The Study Of Mechanical And Electrical Properties Of Individual Poly (3-alkylthiophene) Nanofiber Using Atomic Force Microscopy

Polythiophene conjugated polymers have great potential for applications in a variety of flexible and stretchable electronic devices due to their simple synthesis and processing method,excellent photoelectric properties,and good mechanical flexibility.In recent years,polythiophene nanofibers prepared via crystallization have been widely used in a variety of flexible and stretchable devices.Therefore,in-situ investigation of the mechanical and electrical properties of polythiophene nanofibers during deformation and further establishing the relationship between properties and structure is of great significance for improving the performance and prolonging the service life of flexible devices.However,precise determination of the properties of individual nanofiber using traditional technology remains challenging due to their small size.Atomic force microscopy is one of the powerful tools for studying one-dimensional nanomaterials because of its sub-nanometer spatial resolution,high mechanical accuracy,and in-situ imaging and manipulation.In this thesis,the manipulation of individual poly(3-alkylthiophene)nanofibers in different ways has been realized through a variety of techniques based on atomic force microscopy.The relationship between mechanical,optical and electrical properties of poly(3-alkylthiophene)nanofibers and the molecular structure have been systematically studied.The main findings are as follows:1.A set of effective methods for manipulating and characterizing flexible one-dimensional nanomaterials has been established based on atomic force microscopy.The persistence length of poly(3-hexylthiophene)nanofibers was estimated by thermal shape-fluctuation analysis,and the influence of substrates and imaging environment on the results were studied.The bending behavior of flexible poly(3-hexylthiophene)nanofibers was studied by three-point bending test using the substrate-guided positioning method.The single-molecule mechanical properties of poly(3-hexylthiophene)were investigated by the single molecule force spectroscopy.Finally,we established a new method of stretching a single nanofiber and proved its feasibility in studying the recrystallization of poly(3-hexylthiophene).2.The mechanical properties of poly(3-hexylthiophene)nanofibers were systematically investigated using a combination of single molecule force spectroscopy,thermal shape-fluctuation analysis and three-point bending test.The single-molecule mechanical properties,mesoscopic flexibility,and dynamics of slippage of thiophene rings during fiber deformation were comprehensively studied.The results show that the van der Waals interaction between the side chains stabilizes the structure of nanofiber and affects their modulus and single-molecule mechanical behavior.Meanwhile,the existence of interlayer interaction promotes the slippage of thiophene rings to show the characteristics of"more stable and easy to recover".In addition,poly(3-hexylthiophene)nanofibers show the huge mechanical anisotropy.We attributed this mechanical anisotropy to the?-stacked structure of the conjugated thiophene rings within the nanofibers,and the layer stacking was proved to weaken the mechanical anisotropy.3.The influence of the side chain length on the optical and mechanical properties of poly(3-alkylthiophene)nanofibers has been studied.The results showed that achiral poly(3-alkylthiophene)molecules were assembled in solution by supramolecular chiral assembly to form chiral nanofibers.We have confirmed that the generation of this supramolecular chirality was controlled by the kinetics of nucleation and the alkyl side chain length does not affect the generation of the chiral structure.Poly(3-butylthiophene)chiral nanofibers exhibit circularly polarized luminescence properties.We have realized the construction of a circularly polarized luminescent material by only achiral components.Our results also show that poly(3-hexylthiophene)nanofibers exhibit a higher Young's modulus and strength than other poly(3-alkylthiophene)nanofibers.Moreover,we provide the structural interpretation of the effect of side chain length through UV-vis absorption measurement.We find that the J-aggregate characteristics are more pronounced in poly(3-hexylthiophene)nanofibers,which improves the planarity of thiophene units.These ordered conformations and compact arrangement of polymer chains increases the probability of the one-step unfolding pathway in single molecule force spectroscopy experiments and improve the mechanical properties of nanofibers.4.We have achieved the in-situ monitoring of the mechanical and electrical properties of individual poly(3-hexylthiophene)nanofibers under force-induced deformation.To achieve that,we have prepared the Si O₂ substrate with nano-grooves by using the combination of AFM tip-based nanoscratching and plasma etching.The most suitable conductive probe for collecting both force and electrical signals was identified.The force-displacement and current-displacement curves were obtained at the same time during the three-point bending test on poly(3-hexylthiophene)nanofibers.We have established the relationship between the nanofiber stiffness and current intensity and deformation.Our results show that in conductive three-point bending test,Young's modulus of poly(3-hexylthiophene)nanofiber can be enhanced under the applied voltage(>2 V).In addition,the current intensity of poly(3-hexylthiophene)nanofibers doesn't change under small deformation.