Photoluminescence and raman spectroscopic study of cvd-grown Mos2 monolayers.

Two-dimensional transition metal dichalcogenides (TMDs), particularly molybdenum disulfide (MoS 2 ), have attracted significant attention for next-generation electronic and optoelectronic applications due to their unique structural, electronic, and optical properties.MoS 2 exhibits a transition from an indirect to a direct band gap when confined in a monolayer, leading to enhanced photoluminescence and strong excitonic effects.This thesis presents a comprehensive spectroscopic study of MoS 2 , emphasizing Raman spectroscopy to analyze vibrational modes, as well as photoluminescence (PL) to explore excitonic complexes.The study investigates these properties as a function of doping, temperature, and circular polarization of the excitation beam.Firstly, PL and micro-Raman measurements were conducted on MoS 2 samples coated with 5nm and 10nm thick Al 2 O 3 layers deposited by atomic layer deposition (ALD) on top to investigate the impact of alumina passivation on the optical properties.MoS 2 was synthesized via chemical vapor deposition on Si/SiO 2 substrates, with subsequent passivation using alumina (Al 2 O 3 ) to modulate surface states and electrical properties.Raman spectroscopy reveals a pronounced redshift in the A 1g phonon mode associated with n-type doping induced by Al 2 O 3 , while the E 1 2g mode remains stable, indicating minimal elastic strain.PL spectra show a corresponding redshift of the A exciton peak with increasing alumina thickness, attributed to doping rather than strain,too.Temperature-dependent PL studies (80-340 K) demonstrate a clear redshift of the exciton peak consistent with band gap narrowing from thermal expansion and exciton-phonon interactions.Analysis of the spectra indicates that the strong PL excitonic feature at about 1.8 eV, is composed of two bands attributed to trions(charged excitons) and A excitons.Valley polarization measurements under circularly polarized excitation for MoS 2 monolayers with Pt nanoparticles,indicate suppression of valley coherence at low temperatures.This suggests that, beyond temperature, additional tuning parameters such as the excitation laser frequency are necessary to induce significant modulation.Overall, this work highlights the intricate interplay of doping, temperature, and defect states in determining the optoelectronic and valleytronic properties of MoS 2 , offering insights crucial for future 2D material-based devices in electronics, photonics, and neuromorphic applications.