PROPERTIES OF BUILT-UP FILMS O F BARIUhI STEARATE' KATHARINE B. BLODGETT
Research Laboratory, General Electric C o m p a n y , Schenectady, New Y o r k Received J u n e 22, 1937
In a recent paper Langmuir, Schaefer, and Wrinch (4) described briefly the method which they used to measure the thickness of a monolayer of egg albumin transferred from a water surface to a metal slide. They prepared a chromium-plated slide for this experiment by coating the slide with built-up films of barium stearate which reflected vivid interference colors. They then transferred the monolayer of egg albumin from the water surface to the slide, depositing the albumin on top of the barium stearate, and the change of interference colors produced by the layer of albumin molecules afforded a measure of the thickness of the layer. The thickness was 20 A.U. when the surface pressure exerted on the layer mas 30 dynes per centimeter. This paper will describe some of the optical properties of thin films and will show how these properties have been used to obtain very great sensitivity in the method of measuring small increments of film thickness. The films used in thickness measurements are built by depositing successive monolayers of barium stearate on a chromium-plated slide, by a method which has been described in previous papers (1, 2). The monolayers are formed by spreading stearic acid on water containing a dissolved barium salt, the pH of the water being usually held a t about 7.0 by adding a small concentration of potassium bicarbonate to the solution. The layers are transferred from the water surface to the slide by a dipping process, in which one layer is deposited as the slide travels down into the water and the next layer as it rises from the water. Films covering an area of 5 x 2.5 em. can easily be built a t a rate of 20 layers per minute and smaller areas a t rates of 40 to 60 layers per minute. The films are commonly built in a series of steps of increasing thickness, several steps being built on one slide. This is done by controlling the depth to which the slide is dipped into the water. For example, a series of eleven steps having 35, 3 i , , . . 55 layers is used as a color gauge in many types of measurement, since this series reflects the interference colors 1 Presented at the Fourteenth Colloid Symposium. held a t Minneapolis, Min nesota. June 10-12. 1937.
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KATHARIKE U. BLODGETT
which range from yellow at 35 layers through red at 45 layers to a greenblue at 55 layers. This series is built on a 7.5 x 2.5 cm. slide by holding the slide with the long edge vertical and dipping it to a depth of 5.5 cm. until 35 layers are built up by repeated dips. The depth to which it is allowed t o travel down into the water is then decreased by progressive steps of 0.5 cm. as two more layers arc added on each down-and-up journey. The dividing line between neighboring steps is straight and extremely sharp. The entire series can be built in two to three minutes. The colors of the light reflected by films built on polished chromium are most brilliant when the films are viewed by polarized light at angles near the grazing angle. The increased brilliancy under these conditions is due to multiple reflections in the film and is explained by the curves in figure 1. These curves are a plot of the light intensity reflected by the film
h h
2k 3
11 1
1
1
j
1 i 1'1 1 I I J 0
50
0)
j
1
1
UI
100
1
I Ih
"1
150
~
1
j
U 200
NUMBER O f LAYERS
FIG.1. Intensity minima for monochromatic light reflected from a step-series of barium stearate films built on a polished chromium surface. Angle of reflection is 80". Wave length of light is 5893 A.LTT.
as a function of film thickness when the films are viewed by monochromatic polarized light at an angle of reflection of 80'. The curves were drawn from rough estimates of the intensity made by the eye without the use of a photometer. The source of light was a 6000-lumen sodium vapor lamp, A = 5893 A.U. It was polarized by means of a screen of polarizing material of the type manufactured by the Land-~7heelwright co. The solid line in figure 1 shows the intensity distribution that is seen when a step-series is viewed by light R, polarized in a plane perpendicular to the incident plane, that is, by light having its electric vector vibrating in a direction parallel to the film surface. The step having 49 layers is very dark, nearly black, and the two neighboring steps having 47 and 51 layers are seen to be much brighter. The sharpness of the minimum de-
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pends to a great degree on the angle at which the film is viewed. -At angles of reflection of 80 to 83" only about four two-layer steps can be seen on either side of the minimum. Beyond these steps the intensity distribution curve follows a flat maximum and the separate steps are invisible, since it is the contrast between neighboring steps which renders the steps visible to the eye when monochromatic light is used. When white light illuminates the steps the minima for the various wave lengths appear a t successive film thicknesses. Thus the minimum for blue light of wave length X = 4500 X.U. occurs at 37 layers, so that the color of the light reflected by this step is yellow. Since the minima are sharp for each wave length one obtains a high resolving power for the different colors reflected by a series of steps built in two-layer intervals in the region froin 35 to 55 layers. When the direction of polarization of the light is changed so that this series is illuminated by light R,, polarized parallel to the incident plane, the bright colors disappear and only a yellow tinge is seen in the thicker steps of the series. This condition is represented by the dotted curve in figure 1. The contrast between neighboring steps is then imperceptible in the 35 to 55 layer region, but is very great in the region from 87 to 107 layers. The film thicknesses at which minima occur for the ray R, are proportional to X/4, 3X/4, 5X/4, etc., and for the ray R, are proportional to X/2, A, 3X/2, etc. The difference between the two series of interference minima arises from the fact that light commonly undergoes a phase change when it is reflected at the boundary between two media. In the case of the ray R, at large angles of reflection, the phase change occurring where light is reflected from the upper surface of the film is nearly the same as where light is reflected from the lower surface, whereas for the ray R, the two reflections have a difference of phase of nearly 180". This difference of phase is equivalent to a path difference of nearly one-half a wave length between the ray reflected from the upper surface and the ray reflected from the lower surface of the film. The path difference between the two rays is d = 2nt cos r
(1)
where n is the refractive index, t the thickness of the film, and r the angle of refraction of light in the film. Intensity minima occur at thicknesses given by fzt
cos
I-
=
mX/4
(2)
where m has the values 1, 3, 5,. . . for the R, ray and 2, 4, 6 , . . . for the R, ray. -4more detailed study of this subject has been given in another Paper (2).
