Experimental Guidelines for Producing Molecular Assemblies by Langmuir-Blodgett Techniques M. J. Levine and J. A. k h w a r z t Department of Chemical Engineering and Material Science. Syracuse University, Syracuse, NY 13244 The idea of placing monomolecular layers of molecules onto an ao~rowriatesubstrate in the form of an ordered array was fikt postulated by Irving Langmuir in the 1930's. Basic experiments involving the deposition of monolavers and multilayers (successive monolay&) were performedunder Langmuir by Katharine B. Blodgett using long-chain fatty acids (I). With the ever-increasing need for miniaturization of electrical components for computer hardware, the study into the properties of thin organic films has undergone renewed interest. In anticiwatiou of the continued size reduction of switching elements to the molecular level, new approaches to materials and fahrication techniaues for accomwlishinr lithography a t the molecular scale must be devised. ~ x t i n s i v e studies into the physical orientation and conducting mechanisms of various types of Langmuir-Blodgett films have been conducted with encouraging results by people like Hans Kuhn, of the Max-Planck Institute, and others (2-4). Although fundamentally based on organic chemistry, the field of thin organic films is actually very interdisciplinary in nature involving elements of physical chemistry, physics, biology, as well as chemical engineering and material science. T o the chemical engineerhaterial scientist, thin organic films are a natural extension to extensively studied silicon-based films. The challenge of successfully harnessing the untapped potential of thin organic films is to create microelectronic devices superior to those which can currently he supplied by the silicon chip industry. With all the renewed fervor in this area, there invites a need for basic information for those wishing to break into the realm of thin organic films. The objective of this paper is to define precisely the basic problems and considerations that are not so ohvious, but important, to the heginner in this field. This task will be accomplished by examining the actual difficulties and ~roblemsthat were encountered in the deposition of calci& stearate monolayers on to glass substrates using an easilv fabricated and inexpensive L a m muir-~lodgetttrough in-an attempt to reproduce one of t6e Blodgett experiments (5). Basic Background Information Lanpmuir-Blodcett ( G B .) aliwhatic soap films are easilv . formed on water-bath surfaces. These films consist of molecules that contain h\~drowhilic " . (-COO-) and h\fdroohobic ((CH2)nCH3) groups a t opposite ends of a lo& chain. such films will form a monolayer on the top of a water bath as shown in Figure 1. Common types of films that can be deposited onto a substrate from a-water bath arr shown in the insert of Figure 1. Earh layer o f a film is deposited onto thesuhstrate in juccessive layers by repeatedly dipping the substrate into and out of the water bath. Experimental results, like those in reference I , have demonstrated that different film structures can be produced
-
' The Teflon plugs were cut from broken buret stopcocks. 638
Journal of Chemical Education
III
0
...... ..... .... ..... ....
-CH, (Hydrophob~c)
I
PPP PPP PPP
................ ................ .............. ................. X-Type
............... ................... Y-Type
Carton main
Substrate
2- Type
F gLre 1 The artentat on ot sllphal~emap mo sc.les lormlnga monolayer on a wafer ~ b r f a c e(amv.9) Types of flms commonly oeposwd on a suosfrafe (below).
depending on the method of preparation. The X-type films are those which are onlv deposited on the substrate uwon its descent into the wate; b&h. The Y-type films are those which are deposited on the suhstrate during both the substrate's ascent out of and descent into the water bath. The Ztype films are those which are deposited only during the substrate's ascent out of the water bath (6). An aliphatic soap film is typically formed on the surface of a water bath by depositing drops of long-chain fatty acid (e.g., stearic, palmitic) on the surface of a water bath that is doped with a slight concentration of metal ion (e.g., Naf, Ba2+,Cd2+).The molecules of fatty acid react with the cation in the water to form single soap molecules that are spread across the surface of the water bath. These soap molecules are then compressed into a uniform mouomolecular film (called a monolayer) by exerting a surface pressure on the region of the surface containing the soap molecules (this surface pressure is on the order of 20-30 dyneslcm for calcium stearate a t 20 OC). The soap molecules will transfer to a substrate if and only if they are compressed into a monolayer. This basic description of some of the physical properties of L-B films is highly idealized in nature. Perfect films will not just form as shown in Figure 1 using the above procedures alone. In fact, unless nearly perfect conditions exist in the laboratory, small perturbations like dust, oil, and vibration of the substrate can commonly cause films to be deposited either as films with holes and foreign particles or as small islands of film. In the discussion to follow, this actual deviation from ideality will he demonstrated through the
