Compressive Strength Enhancement of Vertically Aligned Carbon Nanotube Forests by Constraint of Graphene Sheets

We fabricated a 3D sandwich hybrid material composed of graphene and vertically aligned carbon nanotube forests (VACNTs) using chemical vapor deposition. The graphene was first synthesized on Cu foil. Then it was transferred to a substrate which had a pre-deposited catalyst Fe film and a buffer film of Al(2)O(3) for the growth of VACNTs. The VACNTs were grown underneath the graphene and lifted up the graphene. The graphene, with its edges anchored on the Al(2)O(3), provided a constrained boundary condition for the VACNTs and hence affected the growth height and mechanical strength of the VACNTs. We prepared three groups of samples: VACNTs without graphene, VACNTs with graphene transferred once (1-Gr/VACNTs), and VACNTs with graphene transferred twice (2-Gr/VACNTs). A nano-indentation system was used to measure the reduced compressive modulus (Er) and hardness (H). The Er and H of Gr/VACNTs increased with the number of transfers of the anchored graphene. The 2-Gr/VACNTs had the largest Er and H, 23.8 MPa and 912 KPa, which are 6.6 times and 5.2 times those of VACNTs without the anchored graphene, respectively. In this work, we have demonstrated a simple method to increase the mechanical properties and suppress the height of VACNTs with the anchored graphene and number of transfers.