10.1
Introduction
The discovery of carbon nanotubes (CNTs) by Iijima [1] in 1991, initiated an intensive study of one-dimensional (1D) nanomaterials including tubes, wires, cables
and ribbons in general, and of the fundamental properties and potential applications of carbon nanotubes [2–6] in particular. The interest in CNTs stems from
their small diameter (smallest, 4 A ˚ ) enabling unprecedented and exciting opportunities for the study of size- and dimensionality-dependent chemical and physical
phenomena [7–13]. It is believed that these size effects open the door for many
potential applications in nanotechnology, such as high-strength materials [14],
electronic components [15], sensors [16, 17], field emitters [18, 19] and hydrogen
storage materials [20].
The limitations of carbon nanotubes, such as the selective growth of metallic or
semiconducting tubes and the difficulty of achieving controlled doping, motivated
the alternative study of conventional one-dimensional (1D) semiconducting materials. These nanowires do not seem to face these problems, which make them
much more adaptable for volume fabrication of nanodevices. Silicon nanowires
(SiNWs) are of special interest since silicon is the most widely used and studied
semiconducting material. In 1998, Lieber et al. [21] and the CityU team [22] independently reported the bulk-quantity synthesis of SiNWs. At CityU, we proposed
an oxide-assisted growth (OAG) model to explain the growth of SiNWs [23–28],
while Lieber et al. advocated the laser-assisted metal catalyst vapor–liquid–solid
(VLS) growth [21]. In contrast to the conventional metal catalyst VLS growth [21],
the OAG does not require a catalytic metal nanoparticle tip, thus providing a much
‘‘cleaner’’ method for the 1D material fabrication.
With this OAG approach, highly pure, ultra-long and uniform-sized SiNWs in
bulk-quantity could be synthesized by either laser ablation or thermal evaporation
of silicon powders mixed with silicon oxide or silicon monoxide only [23–28]. Section 10.2 discusses the physical chemistry aspects of the OAG. Transmission electron microscopic data and theoretical calculations are used to describe the nucleation and the growth of SiNWs via the OAG process.
In further efforts to achieve controlled growth, SiNWs of varying diameter, phase
purity, morphology, defect density and doping have been obtained. This was achieved by varying the deposition parameters including growth temperature, carrier
gas composition, carrier gas flow, and target composition. Different SiNW diameters were obtained by varying the carrier gas [29]. In contrast to the work of Lieber
et al. [30] and Yang et al. [31] (who used metal nanoparticles of uniform size to
control the diameter of SiNWs via the laser-assisted catalytic VLS growth) we found
that the SiNW diameters had a wide distribution. The OAG method enabled not
only fabrication of Si 1D nanostructures of different morphologies [32, 33], but
also 2D nanostructures, i.e. silicon nanoribbon [34]. Section 10.3 reviews our work
on the control of SiNW structure and size.
Future applications of SiNWs require the production of hybrid structures made
10.1 Introduction 309
Introduction
The discovery of carbon nanotubes (CNTs) by Iijima [1] in 1991, initiated an intensive study of one-dimensional (1D) nanomaterials including tubes, wires, cables
and ribbons in general, and of the fundamental properties and potential applications of carbon nanotubes [2–6] in particular. The interest in CNTs stems from
their small diameter (smallest, 4 A ˚ ) enabling unprecedented and exciting opportunities for the study of size- and dimensionality-dependent chemical and physical
phenomena [7–13]. It is believed that these size effects open the door for many
potential applications in nanotechnology, such as high-strength materials [14],
electronic components [15], sensors [16, 17], field emitters [18, 19] and hydrogen
storage materials [20].
The limitations of carbon nanotubes, such as the selective growth of metallic or
semiconducting tubes and the difficulty of achieving controlled doping, motivated
the alternative study of conventional one-dimensional (1D) semiconducting materials. These nanowires do not seem to face these problems, which make them
much more adaptable for volume fabrication of nanodevices. Silicon nanowires
(SiNWs) are of special interest since silicon is the most widely used and studied
semiconducting material. In 1998, Lieber et al. [21] and the CityU team [22] independently reported the bulk-quantity synthesis of SiNWs. At CityU, we proposed
an oxide-assisted growth (OAG) model to explain the growth of SiNWs [23–28],
while Lieber et al. advocated the laser-assisted metal catalyst vapor–liquid–solid
(VLS) growth [21]. In contrast to the conventional metal catalyst VLS growth [21],
the OAG does not require a catalytic metal nanoparticle tip, thus providing a much
‘‘cleaner’’ method for the 1D material fabrication.
With this OAG approach, highly pure, ultra-long and uniform-sized SiNWs in
bulk-quantity could be synthesized by either laser ablation or thermal evaporation
of silicon powders mixed with silicon oxide or silicon monoxide only [23–28]. Section 10.2 discusses the physical chemistry aspects of the OAG. Transmission electron microscopic data and theoretical calculations are used to describe the nucleation and the growth of SiNWs via the OAG process.
In further efforts to achieve controlled growth, SiNWs of varying diameter, phase
purity, morphology, defect density and doping have been obtained. This was achieved by varying the deposition parameters including growth temperature, carrier
gas composition, carrier gas flow, and target composition. Different SiNW diameters were obtained by varying the carrier gas [29]. In contrast to the work of Lieber
et al. [30] and Yang et al. [31] (who used metal nanoparticles of uniform size to
control the diameter of SiNWs via the laser-assisted catalytic VLS growth) we found
that the SiNW diameters had a wide distribution. The OAG method enabled not
only fabrication of Si 1D nanostructures of different morphologies [32, 33], but
also 2D nanostructures, i.e. silicon nanoribbon [34]. Section 10.3 reviews our work
on the control of SiNW structure and size.
Future applications of SiNWs require the production of hybrid structures made
10.1 Introduction 309