  Layered two-dimensional (2D) semiconductors such as molybdenum disulfide
(MoS2) have recently attracted remarkable attention because of their unique
physical properties. Here, we use photoluminescence (PL) and Raman spectroscopy
to study the formation of the so- called trions in a synthesized freestanding
trilayer MoS2. A trion is a charged quasi-particle formed by adding one
electron or hole to a neutral exciton (a bound electron-hole pair). We
demonstrate accurate control over the transformation of excitons to trions by
tuning the power of the optical pump (laser). Increasing the power of the
excitation laser beyond a certain threshold (~ 4 mW) allows modulation of
trion-to-exciton PL intensity ratio as well as the spectral linewidth of both
trions and excitons. Via a systematic and complementary Raman analysis we
disclose a strong coupling between laser induced exciton-to-trion
transformation and the characteristic phononic vibrations of MoS2. The onset of
such an optical transformation corresponds to the onset of a previously unknown
nonlinear Raman shift of the in-plane (E12g) and out-of-plane (A1g) vibrational
modes. This coupling directly affects the well-known linear red-shift of the
A1g and E12g vibrations due to heating at low laser powers, and changes it to a
nonlinear and non-monotonic dependence with a blue-shift in the high laser
power regime. Local reduction of the electron density upon exciton-to-trion
transformation is found to be the underlying mechanism for the blue-shift at
high laser powers. Our findings enrich our knowledge about the strong coupling
of photonic and phononic properties in 2D semiconductors, and enable reliable
interpretation of PL and Raman spectra in the high laser power regimes.
