The numerous unique properties of layered transition metal chalcogenides have recently sparked significant interest for potential applications in next-generation electronics and optoelectronics. To achieve this objective, several elements of the material chemistry and engineering must be established and effectively controlled, including synthesis and processing, interplay and tunability of composition-structure-property relationship, and stability under external factors. In chapter II, we discuss the scaled-up synthesis of TaSe3 crystals to use as fillers in thin-film composites for potential electromagnetic interference (EMI) shielding applications. We investigate the influence of various synthetic conditions on the composition, size, and quality of TaSe3 crystals and optimize its synthesis to produce a large quantity of impurity-free, high-quality crystals in high yield. In chapter III, we extend synthetic studies to three transition metal trichalcogenides (TMT) alloy systems: NbxTi1-xS3, TixTa1-xSe3, and NbxTa1-xSe3. We optimize the synthesis of large crystals for each mixed-metal alloy system and provide detailed compositional and structural characterization. We also compare the significantly different effects of isoelectronic and non-isoelectronic metal substitution on the TaSe3 lattice. We gain insight into the thermal stability of NbS3 using in situ electron microscopy in chapter IV. We provide a possible mechanismfor the breakdown and transformation of quasi-one-dimensional NbS3 nanoribbons into two-dimensional NbS2 nanosheets which we observe at temperatures above 500 °C. Finally, chapter V investigates the optical properties of MoS2 and WS2 monolayers engineered as compositionally-graded alloys and in-plane heterostructures. Temperature-dependent photoluminescence studies reveal significantly different behaviors as a function of temperature.