N. Wang et al/Materials Science and Engineering R 60(2008)1-51 formation of different morphologies of ZnO nanostructures for them to form the Si-Si, Si-O, ando-o bonds were revealed according to the well-known frontier orbital theory [142]. The er With the presence of carbon, during the reaction in an open- HOMO-LUMO gap for(SiO)n clusters are 2.0-4.5 eV, much end quartz tube(open one end of the tube to air ), Zn vapor or lower than those for(SiO) species, indicating higher chemica droplets can be partially oxidized forming suboxides, which reactivity of (SiO)n clusters. The HOMO mainly localizes on generally have low melting temperatures. The formation of the the Si atoms at the cluster surface, making these regions the suboxides is because the amount of oxygen contributing to the reactive. As the O ratio is less than about 0.62, the reactivity to reaction in the open-end quartz tube is limited. This condition is form a Si-Si bond of two silicon oxide clusters is remarkably reasonable since Zn droplets co-exist with ZnO nanowire larger than to form a Si-O or O-O bond [141], as shown in products in the early stage of the nanowire formation. Either Zn Fig. 18. The combination of these clusters might occur easily droplets or vaporized Zn suboxide droplets could be the nuclei through the Si-Si bonding or Zno nanowires. Similar to the oxide-assisted growt The richer the si atoms in the cluster. the higher will be the mechanism(to be discussed in the next section), Zn suboxides chance for them to form a si-Si bond. However the cohesion are more reactive than Zno and may largely enhance the energy per atom of the silicon-rich clusters is much higher, deposition of Zn oxides at the tips of Zno nanowires during indicating a smaller chance of their presence in the gas phas growth. Due to further oxidation of Zn or Zn suboxides, the The optimum ratio of Si atom to O atom in the silicon suboxide concentration of oxygen in the droplets/tips increases, and thus clusters to achieve the highest yield and formation of Zno deposits on the interface between the droplets and nanowire should be close to l, as also observed experimentally ubstrate, resulting in the growth ZnO nanowires. Zn and Zn-(about 49 at. o of O)[40]. Our recent experiment using silicon rich phases have been observed by HRTEM on the Zno monoxide has given the largest yield of Si nanowires. It is anowire tips grown by the Vs growth(Fig. 17(h))[62]. worthwhile noting that there are also experimental reports or Moreover, Zn-Zno core-shell nanobelts and tubes were also the formation of the crystalline phase of Si nanoclusters from observed [134]. Though the self-catalytic growth mechanisms the deposition of silicon-rich oxide [140, 143 the vs growth are complicated and unclear, many metal The nucleation of Si nanocrystals could be expected to take oxide nanowires and interesting morphologies of nanostruc- place via the combination of small Si suboxide clusters. The tures have been produced by this method [135, 136 formation of Si core begins at n=3 is shown in Fig. 19[144] As shown in the figure, (1)a Si core(represented by the open 3.3. Oxide-assisted growth circles containing stars surrounded by a silicon oxide sheath is involved; (2)the Si-Si bonds prefer to form in the center rather 3.3.1. Kinetics and reactivity of silicon oxide in nucleation than at the cluster surface so as to reduce the strain caused; (3) and growth most of the si atoms in the si have three or four he VLs mechanism, the nucleation and coordinates with Si-Si-Si bond angles close to 109(the value growth of Si nanowires from the oxide-assisted mechanism found in silicon crystal), which is quite different from that of appears to be novel. The oxide-assisted nanowire growth is pure Si clusters of the same size [1451;(4) with increasing described by reactions(3.3. 