In the last decade, lead halide perovskites have revolutionized material science with their remarkable charge mobility, light absorption, and adjustable band gaps, all achieved through low-temperature processing.[1] However, their instability and toxicity limit their broader application. Despite these challenges, perovskites hold great potential for future energy technologies, spurring the development of "perovskite-inspired" materials (PIMs). In this aspect, the scientific interest has recently shifted to alkali metal-based chalcogenides, which represent a new category of semiconducting inorganic compounds and are being explored as potential candidates in the quest for novel energy materials. Recently, the focus has shifted to alkali metal-based chalcogenides as promising semiconductors for energy materials. Ternary alkali-metal dichalcogenides {AMeE (A = Li, Na, K, Rb, Cs; Me= Metals E = S, Se, Te} are identified as potential candidates for energy conversion and storage.[2] While high-temperature solid-state synthesis often results in limited phase control, wet chemical synthesis offers a promising alternative by producing uniform nanoscale particles and providing insights into their formation.[3,4]

Building on several theoretical and experimental studies on cesium copper-based chalcogenides, we present the synthesis of cesium copper selenide on a nanoscale regime with precise control over dimension, morphology, and phase.[5,6] The influence of key reaction variables, such as precursor’s reactivity, ligands, reaction temperature, and reaction time, on the size and shape of the nanocrystals was demonstrated, showcasing the flexibility of the wet chemical synthesis. An ex-situ mechanistic investigation reveals that NC formation is driven by the dissolution of binary Cu_2-xSe, followed by incorporating Cs⁺ to form the ternary CsCu₅Se₃. The current study also reveals that variation in the alkyl chain length of amines influences the size, shape, and formation of distinct phases. The structural, electronic, and thermoelectric characteristics were experimentally evaluated and further corroborated by computational analysis. The experimental results revealed the material's ultralow thermal conductivity of 0.6 W.M^-1.K^-1 and a good thermoelectric figure of 0.3 at 720 K, providing concrete evidence of its potential. The detailed mechanistic insights presented in this study will significantly advance the development of cutting-edge functional materials in the field of alkali metal chalcogenides for various applications.