What it is
Excitons, bound pairs of an electron and a hole, govern how semiconductors respond to light; in nanoscale systems they approach atomic size, yet imaging excitonic processes directly in real space has remained elusive, and in two-dimensional semiconductors such as MoS2 the microscopic origin of defect-related emission and its variability across samples have been debated. The authors use scanning tunnelling microscopy-induced luminescence, in which the microscope's tunnelling current (holes injected from the tip, electrons from the graphene underneath) makes the material emit light right beneath the tip, to read the atomic and optical fingerprints of individual defects in monolayer MoS2. Decoupling the layer from its substrate with bilayer hexagonal boron nitride, which made the emission seven times brighter than on a single boron nitride layer, let them detect sharp excitonic emission (a main line at 1.959 eV, which they tentatively assign to the neutral exciton) and spatially localized defect emission, which they attribute to free-to-bound recombination, and map it with subnanometre-resolved fluorescence imaging. They identify negatively charged carbon-hydrogen complexes as the defect species responsible for a ubiquitous defect emission band in MoS2.
Why it matters
A defect glow common to MoS2, resembling the signature often attributed to sulfur vacancies, is now tied to one atomic defect, imaged a defect at a time. In the spectra, sulfur vacancies and oxygen substitutions suppress the material's own exciton emission without adding defect lines, while in the maps the carbon-hydrogen defect hosts emission localized within 1 to 2 nm, with its main peak about 200 meV below the MoS2 exciton; its identity was confirmed by cleaving off the hydrogen with a voltage pulse, which left a carbon defect with its own characteristic spectrum. The authors say linking atomic structure to its optical fingerprint establishes a microscopic basis for deterministic quantum emitter engineering in two-dimensional materials.
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Filed under2D Materials and Applications, Graphene research and applications, Semiconductor Quantum Structures and Devices
