1110
N.J. HARRICK
Yol. G 1
SCRFACE CHEMISTRY FROM SPECTRAL ANALYSIS OF TOTALLY INTERN14LLY REFLECTED RADIATIOY * BY N. J. HARRICK Philips Laboratories, Zrvington-on-Hudson, New York Received March 7. 1.960
A study has been made of the phenomenon of total internal reflection with the possibility in mind of applying it to the study of the spectra of surfaces of optically transparent materials, particularly semiconductors. It is found that the radiation penetrates the surfaces into the rarer medium to a depth predicted by Maxwell's theory and that this radiation is sensitive to molecular absorption on the surface. The degree of interaction on each reflection is comparable to that observed for a single transmission if the film thickness is equal to or less than the penetration depth. Since many total internal reflections can be used without power loss, this technique should be a sensitive one in the study of the chemistry of surfaces of optically transparent materials and of thin films which can be deposited on optically more dense and transparent dielectrics.
Introduction One of t'he most direct approaches to t,he study of chemisorbed molecules on solid stsate surfaces is to analyze the infrared spectrum of the surface.1a S o t only does t'he spectrum tell us what molecule is on the surface but it also tells us something about the nature of the bonding to t,he surface. This technique has worked successfully for metals.Ia Here the sample was powdered in order to gain surface area. The beam was transmitted through the sample and spectrally analyzed. Thin samples have been spectrally analyzed by placing them on polished metal surfaces and, to ga,in sensitivit'y, multiply reflecting the infrared beam between two such mirrors.lb Even though the reflect,ivity of the metal may be high, the beam is rapidly attenuated for many reflections which may be required to gain sensitirit,y as t,he power in the beam is finally Rn where R is the reflectivity and n the number of reflect>ions. It is particularly important to apply similar techniques t'o the study of semiconductor surfaces in order to correlat'e the chemistry of the surface with our present knowledge of tshe physics of t,he surface. Powdering of the semiconductor has the disadvant'age that the characteristics of the semiconductor may change drast'ically, when the sample is powdered, especially if the powder size is comparable to or less than a Debye length. A beam transmitted through a sample consisting of several separated layers has t'he disadvantage that reflectivity losses are high. The same disadvantage applies to a greater degree to a beam multiply reflected between t.wo parallel plates. It has been suggested2 that total internal reff ection using radia* This work has been reported in Phys. Rea. Letters. 4 , 224 (1960). P. Eischens a n d W. A. Pliskin, "Advances in Catalysis," 10, Academic Press, Inc., New York, N. Y . . 1958; (b) S. A. Francis and A . H. Ellison, J . O p t . SOC..4m., 49, 131 (1959). (2) Discussion b y N. J. Harrick following paper by R. E'. Eischens in session on New Techniques a t Second Conference on Semiconductor Surface, Naval Ordnance Laboratories, White Oak, Rld., Dec. 2-4, 1959. J . Phys. Chem. S o l i d s , to he published (1960). It was brought to the author's attention after the work described here was completed t h a t Dr. J. Fahrenfort of the Royal Dutch Shell Laboratories, Amsterdam, has described what appears to b e a similar technique t o observe t h e spectra of organlc materials on silver chloride, a t the F o u r t h International Congress on Xolecular Spectroscopy, Bologna, Sept. 7-12, 1959. ADDEDK o m : Dr. J. Fahrenfort shows t h a t total internal reflection gives spectra of much higher contrast. t h a n conventional reflection. H e suggests t h a t total internal, rather than conventional, reflection might thus be utilized t o obtain spectra when it is inconvenient or impossible to obtain the spectra from transmission measurements. H e is concerned with a single reflection. Our approach is to utilize total inter(1) (a) R.
