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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Solution-mediated strategies for synthesizing metal oxides, borates and phosphides using nanocrystals as reactive precursors

Henkes, Amanda Erin 15 May 2009 (has links)
Because of their high surface area (and hence, increased reactivity) nanocrystals can be used as reactive precursors in the low-temperature synthesis of solid state materials. When nanocrystals are used as reactants, the temperatures needed for diffusion between them can be significantly lower than for bulk-scale reactions—often at temperatures attainable using solution-based techniques. In the following work, two synthetic strategies are defined and developed for accessing metal oxides, borates and phosphides using nanocrystalline precursors and solution-mediated techniques. Broadly, the strategies involve either 1) the formation of a nano-sized precursor in solution which is post-annealed after isolation to form a target metal oxide or borate or 2) the solution-mediated diffusion of phosphorus into a nanocrystalline metal to form target metal phosphides. To form multi-metal oxides using the first strategy, metal oxide nanoparticle precursors are mixed in stoichiometric ratios in solution to form a nanocomposite. After isolation, the nanocomposite is annealed in air at 700-800 °C to form target ternary metal oxides, including Y2Ti2O7, Eu2Ti2O7, NiTiO3, Zn2SnO4 and CuInO2. As a variation of this method, rare earth borate nanoparticle precursors can be formed in solution by the reaction of RE3+ with NaBH4. After isolation, annealing in air at 700-800 °C crystallizes a range of REBO3 and Al3RE(BO3)4 powders. Using solution-based techniques, metal phosphides can be formed by the reaction of pre-formed metal nanocrystals with trioctylphosphine (TOP), which acts as a mild phosphorus-source, at 300-370 °C. A range of transition metal phosphide nanocrystals are accessible using this strategy, including the polyphosphides PdP2, AgP2 and Au2P3. Furthermore, shape and size of the metal phosphide product can be influenced by the shape and size of the metal precursor, allowing for the templated-design of nanostructured metal phosphides. The utility of this technique is not limited to the nano-regime. Bulk-scale metal powders, wires, foils, thin films and nanostructured metals can be converted to metal phosphides using analogous reactions with hot TOP. The two-fold purpose of this work is to extend these solution-mediated nanocrystal-based synthetic strategies to new classes of materials, and to compliment the existing library of low-temperature methods for making solid state materials.
2

Solution-mediated strategies for synthesizing metal oxides, borates and phosphides using nanocrystals as reactive precursors

Henkes, Amanda Erin 15 May 2009 (has links)
Because of their high surface area (and hence, increased reactivity) nanocrystals can be used as reactive precursors in the low-temperature synthesis of solid state materials. When nanocrystals are used as reactants, the temperatures needed for diffusion between them can be significantly lower than for bulk-scale reactions—often at temperatures attainable using solution-based techniques. In the following work, two synthetic strategies are defined and developed for accessing metal oxides, borates and phosphides using nanocrystalline precursors and solution-mediated techniques. Broadly, the strategies involve either 1) the formation of a nano-sized precursor in solution which is post-annealed after isolation to form a target metal oxide or borate or 2) the solution-mediated diffusion of phosphorus into a nanocrystalline metal to form target metal phosphides. To form multi-metal oxides using the first strategy, metal oxide nanoparticle precursors are mixed in stoichiometric ratios in solution to form a nanocomposite. After isolation, the nanocomposite is annealed in air at 700-800 °C to form target ternary metal oxides, including Y2Ti2O7, Eu2Ti2O7, NiTiO3, Zn2SnO4 and CuInO2. As a variation of this method, rare earth borate nanoparticle precursors can be formed in solution by the reaction of RE3+ with NaBH4. After isolation, annealing in air at 700-800 °C crystallizes a range of REBO3 and Al3RE(BO3)4 powders. Using solution-based techniques, metal phosphides can be formed by the reaction of pre-formed metal nanocrystals with trioctylphosphine (TOP), which acts as a mild phosphorus-source, at 300-370 °C. A range of transition metal phosphide nanocrystals are accessible using this strategy, including the polyphosphides PdP2, AgP2 and Au2P3. Furthermore, shape and size of the metal phosphide product can be influenced by the shape and size of the metal precursor, allowing for the templated-design of nanostructured metal phosphides. The utility of this technique is not limited to the nano-regime. Bulk-scale metal powders, wires, foils, thin films and nanostructured metals can be converted to metal phosphides using analogous reactions with hot TOP. The two-fold purpose of this work is to extend these solution-mediated nanocrystal-based synthetic strategies to new classes of materials, and to compliment the existing library of low-temperature methods for making solid state materials.
3

