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On the Nature of Thin Films Generated during the Laser-Assisted Molecular Beam Deposition of Metal Plasma and Organic Vapors W. M. K. P. Wijekoon,† J. J. Stry, P. N. Prasad,* and J. F. Garvey* Photonics Research Laboratory and Department of Chemistry, NSM Complex, State University of New York at Buffalo, Buffalo, New York 14260-3000 Received October 13, 1995. In Final Form: July 16, 1996X Micrometer thick polymeric films containing embedded metallic nanoparticles have been fabricated via the technique of laser-assisted molecular beam deposition. A gaseous mixture of acetylene/acetone is expanded supersonically into the plasma plume of laser-ablated metallic (copper, titanium) targets and then expanded toward a substrate. The formation of these polymeric films is found to be not simply due to thermal effects of the plasma but also due to catalytic activity of the metal clusters. In the case of copper metal it is found to be physically embedded as metallic nanoparticles. In the case of the titanium, the metal is found to be bonded with oxygen.
Introduction The interaction of a laser-evaporated metal plasma with organic vapors can produce exciting new materials comprised of stable molecular clusters.1-15 One exciting class of such molecular clusters includes metallo-carbohedrenes or so-called met-cars which are typically generated by introducing a suitable hydrocarbon gas into laserablated metallic plasma plumes.1-10 The nature of the interaction between the metal plasma and organic vapor depends on experimental parameters such as laser fluence, expansion condition, the chemical nature of the carrier gas, and the nature of the metal. Under certain conditions the interaction of the metal plasma with a gaseous hydrocarbon results in dehydrogenation or polymerization of the hydrocarbon.10-13 In fact, laser-evaporated energetic metal clusters have been utilized to catalyze polymerization of monomeric isobutylene.14 In our previous paper, we reported the formation of thin films of metal (copper) nanoparticles encapsulated in a poly(hydrocarbon) film as a result of expansion of acetylene (99.6% dissolved in acetone) into a laser-ablated plasma plume of metallic copper.15 This observation was made during our attempts to fabricate thin films of organic: inorganic composite materials via laser-assisted molecular † Present address Applied Materials, 3111 Coronado Dr., Santa Clara, CA 95054. X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) Guo, B. C.; Kerns, K. P.; Castleman, A. W., Jr. Science 1992, 254, 1144. (2) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. Science 1992, 256, 818. (3) Wei, S.; Guo, B. C.; Purnell, J.; Buzza, S.; Castleman, A. W., Jr. J. Phys. Chem. 1993, 96, 4166. (4) Guo, B. C.; Kern, K. P.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1993, 115, 7415. (5) Yamada, Y.; Castleman, A. W., Jr. Chem. Phys. Lett. 1993, 204, 133. (6) Guo, B. C.; Kern, K. P.; Castleman, A. W., Jr. J. Phys. Chem. 1993, 97, 9559. (7) Pilgrim, J. S.; Duncan, A. M. J. Am. Chem. Soc. 1993, 115, 4395. (8) Pilgrim, J. S.; Duncan, A. M. J. Am. Chem. Soc. 1993, 115, 9724. (9) Pilgrim, J. S.; Duncan, A. M. J. Am. Chem. Soc. 1993, 115, 6958. (10) Guo, B. C.; Castleman, A. W., Jr. J. Am. Chem. Soc. 1992, 114, 6152. (11) El-Shall, M. S.; Shriver, K. E.; Wheaten, R. L.; Met-New, M. J. Phys. Chem. 1989, 93, 7969. (12) Vain, W.; El-Shall, M. S. J. Am. Chem. Soc. 1993, 115, 4385. (13) Daly, G. M.; El-Shall, M. S. J. Phys. Chem. 1994, 98, 696. (14) Daly, G. M.; El-Shall, M. S. J. Phys. Chem. 1995, 99, 5283. (15) Wijekoon, W. M. K. P.; Stry, J. J.; Prasad, P. N.; Garvey, J. F. Langmuir 1995, 11, 27.
