Investigation of molecular properties to perform nanocomputing

The need to have more performing electronics devices led the researches to scale down transistors more and more. However, when very small sizes are reached, some problems arise which led to exploration of new technologies in digital electronics. Among them, the most promising one seems to be Quantum-dot Cellular Automata (QCA), an approach in which the device is not used as a current-channel switch, but as a charge container where binary information can be represented. The basic unit of QCA device is a cell composed by four dots (seen as a vessels to host the charge) and the information is encoded depending on where the charge is localized. For the implementation of QCA paradigm, the molecular cell (Molecular Quantum dot Cellular Automata - MQCA) is the most promising trend: the molecules involved in this technology have redox centers (an atom or a group of atoms that can gain or loose an electron) acting as quantum dots. In this work, for the study of the molecular properties, the MosQuiTo (Molecular Simulator Quantum-dot cellular automata Torino) methodology has been used: for the computational chemistry analysis a series of ab initio simulations have been performed in different conditions and several figures of merit have been evaluated (Aggregated Charge, Electric field generated by molecule and so on). The aim is then to find the best molecules suitable for QCA purposes: in order to do that, a MATLAB tool interfacing with ORCA able to characterize the molecules has been used. A characterization of the monostable Bis(trimethylsilyl) Sulfide and its others form (the molecule with thiol, long thiol ecc...) has been performed and the results have been studied: Aggregated Charge, Dipole moment and SCF Energy are some of the outputs of the tool. For having an idea of the impact of the clock on the system, even molecules with 3 DOT have been simulated. Bistable Diallyl butane has been taken as example for the study of the bistable molecules and methods. The idea developed in this thesis is that to make a single phthalocyanine molecule as a full QCA cell. The phthalocyanines are organic compounds composed of four isoindole units linked by a ring of nitrogen atoms; the peculiarity of this class of molecules is that they lies in on a plane having a fourfold symmetry; the aim is then to make each isoindole group as a quantum dot of the QCA cell. In order to achieve this goal Hydrogenphthalocyanine (H2Pc) molecule has been characterized and various disposition of charges around the molecule have been analyzed for emulate the presence of a driver or another molecule. From the simulations, it came out that the best charge disposition is with the charges along the two diagonals of the molecule: a relative good switching has been observed. In addition, an analysis on the polarizability on the most important phthalocyanines has been performed; more than 70 metal phthalocyanines have been prepared (substituting the two central hydrogen atoms with a metal atom, for example Cu, Zn, Co) and by stimulating the molecules with a proper electric field it has been studied their tendency to create a dipole moment. From the analysis carried out, it has been shown that in general the presence of metals improves the molecules polarizability, in particular the highest values of the isotropic polarizability have been encountered in the reduced forms of the molecules. The overall results obtained in this work seems to be promising and renew the will to investigate further these aspects.