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Ultrasonic transducer action of Langmuir-Blodgett films
Thin Solid Films, 99 (1983) 271 275
271
U L T R A S O N I C T R A N S D U C E R A C T I O N OF L A N G M U I R - B L O D G E T T FILMS* J. R. DRABBLE AND S. M. AL-KHOWAILDI
Department o/'Ph.l,sics, University of l£xeter, Exeter, Devon (Gt. Britain ) (Received September 1, 1982; accepted September 22, 1982)
A Langmuir-Blodgett film was deposited onto one face of a Duralumin block and an aluminium film was evaporated onto this to form a capacitor structure. This was connected to a charge amplifier. A quartz transducer bonded onto the opposite face of the block provided a source of longitudinally polarized ultrasound. When the capacitor structure was biased with a d.c. voltage, the output of the charge amplifier showed well-defined peaks corresponding in all respects to the expected arrival of ultrasonic pulses at the structure. Possible reasons for the effect and some associated problems are discussed.
I. INTRODUCTION
The interpretation of measurements of the electrical properties of LangmuirBlodgett films is often complicated by the presence of layers of metal oxide associated with contacts. In, for example, capacitance t or current-voltage 2 studies these layers give contributions to the measurements which have to be allowed for to arrive at the properties of the films themselves. In the present study, we investigated an electrical response associated with the Langmuir-Blodgett film arising from ultrasonic signals arriving at the film from a distant source. The initial significant result is that such a response exists in the presence of a film and is absent in control experiments using as closely as possible an identical arrangement without the film. Thus the effect seems to arise from the film itself and offers the prospect that the technique used can be developed to provide new methods of studying the properties of such films, as well as the possibility of some practical applications. The effect was studied by applying a steady d.c. bias across the film and associated contacts--in effect treating it as a capacitor. The most likely explanation at the present time is in terms of a change in the capacitance as a result of the arrival of the longitudinally polarized ultrasonic signal, this being brought about either by
*Paper presented at the First International Conference on Langmuir-Blodgett Films, Durham, Gt. Britain, September20-22, 1982. 0040-6090/83/0000-0000/$03.00
© ElsevierSequoia/Printed in The Netherlands
272
J. R. I)RABBI.E, S. M. A L - K H O W A I I D I
mechanical compression of the film or a change in electrical polarization, or a combination of these, thus leading to a change in the charge on the capacitor. 2.
E X P E R I M E N T A L DETAILS
A diagram of the specimen and holder used is shown in Fig. 1. A block of Duralumin in the shape of a cube with linear dimensions of approximately 2 cm had two opposing faces which were ground flat and parallel and subsequently polished to an optical finish. An X-cut quartz transducer was acoustically bonded to one of these faces and was excited with a short electrical pulse. It is well known that this arrangement resul.ts in a short pulse of ultrasound which propagates from the transducer through the Duralumin and, owing to partial reflections at the two faces, gives a characteristic echo pattern in which pulses of ultrasound arrive at the opposite face at times T, 3T, 5Tetc., where Tis the acoustic transit time across the block. On the opposite face was a Langmuir-Blodgett film consisting of several monomolecular layers deposited from a stearic acid film grown on a subphase of water. An aluminium film of thickness 5000 A was evaporated onto the top surface of this film and electrical contact to this was made via a mercury drop at the end of a narrow tube. This contact was taken to the input of a sensitive charge amplifier (type EMI 6307) which had a built-in provision for applying a steady d.c. potential difference between the mercury contact and the Duralumin block, i.e. across the Langmuir Blodgett film. The output of the charge amplifier after amplil]cation was displayed on an oscilloscope, triggered by the pulse applied to the quartz transducer. The stearic acid monomolecular layers were prepared in a standard way 3"4 on the surface of freshly distilled water using a polytetrafluoroethylene thread and oleic acid to control the surface pressure. We were unable to lind any references in the literature to the transfer of such films to the surfaces of the bulk solid specimens but, as judged from the motion of the piston thread and the shedding of water as the specimen was raised, transfer to the surface of the Duralumin block took place with no obvious difficulty using a mechanical pulling arrangement at a speed of 3 mm min 1. The whole process was carried out in a dust-free atmosphere provided by a laminar flow cabinet and the Langmuir-Blodgett trough was insulated from mechanical vibrations. After transfer of the appropriate number of layers, the specimens were stored in a dry atmosphere before evaporation of the aluminium electrode and bonding of the quartz transducer on the opposite face. 3. RESULTS When a d.c. voltage was applied across the film, the oscilloscope display showed a set of well-defined peaks whose positions on the trace corresponded in all respects to the expected arrival of acoustic pulses at the face of the Duralumin block onto which the Langmuir-Blodgett film had been deposited. The magnitude of these peaks depended on the value of the d.c. voltage and became zero as this voltage was reduced to zero. Figure 2 shows one of the traces obtained and Fig. 3 shows the first part of this on an expanded time scale. These particular traces were obtained with a five-layer
273
ULTRASONIC TRANSDUCER ACTION
A
I~XXX\\~~k~%'l[~] ~/~
-
TOchargeamplifier
--Ogr;;7;a°oTL.,
SECTIONAA Fig. 1. Diagram of the specimen and holder.