Vol.
tion above the lattice absorption edge might work because on reflection the beam actually penetrates the surface and is pumped into and out of the rarer medium. This beam, if it is sensitive to molecular resonances, should give information regarding the type of impurity on the surface and the nature of the bonding to the surface. Since total internal reflection means that the reflection is loo%, many internal reflections can be used and thus the sensitivity of this technique can be much greater than that of transmission or external reflection. To determine whether application of total internal reflection to the study of the surfaces of optically transparent materials is possible the following questions must be answered : (a) Is this reflected radiation sensitive to molecular absorption? (b) How great is any interaction ('ompared to that iii a traiismission me:tsurement? Before giving the experimental results we will review briefly some aspects oi the phenomenon of reflection. When a beam strikes at normal incidence an interface of a non-conduc.ting medium it is reflected to a degree given hy €i' = (n12 - 1)2 (nl2 1)2,where nI2is the relative index of refraction of the two media. h q the angle of incidence is increased, the reflectivity remains constant until the principal angle. Op = tan-' n21,is reached. For further increase iii incident angle, the reflectivity increases rapidly to unity a t grazing incidence whenthe second medium is more dense than the first. If. however, the second medium is less dense than the first, the reflectivity becomes unity for an angle only slightly larger than the principal niigle which may be much less than 90'. This is called the critical angle and is given by 8, = si1i-I ti21. I'or angles greater than the critical angle many reflections may take place with losses occurring only due to absorption in the bulk and reflectivity loqseh at the entrance and exit surfaces of the sample. The denre medium thus acts as a w a w guide or light pipe. Theie features of the reflection phenomenon are shown in Fig. 1 for the Ge-air iiiterface. I\Iaxwell's theory predicts that the totally reflected heam actually penetrates into the rarer medium (see insert of Fig. 2a) where the electric field intensity falls off exponentially. The depth of penetration, defined as the distance nhere the
+
nal reflection in order to gain sensitivity through many reflections and thus ultimately examine films of the order of a monolayer thick. T h e author nishes t o thank Dr. Fahrenfort for a preprlnt of his paper to be published in the proceedlngs of the Bologna conferenre
Sept., 1960
SPECTRAL h d L Y S I S OF
TOTALLY I N T E R S A L L Y IlKFLECTEU
2'41 2.2
-
I
1111
1l.\UI.\TIoN
0
THEORY EXPERIMENT
I
I
I
I
i
l
/
401
I I I
I
(>
w
--2o 10
1
0
AIR-+Ge
-Be-AIR
IO
1
20
30
40
50
60
70
180
90
80
@P
ANGLE OF INCIDENCE,
Fig. 1.-The
reflectivity of the germanium-air interface as a function of angle of incidence.
* IR
b shapes for total internal reflection measurements.
electric intensity has fallen to one-half of its value at the surfare, is given by3 XI/*
IO
20
30
40
SO
60
TO
80
90
ANGLE OF INCIDENCE.
Fig. 3.-The penetration depth vs. angle of incidence for total internal reflection a t the germanium-polyethylene interface. The experimental point is obtained from the experimental data of Fig. G .
MAGNIFICATION lo4
Fig. 2.-Sample
0
=
0.693 X i 2?r(sinz 0 - n212)'/2
Here XI is the wave length in the denser medium, the angle of incidence and nZ1,which is less than unity, the relative index of refraction of medium 2 to medium 1. The penetration depth a t a wave length of 3.4 as a function of angle of incidence and calculated from this formula is shown in Fig. 3 for germanium, dielectric constant 16, in contact
8
(3) J. A. Stratton, "Electromagnetlo Theory," McGraa-Hill Book Co., Inc., New York, N. Y., 1941. This penetration depth is that for 8 non-absorbing medium.
with polyethylene, dielectric constant 2.3. Except near the critical angle when sin 9" n21. the penetration depth is about one-tenth of a wave length. It should be noted the penetration is proportional to the wave length in the denser medium and becomes less as the dielectric constant of the rarer medium decreases. This penetration into the rarer medium has been demonstrated in numerous experiments. Penetration with displacement as shown in the insert of Fig. 2a was demonstrated in a remarkable experiment by Goos and Hanchen4 n.ho compared total internal reflection with metallic reflection (dotted line in the insert of Fig. 2a) and found after many reflections a displacement of the two beams relative to each other. They found for the displacement distance for a single reflection the expression D
=
Knp
11
(sin2 e
- m1-
where K = 0.52 and n2 = Xo/X2. The displacement is thus directly proportional to the depth of penetration and thus follows a curve as a function of angle of incidence similar to that shown in Fig. 3. The phenomenon of the penetration into the rarer medium of totally internally reflected radiation has been demonstrated in other experiments. Here vi-e list a few such experiments demonstrating this fascinating effect. 1. Schaefer and Gross5 have measured the penetration in the rarer medium for total internal reflection a t the glass-air interface using radiation of 15 cm. wave length. (4) F. Gooa and H. Hinohen Ann. Phva., [ 6 ] 1, 333 (1947). (5) C1. Sohaefer and G. Gross, Ann P h y s t k , 32, 6-18(1910).
