Functionalization of mild oxidized graphene with O-phenylenediamine for highly thermally conductive and thermally stable epoxy composites

Abstract

In this study, an effective and novel method was developed to improve the thermal conductivity of epoxy composites by functionalization of graphene. The functionalization of graphene was carried out with a simple refluxing method using a double N-precursor with O-phenylenediamine (OPD), in which the graphene was first treated with acid (2 : 6 molar H₂SO₄ : HNO₃) to form oxygen containing groups on the graphene surface (O-graphene). Amidation and nucleophilic addition reactions through amine groups in OPD contributed significantly to the doping of nitrogen into the graphene layers. The OPD functionalized graphene (OPD-f-graphene) was highly effective and compatible with an epoxy matrix, resulting in homogenous dispersion of a filler in the matrix. The in-plane and through-plane thermal conductivity of the functionalized graphene filled epoxy composite (fG–epoxy) was significantly increased ∼13 fold and ∼4.8 fold, respectively, in comparison to the neat epoxy composite (G–epoxy) with the addition of 6 wt% of the filler. This improvement in thermal conductivity was attributed to better dispersion of the filler into fG–epoxy which generated phonon conduction pathways.

---

1 Introduction

As the demands of high density electronic devices are increasing, the use of thermally conductive polymers is intensifying for heat dissipation applications.¹ A major drawback of modern electronic devices is the need for removal of excess heat during their operation, which can deplete their performances and stabilities. The use of high cost heat sinks is a common solution but they are often subject to micro-cracking that is induced by thermal fatigue.² Recently, the use of thermally conductive polymers in thermal devices has been increasing due to their low cost, light weight, good mechanical properties and excellent chemical resistance.^3–5 The thermal conductivity of the polymers is usually very low, which limits their potential applications. This provokes a need for highly thermally conductive polymers with low cost and ease of fabrication. Polymers with thermally conductive fillers are one of the most effective ways to deal with thermal management issues.⁶ Various fillers like fibers (carbon nanotubes and carbon nanofibers) and particles (alumina, magnesium oxide, boron nitride and graphene) have been used to enhance the thermal properties of the polymer. Earlier reports showed that different inorganic fillers like boron nitride (BN),⁷ aluminum oxide (Al₂O₃),^8,9 aluminum nitride (AlN),^8,10 and metal particles such as copper¹¹ and silver¹² have been incorporated into polymer matrices to improve their thermal conductivity. However, these fillers are expensive and dense, so this limits their use in lightweight applications, and they rely on large amounts of fillers to achieve a thermal conductivity of 1–2 W m⁻¹ K⁻¹.

In the last few decades, carbon materials have widely become known as next generation materials due to their exceptional properties. Due to their high thermal conductivity at room temperature and low density, along with good mechanical properties, carbon materials could be used as fillers to improve the thermal and mechanical properties of polymers. A single walled carbon nanotube (SWCNT) filled epoxy composite shows substantial four fold improvement in thermal conductivity.¹³ In addition, functionalized multiwalled carbon nanotubes as fillers in epoxy composites improve thermal stability to a certain extent.¹⁴ Carbon fiber,¹⁵ graphite,¹⁶ carbon black¹⁷ and fullerenes¹⁸ have also been studied as thermally conductive fillers to improve the thermal conductivity of polymers. Recently, graphene has received a great deal of attention as a thermally conductive filler due to its high thermal conductivity, low specific gravity, large surface area and high aspect ratio.^19–21 In general, graphene facilitates the formation of bridges and percolating networks in a polymer matrix. Phonon transfer becomes easier and more convenient, resulting in improved thermal conductivity of the composite with graphene. The introduction of 0.25 wt% graphene in polymers improves the thermal conductivity up to 6%.²² The thermal conductivity of the expanded graphene nanoplatelets (EGNP) was observed to increase up to 36% with the addition of 2 wt% of fillers. However, a very high proportion of the fillers (volume fraction > 50%) is required to enhance the thermal conductivity for certain applications.²³ Therefore, the high proportion of filler leads to poor mechanical properties and increases the density of the polymer. The main reason for this is the inhomogeneous dispersion of the thermally conductive particles in the polymer matrix with thermal resistance of the interface (Kapitza resistance (R_K)) that limits heat flow due to differences in the phonon spectra of the two different phases (filler and matrix) and weak contact at the interface.²⁴ In general, thermal energy is transported in graphene through phonons.²⁵ The conduction of phonons over a long distance without any transition from particle to particle is possible in highly thermally conductive fillers. Furthermore, the weak bonding between polymer matrix/fillers and several acoustic phonon mismatches affects the transport of thermal waves that decrease the conductivity even at high filler proportion.

