Young modulus and elastic properties of single-walled boron nitride nanotubes probed by in-situ TEM

In recent years a significant interest has been devoted to boron nitride nanotubes (BNNTs) due to their intriguing physical properties, which make them a possible alternative to their carbon brethren in regards to possible applications [1,2,3]. However, in contrast to CNTs for which many studies have been focused in their properties, including the mechanical ones, there are almost no data for BNNTs [1]. This is the key point for the BNNTs applications in real devices. Only a few years ago it has been shown that in-situ transmission electron microscopy (TEM) is a very powerful and useful technique to investigate the mechanical and electrical properties of nanomaterials. Importantly, the structural information at the nanometer scale during the measurements could be obtained in parallel [4-6]. We carried out all these in-situ TEM studies using an AFM-TEM NanofactoryTM holder on a JEM-3100FEF microscope. In this work, we studied individual and bundled single-walled (SW) BNNTs [6,1]. Figure 1 shows a series of force/displacement curves of an isolated SW-BNNT placed between a silicon cantilever and a gold wire (inset of Fig. 1 (a)). The applied deformation force is ~ 9.5 nN, when a kink forms at the displacement of ~ 11 nm. After that no extra force is necessary to continue the deformation. These experiments have been repeated several times under different cycles (see Fig. 1 (a)). The sequence of images (Fig. 1(b)-(e)) are the snapshots from a video recorded of one of these cycles. The NTs exhibit reversible deformation during loading and unloading processes, indicating that they are extremely flexible. This fact emphasizes the unique deformation behaviour of SW-BNNTs. Applying the elastic beam theory and knowing all required structural/geometrical (length, diameter and thickness) and loading parameters (force versus displacement data), we deduced the stress and the strain values applied to the SW-BNNT. From these values, we calculated the Young Modulus. At this point, it is worth mentioning that there has been a controversy concerning the value of the Young modulus of carbon NTs. And it has been suggested that it is due to the author's interpretation of the wall thickness of the nanotube. In our case we have considered a thickness value for SW-BNNT of 0.07 nm, and them we deduced an elastic modulus of 1.11 ± 0.17 TPa. In this contribution we will discuss about this important aspect. In summary, we have developed in-situ TEM experiments for the first time probing elastic properties of SW-BNNTs. All these experiments indicated that these NTs are very flexible. Furthermore, we determined the Young modulus of an individual SW-BNNT. These studies provide very useful information that has been not acquired before and that sheds a light on the properties of these inorganic nanotubes.