Preparation Of HfO₂ Coated Tip And Its Application In The Study Of Carbazole Anions Electrical Transport Properties In Polar Solvents

The popularization of precise measurement techniques such as Scanning Tunneling Microscopy Break Junction(STM-BJ)have been widely applied to the researches on the relationship between the physical and chemical properties of substances and their structures at the single-molecule level,and provide potentials for designing the functional molecular devices based on the bottom-up strategy.The microenvironment in which the molecules are located has a significant influence on the performance of molecular devices.For example,the polarity of the solvent,the pH of the solution and the water content of the solvent all might affect the configuration of the molecular junction,thereby changing the electrical transport properties of the molecular device.However,the testing of molecular electrical transport properties in relatively harsh environments requires special functionalized STM tips.The Apiezon wax coated tips were often used in the STM-BJ,but there were only suitable for inorganic solvents or weakly polar solvents.The development of a more versatile functional tip can not only improve the test accuracy of the instrument,but also expand the range of research objects.This thesis is based on the hafnium dioxide coated tip prepared by atomic layer deposition technology,which is applied to the STM-BJ,and the relationship between the deprotonation effect of carbazole and the charge transport in polar solvents is systematically studied.The main research contents are as follows:1.Preparation of functional tip:To expand the application range of STM-BJ technology,we used the Atomic Layer Deposition technology to prepare the hafnium dioxide coated tip.By studying and comparing the electrochemical cyclic voltammetry curves of the tip under different preparation methods and parameters,it was found that the tip surface was pretreated with(3-Mercaptopropyl)methyl-dimethoxy silane(MPS)can significantly improve the density of the hafnium dioxide coated layer.Combined with the usage of the tip,125 atomic layer deposition cycles(about 12.5 nm)met the requirements of the electrical test window in the STM-B J experiment,and was the best choice when considering the cost.The test of STM-BJ showed that the coated tip prepared under this process condition met the test condition of the electrical transport properties of terphenyl molecules in polar solvents.2.Study on the electrical transport properties of the electric field catalyzed assembly of terphenyl:We have studied the electrical transport properties of terphenyl molecules under different bias voltages,and found that the probability of low conductance of terphenyl molecules increases under high bias voltages.Theoretical calculations prove that the terphenyl molecules are assembled to form dimers with lower conductance under the catalysis of the strong electric field,which also leads to a change of its electrical transport mode from through-bond to through-space.3.The study on the electrical transport mode of carbazole anion:We studied the effect of deprotonation reaction on the electrical transport properties of carbazole derivatives.First,a series of carbazole derivatives were designed and synthesized.We used the STM-BJ and the hafnium dioxide coated tip to test their electrical transport properties in polar solvents,and the response of molecules connected to different substitution sites to negative charges was studied.The results showed that the existence of negative charges strongly inhibited the electrical transport ability of the para-linked carbazole derivatives,but had no obvious effect on the electrical transport ability of the meta-linked carbazole derivatives.The DFT calculation reveals that the presence of negative charge leads to the localized distribution of HOMO level and the delocalized distribution of LUMO level for the para-carbazole anion,resulting in the destructive quantum interference effect and the formation of the ring current.