11722
Langmuir 2005, 21, 11722-11728
A [4+2]-like Cycloaddition of Methyl Methacrylate on Si(100)-2×1 Jing Yan Huang,†,‡ Hai Gou Huang,† Yue Sheng Ning,† Qi Ping Liu,† Solhe F. Alshahateet,‡ Yue Ming Sun,§ and Guo Qin Xu*,† Department of Chemistry, National University of Singapore, 10 Kent Ridge, Singapore 119260, Institute for Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833, and Department of Chemistry and Chemical Engineering, Southeast University, Nanjing, People’s Republic of China 210096 Received June 13, 2005. In Final Form: September 15, 2005 The attachment of methyl methacrylate (MMA) on Si(100)-2×1 was investigated using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. The HREELS spectra of chemisorbed MMA show the disappearance of characteristic vibrations of CdO (1725 cm-1) and C(sp2)-H (3110, 1400, and 962 cm-1) coupled with the blue shift of the CdC stretching mode by 34 cm-1 compared to those of physisorbed molecules. These results clearly demonstrate that both CdC and CdO in MMA directly participate in the interaction with the surface to form a SiCH2C(CH3)dC(OCH3)OSi species via a [4+2]-like cycloaddition. This binding configuration was further supported by XPS, UPS, and DFT studies.
I. Introduction Recently, organic functionalization of silicon surfaces has received much attention due to its potential applications in developing inorganic/organic hybrid devices.1-3 As the starting points for microelectronic and optical devices, the (100) crystallographic face of the silicon is of particular interest. Upon thermal annealing, the surface reconstructs to a (2×1) structure, in which surface silicon atoms form dimer rows.4,5 The silicon dimer is tilted to decrease the strain energy and produces electron-deficient buckled-down and electron-rich buckled-up silicon atoms.6,7 This asymmetric structure increases the zwitterionic property of the dimer and facilitates the attack of both nucleophilic and electrophilic reagents. The unique electronic and steric properties of Si(100)-2×1 make it a good template for investigating the reaction of organic molecules on semiconductor surfaces. Organic molecules containing a single functional group, such as CdC, CtC, CdO, CtN, or NdN, undergo [2+2]like cycloadditions with Si(100)-2×1, leading to the formation of stable cyclic products.8-14 However, for bifunctional and multifunctional molecules, the reaction * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (65) 6779 1691. † National University of Singapore. ‡ Institute for Chemical and Engineering Sciences. § Southeast University. (1) Bent, S. F. Surf. Sci. 2002, 500, 879. (2) Yates, J. T., Jr. Science 1998, 279, 335. (3) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (4) Schlier, R. E.; Farnsworth, H. E. J. Chem. Phys. 1959, 30, 917. (5) Appelbaum, J. A.; Baraff, G. A.; Hamann, D. R. Phys. Rev. B 1976, 14, 588. (6) Chadi, D. J. Phys. Rev. Lett. 1979, 43, 43. (7) Wolkow, R. A. Phys. Rev. Lett. 1992, 68, 2636. (8) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. J. Chem. Phys. 1987, 87, 7332. (9) Lu, X. J. Am. Chem. Soc. 2003, 125, 6384. (10) Huang, C.; Widdra, W.; Wang, X. S.; Weinberg, W. H. J. Vac. Sci. Technol., A 1993, 11, 2250. (11) Kim, W.; Kim, H.; Lee, G.; Chung, J.; You, S.-Y.; Hong, Y.-K.; Koo, J.-Y. Surf. Sci. 2002, 514, 376. (12) Armstrong, J. L.; White, J. M.; Langell, M. J. Vac. Sci. Technol., A 1997, 15, 1146.
