ARTICLE pubs.acs.org/JPCC
Selective Reactions and Adsorption Structure of Pyrazine on Si(100): HRPES and NEXAFS Study Han-Koo Lee,*,† Jinwoo Park,‡ Ilyou Kim,§ Hyeong-Do Kim,†,§ Byeong-Gyu Park,† Hyun-Joon Shin,†,§ Ik-Jae Lee,† Abhinav Pratap Singh,† Anup Thakur,†,|| and J.-Y. Kim*,†,§ †
Pohang Accelerator Laboratory, San 31 Hyojadong, Namgu, Pohang, Kyung-Buk, 790-784, Republic of Korea Department of Physics, Graphene Research Institute, and Institute of Fundamental Physics, Sejong University, Seoul 143-747, Korea § Department of Physics, POSTECH, San 31 Hyojadong, Namgu, Pohang, Kyung-Buk, 790-784, Republic of Korea UCoE, Punjabi University, Patiala 147-002, Punjab, India
)
‡
ABSTRACT: We have investigated adsorption of pyrazine molecules on the Si(100)-21 surface at 300 K by using highresolution photoemission spectroscopy (HRPES) and near-edge X-ray absorption fine structure (NEXAFS) in partial electron yield mode. The Si 2p, C 1s, and N 1s core-level spectra reveal that pyrazine is directly bonded to silicon surface dimer rows through two para-nitrogen atoms, forming two new SiN σlinkages at 300 K. The π* orbitals of CdC bonds in the carbon K-edge NEXAFS spectra show a good angular dependence providing an average tilt-angle α ∼ 34 ( 5° of the CdC bond for the chemisorbed pyrazine with respect to the surface. The experimentally measured angle of α ∼ 34° agrees reasonably well with the cross-row configuration structure
’ INTRODUCTION The interaction of small organic molecules with Si surfaces has been actively studied to understand the reaction mechanism of organic functionalization on semiconductors.115 These interactions are not only of fundamental interest but also of practical importance in molecular electronics and sensors.1517 Pyrazine is isoelectronic to benzene and pyridine and has properties similar to those of pyridine in the aromatic family. It is a sixmembered aromatic heterocyclic molecule containing two N atoms at the opposite ends of the ring with an inhomogeneous charge distribution within the ring system (see Figure 1). The hexagonal ring of pyrazine maintains the circular π bonds but is slightly modified due to the presence of the substituted N atoms containing the lone-pair electrons. This elemental homogeneity of pyrazine increases the number of possible configurations for molecular adsorption. Therefore, it is very interesting to study the orientation of the adsorbed pyrazine molecule on the Si surface. Numerous theoretical and experimental studies have been focused on the adsorption of a pyrazine molecule on Si(100).1215 Huang et al.13 show that the covalent attachment of pyrazine to Si(100) occurs in a highly selective way, in which the two paranitrogen atoms are directly bonded to the surface to form a 1,4-N, N-dihydropyrazine (DHP)-like surface intermediate. Lu et al.12 show that the N-end-on adsorbed pyrazine would be the primary adspecies at low temperature, whereas at elevated temperatures, the primary adspecies would be the side-on adsorbed pyrazine, which is di-σ-bonded onto the surface dimer through the 2 and 5 r 2011 American Chemical Society
carbon atoms. Jung and Kang performed first-principles density functional calculations for the structural and bonding properties of pyrazine adsorbed on the Si(001) surface and have suggested the cross-row chain adsorption configuration (see the schematic representations in Figure 2) formed on the Si(001) surface.15 Their calculations are well in accordance with the room-temperature (RT) scanning tunneling microscopy (STM) results indicating that C4H4N2 molecules reside between dimer rows, and the experimental result is also in accordance with the earlier experimental data of Huang et al.13,14 Thus, these rather simple heteroatom aromatic molecules appear to interact with Si(100) through much different ways whose mechanisms are not well-understood yet. These wide ranges of different theoretical predictions and experimental observations have motivated us to investigate the interaction mechanism of pyrazine on the Si(100) surface using more sophisticated techniques based on synchrotron radiations. Here, we report the formation and bonding characteristics of a pyrazine layer self-assembled on the Si(100) surface at 300 K. Near-edge X-ray absorption fine structure (NEXAFS) measurements were performed to characterize the unoccupied molecular orbital states and to determine the adsorption geometry of pyrazine on Si(100). High-resolution photoemission spectroscopy (HRPES) has been used to identify the chemical shifts of the Si 2p, C 1s, and N 1s core levels. The NEXAFS measurements used to determine the bonding geometry of pyrazine provide an Received: August 17, 2011 Revised: November 10, 2011 Published: November 18, 2011 722
dx.doi.org/10.1021/jp2078874 | J. Phys. Chem. C 2012, 116, 722–725
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Si 2p spectra (a) from the clean Si(100)-21 surface at 300 K and (b) a pyrazine-covered surface of 2 L exposure at 300 K using synchrotron radiations of energy hν = 130 eV with a takeoff angle θe = 0°. The raw spectra (empty circles) have been best fitted with four components in (a) and one component (SN) in (b). Schematic representation is a pyrazine molecule where the filled large circles (black) denote C atoms and the small circles (red) represent N atoms.
