the direction of the flow. At the same time, overcrowding of the nanowires will
limit the possibility of nanowire propagation in other directions [63]. In addition, it
was found that a smooth plane substrate is very helpful for oriented growth.
10.3.4
Si Nanoribbons
SiNWs are a 1D nanostructure. A distinct feature of the OAG process revealed in
our studies is the variety of different configurations it can form. Some were discussed in the previous sections. Now we present an exciting and unexpected 2D
configuration, nanoribbons, discovered in the course of studying OAG. A 2D configuration is not expected to exhibit the same magnitude of size effects as a 1D
structure, but it may be advantageous in processing and in obtaining signals with
more measurable intensities in single object characterization.
The single-crystal silicon nanoribbons were grown by simple thermal evaporation of silicon monoxide (SiO) heated to 1150
C. No templates or catalysts were
used. The nanoribbons have a thickness of only about 10–20 nm (average 15 nm),
widths of several hundreds of nanometers (50–450 nm), and lengths of many
micrometers [34]. Most of the ribbons have rippling edges (Figure 10.14(a)), and
a small portion of the ribbons has smooth edges (Figure 10.14(b)). Due to their
small thickness the ribbons seem transparent in TEM imaging (Figure 10.14(b))
using 200 keV electrons. Nanoribbons of different width and morphology have a
similar thickness which is constant throughout each individual ribbon. In the typical nanoribbon shown in (Figure 10.14(b)) the thickness is about 14 nm and the
width is about 370 nm. The thickness to width ratio of the SiNWs varies from 4 to
22. The rippling and curling features at the edge of most ribbons also confirm that
the nanoribbons are quasi-2D structures distinctly different in shape from the 1D
SiNWs.
HRTEM imaging of a single nanoribbon revealed that the ribbon has a crystal
core nipped by amorphous layers with atomically sharp interfaces. The in-plane
layers of the nanoribbon were determined to be silicon (110) facet with a perfect
atomic, defect-free, single-crystal structure grown along the h111i direction. This
direction is different from the predominant h112i and h110i direction of SiNWs
synthesized by the OAG method. It is the same as the most abundant growth direction of Si nanowires synthesized by the metal catalyzed VLS method. The wide
part of the ribbon is along the h112i direction. The amorphous edges of the ribbon
consist of silicon oxide (SiO x ), as determined by EDS and EELS attached to the
TEM. The width of the amorphous edge is about 10 nm. Analysis of a number of
nanoribbons with different widths shows that the width of the oxide edges varies
from 3 to 25 nm, similar to the thickness of the amorphous silicon oxide shell of
the nanowires synthesized by OAG. The thickness of the oxide layer covering the
flat surfaces of the ribbons is much less than the width of the oxide edges; that is,
the thickness of the oxide layers is anisotropic. This result may be understood in
terms of the OAG process, in which the silicon oxide shell was formed by the re10 Oxide-Assisted Growth of Silicon and Related Nanowires
330
limit the possibility of nanowire propagation in other directions [63]. In addition, it
was found that a smooth plane substrate is very helpful for oriented growth.
10.3.4
Si Nanoribbons
SiNWs are a 1D nanostructure. A distinct feature of the OAG process revealed in
our studies is the variety of different configurations it can form. Some were discussed in the previous sections. Now we present an exciting and unexpected 2D
configuration, nanoribbons, discovered in the course of studying OAG. A 2D configuration is not expected to exhibit the same magnitude of size effects as a 1D
structure, but it may be advantageous in processing and in obtaining signals with
more measurable intensities in single object characterization.
The single-crystal silicon nanoribbons were grown by simple thermal evaporation of silicon monoxide (SiO) heated to 1150
C. No templates or catalysts were
used. The nanoribbons have a thickness of only about 10–20 nm (average 15 nm),
widths of several hundreds of nanometers (50–450 nm), and lengths of many
micrometers [34]. Most of the ribbons have rippling edges (Figure 10.14(a)), and
a small portion of the ribbons has smooth edges (Figure 10.14(b)). Due to their
small thickness the ribbons seem transparent in TEM imaging (Figure 10.14(b))
using 200 keV electrons. Nanoribbons of different width and morphology have a
similar thickness which is constant throughout each individual ribbon. In the typical nanoribbon shown in (Figure 10.14(b)) the thickness is about 14 nm and the
width is about 370 nm. The thickness to width ratio of the SiNWs varies from 4 to
22. The rippling and curling features at the edge of most ribbons also confirm that
the nanoribbons are quasi-2D structures distinctly different in shape from the 1D
SiNWs.
HRTEM imaging of a single nanoribbon revealed that the ribbon has a crystal
core nipped by amorphous layers with atomically sharp interfaces. The in-plane
layers of the nanoribbon were determined to be silicon (110) facet with a perfect
atomic, defect-free, single-crystal structure grown along the h111i direction. This
direction is different from the predominant h112i and h110i direction of SiNWs
synthesized by the OAG method. It is the same as the most abundant growth direction of Si nanowires synthesized by the metal catalyzed VLS method. The wide
part of the ribbon is along the h112i direction. The amorphous edges of the ribbon
consist of silicon oxide (SiO x ), as determined by EDS and EELS attached to the
TEM. The width of the amorphous edge is about 10 nm. Analysis of a number of
nanoribbons with different widths shows that the width of the oxide edges varies
from 3 to 25 nm, similar to the thickness of the amorphous silicon oxide shell of
the nanowires synthesized by OAG. The thickness of the oxide layer covering the
flat surfaces of the ribbons is much less than the width of the oxide edges; that is,
the thickness of the oxide layers is anisotropic. This result may be understood in
terms of the OAG process, in which the silicon oxide shell was formed by the re10 Oxide-Assisted Growth of Silicon and Related Nanowires
330