Design and Synthesis of Metal Nanoparticles and Metal Oxide Materials for Electronic Devices


Student thesis: Doctoral Thesis

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  • Li ZHOU


Awarding Institution
Award date20 Jun 2016


As a result of the continuous drive to fabricate electronic devices on lightweight, large-area plastic substrates by low-cost processing techniques, organic electronics is currently running in the fast lane. Organic electronics is a promising technology that is presently being introduced in an extensive range of applications. Organic electronics has been the subject of latest attention for energy saving and other innovative technologies, including organic light emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic memories, organic batteries, organic lasers, and organic photo voltaic (OPV). Organic-inorganic nanocomposites are one type of advanced material for organic devices because of the expected synergy between the organic and inorganic components, which lead potentially to new physical properties of the composites and various applications in devices, including thin film transistors, solar cells, phototransistors, memories, and sensors.
The fascinating size-dependent properties of noble metal nanoparticles have created a great promise for their use in a variety of electronic, optical, and biomedical applications. Among them, silver and gold nanoparticles, specifically, have received a great deal of attention due to their unusual physical properties. Initially, spherical gold nanoparticles received the most attention due to the ease of synthesis of such structures. As synthetic capabilities improved, it is necessary to find reaction conditions which can break the propensity towards isotropic growth and instead direct the nanoparticle growth into an anisotropic dimension. Among them, gold nanorods (Au NRs) are of particular interest because this unique metallic nanostructure has two plasmon resonance peaks corresponding to longitudinal and transverse absorption bands, which differ from spherical Au NPs. The longitudinal plasmon resonance band is extremely sensitive to changes in the dielectric properties of surrounding media and can be tuned from the visible to near IR region by adjusting the aspect ratio of the NR. In addition, the plasmon resonance bands of Au@Ag core-shell NRs can be synthetically manipulate by varying the Au-core size or the Ag-shell thickness to achieve a broad spectrum region from visible to near-infrared.
This thesis is mainly concerning studies of the design and synthesis of Au nanoparticles including nanospheres, nanorods and core-shell structures for organic electronic devices. Firstly, A dual plasmonic resonance effect on the performance of poly(3-hexylthiophene) (P3HT):phenyl C61-butyricacid methyl ester (PC61BM) based polymer solar cells (PSCs) has been demonstrated by selectively incorporating 25 nm colloidal gold nanoparticles (Au NPs) in a solution-processed molybdenum oxide (MoO3) anode buffer layer and 5 nm colloidal Au NPs in the active P3HT :PCBM layer. The devices exhibit up to ~20% improvement in power conversion efficiency, which is attributed to the dual effect of localized surface plasmon resonance (LSPR) of Au NPs with enhanced light absorption and exciton generation. Experimental and simulation results demonstrate that the enhancement over the device performance is ascribed to the incorporation of Au NPs into individual layers. Meanwhile, a balanced charge transport is achieved in P3HT:PCBM layer by inserting 5 nm Au NPs hence an improvement over the device performance is realized. This dual LSPR approach might be further used in other solution-processed anode buffer layers and active layer systems to achieve higher efficiencies.
Secondly, we demonstrate an incorporation of Au NRs with alkylamino and pyridine ligand into P3HT matrix based OTFTs for investigating the effect of Au NRs on device performance. Through varying the doping concentration and ligand of Au NRs, remarkable enhancements in filed effect mobility of P3HT based OTFTs have been demonstrated. This significant improvement in mobility is due to the enhanced crystallinity and optimized orientation of P3HT induced by rod-like shape Au nanoparticle doping. In addition, the pyridine ligand capped Au NRs shows more efficient hole conduction is another important reason of the device performance enhancement. The experimental results demonstrate that the nanocomposites with appropriate modification are very useful to the development of polymer devices for a wide range of commercial applications.
Finally, we developed a novel light modulated photonic nonvolatile memory by utilizing solution-processed Ag/Au core/shell nanorods as charge trapping and photoresponse material. By tuning the aspect ratio of Au NRs and thickness of Ag shell, the optical properties of Au@Ag NRs show a plasmon resonance assigned to Ag around 400-600nm and another resonance at ~600-900 nm. The memory characteristics of the devices are systematically tuned through varying the combinations of applied gate pulses and light pulses. Light manipulation is a feasible way to achieve multilevel Vth in a single memory cell. These effects can be explained by LSPR effect of Au@Ag NRs may leading to the enhancement of semiconductor absorption, which enhance the carrier generation in the channel. Therefore, this multilevel data storage based on the LSPR effect may further be employed into other systems to manufacture advanced optoelectronic memory devices.