Surface plasmon spectroscopy of organic monolayer assemblies

Surface plasmon spectroscopy of organic monolayer assemblies

Surface Science 74 (1977) 237-244 0 North-Holland Publishing Company SURFACE PLASMON SPECTROSCOPY OF ORGANIC MONOLAYER ASSEMBLIES I. POCKRAND, J.D. ...

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Surface Science 74 (1977) 237-244 0 North-Holland Publishing Company

SURFACE PLASMON SPECTROSCOPY OF ORGANIC MONOLAYER ASSEMBLIES

I. POCKRAND, J.D. SWALEN, J.G. GORDON II and M.R. PHILPOTT IBM Research Laboratory,

San Jose, California 95193,

USA

Received 8 November 1977

Because the surface plasmon resonance is a sensitive probe of metallic surfaces we have measured quantitatively the changes that occur in surface plasmon resonances from coatings of thin layers of cadmium arachidate of different but known thicknesses on silver films. An ATR (attenuated total reflection) experimental arrangement was used. The dispersion of the refractive index of the organic layer was derived by a least square fit of the measured reflectivity curves and it agrees well with other data obtained by different methods. The influence of a possible intermediate layer between the silver surface and the fust organic monolayer due to the preparation process is also discussed.

1. Introduction

Surface plasma oscillations (SPO’s) are collective oscillations of the free charges at a metal boundary which propagate along the interface. The electromagnetic field connected with the motion of the charges peaks at the boundary and decays exponentially on both sides. Therefore SPO’s are rather sensitive to any modification occurring at the interface. A detailed review on SPO’s and their applications was published recently [ 11. Energy loss experiments with electron beams first showed the pronounced sensitivity of SPO’s to thin dielectric coatings several tens of a thick ([2], for more detail see ref. [3]) and in principle, allow the determination of the optical properties of the coating. However, due to the limited angular resolution of the experiments, the refractive index and the thickness of the coating normally cannot be derived from the energy loss spectrum accurately, especially for the small wavevectors associated with the visible region of the spectrum. For these frequencies SPO’s can be easily excited optically by using evanescent waves in a grating [4] or prism arrangement [5,6]. Experiments on overcoated metal gratings qualitatively showed the expected influence of the coating layer on the SPO properties [7,8], but a quantitative evaluation of the experimental data is rather complicated because of the influence of the corrugated (grating) surface itself on the SPO properties [9,10]. These difficulties are avoided if one user a 237

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prism arrangement as proposed in ref. [S], where the evanescent field penetrating the thin metal fim excites the SPO. The theoretical description of this system is str~~tfo~ard [I I ] . Experiments on silver films coated with thin layers of Ag$ [ 121, LiF and carbon 1131 demonstrated the accuracy with which the optical properties of the overcoating could be determined. Measurements on gold films covered with an organic monolayer assembly were reported recently [ 141. In this paper we extend SPO spectroscopy on silver films to the determination of the dispersion of the refractive index of thin cadmium arachidate monolayer assemblies in the visible region of the spectrum. These monolayer assemblies form regular arrays of molecules perpendicular to the plane of the films but randomly ordered in the plane. Also other molecules can be incorporated in the layers. Since orientation and spacing from the metallic surface can be prescribed, the surface con~guration and properties can be designed. The thicknesses of our films were all known [15] ) resulting in unique dete~inations of optical ~o~tants which are compared with those obtained by other methods. The influence of an intermediate layer between the metal and the first monolayer will also be discussed.

