J. Phys. Chem. 1993, 97, 2649-2663
2649
External Reflection Infrared Spectroscopy at Metallic, Semiconductor, and Nonmetallic Substrates, 1. Monolayer Films Jerzy A. Mielczarski Luboratoire “Environnement et MinCralurgie” et UA 235, INPL- ENSG, B.P. 40, 54501 Vandoeuvre-les-Nancy, Cedex, France Received: June 8, 1992; In Final Form: November 13, 1992
The monolayers structure, orientation, and chemical interaction a t metallic and nonmetallic substrates have been studied by infrared reflection spectroscopy. The reflection spectra of a thin film deposited on different substrates are generally modified by the optical effects as compared to the corresponding transmission spectra of the same film without a substrate support. Distinguishing the changes observed in reflection spectra caused by optical effects is crucial for the interpretation of the spectroscopic data and must be done before relating any differences in band shape, position, and intensity to structural and/or chemical bonding changes in the thin film. Therefore, spectral simulation has been used extensively to determine the optical effects phenomena. It is shown that physical insight into the mechanism of the complex optical phenomena of the interaction of the incident and reflected radiation is gained from the optical consideration of the electric field, ( E 2 ) ,components within the characterized layer over a wide range of optical constants of individual phases in the multilayer system. It was found that the reflection spectra are significantly changed due to optical effects recorded in the conditions in which interaction of the normal to the interface ( E z 2 )component within a thin film dominates, while for the conditions in which the parallel field components, (E,?) and ( E ? ) , dominate, the reflection spectra are similar to the corresponding transmission spectra. In the latter case the negative absorbance bands (reverse spectra) are expected. Experimental and simulated spectra of self-assembled cuprous ethyl xanthate films on copper, cuprous sulfide, and water are discussed for various experimental conditions. Understanding the basis of optical effects allows a more detailed interpretation of the experimental spectroscopic data. More precisely, assignment of the absorbance bands, the concepts of reorganization of molecules in a thin film, or the formation of an adsorbed film having an oriented polymeric-like structure could only be postulated on the basis of a close interpretation of the reflection spectra after carefully analyzing the optical effects. Furthermore, theoretical examination allows one to predict the optimum experimental conditions under which maximum sensitivity can be obtained, which is particularly important for the recording spectra a t submonolayer coverages.
Introduction During the past two decades infrared external reflection technique spectroscopy has been used extensively to examine molecular structure of thin films deposited on metals.’.* Recent improvements in instrumentation and the use of spectral simulation have led to the application of infrared external reflection method in molecular-level studies of monolayers and even submonolayers on other substrates like semiconductor^,^-^ glassy carbon,z,lO coal,Il and water.12s1jThis promising application of the infrared reflection technique to characterize more complex systems having low reflectivity is becoming an increasinglyactive method in surface characterization research on numerous technologically and fundamentally significant areas as catalysis, microelectronic,biology, adhesion, separation, sensors, corrosion inhibition, etc. Scientists applying infrared external reflection spectroscopy to the study of molecular details of adsorption layers, chemically modified surfaces, or monolayers deposited by the LangmuirBlodgett method have to overcome two problems, Le., recording reasonably high quality spectra, which is very difficult especially with a submonolayer coverage, and the proper interpretation of the recorded spectra. The solution to these problems can be tremendously gained by the application of the spectral simulation methods. The aim of scientific effort is not only to determine the type of adsorbed products and their amount deposited on solid (thought very often it is not a trivial problem), but it would also be very helpful in adsorption studies to be able to monitor the evolution of the structure of the adsorbed thin film and its subsequent growth into multilayer deposits. Therefore, to OO22-3654/93f 2091-2649%04.00f 0
formulate fundamental concepts from availabledata, it is essential to utilize very specific experimental and theoretical strategies. The reflection spectra of deposited thin films are in general different from the correspondingtransmissionspectra of the same film without support. The differences are caused by orientation, surface perturbation, and optical effects. The questions are, (i) how much different are they from each other, and (ii) what is the physical basis for thechanges? It was found14J5that therecorded spectra of isotropic and very thin layers with absorption coefficient, k 2 < 0.1, on metallic substrates showed very small changes in the position and shape of isolated absorbance bands, sometimes negligible, while for strongly absorbed surface films, kz > 0.1, the absorbance bands significantly changed position and shape. For nonmetallic substrates, the position and shape of the absorbance bands in recorded reflection spectra of isotropic thin film are also very sensitive to polarization and incident angle of beam.*-’1 As was reported recently,Z-l I these changes are dramatic and they are due to optical effects. Therefore, distinguishing the changes observed in reflection spectra caused by optical effects from those due to changes in structural and chemical bonding is crucial for interpretation of spectroscopicdata and must be done before relating any band shape, position, and intensity differences to structure and/or chemical bonding changes of the surface film. This requires a careful theoreticalconsideration of the investigated system. An advantage as a result of this consideration is the gain of physical insight into mechanism of interaction of the incident and reflected beam for a multilayer system which allows one to optimize sensitivity. This knowledge is especially valuable when performing reflection experiments on low-absorption substrates with submonolayer coverages. 0 1993 American Chemical Society
Mielczarski
2650 The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 It is well-known that in the case of metallic substrates the maximum sensitivitycan be obtainedatgrazinganglesofincidence close to 8 S 0 , and only thevibrations which have transition dipole moment components perpendicular to the interface can be seen, since the electric field only normal to the plane of the interface, ( E Z 2 ) can , practically produce absorbance bands in this system. However, for nonmetallic substrates such as a semiconductor, the angles of 60-80° for p-polarization and close to normal for s-polarization are suggested, and information about vibration of molecular groups in all three directions can be 0btained.~,~-~.l2 A number of papers have discussed the differences between systems with metallic and nonmetallic substrates,10,'2,16,'7 and it has been pointed out that the electric field ( E 2 )at a metal surface is enhanced several times a t grazing angles of incidence, while for nonmetallic samples the enhancement effect is not observed, and even a lowering of the (E2) is found for all incident angles. On the basis of this consideration of the substrate-air interface, it has been concluded that it is easier to record spectra of the monolayer deposited on metals, at a satisfied signal/noise ratio, than on for example water.12J7 Since theelectric fieldcomponents describe the amount of energy which can be absorbed, in fact, the most important is to know the magnitude of the ( E 2 ) components at various points wirhin a characterized medium. All of the ( E 2 )components change magnitude with distance from interface, and most importantly, the normal component ( E Z Z )is not continuous across the boundary and jumps as the boundary is crossed.8-18-22Therefore, only the consideration of the (E2) components within each phase of a multilayer system can gain physical insight into mechanism of absorption of incident radiation in multilayer system. Our theoretical consideration presented in this paper is focused on understanding causes of the differences between the reflection spectra of sample of thin film deposited on metal, semiconductor, and nonmetallic substrates and the corresponding spectra of this sample recorded in transmission mode. Thus, we will discuss the basisof optical effects which can significantly influence the shape of recorded spectrum. Furthermore, we would like to answer the question whether the enhancement effect of the normal component ( E z 2 )at metallic substrates plays an important role in increasing thesensitivityof recorded reflection spectra of thin filmson metals compared with nonmetallic substrates, and how to predict the maximum sensitivity for a system is being investigated. Some examples of experimental results will be discussed in light of the theoretical consideration. Since the conclusions are drawn on the basis of the calculated and experimental results for wide spectrum of optical constants of multilayer systems they should be broadly applicable to a variety of organic (and inorganic) films on metallic and nonmetallic substrates.
Theoretical Consideration and Experimental Verification Reflection spectra of thin films deposited on metals and nonmetallic substrates are generally different from the transmission spectra of these films because of the so-called optical effect. Our meaning of the optical effect includes all of the effects which cause changes in the recorded spectrum except those resulting from the surface perturbation and the changes in structure and chemical bonding. The optical effect has wider meaning than a dispersiondistortion effect due to the frequencydependent refractive index of thin film commonly used for explanation of changes in spectra (e.g. ref 16). This effect cannot explain, for example, the negative spectra observed (e.g., refs 4, 6-8) for thin films on nonmetallic substrates for s-polarization. Absorbance or decrease in reflectivity of the system with and without thin film is a complex function of several components:l9.22 A a n2k2u(E2')d2/cos B (1) where n2 and k2 are refractive index and absorption coefficient
#
EllXJ
I
air thin film "3Ik3
substrate
7k Y
Figure 1. Schematic diagram of multilayer optical system discussed in the paper. Parallel (p) (E11lx.z)and perpendicular (s) ( E , . y ) electric field components are shown for incident radiation in phase 1. Similar spacial distribution of the electric field with indexes 2 and 3 are present within the thin film and substrate, respectively.
