Work function and DC measurements

Schematic setup of the Kelvin Probe technique: The potential differences between the tip and the sensing is measured (CPD). Besides, also the changes in the resistance (constant voltage mode) are measured.

The work function of a semiconductor is the least amount of energy required to remove an electron from the Fermi level to a point far enough not to feel any influence from the material, with zero kinetic energy. As the electron has to move through the surface region, the work function is extremely sensitive towards changes at the surface caused by gaseous interaction. Simultaneous work function and resistance changes measurements in operation conditions for sensors (in air, in the presence of humidity and target gases, with heated sensors, etc) are performed in our lab by using the Kelvin Probe technique. The latter is a non-contact, non-destructive method that uses a vibrating reference electrode to measure the changes of the contact potential difference (CPD) between the sample and the a.m. electrode. Therefore, the Kelvin Probe technique only allows to measure changes of the work function determined by changes in the composition of the ambient atmosphere, not absolute values; variations in the CPD induced by the changes in the test gas atmosphere represent layer’s relative work function variations. By simultaneously measuring the changes in the layer’s resistance one gains insight about the conduction mechanism in the sensing layer and the way in which the surface charge transfer impacts on that one. Besides that, it is possible to de-convolute the different contributions to the work function, namely the band bending and the electron affinity of the surface. In our lab two different set-ups are used, the Besocke Kelvin the McAllister.

Related References

  • Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO2 sensors in the presence of humidity, N. Barsan and U. Weimar, J. Phys. Condens. Matter, 15, 2003, R813-R83.

  • A p- to n- transition on α-Fe2O3-based thick film sensors studied by conductance and work function change measurements, A.Gurlo, M.Sahm, A.Oprea, N.Barsan, U.Weimar, Sensors and Actuators B, 2004.

  • A n- to p- conductivity transition induced by oxygen adsorption on α-Fe2O3, A.Gurlo, N.Barsan, A.Oprea, M.Sahm, T.Sahm, U.Weimar, Applied Physics Letters, 85, 2004, 2280-2282.

  • Complementary Phenomenological and Spectroscopic Studies of Propane Sensing with Tin Oxide Based Sensors, D. Koziej, N. Barsan, V. Hoffmann, J. Szuber, U. Weimar, Sensors & Actuators B,, 108, 2005, 75-83.

  • Fundamental studies on SnO2 by means of simultaneous work function change and conductance measurements, T. Sahm, A. Gurlo, N. Barsan, U. Weimar, L. Mädler, Thin Solid Films, 490, 2005, 43-47.

  • Metal oxide based gas sensor research: how to?, N. Barsan, D. Koziej and U. Weimar, Sensors & Actuators B,, 121, 2007, 18-35

  • Basics of oxygen and SnO2 interaction, work function change and conductivity measurements, T. Sahm, A. Gurlo, N. Barsan and U. Weimar, Sensors and Actuators B, 118, 2006, 78-83.

  • Investigations of conduction mechanism in Cr2O3 gas sensing thick films by ac impedance spectroscopy and work function changes measurements, S. Pokhrel, C.E. Simion, V. Quemener, N. Barsan, U. Weimar, Sensors and Actuators B 133, 2008, 78-83.

  • Work function changes in gas sensitive materials: Fundamentals and applications, A. Oprea, N. Barsan and U. Weimar, Sensors and Actuators B, 142, 2009.

  • Modeling of sensing and transduction for p-type semiconducting metal oxide gased gas sensors, N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Journal of Electroceramics, 25, 2010, 11-19.

  • Influence of humidity on CO sensing with p-type CuO thick film gas sensors, M. Hübner, C.E. Simion, A. Tomescu-Stanoiu, S. Pokhrel, N. Barsan, U. Weimar, Sensors and Actuators B, 2010, in press