Pyroelectric behaviour of Langmuir-Blodgett films containing polar chromophores

Thin Solid Films, 151 (1987) 9-15 ELECTRONICS AND OPTICS

P Y R O E L E C T R I C B E H A V I O U R OF L A N G M U I R - B L O D G E T T FILMS CONTAINING POLAR CHROMOPHORES G. W. SMITH Royal Signals and Radar Establishment, St. Andrews Road, Malvern, Wores. ( U.K.)

N. RATCLIFFEAND S. J. ROSER School of Chemistry, University of Bristol, Cantoek's Close, Bristol BS8 1TS ( U.K.)

M. F. DANIEL Royal Signals" and Radar Establishment, St. Andrews Road, Malvern, Worcs. ( U.K. )

(ReceivedJuly 7, 1986;acceptedMarch 9, 1987) The pyroelectric behaviour of Langmuir-Blodgett films containing several different polar chromophores was studied. Two distinct levels of activity are seen. By comparison with films containing no chromophores it is shown that the lower level of activity is likely to be due to a thermal expansion effect. A possible explanation for the higher level of activity is also suggested.

1. INTRODUCTION

One potential use of Langmuir-Blodgett* (LB) films is in the production of non-centrosymmetric structures for pyroelectric, piezoelectric and second-order non-linear optical applications 2 6. These structures may be produced by one of two possible methods: by the deposition of successive layers of two different materials 2' 3 or by the deposition of a single material under such conditions that a head-to-tail arrangement of molecules results 4 7. We report here the production of pyroelectric LB films produced by the first method. In particular, we deposited films of alternate layers of materials wherein one or both materials contain a polar chromophore situated part way along their length (Fig. 1). The chromophores are oriented in the opposite sense in the two materials so that when the film is assembled the polarities of successive layers add to produce an overall spontaneous polarization Ps (C m-2). The pyroelectric coefficient, p (C m 2 K - *) is simply d P J d T . We have reported 2 the production of polar films, concentrating principally on using different polar headgroups in the two layers to produce the required polarity. Here we shall concentrate mainly on structures where the headgroups of each compound are carboxylic acids, thus minimizing any contribution to P~ by the headgroups. 2. EXPERIMENTAL DETAILS

The structural formulae of the three chromophore-containing compounds are detailed in Fig. 2. The details of synthesis are to be reported separately s. The 0040-6090/87/$3.50

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Fig. 1. Schematic representation of two polar chromophore-containing compounds and their assembly to produce a polar LB film. Fig. 2. The three compounds referred to in the text: (a) 180N4A:(b) 18N04A;(c) 140'04A. c o m p o u n d s were repeatedly recrystallized and their purity monitored by thin layer c h r o m a t o g r a p h y and mass spectroscopy. Dilute solutions (0.2-1.0 mg ml 1) in Aristar ( B D H Ltd., U.K.) grade chloroform were spread on a commercial t w o - c o m p a r t m e n t trough (Joyce Loebl Ltd., U.K.) incorporating the Royal Signals and Radar Establishment gate design ~. Pressure was maintained at + 1 m N m 1 from the set transfer pressures for the materials. These were 25 m N m i for stearylamine and 30 m N m I for the other materials (Fig. 3). The subphase (original resistivity, 18 M ~ cm) contained CdC1 z at a concentration of 2.5 x 10 4 M. The p H was adjusted to 8.5 _+0.2 with dilute N a O H . Substrate speeds of 1 m m s -1 were used. 180N4A was normally transferred on the up stroke, 140'04A, 18N04A and stearylamine on the down stroke. Eicosanoic acid when used as a counter-layer could be transferred on either stroke. The substrates were microscope cover slips with an aluminium electrode stripe I cm wide. T o p electrodes (aluminium, 400 ,~) were evaporated after film deposition. Film thicknesses of 50-100 layers were used. L'O[- Surf o,ce Pressure (mNrn I )

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11

PYROELECTRIC BEHAVIOUR OF LB FILMS

Pyroelectric coefficients were measured by a charge integration technique reported earlier 2. An ambient of dry nitrogen gas was used throughout. Bilayer repeat distances were determined using a low angle diffractometer and Cu K s radiation. Dipole moment measurements were made using the method of Raynes 1°. The materials used were samples of the chromophores terminating in n-butyl chains at the 1,4 positions to avoid association effects due to the carboxylic acid headgroups. 3. RESULTS

Figure 4(a) shows the pyroelectric coefficient p vs. temperature for alternating layer structures of 180N4A, 18N04A and 140'04A with eicosanoic acid as the counter-layer. Also included is the curve for 180N4A with a stearylamine (n-octadecylamine) counter-layer 2 to demonstrate the effect of headgroup asymmetry. The sign of the pyroelectric current was the same for all four structures. Cooling produces a current in the external circuit from the bottom to the top electrode. Therefore Ps increases with decreasing temperature. Reversing the deposition order for the 180N4A/eicosanoic acid film reversed the direction of current flow as expected. Figure 4(b) shows p vs. temperature for alternating layer structures containing chromophores in each layer (Fig. 1). Also included for comparison is the curve for an alternating layer film of eicosanoic acid and stearylamine 2. This film includes no polar chromophores but does have an extreme headgroup asymmetry. It should be noted that p increases with temperature for most of the films. p

