Simultaneous Control of Surface Potential and Wetting of Solids with Chemisorbed Multifunctional Ligands

Abstract

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.

Surface photovoltage phenomena: theory, experiment, and applications

“…A combination of CPD and saturated-SPV measurements was used in recent years by Cahen et al for deducing the systematic effects of various organic ligands to semiconductor surfaces on the bandbending and surface dipole at CuInSe 2 [50,51,54], CdTe [50,51,55,56], CdSe [52], GaAs [53], and Si [57] surfaces. Organic ligands are an attractive alternative to the inorganic treatments usually used for surface control.…”

“…Organic ligands are an attractive alternative to the inorganic treatments usually used for surface control. This is because organic ligands feature structural versatility and¯exibility, may be designed to incorporate several properties simultaneously, and potentially allow systematic modi®cations of one speci®c property independent of the others [54,56].…”

Molecules on Si: Electronics with Chemistry

“…[13,14] We [3,64,109,110] and others [63,[111][112][113] have shown that adsorbing molecules with intrinsic dipoles and/or polar bonding to a solid can change the barrier height for transport in an MS/MIS junction. The theoretical potential drop, C I , can be calculated from: [114,115] . C I and m ?…”

“…m ? is composed of three contributions, [113][114][115] which we illustrate here for SiÀalkyl systems: the intrinsic molecular dipole, m mol ; the SiÀC bond dipole, m Si-C and the ''pillow effect'', m Me/Hg .…”

“…While the pillow effect on the substrate is commonly included in the measured change in substrate work function upon molecule adsorption (by KP or UPS, see Table 1), measuring a pillow effect of the deposited contact is problematic. The reason is that a strong dipole effect is commonly achieved by the terminal groups of the molecules, [17,53,63,114,119] which are the ones most likely to interact with the deposited metal contact, via polarization or even charge transfer (i.e., change of m Me/Hg ). [49,64] One way to study the effect of such direct interactions is to add a second monolayer on the metal side as a dipolar [109] or blocking [120] layer.…”