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beam deposition (LAMBD). We also demonstrated that the formation of the polymeric films was not only a thermal effect of the plasma but also a catalyzing activity of the metal atoms. We have now extended our studies to several other metal:organic gas systems (i.e., titanium:acetylene/ acetone and titanium:1-butene). In this paper we report on the nature of these composite LAMBD films which were studied by using a number of analytical and spectroscopic techniques such as elemental analysis, scanning electron microscopy, X-ray photoelectron spectroscopy, and mass spectroscopy. We find that copper in these films exists as metallic nanoparticals whereas titanium is chemically bonded to oxygen. Experimental Procedure All films were deposited in our LAMBD apparatus, a detailed account of which has been described previously.16,17 Briefly, the metal plasma was generated by pulsed laser (KrF excimer laser; λ ) 248 nm) ablation of pure metallic (titanium, copper) target rods. Coincident with each ablation pulse, organic vapors were supersonically expanded into the metal plasma plume through a pulsed molecular beam valve. Acetylene (99.6% dissolved in acetone) and 1-butene were directly expanded without using a carrier gas. All films were deposited under identical experimental conditions on substrates which were situated within the deposition chamber in the path of the molecular beam. Typical operational pressure of the vacuum chamber fluctuated between (4 and 8) × 10-5 Torr when the backing pressure of the carrier gas was ∼8 psi. Typical laser power levels were ∼170 mJ/pulse with a pulse duration of 20 ns and a repetition rate of 5 Hz. For the surface studies polymeric films were deposited on ITOcoated glass substrates. The ESCA and SEM/EDX studies of the LAMBD films were carried out at our university biomaterial center. The LAMBD films were removed from the vacuum chamber and transported in closed containers. The surface analysis of the films was performed with a Hitachi S-800 SEM system and a PGT IMIX EDXS system. Electron micrographs were recorded at a 25 keV filament voltage and the tilt angle was 45°. The ESCA analysis was made with a SSI-100 small spot ESCA instrument using monochromatized Al KR X-rays (1486.6 eV) as the excitation source and a multichannel plate as the detecting element. The spot size was 1000 µm. The charging of the samples was compensated by a flood gun and controlled with the C(1s) signal. Data were collected at a take-off angle of 45°. The spectral areas were computer integrated and leastsquares fitted to experimental ESCA peaks. (16) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.; Prasad, P. N.; Garvey, J. F. J. Appl. Phys. 1993, 74, 5767. (17) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.; Prasad, P. N.; Garvey, J. F. J. Phys. D: Appl. Phys. 1994, 27, 1548.
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Figure 1. Mass spectrum of the film fabricated by introducing acetylene into a copper plasma. The data were obtained with the desorption electron impact (DEI) technique. 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 appearance of some m-NBA peaks in the FAB experiment). All the other measurements were made either on powders (which were collected by scraping a number of films) or on solutions which were made by dissolving the collected powder. Elemental analysis of the powder material was performed by the Gailbraith Laboratories. Infrared spectra were recorded on a pellet made of spectral grade KBr and the powders. The proton and 13C NMR spectra were recorded using the solutions which were made by dissolving the powder in deuterated toluene. The mass spectra were recorded by using solutions which were made by dissolving the powder in spectral grade hexane. The data were obtained with desorption electron impact (DEI) technique. The solubility of LAMBD films in common solvents is very low. Both mass and NMR spectra were recorded using solutions filtered through a 0.2 µm filter. Mass spectra obtained with fast atom bombardment (FAB) using a m-nitrobenzyl alcohol (mNBA) matrix and DEI techniques show the same fragmentation pattern with the exception of the appearance of some m-NBA peaks in the FAB experiment.
Results The films grown by the LAMBD technique are black in color and consist of a significant amount of inhomogeneities as studied by scanning electron microscopy (SEM).15 A typical film deposited for a period of ∼5 h yielded a thickness of ∼0.6 µm as estimated by edge-on SEM.15 As reported previously, the mass spectrum of the polymer generated by introducing acetylene into copper plasma exhibits a fragmentation pattern which is characteristic of a (poly)hydrocarbon.18 The largest peak in each cluster represents a CnH2n+1 fragment, accompanied by CnH2n and CnH2n-1 fragments (Figure 1). The binding energies of the copper 2p3/2 (932.9 eV) and oxygen 1s (531.9 eV) photoelectron peaks (Figure 2) indicate that neither oxygen nor carbon is chemically bonded to copper.19-23 Therefore, (18) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds; John Wiley & Sons: New York, 1981. (19) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Hand Book of X-ray Photoelectron spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979.
