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Bonding Structure of Phenylacetylene on Hydrogen-Terminated Si(111) and Si(100): Surface Photoelectron Spectroscopy Analysis and Ab Initio Calculations ,^
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Masakazu Kondo,*,†,‡ Thomas E. Mates,‡ Daniel A. Fischer,§ Fred Wudl,‡, Edward J. Kramer‡,^,#
and
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† Chisso Corporation, 2-2-1, Otemachi, Chiyoda-ku, Tokyo 100-8105, Japan, §National Institute of Standards and Technology, Gaithersburg, Maryland 20899, ‡Materials Research Laboratory, Department of Chemistry and Biochemistry, ^Materials Department, and #Department of Chemical Engineering, University of California, Santa Barbara, California 93106
Received May 28, 2010. Revised Manuscript Received September 10, 2010 Interfaces between phenylacetylene (PA) monolayers and two silicon surfaces, Si(111) and Si(100), are probed by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and the results are analyzed using ab initio molecular orbital calculations. The monolayer systems are prepared via the surface hydrosilylation reaction between PA and hydrogen-terminated silicon surfaces. The following spectral features are obtained for both of the PA-Si(111) and PA-Si(100) systems: a broad π-π* shakeup peak at 292 eV (XPS), a broad first ionization peak at 3.8 eV (UPS), and a low-energy C 1s f π* resonance peak at 284.3 eV (NEXAFS). These findings are ascribed to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with theoretical analysis results. A conclusion is drawn that the vinyl group can keep its π-conjugation character on the hydrogen-terminated Si(100) [H:Si(100)] surface composed of the dihydride (SiH2) groups as well as on hydrogen-terminated Si(111) having the monohydride (SiH) group. The formation mechanism of the PA-Si(100) interface is investigated within cluster ab initio calculations, and the possible structure of the H:Si(100) surface is discussed based on available data.
Introduction Fabricating self-assembled monolayers (SAMs) on metal/ semiconductor surfaces has been an active research field not only for scientific interest, but also for electronic device applications.1 Among such interface structures, the silicon-organic monolayer system2-4 is considered as one of the best practical choices for the development of high-performance devices. Of different SAM formation strategies, the surface hydrosilylation reaction between molecules containing alkene/alkyne groups and hydrogen-terminated silicon surfaces is especially promising.2-16 The reaction process generates Si-C bonds, forming a SAM architecture on the silicon *Corresponding author. E-mail:
[email protected]. (1) Schreiber, F. J. Phys.: Condens. Matter 2004, 16, R881. (2) Buriak, J. M. Chem. Rev. 2002, 102, 1271. (3) Shirahata, N.; Hozumi, A.; Yonezawa, T. Chem. Rec. 2005, 5, 145. (4) Leftwich, T. R.; Teplyakov, A. V. Surf. Sci. Rep. 2008, 63, 1. (5) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (6) (a) Sieval, A. B.; Vleeming, V.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 1999, 15, 8288. (b) Sieval, A. B.; Linke, R.; Zuilhof, H.; Sudh€olter, E. J. R. Adv. Mater. 2000, 12, 1457. (7) Effenberger, F.; G€otz, G.; Bidlingmaier, B.; Wezstein, M. Angew. Chem., Int. Ed. 1998, 37, 2462. (8) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (9) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 123, 7821. (10) (a) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Wright, A.; Zuilhof, H.; Sudh€olter, E. J. R. Angew. Chem., Int. Ed. 2004, 43, 1352. (b) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers, M.; Th€une, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudh€olter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514. (11) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225. (12) Aswal, D. K.; Koiry, S. P.; Jousselme, B.; Gupta, S. K.; Palacin, S.; Yakhmi, J. V. Physica E 2009, 41, 325. (13) Sano, H.; Maeda, H.; Ichii, T.; Murase, K.; Noda, K.; Matsushige, K.; Sugimura, H. Langmuir 2009, 25, 5516. (14) Qu, M.; Zhang, Y.; He, J.; Cao, X.; Zhang, J. Appl. Surf. Sci. 2008, 255, 2608.
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surface. A large number of studies have shown that the reaction can be initiated by heat,5,6 ultraviolet light,7,8 and/or visible light.9,10 Other alternatives include surface Grignard reactions11 and electrochemical grafting processes.12 The resulting Si-C bonds are stable in acidic/alkaline conditions as well as in the air.5,13 Additionally, when some reaction centers are introduced in the attached molecules, different molecules can be added after the creation of SAMs to fine-tune device functionality.14-16 A schematic reaction pathway for surface hydrosilylation reactions, which is derived from experimental2 and theoretical17-19 evidence, is shown in Figure 1. First, an unpaired electron is generated on a silicon atom as a result of the cleavage of a bond to hydrogen atom by external perturbations, e.g., by heating or by light irradiation. Then, the electron attacks one of the carbon atoms in an unsaturated vinyl/ethynyl group, creating a Si-C bond and a radical on the other carbon atom. The radical molecule subsequently abstracts a hydrogen atom from Si-H bonds adjacent to the reacted Si to form a closed-shell molecule and a new unpaired electron on the Si. The chain reaction continues until the surface silicon radical is deactivated by a termination agent. In the case of alkenes, hydrogen abstraction reactions stop after the first Si-C bond has been created since carbon atoms are fully saturated in the first step. On the other hand, in the case of alkynes, an additional (15) Dietrich, P.; Michalik, F.; Schmidt, R.; Gahl, C.; Mao, G.; Breusing, M.; Raschke, M. B.; Priewisch, B.; Els€asser, T.; Mendelsohn, R.; Weinelt, M.; R€uck-Braun, K. Appl. Phys. A: Mater. Sci. Process. 2008, 93, 285. (16) Huang, K.; Duclairoir, F.; Pro, T.; Buckley, J.; Marchand, G.; Martinez, E.; Marchon, J.-C.; de Salvo, B.; Delapierre, G.; Vinet, F. ChemPhysChem 2009, 10, 963. (17) (a) Takeuchi, N.; Kanai, Y.; Selloni, A. J. Am. Chem. Soc. 2004, 126, 15890. (b) Kanai, Y.; Takeuchi, N.; Car, R.; Selloni, A. J. Phys. Chem. B 2005, 109, 18889. (18) Martı´ nez-Guerra, E.; Takeuchi, N. Phys. Rev. B 2007, 75, 205338. (19) Pei, Y.; Ma, J. J. Phys. Chem. C 2007, 111, 5486.
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Figure 1. (a) Schematic pathway of the hydrosilylation reaction between alkene/alkyne molecules and hydrogen-terminated silicon surfaces. In the case of alkenes, reactions stop after the first Si-C bond is created. In the case of alkynes, the (b) 1-bond structure can form 2-bond structures using the remaining carbon double bonds: the (c) 1,1-bridge and (d) 1,2-bridge structures.
chain reaction can occur in the attached molecules using the vinyl group generated after the first reaction. When the second reaction proceeds, two possible 2-bond structures, 1,1-bridge and 1,2-bridge structures, can be formed depending on which carbon atom is attacked (Figure 1). It is widely believed that the 1-bond structure is dominant at alkyne-Si(111) interfaces. Cicero, Linford, and Chidsey8 observed an infrared (IR) peak characteristic of the vinyl group for 1-octyne and phenylacetylene (PA) SAMs on Si(111). Furthermore, Kellar, Lin, and co-workers20 thoroughly investigated the interface structure of 4-bromo-phenylacetylene on Si(111) using X-ray reflectivity (XRR), X-ray standing wave, and sum frequency generation techniques, and concluded that a π-conjugation character is retained in the system. By contrast, there is controversy regarding the type of molecular bonding structure formed by alkynes and Si(100) surfaces. Sieval and collaborators21 proposed complete formation of 2-bond configurations in 1-alkyne-Si(100) systems through the interpretation of XRR and surface IR spectra. However, Cerofolini et al.22 alleged from their meticulous X-ray photoelectron spectroscopy (XPS) analysis that an appreciable number of alkyne molecules keep the 1-bond configuration. A similar conclusion was also reached by Cossi, Zanoni, and co-workers23 from cyclic voltammograms and theoretical calculations on some ferrocene-silicon systems. Since π-conjugation length substantially changes the electronic properties of molecular monolayer systems, a better understanding of the organic-silicon interface characteristics may be helpful in realizing functionalized electronic components. Our goal in the present study is to provide additional findings on the interface geometry of alkyne-silicon systems. We focus on differences between 1-bond and 2-bond structures by probing the existence of the vinyl group after reaction of PA, which is one of the (20) (a) Kellar, J. A.; Lin, J.-C.; Kim, J.-H.; Yoder, N. L.; Bevan, K. H.; Stokes, G. Y.; Geiger, F. M.; Nguyen, S. T.; Bedzyk, M. J.; Hersam, M. C. J. Phys. Chem. C 2009, 113, 2919. (b) Lin, J.-C.; Kellar, J. A.; Kim, J.-H.; Yoder, N. L.; Bevan, K. H.; Nguyen, S. T.; Hersam, M. C.; Bedzyk, M. J. Eur. Phys. J. Special Topics 2009, 167, 33. (21) Sieval, A. B.; Opitz, R.; Maas, H. P. A.; Schoeman, M. G.; Meijer, G.; Vergeldt, F. J.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 2000, 16, 10359. (22) (a) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L.; Condorelli, G. G.; Fragala, I. L.; Giorgi, G.; Sgamellotti, A.; Re, N. Appl. Surf. Sci. 2005, 246, 52. (b) Cerofolini, G. F.; Galati, C.; Reina, S.; Renna, L. Surf. Interface Anal. 2006, 38, 126. (23) (a) Cossi, M.; Iozzi, M. F.; Marrani, A. G.; Lavecchia, T.; Galloni, P.; Zanoni, R.; Decker, F. J. Phys. Chem. B 2006, 110, 22961. (b) Zanoni, R.; Cossi, M.; Iozzi, M. F.; Cattaruzza, F.; Dalchiele, E. A.; Decker, F.; Marrani, A. G.; Valori, M. Supperlatt. Microstruct. 2008, 44, 542. (24) Saito, N.; Hayashi, K.; Sugimura, H.; Takai, O. Langmuir 2003, 19, 10632.
