Yueqin Wu

Synthesis of Sintering-Resistant Sorbents for CO2 Capture

Sorbents for high temperature CO2 capture are under intensive development owing to their potential applications in advanced zero emission power, sorption-enhanced steam methane reforming for hydrogen production and energy storage systems in chemical heat pumps. One of the challenges in the development is the prevention of sintering of the sorbent (normally a calcium oxide derivative) which causes the CO2 capture capacity of the material to deteriorate rapidly after a few cycles of utilization. Here we show that a simple wet mixing method can produce sintering-resistant sorbents from calcium and magnesium salts of d-gluconic acid. It was found that calcium oxide was well distributed in the sorbents with metal oxide nanoparticles on the surface acting as physical barriers, and the CO2 capture capacity of the sorbents was largely maintained over multiple cycles of utilization. This method was also applied to other organometallic salts of calcium and magnesium/aluminum and the produced sorbents showed similarly high reversibility.

Superior electrical properties of crystalline Er2O3 films epitaxially grown on Si substrates

Crystalline Er2O3 thin films were epitaxially grown on Si (001) substrates. The dielectric constant of the film with an equivalent oxide thickness of 2.0 nm is 14.4. The leakage current density as small as 1.6 X 10(-4) A/cm(2) at a reversed bias voltage of -1 V has been measured. Atomically sharp Er2O3/Si interface, superior electrical properties, and good time stability of the Er2O3 thin film indicate that crystalline Er2O3 thin film can be an ideal candidate of future electronic devices. (c) 2006 American Institute of Physics.

Nanoscratch-induced deformation of single crystal silicon

The nanoscratching-induced deformation of monocrystalline Si has been investigated using transmission electron microscopy (TEM). The results indicate that amorphization and formation of crystalline defects are two dominant phenomena associated with the scratching processes. TEM analyses reveal that amorphization occurs at extremely small scratching loads. Stacking faults and twins are nucleated at a smaller load than that for dislocation. Dislocations start to nucleate along Si {111} planes when the normal scratching load is greater than a threshold value and penetrate deeper into the Si subsurface with the increasing load. Both normal load and tip radius have significant influence on the deformation, which are somehow different from those associated with nanoindentation and nanogrinding.

Formation mechanism of nanocrystalline high-pressure phases in silicon during nanogrinding

The phase transformations of Si under nanogrinding have been studied by transmission electron microscopy and Raman spectroscopy. Nanocrystalline high-pressure phases (Si-III/Si-XII) were found in the amorphous layer of the subsurface of heavily ground Si. The sequence of the phase transformation in nanogrinding has been found to be different to that in nanoindentation. The formation mechanism of the nanocrystalline high-pressure phases in nanogrinding is proposed based on experimental results.

Fracture Strain of SiC Nanowires and Direct Evidence of Electron‐Beam Induced Amorphisation in the Strained Nanowires

SiC nanowires with diameters ranging from 29 to 270 nm exhibit an average strain of 5.5% with a maximum of up to 7.0%. The brittle fracture of the nano-wires being measured was confirmed by transmission electron microscopy (TEM) analysis. This study demonstrates that amorphisation occurs in the stained SiC nanowires during normal TEM examination, which could be induced by electron irradiation.

Shape change of SiGe islands with initial Si capping

The morphologies of self-assembled Ge/Si(001) islands with initial Si capping at a temperature of 640 °C are investigated by atomic force microscopy. Before Si capping, the islands show a metastable dome shape with very good size uniformity. This dome shape changes to a pyramid shape with {103} facets at a Si capping thickness of 0.32 nm, and then changes to pyramid shapes with {104} and {105} facets at Si capping thicknesses of 0.42 and 0.64 nm, respectively. Noteworthy is that islands with one side retained their dome shape while the other three sides that changed to {103} facets are observed at a Si capping thickness of 0.18 nm. These observations indicate that island shape change with Si capping is a kinetic rather than thermodynamic process. The atomic processes associated with this island shape change are kinetically limited at a low temperature of 400 °C, and no significant change in size and shape of islands is observed when Si capping layers are deposited at this temperature.

Formation of coupled three-dimensional GeSi quantum dot crystals

Coupled three-dimensional GeSi quantum dot crystals (QDCs) are realized by multilayer growth of quantum dots (QDs) on patterned SOI (001) substrates. Photoluminescence spectra of these QDCs show non-phonon (NP) recombination and its transverse-optical (TO) phonon replica of excitons in QDs. With increasing excitation power, peak energies of both the NP and TO peaks remain nearly constant and the width of the TO peak decreases. These anomalous features of the PL peaks are attributed to miniband formation due to strong coupling of the holes and the emergence of quasioptical phonon modes due to periodic scatters in ordered GeSi QDs.

Atomic composition profile change of SiGe islands during Si capping

The 6% Ge isocomposition profile change of individual SiGe islands during Si capping at 640 degrees C is investigated by atomic force microscopy combined with a selective etching procedure. The island shape transforms from a dome to a {103}-faceted pyramid at a Si capping thickness of 0.32 nm, followed by the decreasing of pyramid facet inclination with increasing Si capping layer thickness. The 6% Ge isocomposition profiles show that the island with more highly Si enriched at its one base corner before Si capping becomes to be more highly Si intermixed along pyramid base diagonals during Si capping. This Si enrichment evolution inside an island during Si capping can be attributed to the exchange of capped Si atoms that aggregated to the island by surface diffusion with Ge atoms from inside the island by both atomic surface segregation and interdiffusion rather than to the atomic interdiffusion at the interface between the island and the Si substrate. In addition, the observed Si enrichment along the island base diagonals is attempted to be explained on the basis of the elastic constant anisotropy of the Si and Ge materials in (001) plane. (c) 2006 American Institute of Physics.

Thermal stability of Er2O3 thin films grown epitaxially on Si substrates

The thermal stability of Er2O3 thin films grown epitaxially on Si substrates has been investigated in this paper by x-ray diffraction and high resolution transmission electron microscopy. The Er2O3∕Si(001) films are found to react with Si to form silicates at the temperature of 450°C in N2 ambience, whereas O2 ambience can prevent the silicate formation even at the temperature of 600°C. However, at a high temperature of 900°C in either N2 or O2 ambience, Er2O3 films react with Si, and both silicate and SiO2 are formed in the films. In addition, the Er2O3 films grown on Si(111) substrates show poorer thermal stability than those grown on Si(001) substrates; Er silicide is formed at the interface in the films annealed at 450°C in O2 ambience, which is attributed to that the reaction product hexagonal ErSi2 is formed more easily on Si(111) than on Si(001) due to structure similarity as well as small lattice mismatch.

Fowler-Nordheim hole tunneling in metal-Er2O3-silicon structures

Fowler-Nordheim (FN) tunneling of holes in metal-Er2O3–Si structures is confirmed. The effective mass of holes in Er2O3 films is estimated ranging from 0.068m to 0.092m, where m is the free electron mass. The film shows a high breakdown electric field of about 70MV∕cm for an Er2O3 film thickness of 8.5nm, implying that the film which is epitaxially grown on Si substrate has smooth interface and surface.