2837
Langmuir 1995,11, 2837-2839
Preparation of Ultrathin Metal Oxide Films from Langmuir-Blodgett Layers C. L. Mirley and J. T. Koberstein" University of Connecticut, Department of Chemical Engineering, Institute of Materials Science, U-136, Storrs, Connecticut 06269-3136
Table 1. Summary of XPS Results for Cadmium Arachidate Film" Deposited on Silicon, before and after Exposure to W-Ozone before W-ozone after W-ozpne exposure
C/Cd
c/o
Introduction Recently, Kalachev e t al.' reported that LangmuirBlodgett (LB) films of cadmium arachidate could be converted to cadmium oxide ultrathin films by exposure to a low-pressure (0.04 mbar) and low-temperature (35 "C) oxygen plasma. In our laboratory, we have found that ultrathin metal oxide films can also be prepared by exposing metal soaps of fatty acid LB films to ultraviolet light (W)and ozone. Previously, we reported on the preparation of ultrathin silicon dioxide films from Langmuir-Blodgett (LB) layers of carboxylic acid-terminated polydimethylsiloxane (PDMS) exposed to UV-ozone.2 The process is essentially one where UV radiation (185-254 nm) is used to simultaneously produce ozone from atmospheric oxygen and excite organic molecules. The ozone then oxidizes the organic portion of molecules, producing noncondensing, volatile species such as carbon dioxide and water.3 The advantage of the UV-ozone technique over other methods for preparing metal oxide films, such as direct evaporation, glow-discharge plating or sputtering, and chemical vapor d e p ~ s i t i o nis, ~that it is carried out at room temperature and atmospheric pressure. Additionally, the use of fatty acid LB films with UV-ozone makes it possible to produce metal oxide films with thicknesses on the order of only a few angstroms. Potential applications for ultrathin metal oxide films include magnetic or insulating layers for electronic devices, antireflective optical coatings, and high-temperature boundary lubricants. This note describes results from the analysis of cadmium oxide films prepared from UV-ozone exposure of cadmium arachidate LB films.
Experimental Section Materials. Arachidic acid (99%)was obtained from Sigma Chemical Co. Cadmium chloride (99.999%)was obtained from
Janssen-Chimica. The water subphase in the Langmuir trough was purified with a Millipore Super-Qwater purification system. Substrates for LB film deposition were prepared by vacuum evaporating gold onto glass slides and silicon wafers. Equipment. The LB trough used to prepared the LB films A UVOCS TlOx 101OES UVhas been described previ~usly.~ ozone cleaner was used to treat the deposited fatty acid LB films. Ultraviolet emissions at 185 and 254 nm were generated by a low-pressure mercury-quartz lamp (10 mW/cm2). LB film thicknesses were measured using a variable-anglespectroscopic ellipsometer (J.A. Woollam).6 Film composition was measured
* To whom correspondence should be addressed. FAX: (203)4864745. Telephone: (203) 486-4716. E-mail:
[email protected]. (1)Kalachev, A. A, Mathauer, K.; Hohne, U.; Mohwald, H.; Wegner, G. Thin Solid Films 1993,228, 307. (2) Mirley, C. L.; Koberstein, J. T. Langmuir 1995,11, 1049. (3)Vig, J. In Treatise on Clean Surface Technology; Mittal, K., Ed.; Plenum Press: New York, 1987. (4) Bhushan, B.; Gupta, B. Handbook of Tribology; McGraw Hill, Inc.: New York, 1991. ( 5 ) Mirley, C. L.; Lewis, M. G.; Koberstein, J. T.; Lee, D. H. T. Langmuir 1994,10,2370. (6) Ulman, A. An Introduction to Ultrathin 0rganicFilms;Academic Press Inc.: San Diego, 1991. 0743-7463/95/2411-2837$09.00/0
0.9411
1111c
0.3411
binding energy, eV Cd3d,5/z
Received February 1, 1995. I n Final Form: April 24, 1995
exposure
3511' 405.3
405.2
Film thickness = 351 A. Exposure time was 30 min. Theoretical values, assumingtwo arachidic acid molecules per cadmium ion: CICd = 4011, C/O = 1011. a
using a Perkin Elmer Physical Electronics PHI 5300 X-ray photoelectron spectrometer (XPS),7equipped with a monochromaticAl KaX-ray source (1486.6eV)and hemispherical analyzer. Grazing-incidenceinfrared (GIRY spectra were measured using a Matson Cygnus 100 FTIR having an MCT detector with 4 cm-l resolution. LB Film Preparation. An arachidic acid solution was prepared in chloroform at 4 mg/mL concentration;100 pL of the solution was spread onto the LB trough water subphaseconsisting of CdClz [2.0 x MI and KHC03 [2.4 x lo-* MI; pH = 7.65 and T = 19 "C. The floating LB film was compressed at 4 m d min to a surface pressure of 30 mN/m, where deposition was carried out at a vertical dipping speed of 10 mdmin.
