Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

9A,12C/2004G085

XAFS Study on Co2+- Doped ZnO Nanocrystals
Masatsugu SHIMOJO1, Akihiro NOJIMA1, Jun MIYAWAKI1, Shriwas. Surendra. Ashtaputre2,Sulabha. Kulkarni2 and Toshiaki OHTA1,*
1
Department of Chemistry, School of Science, The University of Tokyo, Tokyo 113-0033, Japan2
Department of Physics, University of Pune, Pune 411007, India
Fourier transform intensities become very weak far from2+2+Co ions. This suggests that Co is mostly distributednear the surface of the nanocrystals. We cannot decide the2+local structure of Co ions only by EXAFS. However,ligand-field electronic absorption spectra(not shown)2+
indicate many of Co ions has tetrahedral coordination.
2+
Thus we can conclude that many Co ions are2+
substitutionally doped at the Zn sites of ZnO anddistributed near the surface of the nanocrystals.
2.5

Absorption (a.u.)

Introduction
Diluted magnetic semiconductors (DMSs) are now
attracting great interest in the field of spintronics. The
development of high-Tc ferromagnetic DMSs remains acentral challenge confronting this field. Roomtemperature ferromagnetism has been reported for some2+2+
Co -doped ZnO(Co :ZnO) synthesized by pulsed laser[1]
deposition technique or other methods, but there are2+[2]
contradicting reports on Co :ZnO . It is still open todiscussion whether the observed ferromagnetism comesfrom DMSs or from other magnetic precipitates.
We adopted a direct chemical synthesis of colloidal
2+
Co :ZnO. A major concern in the preparation of DMS
nanocrystals from solution is dopant extrusion during
synthesis, which may compromise the desired physicalproperties. We investigated the local structure of thedopant of ZnO based DMSs with XAFS to understand the2+distribution of Co in nanocrystals.

2.0Co(OH)2

1.5

5%
3%
1%

1.0
0.5

Results and Discussion
Figure 1 shows Co K-edge XANES spectra of 1%, 3%2+
and 5% Co :ZnO and Co(OH)2. Three spectra havealmost the similar form. In this synthesis, cobalt
hydroxide is the most likely species contained as2+impurities. However, these spectra shows that Co in2+
synthesized Co :ZnO do not have the similar structure toCo(OH)2.
Figure 2 shows the Fourier transform (FT) of Co K2+edge EXAFS spectra of 1%, 3%, 5% Co :ZnO, cobalt
hydroxide and the calculated Fourier transform intensity
2+for Co :ZnO. It was calculated with the FEFF8.0
2+
program for a cluster of wurzite ZnO in which a Co ion
substitutes the central Zn site. This figure shows that0.077007750
Photon Energy (eV)Figure 1. Co K-edge XANES spectra for Codoped ZnO and Co(OH)2.
7060

Intensity (a.u.)

Experimental
2+
Colloidal nanocrystalline Co :ZnO is prepared by themethod described below. 10mL of 0.55M KOH in ethanol
was added dropwise to a 20mL solution of Zn(OAc)2 inDMSO. Transition-metal doping was achieved byaddition of CoCl2 to the precursor solution. The
precipitate was washed with ethanol and dispersed in
0.05M oleilamine in 10mL of ethanol. After stirring for3h, the precipitate was washed with ethanol and dispersed
in ethanol and dried the precipitate to get a sample in thepowder form. XAFS experiments were carried out at 12C
station of Photon Factory in the transmission mode. All
XANES spectra were taken at room temperature andEXAFS spectra at 20K.

504030FEFF20Co(OH)25%
3%1%

1000123456Distance (Å)

Figure 2. Fourier transform intensities for Codoped ZnO, Co(OH)2 and calculated Fouriertransform.
References[1] K. Ueda et al., Appl. Phys. Lett. 79, 988 (2001).
[2] J. H. Kim et al., J. Appl. Phys. 92, 6066 (2002).
* ohta@chem.s.u-tokyo.ac.jp

Users' Report27