Séminaire de George Kioseoglou

Department of Materials Science and Technology, University of Crete, Greece
Institute of Electronic Structure and Lasers – FORTH

Tuning the optoelectronic properties of TMD monolayers
with dielectric environment, photochemical processes, and strain

Monolayers of MX2-type (M=Mo or W and X=S or Se) Transition Metal Dichalcogenides (TMDs),
exhibit promising potential for future 2D nanoelectronics owing to their unique optoelectronic properties.
In this seminar, I will present methods for controlling their optical and electronic characteristics through
the engineering of their dielectric environment, employing photochemical methods, and applying
mechanical strain.
In particular, we investigate WS2 monolayers on pre-patterned Si/SiO2 substrates with cylindrical
wells of 3 μm in diameter, analyzing strained and suspended areas. Raman mapping experiments quantify
strain, revealing a 10-fold enhanced photoluminescence efficiency with strong neutral excitonic emission
in suspended areas. TMD optoelectronic properties are chemically controlled by modulating the Fermi level
using UV-assisted photochlorination processes in WS2 [1,2] and WSe2 [3] monolayers. Systematic shifts
and relative intensities between neutral and charged excitons indicate a controllable decrease in electron
density, switching WSe2 from n-type to p-type semiconductor. Validation of chlorine species and DFT
calculations [4] predict p-type doping through chlorine adsorption on selenium vacancy sites. Investigating
isotropic, biaxial strain at room temperature on WS2 monolayers shows a strong shift on the order of ~130
meV per % of strain in neutral exciton emission and a decrease in circular polarization degree [5]. The
analysis reveals the interplay of energy and polarization relaxation channels, as well as variations in the
exciton oscillator strength affecting long-range exchange interactions.

[1] I. Demeridou, et al. 2D Mater. 6, 015003 (2018)
[2] I. Demeridou, et al. Appl. Phys. Lett. 118, 123103 (2021)
[3] E. Katsipoulaki, et al., 2D Mater. 10, 045008 (2023)
[4] G. Vailakis, et al. Phys. Rev. Mater 7, 024004 (2023)
[5] G. Kourmoulakis, et al., Appl. Phys. Lett. 123, (2023) in print