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Theoretical study of N2O adsorption on clean and partially oxidized Si(1 0 0)-(2 × 1) small clusters
Chemical Physics Letters 436 (2007) 263–267 www.elsevier.com/locate/cplett
Theoretical study of N2O adsorption on clean and partially oxidized Si(1 0 0)-(2 · 1) small clusters Kenji Imamura, Hiroaki Tokiwa
*
Department of Chemistry, Faculty of Science, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan Received 27 September 2005 Available online 18 January 2007
Abstract Theoretical studies of the adsorption and decomposition of nitrous oxide (N2O) on clean and partially oxidized Si(1 0 0) clusters were performed by means of the hybrid density functional theory method. N2O underwent physisorption on the clean surface and chemisorption on the surface modified by molecular oxygen. The difference is clearly explained by means of electron transfer analysis. N2O dissociated to form N2 on surface dimers modified with O, and the activation energies in this process were small. 2007 Published by Elsevier B.V.
1. Introduction Because the elucidation of the initial oxidation process on semiconductor surfaces is important not only in terms of fundamental studies of surface science but also in terms of the development of practical electronic devices, several experimental and theoretical studies of oxidation processes have been performed on Si(1 0 0) and Si(1 1 1) surfaces with an oxygen atom or molecule as the oxidant [1]. Nitrous oxide (N2O) is also frequently used as an oxidant, because it easily releases an oxygen atom on surfaces and because experiments with N2O provide information about the initial oxidation process on the surfaces [2–5]. Using high-resolution electron energy loss spectroscopy (HREELS), Kubo et al. [4] have assigned vibrational modes of Si– O–Si species and stretching modes of two types of N–N and N–O bonds of N2O on a partially oxidized surface. Lee et al. [5] found a unique angular distribution of desorbing N2 molecules after N2O adsorption on clean and partially oxidized Si(1 0 0) surfaces at temperatures below 100 K using a time-of-flight (TOF) technique. The collimation angle was 32 from normal to the surface. Lee et al. proposed a desorption mechanism in which the inclined *
Corresponding author. Fax: +81 3 3985 2394. E-mail address: [email protected] (H. Tokiwa).
0009-2614/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.cplett.2007.01.043
desorption was induced by N2O chemisorbed with the O-end in a down configuration on the partially oxidized surface, since the collimation angle of the molecule was similar to the angle of the symmetric Si–Si dimer. The angular distribution of desorbing N2 molecules released after N2O adsorption has recently been confirmed on clean and partially oxidized Rh(1 1 0) surfaces using both angleresolved thermal desorption spectroscopy [6] and a pressure-jump method [7]. However, the reaction mechanisms have not yet been determined for any of these reactions. We expected that a quantum chemical approach would provide useful information regarding surface reaction dynamics. In this communication, we report on N2O adsorption behavior on both clean and partially oxidized Si(1 0 0) surfaces with an oxygen atom or molecule as an oxidant. 2. Computational details We employed simple cluster models containing nine silicon atoms for both clean and oxygen-modified Si(1 0 0) surfaces. The clean surface was composed of four layers that included a single surface dimer. Hydrogen atoms were used to terminate bulk Si atoms (Si9H12). This cluster model gives energetic and geometric predictions that are in good agreement with experimental results and other theoretical
264
K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
methods at a reasonable computational cost, although the model neglects interactions with adjacent dimers [8]. We adopted two types of oxygen-modified surfaces: one precovered with a single oxygen atom and one pre-covered with an oxygen molecule. The unrestricted-B3LYP (UB3LYP) hybrid density functional [9,10] with the standard 6-31G(d) basis set [11,12] was used for all atoms throughout. Under the supermolecular approach, adsorption energies (Ead) were defined by the following equation:
