Scanning Tunnelling Luminescence Maps Exciton Emission from Single Defects in MoS2

Scanning tunnelling microscopy-induced luminescence has resolved the atomic and optical fingerprints of individual defects in monolayer molybdenum disulfide, linking atomic defect structure directly to its optical emission. The measurements identify negatively charged carbon–hydrogen complexes as the specific defect species responsible for a ubiquitous defect emission band in molybdenum disulfide. These findings provide atomically resolved images of exciton emission within a solid-state material and establish a microscopic basis for deterministic quantum emitter engineering in two-dimensional materials.

Excitons are bound electron–hole pairs that govern the optical response of semiconductors and underpin technologies ranging from photovoltaics to quantum light sources. In nanoscale systems, the spatial extent of excitons approaches atomic dimensions, yet direct real-space imaging of excitonic processes has remained elusive. While two-dimensional semiconductors such as transition metal dichalcogenides provide reduced screening, high light extraction, and atomically defined defect sites, the microscopic origin of defect-related emission and its variability across samples have remained debated.

To overcome substrate interference, the researchers decoupled the monolayer molybdenum disulfide from its underlying graphene substrate using an interlayer of bilayer hexagonal boron nitride. This bilayer hexagonal boron nitride spacer effectively isolates the molybdenum disulfide from the graphene, which increases the transition metal dichalcogenide band gap and extends the charge state lifetime of defect states to tens of picoseconds. This substrate decoupling enabled the detection of sharp excitonic emission alongside spatially localized defect emission attributed to free-to-bound recombination.

The electrical decoupling achieved with ultrathin hexagonal boron nitride allows charge-mediated excitation of the pristine molybdenum disulfide exciton, multiple trion states, and a dark exciton. In proximity to local potential fluctuations induced by nearby atomic defects, the relative emission intensities of these excitonic states exhibit clear nanoscale variations. Subnanometre-resolved fluorescence imaging enabled the direct mapping of these localized optical distributions across the material surface.

The investigation revealed marked behavioral differences among distinct atomic defect types within the molybdenum disulfide lattice. Oxygen substitutions and negatively charged sulfur vacancies suppress pristine molybdenum disulfide emission without producing any additional defect-related emission lines. In contrast, negatively charged carbon–hydrogen complexes host spatially localized emission described by a free-to-bound transition, matching emission previously detected in far-field photoluminescence and now conclusively tied to its atomic origin.

Spatial mapping of the defect recombination revealed ground-state and excited-state transitions confined within a spatial extent of 1 to 2 nanometres. The researchers observed spectral redshifts in the defect emission, which originate from tip-induced band bending within the double-barrier tunnelling junction. This band bending reshapes the local electrostatic potential experienced jointly by the tip-confined electron and the defect-bound hole, correlating local electronic structure with nanoscale optical response.

Sample fabrication relied on a clean-dry transfer method using mechanically exfoliated flakes of monolayer molybdenum disulfide, bilayer hexagonal boron nitride, approximately 10-nanometre graphene, and approximately 30-nanometre hexagonal boron nitride on silicon and silicon dioxide substrates. After stamp pickup and release onto substrates with prepatterned titanium-platinum electrodes or epitaxial graphene on silicon carbide, polymer residues were removed with chloroform, acetone, and isopropanol. Samples underwent contact-mode atomic force microscopy scanning, ultrahigh-vacuum annealing at 350 degrees Celsius for several hours, and mild argon sputtering to induce sulfur vacancies and carbon–hydrogen defects.

Scanning probe and luminescence measurements were conducted using a CreaTec Fischer and Company instrument under ultrahigh vacuum below 2 × 10⁻¹⁰ millibar at liquid helium temperatures below 5 Kelvin. Optical signals were collected through parabolic mirrors with a 0.4 numerical aperture and routed to a spectrograph equipped with a liquid-nitrogen-cooled detector, complemented by in situ laser photoluminescence. The experiments employed tungsten and silver tips conditioned on gold and silver surfaces to optimize plasmonic cavity strength and spectral overlap with molybdenum disulfide and defect emission.

