Ultrathin flexible graphene films with high thermal conductivity and excellent EMI shielding performance using large-sized graphene oxide flakes

As the demand for wearable and foldable electronic devices increases rapidly, ultrathin and flexible thermal conducting films with exceptional electromagnetic interference (EMI) shielding effectiveness (SE) are greatly needed. Large-sized graphene oxide flakes and thermal treatment were employed to fabricate lightweight, flexible and highly conductive graphene films. Compared to graphene films made of smaller-sized flakes, the graphene film made of large-sized flakes possesses less defects and more conjugated domains, leading to higher electrical and higher thermal conductivities, as well as higher EMI SE. By compressing four-layer porous graphene films together, a 14 μm-thick graphene film (LG-4) was obtained, possessing EMI SE of 73.7 dB and the specific SE divided by thickness (SSE/t) of 25 680 dB cm2 g−1. The ultrahigh EMI shielding property of the LG-4 film originates from the excellent electrical conductivity (6740 S cm−1), as well as multi-layer structure composed of graphene laminates and insulated air pores. Moreover, the LG-4 film shows excellent flexibility and high thermal conductivity (803.1 W m−1 K−1), indicating that the film is a promising candidate for lightweight, flexible thermal conducting film with exceptional EMI shielding performance.


Introduction
With the rapid development of information technology in recent years, electromagnetic (EM) pollution caused by electronic components of wearable and foldable equipments, such as exible electrodes, storage devices, and smart sensors, 1-3 has became a serious environmental issue. Electromagnetic interference (EMI) shielding protection is the most effective way to prevent EM pollution. [4][5][6][7] According to electromagnetic theory, the main mechanisms to reduce undesirable EM emissions include reection of EM radiation by impedance mismatching and absorption of the EM wave energies by dielectric or magnetic loss. 8 Electrically conductive materials are reective shielding materials, which could reect most of the incident waves, due to their high electrical conductivity. For the portion of waves able to enter shielding materials, absorption loss is the dominant consumption of this EM energy, which depends on the interaction between conducting parts of shielding materials and EM waves. 7,9,10 Hence, materials with high electrical conductivity, such as metals, carbon materials and conductive polymers, have been applied as effective EMI shielding materials. However, metal shielding materials have the disadvantages of high densities and easy corrosion, limiting their application in lightweight, wearable electronic equipment. With a high specic surface, outstanding thermal and electrical conductivities, carbon nanomaterials have been studied as lightweight and efficient EMI shielding materials. 4,[11][12][13] For instance, the carbon nanotube (CNT) sponge with a thickness of 1.8 mm shows a high EMI shielding effectiveness (SE) of 54.8 dB and specic SE (SSE) of 5480 dB cm 3 g À1 in X band. 5 By adding conductive polymer and magnetic loss materials, the graphene/CNT lm with a thickness of 0.6 mm can achieve EMI SE as high as 133.22 dB. 14 However, in some cases like foldable and exible electronic devices, the thickness of the shielding lms is highly restricted. [15][16][17] Moreover, the shielding lms are also required to be highly thermally conductive, since electronic components of these devices, such as smart sensors and exible electrodes, 15,18,19 produce signicant heat emission during operation. If the heat is not effectively dissipated, it will lead to malfunction of these source electronic devices and reduction of their service life. 20,21 Hence, ultrathin, exible and thermal conducting lms with exceptional EMI SE are demanded.
Graphene has attracted much attention, due to its outstanding thermal conductivity (5000 W m À1 K À1 ) and ultrahigh electrical conductivity ($10 4 S cm À1 ). 22,23 Ultrathin graphene lms could absorb and reect EM waves effectively, which was widely studied as efficient shielding materials. 24,25 Moreover, large-scale production of graphene lms could be achieved through reduction of graphene oxide (GO) lms. 26,27 Based on these merits, reduced graphene oxide (rGO) lms are promising to be applied as ultrathin thermal conducting lms with excellent EMI SE. According to Shen's work, a 8.4 mm-thick rGO lm with EMI SE of 20 dB and in-plane thermal conductivity of 1100 W m À1 K À1 was obtained by thermal annealing of GO lm at 2000 C. 20 Through improving reduction temperature, higher electrical and thermal conductivities, as well as EMI shielding performance were achieved. 13,21 By graphitization of GO lms at 3000 C, Xi et al. has fabricated the foam-like graphene lms, with EMI SE up to 65-105 dB. 28 The ultrahigh EMI SEs of the foam-like graphene lms were attributed to signicantly improved electrical conductivity and "expansion enhancement effect" of insulating space layers.
