Surface potential Kelvin probe method

For calculating the space-charge conductance (j(F,) we need to know the density of states g E) and the surface potential F,. Under favorable conditions, F, of a free surface can be obtained by the Kelvin probe method or from the saturation value of the surface photovoltage. These methods will be illustrated with some preliminary results obtained on a-Si H. We then shall discuss metastable light-induced changes of the surface potential and conductance. 

The state of the polymer surface and the polymer/elec-trolyte interface can be probed by measurement of the work function after initial polarization of the polymer-coated electrode in a liquid electrolyte. The latter is then withdrawn from the electrolyte ( emersed") under potential control, and then the work function measurement is performed [159,160]. The emersion procedure is schematized in Fig. 20.32. A widely used strategy for the work function measurement is based on the vibrating capacitor plate or the Kelvin probe method [161]. These methods rely on the fact that between two conducting and connected surfaces there exists a contact potential difference (CPD) because of the difference in work functions [162]. Changes in capacitance are induced by periodic vibration of one of the capacitor plates around its equilibrium position. The resulting ac current i is detected with a phase-sensitive detector and is given by... 

A further spatially resolved method, also based on work function contrast, is scanning Kelvin probe microscopy (SKPM). As an extended version of atomic force microscopy (AFM), additional information on the local surface potential is revealed by a second feedback circuit. The method delivers information depending on the value (p (p(x) + A x). Here, A(zS(x) is the difference in work function between the sample and the AFM tip and cp(x) is the local electric potential [12]. (p x) itself gives information on additional surface charges due to... 

For all nc-AFM measurements, a Kelvin probe force microscopy (KPFM) feedback controller was additionally activated for simultaneous topographic imaging [19]. In order to compensate for electrically or electronically induced artefacts, an ac voltage was applied between tip and sample and used in combination with lock-in techniques and a feedback controller to compensate for the contact potential difference (CPD) between tip and sample. With this method, nc-AFM is assiued to image the sample topography without any artefacts originating from different local surface potentials [20]. 

The Kelvin probe is a noncontact, nondestructive, vibrating capacitor technique for measuring work functions, or more precisely the difference between the work function of sample and probe. It was first used by Thomson, later Lord Kelvin, in 1862 [93]. This method has been further improved throughout the following decades [94] and is now a well-established method for measuring work functions, or, from a more electrochemical point of view, Volta potentials. Whereas in traditional Kelvin probes, the probe is a small gold plate or mesh of several square millimeters or centimeters, in Scanning Kelvin Probes (SKP), the probe is a small metal tip with a diameter of typically several tens of micrometers, which can be scaimed across the surface of the sample. 

Corrosion protection of conducting polymers is also involved with an inhibition of the oxygen reduction." Oxygen reduction and the formation of OH anions were the reasons for the delaminating of protective films around film defects. A very successful method to investigate delaminating is provided by the Kelvin probe connected with surface potential measurements as developed by Stratman, Rohwerder et... 

The surface potential at the interfacial layer is controlled by the surface excess of ions, their charges and polariTation, packing density, and dipole moment or induced dipole moment it can be determined by special techniques, that is, the vibrating plate method, combined with a Kelvin probe. Experimental data together with the most suitable prediction models may include the ion concentration in the bulk or ionic strength, and, when dealing with a monolayer, the ion number density in the monolayer, the monolayer thickness, and ion diameter. 

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