Controlled Surface Assembly And Electrical Property Of Molecular Chains Composed Of M₂(RCOO)₄ Units

With the development of microelectronics,the miniaturization trend of microelectronic devices is becoming more and more obvious.However,there are still technical challenges in how to further reduce the geometry of chip components.Different from the traditional"top-down"approach,molecular electronics uses a single molecule or molecular group as a raw material,and adopts a"bottom-up"approach to build logic circuits to realize the functions of microelectronic devices.From the perspective of chemical synthesis,the building blocks used can be prepared in large quantities through rational design and organic synthesis,which provides unlimited possibilities for the development of molecular electronics.With the rapid development of this emerging field,various electrical behaviors and charge transport mechanisms of molecular devices have been extensively studied and reported.However,in the design of molecular devices,the structure-activity relationship between the molecular structure and its electrical properties in molecular electronic devices remains to be further explored.At present,the model molecules used to construct molecular electronic device junctions are mostly directly synthesized organic molecules,which are difficult to achieve precise control of their composition and structure,and difficult to synthesize,which is not conducive to establishing a clear relationship between molecular structure and its electrical properties.In this thesis,a bimetallic unit based on carboxylate coordination(Rh₂(O₂CCR₃)₄,R=CH₃,H and F,referred to as Rh₂)was designed and prepared by a layer-by-layer self-assembly method,and the composition,structure and length were controllable and Self-assembled supramolecules with specific energy level structures.The electrical properties of self-assembled supramolecules were studied by UV-vis absorption spectroscopy,cyclic voltammetry curves and conductive probe atomic force microscopy,and the effects of their composition,structure and energy level on the charge transport properties were explored.The structure-activity relationship between molecular structure and charge transport properties was established,and its charge transport mechanism was discussed.The details are as follows:(1)The first work in this thesis is designed with Rh₂as the building block,pyrazine(L_S),4,4'-bipyridine(L_M)and 1,2-bis(4-pyridine)ethylene(L_L)as bridging ligands.A series of self-assembled supramolecules with similar molecular frameworks and tunable chemical structures,namely(Rh₂L)_n@Au,were assembled on the surface of gold electrodes by a layer-by-layer self-assembly method.Ultraviolet-visible spectroscopy,electrochemical characterization and conductive probe atomic force microscopy(CP-AFM)were used to explore the influence of molecular chain composition and structure on its charge transport properties,and to explore its transport mechanism.It was found that the rectification ratio of self-assembled supramolecules gradually decreased with the increase of the conjugation degree of the bridging ligands(L_SML).Furthermore,when the charge transport mechanism shifts from the tunneling mechanism in the dimer to the hopping mechanism in the tetramer,its rectification ratio increases.This work demonstrates that tuning the charge transport behavior of self-assembled supramolecules by changing the chemical structure of the building blocks deepens the understanding of the structure-activity relationship between molecular structures and their electrical properties in self-assembled supramolecules.(2)The second work of this thesis is designed to link the photosensitive molecule(Pyridine,4,4'-(9,10-anthracenediyl)bis-,ABP)to the Rh₂(O₂CCF₃)₄-pyridine molecular framework through coordination bonds to form self-assembly supramolecular ABP-SM.It is found that the self-assembled supramolecules have good photoresponsive conductivity behavior and can be used as a multifunctional molecular photodetector.Under the bias voltage of±1.0 V,the average current ratio of the self-assembled supramolecules(the ratio of the current without UV irradiation to UV irradiation,I_off/I_on)was 259.At the same time,it was operated continuously for 50 times.In addition,ABP-SM is also very sensitive to the power density and wavelength of the incident light.At-1.0 V bias,as the power density of the incident light decreases from 25.09 m W/cm~2to 0.8 m W/cm~2,the average current ratio also decreases from 259 to 15.Moreover,ABP-SM can obtain effective photoresponse conductivity only under short wavelength(?420 nm)incident light irradiation.In this work,we demonstrate a molecular electronic device that utilizes the energy difference between the ground and excited states of the molecule to produce a photoresponsive electrical conductivity difference.This provides a new idea for designing and fabricating molecular electronic devices with desirable functions.