STM-Induced Light Emission: Mechanisms & Applications

• STM-induced light emission is photon emission from inelastic tunneling in an STM junction, enabling nanoscale resolution of plasmonic, excitonic, and molecular transitions.
• Advanced optical setups using high-NA mirrors and deconvolution techniques achieve sub-diffraction spatial mapping of optoelectronic phenomena.
• The technique facilitates studies of quantum light sources, exciton dynamics in 2D materials, and single-molecule spectroscopy with precise electrical control.

STM-induced light emission (STM-LE) refers to photon emission resulting from inelastic processes in a scanning tunneling microscope (STM) junction. STM-LE offers atomic- to nanoscale spatial resolution of optical phenomena, including plasmonic, excitonic, and molecular electroluminescence, by combining local electrical excitation—via tunneling electrons—and high-efficiency photon collection. STM-LE has enabled fundamental advances in single-molecule spectroscopy, quantum light source engineering, the study of carrier and exciton dynamics in 2D materials, and the manipulation and detection of optoelectronic transitions at surfaces and nanostructures.

1. Mechanisms of STM-Induced Light Emission

STM-LE encompasses several classes of light-emission mechanisms that depend on the sample composition, local energy alignment, and excitation conditions:

a) Inelastic electron tunneling and plasmonic emission:

Electrons traversing the nanogap between tip and sample can undergo inelastic scattering, exciting localized surface plasmon (LSP) modes in the metallic nanocavity. Decay of these plasmons emits photons with energy bounded by the applied bias: $\hbar\omega \leq eV$ (Martínez-Blanco et al., 2015, Román et al., 2022). The local plasmonic density of states and Purcell enhancement amplify radiative rates in such junctions.

b) Excitonic and defect-mediated emission in semiconductors:

When the STM tip injects electrons (or holes) into a semiconductor, Coulomb-bound electron–hole pairs (excitons) form and may recombine radiatively. In 2D transition metal dichalcogenides (TMDs), local tunnel injection creates neutral excitons (X$^0$) and trions (X$^-$), whose photoluminescence-like emission can be spatially resolved down to $\sim5$ nm (Román et al., 2022, Laurent et al., 9 Dec 2025, Román} et al., 2022).

c) Molecular electroluminescence:

On molecules adsorbed on insulating films, tunneling electrons can induce direct molecular excitations (excitons) or manipulate charge states (e.g., radical cations, trions), followed by optical transitions (Zhang et al., 2018, Doppagne et al., 2018, Friedrich et al., 2023). Appropriate decoupling layers (e.g., NaCl) are essential to suppress non-radiative quenching and to preserve molecular electronic structure.

d) Stimulated and amplified emission:

In optically pumped molecular junctions under laser and bias, population inversion and optical gain can be achieved, realizing the “spaser” regime—localized, plasmon-enhanced amplified spontaneous emission and lasing in the STM cavity (Braun et al., 2013).

2. Instrumentation: Photon Collection and Spectroscopy

Achieving high-throughput, high-fidelity photon detection in STM-LE requires specialized collection optics and optical paths:

a) High-NA parabolic mirror integration:

An off-axis parabolic mirror, positioned such that the tunneling site is at the focal point, enables $\sim$0.98 NA collection—intercepting 72% of the upper hemisphere solid angle ($\Omega_{\text{mirror}}\approx4.5$ sr out of $2\pi\approx6.28$ sr) (Román et al., 2022). The collimated beam passes through the UHV viewport, with total optical transmission (excluding detector quantum efficiency) measured at $\sim$50%.

b) Étendue conservation:

System é t endue $E_{\text{source}}=n^2A_e\pi\sin^2\theta_{\max}$ is matched through the optical chain down to the spectrometer slit. With NA=0.98 at the sample, collection losses are minimized so long as the emitter area under the tip is $<12\,\mu$m in diameter, guaranteeing optimal coupling into a $^0$0m spectrometer slit (Román et al., 2022).

c) Multi-modal operation:

This configuration supports four principal detection paths: grating spectrometer for high-resolution ($^0$10.5 nm) photon spectroscopy; single-channel detectors (PMT, SPAD) for quantum yield measurements; direct CCD imaging for angle-resolved emission; and beam-splitter-assisted laser injection for in situ PL and Raman spectroscopy.

3. STM-LE on Metals, 2D Semiconductors, and Defect Systems

a) Metallic Surfaces:

On clean Au(111), inelastic tunneling excites cavity plasmons, producing broad luminescence peaks whose cutoff tracks the applied $^0$2. Field emission regime studies on Ag(111) reveal sharp image state resonances (field emission resonances, FERs) and associated photon emission, with resonance energies strongly dependent on tip radius and gap field (Martínez-Blanco et al., 2015).

b) 2D Semiconductors:

STM-LE in monolayer TMDs (WSe$^0$3, WS$^0$4) demonstrates direct observation of neutral and charged exciton recombination at 751–765 nm. The quantum yield is typically $^0$5 photons/electron. The ability to vary the X$^0$6:X$^0$7 emission ratio with tunnel current or bias offers electrical control of emission species at nanometric resolution (Román et al., 2022, Román} et al., 2022, Laurent et al., 9 Dec 2025):

• STM-LE spatial mapping reveals diffusion lengths $^0$8 nm for excitons (Laurent et al., 9 Dec 2025).
• Band alignment and spacer engineering (water, oxide, h-BN) are critical to prevent metallic substrate quenching (Román et al., 2022).

c) Wide-Bandgap and Defect Systems:

On step-bunched, oxidized 4H-SiC, sub-gap electroluminescence is mapped with $^0$930 nm resolution, revealing defect-related emission ($^-$0 eV) localized at surface risers. Correlated STS identifies high-density charge traps as the luminescent centers (Alyabyeva et al., 2022).

d) Quantum Dots and Nanoribbons:

STM-LE has been extended to quantum dots (CdSe/ZnS) and atomically precise graphene nanoribbons, uncovering intricate vibronic structures, topologically protected dark excitons, and emission modulated by nanoribbon length and edge termination (Jiang et al., 2022).

