Electron-Beam-Driven Quantum Tunneling

by Kenan Elibol

Electron microscopy constitutes a central analytical tool for the structural and electronic characterization of materials at the nanoscale. Conventionally, the focused electron beam is regarded as a passive probe that enables high-resolution imaging and spectroscopy without fundamentally altering intrinsic charge transport processes within the investigated specimen.

Recent experimental findings challenge this paradigm. An international collaboration has demonstrated that highly focused electron beams can actively induce quantum tunneling phenomena in plasmonically engineered nanostructures. Specifically, intense electron irradiation can trigger tunneling field emission through the generation of strongly confined electromagnetic near-fields. These results provide direct evidence that the electron beam can act not only as a diagnostic instrument but also as a localized excitation source capable of initiating quantum-mechanical charge transfer processes.

The study reveals that electron-beam excitation of metallic nanostructures can produce highly localized electric field intensities sufficient to lower the potential barrier at metal surfaces, thereby enabling electron emission via quantum tunneling. Such findings establish new perspectives for the development of spatially confined electron sources with ultrafast temporal response and contribute to a deeper mechanistic understanding of electron–matter interactions under non-equilibrium conditions.

 

The Electron Beam as an Active Excitation Source

Field emission is a quantum-mechanical transport process in which electrons traverse a classically forbidden energy barrier as a consequence of strong electric fields that effectively reduce the barrier width. Traditionally, tunneling field emission requires either high static voltages or intense optical excitation, typically provided by ultrafast laser pulses capable of generating strong transient electromagnetic fields.

The present work demonstrates that comparable field strengths can be achieved through electron-beam-driven plasmonic excitation when the nanostructured material system is carefully designed. The investigated emitters consist of lithographically defined gold nanostructures comprising elongated triangular elements coupled to hemispherical nanoparticles positioned on a conductive gold substrate separated by an ultrathin dielectric membrane. This architecture produces geometrical singularities, such as nanoscale gaps and sharply curved apex regions, that strongly enhance local electromagnetic fields through plasmonic confinement.

Upon interaction with the incident electron beam, collective oscillations of conduction electrons, so called localized surface plasmons, are excited within the metallic nanostructures. These oscillatory charge distributions generate highly concentrated near-fields whose magnitude can exceed the threshold required to initiate quantum tunneling of electrons from the metal surface into vacuum.

 

Nanoscale mapping of electric field distributions

Quantitative characterization of the spatial field distribution was achieved using four-dimensional scanning transmission electron microscopy (4D-STEM). This technique enables reconstruction of the local electric field vector by recording momentum-resolved diffraction patterns as a function of beam position. The resulting datasets allow direct visualization of electromagnetic field distributions with nanometer spatial resolution.

The experimental measurements reveal pronounced field localization at the apex regions of the triangular nanostructures and within the nanometer-scale gaps between triangular elements and hemispherical particles. In these regions, electric field strengths exceeding 10 V/nm were experimentally determined. Such field magnitudes fall within the regime required for Fowler–Nordheim tunneling, confirming that the electron-beam-induced plasmonic response can provide sufficient field enhancement to enable quantum-mechanical emission processes.

 

Experimental verification of tunneling emission

A key methodological challenge consisted in distinguishing genuine field emission from competing electron emission mechanisms, in particular secondary electron emission resulting from inelastic scattering processes. To unambiguously identify the dominant emission mechanism, electrical measurements were performed in situ within the electron microscope under controlled bias conditions.

The recorded current–voltage characteristics exhibit the characteristic exponential dependence predicted by Fowler–Nordheim theory, providing strong evidence for tunneling-mediated emission. The measured emission currents significantly exceed values expected from secondary electron processes under comparable irradiation conditions. Moreover, the emission signal vanished immediately upon termination of the electron beam, confirming the direct causal relationship between electron-beam excitation and plasmonically enhanced tunneling emission.

Control experiments further exclude thermal effects and beam-induced structural modifications as primary drivers of the observed phenomenon. The nanostructures exhibited no measurable morphological degradation after repeated exposure cycles. Complementary optical characterization independently confirmed strong plasmonic near-field enhancement at the same spatial locations identified as emission hotspots in the electron microscopy experiments.

 

Implications for nanoscale electron sources

The presented results identify a previously unexplored regime of electron–matter interaction in which the electron beam acts as a localized driver of quantum transport phenomena rather than solely as an observational probe. The demonstrated mechanism enables the realization of highly localized electron sources with potentially ultrafast switching dynamics governed by plasmonic excitation lifetimes.

Such nanoscale emitters may provide new opportunities for the development of next-generation electron microscopy techniques, high-resolution lithographic patterning, and time-resolved studies of ultrafast electronic processes. More generally, the findings highlight the critical role of nanoscale geometry and plasmonic field enhancement in modifying fundamental material responses under electron irradiation.

In conclusion, the interplay between plasmonic nanostructure design and quantum tunneling processes demonstrates that even well-established experimental tools such as electron microscopes can reveal fundamentally new physical behavior when applied to carefully engineered material systems.

 

Publication details

Kenan Elibol et al., Tunneling field emission from nano-optics under electron irradiation. Science Advances 12, eady5421 (2026). DOI: 10.1126/sciadv.ady5421