Physical and electronic structure effects of embedded dipoles in self-assembled monolayers: Characterization of mid-chain ester functionalized alkanethiols on Au{111}

Self-assembled monolayers (SAMs) on Au{111} prepared from the midchain ester functionalized. thiols, HS(CH2)(m)CO2(CH2)(n-1)CH3 with m,n = 10,5; 10,10; 15,5; and 5,10, along with deuterated analogs, were characterized by wetting, ellipsometry, near edge X-ray absorption fine structure spectroscopy, quantitative infrared spectroscopy, high resolution X-ray photoelectron spectroscopy (HRXPS), and density functional theory calculations. The SAMs can be viewed as layered structures in which the bottom -(CH2)- alkyl segment (between the ester group and the substrate) exhibits typical conformational ordering and orientation analogous to long chain alkanethiolate SAMs, while the upper segment (ambient side) is significantly more conformationally disordered. The presence of the ester moiety leads to the formation of a strong electric dipole layer with a component of 1.05(0.09) Debye normal to the surface. This dipole layer exhibits a strong electrostatic effect on the XPS spectra in which the C 1s photoelectron kinetic energies are consistently shifted by 0.85(+/- 0.03) eV between the top and bottom -(CH2)- alkyl segments, regardless of relative lengths. This shift correlates, within error, with the value of 0.81(+/- 0.06) eV predicted via classical electrostatics due to the presence of the ester dipole layer. Overall, these data show that SAMs assembled from molecules with appropriately selected internal groups can be used to prepare internally layered structures with highly controlled electrical characteristics and further demonstrate that simple XPS shifts in core level energies can be used to derive accurate molecular dipole values in structured thin films.