Role of processing parameters in CVD grown crystalline monolayer MoSe₂

The quality of as-synthesized monolayers plays a significant role in atomically thin semiconducting transition metal dichalcogenides (TMDCs) to determine the electronic and optical properties. For designing optoelectronic devices, exploring the effect of processing parameters on optical properties is a prerequisite. In this view, we present the influence of processing parameters on the lattice and quasiparticle dynamics of monolayer MoSe₂. The lab-built chemical vapour deposition (CVD) setup is used to synthesize monolayer MoSe₂ flakes with varying shapes, including sharp triangle (ST), truncated triangle (TT), hexagon, and rough edge circle (REC). In particular, the features of as-synthesized monolayer MoSe₂ flakes are examined using Raman and photoluminescence (PL) spectroscopy. Raman spectra reveal that the frequency difference between the A_1g and E¹ _2g peaks is >45 cm^-1 in all the monolayer samples. PL spectroscopy also shows that the synthesized MoSe₂ flakes are monolayer in nature with a direct band gap in the range of 1.50-1.58 eV. Furthermore, the variation in the direct band gap is analyzed using the spectral weight of quasiparticles in PL emission, where the intensity ratio {I(A⁰)/I(A^-)} and trion binding energy are found to be ∼1.1-5.0 and ∼23.1-47.5 meV in different monolayer MoSe₂ samples. Hence, these observations manifest that the processing parameters make a substantial contribution in tuning the vibrational and excitonic properties.