Preparation of Nanoscale Lithium Niobate for Non-linear Optical Applications

Book excerpt: Lithium niobate (LiNbO3) is a unique photonic material, often referred to as the "silicon of photonics", due to its excellent optical properties. In this thesis, we advanced the development of solution-phase approaches for the preparation of LiNbO3 nanoparticles (NPs) with an average, tunable size from 7 to 100 nm. This solution-phase process results in the formation of crystalline, uniform NPs of LiNbO3 at a reaction temperature of 220 °C with an optimal reaction time as short as 30 h. Advantages of these methods include the preparation of single-crystalline LiNbO3 NPs without the need for further heat treatment or without the need for using an inert reaction atmosphere. The growth of these nanoparticles began with a controlled agglomeration of nuclei formed during a solvolysis step. The reactions subsequently underwent the processes of condensation, aggregation, and Ostwald ripening, which remained the dominant process during further growth of the nanoparticles. These processes did produce single-crystalline nanoparticles of LiNbO3, suggesting an oriented attachment process. Average dimensions of the NPs were tuned from 7 to ~100 nm by either increasing the reaction time or changing the concentration of the lithium salts used in the solvothermal process. The nanoparticles were also confirmed to be optically active for SHG. These NPs could enable further development of SHG based microscopy techniques. In this thesis, we also performed a comparative study on the role of different Li precursors during the synthesis of LiNbO3 NPs. The results of these studies suggest that the type of Li precursor selected plays an important role in nanoparticle formation, such as through controlling the uniformity, crystallinity, and aggregation of LiNbO3 NPs. The average diameter of the resulting NPs can also vary from ~30 to ~830 nm as a function of the Li reagent used in the synthesis. The selection of Li precursors also influences the phase purity of the products. Nanoparticles of LiNbO3 are explored in literature as SHG bioimaging probes for their potential to expand underdeveloped SHG based microscopy techniques. The efficient use of SHG active LiNbO3 NPs as probes does, however, require their surface functionalization with polyethylene glycol and fluorescent molecules to enhance their colloidal stability, chemical stability, and to enable a correlative imaging platform. This surface functionalization approach used functional alcohols to serve as a platform for attaching a variety of reagents, including nonreactive surface coatings (e.g., polyethylene glycol). As a demonstration of this approach to utilizing the surface chemistry derived from the silanol-alcohol condensation reaction, the surfaces of the NPs were covalently functionalized with biologically important molecules such as polyethylene glycol and a fluorescent probe. This strategy in tuning the surface chemistry of the nanoparticles based on covalent bonding to their surfaces reduced aggregation of the NPs, provided chemical stability and enabled a multimodal tracking platform for SHG nanoprobes. We also developed the first porous and monodisperse LiNbO3 NPs that were also verified to be SHG active, which could be used as contrast agents in nonlinear optical microscopy, optical limiters, biosensors, and photocatalysts. The porous nonlinear optical material can also enhance the SHG response by loading the pores with organic guest molecules (e.g., carboxylic acids, anilines). We introduce a hydrothermal method to prepare monodisperse and mesoporous LiNbO3 NPs for enhanced SHG response. This approach forms mesoporous LiNbO3 NPs with diameters of ~600 nm without additional organic additives (e.g., surfactants) to control growth and aggregation of the nanoparticles. The mesopores of the LiNbO3 NPs were loaded with organic molecules such as tartrates that offer better photochemical stability and more acentric molecular alignment to the host material. The loading of tartrate anions onto the surfaces of these nanoparticles provides enrichment of pi-electrons to LiNbO3, which enhances the SHG response of mesoporous LiNbO3 by 4 times.