Shape Anisotropy Research Articles

Surface enhanced raman spectroscopy based sensitive and onsite detection of microplastics in water utilizing silver nanoparticles and nanodendrites.

Plastics, of the order of microns in size, being not visible to the naked eye, are one of the significant contributors to pollution in the environment. Thus, the detection of micron-sized plastics (microplastics (MPs)) is crucial because of its hazardous toxic effects on our surroundings. In this work, we have proposed a quick and on-site detection of MPs, such as, polyvinyl chloride (PVC), polyvinyl alcohol (PVA) and polystyrene (PS) at ultra trace level using surface-enhanced Raman spectroscopy (SERS). To detect and analyse the spectra, two different nanostructures, such as, spherical shaped Ag nanoparticles (NPs), and shape anisotropic Ag nano-dendrites (NDs) were utilised to acquire the SERS spectra. A comprehensive analysis was further performed to check and investigate the amount of enhancements due to the mentioned nanostructures. We observed the Ag NDs exhibited amplified signal intensity compared to the Ag NPs due to the shape anisotropy leading to the surface charge confinement effect to create highly dense hotspots. However, the spherical shaped polystyrene beads of micron size exhibited better enhancement in Raman signal intensity when mixed with Ag NPs due to increased surface adsorption with the NPs. Therefore, the comparative study emphasizes the ability of using solution-based nanostructure as SERS for the onsite detection of microplastics having diverge size range at low concentration.

Adjusting microwave sensing frequency through aspect ratio variation and bending repetitions in Permalloy ellipses

The Ferromagnetic Resonance (FMR) phenomenon, marked by the selective absorption of microwave radiation by magnetic materials in the presence of a magnetic field, plays a pivotal role in the development of radar absorbing materials, high speed magnetic storage, and magnetic sensors. This process is integral for technologies requiring precise control over microwave absorption frequencies. We explored how variations in resonance fields can be effectively modulated by adjusting both the shape and stress anisotropies of magnetic materials on a flexible substrate. Utilizing polyethylene-naphthalate (PEN) as the substrate and Permalloy (Ni79Fe21, noted for its positive magnetostriction coefficient) as the magnetic component, we demonstrated that modifications in the aspect ratio and bending repetitions can significantly alter the resonance field. The results, consistent with Kittel’s equation and the predictions of a uniaxial magnetic anisotropy model, underscore the potential for flexible substrates in enhancing the sensitivity and versatility of RF-based magnetic devices.

Interacting Trimagnetic Ensembles for Enhanced Magnetic Resonance Transverse Relaxivity.

An ensemble of nanosystems can be considered to improve magnetic resonance imaging (MRI) transverse relaxivity. Herein, an interacting superparamagnetic competing structure of an isotropic-anisotropic trimagnetic hybrid nanosystem, γ-Fe2O3@δ-MnO2@NiFe2O4, is considered for MRI relaxivity exploration. The interacting superparamagnetic system reveals fascinating dynamic magnetic behavior, where flower-shaped two-dimensional flakes are decorated over nanoparticles. The hybrid nanosystem exhibits modulated shape anisotropy with spin blocking and energy barrier broadening, which help in achieving faster MR transverse relaxivity. The hierarchical architecture ensemble of the trimagnetic landscape shows effective MR transverse relaxivity with a transverse (r2)/longitudinal (r1) relaxivity of 61.5 and potential cell viability. The competing trimagnetic system with regulated activation energy is found to be the underlying reason for such signal enhancement in MRI contrast efficiency. Hence, this study displays a novel pathway correlating MR transverse relaxivity with dynamic magnetic behavior and competing landscape of hierarchical trimagnetic ensembles.

