Studies On Light-Emitting Diodes Of Two-Dimensional Transition Metal Dichalcogenides MoSe₂ And WSe₂

Two-dimensional transition metal dichalcogenides（TMDCs）have attracted attentions due to their unique optoelectronic properties,such as strong photo-material interaction,spin orbit coupling and spin valley degree of freedom.Due to the advantage of narrow band gap,TMDCs are easy to be used in infrared light-emission.The optoelectronic properties of layered TMDCs mainly depend on the number of layers.The band gap of TMDCs will convert from indirect band gap of the bulk to the direct band gap of a single layer.Currently,the development of 2D TMDCs application is limited by several factors of synthesis and preparation,such as complicated craft,excessive cost,and unmanageable size.Aiming at changing the above circumstances of TMDCs,we proposal our project covering the following four aspects:（1）Large-scaled TMDCs（MoSe₂ and WSe₂）were synthesized by chemical vapor deposition,and the challenge of metal doping in TMDCs was overcome.On one hand,monolayer TMDCs materials MoSe₂with maximal size of 36.0mm was prepared.The controlled growth of monolayer and bilayer MoSe₂ was explored.The product morphology of monolayer MoSe₂ was regulated and controlled by varying the gas-phase reactive parameter.We also analyzed the influence of vapor pressure ratio ofSe and MoO₃ on nucleation density and morphology of MoSe₂.On the other hand,the Yb/Er co-doped bilayer WSe₂ with the size of 25.0mm was prepared using ultrasonic mixing technology combined with molten-salt assisted chemical vapor deposition technology.The different doping content of Yb/Er in bilayer WSe₂ was regulated by the atomic ratio of W:Yb:Er in precursor.（2）For requirement of remote infrared sensing system,we designed and fabricated infrared light-emitting diode（LED）based on MoSe₂.The photoluminescence peak of monolayer MoSe₂ locates at 804 nm,indicating that the band gap energy of monolayer MoSe₂ is 1.54 e V.Ni O was prepared on Si O₂/Si substrate by thermal evaporation and thermal annealing.The monolayer MoSe₂ was transferred onto Ni O to form van der waals heterostructure by using wet transfer method.A LED based on p-Ni O/n-MoSe₂ junction was constructed.The electroluminescence peaks of p-Ni O/n-MoSe₂ LED were located at 812 nm,848 nm and 918 nm.（3）To realize visible light emission of TMDCs,the up-conversion luminescence was achieved on Yb/Er co-doped bilayer WSe₂.The as-synthesized Yb/Er co-doped bilayer WSe₂ was 2H phase and polycrystalline.The photoluminescence peaks of pure WSe₂ bilayer and Yb/Er co-doped bilayer WSe₂ were located at 781 nm and 817nm,indicating their bandgap energy are 1.59 e V and 1.52 e V,respectively.The temperature-dependent photoluminescence spectrum demonstrated that the band gap of Yb/Er co-doped bilayer WSe₂narrowed when the temperature increased.The up-conversion luminescence was realized on Yb/Er co-doped bilayer WSe₂ excited by980 nm laser.The up-conversion luminescence peak at 550 nm originates from the level transition Er³⁺⁴S_3/2→⁴I_15/2.Further,we prepared Yb/Er co-doped bilayer WSe₂/MoSe₂ LED and measured external quantum efficiency of the LED.（4）The flexibility of TMDCs diode is expected to be applied in curved screen display and e-skin communities.It was demonstrated that the plasma resonance enhancement has strong enhancement on the photoluminescence of monolayer MoSe₂.The energy band of monoalyer MoSe₂ and bilayer WSe₂ was estimated using Density Functional Theory.The results showed that the band gap energies were in good agreement with the experimental values.The computed partial density of state indicated the energy band gap of bilayer WSe₂ was mainly determined by 4p electrons ofSe atoms and 5d electrons of W atoms.