Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides (1805.00127v1)

Abstract: Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

Characterization and Control of Intrinsic Defects in Monolayer TMDs for Enhanced Light Emission

The paper investigates the atomic and electronic structure of intrinsic point defects in monolayer transition-metal dichalcogenides (TMDs) and their subsequent impact on the material's optoelectronic properties. This research is particularly relevant due to the potential applications of TMDs in advanced electronics and optoelectronics, owing to their novel optical and electronic properties. However, intrinsic defects in these materials create barriers by thwarting the ideal performance of these semiconductors.

In this paper, single crystals of TMDs, specifically MoSe$_2$ and WSe$_2$, were synthesized by chemical vapor transport (CVT) and self-flux growth techniques. The authors focus on characterizing the primary intrinsic defects, namely metal vacancies and chalcogen antisites, using advanced atomic imaging techniques like scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM).

The paper highlights that self-flux growth can considerably reduce defect densities compared to CVT methods, achieving defect densities below $10^{11} \, \text{cm}^{-2}$. Such reduction in defect density directly correlates with improved material properties, including a dramatic hundredfold enhancement in radiative recombination efficiency. The implication is that controlling defects to this degree allows TMD monolayers to approach their theoretical performance limits, though it does not yet surpass the purity achieved in traditional semiconductors such as silicon.

The synthesis approaches were systematically studied: CVT, post-annealed CVT (t-CVT), and self-flux growth. The examinations reveal that the self-flux growth method significantly minimizes defect densities, facilitating exceptional optoelectronic quality.

Defect impact is quantitatively explored through density functional theory (DFT) to calculate defect formation energies, aligning the theoretical predictions with STM and STS measurements. The paper found that transition-metal vacancies exhibit high formation energies but significantly impact electronic properties by introducing deep states that act as electron acceptors.

Furthermore, photoluminescence (PL) measurements demonstrate the correlation between defect density and optical performance. The self-flux monolayer exhibits remarkable PL emission enhancement and narrower full-width at half-maximum (FWHM) compared to CVT layers, further evidencing the advantages of low defect densities.

A notable contribution of this paper is the exploration of the relationship between defect control and excitonic properties in monolayer TMDs. By examining PL responses, the researchers elucidate the role of exciton localization due to defect sites and non-radiative recombination processes, which must be minimized to achieve high quantum yields.

In conclusion, the authors provide a rigorous analysis that fundamentally connects the synthesis and defect control in monolayer TMDs with their optoelectronic functionality. This work lays an essential foundation for future efforts aimed at improving material quality, moving towards achieving the potential promised by TMDs in real-world devices. Future research is likely to focus on further reducing defect densities and exploring alternative synthetic techniques to more closely match the benchmark set by classical semiconductors.