Yuyuan Zheng

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.

TiCx–Ti2C nanocrystals and epitaxial graphene-based lamellae by pulsed laser ablation of bulk TiC in vacuum

Nonstoichiometric δ-TiCx, δ′/δ′′-Ti2C and 2-D turbostratic graphene-based lamellae were co-synthesized by pulsed laser ablation of bulk δ-TiC in a vacuum and characterized using X-ray diffraction, electron microscopy and optical spectroscopy. The predominant δ-TiCx occurred as submicron-sized particulates and nanocondensates with size dependent shapes (5 nm sphere vs. 10 and 15 nm cubo-octahedra), which tended to coalesce over well-developed (111) as a unit or impinged by the ~(001) vicinal surface to form a [10] tilt boundary. The minor Ti2C nanocondensates occurred as cubo-octahedra for the cubic δ′-phase and triangular plates with well-developed (0001), {102} and {10} faces for the trigonal δ′′-phase, which tended to coalesce over (0001) as a unit. The turbostratic graphene-based lamellae occurred as nanoribbons or an artificial epitaxy shell on δ-TiCx and δ′/δ′′-Ti2C to facilitate carbon vacancy diffusion. The present TiCx–Ti2C-graphene phase assembly with characteristic Raman modes sheds light on their natural dynamic occurrence in presolar settings. The bimodal UV-visible absorbances of this phase assembly in the form of nanoparticles may have potential photocatalytic and abrasive/lubricant applications.

Pulsed laser synthesis of diamond-type nanoparticles with enhanced Si–C solid solubility and special defects

Diamond-type Si–C particles with enhanced solid solubility were synthesized by pulsed laser ablation of 6H-SiC (102) wafer in tetraethyl orthosilicate and characterized using X-ray/electron diffraction and optical spectroscopy. The C-overdoped Si, Si1+xC and Si-overdoped C nanoparticles underwent (hkl)-specific coalescence to form special grain boundaries, i.e. {111} twin boundary, (110) 70.5° twist boundary and [11](123)/(011) tilt boundary, and polycrystals up to several microns in size. Paracrystalline distribution of defect clusters and shock-induced 1-D 4×(111) faulting were also identified in the C-overdoped Si nanocrystals. The co-existent turbostratic graphene with varied Si–O–H content not only formed ribbons surrounding the diamond-type nanoparticles but also spherulites with bimodal basal spacing distribution due to dynamic constitutional supercooling. The overall refractory particles with significant photoluminescence (532, 603 nm) and violet light absorbance (3.1 eV) have potential opto-catalytic/electronic/abrasive applications and shed light on their natural occurrence in dynamic settings.

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.

The basal twin of α-Cr2O3 nanocondensates: occurrence and first-principles calculations

The α-Cr2O3 nanocondensates prepared by pulsed laser ablation were identified by transmission electron microscopy to have a coherent basal twin boundary and well-developed (0001) and (10) surfaces. The cause of such a basal twin with a Burgers vector = 1/3[10] can be rationalized by the (0001)-specific rotation/coalescence of the crystalline nanocondensates to form a 2-D coincidence site lattice with an excess energy of 13 eV nm−2 according to first-principles density functional calculations. The unification/twinning bifurcation of the sesquioxide atom clusters that were previously coalesced can be explained by the critical particle size and the temperature for anchorage release at the interface.

Spontaneous bending of the coalesced α-Cr2O3 nanocondensates: implications for multilayer coating systems

Nanosized α-Cr2O3 bicrystals with a special boundary of mixed planes via (hkil)-specific coalescence were formed by pulsed laser ablation on Cr in oxygen atmosphere. Transmission electron microscopy observations indicated that the bicrystals consist of rhombic (denoted as r) and platy (denoted as p) parts with well-developed

Structure and optical property changes of Nb_{2}O_{5} particles by Q-switched laser pulses in water

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On the densification and hydration of CaCO3 particles by Q-switched laser pulses in water

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