Langmuir 1988,4, 1240-1247
Fabrication of Thin-Film Dielectrics from an Amphiphilic Two-Ring Phthalocyanine Joel D. Shutt,*pt Daniel A. Batzel, Rashmikant V. Sudiwala, Scott E. Rickert,* and Malcolm E. Kenney" Department of Macromolecular Science and Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106 Received October 29, 1987. In Final Form: June 1 , 1988 This paper describes the synthesis of the novel amphiphilic two-ring phthalocyanine (H0)GePcOSiPc(OSi(n-C6H13)3) (where Pc is the phthalocyaninato dianion), the formation of a monolayer of this compound in which the rings are parallel to the plane of the monolayer, and some properties of this monolayer. The paper also describes the preparation by the Langmuir-Blodgett technique of multilayers of the compound in which the rings appear to be parallel to the substrate plane and the fabrication and properties of metal-insulatopmetal devices constructed of aluminum electrodes and films of the compound of varying thicknesses. It is shown that the phase-change pressure of the monolayer is 17 mN/m and thus is sufficiently high to facilitate its deposition on a solid substrate. It is estimated that the relative permittivity of the multilayers is 5.5 f 0.2 and thus is consistent with the permittivities reported for multilayers of porphyrins. The monolayer and multilayers are representatives of a new class of dielectric films. They are almost certainly highly anisotropic and have the potential to be used in a molecular engineering capability as components for complex microelectronic devices.
Introduction Some time ago it was shown that long-chain fatty acids can be deposited as monolayers on water in a film balance' and that these monolayers can be deposited as multilayers on solid substrates.2 More recently it has been shown that aromatic compounds can likewise be deposited as monolayers on water in a film balance and that the monolayers also likewise can be deposited as multilayers on solid substrates. A review dealing in part with these films has appeared r e ~ e n t l y . ~ The aromatic compounds that have been deposited as monolayers and multilayers include those with the anthracene: chlorophyll: porphyrin: and phthalocyanine7 ring systems. The compounds with the phthalocyanine ring system that have been deposited encompass those with hydrogen, silicon, or one of a variety of metals in the center of the ring; with zero or two axial groups; and with zero, three, four, or eight peripheral g r o ~ p s . ~ - ~ ~ In addition, it has also been shown that certain phthalocyanines become conducting when fractionally oxidized (doped).29 This conductivity is generally very dependent on the direction of the current since it arises primarily through a conduction band generated by a-orbital overlap.s0 The known capacity of some phthalocyanines to form multilayers and of others to yield conducting materials when doped, and the stability which is generally characteristic of phthalocyanines, has led to an interest in members of this class for use in the fabrication of microelectronic devices including gas sensors,8*10~18J9 electroluminescent d i o d e ~ > ' ~and J ~ bistable s w i t ~ h e s . ~ ~ *~' Similarly, the film-forming and stability properties of phthalocyanines have led to an interest in them for use in the fabrication of larger electronic devices including photov o l t a i ~ and ~ * ~electrochromic20 ~~ device^.^^-^^ To allow for full flexibility in the design of such devices, phthalocyanine multilayers having rings that are perpendicular to the substrate, parallel to it, or both perpendi'Present address: Georgia Tech Research Institute, Energy & Materials Science Laboratory, Georgia Institute of Technology, Atlanta, GA 30332. *Present address: Nanofilm Corp., 14675 Foltz Ind. Pkwy., Strongsville, O H 44136.
cular and parallel to it in some specified sequence are needed. Although the work done on phthalocyanine films (1)Langmuir, I. J. Am. Chem. SOC. 1917,39, 1848. 1935,57, 1007. (2)Blodgett, K. B. J. Am. Chem. SOC. (3)Honeybourne, C. L. J. Phys. Chem. Solids 1987,48, 109. (4)Peterson, I. R.; Russell, G. J.; Neal, D. B.; Petty, M. C.; Roberts, G. G.; Ginnai, T.; Hann, R. A. Philos. Mag. [Part] B 1986,54, 71. (5)Jones, R.; Tredgold, R. H.; OMullane, J. E. Photochem. Photobiol. 1980, 32, 223. (6)Tredgold, R. H.; Young, M. C. J.; Hodge, P.; Hoorfar, A. IEE Proc., Part I: Solid-state Electron Deuices 1985, 132, 151. (7) Baker, S.;Petty, M. C.; Roberts, G . G.; Twigg, M. V. Thin Solid Films 1983. 99. 