Biomedical Engineering Reference
In-Depth Information
(4)
Limitation of dopants and inability to functionalize materials other than silicon
limit the diversity of semiconductor NWs.
(5)
Single-walled CNTs can exhibit either metallic or semiconducting behavior
depending on the diameter and helicity [38]. The inability to control whether
NTs are semiconducting or metallic makes specific device fabrication largely a
random event.
(6)
Synthesis of Si NWs and CNTs require extreme conditions followed by purifica-
tion before functionalization with appropriate receptor and device fabrica-
tion/assembly.
In an attempt to address some of the above-mentioned challenges, we recently reported
a facile technique for synthesis of high aspect ratio (100 nm wide by up to 13 µm long), den-
dritefree, single and multiple individually addressable CP NWs by electrodeposition
within channels between two electrodes on the surface of silicon wafers [39]. The electrodes
were fabricated on a (100)-oriented silicon wafer with a chemical vapor deposition grown
1 µm thick insulator layer of low-stress Si
3
N
4
film. A Cr adhesion layer and a ~3000 Å thick
Au contact layer were then deposited, with contacts being patterned by lift-off. Thermally
evaporated SiO
2
was then deposited on the wafer at room temperature followed by a coat
of PMMA photoresist. Using e-beam lithographic patterning, the deposited SiO
2
and
PMMA were selectively opened with reactive ion etching to form electrolyte channels. The
widths of the electrolyte channels were fixed at 100, 200, 500, or 1000 nm [40].
Figure 4.4 shows a cross-section of the silicon wafer. The deposition and growth of the
nanowire were based on well-known electrochemical oxidative polymerization, starting
with monomers and dopants. Multiple channels were etched between gold electrode pairs
E-beam patterned nanochannel
SiO
SiO
Au electrode
Au electrode
Silicon substrate
(A)
(B)
FIGURE 4.4
Schematic of e-beam lithographically paterned electrolyte channels with defined channel dimensions, showing
cross-sectional (A) and top-view (B) details.
