Selective Reactions and Adsorption Geometries of a Multifunctional

Sep 29, 2009 - Department of Chemistry, Sookmyung Women's University, Seoul 140-742, Republic of Korea. Received July 14, 2009. Revised Manuscript ...
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Selective Reactions and Adsorption Geometries of a Multifunctional Molecule: cis-2-Butene-1,4-diol on Si(100)-2  1 Sung-Soo Bae,† Ki-jeong Kim,*,‡ Han-Koo Lee,§ Hangil Lee, Tai-Hee Kang,‡ Bongsoo Kim,‡,§ and Sehun Kim*,†

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† Department of Chemistry and Molecular-Level Interface Research Center, KAIST, Daejeon 305-701, Republic of Korea, ‡Beamline Research Division, Pohang Accelerator Laboratory, Pohang, Kyungbuk 790-784, Republic of Korea, §Department of Physics, POSTECH, Pohang, Kyungbuk 790-784, Republic of Korea, and Department of Chemistry, Sookmyung Women’s University, Seoul 140-742, Republic of Korea

Received July 14, 2009. Revised Manuscript Received September 7, 2009 The adsorption geometry of cis-2-butene-1,4-diol (BEDO, HOCH2CHdCHCH2OH) on Si(100)-2  1 was studied using scanning tunneling microscopy (STM), high resolution X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Bias-voltage-dependent STM images exhibited features characteristic of two distinct BEDO adsorption geometries. One feature was a bright protrusion located on the center of a single dimer, indicating an on-top configuration. The low bias-voltage STM image clearly showed dark features indicative of the formation of Si-H bonds on adjacent dimers in the same dimer row. The other feature was a bright protrusion bridged on end between two adjacent dimers in the same dimer row, indicating an end-bridge configuration. Accompanying this feature, two dark features attributed to Si-H bonds were observed on opposite positions to the bridged protrusion. From the XPS results, the Si 2p core level spectra revealed that the dimer atoms are involved in the formation of Si-O and Si-H bonds. On the other hand, carbon K-edge NEXAFS spectra showed that the CdC bond does not participate in the adsorption reaction and remains as an unreacted group. Collectively, the experimental results indicate that the adsorption of BEDO on Si(100)-2  1 occurs through the formation of two Si-O bonds via nucleophilic reaction between the two OH groups of BEDO and two Si-Si dimers. Importantly, the maintenance of the CdC bond means that the CdC functional group can be utilized as a new reaction site for further surface chemical reactions.

Introduction Recently, there has been a surge in interest in the attachment of multifunctional organic molecules to semiconductor surfaces, because the ability to controllably attach organic molecules to such surfaces is a crucial step in areas such as hybrid organicsemiconductor devices and biological recognition.1-8 Since many of the proposed applications require the selective adsorption of molecules, it is essential to understand the principles that govern the competition and selectivity involved in the adsorption of multifunctional organic molecules on semiconductor surfaces. To examine the surface reactivity and selectivity of a multifunctional molecule with two different functional groups, we specifically considered cis-2-butene-1,4-diol (BEDO). BEDO has two distinct types of functional groups: one CdC bond as the central group and two hydroxyl (OH) groups as terminal groups. Both the CdC and OH functional groups are representative nucleophiles in organic substitution reactions. Therefore, it is necessary to understand the unique reactions of the two different functional groups on semiconductor surfaces. A number of previous works have established that unsaturated hydrocarbons containing a CdC bond or a CC bond can be *Corresponding author. E-mail: (S.K.) [email protected]; (K.K.) [email protected].

(1) Yates, J. T. Science 1998, 279, 335–336. (2) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33, 617– 624. (3) Wolkow, R. A. Annu. Rev. Phys. Chem. 1999, 50, 413–441. (4) Lu, X.; Lin, M. C. Int. Rev. Phys. Chem. 2002, 21, 137–184. (5) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830–2842. (6) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73, 1–56. (7) Buriak, J. M. Chem. Rev. 2002, 102, 1271–1308. (8) Bent, S. F. Surf. Sci. 2002, 500, 879–903.

