Langmuir 1990,6, 519-521 Particle size estimation from inelastic scattering has at least two advantages over earlier methods based on the dispersed phase/support phase XPS intensity ratios.lg Specifically, the metal loading and support surface area do not need to be known, and dispersed phase distribution throughout the pore structure of the support need not be uniform. The second consideration is especially appealing, as it is typically very difficult to determine the metals distribution on an atomic scale. A disadvantage of particle size estimation from inelastic scattering is that the presence of contaminant overlayers on the surfaces of interest can have a significant effect on the particle size prediction. Contaminant overlayers increase the probability for inelastic scattering, which
519
increases the measured B(d$)/I(d,EJ ratio and causes d to be overestimated. Moreover, for applications to high area practical catalysts, surface roughness and the presence of a support with high surface area can lead to significant deviations from eq 4-7, which have been derived assuming a flat surface, model catalyst configuration. The role of contaminant overlayers, surface roughness, and a high surface area support will be considered along with example applications in a separate r e p ~ r t . ~
Acknowledgment. Support of this research by the Exxon Research and Engineering Co. is gratefully acknowledged.
Effect of Dipping Rate on Alternating Layer Langmuir-Blodgett Film Structure M. J. Grundy, R. J. Musgrove, R. M. Richardson,* and S. J. Roser School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, England
J. Penfold Neutron Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 OQX, England Received October 26, 1989. I n Final Form: January 5, 1990 X-ray and neutron reflectometry studies on deposited LB films have been performed. “Rockingcurves” have indicated accurate alignment of crystal planes parallel to the substrate. The effect of deposition speed has been investigated and found to lead to intermixing of alternating layer films at slower deposition speeds, having important implications for device fabrication.
Introduction Recently, there has been considerable interest in the Langmuir-Blodgett (LB) technique for fabrication of wellordered thin films. A wide variety of potential applications have been proposed’ including, for example, infra red detectors. Many applications involve synthesis of AB type films2 to propagate a dipole throughout the film. The pyroelectric effect is just one consequence of such a structure. It has now become apparent that conventional ideas of both monolayer and multilayer structure and behavior are sometimes incorrect, and several fundamental aspects of LB film preparation remain poorly characterized. One such area of interest is the deposition process itself. The effects of subphase conditions on properties of monolayers3and multilayers4 have been previously investigated for the cadmium chloride/docosanoic acid system. In addition, detailed descriptions of the principles and theoretical aspects of X-ray and neutron reflection (1) Roberts, G. G. Contemp. Phys. 1984,25,109. (2) See, for example: Daniel, M. F.; Smith, G. W. Mol. Cryst., Liq. Cryst. Lett. 1984, 102, 6-7, 193. (3) Richardson, R. M.; Roser, S. J. Liq. Cryst. 1984,102, 797. (4) Buhaenko, M. R.; Grundy, M. J.; Richardson, R. M.; Roser, S. J. Thin Solid Films 1988,159,253-265. (5) Daniel, M. F.; Dolphin, J. C.; Grant, A. K.; Kerr, K. E. N.; Smith, G. W. Thin Solid Films 1985,133,235.
applied to LB films exist X-ray and neutron reflection are sensitive to the scattering density profile in the 2 direction (Le., in a direction perpendicular to the substrate). In this work, we have deposited alternating layers of hydrogenous and perdeuterated docosanoic acid and used X-ray and neutron reflection to study the structure of the LB film produced. In particular, the effect of varying the dipping rate on the structure has been investigated. For both neutron and X-ray diffraction, the position of the Bragg peaks is determined to a good approximation by the Bragg law
Q ru 2 r / d where d is the repeat distance perpendicular to the layers and Q is the scattering vector (Q = 4 r sin B / X , where 28 is the scattering angle and X is the wavelength) at the 001 reflection from the bilayers. The major difference between the two types of radiation lies in the factors governing the intensity of diffraction. For X-rays, the Bragg intensity depends on the electron density variation across the layers. For neutrons, it is the variation of nuclear scattering length density that determines the Bragg intensity. Since normal hydrogenous fatty acid has a scatter(6) Als Nielson, J. 2.Phys. B 1985, 61, 411.
(7) Grundy, M. J. MSc. Thesis, University of Bristol, 1986.