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well-investigated molecular systems in SAMs on silicon.8,20,24,25 In a low molecular coverage regime, configurations can be determined by height imaging;26 in a densely packed layer case, however, various experimental/theoretical data are required to investigate the SAM structure. The chemical group affects the conjugation length and, therefore, can change electron-excitation features in which π and/or π* orbitals, especially the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), take part. As a means to address PA-silicon interface structures, we utilize photoelectron/photoabsorption spectroscopy methods: XPS,27 ultraviolet photoelectron spectroscopy (UPS),28 and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.29 These techniques give us complementary information from different electron excitation mechanisms: XPS obtains knowledge about the local bonding structures of the system, UPS observes occupied valence electronic states of surface molecules, and NEXAFS contains information on unoccupied electronic states below the ionization edge. We also employ quantum chemical calculations to gain understanding of our spectroscopic data and the electron excitation properties in the PA-silicon surface.
Experimental Section Materials. Single-side-polished, boron-doped Czochralski-
grown silicon wafers [resistivity = 1-10 Ω cm (Si(100) wafer); 525 Ω cm (Si(111) wafer)] were purchased from Universitywafer. com (South Boston, MA). Phenylacetylene (PA, Acros Organics, 98%) was used as received. Acetone (Fischer), isopropanol (Fischer), toluene (Fischer), and a buffered HF solution (Buffer HF Improved, Transene Company, Inc.) were also used as received. Fabrication of Phenylacetylene Monolayers. Unless otherwise noted, silicon surfaces were dried with a flow of nitrogen after each solvent rinsing/cleaning process. Before HF etching, pieces of cut wafers were washed ultrasonically with acetone and isopropanol for 3 min each. Then, the wafers were treated in a PR-100 UV-ozone reactor (Ultra-Violet Products, Upland, CA) for 10 min and rinsed with deionized (DI) water. Next, the cleaned wafers were immersed into a buffered HF solution to remove surface native oxide layers. Ten minutes later, which is enough time to remove several-nanometers-thick oxide layers, the wafers were taken out from the solution and subsequently washed with copious amounts of DI water. The hydrogen-terminated silicon wafers were immersed in neat liquid PA, and then, the system was heated to 150 C for 3 h in a nitrogen atmosphere. Nitrogen was bubbled through the liquid PA for at least 30 min to reduce the concentration of oxygen before the immersion of the wafers. After the thermal reaction, the samples were ultrasonically cleaned several times with toluene and acetone for 5 min each and rinsed with DI water. Using a set of experimental techniques, we confirmed that PA monolayers were successfully fabricated on the hydrogenterminated silicon surfaces (see the Supporting Information). Fourier Transfer Infrared (FT-IR) Spectroscopy. FT-IR measurements were carried out in the attenuated total reflection (ATR) mode with a Nicolet Magna 850 FT-IR spectrometer (Thermo Scientific, Waltham, MA) combined with a GATR accessory (Harrick Scientific Products, Inc., Pleasantville, NY) in order to probe the surface structures of hydrogen-terminated silicon surfaces. Data were collected 2048 times with a resolution of 4 cm-1. UV/ozone-treated wafers before HF etching were used as references. (25) Hiremath, R. K.; Mulimani, B. G.; Rabinal, M. K.; Khazi, I. M. J. Phys.: Condens. Matter 2007, 19, 446003. (26) Pluchery, O.; Coustel, R.; Witkowski, N.; Borensztein, Y. J. Phys. Chem. B 2006, 110, 22635. (27) H€ufner, S. Photoelectron Spectroscopy; Springer: Berlin, 2003. (28) Zahn, D. R. T.; Gavrila, G. N.; Salvan, G. Chem. Rev. 2007, 107, 1161. (29) St€ohr, J. NEXAFS Spectroscopy; Springer: Berlin, 1996.
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X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed using a Kratos Axis Ultra XPS/UPS instrument (Manchester, UK) with monochromated aluminum KR radiation (hν = 1486.6 eV) under ultrahigh-vacuum conditions (10-8 Torr). For survey scans, spectra were collected at the beam intensity of 225 W (15 kV 15 mA) with the pass energy of 80 eV. Two scans were taken and averaged with a step size of 0.5 eV and a dwell time of 100 ms. High-resolution scans were also done for Si 2p, C 1s, and O 1s states in order to study interface structures in detail. In the measurements, the beam intensity was set to 270 W (15 kV18 mA). A pass energy of 10 (20) eV was used for Si 2p (C 1s and O 1s) scans. Eight (for Si 2p) and four (for C 1s and O 1s) scans were carried out with a step size of 0.05 eV and a dwell time of 300 ms. Binding energies for the spectra were calibrated so that the energy position of the Si 2p3/2 state in silicon is 99.40 eV. All the measurements were done at the takeoff angle of 0 with respect to the surface normal. Ultraviolet Photoelectron Spectroscopy (UPS). UPS measurements were made on the same Kratos instrument with helium I radiation (hν = 21.22 eV) and a gate bias voltage of -9 V. Photoelectrons at the 0 takeoff angle were collected at 90 W (6 kV 15 mA) with a pass energy of 10 eV, a step size of 0.05 eV, and a dwell time of 300 ms. Scans were repeated 4 times. Near-Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy. NEXAFS spectroscopy was performed at the NIST/Dow endstation of beamline U7A at the National Synchrotron Light Source at Brookhaven National Laboratory. Photoelectrons (Auger electrons) were collected in the partial electron yield (PEY) mode with an entrance grid bias of -150 V. In our experimental setup, the polarization factor is 0.85. Data were collected with a 0.1 eV energy step and a 1 s dwell time. The raw NEXAFS spectra were divided by the photocurrent from a clean gold mesh placed after the monochromator but before the sample. The data collected were normalized by subtracting a pre-edge background (obtained by averaging 278-282 eV data), followed by the division by the PEY value at 320 eV to provide postedge normalization. This data conversion allows us to compare NEXAFS data for the PA-Si(100) system with that for PA-Si(111). Ab Initio Calculations. The GAMESS program package30 was employed for all the molecular orbital (MO) calculations. Below are the details of the computational process. Model Systems. In the present study, hydrogen terminated Si(111) and Si(100) systems were modeled by cluster molecules in which 26 and 29 Si atoms are included, and all the silicon dangling bonds were terminated with hydrogen atoms. One PA molecule was attached to a proper position of one of the top Si atoms to depict molecular attachment. The electronic structures of two small molecules, styrene (smallest model of the 1-bond structure) and ethylbenzene (a model of the 2-bond structures), were calculated to allow us to compare valence electron excitation features. Geometry Optimization. Structural parameters of molecules and molecular clusters were optimized within density functional theory (DFT), where the hybrid Hartree-Fock/DFT B3LYP functional31-34 and the 6-31G(d) basis set35,36 were chosen. Adsorption energies were estimated from an energy difference between the PA-silicon clusters and the independent PA/silicon cluster molecules. In the energy diagram calculations for radical chain reactions, the unrestricted DFT formalism was employed, (30) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347. (31) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (32) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (33) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (34) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (35) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (c) Hariharan, P. C.; Pople, J. A. Theoret. Chim. Acta 1973, 28, 213. (36) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163.