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KATHARINE B. BLODGETT
The sharpness of the intensity minima supplies the means whereby small increments of film thickness can be made visible. When a film of unknown optical thickness is compared with a color gauge built in two-layer steps, the film and the color gauge being built on separate slides, the eye can measure differences of thickness by means of color with a probable error of about 0.5 layer. The thickness of one layer is 24.4 -4.U. In order to test one's eyesight for the differences which can be detected it is useful to employ vernier color scales which are built of barium palmitate (CH),barium stearate (C18),and barium arachidate (C20).Since the thickness per layer of these substances is closely in the ratio 16:18:20, they provide three different color scales which may be combined in any desired way for use as verniers. Greater sensitivity is obtained by illuminating films with sodium light than with white light. Differences of thickness equal to 0.2 layer of barium stearate have been made plainly visible to the eye by the following procedure: When a slide on which films are built is turned so that the angle of incidence i decreases from 80 to 73", the corresponding angle of refraction ( n = 1.495) changes from r = 41'12' to r = 39"46', and cos increases 2.1 per cent in value. Therefore, if the minimum lies exactly at 49 layers when seen at SO", as shown in curve I1 of figure 2, it shifts to 48 layers a t 73" in accordance with equation 2. The neighboring steps having 47 and 49 layers on curve I then match exactly. The intensities of these steps are shown by the open circles on curve I. If an increment of thickness equal to 0.2 layer is added to the film, these neighboring steps no longer match a t 73", the 49.2-layer step being brighter than the 47.2-layer step. Their intensities are shown on the curve by solid circles. If the slide is turned to an angle slightly greater than 73", the steps again match exactly. It is always possible to find an angle a t which two neighboring steps match. The accuracy with which small increments can be detected is usually greater when the method of curve I is employed, in which the minimum lies halfway between two steps, than when the minimum lies at one of the steps as shown in curve I1 and the series of three steps shown by squares is used to estimate small displacements of the minimum. The experiment in which known differences of thickness equal to 0.2 layer of barium stearate were observed was performed in the following manner: A step-series of barium stearate was built in two-layer intervals by dipping the slide in the usual way with the long edge of the slide perpendicular to the water surface. The slide was then turned and dipped with the long edge horizontal while three steps of barium arachidate having 2, 4, and 6 layers were built on top of the stearate layers, a part of the stearate being left untouched. The boundaries of the arachidate films therefore ran at right angles to the stearate boundaries, forming a checkerboard of color. The steps which contained 45 and 47 layers of
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stearate plus 2 layers of arachidate had thicknesses equal to 47.2 and 49.2 layers of stearate. Similarly in the second row, having 4 layers of arachidate, the 43- and 45-layer steps of stearate were increased to 47.4 and 49.4, and in the third row there were steps equal to 47.6 and 49.6 layers. The slide was held at an angle a t which the 47- and 49-layer steps of the uncoated stearate matched exactly. It could then be seen that the 47.2 and 49.2 steps did not match exactly, the 49.2 being slightly brighter than 47.2, and the contrast was progressively greater in the rows having the 0.4- and 0.6-layer increments. The contrast was sufficiently great to make it possible to estimate thicknesses having a value halfway between these successive increments. This means that adsorbed layers of molecules or atoms having an optical thickness of 5 X.U. or more can be made visible to the eye without the aid of optical apparatus, and can be measured
L
$ k 4 !
4/
1
d3
I
45
I
1
I
I
47 49 51 53 NUMBER OF LAYERS
I
55
!
57
FIG. 2. Intensity minimum for step-series of films of barium barium stearate built on polished chromium. Films seen by ray R. of sodium light a t angles an.gles of incidenc incidence :e i = 80" and 73".
with a probable error of about 2.5 A.U. By the use of a microphotometer to measure the intensities of the steps as a function of the angle of incidence, the accuracy of measurement of thickness could undoubtedly be increased a t least tenfold. The term 'Lopticalthickness" is defined as the product of actual thickness and refractive index of the film. The thickness measured by means of interference colors is the optical thickness given by the product nt in equation 2, and the refractive index is also used to calculate values of cos r in equation 2 from observed values of the angle of incidence i. Therefore, in order to determine the actual thickness it is necessary to know the refractive index. Fortunately the refractive indices of built-up films of many organic substances do not vary greatly from the refractive index of barium stearate. Built-up films of barium stearate are birefringent, the refractive index
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KATHARINE B. BLODGETT
of the ordinary ray, which is the R, ray, being 1.495 for sodium light. The intensity minima seen with the extraordinary ray R, are not ordinarily used for interference measurements, since the refractive index of this ray increases with increasing angles of incidence, the film being a positive crystal. The refractive index of a series of ten monolayers of egg albumin was found to be the same as that of the ordinary ray of barium stearate, within a possible error of 0.5 per cent. The method by which refractive indices are measured is illustrated in figure 3. The graphs show the series of interference minima and maxima
FIG.3. Interference fringes seen with step-series of films built on glass. Itefrac. tive indices of glass, n,, and of film, nl, are measured for = 5893 A.U. Films reflect a-series when n,>nl and (3-series ivhen n,
