Nanowires of TiO 2 have been fabricated in nanoporous alumina membranes by
sol–gel and other chemical methods. TiO 2 nanotubes in AM have been prepared
by the sol–gel method [45]. The sol–gel method has been used to fabricate highly
ordered single crystalline anatase phase TiO 2 nanowire arrays in anodic alumina
membranes [46]. The wires have an average uniform diameter of around 60 nm
and length 25 mm. These wires have broad band visible PL at about 425, 465, and
525 nm that is attributed to self-trapped excitons, and oxygen defect centers. The
TiO 2 nanowires were fabricated using TiO 2 sol made from alkoxide solution prepared by mixing tetrabutyl titanate, ethanol and CH 3 COOH. The templates were
immersed in the sol. The template was then allowed to dry at somewhat elevated
temperature for the sol–gel transition to occur. The final phase formation occurs
after heating to above 600
C.
Nanowires of 60–200 nm anatase TiO 2 were fabricated using liquid phase deposition in a track etched membrane [47]. The acidic precursor solution used was
(NH 4 ) 2 TiF 6 (0.2 M) and H 3 BO 3 (0.1 M) and was adjusted to a pH of around 1.4
with 1:1 ammonia solution.
Arrays of concentric composite nanowires of ZrO 2 with Co cores were made in
an alumina template by the sol–gel method from ZrO 2 sol [48]. The sol was made
from ZrOCl 2 Á8H 2 O, ethanol, HNO 3 and acetylacetone. The alumina membrane
was dipped in the sol for 0.5–1 h and after drying was annealed at around 500
C.
The ZrO 2 template was then filled with Co metal by electrodeposition. This composite nanostructure had an enhanced coercivity compared to that of bulk Co.
Transition metal oxide nanowires have been fabricated using the sol–gel method.
LiNiO 2 nanowires in alumina templates have been fabricated [49]. Highly ordered
LiNi 0:5 Mn 0:5 O 2 nanowires were fabricated from sol made from metal acetate cationic sources, citric acid and ethylene glycol [50]. The template was dipped into the
sol, dried in vacuum and then annealed for 10 h at 600
C. Preparation of nanowires of complex materials using alumina templates and a chemical route such as
the sol–gel route is a profitable exercise. It gives the freedom of fabrication of a
number of functional metallic oxides without changing the basic procedure significantly.
21.3.3
CVD Method
Templates have been used for the growth of semiconductor nanotubes, including
ordered arrays of carbon nanotubes, by chemical and physical vapor deposition.
This is a very promising route because it uses the existing knowledge base of
physical and chemical vapor deposition in general and couples that to a rather
ready made template that leads to an array formation. A schematic of nanowire
formation using the vapor route is shown in Figure 21.12.
Early reports of the formation of carbon nanotube (CNT) arrays in templates are
given in [51]. Aligned CNT with a diameter of about few nm with sufficient
graphitic character were made by chemical vapor decomposition (CVD) using propylene gas. The CNTs were made with Fe and Co catalysts or without catalysts. The
21 Nanolithography and Nanomanipulation
704
sol–gel and other chemical methods. TiO 2 nanotubes in AM have been prepared
by the sol–gel method [45]. The sol–gel method has been used to fabricate highly
ordered single crystalline anatase phase TiO 2 nanowire arrays in anodic alumina
membranes [46]. The wires have an average uniform diameter of around 60 nm
and length 25 mm. These wires have broad band visible PL at about 425, 465, and
525 nm that is attributed to self-trapped excitons, and oxygen defect centers. The
TiO 2 nanowires were fabricated using TiO 2 sol made from alkoxide solution prepared by mixing tetrabutyl titanate, ethanol and CH 3 COOH. The templates were
immersed in the sol. The template was then allowed to dry at somewhat elevated
temperature for the sol–gel transition to occur. The final phase formation occurs
after heating to above 600
C.
Nanowires of 60–200 nm anatase TiO 2 were fabricated using liquid phase deposition in a track etched membrane [47]. The acidic precursor solution used was
(NH 4 ) 2 TiF 6 (0.2 M) and H 3 BO 3 (0.1 M) and was adjusted to a pH of around 1.4
with 1:1 ammonia solution.
Arrays of concentric composite nanowires of ZrO 2 with Co cores were made in
an alumina template by the sol–gel method from ZrO 2 sol [48]. The sol was made
from ZrOCl 2 Á8H 2 O, ethanol, HNO 3 and acetylacetone. The alumina membrane
was dipped in the sol for 0.5–1 h and after drying was annealed at around 500
C.
The ZrO 2 template was then filled with Co metal by electrodeposition. This composite nanostructure had an enhanced coercivity compared to that of bulk Co.
Transition metal oxide nanowires have been fabricated using the sol–gel method.
LiNiO 2 nanowires in alumina templates have been fabricated [49]. Highly ordered
LiNi 0:5 Mn 0:5 O 2 nanowires were fabricated from sol made from metal acetate cationic sources, citric acid and ethylene glycol [50]. The template was dipped into the
sol, dried in vacuum and then annealed for 10 h at 600
C. Preparation of nanowires of complex materials using alumina templates and a chemical route such as
the sol–gel route is a profitable exercise. It gives the freedom of fabrication of a
number of functional metallic oxides without changing the basic procedure significantly.
21.3.3
CVD Method
Templates have been used for the growth of semiconductor nanotubes, including
ordered arrays of carbon nanotubes, by chemical and physical vapor deposition.
This is a very promising route because it uses the existing knowledge base of
physical and chemical vapor deposition in general and couples that to a rather
ready made template that leads to an array formation. A schematic of nanowire
formation using the vapor route is shown in Figure 21.12.
Early reports of the formation of carbon nanotube (CNT) arrays in templates are
given in [51]. Aligned CNT with a diameter of about few nm with sufficient
graphitic character were made by chemical vapor decomposition (CVD) using propylene gas. The CNTs were made with Fe and Co catalysts or without catalysts. The
21 Nanolithography and Nanomanipulation
704