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.

Thermoelectric generators (TEGs), which are fabricated from thermoelectric materials, convert heat into electricity with no moving parts, making them a robust and appealing renewable energy technology, given that approximately two thirds of the energy produced on earth is lost as waste heat. TEGs could be employed to scavenge heat from industrial plants and combustion engines to boost the efficiency of existing processes, or they could be used independently to generate usable electricity from geothermal sources and the sun. The efficiency of a TEG is dependent on the thermoelectric figure of merit, zT, of the materials used to create the device. zT is determined using a material’s Seebeck coefficient (S), temperature (T), electrical resistivity (ρ), and thermal conductivity (κ), according to the relationship: zT= (S^2 T)/ρκ. To make TEGs attractive for large scale applications, thermoelectric materials with high zT need to be designed. This dissertation investigates the chemistry of several potential thermoelectric materials to gain deeper understanding of structure-property relationships that enable the design of highly efficient materials.Layered Zintl phases are a class of materials that inherently exhibit low thermal conductivity due to their complex crystal structures and chemical tunability. They are composed of covalent networks with diverse bonding motifs separated by layers of cations, and the ionic and covalent portions can be tuned separately to optimize thermal and electronic properties. The directional nature of the covalent bonding can also give rise to low dimensional electronic structures, which are beneficial for thermoelectric properties. The first project described in Chapter 2 of this dissertation is a study on defect chemistry in a solid solution of the layered Zintl phase Yb2-xEuxCdSb2. The goal of this research was to understand the unusually low thermal conductivity, high Seebeck coefficient and low electrical resistivity of this promising thermoelectric material by examining changes in defect chemistry as Eu is substituted for Yb using pair distribution function analysis. The local structures of x = 0.2 and 0.3 deviated substantially from the average structure due to large concentrations of Eu and Sb vacancies that give rise to structural distortions that alter electronic properties. Predictions suggest that zT > 2 at 550 K could be achieved for x = 0.3 if the carrier concentration can be tuned via doping. This work highlights the importance of understanding crystallographic disorder and defect structure in thermoelectric materials. This work was published in ACS Chemistry of Materials (2022, 34, 20, 9228–9239). The next project presented in Chapter 3 focuses on Ba2-xEuxZnSb2, a Zintl phase that contains one-dimensional covalent chains and was predicted to exhibit zT > 2 at 900 K. Eu was substituted for Ba (x = 0.2, 0.3, 0.4) to reduce the air sensitivity of this compound, as well as reduce thermal conductivity through alloy scattering. Samples were prepared using binary precursors via ball milling and annealing and then consolidated by spark plasma sintering for thermoelectric property measurement. All compositions exhibited high Seebeck coefficients and low thermal conductivity, as well as high charge carrier mobility, an important quality for high efficiency thermoelectric materials, which is likely due to the one-dimensional chains. Experimental results agreed well with theory and predictions suggest higher zT is achievable through doping. Eu11Zn¬4Sn2As12 is another layered Zintl phase expected to exhibit a low-dimensional electronic structure due to high mobility hexagonal [Zn2As3]5- sheets in two crystallographic directions and ionic bonding in the third. This project presented in Chapter 4 brings theory and experiment together to understand a complex, disordered, modulated crystal structure, and develop a road map for further optimization of this promising thermoelectric material. Experimental results show high Seebeck coefficient and low thermal conductivity and electrical resistivity that decreases 5-fold with Na-doping. Denisty Functional Theory (DFT)computations show that, when doped, this material possesses a two-dimensional Fermi surface, and band structure calculations highlight the presence of multiple bands just below EF, making Eu11Zn¬4Sn2As12 a promising candidate for further optimization. Finally, theory and experiment are employed together again to characterize the electronic and structural transitions in Yb5Sb3Hx as H content varies from x = 0.25 to 1. Yb5Sb3Hx has been known to exist in two polymorphs, a hexagonal and an orthorhombic crystal structure, and previous work has suggested that H content drives the transition. In the study presented in Chapter 5, DFT calculations confirm that at lower H content (x < 0.25), the hexagonal crystal structure is energetically favored, while at x > 0.25, the orthorhombic cell is more stable and the synthesized polycrystalline samples for x = 0.25, 0.5, 0.75, and 1 support this finding. Previous theoretical work also suggests that Yb5Sb3Hx is an electride with localized anionic electrons present in zero-dimensional cavities when x < 1 and a charge balanced semiconductor at x = 1. Thermoelectric properties were used to map this transition, and they show a shift from the low charge carrier mobility associated with highly localized electride states (x = 0.25 and 0.5) to high mobility associated with free electrons in a semiconductor (x = 0.75 and 1). At x = 1, the compound exhibits high Seebeck coefficients and low thermal conductivity up at 775 K, in addition to excellent charge carrier mobility that make it a potential thermoelectric material if charge carrier concentration can be increased.

