Metal Hemisphere Research Articles

Understanding the origin of a second mobility reversal in optoelectrically powered metallo-dielectric Janus particles.

While previous studies indicated the mobility reversal of an electrically-powered metallo-dielectric Janus particle (JP) with increasing frequency, here we report an intriguing second mobility reversal observed in optoelectronically-driven JPs. In contrast to the commonly used setup with parallel ITO-coated glass substrates to induce a uniform electric field orthogonal to the velocity direction, this setup incorporates a thin photoconductive layer deposited on the bottom ITO-coated glass substrate. We have found that the reversal is associated with the asymmetry of the bottom substrate's photoconductivity, localized underneath the JP, resulting from the self-shading effect of the metallic hemisphere under top optical illumination. Numerous control tests, including optical illumination from the bottom, along with numerical simulations, support this hypothesized mechanism.

Asymmetric optical transmission through periodic metallic hemisphere arrays on the transparent substrates

Asymmetric optical transmission is useful in various fields such as electromagnetic shielding, photovoltaic devices, and optical diodes owing to its directional selectivity for light. This work theoretically investigates asymmetric optical transmission by the periodic metallic hemisphere arrays on the transparent substrates. It is found that the formation of asymmetric optical transmission can be attributed to two aspects: the difference in the driving electric field and the first-order diffraction of the arrays. The maximum asymmetric optical transmission results from the match between the resonance wavelength of the arrays and the cut-off wavelength of the first-order diffraction inside the substrates. The bandwidth of asymmetric optical transmission is determined by the cut-off wavelength of the first-order diffraction inside the substrates. In addition, the effects of the structural parameters, including the radius of the metallic hemisphere and the material of the transparent substrate, on the maximum asymmetric optical transmission and the bandwidth are studied. This work provides design guidelines for practical devices to apply asymmetric optical transmission.

Simultaneously improving strength and abradability for abradable seal coatings by localized bonding hollow spheres

Simultaneously improving strength and abradability for abradable seal coatings by localized bonding hollow spheres

Electro-Orientation and Electro-Rotation of Metallodielectric Janus Particles.

The electro-rotation (EROT) and electro-orientation (EOR) behavior of metallodielectric spherical Janus particles (JP) are studied analytically and verified experimentally. This stands in contrast to previous either heuristic or numerically computed models of JP dipoles. First, we obtain frequency-dependent analytic expressions for the corresponding dipole terms for a JP composed of dielectric and metallic hemispheres by applying the "standard" (weak-field) electrokinetic model and using a Fourier-Legendre collocation method for solving two sets of linear equations. EROT and EOR spectra, describing the variation of the JP's angular velocity on the forcing frequency of a rotating and nonrotating spatially uniform electric field, respectively, are explicitly determined and compared against experiments (different JP sizes and solution conductivities). While a favorably good agreement between theory and experimental measurements was found for the frequency response (∼8% difference), there is still a factor of ∼2 difference in the amplitude of the angular velocity, which necessitates further future improvements to the model.

Electrophoresis of metal-dielectric Janus particles with dipolar director symmetry in nematic liquid crystals.

We study the electrophoresis of metal-dielectric Janus particles with dipolar director symmetry in two nematic liquid crystals (LCs) having the same sign of conductivity anisotropy (Δσ) but opposite signs of dielectric anisotropy (Δε). The applied ac electric field is parallel and perpendicular to the director for positive and negative dielectric anisotropy LCs, respectively. We show that the Janus dipolar particles propel faster than the non-Janus dipolar particles in both LCs. The propelling speed of the Janus dipolar particles is also significantly higher compared to the quadrupolar Janus particles studied previously. We map the electroosmotic flow fields surrounding a Janus dipolar particle using microparticle image velocimetry (μ-PIV) and show that the flow on a metal hemisphere is stronger than that on a dielectric hemisphere. Altogether, Janus dipolar particles demonstrate efficient electrophoresis compared to both Janus and non-Janus quadrupolar particles. These findings may be useful for applications in active matter, microrobotic and microfluidic devices.

Enhanced broadband extraordinary terahertz transmission through plasmon coupling between metal hemisphere and hole arrays

Enhanced extraordinary optical transmission (EOT) is generated by a gold hole array with a concentric hemisphere in the terahertz (THz) region. By introducing hemispherical particles and using the plasmon coupling effects of surface plasmon polaritons and localized surface plasmon resonances, it can be found that not only the transmission intensity is greatly enhanced to 0.97 and the bandwidth is nine times wider than that of a non-particle in a hole, but also the size of the structure can be significantly reduced. Additionally, optical characteristics of the hemisphere-in-hole structure are thoroughly analyzed by the schematic diagram, transmission spectra, and optical field distribution. In addition, with the diameter of the hemisphere increasing, the transmission peak maintains at a high value and the peak position redshifts correspondingly. Finally, we alter the shapes and sizes of the central particles to verify the influence on the enhanced THz EOT. Our results provide a reference for theoretical understanding and expand the application prospects for many THz plasmonic devices.