20-
Y
=-
i
10:
1400
I200
1000
cm-1
Figure 2. Optical constants, refractive index, n, and absorptioncoefficient, k, of cuprous ethyl xanthate complex, as a function of wavenumber (adapted from ref 27).
of thin film, u is frequency of incident beam, ( E 2 2 )is the meansquare of electric field in thin film, d2 is the thickness of the film, and B is angle of incidence (Figure 1). Changes in the reflection spectrum caused by optical effects can be predicted by simulation of the spectrum of a characterized medium. Differences between the reflection spectra simulated on the basis of the transmission data and the experimental reflection spectra of thin film were used for determination of the average spatial orientation of the surface structures.2-2'-26 The spectral simulations presented here were made by using the exact equations based on Hansen's formulasI8 for a multilayer system of isotropic and homogeneous phases with parallel interface boundaries. The calculations were performed with FORTRAN routines run on a personal computer. The cuprous ethyl xanthate complex (C2H50CS2Cu) was chosen as an example of an organic thin film since its spectrum in the region 1300-950 cm-I shows three groups of complex absorbance bands with different intensities which represent2' wide changes in the refractive index and absorption coefficient with frequency (Figure 2). The optical constants of copper and cuprous sulfide, n3 = 10.8, 6,27 and k3 = 47.3, 0.19, respectively6,28were assumed constant in the region 1300-950 cm-I. The optical constants for water were interpolated from the data of Downing and Williams.Z9 Formation of copper xanthate complexes on the surface of heavy metal sulfides has a wide industrial application for making sulfide minerals selective hydrophobic. Thin Film on Metal. The simulated spectra of a IO-A thin film of cuprous ethyl xanthate calculated for transmission and reflection (on copper substrate) modes are shown in Figure 3. Since the calculations were made for an isotropic film, the observed
IR Reflection Spectroscopy of Monolayer Films
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2651 1
-
. -
+
uansm.
0
K
* 0
K
N *
-M
A hl
w V
I 1250
1200
1150
1100
1050
loo0
Wavenumber. (cm-I)
Figure 3. Simulated spectra of isotropic 10-A-thick film of cuprous ethyl xanthate complex: (a) transmission spectrum; (b) reflection spectra of the film deposited on copper for p-polarization, a t 0 = 80' (solid line) and 0 = 20° (XSO, dashed line).
changes in shapes, positions, and relative intensities of the absorption bands are caused by the optical effects resulting from the mode of recording the spectra and metallic character of the substrate. This will be discussed later in detail. However, it is noteworthy here that the position and shape of the bands in the reflection spectrum are independent of the angle of incidence (Figure 3b, 0 = 80° solid and 0 = 20° dashed lines); only the intensities of the absorbance bandschange about 50 times between these two spt .a. It can be seen immediately from Figure 3 that the intensities of the absorbance bands of a lo-A thin film in transmission spectrum are very similar to those calculated for reflection spectrum of this film deposited on copper for 0 = 80°. The ideal maximum absorbance is observed for 0 = 88O, and it shows a value about 3 times higher than that calculated for 0 = 80° (Figure 5). The grazing angleof about 80° is a more realistic experimental situation encountered in laboratory practice because of an angular divergence of the incident beam and a finite size of sample studied. It was discussed in a number of papers and reviews (e.g. ref 30) that a gain in sensitivity in a reflection experiment over a corresponding transmission measurement is due to enhancement of the normal electric field component ( E z 2 )at the metal surface, and to increasing the optical path length by l/cos 0 (see eq 1). The latter relationship is especially sensitive at grazing angles. The calculation of the electric field at various points within the thin film and outsideof it (Figure 4) shows that ( E z 2 )component is not continuous across the boundary. In air, just at the airadsorption layer interface (see inset in Figure 4), the electric field strength ((Ez12) has a maximum at 0 = 80° which is 3.67 times higher than that of the incident electric field (it is assumed that incident radiation ( E ? ) and (E,?) are unity and the calculated values are corresponding dimensionless ratios to the incident radiation components). The same shape and magnitude of the (Ez12)were found when the calculation was made for a two-phasesystem, air-copper, without the thin film. These shapes and magnitudes are characteristic for all metals in the infrared2' and change somewhat for metals having an absorption coefficient lower than 30. However, the electric field within the thin film ( E z 2 2 )has a maximum strength of 1.05 at 0 = 80° (Figure 4); hence, it is similar to that of the incident field. Therefore, for this system it can beconcluded that the enhancement in sensitivity observed a t grazing angles in the reflection spectrum (Figure 5 ) over the corresponding transmission spectrum (Figure 3a) can be explained only by the increase in the number of molecules experiencing the field. The magnitude of the (EZ22)within a thin film, as is expected, is strongly dependent on the optical properties of the thin film
Angle of Incidence Figure 4. Electric field intensities ( E 2 )in three-phase system: air (nl = 1, k l = 0)-thin film of cuprous ethyl xanthate (n2 = 1.32, k2 = 0.36 at 1050 cm-I, dz = m)-copper (n, = 10.8, k3 = 47.3), as a function of incident angle. Inset shows the calculation points. x10-3
h
M
0
c
Angle of Incidence Figures. Calculated absorbancecomponents, Al and All, for IO-A-thick film of cuprous ethyl xanthate on copper for the data shown in Figure 4.
in contrast to the normal components in air ( E z 1 2 )and in metal ( E z ~ ~The ) . calculated results of the electric field components in air ( E z 1 2 )and within a thin film (EZ22)as a function of the refractive index and absorption coefficient of hypothetical thin films are shown in Figures 6 and 7, respectively. These results are calculated for the maximum field intensities which were observed at 0 = 80° for the combination of the optical constants. It can be seen immediately that the normal field component in air ( E z 1 2 ) just outside of the adsorption layer is practically independent of the optical constants of the adsorption layer, while
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The Journal of Physical Chemistry, Vol. 97, No. I I, 1993
h
. 2
v
-
M 0
1
1.5
2
2.5
Refractive Index, n2 Figure 6. Electric field ( E ? )components in three-phase system at 0 = 80° for various refractive index of thin film, nz, and the two values of absorption coefficient k ? , = 0 and k l z = 0.36. Other data as in Figure 4. The ( E z j 2 ) component within copper and the (E?) and ( E 1 2 ) components in all calculated points show values below IO-?. 5 4.5
4 E2 1
3.5 3 A N
w
2.5
V
2 1.5 1
0.5
0
0
0.5
1
1.5
Absorption Coefficient, k Figure 7. Electric field ( E ? ) components in three-phase system at 0 = 80° for various absorption coefficients of thin film, k : , and the twovalues of refractive index
= 1.3 and n?? = 1.5. Other data as in Figure 6 .