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Kondo et al. where the doublet spin state was assumed. Negligible spin contamination was observed for all of the systems with S2 values between 0.75 (theoretical value) and 0.79. Vibrational analyses were carried out to confirm that the optimized structures are essentially in a local minimum (no negative frequencies) or a transition state (one negative vibrational frequency). More accurate computations at the B3LYP/6-311þG(d,p)37-39 level were performed using the optimized structures at B3LYP/6-31G(d). Valence Electron Excitation. To evaluate the valence excitation features, time-dependent DFT (TDDFT)40,41 calculations were done on styrene (small model of the 1-bond structure) and ethylbenzene (a model of the 2-bond structures) at the B3LYP/ 6-311þG(d,p)//B3LYP/6-31G(d) level of theory. One hundred excited states were generated for each molecule, where singlet transitions only were considered. Theoretical NEXAFS Spectra. Single-point energy calculations were conducted on molecules in the ground singlet and coreionized doublet states in order to estimate core-ionization energies. The restricted and the restricted-open Hartree-Fock/DFT formalisms were employed in the calculations of singlet and doublet states, respectively. A set of basis functions, hereafter referred to as “Bas1,” was used for the purpose, where “Bas1” means IGLO-III42 for the excited carbon atoms; the effective-core potential of Stevens et al.43 and (31)-contracted double-ζ basis functions with a d-type polarization function and an s/p-type diffuse function for the other carbon atoms and oxygen; and a scaled 31G function of Pople et al.35 with a p-type polarization function for hydrogen. Coefficients and exponents for IGLO-III and hydrogen’s 31G basis sets were taken from the Environmental Molecular Sciences Laboratory Basis Set Exchange Library website.44,45 We found that the two computational approaches gave similar energies for hydrocarbon systems (see the Supporting Information). NEXAFS spectra on some of PA-Si systems were predicted in the static exchange (STEX) approximation.46-48 The reliability of this approach in NEXAFS data interpretation has been proven by previous application studies.28,48,49 Using MOs in the ground and core-ionized states at the Hartree-Fock/Bas1 level, STEX wave functions (STEX MOs) and their eigenvalues (orbital energies) were constructed according to the procedure of A˚gren and collaborators,48 and then oscillator strengths were estimated by using core orbitals in the ground state and the STEX MOs. It is well-known that the STEX approach overestimates core-hole transition energies by several electron volts.48,49 From preliminary tests using small molecules, we confirmed that energies comparable to the experimental ones are obtained by uniformly subtracting 1.5 eV from the computed excitation energies. To do the calculations, several subroutines were implemented in the original codes. (37) Krishnan, R.; Binkley, J. S.; Seegar, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (38) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (39) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (40) Runge, E.; Gross, E. K. U. Phys. Rev. Lett. 1984, 52, 997. (41) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem. Phys. 1998, 108, 4439. (42) Kutzelnigg, W.; Fleischer, U.; Schindler, M. In NMR Basic Principles and Progress; Diehl, P., Fluck, E., G€unther, H., Kosfeld, R., Seelig, J., Eds.; Springer: Berlin, 1991; Vol. 23, p 165. (43) (a) Stevens, W. J.; Basch, H.; Krauss, M. J. Chem. Phys. 1984, 81, 6026. (b) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem. 1992, 70, 612. (c) Cundari, T. R.; Stevens, W. J. J. Chem. Phys. 1993, 98, 5555. (44) Feller, D. J. Comput. Chem. 1996, 17, 1571. (45) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Modeling 2007, 47, 1045. (46) Hunt, W. J.; Goddard, W. A., III Chem. Phys. Lett. 1969, 3, 414. (47) (a) Huzinaga, S. J. Chem. Phys. 1969, 52, 3971. (b) Huzinaga, S.; Arnau, C. J. Chem. Phys. 1971, 54, 1948. (c) Huzinaga, S.; McWilliams, D.; Cantu, A. A. Adv. Quantum Chem. 1973, 7, 187. (48) (a) A˚gren, H.; Carravetta, V.; Vahtras, O.; Pettersson, L. G. M. Chem. Phys. Lett. 1994, 222, 75. (b) Triguero, L.; Pettersson, L. G. M.; Ågren, H. Phys. Rev. B 1998, 58, 8097. (49) Oji, H.; Mitsumoto, R.; Ito, E.; Ishii, H.; Ouchi, Y.; Seki, K.; Yokoyama, T.; Ohta, T.; Kosugi, N. J. Chem. Phys. 1998, 109, 10409.
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Figure 2. ATR FT-IR spectra for (a) H:Si(111) and (b) H:Si(100).
Results and Discussion Structure of Hydrogen-Terminated Silicon Surface. ATR FT-IR spectra for hydrogen-terminated Si(111) [H:Si(111)] and Si(100) [H:Si(100)] surfaces are shown in Figure 2. For H:Si(111), the most intense peak appears at 2084 cm-1, which corresponds to the stretching vibration of monohydride (tSiH) species.50-52 Peak intensities at ∼2100 cm-1 and ∼2140 cm-1, corresponding to Si;H stretching modes in dihydride (dSiH2) and trihydride (;SiH3) species, are weaker, indicating that the surface is predominantly composed of the monohydride structure. In contrast, for H:Si(100), the highest peak exists at 2102 cm-1 and 2112 cm-1, showing that the H:Si(100) surface is mainly composed of the dihydride species. The two peaks are due to the symmetric and antisymmetric Si-H vibration of the dihydride group, respectively. Furthermore, the peak intensity in the 2200-2300 cm-1 range that features Si-H stretching vibrational modes with oxygen in the backbones (oxidized configurations: HnSiO(4-n), n representing the number of chemical bonds)53,54 is much weaker than that in the 2050-2150 cm-1 range. This finding demonstrates that a large amount of the surface region with intermediate oxidation states is not formed. However, our FT-IR spectra are broad compared with those of Higashi et al. [for H:Si(111)]50 and Cerofolini et al. [for H:Si(100)],22b suggesting that the hydrogenterminated silicon surfaces are not atomically flat. In the present study, therefore, we focus more on the local bonding structure than on long-range ordering. We can discuss our experimental and theoretical data in this sense, whereas our NEXAFS data imply that the PA molecules have some orientational order. Structure and Stability of Phenylacetylene-Silicon Interfaces. We discuss here the stability of PA-Si structures using cluster models in order to evaluate the most stable interface configuration for each system. Taking into account the FT-IR data above, we consider monohydride and dihydride structures as the local PA-Si(111) and PA-Si(100) interface geometries, respectively. Optimized geometries of the cluster model systems are shown in Figure 3. When PA creates a Si-C bond on silicon surfaces with a bond length of 1.87-1.88 A˚; the bond length of the ethynyl group is elongated from 1.20 A˚ to 1.35 A˚, a typical carbon-carbon double bond length. For our models, the phenyl
ring stands nearly perpendicular to the surface as reported in some computational studies.17 However, there is a possibility that molecules may be tilted in the actual situation depending on packing patterns between PA molecules.20 Molecular orientations will be further investigated by means of NEXAFS. The formation of another Si-C bond changes the adsorption structure of PA in the following aspects. First, the bond length of the ethynyl group is further extended by 0.2 A˚ compared with that in the 1-bond structure, reaching 1.54-1.58 A˚ and leading to a reduction in conjugation length. Second, the Si-C bond lengths for the 2-bond structures (1.93-1.99 A˚) concomitantly become longer than those for the 1-bond ones (1.87-1.88 A˚). For the 2-bond configurations, the phenyl group is inclined at an angle of ∼50 relative to the surface. Calculated binding energies are listed in Table 1. For all systems, the values are negative, so the attachment of PA molecules onto hydrogen-terminated silicon surfaces is energetically favorable. The relative stability depends strongly on the surface structure. For the PA-Si(111) system, both of the 2-bond structures are less stable than the 1-bond structure. In particular, the 1,1-bridge structure is quite unstable with an energy difference of 20 kcal/mol compared to the 1-bond structure, meaning that the creation of the 1,1-bridge configuration on Si(111) is very unlikely on thermodynamic grounds. These findings reinforce the X-ray data interpretation of Kellar et al.20 that ruled out the creation of 2-bond configurations for similar systems at room temperature. A reason for the instability is that generating a second Si-C bond requires deformation of the atomic configuration on the Si(111) surface, destroying the sp3 character of a bond in solid Si. The stability of the 1,2-bridge structure is relatively close to that of the 1-bond structure with an energy difference of ∼3 kcal/mol. This indicates that the 1,2-bond configuration may be constructed if a second Si-C bond creation reaction can occur in the system. On the other hand, for the PA-Si(100) system, the 2-bond structures are more stable than the 1-bond one. The result supports the computational data of Sieval et al.21 obtained using similar model systems. In our calculations, the energy gain due to bond-formation reactions is estimated to be about 15 and 12 kcal/mol for the 1,1-bridge and 1,2-bridge structures, respectively. Due to a small energy difference for the two systems, both of the 2-bond structures can be generated unless some kinetic barrier exists in the reaction pathways. Surface Coverage Analysis. In the following, we will exhibit our spectroscopy results and consider the structure of the PA layers. First, we take a look at the surface coverage of the PA-Si systems. Table 2 lists the results of XPS atom component analysis in the systems. The values as well as ellipsometry and XRR data (Supporting Information) show that PA monolayers (∼1 nm) are successfully fabricated. The surface coverage ratios of PA and oxygen can be calculated by Table 2 and the use of the following equations:8 NPA NSi, surf ¼
(50) Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Raghavachari, K. Appl. Phys. Lett. 1990, 56, 656. (51) (a) Arima, K.; Endo, K.; Kataoka, T.; Oshikane, Y.; Inoue, H.; Mori, Y. Appl. Phys. Lett. 2000, 76, 463. (b) Endo, K.; Arima, K.; Hirose, K.; Kataoka, T.; Mori, Y. J. Appl. Phys. 2002, 91, 4065. (52) Sakaue, H.; Taniguchi, Y.; Okamura, Y.; Shungubara, S.; Takahagi, T. Appl. Surf. Sci. 2004, 234, 439. (53) Miura, T.; Niwano, M.; Shoji, D.; Miyamoto, N. J. Appl. Phys. 1996, 79, 4373. (54) Michalak, D. J.; Amy, S. R.; Esteve, A.; Chabal, Y. J. J. Phys. Chem. C 2008, 112, 11907.
