J. Phys. Chem. B 2000, 104, 11211-11219
11211
Electronic and Vibrational Properties of Thiophene on Si(100) M. H. Qiao,†,‡ Y. Cao,†,‡ F. Tao,† Q. Liu,†,‡ J. F. Deng,‡ and G. Q. Xu*,† Department of Chemistry, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore, and Department of Chemistry, Fudan UniVersity, Shanghai 200433, People’s Republic of China ReceiVed: June 12, 2000; In Final Form: August 20, 2000
The surface reactions of thiophene on Si(100)-2×1 have been investigated as part of a larger study on the interaction between five-membered heterocyclic aromatic molecules and silicon substrates. Using XPS, UPS, and HREELS, two adsorption states are identified at 120 K, corresponding to physisorbed and chemisorbed thiophene. The former desorbs below 200 K, whereas the latter strongly binds to the surface, showing lower C(1s) and S(2s) binding energies. HREELS reveals that chemisorbed thiophene is a 2,5-dihydrothiophenelike species which exhibits coverage-dependent reorientation from nearly parallel to tilted relative to the surface plane. Based on the frontier molecular orbital theory and work function measurements, an inverse Diels-Alder cycloaddition mechanism for thiophene with Si(100)-2×1 is proposed. Above 400 K, chemisorbed species either desorbs as molecular thiophene or decomposes possibly via R-thiophenyl and Si-H, and metallocycle-like intermediate and atomic S through a sulfur abstraction mechanism. By 1000 K, the S(2s) peak has disappeared, leaving silicon carbide on the substrate.
I. Introduction The growing interest in studying thiophene and its derivatives lies not only in better understanding of the electronic properties of five-membered heteroatom aromatic rings, but in practical realms as well. Thiophene is notorious as a poison for catalysts used in petroleum refining and exhaust gas conversion in automobiles.1,2 On the other hand, thiophene is the building block for polythiophene, a kind of conductive polymer by proper doping, which is of great importance in view of applications such as electrochromic displays, electrooptic devices (color switching and memory), protection of semiconductors against photocorrosion, and energy storage.3-6 The surface chemistry of thiophene has been widely explored over transition metals. Unlike thiophene in organometallic complexes in which thiophene coordinates with a central metal atom in η1 (via S lone pair), η2 (via one CdC moiety), η4 (via two CdC moieties), or η5 (via ring aromatic π system) configurations,7 the bonding of thiophene on metal surfaces is relatively less complicated. Generally at low temperature and low coverage, the π interaction dominates and thiophene exhibits a parallel or nearly parallel configuration similar to that of benzene on metals.8 In some cases, a tilted geometry with respect to the surface plane can be formed at higher coverages due to the compression of the loosely packed overlayer with the replacement of the π interaction by S lone pair as Lewis base.9,10 Decomposition of thiophene on transition metals is usually via three different intermediates as shown in Figure 1.10-18 The ultimate decomposition products are reported to be closely dependent on experimental conditions and substrates. Under ultrahigh vacuum on Ru(0001),14 Mo(100),10 Mo(110),19 Re(0001),20 and Ni(111)15 thiophene primarily decomposes to surface sulfur, carbon, and hydrogen. On Pt(111),11,21 stepped Pt,21 Pd(111),22 W(211),17 and Rh(111),13 trace levels of * To whom correspondence should be addressed. † National University of Singapore. ‡ Fudan University.
Figure 1. Three possible intermediates involved in thiophene decomposition on transition metals: (a) R-H substituted thiophene, (b) metallocycle-like intermediate, and (c) polymerized thiophene.
hydrocarbons are detected. Only at high pressure are large quantities of hydrocarbon products observed on various singlecrystal faces of Mo and Re.23 On semiconductor surfaces, however, limited works have been carried out. On Si(111)-2×1, at low temperature and low coverage, two different adsorption states were identified.24 One is a π-bonded parallel state similar to that on transition metals. The other is a σ-bonded state, in which the thiophene loses a hydrogen atom at its R-position with respect to the sulfur atom and subsequently forms a σ-bond between the carbon atom and a silicon surface atom. At room temperature, an HREELS, XPS, and UPS combined study provided evidence for a desulfurization reaction, by which sulfur atom is directly abstracted from the thiophene ring and the remaining C4H4 moiety does not lose any isolated hydrogen atoms as a lack of discernible Si-H loss peak.24,25 However, splitting of the carbonaceous moiety into smaller parts without C-H bond scission is also possible. On Si(111)-7×7,26 previous work has shown that adsorption of thiophene at room temperature leads to two molecular desorption states which were supposed to involve no C-H bond breakage but rather a σ bond through the lone-pair electrons of sulfur and a π-bonded state. Recently, a combined HREELS and STM study27 demonstrates that thiophene undergoes a [4 + 2] cycloaddition reaction toward the adjacent rest atom-adatom pairs on Si(111)-7×7. The adsorption configuration of thiophene on Si(100)-2×1 at 300 K was investigated using LEED, AES, UPS, and semiempirical PM3 calculations.28 It was found that the (2 ×
10.1021/jp002101p CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000
11212 J. Phys. Chem. B, Vol. 104, No. 47, 2000 1) LEED pattern is sustained after the saturated exposure of thiophene, and the saturation coverage with respect to the atomic density of surface silicon is estimated to be ∼0.6 by AES, suggesting that thiophene molecule is chemisorbed molecularly on the Si(100) surface most likely by σ-bonding between C and Si atoms. UPS spectrum for the chemisorbed species shows binding energy shifts for the π and σ orbitals. Semiempirical calculations based on the cluster model suggested that thiophene molecule adsorbs on the Si(100)-2×1 surface by forming di-σ bonds between one thiophene CdC double bond and Si dimer atoms on the surface by analogy to that of ethylene on Si(100)2×1.29-33 However, further vibrational information will be necessary and helpful in establishing the bonding configuration of chemisorbed thiophene. On the other hand, the mechanistic study of the bonding and thermal stability of organics on semiconductors, especially on the technologically important Si(100) surface, is of great significance in view of transplanting the photoemission, photoabsorption, and