I–V and π−A characteristics of ultrathin Langmuir-Blodgett films

Abstract

The deposition conditions and electrical properties of C 22-quinolium(TCNQ) Langmuir-Blodgett (LB) films have been studied. The optimum conditions of film deposition were examined by a study of surface pressure-area (π− A) isotherms with variations of pH, temperature, barrier moving speed, dipping speed, spreading amount of solution etc. The LB films were deposited on a slide glass substrate and tested with several tools. The π− A isotherm shows that there might be two phase transitions near surface pressures of 30 and 45 mN m −1. It is thought that the phase just below 30 mN m −1 is a liquid phase and that above 30 mN m −1 is a solid phase. The LB films were made under the following conditions; surface pressure, 45 mN m −1; 25°C; pH 5.6; spreading amount 60–70 Å 2 per molecule; and dipping speed, 4 mm min −1. The electrical conductivities and conduction mechanism were investigated by measuring current-voltage ( I–V) characteristics along the vertical and horizontal direction of the film. We observed an anisotropy in conductivity between the vertical and horizontal direction of the film. The conductivity along the film surface direction is about 10 −7 S cm −1 and that of the perpendicular direction is about 10 −14 S cm −1 at room temperature. We saw non-ohmic conducting behaviour in the presence of high electric field along the vertical direction. When the electric field is high in this direction, a breakdown occurs in the I–V characteristics. We analysed the I–V characteristics in detail near the breakdown region by changing the type of applied voltage (step or pulse) as well as the temperature.