Sheng-Wei Lee

Self-assembled nanorings in Si-capped Ge quantum dots on (001)Si

Nanorings with an average height and diameter of 1.2 and 65 nm, respectively, were observed to form in Si-capped Ge quantum dots grown at 600 °C by ultrahigh-vacuum chemical vapor deposition. The nanorings were captured with the rapid cooling of the samples with appropriate amount of Si capping. Based on the results of transmission electron microscopy and Raman spectroscopy, the formation of nanorings is attributed to alloying and strain relief in the Si/Ge/(001)Si system. The self-assembly of nanorings provides a useful scheme to form ultrasmall ring-like structure and facilitates the characterization of the physical properties of unconventional quantum structures.

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si self-assembled quantum dots

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si quantum-dot light-emitting diodes is reported. The devices were fabricated in a mesa-type structure, with a silicon oxide layer on the top for surface/sidewall passivation. Different passivation processes were employed. We found that the integrated electroluminescence intensities were relatively less sensitive to temperature, persisting at nearly the same intensity up to RT. The fabricated device shows an internal quantum efficiency of about 0.015% at RT. The improved emission property is attributed to the reduced nonradiative recombination centers due to the surface passivation and thermal treatment.

Effects of low-temperature Si buffer layer thickness on the growth of SiGe by molecular beam epitaxy

The thickness of a low-temperature silicon (LT-Si) buffer layer has been found to affect the growth of a SiGe overlayer significantly. 300-nm-thick Si0.7Ge0.3 films were grown on 50- to 300-nm-thick LT-Si buffer layers at 450 °C by solid-source molecular beam epitaxy. The threading dislocation density was found to decrease with the thickness of the LT-Si buffer in the thickness range of 50–200 nm. The density remains at the same low level when the thickness was increased from 200 to 300 nm. A relatively dense misfit dislocation network was observed to form at the SiGe/Si interface in samples with the LT-Si buffer layer thickness exceeding 200 nm. It is suggested that the presence of more point defects in the thicker LT-Si buffer layer is more effective to block the propagation of threading dislocations.

Both Enhanced Biocompatibility and Antibacterial Activity in Ag-Decorated TiO2 Nanotubes

In this study, Ag is electron-beam evaporated to modify the topography of anodic TiO2 nanotubes of different diameters to obtain an implant with enhanced antibacterial activity and biocompatibility. We found that highly hydrophilic as-grown TiO2 nanotubes became poorly hydrophilic with Ag incorporation; however they could effectively recover their wettability to some extent under ultraviolet light irradiation. The results obtained from antibacterial tests suggested that the Ag-decorated TiO2 nanotubes could greatly inhibit the growth of Staphylococcus aureus. In vitro biocompatibility evaluation indicated that fibroblast cells exhibited an obvious diameter-dependent behavior on both as-grown and Ag-decorated TiO2 nanotubes. Most importantly, of all samples, the smallest diameter (25-nm-diameter) Ag-decorated nanotubes exhibited the most obvious biological activity in promoting adhesion and proliferation of human fibroblasts, and this activity could be attributed to the highly irregular topography on a nanometric scale of the Ag-decorated nanotube surface. These experimental results demonstrate that by properly controlling the structural parameters of Ag-decorated TiO2 nanotubes, an implant surface can be produced that enhances biocompatibility and simultaneously boosts antibacterial activity.

Diameter-sensitive biocompatibility of anodic TiO2 nanotubes treated with supercritical CO2 fluid.

This work reports on the diameter-sensitive biocompatibility of anodic TiO2 nanotubes with different nanotube diameters grown by a self-ordering process and subsequently treated with supercritical CO2 (ScCO2) fluid. We find that highly hydrophilic as-grown TiO2 nanotubes become hydrophobic after the ScCO2 treatment but can effectively recover their surface wettability under UV light irradiation as a result of photo-oxidation of C-H functional groups formed on the nanotube surface. It is demonstrated that human fibroblast cells show more obvious diameter-specific behavior on the ScCO2-treated TiO2 nanotubes than on the as-grown ones in the range of diameters of 15 to 100 nm. This result can be attributed to the removal of disordered Ti(OH)4 precipitates from the nanotube surface by the ScCO2 fluid, thus resulting in purer nanotube topography and stronger diameter dependence of cell activity. Furthermore, for the smallest diameter of 15 nm, ScCO2-treated TiO2 nanotubes reveal higher biocompatibility than the as-grown sample.

