Van der Waals Interaction between Carbon Nanotubes and Flat Surfaces

Due to their superb electrical, mechanical, and thermal properties, carbon nanotubes (CNTs) hold promise as potential building blocks for future electronics. A major role is played by the substrate on which the CNTs are lying. Until recently, SiO₂ was the most common substrate for CNTFETs. This substrate has many advantages, mainly its compatibility to silicon technology. However, its primary drawback is the reduction of electronic mobility in CNT channels due to charge density fluctuations induced by the impurities presented in the devices.

Recent studies have shown that hexagonal boron nitride (hBN) serves as superb substrates for graphene devices, due to their almost perfect lattice match. Moreover, the twist angle between these two dimensional (2D) van-der Waals materials play an important role in the electrical, mechanical, and optical properties of the resulted hybrid devices.

In this study we investigate van-der Waals interaction between carbon nanotubes and different substrates using atomic-force microscopy, and molecular-dynamics simulations. A lower CNT height was simulated and measured on top of graphene and hBN compared to that on amorphous SiO₂. For small diameter CNTs the resulted height difference is negligible, due to their high curvature and resulted higher bending energy. However, for larger diameters it is more favorable to deform the CNTs on graphene or hBN than on silica due to the higher binding energy between the CNT and these substrates. Energetically it is more likely to deform the CNTs and to increase their contact area on the expanse of the elastic energy loss by their deformed cross sections. These results suggest that electronic devices based on CNTs and two-dimensional materials will operate differently than conventional CNTs transistors.