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attached to Si(100) through the so-called [2 þ 2] and [4 þ 2] cycloaddition reactions. In [2 þ 2] cycloaddition, the π bond of the unsaturated hydrocarbon interacts with the dangling bonds of the Si-Si dimer, producing a four-membered ring with two new Si-C σ bonds. This type of reaction occurs during the adsorption on a Si(100) surface of ethylene, acetylene, 2-butene, 2-butyne, and various cyclic unsaturated hydrocarbons, such as cyclopentene and 1,4-cyclohexadiene.3,9-14 Also, conjugated organic molecules, such as 1,3-cyclohexadiene and 1,3-butadiene, undergo the [4 þ 2] cycloaddition reaction, in which two π bonds are broken to form a six-membered ring with two new Si-C σ bonds.15,16 Studies of the adsorption of alcohols on a Si substrate have revealed common phenomena irrespective of the surface phase. Methanol or ethanol adsorption on Si(100) has been studied by reflection IR, photoemission spectroscopy, and theoretical methods.17-23 When adsorbed on Si(100), these alcohols dissociate into an alkoxy group and a hydrogen atom by O-H bond breaking. The resulting fragments bond to Si surface atoms (9) Mezhenny, S.; Lyubinetsky, I.; Choyke, W. J.; Wolkow, R. A.; Yates, J. T. Chem. Phys. Lett. 2001, 344, 7–12. (10) Lopinski, G. P.; Moffatt, D. J.; Wayner, D. D.; Wolkow, R. A. Nature 1998, 392, 909–911. (11) Kim, K. Y.; Kim, J. H.; Cho, J. H.; Kleinman, L.; Kang, H. J. Chem. Phys. 2003, 118, 6083–6088. (12) Hamaguchi, K.; Machida, S.; Mukai, K.; Yamashita, Y.; Yoshinobu, J. Phys. Rev. B 2000, 62, 7576–7580. (13) Cho, J. H.; Kleinman, L. Phys. Rev. B 2001, 64, 235420. (14) Kim, W.; Kim, H.; Lee, G.; Hong, Y. K.; Lee, K.; Hwang, C.; Kim, D. H.; Koo, J. Y. Phys. Rev. B 2001, 64, 193313. (15) Teague, L. C.; Boland, J. J. J. Phys. Chem. B 2003, 107, 3820–3823. (16) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100–11101. (17) Casaletto, M. P.; Zanoni, R.; Carbone, M.; Piancastelli, M. N.; Aballe, L.; Weiss, K.; Horn, K. Surf. Sci. 2000, 447, 237–244.

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intradimer interaction of a single OH group, it is also possible to develop interdimer interactions in which the other OH group of BEDO interacts with an adjacent dimer (Figure 1c,d). Figure 1c shows the on-top configuration with the two Si-O bonds on the same dimer, while Figure 1d shows the end-bridge configuration with the two Si-O bonds on two neighboring dimers. These interdimer reactions involve proton transfer by O-H dissociation at the reaction sites. In the present study, we examined the reaction competition and selectivity of the CdC bond and the OH groups in BEDO on a Si(100) substrate using scanning tunneling microscopy (STM), high resolution X-ray photoemission spectroscopy (XPS) and near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy. From the experimental results, we were able to determine the most favorable adsorption geometries of BEDO on Si(100). Our findings indicate that the OH groups of the bifunctional BEDO molecule selectively react with the surface, leaving the CdC group available for utilization in further chemical reactions.

Experimental Section

Figure 1. Schemes of possible reaction pathways and configurations for BEDO adsorbed on Si(100)-2  1. (a) intradimer interaction by [2 þ 2] cycloaddition, (b) intradimer interaction by single O-H dissociation, (c) on-top configuration interdimer interaction by dual O-H dissociation, and (d) end-bridge configuration interdimer interaction by dual O-H dissociation.

