10.1007/s11453-008-2014-9

Abstract

In the last decade, a large variety of films with diverse electrical, optical, and magnetic properties have been obtained on the basis of Langmuir-Blodgett technology [1,2]. Interest in these films is due to their promise for use as a material of molecular electronic structures. Langmuir films consisting of phthalocyanine dye molecules are of interest for such practical purposes. The structural, electrical, and optical properties of Langmuir films have not yet been adequately studied. In this paper, we report the results of an investigation into the conductance of ultrathin tin diphthalocyanine (Pc 2 Sn) films. As an active effect on the conductance of Pc 2 Sn films, we chose adsorption of water molecules featuring donor properties during adsorption on semiconductor and insulator surfaces. A significant effect of adsorption on the conductance of phthalocyanines was noticed in [3].Pc 2 Sn films were grown using the Langmuir-Schaefer method at room temperature by removing their monolayers from the water surface at a surface pressure of 7 mN/m and pH 6.0. This technique allows'-type film deposition on one side of the substrate. Pc 2 Sn monolayers were formed on the water surface by introducing a tin diphthalocyanine solution in chloroform with a weight content of (1-3) × 10 -2 %. The film thickness was seven monolayers.A system of aluminum contacts 1 mm 2 in area was deposited in vacuum at room temperature on the filmsubstrate interface and on the outer film surface. The absence of an effect of aluminum contacts on Langmuir film properties was justified in [4]. Using the thermopower method, we determined that the studied Pc 2 Sn films are of n -type conductivity. The conductance was measured with an ac signal in the frequency range f = 5 Hz-10 MHz using an HP 4192A impedance analyzer. During measurements, the sample temperature was maintained at T = 300 K. Figure 1 shows the dependences of the imaginary part of the impedance on the real part, measured in vacuum at a residual pressure of P = 10 -3 Pa (curve 1 ) and after adsorption of water molecules (curve 2 ). We can see that the obtained dependences are similar for the cases of vacuum and adsorption of water molecules. One of the possible equivalent circuits which allows qualitative description of the dependences in Fig. 1 is shown in the inset. It consists of the resistance ( R c ) of contacts, connected in series with a parallel network containing the resistance ( R s ) and capacitance ( C s ) of Abstract -The frequency dependences of the conductance and capacitance of Langmuir-Blodgett tin diphthalocyanine films in vacuum and in the case of adsorption of water molecules were studied. An increase in the film conductance due to adsorption was detected. Possible conduction mechanisms of tin diphthalocyanine in vacuum and water vapor are discussed. The permittivity of tin diphthalocyanine films was estimated from an analysis of the frequency dependence of the capacitance.