| Microelectronics based on silicon transistor has witnessed rapid development in the past few decades and the miniaturization of electronic devices continues ceaselessly. However, due to the limits in production process and physical principles, the development is now facing a severe bottleneck. Using single molecules to build functional electronic devices and replacing traditional silicon transistors has become a widely accepted trend. Molecular switch is an important kind of molecular electronic device. Since it can be used as the basic unit in logic gate and memory circuit, it has always been a hot topic in molecular electronics. This thesis aims at designing novel molecular switches and provides theoretical predictions and explanations. Molecular quantum-dot cellular automata (MQCA) is a kind of molecular switch based on intercellular electrostatic interactions. It can be used to build logic circuits for novel computers, which have higher computing speed, extremely low power dissipation and wide application prospect. Another kind of electro-controllable molecular switch is now attracting much research interest. Since the surface function can be regulated by electrical stimulus, it is expected to be widely used in biological, chemical and microelectronic fields. In order to theoretically design novel MQCA, and investigate the conformational change and switching process of the electro-controllable surface oligopeptide molecular switches, we carry out both quantum chemical calculations and molecular dynamics simulations. The results and conclusions of this thesis are summarized as follows:1. Theoretical design of molecular quantum-dot cellular automataDouble-cage fluorinated fullerenes have extremely strong electron affinities, thus are able to encapsulate an excess electron in either cage and form bistable charge configurations. By applying an external electric field, we can realize intercage electron transfer, i.e., switch between bistable charge configurations. The two equivalent charge configurations can be used to represent logic "0" and logic "1", and thus encode binary information. In this way, an MQCA cell is built.We investigate the electronic structures for several double-cage systems with different kinds of bridges (e-@C20F18(X)2C20F18, X=-NH-,-BH-,-CO-). Using a series of background point charge models, the Coulomb interaction between neighboring MQCA cells is simulated, along with the response to an input driver. The result shows that double-cage fluorinated fullerenes have excellent QCA function, i.e., the encapsulated electron can transfer between the two cages of the double-cage system, thus realizing the transition between bistable states, or logic "0" and logic "1" Therefore, these double-cage fluorinated fullerenes are expected to be promising MQCA candidates.In addition, we investigate a series of carborane cages with different topologies and find that these systems are unable to form bistable charge configurations, limiting their application in the design of MQCA.2. Signal transmission in MQCA arrays on silver surfaceThe substrate supported MQCA arrays with the above double-cage systems are desirable in the further applications to the fabrication of real molecular devices. The cells should be deposited at the predetermined positions and form QCA circuits. At the level of quantum chemistry, we explore the deposition and electronic structure of a single MQCA cell on silver surface. Density functional theory calculations show that the introduction of the silver surface does not cause significant degradation in the QCA function. In addition, through molecular dynamics (MD) simulations, we investigate the formation of ordered MQCA arrays on silver surface and the influence of temperature on the self-assembly of surface molecules. Both Ag(100) and Ag(111) surfaces are adopted as the substrates. It is found that the double-cage molecules can form ordered MQCA arrays in both "side-by-side" and "head-to-tail" styles on Ag(100) surface.With models at different scales, we investigate the signal transmission between MQCA units. The signal transmission in a single MQCA cell is studied by solving time-dependent Schrodinger equation with one-electron Hamiltonian. In addition, a simple electrostatic model is proposed to study the signal transmitting mode and time scale in an MQCA wire, which is composed of a large number of cells. Our work offers a new possible framework for the design and implementation of MQCA in the future.3. Electro-controllable oligopeptide molecular switches on gold surfaceIn recent years, switchable biological surfaces are widely used in drug delivery, cell culture, biosensor and tissue engineering. Surfaces made up of oligopeptides are now attracting much research interest. By using the polarizable force field based on a fluctuating charge model, in combination with MD simulations, we systematically study the switching behavior of a series of oligopeptides on gold surface under the control of electric fields. These oligopeptides (Biotin-nKC) have biotin head as the recognition unit, connected by several lysine residues (K) with NH3+groups as the switching units. A cysteine residue (C) is used to bind the gold surface with the thiol group. The result shows that the oligopeptide chains experience strong polarization under the applied electric fields and the atomic charges change dramatically. Traditional force field methods take fixed atomic charges and thus cannot treat this kind of switching process appropriately.The oligopeptide chains will extend or fold driven by the electric field, showing "ON" and "OFF" switch, which is in agreement with the experiment. The switching behavior for oligopeptides with different chain lengths is also investigated. The result indicates that when building such kind of surface molecular switches, the difference in the lengths of the long and short surface chains should be in a certain range, leading to a difference in the surface activity and thus showing switching function. |
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