biofunctional properties of organic molecules to silicon surfaces.34,35 Due to the diversity of organic compounds, the surface property of silicon can be tailored with ease by selecting suitable organic molecules. In general, the formation of a stable interface between organic thin film and Si(100) can be achieved by the following ways: (1) formation of di-σ bonds between unsaturated olefin and silicon dimer via a [2 + 2] mechanism;36 (2) formation of self-assembled monolayers (SAMs) via strong σ interaction between silicon atom and hydrocarbons;37-40 and (3) formation of a [4 + 2] Diels-Alder cycloadduct with conjugated olefin as diene and silicon dimer as dienophile. The latter was first predicted by Konecny and Doren41 based on B3LYP hybrid Hartree-Fock/density functional method and was readily confirmed by an IR study on adsorption of 1,3-butadiene and 2,3-dimethyl-1,3-butadiene on the Si(100)-2×1 surface.42 The target molecule in the present study, thiophene, however, is much more complicated. Unlike common conjugated molecules, thiophene shows considerable aromaticity; also unlike benzene, it has inhomogeneous electron distribution within the whole ring system and a heteroatom, sulfur, with lone-pair electrons. Thus the interesting bonding behavior of thiophene is expected. Furthermore, there is no report on the thermal evolution of thiophene overlayer on Si(100)-2×1. All these motivate the present XPS, UPS, and HREELS combined study on the adsorption and decomposition behavior of thiophene on Si(100)-2×1. II. Experimental Section The experiments were performed in two ultra-high-vacuum (UHV) chambers both with a base pressure better than 2 × 10-10 Torr. One of them is equipped with a high-resolution electron energy loss spectrometer (HREELS, LK-2000). EELS measurements were taken in a specular geometry. The electron beam with an energy of 5.0 eV impinges on the surface at an incident angle of 60° with respect to the surface normal. A typical instrumental resolution of 5 meV (40 cm-1) is achieved. Photoelectron studies were carried out on the other chamber mainly equipped with an X-ray source, a He discharge lamp, and a concentric hemispherical energy analyzer (CLAM2, VG). XPS spectra were acquired using Al KR radiation (hν ) 1486.6 eV) and 20 eV pass energy. For XPS the binding energy (BE) scale is referenced to the peak maximum of the Si(2p) line (99.3 eV)43 of the clean Si(100) substrate with full width at halfmaximum (fwhm) of less than 1.2 eV. He II (hν ) 40.8 eV) was selected for obtaining valence band spectra due to its low secondary electron background, while He I (hν ) 21.2 eV)
Qiao et al. spectra were mainly used to extract the variation of work function with thiophene exposure. In UPS studies, the pass energy was set at 10 eV. The Si(100) samples were cut from p-type boron-doped silicon wafers (99.999%, 1-30 Ω‚cm, Goodfellow) and were mounted as follows. Two pieces of Si(100) single crystals of the same dimension (18 × 10 × 0.38 mm3) were first covered by evaporation with a thin Ta layer on their unpolished backs for homogeneous heating and cooling. A piece of Ta foil (0.025 mm thick, Goodfellow) was sandwiched between the Ta-covered backs of two silicon samples as a heater. The samples were clamped together using two Ta clips. The Ta foil was then spotwelded to Ta rods connected to the feedthroughs of the manipulator. A 0.003 in. W-5% Re/W-26% Re thermocouple was attached to the center of one silicon sample using a high temperature ceramic adhesive (Aremco516) for temperature measurement and control. Such a mounted silicon sample can be resistively heated to 1400 K and conductively cooled to 120 K using liquid nitrogen. The temperature distribution on the sample was within (10 K at 1000 K as identified by an IR pyrometer. The silicon sample was then thoroughly degassed at 900 K overnight under ultrahigh vacuum. Surface contaminants, such as carbon and oxygen, were removed by repeated Ar+ bombardment and annealing to 1300 K. Surface cleanliness was confirmed by XPS, UPS, and HREELS. Thiophene (99+%, Aldrich) was purified by freeze-pump-thaw cycles prior to use. Dosing was accomplished by backfilling the chamber through a variable leak valve without ion gauge sensitivity calibration. III. Results III.A. Adsorption at 120 K. X-ray photoelectron spectroscopy measurements were employed to investigate the chemical states of thiophene on Si(100). The C(1s) and S(2s) spectra of thiophene following a sequence of exposures at 120 K are shown in Figure 2a and Figure 2b, respectively. The S(2p) spectrum was not offered because of its overlapping with one of the plasmon peaks of the substrate silicon.44 For exposures from 0.1 langmuir (L) (1 langmuir (L) ) 1 × 10-6 Torr‚s) to ∼4.0 L, only one adsorption state is observed for thiophene, which is characterized by a C(1s) peak at 284.6 eV and a S(2s) peak at 227.3 eV. Further dosing of thiophene leads to dominant features at 285.0 and 228.6 eV for C(1s) and S(2s), respectively. For gas-phase thiophene, Gelius et al. reported a single C(1s) peak at ∼290.4 eV relative to the vacuum level (Ev), which is intrinsically composed of two peaks of same intensity with a binding energy separation of 0.34 ( 0.12 eV.45 For condensed thiophene, due to the solid state broadening effect, only one C(1s) peak at 285.0 eV relative to the Fermi level (EF) was resolved.46-48 Thus the C(1s) peak at 285.0 eV at 8.0 L thiophene exposure is attributed to physisorbed thiophene. This is further supported by S(2s) binding energy (BE) of 228.6 eV for bulk thiophene,44,46 which is in excellent agreement with our observation. The assignment of the lower lying state with BE(C(1s)) ) 284.4 eV and BE(S(2s)) ) 227.3 eV is not so straightforward. However, the dissociative adsorption of thiophene on Si(100) at 120 K can be readily excluded. If thiophene decomposed into atomic sulfur and carbonaceous moiety upon adsorption, the S(2s) for atomic sulfur should be at least 1 eV lower in BE.11 In addition, thiophene dissociation into thiolate and other smaller parts would broaden the C(1s) peak as carbons in different chemical environments should exist, which is incon-
Properties of Thiophene on Si(100)
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11213
Figure 3. Coverage-dependent He II (hν ) 40.8 eV) UPS spectra for thiophene on Si(100)-2×1 at 120 K. The bar graph at the bottom of (h) indicates the gas-phase ionization potentials of thiophene (ref 49), which are shifted to align with the multilayer spectrum. Pass energy ) 10 eV.