Structural analysis, relaxor-ferroelectric behavior and optical properties of Mn3+-doped CaBi3NdTi4O15 synthesized via hydrothermal technique

In this study, the synthesis of the Aurivillius Ca1-xBi3+xNdTi4-xMnxO15 (CBNTMx), where x is 0, 0.2, and 0.4, was carried out using the hydrothermal technique. The X-ray diffraction (XRD) data showed that the polar orthorhombic structure in space group of A21am was adopted by all powder samples. All samples possessed anisotropic plate-like particle shapes, and the particle size increased with increase in x value. The phase transition temperature (Tm) is decreased by the ionic substitution owing to the decrease in orthorhombic distortion reflected by the reduction of orthorhombicity. Relaxor-ferroelectric behavior was observed for CBNTM0.4 because of its strong frequency dispersion, while CBNTM0 and CBNTM0.2 only showed a diffused-phase transition (DPT) indicated by the broad dielectric constant at the transition temperature (Tm). The electrical conductivity exhibited an enhancement at higher x value since the ionic substitution with acceptor Mn3+ ions introduced the charge carrier in the lattice. Similarly, the reduction of optical band gap energy from 3.43 eV for the parent CBNTM0 to 2.75 eV and 2.71 eV for CBNTM0.2 and CBNTM0.4, respectively, was described as the contribution of the lower energy level of Mn 3d orbital than that of Ti 3d in the conduction band and also the less distorted structure.

GaAs Cone-Shell Quantum Dots in a Lateral Electric Field: Exciton Stark-Shift, Lifetime, and Fine-Structure Splitting.

Strain-free GaAs cone-shell quantum dots have a unique shape, which allows a wide tunability of the charge-carrier probability densities by external electric and magnetic fields. Here, the influence of a lateral electric field on the optical emission is studied experimentally using simulations. The simulations predict that the electron and hole form a lateral dipole when subjected to a lateral electric field. To evaluate this prediction experimentally, we integrate the dots in a lateral gate geometry and measure the Stark-shift of the exciton energy, the exciton intensity, the radiative lifetime, and the fine-structure splitting (FSS) using single-dot photoluminescence spectroscopy. The respective gate voltage dependencies show nontrivial trends with three pronounced regimes. We assume that the respective dominant processes are charge-carrier deformation at a low gate voltage U, a vertical charge-carrier shift at medium U, and a lateral charge-carrier polarization at high U. The lateral polarization forms a dipole, which can either enhance or compensate the intrinsic FSS induced by the QD shape anisotropy, dependent on the in-plane orientation of the electric field. Furthermore, the data show that the biexciton peak can be suppressed by a lateral gate voltage, and we assume the presence of an additional vertical electric field induced by surface charges.

Designing highly efficient interlocking interactions in anisotropic active particles

Cluster formation of microscopic swimmers is key to the formation of biofilms and colonies, efficient motion and nutrient uptake, but, in the absence of other interactions, requires high swimmer concentrations to occur. Here we experimentally and numerically show that cluster formation can be dramatically enhanced by an anisotropic swimmer shape. We analyze a class of model microswimmers with a shape that can be continuously tuned from spherical to bent and straight rods. In all cases, clustering can be described by Michaelis-Menten kinetics governed by a single scaling parameter that depends on particle density and shape only. We rationalize these shape-dependent dynamics from the interplay between interlocking probability and cluster stability. The bent rod shape promotes assembly in an interlocking fashion even at vanishingly low particle densities and we identify the most efficient shape to be a semicircle. Our work provides key insights into how shape can be used to rationally design out-of-equilibrium self-organization, key to creating active functional materials and processes that require two-component assembly with high fidelity.

New Insights into the Formation of Aggregates of Bidisperse Nano- and Microplastics in Water Based on the Analysis of In Situ Microscopy and Molecular Simulation.

Microplastics (MPs) and nanoplastics (NPs) in water pose a global threat to human health and the environment. To develop efficient removal strategies, it is crucial to understand how these particles behave as they aggregate. However, our knowledge of the process of aggregate formation from primary particles of different sizes is limited. In this study, we analyzed the growth kinetics and structures of aggregates formed by polystyrene MPs in mono- and bidisperse systems using in situ microscopy and image analysis. Our findings show that the scaling behavior of aggregate growth remains unaffected by the primary particle size distribution, but it does delay the onset of rapid aggregation. We also performed a structural analysis that reveals the power law dependence of aggregate fractal dimension (df) in both mono- and bidisperse systems, with mean df consistent with diffusion-limited cluster aggregation (DLCA) aggregates. Our results also suggest that the df of aggregates is insensitive to the shape anisotropy. We simulated molecular forces driving aggregation of polystyrene NPs of different sizes under high ionic strength conditions. These conditions represent salt concentration in ocean water and wastewater, where the DLVO theory does not apply. Our simulation results show that the aggregation tendency of the NPs increases with the ionic strength. The increase in the aggregation is caused by the depletion of clusters of ions from the NPs surface.

