4549
Langmuir 1995,11, 4549-4553
Preparation and Synchrotron X-ray Specular Reflectivity Study of Langmuir-Blodgett Films of the Phthalocyanine [(CsH&SiO (SiPcO)aH] Hong-Ying Wang, J . Adin Mann, Jr., Jerome B. Lando,* Terri R. Clark,+and Malcolm E. Kenneyt Department of Macromolecular Science, Case Western Reserve University, Cleveland, Ohio 44106 Received January 12, 1994. I n Final Form: August 16, 1995@ The preparation by the Langmuir-Blodgett technique of multilayers of the phthalocyanine [(C6H13)3SiO(SiPcO)zH] (Si-Si Pc dimer) is described and compared with a similar compound, [(C6H13)3SiOSiPcOGePcOH](Si-Ge Pc dimer), reported before. The Si-Si Pc dimer monolayer at the air-water interface is stable on a pH = 7 buffered water subphase. The multilayer formed through Y-type deposition has a deposition ratio of 1, assuming that the rings are parallel to the substrate surface. The film was studied by using synchrotron X-ray specular reflectivity experiments. The reflectivity profile obtained gives many Kiessig fringes and two strong Bragg peaks, indicating that the multilayer obtained has a very good quality. The Bragg peaks correspond to a bilayer spacing of 23.7 A, and the average film density obtained from the Kiessig fringes is 0.974 g/cm3. The high degree of ring orientation and film perfection with the low film density give great advantages for the application of this phthalocyanine LangmuirBlodgett film as gas sensors.
Introduction The valuable properties of phthalocyanines (Pc's), including optical and electrical conductivity anisotropy, thermal and chemical stability, and diverse chemistry, have led to a significant interest in their use as active components in optical and microelectronic devices.l For some applications it is desired to deposit thin films with a controlled thickness in which the phthalocyanine molecules are oriented in a well-defined way. This requirement can be achieved by using the LangmuirBlodgett (L-B) film deposition technique. For this purpose, Kenney et al.2,3have developed a n unsymmetrical axially substituted phthalocyanine dimer, [ ( C ~ H ~ ~ ) ~ S ~ ~ S ~ P C O(abbreviated C T ~ P C O HSi-& ] Pc dimer) (Figure 11,where Pc is thephthalocyaninato dianion. This compound incorporates regions of hydrophilicity and hydrophobicity along, the axis of the phthalocyanine molecule, and the two-ring system is built to give the compound a profile that would lead to a substantial collapse pressure. The isotherm from the pure water surface shown in Figure 2 indicates that there are two molecular arrangements during the compression.2 The packing area of a phthalocyanine ring resting on its face is estimated from space-filling models, -160 nm2/ m ~ l e c u l eand , ~ from diffraction studies of vapor-deposited phthalocyanine monolayers, 1.4- 1.9nmz/molecule.5 The initial transition (high-coarea arrangement) pressure of the film is 17 mN/m, and the coarea is 1.70 nmVmolecule indicating that the rings are parallel to the water surface. The low-coarea molecular arrangement has a coarea of
(HO)GePcOSIPc(OSl(n -C,H
3)3)
0 Si (C6H1313
OH
Figure 1. Structure of [CsH13)3SiOSiPcOGePcOH].
' Present address: Allied Signal Corporation,Morristown, NJ.
* Department of Chemistry, Case Western Reserve University.
@
Abstract published in Advance A C S Abstracts, November 1,
1995. (1) For a review, see: Lezniff, C. C., Lever, A. B. P. Eds. Phthalocyanines: Properties and Applications; VCH Publishers, Inc.: New York, 1989. (2) Shutt,J. D.; Batzel, D. A.;Sudiwala, R. V.; Rickert, S. E.; Kenney, M. E. Langmuir 1988,4,1240-1247. (3) Batzel, D. A.; Rickert, S. E.; Kenney, M. E. US.Patent 4 900 817, 1990. (4) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983,99, 53. (5)Buchhloz, J. C.; Somorjai, G. A. J. Chem. Phys. 1977,66, 573.
