Developing seedless growth of atomically thin semiconductor layers: Application to transition metal dichalcogenides

Abstract Controlled growth of atomic monolayers of IV-VII transition metal dichalcogenides (TMDs) has provided unprecedented opportunities to fabricate modern optoelectronic nanodevices. However, synthesis of large-area and high quality two-dimensional TMDs is still challenging. We have synthesized WS 2 and MoS 2 nanosheets by atmospheric pressure chemical vapor deposition (APCVD) at wide-range of processing conditions. The nanostructures were analyzed by optical and confocal microscopy, atomic force microscopy, Raman spectroscopy, and X-ray diffraction to determine the thickness, lateral size and structure of the deposits. Through designing and performing of a set of controlled experiments as well as comparing the attained results with others, we present a general roadmap for APCVD of atomically thin WS 2 and MoS 2 nanosheets. The appropriate working windows for the controlled synthesis are established. For the controlled growth of 2D WS 2 crystals, relatively low growth temperature (750 °C) can be utilized. It is shown that an oxide mass to the gas flow rate of 0.004 favorably results in the formation of 2D WS 2 . Monolayers and few layers might be processed at higher temperatures (≥ 800 °C) at an oxide mass/gas flow rate ratio of 2–3. The seedless growth of MoS 2 nanosheets is more complicated than WS 2 possessing with narrower working window. A minimum temperature of 575 °C is required to provide enough oxide vapor pressure for deposition; otherwise, no deposits are formed. Above 650 °C, achieving 2D nanolayers is tricky due to massive deposition of the metal oxide from the vapor phase forming thick layers. In the intermediate range, however, there is a possibility to attain 2D nanostructures through fine controlling of the oxide mas/gas flow rate. Finally, it is shown that through precise control of the processing parameters, a controllable growth of the nanosheets without pre-seeding at lower temperatures and shorter times is feasible.