二嵌段分子整流器的电荷输运性质及调控机理的理论研究
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摘要
随着信息技术的快速发展,量子效应在半导体集成电路中越来越不容忽视,逐渐成为其中电荷输运的主导效应。微电子学在传统工艺的基础上设计制造集成度更高、性能更优异的电路时,将会面临越来越大的困难,这将使得整个信息技术产业面临硬件层面的停滞。要延续当前集成电路的发展趋势(Moore定律),探索新的技术路线势在必行,而科学界所公认的最具可行性的路线之一就是发展分子电子学——用分子官能团来构建功能器件,组成电路,最终实现“自下而上”自组装地“生长”集成电路。
近30年间,随着STM、MCBJ、AFM、SAM、C60、CNT、Graphene等实验技术和材料的相继问世,分子电子学已经由早期的理论预言发展成为一个规模庞大、成果丰富、涉及物理学、化学、材料科学以及生命科学等领域的交叉学科。在众多实验和理论研究热点中,分子整流效应与分子整流器一直以来受到极大地关注,成为最热门的问题之一
本文首先回顾了IT技术与分子电子学的发展历程,简述了三种分子整流器模型机理与特点,然后简要阐述了本文理论研究所用的理论模型与方法,包括：SSH哈密顿量、格林函数方法、SSH+NEGF方法、第一性原理计算方法。针对文献中实验观测到的分子整流器,采用上述方法分三部分进行了讨论：
一、针对Yu研究组在Au(111)/2Ph2Py/Au(STM Tip)分子整流器的研究,采用SSH+NEGF方法,建立沿电荷输运方向的准一维分子结模型。成功的模拟再现了实验中的分子整流器,讨论分子结的整流机理。然后通过调整参数,模拟对分子结中的电子—晶格相互作用的调制,来实现对分子的电偶极矩的改变,最终可以实现对分子结中的电荷输运性质的调制。发现当分子结的电子—晶格相互作用增强时,分子的电偶极矩近乎线性的下降,进而发现分子结的正向开启偏压随电偶极矩增大而近线性的下降。
二、针对Whitesides研究组基于SAM技术和导电AFM技术的Ag/HSC11Fc/GaIn分子结实验,采用基于DFT+NEGF方法进行了模拟。为了证明分子结的整流效应是HSC11Fc分子内禀属性,我们建立了一系列具有对称电极的Au(111)HSCnFc/Au(111)(n=11.9.7)分子结,并在其中发现明显的整流现象,得到HSC11Fc的整流比RR11～100,而HSC9Fc的整流比RR9<10,这与实验结果符合得很好。我们发现此分子结满足π-σ模型。通过对Whitesides的模型进行修正,成功地解释了在n=11的分子结的整流比明显大于n=9和7的原因。同时指出只有当分子结足够长时,才能有很大的整流比。虽然此类分子结有出色的整流效果,但是,分子结中包含较长的饱和烷链,因而电导率过低,使得开启后的电流仍然太小。所以,我们设计并模拟了由四联苯构成的分子结,通过施加不对称门电压来实现的可控整流方向的分子晶体管。
三、通过DFT+NEGF的方法,改变分子结中的发子构象,更为形象具体地展现了分子结中电子—晶格相互作用对π共轭分子整流器中的电荷输运的影响。电子—晶格相互作用会使得分子中各官能团之间发生相对扭转畸变,从而对电子输运性质产生影响,实现分子结的整流性能。
最后,进行了总结与展望。
As the rapid development of information technology (IT), the quantum effect was playing a more and more important role in the inorganic semiconductor based integrated circuit, which would have been a dominating effect on the charge transport. Therefore, there were heavily increasing numbers of difficulties in designing and manufacturing new applications with higher levels of integration and performance, which lead to the whole IT industry slow down its steps. The new route to the new generation electronics must be found to continue the trend of the advancements, the Moore's Law. The molecular electronics was one of the most popular reasonable routine to solving the embarrassment, which meant a "bottom-up" self-assembly construction of molecular circuits with functional organic groups.
