|Ph.D Student||Gazit Nimrod|
|Subject||Hollow Metallic Nano-Structures Attached to the Substrate|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Eugen Rabkin|
|Full Thesis text|
Hollow metallic nanostructure (nanotubes, nanoparticles, etc.) attract a great deal of attention due to their possible applications in various fields of nanotechnology (drug delivery, energy production and storage, catalysis, etc.). A number of wet chemistry methods of hollow structures synthesis have been proposed1,2, yet these methods result in a high concentration of defects and impurities in the obtained nanostructures. An essential drawback of these methods is the difficulty in controlling the pores nucleation and growth processes.
In this work, a novel method for synthesizing hollow Au nanoparticles (NPs) on sapphire substrates at low homological temperature is proposed. The Ag particles attached to the sapphire substrate are synthesized by the solid-state dewetting of thin Ag film. Afterward, the particles are coated by a thin Au overlayer, and the system is annealed at a low homological temperature of 170 °C, at which the Ag outdiffuses from the particles into the Au film, leaving a hole in the particle. The Ag atoms which left the particles were accumulated on the surface of the Au film, forming a thin layer of the Au-Ag solid solution. The driving force for this hollowing process was the mixing free energy of Au and Ag in the solid solution formed on the top of Au film. This method provides higher microstructure stability of the hollow structure than the nanoscale Kirkendall effect and allows “sculpturing” of the size and shape of the internal pores. Applying this method to the nanoparticles of the Ag-Au alloy did not result in particles hollowing. Instead, partially agglomerated thin Au films with a high area density of holes were obtained. The decreased thermal stability of the Au film was attributed to the thermal grooving at the grain boundaries and their triple junctions in the Au film accelerated by the diffusion flow of Au from the film to the alloy particles. Based on the latter results we concluded that chemical driving forces have to be taken into account in the analysis of the thermal stability of multicomponent thin films.
A similar procedure was also applied to Al@Au core-shell particles and resulted in hollow particles of the AlAu2 intermetallic phase and the formation of homoepitaxially grown alumina at the Au-sapphire interface. The mechanism of this hollowing process was discussed in terms of oxidation of Al diffusing along the Au-sapphire interface by the oxygen diffusing through the grain boundaries of the Au layer.
The above examples illustrate how the short-circuit diffusion along the surfaces, grain boundaries and interfaces can be utilized for the design of complex nanoscale microstructures.