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Si(1 0 0)-2 × 1
Chemical Physics Letters 411 (2005) 75–80 www.elsevier.com/locate/cplett
Coexistence of ketenimine species and tetra-r adduct at acetyl cyanide/Si(1 0 0)-2 · 1 Jing Yan Huang a,b, Hai Gou Huang a, Yue Sheng Ning a, Qi Ping Liu a, Solhe F. Alshahateet b, Yue Ming Sun c, Guo Qin Xu a,b,* a
Department of Chemistry, National University of Singapore, 10 Kent Ridge, Singapore 119260, Singapore Institute for Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833, Singapore Department of Chemistry and Chemical Engineering, Southeast University, Nanjing, 210096, PeopleÕs Republic of China b
c
Received 1 March 2005; in final form 26 May 2005 Available online 24 June 2005
Abstract The covalent binding of acetyl cyanide on Si(1 0 0)-2 · 1 has been investigated using high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The absence of the C@O and C„N stretching modes in the EELS spectra for chemisorbed molecules indicates the direct interactions between carbonyl, cyano groups and the dangling bonds on Si(1 0 0)-2 · 1. The characteristic C@C@N asymmetric stretching mode at 2031 cm 1 and C@N stretching mode at 1667 cm 1 suggest the existence of both ketenimine species and tetra-r adduct at the interface, further supported by XPS results and DFT calculations. 2005 Elsevier B.V. All rights reserved.
1. Introduction There is a growing interest in the field of organic modification of semiconductor surfaces due to its potential applications in molecular electronics, sensors and nanotechnology [1–3]. By introducing the tailorability of organic materials onto semiconductor surfaces, the chemical and physical properties of interfaces can be fine-tuned. To gain the control in incorporating desired organic functionalities into existing device technologies, the growing efforts have been dedicated to the fundamental understanding of chemical attachment of organic molecules on semiconductor surfaces at molecular level. Of particular importance in semiconductor surfaces is the Si(1 0 0) surface, which undergoes a (2 · 1) reconstruction involving a pairing of adjacent silicon atoms into dimers [4]. The two silicon atoms in each dimer *
Corresponding author. Fax: +65 6779 1691. E-mail address: [email protected] (G.Q. Xu).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.008
are linked together through a strong r bond and a weak p bond. The reported values for the weak p bond are around 2–8 kcal/mol [5–7], much smaller than the 64 kcal/mol of traditional alkenes [8]. Thus, the dimer can be viewed as a diradical with each silicon atom containing one unpaired electron. The unique structure of Si(1 0 0)-2 · 1 makes it a good template for studying the interaction of organic molecules with semiconductor surfaces. Carbonyl and cyano are two important organic functionalities, which have been shown to be active in organic functionalization of silicon surfaces [9–13]. By combining these two functional groups in acetyl cyanide, the selectivity and reactivity of cyano and carbonyl groups can be explored in the study of the reaction pathways of acetyl cyanide on Si(1 0 0)-2 · 1. In addition, a functional intermediate may be formed for further organic syntheses in the second layer preparation due to the multifunctionality of this molecule. There are four possible reaction pathways for acetyl cyanide on
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J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
Si(1 0 0)-2 · 1: a [2 + 2]-like cycloaddition through C@O (a) or C„N (b); a [4 + 2]-like cycloaddition with the formation of ketenimine species containing two cumulated double bonds (C@C@N) (c); and two [2 + 2]-like cycloadditions producing tetra-r adduct (d). In this Letter, we investigated the attachment chemistry of acetyl cyanide on Si(1 0 0)-2 · 1 using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations.
The cluster employed in the DFT calculations is Si15H16, which contains two neighboring dimers in the same dimer row. The silicon atoms at the cluster edge are terminated with H-atoms to saturate all silicon atoms except those forming the surface dimers. All calculations in this work were performed using the B3LYP three-parameter gradient corrected hybrid density functional theory (DFT) with the SPARTAN software package. Adsorption energy is defined here as the difference between the total energy of adsorbate/substrate complex and the total sum of the optimized free cluster and free acetyl cyanide.
