Molecular Electrostatics of Conjugated Self-Assembled Monolayers on Au(111) Using Electrostatic Force Microscopy

Abstract

The electrostatic potential produced by a variety of self-assembled monolayers on Au(111) is measured using scanning probe techniques. The molecules chosen for this study contain thiol-terminated end groups and π-conjugated orbitals, making them suitable for molecular electronics applications. We have measured the surface potential of molecules having a symmetric structure and compared these results to those obtained from similar nonsymmetric molecules. The measured potential for nonsymmetric molecules scales with the dipole moment of the molecule comprising the monolayer. For symmetric molecules, the measured surface potential is essentially the same as the substrate. This result suggests that the dipole moment formed by the Au-S bond makes a small contribution to the measured surface potential. The dipole moment of a strong electron accepting molecule was intentionally modified by reaction with a strong electron acceptor. In this case, the surface potential produced by the self-assembled monolayer was found to change polarity after the formation of the charge-transfer complex.

Gold surfaces and nanoparticles are protected by Au(0)–thiyl species and are destroyed when Au(I)–thiolates form

“…In chemical compounds and in molecular materials, Au(I)-thiolate species are well established (1)(2)(3)(4)(5). Bond polarizations can be both measured (27)(28)(29)(30)(31)(32)(33)(34)(35)(36) and calculated (19,29,31,32,(37)(38)(39)(40)(41)(42)(43), leading to the conclusion that the charge on S is of order −0.2 e (1-3). Although such quantities cannot be uniquely defined by either experimental measurement or computational evaluation, the broad range of methods that have been applied yield a recognizable consensus.…”

Observation of surface enhanced IR absorption coefficient in alkanethiol based self-assembled monolayers on GaAs(001)

“…A chain-length dependent dipole potential of about 230 mV is expected for n17-SAMs on GaAs͑001͒, 12 based on measurements of n17-SAMs on Au. 36 In addition, XPS analysis has demonstrated an estimated 1.5 eV reduction in the binding energy of the S 2p line upon thiol chemisorption in n17-GaAs͑001͒ SAMs, 4,7 indicating significant charge transfer from the surface upon covalent bond formation. Other surface potential factors may play a role, such as those stemming from uncompensated semiconductor surface states.…”

Humidity-dependent open-circuit photovoltage from a bacteriorhodopsin/indium tin oxide bioelectronic heterostructure

“…Heat-induced drift caused us to set the tip-sample separation to about 1 µm, rendering the tip itself essentially irrelevant to the voltage measured. In this experiment, only the main cantilever beam contributes significantly to the electrostatic force and, hence, measured photovoltage [30,31]. We estimate that the measured photovoltage represents an average value over an area of a few hundred square microns.…”