mechanisms become complex. Cyclopentadiene reacts with Si(100)-2×1 at room temperature to form a [4+2]-like cycloaddition product,15 while 1,3-cyclohexadiene was found to yield both [4+2]- and [2+2]-like cycloadducts.16 For unsaturated ketones, such as ethyl vinyl ketone and 2-cyclohexen-1-one, ene reactions and [2+2] CdO cycloadditions, along with [4+2]-like cycloadditions, were observed to occur on Si(100)-2×1.17 Two [2+2]-like reactions were reported for acetylethyne (CH3COCtCH) chemisorption on the silicon surface via the binding of CdO and CtC to Si dimers.18 Therefore, a detailed understanding of the attachment mechanism and selectivity of multifunctional molecules on the silicon surface is essential for creating organic functionalized semiconductor surface structures with custom-tailored properties. In this paper, we have investigated the adsorption of methyl methacrylate (MMA) on Si(100)-2×1 using highresolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and density functional theory (DFT) calculations. MMA is an ester with conjugated CdC and CdO groups. By combining these two functional groups in MMA, the selectivity and competition of CdC and CdO groups can be explored in the study of the binding mechanisms of MMA on Si(100)-2×1. On the basis of the molecular structure, there are five possible reaction pathways of MMA on Si(100)-2×1, shown in Figure 1. The interaction of lone-pair electrons at carbonyl/ ether oxygen atoms in MMA with the Si dangling bonds leads to the formation of dative bonds (modes I and II). The [2+2]-like cycloadditions may occur through either (13) Tao, F.; Wang, Z. H.; Qiao, M. H.; Liu, Q.; Sim, W. S.; Xu, G. Q. J. Chem. Phys. 2001, 115, 8563. (14) Ellison, M. D.; Hovis, J. S.; Liu, H. B.; Hamers, R. J. J. Phys. Chem. B 1998, 102, 8510. (15) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 1999, 103, 6803. (16) Kong, M. J.; Teplyakov, A. V.; Jagmohan, J.; Lyubovitsky, J. G.; Mui, C.; Bent, S. F. J. Phys. Chem. B 2000, 104, 3000. (17) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 8990. (18) Huang, H. G.; Cai, Y. H.; Huang, J. Y.; Tang, H. H.; Xu, G. Q. Langmuir 2005, 21, 3384.
10.1021/la051559l CCC: $30.25 © 2005 American Chemical Society Published on Web 10/19/2005
[4+2]-like Cycloaddition of MMA on Si(100)-2×1
Figure 1. Schematic diagrams for the five possible reaction pathways of methyl methacrylate on Si(100)-2×1.
CdC (mode III) or CdO (mode IV). In addition, a [4+2]like reaction via conjugated carbonyl and vinyl groups is presented in mode V. Our experimental and theoretical results indicate that MMA is covalently bonded to Si(100)-2×1 mainly through a [4+2]-like addition reaction to form a stable six-membered cycloadduct with new σ-linkages of Si-C and Si-O.
Langmuir, Vol. 21, No. 25, 2005 11723
Figure 2. HREELS spectra obtained after Si(100)-2×1 was exposed to 5.0 langmuirs of methyl methacrylate at 110 K (a) and the sample in (a) was annealed to 200 K (b) and 300 K (c). Ep ) 5.0 eV, specular mode. were reported in units of langmuirs (1 langmuir ) 10-6 Torr‚s) without calibration of ion gauge sensitivity. XPS was employed to follow the thermal evolution of MMA on the Si surface. The physisorbed MMA was found to completely desorb from the surface at ∼200 K, leaving a chemisorbed layer. Further annealing to 350 K does not lead to obvious changes in the XPS spectra. Thus, the experimental data on chemisorbed molecules were acquired after the multilayer-covered sample was annealed to 300 K.
II. Experimental Section III. Results and Discussion
