Kelvin method

It would clearly be desirable to extend the scope of the Kelvin method to include a range of adsorptives having varied physical properties, especially surface tension, molar volume, molecular shape and size. This would enable the validity of the method and its attendant assumptions to be tested more adequately, and would also allow a variation in experimental technique, for example by permitting measurements at 298 K rather than 77 K. 

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. 

A linear relationship (straight line with unit slope) is observed in Fig. 4-29 between the electrode potential E measured by a conventional method and the electrode potential E-Ekp measured by the Kelvin method for equilibrium... 

The C.P.D. is obtained directly by the magnetron and the capacitor (or Kelvin) methods. Other methods which have proved successful rely on the variation of anode potential in a diode with constant cathode conditions. In this case, since adsorption changes the effective anode potential, the applied potential necessary to restore the anode current to its original value is equal to the C.P.D. between the two surfaces. As considered in Sec. II, a true average work function is measured in the C.P.D. method when the two conductors are separated by a distance which is much greater than the size of the patches on the surface. These conditions are invariably fulfilled in the capacitor and the space-charge-limited diode methods. 

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. 

In the Kelvin method, a planar electrode is vibrated up and down just above the surface. This electrode and the surface form the two plates... 

The detection and compensation of the a.c. current is the classical Kelvin method however, the resulting electrostatic forces, i.e. the corresponding cantilever bending, can also be used to establish a potential sensitive feedback. If an a.c. voltage is applied between the tip and the back electrode of the sample instead of using the dither piezo, the Maxwell stress microscopy (MSM) [379-381] or the electrostatic force microscopy (EFM) [317, 382-393] can be performed. 

Fig. 7.13 Diagram of a modified Kelvin method for measurement of the contact potential difference between two different metals, 1 and 2.

If strong illumination produces flat bands at the surface without changing X, that is, without aflfecting the surface dipole layer, then the saturation value of the surface photovoltage measured by the Kelvin method A should be... 

Hansen and Kolb obtained 4.7 V as VnheCv c. scale) by measuring the work function of an immersed electrode with a NHE by the Kelvin method.52 This value may be affected by contamination of the electrode41 since more recent results show that the surface emerging from the solution is contaminated.53... 

Althongh the Kelvin method is generally used by some authors to study the electronic work function of monocrystals, it is also appropriate for studying metallic or non-metallic polycrystalline materials. 

In the case of a heterogenous surface of polycrystalline materials, the woik function of the grains depends on their crystal orientation it has been demonstrated that the work function measurements obtained by the Kelvin method led to the mean value (p of the work function values of each grain i ... 

Some authors have chosen to work at very low temperatures, while others have used the Kelvin method at a temperature of a few hundred degrees in the framework of studies on catalysts. 

There is a scarcity of studies in the literature for pore size distributions in the range where mercury porosimetry and the BET Kelvin method overlap. However, the pore size distribution of CPG 75 in the 10 nm range as measured using porosimetry was in excellent agreement with that measured using BET surface area, as reported by Porcheron et al. [52] (see Figure 3.12b). 

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. 

Kelvin probe Probe used to measure the contact potential difference between tow materials by Kelvin method... 

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. 

Surphce NA, D Arcy RJ (1970) A critique of the Kelvin method of measuring work functions. J Phys E Sd Instrum... 

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 principle of the Kelvin method and its application in the study of surface properties of compounds have been reported in References 4, 5, and 9. 

For the measurement of work-function differences as they occur, for instance, during adsorption, several methods are in use. The most common ones are the Kelvin method and the diode method. 

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