2. Experimental and computational details 3. Results and discussion Fig. 1 presents HREELS spectra taken after exposing a clean Si(1 0 0)-2 · 1 sample to acetyl cyanide (C3H3C2OC1N). The lower spectrum (Fig. 1a) was obtained by condensing multilayers of acetyl cyanide on Si(1 0 0)-2 · 1 at 110 K. After annealing the multilayer acetyl cyanide-covered sample to 200 or 300 K to drive away all the physisorbed molecules, we obtained the spectra for chemisorbed acetyl cyanide on Si(1 0 0)2 · 1, as shown in Figs. 1b,c. For comparison, Table 1 lists the experimental vibrational frequencies of gaseous and liquid acetyl cyanide together with calculated values [15–17]. In Fig. 1a, the peaks at 2227 and 1730 cm 1 are assigned to the respective C„N and C@O stretching
2964
2031
1667
(b)
2964
(a) 2227
999 1182 1439
1126 1193 1425
0
(c)
1730
-1
78cm
498 593 716 856 991
Acetyl cyanide/Si(100)-2x1
591 721
The experiments were performed in two ultra-high vacuum (UHV) chambers, each having 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). An 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 80 cm 1 was achieved. XPS studies were carried out in another chamber equipped with an X-ray source and a concentric hemispherical energy analyzer (CLAM2, VG). The spectra were acquired using Al Ka radiation (hm = 1486.6 eV) and 20 eV pass energy. All of the XPS data presented here are referenced to the Si (2p) line with a binding energy of 99.3 eV [14]. The Si(1 0 0) samples (22 mm · 8 mm · 0.38 mm) were cut from p-type boron-doped silicon wafers (purity 99.999%, thickness 0.38 mm, Goodfellow). A Ta foil (thickness 0.025 mm, Goodfellow) was sandwiched between two experimental samples held together using Ta clips, and in turn spot-welded to Ta posts at the bottom of a Dewar-type liquid N2 cooled sample holder. The samples can be heated to 1300 K through the resistive heating of the Ta foil and cooled to 110 K using liquid nitrogen. To clean the samples, they were degassed at 850 K overnight in the chamber and then repeatedly heated and cooled until the pressure remains below 1 · 10 9 torr at a sample temperature of 1300 K for 10 min. Sample temperatures were determined by an infrared pyrometer (T > 800 K), which showed a good temperature distribution within ±10 K at 1000 K. The carbon contaminants were removed by cycles of Ar+ bombardment (36 min at 500 eV and 5 lA cm 2) and annealing (1300 K for 5 min). The cleanliness of the samples was verified using XPS and HREELS. The surface structure was examined using STM in a separate chamber, showing a defect density of 5–10%. The acetyl cyanide (95%, Aldrich) were purified by freeze–pump–thaw cycles before dosing onto the Si(1 0 0)-2 · 1 surface through a variable leaking valve. The exposures are reported in the unit of Langmuir (1 L = 10 6 Torr s) without the calibration of ion gauge sensitivity.
500 1000 1500 2000 2500 3000 3500 4000 -1
Frequency(cm ) Fig. 1. HREELS spectra obtained after an exposure of 5.0 L acetyl cyanide on Si(1 0 0)-2 · 1 at 110 K (a) and annealing the sample in (a) to 200 K (b) and 300 K (c). Ep = 5.0 eV; specular mode.
J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
77
Table 1 Assignments of vibrational frequencies (cm 1) for acetyl cyanide (C3H3C2OC1N) adsorbed on Si(1 0 0)-2 · 1 Mode description
MP2/6-31G(d)a
MP2/6-311G(2d,2p)a
IR (gas phase)b
IR (liquid phase)c
Physisorbed acetyl cyanide on Si(1 0 0)-2 · 1
Chemisorbed acetyl cyanide on Si(1 0 0)-2 · 1
CH3 asymmetric stretch CH3 symmetric stretch C„N stretch C@C@N asymmetric stretch C@O stretch C@N stretch CH3 asymmetric bend CH3 symmetric bend C2–C1 stretch C@C@N symmetric stretch C–O stretch CH3 in plane rock C@C@N torsion C2–C3 stretch SiO stretch C@C@N bend C3–C2–C1 bend SiN/SiC stretch C3C2O bend C2C1N bend
3240 3111 2184
3222 3095 2154
3027 2932 2229
2971 3016 2216
2964
2964
1763
1743
1740
1728
1730
1523 1450 1243
1488 1416 1215
1431 1368 1178
1413 1363 1174
1439
1020
998
976
981
999
738
722
712
705
721
597
590
584
436 172
431 170
431 176
a b c
2227 2031
1182
591
1667 1425 1193 1126 1126 991 856 716 716 593 593 498
Ref. [15]. Ref. [16]. Ref. [17].
