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ARTICLES New Motif of Silicon Segregation in Silicon Monoxide Clusters H. Wang,†,‡ J. Sun,‡ W. C. Lu,*,†,‡ Z. S. Li,‡ C. C. Sun,‡ C. Z. Wang,§ and K. M. Ho§ Department of Physics, Qingdao UniVersity, Qingdao, Shandong 266071, People’s Republic of China, Institute of Theoretical Chemistry, State Key Laboratory of Theoretical and Computational Chemistry, Jilin UniVersity, Changchun 130021, People’s Republic of China, and Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State UniVersity, Ames, Iowa 50011 ReceiVed: September 6, 2007; In Final Form: January 14, 2008
Structures of SinOn clusters are of great interest because of the observed growth of oxide-coated Si nanowires from gas-phase SiO. We studied the geometries of SinOn clusters with n ranging from 12 to 18 using firstprinciples density functional calculations. We found a new structural motif which produces structures that are energetically more favorable than those proposed in recent literature. These structures consist of polygonal bipyramidal Si clusters of sizes between 5 and 7 attached to low-energy Si8O12 or Si12O18 wheel structures previously discovered. The segregation of silicon to the side of the cluster is intriguing and contradicts previous models that assumed silicon segregation nucleates in the center of the monoxide clusters. Electronic structure analysis shows that the HOMO and LUMO states of the monoxide clusters are localized on the segregated silicon cluster, indicating that the segregated Si may act as a nucleation site for further nanostructure growth.
I. Introduction The observation1-3 of high-yield silicon nanowire (SiNW) growth from SiO vapor without the assistance of metallic catalysts has stimulated intense interest in the structures of silicon monoxide (SinOn) clusters. However, SiO is one of the most abundant oxygen-bearing species in astronomical molecular clouds; the condensation process of SiO also has important implications on the formation of interstellar and intergalactic silicate dust particles.4 On the basis of both experimental and theoretical results, it has been suggested that silicate formation probably proceeds via the agglomeration of those small molecular species.5,6 Experiments have shown that oxide-sheathed SiNWs can be obtained by either thermal evaporation or laser ablation of Si powder mixed with SiO2 or simply SiO.1-3 Nearly no SiNWs could be achieved with pure SiO2 materials,2 whereas the highest yields are obtained when the chemical composition of silicon and oxygen in the source are equal.1,7,8 Although these experimental discoveries have been reported for several years, the mechanism for oxide-assisted nanowire growth is still not well understood. A systematic study of the structural motif and evolution of silicon oxide clusters, especially the silicon monoxide clusters, would provide useful insights into the nanowire formation process. Unlike many polymorphs of silica, where the structural basis is a three-dimensional network of silicon-centered corner-sharing SiO4 tetrahedra, the structure of the silicon sub-oxide cluster is * To whom correspondence should be addressed. E-mail: wencailu@ jlu.edu.cn. † Qingdao University. ‡ Jilin University. § Iowa State University.
Figure 1. Wheel structures of Si8O12 and Si12O18 clusters proposed in ref 19.