State Transitions and Crystalline Structures of Single Polyethylene Rings: MD Simulations.

This study investigates the structural changes of cyclic polyethylene (PE) single chains during cooling through molecular dynamics simulations. The influence of topological constraint on a ring is examined by comparing it with the results of its linear counterpart. A pseudo phase diagram of state transition for PE rings based on length and temperature is constructed, revealing a consistent chain-folding transition during cooling. The shape anisotropy of short crystallized cyclic chains exhibits oscillations with chain length, leading to a more pronounced odd-even effect in single cyclic chains compared with the linear ones. A honeycomb model is proposed to elucidate the odd-even effect of chain folding in crystalline structures of single linear and cyclic chains, and we discuss its potential to predict surface tension. Analyses of the tight folding model and the re-entry modes demonstrate that a cyclic chain possesses a shorter average crystalline stem length and a more compact folded structure than its linear counterpart. The findings highlight the impact of topological change on crystallization and the odd-even effect of chain length, providing valuable insights for understanding polymer crystallization with different topologies.

The Paradoxical Behavior of Rough Colloids at Fluid Interfaces.

Colloidal particles adsorb and remain trapped at immiscible fluid interfaces due to strong interfacial adsorption energy with a contact angle defined by the chemistry of the particle and fluid phases. An undulated contact line may appear due to either particle surface roughness or shape anisotropy, which results in a quadrupolar interfacial deformation and strong long-range capillary interaction between neighboring particles. While each effect has been observed separately, here we report the paradoxical impact of surface roughness on spherical and anisotropic ellipsoidal polymer colloids. Using a seeded emulsion polymerization technique, we synthesize spherical and ellipsoidal particles with controlled roughness magnitudes and topography (convex/concave). Via in situ measurement of the interfacial deformation around colloids at an air-water interface, we find that while surface roughness strengthens the quadrupolar deformation in spheres as expected by theory, in stark contrast, it weakens the same in ellipsoids. As roughness increases, particles of both shapes become more hydrophilic, and their apparent contact angle decreases. Using numerical predictions, we show that this partially explains the decreased interfacial deformation and capillary interactions between the ellipsoids. Therefore, particle surface engineering has the potential to decrease the capillary deformation by asymmetric particles via changing their capillary pinning, as well as wetting behavior at fluid interfaces.

Faceting effect on magnetism in manganese ferrites nanoparticles

Manganese ferrite (MnFe2O4) stands out among multifunctional spinel ferrites for its attractive magnetic properties, high chemical stability and excellent biocompatibility, which are essential prerequisites for effective performance in various applications, including biomedical. However, very little work has been carried out on the ideal reaction conditions to obtain nanoparticles with high performance in terms of magnetization correlated to facet engineering. This study reports the synthesis of MnFe2O4 nanoparticles by a thermal decomposition process, controlling the decomposition temperature and the ligand/precursor ratio. Indeed, with a decomposition temperature of manganese stearate maintained at 270 °C and a ligand/precursor ratio equal to 2, superparamagnetic behavior and significantly high saturation magnetization were obtained from monodisperse nanoparticles of cubic shape and approximately 7 nm in size. Combining magnetic measurements with Monte Carlo simulations demonstrated that this behavior is attributed to the shape anisotropy of cubic nanoparticles, which favors the orientation of the magnetic moment along planar facets with low surface anisotropy. The strong influence of this faceting on the nanomagnetism of ferrite based on MnFe2O4 should open new avenues for numerous applications, particularly in biomedical imaging.

Circularly Polarized Luminescence Without External Magnetic Fields from Individual CsPbBr3 Perovskite Quantum Dots.