1),(3.3.2)and (3.3.3). The vapor cluster size, the size of the Si core increases and the fraction of phase of Sio and Si,o(x> 1) generated by the thermal effect (thermal evaporation or laser ablation) is the key factor Si(solid)+SiO2(solid)"- 2Sio(gas) (3.3.1 Si-Si 2Sio(gas low temperatur Si(solid)+SiO2(solid) (3.3.2) ix-l(solid)+SiO(solid)(r>1 (3.3.3) Silicon oxide clusters generated and present in the gas phase in i nanowire synthesis play an important role in the nucleation and growth. Small silicon oxide clusters Si,Om(n, m=1-8) studied both experimentally and theoretically [137- 139, 262, 270, 271, 274] revealed that silicon monoxide-like clus ters adopt planar and buckled-ring configurations, while oxy- ren-rich clusters are rhombuses arranged in a chain with djacent ones perpendicular to each other. Si suboxide clusters [OV(O+[Si) atom ratio are highly reactive to bond with other clusters and prefer to form Si-Si bonds [141]. By analysis of the highest occupied HOMo (electron donor) and thus the reactivity(proportional to the inverse of molecular orbitals(HOMOs) and the lowest unoccupied mole the energy difference) for the formation of a Si-Si bond, a Si-O bond, or an O cular orbitals (LUMOs)of silicon oxide clusters, the reactivity o bond between two silicon oxide clusters as a function of the Si: O ratio[1411
formation of different morphologies of ZnO nanostructures [67]. With the presence of carbon, during the reaction in an openend quartz tube (open one end of the tube to air), Zn vapor or droplets can be partially oxidized forming suboxides, which generally have low melting temperatures. The formation of the suboxides is because the amount of oxygen contributing to the reaction in the open-end quartz tube is limited. This condition is reasonable since Zn droplets co-exist with ZnO nanowire products in the early stage of the nanowire formation. Either Zn droplets or vaporized Zn suboxide droplets could be the nuclei for ZnO nanowires. Similar to the oxide-assisted growth mechanism (to be discussed in the next section), Zn suboxides are more reactive than ZnO and may largely enhance the deposition of Zn oxides at the tips of ZnO nanowires during growth. Due to further oxidation of Zn or Zn suboxides, the concentration of oxygen in the droplets/tips increases, and thus ZnO deposits on the interface between the droplets and substrate, resulting in the growth ZnO nanowires. Zn and Znrich phases have been observed by HRTEM on the ZnO nanowire tips grown by the VS growth (Fig. 17(h)) [62]. Moreover, Zn–ZnO core–shell nanobelts and tubes were also observed [134]. Though the self-catalytic growth mechanisms of the VS growth are complicated and unclear, many metal oxide nanowires and interesting morphologies of nanostructures have been produced by this method [135,136]. 3.3. Oxide-assisted growth 3.3.1. Kinetics and reactivity of silicon oxide in nucleation and growth Compared to the VLS mechanism, the nucleation and growth of Si nanowires from the oxide-assisted mechanism appears to be novel. The oxide-assisted nanowire growth is described by reactions (3.3.1), (3.3.2) and (3.3.3). The vapor phase of SiO and SixO (x > 1) generated by the thermal effect (thermal evaporation or laser ablation) is the key factor: SiðsolidÞ þ SiO2ðsolidÞ ! high temperature2SiOðgasÞ (3.3.1) 2SiOðgasÞ ! low temperatureSiðsolidÞ þ SiO2ðsolidÞ (3.3.2) SixOðgasÞ ! low temperatureSix1ðsolidÞ þ SiOðsolidÞ ðx > 1Þ (3.3.3) Silicon oxide clusters generated and present in the gas phase in Si nanowire synthesis play an important role in the nucleation and growth. Small silicon oxide clusters SinOm (n, m = 1–8) studied both experimentally and theoretically [137– 139,262,270,271,274] revealed that silicon monoxide-like clusters adopt planar and buckled-ring configurations, while oxygen-rich clusters are rhombuses arranged in a chain with adjacent ones perpendicular to each other. Si suboxide clusters are highly reactive to bond with other clusters and prefer to form Si–Si bonds [141]. By analysis of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of silicon oxide clusters, the reactivity for them to form the Si–Si, Si–O, and O–O bonds were revealed according to the well-known frontier orbital theory [142]. The HOMO–LUMO gap for (SiO)n clusters are 2.0–4.5 eV, much lower than those for (SiO)2 species, indicating higher chemical reactivity of (SiO)n clusters. The HOMO mainly localizes on the Si atoms at the cluster surface, making these regions the reactive. As the O ratio is less than about 0.62, the reactivity to form a Si–Si bond of two silicon oxide clusters is remarkably larger than to form a Si–O or O–O bond [141], as shown in Fig. 18. The combination of these clusters might occur easily through the Si–Si bonding. The richer the Si atoms in the cluster, the higher will be the chance for them to form a Si–Si bond. However, the cohesion energy per atom of the silicon-rich clusters is much higher, indicating a smaller chance of their presence in the gas phase. The optimum ratio of Si atom to O atom in the silicon suboxide clusters to achieve the highest yield and formation of Si nanowire should be close to 1, as also observed experimentally (about 49 at.