N A N O E X P R E S S
Open Access
Solution-Processed Hybrid Light-Emitting
Devices Comprising TiO
2
Nanorods and
WO
3
Layers as Carrier-Transporting Layers
Tsung-Yan Tsai, Po-Ruei Yan and Sheng-Hsiung Yang
*Abstract
The goal of this research is to prepare inverted light-emitting devices with improved performance by combining titanium dioxide (TiO2) nanorods and tungsten trioxide (WO3) layer. TiO2nanorods with different lengths were
established directly on the fluorine-doped tin oxide (FTO) substrates by the hydrothermal method. The prepared TiO2nanorods with lengths shorter than 200 nm possess transmittance higher than 80% in the visible range.
Inverted light-emitting devices with the configuration of FTO/TiO2nanorods/ionic PF/MEH-PPV/PEDOT:PSS/WO3/Au
were constructed. The best device based on 100-nm-height TiO2nanorods achieved a max brightness of 4493 cd/
m2and current efficiency of 0.66 cd/A, revealing much higher performance compared with those using TiO2
compact layer or nanorods with longer lengths as electron-transporting layers.
Keywords: Titanium dioxide nanorods, Tungsten trioxide, Inverted light-emitting devices, Electron-transporting layers Background
Organic light-emitting devices (OLEDs) have made impressive progress during the past two decades due to high luminance efficiency, wide viewing angle, flexibility, and low cost [1, 2]. They also show highly potential applications in the areas of displays and solid-state lighting. Despite those advantages, there still exist many challenges for organic light-emitting devices to replace liquid crystal displays and inorganic light-emitting diodes due to shorter lifetime at high luminescence and sensitivity to oxidation and humidity.
Conventional light-emitting devices are usually con-structed with the configuration of anode/hole-transport-ing layer (HTL)/active layer/electron-transportanode/hole-transport-ing layer (ETL)/metal cathode. Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) is the most used material as HTL. However, the acidic nature of PEDOT:PSS can lead to the corrosion of the oxide anode; moreover, high-efficiency devices usually require highly active metals as the cathode, e.g., calcium or magnesium, which are easy to oxidize. In view of the
above reasons, inverted light-emitting devices have been proposed and drawn increasing attention mainly due to higher stability compared with conventional ones. The usage of high work function metals such as gold (Au) or silver (Ag) as anode and air-stable metal oxide materials as carrier-transporting layers consequently prolongs lifetime of inverted-type devices. The architecture of inverted light-emitting devices is constructed with the configuration of cathode/ETL/active layer/HTL/metal anode [3, 4]. Manyn-type metal oxide materials, including zinc oxide (ZnO), titanium dioxide (TiO2), and tin dioxide
have been used as ETL [5–7]. On the other hand, p-type materials such as vanadium oxide, nickel oxide, and tungsten trioxide (WO3) can be utilized as HTL [8–10].
Taking TiO2as an example, it is a well-known
semicon-ducting material that possesses high thermal stability, good electron-transporting property, large energy bandgap of 3.0–3.4 eV, and friendly to our living environment [11]. TiO2 is normally a white solid; however, it can be
transparent in thin film or in some nanostructured states. By changing the morphology of TiO2, it is expected that
the optical and/or electrical transporting properties of TiO2are also modified. In fact, nanostructured TiO2have
been applied in many areas, such as ultraviolet photo-detectors [12], OLEDs [13], organic solar cells [14], and
* Correspondence:[email protected]
Institute of Lighting and Energy Photonics, National Chiao Tung University, No. 301, Gaofa 3rd Road, Guiren District, Tainan 71150, Taiwan, Republic of China
© The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
photocatalytic water-splitting technology [15]. Morii et al. reported encapsulation-free hybrid organic/inorganic light-emitting diodes with the configuration of fluorine-doped tin oxide (FTO)/TiO2/F8BT/MoO3/Au [16], using
TiO2as an electron injection layer (EIL) and MoO3as a
hole injection layer (HIL). The device could be operated in air with a lower threshold voltage that provided similar luminance output compared with conventional devices using Ca/Al as cathode. Bolink et al. reported inverted light-emitting devices with the configuration of indium tin oxide (ITO)/ZnO/Cs2CO3/super yellow PPV/MoO3/Au
