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/two-dimensional hybrid tunneling diode. Electrons can tunnel 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 hν > eVSD, indicating upconversion by energies up to ∼1 eV. We show that tunneling 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 upconversion.
Highlights
Two-dimensional supramolecular arrays stabilized by noncovalent interactions provide a highly flexible route to the spatial organization, down to the molecular scale, of functional molecules on a surface.[1−4] While this route to surface patterning has been successful in positioning chemical groups within adsorbed monolayers, the noncovalent nature of the stabilizing interactions has limited the possibilities to explore the transport of charge through the component molecules
The techniques used to fabricate van der Waals heterostructures,[8] such as a tunnel diode formed by placing few-layer hexagonal boron nitride between two graphene layers,[9] offer an alternative approach
In this Letter we show that a similar device architecture may be employed to embed a supramolecular monolayer between two hexagonal boron nitride (hBN) tunnel barriers, forming a hybrid molecular/two-dimensional (2D) device
Summary
We use polymer stamp-assisted van der Waals assembly[10−12] to sequentially pick up flakes of fewlayer graphene (FLG) and hBN. hBN flakes with adsorbed monolayers of organic molecules can be picked up and deposited as part of the assembly process in the same way as pristine flakes, allowing the integration of molecular layers within van der Waals heterostructures. While the architecture of our devices is significantly different to conventional OLEDs, we draw analogies with luminescence generated by the tip of a scanning tunneling microscope[27,28] (STML) for which the electrodes and photon source are in close proximity, and, the widely accepted mechanism for molecular excitation in STML is via inelastic scattering, and there are reports of photon upconversion.[29−31] the efficiency of our devices, typically 10−6−10−8 photons per electron, is similar to that observed in STML experiments.[27] The coupling of tip-induced localized plasmons to an adsorbed molecule is considered to be the most significant STML emission process;[27] this is unlikely to be significant in our devices, since the plasmon energy is much smaller[32] (in the range of 100s of meV) for our FLG contacts.
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