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Surface plasmon polariton studies of highly absorbing Langmuir-Blodgett films
Thin Solid Films, 208 (1992) 269-273
269
Surface plasmon polariton studies of highly absorbing Langmuir- Blodgett films C. R. Lawrence, A. S. Martin and J. R. Sambles Thin Film and Interface Group, Department of Physics, University o["Exeter, Exeter EX4 4QL (U.K.)
(Received May 1, 1991; accepted September 16, 1991)
Abstract In this study, surface plasmon resonance studies of Langmuir Blodgett multilayers have been used to characterize an optically highly absorbing zwitterionic material which would be difficult to otherwise analyse. By fitting reflectivity data with Fresnel theory for a range of wavelengths it has been possible to obtain the real and imaginary parts of the assumed-isotropic dielectric optical permittivity over the visible part of the spectrum. These results show an asymmetric resonance peak which may be interpreted as being a composite of two resonances, one associated with an intermolecular dipole transition and one with an intramolecular dipole.
I. Introduction With the recent developments in the field o f non-linear optical devices the characterization of thin organic films has risen in importance, a knowledge of the optical dielectric constants of such films being fundamental to device construction. Methods such as the L a n g m u i r - B l o d g e t t (LB) multilayer waveguide technique (illustrated by Martin and Sambles [1]) can be used to characterize a material fully, gleaning both film thickness and the full optical dielectric tensor of a material, but it is not always possible to deposit the several hundred layers requisite to this procedure. Also, both this and the more conventional ellipsometric techniques require that the material is only weakly absorbing. Since high non-linear coefficients are often accompanied by a strong resonance in the wavelength region of interest, leading to strong absorption, many of the molecules which are best suited to non-linear optical work are beyond the scope of these methods of characterization. An alternative procedure for characterization involves the excitation of surface plasmon polaritons (SPPs) in a Kretschmann [2] geometry structure of p r i s m / m e t a l / L B film. SPPs are extremely sensitive to modifications of the metallic surface, and the addition of a dielectric overlayer will affect the resonance shape (shown by m a n y workers, such as in refs. 3-5). Provided that the thickness of the LB multilayer film can be determined, then its optical dielectric constants can be obtained by fitting the surface plasmon curve to Fresnel theory, giving an assumed-isotropic refractive index. In this paper, by studying the surface plasmons
0040-6090/92/$5.00
across a range of wavelengths, the inherent resonance of the highly absorbing film was observed in detail and the data obtained were used in comparison with direct absorption studies.
2. Experimental details 2.1. Sample preparation
The LB material used was CI6H33-Q3CN Q (Fig. 1), a zwitterionic molecule that was synthesized at Cranfield Institute of Technology [6], specifically for its non-linear properties. This material is of particular interest owing to its use in molecular rectification studies [7]: knowledge of the dielectric dispersion of the molecule, and hence of the molecular dipole transitions, may well be pertinent to explanations of the processes involved. It has been found that the deposition of high quality multilayer films of this material is difficult with a singlec o m p a r t m e n t LB trough, the transfer ratios (the ratios of the area of film removed from the subphase to the area of substrate covered) often indicating incomplete substrate coverage and bilayer formation. Dipping directly onto a prism would increase the likelihood of uneven coverage due to the geometry, and so it was considered advantageous to deposit the film onto a thin (100 ~tm) glass substrate of identical refractive index to that of the prism, and to use a suitable index-matching fluid to keep the two components in contact with each other. Effectively, the substrate becomes an extension of the prism (Fig. 2). Prior to use, all glassware was immersed in AnalaR grade isopropyl alcohol (IPA) and ultrasonically agitated
© 1992 - - Elsevier Sequoia. All rights reserved
C. R. Lawrence et aL / S P P studies o / highly absorbing LB.fihns
270
i~
C~ H33- - -
lq
'
--
22"
'), /
forming cells where the matching fluid resided; the fluid chosen was methyl benzoate, a low volatility liquid with a refractive index very close to that of the glassware (n~l,51).
,
N Fig. 1. Structure of the CE6H~-Q3CN Q zwitterion.
l
I
2.2.