Zintl phases are an extremely broad class of compounds that are charge balanced semiconductors. They follow simple assumptions where the more electropositive atoms donate their electrons (cations) to the more electronegative atoms (anions) which may or may not form additional bonding networks to satisfy their valence. Given the large scope of this category, this naturally brings about a huge number of possible structures and range of physical phenomenon. The understanding of how the atomic structure relates to the physical properties is fundamental in the design and optimization of new materials. Thermoelectric materials are a unique type of material that can convert heat directly into electricity in a reversible fashion. These materials can be used in power generators to recycle and convert waste heat into usable electricity. Given that there are no moving parts in these thermoelectric generators and are only based on the solid-state materials, they are an extremely reliable source of power and commonly used on long term space missions where other power sources are not realistic or robust enough. Their widespread application is however, currently being held back by their conversion efficiencies. The efficiency of a thermoelectric material is dependent on zT=(α^2 T)/ρκ , the figure of merit, where α is the Seebeck coefficient, ρ is the electrical resistivity, and κ is the thermal conductivity. To maximize zT, a compound would need to have low electrical resistivity, low thermal conductivity and a high Seebeck coefficient. The optimization of these parameters is non-trivial due to their interconnected nature with carrier concentration. Optimization schemes such as a single parabolic band model can provide directions to probe experimentally. The viability of this method will be visited in this work. The first part of this dissertation will focus on the structure and properties of the Yb21Mn4Sb18 phase. This material was predicted to have a promising starting value of zT because of its complex structure. The intrinsic phase was successfully synthesized in both bulk powder and single crystal form and indeed has a high zT prior to optimization. A systematic study was done to explore the effects on the electronic structure through isovalent substitution on all three sites where Yb was replaced with Ca, Mn with Cd, and Sb with Bi site. The carrier concentration was also tuned explicitly through aliovalent doping by introducing Na on the Yb site. An increase in the figure of merit was observed when the carrier concentration of the system increased showing large improvements in the case of Na doping and Cd substitution. The structure was analyzed through synchrotron x-ray diffraction and pair distribution function analysis revealing both long range and local disorder. While the aforementioned studies focused on the high temperature physical properties, the structure-property relationship in the low temperature regime was also visited in two compounds: the new Eu5Al3Sb6 phase and the Yb14MSb11 (M = Zn, Mg, Mn) series. The Eu5Al3Sb6 phase represents a new structure type that features Al4 tetrahedra in the solid state. These kinds of triel clusters are rarely seen with only a few examples in the literature making it a great case study for examining the emergent properties of this unique structure. This compound features antiferromagnetic ordering and metallic-like resistivity. Single crystal x-ray diffraction suggests the possibility of a supercell or modulated structure. From electronic band structure calculations, band crossing has been observed indicating that there is potential for this material to exhibit topological behavior. Careful defect tuning can shift the Fermi level to the appropriate energy to induce this behavior. The intermediate valent nature of the Yb14MSb11 (M = Zn, Mg, Mn) compounds have been confirmed through XANES, heat capacity and magnetic susceptibility measurements showing that they are not strictly valence precise compounds as predicted from a Zintl formalism. The three substitutions provide a systematic study on the effect of d electrons within the system where Mg2+ is d0, Mn2+ is d5 and Zn2+is d10. In all cases, there is a small amount of intermediate valency. The amount of intermediate valency is constant for the Mn and Mg compounds while the Zn compound was found to exhibit temperature dependent behavior. This dissertation contains works that are to be published in the future. Some of the work presented here has already been published in part, or full in the following articles: He, A.; Bux, S. K.; Hu, Y.; Uhl, D.; Li, L.; Donadio, D.; Kauzlarich, S. M. Structural Complexity and High Thermoelectric Performance of the Zintl Phase: Yb21Mn4Sb18. Chem. Mater. 