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Enhanced Deseleniumization of Selenophene Molecules Adsorbed on Si(100)-2 1 Surface Han-Koo Lee,*,† Jinwoo Park,‡ Ki-jeong Kim,† Hyeong-Do Kim,† Ik-Jae Lee,† Hyun-Joon Shin,† Bongsoo Kim,† B. D. Yu,§ Suklyun Hong,*,‡ and J. W. Chung*,|| †
)
Beamline Research Division, 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, University of Seoul, Seoul 130-743, Korea Departments of Physics, POSTECH, San 31 Hyojadong, Namgu, Pohang, Kyung-Buk, 790-784, Republic of Korea ABSTRACT: We report the bonding structure of the selenophene molecules adsorbed on the Si(100)-2 1 surface at 300 K, and its evolution upon annealing investigated by adopting core-level photoemission spectroscopy, near-edge X-ray absorption fine structure (NEXAFS), and ab initio calculations. The Si 2p, C 1s, Se 3d core-level spectra measured at two temperatures, 300 and 350 K, are consistently interpreted in terms of the two major structures suggested by theory, a twisted (T) 2,5-dihydroselenophene (T-DHS) and a T-deseleniumization where the selenium atom is dissociated from the selenophene ring. We find a significantly enhanced deseleniumization of selenophene molecules by mild thermal annealing indicating that these two equally abundant structures at 300 K become a single uniform phase of the T-deseleniumization structures at 350 K by overcoming a relatively low dissociation energy barrier between the two structures. In addition, we obtain an average tilt angle of a selenophene ring at 300 K from our NEXAFS spectra R∼53 ( 3°, which represents an ensemble average of the tilt angles, R = 15° of the T-DHS and 75° of the T-deseleniumization.
’ INTRODUCTION Chemical attachment of organic molecules on semiconductor surfaces has a vast potential application in molecular electronics, biosensors, and nonlinear optical materials14 where Si(100) surface plays as an important substrate in molecular devices. The interaction of small organic molecules with Si surfaces has been actively studied to understand the reaction mechanism of organic functionalization on semiconductors.520,22,23 Some of previous studies have been focused on utilizing simple aromatic molecules containing only one heteroatom such as thiophene, furan, or pyrrole on Si(100).620 It has been known that the adsorption of furan and thiophene on Si(100) involves a [4 + 2] cycloaddition between their R-C atoms and silicon surface dimers, forming 2,5dihydrofuran- and 2,5-dihydrothiophene-like intermediates, respectively.68,19,20 In contrast, the adsorption of pyrrole on Si(100)-2 1 at 300 K occurs dissociatively by breaking its NH bond, to form adsorbed pyrrolyl and H-atoms.1016 These rather simple heteroatom aromatic molecules thus appear to interact with Si(100) through much different ways whose mechanism has not been properly understood yet. Here, we report the bonding characteristics and its evolution with mild annealing of selenophene molecules self-assembled on Si(100) surface illustrating an example where reaction energetics drives the enhanced dissociation of the adsorbed molecules. Selenophene is a five-membered heterocyclic molecule containing one Se atom with an inhomogeneous charge distribution within the ring system. Our analysis of the chemical shifts in the r 2011 American Chemical Society
Si 2p, C 1s, and Se 3d core levels obtained by using high-resolution photoemission spectroscopy (HRPES) reveals evidence that the selenophene molecules are adsorbed in two prominent bonding structures, the twisted (T) 2,5-dihydroselenophene (T-DHS) and the T-deseleniumization, at 300 K with almost equal abundance, which drastically change upon annealing at 350 K where the T-deseleniumization phase becomes dominant. Moreover, our near-edge X-ray absorption fine structure (NEXAFS) data probing the unoccupied molecular orbital states to determine bonding geometry of selenophene provide an average tilting angle R∼53 ( 3° for the plane containing the CdC double bond with respect to the substrate surface.