0743-7463/90/2406-0519$02.50/0 0 1990 American Chemical Society
520 Langmuir, Vol. 6, No. 2, 1990
ing length density of -0.03 X A-2 and the perdeuterated version has +0.73 X lo-' A-2, an alternating layer (HDHD) LB film would be expected to give a very strong 001 reflection from the bilayers. A decrease in this intensity has been used as an indication of intermixing of the H and D molecules during the dipping process.
Letters "
1.
Mr
Experimental Section The substrates were freshly etched 111 crystal plane silicon prepared in the usual way.4 Films were prepared by using a Joyce-Loebl RSRE two-compartment trougha5 The docosanoic acids (Larodan) were spread on a 2.5 x lo-" M CdCl, subphase M solution in Aristar chloroform (BDH). The subfrom phase pH was adjusted to 6.2 because previous work4 indicated that a pure salt film would be deposited at this pH. The surface pressure was 35 f 5 mN m-l during deposition, and all films were 10 bilayers thick to give a total thickness of ca. 1100 A. This allowed interference fringes to be seen in neutron reflection experiments to yield the total film thickness. The films were prepared on consecutive days and stored in ambient conditions. They were treated identically, and the neutron and X-ray reflection experiments were performed within 1 month of fabrication. X-ray experiments were performed on a sealed-tube reflectometer with X = 1.54 A in this laboratory; and the neutron experiments were performed on the CRISP' reflectometer at the ISIS neutron source at the Rutherford Appleton Laboratory (RAL). The CRISP reflectometer is a time of flight instrument (TOF), and this permitted a rocking curve to be determined. The sample was initially aligned so that the scattering vector was perpendicular to the silicon substrate. This was achieved by use of the reflection of a laser beam (which followed the same path as the neutrons) from the silicon wafer into the detector aperture. The narrow beam definition slits permitted this to be done with an accuracy of better than 0.01'. This sample angle ($) was defined as zero, and rocking curves were measured by varying $ from -0.1' to +0.1' in steps of 0.02'. At each sample angle, a reflectivity profile was acquired, and a rocking curve could be obtained as a plot of the Bragg peak intensity versus sample angle. The peak in the rocking curve indicates the mean angle between the layers and the substrate, and its width (full width at half-maximum) is the mosaic spread of the multilayer film about the layer normal.
-
Results and Discussion Rocking Curves. The neutron data were reduced and displayed on a VAX computer by using the GENIE' analysis programs at RAL. Figure 1 shows reflecting profiles for a fast dipped LB film obtained by varying the # angle between &0.06' in 0.02' steps and recording the reflectivity in the region of the 001 Bragg peak. The data have been normalized to the incident beam wavelength distribution. Each scan required ca. 20 min to obtain. The data are for an HDHD LB film, and it can be seen that the peak is centered at ca. 0.082 A-' in Q. The maximum intensity is at # = 0', and a rapid decay occurs with the peak effectively absent at an offset angle of # = 0.06'. T h i s is more clearly illustrated by the rocking curve in Figure 2, which shows the integrated 001 peak intensity at each angle. The maximum is at OD, and the rapid decay in intensity indicates correct alignment, with the layers of the unit cells arranged parallel to the 111 crystal plane silicon substrate. In addition, a narrow mosaic spread of 0.036' is recorded, further indicating very tight alignment of the LB film with the substrate. Previous reflection experiments on coated silicon (8) Penfold, J.; Ward, R. C.; Williams, W. G. J. Phys. E 1987, 20, 1411.
(9) David, W. I. F.; Johnson, M. W.; Knowlee, K. J.; Moreton-Smith, C. M.; Crosbie, G. D.; Campbell, E. P.; Lyall, J. S. Rutherford Appleton Report RAL-86-102, 1986.
0.04
0.05
0.08
0.07
0.08
0.00
0.10 SINCT>/L.IO.+I
0.11
0.12
Figure 1. Neutron reflectivity profiles (displaced for clarity) for HDHD LB films at sample offset angles ($) from -0.06' to +0.06O in equal steps.
"r I
A
7000.
/
\
P . A
0. 0 -0. 80
-0.30
.L
I
0. 30
0.00
PSI
0.en
l..OI.