During sample preparation, the mechanically exfoliated flakes of monolayer molybdenum disulfide, bilayer hexagonal boron nitride, approximately 10-nanometre graphene, and approximately 30-nanometre hexagonal boron nitride were initially identified on silicon and silicon dioxide substrates using an optical microscope based on optical contrast or atomic force microscopy characterization. Assembly utilized a polydimethylsiloxane and polycarbonate film stamp to assemble the heterostructure stack. The substrates supporting the released heterostructures comprised either silicon and silicon dioxide substrates with prepatterned titanium-platinum electrodes or quasi-freestanding epitaxial graphene on silicon carbide. Contact-mode atomic force microscopy scanning was carried out with a Park NX7 system at a contact force set to approximately 300 nN before ultrahigh-vacuum annealing at 350 degrees Celsius. The mild argon sputtering used to induce sulfur vacancies and carbon–hydrogen defects was executed at room temperature for 1 second at a pressure of 3.6 × 10⁻⁶ mbar under a 0.2 kV bias with a submicroamp sputter current.

Coarse positioning of the scanning tip on the heterostructure was achieved using a telescopic camera system that referenced marker flakes or contact pads as visual guides. Optical emissions were collected through a fibre collimator into the Teledyne Princeton Instruments SpectraPro HRS 300 spectrograph, and complementary in situ photoluminescence characterization was carried out using a PicoQuant Prima-405-515-640 laser. Scanning tunneling microscopy topographic acquisitions were conducted in constant-current mode with the bias voltage applied directly to the sample, while scanning tunneling spectroscopy data were collected using a Nanonis Specs lock-in amplifier operating at a modulation frequency between 600 and 900 Hz.

Data acquisition for the scanning tunnelling luminescence measurements relied on an integrated software platform that enabled simultaneous recording of optical and scanning probe data. The scanning tunnelling luminescence spectra in the main text, with the exception of Fig. 4c–i, were acquired using exposure times of 5 minutes (15 seconds) per measurement, recorded at least in triplicate (single), and filtered to remove cosmic ray events, with peak fitting performed according to dedicated fitting procedures.

Decoupling the monolayer from graphene via bilayer hexagonal boron nitride enables charge-mediated excitation of the pristine exciton, denoted X, alongside multiple trion states, denoted X₁,₂⁻, and a dark exciton, denoted XD. Local potential fluctuations induced by nearby atomic defects modulate the relative emission intensities of these excitonic states across nanoscale dimensions. Specific defect designations show that oxygen substitutions, denoted OS, and negatively charged sulfur vacancies, denoted VacS⁻, suppress the pristine exciton emission without introducing additional defect emission peaks. In contrast, negatively charged CHS⁻ defects host the spatially localized free-to-bound transition, denoted (e, A0). Spatial maps resolve the ground-state defect recombination D1 alongside excited-state defect recombinations D2 and D3 within a 1 to 2 nanometre spatial extent.

The direct correlation between local electronic structure and nanoscale optical response provides fundamental insight into exciton–defect interactions in two-dimensional semiconductors. Resolving and assigning the optical fingerprints of individual atomic defects opens opportunities to advance beyond identifying naturally occurring emitters toward deliberately creating and tuning optical quantum states through controlled modification of atomic defects and their local environment. Supporting datasets for the study are available from the corresponding author upon request.

Subject of Research: Atomically resolved exciton emission from single defects in monolayer MoS2

Article Title: Atomically resolved exciton emission from single defects in MoS2

Article References: Huberich, L., Ammerman, E., Gu, Y., Ren, Y., Papadopoulos, S., Dong, C., Robinson, J. A., Watanabe, K., Taniguchi, T., Gröning, O., Novotny, L., Li, T., Wang, S., & Schuler, B. (2026). Atomically resolved exciton emission from single defects in MoS2. Nature Nanotechnology. https://doi.org/10.1038/s41565-026-02314-x

Image Credits: AI Generated

DOI: 10.1038/s41565-026-02314-x

Keywords: MoS2, exciton emission, scanning tunnelling luminescence, atomic defects, transition metal dichalcogenides, quantum emitters, hexagonal boron nitride, nanophotonics, Atomically, resolved, exciton, emission