nected by amorphous SiO 2 , like a chain of pearls. The SiNC consists of knots and
necks with equal distances between them. The average diameters of the knots and
necks are 15 and 4 nm, respectively and the thickness of the SiO 2 sheath surrounding the Si spheres is about 2 nm. The product is uniform with no isolated
particles and consisting of 95% SiNC and 5% SiNWs.
To summarize, both the growth process and the morphology of SiNWs can be
controlled by the growth temperature. The temperature required for a metal catalyst growth (using iron as the catalyst, for gold the temperature would be much
lower) is higher than that required for the OAG. Post-growth annealing results in
spheroidization and structural changes which occur faster at higher temperatures.
The diameter control by temperature is readily possible for the metal catalyst VLS
method but appears to be difficult for the OAG.
10.3.2
Diameter Control of Nanowires [29]
The motivation for the study of nanomaterials stems from the expected size effects; SiNWs are no exception. This makes the issue of diameter control very important. In a previous section we have shown that the growth temperature affects
the diameter of the SiNWs grown by the metal catalyst VLS method but not the
diameter of SiNWs grown by the OAG method. Here we show that the SiNW diameter can be affected by the carrier gas used in the growth process.
The SiNWs were synthesized by laser ablation of a target made by compaction of
a mixed powder of 90% Si and 10% SiO 2 . Different carrier gases were used including He, N 2 , and a mixture of Ar with 5% H 2 . During growth the carrier gas
pressure was about 300 Torr and the flow 50 sccm. The substrate growth temperature was @930
C. No deposition was observed at places where the temperature
was higher than @950
C, while some nanoclusters and amorphous mixtures of Si
and oxygen were deposited at lower temperature regions (for N 2 ). The SiNWs synthesized in He and in Ar (5% H 2 ) consisted almost entirely of nanowires. Some
spherical particles, with diameters ranging from @9 nm to several hundred nm,
composed of a mixture of crystalline Si and amorphous Si oxide, were found to
coexist with the nanowires grown in a N 2 atmosphere, and the quantity of the
spherical particles was a little less than that of the SiNWs. Most of the SiNWs were
smoothly curved with some short straight sections, a few possessed bends and
kinks. The SiNWs synthesized in a He atmosphere possessed many more bends
and curves.
The diameter distributions of the SiNWs were measured from the TEM micrographs as given in Figure 10.12. The SiNWs (up to several mm long) had a distribution of diameters (Si core plus SiO 2 sheath) peaked at 13, 9.5 and 6 nm for carrier gas mixtures of He, (5% H 2 in Ar) and N 2 respectively. The smallest wires
were mixed with spherical particles with diameters ranging from @9 nm to several
hundreds of nm. High-resolution TEM images of several SiNWs produced in He,
Ar (5% H 2 ), and N 2 atmospheres shows that every nanowire consists of a crystalline Si core and an amorphous SiO 2 sheath. The crystalline Si core has many lat10 Oxide-Assisted Growth of Silicon and Related Nanowires
326
necks with equal distances between them. The average diameters of the knots and
necks are 15 and 4 nm, respectively and the thickness of the SiO 2 sheath surrounding the Si spheres is about 2 nm. The product is uniform with no isolated
particles and consisting of 95% SiNC and 5% SiNWs.
To summarize, both the growth process and the morphology of SiNWs can be
controlled by the growth temperature. The temperature required for a metal catalyst growth (using iron as the catalyst, for gold the temperature would be much
lower) is higher than that required for the OAG. Post-growth annealing results in
spheroidization and structural changes which occur faster at higher temperatures.
The diameter control by temperature is readily possible for the metal catalyst VLS
method but appears to be difficult for the OAG.
10.3.2
Diameter Control of Nanowires [29]
The motivation for the study of nanomaterials stems from the expected size effects; SiNWs are no exception. This makes the issue of diameter control very important. In a previous section we have shown that the growth temperature affects
the diameter of the SiNWs grown by the metal catalyst VLS method but not the
diameter of SiNWs grown by the OAG method. Here we show that the SiNW diameter can be affected by the carrier gas used in the growth process.
The SiNWs were synthesized by laser ablation of a target made by compaction of
a mixed powder of 90% Si and 10% SiO 2 . Different carrier gases were used including He, N 2 , and a mixture of Ar with 5% H 2 . During growth the carrier gas
pressure was about 300 Torr and the flow 50 sccm. The substrate growth temperature was @930
C. No deposition was observed at places where the temperature
was higher than @950
C, while some nanoclusters and amorphous mixtures of Si
and oxygen were deposited at lower temperature regions (for N 2 ). The SiNWs synthesized in He and in Ar (5% H 2 ) consisted almost entirely of nanowires. Some
spherical particles, with diameters ranging from @9 nm to several hundred nm,
composed of a mixture of crystalline Si and amorphous Si oxide, were found to
coexist with the nanowires grown in a N 2 atmosphere, and the quantity of the
spherical particles was a little less than that of the SiNWs. Most of the SiNWs were
smoothly curved with some short straight sections, a few possessed bends and
kinks. The SiNWs synthesized in a He atmosphere possessed many more bends
and curves.
The diameter distributions of the SiNWs were measured from the TEM micrographs as given in Figure 10.12. The SiNWs (up to several mm long) had a distribution of diameters (Si core plus SiO 2 sheath) peaked at 13, 9.5 and 6 nm for carrier gas mixtures of He, (5% H 2 in Ar) and N 2 respectively. The smallest wires
were mixed with spherical particles with diameters ranging from @9 nm to several
hundreds of nm. High-resolution TEM images of several SiNWs produced in He,
Ar (5% H 2 ), and N 2 atmospheres shows that every nanowire consists of a crystalline Si core and an amorphous SiO 2 sheath. The crystalline Si core has many lat10 Oxide-Assisted Growth of Silicon and Related Nanowires
326