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

Abstract

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.

Fig. 1. Schematic illustration of preparation process of graphene films (x represents the number of PrGO films were compressed together).

Fig. 2. Morphology and structural characterization of GO and graphene films. (a) Corresponding histograms of GO flakes size distribution; SEM images of: (b) small-size flakes, (c) medium-size flakes and (d) large-size flakes; cross-section SEM images of (e) LGO film; (f) PLG film; top views of (g) PLG film, (h) LG-1 film; cross-section SEM images of (i) LG-8 film; (j) picture of thickness measurement.

Fig. 3. (a) FT-IR spectra of SGO, MGO and LGO; (b) XPS survey spectra, (c) XRD spectra, and (d) Raman spectra of the GO films and graphene films.

Fig. 4. The EMI SEs of (a) the PG films, (b) the graphene films compressed with one PG film, (c) the graphene films made of large-sized flakes; (d) an ideal multi-layer model for PLG and compressed graphene films composed of graphene laminates and insulated air pores; (e) schematic representation of a local multi-layer model composed of two graphene laminates and insulated pores between two graphene laminates; (f) EMI SEs of the LG-4 film after bending and folding.

Fig. 5. SE_ref, SE_abs and SE_total at 10 GHz of (a) the PLG, LG-1, LG-2, LG-4 films, (b) the SG-4, MG-4, LG-4 films; (c) electrical conductivity of GO films and graphene films; (d) EMI SSE/t versus thickness of the graphene films, in comparison to different materials in the previous literature, and t represents thickness.

Fig. 6. (a) In-plane thermal conductivity and (b) out-plane thermal conductivity of compressed graphene films.

Fig. 7. Tensile properties of GO and compressed graphene films: (a) stress–strain curves and (b) tensile strength and elongation at break.