Electronic Materials Letters

The Korean Institute of Metals and Materials (대한금속재료학회)
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Effect of Phonons on Valley Depolarization in Monolayer WSe2

  • Chellappan, Vijila (Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR)) ;
  • Pang, Ai Lin Christina (Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR)) ;
  • Sarkar, Soumya (NUS Nanoscience and Nanotechnology Initiative (NUSNNI), National University of Singapore) ;
  • Ooi, Zi En (Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR)) ;
  • Goh, Kuan Eng Johnson (Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and Research (A*STAR))
  • Received : 2018.04.16
  • Accepted : 2018.07.19
  • Published : 2018.11.10
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Abstract

In this paper, temperature dependence of the excitonic bands in a mechanically exfoliated tungsten diselenide ($WSe_2$) monolayer is studied using photoluminescence and circular dichroic photoluminescence (PL) in the temperature range between 8 and 300 K. The peak energies associated with the neutral exciton (A), charged exciton (trion) and localized excitons are extracted from the PL spectra revealing a trion binding energy of around 30 meV. The circular dichroic PL measured at 8 K shows about 45% valley polarisation that sharply reduces with increasing temperature to 5% at 300 K with photoexcitation energy of 1.96 eV. A detailed analysis of the emission line-width suggests that the rapid decrease of valley polarisation with the increase of temperature is caused by the strong exciton-phonon interactions which efficiently scatter the excitons into different excitonic states that are easily accessible due to the supply of excess photoexcitation energy. The emission line-width broadening with the increase of temperature indicate residual exciton dephasing lifetime < 100 fs, that correlates with the observed rapid valley depolarisation.

Keywords

Acknowledgement

Supported by : A*STAR Pharos

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