Figure 2. (a) C(1s) and (b) S(2s) XPS spectra of thiophene on Si(100)-2×1 as a function of thiophene exposures at 120 K. hν ) 1486.6 eV, pass energy ) 20 eV.
sistent with the virtually invariant C(1s) peak width (fwhm < 1.5 eV) with increasing thiophene exposures. Previous studies demonstrated molecular adsorption character of thiophene on cleaved Si(111)-2×124 at low temperature or on Si(111)-7×726,27 at room temperature. For thiophene adsorption on Si(100)-2×1 at 300 K, molecular desorption was observed at ∼430 K.28 Only room-temperature exposure of thiophene on reactive Si(111)2×1 leads to desulfurization. In this experiment, our XPS result is consistent with the nondissociative nature of chemisorbed thiophene on Si(100) at 120 K. He II valence band spectra of adsorbed thiophene on Si(100)
at 120 K as a function of thiophene exposures are shown in Figure 3. For comparison, gas-phase ionization potentials from ref 49 are rigidly shifted to align with our spectrum of physisorbed multilayer and are displayed at the bottom of Figure 3h in the form of a bar graph. For a clean Si(100) surface (Figure 3a), the peak at 0.7 eV below EF is attributed to the existence of the bonding “dangling bond” (πb) on the surface, according to the STM study of the occupied states of Si(100) by Hamers et al.50 Increasing thiophene exposures leads to the attenuation of the surface state and its total quenching at ∼4.0 L dosage. The loss of the surface state is possibly caused by the redistribution of its electron density in the resulting adsorbatesubstrate complex. Meanwhile, chemisorbed species induced emissions appear with maxima at ∼2.1, 3.3, 6.3, 7.3, 10.0, and 11.5 eV below EF, which are distinctively different from those of gas-phase thiophene. At higher exposures, thiophene multilayer builds up and the features of the chemisorbed layer are attenuated. The peak at 3.6 eV below EF is assigned to the convolution of the 1a2 and 2b1 levels of π character of physisorbed thiophene. In addition, a broad peak extending from 5.0 to 10.0 eV below EF consists of emissions from five overlapping levels of 6a1 nonbonding orbital mainly located on sulfur, 1b1 (π) level, and σ orbitals (σ C-C, C-H, and C-S) of 4b2, 5a1, and 3b2 symmetries. The lower lying feature with two apparent maxima at BEs of ∼11.0 and 12.1 eV is assigned to the molecular orbitals of 4a1, 2b2, and 3a1 symmetries. The close resemblance of the valence band spectrum after 8.0 L thiophene exposure on Si(100) at 120 K with the gas-phase spectrum clearly demonstrates the formation of thiophene multilayer. The work function variation is extracted from measuring the shift of secondary electron cutoffs in the He I spectra for different thiophene exposures at 120 K. Figure 4 shows that the work function decreases sharply upon thiophene exposures below 1.0 L. Above that exposure, it decreases slowly and levels off above 4.0 L with a maximum work function change (∆Φ) of about -0.85 eV, indicating the saturation of chemisorption.
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Figure 4. Coverage-dependent work function variation of thiophene on Si(100)-2×1 at 120 K.
This observation is in line with the coverage-dependent XPS results and the He II UPS spectra shown above. The decrease in work function upon thiophene adsorption on Si(100) suggests electron donation from chemisorbed thiophene to the substrate. By fitting the C(1s) spectrum of 4.0 L thiophene exposure at 120 K, we estimated a saturation coverage of ∼0.44 for chemisorbed thiophene with respect to the surface atomic density of Si(100)-2×1, implying that one thiophene molecule consumes one silicon surface dimer. Figure 5 shows the electron energy loss spectra under specular mode after thiophene adsorption on Si(100) at 120 K. The vibrational frequencies and corresponding assignments for 12.0 L thiophene exposed surface are listed in Table 1. From the table, it can be seen that all of these frequencies agree well with the infrared analysis of vapor-phase thiophene51 and previous EELS studies on transition metals. Two out-of-plane bending modes (B2 symmetry) are found at 468 [γ(ring)] and 724 cm-1 [γ(CH)], with in-plane modes of A1 and B1 symmetries ranging from 816 to 3095 cm-1. At such an exposure, the intensity ratio of the out-of-plane and in-plane CH stretching modes is ∼6, similar to that of multilayer thiophene on Cu(100),9 Ru(0001),14 or Mo(100),10 but somewhat lower than that on Fe(100),18 in which the intensities of these two modes are comparable. However, the features of chemisorbed thiophene at low exposure (Figure 5a) are significantly different. The ν(CH) mode splits into two peaks at 2920 and 3054 cm-1 with identical intensity, attributable to the stretching of sp3 C-H and sp2 C-H bonds, respectively. The rehybridization of some carbons from sp2 to sp3 suggests the loss of aromaticity of the thiophene ring when chemisorbed on Si(100) at 120 K. In addition, a strong loss peak at ∼622 cm-1 is identified and assigned to the outof-plane bending of dC-H. The loss peak at ∼490 cm-1 is attributed to the hindered translation/rotation of chemisorbed thiophene. Although atomic sulfur on Si(111)-7×7 also exhibits a broad and weak feature near 490 cm-1,52 the much higher binding energy of S(2s) (227.3 eV) of chemisorbed species observed in this work compared to atomic S on Pt(111) (226.4 eV)11 or on Si(100) (226.2 eV) shown below makes this assignment unlikely. Detailed spectrum assignment is shown in Table 2. The negligible feature at ∼2051 cm-1 due to ν(SiH)53 rules out the dissociative nature of chemisorbed thiophene by forming a Si-C bonded σ complex and a Si-H bond through the breakage of one R-C-H bond of thiophene. Further, such
Figure 5. HREELS spectra of thiophene on Si(100)-2×1 at 120 K. Ep ) 5.0 eV, specular mode.