Metallic nanovoid and nano hemisphere structures fabricated via simple methods to control localized surface plasmon resonances in UV and near IR wavelength regions

We introduce metal nanovoid (NV) and metal on metal nano-hemisphere (MoNH) structures to tune the localized surface plasmon resonance (LSPR) for wider wavelength ranges. The NV structures were constructed with metal layers on gallium oxide nano hemisphere (NH) structures, and the MoNH structures were constructed with dielectric layers and metal layers on random silver NH structures. These NH structures were fabricated via thermal annealing of the deposited metal thin films without top-down nanofabrication technologies. We verified both experimentally and numerically that NV and MoNH structures enable flexible tuning of the LSPR in the UV and NIR wavelength regions, respectively.

Localized surface plasmon resonance in deep ultraviolet region below 200\xa0nm using a nanohemisphere on mirror structure

Localized surface plasmon resonance (LSPR) was performed in the deep ultraviolet (UVC) region with Al nanohemisphere structures fabricated by means of a simple method using a combination of vapor deposition, sputtering, and thermal annealing without top-down nanofabrication technology such as electron beam lithography. The LSPR in the UV region was obtained and tuned by the initial metal film thickness, annealing temperature, and dielectric spacer layer thickness. Moreover, we achieved a flexible tuning of the LSPR in a much deeper UVC region below 200 nm using a nanohemisphere on a mirror (NHoM) structure. NHoM is a structure in which a metal nanohemisphere is formed on a metal substrate that is interposed with an Al2O3 thin film layer. In the experimental validation, Al and Ga were used for the metal hemispheres. The LSPR spectrum of the NHoM structures was split into two peaks, and the peak intensities were enhanced and sharpened. The shorter branch of the LSPR peak appeared in the UVC region below 200 nm. Both the peak intensities and linewidth were flexibly tuned by the spacer thickness. This structure can contribute to new developments in the field of deep UV plasmonics.

Flexibly tunable surface plasmon resonance by strong mode coupling using a random metal nanohemisphere on mirror

Abstract We propose a unique random metal nanohemisphere on mirror (NHoM) structure to tune the surface plasmon (SP) resonance in a flexible manner. The SP resonance peak was split into two peaks owing to the strong coupling between the SP mode in the metal nanohemisphere and the mirror image mode generated in the metal substrate. This phenomenon is based on the fact that the strong coupling and the induced electromagnetic effects are similar to those pertaining to the Rabi splitting, Fano resonance, and electromagnetically induced transparency, thus providing quantum effect analogies. These phenomena have recently attracted increased attention and have been studied with nanocavities fabricated with top-down nanotechnologies. Compared with previous reports, NHoM structures can be fabricated in a much easier manner and are tunable in rather wider wavelength regions without nanofabrication technologies. The SP resonance peaks were enhanced, sharpened dramatically, and tuned flexibly, based on the optimization of the thickness of the spacer layer between the metal hemisphere and metal substrate. Experimental results were reproduced and were explained based on finite difference time domain (FDTD) simulations. These phenomena have never been observed previously on similar nanosphere on mirror (NSoM) because nanohemispherical structures were required. The NHoM nanocavity structure has a quality factor >200 that is surprisingly high for the localized SP mode of nanoparticles. Flexible tuning of the SP resonance with the use of NHoM is envisaged to lead to the development of new applications and technologies in the field of plasmonics and nanophotonics.

Theoretical investigation of broadband absorption enhancement in a-Si thin-film solar cell with nanoparticles

Abstract Thin-film solar cells have attracted increasing attention due to its low material cost and large flexibility, but they also face the challenge of low solar absorption due to reduced active layer thickness. Through exciting surface plasmon resonance, plasmonic metal nanoparticles are usually placed on the cell front surface to enhance solar absorption. However, if eliminating the unuseful intrinsic absorption in nanoparticles, we find that dielectric ones are better choices to enhance a-Si thin-film solar cell absorption efficiency. Moreover, a composite light trapping structure with dielectric nanoparticles on the front surface and metal hemispheres on the rear surface is proposed to achieve broadband absorption enhancement in both short and long wavelengths, with the aim to get a higher conversion efficiency. The finite-difference-time-domain simulation results show that, compared with bare 100-nm-thick amorphous silicon solar cell, the short-circuit current density and photoelectric conversion efficiency could be respectively improved by 21% and 18% with addition of optimized composite light trapping structure. The general method proposed in this study could provide valuable guidance to light trapping structure design for various kinds of thin-film solar cells.