During the nearly30years, molecular electronics developed quickly, with the inventions of technologies of STM, MCBJ, AFM, SAM, C60, CNT, Grphene and etc. The molecular electronics now had been an interdisciplinary research area, blending concepts and technologies from physics, chemistry, material and biological science. In large number of the research points, the issue on molecular rectification and rectifier were the hottest one and kept attracting eyes of researchers intensively.
In this thesis, after reviewing the progress of the IT industry and the molecular electronics, we depicted three models for molecular rectification. And then, we described the theoretical models and methods, such as the SSH Hamiltonian, Green's Function method, SSH+NEGF method, First Principle Calculation method and DFT+NEGF method. In the next part, we focused and discussed on some excellent experimental results reported by several groups.
Firstly, according to the experiments of Yu et al., we employed an extended SSH type tight-binding Hamiltonian coupled with the NEGF method to simulate the2Ph2Py based molecular rectifier successfully. We explained the rectifying mechanism in this conjugated molecule. Then we found that the inner molecular electric dipole moment could be modulated by electron-lattice coupling, which was a key factor for the charge transport. The dipole moment decreased linearly with the e-1coupling increasing, while the positive threshold bias voltage increased contrarily.
Secondly, according to the Whitesides'AFM experiments on SAM of HSC11Fc molecule, we performed a DFT+NEGF calculation to simulate the junctions. The rectification in the HSC11Fc based junction was confirmed as its inner molecular property by study a series of junctions with two symmetric electrodes (Au(111)/HSCnFc/Au(111), n=11,9,7). Corresponding to the experimental results, the rectifying ratio (RR) of HSC11Fc was larger than100, however, the RR of HSC11Fc was less than10. According to the analysis on its mechanism, we believed that this was a π-σ type rectifier. By revising the Whitesides'explanation, we interpreted the great difference of RRs between the cases of n=9and n=7. Meanwhile, it was pointed out that the giant RR did not work unless the molecule was long enough. On the other hand, because of the saturated alkyl chain in the junction, the conductance was pretty low, therefore, the current was rather poor. This was not good for the future application. As a result, we designed a gate modulating1,4-(phenyl)4based transistor, which could work as an excellent direction reversible rectifier with sufficient conducting ability.
Thirdly, a deeper study of the2Ph2Py molecular junction was performed by taking a DFT+NEGF calculation. The interaction between e-1coupling and charge transfer in this π conjugated molecular rectifier was exhibited. Especially, the lattice distortion was represented concretely by the relative torsions between the phenyl rings, which severely affected the charge transportation. Finally, it could be seen the distortion was of the essence in the rectifying progress.
Finally, we summarized our work and gave an outlook of this field.
近30年间,随着STM、MCBJ、AFM、SAM、C60、CNT、Graphene等实验技术和材料的相继问世,分子电子学已经由早期的理论预言发展成为一个规模庞大、成果丰富、涉及物理学、化学、材料科学以及生命科学等领域的交叉学科。在众多实验和理论研究热点中,分子整流效应与分子整流器一直以来受到极大地关注,成为最热门的问题之一
本文首先回顾了IT技术与分子电子学的发展历程,简述了三种分子整流器模型机理与特点,然后简要阐述了本文理论研究所用的理论模型与方法,包括：SSH哈密顿量、格林函数方法、SSH+NEGF方法、第一性原理计算方法。针对文献中实验观测到的分子整流器,采用上述方法分三部分进行了讨论：
一、针对Yu研究组在Au(111)/2Ph2Py/Au(STM Tip)分子整流器的研究,采用SSH+NEGF方法,建立沿电荷输运方向的准一维分子结模型。成功的模拟再现了实验中的分子整流器,讨论分子结的整流机理。然后通过调整参数,模拟对分子结中的电子—晶格相互作用的调制,来实现对分子的电偶极矩的改变,最终可以实现对分子结中的电荷输运性质的调制。发现当分子结的电子—晶格相互作用增强时,分子的电偶极矩近乎线性的下降,进而发现分子结的正向开启偏压随电偶极矩增大而近线性的下降。
二、针对Whitesides研究组基于SAM技术和导电AFM技术的Ag/HSC11Fc/GaIn分子结实验,采用基于DFT+NEGF方法进行了模拟。为了证明分子结的整流效应是HSC11Fc分子内禀属性,我们建立了一系列具有对称电极的Au(111)HSCnFc/Au(111)(n=11.9.7)分子结,并在其中发现明显的整流现象,得到HSC11Fc的整流比RR11～100,而HSC9Fc的整流比RR9<10,这与实验结果符合得很好。我们发现此分子结满足π-σ模型。通过对Whitesides的模型进行修正,成功地解释了在n=11的分子结的整流比明显大于n=9和7的原因。同时指出只有当分子结足够长时,才能有很大的整流比。虽然此类分子结有出色的整流效果,但是,分子结中包含较长的饱和烷链,因而电导率过低,使得开启后的电流仍然太小。所以,我们设计并模拟了由四联苯构成的分子结,通过施加不对称门电压来实现的可控整流方向的分子晶体管。