Data
acquisition
To excite surface plasmons at the metal-dielectric interface, a beam of p-polarized transverse magnetic (TM) monochromatic light was shone through the prism so as to be incident internally on the metal layer, and its reflectivity measured as a function of angle. Fluctuation in incident beam intensity was accounted for by comparison with a reference beam, split off from the modulated main beam before it entered the sample. Modulation was obtained through the use of a beam chopper set at approximately 2 kHz, and both reference and signal beams were detected by photomultiplier tubes whose signals were passed to lock-in amplifiers. The angle of incidence was varied by placing the sample onto a computer-controlled rotating table, the angular resolution of which was 0.01:'. As already stated, only half of each substrate's surface area was coated with LB film, leaving an area of bare silver. Surface plasmons were excited on both this and the LB-film-covered region at different wavelengths, ranging from 500 nm to 650 nm, and the resultant data of reflected intensity vs. angle stored on disc.
Fig. 2. Sample configuration: see Section 2.1. 3. Analysis for 30 rain. It was then cleaned with lens tissue soaked in chloroform, before being refluxed in Aristar grade IPA for an hour. Substrates were rendered hydrophobic by storing them in hexamethyldisilazane vapour overnight. The metallic layer necessary for surface plasmon excitation was produced by thermally evaporating silver onto the substrates at a rate of 1 n m s i to a thickness of approximately 40 rim, under a vacuum of less than 10 4pa. By dissolving the LB material in Aristar grade dichloromethane, 2 x 10 -s M solutions were prepared, to be spread onto the ultrapure water subphase (resistivity, 18 MD cm) of a polytetrafluoroethlylene movingbarrier LB trough. Here they were compressed to a surface pressure of 23 m N m r and multilayers were deposited onto one half of each substrate's surface area, at a dipping speed of 0 . 2 m m s -~. All samples were stored in a desiccator when not in use. Immediately prior to use, a substrate would be clamped against a prism, silver side outwards, with an additional glass plate of l m m thickness between the two. The latter's purpose was merely to increase the mechanical stability of the final structure, having no other appreciable effect. Each glass surface was separated from the next by 100 gm thick Mylar spacers,
All data obtained are first normalized to the relerence signal, the external angles of incidence that have been measured are converted to the internal angles within the prism, and reflection at the entrance and exit faces of the prism is corrected for. Once this has been done the curves can be fitted to Fresnel theory. The data from the region of bare silver are fitted first, over a range of seven different wavelengths. For deep resonances there are two possible solutions at each wavelength, but as there was only one value for the film's thickness t~g that was the same at all wavelengths, the correct set of solutions soon became apparent. From these solutions for a few wavelengths the averaged metal thickness is 39.0 + 0.6 nm, and this t~g was then used to fit the data obtained at all wavelengths; with only two remaining variables, ~:r and ~:i, the fitting was then much faster. With the silver film characterized at all wavelengths, the glass/silver/LB film data could be fitted in the same manner, but with the LB layer parameters being the only new variables. Again, fits were made for a few wavelengths to obtain consistent values for the thickness of the film in question, and these were averaged. Taking this average t]~o v as being 29.2 + 0.2 nm, all other
271
C. R. Lawrence et al. / SPP studies of highly absorbing LB films
400
III I
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120
]
,_ln(i )
~l
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]
O~') 0 ,I (3 i ItJ
=
-2
where k is the imaginary part of the complex refractive index of the LB film and 2 is the wavelength of the incident light (in vacuo). N o w k may be derived from the e values found at each wavelength using the SPP technique; the two results are compared by plotting a graph of k/2 vs. ln(I/Io), giving a straight line of slope - 4 n d (Fig. 4).
