**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 [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].

### **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

**3**

*Introductory Chapter: 2D Materials*

*DOI: http://dx.doi.org/10.5772/intechopen.86172*

**3. Applications of 2D materials**

give a capacitance as high as 2.5 mF cm<sup>−</sup><sup>2</sup>

any modification of smart phone hardware.

times were ∼51 and ∼88 s, respectively [20].

**3.3 Molybdenum disulfide (MoS2)**

**3.2 Tungsten disulfide (WS2)**

**3.1 Graphene**

temperature tubular furnace with a flow rate of nitrogen and hydrogen under optimal condition to promote the formation of Si3N4 [14]. Fe-Si3N4 composite was also prepared by FeSi75 powder as a precursor under reaction of high purity nitrogen

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

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

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 [19]. Pawbake et al. reported that WS2 nanoparticle was used for photodetector and humidity sensing applications [20]. It was found that the WS2 nanoparticlebased 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

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 [21]. 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

and maintain 95.6% in cyclic stability over

flow via flash combustion at a high temperature of 1450°C [15].

temperature tubular furnace with a flow rate of nitrogen and hydrogen under optimal condition to promote the formation of Si3N4 [14]. 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 [15].