a
2.225
Ead ¼ EC ðEA þ ES Þ where EC, EA, and ES represent the total energies of the complex, the isolated adsorbate, and the surface, respectively. From these definitions, negative Ead values represent a stable adsorption state. These energies include the scaled zero-point-energy (ZPE) corrections calculated at the UB3LYP/6-31G(d) level [13]. And, we use the Counterpoise(CP)-corrected geometry optimization and vibrational analysis [14] in order to correctly handle the unphysical basis set superposition error (BSSE) for evaluations of Ead and ZPE values. Geometry optimizations were performed without any constraint during calculations. All calculations were carried out using the GAUSSIAN 98 and 03 packages [15,16]. 3. Results and discussion
b O 1.738
1.739
R1
R 1 : 2.302 A 1 : 82.9
c
O
3.1. Isolated Si(1 0 0) surface and N2O structures In the fully optimized structure of the clean surface ˚, (Fig. 1a), the bond length of the Si–Si dimer was 2.225 A which is good agreement with the experimental value of ˚ [17]. 2.24 ± 0.08 A When an oxygen atom was adsorbed on the clean surface, a Si–O–Si bridge structure was formed (Fig. 1b). The bonding orbital of the surface dimer was intact, and the Si–O–Si bond angle was 82.9, which is smaller than the usual angle for sp3- and sp2-hybridized orbitals. When an oxygen molecule was adsorbed on the clean surface, we obtained the stable geometry adsorbed molecularly or dissociatively on the surface. These results closely correspond to the results reported by Widjaja and Musgrave [18]. They investigated a symmetrical adsorption configuration for the dimer (not shown here), which showed a structure in which one of O atoms was inserted between the Si atoms of the cleaved surface-dimer (Fig. 1c). The optimized Si–O–Si bond angle was 145, which is in excellent agreement with both experimental [19] and theoretical [20] values of 144. The dissociated structure was energetically more stable than the molecularly adsorbed structure, by about 60 kcal/mol at the UB3LYP level. We used only the dissociated structure as our oxygen-modified model for O2 adsorption, since the surface dimer was saturated in the molecularly adsorbed structure.
A1
1.589
1.818 O
1.547
A1
A 1 : 145.0 Fig. 1. Optimized surface structures at the level of UB3LYP/6-31G(d) on (a) clean surface (Si9H12), and modified surfaces by oxygen (b) atom (Si9H12O) and (c) molecule (Si9H12O2). Purple and light-blue circles represent Si and H atoms, respectively. R1 and A1 represent Si–Si bond length and Si–O–Si bond angle, respectively. The units of bond lengths ˚ ) and (), respectively. (also Figs. 2–5.) and bond angles were given in (A
Our calculations result that isolated N2O has a linear structure with N–N and N–O bond lengths of 1.134 and ˚ , respectively. The scaled vibrational frequencies 1.193 A were 581, 1292, and 2281 cm1 for the N–N–O bending mode and the N–O and N–N stretching modes, respectively. These calculated results were consistent with IR spectroscopy results of 589, 1285, and 2224 cm1, respectively [21].
K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
3.2. N2O adsorption on the clean Si(1 0 0) surface
1.134 N [–0.11] 1.193 N [+0.62] [–0.50] O A1 A2
The CP-corrected fully optimized geometry of N2O adsorbed on the Si(1 0 0) clean surface showed an O-enddown configuration. This geometry clearly indicates that N2O is weakly bounded on the Si atom of the surface. The adsorption energy showed slightly stable value at the UB3LYP/6-31G(d) level (Fig. 1). The intermolecular distance between the O atom and the adjacent Si atom was ˚ because both atoms had negative Mullarger than 4.0 A liken charges. The geometrical parameters for N2O were almost identical to those of the isolated system. We therefore conclude that N2O was physisorbed on the clean surface by means of an electrostatic interaction. This conclusion is consistent with the results of HREELS [4] and TOF [5] measurements. Since Kubo et al. and Lee et al. [4,5] further insisted that N2O partially dissociated on the clean surface even at 90 K, we next discuss the dissociation process of N2O on the clean surface.
