Self-Assembled Monolayers
Self-assembled monolayers (SAMs) provide a versatile means of modifying the chemical and electronic properties of surfaces at the nanoscale using principles of organic, inorganic, and supramolecular chemistry. They also provide a powerful platform for organizing a single, often highly ordered, layer of molecules at an interface with molecular-scale precision. The structure and properties of these assemblies depend not only on how individual molecules interact with the underlying surface, but also on their surface coverage and the extent and nature of intercomponent interactions within the SAM.
Our group is particularly interested in monolayers comprising (i) multiple components and (ii) electrically charged components. We seek to understand how the interplay between molecular structure, assembly conditions, and surface-confined molecule-molecule interactions controls the nanoscale organization and molecular- and macroscopic-scale properties of these two-dimensional systems. Ultimately, we aim to use these insights to control chemical and electronic behavior at functional surfaces, with potential applications in interfacial charge transport, catalysis, and sensing.
We characterize molecular monolayers using complementary electrochemical, spectroscopic, and scanning probe methods. For example, surface voltammetry provides a particularly sensitive probe of the surface coverage and local environment of electroactive molecules in a SAM, while X-ray photoelectron spectroscopy, quartz crystal microbalance measurements, scanning tunneling microscopy, infrared spectroscopy, and surface wettability measurements provide complementary information about surface composition, nanoscale organization, and molecular structure. Together, these measurements allow us to connect molecular-scale features with observed SAM properties.
Figure 1. A schematic of SAM formation. Molecules with appropriate structures and surface-binding groups can self-organize into periodic structures on metal surfaces.
Building multicomponent molecular interfaces
Although they may adopt distinct nanoscale structures, conventional SAMs are often composed of a single molecular component that repeats in a specific pattern across the surface. Incorporating multiple components provides opportunities to create interfaces with much greater chemical complexity, but introduces challenges associated with controlling surface composition and nanoscale phase segregation. We are exploring different strategies to mitigate these issues, for example by using families of SAM components with similar external structures and lateral dimensions but distinct pendant or internal functionality (the "platform" approach). By varying the identity and relative fraction of these components within a SAM, we seek to control the local molecular environment surrounding specific surface-bound functional sites and understand how intermolecular interactions can be leveraged to determine whether different components mix, segregate, or otherwise reorganize following assembly.
One potential application lies in constructing modular "active sites" using surface-bound molecular catalysts. Here, neighboring molecules in the SAM could be exploited to modify the immediate steric, electrostatic, or chemical environment (the secondary coordination sphere) surrounding an immobilized catalyst. This provides an additional level of control beyond the conventional advantages of heterogenizing homogeneous catalysts, an approach that seeks to combine the well-defined and tunable characteristics of solution-phase molecular catalysts with the facile separation and potential reusability provided by affixing them to a surface. Through this work, we aim to establish strategies for controlling the activity, selectivity, and stability of catalyst systems at the electrode-solution interface through rational design of the surrounding molecular environment.
Figure 2. Controlling the local environment within multicomponent self-assembled monolayers. Molecular components based on a common platform (P) can introduce different functional groups (R and R′) into the environment surrounding a surface-bound catalyst or other functional site (C), providing a strategy for modifying its local chemical environment.
Controlling self-assembly through electrostatic interactions
Efforts to control the structure of SAMs through lateral interactions between adjacent components have typically leveraged sterics or supramolecular forces. However, the use of electrically charged components introduces long-range Coulomb interactions that may profoundly influence SAM properties. In contrast to monolayers constructed from charge-neutral components, these systems must also accommodate mobile (freely diffusing) counterions or may instead be charge-balanced by immobile (fixed/localized) surface charges. Together with the anticipated influence of solvent and the local dielectric environment on the screening of electrostatic interactions, the use of charged components provides additional variables for controlling intermolecular spacing, monolayer composition, and stability.
By combining electrochemical and surface characterization with physical models of intermolecular electrostatics, we are using charged coordination complexes and organic salts to explore the fundamental roles of molecular charge, counterion identity, and assembly environment in determining the structure and composition of the resulting SAMs. From this work, we seek to determine whether long-range Coulomb interactions can be deliberately harnessed as a powerful design parameter, beyond steric and supramolecular interactions, for precisely controlling the relative positions of individual molecules on a surface.
Figure 3. Electrostatic interactions in charged self-assembled monolayers. Counterion loss produces charged surface-bound complexes that interact through long-range Coulomb forces (electric field lines are shown schematically).
Selected publications
Template-stripped substrates with solvent-impermeable metal thin films
ACS Nanoscience Au, 2025, 5, 269
Counterion Loss from Charged Surface-Bound Complexes Drives the Formation of Loosely Packed Monolayers
J. Am. Chem. Soc., 2024, 146, 25625
Methyldisulfide groups enable the direct connection of air-stable metal bis(terpyridine) complexes to gold surfaces
Dalton Trans., 2023, 52, 7836