Study on the Electrical Analysis and Physical Mechanism of Nanocrystal Nonvolatile Memory

Wang, Ren-You 01 August 2007 (has links)
The conventional floating gate NVSM will suffer some limitations for continued scaling of the device structure. The floating gate is a continuous semiconductor thin film which charges are stored in and able to move around. With the scaling of tunneling oxide, the thickness is decreased gradually. Once the tunneling oxide has been created a leaky path, all the stored charges in the FG will be lost after numerous counts of write/erase operation. When the tunnel oxide is thinner, the phenomenon happens more easily but the speed of write/erase operation is quicker. Therefore, there is a tradeoff between speed and reliability.Therefore, two approaches, the silicon-oxide-nitride-oxide-silicon (SONOS) and the nanocrystal nonvolatile memory devices, have been investigated to overcome the limit of the conventional floating gate NVSM. In this thesis, the nonvolatile nanocrystal memory structures were proposed for electrical analysis and physical mechanism studied. We proposed two nanocrystal memory, silicon nanocrystal memory and nickel-silicide nanocrystal memory. The silicon nanocrystal memories have standard sample and nitridation sample. The interface between the nitride and Si-dots can offer extra trap cites for electrons storage. And the nickel-silicide dots memory has standard sample and high-k sample. The HfO2 layer for control oxide can increase the electric field of the tunnel oxide to get better programming efficiency.
4

Formation of Co-Si-N nanocrystal for nonvolatile memory application

Liu, Tzu-Chia 25 June 2009 (has links)
Current requirement of nonvolatile memory (NVM) are high density cell, low-power wastage, high speed operation, and good reliability for the scaling down device. In a conventional nonvolatile memory, once the tunnel oxide develops a leaky path under repeated write/erase operation, all the stored charge will be lost. Therefore, the tunnel oxide thickness is incapable to scale down in terms of charge retention and endurance characteristics. Therefore conventional floating gate (FG) nonvolatile memories (NVMs) present critical issues on device scalability beyond the sub-50nm node. The nonvolatile nanocrystal memories are one of promising candidates to substitute for the conventional floating gate (FG) memories, because the nanocrystal memories storage charge by separated node. So it is not major influence of charge lost from partial oxide layer. The thickness of tunnel oxide can be reduce also can maintain good retention, therefore it is key to lowering operating voltages and increasing operating speeds. Also reduce device to increasing the density of device. The advantages of metal nano-dot compared with other material counterparts include stronger coupling with the conduction channel, a wide range of available work functions, and higher density of states around the Fermi level. Because these advantages. It is possibility of metal nanocrystals nonvolatile memory fabricated in industry in practice. In this thesis, an ease and low temperature fabrication technique of Co-Si-N nanocrystals was demonstrated for the application of nonvolatile memory. The nonvolatile memory structure of Co-Si-N nanocrystals embedded in the SiOx layer was fabricated by sputtering a co-mix target (CoSi2) in an Ar/N2 environment at room temperature. It can be considered that the nitrogen plays a critical role during sputter process for the formation of nanocrystal. In addition, the high density (~1012 cm-2) nanocrystal can be simple and uniform to be fabricated in our study. We also proposed a formation of Co-Si-N nanocrystals by sputtering a co-mix target (CoSi2) in the Ar/NH3 environment at room temperature. It was also found that high density Ni-Si-N nanocrystals embedded in the silicon nitride (SiNx) and larger memory effect. A rapid thermal annealing (RTA) with process temperature at 700¢XC¡B800¢XC and short duration (60sec) was used to form nanocrystals. The charge storage layer of nanorystals embedded in SiNx shows larger memory window and better reliability over nanocrystals embedded in SiOx, due to different distributions of electronic field .
5

