Showing papers by "Minghui Hong published in 2010"
TL;DR: The results suggest that the doping-induced structural and size transition, demonstrated here in NaYF4 upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.
Abstract: Doping is a widely applied technological process in materials science that involves incorporating atoms or ions of appropriate elements into host lattices to yield hybrid materials with desirable properties and functions. For nanocrystalline materials, doping is of fundamental importance in stabilizing a specific crystallographic phase, modifying electronic properties, modulating magnetism as well as tuning emission properties. Here we describe a material system in which doping influences the growth process to give simultaneous control over the crystallographic phase, size and optical emission properties of the resulting nanocrystals. We show that NaYF(4) nanocrystals can be rationally tuned in size (down to ten nanometres), phase (cubic or hexagonal) and upconversion emission colour (green to blue) through use of trivalent lanthanide dopant ions introduced at precisely defined concentrations. We use first-principles calculations to confirm that the influence of lanthanide doping on crystal phase and size arises from a strong dependence on the size and dipole polarizability of the substitutional dopant ion. Our results suggest that the doping-induced structural and size transition, demonstrated here in NaYF(4) upconversion nanocrystals, could be extended to other lanthanide-doped nanocrystal systems for applications ranging from luminescent biological labels to volumetric three-dimensional displays.
2,835 citations
TL;DR: In this paper, the authors proposed an approach to reduce the feature size to the nanometer scale by using laser cooling to localize atoms, which brought laser-processing technology to a new era of atomic engineering.
Abstract: Laser precision engineering is being extensively applied in industries for device microfabrication due to its unique advantages of being a dry and noncontact process, coupled with the availability of reliable light sources and affordable system cost. To further reduce the feature size to the nanometer scale, the optical diffraction limit has to be overcome. With the combination of advanced processing tools such as SPM, NSOM, transparent and metallic particles, feature sizes as small as 20 nm have been achieved by near-field laser irradiation, which has extended the application scope of laser precision engineering significantly. Meanwhile, parallel laser processing has been actively pursued to realize large-area and high-throughput nanofabrication by the use of microlens arrays (MLA). Laser thermal lithography using a DVD optical storage process has also been developed to achieve low-cost and high-speed nanofabrication. Laser interference lithography, another large area nanofabrication technique, is also capable of fabricating sub-100 nm periodic structures. To further reduce the feature size to the atomic scale, atomic lithography using laser cooling to localize atoms is being developed, bringing laser-processing technology to a new era of atomic engineering.
169 citations
TL;DR: Wang et al. as discussed by the authors used this environmentally friendly technique to overcome the problems of conventional blasting, which always has intrinsic problems, such as noise, explosion risk, contaminant particles, vibration, and dust.
Abstract: Paint removal is an important part of steel processing for marine and offshore engineering. For centuries, a blasting techniques have been widely used for this surface preparation purpose. But conventional blasting always has intrinsic problems, such as noise, explosion risk, contaminant particles, vibration, and dust. In addition, processing wastes often cause environmental problems. In recent years, laser cleaning has attracted much research effort for its significant advantages, such as precise treatment, and high selectivity and flexibility in comparison with conventional cleaning techniques. In the present study, we use this environmentally friendly technique to overcome the problems of conventional blasting. Processed samples are examined with optical microscopes and other surface characterization tools. Experimental results show that laser cleaning can be a good alternative candidate to conventional blasting.
95 citations
TL;DR: Thermoelectric investigation of these nanocrystals shows that the Bi(3+) doping increases electrical conductivity from 350 to 650 K and changes the Seebeck coefficient sign from positive to negative.
Abstract: We report the synthesis of a series of monodispersed Bi-doped PbTe nanocrystals with tunable morphologies by using a doping precursor of bismuth(III) 2-ethylhexanoate. The as-synthesized Pb1−xBixTe (x = 0.005, 0.010, 0.015, 0.020) nanocrystals are characterized by X-ray diffraction, X-ray photoelectron spectroscopy and Hall measurements. The nanocrystals with controlled spherical, cuboctahedral, and cubic shapes were readily prepared by varying the Bi doping concentration. Thermoelectric investigation of these nanocrystals shows that the Bi3+ doping increases electrical conductivity from 350 to 650 K and changes the Seebeck coefficient sign from positive to negative.
