Showing papers by "Xiaohua Wu published in 2009"

Direct numerical simulation of turbulence in a nominally zero-pressure-gradient flat-plate boundary layer

Abstract: A nominally-zero-pressure-gradient incompressible boundary layer over a smooth flat plate was simulated for a continuous momentum thickness Reynolds number range of 80 ≤ Reθ ≤ 940. Transition which is completed at approximately Reθ = 750 was triggered by intermittent localized disturbances arising from patches of isotropic turbulence introduced periodically from the free stream at Reθ = 80. Streamwise pressure gradient is quantified with several measures and is demonstrated to be weak. Blasius boundary layer is maintained in the early transitional region of 80 < Reθ < 180 within which the maximum deviation of skin friction from the theoretical solution is less than 1%. Mean and second-order turbulence statistics are compared with classic experimental data, and they constitute a rare DNS dataset for the spatially developing zero-pressure-gradient turbulent flat-plate boundary layer. Our calculations indicate that in the present spatially developing low-Reynolds-number turbulent flat-plate boundary layer, total shear stress mildly overshoots the wall shear stress in the near-wall region of 2–20 wall units with vanishing normal gradient at the wall. Overshoots as high as 10% across a wider percentage of the boundary layer thickness exist in the late transitional region. The former is a residual effect of the latter. The instantaneous flow fields are vividly populated by hairpin vortices. This is the first time that direct evidence (in the form of a solution of the Navier–Stokes equations, obeying the statistical measurements, as opposed to synthetic superposition of the structures) shows such dominance of these structures. Hairpin packets arising from upstream fragmented Λ structures are found to be instrumental in the breakdown of the present boundary layer bypass transition.

Selective-area vapour-liquid-solid growth of InP nanowires.

Abstract: A comparison is made between the conventional non-selective vapour-liquid-solid growth of InP nanowires and a novel selective-area growth process where the Au-seeded InP nanowires grow exclusively in the openings of a SiO(2) mask on an InP substrate. This new process allows the precise positioning and diameter control of the nanowires required for future advanced device fabrication. The growth temperature range is found to be extended for the selective-area growth technique due to removal of the competition between material incorporation at the Au/nanowire interface and the substrate. A model describing the growth mechanism is presented which successfully accounts for the nanoparticle size-dependent and time-dependent growth rate. The dominant indium collection process is found to be the scattering of the group III source material from the SiO(2) mask and subsequent capture by the nanowire, a process that had previously been ignored for selective-area growth by chemical beam epitaxy.

Noninjection, One-Pot Synthesis of Photoluminescent Colloidal Homogeneously Alloyed CdSeS Quantum Dots

Abstract: Colloidal photoluminescent CdSeS quantum dots (QDs) were readily prepared via a noninjection, one-pot approach, with air-stable compounds cadmium acetate dihydrate (Cd(OAc)2·2H2O), elemental selenium (Se), and elemental sulfur (S) as Cd, Se, and S source compounds, respectively. The homogeneously alloyed CdSeS QDs were synthesized at 240 °C in a reaction flask containing the source compounds together with myristic acid, 2,2′-dithiobisbenzothiazole (MBTS), and 1-octadecene (ODE). All of these chemicals were loaded at room temperature with this newly developed approach. The effects of Cd/Se/S and S/MBTS feed molar ratios affecting the growth of the CdSeS QDs were investigated, via monitoring the temporal evolution of the optical properties (namely, absorption and emission) of the growing nanocrystals. The resulting ternary nanocrystals can be tuned easily to emit in the range of 470−550 nm of the electromagnetic spectrum; such an emission window is hard to manage with binary CdS and CdSe QDs alone. The band...

Non-Injection and Low-Temperature Approach to Colloidal Photoluminescent PbS Nanocrystals with Narrow Bandwidth

Abstract: Colloidal photoluminescent (PL) PbS nanocrystals have attracted a lot of attention in various applications such as bioimaging and optical telecommunications due to their tunable bandgap in the near-infrared region of the electromagnetic spectrum. Hot-injection processes seem to be the best to engineer high-quality PbS nanocrystals. However, there is a limited body of literature documented on the syntheses, with little information on synthetic parameters affecting the optical properties of the product. Moreover, small PbS nanocrystals with large bandgap greater than 1.38 eV (ca. 900 nm) and narrow bandwidth are rarely reported, due to the fact that high-temperature growth in hot-injection processes leads to large nanocrystals rapidly. This manuscript deals with our noninjection and low temperature approach to small PbS nanocrystal ensembles with bandgap in wavelength shorter than 900 nm and with narrow bandwidth; the growth temperature can be as low as room temperature. For our noninjection approach, syste...

CdS magic-sized nanocrystals exhibiting bright band gap photoemission via thermodynamically driven formation.

