Electroluminescence from a phthalocyanine monolayer encapsulated in a van der Waals tunnel diode

Monolayers of free base phthalocyanine (H2Pc) are grown on monolayer and few-layer exfoliated flakes of hexagonal boron nitride (hBN) which are subsequently integrated into a van der Waals tunnel diode. This heterostructure consists of two thin hBN flakes between which the H2Pc monolayer is sandwiched and also incorporates upper and lower few-layer graphene contacts. When a voltage is applied between the contacts, a tunnel current flows and the embedded molecules can be excited resulting in the emission of photons with wavelengths which are close to the peaks observed in photoluminescence. We also observe electroluminescence at voltages where the energy gained by a tunnelling electron is lower than the energy of the emitted photon implying a multi-electron excitation pathway which we attribute to the formation of an intermediate triplet state. Our results provide insights into the differences in excitation and relaxation of molecules in supramolecular monolayers and bulk crystals and we discuss how the alignment of the energy levels of the molecules and contact layers determine the emission process.

Automated summary

Monolayers of H2Pc are grown on hBN flakes and integrated into a van der Waals tunnel diode. Tunnel current generates photon emission close to the peaks observed in photoluminescence, and the electroluminescence suggests a multi-electron excitation pathway. The study provides insights into the excitation and relaxation of molecules in supramolecular monolayers and bulk crystals, as well as the role of energy level alignment in the emission process.