49 citations
TL;DR: It is found that porous silicon was formed near the Au catalyst during the fabrication of the nanowires, and porous silicon exhibited enhanced oxidation ability when exposed to atmospheric conditions or in wet oxidation ambient.
Abstract: We report a simple and cost effective method for the synthesis of large-area, precisely located silicon nanocones from nanowires. The nanowires were obtained from our interference lithography and catalytic etching (IL-CE) method. We found that porous silicon was formed near the Au catalyst during the fabrication of the nanowires. The porous silicon exhibited enhanced oxidation ability when exposed to atmospheric conditions or in wet oxidation ambient. Very well located nanocones with uniform sharpness resulted when these oxidized nanowires were etched in 10% HF. Nanocones of different heights were obtained by varying the doping concentration of the silicon wafers. We believe this is a novel method of producing large-area, low cost, well defined nanocones from nanowires both in terms of the control of location and shape of the nanocones. A wide range of potential applications of the nanocone array can be found as a master copy for nanoimprinted polymer substrates for possible biomedical research; as a candidate for making sharp probes for scanning probe nanolithography; or as a building block for field emitting tips or photodetectors in electronic/optoelectronic applications.
46 citations
TL;DR: Hybrid moth-eye structures were proposed to enhance broadband antireflection properties in this paper, which achieved an ultralow average reflectance down to 0.11% over the solar spectral range.
Abstract: Hybrid moth-eye structures were proposed to enhance broadband antireflection properties. An ultralow average reflectance down to 0.11% over the solar spectral range has been achieved, which demonstrates a similar to 50% enhancement in broadband antireflection capability as compared with corresponding uniform moth-eye structures. This is attributed to a smaller refractive index gradient with respect to structure height. These hybrid structures can be applied to solar cells for higher light-to-electricity conversion efficiency. (C) 2010 The Japan Society of Applied Physics
37 citations
TL;DR: In this article, a large area of vertically aligned carbon nanofibers with different interfiber-distance-to-fiber-height ratios and fiber aspect ratios were synthesized.
29 citations
TL;DR: FDTD simulation proves that there is a strong mutual coupling between 2 SRRs besides a strong localized electric field at the split gap, which can enhance the electric field up to 364 times for tunable, broad band and high sensitivity THz sensing.
Abstract: Planar hybrid metamaterial with different split ring resonators (SRR) structure dimensions are fabricated on silicon substrates by femtosecond (fs) laser micro-lens array (MLA) lithography and lift-off process. The fabricated metamaterial structures consist of: (a) uniform metamaterial with 4 SRRs at same design and dimension as a unit cell and (b) hybrid metamaterial with 4 SRRs at same design but different dimensions as a unit cell. The electromagnetic field responses of these hybrid and single dimension metamaterial structures are characterized using a terahertz (THz) time-domain spectroscopy. Transmission spectra of these metamaterial show that a broader resonance peak is formed when 2 SRRs are close to each other. FDTD simulation proves that there is a strong mutual coupling between 2 SRRs besides a strong localized electric field at the split gap, which can enhance the electric field up to 364 times for tunable, broad band and high sensitivity THz sensing. Meanwhile, the strong coupling effect could lead to the formation of an additional resonance peak at ~0.2 THz in the THz spectra regime.
24 citations
TL;DR: In this paper, the double square asymmetric split ring resonator (ASRR) arrays are fabricated by femtosecond laser microlens array lithography and the resonance properties of this ASRR under structural symmetry breaking conditions are investigated in the terahertz region by changing gap size at one side.
