11.2 Nanochannel Templates Obtained with Top-Down Synthesis Methods
383
nanocavity by the electrodeposition process without an end-bump formation has also
been reported [200].
Switch of the nanowire growth to nanotube formation. The morphology of the
nanoobject being deposited may change abruptly in situ during the deposition into
a specific porous membrane. This morphological change, an opposite phenomenon
of the nanotube closure with time, is visualized in Fig. 11.8c. The sudden turnover
to nanotube formation is a result of the combined impact of the diffusion-limited
nature of the growth and the high electrical field. In such cases, it might seem that
the deposit appears at the top of the porous layer without an obvious pore filling due
to a very fast penetration of the tiny nanotube along the pore wall [201].
It is easy to understand that the change in the deposit morphology can be observed
as a function of the current density. Even if hydrogen evolution is not considered,
the rod-like growth mode at a small current density can switch to a tube-like growth
mode if the current density is high enough and the deposition is largely impacted
by both the diffusion limitation and the electrical field [202]. However, the current
density effect is not uniform since one can find examples also for the case when the
low and high current density leads to nanotube and nanowire formation, respectively
[203]. Since the diffusion and electrical field effects must not work in this case, one
can speculate that such a current density dependence of the morphological transition
is related to the anomalous change of the current efficiency (which is possible for
the Ni–P alloy produced in the above-mentioned study).
It was also shown, however, that the large electric field is by far not the single
important factor in the nanowire-to-nanotube transient. Instead of this transient, the
large negative electrode potential may lead to anomalously fast nanowire growth
instead of nanotube formation [32].
Effect of the pore diameter. It seems that there is a threshold nanopore diameter
above which nanochannels can accommodate not only nanowires but also nanotubes
[204]. One can hardly find data for electrodeposited nanotubes within templates of
pore diameter less than 150 nm.
Chemical aspects of the deposition process. In the case when the quality of the
substrate covering the pores continuously has an impact on the growth mode, it can
also be assumed that the high nucleation overvoltage on the conducting substrate
plays a role in the inclination to nanotube growth [205]. The same effect may stand
behind the variation of the growth preference when the modification of the growth
mode can be achieved with an additive commonly used for the deposition of metal
films [206].
11.2.9 Templates Prepared from Diblock Copolymers
Diblock copolymers are composite materials made from two polymers that are
prepared separately and are not miscible. The morphological features of diblock
copolymers include a range of segregated structures, such as spherical micelles, cylindrical micelles, vesicles, fcc- and bcc-packed spheres, hexagonally packed cylinders,
383
nanocavity by the electrodeposition process without an end-bump formation has also
been reported [200].
Switch of the nanowire growth to nanotube formation. The morphology of the
nanoobject being deposited may change abruptly in situ during the deposition into
a specific porous membrane. This morphological change, an opposite phenomenon
of the nanotube closure with time, is visualized in Fig. 11.8c. The sudden turnover
to nanotube formation is a result of the combined impact of the diffusion-limited
nature of the growth and the high electrical field. In such cases, it might seem that
the deposit appears at the top of the porous layer without an obvious pore filling due
to a very fast penetration of the tiny nanotube along the pore wall [201].
It is easy to understand that the change in the deposit morphology can be observed
as a function of the current density. Even if hydrogen evolution is not considered,
the rod-like growth mode at a small current density can switch to a tube-like growth
mode if the current density is high enough and the deposition is largely impacted
by both the diffusion limitation and the electrical field [202]. However, the current
density effect is not uniform since one can find examples also for the case when the
low and high current density leads to nanotube and nanowire formation, respectively
[203]. Since the diffusion and electrical field effects must not work in this case, one
can speculate that such a current density dependence of the morphological transition
is related to the anomalous change of the current efficiency (which is possible for
the Ni–P alloy produced in the above-mentioned study).
It was also shown, however, that the large electric field is by far not the single
important factor in the nanowire-to-nanotube transient. Instead of this transient, the
large negative electrode potential may lead to anomalously fast nanowire growth
instead of nanotube formation [32].
Effect of the pore diameter. It seems that there is a threshold nanopore diameter
above which nanochannels can accommodate not only nanowires but also nanotubes
[204]. One can hardly find data for electrodeposited nanotubes within templates of
pore diameter less than 150 nm.
Chemical aspects of the deposition process. In the case when the quality of the
substrate covering the pores continuously has an impact on the growth mode, it can
also be assumed that the high nucleation overvoltage on the conducting substrate
plays a role in the inclination to nanotube growth [205]. The same effect may stand
behind the variation of the growth preference when the modification of the growth
mode can be achieved with an additive commonly used for the deposition of metal
films [206].
11.2.9 Templates Prepared from Diblock Copolymers
Diblock copolymers are composite materials made from two polymers that are
prepared separately and are not miscible. The morphological features of diblock
copolymers include a range of segregated structures, such as spherical micelles, cylindrical micelles, vesicles, fcc- and bcc-packed spheres, hexagonally packed cylinders,