description of the procedures used to produce Y-type films. It is important to note that the lack of experimental detail reported in both the original works of Langmuir and Blodgett, as well as in work since then, complicates the attainment of the nearly perfect conditions, that are required. Similarly it is important to emphasize that the preparation of continuously uniform monomolecular films, while scientifically well established, has remained more an art than a science. Experimental Overview
The experiment involved the deposition of Y-type monolayers of calcium stearate (CaC1sH3fi02) on ordinary glass substrates. The monolayers of calcium stearate were created hy spreading drops of stearic acid in various solvents over the surface of an aqueous solution 1 0 4 M calcium carbonate (Fisher lot #051069) in distilled water. The solution was air saturated with an adjusted pH between 6.2 and 6.6 as measured withaFisher Accumet p H meter (Model 805 MP). T h e calcium stearate film was compressed on the surface of the aqueous solution by use of a "piston oil" that was also spread on the surface. This "piston oil" was either castor oil or oleic acid, which each exert surface pressures of approximately 17 dyneslcm and 30 dyneslcm, respectively, a t 20 'C (7). Compressed monolayers of calcium stearate were then transferred to a glass substrate by successive dipping of the substrate a t a carefully controlled speed and rate. Apparatus Any apparatus used to deposit L-B films must have certain unique properties in its design. First, the trough must have a hydrophobic surface (e.g., Teflon) in order to allow the meniscus of the water bath to lie above the trough's edges. Second, the frame of the trough must be sturdy to avoid any type of shaking motion. Finally, all moving parts must produce essentially no vibrations. The Langmuir-Blodgett trough that was used is shown in Figure 2. The frame of the trough was made of welded aluminum, the moving shafts were made of stainless steel with brass bearings, and the pulleys and barriers were made of Teflon. The motors that were used were right-angle dc reduction motors from Bodine Electric Company. The actual trough itself was a baking pan (36 X 25 X 5 cm) coated with Silverstone, a hydrophobic Teflon-like surface made by E. I. du Pont de Nemours and Co. The entire apparatus was enclosed in a simple hood made of wood and clear acetate. Procedure In this section, the step-by-step procedure that was employed in depositing Y-type films of calcium stearate is described. To our knowledge, many of these individual steps have not been sufficiently documented in the literature. Through trial and error i t was found that the procedures listed below will provide reasonably reproducible films. The beginner is cautioned to pay close attention to the recipe provided, as even slight deviations were found to yield poor films. 1. The trough was first thoroughly cleaned with soap and water and
rinsed with distilled water. 2. The trough was then filled with distilled water and an appropriate amount of calcium bicarbonate solution needed to make the water hath 10-4 M in calcium carbonate (7). The pH of the hath was then adjusted to the appropriate values for either castor oil (pH 6.P6.6) or oleic acid (pH 6.2-6.4) using buffer solution (pH 7, 0.05 M potassium phosphate monohasic sodium hydroxide). Themeniscusof the water bath was above the upper edges of the trough. 3. The surface of the water bath was then swept clean by slowly passing (approximately 36 cmlmin) the barrier lightly over the top of the surface.
Figure 2. The Langmuir-Blodgeti trough used to deposit films.
Figure 3. Orientallon of water bath surlaface during monolayer formation. Drops of Stearlc acid are placed at point A. drops of piston oil ere placed at point 8. and the films are deposited an the slide at point S.
4. The glass substrate to be used was then cleaned. First, the glass
was dipped several times in chromic sulfuricacid (Fisher Cleaning Solution lot #746799) to remove large-size particles and organics. Second, the glass was rinsed with distilled water and then dipped into a solution of ethanol and KOH (11g/L ethanol) a few times to remove organic oils. Third, the glass was rinsed with distilled water (pH 6.2-6.4) and dipped into a solution of 50%sulfuric acid and 30%hydrogen peroxide (40:l by volume) a few times to oxidize any remaining foreign particles on the surfaceof the glass. Finally, the glass was rinsed with distilled water and dated direetlv into the water bath with one side clamped onta'the dipping a&. 5. Next, a string of rolled Teflon tape ('12 X 600 in.) was placed across (in serpentine fashion) half of the trough surface, as shown in Figure 3a, to separate the surface into two halves. 6. Drops of stearic acid in solvent were then placed on the side of the trough containingthe glass substrate, point A on Figure 3b, until the string was noticeably bowed away from the glass substrate. 7. After a 10-min interval to ensure complete evaporation of the solvent, one or two drops of "piston oil" (either castor oil or oleic Volume 65