100
0 60
thickness implies that each layer has a thickness of 2.92 nm. Other work [8] has estimated the thickness of a monolayer of ion-contaminated material deposited at 25 mN m -I to be 2.99 nm. This estimate was determined from LB multilayer waveguide studies [1] of films which were contaminated by metallic ions whilst on the subphase: such films exhibit much lower optical absorbance than those made of uncontaminated material, making such studies possible (the presence of a metallic ion is not expected to alter the length of the molecule appreciably). Thus the 2.92 nm estimate for the film used in the SPP studies is very sensible, especially when it is remembered that this film was deposited at a lower surface pressure (23 m N m -r) and would thus be expected to have a smaller thickness. Now, having both assumed-isotropic dielectric constants and film thickness, it was decided to compare the results with absorption data obtained for similar films on bare glass substrates. If I/Io is the fractional transmission for normal incidence light in a film of thickness d, having corrected for reflections at the interfaces, then
i t
]
4
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'3, 2 Q
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(b) Fig. 3. (a) Dispersion curve: wavelength of incident light (in vacuo) vs. real component of permittivity for LB film. (b) Dispersion curve: wavelength of incident light (in vacuo) vs. imaginary component of permittivity for LB film.
0.20
(.9 0
..~ - 0 . 4 0
O
O.60
L
wavelengths were fitted and the LB film was characterized, giving the relative permittivity data shown in Fig. 3. The transfer ratios obtained during the deposition of the film used in the SPP studies were difficult to interpret owing to indications of bilayer formation and incomplete substrate coverage, making a direct estimation of monolayer thickness impossible. However, we can assume that the film consists of an integer number of monolayers, and we know that this number is greater than 8 and less than 12; taking 10 monolayers, the film
~J
WAVELENGTH INCREASING
~ \\\\
0.80
,\.
1 00 ......... T, . . . . . . . . ~ , , , r~m , , , ~ - . . . . o,oo 2o000000 ~ooooooo 60o0o0.o0
k/wavelength
Fig. 4. Plot of In(transmittance) vs. the imaginary part of the film's refractive index divided by the wavelength of incident light in vacuo. It should be noted that points at or near the resonance ( Q ) have been ignored for the purpose of calculating the gradient.
272
c. R. Lawrence eta/. / SPP studies q[' h~hly absorbing LB.17'lms
Clearly, when the points are joined successively as their wavelength increases, the resultant line has a p r o n o u n c e d loop at the points nearest the resonance peak (2 = 560 585 nm). This effect is due to the shift in resonance peaks between the films used for the SPP and direct absorption studies (see Section 4). It is possible to estimate the film thickness by ignoring these points and determining the gradient from the remainder, all o f which were relatively unaffected by the resonance shift. This gradient suggests that the thickness X~,b~o f the film used in the absorption experiment is 111.9 + 4.7 rim. Since this was nominally a 40 layer film, it gives a thickhess o f approximately 2.8 nm per monolayer; again, this is in close agreement with the previously mentioned value [8]. This total film thickness was then used with the k / 2 values to plot an absorption peak based on the surface plasmon data, thus allowing another comparison o f these data with the direct absorption data.
4. Results and discussion
The graphs o f both ~:r and ~;, vs. wavelength of incident light f r o m the SPP data show a clear resonance (see Fig. 3), with the principal band at a r o u n d 570 nm. Both dispersion plots show asymmetric behaviour. This is most p r o n o u n c e d in the ei case, which shows a " s h o u l d e r " to the resonance that distorts the peak from approximately 600 n m upwards. This behaviour is typical o f two or m o r e resonances in close proximity, and work by Ashwell et al. [9] indicates that this is the case. T h e y suggest that there are two resonances involved in absorption in the zwitterions, one due to an intramolecular charge transfer b a n d at a r o u n d 565 rim, and the other an intermolecular band above 600 nm. It is difficult to give exact values to these transitions since they will vary according to deposition pressure; their study gives the example of a film deposited at 24 m N m ] having its principal band at 561 nm and a b r o a d shoulder above 600 nm, whereas a film deposited at 40 m N m ~ has its main absorption at 614 nm. This difference is due to the 4 0 m N m ~ film having its constituent molecules virtually perpendicular to the substrate: the absorption studies set the incident light o r t h o g o n a l to the substrate, and thus the electric vector is at right angles to the intramolecular transition moment. This means that the intramolecular dipole oscillation is not excited, and only the intermolecular resonance is seen. However, at lower deposition pressures the molecules are tilted, and therefore a c o m p o n e n t o f the intramolecular resonance, that increases as the angle o f tilt increases, is sensed by the normal incidence radiation. F o r the SPP work, the incident light is T M polarized with its electric vector not parallel to the surface, and thus both resonances are detected.
j t ,= o ~;,:, "' ':± i ,~ ,, _~: ,} 4-~ " ,x; | ~O . ::: ;,: 4 . _~ J
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Fig. 5. Superposition of experimentally determined absorption peak ( ) and evaluated data from SPP studies (thickness of film modelled as 112 + 5 nm).