4.511 4.061
[–0.09]
265
2.231
A 1 : 180.0 A 2 : 104.5
3.3. N2O dissociation on the clean Si(1 0 0) surface
E ad :–0.03
Fig. 3 shows the predicted potential energy surface (PES) for N2O dissociation, with structures at each stationary point. As the N–N–O bond angle of N2O was gradually distorted to 163.0 from the angle in the physisorbed structure shown in Fig. 2 (denoted Int.1 in Fig. 3), we found a transition state (TS) structure with one imaginary
Fig. 2. Physisorbed N2O structure on Si(1 0 0) clean surface. Each of values and symbol A inserted in figures represents bond lengths and bond angles, respectively. Symbols of A1 and A2 indicate bond angles of NNO and SiON, respectively. Relative CP-corrected adsorption energy (Ead), the unit of kcal/mol, was represented at the level of UB3LYP/6-31G(d) (also Figs. 3–5). And values in a bracket indicates Mulliken charges (also Figs. 4 and 5).
9
13 [+0.53] 1. 1.218
[–0.43]
O
2.462
[–0.07]
A1
[–0.09]
[–0.04]
[+0.07] [–0.57] O
1.125 A2
1.581 [+0.49]
[+0.07]
A 1 : 163. 0
Relative Energy : ΔE
[–0.07]
1.929
TS Int.1 0.00
2.81
–0.03
≈
≈
A 2 : 141. 0
Int.2 –56.84
Reaction coordinate Fig. 3. Potential energy surface (PES) for N2O dissociation on Si(1 0 0) clean surface with each stationary point structures. Int.1 was same structure denoted in Fig. 2. Several values and symbol A inserted in figures indicate bond lengths and bond angles, respectively. A1 and A2 indicate bond angles of NNO and SiNN, respectively, in TS and Int.2. Arrows in transition structure were denoted transition vector.
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K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
frequency of 188i cm1. The activation energy was only 2.84 kcal/mol from Int.1. Because the barrier was only slightly higher than the energy of the reactants, photo-irradiation or heating of the surface could promote the reaction. As the N–N–O bond angle was further distorted along the reaction coordinate, we found a stable dissociated structure (Int.2) containing two chemical bonds, one between the surface dimer and the terminal nitrogen and one between the dimer and the oxygen atom. We observed an initial oxidation step on the Si(1 0 0) surface.
system. Because this result is inconsistent with the results of previous work, in which N2O was observed to chemisorb on a partially oxidized surface [4,5], we calculated N2O adsorption on a surface modified with an oxygen molecule. In the optimized structure of N2O adsorbed on the O2modified surface, N2O was oriented toward the surface either with the O-end-down or with the terminal N-enddown (Fig. 5). The adsorption energies at the UB3LYP level were estimated to be 5.47 and 3.05 kcal/mol for the O-end-down and N-end-down structures, respectively. These results clearly showed that Ead was significantly
3.4. N2O adsorption on the oxygen-modified Si(1 0 0) surfaces In the optimized structure of N2O adsorbed on the surface modified by an oxygen atom, the adsorbed N2O took on an O-end-down configuration to adjacent Si atom of the surface (Fig. 4). In the case, the adsorption energy was determined to be 0.79 kcal/mol, which was about oneorder larger than that of the clean surface. However, we conclude that N2O was physisorbed even on this surface, because the geometrical parameters for N2O were nearly identical to those of the isolated system and because the intermolecular distances between the surface and N2O were larger than in the case of the clean surface. Furthermore, Kohn–Sham orbital analysis clearly showed that no bonding orbital was formed between the surface and N2O in this
[–0.01]
1.124
N
[+0.67] N A2
[–0.56]
1.218
[–0.54] O A 3
2.240 [+0.65]
O
[–0.65] O
A1
A 1 : 146.0 A 2 : 180.0 A 3 : 114.0
[–0.12] 1.133
N 1.192 N [+0.64] A2 [–0.50] O A3
Ead : –5.47 [–0.43]
3.869 O
[+0.26]
O
1.176
3.032
[+0.69] N A2
[–0.57]
1.139
[–0.60]
[–0.18] N A3
A1
2.356 [+0.66]
O
[–0.63] O
A1
A 1 : 82.7
A 1 :145.0 A 2 :179.0 A 3 :130.5
A 2 : 179.2 A 3 : 99.0
E ad : –0.79 Fig. 4. N2O adsorption structure with O-end-down configuration on modified surface by oxygen atom. Various values in figures represent bond lengths. A1, A2 and A3 indicate bond angles of Si–O–Si, NNO and SiON, respectively.
