| The developments of information technology (IT) ask for larger storage capacity and faster computing speed of computers. During the last ten or more years, microprocessor has undergone striking process of miniaturization. For example, at present, the minimum size of transistors, which have the crucial role in today’s integrated circuits based on silicon, is22nm. According to Moore’s Law, size of transistors will be further reduced, and in about2015, their thickness will be only1～2nm, which is in the size region of atom or molecule. Therefore, it is well known that one of the scientific researching subject in the21th century is decreasing the size of conventional electronic devices based on silicon into nano scale. In this size range, many classical technologies and theories will be greatly limited, while the influence of quantum effects becomes more and more important. As a result, people always try to search for new methods for designing and manufacturing electronic devices with a small enough size. Molecular Electronics is just one of these methods.From a general survey of the history of human civilization, every revolution in science and technology would make a huge contribution to the social development and improvement in life quality, whether from the Steam Age to the Electric Age, or from Vacuum Electronics to Microelectronics. At present, it is also a revolution from Microelectronics to Nanoelectronics. Since atoms and molecules are the smallest units that constructing matter, so this revolution obviously has more significant possible meaning, and more profound influence. With so many difficulties in minimizing electronic devices based on silicon, the famous American physicist Richard P. Feynman made a famous speech,"Plenty of Room at the Bottom", at the annual meeting of the American Physical Society (APS) in Dec.1959, which proposed an idea of building electronic devices from atoms or molecules, then assembling them to make circuits. This idea also pointed out that the research content of Molecular Electronics is the physical phenomena and mechanism in molecular functional materials and corresponding devices, and aiming at building electronics devices used in IT from single molecules, especially small organic molecules, to substitute the conventional silicon-based electronic devices. Compared with the conventional silicon-based devices, using single organic molecules as elementary units has many advantages. For example,(i) The size of organic molecules is small, in the range of about1-10nm, which therefore largely decreases the size of devices, and increases the integration level, and consequently enhances computing speed,(ii) We can make use of the interaction between molecules, namely "molecular recognition", to change the electric properties, achieving molecular sensor or molecular switch,(iii) We can tune the transport properties of molecules by changing their components or geometry,(iv) The cost of manufacturing molecular devices is low, and a large number of molecular devices with identical structures can be obtained.Design idea of molecular devices was first proposed by Ratner and Aviram in1974. They built up a bridge molecule containing electron donor and acceptor between two metal electrodes, when applying a certain finite bias on the electrodes, obvious asymmetric current-voltage curve would be observed. This kind of molecules are called molecular rectifiers, which is the earliest idea for constructing devices by using single organic molecules. However, due to the limitation of experimental technology at that time, there was no corresponding experiment to prove it, and Aviram-Ratner’s idea did not cause enough attention.Afterwards, with the development of experimental technology, Molecular Electronics has aroused more and more research interests, many experimental methods have been developed and used in building molecular devices. For example,(i) Mechanically controllable break junction (MCBJ), which fixes a metal wire with a notch on the basement, controls its bend by precise piezoelectric method, stretches and breaks the metal wire at the notch, and obtains a molecular scale electrode gap. Then two approaches can assemble the organic molecules over this gap. One approach is dropping a drop of solution of target molecule on the surface of basement at the same time of preparing electrode gap, and after volatilization the molecular device is obtained. The other approach is exposing the electrode gap in the gas of target molecule after it is prepared, then the molecule will self-assembled between the two electrodes,(ii) Scanning tunneling microscope (STM) pulling method, which is one of the most reliable and flexible methods at present,(iii) Self-calibration template method, which is the latest ingenious method for manufacturing molecular devices,(iv) In addition, there are also other methods, for example, Electromigration method, Nanoparticle-molecular method, Oblique evaporation method, and so on. All of these experimental methods enable us to manufacture all kinds of molecular devices, and measure their electronic transport properties.On the other hand, with the development of experimental technology in Molecular Electronics, theoretical research faces with numerous challenges. Improvements should be made to describe electronic transport properties in molecular devices more accurately. Presently, the method combining density functional theory (DFT) with non-equilibrium Green’s function (NEGF) is effective for devices in molecular scale, and has been used in studying the transport properties of these devices. One important advantage of this method is its universal, with many types of molecular structures can be treated under the same theoretical framework. Since the transport properties can be calculated without parameter adjustment, therefore, this method is also called "first principles" or "ab initid" method.With the development in experimental technology and theoretical method, many kinds of two-probe single molecular devices are designed and prepared. According to the study of their electronic transport properties, many peculiar phenomena are observed, such as Molecular switching, Molecular rectifying, Negative differential resistance (NDR), and so on. All of these properties indicate the wide application prospect of molecular devices.Considering the research object of Molecular Electronics, the next task is how to effectively control the electronic transport properties of molecular devices. At present, there are two main approaches to achieve this,(i) One is through changes in molecular structures. Over the past few decades, people paid more attention on this approach, and have made great advances,(ii) The other approach is introducing a third electrode. The former has one obvious disadvantage, that changes in molecular structure need a certain response time, which makes this kind of devices have a low operation frequency. Therefore, people turned to the later approach these years. The way of introducing the third electrode also has two patterns, Current-regulation and Voltage-regulation. For Current-regulation, the third electrode connects with the device structure, which is similar to semiconductor triode. While for Voltage-regulation, the third electrode does not contact with the device structure, but through applying vertical electric field, which is like the structure of metal-oxide-semiconductor field-effect transistor (MOSFET). The later has become a research hotpot for many advantages in these days. In this dissertation, we did researches based on both approaches. We propose a design of an all-carbon molecular switch based on double-walled carbon nanotube (DWCNT). A segment of (10,0) single-walled carbon nanotube (SWCNT) is placed concentrically outside a (5,0) SWCNT, namely, a (5,0)@(10,0) DWCNT. It is found that the position, orientation and length scaling of the (10,0) SWCNT have crucial effects on the electronic transport properties of the system. When the (10,0) SWCNT is mechanically pushed forward along the axial direction, alternation of on/off switching behavior under low bias and negative differential resistance behavior under high bias are observed. Significant changes in the electronic transport properties arise when rotating the (10,0) SWCNT around the common axis or adding carbon atom layers in the transport direction.(See details in Chapter4.) This study corresponds to the former approach, namely modulating through changes in molecule structures. The emphasis of this dissertation is focused on the later approach, and research contents are as follows:We investigate the electronic transport properties of Au/BDT/Au structure under gate voltages, and obtain obvious field effect.(See details in Chapter5.) Based on this study, we investigate the influence of functional groups on the electronic transport properties, focus on the orientation of the nitro group with respect to the backbone molecule. The result shows that when the dihedral angle is small, obvious Fano-resonance appears in the transmission spectra, which can be tuned by gate voltages to obtain a molecular switch with large on/off’ratio.(See details in Chapter6.) Then we consider many factors that can affect the performance of molecular field-effect devices, such as the size of gate electrode and different applying positions.(See details in Chapter7. Research on other factors will be carried out one after another, see details in Chapter9.)The ultimate goal of Molecular Electronics is constructing electronic devices needed in IT by using single organic molecules to replace conventional semiconductor devices based on silicon, then assembling them to achieve logical functions, and at last building molecular machines. With the achievement in controlling electronic transport properties in organic molecular devices, the next step is to combine organic molecular devices with controllable electronic transport properties to achieve certain logical functions. So we need to find the combination law under atomic or molecular scale, which will meet quantum interference (QI) effect unavoidably. Therefore, we also do some research on QI in molecular devices, which will provide series-parallel combination and integration of molecular devices with important theoretical directions.(See details in Chapter8and9.)... |
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