11.2 Nanochannel Templates Obtained with Top-Down Synthesis Methods
373
no single-bath solution of the task is available due to the chemical incompatibility
of the bath components to be used. The examples to be shown below will highlight
a few cases where the dimensional constraint applied by the nanowire geometry has
some significance in the process.
Various nanowires synthesized with the multiple-bath methods were used to study
the properties of the junction within the nanowire. Concerning the metal–metal interfaces, it was demonstrated that the crystalline form of Co strongly determines the
coherence of the heterojunction connection to Au, and the fcc–fcc connection proved
to be crack-free as opposed to the hcp–fcc junctions [141]. For metallic heterojunctions between Au and Sn, an asymmetry was demonstrated for the Au–Sn and Sn–Au
deposition order, showing the formation of different intermetallic compounds by the
spontaneous interdiffusion of the components [142]. In these studies, nanowires
proved to be advantageous because the junction of different metals could be studied
without an additional sample preparation step (like cross-sectional cutting and
polishing), hence eliminating the possibility of sample preparation artefacts.
Nanowires also make an ideal geometry to study the junction properties between
organic materials and metals. For instance, photoelectrochemical response of the
metal–polymer junction is difficult to detect for planar films, but it is easily accessible
in nanowires [143]. The study of the molecular conductivity is based on the same
principle. If nanowires partly filling a template are covered with a self-assembled
molecular layer and this layer is plated with another metal, a good “metal 1–organic
layer–metal 2” type sequence is obtained in a well-contained nanocavity whose
conductivity is easy to study [144]. The synthesis of such structures is possible with
the multiple-bath method only.
The double-bath synthesis is a suitable tool in bulk nanowire production, too.
If the nanowires needed are much shorter than the available template, the alternating deposition of the wire segments with desired composition separated by a less
noble sacrificial metal leads to the preparation of a precursor material containing
the nanoobjects needed. After the template synthesis, the sacrificial metal has to be
dissolved, and batches of monodispersed nanorods can be obtained. The demonstration study of this method applied Au as nanorod material and Ag as thin sacrificial
wire segments [145].
Another production-related field of nanowire synthesis is the construction of
multisegmented nanowires with a magnetic segment and one or several non-magnetic
segments that can be used for functionalization. Such nanowires are a promising
candidate for biomedical applications by using the guiding of the nanowires with
external magnetic field for targeted drug delivery.
Double-bath-based nanowire electrodeposition is an interesting synthesis
approach when the co-deposition from a single bath with a sufficient composition control cannot be solved. This was demonstrated for Bi–Tl [146] and Pb–
Bi nanowires [147]. When segmented Pb–Bi nanowires plated from alternating
baths were melted within the PAA membrane, homogeneous Pb–Bi nanowires were
obtained after solidification, in spite of the equilibrium miscibility gap of the components in bulk form. Similar processes have a great perspective to produce materials
at the nanoscale that cannot exist as bulk alloys.
373
no single-bath solution of the task is available due to the chemical incompatibility
of the bath components to be used. The examples to be shown below will highlight
a few cases where the dimensional constraint applied by the nanowire geometry has
some significance in the process.
Various nanowires synthesized with the multiple-bath methods were used to study
the properties of the junction within the nanowire. Concerning the metal–metal interfaces, it was demonstrated that the crystalline form of Co strongly determines the
coherence of the heterojunction connection to Au, and the fcc–fcc connection proved
to be crack-free as opposed to the hcp–fcc junctions [141]. For metallic heterojunctions between Au and Sn, an asymmetry was demonstrated for the Au–Sn and Sn–Au
deposition order, showing the formation of different intermetallic compounds by the
spontaneous interdiffusion of the components [142]. In these studies, nanowires
proved to be advantageous because the junction of different metals could be studied
without an additional sample preparation step (like cross-sectional cutting and
polishing), hence eliminating the possibility of sample preparation artefacts.
Nanowires also make an ideal geometry to study the junction properties between
organic materials and metals. For instance, photoelectrochemical response of the
metal–polymer junction is difficult to detect for planar films, but it is easily accessible
in nanowires [143]. The study of the molecular conductivity is based on the same
principle. If nanowires partly filling a template are covered with a self-assembled
molecular layer and this layer is plated with another metal, a good “metal 1–organic
layer–metal 2” type sequence is obtained in a well-contained nanocavity whose
conductivity is easy to study [144]. The synthesis of such structures is possible with
the multiple-bath method only.
The double-bath synthesis is a suitable tool in bulk nanowire production, too.
If the nanowires needed are much shorter than the available template, the alternating deposition of the wire segments with desired composition separated by a less
noble sacrificial metal leads to the preparation of a precursor material containing
the nanoobjects needed. After the template synthesis, the sacrificial metal has to be
dissolved, and batches of monodispersed nanorods can be obtained. The demonstration study of this method applied Au as nanorod material and Ag as thin sacrificial
wire segments [145].
Another production-related field of nanowire synthesis is the construction of
multisegmented nanowires with a magnetic segment and one or several non-magnetic
segments that can be used for functionalization. Such nanowires are a promising
candidate for biomedical applications by using the guiding of the nanowires with
external magnetic field for targeted drug delivery.
Double-bath-based nanowire electrodeposition is an interesting synthesis
approach when the co-deposition from a single bath with a sufficient composition control cannot be solved. This was demonstrated for Bi–Tl [146] and Pb–
Bi nanowires [147]. When segmented Pb–Bi nanowires plated from alternating
baths were melted within the PAA membrane, homogeneous Pb–Bi nanowires were
obtained after solidification, in spite of the equilibrium miscibility gap of the components in bulk form. Similar processes have a great perspective to produce materials
at the nanoscale that cannot exist as bulk alloys.