Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides (1703.02831v1)

Abstract: Since their discovery single-layer semiconducting transition metal dichalcogenides have attracted much attention thanks to their outstanding optical and mechanical properties. Strain engineering in these two-dimensional materials aims to tune their bandgap energy and to modify their optoelectronic properties by the application of external strain. In this paper we demonstrate that biaxial strain, both tensile and compressive, can be applied and released in a timescale of a few seconds in a reproducible way on transition metal dichalcogenides monolayers deposited on polymeric substrates. We can control the amount of biaxial strain applied by letting the substrate expand or compress. To do this we change the substrate temperature and choose materials with a large thermal expansion coefficient. After the investigation of the substrate-dependent strain transfer, we performed micro-differential spectroscopy of four transition metal dichalcogenides monolayers (MoS2, MoSe2, WS2, WSe2) under the application of biaxial strain and measured their optical properties. For tensile strain we observe a redshift of the bandgap that reaches a value as large as 95 meV/% in the case of single-layer WS2 deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe2 < MoS2 < WSe2 < WS2. Theoretical calculations of these four materials under biaxial strain predict the same trend for the material-dependent rates of the shift and reproduce well the features observed in the measured reflectance spectra.

Analysis of Biaxial Strain Effects on Optical Characteristics of Single-Layer TMDCs

In the paper titled "Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides", the authors provide an in-depth exploration of strain engineering as a means to modulate the optoelectronic properties of transition metal dichalcogenides (TMDCs). Utilizing biaxial strain applied via polymer substrates, this paper investigates four distinct TMDC monolayers (MoS₂, MoSe₂, WS₂, and WSe₂) and their optical responses to strain.

Experimental Approach and Numerical Observations

The application of biaxial tensile and compressive strains on TMDC monolayers was achieved by modifying substrate temperature, utilizing materials like polypropylene (PP) with sizable thermal expansion coefficients. Observations indicated that changing substrate temperature leads to measurable biaxial strain, impacting optical features, primarily exciton shifts. Experimental results recorded the bandgap redshift under tensile strain, notably up to 95 meV/% for WS₂ on polypropylene – marking the largest shift amongst the investigated materials. This comparative ranking was MoSe₂ < MoS₂ < WSe₂ < WS₂, demonstrating significant tunability.

Theoretical Insights and Simulations

The theoretical approach encompassed ab-initio calculations, incorporating density functional theory (DFT) and the Bethe-Salpeter equation, validating the observed experimental effects. Notably, theoretical calculations provided an upper-bound prediction for strain-induced optical shifts. The magnitude and hierarchical arrangement of exciton gauge factors across different TMDCs were consistent with predictions, supporting the experimental trend. Theory exhibited higher strain sensitivity in material electronic structures correlated with strong inter-atomic orbital overlap, implicating conduction band shifts as the dominant factor.

Implications and Future Directions

This paper's demonstration of biaxial strain's effectiveness in controlling TMDC optical properties posits a promising avenue for developing future optical modulators or strain sensors. Real-time modulation and exciton tunability present applications in dynamic optical devices. Further developments may explore higher Young’s modulus substrates to enhance strain transfer efficiency, unlocking more drastic property shifts while maintaining monolayer integrity.

The methodology employed herein can be extended to other 2D material systems, proposing utility in varied optoelectronic applications, particularly where rapid response and fine-tuning optical properties are requisite. Continuing exploration in optimizing substrate selection and enhancing theoretical modeling accuracy will bolster the practical translation of these findings. In essence, this research substantiates the potential of biaxial strain engineering in designing adaptable electronic and optical devices, championing advancements in nanoscale material applications.