Xiaoning Wang

Bio: Xiaoning Wang is an academic researcher from Boston University. The author has contributed to research in topics: Self-healing hydrogels & Iron oxide. The author has an hindex of 6, co-authored 15 publications receiving 118 citations.

Effects of composition and thermal annealing on the mechanical properties of silicon oxycarbide films

Abstract: There is an increasing trend to incorporate silicon carbide (SiC) into silicon oxides to improve the mechanical properties, thermal stability, and chemical resistance. In this work the silicon oxycarbide (SiOC) films were deposited by RF magnetron co-sputtering from silicon dioxide and silicon carbide targets. Subsequently rapid thermal annealing was applied to the as-deposited films to tune the mechanical properties. Energy dispersive spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy and ellipsometry were employed to characterize the compositions and microstructure of the films. The residual stress of the films was calculated from the film–substrate curvature measurement using Stoney's equation. The film stress changed from compressive to tensile after annealing, and it generally increased with carbon contents. The Young's modulus and hardness were investigated by the depth-sensing nanoindentation, which were found to increase with the carbon content and annealing temperature. A thorough microstructural analysis was conducted to investigate the effect of carbon content and annealing temperature on the mechanical properties of SiOC films.

Terahertz radiation-induced sub-cycle field electron emission across a split-gap dipole antenna

Abstract: We use intense terahertz pulses to excite the resonant mode (0.6 THz) of a micro-fabricated dipole antenna with a vacuum gap. The dipole antenna structure enhances the peak amplitude of the in-gap THz electric field by a factor of ∼170. Above an in-gap E-field threshold amplitude of ∼10 MV/cm−1, THz-induced field electron emission is observed as indicated by the field-induced electric current across the dipole antenna gap. Field emission occurs within a fraction of the driving THz period. Our analysis of the current (I) and incident electric field (E) is in agreement with a Millikan-Lauritsen analysis where log (I) exhibits a linear dependence on 1/E. Numerical estimates indicate that the electrons are accelerated to a value of approximately one tenth of the speed of light.

Towards uniformly oriented diatom frustule monolayers: Experimental and theoretical analyses.

Abstract: Diatoms are unicellular, photosynthetic algae that are ubiquitous in aquatic environments. Their unique, three-dimensional (3D) structured silica exoskeletons, also known as frustules, have drawn attention from a variety of research fields due to their extraordinary mechanical properties, enormous surface area, and unique optical properties. Despite their promising use in a range of applications, without methods to uniformly control the frustules’ alignment/orientation, their full potential in technology development cannot be realized. In this paper, we realized and subsequently modeled a simple bubbling method for achieving large-area, uniformly oriented Coscinodiscus species diatom frustules. With the aid of bubble-induced agitations, close-packed frustule monolayers were achieved on the water–air interface with up to nearly 90% of frustules achieving uniform orientation. The interactions between bubble-induced agitations were modeled and analyzed, demonstrating frustule submersion and an adjustment of the orientation during the subsequent rise towards the water’s surface to be fundamental to the experimentally observed uniformity. The method described in this study holds great potential for frustules’ engineering applications in a variety of technologies, from sensors to energy-harvesting devices. A simple technique for harnessing the remarkable properties of algal exoskeletons could lead to advances in nanotechnologies. Frustules, the silica cell walls of diatomic algae, are intricate and multilayered porous structures with extraordinary strength, large surface areas and unique optical characteristics. Controlling the alignment and orientation of the frustules is key to exploiting their attributes but has so far proved challenging, limiting their potential applications. Now, Xin Zhang at Boston University in Massachusetts, United States, and her colleagues have developed an efficient method for generating uniformly oriented frustules. The team pumped nitrogen bubbles under water, on which the dish-shaped frustules floated, forming clusters of closely packed, similarly oriented frustule monolayers on the surface. Their findings demonstrate a scalable process for producing large areas of aligned frustules that could facilitate micro/nanomanufacturing of biotemplated structures for a host of practical technological applications.

