Optical And Optoelectronic Properties Of Quantum Dot/MoS₂ Hybrid Heterostructure

Since the"future material"of graphene has been discovered and widely studied,two-dimensional（2D）transition metal dichalcogenides（TMDs）have attracted great attentions.2D TMDs show unique advantages in the application of optoelectronic devices due to their ultrathin atomic-level thickness,high integration compatibility and excellent optoelectronic properties.TMDs are a family of materials,the typical representative of which is molybdenum disulfide（MoS₂）.MoS₂ has been widely studied because of its unique electronic band structure,excellent photo-electronic properties and unique physical and chemical properties.Single-layer MoS₂ owns 2H-phase atomic structure,single layer thickness less than 1 nm,light absorption in visible wavelengths.However,single-layer MoS₂ only absorbs less than 8%visible photon,which greatly limits its practical applications in optoelectronic devices.In order to improve the light absorption of monolayer MoS₂ and the light response performance of MoS₂-based photodetector,the CdSe@ZnS QD/MoS₂ mixed dimensional heterojunctions were designed and prepared with using the strong light absorption of quantum dots（QD_S）and the property that the light absorption range can be adjusted with the size of quantum dots.The light-matter interactions of heterostructure are enhanced significantly in a limited size space.By performing systematical spectral analyses and measurements,the effect of the overlapping concentration of QDs and the laser-power-dependent charge carriers at the heterojunction interface were explored.The physical mechanism of the performance enhancement of mixed-dimensional optoelectronic devices was further explained.This literature mainly contains the following research contents:1.Preparation and characterization of mixed dimensional heterojunction.The solution of CdSe@ZnS quantum dots was spin-coated on the monolayer MoS₂ surface prepared by chemical vapor deposition.The mixed dimensional heterojunction（0D/2D）with different concentration of quantum dots can be prepared by controlling the appropriate rotational speed in the experiment.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,atomic force microscopy（AFM）and X-ray photoelectron spectroscopy（XPS）were used to characterize the surface morphology and microstructure of monolayers MoS₂,QD_S and 0D/2D heterojunction,respectively.It is proved that the heterojunction materials own rich 0D/2D interfaces for effective charge transfer.2.Study on interfacial charge transfer process of heterojunction.Through the systematical research of QDs-concentration-dependent exciton competition and spectral evolution,the exciton rate equation and the three-level model were constructed to estimate the effective charge doping of 3.9 x10¹³ cm^-2.The single-layer MoS₂,QD_Sand 0D/2D heterojunction were detailed characterized by steady-state and transient spectroscopy.At low power excitation,A exciton and trion radiations are the main contributions in luminescence spectra.The fluorescence intensities of A exciton,trion and QD_S exciton increase continuously with the increase of excitation power.3.Performance of optoelectronic devices and device physics.The mixed-dimensional photoelectronic devices were fabricated by micro-nano fabrication technology.Under the irradiation of 520 nm laser(V_gs=60 V,V_ds=1 V),the Photoresponsivity（R）and detectivity（D*）of the hybrid dimensional heterojunction phototransistor were～1.8×10⁴ AW^-1 and～3×10¹¹ Jones,respectively.Compared with the intrinsic molybdenum disulfide phototransistor,the Photoresponsivity and detectivity are increased by 21.6 and 9.6 times,respectively.The photoelectronic performance of the hybrid device can be operated by adjusting the backgate voltage.The energy band diagram and physical model can provide intuitively understanding in charge-transfer-induced performance improvement.