MOLECULAR ELECTRONIC DEVICES

Calix( 8) arene LB super latt ices: pyroelectric molecular baskets

T. Richardson M.B. Greenwood F. Davis C.J.M.Stirling

Indexing terms: Electron device.c, Supevlattices, Langmuir-Blodgett assemblies

Abstract: Carboxyl and amino groups have been inserted into calix(8)arene rings to yield two materials which, when processed as alternate- layer Langmuir-Blodgett assemblies, exhibit the pyroelectric effect. Such a temperature-dependent electric polarisation arises partially from proton transfer between the acid and amine groups. The calix(8)arene system has been selected because of the extreme thinness of each monolayer (-1 .Onm), the resulting high density of acid-amine pairs and the very high thermal stability exhibited by calixarenes (typically, melting point above 2OOOC).

1 Introduction

The pyroelectric effect [ 11 is the manifestation of the temperature-dependent electric polarisation which exists in certain materials possessing a non-centrosym- metric structure. When electrodes deposited on either side of a slab of a pyroelectric material are short- circuited, an electrical current is observed to flow whenever the rate of change of temperature (dTidt) is finite. Thus, a current is generated only when the tem- perature is changing and no current is observed at con- stant temperature. The need for non-centrosymmetry means that a deposition technique is required which allows molecules to be arranged in a polar structure. One method of achieving this, which aids the investiga- tion of the pyroelectric effect at the nanoscale level. is the alternate layer Langmuir-Blodgett deposition (ALLBD) technique. By alternately depositing floating monolayers of two dipolar materials, A and B, this technique yields an ABABA.. sequence. Thus, the polarity associated with each AB bilayer can be main- tained throughout the multilayer film.

The pyroelectric properties of scveral alternate-layer LB film systems have been studied previously [2-41.

0 IEE, 1997 IEE Proceedings online no. 19971027 Paper received 8th July 1996 T. Richardson and M.B. Greenwood are with the Centre for Molecukdr Mdterials and the Department of Physics, University of Sheffield, Hicks Building, Hounsfield Road, Sheffield S 3 7RH, UK F. Davis and C.J.M. Stirling are with the Centre for Molecular Materials and the Department of' chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield S3 7HF, UK

Most of these have included molecules possessing car- boxylic acid and primary amine groups substituted onto the aliphatic or aromatic chains belonging to either monomer [2, 31 or polymer systems [4]. The car- boxyl groups from monolayer A donate protons to the amino groups from monolayer B, creating a large dipole moment. The degree of proton transfer is tem- perature-dependent and thus provides the pyroelectric mechanism.

In this work, two 4-tert-octylcalix(8)arenes, shown in Fig. 1, have been substituted with carboxyl (1) and amino (2) groups, respectively, to obtain two amphiphilic materials which can be co-deposited via the ALLBD technique. Calixarenes and calixresorcin- arenes are known as molecular baskets due to their ability to complex a range of chemical species [SI. Their use, in this study, arises from the desire to probe the pyroelectric acid-amine interactions within the novel environment of the calix ring, and to develop novel pyroelectric materials whose thermal stability [SI is higher than previously studied materials. To the best of our knowledge, this is the first time that alternate-layer LB multilayers containing two sequentially deposited calixarene monolayers have been produced and the first time that pyroelectric calixarenes have been reported.

2 Method

The synthesis of the calixarenes studied here (Fig. 1) has been described in detail elsewhere [6].

cal ix1 R cal ix 2 R

CH2 CO2 H Fig. 1 R = -C(CH3)2CHSUB2C(CH,),

Culixurenes used in formation of alternate-layer superlattices

Chloroform solutions of each calix(8)arene were spread onto the pure water (Elga UHP system) sub- phase of a constant perimeter Langmuir trough. After evaporation of the solvent, each monolayer was com- pressed at a rate of -l%s-'. An alternate-layer Lang- muir trough possessing a central fixed barrier separating the two spreading zones was used to prepare the calix-licalix-2 (1/2/1/2/1 ....) multilayer LB films.

The substrate (Merck microscope glass slide coated with a 50nm aluminium film) was held by a damp

IEE Proc -Circuztr Device., Syst , Vol 144, No 2, April 1997 108

attached to part of the fixed central barrier, which was able to rotate around the barrier axis and, thus, be exposed to each cahxarene monolayer in an alternating sequence. The deposition surface pressures used were 25m'Vm for both monolayers and the rate of with- drawal was -20mmimin for the upstroke through the calix 1 monolayer and -80mmimin for the downstroke through the calix 2 monolayer.

The samples prepared contained 15 monolayers in total (8 monolayers of calix I alternately deposited with 7 monolayers of calix 2). A Perkin-Elmer 1725X infra-red spectrometer was used to measure the infra- red spectra of the deposited LB films (RAIRS). The uncoated substrate was used as a reference.

The quasistatic pyroelectric assessment technique [4] was atdopted for the characterisation of the pyroelectric behaviour of the calix(8)arene assemblies. This method involves the linear heating and cooling of the sample about a mean temperature which could be varied from -290- 330K. The frequency and amplitude of this saw- tooth temperature cycle was 0.01-0.02Hz and -0.75K, respectively. The current generated as a result of the pyroelectric activity of the sample was measured using a Keithley Instruments 614 electrometer whose ana- logue output could be fed into a chart recorder or a personal computer (via an A/D convertor).

