 401.8 (peak A) and 398.8 eV (peak B) for chemisorbed pyridine. These two peaks do not show observable shifts with the increase of exposures. A further increase in the pyridine exposure leads to the physisorption of pyridine molecules on top of the chemisorbed monolayer. The appearance of an N(1s) peak at 400.0 eV is attributable to physisorbed molecules, consistent with previous studies of pyridine on Ni(110).19 This peak becomes dominant at the exposures higher than 2.5 L because of the screening of the physisorbed multilayer on the photoemission signal from the chemisorbed monolayer. Our temperature-programmed desorption studies show that physisorbed molecules are weakly bound to the surface and completely desorb from the surface at 170 K (starting from 120 K). Upon annealing a multilayer pyridine-covered Si(111)-7 × 7 to 240 K, the two peaks at 401.8 (A) and 398.8 eV (B) are still identifiable, further confirming their chemical bonding natures. For state A, its N(1s) core level of 401.8 eV is ∼3.0 eV higher than that of B and ∼1.8 eV higher than the value (19) Cohen, M. R.; Merrill, R. P. Surf. Sci. 1991, 245, 1-11.
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of physisorbed pyridine (left panel of Figure 2). This unusually high binding energy (BE) suggests a strong electron-deficient environment around the N atom in state A. Compared to the N(1s) core level of 401.6 eV for [(CH3)4N]+Br- 20 and 402.5 eV for [(CH3)4N]+Cl-,21 our N(1s) value of 401.8 eV observed for state A possibly results from the significant transfer of electron density from the N atom of pyridine to surface Si adatoms through the formation of a N f Si dative bond. For state B, the N(1s) BE approximates to the typical values of 398.5-399.3 eV for Si-N σ-bonded linkages formed in the dissociative chemisorption of pyrrole11 on silicon surfaces. The nature of state B will be further demonstrated in our vibrational studies. To investigate the possible conversion between the two states, XPS studies were carried out as a function of the sample temperature between 110 and 300 K (right panel of Figure 2). In curve b of the right panel, the two N(1s) peaks at 401.8 (A) and 398.8 eV (B) are coexistent at 110 K and their area ratio is ∼42%:∼58%. A slight increase of the surface temperature leads to the decrease in the intensity at 401.8 eV (A) but the concurrent growth of the 398.8 eV peak (B). Upon annealing to 180 K, the intensity ratio between the peaks at 401.8 and 398.8 eV changes to ∼22%:∼75% without the major loss of total chemisorbed molecules. At 300 K, only the peak at 398.8 eV can be resolved, indicating that only state A is on the surface. It is also noted that there are no detectable chemical shifts for these two peaks during the annealing process, indicating no new species formed. The evaluation of peak areas for N(1s) at 401.8 (A) and 398.8 eV (B) suggests that state A [N(1s) 401.8 eV] possibly partially converts to B [N(1s) 398.8 eV] in this process. The conversion possibly originates from (1) the weakening of the aromaticity of pyridine as a result of the electron transfer from its N atom to the Si adatom and (2) the availability of a radical-like rest atom at a suitable distance. However, further investigations are required to figure out the possibility of the conversion. For further understanding the binding mechanism of state B, HREELS studies were carried out. Figure 3 presents the vibrational signatures of the physisorbed multilayer of pyridine (3a) and chemically bonded molecules with a pure state B (3b) obtained by annealing the pyridine/Si(111)-7 × 7 to 300 K. The vibrational features of physisorbed molecules are very consistent with the IR spectrum of liquid pyridine.22,23 In contrast, the vibrational spectrum of state B (Figure 3b) presents two resolvable peaks at ∼3055 and ∼2898 cm-1, attributable to the Csp2-H and Csp3-H stretching modes, respectively. Previous studies showed that benzene can be covalently attached onto Si(100) 24 and Si(111)-7 × 7 25 through the formation of two new Si-C σ-bond linkages, evidenced by the doublet at ∼3050 (Csp2-H) and ∼2910 cm-1 (Csp3-H). The involvement of C and N atoms in chemical binding with the surface for state B can be further evidenced by the detection of the vibrational peaks at 656 and 506 cm-1 (Figure 3b) that are related to Si-C and Si-N stretching,24,26 respectively. Therefore, these observations show that state B is a cycloadduct via the (20) Swartz, W. E.; Gray, R. C., Jr.; Carver, J. C.; Taylor, R. C.; Hercules, D. M. Spectrochim. Acta A 1974, 30, 1561-1572. (21) Jack, J. J.; Hercules, D. H. Anal. Chem. 1971, 43, 729-736. (22) Long, D. A.; Thomas, E. L. Trans. Faraday Soc. 1963, 59, 783. (23) Wiberg, K. B.; Walters, V. A.; Wong, K. N.; Colson, S. D. J. Phys. Chem. 1984, 88, 6067-6075. (24) Taguchi, Y.; Fujisawa, M.; Takaokat, T.; Okada, T.; Nishijima, M. J. Chem. Phys. 1991, 95, 6870-6876. (25) Cao, Y.; Wei, X. M.; Chin, W. S.; Lai, Y. H.; Deng, J. F.; Bernasek, S. L.; Xu, G. Q. J. Phys. Chem. B 1999, 103, 5698-5702.