The experiments were carried out in two ultra-high-vacuum chambers each with a base pressure lower than 2 × 10-10 Torr. One is equipped with a high-resolution electron energy loss spectrometer (LK-2000). EELS measurements were taken in a specular geometry. An electron beam with an energy of 5.0 eV impinges on the surface at an incident angle of 60° with respect to the surface normal. A typical instrumental resolution of ∼60 cm-1 was achieved. Photoelectron studies were performed in the other chamber, which is equipped with a dual-anode X-ray source, a He discharge lamp, and a concentric hemispherical energy analyzer (CLAM2, VG). XPS spectra were acquired using Al KR radiation (hν ) 1486.6 eV) and a 20 eV pass energy. The binding energy (BE) scale is referenced to 99.3 eV for the bulk Si(2p) line.19 He II (hν ) 40.8 eV) and a 10 eV pass energy were selected for obtaining the valence band spectra. The Si(100) samples (18 × 8 × 0.38 mm3) were cut from p-type boron-doped silicon wafers with a resistivity of 1-30 Ω‚cm and a purity of 99.999% (Goodfellow). A Ta foil (thickness ∼0.025 mm) was sandwiched between two identical samples held together using Ta clips and in turn spot-welded to Ta posts at the bottom of a dewar-type liquid N2 cooled sample holder. The sample can be heated to 1300 K through the resistive heating of the Ta foil and cooled to 110 K using liquid nitrogen. The temperature distribution on the samples is within (10 K at 1000 K, determined using a pyrometer. The Si(100) sample was cleaned by several cycles of Ar+ bombardment and annealing at 1300 K. The cleanliness of the samples was verified using XPS and HREELS. The surface structure was examined using scanning tunneling microscopy (STM) in a separate chamber. MMA (Aldrich, 98.0+%) was purified by freeze-pump-thaw cycles before being dosed onto the silicon surface through a variable leak valve. The exposures
A. High-Resolution Electron Energy Loss Spectroscopy. Figure 2a shows the HREELS spectrum of Si(100)-2×1 exposed to 5.0 langmuirs of MMA [C1H2d C2(C3H3)C4OOC5H3] at 110 K. Losses at 3110, 2961, 1725, 1630, 1450, 1400, 1317, 1185, 962, 839, 650, and 351 cm-1 can be clearly resolved and are consistent with those in the IR and Raman spectra of gaseous MMA,20 suggesting the formation of physisorbed multilayer MMA on the silicon surface. The peaks at 3110, 1400, and 962 cm-1 are assigned to the corresponding stretching, deformation, and wagging modes of C(sp2)-H. The stretching modes of CdO and CdC are located at 1725 and 1630 cm-1, respectively. The characteristic vibrational features associated with ester bands appear at 1317, 1185, and 839 cm-1, which are attributable to the asymmetric stretching of CCO and asymmetric and symmetric stretching modes of COC, respectively. When the multilayer-MMA-covered sample was annealed to 200 and 300 K to drive away all the physisorbed molecules, EELS spectra for chemisorbed molecules on Si(100)-2×1 (Figure 2b,c) were obtained, which show loss peaks at 2961, 1664, 1450, 1162, 1001, 841, 644, and 520 cm-1. The detailed assignments of the vibrational features for physisorbed and chemisorbed MMA on Si(100)-2×1 are listed in Table 1. The negligible loss in the range of 2000-2150 cm-1 related to ν(Si-H)21 excludes the dissociative chemisorption of MMA on the silicon surface. Compared to the EELS spectrum for physisorbed MMA,
(19) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corp.: Eden Prairie, MN, 1991.
(20) Manley, T. R.; Martin, C. G. Spectrochim. Acta 1976, 32A, 357. (21) Wagner, H.; Butz, R.; Backes, U.; Bruchmann, D. Solid State Commun. 1981, 38, 1155.
11724
Langmuir, Vol. 21, No. 25, 2005
Huang et al.
Table 1. Vibrational Assignments of Physisorbed and Chemisorbed Methyl Methacrylate on Si(100)-2×1a vibrational assignment νas(dCH2) νs(dCH2) νas(O-CH3) νas(CH3) νs(O-CH3) νs(CH3) ν(CdO) ν(CdC) δas(CH3) δas(O-CH3) δ(dCH2) δs(CH3) νas(CCO) - ν(O-C) νas(CCO) + ν(O-C), ω(CH2 ) νas(COC) - ν(C-C) νas(COC) + ν(C-C) νas(CCC) νs(CCC), ν(CH3), τ(R-CH3) ω(O-CH3) ω(R-CH3) ω(dCH2) νs(COC) - ν(C-C) νs(COC) + ν(C-C) τ(dCH2) ω(CdO) νs(CCO) - ν(O-C) νs(CCO) + ν(O-C) r(CdO) δ(COC), δ(CCC) δ(CCO) τ(O-CH3), τ(R-CH3) τ(CCOC) τ(CCCO) Si-C Si-O a
IR20
Raman20
3110 3015 2991 2959 2935 2850 1728 1642 1458 1444 1408 1382 1328 1307 1204 1167 1025 1019 1004 990 945 836 830 820 655 604 597 511 381 358 215 165 60
3105 3019 2995 2961 2930 2849 1720 1640 1450 1441 1405 1378 1323 1302 1200 1163 1030 995 990 935 830 815 657 604 600 512 372 355 220
physisorbed CH2dC(CH3)COOCH3 on Si(100)-2×1
chemisorbed CH2dC(CH3)COOCH3 on Si(100)-2×1
3110 2961
2961
1725 1630 1450
1664 1450
1400 1317 1185
1162
1001 962 839
841
650
351
520 644
All frequencies are in inverse centimeters.