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ðIC =sC Þ dPA λSi, Si FSi exp½- dPA =ðλSi, PA cos θÞ ðISi =sSi Þ λC, PA NC NSi, surf 1 - exp½- dPA =ðλC, PA cos θÞ
ð1Þ
NO ðIO =sO Þ λSi, Si FSi exp½- dPA =ðλSi, PA cos θÞ ¼ ðISi =sSi Þ NSi, surf exp½- dPA =ðλO, PA cos θÞ NSi, surf
ð2Þ
where NSi,surf is the surface atom number density (0.078 A˚-2), FSi is the number of atoms per unit volume (0.050 A˚-3), dPA is the monolayer thickness (∼1 nm), NC is the number of carbon atoms DOI: 10.1021/la103208n
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Figure 3. Optimized geometries of cluster models of (a-c) PA-Si(111) and (d-f) PA-Si(100) interfaces. The color scheme of the atoms is as follows: white, hydrogen; black, carbon; and blue, silicon. Bond lengths between carbon and silicon and the carbon-carbon bond length of the ethynyl group are shown in A˚. The models were drawn with the MacMolPlt software.55 Table 1. Calculated Binding Energies (in kcal/mol) for the 1-Bond, 1,1-Bridge, and 1,2-Bridge Structures in the PA-Si(111) and PA-Si(100) Systems PA-Si(111) level 1a
level 2b
PA-Si(100) level 1a
level 2b
1-bond -42.20 -37.33 -42.15 -37.10 1,1-bridge -21.52 -15.56 -57.70 -52.10 1,2-bridge -39.60 -34.11 -54.78 -49.16 a B3LYP/6-31G(d)//B3LYP/6-31G(d). b B3LYP/6-311þG(d,p)//B3LYP/ 6-31G(d).
Table 2. Atomic Compositions of the PA-Si(111) and PA-Si(100) Systems Evaluated by XPS Survey Analysesa PA-Si(111) component (atom %)b
PA-Si(1) component (atom %)b
Si (Si 2p) C (C 1s) O (O 1s) Si (Si 2p) C (C 1s) O (O 1s) 63 30 7 63 31 6 a For more data, see the Supporting Information (Table S2). b Relative sensitivity factors with respect to carbon calibrated on our XPS instrument are 1.2 (silicon) and 2.2 (oxygen).
in the molecule (8 for PA), λA,B is the inelastic mean free path (attenuation length) of a core electron of element A through media B (λSi,Si = 16 A˚, λC,PA = 36 A˚, λSi,PA = 41.5 A˚, λO,PA = 28 A˚), and the IA/sA value, in which IA and sA are the counts of photoelectrons and the element-specific sensitivity factor for A, is proportional to the atom contribution in the system. For both the PA-Si(111) and PA-Si(100) systems, calculated surface coverage of PA is ∼0.5, indicating that PA molecules construct a densely packed monolayer. In the present analysis, NO/NSi,surf was estimated to be less than 0.9-1.1 (∼1 atom % of oxygen is from (55) Bode, B. M.; Gordon, M. S. J. Mol. Graphics Mod 1998, 16, 133.
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adventitious contamination8,22,56-58 and adsorbed species will also contribute to the intensity). From the value and the coverage ratio for carbon (0.5), we expect that the majority of the remaining surface silicon atoms exist in the form of suboxide with the oxidation states of Si2þand Si1þ. High-Resolution XPS Spectra. To examine the surface bonding structure in detail, high-resolution XPS scans were conducted for the Si 2p, C 1s, and O 1s states. XPS spectra in the Si 2p state in PA-Si(111) and PA-Si(100) are given in Figure 4a,b. Two peaks located at 99.4 and 100.0 eV are the Si 2p doublet in the Si0 state (solid silicon). A photoelectron peak at 103 eV associated with oxidized silicon (Si4þ) is low in intensity even after the thermal reaction, so the silicon surface is presumably stabilized by PA. XPS spectra for C 1s excitation in the PA-Si(111) and PA-Si(100) systems illustrated in Figure 4c,d corroborate this prediction. Both of the spectra include a strong core-ionization peak at ∼285 eV and a shakeup peak at ∼292 eV due to a simultaneous occurrence of core excitation and π-π* transitions. We successfully decomposed the main peak into 4 Gaussian/ Lorentzian functions, the positions of which are determined to be 284.0/284.1 eV, 284.9/284.8 eV, 285.5/285.4 eV, and 286.5/286.6 eV for PA-Si(111)/PA-Si(100). The strongest peak comes from the core ionization of the aromatic carbon atoms in PA, and the lowest energy peak is assigned to core ionization from the carbon atom bonded to the silicon surfaces. The energy split of 0.70.9 eV, which is explained as a chemical shift due to a difference in electronegativity (C, 2.6; Si, 1.9), is in good agreement with computed ionization energy differences (Table S4, Supporting (56) Scheres, L.; Achten, R.; Giesbers, M.; de Smet, L. C. P. M.; Arafat, A.; Sudh€olter, E. J. R.; Marcelis, A. T. M.; Zuilhof, H. Langmuir 2009, 25, 1529. (57) Scheres, L.; Arafat, A.; Zuilhof, H. Langmuir 2007, 1923, 8343. (58) Wallart, X.; de Villeneuve, C. H.; Allongue, P. J. Am. Chem. Soc. 2005, 127, 7871.
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Figure 4. (a,b) High-resolution Si 2p XPS spectra for (a) PA-Si(111) and (b) PA-Si(100). (c,d) High-resolution C 1s XPS spectra for (c) PA-Si(111) and (d) PA-Si(100). (e,f) High-resolution O 1s XPS spectra for (e) PA-Si(111) and (f) PA-Si(100). The CasaXPS software (Casa Software Ltd.)62 was used for data analysis. Table 3. Peak Decomposition Results for the PA-Si Systems PA-Si(111)
PA-Si(100)
assignment
position (eV)
contribution (%)
position (eV)
contribution (%)
C-C C-Si C-O shake-up
284.9 (285.6) 284.1 286.7 291.7
64 (18) 9 3 6
284.8 (285.4) 284.1 286.6 291.7
65 (15) 9 2 9
Information). The 286.5 eV contribution is considered an adventitious creation of C-O bonds, which is often found in XPS spectra for similar systems.22,56-58 The other small component at 285.5 eV may indicate the existence of surface contamination59 or the contribution of small asymmetry in the main peak.60 XPS spectra for O 1s excitation in the PA-Si(111) and PA-Si(100) systems are given in Figure 4e,f. The spectra can be divided into two components whose energy positions are 532.0 and 532.6 eV, respectively. From the peak position, the lower-energy peak is assigned to core excitation from the Si-O group.61 Since the 103 eV peak intensity is very low in the Si 2p spectra (Figure 4a,b), partially oxidized silicon such as Si1þ (100.2 eV) and Si2þ (101.1 eV) states are more likely as suboxide. The other component is associated with the C-O group, as expected from the C 1s XPS spectra. Shown in Table 3 is a more quantitative analysis result of the C 1s XPS peak. The peak decomposition indicates that a nonnegligible number of C-O bonds exist in the PA monolayer, as pointed out in previous experimental studies.22,56-58 The structure of the adventitious C-O bond is not fully elucidated, although the contamination is detected in many experiments. A plausible explanation would be that it is generated when a water and/or oxygen molecule in the reaction system is attacked by a radical electron on the vinyl/ethynyl carbon that does not possess a C-Si bond (Figure 1). In our case, water is more probable as a reaction species due to the high-temperature (150 C) condi(59) Sohn, K. E.; Dimitriou, M. D.; Genzer, J.; Fischer, D. A.; Hawker, C. J.; Kramer, E. J. Langmuir 2009, 25, 6341. (60) Beamson, G.; Clark, D. T.; Kendrick, J.; Briggs, D. J. Electron. Spectrosc. Relat. Phenom. 1991, 57, 79. (61) Dreiner, S.; Sch€urmann, M.; Westphal, C.; Zacharias, H. Phys. Rev. Lett. 2001, 86, 4068.