Figure 5, overlaying the direct and the scaled SPP absorption peaks, shows that the SPP data do not scale accurately to the absorption data. The overriding discrepancy is in the peak heights, the SPP data showing 8% greater absorption at its zenith. This is reasonable; if the molecules have only a slight tilt, making a small angle to the normal, a tangential electric vector, as in the absorption experiment, will excite only a relatively small c o m p o n e n t of the intramolecular dipole oscillation that lies along the molecule's length. However in
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C. R. Lawrence et al./ SPP studies q[" highly absorbing LB films
the SPP experiment both normal and tangential electric fields are present and there is a stronger influence of the intramolecular transition. Thus this resonance will have a greater effect on the resultant optical constants, causing the 8% difference in absorption peak maxima. There is also a shift in the resonance position between the two sets of data, which can be made more obvious if the peak heights are matched by rescaling the SPP data appropriately (Fig. 6). It is then clear that the peak maximum moves from 573 ___1 nm for the direct absorption data to 567___ 3 nm for the SPP data. As already mentioned, Ashwell et al. [9] indicate that the resonance position will vary with deposition pressure, the absorption peak wavelength increasing with the surface pressure of the film; since the films used in the SPP experiments were dipped at 23 m N m - ~, 2 m N m lower than those used in the absorption studies, the shift is as expected.
5. Conclusions By using the optical excitation of surface plasmons it has been possible to characterize a highly absorbing dielectric over a range of wavelengths. The dispersion of the optical constants is consistent with the presence of two optical resonances within the material, comparing favourably with Ashwell et al.'s work [9]. The absorption data derived from the results of the SPP studies closely model data obtained by direct absorp-
273
tion studies, with differences being caused by the differing roles of intermolecule and intramolecule dipole effects in the experiment.
Acknowledgments The authors thank Geoff Ashwell and coworkers at the Cranfield Institute of Technology for the synthesis of the zwitterionic material used, Steve Elston for his help in data acquisition and analysis, and SERC for financial support.
References 1 A. S. Martin and J. R. Sambles, Surf Sci., 225 (1990) 390396. 2 E. K r e t s c h m a n n and H. Raether, Z. NaturJorsch A, 23 (1968) 2135. 3 I. Pockrand, J. D. Swalen, R. Santo, A. Brillante and M. R. Philpon, J. Chem. Phys., 69 (9) (1978) 4001 4011. 4 G. Wahling, Z. NaturJbrsch A, 36 (1981) 588--594. 5 J. G. Gordon II and J. D. Swalen, Opt. Commun., 22 (3) (1977) 374. 6 G. J. Ashwell, Th#l SolM Films, 186 (1990) 155. 7 G. J. Ashwell. J. R. Sambles, A. S. Martin, W. G. Parker and M. Szablewski, J. Chem. Soc., Chem. Comm., 19 (1990) 1374 1376. 8 A. S. Martin and G. Bryan-Brown, personal communication, 1990. 9 G. J. AshwelL E. J. C. Dawnay, A. P. Kuczynski, M. Szablewski, I. M. Sandy, M. R. Bryce, A. M. Grainger and M. Hasan, J. Chem. Soc., Faraday Trans., 86 (1990) 1117.
269
Surface plasmon polariton studies of highly absorbing Langmuir- Blodgett films C. R. Lawrence, A. S. Martin and J. R. Sambles Thin Film and Interface Group, Department of Physics, University o["Exeter, Exeter EX4 4QL (U.K.)