However, the insulating air pores in the graphene lms, formed during graphitization of GO lms, were detrimental to their electrical and thermal conductivities. Moreover, the porous graphene (PG) lms were not mechanically robust enough to meet the requirement of harsh deformations for the wearable and foldable equipments. Cracks were found in the PG lms aer repeated folding or bending ( Fig. S1 and S2 †). Compression of the PG lms could effectively reduce size of these insulated air pores, thus improving their electrical and thermal conductivity signicantly. 27 The effect of reducing size of these insulated air pores on EMI shielding performance of the graphene lms still needs to be studied further, through investigating EMI SEs of both porous graphene lms and compressed graphene lms.
In this work, large-sized graphene oxide akes and compression of porous graphene lms were employed to prepare ultrathin exible graphene lms with ultrahigh EMI SEs and excellent thermal conductivity. Herein, the porous graphene lms were fabricated with the combination of castingevaporation of GO suspension and thermal annealing at 2600 C. Due to their better thermal and electrical performances, the porous lm made of large-sized graphene akes (PLG) shows an increase in EMI SE of 7.8 dB, compared to that made of smaller-sized graphene akes. And four 35.6 mm-thick PLG lms were compressed into a 14 mm-thick lm (LG-4), with an excellent EMI SE of 73.7 dB and specic SE divided by the thickness (SSE/t) of 25 680 dB cm 2 g À1 , as well as outstanding thermal conductivity of 803.1 W m À1 K À1 . Moreover, the LG-4 lm could endure harsh deformation, with tensile strength of 42.61 MPa and elongation at break of 7.85%, proving to be a promising candidate as thermal conducting materials in wearable and foldable electronic devices.

Preparation of graphene lms
Fabrication process of these graphene lms is shown in Fig. 1. The abbreviated symbols and thickness of graphene lms and graphene oxide lms are presented in Table 1.
Three kinds of GO/H 2 O dispersions were purchased from Hangzhou Gaoxi Technology Co., Ltd., with GO akes of different ranges of diameters, i.e. 5-8 mm, 20-30 mm and 40-50 mm, noted as SGO, MGO and LGO, respectively. The GO dispersions were diluted to 8.0 mg ml À1 with water, and dispersed by mechanical stirring for 4 h, then bar-coated on a PET plate (Fig. S3 †). Aer drying at 35 C for 24 h, GO lms were peeled off from the PET substrate. Areal densities of these GO lms were precisely controlled to be around 1.70 mg cm À2 by adjusting the concentration of GO suspension and height of scraper at around 2 mm. Aer graphitization at 2600 C for 4 h,

Material characterization
Sizes of the GO akes, and morphologies of the GO and graphene lms were investigated by scanning electron microscope (SEM, Hitachi S4800, FEI, Japan). Raman spectra were performed on a LabRAM HR Raman Spectroscopy, and a laser excitation of 532 nm was employed. The elemental compositions of the samples were investigated by X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA System, 14.0 kV). X-ray diffraction (XRD) data was collected with a X'Pert Pro (PANalytical) diffractometer using monochromatic Cu Ka1 radiation (l ¼ 1.5406Å) at 40 kV. Fourier transform infrared (FTIR) spectra were recorded by a Nicolet 10 spectrometer. Static uni-axial tensile tests of the lm samples were conducted on a dynamic mechanical analyzer (D8000 DMA, Perkin Elemer, US). Two ends of the sample, with a length of about 6 mm and width of around 3 mm, were gripped by tension clamps tightly (Fig. S3 †). All tensile tests were conducted at 25 C, in controlled force mode at a loading rate of 0.02 N min À1 . Sheet resistance of graphene lms was measured on a Keithley 2450 Source Meter by the four-probe method, under small currents. Electrical conductivity of all the lms can be calculated by the following equation: where, s, R s and t are the electrical conductivity, sheet resistance and thickness of the graphene lms, respectively. Dimension of graphene lms for electrical conductivity testing is 25 mm Â 25 mm. Thermal conductivity can be calculated from the equation: where, l, a, C p , and r are thermal conductivity, thermal diffusivity, specic heat capacity and material density, respectively. Thermal diffusivities of the compressed graphene lms were measured by a light ash system (NETZSCH LFA 447) at room temperature. The specic heat capacities were measured from