Langmuir 1998, 14, 6625-6627
6625
Elastic Properties of Langmuir-Blodgett Films. A New Brillouin Spectroscopic Strategy Rafael J. Jime´nez Riobo´o,† Jorge Souto,*,‡ Jose´ A. de Saja,‡ and Carlos Prieto† Instituto de Ciencia de Materiales de Madrid (CSIC), Cantoblanco, E-28049 Madrid, Spain, and Departamento de Fı´sica de la Materia Condensada, Cristalografı´a y Mineralogı´a, Universidad de Valladolid, E-47005 Valladolid, Spain Received June 1, 1998. In Final Form: September 17, 1998 A new Brillouin spectroscopic strategy has permitted us, for the first time, to obtain direct information about the values of the longitudinal elastic constant in the growth plane and in the growth direction (c11 and c33, respectively) of a Langmuir-Blodgett film of copper arachidate. The obtained results show the elastic isotropy of the film in the growth plane and the clear difference from the value obtained in directions almost parallel to the growth direction (c11 ) 9.4 ( 0.1 GPa and c33 > 10.59 GPa). This nondestructive and contactless method opens new perspectives in the determination of the elastic properties of thin film samples.
Introduction In the last years the interest in thin film materials has increased drastically thanks to the wide application field opened for this kind of material. The necessity of complete characterization of the film materials includes the knowledge of the elastic (mechanical) properties that can be influenced by the existence of stresses in the films. The determination of the elastic properties of thin films on substrates is a difficult task. In fact there are in the literature very few examples of experimental techniques able to give some indication on these properties.1 Obviously, the thickness of these films (d < 1-2 µm) prevents the use of usual techniques such as ultrasound, torsion pendulum, and similar ones. The deposition on a substrate makes it difficult to separate the contributions of substrate and film using habitual micro- and nanoindentation techniques, resonant techniques, or bending techniques.2 Indentation techniques deform irreversibly, the films not being suitable for the study of the temperature or time evolution of the elastic properties. These techniques are continuously evolving in order to improve the outputs obtained.3 In the case of thin film samples, Brillouin spectroscopy has been used almost exclusively to study surface acoustic waves in opaque thin films (metals, semiconductors, and metallic and semiconductor superlattices), obtaining indirect information about the elastic constants of the materials studied.4 As an alternative to the usual experimental techniques, we propose the use of high-resolution Brillouin spectroscopy to obtain direct information on the elastic constants of nonopaque thin films on substrates. This work is based on the results obtained in ref 5 by one of the authors (R.J.J.R.). In the case of isotropic thin films the optical properties can also be obtained simultaneously. As a model substance we have used a Langmuir-Blodgett (LB) film of copper arachidate deposited on a silicon substrate. * To whom correspondence should be addressed. † Instituto de Ciencia de Materiales de Madrid (CSIC). ‡ Universidad de Valladolid. (1) Smith, D. L. Thin-Film deposition. Principles & Practice; McGrawHill: New York, 1995. (2) Pharr, G. M.; Oliver, W. C. Measurement of thin film mechanical properties using nanoindentation MRS Bull. 1992, July, 28. (3) Ja¨tming, A. K.; Bell, J. M.; Swain, M. V.; Schwarzer, N. Thin Solid Films 1997, 308-309, 304. (4) Mutti, P.; Bottani, C. E.; Ghislotti, G.; Beghi, M.; Briggs, G. A. D.; Sandercock, J. R. Adv. Acoust. Microsc. 1995, 1, 249.
The thickness of the deposited film was clearly below 1 µm, approximately 900 nm. Even though a considerable amount of work has been devoted to the study of the structure of thin organic films prepared with the Langmuir-Blodgett method,6,7 the mechanical properties of these systems have rarely been estimated. Some of the scarce efforts in this direction report the behavior of floating Langmuir films when capillary transverse waves are generated at the air-water interface.8,9 The friction and wear properties have also been studied on the surfaces of organic assemblies applying macroscopic techniques, basically by using a pin-on-flat or pin-on-disk apparatus.10 More recently, the different effective elastic properties of segments within a Langmuir-Blodgett film have been inspected by atomic force microscopy (AFM) in the force modulation mode (FMM).11 In most of the reports related to this subject, only the frictional behavior of the surfaces of the films is analyzed, and as practical applications of thin organic films are considered, more information regarding the stability of these structures is a necessary requirement. The present lack of reliable information on this subject may represent a serious limitation in the utilization of these systems in certain potential areas of interest, such as lubricant layers for magnetic mass storage media12 or selective barriers for membranes.13 Experimental Section High-resolution Brillouin spectroscopy (BS) was used to obtain the elastic properties of LB film samples. The experiments were performed at room temperature on a 6-pass in tandem FabryPerot interferometer.14 The light source was an Ar+ laser radiating at the wavelength λ0 ) 514.5 nm. Typically the (5) Kru¨ger, J. K.; Embs, J.; Brierley, J.; Jime´nez, R. Accepted for publication in J. Phys. D: Appl. Phys. (6) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: New York, 1991. (7) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (8) Mo¨bius, D.; Mo¨hwald, H. Adv. Mater. 1991, 3, 19. (9) Mo¨bius, D. Can. J. Phys. 1990, 68, 992. (10) Novotny, V.; Swalen, J. D.; Rabe, J. P. Langmuir 1989, 5, 485. (11) Chi, L. F.; Gleiche, M.; Fuchs, H. Langmuir 1998, 14, 875. (12) Fuchs, H.; Ohst, H.; Prass, W. Adv. Mater. 1991, 3, 10. (13) Pen˜acorada, F.; Reiche, J.; Zetzsche, T.; Dietel, R.; Brehmer, L.; de Saja, J. A. Thin Solid Films 1997, 295, 246. (14) Sandercock, J. R. Topics in applied physics 51. Light scattering in solids III; Springer: Berlin, 1982; Chapter 6.
