Shih-Yun Chen

Towards understanding the electronic structure of Fe-doped CeO2 nanoparticles with X-ray spectroscopy

This study reports on the electronic structure of Fe-doped CeO2 nanoparticles (NPs), determined by coupled X-ray absorption spectroscopy and X-ray emission spectroscopy. A comparison of the local electronic structure around the Ce site with that around the Fe site indicates that the Fe substitutes for the Ce. The oxygen K-edge spectra that originated from the hybridization between cerium 4f and oxygen 2p states are sensitive to the oxidation state and depend strongly on the concentration of Fe doping. The Ce M(4,5)-edges and the Fe L(2,3)-edges reveal the variations of the charge states of Ce and Fe upon doping, respectively. The band gap is further obtained from the combined absorption-emission spectrum and decreased upon Fe doping, implying Fe doping introduces vacancies. The oxygen vacancies are induced by Fe doping and the spectrum reveals the charge transfer between Fe and Ce. Fe(3+) doping has two major effects on the formation of ferromagnetism in CeO2 nanoparticles. The first, at an Fe content of below 5%, is that the formation of Fe(3+)-Vo-Ce(3+) introduces oxygen deficiencies favoring ferromagnetism. The other, at an Fe content of over 5%, is the formation of Fe(3+)-Vo-Fe(3+), which favors antiferromagnetism, reducing the Ms. The defect structures Fe(3+)-Vo-Ce(3+) and Fe(3+)-Vo-Fe(3+) are crucial to the magnetism in these NPs and the change in Ms can be described as the effect of competitive interactions of magnetic polarons and paired ions.

Understanding and tuning electronic structure in modified ceria nanocrystals by defect engineering.

This study investigates the effect of Fe(3+) on the electronic structure of nanocrystalline ceria. Systematic synchrotron X-ray absorption spectroscopy coupled with scanning transmission electron microscopy/electron energy loss spectroscopy was utilized. The oxygen vacancies can be engineered and their number varied with the degree of iron doping. Comparing the local electronic structure around Ce sites with that around Fe sites reveals two stages of defect engineering. The concentration of Ce(3+) and the distribution of defects differ between lower and higher degrees of doping. Charge is transferred between Ce and Fe when the doping level is less than 5%, but this effect is not significant at a doping level of over 5%. This transfer of charge is verified by energy loss spectroscopy. These Fe-modified ceria nanoparticles exhibit core-shell-like structures at low doping levels and this finding is consistent with the results of scanning transmission electron microscopy/electron energy loss spectroscopy. More Fe is distributed at the surface for doping levels less than 5%, whereas the homogeneity of Fe in the system increases for doping levels higher than 5%. X-ray magnetic circular dichroism spectroscopy reveals that Ce, rather than Fe, is responsible for the ferromagnetism. Interestingly, Ce(3+) is not essential for producing the ferromagnetism. The oxygen vacancies and the defect structure are suggested to be the main causes of the ferromagnetism. The charge transfer and defect structure Fe(3+)-Vo-Ce(3+) and Fe(3+)-Vo-Fe(3+) are critical for the magnetism, and the change in saturated magnetization can be understood as being caused by the competition between interactions that originate from magnetic polarons and from paired ions.

Observations on PVP-protected noble metallic nanoparticle deposits upon heating via in situ synchrotron radiation X-ray diffraction.

Through monitoring the evolution of the X-ray diffraction peaks, the phase transformation of PVP-protected Ag and Au nanoparticle deposits (NPDs) on electronic substrates of Cu and Ni upon heating in air was investigated via in situ synchrotron radiation X-ray diffraction. With an increasing temperature, the broad diffraction peak of nano-sized Ag and Au particles with the original average diameters of 4.2 nm and 9.6 nm, respectively, became sharp because of particle coarsening and coalescence. Complex phase transitions among Au, Cu, AuCu(3) and CuO(x) were observed, mainly due to the negative enthalpy of mixing between Au and Cu. The interactions between NPDs and the substrates affected the shift of diffraction peaks to lower angles, caused by thermal expansion and also the temperature for the oxide formation. Compared to Au, Ag NPDs did not form intermetallic compounds with Cu and the formation of copper oxides can also be retarded mainly due to the phase separation feature of the Ag-Cu system.

Spectroscopic study on spontaneously grown silver@ultra-thin cerium oxide nanostructures

Ag@CeO2 nanostructures have been recently reported to show unique catalytic properties but synthetic methods for them are limited. This study investigates microstructural characteristics of Ag@CeO2 nanowires and nanoparticles with a spontaneously-grown ultra-thin ceria shell (∼0.5 nm), which were synthesized on CeO2 substrate without using oxide precursors. Elemental mapping and line scanning by electron energy loss spectroscopy (EELS) suggest that the Ce in the CeO2 substrate dissolved in the molten silver nitrate salt and was repelled to the surface of the Ag nanostructures to form continuous oxide shells during the growth of Ag single-crystals. X-ray absorption near edge structure (XANES) spectra verify that the valence of the Ce ions in the oxide layer was between Ce3+ and Ce4+.

The application of Cu-Ag submicron composite particles in microelectronic bonding

In this study, Cu-Ag composite pastes with the mixture of Cu@Ag core-shell particles and Ag nanoparticles prepared by one-step spray pyrolysis are developed for microelectronic bonding applications. Cu to Cu bonding can be successfully achieved by thermal compression with the Cu/Ag pastes at 250°C. The joint strength reaches 28.2 MPa and 32.4MPa respectively when bonding pressure holds at 5 MPa and 10MPa at 275°C. It has been demonstrated the joints thus formed remain robust at elevated temperatures up to 250°C and excellent reliability subjected to thermal cycling ranged from -65 °C~150 °C.

Cu to Cu bonding with Cu@Ag core-shell nanoparticles

In this study, PVP-protected Cu@Ag core-shell nanoparticles with different Ag shell thicknesses were synthesized and applied to Cu to Cu bonding. The phase transformation of Cu@Ag core-shell nanoparticles (about 80 nm in average particle diameter and 10 nm in shell thickness) upon heating in air was investigated via in situ synchrotron radiation X-ray diffraction. The formation temperatures of copper oxides (CuO and Cu2O) were higher than those of pure Cu nanoparticles. This verifies that the oxidation of Cu nanoparticles can be effectively retarded by the Ag shell. It is also demonstrated that firm bonding between two Cu pads could be obtained using Cu-Ag particle pastes after thermal compression at 250 °C.