a σ-bonded species is not consistent with the observed interruption of π conjugation in the present study, evidenced in the splitting of the CH stretching mode. It is interesting to note that at 1.0 L thiophene exposure, the intensity ratio between the out-of-plane and in-plane CH stretching modes increases to ∼25 from ∼6 for multilayer thiophene. Thiophene molecules are expected to be randomly packed in the multilayer. Since the surface selection rule can be applied for semiconductors with a high dielectric constant,54 the increase in the intensity ratio between γ(CH) and ν(CH) modes suggests that the chemisorbed species bears a nearly parallel configuration relative to the surface plane at 1.0 L thiophene exposure. III.B. Thermal Evolution. Shown in Figure 6 are the C(1s) and S(2s) XPS spectra after exposing 8.0 L of thiophene onto the surface at 120 K and annealing it to various temperatures. When the surface is annealed to 150 K at a ramping rate of 1.5 K‚s-1, features for physisorbed thiophene decrease substantially with the appearance of the peaks at 284.7 and 227.4 eV for C(1s) and S(2s) respectively, corresponding to the chemisorbed species. Complete desorption of physisorbed molecules occurs at 200 K. Further annealing to 350 K does not lead to obvious changes in either peak intensity or peak shape of the C(1s) and S(2s) except for a continuous but small shift to lower BEs for both peaks. At 450 K, there is about 9% overall intensity loss for C(1s) or S(2s) as referenced to the saturated chemisorption spectra at 200 K, attributable to the molecular thiophene desorption. Accompanying molecular thiophene desorption, the C(1s) and S(2s) peaks are somewhat broadened, with the emergence of new features at the low binding energy side. The broadening of the photoemission peaks becomes evident at 500 K, while further annealing to 735 K leads to the formation of carbonaceous moiety with C(1s) of 284.0 eV and atomic sulfur with S(2s) of 226.2 eV. Finally at 1000 K, the C(1s) peak shifts to 282.7 eV due to the formation of silicon carbide,55 while the S(2s) peak virtually vanishes due to the possible desorption of SiS species.56
Properties of Thiophene on Si(100)
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TABLE 1: Assignment of the Vibrational Frequencies for Thiophene Physisorbed on Si(100)-2×1 at 120 K (Unit in cm-1) gas-phase thiophene51 452 608 712 839 1036 1083 1256 1360 1409 1503 3086, 3098, 3126
Cu(100) multilayer9
Ru(0001) multilayer14
Mo(100) multilayer10
Fe(100) multilayer18
Si(100) multilayer
450
468 626 724 816
460 730 830
760 880
735 840
720 860
1090 1270 1370 1430
1065 1250
1090 1265
1070
1088 1267
1410
1390
3130
3125
1435 1600 3150
1418 1597 3095
3080
assignment γ(ring) (B2) ν(ring) (A1) γ(CH) (B2) ν(ring) (A1) δ(CH) (A1) δ(CH) (A1) δ(CH) (B1) ν(ring) (A1) ν(ring) (A1) ν(CC) (B1) ν(CH) (A1)
TABLE 2: Comparison of the Vibrational Modes of 1.0 L Thiophene Exposure on Si(100)-2×1 at 120 K with Those of Gaseous 2,5-Dihydrothiophene (2,5-DHT) and 2,5-DHT on Mo(110)-(4×1)-S (Unit in cm-1) 2,5-dihydrothiophene gas phase60 641 669 716 824 953 961 1112 1114 1119 1227 1273 1343 1451 1647 2866 2936 3065
submonolayer 2,5-DHT on Mo(110)-(4×1)-S61
submonolayer thiophene on Si(100)-2×1
assignment
675
492 622
hindered translation/rotation )C-H out-of-plane bending
800
751
C-S-C stretching
950
946
C-C stretching
1100
1088
)C-H in-plane bending
1225
1180
)C-H in-plane bending
1325
CH2 wagging
1425 1625 2900
2920
CH2 deformation CdC stretching -C-H stretching
3050
3054
)C-H stretching
The thermal effect on the He II UPS spectra of thiophene/ Si(100) was also studied, and the results are shown in Figure 7. Figure 7a is obtained after annealing the 8.0 L thiophene exposed surface to 150 K. As expected from the XPS result, the chemisorbed thiophene is dominant at this temperature. However, the features induced by physisorbed thiophene, e.g., π1a2 and π2b1 levels at ∼3.6 eV do not disappear until 200 K. From 200 to 350 K, the valence band features of chemisorbed molecules remain unchanged. Above 400 K, there is an overall attenuation of emissions due to the partial desorption of the chemisorbed species in the form of molecular thiophene. All chemisorbed species related features become very weak at 500 K. At 735 K, the broad peaks centered around 3.6 and 7.0 eV below EF can be ascribed to atomically adsorbed S and carbonaceous species,57,58 consistent with our XPS results shown above. Upon annealing to 1000 K, these two broad peaks disappear, and only a small hump at ∼7.2 eV can be identified, which is attributable to the SiC species.58 The dissociation process was also monitored using HREELS, as shown in Figure 8. Figure 8b was taken after annealing a multilayer thiophene covered surface to 165 K. The most important change is the weakening of the in-plane CH stretching mode at 3095 cm-1 for physisorbed thiophene and the appearance of chemisorbed features (Figure 5b). The spectrum taken after annealing to 200 K (Figure 5c) shows the vibrational features of a saturated chemisorption monolyer, completely desorbing physisorbed thiophene. Further comparing Figure 8c with Figure 5a gives additional information on the evolution of the orientation of chemisorbed species. For the saturated chemisorption monolayer, the ratio of the out-of-plane and inplane CH vibration modes is about 7 (Figure 8c), much smaller
than that (∼25) of the submonolayer covered surface (Figure 5a). As the surface species is the same and the only difference is the coverage of chemisorbed species, this observation suggests the change of the orientation of chemisorbed species from nearly parallel to tilted with respect to the surface plane with increasing thiophene coverage. Above 200 K, the loss features remain constant until 450 K, at which temperature all features related to chemisorbed species except the loss peak at ∼509 cm-1 are weakened and the ν(CH) mode at the higher loss energy side of the doublet is decreased, indicating the occurrence of desorption and decomposition of chemisorbed species. It is worth noting that at 450 K there is a distinct ν(SiH) loss peak at 2080 cm-1, implying C-H bond scission in chemisorbed species or further decomposition of the carbonaceous moieties, leading to Si-H bond formation. Annealing to 700 K leads to the disappearance of the ν(CH) tail at higher loss energy. The broad loss features left at 519, 714, 1014, 2119, and 2948 cm-1 are mainly due to the modes related to Si-C, Si-S, Si-H, and C-H vibrations, respectively. At 1000 K, the EELS spectrum is dominated by the peak at 864 cm-1, corresponding to the ν(SiC) mode of silicon carbide.59 IV. Discussion IV.A. Bonding of Chemisorbed Thiophene on Si(100)-2×1. The bonding configurations of thiophene on silicon substrates have been explored by several research groups. In the present study, besides the physisorbed thiophene that desorbs below 200 K, the electronic and vibrational properties of the chemisorbed species have been characterized using XPS, UPS, and HREELS techniques in detail. In the C(1s) and S(2s) spectra