Chemical functionalization is the most suitable route to improve the interface between the matrix and fillers.²⁶ Functionalization and surface modification of the filler or matrix may help to generate strong interconnecting networks, and better thermal transport bridges. So far, chemical functionalization in graphene is basically carried out from graphene oxide.²⁷ The use of strong oxidizing agents alters the properties of the pristine graphene, and requires multiple steps during synthesis.

In this paper, we report on the functionalization of graphene through a novel and innovative method in which the graphene is first shear exfoliated in volatile solvent (1-butanol). The multilayered graphene is mildly treated with a molar mixture of H₂SO₄ : HNO₃ to attach some oxide groups to the surface of the graphene. O-Graphene is further treated with O-phenylenediamine (OPD) to substitute the oxide groups with amine groups via amidation. The substitution of oxide by OPD significantly improved the homogeneous dispersion of the fillers and effective thermal transport in the composite. Furthermore, OPD-f-graphene exhibits good dispersion, and stability under a continuous flow of heat, which results in improved thermal conductivity of the epoxy composite.

2 Experimental

2.1 Materials

The epoxy resin based on a diglycidyl ether of bisphenol A (DGEBPA, YD-128), and hardener of modified aliphatic amine (KH-602) were purchased from Kukdo Chem. Co. Graphite powder (<20 μm) and O-phenylenediamine (OPD) were purchased from Sigma Aldrich. 1-Butanol, sulfuric acid (H₂SO₄, 95%) and nitric acid (HNO₃, 60%) were purchased from Daejung chemicals Co. Ltd. These chemicals were used as received without any purification.

2.2 Synthesis of graphene and O-graphene

Few-layered graphene was synthesized through shear exfoliation in 1-butanol, as reported elsewhere.²⁸ In a typical procedure, 15 g of natural graphite was dispersed in 150 ml of 1-butanol in a 250 ml beaker and was exfoliated at 4500 rpm for 45 min. The dispersed graphene was separated through centrifugation and dried at 130 °C overnight. The oxidation of graphene was carried out with a molar mixture of H₂SO₄ : HNO₃. In a typical procedure 100 mg of graphene was dispersed in 100 ml of a 2–6 molar solution of H₂SO₄ : HNO₃ in a 2-necked round bottom flask with a condenser, thermo-controller and magnetic stirring bar. The reactor was placed in a water bath and heated up to 70 °C for 48 hours under continuous stirring at 200 rpm. The reactor was cooled to room temperature and washed several times with water until the pH of the filtrate became neutral and then further washed with ethanol to remove the remaining impurities. The oxidized graphene (O-graphene) was vacuum dried at 80 °C overnight.

2.3 Functionalization of graphene

The O-graphene was treated with O-phenylenediamine (OPD) by simple refluxing. The functionalization of graphene was carried out in a 2-necked round bottom flask. First, 50 mg of O-graphene was dispersed in 100 ml of DI water through sonication for 30 min. Subsequently, 500 mg (1 wt equivalent) of OPD was added to the reactor. Additionally ∼400 μL of NH₃ was also added to the reactor. The reaction mixture was refluxed at 95 °C for 48 hours in an oil bath. Finally, the reaction was filtered using a 0.45 μm nylon membrane and washed several times with ethanol and water until all of the unreacted OPD was removed. The OPD-f-graphene was vacuum dried at 80 °C overnight.