0743-7463/95/2411-4549$09.00/0
0 0
'
0:4
0.4
1.2
'
I 2
Rolecular A r e a (nm /molecule)
Figure 2. Isotherms of [CsH13)3SiOSiPcOGePcOHl films at various temperatures ("C). (from ref 2).
approximately 1.0 nmVmolecule and a collapse pressure of 60 mN/m. It is speculated that the transition plateau is due to bilayer formation.6 The monolayer a t the highcoarea arrangement can be transferred to solid substrates 0 1995 American Chemical Society
4550 Langmuir, Vol. 11, No. 11, 1995
Wang et al.
through Y-type deposition with a deposition ratio of 1, which indicates that the phthalocyanine rings are parallel to the substrate surface as has been demonstrated by electron and X-ray d i f f r a c t i ~ n . ~The - ~ deposited Si-Ge Pc multilayer has been well characterized and used successfully as gas s e n s o r ~ . ~ - lA l similar compound, [ ( C G H ~ ~ ) ~ S ~ O ( S(Si-si ~ P C ~Pc ) ~dimer), H] has also been synthesized by Kenney’s g r ~ u p . ~The ~ Jgreat ~ advantage of the Si-Si Pc dimer over the Si-Ge Pc dimer is that it is much easier to synthesize. In this paper, we present our work on the preparation and characterization of the Si-Si Pc dimer LangmuirBlodgett films.
Experimental Section
- --I. 80
3
70
0)
v
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20
+
5
I
10
’
’
v)
0’ 0.0
0.5
1.0
1.5
I 2.5
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M o l e c u l a r A r e a (nm / m o l e c u l e )
The Si-Si Pc dimer was obtained from Kenney’s group. A commercial Lauda film balance was used to contain and manipulate the monolayer at the air-water interface. The balance was placed on a vibration isolation table under a class 10 laminar flow area in a class 100 clean room. An IBM personal computer interfaced to the balance was employed for data acquisitiodmanipulation and dipper control. The software and interface for the computer were developed by J. D. Shutt.14 The ultrapure water supply system included a water softener, an activated carbon tank, a commercial reverse osmosis unit (Milli-RO 1201, and a commercial water purification unit (Milli-Q Plus, QPAK1). The water produced has a resistivity of 18.2 MQ. cm. In addition, a single “shake test” was used to show the complete absence of any tendency to foam. The spreading solutions had concentrations of 0.2-0.4 mg/ mL in chloroform (Aldrich; HPLC grade, stabilized with 0.75% ethanol). Typically, 20 min was allowed before compression of the monolayer to ensure solvent evaporation. The pressurearea isotherm was collected a t a compression rate of 1c d m i n . The solution of Si-Si Pc dimer was used within a day because the siloxy group of the compound may undergo cleavage after long times in the HPLC grade chloroform solvent stabilized with ethanol. Stability of the monolayer a t the air-water interface was studied by running a compressive creep test a t a surface pressure of 7 mN/m. Because of special requirements for the gas sensor dipping (many layers on a large substrate at a slow dipping speed and long draining time), long time stability of the film on the water surface was desired and the creep test was run for a length of 6-10 h. Y-type vertical dipping was achieved by passing the substrate through the compressed monolayer a t 1m d m i n in the first round trip and 5 m d m i n in the remaining round trips a t a pressure of 7 mN/m. The substrate was held 10 mm above the water for 30 min during the first round trip and 10 min for subsequent trips to allow complete drainage. The hydrophobic glass substrates were prepared by putting the substrates in concentrated nitric acid for more than 24 h and then in a glass desiccator containing hexamethyldisilazane (HMDS) vapor for more than 24 h after rinsing and drying. The buffered subphases were prepared by adding the appropriate volumes of 0.1 M NaH2PO4.H20 and NaOH plus water to a 1 L final volume of mixture.15 (6) Chiang, C. W. