Carbon nanotubes (CNTs) have remarkable mechanical and electrical properties but so far their growth has been restricted to substrates made of flat and brittle dielectric oxide materials. This has prevented their use in devices requiring flexible and durable surfaces with good electroconductive properties. A team of scientists from Korea and the US led by Sang Ouk Kim from KAIST in Seoul, Korea1, have now provided a solution by demonstrating that reduced thin graphene oxide films provide an excellent platform for CNT growth. The team has also shown that the resulting graphene–CNT hybrid films may be suitable for a wide range of applications including flexible displays, solar cells and sensors.

One of the requirements for successful commercialization of hybrid films is their suitability for large-area fabrication. Preparing large-area graphene films using the conventional micromechanical exfoliation or chemical vapor deposition (CVD) methods is challenging. However, as Kim explains, “Spin casting of graphene films using oxygenated natural graphite is a viable technique.” By this approach, graphene oxide platelets are first dispersed in water, then extracted on a silicon-dioxide wafer to form an ultrathin, large-area film with good electroconductivity, mechanical flexibility and optical transparency.

The team grew CNTs on top of the graphene oxide film using plasma-enhanced CVD from arrays of iron catalyst nanoparticles, resulting in a ‘forest’ of CNTs. The height and thickness of the nanotubes could be sensitively tuned by adjusting the growth conditions and the size of the catalyst particles.

However, as Kim notes, “As the CNT arrays are grown vertically on the graphene film, the structure could potentially be distorted by continuous external deformations.” To prevent this from occurring, the researchers infiltrated the three-dimensional matrix of CNTs with an elastomer, which stabilized the CNT arrays and enhanced the mechanical robustness of the hybrid film.

Fig. 1: Cross-sectional scanning electron microscopy image of a hybrid graphene–CNT film attached to a flexible poly(ethylene terephthalate) (PET) film (upper). The hybrid film has excellent mechanical compliance and stays attached to the PET film even after repeated stretching and bending (lower).

The carbon hybrid films displayed excellent structural integrity when subjected to various types of mechanical deformation, such as bending (Fig. 1, upper) and stretching. Crucially, the films could be attached to and detached from nonplanar substrates, and subjected to repeated bending and stretching cycles with minimal damage and little change to their electrical properties, while remaining attached to the host films (Fig. 1, lower).

According to Kim, the carbon hybrid films can be readily fabricated on an industrial scale. However, he notes that further development is need on the method for transferring the hybrid film from the underlying silicon dioxide wafer, which was achieved in their study by chemical etching.