Silicon eccentric shell nanoparticles fabricated by template-assisted deposition for Mie magnetic resonances enhanced light confinement.

We report a structure of silicon eccentric shell particles array, fabricated by the SiO2particles monolayer array assisted deposition of amorphous Si, for high-efficiency light confinement. The SiO2particles monolayer array is tailored to regulate its interparticle distance, followed by silicon film deposition to obtain silicon eccentric shell arrays with positive and negative off-center distancee. We studied the Mie resonances of silicon solid sphere, concentric shell, eccentric shell and observed that the eccentric shell with positive off-centeresupports superior light confinement because of the enhanced Mie magnetic resonances. Spectroscopic measurements and finite difference time domain simulations were conducted to examine the optical performance of the eccentric shell particles array. Results show that the Mie magnetic resonance wavelength can be easily regulated by the size of the inner void of the silicon shell to realize tunable enhanced light confinement. It was found silicon shell withD= 460/520 nm offered high enhanced light absorption efficiency at wavelength ofλ= 830 nm, almost beyond the bandgap of the amorphous silicon.

Progressive algorithm for the scattering of electromagnetic waves by a multilayered eccentric sphere.

This paper presents a general progressive algorithm for the computational study of electromagnetic wave scattering by a multilayered eccentric nanoparticle. The presented methodology is based on a combination of the vector addition theorem for spherical wave functions and an efficient progressive algorithm that matches the boundary conditions of every two adjacent shell layers from the outmost to the innermost layer. As a result, only a solution of small-sized matrices is required rather than solving a large set of system equations as reported in other works. With the developed approach, explicit expressions of the Mie scattering coefficients of the eccentric particle can be obtained. Moreover, the Mie coefficients of a specific inner layer could be calculated selectively, instead of having to compute those of all layers of the entire particle as required by other algorithms. The presented methodology can be used to study practically any type of spherical particle inclusions and the most widely studied cases such as scattering by solid particles, concentric particles, and inclusions with centers displaced along a straight line are just special cases of the algorithm presented. Computed results are also presented, illustrating that the eccentric structure allows extra freedom in the design of multilayered nanoparticles for optical applications.

Bubble rising and interaction in ternary fluid flow: a phase field study.

Bubble-droplet interaction is essential in the gas-flotation technique employed in wastewater treatment. However, due to the limitations of experimental methods, the details of the fluid flow involved have not been fully understood. Therefore, a phase field model for a three-phase flow was developed to study the rise of a single bubble and bubble-droplet interactions. The fluid-fluid interfaces are tracked by the Cahn-Hilliard equation, which is coupled with the Navier-Stokes equations with an equivalent volumetric force substituted for interfacial tensions. The model was discretized using an explicit finite difference method on a half staggered grid, and the pressure velocity coupling was tackled using the projection method. The in-house code was written in Fortran and run with the help of OpenMP, a shared memory parallelism. The model was validated against experiments with gratifying agreement achieved. Bubble-droplet interaction was simulated in two distinct situations: the first features a gas bubble crossing the interface between two other phases, and the second features a gas bubble chasing from behind an oil droplet in a surrounding fluid of the third phase.

Efficient progressive algorithm for light scattering of a multilayered concentric nanoparticle.

An efficient progressive methodology is presented for the computation of multi-scattering of electromagnetic waves by a multilayered concentric nanoparticle. Instead of solving a large set of system equations as reported in other works, the proposed approach utilizes a progressive algorithm which considers two adjacent shell layers at a time, marching progressively from the innermost to the outmost layer, and requires only multiplication of 4×4 matrices. The progressive algorithm yields the analytical expression for the scattering parameter of the concentric particle. Moreover, the progressive algorithm allows the scattering coefficients of a specific internal layer to be computed selectively, rather than having to calculate those of all layers of the entire particle as required by other algorithms. We show that the presented progressive method has equivalent accuracy to the well-known recursive algorithm, but it is more attractive due to its lower complexity in implementation. It is shown that light scattering of both a single solid sphere and two-layered concentric shell are special cases of the proposed methodology. Case study demonstrates that the presented methodology is useful in assisting the design of a multilayered core/shell structure with maximum forward scattering feature, indicating it is applicable to the exploration of optical phenomena of nanoparticles with numerous layers. Moreover, the present progressive algorithm is further extended to the electromagnetic scattering by an eccentric multilayered particle with inner cores displaced along a line defined by the centers of the spheres, which provides extra freedoms for the design of optical core shell spherical particles.

