Open access

Introductory Chapter: 2D Materials

Written By

Yotsarayuth Seekaew and Chatchawal Wongchoosuk

Submitted: 22 March 2019 Published: 09 October 2019

DOI: 10.5772/intechopen.86172

From the Edited Volume

2D Materials

Edited by Chatchawal Wongchoosuk and Yotsarayuth Seekaew

Chapter metrics overview

1,172 Chapter Downloads

View Full Metrics

1. Overview

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.

Figure 1.

Number of publications versus publication years based on Scopus database (search by keyword “fullerene,” “carbon nanotube,” and “graphene” on March 18, 2019).


2. Synthesis methods of 2D materials

2.1 Graphene

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.

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 [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.

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 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].


3. Applications of 2D materials

3.1 Graphene

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−2 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.

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

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 [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 1000 cycles [22]. MoS2 was also synthesized to form hydrangea-like flowers or clusters comprising MoS2 nanosheet for high-dielectric and electrical energy storage applications [23]. Moreover, Yin et al. synthesized the biocompatible nanoflowers between MoS2 with polyethylene glycol (PEG) for antibacterial applications [24].

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 [25] and biomedical applications [26]. Also, Si3N4 owns good optical properties. The Si3N4 was fabricated as photonic circuits to spectroscopic sensing [27]. The Si3N4 was used for nonlinear signal processing applications [28]. Furthermore, Si3N4 was microfabricated as the waveguides and grating couplers for new nanophotonic approach of light delivery for optogenetic applications [29].


4. Conclusion

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.


