Microchimica Acta, 1996, suppl. 13, pp. 444-451. STUDY OF QUASI-FRACTAL MANY-PARTICLE-SYSTEMS AND PERCOLATION NETWORKS BY ZERO-LOSS SPECTROSCOPIC IMAGING, ELECTRON ENERGY-LOSS SPECTROSCOPY AND DIGITAL IMAGE ANALYSIS Vladimir P. Oleshko 1*/** , Volodymyr V. Kindratenko 2 , Renaat H. Gijbels 2 , Pierre J.M. Van Espen 2 , and Wim A. Jacob 3 1 N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 117421 Moscow, Russia. 2 Department of Chemistry and 3 Department of Medicine, University of Antwerp (U.I.A.), B-2610 Wilrijk-Antwerpen, Belgium. Abstract. Submicron colloidal Ag particles and nano-sized filaments forming a statistical percolation network during "in situ" development of double structure tabular microcrystals of AgBr(I) emulsions have been studied by electron energy-loss spectroscopy and zero-loss electron spectroscopic imaging (EELS/ ZLESI). Image analysis has shown that random quasi-fractal clusters were formed in the colloid. ZLESI has been applied to characterise the morphology and defect structure of aggregated particles and filaments. Their energy-loss spectra revealed plasmon excitations and interband 4d electron transitions between 4-32 eV energy-loss. To study the cluster structure and its relation to the physical properties, fractal analysis including estimations of cluster fractal dimensions and of density autocorrelation functions has been performed. Mechanisms of fractal aggregation based on known models of diffusion limited aggregation, cluster-cluster aggregation and percolation are discussed. Key words: small particles, EELS/ZLESI, image analysis, fractal clusters, percolation networks. Introduction Nowadays coagulated fractal and quasi-fractal structures have been an area of active research [1-3]. They can result from a variety of different processes: film deposition on a solid surface, adsorption of molecules by porous matter, solidification of colloids, crystallization of ultradisperse powders, various biophysical processes, etc. As a rule, fractal properties at objects having porous, ramified, rough or sparse and tenuous structures are usually associated with nonequilibrium growth [4]. The unique physical and chemical properties, different from those of both gases and condensed media make fractal clusters of great fundamental interest. Moreover, fractal properties can be used for the characterisation of many-particle-systems and microanalysis of surfaces and various disperse substances as well. Volume and surface plasmon excitations, in particular, are determined both by properties of the individual particles and by collective effects due to interaction among the particles densely packed in aggregates. To understand how fractal geometry affects electronic properties of aggregated matter, electron-optical and optical methods can be applied [3,5]. Here we present some data on formation of quasi-fractal clusters in aggregated Ag colloids and of percolation filament networks during development of composite tabular microcrystals of Ag(Br,I) emulsions studied by electron energy-loss spectroscopy (EELS), zero-loss electron spectroscopic imaging (ZLESI) and image analysis. Experimental A suspension of colloidal Ag particles was obtained by reduction of a diluted aqueous solution of AgNO 3 in the presence of 0.1% polyvinyl alcohol (PVA). In another experiments statistical networks * Present address: Department of Chemistry, University of Antwerp (U.I.A.), Universiteitsplein 1, B-2610 Antwerpen-Wilrijk, Belgium. ** To whom correspondence should be addressed.