J. Phys. Chem. B 2004, 108, 18179-18184
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Characterization of Nanoparticles Fabricated by Oxidation of Ni80Fe20 and Co80Fe20 Thin Films during Imidization Sung K. Lim, Chong S. Yoon,* Chang K. Kim, and Young H. Kim Department of Materials Science and Engineering, Hanyang UniVersity, HaengDang-Dong, SeongDong-Ku, Seoul, 133-791, Korea ReceiVed: July 14, 2004; In Final Form: September 2, 2004
Metallicnanoparticles embedded in a polyimide (PI) matrix were fabricated through selectively oxidizing a layer of Ni80Fe20 metal film sandwiched between two PI precursor layers. Ni nanoparticles, formed in a monolayer between two PI layers, had an average particle size of ∼5 nm. X-ray photoelectron spectroscopy confirmed that Fe in the film was preferentially consumed, resulting in formation of Ni nanoparticles. Similar experiments to produce uniform-sized Co nanoparticles failed to produce uniform-sized Co nanoparticles when a Co80Fe20 film was inserted because, unlike Ni, Co was partially oxidized during imidization. Transmission infrared spectroscopy suggested that thermal degradation of the PI matrix was catalyzed by the Co80Fe20 film, leading to the Co oxidation together with Fe.
1. Introduction An array of magnetic nanoparticles arranged in a regular manner is increasingly envisioned as an alternative system for next generation of magnetic data storage beyond 100 Gbit/in2, which is thought to be unattainable using conventional continuous magnetic thin films.1 An array of self-assembled magnetic nanoparticles using Co and FePt have already been demonstrated;2-4 however, the self-assembling process often requires organic additives to protect the nanoparticles from oxidation and aggregation. This process can be expensive and it is also questionable whether such methods of producing nanoparticles can be scaled up for robust commercial applications. Even if it does, the self-assembled system may not be sufficiently robust for commercial applications. We have previously demonstrated that magnetic oxide (γFe2O3) nanoparticles can be produced as a byproduct of the imidization reaction of polyimide (PI).5 These nanoparticles were both physically and magnetically isolated from each other in the as-prepared state without requiring further processing. We propose a new method of fabricating metallic nanoparticles based on the selective oxidation of an alloy film. In Ni80Fe20 and Co80Fe20 thin films, because the Gibbs free energy of formation for the respective oxides of the constituent metal elements is lowest for Fe,6 Fe would be preferentially oxidized in both alloy films during imidization, leaving a layer of unreacted metal particles. In this paper, we compare metallic nanoparticles produced this way from two different alloy films. Their structures and magnetic properties were characterized as a function of the curing condition of the polyimide matrix. 2. Experimental Procedure Nanoparticles embedded in the PI matrix were produced by oxidizing a thin film of Ni80Fe20 or Co80Fe20 sandwiched * Corresponding author. E-mail address:
[email protected]. Fax: +82-2-2290-1838. Telephone: +82-2-2290-0384.