[17], using ZnO as an EIL and Cs2CO3as a hole-blocking
layer. High luminance and current efficiency of 12,000 cd/ m2 and 6.5 cd/A were achieved that is comparable with conventional devices. Park et al. proposed inverted light-emitting devices with the configuration of ITO/ZnO/ Cs2CO3/F8BT/PEDOT:PSS/Ag [18]. The Cs2CO3 layer
was spin-cast into a thin film from its solution, not by thermal evaporation. A max luminance of 3399 cd/m2and max current efficiency of 0.81 cd/A around 14–16 V were obtained. Song et al. further modified a device structure with the configuration of FTO/ZnO/FPQ-Br/F8BT/ MoO3/Au [19], using an ionic polyfluorene (PF) derivative
FPQ-Br tethering Br−counterions as the wetting agent to improve contact between inorganic ZnO and organic F8BT. Wen et al. employed a self-assembled monolayer PEDA-TMS to modify TiO2surface for tuning its
conduc-tion band to match the energy level of high-yellow phenyl-substituted poly(para-phenylenevinylene) copoly-mer (HY-PPV). Inverted devices with the configuration of ITO/TiO2/PEDA-TMS/HY-PPV/MoO3/Au/Ag were
fab-ricated, revealing much higher brightness of 3148 cd/m2 than that of the bare TiO2device [20]. From literature
sur-vey, we notice that the above metal oxide materials were prepared and utilized as a thin film; TiO2nanorods has
not been reported in the light-emitting area so far. In addition to TiO2and ZnO thin films, TiO2nanorods are
regarded as a potential candidate for the construction of light-emitting devices based on the following reasons. As mentioned in the previous part, TiO2is a cheap, thermally
stable, and non-toxic semiconducting material that is favorable for device fabrication and mass production. Moreover, nanorod-type TiO2can provide one-dimensional
pathways for carrier transportation. This is particularly beneficial for electrons to enter the device and to improve recombination rate of carriers. The transmittance of TiO2
in the visible region can also be tuned by experimental controls. From a scientific and industrial viewpoint, developing alternatives or new type of materials is an essential issue to expand the diversity of research instead of adopting limited materials.
In this research, we demonstrate the preparation and characterization of TiO2 nanomaterials, including
nano-particles, nanorods, and compact layer, which can be used
as an ETL for the fabrication of hybrid-inverted light-emitting devices. TiO2nanorods and nanoparticles were
prepared on the FTO substrates by the hydrothermal method without using either templates or seeds. This is because the FTO substrate also has the tetragonal rutile structure, and the lattice mismatch between the tetragonal FTO (a = b = 0.4687 nm) and the rutile TiO2 (a = b =
0.4593 nm) is only 2% [21]. To further improve device performance, ultra-thin layers of tungsten trioxide (WO3)
and ionic PF derivative are incorporated. WO3has been
reported to serve as the HIL for application in organic light-emitting and photovoltaic devices [22–24]. The ionic PF material carrying hexafluorophosphate (PF6−)
counter-ions as wetting agent was synthesized and demonstrated in this study. Inverted light-emitting devices with the con-figuration of FTO/TiO2 nanorods/ionic PF/MEH-PPV/
PEDOT:PSS/WO3/Au were fabricated and evaluated. The
illustration of the device structure is shown in Fig. 1a, and the energy level diagram of the whole device is illustrated
Fig. 1 a Device architecture and b energy level diagram of the whole device
in Fig. 1b. It is seen that electron injection from FTO to TiO2layer is undisturbed [25]. To overcome the large
en-ergy barrier between TiO2and the active layer MEH-PPV,
a thin layer of ionic PF was introduced; besides, this ionic PF also serves as the wetting layer to increase contact between inorganic TiO2 and organic MEH-PPV layers.
On the other hand, the valence band of WO3lies between
the highest occupied molecular orbital of PEDOT:PSS and the work function of gold electrode that is favored for hole injection from the anode [22]. The PEDOT:PSS layer is incorporated between WO3 and the active layer to
increase hole transfer. The recombination of electrons and holes in the active layer MEH-PPV results in electro-luminescence (EL) under bias operation. The features of inverted architecture and solution process for deposition of organic and inorganic layers in this study provide a promising way to low-cost manufacturing in the future.