Fig. 2. Trace obtained with one specimen (details in the text) (time scale, 5 Its division 1). Fig. 3. Left-hand side of Fig. 2 on an expanded time scale (time scale, 1 Its division - ~). film d e p o s i t e d from a stearic acid m o n o l a y e r on distilled water. The bias across the film was 3 V a n d a 10 M H z q u a r t z t r a n s d u c e r was used. In Fig. 2 the first peak is due to electrical b r e a k t h r o u g h of the pulse used to fire the transducer. Thereafter peaks occur in a c c o r d a n c e with the expected arrival of acoustic pulses, as described in Section 2, with an acoustic transit time of a b o u t 2.94 rts. As the d.c. bias was increased from zero, the peaks a p p e a r e d a n d grew in size but eventually seemed to saturate. W i t h further increase in bias, the echo p a t t e r n eventually b r o k e d o w n a n d the peaks d i s a p p e a r e d but the trace could be recovered on r e d u c t i o n of the bias. The m a x i m u m bias that could be a p p l i e d was sensitive to the h u m i d i t y of the a t m o s p h e r e and, for this reason, the specimen a n d holder were enclosed in a glove box which c o n t a i n e d a d r y i n g agent and could be flushed with d r y nitrogen.
274
J. R. I)RABBI,E, S. M. A I , - K t t O W A I I D I
Similar results were obtained with a stearic acid specimen containing 15 layers and with a specimen obtained from a stearic acid film deposited from a water subphase containing 5 × 10 3M CdC12 in which presumably the Langmuir Blodgett film was cadmium stearate. No significant differences were observed when the transducer was replaced by a 5 M H z transducer. Measurements of capacitance were made but these were variable, depending on the position of the mercury contact and to some extent on the area of this contact, indicating that only part of the structure was contributing to the results. This is borne out by the low values measured which were in the region of 100 pF. These values, however, may not relate directly to the monolayers owing to the possible presence of oxide layers. Control experiments using a similar arrangement in which the Langmuir Blodgeit film was omitted but which presumably had an oxide film on the surface of the block showed no trace of an echo pattern. It thus appears that the presence of a film is essential for the effect to be observed. 4. DISCUSSION The motive for the present investigation was the possibility of improving the performance of an ultrasonic capacitance transducer. The sensitivity of such a transducer is inversely proportional to {2 where { is the plate separation 5. However, it becomes increasingly difficult to control both ~ and the uniformity of the gap by mechanical means at { values below about 2 lam. Thus, although the arrangement used in the present experiments differs somewhat from that of the standard form of capacitance transducer, it seemed worthwhile to investigate the use of a Langmuir Blodgett film to provide small spacings and uniform gaps, relying on the mechanical inertia of the upper evaporated electrode to allow the necessary variation in spacing with the arrival of the ultrasonic signal. It does in fact seem likely that the observed effect is due to a change in capacitance brought about by the arrival of the longitudinally polarized ultrasonic signal at the face of the block forming one electrode of the capacitance structure. The associated mechanical displacement, if not transmitted directly to the other electrode formed by the aluminium film, would lead to compression of the Langmuir-Blodgett film and hence to a capacitance change. An indirect secondary contribution to such a change could be a change in dielectric constant with compression. However, although the expected effect has been observed, the measurements we have made so far only partly support this explanation and leave a number of problems unsolved. A major such problem arises from the low values of capacitance measured in the experiments. The area of the evaporated aluminium film was of the order of 1 cm 2 and this should have given capacitance values of the order of 0.1 laF. The measured values are some two orders of magnitude below this and are variable over the electrode, indicating that only a small local area in the region of the mercury contact is contributing to the effect. In several references in the literature, e.g. Ginnai et al. 6, it is suggested that this could be due to poor film quality. Also, it is certain that the Duralumin surface was extremely rough on the scale of the thickness of a typical Langmuir Blodgett film and this would give rise to problems of deposition which might well lead to only localized uniformity of structure.