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1988. (7) Wang, H. Y. Ph.D. Thesis. Case Western Reserve Universitv. Cleveland,-OH, 1995. (8) Wang, H. Y.; Lando, J. B. To be submitted. (9) KO,W. H.; Fu, C. W.; Wang, H. Y.; Batzel, D. A,; Kenney, M. E.; Lando, J. B. Sens. Mater. 1990,Z (11, 39-55. (10)Wang, H. Y.; KO,W. H.; Batzel, D. A.; Kenney, M. E.; Lando, J. B. Sens. Actuators 1990,B l , 138-141. (11)Wang, H. Y.; Lando, J. B. Langgmucr 1994, 10 (31, 790. (12) Batzel, D. A. Ph.D. Thesis, Case Western Reserve University, Cleveland. OH. 1990. (13)Clark, T . R. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1993. (14) Shutt, J. D. Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1988. (15) Weast, R. C., Astle, M. J., Beyer, W. H., Eds. CRC Handbook of Chemistry and Physics, 66th ed.; CRC Press: Boca Raton, FL, 1985; p D-149. “
I
Figure 3. Isotherm of a [CsH13)3SiO(SiPcO)zH]film on a pure water surface at 20 “C. The deposited Si-Si Pc multilayers were studied by synchrotron X-ray specular reflectivity experiments16 a t the Naval Research Laboratory beam line X-23B of the National Synchrotron Light Source at Brookhaven National Laboratory. The sample was radiated at a grazing incident angle and scanned off-plane with the incident glancing angle e,, being equal to the exit glancing angle, while the surface normal, the incident, and the scattered wave vectors are all in the same plane. The wave vectors are defined as 2n/I and qr = (4n/L) sin e,,. A four-circle Hubergoniometer was used to adjust the orientation ofthe sample to bring any desired plane into reflecting position. The output energy of the synchrotron X-ray beam was set a t approximately 7600 eV which corresponded to a wavelength of 1.63 and was calibrated using sodium chloride (NaCl) as the standard crystal to have a wavelength of 1.621 A detailed description of the synchrotron X-ray experimental setup and procedures is given in ref 7 .
A
A.
Results and Discussion 1. Preparation of the Si-Si Pc L-B films. The isotherm ofthe Si-Si Pc dimer is shown in Figure 3.Again, two structural arrangements occurred during the compression, which was the same as in the isotherm of the Si-Ge Pc dimer. The initial transition pressure of the film is 13 mN/m, and the coarea is 1.55 f 0.01 nm2/ molecule, also indicating that the rings are parallel to the water surface. Please note that because of compressive creep the “coareas” vary with time. The value f O . O 1 indicates the inherent accuracy of the measurement for a stable monolayer. The coarea for the second transition is 1.01 f 0.01 nm2/molecule, showing that the transition between the two molecular arrangements is shorter for Si-Si than for Si-Ge. The lower initial transition pressure and coarea, which lead to a smaller area under the isotherm curve of Si-Si than that of Si-Ge, may be a n indication that the film of Si-Si on the water surface is less stable than that of Si-Ge. As pointed out by Biddle et al.17 the isotherm may be thought of as a twodimensional compression stress-strain test. The collapse
point represents the relative strength of the monolayer, and the amount of decline after collapse is indicative of the relative toughness of the film. Since the area under the curve represents the energy required for the film to “fail”,stability is seen to decrease as this area decreases. The easier transition from the first molecular arrangement to the second one for the Si-Si than Si-Ge also shows that the film of Si-Si on the water surface at the first (16) For reviews, see: (a) Als-Nielsen, J. In Surface and Dynamics of Surfaces: Topics in current Physics, Eds. Schommers, W., Blanckenhagen, P., Eds.; Springer: New York, 1986; p 181. (b)Russell, T. P. Mater. Sci. Rep. 1990, 5, 171. (17)Biddle, M. B.; Rickert, S.E.; Lando, J. B. Thin Solid Films 1986, 134, 121.