Abstract: Large-area asymmetric split ring resonator (ASRR) arrays are fabricated by femtosecond laser microlens array lithography. The double square ASRR consists of two gaps at the outer edge. The resonance properties of this ASRR under structural symmetry breaking conditions are investigated in the terahertz region by changing gap size at one side. The resonance of the ASRR shows blueshift from 0.85 to 0.98 THz when the one gap size increases from 2 to 14 μm. Numerical simulations reveal that the electric field enhancement at the fixed gap increases from 152 to 206 times as the degree of asymmetry increases.
23 citations
TL;DR: In this paper, super-hydrophobic surfaces were fabricated by femtosecond laser micro-machining and chemical vapor deposition to constitute hybrid scale micro/nano-structures formed by carbon nanotube (CNT) clusters.
Abstract: In this work, super-hydrophobic surfaces were fabricated by femtosecond laser micro-machining and chemical vapor deposition to constitute hybrid scale micro/nano-structures formed by carbon nanotube (CNT) clusters. Nickel thin-film microstructures, functioning as CNT growth catalyst, precisely control the distribution of the CNT clusters. To obtain minimal heat-affected zones, femtosecond laser was used to trim the nickel thin-film coating. Plasma treatment was subsequently carried out to enhance the lotus-leaf effect. The wetting property of the CNT surface is improved from hydrophilicity to super-hydrophobicity at an advancing contact angle of 161 degrees. The dynamic water drop impacting test further confirms its enhanced water-repellent property. Meanwhile, this super-hydrophobic surface exhibits excellent transparency with quartz as the substrate. This hybrid fabrication technique can achieve super-hydrophobic surfaces over a large area, which has potential applications as self-cleaning windows for vehicles, solar cells and high-rise buildings.
22 citations
TL;DR: In this paper, a laser interference lithography (LIL) was adopted to fabricate micro/nanostructures in quartz by combining the following dry-etching process either reactive ion etching (RIE) or inductively coupled plasma (ICP).
Abstract: Laser interference lithography (LIL) has the capability to fabricate large-area microstructures on the photoresist with only a couple of minutes’ exposure and development. In this study, LIL was adopted to fabricate micro/nanostructures in quartz by combining the following dry-etching process either reactive ion etching (RIE) or inductively coupled plasma (ICP). A layer of gold film was coated on the quartz to act as a hard mask during the dry-etching process. A microhole array in quartz with a thin gold film covered on the surface was fabricated when choosing RIE. Each hole in the microhole array was surrounded with gold nanoparticle capped silica (Au/SiO2) cones when using ICP instead of RIE. This is due to the thin gold film that serves as the mask for creating the surface roughness required for creating the silica cone structure.
TL;DR: In this article, a large-area split ring resonator (SRR) array was fabricated by femtosecond laser micro-lens array lithography at a fast speed.
Abstract: Large-area split ring resonator (SRR) array was fabricated by femtosecond laser micro-lens array lithography at a fast speed. Transmission spectra of the SRR arrays at different incident terahertz wave polarization states were characterized by terahertz time domain spectroscopy. A polarization-dependent transmission property of the SRR array was observed. Polarization-dependent loss (PDL) spectrum was characterized to investigate the polarization properties of the terahertz metamaterials. The PDL characterization can eliminate the substrate effect and provide a flexible platform to study the characteristics of free-standing terahertz metamaterials.
TL;DR: This work postulates that the motion of the bottom (larger) catalyst is driven by etching of the graphitic walls and 'cups' inside the CNF, which converts the carbon nanofibers to the final nanoneedle shape.
Abstract: Carbon nanoneedles (CNNs) were grown using a plasma-enhanced chemical vapor deposition process in which the source gas (C2H2) was turned off 10 min before the NH3 flow and plasma were turned off. It is demonstrated that tubular carbon nanofibers (CNFs) grow while the source gas is on. However, once the source gas is turned off, the Ni catalyst at the top of each CNF splits to form a small catalyst that remains at the top of the tube and a larger catalyst that travels down the interior of the tube. We postulate that the motion of the bottom (larger) catalyst is driven by etching of the graphitic walls and ‘cups’ inside the CNF. This process, combined with slowing growth of the CNFs and etching of the material above the bottom catalyst, converts the carbon nanofibers to the final nanoneedle shape.
