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

12C/2002G237

EXAFS Study of Semimetal-Semiconductor Transition of Bismuth Clusters
Hiroyuki Ikemoto*1, Takafumi Miyanaga2, Taichi Kiuchi1, Keisuke Deto1 and Kiyofumi Nitta2
1
Faculty of Science, Toyama University, Gofuku 3190, Toyama 930-8555, Japan
2
Faculty of Science and Technology, Hirosaki University, Hirosaki, Aomori 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 increasewith 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.
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 Biclusters 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 Biclusters 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.
Result and Discussion
The XAFS χ(k), as a function of wave number k, forseveral temperatures of the 0.5 nm thick film are shown inFig. 1. The phases in oscillations of χ(k) are observed up-1
to 20 Å . If the phase difference between XAFS ofdifferent temperatures is negligible, then increasing
temperatures simply attenuates the amplitude of theXAFS signal and the XAFS will have zero crossing at thesame wave number where the background must passthrough the absorption data itself. The temperature
variations indicate success of evaluations of χ(k).
Figure 2 shows Fourier transform of kχ(k) data of the0.5 and 300 nm thick films at 23K. As can be noted in Fig.
2, the major contribution to the XAFS is from the firstshell. The large peaks around 3.0 Å are the contribution
from the first shell of Bi neighbors within a layer. Thepeaks locate at 3.03 and 2.97 Å for the 0.5 and 300 nm

thick films, respectively. The shift of first peak indicatesshrincage of covalent bond with decreasing size. There isa second peak around 3.6 Å in 300 nm thick films. Thepeak dumps with increasing temperature, suggesting thatthe peak originates from interlayer correlation. It is notobserved in the 0.5 nm thick film. The disappearance ofthe peak around 3.6 Å suggests loss of interlayerinteraction. We suppose that the loss of the overlap
between the orbitals in adjacent layers gives rise to thebond shrinkage and the semiconducting nature.
References[1] M. G. Mitch, S.J.Chase, J Fortner, R.Q. Yu, and J. S.
Lannin, Phys. Rev. Lett., 67 (1991) 875-878* ikemoto@sci.toyama-u.ac.jp

Fig. 1 kχ(k) for the 0.5 nm thick film measured at severaltemperatures.

Fig. 2 Fourier transform of kχ(k) for the 0.5 and 300 nm
thick films measured at 23K.

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