Effect of Molecule-Molecule Interaction on the Electronic Properties of Molecularly Modified Si/SiO x Surfaces Olga Gershewitz, Miri Grinstein, and Chaim N. Sukenik Department of Chemistry, Bar-Ilan UniVersity, Ramat Gan, Israel 52900 Keren Regev, Jamal Ghabboun, and David Cahen* Department of Materials & Interfaces, Weizmann Institute of Science, RehoVoth, Israel 76100 ReceiVed: June 21, 2003; In Final Form: October 23, 2003 We use the adsorption of systematically substituted silanes with either simple alkyl or alkyl phenyl ether chains onto oxidized Si to study the electronic effects of such molecular monolayers on Si. While there is no significant effect of distance of the substituents from the surface, a strong effect of what we interpret as depolarization is found for layers made up of molecules with high (>5 D) free molecule dipole moment. This is also apparent from differences in UV-visible and Fourier transform infrared (FTIR) spectral features, suggesting changes in molecular conformation, and, especially, from the measured contact potential differences. These reflect the modified surface’s electron affinity and, thus, the effective dipole moment of the monolayer. The effect is ascribed to the system’s response to the energetic price of dipole-dipole repulsion. Introduction Control over the surface chemistry and physics of a variety of solids can be achieved by the self-assembly of variously functionalized organic molecules onto their surfaces. 1,2 Silanes chemisorb onto an oxidized Si surface and form monolayers via self-assembly. 3,4 By systematically varying the functionality of these molecules, one can examine their effect on the surface’s electronic properties [i.e., work function (WF), electron affinity (EA), band bending (BB), and surface recombination velocity 5 ]. For oxidized Si this has been demonstrated in a preliminary study of Cohen et al. 6 on Si and in a number of studies on other semiconductors. 7 In principle, the most straightforward effect is that of a molecular dipole on the semiconductor’s electron affinity (or on the metal’s work function). As long as we deal with a layer of dipoles with domains whose lateral dimensions (L) are much larger than the layer’s width (d), the effect of the dipole layer on the measured surface potential can be gotten from the Helmholtz equation, 8,9 which gives the potential drop across the dipole film: In this expression, N is the dipole density (per square centime- ter), µ is the dipole moment (debyes), 10 θ is the aVerage angle the dipole makes with the surface normal, ǫ is the effective dielectric constant of the molecular film (better viewed, for these monolayers, via the Clausius-Mosotti relation as an expression of the molecular polarizabilities), and ǫ 0 is the permittivity of vacuum. 9 For the type of monolayers that are obtained by self-assembly of silanes on oxidized Si, L . d will hold true. 11-13 In that case, one could argue that the actual distance of the dipole layer from the semiconductor surface is not important (cf. discussion in ref 14). While some of the earlier work that considered the effect of chain length of adsorbed saturated (alkyl) molecules on the resulting (Au) surface potential did find a clear dependence, 1,15 the results were less clear for arenethiols on Au. 16 As part of an effort to optimize the design of maximally effective molecular absorbates, we focused the present study on a system that would allow convenient synthetic variation in creating appropriate building blocks for ordered siloxane- anchored monolayer films. Molecules of structure 1 mostly form well-packed siloxane-anchored monolayers, and the range of commercially available precursors provides for diverse func- tionality (see Chart 1). Homologues of 1, in the form of molecules 2 and 3, allow for systematic variation of the distance between the dipolar chromophore and the silicon surface. Molecules 4 provided for films of thickness comparable to those derived from molecules 1 with surfaces of comparable composi- tion but lacking the aromatic ether moiety and its highly polarizable π-electron system. Self-assembly of such molecules differs from the in situ displacement chemistry approach taken previously by us 6 and others, 17,18 where such chemistry provided access to a range of functionalized arrays. Both approaches have their advantages. CHART 1. Molecules Used for Preparation of Monolayers on SiO x ΔV ) Nµ cos θ/ǫǫ 0 (1) 664 J. Phys. Chem. B 2004, 108, 664-672 10.1021/jp035764q CCC: $27.50 © 2004 American Chemical Society Published on Web 12/13/2003