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4. DISCUSSION

4.1. Low pyroelectric coefficient films It is possible to write the total pyroelectric coefficient PT as PT = Pe + PS

11

12

o . w . SMITH et al.

a sum of primary and secondary coefficients. In this context, the primary coefficient pp arises from a change in the dipole m o m e n t per molecule or a change in the molecular orientation whereas Ps arises from the change in P~ resulting from thermal expansion of the film. It is possible to show 12 that uniform expansion along the film normal leaves P~ unchanged; therefore we need only consider expansion in the film plane. Ps for alternating layer films of acids and amines has been estimated 2 to be 4 x 10 2 C m 2. The LB film is thin c o m p a r e d with its substrate and is therefore mechanically much weaker, so it is likely that the LB film will be constrained to expand thermally at a rate determined by the substrate. The linear expansion coefficient for s o d a - l i m e glass is ~ ~ 10 5 K ~. Calculating p on this basis yields p ~ 8 x 10 v C m 2 K 1. The agreement with the value obtained experimentally is striking. A similar calculation can be made for 140'04A with an eicosanoic acid counterlayer. The dipole m o m e n t /~ of the c h r o m o p h o r e was measured as 1.20+0.03 x 10-29 C m. The area per molecule A--m-~at the deposition pressure was 2.45 x 10 19 m 2. The bilayer repeat distance for an alternating film with eicosanoic acid was 5.52 nm (Table I). Assuming that an eicosanoic acid molecule is 2.63 nm long and oriented perpendicular to the film plane and that the 140'04A was fully extended (3.30 nm from molecular models), then the 140'04A was tilted at 28.9 ~ (299 to the film normal. This is qualitatively confirmed by a polarized IR absorption measurement. The spontaneous polarization P~ is p, _ It cos 0

Vm where cos 0 is the mean cosine of the angle between the dipole m o m e n t and the film normal and Vm is the molecular volume, taking into account the volume of the inert spacer layer of fatty acid. Therefore for 140'04A/eicosanoic acid P,=7.9x10

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Again the agreement with experiment is good. A similar calculation can be made for 18N04A and gives similar results. In this case no accurate repeat distance has been determined. However, the material is sufficiently similar to 180N4A, so the assumption that the molecular volumes of the two materials are similar will not introduce a gross error in the calculation. The area per molecule for 180N4A was 2.78 x I0 19 m 2 (for 18N04A ~-m = 2.64 X 10 19 m 2) and the repeat distance of an 180N4A/eicosanoic acid film was 5.4 nm (Table I). This TABLE l BILAYER REPEAT DISTANCES EOR T W O FILM STRUCTE RES

Film structure

Bilaver repeat distance (nm)

140'04A/eicosanoicacid 180N4A/eicosanoic acid

5.52 5.40

PYROELECTRIC BEHAVIOUR OF LB FILMS

13

yields a molecular volume, including the acid counter-layer, of 1.50 × 10-27 m 3. It will be shown later that the repeat distance measurement implies a tilt of the 180N4A of 43 °. Assuming that the same is true of 18N04A then Ps = 9.3x10 3 C m 2 (/1 = 1.9 x 1 0 - 29 C m) and p ~ 1 . 9 x l 0 - V C m - 2 K -1. This is again similar to the value obtained. It therefore appears that the pyroelectric response of the low coefficient films including the acid/amine films can be explained as a secondary response determined by the thermal expansion coefficient of the substrate rather than that of the film.

4.2. High pyroelectric coefficientfilms These all contain 180N4A. The pyroelectric coefficients vary in the range ( 1 - 2 . 5 ) x 1 0 - 6 C m - 2 K -1, about 10 to 20 times greater than those of the low coefficient films. Obviously the argument used above cannot explain these results. The most likely explanation is a change in tilt of the chromophore with temperature, thus changing the projection of the molecular dipole moment on the film normal. The required change in tilt is small. X-ray data give the bilayer repeat distance of an 180N4A/eicosanoic acid film as 5.4 nm. Assuming that the eicosanoic acid is perpendicular to the film plane and fully extended yields a thickness for the 180N4A layer of 2.64 nm. The fully extended length of a single molecule of 180N4A as determined by measurement of molecular models is 3.6 nm. The simplest explanation of the 180N4A layer thickness is that the molecule is fully extended and at an angle of 43 ° to the film normal, giving Ps = 9.3 × 10 - 3 C m -2. The total measured change in Ps over the range 240-300 K was - 9 . 4 × 1 0 5 C m 2. Changing the angle of tilt to 44 ° gives P~ = - 1.52 × 10 -4 C m-2, far in excess of what is required. Replacing the eicosanoic acid layer with a layer of stearylamine gave a film with a somewhat higher pyroelectric response, increased by an amount very similar to the response of an eicosanoic acid/stearylamine film. This clearly demonstrates that the large pyroelectric response with 180N4A is not due to this material being somehow mechanically decoupled from the substrate and expanding at a much higher rate. If this were the case the effect of replacing the acid with the amine would be expected to be much larger. Two different structures containing chromophores in each layer have been made, 140'04A/180N4A and 18N04A/180N4A (Fig. 3(b)). The pyroelectric response of the former was approximately twice that of the latter. Why this should be is not immediately clear, since both structures presumably derive their pyroelectric response from 180N4A. It may simply be that changes in the counter-layers produced changes in the orientation of the 180N4A molecules. 5. CONCLUSIONS