Targeted Nanoscale 3D Thermal Imaging of Tumor Cell Surface with Functionalized Quantum Dots.

Measuring the changes in tumor cell surface temperature can provide insights into cellular metabolism and pathological features, which is significant for targeted chemotherapy and hyperthermic therapy. However, conventional micro-nano scale methods are invasive and can only measure the temperature of cells across a single plane, which excludes specific organelles. In this study, fluorescence quantum dots (QDs) are functionalized with the membrane transport protein transferrin (Tf) as a thermo-sensor specific for tumor cell membrane. The covalent conjugation is optimized to maintain the relative fluorescence intensity of the Tf-QDs to >90%. In addition, the Tf-QDs undergo changes in the fluorescence spectra as a function of temperature, underscoring its thermo-sensor function. Double helix point spread function imaging optical path is designed to locate the probe at nanoscale, and 3D thermal imaging technology is proposed to measure the local temperature distribution and direction of heat flux on the tumor cell surface. This novel targeted nanoscale 3D thermometry method can be a highly promising tool for measuring the local and global temperature distribution across intracellular organelles.

Interdigital electrodes of air@NiO porous nanoshells for high performance microsupercapacitors by thermally-assisted 3D printing.

Microsupercapacitors of air@NiO porous nanoshells are manufactured by a novel thermally-assisted 3D printing process. It entails the use of printing inks of the moderate solid content of CNT-PS@Ni-precursor-nanoparticle mixture, a real-time heating substrate to print 3D interdigital electrodes, and subsequent thermal annealing to convert PS@Ni-precursor particles into air@NiO porous nanoshells. The microstructure of 3D printed electrodes is characterized by air@NiO porous nanoshells being well dispersed in the CNT network. The CNT network provides a fast electronic migration path and meanwhile ensures the mechanical integrity of electrodes to prevent the fracture and/or collapsing of electrode structures during 3D printing manufacturing and charging/discharging cycles. The air@NiO porous nanoshells, manufactured in our labs, consist of randomly oriented nanosheets and offer superb charge storage via redox reactions. The metal layer is sputtered indiscriminately on the surface of interdigital electrodes and substrate before it is peeled off with electrolyte film and electrodes. The proposed tactic resolves problems connected with the tedious courses of traditional lithography and the delamination at the interface of active materials and collectors from mechanical stress. Experiments were conducted to study the performance of the microsupercapacitors (i.e. areal capacitances, energy and power densities) as a function of printing parameters, such as electrode heights, embedded amount of air@NiO porous nanoshells and the thickness of the metal layer on the electrochemical characteristics. The thickness of as-printed electrodes reaches up to 117 µm, which is vital in ensuring high energy density and is beyond the reach of any other technology. Moreover, the 3D printedmicrosupercapacitors of air@NiO porous nanoshells show excellent cycle stability and deliver an excellent areal capacitance of 56.7 mF cm-2, about a magnitude or two higher than that of C-based counterparts.

Hybrid nanostructure of SiO2@Si with Au-nanoparticles for surface enhanced Raman spectroscopy.

In this study, a structure of large-area orderly-arranged SiO2@Si core-shell nanoparticles decorated with Au nanoparticles was fabricated for surface-enhanced Raman spectroscopy (SERS). This hybrid structure features light confinement in the Si shells and a uniform distribution of localized electric hot spots. FDTD simulations were carried out to examine the near-field enhancement response of this structure. Results indicate that the strongly enhanced local electric field is attributed to the WGM-LSPR coupling, that is, the coupling of the whispering gallery mode (WGM) of Si nanoshells with the localized surface plasmon resonance (LSPR) of Au nanoparticles. The excitation of WGM comes primarily from the magnetic response of the Si shell with a minor modification by its electric response. The WGM-LSPR coupling of the structure is tunable through the change of geometric parameters of SiO2@Si particles. Raman scattering measurements were conducted on the samples fabricated, which agree well with the simulated results. The measured data gave a SERS G factor of ∼2 × 108 and showed highly sensitive and reproducible SERS signals of R6G with a high spatial uniformity on a 2 × 2 cm2 substrate consisting of an array of SiO2@Si (D = ∼220 nm/290 nm) particles whose outer surfaces were scattered with d = ∼20 nm Au particles.

Molecular Dynamics Simulation on the Electrowetting Behaviors of the Ionic Liquid [BMIM][BF4] on a Solid Substrate.

