Scanning Kelvin probe technique

G. Williams, H. N. McMuiray, and D. A. Worsley, Cerium(III) inhibition of corrosion-driven organic coating delamination studied using a scanning Kelvin probe technique , J. Electrochem. Soc., 149, B154 (2002). 

H. N. McMurray, D. Wilhams, G. Williams and D. Worsley, Inhibitor pietreatment synergies demonstrated using a scanning Kelvin probe technique . Corrosion Engineering, Science and Technology, 38,112 (2003). 

The scanning Kelvin probe, which measures the Volta potential difference between a specimen and the calibrated sensing probe, is introduced as the only electrochemical technique which allows nondestructive, real-time measurements of electrode potentials at adhesive/metal oxide interfaces in situ, even if they are covered with an adhesive layer. 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

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. 

The scanning Kelvin probe (SKP,. similar to the EFM technique) permits determination of the work function of CP films. This technique has been used to characterize delamination of a polyaniline containing primer combined with an epoxy topcoat [63] and also to probe the doping-level distribution (from the work function variation) in a conducting poly(2,2 -bithiophene) film [64]. 

The scanning Kelvin probe (SKP) provides a measure of the Volta potential (work function) that is related to the corrosion potential of the metal, withont touching the corroding surface [24]. The technique can give a corrosion potential distribution, with a spatial resolution of 50 to 100 pm, below highly isolating polymer films. The SKP is an excellent research tool to study the initiation of corrosion at the metal/poly-mer interface. 

Other than for electrodes immersed in bulk electrolyte, on electrodes covered by ultrathin layers the electrode potential may differ significantly across the electrode surface. Hence, localised measurements are of interest, being performed by scanning the tip across the sample. This was first applied for organic coated metals where the coating was electrochemically delaminating, driven by corrosion [12-14, 29], Even on the submicron scale the Kelvin probe technique can be applied for such studies, and then based on a modified atomic force microscope, see [34, 35]. Recent developments are the combination of Kelvin probe and SECM [36] and the use of Kelvin probe for hydrogen detection [37]. 

Detail at the atomic level of spatial resolution concerning chemical states and composition is available from STM operated in its various modes—scanning Kelvin probe and scanning tunneling spectroscopy (STS) (Table 2). Single-atom resolution and site identification with these techniques are applicable only to reasonably conductive samples. The alternative is LEIS from which a combination of local atomic structure and quantification of surface composition, especially for light elements, can be obtained [14]. 

Guillaumin, V., Schmutz, P, Frankel, G. S. Characterization of corrosion interfaces by the scanning Kelvin probe force microscopy technique. Journal of the Electrochemical Society 2001, 148, B163-B173. 

Many-pass techniques Electric Force Microscopy (EFM) Scanning Capacitance Microscopy (SCaM) Kelvin Probe Microscopy (SKM) DC Magnetic Force Microscopy (DC MFM) AC Magnetic Force Microscopy (AC MFM) Dissipation Force Microscopy-Scanning Surface Potential Microscopy (SSPM) Scanning Maxwell Stress Microscpy (SMMM) Magnetic Force Microscopy (MFM) Van der Waals Force Microscopy (VDWFM)... 

While the previously described techniques both require extrapolation of measured data in order to calculate the contact resistance, Kelvin probe force microscopy (KFM, also known as scanning surface potential microscopy or scanning potenti-ometry) can be used to determine the source and drain contributions to the contact resistance directly. In KFM, a conductive atomic force microscope (AFM) tip is scanned over the operational OFET channel twice. On the first pass, the topography... 

Characterizing these many aspects of microstructure is necessary to establish relationships between primary chemical structure, processing, and performance. Currently, the most commonly used methods are scanning probe microscopy techniques such as atomic force microscopy (AFM) or kelvin probe force microscopy... 

Under certain circumstances, the electrode potential of a surface determines its Volta potential, and thus SKP microscopy allows measurement of local electrode or corrosion potentials. Conventional scanning electrochemical reference electrode techniques require a finite electrolytic resistance between sample and reference electrode, whereas the Kelvin Probe operates across a dielectric medium of infinite... 

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