modes, which are the characteristic features of cyano and carbonyl groups. In addition, CH3 stretching (2964 cm 1), bending (1439 cm 1) and in-plane rocking (999 cm 1) modes are resolved. The other three peaks at 1182, 721 and 591 cm 1 are attributed to C2–C1, C2–C3 stretching and C3–C2–C1 bending modes, respectively. As shown in Table 1, all these vibrational frequencies agree well with those of the gaseous and liquid acetyl cyanide. However, the vibrational features of chemisorbed acetyl cyanide (Figs. 1b,c) are significantly different. Losses at 2964, 2031, 1667, 1425, 1193, 1126, 991, 856, 716, 593 and 498 cm 1 can be clearly resolved. Careful examination and comparison of the EELS spectra for physisorbed and chemisorbed molecules reveal some important information about the chemical binding of acetyl cyanide on Si(1 0 0)-2 · 1. The complete attenuation of the C„N stretching at 2227 cm 1 and C@O stretching at 1730 cm 1 upon chemisorption suggests the concurrent involvement of both the C„N and C@O portions of the molecule in surface reaction. The new feature appearing at 2031 cm 1 in the spectra for the chemisorbed acetyl cyanide can be related to the asymmetric stretching mode of ketenimine species (C@C@N) [18–22]. This assignment is further supported by the observation of other related features for cumulative double bonds, such as C@C@N symmetric stretching (1126 cm 1), torsion (856 cm 1) and bending (593 cm 1) modes [21,22]. All these data correlate well
with those of a similar C@C@N structure formed for acrylonitrile binding on Si(1 1 1)-7 · 7 [11]. Apart from these, another new peak at 1667 cm 1 is also resolved in the EELS spectra for chemisorbed acetyl cyanide, similar to the value (1663 cm 1) obtained for the C@N stretching mode in the EELS spectra for chemisorbed acetonitrile on Si(1 1 1)-7 · 7 through a side-on di-r binding mode [9]. The presence of this peak, together with the absence of both C@O and C„N corresponding features, suggests a tetra-r binding mechanism on the surface. Both C@O and C„N functional groups of acetyl cyanide directly participate in binding with the two adjacent dimers on Si(1 0 0)-2 · 1 to form C–O and C@N bonds, respectively. A similar tetra-r binding mechanism was found in the adsorption of allyl cyanide on Si(1 1 1)-7 · 7 [12]. Thus, our EELS results demonstrate that there are two binding modes in the reaction of acetyl cyanide with Si(1 0 0)-2 · 1, as shown in Fig. 2. Besides ketenimine species via a [4 + 2]-like cycloaddition, a tetra-r adduct through two [2 + 2]-like reactions coexists at the interface of acetyl cyanide/ Si(1 0 0)-2 · 1. Figs. 3 and 4 show the O1s and N1s XPS spectra following a sequence of exposure at 110 K. At low exposures (60.8 L), main features centered at 532.2 eV (O1s) and 398.8 eV (N1s) are observed, assigned to the chemisorbed acetyl cyanide on Si(1 0 0)-2 · 1. Further dosing acetyl cyanide leads to the appearance of new peaks at higher binding energies, together with a gradual attenuation of the chemisorption features. At a high
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J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
Fig. 2. Schematic presentation of two adsorption products of acetyl cyanide on Si(1 0 0)-2 · 1.
O(1s)
N(1s)
CH3COCN
532.2
Si(100)-2x1,110K
CH3COCN Si(100)-2x1,110K
398.8
j 300K
j 300K
533.6
400.4
l 20.0L
i 20.0L
h13.0L g 5.0L f 3.0L e 1.6L d 0.8L c 0.4L b 0.2L a 0.0L
h13.0L g 5.0L f 3.0L e 1.6L d 0.8L c 0.4L b 0.2L a 0.0L 526
528
530
532
534
536
538
540
394
396
398
400
402
404
406
408
Binding Energy (eV)
Binding Energy(eV)
Fig. 3. O1s XPS spectra of adsorbed acetyl cyanide on Si(1 0 0)-2 · 1 as a function of exposure at 110 K. j is O1s XPS spectrum of saturated chemisorption monolayer obtained by annealing the multilayer acetyl cyanide-covered sample (20.0 L) to 300 K to drive away all physisorbed molecules.
Fig. 4. C1s XPS spectra of adsorbed acetyl cyanide on Si(1 0 0)-2 · 1 as a function of exposure at 110 K. j is C1s XPS spectrum of saturated chemisorption monolayer obtained by annealing the multilayer acetyl cyanide-covered sample (20.0 L) to 300 K to drive away all physisorbed molecules.