Compared with traditional aqueous solutions, ionic liquids have important application prospects in the field of wetting and electrowetting due to the advantages of high electric conductivity, long liquid range, and low volatility. In this paper, molecular dynamics method was employed to investigate the wetting and electrowetting behaviors of the nanodroplet of ionic liquid on a solid substrate, as well as the distribution of ionic groups. The ionic liquid is 1-butyl-3-methyl tetra-fluoroborate and coarse grained to simplify the molecular simulation model. The results show that the anion and cation groups are distributed in layers above the wall, and the peaks are different corresponding to different ionic groups. Due to the attraction of the solid substrate and the electrostatic force between anions and cations, the contact angle tends to increase slightly with the increase of ionic liquid pairs. To investigate the electrowetting behaviors of ionic liquid droplet, several electric fields of different strengths and directions have been applied to the system, respectively. The results show that the static contact angles decrease obviously with the increase of electric field, and the ionic liquid droplet wets the solid surface asymmetrically under electric fields in positive and negative directions due to different diffusion abilities of cationic and anionic coarse particles. However, for a hydrophilic surface (ε = 2.0 kcal/mol), the ionic liquid droplet wets symmetrically under the electric field E = ±0.18 V/Å because of the strong interaction from the solid surface. Thus, the wetting and electrowetting behaviors are determined by the combine effect of electric field, interaction among cationic/anionic coarse particles, and solid substrates.

Multilayered Dual Functional SiO2@Au@SiO2@QD Nanoparticles for Simultaneous Intracellular Heating and Temperature Measurement.

This paper discusses synthesis and application of dual functional SiO2@Au@SiO2@QD composite nanoparticles for integrated intracellular heating with temperature motoring. The particles are of multilayered concentric structure, consisting of Au nanoshells covered with quantum dots, with the former for infrared heating through localized surface plasma resonance while the later for temperature monitoring. The key to integrate plasmonic-heating/thermal-monitoring on a single composite nanoparticle is to ensure that the quantum dots be separated at a certain distance away from the Au shell surface in order to ensure a detectable quantum yield. Direct attachment of the quantum dots onto the Au shell would render the quantum dots practically functionless for temperature monitoring. To integrate quantum dots into Au nanoshells, a quantum quenching barrier of SiO2 was created by modifying a Stöber-like process. Materials, optical and thermal characterization was made of these composite nanoparticles. Cellular uptake of the nanoparticles was discussed. Experiments were performed on simultaneous in vitro heating and temperature monitoring in a cell internalized with the dual-functional SiO2@Au@SiO2@QD composite nanoparticles.

Nanoscale 3D temperature gradient measurement based on fluorescence spectral characteristics of the CdTe quantum dot probe.

The existing quantum dot temperature measurement techniques can only measure the planar temperature in the cell but fails in 3D temperature investigation. We present a novel method of measuring the 3D temperature field on nano scale, combining fluorescence spectral characteristics of the CdTe quantum dot probe with optical spatial positioning. Based on dual-helix point spread function, a 3D temperature optical measurement system with a resolution of 0.625 °C is established, providing a new perspective of 3D temperature measurement inside the cell. We thus offer an original research tool for further revealing the evolution process of secretions in cell metabolism.

Quantum 3D thermal imaging at the micro-nanoscale.

Real-time and accurate measurement of three-dimensional (3D) temperature field gradient maps of cells and tissues would provide an effective experimental method for analyzing the coupled correlation between metabolism and heat, as well as exploring the thermodynamic properties of nanoparticles under complex environments. In this work, a new principle of quantum 3D thermal imaging is proposed. The photoluminescence principle of quantum dots is expounded and CdTe QDs are prepared by aqueous phase synthesis. Fluorescence spectral characteristics of QDs at different temperatures are studied. The optimized algorithm of the optical spot double helix point spread function is proposed to improve the imaging, where optimized light energy increased by 27.36%. The design scheme of a quantum 3D thermal imaging system is presented. The measurement range is (-8 mm, +8 mm). The temperature is calculated according to the temperature-heat curve of quantum dots. The double helix point spread function has converted the defocus distance of QDs into the rotation angle of the double optical spot, thereby determining its position. The experimental results reveal that real-time 3D tracking and temperature measurements of quantum dots at the micro-nanoscale are achieved. Overall, the proposed nano-scale 3D quantum thermal imaging system with high-resolution may provide a new research direction and exploration of many frontier fields.

Wetting Behaviors of a Nano-Droplet on a Rough Solid Substrate under Perpendicular Electric Field.

