SIMULTANEOUS CONTROL OF SURFACE-POTENTIAL AND WETTING OF SOLIDS WITH CHEMISORBED MULTIFUNCTIONAL LIGANDS

Homogeneously mixed molecular assemblies of defined stoichiometry were created by adsorption of asymmetric, trifunctional ligands on gold an d CuInSe2 (CISe). The ligands rely on cyclic disulfide groups for bind ing to the substrate and can in addition-possess two different substit uents, one polar substituent (p-cyanobenzoyl or anisoyl) and one long- chain, aliphatic residue (palmitoyl). Because the substituents are cov alently connected, no phase segregation will occur upon surface bindin g. Adsorption of these ligands on conducting surfaces changed both the surface potential (because of the polar substituent) and hydrophobici ty (because of the aliphatic residue). Larger changes of surface poten tial were obtained by adsorption of the symmetric, dipolar ligands tha n by adsorption of the asymmetric ligands, and larger changes occurred on gold than on CuInSe2 (up to 1.2 V between extreme modifications on Au and 0.3 V on CISe). The magnitude and direction of the observed co ntact potential difference changes were found to depend on the extent of coverage (as derived from electrochemical and contact angle measure ments) and on the orientation of the ligands (estimated from ellipsome try and FTIR data) and could also be reconstructed using a simple, ele ctrostatic model. These findings demonstrate that the present methodol ogy enables simultaneous grafting of two desired properties onto solid surfaces and illustrate the predictive power of a simple, electrostat ic model for molecule-controlled surface engineering.