RESUMEN
Formation of Au, Pt, and bimetallic Au-Pt nanostructures by thermal dewetting of single-layer Au, Pt and bilayer Au-Pt thin films on Si substrates was systematically studied. The solid-state dewetting of both single-layer and bilayer metallic films was shown to go through heterogeneous void initiation followed by void growth via capillary agglomeration. For the single-layer of Au and Pt films, the void growth started at a temperature right above the Hüttig temperature, at which the atoms at the surface or at defects become mobile. Uniformly distributed Au (7 ± 1 nm to 33 ± 8 nm) and Pt (7 ± 1 nm) NPs with monodispersed size distributions were produced from complete dewetting achieved for thinner 1.7-5.5 nm thick Au and 1.4 nm thick Pt films, respectively. The NP size is strongly dependent on the initial thin film thickness, but less so on temperature and time. Thermal dewetting of Au-Pt bilayer films resulted in partial dewetting only, forming isolated nano-islands or large particles, regardless of sputtering order and total thin film thickness. The increased resistance to thermal dewetting shown in the Au-Pt bilayer films as compared to the individual Au or Pt layer is a reflection of the stabilizing effect that occurs upon adding Pt to Au in the bimetallic system. Energy dispersive x-ray spectroscopic analysis showed that the two metals in the bilayer films broke up together instead of dewetting individually. According to the x-ray diffraction analysis, the produced Au-Pt nanostructures are phase-segregated, consisting of an Au-rich phase and a Pt-rich phase.
RESUMEN
Two different dewetting methods, namely pulsed laser-induced dewetting (PLiD)-a liquid-state dewetting process and thermal dewetting (TD)-a solid-state dewetting process, have been systematically explored for Ag thin films (1.9-19.8 nm) on Si substrates for the fabrication of Ag nanoparticles (NPs) and the understanding of dewetting mechanisms. The effect of laser fluence and irradiation time in PLiD and temperature and duration in TD were investigated. A comparison of the produced Ag NP size distributions using the two methods of PLiD and TD has shown that both produce Ag NPs of similar size with better size uniformity for thinner films (<6 nm), whereas TD produced bigger Ag NPs for thicker films (≥8-10 nm) as compared to PLiD. As the film thickness increases, the Ag NP size distributions from both PLiD and TD show a deviation from the unimodal distributions, leading to a bimodal distribution. The PLiD process is governed by the mechanism of nucleation and growth of holes due to the formation of many nano-islands from the Volmer-Weber growth of thin films during the sputtering process. The investigation of thickness-dependent NP size in TD leads to the understanding of void initiation due to pore nucleation at the film-substrate interface. Furthermore, the linear dependence of NP size on thickness in TD provides direct evidence of fingering instability, which leads to the branched growth of voids.
RESUMEN
This work reports the influence of the substrate in the pulsed laser-induced dewetting (PLiD) of Au thin films for the fabrication of nanoparticle (NP) arrays. Two substrates were studied, i.e., polished silicon and porous silicon (PS), the latter being fabricated via electrochemical anodization in HF-containing electrolytes. The effect of both PLiD and substrate preparation parameters was explored systematically. On polished silicon substrates, it has been shown that uniform, randomly arranged NPs between 15 ± 7 nm and 89 ± 19 nm in diameter are produced, depending on initial thin film thickness. On PS however, there are topographical features that lead to the formation of ordered NPs with their diameters being controllable through laser irradiation time. The presence of surface pores and the appearance of surface ripples under low HF concentrations (<9.4 wt%) during electrochemical anodization results in this unique dewetting behaviour. Through AFM analysis, it has been determined that the ordered NPs sit within the valleys of the ripples, and form due to the atomic mobility enabled using the PLiD approach. This work has demonstrated that the utilization of topographically complex PS substrates results in size controllable and ordered NPs, while the use of polished Si does not enable such control over array fabrication.