2.4 Fabrication of epoxy nanocomposite

The epoxy nanocomposite containing the graphene filler was synthesized by the following procedure. The desired amount of OPD-f-graphene was mixed with the epoxy by centrifugal mixing using a Thinky mixer (ARE-310) at 1000 rpm for 15 min, followed by sonication at 60 °C for 30 min. Then the mixture was degassed under vacuum for ∼10 min. The addition of curing agents was done by mechanical mixing followed by sonication at room temperature for 5 min. The weight ratio of the epoxy resin to curing agent was 100 : 25. The mixture was degassed again to remove trapped air bubbles and then cast into a Teflon mold. The curing was done at 80 °C for 30 min and the mold was cured at room temperature for 48 hours. In addition, the control graphene samples were also used to prepare epoxy nanocomposites (G–epoxy) to compare the thermal conductivity. The filler proportions are listed in Table 1.

Table 1 Filler weight fractions of epoxy nanocomposites filled with the control graphene and OPD-f-graphene

Sample #	Sample	Filler wt fraction (%)
1	Neat epoxy	—
2	Graphene filled epoxy (G–epoxy)	0.5	1	2	3	4	5	6
3	OPD-f-graphene filled epoxy (fG–epoxy)	0.5	1	2	3	4	5	6

---

2.5 Characterization

The in-plane thermal conductivity of the epoxy thin film (sample dimensions: 30 × 5 × 0.30 mm) was characterized by a thermal diffusivity analyzer (ULVAC LPIT (Laser-PIT-M2, Japan)). The through plane thermal conductivity (sample dimensions: 10 × 10 × 0.50 mm) was measured by a Nanoflash thermal diffusivity analyzer (LFA-447, NETZSCH, Germany). Thermal stability was measured by using Thermogravimetric analyzer (TGA-Q50, USA). TEM images were obtained by high resolution transmission electron microscopy (JEOL 2200 FS, USA) operating at 200 kV. Fourier transform infrared (FT-IR) spectra were recorded and analyzed on a Fourier transform-infrared spectrophotometer (JASCO FT/IR-4100, Japan). An X-ray photoelectron spectrometer (XPS) system (K-Alpha⁺, Thermo Scientific, USA) was used to characterize the surface composition of graphene. Raman scattering was performed on a Raman confocal spectrometer (Nanofinder®, Japan). The surface morphology and elemental analysis were recorded on a field emission transmission electron microscope (FE-SEM) (Carl Zeiss Supra 40VP, Germany). Optical images were taken through a Nikkon Eclipse LV-100 microscope, Japan. The dispersion of the filler in the matrix was studied using a scanning probe microscope, Nanoscope IV multimode AFM (Bruker, Germany).

3 Result and discussion

3.1 The synthesis and functionalization of graphene

The graphene structure is chemically inert. Chemical oxidation is an efficient way to make possible the generation of functional groups on graphene. The oxidation forms hydroxyl and carbonyl groups on the graphene. Herein, these oxide groups are replaced with the amine groups of O-phenylenediamine (OPD) through simple refluxing in water. The dark black color of the graphene turning to blackish brown is the indicator for substitution of the oxides by amines. The functionalization of graphene with OPD was confirmed by X-ray photon spectroscopy (XPS), as shown in Fig. 3. The XPS studies reveal the formation of chemical bonds on the graphene after treatment with OPD. A survey spectrum of OPD-f-graphene shows the presence of carbon (∼284.6 eV), nitrogen (∼398 eV) and oxygen (∼530 eV), whereas the O-graphene displays only carbon and oxygen elements, as shown in Fig. 3(a). Fig. 3(b) shows the C 1s spectrum of O-graphene, which can be deconvoluted into three binding energies at 284.6 eV, 286.1 eV and 289.4 eV, corresponding to C–C, C–OH in a hydroxyl group and O–C=O in a carbonyl group, respectively. The appearance of the O–C=O and C–OH bonds is attributed to the oxidation of the graphene.²⁹ As seen in Fig. 3(c), OPD-f-graphene exhibits an identical spectrum to O-graphene along with a new binding energy at 285.6 eV which originates from C–N after OPD functionalization.^30,31 The existence of C–N peaks confirms the attachment of amine groups in O-graphene after the OPD functionalization. Fig. 3(d) shows the N 1s spectrum of OPD-f-graphene which shows two peaks at 398.8 eV and 399.8 eV. The peak at 399.8 eV is associated with N–H bonds.³² This shows that the nitrogen is doped with carbon atoms after the treatment with OPD. The amine groups present in OPD replace the carboxylic groups on the edges of O-graphene by an amidation reaction and furthermore, the amine groups react with epoxide groups in the graphene to form phenazine.³³ The elemental analysis of the XPS shows that the oxygen content in OPD-f-graphene is reduced after treatment with OPD, which indicates that most of the oxygen functional groups are removed after the reduction.

