Nanomaterials composed of metals and semiconductors have been studied as powerful tools for various applications in the fields of diagnostics, therapy, sensing, and photovoltaics. Based on the specific application, these nanomaterials can be tuned and optimized to achieve their best performance. Modifications can be made to alter the size, shape, surface morphology, and surface passivation of these nanoparticles. In this dissertation, the effect of these changes on the optical characteristics necessary for effective performance of hollow plasmonic nanoparticles and perovskite nanocrystals are investigated. In chapter 1, a comprehensive literature review was completed on hollow plasmonic nanoparticles, their synthetic procedures, and their applicability to the biomedical field. This chapter, describes the physical properties of hollow plasmonic structures, such as hollow gold nanospheres (HGNs), by discussing SPR and Mie theory. Numerous synthetic methods leading to hollow structures, including galvanic exchange, and ways to effectively tune size, shell thickness, and surface properties were described. Finally, hollow plasmonic particles’ ability to sense, image, diagnose, and treat diseases were assessed based off the changes in their physical properties. In chapter 2, HGNs, hollow gold nanostars (HNSs), and silver coated hollow gold nanostars (AgHNSs) were synthesized and compared for their SERS applicability. It was demonstrated in this work that both HNSs and AgHNSs exhibit tunable structural and optical properties. Using rhodamine 6G as a probe molecule, it was determined that HNSs reported an order of magnitude higher relative enhancement over previously reported HNSs and AgHNSs have a four-fold increase in SERS signal compared to HNSs reported herein. Theoretical analyses were compared to the experimental findings, which suggested that the increase in enhancement was not completely due to the addition of silver, but also potentially the branching of individual spikes or increased binding efficacy of the analyte to the nanoparticle. To further explore the applicability of these nanoparticles, capping ligand exchange was completed from citrate to pentanethiol to increase their interaction with negative analytes like bovine serum albumin. In chapter 3, methylammonium lead bromide magic sized clusters (MSCs) and quantum dots (QDs) were synthesized using a new heated ligand assisted reprecipitation method. A mixture of QDs and MSCs were tuned to monodispersed MSCs by increasing the temperature of the precursor solution. Additionally, the size of the MSCs themselves was tuned by increasing the temperature of the precursor solution and increasing the concentration of capping ligand. The MSCs were analyzed with Raman spectroscopy and determined to be similar to bulk perovskite. Using IR, mass spectrometry, and control experiments it was determined that oleylamine was the primary capping ligand on the surface of the magic-sized cluster and therefore that the surface must be cationic in nature. In chapter 4, a comparison of the stability of previous ligand assisted reprecipitation method and newer temperature dependent synthesis of MSCs was made. The MSCs were placed under ambient conditions and exposed to water and observed until they no longer emitted light. Then, they were stabilized with noncoordinating polymer matrix, paraffin, and the solution was then similarly exposed to water and observed until both particles degraded. The paraffin MSCs mixtures were then dried as a film. The LARP MSCs degraded after 3 hours, but the HLARP MSCs remained luminous for 8 days when stabilized with paraffin. Both solids were analyzed with Raman spectroscopy. HLARP MSCs were then characterized using X-ray diffraction and TEM. It was determined that HLARP MSCs are more ordered that LARP MSCs and have an orthorhombic quasi-crystalline structure.