ULTRASONIC TRANSDUCER ACTION
275
A second unexplained feature is the unexpected form of the peaks, shown in Fig. 3. It is well known from other experiment~ using quartz transducers on both sides of the block that the ultrasonic pulse contains several cycles of oscillation with the frequency of oscillation of the quartz transducer, but nearly all of these are absent when the present means of detection is used. Several experiments which it is hoped will resolve these problems are in hand. The improvement of film quality and of the surface is clearly a first priority since ifa larger area of the structure can be brought in to contribute to the effect the sensitiyity can be increased proportionally. It is hoped that this, coupled with investigations into the frequency response and the effects of varying the number of layers, will lead to an increased understanding of the mechanisms involved. ACKNOWLEDGMENTS
One of us (S.M.A.) thanks the University of the United Arab Emirates for leave of absence during which the work was carried out and the United Arab Emirates Ministry of Education for financial assistance. We also thank Mr. E. Harris for assistance with the construction of the apparatus and for several helpful suggestions. REFERENCES
1 H.M. MillanyandA. K. Jonscher, ThinSolidFilms, 68(1980)257-273. 2 G . G . Roberts, T. M. McGinnity, W. A. Barlow and P. S. Vincen, Thin Solid Films, 68 (1980) 223-232. 3 K.B. Blodgett, J. Am. Chem. Soc., 57(1935) 1007-1022. 4 M.H. Nathoo, Thin Solid Films, 16(1973)215-226. 5 C.B. ScrubyandH. N.G. Wadley, J. Phys. D, ll(1978) 1487-1494. 6 T.M. Ginnai, D. P. Oxley and R. G. Pritchard, Thin Solid Films, 68 (1980) 241-256.
271
U L T R A S O N I C T R A N S D U C E R A C T I O N OF L A N G M U I R - B L O D G E T T FILMS* J. R. DRABBLE AND S. M. AL-KHOWAILDI
Department o/'Ph.l,sics, University of l£xeter, Exeter, Devon (Gt. Britain ) (Received September 1, 1982; accepted September 22, 1982)
A Langmuir-Blodgett film was deposited onto one face of a Duralumin block and an aluminium film was evaporated onto this to form a capacitor structure. This was connected to a charge amplifier. A quartz transducer bonded onto the opposite face of the block provided a source of longitudinally polarized ultrasound. When the capacitor structure was biased with a d.c. voltage, the output of the charge amplifier showed well-defined peaks corresponding in all respects to the expected arrival of ultrasonic pulses at the structure. Possible reasons for the effect and some associated problems are discussed.
I. INTRODUCTION
The interpretation of measurements of the electrical properties of LangmuirBlodgett films is often complicated by the presence of layers of metal oxide associated with contacts. In, for example, capacitance t or current-voltage 2 studies these layers give contributions to the measurements which have to be allowed for to arrive at the properties of the films themselves. In the present study, we investigated an electrical response associated with the Langmuir-Blodgett film arising from ultrasonic signals arriving at the film from a distant source. The initial significant result is that such a response exists in the presence of a film and is absent in control experiments using as closely as possible an identical arrangement without the film. Thus the effect seems to arise from the film itself and offers the prospect that the technique used can be developed to provide new methods of studying the properties of such films, as well as the possibility of some practical applications. The effect was studied by applying a steady d.c. bias across the film and associated contacts--in effect treating it as a capacitor. The most likely explanation at the present time is in terms of a change in the capacitance as a result of the arrival of the longitudinally polarized ultrasonic signal, this being brought about either by
*Paper presented at the First International Conference on Langmuir-Blodgett Films, Durham, Gt. Britain, September20-22, 1982. 0040-6090/83/0000-0000/$03.00
© ElsevierSequoia/Printed in The Netherlands
272
J. R. I)RABBI.E, S. M. A L - K H O W A I I D I
mechanical compression of the film or a change in electrical polarization, or a combination of these, thus leading to a change in the charge on the capacitor. 2.
