broadly reported is the sol-gel method, which has several advantages, such as the low-

cost, simple, and highly controlled approach [30].

Lin et al. [31] used the sol-gel technique to develop a ZnO thin film for organic solar

cells on flexible plastics substrates. First, it was prepared as a ZnO sol-gel by dissolving

zinc acetate dihydrate (ZAD) and ethanolamine (MEA) in ethylene glycol monomethyl

ether (EGME). It was investigated the molar ratio of ZAD to MEA, and the characteristics

of the resulting ZnO thin films. Feng et al. [32] demonstrate for the first time the fabri­

cation of SiC nanowires with precise growth control by tailoring the cooling rates based

on the VLS process, aiming to provide a new strategy for the growth of one-dimension

nanowires with well-controlled morphology. Low et al. [33] synthesized GaN nanowires

on Ni-coated sapphire substrate using the CVD method at different growth temperatures,

which studied its influence on the morphological, structural, and optical characteristics of

the nanowires.

13.4.2.2 Top-Down Growth

Top-down growth consists basically of a subtractive process through mostly physical

methods, such as lithography and derivatives, focused ion beam (FIB), electron beam li­

thography (EBL), gas-phase condensation, and others, combined with another process [2].

In the same way as the previous technique, this one has this name because it starts from

larger structures, converts into smaller ones (e.g., powder), and then into nanostructures

with control of some parameters, such as shape and size [29].

Conventional lithography creates patterns on the surface through some processes like

coating, irradiation (and because of this, it is commonly named photolithography), and

etching [34]. The FIB lithography technique is one of the most advanced and efficient

processes for surface modifications and fabrication of semiconductor nanomaterials. It

consists of an application of highly focused ion beams (Figure 13.7a) aiming to modify the

surface of the target via the sputtering method [23]. Another similar technique widely

reported is the EBL, in which the patterns are formed by scanning a focused beam of

electrons on a treated surface (Figure 13.7b).

Since the presented mechanisms in this chapter have great advantages and some draw­

backs, all these methods can be combined into a hybrid fabrication technique, which offers

an improved alternative to prepare WBG semiconductors for bioelectronics. Phan et al. [35]

reported the fabrication of silicon nanowires for microelectromechanical systems applica­

tions through the FIB process followed by wet etching and thermal annealing. The man­

ufactured materials in this study had an electrical conductivity magnitude enhanced

FIGURE 13.7

Schematic mechanism of a (a) focused ion beam and (b) electron beam lithography techniques.

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Bioelectronics