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A surface Acoutic Wave Study of the Structural Properties of Langmuir-Blodgett Films
P. Koštial, J. Cirák, D. Barančok, P. Tomčik, S. Hunklinger, M. Schickfus, D.
Binz
To cite this version:
P. Koštial, J. Cirák, D. Barančok, P. Tomčik, S. Hunklinger, et al.. A surface Acoutic Wave Study of the Structural Properties of Langmuir-Blodgett Films. Journal de Physique III, EDP Sciences, 1995, 5 (3), pp.333-338. �10.1051/jp3:1995129�. �jpa-00249314�
Classification Physics Abstracts
68.35G 61.65
A Surface Acoustic Wave
Study
of the StructuralProperties
ofLangmuir-Blodgett
FilmsP. Ko§tial (~>*), J. Cir6k (~), D. Barantok (~), P. Tomtik (~), S. Hunklinger (~),
M. V. Schickfus (~) and D. Binz (~)
(~) Department of Physics, Technical University of Transport and Communications Engineering, Vel'kf Diel, 01026 iilina, Slovak Republic
(~)Department of Physics, Faculty of Electrical Engineering, Slovak Technical University, 81219 Bratislava, Slovak Republic
(~) Institute for Applied Physics, Albert flberle str. 3-5, 69120 Heidelberg, Germany (Received 9 June 1994, revised 22 November 1994, accepted 25 November 1994)
Abstract. We have investigated the propagation of surface acoustic waves in ultrathin
or-
dered organic films. For this purpose cadmium stearate Langmuir -Blodgett films
were deposited
on iZ LiNb03 crystals. By measuring the wave attenuation~ changes in the configuration of the film could be detected. The results are explained qualitatively within a microscopic model.
Introduction
The preparation of ultrathin organic monomolecular films developed by Langmuir and Blodgett [Ii attracts much attention due to its applicability in many branches of material sciences. The
Langmuir-Blodgett (LB) technique is the following: a monomolecular layer is formed at the interface between water and air and then transferred onto a solid substrate. These LB films
are excellently suited for the investigation of model structures at the molecular level.
During the last ten years a variety of physical methods have been applied in order to char- acterize the structure of the LB films and to investigate their properties. However, only a few reports on the propagation of acoustic surface waves (SAW) have been published. The
authors in [2] confirmed the considerable suitability of SAW velocity measurements for the
development of gas sensing devices. Certainly, the SAW technique together with LB structure will result in the development of a new kind of sensors fully exploiting the unique properties of
organic molecules. But a deficiency of SAW velocity measurements is that they are insensitive to subtle conformation changes of the molecules forming the LB film. On the other hand it is well known that the measurements of ultrasonic attenuation can provide such information.
(*) From October l~~until December l~~ 1993, the studies were effected in the Institute for Applied Physicsj Heidelberg, Germany
© Les Editions de Physique 1995
334 JOURNAL DE PHYSIQUE III N°3
Measurements of SAW attenuation have been applied to structural studies repeatedly and with great success [3,4].
This paper presents the results of SAW attenuation measurements. These results have been obtained by coating samples of cadmium stearate LB films on the YZ cut of LiNb03 A special arrangement originally developed for studying liquids [3] was employed. This arrangement
meets very well with the specific requirements of sample preparation by the LB method.
Experimental
The experimental arrangement is illustrated in Figure 1 [3]. The interdigital transducer (IDT)
with working frequency of 21 MHz was prepared on the polished surface of YZ LiNb03 slab.
A Matec 7700, ultrasonic transmiter/receiver was used to transmit and detect the ultrasonic pulses. The measurements of the SAW attenuation were performed with a Matec 2470 A
attenuation recorder.
The LiNb03 substrate with one IDT was fixed in the holder of the LB apparatus and moved in the vertical direction. The sample of LB film was deposited on the free end of the delay line
(see Fig. I).
The p-a- (pro analysis) purity grade stearic acid was purchased from Lachema Brno. Before
applying the LB film the surface of the substrate was cleaned by the following procedure. The samples were washed in trichlorethylene, successively sonicated in methanol, rinsed in 10 Mfl
deionized water and finally dried in N2 atmosphere.
Monomolecular films were spreaded from 0.5 mmol/I solution of stearic acid in chloroform onto bidistilled water into which CdC12 was dissolved at concentration of I mmol/I. For the LB deposition a computer~controled film balance of Nima Technology was used. The film
was transfered at a surface presure of 25 mN/m and at vertical dipping rate of10 mm/min.
The subphase temperature was 22 °C. The prepared films contained 21 monolayers which corresponds to the thickness of approximately 50 nm.
absorber DT
7700 2~70A
[motion of the sample
LB film of stearic acid
steariz acid
@fifififi
j~~i water
-q=-==_=
Fig. I. Experimental arrangement demonstrating SAW delay line with
one interdigital transducer
prepared on the surface YZ LiNb03 with the cadmium stearate LB film deposited on the free end of the delay line. The arrow show the movement of the delay line in the process of the LB film
preparation.
