SKP101: An Introduction to Scanning Kelvin Probe

What is Scanning Kelvin Probe?

Scanning Kelvin Probe (SKP) is a highly sensitive, non-contact scanning probe technique used for the measurement of the local work function of a material. Unlike traditional Kelvin probe measurements, the use of SKP allows for work function changes across a material to be spatially resolved. SKP has found widespread use due to its ability to sensitively measure changes in the sample surface state.

The Scanning Kelvin Probe technique is based on work carried out in the late 1800s by Lord Kelvin [1], and expanded on in the 1930s to demonstrate that the contact potential difference between two electrically connected metals could be measured [2]. While SKP allows sample work function to be found, it actually uses a vibrating capacitor probe to measure its contact potential difference with the sample located directly under the probe. The contact potential difference can then be easily converted to the work function by measurement of a sample with a known, stable, work function value. If the corrosion properties of a material are of interest it is also possible to convert the measured contact potential difference to the corrosion potential (E_corr).

What is work function?

The work function of a material is the minimum energy required to remove an electron from the surface of a material to vacuum. It is dependent not only on the bulk properties of a material but also its surface state, including the presence of oxide layers, adsorbates, and surface contaminants. The work function of a material can be correlated to its corrosion potential [3,4].

How does Scanning Kelvin Probe work?

Scanning Kelvin Probe allows the measurement of the contact potential difference of a sample directly connected to the SKP probe. The SKP probe is held parallel to the sample surface forming a parallel plate capacitor. This occurs because when two different materials, typically metals, have electrical contact a flow of charge between the two materials to equalize their Fermi levels occurs. The result of this equilibration is a potential difference giving rise to a surface charge at the materials, and the formation of a capacitor. The potential difference between the two materials is the contact potential difference. To determine the contact potential difference, a backing potential must be applied between the probe and sample to null the surface charge. The potential at which the surface charge nulls is equal to the work function difference between the probe and the sample. The progression of the Fermi level and charging of the probe and sample can be followed in Fig. 1.

Figure 1: When Scanning Kelvin Probe and sample do not have an electrical connection each has their own distinct Fermi level E_xp and work function W_xp. In an SKP measurement the probe and sample are electrically connected (b), leading to charge flow from sample to probe (c). This gives rise to a surface charge between the probe and sample with a potential difference known as the contact potential difference V_cp. In SKP a backing potential V_b, equal, but opposite in sign, to the contact potential difference is applied between the probe and the sample (e), to null the surface charge and return the Fermi levels to their original positions.

When considering the backing potential to be applied between the probe and the sample it is important to consider the relation between the charge which has developed and the probe to sample distance. The charge, Q, which develops is given by the equation:

$$C=\varepsilon_0\varepsilon_r\frac{A}{d}$$

$Q=CV$

Where ε₀ is the permittivity of free space, ε_r the relative permittivity, A the area of the capacitor, d the distance between the capacitors, and V the potential between the capacitors. From this it is apparent that capacitance between the probe and the sample is inversely proportional to the distance between the probe and the sample. When all other factors are kept constant, changing the probe to sample distance changes the capacitance between the two surfaces. To determine the contact potential difference through Scanning Kelvin Probe, the probe is vibrated perpendicular to the sample surface. This results in a sinusoidal change in the capacitance, giving rise to an ac signal. The ac signal is fed to a Lock-In Amplifier (LIA), which demodulates the ac signal into the dc potential of interest, allowing the contact potential difference to be found. Calibration of the SKP probe against a known sample allows the contact potential difference to be converted into a work function or corrosion potential value, depending on the application area of the measurement.

Although Scanning Kelvin Probe is typically used to determine contact potential difference, it can also be used to determine the probe to sample distance, and therefore the sample topography. In this case the aim is no longer to null the charge which has developed between the probe and sample. Instead a known potential is applied between the probe and the sample. Because the capacitance will vary according to Equation 1 changes in capacitance can be correlated to changes in probe to sample distance. This can either be plotted as is, or the probe can be moved in z to maintain a set capacitance, and the change in probe position plotted. More information on using Scanning Kelvin Probe to measure topography can be found in SCAN-Lab AN#1.

What are the components of a Scanning Kelvin Probe?

In Fig. 2 the components of a Scanning Kelvin Probe instrument are annotated. The control electronics are required to interface between the software and computer, and the various electronics required by the system. An x,y,z scanning stage is used to allow the probe to be approached to the sample in z, as well as follow sample topography if desired, and to raster scan the surface in x, and y. These are automated to allow area maps of the contact potential difference of the sample to be produced. A piezo vibration actuator is required to vibrate the probe at known, repeatable amplitudes throughout the measurement. When the null SKP technique is used a Lock-In Amplifier is required to demodulate the ac signal resulting from probe vibration. As one of the most sensitive detection methods the Lock-In Amplifier is used because it offers superior performance to other signal detection methods.  The electrometer is required to measure the electrical potential of the system. The backing potential controller applies the potential required to null the charge between the probe and sample. This is done in real time using a PID control loop. Finally of great importance is the SKP probe. Because the contact potential difference measured is relative to the probe the material of the probe is an important consideration. The diameter of the probe is also important. The probe diameter affects the ultimate resolution of the measurement. Typically for Scanning Kelvin Probe the maximum resolution is equal to the probe size. It should be noted though that this relationship does not scale indefinitely. Instead as the probe is made smaller and smaller, stray capacitance causing fringe effects has a large effect on the measurement, which can lead to measurement artefacts [5].