E X P E R I M E N T A L DETAILS
A diagram of the specimen and holder used is shown in Fig. 1. A block of Duralumin in the shape of a cube with linear dimensions of approximately 2 cm had two opposing faces which were ground flat and parallel and subsequently polished to an optical finish. An X-cut quartz transducer was acoustically bonded to one of these faces and was excited with a short electrical pulse. It is well known that this arrangement resul.ts in a short pulse of ultrasound which propagates from the transducer through the Duralumin and, owing to partial reflections at the two faces, gives a characteristic echo pattern in which pulses of ultrasound arrive at the opposite face at times T, 3T, 5Tetc., where Tis the acoustic transit time across the block. On the opposite face was a Langmuir-Blodgett film consisting of several monomolecular layers deposited from a stearic acid film grown on a subphase of water. An aluminium film of thickness 5000 A was evaporated onto the top surface of this film and electrical contact to this was made via a mercury drop at the end of a narrow tube. This contact was taken to the input of a sensitive charge amplifier (type EMI 6307) which had a built-in provision for applying a steady d.c. potential difference between the mercury contact and the Duralumin block, i.e. across the Langmuir Blodgett film. The output of the charge amplifier after amplil]cation was displayed on an oscilloscope, triggered by the pulse applied to the quartz transducer. The stearic acid monomolecular layers were prepared in a standard way 3"4 on the surface of freshly distilled water using a polytetrafluoroethylene thread and oleic acid to control the surface pressure. We were unable to lind any references in the literature to the transfer of such films to the surfaces of the bulk solid specimens but, as judged from the motion of the piston thread and the shedding of water as the specimen was raised, transfer to the surface of the Duralumin block took place with no obvious difficulty using a mechanical pulling arrangement at a speed of 3 mm min 1. The whole process was carried out in a dust-free atmosphere provided by a laminar flow cabinet and the Langmuir-Blodgett trough was insulated from mechanical vibrations. After transfer of the appropriate number of layers, the specimens were stored in a dry atmosphere before evaporation of the aluminium electrode and bonding of the quartz transducer on the opposite face. 3. RESULTS When a d.c. voltage was applied across the film, the oscilloscope display showed a set of well-defined peaks whose positions on the trace corresponded in all respects to the expected arrival of acoustic pulses at the face of the Duralumin block onto which the Langmuir-Blodgett film had been deposited. The magnitude of these peaks depended on the value of the d.c. voltage and became zero as this voltage was reduced to zero. Figure 2 shows one of the traces obtained and Fig. 3 shows the first part of this on an expanded time scale. These particular traces were obtained with a five-layer
273
ULTRASONIC TRANSDUCER ACTION
A
I~XXX\\~~k~%'l[~] ~/~
-
TOchargeamplifier
--Ogr;;7;a°oTL.,
SECTIONAA Fig. 1. Diagram of the specimen and holder.
Fig. 2. Trace obtained with one specimen (details in the text) (time scale, 5 Its division 1). Fig. 3. Left-hand side of Fig. 2 on an expanded time scale (time scale, 1 Its division - ~). film d e p o s i t e d from a stearic acid m o n o l a y e r on distilled water. The bias across the film was 3 V a n d a 10 M H z q u a r t z t r a n s d u c e r was used. In Fig. 2 the first peak is due to electrical b r e a k t h r o u g h of the pulse used to fire the transducer. Thereafter peaks occur in a c c o r d a n c e with the expected arrival of acoustic pulses, as described in Section 2, with an acoustic transit time of a b o u t 2.94 rts. As the d.c. bias was increased from zero, the peaks a p p e a r e d a n d grew in size but eventually seemed to saturate. W i t h further increase in bias, the echo p a t t e r n eventually b r o k e d o w n a n d the peaks d i s a p p e a r e d but the trace could be recovered on r e d u c t i o n of the bias. The m a x i m u m bias that could be a p p l i e d was sensitive to the h u m i d i t y of the a t m o s p h e r e and, for this reason, the specimen a n d holder were enclosed in a glove box which c o n t a i n e d a d r y i n g agent and could be flushed with d r y nitrogen.
