Langmuir-blodgett films of organic charge-transfer complexes

Abstract

This thesis is concerned with the preparation of floating films of electrically conductive organic charge-transfer complexes and their subsequent transfer to solid substrates by the Langmuir-Blodgett (LB) technique. Characterisation of the morphology (using ellipsometry, surface profiling, optical and scanning electron microscopy, energy dispersive spectroscopy and optical absorption spectroscopy) and electrical properties (at room temperature and low temperature) of the resulting multilayer structures is discussed. Three different systems were investigated: (i) mixed films containing the long chain tetrathiafulvalene (TTF) derivative octadecanoyl-TTF (ODTTF) and either octadecanoic acid (OA) or pentacosa-10,12-diynoic acid (PA); (ii) pure films of the charge-transfer complex Ν-octadecylpyridiniura-bis-(4,5-dimercapto-1,3-dithiole-2- thione) palladium (Ci8Py-Pd(dmit)2); and (iii) pure films of the charge-transfer complex N-octadecylpyridinium-bis-(4,5-dimercapto-1,3-dithiole-2-thione) nickel (CigPy- Ni(dmit)2). Some interesting electrical properties were observed in these films. Also, Ci8Py-Ni(dmit)2 has been incorporated as the active layer in a thin film field effect transistor structure. Carrier mobility values of 1.9 ± 0.5 x l0(^-5) cm(^2) V(^-1) S(^-1) and 0.3 ± 0.1 cm(^2) V(^-1) s-(^-1) were calculated from the device characteristics, before and after doping with iodine, respectively In the case of films containing ODTTF, a maximum room temperature in-plane dc conductivity after iodine doping of 2±lxl0(^-2) S cm(^-1) was recorded. This conductivity was found to be strongly dependent on the molar ratio of the two components present in the film. The behaviour has been explained using two-site percolation models. For C(_18)Py-Pd(dmit)(_2) and C(_18)Py-Ni(dmit)(_2), the properties of the floating layers and transferred films were found to be influenced by the exact experimental conditions. Ci8Py-Pd(dmit)(_2) films were conductive as deposited, with a stable maximum room temperature in-plane dc conductivity value of 1.5±1.0xl0(^-l) S cm(^-1). C(_18)Py-Ni(dmit)(_2) samples became conductive after exposure to iodine vapour, with a stable peak conductivity value of 1.3±0.8 x10(^-1)S cm(^-1).