Vibrating capacitor-techniques

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. 

These important equations have been established using both the Kelvin probe (vibrating capacitor) technique [11,140] and UPS [89] (electron cutoff energy). At the molecular level the variation of <5 with catalyst-electrode potential Uwr is due to the spillover-backspillover of (or Na ) from the solid electrolyte onto the catalyst-gas interface. 

An important step in the understanding of the origin of NEMCA was the realization that solid electrolyte cells with metal electrodes are both work function probes and work function controllers for the gas-exposed surfaces of their electrodes [7,27]. Both theory [9,14] and experiments via the Kelvin probe (vibrating capacitor) technique [7,27] and more recently via UPS [35] have shown that ... 

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. 

Table II compares the work function of a series of polymers with the ionization potential estimated from the ionization potential of similar monomers. The work function was obtained by direct measurement of contact potential of a polymer film using the vibrating capacitor technique under vacuum. The reference electrode was gold, assumed to be 4.7 eV. Calculation of the ionization potential is based on the values of structurally similar model compounds given by Turner. The relationship is reasonably linear but certainly not the 1 to 1 relationship which might be expected. Reasons for the small variation in work function are currently unexplained.

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... 

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. 

A widely used technique consists in building a vibrating capacitor, charge variations of which create a current, which is reduced to zero by insertion of an external bias equal and opposite to V. ... 

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]. 

Kelvin probe force microscopy (KFM) [113-116] is a well-established technique for measnring the contact potential differences (CPDs) between a reference electrode and a sample its basic operation is similar to EFM. In KFM, two conductors (sample and tip) are arranged as a parallel plate capacitor with a small spacing. In a simple model, the contact potential between the two materials is Vcpd = -(4 i - where 4>i and O2 are the work functions of the conductors, including changes due to the adsorbed layers. A periodic vibration between the two plates at a frequency co gives an alternating cnrrent (AC) with the same frequency co when the two plates have different work functions. 

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