Study On The Temperature Dependent Luminescence Characteristics Of Two-dimensional WS₂/WSe₂ Semiconductors

2D transition metal dichalcogenides?2D TMDs?with energy bandgaps covering the visible and near infrared spectral region have enormous advantages in various photonic and optoelectronic applications,such as light emitting devices,transistors,photovoltaic devices,photodetectors,and nano cavity lasers.Understanding the mechanisms and controlling the optical properties of 2D TMDs is of crucial importance for further applications in integrated photonics.WS₂ and WSe₂which have superior luminescent quantum efficiency and larger spin-orbit coupling strength have gained a lot attention.Heating is usually taken as a simple and efficient strategy to tune the optical properties of materials.In this work,the optical properties of tungsten sulfide and tungsten selenide are tuned by temperature.Temperature induced fluorescence emission enhancement and energy band structure changing are observed in ML-WS₂ and ML-WSe₂.The carrier dynamic behaviors of ML-WS₂ and ML-WSe₂ at different temperatures are discussed.The specific study and the main conclusions are as follow:1.A remarkable high-temperature induced fluorescence emission enhancement?³00-fold compared with room temperature?from ML-WS₂ nanoflakes is observed for the first time.Temperature-dependent Raman studies and electronic structure calculations reveal that this observation cannot be fully explained by a common mechanism of thermal-expansion-induced interlayer decoupling.2.A theoretical model involving thermo-driven inter-valley transfer of photo-carriers,is proposed to understand the high-termperature luminescence enhancement of multilayer WS₂.Population ratio calculations and multiple spectral characterizations?including temperature-dependent and time-resolved spectra?confirm the validity and reliability of this established inter-valley transfer model.3.By studying the TD-PL of ML-WSe₂ nanoflakes,the competition of conduction band valleys in ML-WSe₂ is investigated.We compare the temperature-dependent shift in the optical transition energies to that predicted by density functional theory?DFT?calculations and conclude the origin of the indirect transition.The results indicate that the CBM is located at the K point for ML-WSe₂.