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Figure 7. He II (hν ) 40.8 eV) UPS spectra after an 8.0 L thiophene exposure at 120 K and annealing the sample to various temperatures. Pass energy ) 10 eV.
Figure 6. (a) C(1s) and (b) S(2s) spectra obtained after annealing the Si(100) preexposed to 8.0 L of thiophene at 120 K to various temperatures. hν ) 1486.6 eV, pass energy ) 20 eV.
(Figure 2), both the C(1s) and S(2s) signals show a single peak at 284.6 and 227.3 eV, respectively, which are about 0.4 and 1.3 eV lower in BE than that of physisorbed thiophene. Comparing our HREELS spectra of the chemisorbed species with that of chemisorbed thiophene on Si(111)-2×1 may shed some light on the understanding of the reactivity of Si(100)2×1 toward thiophene. For thiophene on Si(111)-2×1 at 85 K, HREELS spectra for exposures from 1 to 20 L give a distinct ν(SiH) mode at 2015 cm-1 and a multiple ν(CH) mode peaked at 2900 and 3200 cm-1 with the peak intensity of the latter being about 2 times stronger than the former.24 We do observe a doublet for the ν(CH) mode, however, peaking at 2920 and
Figure 8. HREELS spectra after an 8.0 L thiophene exposure at 120 K and annealing the sample to various temperatures. Ep ) 5.0 eV, specular mode.
3054 cm-1 with comparable intensities. Moreover, the bonding of thiophene on Si(100)-2×1 at 120 K does not involve the breakage of C-H bond and formation of Si-H bond, as there is no distinct SiH stretching mode either at low thiophene exposures (Figure 5a) or for the surface after multilayer thiophene desorption at 200 K (Figure 8c). The bonding models for unsaturated hydrocarbons on Si(100)2×1 have been proposed based on previous experimental facts
Properties of Thiophene on Si(100) and theoretical calculations. Two generally acknowledged bonding schemes are termed as [2 + 2] and [4 + 2] cycloaddition, forming a Si2C2 four-membered ring between one CdC double bond and one surface silicon dimer, and a Si2C4 sixmembered ring with the conjugated olefin as diene and silicon dimer as dienophile, respectively. It is expected that thiophene can react in a similar way with silicon dimer. Jeong et al.28 interpreted their experimental results using the framework of [2 + 2] cycloaddition, which can justify the appearance of the sp3 hybridized carbon related ν(CH) mode at 2920 cm-1 in our study. However, the close resemblance of the vibrational features of chemisorbed thiophene with that of gaseous 2,5-dihydrothiophene60 or 2,5-dihydrothiophene condensed on Mo(110)-(4×1)-S,61 as illustrated in Table 2, strongly favors the formation of the [4 + 2] cycloadduct between thiophene and silicon dimers with two R-carbons of thiophene rehybridized from sp2 to sp3 and bonded to a silicon dimer by forming two Si-C σ bonds. Such bonding configuration of thiophene on Si(100)-2×1 can rationalize the lack of CdC stretching mode in Figure 5a that otherwise should appear at around 1647 cm-1. The resulting [4 + 2] cycloadduct is expected to have a Cs symmetry with its symmetric plane perpendicular to the Si-Si dimer bond and the CdC double bond between two β-carbons while passing through the sulfur atom, which is confirmed by the PM3 semiempirical calculation.62 Thus the CdC bond is parallel to the surface plane with no vertical vibrational component with respect to the surface plane. Furthermore, the C(1s) binding energy of chemisorbed thiophene at 284.6 eV is consistent with that for 2,5-dihydrothiophene on Mo(110)-(4×1)-S,61 lending further support to the formation of the 2,5-dihydrothiophenelike cycloadduct. Based on the above argument, we attributed the loss peak at 492 cm-1 for chemisorbed thiophene to hindered translation/rotation of the 2,5-dihydrothiophene-like cycloadduct similar to that of the di-σ-bonded ethylene or acetylene on Si(100)-2×1.33 The UPS results provide further information about the orbitals involved in the formation of the 2,5-dihydrothiophene-like cycloadduct. The quenching of the silicon bonding “dangling bond” (πb) state and the lack of the thiophene π1a2 and 2b1 levels of π character strongly suggest that chemisorption of thiophene on Si(100) mainly involves the thiophene π bonds and the silicon dangling bonds. Instead of the elimination of the π features, a new band at ∼2.1 eV below EF is formed. For benzene chemisorption, a new peak at 2.3 eV below EF is observed on cleaved Si(111)-2×163 or Si(100)-2×1,64 and at 3.0 eV on Si(111)-7×7.65 The occurrence of such a much shallower occupied molecular orbital is interpreted as splitting of the degenerated π1e2u orbital by electron donation from the silicon substrate. This argument is compatible with the frontier molecular orbital (FMO) theory, in which the reactant pair that has the lowest energy difference between their frontier orbitals should be the most reactive.66 As illustrated in Figure 9, for benzene the highest occupied molecular orbital (HOMO), π1e1g, lies at 9.2 eV below Ev while the lowest unoccupied molecular orbital (LUMO), π1e2u, lies at 4.2 eV below Ev.65 By taking into account of the work function of 4.85 eV for Si(100)-2×1, the bonding “dangling bond” (πb) level locates at 5.65 eV below Ev while the antibonding “dangling bond” (π*a) level locates at 4.50 eV below Ev.50 For symmetry consideration, only overlapping between LUMObenzene and πb,Si or π*a,Si and HOMObenzene can result in stable chemical bonds. Referring to their energy differences ∆E’s (∆E(LUMObenzene - πb,Si) and ∆E(π*a,Si - HOMObenzene)) of 1.45 and 4.70 eV, respectively,
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11217
Figure 9. Schematic orbital energy correlation diagram of frontier orbitals (HOMO and LUMO) of benzene and thiophene with Si(100)2×1 bonding “dangling bond” (πb) and the antibonding “dangling bond” (π*a) orbitals.
