Simple bipolar molecules constructed from biphenyl moieties as host materials for deep-blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/chem.201103703

Simple is good! Based on biphenyl molecules, two bipolar host materials with high triplet energies have been rationally designed, synthesized, and fully characterized. Deep blue phosphorescent organic light-emitting diodes, which employ the new hosts and

Full text of DOI:10.1002/chem.201103703

DOI: 10.1002/chem.201103703
Simple Bipolar Molecules Constructed from Biphenyl Moieties as Host
Materials for Deep-Blue Phosphorescent Organic Light-Emitting Diodes
Cong Fan,[a] Fangchao Zhao,[b] Pei Gan,[a] Sifen Yang,[a] Tengxiao Liu,[a] Cheng Zhong,[a]
Dongge Ma,*[b] Jingui Qin,[a] and Chuluo Yang*[a]
Phosphorescent organic light-emitting diodes (PhOLEDs)
that have heavy-metal complexes as phosphorescent emit-
ters can harvest both singlet and triplet excitons by spin-
orbit coupling in the emitting layer to achieve 100% inter-
nal quantum efficiency in theory.[1] This makes OLEDs very
promising candidates for the next commercial full-color,
flat-panel, solid-state displays and lighting products.[2] To
avoid the triplet–triplet (T–T) annihilation and concentra-
tion quenching, phosphorescent emitters of heavy-metal
complexes are usually doped in host materials to guarantee
the device performance.[3] Of the three primary colors, green
and red PhOLEDs have achieved satisfactory efficiency and
stability.[4] However, the efficiency and stability of blue
PhOLEDs are still far away from practical application.
Nowadays, most research about blue PhOLEDs is focused
on the development of new host materials and blue phos-
phorescent emitters. Significant advances have been made
prevent energy back from the blue phosphors to the hosts.[7]
Simultaneously, the bipolar host materials should have
matched HOMO/LUMO levels with adjacent layers to de-
crease carrier-injection barriers, as well as high carrier drift
mobility to increase current and power efficiency. However,
introducing hole- and electron-transporting moieties into
a single molecule usually brings about intramolecular charge
transfer, causing a depreciation of the triplet energy. A
design strategy to overcome these problems is to employ in-
sulating linkages, such as adamantane and tetraaryl silane,
to interrupt the p conjugation. So far, only a few bipolar
host materials for FIr6-based deep-blue PhOLEDs have
been reported.[8]
The biphenyl molecule has high triplet energy of
2.84 eV,[9] but direct functionalization usually brings down
the triplet energy due to the enlarged p conjugation, such as
4,4’-(N,N’-dicarbazolyl)biphenyl (CBP) with ET =2.56 eV
and 4,4’-bis(diphenyloxophosphinoyl)biphenyl (PO1) with
ET =2.72 eV.[10] Interestingly, when bulky groups are placed
at the 2,2’-positions of the biphenyl molecule, a twist config-
uration of biphenyl molecule can reduce the p conjugation
and therefore increase the triplet energy, for example, 4,4’-
(N,N’-dicarbazolyl)-2,2’-dimethylbiphenyl (CDBP) with ET =
3.0 eV.[11] In this paper, with the aim to develop simple bipo-
lar host materials with high triplet energy, we introduce the
hole-transporting carbazole moiety and the electron-trans-
porting diphenylphosphine oxide moiety into the 2,2’-posi-
tions of a biphenyl molecule. We anticipate that the carba-
zole group and the diphenylphosphine oxide group will
cause biphenyl molecule to twist through steric effects, and
thus minimize the p conjugation of the two moieties. Mean-
while, the different spatial orientations of the two groups
can avoid intramolecular charge transfer, and thus maintain
their individual functionality. The two new hosts, POBPCz
and POBPtCz, show triplet energies of approximately
3.0 eV, which are significantly higher than that of biphenyl
molecule itself and qualify their application as bipolar hosts
to fabricate deep blue PhOLEDs.