Asymmetric PdPtCu mesoporous hemispheres on nitrogen-functionalized graphene for methanol oxidation electrocatalysis

This work reports an easy solution-phase synthesis of asymmetric metallic mesoporous hemispheres on nitrogen-functionalized graphene for fuel cell electrooxidation.

Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride

Single photon emitters in two-dimensional materials are promising candidates for future generation of quantum photonic technologies. In this work, we experimentally determine the quantum efficiency (QE) of single photon emitters (SPE) in few-layer hexagonal boron nitride (hBN). We employ a metal hemisphere that is attached to the tip of an atomic force microscope to directly measure the lifetime variation of the SPEs as the tip approaches the hBN. This technique enables non-destructive, yet direct and absolute measurement of the QE of SPEs. We find that the emitters exhibit very high QEs approaching $(87 \pm 7)\,\%$ at wavelengths of $\approx\,580\,\mathrm{nm}$, which is amongst the highest QEs recorded for a solid state single photon emitter.

Adhesion strength of a living cell to various substrates measured using a cup-attached atomic force microscopy chip

Cell adhesion strengths to various substrates were quantitatively measured using atomic force microscopy (AFM). A cup-shaped metal hemisphere was attached to the apex of the AFM cantilever, the “cup-chip” approached a cell (FP10SC2) to pick it up, the captured cell approached any one of six different substrates [gold (Au), nickel (Ni), bovine serum albumin (BSA), an amino group (NH2), poly(tetrafluoroethylene) (PTFE), and structured PTFE (sPTFE)], and the cell adhesion strength at the initial contact period was evaluated by detaching the cell from the substrate. The results obtained showed that the force needed to detach the cell from the NH2 substrate was more than 3-fold larger than that of metal substrates (Au and Ni), more than 15-fold larger than that of biochemically treated substrates (BSA), and more than 20-fold larger than that of hydrophobic substrates (PTFE and sPTFE). Using differences in adhesion strengths, a cell on a sPTFE substrate was picked up using a BSA-coated cup-chip, placed on a NH2 substrate, repeating this cell manipulation five times, and line patterning of cells was achieved. These results indicate that measurements of cell adhesion strength are fundamental to fabricate desired cell networks and the cup-chip is a useful tool for achieving easy cell manipulation.

Quantitative measurements of intercellular adhesion between a macrophage and cancer cells using a cup-attached AFM chip

Intercellular adhesion between a macrophage and cancer cells was quantitatively measured using atomic force microscopy (AFM). Cup-shaped metal hemispheres were fabricated using polystyrene particles as a template, and a cup was attached to the apex of the AFM cantilever. The cup-attached AFM chip (cup-chip) approached a murine macrophage cell (J774.2), the cell was captured on the inner concave of the cup, and picked up by withdrawing the cup-chip from the substrate. The cell-attached chip was advanced towards a murine breast cancer cell (FP10SC2), and intercellular adhesion between the two cells was quantitatively measured. To compare cell adhesion strength, the work required to separate two adhered cells (separation work) was used as a parameter. Separation work was almost 2-fold larger between a J774.2 cell and FP10SC2 cell than between J774.2 cell and three additional different cancer cells (4T1E, MAT-LyLu, and U-2OS), two FP10SC2 cells, or two J774.2 cells. FP10SC2 was established from 4T1E as a highly metastatic cell line, indicates separation work increased as the malignancy of cancer cells became higher. One possible explanation of the strong adhesion of macrophages to cancer cells observed in this study is that the measurement condition mimicked the microenvironment of tumor-associated macrophages (TAMs) in vivo, and J774.2 cells strongly expressed CD204, which is a marker of TAMs. The results of the present study, which were obtained by measuring cell adhesion strength quantitatively, indicate that the fabricated cup-chip is a useful tool for measuring intercellular adhesion easily and quantitatively.

Microgravity spreading of water spheres on hydrophobic capillary plates

We create nearly perfect centimetric spheres of water by splitting a cavity consisting of two metal hemispheres coated with a hydrophobic paint and under-filled with liquid, while releasing the apparatus in free-fall. A high-speed camera captures how water spread on hydrophobic aluminum and polycarbonate plates perforated with cylindrical capillaries. We compare observations at the ZARM drop tower in Bremen with Lattice-Boltzmann numerical simulations of Frank, Perre and Li for the inertial phase of imbibition.

