Graphene Oxide

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Cationic Water-Soluble Conjugated Polyelectrolytes/ Graphene Oxide Nanocomposites as Efficient Green Hole Injection Layers in OLEDs Afsoon Fallahi, Masoud Alahbakhshi, Ezeddin Mohajerani, Faramarz Afshar Taromi, Alireza Mohebbi, and Mohsen Shahinpoor J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00863 • Publication Date (Web): 20 May 2015 Downloaded from http://pubs.acs.org on May 22, 2015

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The Journal of Physical Chemistry

Cationic Water-Soluble Conjugated Polyelectrolytes/ Graphene Oxide Nanocomposites as Efficient Green Hole Injection Layers in OLEDs

Afsoon Fallahia,d, Masoud Alahbakhshib, Ezeddin Mohajeranib*, Faramarz Afshar Taromia, Ali Reza Mohebbic, Mohsen Shahinpoord a

Department of Polymer Engineering and Color Technology, Amirkabir University of Technology (Tehran

Polytechnic), 424 Hafez Avenue, P.O. Box 15875-4413, Tehran, Iran b

Laser and Plasma Research Institute (LAPRI), Shahid Beheshti University, Tehran, Iran,

[email protected] c

Department of Chemistry, Northeastern Illinois University, Chicago, IL 60625.

d

Department of Mechanical Engineering, University of Maine, Orono, ME 04469-5711, USA

1 Environment ACS Paragon Plus

*

Email: e-


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ABSTRACT The current research presents using a nanocomposite comprising of a cationic conjugated polyelectrolyte

(CPE),

Poly[(2,5-bis(2-(N,N-diethylammonium

bromide)ethoxy)-1,4-

phenylene)-alt-1,4-phenylene] or (PPPNEt2.HBr), with graphene oxide (GO) as a new hole injection layer (HIL) for organic light emitting diodes. It is demonstrated that using the designed ionically functionalized water soluble conjugated polymers instead of polyethylene dioxythiophene: polystyrene sulfonate (PEDOT:PSS) is a promising approach to overcome strong acidic nature of PEDOT:PSS besides excluding its non-conductive PSS part. As the other aspiration of this work, we introduce a good partner for dissolving and spin-casting of GO as a simple and economic technique to use the hole conductive and electron blocking nature of GO in hole injection portion of assembled devices. Using this new binary blend showed enhanced charge carrier mobility, good electroluminescent and J-V characteristics in comparison with the conventional devices. Such improvement is interpreted with induced ion space charge of HIL at the interface and resulting electric field screening effect due to ion migration.

INTRODUCTION Conjugated polymer semiconductors offer advantageous properties such as flexibility, low weight, good processability in solution that make them useful for optoelectronic and electrochemical devices including polymer light-emitting diodes (PLEDs),1,2 polymer field effect transistors (PFETs),3-7 supercapacitors, chemical and biochemical sensors, photodetectors, polymer photovoltaic cells (PPVCs)8-13 and chemo14,15 and biosensors.16 In order to design efficient electronic devices, it is necessary to tune their properties, such as their band gap, molecular energy level, solubility, absorbance, luminescence, and so on. An ideal organic light emitting diode (OLED) device needs to apply easily processable polymers with high fluorescence efficiency, effective and balanced injection of carriers and sufficient high stability under loading bias.17-19 As such, considerable research has been devoted to design and test new kinds of polymeric hole and electron injection materials as well as polymers used in the emissive layer.20,21 Moreover, direct hole-injection from ITO is not efficient