Figure 2. (a) C 1s and (b) N 1s spectra of the pyrazine-coverd surface. Pyrazine molecules were dosed to 2 L on the Si(100)-21 surface and synchrotron radiations of energy hν = 480 eV were employed for the measurements. The experimental data are represented by the empty circles, and the blue solid curve represents the fitted data.
measurements. We also obtained several core-level spectra at the 3A1 beamline of the PLS with an incident beam with an energy of 130 eV for Si 2p and 480 eV for C 1s and N 1s core levels with ΔE = 200 meV. The binding energies and the spectral resolution were referenced to the Au 4f7/2 core level and the Fermi level using sputtered Au film.
average tilting angle of α ∼ 34 ( 5° for the CdC double bond with respect to the substrate surface.
’ EXPERIMENTAL SETUP Boron-doped Si(100) with dimensions of 14 4 0.5 mm3 with a resistivity F ∼ 912 Ω 3 cm was used as the substrate. The substrate was thoroughly degassed by resistive heating at 900 K for about 12 h and then cleaned by repeated flashings at 1500 K. During the cleaning process, the base pressure of the experimental chamber was below 2 1010 Torr. The surface cleanliness and the crystallographic ordering of the Si(100)-21 phase was verified using photoemission spectra and low-energy electron diffraction (LEED), respectively. Pyrazine (99%, Aldrich) was purified by the freezepumpthaw cycles before use. Dosing of pyrazine to 2 L (L = langmuir; 1L = 1 106 Torr 3 s) was made by backfilling the chamber through a variable leak valve, as reported earlier for furan.9 All the NEXAFS measurements at 300 K were performed at the 2B1 Beamline18,19 of the Pohang Light Source (PLS).20 Upon annealing up to 450 K, we have not observed any appreciable spectroscopic changes. We have used the partial-electron-yield detection mode for NEXAFS spectra by recording the sample current normalized to a signal current measured simultaneously using a gold mesh in ultrahigh vacuum. We used a polarized (p-polarized) synchrotron photon beam (∼85%) with an energy in the range of 280320 eV with a spectral energy resolution of ΔE = 350 meV and the probing depth of ∼20 Å for surface-sensitive
’ RESULTS AND DISCUSSION In Figure 1, we present surface-sensitive Si 2p core-level spectra obtained using synchrotron X-rays of 130 eV (a) from a clean and (b) from a pyrazine-covered Si(100) surface with an exposure of 2 L at 300 K. As reported previously,21 the clean Si 2p peak has been fitted with one bulk component B and four surface components, Su, Sd, SS, and D, which are associated with up-Si, down-Si, second-layer Si and defects, respectively, using the spin orbit split Voigt functions. We find that the binding energies of these surface components, Su, Sd, SS, and D are shifted by 0.56 ( 0.02, 0.06 ( 0.02, 0.26 ( 0.02, and 0.26 ( 0.02 eV, respectively, relative to B, in agreement with previous reports.9,21 With a branching ratio of 0.5 and a spinorbit splitting of 0.6 eV, the Gaussian width appears to be 0.30 eV for all components. The Si 2p spectra from the pyrazine-covered surface in Figure 1b, however, exhibit only one prominent surface component SN (green) shifted by an energy difference of 0.69 eV from the bulk component B (blue). The surface component SN has been well identified as the contribution from the SiN bond.22 This implies that the binding of pyrazine with surface Si dimers does not involve the C atoms 723
dx.doi.org/10.1021/jp2078874 |J. Phys. Chem. C 2012, 116, 722–725
The Journal of Physical Chemistry C
ARTICLE
Figure 4. Change of peak intensity of the π* resonance in Figure 3 versus cos2 θ. The blue solid line represents the best fit of experimental data (empty circles). The two dotted lines with α = 29° (red) and 39° (black) show the error range for the tilt angle α = 34° determined in this work. Here, the tilt angle (α) is the angle between planes A and B. Plane A is parallel to the Si surface and plane B contains the two para-N atoms and the CdC double bond, as shown in the figure.
