Crystallography-Derived Young's Modulus and Tensile Strength of AlN Nanowires as Revealed by in Situ Transmission Electron Microscopy.

Aluminum nitride (AlN) has a unique combination of properties, such as high chemical and thermal stability, nontoxicity, high melting point, large energy band gap, high thermal conductivity, and intensive light emission. This combination makes AlN nanowires (NWs) a prospective material for optoelectronic and field-emission nanodevices. However, there has been very limited information on mechanical properties of AlN NWs that is essential for their reliable utilization in modern technologies. Herein, we thoroughly study mechanical properties of individual AlN NWs using direct,  in situ bending and tensile tests inside a high-resolution TEM. Overall, 22 individual NWs have been tested, and a strong dependence of their Young's moduli and ultimate tensile strengths (UTS) on their growth axis crystallographic orientation is documented. The Young's modulus of NWs grown along the [101̅1] orientation is found to be in a range 160-260 GPa, whereas for those grown along the [0002] orientation it falls within a range 350-440 GPa. In situ TEM tensile tests demonstrate the UTS values up to 8.2 GPa for the [0002]-oriented NWs, which is more than 20 times larger than that of a bulk AlN compound. Such properties make AlN nanowires a highly promising material for the reinforcing applications in metal matrix and other composites. Finally, experimental results were compared and verified under a density functional theory simulation, which shows the pronounced effect of growth axis on the AlN NW mechanical behavior. The modeling reveals that with an increasing NW width the Young's modulus tends to approach the elastic constants of a bulk material.