differential scanning calorimeter (DSC, Mettler), and densities of the graphene lms were calculated based on sample weight and volume. Diameter of graphene lms for thermal conductivity testing is around 25 mm. S-parameters of the samples, including the reection (S 11 or S 22 ) and transmission (S 12 or S 21 ) of a transverse EM wave, were measured by a vector network analyzer (AV3672E, CETE-41) using the wave-guide method in X-band. The standard sample dimensions were 22.86 mm Â 10.16 mm, and the tested samples were made by attaching slightly larger graphene lms onto a polyurethane (PU) foam substrate. As shown in Fig. S4, † dimension of PU foam is 22.8 mm Â 10.2 mm Â 3 mm approximately. The EMI shielding effectiveness (SE) was calculated by the following formulas: The A, R and T are the absorption, reection and transmission coefficients, respectively. SE total , SE abs and SE ref are the total, absorptive and reective EMI SE, respectively.

Morphologies of graphene lms
Sizes of graphene oxide akes, and morphology of the LGO lm were observed by SEM, as shown in Fig. 2a-e. The average sizes of graphene oxide akes were calculated by measuring the longest end to end point of more than 50 akes. And the size distribution histograms of SGO, MGO and LGO are shown in Fig. 2a. The average akes size of LGO is around 50 mm, which is much larger than that of SGO ( Fig. 2b-d). As shown in Fig. 2e, the LGO lm shows well-order layered structure, formed during evaporation of GO suspension aer casting. Aer graphitization, the expanded graphene lm shows porous multi-layer structure, composed of graphene laminates and insulated air pores, which range from several to tens of micrometers (Fig. 2f), due to gas releasing between the graphene laminates. Though the porous graphene lms expanded signicantly on the thickness direction, the graphene laminates still formed continuous structure, owing to the overlaps among graphene akes. As shown in Fig. 2f and g, the graphene laminates are closely compacted and non-defective. Aer compression, the microfolds, which are essential to the foldability and exibility of graphene lms, could be observed on the surface of the LG-1 lm (Fig. 2h). Moreover, due to the highly ordered structure, several porous graphene lms are easily to be compressed compactly. As shown in Fig. 2i, a 28 mm-thick lm was obtained by the compression of 8 PLG lms together, which is still exible and foldable with compacted structure along the crosssection direction. Thickness of the LG-8 lm is measured by the micrometer, which is in good agreement with that determined by SEM images (Fig. 2i and j).
Characterization of the modied graphene lms XPS, XRD, Raman and FTIR spectra of as-prepared graphene oxide and graphene lms. The chemical compositions and molecular structures of GO and graphene lms with varied akes sizes are systematically characterized by FT-IR, XPS, XRD and Raman spectroscopy. As shown in Fig. 3a, FT-IR spectra of all GO samples exhibit peaks of functional groups, corresponding to hydroxyl stretching vibrations ($3430 cm À1 ), carboxyl stretching vibrations ($1725 cm À1 ), aromatic carbon bonds ($1615 cm À1 ), and epoxy and alkoxy bonds ($1065 cm À1 ). From Fig. 3a, it can be drawn that the intensity ratios of aromatic carbon to carboxyl bonds increase, when sizes of graphene akes increase, which is further conrmed by XPS tests.
The XPS spectra of all GO and porous graphene lms is shown in Fig. 3b. Generally, the larger the size of the graphene akes, the higher the atomic ratios of carbon to oxygen (C/O). In this work, C/O ratio of the SGO lm is 1.85, which increases to 2.12 for the LGO lm, indicating that the LGO lm has relatively less oxygenated groups. Aer graphitization, as seen in the survey spectrum, oxygen can be barely observed for all reduced graphene lms, owing to the elimination of most oxygenated groups. C/O ratio of the SG-1 lm is 87.2, which is lower than that of the LG-1, indicating a few remaining defects and oxygenated groups for the SG-1 lm. LG-1, respectively. The shi of 2q suggests that the GO lms were well reduced during thermal treatment. Moreover, d-spacing of these compressed graphene lms are slightly greater than that of the natural graphite, attributed to residual structural defects. Due to higher degree of compactness and more ordered structure during assembling of graphene akes, d-spacing of the LG-1 lm (3.352Å) is a bit smaller than that of the SG-1 lm (3.368Å).