one of the important changes is the disappearance of ν(Cd O) in the spectra for chemisorbed molecules, which suggests the involvement of the carbonyl group in the reaction and thereby rules out the formation of datively bonded products (modes I and II) and the [2+2] CdC cycloadduct (mode III) on the surface. Furthermore, the observation of an intensity at 1664 cm-1 attributable to the CdC stretching implies the presence of CdC in chemisorbed species. It is noted that C1dC2 and C2dC4 appear in the possible binding modes IV and V, respectively. However, the disappearance of the three characteristic peaks of C(sp2)-H at 3110, 1400, and 962 cm-1 in the vibrational features of chemisorbed molecules clearly rules out the [2+2]-like cycloaddition occurring through the CdO group (mode IV) with the retention of C1H2d C2(C3H3)-. In fact, our HREELS results are very consistent with a [4+2]-like cycloaddition in the interaction of MMA with Si(100)-2×1 (mode V), which involves the formation of a new double bond between C2 and C4, together with the breakage of C1dC2 and C4dO double bonds. Compared to the conjugated C1dC2 group in physisorbed MMA, the newly formed C2dC4 in chemisorbed species experiences a blue shift of 34 cm-1.22 Since there are no H atoms directly attached to C2 and C4 atoms in the [4+2]-like cycloadduct [SiC1H2C2(C3H3)dC4(O2C5H3)O1Si], the observation of the vibrational features associated with C(sp2)-H is not expected. The formation of new Si-C and Si-O linkages is confirmed in the appearance (22) Socrates, G. Infrared and Roman Characteristic Group Frequencies: tables and charts; John Wiley & Sons: New York, 2001.
of two peaks at 520 and 644 cm-1, which are ascribed to ν(Si-C) and ν(Si-O), respectively.22,23 B. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy was employed to investigate the C1s and O1s core levels of MMA [C1H2dC2(C3H3)C4O1O2C5H3] adsorbed on Si(100)-2×1. The photoemission features of physisorbed molecules are shown in Figures 3a (C1s) and 4a (O1s). The C1s intensity of physisorbed MMA can be deconvoluted into three peaks centered at 285.6, 287.2, and 289.6 eV, consistent with C1s binding energies of gaseous MMA,24 considering the different reference levels. The peak at 285.6 eV accounts for the total contribution from three alkyl carbon atoms (C1, C2, and C3). Furthermore, the features at 287.2 and 289.6 eV with a separation of 2.4 eV can be assigned to C5 and C4, respectively. The C1s XPS spectrum for multilayer methyl formate (HCOOCH3) on Ag(111) shows a similar BE separation of 2.6 eV between methyl and formyl carbon atoms.25 In Figure 4a, the O1s photoemission feature for condensed MMA on Si(100)-2×1 can be resolved into two peaks at 532.6 (O1) and 534.2 (O2) eV with an intensity ratio of approximately 1:1.26 The separation (1.6 eV) between these two features agrees well with the value obtained in the O1s XPS spectrum of gaseous MMA.24 (23) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf. Sci. 1985, 154, 35. (24) Naves de Brito, A.; Keane, M. P.; Correia, N.; Svensson, S.; Gelius, U.; Lindberg, B. J. Surf. Interface Anal. 1991, 17, 94. (25) Schwaner, A. L.; Fieberg, J. L.; White, J. M. J. Phys. Chem. B 1997, 101, 11112. (26) Lo´pez, G. P.; Castner, D. G.; Ratner, B. D. Surf. Interface Anal. 1991, 17, 267.
[4+2]-like Cycloaddition of MMA on Si(100)-2×1
Figure 3. Deconvoluted C1s XPS spectra of physisorbed (a) and chemisorbed (b) methyl methacrylate on Si(100)-2×1. For obtaining a saturated chemisorption monolayer on Si(100)-2×1, the sample was preexposed to 20.0 langmuirs of methyl methacrylate at 110 K and then annealed to 300 K to drive away all of the physisorbed molecules.
Figure 4. Fitted O1s XPS spectra for physisorbed (a) and chemisorbed (b) methyl methacrylate on Si(100)-2×1. For obtaining a saturated chemisorption monolayer on Si(100)-2×1, the sample was preexposed to 20.0 langmuirs of methyl methacrylate at 110 K and then annealed to 300 K to drive away all of the physisorbed molecules.
