Spheres on pillars: Nanobubbling based on attogram mass delivery from metal-filled nanotubes

We report an experimental investigation into the controlled fabrication of metallic nanospheres on the tip of nanotubes. The fabrication process, nanobubbling, is based on nanofluidic mass delivery at the attogram scale using metal-filled carbon nanotubes (CNTs). Two methods have been investigated including electron-beam-induced bubbling (EBIB) and electromigration-based bubbling (EMBB). Under the irradiation of a high energy electron beam of a transmission electron microscope (TEM), the encapsulated metal is melted and extruded out from the tip of the nanotube; generating a metallic sphere. Our investigation showed that several factors including temperatures, nanotube tip breaking/opening, and electron-beam-induced reconstruction of the carbon shells are responsible to the sizes and shapes of the metallic spheres on the nanotubes. In the case that the encapsulated materials inside the CNT has a higher melting point than that of the beam energy can reach, electromigration-based mass delivery is an optional process to apply. Results show that under a low bias (2-2.5V), spherical nanoparticles can be formed on the tips of nanotubes. EMBB is a further development of the nanorobotic spot welding technique and a fundamental technology for thermal dip pen nanolithography. The sphere-on-pillar structures and the nanobubbling processes proposed will enable devices such as nanooptical antennas, scanning near-field optical microscope (SNOM) probes, scanning anodes for field emitters, and single molecule detectors, which can find applications in bio-sensing, molecular detection, and high-resolution optical microscopy.