Nonlinear optical properties of several siloxane polymer Langmuir-Blodgett films

PT~CAL

OPTICALMATERIALS 1(1992) 259-265 North-Holland

Nonlinear optical properties of several siloxane polymer Langmuir—Blodgett films N. Kalita a J.P. Cresswell a M.C. Petty M.J. Goodwin C and N. Carr b

a

A. McRoberts

b

D. Lacey 12 G. Gray

b

Molecular Electronics Research Group, School of Engineering and Computer Science, University of Durham, Durham DHJ 3LE, UK Department of Chemistry, University of Hull, Hull HU6 7RX, UK GEC-Marconi Materials Technology Ltd., Caswell, Northants NNJ2 8EQ, UK

Received 15 May 1992

The Langmuir—Blodgett film formation and the nonlinear optical properties ofthree polysiloxane polymer materials have been investigated. All three compounds could be built-up as Z-type layers on a variety of substrates. Experiments on the linear electrooptic (Pockels’) effect revealed a second-ordernonlinear susceptibility x (2) (— w; w, 0) of 10.6 pmV~for one of the materials; the order ofmagnitude of this result was confirmed using second-harmonic generation. For multilayer films the second-harmonic intensity was found to increase with the thickness of the LB structure. However, a linear dependence upon film thickness was evident, rather than the expected quadratic variation.

1. Introduction The essential criteria for an organic material to exhibit large second-order optical nonlinearities are now well understood [11. Generally, the constituent molecules should possess a large difference in dipole moment between the ground and excited states and must be arranged in a noncentrosymmetric manner. The former requirement may be met by separating (electron) donor and acceptor molecular groups by a conjugated electron system. For device development it is often convenient to have the nonlinear optical material in the form of a thin (micrometre dimension) film. In the case of polymeric materials, this may be achieved using methods such as spinning, dip coating or casting. Unfortunately, these methods invariably result in a centrosymmetric arrangement of the nonlinear moieties and a preferred orientation must be built-in by an electric field poling process. An alternative approach is to use the Langmuir— Bbodgett (LB) technique to build-up noncentrosymmetric thin films on nb materials. A very large number of amphiphilic dye molecules has now been synthesised and the more successful LB monolayer and

multilayer structures studied for their nb behaviour [2]. Monolayers of many of these materials exhibit very high second-order nonlinear susceptibilities (e.g., second-harmonic d coefficients of 100’s pmV1). A major problem has always been that of building of micrometre dimension films while retaming these high nb coefficients; the molecular order is often lost as the film thickness is progressively increased. Another drawback has been the poor optical quality of most (micrometre) thick films. However, very recent work has indicated that both these difficulties may be overcome [3,51.Moreover, simple electro-optic structures based on LB layers are now being demonstrated in the laboratory [6,7]. Of course, for any commercial LB film application, it will be essential that the organic layer is stable over a period of time. There has therefore been considerable interest in the development of polymer and oligomer LB materials [21. In a previous publication the formation and nb properties of monolayers of a preformed polysiloxane polymer were reported [8]. In this work we discuss the LB film formation and the optical and nonlinear optical properties of a series of related compounds.

0925-3467/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

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2. Experimental The polymers, whose formulae are given in fig. 1, were synthesised at Hull by random substitution of chromophoric side groups onto the short (approximately 16 atoms) siboxane spine [81. This spine renders the polymer water-insoluble and acts as a substitute for the more conventional hydrocarbon chain; the siboxane backbone also results in increased structural stability of the deposited film. The alcohol head groups tend to improve monolayer orientation while the methylene spacer units between the dye moiety and the backbone permit the chromophores some degree of movement. The chromophores themselves consist of donors (the nitromethyl and ether oxygen groups) separated by a conjugated aromaticring from acceptor groups (the azo-phenyl groups). The degree of substitution was limited to 50% to avoid steric hindrance. All floating monolayer studies were undertaken on a constant-perimeter barrier trough, which has been described previously [9]. In the bulk, the materials were red tar-like substances. Each compound was dissolved in chloroform (BDH, Aristar grade, concentration 1 g1’) and spread onto a pure water subphase (obtained by reverse osmosis, deionization and UV sterilization). The subphase pH remained between 5.5 and 5.7 during the isotherm measurement and film deposition; the temperature was 18±2°C. A “suction test” consisted of compressing a film to

