High reactivity of silicon suboxide clusters

Abstract

The recent silicon-oxide-assisted formation of Si nanostructures has been studied based on quantummechanical calculations of Si n O m (n,mϭ1 -8) clusters. We found that ͑1͒ energetically the most favorable small silicon-oxide clusters have O atomic ratios at around 0.6, and ͑2͒ remarkably high reactivity at the Si atoms exists in silicon suboxide Si n O m clusters with 2nϾm. The results show that the formation of Si-Si bonds is preferred and thus facilitates the nucleation of Si nanostructures when silicon suboxide clusters come together or stack to a substrate. DOI: 10.1103/PhysRevB.64.113304 PACS number͑s͒: 61.46.ϩw, 73.22.Ϫf Silicon oxide has found important applications in wide fields such as microelectronics, optical communications, and thin-film technology. 1-3 Recent work 4 further showed that silicon oxide could also play an important role in the fabrication of an important nanometer material, the Si nanowire, 5,6 which has attracted much attention for its interesting quantum confinement effect as well as useful electrical, optical, mechanical, and chemical properties. The formation of Si nanowires was achieved by using Si powder sources mixed with SiO 2 . 4 A high yield of nanowires has been obtained when the chemical compositions of Si and O in the source are approximately equal. 7 On the contrary, the important role of oxide is not needed in the conventional growth mechanisms of nanowires. 8,9 To understand the oxide-assisted formation mechanism, the exploration of silicon oxide clusters about their geometric, electronic, and chemical properties would provide useful information.To promote various technological applications based on silicon oxide, theoretical investigations have been performed on small silicon oxide clusters. Systems, such as isolated SiO 2 , gas-phase oligomers (SiO 2 ) n ͑nр8, nϭ18͒, 10 Si(SiO 2 ) n (nϭ2,3), (SiO 2 ) n (nϭ1 -4), (SiO 2 ) n (n ϭ3 -5), 11 Si 3 O m (mϭ1 -6), 12 Si 2 O 4 2Ϫ , Si 2 O 5 2Ϫ , 13 charged and neutral Si n O m ͑nр6, mр12͒, 14 and (SiO) n (nр5), 15 have been studied using ab initio theories. Information achieved includes geometric structures, energetics, bonding, energy gaps, and valence electronic structures of these species. However, understanding of the oxide-assisted formation of Si nanowires in which silicon oxide acts as a special ''catalyst'' would require a systematic study of Si n O m . In this work, we have made such a systematic study of Si n O m (n,mϭ1 -8) at the quantum-mechanical level, aiming at elucidating the oxide-assisted formation mechanism of silicon nanowires.Energetically, the more favorable structure has been searched for each composition of Si n O m (n,mϭ1 -8) by means of calculations with density functional theory ͑DFT͒. The DFT calculations in this work used the popular B3LYP method, which is based on the Becke-type three-parameter density functional theory. 16 An economic basis set, where 3-21G and 6-31G* were selected to describe Si and O, respectively, according to their electronegativities and charge transfers, 17 was used in ...

Structures and Fragmentations of Small Silicon Oxide Clusters by ab Initio Calculations

“…Recently, Chu et al have performed a detailed computational study of the structures of Si n O m (n, m ) 1-8) clusters using the DFT-B3LYP method and suggested several new geometries for these clusters. 18,19 Despite many efforts, our understanding of the structures and properties of silicon oxide clusters is still far from complete. In this paper, we present a systematic study on the structures and stabilities of SiO n (n ) 1-4), Si 2 O n (n ) 1-5), Si 3 O n (n ) 1-7), Si 4 O n (n ) 1-9), and Si 5 O n (n ) 1-11) clusters.…”

First Principles Study on Sino (N=14-18) and Si10-Mom(M=1-8) Clusters

“…In recent years, the oxide silicon was investigated in both experiment [1,4,5,[10][11][12][13] and theory [10][11][12][13][14][15][16][17][18][19][20][21] for its broad prospect in the application. Desjardin and co-workers [1] studied oxidation of Si (111)-(7×7) surface by STM.…”

Birth of silicon nanowires covered with protective insulating blanket

“…[21,22] The bond lengths of Si-Si and Si-O are 230 p.m. and 153.3 p.m., respectively, while their bond energy is 226 and 465 kJ/mol, respectively. [20,23,24] Si-O is more stable bond considering the shorter bond length with higher bond energy compared with Si-Si. Hence, the precipitated NW will have more Si-O bonds that form a thick shell of SiO x compared with the Si core.…”

“…According to the frontier orbital theory, the reactivity of SiO x clusters indicates the formation of Si-Si, O-O, and Si-O bonds where small clusters join to form larger clusters. [24] The smaller gap of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals in SiO x clusters indicate higher chemical reactivity, especially localization of HOMO on the Si atoms at the surface of the cluster leads to a highly reactive region that attracts other clusters to grow. The cohesive energy (energy gained when a surface is created by the formation of the bond from free atoms) per atom in Si-rich clusters at mentioned concentration is very high, signifying less chance of their presence in gaseous phase.…”

“…The cohesive energy (energy gained when a surface is created by the formation of the bond from free atoms) per atom in Si-rich clusters at mentioned concentration is very high, signifying less chance of their presence in gaseous phase. [24] Based on this energetic favorability, the clusters that spread out to form NWs at L-S interface is likely to be in a solid phase.…”