Tuning Charge Transport In Single-Molecule Junction Based On Molecular Design

The manufacturing process of the semiconductor industry of integrated circuits has been developed in a top-down manner according to Moore's Law until now.After development of several decades,the size of the solid devices is approaching their physical limits.To overcome the challenges,the organic molecules are considered as promising candidates for future electronic components,which is expected to further reduce the feature sizes and increase the integration level of integrated circuits.Molecular electronics mainly studies the electrical properties of single molecules,provides the basis of building integrated circuits for single-molecule electronic devices in a bottom-up manner.Fabrication of stable single-molecule junctions as well as the accurate measurement of their charge transport properties and structure-function relationship,are the main scientific issues in molecular electronics studies.To investigate the structure-function relationship in charge transport through single-molecule junction,the coupling interaction between the various building blocks plays a critical role.With focus on the structure-function relationship of the charge transport through molecules,this thesis studies the regulation of coupling interactions between each building blocks in molecular electrical transport using scanning tunneling microscopy break junction technique.The main contents of this thesis are as follows:1.We introduced the basic principle,experimental setup,experimental methods and data processing methods of scanning tunneling microscope break junction technology.The modeling and calculation methods of charge transport in single-molecule junctions are discussed,as well as the result analysis of the transmission spectra.2.By designing a large inter-ring twisted angles between building blocks to localize electronic structure of the molecules,modularized tuning of charge transport in single-molecule junction can be achieved,for which a single building block of molecules can be modified individually without perturbation from other building blocks.By modification of the core module,tuning of the charge transport through the single-molecule junction can be achieved by changing the coupling interaction between the main charge transport channel and the metal electrode,which inserts a charge transport channel with strong coupling with the electrode near the Fermi level.3.By adding hydroxyls in the molecules to introduce intramolecular hydrogen bonds in the molecules,the resonance structure of the molecule can be changed,for which selective opening or closing a specific transmission pathways can be achieved.Thereby the coupling interaction between the frontier molecular orbitals can be tuned,for which quantum interference can be introduced or suppressed.We suppress quantum interference by blocking specific transmission path,which is also the first direct evidence of quantum interference in multipath electrical transport.4.By introducing unpaired electrons into the molecule by free radicals,the molecule has an open-shell electronic structure,and the major charge transport channel of the molecule becomes SOMO orbits with strong coupling with the electrode,which is significantly different with the molecules with closed-shell electronic structure.The introduction of SOMO orbits breaks the structure-function relationship rules of quantum interference in the molecular charge transport process of conventional molecules with closed-shell electronic structures,and builds up a new spin-based charge transport rule.In addition,from the measurement of current-voltage curves,we found that molecules with open-shell electronic structure can exhibit negative differential resistance or Coulomb blockade at room temperature,opening the new avenue towards open-shell molecular charge transport.5.We summarize this thesis,and a theory of molecular charge transport based on electrical excitation of molecule was developed according to our understandings on the nature of charge transport in single-molecule junction.