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due to the energy level disparity at the interface.22 Therefore, high operating voltages are needed to overcome the injection barrier, resulting in reduced efficiency. Various hole injection layers (HILs) have been incorporated at the ITO-organic interface to improve HI with a view of energy level matching, such as polyethylene dioxythiophene: polystyrene sulfonate (PEDOT:PSS).23 PEDOT:PSS can cause degradation in OLED devices due to its acidic nature and the presence of moisture and non-conductive PSS part, leading to reduced device lifetime. The high acidity of PEDOT:PSS (pH=1-2) is believed to etch the surface of ITO liberating oxygen and metal ions, contaminating emissive layer and reducing the efficiencies and lifetime. Moreover, to achieve a high enough conductivity, the transparency of the polymer film is often compromised which is another problem of using PEDOT:PSS.24-26 Among different approaches, using ionically functionalized water soluble conjugated polymers27 (also known as conjugated ionomers28,29 or conjugated polyelectrolytes (CPEs)30 ) instead, have attracted special attention in recent years.31 These helical π-conjugated polymers32,33 have been of interest, which is because of their polarized photo- and electroluminescence34,35 as well as enantioslective sensing,36-38 compatibility with biological systems and allowing the development of new biosensors 39,40 even in considerably low chemical and biological analytes concentrations based on the superquenching mechanism.41-44 A lot of CPEs have been reported in fabricating OLED45,46 and solar cell devices47-52 including water-soluble

polythiophenes,53,54

poly(p-phenylene),55,56

poly-(phenylenevinylene),57,58

poly(phenylene ethylene)59,60 and polyfluorene derivatives.61-65 Briefly, several of these studies are about using CPEs in electron injection layers (EILs).66-69 The CPE function in these OLEDs is that of electron injection/transport layers (EILs/ETLs) and allows the use of stable metals as cathodes, potentially simplifying the encapsulation process.70,71 This study reports using a water soluble poly(p-phenylene) or PPP72,73 for the first time as HIL in OLED devices. The most important criteria for a HIL are: The HOMO of HIL must be at an energy level close to the emissive semiconducting polymer and the solvent used for casting must not dissolve next emissive polymer. In this view, the developed polymers have environmental friendly watersoluble nature which eliminate the problem of interface interaction with other layers which are dissolved in organic solvents. In this contribution, a CPE namely [Poly[(2,5-bis(2-(N,N-diethylammonium bromide) ethoxy)1,4-phenylene)-alt-1,4-phenylene] (PPPNEt2.HBr)], Scheme S1(3), in blend with GO as dopant



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is proposed as a new HIL for OLED devices. Our recent study74,75 has revealed that using graphene oxide in HIL enhances efficiency of OLED device due to its remarkable properties including high electrical and proton conductivity, mechanical strength, excellent chemical stability and effective tuning of work function. To improve device performance using GO in HIL, we need a water-soluble polymer containing ionic and/or polar bonds for ionic and hydrogen bonding formation which PPPNEt2.HBr would be a good candidate.76 It is demonstrated that such a blend of GO and a water soluble PPP derivative is promising to be used between the emitting layer and the ITO anode, leading to a significant enhancement in device performance. A possible mechanism about this enhancement is attributed to the migration of mobile positive ions under the applied bias. So, the accumulation of positive ammonium ions besides injected holes near the emitting layer induced ion space charge in the emissive layer interface. As a result, the effective injection barrier could be reduced by large space charge between the emitting layer and Ag cathode with modified electron injection and recombination at the interface. This is a kind of electric field screening effect due to ion migration.

EXPERIMENTAL SECTION Materials and Methods All solvents, starting materials and 1,4-dibromo-2,5-dimethoxybenzene, 1,4-benzene diboronic acid were obtained from Aldrich Chemical Co. and were used without further purification. [Poly[(2,5-bis(2-(N,N-diethylammonium bromide) ethoxy)-1,4-phenylene)-alt-1,4-phenylene] was purchased from Aldrich, 678074. Thermal properties were measured by using differential scanning calorimetry (DSC) -Perkin Elmer Pyris Diamond- in alumina caps under a nitrogen flow at a scan rate of 15°C min−1 over the temperature range of 35 to 430°C. Thermo gravimetric analysis (TGA) was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT) to monitor sample weight loss as a function of temperature. The heating rate was 10°C/min ranging from 50 to 450°C under nitrogen atmosphere to avoid oxidization. FTIR spectra were recorded on a Bruker Tensor 37 spectrometer in its high resolution reflection mode for thin films. Thickness measurements were performed by DekTak 8000; UV-vis, EL and Photoluminescence (PL) of fabricated OLEDs were performed by


