0 Copyright 1992 American Chemical Society
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Surfaces and Colloids
The ACS Joumal of
AUGUST 1992 VOLUME 8, NUMBER 8
Letters Formation of Silver Islands on Langmuir-Blodgett Films As Investigated by X-ray Reflectometry G.Reiter, C. Bubeck, and M. Stamm* Max-Planck-Institut fur Polymerforschung, Postfach 3148, 6500 Mainz, FRG Received February 26, 1992. I n Final Form: May 15, 1992 Silver overlayers on Langmuir-Blodgett multilayers of cadmium arachidate (CdA),a substituted polyglutamate (PG),and a polymeric phthalocyanine derivative (PCPS)were studied by X-ray reflectometry. From fits to the measured reflectivity curves electron density profiles are obtained for the films with and without silver overlayers. The initial sticking ratios of silver on CdA, PG, and PCPS are 0.37,0.72,and 0.93,respectively. The deposited silver forms islands on top of the multilayers with a maximum penetration depth into the LB films of one to two monolayers only depending on the material of the film.
Introduction The formation of metal overlayers on organic materials especially on polymers is of basic and technological interest.' The thermal evaporation of metals on top of Langmuir-Blodgett (LB) films offers the opportunity to study the initial stages of metal film growth and the basic metalpolymer interactions at reasonably well-defined model surfaces. The technological and applied science perspective of such studies is related to the fundamental questions of sticking and desorption probabilities for the incident metal atoms at the sample surface and to the question of how far metal atoms can diffuse or metal islands can penetrate into various organic materials, respecti~ely.~-~ Recently the nucleation and island formation of silver was studied on LB films that were prepared from cadmium arachidate (CdA), an alkyl-substituted polyglutamate statistical copolymer (PG), and a polymeric phthalocyanine (PCPS).k7 The chemical structures of the polymers are displayed in Chart I. Both polymers belong to the class of rigid rod polymers with substituted alkyl side chains,
Chart I. Chemical Structures of the Phthalocyanine and Polyglutamate Derivatives PCPS and PG, Respectively
-
-
PG
I
( ~ ~ z ) z
-
I
( y 2 ) z
c=o I
c=o I
0
0
RZ?
(1) Sacher, E.; Pireaux, J. J.; Kowalczyk, S. P. Metallization of Polymers; ACS Symposium Series, 440; American Chemical Society: Washington, DC, 1990. (2) LeGones, F. K.; Silverman, B. D.; Ho, P. S. J. Vac. Sci. Technol. 1988, A6, 2200. (3) FauDel,F.: GUD~A, D.: Silverman, B. D.; Ho, P. S. A_ d_ . Phvs. - Lett. 1989,55, $57. (4) Payne, R. S.; Swann, A,; Mills, P. J. J . Mater. Sci. 1990,25,3133. (5) Bubeck, C. Thin Solid Films 1989, 178, 483. (6) Bubeck, C. Adu. Mater. 1990, 2, 537. (7) Bubeck, C. Thin Solid Films 1992,210/211, 674.
-
- -HN-CH-CO- - -HN-CH-CO- -
R, =
which are very well suited for the LB technique due to their good transfer properties and the high thermal
0743-7463/92/2408-1881$03.00/00 1992 American Chemical Society
Letters
1882 Langmuir, Vol. 8, No. 8, 1992
stability of the film^.^^^ By means of electron microscopy and optical absorption spectroscopy of the plasmon resonance of silver islands, it was concluded that the amount of silver that sticks on LB filmsvaries significantly with the chemicalstructure and thickness of the LB films?@ However,there is still a need to quantitatively characterize the total amount, thickness, and density profile of ultrathin silver overlayers on model surfaces since, in particular, previous studies did not yield information on the distribution of the silver islands perpendicular to the surface. The technique of X-ray reflectivity is well suited for the investigation of ultrathin metal films on organic substrates.1° First, it has an excellent depth resolution, of the order of angstroms,ll to determine surface roughness, interface width, or the density profile of thin films. Second, it shows a very good sensitivity to the detailed investigation of a metal layer in or on an organic substrate, since a high electron density contrast exists in this case. This has recently been demonstrated in an X-ray reflectometry study of gold marker