Thin Solid Films, 210/211 (1992) 231-233 231

Surface coverage by adsorption or Langmuir-Blodgett technique: a comparative study using second harmonic generation

M. Pinnow, G. Marowsky, F. Sieverdes and D. M6bius Abteilung Laserphysik, Max-Planck-lnstitut ffir Biophysikalische Chemie, Postfach 28 41, D-3400 G6ttingen (Germany)

C. Kr6hnke and D. Neuschfifer CIBA Geigy AG, CH-Basel (Switzerland)

Abstract

Optical second harmonic generation was used to compare the molecular orientation in Langmuir-Blodgett and adsorbate films of hemicyanine molecules at CaF2-substrate. Small variations of the coverage density in LB films result in a change of the tilt angle of the molecules in the monolayer. The adsorbate films are built up by a stack of molecules with large variation of the average tilt angle depending on the concentration of the wetting solution.

I. Introduction

Optical second harmonic generation (SHG) has re- cently been developed into a very sensitive tool for surface and interface studies [1]. Symmetry require- ments restrict SHG to noncentrosymmetric structures [2]. Hence, SH signals are either due to the surface itself (where inversion symmetry is necessarily broken), or the near-surface quadrupolar effects, or to thin film coverages [3]. Molecules of known high second-order polarizability permit the study of thin films irrespective of the origin of the substrate susceptibilities. Hence the SHG technique can be used to study the influence of different preparation conditions of the thin film struc- tures.

P1 X/2

LQser tp

/ C18H37 ® N

H 3 C - N ~ ~C18N37

13= 2.5 * 10- 27esu

~ P 2 {q4

Fig. 1. Experimental setup together with the chemical structure of the hemicyanine dye used for adsorbates and LB films.

2. Preparation and experimental setup

All experiments were performed with CaFz prisms covered with hemicyanine molecules (Fig. 1), by either the Langmuir-Blodget t (LB) technique or by simple wetting. Adsorbate layers were prepared by wetting the hypotenuse of the prism under reproducible conditions (wetting speed 5 mm/min) in a well-defined solution and subsequent evaporation of the solvent. The wetting technique permits the use of a wide concentration range of hemicyanine in chloroform from 10 -8 to 10 -2 M. The LB monolayers were prepared by spreading a 10-3 M solution of hemicyanine in chloroform on pure water. The surface potent ia l -area curve indicates the orientation of the molecules starting at a surface cover- age of 1.23nmZ/molecule, while an increase of the

surface pressure is detectable only at values smaller than 0.8 nm2/molecule (Fig, 2). The LB films were deposited hydrophilically onto the prism hypotenuse at different surface pressures with a dipping speed of 5 mm/min.

Figure 1 shows experimental details of the setup used for SHG measurements. The fundamental of a N d : Y A G laser first passes a fixed polarizer P1 and the rotable 2/2-plate and is then slightly focused onto the prism at an angle of incidence of ~ = 50 ° to permit total reflection. The SH radiation generated in the adsorbate is sent through a second polarizer P2 and finally recorded by a monochromator-photomultiplier combination after additional rejection of the fundamental by appropriate filters. S- and p-polarized signals were selected by adjust- ing the polarizer P2. The photomultiplier signals .were

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2 3 2 M. Pinnow et al. / Surface cooerage by adsorption or LB technique

60 - 2.0

~ 50 z IE 40 8 O9 O9

\

3 0 -

2 0 -

10-

o

-1.5

1.0

t~

E

0 & .

t~

0.5 o9

L D.0 I I I

0.5 1.0 1.5 Area / Molecule [nm2]

Fig. 2. Surface pressure (it) and surface potential (AV) area curves for hemicyanine at pure water.

sampled by a boxcar integrator and processed by a computer. The total reflection geometry was preferred since its SH signals are at least one order of magnitude larger compared with the signal level of any other geometry [4, 5].

