A Novel Methodology of Using Nonsolvent in Achieving Ultraclean Transferred Monolayer MoS₂

Abstract

the research community because of its intriguing optical and electronic properties. [1] The optical property of a monolayer is distinguishably different from its bulk counterparts. [2] The direct bandgap ≈1.82 eV of 1L-MoS 2 makes it a suitable material for electronic and optoelectronic devices. [3,4] Additionally, the excitons in 1L-MoS 2 possess an extraordinarily high binding energy because of strong quantum confinement and the reduced screening effect. [5] The presence of excitons at ambient temperature enriches the exciton physics and finds application in macroscopic quantum phenomena. [6] Furthermore, the spin-valley coupling and breaking of the spatial inversion symmetry extend the 1L-MoS 2 application from spintronics to valleytronics. [7] By transferring the monolayer flakes on the transparent and flexible substrates, 1L-MoS 2 can expand its utility to the flexible optoelectronic devices such as flexible field-effect transistors (FETs), piezoelectric nanogenerators, organic light-emitting diode displays, foldable displays, and memory devices. [8] Large-area 1L-MoS 2 flakes are necessary to develop the monolayer-based flexible devices, In this context, chemical vapor deposition (CVD) is a proven method for the wafer-scale growth of 1L-MoS 2 . [9] Moreover, CVD is the most viable technique because of its effectiveness in scalability, reproducibility, and production of high optical quality materials. [10] However, direct growth of large-area 1L-MoS 2 onto flexible substrates such as polyethylene-terephthalate (PET) and polyethene-naphthalate (PEN) using the CVD is precluded due to its high growth temperatures ≥ 700 °C. Thus, the transfer of large-area 1L-MoS 2 grown by CVD onto flexible substrates is an inescapable step. [11] The transfer process of 1L-MoS 2 is also exploited in multiple applications. [8] First, the transfer process is used in transferring the as-grown films from the insulating substrates (like sapphire) onto the dielectric substrates (like SiO 2 ) for fabricating the complementary metal-oxide-semiconductor (CMOS) devices. [12] In addition, the transfer process is also extended to the transmission electron microscopy (TEM) sample preparation, [13] and the fabrication of vertical 2D heterostructures. [14] Among 2D material transfer methods, almost all techniques need a supporting layer for the large-area transfer and toThe transfer of monolayer molybdenum disulfide (1L-MoS 2 ) onto any target substrates is inevitable for the next generation of optoelectronic devices such as flexible electronics. However, the existing post-transfer treatments are ineffective for the complete removal of poly(methyl methacrylate) (PMMA) polymer, which is usually used as carrier polymer in the wet-transfer method. The presence of PMMA residues seriously degrades the intrinsic properties of any 2D materials. Several new cleaning methods such as annealing, ozone cleaning, and acetone treatment adopted in this report are found to be ineffective for the complete removal of PMMA residues on the transferred 1L-MoS 2 f...