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acid) were then placed on the other side of the string, point B on Figure 3e, to compress the monolayer of calcium stearate that was formed. 8. After the manolayer was compressed (the string no longer moved), the dipping operation was commenced. 9. The dipping operation was continued until the string moved to within 1cm of the slide at point S in Figure 3d. The tvoes of substrates used were standard microscope slides from Fisher Scientific Co. (75 X 35 mm, and standaid proiector slides from Kudak Co. (60 x 60 mm). The types of sol;ents used were hexanes, petroleum ether, and benzene, with concentrations of stearic acid from 1g/L to 0.05 g/L. The pH of the water bath was varied from6.4 to6.6 for castor oil as the piston oil and from 6.2 to 6.4 for oleic acid as the niston oil. All chemicals used were Certified ACS from the the Fisher Scientific Co. The temperature of the bath was not monitored but simply operated at room temperature (25 OC). The dipping speed was varied from 2 cmlmin to 10 cm/ min with out-of-bath drying times of approximately 15 min. Results and Dlmusslon Although there are many papers that describe individual studies of particular aspects of L-B films, most are concerned mainly with results and not with the precise conditions needed to obtain those results. T o the beeinner in this field, this presents the problem of having to l e a k the particulars of film deposition through the age-old technique of trialand error. t his is what washone in t6eexperimenis that were performed. T o minimize the time required for reduction of art to practice, we have documented those procedures that produce reliable monomolecular films of suitable integrity for subsequent study. The best results in terms of the most easilv and visiblv uniform films were observed to . denosited . occur with the Kodak projrrtor shdes when stearic arid was used at 0.25 e/L in netroleurn ether or hexanes. with castor oil as the oii, the pH between 6.5 and 6.6, and the dipping speed kept constant a t about 2.5 cmlmin. The first and most important problem that was encountered was the prevalence of dust in the surroundings around the apparatus. In the preliminary design phase of these experiments, it was naively thought that the presence of a hood around the apparatus would be sufficient to keep out much of the dust in the air. However, it became obvious, after only a few experimental runs, that this was definitely an error in judgement. I t was specifically found that there would always he a t least a few dust particles that would visibly accumulate on the surface of the water bath over the course of an experimental run. The use of simple exhaust fans for both positive and negative pressure were tried. However. these nrocedures simnlv . .caused more dust to be blown around while simultaneously causing ripples on the bath surface. I t was concluded from these experiences that an improved hood device would be one that surrounds the apnaratus on four sides (with access for the experimentalist's finely filtered ai; pressure from hands) and exerts a directly overhead. Dirt, defined as any impurity foreign to the purity of the reaaents used, and specifically oils, from hands and surrou>dings, were also found to he a major experimental prohlem. IXrt and oils were firat observed to cause problems with the substrate. I t was found that commercial slides that were packaged as "precleaned slides" were anything hut clean; oils. used on elass cuttine machines in the glass industrv, coated all of ;he slides t i a t were purchased-and attracted dirt and dust from the surrounding laboratory upon removal of the slides from their packaging..~hisposed the important problem of how adeauatelv torlean the slides that needed to be perfectly clean to pick up monolayers. Many different cleaning procedures were tried including vapor degreasing with methanol and trichloroethylene, commercial window 640
Journal of Chemical Education
cleaners, acid and base treatments, as well as combinations of all of the above. I t was finally concluded that the acidbase-acid dippine procedure (used to clean s ~ e c t r o a r a ~ h i c slides,, desc;i'bedh the Experimental ~ m c e d u r eieciion, worked best. Dirt and oils were also found to build up on the surface of the trough, especially after extended uae. This caused nonremovahle oil films to form on the surface of a prepared water bath before any stearic acid was deposited. Thus, it became impossible to spread stearic acid over the surface of the bath; a drop of stearic acid remained as a drop as it slowly evaporated from the surface of the water bath leaving behind a "clump" of stearic acid. A similar situation resulted if the same Teflon string (used to separate the calcium stearate from the piston oil) was used for more than oneexperimental run,es,el; with cleaning.Thus, it u'asfound that the trough had to he scruhbed dean and n ntu string. prepared before each experimental run. Still another important problem that was encountered occurred durine the de~ositionof d r o ~ of s stearic acid and piston oil. The yeflon &ring that was ;sed on the surface of the water bath floated liehtlv " .on ton of the water due to its hydrophobic nature. During many experimental runs, this enabled the stearic acid/calcium stearate and mostly piston oil to leak underneath the Teflon string. This leakage was a major problem, for i t rendered compression of the calcium stearate mouolayer impossible. Specifically, with castor oil as the piston oil, approximately 20% of the experimental runs faiied because of leakage ofthe castor oil (which generally spread slowly-2 min to reach equilibrium). Oleic acid, which had ereater surface nressure than the castor oil. consequently c a k e d many m&e problems; it was found that greater than 95% of the experimental runs with oleic acid as the piston oil failed on the first dipping and the remaining 5% failed after the followine two successive dippinas. I t was concluded that the higher surface pressure ofthe oleic acid could not be contained by the hydrophobic Teflon string that