Figures 3b and 4b show the fitted C1s and O1s XPS spectra for chemisorbed MMA. The significant changes in photoemission features upon chemisorption can be reasonably interpreted by the [4+2]-like cycloaddition of MMA on Si(100)-2×1. The C1s spectrum of chemisorbed molecules is fitted into two intensities at 284.7 and 287.0 eV with an area ratio of ∼3:2, assigned to C1, C2, C3 and C4, C5 in the proposed [4+2]-like cycloadduct [SiC1H2C2(C3H3)dC4(O2C5H3)O1Si], respectively. In chemisorption, the breakage of the double bond between the C4 and O1 atoms reduces the ability of O1 to withdraw electrons from C4. Therefore, the binding energy of C4 shows a drastic downshift of 2.6 eV referenced to the value of physisorbed molecules. We can also observe a BE downshift of 0.9 eV for the intensity related to C1, C2, and C3 in the [4+2]-like cycloadduct, which may be caused by the changing
Langmuir, Vol. 21, No. 25, 2005 11725
Figure 5. UPS spectra for methyl methacrylate on Si(100)2×1 as a function of exposure at 110 K. The inset shows the influence of methyl methacrylate exposure on the dangling bond surface states of Si(100)-2×1.
chemical environment of C1 and C2. The rehybridization of the C1 atom from sp2 to sp3 upon chemisorption and its direct binding to a silicon atom with a lower electronegativity increase its electron density, thereby producing a lower BE. When cyclohexene undergoes a [2+2]-like cycloaddition on Si(111)-7×7, carbon atoms in CdC show a similar BE change.27 In physisorbed molecules, the strong inductive effect of the C4dO group on the adjacent C1dC2 reduces the electron density at the C2 atom. However, this effect is not present in chemisorbed species, thereby resulting in a higher electron density around the C2 atom and its lower C1s BE. The O1s XPS spectrum for chemisorbed molecules (Figure 4b) can be fitted into two features at 532.1 (O1) and 533.3 (O2) eV, suggesting the existence of two chemically inequivalent oxygen atoms at the surface. Compared to the physisorbed MMA, the loss of the inductive effect from the neighboring carbonyl group on O2 in the chemisorbed product can also account for the downshift of BE for this oxygen atom from 534.2 to 533.3 eV. Furthermore, O1 is attached to the silicon atom directly and shows a BE at 532.1 eV, in agreement with the value obtained for the O in the Si-O bond formed through the [2+2]-like cycloaddition of acetone on Si(100)-2×1.12 The detailed assignments of the XPS features for physisorbed and chemisorbed molecules are listed in Table 2. The large downshifts of C4 and O1 of the carbonyl group and C1 and C2 of the vinyl group strongly suggest that both C4dO1 and C1dC2 functionalities are involved in the [4+2]-like cycloaddition reaction of MMA with Si(100)-2×1, which is consistent with the evidence from our HREELS experiments. C. Ultraviolet Photoelectron Spectroscopy. To further understand the chemical nature of adsorbed MMA on Si(100)-2×1, the UPS spectra as a function of exposure at 110 K were collected (Figure 5). The inset in Figure 5 indicates that increasing exposure leads to the attenuation of dangling bond surface states (0.7-0.8 eV)28,29 and a total quenching at ∼1.0 langmuirs of MMA, demonstrating the interaction of the dangling bonds on Si(100)-2×1 with (27) Tao, F.; Wang, Z. H.; Xu, G. Q. Surf. Sci. 2003, 530, 203. (28) Himpsel, F. J.; Eastman, D. E. J. Vac. Sci. Technol. 1979, 16, 1297. (29) Hamers, R. J.; Avouris, Ph.; Bozso, F. Phys. Rev. Lett. 1987, 59, 2071.
11726
Langmuir, Vol. 21, No. 25, 2005
Huang et al.
Table 2. Fitted Results of XPS Spectra for Physisorbed and Chemisorbed Methyl Methacrylate [C1H2dC2(C3H3)C4O1O2C5H3] on Si(100)-2×1a
a
core level
physisorption
chemisorption
O1 O2 C1, C2, C3
532.6 534.2 285.6
532.1 533.3 284.7
core level
physisorption
chemisorption
C4 C5
289.6 287.2
287.0 287.0
All energies are in electronvolts.
Table 3. Peak Assignments for the UPS Spectrum of Physisorbed Methyl Methacrylate and the Difference Spectrum of Chemisorbed Molecules on Si(100)-2×1a
a
assignment
IP(calcd)30
π(CdC) (5a′′) n0 (a′) n0 (4a′′) σ(C-H) (a′) π(CdO) (3a′′) σ(C-C) (a′) π(Me) (2a′′) π(Me) (1a′′)
9.85 11.31 12.37 13.16 13.93 14.99 15.74 17.33
vert IP(exptl)30 10.06 10.36 11.06 12.56 13.54 14.66 16.07 16.61
}
av
physisorption peak energy
chemisorption peak energy
10.49
A
5.9
A′
5.7
}
13.05
B
8.3
B′
7.4
}
14.66
C
10.0
C′
10.0
16.34
D
11.6
D′
11.6
All energies are in electronvolts.
