Yujun Song

Microfluidic Synthesis of Nanomaterials

An overview of the current information and analyses on the microfluidic synthesis of different types of nanomaterial, including metallic and silica nanoparticles and quantum dots, is presented. Control of particle size, size distribution, and crystal structure of nanomaterials are examined in terms of the special features of microfluidic reactors.

Stable Amorphous Cobalt Nanoparticles Formed by an in Situ Rapidly Cooling Microfluidic Process

The controlled synthesis of nanoparticles (NPs) with stable crystal structures and stable physical and chemical properties is a key issue for commercial applications. The use of a microfluidic reactor (MR) process has proven to be a flexible approach to control the fine crystal structures and the magnetic properties during the ripening and aging of the NPs. We have developed an in situ rapidly cooling microfluidic process (IRCMP) in which Co NPs with stable crystal structures and magnetic properties are synthesized by using elevated reaction temperatures followed by rapid quenching of the colloids to reduced temperatures. The Co NPs that are obtained by this process demonstrate stable crystal structures and stable magnetic properties for a much longer period of time (at least 3 months) than for Co NPs obtained by performing the reaction and the quenching processes at room temperature or under sonication.

Nearly Monodispersion CoSm Alloy Nanoparticles Formed by an In-situ Rapid Cooling and Passivating Microfluidic Process.

An in siturapid cooling and passivating microfluidic process has been developed for the synthesis of nearly monodispersed cobalt samarium nanoparticles (NPs) with tunable crystal structures and surface properties. This process involves promoting the nucleation and growth of NPs at an elevated temperature and rapidly quenching the NP colloids in a solution containing a passivating reagent at a reduced temperature. We have shown that Cobalt samarium NPs having amorphous crystal structures and a thin passivating layer can be synthesized with uniform nonspherical shapes and size of about 4.8 nm. The amorphous CoSm NPs in our study have blocking temperature near 40 K and average coercivity of 225 Oe at 10 K. The NPs also exhibit high anisotropic magnetic properties with a wasp-waist hysteresis loop and a bias shift of coercivity due to the shape anisotropy and the exchange coupling between the core and the thin oxidized surface layer.

A General Strategy for Nanohybrids Synthesis via Coupled Competitive Reactions Controlled in a Hybrid Process

A new methodology based on core alloying and shell gradient-doping are developed for the synthesis of nanohybrids, realized by coupled competitive reactions, or sequenced reducing-nucleation and co-precipitation reaction of mixed metal salts in a microfluidic and batch-cooling process. The latent time of nucleation and the growth of nanohybrids can be well controlled due to the formation of controllable intermediates in the coupled competitive reactions. Thus, spatiotemporal-resolved synthesis can be realized by the hybrid process, which enables us to investigate nanohybrid formation at each stage through their solution color changes and TEM images. By adjusting the bi-channel solvents and kinetic parameters of each stage, the primary components of alloyed cores and the second components of transition metal doping ZnO or Al2O3 as surface coatings can be successively formed. The core alloying and shell gradient-doping strategy can efficiently eliminate the crystal lattice mismatch in different components. Consequently, varieties of gradient core-shell nanohybrids can be synthesized using CoM, FeM, AuM, AgM (M = Zn or Al) alloys as cores and transition metal gradient-doping ZnO or Al2O3 as shells, endowing these nanohybrids with unique magnetic and optical properties (e.g., high temperature ferromagnetic property and enhanced blue emission).

Shell-driven Fine Structure Transition of Core Materials in Co@Au Core-shell Nanoparticles

Co@Au core shell nanoparticles (NPs) of different shell thicknesses were fabricated by a combination of the displacement process and the reduction-deposition process in a microfluidic reactor. The effect of the shell thickness on the fine structures (local atom arrangement) of core materials was investigated by X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). The results indicate that the shell thickness affects the fine structure of the core materials by causing atomic re-arrangement between the hexagonal close pack (hcp) and the face centered cubic (fcc) structure, and forming Co-Au bonds in the core-shell interface.

