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Giant Photoconductivity in Organic
Materials by UV Irradiation
Toshio Naito
Ehime University
・ Found in Se (1870’s)
Photoconductors
・ Explained (1930’s)
ex. CdS, PbS, etc
・ Applied to Electronics (1950’s)
e-
Conduction e
Band
Photoconductor
hn (> Eg)
Battery
Battery
Valence Band
Applied to ex.) Laser Printer, Photocopy, Solar Cell, Sensor, CMOS, etc
various imaging & sensor devices
New (additional) Functions
Information / Communication Technology
Purpose
New PCs with additional functions*
• Wavelength-selectivity of response
New Mechanism
• Different responses to different wavelengths
*
• Photo-magnetic-conductors
etc
・
・
・
・
・
・
・
・
Standard mechanism of photoconductivity ・2+
・(2+d)+
A
A
de
Charge disproportionation
(e.g.)
AX
2+
A (2-d)+
A
Cation A
or
Anion X
e-
LUMO
hn
Cation A
or
Anion X
h+
A (2+d)+
A 2+
HOMO
A 2+
hn
A (2-d)+
A 2+
A (2+d)+
A 2+
A 2+
A (2-d)+
A (2+d)+
・
・
・
・
・
・
・
・
Our strategy
Merits
NO disproportionation
(for high conductivity)
Standard mechanism
Cation
or
Anion
e-
Proposed (New) mechanism
Cat (1-d)+
Anion
or
Cation
An (1+d)hn
hn
Cation
or
Anion
h+
Cat (1+d)+
A-A or C-C Charge Transfer (CT)
Cation
or
Anion
(Localized spins)
e-
h+
Cat (1+d)+
C-A Charge Transfer (CT)
C[Ni(dmit)2]X
Abbre.
C (= Cations)
X
ref
MV
2
（a-, b-）
BPY
2, 6
JACS
18656 (2012)
NMQ
1
（a-, b-）
Eur. J. Inorg.
Chem. (2014)
DiCC
1
（ a-, b- ）
Chem. Lett.
1119 (2014)
Adv. Mater.,
6153 (2012)
[Ni(dmit)2] salts
Conductivity Ratio ( RC )
Carrier Doping Photoconductivity
Charge Disproportionation
(sph)
de
- de A(1-d)- A(1+d)-)
(A- C+ A(1-d)- C(1-d)+
)
(A
A
Dark conductivity (sdark)
-
=
C-A CT
Cations
MV
-
A-A CT
(Almost) No CT
NMQ
DiCC
(n-C4H9)4N
Ru(bpy)3
BPY
・・・・
RC
10-1000
40-880
< 2-3
[Ni(dmit)2]-
BPY2+
C-A CT
[Ni(dmit)2]- = conduction
b
b
a
c
0
front view
top view
c
a
0
BPY[Ni(dmit)2]2 Band Calculation（ Extended Hückel Method ）
Red（16 bands）；
（e.g.）～37% [Ni(dmit)2]Green； ～100% BPY2+
Black； ～100% [Ni(dmit)2]-
Anion-Cation Mixed Bands
UV
(～ a few eV)
BPY[Ni(dmit)2]2
Solution
（Absorption）
Solid State
（Diffuse
Reflection）
Solid State & Solution Spectra
A[Ni(dmit)2]2
Conduction
Ground state = insulating
(A = BPY, MV) （Single Crystal）
（dark）
+
+
MV
Ea = 0.12 eV
(rRT = 63 W cm)
Ea = 0.28 eV
(rRT = 2×105 W cm)
Thermally activated behavior
A[Ni(dmit)2]2
(A = BPY, MV)
（Polycrystal）
Mag. Susceptibility
Ground state = diamagnetic
(non-magnetic)
dark
+
+
MV
c ~ - 1 × 10-3 emu mol-1
JAF ～ 110 K
A[Ni(dmit)2]2 ; photoconductivity
（Single Crystal）
（375 nm, 11.6 mW cm-2 , 300 K, in vacuo）
A = BPY2+
A = MV2+
UV
UV
Photoconduction is negligible
UV
in many Ni(dmit)
2 salts.