with a sharp peak at 398.4 eV and fwhm = 0.97 eV, which can be assigned to the SiNC bond, thus confirming the presence of the pyrazine ring. However, the binding energy of this peak is lower than the reported typical value (398.8399 eV).25 Because the nitrogen atoms of chemisorbed pyrazine are now in trigonal coordination (sp3), the extra lone-pair electrons will contribute more effectively to the screening of the 1s orbitals, and this should lead to the observed decrease of the N 1s binding energy.22,23 The C 1s and N 1s core-level results clearly suggest that the attachment occurs exclusively through the bonding of the two para-nitrogen atoms with the surface without the involvement of the carbon atoms, as evidenced from the presence of the CdC bond of the pyrazine ring and a significant down shift of ∼0.6 eV in the binding energy of N 1s. These conclusions, along with the Si 2p spectra results, are consistent with the reported experimental and theoretical results.1315 To be more specific on the bonding geometry, we have measured NEXAFS spectra from the pyrazine-covered Si(100) surface of 2 L at 300 K by varying the angle of incidence of the X-ray beam from the surface plane, and they are presented in Figure 3. We identify the features in the NEXAFS spectra as the π* (CdC) orbital at 285.9 eV and the several σ* orbitals in the range of 292307 eV.26 We have fitted the spectra using a nonlinear least-squares routine with Gaussian functions for the π* resonant features, a Gaussian broadened step function for the edge jump, and asymmetrically broadened Gaussian functions for the σ* resonant features as reported earlier.27,28 Because NEXAFS is known to be a powerful tool to measure the tilt angle in the reaction intermediate, we have attempted to determine the tilt angle, α, between the CdC double bond and the Si(100)-21 surface assuming a 4-fold symmetry of the Si(100) surface, described previously.11 Such assumptions become reasonable considering the size of the photon beam used (∼0.5 1.0 mm2). We have then fitted the NEXAFS spectra with the formula
Figure 3. Variation of NEXAFS spectra obtained from the pyrazinecovered Si(100)-21 surface as a function of the polarization angle θ of the incident photon beam with respect to the surface normal.
