Investigation Of Charge Transport Through Supramolecular Interaction By Single-Molecule Break Junction Techniques

Molecular electronics is proposed to fabricate various functional molecular devices and even electronic circuits using a single-molecule or a few molecules as components.With the development of a series of single-molecule electrical characterization technologies,it not only meets the technical requirements of direct fabrication and investigation of molecular electronic devices,but also provides a platform for studying the intrinsic properties of materials at molecular scale.At present,Mechanically Controllable Break Junction(MCBJ)and Scan Tunneling Microscope Break Junction(STMBJ)have been widely used to characterize the electrical transport properties of single-molecules.The acquisition of the electrical transport properties of singlemolecule also extends the research scope of single-molecule electronics to more complex systems.It has been developed from the initial quantum conductance measurement of metal atomic wires to the study of electrical transport properties of organic small molecules,nanoclusters,polypeptide molecules,DNA sequences,and even biological macromolecules.In addition to the measurement of single molecule,the use of single-molecule electrical characterization technology to study the electrical transport of supramolecular assemblies has become a frontier in the field of singlemolecule science recently.The investigation of transport properties through noncovalent self-assemblies at single-molecule scale can guide the rapid assembly of functional molecular devices through supramolecular interaction,thus avoiding complex organic synthesis,and helping to explore the mechanism of supramolecular interaction and discover its unique effects.Current studies have proved that supramolecular interactions can not only be used to assemble stable molecular junctions,but also provide the comparable transport capacity relative to covalent bonds.Molecular devices based on supramolecular interactions,such as molecular rectifiers,molecular transistors,have also been reported.With the development of supramolecular chemistry,exploring the transport properties of non-covalent supramolecular interactions is of great importance for the design and assembly of molecular devices.However,there are few reports on the transport of supramolecular assemblies at singlemolecule scale.Since the supramolecular assemblies are assembled based on weak interaction,the characterization of electrical properties through supramolecular assemblies are more difficult than that through single-molecule systems with covalent bonds.It poses a new technical challenge to the development of characterization technology.At the same time,how to select a highly suitable system for assembly is also a key scientific problem in this field.This thesis is based on the single-molecule electrical measurement platform and data statistical analysis method independently developed by the research team from author's supervisor.The electrical transport properties of a series of supramolecular assemblies were studied at single-molecule scale through MCBJ and STMBJ techniques.The main research contents and results are as follows:1.Supramolecular junctions bridged by hydrogen bonds between hydroxyl groups were designed and constructed based on the home-built STMBJ single-molecule conductivity testing system.Quantitative characterization of transport properties through hydrogen bond was realized based on the self-built STMBJ single-molecule conductivity testing system and corresponding statistical analysis methods.The charge transport properties of a series of hydrogen-bonded supramolecular junctions with different assembly lengths have been studied.It is found that the insertion of hydrogen bonds reduces the decay constant of conductivity with decreasing length compared with that of alkane chains.The electrical transport properties of supramolecular junctions constructed by hydrogen bond bridging between carboxylic groups have also been studied,and the mechanism of lower decay constant caused by hydrogen bonding has been revealed by DFT calculation.2.A series of diketopyrrolopyrole(DPP)molecules with different aromatic rings and intramolecular hydrogen bonds were designed and connected to the metal electrodes to form the single-molecule junctions.The strength of intramolecular hydrogen bond between pyrrole and aromatic substituents could be effectively regulated by the steric hindrance of the substituted aromatic rings.Using the MCBJ single-molecule conductivity test platform built by ourselves,the single-molecule conductivities of these molecules were tested by the notched gold wire chip.It was found that the singlemolecule conductivity of DPP unit was regulated by different aromatic rings with up to two orders of magnitude.The important role of intramolecular hydrogen bonds in charge transport of micro-components is confirmed by theoretical analysis of molecular structure,and the regulation of electrical transport properties of molecular wires is realized based on this.3.Van der Waals interaction based graphene/fullerene/graphene device are fabricated and their charge transport properties are characterized.Based on the developed MCBJ system and graphene chip,we have constructed graphene/fullerene/graphene heterojunction,which is a single-molecule scale 0D/2D mixed full-carbon molecular junction with a series of fullerenes.The conductance of all carbon molecular junctions assembled by fullerenes(such as C60,C70,C76 and C90)were characterized.The results show that the fullerenes can be bridged between the two electrodes of CVD(Chemical Vapor Deposition)-grown graphene to form a single fullerene junction through van der Waals interaction.More importantly,the conductance of graphene/fullerene/graphene molecular junction can be adjusted by changing the band gap through inserting different fullerenes,and the range of conductance regulation is more than one order of magnitude.In addition,we further regulate electrical transport by breaking the conjugation of the system and doping fullerenes with N atoms.The results of conductivity measurement of all carbon molecular junctions were verified by DFT calculations.