Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

TL;DR

This study demonstrates how alloying in transition metal dichalcogenide monolayers allows for precise engineering of spin-orbit coupling and optical properties, enabling enhanced control over valley polarization and excitonic states for optoelectronic applications.

Contribution

It introduces the synthesis and analysis of Mo$_{(1-x)}$W$_{x}$Se$_2$ alloys, revealing non-linear tuning of spin-orbit effects and optical behaviors, advancing the understanding of alloy-based TMDCs.

Findings

Valley polarization increases non-linearly with tungsten concentration.

40% tungsten achieves valley polarization comparable to WSe2.

Temperature-dependent PL behavior varies significantly between MoSe2 and WSe2.

Abstract

Transition metal dichalcogenide (TMDC) monolayers are newly discovered semiconductors for a wide range of applications in electronics and optoelectronics. Most studies have focused on binary monolayers that share common properties: direct optical bandgap, spin-orbit (SO) splittings of hundreds of meV, light-matter interaction dominated by robust excitons and coupled spin-valley states of electrons. Studies on alloy-based monolayers are more recent, yet they may not only extend the possibilities for TMDC applications through specific engineering but also help understanding the differences between each binary material. Here, we synthesized highly crystalline Mo$_{(1-x)}$W$_{x}$Se$_2$ to show engineering of the direct optical bandgap and the SO coupling in ternary alloy monolayers. We investigate the impact of the tuning of the SO spin splitting on the optical and polarization properties. We show a non-linear increase of the optically generated valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2. We also probe the impact of the tuning of the conduction band SO spin splitting on the bright versus dark state population i.e. PL emission intensity. We show that the MoSe2 PL intensity decreases as a function of temperature by an order of magnitude, whereas for WSe2 we measure surprisingly an order of magnitude increase over the same temperature range (T=4-300K). The ternary material shows a trend between these two extreme behaviors. These results show the strong potential of SO engineering in ternary TMDC alloys for optoelectronics and applications based on electron spin- and valley-control.