spontaneously at room temperature to form Si-H and Si-OCH3 (or Si-OCH2CH3). Two main potential reaction pathways can be considered for the adsorption of BEDO on a Si (100)-2  1 surface: (1) intradimer interaction and (2) interdimer interaction by interaction between two functional groups and Si surface atoms (Figure 1). If the CdC bond of BEDO reacts with the electrophilic down-Si atom of a dimer, [2 þ 2] cycloaddition is induced by nucleophilic/ electrophilic interaction (Figure 1a). The reaction occurs via intradimer adsorption on a single dimer. On the other hand, if the OH groups of BEDO react with the down-Si atoms of a dimer, various geometries with Si-O bonds are induced, as shown in Figure 1b-d. Figure 1b shows the intradimer interaction of a single OH group. Subsequent O-H dissociation with proton transfer is also possible on the same dimer. In addition to the (18) Lu, X.; Zhang, Q.; Lin, M. C. Phys. Chem. Chem. Phys. 2001, 3, 2156–2161. (19) Kato, T.; Kang, S. Y.; Xu, X.; Yamabe, T. J. Phys. Chem. B 2001, 105, 10340–10347. (20) Zhang, L. H.; Carman, A. J.; Casey, S. M. J. Phys. Chem. B 2003, 107, 8424–8432. (21) Eng, J.; Raghavachari, K.; Struck, L. M.; Chabal, Y. J.; Bent, B. E.; Flynn, G. W.; Christman, S. B.; Chaban, E. E.; Williams, G. P.; Radermacher, K.; Manti, S. J. Chem. Phys. 1997, 106, 9889–9898. (22) Carbone, M.; Piancastelli, M. N.; Paggel, J. J.; Weindel, C.; Horn, K. Surf. Sci. 1998, 412-13, 441–446. (23) Casaletto, M. P.; Zanoni, R.; Carbone, M.; Piancastelli, M. N.; Aballe, L.; Weiss, K.; Horn, K. Surf. Sci. 2002, 505, 251–259.

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BEDO was purchased from Fluka (>96.0% purity). BEDO was further purified through several freeze-pump-thaw cycles to remove all dissolved gases before being exposed to clean Si(100)-2  1. The purity of BEDO was verified using in situ mass spectrometry in a vacuum chamber. BEDO was exposed onto Si(100)-2  1 at room temperature through a direct doser controlled by a variable leak valve. The pressure during BEDO exposure was determined from the uncorrected ion gauge reading. The exposure unit is Langmuir (L) (1 L=1  10-6 Torr s). STM measurements were carried out in an ultrahigh vacuum (UHV) chamber with an OMICRON variable-temperature scanning tunneling microscope (VT-STM). The base pressure was less than 1.0  10-10 Torr. A P-doped Si(100) wafer (R=1-10 Ω cm) was used, which was cut to a size of 10  2 mm2 for the STM measurements. The surface was cleaned by outgassing for 12 h at ∼900 K and flash annealing to ∼1350 K under vacuum without ex-situ sample pretreatment. The Si(100)-2  1 surface was confirmed to be clean and ordered by STM. All STM measurements were performed at room temperature using electrochemically etched W-tips with subsequent annealing in a vacuum. STM images were recorded in constant current mode with a tunneling current of It=0.1 nA. Sample bias voltages of Vs=-1.0 to -2.0 V and þ1.0 to þ2.0 V were employed to obtain the images of the organic molecules. For XPS and NEXAFS experiments, a B-doped Si(100) wafer (R=9-12 Ω cm) was used and cut to a size of 12  4 mm2. After the cleaning procedure, the cleanness and the reconstruction to a 2  1 surface were confirmed by XPS. The XPS and NEXAFS experiments were performed at the 2B1 (spherical grating monochromator) beamline at the Pohang Light Source (PLS) Storage Ring.24,25 Photons impinged on the surface at 45° to the surface normal. The Si 2p core level spectra were recorded using a photon energy of 130 eV with a total resolution of 200 meV. The C 1s and O 1s spectra were obtained at photon energies of 320 and 650 eV with a spectral resolution of 500 meV, respectively. The binding energy and spectral resolution were determined by measuring the Au 4f7/2 core level and the Fermi level of a sputtered Au film. The NEXAFS spectra were recorded using partial electron yield detection by a channel electron multiplier.26 The spectra were normalized by the incident photon flux using the spectrum of a clean gold reference grid, which was measured at the same time as

(24) Ko, I. S.; Huang, J. Y.; Seon, D. K.; Kim, C. B.; Lee, T. Y. J. Korean Phys. Soc. 1999, 35, 411–415. (25) Kim, K. J.; Kang, T. H.; Kim, B. S.; Bang, J. H.; Lee, M. H. J. Korean Phys. Soc. 1997, 30, 148–151. (26) Lee, H. K.; Kim, K. J.; Han, J. H.; Kang, T. H.; Chung, J. W.; Kim, B. Phys. Rev. B 2008, 77, 115324.