2. Experimental technique

Fig. 1 shows the experimental arrangement and the investigated layersystem. A p-polarized (TM) laser beam (HeNe, Cd, or Ar laser with a dye cell) was directed on the base of a 90” glass prism (ns = 1.5170 for A=6328 a and ns= I.5281 for h= 4416 a) which supported the thin silver film. Rotating the prism varied the angle of incidence, Ip, and changed the component of the wavevector of the incident light parallel to the prism base. Surface plasma oscillations at the metal surface opposite to the prism were excited by the evanescent field inside the metal when this component matched the real part of the SPO wavevector. In our case this was a complicated function of the refractive indices and thicknesses of the layers (see for example ref. f13]). The excitation of SPO’s was detected by a pronounced minimum in the reflected intensity caused by the strong, resonantly enhanced absorption in the silver film (see top right inset fig. I). The position and shape of the minimum depended strongly on the optical properties of the overlayer. The layer system was prepared on glass slides with approximately the same refractive index as the prism. First a silver film of approximately 500 a thickness was deposited by vacuum evaporation @ = 10e5 Torr, evaporation rate about 10 Btfsec). Immediately after removaf of the slide from the vacuum chamber, part of the silver fdm was covered with the prescribed number of monolayers of cadmium arachidate ((n-CHa(CH,),aCOO)zCd++) by using the Langmuir-Blodgett dipping technique 115,161. The monolayers consisted of a closely packed parallel arrangement of rod-like arachidate ions with their associated counterions, each monolayer having a well defined, reproducible thickness of 26.8 A. The shdes with the layer system were brought into optical contact with the

I. Pockrand et al. 1 SPS of organic monolayer assemblies

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Cd-Arkzhidate

Fig. 1. Experimental arrangement and schematic of layer system. Each molecule of Cd-arachidate is represented as a bar for the hydrocarbon portion and an open circle for the carboxylate and associated counterions.

prism base with an index matching fluid and measured within a few hours after preparation. We first recorded the critical angle of the p-polarized reflected light at an unsilvered part of the slide; then the SPO resonances were measured on both the bare and the Cd-arachidate coated silver film. All measurements were performed without moving the slide with respect to the prism base in order to minimize inaccuracy in the angle determination; the whole prism arrangement could be translated perpendicular to the plane of incidence of the light beam. The first measurements fixed the angular calibration (derived from the critical angle for total reflection) and the total reflectivity. The subsequent measurement, divided by the first, gave the relative SPO resonance curves with a maximum error of approximately 2% in the reflectivity. The dielectric function and the thickness of the silver film were determined from the experimental reflectivity curves by a least squares fit to the exact Fresnel reflection formula in an APL computer program. Then, with the silver values fixed, the refractive index of the overcoating was varied to generate the best fit to the experimental curves for the coated film. Calculations were done for both an isotropic and a uniaxial anisotropic (axis perpendicular to the surface) refractive index. The

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organic layer was assumed to be nonabsorbing the number of monolayers.

and to have a thickness 26.8 A times

3. Results and discussion Fig. 2 shows the dependence of the SPO resonance minimum on the Cd-arachidate thickness (h = 5 145 b;, similar results were obtained for the other wavelengths used). As expected, the resonance position shifts to greater angles of incidence and the width increases with growing thickness of the Cd-arachidate coating (compare with ref. [ 141). These effects become more pronounced with decreasing wavelength because the SPO electromagnetic field is concentrated to a narrower region around the silverCd-ara~~date boundary. Fig. 3 displays the influence of the coating thickness on the resonance position; a curve is shown for each wavelength of the incident light. For example, at X = 4416 A a coating only one monolayer thick (26.8 is) causes a shift of the resonance of 0.95” (internal angle @,see fig. 1). To see the high sensitivity of SPO spectroscopy one only has to compare this value with the angular resolution of our experimental apparatus (better than .5 X IO-* deg); therefore, even coatings only a few A thick could be detected with this method. The measured SPO resonance curves were used to determine the refractive index of the Cd-arachidate monolayer assemblies (see fig. 2; note the very good agreement between calculated and experimental curves). The results are summarized and compared to data obtained by other methods in fig. 4. The full circles and the solid line represent the values derived from the least squares fit to the reflectivity curves, External 38

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Angle of Incidence B(degj 36

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Cd-Arachidate

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on Silver

Fig. 2. Attenuated total reflectivity curves for silver films covered with different numbers of Cd-arachidate monolayers, as indicated (X = 5145 A). Symbols are experimental points (the size representing the experimental error); the solid line is the theoretical curve. Note that the reference curves (bare silver fdm) are slightly different for the one and three (dashed) and six and twelve monolayer assemblies (solid line).