(Received May 1, 1991; accepted September 16, 1991)
Abstract In this study, surface plasmon resonance studies of Langmuir Blodgett multilayers have been used to characterize an optically highly absorbing zwitterionic material which would be difficult to otherwise analyse. By fitting reflectivity data with Fresnel theory for a range of wavelengths it has been possible to obtain the real and imaginary parts of the assumed-isotropic dielectric optical permittivity over the visible part of the spectrum. These results show an asymmetric resonance peak which may be interpreted as being a composite of two resonances, one associated with an intermolecular dipole transition and one with an intramolecular dipole.
I. Introduction With the recent developments in the field o f non-linear optical devices the characterization of thin organic films has risen in importance, a knowledge of the optical dielectric constants of such films being fundamental to device construction. Methods such as the L a n g m u i r - B l o d g e t t (LB) multilayer waveguide technique (illustrated by Martin and Sambles [1]) can be used to characterize a material fully, gleaning both film thickness and the full optical dielectric tensor of a material, but it is not always possible to deposit the several hundred layers requisite to this procedure. Also, both this and the more conventional ellipsometric techniques require that the material is only weakly absorbing. Since high non-linear coefficients are often accompanied by a strong resonance in the wavelength region of interest, leading to strong absorption, many of the molecules which are best suited to non-linear optical work are beyond the scope of these methods of characterization. An alternative procedure for characterization involves the excitation of surface plasmon polaritons (SPPs) in a Kretschmann [2] geometry structure of p r i s m / m e t a l / L B film. SPPs are extremely sensitive to modifications of the metallic surface, and the addition of a dielectric overlayer will affect the resonance shape (shown by m a n y workers, such as in refs. 3-5). Provided that the thickness of the LB multilayer film can be determined, then its optical dielectric constants can be obtained by fitting the surface plasmon curve to Fresnel theory, giving an assumed-isotropic refractive index. In this paper, by studying the surface plasmons
0040-6090/92/$5.00
across a range of wavelengths, the inherent resonance of the highly absorbing film was observed in detail and the data obtained were used in comparison with direct absorption studies.
2. Experimental details 2.1. Sample preparation
The LB material used was CI6H33-Q3CN Q (Fig. 1), a zwitterionic molecule that was synthesized at Cranfield Institute of Technology [6], specifically for its non-linear properties. This material is of particular interest owing to its use in molecular rectification studies [7]: knowledge of the dielectric dispersion of the molecule, and hence of the molecular dipole transitions, may well be pertinent to explanations of the processes involved. It has been found that the deposition of high quality multilayer films of this material is difficult with a singlec o m p a r t m e n t LB trough, the transfer ratios (the ratios of the area of film removed from the subphase to the area of substrate covered) often indicating incomplete substrate coverage and bilayer formation. Dipping directly onto a prism would increase the likelihood of uneven coverage due to the geometry, and so it was considered advantageous to deposit the film onto a thin (100 ~tm) glass substrate of identical refractive index to that of the prism, and to use a suitable index-matching fluid to keep the two components in contact with each other. Effectively, the substrate becomes an extension of the prism (Fig. 2). Prior to use, all glassware was immersed in AnalaR grade isopropyl alcohol (IPA) and ultrasonically agitated
© 1992 - - Elsevier Sequoia. All rights reserved
C. R. Lawrence et aL / S P P studies o / highly absorbing LB.fihns
270
i~
C~ H33- - -
lq
'
--
22"
'), /
forming cells where the matching fluid resided; the fluid chosen was methyl benzoate, a low volatility liquid with a refractive index very close to that of the glassware (n~l,51).
,
N Fig. 1. Structure of the CE6H~-Q3CN Q zwitterion.
l
I
2.2.
Data
acquisition
To excite surface plasmons at the metal-dielectric interface, a beam of p-polarized transverse magnetic (TM) monochromatic light was shone through the prism so as to be incident internally on the metal layer, and its reflectivity measured as a function of angle. Fluctuation in incident beam intensity was accounted for by comparison with a reference beam, split off from the modulated main beam before it entered the sample. Modulation was obtained through the use of a beam chopper set at approximately 2 kHz, and both reference and signal beams were detected by photomultiplier tubes whose signals were passed to lock-in amplifiers. The angle of incidence was varied by placing the sample onto a computer-controlled rotating table, the angular resolution of which was 0.01:'. As already stated, only half of each substrate's surface area was coated with LB film, leaving an area of bare silver. Surface plasmons were excited on both this and the LB-film-covered region at different wavelengths, ranging from 500 nm to 650 nm, and the resultant data of reflected intensity vs. angle stored on disc.