Ead : –3.05 Fig. 5. Stable structures of N2O on modified surface by oxygen molecule. (a) O-end-down, (b) N-end-down structures, respectively. Each value indicates bond lengths. A1, A2 and A3 indicate bond angles of inserted Si– O–Si, NNO and SiON in (a) or SiNN in (b), respectively.
K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
larger than the corresponding values for the clean surface. The intermolecular distances between the terminal O and N atoms in N2O and the adjacent Si atom were 2.240 and ˚ , respectively. Note that the former is 1.8 A ˚ shorter 2.356 A than the distances calculated for the clean surface. Moreover, the N–O bond was remarkably stretched by more ˚ , and the N–N bond was contracted by than 0.025 A ˚ in the O-end-down configuration. approximately 0.01 A Because a bonding orbital was formed between the surface and the N2O molecule, we conclude that N2O chemisorbed on this surface. This result is consistent with the results of previous experiments [4,5]. Finally, we explain why the adsorption energy increased on the oxygen-modified surfaces. Analysis of electron transfer from N2O to the surfaces (that is, the electron losses from N2O) showed the following relationship at the UB3LYP/6-31G(d) level: Ot =O2 mod: > Nt =O2 mod: > Ot =O mod: > Ot =clean surface where Ot and Nt designate the O-end-down and N-end-down configurations, respectively; and O2-mod. and O-mod. designate the surfaces modified by an oxygen molecule and an oxygen atom, respectively. The order for electron loss and the Ead values turned out to be related. Further investigations – such as vibrational frequency analysis and studies of how the N2O adsorption state changes with oxidation state, cluster size, and coverage – are now in progress [22]. 4. Conclusions Nitrous oxide adsorption and decomposition on several Si(1 0 0) surfaces were investigated using quantum chemical calculations. We clearly showed that clean, O-modified, and O2-modified surfaces exhibited different adsorption states, as indicated by their adsorption energies. Furthermore, our results revealed the relationship between electron transfer and adsorption energy. We also predicted the potential energy surface for the N2O dissociation pathway, which showed small activation barriers. Although our results are totally consistent with experimental observa-
267
tions were insufficient owing to the use of small clusters, so we recognize that further study using larger clusters will be required to give more-explicit descriptions of the reaction dynamics, including the angular distribution of desorbing N2 molecules. Acknowledgements This work was supported by research fellowships from the Japan Society for the Promotion of Science (JSPS) for Young Scientists and from the CREST project of the Japan Science and Technology Agency (JST). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