Figure 6. UPS spectrum of physisorbed methyl methacrylate (a) and the difference spectrum of chemisorbed molecules (b) on Si(100)-2×1 obtained by subtracting the photoemission of a clean Si(100)-2×1 surface from that of fully chemisorbed methyl methacrylate/Si(100)-2×1. The bar graph below (a) is the orbital energy levels of gaseous methyl methacrylate, shifted to account for work function and final-state relaxation effects when condensed on solid-state surfaces.
MMA molecules. Upon 10.0 langmuirs of exposure, the photoemission from the valence band of chemisorbed species is completely screened by the physisorbed multilayer, resulting in four dominant peaks centered at 5.9 (A), 8.3 (B), 10.0 (C), and 11.6 (D) eV below the Fermi level. From Table 3, it can be seen that the energy separations between the two adjacent energy levels are consistent with those in the UPS spectrum for gaseous MMA.30 The peak broadening and spectrum shift in energy are due to solid-state effects and a change in reference levels. The four bands A, B, C, and D shown in our UPS spectrum for physisorbed MMA on Si(100)-2×1 can be ascribed to [πCdC(5a′′) + n0(a′) + n0(4a′′)], [σCH(a′) + πCd O(3a′′)], σC-C(a′), and [πMe(2a′′) + πMe(1a′′)], respectively. (30) Dam, H. V.; Oskam A. J. Electron Spectrosc. Relat. Phenom. 1978, 13, 273.
Upon annealing the multilayer-MMA-covered surface to 300 K, the UPS spectrum of chemisorbed molecules was obtained, which shows four features at 5.7 (A′), 7.4 (B′), 10.0 (C′), and 11.6 (D′) eV. For comparison, the valence level features for physisorbed and chemisorbed MMA are both presented in Figure 6, together with a bar graph showing gaseous MMA molecular orbitals. Compared to physisorbed molecules, the major changes in the spectrum for chemisorbed molecules are the BE downshift (0.9 eV) and intensity reduction of band B, indicating that at least one of the two σCH and πCdO orbitals associated with band B may participate in the interaction of MMA with Si(100)2×1. The absence of the vibrational feature of Si-H at ∼2000-2150 cm-1 in the HEEELS spectra for chemisorbed molecules rules out the possibility of the C-H bond breakage and thereby suggests the preservation of σCH in chemisorbed species. In fact, this change in band B′ can be accounted for considering the disappearance of πCdO in the [4+2]-like cycloaddition of MMA on the silicon surface. In addition, there is a noticeable shift of 0.2 eV between peaks A and A′. While C1dC2 conjugates with C4dO in physisorbed MMA, the newly formed C2dC4 in chemisorbed species is an isolated double bond, expected to have a slightly lower binding energy for its πCdC. D. Density Functional Theory Calculations. The DFT theoretical calculations were carried out to obtain the optimized geometric structures and energies for possible adsorption configurations. Three cluster models were employed to represent the Si(100)-2×1 surface. The first one is the simplest single-dimer Si9H12 cluster, where the top layer consists of one Si dimer. The other two clusters are Si15H16 and Si23H24, which involve two neighboring Si dimers within the same row and in adjacent rows at the top layer, respectively. Except the silicon atoms in the first layer, all other layers are terminated with H atoms. These three clusters were proven to be successful in predicting the adsorption energies of organic compounds on Si(100)-2×1.13,31-33 Both geometry optimizations and adsorption energy calculations were performed at the B3LYP/6-31G** level of DFT with the SPARTAN software package. The adsorption energy is defined as the difference (31) Tao, F.; Sim, W. S.; Xu, G. Q.; Qiao, M. H. J. Am. Chem. Soc. 2001, 123, 9397. (32) Lu, X.; Xu, X.; Wang, N. Q.; Zhang, Q.; Lin, M. C. J. Phys. Chem. B 2001, 105, 10069. (33) Lu, X.; Lin, M. C. Phys. Chem. Chem. Phys. 2000, 2, 4213.