Fig. 2. Sample configuration: see Section 2.1. 3. Analysis for 30 rain. It was then cleaned with lens tissue soaked in chloroform, before being refluxed in Aristar grade IPA for an hour. Substrates were rendered hydrophobic by storing them in hexamethyldisilazane vapour overnight. The metallic layer necessary for surface plasmon excitation was produced by thermally evaporating silver onto the substrates at a rate of 1 n m s i to a thickness of approximately 40 rim, under a vacuum of less than 10 4pa. By dissolving the LB material in Aristar grade dichloromethane, 2 x 10 -s M solutions were prepared, to be spread onto the ultrapure water subphase (resistivity, 18 MD cm) of a polytetrafluoroethlylene movingbarrier LB trough. Here they were compressed to a surface pressure of 23 m N m r and multilayers were deposited onto one half of each substrate's surface area, at a dipping speed of 0 . 2 m m s -~. All samples were stored in a desiccator when not in use. Immediately prior to use, a substrate would be clamped against a prism, silver side outwards, with an additional glass plate of l m m thickness between the two. The latter's purpose was merely to increase the mechanical stability of the final structure, having no other appreciable effect. Each glass surface was separated from the next by 100 gm thick Mylar spacers,
All data obtained are first normalized to the relerence signal, the external angles of incidence that have been measured are converted to the internal angles within the prism, and reflection at the entrance and exit faces of the prism is corrected for. Once this has been done the curves can be fitted to Fresnel theory. The data from the region of bare silver are fitted first, over a range of seven different wavelengths. For deep resonances there are two possible solutions at each wavelength, but as there was only one value for the film's thickness t~g that was the same at all wavelengths, the correct set of solutions soon became apparent. From these solutions for a few wavelengths the averaged metal thickness is 39.0 + 0.6 nm, and this t~g was then used to fit the data obtained at all wavelengths; with only two remaining variables, ~:r and ~:i, the fitting was then much faster. With the silver film characterized at all wavelengths, the glass/silver/LB film data could be fitted in the same manner, but with the LB layer parameters being the only new variables. Again, fits were made for a few wavelengths to obtain consistent values for the thickness of the film in question, and these were averaged. Taking this average t]~o v as being 29.2 + 0.2 nm, all other
271
C. R. Lawrence et al. / SPP studies of highly absorbing LB films
400
III I
I
0
qP 3 5 0 ££ O 'o0 C~ L~ 3.00
i i
2.50 ~L~-TTq 450.00
i i i i i i i i j i i b i r i L J I I I t 1--VWTT~TT~I
500.00
550.00
I I I I I I I I I I I I I
600.00
650.00
700.00
WaveJength (nm)
(a) 1.40 ~r
120
]
,_ln(i )
~l
4nd
rO~,0 8.0
]
O~') 0 ,I (3 i ItJ
=
-2
where k is the imaginary part of the complex refractive index of the LB film and 2 is the wavelength of the incident light (in vacuo). N o w k may be derived from the e values found at each wavelength using the SPP technique; the two results are compared by plotting a graph of k/2 vs. ln(I/Io), giving a straight line of slope - 4 n d (Fig. 4).
