Cross-Correlated AFM and TERS Imaging Reveals Nanoscale Defects and Composition Misconceptions in MoSSe/MoSeS Janus Crystals

Since the discovery of graphene, many more 2D materials have been identified and isolated in monolayer form. Transition metal dichalcogenides (TMDs) are probably the most researched 2D semiconductors due to their electrical properties and a plethora of fascinating optoelectronic phenomena in TMDs and their vertical and lateral heterostructures mediated by tightly bound excitons that survive at room or even elevated temperature. Recently, the naturally occurring TMDs have been complemented by a new, man-made class of monolayers, Janus TMDs, in which the layer of transition metal atoms is sandwiched between two layers of inequivalent chalcogen atoms. Due to the lack of inversion symmetry and the embedded vertical dipole moment, Janus TMDs possess several unique properties that are not observed in conventional TMDs, and which are promising for the future applications in non-linear optics, energy harvesting, catalysis etc. Therefore, perfecting the synthesis of Janus TMDs and understanding and controlling defects in these man-made monolayers is of great importance for the 2D material community.<br/>Using tip enhanced Raman spectroscopy (TERS) imaging cross-correlated with other AFM channels like topography, surface potential, and photocurrent, we identify the typical defects in Janus Mo-based TMDs produced by plasma-assisted selenization/sulfurization at room-temperature. We further demonstrate that the morphology and the specifics of TERS response of the end product such as the band splitting in A₁¹ and A₁² modes, strongly depend on whether the conversion starts from a MoS₂ or MoSe₂ monolayer. Additionally, the high spatial resolution of TERS imaging (&lt; 10 nm in air) reveals that the residual Raman signal of the precursor monolayer (MoS₂ or MoSe₂) is often related to the presence of bilayer islands that can be as small as a few tens- to a few hundreds of nanometers in width, rather than to the incomplete Janus conversion process.<br/>Through correlated TERS and AFM imaging, we demonstrate that excessive tensile strain in MoSeS Janus crystals converted from MoSe₂ mechanically breaks these crystals into approximately 1 μm domains separated by 10-20 nm gaps. Conversely, MoSSe converted from MoS₂ experiences compressive strain which leads to the formation of wrinkles. From morphological characterization, these wrinkles have the appearance of cracks upon the gold- or silver-assisted transfer, but TERS confirms the continuity of the Janus crystal and reveals strong TERS enhancement over these inverted wrinkles. Moreover, we identified that MoSSe crystals synthesized on fused silica feature very smooth topography, mostly free from wrinkles and cracks, and very similar TERS/Raman response as compared to crystals grown on Si/SiO₂ substrates. This opens the possibilities for controlling the desired morphology of Janus monolayers which may be quite different for applications in optoelectronics (smooth topography) or catalysis (with increased number of wrinkles or edges).