基于Lindqvist型钼酸盐衍生物分子器件的性质和应用的理论研究
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摘要
分子电子学是电子纳米科学的一个重要分支，属于一个多学科交叉领域，主要以研究分子为对象，或者说在分子水平上研究分子在电子学方面的应用。近年来，得益于实验技术的不断革新和量子输运理论的发展和完善，分子电子学研究领域得到了飞速发展。在开始研究分子电子学时，人们主要聚焦于有机分子，而随着对分子器件性质的认识深入和实验技术手段上的进步，为弥补器件中的缺点和不断完善器件的性能，探索新型分子在分子电子学中的潜在应用已成为人们发展的重要部分，其中研究的对象包括有机小分子，无机化合物，以及生物蛋白分子(DNA)等，研究范围已得到了极大的丰富。运用量子输运理论方法在分子尺度上进行数值计算模拟，可加深认识电子传输的基本物理过程。通过不断探索器件的性质和分子结构之间的关系，有助于寻找性能更优异、功能更丰富的分子电子学元件。多金属氧酸盐（简称多酸）作为一类无机类化合物，由于其优异的化学物理特性已在分子电子学领域显示出重要的应用前景。目前在实验上已对多酸化合物在分子电池和分子存储等方面进行了大量探索研究，但是目前相应的理论研究还很缺乏，特别是纯理论研究多酸分子的电子传输机理还未见报道。另外，对于此类化合物，由于结构的特殊性、实验成膜的相对复杂性以及实验表征手段的局限性等，在分子电子学的实验研究方面尚处于起步阶段。为进一步认识和了解这类化合物的性质，从理论上对该类化合物进行模拟研究，以辅助实验研究，就显得紧迫和重要。特别是近年来，由于量子输运理论的发展完善和计算机硬件的不断提升，促使分子电子学在数值模拟方面得到了迅速发展。通过大量的理论模拟分析，已可有效地预测和分析分子的相关电子学性质，这对认识和理解实验现象和结果也起到了积极作用，理论与实验相结合的方法已成为研究分子电子学的重要手段。
本文以Lindqvist型钼酸盐及其衍生物分子为基础，采用最新发展的DFT+NEGF方法，在理论上对多酸分子在分子电子学中的潜在应用进行了探讨研究，其目的在于：(1)分析解释多酸分子的电子输运特征；(2)设计基于多酸分子的新型有机-无机杂化分子材料，预测其在分子电子学中的应用；(3)分析多酸分子中抗衡离子，如H+在电子传输方面的作用，特别是在分子整流性质方面的调节作用，为分子在制备器件时对实验化学环境的影响提供参考，以助于优化设计器件，以便控制调整相应电子学元件的功能。
本论文的第一部分对分子电子学，特别是分子存储器和分子整流器的研究进行简要介绍，对多酸化合物的电化学性质进行了小结，并对其在分子电子学中的研究现状进行了概述，重点介绍了其在分子存储方面的实验进展，然后对非平衡格林函数方法在电子学应用方面进行简要介绍，最后对计算程序进行简介。第二部分是本研究工作的主体部分，主要包括以下四个方面：
(1)采用DFT+NEGF方法对Lindqvist型多酸化合物[H_2M_6O_(19)](M=Mo, W)的电子输运特性进行理论研究，分析了H+在电子输运性质方面的影响。结果表明：钼和钨酸盐分子具有相似的电子输运特性，表现出不同于有机分子的传输机理，主要以电子隧穿效应为主要特征。不同位置的H离子对于电子输运的总体特征的影响较小。
(2)采用DFT+NEGF方法对单有机胺取代的Lindqvist型钼酸盐的电子输运特征，特别是其在分子二极管方面的应用进行了理论研究与预测。结果表明：此类有机-无机杂化材料具有优异的电子整流比性质，相比有机分子，其整流比有较大的提高。这也是第一类基于多酸的有机-无机杂化的分子二极管模型。
(3)采用DFT+NEGF方法对反式双有机胺取代的Lindqvist型钼酸盐化合物H_2{trans-[Mo_6O_(17)(NAr)_2]}及衍生物的电子输运性质进行理论分析。结果表明：增加反式取代的共轭链长度可得到明显分子二极管特征，另外通过调节不同的质子位置可得到整流方向和大小可调的分子二极管。
(4)采用周期性DFT方法，从理论上对Keggin型钨酸盐化合物与石墨烯的相互作用进行了讨论，研究了[PW_(12)O_(40)]~(3-)与石墨烯之间的相互作用形式和可能存在分子结构。通过分析分子间的结合能，电子结构特征和电荷转移性质等，解释了多酸与石墨烯之间的电荷转移特征，并对多酸吸附于石墨烯表面后对其电子传输性质进行了探讨，这也是首次在理论上研究多酸与石墨烯之间的相互作用。
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.
本文以Lindqvist型钼酸盐及其衍生物分子为基础，采用最新发展的DFT+NEGF方法，在理论上对多酸分子在分子电子学中的潜在应用进行了探讨研究，其目的在于：(1)分析解释多酸分子的电子输运特征；(2)设计基于多酸分子的新型有机-无机杂化分子材料，预测其在分子电子学中的应用；(3)分析多酸分子中抗衡离子，如H+在电子传输方面的作用，特别是在分子整流性质方面的调节作用，为分子在制备器件时对实验化学环境的影响提供参考，以助于优化设计器件，以便控制调整相应电子学元件的功能。
本论文的第一部分对分子电子学，特别是分子存储器和分子整流器的研究进行简要介绍，对多酸化合物的电化学性质进行了小结，并对其在分子电子学中的研究现状进行了概述，重点介绍了其在分子存储方面的实验进展，然后对非平衡格林函数方法在电子学应用方面进行简要介绍，最后对计算程序进行简介。第二部分是本研究工作的主体部分，主要包括以下四个方面：
(1)采用DFT+NEGF方法对Lindqvist型多酸化合物[H_2M_6O_(19)](M=Mo, W)的电子输运特性进行理论研究，分析了H+在电子输运性质方面的影响。结果表明：钼和钨酸盐分子具有相似的电子输运特性，表现出不同于有机分子的传输机理，主要以电子隧穿效应为主要特征。不同位置的H离子对于电子输运的总体特征的影响较小。
(2)采用DFT+NEGF方法对单有机胺取代的Lindqvist型钼酸盐的电子输运特征，特别是其在分子二极管方面的应用进行了理论研究与预测。结果表明：此类有机-无机杂化材料具有优异的电子整流比性质，相比有机分子，其整流比有较大的提高。这也是第一类基于多酸的有机-无机杂化的分子二极管模型。
(3)采用DFT+NEGF方法对反式双有机胺取代的Lindqvist型钼酸盐化合物H_2{trans-[Mo_6O_(17)(NAr)_2]}及衍生物的电子输运性质进行理论分析。结果表明：增加反式取代的共轭链长度可得到明显分子二极管特征，另外通过调节不同的质子位置可得到整流方向和大小可调的分子二极管。
(4)采用周期性DFT方法，从理论上对Keggin型钨酸盐化合物与石墨烯的相互作用进行了讨论，研究了[PW_(12)O_(40)]~(3-)与石墨烯之间的相互作用形式和可能存在分子结构。通过分析分子间的结合能，电子结构特征和电荷转移性质等，解释了多酸与石墨烯之间的电荷转移特征，并对多酸吸附于石墨烯表面后对其电子传输性质进行了探讨，这也是首次在理论上研究多酸与石墨烯之间的相互作用。
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.
引文
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