ReS₂ Maintains Layer-Independent Defect States from Monolayer to Bulk, Enabling Stable Single-Photon Emission

Defects inevitably arise in two-dimensional materials and significantly influence their performance in electronic and optoelectronic devices. Nikhilesh Maity, Shibu Meher, Manoj Dey, and colleagues at the Materials Research Centre, Indian Institute of Science, now demonstrate a remarkable property of rhenium disulfide (ReS2), revealing that certain defect energy levels remain surprisingly consistent regardless of the material’s thickness. This invariance, stemming from a balance between electronic energy minimization, structural relaxation, and weak interlayer coupling, sets ReS2 apart from other similar materials. The team’s findings establish ReS2 as a promising platform for developing thickness-independent optoelectronic and photonic devices, including potential single-photon emitters, and provide a fundamental understanding of how defects behave in layered semiconductors.

ReS2 Defect Levels Persist Across Layer Thickness

Defects in two-dimensional (2D) semiconductors profoundly influence their electronic, optical, catalytic, and quantum properties. This research investigates the behaviour of defects in ReS2, examining how their energy levels change as the material’s thickness varies from a single atomic layer to bulk material. The study demonstrates that certain defect levels remain largely unchanged by layer thickness, while others shift in energy, revealing a complex interplay between defect formation, layer-dependent screening, and quantum confinement effects. These findings provide valuable insights into the behaviour of defects in 2D materials and offer a pathway towards controlling their properties for advanced applications.

Researchers report the persistence of layer-tolerant defect levels in rhenium disulfide (ReS2), where both donor- and acceptor-type charge transition levels remain nearly unchanged from monolayer to bulk in both stacking arrangements. The associated two-level quantum system also retains its character across different thicknesses, enabling ReS2 to serve as a platform for layer-tolerant single-photon emitters. This invariance arises from the interplay between electronic energy minimization and structural relaxation, which together counteract quantum confinement and reduced dielectric screening. Additionally, the intrinsically weak interlayer coupling in ReS2 plays a crucial role.

Layered ReS2, Defects and Anisotropy

This research focuses on understanding the properties of 2D materials, particularly transition metal dichalcogenides like ReS2, and how defects affect them. The studies involve calculations using Density Functional Theory (DFT) to investigate these materials, focusing on how the number of layers impacts their electronic, optical, and structural properties, including anisotropy and excitonic behaviour. A key aspect of the research is understanding the formation, behaviour, and impact of defects, such as missing atoms or impurities, on the electronic structure and optical properties of 2D materials, accurately calculating their energy and ionization levels.

The research also focuses on developing and refining computational methods for accurately calculating defect properties in 2D materials, addressing challenges like finite model size, charge imbalances, and appropriate exchange-correlation functionals. The optical properties and anisotropy of 2D materials are investigated, with a focus on how these properties are influenced by layer number, stacking order, and the presence of defects. The foundational PAW method and PBE functional are employed, alongside Van der Waals density functionals crucial for accurately describing interlayer interactions.

The studies establish the fundamental properties of monolayer and few-layer ReS2, including its semiconducting behaviour, in-plane anisotropy, and optical properties, highlighting its unique characteristics compared to other transition metal dichalcogenides. Experimental synthesis and characterization of ReS2 and ReSe2 crystals are performed, with in-plane anisotropy investigated using Raman spectroscopy and transmission electron microscopy.

Computational accuracy is paramount, as the emphasis on DFT methodology and correction schemes highlights the importance of obtaining accurate results for defect properties. Layer-dependent behaviour is clearly demonstrated, as the properties of 2D materials are strongly dependent on the number of layers and stacking order. Defects play a crucial role in modifying the electronic and optical properties of 2D materials, and understanding their behaviour is essential for tailoring these materials for specific applications.

Rhenium Disulfide Exhibits Layer-Tolerant Defect Stability

Researchers have demonstrated that rhenium disulfide (ReS2) exhibits remarkably stable defect levels as its thickness varies from a single layer to bulk material. The energy levels associated with these defects remain nearly constant regardless of the number of layers, a behaviour termed layer-tolerance, and the energy gap between key states within the defect system persists at approximately 0. 40 to 0. 39 electron volts across different thicknesses. This invariance arises from a balance between electronic and structural relaxation within the material, counteracting the effects of confinement and reduced dielectric screening. The team attributes this unique stability primarily to the weak interlayer coupling present in ReS2, distinguishing it from other two-dimensional materials.

This finding establishes ReS2 as a promising platform for applications requiring consistent defect properties, regardless of layer number, particularly in optoelectronic and photonic devices. The researchers acknowledge that their work focuses on specific stacking configurations and that further investigation may be needed to fully understand the behaviour of defects in all possible arrangements. Future research could explore the potential of manipulating these stable defects for quantum technologies and advanced device designs.