2019, 31, 8076 – 8086. He, A.; Cerretti, G.; Kauzlarich, S. M. The impact of site selectivity and disorder on the thermoelectric properties of Yb21Mn4Sb18 solid solutions: Yb21Mn4-xCdxSb18 and Yb21-yCayMn4Sb18. Mater. Adv., 2021, 2, 5764 – 5776. He, A.; Wille, E. L. K; Moreau, L. M.; Thomas, S. M.; Lawrence, J. M.; Bauer, E. D.; Booth, C. H.; Kauzlarich, S. M. Intermediate Yb valence in the Zintl phases Yb14MSb11 (M = Zn, Mn, Mg): XANES, magnetism, and heat capacity. Phys. Rev. Mater., 2020, 4, 114407.

Below are other published manuscripts that I have worked on but are not included in this dissertation:Shang, R.; Nguyen, A. T.; He, A.; Kauzlarich, S. M. Crystal structure characterization and electronic structure of a rare-earth-containing Zintl phase in the Yb-Al-Sb family: Yb3AlSb3. Acta Cryst. C. 2021, C77, 281 – 285.

The rapidly increasing interest in Ge-based nanostructures is motivated by important advantages of this material compared to other semiconductors commonly investigated in the broad field of nanotechnology. Ge nanoparticles are promising materials for many applications such as photovoltaics, transistors, light emitting devices, bioimaging, and energy storage devices.Due to the strong covalent bonding nature, synthesis of high-quality Ge nanoparticles with uniform size, well-defined morphology, composition, and surface chemistry is a main scientific challenge in the colloidal chemistry community and finding a reliable, scalable, cost-effective, and environmentally friendly synthetic route is of key importance for their further applications. Besides technical interest, access to defined nanoscale structures and compositions is also essential for uncovering their intrinsic properties and expanding the chemistry of group 14 nanomaterials. In this dissertation, both direct synthetic methods and post transformation approaches are explored for rational synthesis of germanium-based nanomaterials with great control of the size, shape, and composition. The detailed reaction mechanisms are revealed through a systematic study on the reaction conditions such as temperature, precursor concentrations, reaction time, etc. Amorphous solid Ge nanoparticles and hollow Ge nanoparticles are synthesized via galvanic replacement reaction with Ag NPs serving as a sacrificial template. Ag@Ge core-shell nanoparticles with controlled shell thickness are also successfully synthesized as an intermediate. Their structural, optical, and electrical properties have been investigated in detail by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and transmission electron microscopy (TEM), and supported by UV-Vis, Powder X-ray Diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), etc. In addition, we provide an in situ method to observe the phase transition process of amorphous nano-germanium through a combination of both joule heating and high-energy electron beams at atomic resolution. The dynamic process of thermally induced structural evolution of the Ag@Ge binary system is also studied by TEM in real-time. The insights developed here can be extended to other metal semiconductor systems of interest and are crucial to developing the materials necessary for rigorous mechanistic studies of the effect of ligands and precursors on reactivity and selectivity. The two different synthetic approaches represent a way for achieving nanoparticles with strong covalent bonds as well as complex morphology, which would in turns increase the reproducibility of colloidal synthesis and prepare for scale-up and future practical applications.