with sky-blue PhOLEDs based on iridiumACTHNUTRGNE(NUG III) bis(4,6-di-
fluorophenylpyridinato-N,C2’) picolinate (FIrpic) that have
a maximum external quantum efficiency of over 20% and
a maximum power efficiency of over 35 lmWÀ1 [5]
However,
.
the FIrpic-based sky-blue PhOLEDs do not satisfy the blue
color standards recommended by the National Television
Committee. Deep-blue phosphors, such as, [bis(4,6-difluoro-
phenylpyridinato-N,C2’)]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
easy to obtain high efficiencies for FIr6-based deep-blue
PhOLEDs by virtue of the scarcity of host materials, espe-
cially bipolar hosts that possess both hole- and electron-
transporting abilities. The key problem to design bipolar
host materials for deep blue PhOLEDs is how to maintain
high triplet energy (ET) above 2.8 eV, which is necessary to
[a] C. Fan, P. Gan, S. Yang, T. Liu, C. Zhong, Prof. J. Qin, Prof. C. Yang
Department of Chemistry
Hubei Key Lab on Organic and Polymeric Optoelectronic Materials
Wuhan University, Wuhan, 430072 (P. R. China)
Fax : (+86)27-68756757
E-mail: clyang@whu.edu.cn
[b] F. Zhao, Prof. D. Ma
The two host materials were synthesized according to
Scheme 1. Firstly, the intermediate (2-bromo-2’-diphenylox-
ophosphinoyl)biphenyl was prepared from three steps: re-
gioselective lithiation of 2,2’-dibromobiphenyl, coupling with
chlorodiphenylphosphine, and finally oxidation with hydro-
gen peroxide to afford (2-bromo-2’-diphenyloxophosphi-
noyl)biphenyl with a overall yield of 50%. Secondly, carba-
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun, 130022 (P. R. China)
Fax : (+86)431-85262357
E-mail: mdg1014@ciac.jl.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103703.
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COMMUNICATION
terization of the compounds
can be found in Supporting In-
formation.
Figure 2 shows the absorption
and photoluminescence spectra
of POBPCz and POBPtCz in
CH2Cl2. Both compounds show
two characteristic peaks of ab-
sorption in the range of 320–
350 nm, which can be attributed
to n–p* transition of the carba-
zole moiety and the p–p* tran-
sition of biphenyl moiety. Their
energy gaps (Eg), calculated
from the onsets of absorption
spectra, are 3.57 and 3.50 eV
for POBPCz and POBPtCz, re-
Scheme 1. Synthesis of the intermediate (2-bromo-2’-diphenyloxophosphinoyl)biphenyl (1) and the final prod-
spectively, and they exhibit
emission at 399 and 418 nm, re-
ucts POBPCz and POBPtCz.
zole or 3,6-di-tert-butylcarbazole was coupled with (2-
bromo-2’-diphenyloxophosphinoyl)biphenyl by means of the
Ullmann reaction to get the final POBPCz and POBPtCz
with yields of 83 and 80%, respectively. All the compounds
1
were fully characterized by H and 13C NMR spectroscopy,
mass spectrometry and elemental analysis. The structure of
POBPCz was further confirmed by single-crystal X-ray dif-
fraction analysis. As shown in Figure 1, the C13-C18-C19-
Figure 2. UV/Vis absorption spectra, PL spectra of POBPCz (&) and
~
POBPtCz ( ) in CH2Cl2 solution at room temperature, and their phos-
phorescence spectra in 2-MeTHF at 77 K (inset).
spectively, in CH2Cl2. Both absorption and emission spectra
of POBPtCz show a red-shift relative to POBPCz. The inset
of Figure 2 depicts the phosphorescence spectra of the two
compounds measured from a frozen 2-methyltetrahydrofur-
an (2-MeTHF) matrix at 77 K. Their triplet energies (ET)
are determined to be 3.01 and 2.98 eV for POBPCz and
POBPtCz, respectively, from the highest-energy vibronic
sub-band of the phosphorescence spectra. The triplet ener-
gies of POBPCz and POBPtCz are significantly higher than
that of biphenyl molecule itself (ET =2.84 eV), which is
caused by minimizing the p conjugation of biphenyl mole-
cule.
