materials Article Controlling Equilibrium Morphologies of Bimetallic Nanostructures Using Thermal Dewetting via Phase-Field Modeling Taejin Kwak and Dongchoul Kim *   Citation: Kwak, T.; Kim, D. Controlling Equilibrium Morphologies of Bimetallic Nanostructures Using Thermal Dewetting via Phase-Field Modeling. Materials 2021, 14, 6697. https:// doi.org/10.3390/ma14216697 Academic Editor: Alexander Vul Received: 12 October 2021 Accepted: 5 November 2021 Published: 7 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). School of Mechanical Engineering, Sogang University, Seoul 04107, Korea; xowls4189@sogang.ac.kr * Correspondence: dckim@sogang.ac.kr Abstract: Herein, we report a computational model for the morphological evolution of bimetallic nanostructures in a thermal dewetting process, with a phase-field framework and superior optical, physical, and chemical properties compared to those of conventional nanostructures. The quantita- tive analysis of the simulation results revealed nano-cap, nano-ring, and nano-island equilibrium morphologies of the deposited material in thermal dewetting, and the morphologies depended on the gap between the spherical patterns on the substrate, size of the substrate, and deposition thickness. We studied the variations in the equilibrium morphologies of the nanostructures with the changes in the shape of the substrate pattern and the thickness of the deposited material. The method described herein can be used to control the properties of bimetallic nanostructures by altering their equilibrium morphologies using thermal dewetting. Keywords: phase-field simulation; thermal dewetting; bimetallic nanostructure; equilibrium morphology 1. Introduction Thermal dewetting on patterned substrates yields metallic nanostructures [1] with a controlled size and arrangement [2] and low cost [3] for various applications, such as high- density recording media and storage devices [4–7], electron transporting materials [8–10], catalysts for the growth of nanowires and nanotubes [11–13], phase-change devices using plasmonic nanogaps [14], plasmonic devices for photodetection [15], plasmon-resonance devices [8,16], electrochemical sensing [17,18], and nano-plasmonic polymerase chain re- action (PCR) [19,20]. The properties of nanoparticles, such as dimension, configuration, arrangement, gap, and uniformity, must be predictable and precisely controlled to im- prove their optical, catalytic, and electronic performances in these applications [21]. For instance, the localized surface plasmon resonance can be tuned by controlling the surface morphology, nanoparticle size, and space between nanoparticles to obtain an optimum surface-enhanced Raman spectroscopy signal from the metal nanostructures at the target wavelength [22]. Additionally, plasmonic hotspots can be intensified using nanostructures with narrow gaps on a fiber-top surface [23]. Thermal dewetting has been used to control and predict the arrangement, spacing, and uniformity of nanoparticles. Although a substrate with separated nano-templates comprising uniformly arranged nanoparticles were fabricated on a flat plate using thermal dewetting [24], additional processes, such as lithographic patterning, reactive ion etching, and wet etching, were required during the process. Gold nanoparticles were previously fabricated on an aluminum dimple array via thermal treatment and the transformation behavior was characterized using thermal dewetting [25]. However, the control of the parameters, such as the gap, arrangement, and size of the nanoparticles, has not been stud- ied yet. Although attempts were made to reduce the gap in the nanostructure, this could only be achieved using an additional thermal dewetting process [26]. The nano-caps and nano-aperture arrays fabricated using a nanopatterned template during thermal dewetting required lithography and allowed only longitudinal control [27]. Although a TiO 2 nan- otube fabricated using thermal dewetting yielded uniform spherical nanoparticles [28], the Materials 2021, 14, 6697. https://doi.org/10.3390/ma14216697 https://www.mdpi.com/journal/materials