TL;DR: In this article, a wire-grid terahertz polarizer fabricated on quartz substrate working in broadband is demonstrated, showing the good performance of this polarizer in the frequency range up to 3.5 THz.
Abstract: A wire-grid terahertz polarizer fabricated on quartz substrate working in broadband is demonstrated. The transmission spectra of the fabricated wire-grid polarizer are characterized by terahertz time domain spectroscopy, showing the good performance of this polarizer in the terahertz frequency range up to 3.5 THz. The transmission of the peak in the time domain is 80% when the incident terahertz wave polarization is perpendicular to the wire-grid. The transmission of the peak is lower than 10% when the incident polarization is parallel to the wire-grid. The polarization factor ranges from 0.92–0.99 with the mean value of 0.97 in the frequency range from 0.1 to 3.5 THz, which is better than that of the free-standing wire-grid polarizer.
TL;DR: In this article, carbon nanoparticles were prepared by laser ablation of a carbon target in tetrahydrofuran (THF), and optical limiting properties of the samples were studied with 532-nm laser light, which is in the most sensitive wavelength band for human eyes.
Abstract: Nonlinear optical limiting materials have attracted much research interest in recent years. Carbon nanoparticles suspended in liquids show a strong nonlinear optical limiting function. It is important to investigate the nonlinear optical limiting process of carbon nanoparticles for further improving their nonlinear optical limiting performance. In this study, carbon nanoparticles were prepared by laser ablation of a carbon target in tetrahydrofuran (THF). Optical limiting properties of the samples were studied with 532-nm laser light, which is in the most sensitive wavelength band for human eyes. The shape of the laser pulse plays an important role for initializing the nonlinear optical limiting effect. Time-resolved analysis of laser pulses discovered 3 fluence stages of optical limiting. Theoretical simulation indicates that the optical limiting is initialized by a near-field optical enhancement effect.
TL;DR: In this article, a maskless surface nanostructuring technique enables thousands to millions of identical nano-features to be patterned in a couple of laser pulses, and the simulation results of the energy flux distribution at the focal plane of the microlens arrays are discussed.
Abstract: In the past decade, the development of nanoelectronics and nano-optics has attracted much interest in surface nanostructuring of semiconductor materials. The irradiation of a microlens array by a laser beam generates many focused light spots, which can act as a direct writing tool on photo-polymer materials. This maskless surface nanostructuring technique enables thousands to millions of identical nano-features to be patterned in a couple of laser pulses. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images show that nano-features were patterned uniformly on the substrate surface, which suggests a versatile way of parallel surface nanostructuring over a large area. The simulation results of the energy flux distribution at the focal plane of the microlens arrays will also be discussed.
01 Jan 2010
TL;DR: In this article, the results of removing millscales using a pulsed high-power fiber laser are presented, including scanning speed, overlap of scanning lines, scanning pattern, repetition rate of laser pulses, laser power and laser pulse energy.
Abstract: Surface preparation is an important step prior to welding and coating procedures for building a new ship. The conditions of surface preparation directly determines the quality of welding and painting works, which could further affect the occurrence of corrosion and fouling. Removal of millscales from raw steel plates using abrasive blasting method is often a significant process of surface preparation for shipbuilding work. It is a standard and effective procedure used by worldwide shipyards. But it always causes environmental problems of air pollution and secondary wastes. The increasingly widespread use of lasers on surface treatment provides an option for surface preparation with ecological advantages. In this report, we present the results of removing millscales using a pulsed high-power fibre laser. Laser parameters, including scanning speed, overlap of scanning lines, scanning pattern, repetition rate of laser pulses, laser power and laser pulse energy, are varied and optimized to attain the complete removal of millscales. The mechanism of millscale removal is discussed.Surface preparation is an important step prior to welding and coating procedures for building a new ship. The conditions of surface preparation directly determines the quality of welding and painting works, which could further affect the occurrence of corrosion and fouling. Removal of millscales from raw steel plates using abrasive blasting method is often a significant process of surface preparation for shipbuilding work. It is a standard and effective procedure used by worldwide shipyards. But it always causes environmental problems of air pollution and secondary wastes. The increasingly widespread use of lasers on surface treatment provides an option for surface preparation with ecological advantages. In this report, we present the results of removing millscales using a pulsed high-power fibre laser. Laser parameters, including scanning speed, overlap of scanning lines, scanning pattern, repetition rate of laser pulses, laser power and laser pulse energy, are varied and optimized to attain the complete...
