Journal of Magnetism and Magnetic Materials 310 (2007) e335–e336
Fabrication and X-ray absorption spectroscopy in layered cobaltate
Na
x
CoO
2
thin films
W.J. Chang
a,, J.-Y. Lin
b, T.Y. Chung
a, J.M. Chen
c, C.-H. Hsu
a, S.Y. Hsu
c, T.M. Uen
a,
K.H. Wu
a, Y.S. Gou
a,d, J.Y. Juang
a,daDepartment of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan bInstitute of Physics, National Chiao Tung University, Hsinchu 30010, Taiwan cNational Synchrotron Radiation Research Center (NSRRC), Hsinchu 30076, Taiwan
dDepartment of Physics, National Taiwan Normal University, Taipei 11677, Taiwan
Available online 3 November 2006
Abstract
NaxCoO2(x0.68) thin films were fabricated on sapphire (0 0 0 1) substrates via the lateral diffusion of sodium into Co3O4(1 1 1) films,
which were grown by pulsed-laser deposition. From the results of X-ray diffraction and in-plane resistivity rab, the single phase and the
metallic behaviors of these NaxCoO2films can be identified. For the same sodium content x, rabis consistent with that of single crystals.
In addition, the O 1s X-ray absorption near edge spectra of thin films are measured and compared with those of single crystals. r2006 Elsevier B.V. All rights reserved.
PACS: 81.15.z; 71.27.+a; 78.70.Dm
Keywords: NaxCoO2thin film; Layered cobaltates; Lateral diffusion; X-ray diffraction; X-ray absorption spectroscopy
Recently, the layered cobaltate NaxCoO2has generated
much interest of researchers due to its superconductivity in hydrated compounds at x ¼ 0.35[1], a large thermoelectric power[2], a charge ordered insulator at x ¼ 0.5, and the antiferromagnetic metal at x ¼ 0.75 [3]. Many theoretical and experimental works are focused on the fantastic ground states of NaxCoO2 due to its two-dimensional
triangular lattice. Because of the high vapor pressure of Na in these materials, it is difficult to grow single phase and high-quality epitaxial NaxCoO2thin films by pulsed-laser
deposition (PLD). For this reason, reactive solid-phase growth via lateral diffusion of sodium was applied on the growth of the epitaxial NaxCoO2thin films[4].
First, Co3O4 (1 1 1) thin films were grown on Al2O3
(0 0 0 1) substrates by PLD, KrF excimer laser (248 nm), with Tsubstrate¼680 1C, Poxygen¼0.2 Torr, and d ¼ 125 nm. To
keep the surface clean in the process of thermal diffusion, Co3O4(1 1 1) thin films were capped by Al2O3 substrates
when placing specimens in a ceramic boat with Na2CO3
powder. They were then put on the ceramic boat into the quartz tube oven for thermal diffusion. The diffusion process was operated at 700 1C for 10 h in air with the heating (and cooling) rateo10 1C/h. After the lateral diffusion of sodium, Co3O4(1 1 1) thin films became NaxCoO2(0 0 1) epitaxial thin
films with the thickness 250 nm.
To check the crystal structure of Co3O4and NaxCoO2thin
films, we have performed the X-ray diffraction (XRD) measurements, as shown inFig. 1. From the results of XRD, Co3O4 (1 1 1) are epitaxial growth on sapphire (0 0 0 1)
substrates, which are similar to the results of Ohta et al. [4] and Venimadhav et al. [5]. The lateral-diffused NaxCoO2
thin films also show single phase and are epitaxial to the substrates with (0 0 1) orientation. Because of the affiliated mutuality between the crystal structures and Na content of NaxCoO2materials, we judge the Na content by comparing
the XRD results with the structure phase diagram[6]. From
the c-axis lattice constant of NaxCoO2 thin films,
10.9328(8) A˚, we infer x0.68 in our specimens.
As shown in the inset (a) of Fig. 2, the roughness of layered cobaltate thin films was measured by the atomic force microscopy (AFM) after thermal-diffusion process.
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Corresponding author. Tel.: +886 3 5731653; fax: +886 3 5720728. E-mail address:[email protected] (W.J. Chang).
