Direct Observation of the Molten State of Nanometer-sized Particles With an Atomic Force Microscope: A Feasibility Study

An atomic force microscope (AFM) was used to directly examine the physical state of nanometer-sized particles. The critical diameter of indium particles, where evidence of melting at room temperature was observed, was 7.8 ± 1.2 nm. This conclusion is based on a method relying on the manipulation of particles in ambient air and at constant temperature. This method involves a simple set-up that permits a combination of both manipulation and imaging of individual particles. To determine whether a particle is molten, three criteria are used: the merging of particles to form bigger spherical particles, a tip-induced shape change, and the formation of nanofibers. All three criteria have been checked using other particle materials. An attempt at 56°C revealed oxidation of the indium particles as the major problem for melting investigation. Manipulations under high-purity nitrogen atmosphere support the validity of the findings. The use of the AFM to determine whether a nanoparticle is molten is, however, complicated by the oxidation issue.