[ 1

[ 1 3

~ CK3

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J2

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02N AMCR22

R~—l[H2)5NC(13

AMCR23

R

AMCR24

P

Q

N~N

~H2)5 N(H3~~N°N

Q 0

CH2CH2OH

CH2CH20H

o =

~

ab =11

a°b~16

Fig. 1. Polysiloxane polymer materials studied in this work,

260

the dipping pressure and rapidly removing a small portion of the floating monolayer; if the film Femained fluid, the trough barriers responded by immediately reducing the film area (a “rapid” response). In contrast, if the floating layer was too rigid, the area remained largely unchanged on removal of part of the film. Second-harmonic generation (SHG) experiments were performed at GEC-Marconi, Caswell using a linearly-polarized Q-switched Nd-YAG laser tuned to the 1.06 ~.tmfundamental wavelength with pulse energy 7.5 mJ and duration 30 ns [8]. Samples consisted of monolayer and Z-type multilayer LB films deposited on soda or Corning 7059 glass microscope slides. The incident beam was set to 45 degrees and p-polarized with respect to the substrate normal. The intensity of the SHG signal was compared to that of a monolayer of a known material an amphiphilic hemicyanine dye which has a second-order molecular hyperpolansability (/3 coefficient) of (95 ±37) x 10~50Cm3V2 [2,101. The Pockels’, or linear electro-optic effect was observed using equipment designed at Durham based on an attenuated total reflection (ATR) cell in the Kretschmann configuration, as first used by Cross Ct al. [11]. A sample consisted of a soda glass slide (refractive index 1.52) overcoated with 50 nm of silver and the LB film on the top. An applied ac electric field (35 Vpk, 3 kHz) modulated the cell reflectivity (at 633 nm) by altering the permittivity of the polymer monolayer via the nonlinear susceptibility x (2) ( w; w, 0). This reflectivity change is most clearly observed —

~H

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November 1992

in a region where the reflectivity varies rapidly with the angle of incidence, such as near the surface plasmon resonance of the silver/LB assembly. The nonlinear susceptibility was estimated by curve fitting of the experimental results to theoretical reflectivity changes [12]. UV/visible absorption spectra of LB films were recorded by a Perkin-Elmer )~ 19 double-beam spectrophotometer using mono- or multilayer samples on soda or Corning 7059 glass microscope slides. The reference chamber remained empty, although each sample had an uncoated portion whose absorbance was recorded and subsequently used to correct the spectrum of the LB film coated area. Ellipsometric measurements were all performed on a Rudolph Research Auto EL IV nulling ellipsometer operating at

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OPTICAL MATERIALS

633 nm. Typically, each thickness was the result of an average over at least ten measurements.

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3. Results and discussion R 25

3.1. Film deposition

20~

Pressure versus area isotherms of AMCR22 and AMCR23 consisted of two distinct sections; an expanded region at large molecular areas, followed by a plateau at a surface pressure of approximately 25 mNm~.This is illustrated by fig. 2 which shows a family of AMCR23 isotherms recordedat successive half-hourly intervals (curve (a) being the first) after monolayer spreading; the compression rates were approximately 10—i nm2 (repeat unit) — ‘s —‘. In the plateau region, no areas typical of collapsed monolayers were evident on the water surface. We therefore suggest that this part of the pressure versus area curve is indicative of some sort of molecular re-arrangement. In contrast, floating monolayers of AMCR24 did not exhibit a plateau in its isotherm up to surface pressures of 40 mNm (fig. 3). The subsequent recompressions for this material were much more reproducible than those for AMCR22 and AMCR23 and the isotherm was quite similar to that previously reported for a closely related polysiboxane [8]. The monolayers of all three compounds were found to be reasonably stable, with area half-lives ‘