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N. J. IIARRICK
2. If phosphor is placed on an optical surface where total reflection is taking place, the phosphor will light up indicating a penetration into the rarer medium.6 The objection that the condition of total reflection has been disturbed when the phosphor makes contact is ruled out if this experiment is done properly. 3. If a convex lens makes contact to a surface where total reflection is taking place, a spot larger than the point of contact and without fringes app e a r ~ . ~The size of the spot indicates that transmission of light through the lens is not limited to the point of contact. If the spot is viewed through the lens, it appears white with a reddish border; whereas if it is viewed through the prism, it appears black with a bluish border. This color separation is an indication of the wave length dependence of the penetration into the rarer medium. 4. If a razor blade is brought down to a surface of a light pipe, the edge of the blade will become illuminated just before contact.' 5. The frustrated total internal reflection filter takes advantage of the penetration of the radiation into the rarer medium.* 6. If metal strips forming a grating are evaporated on the reflecting surface of a prism an interference pattern can be observed a t grazing incidence outside and behind the reflecting surface even though the incident angle is greater than the critical angle so that condition for total internal reflection is met.9 7. When total internal reflection is occurring and a metal is brought into contact with the reflecting surface, the reflectivity of the interface becomes less than total. Measurements of the reflectivity in this case can provide a measure of the conductivity of the material making contact to the dielectric. Such measurements have been made for the Ge-Hg interface.'O The object of the present experiment is to place known molecules on the surface of the optical wave guide and to attempt t o detect their presence through analysis of the spectrum of the reflected radiation. Experimental The sample was cut, lapped and diamond polished in one piece from 30 ohm-cm., p-type germanium having the shape s h o r n in Fig. 2a. This particular sample shape has the advantage that the exit beam is axial with the entrance beam. This leads t o fewer complications in the optics of the spectrometer. The dimensions were such as t o give eight reflections with the entrance face having dimenThe angle of incidence was made 45" sions of l/*'! X "4". because this results in the largest usable aperture for the sample and makes machining of the angles involved easier. The sample was placed in a cell so that all of the surfaces except the entrance and exit faces could be surrounded by a liquid or a gas. If narrow spectrometer slits are used, the sample can be made thinner and the number of reflections can thus be increased. Figure 2b shows how the surface might be curved to inrrease the number of reflections and thus increase the sensitivity. For the example shown, ( 6 ) R. W. Wood, "Physical Optics," The Macmillan Co., New
York,
N. Y., 19.56. (7) Jenkins and White, "Fundamentals of Optics," McGraw-Hill Book Co., Inc., New York. N. Y., 1957. ( 8 ) P. J. Leurgsns and A. F. Turner, J . Opt. SOC.Am., 97, 983(A) (1947). The term "frustrated reflections" was coined here. (9) P.J. Leurgans, private communication. (10) N. J. Harrick, J . Opt. Soc. Am., 49, 376 (1959).
Vol. 64
1
* WAVELENGTH (MICRONS),
ectra of beam I R of Fig. 2a. The lines attributed to 80, indicate its presence in the at,mosphere.
Fig. 4.-S
TRANSMISSIW
3 4P
REFLECTION
I
7
27
43
23
34
43 23 WAVELENGTH (MICRONS1
34
I
43
Fig. 5.-The spectrum of a 1.5 x 10-4 cm. polyethylene film from reflection and transmission measurements.
s'90.
:: 2
8
75-
Y
0
POLYETHYLENE ABSMIPTION BAN0 AT 3.4p x x TWO TRANSUSSIONS * * EIGHT REFLECTIONS 0 0 TWO REFLECTIONS
860.