Langmuir-Blodgett Films of [(C&&SiO(SiPcO)zH]
Langmuir, Vol. 11, No. 11, 1995 4551 100
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Figure 4. Compressivecreep test ofa [ ( C ~ H ~ ~ ) ~ S ~ ~ ( S ~ P CFigure O ) Z H5. ] Compressive creep test of a [ ( C ~ H ~ ~ ) ~ S ~ O ( S ~ P C O ) Z H ] film on a pH = 7 buffered subphase (buffer concentration 0.1 film on a pure water surface at 20 "C. M) at 20 "C. molecular arrangement may be less stable than that of Si-Ge at the same molecular arrangement. The creep test shown in Figure 4 demonstrates that the Si-Si Pc monolayer is less stable on the pure water surface than the Si-Ge Pc monolayer. After 6 h, the film underwent 20% loss of the initial area a t 7 nWm, while Si-Ge underwent less than 1%loss of initial area a t 10 40 mN/m.ls It was noticed that the shape of the creep curve al was convex instead of concave as usually observed in the a' 3 0 1 creep tests. During the first 3 h the area loss was only 4%, but during the next 3 h the area loss dramatically increased to 16%,and reached a total 40%loss ofthe initial area for a 10 h testing period. This led to speculation that the pH ofthe water subphase 0.0 0.5 1.0 1.5 2.0 2.5 may play an important role in the Si-Si monolayer behavior. Freshly processed water will equilibrate to a Molecular A r e a ( n m / m o l e c u l e ) pH of about 5.5 as a result of atmospheric COZ absorption. Figure 6. Isotherm of a [(CsH13)3SiO(SiPc0)2H]film on a This slight subphase acidity is apparently unimportant M) at 20 buffered pH = 7 subphase (buffer concentration to the Si-Ge Pc monolayer.2 However, since the Si-0 "C. bond is stronger than the Ge-0 bond, the Si-Si monolayer may undergo a change in solvation in the acidic subphase arrangement to the second one is much longer than that unlike the Si-Ge, which may result in the film instability. of the film on a pure water subphase (compared with At the beginning, since the pure water subphase was Figure 3), indicating that the film a t the first molecular freshly processed and neutral, the amount of film creep arrangement on buffered pH = 7 subphase is much more was small. With the increase of COZ absorption, the stable than that on a pure water subphase, as demonsubphase became more and more acidic and finally strated by the creep test. The coarea is 1.72 f 0.01 nm2/ saturated a t a p H of around 5.5, leading to the acceleration molecule for the first molecular arrangement and 0.71 f of the Si-Si film creep. 0.01 nmz/molecule for the second arrangement. Isotherms To prove this speculation, a subphase with pH = 7 was on pH = 6 and 8 buffered subphases (low3M) were used buffered with 500 mL of 0.1 M NaH2PO4.H20 plus collected, and no significant difference between isotherms 291 mL of 0.1 M NaOH to a 1L final volume of the mixture in this pH range (pH = 6-81 was found within experiand measured with a pH meter. The result is shown in mental error. Figure 5, which indicates that the use of a buffered pH The Si-Si monolayer on a pH = 7 buffered subphase = 7 subphase significantly improved the film stability on was transferred to solid substrates with a deposition ratio the subphase surface with a less than 3%loss ofthe initial of 1, indicating that the multilayer obtained has good area after 6 h. Considering that a high buffer concentraquality and the film on the substrate surface has the same tion may bring impurities to the subphase, the concentraconfiguration as that ofthe film on the water surface, i.e., tion of the buffer should be reduced as much as possible. the rings are parallel to the substrate surface. This Thus a buffer concentration of M was tried and more conclusion is supported by the work on the film crystal than 12%area loss after 6 h was