The Self-Assembly And Photoinduced Reaction Of PCDA On The Surfaces Of Oxides And MoS₂/Oxides

Integrating two or more different materials into one compositional system has been a widely applied strategy to realize expanded functions or brand-new properties.Transition-metal dichalcogenide(TMD)is a typical two-dimensional material family with attractive properties which finds wide applications and thus receives intensive attention.While combining semiconductive metal oxide(MO)with TMDs can effectively tailor the properties of the latter,the proper addition of functional organic materials can largely expand the applications of the compositional system with unlimited flexibility.In this thesis,we have taken the as-prepared single layer MoS2 on TiO2 and SrTiO3(110)single crystal substrate as the model systems,and investigate the interactions of a prototypical organic molecule,PCDA,with these surfaces from the atomic level.We mainly applied the atomic force microscopy(AFM)to characterize the detailed assembled structures of PCDA on both the bare TiO2(110)and SrTiO3(001)surfaces,and the MoS2-covered MoS2/TiO2(110)and MoS2/SrTiO3(001)surfaces.These results not only help us to compare the interactions of PCDA with the different material surfaces,but also shed new lights on the tailoring effects of the oxide substrate over the interactions between organic molecules and the supported TMD surfaces.The main results are summarized as following:1.We successfully prepared the atomically flat surfaces on the rutile TiO2(110)and SrTiO3(001)single crystals through an solution-etching procedure followed by annealing to high temperatures under inert atmosphere.The assembly films of PCDA were deposited on these two surfaces via drop-casting methods and investigated systematically with both scanning electron microscopy(SEM)and AFM measurements.It is found that the assembly behaviors of PCDA on the rTiO2(110)and SrTiO3(001)surfaces are very similar to each other.The PCDA molecules mainly assemble into a layered structure which is composed by stacked layers of either tilting or flat-lying PCDA molecules.In the former layer,PCDA molecules assemble side by side and tilt with an angle of 48.5° against the surface normal,while in the latter case the PCDA molecules also assemble into a lamellae structure but orientate parallel to the surface.The tilting and flat-lying layers stagger randomly in the vertical direction.These structures were actually also found on other oxide surfaces as reported in literature,indicating an independence against the atomic structures of the oxide surface.Aside from these staggered layer-structure,PCDA molecules were also found to form a single-layer film which has a chain-like structure and strongly bound on both the TiO2 and SrTiO3 surfaces.This structure was never found in the literatures and may rely on the TiOx termination of our investigated systems.2.On the cleaned TiO2(110)and SrTiO3(001)surface we further grew single layer MoS2 nanosheets through a conventional chemical vapor deposition(C VD)method,thus successfully constructed ideal MoS2/TiO2 and MoS2/SrTiO3 model systems.PCDA molecules were then deposited on these surfaces by drop casting methods and the resulted films were investigated by detailed AFM measurements.We found that PCDA molecules can simultaneously assemble on both the MoS2 flakes and the uncovered TiO2 surfaces.In both cases the assembling behavior of PCDA were not found very different from that on the pure MoS2 and TiO2 materials.In contrast,the assembly of PCDA on the MoS2 flakes of MoS2/SrTiO3 presents much less ordered than that on the MoS2 single crystal surface,possibly due to the presence of the defects on the MoS2 flakes of the compositional systems.We further shed ultraviolet(UV)light on these PCDA films to trigger polymerization reactions.Interestingly,under the irradiation of 365 nm laser,polymerization of the PCDA molecules was indeed observed on the MoS2/TiO2(110)surface whereas not on the MoS2/SrTiO3 surface.The low reactivity of the latter may possibly be attributed to the worse ordering of the assembled PCDA molecules as well as the specific deactivating effect originated from the MoS2/SrTiO3 heterostructure.