exposure of 20.0 L, two new peaks centered at 533.6 eV (O1s) and 400.4 eV (N1s) become dominant, indicating the formation of multilayer physisorbed acetyl cyanide on the silicon surface. The BEs of the O1s and N1s core level for physisorbed acetyl cyanide on Si(1 0 0)-2 · 1 agree well with the calculated results of gaseous acetyl cyanide, referenced to the spectrometer work function [23]. In addition, chemisorbed monolayer was also obtained by annealing the multilayer acetyl cyanidecovered surface to drive away all physisorbed molecules. Chemisorbed acetyl cyanide on Si(1 0 0)-2 · 1 gives O1s
at 532.2 eV (Fig. 3j) and N1s at 398.8 eV (Fig. 4j), in excellent agreement with the values obtained at low exposures. The chemisorption results in significant downshifts of 1.4 and 1.6 eV for BEs of O1s and N1s, respectively, referenced to the data of physisorbed acetyl cyanide. These large chemical shifts suggest the involvement of both oxygen and nitrogen atoms in the interaction of acetyl cyanide with Si(1 0 0)-2 · 1, consistent with the EELS results. Compared to physisorbed acetyl cyanide, higher electron densities are expected at the O and N atoms due to their bonding with Si-atoms with a smaller electronegativity, leading to lower BEs for the
J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
79
Table 2 Adsorption energies of the local minima in the acetyl cyanide/Si15H16 model systems on Si(1 0 0)-2 · 1 Binding mode
Mode I
Mode II
Mode III
Mode IV
Reaction pathway Adsorption energy
[2 + 2] via C@O 38.2
[2 + 2] via C„N 35.6
[4 + 2]
Two [2 + 2] via C@O and C„N 33.5
49.3
All energies are in kcal/mol.
[2 + 2]-like reactions. Although the tetra-r addition is less energetically preferred than the [4 + 2] cycloaddition, the coexistence of these two reaction mechanisms found in our experimental studies suggests that the surface reaction may be kinetically controlled. Detailed theoretical work is required to gain further understanding on the reaction mechanisms of acetyl cyanide on Si(1 0 0)-2 · 1.
4. Conclusions
Fig. 5. Cluster models of acetyl cyanide (C3H3C2OC1N) on Si(1 0 0)2 · 1 optimized by DFT calculations.
O1s and N1s. The 532.2 eV O1s binding energy for chemisorbed acetyl cyanide on the silicon surface is close to that for the oxygen atom in a Si–O bond formed by the dissociative reaction of formic acid on Si(1 1 1)7 · 7 [24]. Moreover, the N1s XPS feature located at 398.8 eV is well consistent with the N1s value of the resulting (Si)CH2–CH@C@N(Si) intermediate in chemisorption of acrylonitrile on Si(1 1 1)-7 · 7 [11]. The DFT calculations focus on the geometry optimization and adsorption energy for the proposed adsorption configurations to obtain a better understanding of the experimental conclusion. Fig. 5 shows the optimized adsorption geometries for the possible binding modes, including [2 + 2] cycloadduct through either carbonyl or cyano (Modes I and II), ketenimine species (Mode III) and tetra-r adduct (Mode IV). The calculated adsorption energies for the configurations are listed in Table 2. From the table, it is noticed that the adsorption energies for Modes I, II and IV are very close, with values of 38.2, 35.6 and 33.5 kcal/mol, respectively. However, the binding energy of ketenimine species is significantly larger by about 11–16 kcal/mol, indicating that the [4 + 2] cycloaddition is thermodynamically favored compared with the other
HREELS experimental results show the coexistence of two surface intermediates of a ketenimine-like species and a tetra-r adduct at acetyl cyanide/Si(1 0 0)-2 · 1 interface, which is further supported by XPS studies and DFT calculations. The ketenimine species is an active intermediate and can be used as a precursor for the further organic syntheses on silicon surfaces.
References [1] J.T. Yates Jr., Science 279 (1998) 335. [2] T. Strother, R.J. Hamers, L.M. Smith, Nucleic Acids Res. 28 (2000) 3535. [3] M.C. Hersam, N.P. Guisinger, J.W. Lyding, Nanotechnology 11 (2000) 70. [4] C.B. Duke, Chem. Rev. 96 (1996) 1237. [5] C.J. Wu, E.A. Carter, Chem. Phys. Lett. 185 (1991) 172. [6] P. Nachtigall, K.D. Jordan, C. Sosa, J. Phys. Chem. 97 (1993) 11666. [7] M.C. Flowers, N.B.H. Jonathan, Y. Liu, A. Morris, J. Chem. Phys. 99 (1993) 7038. [8] F.A. Carey, R.J. Sundberg, Advanced Organic Chemistry, Plenum Publishing Corporation, New York, 2001. [9] F. Tao, X.F. Chen, Z.H. Wang, G.Q. Xu, J. Phys. Chem. B 106 (2002) 3890. [10] C. Hamai, A. Takagi, M. Taniguchi, T. Matsumoto, T. Kawai, Angew. Chem. Int. Ed. 43 (2004) 1349. [11] F. Tao, X.F. Chen, Z.H. Wang, G.Q. Xu, J. Am. Chem. Soc. 124 (2002) 7170. [12] F. Tao, Y.J. Dai, G.Q. Xu, Phys. Rev. B 66 (2002) 35420. [13] J.L. Armstrong, E.D. Pylant, J.M. White, J. Vac. Sci. Technol. A 16 (1998) 123. [14] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin–Elmer Corporation, Minnesota, 1991. [15] R.J. Horwitz, J.S. Francisco, J.A. Guest, J. Phys. Chem. A 101 (1997) 1231.