53. (8)Baker, S.; Roberts, G . G.; Petty, M. C. IEE Proc., Part I: SolidState Electron Devices 1983, 130, 260. (9)Batey, J.; Petty, M. C.; Roberts, G. G.; Wight, D. R. Electron Lett. 1984, 20, 469. (10)Snow, A. W.; Barger, W.; Jarvis, N.; Wohltjin, H. Natl. SAMPE Tech. Conf. 1984,16 (Hi-Tech. Rev. 1984), 388. (11)Snow, A. W.; Jarvis, N. L. J. Am. Chem. SOC. 1984, 106, 4706. (12)Barger, W.R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985, 133, 197. (13)Dilella, D. P.; Barger, W. R.; Snow, A. N.; Smardzeweki, R. R. Thin Solid Films 1985,133, 207. (14)Fowler, M. T.; Petty, M. C.; Roberta, G. G.; Wright, P. J.; Cockayne, B. J. Mol. Electron. 1985, 1, 93. (15)Fryer, J. R.; Hann, R. A.; Eyres, B. L. Nature (London) 1985,313, 382. (16)Hann, R.A.; Gupta, S. K.; Fryer, J. R.; Eyres, B. L. Thin Solid Films 1985, 134, 35. (17)Kovacs, G. J.;Vincett, P. S.; Sharp, J. H. Can. J. Phys. 1985,63, 346. (18)Roberts, G. G.; Petty, M. C.; Baker, S.; Fowler, M. T.; Thomas, N.J. Thin Solid Films 1986, 132, 113. (19)Wohltjen, H.; Barger, W. R.; Snow, A. W.; Jarvis, N. L. IEEE Trans. Electron Devices 1985, 32, 1170. (20)Yamamoto, H.; Sugiyama, T.; Tanaka, M. Jpn. J. Appl. Phys., Part 2 1985,24, L 305. (21)Cook, M. J.; Daniel, M. F.; Dunn, A. J.; Gold, A. A.; Thomson, A. J. J. Chem. SOC.,Chem. Commun. 1986, 863. (22)Hua, Y. L.;Roberts, G. G.; Ahmad, M. M.; Petty, M. C.; Hanack, M.; Rein, M. Philos. Mag., [Part] B 1986, 53, 105. (23)Kalina, D. W.; Crane, S. W. Thin Solid Films 1986, 134, 109. (24)Yoneyama, M.; Sugi, M.; Saito, M.; Ikegami, K.; Kuroda, S.; Iizima, S. Jpn. J. Appl. Phys., Part 1 1986, 25, 961. (25)Cook, M. J.; Daniel, M. F.; Harrison, K. J.; McKeown, N. B.; Thompson, A. J. J. Chem. SOC.,Chem. Commun. 1987, 1148. (26)Fujiki, M.; Tabei, H.; Imamura, S. Jpn. J. Appl. Phys., Part Z 1987, 26, 1224. (27)Hua, Y.L.;Petty, M. C.; Roberta, G. G.; Ahmad, M. M.; Hanack, M.;Rein, M.Thin Solid Films 1987, 149, 163. (28)Nichogi, K.; Machida, Y.; Taomoto, A.; Asakawa, S. Nippon Kagaku Kaishi 1987, 2131.
0743-7463/88/2404-1240$01.50/0 0 1988 American Chemical Society
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Langmuir, Vol. 4, No. 6, 1988 1241
protection from light. These are indicated with the letter p. Under Ar a mixture of CH3GeC13(5.69 g), diiminoisoindoline (7.64 g), and distilled, deoxygenated quinoline (60 mL) was heated to reflux over a 30-min period (p), refluxed for 1h (p), cooled (p), and filtered (p). The solid was washed with MeOH, air dried, and weighed (5.64 g). A like product (22.7 g) was mixed with concentrated H2SO4 (600 mL), and the mixture formed was kept cold (0 "C) for 2 h while being stirred (p). The resulting solution was poured on ice (1500 g) (p), and the suspension obtained was filtered. The solid was washed with water, a warm (-50 OC) concentrated NH40H-pyridine solution (1:l)(in a separate vessel with much stirring), and an ethanol-H20 solution (25) (in a separate vessel with sonication), air dried, and weighed (20.6 9). A mixture of some of this product (3.60 g), HOSi(n-C,&d3 (3.51 g), and toluene (250 mL) was refluxed for 1h (p) and filtered (p). I The residue was washed with toluene (p), and the washings and I OH the filtrate were combined and vacuum evaporated (-85 "C) to a viscous suspension (p). The suspension was mixed with pentane Figure 1. Structure of (HO)G~PCOS~PC(OS~(~-C~H~~)~). (50 mL) (p), and the mixture formed was cooled to 0 "C (p) and filtered (p). The solid was vacuum dried (-85 "C) and weighed to date has given a monolayer on a water surface in which (3.69 9). the rings are parallel to the water it has not Some of this solid (1.50 g) was mixed with glacial HOAc (3 mL), yielded a multilayer of monolayers in which the rings are H20 (3 mL), and toluene (50 mL) (p), and the suspension formed parallel to the substrate surface. was refluxed for 30 min (p) and filtered (p). The solid was washed In the present paper, we describe the synthesis of a novel with MeOH, air dried, and mixed with concentrated NH40H (5 amphiphilic two-ring phthalocyanine, the formation on mL) and pyridine (50 mL). The resulting suspension was refluxed water of a monolayer of this compound in which the rings for 15 min and filtered. After being washed with MeOH the solid are parallel to the water surface, and some properties of was vacuum dried (-85 "C) and weighed (0.814 g). An intimate mixture of a similarly prepared product (2.17 g) this monolayer. Also described are the preparation of and S ~ P C ( O H ) ( O S ~ ( ~ - C(3.15 ~ H g)39 ~ ~ )was ~ ) suspended in 1,2,4multilayers of the compound on solid substrates in which trimethylbenzene (150 mL), and the suspension was refluxed for the rings appear to be parallel to the substrate plane and 4 h (p) and filtered (p). The residual solid was washed with toluene the fabrication and properties of metal-insulator-metal (p), and the washings and the