3. Theory

The second-order nonlinear polarization responsible for SH emission from a polarization sheet represented by a thin film [6] can be calculated from

P(2co) = Z(2):E(~o)E(~o) (1)

Following the usual convention, g ~2) is defined with respect to the electric field taken just inside the film carrying linear substrate [7]. For experiments employ- ing rotations of optical components such as 2/2-plates, it is convenient to rearrange the Cartesian field compo- nents of the incoming fundamental E(og) = {E,-, E,., E__ } in terms of only two components Ep{E~.,E__} and E~ { E,' } where

Ep = E(~o) cos 2~p (2a)

E~ = E(~o) sin 2q~ (2b)

The angle ~o denotes the direction of the fundamental with respect to the p-direction. For adsorbates exhibit- ing isotropy around the surface normal ( i .e . the z-direc- tion) the Z(2)-susceptibility tensor reduces to the following five non-vanishing components: ,~_,:,v (2) Z:;,y,(2) zt2) v (2~ and Z!,.2,~.. Assuming a rod-like structure, with ,TX.X-, /~ y~V '

the molecular hyperpolarizability fl(:) orientated along the axis of the molecule, the susceptibility components Z ( 2 ) 0k can be expressed by an average coverage density N, and tilt angle ~ with respect to the surface normal [8, 9].

-4 d

3

g

T "1- 09

i

0 90 i

180 2-/0 Rotation Angle qo

mN/m ~, -35.8

+ -9.8

+

~ -2.0

~ -0.1

360

Fig. 3. Rotation patterns l~,p(2(o) representing SH response of LB films deposited at different surface pressures. (The curves are shifted along the /p-axis at arbitrary units.)

Introducing the geometry-dependent prefactors F0k, incorporating both, linear and nonlinear Fresnel factors as discussed in detail in ref. 4 into the susceptibilities 2(2) p-polarized SH intensity will be denoted by /jk,

I~.p(2~o) = r (2) 2 (2) 2 ~Z_-,., (,9)Ep + Z->:,.(0)E~

12) 2 + Z---(9)Ep + Z!,.2_-!,.(~)E2 } 2 (3a)

= E4{A c0s22~0 + B sin22~o} 2 (3b)

A = z~.),. + z ~ ' + ~(~) 8 = z(=,~ (3c) 22-- - - A A'Z.V ~ z ' '

(2) ~,(2) . F~ k (3d) Ok ~ ,~ ijk

AS has been discussed in ref. 4, the set of prefactors F, Tk turns out to be complex in total reflection geometry. We have inserted all the necessary data concerning the substrate material CaF 2 and an angle of incidence

= 50 ° for the fundamental into eqn. (3) and consid- ered the average tilt angle 6~ as the variable. The shape of the resulting rotation patterns is quite different, depending on the actual value for 0. The rotation patterns predicted according to eqn. (3a) are in good agreement with experimental rotation patterns as shown in Fig. 3.

This behavior indicates the high sensitivity of the film preparation technique and hence the resulting orienta- tion of molecules for the determination of Z C2) tensor components. According to the angular dependence of

(2) Z0k, their ratio can be directly interpreted in terms of a molecular tilt angle

( 2 ) / y ( 2 ) I .... y . . . . . = ~ tan23 . Em((,0 ) -2 (4)

Equation (4) incorporates a necessary relative suscepti- bility value Em (~o) in order to correctly apply the polar- ization-sheet model for surface SH emission as discussed in refs. 6, 10. A value Em(~O) = 2.5 has been taken from ref. I1 to account for an angle-of-incidence independent set of Z (2) susceptibility components [12].

M. Pinnow et al. / Surface coverage by adsorption or LB technique 233

4. Results and discussion

The LB films were deposited onto the prism sub- strates at surface pressures of 35.8, 9.8, 2.0 and 0.1 mN/ m, corresponding to an area covered by one molecule of 0.52, 0.56, 0.62 and 0.68 nm 2, respectively. With decreasing surface pressure the ratio I¢,p (~o = 45°)/I~,p (tp = 0 °) increases from nearly 0.5 to 1.0 (Fig. 3) indi- cating an increase of the average tilt angle O from 31.7 ° to 39.1 ° following an evaluation of eqn. (4).

The good agreement between theoretical prediction according to eqn. (3a) and the measured data points indicates the achievable precision in the total reflection geometry and the absence of any anisotropic Z~2~-tensor components. For LB films prepared from hemicyanine dyes, no background contribution from the prism mate- rial was discernible. In fact, we estimate a susceptibility component g ~ as large as 10-t3esu practically inde- pendent of the observed small variations in coverage density. The hemicyanine adsorbates produced by the wetting technique with 10-~8-10-2M solutions show quite different behavior of the SH intensities and varia- tion of the tilt angle ~9, suggesting a subdivision of the whole concentration range:

(i) 10-8-10 -6 M: the SH signal is governed mainly by the p-polarized background contribution. No s-po- larized SH signal can be identified.

