Functionalized Nano-porous Silicon Surfaces for Energy …
379
increased energy density compare to uncoated samples. This chapter detailed about
the synthesis and characterization of PS and how the functionalization of PS can
enhance the overall properties that are required to store energy efficiently.
2 Synthesis of Porous Silicon with Different Pore Diameter
A material that has a small hole in it through which water, liquid, vapors, and gas
can be passed and provide large surface to volume ratio in the order of 500 m
2 /cm
3
called porous materials. Porous silicon (PS) which has accidentally discovered while
Uhlir Jr. and Ingeborg Uhlir in 1956 at the Bell labs in U.S. were developing a
technique for polishing and shaping the surface of silicon and germanium (Ulhir
1956; Zhang 2004). During that process under several conditions they observed thick
black, brown, and red film on the surface of the materials. But at that time, the finding
was described as porous silicon and only published in Bell lab’s technical notes. The
scientific community was not interested in porous silicon despite the discovery of
porous silicon in 1956 till Leigh Canham published an experimental result in 1990
on photoluminescence appearance in PS (Canham 1990). Since the discovery of
photoluminescence in PS, it has triggered large-scale investigation for its use in
technological applications. Several methods have been used to prepare the PS that
include electrochemical etching, stain etching, and hydrothermal etching (Abramof
et al. 2006; Liua and Wang 2005; Smith and Collins 1992) which allows to vary
physical and chemical properties of PS as per the technical and scientific demands.
The physical properties, e.g., specific surface area, porosity, pore diameter, and pore
orientation of PS can be varied by changing etching parameters such as current
density, type of doping, level of doping, or crystallographic orientation of the silicon
wafers used (Herino et al. 1987). The well-defined PS morphology is ranged from
microporous with pore size <10 nm and branchy mesoporous silicon with pore size
10–50 nm to the classical macroporous silicon of pore size 50–20 µm.
2.1 Pore Formation in Silicon
Several methods can be used to synthesis porous silicon as discussed in the introduction part. However, in this chapter the electrochemical etching of Si in aqueous HF
solution was discussed. The holes are prerequisite to create pores in Si. The pores
in Silicon form during anodic polarization in aqueous HF solution, depending on
applied potential between electrode, doping level in Si wafer and HF concentration.
Several mechanism for pore formation have been discussed in the literature (Smith
and Collins 1992; Zhang 2004). The pore formation in Si is the results of inhomogeneous dissolution of the Si surface in HF-based electrolyte due to competing
reactions lead to silicon oxide formation followed by dissolution of the oxide by
HF. The overall process during pore formation in Si can be expressed by following
379
increased energy density compare to uncoated samples. This chapter detailed about
the synthesis and characterization of PS and how the functionalization of PS can
enhance the overall properties that are required to store energy efficiently.
2 Synthesis of Porous Silicon with Different Pore Diameter
A material that has a small hole in it through which water, liquid, vapors, and gas
can be passed and provide large surface to volume ratio in the order of 500 m
2 /cm
3
called porous materials. Porous silicon (PS) which has accidentally discovered while
Uhlir Jr. and Ingeborg Uhlir in 1956 at the Bell labs in U.S. were developing a
technique for polishing and shaping the surface of silicon and germanium (Ulhir
1956; Zhang 2004). During that process under several conditions they observed thick
black, brown, and red film on the surface of the materials. But at that time, the finding
was described as porous silicon and only published in Bell lab’s technical notes. The
scientific community was not interested in porous silicon despite the discovery of
porous silicon in 1956 till Leigh Canham published an experimental result in 1990
on photoluminescence appearance in PS (Canham 1990). Since the discovery of
photoluminescence in PS, it has triggered large-scale investigation for its use in
technological applications. Several methods have been used to prepare the PS that
include electrochemical etching, stain etching, and hydrothermal etching (Abramof
et al. 2006; Liua and Wang 2005; Smith and Collins 1992) which allows to vary
physical and chemical properties of PS as per the technical and scientific demands.
The physical properties, e.g., specific surface area, porosity, pore diameter, and pore
orientation of PS can be varied by changing etching parameters such as current
density, type of doping, level of doping, or crystallographic orientation of the silicon
wafers used (Herino et al. 1987). The well-defined PS morphology is ranged from
microporous with pore size <10 nm and branchy mesoporous silicon with pore size
10–50 nm to the classical macroporous silicon of pore size 50–20 µm.
2.1 Pore Formation in Silicon
Several methods can be used to synthesis porous silicon as discussed in the introduction part. However, in this chapter the electrochemical etching of Si in aqueous HF
solution was discussed. The holes are prerequisite to create pores in Si. The pores
in Silicon form during anodic polarization in aqueous HF solution, depending on
applied potential between electrode, doping level in Si wafer and HF concentration.
Several mechanism for pore formation have been discussed in the literature (Smith
and Collins 1992; Zhang 2004). The pore formation in Si is the results of inhomogeneous dissolution of the Si surface in HF-based electrolyte due to competing
reactions lead to silicon oxide formation followed by dissolution of the oxide by
HF. The overall process during pore formation in Si can be expressed by following