’ EXPERIMENTAL DETAILS We have used a boron-doped Si(100) sample of dimension 14 4 0.5 mm3 with a resistivity F ∼ 912Ω 3 cm. The substrate was thoroughly degassed by resistively 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 has been maintained below ∼5 1010 Torr. The surface cleanness and the crystallographic ordering of the Si(100)-2 1 phase have been verified by utilizing photoemission spectra and low-energy-electron diffraction (LEED). Selenophene Received: April 27, 2011 Revised: July 10, 2011 Published: July 20, 2011 17856
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Figure 1. Si 2p spectra (a) from a clean Si(100)-2 1 surface at 300 K, (b) a selenophene-covered surface of exposure 3 L at 300 K, and (c) at 350 K obtained by using synchrotron photons of energy hν = 130 eV with a takeoff angle θe = 60°. The raw spectra (empty circles) have been best fitted with four components in (a) and three components in (b) and (c) as explained in text. Insert depicts a schematic drawing of a selenophene molecule where filled small circles denote C atoms and a large green circle a selenium atom.
(97%, Aldrich) was purified by the freezepumpthaw cycles before use. Dosing of selenophene to 3 L was made by backfilling the chamber through a variable leak valve as done earlier for furan.19 All of the NEXAFS measurements were made at the 2B1 Beamline36,37 of the Pohang Light Source (PLS)38 with the sample at 300 K. We have used a 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%) of energy in the range 280320 eV with a spectral energy resolution ΔE = 350 meV producing the probing depth of ∼20 Å for surface-sensitive measurements. We also obtained several core-level spectra at the 3A1 and 8A2 beamline of the PLS with photons of energy 130 eV for Si 2p and 320 eV for C 1s and Se 3d 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 of sputtered Au film. To determine adsorption energies and tilt angles of selenophene on Si(100), we have performed density functional theory (DFT) calculations within the generalized gradient approximation (GGA)39 using the Vienna ab initio simulation package (VASP).4042 Plane waves up to an energy of 400 eV were included to expand the wave functions, and the ions were represented by the projector-augmented-wave (PAW) potentials,43 as implemented in VASP. The Si(100) surface with adsorbed selenophene was modeled as a slab with a p(4 2) surface supercell, which was composed of an adsorbed selenophene molecule, seven Si layers, and a passivating H layer. For the Brillouin-zone integration we used a 4 4 1 k-point grid. All coordinates are fully relaxed
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Figure 2. Optimized bonding structures of a selenophene molecule on the Si(100)-2 1 surface; (a) [2 + 2] cycloaddition, (b) [4 + 2] cycloaddition, (c) T-DHS, (d) T-THS, and (e) T-deseleniumization.
except the two bottom Si layers and the passivating H layer until the HellmannFeynman forces are lower than 0.02 eV/Å.
’ RESULTS AND DISCUSSION In Figure 1, we present surface-sensitive Si 2p core-level spectra obtained by using synchrotron photons of 130 eV (a) from a clean, and (b) from a selenophene-covered Si(100) surface with exposure of 3 langmuirs (1 L = 1 106 Torr 3 s) at 300 K and (c) at 350 K. As done earlier, the clean Si 2p peak has been fitted with one bulk component B and four surface components, Su, Sd, SS, and D, which associate with up-Si, down-Si, second layer Si, and defects respectively using the spinorbit split Voigt functions.21 We find that the binding energies of these surface components, Su, Sd, SS, and D are shifted by 0.56 ( 0.02 eV, 0.06 ( 0.02 eV, 0.26 ( 0.02 eV, and by 0.26 ( 0.02 eV respectively relative to B, in agreement with previous reports.19,21 With a branching ratio of 0.5 and a spinorbit splitting of 0.60 eV, the Gaussian width appears to be 0.30 eV for all components. The Si 2p spectra from the selenophene-covered surface in parts b and c of Figure 1 however exhibit three prominent surface components, SC (red), SSe (green), and SO (gray), at 0.20 eV, 0.40 eV, and 1.0 eV respectively away from the bulk component B (blue). Because the surface components SC and SO have been well identified as the contribution from the SiC bonds and SiO bonds, we ascribe the SSe component to the SeSi bonds of the T-deseleniumization, where Se is dissociated from the selenophene ring. Such an assignment for SSe appears to be consistent with the corresponding changes observed in the C 1s and Se 3d core levels as discussed below. It is quite interesting to note that such a mild annealing converts much of the T-DHS structures into the T-deseleniumization by freeing the Se atom from its selenophene ring. The presence of a small oxygen-induced component was inevitable due to the rearrangement of the sample manipulator after thermal treatment of the sample at the synchrotron beamline. To find stable bonding geometries of selenophene on the Si(100)-2 1 surface, we have carried out total energy calculations 17857
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Table 1. Calculated Results for Binding Energies (Ea) and Tilt Angles of Selenophene on Si(100)-p(4 2) configuration
[2 + 2] [4 + 2] T-DHS T-THS T-deseleniumization
Ea (eV)
1.81
2.06
2.23
tilt angle (degrees)
30
16
15
2.32
3.36 75
Figure 4. Spectroscopic change of Se 3d spectrum upon annealing, (a) at 300 K and (b) at 350 K. Selenophene exposure was 3 L and synchrotron photons of energy hν = 320 eV were used. The blue curves are the best fit-curves for experimental data (empty circles). The blue (yellow) component stems from the T-deseleniumization (T-DHS).