I. Pockrand et al. / SPS of organic monolayer assemblies I

4416 B,



64880

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B

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Cd-Arachidate Thickness d,(A)

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1234

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Fig. 3. Shift, [email protected], of the SPO resonance minimum as a function of Cd-arachidate thickness, d,, for various wavelengths of the incident light. Symbols represent experimental results, curves were calculated by using the average refractive index of Cd-arachidate determined from the SPO-resonances (see fig. 4).

where we assumed an isotropic optical behavior for the Cd-arachidate layer. Each point is the average of the results obtained for coatings of 1, 2, 3, 4, 6, 8, and 12 monolayers (X = 6328,.5145,4880, and 4416 A) or 1,3,6, and 12 monolayers (X = 6041, 5840, and 4579 A). Here we included the influence of an intermediate layer between silver and the organic coating as described in the following. As pointed out in ref. [17] one usually observes a thickness dependence of the refractive index for the first few layers of Cd-arachidate if the uncovered silver (or gold) surface is taken as a reference. This observation is confirmed by our results: fig. 5 shows the refractive index of Cd-arachidate as a function of the number of monolayers (open circles and dashed line, h = 6328 A) where we took the values determined at the bare silver film as a reference. The refractive index of the Cdarachidate layer assembly decreases with increasing thickness. This effect can be explained by assuming a thin layer of adsorbed atoms between the silver and the first Cd-arachidate monolayer caused by the preparation process. ESCA measurements on bare silver films dipped into the tank used for the Cd-

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oforganic monolayer assemblies

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Fig. 4. Refractive index of Cd-arachidate monolayer assemblies as a function of wavelength derived from a least squares tit to the experimental SPO resonance curves (0 and solid line, isotropic fit; o and dashed lines, uniaxial anisotropic fit). Our results are compared to values obtained with other methods, where the full symbols represent data assuming an isotropic film, the open symbols assuming a uniaxial anisotropic film.

arachidate deposition showed a thin film (3-8 A thick) of adsorbed material which consists mainly of oxide and hydroxyl groups (from water and air), hydrocarbons (from air) and cadmium probably present as Cd(OH), (from the tank solution). Note that a 10% increase in thickness of the first monolayer would also explain the effect but this is unrealistic since the assumed thickness already corresponds to a fully extended arachidate chain. 1.64 E 1.62 -

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Fig. 5. Influence of an intermediate layer between silver aqd the first monolayer on the calculated (isotropic) refractive index of Cd-arachidate as a function of the number of monolayers (h = 6328 A). Open circles and dashed line: No intermediate layer. Full circles and solid line: Intermediate layer 3 A thick with refractive index 1.534.

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An intermediate layer 3 A thick with a refractive index of 1 S34 gave the best, thickness independent, refractive index of Cd-arachidate (see fig. 5, full circles, X = 6328 A). The average of the values obtained for the various Cd-arachidate layers (given by the solid line in fig. 5) is plotted in fig. 4. Fig. 4 also shows the results obtained by assuming uniaxial anisotropy of the Cdarachidate film (axis perpendicular to the surface) which one expects because of the structure of the layer (see fig. 1). The anisotropic fit was performed with a fixed, wavelength independent anisotropy of nl - nil = 0.043 because measurements done with guided wave techniques [ 181 showed this to be approximately true. The calculated indices are given by the open circles and dashed lines in fig. 4. The anisotropic calculation does not appear to tit the experimental reflectivity curves any better than the isotropic calculation; SPO resonances at metal surfaces coated with thin anisotropic films can generally be described by an isotropic coating of the same thickness with an intermediate refractive index. Therefore, SPO resonance measurements alone do not allow us here to distinguish between anisotropic and isotropic coatings! Fig. 4 also summarizes the refractive indices of Cd-arachidate monolayer assemblies obtained with guided wave techniques [ 181 or other optical methods [ 19-22, 241, especially ellipsometric techniques [ 171, [23]. Although the influence of an intermediate layer was not taken into account, these values are in good agreement with our data because they were derived from measurements on thick Cd-arachidate films, where the effect on an intermediate layer becomes negligible (compare fig. 5). 4. Summary We have determined the dispersion of the refractive index of thin Cd-arachidate monolayer assemblies of known thickness on silver films in the visible wavelength region using SPO excitation. The calculated data - derived from a least square fit to the experimental curves - agree well with those obtained by other methods. The influence of an intermediate layer between the silver surface and the first monolayer becomes important for very thin Cd-arachidate films (only a few monolayers) and was taken into account for the quantitative evaluation of the experiments. Generally SPO’s represent a highly sensitive tool for investigations of thin films on metal surfaces. However, it should be emphasized that SPO’s do not provide enough information for a complete quantitative description of anisotropic films: It is usually not possible to determine both thickness and the refractive indices from SPO measurements alone; one must combine SPO spectroscopy with other measurements. For example, one could determine the in plane refractive index nil from reflectivity measurements and then calculate nl and the thickness from the SPO curves at several wavelengths. The utility of SPO spectroscopy for the quantitative study of thin film coatings on metals has, however, been clearly demonstrated with our optically well-characterized layers.