largely unknown. Although there have been extensive studies on small silicon oxide clusters,9-13 the studies of medium-sized silicon sub-oxide clusters are still very limited.14-16 In a recent paper,15 Zhang et al. proposed that silicon monoxide clusters
10.1021/jp077159j CCC: $40.75 © 2008 American Chemical Society Published on Web 04/15/2008
7098 J. Phys. Chem. C, Vol. 112, No. 18, 2008 facilitate the nucleation and growth of silicon nanostructures through the formation of sp3 silicon cores surrounded by silicon oxide sheaths. On the basis of the structures of SinOn (n ) 1-12) from their calculations, Reber et al. also suggested that there is a tendency for SinOn clusters to segregate into Si-rich cores and O-rich, particularly SiO2, outer shells.16 However, in this work, we propose another structural motif that yields much lower energies for medium-sized silicon monoxide clusters SinOn, with n raging from 12 to 18. Instead of silicon cores surrounded by silicon oxide sheaths, these structures consist of polygonal bipyramid Si clusters with sizes ranging from 5 to 7 attached to low-energy Si8O12 or Si12O18 wheel structure discovered previously by Lu et al.19 (Figure 1). Electronic structure analysis indicates that the segregated silicon can act as a nucleation seed for further Si nanostructure growth. II. Computational Methods Classical potentials available for Si-O systems are usually adopted to study silicon dioxide systems.17,18 They are not suitable for silicon monoxide systems, and thus, the global optimization of SinOn clusters using classical potentials is not feasible at the present. In this work, starting from our previous systematic study on silicon oxide clusters, we designed several structural motifs to estimate the lower-energy structures, e.g., we constructed a series of SinOn structures based on the magic wheel structures of Si8O12 and Si12O18 as shown in Figure 1, which have been shown to be energetically very favorable,19 and additional silicon atoms are added to various locations of these stable silicon oxide fragments. The ab initio calculations were carried out using GAUSSIAN 03 package.20 Initial structure optimizations were performed with B3LYP (Becke-3-Lee-Yang-Par) and 6-31G(d) basis set. B3LYP/ 6-31G(d) has been widely used for studying the structures and properties of SinOm clusters in the literature.10-13 A larger basis set of 6-311G(3df) was then applied for the single-point energy calculations at the DFT level. Moreover, in order to ensure that the structures obtained in our optimizations are indeed stable structures, the isomers are further examined by analyzing the vibration frequencies at the B3LYP/6-31G(d) level. III. Results and Discussion The lowest-energy structures of each size obtained from our calculations are plotted in Figure 2, and the corresponding binding energies of from our calculations are listed in Table 1. For the purpose of comparison, the structural motifs previously proposed in ref 15 were also studied using the same level of calculation method and basis, and their structures and energies are also included in Figure 2 and Table 1, respectively. As one can see from Figure 2, the structures obtained from our calculations have a different motif in comparison with previously proposed structures with silicon cores surrounded by silicon oxide sheaths.15 Instead of segregating to the core, in our silicon monoxide clusters, silicon segregation prefers to form at the side of a stable silicon oxide fragment in the form of Si8O12 or Si12O18 wheel. As shown in Figure 3, the binding energies, defined by EBE ) (Etotal - nESiO)/n, of the structures from our present study are obviously better than those of the Si-core structures of ref 15. Note that the binding energies of the Siedge structures increase as the cluster size n increases. The growth pattern of the SinOn (n ) 12-18) clusters obtained from our calculations is very interesting. Starting from n ) 12 until n ) 16, the most stable structures of SinOn as shown in Figure 2 (parts 1a-5a) are based on the Si8O12 wheel motif. In Si12O12, 8 Si atoms and 12 O atoms have been used
