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Chemistry Letters Vol.36, No.8 (2007)
Chemical States of Ag in Ag(DMe-DCNQI) Photoproducts
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and a Proposal for Its Photoinduced Conductivity Change Mechanism
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Takeshi Miyamoto, Hironobu Niimi, Wang-Jae Chun, Yoshinori Kitajima, Hideyuki Sugawara,
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ꢀ1;3
Tamotsu Inabe, Toshio Naito, and Kiyotaka Asakura
Department of Quantum Science and Engineering, Graduate School of Engineering,
Hokkaido Unversity, 21-10, Sapporo 001-0021
CREST JST, 21-10, Sapporo 001-0021
Catalysis Research Center (CRC), Hokkaido University, 21-10, Sapporo 001-0021
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Photon Factory, Institute of Materials Structure Sciece, High Energy Accelerator Research Organization,
-1 Oho, Tsukuba 305-0801
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Division of Chemistry, Graduate School of Science, Hokkaido University, 10-8, Sapporo 060-0810
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Creative Research Initiative ‘‘Sousei’’ (CRIS), Hokkaido University, 21-10, Sapporo 001-0021
(Received May 17, 2007; CL-070540; E-mail: askr@cat.hokudai.ac.jp)
UV–vis light converts Ag(DM)2 (DM = 2,5-dimethyl-
establish the mechanism, the chemical state of Ag is a key issue.
In this study, we examined chemical states and structures of Ag
in the photoproducts of Ag(DM)2 using X-ray diffraction (XRD)
and X-ray absorption near edge structure (XANES). The result
demonstrated that the phototunable conductivtity change mech-
anism of Ag(DM)2 could not be explained solely by the simple
redox mechanism above. We will propose more detailed mech-
anism. This will be helpful for further development of devices
made of Ag(DM)2. Ag(DM)2 was synthesized using a previously
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N,N -dicyanoquionediimine; DMe-DCNQI) to several solids
with metallic, semiconducting, or insulating conductivities
depending on the irradiation conditions. Ag valence state in
each photochemical product was determined by Ag L3-edge
XANES. The XANES result requires the correction of the redox
mechanism to explain the photoinduced conductivity change of
Ag(DM)2 photoproducts.
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reported method. A 200-W Hg/Xe lamp was used as a UV–vis
Organic electronic devices attract much attention because of
their lightness and versatility. A silver salt of 2,5-dimethyl-di-
cyanoquinonediimine (DM), Ag(DM)2, shows one-dimensional
light (200–1100 nm) source. XRD patterns were measured using
an X-ray diffractometer (RINT-Ultra+; Rigaku Corp.) with
Cu Kꢁ radiation. The X-ray absorption experiments were
performed using the beam line 11B at the Photon Factory of
Institute of Material Structure Science (KEK-PF) (PF Proposal
No. 2004G062). The Ag L3-edge XANES spectra were recorded
in a total electron yield mode. The X-ray spectra were measured
under UHV conditions. In order to have homogeneous sample,
we ground samples and they were fixed on conducting carbon
tapes. The spectra did not change even after prolonged X-ray
irradiation.
We found four photoproducts that were classified by their
conductivity as well as their appearances. Table 1 summarizes
the properties of the pristine material and the photoproducts.
Including the pristine material, we designate them respectively
as ꢁ (pristine), ꢂ, ꢃ, ꢄ, and ". Among them, ꢂ is formed by a
weak and short irradiation. The others, ꢃ, ꢄ, and " are produced
with more intense light in this order. ꢁ, ꢂ, and ꢃ retained the
original needle crystalline shape and composition, whereas ꢄ
and " became powdery. The ꢁ, ꢂ, and ꢃ solids are interesting
for device applications.
(
DM anion column. The salt has a 1/4-filled electronic system.
1D) metallic conduction through the ꢀ-electronic band of the
1–3
It shows interesting electronic phases: metallic, CDW, and spin
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Peierls phases. These phases can be transformed reversibly into
each other using appropriate temperature and pressure. The salt
also shows photoinduced conductivity change; the electric con-
ductivity changes dramatically from metallic to semiconducting
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or insulating ones only by UV–vis illumination. This property
is useful to fabricate electronic devices from a single crystal of
Ag(DM)2 directly by photolithography. Actually, Yamamoto
et al. prepared single crystals of Ag(DM)2 between the elec-
trodes patterned on silicon substrates and made rectifiers with il-
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lumination of UV–vis light. Understanding this conductivity
change mechanism is very important to tune the illumination
conditions to fabricate such devices. As for a conductivity
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change mechanism, the redox mechanism has been proposed,
þ
in which solid-state charge transfer occurs between the Ag and
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ꢁ
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DM ions under illumination to create neutral Ag and DM
species. Accordingly, the number of unpaired electrons on the
DM column is reduced, thereby decreasing conductivity. Similar
redox mechanisms were proposed to explain the photoinduced
Figure 1 shows the XRD patterns of ꢁ, ꢂ, ꢃ, ꢄ, and " togeth-
er with Ag foil and neutral DM. In ꢄ and ", we found peaks in the
corresponding to Ag metal, indicating metallic Ag formation.
The broad peaks in ꢄ and " indicate the presence of smaller
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metal-to-insulator transitions in AgTCNQ and AgTNAP. To
Table 1. The properties of Ag(DM)2; ꢁ and photo-induced products ꢂ, ꢃ, ꢄ, and "
Product
ꢁ (pristine)
ꢂ
ꢃ
ꢄ
"
Color
Appearance
Stoichiometry [DM/Ag]
Conductivity
Black
Needle (10 mm ꢂ 2 mm)
Black
Needle
2
Brown
Black
Silvery white
Powder
ꢃ0
ꢅ
Needle
2
Insulating
Powder
1.5
Insulating
2
Metallic
Semiconducting
Metallic
Copyright ꢀ 2007 The Chemical Society of Japan