pared to partially hydrogenated ones. Removal of up to 50% of the total terminating H atoms only causes distortion to the crystal structure, while the tetrahedral
structures are retained. By removing more than 70–80% of the total terminating H
atoms, the clusters evolve to more compact structures (Figure 10.47).
10.8.3.2 Size-Dependent Oxidation of Hydrogenated Silicon Clusters
We have shown explicitly the size-dependent chemical reactivity of hydrogenated
silicon clusters towards water [89]. A unique trend of decreasing reactivity with
decreasing cluster size has been deduced from reaction energetics, frontier orbital
analysis, and chemical reaction rates determined by the transition state theory in
conjunction with ab initio calculations at Hartree–Fock and Møller–Plesset perturbation levels for water reaction with both dihydride and trihydride silicon configurations, as shown in Figure 10.48. This study indicates the possibility of fabricating stable hydrogenated silicon structures with sizes close to nanometers. We
predict that, with the nanosized hydrogenated silicon structures, it is possible to
fabricate nonreactive, stable nanodevices.
10.9
Summary
A new method based on OAG has been developed that is capable of producing
high-quality and bulk-quantity of various semiconductor nanowires. The presence
of oxides in the target is a common and essential ingredient for the synthesis using
laser ablation or thermal evaporation, so that the targets are capable of generating
semiconductor oxides in the vapor phase. Subsequent decomposition of the vapor
phase oxides at high temperature and defect structures play crucial roles in the
nucleation and growth of high-quality nanowires. The developed OAG approach
has been applied to grow a host of semiconducting nanowires such as Ge, GaN,
GaAs, GaP, SiC, ZnO (whiskers), and carbon nanowires, by either laser-ablation,
HFCVD or thermal evaporation. Large-area, aligned, and long SiNWs via flow
control, and diameter and morphology control by temperature have been achieved.
High-quality SiNW-based nanocables and Si nanoribbons using oxide-assisted
methods have been additionally grown. Oxide-stripping and hydrogen-termination
along with nanowire dispersion enabled the study of SiNW surfaces by STM and
STS. The quantum size effect of the SiNW diameter on the electronic bandgap
was demonstrated for the first time. The nanochemistry of SiNWs has been investigated by performing surface reactions with Ag and Cu ions on the SiNWs.
Nanosized ligated metal clusters on a SiNW surface were realized to achieve good
metal–nanowire contact. The surface reactivity of SiNWs was exploited for growing
carbon-based nanostructures on a SiNW template (liquid solution) and for chemical sensing in the gas phase. Our synthesized highly-oriented SiC nanowires were
shown to be excellent field emitters with large field emission current densities at
very low electric turn-on and threshold fields. Modeling of SiNW structures, nu10.9 Summary 365
structures are retained. By removing more than 70–80% of the total terminating H
atoms, the clusters evolve to more compact structures (Figure 10.47).
10.8.3.2 Size-Dependent Oxidation of Hydrogenated Silicon Clusters
We have shown explicitly the size-dependent chemical reactivity of hydrogenated
silicon clusters towards water [89]. A unique trend of decreasing reactivity with
decreasing cluster size has been deduced from reaction energetics, frontier orbital
analysis, and chemical reaction rates determined by the transition state theory in
conjunction with ab initio calculations at Hartree–Fock and Møller–Plesset perturbation levels for water reaction with both dihydride and trihydride silicon configurations, as shown in Figure 10.48. This study indicates the possibility of fabricating stable hydrogenated silicon structures with sizes close to nanometers. We
predict that, with the nanosized hydrogenated silicon structures, it is possible to
fabricate nonreactive, stable nanodevices.
10.9
Summary
A new method based on OAG has been developed that is capable of producing
high-quality and bulk-quantity of various semiconductor nanowires. The presence
of oxides in the target is a common and essential ingredient for the synthesis using
laser ablation or thermal evaporation, so that the targets are capable of generating
semiconductor oxides in the vapor phase. Subsequent decomposition of the vapor
phase oxides at high temperature and defect structures play crucial roles in the
nucleation and growth of high-quality nanowires. The developed OAG approach
has been applied to grow a host of semiconducting nanowires such as Ge, GaN,
GaAs, GaP, SiC, ZnO (whiskers), and carbon nanowires, by either laser-ablation,
HFCVD or thermal evaporation. Large-area, aligned, and long SiNWs via flow
control, and diameter and morphology control by temperature have been achieved.
High-quality SiNW-based nanocables and Si nanoribbons using oxide-assisted
methods have been additionally grown. Oxide-stripping and hydrogen-termination
along with nanowire dispersion enabled the study of SiNW surfaces by STM and
STS. The quantum size effect of the SiNW diameter on the electronic bandgap
was demonstrated for the first time. The nanochemistry of SiNWs has been investigated by performing surface reactions with Ag and Cu ions on the SiNWs.
Nanosized ligated metal clusters on a SiNW surface were realized to achieve good
metal–nanowire contact. The surface reactivity of SiNWs was exploited for growing
carbon-based nanostructures on a SiNW template (liquid solution) and for chemical sensing in the gas phase. Our synthesized highly-oriented SiC nanowires were
shown to be excellent field emitters with large field emission current densities at
very low electric turn-on and threshold fields. Modeling of SiNW structures, nu10.9 Summary 365