Investigating the impact of ITO substrates on the optical and electronic properties of WSe2 monolayers

Two-dimensional (2D) materials, particularly transition metal dichalcogenides (TMDs), have gathered significant attention due to their interesting electrical and optical properties. Among TMDs, monolayers of WSe2 exhibit a direct band gap and high exciton binding energy, which enhances photon emission and absorption even at room temperature. This study investigates the electronic and optical properties of WSe2 monolayers when they are mechanically transferred to indium tin oxide (ITO) substrates. ITO is a transparent conducting electrode (TCE) used in many industrial optoelectronic applications. Samples were mechanically transferred under ambient conditions, consequently trapping an adsorbate layer of atmospheric molecules unintentionally between the monolayer and the substrate. To reduce the amount of adsorbates, some samples were thermally annealed. Atomic force microscopy confirmed the presence of the adsorbate layer under the TMD and its partial removal after annealing. X-ray photoelectron spectroscopy confirmed the presence of carbon species among the adsorbates even after annealing. Photoluminescence measurements show that WSe2 remains optically active on ITO even after annealing. Moreover, the luminescence intensity and energy are affected by the amount of adsorbates under the WSe2 monolayer. Scanning tunnelling spectroscopy reveals that the TMD monolayer is n-doped, and that its band edges form a type I band alignment with ITO. Surface potential measurements show a polarity change after annealing, indicating that polar molecules, most likely water, are being removed. This comprehensive study shows that a TCE does not quench WSe2 luminescence even after a prolonged thermal annealing, although its optical and electronic properties are affected by unintentional adsorbates. These findings provide insights for better understanding, controlling, and design of 2D material heterostructures on TCEs.