Nonvolatile Memory based on NiSi2/SiNX compound nanocrystals

Chen, Yu-Ting 26 June 2009 (has links)
Current requirements of nonvolatile memory (NVM) are the high density cells, low-power consumption, high-speed operation and good reliability for next-generation NVM application. However, all of the charges stored in the floating gate will leak into the substrate if the tunnel oxide has a leakage path in the conventional NVM during endurance test. Therefore, the tunnel oxide thickness is difficult to scale down in terms of charge retention and endurance characteristics. Nanocrystals (NCs) NVMs are one of the promising candidates to substitute for conventional floating gate memory since the discrete storage nodes as the charge storage media can effectively enable the improvement of data retention for the scaling down device. In this thesis, we try to overcome the limitation of conventional NVMs during the scaling down process and further increase the retention time by means of changing the structure of Nanocrystals NVMs. Firstly, we deposit a NiSi2 layer as the nanocrystal self-assembled layer and thereby apply it to Nanocrystals NVMs. In room temperature, we bombard NiSi2 target to form single layer and double layer charge trapping layer through sputtering system layer by layer, and the two charge trapping layers are separated by 30 Å silicon-oxide (SiO2). Next, we also deposit silicon oxide as control oxide. According to rapid thermal anneal (RTA) mix oxide gas, we improve the oxide quality and supply NiSi2 sufficient energy to reach the smallest Gibbs free energy so as to form uniform and high density NiSi nanocrystal. On account of the increasing of trapping center and the coulomb repulsion power, the double layer structure NiSi Nanocrystals NVMs has better memory window and retention than the single layer one. In the similar process, we sputter NiSi2 target with Ar gas mixes NH3 gas to form silicon-nitride compound layers. Then, we use the same RTA process to form nanocrystal and improve the oxide quality. In the light of TEM and XPS analysis, we may infer that the nanocrystal is formed by NiSi2 and SiNX compound. Further, based on our electronic analysis, we can observe that the retention of NiSi2/SiNX compound Nanocrystal NVMs after 104 sec rises from 50% to 72% in comparison with the traditional one due of the quantum well band structure contributes by NiSi2 and SiNX compound nanocrystals. The retention of NiSi2/SiNX compound Nanocrystal NVMs after 104 sec is even better than the double layer without NH3 mixed one, 68%. Furthermore, the threshold voltage of NiSi2/SiNX compound Nanocrystal NVMs has not been subject to change after endurance with 104 programming and erasing cycles continuously. Thus, by means of depositing nanocrystal charge trapping layer mixed with NH3 gas, we achieve the objective of simplifying the fabrication process. These fabrication techniques for the application of nonvolatile nanocrystal memory can also be applicable to the current manufacture process of the integrated circuit manufacture.
6

Formation of Nanocrystalline Germanium via Oxidation of Si₀.₅₄Ge₀.₄₆ for Memory Device Applications

Kan, Eric Win Hong, Leoy, C.C., Choi, Wee Kiong, Chim, Wai Kin, Antoniadis, Dimitri A., Fitzgerald, Eugene A. 01 1900 (has links)
In this work, we studied the possibility of synthesizing nanocrystalline germanium (Ge) via dry and wet oxidation of both amorphous and polycrystalline Si₀.₅₄Ge₀.₄₆ films. In dry oxidation, Ge was rejected from the growing SiO₂ forming a Ge-rich polycrystalline layer. As for wet oxidation, Ge was incorporated into the oxide, forming a layer of mixed oxide, SixGe₁₋xOy. Formation of nanocrystalline Ge was observed when the layer of SixGe₁₋xOy was annealed in a N₂ ambient. We have fabricated a metal-insulator-semiconductor structure with nanocrystalline Ge embedded within the insulator layer to study its feasibility as a memory device. / Singapore-MIT Alliance (SMA)
7

Nanocrystal-based optoelectronic devices in plamonic nanojunctions

Evans, Kenneth 05 June 2013 (has links)
Optical trapping is an important tool for studying and manipulating nanoscale objects. Recent experiments have shown that subwavelength control of nanoparticles is possible by using patterned plasmonic nanostructures, rather than using a laser directly, to generate the electric fields necessary for particle trapping. In this thesis we present a theoretical model and experimental evidence for plasmonic optical trapping in nanoscale metal junctions. Further, we examine the use of the resultant devices as ultrasmall photodectors. Electromigrated nanojunctions, or “nanogaps”, have a well-established plasmon resonance in the near-IR, leading to electric field enhancements large enough for single-molecule sensitivity in Surface-Enhance Raman (SERS) measurements. While molecule-based devices have been carefully studied, optically and electrically probing individual quantum dots in nanoscale metal junctions remains relatively unexplored. Plasmon-based optical trapping of quantum dots into prefabricated structures could allow for inexpensive, scalable luminescent devices which are fully integrable into established silicon-based fabrication techniques. Additionally, these metal-nanocrystal-metal structures are ideal candidates to study optoelectronics in ultrasmall nanocrystals-based structures, as well as more exotic nanoscale phenomena such as blinking, plasmon-exciton interactions, and surface-enhanced fluorescence (SEF). We present experimental data supporting plasmon-based optical trapping in the nanogap geometry, and a corresponding numerical model of the electric field-generated forces in the nanogap geometry. Further, we give proof-of-concept measurements of photoconductance in the resultant quantum dot-based devices, as well as challenges and improvements moving forward.
8