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USB2000 and HR4000 Ocean Optics. The current–voltage–luminance characteristics by Keithley source meter 2400 model and optical meter Mastech-MS6612, respectively. Conductivity was measured by a four point probe setup using a source measurement unit (Keithley 2400). All glass ITO substrates were washed in solvents and detergent baths, and UV-ozone treatment. Briefly, Indium tin oxide (ITO)-coated glass substrates (Sigma Aldrich) were cleaned sequentially in ultrasonic bathes of phosphate-free detergent, water, iso-propanol, water, and acetone, and then dried at 60 °C in a vacuum oven for an hour. After drying, the substrates were exposed to UV ozone, and the spin-coating process followed immediately after the UV ozone cleaning. The substrates were spin-coated with two HILs at 3000 rpm: 70 nm poly[3,4-ethylene dioxythiophene] blended with poly[styrene sulfonate] (PEDOT:PSS) (Sigma-Aldrich) or Poly[(2,5-bis(2-(N,N-diethylammonium

bromide)ethoxy)-1,4-phenylene)-alt-1,4-phenylene]

(PPPNEt2.HBr) or (PPPNEt2.HBr: GO); 30nm of hole- transporting layer (HTL) poly[Nvinylcarbazole] (PVK, Aldrich, Mw =1,100,000). The Alq3 was evaporated directly onto the PVK. The 65 nm aluminum tri-8-hyroxyquinoline (Alq3, sublimed grade, Sigma Aldrich) was deposited onto the thin films at pressures below 10-5 mbar.77-79 Finally, the cathode was made by thermally evaporating 150 nm films of aluminum under the same conditions. Al was evaporated through a shadow mask of the resultant devices with the architecture of: ITO/HIL/PVK/Alq3/Al.

RESULTS and DISCUSSION The cationic polymer, PPPNEt2.HBr, is freely soluble in water, DMSO and methanol. For full synthesis mechanism please follow reference 80 and Scheme S1. To fabricate thin films comprising PPPNEt2.HBr and GO, we synthesized and characterized GO based on our previous report81 and blended 0.05 wt.% GO into the PPPNEt2.HBr solution.82-84 After sonicating the blend for 15 minutes and preparing spin-coated solid-state films (3000rpm, 30sec), PPPNEt2.HBr:GO thin films were annealed up to 120°C. GO is reduced in this temperature to some extent. All resistances [Ω/sq] were measured by four-point probe method. For all composites of different weight ratios, before and after thermal treatment, sheet resistances were greater than 10 MΩ/sq; beyond the measurable range of our setup.