T. Engel, Surf. Sci. Rep. 18 (1993) 91, and references there in. E.G. Keim, L. Wolterbeek, A. van Silfhout, Surf. Sci. 180 (1986) 565. E.G. Keim, A. van Silfhout, Surf. Sci. 216 (1989) L337. T. Kubo, T. Ema, A. Atli, T. Aruga, N. Takagi, M. Nishijima, Surf. Sci. 382 (1997) 214. J. Lee, H. Kato, K. Sawabe, Y. Matsumoto, Chem. Phys. Lett. 240 (1995) 417. K. Imamura et al., Surf. Sci. 566–568 (2004) 1076. K. Imamura, T. Matsushima, Catal. Lett. 97 (2004) 197. R. Konecny, D.J. Doren, Surf. Sci. 417 (1998) 169, and references there in. A.D. Becke, J. Chem. Phys. 98 (1993) 5648. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. P.C. Hariharan, J.A. Pople, Theoret. Chim. Acta 28 (1973) 213. M.M. Francl, W.J. Pietro, W.J. Hehre, J.S. Binkley, M.S. Gordon, D.J. DeFree, J.A. Pople, J. Chem. Phys. 77 (1982) 3654. A.P. Scott, L. Radom, J. Phys. Chem. 100 (1996) 16502. S. Simon, M. Duran, J.J. Dannenberg, J. Chem. Phys. 105 (1996) 11024. M.J. Frisch et al., GAUSSIAN 98, Revision A.11, Gaussian, Inc., Pittsburgh PA, 2001. M.J. Frisch et al., GAUSSIAN 03, Revision D.01, Gaussian, Inc., Pittsburgh PA, 2005. H. Over, J. Wasserfall, W. Ranke, C. Ambiatello, R. Sawitzki, D. Wolf, W. Moritzet, Phys. Rev. B 55 (1997) 4731. Y. Widjaja, C.B. Musgrave, J. Chem. Phys. 116 (2002) 5774. L. Mozzi, B.E. Warren, J. Appl. Crystallogr. 2 (1969) 164. P. Ordejo´n, F. Yndura´in, Phys. Rev. B 43 (1991) 4552. G. Herzberg, Molecular Spectra and Molecular Structure, vol. 2, Van Nostrand, New York, 1945. K. Imamura, H. Tokiwa, in preparation.
Theoretical study of N2O adsorption on clean and partially oxidized Si(1 0 0)-(2 · 1) small clusters Kenji Imamura, Hiroaki Tokiwa
*
Department of Chemistry, Faculty of Science, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan Received 27 September 2005 Available online 18 January 2007
Abstract Theoretical studies of the adsorption and decomposition of nitrous oxide (N2O) on clean and partially oxidized Si(1 0 0) clusters were performed by means of the hybrid density functional theory method. N2O underwent physisorption on the clean surface and chemisorption on the surface modified by molecular oxygen. The difference is clearly explained by means of electron transfer analysis. N2O dissociated to form N2 on surface dimers modified with O, and the activation energies in this process were small. 2007 Published by Elsevier B.V.
1. Introduction Because the elucidation of the initial oxidation process on semiconductor surfaces is important not only in terms of fundamental studies of surface science but also in terms of the development of practical electronic devices, several experimental and theoretical studies of oxidation processes have been performed on Si(1 0 0) and Si(1 1 1) surfaces with an oxygen atom or molecule as the oxidant [1]. Nitrous oxide (N2O) is also frequently used as an oxidant, because it easily releases an oxygen atom on surfaces and because experiments with N2O provide information about the initial oxidation process on the surfaces [2–5]. Using high-resolution electron energy loss spectroscopy (HREELS), Kubo et al. [4] have assigned vibrational modes of Si– O–Si species and stretching modes of two types of N–N and N–O bonds of N2O on a partially oxidized surface. Lee et al. [5] found a unique angular distribution of desorbing N2 molecules after N2O adsorption on clean and partially oxidized Si(1 0 0) surfaces at temperatures below 100 K using a time-of-flight (TOF) technique. The collimation angle was 32 from normal to the surface. Lee et al. proposed a desorption mechanism in which the inclined *
Corresponding author. Fax: +81 3 3985 2394. E-mail address: [email protected] (H. Tokiwa).
