Title: Improved optical and electrical properties of MoSe₂ and WSe₂ via reduction of point defects
Description: Transition metal dichalcogenides (TMDs) have displayed a host of novel physical phenomena, which opens-up promising future applications in electronics, optoelectronics, spintronics and valleytronics. However, the high defect density of 10¹² - 10¹³ cm-² in commercially available TMDs may hinder the observation of their intrinsic properties. In this thesis, the defect density of MoSe₂ and WSe₂ has been reduced by ~10x - 1000x using flux method. The reduced defect denstiy of MoSe₂ and WSe₂ enables to observe optical and electrical properties approaching their intrinsic properties.First of all, photocurrent measurements on the ultra-clean WSe₂ unveil the effect of point defects on photo-response. Substantial improvement of AC photocurrent in the ultra-clean WSe2 indicates that free carriers are likely to non-radiatively decay at atomic defects at room temperature. Then, time-resolved photoluminescence measurements on the ultra-clean MoSe₂ samples allow for direct determination of both the intrinsic (radiative) and defect-dependent (non-radiative) lifetimes of trions.

In the cleanest MoSe₂, the trion quantum yield approaches unity. The long lifetime of 230 ps of trions allows direct observation of their diffusion, conclusively demonstrating that trions are free particles. Both the long radiative and non-radiative lifetime of trions can be attributed to Pauli blocking effects. Morover, transport measurements of ultra-clean WSe2 provide Hall mobility exceeding 10,000 cm²V-¹s-¹ and long mean free path over 200 nm, which are nearly three times higher than those in previous study. This improved mobility and mean free path in the ultra-clean WSe₂ indicate that the electrical properties have been limited by defect scattering. Finally, WSe₂ has been a decent platform to generate single photon emitters. However, the microscopic origin of the single photon emitter has been debated. From power- and gate-dependent photoluminescence of ultra-clean WSe₂, emerging defect bound excitons are observed, which is likely formed from the interaction between donor defects and excitons.