% of O) [40]. Our recent experiment using silicon monoxide has given the largest yield of Si nanowires. It is worthwhile noting that there are also experimental reports on the formation of the crystalline phase of Si nanoclusters from the deposition of silicon-rich oxide [140,143]. The nucleation of Si nanocrystals could be expected to take place via the combination of small Si suboxide clusters. The formation of Si core begins at n = 3 is shown in Fig. 19 [144]. As shown in the figure, (1) a Si core (represented by the open circles containing stars surrounded by a silicon oxide sheath is involved; (2) the Si–Si bonds prefer to form in the center rather than at the cluster surface so as to reduce the strain caused; (3) most of the Si atoms in the Si core have three or four coordinates with Si–Si–Si bond angles close to 1098 (the value found in silicon crystal), which is quite different from that of pure Si clusters of the same size [145]; (4) with increasing cluster size, the size of the Si core increases and the fraction of Fig. 18. The inverse of the energy difference DE = LUMO (electron acceptor)– HOMO (electron donor) and thus the reactivity (proportional to the inverse of the energy difference) for the formation of a Si–Si bond, a Si–O bond, or an O– O bond between two silicon oxide clusters as a function of the Si:O ratio [141]. 16 N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51
N. Wang et al. /Materials Science and Engineering R 60 (2008)1-51 17 nanowires [146]) by assuming the process is at equilibrium and described by the Boltzmann factor exp(E/kT), where k is the Boltzmanns constant, E the energy difference, and T is the temperature in Kelvin. The results shown in the inset in Fig. 20 (a)n=3(b) confirm that the structures containing Si cores still play the major role at such a high temperature starting at a size as small as n=8. The formation of sp Si core inside the silicon oxide clusters contributes to the nucleation of the Si nanocrystals. Because of their high chemical reactivity, a combination of these clusters may easily take place, forming clusters with a (e)n=8 (f)n=9 (g)n=11 large sp Si core via subsequent reconstruction andO migration from the center to the surface of the clusters. The crystalline Si cores thus formed can act as nuclei and precursors for subsequent growth of Si nanostructures Fig. 21 shows three different isomers of the(SiO)1 cluster with an o atom locating in different sites from the center to the surface of the cluster. The most stable configuration is the one with O located on its surface, and the total binding energy is 211.74 eV. However, the binding energy decreases as the o ()n=18 atom moves from the surface into the cluster The o atom could Fig 19. The most favorable structures of silicon monoxide clusters(SiO)n for migrate from the center of the silicon monoxide cluster to its 21[144] surface via bond switching For the(sio)s cluster, the estimated migration barrier is about 1.79 eV. The high strain involved in Si atoms with three and four coordinates increases correspond the migration ofo atom from ingly, making the cluster more stable; and(5)starting at n=18 the inside to the surface, leading to the formation of a Si core all of the Si atoms in Si cores are four-coordinated, indicating The nuclei containing a Si core would grow larger with the the formation of sp Si cores similar to the configuration in the assistance of o diffusion from the core to the surface laye Si crystal. Fig 20 depicts the binding energies of (SiO)n clusters during deposition containing Si cores as a function of n, together with those In an experiment using Sio powder or a mixture of