Molecular electronics : a theoretical study of electronic structure in molecular crystals and surfaces

Abstract:   This thesis deals with theoretical studies of the electronic structures of molecules in organic crystal and molecules adsorbed on metal surfaces. Both studies with first principle calculations and classical mechanics were performed. The materials include pentacene and bis(9-cyclohexylimino-l-oxyphenalenyl)boron and theTetrakis( dimethylamino )ethylene (TDAE) molecule chemisorbed on a gold surface. Molecular crystals of pentacene are reported to have the highest field-effect mobility values for organic thin film field effect transistors, and are therefore suitable to be the active material in such devices. The increase in conductivity obtained experimentally by doping with alkali metals showed great promise, and we therefore performed calculations on pentacene crystals doped with potassium. Phase transitions occured upon doping, and the herringbone arrangement of the pristine pentacene molecules relaxes towards a 1r stacked structure. Charge transfer between the crystal and the potassium occurs, which shifts the Fermi level into the lowest unoccupied molecular orbitals bands and turns the crystal metallic. The study of bis(9-cyclohexylimino-l-oxyphenalenyl)boron, a biphenalenyl based neutral radical, was performed in order to investigate the electronic properties since experimental studies had shown simultaneous electrical, optical and magnetical bistability. The system is quasi one-dimensional, as shown from band structure calculations.. The injection of charge carriers from metals to organic semiconductors is one of the most central processes when it comes to the performance of organic devices. The interface between the metal and the semiconductor is one of the device parameters that will significantly influence the device performance. The addition of a molecular layer on the interface can, by the introduction of a dipole, reduce the energy barrier for charge injection. ·we found that a molecular layer of TDAE deposited on a gold surface results in a charge transfer from the molecule to the surface which reduced the charge injection barrier by as much as 2.4 eV.