Adsorption state and morphology of tetracyanoquinodimethane deposited from solution onto the atomically smooth native oxide surface of Al(111) films studied by X-ray photoelectron spectroscopy and atomic force microscopy

The adsorption state and morphology of tetracyanoquinodimethane (TCNQ) deposited from acetonitrile solutions onto atomically smooth native oxide surfaces of Al(1 1 1) films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The analysis of the XP spectra of the deposited TCNQ showed that it is adsorbed as both a neutral and ionized state on the surfaces and the intensity for the former increases more rapidly than that for the latter as the concentration increases. The atomically smooth surfaces of the Al films allowed the detailed observation and analysis of the morphology of the deposited TCNQ. The surface height analysis (bearing analysis) of the AFM images of the deposited TCNQ showed that it is adsorbed as both a uniform film on a nanometer scale and micrometer-sized particles with height ranging from 10 to 100 nm above the surfaces. The volumes of the thin uniform film and the large particles of the deposited TCNQ could be obtained separately from the analysis of the bearing histograms and bearing area curves of the AFM images. A similarity in the dependence of both the volumes of the large and small particles of the deposited TCNQ obtained by AFM and in that of the surface concentrations of neutral and ionized TCNQ obtained by XPS on the concentration of TCNQ in solution clarified the adsorption mechanism. It is concluded that the large particles are identified as microcrystallites of neutral TCNQ and the thin uniform film results predominantly from corrosion of the oxide surface by the TCNQ anion formation reaction.