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Acknowledgment We would like to thank Mr. R. Santo for the careful preparation films.

of the organic

References [ 11 H. Raether, Surface Plasma Oscillations and Their Applications, in: Physics of Thin Films, Vol. 9 (Academic Press, New York, 1977) p. 145. [2] J.J. Powell and J.B. Swan, Phys. Rev. 118 (1960) 640. [3] T. Kloos, Z. Physik 208 (1968) 77. [4] Y. Teng and E.A. Stern, Phys. Rev. Letters 19 (1967) 511. [5] E. Kretschmann and H. Raether, Z. Naturforsch. 23a (1968) 2135. [6] A. Otto, Z. Physik 216 (1968) 398. [ 71 J.J. Cowan and E.T. Arakawa, Z. Physik 235 (1970) 97. [ 81 I. Pockrand, J. Phys. D (Appl. Phys.) 9 (1976) 2423. [9] I. Pockrand, Phys. Letters 49A (1974) 259. [lo] I. Pockrand and H. Raether, Appl. Opt. 16 (1977) 1784. (111 E. Kretschmann, Z. Physik 241 (1971) 313. [ 121 F. Abel&, Surface Sci. 56 (1976) 237. [ 131 I. Pockrand, Surface Sci. 72 (1978) 577. [ 141 J.G. Gordon II and J.D. Swalen, Opt. Commun. 22 (1977) 374. [ 151 H. Kuhn, D. Mobius and H. Bticher, Spectroscopy of Monolayer Assemblies, in: Techniques of Chemistry, Vol 1, Physical Methods of Chemistry, Part IIB, Eds. A. Weissberger and B.W. Rossiter (Wiley-Interscience, New York, 1972) ch. VII. [ 161 G.L. Gaines, Jr., Insoluble Monolayers at Liquid-Gas Interfaces (Wiley-Interscience, New York, 1966) ch. 8. [17] R. Steiger, Helv. Chim. Acta 54 (1971) 2645. [ 181 J.D. Swalen, K. Rieckhoff and M. Tacke, Opt. Commun., submitted. [19] K.B. Blodgett, Phys. Rev. 55 (1939) 391. [20] K.H. Drexhage, Habilitationschrift, University Marburg (1966). [21] H. Forster, Diplomarbeit, University Marburg (1967). [22] M. Fleck, Thesis, University Marburg (1969). [23] D. den Engelsen, J. Opt. Sot. Am. 61 (1971) 1460. [24] J.G. Gordon II and J.D. Swalen [ 141 subsequently obtained more accurate data for Cdarachidate on gold. These were analyzed by I. Pockrand to give isotropic refractive indices of 1.538 (2 monolayers), 1.539 (4 monolayers), 1.547 (6 monolayers), 1.547 (8 monolayers), and an average of 1.543 f 0.004 (h = 6328 A).