Shuei-Yuan Chen

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.

Defect generation of anatase nanocondensates via coalescence and transformation from dense fluorite-type TiO2

The anatase nanocondensates produced by energetic pulse laser ablation on Ti target under oxygen background gas have been observed by transmission electron microscopy. They were found to coalesce in approximately parallel orientation on the (001) plane to form dislocations and in twin orientation on the (112) plane to form faulty and twinned crystals. The anatase phase (denoted as a) was also derived from the dense fluorite-type (denoted as f) condensates by a martensitic-type transformation to form twin variants following the crystallographic relationship ⟨001⟩f‖[100]a with a habit plane close to {100}f‖(001)a for a beneficial lower interfacial strain energy. The f‐a transformation route of TiO2 can be rationalized by a metastable phase change in terms of the intersection of the internal energy versus cell volume curves of the polymorphs.

Laser ablation condensation of polymorphic ZrO2 nanoparticles: Effects of laser parameters, residual stress, and kinetic phase change

Nd-YAG laser ablation on a Zr target under optimum oxygen flow rate and a very high-temperature route was used to fabricate fluorite-type related ZrO2 nanoparticles, which are of monoclinic (m) and tetragonal (t) symmetries when the laser power density was below ca. 1×107W∕cm2, but t plus cubic (c) symmetry when above this threshold. Electron diffraction indicated that the dense c and t phases with specific size and residual stress were allowed to relax and/or kinetically phase change into lower-energy state as constrained by the intersections of the internal energy versus the cell volume plot of the two polymorphs. The m-ZrO2 can be rationalized as a derivative of the coarsened/coalesced nanoparticles with defects vulnerable to martensitic t−m transformation.

Condensation and relaxation/transformation of dense t-ZrO2 nanoparticles

Dense tetragonal (t)-ZrO(2) nanocondensates were synthesized under very rapid heating and cooling by pulsed Nd-YAG (YAG--yttrium aluminum garnet) laser ablation with oxygen background gas and characterized by electron diffraction. The t-ZrO(2) nanoparticles with a residual stress up to about 5 GPa tended to form deformation twins/faults upon local electron dosage. By contrast, the t-ZrO(2) nanoparticles formed at an-order-of-magnitude higher power have a residual stress above 6 GPa and tended to transform into a metastable cubic (c) phase vulnerable to amorphization. The relaxation/transformation of the self-constrained t-ZrO(2) nanoparticles can be rationalized by a lowering of internal energy under the influence of resolved shear stress and local electron heating.

Tapered ZnO Whiskers: {hkil}-Specific Mosaic Twinning VLS Growth from a Partially Molten Bottom Source

Zn particulates overlaid with wurtzite (W)-type ZnO condensates having nearly orthogonal and facets were found to self-catalyze unusual tapered W-ZnO whiskers upon isothermal atmospheric annealing, i.e., thermal oxidation, at 600 °C. Analytical electron microscopic observations indicated that such whiskers formed tapered slabs having mosaic and twinned domains. The tapered whiskers can be rationalized by an alternative vapor–liquid–solid growth, i.e., {hkil}-specific coalescence twinning growth from the ZnO condensates taking advantage of a partially molten bottom source of Zn and the adsorption of atoms at the whisker tips and ledges under the influence of capillarity effect. The tapered whiskers having strong photoluminescence at 391 nm and with a considerable flexibility could have potential applications.

Special grain boundaries of anatase nanocondensates by oriented attachment

Anatase nanocondensates with a 1-D commensurate superstructure as prepared by the pulsed laser ablation of Ti in tetraethyl orthosilicate were identified by transmission electron microscopy to have special grain boundaries, i.e. symmetrical [100] tilt boundaries with (001) or (01) interfaces and an asymmetrical [10] tilt boundary with a (001)/(11) heterointerface. The [100] tilt boundaries are in fact due to basal twinning with a coherent (001) and a semi-coherent (01) interface, respectively, both following the Burgers vector = 1/2[00] + 1/4[00]. The (001)/(11) heterointerface is decorated with {101} ledges, both having a primitive coincidence site lattice for fair lattice coherency. These special grain boundaries can be rationalized by the (hkl)-specific rotation/coalescence of the crystalline nanocondensates.

Pulsed laser synthesis of carbon-overdoped tungsten with a body-centered orthorhombic structure and planar defects

C-overdoped W crystallites with a new body-centered orthorhombic (BCO) structure were synthesized by inward diffusion of C from a substrate upon pulsed laser heating in vacuum. Analytical electron microscopic observations indicated that the BCO-type W has occasional long-range ordering of atoms for the particulates, whereas {110}-shuffling for the nanocondensates tended to form a (111) 30° twist boundary and a (01)/(101) interface with a fair coincidence site lattice upon (hkl)-specific coalescence. The co-existent bcc-W nanocondensates have well-developed ~{110} and ~{112} vicinal surfaces to adjoin as tilt boundaries. The refractory phase assemblages have bimodal UV-visible absorptions for potential machining and optoelectronic/catalytic applications in a harsh environment.

C–H doped anatase nanospheres with disordered shell and planar defects synthesized by pulsed laser ablation of bulk Ti in tetraethyl orthosilicate

C–H doped anatase nanospheres with disorder-engineered surface layers were synthesized by pulsed laser ablation of Ti in tetraethyl orthosilicate and characterized using X-ray/electron diffraction and optical spectroscopy. The anatase (Ant) has (hkl)-specific twinning/faulting/ordering and follows a specific crystallographic relationship with the TiCxOy and β-Ti nuclei both having paracrystalline distribution of defect clusters due to solute trapping. The colloidal suspension containing composite nanocondensates with an amorphous C1−xSix:H shell showed visible absorbance indicating a minimum band gap of ca. 2 eV for potential photocatalytic applications.

Pulsed laser ablation synthesis of magnesiowüstite based phases with special defect clusters, interfaces and internal stress: implications for natural occurrence and engineering applications

Nanocondensates of rocksalt-type (R) magnesiowustite and minor spinel-type (S) magnesioferrite with paracrystalline distribution of defect clusters were fabricated by pulsed laser ablation of MgO–Fe2O3 solid solution (9 : 1 molar ratio) under specified power densities in air and characterized by X-ray/electron diffraction and optical absorption spectroscopy. The predominant magnesiowustite nanocondensates have well-developed {100}, {110}, {111} facets for (hkl)-specific coalescence as unity and bicrystal with exact/almost {110} 70.5° twist boundary, {111} twin boundary, (1)/(001) heterointerface, and (100)R/(310)S interphase interface constrained by [01]R//[001]S. The magnesiowustite nanocondensates have a minimum band gap of ca. 3 eV for potential opto-electronic catalytic applications and a significant internal compressive stress of ca. 2.5 GPa when fabricated at a relatively high power density, shedding light on its natural high-pressure occurrence.