NANO LETTERS
Controlled Generation of Ni Nanoparticles in the Capping Layers of Teflon AF by Vapor-Phase Tandem Evaporation
2003 Vol. 3, No. 1 69-73
A. Biswas, Z. Marton, J. Kanzow, J. Kruse, V. Zaporojtchenko, and F. Faupel* Lehrstuhl fu¨ r MaterialVerbunde, Technische Faku¨ lta¨ t der CAU, Kaiserstrasse 2, D-24143 Kiel, Germany
T. Strunskus Lehrstuhl fu¨ r Physikalische Chemie I, Ruhr-UniVersita¨ t Bochum, UniVersita¨ tsstrasse 150, NC 5/28, D-44780 Bochum, Germany Received September 24, 2002; Revised Manuscript Received October 24, 2002
ABSTRACT A promising solvent-free route to vapor-phase tandem evaporation is employed to produce nanoclusters of reactive Ni protected between the capping layers of Teflon AF. Independent control over the homogeneously distributed particle size (∼3−10 nm) is demonstrated while increasing the cluster-volume filling from ∼1 to 20% by following codeposition to many-sequential thermal evaporation of the two components. TEM investigations have been applied to characterize the nanocomposites.
Nanocomposite two-component materials consisting of finely dispersed mixtures of 3d transition-metal clusters and an insulating polymer with a tailored single-domain1 cluster size, on the nanometer scale, are of considerable interest because of a large variety of technological applications. These composites with appropriate magnetic volume are promising candidates for magnetic high-frequency or data storage applications2 such as low-loss tunable microwave devices3 or microinductors.4 The problems in the production of nanoscale particles arise from the facility to aggregrate and the difficulties in controlling their size in different matrices.5 However, an effective way to control the size and size distribution of growing particles is to make use of organic polymers.6 In addition, proper carbon encapsulation in the form of an airtight cage can completely prevent the oxidation of 3d clusters,7 as required to minimize ferromagneticantiferromagnetic exchange coupling in order to preserve the magnetic properties of the composites. Accordingly, the selection of typical polymers as hosts (e.g., Teflon AF offering a high degree of resistance to chemical attack is desired in order to protect the metallic nanoclusters). Besides, the insulating polymer matrix can be extremely beneficial in minimizing eddy current losses8 in the composite, one of the key requirement,2 particularly for the above applications. * Corresponding author. E-mail:
[email protected]. Phone: (+49)431880-6225. Fax: (+49)431-880-6229. 10.1021/nl020228f CCC: $25.00 Published on Web 11/22/2002
© 2003 American Chemical Society
Concerning the preparation of such composites, however, there is no existing straightforward or easy synthesis route in general to obtain nanoclusters of an appropriate narrow size distribution with a high concentration as well as a reactive 3d metal protected in an insulating matrix. Normally, in such cases, a suitable reducing agent9 is needed to avoid metal oxide formation. Several approaches usually consisting of solvent-based multiple complex preparation steps have been used to produce nanocomposite materials containing magnetic nanoclusters with different size distributions embedded in several matrices such as sol-gel,10,11 colloidal synthesis,9 or sputtering,12 et cetera. It has also been observed13-15 that the average size distribution depends on the metal volume fraction, which does not facilitate the control of the nanoparticle size independently of the volume fraction. However, an uncomplicated and completely solventfree technique of vapor-phase deposition of metal and polymer16,17 is advantageous and provides easy control over nanocluster growth. However, as one prime limitation of this technique, because of normally encountered low condensation coefficients of metals, particularly on low surface-energy polymers such as Teflon AF,18 it generates low metal cluster volume filling in the polymer. So engineering the air-stable desired nanocluster size along with controlling the size distribution independently while generating high 3d-metal cluster-volume filling in the
Figure 1. Schematic representations of the nanoengineering: (a) codeposited metal nanoparticles and polymer components; (b) many-sequential evaporation of the two components resulting in the production of metallic clusters capped by various layers of polymers, with the light background shown as the polymer matrix; and (c) chemical structure of the employed Teflon AF in the form of the combination of two monomers.
