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

11B/2004G062

Chemical States of Silver Species in an Organic Charge Transfer Salt at Different
Stages during UV-VIS Illumination
Toshio NAITO*1,2, Yoshinori KITAJIMA3, Takeshi MIYAMOTO4, Hironobu NIIMI4, Kiyotaka
4
ASAKURA
1
CRIS, Hokkaido Univ., Kita-ku, Sapporo 001-0021, Japan
2
Graduate School of Science, Hokkaido Univ., Kita-ku, Sapporo 060-0810, Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
4
CRC, Hokkaido Univ., Kita-ku, Sapporo 001-0021, Japan

Experimental
XAFS was measured on the pressed pellet samples
around LI (3829 eV), LII (3548 eV) and LIII (3374 eV)edges at room temperature (RT) using total electron yield
method. The single crystals of Ag(DM)2 were wellground to fine powder and illuminated using Hg/Xe-lamp
with intermittent stirring until the whole powder (darkblue) turned brown. On the other hand, a bundle of single
crystals of Ag(DM)2 turned black by much stronger
illumination. Along with silver foil (99.95%, Alfa Aesar),
we measured XAFS of the pristine, the brown, and theblack samples to determine the chemical states of silver
species in each solid state. Measurements were repeated
to confirm that the result was not affected by X-ray
irradiation during the measurement. We observed
identical XAFS spectra after the several repetitions.
Results and Discussion
Comparison of the obtained spectra (Fig. 1)
unambiguously indicates that the silver species containedin the “black” samples is bulk silver, and that silver in the“brown” sample closely resembles bulk silver. Alongwith the results of infrared spectra, elemental analyses
and XPS, the XAFS results are consistently explained
supposing that the illumination progressively induced an26

Users' Reportelectron transfer (redox reaction) between the Ag(I) ionsand the DM radical-anion species. From our previous
work [1], continuous illumination of the sample finallydeposited a silver solid on its surface. Therefore thephoto-induced electron-transfer deprive the Ag ion of thecrystal lattice of Ag(DM)2, and the free Ag ionsspontaneously gathered to become bulk silver via“brown” and “black” states. X-ray powder diffraction
revealed that the averaged diameter of the silver prticles
in the “black” sample is ~9 nm, while the “brown”
sample is amorphous [2]. Therefore, it is found fromXAFS that the “brown” and the “black” samples bothcontain bulk silver and that the difference among themlies in the average diameter of the Ag particles. Also wehave found an illumination stage, where the original
crystal lattice remains yet the conduction behavior differsfrom the pristine crystals. A preliminary XAFS of thesample in this state did not reveal a clear difference inspectra from the pristine sample. Based on the XANES,XPS, elemental analysis, IR, XRD, SEM, DSC and otherdata, this state cannot be explained as a mixture of thepristine and the brown states. Evidently, this state should
be further studied in more detail.

0.8µ (A.U.)

Introduction
It is indispensable for miniaturization of various
electronic products to develop nanodevices based on
conducting organic compounds. A direct fabrication ofcircuits and nanodevices on a chip will be possible if we
can control the electrical conduction at arbitrary limitedparts of organic solids by illumination.
We have recently found that an organic charge transfer
salt, Ag(DM)2 (DM = 2,5-dimethyl-dicyanoquinonediimine), is a prospective candidate for the abovementioned control [1]. The electrical conductivity occurs
solely through the DM radical anion species in thismaterial. However, it is suggested that we should control
the formal charge on DMδ by UV-VIS illumination due to+the photo-induced oxidation reactivity of Ag . In order toclarify the mechanism of the photo-induced change in
conducting behavior observed in Ag(DM)2, we measured
a series of Ag L-edge XAFS of the sample at different
stages during UV-VIS illumination.

0.6
0.4

pristine"brown"" black"
Ag foil

0.234503400
3500
Energy (eV)Fig. 1. Ag L-edge XAFS spectra for pristine, “brown”,
and “black” states of Ag(DM)2 during UV-VIS
illumination with that of authentic Ag foil as a reference.
3300

3350

References[1] T. Naito et al., Adv. Mater. 16, 1786 (2004).
[2] T. Naito et al., manuscript in preparation.
* tnaito@mail.cris.hokudai.ac.jp