Wang et al. in the construction of the Si8O12 wheel, the remaining 4 Si atoms are attached to one of the edge Si atoms of the Si8O12 wheel to form a trigonal bipyramid Si5 cluster (which is the lowest-energy structure of Si5 cluster). When one more SiO unit is added, it inserts into the bridge site between the edge Si atom of Si8O12 and one of the top Si atoms of Si5, resulting in the structure of Si13O13 as shown in Figure 2 (2a). From Si13O13 to Si14O14, the additional SiO unit allows a Si2O2 rhombus to form between the Si6 cluster and the Si8O12 wheel as shown in Figure 2 (part 3a). As SiO units continue to add, the bridging Si2O2 rhombus changes into a Si3O3 ring in Si15O15, and two perpendicular Si2O2 rhombuses in Si16O16. Meanwhile, the Si cluster grows into the pentagonal bipyramid Si7 in Si16O16. The growth motif described above ensures that oxygen atoms always occupy the bridge site between two Si atoms in the oxide part of the cluster, while the excess Si atoms are segregated and form the most stable bipyramid clusters of Si5, Si6, and Si7 attached to the Si8O12 wheel structure. For Si17O17 and Si18O18, the pentagonal bipyramid Si7 cluster remains, but the framework of silicon oxide fragment changes from the Si8O12 wheel to the Si11O17 wheel-like and the Si12O18 wheel structures, as one can see from Figure 2 (parts 6a and 7a). The Si11O17 wheel-like structure is very similar to that of the Si12O18 wheel structure, except that one of Si3O3 rings in the Si12O18 is replaced by a Si2O2 rhombus. As one more SiO unit is added to Si17O17, the perfect Si12O18 wheel fragment is resorted. We also found that for Si18O18, attaching a Si7 pentagonal bipyramid to a Si12O18 wheel is better than connecting a bigger Si cluster to the Si8O12 wheel. This is consistent with the fact that Si7 is a magic silicon cluster. As the size of the SinOn cluster increases, we expect bigger silicon clusters will be formed. Thus, the observed structural motif might serve as a guide for a mechanism for further growth of the silicon monoxide nanocluster into nanowires or nanocrystals. In order to further understand the stability and chemical reactivity of the SinOn clusters, we have calculated the gaps between the highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), the vertical ionization potential (VIP), and the vertical electron affinity (VEA) for these SinOn clusters. The calculations were performed at the B3LYP level of theory. For the clusters with n ) 1215, the basis set used in the calculations is 6-311G(3df), whereas for n ) 16-18, the basis set is changed to be 6-311G(2d) in order to save computational time. The VIP, VEA, and HOMO-LUMO gap of the SinOn clusters as a function of cluster size n are listed in Table 1 and plotted in Figure 4. The HOMO-LUMO gaps are about 3 eV, and the VIPs are about 8 eV. VEAs show some variation from 2.26 eV in Si12O12 to about 1.81 eV in Si18O18. These results indicate that the SinOn structures obtained from our calculations are stable against losing or adding electrons. It is also very interesting to note that for all of the clusters, the HOMOs and LUMOs are mostly located at pure silicon clusters as one can see from Figure 5, thus facilitating high reactivity in this region. With this high reactivity, the Si-cluster would act as the nucleation seed for further growth of pure Si structures out of the silicon oxide network. Therefore, the Si clusters attached to the silicon oxide fragment might contribute to the further growth of SiNWs. We also studied the fragmentation pathways and dissociation energies of the clusters to provide further understanding of the stability of the clusters. The dissociation energy for fragmentation pathway SinOn f SikOl + Sin-kOn-l is defined as DE ) E(SikOl) + E(Sin-kOn-l) - E(SinOn), in which the energies of SinOn clusters (n ) 12-18), and the fragmentation products SikOl and Sin-kOn-l, were obtained from the DFT calculations
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Figure 2. Geometric structures of SinOn clusters (n ) 12-18). The letters (a and b) denote our results and the corresponding structures in ref 15, respectively. The numbers under structures b in the right column is the energy difference (in eV) between our new structure and the structure of the corresponding size proposed in ref 15, i.e., Eb - Ea.
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TABLE 1: Binding Energies (BE), Vertical Ionization Potentials (VIPs), Vertical Electron Affinities (VEAs), and the HOMO-LUMO Gaps (all in eV) of the SinOn (n ) 12-18) Clustersa clusters
BE
VIP
VEA
HOMOLUMO gap
Si12O12 Si13O13 Si14O14 Si15O15 Si16O16 Si17O17 Si18O18
2.234 (2.140) 2.254 (2.122) 2.327 (2.154) 2.363 (2.175) 2.290 (2.249) 2.359 (2.165) 2.421 (2.164)
7.84 (7.49) 7.61 (7.45) 7.77 (7.39) 7.62 (7.51) 7.92 (7.61) 7.81 (7.12) 7.79 (7.85)
-2.26 (-1.69) -1.83 (-1.90) -1.29 (-1.70) -1.52 (-1.60) -1.91 (-1.42) -1.83 (-1.73) -1.81 (-1.25)
2.42 (2.83) 2.73 (2.67) 3.19 (3.26) 3.91 (3.33) 3.04 (3.67) 3.03 (2.61) 3.03 (4.22)
a
Numbers in parentheses are for the corresponding structures of ref
15.