filtrate were combined (p) and devices made with aluminum electrodes and multilayers vacuum evaporated (-70 "C) to a viscous suspension (p). The of the compound of varying thicknesses. suspension was mixed with pentane (100 mL), and the mixture Phthalocyanine Utilized. The phthalocyanine used formed was filtered. The solid was vacuum dried (-85 "C) and to make the films, (HO)G~P~OS~PC(OS~(~-C~H~~)~), where weighed (3.91 g). Pc is the phthalocyaninatodianion, Figure 1, was specifSome of this product (3.14 g) was dissolved in toluene (3 L), ically designed and prepared for this purpose. Several and the solution formed was irradiated with direct sunlight for 75 min and then vacuum evaported (-30 "C) to dryness. The preliminary descriptions of it have been solid remaining was vacuum dried (-85 "C) and weighed (2.77 The OH group in this compound was incorporated in 9). it to give it a hydrophilic head group. The trihexylsiloxy A portion of this product (1.46 g) was chromatographed (dry group was incorporated to give it a matching hydrophobic loading on Celite 545, A1203(V),CHC13) (0.839 g), and some of tail group and, at the same time, confer upon it solubility the resultant (0.394 g) was rechromatographed (solvent loading, in organic solvents. To give the compound a profile that Al,O,(V), toluene-CHC13 (1:l))(0.141 g). Part of this (0.072 g) would lead to a substantial collapse pressure, the oxywas mixed with pyridine (20 mL) and water (5 mL), and the gen-bridged ring stack was build into it. (The Ge-0-Si resulting suspension was kept warm (-60 "C) for 1h while being backbone was incorporated in the compound not to give stirred. It was then filtered, and the solid was washed with H20, vacuum dried (-85 "C), and weighed (0.059 g): IR (Nujol mull) it added amphiphilic character but rather to meet the 3500 (m, 0-H), 1250 (w, Si-CH2), 1025 (s, Si-0-Si), 940 (m, demands of the synthetic approach used.) Si-0-Ge) cm-'; 'H NMR (200 MHz, CDC13, 20 "C, 4.1 X M) 6 8.92 (m, 1,4-SiPcH),8.87 (m, 1,4-GePcH),8.31 (m, 2,3-GePcH), Experimental Section 8.25 (m,2,3-SiPcH),0.42(m, c-CH2,t-CH&,-0.20 (m, 6-CHz),-0.65 Synthesis of (HO)GePcOSiPc(OSi(n-CBH13)3). Some of (m, -y-CH2),-2.29 (m, P-CH,), -3.53 (m, cu-CH2);MS-FAB m / z the procedures used in this synthesis were carried out with 1442 (M - OH)', 1143 (M - OSi(n-C6H13)3)+, 840 (M - OGePcOH)', 603 (M - O S ~ P C O ( S ~ ( ~ - C ~ H558 ~ ~(M ) ~ -) )OGePc+, (oSi(n-C6H&)+. (29) Curry, J.; Cassidy, E. P. J. Chem. Phys. 1962, 37, 2154. Anal. Calcd for C82H72N1BSi2Ge03: C, 67.54; H, 4.98; Ge, 4.98. (30) Schra", C. J.; Scaringe, R. P.; Stojakovic, D. R.; Hoffman, B. Found: C, 67.12; H, 5.03; Ge, 4.52. M.; Ibers, J. A.; Marks, T. J. J. Am. Chem. SOC.1980,102, 6702. (31) For recent reviews dealing with microelectronicdevices based on The compound is deep blue when finely divided. It is soluble Langmuir-Blodgett films, we papers by Vicett,sz Roberta,w and SugLs in CHC13 and slightly soluble in toluene and pyridine. (32) Vincett, P. S.;Roberta, G. G. Thin Solid Films 1980, 68, 135. Film Studies. The film balance and the water purification (33) Roberta, G. G. Sem. Actuators 1983,4, 131. system used in these studies have been described p r e v i ~ u s l y . ~ ~ ~ ~ (34) Roberta, G. G. Contemp. Phys. 1984,25,109. The water had a pH of 5.5-6.0. (Its slight acidity, due to at(35) Sugi, M. J. Mol. Electron. 1985, 1, 3. (36) Fu, C. W.; Batzel, D. A.; Rickert, S.E.; KO,W. H.; Fung, C. D.; mospheric COz,was deemed unimportant since it has been shown Kenney, M. E. Digest of Technical Papers, The 4th International Conthat monolayers of materials like those dealt with here are not ference on Solid State Sensors and Actuotors; Tokyo, Japan; Institute affected by a slight subphase acidity.') All layers were spread of Electrical Engineering of Japan: Tokyo, Japan, 1987; p A-8.4. on water that had been freshly purified and had been swept with (37) Fu, C. W.; Batzel, D. A.; Rickert, S. E.; KO,W. H.; Kenney, M. the moving barrier prior to use. E. Abstracts of Papers, The IUPAC International Symposium on Polymers for Aduanced Technologies; Jerusalem, Israel; International Union of Pure And Applied Chemistry: Oxford, U.K., 1987. (38) Batzel, D. A.; Fu, C. W.; Rickert, S. E.; KO,W. H.; Kenney, M. E. Abstracts of Papers, International Topical Workshop, Advances in Silicon-Based Polymer Science; Makaha, HI; Amercian Chemical Society: Washington, DC, 1987; p 53-5.
(39) DeWulf, D. W.; Leland, J. K.; Wheeler, B. L.; Bard, A. J.; Batzel, D. A.; Dininny, D. R.; Kenney, M. E. Inorg. Chem. 1987, 26, 266. (40) Shutt, J. D. PbD. Thesis, Case Western Reserve University, 1988. (41) Shutt, J. D.; Burkhart, C. W. Colloids Surf. 1988, 29, 233.
