Biaxial strain tuning of the optical properties of single-layer transition metal dichalcogenides

• Published in: NPJ 2D materials and applications Vol. 1; no. 1; pp. 1 - 7
• Main Authors: Frisenda, Riccardo, Drüppel, Matthias, Schmidt, Robert, Michaelis de Vasconcellos, Steffen, Perez de Lara, David, Bratschitsch, Rudolf, Rohlfing, Michael, Castellanos-Gomez, Andres
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25-05-2017; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/357/1018; 639/925/357/1018; Chalcogenides; Chemistry and Materials Science; Compressing; Compressive properties; Emitters; Energy gap; Materials Science; Mechanical properties; Molybdenum compounds; Molybdenum disulfide; Monolayers; Nanotechnology; Optical properties; Optoelectronics; Photovoltaic cells; Red shift; Solar cells; Spectrum analysis; Substrates; Surfaces and Interfaces; Tensile strain; Thermal expansion; Thin Films; Time compression; Transition metal compounds; Tungsten disulfide; Tuning; Two dimensional materials
• ISSN: 2397-7132; 2397-7132
• DOI: 10.1038/s41699-017-0013-7

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 (MoS
2
, MoSe
2
, WS
2
, WSe
2
) 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 WS
2
deposited on polypropylene. The observed bandgap shifts as a function of substrate extension/compression follow the order MoSe
2
 < MoS
2
 < WSe
2
 < WS
2
. 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.
Strain engineering: Tuning the bandgap of 2D materials
The bandgap of two-dimensional semiconducting materials can be easily tuned in real time by stretching or compressing them. An international team of researcher led by Dr. Andres Castellanos-Gomez at IMDEA Nanoscience, Spain, studied the optical properties of single-atom thick two-dimensional semiconductors under the application of tensile or compressive biaxial strain. In order to apply the strain the researchers exploited the thermal expansion or compression of the different substrates carrying the atomically thin materials and then compared their results to atomistic simulations. This strain method can be applied in a fast and reversible way and it leads to large changes in the band structure of these semiconducting materials. Research into strain engineering two-dimensional materials may help us in fabricating novel devices like color-changing light emitters or novel and more efficient solar cells.