Observation of quantum tunneling between two plasmonic nanoparticles

Area of Science:

• Plasmonics
• Quantum mechanics
• Nanotechnology

Background:

• Metallic nanoparticle dimers exhibit plasmon resonances crucial for sensing, spectroscopy, and nanoantennas.
• Classical electrodynamics predicts increasing plasmon strength with decreasing particle gap.
• Quantum mechanics suggests electron tunneling diminishes plasmon strength at subnanometer separations.

Purpose of the Study:

• To directly observe plasmon resonances in coupled metallic nanoparticles as gap size approaches atomic dimensions.
• To investigate the transition from classical to quantum plasmonic behavior in nanoparticle dimers.
• To map the evolution of dimer plasmon resonances during controlled nanoparticle manipulation and coalescence.

Main Methods:

• Utilizing a scanning transmission electron microscope (STEM) electron beam for precise manipulation of silver nanoparticle dimers.
• Employing electron energy-loss spectroscopy (EELS) for in situ dynamic spectroscopy of plasmonic properties.
• Controlling nanoparticle convergence and coalescence to study plasmon evolution across various gap sizes.

Main Results:

• Observed redshift in the dominant dipolar peak as nanoparticle separation decreased, consistent with classical predictions.
• Noted reduced intensity of the dipolar peak at gaps <0.5 nm, supporting quantum theories of electron tunneling.
• Documented the disappearance of the bonding dipolar mode and emergence of a charge transfer mode upon particle overlap.

Conclusions:

• Demonstrated the first full spectral mapping of dimer plasmon evolution from classical to quantum regimes.
• Provided direct experimental evidence for quantum mechanical effects, specifically electron tunneling, influencing plasmon resonances.
• Opened new avenues for in situ nanoassembly and quantum regime analysis of nanostructures.