Citation: Irfan, S.; Haleem, Y.A.; Irshad, M.I.; Saleem, M.F.; Arshad, M.; Habib, M. Tunability of the Optical Properties of Transition-Metal-Based Structural Phase Change Materials. Optics 2023, 4, 351–363. https:// doi.org/10.3390/opt4020026 Academic Editor: Marco Gandolfi Received: 22 February 2023 Revised: 13 April 2023 Accepted: 8 May 2023 Published: 24 May 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/). Review Tunability of the Optical Properties of Transition-Metal-Based Structural Phase Change Materials Sheheera Irfan 1 , Yasir A. Haleem 1, *, Muhammad Imran Irshad 1 , Muhammad Farooq Saleem 2 , Muhammad Arshad 3 and Muhammad Habib 4 1 Institute of Physics, Khwaja Fareed University of Engineering and Information Technology, Rahim Yar Khan 64200, Pakistan 2 GBA Branch of Aerospace Information Research Institute, Chinese Academy of Sciences, Guangzhou 510700, China 3 Nano Sciences and Technology Department, National Centre for Physics, Quaid-e-Azam University, Islamabad 45320, Pakistan 4 Department of Physics, COMSATS University of Islamabad, Lahore 5400, Pakistan * Correspondence: hiyasir@mail.ustc.edu.cn Abstract: Phase transitions are an intriguing yet poorly understood aspect of transition-metal-based materials; these phase transitions can result in changes to the refractive index, absorption coefficient, and other optical properties of the materials. Transition-metal-based materials exist in a variety of crystalline phases and also have metallic, semi-metallic, and semi-conducting characteristics. In this review, we demonstrate that alloyed W- and Mo-based dichalcogenides enable phase transitions in structures, with phase transition temperatures that are tunable across a wide range using various alloy models and modern DFT-based calculations. We also analyze the tuning the optical bandgap of the metal oxide nanoparticles through doping of the transition metal in a manner that is suitable for optical switching and thermal imaging. After the introduction and a brief illustration of the structures and their exceptional properties, we discuss synthetic methodologies and their application as part of important strategies toward the enhanced performance of transition-metal-based dichalcogenides and oxides. In the end, our conclusion highlights the prospects of 2D materials as phase transition materials due to their advantages in terms of scalability and adaptability. Keywords: transition metal; phase change; thermal imaging; optical switching 1. Introduction A number of decades ago, researchers from all over the world began focusing on a broad category of two-dimensional (2D) transition materials that presented distinct challenges to our understanding of materials chemistry and physics. Two-dimensional nanomaterials are a broad category of crystals that range from complex compounds to elemental allotropes [1]. Transition metal dichalcogenides and oxides are the most promi- nent groups of 2D crystalline compounds [2]. Investigations of 2D materials have been stimulated by their intriguing optical, catalytic, and electronic characteristics. These charac- teristics can be tuned not only by functionalization, composition, and size but also by the phase and structure of the crystal. Two-dimensional materials often exhibit, for a given ma- terial, crystal phases with substantially diverse optical and electronic properties, a diversity that allows for unique phenomena [3]. As a result of their expectational characteristics and lower energetic switching costs, transition-metal-based dichalcogenides with the ability to switch between semi-metallic, metallic, and semi-conducting phases are of great interest for next-generation electronics, including those incorporating phase change memory and its corresponding applications [4]. Most research has focused on transition-metal-based dichalcogenides with semi- metallic distorted octahedral coordinate 1T ′ phases, metallic octahedral coordinate 1T Optics 2023, 4, 351–363. https://doi.org/10.3390/opt4020026 https://www.mdpi.com/journal/optics