Radioisotope thermoelectric generators (RTG) have been providing reliable, long-term power to space missions for over 50 years. These devices consist of a radioisotope heat source at the core which is surrounded by many p- and n-type thermoelectric legs. Radiative fins on the outside of the enclosure provide the cold side of thermal gradient across the thermoelectric legs. The efficiency at which these materials then covert that thermal gradient into usable electricity is related to the unitless thermoelectric figure of merit, zT = σS2T/κ, where sigma (σ) is the electrical conductivity, S is the Seebeck coefficient, or voltage per degree of thermal gradient, T is the absolute temperature, and kappa (κ) is the thermal conductivity.Yb14MnSb11 and its analog, Yb14MgSb11, are two p-type thermoelectric materials which have zT’s greater than 1.0 at high temperatures. Because of this, these materials are of interest for use in the next generation of RTG However, polycrystalline samples of these materials can be difficult to make with high purity and those impurities tend to negatively impact the zT. In addition to impacts on thermoelectric properties, impurities can also have an impact on physical and chemical properties such as thermal stability and oxidation. In the first work presented, the metal hydrides, YbH2 and MgH2, were investigated for use as reactive precursors to make Yb14MgSb11. While YbH2 proved effective in reducing the cold-welding of Yb during ball milling, a large excess of Mg (50%) was required to make high purity product due to Mg being 1/26 atoms within the structure. MgH2 provided a higher dispersity Mg source allowing for a reduction in Mg excess to 20%, ultimately leading to improved thermoelectric figure of merit, reaching a peak zT of 1.26 at 1200 K. To better understand the reaction pathway taken by polycrystalline reactions to form Yb14MSb11 (M = Al, Mn, Mg) the next work utilizes a series of quenching studies to investigate the phases formed after ball milling and during heating before the final product forms. These studies revealed that utilization of the binary intermediate Yb4Sb3 as a precursor with YbH2 and either MnSb or Mg3Sb2 allowed for direct formation of high purity Yb14MnSb11 and Yb14MgSb11 through balanced, stoichiometric reactions. The improved synthetic route yielded Yb14MgSb11 and Yb14MnSb11 with further improved thermoelectric properties, reaching peak zT’s of 1.28 at 1175 K and 1.24 at 1275 K respectively. While Yb14MgSb11 and Yb14MnSb11 have been proven to be high performing thermoelectric materials, another analog, Yb14ZnSb11, was initially reported as having very poor thermoelectric performance. As only 1/26 atoms are being exchanged in Yb14ZnSb11, the overall bonding and resultant density of states (DOS) within the structure should change very little. In this work, computed DOS and band structures for Yb14ZnSb11 and other analogs are presented. As expected, the band structure of Yb14ZnSb11 was shown to indeed have the light band at gamma and the highly degenerate (Nv = 8) pocket of bands between N and P which are responsible for the high thermoelectric performance of the Mn and Mg containing analogs. To investigate whether an improved synthetic route could bring the predictions of high zT into a reality, the route previously developed for Yb14MgSb11 and Yb14MnSb11 was adapted to make Yb14ZnSb11. Pure phase Yb14ZnSb11 was obtained through reactions with Yb4Sb3, YbH2, and ZnSb in a matter of hours via spark plasma sintering (SPS). This high purity Yb14ZnSb11 confirmed the predicted high thermoelectric performance (zTmax = 1.2 at 1175 K), placing Yb14ZnSb11 amongst the other high performing analogs of this structure type and suggests that other less explored analogs may have similar performance when made in high purity. The final three studies within this dissertation focus on the high temperature oxidation of Yb14MnSb11, Yb14MgSb11, and Yb14ZnSb11 along with chemical substitution of Lu for Yb within Yb14-xLuxMnSb11 and Yb14-xLuxZnSb11 (x = 0 – 0.9) to investigate the possibility of improving oxidation properties along with its effect on thermoelectric properties.

