Semiconductor nanotubes partially filled with metals of low melting temperature are of interest due to potential applications as metal-semiconductor heterostructures in nanoelectronics or in nanoelectromechanical systems. In this study, we have investigated ZnO nanotube structures with Sn-rich core regions that have been grown by a thermal evaporation-deposition method from a mixture of SnO2 and ZnS powders as precursors [1]. The microstructure has been characterized by diffraction contrast transmission electron microscopy (TEM) and by high-angle annular dark-field imaging in scanning TEM (STEM-HAADF) combined with energy-dispersive X-ray spectroscopy (EDXS). Furthermore, in situ TEM experiments were performed to investigate the melting and the behavior of the molten Sn-rich core material.
Most of the ZnO nanorod structures possess hexagonal cross-sections and diameters in the range of tens of nanometers. Typical of many of these ZnO nanomaterials are tubular core regions that are either largely empty or are locally filled with Sn or a Sn-rich phase as shown by 2-dimensional EDXS mappings of the elemental distributions. In situ TEM experiments confirm the observation of a core material of low melting temperature. Applying variable thermal load by changing the electron beam flux allows inducing locally phenomena that are related to the melting and reversible thermal expansion and contraction of the Sn-rich core material. For ZnO nanotubes with open tip regions, for instance, the melting of the Sn core and the subsequent expansion lead to a nanodrop emerging from the tip and, upon removing the heat load, its contraction and re-solidification. This phenomenon occurs reversibly for multiple cycling. The emergence of the nanodrop and its projected radius of curvature can be controlled to a certain degree by focusing the electron beam. Isolated Sn nanodrops can form as well, containing an estimated volume of 10-18 - 10-20 liters of liquid Sn at a radius of few tens of nanometers. These observations suggest potential applications for soldering at nanometer scale. Furthermore we describe some phenomena related to structure modifications during extended electron beam exposure.
[1] Y.Ortega, Ch. Dieker, W. Jäger, J. Piqueras and P. Fernández. Nanotechnology 21 (2010) 225604