(ii) 10-6-6 × l0 -4 M: both s-polarized and p-polar- ized SH signals are present. They increase approximately with the square of the surface coverage density N s. The coverage density corresponding to the chemisorption of a full monolayer required a 3 × l0 -5 M concentration of the wetting solution. The SH signals of the LB film are more than two orders of magnitude larger than the corresponding signals of the adsorbates.

(iii) 6 x 10-4-10 -2 M: there is only a slight increase of the SH signals with concentration, indicating uncon- trolled, unpolar stacking of the adsorbed molecules.

The clean CaF2 apparently offers a few selected sites for chemisorption with a unique polar ordering of the surface-integrated dye molecules. Their polar heads point away from the bulk. In addition the clean surface imposes a preferential direction on the molecular align- ment, giving rise to a perfect polar ordering parallel to the surface normal. At l 0 -6 M dye concentration all preferred sites are occupied and additional molecules tend to absorb more or less irregularly with 9 = 51 ° and reversed polarity. These molecules tend to have their hydrophilic polar groups attached in direction to the substrate and their hydrophobic hydrocarbon tails pro- trude up into the air. This directional arrangement has been identified by phase measurements [13, 14] and by

comparison with the well-known orientational behavior of LB films.

The fact that the p-excited p-polarized SH signals dominate again for high hemicyanine concentrations (range (iii)) indicates that the tilt angle has decreased (~ = 35°). One should, however, bear in mind that for adsorbate coverage, densities exceeding the absorption of a full monolayer, as obtained from dipping concen- trations exceeding 10 -4 M , the adsorption saturates as discussed in detail in ref. 15 and the tilt angle evalua- tion merely indicates an effective susceptibility ratio of the bulk-type adsorbate probed by the evanescent fun- damental wave.

Hence, it can be concluded that the SHG technique unravels small variations of the coverage density of LB films and the resulting changes of the tilt angle of the hemicyanine molecules in contrast to the constant tilt angle in polymeric monolayers [16]. The change of the tilt angle could be observed, when the magnitude of the SH intensity and therefore the susceptibility does not change significantly. The adsorbates produced by the wetting technique cannot be considered to be of monomolecular structure. They are built up by a stack of molecules with a large variation of the average tilt angle depending on the concentration of the wetting solution.

References

1 Y. R. Shen, Nature, 337(1989) 519. 2 Y. R. Shen, The Principles of Nonlinear Optics, Wiley, New York,

1984. 3 J. E. Sipe, D. J. Moss and H. M. van Driel, Phys. Rev. B, 35

(1987) 1129. 4 F. Sieverdes, G. Liipke, G. Marowsky, A. Bratz and U. Felderhof,

NA TO A SI Series E, Applied Sciences, 194 (1990) 137. 5 V. Mizrahi and J. E. Sipe, J. Opt. Soc. Am. B, 5(1988)660. 6 B. U. Felderhof and G. Marowsky, Appl. Phys. B, 44 (1987) 11. 7 J. E. Sipe, V. Mizrahi and G. I. Stegeman, Phys. Rev. B, 35(1987)

9091. 8 G. Marowsky, R. Steinhoff, D. Erdmann and D. Dorsch, Opt.

Commun., 63 (1987) 109. 9 T. F. Heinz, H. W. K. Tom and Y. R. Shen, Phys. Rev. A, 28

(1983) 1883. 10 B. U. Felderhof and G. Marowsky, Appl. Phys. B, 43 (1987) 161. I1 B. Mann and H. Kuhn, J. Appl. Phys., 42(1971) 4398. 12 F. Sieverdese, M. Pinnow and G. Marowsky, Appl. Phys. B53

(1992) 2255~ 13 K. Kemnitz, K. Bhattacharyya, J. M. Hicks, G. R. Pinto, K. B.

Eisenthal and T. F. Heinz, Chem. Phys. Lett., 131 (1986) 285. 14 G. Berkovic, Y. R. Shen, G. Marowsky and R. Steinhoff, J. Opt.

Soc. Am. B, 6(1989) 205. 15 G. Marowsky, L. F. Chi, D. M6bius, R. Steinhoff, Y. R. Shen, D.

Dorsch and B. Rieger, Chem. Phys. Lett., 147(1988)420. 16 G. L. Gaines, Langmuir, 7 (1991) 834.