Biocompatible, micro- and nano-fabricated magnetic cylinders for potential use as contrast agents for magnetic resonance imaging ☆

Abstract: This paper reports novel MEMS and NEMS-based fabrication processes for biocompatible, hollow cylindrical ferromagnetic structures for potential use as contrast agents for magnetic resonance imaging (MRI). Compared to previous works on Ni-based cylindrical-nanoshells and Fe-based double-disk particles, biocompatibility and yield issues were strongly considered in this development of a simplified fabrication process incorporating iron oxide thin films. The novel, simplified fabrication processes developed herein yield robust, reproducible fabrication methodologies for the further development of this new class of MRI contrast agents. Specifically, both micron- and nano-scale hollow cylindrical agents were successfully fabricated, the size regimes of which enable a wide array of potential imaging applications. The use of top-down engineering approaches to MRI contrast agent design such as reported herein offers the capacity for multiplexed imaging which may dramatically potentiate the capabilities of MRI imaging.

Current investigations into magnetic nanoparticles for biomedical applications.

Abstract: It is generally recognized that nanoparticles possess unique physicochemical properties that are largely different from those of conventional materials, specifically the electromagnetic properties of magnetic nanoparticles (MNPs). These properties have attracted many researchers to launch investigations into their potential biomedical applications, which have been reviewed in this article. First, common types of MNPs were briefly introduced. Then, the biomedical applications of MNPs were reviewed in seven parts: magnetic resonance imaging (MRI), cancer therapy, the delivery of drugs and genes, bone and dental repair, tissue engineering, biosensors, and in other aspects, which indicated that MNPs possess great potentials for many kinds of biomedical applications due to their unique properties. Although lots of achievements have been obtained, there is still a lot of work to do. New synthesis techniques and methods are still needed to develop the MNPs with satisfactory biocompatibility. More effective methods need to be exploited to prepare MNPs-based composites with fine microstructures and high biomedical performances. Other promising research points include the development of more appropriate techniques of experiments both in vitro and in vivo to detect and analyze the biocompatibility and cytotoxicity of MNPs and understand the possible influencing mechanism of the two properties. More comprehensive investigations into the diagnostic and therapeutic applications of composites containing MNPs with "core-shell" structure and deeper understanding and further study into the properties of MNPs to reveal their new biomedical applications, are also described in the conclusion and perspectives part.

Mechanical Response of He-Implanted Amorphous SiOC/Crystalline Fe Nanolaminates.

Abstract: This study investigates the microstructural evolution and mechanical response of sputter-deposited amorphous silicon oxycarbide (SiOC)/crystalline Fe nanolaminates, a single layer SiOC film, and a single layer Fe film subjected to ion implantation at room temperature to obtain a maximum He concentration of 5 at. %. X-ray diffraction and transmission electron microscopy indicated no evidence of implantation-induced phase transformation or layer breakdown in the nanolaminates. Implantation resulted in the formation of He bubbles and an increase in the average size of the Fe grains in the individual Fe layers of the nanolaminates and the single layer Fe film, but the bubble density and grain size were found to be smaller in the former. By reducing the thicknesses of individual layers in the nanolaminates, bubble density and grain size were further decreased. No He bubbles were observed in the SiOC layers of the nanolaminates and the single layer SiOC film. Nanoindentation and scanning probe microscopy revealed an increase in the hardness of both single layer SiOC and Fe films after implantation. For the nanolaminates, changes in hardness were found to depend on the thicknesses of the individual layers, where reducing the layer thickness to 14 nm resulted in mitigation of implantation-induced hardening.

Nonlinear Terahertz Metamaterials via Field-Enhanced Carrier Dynamics in GaAs

Abstract: We demonstrate nonlinear metamaterial split ring resonators (SRRs) on GaAs at terahertz frequencies. For SRRs on doped GaAs films, incident terahertz radiation with peak fields of ~20-160 kV/cm drives intervalley scattering. This reduces the carrier mobility and enhances the SRR LC response due to a conductivity decrease in the doped thin film. Above ~160 kV/cm, electric field enhancement within the SRR gaps leads to efficient impact ionization, increasing the carrier density and the conductivity which, in turn, suppresses the SRR resonance. We demonstrate an increase of up to 10 orders of magnitude in the carrier density in the SRR gaps on semi-insulating GaAs. Furthermore, we show that the effective permittivity can be swept from negative to positive values with an increasing terahertz field strength in the impact ionization regime, enabling new possibilities for nonlinear metamaterials.