Kelvin Probe investigations

The Kelvin Probe (KP) is an electrostatic set-up devised and employed by William Thompson (Lord Kelvin) to measure the contact potential difference (CPD) of distinct metals. To do this, the instruments exactly compensates the work function (WF) difference between dissimilar conductors by applying a “backing potential” and reading it out [1]. Therefore, the KP is a very useful tool for the investigation of the conducting materials or structures which respond to the changes in the ambient gas composition through work function modifications.

While the meaning and evaluation of the KP signals in the case of the conductors are straightforward, the interpretation of the results obtained on structures including insulators is more subtle; it usually requires additional/combined investigation/evaluation methods.

The metal phthalocyanines (MPc) are organic semiconductors with very high sensitivity to oxidizing gases. MPc-gas interaction affects many material parameters, among them being the WF. Fig. 1 [2] gives an example of KP evaluation performed on CuPc layers. The “Nernstian” aspect of the CPD-NO₂ concentration dependency is due to the formation of fractional charge transfer complexes [Janata] influencing the chemical potential value (Fermi level position), and by this the value of the WF

Fig. 1. Typical KP signal (left) and calibrations curves (right) for CuPc sensing films. The values of the NO2 concentration during the exposure periods were in order: 100 ppb, 300 ppb, 1 ppm, 3 ppm; rh denotes the relative humidity. (after [2])

Heterogeneous structures of the type “polymer film – conducting substrate / electrode” based on polyacrylic acid (PAA) have been also investigated [3, 4] (Fig. 2) [5].

Fig. 2. KP signal (left) and mechanism sketch (right) for PAA/AU sensing structures. NH3 concentration during the exposures is indicated in the figure; rh denotes the relative humidity. (after [4,5])

The PAA-gold bimorph response to ammonia is coming from the structure interface, which modulates the Au-WF upon gas exposure. The whole sensing mechanism is more complicated [6-8], involving the control of the water absorption in the polymer by the analyte (NH₃) concentration.

References

[1] A. Oprea, N. Barsan, U. Weimar, Work function changes in gas sensitive materials: fundamentals and applications. Sensors and Actuators, B, 142 (2009) 470-493.

[2] A. Oprea, H.P. Frerichs, C. Wilbertz, M. Lehmann, U. Weimar, Hybrid gas sensor platform based on capacitive coupled field effect transistors: ammonia and nitrogen dioxide detection. Sensors and Actuators B, 127 (2007) 161-167.

[3] A. Oprea, N. Barsan, U. Weimar, Ammonia detection mechanism with polyacrylic acid sensitive layers: field effect transduction. Sensors and Actuators B, 111-112 (2005) 577-581.

[4] M. Hoerter, A. Oprea, N. Barsan, U. Weimar, Kelvin Probe measurements of polymer coated gold substrates: mechanism studies. Sensors and Actuators B, 134 (2008) 266-272.

[5] A. Oprea,U. Weimar, High sensitivity polyacrylic acid films for ammonia detection with field effect devices. Sensors and Actuators B, 111-112 (2005) 572-576.

[6] M. Sahm, A. Oprea, N. Barsan, U. Weimar, Water and ammonia influence on the conduction mechanisms in polyacrylic acid films. Sensors and Actuators B, 127 (2007) 204-209.

[7] M. Sahm, A. Oprea, N. Barsan, U. Weimar, Interdependence of ammonia and water sorption in polyacrylic acid layers. Sensors and Actuators B, 130 (2008) 502-507.

[8] M. Hoerter, A. Oprea, N. Barsan, U. Weimar, Chemical interaction of gaseous ammonia and water vapour with polyacrylic acid layers. Sensors and Actuators B, 134 (2008) 743-749.