Methods
Polymer Materials
The light-emitting polymer poly(2-methoxy-5-(2′-ethylhexy-loxy)-1,4- phenylenevinylene) (MEH-PPV) was synthesized according to the literature [26]. PEDOT:PSS aqueous solution (CleviosTM P VP AI 4083) was purchased from Heraeus Precious Metals GmbH & Co. KG. Titanium(IV) n-butoxide and Titanium(IV) chloride was purchased from Alfa. Ammonium hexafluorophosphate was purchased from Matrix Scientific. Hydrochloric acid
was purchased from ECHO Chemical. The solvents including methanol, acetonitrile, toluene, and ethanol were also purchased from ECHO Chemical. These reagents and solvents were used as received without further purification. The ionic PF carrying PF6− groups
was synthesized by ionic exchange from its precursor PF-Br according to the previous literatures [27]. The detailed synthetic procedure of this ionic PF is de-scribed as follows. To a solution of PF-Br (100 mg) in methanol (20 mL) was slowly added a solution of ammonium hexafluorophosphate (0.4 g, 4.8 mmol) in methanol (20 mL). The mixture was stirred at room temperature for 48 h, followed by removing the solvent by rotary evaporation. The previous procedure was repeated for 4 or 5 times to achieve high percentage of ionic exchange from Br−to PF6−. The final product was collected
and dried in an oven to give a yellow solid (90 mg, 75%). The chemical structures of the above materials are shown in Fig. 2a.
Preparation of TiO2Nanomaterials
The schematic illustration of the growth of TiO2
nanoparticles and nanorods on FTO substrates by the hydrothermal method [28] is shown in Fig. 2b, and the detailed preparation is described as follows. Thirty-seven percent Hydrochloric acid (20 mL) was added in 20 mL of deionized water and stirred for 10 min. Titanium(IV) n-butoxide (0.7 mL) was then added and stirred for an
additional 10 min. The solution mixture was transferred into the autoclave in which the FTO substrates were placed at an angle against the wall of the teflon-liner. The autoclave was placed in a preheated oven at 170 °C. The growth time was set to 100, 110, 120, and 130 min. TiO2nanoparticles were formed with the growth time of
100 min, while nanorods with lengths of 100, 200, and 300 nm were obtained with the growth time of 110, 120, and 130 min, respectively. The substrates were taken out, rinsed thoroughly with deionized water, and cal-cined at 450 °C in air for 1 h. The TiO2compact layer
was also prepared via the chemical bath deposition for comparison in this study [29]. A solution of 1 M TiCl4
in toluene (1 mL) was added in 4 mL of deionized water dropwise at 0 °C and stirred for 5 min. The FTO sub-strates were then immersed into this solution and placed in an oven at 70 °C for 1 h. The substrates were taken out, washed with deionized water and ethanol, and calcined at 450 °C in air for 1 h.
Fabrication of Inverted Light-Emitting Devices
Inverted light-emitting devices with the configuration of FTO/TiO2 nanomaterials/ionic
PF/MEH-PPV/PED-OT:PSS/WO3/Au were fabricated. The device
architec-ture is shown in Fig. 1a. The ionic PF in acetonitrile (1 mg/1 mL) was spin-coated on top of the TiO2layer,
followed by drying in a vacuum oven at 90 °C for 30 min. MEH-PPV film (150 nm) was spin-cast from its toluene solution (15 mg/1 mL) and dried in a vacuum oven at 90 °C for 30 min. PEDOT:PSS layer (50 nm) was spin-cast on top of MEH-PPV and baked in a vac-uum oven at 90 °C for 30 min. WO3layer (5 nm) was
prepared by spin-coating from its precursor solution (1 mg of W(OEt)5in 1 mL of anhydrous ethanol) and
placed in ambient environment for 15 min. Finally, Au electrodes were deposited by thermal evaporation at a base pressure of 10−6 torr. The active area of each de-vice is 4 mm2.
