Atomic force microscopy observation of the morphology of tetracyanoquinodimethane (TCNQ) deposited from solution onto the atomically smooth native oxide surface of Al(111) films

Abstract

The morphology of tetracyanoquinodimethane (TCNQ) and of benzoic acid deposited from solutions onto atomically smooth native oxide surfaces of Al(111) films was observed by atomic force microscopy (AFM). A surface height analysis (bearing analysis) of the AFM images of TCNQ-deposited atomically smooth oxidized Al films gave a bimodal height distribution of the adsorbed TCNQ on the surfaces. The height distribution showed that TCNQ is adsorbed on the oxide surface as both a uniform film on a nanometer scale and micrometer-sized particles with the height ranging from 10 to 100 nm above the surface. The large particles are easily identified by AFM as microcrystallites of neutral TCNQ and they have many morphologies. The uniform film with the nanometer scale height distribution results predominantly from corrosion of the oxide surface by the TCNQ anion formation reaction. The volumes of the large particles and of the thin uniform film were obtained separately from the analysis of the bearing histogram and bearing area curve. Generally, the volume (approx. 2 μm 3 for a 30×30-μm 2 area) of the thin uniform film is 4–5 times larger than the volume of one monolayer of TCNQ or TCNQ salt. Benzoic acid treated surfaces, on the other hand, did not show a bimodal distribution nor large particles, and surface roughness is consistent with monolayer formation of aluminum benzoate. Discrepancies in inelastic electron tunneling spectroscopy and X-ray photoelectron spectroscopy results for TCNQ adsorption on alumina were discussed.