Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides with experiment and theory (1810.03364v2)

Abstract: Chalcogen vacancies are considered to be the most abundant point defects in two-dimensional (2D) transition-metal dichalcogenide (TMD) semiconductors, and predicted to result in deep in-gap states (IGS). As a result, important features in the optical response of 2D-TMDs have typically been attributed to chalcogen vacancies, with indirect support from Transmission Electron Microscopy (TEM) and Scanning Tunneling Microscopy (STM) images. However, TEM imaging measurements do not provide direct access to the electronic structure of individual defects; and while Scanning Tunneling Spectroscopy (STS) is a direct probe of local electronic structure, the interpretation of the chemical nature of atomically-resolved STM images of point defects in 2D-TMDs can be ambiguous. As a result, the assignment of point defects as vacancies or substitutional atoms of different kinds in 2D-TMDs, and their influence on their electronic properties, has been inconsistent and lacks consensus. Here, we combine low-temperature non-contact atomic force microscopy (nc-AFM), STS, and state-of-the-art ab initio density functional theory (DFT) and GW calculations to determine both the structure and electronic properties of the most abundant individual chalcogen-site defects common to 2D-TMDs. Surprisingly, we observe no IGS for any of the chalcogen defects probed. Our results and analysis strongly suggest that the common chalcogen defects in our 2D-TMDs, prepared and measured in standard environments, are substitutional oxygen rather than vacancies.

• The paper demonstrates that substitutional oxygen is the prevailing point defect in monolayer TMDs rather than chalcogen vacancies.
• It integrates low-temperature nc-AFM, STM/STS, and first-principles DFT/GW simulations to challenge traditional defect interpretations.
• The study reveals the absence of deep in-gap states, prompting a reevaluation of defect engineering for improved optoelectronic properties.

Identifying Substitutional Oxygen as a Point Defect in Monolayer TMDs

The paper "Identifying substitutional oxygen as a prolific point defect in monolayer transition metal dichalcogenides (TMDs) with experiment and theory" published in Nature Communications explores the nature of point defects in two-dimensional transition metal dichalcogenides (2D-TMDs), specifically focusing on monolayer MoSe$_2$ and WS$_2$. Through a combined approach involving low-temperature non-contact atomic force microscopy (nc-AFM), scanning tunneling microscopy and spectroscopy (STM/STS), and first-principles calculations using density functional theory (DFT) and many-body perturbation theory within the GW approximation, the paper challenges the paradigm attributing unexpected electronic properties in TMDs primarily to chalcogen vacancies.

Traditionally, chalcogen vacancies in 2D-TMDs have been considered prevalent due to their low formation energy, as observed in transmission electron microscopy (TEM) images. These vacancies were thought to introduce deep in-gap states (IGS) affecting the optical, transport, and catalytic properties of TMDs. However, the authors present a compelling alternative, identifying substitutional oxygen defects rather than vacancies as the main defect type under standard preparation conditions. This conclusion is strongly supported by the absence of IGS in their detailed spectroscopic measurements and the agreement of their empirical results with simulations.

The paper demonstrates that the prevalent view of chalcogen vacancies as point defects is incomplete, and substitutional oxygen may not only remove these supposed vacancy states but also plays a critical role in the functional properties of TMDs. Their experimental analysis revealed the lack of deep in-gap states via STM/STS for point defects in samples grown by molecular beam epitaxy (MBE) for MoSe$_2$ and chemical vapor deposition (CVD) for WS$_2$. This observation aligns with DFT and GW theoretical predictions, wherein oxygen substituting for chalcogen atoms does not give rise to in-gap states due to its isoelectronic nature with chalcogen atoms.

Another significant outcome from this work includes simulated nc-AFM images that showed close resemblance to those from experiments, which supported the conclusion of substitutional oxygen defects. Furthermore, these defects' electronic structures under the GW approach confirmed the lack of deep in-gap states contrasting with the traditional expectations for chalcogen vacancies.

Implications from this paper are multifaceted; practically, they suggest a reevaluation of defect engineering strategies within TMDs, highlighting the role of atmospheric and processing conditions on defect formation. Oxygen substitution, which is frequently overlooked in TEM studies due to its weak contrast compared to vacancy defects, should be considered as a controlling factor in the optoelectronic properties of TMDs. Theoretically, it calls for improved understanding and characterization of defects in layered materials, challenging the community to develop and optimize material synthesis techniques to tailor desired electronic properties.

Overall, the findings contribute vital insights into defect characterization in 2D-TMD systems, providing groundwork that can influence future research directions in the role of defects in modulating TMD-related devices' electrical and optical properties.