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Figure 1 shows the UV-vis absorption and PL emission spectra of PPPNEt2.HBr and its composite with GO. HOMO and LUMO levels of the compounds are calculated from Figure S2 and absorption edges in Figure 1a. For PPPNEt2.HBr, the onset of the absorption represents the low-energy edge in solid film absorption spectrum (Figure 1a). The observed band gap of the polymer estimated from the onset of the absorption (391 nm) is 3.17 eV and for its blend with GO (5mg/ml) is about 3.14eV (395nm). As can be observed from Figure 1b, this polymer is really transparent in comparison with PEDOT:PSS without significant absorption in the visible light region. Even after doping 0.05 wt.% GO into the polymer, there is a slight reduction in transparency around 1-2%, still with more transparency than PEDOT:PSS. Therefore, it can be a good candidate among polymeric backbones as HIL in OLED devices. To compare with glass ITO transparency, please see Figure S1 in the Supporting information. As illustrated in Figure 2, PPPNEt2.HBr higher LUMO value (in comparison with PEDOT:PSS) results in promoted electron blocking effect in HIL. Also with the introduction of 0.05 wt.% graphene oxide in the PPPNEt2.HBr, the energy barrier between ITO work function and HIL HOMO lowered which facilitates the injection of holes from anode and led to the reduction of the turn-on voltage of corresponding device in addition to ions movements. The thermal properties of the polymers were investigated by TGA and DSC at a heating rate of 10 °C/min under nitrogen atmosphere. As illustrated in Figure 3, polymer showed good thermal stability and the onset degradation temperature was in the range of 250–271°C. This decomposition corresponds to the degradation of the alkyl groups on the polymer backbone which will degrade completely at around 370°C. According to DSC thermogram, there is a visible phase transition around 100-130°C. Hence, we used 120°C as its post-treatment temperature for curing hole injection spin-coated layer. Conjugated polymers consist of two types of regions: the ordered regions have metallic conductivity and the disordered ones exhibit hopping type conductivity. In such a model, the charges can travel successively over macroscopic distances across both regions in the sample. The hole conductivity of the blend is also attributed to the ability of GO as a dopant to interact with polymer chain through van der Waals forces and form hydrogen bonds; according to this hypothesis, the hole conductivity in this blend can be described as a sum of following terms: hopping process, ion movements and minor r-GO metallic regions.85-87


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As shown in Figure 4, the blend shows effective hydrogen bonding resulting in better solubility of GO in electrolyte and a more uniform solution in aqueous medium. As a result, rings are twisted into a more planar conformation which leads to an increased conjugation length and red shifts in EL spectra. Also there is possibility of constituting π-π stack bondings between the water soluble polymer and GO with more efficient carrier injections due to better film interface forming and better hopping. PPPNEt2.HBr:GO binary blend is a kind of geometric reorientation of polymer backbone and adjustment of conjugation length to a more expanded conformation. FT-IR experiments were carried out to investigate the interaction between GO and conjugated polymer. The FT-IR spectra of PPPNEt2.HBr, GO and their composite are depicted in Fig. 5. The characteristic absorption bands related to GO are observed at 1230 cm-1 (epoxy C–O), 1070 cm-1 (alkoxy/alkoxide C–O) and 1740 cm-1 (carboxyl C=O).88,89 The weaken carboxyl C–O stretch, combined with the shifting of alkoxy/alkoxide C–O stretches to lower frequency, indicate that the hydrogen bonding is formed between oxygen-contained groups from GO and PPPNEt2.HBr. According to Figure 6, under an applied external voltage to the fabricated OLED devices, anions drift to the positive electrode and cations to the interface of HIL and emitting layer until drift and diffusion currents are equalized. Because of the size of ionic charges in a typical device, small movements of these ions can give rise to very large electric fields.90 Under conditions of constant applied bias, ionic charge redistribution occurs throughout the bulk of the polyelectrolyte thin film. Just after cancelling the local electric field, a limited electric field can only be sustained at the interface of the emitting layer, and the motion of Br ions will be blocked by ITO electrode side. This mechanism can help for higher hole injection and therefore, better efficiencies and superior hole-electron recombination at the interfaces under very low bias voltages. Besides, its lower acidic nature in comparison with PEDOT:PSS could be considered as an alternative approach to achieve

better film properties, higher HIL lifetime and overcome the degradation of the devices.91,92 (Figure S4) The electroluminescence spectra of these devices are shown in Figure 7. Based on the concentration of PPPNEt2.HBr, a small shift can be observed in EL spectra as described before. Just in the case of 3mg/ml, the hole injection was not sufficient enough to have an emission in its OLED device.