Shutt et al.
Langmuir, Vol. 4, No. 6, 1988 Generator
I y channel
mu it i layer bottom electrode 4-giass
Figure 3. Electrical measurements circuit.
Figure 2. Metal-Langmuir film-metal device structure. The solvent used in the stock spreading solution was HPLC grade CHC13 stabilized with 0.75 % ethanol. The concentration of the solution was 9.6 X lo4 M (1.4 mg/mL). When not in use, the solution was stored at 5 “C in amber glass vials that were tightly sealed with Teflon-lined caps. The solution was handled and stored under UV-filtered light at all times (to reduce the formation of HC1 and thus to avoid significant cleavage of the siloxy group of the compound). The films used in the pressurearea isotherm studies were made by spreading 35 pL of the stock solution (3.4 X lo4 mol) on the water with a single shot from a Dnunmond micropipet at an initial area of 3.8 nm2/molecule. After 4-5 min, the resulting films were compressed at a rate of 15 cm2/min (0.074 nm2/(molecule.min)), and pressure-area data were collected. The f i s used in the compressive creep studies were prepared by spreading the stock solution in the same way. However, these films were then brought to a given pressure, and area-time data were collected at this pressure. For the relaxation studies, initial films were again prepared in the same way and then brought to a given pressure. Pressure-time data were then collected while the area was being kept constant. Multilayer and Device Studies. The monolayers used for deposition were made by spreading the stock solution on the water at an initial molecular area of 2.0 nm2/molecule. They were then compressed to a pressure of 10 mN/m. The substrates used were glass microscope slides upon which 21 aluminum electrodes had been shadowed. These electrodes were 200 nm thick, 1 mm wide, and 4 mm long. They were positioned so that their long axes were parallel to the long axes of the slides. Details of the procedure used to fabricate the substrates are given Before the substrates were used, they were ultrasonically cleaned for 15-30 min with HPLC grade methanol, flushed with running ultrapure water, and dried at 120 OC in a Class 100 oven. Their surfaces were uniformly hydrophilic. In the monolayer deposition process itself, first the substrate was mounted in the film balance with its long axis perpendicular to the surface of the water, and then it was submerged in the water. Next, a film of the compound was spread on the water and compressed, and the substrate was withdrawn from the water at a rate of 5 mm/min until its bottom edge was -2 mm above the water surface. The substrate was held in this position for 2 min and then was lifted into a chamber that was being flushed with a laminar flow of filtered, dry nitrogen. It was held in the chamber for 15 min and following this was dipped into the water at a rate of 5 mm/min until all the electrodes had been submerged. Next it was withdrawn from the water at the same rate until it was above but still in contact with the water and was held in this position for 2 min. This dipping-withdrawal procedure was repeated as many times as necessary to build up the desired multilayer. With these conditions the transfer ratio of the monolayer was 1:l. Exploratory tests indicated that after the first few layers had been deposited a faster rate of substrate movement could have (42)Biddle, M. B. Ph.D. Thesis, Case Western Reserve University, 1988.
Figure 4. Relationship between i+, i-, and V on a typical loss capacitor trace. been used. Details of the deposition apparatus and related microcomputer control programs are given elsewhere.40 After the deposition had been completed, the coated slide was stored in a nitrogen-filled desiccator box for at least 24 h. It was then shadowed with a second set of electrodes, each of which was 100 nm thick, 1 mm wide, and 4 mm long. These electrodes were positioned above the first set and oriented at right angles to them. One of the resulting devices is shown schematically in Figure 2. The active device area is at the overlap of the electrodes and is thus 1 mm2. Tungsten point probes were placed in contact with the top electrodes, and 0.5” gold J-shaped wires were placed in‘contad with the bottom electrodes. Both the probes and the wires were positioned with the aid of micromanipulator controls and a microscope. The contacts were tested by measuring the resistance between opposite ends of the electrodes. They were taken as being satisfactory if the resulting dc resistance was 2 Q or less. Capacitance and conductance data for the devices under various conditions were obtained by means of an ac oscillographic technique,43Figure 3. The oscilloscope (Gould OS3600) was operated in an X-Y mode. To obtain the necessary measurements for a device, a wave of amplitude V, and frequency f was applied by the signal generator (Hewlet-Packard 3325A) to the device modeled in Figure 3 as device resistance R in parallel with device capacitance C. The resulting current was measured as a voltage drop across the measuring resistor h.This voltage drop was kept much smaller than the voltage drop across the device by makiig the resistance of R, suitably small. With this arrangement, both the capacitance and the parallel resistance of the device could be obtained simultaneously for a triangular or sinusoidal voltage input at a frequency f and a bias V. For a triangular voltage input, the capacitance at a particular frequency, C(0, was obtained with the equation
where i+ and i- are the currents measured in the rising and falling (43) Twarowski, A. J.; Albrecht, A. C. J.Chem. Phys. 1979, 70,2255.
Fabrication of Thin-Film Dielectrics
Langmuir, Vol. 4, No. 6, 1988 1243
a a m
0 20 40 60 Mo I ec u I a r A r e a ( nm2/m o I ec ul e 1 Time (min) Figure 5. Isotherms of ( H O ) G ~ P C O S ~ P C ( O S ~ ( ~films -C~H~~)~) at various temperatures ("C). Figure 7. Stress relaxation curves of (HO)GePcOSiPc(OSi(n-
C6H13)$) films at various pressures.