Synthesis and design of nanocrystalline metal oxides for applications in carbon nanotube growth and antioxidants

Lee, Seung Soo 16 September 2013 (has links)
Synthesis of size tunable nanomaterials creates distinct chemo-physical properties. Recently, the popularity of magnetic iron oxide and cerium oxide (CeO2) nanocrystals enables researchers to use magnetic iron oxides (magnetite and ferrites) in size dependent magnetic separation and CeO2 as an automobile exhaust gas catalyst. This research shows production of diameter-controlled monodisperse magnetic iron oxide (ranging from 3 to 40 nm in diameter) and CeO2 (from 3 to 10 nm in diameter) nanocrystals with exceptional narrow diameter distribution (σ<10%). The morphology and composition of the nanocrystals were varied by use of diverse metal precursors, reaction temperature, time, cosurfactants, and molar ratio between metal salt and surfactant. Now the narrow diameter distributions of preformed magnetic iron oxide nanocrystals made it possible to grow diameter controlled uniform CNTs. The correlation between aluminum ferrite nanocrystal diameter and CNT diameter was nearly one. Additionally, we could synthesize the highest percentage (60%) of single walled CNTs from the smallest aluminum ferrite nanocrystals (4.0 nm). Because of the synthesis of uniform nanocrystalline CeO2, we could study diameter dependent antioxidant properties of nanocrystalline CeO2; antioxidant capacity of CeO2 was nine times higher than a known commercial standard antioxidant, Trolox. In addition, the smallest CeO2 nanocrystal (4 nm) decreased the oxidative stress of human dermal fibroblasts (HDF) exposed to hydrogen peroxide. These works suggest better understanding of monodisperse nanocrystal synthetic mechanism and potential uses of the materials, such as high quality CNT growth using magnetic iron oxides as precursor catalysts and the reduction of oxidative stress in cells using monodisperse CeO2 nanocrystal as an antioxidant for reactive oxygen species in biological media.
9

Investigation of the correlation between the structure and fluorescence properties of semiconductor quantum dots

Lin, Wen-Bin 05 August 2005 (has links)
Quantum confinement structures are attractive for their unconventional size dependence of the optical and electrical properties. There are still challenges to control the size uniformity for the application. The thesis studies the correlation between the size distribution of CdSe/ZnS quantum dots (QD), and the fluorescence properties to understand the shape and size influences of their fluorescence properties. Results from the transmission electron microscopy (TEM) provide the structure and size distribution of the samples. Excitation dependent fluorescence spectra as well as the PL excitation at various emission wavelength confirm that the inhomogeneous distribution of the samples. The results show that the samples are mostly composed of QDs with quasi-spherical structure (aspect ration between 1.1 and 1.5 ;76%) and spherical structure ( aspect ratio < 1.1; 12.8%). In addition, it exhibits a distribution of the long axis of 5.4nm¡Ó1.3nm. By measuring the fluorescence spectra of individual QDs, we construct the distribution. The peaks of the fluorescence spectra show a Gaussian distribution with center at 615.7 nm and width 13.8 nm. In addition, the spectra exhibit a width of 19.7¡Ó8.0nm. This is consistent with the ensemble measurement of the fluorescence from a solution (peak at 616 nm, and width 25 nm). Results of the fluorescence lifetime on the individual QDs indicate the lifetime distribution of 10.3¡Ó5.6ns. Further analyze the size distribution by constructing the size ¡V fluorescence spectrum relationship. By analysis the distribution of the fluorescence spectra, it results the corresponding size distribution of width 0.7 nm. This is much narrower than the size distribution of the long axis measured by TEM, but is more consistent with the corrected size distribution considering the short axis contribution. We conclude that the deviation results from the non-spherical structures in the samples.
10

Activity of Nanocrystalline Gold and Silver Alloys

Unrau, Kevin R Unknown Date
No description available.

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