In this contribution we report on a transmission electron microscopy study of gold elongated nanoparticles (NPs) embedded in SiO2 produced via deposition and swift heavy ion irradiation. For this Si3N4/Au/SiO2/Si3N4 films were deposited on Si (001). RTA at 950 ºC in N2 atmosphere was then employed to induce the breakdown of the continuous Au film into a system of discrete metal NPs. The ion beam shaping process was performed by swift heavy ion irradiation with 185 keV Au ions at fluences ranging from 1.0×1013 - 3.0×1014 ions/cm2 in normal incidence.
The TEM results demonstrate that after the RTA an array of Au NPs has been formed at the interface between the bottom silicon nitride layer and the silicon dioxide layer. The particle size distribution is characterized by a mean particle size of ~ 35.0 ± 9.0 nm. Upon ion irradiation nanoparticle elongation is already noticeable for fluences as low as 3.0×1013 ions/cm2. The elongation process progress with increasing irradiation fluence. We observe, however, that even after irradiation with 3.0 × 1014 ions/cm2 the elongated NPs are still confined within the silicon dioxide layer.
The obtained results are discussed based on the Thermal Spike (TS) model. During ion irradiation, the energy deposited in the lattice results in a fast increase in the local temperature. This temperature increase resulting from TS is capable to exceed the melting temperature on the material in a narrow region surrounding the ion pathway. In this scenario, the short-lived TS in silicon nitride limits the shaping of the Au nanoparticles in this region hence confining the elongation process to the SiO2 layer.
In conclusion, the difference in thermophysical properties between silicon nitride and silicon dioxide, particularly thermal conductivity and electron phonon coupling, can be used to confine the ion shaping process to the silicon dioxide intermediate layer.