phys. stat. sol. (a), 202, No. 8, 1698 – 1702 (2005) / DOI 10.1002/pssa.200461230
Magnetic nanoparticles – porous silicon composite material S. Balakrishnan1, Yurii K. Gun’ko*, 1, T. S. Perova2, M. Venkatesan3, E. V. Astrova4, and R. A. Moore2 1 2
3 4
Department of Chemistry, Trinity College, University of Dublin, Dublin 2, Ireland Department of Electronic & Electrical Engineering, Trinity College, University of Dublin, Dublin 2, Ireland Department of Physics, Trinity College, University of Dublin, Dublin 2, Ireland Ioffe Physico-Technical Institute, St. Petersburg, Russia
Received 25 July 2004, revised 4 October 2004, accepted 27 January 2005 Published online 8 June 2005 PACS 61.46.+w, 68.37.Hk, 75.75.+a, 78.30.Ly New composite magnetite – porous silicon materials have been prepared. The materials have been studied using various characterisation techniques such as Raman, XRD, magnetic measurements and SEM. Selforganisation of magnetic particles into linear micro-assemblies on the porous silicon surface has been observed. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Surface modification of silicon has been a subject of interest from the last two decades. Much attention has been given to porous silicon after the room temperature photoluminescence was discovered in porous silicon [1]. There are various reports on the modification of silicon, both flat and porous using different chemical precursors. In one such attempt, attention was focused on nanoparticles-silicon composite materials. There are a few reports on Fe2O3 nanoparticles incorporated onto flat silicon wafers leading to multiple light emission and multiple functionality [2], in which the authors were describing about a “plug and play” approach where externally synthesised nanoparticles of desired functions and size are incorporated into the semiconductor, followed by manipulation of surface chemical bonds. Magnetic nanoparticles are widely studied for their application in various fields such as information storage [3], colour imaging [4], bioprocessing [5] and in controlled drug delivery [6]. Magnetic particles when reduced to their nanometers size exhibit a number of physical properties such as giant magnetoresistance, large coercivities, super-paramagnetism and high Curie temperature [7]. Magnetite (Fe3O4) has been studied widely, a large number of publications can be seen in this area of research. In the present work we report on the composite material of porous silicon (PS) and magnetic nanoparticles by introducing magnetite nanoparticles to the porous silicon substrate. The properties of the as such prepared composite material were studied by various characterisation methods such as Raman spectroscopy, SEM, XRD and magnetic measurements.
2
Experiment
The macroporous silicon used in this study has a system of regular cylindrical pores of micrometer diameter and high aspect ratio [8]. The starting material was single-crystal (100)-oriented Czochralski*
Corresponding author: e-mail:
[email protected], Phone: +353 1 6083543, Fax: +353 1 6712826 © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Original Paper
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grown n-type silicon with resistivity ρ = 15 Ω cm. A standard photolithographic process was employed to form pits spaced 12 µm apart on the polished surface of the silicon wafer. Deep pores were etched electrochemically in a 2.5% aqueous-ethanol solution of HF for 300 to 450 min under backside illumination [8] at a voltage of 5 V and a constant current density of j = 3 mA/cm2. The pore depth was 200 – 250 µm and pore diameter d = 3 – 4.5 µm, which corresponds to the porosity of our triangular lattice 5.7 – 12.8%. The porous silicon sample was first etched with HF (2 wt%) and hydroxyl functionalised as reported earlier [9]. The magnetite nanoparticles were prepared as reported earlier [10] with slight modifications. The magnetic nanoparticles were prepared under argon atmosphere. In brief, FeCl2 and FeCl3 were weighed by maintaining the stoichiometry of Fe3+/Fe2+ = 2. Both were dissolved in 100 ml of 1.0 M NaCl aqueous solution (deoxygenated water). Sodium chloride (NaCl) was used to adjust the ionic strength of the iron solutions. The above solution were placed in a water-bath preheated to 25 °C with sonicating. Concentrated ammonia solution was added drop-wise until the pH reached 9. Stirred for two days. The hydroxyl functionalised silicon samples were introduced to this magnetite solution. The dark precipitate together with the silicon samples were washed with deoxygenated water for three times followed by diethyl ether and ethanol. Dried under vacuum for 9–10 hours. The PS-magnetic composites were further annealed under high vacuum (10–5 Torr) at a temperature of 500 °C for three hours. 2.1 Raman spectroscopy Room temperature raman spectra were measured with a Renishaw 1000 micro-Raman system. The excitation wavelength was 514.5 nm from an Ar+ ion laser (Laser Physics Reliant 150 Select Multi-Line) with a typical laser power of ~10 mW in order to avoid excessive heating. The 50×-magnifying objective of the Leica microscope focused the beam into a spot of about 1 µm in diameter. 2.2
Scanning Electron Microscopy (SEM)
Annealed samples were fractured for observation of the microstructure at the interface. The SEM equipment used was an S-3500N variable pressure scanning electron microscope (Hitachi, Japan) which was operated at 20.0 kV. 2.3 Magnetic measurements Magnetic measurements were carried out both at 5 K and 300 K using a MPMS superconducting quantum interference device (SQUID) magnetometer. Magnetoresistance (MR) measurements were performed by the linear four probe method at room temperature using a MULTIMAG variable flux source in a magnetic field of up to 2 T. 2.4 XRD measurements The XRD in this work was carried out using a Siemens D-500 X-ray diffractometer. X-ray patterns from powder samples were taken in reflection mode.