polymer is a conflicting process and a matter of interest both from the fundamental and technological aspects in order to explore several new nanomaterial properties, as mentioned in the begining. In this preliminary communication, we have employed coevaporation with tandem deposition of a reactive 3d metal (Ni) and polymer (Teflon AF). It is demonstrated that the average metal nanocluster size can be controlled independently while generating low to relatively high cluster-volume filling in the polymer. The so-produced Ni clusters are also shown to be uniform in size in general and are protected and distributed homogeneously in the capping polymer layers. The production of the nanocomposite films was carried out in vacuum (∼10-6 to 10-7 mbar) by an ingeniously developed evaporation chamber16 consisting of two separate evaporation sources for metal and polymer. Nanocomposite films (∼100-140 nm) were prepared by coevaporation and many-sequential thermal evaporation of Teflon AF 2400 (procured from DuPont and used in the granulate form as received) and Ni (99.99%, metallic wire procured from Good Fellow Industries, U.K.) on a carbon-covered Cu TEM grid. Figure 1a and b shows schematically the different methods of nanoengineering and generating Ni clusters dispersed in a Teflon AF matrix on the basis of simultaneous evaporation and tandem deposition of the two components. Upon pyrolysis of Teflon AF granulates containing monomers of 2,2-bitrifluormethyl-4,5-difluor-1,3-dioxol (PDD) (66 mol %) and tetrafluoroethylene (TFE) (34 mol %), vapors of selfstabilized monomers finally resulted in repolymerization on the substrate in the form of highly amorphous material. The chemical structure of the employed Teflon AF is shown in Figure 1 c. The various properties of the vapor-deposited Teflon AF do not vary, in general, with those of the original material.19 Moreover, XPS results on the vapor-phasedeposited and spin-cast Teflon AF did not indicate any difference in the fluorine content of the polymer, as such, prepared by separate methods. Deposition rates of metal and 70
polymer were power-controlled to maintain steady state rates of evaporation. The equivalent metal thickness from each deposition was kept in the distinctly low nanometer range (∼5 nm) to avoid layering the material so that metal formed clusters with sizes dependent on the postdeposition thermal treatment. A quartz crystal monitoring system installed in the evaporation chamber was used to measure the growing thickness of the evaporated components. The prepared nanocomposites were annealed in the same vacuum for a relatively prolonged duration (∼3-4 h, 220-230 °C) to induce mobility even to the deeply buried nickel atoms in the Teflon AF (Tg of evaporated film ∼200 °C, and hence the heating was above Tg) matrix so that the material would agglomerate into microscopically visible clusters. In the absence of any substrate cooling arrangement in the present case, a substrate temperature rise (∼70 °C) was frequently observed during deposition, which essentially witnessed the relatively hot landing of the nickel atoms. The prepared annealed composite films containing Teflon AF and nickel nanoclusters were exposed for several weeks at ambient conditions without the use of a desiccator before being examined for microstructure using a Philips CM 30 transmission electron microscope. The crystal structure of the soprepared nickel clusters was investigated by TEM diffraction patterns. Figures 2, 3, and 4a and b represent TEM bright-field images along with selected-area electron diffraction (SAED) and energy-dispersive X-ray analysis (EDAX) for the composites prepared by the two deposition methods and annealed at different temperatures. In these micrographs, the Ni clusters appear as dark objects whereas the Teflon AF matrix appears as a light background. Since the elastic scattering from the Ni clusters is strongly orientationdependent, the individual Ni clusters therefore appear with different intensities based on the random crystallographic orientation with respect to the sample surface. Codeposited nanocomposites (∼100 nm) annealed at 230 °C for ∼4 h produced clusters with a size distribution of ∼10-15 nm and with a cluster-volume filling of about 1% (Figure 2a. However, tandem-deposited samples (∼120 nm) upon annealing at the same temperature for ∼3 h generated clusters with a size distribution of ∼7-10 nm with a cluster-volume filling of ∼10 ( 5% (Figure 3a). Furthermore, a relatively lower annealing temperature (∼220 °C for ∼4 h) produced clusters with a size distribution of ∼3-11 nm having a cluster-volume filling of 20 ( 5% for a comparatively thicker film (∼140 nm) (Figure 4a). In view of possible overlapping images of Ni particles at different depths in the film, the estimation of the cluster-volume filling is approximated by taking into account the realistic number of overlapped layers. However, normal bright field TEM micrographs for such relatively thick films (∼120 and 140 nm) are adequate for determining the particle-size distribution and the concentration, in general. Establishing particle shape more precisely would require stereoscopic pairs of TEM micrographs at various angles of incidence, which is not the main task of the present work. The observed, significantly low Ni cluster-volume filling Nano Lett., Vol. 3, No. 1, 2003
Figure 2. (a) TEM micrographs of the codeposited nanocomposite (∼100 nm) with a cluster size distribution of ∼10-15 nm and a volume filling of about 1%. (b) EDAX spectra showing quite a low Ni/C ratio.