Figure 1. Oak Ridge thermal ellipsoidal plot (ORTEP) diagram of
POBPCz.
C24 torsion angle in the biphenyl molecule is almost 608,
À
consequently the free rotation through the C18 C19 bond is
partially suppressed by the two bulk groups at the 2,2’-posi-
À
tions. The rotation through C13 N1 bond is also suppressed
due to the molecular configuration, as indicated by the dif-
ferent chemical shift of two tert-butyl groups of POBPtCz in
the H NMR spectra. The detailed preparation and charac-
The thermal stability of the two host materials, POBPCz
and POBPtCz, is indicated by the high decomposition tem-
1
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C. Yang, D. Ma et al.
peratures (Td, corresponding to 5% weight loss) of 3528C
for POBPCz and 3468C for POBPtCz through thermogravi-
metric analysis (TGA; Figure S1 in the Supporting Informa-
tion). Their glass transition temperatures (Tg) appear at
1708C for POBPCz and 1408C for POBPtCz in the differen-
tial scanning calorimetry (DSC) thermograms (inset of Fig-
ure S1). The Tg temperatures of POBPCz and POBPtCz are
much higher than that of CBP (628C).[12] Both high decom-
position temperatures and high glass transition temperatures
implicate that the two host materials form thermally durable
and morphologically stable amorphous thin-films, which is
essential for fabrication of PhOLEDs during thermal evapo-
ration
The electrochemical behaviors of POBPCz and POBPtCz
were examined by the cyclic voltammetry (CV; Figure S2 in
the Supporting Information). POBPCz exhibits a quasi-re-
versible oxidation process, whereas POBPtCz shows a rever-
sible oxidation process, because the active sites of carbazole
are blocked by the tert-butyl groups. Their HOMO energy
levels determined from the oxidation potentials are À5.69
and À5.51 eV for POBPCz and POBPtCz, respectively. The
higher HOMO level of POBPtCz can be attributed to the
electron-denoting property of tert-butyl groups. Their
LUMO energy levels, deduced from the HOMO energy
levels and the energy gaps, are À2.12 and À2.01 eV for
POBPCz and POBPtCz, respectively.
phenylamine (TCTA) and electron-transporting material
1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (Tm) were used for
blocking excitons due to their high triplet energies.[15] 1,3,5-
Tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) was utilized
as electron transporting material; FIr6 was used as the deep
blue triplet emitter, with optimized doping level at 10 %;
MoO3 and LiF served as hole- and electron-injecting materi-
als, respectively.
Current-density–voltage–luminance (J–V–L) characteris-
tics and current efficiency, power efficiency versus current
density of the devices are shown in Figure 3. Both devices
show typical emission from the deep blue phosphor of FIr6
(inset of Figure 3c,d). Device A with POBPCz as host dis-
plays a turn-on voltage of 3.1 V and achieves a maximum
current efficiency of 40 cdAÀ1, a maximum power efficiency
of 36 lmWÀ1 and a maximum external quantum efficiency of
19.5
%
at 0.01 mAcmÀ2. At practical brightness of
100 cdmÀ2 (0.41 mAcmÀ2), device A shows a current effi-
ciency of 27 cdAÀ1 and power efficiency of 17 lmWÀ1. At
brightness of 1000 cdmÀ2 (6.4 mAcmÀ2), device A still shows
current efficiency of 16 cdAÀ1 and power efficiency of
7 lmWÀ1. Device B with POBPtCz as host shows a turn-on
voltage of 3.8 V and reveals a maximum current efficiency
of 38 cdAÀ1, a maximum power efficiency of 31 lmWÀ1 and
a maximum external quantum efficiency of 19.5% at
0.01 mAcmÀ2. At brightness of 100 (0.35) and 1000 cdmÀ2
(7.26 mAcmÀ2), device B exhibits current efficiency of 27
and 14 cdAÀ1 and power efficiency of 17 and 6 lmWÀ1, re-
spectively. To the best of knowledge, these efficiencies of
the devices are comparable with the best results recently re-
ported,[8] and even better than the device with p–i–n struc-
ture.[16]
The geometrical configurations of POBPCz and POBPtCz
were optimized at B3LYP/6-31g(d) level and their triplet en-
ergies were calculated at wB97x/6-311+gACTHNUGRTENUNG(d,p) level using
a polarized continuum solvent model (IEFPCM) with THF.