FMO theory readily justifies the direction of electron donation from the silicon πb level to the benzene empty π1e2u orbital, in agreement with the previous interpretation. If we take benzene as diene and silicon dimer as dienophile, the formation of the 1,4-cyclohexadiene-like adsorption complex for benzene on Si(100)-2×1 at 85 K64 can be termed as normal Diels-Alder reaction. However, it is not the case for thiophene chemisorption on Si(100). In Figure 9 we also show the HOMO and LUMO energies for gaseous thiophene molecule. Similarly, we obtain the energy differences of 6.80 and 4.46 eV for ∆E(LUMOthiophene - πb,Si) and ∆E(π*a,Si - HOMOthiophene), respectively.49,67 Thus, an inverse Diels-Alder reaction may occur for the thiophene/ Si(100) system, which results in electron flow from the adsorbate to the substrate. This is not surprising, as five-membered heterocyclic aromatic molecules such as pyrrole, furan, and thiophene are excessive in π electrons as compared to benzene.68 This approach is consistent with the work function variation with respect to thiophene exposures at 120 K. Thus we ascribe the newly formed low-lying valence band feature as a downshift of the antibonding “dangling bond” level of silicon dimers in the present case. IV.B. Reorientation of Chemisorbed Thiophene on Si(100)-2×1. HREELS study uncovers that the resulting [4 + 2] cycloadduct can reorient with the variation of surface coverage. In multilayer thiophene molecules are randomly packed with an intensity ratio of ∼6 between the out-of-plane and in-plane CH stretching modes, similar to that of multilayer thiophene on Cu(100),9 Ru(0001),14 or Mo(100).10 However, the ratio is ∼25 for the submonolayer-covered Si(100) obtained after 1.0 L thiophene exposure at 120 K (Figure 5a), which decreses to about 7 for a saturated chemisorption layer (Figure 8c). This is attributable to the possible reorientation of chemisorbed species from nearly parallel to tilted to minimize the lateral repulsion within the chemisorbed layer with increasing surface coverage. It has been recognized that the orientation of thiophene with respect to transition metal surfaces is sensitive to surface coverage. On most surfaces, a parallel geometry is favored at low coverages, but converts to a more perpendicular disposition of the ring at high coverages. For thiophene on Cu(100) at 80 K,9 prior to multilayer formation, it exhibits two distinct adsorption states with the low-temperature state bearing strong activity of all the in-plane vibrations. The change in the average tilting angle of the rings with coverage was experimentally demonstrated. A “compression” model similar to that of Demuth et al. for pyridine on Ag(111)69 was then proposed. An NEXAFS study by Sto¨hr et al. found that after multilayer desorption from
11218 J. Phys. Chem. B, Vol. 104, No. 47, 2000 Pt(111) at 150 K thiophene is oriented with the ring plane tilted by ∼40° relative to the surface plane. Although the submonolayer spectra obtained at 180 K are also dominated by the same resonances as those at 150 K, the angular dependence is more pronounced. The strong and opposite angular dependence of the π and σ resonances clearly shows that thiophene lies flat on the surface.11 The exceptions are Rh(111)13 and Cu(111),70 for which there are parallel geometries at all coverages studied. Such a coverage-dependent phenomenon may be analogous to the steric interactions between neighboring ligands in a discrete cluster or complex.8 IV.C. Thermal Dissociation of Thiophene on Si(100)-2×1. Previous work demonstrated several dissociation pathways of thiophene on transition metal surfaces. It has been suggested that thiophene desulfurizes and forms metallocycles on metal surfaces, such as Pt(111),11 Rh(111),13 Ru(0001),14 and Ni(100).15 The metallocyclic intermediate resembles the thiophene ring structure with the sulfur atom replaced by surface metal atom and decomposes at higher temperatures. On W(211)17 and Mo(100)10 surfaces, however, it was proposed that electrophilic attack at the R position results in the formation of carbon-bonded surface species. Moreover, polymerized thiophene was found on Ni(111)16 and Fe(100)18 surfaces, following electrophillic attack at the R position. These mechanistic studies show that thiophene behaves very differently on different transition metal surfaces. On the other hand, thiophene adsorbs molecularly on Si(111)7×7 surface,26,27 while on cleaved Si(111)-2×1 a desulfurization process was observed even at room temperature with no C-H bond breakage.24,25 In the present study, a desulfurization reaction occurs at ∼450 K and is completed at ∼700 K, characterized by the S(2s) peak at 226.2 eV for atomic S and the ν(SiS) mode at ∼520 cm-1. Further heating to 1000 K leads to the desorption of S-related species and the formation of silicon carbide. However, to unambiguously determine the reaction intermediate(s) seems infeasible due to the coexistence of 2,5dihydrothiophene-like cycloadduct and the dissociation products of the reaction intermediate. In addition, the involvement of the C-H bond scission makes the assignment even more difficult. However, it is worth noting that, at 700 K, the losses in the range of 840-1400 cm-1 corresponding to δ(ring) and ν(ring) modes are still discernible, suggesting