Structural Characteristics of Ag Colloid−Adsorbate Films Determined from Transmission Electron Microscopic Images: Fractal Dimensions, Particle Size and Spacing Distributions, and Their Relationship to Formation and Optical Responses of the Films

Abstract

Ag colloid−adsorbate films (similar in appearance to MELLFs) containing 2,2‘-bipyridine (bpy), 2,2‘,6,6‘-bipyrimidine (bpm), 2,3,5,6-tetrakis(2‘-pyridyl)pyrazine (tppz), 5,10,15,20-tetrakis(4‘-pyridyl)porphine (TPyP), and 5,10,15,20-tetraphenylporphine (TPP) adsorbates, respectively, were prepared at the interface between the aqueous Ag colloid and the solution of adsorbate in dichloromethane, deposited on carbon-coated copper grids and imaged by transmission electron microscopy (TEM). The films are mostly two-dimensional (monolayer) structures providing good quality TEM images suitable for image analysis. Fractal dimensions (D), Ag colloidal particle-size distributions, and relative particle-spacing distributions of the films were analyzed. Both the fractal (D = 1.23 for Ag−TPP) and nonfractal (D = 1.97 for Ag−TPyP and D = 1.93 for Ag−tppz) film structures were observed. Fractality of the Ag colloid−adsorbate films appears to be a function of the hydrophobic/hydrophilic character of the adsorbates. Further on, preparation and TEM imaging of nonfractal, monolayer metal−adsorbate films (such as Ag−TPyP an Ag−tppz) are proposed as new suitable techniques for particle-size distribution analysis of metal colloids. Furthermore, all the films investigated prove themselves to be SERS-active. In particular, a SERS spectrum of the real TEM microscopic sample of Ag−bpm film deposited on carbon-coated copper grid is reported and analyzed.