The electronic properties of the compounds were studied by
density functional theory (DFT) calculations by using
B3LYP hybrid functional. All the computations were carried
out with Gaussian 09 program.[13] The calculated triplet ener-
gies are 3.03 and 2.99 eV for POBPCz and POBPtCz, re-
spectively, very close to their measured values. As shown in
Figure S3 in the Supporting Information, the HOMO orbi-
tals of the two molecules are mainly located on carbazole
moiety, while their LUMO orbitals are distributed on bi-
phenyl moiety and phenyl groups of diphenyl phosphine
oxide. The complete separation between HOMO and
LUMO levels is favorable for efficient hole and electron in-
jection and thus benefit to the electroluminescence perfor-
mance of the devices.
In conclusion, we rationally designed two new and simple
bipolar host materials, POBPCz and POBPtCz, based on the
biphenyl molecule. The molecular structure of POBPCz and
POBPtCz shows a twist configuration due to the steric
effect, and thus can minimize the p conjugation of biphenyl
molecule. Meanwhile, the hole-transporting carbazole group
and the electron-transporting diphenylphosphine oxide
group in the two molecules still retain their individual attri-
butes on account of their different spatial orientations. Both
POBPCz and POBPtCz show high triplet energies of about
3.0 eV and can be used as hosts to fabricate deep blue PhO-
LEDs. The FIr6-based deep blue device A with POBPCz as
host achieves a maximum current efficiency of 40 cdAÀ1,
a maximum power efficiency of 36 lmWÀ1, and a maximum
external quantum efficiency of 19.5%, which are among the
best results ever reported. The impressing efficiencies are
mainly attributed to the high triplet energies of hosts and
their bipolar carrier-transporting abilities.
The triplet energies of POBPCz and POBPtCz are high
enough to host the deep-blue phosphorescent emitter
[bis(4,6-difluorophenylpyridinato-N,C2’)]
ACHTUNGTRENNUNG
pyrazolyl)borate]iridiumACHTGNUTRENNUNG
2.72 eV);[14] hence we fabricated devices with the following
configurations: ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA
(5 nm)/POBPCz (device A) or POBPtCz (device B): FIr6
(20 nm)/Tm (5 nm)/TPBi (30 nm)/LiF (1 nm)/Al (120 nm).
The device structure and energy level diagram of materials
used in the devices are shown in Figure S4 in the Supporting
Information. In these devices, 1,4-bis(1-naphthylphenylami-
no)biphenyl (NPB) was used as the hole-transporting mate-
rial; hole-transporting material 4,4’,4’’-tri(N-carbazolyl)tri-
Acknowledgements
We thank the National Science Fund for Distinguished Young Scholars
of China (No. 51125013), the National Basic Research Program of China
(973 Program 2009CB623602 and 2009CB930603), the National Natural
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Organic Light-Emitting Diodes
COMMUNICATION
Figure 3. J–V–L characteristics of a) device A and b) device B. Current efficiency and power efficiency versus current density of c) device A (EL spec-
trum inset) and d) device B (EL spectrum inset).
