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 and CuInSe2 (CISe). The ligands rely on cyclic disulfide groups for binding to the substrate and can in addition-possess two different substituents, one polar substituent (p-cyanobenzoyl or anisoyl) and one long-chain, aliphatic residue (palmitoyl). Because the substituents are covalently connected, no phase segregation will occur upon surface binding. Adsorption of these ligands on conducting surfaces changed both the surface potential (because of the polar substituent) and hydrophobicity (because of the aliphatic residue). Larger changes of surface potential were obtained by adsorption of the symmetric, dipolar ligands than 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 contact potential difference changes were found to depend on the extent of coverage (as derived from electrochemical and contact angle measurements) and on the orientation of the ligands (estimated from ellipsometry and FTIR data) and could also be reconstructed using a simple, electrostatic model. These findings demonstrate that the present methodology enables simultaneous grafting of two desired properties onto solid surfaces and illustrate the predictive power of a simple, electrostatic model for molecule-controlled surface engineering.