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Figure 2. (a) A filled-state STM image (17 nm  17 nm, Vs = -2.0 V, It = 0.1 nA) before exposure of Si(100)-2  1 to BEDO at room temperature. (b) A filled-state STM image (17 nm  17 nm, Vs = -1.6 V, It = 0.1 nA) after exposure of Si(100)-2  1 to 0.0025 L BEDO at room temperature. The line profile analysis of (c) feature A and (d) feature B. the incident photon intensity was recorded. The photon resolution was 250 meV near the carbon K-edge region.

Results and Discussion A filled-state STM image (17 nm  17 nm, Vs=-2.0 V, It=0.1 nA) recorded before exposure of a Si(100)-2  1 substrate to BEDO is shown in Figure 2a. The image shows a clean Si(100) surface with 2  1 reconstruction structure. The dark features observed in the image may correspond to missing dimers or C-type defect by contaminants. Figure 2b shows a filled-state STM image (17 nm  17 nm, Vs =-1.6 V, It =0.1 nA) recorded after exposure of a Si(100)-2  1 substrate to 0.0025 L BEDO at room temperature. Two types of bright protrusion (denoted A and B) can be discerned in this image. Although features A and B are similar in size and brightness, their positions clearly differ. Feature A is located on top of a single dimer, whereas feature B is located between two adjacent Si-Si dimers in the same dimer row. From statistical analysis of numerous STM images of different areas of the surface, the population ratio of the two types of feature is A/B = 7:1. In the STM images, bright protrusions were only observed at the positions of features A and B, thus ruling out other possible adsorption configurations such as interrow adsorption. On the other hands, the clean surface has a 15% defect density while the BEDO adsorbed surface has a 21% defect density. There is a slight increase of defect after exposure of the surface to the BEDO. This is probably due to the adsorption of impurities or contaminants. From the analysis of adsorption features by comparison of BEDO adsorbed surface with Langmuir 2010, 26(2), 1019–1023

the clean surface, it is conclude that there is no effect of the original defect in adsorption geometry of BEDO. The exact adsorption positions of BEDO molecules on the Si(100) surface were confirmed by line profile analysis of STM images. The line profile analysis of features A and B in Figure 2b is shown in Figure 2c,d, respectively. The R-R0 and β-β0 lines revealed that the feature A protrusion is symmetrically located on the center of a reacted dimer. On the other hand, as shown in the γ-γ0 line perpendicular to the dimer row, feature B is asymmetrically located toward one side of the reacted dimer. However, the δ-δ0 line along the dimer row shows that feature B is symmetrically located between two adjacent dimers. The results of this line profile analysis thus establish that each BEDO molecule adsorbs on a single dimer row without molecular interaction between dimer rows. The bonding character of the adsorption features was verified by examining empty-state STM images such as that in Figure 3. The bright feature A located on the center of a dimer in the filled-state STM image (Figure 3a) corresponds to the bright feature A0 on the center of the same dimer in the empty-state STM image (Figure 3b). Feature A0 in the empty-state image is comparable to the on-top feature of acetylene chemisorbed on Si(100).14 The cited study found that, after acetylene was adsorbed on a Si-Si dimer by two Si-C σ bonds, the empty-state STM image showed that the adsorbed configuration has a CdC antibonding orbital character. This supports the hypothesis that, in the emptystate STM images of BEDO adsorbed on Si(100), each bright protrusion is due to an unreacted CdC bond in a BEDO molecule DOI: 10.1021/la902570y