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[16] S. Bell, G.A. Guirgis, J. Lin, J.R. Durig, J. Mol. Struct. 238 (1990) 183. [17] M. Sugie´, K. Kuchitsu, J. Mol. Struct. 20 (1974) 437. [18] G. Socrates, Infrared and Raman Characteristic Group Frequencies: Tables and Charts, John Wiley & Sons Ltd, New York, 2001. [19] M. Prober, J. Am. Chem. Soc. 78 (1956) 2274.
[20] R. West, G.A. Gornowicz, J. Am. Chem. Soc. 93 (1971) 1714. [21] Y. Amatatsu, Y. Hamada, M. Tsuboi, J. Mol. Spectrosc. 123 (1987) 476. [22] M.E. Jacox, Chem. Phys. 43 (1979) 157. [23] D.P. Chong, C.H. Hu, J. Chem. Phys. 108 (1998) 8950. [24] J.Y. Huang, H.G. Huang, K.Y. Lin, Q.P. Liu, Y.M. Sun, G.Q. Xu, Surf. Sci. 549 (2004) 255.
Coexistence of ketenimine species and tetra-r adduct at acetyl cyanide/Si(1 0 0)-2 · 1 Jing Yan Huang a,b, Hai Gou Huang a, Yue Sheng Ning a, Qi Ping Liu a, Solhe F. Alshahateet b, Yue Ming Sun c, Guo Qin Xu a,b,* a
Department of Chemistry, National University of Singapore, 10 Kent Ridge, Singapore 119260, Singapore Institute for Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833, Singapore Department of Chemistry and Chemical Engineering, Southeast University, Nanjing, 210096, PeopleÕs Republic of China b
c
Received 1 March 2005; in final form 26 May 2005 Available online 24 June 2005
Abstract The covalent binding of acetyl cyanide on Si(1 0 0)-2 · 1 has been investigated using high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. The absence of the C@O and C„N stretching modes in the EELS spectra for chemisorbed molecules indicates the direct interactions between carbonyl, cyano groups and the dangling bonds on Si(1 0 0)-2 · 1. The characteristic C@C@N asymmetric stretching mode at 2031 cm 1 and C@N stretching mode at 1667 cm 1 suggest the existence of both ketenimine species and tetra-r adduct at the interface, further supported by XPS results and DFT calculations. 2005 Elsevier B.V. All rights reserved.
1. Introduction There is a growing interest in the field of organic modification of semiconductor surfaces due to its potential applications in molecular electronics, sensors and nanotechnology [1–3]. By introducing the tailorability of organic materials onto semiconductor surfaces, the chemical and physical properties of interfaces can be fine-tuned. To gain the control in incorporating desired organic functionalities into existing device technologies, the growing efforts have been dedicated to the fundamental understanding of chemical attachment of organic molecules on semiconductor surfaces at molecular level. Of particular importance in semiconductor surfaces is the Si(1 0 0) surface, which undergoes a (2 · 1) reconstruction involving a pairing of adjacent silicon atoms into dimers [4]. The two silicon atoms in each dimer *
Corresponding author. Fax: +65 6779 1691. E-mail address: [email protected] (G.Q. Xu).
0009-2614/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.06.008
are linked together through a strong r bond and a weak p bond. The reported values for the weak p bond are around 2–8 kcal/mol [5–7], much smaller than the 64 kcal/mol of traditional alkenes [8]. Thus, the dimer can be viewed as a diradical with each silicon atom containing one unpaired electron. The unique structure of Si(1 0 0)-2 · 1 makes it a good template for studying the interaction of organic molecules with semiconductor surfaces. Carbonyl and cyano are two important organic functionalities, which have been shown to be active in organic functionalization of silicon surfaces [9–13]. By combining these two functional groups in acetyl cyanide, the selectivity and reactivity of cyano and carbonyl groups can be explored in the study of the reaction pathways of acetyl cyanide on Si(1 0 0)-2 · 1. In addition, a functional intermediate may be formed for further organic syntheses in the second layer preparation due to the multifunctionality of this molecule. There are four possible reaction pathways for acetyl cyanide on
76
J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
Si(1 0 0)-2 · 1: a [2 + 2]-like cycloaddition through C@O (a) or C„N (b); a [4 + 2]-like cycloaddition with the formation of ketenimine species containing two cumulated double bonds (C@C@N) (c); and two [2 + 2]-like cycloadditions producing tetra-r adduct (d). In this Letter, we investigated the attachment chemistry of acetyl cyanide on Si(1 0 0)-2 · 1 using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations.