Comparison
(375 nm)
200 K
（BPY salt）
300 K
(n-C4H9)4N salt
~ 10-10 S
（sph/sdark  2）
A[Ni(dmit)2]2 ESR （single crystal, H ^ [010] ）
A = BPY2+
under dark
under UV
BPY2+
153 K
[Ni(dmit)2]-
(integrated)
-
BPY2+ [Ni(dmit)2]
hn
hn
A[Ni(dmit)2]2 ESR （single crystal, H ^ [010] ）
A = BPY2+
< under dark vs. under UV >
BPY2+
153 K
[Ni(dmit)2]-
-
BPY2+ [Ni(dmit)2]
(integrated)
hn
hn
e-
[Ni(dmit)2] + BPY2+
UV
[Ni(dmit)2](1-d)- + BPY(2-d)+
d @ 0.1 (Consistent with band calc. & UV spectra)
A[Ni(dmit)2]2 ; photoconductivity – I- & T-dependences
（Single Crystal） （375 nm, in vauo）
C-A CT type salts
New Features in Photoconduction s
(1)
s = sdark + aI + bI2
(2)
Wavelength selectivity (Responsive ONLY to ～250-450 nm)
(I : light intensity)
C-A CT bands
Photoconduction mechanism is different from the known mechanism
[Ni(dmit)2] salts
Conductivity Ratio ( RRCC)
Photoconductivity (sph)
Dark conductivity (sdark)
C-A CT
Cations
MV
A-A CT
(Almost) No CT
NMQ
DiCC
(n-C4H9)4N
Ru(bpy)3
BPY
・・・・
RC
10-1000
40-880
(RT) (200 K)
< 2-3
A-A CT-based PC
( A = Anion, C = Cation )
(A)
NMQ[Ni(dmit)2]*
Unique PC
Sharp wavelength-selectivity
(Only ~ 375 nm)
unusually
(C)
Large sUV / sdark
(eg. ~ 40 @300 K, ~ 880 @200 K)
* (in regard to synthesis, crystal structure, and dark conductivity)
J. P. Cornelissen, et al. Inorg. Chim. Acta 1991, 185, 97-102.
Crystal Structure
[Ni(dmit)2]- NMQ+
b
c
Negligible C-A interactions
ca. 0.121 eV
ca 0.012 eV
Band Structure & Conductivity
Ni(dmit)2 band
NMQ
band
Current x 107 / A
UV ON
OFF
2.5
300 K
375 nm
11.6 mWcm-2
DC 2 V
2-probe
2.0
X 40
1.5
1.0
0.5
0
40
80
120
Time / s
Ea (dark) = 0.20 eV
375 nm
(3.3 eV)
Ea (UV) = 0.12 eV
XPS & UV-Vis-NIR Spectra
Ni 2p
S 2s
(375 nm)
C-A
S 2p
A-A
EF
C-C
Under UV-irradiation (375 nm)
Under dark
XPS (Ni, S, EF) do NOT change
(under dark and UV)
NMQ[Ni(dmit)2] (powder)
(C4H9)4N[Ni(dmit)2] (in CH3CN)
NMQ·I (in CH3CN)
CT between A-A (not C-A or C-C)
New contribution in PC
(I)
RC @ 880
𝛔𝐬𝐪 = 𝒂 + 𝒃𝑰 + 𝒄𝑰𝟐 + 𝒅𝑰𝟑
s dark
s Ph, new
s Ph, std
2nd
s sq
S
R =
photoconduction
C
s Ph, new
s Ph, std
s dark
s dark
( I = 15.71 mW cm-2 )
Summary; A-A CT
On the verge (orNMQ[Ni(dmit)
in the middle) ]of melting of charge-ordered state?
2
s (ph) / s (dark) = 40 (RT) - 880 (200 K)
(15.7 mWcm-2)
Remains to be clarified
Coexistence of three kinds of (photo)conduction
𝛔𝐬𝐪 = 𝒂 + 𝒃𝑰 + 𝒅𝑰𝟑
s Ph, new
s Ph, std
s dark
Optical Doping
YBa2Cu3O7-d (TC = 90 K)
MV[Ni(dmit)2]2
Chemical doping → Metastable State
Irradiation → Photoexcited State
（930 ºC、5 h ） → Oxygen deficiency （irrev.）
→ CT trans. （rev.）
PHOTOMAGNETIC CONDUCTORS
 T2
 log T
(Possible Kondo Effect under UV)
T. Naito et al, Adv. Mater., 24 (46), 6153-6157 (2012)
T. Naito et al, J. Am. Chem. Soc., 134(45), 18656-18666 (2012)
Acknowledgments
Collaborators
Thank you
Mr. T. Karasudani, Dr. S. Mori, Porfs. K. Ohara, K. Konishi
& T. Yamamoto (Ehime University)
Mr. T. Takano, Dr. Y. Takahashi, Prof. T. Inabe (Hokkaido University)
Profs. S. Nishihara & K. Inoue (Hiroshima University)
Profs. K. Furukawa* & T. Nakamura (IMS) (*present Niigata Univ.)
Financial Support
JSPS (No. 23540432)
Ito Science Foundation
Japan Securities Scholorship
Ehime Univ. GP
Ehime Univ. Grant for Interdisciplinary Research
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