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30 E

~ 25 ~ 20

~

15 10 5 0

10

iS

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25 30 35 40 452) 50 Area per repeat unit nm

55

60

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Fig. 2. Pressure versus area curves for AMCR23. Temperature is 18 ±2 C, pH is 5.6, compression rate is 1 Ø3 nm2 (repeat unit) - I s~1, The different curves correspond to a series of compressions measured at 30 mm intervals ((a) first; (e) last)).

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10 15 20 25 3,0 35 40 45 5.0 55 60 65 70 75 2) Area per repeat unit mm Fig. 3. Pressure versus area curves for AMCR24. Conditions as for figure 2.

z~ 12— the time taken for the area of the floating film to halve — in excess of 11 hours (measured just below this plateau for AMCR22 and AMCR 23 and at 35 mNm~for AMCR24). Because of the very expanded nature of the isotherms shown in figs. 2 and 3, it is not particularly meaningful to measure (or quote) the area per molecule (in our case, per repeat unit) obtained by extrapolating the pressure versus area curves back to zero surface pressure. The film areas obtained for the three materials studied in this work are similar to that of the related polysiboxane material and are consistent with a molecular arrangement in which the polymer backbone is in a plane parallel to the subphase surface, with the pendant groups pointing towards the water [8]. All three materials showed a “rapid” response to films (i.e., film transfer from the water surface on the substrate up-stroke only) onto a variety of solid substrates using the LB technique; the substrates used included hydrophobic glass slides, single crystal sibthe test and could transferred as deposiZ-type iconsuction and evaporated silver.beFor the Z-type tion, the floating monolayer was expanded before each downstroke of the substrate and compressed with the substrate immersed in the subphase. The deposition onto glass are given in table 1. Fromcharacteristics the three materials studied, AMCR23 was found to produce the best quality LB films (as judged by the deposition ratio and by visual inspection). It should be noted that the closely related polysiboxane previously reported was found to deposit 261

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Table 1 Properties of floating monolayers and dipping characteristics of polysiloxane materials. Pure water subphase; temperature 18 ±2°C. 11= deposition pressure. Material

AMCR22

AMCR23

AMCR24

Mol. Wt (g. Mol) pH Suction test response

~4I30 5.6±0.1 rapid after 14 hours

~3544 5.6±0.1 rapid after 12 hours

~40l6 5.6±0.1 rapid after 4 hours

1320 20 30 Z 95 slightly patchy at start

850 21 30 Z 100 excellent

~lO000 20 30 Z 90 very patchy

30

40

10

t

112 (mm) ll(mNm’) Dip rate Qtms’) Monolayer pickup mode Transfer ratio (%) ±5 Film quality Max. no. of layers investigated

as Y-type layers; thus subtle changes in molecular structure appear to alter the deposition characteristics of these materials quite markedly [8]. 3.2. Linear optical properties Ebbipsometry of stepped structures on a previously characterized substrate is a quick and precise means of estimating film thickness and refractive index, and thus reproducibility. Samples were stepped structures of 0, 10, 20 and 30 Z-type monolayers on sibicon. The average thickness and refractive index values for AMGR22 were 1.56 nm and 1.62, respectively. Although the deposition of AMCR23 seemed very good (as judged by the deposition ratio and the optical quality), the elbipsometric experiments revealed a decreasing thickness per monolayer with increasing number of LB layers. Figures for the 10-, 20- and 30-layer films were 1.8 nm, 1.24 nm and 1.12 nm, respectively; the corresponding refractive indices were 1.92, 1.61 and 1.60. It was not possible to make a reliable measurement for AMCR24 because of the poor quality of the film deposition. The above thickness values obtained for both polymers are mostly smaller than the length of the pendant (dye) groups. This suggests some degree of tilting and/or interdigitation for the chromophores in the multilayer array, which is not inconsistent with data for obtained for other LB structures containing dye moieties [13]. (However, previous work on the rebated polysiboxane enabled a thickness of 2.2 nm to be obtained, implying the tilt angle for this material was close to zero [14].) Assuming that there is no 262