the nanoparticles of copper are physically embedded within the polymer. The oxygen 1s photoelectron signal seen in the ESCA spectrum can be best assigned to chemisorbed oxygen in the film.17,22 Although the oxygen 1s photoelectron peak occurs at the same energy (531.8 eV) as for acetone chemisorbed on Cu surface,24 it is highly unlikely to expect any molecular acetone under the used experimental conditions. Most probably this oxygen may result from adsorbed water. The oxidation state of Cu was found to be Cu0, as determined with modified Auger parameters (R′) which were calculated from the binding energy of the Cu 2p3/2 photoelectron line and the kinetic energy of the Cu L3M4,5M4,5 Auger line.25 Furthermore, no polymer film was generated when acetylene/acetone was expanded into laser-ablated carbon (Figure 3). Hence, the formation of the polymeric products is not just due to thermal effects of the plasma. The process is also being catalyzed by the presence of the metal atoms. However, the chemical nature of the products generated by the expansion of acetylene into the titanium plasma is somewhat different from that of the corresponding products formed during the expansion of acetylene into the copper plasma. As seen in the ESCA results (Figure 4), two types of carbon and oxygen species are present in the film fabricated by introduction of acetylene into the titanium plasma. The C(1s) photoelectron peak at 286.4 eV suggests that one of the carbon species (∼11% of the full carbon content) is bonded to oxygen (Figure 4C). Both the infrared spectrum (peak at 1730 cm-1) and the 13C NMR spectrum (peak at δ ) 172 ppm) (which are shown in Figures 5 and 6, respectively), indicate the presence of a >CdO functional group in the film. Also, the 13C NMR (20) Fleisch, T. H.; Mains, G. J. Appl. Surf. Sci. 1982, 10, 5. (21) Siriwardhane, R.; Paston, J. A. Appl. Surf. Sci. 1993, 68, 65. (22) Roberts, T.; Bartel, M.; Offergeld, D. Surf. Sci. 1972, 33, 123. (23) Kim, K. S. J. Electron Spectrosc. Relat. Phenom. 1974, 5, 259. (24) Prabhakaran, K.; Rao, C. N. R. Appl. Surf. Sci. 1990, 44, 205. (25) Wijekoon, W. M. K. P.; Lyktey, M. Y. M.; Prasad, P. N.; Garvey, J. F. Manuscript under preparation.
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Figure 3. Proton NMR spectra of films obtained by introducing a supersonic expansion of (A) acetylene into a ablated carbon rod, (B) acetylene into a copper plasma, and (C) 1-butene into a titanium plasma. All the spectra were recorded in deuterated toluene by using filtered solutions.
Figure 2. High resolution ESCA data of (A) carbon 1s, (B) oxygen 1s, and (C) copper 2p photoelectron peaks of the polymer film deposited by introducing a supersonic expansion of acetylene into a copper plasma. Data indicate the presence of only one type of carbon in the film.
spectrum (peak at δ ) 65 ppm) as well as the proton NMR spectrum (peak around δ ) 4 ppm) give evidence for the presence of a -OCH2 group (Figure 6B). The existence of a -OCH2 group is also indicated by the infrared peak at 1157 cm-1.26 We assign the broad infrared peak at 1450 cm-1 to asymmetric and symmetric deformation of oxygen bound methylene groups.26 All of these results indicate the presence of one or more oxygen bound carbon species in the products. However, it should be noted, although the infrared spectrum is indicative of the presence of oxygen bound vinyl carbon (infrared peat at 1630 cm-1), neither mass spectra nor NMR spectra provide such information. Therefore this infrared peak may be originating from the products which are not soluble. As evident by the results of elemental analysis (C, 71.6; Ti, 8.84; H, 8.35) and ESCA studies (C, 64.7; Ti, 7.9; O, 26.9), these films contain a high fraction of carbon. It should be noted that the percentage of oxygen in the film could not be determined by elemental analysis due to interference from titanium. Although ESCA is essentially a surface analytical probe, both techniques result in approximately the same values for carbon and titanium. As judged by the ESCA results, the amounts of the two types of oxygen present in these films are approximately the same (52% and 48%). The binding energies of the titanium 2p3/2 (459 eV) and the oxygen 1s (530.6 eV) (26) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: London, 1990.
photoelectron peaks indicate that only one type of oxygen is bonded to titanium. The other type of oxygen (binding energy at 532.3 eV) present in the film can be best ascribed to the adsorbed water or oxygen bonded to carbon. In order to verify the presence of titanium bound oxygen, we studied a film by the secondary ion mass spectroscopy (SIMS). For the SIMS investigation the film was directly deposited on the tip of the probe. As shown in Figure 7 the SIMS data clearly demonstrate the Ti-O fragmentation (m/z ) 64) confirming a presence of the Ti-O bond in the film. However, the SIMS spectra provided no information for the presence of TiC or CuC in these films. The mass spectrum of the dissolved film (Figure 8) is very similar to that of the copper/acetylene film in the sense that both spectra exhibit a -(CH2)n- fragmentation pattern. Nevertheless, in the mass spectrum of the titanium:acetylene film some additional peaks (for example, m/z ) 515) are clearly visible. These peaks may arise due to the presence of oxygenated species in the products. Discussion and Conclusion As indicated earlier, the solubility of these LABMD films in common organic solvents is very low. When the solutions in toluene and in hexane are filtered through a 0.2 µm filter, black residues are clearly visible in the filter paper indicating a significant amount of insoluble products in the films. Since the NMR and mass spectra were obtained by using the filtered solutions, they only account for the soluble products. As seen in the elemental analysis and ESCA data the carbon content in these films is fairly high. Therefore, these films contain other products which are not directly accounted for in the mass spectra and in the NMR spectra. Neither ESCA nor mass spectra can provide any evidence for the presence of metal bound oxygen in the films obtained by the introduction of acetylene (99.6% dissolved in acetone) into a copper plasma. In contrast, such data clearly demonstrate the presence of titanium bound oxygen in films obtained by
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Figure 6. (A) 13C NMR spectrum and (B) proton NMR spectrum of products obtained by introducing acetylene into a titanium plasma.
