Sub-Nanometer STML Mapping

• Sub-nanometer STML maps are high-resolution imaging techniques that reveal the spatial variations in optical emissions and electronic states within 2D materials.
• They employ a confined tunneling current from an STM tip to excite localized optical transitions, enabling the visualization of defect-bound excitons and interfacial band bending.
• This method has practical applications in engineering quantum emitters and designing tunable optoelectronic devices by correlating atomic defect structures with electronic properties.

Sub-nanometer-resolved scanning tunneling microscope luminescence (STML) maps quantify the spatial variations in optical emission—and their correlation with electronic states—at atomic length scales, typically within two-dimensional (2D) materials or heterostructures. Such maps are generated by collecting the local luminescence induced via tunneling electrons in a scanning tunneling microscope, and simultaneously resolving the structural and electronic origin of fluctuating emissive features. This approach has enabled direct visualization of defect-bound excitons, interfacial band bending, and quantum emitter behavior in atomically thin semiconductors.

1. Principles of Sub-Nanometer STML Mapping

Sub-nanometer STML mapping exploits the atomic-scale confinement of the electron tunneling junction between an STM tip and a target material. When a bias voltage is applied, electrons tunnel across the gap and non-radiatively excite optical transitions—generating photons from localized recombination processes, such as exciton or trion emission. The spatial origin of light emission is then mapped by scanning the STM tip with sub-angstrom lateral increments while concurrently recording both topographical (STM) and photon signals.

Key characteristics:

• Optical emission is sensitive to atomic defect states, local potential gradients, and electronic band structure deformations.
• The emission energy ($E_X$ for neutral excitons, $E_{X^-}$ for charged trions or defect-bound complexes) is locally modified by defect-induced changes in band gap ($E_g$), binding energy ($E_b$), and trion binding ($E_{tr}$):

$$E_X = E_g - E_b,\qquad E_{X^-} = E_g - E_b - E_{tr}$$

• Sub-nanometer spatial resolution arises from the spatially confined tunneling current and highly localized electromagnetic field near the tip apex.

2. Experimental Approach and Decoupling Effects

In practice, pristine MoS$_2$ flakes are transferred onto a bilayer of hexagonal boron nitride (hBN), which itself is situated above a graphene substrate. The hBN serves as an ultrathin decoupling layer, reducing hybridization with substrate states and quenching channels for excitonic emission. As a result:

• The MoS$_2$ band gap is renormalized to higher energy due to diminished screening.
• Charge state lifetimes associated with defects are extended, enabling time-resolved studies and sharper spectral features.
• Emission arising solely from intrinsic and defect-bound excitons can be isolated, unmasking their sensitivity to atomic-scale variations.

3. Optical Fingerprints of Atomic Defects

Sub-nanometer-resolved STML maps differentiate the optical response of various point defects in MoS$_2$, notably:

• Sulfur vacancies (Vac$_\text{S}^-$): act as negatively charged sites, introduce in-gap defect orbitals, and red-shift exciton emission peaks by hundreds of meV.
• Oxygen substitutions (O$_\text{S}$): alter local atomic symmetry, typically suppress but do not shift the pristine excitonic emission.
• Carbon–hydrogen complexes (CH$_\text{S}^-$): form defect-bound exciton complexes ($A^-X$), observed as sharp peaks ∼200 meV below the neutral MoS$_2$ exciton.

These optical fingerprints correlate to STM topography and local electronic states measured via scanning tunneling spectroscopy (STS), cementing atomistic assignment to individual luminescence peaks.

4. Band Bending and Electric Field Effects

When the STM tip approaches the sample (down to sub-nanometer distances), its local electric field induces measurable band bending within the MoS$_2$ layer, shifting both defect and excitonic emission energies through the tip-induced Stark effect:

$\Delta E = \alpha \, \Delta d$

where $\Delta d$ is the change in tip distance, and $\alpha$ is a proportionality constant determined empirically (meV/nm). This effect causes smooth spectral variations across the scan area, superimposed on abrupt jumps near atomic defects.

5. Applications: Engineering Quantum Emitters and Electronic Structure

The deterministic imaging and spectral assignment of single-defect emitters in 2D materials enables several concrete applications:

• Quantum photonics: Defect-bound excitons with extended charge lifetimes and sharp emission lines can function as room-temperature quantum emitters, essential for single-photon sources.
• Structure-emission correlation: The ability to link band structure, defect configuration, and optical response at the atomic level facilitates deliberate engineering of optoelectronic properties.
• Local gating: The STM tip, via proximity and voltage, can modulate emission energy—enabling dynamic control over quantum states.
• Probing many-body effects: Atomically resolved luminescence allows investigation of exciton-defect interactions and strong correlation phenomena unique to 2D semiconductors.

6. Limitations and Technical Considerations

Sub-nanometer-resolved STML mapping faces several intrinsic challenges:

• The photon count rate is low; statistical averaging or long integration times may be required for precise spectral attribution.
• Tip stability and the exact tip apex shape critically determine spatial resolution and spectral broadening.
• The decoupling efficacy of hBN is material dependent; some substrates may still allow hybridization effects or dielectric screening, potentially complicating band gap measurements.
• Tip-induced electric field can artificially modify local energy landscapes, necessitating calibration or correction in quantitative analyses.

7. Future Directions

Continued advances in sub-nanometer STML mapping are expected to enable:

• Deterministic placement and activation of single-photon sources via site-selective defect engineering.
• Time-resolved studies of exciton dynamics, recombination, and nonradiative decay modulated by atomic environment.
• Extension to other 2D materials and van der Waals heterostructures, enabling customizable quantum emitter arrays and mapping of moiré potential landscapes.
• Integration with complementary techniques (e.g., proximity-induced strain mapping or combined electronic transport and luminescence measurements) for comprehensive quantum device characterization.

In summary, sub-nanometer STML maps facilitate atomistically precise visualization of excitonic emission and its modulation by individual defects in MoS$_2$ and other 2D semiconductors, forming an essential foundation for quantum emitter engineering and nanoscale optoelectronic studies (Huberich et al., 17 Oct 2025).