Science Foundation of China (No. 90922020), the Fundamental Research
Funds for the Central Universities of China, and the Open Research
Fund of State Key Laboratory of Polymer Physics and Chemistry, Chang-
chun Institute of Applied Chemistry, Chinese Academy of Sciences for fi-
nancial support.
[3] a) C. Fan, Y. Chen, Z. Jiang, C. Yang, C. Zhong, J. Qin, D. Ma, J.
Mater. Chem. 2010, 20, 3232–3237; b) C. Fan, Y. Chen, P. Gan, C.
Yang, C. Zhong, J. Qin, D. Ma, Org. Lett. 2010, 12, 5648–5651; c) S.
Gong, Y. Chen, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 2010,
22, 5370–5373; d) Y. Tao, C. Yang, J. Qin, Chem. Soc. Rev. 2011, 40,
2943–2970; e) S. Gong, Q. Fu, Q. Wang, C. Yang, C. Zhong, J. Qin,
D. Ma, Adv. Mater. 2011, 23, 4956–4959; f) S. Gong, Y. Chen, J.
Luo, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Funct. Mater. 2011, 21,
1168–1178.
[4] a) Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin, D.
Ma, Angew. Chem. 2008, 120, 8224–8227; Angew. Chem. Int. Ed.
2008, 47, 8104–8107; b) C.-H. Chien, F.-M. Hsu, C.-F. Shu, Y. Chi,
Org. Electron. 2009, 10, 871–876; c) W-Y. Wong, C-L. Ho, Coord.
Chem. Rev. 2009, 253, 1709–1758; d) W-Y. Wong, C-L. Ho, J. Mater.
Chem. 2009, 19, 4457–4482; e) Y. Tao, Q. Wang, C. Yang, C. Zhong,
J. Qin, D. Ma, Adv. Funct. Mater. 2010, 20, 2923–2929; f) G. Zhou,
W-Y. Wong, S. Suo, J. Photochem. Photobiol. C 2010, 11, 133–156;
g) G. Zhou, W-Y. Wong, X. Yang, Chem. Asian J. 2011, 6, 1706–
1727; h) S. Gong, Y. Zhao, M. Wang, C. Yang, C. Zhong, J. Qin, D.
Ma, Chem. Asian J. 2010, 5, 2093–2099; i) S. Gong, X. He, Y. Chen,
Z. Jiang, C. Zhong, D. Ma, J. Qin, C. Yang, J. Mater. Chem. 2012,
22, 2894–2899.
Keywords: biphenyls · carbazoles · iridium · organic light-
emitting diodes · phosphine oxides
[1] a) M. A. Baldo, D. F. OꢁBrien, Y. You, A. Shoustikov, M. E. Thomp-
son, S. R. Forrest, Nature 1998, 395, 151–154; b) C. Adachi, R. C.
Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett. 2001, 79, 2082–2084; c) E. Holder,
B. M. W. Langeveld, U. S. Schubert, Adv. Mater. 2005, 17, 1109–
1121; d) S. Chen, L. Deng, J. Xie, L. Peng, L. Xie, Q. Fan, W.
Huang, Adv. Mater. 2010, 22, 5227–5239; e) Y. J. Cho, J. Y. Lee,
Adv. Mater. 2011, 23, 4568–4572.
[2] a) C. W. Tang, S. A. VanSlyke, J. Appl. Phys. 1987, 51, 913–915;
b) Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R.
Forrest, Nature 2006, 440, 908–912; c) H. Wu, G. Zhou, J. Zou, C.-
L. Ho, W.-Y. Wong, W. Yang, J. Peng, Y. Cao, Adv. Mater. 2009, 21,
4181–4184; d) G. Zhou, Q. Wang, C.-L. Ho, W.-Y. Wong, D. Ma, L.
Wang, Chem. Commun. 2009, 3574–3576; e) S. Reineke, F. Lindner,
G. Schwartz, N. Seidler, K. Walzer, B. Lussem, K. Leo, Nature 2009,
459, 234–238.