the ( E z ~ component ~) within thin film varies significantly. It should be noted here that the electric field across the lo-A thin film, with fixed optical constants, shows practically a constant value (data not shown) which agrees with previously reported finding.21.2?Figures 6 and 7 show that for thin films with a refractive index above 1.4 the electric field strength within the film ( E & ) is lower than that of the incident beam for the entire range of absorption coefficient from transparent to strongly absorbing. It is important to note that for the latter film (Figure 6 , curve for k2 = 0.36) the breakeven value of n is even lower than
"
1
1.5
2
2.5
Refractive Index, n2 Figure 8. Calculated absorbance for A L , versus refractive index for the data set as in Figure 4. Absorbance for s-polarization,,411,shows values below 10 6 . 1.4. Hence, it can beconcluded that the enhancement in sensitivity due to the gain of magnitude of the electric field at a metallic interface is not observed practically for the majority of organic thin films. Other very important observations can be made based on Figures 6 and 7. The electric field in the thin film shows a dramatic increase for the refractive index range n2 < 1.2 and especially below 1.0, while for n2 > 1.5 a slow decrease in the (EZ22)is observed (Figure 6 ) . The absorption coefficient does not show any significant changes in the electric field, ( E z ~ ~for) k2 , < 0.25; however, there is a significant decrease in the ( E z z 2 ) for k2 > 0.25 (Figure 7). These relationships (Figures 6 and 7) are responsible for changes in the reflection spectra caused by optical effects. Therefore, it is very useful to compare these relationships with the changes in the calculated values of absorbance, which are in fact the measured values in spectroscopic experiments. The calculated absorbancies for the set of data similar to those used in the calculation of the electric field components are shown in Figures 8 and 9. It is easily seen from a comparison of these results and those presented in Figures 6 and 7 that the electric field plays a prominent role in determining the magnitude of the absorbance observed in reflection spectra. Hence, similar relationships are observed in the case of the absorbance (Figure 8) and the electric field (Figure 6 ) versus nz. A strong increase in absorbance with decreasing refractive index, especially for nz < 1.2, is observed although the absorbance is proportional to n2 as shown by eq 1. A similar observation can be made from the comparison of Figures 9 and 7 . The absorbance increases linearly with the absorption coefficient in the range of kz < 0.25, while the electric field remains almost constant. However, for k z > 0.25 the absorbance shows nonlinear relationship, it passes through a maximum, and then decreases since the ( E ? )decreases significantly over this range. This nonlinearity followed by a lowering in the absorbance contrasts with the proportional relationship with k2 given by eq 1, even though the absorbance behaves proprotionally when k2 < 0.25. Therefore, on the basis of the above discussion, it can be concluded that knowledge of the electric field within a thin film is essential to understanding the optical phenomena taking place in reflection
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The Journal of Physical Chemistry. Vol. 97, No. 11, 1993 2653
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of the intensity of this band. However, the band at 1048 cm-1, with almost the same absorption strength (absorption coefficient) as the band at 1010 cm-I, is shifted 2 cm-1and shows an increase in intensity. These result from the changes in refractive index which are even less significant in the case of the band at 1048 cm-I than that one at 1010 cm-' but take place in the range of n2 = 1.2-1.3 (Figure 2), where the electric field versus n2 shows a higher sensitivity and a considerable increase in magnitude. The band at 1034 cm-I shifts 6 cm-' and shows a strong reduction in intensity. The smaller shift of this band compared with the band at 1196 cm-' is due to a smaller decrease of the n2, which at the minimum reaches the value of 1.20, resulting in a lower enhancement effect of the electric field than that observed for the band at 1196 cm-I. The strong decrease in the intensity of this band results from two overlapping effects, the increase of n2above 2.0 and kz to 1.36. As a result of this, a strong reduction of the electric field in the adsorption layer (Figures 6 and 7), and consequently a strong lowering in the absorbance are observed. Since the intensities of the bands at 1050 and 1040 cm-I in the reflection spectrum are very similar, this suggests, on the basis of close inspection of Figure 9, that the effect of a very large value of kz has major contribution in decreasing the intensity of the band at 1034 cm-1. In concluding this discussion of electric field effect on distortion of band shapes in reflection spectra, e.g., optical effects, it should Absorption Coefficient, k2 be noted that for low intensity bands, k2 < 0.1 (e.g. the band at Figure 9. Calculated absorbance for A l , versus absorption coefficient 1 156 cm-I, Figure 3) the positions and shapes of the bands in the for the data set as in Figure 4. Absorbance for s-polarization, All, shows reflection and transmission spectra are the same, as was also values below 10 5 . concluded recently by Allara and co-workers.I5 However, for k2 > 0.1 and k2 >> 0.1 the magnitude of the changes observed in experiment for a multilayer system, and allows one to distinguish reflection spectra, which are due to dispersion of refractive index between the optical effects phenomena and the others caused by and increase in the absorption coefficient, are very sensitive to structural and chemical bonding changes. The importance of the role of the electric field magnitude within a characterized the range in which the optical constants change. As shown above, medium in determining optical phenomena in a multilayer system, for even a strong and relatively broad absorbance band (band at compared with other parameters which can be calculated from 1010 cm-1) both the position and shape of the band do not change Maxwell's equation, was pointed out by Hansen18-21and at all, since the changes of refractive index (even so considerably) McIntyre22 and used for explaining optical phenomena taking take place in the range of values above 2.0, a t which the ( E 2 ) place in interna119-21.3 and external r e f l e ~ t i o n ~ ~systems. * J ~ . ~ ~ ~ ~changes ~ insignificantly. Therefore, the best way to understand On the basis of the above discussion and knowing the optical the physical meaning of the observed phenomena and to predict constants of a thin film of cuprous ethyl xanthate as a function optical effects is the careful consideration of the changes in the of wavenumber (Figure 2), we are able to discuss differences electric field within the characterized film. between simulated reflection and transmission spectra of the same The experimental reflection spectra of the thin films of cuprous film presented in Figure 3. This gives us a better understanding ethyl xanthate on copper produced by chemical adsorption under of optical effects phemomena which dramatically change the open circuit potential from M xanthate solution for various profile of a reflection spectrum. The assignment of the absorbance adsorption time34are shown in Figure 10 (note different reflection bands of cuprous ethyl xanthate complex which forms a thin film scale). Allof the recorded spectra are different from the simulated has been reported r e ~ e n t l y . 6 J ~The . ~ ~reflection spectrum (Figure spectrum of an isotropic 10-8, film of cuprous ethyl xanthate 3b) differs from the transmission spectrum (Figure 3a) by a shift complex on copper (Figure 3b). Since the observed differences of a majority of the absorbance bands to higher frequency and cannot be explained by optical effects, it can be assumed that changes in the relative intensity ratios of the bands. The shift they are caused by the chemisorption type bonding of xanthate of the band at 1196 cm-I to 1204 cm-I is due to the lowering of molecules with copper surface and/or the orientation of molecules n2 to 1.05 at 1205 cm-I (Figure 2). This involves a dramatic in the adsorption layer. For the lowest amount of xanthate enhancement of the electric field at this frequency, to near 3.6 adsorbed, below monolayer coverage, the bands at 1220, 1198, (Figure6),resultingina strong increasein theabsorbance (Figure 1122, and 1034 cm-1 are observed (Figure loa). The band at 8) at this wavenumber. In contrast, sharp increase of n2 to the 1220 cm-I, unpredicted by simulation (Figure 3b), is assigned to value of 2.17 at 1180 cm-I reduces the electric field to about 0.2 the asymmetric stretching vibration of the (SCS) which is shifted which involves a strong decrease in the absorbance of the band to higher frequency resulting from direct bonding of xanthate on its low-frequency side and also in the intensity of the second molecules to surface copper atoms.34 Upon increasing the component of the band at 11 88 cm-I resulting in low intensity adsorption time and the thickness of the adsorption layer, the shoulder at 1188 cm-I. The band at 1124 cm-I is only shifted 2 reflection spectra (Figure 1Ob-d) feature three dominant bands cm-1 although the k2 value reaches 0.64 and the refractive index at 1197, 1127, and 1050 cm-I with continuously increasing shows strong dispersion from 1.49 to 2.13. This observation intensities. These three bands are assigned to the v,(COC) corresponds well with the relatively small changes in the electric stretching mode,)4.35indicating a close to perpendicular orientation field with refractive index for n2 > 1.5 (Figure 6). The same of this molecular group to the copper interface. The absence of reason can be given for explanation of lack of the shift of the band thebandsat 1034and 1010cm-1assigned tothev,(SCS)stretching at 1010 cm-I, although the band shows strong absorption with m ~ d e ,but ~ .observed ~~ in the simulated spectrum of an isotropic k2 = 0.36, since the dispersion of n2 takes place above 2.0. Furthermore, these high valuesof n2result in a substantial lowering film (Figure 3b), indicates that these molecular groups areoriented I
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1
a
c
0
F
E
-m I
I200 I
PI
s c
1400
1000
c m-
Figure 10. Reflection spectra of the adsorbed thin layer of cuprous ethyl xanthateoncopper for ppolarizationandate = 80°,for variousadsorption times: (a) 10 s; (b) 90 s; (c) 9 min; (d) 30 min; (e) 120 min (adapted from ref 34).
a)
1000 cm’
Figure 12. Reflection spectra of adsorbed molecules before and after reorientation: (a) after 5 min xanthate adsorption from stirred solution M, pH 6.5; (b) sample a after 20 min in of concentration of 1.7 X water.
b)
Figure 11. Proposed molecular orientation of ethyl xanthate complex on copper and cuprous sulfide. For details see text.
