Imposing biaxial strain on 2D layered materials by liquid-induced swelling of supporting polymer

Abstract

2D layered materials promise to revolutionize the field of electronics, photonics, optoelectronics, energy storage, and sensing, etc. 2D materials have exceptional mechanical properties, with critical elongation >10%. Employing the strain to manipulate the electronic structure of these 2D materials could lead to further improvement of their implementation in many aspects. The ease of manipulation of their electronic structure can be one of the critical factors for their utilization in photonic devices. Apart from the strain, which decreases (increases) the bandgap energy at the rate of ~100 meV under 1% of biaxial tension (compression), also the layer number causes bandgap energy change of, e.g., 0.5 eV between bulk (1.3 eV) and monolayer MoS2 (1.8 eV). In our work, we focus on using the swelling behavior of PMMA/SU8 polymer in methanol to impose the strain on 2D layered materials. In the first trials, we have shown that it is possible to reach a strain gradient from 0 to ~0.5% of biaxial strain via simple swelling of polymer substrates, both for graphene [1] and transition metal di-chalcogenides (TMDC) like MoS2. Raman spectroscopy was used to probe the lattice strain in the materials through measuring changes of vibrational frequencies, and photoluminescence was used to probe the strain-induced bandgap character and energy in TMDC at room temperature. The surface corrugation of the 2D material after the soaking was recorded with the help of atomic force microscope (AFM).2D layered materials promise to revolutionize the field of electronics, photonics, optoelectronics, energy storage, and sensing, etc. 2D materials have exceptional mechanical properties, with critical elongation >10%. Employing the strain to manipulate the electronic structure of these 2D materials could lead to further improvement of their implementation in many aspects. The ease of manipulation of their electronic structure can be one of the critical factors for their utilization in photonic devices. Apart from the strain, which decreases (increases) the bandgap energy at the rate of ~100 meV under 1% of biaxial tension (compression), also the layer number causes bandgap energy change of, e.g., 0.5 eV between bulk (1.3 eV) and monolayer MoS2 (1.8 eV). In our work, we focus on using the swelling behavior of PMMA/SU8 polymer in methanol to impose the strain on 2D layered materials. In the first trials, we have shown that it is possible to reach a strain gradient from 0 to ~0.5% of biaxial strain via simple swelling of polymer substrates, both for graphene [1] and transition metal di-chalcogenides (TMDC) like MoS2. Raman spectroscopy was used to probe the lattice strain in the materials through measuring changes of vibrational frequencies, and photoluminescence was used to probe the strain-induced bandgap character and energy in TMDC at room temperature. The surface corrugation of the 2D material after the soaking was recorded with the help of atomic force microscope (AFM).