We have demonstrated the possibility of LB film pyroelectric devices based on the incorporation of highly polar chromophores. Incorporation of the best chromophore gave an improvement by a factor of 3-5 in pyroelectric response over that attainable using asymmetric headgroups alone. Incorporation of other chromophores which were just as polar gave a much smaller response.

]4

G . W . SMITH et al.

It has been shown that the lowest level of pyroelectric response to be expected from any polar film is that resulting from its expansion at a rate determined by its substrate. It is worth noting that for many organic substances the linear expansion coefficient is 10-4 K i about ten times that of glass. If it were possible to make a free-standing LB film or perhaps to use a substrate with :~similar to that for the film then for acid/amine films p would be about 8 × 10 6 C m 2 K 1 This is still a factor of 30 less than that attainable using conventional pyroelectrics such as triglycine sulphate. It is difficult to see how a film with a significantly higher P,, and thus p, could be made. A more useful level of response appears to be available from films incorporating polar chromophores. Temperature-dependent tilting of chromophores yields a much larger value ofp than simple expansion. Simple calculations indicate that for a hypothetical molecule with/~ = 1.5 × 10 2, C m and Vm = 5 × 10- ZS m s then tilting from 0 = 0'-'to 0 = 20 ~ yields P~ = 1.8 × 10 3 C m 2. If this tilt change were to occur over a 3 0 K temperature interval then this would give an average p of 6 x l 0 5 C m 2K I about one-fifth of the value for triglycine sulphate. Any changes in volume associated with the change in tilt will obviously affect the final result. At present it is not clear why some chromophores (180N4A) increase the pyroelectric response whilst others (18N04A: 140'04A) do not. The differences between 180N4A and 140'04A are marked. The latter gives a more strongly ordered and close-packed film (X-ray studies) with the chromophores oriented nearly vertically (X-ray and IR studies). In contrast, 180N4A has lower order (X-ray and IR studies) and is less close packed (larger area per molecule). Tilting of molecules in 140'04A may therefore require a cooperative motion whilst in 180N4A the chromophores may have sufficient room to allow them to move individually. The resolution of this question and the orientation of 18N04A awaits more detailed structural studies. ACKNOWLEDGMENTS

.The authors would like to thank Mr. R. C. O. Hart and Mr. C. P. Burton for assistance in the preparation of the LB films. Thanks are also due to Dr. R. R. Richardson for assistance in making the X-ray measurements and Dr. J. W. Barton for the helpful discussions on synthetic routes. The permission of the Controller, HMSO, London, to reproduce this work is gratefully acknowledged. REFERENCES 1 K.B. Blodgen, J. Am. (~7zem. Soe., 57 (1935) 1007. 2 G . W . Smith, M. F. Daniel, J. W. Barton and N. Ratcliffe, Thin Solid Fihns, 132 (1985) 125. 3 M . F . Daniel and G. W. Smith, Mol. Cryst. Liq. Cryst. (Lett. ~, 102 (1984) 193. 4 L . M . Blinov, N. M. Davydova, V. V. kazarev and S. G. Yudin, Sor. Phys. Solid State, 24 (9) (1982) 1523. 5 L . M . Blinov, N . V . Dubinin, L.V. MikhnevandS. G. Yudin, ThinSolidFilms, 12011984) 161. 6 L.M. Blinov, L. V. Mikhnev, E. B. Sokolova and S. G. Yudin, Soz. Tech. Phys. Lett., 9 (1983) 640.

PYROELECTRIC BEHAVIOUR OF LB FILMS

15

P. Christie, G. G. Roberts and M. C. Petty, Appl. Phys. Lett., 48 (1986) 1101. N. Ratcliffe and J. Barton, to be published. M . F . Daniel, J. Dolphin, A. J. Grant, K. E. N. Kerr and G. W. Smith, Thin Solid Films, 133 (1985) 235. 10 E.P. Raynes, Mol. Cryst. Liq. Cryst. (Lett.), 1 (1985) 69. I 1 J.F. Nye, Physical Properties of Crystals, Clarendon, Oxford, 1967. 12 J.D. ZookandS. T. Liu, J. Appl. Phys.,49(1978)4604. 7 8 9