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tion and the bubbling treatment before the reaction. Another possibility may be the creation of a Si-O-C framework between a suboxide (Si-OH) and the vinyl/ethynyl group; however, the mechanism would be less likely because the O-H bond dissociation energy (BDE) is higher than the Si-H BDE; for example, theoretical BDEs in silanol are 119.3 kcal/mol for O-H and 92.6 kcal/mol for Si-H.63 Consequently, the adventitious C-O group should exist in the form of a Si-C-C-OH structure. Additionally, with the help of quantum chemical calculations, we assign the adventitious C-O peak predominantly to the core ionization of monohydroxyl carbon atoms [-CH(OH)-]; BDE shift values in monohydroxyl and dihydroxyl carbon atoms [-C(OH)2-] relative to one in the methyl carbon are ∼1.6 eV and ∼3.2 eV (Table S5 in the Supporting Information), corresponding binding energies being 286.6 and 288.2 eV, respectively (285 eV is a typical core ionization energy for hydrocarbon systems). Under the assumption discussed above and from the peak intensity ratio between the C-Si and the C-O peaks, we estimate that about 1/3 of the PA molecules contain a monohydroxyl carbon atom; the amount is comparable with that of Cerofolini et al.22b XPS Shakeup Feature. The main C 1s XPS peak indicates the existence of Si-C bonds, but it is difficult to extract the type of bonding structure (1-bond or 2-bond). As shown in Figure 4, we observe a broad peak at ∼292 eV, which is assigned to a shakeup peak, for both of the PA-Si systems. This shakeup feature originates from concomitant π-π* transitions in the core excitations (62) Fairley, N.; Carrick, A. The Casa Cookbook; Acolyte Science: Cheshire, 2005. (63) Lucas, D. J.; Curtiss, L. A.; Pople, J. A. J. Chem. Phys. 1993, 99, 6697.
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Figure 5. (a) Shakeup peaks for the PA-Si systems. A shakeup feature for polystyrene is also shown as a reference. Calculated valence-electron excitation spectra for (b) styrene and (c) ethylbenzene within TD-B3LYP/6-311þG(d,p)//B3LYP/6-31G(d).
of aromatic molecules. Since differences in π-conjugation length change the energy position and shapes of π electronic states, important structural information may be gained regarding the bonding structure in the PA SAM systems. Figure 5a shows the magnified shakeup spectra of the PA-Si systems, in which that for polystyrene64 is added as a reference. Fitting the shakeup peaks with one symmetric product Gaussian/ Lorentzian function gave full width at half-maximum (fwhm) values of 3.4 eV [for PA-Si(111)] and 4.1 eV [for PA-Si(100)]. These values are larger than that of polystyrene having a fully saturated ethyl structure (fwhm = 2.1 eV). Because the π-conjugation structure for the 2-bond configurations is similar to that for polystyrene, the shakeup features for the PA-Si system should be similar to those of polystyrene if the majority of PA molecules have 2-bond structures. However, the PA-Si(100) and PA-Si(111) systems give a rather broad shakeup structure. Considering that the 1-bond structure should be the majority one for the PA-Si(111) system,20 the result implies that a considerable number of the PA molecules will have the 1-bond configuration even at the PA-Si(100) interface. To interpret the spectral shape at a molecular level, we theoretically analyze an electron-transition mechanism within TDDFT. Computed TDDFT spectra for styrene (model of the 1-bond structure) and ethylbenzene (model of the 2-bond structures) are presented in Figure 5b,c. There exist several strong peaks in a wide energy range of 5-8 eV for styrene (corresponding to the 1-bond structure), which predict the broad shakeup feature observed for the PA-Si systems. On the other hand, the majority of the strong peaks exist in a narrower energy range (6.5-7.0 eV) for ethylbenzene (2-bond structures/polystyrene). MO analysis reveals that the strong peaks are mainly due to π-π* transitions (some of the peak assignments are given in the Supporting Information). The difference suggests that the lack of π-conjugation lowers the width of the shakeup peak; in fact, the fwhm value for polystyrene is relatively small and the peak maximum is higher than those for the PA-Si systems. Also, an estimate of the shakeup peak of ∼(285 þ 7) = 292 eV is comparable to the experimental shakeup peak position. (64) The polystyrene samples were prepared by heating hydrogen terminated silicon wafers in a 10 wt % styrene/xylenes solution (styrene, Sigma Aldrich; xylenes, Fisher) at 150 C for 10 h followed by sonication with toluene and acetone (5 min each). XPS survey spectra exhibit a lower Si 2p intensity (atomic ratio: Si, 25%; C, 69%; O, 6%) compared to those for PA-Si (Si, 63%; C, 30%; O, 7%), indicating that a thin polystyrene layer was fabricated.
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Figure 6. Orbital interactions in styrene. White and black circles represent π atomic orbitals with different phases.
The shakeup feature is qualitatively understood in terms of the concept of MO interactions.65 Figure 6 illustrates a MO interaction scheme between π fragment MOs (FMOs) of benzene and those of ethylene in styrene. As a result of orbital interactions between one of the doubly degenerate HOMOs of benzene (h1) and the HOMO of ethylene (he), two different MOs, H and H-2, are generated. MOs H-2 and H are the bonding and antibonding combinations of the two FMOs, respectively, and have large energy splitting. The other HOMO of the benzene fragment, h2, rarely interacts with π MOs of the ethylene component due to a zero MO coefficient on the carbon atom to which the ethylene structure is connected. So, the MO level of H-1 is close to that of h2 and lies in between H and H-2. In the case of ethylbenzene, on the other hand, both of the HOMOs of the benzene fragment do not significantly interact with the σ FMOs because of a large energy difference between the two FMOs; therefore, the extent of the energy splitting between MOs corresponding to h1 and h2 is small. The energy level splitting is explained in the same fashion also in the LUMO case. The resulting orbital energy diagram leads to a broader range of π-π* absorption. UPS. UPS spectra for the PA-Si(111) and PA-Si(100) systems are reported in Figure 7a. The spectrum for a polystyrene sample is also included in the figure for comparison. We see two peaks, A and B, at the binding energy (BE) of 3.8 and 6.2 eV, respectively, for both of the PA-Si systems. The spectra for the PA-Si systems have essentially the same line shape; however, for polystyrene, peak A is sharp compared to those for the PA-Si systems. Energy differences between the position of the peak maximum and the lower-energy position giving the halfmaximum are 0.6 eV for polystyrene, 1.0 eV for PA-Si(111), and 1.0 eV for PA-Si(100). We attribute the broadness of peak A in PA SAMs on Si to a peculiar electronic structure rather than background from the silicon substrates, since no clear peak is observed in UPS spectra in 1-alkenes on Si(111) in the binding energy range 0.0-4.5 eV.66 The spectral difference between the PA-Si systems and polystyrene indicate that molecular structures (65) Albright, T. A.; Burdett, J. K.; Whangboo, M.-H. Orbital Interactions in Chemistry; Wiley: New York, 1985. (66) Segev, L.; Salomon, A.; Natan, A.; Cahen, D.; Kronik, L.; Amy, F.; Chan, C. K.; Kahn, A. Phys. Rev. B 2006, 74, 165323.
