Doctor of Philosophy

dissertationA detailed understanding of the conformation of adsorbed molecules and regional surface functionalization of metal nanoparticles (MNPs) is challenging for nanometersize (10 - 100 m) materials and necessary for fundamental studies and applications. The studies are motivated by open questions related to surface chemistry of noble MNPs. Although citrate-stabilized gold NPs (AuNPs) have been widely used, the citrate layer is not well-understood. Thiols have been suggested to displace citrate anions adsorbed on metal surfaces due to strong gold-sulfur interaction, but quantitative experimental evidence of the extent of ligand-exchange has not been reported. Whereas asymmetrically-functionalized AuNPs are utilized for nanoparticle assembly due to the interparticle coupling of localized surface plasmons, the interface between asymmetric nanoparticles in single assemblies has not been studied. Noble MNPs with sizes smaller than citrate-stabilized AuNPs also need to be surface-modified for stability in water for biological applications. The dissertation presents investigations of the chemical and physical properties of gold and silver NPs (AgNPs) related to ligand adsorption at the metal surface. Firstly, self-assembled layers of citrate adsorbed on AuNP (111), (110), and (100) surfaces were proposed, based on geometric considerations and spectroscopic investigations by infrared (IR) and X-ray photoelectron spectroscopy (XPS). Adsorption characteristics of citrate are the unique structure of adsorbed species, intermolecular interactions through hydrogen bonds and van der Waals attractions, bilayer formation, surface coverage, nanoparticle-stabilization role, and chirality. Secondly, IR and XPS studies showed coadsorption of thiolate on the surface of citrate-stabilized AuNPs. Steric, chelating effects and intermolecular interactions are the origins of the strong adsorption of citrate on AuNP surfaces. Surface coverage was determined from XPS analyses. Thirdly, an exclusive placement of Raman probe molecules (4-nitrobenzenethiol) at junctions (hot-spots) of AuNP dimers was achieved through an asymmetric functionalization approach. The orientation of asymmetric nanoparticles in dimers and the location of Raman probes were investigated using surface-enhanced Raman scattering (SERS). A linear correlation of SERS signal with hot-spot population and a SERS enhancement factor are presented. Lastly, AuNPs and AgNPs of small sizes (< 5 nm) were synthesized in water using poly(allylamine). Ligand-exchange by thiols was demonstrated by IR spectroscopy and transmission electron microscopy analyses

Pressure Transmission in the Compaction Process for Nickel Powder using Finite Element Methods

The compression process is one of the more widely used industrial manufacturing methods for fabricating desired shape of specimens with various materials such as metals and ceramics. In the compaction process, the upper punch moves into the powder, and force is transmitted between particles, then achieving densification. In this process, the powder can be considered to be in a particulate state, which means that while the powder consists of solids, it has characteristics quite similar to the fluid. Therefore, particles in the process can be seen as responding to hydrostatic pressure, and it can be assumed that the pressure is constant. However, the forces acted on the inter-particle continue to change during the process. Many parameters affect the force change, including compaction speed and the contact angle between particles. However, it is very difficult to verify these effects through experiments because it is impossible to arrange the inter-particle angle. Therefore, in this study, the force transmission mechanism was simulated in the compaction process using FEM simulation. To examine the contact angle and force transmission between the particles, a green compact was modeled as individual particles rather than as a continuum green compact. Finally, it was confirmed through analysis that the pressure transmission between the particles remained constant during the compression process.11Ysciescopu

The Guidelines of Material Design and Process Control on Hybrid Fiber Metal Laminate for Aircraft Structures

Fiber metal laminate (FML) is a hybrid material system that consists of thin metal sheets bonded into a laminate with intermediate thin fiber reinforced composite layers. The aerospace industry has recently increased their use of FMLs due to the considerable weight reduction and consequent benefits for critical load-carrying locations in commercial aircraft, such as upper fuselage skin panels. All FML materials and their processes should be qualified through enough tests and fabrication trials to demonstrate reproducible and reliable design criteria. In particular, proper surface treatment technologies are prerequisite for achieving long-term service capability through the adhesive bonding process. This chapter introduces a brief overview of design concept, material properties and process control methodologies to provide detailed background information with engineering practices and to help ensure stringent quality controls and substantiation of structure integrity. The guidelines and information found in this chapter are meant to be a documentation of current knowledge and an application of sound engineering principles to the FML part development for aerospace usage

The 750 GeV Diphoton Excess May Not Imply a 750 GeV Resonance

We discuss non-standard interpretations of the 750 GeV diphoton excess recently reported by the ATLAS and CMS Collaborations which do not involve a new, relatively broad, resonance with a mass near 750 GeV. Instead, we consider the sequential cascade decay of a much heavier, possibly quite narrow, resonance into two photons along with one or more invisible particles. The resulting diphoton invariant mass signal is generically rather broad, as suggested by the data. We examine three specific event topologies - the antler, the sandwich, and the 2-step cascade decay, and show that they all can provide a good fit to the observed published data. In each case, we delineate the preferred mass parameter space selected by the best fit. In spite of the presence of invisible particles in the final state, the measured missing transverse energy is moderate, due to its anti- correlation with the diphoton invariant mass. We comment on the future prospects of discriminating with higher statistics between our scenarios, as well as from more conventional interpretations.Comment: Discussion about the ATLAS Moriond EW2016 added. Matched to PRL accepted versio

Influencing Factors on Knowledge Adoption of Mobile Game Developers in Online Communities

Recently, with the advance of wireless Internet access via mobile devices, a myriad of game development companies has forayed into the mobile game market, leading to intense competition. To survive in this fierce competition, mobile game developers often try to get a grasp of the rapidly changing needs of their customers by operating their own official communities where game users freely leave their requests, suggestions, and ideas relevant to focal games. Based on the heuristic-systematic model (HSM) and data quality (DQ) framework, this study derives key content, non-content, and hybrid cues that can be utilized when game developers accept suggested postings in these online communities. The results of the hierarchical multiple regression analysis show relevancy, timeliness, amount of writing, and the number of comments are positively associated with mobile game developers’ knowledge adoption. In addition, title attractiveness mitigates the relationship between amount of writing/the number of comments and knowledge adoption