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9 30u)
POLYETHYLENE THICKNESS (MICRWS)
Fig. 6.-Comparison of the signal strength for transmission and reflection beams, IT and I R of Fig. 2a, for the polyethylene absorption line a t 3.4 g. The spectrometer slit width was 0.5 mm. for these measurements. the number of reflections has been increased from four to fourteen by curving the surface. Care must be taken in such a sample that the angle of incidence does not decrease to a value less than the critical angle otherwise the beam will be lost. The exit infrared beam was analyzed with the aid of a Perkin-Elmer Model 12C Spectrometer. Because of the lack of sensitivity of the present infrared set-up, we chose to work with absorption bands in the two to four micron range where the radiation intensity of the globar is high. The polyethylene was placed on the sample surface by first dissolving i t in xylene to a dilution of 0.04% by weight and spraying it on the surface of the sample. It was thus possible to control the thickness of the polyethylene film. The signal intensity for transmission and reflection were compared by analyzing the beams ITand ZR of Fig. 2a.
Sept., 1N;O
SPECTRAL ANALYSISOF TOTALLY IFTERNALLY REFLECTED RADIATION
Results Water.-The experimental results for mater are shown in Fig. 4. The solid line represents the transmission of the sample after eight reflections. The COOabsorption bands, due to the surrounding atmosphere are detectable. The dotted lines show the deviation from the solid curve when the sample is surrounded by mater. The water absorption bands a t 3 and 6 p are clearly visible. A rough check of the depth of penetration of the 3 p radiation into the water was made by comparing i,he signal strength in Fig. 4 to that observed from a film of mater of known thickness. The signal in Fig. 4, assuming a comparable strength of interaction in the transmission and reflection experiments, was calculated to correspond to a penetration depth of about 0.15 p, in agreement with theory. 2. Polyethylene.-T$e experimental results for the polyethylene absorption band a t 3.4 p are shown in Figs. 5 and 6. Figure 5 compares the spectra observed for reflection and transmission for a film thickness of 1.5 p. Figure 6 shows how the signal strengths behave as the film thickness is changed. For a film thickness less than 0.5 p, the signal from two transmissions is less than that due to eight reflections. The reflection signal is independent of film thickness except for films less than 0.16 p thick. This thickness is thus a measure of the depth of the penetration of the radiation in the polyethylene. This experimental point falls right on the theoretical curve given in Fig. 3 and calculated from the formula given in the text for the germanium-polyethylene interface. When two reflections are compared to two transmissions it is evident that the signal strengths are about the same for film thicknesses less than a penetration depth. 1.
Discussion This experiment has provided another demonstration and test of Maxwell’s theory on the penetration of radiation into the rarer medium on total internal reflection. It has been shown that this radiation is sensitive to molecular absorption to about the same degree that is observed in transmitted radiation for film thicknesses less than a penetration depth. The measured depth of penetration agiees with that calculated from theory. The resulta show that the technique outlined here can be used to investigate surface impurities on optically t ransparent materials. A particularly important application of the technique should be found in the study of semiconductor surfaces. Another uqeful application can be found in the study of thin films. The latter application is important is it is known that thin films often have properties different from the bulk. From the present results we would estimate that through tke use of more reflections and a more sensitive spectrometer one to ten molecular layers can be detected using this technique. We reach this conclusion from the observation that an absorption b:tnd in a film 0.03 M thick was readily detected (see Fig. G) through the use of only eight reflections and 3 rather insensitive spectrometer set-up.