between two polyimide precursor layers. The PI precursor used in this experiment was the p-phenylene biphenyltetracarboximide (BPDA-PDA) type polyamic acid (Dupont, PI2610) dissolved in N-methyl-2-pyrrolidinone. Thin films of Ni80Fe20 and Co80Fe20 with thicknesses varying from 3.5 to 9 nm were deposited using magnetron sputtering. The thickness of the spincoated PI precursor were controlled so that the final cured thickness of both top and bottom PI layers was 40 nm. Detailed procedure of preparation of the nanoparticles was described elsewhere.5,7 The PI/Ni80Fe20(Co80Fe20)/PI samples were cured at different temperatures (300-600 °C) for 1 h under vacuum (10-3 Pa). Transmission electron microscopy (TEM, JEOL 2010) was used to characterize the synthesized nanoparticles. TEM samples were prepared by mechanical grinding and an ion mill equipped with a cold stage. For X-ray photoelectron spectroscopy (XPS, PH 5400), monochromatic X-ray generated from Al KR (15 kV) was used. The FT-IR spectra of the samples were measured by Nicolet Magna IR-760 spectrometer. 3. Results and Discussion 3.1. Ni80Fe20 Thin Films. Shown in Figure 1a is the TEM bright field image of the PI/Ni80Fe20(3.5 nm)/PI sample cured at 300 °C. The Ni80Fe20 film broke up into particles whose average size was 5.4 ( 1.1 nm standard deviation, and the volume fraction was ∼25%. The formation mechanism for the nanoparticles was described in detail elsewhere.5,8 Similar to the oxide particles formed in previous experiments,5,7 the nanoparticles were arranged in a monolayer as can be seen from the cross-sectional TEM image of the same sample in Figure 1b. The selected area diffraction pattern from the nanoparticles in Figure 1c was tentatively indexed to a face-centered cubic (fcc) structure with a ≈ 3.6 Å because of the line broadening of the diffraction peaks resulting from the small particle size. Although NiO (a ) 2.955 Å) [JCPDS 44-1159], γ-Fe2O3 (a ) 8.351 Å) [JCPDS 39-1306], and Fe3O4 (a ) 8.396 Å) [JCPDS 19-0926] all have similar fcc structures, their lattice parameters are either too larger or too small compared to the estimated value. Hence, the nanoparticles appears to belong to metallic Ni (a ) 3.523 Å) [JCPDS 04-0850] or Ni80Fe20 (a ) 3.542
10.1021/jp0468758 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/03/2004
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Figure 1. (a) TEM image of the PI/Ni80Fe20(3.5 nm)/PI sample cured at 300 °C. (b) Cross-sectional TEM image of the sample in part a. (c) Indexed electron diffraction patterns of part a, showing a fcc structure [A, (111); B, (200); C, (220); D, (311); E, (222)]. (d) PI/Ni80Fe20(9 nm)/PI sample cured at 300 °C.
Figure 2. (a) TEM image of the PI/Ni80Fe20(3.5 nm)/PI sample cured at 600 °C. (b) particle size distributions for the PI/Ni80Fe20(3.5 nm)/PI sample cured at different temperatures.
Å).9 Unlike our previous results in which the imidization led to oxidation of the adjacent Fe thin film, in the case of Ni80Fe20, the particles appeared to remain metallic during imidization. This suggests that the polyamic acid selectively reacted with Fe while residual Ni, being more oxidation-resistant, remained metallic and coalesced to form spherical particles. When the thickness of the initial Ni80Fe20 film was increased to 9 nm, instead of spherical particles, irregularly shaped islands of metallic Ni, composed of multiple grains, were observed as shown in Figure 1d.
The curing temperature was increased up to 600 °C in order to study its effect on the particles size distribution and morphology. As can be seen from the TEM image of the PI/ Ni80Fe20(3.5 nm)/PI sample cured at 600 °C in Figure 2a, the curing temperature hardly had any effect either on the particles shape or on the size distribution. In fact, the average particle size was still 5.8 ( 1.1 nm when cured at 600 °C. The particle size distributions in Figure 2b better demonstrate the thermal stability of the nanoparticles as the average particle nearly remained constant within the measurement error. The constancy
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Figure 3. XPS spectra for the PI/Ni80Fe20(3.5 nm)/PI sample cured at 300 °C and 600 °C: (a) Ni 2p; (b) Fe 2p with the peak-separated curves showing the contributions from Fe2O3 and Fe. (SAT indicates the satellite peaks.)