三、通过DFT+NEGF的方法,改变分子结中的发子构象,更为形象具体地展现了分子结中电子—晶格相互作用对π共轭分子整流器中的电荷输运的影响。电子—晶格相互作用会使得分子中各官能团之间发生相对扭转畸变,从而对电子输运性质产生影响,实现分子结的整流性能。
最后,进行了总结与展望。
As the rapid development of information technology (IT), the quantum effect was playing a more and more important role in the inorganic semiconductor based integrated circuit, which would have been a dominating effect on the charge transport. Therefore, there were heavily increasing numbers of difficulties in designing and manufacturing new applications with higher levels of integration and performance, which lead to the whole IT industry slow down its steps. The new route to the new generation electronics must be found to continue the trend of the advancements, the Moore's Law. The molecular electronics was one of the most popular reasonable routine to solving the embarrassment, which meant a "bottom-up" self-assembly construction of molecular circuits with functional organic groups.
During the nearly30years, molecular electronics developed quickly, with the inventions of technologies of STM, MCBJ, AFM, SAM, C60, CNT, Grphene and etc. The molecular electronics now had been an interdisciplinary research area, blending concepts and technologies from physics, chemistry, material and biological science. In large number of the research points, the issue on molecular rectification and rectifier were the hottest one and kept attracting eyes of researchers intensively.
In this thesis, after reviewing the progress of the IT industry and the molecular electronics, we depicted three models for molecular rectification. And then, we described the theoretical models and methods, such as the SSH Hamiltonian, Green's Function method, SSH+NEGF method, First Principle Calculation method and DFT+NEGF method. In the next part, we focused and discussed on some excellent experimental results reported by several groups.
Firstly, according to the experiments of Yu et al., we employed an extended SSH type tight-binding Hamiltonian coupled with the NEGF method to simulate the2Ph2Py based molecular rectifier successfully. We explained the rectifying mechanism in this conjugated molecule. Then we found that the inner molecular electric dipole moment could be modulated by electron-lattice coupling, which was a key factor for the charge transport. The dipole moment decreased linearly with the e-1coupling increasing, while the positive threshold bias voltage increased contrarily.
Secondly, according to the Whitesides'AFM experiments on SAM of HSC11Fc molecule, we performed a DFT+NEGF calculation to simulate the junctions. The rectification in the HSC11Fc based junction was confirmed as its inner molecular property by study a series of junctions with two symmetric electrodes (Au(111)/HSCnFc/Au(111), n=11,9,7). Corresponding to the experimental results, the rectifying ratio (RR) of HSC11Fc was larger than100, however, the RR of HSC11Fc was less than10. According to the analysis on its mechanism, we believed that this was a π-σ type rectifier. By revising the Whitesides'explanation, we interpreted the great difference of RRs between the cases of n=9and n=7. Meanwhile, it was pointed out that the giant RR did not work unless the molecule was long enough. On the other hand, because of the saturated alkyl chain in the junction, the conductance was pretty low, therefore, the current was rather poor. This was not good for the future application. As a result, we designed a gate modulating1,4-(phenyl)4based transistor, which could work as an excellent direction reversible rectifier with sufficient conducting ability.
Thirdly, a deeper study of the2Ph2Py molecular junction was performed by taking a DFT+NEGF calculation. The interaction between e-1coupling and charge transfer in this π conjugated molecular rectifier was exhibited. Especially, the lattice distortion was represented concretely by the relative torsions between the phenyl rings, which severely affected the charge transportation. Finally, it could be seen the distortion was of the essence in the rectifying progress.
