3FTFBSDI

X-ray Irradiation-induced Carrier Doping in
Organic Mott Insulators

X-ray irradiation usually induces disorders in a crystal, forexample, displacement of the atomic position. In the case ofmolecular materials, the irradiation produces the molecular
defects, which are radiolysed by ionizing radiation. This kindof molecular defect permanently remains, while the irradiation
damage in inorganic materials in general is only due to atomic
displacements which can be restored by a proper heat
treatment. These defects and disorders make the electricalconductivity worse because of increasing the electron
scattering [1]. Increase of the conductivity by X-ray irradiation,
however, has been found in organic Mott insulators [2]. The
irradiation-induced defects expected at the donor molecule
sites might cause a local imbalance of the charge-transfer inthe crystal. Such local modulation of the charge-transfer
comes into being the effective doping of carriers into the halffilled Mott insulators.
Figure 1 shows a schematic crystal structure of κ-type
BEDT-TTF molecule based organic conductor. The anionand donor molecule layers are stacked alternatively. Chargetransfer between donor and anion molecules introduces ahole carrier for a BEDT-TTF dimer. In the case of the Mott
insulating state that the band width W is smaller than theeffective on-site Coulomb energy U, the hole is localized at
the dimer site. When the sample is irradiated by X-ray, somemolecular defects could be introduced into the donor layer.
The expected defects at donor molecule sites might cause alocal imbalance of the charge transfer between BEDT-TTF

C u[N(C N) 2 ]C l insulating layer
B E DT -T T F conducting layer

X-ray irradiation
Local imbalance of
charge transfer

donor and anion molecules. Such molecular defects result in
the effective carrier doping into the half-filled dimer-Mott
insulators.
As shown in Figure 2, the in-plane resistivity of the organic
Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl decreases
drastically by X-ray irradiation with the tungsten tube (40kV,
20mA). It is noted, moreover, that a metal-like temperaturedependence appears with rather small irradiation dose.
These large changes of the transport properties may indicatethat the carrier-doping by X-ray irradiation breaks down of
the Mott insulating state due to small shift from half-filling of
the conduction band. By controlling the molecular defects by
X-ray irradiation, we could tune the carrier doping continuously
for the Mott insulator, which has not been possible for theorganic system by chemical substitution and other methods.
This new way of carrier-doping technique will be useful not
only for studying the Mott physics but also for moleculardevice fabrications.

104ρ (Ωcm)Modulation of the band-filling in the Mott insulator could
produce the novel interesting strongly correlated electronic
states like as High-Tc superconductors and non-Fermi liquidstates. X-ray irradiation for the molecular materials could bea new way of the carrier-doping into the Mott insulatingstate.

κ-(B E DT -T T F ) 2 C u[N(C N) 2 ]C l

103 4 orders decrease
before irradiation
102
after irradiaion
1(340 hours )
10
100
10-1
10

40% decrease
50 100T (K )

500

Fig.2 Temperature dependence of the resistivity of κ-(BEDTTTF)2Cu[N(CN)2]Cl before and after X-ray irradiation at room
temperature. After irradiation, the resistivity drasticallydecreases by about 40 % at room temperature and fourorders of magnitude at low temperatures in comparison to
the resistivity before irradiation. Moreover the resistivity
shows a metallic temperature dependence above 50 K.

References[1] J. G. Analytis, A. Ardavan, S. J. Blundell, R. L. Owen, E.
F. Garman, C. Jeynes and B. J. Powell, Phys. Rev. Lett. 96,
177002 (2006).
[2] T. Sasaki, H. Oizumi, N. Yoneyama and N. Kobayashi, J.
Phys. Soc. Jpn. 76, 123701 (2007).

effective carrier doping

Fig.1 Schematic crystal structure of organic dimer-Mott
insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl. X-ray irradiation
produces molecular defects in the BEDT-TTF layer. The
molecular defects will act as a carrier-doping site due to local
imbalance of charge transfer.