Fabrication of multi-level 3-dimension microstructures by phase inversion process

One process based on phase inversion of fillers in microstructures for the fabrication of multi-level three-dimensional (3-D) microstructures is described using SU-8, a kind of epoxy photoresist, as the model constructing materials. This process is depicted by use of the routine photolithography technique to construct the top layer of 3-D microstructures on the bottom layer of 3-D microstructures layer by layer. This process makes it possible to fabricate multi-level 3-D microstructures with connectors at desired locations, and to seal long span microstructures (e.g. very shallow channels with depth less than 50 μm and width more than 300 μm) without blockage. In addition, this process can provide a sealing layer by the solidification of a liquid polymer layer, which can be as strong as the bulk constructing materials for microstructures due to a complete contact and cross-linking between the sealing layer and the patterned layers. The hydrodynamic testing indicates that this kind of sealing and interconnection can endure a static pressure of more than 10 MPa overnight and a hydrodynamic pressure drop of about 5.3 MPa for more than 8 hours by pumping the tetrahydrofuran solution through a 60 μm wide micro-channels.

Effect of Oxidation, Etching, and Thin-Film Deposition on Silicon Photonic Crystals

The effects of oxidation, partial oxide etching, and thin-film deposition on the photonic bandgap of two-dimensional triangular photonic crystals are studied. Two structures, air cylinders in silicon background and silicon cylinders in air background, are analyzed by the plane wave expansion method. Fine tuning of the bandgap is demonstrated by varying the thickness of the interfacial layers of SiO 2 grown by thermal oxidation and Al 2 O 3 produced by atomic layer deposition (ALD). In the air cylinder structure the absolute bandgap can be tuned from 0 to 0.0365 a/λ during oxidation by varying the oxide thickness from 0 to 0.2125a, where a is the lattice constant. During etching it can be tuned from 0 to 0.0759 a/λ. In a silicon cylinder structure the transverse magnetic bandgap can be tuned from 0 to 0.15 a/λ during oxidation and from 0 to 0.16 a/λ during etching. Tuning of the defect frequency of a single defect formed by selective oxidation is demonstrated, and the dependence of the defect frequency on the thickness of the oxide is investigated. Results indicate that the effect of complementary metal oxide semiconductor compatible processes such as thermal oxidation and ALD on the photonic bandgap is significant.

FUNCTIONAL MICRO DEVICES USING 'NANOPARTICLE-PHOTORESIST' COMPOSITES

We have developed two fabrication routes for functional micro devices using SU-8/nanoparticle composites. In the first route, patterning of a 0.5% nanoparticle/SU-8 composite resulted in magnetic and luminescent polymeric microstructures. In the second route, a 42% nanoparticle/SU-8 composite was patterned followed by sintering to obtain free standing ceramic microstructures with aspect ratios as high as 16.

Waste-Reducing Catalytic Oxidation of m-Xylene to m-Toluic Acid

Catalytic performances of soluble Co, Co/Mn, Co/Mn/Br, and Co/Mn/Ce catalysts for m-xylene (MX) oxidation without acetic acid (HAc) solvent were studied, comparing with Co/HAc catalysts. In m-toluic acid (MTA) production, a Ce-salt promoter could effectively substitute for bromide salts and the recycle alcohol/aldehyde liquid mixture acts as a more efficient promotor than either peroxides or MTA itself. The Co/Mn/Ce system, free of acetic acid and Br−, was a superior catalyst in terms of activity and selectivity to MTA. Just as for p-xylene oxidation using Co/Mn/Br catalysts, both the MX reaction rate and the MTA selectivity increased with temperature when using Co/Mn/Ce catalysts. Unlike p-xylene oxidation however, MX oxidation resulted in more coupling products. For MX oxidation catalyzed by a supported Co-imide complex in supercritical CO2, the activity was lower by more than an order of magnitude, but the MTA selectivity was greater.Graphical AbstractThe selectivity behavior of the catalyst systems indicate that the Co/Mn/Ce system, free of acetic acid solvent and corrosive Br−, was a superior catalyst, both in terms of overall activity and selectivity for MTA.

Strong nonlinear current–voltage behaviour in iron oxyborate

Strong nonlinear resistance has been found in the charge ordered ferroelectric iron oxyborate (Fe2OBO3) with a high dielectric constant and giant converse magnetoelectric effect. In low temperature range the I-V nonlinearity increases quickly with decreasing temperature. Transport measurements on polycrystalline and single crystal Fe2OBO3 indicate that the nonlinearity is not induced by grain boundaries. The nonlinear I-V behavior is intrinsically correlated with the charge order phase melting in Fe2OBO3 by detailed in-situ TEM investigations. These results provide an insight into structure-activity relationship of resistance switching effects at atomic and electric scales, which is essential for its potential application as varistors and storage media.