Asymmetry of the Junction Line Defect Distribution in WS₂-WSe₂ Lateral/Vertical Heterostructures Revealed by TERS Imaging

2D semiconductors attracted significant attention of the research community in recent years due to combination of unique optoelectronic properties and great promises in diverse applications like optoelectronics, light harvesting and quantum computing. Relative ease of vertical stacking of these Van der Waals materials and possibility of the synthesis of their lateral heterosctructures open new horizons in the control of the optoelectronic properties of such hybrid materials and rational engineering of new unique states like vertical or lateral interlayer excitons, defect-bound excitons, quantum localization of the excitons due to extreme mechanical strain etc. In particular, the 1D interface in lateral heterostructures as well as the vertical multilayers comprised of different 2D semiconductors look specifically attractive in terms of rational design of optoelectronic properties of such hybrid materials. Characterization of the optical properties and chemical composition of the 1D/2D interfaces at proper length scale, which is often of the order of few nanometers or less, becomes a crucial necessity for further progress in understanding their properties and rational device engineering.<br/>In presented work we report deep subdiffractional Raman imaging achieved by means of the tip enhanced Raman spectroscopy (TERS) of lateral and vertical heterostructures of WS₂ and WSe₂ transferred to gold.<br/>Thanks to dramatically enhanced sensitivity and spatial resolution enabled by the gap mode TERS, we first observed that within 20-50 nm at the junction line in WS₂-WSe₂ lateral heterostructures that grew on top of the bilayer WS₂, TERS spectra were clearly different from the core WSe₂@2LWS₂vertical heterotrilayer showing noticeable intensity of two new peaks at ~ 290 cm^-1and ~ 330 cm^-1. The positions of these two junction-specific peaks were strikingly reminiscent of the A₁¹and E² modes correspondingly of Janus WSSe structures¹. Formation of the Janus –type junction was a rather non-trivial, though not totally impossible hypothesis.<br/>In order to probe it, we collected TERS maps of the junction in lateral WS₂-WSe₂ heteromonolayers and it turned out that the junction-specific spectra actually occurred on the WSe₂ side of the lateral heterostructure. With certain degree of confidence we can guess that the ~ 290 cm^-1 band was the second overtone of the mode at ~145 cm^-1 as the latter was also evident in the junction specific spectra and in some cases we even observed a weak peak at ~435 cm^-1 that would be the third overtone of the 145 cm^-1 band. TERS spectra on the WS₂ side of the lateral WS₂-WSe₂ heteromonolayer junction remained practically the same as the TERS spectra of the core WS₂.<br/>Junction specific peaks in WSe₂ spectra look very similar to the longitudinal acoustic modes at M point which are associated with defects in this material, therefore we can make a conclusion that at the junction in lateral WS₂-WSe₂heterostructures defects are preferentially concentrated on WSe₂ side. How such asymmetry affects the electronic and optoelectronic properties of the heterostructures remains the question for further investigation.<br/>(1) Petrić, M. M.; Kremser, M.; Barbone, M.; Qin, Y.; Sayyad, Y.; Shen, Y.; Tongay, S.; Finley, J. J.; Botello-Méndez, A. R.; Müller, K. Raman Spectrum of Janus Transition Metal Dichalcogenide Monolayers WSSe and MoSSe. <i>Physical Review B</i> <b>2021</b>, <i>103</i> (3). https://doi.org/10.1103/PhysRevB.103.035414.