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Figure 7. (a) Experimentally obtained UPS spectra for the PA-Si systems and polystyrene on Si. (b) Density of occupied states (DOOS) of styrene. The curved line was generated by broadening MO levels at B3LYP/6-311þG(d,p)//B3LYP/6-31G(d) with a fwhm value of 1.2 eV. The GaussSum software (v 2.1.6)68 was used for the manipulation. Vertical bars correspond to the energy positions of valence MOs. Symbols H, H-1, and H-2 are the abbreviations of the HOMO, HOMO-1, and HOMO-2, respectively. (c) DOOS of ethylbenzene. The spectrum was generated in the same manner as (b). A fwhm value for peak A in polystyrene is estimated to be ∼0.6 2=1.2 eV (see text). The fwhm value of 1.0-1.2 eV is reasonable in our experimental setup.
at the molecule-silicon interface may be similar to each other for PA-Si, but different from that of polystyrene (2-bond structures) as inferred from the above XPS results. To garner more in-depth insight into the UPS spectra, we performed density of occupied states (DOOS) analyses for styrene and ethylbenzene. From Koopmans’ theorem, the DOOS spectra are related to the valence ionization properties; previous studies have proven that this kind of approach gives us important structural information on molecular thin film systems.28 The electronic structure of molecular films is determined mainly by intramolecular (orbital) interactions, and weak intermolecular (van der Waals) interactions do not significantly perturb the electronic states of the molecules. Hence, valence electronic properties of SAMs are close to those of the individual molecules in a first approximation. Computed DOOS spectra for styrene and ethylbenzene are presented in Figure 7b,c, respectively. The spectral features are in agreement with those for styrene/ethylbenzene reported by Ranke and Weiss,67 corroborating that aromatic (PA) molecules are attached on the silicon surfaces. Although Figure 7c has nearly an identical line shape to Figure 7b, we notice two different aspects between these two spectra in the lowest energy peak. One is that the highest-energy peak for ethylbenzene is sharper than that for styrene. This is rationalized from the energy difference between the HOMO and HOMO-1; the HOMO-1 lies closer to the HOMO in ethylbenzene (0.2 eV) than in styrene (0.8 eV). The other is the depth of a dip at -8 eV; the spectrum of ethylbenzene has a deeper dip than that of styrene because of the energy gap between HOMO-1 and HOMO-2 (2.1 eV for ethylbenzene; 1.2 eV for styrene). Figure 7b reproduces especially well the experimental line shapes of the PA-Si system, and Figure 7c is similar in shape to the UPS spectrum of polystyrene, which supports the possible existence of the 1-bond configuration at the PA-Si interface. NEXAFS. NEXAFS spectroscopy gives complementary findings about the molecular structure at the surface from information on the unoccupied MOs, in contrast to UPS that mirrors the valence electronic structure. Furthermore, the average molecular (67) Ranke, W.; Weiss, W. Surf. Sci. 2000, 465, 317.
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Figure 8. Computed core-hole excitation properties for PASi(100) having the (a) 1-bond and (b) 1,1-bridge configurations. The magic angle, at which an incidence-angle independent NEXAFS spectrum is obtained, is assumed in the calculation of oscillator strengths. Red vertical bars represent the oscillator strengths for the carbon atom directly attached to the silicon surface. Contributions from the other carbon atoms are shown in green bars. MOs inserted in the graphs are STEX eigenvectors (STEX MOs) for specific peaks X, Y, and Z. Theoretical NEXAFS spectra (blue lines) were generated by broadening the oscillator strengths with two kinds of Gaussian fwhm values: in the energy range less than the core ionization energies (IEs), we used a constant value of 0.6 eV, and above the IEs, a linear function [0.3414 (Photon Energy) - 95.106] eV proposed by Outka and St€ ohr70 was used.
orientations can be evaluated from the incidence angle dependence of the NEXAFS spectra.29,69 Before discussing experimental NEXAFS data, we show theoretical NEXAFS spectra for the PA-Si(100) clusters in order to comprehend differences in NEXAFS spectral features between the 1-bond and 2-bond structures. Computed core-excitation spectra for the two structures are presented in Figure 8. Since the 1,2-bond configuration gave almost the same spectral shape as the 1,1-bridge one, only the result for the 1,1-bridge structure is presented. For the 1-bond geometry, we see a shoulder (peak X) at 284.5 eV and a strong peak (peak Y) at 285.5 eV. Corresponding eigenvectors (STEX MOs) reveals that both of X and Y are due to electron excitations from the carbon 1s atomic orbital to a delocalized LUMO-like π* orbital; more specifically, core excitations from the carbon atom forming C-Si bonds (Cv1) lead to peak X and those from the other carbon atoms contribute to peak Y. A reason for this peak split is a chemical shift in the core ionization energy (IE); IE for Cv1 (288.9 eV) is energetically lower than those for the other carbon atoms (289.8-290.2 eV) because of the lower electronegativity in silicon. On the other hand, for the 1,1-bridge structure, there exists one sharp peak at 285.8 eV (peak Z) ascribed to the C 1s f π* LUMO resonances, but no peak at 284.5 eV. This result is reasonable considering that the Cv1 atom is fully saturated and the atomic orbitals do not participate in the π* orbitals. Peak X is peculiar to the 1-bond configuration, in contrast to the 284 eV shoulder in the XPS spectra that may be observed for the 2-bond structures as well as the 1-bond structure. The peak corresponding to X therefore helps us judge structural differences (1-bond or 2-bond structures) in the system. Now, we turn to experimental core-hole excitation properties in the PA-Si systems. Figure 9a,b illustrates NEXAFS spectra (68) O’Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. J. Comput. Chem. 2007, 29, 839. (69) (a) DeLongchamp, D. M.; Sambasivan, S.; Fischer, D. A.; Lin, E. K.; Chang, P.; Murphy, A. R.; Frechet, J. M. J.; Subramanian, V. Adv. Mater. 2005, 17, 2340. (b) DeLongchamp, D. M.; Kline, R. J.; Lin, E. K.; Fischer, D. A.; Richter, L. J.; Lucas, L. A.; Heeney, M.; McCulloch, I.; Northrup, J. E. Adv. Mater. 2007, 19, 833.
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peak position (eV)
assignment
284.3 ( 0.1 285.5 ( 0.1 286.5 ( 0.2
C 1s [C-Si (1-bond)] f π* LUMO C 1s (others) f π* LUMO C 1s f σ*(C-Si); C 1s f σ*(C-C); C 1s f higher π* D 287.4 ( 0.2 C 1s f σ*(C-H); C 1s f higher π* E 288.2 ( 0.3 C 1s f higher π* (and C 1s f Rydberg) a The assignment was done based on the STEX MOs. A B C
Figure 10. Integrated intensities, which are defined as a sum of the
Figure 9. Experimental C K-edge NEXAFS spectra for (a) PASi(111) and (b) PA-Si(100). (c) Example of a peak decomposition for the PA-Si system.
for the PA-Si(111) and PA-Si(100) systems, respectively. Overall spectral shapes are reproduced by the STEX MO calculations, although theory tends to underestimate the intensity in the photon energy range 287-288 eV. A distinguishing shoulder is found at 284.3 eV for both of the PA-Si systems; this peak is attributed to C 1s f π* resonances for the core orbital of the carbon atom bonded to the silicon surface to a LUMO-like π* MO, as revealed by the ab initio calculations. This peak is expected for the PA-Si(111) system from previous studies.20 The 284.3 eV peak for the PA-Si(100) system demonstrates the existence of the 1-bond geometry; the data support our interpretation of the XPS shakeup features and the UPS spectra. We further investigate the experimental data by decomposing the NEXAFS spectra following the standard prescription of Outka and St€ohr.70 Core-ionization features before the ionization edge were expressed by symmetric Gaussian functions; a combination of a step-like error function and an exponential decay function was used to describe the continuous ionization state; and broad peaks after the ionization edge were fitted using asymmetric Gaussian functions in which the fwhm is assumed to be a linear function of the photon energy with the slope and intercept of 0.3414 and -95.106, respectively. An example of peak decomposition is shown in Figure 9c. A reasonable fit before the ionization edge was obtained using 5 Gaussian functions. With the help of the STEX MO calculations, we assigned the core-hole excitation peaks as described in Table 4. Components A and B are due to the C 1s f LUMO resonances. For components C, D, and E, located at 286.5 eV, 287.4 eV, and 288.2 eV, respectively, σ* resonances, as well as π* virtual MOs, contribute to the peaks. Incident angle dependence of the intensity of C 1s f π* resonances gives us information on the average orientation of phenyl rings (see the Supporting Information for theoretical background). Because peaks A and B are pure C 1s f π* resonances and easier 17008 DOI: 10.1021/la103208n
volume of peaks A and B (Figure 9c) as a function of sin2 θ (θ= 20, 30, 40, 50, 60, 70, 80, and 90): (a) PA-Si(111) and (b) PA-Si(100).
to analyze than the others, we use the two components for the molecular orientation analysis. PEY intensity-sin2 θ plots for the C 1s f LUMO resonances are shown in Figure 10. In both cases, plots are reasonably fitted by a linear function. From the slope and intercept of the function, average tilt angles of the conjugated molecule with respect to the surface normal for the PA-Si(111) and PA-Si(100) systems were estimated to be 25 and 29, respectively. The molecular orientation of 25 for PA-Si(111) was larger than that obtained by Kellar et al. (14)20a for a similar molecular system, part of which would be accounted for by surface roughness at the PA-Si(111) interface. Possible creation of the 2-bond configurations cannot be ruled out in our case; nevertheless, molecular tilt of at 10-20 is energetically accessible, since the destabilization of the tilt can be compensated by the intermolecular interaction energy between aromatic molecules even for the 1-bond geometry. For a single molecule attached on Si(111), the upright structure is most stable, and its energy increases as the molecule tilts toward the surface: a 20 tilted structure is less stable than the perpendicularly standing one by 3.36 kcal/mol (at the B3LYP/6-311þG(d,p) level). However, when many molecules stack at the interface, a tilted configuration may be more stable by van der Waals or π-π stacking interactions acting on each molecule. High-level ab initio calculations of Tsuzuki et al.71 estimated that a parallel-displaced conformation is energetically more stable than the completely stacked one by ∼1 kcal/mol in the case of a benzene dimer. The order of stabilization (2-3 kcal/mol) is comparable to that of destabilization due to a ∼20 tilt [3.36 kcal/mol at B3LYP/6-311þG(d,p)] and the distance between nearest Si atoms on the Si(111) surface is 3.9 A˚, which is close to the intermolecular distance for a benzene dimer (3.8 A˚). The observed deviation from the vertical orientation therefore is physically possible. A larger average tilt angle for PA-Si(100) may suggest that several configurations exist at the (70) Outka, D. A.; St€ohr, J. J. Chem. Phys. 1988, 88, 3539. (71) Tsuzuki, S.; Honda, K.; Azumi, R. J. Am. Chem. Soc. 2002, 124, 12200.
