Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

12C/2004G219

EXAFS Study of Semimetal-Semiconductor Transition of Bismuth Clusters
Juli Sogoh1, Shinji Yoshida1, Shinnosuke Furuta2, Hiroyuki Ikemoto*1 and Takafumi Miyanaga2
1
Faculty of Science, University of Toyama, Gofuku 3190, Toyama 930-8555, Japan
2
Faculty of Science and Technology, Hirosaki University, Hirosaki 036-8561, Japan
Introduction
Raman-scattering measurements of bismuth clusters
exhibit a phase transition from rhombohedral Bi
nanocrystalline to amorphouslike clusters depending oncluster size[1]. They also suggest that amorphous clusters
are semiconducting and covalent interactions increase
with decreasing size.
The Raman studies are good tools but indirect
evidences for the transition. So it is very important toinvestigate the structure to reveal the mechanism of thephase transition. In the present study we report results ofextended X-ray absorption fine structure (EXAFS) of Biclusters.

Bi2O3 or BiCl3. But bond lengths are 2.40 and 2.48 Å for
Bi-O and Bi-Cl, respectively, so they may be suspectedpeaks. The main peak locates at 2.97 Å, whih is shoterthan that of the 300 nm thick film. It is interesting that thepeak originated from the interlayer correlation disappears.
The results of EXAFS indicate slight shrinkage ofcovalent bond within the layer and the disappearance ofinterlayer correlation.

Experimental
Bismuth of 99.999 % purity was slowly deposited
onto the substrates from a tungsten boat. The Bi film wasdiscontinuous with isolated island formation. Then, NaClof 99.99 % purity was deposited to cover the Bi islands.
By repeating these procedures, a sample of Bi clusters
isolated in the NaCl matrix was obtained. Since the Bi
clusters are formed in thin films, samples are represented
by their average thickness of the Bi thin films in thisreport.
EXAFS measurements were carried out for the Bi
clusters isolated in the NaCl matrix, using thespectrometer installed at BL-12C of the Photon Factory(PF). X-ray absorption spectra were measured for Bi LIIIedge (13.404 keV). The intensity of incident beam andthe transmitted beam were monitored by ionizationchambers.

Ikemoto@sci.u-toyama.ac.jp

Result and Discussion
The experimental XAFS signals are Fouriertransformed (FT) with a Hamming window in the range-1
4 to 18 Å . For the 300 nm thick films the evolution ofFT with the temperatures (23K, 200K and roomtempearure (RT)) are presented in Fig.1. The large peakat 3.03 Å is the contribution from the first shell of Bi
neighbors within the layer. The intensities of the main
peak are very large compared to other peaks at alltemperatures. There is a second peak around 3.6 Å which
is close to the first inter atomic distance between layers.
The peak dumps with increasing temperature anddisappears at RT, suggesting that the peak originates frominterlayer correlation.
Fig.2 shows FT of the 0.5 nm thick films. Some peaks
exist in the shorter distance region compared to the main
peak. There is possibility that they are originated from

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Users' Report

References[1] M. G. Mitch, S.J.Chase, J Fortner, R.Q. Yu, and J. S.
Lannin, Phys. Rev. Lett., 67 (1991) 875-878

Fig. 1 Fourier transform of kχ(k) for the 300 nm thickfilms at 23K, 200K and RT.

Fig. 2 Fourier transform of kχ(k) for 0.5 nm thick films
at 23K, 200K and RT.