Synthesis And Second Harmonic Generation Of Inversion Symmetry Broken Low-Dimensional Semiconductors

Nonlinear optics,which usually occurs at strong optical field interacting with nonlinear materials,is a unique branch of optics.Second harmonic generation(SHG),also called frequency doubling,is the most common non-linear optical process,where two identical photons interacting with non-linear optical materials will generate one photon with twice the energy and half the wavelength of the initial photons.As a typical non-linear optical parametric process,SHG can stablly converse frequency of input laser,which leads to its fast development in biochemical image,frequency doubling crystal and large femtosecond laser.Recently,owing to the progress of nano-optics and the urgent demand of nonlinear optical devices with small volume,low loss and multifunction,the attention to the low dimensional semiconductors is increasing.In the last decades,low dimensional materials such as Mo S₂ as well as metal halide perovskite nanomaterials have shown great perspective in photonic and optoelectronic applications,and have the potential being the next generation of ultra-thin non-linear optical divices.The generation of SHG requires the broken inversion symmetry of materials.However,metal halide perovskites show intrinsic structure symmetry,which prohibits of their SHG generation.Meanwhile,mechanical exfoliation obtained Mo S ₂nanosheets usually have a small lateral size,and with the increase of thickness,their SHG signal s exhibit diminishing the oscillation trend,which is disadvantage of application in high efficient on-chip non-linear optical divices.Actually,different from their bulk counterparts,the optical signals from low-dimensional semiconductors are sensitive to the surface appearance and atomic stacking,because of quantum confinement effect.Accordingly,if it is possible to change atomic stacking or surface structure of low dimensional materials and make them generate high efficient SHG,there would pave the way to the application of non-linear frequency doubling devices in the future.Based on this background,this thesis focuses on the atomic arrangement,stacking structure and surface appearance of low-dimensional materials,where several inversion symmetry broken low-dimensional materials with high SHG conversion efficiency were controllablly synthesized.Meanwhile,their growth mechanism and the origination of high SHG conversion efficiency were explicitly explained.In the introduction,several common two-dimensional materials including transition metal dichalcogenides(TMDCs)were discussed,and their development history,usual preparation methods as well as basic optoelectronic properties were illustrated.The preparation methods consist of top-down exfoliation ways and bottom-up synthesization routes.Owing to their unique optoelectronic properties,different applications varied from LED to photodetector were realized.Later,the generation and requirement conditions of SHG were deduced based on coupled wave functions,and the fundamental theories of the phase matching conditions as well as polarization selection rules were fully explained.Finally,some SHG related phenomena and application in TMDCs were introduced.3-rhombohedral(3R)phase TMDCs materials as a symmetrically broken nano-semiconductors are metastable,and hence they are hard to preparation in large area.In the second chapter,a temperature selected physical vapor deposition(PVD)method was developed,where 2H and 3R phase WS₂ and WSe₂ nanosheets were synthesized.By TEM selected area electron diffraction(SAED)characterizations,a basic deposition phenomenon was found,where two phases exhibited respectively different shape s and deposited in different temperature section s.Later,by steady state photoluminescence(PL)and Raman measurements,and ultrafast PL characterizations,stacking induced different interlayer coupling in two phases was associated with deposition temperature sections and growth shapes.A hypothsis was introduced to explain these shape and deposition temperature difference in two phases.Finally,by understanding growth conditions,large area 3R phase W-based TMDCs nanomaterials with different thicknesses were controllably synthesized.Based on this PVD route,by changing growth parameters,other series of inversion symmetry broken structures including 3R like WS₂(WSe₂)nanosheets,WS₂(WSe₂)spirals and WSe₂/WS₂vertical lateral heterostructures were further synthesized.The success in synthesization o f these inversion symmetry broken TMDCs nanomaterials advances the preparation of large area