100
0 60
thickness implies that each layer has a thickness of 2.92 nm. Other work [8] has estimated the thickness of a monolayer of ion-contaminated material deposited at 25 mN m -I to be 2.99 nm. This estimate was determined from LB multilayer waveguide studies [1] of films which were contaminated by metallic ions whilst on the subphase: such films exhibit much lower optical absorbance than those made of uncontaminated material, making such studies possible (the presence of a metallic ion is not expected to alter the length of the molecule appreciably). Thus the 2.92 nm estimate for the film used in the SPP studies is very sensible, especially when it is remembered that this film was deposited at a lower surface pressure (23 m N m -r) and would thus be expected to have a smaller thickness. Now, having both assumed-isotropic dielectric constants and film thickness, it was decided to compare the results with absorption data obtained for similar films on bare glass substrates. If I/Io is the fractional transmission for normal incidence light in a film of thickness d, having corrected for reflections at the interfaces, then
i t
]
4
[ ]
'3, 2 Q
I I
: 0 ' 0 0
~
40b00
r
T V ~ V
] - r ~
b00.00
r i i
, , , r~FT~[
r y ~
• i TT~r
550.00 600.00 6b0.00 ',,",,'L] v @ ~ " ,'..:] [ t-, ( r ~ q :,
°°°I
~ 7
70030
(b) Fig. 3. (a) Dispersion curve: wavelength of incident light (in vacuo) vs. real component of permittivity for LB film. (b) Dispersion curve: wavelength of incident light (in vacuo) vs. imaginary component of permittivity for LB film.
0.20
(.9 0
..~ - 0 . 4 0
O
O.60
L
wavelengths were fitted and the LB film was characterized, giving the relative permittivity data shown in Fig. 3. The transfer ratios obtained during the deposition of the film used in the SPP studies were difficult to interpret owing to indications of bilayer formation and incomplete substrate coverage, making a direct estimation of monolayer thickness impossible. However, we can assume that the film consists of an integer number of monolayers, and we know that this number is greater than 8 and less than 12; taking 10 monolayers, the film
~J
WAVELENGTH INCREASING
~ \\\\
0.80
,\.
1 00 ......... T, . . . . . . . . ~ , , , r~m , , , ~ - . . . . o,oo 2o000000 ~ooooooo 60o0o0.o0
k/wavelength
Fig. 4. Plot of In(transmittance) vs. the imaginary part of the film's refractive index divided by the wavelength of incident light in vacuo. It should be noted that points at or near the resonance ( Q ) have been ignored for the purpose of calculating the gradient.
272
c. R. Lawrence eta/. / SPP studies q[' h~hly absorbing LB.17'lms
Clearly, when the points are joined successively as their wavelength increases, the resultant line has a p r o n o u n c e d loop at the points nearest the resonance peak (2 = 560 585 nm). This effect is due to the shift in resonance peaks between the films used for the SPP and direct absorption studies (see Section 4). It is possible to estimate the film thickness by ignoring these points and determining the gradient from the remainder, all o f which were relatively unaffected by the resonance shift. This gradient suggests that the thickness X~,b~o f the film used in the absorption experiment is 111.9 + 4.7 rim. Since this was nominally a 40 layer film, it gives a thickhess o f approximately 2.8 nm per monolayer; again, this is in close agreement with the previously mentioned value [8]. This total film thickness was then used with the k / 2 values to plot an absorption peak based on the surface plasmon data, thus allowing another comparison o f these data with the direct absorption data.
4. Results and discussion
The graphs o f both ~:r and ~;, vs. wavelength of incident light f r o m the SPP data show a clear resonance (see Fig. 3), with the principal band at a r o u n d 570 nm. Both dispersion plots show asymmetric behaviour. This is most p r o n o u n c e d in the ei case, which shows a " s h o u l d e r " to the resonance that distorts the peak from approximately 600 n m upwards. This behaviour is typical o f two or m o r e resonances in close proximity, and work by Ashwell et al. [9] indicates that this is the case. T h e y suggest that there are two resonances involved in absorption in the zwitterions, one due to an intramolecular charge transfer b a n d at a r o u n d 565 rim, and the other an intermolecular band above 600 nm. It is difficult to give exact values to these transitions since they will vary according to deposition pressure; their study gives the example of a film deposited at 24 m N m ] having its principal band at 561 nm and a b r o a d shoulder above 600 nm, whereas a film deposited at 40 m N m ~ has its main absorption at 614 nm. This difference is due to the 4 0 m N m ~ film having its constituent molecules virtually perpendicular to the substrate: the absorption studies set the incident light o r t h o g o n a l to the substrate, and thus the electric vector is at right angles to the intramolecular transition moment. This means that the intramolecular dipole oscillation is not excited, and only the intermolecular resonance is seen. However, at lower deposition pressures the molecules are tilted, and therefore a c o m p o n e n t o f the intramolecular resonance, that increases as the angle o f tilt increases, is sensed by the normal incidence radiation. F o r the SPP work, the incident light is T M polarized with its electric vector not parallel to the surface, and thus both resonances are detected.
