We started the project with the first successful demonstration of valley polarisation of charge carriers in a TMDC material via electrical injection of spin-polarized charge carriers [Nano Lett. 16, 5792–5797 (2016), DOI:10.1021/acs.nanolett.6b02527] which was detected optically. Following was the inverse experiment, in which circularly polarised light was used to generate valley polarized charge carriers in TMDs which was successfully transferred into graphene and detected electrically [ACS Nano 11, 11678–11686 (2017), DOI:10.1021/acsnano.7b06800] This is one of two simultaneously published experimental papers that started the field of optospintronics of 2D materials. On a pure electrical device, we also showed that quantum point contacts in MoS2 and by extension, semiconducting TMDCs carry current via valley/spin polarised conduction modes that can be electrically selected and therefore could, in principle be used for electrically generating valley polarized currents [Nature Communications 8, (2017), DOI:10.1038/s41467-017-02047-5]. This was the first surprise of the projects, since we originally thought that we would need to use quantum dots for this purpose but it turned out that a simpler structure could serve the same purpose. We also discovered that PtSe2 is magnetic, with magnetism related to Pt vacancies and that depending on the number of layers, we can tune the magnetic response from a ferromagnetic to an antiferromagnetic one [Nature Nanotechnology 14, 674–678 (2019), DOI:10.1038/s41565-019-0467-1]. PtSe2 could then be an interesting material for injecting spin polarized charge carriers which could then result in valley polarisation of charge carriers in TMDCs.
While working on TMDC semiconducting heterostructures, we have been successful in achieving and controlling exciton transport, which was a way to achieve the main goal of this project, since it achieved valley currents in the absence of charge currents. We have achieved this using excitons instead of single charges as originally planned. While pursuing this direction, we first managed to build the first room-temperature exciton transistor [Nature 560, 340–344 (2018), DOI:10.1038/s41586-018-0357-y] which attracted a lot of attention. This is a device analogous to a field effect transistor but with electrical control over the currents of excitons, instead of individual charge carriers. We continued by demonstrating purely electrical control over the circular polarisation of light emitted from the device [Nature Photonics 13, 131–136 (2019), DOI:10.1038/s41566-018-0325-y] and valley polarized excitonic currents and transistors [Nat. Nanotechnol. 14, 1104–1109 (2019), DOI:10.1038/s41565-019-0559-y].
