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THE CHEMISORPTION OF OXYGEN ON (100)
LEAD SULFIDE
R. Lee
To cite this version:
JOURNAL DE PHYSIQUE Colloque C 4, supplkment au no 11-12, Tome 29, Novembre-Dkcembre 1968, page C 4 - 43
THE
CHEMISORPTION OF OXYGEN ON
(100)
LEAD
SULFIDE
R.
N. LEE (*)Battelle Memorial Institute, Columbus Laboratories, Columbus, Ohio
Rksumk. - Nous avans etudie la chirnisorption d'oxygene sur la surface clivee du PbS par diffraction d'electrons lents. Des expositions B l'oxygkne jusqu'a 5 0 torrs-heures ont seulement rkduit de peu I'intensitk des faisceaux diffract& et nous en avons conclu que la couche d'oxygkne produite n'excedait pas quinze pour cent d'une monocouche. Les variations du travail de sortie mesurees dans le tube de diffraction ont rev616 un changement lent B mesure de l'adsorption de l'oxygbne. Une exposition de 36 torrs-heures produit un changement de 0.3 eV. Ces resultats sont inattendus si l'on en juge par les experiences sur des films polycristallins et doivent &re relies a l'interpretation des mesures de transport dans des couches minces monocristallines.
Abstract.
-
We have studied the chemisorption of oxygen on the cleavage surface of PbS by low-energy electron diffraction. Oxygen exposures up to 5 0 torr-hours resulted only in a slight reduction in the diffraction beam intensities, and it was concluded that the resulting oxygen coverage was no greater than fifteen percent of a monolayer. Work function changes measured in the diffrac- tion tube revealed a slow change with oxygen adsorption, with a 36 torr-hour exposure producing a change of 0.3 eV. These results are in disagreement with the results expected on the basis of experi- ments with polycrystalline films and are pertinent to the interpretation of transport data obtained from single crystal thin films.We have studied oxygen adsorption on (100) PbS
by low energy electron diffraction (LEED) techniques, using an instrument which is patterned after that described by Park and Farnsworth [l]. Data are obtai- ned either in the form of a display of the diffraction pattern or a plot of the energy dependence of the intensity of a given diffraction beam (i. e., the intensity distribution of the beam). Contact potential difference (CPD) measurements can also be made to provide a determination of any work function changes.
The PbS sample was prepared from a large n-type (n = 4 X 1018 vapor grown crystal. A diffrac- tion pattern which was characteristic of the bulk (100) plane of PbS was obtained from the freshly cleaved surface, and the surface was characterized by taking intensity distributions of various diffraction beams. The contact potential difference between the fresh surface and the tungsten cathode of the electron gun was also measured.
Throughout the investigation, the original surface condition (as characterized by the above measure- ments) could always be restored by heating the crystal between 200 and 300 OC, and it was concluded that (*) Present address : U. S. Naval Ordnance Laboratory, White Oak, Silver Spring, Maryland, U. S. A.
the initial surface was indeed a clean surface. The observed ease of cleaning PbS by heat treatment in vacuum is in agreement with previous LEED studies of this material [2, 31. Any changes in the surface
stoichiometry which might have resulted from heat treatments at less than 300 OC were too small to be observed by LEED techniques.
The only observed changes in the clean surface diffraction pattern resulting from oxygen exposures at pressures between l O W 4 and 2 torr was a uniform decrease in beam intensities which occurred at expo- sures in the torr-hour range. No new beams were formed, and there was no detectable increase in the background scattering. The extent and character of the intensity decrement is illustrated in figure 1, where curve a is the intensity distribution of the (10) beam after 48 torr-hours 0, exposure and curve b is the intensity distribution of the same beam for the clean surface. The only significant difference between these two curves is the absolute value of the intensity. The rate of decrease of beam intensities at a given pressure showed that the rate of oxygen uptake on PbS is a rapidly decreasing function of coverage.
From the magnitudes of the intensity decrements, we conclude that oxygen exposures as high as 48 torr- hours result in a surface coverage of no more than
R. N. LEE
V'-
ELECTRON ENERGY-eV80 oxygenated surface appeared to reduce the work
function by about 50 meV. No lower limit on the CO exposure required to produce this change was esta-
U) blished, but it was found that flashing an ion gauge
G
X filament subsequent to an oxygen exposure produced
40 enough CO to provide this apparent drop in work
l=
function. Again, the uncertainties in the CPD mea-V)
z surements were such that firm conclusions must
W
I- await more precise data. No changes either in work
5
function or diffraction data were observed subsequent1 0 to exposure of the clean surface to CO.
a
W These results pose some questions about other
FIG. 1. - Intensity distributions for the (10) beam.
