Thin Solid Films 259 (1995) 118- 124 An investigation of the influence of film thickness on the electronic and vibrational properties of trans -polyacetylene thin films W. Graupnef , M. Mauri”, G. Leditzky”, G. Leising”, W. Fischerb, F. Stelzerb zyxwvutsrqponm “Institut fiir Festkiirperphysik, Technische Universitiit Graz, Petersgasse 16, A-8010 Graz, Austria “Institut fiir Chemische Technologie organischer Stoffe, Technische Universitiit Graz, Stremayrgasse 16, A-8010 Graz, Austria Received 19 July 1994; accepted 28 October 1994 Abstract Upon varying the thickness of [vans-polyacetylene layers on different substrates we find a remarkable dependence of the electronic and vibronic properties on the optical density of the films. This behaviour is unaffected by the choice of the substrate or the film-producing process. The spectroscopic results in the ultraviolet, visible and in the infrared (IR), as well as photomodulated absorption measurements in the IR, reveal an improved order in the samples with decreasing film thickness. We find a narrowing of the distribution of the effective conjugation lengths (I,. efl) and a shift towards longer I,, efffor very thin films ( < 60 nm). To find quantitative relations between I,, eff and the electronic properties of our samples we compare them with data on polyenes. Keywords: Dielectric properties; Infrared spectroscopy; Optical properties; Organic substances 1. Introduction The electronic properties of trans-polyacetylene (PA) depend on the synthesis route and the subsequent treat- ment of the samples (i.e. doping [ 11)to reach the highly conducting form or stretching [2, 31 which yields ori- ented samples. All measurements made on PA reflect data obtained on a system characterized by a distribu- tion of I,, eff, which is defined as the length of the polyene or PA segment along which the delocalization of z electrons can take place [4]. By choosing the term effective conjugation length (/c,eR) we want to express that the conjugation length found in real PA is smaller than the degree of polymerization which we would call the real length of the chain. By conjugation length we mean the length of a completely undisturbed PA seg- ment, being planar and allowing maximum overlap of n electrons [4]. The same terminology is used by Knoll et al. [5] who describe the Raman spectra of polyacetylene (PA) within the framework of the conjugation length model [6-91. They speak of a distribution of effective conjugation lengths [5]. From measurements on finite polyenes [ IO- 131 and comparatively early theoretical work by Kuhn [ 141 it is well established that the energy 0040-6090/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDI 0040-6090(94)06424-5 of the rc-rc* transition of molecules increases with decreasing conjugation length. Extrapolated to PA this means that the bandgap increases with decreasing mean &r. Therefore, low energies for the peak of the linear absorption of PA correspond to high values for the mean L, eff in the sample. Furthermore, a narrow distri- bution of l_ff in the sample is related to a narrow Vis absorption peak. Due to the degenerate ground state of PA, movement of domain walls between two (energetically equivalent but geometrically different) ground states is predicted by theory for this material [ 151. The disturbed periodic- ity of the repeat units leads to electronic states within the gap of the one-dimensional ( 1D) semiconductor PA. The number of these electronic defect states can be increased by photons of a minimum energy (2/7c)E,, where Eg is the gap of PA [ 161 (and Refs. [21 and 221 therein). The process of their genesis can be described as a creation of an electron-hole pair and subsequently a lattice relaxation via electron-phonon coupling, which changes the positions of the CH units with respect to their ground state positions. Once created, these states can give rise to an additional absorption (photoinduced absorption (PIA)), and can be identified