9 6 / , , 0
Time (min) Figure 6. Compressive creep curves of (HO)GePcOSiPcOSi(nC&I13)3) films at various pressures. sections of the applied voltage wave, respectively, Figure 4. For a sinusoidal voltage input, Ccf, was obtained with the equation Ccf, = (i+
- i-) /47rf(
+++ +++ +++ +++ 2.7
where V is the voltage input at which i+ and i- are measured. In both cases, the resistance R ( f )was obtained with the equation Rcf, = 2V/(i+ + i-) (3) The conductance Gcf, was obtained as the reciprocal of Rcf,. In the experiments carried out, Ccf, and Ref, were measured across a frequency range of 1 Hz to 1 MHz.
Results Film Properties. Pressure-area isotherms for the monolayer a t various temperatures are shown in Figure 5. These indicate that two molecular arrangements occur during its compression. In addition, they indicate that both the phase-change pressure (see below) of the highco-area arrangement and the co-area of the low-co-area arrangement vary inversely with temperature. They further suggest that the collapse pressure of the low-co-area arrangement and the co-area of the high-co-area arrangement also vary inversely with temperature. The 20 "C isotherm gives a co-area of 1.7 nm2/molecule and a phase-change pressure of 17 mN/m for the high-co-area arrangement and a co-area of 1.0 nm2/molecule and a collapse pressure of 60 mN/m for the low-co-area arrangement. Compressive creep curves for films of the compound are shown in Figure 6. Stress relaxation curves for them are shown in Figure 7.
Figure 8. Capacitance of 15-layerdevices as a function of relative position on the slide.
Multilayer Formation and Properties. Monolayers of the high-co-area arrangement were satisfactorily deposited at 20 "C. The resulting multilayers were blue. This blue was uniform in intensity when the multilayers had the same number of layers throughout but was sharply stepped in intensity when they had a stepped structure. During the upward trips in the deposition process, it wm possible to observe the drainage front on the edge of the slide with the aid of the balance miroscope. At a withdrawal rate of 5 mm/min, it was found that this front occupied a roughly constant position and had a parabolic shape. At somewhat higher withdrawal rates, it moved higher as the slide advanced and eventually broke. When this happened, the multilayer had irregularities and bore visible water on its surface. (A discussion of the limitations on the speed at which thin films can be deposited that are imposed by drainage front movement had been given by Peter~on.)~~,~~ No conditions were found that allowed the formation of multilayers of the low-co-area arrangement. (44)Peterson, I. R.; Russell, G. J.; Roberta, G. G. Thin Solid Films 1983, 109, 371. (45) Veale, G.; Girling, I. R.; Peterson, I. R. Thin Solid Films 1985, 127, 293.
Shutt et al.
1244 Langmuir, Vol. 4 , No. 6, 1988
mA r 0.6
Figure 9. Current-voltage trace of a five-layerdevice with an applied triangular wave at 10 KHz. mA
- 2@ "k;
Figure 11. Current-voltage trace of a five-layer device with an applied sinusoidal wave at 10 kHz. 10
Figure 10. Current-voltage trace of a five-layer device with an applied triangular wave at 1 Hz.
Device Properties. Devices with 1-29 monolayers of the high-co-area arrangement were made. None of these devices was initially short circuited. The capacitance of 15-layer devices as a function of their position on the slide was determined by studying the capacitance of a slide carrying 18 15-layer devices. The results obtained are given in Figure 8. From these results it can be seen that the capacitance of the devices near the top of the slide is similar, while that of those near its bottom is substantially higher. This is probably attributable to variations in the multilayer caused by incomplete drainage of the slide during the film deposition process. Because of this variation, all further testing reported here was done on devices that were situated near the tops of the slides. The shape of the current-voltage traces of a five-layer device for applied triangular voltage inputs of various frequencies was also determined. At frequencies above about 100 Hz, the shape was essentially rectangular (excluding rounding due to the size of the measuring resistance, R,, used), Figure 9. At lower frequencies it was rounded, Figure 10 (a reference ceramic capacitor treated under the same conditions gave a rectangular trace as expected). Similarly, the shape of the current-voltage traces of a five-layer device for applied sinusoidal voltage inputs for frequencies above 10 kHz was determined. These were found to be approximately circular, Figure 11. The capacitance of devices with various numbers of layers was measured as a function of the number of layers at various frequencies as well. A t a frequency of 5000 Hz, a linear relationship was found between inverse capacitance and the number of layers, Figure 12. The same type of relationship was found for frequencies up to 10000 Hz and down to 500 Hz. From the capacitance-thickness data a t 5000 Hz, the thickness of the aluminum oxide layer on the electrodes and the relative permittivity of the multilayers were estimated. For these estimations, it was assumed that the inverse capacitance of the aluminum oxide layer is given
N Figure 12. Inverse capacitance of devices as a function of number of layers at 5 kHz. 12
*a 15 layers
01 0 1
ci) ( r a d / s )
Figure 13. Capacitance of devices with varying numbers of layers as a function of frequency. by the intercept of the line drawn in Figure 12. Further, it was assumed that the relative permittivity of the oxide layer was 8.3.46The thickness of the aluminum oxide layer was calculated with these assumptions and the equation
(4) where C is the total capacitance, do, is the thickiness of the aluminum oxide layer, eo: is the relative permittivity of the aluminum oxide layer, N is the number of monolayers, d is the thickness of a monolayer, tLris the relative permittivity of the multilayer perpendicular to the multilayer plane, e,, is the permittivity of a vacuum, and A is the area of the capacitor. The value obtained for this thickness is 3.9 nm. This value, the value for the thickness of the monolayer, 1.16 nm:' and eq 4 were then used to estimate the relative permittivity of the multilayers. The value obtained is 5.5 0.2. (46) Argall, F.; Jonscher, A. K. Thin Solid Films 1968,2, 185. (47) Chiang, C. W.; Lando, J. B., Case Western Reserve University, personal communication.
