**3.1 Graphene**

*2D Materials*

**2.1 Graphene**

**2. Synthesis methods of 2D materials**

**2.2 Tungsten disulfide (WS2)**

**2.3 Molybdenum disulfide (MoS2)**

**2.4 Silicon nitride (Si3N4)**

Graphene can be synthesized by several methods depending on the required quality and quantity. (I) Chemical exfoliation method by modified Hummers method [1] is one of the popular methods for graphene oxide growth based on suitable oxidizing agents from graphite oxide. This method offers a large amount of graphene products and is of low cost. (II) Electrochemical exfoliation method is based on formation of graphene product from graphite rod or highly orientated pyrolytic graphite (HOPG) by using electricity for exfoliation of the graphite rod or HOPG immersed into electrolyte solutions [2]. (III) Chemical vapor deposition (CVD) method provides high-quality graphene products with controllable graphene layers over a large-scale area [3, 4]. Usually, methane (CH4) and acetylene (C2H2) were used as carbon source for graphene growths on

copper (Cu) or nickel (Ni) foam under high temperature around 1000°C.

The synthesis of tungsten disulfide (WS2) can be done by three main methods, namely hydrothermal method, atomic layer deposition (ALD), and CVD. A simple hydrothermal method was used to form WS2/C composite using Na2WO4·2H2O and CH3CSNH2 as raw materials, polyethylene glycol as dispersant, and glucose as the carbon source under annealing at a low temperature in argon atmosphere [5]. ALD was employed to form mono-, bi-, and multilayer WS2 nanosheets by controlling the number of cycles of ALD WO3 with plasma enhancement using WH2 (iPrCp)2 and oxygen [6]. The synthesis process of large-area WS2 films based on CVD can be described as follows [7]: (I) the Na2WO4 precursor coated on SiO2/Si substrate was loaded into quartz tube of CVD process. (II) Argon was flowed into the quartz tube until temperature reached 850°C. (III) A liquid phase of dimethyl disulfide ((CH3)2S2, DMDS) was introduced with a bubbling system for 30 min to form the WS2 film.

MoS2 can be synthesized by using mechanical and chemical methods. For example, single-layer and multilayer MoS2 nanosheets were formed by using adhesive Scotch tape from transition metal dichalcogenide (TMD) materials [8]. MoS2 nanosheets were synthesized from NaBH4 as a reductant by chemical exfoliation [9] and liquid-phase exfoliation method with N-methyl-2-pyrrolidone (NMP) solvents [10]. Moreover, MoS2 can be prepared via hydrothermal method, ALD, and CVD. For example, MoS2 nanospheres were formed with Na2MoO4·2H2O dissolved in DDW by hydrothermal method [11]. MoS2 atomic layers were synthesized from MoO3 and pure sulfur in a vapor-phase-deposition process with a reaction temperature of 850°C [12]. Based on CVD, the synthesis of MoS2 was prepared from high purity MoO3 powder and S powder in two separate Al2O3 crucibles and placed into quartz tube of CVD process. The SiO2/Si substrates were faced down and placed on the crucible of MoO3 powder together with annealing at 650°C for 15 min and N2

flow (1 sccm) at ambient to obtain 2D-MoS2 on Si substrates [13].

Si3N4 has been widely synthesized by using carbothermal and nitriding reactions. For example, SiO2/C mixture on alumina boat was placed in a high

**2**

Graphene has been widely used for various applications including energy storage, solar cells, and gas sensor. Abdelkader et al. [16] reported the fabrication of flexible printed graphene supercapacitor device for wearable electronics by using graphene oxide ink and a screen-printing technique. The supercapacitor device can give a capacitance as high as 2.5 mF cm<sup>−</sup><sup>2</sup> and maintain 95.6% in cyclic stability over 10,000 cycles. Shin et al. [17] reported the fabrication of graphene/porous silicon Schottky-type solar cells by doping with silver nanowires (AgNWs) into graphene/ porous silicon nanocomposite. Moreover, graphene has been widely applied in sensing application. For example, graphene was combined with carbon nanotubes to form as the 3D carbon nanostructures or the pillared graphene structures for toluene-sensing applications at room temperature [18]. We reported fabrication of various layer graphene gas sensors for NO2 detection and investigated the layer effect of graphene to NO2 detection. We found that bilayer graphene gas sensor exhibited the highest response and highest sensitivity to NO2 at room temperature due to accessible active surface area and unique band structure of bilayer graphene [3]. Very recently, we demonstrated a new type of graphene gas sensor based on AC electroluminescent (EL) principle [4]. This device can monitor carbon dioxide (CO2) at room temperature via changing El emission upon CO2 gas concentration. Advantage of our graphene-based electroluminescent gas sensor over typical current gas sensor is to directly integrate with a smart phone via light sensor without any modification of smart phone hardware.