Results and Discussion Ellipsometer measurements on multilayer cadmium arachidate LB films indicated that the thickness of a single layer was in the range 27-29 A, which was in good agreement with the published values9 Before UV-ozone exposure, XPS measurement of a 13-layer cadmium arachidate film on silicon gave a C/Cd atomic ratio of 351 1. This value was slightly lower than the calculated C/Cd value of 40/1 assuming two arachidic acid chains per cadmium ion. After 30 min of UV-ozone exposure, the C/Cd atomic ratio dropped to 0.9411. representing a 97% loss in carbon atoms. The actual percentage carbon loss is probably higher since we have found that 2-3% of adventitious carbon is typically found on most sample surfaces that are exposed to air prior to XPS measurement. The binding energy ofthe CdSd5,2 photoelectron after UVozone exposure of the cadmium arachidate was found to be 405.2 eV, consistent with the expected value for cadmium in CdO.1° Table 1summarizes the XPS results for the 13-layer cadmium arachidate film deposited on silicon before and after UV-ozone exposure. The time dependence of the UV-ozone reaction for cadmium arachidate films was monitored using grazingincidence FTIR (GIR). Figure 1ghows the spectra for a cadmium arachidate film (240 A deposited on gold) at various time intervals after UV-ozone exposure. At time zero, the most evident absorbance bands in the cadmium arachidate GIR spectrum were the COz symmetric and asymmetric stretches at 1433and 1549 cm-l, respectively. The positions of these absorbance bands and the absence of a strong band at 1700 cm-l indicated that the arachidic acid monolayers were transferred to the gold substrate as the cadmium salt and not the free acid form.ll Other absorbance bands present in the GIR spectrum were the (7) Andrade, J. Surface and Interfacial Aspects of Biomedical Polymers; Plenum Press: New York, 1985. (8) Garton, A. Infrared Spectroscopy ofpolymer Blends, Composites, and Surfaces; Hanser Publishing: New York, 1992. (9) Gaines, G. Insoluble Monolayers; Interscience Publishing: 1966. (10)Wagner, C.; Riggs, W.; Davis, L.; Moulder, J.; Mullenberg, G. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1974. (11)Rabolt, J. F.; Burns, F. C.; Schlotter, N. E.; Swalen, J. D. J. Chem. Phys. 1983,78 (3), 946.
0 1995 American Chemical Society
Notes
2838 Langmuir, Vol. 11, No. 7, 1995
D
C
0.03
n 0.02E
B
A 0 . 00 -I
,
1
I
*,I.,,~...I,,.I...(...I.,,~.tiL .. 9 8
-.
--
e
e
CH2 symmetric (2851 cm-l) and asymmetric stretches (2918 cm-l) and the CH3 symmetric (2874 cm-l) and asymmetric stretches (2961 cm-l). After 5 min of UV-ozone exposure, it was observed that the absorbance of the carboxylate salt band a t 1433 cm-l decreased significantly while new bands a t 1728 and 3240 cm-l appeared. The latter two peaks were most likely due to COOH and OH groups formed during oxidation of the arachidic acid molecules. The peak wavenumbers of the CH2 and CH3 bands shifted after 5 min of W-ozone exposure such that the GIR spectrum for cadmium arachidate resembled that of an IR transmission spectrum for bulk cadmium arachidate.6 This indicated that the oxidation reaction brought about by W-ozone exposure changed the molecular arrangement of the cadmium arachidate molecules in the deposited LB film from one that was well-ordered to one that was more random. After 15min of exposure to W-ozone, the cadmium arachidate film showed almost a complete loss of the carbonaceous groups, leaving only the cadmium oxide on the surface of the gold substrate. The etch rate of cadmium arachidate LB films by Wozone was measured by ellipsometry. The etch rate was
Table 2. UV-Ozone Etch Rate of Cadmium Arachidate LB Films as Measured by Ellimometry ~
~
~~
~
initial film thickness (A)
UV-ozone exposure time (min)
etch rate (fUmin).
190 240 1420
3 5 10 (first exposure) 10 (second exposure)
16 53 34
a
8
Etch rate = (tln,tla1 - tnnal/exposuretime), where t is the film
thickness. defined as the difference between the initial average film thickness and the final average film thickness after exposure, divided by the exposure time. Table 2 shows the effects offilm thickness and W-ozone exposure time on the etch rate of cadmium arachidate films deposited on silicon. When the W-ozone exposure time was on the order of a feqminutes, the etch rate was found to be fairly low a t 8 Nmin. When the exposure tim? was increased to 10 min, the etch rate increased to 53 Nmin or about 2 monolayerdmin for cadmium arachidate. This phenomenon could be due to the fact that an induction period is needed to build up a sufficient concentration of
Langmuir, Vol. 11, No. 7, 1995 2839
Notes ozone in the W-ozone cleaner or for ozone to penetrate and react with the cadmium arachidate LB film. Table 2 also shows that, after a n initial UV-ozone exposure of 10 min, the etch rate for another 10 min of W-ozone exposure was lower than the initial etch rate. This was probably due to the fact that, after the first 10 min of W-ozone, a CdO film formed on top of the cadmium arachidate film, limiting the ozone permeability during the second exposure period, thereby giving a lower etch rate. Figure 2 demonstrates how metal oxide films can be loaded onto a substrate surface simply by increasing the number of deposited LB layers prior to W-ozone exposure. Along with the XPS spectra for a 5-layer cadmium arachidate film after 30 min of W-ozone exposure, Figure 2 also shows a plot of the CdAu atomic ratio for CdO films formed on a gold surface as a function of the number of deposited cadmium arachidate layers before UV-ozone exposure. It was observed that the C d
Au atomic ratio increased linearly as the number of cadmium arachidate layers increased from a single monolayer to 13 layers. Results similar to those described above for the formation of metal oxide films from W-ozone exposure of arachidic acid LB layers condensed with cadmium counterions have been obtained by using manganese and aluminum counterions. Therefore a variety of metal oxide films can be produced simply by changing the metal counterion added to the trough water subphase during LB film preparation.
Acknowledgment. C.L.M. wishes to thank the Eastman Kodak Fellows Program, the US.Army Research Office, and the Connecticut Department of Higher Education (Grant No. 631606) for their financial support during the preparation of this work. LA950073+