obtained, indicating that structure determination by X-ray and electron diffraca buffer concentration of M was not strong enough t i ~ n . ~ ~ ~ to maintain the stability ofthe Si-Si monolayer. Abuffer To check whether the monolayer was contaminated by M gave as good a result as that concentration of 5 x the subphase buffer, X-ray photoelectron spectra ( X P S ) with a buffer concentration of 0.1 M. Therefore buffer were collected for 10layers ofboth buffered and unbuffered concentrations of 5 x 10-3-10-2 M were used for the Si-Si multilayers on hydrophobic Corning microscope subsequent Si-Si Pc dimer dipping. glass slides. The spectra were obtained by the Center for The isotherm of a Si-Si Pc dimer on a pH = 7 subphase Surface Analysis of Materials, Case Western Reserve with a M buffer concentration is shown in Figure 6. University, on a Phi 5400 Electron Spectroscopy for It can be seen that the transition from the initial molecular Chemical Analysis Electron Spectrometer (Perkin-Elmer) equipped with Mg and Al sources. The results showed (18)Fu, C. W. Ph.D. thesis, Case Western Reserve University, that the buffered and unbuffered films gave essentially Cleveland, OH, 1988.
4552 Langmuir, Vol. 11, No. 11, 1995
Wang et al. was too big. The width of the fringes is inversely proportional to the film thickness by the following relationship:16b
c n V."
5.0
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4.0
'
3.0
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C 3 0
0 0
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2 theta (degrees)
Figure 7. Reflectivity profile for 50 layers of Si-Si Pc dimer on a Si substrate at a 28 scan increment of 0.075" (1 = 1.621
A).
1.0
where Y = U2t, n is the order of the interference fringe, 8, is the reflectance angle of the nth fringe, 8, is the critical angle, t is the film thickness, and A is the wavelength. Plotting On2 versus (8,+12 - 8,-12)2 yields 8,2 from t h e y intercept and t from the slope. The average electron density can be calculated from the 82 by the following relation:23
'
J
0
2
6
4
10
8
2 t h e t a (degrees)
Figure 8. Reflectivity profile for 50 layers of Si-Si Pc dimer on a Si substrate at a 28 scan increment of 0.015' (1= 1.628
A,.
7.1
-
5.4
-
0 0 0
h
f5
0 0
3.6
-
1.8
-
0 0
0
"."
I
0 .o
where t is the film thickness and q = (4n/A)sin 8. Thus for the relatively thick 50 layer film, the fringes would be very narrow. We scanned the same sample a t a 28 increment of 0.015", and the wavelength of the beam in this experiment was 1.628 A. The result is shown in Figure 8, which gives excellent Kiessig fringes and demonstrates that the deposited film has good quality. Since the sample had been exposed to the synchrotron beam many times in the first experiment and was over 3 months old, the (002) Bragg peak no longer appeared. The Kiessig fringes were evaluated using the following equati~nl~-~~:
0 0
-0 OoU
0.6
1 .2
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2.4
3.0
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Figure 9. Plot of (e,+? - 0,-12)2versus On2. identical spectra, indicating that there was no contamination of the monolayer from the subphase buffer. 2. SynchrotronX-raySpecular Reflectivity Study. Fifty layers of Si-Si Pc L-B film on a silicon substrate were prepared from the 5 x M pH = 7 buffered subphase a t a surface pressure of 7 mN/m. The specular reflectivity profile obtained is shown in Figure 7. It gives many Kiessig fringes and two strong Bragg peaks. The (001)and (002)Bragg peaks correspond to a d-spacing of d(001)= 23.70 A. However, in the first experiment, the Kiessig fringes did not show up nicely, which was found to be caused by the increment of the 28-8 scan (A(28)= 0.075") which
0 : n Qe=F
