H. M. Fan

Reduced Graphene Oxide Conjugated Cu2O Nanowire Mesocrystals for High-Performance NO2 Gas Sensor

Reduced graphene oxide (rGO)-conjugated Cu(2)O nanowire mesocrystals were formed by nonclassical crystallization in the presence of GO and o-anisidine under hydrothermal conditions. The resultant mesocrystals are comprised of highly anisotropic nanowires as building blocks and possess a distinct octahedral morphology with eight {111} equivalent crystal faces. The mechanisms underlying the sequential formation of the mesocrystals are as follows: first, GO-promoted agglomeration of amorphous spherical Cu(2)O nanoparticles at the initial stage, leading to the transition of growth mechanism from conventional ion-by-ion growth to particle-mediated crystallization; second, the evolution of the amorphous microspheres into hierarchical structure, and finally to nanowire mesocrystals through mesoscale transformation, where Ostwald ripening is responsible for the growth of the nanowire building blocks; third, large-scale self-organization of the mesocrystals and the reduction of GO (at high GO concentration) occur simultaneously, resulting in an integrated hybrid architecture where porous three-dimensional (3D) framework structures interspersed among two-dimensional (2D) rGO sheets. Interestingly, super-mesocrystals formed by 3D oriented attachment of mesocrystals are also formed judging from the voided Sierpinski polyhedrons observed. Furthermore, the interior nanowire architecture of these mesocrystals can be kinetically controlled by careful variation of growth conditions. Owing to high specific surface area and improved conductivity, the rGO-Cu(2)O mesocrystals achieved a higher sensitivity toward NO(2) at room temperature, surpassing the performance of standalone systems of Cu(2)O nanowires networks and rGO sheets. The unique characteristics of rGO-Cu(2)O mesocrystal point to its promising applications in ultrasensitive environmental sensors.

SINGLE-CRYSTALLINE MFE2O4 NANOTUBES/NANORINGSSYNTHESIZED BY THERMAL TRANSFORMATION PROCESS FOR BIOLOGICAL APPLICATIONS

We report a general thermal transformation approach to synthesize single-crystalline magnetic transition metal oxides nanotubes/nanorings including magnetite Fe(3)O(4), maghematite gamma-Fe(2)O(3), and ferrites MFe(2)O(4) (M = Co, Mn, Ni, Cu) using hematite alpha-Fe(2)O(3) nanotubes/nanorings template. While the straightforward reduction or reduction-oxides process was employed to produce Fe(3)O(4) and gamma-Fe(2)O(3), the alpha-Fe(2)O(3)/M(OH)(2) core/shell nanostructure was used as precursor to prepare MFe(2)O(4) nanotubes via MFe(2)O(4-x) (0 < x < 1) intermediate. The transformed ferrites nanocrystals retain the hollow structure and single-crystalline nature of the original templates. However, the crystallographic orientation-relationships of cubic spinel ferrites and trigonal hematite show strong correlation with their morpologies. The hollow-structured MFe(2)O(4) nanocrystals with tunable size, shape, and composition have exhibited unique magnetic properties. Moreover, they have been demonstrated as a highly effective peroxidase mimic catalysts for laboratory immunoassays or as a universal nanocapsules hybridized with luminescent QDs for magnetic separation and optical probe of lung cancer cells, suggesting that these biocompatible magnetic nanotubes/nanorings have great potential in biomedicine and biomagnetic applications.

Controlled synthesis of monodispersed CuO nanocrystals

Controlling the size, shape and structure of nanocrystals is technologically important because of the strong effect of size and shape on optical, electrical, and catalytic properties. Monodispersed nanocrystals of copper oxide were prepared by the precipitation?pyrolysis method. By controlling the starting materials, reactive concentration and annealing temperature, we can obtain spherical monodispersed CuO nanocrystals of different sizes or rodlike CuO nanocrystals. The particle sizes of CuO monodispersed nanocrystals can be tuned in the range between and 30?nm. The products have been characterized by x-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy and the UV?visible absorption spectrum. The absorption spectra of CuO nanocrystals show clear evidence of the quantum size effect. The possible formation mechanism of monodispersed CuO nanocrystals is discussed.

Quantum Dot Capped Magnetite Nanorings as High Performance Nanoprobe for Multiphoton Fluorescence and Magnetic Resonance Imaging

In the present study, quantum dot (QD) capped magnetite nanorings (NRs) with a high luminescence and magnetic vortex core have been successfully developed as a new class of magnetic-fluorescent nanoprobe. Through electrostatic interaction, cationic polyethylenimine (PEI) capped QD have been firmly graft into negatively charged magnetite NRs modified with citric acid on the surface. The obtained biocompatible multicolor QD capped magnetite NRs exhibit a much stronger magnetic resonance (MR) T2* effect where the r2* relaxivity and r2*/r1 ratio are 4 times and 110 times respectively larger than those of a commercial superparamagnetic iron oxide. The multiphoton fluorescence imaging and cell uptake of QD capped magnetite NRs are also demonstrated using MGH bladder cancer cells. In particular, these QD capped magnetite NRs can escape from endosomes and be released into the cytoplasm. The obtained results from these exploratory experiments suggest that the cell-penetrating QD capped magnetite NRs could be an excellent dual-modality nanoprobe for intracellular imaging and therapeutic applications. This work has shown great potential of the magnetic vortex core based multifunctional nanoparticle as a high performance nanoprobe for biomedical applications.

Raman spectroscopic investigation of carbon nanowalls.

