Measurement of Surface Potential or Contact Potential DifferencesArthur A. Frost Citation: Review of Scientific Instruments 17, 266 (1946); doi: 10.1063/1.1770482 View online: http://dx.doi.org/10.1063/1.1770482 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/17/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improved Kelvin method for measuring contact potential differences between stepped gold surfaces inultrahigh vacuum Rev. Sci. Instrum. 66, 5544 (1995); 10.1063/1.1146082 MEASUREMENT OF CONTACT POTENTIAL DIFFERENCES BY ELECTRON INTERFEROMETRY Appl. Phys. Lett. 5, 209 (1964); 10.1063/1.1723591 Ionization Method of Measuring Contact Potential Differences Rev. Sci. Instrum. 35, 1160 (1964); 10.1063/1.1718986 A Modified Kelvin Method for Measuring Contact Potential Differences Rev. Sci. Instrum. 17, 15 (1946); 10.1063/1.1770387 A NEW METHOD OF MEASURING CONTACT POTENTIAL DIFFERENCES IN METALS Rev. Sci. Instrum. 3, 367 (1932); 10.1063/1.1748947

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THE REVIEW OF SCIENTIFIC INSTRUMENTS VOLuME 17, NUMBER 7 JULY, 1946

Measurement of Surface Potential or Contact Potential Differences

ARTHUR A. FROST

Department of Chemistry, Northwestern University, Evanston, Illinois

(Received April 10, 1946)

The term "surface potential" is recommended as preferable to "contact potential" when surface effects are being studied. The use of a pH meter for measurement of surface potentials is described.

T HE recent descriptions of two closley related methods for determining contact potential

differences by Rosenfeld and Hoskins! and by Meyerhof and Miller2 show the growing interest in such measurements as a tool in solving various problems involving surfaces. Similar methods have been used by the author3 to study surface potential changes due to adsorption of gases at solid surfaces.

The method described here is a modified Kelvin method having the advantage over other de­scribed methods, in that it makes use of electrical apparatus commonly found in chemical or bio­logical laboratories.

NOMENCLATURE

Potential differences measured by the Kelvin method and similar methods are most often

generalize this most descriptive term and apply it also to potential changes at solid surfaces in place of "contact potential" which falsely implies that the observed potential is a property of the con tact between the conductors rather than a property of their surfaces. If the two plates are of the same metal but with surfaces treated differently, a potential difference is observable without there being any contact, hence the absurdity of the term "con tact poten tial."

APPARATUS

In the Kelvin method the two conductors, whose surface potential difference is to be measured, are made the opposing plates of a parallel plate condenser. If there is an electric field between the surfaces of the conductors, any change in the distance between them will cause a change in capacity of the condenser and therefore a change in potential or a flow of current in the circuit to which the condenser is attached. The effect is easily detected by means of a vacuum tube electrometer.

The measurement of the surface potential difference is carried out by introducing an opposing potential from a potentiometer until a null effect is obtained on moving one plate with respect to the other. .

described as contact potentials or Volta po­tentials. These names apply particularly to results obtained with metal surfaces thoroughly cleaned and in a high vacuum. However, there is an increasing amount of work w4ere the interest is in effects caused by changes in the condition of the surface. A monomolecular or multimolecular film or adsorbed layer of molecules on the surface of a conductor would be expected to give rise to a change in potential owing to the creation of an electrical double layer. For the case of mono­molecular layers on water the term "surface potential" has been used.4 It is desirable to

Commercial "pH meters" of the potentiometer type are well adapted for these measurements since they contain both the potentiometer and

1 S. Rosenfeld and W. M. Hoskins, Rev. Sci. Inst. 16, vacuum tube electrometer in one unit. A Beckman 343 (1945). Model G pH meter is used by the author. A

2 W. E. Meyerhof and P. H. Miller, J r., Rev. Sci. Inst. 17, 15 (1946). shielded cable connects the electrodes of the

3 A. A. Frost and V. Hurka, J. Am. Chern. Soc. 62, 3335 . 11 hId . f h (1940). A. A. Frost, Trans. Electrochem. Soc. 82, 259 measunng ce to tee ectro e connectlOns 0 t e (1942). pH meter. In order to have sufficient stability

4 For example see W. D. Harkins in A. Weissberger, Physical Methods of Organic Chemistry (Interscience when the grounding key was depressed (circuit Publishers, Inc., New York, 1945), Vol. I, p. 235 or W. D. opened) it was found necessary to introduce a Harkins in J. Alexander, Colloid Chemistry (Reinhold Publishing Corporation, New York, 1944), Vol. V, p. 42. 1O,OOO-megohm resistor or grid leak between

266

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MEASUREMENT OF SURFACE POTENTIAL 267

ground and the grid connection of the electrome­ter tube. This did not appreciably reduce the sensitivity. The resistor can be placed in the electrode compartment of the pH meter, con­necting it between the metal case and the upper pin-jack.

