Periodic Reporting for period 4 - Valleys (Valley and spin devices based on two-dimensional semiconductors)
Période du rapport: 2021-01-01 au 2021-06-30
The main objective of this research proposal is to realize new types of electronic devices based on the valley/spin degree of freedom in two-dimensional semiconductors from the transition metal dichalcogenide family. These materials are analogous to graphene but have a direct band gap. Together with the unique band structure, this allows manipulating the spin and valley degrees of freedom interchangeably. In addition, it can give rise to a mechanism for protecting the spin which could in future result in very high spin relaxation lengths. This proposal will explore various spin/valley injection mechanisms as well as detection mechanisms with the goal of realizing an all-electric valleytronic device. Various new device architectures will be realized in the central part of the proposal.
The research we propose here will address practical applications and fundamental questions related to the main feature that distinguishes 2D TMDC materials from other semiconductors: the valley/spin degree of freedom. The lack of inversion symmetry could lead to interesting new physics due to strong spin-orbit and spin-valley couplings that could be exploited for the construction of an entirely new type of electronics, called valleytronics. Results of this research will enrich the applications of 2D materials and possibly result in a new paradigm for computing.
The research we propose here will address practical applications and fundamental questions related to the main feature that distinguishes 2D TMDC materials from other semiconductors: the valley/spin degree of freedom. The lack of inversion symmetry could lead to interesting new physics due to strong spin-orbit and spin-valley couplings that could be exploited for the construction of an entirely new type of electronics, called valleytronics. Results of this research will enrich the applications of 2D materials and possibly result in a new paradigm for computing.
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].
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].
The work until the end of the project will focus on realizing practical devices based on the valley degree of freedom, including the newly opened avenue of excitonic devices with offer new opportunities for realizing the main goals of the project. Materials-related aspects will also be explored with the aim of identifying and addressing limits to these devices that are imposed by the material quality.
Results of this research will enrich the applications of 2D materials and possibly result in a new paradigm for computing.
Results of this research will enrich the applications of 2D materials and possibly result in a new paradigm for computing.