interdigitation 01 me AIv1L!c..~molecules, Lne aoove results suggest that the first 10 layers are deposited at an angle ofabout 40°to the substrate normal and thereafter at approximately 70° (taking the molecular length to be 2.3 nm). This does not appear to happen with the AMCR22 for which the tilt angle appears to be constant at approximately 47°. The only difference between AMCR22 and AMCR23 is the presence of the NO2 group in the former molecube. A possible explanation is that this group somehow fixes the tilt angle in AMCR22 multilayers, regardless offilm thickness, through cohesive, steric or rotational effects. The optical absorption spectra of the three materials all exhibited a broad absorption in the bbue/ green region of the visible spectrum, giving the layers a characteristic yellow/brown appearance. Figure 4 shows the absorption at 430 nm versus film thickness for Z-type LB layers of AMCR23 deposited onto soda glass. The full spectrum is given as the inset to this figure. The approximately linear increase in absorption with film thickness indicates reasonably reproducible LB deposition up to 30 layers (deposited on each side of the glass slide). A similar result was obtained for AMCR22 [15]. Surface plasmon resonance studies of an AMCR23 monolayer deposited onto 49 nm of silver (evaporated onto glass slide) produced the data shown in fig. 5, which is an overlay of experimental points (stars) and a fitted theoretical reflectivity curve (solid line). The above ellipsometric thickness for AMCR23 was used as a guide for this fit. The best beast squares fit was found for a thickness per layer

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OPTICAL MATERIALS

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of 1.7

A preliminary study of the nonlinear optical properties of the polysiloxane monolayer and multilayers was undertaken using the techniques of SHG and the Pockebs effect. The results of the Pockebs experiment for AMCR22 are given in fig. 6. For comparison, a curve is also shown for one monolayer of the amphiphibic hemicyanine dye. In both cases the points are experimental and the solid lines represent the best (least squares) fit to the data. The nonlinear susceptibility is given by 2~(—w; 0), 0) = ((~C~frt)/2V,

x~

06

&

that the SPR experiments produced identical thicknesses for the AMCR22 and AMCR23 compounds, in contrast to the elbipsometric results. It must be remembered that the SPR figures are obtained for monolayer samples, whereas the ellipsometric thicknesses are average values measured for 10-, 20- and 30-layer LB structures. Thus, the SPR results support our earlier suggestion that the chromophores for AMCR23 tilt further (or interdigitate) as the multibayer structure is built-up. Another important consideration is the fact that the materials are deposited onto different substrates for the ellipsometry and SPR studies; it is unlikely that the orientation of the first monolayer will be identical in both cases.

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November 1992

nm,

yielding a relative permittivity A similar procedure for AMCR22 also gave a thickness per layer of 1.7 nm and ~r 1.99+0.038i. These thickness values imply a tilt angle, with respect to the substrate normal, of approximately 41°,which compares well to the values for the AMCR22 (47°) and the first 10 AMCR23 layer (40°)discussed above. It is perhaps significant 6r=3.4~0A21

where ~ is the field-induced permittivity change, ~IC is the low frequency relative permittivity (assumed to be the same as the optical value in the calculation), the air gap 1= 10.5 jim and V is the applied (rms) voltage [111. Using the above SPR data for the curve fitting, the nonlinear susceptibility of AMCR22 was evaluated x12~(—°~ w, 0)= —0.76 1 .Oi pmV The corresponding value for AMCR23 was found to be — 10.6 + 6.79i pmV The lower figure for AMCR22 does not reflect the presence of the stronger acceptor group in this material. It is most likely associated with the relatively poor order for one monolayer of this polymer deposited onto silver. AMCR24 did not adhere well enough to the silverto produce SPR and Pockels’ curves of sufficient quality to allow reliable measurements to be made. The hemicyanine monolayer gave ~ (—w; w, 0)=67.5+35.4i pmV~which compares reasonably well to the value of 61.1 + 1 4.4i pmV mea—

.