10.1021/la9806350 CCC: $15.00 © 1998 American Chemical Society Published on Web 10/21/1998
6626 Langmuir, Vol. 14, No. 23, 1998
Letters q2RA )
4π sin(R) ; λ0
q180 )
4πni λ0
(1)
where ni is the relevant refractive index of the sample at the laser wavelength (in the case of optic isotropic samples there is only one refractive index n). From the dispersion relation between the phonon energy and the wave vector, valid in the case of no acoustic dispersion and for very small q values, one obtains the sound velocity for the 2RA scattering geometry as
2 ω ) vq w f ) v sin(R) λ0
and
v)
λ0 ω ) fΛ ) f q 2 sin(R) (2a)
and that for the 180 scattering geometry as
Figure 1. Schematic representation of the experimental scattering geometry. The sample (S) is deposited on the reflectant substrate (R); N is the axis of rotation for the azimutal angle; c is the growth direction; a,b indicates the growth plane; the dashed arrow represents the virtual light path; R is the outer scattering angle; ki and ks are the incident and scattered light wave vectors, respectively; q2RA and q180 are the phonon wave vectors involved in the scattering processes; 180 and 2RA stand for the scattered light belonging to the two simultaneous scattering geometries; the scattered light is analyzed by Brillouin spectrometry (BS). scattering volume is of the order of 10-7 cm3. An exhaustive description of the experimental setup can be found in ref 15. Arachidic acid (99%) was purchased from Aldrich and dissolved in chloroform stabilized with ethanol. A computer-controlled KSV 5000 thermostated system with symmetrical compression was used for the formation of the organic layers. The aqueous subphase was obtained by dissolving SO4Cu (Aldrich) in MilliQ ultrapure water. The salt concentration was 3 × 10-4 M. Arachidic acid (100 µL of a 2.66 × 10-3 M solution) was spread onto the subphase, kept at 20 ( 0.5 °C, to form the floating monolayer. The floating film was transferred to hydrophobic silicon substrates at the surface pressure 30 mN/m and at the constant rate 5 cm/min. The silicon wafers were cleaned with hot chromosulfuric acid. This treatment renders hydrophilic substrates. A hydrophobic surface was obtained by exposing the wafers to hexamethyldisilazan vapors.16 Y-type films with a transfer ratio higher than 95% were formed on the substrates. Films with thicknesses of up to 360 monolayers were built. The refraction index of the samples was determined with a PLASMOS SD-2000 rotating analyzer ellipsometer (RAE) coupled to a 12 bit 6502 microprocessor. The backscattering setup used for BS is schematically shown in Figure 1. When the LB film sample is deposited on a reflecting substrate, the backscattering setup gives rise to the usual 180 scattering geometry and to a supplementary scattering geometry. Due to the similarity to the 90A scattering geometry, the supplementary scattering geometry will be denoted as 2RA (2R is the outer scattering angle). The capital A in the notation of the scattering geometry (90°) indicates that, in the case of transparent samples, these are placed so as to avoid the reflection of the incident laser beam at the entrance of the spectrometer. Under these circumstances, the outer scattering angle is kept at 90° and the phonon wave vector is independent of the refractive index of the sample. In our case, the independence of the refractive index17,18 and the fact that at R ) 45° we obtain the same 90A scattering geometry defined for transparent samples are the reasons why we denote the supplementary scattering geometry as 2RA. The phonon wave vectors (q) involved in both scattering processes have the absolute values (15) Jime´nez Riobo´o, R. J.; Garcı´a-Herna´ndez, M.; Prieto, C.; FuentesGallego, J. J.; Blanco, E.; Ramı´rez-del-Solar, M. J. Appl. Phys. 1997, 81, 7739. (16) Souto, J.; Rodrı´guez-Me´ndez, M. L.; Pen˜acorada, F.; Reiche, J. Mater. Sci. Eng. C 1997, 5, 59. (17) Kru¨ger, J. K.; Peetz, L.; Pietralla, M. Polymer 1978, 19, 1397. (18) Kru¨ger, J. K.; Marx, A.; Peetz, L.; Unruh, H.-G. Colloid Polym. Sci. 1986, 264, 403.
