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 high temperature requirements for heating are definitely a big concern for the purpose of designing a low cost process. The relatively low temperature of 600??C required for condensation on a substrate is also difficult to achieve in small furnace set ups where most of the furnace would be at one temperature or at an indeterminable range of high temperatures.
Thus, this method too does not fulfill all our requirements and further methods are to be investigated.
4.4 Chemical Vapour Deposition
CVD methods mainly comprise of sulphurization of Mo reactants in various forms. Different forms of Mo compounds can be used as precursors for CVD. One may use its oxide as MoO3, metal form Mo, or even MoCl5. These may appear in different physical forms, such as a thin metal film deposited on a substrate, or as nanorods. Here, an insight is provided at specific aspects of the CVD growth process that are common amongst many research groups attempting to fabricate monolayer MoS2.
4.4.1 Seeding and Controlled Nucleation
Around the globe, several research groups have employed seeding methods to enhance the nucleation and growth of MoS2.
In a method employed by Y. Lee et al. 14, seeding agents such as, reduced graphene oxide (rGO), perylene-3,4,9,10- tetracarboxylic dianhydride (PTCDA) and perylene- 3,4,9,10-tetracarboxylic acid tetrapotassium salt (PTAS) were used to treat the substrate prior to the growth process. For rGO, a GO solution is mixed with hydrazine and heated to cause the reduction, before a drop of rGO-hydrazine is spin coated onto the substrate. PTAS and PTCDA were also used similarly in separate experiments in order to spin coat the substrate prior to the CVD growth and successfully promote MoS2 flake growth15.
One group headed by S. Najmaei16 chose to manipulate the substrate surface, in order to provide low energy locations for easier nucleation sites for MoS2. They used lithography to pattern the rectangular SiO2 pillars to achieve a high density of nucleation sites and subsequently, the formation of large-area MoS2 films.
Another group made use of the intermediate reduction product of MoO3 to MoO2 as the nucleation structures. X.Wang et al.17 allowed the MoO2 rhomboidal microplates to form, before further annealing was carried out on their surfaces and reduced to MoS2 with multiple layers which depended on the annealing time.
Moreover, there were a few groups which focussed on using clean substrates and not using any seeding agents whatsoever4.
4.4.2 Precursor, Reactant and Gas Flow
In terms of reactants, a majority of groups were found to adhere to the use of high purity MoO3 and sulphur powder respectively4,15-17. However, there were certain groups which used different starting precursors.
X.Shi et al. 19, used ammonium thiomolybdate in vapour form as a precursor, and which is carried to a graphene surface with the aid of a carrier gas prior to annealing for thermal decomposition of the precursor into epitaxial MoS2 flakes on graphene.
MoCl5 powder and sulphur powder were used as precursor and reactant respectively by Y. Yu et al.20. They grew high quality thin film of MoS2 at a high temperature of 850??C and a low pressure of 2 Torr. Also, they varied the number of layers grown by altering the amount of precursor used or the total pressure exerted.
Y.Zhan et al. 21 is reported to have deposited Mo thin films onto SiO2 substrates and sulphurized them directly with sulphur vapour.
4.4.3 Temperature and Pressure
It has been observed that the research groups that simply stuck to use high purity MoO3 and S powders had to use slightly elevated furnace temperatures of approximately 750??C to 950??C 4,15-17. However, when seeding was involved or when the precursor used is a less conventional form like Mo thin film or ammonium thiomolybdate, then the growth process usually requires a relatively lower temperature between 400??C to 750 ??C 15,19,21 approximately..