Lead halide perovskite quantum dots (QDs), the latest generation of the colloidal QD family, exhibit outstanding optical properties, which are now exploited as both classical and quantum light sources. Most of their rather exceptional properties are related to the peculiar exciton fine-structure of band-edge states, which can support unique bright triplet excitons. The degeneracy of the bright triplet excitons is lifted with energetic splitting in the order of millielectronvolts, which can be resolved by the photoluminescence (PL) measurements of single QDs at cryogenic temperatures. Each bright exciton fine-structure-state (FSS) exhibits a dominantly linear polarization, in line with several theoretical models based on the sole crystal field, exchange interaction, and shape anisotropy. Here, we show that in addition to a high degree of linear polarization, the individual exciton FSS can exhibit a non-negligible degree of circular polarization even without external magnetic fields by investigating the four Stokes parameters of the exciton fine-structure in individual CsPbBr3 QDs through Stokes polarimetric measurements. We observe a degree of circular polarization up to ∼38%, which could not be detected by using the conventional polarimetric technique. In addition, we found a consistent transition from left- to right-hand circular polarization within the fine-structure triplet manifold, which was observed in magnetic-field-dependent experiments. Our optical investigation provides deeper insights into the nature of the exciton fine structures and thereby drives the yet-incomplete understanding of the unique photophysical properties of this class of QDs for the benefit of future applications in chiral quantum optics.

Lateral Confinement in 2D Nanoplatelets: A Strategy to Expand the Colloidal Quantum Engineering Toolbox

AbstractAmong colloidal nanocrystals, 2D nanoplatelets offer a unique set of properties with exceptionally narrow luminescence and low lasing thresholds. Furthermore, their anisotropic shape expands the playground for the complex design of heterostructures where spectra but also scattering rates can be engineered. A challenge that still remains is to combine shell growth which makes NPLs stable, with spectral tunability. Indeed, most reported shelled nanoplatelets end up being red emitters due to a loss of quantum confinement. Here, the combination of both lateral and in‐plane confinements within a single heterostructure is explored. A CdS/CdSe/CdS/CdZnS core–crown–crown shell structure that enables yellow emission is grown and that is responsive to a large range of excitation including visible photons, X‐ray photons, electron beams, and electrical excitations. k.p simulations predict that emission tunability of up to several 100 s of meV can be obtained in ideal structures. This material also displays stimulated emission resulting from bi‐exciton emission with a low threshold. Once integrated into an LED stack, this material is compatible with sub‐bandgap excitation and exhibits high luminance. Scaling of the electroluminescence properties by downsizing the pixel size is also investigated.

Multiple Phase Structures and Enhanced Dielectric Properties of Side-Chain Liquid Crystalline Polymer Containing Unique Biaxial Mesogen with Large Dipole Moment

To achieve all-organic polymer with high dielectric performances, we have designed a novel side-chain liquid crystalline polymer (P7) with strong polar mesogen of (Z)-4-(2-cyano-2-phenylvinyl)benzonitrile (CSCN) attached to polycyclooctene backbone. The bis-cyano-substituted CSCN is board-shaped and exhibits a large dipole moment (8.54 D) which tilts ∼34.2° away from its molecular long axis. Consequently, CSCN shows unique dual molecular anisotropy: one from biaxial shape anisotropy and the other from polarization anisotropy. The complex phase behaviors of P7 were investigated employing mainly the techniques of differential scanning calorimetry and X-ray diffraction. Four liquid crystal (LC) phases are identified as K0, K1, K2 and K3, which are SmA, highly-ordered biaxial SmA, B5-like and B7-like, respectively, with the thermal stability increased in sequence. The experimental results indicate that the different LC phases are arisen from the competition and balance between π-π stacking and dipole-dipole interaction. While the face-to-face π-π stacking is dominant in K0 and K1, optimizing the dipole-dipole interaction causes the CSCN mesogens within the smectic layer to tilt and rotate, resulting in K2 and K3. We further investigated the dielectric properties of P7 films using polarization-electric field loops test. The dielectric constant (εr) of P7 is found to be LC structure dependent, which is increased when the LC phase is varied from K0 to K3. With an average εr of 9.7 achieved in K3 and the low dielectric loss (tan δ = 0.001), P7 film offers a promising material in advanced applications like energy storage and electronic devices.

Effect of shape anisotropy on percolation of aligned and overlapping objects on lattices.