Langmuir, Vol. 4,No. 6, 1988 1245
Fabrication of Thin-Film Dielectrics
03 (rad/s) Figure 14. Conductance of a five-layer device as a function of
W (radls) Figure 15. Loss tangent of a five-layer device as a function of frequency.
In addition, the capacitance and conductance of devices which had varying numbers of layers were determined as a function of frequency. For capacitance, as is shown in Figure 13, the dependence was found to be nonlinear. For conductance, as is illustrated in Figure 14, it was found to follow approximately the equation log G = n log w (5) where G is the conductance,n is a proportionality constant, and w equals 2rf and is the angular frequency. The value of n was found to be a function of the number of layers; Le., for 1,5, and 15 layers n was found to be 1.00,0.86, and 0.66, respectively. From the capacitance-conductancedata for the five-layer device, the loss tangent, G/Co,for the device was determined as a function of frequency, Figure 15.
Discussion Synthesis of (HO)GePcOSiPc(OSi(n-C&13)3). While the intermediates in the synthesis of (H0)GePcOSiPc(0Si(n-C6Hl3),) have not been fully identified, it is concluded on the basis of NMR and IR spectra taken during the synthesis and on the basis of data for relevant silicon phthalocyanine^^^^^ that the synthesis can be summarized as shown in Scheme I. It is of interest that the GePc-CH, bond is stable to concentrated H2S04but is readily photolyzed. In this respect, it is like the analogous SiPc-CH3 bond.48 Attempts to make the corresponding silicon two-ring compound by a parallel route were abandoned because the necessary ring-to-ring bridging reaction did not take place sufficiently. Apparently the OH group of SiPc(CH,)(OH) (48)Esposito, J. N.; Lloyd, J. E.; Kenney, M. E. Inorg. Chem. 1966, 5, 1979.
is less reactive toward S ~ P C ( O H ) ( O S ~ ( ~ - Cthan ~ H is ~~)~) the OH group of GePc(CH3)(OH). In accordance with expectations, the NMR spectrum of the compound shows large ring-current effects. Film Phases. As indicated above, it is clear that two molecular arrangements occur during compression of the initially formed film. From the data shown in Figure 5, it appears that the transition between them is not first order.49 This suggests that these arrangements may not be phases in a strict thermodynamic sense. However, they will be referred to as phases in the present discussion. On the basis of the data for the co-area of the high-coarea phase and data for the packing area of a phthaloit is concluded cyanine ring resting on its face, 170 that the rings in the high-co-area phase are parallel to the water surface. The arrangement of the rings in the lowco-area phase is not immediately apparent. It may involve tilted or stacked rings. As is seen, Figure 5, the co-area of the low-co-area phase varies inversely with temperature at a constant rate of change of pressure with area. Accordingly, its surface compressional modulus ks,a quantity given by the equation
ks = -A(dr/dA)T
where A is the area of the molecule, ( d ~ / d A is ) ~the isothermal rate of change of surface pressure with area, and T is the temperature,62also varies inversely with temperature. Although the surface compressionalmodulus of the high-co-area phase may vary likewise, this is not clearly shown by the data. Additional data are required for the development of a detailed understanding of the structures of the phases and the transition between them. Efforts to gather such data are in progress. Film Relaxation Behavior. The creep results at 10 and 15 mN/m, Figure 6, show that a sharp decrease in the stability of the film occurs when the pressure on it is increased from 10 to 15 mN/m. This decrease is attributed to the onset of the high to low-co-area phase change at the higher pressure. The creep results at 50 mN/m show that the rate of creep is initially high at this pressure but then decreases. This change may also be associated with the high to low-co-area phase change. The stress relaxation results on the film, Figure 7, reveal that at an initial pressure of 10 mN/m most of the initial pressure, 90%, remains even after 30 min. From this it is clear that no relaxation processes for the high-co-area phase are readily available under these conditions. This is believed to be one of the factors that leads to the good (49)Pallas, N.R. Ph.D. Thesis, Clarkson College of Technology, 1983. (50)Buchholz, J. C.; Somorjai, G. A. J . Chem. Phys. 1977, 66,573. (51)Harada, Y.; Ozaki, H.; Ohno, K.; Kajiwara, T. Surf. Sci. 1984,147, 356. (52)Gaines, G.L. Insoluble Monolayers at Liquid-Gas Interfaces; Wiley: New York, 1966;p 24.