34

Innovative Electronic Materials

Contact toTakahiko Sasaki (Low Temperature Physics Division)e-mail: takahiko@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2008

3FTFBSDI

Organic Light Emitting Transistor
Organic transistor is one of the key devices in organic
electronics, which has a function of the electrical current
switch. Here we demonstrate a new function of organic
transistor, the first single crystal ambipolar light emitting
transistor (LET), which is capable of carrying high current
density, providing a novel opportunity of challenge to realize
current-driven organic lasers.
2007 was an epoch-making year for organic electronics,
since OLED-TV (television set using organic light-emitting
diodes) has come to the market. This achievement isaccelerating the research on the other organic electronic
devices, such as transistors and solar cells. Particularly,organic field effect transistors (OFETs) are attracting a
growing interest, since the mobility in OFETs are now higher
than that in amorphous Si, and hence the application of
OFETs to flexible, low-cost and environmental-friendly
electronic devices are becoming more and more realistic.
A unique feature of organic FETs that has been uncoveredby fundamental research is that the polarity of the device, i. e.
the sign of carriers can be simply changed by the sign of gate
voltages. In 2006, we found poly(methylmethacrylate) (PMMA)
is a very good insulator to promote electron conduction, andrealized a first ambipolar FET of organic single crystals using
rubrene in an anaerobic atmosphere [1].
Using the ambipolar nature of OFETs, one is able toinject both electron and holes from source and drainelectrodes. Injected charges meet at the center of thetransistor channel, forming a kind of electric field-induced PN
junctions, where charge recombination takes place to emit
light. This light emitting device, which is called light emitting
transistor (LET) provides a novel possibility of organic
materials, which is a realization of current-driven organic
laser. The conventional organic light emitting device, OLED,
is a sandwich structure of hole-transport, light-emitting, andelectron-transport layers (Fig.1 (a)). Injected electron andholes are transferred through the corresponding transporting
layers to the light-emitting layer. One of the crucial
requirements for realizing a current-driven organic laser is alarge current density, higher than 103 A/cm2. However, in theFig.2 LET using an organic single crystal: (bottom) Schematic
device structure and the carrier density distribution of tetracene
single crystal LETs. (Top) Emission intensity distribution map.
conventional OLED structures, the highest current density
has been so far limited to 10 A/cm2. This is basically because
the resistivity of both carrier transport layers are too high tocarry such a large current density, because these transportlayers have been made of basically insulating materials.
The lateral structure of LET devices (Fig. 1(b)), on theother hand, is disadvantageous because the carrier paths
are extremely long in length comparing to that in the LED
structure. However, since the carrier transport zones in LETs
are doped by the gate electric field, the resistivity of thesezones is dramatically low comparing to the carrier transportlayer of OLEDs. Particularly, since we are utilizing organic
single crystals, the mobility is two or three orders of magnitudelarger, and thus the electrical current in LET using single
crystals are as high as 102-103 A/cm2 [2].
Using tetracene and rubrene single crystals, we
succeeded in realizing a first organic LET device using
tetracene single crystals (Fig. 2)[3], where carrier density hasreached 103 A/cm2, which is nothing but what is required for
current-driven organic lasers. These findings might pave
ways for ambipolar LETs based upon organic single crystals
as a possible route for pursuing the realization of anelectrically-driven laser oscillation from organic materials.
References[1] T. Takahashi, T. Takenobu, J. Takeya, and Y. Iwasa,
Appl. Phys. Lett. 88, 033505 (2006).
[2] T. Takenobu, S. Z. Bisri, T. Takahashi, M. Yahiro, C.
Adachi, and Y. Iwasa, Phys. Rev. Lett. 100, 066601 (2008).
[3] T. Takahashi, T. Takenobu, J. Takeya, and Y. Iwasa, Adv.
Funct. Mater. 17, 1623 (2007).

Fig.1 Comparison of organic light emitting devices: (a) Lightemitting diodes, where carriers are transported in a verticaldirection. (b) Light emitting transistors, where lateral carriertransport is occurring.

KINKEN Research Highlights 2008 IMRContact toYoshihiro Iwasa (Low Temperature Condensed State Physics
Division)e-mail: iwasa@imr.tohoku.ac.jp

