identified by electron diffraction and high-resolution transmission electron microscopy (HREM) in Figure 8.29(c), the axis of the nanowires was generally along
the h112i direction and the {111} surfaces of Si crystalline cores were parallel to
the axis of the nanowire.
Oxide-Assisted Growth In contrast to the well-established VLS mechanism, Lee
et al. have recently proposed a new nanowire growth route called oxide-assisted
nanowire growth. They report the synthesis and optical characterization of GaAs
nanowires obtained by oxide-assisted laser ablation of a mixture of GaAs and
Ga 2 O 3 [294a]. The GaAs nanowires have lengths up to tens of micrometers and
diameter in the range 10–120 nm, with an average of 60 nm. The nanowires have a
thin oxide layer covering and a crystalline GaAs core with a [111] growth direction.
The oxide-assisted nanowire growth mechanism was further applied to the production of Si nanowires [293, 295]. The growth of Si nanowires was greatly enhanced when SiO 2 -containing Si powder targets were used. Wang et al. have synthesized Si nanowires with uniform size by laser ablation of highly pure Si powder
targets mixed with SiO 2 [295a]. A large quantity of Si nanowires was obtained by
mixing 30%–70% of SiO 2 into the Si powder target. SiO 2 played a crucial role in
enhancing the formation and growth of the Si nanowires. In this proposed oxideassisted growth mechanism, the vapor phase of Si x O ðx > 1Þ generated by thermal
evaporation or laser ablation is the key factor.
Si x OðsÞ ! Si xÀ1 ðsÞ þ SiOðsÞ ðx > 1Þ
2SiOðsÞ ! SiðsÞ þ SiO 2 ðsÞ
The Si nanowires synthesized by using different SiO 2 contents in the targets are
similar in structure except that the outer Si-oxide surfaces of the nanowires synthesized from targets with high SiO 2 content are rough. The diameter of the
nanowires measured from the TEM image ranges from 9 to 12 nm. Most nanowires are quasi-aligned. Obviously, the intensity of the cubic Si (111) diffraction
ring exhibits a strong texture feature of the Si crystals in the nanowires. This indicates that the crystals in the nanowires should have a similar orientation, i.e., the
Si nanowires should have a similar growth direction.
Vapor–Solid Growth Besides the VLS mechanism, the classical vapor–solid (VS)
method for whiskers growth also merits attention for the growth of nanometer 1D
materials [278]. In this process, the vapor is first generated by evaporation, chemical reduction or gaseous reaction. The vapor is subsequently transported and condensed onto a substrate. The VS method has been used to prepare oxide, metal
whiskers with micrometer diameters. Hence it is possible to synthesize the 1D
nanostructures using the VS process if one can control its nucleation and subsequent growth process. Using the VS method, synthesis of nanowires for oxides of
Zn, Sn, In, Cd, Mg and Ga has been attempted. Seo et al. [293c] synthesized single
8.4 Nanowires 259
the h112i direction and the {111} surfaces of Si crystalline cores were parallel to
the axis of the nanowire.
Oxide-Assisted Growth In contrast to the well-established VLS mechanism, Lee
et al. have recently proposed a new nanowire growth route called oxide-assisted
nanowire growth. They report the synthesis and optical characterization of GaAs
nanowires obtained by oxide-assisted laser ablation of a mixture of GaAs and
Ga 2 O 3 [294a]. The GaAs nanowires have lengths up to tens of micrometers and
diameter in the range 10–120 nm, with an average of 60 nm. The nanowires have a
thin oxide layer covering and a crystalline GaAs core with a [111] growth direction.
The oxide-assisted nanowire growth mechanism was further applied to the production of Si nanowires [293, 295]. The growth of Si nanowires was greatly enhanced when SiO 2 -containing Si powder targets were used. Wang et al. have synthesized Si nanowires with uniform size by laser ablation of highly pure Si powder
targets mixed with SiO 2 [295a]. A large quantity of Si nanowires was obtained by
mixing 30%–70% of SiO 2 into the Si powder target. SiO 2 played a crucial role in
enhancing the formation and growth of the Si nanowires. In this proposed oxideassisted growth mechanism, the vapor phase of Si x O ðx > 1Þ generated by thermal
evaporation or laser ablation is the key factor.
Si x OðsÞ ! Si xÀ1 ðsÞ þ SiOðsÞ ðx > 1Þ
2SiOðsÞ ! SiðsÞ þ SiO 2 ðsÞ
The Si nanowires synthesized by using different SiO 2 contents in the targets are
similar in structure except that the outer Si-oxide surfaces of the nanowires synthesized from targets with high SiO 2 content are rough. The diameter of the
nanowires measured from the TEM image ranges from 9 to 12 nm. Most nanowires are quasi-aligned. Obviously, the intensity of the cubic Si (111) diffraction
ring exhibits a strong texture feature of the Si crystals in the nanowires. This indicates that the crystals in the nanowires should have a similar orientation, i.e., the
Si nanowires should have a similar growth direction.
Vapor–Solid Growth Besides the VLS mechanism, the classical vapor–solid (VS)
method for whiskers growth also merits attention for the growth of nanometer 1D
materials [278]. In this process, the vapor is first generated by evaporation, chemical reduction or gaseous reaction. The vapor is subsequently transported and condensed onto a substrate. The VS method has been used to prepare oxide, metal
whiskers with micrometer diameters. Hence it is possible to synthesize the 1D
nanostructures using the VS process if one can control its nucleation and subsequent growth process. Using the VS method, synthesis of nanowires for oxides of
Zn, Sn, In, Cd, Mg and Ga has been attempted. Seo et al. [293c] synthesized single
8.4 Nanowires 259