shifted from the bulk. This is explained as arising due to confinement of the phonon modes.
21.3.2
Sol–Gel Method
The sol–gel method is a low temperature synthesis route for complex oxides [42].
It can be used to make complex functional oxide nanowires inside the pores of
templates. In addition to the sol–gel method precursor-based solution deposition
routes can also be used for nanostructure formation [43]. In both cases a postdeposition high temperature anneal (>500–600
C) is needed to form the required
stoichiometric phase. Due to the requirement of a high temperature anneal, alumina templates are used as the polycarbonate membranes decompose at a much
lower temperature. For chemical solution deposition the membrane is dipped directly into the precursor solution. For sol–gel growth generally the required sol is
prepared and the template is put into the sol for a required period (e.g. 0.5–1 h).
After removing the membrane from the sol it is dried and then annealed at higher
temperature before the required phase is formed. A schematic of the sol–gel route
is shown in Figure 21.10.
An example of the formation of colossal magnetoresistive manganite nanowires
is the formation of La 0:7 Ca 0:3 MnO 3 (LCMO) in alumina templates [44]. These materials possess perovskite structure and the cell volumes are generally larger than
those of metals and semiconductors. Hence it is comparatively difficult to structure
them in the nanocrystalline form. Moreover, achieving the right stoichiometry of
these multicomponent oxides at the nano-level is also a challenging task. The
sol–gel technique is a powerful approach for preparing multicomponent inorganic materials as it is possible to precisely control the molecular microstructure,
stoichiometry, high purity, and all at relatively low processing temperatures.
The sol used in the fabrication of LCMO nanowires was prepared by dissolving a
stoichiometric ratio of lanthanum, calcium and manganese nitrates in an equal
amount of water and adding ethylene glycol. The mixture was then heated on a hot
plate till a sol of desired viscosity formed. Nanowires were fabricated by immersing
the AM in the sol for 30–60 min and then heating to higher temperatures after
wiping both surfaces clean. The membranes heated at different temperatures were
subjected to XRD analysis to check the phase formation. Heating the membranes
at 700
C was sufficient to get the desired phase. Scanning electron microscopy
was carried out on the nanowires embedded within the alumina membrane, both
on the top surface and along the cross-section. The membranes were dissolved by
immersing in 2 M NaOH for 30 min. The solution was then rinsed in triple distilled water several times before dispersing on the carbon grid to carry out transmission electron microscopy or on a metal substrate for SEM. An SEM of such a
wire fabricated in AM is shown in Figure 21.11. In this case a metal was attached
to the template before the sol–gel growth. The nanowires grow in an aligned
fashion and retain their structure even after the template is etched away.
21 Nanolithography and Nanomanipulation
702
21.3.2
Sol–Gel Method
The sol–gel method is a low temperature synthesis route for complex oxides [42].
It can be used to make complex functional oxide nanowires inside the pores of
templates. In addition to the sol–gel method precursor-based solution deposition
routes can also be used for nanostructure formation [43]. In both cases a postdeposition high temperature anneal (>500–600
C) is needed to form the required
stoichiometric phase. Due to the requirement of a high temperature anneal, alumina templates are used as the polycarbonate membranes decompose at a much
lower temperature. For chemical solution deposition the membrane is dipped directly into the precursor solution. For sol–gel growth generally the required sol is
prepared and the template is put into the sol for a required period (e.g. 0.5–1 h).
After removing the membrane from the sol it is dried and then annealed at higher
temperature before the required phase is formed. A schematic of the sol–gel route
is shown in Figure 21.10.
An example of the formation of colossal magnetoresistive manganite nanowires
is the formation of La 0:7 Ca 0:3 MnO 3 (LCMO) in alumina templates [44]. These materials possess perovskite structure and the cell volumes are generally larger than
those of metals and semiconductors. Hence it is comparatively difficult to structure
them in the nanocrystalline form. Moreover, achieving the right stoichiometry of
these multicomponent oxides at the nano-level is also a challenging task. The
sol–gel technique is a powerful approach for preparing multicomponent inorganic materials as it is possible to precisely control the molecular microstructure,
stoichiometry, high purity, and all at relatively low processing temperatures.
The sol used in the fabrication of LCMO nanowires was prepared by dissolving a
stoichiometric ratio of lanthanum, calcium and manganese nitrates in an equal
amount of water and adding ethylene glycol. The mixture was then heated on a hot
plate till a sol of desired viscosity formed. Nanowires were fabricated by immersing
the AM in the sol for 30–60 min and then heating to higher temperatures after
wiping both surfaces clean. The membranes heated at different temperatures were
subjected to XRD analysis to check the phase formation. Heating the membranes
at 700
C was sufficient to get the desired phase. Scanning electron microscopy
was carried out on the nanowires embedded within the alumina membrane, both
on the top surface and along the cross-section. The membranes were dissolved by
immersing in 2 M NaOH for 30 min. The solution was then rinsed in triple distilled water several times before dispersing on the carbon grid to carry out transmission electron microscopy or on a metal substrate for SEM. An SEM of such a
wire fabricated in AM is shown in Figure 21.11. In this case a metal was attached
to the template before the sol–gel growth. The nanowires grow in an aligned
fashion and retain their structure even after the template is etched away.
21 Nanolithography and Nanomanipulation
702