Chang-Ning Huang

Nonstoichiometric Titanium Oxides via Pulsed Laser Ablation in Water

Titanium oxide compounds TiO,Ti2O3, and TiO2 with a considerable extent of nonstoichiometry were fabricated by pulsed laser ablation in water and characterized by X-ray/electron diffraction, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. The titanium oxides were found to occur as nanoparticle aggregates with a predominant 3+ charge and amorphous microtubes when fabricated under an average power density of ca. 1 × 108W/cm2 and 1011W/cm2, respectively followed by dwelling in water. The crystalline colloidal particles have a relatively high content of Ti2+ and hence a lower minimum band gap of 3.4 eV in comparison with 5.2 eV for the amorphous state. The protonation on both crystalline and amorphous phase caused defects, mainly titanium rather than oxygen vacancies and charge and/or volume-compensating defects. The hydrophilic nature and presumably varied extent of undercoordination at the free surface of the amorphous lamellae accounts for their rolling as tubes at water/air and water/glass interfaces. The nonstoichiometric titania thus fabricated have potential optoelectronic and catalytic applications in UV–visible range and shed light on the Ti charge and phase behavior of titania-water binary in natural shock occurrence.

Correlation between In content and emission wavelength of InxGa1−xN/GaN nanowire heterostructures

GaN nanowire ensembles with axial In(x)Ga(1-x)N multi-quantum-wells (MQWs) were grown by molecular beam epitaxy. In a series of samples we varied the In content in the MQWs from almost zero to around 20%. Within the nanowire ensemble, the MQWs fluctuate strongly in composition and size. Statistical information about the composition was obtained from x-ray diffraction and Raman spectroscopy. Photoluminescence at room temperature was obtained in the range of 2.2 to 2.5 eV, depending on In content. Contrary to planar MQWs, the intensity increases with increasing In content. We compare the observed emission energies with transition energies obtained from a one-dimensional model, and conclude that several mechanisms for carrier localization affect the luminescence of these three-dimensional structures.

In situ doping of catalyst-free InAs nanowires with Si: Growth, polytypism, and local vibrational modes of Si

Growth and structural aspects of the in situ doping of InAs nanowires with Si have been investigated. The nanowires were grown catalyst-free on Si(111) substrates by molecular beam epitaxy. The supply of Si influenced the growth kinetics, affecting the nanowire dimensions, but not the degree of structural polytypism, which was always pronounced. As determined by Raman spectroscopy, Si was incorporated as substitutional impurity exclusively on In sites, which makes it a donor. Previously unknown Si-related Raman peaks at 355 and 360 cm−1 were identified, based on their symmetry properties in polarization-dependent measurements, as the two local vibrational modes of an isolated Si impurity on In site along and perpendicular, respectively, to the c-axis of the wurtzite InAs crystal.

Coalescence-induced planar defects in GaN layers grown on ordered arrays of nanorods by metal–organic vapour phase epitaxy

The planar defect structure of coalesced GaN layers fabricated on ordered arrays of nanorods and grown by metal–organic vapour phase epitaxy has been studied using conventional and high-resolution transmission electron microscopy. During the process of coalescence, a boundary was created between two pyramids, where I1-type basal plane stacking faults propagating through the overgrown layers are terminated by Frank-Shockley partial dislocations. According to multislice HRTEM simulations of experimental observed images in the [ ] zone axis, the step-and hairpin-shaped basal prismatic stacking faults with inclined { } plane are consistent with Drum’s structural model, which has a lower formation energy compared with the model proposed by Amelinckx. Based on the observation that there are no stacking faults in the overgrown layers prior to the nanopyramid merging, the mechanism of coalescence induced stacking faults is proposed. This research contributes to the understanding of planar defect formation in III-nitride semiconductor grown by a coalescence process.

Water-Driven Assembly of Laser Ablation-Induced Au Condensates as Mesomorphic Nano- and Micro-Tubes

Reddish Au condensates, predominant atom clusters and minor amount of multiply twinned particles and fcc nanoparticles with internal compressive stress, were produced by pulsed laser ablation on gold target in de-ionized water under a very high power density. Such condensates were self-assembled as lamellae and then nano- to micro-diameter tubes with multiple walls when aged at room temperature in water for up to 40 days. The nano- and micro-tubes have a lamellar- and relaxed fcc-type wall, respectively, both following partial epitaxial relationship with the co-existing multiply twinned nanoparticles. The entangled tubes, being mesomorphic with a large extent of bifurcation, flexibility, opaqueness, and surface-enhanced Raman scattering, may have potential encapsulated and catalytic/label applications in biomedical systems.

Mesomorphic Lamella Rolling of Au in Vacuum

Lamellar nanocondensates in partial epitaxy with larger-sized multiply twinned particles (MTPs) or alternatively in the form of multiple-walled tubes (MWTs) having nothing to do with MTP were produced by the very energetic pulse laser ablation of Au target in vacuum under specified power density and pulses. Transmission electron microscopic observations revealed (111)-motif diffraction and low-angle scattering. They correspond to layer interspacing (0.241–0.192 nm) and the nearest neighbor distance (ca. 0.74–0.55 nm) of atom clusters within the layer, respectively, for the lamella, which shows interspacing contraction with decreasing particle size under the influence of surface stress and rolls up upon electron irradiation. The uncapped MWT has nearly concentric amorphous layers interspaced by 0.458–0.335 nm depending on dislocation distribution and becomes spherical onions for surface-area reduction upon electron dosage. Analogous to graphene-derived tubular materials, the lamella-derived MWT of Au could have pentagon–hexagon pair at its zig-zag junction and useful optoelectronic properties worthy of exploration.

On the twinning and special grain boundaries of bimetallic particles via pulsed laser ablation of bulk AuCu in a vacuum

Bimetallic particles were formed by pulsed laser ablation of bulk AuCu in a vacuum and characterized by X-ray diffraction and transmission electron microscopy. The fcc-type AuCu3 and Cu-doped Au nanocondensates ca. 5 nm in size were coalesced over {001} and ~{111} vicinal surfaces to form quasi multiple twinned particles with solute/vacancy entrapped [110]{hkl} tilt boundaries with a symmetric {111}/{111} interface as well as asymmetric {001}/{111} and {331}/{112} interfaces. Polysynthetic {111} twinning due to strain relaxation and the shock effect was also observed for the alloyed particulates and nanocondensates.