Revealing the Local Band Structures of WS₂/MoS₂ Lateral Heterojunction and W_xMo_1-xS₂ Alloy by Near-Field Optical Imaging

Chi Chen^1*

¹Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan

* Presenter:Chi Chen, email:chenchi@gate.sinica.edu.tw

With the development of various chemical vapor deposition (CVD) methods, many artificial 2D semiconductors have been synthesized, which increases the chances of forming disrupted interfaces and non-periodic or small-size systems (defects and grain boundaries). All such systems create local electronic band structures within a finite scale, which cannot be readily explained by the solid-state band theory nor be probed easily by confocal microscopes and macroscopic transport.
Here, we investigated the abrupt heterojunction and the graded alloy between two transition metal dichalcogenides (TMD), which both involve non-periodic band structures and require high spatial resolution. We employed near-field photoluminescence (NF-PL) imaging to study the atomically sharp 1D interfaces between the WS2 and the MoS2. With an optical resolution of 68 nm, a 105 nm-wide region for quenched PL is confirmed with the NF-PL imaging. We further developed the near-field broadband absorption (or transmittance, NF-tr) imaging method to overcome the limitation of NF-PL in the case of low-quantum-yield materials. NF-tr technique provides the abbreviation-free and nanoscale-resolution imaging capability of the whole conduction bands over highly lateral inhomogeneity. We utilized NF-tr microscopy to investigate the varying bandgap and the bowing factor of the single-layered WxMo1-xS2 alloy. For bilayer WxMo1-xS2 alloy, the energy contour maps present the bandgap evolution in the alloy and reveal the bilayer coupling between the top and bottom layers. High-spatial-resolution spectral capability is essential for analyzing such compositional and location-dependent bandgap evolution.

Keywords: 2D materials, Transition metal dichalcogenide , Near-feild optics, Heterojunction, Alloy