The instrument (at Solid state physics).

Many things have been named after lord Kelvin.

Some other things named after Kelvin: Kelvin contact, Kelvin clip, Kelvin connection, and of course, the absolute temperature scale.

These are distinct techniques:

• Kelvin probe – measures work function or surface voltage.
• Four-point probe – sheet resistance of thin films.

The work function

The least amount of energy required to lift an electron from the surface a conducting material to a point just outside the material with zero kinetic energy.

To leave the material, the electron has to move through the surface. The work function thus depends on the chemical, optical, electric and mechanical properties of the surface.

The work function is the distance fromt the vacuum level (at the surface) to the Fermi level (i.e., the electrochemical potential of electrons).

The work function is an extremely sensitive indicator of surface condition. Easily affected by

• adsorbed or evaporated layers,
• surface reconstruction,
• surface charging,
• oxide-layer imperfections,
• surface and bulk contamination, etc.

The concept of absolute electrode potential

Absolute potential is defined in terms of the electron potential in vacuum.

The problem lies in how to relate that to aqueous electrochemistry.

Many prominenent scientists have made important contributions, defining and quantifiying the absolute potential concept (Bockris, Gerischer, Trasatti, among others).

IUPAC recommendation (1986) on relationship between absolute potential scale and aqueous SHE:

In practice

Measuring the work function

• Invented by lord Kelvin in 19th century.
• Bringing two different metals into contact will create an electric field at the junction.
• The traditional Kelvin probe forms a simple parallel-plate capacitor.

An indirect technique

• Electrons are not extracted directly from the surface. Electrons flow back and forth in the external circuit as the tip vibrates.
• The work function difference (CPD) is determined by addition of an external voltage, \(V_b\).

A typical experiment

A typical experiment

• Measure CPD between tip and reference material (gold). This gives the tip’s WF.
• Measure CPD between tip and your samples. Now you can easily calculate WF.

“Glovebox” gives better repeatability

• Kelvin probe sits inside a relative humidity chamber.
• Chamber connected to N₂ flow.
• Oxygen probe tracks O₂ concentration (in percent).
• Humidity controller (but not often used).

The Kelvin probe itself

• Scanning stage 50 mm by 50 mm, step size 635 nm.
• Resolution: 3 meV.
• Manufactured by KPTechnology, uses their “gradient tracking system” to keep tip – specimen distant almost constant during measurement.

Some data obtained with the Kelvin probe

Scanning Kelvin probe (sc in darkness)

Scanning Kelvin probe (sc under illumination)

Thanks for listening!