Theoretical Study Of Properties And Applications Of Lindqvist-type Molybdate And Its Derivatives In Molecular Device

The molecular electronics is a new concept developed from nanoscience. It is an emerging,and multidisciplinary field with focus on the single molecules. In recent years, mainly due tothe fast development of high-tech innovation, especially the rapid developments of quantumtransport theory result in great progress in molecular electronics. In experimental aspect, peo-ple have constantly explored new classed of molecules from small organic (inorganic) mole-cules, to the biological protein molecule. The relationship between the properties and perfor-mance has gained great attention. The POM compounds due to the special stability, diversityand unique chemical and physical properties, have potential application in analytical chemis-try, catalyst, and material science. Using Density Functional Theory (DFT) calculations to re-veal the relationship between the structure and their properties would be helpful to understandthe area and to explore new field applications. Due to the experimental characterization limits,the POM, whose applications on molecular cell and storage (memory) are still in its primarystage. The phenomena and results in experiment have been in-depth understood from the the-oretical aspect. The role of numerical simulation at the molecular scale level has played animportant role in understanding the basic physical processes.In current status, very seldom theoretical studies have been carried out on POM based mo-lecular devices. It is important to perform the theoretical investigation of this type of com-pounds to further development of its molecular electronics, and guide the further developmentin molecular electronics. With the development of quantum transport theory and computertechnology, it is possible to perform more efficient numerical simulation and predict theproperties of the molecules. This should be helpful to provide a guide for the experimentalexploration.In the present thesis, using DFT+NEGF method, we carried out systematic theoreticalstudies on Lindqvist type molybdate molecule and its derivatives based molecular junction toexplore their application on molecular electronic. The aim of this thesis is that:(1) to explainthe electron transport property of Lindqvist type POM,(2) to design a new type of moleculardevice based on the organic-inorganic hybrid system,(3) to explore the role of H+in the elec-tron transport, especially the effect on molecular rectification.Part one of this thesis is a review of molecular electronics, mainly concert in the molecularmemory and rectifier. Then, the electrochemical properties of POM and recent developmentsin molecular electronics have been summarized. Finally, a brief introduction of NEGF methodis derived. The second part is the main body of this thesis; it includes the following four as- pects:(1) We report a first study of Lindqvist polyoxometalates (Mo and W) based molecularjunctions by using the DFT+NEGF method. We find that protonation weakly affects thetransport properties, which are also similar for both Mo-and W-based complexes. In particu-lar, the transmission at the Fermi level is dominated by a tunneling mechanism with the firstof the molecular levels available for resonant transport being at least1eV away from Fermienergy of electrode.(2) We have designed a new type of molecular diode based on organoimido derivatives ofhexamolybdates and investigated the transport behavior using DFT combined with the NEGFformalism. These new types of inorganic-organic hybrid systems are predicted to show a highand robust rectification ratio, which can be maintained in a large bias range. It is possible toenhance the rectification ratio by increasing the conjugate length of organic part.(3) We have studied the molecular junction based on the trans-diorganoimido derivatives ofhexamolybdates by exploring the transport properties using DFT+NEGF. The asymmetriccurrent-voltage characteristics were obtained for the model with different protonation sites.The current magnitude and direction of molecular rectification can be tunable with protona-tion. To increase the conjugate length of one organic group of hexamolybdate could also re-sult in rectifying behavior.(4) We have carried out ab initio study on the structure of PW12deposited on a pristinegraphene with periodic DFT and pseudopotential theory. The charge transfers, adsorption en-ergy and electronic structure characteristics between POM and grapheme have been analysedand discussed.