Figure 2: The components of a Scanning Kelvin Probe instrument are annotated.

Why use Scanning Kelvin Probe?

Scanning Kelvin Probe has a number of advantages which have seen it applied to a wide variety of applications. Of particular importance is the fact that Scanning Kelvin Probe is one of the only techniques which allows the work function of a sample to be found. This characteristic is important in understanding the electronic properties of materials. Unlike other techniques to measure work function SKP can be performed with varied environmental conditions, including changes to the light exposure of the sample. Furthermore, the work function of a sample is greatly influenced by the surface state of a material, as a result this makes SKP a highly sensitive measurement to investigate surface properties, like oxide layers and contaminants, and changes to these properties over time. As long as the underlying sample is conductive, or semi-conductive, it can even be used in the investigation of the effect of insulating layers on the sample. Like many other scanning probe electrochemistry techniques SKP is a non-destructive technique. It has the particular advantage, however, that the sample is not exposed to electrolyte. This is useful for samples which may degrade in the time frame of the measurement if the measurement were performed in solution. Finally, SKP can be used to perform topography measurements on a sample in non-contact manner, which can also be used to perform height tracking SKP measurements.

Figure 3: The fields Scanning Kelvin Probe has been used in are shown.

Scanning Kelvin Probe can be applied to any application in which the work function, corrosion potential or surface charge of a sample is of interest, as illustrated in Fig. 3. While it can be used to perform single point measurements needed to find, for example, the average surface work function, SKP is of particular interest when these local characteristics are required. Some examples of applications of SKP are:

• Determination of the work function of photovoltaic materials [6]
• Screening of Si wafers, with a focus on contaminants [7]
• Detection of fingerprints in forensic studies [8]
• Studies of the effects of surface treatment on the activity of catalytic materials [9]
• Examining the self-healing abilities of smart coatings [10]
• Following coating breakdown [11]
• Visualizing local corrosion [12]

The applications of Scanning Kelvin Probe, and the other scanning probe electrochemistry techniques, have been covered in detail in a series of Learning Center articles. The most recent of these articles focuses on the questions which can be answered using scanning probe electrochemistry in fuel cell research.

Further Information

To further your understanding of the applications of Scanning Kelvin Probe BioLogic offers a number of resources. For more information on the use of SKP in corrosion SCAN-Lab AN#9 should be consulted. Further information about the use of SKP in photovoltaics can be found in a recent Learning Center article. For information on using Scanning Kelvin Probe while maintaining a constant probe to sample distance please see SCAN-Lab AN#1. And keep checking back at our Learning Center, which we update regularly with new articles.

Glossary

Contact potential difference: The potential difference arising between two materials in electrical contact after their Fermi levels equalize.

Fermi level: The highest occupied energy level of a material at 0 K.

Work function: The minimum energy required to remove an electron from the surface of a material.

References

1. Lord Kelvin, Philos. Mag., 46 (1898) 82-120
2. A. Zisman, Rev. Sci. Instrum., 3 (1932) 367-370
3. Stratmann, H. Streckel, Corros. Sci., 30 (1990) 681-696
4. Stratmann, H. Streckel, Corros. Sci., 30 (1990) 697-714
5. Wicinski, W. Burgstaller, A. W. Hassel, Corros. Sci. 104 (2016) 1–8
6. Liu, X. Cheng, S. Wang, K. Zhang, Y. Gu, Phys. Chem. Chem. Phys. 17 (2015) 17041-17052
7. Cavalcoli, A. Cavallini, M. Rossi, S. Binetti, F. Izzia, S. Pizzini, J. Electrochem. Soc. 150 (2003) G456-G460
8. Williams, H. N. McMurray, D. Worsley, J. Forensic Sci. 46 (2001) 1085-1092
9. S. Zance, S. Ravichandran, Appl. Phys. A 125 (2019) 456
10. Fan, Y. Zhang, W. Li, W. Wang, X. Zhao, L. Song, Chem. Eng. 368 (2019) 1033-1044
11. J. Borth, E. B. Iezzi, D. S. Dudis, D. C. Hansen, CORROSION 75 (2019) 457-464
12. Zhang, F. Zhou, K. Xiao, T. Cui, H. Qian, X. Li, J. Mater. Eng. Perform. 24 (2015) 2688-2698