274
J. R. I)RABBI,E, S. M. A I , - K t t O W A I I D I
Similar results were obtained with a stearic acid specimen containing 15 layers and with a specimen obtained from a stearic acid film deposited from a water subphase containing 5 × 10 3M CdC12 in which presumably the Langmuir Blodgett film was cadmium stearate. No significant differences were observed when the transducer was replaced by a 5 M H z transducer. Measurements of capacitance were made but these were variable, depending on the position of the mercury contact and to some extent on the area of this contact, indicating that only part of the structure was contributing to the results. This is borne out by the low values measured which were in the region of 100 pF. These values, however, may not relate directly to the monolayers owing to the possible presence of oxide layers. Control experiments using a similar arrangement in which the Langmuir Blodgeit film was omitted but which presumably had an oxide film on the surface of the block showed no trace of an echo pattern. It thus appears that the presence of a film is essential for the effect to be observed. 4. DISCUSSION The motive for the present investigation was the possibility of improving the performance of an ultrasonic capacitance transducer. The sensitivity of such a transducer is inversely proportional to {2 where { is the plate separation 5. However, it becomes increasingly difficult to control both ~ and the uniformity of the gap by mechanical means at { values below about 2 lam. Thus, although the arrangement used in the present experiments differs somewhat from that of the standard form of capacitance transducer, it seemed worthwhile to investigate the use of a Langmuir Blodgett film to provide small spacings and uniform gaps, relying on the mechanical inertia of the upper evaporated electrode to allow the necessary variation in spacing with the arrival of the ultrasonic signal. It does in fact seem likely that the observed effect is due to a change in capacitance brought about by the arrival of the longitudinally polarized ultrasonic signal at the face of the block forming one electrode of the capacitance structure. The associated mechanical displacement, if not transmitted directly to the other electrode formed by the aluminium film, would lead to compression of the Langmuir-Blodgett film and hence to a capacitance change. An indirect secondary contribution to such a change could be a change in dielectric constant with compression. However, although the expected effect has been observed, the measurements we have made so far only partly support this explanation and leave a number of problems unsolved. A major such problem arises from the low values of capacitance measured in the experiments. The area of the evaporated aluminium film was of the order of 1 cm 2 and this should have given capacitance values of the order of 0.1 laF. The measured values are some two orders of magnitude below this and are variable over the electrode, indicating that only a small local area in the region of the mercury contact is contributing to the effect. In several references in the literature, e.g. Ginnai et al. 6, it is suggested that this could be due to poor film quality. Also, it is certain that the Duralumin surface was extremely rough on the scale of the thickness of a typical Langmuir Blodgett film and this would give rise to problems of deposition which might well lead to only localized uniformity of structure.
ULTRASONIC TRANSDUCER ACTION
275
A second unexplained feature is the unexpected form of the peaks, shown in Fig. 3. It is well known from other experiment~ using quartz transducers on both sides of the block that the ultrasonic pulse contains several cycles of oscillation with the frequency of oscillation of the quartz transducer, but nearly all of these are absent when the present means of detection is used. Several experiments which it is hoped will resolve these problems are in hand. The improvement of film quality and of the surface is clearly a first priority since ifa larger area of the structure can be brought in to contribute to the effect the sensitiyity can be increased proportionally. It is hoped that this, coupled with investigations into the frequency response and the effects of varying the number of layers, will lead to an increased understanding of the mechanisms involved. ACKNOWLEDGMENTS
One of us (S.M.A.) thanks the University of the United Arab Emirates for leave of absence during which the work was carried out and the United Arab Emirates Ministry of Education for financial assistance. We also thank Mr. E. Harris for assistance with the construction of the apparatus and for several helpful suggestions. REFERENCES
1 H.M. MillanyandA. K. Jonscher, ThinSolidFilms, 68(1980)257-273. 2 G . G . Roberts, T. M. McGinnity, W. A. Barlow and P. S. Vincen, Thin Solid Films, 68 (1980) 223-232. 3 K.B. Blodgett, J. Am. Chem. Soc., 57(1935) 1007-1022. 4 M.H. Nathoo, Thin Solid Films, 16(1973)215-226. 5 C.B. ScrubyandH. N.G. Wadley, J. Phys. D, ll(1978) 1487-1494. 6 T.M. Ginnai, D. P. Oxley and R. G. Pritchard, Thin Solid Films, 68 (1980) 241-256.