Table 2. Fit-Results of C 1s Core-Level Spectra for Selenophene-Covered Si(100) Surface Figure 3. Spectroscopic change of C 1s core-level spectrum upon annealing; (a) at 300 K and (b) at 350 K. Selenophene molecules were dosed to 3 L on the Si(100)-2 1 surface and synchrotron photons of energy hν = 320 eV were adopted. In (a), three components, P1 (blue), P2 (red), and P3 (yellow) represent contributions from the T-deseleniumization, the T-DHS, and the CdC bonds not associated with either Se or Si, respectively. The blue solid curves best represent the experimental data (empty circles).
using density functional theory (DFT) for five possible bonding structures; (a) [2 + 2] cycloaddition, (b) [4 + 2] cycloaddition, (c) T-DHS-like, (d) T-bridge tetrahydroseleophene (T-THS)-like, and (e) T-deseleniumization structures. Their optimized structures are shown in Figure 2. The adsorption energy Ea is defined by Ea ¼ Eselenophene=Sið100Þ Eselenophene ESið100Þ where Eselenophene/Si(100), Eselenophene, and ESi(100) are total energies of selenophene/Si(100), selenophene molecule, and clean Si(100), respectively. The adsorption energies Ea per selenophene thus calculated are listed in Table 1 along with tilt angles of the plane containing the CdC double bond with respect to the Si(100)-2 1 surface. Three structures, the TDHS, the T THS, and the T deseleniumization, are found to be relatively stable on the Si(100)-2 1 surface at 300 K with the T-deseleniumization being the most favored. We, however, find that only two structures, the T-DHS and the T-deseleniumization, are compatible with our core-level data of C 1s in Figure 3 and Se 3d in Figure 4 as discussed in the following paragraphs. We find no evidence for the presence of the T-THS structure having no CdC double bonds despite the slightly favored adsorption energy over the T-DHS as seen in Table 1. On the other hand, we calculate the activation energy barrier from the T-DHS to the T-deseleniumization structures using the nudged elastic band
Temp. 300 K/350 K binding energy (eV)
fwhm (eV)
peak area (%)
origin
P1
283.5
0.69/0.80
24/43
CSi
P2
284.2
0.70/0.80
20/2
SiCSe
P3
284.5
0.70/0.79
56/55
CdC
(NEB) method.44 The calculated dissociation barrier of selenium atom from the selenophene ring is about 0.6 eV, which may explain the conversion of much of the T-DHS to the Tdeseleniumization structures upon annealing. Now we look into our C 1s and Se 3d core-level spectra. In Figure 3, we present two C 1s core-level spectra obtained; (a) from the surface with selenophene exposed to 3 L at 300 K, and (b) from the same sample after annealing at 350 K. The highly asymmetric C 1s spectrum in part a of Figure 3 has been best fitted with three components P1, P2, and P3 of Gaussian line shape. Previous study suggests that P1 comes from the CSi bonds,22,24,25 whereas P3 from the CdC bonds.22,2426 The best fit-parameters are shown in Table 2. Fitting with only two components results in a four-times degraded value for the goodnessof-fit χ2. We thus identify origins of the three components as follows: P1 from the CSi bonds of the T deseleniumization, P2 from the SiCSe bonds, a characteristic of the T-DHS, and P3 from the CdC bonds not associated with either Si or Se. Here, we also notice the presence of the T-deseleniumization structure as indicated in the Si 2p spectra on the surface at 300 K. Similar dissociation has been also seen from thiophene adsorbed on other semiconducting surfaces.2729 The binding energy of P2 (E2 = 284.2 eV) appears closer to E3 = 284.5 of P3 than to P1 (E2 = 283.5 eV), which makes a sense considering the slightly negatively charged silicon atoms (Pauling electronegativity ε = 1.90) compared to carbon or selenium (ε = 2.55). 17858
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Figure 6. Change of peak intensity of the π* resonance in Figure 5 versus cos2 θ. The blue solid line represents the best fit of experimental data (empty circles). The two dotted lines with R = 50° (red) and 56° (black) show the error range for the tilt angle R = 53° determined in this work. Figure 5. Variation of NEXAFS spectra obtained from the selenophene-covered Si (100)-2 1 surface as a function of the polarization angle θ of incident photon beam with respect to the surface normal.