The cluster employed in the DFT calculations is Si15H16, which contains two neighboring dimers in the same dimer row. The silicon atoms at the cluster edge are terminated with H-atoms to saturate all silicon atoms except those forming the surface dimers. All calculations in this work were performed using the B3LYP three-parameter gradient corrected hybrid density functional theory (DFT) with the SPARTAN software package. Adsorption energy is defined here as the difference between the total energy of adsorbate/substrate complex and the total sum of the optimized free cluster and free acetyl cyanide.
2. Experimental and computational details 3. Results and discussion Fig. 1 presents HREELS spectra taken after exposing a clean Si(1 0 0)-2 · 1 sample to acetyl cyanide (C3H3C2OC1N). The lower spectrum (Fig. 1a) was obtained by condensing multilayers of acetyl cyanide on Si(1 0 0)-2 · 1 at 110 K. After annealing the multilayer acetyl cyanide-covered sample to 200 or 300 K to drive away all the physisorbed molecules, we obtained the spectra for chemisorbed acetyl cyanide on Si(1 0 0)2 · 1, as shown in Figs. 1b,c. For comparison, Table 1 lists the experimental vibrational frequencies of gaseous and liquid acetyl cyanide together with calculated values [15–17]. In Fig. 1a, the peaks at 2227 and 1730 cm 1 are assigned to the respective C„N and C@O stretching
2964
2031
1667
(b)
2964
(a) 2227
999 1182 1439
1126 1193 1425
0
(c)
1730
-1
78cm
498 593 716 856 991
Acetyl cyanide/Si(100)-2x1
591 721
The experiments were performed in two ultra-high vacuum (UHV) chambers, each having 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). An 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 80 cm 1 was achieved. XPS studies were carried out in another chamber equipped with an X-ray source and a concentric hemispherical energy analyzer (CLAM2, VG). The spectra were acquired using Al Ka radiation (hm = 1486.6 eV) and 20 eV pass energy. All of the XPS data presented here are referenced to the Si (2p) line with a binding energy of 99.3 eV [14]. The Si(1 0 0) samples (22 mm · 8 mm · 0.38 mm) were cut from p-type boron-doped silicon wafers (purity 99.999%, thickness 0.38 mm, Goodfellow). A Ta foil (thickness 0.025 mm, Goodfellow) was sandwiched between two experimental samples held together using Ta clips, and in turn spot-welded to Ta posts at the bottom of a Dewar-type liquid N2 cooled sample holder. The samples can be heated to 1300 K through the resistive heating of the Ta foil and cooled to 110 K using liquid nitrogen. To clean the samples, they were degassed at 850 K overnight in the chamber and then repeatedly heated and cooled until the pressure remains below 1 · 10 9 torr at a sample temperature of 1300 K for 10 min. Sample temperatures were determined by an infrared pyrometer (T > 800 K), which showed a good temperature distribution within ±10 K at 1000 K. The carbon contaminants were removed by cycles of Ar+ bombardment (36 min at 500 eV and 5 lA cm 2) and annealing (1300 K for 5 min). The cleanliness of the samples was verified using XPS and HREELS. The surface structure was examined using STM in a separate chamber, showing a defect density of 5–10%. The acetyl cyanide (95%, Aldrich) were purified by freeze–pump–thaw cycles before dosing onto the Si(1 0 0)-2 · 1 surface through a variable leaking valve. The exposures are reported in the unit of Langmuir (1 L = 10 6 Torr s) without the calibration of ion gauge sensitivity.
500 1000 1500 2000 2500 3000 3500 4000 -1
Frequency(cm ) Fig. 1. HREELS spectra obtained after an exposure of 5.0 L acetyl cyanide on Si(1 0 0)-2 · 1 at 110 K (a) and annealing the sample in (a) to 200 K (b) and 300 K (c). Ep = 5.0 eV; specular mode.