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According to J-V curves in Figure 7, it is observed that hole injection from anode in 3mg/ml was the best but there were no detectable EL for the device which may because of the diminished hole concentration in this layer and poorer mobility from anode. Figure 7 showed EL spectra of 520, 528 and 540 nm for 5, 7 and 10 mg/ml of PPPNEt2.HBr as HIL, respectively. Based on the least EL red shift in the sample of 5mg/ml and its lower turn-on voltage (3.5 V) in comparison with others (~4.8 V), this concentration was selected as an optimized for preparing the blend for the next device. To compare the efficiency of our devices, the reference OLED device is shown in Figure 8. The previous typical device was composed of following layers: ITO/PEDOT:PSS/PVK/Alq3/Al, with turn-on voltage of 8 volts and maximum EL of 519nm. Using GO as dopant can solve the problem of barrier height with optimizing the HOMO level of the blend. Based on EL intensities and turn on voltages, we had selected polymer with 5mg/ml. Here is the comparison between J-V of

this

device

(ITO/

HIL

/PVK/Alq3/Al)

with

our

reference

device

(ITO/PEDOT:PSS/PVK/Alq3/Al). Based on the reference device data shown in Figure 8, the EL of PPPNEt2.HBr+GO blend system vs. PEDOT:PSS and pure PPPNEt2.HBr showed a red shift of about 8nm without significant exciton-polaron quenching or excimer formation. Based on the J-V characteristics of PPPNEt2.HBr blend OLED shown in Figure 8, lower driving voltages of ~ 3.5 V was observed versus pure Alq3 OLED with ~ 8V. However, with increasing applied voltage, a significant increase in current density of blend OLED observed which was much faster than those of pure Alq3 OLED device. For instance, the driving voltage at a current density of 50 mA cm-2 is 12.5 V for PPPNEt2.HBr blend OLED while for PEDOT:PSS OLED is 15 V. Therefore, it is exhibited that the J-V curve has a tendency to increase with doping GO. GO doped PPPNEt2.HBr layer can create a blocking layer against singlet exciton quenching as well as improving hole injection and electron blocking. To ensure that the improved performance is from the blend itself, a device composed of a pure GO layer (0.05 wt.%) was fabricated to compare against the polyelectrolyte-GO blend. This device did not work at all. These findings imply that the carriers are confined into the EML interface after an efficient injection, resulting in high exciton formation efficiencies even in high current densities. The presented results demonstrate that devices with PPPNEt2.HBr have significantly lower turn-on voltages, and good luminous efficiency with respect to those with PEDOT:PSS. Using x, y as the coordinates (reported in Table 1), a two-dimensional chromaticity diagram (the CIE 1931 color space


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diagram) can be plotted as shown in Figure 9. It can be observed that all emissions are in yellow green part of the diagram.

CONCLUSIONS Here, we have successfully used the water soluble PPPNEt2.HBr, and its blend with GO as a new potential hole injecting candidate and electron blocking layer to be used in OLED devices. This layer will not damage or mix with beyond organic soluble polymer film. In fact, this hydrophilic layer can show a super wetting feature for the next hydrophobic layer. The devices produced a substantially pure and strong EL. Due to the hydrogen bonding nature of the PPPNEt2.HBr +GO blend, making better solubility and more homogeneous solution and also ion migrations through HIL, devices can offer better charge carrier mobilities and enhanced recombination at their interface which will lower the driving-voltages dramatically. As a simple strategy, GO is a promising material to use in HIL to maximize hole injection. Electrons can be blocked and singlet exciton quenching can be reduced by this new HIL resulting in more efficient radiative recombination between holes and electrons inside the emitting layer. We believe although this strategy is both simple and effective, but should be explored further.

♣ Acknowledgment All supports from the Research Centers at Amirkabir University of Technology and Shahid Beheshti University are gratefully acknowledged.

♣ Supporting Information Description Transmission spectra and cyclic voltammograms, PPPNEt2.HBr synthetic route, pH comparison. This material is available free of charge via the Internet at http://pubs.acs.org.

♣ Author Information Corresponding Author * Email: [email protected]

Notes



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The authors declare no competing financial interest.

Abbreviations GO, Graphene oxide; OTFT, Organic thin film transistor; OLED, Organic light emitting diode; EL, Electroluminescence; PL, Photoluminescence; CV, Cyclic voltammetry.

