Citation: Lapi ´ nski, M.; Dró ˙ zd˙ z, P.; Gol˛ ebiowski, M.; Okoczuk, P.; Karczewski, J.; Sobanska, M.; Pietruczik, A.; Zytkiewicz, Z.R.; Zdyb, R.; Sadowski, W.; et al. Thermal Instability of Gold Thin Films. Coatings 2023, 13, 1306. https://doi.org/10.3390/ coatings13081306 Academic Editors: Roberto Montanari and Philipp Vladimirovich Kiryukhantsev-Korneev Received: 26 June 2023 Revised: 14 July 2023 Accepted: 21 July 2023 Published: 25 July 2023 Copyright: © 2023 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/). coatings Article Thermal Instability of Gold Thin Films Marcin Lapi ´ nski 1, * , Piotr Dró ˙ zd˙ z 2 , Mariusz Gol ˛ ebiowski 2 , Piotr Okoczuk 1 , Jakub Karczewski 1 , Marta Sobanska 3 , Aleksiej Pietruczik 3 , Zbigniew R. Zytkiewicz 3 , Ryszard Zdyb 2 , Wojciech Sadowski 1 and Barbara Ko´ scielska 1 1 Institute of Nanotechnology and Materials Engineering, Advanced Materials Center, Gdansk University of Technology, 80-233 Gda´ nsk, Poland; piotr.okoczuk@pg.edu.pl (P.O.); jakkarcz@pg.edu.pl (J.K.); wojciech.sadowski@pg.edu.pl (W.S.); barbara.koscielska@pg.edu.pl (B.K.) 2 Institute of Physics, Maria Curie-Sklodowska University, 20-031 Lublin, Poland; piotr.drozdz@mail.umcs.pl (P.D.); mariusz.golebiowski@mail.umcs.pl (M.G.); ryszard.zdyb@umcs.pl (R.Z.) 3 Institute of Physics, Polish Academy of Sciences, 02-668 Warsaw, Poland; sobanska@ifpan.edu.pl (M.S.); petruc@ifpan.edu.pl (A.P.); zytkie@ifpan.edu.pl (Z.R.Z.) * Correspondence: marcin.lapinski@pg.edu.pl; Tel.: +48-583486616 Abstract: The disintegration of a continuous metallic thin film leads to the formation of isolated is- lands, which can be used for the preparation of plasmonic structures. The transformation mechanism is driven by a thermally accelerated diffusion that leads to the minimalization of surface free energy in the system. In this paper, we report the results of our study on the disintegration of gold thin film and the formation of nanoislands on silicon substrates, both pure and with native silicon dioxide film. To study the processes leading to the formation of gold nanostructures and to investigate the effect of the oxide layer on silicon diffusion, metallic film with a thickness of 3 nm was deposited by molecular beam epitaxy (MBE) technique on both pure and oxidized silicon substrates. Transformation of the thin film was observed by low-energy electron microscopy (LEEM) and a scanning electron micro- scope (SEM), while the nanostructures formed were observed by atomic force microscope (AFM) method. Structural investigations were performed by low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) methods. Our experiments confirmed a strong correlation between the formation of nanoislands and the presence of native oxide on silicon substrates. Keywords: thin films; MBE; dewetting; thermal disintegration; gold nanostructures 1. Introduction Continuous solid metallic nanometric thin films can be meta- or unstable after de- position. This results in disintegration (dewetting) of the continuous layer into isolated nanoislands [1–4]. That process can be observed at temperatures well below the melting point of the nanolayer material, even at room temperature. The transformation mechanism is driven by a thermally accelerated diffusion that leads to the minimalization of surface free energy in the system [5–8]. Thin films, especially those with nanometer-scale thicknesses, have a relatively high surface energy and a large interfacial area with the substrate. This can create a driving force for the film to undergo dewetting and finally to form isolated islands. This process is governed by the balance between the energy required to create new surfaces (surface tension) and the energy gained by reducing the total interfacial area (interfacial tension) [9–12]. However, even if the thermodynamics of the system are known, there is no clear step-by-step explanation of the microscopic mechanism and kinetics of the dewetting process. Drawing from our previous experiments with gold, silver, and bimetallic Au-Ag alloyed nanostructures, we can explain the growth of metallic islands from thin films as the nucleation of holes on the film’s surface [13–15]. These holes appear at the grain borders, preferably at the triple junctions, so-called T-junctions, of boundaries [16–19]. Additionally, the nucleation of holes can be accelerated by surface melting or premelting, especially in a Coatings 2023, 13, 1306. https://doi.org/10.3390/coatings13081306 https://www.mdpi.com/journal/coatings