01 Jan 2010
TL;DR: In this article, the authors focused on laser removal of the crude oil from steel plates in water and found that the underwater cleaning is different from that of dry laser cleaning that is based on combustion.
Abstract: Underwater constructions such as offshore platforms and marine structures often need on-site processing for building and repair works. The surface contaminants such as oil, rust, fouling material, biofilm and deteriorated paint can seriously affect the underwater work. Underwater surface preparation is always necessary to remove contaminants for achieving good processing quality. Most of the conventional cleaning techniques are limited by the critical conditions of the environment underwater. Laser cleaning method can be a suitable approach for underwater work with significant advantages such as ecological benefit, localized treatment, noncontact working mode, and the flexibility for processing complicated geometries. The advance of high-power fibre laser techniques allows cleaning work being done by lasers in critical underwater conditions. This work focuses on laser removal of the crude oil from steel plates in water. It is found that the underwater cleaning is different from that of dry laser cleaning that is based on combustion. The property of laser pulses signifies an important role for underwater laser cleaning process.Underwater constructions such as offshore platforms and marine structures often need on-site processing for building and repair works. The surface contaminants such as oil, rust, fouling material, biofilm and deteriorated paint can seriously affect the underwater work. Underwater surface preparation is always necessary to remove contaminants for achieving good processing quality. Most of the conventional cleaning techniques are limited by the critical conditions of the environment underwater. Laser cleaning method can be a suitable approach for underwater work with significant advantages such as ecological benefit, localized treatment, noncontact working mode, and the flexibility for processing complicated geometries. The advance of high-power fibre laser techniques allows cleaning work being done by lasers in critical underwater conditions. This work focuses on laser removal of the crude oil from steel plates in water. It is found that the underwater cleaning is different from that of dry laser cleaning ...
TL;DR: In this article, a phase shift mask for UV exposure was used using Reactive ion etching (RIE) and laser microlens array (MLA) lithography.
Abstract: Laser microlens array (MLA) lithography was used to fabricate arbitrary periodic array of patterns on photoresist. Reactive ion etching (RIE) was then utilized to etch and transfer the patterns down to the fused silica substrate. By controlling the etching process, the etched surface was 180° out of phase with the unetched surface. Subsequently, the patterns were used as a phase shift mask for UV exposure. Destructive interference occurred at the edges of the phase shift structures, resulting in regions of low light intensities at the edges. It formed periodic array of smaller patterns on the unexposed photoresist, with feature sizes around 290 nm. Simulation verified the experimental result.
TL;DR: In this article, a self-similar cascade of three nanocrescents with progressively decreasing sizes and separations was proposed, and the maximum E-field enhancement factor of over 105 was obtained with both polarizations of perpendicular and parallel to the axis of the chain, which is larger than the single crescent.
Abstract: A self-similar cascade of three nanocrescents with progressively decreasing sizes and separations were proposed. Electric field was obtained in this structure using 3D-FDTD simulation. The maximum E-field enhancement factor of over 105 was obtained with both polarizations of perpendicular and parallel to the axis of the chain, which is larger than the single crescent. The location of the hottest spots and the maximum E-field value depend on the polarization of incident light. These results show that this structure holds a great promise in application of surface-enhanced Raman scattering (SERS) substrate for nanooptical detection, and nanooptical antenna etc.