[5] a) S. Su, H. Sasabe, T. Takeda, J. Kido, Chem. Mater. 2008, 20,
1691–1693; b) F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C.
Hsieh, K.-W. Wang, P.-T. Chou, Adv. Funct. Mater. 2009, 19, 2834–
2843; c) C. Han, G. Xie, H. Xu, Z. Zhang, D. Yu, Y. Zhao, P. Yan,
Z. Deng, Q. Li, S. Liu, Chem. Eur. J. 2011, 17, 445–449; d) D. Yu, Y.
Zhao, H. Xu, C. Han, D. Ma, Z. Deng, S. Gao, P. Yan, Chem. Eur. J.
2011, 17, 2592–2596.
Chem. Eur. J. 2012, 18, 5510 – 5514
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5513

C. Yang, D. Ma et al.
[6] a) J. Li, P. I. Djurovich, B. D. Alleyne, I. Tsyba, N. N. Ho, R. Bau,
M. E. Thompson, Polyhedron 2004, 23, 419–428; b) X. Ren, J. Li,
R. J. Holmes, P. I. Djurovich, S. R. Forrest, M. E. Thompson, Chem.
Mater. 2004, 16, 4743–4747.
[7] a) S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho, S.-
F. Hsu, C.-H. Chen, Adv. Mater. 2005, 17, 285–289; b) M.-H. Tsai,
H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang, Y.-L. Liao, K.-
T. Wong, C.-I. Wu, Adv. Mater. 2006, 18, 1216–1220; c) P.-I. Shih,
C.-H. Chien, C.-Y. Chuang, C.-F. Shu, C.-H. Yang, J.-H. Chen, Y.
Chi, J. Mater. Chem. 2007, 17, 1692–1698.
[8] a) R. J. Holmes, B. W. D’Andrade, S. R. Forrest, X. Ren, J. Li, M. E.
Thompson, Appl. Phys. Lett. 2003, 83, 3818–3820; b) H. Fukagawa,
N. Yokoyama, S. Irisa, S. Tokito, Adv. Mater. 2010, 22, 4775–4778;
c) H.-H. Chou, C.-H. Cheng, Adv. Mater. 2010, 22, 2468–2471.
[9] P. J. S. Gomes, C. Serpa, L. G. Arnaut, J. Photochem. Photobiol. A
2006, 184, 228–233.
[13] Gaussian 09, Revision B.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V.
Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X.
Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Son-
nenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J.
Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A.
Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ꢂ.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford CT, 2009.
[10] a) R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong, J. J. Brown,
S. Garon, M. E. Thompson, Appl. Phys. Lett. 2003, 82, 2422–2424;
b) P. E. Burrows, A. B. Padmaperuma, L. S. Sapochak, P. Djurovich,
M. E. Thompson, Appl. Phys. Lett. 2006, 88, 183503.
[11] S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, F. Sato, Appl.
Phys. Lett. 2003, 83, 569–571.
[12] M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu, A. Mato-
liukstyte, J. Simokaitiene, S. Grigalevicius, J. V. Grazulevicius, C.-P.
Hsu, Adv. Mater. 2007, 19, 862–866.
[14] S.-H. Eom, Y. Zheng, E. Wrzesniewski, J. Lee, N. Chopra, F. So, J.
Xue, Appl. Phys. Lett. 2009, 94, 153303.
[15] a) S. Su, T. Chiba, T. Takeda, J. Kido, Adv. Mater. 2008, 20, 2125–
2130; b) Q. Wang, J. Ding, D. Ma, Y. Cheng, L. Wang, X. Jing, F.
Wang, Adv. Funct. Mater. 2009, 19, 84–95.
[16] S.-H. Eom, Y. Zheng, N. Chopra, J. Lee, F. So, J. Xue, Appl. Phys.
Lett. 2008, 93, 133309.
Received: November 24, 2011
Published online: March 20, 2012
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