Ultrafast Spectroscopy And Dynamics Studies Of Several Low-Dimensional Material Model Systems

Low-dimensional materials,such as 0D quantum dots,1D nanorods,and 2D nanosheets,exhibit broad application prospects because of their unique thermal,optical,electrical,magnetic,and mechanical properties.Affected by surface and quantum-size effects,the excited-state optical properties of low-dimensional materials usually change significantly with respect to their bulk counterparts.In order to obtain important information about the evolution of various excited states involved in low-dimensional materials,ultrafast spectroscopy with femtosecond time resolution is usually used,which enables mechanistic understanding of excited-state dynamics such as charge/energy transfer,electron-hole recombination,elementary excitations,heat transport,and surface/interface phase transition.Generally,the excited-state dynamics at the microscopic level are rather complex,which are closely related to the physicochemical properties of materials,the intensity/wavelength/polarization of excitation light sources,and even the external environments.Such complexities often bring great challenges to the interpretation of experimental phenomena.By regulating such key factors as structure,size,and functional groups of low-dimensional materials,one can scrutinize their influences on the photo-induced dynamics.This can help determine the lifetimes of various excited species such as carriers,excitons,and plasmons as well as the related relaxation channels,thereby allowing for rational modeling and extraction of pertinent microscopic mechanisms.Such kind of mechanistic investigations will provide crucial information and guidance for practical research and development of low-dimensional materials.In view of this situation,this dissertation is devoted to the mechanistic study of several designed low-dimensional material model systems,mainly including the metalloid MXene material and several semiconductor nanomaterials,from which some new insights into excited-state dynamics have been gained.Additionally,addressed also in the dissertation are the photoluminescence negative/zero-thermal quenching effects in MXene and a coordination polymer material.The specific research contents are briefly described as follows:1.Explosive boiling dynamics at the interface of the low-dimensional metalloid MXene materialMXene has excellent photothermal conversion properties.We deliberately designed an alcohol/MXene liquid-solid interfacial system,and used the MXene photoexcited lattice dynamics monitored by ultrafast spectroscopy as a "probe" to track the explosive boiling process at the alcohol/MXene interface in real time.On the basis of the evolution of the MXene lattice dynamics,we identified three stages during the explosive boiling of alcohol at the MXene interface,namely,the initial vapor-layer formation(0-1 ns),the subsequent phase explosion(1-6 ns),and the eventual returning to equilibrium(>6 ns).Starting from a thermal-transport mechanism of "light-MXenealcohol" at the interface,we performed photothermal modeling to rationally acquire the microscopic conditions of explosive boiling at the interface of different alcohols with MXene,which turned out to agree well with the experimental observations.Furthermore,it was estimated that,when explosive boiling happens,～17-25 layers of alcohol molecules participate in this rapid liquid-vapor phase transition.2.Photoexcited lattice dynamics in the functional group-regulated MXeneBoth the structural and electronic properties of MXene are strongly influenced by surface functional groups.In order to explore the influence of functional groups on dynamics and optical properties,we designed and constructed two MXene samples with and without-F functional group.As revealed by ultrafast spectroscopy,the MXene samples(i.e.,one in water and the other in the form of thin film)showed different relaxation dynamics in different environments that can be regulated by functional groups.Additionally,an interesting effect of fluorescence negative-thermal quenching was observed in the thin-film samples,which can be ascribed to trap states in MXene.3.Effect of phosphorescence zero-thermal quenching in a low-dimensional cluster coordination polymer materialLuminescent materials commonly face the challenge of high-temperature thermal quenching.In this collaborative study,it was found that the synthesized fluorescent material of cluster coordination polymer Cu4I4(bpp)2 can resist thermal quenching within a certain temperature range and that the variation of electronic structure can be reversible due to the dynamic Cu-Cu interaction.By means of temperature-dependent photoluminescence spectroscopy and ultrafast transient absorption spectroscopy,we ruled out the possibility of thermally-activated delayed fluorescence and also demonstrated the high thermal stability of photoluminescence in such a lowdimensional cluster coordination polymer material.4.Photoinduced carrier dynamics and plasmonic effect in low-dimensional semiconductor materialsApart from the ultrafast dynamics study of the low-dimensional metalloid MXene material,we also conducted two collaborative investigations aiming at low-dimensional semiconductor material WO3:(1)As for the WO3/CdS redox heterojunction system,our ultrafast spectroscopy identified that,with respect to the bare CdS system,the WO3/CdS heterojunction system can promote photogenerated electron transfer due to the presence of a built-in electric field at the coupling interface,leading to improved performance in photocatalytic nitrogen fixation.(2)As for the SERS system based on alkali-doped WO3,our ultrafast spectroscopy verified that the efficient charge transfer(between Lidoped WO3 and R6G)and the plasmonic effect can synergistically result in the enhancement of SERS performance.5.Independent establishment of a platform for preparation of high-quality lowdimensional materialsThe strength of our laboratory mainly lies in ultrafast spectroscopy/dynamics characterizations,but with an apparent shortcoming in the independent preparation of high-quality low-dimensional materials,which sets a hurdle to our mechanistic studies.To overcome such a shortcoming,I have made great efforts to independently establish a chemical vapor deposition(CVD)platform aiming to prepare high-quality lowdimensional materials that are highly desirable to our research.My exploration in this regard has achieved success.Taking the first successful preparation of high-quality monolayer MoS2 film samples as an example,I have not only grasped the optimization strategies for film-growth regulation,but also gleaned useful information about filmgrowth mechanisms.Such an independent establishment of CVD platform made a valuable contribution to our laboratory,opening new possibilities for the ongoing multidomain high-resolution ultrafast spectroscopy/dynamics studies in our laboratory.