Colloidal Synthesis of Ge Nanocrystals and Bi2Te3, Sb2Te3 and Bi2Se3 Nanoplates

AbstractColloidally synthesized nanocrystals (NCs) play important roles in research studies and industry. Nanotechnology as a field of research has provided new materials for various applications due to their electrical, optical, and magnetic properties which depend on their size and shape. For example, nanoparticles are a wide class of materials whose dimensions are less than 100 nm and spherical in shape. Many nanoparticles are of interest for their optical or optoelectronic properties. Nanoplates are 2D materials that have anisotropic properties that may be of importance for electronic, magnetic, and thermal properties. During my PhD, different studies have been carried out on optimizing the colloidal synthesis of Ge NCs, exploring the synthesis and thermoelectrical properties of Bi2Te3/Sb2Te3 core-shell nanoplates, and discovering the ambipolar effect on solution-synthesized Sb-doped Bi2Se3 nanoplates with high carrier mobility. Both fundamental and advanced reaction principles and characterizations were investigated. Chapter 1 provides an overview of fundamental concepts in the synthesis, nucleation, growth processes, and composition manipulation of NCs and a summary of Ge NCs. Fundamental concepts of property measurements of thermoelectric materials and topological insulators are also briefly discussed. Chapter 2 presents a two-step microwave-assisted reaction that produced single crystalline and monodispersed Ge NCs. The as-synthesized Ge NCs showed high crystallinity with single crystal nature as indicated by powder X-ray diffraction, selected area electron diffraction and high-resolution transmission electron microscopy. The Tauc plot derived from photothermal deflection spectroscopy of Ge NCs thin films showed an increased bandgap of the Ge NCs obtained from GeI2 compared with that from GeI4 with similar particle size, indicating the single crystal nature of the particles prepared via a two-step reaction from GeI4. Solutions involved in this two-step reaction were investigated with 1H NMR spectroscopy, high-resolution mass spectrometry (MS). One possible reaction pathway is proposed to unveil the details of the reaction involved GeI4 and oleylamine (OAm). This two-step synthesis produced high quality Ge NCs and provided new insight on nanoparticle synthesis of covalently bonding semiconductors. Chapter 3 discusses the thermoelectric properties of successfully synthesized Bi2Te3/Sb2Te3 (BTST) nanostructured heterojunctions via a two-step solution route. Samples with different Sb2Te3 to Bi2Te3 ratios could be synthesized by controlling the reaction precursors. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy were used to study the nanostructure and composition. The powder samples were pressed into pellets by spark plasma sintering (SPS). Thermoelectric properties were measured with two different directions, in-plane and out-of-plane, and show anisotropic properties due to the nanostructure alignment of the nanoplates within the pellets. The highest overall zT was observed with BTST1-3 (1-3 represents the ratio of Bi2Te3 (BT) to Sb2Te3 (ST)) sample in the out-of-plane direction at 500 K. Chapter 4 presents a solution-synthesized Sb-doped Bi2Se3 nanoplate with enhanced electronic transport properties. Sb doping was used to suppress the bulk carriers, and an atomic percentage ~ 6% of Sb was demonstrated by energy dispersive X-ray spectroscopy (EDS). The 2D electron carrier concentration for Sb-doped Bi2Se3 nanoplates was lowered to 5.5 × 1012 cm-2, reducing the concentration by a factor of 3 compared to the undoped Bi2Se3 nanoplate sample with an average 2D carrier concentration of 16 × 1012 cm-2. At 2 K, pronounced ambipolar field effect was observed on the low-carrier-density Sb-doped Bi2Se3 nanoplates, further demonstrating the flexible manipulation of carrier type and concentration for these single-crystal nanoplates