Versatile 1-D Nanostructures for Green Energy Conversion …
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deposition and so on (Tiwari et al. 2012). Chemical techniques can give scalable products and through proper selection of precursors and some optimizations one can adopt
completely green synthesis route. One-dimensional nanomaterials can normally be
synthesized either by top-down or bottom-up approach and both of them have their
own set of advantages and disadvantages. Both these approaches are discussed in
detail below.
2.1 Top-Down Approach
In this method, a bulk material is taken and then broken down into finer pieces by
chemical or mechanical energy known as physico-chemical etching of bulk crystals
to nanodimensions using masks. Thus, it is a technique which retains the bulk order
at highly reduced scales with structures that are also sometimes termed as “selfdetermined”. If the mask is used conventionally, then it is termed as photolithography
while the other process is called as e-beam lithography or imprint lithography if the
mask is not used directly (Ganguly et al. 2014). This process is not cheap and is not
suitable for large-scale production.
One of the chemical methods in the top-down approach is metal-assisted chemical
etching. Metal-assisted chemical etching is a technique which is mainly used for
fabricating silicon and germanium nanowires. The silicon and germanium is etched
by using hydrogen peroxide and highly hazardous hydrogen fluoride in the presence
of noble metals such as platinum, silver, gold and so on. The etching mechanism
is dependent on various factors which are the etchant, illumination, noble metal
used, doping type and the concentration of the substrate taken (Huang et al. 2011).
As this method includes hazardous enchants and expensive metals, the method is
inconvenient to use and less popular. The difference in top-down and bottom-up
approach can be easily understood from Fig. 3.
2.2 Bottom-up Approach
In this method the material is synthesized via chemical reactions in such a way that
the atoms or molecules aggregate into clusters which allow the precursor particles to
grow in size, and it known as nucleation. The nucleation process generates clusters
like “seeds” which further generate the long range order needed for crystallization.
Thus, in this technique, self-assembly of crystalline order takes from disordered
vapor (which may be atoms, molecules or ions), solid or liquid phase through least
machine or human interference (Ganguly et al. 2014). Typical examples are quantum
dot formation during epitaxial growth and formation of nanoparticles from colloidal
dispersion. The process is less expensive and large amount of nanomaterials can be
synthesized easily as compared to top-down approach. It can be classified mainly
into two types.
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deposition and so on (Tiwari et al. 2012). Chemical techniques can give scalable products and through proper selection of precursors and some optimizations one can adopt
completely green synthesis route. One-dimensional nanomaterials can normally be
synthesized either by top-down or bottom-up approach and both of them have their
own set of advantages and disadvantages. Both these approaches are discussed in
detail below.
2.1 Top-Down Approach
In this method, a bulk material is taken and then broken down into finer pieces by
chemical or mechanical energy known as physico-chemical etching of bulk crystals
to nanodimensions using masks. Thus, it is a technique which retains the bulk order
at highly reduced scales with structures that are also sometimes termed as “selfdetermined”. If the mask is used conventionally, then it is termed as photolithography
while the other process is called as e-beam lithography or imprint lithography if the
mask is not used directly (Ganguly et al. 2014). This process is not cheap and is not
suitable for large-scale production.
One of the chemical methods in the top-down approach is metal-assisted chemical
etching. Metal-assisted chemical etching is a technique which is mainly used for
fabricating silicon and germanium nanowires. The silicon and germanium is etched
by using hydrogen peroxide and highly hazardous hydrogen fluoride in the presence
of noble metals such as platinum, silver, gold and so on. The etching mechanism
is dependent on various factors which are the etchant, illumination, noble metal
used, doping type and the concentration of the substrate taken (Huang et al. 2011).
As this method includes hazardous enchants and expensive metals, the method is
inconvenient to use and less popular. The difference in top-down and bottom-up
approach can be easily understood from Fig. 3.
2.2 Bottom-up Approach
In this method the material is synthesized via chemical reactions in such a way that
the atoms or molecules aggregate into clusters which allow the precursor particles to
grow in size, and it known as nucleation. The nucleation process generates clusters
like “seeds” which further generate the long range order needed for crystallization.
Thus, in this technique, self-assembly of crystalline order takes from disordered
vapor (which may be atoms, molecules or ions), solid or liquid phase through least
machine or human interference (Ganguly et al. 2014). Typical examples are quantum
dot formation during epitaxial growth and formation of nanoparticles from colloidal
dispersion. The process is less expensive and large amount of nanomaterials can be
synthesized easily as compared to top-down approach. It can be classified mainly
into two types.