parallel to the interface. Therefore, it is concluded that for close to monolayer coverage (Figure 1Ob,c) the molecular arrangement in the adsorption layer is proposed to be similar to that presented in Figure 1la. Close inspection of the experimental and simulated spectra allows the correction of our previous assignment of the bands at about 1 125 cm-1 6 ~ 3 which 4 are not caused by different absorbance bands. This is a single band, which is due to the complex vibration ofthe (COC) and (SCS)groups with a net dipole moment inclined to the chain axis. This bond shifts in the p-polarized spectrum to higher frequency because of optical effects. Our better understanding of the optical effects’ influence on reflection spectrum allows us to propose an assignment for the complex bandat about 1200cm-I. It iseasilyseenin thereflectionspectrum of a well-oriented adsorption layer (Figure 10) that the sharp and symmetric band at 1197 cm-’ is observed at the same position found in the transmission spectrum (Figure 3a). As discussed above, the optical effects cause a strong shift of this band to higher frequency. These observations indicate that the band observed in the experimental spectrum at 1197 cm-’ is most probably the shoulder a t 1188 cm-I shifted to higher frequency, and it is due to the asymmetric stretching vibration of the (COC) group. The asymmetric stretching vibration of the (SCS) group is expected to produce a band at 1204 cm-l (Figure 3b) which does not appear in the reflection spectra of a well-oriented layer since the EX^) parallel component to the interface shows values
below 10-2, which is not sufficient for producing notable intensity bands. This phenomenon is known as the surface selection rule. An absorbance component at a frequency above 1200 cm-’ was observed at submonolayer coverage (Figure loa) when the adsorbed molecules are inclined to the surface of copper and do not show a preferential orientation. Therefore, the observed shift of the band to the position of 1220 cm-I is assigned to optical effects and surface perturbation of the v,(SCS) group. Hence, it can be concluded that the two components a t 1196 and 1188 cm-I found in the transmission spectrum of cuprous ethyl xanthate complex (Figure 3a) can be assigned to the v,(SCS) and v,(COC) stretching vibrations, respectively. It is worth noting here that even for multilayer coverage the adsorbed film shows a well-oriented structure (Figure 10e). Interestingly, the adsorption layer on copper obtained from xanthate solution of high concentration is not so well oriented like that formed from xanthate solution at a concentration of 10-4 M and below, even at close to statistical monolayer coverage. It was found that the molecules in the adsorption layer can reorganize. For example, a not well-oriented film obtained during the adsorption from solution of higher concentration shows almost perfect orientation after holding in clean water for period of 20 min, as shown in Figure 12. This observation is very essential from both a practical and application point of view. This finding indicates that thestructureof theadsorption layer can be modified even after its formation on a solid surface, and furthermore, it shows one possible way to improve the organization in the adsorption layer. Reorganization of a monolayer at a surface has been recently proposed by Evans and c o - ~ o r k e r for s ~ ~the explanation of the storage effect on contact angle changes. The results presented here support this suggestion. The growth of the adsorption layer of cuprous ethyl xanthate can be controlled electrochemically. This method was used for the preparation of thin films of adsorbed ethyl xanthate which vary in coverage and ~ t r u c t u r e . Infrared ~ reflection spectra of a copper electrode contacted with 1.6 X IO4 M ethyl xanthate solution a t different potentials are shown in Figure 13. The electrode was polarized a t -800 mV (SHE)for 5 min prior to 15-min polarization at several different potential values. More detailed description of the experiments can be found el~ewhere.~ At potentials of -350 mV and below, only a very broad band is observed a t about 1150 cm-l, which has been recently assigned to the adsorption of xanthate decomposition product^.^ The first xanthate molecules on the electrode surface are observed at potential about -330 mV and clearly seen at -310 mV. The
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The Journal of Physical Chemistry, Vol. 97, No. I I, 1993 2655
N
r3
200
-200
-260
Angle of Incidence
I 1200 1000 em-1 Figure 13. Infrared reflection spectra of copper electrode in 0.05 M sodium borate with 1.6 X M ethyl xanthate after 5-min reduction at -800 mV, followed by 15-min polarization a t several characteristic potentials, under stirred conditions (ref 9). spectra of the first adsorbed xanthate molecules in this region (Figure 13b,c) show very broad bands. The band near 1200 cm-1 is shifted by 20 cm-' to a higher frequency while the bands a t 1036 and 1010 cm-' show new shapes and positions (Figure 13c). All three of these bands are due to the v, of the SCS group. Moreover, the band at 1050 cm-I does not appear at all. These results indicate that the xanthate molecules are bound directly to the copper surface atoms (i.e., surface perturbation of the bands due to the SCS group is observed), and they are inclined to the plane of surface. The intensity of the absorption bands in the spectra for the first adsorbed xanthate molecules (Figure 13c) is lower than the intensity predicted on the basis of a theoretical calculation for an isotropic monolayer (Figure 3b). The amount of adsorbed cuprous ethyl xanthate increases by approximately an order of magnitude with increasing potential, as evidenced by the increase in the intensity of the bands (Figure 13c-f). Monolayer coverage is most likely achieved in the potential region between -260 and -200 mV. At the very high potential of 350 mV (Figure 12g), where copper oxidation is severe, the amount of xanthate adsorbed was found to be close to an isotropic monolayer coverage and the molecules exhibited near-random orientation with some preference to a parallel position since the intensity ratio of the 1039-cm-1 band to the 1050-cm-l band is higher than that predicted for an isotropic film (Figure 3b). The degree of orientation of the molecules in the adsorption layer was found to be strongly dependent on the xanthate concentration in solution and stirring, even at similar statistical coverage. Therefore, the molecular orientation of adsorbed xanthate a t the interface as shown in Figure 1l a is proposed for films produced a t open circuit potential in low concentration, unstirred solution (Figure lo), whereas the structure of the film obtained under potential control and stirred conditions shows a small anisotropy (Figure 13). It can be concluded from these observations that there exists a strong tendency toward an increase
Figure 14. Electric field intensities ( E 2 ) in three-phase system: airIO-.&thick film of cuprous ethyl xanthatexuprous sulfide (n3 = 6.27, k , = 0.19). Other data as in Figure 4. Inset shows the calculation points. Ex and E v exhibit the calculated values for all three points.
in anisotropy (orientation) of the monolayer film with decreasing adsorption rate. The film of poly(perfluoropropy1ene oxide) on gold is another excellent example which can confirm the advantage of the use of the electric field consideration to explain optical effects. The simulated and experimental reflection spectra5J7 of the isotropic thin films show similar line shapes with two dominated bands near 1270and 1320cm-',e.g.,at thefrequencies where theelectric field shows two maxima resulting from changes of the refractive index of the thin film which reaches two minima n21 = 0.8551 and n22 = 0.8387, respectively. Certainly, these spectra are very different compared with the transmission one. Thin Films on Nonmetallic Substrates. A similar case can be made for the optical consideration for nonmetallic substrates, as in the case of copper. It was shown in a number of papers6.8~I0.i2~17~20~22 that if the substrate is not a perfect conductor, but weakly metallic (semiconductor) or nonmetallic, then all three electric field components (Ex, Ev,E Z ) show significant values at the interface, and these components interact with the deposited molecules resulting in complex spectra. In this section the thin films on semiconductor and water substrates are discussed. Semiconductor (CuprousSulfide). Figure 14 shows thechanges in the electric field components in three phases for an isotropic thin film on a semiconductor surface. For this weakly metallic substrate, cuprous sulfide, which is characterized by a high refractive index and relatively low absorption coefficient, the normal components ( E z 2 )in three phases show a discontinuity across the boundary similar to that observed for copper (Figure 4). Nevertheless, thiscalculation shows two important differences compared with a copper substrate: (i) a lower magnitude of the ( E z z ) components and (ii) a shift of the maximum values for the ( E z ,2 , and ( EzZ2) components to lower angle of incidence, 0 = 63O. The electric field components (Ev2) and ( E X * )which , are parallel to the interface, are continuous across the boundaries and within the thin film. Hence, they show a behavior similar to that observed in the transmission mode. These parallel components are nearly constant for the calculated points in three phases showing an independence on the optical properties of the thin film. Moreover, they reach the same values at angles close
2656 The Journal of Physical Chemistry, Vol. 97, No. 11, I993
x10-4 8
6
4 h
$M
-
2
0
0
-2
0
50
90
Angle of Incidence Figure 15. Calculated absorbance components for IO-A-thick film of
cuprous ethyl xanthate on cuprous sulfide for the data shown in Figures 4 and 14, obtained with the use of exact equations, A l and Ail (solid lines), and approximate equations, A l y, A+.z, Aljx, A,IZ(dashed lines).