—

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xlOW ________________________ I I I I I I 4

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lysiboxane materialoutput yieldedpower a value of 15 [8]. The second harmonic in0.transmission through a thin sample (one too thin for the fundamental and second harmonic to develop significant phase mismatch) is given by [16] d2L2I~,

I

‘2w

Hem~cyanine

i

O 4

‘2w=

-12 .

420 I

43.0 I

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450

Incident Angle

12

1111

0

~ !~AM[R22

I

x~2~( —2w w,

]

~°

ond-harmonic coefficient and n~,n2~are the refractive indices at the fundamental and the second harmonic, respectively. d is related to ~ (2w; w, 0) by 2 1~(—2w;w,0)=2d. xFor constant input intensity and ignoring dispersion

I1XIII1I

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0c (nm) n2~ where L is the film thickness, d is the effective sec-

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Fig. 6. Differential reflectivity data forLB monolayers deposited onto silver. Air gap= 10.5 tom; applied voltage is 35 Vpk, 3kHz. Crosses — experimental data; full curve — least squares theoretical fit. Top — hemicyanine monolayer; bottom — AMCR22 monolayer. A=633 nm.

0)

The above equation was used to relate the nonlinear susceptibilities of the polysiloxanes to that of the hemicyanine using the refractive indices and film .

sured originally by Cross eb ab. [11] and to 49+22i pmV~ measured on a separate occasion (on a different hemicyanine sample and using the same thickness and permittivities as reported by Cross et al. [11]) by our group [12]. The signs of the real parts of ~ (—w; w, 0) for the polysiboxane and hemicyanine materials are consistent with the expected molecular orientation of the silver: for AMCR22 and AMCR23, the acceptor groups of the molecule are expected to be adjacent to the substrate surface whereas, for the hemicyanine monolayer, the donor groups will be closest to the substrate. The measurements of SHG output power ranked the three materials approximately in terms of optical nonlinearity. The magnitudes of the second-harmonic intensities per monolayer, compared to the hemicyanine monolayer were i~i0.0l5(AMCR22), ~0.02 (AMCR23) and ~0.0l in the case of AMCR24. A previous experiment on the related p0-

. .

thicknesses obtained from the SPR data (in the case of AMCR24, for which no reliable SPR data was obtamed, the permittivity and thickness values for AMCR22 were used). The resulting ratios for 2 ~

264

=X~rnl)( —2w; w,

x \j /7~ L(h~~))~ ‘2w(hemi) L \j fl~hemj)

-8

-12

0)

2

~(—2w; w, w)/x~~,~

1(—2w; w, w) were 0.09 (AMCR22), 0.15 (AMCR23) and 0.03 (AMCR24). For AMCR23, this figure is roughly in line with that obtained from the Pockels’ experiment. Figure 7 shows how the intensity of the second harmonic varies with the number of monobayers for the three materials; in the case of AMCR24, measurements on multilayer samples were not possible due to the poor quality of the LB film deposition. As shown in the figure, the SH output for AMCR22 and AMCR23 scales approximately linearly with number of layers for the range of film thicknesses studied. For the ideal case, the SHG power should be proportionab to the square of the film thickness. However, many workers have reported less than quadratic dependence for mubtilayer LB structures, particularly for Z-type layers, of dyes; it appears that the order of the chromophores is progressively lost as the film thickness is increased [17,18]. A better approach may be to attempt to incorporate the p0lysiboxane materials in a Y-type alternate-layer struc-

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neering Research Council for supporting this work.

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0.11

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References

0.10 0.09

[I] D.S. Chemla and J. Zyss, eds., Nonlinear Optical Properties of Organic Materials and Crystals (Academic Press, New York, 1987). [2] MC. Petty, Thin Solid Films 210/211 (1992) 417.