[4+2]-like Cycloaddition of MMA on Si(100)-2×1
Figure 7. Optimized C5H8O2/Si9H12 clusters corresponding to the five possible attachment modes through dative bonding (modes I and II) and [2+2]-like (modes III and IV) and [4+2]like (mode V) addition reactions.
between the energy of the adsorbate/substrate complex and the total sum of the optimized free cluster and free MMA. As mentioned earlier in the Introduction, five chemisorption processes are possible for MMA binding on Si(100)-2×1, including two dative bondings via carbonyl/ ether oxygen atoms, two [2+2]-like cycloadditions through CdC/CdO, and one [4+2]-like addition (Figure 1). We have considered all these reaction pathways in our calculations using the Si9H12 cluster model. Figure 7 presents the five optimized geometries of the local minima for the C5H8O2/Si9H12 model system. Their adsorption energies are summarized in Table 4. Due to the zwitterionic character of Si dimers, MMA may form a dative bond with the surface by donating the lone pair electrons of the oxygen atom in either the carbonyl or the ether to the electron-deficient buckleddown Si atom, as shown in modes I and II of Figure 7. Previous works experimentally established the formation of stable datively bonded species on Si(100)-2×1 through N f Si.34-36 Acetone was also found to form a datively bonded state at low temperature (115 K) on Ge(100).37 For the two datively bonded states of MMA on Si(100)2×1 (modes I and II), the calculated binding energies are 10.9 and 7.6 kcal/mol, respectively. In mode I, the bond length of the newly formed O-Si is 1.911 Å, much shorter than that (3.568 Å) of O-Si in mode II. The smaller O-Si separation and the higher binding energy of mode I suggest that the formation of the datively bonded structure via the carbonyl O is more favorable. The [2+2]-like cycloaddition of MMA on Si(100)-2×1 through its CdC leads to a four-membered species (mode III) with a binding energy of 41.2 kcal/mol, similar to those previously reported for [2+2]-like reactions of alkenes on the Si surface.15,38-40 In mode III, the two newly formed C-Si bond lengths are 1.941 and 2.003 Å. The Si-Si bond (34) Cao, X. P.; Hamers, R. J. J. Am. Chem. Soc. 2001, 123, 10988. (35) Tao, F.; Lai, Y. H.; Xu, G. Q. Langmuir, 2004, 20, 366. (36) Tao, F.; Qiao, M. H.; Wang, Z. H.; Xu, G. Q. J. Phys. Chem. B 2003, 107, 6384. (37) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2001, 105, 12559. (38) Konecny, R.; Doren, D. J. Surf. Sci. 1998, 417, 169.
Langmuir, Vol. 21, No. 25, 2005 11727
length is increased by 0.135 Å upon molecular binding. More importantly, the C1-C2 bond is elongated to 1.574 Å, which is typical for a C-C single bond.41 By comparison, the [2+2]-like cycloaddition via CdO is exothermic by 22.6 kcal/mol, which is thermodynamically less favorable than the [2+2]-like reaction through CdC. In the [2+2] CdO product (mode IV), the lengths of the O1-Si and C4-Si bonds are 1.715 and 2.043 Å, respectively. The SiSi and O1-C4 bonds are lengthened to 2.335 and 1.432 Å, showing their single-bond characters.41 The [4+2]-like cycloaddition involves conjugated Cd C/CdO and gives rise to a six-membered adduct (mode V). In this chemisorption process, the Si-Si bond length is elongated by 0.146 Å with respect to the unreacted Si dimer with a separation of 2.218 Å. The lengths of the resulting O1-Si and C1-Si bonds were found to be 1.715 and 1.930 Å, respectively. Compared to the value of 1.498 Å for C2-C4 in free MMA, the separation between C2 and C4 is shortened to 1.349 Å in mode V, indicating the conversion from C2-C4 to C2dC4 upon chemisorption. Furthermore, the bond lengths of C1-C2 (1.519 Å) and C4-O1 (1.378 Å) in chemisorbed MMA clearly demonstrate their single-bond nature.41 The calculation studies reveal that the [4+2]-like cycloaddition with a binding energy of 56.3 kcal/mol is thermodynamically favored compared to both dative bonding and [2+2]-like cycloadditions. In fact, both computational and experimental results demonstrate the occurrence of a [4+2]-like cycloaddition for MMA binding on Si(100)-2×1. The absence of an O1s XPS feature at a binding energy higher than the value (534.2 eV) for physisorbed molecules34 rules out the presence of a stable datively bonded surface intermediate. This is indeed consistent with relatively low adsorption energies (10.9 and 7.6 kcal/mol) for the dative bonding configurations. The datively bonded state through the