to normal. These observations suggest that for very low angles of incidence the reflection spectra recorded for s- and p-polarizations have to be almost identical, and most importantly they should show the same shapes and positions of the absorbance bands as the coresponding transmission spectrum. The calculations of the absorbance components for s-polarization, A I,and p-polarization, All, for various angles of incidence (Figure 15, solid lines) confirm the above conclusion according to the identityof thespectra for both polarizationat lower incident angles. Moreover, the calculations disclose additional features which cannot be evident from the consideration of the changes of the electric field components alone. It is shown (Figure 15, curves A L ) that for s-polarization only the negative absorbance band are expected, and their intensities reach a maximum when the incident radiation is vertical to the interface. It is also worth mentioning that, as recently reported,* the magnitude of the absorbance for s-polarization, A I is almost insensitive to significant changes in n2, however, it shows a nearly proportional increase with increasing k2. For p-polarization and higher angles of incidence, the normal component, ( E z z 2 ) ,has a magnitude several times higher than the parallel component, ( E ? ) (Figure 14). Therefore, it is expected that the reflection spectra at these angles will be dominated by the interaction of the ( E z z 2 )component with the thin film. This will result in significant changes in the spectra caused by the optical effects. The relationships between the normal component of the electric field ( E z 2 ) and the optical properties, n2 and k2, of the thin film deposited on a semiconductor have been founds to be similar to those observed for a metal (Figures 6 and 7), and therefore they are not shown here. Also, the absorbance values versus n2 and k2 show similar relationships8 to those which have been found for a copper substrate (Figures 8 and 9). As a final result, the p-polarized reflection spectrum of an isotropic thin film of cuprous ethyl xanthate on cuprous sulfide recorded a t angles about 70' should be similar to that
Mielczarski calculated for a metallic substrate (Figure 3b). This will be discussed later in detail. For p-polarization, the absorbance (Figure 15) reaches a maximum (positiveabsorbance) a t an angleof incidencesomewhat below the Brewster angle, OB = 80'. For lower angles, the absorbance becomes negative and reaches the values characteristic for s-polarization. These observations, together with previously published studies,8 suggest that for a semiconductor, and for angles of incidence lower than the OB,the interaction of the ( E 2 )electric field components with the adsorbed molecules produces negative absorption bands, while theinteraction with the ( Ez2)component results in positive bands in the reflection spectra. This conclusion is supported by the theoretical calculation, for p-polarization, of the two absorbance components, Allx and Allz, separately, using the simplified equatiom6 These calculations show (Figure 15, dashed lines) that the parallel component, Allx, has negativevalues while the vertical component, Allz is positive at all angles below the OB. It is noteworthy that these approximated data differ from the results obtained with the use of the exact equations ( A l , All) only slightly at angles close to the OB and indicate that very good approximations were obtained. For angles higher than the OB the reverse behavior is observed (Figure 15). On the basis of above discussion, it can also be found that for anisotropic films positive, as well as, negative absorption bands are expected in the ieflection spectra for p-polarization and at angles of incidence just below the OB, The molecular groups of the adsorbed species having transition dipole moments normal to the interface will appear as positive absorbance bands (Allz) in the reflection spectra, while thoseoriented parallel to the interface will appear as negative absorbance bands (Ailx). These phenomena have been observed in recent experiment~.~-~,l' On the basis of the above theoretical examination, it is also possible to predict the optimum experimental conditions under which maximum sensitivity can be obtained for a semiconductor substrate. For s-polarization the maximum sensitivity can be obtained at the normal angle of incidence, and very similar sensitivity is observed over the range of incident angles up to 25O, in accordance with the changes in the ( E n 2 )component. For higher angles of incidence, although the amount of molecules which interact with the incident beam increases, there is a significant decrease in the ( E n 2 ) component which involves a similar decrease in the absorbance. For p-polarization two maxima of sensitivity are observed at angles close to the OB. The positive maximum of absorbance is found just below the d B at O = 79.5'. This results from the increase of both the ( E Z 2 2 ) component and the amount of molecules which interact with the incident radiation for an increasing angle of incidence. At angles above the OB, very strong negative absorbance is expected with the optimum at I9 = 81O. Furthermore, it is known that for lowabsorbing materials the reflectivity of the p-polarized radiation shows a minimum at the OB which involves a specific sensitivity in this region. In other words, small changes in the reflectivity of the system with thin film results in strong changes in the R / R o ratio, because of significant lowering of the R, value near the Brewster angle. The dramatic decrease in sensitivity at grazing angles is due to lowering of the magnitude of the ( Ez22)component although theamount of molecules probed by theincident radiation significantly increases. In concluding this discussion the maximum theoretical sensitivity can be obtained at O = 0 ' for s-polarization and I9 = 81' for p-polarization. For both cases negative absorbance bands are expected. The practical experimental setup for achieving the maximum sensitivity are for p-polarization I9 = 70' and for s-polarization O = 25' because of the experimental limitation in optic geometry and angular dispersion of the incident beam. Furthermore, under these experimental conditions the recorded reflection spectra are expected to be less complex. The complexity may be caused by mixing of different optical phenomena taking place at angles
IR Reflection Spectroscopy of Monolayer Films
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2657
s-pol..25
s - polarization
- polarization
p
a
O
1200
1250
1150
1050
1100
loo0
Wavenumber. (cm-I)
Figure 16. Simulated spectra of isotropic IO-&thick film of cuprous ethyl xanthate complex: (a) reflection spectrum of the film deposited on cuprous sulfide for s-polarization at 0 = 25'; (b) transmission spectrum; (c) reflection spectrum of the film for p-polarization at 0 = 70'. In parentheses the position of the negative band.
-
-
p polarization
s polarization
I
5x104
]23x10.
!
Ooo5
0
F
e
-B
R
1200
I
!
I
I
I'
I
L
I
l
l
I
I
"2
30"
I
1200 1000 " 1200 i ooocm Figure 17. Simulated reflection spectra of isotropic lO-A-thick film of cuprous ethyl xanthate complex on cuprous sulfide at various angles of incidence with p- and s-polarizations.
very close to the OB. The recorded reflection spectra of a thin film are predicted to show intensities of positive absorption bands for p-polarization an order of magnitude higher than those of negative bands for s-polarization. Simulated reflection spectra of an isotropic 10-Afilm of cuprous ethyl xanthate on cuprous sulfide, and for comparative purposes, transmission spectrum of this film are presented in Figure 16. The simulation was made on the basis of optical constants recently reported2' for the experimental conditions, which, as discussed above, can ensure high sensitivity. It is readily seen that the simulated reflection spectrum for s-polarization (Figure 16a) is an inversion of the transmission spectrum (Figure 16b). The shapes, positions, and relative intensities of the bands are the
1000
V
1200
1000 c
Figure 18. Experimental reflection spectra of approximately monolayer of cuprous ethyl xanthate complex on cuprous sulfide at various angles of incidence with p- and s-polarizations (ref 6).
same. This reflection spectrum does not exhibit any changes which can be explained by dispersion of n2 in the region of the absorbance band; the inversion and the lower intensity of the bands are the only optical effects observed. However, the reflection spectrum recorded for p-polarization at 0 = 70° (Figure 16c) shows typical features of optical effects resulting from strong sensitivity of the normal component of electric field, ( Ez22), to the changes of the optical constants of the thin film in the region of absorbance. (See discussion above for copper substrate.) Therefore, this spectrum is very similar to the simulated reflection spectrum of the thin film on copper (Figure 3b). Nevertheless, close inspection of these spectra (Figures 16c and 3b) discloses small but distinct differences which arevery important in understanding the optical effects phenomena. In the simulated spectrum of the thin film on a semiconductor the shoulder at 1188 cm-I and the band at 1010 cm-I almost disappear. Moreover, a very low intensity negative band at 1032 cm-I is observed. These features support the previous conclusion that this reflection spectrum is the result of the addition of the low intensity negative spectrum produced by the interaction of the thin film with the EX^^) component and the more intensive positivespectrumcreated by the ( Ezz2)component. Simplifying, this reflection spectrum results from an addition of the spectra presented in Figures 16a and 3b with suitable relative intensity. The relative intensities of these two spectral components vary with angle of incidence similarly to the changes in electric field (Figure 14) and absorbance (Figure 15) components. Figure 17 shows a graduated evolution of the simulated p-polarized reflection spectra from strongly positive to weakly negative with decreasing angle of incidence. For s-polarization, however, as predicted (Figure 15), only negative absorbance bands are observed with
Mielczarski
2658 The Journal of Physical Chemistry, Vol. 97, No. I I, 1993 decreasing intensities for higher angles of incidence. The positions and shape of these negative absorbance bands are found to be the same for various angles. Comparison of the simulated reflection spectra (Figure 17) with the experimental results which have been recently published? and are presented for comparative purposes in Figure 18, shows in general a good agreement between these results. The major features of the experimental reflection spectra of the same sample recorded a t various angles have been predicted by theoretical calculation and explained by changes in the electric field components within the thin film. Nevertheless, certain important differences are observed between experimental and simulated results which must be explained by more than just the optical effects. The most notable differences are observed between the recorded and simulated p-polarized spectra at 0 = 70°. They are (i) a negative band at 1035 cm-1 instead of a positive band at 1040cm-1, (ii) shift ofthe band from 1204 to 1197 cm-I, and (iii) lowering of its intensity in relation to the other bands. These indicate6 that the transition moment of the asymmetric stretching vibration of the COC group (bands at 1197, 1051 cm-I, Figure 18) has a position close to vertical at the interface while the transition moment of the asymmetric stretching vibration of the SCSgroup (bands at 1035,lOlOcm-I) is parallel to the interface plane. Hence, the molecules in the thin film most probably show a molecular arrangement similar to that shown in Figure 1la. Other features, e.g. the disappearance of the band at 1048 cm-I, assigned to the v, of the COC group, in the spectra recorded for s-polarization and the presence of the band in the simulated spectra confirms this conclusion. If the assumption that the experimental spectra (Figure 18) corresponds to monolayer coverage is true, then the spectra display absorbance bands which are several times higher than those predicted by simulation. One of the reasons for this is that the simulation was made for an isotropic thin film while the experimental spectra show anisotropic character of the adsorbed thin layer. This results in a 3-fold increase in the intensities of the observed bands.24 Obviously, the experimental spectra might represent data for a surface film that is somewhat thicker than monolayer. A more detailed discussion of this problem can be found elsewhere.6 The spectra presented in Figure 19 yield information on the kinetic of thin film formation and the changes in the structure of the adsorption layer with increasing coverage. On the basis oftheabovediscussion,it iseasy tofind that thespectrumrecorded for p-polarization at close to monolayer coverage (Figure 19c) shows anisotropic character since the negative bands a t 1035 and 1010 cm-’ due to the va of the (SCS) group and the positive bands at 1197 and 1051 cm-I assigned to the V, of the (COC) group are observed. At submonolayer coverage in the p-polarized spectra (Figure 19a), the negative bands are not observed which indicates random orientation of the adsorbed molecules. The shoulder near 1220 cm-I, as has been explained for the case of copper, is due to a surface-induced effect. This effect is also observed for s-polarization spectra at submonolayer coverage (Figure 19a) where some broadening of the band on the high frequency side is observed. However, at monolayer and higher coverage the s-polarized spectra do not show any changes in positions and shapes of the bands compared with the simulated spectra, with the exception that the band at 1048 cm-I almost disappears. Some changes in the shapes and band positions resulting from the surface-induced effect are expected in s-polarized spectra for close to monolayer coverage, especially for the bands at 1197, 1036, and 1010 cm-l which are assigned to the va of the (SCS) group which is bound directly to surface copper atoms of cuprous sulfide. The lack of these differences could be interpreted as a result of different structure formation of the adsorbed layer a t coverages below monolayer. It is likely that with increasing surface coverage an oriented polymeric chain structure in which
s - polarization
p - polarization
I
c
e
d
-a \
0
0 I
I,, 5x10.’