1246 Langmuir, Vol. 4,No. 6,1988 deposition properties of this phase; for if such internal relaxation processes were present, they would compete with the relaxation process provided by the deposition and thus interfere with it. As is seen, Figure 7, a plateau occurs in the 15 mN/m stress relaxation curve when the pressure drops to -11 mN/m or -75% of its initial value. This indicates that the film achieves a temporary stability as the pressure is released. Since the 50 mN/m curve runs nearly parallel to the 15 mN/m curve until the latter reaches its plateau, it appears that the initial relaxation mechanism is similar for the film at both pressures. Multilayer and Device Properties. As is clear from the experimental section, the structure of the multilayers was not directly investigated by polarized optical methods or other direct methods. However, in view of the molecular shape of (HO)GePcOSiPc(OSi(n-C6H&, the properties of its monolayer, and the 1:l transfer ratio characteristic of the deposition process of the monolayer, it is concluded that the rings in the deposited multilayer are parallel to the substrate plane. The apparent parallel ordering of the rings in the multilayers suggests that the monolayer has enough fluidity to deposit well. This is significant because not all monolayers possess this character is ti^.^ The shapes of the current-voltage traces of the five-layer device with a triangular voltage input at frequencies above about 100 Hz, Figure 9, show that it has a finite parallel conductance but no voltage dependence on capacitance or conductance under these conditions. With the same type of voltage input at low frequencies, Figure 10, the shapes of its traces show that it does have a voltage dependence in capacitance and conductance under these conditions. Thus the behavior of the device with a triangular voltage input is nearly ideal at high frequencies but not at low frequencies. The shapes of the traces of the device with a sinusoidal voltage input over the range examined, Le., above 10 KHz, show, Figure 11, that under these conditions it again has a finite parallel conductance but no voltage dependences and thus again has nearly ideal behavior. As shown earlier, the real part of the relative permittivity of the multilayers as estimated from the inverse capacitance-thickness curve is 5.5 f 0.2. This is somewhat larger than the real part of the relative permittivities of deposited multilayers of certain porphyrins as estimated by the technique used with the present m~ltilayers.5~9”It is also somewhat larger than the real part of the relative permittivities of bulk polycrystalline H,Pc and CuPc as estimated by a difference technique.55 However, in the porphyrin multilayers the rings were at an angle to the substrate, and in the bulk phthalocyanines, since the crystallites were at all angles, the rings were at all angles. These orientation differences combined with the expected anisotropies of the rings may well account for much of the observed differences. The value for the thickness of the aluminum oxide layer estimated from the inverse capacitance-thickness curve, 3.9 nm, is similar to a previously reported value for the thickness of thermally grown oxide layers on aluminum.@ It thus appears reasonable. Decreases in capacitance at high frequency similar to the decreases observed with the one- and five-layer devices, (53) Jones, R.;: Tredgold, R. H.; Hodge, P. Thin Solid Films 1983,99, 25. -_
(54) Jones, R.; Tredgold, R. H.; Hoorfar, A,; Hodge, P. Thin Solid
(55) Abkowitz, M. A.; Lakatos, A. I. J . Chem. Phys. 1972,57,5033. (56) Tredgold, R. H.; Jones, R.: Evans, S. D.: Williams. P. I. J . Mol.
Shutt et al.
Figure 13, have been observed in similar lead stearate57 and p ~ l y b u t a d i e n edevices. ~~ With the polybutadiene devices, it is suggested that the decreases are the result of dipolar relaxation in the multilayer. With the lead stearate devices it is shown that the decreases result trivially from the series combination of the aluminum oxide and the multilayer capacitances. For an ideal series combination capacitor of the aluminum oxide multilayer type, the capacitance should level off to a final value at high frequency. This value, Cf, is given by the equation (7) where C,, is the capacitance of the oxide layer and CL is the capacitance of the multilayer. Since in the present case the decrease in capacitance are much larger than those predicted by this equation, it appears that they may be due to both dipolar relaxation and series combination effects. The essential lack of change in capacitance at low frequency seen with the one-layer device, Figure 13, is similar to that seen with a one-layer porphyrin device,” while the increase in capacitance seen with the 5- and 15-layer devices is similar to the increase found with multilayer porphyrin,” polybutadiene,58and lead stearate5’ multilayer devices. The increase in these latter devices have been ascribed variously to ionic conduction, electron hopping between traps, and space charge effects. Since aluminum oxide itself shows a similar dispersion at low frequencie~:~ it appears that the dispersion observed with the present multilayer devices is attributable to a combination of multilayer and aluminum oxide layer effects. As noted above, a power law dependence of conductance on frequency is found for the devices. This type of dependence is known to be characteristic of most dielectrics including thin-film dielectrics.m With such dielectrics the exponent of the power law pertaining to them is 1if the response is a “lattice” response and 0.3-0.8 if it is a “carrier” or hopping response.%gW2 Thus it appears that the conductance of the monolayer device studied here is dominated by a lattice response associated with the aluminum oxide layer, and the conductance of the multilayer devices is influenced more and more by hopping conduction as the number of layers increases. From Figure 14 it is seen that there is a dip in the log G-log w curve for the five-layer device at about 10 KHz, and from Figure 15 it is seen that there is a corresponding dip in the loss tangent-log w curve for the device at this frequency. Earlier it has been found that the log G-log w curves of devices with aluminum oxide dielectric layers have a dip at a slightly higher frequency, -10.5 K H Z . ~ ~ Thus it appears that the dips observed in the present curves are attributable to the aluminum oxide layers. Data from devices with electrodes with no oxide layers, e.g., with gold electrodes, could confirm or refute this interpretation.