Characterization Methods
The transmission and absorption spectra of TiO2
nano-materials were measured with a Princeton Instruments Acton 2150 spectrophotometer. The surface morphology of the materials was studied using the tapping-mode Bruker Innova atomic force microscopy (AFM). Cross-sectional scanning electron microscopy (SEM) micro-graphs of devices were performed with a JEOL 6700 F SEM. X-ray diffraction (XRD) patterns were obtained from a Rigaku D/MAX2500 diffractometer. The electrical and emission characteristics of light-emitting devices were measured using an Agilent 4155C semiconductor analyzer and a calibrated silicon photodiode.
Results and Discussion
Morphological Studies of TiO2Nanomaterials
The SEM micrographs of TiO2 nanomaterials on the
FTO substrates are shown in Fig. 3. TiO2nanoparticles
under growth time of 100 min are sparsely dispersed on the surface of FTO, and the diameters of those TiO2
nanoparticles are estimated to be 20–30 nm, as verified in Fig. 3a and f. The growth and dispersion of TiO2
nanoparticles on the FTO is caused by the homogeneous nucleation of TiO2[30, 31]. When the growth time
pro-longs to 110 min, TiO2 nanoparticles can serve as the
seeds for further growth of nanorods as well as the heterogeneous nucleation of TiO2. It is seen that TiO2
nanorods are mainly grown upwards, with some nano-rods tilt on the FTO substrates. From the SEM side view in Fig. 3b–d, the lengths of TiO2nanorods are estimated
to be 100, 200, and 300 nm when the growth time is set to 110, 120, and 130 min, respectively. From the SEM top view in Fig. 3g-i, the diameters of TiO2nanorods is
determined in the range of 40–80 nm. The lengths and diameters are concluded to be dependent on the growth time [25], i.e., nanorods grow longer and wider as the growth time increases. Besides, the FTO substrates are not fully covered by TiO2 nanoparticles or nanorods
when the growth time is set shorter than 120 min. The SEM side view and top view images of TiO2 compact
layer are shown in Fig. 3e and j, respectively. Large amounts of crystalline nanoparticles with total thickness of 100 nm are clearly observed for TiO2 compact layer,
which is very different from those nanorods. The chem-ical stoichiometry of Ti and O atoms in the nanorods is examined with an energy dispersive X-ray spectroscopy (EDX) analyzer and shown in Fig. 3k. It indicates the presence of Ti and O with an atomic ratio of 1:2.25 that is in agreement with the stoichiometric composition of TiO2 [30]. The total surface area of TiO2nanorods are
supposed to be larger than the compact layer due to high aspect ratio of nanorods, which might show posi-tive effect on carrier injection and transportation. Due to different surface properties (TiO2 is hydrophilic and
organic materials are hydrophobic), an ionic PF was introduced between TiO2 and organic layer to serve as
the wetting layer. The AFM topographic images of TiO2
nanorods and compact layer are shown in Fig. 4. Rod-like and particle-like morphologies are observed for TiO2 nanorods and compact layer, with surface
roughness (Ra) of 26.8 and 17.7 nm, respectively.
Optical Properties of TiO2Nanomaterials
The transmission and absorption spectra of TiO2
nano-materials on the FTO substrates were measured and shown in Fig. 5. The transmittance of TiO2 nanorods
drops sharply in the range of 330–380 nm, arising from the nature of TiO2 absorption. For TiO2 nanoparticles,
Fig. 4 AFM topographic images of TiO2a nanorods and b compact layer
Fig. 3 Side view SEM images of TiO2a nanoparticles, b 100, c 200, and d 300-nm-height nanorods, and e compact layer; top view SEM images
compact layer, and nanorods with lengths of 100 and 200 nm, the transmittance reaches over 80% in the vis-ible range of 380–700 nm. As for TiO2nanorods with a
length of 300 nm, the transmittance is significantly lower than others due to longer height of nanorods and full coverage of FTO substrate. Nevertheless, moderate to high transparent property of TiO2nanomaterials is
suit-able for optoelectronic applications. The main absorp-tion band of TiO2is observed from 300 to 380 nm. The
absorption edges of TiO2 nanorods and compact layer
are found at 400 and 380 nm, respectively. The band gap of the TiO2nanorods and compact layer were then
calculated from their absorption edges to be were 3.10 eV and 3.25 eV, respectively, which is in good accordance with the previous reports [32].
