Work Functions of Conductive Coatings on GlassJay Burns and Edward Yelke Citation: Review of Scientific Instruments 40, 1236 (1969); doi: 10.1063/1.1684209 View online: http://dx.doi.org/10.1063/1.1684209 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/40/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Correlated conductivity and work function changes in epitaxial graphene Appl. Phys. Lett. 100, 092113 (2012); 10.1063/1.3691628 Behavior of Cesium Oxide as a Low WorkFunction Coating J. Appl. Phys. 41, 4505 (1970); 10.1063/1.1658489 Work Function Variation of Metals Coated by Metallic Films J. Appl. Phys. 33, 67 (1962); 10.1063/1.1728530 Work Functions of GasCoated Tungsten and Silver Surfaces J. Appl. Phys. 24, 472 (1953); 10.1063/1.1721304 Work Functions and Conductivity of OxideCoated Cathodes J. Appl. Phys. 20, 197 (1949); 10.1063/1.1698332

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1236 NOTES

FIG. 1. The indexed worm drive on the left is connected to the step­ping motor on the right through a universal joint and an aircraft­type gear reducer (whose label can be seen).

commercial diffractometers, for which the precIsIOn of positioning is of the order of ± 7 -10" at best. This draw­back can be simply overcome, as shown in Fig. 1, by in­serting an aircraft-type gear reducer between the drive wheel and a step-scanning device based on a Slo-Syn motor'! (This step-scanning device is available from Superior Electric Co.) A universal joint mates the two. With a 30: 1 gear reduction, the minimum step with the Superior Electric Co. stepper (18/1 in 28) is reduced to 0.6". The first crystal is held in a monochromator mount at the x-ray tube. The second is placed in a goniostat at the center of the goniometer. Reproducibility of profile shapes and integrated intensities is excellent. Histograms quite similar in appearance to chart recordings can be obtained by keeping the time at each step to the order of 1 sec.

This work was part of an advanced course in diffraction at Northwestern University.

1 M. H. Mueller, L. Heaton, and E. W. Johanson, Rev. Sci. lnstrum. 32,456 (1961).

Work Functions of Conductive Coatings on Glass*

JAY BURNS AND EDWARD YELKE

Laboratory for Experimental Astrophysics, Lindheimer Astronomical Research Center, Northwestern University,

Evanston, Illinois 60201

(Received 12 November 1968)

FREQUENT use is made in experimental electron physics of conductive coatings of stannous chloride

and of colloidal graphite ("Dag") on glass apparatus for various purposes such as electrostatic shielding, stabiliza­tion of wall potentials, application of accelerating or retarding potentials, etc. When these conductive coatings are used in experiments involving low velocity electrons, e.g., in photoemission or secondary emission research, it becomes important to know what degree of uniformity in work function can be expected from these materials under typical ultrahigh vacuum conditions, for sizable variations in work function produce corresponding contact

potential differences that can prove insidiously disturbing to many experimental results.

Parker and Warren,! using the Kelvin method, have investigated the uniformity of Dag coatings, but they did not report a value for the work function of this material. Neither the work function uniformity nor its value for stannous chloride coatings seems to have been reported in the literature. The purpose of this note is to present the results of an investigation of work function variations from point to point on a given specimen and from sample to sample in a series of stannous chloride and Dag coatings on Pyrex and to report values for the work functions of these materials.

Anderson has shown2 that no significant difference is expected between work function values determined by the Kelvin method and the electron beam method. For reasons of convenience we have used the electron beam methodS to obtain the work functions for these materials. To determine the work function from the measured contact potential differences a gold reference surface was used. Although barium is a suitable material and is in wide use,4 if the highest precision is not required, gold is ac­ceptable and is easier to work with. The work function value for gold reported by Anderson5 was used to obtain values for the conductive coatings.

Experimentally, all the stannous chloride coatings were prepared on Pyrex glass using Gomer's method. 6 The colloidal graphite was used in the form of an alcoholic suspension7 either brushed or sprayed on, and the gold reference consisted of a 99.99% pure gold foil substrate onto which a layer of gold (Johnson-Matthey spectro­scopic standard) was evaporated just prior to making the measurements in the final vacuum (10-10 Torr range) allowing only the middle fraction of the evaporated sample to fall on the substrate.

The experimental tube contained three samples in­cluding the gold reference mounted on a gold covered stainless steel carrier. The carrier could be moved by means of a magnetic slug to position it over an aperture through which the electron beam passed. The aperture was located in the bottom side of a U-shaped gold covered shield in which the sample holder was centered. This served to shield all the samples but the one under study from the electron beam. The sample carrier moved suffi­ciently close to the shield to prevent scattered electrons from reaching the sample. The spacings and dimensions chosen afforded a planar geometry, insured against edge effects, and provided a spatial resolution of about 1 mm, i.e., this was roughly the size of the region over which the contact potential difference was measured.

