Browsing by Subject "nanocrystals"
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Item Energy Transfer Dynamics and Dopant Luminescence in Mn-Doped CdS/ZnS Core/Shell Nanocrystals(2012-11-13) Chen, Hsiang-YunMn-doped II-VI semiconductor nanocrystals exhibit bright dopant photoluminescence that has potential usefulness for light emitting devices, temperature sensing, and biological imaging. The bright luminescence comes from the 4T1?6A1 transition of the Mn2+ d electrons after the exciton-dopant energy transfer, which reroutes the exciton relaxation through trapping processes. The driving force of the energy transfer is the strong exchange coupling between the exciton and Mn2+ due to the confinement of exciton in the nanocrystal. The exciton-Mn spatial overlap affecting the exchange coupling strength is an important parameter that varies the energy transfer rate and the quantum yield of Mn luminescence. In this dissertation, this correlation is studied in radial doping location-controlled Mn-doped CdS/ZnS nanocrystals. Energy transfer rate was found decreasing when increasing the doping radius in the nanocrystals at the same core size and shell thickness and when increasing the size of the nanocrystals at a fixed doping radius. In addition to the exciton-Mn energy transfer discussed above, two consecutive exciton-Mn energy transfers can also occur if multiple excitons are generated before the relaxation of Mn (lifetime ~10^-4 - 10^-2 s). The consecutive exciton-Mn energy transfer can further excite the Mn2+ d electrons high in conduction band and results in the quenching of Mn luminescence. The highly excited electrons show higher photocatalytic efficiency than the electrons in undoped nanocrystals. Finally, the effect of local lattice strain on the local vibrational frequency and local thermal expansion was observed via the temperature-dependent Mn luminescence spectral linewidth and peak position in Mn-doped CdS/ZnS nanocrystals. The local lattice strain on the Mn2+ ions is varied using the large core/shell lattice mismatch (~7%) that creates a gradient of lattice strain at various radial locations. When doping the Mn2+ closer to the core/shell interface, the stronger lattice strain softens the vibrational frequency coupled to the 4T1?6A1 transition of Mn2+ (Mn luminescence) by ~50%. In addition, the lattice strain also increases the anharmonicity, resulting in larger local thermal expansion observed from the nearly an order larger thermal shift of the Mn luminescence compared to the Mn-doped ZnS nanocrystals without the core/shell lattice mismatch.Item High-K Based Non-Volatile Memory Devices with the Light Emitting Application(2014-09-05) Lin, Chi-ChouThe zirconium-doped hafnium oxide (ZrHfO) high-k gate dielectric films with and without the embedded nanocrystals have been studied for the applications of the nonvolatile memory and light emitting devices. By replacing the polycrystalline Si with the novel discrete nanocrystal embedded high-k ZrHfO structure, the promising memory functions can be expected. On the other hand, by using the same metal oxide semiconductor (MOS) capacitor structure with ZrHfO gate dielectric layer but different operating gate voltage (Vg) ranges, e.g., when Vg is larger than the breakdown voltage (VBD), the device starts emitting the white light. This new solid state incandescent light emitting device (SSI-LED) unveils a new concept for the future LED evolution. The nanocrystals cadmium selenide (nc-CdSe) and molybdenum oxide (nc-MoO3) embedded ZrHfO on the p-type silicon wafer have been fabricated by self-assembly process and studied for their charge trapping, detrapping, and retention characteristics. Moreover, the temperature effect on the memory function has been investigated on the nc-MoO3 embedded device. More than half of the originally trapped holes can be retained in the CdSe nanocrystals for more than 10 years. For the temperature test, with the increase of temperature, the memory window was enlarged and the Coulomb blockade effect was suppressed in the nc-MoO3 embedded ZrHfO memory device. At the same time, the interface quality was deteriorated, the leakage current was increased, and the lifetime was shortened. The light emission characteristics of the new SSI-LED composed of the ZrHfO or WO3 thin film have been investigated. The light emitting principle is based on the thermal excitation of the conductive paths formed after the dielectric breakdown, which is different from the electron-hole or exciton radiative recombination mechanism in conventional LEDs. The emission spectrum covers the visible to the near IR wavelength range with the color rendering index of 98.4. The light intensity can be enhanced by embedding CdSe nanocrystals into the ZrHfO dielectric layer due to the increase of the defect density which causes the enhancement of the leakage current. The SSI-LED has a very long lifetime of > 5,664 hours in the atmosphere. Lastly, the additive gas effect of a plasma-based process for etching the copper film over a near-vertical step has been investigated. A new process that minimizes the excessive attacks of the cusp region was developed.