Fig. 1 Schematic illustration of the functionalization of graphene.

Fig. 2 Schematic illustration of the interface mechanism of the fG–epoxy nanocomposite.

Fig. 3 XPS spectra: (a) survey spectra of O-graphene and OPD-f-graphene; high resolution C 1s spectra of (b) O-graphene and (c) OPD-f-graphene; (d) N 1s spectra of OPD-f-graphene.

In addition, the functionalization of O-graphene was also studied by FT-IR spectroscopy, as shown in Fig. 4. The O-graphene contains IR peaks at 1702 cm⁻¹ and 1237 cm⁻¹, which are attributed to the carbonyl stretching (C=O) and vibration modes, respectively, of the epoxide group (C–O–C). The band at 1640 cm⁻¹ is due to the C=C bonds of the aromatic ring. After the treatment of O-graphene with OPD, the characteristic IR bands at 1237 cm⁻¹ and 1702 cm⁻¹ decreased, which is an indication of the reduction of the oxide groups with amines. The appearance of some new peaks at 758 cm⁻¹, 1233 cm⁻¹, 1494 cm⁻¹ and 1542 cm⁻¹ again confirmed the reduction of the oxides after functionalization. The peak at 1233 cm⁻¹ is due to the C–N stretching vibration, and this shows the formation of C–NH–C from the grafting of amine groups on the graphene. The bands at 1494 cm⁻¹ and 1542 cm⁻¹ are due to the asymmetric stretching of C–N³⁴ and the skeletal vibration of the benzoid rings in phenazine,^35,36 respectively. The peak at 758 cm⁻¹ is the primary peak of the N–H bond and is assigned as the characteristic peak of phenazine.^37,38 Thus, these observations enable the deduction that the nucleophilic reduction of the epoxide in O-graphene with the amine group in OPD has taken place, confirming the nitrogen doping on graphene by refluxing. On the basis of structural observations, the functionalization of O-graphene has been explained by a proposed mechanism as illustrated in Fig. 1. Pristine graphene is first oxidized by acid treatment to generate the carboxyl and epoxide groups on the surface of graphene. With the functionalization of O-graphene by OPD, these oxidized groups are substituted with electron rich amine groups in a nucleophilic addition reaction. The amine groups of OPD firstly react with the peripheral carboxylic groups to form a strong amide bond. Simultaneously, the electron rich amine groups of OPD might break the epoxide bonds in the O-graphene structure, and produce a stable phenazine structure, as evidenced by the XPS and FTIR results.

Fig. 4 (a) FTIR-spectra of graphene, O-graphene and OPD-f-graphene and (b) magnified spectrum of OPD-f-graphene.

3.2 The analysis of graphene structure and surface morphology of composite

To study the surface and structural changes in graphene after functionalization, both graphene materials are characterized by FESEM and TEM analysis, as shown in Fig. 5. A TEM image of exfoliated graphene (Fig. 5(a)) reveals a typical transparent glassy sheet-like graphene surface. After OPD functionalization (Fig. 5(b)), non-uniform sheets are seen in the morphology of graphene due to the significant doping of OPD into the surface of graphene. Moreover, Fig. 5(c and d) shows the surface morphology of graphene before and after functionalization, measured by FESEM analysis. Fig. 5(c) exhibits a well oriented three dimensional graphene structure, whereas OPD-f-graphene shows a slightly rough surface which might usually happen because of the presence of amine dopants in graphene.³⁹ To find out the degree of deformation in the structure of graphene, the samples were analyzed using Raman spectroscopy, as depicted in Fig. 6. Two prominent peaks at 1340 cm⁻¹ and 1580 cm⁻¹ are observed, corresponding to the D and G bands of graphene, respectively. The intensity ratio of D-band to G-band, I_D/I_G, increased in O-graphene after treatment with mild acid, in comparison to pristine graphene. However, the intensity is not very significant and most of the pristine properties are maintained after oxidation. In OPD-f-graphene, the G peak is down shifted to 1573 cm⁻¹ and the I_D/I_G ratio slightly increased from 0.83 to 0.89, suggesting the attachment of the nitrogen atom to graphene. Moreover, a shoulder of lower frequency at 1516 cm⁻¹ is seen in OPD-f-graphene, which is due to the generation of the phenazine structure.³⁷ From the Raman results, it was established that the integrated intensity ratio (I_D/I_G) slightly increased from 0.83 to 0.89 after functionalization, due to edge plane exposure caused by heterogeneous nitrogen atom incorporation into the graphene layers.^40,41 However, it is supposed that this cannot disrupt the thermal conjugation of graphene. Moreover, during the composite fabrication, the amine group reacts with epoxy moieties, which restores the sp² C–C bond to its original position and enhances the thermal conductivity. These results are consistent with previously reported work.^26,27,42,43

