Fabrication of Ternary AgPdAu Alloy Nanoparticles on c-Plane Sapphire by the Systematical Control of Film Thickness and Deposition Sequence SUNDAR KUNWAR, PURAN PANDEY, MAO SUI, SUSHIL BASTOLA, and JIHOON LEE In this work, a systematic study on the fabrication of ternary AgPdAu alloy nanoparticles (NPs) on c-plane sapphire (0001) is presented and the corresponding structural and optical characteristics are demonstrated. The metallic trilayers of various thicknesses and deposition orders are annealed in a controlled manner (400 °C to 900 °C) to induce the solid-state dewetting that yields the various structural configurations of AgPdAu alloy NPs. The dewetting of relatively thicker trilayers (15 nm) is gradually progressed with void nucleation, growth, and coalescence, isolated NP formation, and shape transformation, along with the temperature control. For 6 nm thickness, owing to the sufficient dewetting of trilayers along with enhanced diffusion, dense and small spherical alloy NPs are fabricated. Depending on the specific growth condition, the surface diffusion and interdiffusion of metal atoms, surface and interface energy minimization, Rayleigh instability, and equilibrium configuration are correlated to describe the fabrication of ternary alloy NPs. Ternary alloy NPs exhibit morphology-dependent ultravio- let–visible–near infrared (UV–VIS–NIR) reflectance properties such as the inverse relationship of average reflectance with the surface coverage, absorption enhancement in specific regions, and reflectance maxima in UV and NIR regions. In addition, Raman spectra depict the six active phonon modes of sapphires and their intensity and position modulation by the alloy NPs. https://doi.org/10.1007/s11661-018-4573-8 Ó The Minerals, Metals & Materials Society and ASM International 2018 I. INTRODUCTION METALLIC alloy nanoparticles (NPs) can offer great potential in optoelectronic, photonic, catalysis, sensing, energy, and biomedical applications due to their multifunctionality, elemental diversity, and composi- tional sensitivity. [1–6] The integration of two or more metallic elements in a single NP can offer an additional route to engineer the physical, electronic, catalytic, and magnetic properties by the appropriate control of structural configuration as well as elemental composi- tion. [7–11] For example, the Pt/Ni and Pt/Ru/Ni alloy NPs have shown improved catalytic activities as com- pared to the monometallic Pt NPs in the oxidation methanol fuel cell, which is correlated to the modulation of binding energy and electronic structure of alloy NPs. [7] The control of shape, size, density, crystal structures, and elemental composition of alloy NPs has been identified as the key element to tune the localized surface plasmon resonance (LSPR) frequency, target sensitivity, optical coupling, and chemical reac- tivity, which directly determines the corresponding device performance. As an example, the Au-Pd NPs showed a large red shift of LSPR peak from 440 to 732 nm along with the increased Pd content. [12] Among the various metal elements, monometallic Au, Pd, and Ag exhibit interesting plasmonic, catalytic, and structural properties, which have been widely investigated and applied in many practical applications. [13–18] However, the systematic investigation of composite AgPdAu alloy NPs with the tunable elemental and physical properties, which can be of importance for many applications, has not been reported yet. In this work, the fabrication of ternary AgPdAu alloy NPs on sapphire (001) is demon- strated by the solid-state dewetting of sequentially deposited trimetallic layers of Ag, Au, and Pd. Thermal annealing of trilayers yields the dewetting of uniform film into composite AgPdAu alloy NPs by the diffusion and interdiffusion of metal atoms. Due to the altered dewetting behavior, various surface morphologies of SUNDAR KUNWAR, PURAN PANDEY, MAO SUI, and SUSHIL BASTOLA are with the College of Electronics and Information, Kwangwoon University, Seoul 01897, South Korea. JIHOON LEE is with the College of Electronics and Information, Kwangwoon University and the Institute of Nanoscale Science and Engineering, University of Arkansas, Fayetteville AR 72701. Contact e-mail: jihoonleenano@gmail.com Manuscript submitted September 26, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A