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Figure 3. (a) Filled-state (Vs = -1.8 V) and (b) empty-state (Vs = þ1.8 V) STM images (15 nm  10 nm, It = 0.1 nA) of BEDO adsorbed on Si(100)-2  1. Feature A in the filled-state image corresponds to feature A0 in the empty-state image.

whose two terminal OH groups interact with Si surface atoms via O-H bond dissociation. To verify that the adsorption geometry corresponds to that expected for O-H bond dissociation, sequential bias-voltagedependent STM measurements were performed, as shown in Figure 4. The sizes of features A and B gradually decreased as the bias voltage was decreased from -1.8 to -1.0 V. At Vs = -1.0 V, dark areas are clearly identified as saturated sites, indicating Si-H bonds (denoted by arrows). That is, the bright protrusion corresponding to feature A (Figure 4a) is due to the Si-O-CH2-CHdCH-CH2-O-Si on-top configuration on a single dimer, and the dark area corresponds to two Si-H bonds formed on the next dimer in the same dimer row. For feature B (in Figure 4b), by contrast, the bright protrusion is due to the Si-O-CH2-CHdCH-CH2-O-Si end-bridged configuration on one side of two adjacent dimers, and the dark area corresponds to two Si-H bonds formed on unreacted Si atoms of the same dimers. These findings thus indicate that all adsorptions of BEDO involve two adjacent dimers as reactive sites, with two Si-H bonds being created by dual O-H bond dissociation. To investigate the bonding nature of the adsorbed molecules and the bonding geometry in detail, NEXAFS measurements were performed. Figure 5 shows the carbon K-edge NEXAFS spectra of Si(100)-2  1 exposed to 10 L BEDO at different photon incident angles (θ). Two major peaks are observed: one at 285 eV originating from π* (CdC) and another at 288 eV originating from σ* (C-H). The π* (CdC) peak intensity shows a dependence on the incident angle of photon beam. Specifically, the π*(CdC) resonance intensity is strong when the incident angle of the photon beam is θ=20° (glancing incidence), whereas it is weak when the incident angle of photon beam is θ=90° (normal incidence). These results indicate that π* (CdC) does not participate in the bonding interaction with the Si(100) surface. The bonding state of BEDO adsorbed on Si(100)-2  1 was characterized by XPS measurements. Figure 6 shows the Si 2p, C 1s, and O 1s spectra of Si(100)-2  1 exposed to BEDO. These spectra were fitted by a standard curve fitting procedure using spin-orbit split Voigt functions for quantitative analyses.26 The Si 2p spectrum of the clean Si(100)-2  1 surface is shown in Figure 6a. The Si 2p peak of the clean Si(100)-2  1 is decomposed into four peaks that are related to the surface states of the dangling bond (Su), the second layer (S), defect-originated components (D), and the bulk (B). After Si(100)-2  1 is exposed to 1 L BEDO, the contributions of Su and D decrease. At the same time, however, two new components develop at surface core-level shifts (SCLSs) of þ0.330 eV and þ0.965 eV, as expressed in the relative binding energy (RBE) from the bulkoriginated peak B. The peak localized at þ0.330 eV RBE is assigned to surface silicon atoms bonded to hydrogen (Si-H component), in (27) Himpsel, F. J.; McFeely, F. R.; Talebibrahimi, A.; Yarmoff, J. A.; Hollinger, G. Phys. Rev. B 1988, 38, 6084–6096.

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Figure 4. Sequential bias voltage dependent STM images and schematic bonding configurations of (a) feature A and (b) feature B of BEDO adsorbed on Si(100)-2  1. The sample bias voltage Vs varies from -1.8 to -1.0 V.