in the particles size distribution is quite remarkable since the curing temperature was high enough to destroy the polymer structure of the organic matrix (verified using FT-IR) and expedite the diffusion of Ni necessary for particle coarsening. It is speculated that the constancy in the size distribution may have stemmed from the columnar microstructure of the asdeposited Ni80Fe20 film. Since grain boundaries are more susceptible to oxidation, the particle size may just reflect the original grain size of the Ni80Fe20 film. Changes in the chemical states of Ni and Fe were studied using XPS. The Ni 2p absorption edge from the samples cured at 300 and 600 °C shown in Figure 3a provides further evidence that nanoparticles in Figure 1a and Figure 2a were made from metallic Ni. Both the Ni 2p binding energy (852.7 eV)10 and the energy width between 2p3/2 and 2p1/2 peaks well matched the standard values for metallic Ni. The spectra were also peakseparated to detect the presence of any overlapping peaks. As indicated in the figure, no oxide peak was found. In contrast, Fe 2p spectra in Figure 3b obtained from the same set of samples showed relatively broad peaks, composed of both metal and oxide spectra. The peak separation of spectra clearly attests to the preferential oxidation of Fe from the alloy film as suggested earlier. Presence of metallic Fe indicates that the particles retained a fraction of Fe still dissolved in the lattice. Comparing the relative intensity of the metallic and oxide peaks, it was estimated that 62% of Fe in mole fraction was oxidized during imidization at 300 °C. However, as the curing temperature was raised to 600 °C, this value decreased to 43% as evidenced by the increased metallic Fe peak at 600 °C. It appears that the curing temperature of 600 °C was too high so that the polyamic structure was substantially damaged, reducing the reaction rate with Fe. The transmission FT-IR was used to verify the structure of the PI matrix cured at different temperatures. Figure 4 shows spectra obtained from a PI film and the PI/Ni80Fe20(3.5 nm)/PI sample cured at different temperatures. Detailed analysis of the major peaks in the PI film can be found elsewhere.11 Figure 4a clearly indicates that the structure of the PI film changed significantly when cured in vicinity of Ni80Fe20 film. The
Figure 4. FT-IR spectra for the PI/Ni80Fe20(3.5 nm)/PI sample cured at different temperatures. (- - - lines show the PI cured without the metal film for comparison.)
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Figure 5. Magnetic hysteresis loops measured at room temperature from the nanoparticles prepared at different curing temperatures. Also included is the hysteresis loop from the PI/Fe(3.5 nm)/PI sample cured at 400 °C for comparison.
intensity loss of 1770, 1712, and 1621 cm-1 bands reflects the formation of Fe carboxylate group as the CdO cabonyl stretch is responsible for these bands in agreement with the XPS data. Similar observations were also found when Cu, Cr, and Co films were deposited on a cured PI film.12-13 In addition to that, the bands ranging from 1600 to 1400 cm-1, corresponding to the aromatic ring vibration, also experienced a significant loss in intensity. This loss suggests that Fe also reacted with the aromatic rings, forming a metal complex during imidization at 300 °C. The 1360 cm-1 band from the C-N stretch remained unchanged, indicating that the chain structure of the PI was not yet destroyed at this curing temperature. However, increasing the curing temperature to 500 °C, all the bands were severely reduced. The PI structure was effectively destroyed by the
Lim et al. presence of a metallic film at this curing temperature. It is also noted that the single PI film still exhibited a significant intensity near the aromatic ring vibration band around 1500 cm-1 even at 600 °C in contrast to the PI/ Ni80Fe20/PI sample. The observation indicates that the Ni80Fe20 film enhanced the deterioration of the PI film. Similar observation was also made for the Cu/PI and Co/PI films.12 The FT-IR result showed that there was an extensive reaction between metal and polyamic acid when the acid was cured in the presence of the Ni80Fe20 film. The XPS data verified that this reaction was mainly confined to Fe so that Fe was preferentially consumed from the Ni80Fe20 film. In Figure 5a are the magnetic hysteresis loops obtained from the PI/Ni80Fe20(3.5 nm)/PI samples cured at different temperatures as well as from the PI/Fe(3.5 nm)/PI sample for comparison. The loops were measured using superconducting quantum interference device (SQUID) at room temperature. The PI/Ni80Fe20(3.5 nm)/PI samples cured at 300-500 °C were ferromagnetic, and the magnetization became saturated at ∼2000 Oe. The saturation magnetization remained unaltered as the curing temperature was increased to 500 °C. The hysteresis loop obtained from the sample cured at 600 °C did not, however, saturate at the maximum applied field, suggesting presence of paramagnetic phase in the sample. The paramagnetic phase may have originated from the decomposition products of the PI matrix itself or metal complexes. Comparing the saturation magnetization of the Ni80Fe20 sample with that of the pure Fe film, the saturation magnetization from the Ni80Fe20 film was approximately four times higher than that of the pure Fe sample.