Conclusions The novel phthalocyanine (HO)GePcOSiPc(OSi(nC6H&J gives monolayers on a water surface in which the (57) Honig, E. P.; de Koning, B. R. J . Phys. C 1978,11, 3259. (58) Christie, P.; Petty, M. C.; Roberta, G. G.; Richards, D. H.; Service, D.; Stewart, M. J. Thin Solid Films 1985,134,75. (59) Millany, H. M.; Jonscher, A. K. Thin Solid Films 1980, 68, 257. (60) Jonscher, A. K. Nature (London) 1977,267,673. (61)Careem, M.; Jonscher, A. K. Philos. Mag. 1977,35,1489. (62) Ngai, K. L.; Jonscher, A. K.; White, C. T. Nature (London)1979, 277, 185.
Langmuir 1988,4, 1247-1251 rings are parallel to the water surface and multilayers on solid substrates in which the rings appear to be parallel to the substrate surface. It has “head” and “tail” characteristics arising from the combination of its trihexylsiloxy and hydroxy groups and organic solubility because of its trihexylsiloxy group. The compound has the ability to be compressed into a monolayer that is robust enough to be formed into multilayers due, in part, to its thick, stiff ring stack. The multilayers, formed by the compound are almost certainly highly anisotropic. In addition, they are thinner than aliphatic multilayers with comparable numbers of layers because their monolayers are thinner than those of the aliphatic multilayers. For the same reason the thickness of the new multilayer can be adjusted more precisely than can the thickness of the aliphatic multilayers. The monolayer and multilayers are stable dielectrics. It appears that the conduction mechanism in the multilayers
is a hopping-type mechanism. It further appears that the overall conduction mechanism in the devices is a hopping-type mechanism modified by effects due to the oxide layers on the electrodes. The monolayer and multilayers are members of a new class of dielectrics having the potential to yield microelectronic devices engineered to a high level of precision. Among the types of devices for which they appear suited are those in which semiconductivity and photoconductivity play a role.
Acknowledgment. We thank Professors J. B. Lando, J. A. Mann, and W. H. KO of Case Western Reserve University for help with the analysis of the data. The generous support of DARPA and the Office of Naval Research (Grant N00014-83K-0246) is gratefully acknowledged. Registry No. (HO)G~PCOS~PC(OS~(~-C~H~~)~), 115461-96-6.
Effect of Added Sodium and Lithium Chlorides on Intermicellar Interactions and Micellar Size of Aqueous Dodecyl Sulfate Aggregates As Determined by Small-Angle Neutron Scattering Stuart S. Berr*>tand Richard R. M. Jonesf Magnetic Resonance Imaging Facility, Box 190, Medical Center, University of Virginia, Charlottesville, Virginia 22908, and Industrial and Consumer Sector Research Laboratory, 3M Company, 3M Center-201-4N-01, St. Paul, Minnesota 55144 Received January 9, 1987. I n Final Form: June 9, 1988 The effect of added electrolyte on ionic micelle size and interparticle interactions has been studied by small-angle neutron scattering (SANS). The surfactants, lithium and sodium dodecyl sulfate (LDS and SDS), were kept at a fixed concentration (0.05 M), while their respective chloride concentrations (LiC1 and NaCl) were increased. As the concentration of added salt was raised, intermicellar Coulombic repulsions decreased to the point where attractive (possibly van der Waals) forces could dominate. Furthermore, the added electrolyte appeared to screen intramicellar head-group repulsions, allowing the micelles to grow in terms of an increased aggregation number, N . Sodium was found to be much more effective than lithium at screening charge. N for SDS micelles increased from 54 to 928, and the contact potential decreased from 16 to -221 kJ/mol when [NaCl] was increased from 0.0 to 0.6 M. N for LDS, on the other hand, went from 53 to 91, and the contact potential decreased from 14 to -2.5 kJ/mol as [LiCl] was increased from 0.0 to 0.8 M.
Introduction The interactions between charged micelles are dominated by Coulombic repulsive interactions when the ionic strength of the solution is low. These interactions prevent micelles from approaching closely enough to experience strong van der Waals attractions. The ionic strength of the solution can be increased through the addition of an electrolyte. The effect of doing so has been previously studied by small-angle neutron scattering (SANS) for large concentrations of lithium dodecyl sulfate (LDS) with added LiC1’ and by static and dynamic light scattering for t Magnetic Resonance Imaging Facility, University of Virginia.
This paper was drawn from work done at Wake Forest University, Winston-Salem,NC, and Oak Ridge Associated Universities, Oak Ridge National Laboratory, Oak Ridge, TN, as partial fulfillment of requirements for the Ph.D. degree. BITNET address is ssb4ba Virginia. *Industrialand Consumer Sector Research Laboratory, 3M Company.
sodium dodecyl sulfate (SDS) at just above the cmc with added NaC1.2 In this work, an effective intermicellar potential is used to account for the small-angle neutron scattering that is due to the interparticle structure as described by the function S(Q). Once S(Q) has been taken into account, the intraparticle form factor, F(Q),is calculated and used to determine the micellar size. The basic micelle model utilized consists of an elliptical hydrocarbon core, surrounded by a shell of constant thickness that contains the sulfate head groups, associated counterions and water, and possibly some surfactant hydrocarbon.
Experimental Section Surfactants. The preparation of SDS-I,I-d2and LDS-I,I-d2 was carried out as previously described?~~ These surfactants were (1)Bendedouch, D.; Chen, S.-H. J. Phys. Chem. 1984,88, 648. (2) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 711.
0 1988 American Chemical Society