Processing and Characterization of 2D Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are a diverse group of materials that has a variety of chemical and electronic properties. This group contains insulators, semiconductors, and metals, allowing for wide applications in catalysis, energy storage, sensing, and electronic devices^[1]. One particular characteristic for many TMDs is that they are layered structures, with the layers only bounded with each other by Van der Waals forces. This layered crystal structure with strong planar forces but weak orthogonal forces is promising in 2D materials applications, where mono- to few layer TMDs have altered electronic properties due to decreased dimensionality^[1]. These films are not well explored; therefore, we have focused on processing and characterization of TMDs to explore the innovation of 2D materials beyond graphene.

Different approaches have been successfully employed to make 2D TMDs, including chemical/mechanical exfoliation of monolayers from bulk crystals, thin film conversion, chemical vapor deposition (CVD), metal-organic CVD, and atomic layer deposition (ALD). Among them, sulfidation of oxide or metal seed layers (thin film conversion) enables a simple approach for the growth of large-area 2D TMDs. A common approach to make metal or oxide films is sputtering, which utilizes momentum transfer from ions (plasma) to a target’s surface atoms^[2], knocking out atoms from the target to deposit thin films onto substrates. Thin film conversion can then be done by heating flowing chalcogen vapor over the films, converting them to metal dichalcogenides^[3]. In this study, DC magnetron sputtering was utilized to deposit 1 nm molybdenum thin films onto Si/SiO₂/Al₂O₃ substrates, with film thicknesses measured by crystal monitor. These films were then sulfidized at 750°C under atmospheric pressure for 20 minutes to synthesize MoS₂. I characterized the TMD films using scanning electron microscopy (SEM) and Raman spectroscopy, then further interpreted my findings using data I received on the films’ morphology and surface chemistry from atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. SEM and AFM analysis revealed that the produced MoS₂ thin films had a uniform smoothness and fine grains across their surfaces. XPS analysis revealed the composition of the film surface and confirmed the successful conversion to MoS₂. Raman analysis identified compounds present on the film’s surface; large peak intensity readings at 383.5 cm^-1 and 406.7 cm^-1 were identified as E_1g and A_1g Raman shifts characteristic of MoS₂. The peak splitting of 23.2 cm^-1 suggests 3-layer thickness of synthesized MoS₂ films.

References:

[1] Chhowalla, M., Shin, H. S., Eda, G., Li, L.-J., Loh, K. P., & Zhang, H. The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets. Nature Chemistry 5(4), 263–275 (2013).

[2] Ohring, M. Materials Science of Thin Films: Deposition and Structure (2^nd ed.). San Diego, CA: Academic Press. (2002).

[3] EM Vogel, JA Robinson; Two-dimensional layered transition-metal dichalcogenides for versatile properties and applications; MRS Bulletin 40(07), 558-563 (2015).