Figure 6. TEM images of the PI/Co80Fe20(9 nm)/PI sample cured as follows: (a) at 400 °C (The arrow indicates a particle with multiple grains.); (b) at 600 °C. Indexed electron diffraction patterns of the same sample cured as follows: (c) at 400 °C; (d) at 600 °C, showing both CoO and hexagonal Co phases in the samples [1, Co(100); 2, Co(002); 3, Co(101); 4, Co(102); 5, Co(110); 6, Co(103); 7, Co(200); a, CoO(111); b, CoO(220); c, CoO(311)].
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Figure 8. Comparison of the FT-IR spectra from the PI/Ni80Fe20(3.5 nm)/PI and PI/Co80Fe20(9 nm)/PI samples cured at 400 °C.
Figure 7. XPS Co 2p spectra for the PI/ Co80Fe20 (9 nm)/PI sample cured at 300 and 600 °C with the peak-separated curves showing the contributions from CoO and Co. (SAT indicates the satellite peaks.)
It was previously shown that the pure Fe film oxidized to form ferrimagnetic γ-Fe2O3 particles.5 Since Ni has a much higher bulk magnetic moment than those of γ-Fe2O3 , or Fe3O4, Figure 5 once again proves that the particles should be metallic Ni. 3.2. Co80Fe20 Thin Films. Parts a and b of Figure 6 show the PI/Co80Fe20(9 nm)/PI sample cured at 400 and 600 °C, respectively. The particles at 400 °C were rather irregularly shaped, and the particle size ranged from 4 to 12 nm. Some of the particles consisted of several grains. At 600 °C, the particle size distribution became even wider with particles as large as 15 nm interspersed among much finer particles. Respective electron diffraction patterns were indexed in Figure 6, parts c and d. The diffraction patterns indicate coexistence of crystalline CoO and hexagonal Co in both samples. Unlike the Ni80Fe20 samples, Co in these samples was oxidized together with Fe, which explains the irregular shapes of the particles. Instead of the selective oxidation observed in the Ni80Fe20 film, it seems that the entire surface of the Co80Fe20 film partly reacted with the polyamic acid during oxidation. In fact, in the sample with 3.5 nm-thick Co80Fe20 film, the TEM image appeared as if the entire metal film was dissolved during imidization with no sharp contrasts as seen in Figure 1a. The XPS spectra in Figure 7 further confirms the oxidation of Co in the Co80Fe20 film. Although both spectra had their 2p3/2 peaks at 778.3 eV,10 exactly matching the binding energy of metallic Co, the spectra contained a shoulder near 780 eV, corresponding to the CoO binding energy.14-15 The peak separation clearly shows that Co in the Co80Fe20 film was partially oxidized. The Gibbs free energy of formation at 300 K for CoO (-216.5 kJ/mol) and NiO (-211.9 kJ/mol)6 are nearly the same so that the tendency for oxidation for Ni and Co should not be
too different. It is not clear why Co in Co80Fe20 oxidized during imidization whereas Ni in Ni80Fe20 did not. We have also prepared samples with pure Co and Ni thin films. In those samples, reaction of the polyamic acid with Co was much more extensive than with Ni. It was also shown that thermal degradation of PI in air was catalyzed by a thin film of Co whereas the catalytic effect was not observed with Ni.12 It appears that there is a complex chemistry involving the polyamic acid and alloy films which made Ni especially inert. As can be