carbonyl O was considered as an important precursor to [4+2]-like and [2+2]-like CdO cycloadditions.17,42 The fact that the datively bonded species were not experimentally observed implies that their residence time may be short compared to the experimental measurements. It also points out that the conversion from a datively bonded state to a di-σ-bonded [4+2]-like cycloadduct (mode V) is a rapid process, suggesting a low energy barrier in the reaction pathway. Compared to the [4+2]-like cycloaddition, the process to form four-membered [2+2]-like adducts may involve a greater structural strain, possibly implying higher transition states. This agrees with the reaction selectivity for the [4+2]-like cycloaddition in this experiment. Recently, a number of experimental and theoretical studies have clearly shown that the cycloadditions of unsaturated organic molecules on Si(100)-2×1 also involve two neighboring dimers from either the same row or two adjacent rows.17,32,33,43-46 To further investigate the interdimer [4+2]-like cycloadditions, clusters (Si15H16 and Si23H24) containing two Si dimers were built. Figure 8 shows the intradimer products (A) and interdimer adducts involving two Si atoms in the same row (B) and in two adjacent rows (C). The adsorption energy of product A (39) Choi, C. H.; Gordon, M. S. J. Am. Chem. Soc. 1999, 121, 11311. (40) Liu, Q.; Hoffmann, R. J. Am. Chem. Soc. 1995, 117, 4082. (41) Smith, M. B.; March, J. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure; John Wiley & Sons: New York, 2001. (42) Lu, X.; Zhang, Q.; Lin, M. C. Phys. Chem. Chem. Phys. 2001, 3, 2156. (43) Sorescu, D. C.; Jordan, K. D. J. Phys. Chem. B 2000, 104, 8259. (44) Teague, L. C.; Boland, J. J. J. Phys. Chem. B 2003, 107, 3820. (45) Lu, X.; Zhu, M. P.; Wang, X. L. J. Phys. Chem. B 2004, 108, 7359. (46) Lu, X.; Zhu, M. P. Chem. Phys. Lett. 2004, 393, 124.
11728
Langmuir, Vol. 21, No. 25, 2005
Huang et al.
Table 4. Adsorption Energiesa and Selected Geometric Parameters of the Local Minima in the C5H8O2/Si9H12 Model System on Si(100)-2×1 products Si-Si (Å) C1-Si (Å) C2-Si (Å) C4-Si (Å) O1-Si (Å) O2-Si (Å) C1-C2 (Å) C2-C3 (Å) C2-C4 (Å) C4-O1 (Å) C4-O2 (Å) C5-O2 (Å) ∆E (kcal/mol) a
reactants
mode I
mode II
mode III
mode IV
mode V
2.218
2.373
2.235
2.353 1.941 2.003
2.335
2.364 1.930
1.911 1.338 1.506 1.498 1.215 1.355 1.435
1.338 1.510 1.492 1.251 1.316 1.447 10.9
3.568 1.338 1.506 1.496 1.214 1.361 1.438 7.6
1.574 1.535 1.510 1.219 1.353 1.439 41.2
2.043 1.715
1.715
1.336 1.509 1.526 1.432 1.423 1.417 22.6
1.519 1.508 1.349 1.378 1.363 1.432 56.3
Adsorption energy ∆E ) E(Si9H12) + E(C5H8O2) - E(C5H8O2/Si9H12).
adducts may have a greater structural strain. Furthermore, the interdimer reactions involve the breakage of the π bonds in two Si dimers. Our calculation results lead to the conclusion that the [4+2]-like intradimer cycloadduct (product A) is the most thermodynamically favorable. Detailed theoretical work involving transition states is required to gain further understanding of the reaction mechanisms of MMA on Si(100)-2×1 from a kinetic view. Figure 8. Optimized cluster models corresponding to the [4+2]like cycloaddition of C5H8O2 on two neighboring dimers of Si(100)-2×1.
(56.9 kcal/mol) is close to the value of 56.3 kcal/mol obtained for a smaller silicon cluster, Si9H12 (mode V, Figure 7). This consistency indicates that the size of the cluster may not have significant effects on the molecular binding energy. However, the interdimer binding configurations of products B and C are less stable by ∼5 and 13 kcal/mol, respectively, compared to that of product A. The energy differences between intradimer and interdimer products may arise from the changes in the site distances. The separations of the two reactive sites across two dimers are 3.8 Å (in one row) and 5.3 Å (two adjacent rows), much larger than that (2.3 Å) in one dimer.47 Thus, interdimer
IV. Conclusions The adsorption of MMA onto Si(100)-2×1 was found to occur in a highly selective pathway, through a [4+2]-like cycloaddition between conjugated CdC/CdO groups and the dangling bonds on Si dimers. The chemisorbed species containing one CdC double bond may be employed as an intermediate for molecule synthesis in a vacuum by reacting with chosen organic functionalities through typical vinyl reactions including addition, cyanoethylation, and polymerization. LA051559L (47) Kubby, J. A.; Boland, J. J. Surf. Sci. Rep. 1996, 26, 61.
