Triplet Excitation and Electroluminescence from a Supramolecular Monolayer Embedded in a Boron Nitride Tunnel Barrier

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 h > 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 solidstate 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

Efficient Full-Color Boron Nitride Quantum Dots for Thermostable Flexible Displays

“…Graphene and beyond two-dimensional (2D) materials, with extraordinary physical and chemical properties, have received a wide range of interest in electronics, optoelectronics, energy conversion, and catalysis. − Among them, hexagonal boron nitride nanosheets (BNNSs) could yield good electrical insulation, high thermal conductivity, superb antioxidation stability, prominent biocompatibility, and low toxicity, holding great promise in a wide range of applications, such as semiconductor devices, insulating thermal management, biomedical and photoelectric field. − Up until now, optical aspects of BN have not been fully explored.…”

Fluorescence and Electroluminescence of J-Aggregated Polythiophene Monolayers on Hexagonal Boron Nitride

“…We estimate τ T ∼ 0.13 μs from the measured current density (0.37 pA nm –2 ) close to the threshold for EL, assuming an effective fluorophore area of 4 nm 2 . In common with recent studies of EL from organic molecules using scanning tunnelling luminescence, ,, and molecular/2D hybrid tunnel devices, we identify the intermediate state as the spin triplet, T 1 , since other excited states, for example vibrationally excited states, are expected to have much shorter lifetimes. In this scenario, a molecule is excited into a triplet state, T 1 , via inelastic scattering of a tunnelling electron followed by a further excitation to an excited singlet, S 1 , state which relaxes through the emission of a photon.…”

“…The current–voltage dependence of a completed device consisting of hBN tunnel barriers with thicknesses of 1 ML (top) and 2 ML (bottom), respectively, was acquired at room temperature and is highly nonlinear, as expected for a tunnel diode. ,− As the bias, V , applied to the device was increased, electroluminescence was observed. The electroluminescence (EL) and photoluminescence (PL) spectra of the encapsulated P3DT show dominant peaks at E EL = 1.93 eV and E PL = 1.95 eV, respectively, which are broadened with respect to uncapped P3DT monolayers, with the full width half-maximum increasing from 68 to 140 meV between uncapped and capped room temperature photoluminescence measurements; this increase is likely due to variations in the local environment of the encapsulated P3DT within the heterostructures, for example, due to structural defects such as wrinkles and blisters which are common in van der Waals heterostructures .…”

“…It is also possible to incorporate P3DT monolayers on hBN, similar to those discussed above, into van der Waals heterostructures to produce a hybrid polymer/2D electrical device. We have adapted a device architecture, which was developed recently to form analogue structures with embedded small organic molecules, to form a heterostructure in which a P3DT monolayer is embedded between two few-layer hBN tunnel barriers and overlapping few-layer graphene (FLG) contacts (see Figure a) . This device structure is further encapsulated between two thicker (>10 nm) hBN flakes.…”

“…It has recently been shown that H- and J-type aggregation, and therefore the optical properties, of small organic molecules can be controlled through the variation of their relative position and orientation within self-assembled domains which form on the surface of hexagonal boron nitride (hBN) and are stabilized, for example, through hydrogen-bonding interactions. − Molecular monolayers adsorbed on supporting flakes of hBN, an insulating 2D material, can also be integrated into hybrid van der Waals heterostructures, forming tunnel diodes, which enable the electroluminescence from the molecular layers to be explored, , resulting in various effects such as photon up-conversion and selective spin triplet excitation. − Polythiophene can also be adsorbed on the surface of hBN; the polymer backbone can be resolved within monolayer-thick islands using atomic force microscopy under ambient conditions allowing the molecular scale characterization of these self-assembled monolayers . In light of these results, it is interesting to consider whether the order imparted by adsorption influences the optical properties of a polymer as previously reported for small molecules. , …”