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Top-Down Strategies Synthesis of 2D Nanomaterial

Pranjyan Dash1 and Pradeep Kumar Panda2*

1National Taipei University of Technology, Taipei, Taiwan

2Department of Packaging & Logistics, Yonsei University, Gangwon-do, South Korea

Abstract

The nanotechnology field originated in the 21st century. Especially 2D-based nanomaterials have received a lot of attention since they are easily affordable, nontoxic, and have excellent electrical, optical, thermal, and mechanical properties. Moreover, it is simple to synthesize and can be applied to a wide range of applications. The present chapter mainly focuses on various types of synthesis methods for 2D nanomaterials using top-down strategies. Many top-down strategies have been developed to synthesize 2D nanomaterials, such as etching, mechanical milling, sputtering, and laser ablation. In all methods, we would introduce their synthesis parameters, advantages, disadvantages, and applications. Moreover, its characterization and toxicity were briefly introduced.

Keywords: Nanomaterials, synthesis, 2D nanomaterials, top-down strategy, application of nanomaterials

1.1 Introduction

Nowadays, nanomaterials (NMs) are diverging materials among all research fields [1–4]. Nanomaterials are tiny-sized materials with external diameters up to 100 nm. The nanomaterials are primarily used to make the tubes, rods, and fibers. In addition, the nanoparticles found their physical existence in nature [5]. Nanomaterials possess different physical properties as well as chemical properties to form the bulk of their counterparts. As the size of the nanomaterials is too tiny, they cannot be seen with the naked eye. These nanomaterials are added to different materials, such as cloth, cement, and other materials. The tiny size of these materials also makes them useful in electronics, environmental remediation, and neutralizing toxins. The emergent properties of nanomaterials make them beneficial and impart great impacts in electronics, medicine, and other fields [6–8]. The chemical and physical properties of NMs highly depend on the surface atoms. As per the applications, NM size can be controlled by various techniques, such as modification of surface and micelle concentration [9, 10]. Dimensionally, NMs are divided into zero-dimensional (0D), onedimensional (1D), two-dimensional (2D), and three dimensional (3D). In 0D, all three dimensions merge into a nanoscale range. A schematic diagram of dimension-based NMs is provided in Figure 1.1. In this category, nanospheres, quantum dots, and nanoclusters are included. In 1D, two dimensions merge into one. In this category, nanotubes and nanorods are included. In the 2D category, one dimension is at the nanoscale and the other dimension is outside [11]. In this category, nanofilms and nanolayers are included. 3D NMs are bulk NMs with diameters greater than the nanoscale (1–100 nm). The building blocks for 3D NMs are 0D, 1D, and 2D NMs. Core shells, nanowire bundles, nanotube bundles, and multinanolayers are included in this category [12].

Among these, more and more attention has been paid to twodimensional (2D) NMs due to their unique properties, such as excellent electrical, optical, thermal, and mechanical properties [14–17]. Many strategies have been developed to synthesize it for a specific application. These synthesis methods are broadly divided into two types of strategies: top-down and bottom-up approaches. Figure 1.2 displays an illustration of NM top-down and bottom-up strategies. The distinction between these two general classifications is based on the processes involved in the creation of nanometer-sized structures, and the choice of method depends on the specific requirements of the desired end product and the available techniques and technologies [18]. In the bottom-up approach, nanoscale materials are constructed from atomic or molecular precursors that are allowed to react, grow in size, or self-assemble into more complex structures [19]. By contrast, the top-down approach carves nanoscale structures by controlling the removal of materials from larger or bulk solids [20]. Each strategy has its own advantages and disadvantages [21]. In bottom-up strategies, nanoshells, ultrafine nanoparticles, and even nanotubes can be produced with a size of 1–20 nm. However, massive production is not possible, and synthesized nanomaterials need chemical purification. In a topdown strategy, nanomaterials can be produced massively, and purification is not needed. However, with this strategy, it is difficult to optimize the synthesized parameters [22]. This strategy is also not cost-effective.

Figure 1.1 NM classification based on dimensions [13].

Figure 1.2 Nanoparticle’s production strategies [23].

In this chapter, we mainly emphasize the synthesis of 2D nanomaterials using a top-down strategy. In this strategy, we introduce the synthesis parameters, advantages, disadvantages, and applications for all methods. Eventually, the characterization and toxicity of 2D materials will be proposed.

1.2 Top-Down Strategy Synthesis Method

In a top-down strategy, bulk material is first converted into powder-based materials, which are then converted into nanomaterials [20]. There are mainly four methods available in this strategy (Figure 1.3), such as etching, mechanical milling, sputtering, and laser ablation. Each method’s advantages, disadvantages, and own perspectives on applications are critically discussed. A summary of 2D NMs is listed in Table 1.1 using different topdown strategy methods.

Figure 1.3 Top-down synthesis method of 2D nanomaterials.

Table 1.1 Synthesis of 2D NMs by different methods of top-down strategy.

1.2.1 Etching

In this method, the 2D NM surface is modified for enhanced physical and chemical properties. Mainly, this method of obtaining materials is applicable to the semiconductor industry. In this approach, a bulk material is treated with an etchant that selectively removes or dissolves certain layers or regions, leaving behind thin layers or flakes with desired 2D NM properties. This method has been used to synthesize 2D NMs such as graphene oxide (GO), transition metal dichalcogenides (TMDs), and boron nitride by selectively oxidizing or etching away layers from their respective bulk materials [24]. In this method, it is necessary to understand growth mechanisms [25]. Etching can be used to create patterns, structures, or features on the nanoscale, and it is a crucial step in the fabrication of various nanodevices and nanosystems [26]. Depending on the material and the desired outcome, several etching techniques can be used for 2D nanomaterials. Here are some commonly used etching techniques for 2D nanomaterials:

Wet etching: Wet etching involves the use of liquid chemicals to dissolve or remove material from a substrate selectively. For 2D nanomaterials, wet etching can be performed by immersing the substrate containing the nanomaterial in a chemical solution that selectively reacts with the material to be etched while leaving other parts of the substrate untouched [27]. Wet etching is relatively simple and can be used for a wide range of 2D NMs, including graphene, transition metal dichalcogenides (TMDs), MoS2, and WS2.

Dry etching: Dry etching, also known as plasma etching, involves using reactive gases and plasma to remove material from a substrate. Dry etching can be used for 2D NMs by exposing the substrate to a reactive gas, typically in a plasma chamber, which reacts with the material being etched and removed [28]. Dry etching offers higher precision and control over the etching...