Propulsion of Active Colloids by Self-Induced Field Gradients.

Previously, metallodielectric Janus particles have been shown to travel with their dielectric hemisphere forward under low frequency applied electric fields as a result of asymmetric induced-charge electroosmotic flow. Here, it is demonstrated that at high frequencies, well beyond the charge relaxation time of the electric double layer induced around the particle, rather than the velocity decaying to zero, the Janus particles reverse direction, traveling with their metallic hemisphere forward. It is proposed that such motion is the result of a surface force, arising from localized nonuniform electric field gradients, induced by the dual symmetry-breaking of an asymmetric particle adjacent to a wall, which act on the induced dipole of the particle to drive net motion even in a uniform AC field. Although the field is external, since the driving gradient is induced on the particle level, it may be considered an active colloid. We have thus termed this propulsion mechanism "self-dielectrophoresis", to distinguish from traditional dielectrophoresis where the driving nonuniform field is externally fixed and the particle direction is restricted. It is demonstrated theoretically and experimentally that the critical frequency at which the particle reverses direction can be characterized by a nondimensional parameter which is a function of electrolyte concentration and particle size.

Study and Design of a Hemispherical Reconfigurable Antenna

This paper, presents a novel design for a reconfigurable beam-switching antenna operating at 2.45 GHZ (ISM) for the wireless sensor network node. The geometry of this antenna is designed as a hemisphere and composed of three major parts. One is an omnidirectional monopole antenna in the focus of the structure as an excitation source, the second part is a metal hemisphere with three slots and the third part is three diodes centered on slots in order to switch the radiation pattern through it. A slot is chosen with care. When the diode is turned on, it behaves like a short circuit as a small resistance and slot became blocked and when the diode is turned off, it behaves like an open circuit as a very high resistance and the radiation pattern pass throughout the slot. So the radiation pattern of the structure is determined by the number of the off and on state diode. The parameters performance considered in this work is the resonant frequency, return loss, efficiency, bandwidth, radiation pattern, gain and the electromagnetic field of the proposed antenna. All results of the simulations were done by CST Microwave Studio simulation software and validated by HFSS software.

Plasmonic properties of metal nanoislands: Practical guide

We propose a simple approach to describe plasmonic properties of a metal hemisphere placed on a dielectric substrate and covered by another dielectric. Very simple analytical expressions and numerical data presented allow avoiding numerical calculations in finding the spectral position of the surface plasmon resonance and the dispersion of the polarizability of the hemisphere. The expressions are obtained using semi-analytical solution of the related problem in the quasistatic approximation. They are valid for most of the metals and for a wide variety of dielectric substrate and cover materials. The maps allowing to find the surface plasmon resonance wavelength and spectral dispersion of the polarizability of Ag, Au, Cu, and Al hemispherical nanoparticles are also presented. The maps are plotted in the coordinates of the substrate and the surrounding media dielectric permittivity. The results of the study can be applied to describe the polarizability of isolated or weakly interacting metal nanoislands and metal island films.

Second harmonic generation from hemispherical metal nanoparticle covered by dielectric layer

Using hydrodynamic theory of electron gas motion in metals, we obtain hyperpolarizability of the metal hemisphere in the framework of the quasistatic approach. For a silver hemisphere placed on a glass substrate and covered with TiO2 shell, we demonstrate analytically that conduction electrons in the vicinity of the hemisphere sharp edge dominate the nonlinear optical response of the nanoparticle. The developed theory is verified by numerical simulation in COMSOL. Numerical analysis reveals that rounding of the sharp edge affects the linear polarizability and first hyperpolarizability of the hemisphere differently. We also discuss dependence of the hyperpolarizability on the dielectric shell thickness and show that both lacking of the inversion symmetry and presence of the glass–air-TiO2 interface essentially contribute to the polarizability of the hemisphere at the frequency of the second harmonic.

Observing Electrokinetic Janus Particle-Channel Wall Interaction Using Microparticle Image Velocimetry.

Three-dimensional/two-component microparticle image velocimetry is used to examine the hydrodynamic flow patterns around metallodielectric Janus particles 15 μm in diameter adjacent to insulating and conducting walls. Far from the walls, the observed flow patterns are in good qualitative agreement with previous experimental and analytical models. However, close to the conducting wall, strong electrohydrodynamic flows are observed at low frequencies, which result in fluid being injected toward the particle. The proximity of the metallic hemisphere to the conducting wall is also shown to produce a localized field gradient, which results in dielectrophoretic trapping of 300 nm polystyrene particles across a broad range of frequencies.