Fig. 5 TEM images of (a) exfoliated graphene and (b) OPD-f-graphene; FESEM images of (c) exfoliated graphene and (d) OPD-f-graphene.

Fig. 6 (a) Raman spectra of graphene, O-graphene and OPD-f-graphene. (b) Magnified view of the G-band.

The morphology of the epoxy nanocomposite was studied by FESEM. The cross sectional FESEM images of the 6 wt% controlled graphene filled nanocomposites are shown in Fig. 7(a–c). Fig. 7(a) shows the FESEM image of non-functionalized graphene, exhibiting a lot of agglomeration. After functionalization, the well oriented and interconnected graphenes with the epoxy are evidenced in Fig. 7(d–f), and thus it can be deduced that the functionalization helps in forming better and well dispersed graphene in the polymer matrix. Moreover, EDX analysis (Fig. 8(a)) also confirms the presence of the desired content of nitrogen in the epoxy composite with OPD-f-graphene, which is comparable with the XPS results. Elemental mapping (Fig. 8(b–e)) reveals the homogenous dispersion of nitrogen all over the matrices. To study the dispersion of graphene in the epoxy matrix, the composite was examined using optical microscopy and electronic force microscopy (EFM). Fig. 9(a and b) shows optical images of the epoxy composite. G–epoxy shows a lot of agglomeration of the graphene sheets, as seen in Fig. 9(a). However, the fG–epoxy possesses a less agglomerated surface, indicating the homogenous dispersion of OPD-f-graphene fillers in the epoxy matrix. Furthermore, since graphene has good electronic properties as compared to epoxy, EFM has been used to further analyze the dispersion of graphene on the epoxy matrix. Fig. 9(c and d) depicts the EFM images of G–epoxy and fG–epoxy. The fG–epoxy shows a better filler dispersion in comparison with the G–epoxy composite, which is in good agreement with optical microscopy and the FESEM images. Moreover, a low surface roughness of the composite is normally related to a high thermal conductivity.⁴⁴ From roughness analysis, fG–epoxy presents a low roughness value of 2.05 nm, while a high roughness value has been observed for G–epoxy (∼4.07 nm). Thus, the introduction of the OPD-f-graphene filler in the matrix improves the dispersion and lowers the roughness via a better interface between the matrix and filler, and this might result in the enhanced thermal conductivity.

Fig. 7 (a–c) FE-SEM images of the pristine graphene–epoxy composite (G–epoxy) and (d–f) OPD-f-graphene–epoxy composite (fG–epoxy).

Fig. 8 (a) EDX spectrum and elemental analysis of fG–epoxy (6 wt% filler); (b–e) carbon, oxygen and nitrogen mapping images of the fG–epoxy composite.

Fig. 9 (a and b) Optical images at 20X of G–epoxy and fG–epoxy; EFM images of G–epoxy (c) and fG–epoxy (d).

