Temperature and power-dependent photoluminescence spectroscopy in suspended WSe₂ monolayer

This work, the photoluminescence (PL) 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 demonstrated the monolayer characteristic of the sample due to the absence of the B¹ 2 g 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 (X_T), where X₀ showed a redshift with temperature variation from 10 K to 310 K, 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 (X_T). 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 X_T. 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 an increase 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 PL 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.