Raman spectroscopy of the GO lms and graphene lms are shown in Fig. 3d, from which two noticeable peaks could be observed at about 1354 cm À1 and 1592 cm À1 for GO lms, corresponding to D band and G band. The peak intensity ratio of D band to G band (I D /I G ) is used to characterize structural defects and sp 2 hybridized domains. The SGO lm possesses the I D /I G of 1.74, which is higher than that of the MGO (1.65) and the LGO (1.53). The results demonstrate that larger-sized graphene akes has less oxygenated groups and fewer defects, which might lead to higher electrical conductivity and better EMI shielding performance. Aer graphitization, a new peak appears at 2718 cm À1 for the compressed graphene lms, corresponding to the 2D peak, which indicates the formation of graphite structure during thermal treatment.
EMI sheilding performance of graphene lms. EMI shielding performance of the PG lms and compressed graphene lms made of varied akes sizes were investigated. As shown in Fig. 4a and b, both the PLG and LG-1 lms show higher EMI SEs than the lms made of smaller-sized akes. For instance, EMI SE of the PLG lm is 7.8 dB higher than that of the PSG lm. Aer compression, the SG-1 lm possesses EMI SE of 26.4 dB, which increases to 29.3 dB for the MG-1 lm and 33.1 dB for the LG-1 lm, respectively. Interestingly, EMI SE of the PLG lm decreased signicantly from 62.0 dB to 33.1 dB aer compression, which is mainly attributed to compression of insulated air pores in the porous multi-layer structure. As shown in Fig. 2f, the PLG lm expanded along the thickness direction, resulting in porous multi-layer structure composed of graphene laminates, insulated air pores between graphene laminates and interfaces of graphene laminates and air pores.
A multi-layer model and electromagnetic theories are applied to study the effect of air pores on EMI shielding performance of graphene lms. 28 As shown in Fig. 4d, it is assumed that graphene lms are composed of homogenous graphene laminates and insulated pores. And the average thickness of air pores is d, which decreased signicantly for LG-1 and LG-2 aer compression. As shown in Fig. 4e, A local model was extracted from Fig. 4d, which is composed of two graphene laminates, insulated pores and two interfaces. At the interface of graphene laminate and air pores, a portion of waves would reect and the others will transmit through the interface. Then the reected waves would be reected to another interface and experience reection and transmission again. Therefore, the incident waves will be reected and transmitted innitely between two graphene laminates. The effective transmission coefficient (s eff ) is dened as the ratio of total transmitted EM waves to incident waves, which could be calculated by the eqn (7): 29 where, Z 1 and Z 2 are wave impedance of free space and graphene materials, r is propagation constant of free space, and d is average distance between adjacent graphene laminates. On  the condition that d ranges from 0 to 100 mm and f is from 8.2 to12.2 GHz, s eff is obviously lower than 1. Therefore, the air pores contribute signicantly to shielding performance. With smaller d, compressed graphene lms show much lower EMI SE, compared with porous graphene lms. On the condition that d ¼ l/2 and r ¼ j Â 2 Â p/l (l is wavelength), s eff is calculated as À1, indicating that destructive interference of waves might happen. Hence, the excellent EMI shielding performance of the porous graphene lms is probably because of the interference among the component waves, which is not related to the number of graphene laminates in the multi-layer structure, but related to the size of air pores. For the PLG lms, although insulated air pores could not consume waves directly, these air pores produce multiple interfaces of graphene laminates and air pores, contributing to dispersing extra waves due to the wave interference.