.

0.08’ 0.07 E

006

x

0.1)5 0.04 003

~ X

0)2

x

0 01

• I

I

~=MlcR22

I

=~IcR23

I

I

S

M~CR24

I

I

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I 12345678910 Number of Monotayers Fig. 7. Second harmonic intensity (relative to one monolayer of hemicyanine dye) %ersus number of LB monolayers for polysiloxane LB materials.

[3] G. Decher, B. Tieke, C. Bosshard and P. GOnter, J. Chem. Soc. Chem. Commun. (1988) 933. [4] G.J. Ashwell, E.J.C. Dawnay, A.P. Kuczynski and P.J. Martin, Proc. SPIE Conf., Aachen (1990) 1361. [5] T.L. Penner, N.J. Armstrong, CS. Willand, J.S. Schildkraut and D.R. Robello, Proc. SPIE Conference on Nonlinear optical properties of organic materials IV, San Diego, CA (1991). [6] C. Bossard, M. Kupfer, P. Günter, C. Pasquier, S. Zahir and M. Seifert, AppI. Phys. Lett. 56 (1988) 933. [7] J.P. Cresswell, G.H. Cross, D. Bloor, Wi. Feast and MC. Petty, Thin Solid Films 210/211 (1992) 216. [8] N. Can, M.J. Goodwin, A.M. McRoberts, G.W. Gray, R. Marsden and R.M. Scrowston, Makromol. Chem., Rapid

ture

Commun. 8 (1987) 487. [9] MC. Petty and WA. Barlow, in: Langmuir—Blodgett Films, ed. G.G. Roberts (Plenum Press, New York, 1990) p. 93. [10] I.R. Girling, NA. Cade, P.V. Kolinsky, J.D. Earls, G.H. Cross andl.R. Peterson, Thin Solid Films 132 (1985)101.

4. Conclusions

[11] G.H. Cross, I.R. Girling, I.R. Peterson and N.A. Cade, Electron Lett. 22 (1986)1111; there is an error in this paper in the calculation of x (2) (— co; w, 0) — the quoted value is a factor of 2 too large; this error is perpetuated by most other workers reporting on the Pockels’ effect in LB films. [12] J. Cresswell, PhD thesis, University of Durham (1992). [13] Y.P. Song, J. Yarwood, J. Tsibouklis, W.J. Feast, J. Cresswell and MC. Petty, Langmuir 8 (1992) 262. [14] J. Brettle, N. Can, R. Glenn, M. Goodwin and C. Trundle, SPIE Proc. 824 (1987) 171.

(alternating with a complementary active mobecule or by using a passive spacer layer) or by concentrating on the development of composite systems in which the active LB layers are sandwiched between passive dip-coated polymer films [7].

A series of pobysiboxane materials have been investigated for LB film formation. Two of these, AMCR22 and AMCR23, could be built-up into Z type mubtilayer films. A monolayer of AMCR23 was found to possess a nonlinear optical susceptibility ~t2~ ( —w, w, 0)= 10.6 pmV about one fifth ofthat for the often reported hemicyanine dye compound. ~,

Acknowledgements

[15] N. Kalita, PhD thesis, University of Durham (1991).

[16] Ch. Bossard, B. Tieke, M. Seifert and P. Günter, Inst. Phys. Conf. Ser. 103 (1989) 181. [171 L.M. Hayden, B.L. Anderson, J.Y.S. Lam, B.G. Higgins, P. Stroeve and ST. Kowel, Thin Solid Films 160 (1988) 379. [18] J. Tsibouklis, J.P. Cresswell, N. Kalita, C. Pearson, P.J. Maddaford, H. Anceling, J. Yarwood, M.J. Goodwin, N. Can, W.J. Feast and MC. Petty, J. Phys. D: AppI. Phys. 22 (1989) 1608.

We should like to thank the Science and Engi-

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