ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e1285–e1287
Magnetic nanostructures obtained by colloidal crystallization onto patterned substrates O. Crisana,b, M. Angelakerisa,*, N. Vouroutzisa, A.D. Crisana,b, E. Pavlidoua, I. Kosticc, N. Sobald, M. Giersigd, N.K. Flevarisa a
Department of Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece b National Institute for Materials Physics, PO Box MG-7, Bucharest 76900, Romania c Institute of Informatics, SAS, Bratislava, Slovakia d Hahn Meitner Institute, Berlin, Germany
Abstract Colloidal solutions of magnetic nanoparticles are regularly dispersed onto patterned substrates in order to form novel magnetic nanostructures. The morphology of these nanostructures is investigated by atomic force microscopy (AFM) and scanning electron microscopy (SEM) and their structure is correlated with magnetic properties. It is shown that, depending on the nature of the substrate, different nanoparticle growth modes are identified during the colloidal crystallization. r 2003 Elsevier B.V. All rights reserved. PACS: 81.07. b; 81.16.Dn; 75.30. m Keywords: Colloidal crystallization; Magnetic nanoparticles; Nanostructured material; Patterned substrate; Atomic force microscopy
Recent development of lithographic techniques as well as improved chemical synthesis methods allows researchers to engineer novel nanostructured materials consisting of arrays of plots, self-organized nanocrystals and multilayers grown as patterns on different substrates. The advance of lithography has allowed in recent years fabrication of arrays of plots for nanoelectronics [1], nanocrystals for magnetic recording media [2] and patterning substrates in different geometries [3] required for various applications like magnetic heads, GMR sensors, etc. Besides the technological applications, patterning is also very useful for providing sizemodulated systems for fundamental studies of intrinsic effects such as electron transport or spin polarization in preferred orientation. The present paper proposes the combination of selfassembly techniques with the use of patterned substrates in an effort to manipulate the local arrangement of the *Corresponding author. Tel.: +30-2310998172; fax: +302310998095. E-mail address:
[email protected] (M. Angelakeris).
nanoparticles and produce well-defined large-scale arrays of magnetic nanoentities. The colloidal solutions were deposited on different substrates such as: Si, Pd thin film, multilayers and patterned substrates under different conditions while various imaging techniques have been employed to study the as-obtained nanostructures: optical microscopy, electron microscopy techniques (SEM, TEM, HRTEM) and atomic force microscopy (AFM). The AgCo nanoparticles have been obtained by colloidal chemistry technique [4] as a ferrofluid with good oxidation stability. The nanoparticles are coated with organic surfactant and dispersed in toluene. The chemical composition of nanoparticles obtained by energy-dispersion spectroscopy (EDS) is shown to be Ag30Co70, their structure has been proven to be multiphase, composed of FCC Ag and HCP Co and the single nanoparticle exhibits multiple twinning between crystalline entities. To organize the nanoparticles into 2D arrays or 3D packing structures and to prevent the aggregation during the self-assembly is rather challenging. The self-assembly arises from the balance between
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.184
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O. Crisan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1285–e1287
magnetic dipolar interparticle interaction, van der Waals attraction and the repulsive steric interaction due to the surfactant layer. Additionally the substrate is a key factor to this procedure. Fig. 1 shows the 2D AFM image of the morphology of four different substrates with one drop of colloidal solution of AgCo nanoparticles dispersed onto and allowed to dry freely in air. The substrates presented in this figure are (a) Si(1 0 0) wafer, (b) Co/Pt multilayer Pt37nm/(Co3nm/Pt0.2nm)40/Pt60nm deposited on Si, (c) Pd thin film of 80 nm deposited on Kapton foil and (d) prepatterned substrate consisting of 100 nm in height Pt stripes on a Si wafer. In top images 1(a) and 1(b) larger clusters are formed leading to an undesirable disordered state. It is worth mentioning that the colloidal crystallization in this case provides a more compact, bulk-like, growth mode. The AgCo nanoparticles are agglomerated with significantly bigger grain size and the film deposition is more homogeneous. It seems, though, that the bimodal size distribution and consequently the two-phase feature, is preserved. On the other hand just by using a Pd thin film of 80 nm as a substrate (image 1(c)) uniform dispersion seems to retain but smaller grains are formed. Additionally a patterned substrate (the darker area corresponds to a Pt stripe) shown in image 1(d) with physical boundaries in the "ım scale also seems to prevent
aggregation during self-assembly while the periodicity of the initial patterning of the substrate is still preserved. This is clearly shown in Fig. 2 where the 3D representation of a pre-patterned Pt7nm/PMMA200nm/Si substrate with one drop of AgCo nanoparticles is shown. By direct comparison of top and bottom (original substrate) insets one may see that after the colloidal crystallization of nanoparticles, the patterning is preserved and the film deposition is rather homogeneous over the whole substrate (stripes still well defined). Few clusters of nanoparticles are nevertheless formed. From a magnetic point of view, it is expected that such sample will exhibit two-phase behavior, with a ferromagnetic component, given by the big agglomerated nanoparticles and a superparamagnetic one, given by the very small and dispersed nanoparticles [5]. From AFM results it is inferred that even if the growth mode is strongly dependent of the nature of substrate, the colloidal crystallization of AgCo nanoparticles tends to provide size-selective arrangements, independently of the choice of substrate, that ultimately leads to at least two magnetically different phases in the sample. In conclusion, by using prepatterned substrates we may provide an extra manipulation factor that facilitates large areas’ ordering of self-assembled magnetic nanoentities.
Fig. 1. 2D AFM image of AgCo nanoparticles dispersed onto (a) Si(1 0 0) wafer, (b) Co/Pt multilayer deposited on Si, (c) Pd thin film of 80 nm deposited on Kapton foil and (d) prepatterned substrate consisting of 100 nm Pt on a Si wafer.
ARTICLE IN PRESS O. Crisan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e1285–e1287
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This work has been financially supported by the EU funded RTN program HPRN-CT-1999-00150. Partial support has also been provided by Romanian Ministry of Education and Research via CERES projects no. 48/ 13.11.2002 and 24/15.10.2001.
References
Fig. 2. 3D AFM image of AgCo nanoparticles dispersed onto 7 nm Pt/200 nm PMMA/Si substrate. Top inset: 15 mm 15 mm 2D detail view of the same image, bottom inset: 15 mm 15 mm SEM image of the substrate alone where the original rectangular patterns may be seen.
[1] C.B. Murray, C.R. Kagan, M.G. Bawendi, Science 270 (1995) 1335. [2] S. Sun, C.B. Murray, D. Weller, L. Folks, A. Moser, Science 287 (2000) 1989. [3] S.Y. Chou, P.R. Krauss, P.J. Renstrom, J. Vac. Sci. Tech. B 14 (1996) 4129. [4] N.S. Sobal, M. Hilgendorff, H. Mohwald, M. Giersig, M. Spasova, T. Radetic, M. Farle, Nano Lett. 2 (2002) 621. [5] O. Crisan, et al., Proceedings of ICM, Rome, 2003.