1200
, * ” 1000
[
10-3
1200
iooocm+
Figure 19. Reflection spectra of thin films of the cuprous ethyl xanthate on cuprous sulfide recorded for each sample with p-polarization at 0 = 70° and s-polarization at 0 = 25’ after the following adsorption times: (a) 2 min, (b) 5 min, (c) 25 min, (d) 150 min.
one copper atom is bound to two xanthate molecules is formed (Figure 1 lb). The differences in the configuration of the SCS group between the two molecular arrangements proposed in Figure 11 will modify the transition moment of the v, of the SCS group resulting in changes in the recorded spectra. Hence, a shift of the band positions to lower frequency could explain the experimental results for s-polarization. Another possible explanation for the lack of the expected changes in the (SCS) bands is that the structure of precipitated cuprous ethyl xanthate complex compared with that adsorbed on the surface of cuprous sulfide at monolayer coverage is thesame. This suggests a sharpdecrease in the influence of the surface induced effect on the recorded spectra with increasing coverage ranging up to a monolayer. The growth of the adsorption layer of cuprous ethyl xanthate on cuprous sulfide can be controlled electrochemically. This method can be used for the preparation of thin films which vary in coverage and s t r ~ c t u r e .The ~ infrared p-polarized reflection spectra of the cuprous sulfide electrode contacted with 1.6 X 10-4 M ethyl xanthate solution at different potentials are shown in Figure 20. The electrode was polarized at -350 mV (SHE) for 5 min prior to 15-min polarization at several different potential values. At potentials of -75 mV and below, only very broad bands are observed which are characteristic for the adsorption of xanthate decomposition products, as recently reportedS9More details about the electrochemical studies can be found el~ewhere.~ The first xanthate molecules on the electrode surface are observed a t potential about -75 mV and clearly seen at -60 mV. The spectra are dominated by negative absorbance bands at 1030, 1115, and 1195 cm-1 although for an isotropic film only positive absorbances are predicted (Figure 17). This indicates that the SCS groups show a position close to parallel to the interface. The relatively low intensity of the positive bands at 1045 and 1226 cm-1 indicates that the (COC) groups are inclined to the cuprous sulfide surface resulting in the slightly higher absorbance component Anx compared with the Allz component. This observation is in contrast to the predicted relative intensity ratiobetween these components for isotropic film (Figure 15) which suggests
IR Reflection Spectroscopy of Monolayer Films
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2659
3
I
ul
E! N (D
N
-20mV
2.5
2 -60mV
h(
-
A
1.5
cy
E
1 -75mV
1
0.5
1400
1200
1000
0
cm-1
1400
Figure 20. Infrared reflection spectra, recorded for p-polarization a t -9 = 70°,of cuprous sulfide electrode in 0.05 M sodium borate with 1.5 X M ethyl xanthate after 5 min prereduction at -350 mV, followed by 15 min polarization a t several characteristic potentials under stirred conditions: (a) -75 mV (SHE); (b) -60 mV; (c) -20 mV (for details see ref 9).
that the majority of xanthate molecules in the adsorption layer are oriented almost parallel to the interface. The observed shifts of the positive bands to higher and the negative bands to lower frequencies are caused by the overlapping and cancellation of the intensities of the negative and positive broad bands. Nevertheless, the positive band at 1226 cm-I shows a somewhat larger than expected the blue shift and the intensity. These could be interpreted as the formation of two different domains on the electrode surface: one at submonolayer coverage with random orientation and the other oriented close to parallel to surface. It is possible that a support for this explanation can be obtained using simulation procedure proposed recently by Parikh and Allara.2 Although the amount of the adsorbed molecules is difficult to estimate precisely, since the different domains present at the surface, it can be concluded at a first approximation that the intensities of the negative bands (Figure 20b) are similar to those which can be predicted by the simulation (e.g., Figure 15, theAllxcomponent) for about 5-thickf film. These experimental observations show that different structures of a film formed by the same molecules can be obtained under different preparation conditions and that valuable information can be obtained by infrared reflection spectroscopy about the film structure and the amount of the adsorbed molecules, although extraction of the details is sometimes a difficult task. Silicon dioxide on silicon system is another excellent example which confirms, as proposed here, the usefulness of applying the electric field consideration to explain the changes in the reflection spectra. The reflection spectra of a 30-nm-thick film of silicon dioxide on silicon have been reported r e ~ e n t l y . ~Since, as concluded above, the electric field for s-polarization is almost insensitive to dispersion of the refractive index of a thin film, the recorded4 s-polarized reflection spectra show that the band near 1078 cm-1 has a shape and position similar to the absorption coefficient, and they are also similar to those observed in the transmission mode, although the refractive index showsvery strong dispersion (Figure 21a). For p-polarization two bands are observed a t about 1260 and 1 100 cm-1 and the first onedominates at higher angles of i n ~ i d e n t . For ~ comparative purposes, the simulated spectrum of a 30-nm-thick silicon oxide film on silicon at 0 = 60° is presented here (Figure 21b) which is very similar
1200
1000
800
Wavenumber, (cm-1)
h
2 .
1400
1200
1000
800
Wavenumber, (cm- 1) Figure 21. Thin film of a silicon dioxide on silicon: (a) optical constants of silicon dioxide, n2 (dashed line), kz (solid line) interpolated from the data of Philipp;38 (b) simulated spectra of the 30-nm-thick film a t 0 = 60° for p- and s-polarizations.
