Electrical and optical properties of transition metal dichalcogenides on talc dielectrics.

Advanced van der Waals (vdW) heterostructure devices rely on the incorporation of high quality dielectric materials which need to possess a low defect density as well as being atomically smooth and uniform. In this work we explore the use of talc dielectrics as a potentially clean alternative substrate to hexagonal boron nitride (hBN) for few-layer transition metal dichalcogenide (TMDC) transistors and excitonic TMDC monolayers. We find that talc dielectric transistors show small hysteresis which does not depend strongly on sweep rate and show negligible leakage current for our studied dielectric thicknesses. We also show narrow photoluminescence linewidths down to 10 meV for different TMDC monolayers on talc which highlights that talc is a promising material for future van der Waals devices.

Electrical tuning of optically active interlayer excitons in bilayer MoS2.

Interlayer (IL) excitons, comprising electrons and holes residing in different layers of van der Waals bonded two-dimensional semiconductors, have opened new opportunities for room-temperature excitonic devices. So far, two-dimensional IL excitons have been realized in heterobilayers with type-II band alignment. However, the small oscillator strength of the resulting IL excitons and difficulties with producing heterostructures with definite crystal orientation over large areas have challenged the practical applicability of this design. Here, following the theoretical prediction and recent experimental confirmation of the existence of IL excitons in bilayer MoS2, we demonstrate the electrical control of such excitons up to room temperature. We find that the IL excitonic states preserve their large oscillator strength as their energies are manipulated by the electric field. We attribute this effect to the mixing of the pure IL excitons with intralayer excitons localized in a single layer. By applying an electric field perpendicular to the bilayer MoS2 crystal plane, excitons with IL character split into two peaks with an X-shaped field dependence as a clear fingerprint of the shift of the monolayer bands with respect to each other. Finally, we demonstrate the full control of the energies of IL excitons distributed homogeneously over a large area of our device.

Heterostructures formed through abraded van der Waals materials.

To fully exploit van der Waals materials and their vertically stacked heterostructures, new mass-scalable production routes which are low cost but preserve the high electronic and optical quality of the single crystals are required. Here, we demonstrate an approach to realise a variety of functional heterostructures based on van der Waals nanocrystal films produced through the mechanical abrasion of bulk powders. We find significant performance enhancements in abraded heterostructures compared to those fabricated through inkjet printing of nanocrystal dispersions. To highlight the simplicity, applicability and scalability of the device fabrication, we demonstrate a multitude of different functional heterostructures such as resistors, capacitors and photovoltaics. We also demonstrate the creation of energy harvesting devices, such as large area catalytically active coatings for the hydrogen evolution reaction and enhanced triboelectric nanogenerator performance in multilayer films. The ease of device production makes this a promising technological route for up-scalable films and heterostructures.