directly, which is consistent with the high-resolution electron energy loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS) results.13 Figure 2 shows the C 1s and N 1s core-level spectra obtained from the surface exposed to 2 L with pyrazine at 300 K. We notice that the C 1s spectrum (Figure 2a) is quite symmetric in its line shape. The Voigt function and the least-squares approach were employed to determine the quality of the fit. It was found that it consists of three components, one with a sharp peak at 284.8 eV and a full width at half-maximum (fwhm) = 1.0 eV, which can be assigned to the CdC bond, and two weak peaks at 286.4 and 228.9 eV.9,22,23 This confirms the presence of the pyrazine ring, as expected for a surface-bound pyrazine, forming a cross-row configuration linkage between the substrate silicon dimer rows and the two para-nitrogen atoms of the pyrazine. The two weak (high and low binding energy) components have been attributed to the CdN and SiC species resulting from the fragments of pyrazine due to the photon irradiation effects. However, considering their low intensity (∼10%), they are neglected as minority species on the surface, as observed for some related compounds on the Si(100) surface.10,24 The N 1s core-level spectrum, recorded at a photon energy of 480 eV, as shown in Figure 2b, in fact reveals more convincing evidence for the formation of cross-row configuration. It consists of a single component
Iv µ
P 1 ð1 PÞ 2 1 þ ð3 cos2 θ 1Þð3 cos2 α 1Þ þ sin α 3 2 2
ð1Þ 724
dx.doi.org/10.1021/jp2078874 |J. Phys. Chem. C 2012, 116, 722–725
The Journal of Physical Chemistry C where θ is the polarization angle of the incident beam with respect to the surface normal and P is the degree of polarization, which is set to be equal to 0.85 in the present case.29 We find that the peak intensity of the π* resonance decreases with increasing θ, as shown in Figure 4, thus revealing a flat-lying orientation of the molecules. From the best fit (solid blue line) in Figure 4, we obtained an average tilt angle α ∼ 34 ( 5°. It suggests that the chemisorbed layer may not be needed for having a planar geometry because of the loss of aromaticity due to the formation of the SiN bond. The tilt angle α = 34° obtained from NEXAFS measurements excludes the possibility of the other adsorption configurations suggested for this system, except for the two models: the DHPlike surface intermediate,13 where two opposite N atoms of pyrazine are bonded to two Si atoms of a single dimer, and the cross-row configuration model.14,15 However, the DFT calculations of Jung et al.15 show that the cross-row configuration has a relatively lower energy barrier of 0.47 eV, which is more stable as compared to the DHP-like surface intermediate model by 0.35 eV. Moreover, as we have already mentioned, from the combined work of STM, PED, and DFT study of Shimomura et al.,14 we can suggest that the cross-row configuration is more preferable. This model is further supported by the earlier high-resolution electron energy loss spectroscopy experimental result of Huang et al.13
ARTICLE
’ REFERENCES (1) Ma, Z.; Zaera, F. Surf. Sci. Rep. 2006, 61, 229–281. (2) Leftwich, T. R.; Teplyakov, A. V. Surf. Sci. Rep. 2008, 63, 1–71. (3) Tao, F.; Bernasek, S. L.; Xu, G.-Q. Chem. Rev. 2009, 109, 3991– 4024. (4) Lee, H.-K.; Kim, K.-j.; Han, J.-h.; Kang, T.-H.; Chung, J. W.; Kim, B. Phys. Rev. B 2008, 77, 115324. (5) Qiao, M. H.; Tao, F.; Cao, Y.; Li, Z. H.; Dai, W. L.; Deng, J. F.; Xu, G. Q. J. Chem. Phys. 2001, 114, 2766–2774. (6) Konecny, R.; Doren, D. J. J. Am. Chem. Soc. 1997, 119, 11098– 11099. (7) Cao, X.; Coulter, S. K.; Ellison, M. D.; Liu, H.; Liu, J.; Hamers, R. J. J. Phys. Chem. B 2001, 105, 3759–3768. (8) Wang, G. T.; Mui, C.; Tannaci, J. F.; Filler, M. A.