  Beyond-diffraction-limit optical absorption spectroscopy provides profound
information on the graded band structures of composition-spread and stacked
two-dimensional materials, in which direct/indirect bandgap, interlayer
coupling, sliding, and possible defects significantly modify their
optoelectronic functionalities such as photoluminescence efficiency. We here
visualize the spatially-varying band structure of monolayer and bilayer
transition metal dichalcogenide alloys for the first time by using near-field
broadband absorption microscopy. The near-field-spectral and -spatial diagrams
manifest the excitonic band shift that results from the interplay of
composition spreading and interlayer coupling. These results enable us to
identify the top layer of the bilayer alloy as pure WS2. We also use the
aberration-free near-field transmittance images to demarcate the exact
boundaries of alloyed and pure transition metal dichalcogenides. This
technology can offer new insights on various layered structures in the era of
stacking science in quest of novel quantum optoelectronic devices.