4. Single-Molecule STM-LE: Exciton Dynamics and Quantum Light Sources

On individual molecules decoupled on insulators, STM-LE provides access to photon emission from discrete electronic states, charge states, and their vibronic structure:

a) Cascade dynamics:

STM-LE can probe transitions such as neutral excitonic (S$^-$1S$^-$2), trionic (D$^-$3D$^-$4), and can reveal electrically driven cascaded single-photon emission with sub-nanosecond cross-correlation signatures (Kaiser et al., 2024).

b) Optical control via energy alignment and charging:

By tuning the substrate work function and tip-induced voltage drop, STML of single porphyrins can be toggled between “dark” and “bright” regimes—activating or suppressing optical cascades (neutral ground $^-$5 ion $^-$6 excited state $^-$7 ground + photon) (Ammerman et al., 1 Nov 2025). Rate-equation and polaron models accurately capture the gating and reorganization effects.

c) Fine manipulation of emission via plasmons and field effects:

Plasmon–exciton coupling can be studied by varying tip–molecule position, revealing Fano resonances, Purcell-enhanced rates, and even internal (point-charge-generated) Stark effect shifts upon molecular deprotonation (Vasilev et al., 2021, Zhang et al., 2018, Imada et al., 2016, Friedrich et al., 2023). The transition dipole orientation with respect to the local plasmonic field enables orbital-selective excitation and emission (Imada et al., 2016).

d) Vibronic and spin-selective phenomena:

STM-LE resolves the full vibronic progression of neutral and charged states, allows for mapping singlet–triplet intersystem crossing, and, on organic phosphors (PtPc), records both fluorescence and phosphorescence at molecular scale, elucidating decay lifetimes and triplet branching ratios (Doppagne et al., 2018, Grewal et al., 14 Feb 2025).

5. Data Analysis: Beyond the Optical Diffraction Limit

STM-LE enables spatially correlated optical spectroscopy with a local excitation volume set by the tip radius ($^-$81–5 nm), but optical collection resolutions are limited by $^-$9. To circumvent this diffraction barrier:

Richardson–Lucy (RL) deconvolution is used to extract sub-diffraction spatial distributions, $\sim5$0, from measured wide-field emission images $\sim5$1 via iterative 2D convolution with an instrument-calibrated point spread function (PSF). The RL update equation is

$\sim5$2

After 200–500 iterations, emitter distributions with effective $\sim5$3 nm (at 600 nm) are retrieved, enabling quantification of exciton transport and drift on sub-wavelength scales (Laurent et al., 9 Dec 2025).

6. Control Parameters, Limitations, and Applications

a) Control parameters affecting STM-LE:

• Tunnel current ($\sim5$4): Larger $\sim5$5 results in broadening of emitter spatial profiles via electric field–induced drift and carrier interaction (Laurent et al., 9 Dec 2025).
• Bias voltage ($\sim5$6): Controls band alignment and enabling of specific emission pathways; determines onset and species (e.g., trion vs. exciton) in TMDs (Román et al., 2022).
• Tip–sample geometry and plasmon resonance: Directly governs field enhancement, local density of optical states, Purcell factors, and thus photon yield and spectra (Román et al., 2022, Martínez-Blanco et al., 2015).
• Decoupling layer and local environment: Insulating spacers (NaCl, h-BN, water) prevent non-radiative quenching; electrostatic environment shifts emission via Stark and image-charge effects (Friedrich et al., 2023, Vasilev et al., 2021).
• Sample preparation: Defect density, surface steps, or edges determine locations of emission hot spots, especially for defect-based and sub-gap emission (Alyabyeva et al., 2022, Jiang et al., 2022).

b) Applications and implications:

• Electrically driven quantum light sources and single-photon emitters are realized at the nanoscale, including cascaded two-color photon emission (Kaiser et al., 2024), vibronic-resolved sources (Doppagne et al., 2018), and room-temperature candidates via hybrid plasmonic excitation (Braun et al., 2013).
• Mapping and engineering exciton diffusion, carrier trapping, and field-driven drift are possible in 2D semiconductors, informing the design of ultracompact LEDs and excitonic circuitry (Laurent et al., 9 Dec 2025).
• STM-LE platforms support correlative studies: mapping PL, CL, Raman, and electronic structure concurrently with atomic precision (Román et al., 2022).
• Internal charge manipulation (protonation, local gating) provides a scalable strategy for color tuning and mapping nano- to atomic-scale electric fields via emission shifts (Vasilev et al., 2021).

c) Limitations and challenges:

• High voltages and currents risk damaging sensitive surfaces or perturbing molecular systems; proper calibration of tip geometry is essential for quantitative interpretation (Alyabyeva et al., 2022).
• Light collection efficiency in traditional configurations is often $\sim5$7%; integration of parabolic mirrors and étendue matching improves detection limits dramatically (down to 10 photons/s) (Román et al., 2022).

STM-induced light emission thus constitutes a unified platform for atomic-scale photonics, combining exquisite spatial control with the spectroscopic sensitivity to probe and manipulate quantum states of matter. The methodology is extensible to a wide range of materials, from metals and semiconductors to molecules and nanostructures, enabling fundamental and applied advances in optoelectronics, quantum optics, and materials science.