Abstract: CdS magic-sized nanocrystals (MSNs) exhibiting both band gap absorption and emission at 378 nm with a narrow bandwidth of approximately 9 nm and quantum yield (QY) of approximately 10% (total QY approximately 28%, in hexane) were synthesized via a one-pot noninjection approach. This CdS MSN ensemble is termed as Family 378. It has been acknowledged that magic-sized quantum dots (MSQDs) are single-sized, and only homogeneous broadening contributes to their bandwidth. The synthetic approach developed is ready and highly reproducible. The formation of the CdS MSQDs was carried out at elevated temperatures (such as 90-140 degrees C) for a few hours in a reaction flask containing bis(trimethylsilyl)sulfide ((TMS)(2)S) and Cd(OAc)(OA) in situ made from cadmium acetate dihydrate (Cd(OAc)(2).2H(2)O) and oleic acid (OA) in 1-octadecene (ODE). Low OA/Cd and high Cd/S feed molar ratios favor this formation, whose mechanism is proposed to be thermodynamically driven. (13)C solid-state cross-polarization magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) demonstrates that the capping ligands are firmly attached to the nanocrystal surface via carboxylate groups. With the cross-polarization from (1)H of the alkyl chains to surface (113)Cd, (113)Cd NMR is able to distinguish the surface Cd (471 ppm) bonding to both -COO(-) and S and the bulk Cd (792 ppm) bonding to S only. DOSY-NMR was used to determine the size of Family 378 ( approximately 1.9 nm). The present study provides strategies for the rational design of various MSNs.

Photoluminescent Colloidal CdS Nanocrystals with High Quality via Noninjection One-Pot Synthesis in 1-Octadecene

Abstract: High-quality colloidal photoluminescent CdS quantum dots (QDs) were synthesized via a noninjection one-pot approach in noncoordinating solvent 1-octadecene. This synthetic approach uses cadmium acetate dihydrate and elemental sulfur as Cd and S sources, respectively, together with one long-chain fatty acid (CH3−(CH2)n−COOH) as surface ligands and 2,2′-dithiobisbenzothiazole (MBTS) to increase sulfur activity. The CdS QDs were synthesized at elevated temperatures such as 240−300 °C, and the kinetics of nucleation/growth was monitored via the temporal evolution of the optical properties of the growing CdS QDs. Various synthetic parameters were investigated, such as the feed molar ratios of (0.5−8)Cd/1S and (2−64)S/1MBTS, reactant concentrations of 5−80 mmol/Kg, and growth temperature of 220−350 °C. The feed molar ratios of (1−2)Cd/1S and (8−32)S/1MBTS are suggested to be the optimal synthetic window, together with the S feed concentration of 10−20 mmol/Kg and the growth temperature of 240−260 °C. Moreover, ...

Single-Sized CdSe Nanocrystals with Bandgap Photoemission via a Noninjection One-Pot Approach

Abstract: The manuscript addresses the synthesis and characterization of one colloidal CdSe single-sized nanocrystal ensemble exhibiting bandgap absorption and emission peaking at 463 and 465 nm, respectively, with full width at half-maximum of ca. 8 nm. This ensemble is called Family 463; also, these single-sized quantum dots (QDs) are termed as magic-sized quantum dots (MSQDs). A magic-sized quantum dot (MSQD) ensemble is similar to a regular quantum dot (RQD) ensemble but consisting of single-sized nanocrystals; meanwhile, MSQDs do not grow in size any more with longer reaction periods or higher temperature, after their well development during synthesis. The CdSe MSQDs were synthesized via a noninjection one-pot approach with long reaction periods at elevated temperature, with cadmium acetate dihydrate (Cd(OAc)2·2H2O) and elemental selenium as Cd and Se sources, respectively, one long-chain fatty acid as ligands, and 1-octadecene (ODE) as a reaction medium. Different carboxylic acids with the carbon-chain length...

Homogeneously-Alloyed CdTeSe Single-Sized Nanocrystals with Bandgap Photoluminescence

Abstract: Homogeneously alloyed ternary CdTeSe magic-sized nanocrystals (MSNs) with bandgap emission were synthesized in 1-octadecene (ODE) via a noninjection one-pot approach featuring synthetic reproducibi...

Magic-Sized Cd3P2 II-V Nanoparticles Exhibiting Bandgap Photoemission

Abstract: The very first single-sized Cd3P2 II−V nanoparticles were synthesized via a non-injection-based approach which was designed to be thermodynamically driven. The Cd3P2 nanoparticles were synthesized in a pure form and exhibited bright bandgap photoemission peaking at 455 nm with a full width at half-maximum (fwhm) of only 17 nm and narrow bandgap absorption peaking at 451 nm. Compared to those reported before with a fwhm of 50−150 nm, the newly developed Cd3P2 nanoparticles represent significant progress in synthesis with better design and control. Cadmium acetate dihydrate (Cd(OAc)2·2H2O) and tris(trimethylsilyl)phosphine ((TMS)3P) were used as Cd and P source compounds, respectively; the synthesis was carried out in 1-octadecene (ODE), a noncoordinating solvent. The novel Cd3P2 nanoparticles were further characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), and 113Cd and 31P solid-state NMR. These single-sized Cd3P2 nanopartic...

Multimodal Plasmonic Nanosensor for the Detection of Pathogenic Bacteria

Abstract: Multi-modal sensing scheme significantly improves the detection accuracy but can also introduce extra complexity in the overall design of the sensor. We overcome this difficulty by utilizing the plasmonic properties of metallic nanoparticles. In this study, we will present a simple dual optical sensing mechanism which harvests signals of the resonantly excited metallic nanostructure in the form of surface enhanced Raman scattering (SERS) and resonant Rayleigh scattering. Silver and gold nanoparticles labeled with appropriate antibodies act as signal transduction units and upon exposure to the targeted pathogen render the targeted species optically active. We demonstrate that detection of a single pathogen cell is easily attainable with the dual detection scheme. Furthermore, we explore the markedly different SERS intensity observed from the use of two very different antibody recognition units during the pathogen labeling process.