seen from the comparison of the IR spectra from the Co80Fe20 and Ni80Fe20 samples cured at 400 °C in Figure 8, intensities of the bands at 1712 and 1361 cm-1 for the Co80Fe20 sample were substantially reduced compared to the Ni80Fe20 sample. The reduced intensity at 1712 cm-1 indicates stronger reaction of the carboxylic group with Co80Fe20; however, the reduced intensity of the 1361 cm-1 band arising from the cyclic imide C-N stretch12 points out that a fraction of the polymer chains were severed in the presence of the Co80Fe20 film. From the IR spectra in Figure 4, the intensity of the 1361 cm-1 band in the Ni80Fe20 sample was comparable to that for the pure PI cured at the same temperatures. This result suggests that in the Co80Fe20 sample the structure of PI was damaged, opening up imide bonds which provided sites for oxidation of Co. 4. Summary We have demonstrated that metallic nanoparticles embedded in a polymer matrix can be produced by utilizing the relative difference in chemical reactivity of metal elements in an alloy film. In Ni80Fe20 film, Fe was preferentially attacked by the PI precursor, generating Ni nanoparticles. However, the Co80Fe20 film failed to produce uniform-sized metal particles due to the deterioration of the polymer matrix catalyzed by Co. At this point, it is not clear why Co was more effective in decomposing the polymer compared to Ni. Investigation is underway to test different metallic elements and their alloys as well as different types of polyimide. Acknowledgment. This research was supported by a grant (Code No. 04K1501-01210) from “Center for Nanostructured Materials Technology” under “21st Century Frontier R&D Programs” of the Ministry of Science and Technology, Korea. References and Notes (1) Moser, A.; Weller, D. The Physics of Ultra-High-Density Magnetic Recording; Springer-Verlag: Berlin, 2001. (2) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805.
18184 J. Phys. Chem. B, Vol. 108, No. 47, 2004 (3) Chushkin, Y.; Ulmeanu, M.; Luby, S.; Majkova, E.; Klang, I. P.; Holy, V.; Bochnicek, Z.; Giersig, M.; Hilgendorff, M.; T. H. Metzger J. Appl. Phys. 2003, 94, 7743. (4) Harrel, J. W.; Wang, S.; Nikles, D. E.; Chen, M. Appl. Phys. Lett., 2001, 79, 4393. (5) Lim, Sung, K.; Chung, Keum Jee; Kim, Young-Ho; Kim, C. K.; Yoon, C. S. J. Colloid Interface Sci., 2004, 273, 517. (6) Samsonov, G. V. The Oxide Handbook, 2nd ed.; IFI/Plenum: New York, 1982. (7) Chung, Y.; Park, H. P.; Jeon, H. J.; Yoon, C. S.; Lim, Sung, K.; Kim, Y. H. J. Vac. Sci. Technol. B. 2003, 13, L9. (8) Kowalczyk, S. P.; Kim, Y.-H.; Walker, G. F.; Kim, J. Appl. Phys. Lett., 1988, 52, 375.
Lim et al. (9) Meyer, Dirk C.; Paufler, P. J. Alloys Compd. 2000, 298, 42. (10) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Chanhassen, MN, 1995. (11) Pireaux, J. J.; Vermeersch, M.; Gre´goire, C.; Thiry, P. A.; Caudano, R. J. Chem. Phys., 1988, 88, 3353. (12) Shih, Da-Yuan; Klymko, N.; Flitsch, R.; Paraszczak, J.; Nunes, S. J. Vac. Sci. Technol. A. 1991, 9, 2963. (13) Dunn, D. S.; Grant, J. L. J. Vac. Sci. Technol. A, 1989, 7, 253. (14) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley & Sons: New York, 1990; Vol. 1. (15) Galtayries, A.; Grimblo, J. J. Electron Spectrosc. Relat. Phenom. 1999, 98-99, 267.
