Silicon nanocrystals (SiNCs) have become an important class of materials in the fields of photovoltaics, thermoelectrics, lighting, and medicine. Impurities within SiNCs dramatically alter the electrical and optical properties of the host material, whether the impurity is intentionally added in an attempt to manipulate properties, or is inherent to the material and its natural state. Despite such remarkable changes, impurity incorporation within SiNCs remains poorly understood, since concepts applied to understanding impurities in bulk materials may not completely translate to nanomaterials. Understanding the effect of SiNC impurities requires new technologies to produce materials suitable for study combined with new insights to expound the differences in the nanoscale physics. Nonthermal plasma-assisted gas-phase synthesis provides an excellent route to producing and investigating impurities within SiNCs due to the unique chemical reaction environment of the plasma. The robustness of such a technique allows for the production of very pure SiNCs or SiNCs with added impurities simply by adding different chemicals to the plasma. The chapters in this document focus on the effect that different impurities have on the properties of SiNCs. Chapter 2 focuses on heavily P-doped SiNCs exhibiting the first known observation of a unique electrical and optical property known as localized surface plasmon resonance (LSPR) within free-standing SiNCs. Chapter 3 explains the synthesis of B- and P-doped SiGeNC alloys and their deposition into thin films for thermoelectric applications. Chapter 4 highlights research which uses P-doped SiNCs to form emitter layers for pn-junction type solar cells, including device fabrication and optical characterization. Chapter 5 examines inherent impurities in the form of dangling bond defects which may be responsible for the quenching of SiNC photoluminescence, and their evolution during the process of air-ambient oxidation. Several appendices at the end of the document detail some of the fabrication processes used throughout this work, as well as brief reports of some side projects that may be of interest to researchers intent on studying SiNC synthesis and deposition technologies.