Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

2A/2003G246

A Development of a New EXPEEM System (3) - Chemical Mapping of TaOx
Tetsuya TSUTSUMI1, Takahiro KAWASAKI1, Takeshi MIYAMOTO1, Hironobu NIIMI1, Shushi
SUZUKI1, Wang-Jae CHUN1,2, Kiyotaka ASAKURA1*, Yoshinori KITAJIMA3 and Yasuhiro
IWASAWA4
1Catalysis Research Center, Hokkaido University and 2CREST JST, Kita-ku, Sapporo 060-0811,Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
4
Department of Chemistry, The University of Tokyo, Hongo, Tokyo 113-0033, Japan..
Introduction
It is quite important to obtain the real time chemical
imaging of the surface in order to understand thechemical processes accompanying the diffusion and thereaction between different phases. EXPEEM(Energyfiltered Photoemission Electron Microscopy) is a newspectromicroscopy which can visualize the surface
chemical state and element distributions by selecting the
kinetic energies of photoelectrons.
Previously wereported the first EXPEEM image obtained by a Wien
filter energy analyzer using a high energy X-ray emitted
from a bending magnet.[1]In the previous work we
could obtain elemental distribution in a µm order. In thiswork we tried to obtain a chemical distribution based on adifferent binding energy of core electrons.
Experimental
The details of EXPEEM instrument are described
thelsewhere.[1] In this work we used the 5 order higher
harmonics emitted from an undulator at BL2A. The Xray was monochromatized by a Ge(111) double crystal
monochromator.
The ejected photoelectrons werecollected by a cathode objective lens and filtered by aWien filter.
In order to decrease the sphericalaberrations we put an angle limiting aperture with a 25µm opening.
In addition, a narrower energy selecting
slit than reported before[1] was used after the Wien filter
to get a higher energy resolution(∆E=1 eV). The sample
was Ta sheet with 10 µm Au island deposited. The
separation of Au islands was 25 µm.
PEEM wasmeasured using a Deuterium lamp(HAMAMATSU
L1314).
The sample was irradiated through a BaF2window.
Results and discussion
Fig. 1 shows the PEEM picture of Au on TaOx during
the heating process.
At room temperature, Au wasbrighter than the rest because of the Ta surface was fully
oxidized and the work function of Ta region was larger
than that of Au. When the sample was heated up to 723
K, the Ta area became brighter than the Au area.
Especially periphery of Au island was the brightest. Theline profile of the gray scale across the Au island clearlyindicates that the gray scale was almost constant in the Au
region and the Ta region became brighter. The periphery
was brighter than the rest. If the Ta region at theperiphery was really bright, the oxygen was removed firstfrom the peripheral region. But PEEM contrast often
suffers from the geometrical effects.[2] We carried out
the EXPEEM analyses on the same sample, using Ta 3d5/2
photoelectrons with 1 eV energy step. Figure 2 shows theEXPEEM image using different photoelectron kineticenergies. The Ta region including the periphery of Au
island became brightest at the photon energy of 654 eV.
This demonstrates that the bright periphery around the Au
region arises not due to the chemical reason but due to thegeometrical reason. EXPEEM is more sensitive to thechemical information.
[1] H. Yasufuku, Y. Ohminami, T. Tsutsumi, K. Asakura,M. Kato, Y. Sakai, Y. Kitajima, and Y. Iwasawa,Chem.Lett., (8), 842-843 (2002);H. Yasufuku, Y.
Ohminami, T. Tsutsumi, H. Niimi, N. Matsudaira, K.
Asakura, M. Kato, Y. Sakai, Y. Kitajima, and Y. Iwasawa,
Jpn.J.Appl.Phys. 43, (11), 7682-7688 (2004).
[2] S. A. Nepjiko, N. N. Sedov, G. Schonhense, J.
Microscopy, 203 269-276 (2001).
RT723 K

773 K

873 K

Fig. 1 PEEM picture of Au on Ta with different
temperatures

Figure 2 EXPEEM of Au on Ta measured with 1
eV energy step.
askr@cat.hokudai.ac.jp

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