Innovative Electronic Materials

35

3FTFBSDI

Ab initio Simulation Study on the Control of Electron Transportin Molecular Device
We present the controlling factors of the electrontransport in the molecular junctions through the deep
understanding of the electronic and transport processes atthe single molecular level. In parallel, we show that thedesigned junctions with single-molecule or low-dimensional
carbon allotropes open the door to the design of nanogadgetry
embodied with interesting device characteristics, namely“Computer-Aided Nanodesign”.
“Molecular electronics” has emerged as a next-generation
technology because of the fact that useful devices can bebuilt on the basis of individual molecules. Since the advent ofmolecular electronics, an abundance of organic functional
molecules have been designed and investigated ascomponents in electronic devices such as molecular wires,
rectifiers, and switches. Accompanied with the synthesis offunctional organic molecules, the advancement of techniques
for characterizing and manipulating individual molecules andthe availability of first-principles methods to describe electron
tunneling through atomic chains or single molecules, make itpossible to realize molecular electronic devices.
In this work, a series of theoretical analyses were devoted
to the designed molecular devices with single molecule ormolecular strands in order to have the basic physics in themolecular electronic devices and to determine the factors
controlling the electron transport. The important featureemerging from this work is that the individual features ofelectron transport characteristics originate from the distinctalignment of molecular levels to the Fermi level according to
the molecule-contact coupling and applied bias voltage. In
addition, the alignment of molecular levels is closely related tothe intrinsic nature of molecule. In this work, we propose four
controlling factors of the electron transport: 1) aromaticity, 2)
π-conjugated framework, 3) doping, and 4) dipole moment.
The incorporation of different types of aromaticcomponents induces unique electron transport characteristics
through the distinct electronic structure and the response to
the molecule-contact coupling and applied bias.[1] In themultiporphyrinic systems, we showed that the distinct
arrangement of π-conjugated framework gives the switching
behavior as a single-molecule switch.[2] And we could knowthat the tunneling barrier can be controlled by the strength of
dipole moment and the rectifying behavior as a Schottky-like

diode can be improved by incorporating the peptide linkages
to the metallic/semiconducting CNT junctions, where thedirection of the dipole moment plays an important role in thedetermination of the rectifying behavior.[3,4] Finally, we
showed that the doped nitrogen atoms modify the molecularorbitals so as to generate a conducting channel in thedesigned CNT junctions by inducing a negative differential
resistance (NDR) behavior, which is a characteristic featureof the Esaki-like diode, i.e. tunneling diode.
In parallel, we showed that the designed junctions withsingle-molecule and low-dimensional carbon allotropes(namely fullerene, CNT, and recently graphene) open the door
to the design of nanogadgetry embodied with interesting
device characteristics, namely “Computer-Aided Nanodesign”.
When we inserted Zn metal layers within metallic CNTelectrodes thereby providing 1D heterojunctions that can act
as a wire-like, NDR, or varistor-type nanoscale device.[5]In general, the preceded theoretical investigations allow
us to avoid starting costly experiments by choosing
preferentially the way which has no serious objections from atheoretical point of view. Therefore, we believe that our
results will give an insight into the design and implementationof various electronic logic functions for applications in thefield of nanoelectronics.

Fig. 2 Designed nanogadgetry using carbon nanotube with
Zn metal layers
References[1] S. U. Lee, R. V. Belosludov, H. Mizuseki and Y.Kawazoe,
J. Phys. Chem. C. 111, 15397 (2007).
[2] S. U. Lee, R. V. Belosludov, H. Mizuseki and Y.Kawazoe,
Small. In press (2008).
[3] S. U. Lee, M. Khazaei, F. Pichierri, and Y.Kawazoe,
Phys. Chem. Chem. Phys. In press (2008).
[4] M. Khazaei, S. U. Lee, F. Pichierri, and Y. Kawazoe,
J. Phys. Chem. C. 111, 12175 (2007).
[5] M. Khazaei, S. U. Lee, F. Pichierri, and Y. Kawazoe,
ACS Nano 2, 939 (2008).
Contact toSang Uck Lee (Materials Design by Computer Simulation Division)e-mail: sulee@imr.edu
Yoshiyuki Kawazoe (Materials Design by Computer Simulation
Division)e-mail: kawazoe@imr.edu

Fig. 1 Controlling factors of electron transport at the molecular device

36

Innovative Electronic Materials

IMR KINKEN Research Highlights 2008

3FTFBSDI

Conducting Polymer / ZnO Schottky Junction Toward
Transparent Oxide Electronics
An oxide semiconductor ZnO is one of the promising
semiconductors for future electronics. In this study, highquality Schottky junctions were realized on a ZnO (0001)
bulk single crystal simply by spin-coating a commercial
conducting polymer as the metal electrode. This polymer
electrode can be used for implementations of various
functional oxide electronic devices.