2n λ0
ω ) vq w f ) v
and
v)
λ0 ω ) fΛ ) f q 2n
(2b)
where f is the Brillouin frequency shift. The elastic constant is then
c ) Fv2
(3)
with F as the mass density of the sample. In the case of acoustic isotropic samples and in absence of acoustic dispersion, the value of the sound velocity is independent of the propagation direction in the material. Combining the 180 and 2RA scattering geometries, it is straightforward to see that the refractive index of the sample is
n)
f 180 sin(R) f 2RA
(4)
Results and Discussion The Brillouin spectra obtained for different scattering angles (sagital angles) are shown in Figure 2. The two contributions, from the 2RA and 180 scattering geometries, can be clearly seen. The quality of the spectra is influenced by the poor optical quality of the sample. The angle dependence of the Brillouin frequency is shown in Figure 3. The straight line behavior indicates the absence of acoustic dispersion, and from the value of the slope we obtain the sound velocity parallel to the a direction in the film plane after eq 2a: va ) 2759.29 m/s. The value of the Brillouin frequency shift for the b direction (perpendicular to the a direction but contained in the film plane) at the sagital angle R ) 55° is fb ) 8.703 GHz. The corresponding sound velocity obtained from eq 2a is vb ) 2733.1 m/s. Taking into account the optical quality of the sample, the difference between these two values lies within the margin of error we have in the determination of the sound velocity. The experimental data suggest elastic isotropy in the growth plane with a sound velocity of v ) 2746.2 ( 13 m/s. To evaluate the elastic constant in the growth plane (c11), we have made an estimation of the density of the Langmuir-Blodgett film by considering the molecular weight of the substance and assessing the volume occupied by single molecules, and the value 1.25 ( 0.1 g/cm3 was obtained. The value of the longitudinal elastic constant in the growth plane will be c11 ) 9.4 ( 0.1 GPa. To evaluate the sound velocity obtained in directions almost perpendicular to the growth plane (the 180 peaks), it is necessary to count the refractive index of the sample as shown in eq 2b. Ellipsometry was used to estimate the refractive index of the sample in the growth plane, and the value obtained was n ) 1.45 ( 0.02. The values of the 180 (19) Kru¨ger, J. K. In Optical techniques to characterize polymer systems; Ba¨ssler, A., Ed.; Elsevier: Amsterdam, 1989.
Letters
Langmuir, Vol. 14, No. 23, 1998 6627
Figure 3. Angular dependence of the Brillouin frequency shift (2RA scattering geometry). The sin(R) value is proportional to the 2RΑ scattering wave vector. The straight line is the result of a least-squares fit of the function f ) v(2/λ0) sin(R) to the experimental data.
corresponding sound velocities are v55 ) 2803.6 m/s and v45 ) 2911.3 m/s. These values are clearly higher than the values obtained in the growth plane. For a sagital angle of 55° the scattering wave vector lies 34.4° apart from the growth direction of the film (normal to the film plane), and for a sagital angle of 45° the scattering wave vector lies at 29.2°. The increase in sound velocity when approaching the growth direction indicates the clear anisotropy of the LB film, differentiating between growth plane and growth direction. The expected value of the elastic constant in the growth direction will be higher than the value c33 ) 10.59 GPa obtained for the sagital angle R ) 45°. The higher value of the elastic constant in the growth direction (c33 > 10.59 GPa) compared to the value in the growth plane (c11 ) 9.4 GPa) confirms the stacking of the molecules in the LB film even for the relatively large thickness studied (d ) 900 nm), being similar to that for oriented polymers.19 Conclusions
Figure 2. Brillouin spectra for different sagital angles R. (a) R ) 65° (upper) and R ) 60° (lower). The 2RA wave vector lies within the growth plane and in the same direction in both cases. (b) R ) 55° in both cases but the 2RΑ vector points in perpendicular directions contained in the growth plane.
frequency for the sagital angles R ) 55° and R ) 45° are respectively f 55 ) 15.803 GHz and f 45 ) 16.41 GHz. The
The results obtained by high-resolution Brillouin spectroscopy show the elastic anisotropy of the copper arachidate LB films, differentiating the growth plane from the growth direction, and the capability of this technique to determine the elastic properties of thin films on substrates. This nondestructive and contactless technique opens new and promising perspectives in the field of determination of the elastic properties even in the “in situ” study of thermal and temporal evolution. Acknowledgment. The authors are grateful to Mr. Eugenio de la Rosa for performing the ellipsometry measurements. This research was supported by CICYT of Spain (Grants MAT95-0544 and MAT97-7250). LA9806350
