of SiNWs integrated with nanostructures of other materials. We treated this issue
by using multistep processes. One example was to grow nanocables. Metallization
of the SiNW surfaces is another example of a hybrid configuration. We addressed it
by ion implantation of SiNWs with metal ions. An additional route for incorporation of SiNWs with other materials/structures is the transformation of one nanowire to another by chemical reactions. We demonstrate this approach by our work
on SiC nanowires. Our works on compound structures are detailed in Section 10.4.
The OAG is a generic method capable of producing different nanostructures
from a variety of materials. We have extended the oxide-assisted approach to successfully synthesize a host of semiconducting materials, including Ge [35], GaN
[36, 37], GaAs [38, 39], SiC [40], GaP [41], and ZnO (whiskers) [42]. Section 10.5
reviews these works.
The motivation for studying nanoscience and nanotechnology stems from the
exciting properties predicted for nanomaterials due to size effects. Section 10.6 details our work on the chemical properties of SiNWs. We first discuss the stability of
the hydrogen-terminated SiNWs produced by HF dipping which removes the SiO 2
sheath surrounding the crystalline Si core. This issue is central to the silicon wafer
technology. A second study was dedicated to the reduction properties of the SiNWs
in liquid solutions containing metal ions. Surface interactions of SiNWs with gases
enable their use in chemical sensing. This was demonstrated for ammonia and
water vapors in air or nitrogen. The reactivity of SiNWs in a liquid solution can be
exploited for their use as templates to grow carbon nanostructures, which is the
final topic of this section.
The optical and electrical properties of the nanowires (Section 10.7) have been
characterized systematically by Raman scattering, photoluminescence and field
emission [43, 44]. Understanding the atomic structure and electronic properties of
SiNWs, including the dopant-induced conductivity, is an essential step towards the
application of the nanowires. Although the structures and electronic properties of
boron-doped silicon wafers have been investigated extensively, the corresponding
study for SiNWs is relatively lacking, due to the insulating nature of the oxide sheath
on most semiconductor nanowires and the difficulty in dispersing them. We have
succeeded in removing the oxide layer of the SiNWs, obtaining atomically resolved
STM images of H-terminated surfaces of SiNWs with diameters ranging from 1 to
7 nm. This enabled reliable scanning tunneling spectroscopy (STS) measurements
of these wires, from which the electronic density of states and energy band gaps
could be derived. The energy band gaps indeed increase from 1.1 eV for a 7 nm
diameter SiNW to 3.5 eV for a 1.3 nm diameter SiNW, in accord with theoretical
predictions, demonstrating the quantum size effect in SiNWs. In Section 10.7 we
review our scanning tunneling microscopy (STM)/STS study on boron-doped and
undoped SiNWs [45] and on the quantum size effect in SiNWs as well as our
characterization work on other electrical and optical properties of SiNWs.
Modeling of SiNW structures, nucleation and growth processes and properties
was done in parallel with the experimental work. The modeling work is most valuable in providing additional insight into the nature of the OAG and in explaining
our experimental results. Our modeling efforts are described in Section 10.8.
10 Oxide-Assisted Growth of Silicon and Related Nanowires
310
by using multistep processes. One example was to grow nanocables. Metallization
of the SiNW surfaces is another example of a hybrid configuration. We addressed it
by ion implantation of SiNWs with metal ions. An additional route for incorporation of SiNWs with other materials/structures is the transformation of one nanowire to another by chemical reactions. We demonstrate this approach by our work
on SiC nanowires. Our works on compound structures are detailed in Section 10.4.
The OAG is a generic method capable of producing different nanostructures
from a variety of materials. We have extended the oxide-assisted approach to successfully synthesize a host of semiconducting materials, including Ge [35], GaN
[36, 37], GaAs [38, 39], SiC [40], GaP [41], and ZnO (whiskers) [42]. Section 10.5
reviews these works.
The motivation for studying nanoscience and nanotechnology stems from the
exciting properties predicted for nanomaterials due to size effects. Section 10.6 details our work on the chemical properties of SiNWs. We first discuss the stability of
the hydrogen-terminated SiNWs produced by HF dipping which removes the SiO 2
sheath surrounding the crystalline Si core. This issue is central to the silicon wafer
technology. A second study was dedicated to the reduction properties of the SiNWs
in liquid solutions containing metal ions. Surface interactions of SiNWs with gases
enable their use in chemical sensing. This was demonstrated for ammonia and
water vapors in air or nitrogen. The reactivity of SiNWs in a liquid solution can be
exploited for their use as templates to grow carbon nanostructures, which is the
final topic of this section.
The optical and electrical properties of the nanowires (Section 10.7) have been
characterized systematically by Raman scattering, photoluminescence and field
emission [43, 44]. Understanding the atomic structure and electronic properties of
SiNWs, including the dopant-induced conductivity, is an essential step towards the
application of the nanowires. Although the structures and electronic properties of
boron-doped silicon wafers have been investigated extensively, the corresponding
study for SiNWs is relatively lacking, due to the insulating nature of the oxide sheath
on most semiconductor nanowires and the difficulty in dispersing them. We have
succeeded in removing the oxide layer of the SiNWs, obtaining atomically resolved
STM images of H-terminated surfaces of SiNWs with diameters ranging from 1 to
7 nm. This enabled reliable scanning tunneling spectroscopy (STS) measurements
of these wires, from which the electronic density of states and energy band gaps
could be derived. The energy band gaps indeed increase from 1.1 eV for a 7 nm
diameter SiNW to 3.5 eV for a 1.3 nm diameter SiNW, in accord with theoretical
predictions, demonstrating the quantum size effect in SiNWs. In Section 10.7 we
review our scanning tunneling microscopy (STM)/STS study on boron-doped and
undoped SiNWs [45] and on the quantum size effect in SiNWs as well as our
characterization work on other electrical and optical properties of SiNWs.
Modeling of SiNW structures, nucleation and growth processes and properties
was done in parallel with the experimental work. The modeling work is most valuable in providing additional insight into the nature of the OAG and in explaining
our experimental results. Our modeling efforts are described in Section 10.8.
10 Oxide-Assisted Growth of Silicon and Related Nanowires
310