associated with the codeposition route may be attributed to the low rates of generation of the nucleation centers or energetically preferred cites offered by the growing Teflon AF matrix to the simultaneously arriving Ni atoms. Metal atoms diffusing on the polymer surface that do not become trapped at defect sites or at stable clusters will eventually desorb from the polymer surface.6 Hence, in the case of insignificant metal coverage, the Ni atoms that arrive on the polymer surface eventually produce low sticking due to very weak interactions between two dissimilar materials. In the case of the tandem deposition process, metal atoms impinge on metal surfaces that are already settled in the polymer layers, thereby increasing the Ni condensation coefficient (ratio of the number of adsorbed metal atoms to the total number of metal atoms arriving at the surface), which further increases with relatively higher metal coverage. Clusters already formed in this case act as trapping centers for the arriving metal atoms. Therefore, the problem of normally encountered low metal condensation coefficients on polymers associated with insignificant cluster-volume filling could be overcome by employing many-sequential evaporation of the two components. TEM SAED patterns with designated rings suggest the crystal structure of the grown Ni clusters to be fcc (Figure Nano Lett., Vol. 3, No. 1, 2003
Figure 3. (a) TEM micrographs of the tandem-deposited nanocomposite (∼120 nm) with a cluster size distribution of ∼7-10 nm and a volume filling of about 10%. The inset shows a SAED pattern of several designated fcc rings of an assembly of such clusters. (b) EDAX spectra suggest high a Ni/C ratio. The fluorine and oxygen peaks are due to the polymer (PDD + PTFE) matrix, with the Cu line originating from the TEM grid.
3a). SAED measurements were carried out on the composite films that were exposed for a couple of weeks. The estimated crystal constant (3.501 ( 0.001 Å) for the (111) ring matches the bulk Ni crystal parameter (3.524 Å) closely and differes appreciably from the standard crystal constant of NiO (4.177 Å). The reason for the apparent oxidation stability of the produced clusters, which could be due to the carbon encapsulation in the form of airtight cage7 protecting the clusters in the capping layers of Teflon AF, is not yet clear. However, Teflon AF has a relatively high oxygen permeability too, in spite of its well-known resistance to chemical attack. The Ni clusters are probably protected by a thin passivating oxygen layer, which is too thin to be seen in the SAED patterns. EDAX spectra show the elemental analysis of the relative Ni/C ratio from low to high corresponding to the different cluster-volume filling, where fluorine and oxygen peaks, which are due to the Teflon AF polymer (PDD + TFE) matrix, can also be seen (Figures 2, 3, and 4b). A size distribution curve of volume fraction (%) as a function of size for generated Ni nanoparticles by the tandem 71
Figure 5. Size distribution of generated Ni nanoparticles in Teflon AF by the tandem evaporation route.
sequential depositions of the two components. Furthermore, the studied crystal structure based on TEM diffraction patterns suggests the so-produced Ni clusters embedded in Teflon AF layers to be fcc. The applied method opens up the possibility of controlling and producing 3d metal nanoclusters embedded in polymers with the desired clustervolume filling corresponding to the single-domain and superparamagnetic regimes separately for different applications.