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I n applying this technique one should be aware of the following possible difficulties. The index of refraction may change rapidly in the vicinity of molecular resonances if the damping effects are small. When this is the case, the condition of total reflection a t the interface may beno longer met and the radiation can then escape the light pipe. It would return to the light pipe if total reflection occurs a t the outside boundary of the film” on the light pipe and if the radiation is not completely absorbed by the film. That the radiation does not escape the light pipe in our experiment is immediately evident from the ob,‘ervation in Fig. 6 that the signal strength is independent of film thickness beyond a thickness of 0.16 p. Hence, any change of index of refraction is not great enough to violate the condition of total internal reflection at 45’ angle of incidence used in the present experiment. Such an effect could undoubtedly be observed by adjusting the angle of incidence. Furthermore, such measurements could be used to provide a sensitive determination of the index of refraction in the vicinity of molecular resonance. In analyzing the spectrum of the impurities on the surface it is necessary to take into account any absorptions in the bulk: e.g., oxygen in silicon,I2 and the spectrum of the free carriers, e.g., h01es.13 Although the absorption coefficient of the free carriers in a semiconductor is low cm.2 per free carrier), it is advisable to keep their density low especially if the sample is traversed many times when many reflections are used to increase the sensitivity. The density of free carriers can be kept a t a low value by using either wide band gap or near intrinsic semiconductors and by working at lorn temperatures if necessary. High surface barriers, when the number of free carriers may not be negligible, should also be avoided since the carriers in the space charge region also contribute to the absorption of the infrared radiation.14 The author is indebted to Mr. R. C. Hughes for the germanium crystals and to N r . 13. W. IZeese for technical assistance. DISCUSSION DIETRICHSCHULTZE (Mellon Institute).-How is i t possible to distinguish between actual extinction due to absorbed molecules and interference extinction of the light beam?
N. J. HARRICK.-Firatly, the dimension of the sample and the angle of incidence are such that it is unlikely that any interference will occur; secondly, it is generally a simple matter to recognize an interference pattern because of its symmetrical feature. J .I\. R ~ S H K I(E. H I. du Pont de Nemours Go ).-In the calculation of the penetration depth for a monolayer adsorbed on the surface, where you use the refractive index of the rarer medium, which refractive index do you use? The “refractive index” of the adsorbed molecules? The rerefractive iiidex? (11) It should be realized t h a t if the constants of the media 1 and 2 are such a8 t o give total reflection it makes no difference whether the eonstanta change gradually or abruptly. (12) W. Kaiser, P. H . Keek and C. F. Lange. Phgs. Rm.,101. 1264 (1950).
(13) W .Kaiser, R.C. Collins and H. Y.Fan, tbid., 91, 1380 (1953). (14) N. J. Harrick. J. Phys. Cham. Solida, 8, 106 (1959).
1114
ROBERT J. L E A K
Ah'D
K. J. HARRICK.-%Stly, the penetration dcpth is considerably larger than one monolayer and thus it does not make much difference whether the reflection is considered to take place on the inside or the outside of the monolayer. Secondly, the index of refraction is a macroscopic quantity and is thus really undefined for a monolayer or a single molecule. In dealing with such thin layers, RIaxmll's theory cannot be used and it is necessary to go t o an atomic model. The condition of total reflrction can be imsatisfied a t the surface of the light pipe yet this technique should work since the radiation is not necessarily lost from the system-see footnote (11i . DON.4LD GRAH.431 (E. I. du Pont de Semours & CO.).To what extent would this technique be applicable to silica
P. TJ'. SELWOOD
T'ol. 64
or alumina as represented by fused qiiartz or sapphire? N. J. HARRIcK.-This technique should work for any transparent material. It is convenient t o have a high index of refraction so that the angle of incidence can be made small and thus increase the number of reflections per imit sample length. DONALD GRAHAM-ISit not possihlr that grinding, lapping, and diamond polishing of hard surfaces could introdurr surface defects making the swfacr ronipirrahle t o that of sninll particlrsP Tu'. J. HARRWK.-yeS. To avoid this i,ffect thfl surface should be etched, cleaved, or annealed. 'l'lie s:tmple mag also be initially grown to the shape desired, e . g . , dendrite?.
THE CHENIISORPTIOX OF OXYGEX ON YICKEI,' BY
ROBERT J. LEAK-4ND P. Jv. SELWOOD
Chemical Laboratory of Northuestern University, Evanston, Illinois Received March 7 , 1960
The chemisorption of oxygen on nickel-silica catalyst systems has been investigated by the low frequency a.c. permeameter method and volumetric gas adsorption techniques. The nickel particles were in the superparamagnetic range or slightly larger. A method has been developed for distinguishing between true chemisorption of oxygen on nickel as contrasted with surface oxide formation. The method makes use of what appears to be a change of magnetic anisotropy energy in particles of nickel in the 100 A. diameter range. A possible explanation is discussedfor theso-called "hydride anomaly" in the chemisorption of hydrogen on nickel a t low temperatures.
