Characterization And Manipulation Of Ion Beam Synthesized Germanium Nanocrystals

Author : Katayoon Tabatabaei
Publisher :
ISBN 13 : 9781392664070
Total Pages : pages
Book Rating : 4.6/5 (64 download)

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Download or read book Colloidal Synthesis and Characterization of Pristine and Compositionally Manipulated Germanium Nanocrystals written by Katayoon Tabatabaei and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: A tremendous amount of research efforts were focused on conventional compound semiconductor nanomaterials. Group IV (Si and Ge) semiconducting materials in nanoscale are of significant research interest due to their unique and promising properties in a broad range of technological applications. While during the last two decades, Ge NCs as non-toxic alternatives over metal chalcogenides or group III-V quantum dots were studied by different research group to tune their optical and electronic properties by controlling their size, morphology, surface functionality and composition, still insufficient understanding is at their disposal to design and achieve well-defined and high quality Ge nanostructures for the targeting applications. Nanogermanium is a material that has great potential for technological applications and doped and alloyed Ge nanocrystals (NCs) are actively being considered. The presented work is focused on the microwave-assisted solution-based synthesis of germanium nanocrystals with insights to their formation, manipulating their composition using Group V element and achieving a better understanding of the synthetic chemistry of these materials.Chapter 1 provides an overview of fundamental concepts in the synthesis, nucleation, growth processes, surface chemistry and composition manipulation of NCs and in particular, germanium nanocrystals (Ge NCs). Chapter 2 presents the incorporation of bismuth (Bi), an n-type dopant, within or at the surface of Ge NCs. Bi classically shows no solubility in crystalline Ge. However, Ge could be doped kinetically with Bi in the nanoregime. The first colloidal synthesis in a microwave-assisted solution route and characterization of Bi-doped Ge NCs have been presented. The oleylamine capping ligand can be replaced by dodecanethiol without loss of Bi. A positive correlation between the lattice parameter and the concentration of Bi content (0.5 - 2.0 mol %) has been shown via PXRD and SAED. XPS, TEM, STEM and ICP-MS are consistent with the Bi solubility up to 2 mol %. The NC size increases with increasing amount of bismuth iodide employed in the reaction. Absorption data show that the band gap of the Bi-doped Ge NCs is consistent with the NC size. This work shows that a new element can be doped into Ge NCs via a microwave-assisted route in amounts as high as 1-2 mol % and leads to increased carriers. Colloidal chemistry provides an inroad to new materials not accessible via other means. Chapter 3 discusses a finer control of absolute size and crystallinity that can be achieved by the addition of molecular iodine (I2) and bromine (Br2) to germanium(II) iodide (GeI2). I2 and Br2 are shown to oxidize GeI2 to GeI4 in-situ, providing good control over size and crystallinity. The kinetics of Br2 oxidation of GeI2 are slightly different, but both I2 and Br2 provide size control of the Ge NCs. The samples are highly crystalline as indicated by powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM), and Raman spectroscopy. The solutions of I2, GeI2, and colloidal Ge NCs were investigated with Fourier- transform infrared (FTIR) and proton nuclear magnetic resonance (1H NMR) and showed no evidence for imine, hydrazine, or nitrile formation. Hydrogen and ammonia gases were detected after the reaction by gas chromatography (GC) and high-resolution mass spectrometry (HRMS). The presence of a germanium amine iodide complex was also confirmed with no evidence for a hydrazine-like species. These results suggest an efficient fine-tuning of size and crystallinity of Ge NCs using halogens in addition to the mixed-valence precursor synthetic protocol previously reported. Chapter 4 considers Bi halide precursors (BiCl3 and BiBr3) along with an organobismuth precursor (Bi(OTf)3 for their impact on size control of Bi-doped Ge NCs. In this work, using Bi halide precursors regardless of halide anion nature alters the NCs size. Higher precursor concentration also results in greater anisotropy in morphology. While BiI3 compared to BiCl3 and BiBr3 provides larger NCs sizes, especially in high concentration regime, all three halide precursors lead to size variation. However, using the organobismuth Bi(OTf)3 as a precursor maintains a relatively constant absolute size of the Ge NCs with low Bi precursor concentrations (0.0-1.0 mol %) and more significant changes can be observed with higher Bi content. The successful incorporation of Bi using Bi(OTf)3 into Ge NCs was determined by elemental mapping performed using scanning electron microscopy (SEM)-energy dispersive X-ray spectroscopy (EDS). It shows using the organobismuth precursor (Bi(OTf)3) promises to prevent a dramatic change of NCs sizes even when varying precursor concentration which is of paramount importance for many applications requiring a narrow and uniform size regime of doped Ge NCs. Finally, Chapter 5 presents the composition manipulation of Ge NCs, with another n-type group V element, antimony (Sb), a more common dopant for different semiconductor materials. Sb shows negligible solubility in bulk Ge, however, the kinetically controlled colloidal synthesis allows it to be incorporated in Ge or to reside on its surface. Oleylamine (OAm), OAm-trioctylphosphine (TOP)-capped Sb-doped Ge NCs have been synthesized for the first time by a microwave-assisted colloidal route. An enhancement of the lattice parameter of Ge NCs with increasing Sb concentration (0.0-1.0 mol %) is observed by PXRD. An increase in NCs diameters with higher content of SbI3 is shown by TEM. XPS and EDS confirm the presence of Sb before and after removal of OAm, TOP through hydrazine treatment and exchanging the Ge NCs surface with dodecanethiol suggesting either a strong Sb interaction with Ge surface or its incorporation within the lattice. Passivating the Ge surface by a binary ligand system of OAm, TOP results in formation of consistently larger NCs compared to OAm only. The TOP coordination to the Ge surface is confirmed by 31P NMR and SEM-EDS measurement. This Chapter presents successful synthesis of Sb-doped Ge NCs through colloidal chemistry and opens up new pathways to expand the composition chemistry of group IV semiconducting materials.