Figure 2. Rocking curve showing the integrated intensity of the 001 Bragg peak as a function of sample offset angle $.
wafers" have required a resolution parameter of 0.035' to fit the data to a theoretical model. This effective divergence is a combination of the incident beam divergence and any curvature of the silicon substrate. The instrumental resolution from the slits used was calculated to be 0.OlD,suggesting that the effective beam divergence is arising from the curved nature of the silicon substrate. The previously measured effective divergence of 0.035' is close to the value for mosaic spread recorded in these experiments. This indicates that the mosaic spread is dominated by the substrate, and the value of 0.036' is an upper limit for the mosaic spread of the "LB crystal". Measurements were made to determine rocking curves both perpendicular and parallel to the dip direction, and similar data were obtained in both configurations. Deposition Speed. To examine the effect of deposition speed on 001 Bragg intensity, all measurements were made parallel to the dip direction. Figure 3 shows the neutron reflectivity profile in the region of the 001 Bragg peak as a function of deposition speed, recorded on CRISP; the integrated intensities are given in Table I. Figure 4 shows the first four 001 reflections from the same films determined by X-ray reflectometry. Comparison between Figures 3 and 4 indicates that while the three films show very similar 001 peaks determined by X-rays, only the fast dipped sample shows a significant (10)Grundy, M. J.; Richardson, R. M.; Roser, S. J.; Beamaon, G.; Brennan, W. J.; Howard, J.; O'Neil, M.; Penfold, J.; Shackleton, C.; Ward, R. C. Thin Solid Films 1989,172, 269.
Langmuir, Vol. 6, No. 2, 1990 521
Letters
0. 10
0. 00
0. 10
0. 20
0.30
0. 40
S I N (1)/L*IO-l
Figure 3. Neutron reflectivity profile for HDHD LB films deposited at (from below) 7, 17, and 120 mm min-'. Table I. Integrated HDHD Peak Intensity deposition integrated intensity speed, mm min-' (8000-1200 ps) 7400 f 86 120 f 5 695 f 26 17 f 2 7f2 521 i 23
001 Bragg peak with neutrons. It must also be noted that the CRISP reflectometer allows exact values of reflectivity to be calculated, and even for the fast dipped film the intensity of the Bragg reflection is less than would be expected from a theoretical calculation. The X-ray data show that LB films have been produced in all cases with identical d spacings of 60 A. The profiles show that the films are approximately equivalent in structure (perpendicular to the substrate) and the Bragg peaks, which result from the head group to head group repeat distance, are reasonably intense. This would appear to indicate that for the conditions used a wellordered LB film can be produced independent of deposition speed within the range covered. Conventional wisdom may further suggest that deposition should proceed "slowly" to minimize the possibility of inconsistency arising in the film. The neutron data in Figure 3 are quite different. It is only the sample dipped at 120 mm min-' that has an intense 001 Bragg reflection. Neither of the two slower dipped samples shows an intense Bragg peak, and two possibilities have been considered. The first is that some intermixing of the molecules in the monolayer with those in the uppermost deposited layer occurs. This would take place at the point of deposition, and presumably there would be more time for it to proceed during slow deposition. A second mechanism by which molecules in suc-
Figure 4. X-ray reflectivity profiles for the same films as in Figure 3.
cessive layers may become intermixed is that some rearrangement of deposited monolayer into patches of bilayer takes place while the substrate is under water. Since faster dipping speeds also imply less time under water, there would be much less time for such a rearrangement to happen. Such a mechanism of underwater rearrangement has been suggested by Honig," although it also predicts transfer ratios less than unity which were not observed. Further experiments are planned to investigate the detailed mechanism of layer intermixing and to study the effect for different subphase pH values.
Conclusion We have verified that LB films are tightly aligned with layers parallel to the substrate. There is no suggestion of a structure consisting of molecules in layers which lie at an angle to the substrate. The low value for mosaic spread is applicable both parallel and perpendicular to the dip direction. Combination of X-ray and neutron data has shown LB films can be deposited for docosanoic acid at a = 35 mN m-l on CdC12 subphase independent of deposition speed. We have, however, detected a substantial degree of intermixing of molecules in successive layers which is greatest for low dipping rates. This has been shown to destroy the alternating ABAB type films and suggests that "fast" deposition speed should be more successful for device fabrication. Acknowledgment. We are grateful to the SERC for provision of neutron beam time. S.J.R. and R.J.M. thank SERC for financial support. IC1 and SERC are also acknowledged for a CASE studentship to M.J.G. (11) Honig, E.P.Langmuir 1989,5, 882.
