Application of atomic force microscopy to particle sizing

AFM has been applied for studying morphology and size distribution of nanom eter-sized particles adsorbed on flat surfaces. For the quantitative evalua tion of the images an algorithm for automatical particle detection and volu me calculation has been developed. In this way a large number of particles can be automatically evaluated in order to derive size distributions or sur face coverages. The method has been successfully applied to determine size distributions of environmental aerosol particles collected with an 11-stage low pressure impactor. The first four stages with average aerodynamic equi valent diameters (aed) ranging from 21 to 170 nm were investigated. The cal culated aed values were in good agreement with the predicted aed for each s tage. Additionally, it could be shown that sampling conditions and storage time affect the derived size distributions. Furthermore, AFM has been appli ed as reference method for conventional particle sizing techniques. For thi s purpose technologically relevant powders as titanium oxide powder and tun gsten carbide powder were investigated using AFM and the results were compa red with conventional techniques such as high resolution SEM and a light sc attering method. The derived cumulative size distributions were in good agr eement. The results clearly show that AFM constitutes a convenient tool for size determination, not only for ultrafine particles exploiting the high r esolving power, but also in the case of larger particles.