Finally, we summarized our work and gave an outlook of this field.
引文
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1 Aviram, A.& Ratner, M. A. Molecular rectifiers. Chemical Physics Letters 29,277-283, doi:10.1016/0009-2614(74)85031-1(1974).
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2 Ghosh, S. et al. Device structure for electronic transport through individual molecules using nanoelectrodes. Applied Physics Letters 87,233509, doi:10.1063/1.2140470 (2005).
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28 Nijhuis, C. A., Reus, W. F., Siegel, A. C.& Whitesides, G. M. A Molecular Half-Wave Rectifier. Journal of the American Chemical Society 133,15397-15411, doi:10.1021/ja201223n(2011).
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1 Aviram, A.& Ratner, M. A. Molecular rectifiers. Chemical Physics Letters 29,277-283, doi:10.1016/0009-2614(74)85031-1(1974).
2 Simmons, J. G. Generalized Formula for the Electric Tunnel Effect between Similar Electrodes Separated by a Thin Insulating Film. Journal of Applied Physics 34,1793-1803 (1963).
3 Ghosh, S. et al. Device structure for electronic transport through individual molecules using nanoelectrodes. Applied Physics Letters 87,233509, doi:10.1063/1.2140470 (2005).
4 Xiao, Xu& Tao, N. J. Measurement of Single Molecule Conductance:Benzenedithiol and Benzenedimethanethiol. Nano Letters 4,267-271, doi:10.1021/n1035000m (2004).
5 Chen, X. et al. Chemical fabrication of heterometallic nanogaps for molecular transport junctions. Nano Letters 9,3974-3979, doi:10.1021/n19018726 (2009).
6 Pan, S., Zhao, A., Wang, B., Yang, J.& Hou, J. Controlling Electronic States and Transport Properties at the Level of Single Molecules. Advanced Materials 22,1967-1971, doi:10.1002/adma.200903795 (2010).
7 Song, H., Reed, M. A.& Lee, T. Single Molecule Electronic Devices. Advanced Materials 23, 1583-1608, doi:10.1002/adma.201004291(2011).
8 Tsutsui, M., Teramae, Y., Kurokawa, S.& Sakai, A. High-conductance states of single benzenedithiol molecules. Applied Physics Letters 89,163111-163113 (2006).
9 Armstrong, N., Ho ft, R. C, McDonagh, A., Cortie, M. B.& Ford, M. J. Exploring the Performance of Molecular Rectifiers:Limitations and Factors Affecting Molecular Rectification. Nano Letters 7,3018-3022, doi:10.1021/n10714435 (2007).
10 Stadler, R., Geskin, V.& Cornil, J. A theoretical view of unimolecular rectification. Journal of physics. Condensed matter:an Institute of Physics journal 20,374105, doi:10.1088/0953-8984/20/37/374105(2008).
11 He, J.& Lindsay, S. M. On the Mechanism of Negative Differential Resistance in Ferrocenylundecanethiol Self-Assembled Monolayers. Journal of the American Chemical Society 127,11932-11933, doi:10.1021/ja0532279 (2005).
12 Tivanski, A. V.& Walker, G. C. Ferrocenylundecanethiol Self-Assembled Monolayer Charging Correlates with Negative Differential Resistance Measured by Conducting Probe Atomic Force Microscopy. Journal of the American Chemical Society 127,7647-7653, doi:10.1021/ja0514491 (2005).
13 Zhai, Y. etal. Negative differential resistance in molecular devices:the role of molecule-electrode coupling. Science China Physics, Mechanics and Astronomy 54,1455-1460, doi:10.1007/s11433-011-4406-x(2011).
14 Ji, G. et al. Rectifying behaviors of an Au/(C20)2/Au molecular device induced by the different positions of gate voltage. RSC Advances 2,11349, doi:10.1039/c2ra21146g (2012).
15 Diez-Perez, I. et al. Rectification and stability of a single molecular diode with controlled orientation. Nat Chem 1,635-641, doi:http://www.nature.com/nchem/journal/v1/n8/suppinfo/nchem.392 S1.html (2009).
16 Hu, G. C, Wei, J. H.& Xie, S. J. Bias-induced orbital hybridization in diblock co-oligomer diodes. Applied Physics Letters 91,142115-142113 (2007).
