Chemistry & Biochemistry Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2752
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Item Solution Processing of Long Carbon Nanotubes: from Fundamentals to Applications(2019) Wang, Peng; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Single-walled carbon nanotubes (SWCNTs) are one of the most intensively studied nanomaterials due to their extraordinary mechanical, electrical, and optical properties. Attaining aqueous solutions of individual SWCNTs is the critical first step for harnessing their outstanding properties and applying them in many applications and further processing, such as sorting, imaging, and sensing. However, the current ultrasonication-then-ultracentrifugation approach inevitably introduces defects to SWCNTs and cuts the nanotubes into smaller pieces, compromising the electrical and mechanical properties of this otherwise remarkable material. In this dissertation, we introduce an unexpectedly simple approach that completely eliminates the need for ultrasonication, and nondestructively disperses SWCNTs in aqueous solution, so that the synthetic lengths of SWCNTs can be preserved. The dispersion is achieved by using surfactants to wrap and stabilize the protonated SWCNTs by simple acid-base neutralization reactions. The result is that the protons on SWCNTs are replaced by surfactants, and thus, we name this method “superacid-surfactant exchange (S2E).” In chapters 2-4, we demonstrate the length of dissolved SWCNTs by S2E can be 4-10 times longer than the sonicated controls, thereby significantly improving the optical, electrical and electromechanical properties. We further find that by tuning the concentrations of SWCNTs in this S2E process, short nanotubes can be selectively extracted out, allowing separation of the long carbon nanotubes (>10 µm). In chapter 5, we show that long SWCNTs can behave like mechanical reinforcing structures that enhance the mechanical strength of graphene through π-π interactions without sacrificing much of the outstanding transparency of graphene. This fact has enabled the fabrication of the mechanically strong yet ultrathin graphene/SWCNTs hybrid structure (G+T) for operando probing of the electrical double layer at the electrode-electrolyte interface by X-ray photoelectron. Finally, as a ramification result from the S2E process, chapter 6 describes the scalable synthesis of organic-color-center tailored SWCNTs.Item EXCITON ENGINEERING THROUGH TUNALBLE FLUORESCENT QUANTUM DEFECTS(2016) Kwon, Hyejin; Wang, YuHuang; Chemistry; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis demonstrates exciton engineering in semiconducting single-walled carbon nanotubes through tunable fluorescent quantum defects. By introducing different functional moieties on the sp2 lattice of carbon nanotubes, the nanotube photoluminescence is systematically tuned over 68 meV in the second near-infrared window. This new class of quantum emitters is enabled by a new chemistry that allows covalent attachment of alkyl/aryl functional groups from their iodide precursors in aqueous solution. Using aminoaryl quantum defects, we show that the pH and temperature of complex fluids can be optically measured through defect photoluminescence that encodes the local environment information. Furthermore, defect-bound trions, which are electron-hole-electron tri-carrier quasi-particles, are observed in alkylated single-walled carbon nanotubes at room temperature with surprisingly high photoluminescence brightness. Collectively, the emission from defect-bound excitons and trions in (6,5)-single walled carbon nanotubes is 18-fold brighter than that of the native exciton. These findings pave the way to chemical tailoring of the electronic and optical properties of carbon nanostructures with fluorescent quantum defects and may find applications in optoelectronics and bioimaging.Item THERMAL IMAGING OF MULTIWALLED CARBON NANOTUBES(2010) Baloch, Kamal Hussain; Cumings, John P; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Since early days of their discovery, it has been realized that Carbon Nanotubes (CNTs) have an unusually high thermal conductivity. Unfortunately, the amount of heat they can transfer from one medium to another can be limited by their thermal contact resistance, Rc, which in the worst case can result in thermally insulating bulk materials. Prior studies on individual nanotubes have reached various disparate conclusions, partly because many techniques employed for measuring such small samples rely on uncharacterized heat sources thus leaving fundamental uncertainties in the measurements. This has caused concerns that the true potential of these extraordinary thermal conductors will remain untapped. Relying on solid to liquid phase transition of sub-200nm Indium islands for thermometry, we report direct measurement of Rc by employing an independently characterized metallic heat source. Also we demonstrate that this contact resistance can be reduced by almost two orders of magnitude if a CNT is imbedded into metal contacts. Additionally in our preliminary data on a self-heated CNT we observe that the substrate gets hot while the CNT itself remains cold when electric current is passed through it. This observation cannot be explained by assuming joule heating to be the primary source of heat transfer. We can qualitatively explain these results by assuming that hot electrons flowing through the biased CNT can be scattered off the phonons of a dielectric substrate. Principles of the novel measurement technique, experimental results and simulations are presented in this dissertation.