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interface. Two possibilities can be considered as the reason. The first is that a large number of the 2-bond configurations having a larger tilt angle exist on the surface. However, we do not find a sizable difference in the intensity ratios of peaks A and B [I(A)/ I(B)] (I(A)/I(B) values for both of the systems lie between 0.09 and 0.11); furthermore, the XPS/UPS spectral features are similar to each other as discussed earlier. So, it is difficult to attribute the tilt angle only to the creation of the 2-bond structures. The second possibility is that tilted orientations are possible even for the 1-bond configuration; we will mention the possibility of tilted structures in a later section. Possible Mechanism for the Creation of a 1-Bond Structure at the Hydrogen-Terminated Si(100) Surface. Herein, we summarize the experimental findings and an interpretation based on quantum chemical calculations. Some common spectral features were observed for both the PA-Si(111) and PA-Si(100) systems in the present photoelectron/photoabsorption spectroscopy measurements. In XPS, a broad shakeup peak, which corresponds to low-lying π-π* transitions in aromatic hydrocarbon molecules, was gained at the binding energy of 292 eV. Also, in UPS, a low-energy peak at 3.8 eV was found to be relatively broad. We found from theoretical calculations for styrene (a model of the 1-bond configuration) and ethylbenzene (a model system of the 2-bond structures) that the broadness is reasonably understood by the electronic structure of the former system. In terms of a MO interaction scheme, it is qualitatively explained by characteristic π/π* MOs in styrene; the degenerate HOMOs/LUMOs of the benzene component are split as a result of MO interactions with the HOMO/LUMO of the ethylene one, leading to the broader range of transition/ionization energies. From the XPS and UPS data, we assume a considerable number of PA molecules in the 1-bond configuration at the PA-Si interfaces. Furthermore, in NEXAFS, a peak was detected at the photon energy of 284.3 eV, as well as an intense peak at 285.2 eV due to C 1s f π* resonances. Employing core-hole excitation calculations within the STEX approximation, we attributed the 284.3 eV peak to a C 1s f LUMO resonance at the carbon atom having a C-Si bond in the 1-bond configuration (peak X in Figure 8a). Taking into account that peak X is not seen in the 2-bond structures, the finding supports the hypothesis inferred from the XPS/UPS spectra. The experimental results above show that a considerable number of PA molecules retain the 1-bond structure even at the PASi(100) interface, which supports the conclusion of Cerofolini et al.22 and Cossi et al.23 rather than that of Sieval et al.21 However, from the theoretical viewpoint, the 2-bond structures are energetically much more stable than the 1-bond structure, and should be the dominant ones unless the second Si-C bond creation reactions are prevented. To understand the interface structure in more detail, an atomistic description of the reaction mechanism of surface hydrosilylation is necessary. Consider the hydrosilylation reaction at the H:Si(100)-(1 1) surface (Figure 11a), in which dihydride groups form a square structure with the side length of 4 A˚. For the surface structure, two different mechanisms of the hydrogen abstraction reaction can be presumed. One is that the radical abstracts a hydrogen atom from an adjacent dihydride species [path (A) in Scheme 1]. The other is that the radical attacks the remaining hydrogen atom in the same dihydride species [path (B)]. Some other superlattice structures are known as dihydride surfaces; for example, Arima et al.51 and Sakaue et al.52 proposed by scanning tunneling microscopy and FT-IR observations that a 21 structure (Figure 11b) is generated by a combination of HF etching and subsequent rinsing with water or hot NH4F. The superlattice has the 1 1 dihydride Langmuir 2010, 26(22), 17000–17012
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Figure 11. Surface structures of dihydride species in H:Si(100):
(a) 1 1 structure and (b) 2 1 superlattice structure. Black circles represent hydrogen atoms; the others correspond to silicon atoms in different (100) layers. Scheme 1
structure in the [011] direction and another structure with a longer distance between two dihydride species (∼8 A˚) in the [011] direction. The 2-bond structures could be constructed along the [011] direction (corresponding 1,2-bridge structure is energetically less stable by 1.4 kcal/mol than the 1-bond structure at the B3LYP/6-31G(d) level), but the second Si-C bond creation reactions may be precluded in the [011] direction because of the separated dihydride structures. In this case, path B is the viable mechanism. Martı´ nez-Guerra and Takeuchi18 investigated the energy diagram of path A in the case of Si(100)-11 using ethylene as a molecular system and concluded that the creation of the 2-bond structure is energetically possible. However, the relative reaction rates for paths A and B were not compared in their theoretical study. Here, we systematically investigate the energy profiles of paths A and B within a cluster model, employing a hybrid DFT approach. In the following theoretical calculations, we assume that the surface hydrosilylation reactions occur in accordance with the mechanism drawn in Figure 1. The mechanism of the thermal hydrosilylation reaction is not well understood in contrast to that of the UV-initiated hydrosilylation reaction. One open question is how a radical electron is generated (or how a Si-H bond is homolytically cleaved) by heat. As an alternative to the radicalmediated reaction route (Figure 1), Coletti et al.72 recently proposed a nonradical four-center concerted mechanism based on ab initio electronic structure calculations. Further research is needed to gain a deeper understanding of the mechanism; nevertheless, we expect that the radical chain propagation should contribute to the formation of organic monolayers in the light of a recent FT-IR study by Holm and Roberts.73 Their FT-IR measurements on functionalized nanoparticles prepared by thermal reactions between 1-alkene and deuterium-terminated silicon nanoparticles exhibited an absorption peak at 2140 cm-1 attributable to the stretching vibrations of the carbon-deuterium bond. The result indicates that the radical chain propagation mechanism described (72) Coletti, C.; Marrone, A.; Giorgi, G.; Sgamellotti, A.; Cerofolini, G. F.; Re, N. Langmuir 2006, 22, 9949. (73) Holm, J.; Roberts, J. T. Langmuir 2009, 25, 7050.
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Figure 12. Computed energy diagram for the creation of the 1,1-bridge structure at a PA-Si(100) interface [path (A), at B3LYP/6-311þG(d,p)// B3LYP/6-31G(d), zero point energy correction not considered]. The same silicon cluster shown in Figure 3 was used as a model of the Si(100) surface. Two vinyl carbon atoms are labeled Cv1 and Cv2, where Cv1 is the carbon atom that has a C-Si bond. Some of the interatomic distances are inserted in the figure. The energies are relative to the dissociation limit, independent PA molecule and the silicon cluster radical. The units of energies and lengths are in kcal/mol and A˚, respectively. More data on the energies will be found in the Supporting Information.