j t ,= o ~;,:, "' ':± i ,~ ,, _~: ,} 4-~ " ,x; | ~O . ::: ;,: 4 . _~ J
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Fig. 5. Superposition of experimentally determined absorption peak ( ) and evaluated data from SPP studies (thickness of film modelled as 112 + 5 nm).
Figure 5, overlaying the direct and the scaled SPP absorption peaks, shows that the SPP data do not scale accurately to the absorption data. The overriding discrepancy is in the peak heights, the SPP data showing 8% greater absorption at its zenith. This is reasonable; if the molecules have only a slight tilt, making a small angle to the normal, a tangential electric vector, as in the absorption experiment, will excite only a relatively small c o m p o n e n t of the intramolecular dipole oscillation that lies along the molecule's length. However in
•
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Fig. 6. Superposition of experimentally determined absorption peak ( ) and second model from SPP data•
C. R. Lawrence et al./ SPP studies q[" highly absorbing LB films
the SPP experiment both normal and tangential electric fields are present and there is a stronger influence of the intramolecular transition. Thus this resonance will have a greater effect on the resultant optical constants, causing the 8% difference in absorption peak maxima. There is also a shift in the resonance position between the two sets of data, which can be made more obvious if the peak heights are matched by rescaling the SPP data appropriately (Fig. 6). It is then clear that the peak maximum moves from 573 ___1 nm for the direct absorption data to 567___ 3 nm for the SPP data. As already mentioned, Ashwell et al. [9] indicate that the resonance position will vary with deposition pressure, the absorption peak wavelength increasing with the surface pressure of the film; since the films used in the SPP experiments were dipped at 23 m N m - ~, 2 m N m lower than those used in the absorption studies, the shift is as expected.
5. Conclusions By using the optical excitation of surface plasmons it has been possible to characterize a highly absorbing dielectric over a range of wavelengths. The dispersion of the optical constants is consistent with the presence of two optical resonances within the material, comparing favourably with Ashwell et al.'s work [9]. The absorption data derived from the results of the SPP studies closely model data obtained by direct absorp-
273
tion studies, with differences being caused by the differing roles of intermolecule and intramolecule dipole effects in the experiment.
Acknowledgments The authors thank Geoff Ashwell and coworkers at the Cranfield Institute of Technology for the synthesis of the zwitterionic material used, Steve Elston for his help in data acquisition and analysis, and SERC for financial support.
References 1 A. S. Martin and J. R. Sambles, Surf Sci., 225 (1990) 390396. 2 E. K r e t s c h m a n n and H. Raether, Z. NaturJorsch A, 23 (1968) 2135. 3 I. Pockrand, J. D. Swalen, R. Santo, A. Brillante and M. R. Philpon, J. Chem. Phys., 69 (9) (1978) 4001 4011. 4 G. Wahling, Z. NaturJbrsch A, 36 (1981) 588--594. 5 J. G. Gordon II and J. D. Swalen, Opt. Commun., 22 (3) (1977) 374. 6 G. J. Ashwell, Th#l SolM Films, 186 (1990) 155. 7 G. J. Ashwell. J. R. Sambles, A. S. Martin, W. G. Parker and M. Szablewski, J. Chem. Soc., Chem. Comm., 19 (1990) 1374 1376. 8 A. S. Martin and G. Bryan-Brown, personal communication, 1990. 9 G. J. AshwelL E. J. C. Dawnay, A. P. Kuczynski, M. Szablewski, I. M. Sandy, M. R. Bryce, A. M. Grainger and M. Hasan, J. Chem. Soc., Faraday Trans., 86 (1990) 1117.