non-linear optical two-dimensional materials.Two-dimensional nanomaterials with inverision symmetry broken 3R phase show intrinsic advantage in generation of high efficient SHG signal s.By using pulse infrared laser combined with piezoelectric scanning,SHG imagings of grown WS₂ nanosheets in different phases were successfully revealed,where 2H and 3R phases had different trends with the increase of layer numbers.Other than grown and exfoliated 2H phase two-dimensional materials exhibiting intrinsic diminishing the oscillation SHG,symmetry broken 3R phase layers display continuously enhanced SHG signal with the increase of layer numbers.Further fitting demonstrates that 3R phase layers have a quadratically increased SHG intensity with the respect to layer numbers,which greatly enhancing their SHG conversion efficiency compared to 2H phases.Later,based on the crystal symmetry and SHG properties,a series of 3R(2H)like structures as well as 3R,2H mixed structures were further characterized by SHG imaging,where the complicated stacking of 3R,2H mixed structures can be analyzed by their SHG intensity of different layers.Furthermore,in the WSe₂/WS₂vertical lateral heterostructures,SHG imagings showed the enhanced edges intensity in the second top layer,which were attributed to the nucleation center of the heterostructures.Compared with PL,Raman and other common optical signals,the defects sensitive property of SHG could help analyze nanomaterials growth more effectively.Besides atomic stacking,SHG in TMDCs also demostrates sensitive to the crystal orientations and accordingly,polarized SHG measurements were further conducted.In angle-resolved SHG characterizations,two phases in different shapes displayed similar six-fold polarization diagram,and the petals corresponded to their armchair orientations.While in WSe₂/WS₂vertical lateral heterostructure,a same polarization trand was observed in different top and bottom layers,which indicated a high mismatch tolerance of van der Waals heterostructure.Because of just having one or several atomic layers,TMDCs materials usually show strong spin-orbit coupling and display valley related physics.Valley is the peak or hollow in Bloch electronic band.In SHG process,valley and spin are coupled,leading to a series of allowed and forbidden optical band.By circularly polarized laser,3R and spiral WS₂(WSe₂)nanosheets not only exhibited strong SHG intensity but also had almost one hundred percent inverted circular SHG polarization.If taking this opposite SHG polarization as“0”and“1”in nano logic circuit,spin-orbit coupled SHG in two-dimensional layered materials will make the application of valley logic devices possible.Hybrid oganic-inoganic metal halide pervoskites are other promising low dimensional semiconductors,which have great advantage among large light absorption cross section and high defects tolerance.In chapter four,considering the situation in intrinsic inversion symmetry of metal halide pervoskites generation faint SHG signal,chiral(R/S)-(+/-)-?-methylbenzylamine(R/S-MBA)were added and lead-free chiral perovskite(R-MBA)₄Bi₂Br₁₀ and(S-MBA)₄Bi₂Br₁₀ microplates were synthezied by a cooling-induced crystallization method.These lead-free chiral perovskite microplates not only exhibited notable circular dichroism signals in absorption spectrum,but also had high efficient SHG with the excitation of 1200nm infrared laser.Further circularly polarized SHG characterizations showed that different chir al perovskite microplates had distinct responses to the left and right circularly polarized laser,and their degree of polarization was around 12.5%and 13.2%respectively.In addition,by optimizing growth conditions,spiral structures could appear at the surface of chiral perovskite microplates,whose step hight were determined to be 1.4nm by AFM measurement,corresponding to the thickness of a single chiral perovskite mol ecular layer.Comparing of perovskite microplates with and without surface spiral structure and chiral amines,spiral samples displayed larger SHG conversion efficiency,and could generate SHG signal even without adding chiral molecular.Finally by compar ing grown chiral perovskite microplates with standard commercial non-linear crystal Li Nb O₃,4.72pm/V second order non-linear susceptibility was obtained at the excitation of 1200nm,which is around one order of magnitude larger than previously reported value in chiral perovskite nanowires.Understanding the influence based structure and doping in the generation of SHG paves the way to the development of perovskites multifunctional frequency doubling device.