The RMS roughness is about 1.67 nm. The rab–T
measurements have been done by PPMS of Quantum Designs. In Fig. 2 (b), the rab–T curve of thin films is
identical to that of single crystals[3]around x ¼ 0.68, and
with a much smaller residual resistivity, r(300 K)/
r(0.4 K) ¼ 145, indicating that there are very few impu-rities or defects in the thin films.
Beside the electric transport measurements, X-ray absorption near edge spectroscopy (XANES) has been a powerful tool to investigate the electronic state of these compounds, as in the case of perovskite manganites [7].
The main panel of Fig. 2 illustrates the O K-edge
fluorescent spectrum, which characterize the hybridization of Co 3d and O 2p of the NaxCoO2(0 0 1) thin film at room
temperature. At the pre-edge, the O K-edge XANES shows three absorption peaks of Co3+ and Co4+ 3d bands, in which the t2g orbitals split into a1gand eg0 orbitals under
trigonal crystal field [8,9]. All the three pre-edge peaks resembles those of the x ¼ 0.67 single crystal with respect to the peak energy and the relative intensity [8]. These results suggest that the NaxCoO2 (0 0 1) thin film has an
electronic structure similar to that of the single crystal with the same Na content. Currently, the anisotropy of NaxCoO2from the orbital symmetry of a1gand eg0orbitals
remains controversial. Further investigation of polarized-dependant XANES is certainly desirable.
In summary, we have successfully fabricated NaxCoO2
thin films with x ¼ 0.68 via the lateral diffusion of sodium. Judged from the results of XRD and rab–T curve, the thin
films quality is as decent as that of single crystals. XANES of NaxCoO2thin film shows an electronic structure similar
to that of the single crystal, too.
This work was supported by the National Science Council of Taiwan, under Grant Nos. 009-006, 009-007, and NSC-94-2112-M-009-008.
References
[1] K. Takada, H. Sakurai, E. Takayama-Muromachi, F. Izumi, R.A. Dilanian, T. Sasaki, Nature 422 (2003) 53.
[2] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685.
[3] M.L. Foo, Y. Wang, S. Watauchi, H.W. Zandbergen, T. He, R.J. Cava, N.P. Ong, Phys. Rev. Lett. 92 (2004) 247001.
[4] H. Ohta, S.-W. Kim, S. Ohta, K. Koumoto, M. Hirano, H. Hosono, Cryst. Growth Design 5 (2005) 25.
[5] A. Venimadhav, A. Soukiassian, D.A. Tenne, Q. Lia, X.X. Xi, D.G. Schlom, R. Arroyave, Z.K. Liu, H.P. Sun, X. Pan, M. Lee, N.P. Ong, Appl. Phys. Lett. 87 (2005) 172104.
[6] Q. Huang, M.L. Foo, R.A. Pascal Jr., J.W. Lynn, B.H. Toby, T. He, H.W. Zandbergen, R.J. Cava, Phys. Rev. B 70 (2004) 184110. [7] W.J. Chang, J.Y. Tsai, H.-T. Jeng, J.-Y. Lin, K.Y.-J. Zhang, H.L. Liu,
J.M. Lee, J.M. Chen, K.H. Wu, T.M. Uen, Y.S. Gou, J.Y. Juang, Phys. Rev. B 72 (2005) 132410.
[8] W.B. Wu, D.J. Huang, J. Okamoto, A. Tanaka, H.-J. Lin, F.C. Chou, A. Fujimori, C.T. Chen, Phys. Rev. Lett. 94 (2005) 146402.
[9] T. Mizokawa, L.H. Tjeng, H.-J. Lin, C.T. Chen, R. Kitawaki, I. Terasaki, S. Lambert, C. Michel, Phys. Rev. B 71 (2005) 193107.
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Fig. 1. X-ray diffraction of NaxCoO2(0 0 1) after sodium lateral diffusion
(upper panel) and Co3O4(1 1 1) thin films on sapphire (0 0 0 1) substrates
(lower panel). The results indicate that the both of thin films are in single phase.
a
b
Fig. 2. Main panel: Isotropic O K-edge X-ray absorption spectrum, (IE//c+IE?c)/2, of the NaxCoO2thin film at room temperature. Inset (a):
AFM image (5 mm 5 mm) of the NaxCoO2thin film shows the layered
structure of this cobaltate. Inset (b): The rab–T curve illustrates the high
quality of this thin film.
W.J. Chang et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e335–e336 e336