Matrix and quantum confinement effects on optical and thermal properties of Ge quantum dots

The influence of SiO_2 and Si_3N_4 dielectric matrices on the structural, phonon, luminescence and thermal properties of Ge quantum dots (QDs) has been experimentally investigated. Compared with the case of QDs in SiO_2 layers, Si_3N_4 matrix imposes large interfacial surface energy on QDs and enhances their Ostwald ripening rate, appearing to be conducive for an improvement in crystallinity and a morphology change to a more perfectly spherical shape of Ge QDs. Quantum confinement induced electronic structure modulation for Ge QDs is observed to be strongly influenced not only by the QD size but also by the embedded matrix. Both matrix and surface effects offer additional mechanisms to QD itself for controlling the optical and thermal properties of the QDs.

Induction of apoptosis by high-dose gold nanoparticles in nasopharyngeal carcinoma cells

OBJECTIVE Nasopharyngeal carcinoma (NPC) is a rare malignancy in most parts of the world, but is a common cancer in southern Asia. Local recurrent disease and distant metastasis of NPC are still the unsolved problems. Recently, gold nanoparticles (AuNPs) have been developed as potential in vivo diagnostic and therapeutic agents. However, their role on nasopharyngeal cancer remains unknown. The object of this study is to investigate if AuNPs can be used as a new therapeutic agent for NPC by evaluating their anti-tumor effect in vitro. METHODS The AuNPs were prepared by the reduction of chloroauric acid to neutral gold. Their size distribution and microstructures were characterized by transmission electron microscopy (TEM). To evaluate their cytotoxic effect, NPC cell line TW01 and Human Nasal Epithelial Cells (HNEpC) were cultured in various concentrations of AuNPs for 3 days. Cell viability was evaluated by Trypan Blue viability assay while morphologic findings were observed via light microscopy. Terminal deoxynucleotidyltransferase-mediated dUPT nick end labeling (TUNEL) assay was used to detect apoptosis. RESULTS AuNPs prepared in this study had an average diameter of 20.5nm and they were observed under light microscopy as dark material aggregated in the cells after treatment. Contrary to the HNEpC, the AuNPs reduced cell viability of NPC cell in a concentration-dependant manner by Trypan Blue assay, especially at high concentration. Besides, cell apoptosis was demonstrated by positive TUNEL assay. CONCLUSIONS The AuNP possesses specific imaging properties and is cytotoxic to NPC cells at high concentrations.

Synthesis of blue-light-emitting Si1-xGex oxide nanowires

Blue-light-emitting Si1−xGex oxide nanowires have been grown on epitaxial Si0.8Ge0.2 alloys on silicon by thermal annealing in a quartz tube furnace in N2 ambient. The photoluminescence spectrum of Si1−xGex oxide nanostructures exhibits the blue-light emission with a peak at 415 nm, compared with the Si oxide nanowires with a peak at 470 nm. Nanowires with uncommon shapes, such as sunflowerlike and radiolarialike shape, have been observed. A field emission scanning electron microscope was used to monitor the growth of nanowires on the same patterned catalytic Au region. The growth can be understood in term of vapor-liquid-solid mechanism.

Ionic liquid electrolytes for high-voltage rechargeable Li/LiNi0.5Mn1.5O4 cells

A high-voltage LiNi0.5Mn1.5O4 cathode material with a cubic spinel structure is synthesized using a citric-acid-assisted sol–gel process. Butylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI)-based ionic liquids (ILs) with various kinds of Li salts, namely LiTFSI, LiPF6, and their mixtures, are used as electrolytes for Li/LiNi0.5Mn1.5O4 cells. The IL electrolytes show high thermal stability (>400 °C) and non-flammability, and are thus ideal for high-safety applications. At 25 °C, LiTFSI is more suitable than LiPF6 as an IL electrolyte in terms of cell capacity, rate capability, and cyclic stability. The IL electrolytes clearly outperform the conventional organic electrolytes at 50 °C, since the latter decomposes at high voltage and corrodes both the Al current collector and LiNi0.5Mn1.5O4, degrading the electrode performance. At such an elevated temperature, using LiPF6 to partially substitute LiTFSI in the IL electrolyte can effectively suppress Al pitting corrosion and thus improves the cell performance. In the 0.4 M LiTFSI/0.6 M LiPF6 mixed-salt IL electrolyte, an LiNi0.5Mn1.5O4 discharge capacity of 115 mA h g−1 (at 0.1 C) is obtained at 50 °C with a high cell voltage of ∼4.7 V.

SiGe nanorings by ultrahigh vacuum chemical vapor deposition

Formation of SiGe nanorings from Si capped Si0.1Ge0.9 quantum dots (QDs) grown at 500 °C by ultrahigh vacuum chemical vapor deposition was investigated. SiGe nanorings have average diameter, width, and depth of 185, 30, and 9 nm, respectively. Based on both Raman and x-ray diffraction results, the formation of SiGe nanorings can be attributed to Ge outdiffusion from central SiGe QDs during in situ annealing. Moreover, the depth of SiGe nanorings can be controlled by Si cap thickness. The Si cap is essential for nanorings formation.