0 16 -

3 Results and discussion

I U1

Y -- 0 , I 3 B + U

0 12-

The detailed analysis of the surface pressureiarea iso- therms of calixarenes 1 and 2 is described elsewhere [6]. Fig. 2 shows the FTIR spectrum for the 15-layer alter- nate layer LB film (RAIRS). The main peaks are iden- tified in the Figure. The presence of two carbonyl peaks at 1717cm-' and 1600cm-' suggests (by compari- son with cast films of 1 and its sodium salt) that proton transfer has occurred to a large but not complete extent. Such proton transfer leads to a large dipole moment. The degree of proton transfer throughout the film is, known to depend on temperature and to provide a possible mechanism for pyroelectricity [2].

-

oc'lol ! 0

a" 2 -

1 -

d

3200 2600 2000 1600 1200 wavenumber V , cm-I

Fig. 2 U C-H (stretch), h C = 0 (acid). c C = 0 (salt), d C-H (bend)

FTIR (RAIRS) pe trun? jor calix-l/culix-2 .ruperlaltice

Theoretically, the peak-to-peak current Aip should follow the relationship A$, = FA ( d T / d ) , where r is the pyroelectric coefficient, A is the electrode area and dTldt is the total (heating and cooling) rate of change of temlperature. Fig. 3u depicts the relationship between Ai,, and dTldt (the sum of the magnitudes of the heating and cooling rates). The linear function confirms that the phenomenon is indeed pyroelectricity. The gradient of this graph yields the pyroelectric coefficient.

IEE Proc.-Circuits Devices Sys . , Vol. 144, No. 2. April I997

- 0 O I L I 5 t

0 . I l l

0 10 i 0.09 L

5

I I

/ /

/ /$

l ; , , , , ,

7 8 9 10 11 6 pyroelectric current, p A

a

,

30 t,s 60

b Fig. 3 U dTldt against Ai for calixarcne pyroelectric device h Current and tedperature profiles for the device

@ix(8)arene II-layer pyroelectric mponse at 25°C

P

01 20 25 30 35 LO L5 50 55

temperat u re ,T Fi .4 Variation o j pyroeleclric activity ~. i t l i tenTyerature fbr l l - layu cafxi8,arene jilnz

The current generated in response to the sawtooth tem- perature cycle imposed on the sample (296K) is shown in Fig. 3h. The current response is a square wave as expected from theory. Fig. 4 shows how the pyroelec- tric coefficient varies as a function of temperature over the range 293-328K. The values obtained ( - 6 ~ C m - ~ K-') are very high when compared to long-chain fatty acid/fatty amine pyroelectric LB films described in the literature [2]. This is probably a result of the higher

109

packing density of the pyroelectric acidlamine pairs within the calix(8)arene system compared to that found in the long-chain monomer system which results from the extreme thinness of the calix(8)arene monolayers (-1 .Onm per monolayer).

4 Conclusion

Calix(8)arene alternate-layer LB assemblies have been prepared which yield a strong pyroelectric effect caus- ing by the inclusion of carboxyl and amino groups attached within the calix(8)arene rings of adjacent monolayers. Proton transfer between the carboxyl and amino groups has been identified using Fourier trans- form infra-red spectroscopy. The degree of proton transfer is known to be dependent on temperature [2], suggesting that this mechanism provides the origin of the pyroelectric effect in these materials. The extremely high pyroelectric coefficient (for LB systems) coupled to the high thermal stability of calixarenes makes these

materials exciting candidates for future heat sensing devices.

5 References

1 PETTY, M.C. (Ed.): ‘An introduction to molecular electronics’ (Edward Arnold, London, 1995), Chap. 3

2 CHRISTIE, P., JONES, CA., ROBERTS, G.G., and PETTY, M.C.: ‘Dynamic pyroelectric response of Langmuir-Blodgett film infrared detectors’, J. Phys. D, 1986, 19, pp. L1677L172

3 RICHARDSON, T., TOPACLI, A., MA~ID, w.H.A., GREEN- WOOD, M.B., BRUCE, D.W., THORNTON, A., and MARSDEN, R.: ‘Langmuir-Blodgett films of stilbazole com- plexed of iridium(1) and rhodium(I)’, Adv. Mat. Opt. Electron., 1994,4, pp. 242-251

4 RICHARDSON. T.. MAJID. W.H.A.. CAPAN. R.. LACEY. D., and HOLDER, S.: ‘Molecular engineering of pyroelectric polysiloxane LB superlattices’, Supramolecular Science, 1994, 1, (11, PP. 39

5 GUTSCHE, C.D.: ‘Calixarenes’. Royal Society of chemistry, Monographs in Supramolecular Chemistry, 1989

6 RICHARDSON, T., DAVIS, F., and STIRLING, CJ.M.: ‘Pyro- electric molecular baskets: Temperature-dependent polarisation from substituted calix(8)arene LB films’, Langmuir, 1995, 11, (12), pp. 46234625

110 IEE Proc.-Circuits Devices Sysl., Vol. 144, No. 2, April 1997