3.3 Thermal properties of epoxy composites

The thermal stability of the epoxy composites was studied using thermo-gravimetric analysis (TGA) as presented in Fig. 10. The main loss in weight of the composites was recorded at around 300–500 °C, and this was due to the degradation of the epoxy composite.⁴⁵ It is noted that the decomposition temperature T_d in the fG–epoxy composite was improved to 5 °C as compared to the pristine graphene filled epoxy, as shown in Fig. 10(b). This shows the high compatibility of functionalized graphene with the epoxy matrix. The overall thermal stability of the composite was improved up to 10% with respect to the neat epoxy. Generally, a high amount of filler is required to avail the high thermal conductivity of the epoxy composites. Thermal conductivity is affected by the filler content, homogenous dispersion and thermal interface resistance.^46,47 To this end, the thermal conductivity is improved by reducing the thermal interface resistance and generating thermally conductive pathways. In this work, the laser flash method was used to measure in-plane and through plane thermal conductivity of the epoxy composite with OPD-f-graphene at room temperature. The thermal diffusivity has been measured in order to calculate the thermal conductivity of the epoxy composite. Fig. 11 shows that the thermal conductivity is enhanced upon increasing the amount of filler. This enhancement occurs due to the high surface area of graphene and, in contact with the polymer layers, reduces the transport barrier in the composite. The thermal conductivities in both directions, i.e. the in-plane and through-plane directions, are noticeably high in G–epoxy with 0.5 wt% pristine graphene. From Fig. 11, the in-plane and through-plane thermal conductivities of G–epoxy with 0.5 wt% pristine graphene are estimated to be 0.721 W m⁻¹ K⁻¹ and 0.301 W m⁻¹ K⁻¹, respectively. In comparison with neat epoxy (0.2 W m⁻¹ K⁻¹), both of the thermal conductivities increased considerably ∼3.6 fold and ∼1.5 fold. However, the fG–epoxy composite with 0.5 wt% OPD-f-graphene showed extensive enhancement in the thermal conductivity of ∼5.6 fold (in-plane thermal conductivity) and ∼1.8 fold (through-plane thermal conductivity). The in-plane and through-plane thermal conductivity values of the composite filled with 6 wt% of OPD-f-graphene were 2.671 and 0.976 W m⁻¹ K⁻¹ respectively. The in-plane and through-plane thermal conductivities were improved by an enhancement factor of ∼12.35 and ∼3.88, respectively, as shown in Fig. 11(c and d), when compared to the thermal conductivity value of the neat epoxy (0.2 W m⁻¹ K⁻¹). It can be noticed that the in-plane thermal conductivity of the OPD-f-graphene fillers in epoxy is approximately three times higher than the through-plane thermal conductivity for the graphene filled epoxy. In general, the graphene materials present high thermal conductivity in the in-plane direction due to their 2D carbon structure. Therefore, the thermal conductivity of the in-plane direction for graphene or graphene composites is usually higher than the thermal conductivity measured in the through-plane direction.⁴⁸ The in-plane thermal conductivity of graphene nanoplates (GNP) is 3000 W m⁻¹ K⁻¹, whereas the through plane thermal conductivity is 500 times lower than the in-plane thermal conductivity, i.e. only 6 W m⁻¹ K⁻¹. The observed in-plane thermal conductivity values are comparatively high, whereas the through-plane thermal conductivity values are comparable with previously reported work.^{26,27,41,49–51} The OPD functionalization of graphene significantly plays an important role in interconnecting the graphene with the epoxy. It is supposed that the amino groups in the functionalized graphene react with the epoxy matrix in a quite similar manner and this strengthens the interface between OPD-f-graphene and the epoxy matrix, as illustrated in Fig. 2. This interfacial improvement might help in the generation of a heat conductive path, which might result in a low resistance to heat flow.

Fig. 10 TGA plot of (a) neat epoxy, G–epoxy and fG–epoxy (6 wt% filler); (b) magnified view.

Fig. 11 Thermal conductivity of G–epoxy and fG–epoxy (a) in-plane and (b) through-plane; enhancement in thermal conductivity as a function of graphene content (c) in-plane and (d) through-plane.

4 Conclusion

In summary, we have demonstrated an efficient and novel route to functionalize graphene while maintaining important properties of its pristine form. The graphene is functionalized with a double-N dopant (O-phenylenediamine) which replaces carbonyl groups at the edges of graphene, and amine groups substitute epoxides to form a phenazine-N structure. The appropriate doping of nitrogen species by OPD results in the homogenous dispersion of fillers in the epoxy matrix. The compact structure of the fG–epoxy composite enhances phonon transportation, decreasing Kapitza resistance (R_K) which increases the transport of phonons, resulting in higher thermal conductivity of the fG–epoxy composite. The thermal conductivity is recorded to be very high with an improvement of ∼13 fold in the in-plane direction and ∼4 fold in the through-plane direction, in the fG–epoxy composite with 6 wt% of filler as compared to the neat epoxy. Thus, this approach is quite useful for applications in thermal interface materials (TIMs) and thermal management applications.