The tube was pumped and vacuum baked at 450°C in a conventional manner. Both titanium and molybdenum getters were used, the former being evaporated just prior to sealing the tube off from the diffusion pump, and the

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NOTES 1237

latter being used later after further pumping with an ion appendage pump. The final pressure was below 10-9 Torr and did not rise above this value during the measurements. Tests of the partial pressure of residual adsorbable gases using the flash filament technique showed that the time for formation of a monolayer of gas on clean tungsten was at least one week. These are fairly typical ultrahigh vacuum conditions under which electron physics experi­mental work is done, and the pumping and bakeout schedules to reach these conditions were those in regular use in this laboratory and are believed to be similar to those commonly used elsewhere, so the histories of the conductive coatings prior to measurement are considered to be quite typical.

Measurements were made by a retarding potential method, as usual with the electron beam technique.3 The resulting curves of target current vs potential have toe regions identical in shape to each other and to the toe of the gold reference curve if experimental conditions are correct,. as was the case in the present measurements, and it is only necessary to determine how far the sample curve must be translated along the potential axis to coincide with the gold reference curve to find its contact potential difference against gold. Seven specimens of each type of coating were measured, and uniformity was checked by measuring at eight to 12 positions on each specimen. Actual values of work function were obtained using Anderson's value of 4.83±0.02 eV for gold."

The results showed that both Dag and stannous chloride have comparable uniformity of work function as prepared and vacuum processed in this work. Spatial variations of work function for both types of coating averaged ±1O m V and variations from specimen to specimen did not exceed 0.10 V for the seven samples of each that were measured. This result for graphite was comparable to that reported by Parker and Warren. On the other hand, we found much less variation over a gold surface than did Parker and Warren. The average variation in the present work was found to be ± 7 m V for gold. The differ­ence between the two results is probably attributable to the very good vacuum conditions under which the gold was deposited in the present experiments, whereas Parker and Warren used electroplated gold giving a coating of somewhat indeterminate purity and structure.

The work function for stannous chloride coatings was found to be 4.79±0.07 eV. For colloidal graphite the value is 4.8S±0.08 eV. These values are close to the value 4.83 e V for pure gold given by Anderson, and the negligible differences between the three values have the fortunate practical consequence that all three can be used in ultra­high vacuum apparatus without the necessity of having to balance out contact potential differences between the different materials and with the expectation that patch

effects due to work function variations over the surfaces will be negligible for most experimental work.

The authors should like to acknowledge the helpful assistance of Robert Pernic in the present work and of Charles Vossler (University of Chicago) in earlier pre­liminary measurements by the Kelvin method.

* Work supported by Air Force Office of Scientific Research. 1 J. H. Parker and R. W. Warren, Rev. Sci. Instrum. 33, 948 (1962). 2 P. A. Anderson, Phys. Rev. 88, 655 (1952). 3 P. A. Anderson, Phys. Rev. 76, 388 (1949); 75, 1205 (1949). 4 P. A. Anderson, Phys. Rev. 57, 122 (1940). 6 P. A. Anderson, Phys. Rev. 115,553 (1959). 6 R. Gomer, Rev. Sci. Instrum. 24, 993 (1953). 7 Dag dispersion No. 154 from Acheson Colloids Co., Port Huron,

Michigan.

Evaporated Silver-Aluminum Thermocouples* for Low Temperature Measurement

SHEILA B. BAILEY, Roy T. RICHARD,t AND E. ·N. MITCHELL

The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27514

(Received 4 March 1969; and in final form, 28 April 1969)

T HE thermocouples discussed here were used to measure the temperature of Permalloy films on

glass substrates. It was necessary to know the temperature of the Permalloy surface during cycling between 300 and 4.2 K. Since this was done repeatedly it was imperative that the thermocouple recycle accurately. The silver­aluminum thermocouple was found to have this stability as well as being easy to evaporate, reproducible, and relatively linear above 20 K.

The thermocouple was prepared by evaporating,r.~at 10-6 Torr, intersecting Ag and Al strips on the Permalloy film. An indium spot was evaporated at the end of each strip to facilitate wire connections to the evaporated

10,----------------------------------------,

.8

.6

~ .4

LL ::; w .2

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-.2

Temperature ( K)

FIG. 1. Calibration data for two different Ag-AI thermocouples. The curve is drawn through the data of SBB (0); .-data by RTR.

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