As in the synthesis of MoS2, MoSe2, the main precursors we use are MoO3 and S/Se powders. But in different papers, scientists use different carrier gases without any specific explanation. As Ar gas creates an inert environment, H2 gas helps in reduction of temp. and nucleation part. what is the main significance of using different gases in CVD method ?
The carriers influence the plasma in two ways. One, as you already said, is due to different chemical properties of the gas. To you list I can say that oxygen is used to create an environment where oxydation is easier, and nitrogen is almost inert as argon.
The other way is the difference in the eedf and as a consequence in the energy transferred to the molecules from the electric field. For example nitrogen, being a molecular gas, reduce the electron energy because of vibrational collisions. In the case of Ar, the first excitation is at about 10 eV and moreover ther is a very low minimum in the cross section, and therefore the electron temperature is higher. In the case of oxygen, you have also the contribution of dissociative attachment and the appearance of negative ions with the consequent reduction of the electron density.
Moreover, you have different ionization potential and dissociation energy (for molecules) determining the fractions of ions and atoms.
In conclusion, the choice of the carrier is another parameter to tune, together with pressure, temperature and electric field, to have a plasma with the desired properties.
Dear Priya Pradeep Kumar ;
in the growth of monolayer MoS2 crystals Before heating, the chamber was pumped to 0.5 Pa and refilled with ultrapure argon gas as a carrier gas. the SiO2/Si substrates were dried with compressed argon gas, while hydrogen gases were introduced as carrier and reaction gases in the process of growth of monolayer MoSe2 crystals. as detailed in the following reference;
https://iopscience.iop.org/article/10.1088/1757-899X/274/1/012085/pdf
In addition to the previous answers, the following points shall be considered from pure chemistry point of view: If you have an oxide precursor and want to reduce it to obtain a non-oxide product, then having hydrogen gas will help with the reaction. Nitrogen or ammonia is used for synthesizing nitrides; nitrogen for maintaining a non-oxidizing atmosphere; and argon for an inert atmosphere; carbonaceous gases (CH4, C2H2 etc.) for carbothermal reaction.
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