to the experimentally recorded4 one. The optical constants of silicon dioxide for these two frequencies 1260 and 1100 cm-I are n21 = 0.4677, k z l = 0.216 and n22= 1.043, k22= 2.55, respectively. Hence, at both of these frequencies silicon dioxide shows low values of n2which involvea sharp increase in the (Ezz2)component (Figure 6) and a corresponding increase in the absorbance (Figure 8). However, a very high value of k22 causes an inverse effect ~ (Figures 7 and 9). The calculated values of the ( E z )~component for 0 = 60° are 15.32 at 1260 cm-1 and 0.0217 a t 1100 cm-I; however, the ( E m 2 ) component at these frequencies has values 0.1606 and 0.127 1, respectively. These calculated data help to understand the observed changes in the experimental spectra. A very strong positive band observed at 1260 cm-I (Figure 21b) is caused by the dramatic increase in the normal electric field
2660
Mielczarski
The Journal of Physical Chemistry, Vol. 97, No. 11, I993
component within silicon dioxide, while a much lower intensity negative band at about 1100 cm-’ results from the domination of the parallel electric field, ( E , y 2 2 )at , this frequency. It is noteworthy here that a very low value of the refractive index of the thin film is not a sufficient condition to produce a strong enhancement effect of the electric field. Silicon dioxide also shows a very low value of refractive index, about 0.3565 at 11 30 cm- I, but because of a very high value of the absorption coefficient at this frequency, equal to 1.53, the calculated electric field components ( E z ~ and ~ ) ( E x z 2 )are 0.200 and 0.132, respectively. As a result, only a very low intensity band/shoulder is expected (Figure 21b) and it is fully confirmed in the experimental data. At angles of incident nearly normal, the parallel to interface electric field component, ( E X 2 2 )dominates , and, as is expected, the p-polarized reflection spectrum is very similar to that recorded for s-polarization, hence also to the transmission spectrum. The optical effects observed in the reflection spectra of thin films of poly(perfluoropropy1ene oxide) on silica5 can also be interpreted in this way. These observations confirm again that the enhancement effect in the absorbance can be observed only for certain combination of n and k values (complex refractive index, n), and the best description of the situation in multilayer systems can be obtained by consideration of the electric field components within the characterized thin film. Water. The first experimental infrared reflection spectra of monolayers on water obtained by Dluhy et a1.I2are probably the most spectacular application of the infrared external reflection method. As it was argued,l2.l7 water shows a much lower reflectivity compared with metallic substrates and there is a significant decrease in the electric field magnitude. The latter parameter essentially describes the energy available for interaction with examined sample and obviously determines the quality of the recorded spectra. Unfortunately, the considerations of the electric field intensities have been on the basis of the data calculated only in air, just at the boundary with metal, water or a thin film for two or three phase systems. In fact, as discussed above for other substrates, the electric field component calculated within a thin film solely describes the interaction between the incident beam and the characterized medium. Figure 22 presents the changes of the electric field components for a three phase system with a 10-A-thick film of cuprous ethyl xanthate on water at X = 9.54 pm. The electric field components in air, just at the air-thin film interface, reach the maximum values somewhat below 0.8 for the parallel components while the value for the vertical component is about 0.65. The same shape and magnitude of the ( E 2 )components versus angles of incidence have been found at the water interface in the absence of a thin film. These results are also very similar to those reported for the water interface at X = 3.33 fim.I2.l7 Figure 22 shows dramatic changes in the normal ( EZ2) component after crossing the airthin film boundary (point 2). The maximumvaluesof theelectric field within the thin film, ( E z z 2 )reaches 0.18 at 0 = 59.5’. For a typical IO-A-thick organic film (nz = 1.5, kl = 0.1) on water a t X = 3.33 pm this value is equal to 0.14. In both cases it is only a small part of the incident energy, and even less than the value of the (EZ32)component in water just after crossing the thin film-water boundary (Figure 22, point 3). However, the field components, ( E ? ) and ( E g ) ,which are parallel to the interface, are continuous across the boundaries as in the case of the transmission mode. It can be seen immediately from Figure 22 that they remain practically constant while crossing the boundaries from air to water and within the 10-A-thick film. These observations indicate that only the normal field component in the thin film, ( E z z ? ) ,is sensitive to the changes in optical properties of the thin film. This is confirmed by the results from a calculation of the electric field components for a wide range of values of refractive index, n2 and absorption coefficient, k 2 of the thin film
1
-
1
0.9
0.8 0.7
0.6 A N
w
0.5
V
0.4
0.3 0.2 0.1
0 0
50
90
Angle of Incidence Figure 22. Electric field intensities ( E 2 ) in three-phase system: air-
IO-A-thick film of cuprousethyl xanthate-water (n3 = 1.241, k3 = 0.0441 at 1050 cm-I). Other data as in Figure 4. Inset shows the calculation points. E.Yand E Y exhibit the calculated values for all three points.
0.8 0.7 0.6 A
0.5
N
w 0.4
0.3
\
\
0.2 0.1
0
1
1.5
2
2.5
Refractive Lndex, n Figure 23. Electric field ( E 2 )components in three-phase system at 0 = 5 9 O for various refractive index of thin film, nl,and absorption coefficient kzl = 0.36. Other data as in Figure 22.
(Figures 23 and 24). The relationship between the ( E z ~and ~) the optical properties of thin film are very similar to those found for a metal substrate (Figures 6 and 7), although the magnitude of this component is much lower in the case of water. Hence, this suggests that similar changes would be expected in the recorded spectra for p-polarization as been found for a metallic substrate. Indeed, for thecase of a water substrate, the p-polarized reflection spectra of the thin film are more complex than those observed in the case of a metallic substrate since as can be immediately seen from Figure 22, the field component, ( EX12)which is parallel to the interface practically dominates over the almost full range
IR Reflection Spectroscopy of Monolayer Films
o'811
0.7
E21
I
0.6
I
The Journal of Physical Chemistry. Vol. 97, No. 11, 1993 2661
: I
I
hl
w
I
0.4
0.3
o'21 0.1
"
\: 0
1
0.5
1.5
Absorption Coefficient, k 2 Figure 24. Electric field ( E ? )components in three-phase system at 0 = 5 9 O for various absorption coefficients of thin film, kz, and refractive index n?l = 1.32. Other data as in Figure 22. x10-7
-3 -4
i
0
.,
\ ,
so
90
Angle of Incidence Figure 25. Calculated absorbance components for IO-A-thick film of cuprous ethyl xanthateon water, for thedatashown in Figure 22,obtained with theuseofapproximateequations,A l andAi1 (solid lines),Aii 4 (dashed line), All/ (dashed-dotted line).
of incident angles. Moreover, as discussed earlier, at angles below 0s the ( E X 2 *component, ) similarly to the ( E n 2 )one, produces a negative absorbance band (inversion of the band). Therefore, the p-polarized spectrum a t 0 < OB is expected to be the result of the addition of a strongly negative absorbance component due to the ( E X 2 2 )field and a low intensity positive absorbance component caused by the ( E z 1 2 )electric field (Figure 25). The latter one is distorted in similar fashion as was found for copper. For angles of incidence above the 0s the situation is reversed (Figure 25). The ( EX22)component produces positive absorbance
bands, whereas the ( Ez2*) component produces negative bands. The most complex spectra are expected at higher angles where the ( E x ? ? )and ( Ez22) show similar values which could result in no absorbance bands being observed at all. However, any anisotropy will result (see above) in negative and/or positive absorbance bands in the recorded spectra. For lower angles the ( E12?) component shows a significant increase reaching values near 0.8 while ( E Z 2 2 )decreases to almost zero (Figure 22). This causes the recorded spectra for both polarizations to be the same since only the electric field components parallel to the interface are available. Apparently, such identity in the recorded spectra is possible for samples isotropic in the plane of the interface. Any anisotropy in the x and y directions will produce differences in the recorded spectra. It is noteworthy here that the magnitude of energy, ( E 2 ) , available for interaction with thin film on water is very similar to that found for copper and cuprous sulfide (Figures 22,4, and 14) though at different angles of incidence; for water close to normal, for copper at 0 = 80° and for cuprous sulfide at 0 = 63O. Hence, the magnitude of electric field within the thin film is not a reason for experimental difficulties found while recording spectra of thin film on water. Based on thecalculated results presented in Figure 25, a spectral simulation was performed for a few characteristic incidence angles 10°,450,and 5 5 O . Theresultsareshownin Figure26andconfirm the predictions made on the basis of the consideration of the magnitude of the electric field components within the thin film. 1411the spectra, both negative and positive are very similar in shapes and band positions to the corresponding transmission spectrum. This correlates well with the domination of the parallel field components, ( E x 2 ? )within , the thin film on water for all !.he angles of incidence. The largest differences, although they are relatively small modifications, are found for p-polarization iat 0 = 5 5 O (Figure 26). There is a negative band at 1206 cm-', ;and a lowering of the intensity of the band at 1050 cm-I, which also shifts to 1046 cm-I. These observations confirm that the p-polarized reflection spectrum results from the addition of two absorbancecomponents, and at this particular angle, 0 = 5 5 O , the negative absorbance component produced by the normal field, ( Ezz2),is responsible for these small changes. Close inspection of these spectra reveals that the negative absorbance component is a spectrum with shapes, band positions and relative intensities very similar to that shown in Figure 3b for copper substrate, again supporting the conclusion about the strong modification of the normal field component by optical effects. Simplifying, a similar net spectrum could be produced by adding a relatively strong positive transmission spectrum (Figure 3a) to a low intensity negative spectrum with shapes and band positions similar to those observed for copper substrate (Figure 3b). Hence, for this case the reverse situation is observed as compared with that for a semiconductor substrate (Figure 16c). Some part of the reflection spectra, e.g., spectrum recorded at 45O for frequency below 1100 cm-1, show similar to derivativelike line shapes for absorbance bands. This modification cannot be explained by the simple addition of two absorbance components as discussed above. Derivative-like line shapes were observed by Porter et a1.I0 in s-polarized and at low angles for p-polarized reflection spectra of poly(methy1 methacrylate) on glassy carbon. It is interesting to note that the derivative-like line shapes depend on the conditions of recording the spectra (Figure 26). It was found that they are almost invisible in the simulated spectrum when the refractive index of water was assumed to be a constant value of 1.29; however, it is noticeable for n3 = 1.22. Since the optical constants of water change in this range with wavenumber, this type of perturbation is almost not observed for the bands near 1200 cm-I while it is distinct for the group of the bands near 1030 cm I . Theseobservationssuggest that a certain combination of optical constants of the thin film and substrate is responsible
5 - transm.