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2003, 107, 4982–4996. (9) Lee, H.-K.; Kim, K.-j.; Kang, T.-H.; Chung, J.; Kim, B. Surf. Sci. 2008, 602, 914–918. (10) Liu, H.; Hamers, R. J. Surf. Sci. 1998, 416, 354–362. (11) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Chem. Phys. 1998, 108, 4599–4606. (12) Lu, X.; Xu, X.; Wu, J.; Wang, N.; Zhang, Q. New J. Chem. 2002, 26, 160. (13) Huang, H. G.; Huang, J. Y.; Ning, Y. S.; Xu, G. Q. J. Chem. Phys. 2004, 121, 4820–4825. (14) Shimomura, M.; Ichikawa, D.; Fukuda, Y.; Abukawa, T.; Aoyama, T.; Kono, S. Phys. Rev. B 2005, 72, 033303. (15) Jung, S. C.; Kang, M. H. Phys. Rev. B 2009, 80, 235312. (16) Neergaard Waltenburg, H.; Yates, J. T. Chem. Rev. 1995, 95, 1589–1673. (17) Yates, J. T. Science 1998, 279, 335–336. (18) Rah, S.; Kang, T.-H.; Chung, Y.; Kim, B.; Lee, K. Rev. Sci. Instrum. 1995, 66, 1751–1753. (19) Kim, K. J.; Kang, T. H.; Kim, B. J. J. Korean Phys. Soc. 1997, 30, 148. (20) Ko, I. S.; Huang, J. Y.; Seon, D. K.; Kim, C. B.; Lee, T.-Y. J. Korean Phys. Soc. 1999, 35, 411. (21) Landemark, E.; Karlsson, C. J.; Chao, Y.-C.; Uhrberg, R. I. G. Phys. Rev. Lett. 1992, 69, 1588–1591. (22) Chung, Y. D.; Kim, J. W.; Whang, C. N.; Yeom, H. W. Phys. Rev. B 2002, 65, 155310. (23) Lee, H.-K.; Kim, K.-j.; Kang, T.-H.; Kim, S.; Chung, J.; Kim, B. J. Electron Spectrosc. Relat. Phenom. 2008, 164, 44–47. (24) Fink, A.; Widdra, W.; Wurth, W.; Keller, C.; Stichler, M.; Achleitner, A.; Comelli, G.; Lizzit, S.; Baraldi, A.; Menzel, D. Phys. Rev. B 2001, 64, 045308. (25) Leftwich, T. R.; Teplyakov, A. V. J. Electron Spectrosc. Relat. Phenom. 2009, 175, 31–40. (26) Dudde, R.; Frank, K.-H.; Koch, E. J. Electron Spectrosc. Relat. Phenom. 1988, 47, 245–255. (27) St€ohr, J. NEXAFS Spectroscopy; Springer: Berlin, 1992. (28) Outka, D. A.; St€ohr, J. J. Chem. Phys. 1988, 88, 3539–3554. (29) Krinsky, S.; Perlman, M. L.; Watson, R. E. In Handbook on Synchrotron Radiation; Koch, E. E., Ed.; North Holland: Amsterdam, 1983.
’ CONCLUSIONS In summary, we investigated the Si 2p, C 1s, and N 1s core level and NEXAFS for a pyrazine-coverd Si(100)-21 single-domain surface. The results indicate that pyrazine is nondissociatively chemisorbed on the Si(100)-21 surface at 300 K. The Si 2p core-level results suggest the formation of SiN bonds between the pyrazine and the silicon surface. C 1s and N 1s core-level results are consistent with the formation of the cross-row configuration structure through the two Si dimer rows of the Si(100) substrate and the two para-nitrogen atoms of pyrazine molecules by a self-assembled process with high selectivity. The sensitive dependence of the π* (CdC) resonance peak in NEXAFS spectra on the polarization angle of the incident photon beam with respect to the surface normal yields an average tilt-angle α ∼ 34 ( 5°. Such a selective chemisorption of pyrazine on the silicon surface may serve as a new way to the surface functionalization of the Si(100)-21 surface. ’ AUTHOR INFORMATION Corresponding Author
*Phone: 82-54-2791380. Fax: 82-54-2791599. E-mail: hangulee@ postech.ac.kr (H.-K.L.),
[email protected] (J.-Y.K.).
’ ACKNOWLEDGMENT We thank Dr. Bongsoo Kim and Ki-jeong Kim for supporting the NEXAFS experiments at the 2B1 beamline in the Pohang Accelerator Laboratory. This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (Nos. 2009-0087060, 20090088969, 2010-0023494, and 2011-0006744). Experiments at PLS were supported, in part, by MEST and POSTECH. 725
dx.doi.org/10.1021/jp2078874 |J. Phys. Chem. C 2012, 116, 722–725