0009-2614/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.cplett.2007.01.043
desorption was induced by N2O chemisorbed with the O-end in a down configuration on the partially oxidized surface, since the collimation angle of the molecule was similar to the angle of the symmetric Si–Si dimer. The angular distribution of desorbing N2 molecules released after N2O adsorption has recently been confirmed on clean and partially oxidized Rh(1 1 0) surfaces using both angleresolved thermal desorption spectroscopy [6] and a pressure-jump method [7]. However, the reaction mechanisms have not yet been determined for any of these reactions. We expected that a quantum chemical approach would provide useful information regarding surface reaction dynamics. In this communication, we report on N2O adsorption behavior on both clean and partially oxidized Si(1 0 0) surfaces with an oxygen atom or molecule as an oxidant. 2. Computational details We employed simple cluster models containing nine silicon atoms for both clean and oxygen-modified Si(1 0 0) surfaces. The clean surface was composed of four layers that included a single surface dimer. Hydrogen atoms were used to terminate bulk Si atoms (Si9H12). This cluster model gives energetic and geometric predictions that are in good agreement with experimental results and other theoretical
264
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methods at a reasonable computational cost, although the model neglects interactions with adjacent dimers [8]. We adopted two types of oxygen-modified surfaces: one precovered with a single oxygen atom and one pre-covered with an oxygen molecule. The unrestricted-B3LYP (UB3LYP) hybrid density functional [9,10] with the standard 6-31G(d) basis set [11,12] was used for all atoms throughout. Under the supermolecular approach, adsorption energies (Ead) were defined by the following equation:
a
2.225
Ead ¼ EC ðEA þ ES Þ where EC, EA, and ES represent the total energies of the complex, the isolated adsorbate, and the surface, respectively. From these definitions, negative Ead values represent a stable adsorption state. These energies include the scaled zero-point-energy (ZPE) corrections calculated at the UB3LYP/6-31G(d) level [13]. And, we use the Counterpoise(CP)-corrected geometry optimization and vibrational analysis [14] in order to correctly handle the unphysical basis set superposition error (BSSE) for evaluations of Ead and ZPE values. Geometry optimizations were performed without any constraint during calculations. All calculations were carried out using the GAUSSIAN 98 and 03 packages [15,16]. 3. Results and discussion
b O 1.738
1.739
R1
R 1 : 2.302 A 1 : 82.9
c
O
3.1. Isolated Si(1 0 0) surface and N2O structures In the fully optimized structure of the clean surface ˚, (Fig. 1a), the bond length of the Si–Si dimer was 2.225 A which is good agreement with the experimental value of ˚ [17]. 2.24 ± 0.08 A When an oxygen atom was adsorbed on the clean surface, a Si–O–Si bridge structure was formed (Fig. 1b). The bonding orbital of the surface dimer was intact, and the Si–O–Si bond angle was 82.9, which is smaller than the usual angle for sp3- and sp2-hybridized orbitals. When an oxygen molecule was adsorbed on the clean surface, we obtained the stable geometry adsorbed molecularly or dissociatively on the surface. These results closely correspond to the results reported by Widjaja and Musgrave [18]. They investigated a symmetrical adsorption configuration for the dimer (not shown here), which showed a structure in which one of O atoms was inserted between the Si atoms of the cleaved surface-dimer (Fig. 1c). The optimized Si–O–Si bond angle was 145, which is in excellent agreement with both experimental [19] and theoretical [20] values of 144. The dissociated structure was energetically more stable than the molecularly adsorbed structure, by about 60 kcal/mol at the UB3LYP level. We used only the dissociated structure as our oxygen-modified model for O2 adsorption, since the surface dimer was saturated in the molecularly adsorbed structure.
A1
1.589
1.818 O
1.547
A1
A 1 : 145.0 Fig. 1. Optimized surface structures at the level of UB3LYP/6-31G(d) on (a) clean surface (Si9H12), and modified surfaces by oxygen (b) atom (Si9H12O) and (c) molecule (Si9H12O2). Purple and light-blue circles represent Si and H atoms, respectively. R1 and A1 represent Si–Si bond length and Si–O–Si bond angle, respectively. The units of bond lengths ˚ ) and (), respectively. (also Figs. 2–5.) and bond angles were given in (A
Our calculations result that isolated N2O has a linear structure with N–N and N–O bond lengths of 1.134 and ˚ , respectively. The scaled vibrational frequencies 1.193 A were 581, 1292, and 2281 cm1 for the N–N–O bending mode and the N–O and N–N stretching modes, respectively. These calculated results were consistent with IR spectroscopy results of 589, 1285, and 2224 cm1, respectively [21].