  1. 1. Hummers WS Jr, Offeman RE. Preparation of graphite oxide. Journal of the American Chemical Society. 1958;80:1339-1339
  2. 2. Rao KS, Senthilnathan J, Liu Y-F, Yoshimura M. Role of peroxide ions in formation of graphene nanosheets by electrochemical exfoliation of graphite. Scientific Reports. 2014;4:4237
  3. 3. Seekaew Y, Phokharatkul D, Wisitsoraat A, Wongchoosuka C. Highly sensitive and selective room-temperature NO2 gas sensor based on bilayer transferred chemical vapor deposited graphene. Applied Surface Science. 2017;404:357-363
  4. 4. Seekaew Y, Wongchoosuka C. A novel graphene-based electroluminescent gas sensor for carbon dioxide detection. Applied Surface Science. 2019;479:525-531
  5. 5. Yuan Z, Jiang Q , Feng C, Chen X, Guo Z. Synthesis and performance of tungsten disulfide/carbon (WS2/C) composite as anode material. Journal of Electronic Materials. 2018;47:251-260
  6. 6. Song J-G, Park J, Lee W, Choi T, Jung H, Lee CW, et al. Layer-controlled, wafer-scale, and conformal synthesis of tungsten disulfide nanosheets using atomic layer deposition. ACS Nano. 2013;12:11333-11340
  7. 7. Choi SH, Boandoh S, Lee YH, Lee JS, Park J-H, Kim SM, et al. Synthesis of large-area tungsten disulfide films on pre-reduced tungsten suboxide substrates. ACS Applied Materials & Interfaces. 2017;9:43021-43029
  8. 8. Li H, Wu J, Yin Z, Zhang H. Preparation and applications of mechanically exfoliated single-layer and multilayer MoS2 and WSe2 nanosheets. Accounts of Chemical Research. 2014;47:1067-1075
  9. 9. Guardia L, Paredes JI, Munuera JM, Villar-Rodil S, Ayan-Varela M, Martinez-Alonso A, et al. Chemically exfoliated MoS2 nanosheets as an efficient catalyst for reduction reactions in the aqueous phase. ACS Applied Materials & Interfaces. 2014;6:21702-21710
  10. 10. Gupta A, Arunachalam V, Vasudevan S. Liquid-phase exfoliation of MoS2 nanosheets: The critical role of trace water. Journal of Physical Chemistry Letters. 2016;7:4884-4890
  11. 11. Chung DY, Park SK, Chung YH, Yu SH, Lim DH, Jung N, et al. Edge-exposed MoS2 nano-assembled structures as efficient electrocatalysts for hydrogen evolution reaction. Nanoscale. 2014;6:2131-2136
  12. 12. Najmaei S, Liu Z, Zhou W, Zou X, Shi G, Lei S, et al. Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers. Nature Materials. 2013;12:754-759
  13. 13. Lee YH, Zhang XQ , Zhang W, Chang MT, Lin CT, Chang KD, et al. Synthesis of large-area MoS2 atomic layers with chemical vapor deposition. Advanced Materials. 2012;24:2320-2325
  14. 14. Ortega A, Alcalá MD, Real C. Carbothermal synthesis of silicon nitride (Si3N4): Kinetics and diffusion mechanism. Journal of Materials Processing Technology. 2008;195:224-231
  15. 15. Li B, Li G, Chen H, Chen J, Hou X, Li Y. Reaction and formation mechanism of Fe-Si3N4 composite prepared by flash combustion synthesis. Ceramics International. 2018;44:22777-22783
  16. 16. Abdelkader AM, Karim N, Vallés C, Afroj S, Novoselov KS, Yeates SG. Ultraflexible and robust graphene supercapacitors printed on textiles for wearable electronics applications. 2D Materials. 2017;4:035016
  17. 17. Shin DH, Kim JH, Kim JH, Jang CW, Seo SW, Lee HS, et al. Graphene/porous silicon Schottky-junction solar cells. Journal of Alloys and Compounds. 2017;715:291-296
  18. 18. Seekaew Y, Wisitsoraat A, Phokharatkul D, Wongchoosuk C. Room temperature toluene gas sensor based on TiO2 nanoparticles decorated 3D graphene-carbon nanotube nanostructures. Sensors and Actuators B: Chemical. 2019;279:69-78
  19. 19. Feng C, Huang L, Guo Z, Liu H. Synthesis of tungsten disulfide (WS2) nanoflakes for lithium ion battery application. Electrochemistry Communications. 2007;9:119-122
  20. 20. Pawbake AS, Waykar RG, Late DJ, Jadkar SR. Highly transparent wafer-scale synthesis of crystalline WS2 nanoparticle thin film forphotodetector and humidity-sensing applications. ACS Applied Materials & Interfaces. 2016;8:3359-3365
  21. 21. Park M, Park YJ, Chen X, Park Y-K, Kim M-S, Ahn J-H. MoS2-based tactile sensor for electronic skin applications. Advanced Materials. 2016;28:2556-2562
  22. 22. Wang J, Wu Z, Hu K, Chen X, Yin H. High conductivity graphene-like MoS2/polyaniline nanocomposites and its application in supercapacitor. Journal of Alloys and Compounds. 2015;619:38-43
  23. 23. Jia Q , Huang X, Wang G, Diao J, Jiang P. MoS2 nanosheet superstructures based polymer composites for high-dielectric and electrical energy storage applications. Journal of Physical Chemistry C. 2016;120:10206-10214
  24. 24. Yin W, Yu J, Lv F, Yan L, Zheng LR, Gu Z, et al. Functionalized nano-MoS2 with peroxidase catalytic and near-infrared photothermal activities for safe and synergetic wound antibacterial applications. ACS Nano. 2016;10:11000-11011
  25. 25. Bock RM, McEntire BJ, Bal BS, Rahaman MN, Boffelli M, Pezzotti G. Surface modulation of silicon nitride ceramics for orthopaedic applications. Acta Biomaterialia. 2015;26:318-330
  26. 26. Zhao S, Xiao W, Rahaman MN, O’Brien D, Sampson JWS, Bal BS. Robocasting of silicon nitride with controllable shape and architecture for biomedical applications. International Journal of Applied Ceramic Technology. 2017;14:117-127
  27. 27. Ananth Z et al. Silicon and silicon nitride photonic circuits for spectroscopic sensing on-a-chip. Photonics Research. 2015;3:47-59
  28. 28. Lacava C, Stankovic S, Khokhar AZ, Bucio TD, Gardes FY, Reed GT, et al. Si-rich silicon nitride for nonlinear signal processing applications. Scientific Reports. 2017;7:22
  29. 29. Shim E, Chen Y, Masmanidis S, Li M. Multisite silicon neural probes with integrated silicon nitride waveguides and gratings for optogenetic applications. Scientific Reports. 2016;6:22693

Written By

Yotsarayuth Seekaew and Chatchawal Wongchoosuk

Submitted: 22 March 2019 Published: 09 October 2019