3
Results and discussion
The hydroxylated porous silicon samples were treated with magnetic nanoparticles prepared from iron (Fe3+/Fe2+) salts taken in the stoichiometric ratio (Fe3+/Fe2+ = 2) and co-precipitated with ammonia solution. The black precipitate together with silicon samples were washed with organic solvents and dried under vacuum. A schematic representation of the process is given in Scheme 1. The as-prepared samples were sintered under high vacuum (10–5 Torr) at a temperature of 500 °C for 3 hours. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Balakrishnan et al.: Magnetic nanoparticles – porous silicon composite material
Fe 3 O4 Fe 2+
porous Si
/ Fe 3 +
porous Si
NH4 OH Scheme 1 Schematic representation of the preparation of PS – magnetite composite.
(a) commercial
Magnetite
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10
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2 theta (deg.)
Fig. 1 (online colour at: www.pss-a.com) XRD patterns of the magnetite samples, (a) commercial, (b) as-prepared.
5K 300 K
60
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-1
Magnetization (Am kg )
40 20 0 -20 -40
Fig. 2 (online colour at: www.pss-a.com) Magnetization curves of magnetite nanoparticles measured at 300 K and 5 K.
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-4
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0 µ0H (T)
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Raman Intensity (A.U)
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-1
Raman shift (cm )
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Fig. 3 (a) Raman spectrum of PS-magnetite composite, (b) Raman spectrum of magnetite nanoparticles.
Figure 1 shows the XRD patterns of the magnetite samples which were as-prepared sample compared with that of the commercial magnetite sample. The XRD pattern shows that the peak positions of the asprepared samples were in good agreement with the commercial magnetite samples. The magnetization versus field (M– H) curves measured at 300 K and 5 K are compared in Fig. 2. The magnetization is unsaturated even at 5 T. The saturation magnetization at 5 K is 2.4µB/f.u., much less than the value of 4.0µB/f.u. anticipated for a half-metallic ferrimagnetic configuration of Fe3+ and Fe2+ ions. The reduction and lack of saturation could be attributed to the particle size effect.
Fig. 4 SEM images of PS-magnetite annealed at 500 °C for 3 hrs. © 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Balakrishnan et al.: Magnetic nanoparticles – porous silicon composite material 0.0
Fig. 5 (online colour at: www.pss-a.com) Magnetoresistance of the annealed PS-magnetite sample at room temperature.
∆ρ /ρ(0) (%)
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-0.2
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The PS-magnetite samples were characterised using Raman spectra, which is a good tool for characterisation of magnetite nanoparticles. Raman spectrum of the PS-magnetite sample (Fig. 3(a)) shows a peak positions at 520 cm–1 which is characteristic of silicon [11] and a peak at 667 cm–1, a characteristic peak for magnetite [12]. The Raman spectrum of the magnetite sample alone shows a peak at 667 cm–1 in Fig. 3(b). SEM images of the PS-magnetite annealed at 500 °C for 3 hours under high vacuum are presented in Fig. 4. The SEM images showed an orderly arrangement of nanoparticles after the annealing procedure. Initially-prepared PS-magnetite samples were insulating and showed no magnetoresistance. After heating the sample in vacuum at 500 °C for 3 hours, the sample became conducting at room temperature. The magnetoresistance in annealed samples was of 0.35% MR ratio at room temperature in 2 T field compared to the value of 1– 2% normally observed in epitaxial magnetite thin films [13].
4
Conclusion
Thus in the present work PS-magnetic composite material have been prepared and their magnetic properties have been studied. A self-assembly of magnetite particles on porous silicon substrates was observed after the annealing of the samples. Annealed magnetite-porous silicon composites demonstrated magnetoresistance of 0.35% MR ratio at room temperature. Acknowledgements We gratefully acknowledge Enterprise Ireland (Basic Research Grant Scheme, Grant SC/2001/209) and HEA PRTLI programme for financial support and staff members of the Electron Microscopy Unit of TCD for SEM images. We also thank Prof. J. M. D. Coey for providing an access to his lab facilities.
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