-
a) After 48 torr-hours 0 2 exposure. b) Clean surface. fifteen percent of a monolayer. New beams arising from this degree of coverage should be easily detectable, and the major adsorption product on the surface at this stage of oxidation must therefore be disordered.
Oxygen adsorption was always accompanied by a uniform increase in work function which appeared to be proportional to surface coverage. After a 48 torr-hour 0, exposure, the net work function increase was about 0.3 eV, which is comparable to the 0.4 eV band gap for PbS. There was a reproducible indica- tion that about 50 meV of the work function change occurred for less than 10-3 torr-minutes 0, exposure. This would indicate that there is an initial fast chemi- sorption step which is distinct from the later slow oxygen uptake and may be related to the filling of sulphur vacancies with oxygen. However, the uncer- tainty in the determination of work function change was itself about
+
50 meV, and more precise measu- rements must be made before firm conclusions can be drawn.observations on the interaction of oxygen with the lead salts. Hillenbrand has studied the kinetics of the room temperature oxidation of sublimed films
m
W
2
Q
A number of attempts were made to remove adsor- bed oxygen from the surface by exposing the oxyge- nated surface to CO at room temperature. No detec- table changes in the diffraction data occurred with CO exposures in the Torr-hour range. However, it is interesting to note that CO exposure of the freshly
-1
of PbS 141 and found that the oxygen uptake is quite slow, varying logarithmically with the pressure-time
\ product. However, our results are in quantitative
H disagreement with his conclusion that a 48 torr-hour
oxygen exposure results in a surface coverage of about one-half, assuming adsorption onto the (100)
o 50 surface. The maximum coverage consistent with
our results for a 48 torr-hour oxygen exposure (15 percent of a monolayer) should be obtained at about 5 torr-hours according to Hillenbrand's interpretation of his data.
We suggest here that this discrepancy is due to diffusion processes in the high surface area films used in adsorption kinetics experiments. Diffusion along grain boundaries in the polycrystalline films is consistent with electronic transport data obtained by Hillenbrand and would result in an exaggerated estimate of surface coverage in his analysis.
We now turn to recent results by Brodsky and Zemel 151, which indicate that exposure of the lead salts to oxygen leads to an excess surface charge of the order of 5 X 1013 cm-'. Their observations also suggest that this surface charge is removed by CO. (Other investigators have reported that CO reverses the electronic effects of oxygen adsorption [ 6 ] ) . Zemel has considered both a surface state model and a space- charge limited oxide growth model as possible mecha- nisms for the incorporation of such high charge densities [7]. He concludes that Coulomb interactions preclude the surface state model and finds that a 5 X 10- l 3 cm-2 excess surface charge is consistent
with a 10
A
oxide layer.The failure of CO to significantly alter the oxygena- ted surface in the present study suggests that the interpretation of the thin film transport data obtained by Brodsky and Zemel is considerably complicated by the surface chemistry. Their films were exposed -
THE CHEMISORPTION OF OXYGEN ON (100) LEAD SULFIDE C 4
-
45to the atmosphere during preparation, and it is likely that the film surfaces were entirely composed of the amorphous oxidation product which was observed to form in the present study. (Recent results in our laboratory indicate that the surface is completely covered with an amorphous layer after exposure of the crystal to the atmosphere for a period of weeks.) Thus, the system which provides the excess surface charge and is sensitive to CO is probably oxygen on an oxidation product rather than oxygen on PbS. The author would like to thank Professor J. N. Zemel for a number of invaluable discussions and Dr. L. J. Hillenbrand for providing access to his data prior to publication.
References
[ l ] PARK (R. L.) and FARNSWORTH (H. E.), Rev. Sci. Znstr., 1964, 35, 1592.
[2] JOHNSON ( D . C.) and MACRAE (A. U.), J. Appl. Phys.,
1967, 37, 2298.
[ 3 ] ZYRYANOV (G. K.), Bull. Leningrad University, 1966, 22, 80.
[4]
HILLENBRAND
( L . J.), J. Chem. Phys. (To be published).[ S ] BRODSKY (M.) and ZEMEL (J. N.), Phys. Rev., 1967, 155,780.
[6] BEREZHNAYA ( I . A.), Russian J. Phys. Chem., 1962, 36, 1500.
[7] ZEMEL (J. N.), Battelle Colloquium on Molecular Proces- ses on Solid Surfaces ; Kronberg, Germany, 1968