Introduction There have been many studies of the interaction of oxygen with nickel surfaces, but there are two basic points which have not been elucidated. First, there is no clear distinction as to where surface chemisorption ends and where bulk oxidation begins. =Ind secondly, there has been some confusion as to the effect of chemisorbed oxygen on the magnetization of nickel. The first point has been further confused by loose terminology. "Chemisorption" is sometimes used when actually bulk oxidation probably occurred. Chemisorption of oxygen and oxidation are easily confused because of the ease with which the former can be converted into the latter. Odaz showed that penetration of oxygen into the nickel lattice occurred even at -183'. His work on evaporated films showed that the oxygen "sorption" proceeded until three molecules of oxygen were taken up per surface nickel atom. Further slow "sorption" also occurred. Scheuble3 reported similar studies a t room temperature on evaporated films. He found a "sorption" of oxygen equivalent to 9.5 times that needed for a monolayer if the surface werc smooth. Beeck and co-~vorkers~ found that oxygen was taken up a t 23" to the extent of two molecules per lattice site. Moreover the oxygen adsorption layer will diffuse into the interior even if no additional oxygen IS amilable from the gas phase. Although riot so stated, this amounts to oxidation, (1) Taken i n p a r t from the thesis of Robert J Leak submitted t o t h e Graduate School of Northnestern Unir ersity in partial fulfillmeiit of t h e requirements for t h e degree of Doctor of Philosophy. (2) Z . Oda, Bull Chem. Soc Japan, 27, 465 (1951). ( 3 ) IT' Scheuble, Z Physak, 136, 125 (1953). (4) 0 Beeck, A. E Smith a n d .4.Wheeler. Proc. Roy Soc (London) 8177, 62 (1940).
not simply chemisorption. Stone and co-workers5 showed the limit of oxygen uptake by nickel powders depends not only on temperature, but also on pressure and porosity. These two factors are important because they determine the efficiency of the dissipation of the heat of react'ion. Further evidence concerning this matter is given by Higuchi, Ree and Eyring6 who shom a wide discrepancy between the calculat,ed and observed heat of adsorption of oxygen on nickel. The complexity of the situation has been shown by Zettlemoyer and eo-workers' mho found four layers of oxygen, namely, oxide, chemisorbed 0-, strongly physically adsorbed 02, and weakly physically adsorbed 02. The reaction was not stopped bet'weerl the transition of the chemisorbed layer into t'he oxide, but at least the existence of two chemically bound layers was demonstrated. There has been some experiment,alevidence shoming a distinction between chemisorpt,ion of oxygen and oxidation. Farnsworth and Schliers showed by electron diffraction experiments on single nickel cryst'als that a monolayer of oxygen is chemisorbed a,t room t,emperature aft.er a pressure-t,ime exposure of 2 X 10" mm.-min. Above 10-j mm.-min. a nickel oxide layer is formed. Shurmorskaya and Burshteing have shown by contact' potent,ial measiirement,s that, at 35' oxygen ( 5 ) R. hI. Dell. D. F. Rlemperer and F. S.Stone. THISJOURNAL, 60, 1586 (1956). ( 6 ) I. Higuchi, T. Ree a n d H. Eyring, .I. I m . Chem. Soc., 1 9 , 1330 (1957). (7) A. C. Zettlemoyer, Y. F. Yii, J. J. Chessick a n d F. H. Healey, THISJOCPSAL, 61, 1319 (1957). (8) R. E. Schlier a n d I-I. E. Farnsworth, "Advances in Catalysis," Tol. IX, Edited by D. D . Eley, W.G. Frankenburg and V. I. Komarewsky, Academic Press, Inc., 1 - e York, ~ X. Y.. 1937. p. 431. (9) N. A. Shurmovskaya and R. K h . Burshtein. Zhur. P i z . K h i m . . 31, 1150 (1957).