movement in a matrix of polystyrene.12 Thus, it is possible to determine the electron density profile of a silver layer on a Langmuir-Blodgett film, reflecting the distribution of silver particles within the organic film. Several investigations of LangmuirBlodgett films by reflectivity techniques have been reported where mainly the density profile perpendicular to the surface is examinedl3-I5while by scattering investigations using synchrotron radiation16-18also information on the lateral structure is resolved. From reflectivity experiments on multilayer systems like LB films one obtains Bragg peaks due to the periodic density modulation of the layer structure and Kiessig fringes13-15 corresponding to the overall film thickness. Here we will concentrate on the latter and restrict the investigations to small angles and thin films where Bragg peaks do not significantly contribute to the reflectivity curve. We obtain from measurements of LB films prior to silver deposition the density profiles of the films. These profiles will of course change when silver is evaporated onto the films, and we will be able to determine the quantity of silver on the LB films, its penetration into the layer, and generally, its density profile. We can specify whether silver is preferentially incorporated between the LB layers or whether it will be located mainly on the surface of the film. The technique is complementary to electron microscopy which gives a picture of the lateral distribution of silver islands on the film. Experimental Section The synthesis of poly(y-methyl L-glutamate-co-y-octadecyl L-glutamate) (PG) and tetramethoxytetraoctoxyphthalocyaninato-polysiloxane (PCPS) (seeChart I) and their LB film forming (8)Orthmann, E.;Wegner, G. Angew. Chem., Znt. Ed. Engl. 1986,25, 1105. (9)Duda, G.; Schoutens, A. J.; Arndt, T.; Lieser, G. Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid F i l m 1988,159,221. (10)S t a " , M. Adu. Polym. Sci. 1992,100,357. (11)Foster, M.; Sta", M.; Reiter, G.; Huttenbach, S. Vacuum 1990, 41,1441. (12)Reiter, G.; HUttenbach. S.; Foster, M.; Stamm, M. Macromolecules 1991,24,1179. (13)Rieutard, F.; Benattar, J. J.; Bosio, L.; Robin, P.; Blot, C.; de Kouchkovskv. R. J. Phvs. (Paris) 1987.48.679. (14)Ne&; L.; Pard;, B:; Corno, J. kev.'Phys. Appl. 1988,23,1675. (15)Pommerantz, M. Thin Solid Films 1987,152,165. (16)Benattar, J.J.;Daillant, J.; Bosio, L.; Leger, L. Colloq. Phys. 1989, c7, 39. (17)Kjaer, K.; Ala-Nielson, J.; Kenn, R. M.; Bohm, C.; TippmannKrayer, P.; Peterson, I. R.; Bibo, A. M.; Helm, C. A.; MBhwald, H.; Leveiller, P.; Jacquemain, D.; Weinbach, S.; Leiserowitz, L.; Deutsch, M. Makromol. Chem., Macromol. Symp. 1991,46,89.Tippmann-Krayer, P.;Mdhwald, H. Langmuir 1991,7, 2303. (18)Barton, S. W.; Thomas, B. N.; Flom, E. B.; Rice, S. A.; Liu, B.; Peng, J. B.; Kettesson, J. B.; Dutta, J. J. Chem. Phys. 1988,89,2257.
Table I. Data of Silver Layers on Different LB Films As Determined by Means of X-ray Reflectometry. LB film
PCPS PG CdA
do
m 8 8
(nm) 17.6
6
16.5
16.8
dm from SAXS (nm) 2.10'9 1.759 2.8022
(dA&
(fwhm) (nm) 7.6 6.4 6.8
d,(Ag)
intoLB (nm) 2.6 3.2 4.5
pelpl p h
md
0.61 0.56 0.27
0.93
mi 0.72
0.37
a Errors are of the order of 10%. The monolayer thicknesses are taken from the literature: Sauer et al.,19Duda et al.: and Mann and KuhmZ2See text for definitions of the symbols.
properties have been described recently.s,9J9s20 The LB films with thicknesses given in Table I were prepared on silicon wafers. The wafers were cut to a size of approximately 25 X 75 X 0.5 mm3. They were rendered hydrophobic by a dip into HF solution prior to the LB film transfer. The typical surface roughness was less than 1nm as determined from phase measurement interference microscopy and X-ray reflectometry." The X-ray reflection measurements were performed on a reflectometer with an 18-kW rotating anode generator in Mainz" under atmospheric conditions and at a wavelength of 0.154 nm. The magnitude of the incident wave vector, k = (27r/A) sin 0, was determined by the glancing angle 0 and the wavelength A. Samples were measured after LB film formation and again after silver deposition on top. Silver was evaporated at a pressure of