17 Reus, W. K, Thuo, M. M., Shapiro, N. D., Nijhuis, C. A.& Whitesides, G. M. The SAM, Not the Electrodes, Dominates Charge Transport in Metal-Monolayer//Ga2O3/Gallium-Indium Eutectic Junctions. ACS Nano 6,4806-4822, doi:10.1021/nn205089u (2012).
18 Keren, K., Berman, R. S., Buchstab, E., Sivan, U.& Braun, E. DNA-Templated Carbon Nanotube Field-Effect Transistor. Science 302,1380-1382, doi:10.1126/science.1091022 (2003).
19 Datta, S. S., Strachan, D. R.& Johnson, A. T. C. Gate coupling to nanoscale electronics. Physical Review B 79,205404, doi:10.1103/PhysRevB.79.205404 (2009).
20 Xu, Y. et al. Gated electronic currents modulation and designs of logic gates with single molecular field effect transistors. Applied Physics Letters 99,043304, doi:10.1063/1.3615691 (2011).
21 Stadler, R. Quantum interference effects in electron transport through nitrobenzene with pyridil anchor groups. Physical Review B 80,125401, doi:10.1103/PhysRevB.80.125401 (2009).
22 Fang, C. et al. Effect of different electrodes on Fano resonance in molecular devices. Applied Physics Letters 100,023303-023304, doi:10.1063/1.3676190 (2012).
23 Nijhuis, C. A., Reus, W. F.& Whitesides, G. M. Molecular Rectification in Metal-SAM-Metal Oxide-Metal Junctions. Journal of the American Chemical Society 131, 17814-17827, doi:10.1021/ja9048898 (2009).
24 Nijhuis, C. A., Reus, W. F.& Whitesides, G. M. Mechanism of Rectification in Tunneling Junctions Based on Molecules with Asymmetric Potential Drops. Journal of the American Chemical Society 132,18386-18401,doi:10.1021/ja108311j (2010).
25 Nijhuis, C. A., Reus, W. F., Siegel, A. C.& Whitesides, G. M. A Molecular Half-Wave Rectifier. Journal of the American Chemical Society 133,15397-15411, doi:10.1021/ja201223n(2011).
26 Yoon, H. J. et al. The Rate of Charge Tunneling through Self-Assembled Monolayers Is Insensitive to Many Functional Group Substitutions. Angewandte Chemie 124,4736-4739, doi:10.1002/ange.201201448(2012).
27 Xiao, Xu& Tao, N. J. Measurement of Single Molecule Conductance:Benzenedithiol and Benzenedimethanethiol. Nano Letters 4,267-271, doi:10.1021/nl035000m (2004).
28 Morales, G. M. et al. Inversion of the rectifying effect in diblock molecular diodes by protonation. JAm Chem Soc 127,10456-10457, doi:10.1021/ja051332c (2005).
29 Huang, J., Li, Q., Li, Z.& Yang, J. Rectifying Effect in Polar Conjugated Molecular Junctions: A First-Principles Study. Journal of Nanoscience and Nanotechnology 9,774-778, doi:10.1166/jnn.2009.C022 (2009).
30 Xue, Y.& Ratner, M. Microscopic study of electrical transport through individual molecules with metallic contacts. Ⅰ. Band lineup, voltage drop, and high-field transport. Physical Review B 68,115406, doi:10.1103/PhysRevB.68.115406 (2003).
31 Xue, Y.& Ratner, M. Microscopic study of electrical transport through individual molecules with metallic contacts. Ⅱ. Effect of the interface structure. Physical Review B 68,115407, doi:10.1103/PhysRevB.68.115407 (2003).
32 Sun, Z., Li, Y, Xie, S., An, Z.& Liu, D. Scattering processes between bipolaron and exciton in conjugated polymers. Physical Review B 79, doi:10.1103/PhysRevB.79.201310 (2009).
33 toolkit. A. Atomistix toolkit, http://www.quantumwise.com/.
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35 Diez-Perez, I. et al. Controlling single-molecule conductance through lateral coupling of pi orbitals. Nature nanotechnology 6,226-231, doi:10.1038/nnano.2011.20 (2011).