Though the PG lms possess higher EMI SEs, the insulated air pores would signicantly decrease their electrical and thermal conductivities. Hence, compression of PG lms is necessary to obtain ultrathin, exible thermal conducting lms. To improve the relatively low EMI SEs of compressed graphene lms, adding the thickness of shielding materials is simple and efficient. Four layers of PLG lms were compressed together to obtain the 14 mm-thick LG-4 lm, leading to ultrahigh EMI SE of 73.7 dB (Fig. 4c). And, EMI SE of the ultrathin LG-4 lm, with smaller air pores, surpasses that of the FLG lm, which is mainly attributed to increased graphene laminates and increased interfaces between graphene laminates and pores (Fig. 4d). Besides, the 14 mm-thick LG-4 lm is still mechanically robust, which can be even folded into complicated shape without any breakage or cracks. Aer repeated bending (bending speed of $0.037 Hz and bending radius from N to 0 mm, as shown in Fig. S5 †) or folding for 100 times, EMI SEs of the LG-4 lm are almost unchanged (Fig. 4f), indicating that these harsh deformations have little inuence on its EMI shielding performance.
To further study the enhanced mechanism of EMI shielding performance of graphene lms, the SE total , SE ref and SE abs at 10 GHz of the graphene lms made of large-sized akes and the SG-4, MG-4 lms were calculated ( Fig. 5a-b). As shown in Fig. 5a, SE ref of the PLG lm is the highest among these lms, indicating that the PLG lm reects more EM radiation than other lms. Aer compression of the PLG lm, SE total of LG-1 lm is signicantly reduced, mainly attributed the remarkable decrease of SE abs and slight decrease of SE ref . According to eqn (7), the decreasing distance between graphene laminates aer compression results in decrease of shielding performance for the PLG lm. Comparing with the PLG lm, the LG-4 lm possesses better EMI SE, because of higher SE abs , which is proportional to increased thickness of graphene laminates. Obviously, SE abs of the LG-4 lm is several times higher than its reection loss, which indicates that the LG-4 lm is a absorption-dominant EMI shielding material, consistent with the reported results of carbon-based materials. [30][31][32] Moreover, as shown in Fig. S6, † the absorption and reection coefficients of the LG-4 lm are around 0.160 and 0.839, respectively. And the transmission coefficient is close to zero, meaning that most of electromagnetic waves are blocked and absorbed by the LG-4 lm. As displayed in Fig. 5b, reection loss of these graphene lms compressed with four PG lms are nearly the same. Therefore, the higher EMI SE for the LG-4 lm results from the increased SE abs loss and smaller portion of EM radiation entering into shielding materials, which is mainly attributed to higher electrical conductivity of the large-sized graphene lms.
The electrical conductivity of GO and graphene lms were investigated, as shown in Fig. 5c. As the size of graphene akes increases, the sheet resistance decreases from 26.1 Â 10 8 (SGO) to 8.1 Â 10 8 U (LGO), due to less defects and functional groups. Aer graphitization, electrical conductivity of the porous graphene lms show a remarkable increase, due to defects restoration during thermal treatment. The electrical conductivity of the PSG lm is measured as 1170 S cm À1 , which increases to 1270 S cm À1 for the PLG lm. Aer compression, electrical conductivity of the LG-4 lm increases several times, up to 6740 S cm À1 . Compared with graphene lms made of smallersize akes, large-sized graphene lms possess higher electrical conductivity, leading to better EMI shielding performance. Moreover, a small increase of electrical conductivity would lead to a signicant decrease in the skin depth of a shield. 33,34 And the decrease in the skin depth for the LG-4 lm results in the signicant enhancement of SE abs . The skin depth (d) is calculated according to the following equation: where, s is the electrical conductivity, f is the frequency and m is the magnetic permeability. On the conditions that the electrical conductivity of LG-4 is 6960 S cm À1 , m keeps constant to be m 0 , and f is at 10 GHz, the calculated value of d is 6.0 mm based on eqn (8).
