Tuneable physical properties of MoS₂ for optoelectronic devices induced by strain via heat treatment

In the last decades, the number of two-dimensional (2D) materials has grown exponentially, increasing the interest in studying the properties of nanostructures with different natures1. Transition metal dichalcogenides (TMD) represent one of the biggest families of 2D materials with semiconductive properties, their application is promising as building blocks of new nano-electronic devices based on semiconductor 2D materials. The most famous and studied TMD is the molybdenum disulphide (MoS2), a material easy to exfoliate which shows unexpected optical properties when it is in a single-layer structure (1L-MoS2)2. Due to a reduction of the interaction between the layers, 1L-MoS2 shows a different energy band structure that turns the material into a direct band-gap semiconductor in which it is possible to observe an excitonic recombination coupled to an emission of about 1.8 eV. The optical and conduction properties of this material are the most interesting features for building an optoelectronic nano-size device. Their improvement and control are a challenge nowadays. The sensitivity of MoS2 in strain and doping changes allows us to tune the physical properties by applying external stress in controlled environment conditions. Furthermore, different types of synthesis procedures, like mechanical exfoliation or Chemical Vapour Deposition, could bring some defects that reduce the conduction and optical properties efficiencies. To overcome these issues, this work explores the MoS2 single layer supported by different substrates (Au, SiO2/Si and GaN) obtained by different production techniques, Gold-Assisted Exfoliation, Gold-Assisted Transfer3, and Chemical Vapour Deposition (CVD)4 to compare the differences in strain and doping of different synthesis and substrates. In this way, there is a chance to compare defects from MoS2 flakes with various conditions. We performed thermal treatments in a controlled environment to tailor and modify the strain and doping condition of the 1L-MoS2. Applying a treatment from 150 to 300 °C under 2 atm of an Ar or O2 atmosphere makes it possible to modify the sample and observe a significant change in its luminescence. To perform this study micro-Raman spectroscopy, Atomic Force Microscopy, Scanning Tunnelling spectroscopy, Transmission Electron Microscopy and steady-state micro-photoluminescence are performed, to understand the starting condition and the changes applied to the samples. This work shows how it is possible to tailor strain and doping properties of 1L-MoS2 using a gentle treatment, intending to use it as an improved material for nano-optoelectronic devices For this work, the Italian MUR-PNRR project SAMOTHRACE (ECS00000022) and PRIN2022 project “2DIntegratE” (2022RHRZN2) is gratefully acknowledged. 1. Iqbal et al. Microelectron Eng 219, 111152 (2020). 2. Splendiani, A. et al. Nano Lett 10, (2010). 3. Panasci, S. E. et al. Appl Phys Lett 119, 93103 (2021). 4. Esposito, F. et al. Appl Surf Sci 639, 158230 (2023).