Molecular dynamic simulations were adopted to study the wetting properties of nanoscale droplets on rough silicon solid substrate subject to perpendicular electric fields. The effect of roughness factor and electric field strength on the static and dynamic wetting behaviors of a nano-droplet on a solid surface was investigated at the molecular level. Results show that the static contact angle tends to decrease slightly and show small difference with the increase of roughness factor, while it shows an obvious increase for the ramp-shaped surface because the appearing bottom space reduces the wettability of solid surface. Additionally, under the electric field, a nano-droplet was elongated in the field direction and the equilibrium contact angle increases with the increase of electric field strength. The nano-droplet was completely stretched to be column-shaped at a threshold value of the field. Besides, accompanied by the shape variation of water droplets, the molecular dipole orientations of water molecules experience a remarkable change from a random disordered distribution to an ordered profile because of the realignment of water molecules induced by electric fields.

Electro-wetting of a nanoscale water droplet on a polar solid surface in electric fields.

Molecular dynamics simulations were applied to study the wetting properties of nanoscale droplets on a polar silica solid substrate subjected to constant and alternative electric fields with various field frequencies. Results show that the external applied electric fields have significant effects on the wetting of the nanoscale droplet on a polar solid substrate. The droplet spreads asymmetrically under the effect of the external applied field, and this asymmetry culminates to the maximum when the electric field equals to 0.45 V nm-1. For an electric field of 1.0 V nm-1, the dynamic electro-wetting process undergoes two stages even with a symmetric equilibrium spreading state. The stage A-B transition happens suddenly when molecules on the leading edge drop onto the solid surface due to the strong attraction of the solid substrate. Furthermore, under the alternative electric field with a different GHz frequency range, it was observed that the spreading asymmetry was weakened by increasing the field frequency and the nanoscale water droplet shape changes very slightly above a threshold frequency. Accompanied by the shape variation of water droplets, the molecular dipole orientations of water molecules experience a remarkable change from a random disordered distribution to an ordered profile because of the realignment of water molecules induced by electric fields. In addition, the polar solid surface has significant effects on the rearrangement of water molecules compared with a single droplet. Thus, the electro-wetting behaviors of water droplets on a silica solid surface are determined by the competing intermolecular forces among water, solid and the electric field.

Parametric Study on Electric Field-Induced Micro-/Nanopatterns in Thin Polymer Films.

Electric field-induced micro-/nanopatterns in thin polymer films, sometimes referred as electrohydrodynamic patterning, is a promising technique to fabricate micro-/nanostructures. Extensive attention has been attracted because of its advantages in microcontact (easy demolding) and low cost. Although considerable work has been done on this technique, including both experimental and theoretical ones, there still appears a requirement for understanding the mechanism of electrohydrodynamic patterning. Thus, we systematically studied the effect of different parameters on electrohydrodynamic patterning with a numerical phase field model. Previous researchers usually employed lubrication approximation (i.e., long-wave approximation) to simplify the numerical model. However, this approximation would lose its validity if the structure height is on the same scale or larger than the wavelength, which occurs in most cases. Thus, we abandoned the lubrication approximation and solved the full governing equations for fluid flow and electric field. In this model, the deformation of polymer film is described by the phase field model. As to the electric field, the leaky dielectric model is adopted in which both electrical permittivity and conductivity are considered. The fluid flow together with electric field is coupled together in the framework of phase field. By this model, the effect of physical parameters, such as external voltage, template structure height, and polymer conductivity, is studied in detail. After that, the governing equations are nondimesionalized to analyze the relationship between different parameters. A dimensionless parameter, electrical Reynolds number ER, is defined, for which, a large value would simplify the electric field to perfect dielectric model and a small value leads it to steady leaky model. These findings and results may enhance our understanding of electrohydrodynamic patterning and may be a meaningful guide for experiments.

Nano-fabrication of depth-varying amorphous silicon crescent shell array for light trapping.

We report a new structure of depth controllable amorphous silicon (a-Si) crescent shells array, fabricated by the SiO2 monolayer array assisted deposition of a-Si by plasma enhanced chemical vapor deposition and nanosphere lithography, for high-efficiency light trapping applications. The depth of the crescent shell cavity was tailored by selective etching of a-Si layer of the SiO2/a-Si core/shell nanoparticle array with a varied etching time. The morphological changes of the crescent shells were examined by scanning electron microscopy and atomic force microscopy. A simple model is developed to describe the geometrical evolution of the a-Si crescent shells. Spectroscopic measurements and finite difference time domain simulations were conducted to examine the optical performance of the crescent shells. Results show that these nanostructures all have a broadband high efficiency absorption and that the light trapping capability of these crescent shell structures depends on the excitation of depths-regulated optical resonance modes. With an appropriate selection of process parameters, the structure of crescent a-Si shells may be fine-tuned to achieve an optimal light trapping capacity.

