Preparation and characterization of dense films of poly(amidoamine) dendrimers on indium tin oxide

Indium tin oxide (ITO) substrates were modified with a layer of poly(amidoamine) (PAMAM) dendrimers to change their surface properties and, in particular, the substrates' work function. The functionalization procedure involved the electrostatic adsorption of positively charged PAMAM dendrimers of generation five onto negatively polarized ITO surfaces. Three different PAMAM dendrimers were used: PAMAM-NH2 and PAMAM-OH with terminal amine and hydroxyl groups, respectively, as well as Q-PAMAM-NH2, which had been prepared from PAMAM-NH2 by quaternization of the dendrimer's terminal and internal amine groups with methyl iodide. The resulting organic films were analyzed by contact angle goniometry, X-ray photoelectron spectroscopy, ellipsometry, and Kelvin probe force microscopy to confirm the presence of a dense layer. A Langmuir isotherm was derived from surface densities of fluorescence-labeled PAMAM-NH2 dendrimers from which we deduced an equilibrium binding constant, K-eq, of (1.3 +/- 0.3) x 10(5) M-1. Kelvin probe measurements of the contact potential difference revealed a high reduction of the work function from 4.9 eV for bare ITO to 4.3 eV for ITO with a dense film of PAMAM-NH2 of generation five. PAMAM-OH and Q-PAMAM-NH2 resulted in slightly smaller work function changes. This study illustrates that the work function of ITO can be tuned by adlayers composed of PAMAM dendrimers.