K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
3.2. N2O adsorption on the clean Si(1 0 0) surface
1.134 N [–0.11] 1.193 N [+0.62] [–0.50] O A1 A2
The CP-corrected fully optimized geometry of N2O adsorbed on the Si(1 0 0) clean surface showed an O-enddown configuration. This geometry clearly indicates that N2O is weakly bounded on the Si atom of the surface. The adsorption energy showed slightly stable value at the UB3LYP/6-31G(d) level (Fig. 1). The intermolecular distance between the O atom and the adjacent Si atom was ˚ because both atoms had negative Mullarger than 4.0 A liken charges. The geometrical parameters for N2O were almost identical to those of the isolated system. We therefore conclude that N2O was physisorbed on the clean surface by means of an electrostatic interaction. This conclusion is consistent with the results of HREELS [4] and TOF [5] measurements. Since Kubo et al. and Lee et al. [4,5] further insisted that N2O partially dissociated on the clean surface even at 90 K, we next discuss the dissociation process of N2O on the clean surface.
4.511 4.061
[–0.09]
265
2.231
A 1 : 180.0 A 2 : 104.5
3.3. N2O dissociation on the clean Si(1 0 0) surface
E ad :–0.03
Fig. 3 shows the predicted potential energy surface (PES) for N2O dissociation, with structures at each stationary point. As the N–N–O bond angle of N2O was gradually distorted to 163.0 from the angle in the physisorbed structure shown in Fig. 2 (denoted Int.1 in Fig. 3), we found a transition state (TS) structure with one imaginary
Fig. 2. Physisorbed N2O structure on Si(1 0 0) clean surface. Each of values and symbol A inserted in figures represents bond lengths and bond angles, respectively. Symbols of A1 and A2 indicate bond angles of NNO and SiON, respectively. Relative CP-corrected adsorption energy (Ead), the unit of kcal/mol, was represented at the level of UB3LYP/6-31G(d) (also Figs. 3–5). And values in a bracket indicates Mulliken charges (also Figs. 4 and 5).
9
13 [+0.53] 1. 1.218
[–0.43]
O
2.462
[–0.07]
A1
[–0.09]
[–0.04]
[+0.07] [–0.57] O
1.125 A2
1.581 [+0.49]
[+0.07]
A 1 : 163. 0
Relative Energy : ΔE
[–0.07]
1.929
TS Int.1 0.00
2.81
–0.03
≈
≈
A 2 : 141. 0
Int.2 –56.84
Reaction coordinate Fig. 3. Potential energy surface (PES) for N2O dissociation on Si(1 0 0) clean surface with each stationary point structures. Int.1 was same structure denoted in Fig. 2. Several values and symbol A inserted in figures indicate bond lengths and bond angles, respectively. A1 and A2 indicate bond angles of NNO and SiNN, respectively, in TS and Int.2. Arrows in transition structure were denoted transition vector.
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K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
frequency of 188i cm1. The activation energy was only 2.84 kcal/mol from Int.1. Because the barrier was only slightly higher than the energy of the reactants, photo-irradiation or heating of the surface could promote the reaction. As the N–N–O bond angle was further distorted along the reaction coordinate, we found a stable dissociated structure (Int.2) containing two chemical bonds, one between the surface dimer and the terminal nitrogen and one between the dimer and the oxygen atom. We observed an initial oxidation step on the Si(1 0 0) surface.
