Book Description

To access novel and complex nanostructures spanning the wide variety of chemical composition and crystal conformation has significant impact on the next technology revolution in the sense that the ability to produce advanced materials underpins the development of the future devices. Solution-based chemical synthesis of nanomaterials in the colloidal solution has attracted numerous attentions in the last few decades because the huge potential scientists have seen in this method to achieve unprecedented control over the materials characteristics such as morphology, composition, size, and uniformity demonstrated by the successful synthesis of quantum dots to the construction of complex hierarchical structures. Layered metal chalcogenides is a family of compounds that when reducing the layer thickness into nanoscale becomes a good analog to graphene, but with much more plentiful choices of chemical composition and properties. The application of colloidal synthesis into making 2D materials based on the layered metal chalcogenides is an exciting research direction but still in its infancy. In this dissertation we describe how to control the reaction parameters in colloidal synthesis to make meta-stable and complex nanostructures with interesting properties that could have potential application in the field of energy storage and conversion. First, we discuss the colloidal synthesis of amorphous germanium iron alloy nanoparticles and their electrochemical performance as anode materials for lithium ion batteries. The meta-stable amorphous state of the particles was achieved by a fast quenching step following the crystal nucleation and growth. Both thermodynamic and kinetic factors are evaluated through aliquot study to elucidate the growth pathways. The as-prepared sample was tested for the half-cell and acquired good specific capacity and cycling stability. The addition of iron into the germanium is believed to effectively alleviate the volume change during the lithiation/delithiation process of germanium and possibly has a good impact on the overall electrical conductivity of the material. Introducing earth-abundant elements into the silicon-related materials is a promising way to reduce the cost of the next generation lithium ion batteries while still maintain a good performance. Next, the principles we learned in the colloidal synthesis of metal alloys are adopted and modified to successfully make MoSe2 nanoflowers that comprise of poly-crystalline few-layered nanosheets. Besides the reaction kinetics, precursor choices that affect the reactivity of the chalcogen entities in the solution have been identified as the key parameters to determine both the morphology and crystallinity of the final product. Characterization techniques like powder XRD and high-resolution TEM have been employed to reveal a slight deviation of the crystal structure of the nanoflowers from the bulk counterpart, which we believe can be attributed to the few-layer nature of these flowers. Raman spectroscopy is used to probe the interlayer decoupling behavior of the flowers with different size and layer thickness compared with the bulk MoSe2. We found out that the interlayer interaction can be modulated through laser heating, thermal, as well as nanostructuring effect and especially the laser modulation could result fast and reversible response. This study presents the possibility and feasibility of using colloidally synthesized TMDs as the platform to understand the 2D properties of these materials.Chapter 4 takes the knowledge we learn in the previous two studies into the exploration of novel and under-studied ternary metal chalcogenides using colloidal synthesis. By a facile one-pot heat-up method, we have successfully obtained a ternary In4SnSe4 with a unique crystal structure that is drastically different from the well-known binary metal chalcogenides crystal structures of zincblende or wurtzite that are both derivative of diamond structure. The as-prepared microwires of In4SnSe is proven by high-resolution TEM and STEM-EDS mapping to have a surface Si-contained oxide layer of about 10~20 nm. Bandgap calculation of the In4SnSe predicts an electronic band structure with a direct band gap of 2.0 eV, which matches well with the solar spectrum and make it a promising candidate material in the photovoltaic devices. The optical bandgap of the as-prepared sample was also measured by diffuse reflectance UV-Vis spectroscopy and yielding a value of 1.57 eV, which matches well with the photoluminescence peak located around 1.54 eV. Both theoretical and experimental result corroborate on the possession of a direct bandgap of ~1.5 eV for the In4SnSe4, which could attract more studies on this family of materials that have similar crystal structure. Finally, the GeSe and SnSe from the layered metal chalcogenides family are used as a model system to study the possibility of making 2D heterostructures in colloidal solution. We have employed both heat-up and continuous hot-injection method to test various reaction parameters such as precursor concentration and adding sequence and are able to obtain five different samples of 2D heterostructures, one of which realized a full coverage of SnSe on top of the entire GeSe hexagonal sheet. These 2D heterostructures are in the scale of few micron meters, which has never been achieved in any other 2D heterostructures before. By comparing the morphologies of the five samples, we propose a growth pathway that affected by both thermodynamics and kinetics, involving the competition between homogeneous nucleation/growth and the heterogeneous nucleation/growth. The methodology in this study can be potentially applied to other 2D systems with more imminent technical significance.