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Figure 4. Vibrational spectra for physisorbed-2-d1 pyridine (a) and chemisorbed molecules (state B) obtained after annealing pyridine-exposed Si(111)-7 × 7 to 300 K (b). Figure 3. Vibrational spectra for physisorbed pyridine (a) and chemisorbed molecules (state B) obtained after annealing pyridine-exposed Si(111)-7 × 7 to 300 K (b).
rehybridization of the Csp2 atom and N atom, forming covalent Si-C and Si-N σ linkages, suggesting the possible [2 + 2]-like addition at N1 and C1 or [4 + 2]-like addition at N1 and C3. The similarity in vibational features of state B (Figure 3b) with those of the [4 + 2]-like cycloadduct of benzene on Si(111)-7 × 725 and liquid 1,3cyclohexadiene27-29 implies a [4 + 2]-like addition of N1 and C3 atoms of pyridine to an adjacent adatom-rest atom pair. To further determine the addition mechanism and binding structure of state B, pyridine-2-d1/Si(111)-7 × 7 was also studied. Parts a and b of Figure 4 are the vibrational features of physisorbed pyridine-2-d1 at 110 K and state B obtained after annealing the sample to 300 K, respectively. In the vibrational signatures of physisorbed molecules, the peaks at 3067 and 2282 cm-1 (Figure 4a) are ascribed to the Csp2-H and Csp2-D stretching modes, respectively. Compared to physisorbed pyridine2-d1, vibrational signatures of state B present the following differences: (a) the peak for C-H stretching becomes a doublet, suggesting the rehybridization of one or more C atoms from sp2 to sp3; (b) the peak attributed to the Csp2-D stretching mode does not display obvious shift, showing that C1 and C5 atoms do not directly take part in the (26) Qiao, M. H.; Cao, Y.; Deng, J. F.; Xu, G. Q. Chem. Phys. Lett. 2000, 325, 508-512. (27) Stidham, H. D. Spectrochim. Acta 1965, 21, 23-32. (28) Hagemann, H.; Bill, H.; Joly, D.; Muller, P.; Pautex, N. Spectrochim. Acta, Part A 1985, 41, 751-756. (29) Carreira, L. A.; Cater, R. O.; Durig, J. R. J. Chem. Phys. 1973, 59, 812.
addition reaction with the Si surface. In addition, the concurrent participation of the C and N1 atoms in binding with the surface Si atoms is further confirmed by the observation of Si-C and Si-N1 stretching at 634 and 506 cm-1, respectively. The vibrational results rule out the possibility of the [2 + 2]-like addition reaction occurring through both N1 and C1, both N1 and C5, both C1 and C2, or both C5 and C4; in addition, the photoemission feature of the N(1s) core level excludes out the possible reaction channels of [2 + 2]-like addition at both C1 and C2, both C2 and C3, both C3 and C4, or both C4 and C5. In fact, both vibartional signatures in Figures 3 and 4 strongly support the [4 + 2]-like addition mechanism involving both N1 and C3. Moreover, the rehybridization of the N1 atom from sp2 into sp3 is expected to result in significant changes in its electronic structures, consistent with an observed chemical downshift of ∼1.2 eV for N(1s) compared to physisorbed molecules (Figure 2). Thus, our experimental studies show that the state B with a similar structure to 1,4-dihydropyridine is formed through the [4 + 2]-like addition involving both the N1 and C3 atoms. In summary, we have demonstrated the coexistence of dative- and covalent-bonded pyridine molecules on Si(111)-7 × 7. Pyridine and Si(111)-7 × 7 constitute an excellent electronic donor-acceptor pair by the significant electronic transfer from the rich electronic N atom of pyridine to the Si surface to form the N f Si dative bond. Our results provide new insight into the chemical attachment of aromatic molecules on semiconductor surfaces. Moreover, the formation of dative bonds between aromatic organic molecules and semiconductor surfaces may open up novel routes for the creation of molecular architectures on semiconductor surfaces. LA035900I