Two-dimensional (2D) nanomaterials are of relevance to many energy and technologically-related fields of research due to their remarkable physical and electronic properties. Solution colloidal synthesis offers the ability to precisely tune the structure, composition, and morphology of these 2D nanomaterials for desired applications, including thermoelectric energy conversion. Many 2D nanomaterials have proven to be high-performing thermoelectric materials, including the tetradymite structures, Sb2Te3 and Bi2Te3. During my Ph.D., I have carried out various research studies involving the synthesis, characterization, and thermoelectric property measurements of Sb2Te3 and Bi2Te3 nanoplate composites, Bi2Te3 nanoplates with a single nanopore, and Cu-intercalated Sb2Te3 nanoplates.Chapter 1 outlines the recent advances in colloidal polyol synthesis of 2D nanomaterials and provides perspectives on the similarities and differences in various syntheses. Various materials classes are presented and discussed, including metals, oxides, chalcogenides, and halides, that can be synthesized as 2D nanomaterials via a polyol process. This work has resulted in one perspective article that has been submitted to Dalton Transactions. Chapter 2 discusses the thermoelectric properties of Sb2Te3 and Bi2Te3 nanoplate composites that were synthesized in solution and consolidated via spark plasma sintering. The as-synthesized Sb2Te3 and Bi2Te3 vary drastically from one another in their lateral and vertical dimensions as revealed by scanning electron microscopy and atomic force microscopy. Thermoelectric properties in the parallel and perpendicular directions were measured, revealing strong anisotropy with a significant reduction to thermal conductivity in the perpendicular direction due to increased phonon scattering at nanoplate interfaces. The Seebeck coefficient is also increased dramatically in the nanocomposites, the highest reaching 210 μV/K for 15% Bi2Te3. The increase in Seebeck is attributed to energy carrier filtering at the nanoplate interfaces. Overall, these enhanced thermoelectric properties lead to a drastic increase in the thermoelectric performance in the perpendicular direction, with zT ~1.26, for the 15% Bi2Te3 nanoplate composite at 450 K. These groundbreaking results are demonstrated to be one of the highest for this system, where many similar materials have zT ~1. This work has resulted in one publication in ACS Applied Electronic Materials. Chapter 3 presents the synthesis, growth mechanism, and thermoelectric properties of 2D Bi2Te3 nanoplates with a single nanopore in the center. Analysis of the reaction products during the colloidal synthesis reveals that the reaction progresses via a two-step nucleation and epitaxial growth: first of elemental Te nanorods, then the binary Bi2Te3 nanoplate growth. The rates of epitaxial growth can be controlled during the reaction, thus allowing the formation of a single nanopore in the center of the Bi2Te3 nanoplates. Thermoelectric properties were measured in the parallel and perpendicular directions. These properties reveal strong anisotropy with a significant reduction to thermal conductivity and increased electrical resistivity in the perpendicular direction due to the higher number of nanoplate and nanopore interfaces. Furthermore, Bi2Te3 nanoplates with a single nanopore exhibit ultralow lattice thermal conductivity values, reaching values of ~ 0.21 Wm-1K-1. The lattice thermal conductivity was found to be systematically lowered with pore size, allowing for the realization of a thermoelectric figure of merit, zT ~ 0.75 at 425 K for the largest pore size. Although this is a relatively low figure of merit, this system demonstrates an enhancement through nanoscale voids on the thermoelectric performance, and can be further optimized via doping. This work has resulted in one article that has been accepted for publication in Chemistry of Materials. Chapter 4 discusses the synthesis and solution route intercalation of zerovalent copper into Sb2Te3 nanoplates. The copper intercalant is homogeneously distributed throughout the nanoplates, confirmed by scanning transmission electron microscopy coupled to energy dispersive x-ray spectroscopy. The copper composition was shown to be 2.6 at. % by x-ray photoelectron spectroscopy. There is a slight shift to lower binding energies for the Sb and Te 3d photoelectron peaks, which may indicate higher electron density within the Sb2Te3 nanoplates due to electron donation from copper. Powder x-ray diffraction shows good agreement with the Sb2Te3 bulk phase, with no structural transitions observed upon copper intercalation. Selected area electron diffraction displays the appearance of superlattice reflections in the Cu-intercalated Sb2Te3 nanoplates, which is indicative of copper ordering within the Van der Waals gaps of the nanoplates. Preliminary electronic transport measurements of single nanoplate devices show high conductance.