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♣ Table of Contents Graphic and Synopsis:

This study presents using an innovative water-soluble hole injection layer consisting of a cationic conjugated polyelectrolyte (CPE) and graphene oxide (GO) with enhanced hole mobility and reduced turn-on voltages in comparison with PEDOT:PSS.



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♣ Tables Table 1. Summary of device performances with different HILs

Table 1.

Anode/HIL

ITO/PEDOT:PSS ITO/

PPPNEt2.HBr

(3mg/ml) ITO/

PPPNEt2.HBr

(5mg/ml) ITO/

PPPNEt2.HBr

(7mg/ml) ITO/

PPPNEt2.HBr

(10mg/ml) ITO/

PPPNEt2.HBr:

(5mg/ml: 0.05wt.%)

GO

Work

Max.

Max

functions V turn-on

Luminance CIE (X,Y)

EL

(eV)

(cd m-2)

(nm)

4.7/5.2

8

3000

(0.31,0.52)

519nm

4.7/5.39

3.8

---

---

---

4.7/5.39

4.7

2000

(0.32,0.51)

520nm

4.7/5.39

4.8

1800

(0.35,0.53)

528nm

4.7/5.39

4.8

2200

(0.41,0.47)

540nm

4.7/5.35

3.5

3500

(0.40,0.49)

528nm


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♣ Figures & Captions Figure 1. a) Transmittance, UV-vis absorption and PL spectra for PPPNEt2.H2O solution. b) Transmittance comparison between PEDOT:PSS, PPPNEt2.HBr and PPPNEt2.HBr:GO thin films (thickness: 50nm). Figure 2. Schematic energy level diagram for the PPPNEt2.HBr/graphene nanocomposite in comparison with pure PPPNEt2.HBr and PEDOT:PSS. Figure 3. DSC diagram of PPPNEt2.HBr, Inset: The thermogravimetric analysis (TGA) profile of PPPNEt2.HBr with a heating rate of 10°C/min in air. Figure 4. (a) PPPNEt2.HBr dissolved in water-methanol solvent with the concentrations of: (1)3, (2)5, (3)7 and (4)10 mg/ml. (5) the optimized concentration of PPPNEt2.HBr (5mg/ml) +GO (0.05wt.%) binary blend; (b) Device scheme and 3D schematic picture of hydrogen bond forming between GO and PPPNEt2.HBr in aqueous medium. Figure 5. FTIR spectra of GO, PPPNEt2.HBr and PPPNEt2.HBr +GO. Figure 6. (a) A classic OLED device with PEDOT:PSS HIL; (b) The new OLED device with CPE HIL and (c) The proposed ion and charge migration diagram for the designed OLED device. Positive ionic space charge accumulates close to the emitting layer interface. Negative ionic space charge accumulates close to the anode. This redistributes the electric field away from the bulk of the HIL towards the interfaces and Br ions may act as electron blocking fractions. The steady-state shape of the barriers for hole injection can be set by solutions to Poisson’s equation and Boltzmann statistics.93-95 Figure 7. (a) EL and (b) J-V curves for different molar ratios of PPPNEt2.HBr as HIL. Figure 8. (a) Normalized EL intensities for devices with PPPNEt2.HBr, PPPNEt2.HBr+GO and PEDOT:PSS as HILs, [Inside: Photographs showing the color of solutions of pure PPPNEt2.HBr (left) and PPPNEt2.HBr+GO stable blend after 4 days (right) under ambient light]; (b) J-V curves for

reference

device

ITO/PEDOT:PSS/PVK/Alq3/Al

in

comparison

with

PPPNEt2.HBr+GO/PVK/Alq3/Al. All spectra are shown in Figure S3 for comparison. Figure 9. Comparison between devices CIE chromaticities in the CIE diagram


ITO/


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Figure 1.


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Figure 2.



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Figure 3.


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Figure 4.



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Figure 5.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6.


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Figure 7.



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Figure 8.


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Figure 9.


Graphene Oxide

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