Figure 4. High-resolution ESCA data of (A) oxygen 1s, (B) titanium 2p, and (C) carbon 1s photoelectron peaks for the polymer film deposited by introducing a supersonic expansion of acetylene into a titanium plasma. Data indicate the presence of two types of carbon and two types of oxygen in the products.
Figure 7. Mass spectrum of the film fabricated by introducing acetylene into a titanium plasma. Film was deposited on the tip of the probe, and the data were obtained via SIMS technique.
Figure 5. Infrared spectrum of the powder obtained by scraping a film fabricated by introducing acetylene into a titanium plasma. The spectrum was recorded in a KBr matrix.
expansion of acetylene (99.6% dissolved in acetone) into a titanium plasma. On the basis of the NMR and mass spectral data (Figure 3) it can be fairly assumed that the chemical nature of the soluble products in films obtained by expanding 1-butene into the titanium plasma and acetylene (99.6% dissolved in acetone) into the copper plasma is very similar. The
mass spectra of these films show the same -(CH2)nfragmentation pattern. The peak at m/z ) 515 is only seen in the mass spectra of films fabricated by expansion of acetylene (99.6% dissolved in acetone) into the titanium plasma. As seen in the spectra this peak does not belong to the -(CH2)n- fragmentation pattern. The mass spectra of none of the films indicate the presence of titanium or copper atoms in the filtered solutions. Therefore, the peak at m/z ) 515 (Figure 8) may be originating from an oxygencontaining species. In the case of copper, the nanoparticles are physically embedded in the polymer without being chemically bonded to any other elements. In contrast, the titanium found in the film is bonded to oxygen. However, the mass spectra of the filtered solutions do not show any evidence for the presence of either copper or titanium in the solutions. This again indicates the insolubility of some of the generated products.
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Figure 8. Mass spectrum of the film fabricated by introducing acetylene into a copper plasma. The data were obtained with desorption electron impact (DEI) technique.
Most probably the source of oxygen found in some of these films is chemisorbed water which is incorporated into films during film transfer and transportation. We fabricated several films by expending 1-butene into a laserevaporated titanium plasma. The proton-NMR spectrum (Figure 3C) and mass spectrum of this film were almost identical to that fabricated with the copper plasma and acetylene/acetone. The ESCA results of those films show no indication of the presence of titanium bound oxygen. The SIMS spectra of LAMBD films show no evidence for the presence of TiC or CuC in these films, although one might expect such species to be found in these films. Because of this reason ESCA data were fitted without considering the presence of carbidic carbon. Although SIMS spectra clearly indicated a presence of a Ti-O bond, no evidence was found for the presence of TiO2. However, a series of SIMS spectra exhibited a peak at m/z ) 81 which could not be assigned to TiO2, which has a m/z value of 80. It is not possible to understand from the present data either the exact mechanism through which these polymeric products are generated or the exact structure of these polymers. Several studies have shown that laser-
evaporated Ti+ catalyzes polymerization of some unsaturated hydrocarbons (ethylene10 and isobutylene13) in the gas phase. For example, in the case of ethylene as many as 20 ethylene molecules have been observed to be bound with Ti+, and the gas phase polymerization is supposed to be initiated by the formation of a six-coordinated Ti+ complex. In the case of isobutylene, the reaction pathways depend on the pressure of isobutylene. While the elimination of H2 and the addition of isobutylene onto TiC4H6+ are the predominant processes at low pressure, at higher pressure the polymeric ion, C4H9+(C4H8)n, is formed due to opening of a new condensation channel with the quenching of the elimination reaction.15 Such gas phase studies should provide some insight into the understanding of the reaction pathways of the present study. We plan to continue to pursue these gas phase studies of the present metal/organic systems in the future. Acknowledgment. This research was supported by The National Science Foundation solid state chemistry program (Grant No. DMR9213907). LA950868G
