Amidst the numerous methods reported to grow or fabricate monolayer MoS2, some of them have been very successful attempts. With reference to our aim to implement a controllable growth method, focus was on the bottom-up approaches, leaving aside the top-down approaches like lithium intercalation assisted exfoliation and mechanical exfoliation which exercise minimal control on the thickness of layers. Hence, a detailed study of the pros and cons of these bottom-up methods lead to the conclusion of chemical vapour deposition as the chosen chemical route for the synthesis procedure adopted by this project.
There are several deposition techniques which take place in the liquid phase, where ionic precursors and reactants in aqueous form are used to synthesize monolayer MoS2. Such techniques include thermolysis, electrochemical and hydrothermal methods. Two of the more simple liquid phase methods are briefly enumerated below.
4.1 Electrochemical Synthesis
A two-step electrochemical/chemical synthesis of 2H-MoS2 was designed by Q. Li et al. 11 It was done by electrodeposition of MoO2 nanowires onto highly oriented pyrolytic graphite surfaces, prior to exposing them to hydrogen sulphide at elevated temperatures in order to reduce MoO2 into the desired MoS2 form. The risk factor involved in this method is high as the use of H2S at high temperatures has to be carried out with utmost caution, if it were to be adopted as an industrial process. H2S in high quantities is highly poisonous and is a strong factor to consider in choosing an appropriately safe growth method.
4.2 Thermolysis of Ammonium Thiomolybdates
Figure 3-1 Two-step thermolysis of ammonium thiomolybdate.12
K. Liu et al.12 developed this two step thermolysis method of ammonium thiomolybdate. Here, ammonium thiomolybdate coated on an insulating substrate like SiO2/Si is annealed twice to facilitate reduction in order to form MoS2 trilayers. Though this method appears reliable and simple, the requirement for high temperatures and formation of multilayer MoS2 during synthesis poses as a drawback.
As the intention is to fabricate monolayer MoS2 with relatively lower temperatures and simpler pressure requirements , these methods do not serve the purpose. Hence, other more suitable methods are sought after.
4.3 Physical Vapour Deposition
There are also techniques which require reactants and precursors in the vapour phase, namely physical or chemical vapour deposition. Physical vapour deposition encompasses pulsed layer deposition, thermal or electron beam evaporation, van der Waals , molecular beam epitaxy and sputtering. However, only thermal evaporation has been found to be widely used because of its low equipment requirements and relatively simple experimental set up. Following are some of the more feasible techniques considered during the course of selection of synthesis route.
In a method followed by C. Gong et al. 13, a source of MoS2 powder is placed in a quartz tube which is residing in a furnace, with the insulating substrate placed downstream. The temperature of the furnace is around 650??C. The source is heated for approximately 15-20min at a temperature of about 900??C with Ar gas flowing at base pressure of 20mTorr to maintain an inert atmosphere. Such …
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