was employed. There were also problems encountered with the harrier system that was used. The motor-pulley drag system was originally designed to pull the barrier across the trough in a smooth linear fashion: however. this was not the case exnerimentally. ~pecificall{, the harrier tended to move in a honlinear fashion that subseouentlv caused rinnles in the bath. I t was found that it was much easier and c&ed no problems if the barrier was carefullv moved bv hand to clean the surface of the trough. The tiarrier had k he mo\.ed carefully toavoid splashingand spillage (destruction of the meniscus, of the water with special consideration for the som~what rounded edresof the trough that was used Theadvantare - of having a square trough was recognized a t this point. There were no serious prohlems with vibrations from the surroundings in which the trough was operated (the trough was on an optical table). However, there were some problems with the vibrations in the dipping arm that were caused by the lack of sensitivity of the reduction motor. At low speeds, the motor tended to work with some hesitation extremely smooth motion of the dipping arm. It was not completely clear what consequences resulted from these vibrations, hut they probably caused nonuniform deposition of the monolayers in that some of the calcium stearate molecules were deposited on the surface of the substrate while some were desorbed from the substrate during ascent. The choice of substrate was found to be a c h a l factor in the successful deposition of films. Specifically, it was found that it was much more difficult to deposit films on the microscope slides than it was to deposit films on the projector slides. I t was concluded that the microscope slides were smoother than the projector slides, making i t harder for the films to adsorb effectivelv onto the elass surface. The choice of solvent, withregardio volatility, also proved to be a major factor in the successful deposition of films.
With hexanes as the solvent, the drops of stearic acid spread slowly and visibly, thus facilitating easy deposition of films. Similarly, with petroleum ether, drops of stearic acid spread visibly hut faster, which resulted in uniform deposition of films. With henzene,however, evaporation took place quickly such that the stearic acid tended to remain in a "clump" where the drop was first placed. Conclusions
Although Y-type films of calcium stearate were produced with a relativelv inexvensive laboratorv- apparatus. the over.. all quality of these f h n s was not very good. The films that were produced in these experiments contained imperfections mainly due to the reasons listed above. Detailed study and characterization of films that are nonuniform due to such problems as dust and vibrations would likely lead to nonreproducible results. The results of these studies provide design criteria and specific experimental guidelines. The following list of design considerations is suggested: 1. The trough has to be hydrophobic in nature and square. This can be accommodated by machining a block of Teflon into a square trough with a depth of less than % in. with a square depression of 2. 3.
4.
5.
appropriate depth where the substrate is to be dipped. The dipping device should move linearly in a vertical direction only. The speed of the device should be precisely controllableand continuous at velocities approaching zero. The frame of the trough should be extremely sturdy with no rustable parts. The barrier should he sturdy and completely linear in its vihrationless journey across the trough surface. The compressionof the monolayers should be done mechanically to avoid the leakage and contamination problems associated with piston oils.
6. The hoodshould completely surround the apparatus while delivering a cantrallahle positive pressure of filtered air. The hood
should also contain adequate light so that the surface conditions can be easily observed. The lessons to be learned from this experiment can be summed up in the two cliches .'cleanliness is next to godlinessVand"patience is avirtue". Extensive care must he taken by one triing to construct monomolecular films to ensure that the environment around a L-B trough is relatively dust free and free of air-borne contaminants. It must also be remembered that all the care in the world can be rendered completely in vain if patience is not exercised in learning the "art" of depositing good films. Manv of vroblems associated with L-B film devosition which have not been mentioned here can be found refs. 8 and 9. Many of the problems specifically dealing with mechanical vibration and control of the dipping mechanism as well as piston oil leakage can be alleviated with the purchase of commercially available G B troughs. Acknowledgment
Portions of this work were supported by the Office of Naval Research under contract number NO00 14-86-K-0508. Literature Clted
1. Blodgett, K. B. J. Am. Chem. Sac. 1935,57,1007. 2. Robert., G. G.; Pitt, C. W . Longmuir-Rlodpett Fiimr, 1982: Elaevier: New York, 1983. 3. Agarwa1.V.K.Eleelroromp. Sci. Tsehnol. 13'75.2,l-31. 4. Nathoo.M. H..ThinSolidFilmr 1373,16,ZL5-226. 5. Blodgett.K.B.J.Am. Chsm. Soc. 1935,J7,1010-1011. 6. Agarwa1.V. K.E!~elrocomp. Sci. Teehnol. 1375.2.7. 7. B1odgett.K.B.J . A m C h r m Soc. 1335,57,1011. 8. Agarwal. V. K. Electroromp. Sci. Techno!. 1975.2.5, 9. Gains, G. L., Jr. Insoluble Monoioyars O f Liquid-C., Inlerloces: wi1ey: New York, 1966.
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