Araujo et al. [27] show the use of hydrothermal method assisted by microwave ra­

diation, aiming to reduce the reaction time, where ZnO nanorods are grown on a paper

substrate at low temperature and faster than the commonly used typical method. Wang

et al. [28] reported a low temperature operated direct growth method as hydrothermal

assisted by a ZnO seed layer to fabricate nanorods, which will be used as a flexible micro-

supercapacitor.

13.4.2 Fabrication Methods of Nanostructures Followed by Transferring Processes

The materials can be synthesized by different techniques (including those already men­

tioned) but followed by an additional step of transferring it to a flexible substrate. Fabrication

device techniques can be divided mainly into two types that will be described below.

13.4.2.1 Bottom-Up Growth

It consists of physical and/or chemical methods to promote the growth of micro- or

nanostructures. The methodology has this name because it starts with small structures

such as atoms and molecules, followed by clusters, and sophisticated nanostructures are

obtained in the end [29].

The most widely exploited technique is the vapor-liquid-solid (VLS) mechanism [23],

which occurs in three stages: alloying, nucleation, and axial growth stage (Figure 13.5).

This methodology has a disadvantage in the choice of a specific catalyst. If the catalyst is

poorly chosen, it can promote the formation of defects that directly and significantly

affect the properties of the manufactured nanomaterial. To avoid this challenge, some

catalyst-free methods can be used, such as chemical vapor deposition (CVD) (Figure 13.6),

thermal evaporation, molecular beam epitaxy, and others [23,26]. Another technique

FIGURE 13.5

Schematic mechanism of the preparation of WBG semiconductors through the Vapor-Liquid-Solid (VLS) technique.

FIGURE 13.6

Schematic mechanism of the preparation of WBG semiconductors through the Chemical Vapor Deposition

(CVD) technique.

Wide Bandgap Semiconductors

213