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.

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.

The formation of alloyed Au–Ag bimetallic nanoparticles (NPs) by the solid-state thermal dewetting of Au–Ag bilayer thin films on Si substrates was reported in this work. Complete dewetting of the bilayer thin films to form alloyed Au–Ag bimetallic NPs can be achieved at temperatures above the Tammann temperature of at least one of the metal components. The NP size depends heavily on the total thickness of the bilayer thin films, whereas the atomic ratio of Au/Ag and the sputtering order in the bilayer films do not affect the NP size significantly. It has been demonstrated by X-ray photoelectron spectroscopy (XPS) analysis that the produced Au–Ag bimetallic NPs are alloyed and the sputtering order of Au and Ag in the initial bilayer films has no impact on the final configuration of the produced NPs. To further characterize the surface plasmon resonance (SPR) of Au–Ag NPs, a procedure involving the use of poly­(vinyl alcohol) (PVA) and poly­(methyl methacrylate) (PMMA) was developed to remove the bimetallic NPs from the optically opaque Si substrates. The SPR peak wavelength of the bimetallic Au–Ag alloy NPs has been shown to vary linearly with the atomic percentage of Au in the NPs, allowing for the tuning of the resonance wavelength by changing the alloy composition. The ability to produce uniform Au–Ag alloy NPs by thermal dewetting, the successful removal of the Au–Ag NPs from the Si substrates into the colloidal solution, the tunable SPR, and the excellent long-term stability of the alloy NPs in solution open up many opportunities for the potential applications of these Au–Ag alloy NPs, for example, for surface functionalization, sensing, and catalysis.

Non-stoichiometric Co2FeAl nanoparticles are formed by the in-solution thermal decomposition of the corresponding metal acetylacetonate complexes in the presence of capping ligands followed by reduction of the obtained material under an H2-containing atmosphere. Transmission electron microscopy indicates that sub-100 nm nanoparticles are obtained, with reasonable size control. Magnetic measurements indicate that the saturation magnetization, Bloch behavior, magnetization reversal, spin-wave stiffness, and exchange stiffness are all comparable to those observed for bulk and thin-film Co2FeAl, indicating that these nanomaterials are promising for use in nanoparticle-based spintronic devices.

β-Ga2O3 nanostructures, including nanowires (NWs), nanosheets (NSHs), and nanorods (NRs), were synthesized using thermally dewetted Au nanoparticles as catalyst in a chemical vapor deposition process. The morphology of the as-grown β-Ga2O3 nanostructures depends strongly on the growth temperature and time. Successful growth of β-Ga2O3 NWs with lengths of 7–25 μm, NSHs, and NRs was achieved. It has been demonstrated that the vapor–liquid–solid mechanism governs the NW growth, and the vapor–solid mechanism occurs in the growth of NSHs and NRs. The X-ray diffraction analysis showed that the as-grown nanostructures were highly pure single-phase β-Ga2O3. The bandgap of the β-Ga2O3 nanostructures was determined to lie in the range of 4.68–4.74 eV. Characteristic Raman peaks were observed with a small blue and red shift, both of 1–3 cm−1, as compared with those from the bulk, indicating the presence of internal strain and defects in the as-grown β-Ga2O3 nanostructures. Strong photoluminescence emission in the UV-blue spectral region was obtained in the β-Ga2O3 nanostructures, regardless of their morphology. The UV (374–377 nm) emission is due to the intrinsic radiative recombination of self-trapped excitons present at the band edge. The strong blue (404–490 nm) emissions, consisting of five bands, are attributed to the presence of the complex defect states in the donor (VO) and acceptor (VGa or VGa–O). These β-Ga2O3 nanostructures are expected to have potential applications in optoelectronic devices such as tunable UV–Vis photodetectors.