One notes only slightly increased full-width at half-maximum (fwhm) for all components upon annealing. We also find that the CSi bonds and the SiCSe bonds are almost equally abundant on the surface as indicated by the integrated intensity ratio IP1/IP2 ∼ 1.2 at 300 K. Thus the strong peak in the raw spectrum at higher energy side in part a of Figure 3 has contributions equally from both P2 and P3 considering that each structure contains a pair of carbon atoms to produce both components (Figure 2). We, however, notice that P2 becomes vanishingly small upon annealing at 350 K transferring nearly all of its intensity to P1. This indicates that most of the T-DHS are converted into the T-deseleniumization structures upon annealing by dissociating Se atoms from selenophene rings as Si 2p core level also suggests similar behavior in Figure 1. The behavior of Se 3d core level presented in Figure 4, in fact, reveals more convincing evidence for the enhanced deseleniumization of the selenophene molecules upon annealing. The Se 3d spectrum in part a of Figure 4 from the unannealed selenophene layer exposed to 3 L shows two spinorbit split components, blue and yellow, separated by 0.80 eV, a clear signature for the presence of two different kinds of Se atoms, that is the SiCSe of the T-DHS and the SeSi of the T-deseleniumization, respectively. Similar separation about 1.0 eV of two spin orbit components has also been seen for thiophene on the Pt(111) surface.30 By fitting the Se 3d spectrum with the two spinorbit split components of symmetric Voigt functions with a branching ratio 3:2 (3d5/2:3d3/2) and a Shirley-type background, we locate the two components, a slightly stronger one from the more abundant T-deseleniumization at 54.4 eV (blue) and another from the T-DHS at 55.2 eV (yellow), each with a spinorbit splitting of 0.85 eV.26 The energy scale has been referenced to the binding energy Eb = 84 eV of the Au 4f7/2 peak. We now observe a drastic change upon annealing at 350 K as shown in part b of Figure 4, where most of the T-DHS structures have been converted into the more stable T-deseleniumization structures. Because all three core-level spectra, Si 2p, C 1s, and Se
3d, consistently show the enhanced deseleniumization of the selenophene molecules on the Si(100)-2 1 upon annealing at 350 K, we conclude that the two selenophene bonded structures, the T-DHS and the T-deseleniumization, nearly equally abundant at 300 K become a single uniform phase of the Tdeseleniumization structures at 350 K. In order to estimate the tilt angle of the CdC double bond, we have measured NEXAFS spectra from the selenophene-covered Si(100) surface of 3 L at 300 K by varying the angle of incidence of the synchrotron photon beam from the surface plane as presented in Figure 5. We identify the features in our NEXAFS spectra as the π* (CdC) orbital at 284.9 eV, the σ* (CSe) mixed with Rydberg orbitals around 287291 eV, and the several σ* orbitals in the range 292307 eV.31 We have fitted the spectra by 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 done earlier.32,33 Because NEXAFS is known as a powerful way to measure a tilt-angle in the reaction intermediate, we have attempted to determine the tilt angle R between the plane containing the CdC double bond and the Si(100)-2 1 surface assuming a 4-fold symmetry of the substrate with the two different bonding structures described above.23 Such assumptions become reasonable considering the size of photon beam used ∼0.5 1.0 mm2. We have then fitted our NEXAFS spectra with a formula Iv µ
