Vibrating capacitor method Kelvin

Fig. 4-28. Schematic layout of Kelvin s vibrating capacitor method to measure relative electrode potential of (a) electrode immersed in aqueous solution and of (b) electrode emersed from solution KP = Kelvin s probe 4 s = outer potential of aqueous solution 4>kp = outer potential of Kelvin s probe V and = applied voltages to cancel out a difference in the outer potential.

The vibrating capacitor method known as the Kelvin-Zisman method was developed by Zisman based on the previous method. Instead of simply distancing the electrodes from each other, Zisman periodically vibrates an electrode, thus creating an alternating current that is easier to detect. 

The vibrating capacitor method, or Kelvin-Zisman method, is a non-destructive method that does not disturb the surface using photon or electron beams. It can be used for a large number of materials, and in a wide temperatnre and pressure range. It is therefore highly appropriate for the stndy of snrface properties of polyciystalline metal oxides. 

Principles and Characteristics The vibrating capacitor or Kelvin method [362, 363] is a well-established experimental technique for measuring the contact potential difference (CPD) or work function for a variety of materials, including polymers [364] and carbon-black [365]. Here, the sensitivity of the CPD to the appearance of electronic surface states and surface charges is used. Scanning Kelvin microscopy (SKM) allows for mapping of the two-dimensional CPD distribution on sample areas of 1 cm with /u.m resolution without extensive experimental requirements [366]. 

The vibrating capacitor method is based on the work of Lord Kelvin [1898K] and of Zisman [32Z]. A condensor is formed of the surface to be studied and a reference electrode in front of it which are connected by a ammeter and a variable voltage source. If the capacitance between the plates (sample and reference electrode) is changed, e.g., by changing their distance, a current will flow. By compensating the... 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

The relative work function and the relative electrode potential of electrodes in aqueous solutions and in inactive gases can be measured by a vibrating capacitor technique called Kelvin s method [Samec-Johnson-Doblhofer, 1992]. The Kelvin method estimates the difference in the work function between a test electrode and a Kelvin probe (KF) by measuring the applied voltage V at which the difference in the outer potential ij s- l KP between the test electrode and the Kelvin probe becomes zero (V = liJs - i Kp) as shown in Pig. 4—28. 

Experimentally, the work function itself can be measured with, among other methods, photoemission, since the work function appears as a clearly distinguishable threshold energy there. Changes in work function are often measured by the Kelvin method, which uses a vibrating capacitor. 

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. 

For the studies of LB monolayer assemblies, information about the orientation of amphiphilic molecules in the monolayers is important, as well as determination of the chemical functionalities. The Kelvin method [104] is a well-established technique for determining the contact potential differences (CPDs) between reference electrode and a metal sample. The CPD for clean metal surfaces is given by the difference in the work functions of the two materials. The work functions can be changed by adsorption of molecules with different dipole moments. Therefore, the photoin-duced dipole change of the triad molecules unidirectionally oriented in LB monolayers on electrode surface can be easily detected as a change of CPD by Kelvin method. The Kelvin method of measuring the CPD makes use of the vibrating capacitor. 

SECM is discussed elsewhere in this handbook. The Scanning Kelvin Probe is an updating and miniaturization of the classic method that was first postulated by Lord Kelvin in 1861. It is essentially a non-contact vibrating capacitor... 

Classical work-function-type gas sensors are specific devices based on the Kelvin method. The Kelvin method ( vibrating capacitor/condenser method, capacitive probe method ), using a so-called Kelvin probe (KP) or Kelvin oscillator, is an established tool for measuring the work function of a sample or, more precisely, the contact potential between the sample and a reference. 

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... 

The Kelvin method is based on the vibrating capacitor principle an inert electrode (a small piece of sheet metal or a grid is placed in front of the surface at a distance of d 10-100 (tm. As discussed above, the Fermi level aligns at equilibrium. Hence, different work functions of the two electrodes give rise to an electric field between them and accordingly to a potential difference (contact potential difference). The electrostatic potential difference eUis associated with a charge q on the electrodes ... 

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