  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.
