11.2 Nanochannel Templates Obtained with Top-Down Synthesis Methods
375
Morphology, crystallinity, structure and texture. It is generally believed that the
nearly cylindrical side surface of the compact nanowires is a good replica of the
template that it is embedded in. Although the above statement can be taken as a
sufficient first approximation, a great variety of information has been published
in the relevant literature that makes this picture rather sophisticated. For instance,
nanowires deposited into track-etched polymer membranes were found not to by
cylindrical but “cigar-shaped”, having a central section thicker than the ends [20, 24,
109]. When the nanopores were just partly filled, a thinner diameter was found at the
substrate side [114]. It is not fully clear whether this shape is a consequence of either
the membrane preparation (overetching the central part during the pore widening)
or the shape change of the soft template during the deposition process. Since the
diameter measured for electrodeposited nanowire is sometimes significantly larger
than the nominal diameter of the nanopores in which they were synthesized [114], it
is suspected that polymer membranes can be either mechanically deformed or further
etched during some electrodeposition processes.
It is also common that the side surface roughness of the nanowires composed of
the same material differs as a function of their deposition potential, and nanowires
of different materials in the same template exhibit various side surface morphologies
[165]. The side surface roughness of the electrodeposited nanowires nearly always
changes if potentiostatic mode is replaced with pulse plating, so much that even
diameter-modulated sectioned nanowires can be synthesized from the same material
(Pt) [33]. It is also possible that the nanowire (here, Bi) diameter is constant but
significantly smaller than that of the nanochannels as pulse-plating parameters are
varied [49].
The morphological change along the nanowire length is even more obvious when
the nanowire is multilayered or segmented. In the case of segmented nanowires,
the formation of smooth side surfaces [141, 149, 163], a small change in diameter at the boundary of layers [146, 148, 157, 160, 166] and periodical diameter
oscillations in accord with the composition change [161, 162, 164, 167] are all
possible. Since the periodic oscillation occurs equally for both soft polymer and
PAA templates, this phenomenon cannot be attributed to the local deformation of the
template; rather, a different adhesion of the components to the template side wall may
be behind the diameter change. For multilayered nanowires, the layers are seldom
planar but the nanowire is undulated, lending a “potato chip pack in a cylindrical
box” morphological character to the nanolayer stacking [154, 155, 157, 159].
Nanowires composed of one single metallic element are crystalline, but both the
crystallite size and the preferred orientation show a great variety. The success of the
production of single-crystalline nanowire can be a function of, e.g., the bath used [43].
Even though the application of the potentiostatic deposition can also lead to singlecrystalline nanowires [61], the application of both pulse [54, 55, 72] and reverse
pulse plating [43, 63] is very common to achieve the growth of single-crystalline
nanowires. The change in the deposition mode is nearly always accompanied with
the modification of the preferred growth direction [60]. The variation of the texture
and grain size are also related to the pore diameter [42], which means that nearly
each system needs a unique optimization means to achieve the desired growth mode.
375
Morphology, crystallinity, structure and texture. It is generally believed that the
nearly cylindrical side surface of the compact nanowires is a good replica of the
template that it is embedded in. Although the above statement can be taken as a
sufficient first approximation, a great variety of information has been published
in the relevant literature that makes this picture rather sophisticated. For instance,
nanowires deposited into track-etched polymer membranes were found not to by
cylindrical but “cigar-shaped”, having a central section thicker than the ends [20, 24,
109]. When the nanopores were just partly filled, a thinner diameter was found at the
substrate side [114]. It is not fully clear whether this shape is a consequence of either
the membrane preparation (overetching the central part during the pore widening)
or the shape change of the soft template during the deposition process. Since the
diameter measured for electrodeposited nanowire is sometimes significantly larger
than the nominal diameter of the nanopores in which they were synthesized [114], it
is suspected that polymer membranes can be either mechanically deformed or further
etched during some electrodeposition processes.
It is also common that the side surface roughness of the nanowires composed of
the same material differs as a function of their deposition potential, and nanowires
of different materials in the same template exhibit various side surface morphologies
[165]. The side surface roughness of the electrodeposited nanowires nearly always
changes if potentiostatic mode is replaced with pulse plating, so much that even
diameter-modulated sectioned nanowires can be synthesized from the same material
(Pt) [33]. It is also possible that the nanowire (here, Bi) diameter is constant but
significantly smaller than that of the nanochannels as pulse-plating parameters are
varied [49].
The morphological change along the nanowire length is even more obvious when
the nanowire is multilayered or segmented. In the case of segmented nanowires,
the formation of smooth side surfaces [141, 149, 163], a small change in diameter at the boundary of layers [146, 148, 157, 160, 166] and periodical diameter
oscillations in accord with the composition change [161, 162, 164, 167] are all
possible. Since the periodic oscillation occurs equally for both soft polymer and
PAA templates, this phenomenon cannot be attributed to the local deformation of the
template; rather, a different adhesion of the components to the template side wall may
be behind the diameter change. For multilayered nanowires, the layers are seldom
planar but the nanowire is undulated, lending a “potato chip pack in a cylindrical
box” morphological character to the nanolayer stacking [154, 155, 157, 159].
Nanowires composed of one single metallic element are crystalline, but both the
crystallite size and the preferred orientation show a great variety. The success of the
production of single-crystalline nanowire can be a function of, e.g., the bath used [43].
Even though the application of the potentiostatic deposition can also lead to singlecrystalline nanowires [61], the application of both pulse [54, 55, 72] and reverse
pulse plating [43, 63] is very common to achieve the growth of single-crystalline
nanowires. The change in the deposition mode is nearly always accompanied with
the modification of the preferred growth direction [60]. The variation of the texture
and grain size are also related to the pore diameter [42], which means that nearly
each system needs a unique optimization means to achieve the desired growth mode.