Triplet excitation and electroluminescence from a supramolecular monolayer embedded in a boron nitride tunnel barrier

TL;DR

This paper demonstrates a novel van der Waals heterostructure where tunneling electrons excite triplet and singlet states in organic molecules embedded in a boron nitride barrier, enabling electroluminescence and up-conversion.

Contribution

It introduces a new molecular/2D hybrid tunneling diode that enables controlled excitation of triplet states via electron tunneling, a process not achievable with optical methods.

Findings

Electrons tunnel through the hBN/molecular barrier under bias.

Molecular electroluminescence occurs with photon energy > eVSD.

Triplet states are directly and indirectly excited via inelastic scattering.

Abstract

We show that ordered monolayers of organic molecules stabilized by hydrogen bonding on the surface of exfoliated few-layer hexagonal boron nitride (hBN) flakes may be incorporated into van der Waals heterostructures with integral few-layer graphene contacts forming a molecular/2D hybrid tunneling diode. Electrons can tunnel from through the hBN/molecular barrier under an applied voltage VSD and we observe molecular electroluminescence from an excited singlet state with an emitted photon energy > eVSD, indicating up-conversion by energies up to ~ 1 eV. We show that tunnelling electrons excite embedded molecules into singlet states in a two-step process via an intermediate triplet state through inelastic scattering and also observe direct emission from the triplet state. These heterostructures provide a solid-state device in which spin-triplet states, which cannot be generated by optical transitions, can be controllably excited and provide a new route to investigate the physics, chemistry and quantum spin-based applications of triplet generation, emission and molecular photon up-conversion.