"Nanotechnology and nanomaterials"

Exciton and trion photoluminescence dependent on temperature in few-layer MoS2
Irfan I.1, Golovynskyi S.1,2, Bosi M.3, Seravalli L.3, Datsenko O.I.4, Li Baikui 1, Qu Junle 1
1 Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
Email: iqra@szu.edu.cn
2 Institute of Semiconductor Physics, NAS of Ukraine, Kyiv 03028, Ukraine
3 Institute of Materials for Electronics and Magnetism, IMEM-CNR, I-43124 Parma, Italy
4 Department of Physics, Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine

We show the temperature-dependent variation of photoluminescence (PL) emission and structural properties of few-layer MoS2 flakes (2 and 10 monolayer-thick) grown on SiO2/Si by chemical vapor deposition. The 2D material is also characterized by electron and atomic force microscopies and µ-Raman spectroscopy.
The PL spectra show the bands that correspond to exciton (A0) and trion (A-). Exciton shows two energy bands A and B related to, respectively, lower and higher energy exciton due to spin orbit coupling. Furthermore, some bands related to defect levels are observed in the spectra below 1.6 eV.
We show the behavior of exciton and trion peaks in few layer MoS2 at different temperatures. When temperature increases, exciton and trion peaks show a red-shift due to thermal expansion of lattice constant and interlayer electron-phonon coupling. The red-shift of the trion peak position with increasing temperature is due to the nozn-zero momentum, resulting in an asymmetrical broadening. Withal, intensities of all the bands decrease. This can be attributed to the thermally activated non-radiative recombination through the defect levels. By fitting the temperature dependence of the main spectral band intensity (trion and A exciton) with the Mott formula, we have obtained the quenching activation energy of 28  8 meV. 
The band behavior for trion separated from exciton can be used in valleytronics and spintronics devices based on transition metal dichalcogenides.