Volume 5 Issue 1 - June 27, 2008
Development and investigation of magnetic nanowire arrays in mesoporous matrices
Chuan-Pu Liu1,*, Shu-Fang Chen1, Yuri. Tretyakov2

1DMSE, National Cheng-Kung University
2DMS, Moscow state University

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1.Introduction and objective

One of the most important challenges in physics and materials science today is the preparation of ordered nanostructure arrays with the controlled properties and dimensions. The most challenging nanosystems are nanowires owing to highest anisotropy parameters in them, which could certainly increase functional properties of nanomaterials. The way for preparation of nanoparticle arrays usually involves synthesis in “nanoreactors” formed by colloidal species such as reversed micelles or liquid crystals. Variation of conditions of the synthesis allows regulate the size and morphology of micellar systems and, consequently, the size and morphology of nanostructures formed. The most promising systems possessing ordered one-dimensional porous structure include MFI- and LTL-type zeolites, mesoporous silica, mesoporous aluminosilicates and anodic alumina. In the frames of the present project we intend to use these matrices as nanoreactors for formation of ordered magnetic nanowire arrays in a wide range of length/diameter ratios (the diameters will be varied from 0.5 to 100 nm).

2. Subjects

The project will address following scientific issues:
(1) Fabrication of porous templates with different pore diameters
(2) Synthesis and investigation of magnetic nanowires in porous alumina membranes
(3) Characterization of magnetic structures in (2)~(4) by TEM techniques including HRTEM, EELS, and holography.

At Moscow State University, the research will be performed by the group of Prof. Yu.D. Tretyakov, the Full Member of Russian Academy of Sciences. The team is well experienced in preparation of nanopowders and nanocomposites, synthesis of mesoporous phases; experimental characterization of nanomaterials.

At National Cheng Kung University, Department of Materials Science and Engineering (DMSE NCKU team) of Tainan, the research will be performed under the supervision of Prof. Chuan-Pu Liu. The team has a deep knowledge and experience in the fields of characterization and synthesis of nanomaterials, including quantum dots and nanowires. DMSE NCKU team would be focused on morphological characterization of magnetic nanophase particles, which is one of the major points for realization of the project and developing magnetization reversal model in one-dimensional nanostructure arrays. The DMS MSU team will focus on experimental characterization and theoretical modelling of the magnetic properties of nanocomposites. For this purpose, computer simulations will be carried out and compared with measurements of the magnetization curve, and magnetic susceptibility. DMSE NCKU team would be focused on morphological characterization of magnetic particles by HRTEM.

Fig. 1 TEM image of Ni nanowire. the voltage is 0.8V, and the growth time is 2.5hrs.
3. Results

(1) Ni nanowires

Random arrays of ferromagnetic nanowires were synthesized by electrodeposition of Ni into the pores of homemade AAO templates. For the nanowire growth, a thin noble metal film was deposited by thermal evaporation on one side of the porous membranes to serve as a cathode. The filling of the pores was then performed by electrodeposition of Ni under potentiostatic control. A structural characterization of the Ni nanowire arrays was performed using transmission electron microscopy (TEM). In this work, we use AAO templates with 4 different parameters to fabricate Ni nanowires, which are 0.8V (potemtial)、2hrs (growth time), 0.8V、2.5hrs, 0.8V、6hrs and 1V、1hr, respectively. Figure 1 shows the typical TEM image of Ni nanowires liberated from anodic films, revealing that the nanowires are cylindrical with diameter around 50nm. Table 1 shows size distribution of the nanowires. The pores of the anodic alumina were widened by immersing the as-made template in phosphoric acid for different lengths of time, and cause the fluctuation of the size distribution. In addition, there is a thin layer covered on the surface of Ni nanowire as shown in Fig.1, and the width of this layer is different from the wall width of AAO. So that it is not from the residual AAO. Fig. 2 shows the HRTEM image and the corresponding diffraction pattern of sample 2. Ni nanowire is single crystalline as revealed by high-resolution imaging and electron diffraction. The lattice constant of nanowire is estimated from HRTEM image, which is 0.356nm. the lattice mistmatch between bulk and nanowire is about 1.13%. From diffraction pattern, the nanowire structure is FCC, and the growth direction is along (110). Moreover, from the FFT results, the thin layer mentioned previously is NiO. We use EELS technique for composition analysis. Fig. 3 shows the EELS spectra for all 4 samples. The Ni L edge locates at 854 eV, and O K edge locates at 532 eV. Both Ni and O signal are detected, thus we confirm that the nanowire is composed of Ni and is oxidized.
Fig. 3 EELS spectra of Ni nanowires.
Fig. 2 HRTEM image and corresponding diffraction pattern of Ni nanowire. the voltage is 0.8V, and the growth time is 2.5hrs. The insets are the FFT images.

Table 1 Size distribution for Ni nanowires and the wall width of AAO template

For the electrical characterization, we have designed a devise to measure the resistance of a single nanowire. The schematic diagram for the device is shown in fig. 4. The lead is made by FIB. The R-T curve is obtained by PPMS. Fig. 5 shows both the SEM image of the device and the R-T curve of sample 2. After curve fitting, we calculate the activation energy of the single nanowire is 0.175meV.   
Fig. 5 SEM image of the device for electrical measurement and the R-T curve of Ni nanowire.
Fig. 4 Schematic diagram for electrical measurement on single Ni nanowire.

(2) Ni/Cu multilayered nanowires

The growth method for Ni/Cu multilayered nanowires is the same as that for Ni nanowires. Fig. 6 shows TEM images of Ni/Cu multilayered nanowiwire. TEM figures clearly show a layered structure was fabricated. It is interesting to see that there are two kinds of interfaces. One is straight (shown in fig. 6(a)), and the other is oblique (shown in Fig. 6(b)). Fig. 7 shows the bright-field TEM image and corresponding diffraction pattern for multilayered nanowire. The structure is FCC. We use STEM-EELS to confirm the composition of the light band and dark band in the multilayered structure. Fig. 8 shows the HAADF images, the spectrum imaging, and the corresponding STEM-EELS spectra for the Ni/Cu nanowire. From the EELS spectra, the intensity of Ni signal is decreasing from point 1 to point 3, which indicated that the light layers (~10 nm) are represented by Cu, and the dark layers (~40 nm) are represented by the Ni layer. This particular wire diameter is about 100 nm.
Fig. 7 TEM image and corresponding diffraction pattern of Ni/Cu nanowire.
Fig. 6 TEM image for Ni/Cu nanowire of different interface. (a)straight (b)oblique

Fig. 8 HAADF images and spectrum imaging of Ni/Cu nanowire.

(3) CdSe nanoparticles

CdSe nanoparticles are grown in mesoporous silica. Fig. 8 shows the HRTEM image and diffraction pattern of CdSe nanoparticles. As shown in fig. 8, the CdSe nanoprticles (marked with red circles) are embed in the matrix. The nanoparticle size is about 2~3nm. From the diffraction pattern, the structure is FCC. Moreover, the lattice mismatch between bulk and the nanoparticle is about 1.26%.
Fig. 9 HRTEM image and corresponding diffraction pattern of CdSe nanoparticles.
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