Langmuir 1995,11, 27-29
27
Formation of Polymerized Thin Films during Laser Assisted Molecular Beam Deposition of Cu and Acetylene/ Acetone W. M.K. P. Wijekoon, J. J. Stry, P. N. basad,* and J. F. Garvey* Photonics Research Laboratory and Department of Chemistry, State University of New York ut Buffalo, Buffalo,New York 14260-3000 Received August 31, 1994. In Final Form: November 16, 1994@ Micrometer thick polymeric films containing embedded copper nanoparticles have been generated via laser assisted molecular beam deposition. A gaseousmixture of acetylendacetoneis expandedsupersonically into the plasma plume of a laser-ablated copper target and then directed toward a substrate. It is found that the formation of the resulting black polymeric film is not simply due to thermal effects of the plasma but is catalyzed by the presence of the copper clusters. We report the observation that a laser evaporated metal plasma can interact with an organic vapor to generate novel polymeric materials which can then be deposited as thin films. Previously, Castleman and co-workers,1-8and later Duncan and c o - w ~ r k e r s , utilized ~ - ~ ~ laser-ablated plasma of several different metals (namely Ti, V, Zr, Hf, Cr, Mo, and Fe) to generate a series of gas-phase metallocarbohedrenes which are referred to as met-cars (i.e., M8C12 where M stands for the metal). Met-cars are typically observed by introducing an appropriate hydrocarbon directly into laser-ablated metallic plasma plumes. The nature of the interaction between the metal plasma and the organic vapors depends on experimental conditions such as laser fluence, expansion conditions, and the nature of the carrier gas. In some situations the interaction of the metal plasma with gaseous hydrocarbons results in dehydrogenation or polymerization of the hydrocarbon.12-16 I t is interesting to note that El-Shall and co-workers14 have reported the generation of polyisobutylene bulk polymer by laser evaporation of a metallic target (Ti, Zr, or Sn) and injection of the metal atoms into the cooled liquid of the isobutylene monomer. Recently, we have utilized laser-evaporated metal plasmas in the fabrication of thin films of composite material via laser-assisted molecular beam deposition (LAMBD).17-19 We report evidence for the formation of polymerized products by the reaction of a copper Abstract published inAdvance ACSAbstracts, January 1,1995. (1) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Science 1992,254, 1144. (2) Guo, B. C.; Wei, S.;Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992,256,516. (3) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992,256 818. (4) Guo, B. C.; Wei, S.;Chen, Z.; Kern, K. P.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. J . Chem. Phys. 1993,97, 5243. (5) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.;Castleman, A. W., Jr. J . Phys. Chem. 1993,96, 4166. (6) Guo, B. C.; Kern, K. P.; Castleman, A. W., Jr. J . Am. Chem. SOC. 1993,115, 7415. (7) Yamada, Y.; Castleman, A. W., Jr. Chem. Phys. Lett. 1993,204, 133. (8) Guo, B. C.; Kern, K. P.; Castleman, A. W., Jr. J . Phys. Chem. 1993,97,9559. (9) Pilgrim, J. S.; Duncan, A. M. J . Am. Chem. SOC.1993,115,4395. (10)Pilgrim, J. S.; Duncan,A. M. J.Am. Chem.SOC.1993,115,9724. (11) Pilgrim, J. S.; Duncan, A. M. J . A m . Chem.SOC.1993,115,6958. (12) . Guo.. B. C.: Castleman. A. W., Jr. J . Am. Chem. SOC.1992,114, . . 6152. (13) El-Shall, M. S.;Schriver, K. E.; Whetten, R. L.; Meot-Ner, M. J . Phys. Chem. 1989,93,7969. (14) Vann, W.; El-Shall, M. S. J . Am. Chem. SOC.1993, 115,4385. (15)Daly, G. M.; El-Shall, M. S. J . Phys. Chem. 1994,98, 696. (16)Al-Noori, M. K.; Saleh, J. M. J. Chem. SOC.,Faraday Trans 1 1973,69, 2140. (17) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.;Prasad, P. N.; Garvey, J. F. J . Appl. Phys. 1993, 74, 5767. @
plasma with acety1ene:acetone heteroclusters. The films grown by the LAMBD techinque are black in color and their scanning electron micrographs show that the film surface is very inhomogeneous and contains scattered island-like structures (Figure 1). A LAMBD film grown for a period of 5 h yielded a thickness of -0.6 pm as estimated by edge-on scanning electron microscopy. The films were dissolvedin either hexane or deuterated toluene and filtered through a 0.2-pm filter. The resulting mass spectrum (Figure 2) exhibits a fragmentation pattern which is characteristic of a (poly)hydrocarbon.20 The largest peak in each cluster represents a C,Hb+l fragment, accompanied by C,Hb and CnH2,-1 fragments. The proton N M R spectrum of the LAMBDfilm dissolved in