the possible formation of a metallocycle-like intermediate with the substrate after complete desulfurization. As there is no distinct signal available to aid in the identification of the product after C-H bond scission, we suppose that this reaction pathway is only a minor process. By analogy to thiophene on Si(111)-2×1 surface at low temperatures,24 we tentatively attribute it to be the formation of a small amount of the σ-bonded state, in which chemisorbed thiophene loses a hydrogen atom at the R position and subsequently forms R-thiophenyl through σ bonding between the R-carbon atom and a silicon surface atom. It is also suggested that formation of such a σ-bonded state needs additional active sites, possibly provided by the desorption of molecular thiophene at 450 K. As the active sites are limited (only about 9% molecular thiophene desorbed), the dissociation process is then dominated by direct desulfurization. V. Conclusions (1) Adsorption of thiophene on Si(100)-2×1 at 120 K leads to the physisorbed thiophene and chemisorbed 2,5-dihydrothiophene-like species. The former desorbs from the surface by 200 K while the latter remains intact below 450 K. (2) The absolute saturation coverage of chemisorbed species with respect to the surface atomic density of Si(100)-2×1 is
Qiao et al. about 0.44 monolayer. Based on the FMO theory and work function measurements, an inverse Diels-Alder cycloaddition mechanism for thiophene with silicon dimers on Si(100)-2×1 is proposed. The resulting cycloadduct can reorient from nearly parallel to tilted with respect to the surface plane with increasing surface coverage. (3) Upon annealing, about 9% of the saturated monolayer desorbs molecularly while the remaining part undergoes further reaction. Above 450 K, two possible reaction pathways are suggested to be involved in the dissociation process. One is by the formation of R-thiophenyl and Si-H, and the other is to produce a metallocycle-like intermediate after atomic S abstraction. At 1000 K about 60% of the carbon from chemisorbed thiophene is left on the surface, forming silicon carbide. Acknowledgment. This work was supported by the National University of Singapore under Grant No. RP3981644. References and Notes (1) Gates, B. C. Catalytic Chemistry; Wiley: New York, 1992. (2) Satterfield, C. N. Heterogeneous Catalysis in Practice, 2nd ed.; McGraw-Hill: New York, 1991. (3) Koezuda, H.; Etoh, S. J. Appl. Phys. 1983, 54, 2511. (4) Tsumura, A.; Tossi, L.; Katz, H. E. Science 1995, 268, 270. (5) Handbook of Conducting Polymers; Skotheim, T. Ed.; Dekker: New York, 1986. (6) Fujimoto, H.; Nagashima, U.; Inokuchi, H.; Seki, K.; Cao, Y.; Nakahara, H.; Nakayama, J.; Hoshino, M.; Fukuda, K. J. Chem. Phys. 1990, 92, 4077. (7) Angelici, R. J. Bull. Soc. Chim. Belg. 1995, 104, 265. (8) Wiegand, B. C.; Friend, C. M. Chem. ReV. 1992, 92, 491. (9) Sexton, B. A. Surf. Sci. 1985, 163, 99. (10) Zaera, F.; Kollin, E. B.; Gland, J. L. Surf. Sci. 1987, 184, 75. (11) Sto¨hr, J.; Gland, J. L.; Kollin, E. B.; Koestner, R. J.; Johnson, A. L.; Muetterties, E. L.; Sette, F. Phys. ReV. Lett. 1984, 53, 2161. (12) Zaera, F.; Kollin, E. B.; Gland, J. L. Langmuir 1987, 3, 555. (13) Netzer, F. P.; Bertel, E.; Goldmann, A. Surf. Sci. 1988, 201, 257. (14) Heise, W. H.; Tatarchuk, B. J. Surf. Sci. 1989, 207, 297. (15) Huntley, D. R.; Mullins, D. R.; Wingeier, M. P. J. Chem. Phys. 1996, 100, 19620. (16) Schoof, G. R.; Preston, R. E.; Benziger, J. B. Langmuir 1985, 1, 313. (17) Preston, R. E.; Benziger, J. B. J. Phys. Chem. 1985, 89, 5010. (18) Cheng, L. C.; Bocarsly, A. B.; Bernasek, S. L.; Ramanarayanan, T. A. Surf. Sci. 1997, 374, 357. (19) Roberts, J. T.; Friend, C. M. Surf. Sci. 1987, 186, 201. (20) Kelly, D. G.; Odriozola, J. A.; Somorjai, G. A. J. Phys. Chem. 1987, 91, 5695. (21) Lang, J. F.; Masel, R. I. Surf. Sci. 1987, 183, 44. (22) Gentle, T. M. Energy Res. Abstr. 1984, 9, 38058. (23) Bussell, M. B.; Gellman, A. J.; Somorjai, G. A. J. Catal. 1988, 110, 426. (24) Piancastelli, M. N.; Kelly, M. K.; Margaritondo, G.; Frankel, D. J.; Lapeyre, G. J. Phys. ReV. B 1986, 34, 3988. (25) Piancastelli, M. N.; Zanoni, R.; Kelly, M. K.; Kilday, D. G.; Chang, Y.; McKinley, J. T.; Margaritondo, G.; Perfetti, P.; Quaresima, C.; Capozi, M. Solid State Commun. 1987, 63, 85. (26) Hu, D. Q.; MacPherson, C. D.; Leung, K. T. Solid State Commun. 1991, 78, 1077. MacPherson, C. D.; Leung, K. T. Phys. ReV. B 1995, 51, 17995. (27) Cao, Y.; Yong, K. S.; Wang, Z. Q.; Chin, W. S.; Lai, Y. H.; Deng, J. F.; Xu, G. Q. J. Am. Chem. Soc. 2000, 122, 1812. (28) Jeong, H. D.; Lee, Y. S.; Kim, S. J. Chem. Phys. 1996, 105, 5200. (29) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. J. Chem. Phys. 1987, 87, 7332. (30) Cheng, C. C.; Wallace, R. M.; Taylor, P. A.; Choyke, W. J.; Yates, J. T., Jr. J. Appl. Phys. 1990, 67, 3693. (31) Clemen, L.; Wallace, R. M.; Taylor, P. A.; Dresser, M. J.; Weinberg, W. H.; Choyke, W. J.; Yates, J. T., Jr. Surf. Sci. 1992, 268, 205. (32) Widdra, W.; Huang, C.; Weinberg, W. H. Surf. Sci. 1995, 329, 295. (33) Widdra, W.; Huang, C.; Yi, S. I.; Weinberg, W. H. J. Chem. Phys. 1996, 105, 5605. (34) Yates, J. T., Jr. Science 1998, 279, 335. (35) Amato, I. Science 1998, 282, 402. (36) Hamers, R. J.; Hovis, J. S.; Lee, S.; Liu, H.-B.; Shan, J. J. Phys. Chem. B 1997, 101, 1489.
