Screening, Defects, and Dangling Bonds Induced Optical Damage Threshold in Monolayer MoS2

Abstract

An optoelectronic device based on the TMDs should be versatile in its applications and be able to endure high optical intensities. In this thesis, I have shown the experimentally-measured damage threshold intensity range of 75 to 100 kW/cm2 of a monolayer MoS2 at room temperature. The corresponding photo-excited carrier density, for the CW photo-excitation, is ~1.6 x 10^10 cm^(-2). While the goal has been to find the damage threshold intensity, certain patterns that have emerged from the optical response of the monolayer MoS2 in the vicinity of the optical damage threshold needed to be thoroughly examined. I have quantified the experimental results and have attempted to qualitatively understand the underlying physics. The many-body effects play a crucial role in all these processes such that there is a complicated correlation. While explaining the physics through experimental results, I have attempted to disentangle the screening-related effects and damage and/or defect states related effects from the experimental dependencies, viz., optical power, laser irradiance time, and beam position. The measurements have been done by collecting the photoluminescence (PL) signals. The parameters such as excitonic peak amplitude, area, FWHM, and the central wavelength have been extracted from the curve fitting of the PL spectrums. In the optical power dependence measurement, I have compared the optical responses of the material by employing two different methods of measurement, viz., the Direct and Indirect PL measurement methods. In both these methods of measurement, I increase the excitation intensity step-wise but the method of collecting the PL signal differs. We will see that this slight difference in methodology provides us with a strikingly different optical response. In the laser irradiance time dependence measurement, I have exposed the sample continuously for 62 minutes in total. With this, the charge accumulation and resultant changes in the optical response of the material have been demonstrated. We will see that my experiments concretely debunk the claim of laser-induced atomic healing of defects. In the beam position dependent measurement, I deliberately create damage and optically scan the sample through pristine, within the damage, and at the damaged edge by collecting the PL signal from these locations. The characteristic enhancement in the PL signal is evidently found to be a real feature. From these measurements, salient features have been consistently observed such that these can be marked as the signatures of the damage incurrence. Signatures such as 2x to 4x times the increment in the peak amplitude, a similar increment in the area, changes in PL efficiency, 30% to 50% shrinkage in the FWHM, and 4 nm to 15 nm of a spectral blue shift in the A exciton peak in the PL spectrum. In this thesis, I will further demonstrate that all these reversible and irreversible changes are from the contributions of the screening effects such as Bandgap Renormalization (BGR) and Coulomb screening, defects and dangling bonds induced damage threshold in the monolayer MoS2.