XRD Patterns of TiO2Nanomaterials
The XRD patterns of the 300-nm-height TiO2nanorods
and 100-nm-thick compact layer on FTO substrate is presented in Fig. 6a, which is in good accordance with that of the rutile phase (JCPDS No. 88–1175) [30]. These diffraction peaks are sharp and strong, indicating high-degree crystallization of the prepared TiO2
nanomaterials in this research. The five main diffraction peaks located at 2θ = 36.07°, 41.24°, 54.33°, 62.75°, and 29.3° are assigned to (101), (111), (211), (002), and (112) planes, respectively [30]. The highly intense (101) peak along with the enhanced (002) peak in the nanorods re-veals that the rutile crystal grows with (101) plane paral-lel to the FTO substrate, and the nanorods are oriented along the (002) direction [33]. The diffraction peaks from the FTO substrates are also indicated in Fig. 6. Only two diffraction peaks assigning to (101) and (211) planes can be observed for TiO2compact layer. The
dif-ference in XRD intensity between TiO2 nanorods and
compact layer arises from different thickness of the layer. Besides, the mobility of rutile TiO2nanorods is
re-ported to be 1 cm2/Vs, which is two-order higher than that of TiO2 nanoparticles layer [33, 34]. The higher
electron-transporting properties brought by the nanorod form are beneficial for device performance.
Device Performance Based on TiO2Nanomaterials
Inverted light-emitting devices with the configuration of FTO/TiO2/ionic PF/MEH-PPV/PEDOT:PSS/WO3/Au
were fabricated and evaluated, using TiO2 as
electron-transporting layer, ionic PF as wetting layer, MEH-PPV as active layer, PEDOT:PSS as hole-transporting layer, and WO3as hole-injecting layer. The brightness-voltage
(B-V), current efficiency-current density (E-J) character-istics, and EL spectra of all devices are depicted in Fig. 7a–c. The overall device performance based on different types of TiO2is summarized in Table 1. The max
brightness and current efficiency of the device based on TiO2 nanoparticles reached 333 cd/m2 and 0.08 cd/A,
respectively. By using TiO2 compact layer as an ETL,
the device performance was promoted with max bright-ness of 790 cd/m2 and current efficiency of 0.15 cd/A.
(a)
(b)
Fig. 5 a Transmission and b absorption spectra of TiO2nanoparticles,
nanorods with a height of 100–300 nm, and compact layer
Fig. 6 XRD patterns of 300-nm-height TiO2nanorods and 100-nm-thick
The reason to the improved performance is due to full coverage of FTO surface by the TiO2 compact
layer that reduces charge trapping and quenching on FTO anode. Turing to TiO2 nanorods, the max
brightness achieved 4493, 2589, and 1090 cd/m2for the devices based on 100, 200, and 300-nm-height nanorods, respectively. Furthermore, the current efficiency was de-creased from 0.66 to 0.05 cd/A as the length of TiO2
nanorods was increased. The reason to this phenomenon can be explained as follows. First, TiO2 nanorods with
length of 100 nm own the highest transmittance among three lengths of nanorods, which is beneficial for light
output. Second, 100-nm TiO2 nanorods provide shorter
pathways for carrier injection to the active layer to generate light. We also notice that the devices based on TiO2nanorods revealed much better device performance
than others using TiO2nanoparticles or compact layer as
an ETL. This is due to higher mobility of TiO2nanorods
that is favored for carrier transport than the other two TiO2nanomaterials, as described in the previous section.
Figure 7c shows the original EL spectra of the inverted devices based on TiO2 nanomaterials operated at 12 V.