Figure 5. Carbon K-edge NEXAFS spectra of Si(100)-2  1 exposed to 10 L BEDO at different photon incident angles (θ). The π* orbital of the CdC bond shows an angle dependence in the C K-edge NEXAFS spectra.

analogy with the case of methanol and ethanol adsorption on Si(100)-2  1.17,23,27 On the same basis, the peak at þ0.965 eV RBE is attributed to silicon atoms bonded to oxygen (Si-O Langmuir 2010, 26(2), 1019–1023

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Figure 6. Photoemission spectra of BEDO adsorbed on Si(100)2  1. (a) Si 2p spectra. As BEDO exposure increases, the Su state decreases and Si-O and Si-H states increase. (b) C 1s spectra. These spectra are attributed to two kinds of chemical species: C1 is due to the C3 and C4 atoms and C2 is due to the C2 and C5 atoms in the O1-C2-C3dC4-C5-O6 molecular structure. (c) O 1s spectra. The single peak originates from the Si-O bonding character.

component), in agreement with literature reports on Si-O bond formation.17,23,28 After Si(100)-2  1 is exposed to 10 L BEDO, the Su component completely disappears. On the other hand, peak S is retained with the same intensity after BEDO exposure, and Si-H and Si-O features increase in intensity with increasing BEDO exposure. These findings indicate that BEDO is predominantly adsorbed to the dangling states on Si(100)-2  1. Concurrently, C 1s (Figure 6b) and O 1s spectra (Figure 6c) were analyzed to investigate the bonding configuration of the adsorbed molecules. The C 1s spectra are decomposed with two peaks, denoted C1 and C2. C1, at a binding energy of 284.2 eV, originates from the C3 and C4 atoms, and C2, at a binding energy of 285.3 eV, originates from the C2 and C5 atoms in the O1-C2-C3dC4-C5-O6 molecular structure shown in Figure 1. If the adsorbed molecule were to undergo further fragmentation or formation of Si-C bonding, another peak would be expected to appear at a point shifted by ∼2 eV toward lower binding energy.23,29 However, no such peaks related to Si-C were observed, indicating that the CdC double bond does not (28) Yamamoto, K.; Hasegawa, M. J. Vac. Sci. Technol. B 1994, 12, 2493–2499. (29) Carbone, M.; Piancastelli, M. N.; Zanoni, R.; Comtet, G.; Dujardin, G.; Hellner, L. Surf. Sci. 1997, 370, L179–L184.

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participate in the reaction with Si(100) and is maintained without further fragmentation. The peak in the singlet O 1s spectra at a binding energy of 532 eV is attributed to the Si-O bond. Therefore, Si-O bonding and maintenance of the CdC bond are involved in the final bonding configuration. The NEXAFS and XPS results indicate that BEDO adsorbs on Si(100)-2  1 with two adsorption geometries: an intradimer interaction by nucleophilic reaction of a single OH group (Figure 1b) and an interdimer interaction by nucleophilic reaction of two OH groups (Figure 1c,d). On the other hand, the lack of evidence of the Si-C component in XPS and the angle dependency of the CdC bond peak in NEXAFS indicate that [2 þ 2] cycloaddition by nucleophilic interaction of the CdC bond is not favored. The confirmation of the existence of Si-O bonding by high resolution XPS well supports the UHV-STM results showing two adsorption features with Si-O bonding configurations. Also, the NEXAFS results indicative of the presence of a CdC bond support the CdC bonding character observed in the empty-state STM image of adsorbed BEDO. Therefore, the experimental results collectively show that, at room temperature, the most favorable BEDO adsorption structures are on-top and end-bridge structures involving dual O-H bond dissociations, as shown in Figure 1c,d, respectively.

Conclusions In summary, the adsorption of BEDO on Si(100)-2  1 at room temperature occurs through dual O-H bond dissociations, and the interaction of the resulting molecular fragments with Si surface atoms. As a result of the selective dissociation reaction of the two OH groups of BEDO, the adsorption bonding geometries are on-top and end-bridge configurations. Their bonding characters are attributed to Si-O and Si-H bonding by two interdimer interactions on the same dimer row. On the other hand, the CdC bond in the molecule does not participate in the adsorption. Therefore, following the adsorption of BEDO on Si(100)-2  1, the CdC bond remains intact, and hence can potentially be used as a new reactive site for further chemical reactions such as surface cycloaddition. Acknowledgment. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (2009-0083525). The XPS and NEXAFS experiments at PAL were supported in part by MEST and POSTECH. This work was also supported by KOSEF through CNNC at SKKU (Grant No. R11-2001-00002-0).

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