Although the thickness of LG-1 lm (0.0035 mm) is thinner than the calculated d, EMI SE of LG-1 still reaches to 33.1 dB, due to the consumption of more EM energy by the multi-layer structure of shielding materials. With nearly the same thickness, EMI SE of the LG-4 lm is higher than that of the SG-4 and MG-4 lms, due to smaller s for the LG-4 lm. Complex permittivity (real part 3 0 and imaginary part 3 00 ) and loss tangent (tan d) are analyzed to further study shielding mechanism, as presented in Fig. S9. † To evaluate lightweight EMI shielding materials, like foldable or wearable electronic devices, the specic shielding effectiveness divided by thickness (SSE/t) has been widely used, when taking density and weight into account. Notably, the-14 mm-thick LG-4 lm possesses EMI SE of 73.7 dB, which is higher than the 10 mm-thick copper foil (70 dB, Table S6 †). And SSE/t of the LG-4 lm is nearly 3 times higher than that of the copper foil. In our work, SSE/t of the LG-4 lm reaches to 25 680 dB cm 2 g À1 , superior to most of the reported works (Fig. 5d). 1,4,5,11,13,19,20,28,31,32,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] This nding is noteworthy because the LG-4 lm would satisfy several commercial requirements for an EMI shielding material, for example ultrahigh EMI SE (73.7 dB), low density (2.05 g cm À3 ), ultrathin thickness (0.014 mm), anti-corrosion, high exibility and easy fabrication.
Thermal conductivity analysis. The excellent electrical conductivity of compressed graphene lms implied that these lms would show high thermal conducting property. A light ash system was adopted to determine in-plane thermal conductivity of the graphene lms. As shown in Fig. 6a, the SG-1 lm possessed an excellent thermal conductivity of 628.9 W m À1 K À1 , which is much higher than copper (one of the best heat conductors, $400 W m À1 K À1 ). 20 Compared with that of the SG-1 lm, a 27.9% improvement of thermal conductivity was observed for the LG-1 lm, which is attributed to the reduced phonon-boundary scattering resulted from more compacted and ordered structure of graphene lms made of large-sized graphene akes with fewer defects. With the size of graphene akes increasing, acoustic phonons with longer wavelengths are available for heat transfer, leading to higher thermal conductivity. 52,53 However, the out-plane thermal conductivities of graphene lms are quite low, compared with their in-plane thermal conductivities (Fig. 6b). Due to fewer defects of graphene sheets, the LG lm possesses higher out-plane thermal conductivity than the SG and MG lms.
Mechanical property analysis. The ultrathin graphene lms in this research have not only superior electrical and thermal properties, also excellent mechanical performance. Representative stress-strain curves of GO and graphene lms are shown in Fig. 7a. According to these curves, when size of graphene akes increases, both tensile strength (s) and elongation at break (3 b ) are improved for these GO and compressed graphene lms. For example, the s of the SGO lm is 35.6 MPa, and the 3 b is 1.15%. Those of the LGO lm are signicantly improved to 52.6 MPa and 1.93%, when the average akes sizes increased from 5-8 mm to 40-50 mm. This size effects induced enhancement is due to less defects and stronger p-p interaction between graphene akes for the LGO lm. 9 Aer graphitization, a slight decrease of s is observed for all graphene samples, while 3 b of graphene lms is dramatically increased. As shown in Fig. 7b, the LG-4 lm shows a high 3 b of 7.85%, which is in good agreement with its high exibility and foldability. And the SEM images ( Fig. 2h-i) reveal that the excellent mechanical property of graphene lms should be attributed to microfolds of the compressed graphene lms, which enable graphene lms to recover the original structure aer repeated deformation without any crack or breakage.
The above results demonstrate that the LG-4 lm is promising to be used as thermal conducting lms in wearable or foldable electronic devices, which require high EMI SE, low density, ultrathin thickness, excellent thermal conductivity and high exibility.

Conclusion
In conclusion, ultrathin and exible graphene lms were fabricated with large-sized graphene akes, which displayed outstanding EMI shielding performance, thermal conductivity and mechanical properties. The 14 mm-thick LG-4 lm possesses EMI SE of 73.7 dB and the SSE/t of 25 680 dB cm 2 g À1 , which is one of the highest among the reported values. The superior EMI shielding performance is attributed to less defects, excellent electrical conductivity and multi-layer structure of the LG-4 lm. The LG-4 lm also shows high thermal conductivity of 803.1 W K À1 m À1 , and excellent mechanical exibility with a elongation at break of 7.85%. The above results indicate that the LG-4 lm shows great potential as excellent thermal conducting lms applied in wearable or foldable electronic devices, which require lightweight, high exibility, thermal conductivity, and efficient EMI shielding performance.

Conflicts of interest
There are no conicts to declare.