12.4 Deposition with the Help of Tips
463
methods, the sizes of the dots deposited are close to each other, and no superiority of
either of these methods can be established at the technical level presently available.
It is essential to mention that microtechnology also applies similar electrode
arrangements, although the size of the features deposited is above the micrometre.
By using a laterally moving micronozzle with the permanent maintenance of the
meniscus-confined configuration, microwires can be drawn at the substrate surface
[151]. The writing process has to start at a conductive spot of the substrate (which is
connected to the working electrode input of the process controller), but the conductivity of the wire already plated to the surface can be taken advantage of later; therefore, the writing process can be extended to non-conductive parts of the substrate
surface. Deposition of microdots can be made more efficient by using microcanteliver
arrays [152, 153].
Similarly to the methods described in Sect. 12.4.1, micropipette-based methods
are also suitable for pillar fabrication. However, as compared to the processes
described earlier when the major driving force of the pillar formation was the electric
field, pipette-based methods in the meniscus-confined configuration are also suitable
for nanowire deposition (see Fig. 12.11c). This method was called as electrochemical fountain pen nanofabrication (EC-FPN) [154]. On the one hand, the deposition
setup is very simple, being composed of a glass capillary with about 100 nm diameter, the solution filling it and a counter electrode immersed into its upper part. On
the other hand, the actuator of the nanopipette operated in a feedback-controlled
mode makes the setup thoroughly controlled. This deposition mode is a complex
process in which electrodeposition, fluid dynamics, and mass and heat transfer are
simultaneously involved. The environmental humidity, the evaporation rate of the
liquid as well as the nozzle diameter and the pulling rate were found to have a crucial
importance, as shown in multi-physics simulation of the process [155, 156].
Free-standing Pt nanowires were continuously grown off the substrate by moving
the nanopipette away from the substrate while maintaining a stable meniscus between
the nanopipette and the nanowire growth front. In the introductory study of this
technique [154], the cell voltage was controlled (1 V), and the current was about
0.5 nA. The withdrawal rate of the pipette was 50 nm s
−1 , which must be strictly
equal to the wire growth rate for maintaining the stable meniscus configuration and to
avoid a thickness fluctuation due to the varying conditions. Free-standing nanowires
up to 30 µm length could be produced with this technique. It was found to be typical
that the wire diameter is larger than the internal diameter of the pipette, which is due
to the extension of the droplet diameter at the end surface of the growing nanowire.
Pt nanowires thus produced were later tested as working electrodes in voltammetric
electroanalytical techniques [157].
By improving the meniscus-confined nanopipette pull-up deposition method
[158], the deposit choice was extended to Cu. Here, the solution concentration was
an order of magnitude larger than earlier applied for Pt, which led to a significant
increase in the growth rate. Also, the nanowire diameter could be reduced to 80 nm,
and empirical trends could be established for the dependence of the wire diameter
on the pulling rate. The wire diameter was inversely proportional to the pulling rate
463
methods, the sizes of the dots deposited are close to each other, and no superiority of
either of these methods can be established at the technical level presently available.
It is essential to mention that microtechnology also applies similar electrode
arrangements, although the size of the features deposited is above the micrometre.
By using a laterally moving micronozzle with the permanent maintenance of the
meniscus-confined configuration, microwires can be drawn at the substrate surface
[151]. The writing process has to start at a conductive spot of the substrate (which is
connected to the working electrode input of the process controller), but the conductivity of the wire already plated to the surface can be taken advantage of later; therefore, the writing process can be extended to non-conductive parts of the substrate
surface. Deposition of microdots can be made more efficient by using microcanteliver
arrays [152, 153].
Similarly to the methods described in Sect. 12.4.1, micropipette-based methods
are also suitable for pillar fabrication. However, as compared to the processes
described earlier when the major driving force of the pillar formation was the electric
field, pipette-based methods in the meniscus-confined configuration are also suitable
for nanowire deposition (see Fig. 12.11c). This method was called as electrochemical fountain pen nanofabrication (EC-FPN) [154]. On the one hand, the deposition
setup is very simple, being composed of a glass capillary with about 100 nm diameter, the solution filling it and a counter electrode immersed into its upper part. On
the other hand, the actuator of the nanopipette operated in a feedback-controlled
mode makes the setup thoroughly controlled. This deposition mode is a complex
process in which electrodeposition, fluid dynamics, and mass and heat transfer are
simultaneously involved. The environmental humidity, the evaporation rate of the
liquid as well as the nozzle diameter and the pulling rate were found to have a crucial
importance, as shown in multi-physics simulation of the process [155, 156].
Free-standing Pt nanowires were continuously grown off the substrate by moving
the nanopipette away from the substrate while maintaining a stable meniscus between
the nanopipette and the nanowire growth front. In the introductory study of this
technique [154], the cell voltage was controlled (1 V), and the current was about
0.5 nA. The withdrawal rate of the pipette was 50 nm s
−1 , which must be strictly
equal to the wire growth rate for maintaining the stable meniscus configuration and to
avoid a thickness fluctuation due to the varying conditions. Free-standing nanowires
up to 30 µm length could be produced with this technique. It was found to be typical
that the wire diameter is larger than the internal diameter of the pipette, which is due
to the extension of the droplet diameter at the end surface of the growing nanowire.
Pt nanowires thus produced were later tested as working electrodes in voltammetric
electroanalytical techniques [157].
By improving the meniscus-confined nanopipette pull-up deposition method
[158], the deposit choice was extended to Cu. Here, the solution concentration was
an order of magnitude larger than earlier applied for Pt, which led to a significant
increase in the growth rate. Also, the nanowire diameter could be reduced to 80 nm,
and empirical trends could be established for the dependence of the wire diameter
on the pulling rate. The wire diameter was inversely proportional to the pulling rate