We investigate the percolation transition of aligned, overlapping, anisotropic shapes on lattices. Using the recently proposed lattice version of excluded volume theory, we show that shape-anisotropy leads to some intriguing consequences regarding the percolation behavior of anisotropic shapes. We consider a prototypical anisotropic shape-rectangle-on a square lattice and show that, for rectangles of width unity (sticks), the percolation threshold is a monotonically decreasing function of the stick length, whereas, for rectangles of width greater than two, it is a monotonically increasing function. Interestingly, for rectangles of width two, the percolation threshold is independent of its length. We show that this independence of threshold on the length of a side holds for d-dimensional hypercubiods as well as for specific integer values for the lengths of the remaining sides. The limiting case of the length of the rectangles going to infinity shows that the limiting threshold value is finite and depends upon the width of the rectangle. This "continuum" limit with the lattice spacing tending to zero only along a subset of the possible directions in d dimensions results in a "semicontinuum" percolation system. We show that similar results hold for other anisotropic shapes and lattices in different dimensions. The critical properties of the aligned and overlapping rectangles are evaluated using Monte Carlo simulations. We find that the threshold values given by the lattice-excluded volume theory are in good agreement with the simulation results, especially for larger rectangles. We verify the isotropy of the percolation threshold and also compare our results with models where rectangles of mixed orientation are allowed. Our simulation results show that alignment increases the percolation threshold. The calculation of critical exponents places the model in the standard percolation universality class. Our results show that shape anisotropy of the aligned, overlapping percolating units has a marked influence on the percolation properties, especially when a subset of the dimensions of the percolation units is made to diverge.

Hydrogen/deuterium exchange in 1-alkyl-3-methylimidazolium-based water and methanol solutions

The hydrogen/deuterium (H/D) exchange of ionic liquid (IL) occurred in the deuterated solutions. Kinetic H/D exchange was investigated via Raman spectroscopy and pH measurements. The ILs employed were amphiphilic 1-alkyl-3-methylimidazolium triflate ([Cnmim][OTf] (n = 2, 4, 6, and 8)) and hydrophilic [C2mim][X] (X = Cl, Br, I, and BF4). The H/D exchange occurred in the [C2mim][OTf]-deuterated methanol (MeOD-d4) solutions and was dependent on the additive concentration. For [C2mim][X]-D2O (X = Cl, Br, and I), which did not possess nano-heterogeneity and the anionic anisotropic shape factor, significant relationships were observed among the H/D exchange rate, pD, and ion-pairing strength.

Temperature-driven spin reorientation transition in van der Waals Cr1.7Te2 ferromagnet

The phenomenon of spin reorientation transition in magnetic materials is truly captivating, as it involves a fascinating change in the direction of magnetic moments. However, the research on spin reorientation transition in two-dimensional (2D) magnetic materials has received limited attention, thus hindering its immense potential for significant advancements in various device applications. In this study, we present a discovery of a spin reorientation transition from an in-plane to an out-of-plane direction in the van der Waals ferromagnet Cr1.7Te2 (Tc = 300 K). This transition occurs at 70 K when the temperature ranges from 3 to 300 K, which is evidenced by the temperature-dependent Hall effect and magnetic anisotropy energy measurements. Notably, the anisotropic evolution observed reveals that the shape anisotropy effect surpasses the magnetocrystalline anisotropy in van der Waals ferromagnet at low temperatures, which is distinct from reported ferromagnetic materials. Furthermore, temperature-dependent x-ray diffraction characterizations confirm that no structural phase transition occurs during this intriguing spin reorientation transition process. These findings establish a strong and solid foundation, offering a promising platform for the design and development of cutting-edge 2D spintronic devices.

Temperature dependent structural and magnetic properties of permalloy (Ni80Fe20) nanotubes

One dimensional Permalloy (Ni80Fe20) nanotubes (NTs) were successfully fabricated using Anodic Aluminum Oxide (AAO) templates with 100 nm diameter by employing a low-cost electrodeposition route at room temperature. As prepared Ni80Fe20 NTs morphology, elemental analysis and structural details have been investigated by using field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS) and X-Ray diffraction (XRD) respectively. The structure of prepared Permalloy nanostructures has face centered cubic (FCC) phase with polycrystalline in nature. The Ni80Fe20 NTs were annealed at 200 °C, 400 °C and 600 °C. The vibrating sample magnetometer (VSM) measurements showed that all the samples exhibit ferromagnetic nature. Furthermore, VSM has been employed to study the saturation magnetization (MS), Squareness (SQ) and Coercivity (HC) as the function of annealing temperature. A comparative study of magnetization reversal mechanism has also been conducted. The easy axis lies in the direction perpendicular to the NTS axis due to the dominance in shape anisotropy. Annealing temperature improved the magnetic properties of Permalloy (PA) NTs. This work will provide significant applications in the field of spintronics and high-density data storage devices.

