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Layered two-dimensional (2D) semiconductors such as molybdenum disulfide(MoS2) have recently attracted remarkable attention because of their uniquephysical properties. Here, we use photoluminescence (PL) and Raman spectroscopyto study the formation of the so- called trions in a synthesized freestandingtrilayer MoS2. A trion is a charged quasi-particle formed by adding oneelectron or hole to a neutral exciton (a bound electron-hole pair). Wedemonstrate accurate control over the transformation of excitons to trions bytuning the power of the optical pump (laser). Increasing the power of theexcitation laser beyond a certain threshold (~ 4 mW) allows modulation oftrion-to-exciton PL intensity ratio as well as the spectral linewidth of bothtrions and excitons. Via a systematic and complementary Raman analysis wedisclose a strong coupling between laser induced exciton-to-triontransformation and the characteristic phononic vibrations of MoS2. The onset ofsuch an optical transformation corresponds to the onset of a previously unknownnonlinear Raman shift of the in-plane (E12g) and out-of-plane (A1g) vibrationalmodes. This coupling directly affects the well-known linear red-shift of theA1g and E12g vibrations due to heating at low laser powers, and changes it to anonlinear and non-monotonic dependence with a blue-shift in the high laserpower regime. Local reduction of the electron density upon exciton-to-triontransformation is found to be the underlying mechanism for the blue-shift athigh laser powers. Our findings enrich our knowledge about the strong couplingof photonic and phononic properties in 2D semiconductors, and enable reliableinterpretation of PL and Raman spectra in the high laser power regimes.

Nonlinear Raman Shift Induced by Exciton-to-Trion Transformation in Suspended Trilayer MoS2