Exciting Optoelectronic Properties Of Stacks Of Two-Dimensional Crystals

A team of scientists from the Faculty of Physics, University of Warsaw, the French National High Magnetic Field Laboratory CNRS Grenoble, the Universities of Manchester and Exeter as well as the National Institute for Materials Science Tsukuba reported an intriguing behavior of electrically-driven light emission from van der Waals heterostructures. They observed that light emission sets in at much lower applied voltages than the corresponding energy of emitted photons. In general, for light emitting diodes it is expected that one has to roughly apply the voltage corresponding to the energy of the emitted photons to observe emission. In case of the investigated heterostructures, an emission of photons with an energy of 1.9 eV emerges at much lower applied voltages of around 1.3 V, which is a remarkable upconverted emission. The results were published in Nature Communications. .

The extensive research on graphene that started more than a decade ago gave rise to a new vibrant field of research in condensed matter physics, dedicated to the so called two-dimensional crystals. Alike graphene, there is a broad range of other layered materials that can be thinned down to a single layer. These single layers can show drastically different properties compared to its few-layer or bulk counterparts. The portfolio of available materials includes a large variety of insulating, semiconducting, metallic and even superconducting materials. Such two dimensional crystals are being and will be intensely studied in the future. The recently introduced concept of so called van der Waals (vdW) heterostructures, for which one places many two-dimensional crystals on top of each other to form a kind of artificial crystal, however, opens up an even larger playground for applications and basic studies. The structures are called van der Waals heterostructures, since there are no covalent bonds between different crystals in the out-of plane direction leaving only van der Waals interaction between different layers. Importantly, the lack of covalent bonds lifts the strong limitations regarding the combination of different materials (due to their different lattice constants), so that with such structures, in principle, an arbitrary combination of different materials becomes possible. We hence deal with structures built in analogy to Lego building blocks, but on the nanoscale. Such a NanoLego opens up completely new functionalities on the nanoscale – with great prospects for innovative solutions in fields like flexible electronics, optoelectronics or solar energy conversion.

A team of scientists from the Faculty of Physics, University of Warsaw, the French National High Magnetic Field Laboratory (Grenoble, France), the University of Manchester (UK), the University of Exeter (UK) and the National Institute for Materials Science (Tsukuba, Japan) now investigated the electrically-driven light emission of a structure consisting of two different semiconducting materials (MoS₂, WSe₂) separated by hexagonal boron nitride (hBN) layers. The structure was fabricated in Manchester by the group of Nobel prize winner Prof. Konstantin Novoselov and studied optoelectrically in Warsaw and Grenoble. These complex prototype structures are fabricated using an optimized version of the so called scotch-tape method for which the layers are exfoliated using a special tape and consecutively transferred from the tape to a different substrate under an optical microscope to form a van der Waals heterostructure.

In their recent study published in Nature Communications (which was chosen to be included in the Editor Highlights), the team observed light emission at a much lower voltage than the corresponding energy of the emitted photons. In general, for light emitting diodes (LEDs) one expects that one has to roughly apply the voltage corresponding to the energy of the emitted photons to observe emission, that is why for a red LED a much lower voltage is required to observe emission than for a blue LED. However, in their paper J. Binder and co-authors observe light emission of photons with an energy of 1.9 eV (corresponding to a wavelength of 650 nm) with applied voltages as low as 1.3 V, which is a remarkable upconverted emission. There is however no conflict with the law of conservation of energy. This astonishing result was explained by the authors of the publication to be due to the interaction of strongly bound electron-hole pairs (excitons) for which the electron is situated in one of the two-dimensional semiconducting crystals (MoS₂) and the hole in the other (WSe₂). These so called interlayer excitons are long-lived and can recombine either radiatively (emitting a photon) or non-radiatively (without the emission of a photon). In their publication, the authors could ascribe the emission at higher than expected energies to the excitonic Auger effect, for which one interlayer exciton recombines non-radiatively and transfers the energy and momentum to the other. This way the excitons may reach higher energies as would result from the voltage applied to the heterostructure. This finding is important for future two-dimensional crystal light emitting device engineering as well as for future attempts towards the observation of fundamental phenomena like superfluidity or Bose-Einstein condensation of interlayer excitons in van der Waals heterostructures.