The measuring cell is shown in Fig. 1. I twas constructed with a brass housing of i" thickness which serves both as electrostatic shielding and also permits the cell to be evacuated or to receive various gases or :vapors through the openings, G. The parallel plates, A and B, are of brass, the upper one being insulated by amphenol, I, and adjustable in position by turning the screw, S. The shielded lead, D, goes in through the metal shielding, T, to connect with S and the upper plate. The shielding of the cable connects with the case of the cell and serves as the conductor for the lower plate. The lower plate is moved up and down through a distance of about 3 mm by turning the crank which operates a cam, C. M is a sheet or plate of the metal or conductor, the upper surface of which is to be studied. Sufficient electrical contact is usually provided by M merely resting on plate B. A is the reference electrode and is adjusted so that the closest distaqce of approach of the two surfaces is about 0.5 mm. A is 37 mm in diameter. M is somewhat larger so as to avoid any contamination of the measured surface through handling. A fingerprint on the surface usually causes a marked change in potential and vitiates all succeeding results. The front of the cell is covered with a brass door which may easily be removed for changing the sample.

MEASUREMENT

A measurement is taken by first balancing the pH meter circuit in the usual way when switched to the millivolt scale. Then, with the grounding key locked in the depressed position, the crank is turned a few revolutions. Fluctuation of the galvanometer needle indicates lack of balance. The potentiometer is adjusted until no fluctua­tion occurs and the reading is then taken. The Beckman pH meter galvanometer responds rapidly enough so that a cranking rate of one or two revolutions per second is convenient: Further­more, the grid leak introduced, as explained above, is such that if the potentiometer dial is

turned with the key locked in the depressed position, the galvanometer needle returns to its normal position within a second or two.

Potentials determined with this apparatus may be read to the nearest 5 millivolts over a range from -1.3 to + 1.3 volts. Surfaces exposed to the atmosphere however have potentials usually reproducible to no better than 20 mv.

APPLICATIONS

The measurement of surface potentials consti­tutes a tool for the study of surfaces and surface phenomena. In this respect it may be compared with electron diffraction. A surface potential is a single parameter associated with a given surface in a given condition and so cannot give the detailed information that is obtainable from electron diffraction. However, the former is ex­pected to be more sensitive to surface changes than the latter since a single layer of atoms can change the potential without being noticeable in

D

FIG. 1. Surface Potential Cell.

the electron diffraction pattern. Furthermore surface potential measurements are not restricted to a vacuum but may be carried out in the pres­ence of different gases or vapors.

Applications have been made to monomolecular films on water,4 multimolecular layers on solids,4 adsorption of vapors on solids,3 as well as the more classical experiments on work function. 5

6 e.g., J. H. de Boer, Thermionic Emission and Adsorption Phenomena (Cambridge University Press. New York, 1935).

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268 R. H. DICKE

The work being carried on at present in this laboratory is a study of passivity of iron and other metals. Preliminary results show with iron a surface potential change of about 0.2 volt in the negative direction upon passivation in concen­trated nitric acid followed by a reverse change when made active with hydrochloric acid. This is in the direction that would be expected on the basis of an oxide layer. However, activation by

THE REVIEW OF SCIENTIFIC INSTRUMENTS

dilute nitric acid gives a more negative value than the passive iron, a result which perhaps means the presence of a different oxide or possibly a basic nitrate.

Acknowledgments are due Doctors L. P. Gotsch and Laubscher for suggesting the use of a pH meter for this purpose and Mr. Robert Busch for laboratory assistance. This project was sup­ported in part by the Line Material Company.

VOLUME 17, NUMBER 7 JULY, 1946

The Measurement of Thermal Radiation at Microwave Frequencies

R. H. DICKE*

Radiation Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts**

(Received April 15, 1946)

The connection between Johnson noise and blackbody radiation is discussed, using a simple thermodynamic model. A microwave radiometer is described together with its theory of opera­tion. The experimentally measured root mean square fluctuation of the output meter of a microwave radiometer (OA°C) compares favorably with a theoretical value of OA6°C. With an r-f band width of 16 me/sec., the O.4°C corresponds to a minimum detectable power of 10-16

watt. The method of calibrating using a variable temperature resistive load is described.

INTRODUCTION

SINCE radio waves may be considered infra­red radiation of long wave-length, a hot body

would be expected to radiate microwave energy thermally. In order to be' a good radiator of microwaves, a body must be a good absorber and the best thermal radiator is the "blackbody."

Although their discoveries were historically unconnected, there is a very close connection between "Johnson noise" of resistors and ther­mal radiation. The thermal fluctuations of elec­trons in a resistor set up voltages across the resistor. These "noise voltages" are of such a magnitude that a noise power per unit frequency of kT can be drawn from the resistor. k is Boltz­mann's constant; T is the absolute temperature of the resistor.

The comJ.ection between thermal radiation and

* Now at Palmer Physical Laboratory, Princeton Uni­versity, Princeton, New Jersey.

** This paper is based on work done for the Office of Scientific Research and Development under contract OEMsr-262 with the Massachusetts Institute of Tech­nology.

Johnson noise can best be shown by considering the system of Fig. 1.

An antenna is connected to a transmission line which is in turn terminated by a resistor. The radiation impedance of the antenna is as­sumed to be equal to the characteristic impedance of the coaxial line, i.e., the antenna is "matched" to the line. Also the resistor is assumed to "match" the line. When a transmission line is terminated by a "matched" load, the running waves in the transmission line incident on this load are completely absorbed without reflection. The antenna is completely surrounded by black

R"is!or T.m".rolu"

~~~~~r

Ene/Dlur. WalnS/ack .m".rtlfur. T

FIG. 1. Antenna system in black enclosure.

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