Acknowledgements

This work was supported by the research funds of Chonbuk National university in 2013.

References

1. O. Eksik, S. F. Bartolucci, T. Gupta, H. Fard, T. Borca-Tasciuc and N. Koratkar, Carbon, 2016, 101, 239–244.
2. S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794–797.
3. M. Li, Y. Wan, Z. Gao, G. Xiong, X. Wang, C. Wan and H. Luo, Mater. Des., 2013, 51, 257–261.
4. J. Ordonez-Miranda and J. J. Alvarado-Gil, Compos. Sci. Technol., 2012, 72, 853–857.
5. J. C. Lötters, W. Olthuis, P. H. Veltink and P. Bergveld, J. Micromech. Microeng., 1997, 7, 145.
6. S. Ganguli, A. K. Roy and D. P. Anderson, Carbon, 2008, 46, 806–817.
7. K. C. Yung and H. Liem, J. Appl. Polym. Sci., 2007, 106, 3587–3591.
8. S. Choi and J. Kim, Composites, Part B, 2013, 51, 140–147.
9. S. A. Putnam, D. G. Cahill, B. J. Ash and L. S. Schadler, J. Appl. Phys., 2003, 94, 6785–6788.
10. E.-S. Lee, S.-M. Lee, D. J. Shanefield and W. R. Cannon, J. Am. Ceram. Soc., 2008, 91, 1169–1174.
11. Y. P. Mamunya, V. V. Davydenko, P. Pissis and E. V. Lebedev, Eur. Polym. J., 2002, 38, 1887–1897.
12. X. Huang, P. Jiang and L. Xie, Appl. Phys. Lett., 2009, 95, 242901.
13. M. J. Biercuk, M. C. Llaguno, M. Radosavljevic, J. K. Hyun, A. T. Johnson and J. E. Fischer, Appl. Phys. Lett., 2002, 80, 2767–2769.
14. J. Xiong, Z. Zheng, X. Qin, M. Li, H. Li and X. Wang, Carbon, 2006, 44, 2701–2707.
15. K. Gharagozloo-Hubmann, A. Boden, G. J. F. Czempiel, I. Firkowska and S. Reich, Appl. Phys. Lett., 2013, 102, 213103.
16. H. Fukushima, L. T. Drzal, B. P. Rook and M. J. Rich, J. Therm. Anal. Calorim., 2006, 85, 235–238.
17. F. El-Tantawy, K. Kamada and H. Ohnabe, Mater. Lett., 2002, 56, 112–126.
18. M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science of fullerenes and carbon nanotubes: their properties and applications, Academic press, 1996.
19. A. A. Balandin, Nat. Mater., 2011, 10, 569–581.
20. S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282–286.
21. B. Shen, W. Zhai, D. Lu, J. Wang and W. Zheng, RSC Adv., 2012, 2, 4713–4719.
22. R. Verdejo, F. Barroso-Bujans, M. A. Rodriguez-Perez, J. Antonio de Saja and M. A. Lopez-Manchado, J. Mater. Chem., 2008, 18, 2221–2226.
23. C. Seran, I. Hyungu and K. Jooheon, Nanotechnology, 2012, 23, 065303.
24. B. Baudouy and J. Polinski, Cryogenics, 2009, 49, 138–143.
25. L. N. Denis and A. B. Alexander, J. Phys.: Condens. Matter, 2012, 24, 233203.
26. C.-C. Teng, C.-C. M. Ma, C.-H. Lu, S.-Y. Yang, S.-H. Lee, M.-C. Hsiao, M.-Y. Yen, K.-C. Chiou and T.-M. Lee, Carbon, 2011, 49, 5107–5116.
27. S. H. Song, K. H. Park, B. H. Kim, Y. W. Choi, G. H. Jun, D. J. Lee, B.-S. Kong, K.-W. Paik and S. Jeon, Adv. Mater., 2013, 25, 732–737.
