Spin-orbit engineering in transition metal dichalcogenide alloy monolayers

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.

Spin-Orbit Engineering in Transition Metal Dichalcogenide Alloy Monolayers

Transition metal dichalcogenide (TMDC) monolayers, particularly those composed of molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂), have garnered considerable attention due to their semiconducting properties which are advantageous for electronic and optoelectronic applications. Traditionally, studies have focused on binary TMDC monolayers, which are characterized by direct optical bandgaps, significant spin-orbit (SO) splittings, and robust excitonic effects. However, a growing interest in alloy-based monolayers seeks to exploit the tunable properties of these materials to enhance their function in various applications.

The paper by Gang Wang and colleagues introduces the synthesis and characterization of Mo${1-x}$W${x}$Se₂, a ternary alloy monolayer, designed for engineering both the direct bandgap and SO coupling. The researchers demonstrate that with increasing tungsten (W) composition, the material properties, such as the valley polarization and photoluminescence (PL) intensity, can be significantly altered. These alterations highlight the potential of Mo${1-x}$W${x}$Se₂ monolayers as versatile components for SO-engineered Van der Waals heterostructures.

The study's experimental methodology includes low-temperature PL spectroscopy, nano-resolution X-ray photoelectron spectroscopy (nano-XPS), and Raman spectroscopy. Key findings demonstrate high-quality crystalline samples with accentuated optical properties: narrow PL linewidths that indicate material purity and distinct shifts in excitonic transitions as a function of tungsten content.

Notably, the PL studies reveal a non-linear increase in valley polarization at higher tungsten concentrations, achieving substantial polarization with only 40% tungsten content, comparable to that of pure WSe₂ monolayers. The observed SO bowing further accentuates how the combination of Mo and W in the lattice affects the electronic structure, which emerges as a promising characteristic for applications in spintronic devices.

The theoretical implications of these findings suggest that adopting different transition metal ratios in TMDC alloys not only enables adjustment of the bandgap and excitonic properties but also provides insights into the fundamental SO coupling mechanisms. Consequently, these alloys exhibit potential for enhancing spin-Hall effects and reducing Auger recombination processes in optoelectronic devices.

Future research could focus on further refining synthesis techniques to reduce defect densities, thereby optimizing the electronic and optical properties of these alloy monolayers. Additionally, comprehensive theoretical models incorporating the observed SO bowing effects will be invaluable for predicting and tailoring the properties of related TMDC systems. This research establishes a foundation for the next generation of materials that leverage the intrinsic properties of TMDC alloys, promising advancements in both practical applications and theoretical understanding.