The emission maximum wavelength and shoulder emission are located at 588 and 630 nm, respectively,
(a)
(c)
(d)
(b)
Fig. 7 a B-V, b E-J, and c EL spectra of inverted light-emitting devices based on TiO2nanoparticles, nanorods with a height of 100–300 nm, and
compact layer. d Snapshot of the device based on 100-nm-height TiO2nanorods driven at 15 V
Table 1 Performance of inverted light-emitting devices based on TiO2nanomaterials
TiO2type Turn-on voltage (V) Max brightness (cd/m2) Max current efficiency (cd/A)
Compact layer 14 790 0.15
Nanoparticles 13.5 333 0.08
Nanorods (100 nm) 4.5 4493 0.66
Nanorods (200 nm) 5.0 2589 0.15
revealing an orange-red light. A very bright inverted light-emitting device based on 100-nm-height TiO2 nanorods
under driving bias of 15 V is shown in Fig. 7d. Figure 8a shows the relationship between the transmittance of TiO2
nanorods and the brightness of devices. The value of transmittance was chosen at 588 nm, since the device emitted light at the same wavelength. It is seen that TiO2
nanorods with higher transmittance bring higher brightness. To clarify the relationship between nanorod length and transmittance/brightness, the characteristics transmittance-nanorods and length-brightness are depicted and shown in Fig. 8b. As the nanorod length is increased, both transmittance and brightness are decreased. It is noted that the transmittance as well as the brightness of the devices in the whole visible region should be taken into account to completely understand the transmittance effect on the brightness of the devices, not only at 588 nm. Figure 9 shows the luminance decay curve as a function of time for the device based on 100-nm-height TiO2
nano-rods. The device was monitored at a constant voltage of 10 V in ambient environment without encapsulation. The lifetime of the device is defined as the time when the luminance is decreased to a half of its initial luminance,
which is determined about 40 h. The stability test of inverted OLEDs is seldom reported in the literature. G. He et al. reported the inverted OLEDs with the configuration of ITO/Cs2CO3/Bphen/Alq3/NPB/MoO3/Au [35]. The
de-vice lifetime was only 20 h when using Cs2CO3as the EIL.
By inserting a thin layer of aluminum between ITO and Cs2CO3, the luminance of the device decayed to 80% of its
initial value during an operation time of 70 h. In our case, the usage of TiO2nanorods as the ETL can provide
moder-ate stability in ambient condition. To the best of our know-ledge, this is the first demonstration of inverted light-emitting devices using TiO2nanorods as ETL. These results
suggest that TiO2nanorods may possess potential use in
light-emitting applications.
Conclusions
TiO2 nanoparticles with diameter of 20–30 nm and
nanorods with different lengths of 100–300 nm were prepared by the hydrothermal method. TiO2 compact
layer was prepared by chemical bath deposition for comparison. The results showed that rutile TiO2 was
obtained, and nanorods with shorter length of 100– 200 nm showed higher transmittance in the visible range of 380–700 nm. Inverted light-emitting devices using different TiO2nanomaterials as ETL were fabricated and
compared. The device based on 100-nm-height TiO2
nanorods revealed the best device performance among all devices, with a max brightness and max current efficiency of 4493 cd/m2and 0.66 cd/A, respectively.
Abbreviations
AFM:Atomic force microscopy; B-V: Brightness-voltage; EDX: Energy dispersive X-ray spectroscopy; EIL: Electron injection layer; E-J: Current efficiency-current density; EL: Electroluminescence; ETL: Electron-transporting layer; FTO: Fluorine-doped tin oxide; HIL: Hole injection layer; HTL: Hole-transporting layer; ITO: Indium tin oxide; MEH-PPV: Poly(2-methoxy-5-(2′ ethylhexyloxy)-1,4-phenylenevinylene); OLEDs: Organic light-emitting devices;
(a)
(b)
Fig. 8 a Relationship between transmittance of TiO2nanorods and
brightness of devices; b characteristics of transmittance of TiO2nanorods
and brightness of devices as a function of TiO2nanorod length
Fig. 9 Luminance decay curve of the device based on 100-nm-height TiO2nanorods as a function of time
PEDOT:PSS: Poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate); SEM: Scanning electron microscopy; XRD: X-ray diffraction
Acknowledgements
The authors thank the Ministry of Science and Technology (MOST 104-2221-E-009-187) of the Republic of China for the financial support of this work.
Funding
This work was supported by the Ministry of Science and Technology of the Republic of China under Contract No. MOST 104-2221-E-009-187.
Authors’ Contributions
TYT proposed the research idea, performed the experiments, and drafted the manuscript. PRY participated in the sample preparation. SHY contributed to the data interpretation and the manuscript writing and supervised the research. All authors read and approved the final manuscript.
Competing Interests
The authors declare that they have no competing interests.
Ethics Approval and Consent to Participate Not applicable.
Received: 11 September 2016 Accepted: 14 November 2016
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