Figure 4. (a) TEM micrographs of the tandem-deposited nanocomposite (∼140 nm) with a cluster size distribution of ∼3-11 nm and a volume filling of about 20%. (b) EDAX spectra showing a significantly higher Ni/C ratio.
evaporation route shows a narrow particle-size distribution, which is desirable for many technological applications (Figure 5). Apparently, the employed tandem deposition method could also be worked out for more than a two-component system, with the possibility of preparing large-area metallized polymers consisting of uniform composition and thickness for different applications. The results also point out the possibility of generating 3d clusters and alloys embedded in polymers with independent variable cluster-volume filling corresponding to the single domain and much smaller superparamagnetic1 regimes. Future work is directed at investigating the degree of resistivity and magnetic properties of such composites. In summary, the employed tandem vapor-phase thermal evaporation method in the form of many sequential depositions of Ni and Teflon AF provides a good possibility for control over nanocluster size independently of varying cluster-volume filling from low to relatively high. In addition, the Ni clusters that are produced are uniform in size and distributed homogeneously, in general, within the Teflon AF matrix. The problem of low metal condensation coefficients on the polymer could also be overcome by employing many 72
Acknowledgment. Authors are thankful to Stefan Rehders for his technical assistance in developing the evaporation chamber. One of us (A. Biswas) would like to gratefully acknowledge the financial support for working as a postdoctoral fellow at the Lehrstuhl fu¨r Materialverbunde, University of Kiel. The help provided by Dr. B. S. S. Daniel on size distribution calculations is acknowledged.We gratefully acknowledge financial support from TechnologieStiftung, Schleswig-Holstein. References (1) Handley, O.; Robert, C. Modern Magnetic Materials: Principles and Applications; Wiley & Sons: New York, 2000; p 432. (2) Ohnuma, S.; Fujimori, H.; Mitani, S.; Masumoto, T. J. Appl. Phys. 1996, 79, 5130. (3) Mingzhong, W.; Zhang, D. Y.; Hui, S.; Xiao, T. D.; Ge, S.; Hines, W. A.; Budnick, J. I.; Tayler, G. W. Appl. Phys. Lett. 2002, 80, 4404. (4) Korenivski, V.; van Dover, R. B. J. Appl. Phys. 1997, 82, 5347. (5) Castro, J.; Ramos, A.; Millan, J.; Gonzalez-Calbet, F. P. Chem. Mater. 2000, 12, 3681. (6) Zaporojtchenko, V.; Strunskus, T.; Behnke, K.; Von Bechtolsheim, C.; Keine, M.; Faupel, F. J. Adhes. Sci. Technol. 2000, 14, 467. (7) Flahaut, E.; Agnoli, F.; Sloan, J.; Connor, C. O.; Green, M. L. H. Chem. Mater. 2002, 14, 2553. (8) Bertotti, G.; Magni, A.; Mayergoyz, I. D.; Serpico, C. J. Appl. Phys. 2002, 91, 7559. (9) Murray, C. B.; Sun, S.; Doyle, H.; Betley, T. Mater. Res. Soc. Bull. 2001, 26, 985. (10) De, G.; Tapfer, L.; Catalano, M.; Battaglin, G.; Caccavale, F.; Gonella, F.; Mazzoldi, P.; Haglund, F. Appl. Phys. Lett. 1996, 68, 3820. (11) Hosoya, Y.; Suga, T.; Yanagawa, T.; Kurokawa, Y. J. Appl. Phys. 1997, 81, 1475. (12) Briggs, L. M.; McKenzie, D. R.; MaPhedran, R. C. Sol. Energy Mater. 1982, 6, 455. Nano Lett., Vol. 3, No. 1, 2003
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