EVAC

A wide-gap semiconductor zinc oxide (ZnO) is one of thepromising compounds for realizing transparent oxide
electronics. Recently, the light emission of ZnO p-n
homojunction and the observation of quantum Hall effect in
ZnO heterostructure were realized [1,2]. In order to advancethe ZnO-based electronics further, it is indispensable toimplement high quality electrode contact on ZnO for theperfect control of carrier dynamics in well-designed energy
band structures. For example, a metal / semiconductor
Schottky junction is very useful not only for evaluating theelectronic states of semiconductor, but also for practicalapplications such as photodetector and field-effect transistor.
Many groups have tried to develop ZnO Schottky junction
using various noble metals such as Au and Pt, however, highdevice performance is difficult to be achieved mainly becauseof the poor interface quality. Recently, we realized a highquality ZnO Schottky junction by using a commercial
conducting polymer, poly(3,4-ethylenedioxy- thiophene)poly(styrenesulfonate) (PEDOT:PSS), as a metal electrode[3]. Simple spin-coating process of the polymer in air at room
temperature is sufficient to make a good Schottky contact.
From this Schottky junction, it is possible to demonstrate thefield effect transistor, the solar/visible-blind photodetector
and the carrier profiling in two-dimensional electron gas in
ZnO heterostructures.
Figure 1 shows a schematic energy band profile of PEDOT:PSS/ZnO Schottky junction. ZnO is n-type semiconductor andits electron carriers are depleted at the interface leading to the
upward band bending that serves as a Shottky barrier for theelectron flow. The chemical structure of PEDOT:PSS is shownin bottom of Fig. 1. When raising the Fermi level of ZnO by
applying bias voltage (forward bias), the Schottky barrier
decreases exponentially hence the electron flow from ZnO to
PEDOT:PSS increases according to thermionic emission
model expressed as J ∝ T2 exp(-φb / kB T) exp(q V/(n kB T)). J
is current density, T temperature, φb Schottky barrier height, kB
Boltzman constant, V bias voltage, and n ideality factor
representing a deviation from the ideal model. For reverse
bias, the electron flow is significantly suppressed, and theinterface can serve as a gate capacitor. The capacitance C isa function of the bias voltage: 1/C2 ∝ Vbi - V, where Vbi is builtin potential (see Fig. 1).
Figure 2 shows bias voltage dependence of current
density and inverse square of capacitance. The current
density at positive bias is much larger than that at negative
bias, representing very high rectification ratio of the Schottky
junction. The ideality factor n is close to an ideal value 1, thatis one of the smallest values among the existing reports. The
1/C2 vs. V relation shows very high linearity without thefrequency dependence up to 1 MHz. Both J vs. V and 1/C2
vs. V curves show no hysteresis suggesting high quality of
the interface between ZnO and PEDOT:PSS. It is noted thatthese ideal Schottky behaviors are achieved by simple spincoating method with mild conditions, and such easy-to-use
approach without any damage of the interface enables us toobtain the reproducible junction performance among large
area production. It is expected that this PEDOT:PSS electrodecan be applied to various functional oxides toward realizationof next-generation transparent oxide electronics.

EF

KINKEN Research Highlights 2008 IMRe-Forward
Reverse

φm

qVbi

φb
Metal
(PEDOT:PSS)
OSO

Semiconductor
(ZnO)

O

O
S

EC
EF

O
+
S

O

n

m

SO3- SO3H SO3H

PEDOT

PSS

10

2

10

0

ReverseAu
Ti

ForwardAu
PEDOT:PSS
ZnO (0001)

Au
Ti

n=1.2

5x1014
4

φ b=0.9 eV

10-2

3

10-4

2

10-6
10-8
-3

qVbi=0.6 eV
1
ND=1.4X1017cm-3

300K
-2

1/C2 [F-2 cm4]

Current density [A/cm2]

Fig. 1. A band profile of Schottky junction (upper panel).
Electrons flow from semiconductor to metal under forward
bias and vice versa. EVAC, EF, φm, φb, EC, and qVbi are vacuum
level, Fermi energy, work function of metal, Schottky barrierheight, conduction band bottom of semiconductor, and built-in
potential, respectively. Bottom panel shows chemical structureof a conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), used as a Schottky
metal in this study. This Schottky metal can be formed onoxide semiconductors just by spin-coating method.

-10Voltage [V]102

Fig. 2. The current density (J)–voltage (V) and the 1/C2 (C:capacitance)-V relations at 500 kHz for a PEDOT:PSS/ZnO
Schottky junction. The ideality factor (n) and the Schottky
barrier height (φb) deduced from the J-V curve, the built-in
potential (Vbi) and the ionized donor concentration in depletion
layer (ND) deduced from the 1/C2-V curve are also shown.
References[1] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino,
M. Sumiya, K. Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H.
Ohno, H. Koinuma and M. Kawasaki, Nature Mater. 4, 42(2005).
[2] A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno and
M. Kawasaki, Science 315, 1388 (2007).
[3] M. Nakano, A. Tsukazaki, R. Y. Gunji, K. Ueno, A. Ohtomo,
T. Fukumura and M. Kawasaki, Appl. Phys. Lett. 91, 142113(2007).
Contact to
Masashi Kawasaki (Superstructured Thin Film Chemistry
Division)e-mail: kawasaki@imr.tohoku.ac.jp

Innovative Electronic Materials

37