system. Because this result is inconsistent with the results of previous work, in which N2O was observed to chemisorb on a partially oxidized surface [4,5], we calculated N2O adsorption on a surface modified with an oxygen molecule. In the optimized structure of N2O adsorbed on the O2modified surface, N2O was oriented toward the surface either with the O-end-down or with the terminal N-enddown (Fig. 5). The adsorption energies at the UB3LYP level were estimated to be 5.47 and 3.05 kcal/mol for the O-end-down and N-end-down structures, respectively. These results clearly showed that Ead was significantly
3.4. N2O adsorption on the oxygen-modified Si(1 0 0) surfaces In the optimized structure of N2O adsorbed on the surface modified by an oxygen atom, the adsorbed N2O took on an O-end-down configuration to adjacent Si atom of the surface (Fig. 4). In the case, the adsorption energy was determined to be 0.79 kcal/mol, which was about oneorder larger than that of the clean surface. However, we conclude that N2O was physisorbed even on this surface, because the geometrical parameters for N2O were nearly identical to those of the isolated system and because the intermolecular distances between the surface and N2O were larger than in the case of the clean surface. Furthermore, Kohn–Sham orbital analysis clearly showed that no bonding orbital was formed between the surface and N2O in this
[–0.01]
1.124
N
[+0.67] N A2
[–0.56]
1.218
[–0.54] O A 3
2.240 [+0.65]
O
[–0.65] O
A1
A 1 : 146.0 A 2 : 180.0 A 3 : 114.0
[–0.12] 1.133
N 1.192 N [+0.64] A2 [–0.50] O A3
Ead : –5.47 [–0.43]
3.869 O
[+0.26]
O
1.176
3.032
[+0.69] N A2
[–0.57]
1.139
[–0.60]
[–0.18] N A3
A1
2.356 [+0.66]
O
[–0.63] O
A1
A 1 : 82.7
A 1 :145.0 A 2 :179.0 A 3 :130.5
A 2 : 179.2 A 3 : 99.0
E ad : –0.79 Fig. 4. N2O adsorption structure with O-end-down configuration on modified surface by oxygen atom. Various values in figures represent bond lengths. A1, A2 and A3 indicate bond angles of Si–O–Si, NNO and SiON, respectively.
Ead : –3.05 Fig. 5. Stable structures of N2O on modified surface by oxygen molecule. (a) O-end-down, (b) N-end-down structures, respectively. Each value indicates bond lengths. A1, A2 and A3 indicate bond angles of inserted Si– O–Si, NNO and SiON in (a) or SiNN in (b), respectively.
K. Imamura, H. Tokiwa / Chemical Physics Letters 436 (2007) 263–267
larger than the corresponding values for the clean surface. The intermolecular distances between the terminal O and N atoms in N2O and the adjacent Si atom were 2.240 and ˚ , respectively. Note that the former is 1.8 A ˚ shorter 2.356 A than the distances calculated for the clean surface. Moreover, the N–O bond was remarkably stretched by more ˚ , and the N–N bond was contracted by than 0.025 A ˚ in the O-end-down configuration. approximately 0.01 A Because a bonding orbital was formed between the surface and the N2O molecule, we conclude that N2O chemisorbed on this surface. This result is consistent with the results of previous experiments [4,5]. Finally, we explain why the adsorption energy increased on the oxygen-modified surfaces. Analysis of electron transfer from N2O to the surfaces (that is, the electron losses from N2O) showed the following relationship at the UB3LYP/6-31G(d) level: Ot =O2 mod: > Nt =O2 mod: > Ot =O mod: > Ot =clean surface where Ot and Nt designate the O-end-down and N-end-down configurations, respectively; and O2-mod. and O-mod. designate the surfaces modified by an oxygen molecule and an oxygen atom, respectively. The order for electron loss and the Ead values turned out to be related. Further investigations – such as vibrational frequency analysis and studies of how the N2O adsorption state changes with oxidation state, cluster size, and coverage – are now in progress [22]. 4. Conclusions Nitrous oxide adsorption and decomposition on several Si(1 0 0) surfaces were investigated using quantum chemical calculations. We clearly showed that clean, O-modified, and O2-modified surfaces exhibited different adsorption states, as indicated by their adsorption energies. Furthermore, our results revealed the relationship between electron transfer and adsorption energy. We also predicted the potential energy surface for the N2O dissociation pathway, which showed small activation barriers. Although our results are totally consistent with experimental observa-
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tions were insufficient owing to the use of small clusters, so we recognize that further study using larger clusters will be required to give more-explicit descriptions of the reaction dynamics, including the angular distribution of desorbing N2 molecules. Acknowledgements This work was supported by research fellowships from the Japan Society for the Promotion of Science (JSPS) for Young Scientists and from the CREST project of the Japan Science and Technology Agency (JST). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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