MoS₂ Monolayer Physical Properties—Engineering in Temperature-Controlled Environments

The growing request for high-performance and low-power consumption devices¹ with nanoscale dimensions has led to consider Two-Dimensional (2D) materials as the new building bricks for nano-devices based on semiconductors. Thanks to the surprising new properties that matter shows at the nanoscale, they represent the best candidates to overcome the shrinking limit of electronic devices. Among the wide family of 2D materials, Transition Metal Dichalcogenides (TMDs) are one of the most studied and promising groups displaying semiconductive properties. In particular, Molybdenum Disulphide (MoS2) represents the most suitable TMD for optoelectronic and high-current applications¹, thanks to the interesting properties that the material has as a single-layer (1L-MoS2) structure. When the material is reduced to just one layer, it is possible to observe a rearrangement of the band structure which turns the 1L-MoS₂ into a direct band-gap semiconductor. In this configuration, direct exciton recombination occurs and a fluorescence emission of about 1.8 eV appears, which is useful for optoelectronic applications². Despite the broad study of the physical properties of this material and some device applications, there is a lack of knowledge and a clear interpretation of how the substrate can influence the conduction and optical properties of 1L-MoS₂. In addition, the synthesis procedure has a significant role in defect formation, affecting the physical features of the flakes. Furthermore, applying external stress in controlled conditions gives a chance to tune the strain and doping³, due to a re-arrangement of the interaction between the 1L-MoS₂ and the substrate.<br/>To better understand all these aspects, we studied the interaction of a MoS₂ single-layer with different substrates (gold, silicon dioxide, gallium nitride) obtained via Gold-Assisted Transfer (GAT), Gold-Assisted Exfoliation⁴ and Chemical Vapour Deposition (CVD)⁵. This approach allows us to compare the effects of several supporting materials and the pros and cons of all the syntheses performed. To tailor the conduction and optical properties, we performed thermal treatments from 150 to 300 °C under controlled conditions, using 2 atm of Ar or O₂ gasses during two hours of treatment. The modification allows us to tune strain-doping conditions using a gentle process that significantly enhances the photoluminescence. In this work, we performed micro-Raman spectroscopy, Atomic Force Microscopy, Transmission Electron Microscopy, Scanning Tunneling Spectroscopy and micro-photoluminescence to investigate strain-doping modifications, characterize the morphology and have a wide overview of the defects that are present and the interaction of the substrates with 1L-MoS₂.<br/>This study will help to determine the best conditions to use 1L-MoS₂ as the building blocks of the next generation of nano-devices based on semiconductors for electronic and optoelectronic applications.<br/><br/>For this work, the Italian MUR-PNRR project <b>SAMOTHRACE (ECS00000022) </b>and PRIN2022 project <b>“2DIntegratE” (2022RHRZN2)</b> are gratefully acknowledged.<br/><br/>1. Huang, X., Liu, C. & Zhou, P. 2D semiconductors for specific electronic applications: from device to system. <i>NPJ 2D Mater Appl</i> <b>6</b>, (2022).<br/>2. Splendiani, A. <i>et al.</i> Emerging photoluminescence in monolayer MoS2. <i>Nano Lett</i> <b>10</b>, (2010).<br/>3. Iqbal, M. W., Shahzad, K., Akbar, R. & Hussain, G. A review on Raman finger prints of doping and strain effect in TMDCs. <i>Microelectron Eng</i> <b>219</b>, 111152 (2020).<br/>4. Panasci, S. E. <i>et al.</i> Substrate impact on the thickness dependence of vibrational and optical properties of large area MoS2 produced by gold-assisted exfoliation. <i>Appl Phys Lett</i> <b>119</b>, 93103 (2021).<br/>5. Esposito, F. <i>et al.</i> Role of density gradients in the growth dynamics of 2-dimensional MoS2 using liquid phase molybdenum precursor in chemical vapor deposition. <i>Appl Surf Sci</i> <b>639</b>, 158230 (2023).