in Figure 1 should take place even in the thermal-mediated hydrosilylation reaction. First, we revisit the radical-mediated second C-Si bond creation reaction via path A. A computationally predicted energy diagram is shown in Figure 12. After a PA molecule binds to a monohydride 3 SiH radical, the spin density on the silicon surface moves to PA. An atomic spin population analysis in Ra1 reveals that the spin density is the highest on Cv2 (0.59), meaning this carbon atom is most likely the reaction center. After the adsorption, the radical abstracts one hydrogen atom from the dihydride structure next to the adsorbed molecule, forming a neutral styrene-like structure and a monohydride radical (Ra2). According to the description, Cv2 loses the chemical activity whose spin density is reduced to -0.03, and simultaneously, the spin density of the reacted silicon atom becomes much higher (0.01 f 1.03). The transition state (TSa1) is located below the dissociation limit and the transition energy is low enough to be overcome (8.4 kcal/ mol); therefore, this reaction step should proceed easily even at room temperature. In the next step, Ra2 f Ra3, the silicon radical creates a chemical bond with a carbon atom in the vinyl group. By comparison of the structure of the reactant (Ra2) with that of the transition state (TSa2) and the product (Ra3), we find that the reaction is divided into two steps. First, the adsorbed PA molecule rotates with respect to the C-Si bond. During the rotation process, the C-Si bond length is slightly increased from 1.87 A˚ to 1.89 A˚, and the interatomic distances between the radical Si and Cv1 are decreased from 4.13 A˚ to 3.66 A˚. In this stage, The Si atom holds a radical character with a slight change of the atomic spin population from 1.03 to 0.96. No change is observed in the Cv1-Cv2 double bond distance [1.35 A˚ (Ra2/TSa2)]. The reaction barrier is estimated to be very low with a transition energy of 2.3 kcal/mol, which is in line with that of Martı´ nez-Guerra and Takeuchi on an acetylene-Si(100) system.18 Then, after reaching TSa2, the Si radical attacks Cv1 to form a 2-bond structure Ra3. 17010 DOI: 10.1021/la103208n
In this phase, the existing C-Si bond is further elongated to 1.95 A˚, and another C-Si bond with a bond length of 1.94 A˚ is created. At the same time, the Cv1-Cv2 bond becomes close to a carbon-carbon single bond (1.49 A˚), and the spin population moves toward the Cv2 atom [radical Si, 0.96 (TSa2) f 0.01 (Ra3); Cv2, -0.04 (TSa2) f 0.68 (Ra3)]. The product Ra3 is more stable than Ra2 by 15.2 kcal/mol; consequently, the reverse (bond dissociation) reaction is predicted to be slower than the forward reaction. Since the Si 3 3 3 Cv1 distance (3.66 A˚) is very close to the Si 3 3 3 Cv2 one (3.77 A˚), the 1,2-bridge configuration as well as the 1,1-bridge one should be formed on the realistic nonzero temperature condition. The third reaction (Ra3 f Pa) is an important step so that the chain hydrosilylation reaction process contributes to propagation. From the structure of TSa3, this process is regarded as a typical concerted mechanism: a C-H bond and a Si-H bond are generated and cleaved, respectively, via a fourcenter transition state. At TSa3, the Cv1-Cv2 length is further elongated and becomes closer to the C-C bond length (1.52 A˚). The transition energy for the reaction pathway (31.7 kcal/mol) is quite high compared with those for the preceding two steps, indicating that the third step is the rate-limiting process. The energy difference between Ra3 and Pa is competitive, so the reverse reaction can take place unless another PA molecule is attached to the radical; the molecular attachment should smoothly proceed taking the energy gain of the attachment of PA into account. When the 1,2-bridge structure is constructed in the second step, the subsequent reaction would not occur because Cv2 is fixed through the C-Si bond. In this case, part of the unpaired electrons may be capped with radical termination agents such as OH, resulting in adventitious C-O contamination that is pointed out in similar surface functionalization studies. Next, we take a look at the reaction process of path B in Scheme 1. Figure 13 illustrates an energy diagram for that path. The reaction profile is similar to that of the third step in path A Langmuir 2010, 26(22), 17000–17012
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Figure 13. Computed energy diagram for the creation of the 1-bond structure at a PA-Si(100) interface through path (B) [at B3LYP/6-311þG(d,p)//B3LYP/6-31G(d), zero point energy correction not considered]. The same silicon cluster shown in Figure 3 was used as a model of the Si(100) surface. The radical carbon atom in Rb is labeled Cv. Some of the interatomic distances are inserted in the figure. The energies are relative to the dissociation limit, independent PA molecule, and the silicon cluster radical. The units of energies and lengths are in kcal/mol and A˚, respectively. More data on the energies will be found in the Supporting Information.
(Ra3 f TSa3 f Pa), so an important aspect of the mechanism is repeated. As a result of the adsorption of a PA molecule on a silicon radical, which is energetically more favorable by 19.8 kcal/ mol than the dissociation limit, a reactive radical Rb is generated on Cv with spin density of 0.59. The radical attacks the hydrogen atom on the silicon atom to form a four-centered transition structure (TSb); then, the hydrogen atom is abstracted and another unpaired electron is simultaneously created on the silicon atom (Pb). This new radical center can react with phenylacetylene molecules in the system like the first step. Since the reaction barrier for path B (25.9 kcal/mol) is higher than that of path A (8.4 kcal/mol for the first step), the 2-bond creation reaction should be more likely at the PA-Si(100)-(1 1) interface. However, the energy value indicates that surface reactions through path B are thermodynamically possible at a relatively high temperature (150 C or higher). Our cluster model calculations as well as the first-principles band structure computations of Martı´ nez-Guerra and Takeuchi18 show that the 2-bond structures should be generated when the dihydride structure exists next to the reaction center. This means that the 2-bond configuration will be a predominant one if the H: Si(100) is composed of only the 1 1 structure. So, the existence of another surface structure should be assumed in our case. The most probable surface morphology is the 2 1 superlattice structure shown in Figure 11b, considering that we treated the hydrogen-terminated silicon wafers with DI water after the immersion in buffered HF solution. In this case, termination of the hydrosilylation reactions in the first step is physically reasonable. Since the details of the surface silicon hydride structure depend strongly on the method of HF treatment, it is difficult to judge the ratio of the 2 1 superlattice for samples of different groups.21-23 However, we conclude at least that the π-conjugation character can be retained even on the Si(100) surface. When hydrogen abstraction reactions occur through path B, another alkyne molecule can be attached to the silicon radical. Figure 14 illustrates energy-tilt angle (β) profiles in two possible situations obtained at the B3LYP/6-311þG(d,p) level of theory. In case that one alkyne molecule is attached to the dihydride group, the upright configuration is the most stable, but the energy difference between the upright configuration (β=0) and a tilted Langmuir 2010, 26(22), 17000–17012
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Figure 14. Computed energy profile of molecular orientations on a Si(100)-(2 1) superlattice surface: (a) when one molecule is attached on the surface; (b) when two molecules are attached on the surface. The color scheme of the atoms is as follows: white, hydrogen; black, carbon; and blue, silicon.
one at the β value of 30 is relatively low (1.2 kcal/mol). On the other hand, in the case of the attachment of two molecules (Figure 14b), each molecule tends to be inclined at an angle of around 20 to reduce repulsive interactions between hydrogen atoms on the molecules. Considering π-π interactions in aromatic molecules and a lower tilt energy, the molecular orientation at the PA-Si(100) interface is more flexible than that for PASi(111), qualitatively explaining a larger molecular tilt angle.
Summary We have investigated the interface structure of phenylacetylene (PA) on two different silicon surfaces, Si(111) and Si(100), using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and ab initio molecular orbital calculations. We observe some spectral features for both the PA-Si(111) and PA-Si(100) systems in common: a broad π-π* shakeup peak (XPS), a relatively broad first ionization peak from the HOMO/HOMO-1 (UPS), and a peak at 284.3 eV originating from a C 1s f π* resonance (NEXAFS). We ascribe these findings to a styrene-like π-conjugated molecular structure at the PA-Si interface by comparing the experimental data with computational results and, accordingly, conclude that the PA molecules can keep their π-conjugation character in the form of styrene (1-bond structure) even on the H:Si(100) surface composed of the dihydride groups as well as on H:Si(111). A detailed reaction pathway analysis shows that the 1-bond structure can easily be changed into 2-bond structures having two Si-C bonds in the system at the interface between PA and H:Si(100)-(11), which is contradictory to our experimental findings. We propose a 2 1 superlattice structure as an alternative to the 1 1 lattice in our experimental setup. The superlattice structure can explain the present interpretation, since the 2-bond creation reactions are suppressed due to a large distance between two dihydride species. The present study suggests that how one prepares the H:Si(100) surface may be an important factor in optimizing some physical properties such as electron conduction through monolayers on silicon. Acknowledgment. We would like to thank Michael D. Dimitriou, and Drs. Kristin Schmidt and Cherno Jaye for their help in DOI: 10.1021/la103208n
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NEXAFS experiments and data analysis. M.K. expresses his sincere gratitude to Dr. Tirtha Chatterjee for stimulating discussions and invaluable comments on the manuscript. This work made use of central facilities at the MRL at UCSB, which is funded by the MRSEC program of the NSF under grant number DMR05-20415, and from use of the nanofabrication facilities, which is supported by the NSF-NNIN under Award No. 44771-7475.
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Supporting Information Available: Additional experimental data (ellipsometry, contact angle, XPS, and XRR) for the confirmation of the quality of monolayers, calculated core ionization energies, assignments of valence electron spectra at the TDDFT level, brief explanation for molecular orientation analyses using NEXAFS data, and Cartesian coordinates of the optimized geometries. This material is available free of charge via the Internet at http://pubs.acs.org.
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