J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
77
Table 1 Assignments of vibrational frequencies (cm 1) for acetyl cyanide (C3H3C2OC1N) adsorbed on Si(1 0 0)-2 · 1 Mode description
MP2/6-31G(d)a
MP2/6-311G(2d,2p)a
IR (gas phase)b
IR (liquid phase)c
Physisorbed acetyl cyanide on Si(1 0 0)-2 · 1
Chemisorbed acetyl cyanide on Si(1 0 0)-2 · 1
CH3 asymmetric stretch CH3 symmetric stretch C„N stretch C@C@N asymmetric stretch C@O stretch C@N stretch CH3 asymmetric bend CH3 symmetric bend C2–C1 stretch C@C@N symmetric stretch C–O stretch CH3 in plane rock C@C@N torsion C2–C3 stretch SiO stretch C@C@N bend C3–C2–C1 bend SiN/SiC stretch C3C2O bend C2C1N bend
3240 3111 2184
3222 3095 2154
3027 2932 2229
2971 3016 2216
2964
2964
1763
1743
1740
1728
1730
1523 1450 1243
1488 1416 1215
1431 1368 1178
1413 1363 1174
1439
1020
998
976
981
999
738
722
712
705
721
597
590
584
436 172
431 170
431 176
a b c
2227 2031
1182
591
1667 1425 1193 1126 1126 991 856 716 716 593 593 498
Ref. [15]. Ref. [16]. Ref. [17].
modes, which are the characteristic features of cyano and carbonyl groups. In addition, CH3 stretching (2964 cm 1), bending (1439 cm 1) and in-plane rocking (999 cm 1) modes are resolved. The other three peaks at 1182, 721 and 591 cm 1 are attributed to C2–C1, C2–C3 stretching and C3–C2–C1 bending modes, respectively. As shown in Table 1, all these vibrational frequencies agree well with those of the gaseous and liquid acetyl cyanide. However, the vibrational features of chemisorbed acetyl cyanide (Figs. 1b,c) are significantly different. Losses at 2964, 2031, 1667, 1425, 1193, 1126, 991, 856, 716, 593 and 498 cm 1 can be clearly resolved. Careful examination and comparison of the EELS spectra for physisorbed and chemisorbed molecules reveal some important information about the chemical binding of acetyl cyanide on Si(1 0 0)-2 · 1. The complete attenuation of the C„N stretching at 2227 cm 1 and C@O stretching at 1730 cm 1 upon chemisorption suggests the concurrent involvement of both the C„N and C@O portions of the molecule in surface reaction. The new feature appearing at 2031 cm 1 in the spectra for the chemisorbed acetyl cyanide can be related to the asymmetric stretching mode of ketenimine species (C@C@N) [18–22]. This assignment is further supported by the observation of other related features for cumulative double bonds, such as C@C@N symmetric stretching (1126 cm 1), torsion (856 cm 1) and bending (593 cm 1) modes [21,22]. All these data correlate well
with those of a similar C@C@N structure formed for acrylonitrile binding on Si(1 1 1)-7 · 7 [11]. Apart from these, another new peak at 1667 cm 1 is also resolved in the EELS spectra for chemisorbed acetyl cyanide, similar to the value (1663 cm 1) obtained for the C@N stretching mode in the EELS spectra for chemisorbed acetonitrile on Si(1 1 1)-7 · 7 through a side-on di-r binding mode [9]. The presence of this peak, together with the absence of both C@O and C„N corresponding features, suggests a tetra-r binding mechanism on the surface. Both C@O and C„N functional groups of acetyl cyanide directly participate in binding with the two adjacent dimers on Si(1 0 0)-2 · 1 to form C–O and C@N bonds, respectively. A similar tetra-r binding mechanism was found in the adsorption of allyl cyanide on Si(1 1 1)-7 · 7 [12]. Thus, our EELS results demonstrate that there are two binding modes in the reaction of acetyl cyanide with Si(1 0 0)-2 · 1, as shown in Fig. 2. Besides ketenimine species via a [4 + 2]-like cycloaddition, a tetra-r adduct through two [2 + 2]-like reactions coexists at the interface of acetyl cyanide/ Si(1 0 0)-2 · 1. Figs. 3 and 4 show the O1s and N1s XPS spectra following a sequence of exposure at 110 K. At low exposures (60.8 L), main features centered at 532.2 eV (O1s) and 398.8 eV (N1s) are observed, assigned to the chemisorbed acetyl cyanide on Si(1 0 0)-2 · 1. Further dosing acetyl cyanide leads to the appearance of new peaks at higher binding energies, together with a gradual attenuation of the chemisorption features. At a high
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J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
Fig. 2. Schematic presentation of two adsorption products of acetyl cyanide on Si(1 0 0)-2 · 1.
O(1s)
N(1s)
CH3COCN
532.2
Si(100)-2x1,110K
CH3COCN Si(100)-2x1,110K
398.8
j 300K
j 300K
533.6
400.4
l 20.0L
i 20.0L
h13.0L g 5.0L f 3.0L e 1.6L d 0.8L c 0.4L b 0.2L a 0.0L
h13.0L g 5.0L f 3.0L e 1.6L d 0.8L c 0.4L b 0.2L a 0.0L 526
528
530
532
534
536
538
540
394
396
398
400
402
404
406
408
Binding Energy (eV)
Binding Energy(eV)
Fig. 3. O1s XPS spectra of adsorbed acetyl cyanide on Si(1 0 0)-2 · 1 as a function of exposure at 110 K. j is O1s XPS spectrum of saturated chemisorption monolayer obtained by annealing the multilayer acetyl cyanide-covered sample (20.0 L) to 300 K to drive away all physisorbed molecules.
