Spectroscopy of Interfacial Phenomena

Motivation:

Controlling the physics and chemistry of interfaces is important in a wide range of technological applications such as electrochemical energy storage, photovoltaics, sensor devices, and separation technologies. An important property of the interface between two materials is the local electric field that develops near the junction on the microscopic scale. Rather than being merely a passive bystander, the interfacial field is often the key functional element of a junction. Prime examples are the built-in fields in photovoltaic junctions that act as the main functional elements for charge separation, and fields at the junction of electrodes and electrolytes that drive redox reactions. The existence of interfacial fields is often inferred from bulk transport measurements. Their details at the molecular scale, especially under non-equilibrium conditions, are often elusive. This makes their direct spectroscopic measurement imperative.

Electric Fields Due to Solvation at Interfaces:

We have both measured and modeled the solvation electric field at an interface. Solvation field is the electric field that is generated by a molecule due to its interaction with its surrounding and is critically important in charge transfer processes. We have taken the approach of measuring the local fields through their influence on the vibrational frequency of molecules tethered on the surface (Stark shift spectroscopy). Our work was enabled by sum-frequency generation which is a laser spectroscopy technique well-suited for this problem. Prior to our work a solvation theory for a dipolar molecule at a metal-dielectric junction did not exist. Our experimental and theoretical work resulted in a critical modification of the Onsager’s solvation theory that renders it applicable to the interface.

(link to our published work on interfacial solvation fields)

Electric Fields Near a Biased Electrode:

We have extended the vibrational Stark shift work to directly measure electric fields at an active electrochemical interface. We have identified the influence of ionic concentration and electrode potential on local fields and have shown their inter-relationship with a relatively simple model. Direct and local observation of fields under current flowing conditions at electrochemical interfaces has enabled us to see behavior that is otherwise not possible to infer from conventional electrochemical measurements.

(link to our published work on this topic)

What is next?

We are working on experiments that rigorously link interfacial electric fields with interfacial chemistry, such as protonation equilibria.