3D Printing of Carbon Nanotubes-Based Microsupercapacitors.

A novel 3D printing procedure is presented for fabricating carbon-nanotubes (CNTs)-based microsupercapacitors. The 3D printer uses a CNTs ink slurry with a moderate solid content and prints a stream of continuous droplets. Appropriate control of a heated base is applied to facilitate the solvent removal and adhesion between printed layers and to improve the structure integrity without structure delamination or distortion upon drying. The 3D-printed electrodes for microsupercapacitors are characterized by SEM, laser scanning confocal microscope, and step profiler. Effect of process parameters on 3D printing is also studied. The final solid-state microsupercapacitors are assembled with the printed multilayer CNTs structures and poly(vinyl alcohol)-H3PO4 gel as the interdigitated microelectrodes and electrolyte. The electrochemical performance of 3D printed microsupercapacitors is also tested, showing a significant areal capacitance and excellent cycle stability.

Thermal sensing with CdTe/CdS/ZnS quantum dots in human umbilical vein endothelial cells.

An experimental methodology is presented to measure the temperature variation in cells with the usage of CdTe/CdS/ZnS core/shell/shell quantum dots as nanothermometers. The photoluminescence spectral shifts from the endocytosed quantum dots were measured and analyzed to show heat generation in the human umbilical vein endothelial cell following Ca2+ stress. Cytotoxicity evaluation has demonstrated the CdTe/CdS/ZnS QDs are biocompatible to cells. The measured data show that the thermal sensibility of the core/shell/shell nanocrystals has been calibrated and has a linear correlation of 0.16 nm °C-1 along with temperature variation. The photoluminescence spectral shift of QD uptake in the cell indicates a thermogenesis of 3.125 °C.

Deformation Hysteresis of Electrohydrodynamic Patterning on a Thin Polymer Film.

Electrohydrodynamic patterning is a technique that enables micro/nanostructures via imposing an external voltage on thin polymer films. In this investigation, we studied the electrohydrodynamic patterning theoretically and experimentally, with special interest focused on the equilibrium state. It is found that the equilibrium structure height increases with the voltage. In addition, we have observed, and believe it to be the first time, a hysteresis phenomenon exists in the relationship between the voltage and structure height. With an increase in the voltage, a critical value (the first critical voltage) is noticed, above which the polymer film would increase dramatically until it comes into contact with the template. However, with a decrease in the voltage, a smaller voltage (the second critical voltage) is needed to detach the polymer from the template. The mismatch of the first and second critical voltages distorts the voltage-structure height curve into an "S" shape. Such a phenomenon is verified for three representative templates and also by experiments. Furthermore, the effects of some parameters (e.g., polymer film thickness and dielectric constant) on this hysteresis phenomenon are also discussed.

Step-Controllable Electric-Field-Assisted Nanoimprint Lithography for Uneven Large-Area Substrates.

Large-area nanostructures are widely used in various fields, but fabrication on large-area uneven substrates poses a significant challenge. This study demonstrates a step-controllable electric-field-assisted nanoimprint lithography (e-NIL) method that can achieve conformal contact with uneven substrates for high fidelity nanostructuring. Experiments are used to demonstrate the method where a substrate coated with liquid resist is brought into contact with a flexible template driven by the applied electric field. Theoretical analysis based on the elasticity theory and electro-hydrodynamic theory is carried out. Effective voltage range and the saturation voltage are also discussed. A step-controllable release of flexible template is proposed and demonstrated to ensure the continuous contact between the template and an uneven substrate. This prevents formation of air traps and allows large area conformal contact to be achieved. A combination of Vacuum-electric field assisted step-controllable e-NIL is implemented in the developed prototype. Finally, photonic crystal nanostructures are successfully fabricated on a 4 in., 158 µm bow gallium nitride light-emitting diode epitaxial wafer using the proposed method, which enhance the light extraction property.

Long-wave approximation for hybridization modeling of local surface plasmonic resonance in nanoshells.

A hybridization model for the localized surface plasmon resonance of a nanoshell is developed within the framework of long-wave approximation. Compared with the existing hybridization model derived from the hydrodynamic simulation of free electron gas, this approach is much simpler and gives identical results for a concentric nanoshell. Also, with this approach, the limitations associated with the original hybridization model are succinctly stated. Extension of this approach to hybridization modeling of more complicated structures such as multiplayered nanoshells is straightforward.