P 1 ð1 PÞ 2 1 þ ð3cos2 θ 1Þð3cos2 R 1Þ þ sin R 3 2 2
where θ is the polarization angle of the incident synchrotron light with respect to the surface normal, and the degree of polarization P = 0.85 is used.34 We find that the peak intensity of the π* resonance decreases with increasing θ as shown in Figure 6. From the best fit (solid blue line) in Figure 6, we obtain an average tilt angle R ∼ 53 ( 3°, which appears to be quite close to a magic angle satisfying 3cos2R = 1.32 As seen in our calculation (Table 1), the T-DHS has R = 15°, whereas the T-deseleniumization has R = 75°. Considering the nearly equal abundance of 17859
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’ CONCLUSIONS In summary, our analysis of the Si 2p, C 1s, Se 3d core-level spectra combined with ab initio calculations suggests that the two nearly abundant stable bonding structures of selenophene molecules on the Si(100)-2 1 surface at 300 K, the T-DHS and the T-deseleniumization, converge into a single phase of the Tdeseleniumization structures upon annealing at 350 K, thus showing a thermally driven dissociation of selenium atom from the selenophene ring. The average tilt angle, R ∼ 53 ( 3°, of a selenophene ring containing the CdC bonds at 300 K estimated from our NEXAFS spectra matches well with an average of the calculated tilt angles of the two structures. Such information on bonding structures and thermal behavior of selenophene molecules on the Si(100)-2 1 surface may significantly promote the functionalization of semiconductor surfaces using simple aromatic molecules containing only one heteroatom such as thiophene, furan, pyrrole, and selenophene. ’ AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected] (H.-K.L.),
[email protected] (S.H.),
[email protected] (J.W.C.); Phone: 82-54-2792071; Fax: 82-54-2793099.
’ ACKNOWLEDGMENT This research was supported by the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (No. 2009-0087060 and 2011-0006744), and also in part by NCRC grant R15-2008-006-01001-0. Experiments at PLS were supported in part by MOST and POSTECH. Hong and Park acknowledge the support by Priority Research Centers Program (2011-0018395), the Converging Research Center Program (2011K000620), and the Basic Science Research Program (KRF-2008-314-C00169, KRF-2008-313-C00217) through NRF/MEST. ’ REFERENCES (1) Hamers, R. J. Nature 1998, 412, 489–490. (2) Yates, J. T., Jr. Science 1998, 279, 335–336. (3) Bent, S. F. Surf. Sci. 2002, 500, 879–903. (4) Meyer zu Heringdorf, F.-J.; Reuter, M. C.; Tromp, R. M. Nature 2001, 412, 517–520. (5) Lee, H. K.; Kim, K. J.; Han, J. H.; Kang, T. H.; Chung, J. W.; Kim, B. Phys. Rev. B 2008, 77, 115324. (6) 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. (7) Qiao, M. H.; Tao, F.; Cao, Y.; Li, Z. H.; Dai, W. L.; Deng, J. F.; Xu, G. Q. J. Phys. Chem. B 2000, 104, 11211. (8) Konecny, R.; Doren, D. J. J. Am. Chem. Soc. 1997, 119, 11098. (9) Qiao, M. H.; Tao, F.; Cao, Y.; Xu, G. Q. Surf. Sci. 2003, 544, 285–294. (10) Cao, X.; Coulter, S. K.; Ellison, M. D.; Liu, H.; Liu, J.; Hamers, R. J. J. Phys. Chem. B 2001, 105, 3759. (11) Wang, G. T.; Mui, C.; Tannaci, J. F.; Filler, M. A.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2003, 107, 4982. (12) Qiao, M. H.; Cao, Y.; Deng, J. F.; Xu, G. Q. Chem. Phys. Lett. 2000, 325, 508.
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