Upconversion electroluminescence in 2D semiconductors integrated with plasmonic tunnel junctions

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Conventional light-emitting diodes (LEDs) rely on electron-hole recombination for light emission.
• Plasmonic tunnel junctions offer an alternative electroluminescent system driven by electron tunneling and plasmonic fields.
• Previous research demonstrated upconversion electroluminescence in nanoscopic junctions with luminescent molecules.

Purpose of the Study:

• To investigate upconversion electroluminescence in macroscopic van der Waals plasmonic tunnel junctions.
• To explore the underlying mechanisms of light emission in these novel devices.
• To understand the role of semiconductor excitons and plasmonic interactions.

Main Methods:

• Fabrication of macroscopic van der Waals tunnel junctions using gold and few-layer graphene electrodes.
• Incorporation of a hexagonal boron nitride (hBN) tunnel barrier (~2 nm) and a monolayer semiconductor.
• Characterization of electroluminescence under low conductance and low power density conditions.
• Analysis of emission intensities and scaling relationships.

Main Results:

• Observed upconversion electroluminescence in macroscopic junctions, where emitted photon energy exceeds excitation electron energy.
• Triggering of semiconductor ground exciton emission at electron energies below the optical gap.
• Operation in a low conductance (<10^-6 S) and low power density (<10² W cm^-2) regime.
• Identification of inelastic electron tunneling dipoles and ultrafast hot carrier transfer as key mechanisms.

Conclusions:

• Upconversion electroluminescence is achievable in macroscopic van der Waals plasmonic tunnel junctions.
• The observed phenomenon is driven by inelastic electron tunneling and hot carrier transfer, not conventional recombination.
• These findings open new avenues for designing advanced light-emitting devices based on plasmonic and quantum effects.