Si and containing buckled structures. It is clear that: (1)the Sio2 powder as the source, the evaporated (SiO)n clusters configurations containing Si cores become energetically more deposited on a substrate would be anchored due to their high favorable than the buckled structures for n=5 and larger and reactivity at Si sites. The deposited clusters would act as the (2)the cluster becomes increasingly more stable with nuclei to absorb(SiO)n clusters from the vapor because of their increasing Si core size. As the two structures from n=5 to n=8 in Fig. 20 are close in energy, we further estimate their Si relative population at 900C(the growth temperature of si 88 Number of (SiO)units △ △△A△ △ 20842eV Number of( SiO)units 20 20943eV 211.74eV 20. Binding energy(e v/atom)of (SiO)n clusters vs n. The up triangles are urrounded by a silicon
Si atoms with three and four coordinates increases correspondingly, making the cluster more stable; and (5) starting at n = 18 all of the Si atoms in Si cores are four-coordinated, indicating the formation of sp3 Si cores similar to the configuration in the Si crystal. Fig. 20 depicts the binding energies of (SiO)n clusters containing Si cores as a function of n, together with those containing buckled structures. It is clear that: (1) the configurations containing Si cores become energetically more favorable than the buckled structures for n = 5 and larger and (2) the cluster becomes increasingly more stable with increasing Si core size. As the two structures from n = 5 to n = 8 in Fig. 20 are close in energy, we further estimate their relative population at 900 8C (the growth temperature of Si nanowires [146]) by assuming the process is at equilibrium and described by the Boltzmann factor exp(E/kT), where k is the Boltzmann’s constant, E the energy difference, and T is the temperature in Kelvin. The results shown in the inset in Fig. 20 confirm that the structures containing Si cores still play the major role at such a high temperature starting at a size as small as n = 8. The formation of sp3 Si core inside the silicon oxide clusters contributes to the nucleation of the Si nanocrystals. Because of their high chemical reactivity, a combination of these clusters may easily take place, forming clusters with a large sp3 Si core via subsequent reconstruction and O migration from the center to the surface of the clusters. The crystalline Si cores thus formed can act as nuclei and precursors for subsequent growth of Si nanostructures. Fig. 21 shows three different isomers of the (SiO)21 cluster with an O atom locating in different sites from the center to the surface of the cluster. The most stable configuration is the one with O located on its surface, and the total binding energy is 211.74 eV. However, the binding energy decreases as the O atom moves from the surface into the cluster. The O atom could migrate from the center of the silicon monoxide cluster to its surface via bond switching. For the (SiO)5 cluster, the estimated migration barrier is about 1.79 eV. The high strain involved in the large (SiO)n cluster may cause the migration of O atom from the inside to the surface, leading to the formation of a Si core. The nuclei containing a Si core would grow larger with the assistance of O diffusion from the core to the surface layer during deposition. In an experiment using SiO powder or a mixture of Si and SiO2 powder as the source, the evaporated (SiO)n clusters deposited on a substrate would be anchored due to their high reactivity at Si sites. The deposited clusters would act as the nuclei to absorb (SiO)n clusters from the vapor because of their Fig. 19. The most favorable structures of silicon monoxide clusters (SiO)n for n = 3–21 [144]. Fig. 20. Binding energy (eV/atom) of (SiO)n clusters vs. n. The up triangles are (SiO)n with the Si-cored structure surrounded by a silicon oxide sheath, and open circles are those with buckled-ring structure. The inset shows the relative population of the former (ND) and the latter (NO) structures at 900 8C. Fig. 21. Possible path of O atom migration from the center of a (SiO)n cluster to its surface: (a) (SiO)5 and (b) (SiO)21 [144]. N. Wang et al. / Materials Science and Engineering R 60 (2008) 1–51 17