Results and Discussion
The temperature dependence of the SAW attenuation for virgin LB films is plotted in Figure
2. Two significant attenuation changes can be detected, the first being situated near 40 °C and second at about 60 °C. The second attenuation step corresponds to a melting of crystalline
stearic acid which can also be detected by Diiferential Thermal Analysis (DTA) as shown by
endothermic peak at the same temperature (insert in Fig. 2). In Figure 3 the irreversibility of above mentioned structural changes is demonstrated by the attenuation in two consecutive
attenuation cycles. In both cycles the temperature is increased from 20 °C to 70 °C at rate I K/min. Each successive measurements cycle shows a continuous attenuation increase with
an increasing amount of stochastic features. The increase of the slope between 20 °C and 40 °C with a number of measuring cycles is shown in the insert of Figure 3.
The penetration of SAWS into the material investigated depends on as product of the wave number k and the thickness of the layer h [5]. The larger value of the product, the larger
interaction volume and consequently the more acoustic energy is travelling at the film. In
our case a value of this product is about 0.004; therefore major part of the SAW energy is confined to the substrate. Therefore the SAW traveling at the free end of the sample and back under LB film must be extremely sensitive to any structural transformation of the film, such
as monitored by the SAW attenuation.
The SAW attenuation is strongly influenced by the viscosity of the medium [7]. For a leaky SAW the following relation for the SAW attenuation was derived [7]
20 30 40 50 60 70 80
§~ TPCJ
3
20 30 40 50 60 70 T1°C7
Fig. 2. The temperature dependence of SAW attenuation for virgin LB film of stearic acid salts.
Insert shows the DTA scan of crystalline stearic acid.
336 JOURNAL DE PHYSIQUE III N°3
a = A w~/~ l (n s)~/~ [dB], (I)
A is the constant, is the wave-liquid interaction length, w is the angular frequency, n is the viscosity, s is density. In [3,8] it is shown that an almost linear dependence of SAW attenuation
on the viscosity is to be expected for Rayleigh modes.
Our measurements of the SAW attenuation in LB films display that the SAW temperature dependence has an almost linear form. The LB films used in the measurements are dielectric
materials. It can therefore be assumed that the observed change in the SAW attenuation is mainly due to changes in the viscosity of the film. From Figure 3 one can see that the
temperature dependence of the attenuation (viscosity) increases after each annealing cycle.
Unfortunately, the absolute value of the SAW attenuation could not be measured.
Presumably the increase in temperature, I-e- a supply of thermal energy to the hydrocarbon
chain will interrupt its straight (all-trans) configuration at a certain point and produce either
an isolated gauche rotation or coupled rotations (kink).
The hydrophylic part of the molecule remains unchanged and is arranged in a two-dimensional lattice within the layer. The melting or disordering of the chain configurations in the direction
perpendicular to the plane of the layer (transverse order) is coupled to the disordering of the lattice in the monolayer plane (lateral order). These interruptions of the chain configuration resulting in the main phase transition of the monolayer (see Fig. 4) cause a shortening of the
average chain length as well as an increase in the cross-sectional area. The main transition point depends on the chain length and is situated at 63 °C [9] in the case of a hydrocarbon
chain consisting of 16 carbon atoms. These processes are likely to have a major influence on
~ so #1°cJ
2 number at annael,ng cycles
~
&J 1
x rn
« Lw
2a
Fig. 3. The temperature dependence of the SAW attenuation in the same sample after one and two annealing cycle up to 70 °C. Insert presents a plot of the curve slope taken from the temperature
interval between 20 and 40 °C (tg a) versus number of annealing cycles.
jai ib) (cl
Fig. 4. a) Structure of a stearic acid molecule standing perpendicularly to the air-water interface.
b) Schematic representation of CdSt2 bilayers at temperature below 62 °C. c) Schematic representation
of CdSt2 bilayers at temperature above 62 °C. The carboxyl headgroup with an attached Cd~+ ion is represented either by a straight line (for an all-trans chains) or by a bent line (for a chain containing
a gauche).
the SAW attenuation. The changes observed when the multilayers are heated to a temperature above the main phase transition are irreversible.
When the temperature is decreased the alkyl chains remain trapped in a supercoled state even though the free energy strongly favours an all-trans conformations [10]. The increase in
attenuation (viscosity) detected by SAW attenuation shows that the motion of the chains must be highly hindered. It therefore takes a very long time to reach a thermodynamic equilibrium
and a corresponding release of the entangled chains. Therefore the subsequent scans did not reveal the main transition change.
The trans-gauche isomerization is likely to induce defects in the two-dimensional arrangement in the plane of the layer or even to create domains of different size [iii. This process will
strongly affect the viscous SAW loss as described above (viscosity and SAW attenuation) and it takes a very long time to reach a thermodynamic equilibrium and a corresponding release of the entangled chains. Therefore the subsequent scans did not reveal the main transition
change.
The trans-gauche isomerization is likely to induce defects in two-dimensional arrangement in the plane of the layer or even create domains of different size ii Ii. This process will strongly
affects the "viscous" SAW loss as described above.
Conclusions
Measurements with surface acoustic waves result in interesting information about the LB films structure and especialy the changes induced by thermal cycling. With an analysis based on the interaction of acoustic surface waves with a film that is liquid-like in the plane of the substrate,
we found a consistent interpretation of experimental results. Our experiment shows that the SAW technique is a promissing tool for the structural investigations of Langmuir-Blodgett
films.
338 JOURNAL DE PHYSIQUE III N°3
This work was supported by grants of Slovak ministry of the education and science No
1/312/92 and 1/990176/93.
Acknowledgments
The authors wish to thank to Mr Franti§ek isemobila for the help with experiments and for stimulating discussions.
References
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