Two-dimensional (2D) materials are a class of nanomaterials that have two dimensions (XY plane) outside of the nanometric size range and atomic-scale thicknesses (Z dimension). The first well-known 2D material is graphene consisting of a single layer of carbon atoms arranged in a hexagonal lattice. To compare with 0D material (fullerene) and 1D material (carbon nanotube), the researches related to 2D material (graphene) have grown up quickly over other carbon allotropes as shown in Figure 1. Based on Scopus database (search by keyword “graphene” on March 18, 2019), publications on graphene increased from 3772 papers in 2010 to 21,439 papers in 2018. The total number of graphene-related publications is 132,628 documents. However, it is not only 2D graphene that has been widely applied in a large variety of potential applications but also other 2D materials such as tungsten disulfide, molybdenum disulfide, and silicon nitride open up new opportunities for the future devices. In this chapter, synthesis and applications of these 2D materials have been introduced and presented in brief.
2. Synthesis methods of 2D materials
Graphene can be synthesized by several methods depending on the required quality and quantity. (I) Chemical exfoliation method by modified Hummers method  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 . (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.
2.2 Tungsten disulfide (WS2)
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 . 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 . The synthesis process of large-area WS2 films based on CVD can be described as follows : (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.
2.3 Molybdenum disulfide (MoS2)
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 . MoS2 nanosheets were synthesized from NaBH4 as a reductant by chemical exfoliation  and liquid-phase exfoliation method with N-methyl-2-pyrrolidone (NMP) solvents . 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 . MoS2 atomic layers were synthesized from MoO3 and pure sulfur in a vapor-phase-deposition process with a reaction temperature of 850°C . 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 .
2.4 Silicon nitride (Si3N4)
Si3N4 has been widely synthesized by using carbothermal and nitriding reactions. For example, SiO2/C mixture on alumina boat was placed in a high temperature tubular furnace with a flow rate of nitrogen and hydrogen under optimal condition to promote the formation of Si3N4 . Fe-Si3N4 composite was also prepared by FeSi75 powder as a precursor under reaction of high purity nitrogen flow via flash combustion at a high temperature of 1450°C .
3. Applications of 2D materials
Graphene has been widely used for various applications including energy storage, solar cells, and gas sensor. Abdelkader et al.  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−2 and maintain 95.6% in cyclic stability over 10,000 cycles. Shin et al.  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 . 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 . Very recently, we demonstrated a new type of graphene gas sensor based on AC electroluminescent (EL) principle . 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.
3.2 Tungsten disulfide (WS2)
WS2 nanoflakes were used for lithium ion battery applications. They showed reversible capacity of 680 mA h/g and 86.2% of the initial capacity after 20 cycles . Pawbake et al. reported that WS2 nanoparticle was used for photodetector and humidity sensing applications . It was found that the WS2 nanoparticle-based humidity sensor exhibited sensitivity of 469%, response time of ∼12 s, and recovery time of ∼13 s. In case of based photodetection application, WS2 showed a sensitivity of ∼137% under white light illumination. The response and recovery times were ∼51 and ∼88 s, respectively .
3.3 Molybdenum disulfide (MoS2)
MoS2 have been extensively applied in sensor, optical, energy device, and electronics. For example, tactile sensor was fabricated from MoS2 for electronic skin applications. MoS2 owns its outstanding properties such as good optical transparency, mechanical flexibility, and high gauge factor compared with conventional strain gauges . Wang et al. studied the conductivity and thermal stability of the MoS2/polyaniline (PANI) nanocomposites with increasing the amount of MoS2 for supercapacitor application. The results showed that the MoS2/PANI of 38 wt% exhibited specific capacitance up to 390 F/g and retained capacitance of 86% over 1000 cycles . MoS2 was also synthesized to form hydrangea-like flowers or clusters comprising MoS2 nanosheet for high-dielectric and electrical energy storage applications . Moreover, Yin et al. synthesized the biocompatible nanoflowers between MoS2 with polyethylene glycol (PEG) for antibacterial applications .
3.4 Silicon nitride (Si3N4)
Most applications of Si3N4 have been used in terms of the improvement of properties such as surface modulation for orthopedic applications  and biomedical applications . Also, Si3N4 owns good optical properties. The Si3N4 was fabricated as photonic circuits to spectroscopic sensing . The Si3N4 was used for nonlinear signal processing applications . Furthermore, Si3N4 was microfabricated as the waveguides and grating couplers for new nanophotonic approach of light delivery for optogenetic applications .
In summary, the emerging 2D materials provide high impacts for science and advanced technologies. They own unique physical, optical, mechanical, and electrical properties. Therefore, 2D materials have become one of the hottest topics in this era due to their potential various applications such as gas/chemical sensors, healthcare monitoring, biomedicine, electronic skin, wearable sensing technology, flat panel displays, optoelectronics, photodetector, catalysis, electrochemical sensing, bio sensing, water/air purification, supercapacitor, batteries, fuel cells, and advanced electronics devices.
This work was supported by the Kasetsart University Research and Development Institute (KURDI). Y.S. acknowledges the Ph.D. Graduate Program Scholarship from the Graduate School, Kasetsart University and the National Research Council of Thailand (NRCT) as of fiscal year 2018.