28. M. Wasim Akhtar, C. W. Park, Y. S. Kim and J. S. Kim, J. Nanosci. Nanotechnol., 2015, 15, 9624–9629.
29. S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen and R. S. Ruoff, Carbon, 2007, 45, 1558–1565.
30. N. I. Kovtyukhova and T. E. Mallouk, J. Phys. Chem. B, 2005, 109, 2540–2545.
31. W. Ai, W. Zhou, Z. Du, Y. Du, H. Zhang, X. Jia, L. Xie, M. Yi, T. Yu and W. Huang, J. Mater. Chem., 2012, 22, 23439–23446.
32. C. Ronning, H. Feldermann, R. Merk, H. Hofsäss, P. Reinke and J. U. Thiele, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58, 2207–2215.
33. I. Losito, C. Malitesta, I. De Bari and C.-D. Calvano, Thin Solid Films, 2005, 473, 104–113.
34. N. Sundaraganesan, K. Sathesh Kumar, C. Meganathan and B. Dominic Joshua, Spectrochim. Acta, Part A, 2006, 65, 1186–1196.
35. P. Muthirulan, N. Kannan and M. Meenakshisundaram, J. Adv. Res., 2013, 4, 385–392.
36. S.-J. Tang, A.-T. Wang, S.-Y. Lin, K.-Y. Huang, C.-C. Yang, J.-M. Yeh and K.-C. Chiu, Polym. J., 2011, 43, 667–675.
37. R. H. Sestrem, D. C. Ferreira, R. Landers, M. L. A. Temperini and G. M. do Nascimento, Eur. Polym. J., 2010, 46, 484–493.
38. L. Zhang, Z. D. Zujovic, H. Peng, G. A. Bowmaker, P. A. Kilmartin and J. Travas-Sejdic, Macromolecules, 2008, 41, 8877–8884.
39. H.-L. Ma, Y. Zhang, Q.-H. Hu, D. Yan, Z.-Z. Yu and M. Zhai, J. Mater. Chem., 2012, 22, 5914–5916.
40. J. Yan, Y. Xiao, G. Ning, T. Wei and Z. Fan, RSC Adv., 2013, 3, 2566–2571.
41. D. Geng, S. Yang, Y. Zhang, J. Yang, J. Liu, R. Li, T.-K. Sham, X. Sun, S. Ye and S. Knights, Appl. Surf. Sci., 2011, 257, 9193–9198.
42. H. S. Kim, H. S. Bae, J. Yu and S. Y. Kim, Sci. Rep., 2016, 6, 26825.
43. G. Xin, H. Sun, S. M. Scott, T. Yao, F. Lu, D. Shao, T. Hu, G. Wang, G. Ran and J. Lian, ACS Appl. Mater. Interfaces, 2014, 6, 15262–15271.
44. P. Martin, Z. Aksamija, E. Pop and U. Ravaioli, Phys. Rev. Lett., 2009, 102, 125503.
45. S.-J. Park and F.-L. Jin, Polym. Degrad. Stab., 2004, 86, 515–520.
46. M. B. Bryning, D. E. Milkie, M. F. Islam, J. M. Kikkawa and A. G. Yodh, Appl. Phys. Lett., 2005, 87, 161909.
47. C.-W. Nan, G. Liu, Y. Lin and M. Li, Appl. Phys. Lett., 2004, 85, 3549–3551.
48. S. Y. Kim, Y. J. Noh and J. Yu, Composites, Part A, 2015, 69, 219–225.
49. F. Yavari, H. R. Fard, K. Pashayi, M. A. Rafiee, A. Zamiri, Z. Yu, R. Ozisik, T. Borca-Tasciuc and N. Koratkar, J. Phys. Chem. C, 2011, 115, 8753–8758.
50. L. Cao, X. Liu, H. Na, Y. Wu, W. Zheng and J. Zhu, J. Mater. Chem. A, 2013, 1, 5081–5088.
51. J. Kim, B.-S. Yim, J.-M. Kim and J. Kim, Microelectron. Reliab., 2012, 52, 595–602.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17946k

---