--t '
Mielczarski
The Journal of Physical Chemistry, Vol. 97, No. 11, I993
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'
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absorbance, as concluded above, is not caused by enhancement of the electric field within the thin film or an increase in the amount of molecules which interact with the incident beam. The reason is attributed to a very low reflectivity of water itself, especially for p-polarization near the Brewster angle. As a result, the small changes in reflectivity of the system with the thin film involve strong changes in the R / R o ratio. For practical use the region of incidence angles close to the Be for p-polarization is not easy to apply. The amount of energy that reaches the detector is so low that it is very difficult to record a spectrum with satisfactory signal/noise ratio. Moreover, even if we will record the p-polarized spectrum of a thin film on water this spectrum will be dominated, as shown, by the component parallel to interface. As a consequence, the information obtained will be similar to that from s-polarization experiment since the normal component of p-polarized spectrum will be very difficult to distinguish from the high noise level. Hence, the experimental effort will not bring any new information about the structure of the thin film. Higher quality experimental spectra can be recorded at lower angles for both polarizations or for an angle close to the Be but for s-polarization. In the latter case, in fact, the energy ( Ev2*)which interact with the characterized mediumis sacrificed but at the same time the amount of energy that is reflected and reaches a detector is significantly higher which results in a lower noise level. The optimum angle of incidence for s-polarized reflection studies depends on the characteristics of the detector used. In concluding this discussion of reflectivity effects on the quality of the recorded spectra of a thin film on water, it should be noted that at the present infrared reflection spectroscopy can provide reasonably satisfying information about the molecular group having a transition moment parallel to the interface, even when both polarizations are used.
I 1250
1200
I150
1100
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1000
Wavenumber. (cm-I)
Figure 26. Simulated reflection spectra of an isotropic IO-A-thick film ofcuprous ethyl xanthate complex on water at IOo, 45', and 55' angles of incidence with p- (solid line) and (dashed line) s-polarizations. The transmission spectra of corresponding thin film, which were added for comparative purposes, are in the same scale as respective reflection spectra, and they are moved up for clarity. In parentheses is a position of negative band.
for the derivative-like line shapes rather than dispersion of the refractive index or a high absorption index of the substrate as suggested recently. The simulated spectra of a thin film on water (Figure 26) show a much stronger absorbance bands than those calculated for the same thin film on metallic and semiconductor substrates (Figures 3 and 16). The predicted absorbance values are of the order of IO-*; hence, they should be easy to measure. This relatively strong
It was shown that for different types of substrates, either metallic, semiconductor, or nonmetallic, the optical effects, ie., the changes in position, shape, and intensity of bands in reflection spectra, are closely related to changes in the magnitude of the electric field components within the characterized thin film. The effect of the refractive index dispersion on the changes in reflection spectra, which was discussed in a number of papers, is only a rough approximation. It was found that there is a much more complex relationship than that which has been suggested previously. The observed optical effects are sensitive not only to the magnitude of the refractive index dispersion but also to the range over which the changes take place and to the value of the absorption coefficient. The consideration of the electric field components allows one to gain physical insight into the mechanism of the interaction of incident radiation in a multilayer system. Through a better understanding of the basis of optical effects, a more detailed interpretation of experimental spectroscopicdata is possible. More precisely, assignment of absorbance bands, reorganization of molecules in a thin film, or the formation of an ordered polymerlike structure of adsorbed film could only be postulated on the basis of a close interpretation of the reflection spectra after carefully analyzing the contribution of optical effects. Acknowledgment. Thanks are due to Dr. Z. Xu for writing the computer program which was used t o calculate some data presented in the paper. This work was partially supported by Phygis Program from the Ministbre de La Recherche. References and Notes ( I ) Greenler, R. D. J . Chem. Phys. 1966, 44, 310. Mclntyre, J. D. E.; Aspnes, D. E. Sur/. Sei. 1971,24,417. Hoffman, F. M . SurJ Sci. Rep. 1983, 3 , 107. Swalen, J . D.; Rabolt, J . F. In Fourier Transform Injrared Specrroscopy; Academic Press: New York, 1985; Vol. 4, Chapter 7. Swalen,
IR Reflection Spectroscopy of Monolayer Films J. D. J. Mol. Electron. 1986, 2, 155. Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.;Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, M. Langmuir, 1987,3,932. Hayden, B. E. In Vibrational Spectroscopy of Molecules on Surfaces; Yates, Jr., J . T., Madey, T. E., Eds.; Plenum Press: New York, 1987; Chapter 7 and references therein. Parikh, A. N.; Allara, D. L. J. Cbem. Pbys. 1992, 96, 15. Udagawa, A.; Matsui, T.; Tanaka, S.; Appl. Spectrosc. 1986, 40, Wong, J. S.; Yen, Y. S. Appl. Spectrosc. 1988, 42, 598. Yen, Y. S.; Wong, J. S.J. Pbys. Cbem. 1989, 93, 7208. Mielczarski, J. A.; Yoon. R. H. J. Phys. Cbem. 1989, 93, 2034. Mielczarski, J. A. SPlE Fourier Transform Spectrosc. 1989, 1 / 4 5 . (8) Mielczarski, J. A.; Yoon, R. H. Langmuir, 1991, 7, 101. (9) Mielczarski, J. A.; Zachwieja, J.; Yoon, R. H. 1990. 119th Annual AlME Meeting, Salt Lake City, Utah, Preprint 90-174. (IO) Porter, M. D.; Bright, T. B.; Allara, D. L.; Kuwana, T. Anal. Chem. 1986, 58, 2461. ( I 1) Mielczarski, J. A.; Yordan, J . L.; Yoon, R. H. Energy Fuels (special issue - Prear. Pan. - Am. Chem. Soc.. Div. Fuel Chem. 1988. 33(4). 729. . \ , (12) s;hy, R. A. J. Pbys. Cbem. 1986, 90, 1373. (13) Mitchell, M. L.; Dluhy, R. A. J. Am. Cbem. SOC.1988, 110, 712. (14) Greenler, R. D.; Rahn, R. R.;Schwartz, J. P. J Carol. 1971,23,42. (IS) Allara, D. L.; Baca, A.; Pryde, C. A. Macromolecules 1978, I I , 1215. (16) Porter, M. D. Anal. Cbem. 1988, 60, A1143. (17) Fina, L. J.; Tung, Y. S. Appl. Spectrosc. 1991, 45, 986. ( I 8) Hansen, W. N. J. Opt. SOC.Am. 1968, 58, 380. (19) Hansen, W. N.; Hansen, G.J. Appl. Spectrosc. 1987, 41, 553. (20) Hansen, W. N . In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Eds.; Wiley: New York, 1973; Vol. 9, Chapter I .
The Journal of Physical Chemistry, Vol. 97, No. 11, 1993 2663 (21) Hansen, W. N. J. Opt. SOC.Am. 1979, 69, 264. (22) Mclntyre, 1. D. E. In Advances in Electrochemistry and Electrochemical Engineering; P., Delahay, C. W., Tobias, Eds.; Wiley: New York, 1973; Vol. 9, Chapter 2. (23) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (24) Allara, D. L.; Nuzzo, R. G.Langmuir 1985, I, 52. (25) Nuzzo, R. G.;Dubois, L. H.; Allara, D. L. J. Am. Cbem. SOC.1990. 112, 558. (26) Ihs, A.; Liedberg, B.; Uvdal, K.; Tornkvist, C.; Bodo, P.; Lundstrom, I . J . Colloid Interface Sci. 1990, 140, 192. (27) Mielczarski, J. A.; Milosevic, M.; Berets,S. L. Appl. Spectrosc. 1992, 46, 1040. (28) Lynch, D. W.; Hunter, W. R. In Handbook of Optical Constants of Solid; Palik, E. D., Ed.; Academic Press: New York, 1985; p 280. (29) Downing, H. D.; Williams, D. J . J. Geophys. Res. 1975, 80, 1656. (30) Pritchard, J.; Catterick, T. In Experimental Methods in Catalytic Research; Anderson, R. B., Dawson, P. T., Eds.; Academic Press: New York, 1976; Vol. 3, Chapter 7. (31) Ishino, Y.; Ishida. H. Appl. Spectrosc. 1988, 42, 1296. (32) Khoo, C. G.L.; Ishida, H. Appl. Spectrosc. 1990, 44, 512. (33) Sellitti, C.; Koenig, J. L.; Ishida, H. Appl. Spectrosc. 1990,44,830. (34) Mielczarski, J. A.; Leppinen, J . Surf. Sci. 1987, 187, 526. (35) Ray, A.; Sathyanarayana, D. N.; Prasad, G. D.; Patel, C. C. Spectrocbim. Acta 1973, 29A, 1579. (36) Evans, S.D.; Sharma, R.; Ulman, A. Langmuir, 1991, 7, 156. (37) Pacansky, J.; England, C. D.; Waltmann, R. Appl. Spectrosc. 1986, 40, 8. (38) Philipp, H. R. In Handbook of Optical Constants of Solid; Palik, E. D., Ed.; Academic Press: New York, 1985; p 749.
