Nanoscale bimetallic strip: Atomistic bending mechanisms of AuPd bimetallic nanowhiskers

The bimetallic strips combining two metals with different thermal expansion coefficients are widely employed for temperature sensing, actuation and control. In the present study, we fabricated nanoscale bimetallic strip by one-sided coating with Pd of ultra-strong single crystalline Au nanowhiskers grown by molecular beam epitaxy. The as-fabricated bimetallic nanowhiskers were slightly bent due to heteroepitaxial stresses in the single crystalline Pd coating. The temperature-induced shape changes of these bimetallic Au-Pd nanowhiskers were studied via in-situ thermal actuation in scanning electron microscope. Annealing of the bimetallic nanowhisker resulted in a change in the amplitude and sign of the curvature. At low temperatures, the bending was limited and reversible, indicating that, like in the classical bimetallic strip, it is caused by thermal stresses due to the mismatch of thermal expansion coefficients of Au and Pd. At higher temperatures, the bending was irreversible and in opposite direction as compared to thermal bending. This type of bending was attributed to the coherency loss at the semi-coherent Au-Pd interface, and to the interdiffusion of Au and Pd and concomitant change of the lattice parameter of the formed Au-Pd alloy. Our results demonstrate that single crystalline metallic nanowhiskers with controlled curvature, chemical composition and thermal behavior can be produced employing interface engineering and chemical interdiffusion.