Two-dimensional carbon nanowalls (CNWs) were prepared by microwave plasma-enhanced chemical-vapor deposition and scanning electron microscopy was used to observe their morphologies. The Raman observations of different sample orientations and polarizations show that CNWs are well crystallized. Micro-Raman scattering measurements were also carried out with different excitation laser lines (325, 488, 514, 532, and 633 nm). The D band shows a very strong shift of 46.19 cm(-1)eV with excitation laser energy and this has been explained by the double resonance effect. The decreasing intensity ratios IDIG and ID/IG with increasing laser excitation energy were detected and discussed.

An effective surface-enhanced Raman scattering template based on a Ag nanocluster–ZnO nanowire array

An effective surface-enhanced Raman scattering (SERS) template based on a 3D hybrid Ag nanocluster (NC)-decorated ZnO nanowire array was fabricated through a simple process of depositing Ag NCs on ZnO nanowire arrays. The effects of particle size and excitation energy on the Raman scattering in these hybrid systems have been investigated using rhodamine 6G as a standard analyte. The results indicate that the hybrid nanosystem with 150 nm Ag NCs produces a larger SERS enhancement factor of 3.2 x 10(8), which is much higher than that of 10 nm Ag NCs (6.0 x 10(6)) under 532 nm excitation energy. The hybrid nanowire arrays were further applied to obtain SERS spectra of the two-photon absorption (TPA) chromophore T7. Finite-difference time-domain simulations reveal the presence of an enhanced field associated with inter-wire plasmon coupling of the 150 nm Ag NCs on adjacent ZnO nanowires; such a field was absent in the case of the 10 nm Ag NC-coated ZnO nanowire. Such hybrid nanosystems could be used as SERS substrates more effectively than assembled Ag NC film due to the enhanced light-scattering local field and the inter-wire plasmon-enhanced electromagnetic field.

Thiol-Capped ZnO Nanowire/Nanotube Arrays with Tunable Magnetic Properties at Room Temperature

The present study reports room-temperature ferromagnetic behaviors in three-dimensional (3D)-aligned thiol-capped single-crystalline ZnO nanowire (NW) and nanotube (NT) arrays as well as polycrystalline ZnO NT arrays. Besides the observation of height-dependent saturation magnetization, a much higher M(s) of 166 microemu cm(-2) has been found in NTs compared to NWs (36 microemu cm(-2)) due to larger surface area in ZnO NTs, indicating morphology-dependent magnetic properties in ZnO NW/NT systems. Density functional calculations have revealed that the origin of ferromagnetism is mainly attributed to spin-polarized 3p electrons in S sites and, therefore, has a strong correlation with Zn-S bond anisotropy. The preferential magnetization direction of both single-crystalline NTs and NWs lies perpendicular to the tube/wire axis due to the aligned high anisotropy orientation of the Zn-S bonds on the lateral (100) face of ZnO NWs and NTs. Polycrystalline ZnO NTs, however, exhibit a preferential magnetization direction parallel to the tube axis which is ascribed to shape anisotropy dominating the magnetic response. Our results demonstrate the interplay of morphology, dimensions, and crystallinity on spin alignment and magnetic anisotropy in a 3D semiconductor nanosystem with interfacial magnetism.

Pattern-Dependent Tunable Adhesion of Superhydrophobic MnO2 Nanostructured Film

Tuning the adhesive force on a superhydrophobic MnO(2) nanostructured film was achieved by fabricating different patterns including meshlike, ball cactus-like, and tilted nanorod structures. The marvelous modulation range of the adhesive forces from 130 to nearly 0 μN endows these superhydrophobic surfaces with extraordinarily different dynamic properties of water droplets. This pattern-dependent adhesive property is attributed to the kinetic barrier difference resulting from the different continuity of the three-interface contact line. This finding will provide the general strategies for the adhesion adjustment on superhydrophobic surfaces.

Room temperature ferromagnetism in N-doped rutile TiO2 films

Room temperature ferromagnetism has been experimentally observed in TiO2:N films prepared by pulse laser deposition under N2O atmosphere. The ferromagnetism appears when the N2O partial pressure is higher than 10−5 Torr. XPS study has revealed that N substitutes O at the partial pressure of 10−5 Torr, whereas additional N atoms occupy interstitial sites besides substituting N at higher N2O partial pressures. Our study indicates that the origin of the ferromagnetism is the O substitution with N. Each substituted N has a magnetic moment of approximately 0.9 μB. The substitution of O also resulted in p-type behavior, accompanied with magnetoresistance and anomalous Hall effect.

Phonon-assisted stimulated emission in Mn-doped ZnO nanowires

Well-crystallized Mn-doped ZnO nanowires were synthesized by a high-temperature chemical reduction method. The microscopic and macroscopic photoluminescence (PL) spectra of these Mn-doped ZnO nanowires were studied. Both the free-exciton emission band and the second-order longitude optical (2LO) phonon-assisted emission band were observed in the microscopic PL spectra. Furthermore, from the power dependent macroscopic PL spectra, the stimulated emission was observed around the 2LO phonon-assisted emission band with an increase in pump fluence, which relates to the strong exciton–phonon couplings along the c-axis in these Mn-doped ZnO nanowires. The nonresonant micro-Raman scattering spectrum demonstrates the strong 2LO phonon mode, which is enhanced by Mn doping. Resonant micro-Raman scattering shows multi-LO modes, due to the strong exciton–phonon coupling via a deformation potential induced by the Mn ions in these Mn-doped ZnO nanowires. Moreover, the low-temperature UV PL band is discussed to demonstrate the activity of phonons in the Mn-doped ZnO nanowires.