Fig. 4. C1s XPS spectra of adsorbed acetyl cyanide on Si(1 0 0)-2 · 1 as a function of exposure at 110 K. j is C1s XPS spectrum of saturated chemisorption monolayer obtained by annealing the multilayer acetyl cyanide-covered sample (20.0 L) to 300 K to drive away all physisorbed molecules.
exposure of 20.0 L, two new peaks centered at 533.6 eV (O1s) and 400.4 eV (N1s) become dominant, indicating the formation of multilayer physisorbed acetyl cyanide on the silicon surface. The BEs of the O1s and N1s core level for physisorbed acetyl cyanide on Si(1 0 0)-2 · 1 agree well with the calculated results of gaseous acetyl cyanide, referenced to the spectrometer work function [23]. In addition, chemisorbed monolayer was also obtained by annealing the multilayer acetyl cyanidecovered surface to drive away all physisorbed molecules. Chemisorbed acetyl cyanide on Si(1 0 0)-2 · 1 gives O1s
at 532.2 eV (Fig. 3j) and N1s at 398.8 eV (Fig. 4j), in excellent agreement with the values obtained at low exposures. The chemisorption results in significant downshifts of 1.4 and 1.6 eV for BEs of O1s and N1s, respectively, referenced to the data of physisorbed acetyl cyanide. These large chemical shifts suggest the involvement of both oxygen and nitrogen atoms in the interaction of acetyl cyanide with Si(1 0 0)-2 · 1, consistent with the EELS results. Compared to physisorbed acetyl cyanide, higher electron densities are expected at the O and N atoms due to their bonding with Si-atoms with a smaller electronegativity, leading to lower BEs for the
J.Y. Huang et al. / Chemical Physics Letters 411 (2005) 75–80
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Table 2 Adsorption energies of the local minima in the acetyl cyanide/Si15H16 model systems on Si(1 0 0)-2 · 1 Binding mode
Mode I
Mode II
Mode III
Mode IV
Reaction pathway Adsorption energy
[2 + 2] via C@O 38.2
[2 + 2] via C„N 35.6
[4 + 2]
Two [2 + 2] via C@O and C„N 33.5
49.3
All energies are in kcal/mol.
[2 + 2]-like reactions. Although the tetra-r addition is less energetically preferred than the [4 + 2] cycloaddition, the coexistence of these two reaction mechanisms found in our experimental studies suggests that the surface reaction may be kinetically controlled. Detailed theoretical work is required to gain further understanding on the reaction mechanisms of acetyl cyanide on Si(1 0 0)-2 · 1.
4. Conclusions
Fig. 5. Cluster models of acetyl cyanide (C3H3C2OC1N) on Si(1 0 0)2 · 1 optimized by DFT calculations.
O1s and N1s. The 532.2 eV O1s binding energy for chemisorbed acetyl cyanide on the silicon surface is close to that for the oxygen atom in a Si–O bond formed by the dissociative reaction of formic acid on Si(1 1 1)7 · 7 [24]. Moreover, the N1s XPS feature located at 398.8 eV is well consistent with the N1s value of the resulting (Si)CH2–CH@C@N(Si) intermediate in chemisorption of acrylonitrile on Si(1 1 1)-7 · 7 [11]. The DFT calculations focus on the geometry optimization and adsorption energy for the proposed adsorption configurations to obtain a better understanding of the experimental conclusion. Fig. 5 shows the optimized adsorption geometries for the possible binding modes, including [2 + 2] cycloadduct through either carbonyl or cyano (Modes I and II), ketenimine species (Mode III) and tetra-r adduct (Mode IV). The calculated adsorption energies for the configurations are listed in Table 2. From the table, it is noticed that the adsorption energies for Modes I, II and IV are very close, with values of 38.2, 35.6 and 33.5 kcal/mol, respectively. However, the binding energy of ketenimine species is significantly larger by about 11–16 kcal/mol, indicating that the [4 + 2] cycloaddition is thermodynamically favored compared with the other
HREELS experimental results show the coexistence of two surface intermediates of a ketenimine-like species and a tetra-r adduct at acetyl cyanide/Si(1 0 0)-2 · 1 interface, which is further supported by XPS studies and DFT calculations. The ketenimine species is an active intermediate and can be used as a precursor for the further organic syntheses on silicon surfaces.
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