A novel bio-inspired design method for porous structures: Variable-periodic Voronoi tessellation

This paper introduces a novel approach, namely Variable-Periodic Voronoi Tessellation (VPVT), for the bio-inspired design of porous structures. The method utilizes distributed points defined by a variable-periodic function to generate Voronoi tessellation patterns, aligning with a wide diversity of artificial or natural cellular structures. In this VPVT design method, the truss-based architecture can be fully characterized by design variables, such as frequency factors, thickness factors. This approach enables the optimal design of porous structures for both mechanical performance and functionality. The varied, anisotropic cell shapes and sizes of VPVT porous structures provide significantly greater design flexibility compared to typical isotropic porous structures. In addition, the VPVT method not only can design micro-macro multiscale materials, but is also applicable for the design of meso-macro scale truss-based porous structures, such as architecture constructions, biomedical implants, and aircraft frameworks. This work employs a Surrogate-assisted Differential Evolution (SaDE) method to perform the optimization process. Numerical examples and experiments validate that the proposed design achieves about 51.1% and 47.8% improvement in compliance performance and damage strength, respectively, than existing studies.

Control of Magnetic Shape Anisotropy by Nanopillar Dimensionality in Vertically Aligned Nanocomposites.

Perpendicular magnetic anisotropy forms the foundation of the current data storage technology. However, there is an ever-increasing demand for higher density data storage, faster read-write access times, and lower power consuming storage devices, which requires new materials to reduce the switching current, improve bit-to-bit distributions, and improve reliability of writing with scalability below 10 nm. Here, vertically aligned nanocomposites (VANs) composed of self-assembled ferromagnetic La0.7Sr0.3MnO3 (LSMO) nanopillars in a surrounding ZnO matrix are investigated for controllable magnetic anisotropy. Confinement of LSMO into nanopillar dimensions down to 15 nm in such VAN films aligns the magnetic easy axis along the out-of-plane (i.e., perpendicular) direction, in strong contrast to the typical in-plane easy axis for strained, phase pure LSMO thin films. The dominant contribution to the magnetic anisotropy in these (LSMO)0.1(ZnO)0.9 VAN films comes from the shape of the nanopillars, while the epitaxial strain at the vertical LSMO:ZnO interfaces exhibits a negligible effect. These VAN films with their large, out-of-plane remnant magnetization of 2.6 μB/Mn and bit density of 0.77 Tbits/inch2 offer an interesting strategy for enhanced data storage applications.

Effective thermal and electromagnetic properties of composite materials containing ellipsoidal-coated fillers oriented randomly

This study aims to derive explicit solutions for the effective thermal and electromagnetic properties of composite materials containing coated fillers oriented randomly. In this study, the coated fillers are modeled as double inhomogeneous inclusions with anisotropic properties and ellipsoidal shapes, without loss of generality. This model is analyzed by combining the double inclusion method with the self-consistent method or the Mori–Tanaka method. The derived solutions for the effective properties are expressed in explicit form by the physical properties of constituents and the difference in shape between the inner and outer regions of the coated fillers, and confirmed to be consistent in the limit of no coating layer. In addition, the solutions of effective properties are also derived for composite materials containing many kinds of coated fillers with different shapes and physical properties. The solutions allow the prediction of thermal conductivity, permittivity, electrical conductivity, and magnetic permeability of a wide range of composite materials by analogy with thermal and electromagnetic properties. These solutions are applied to the following two composites: diamond fillers coated with TiC/Al, and Co fillers coated with Co 3 O 4/Mg. The effect of the thickness of the coating layer on the effective thermal conductivity of these materials is investigated. As a result, our analytical results for these composite materials generally agree with the experimental results. Moreover, from the viewpoint of maximizing the thermal conductivity of diamond fillers coated with TiC/Al, the optimal combinations of the thickness of the coating layer and the shape of the diamond fillers are identified. For Co coated with Co 3 O 4/Mg, we find that the shape of Co fillers should be long fibrous from the viewpoint of giving high magnetic permeability while maintaining high thermal conductivity due to Mg. Thus, the analytical solutions are demonstrated to be powerful tools for designing composite materials containing coated fillers.