deuterated toluene (Figure3) indicates the presence of onlymethylene and methyl hydrogens in the LAMBD film. The ratio of CHdCHs protons is approximately 7:3. The ESCA spectrum of the LAMBD film (Figure 4) shows the presence of copper, carbon, and oxygen in the film. The binding energies of the copper 2~312(932.9 eV) and oxygen 1s (531.9ev) photoelectron peaks suggest that neither oxygen nor carbon is chemically bonded to copper.18i21-24This suggests that nanoparticles of copper are physically embedded within the film, as opposed to being a chemically bound species. The oxygen 1s signal seen in the ESCA spectrum can best be assigned to chemisorbed oxygen in the film.18,23This oxygen may originate from the acetone since the oxygen 1s photoelectron peak of acetone adsorbed on a Cu surface occurs at the same position (531.8 eVhZ6 No polymer film was generated when acetylendacetone was expanded into laser-ablated carbon generated when a carbon rod was used as the target. This suggests that the formation of the polymeric film is not simply due to thermal effects of the plasma but is catalyzed by the presence of the metal atoms. Since no polymer was generated when acetone was not present in the molecular beam, the acetone may be the source of the extra hydrogen present in the polymer. (18) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.; Prasad, P. N.;Garvey, J. F. J . Phys. D: Appl. Phys. 1994,27, 1548. (19) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.; Prasad, P. N.; Garvey, J. F. Submitted for publication in Appl. Phys. Lett. (20) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley & Sons: New York, 1981. (21) Fleisch, T. H.; Mains, G. J. Appl. Su$. Sci. 1982, 10, 5 . (22) Siriwardhane, R.; Paston, J. A. Appl. Su$. Sci. 1993, 68, 65. (23) Roberts, T.; Bartel, M.; Offergeld, D. Surf.Sci. 1972,33, 123. (24) IQm, K. S. J . Electron. Speclrosc. Related Phenom. 1974,5,259. (25) Prabhakaran, K.; b o , C. N. R. Appl. Surf. Sci 1990,44, 205.
0743-746319512411-0027$09.0010 0 1995 American Chemical Society
Letters
28 Langmuir, Vol. 11, No. 1, 1995
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Figure 3. Proton NMR spectrum of the LAMBD film in deuterated toluene.
I100
Figure 1. The scanning electron micrograph of a typical LAMBD film. In our experiment the gas-phase aggregates of metallic copper were produced via pulsed ablation of a copper cluster source. Coincident target in a modified Smc~ZZey-type~~*~~ with each ablation laser shot, a pulse (-1 ms) of acetylene (Scott, 99.6% dissolved in acetone) gas was supersonically expanded into the hot copper plasma plume by a pulsed molecular beam valve with a 0.5-mm nozzle orifice. The LAMBD film was grown on a NaCl disk (25 x 4 mm) which was situated in the path of this molecular beam -3 cm away from the target. The operational pressure of the vacuum chamber fluctuated between (5 and 10) x Pa when the backing pressure of acetylene was -70 Wa. Typical laser power level impinged on the copper target was -160 mJ/pulse with a pulse duration of 20 ns and a repetition rate of 5 Hz. The SEM micrograph was recorded at 25 keV filament voltage and the tilt angle was 45". 1881
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Figure 2. Mass spectrum of the LAMBD film dissolved in spectral grade hexane obtained with desorption electron impact (DEI) technique. The solubility of LAMBD films in common solvents is very low. Mass and NMR spectra were recorded using filtered solutions. Mass spectra obtained with fast atom bombardment (FAB) using a m-nitrobenzyl alcohol (m-NBA) matrix and DEI techniques show the same fragmentation pattern (with the exception of the apperance of some m-NBA peaks in the FAB experiment).
It is not possible to determine, from the present data, either the exact mechanism through which the saturated polyhydrocarbon film is generated or the exact structure
Binding Energy (eV)
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Figure 4. Elemental survey ESCA spectrum of the LAMBD film deposited on a 25 x 4 mm NaCl substrate. The ESCA spectrum was recorded with a small spot ESCA instrument using monochromatized Al K a X-rays (1486.6 eV) with a spot size of 1000pm. Data were collected at a take-off angle of 45".
of the polymer. Acetylene is known to adsorb molecularly As the temperature of on Cu at low temperat~res.~~-~O the Cu surface increases, adsorbed acetylene undergoes cyclization, disproportion, and dehydrogenation which generates ethylene, benzene, hydrogen, and carbonaceous species. However, the temperature at which these processes take place (
