Temperature and power-dependent photoluminescence spectroscopy in suspended WSe₂ monolayer

Abstract

This work, the photoluminescence of a suspended monolayer of WSe2 was investigated to avoid the substrate effect, and its evolution with temperature and excitation energy. Raman spectroscopy and PL (photoluminescence) demonstrated the monolayer characteristic of the sample due to the absence of the B12g peak along with a symmetric PL peak centered at approximately 1.65 eV. The PL spectrum exhibited two main emission bands attributed to a neutral exciton (X0) and a trion (XT ), where X0 showed a redshift with temperature variation from 10K to 310K, and the intensity ratio between the bands varied from 2.68 to 3.96 eV. The energy evolution
of the bands as a function of temperature was analyzed using the modified Varshni equation, yielding an electron-phonon coupling constant with a value of 1.54 (2.11) for X0 (XT ). The intensity behavior was studied using the multi-level model, which provided activation energies of 58.0 meV for X0 and 94.6 meV for XT . The redshift of the mentioned bands is explained by the Urbach formalism, attributing this shift to an increase in phonon density in the material. Computational calculations demonstrated an increase in the material’s lattice parameter with rising temperature, resulting in anincrease in the W-Se bond length and a decrease in the distance between Se atoms from different sublattices, causing the direct transition to decrease with temperature. In conclusion, this study showed that the observed evolution in the photoluminescence spectrum of the suspended WSe2 monolayer could be related to the increase of phonon density in the material which is important for the future applications of 2D materials in optoelectronics.