Properties of Thiophene on Si(100) (37) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (38) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. J. Am. Chem. Soc. 1996, 118, 7225. (39) Song, J. H.; Sailor, M. J. J. Am. Chem. Soc. 1998, 120, 2376. (40) Bergerson, W. F.; Mulder, J. A.; Hsung, R. P.; Zhu, X.-Y. J. Am. Chem. Soc. 1999, 121, 454. (41) Konecny, R.; Doren, D. J. J. Am. Chem. Soc. 1997, 119, 11098. (42) Teplyakov, A. V.; Kong, M. J.; Bent, S. F. J. Am. Chem. Soc. 1997, 119, 11100. (43) Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, 1992. (44) He, J.; Patitsas, S. N.; Preston, K. F.; Wolkow, R. A.; Wayner, D. D. M. Chem. Phys. Lett. 1998, 286, 508. (45) Gelius, U.; Allan, C. J.; Johansson, G.; Siegbahn, H.; Allison, D. A.; Siegbahn, K. Phys. Scr. 1971, 3, 237. (46) Clark, D. T.; Lilley, D. M. J. Chem. Phys. Lett. 1971, 9, 234. (47) Land, T. A.; Hemminger, J. C. Surf. Sci. 1992, 268, 179. (48) Tourillon, G.; Jugnet, Y. J. Chem. Phys. 1988, 89, 1905. (49) Kishimoto, N.; Yamakado, H.; Ohno, K. J. Phys. Chem. 1996, 100, 8204. (50) Hamers, R. J.; Avouris, Ph.; Bozso, F. J. Vac. Sci. Technol. A 1988, 6, 508. (51) Rico, M.; Orza, J. M.; Morcillo, J. Spectrochim. Acta 1965, 21, 689. (52) Chakarov, D. V.; Ho, W. Surf. Sci. 1995, 323, 57. (53) Wagner, H.; Butz, R.; Backes, U.; Bruchmann, D. Solid State Commun. 1981, 38, 1155. (54) Ibach, H.; Mills, D. L. Electron Energy Loss Spectroscopy and Surface Vibrations; Academic Press: New York, 1982.
J. Phys. Chem. B, Vol. 104, No. 47, 2000 11219 (55) Rochet, F.; Jolly, F.; Bournel, F.; Dufour, G.; Sirotti, F.; Cantin, J.-L. Phys. ReV. B 1998, 58, 11029. (56) Papageorgopoulos, A.; Kamaratos, M. Surf. Sci. 1996, 352/354, 364. (57) Schro¨der-Bergen, E.; Ranke, W. Surf. Sci. 1990, 236, 103. (58) Bu, Y.; Breslin, J.; Lin, M. C. J. Phys. Chem. B 1997, 101, 1872. (59) Froitzheim, H.; Ko¨hler, U.; Lammering, H. Phys. ReV. B 1984, 30, 5771. (60) Green, W. H.; Harvey, A. B. Spectrochim. Acta 1969, 25A, 723. (61) Xu, H.; Friend, C. M. J. Phys. Chem. 1993, 97, 3584. (62) Qiao, M. H.; Tao, F.; Deng, J. F.; Xu, G. Q. Unpublished results. (63) Piancastelli, M. N.; Kelly, M. K.; Chang, Y.; McKinley, J. T.; Margaritondo, G. Phys. ReV. B 1987, 35, 9218. (64) Gokhale, S.; Trischberger, P.; Menzel, D.; Widdra, W.; Dro¨ge, H.; Steinru¨ck, H.-P.; Birkenheuer, U.; Gutdeutsch, U.; Ro¨sch, N. J. Chem. Phys. 1998, 108, 5554. (65) Carbone, M.; Piancastelli, M. N.; Zanoni, R.; Comtet, G.; Dujardin, G.; Hellner, L. Surf. Sci. 1998, 407, 275. (66) Fleming, I. Frontier Orbitals and Organic Chemical Reactions; Wiley: London, 1976; Chapter 4. (67) Jones, D.; Guerra, M.; Favaretto, L.; Modelli, A.; Fabrizio, M.; Distefano, G. J. Phys. Chem. 1990, 94, 5761. (68) Eicher, T.; Hauptmann, S. The Chemistry of Heterocycles; Thieme: Stuttgart, 1995. (69) Demuth, J. E.; Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980, 76, 201. (70) Richardson, N. V.; Campuzano, J. C. Vacuum 1981, 31, 449.