TABLE 2: Total Energies (a.u.) of the Isomers of Si12O12 Clustersa B3LYP/6-31G(d)
Si12O12
Si-core (a)
Si-core (b)
-4377.5811
-4377.5494
-4377.5014
a
“Si-core (a)” and “Si-core (b)” are the relevant structures presented in refs 15 and 16, respectively.
Figure 3. Binding energy defined as EBE ) (Etotal - nESiO)/n (in eV)of SinOn (n ) 12-18) clusters versus number of Si atoms. The solid line shows the binding energies from the structures obtained from our present study as shown in Figure 2, and the dashed line displays the energies from the structures proposed in ref 15.
Figure 4. Vertical ionization potentials (VIPs), vertical electron affinities (VEAs) and HOMO-LUMO gaps vs the number of silicon atoms for SinOn (n ) 12-18) clusters obtained in this work.
as discussed above. The fragmentation products and the corresponding dissociation energies are listed in Table 3. We can see from Table 3 that one of the two fragmentation products of each silicon monoxide cluster, with the exception of Si13O13 and Si15O15, is a small pure silicon cluster Sin (n ) 3-6). Small silicon clusters have been proven to be stable fragmentation products of small silicon-rich oxide clusters.10-13 Besides the small pure silicon clusters, the larger parts of the products are the magic silicon oxide clusters, e.g., the Si8O12 or Si12O18 cluster. For Si13O13 and Si15O15, Si5O and Si6O2 are also favorable fragmentation products, which can lead to smaller
Figure 5. HOMOs and LUMOs of the SinOn (n ) 12-18) clusters. The isovalue of the contour map is 0.02.
silicon clusters by further dissociations. As listed in Table 3, the fragmentation energies of SinOn (n ) 12-18) ranges from 0.83 to 3.31 eV. The fragmentation analysis indicates that the
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TABLE 3: Fragmentation Channels and Dissociation Energies of SinOn (n ) 12-18) Clusters SinOn f
SikOl
Si12O12 Si13O13 Si14O14 Si15O15 Si16O16 Si17O17 Si18O18
Si4 Si5O Si6O2 Si5 Si6 Si6 Si6
+
Sin-kOn-l Si8O12 Si8O12 Si8O12 Si10O15 Si10O16 Si11O17 Si12O18
the Director for Energy Research, Office of Basic Energy Sciences through Ames Laboratory.
DE (eV) 0.83 2.34 3.31 2.04 1.03 0.98 0.97
most possible dissociation pathway of SinOn (n ) 12-18) would produce a small pure silicon cluster or Si-rich oxide cluster and a stable silicon oxide fragment. The small Si clusters generated from the fragmentations might play an important role in the further growth of silicon nanostructures. IV. Conclusions Using the DFT method, we have studied the new motif structures of SinOn clusters with n ranging from 12 to 18. On the basis of these new structures, we have proposed a growth pattern for the geometric structures of silicon monoxide clusters. From our calculated results, the structures of (SiO)12-16 are consisted of the stable silicon oxide fragment, the Si8O12 wheel consisted of four Si3O3 rings, and a pure silicon cluster attached at the edge of the Si8O12 wheel. For Si18O18, the stable silicon oxide fragment is changed from the Si8O12 wheel to the Si12O18 wheel which contains six Si3O3 rings. The additional Si atoms in the SinOn clusters tend to be segregated as a pure Si cluster of size 5-7 and attach from the side to a silicon oxide fragment, Si8O12 or Si12O18. The new structures obtained from our calculations have been shown to be energetically more favorable than the previously proposed structures. Our studies of these new motifs also suggest that the pure Si cluster attached to the silicon oxide fragment is relatively easy to break away from the silicon oxide fragment. It may also play an important role in the growth of Si nanostructures such as SiNWs or Si nanocrystals in the SiO growth environment. Acknowledgment. This work is supported by the Chinese Natural Science Foundation under Grant Nos. 20473030, 20773047, and 60028403 and the Foundation of Innovation by Jilin University. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. This work was also supported by
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