Small Interparticle Distances Research Articles

Realization of ultrastrong coupling between LSPR and Fabry–Pérot mode via self-assembly of Au-NPs on p-NiO/Au film

Abstract The realization of higher coupling strengths between coupled resonant modes enables exploration of compelling phenomena in diverse fields of physics and chemistry. In this study, we focus on the modal coupling between localized surface plasmon resonance (LSPR) of Au nanoparticles (Au-NPs) and Fabry–Pérot mode (p-NiO/Au film). The effects of nanoparticle size, projected surface coverage (PSC), interparticle distance (IPD), and arrangement to the coupling strength between the two modes are theoretically investigated using finite-difference time-domain (FDTD) method. Au-NPs/p-NiO/Au film (ANA) nanostructures with NPs size of 10 nm, 30 nm, and 50 nm are considered. Numerical calculations point to larger size and higher projected surface coverage (also smaller IPD) of NPs as pre-eminent factors in enhancing the strength of modal coupling. ANA nanostructure with NPs size of 30 nm (ANA-30) and 50 nm (ANA-50) are experimentally fabricated via a facile air–liquid interface self-assembly. The fabricated nanodevices exhibit immense Rabi splitting energies of 655 meV (ANA-30) and 770 meV (ANA-50), and thus fulfill the ultrastrong coupling condition with remarkable splitting energy to bare (plasmon) energy ratio of 0.35 (ANA-30) and 0.4 (ANA-50). The physical insights presented in this study, together with the simple and scalable fabrication process, establish a viable approach to realize stronger coupling between LSPR and Fabry–Pérot mode in metal NPs/dielectric/metal film systems. This will be vital to take advantage of the promising performance enhancements of plasmonic-based nanostructures under strongly coupled regimes in areas such as solar to fuel conversion, sensing, opto-electronics, and quantum applications.

Study of the calorimetric effect in ferrogels subjected to the high-frequency rotating magnetic field

This study investigates the calorimetric effect in iron oxide-containing ferrogels when exposed to a high-frequency rotating magnetic field. The primary heat generation mechanisms were magnetic relaxation (Néel and Brownian) and magnetic hysteresis. Temperature was monitored over time for various magnetic field amplitudes, enabling the determination of the contributions of both heat release mechanisms as a function of particle concentration and field strength. Experiments were performed using agar-based tissue mimicking phantoms with magnetic nanoparticle weight concentrations ranging from 5 % to 20 %. The magnetic fields applied varied from 1 kA/m to 5 kA/m, with the equipment having a capability of up to 7.5 kA/m at a frequency of 200 kHz. The experimental setup employed a two-phase magnetic system enclosed in an external core where magnetizing coils and parallel connected inductors and capacitors generated spatially and phase shifted fluxes by 90 degrees, resulting in a constant amplitude of rotating magnetic field.The heating efficiency of the rotating magnetic field was found to be approximately twice as effective as an equivalent alternating field in the experiments. The specific loss power exhibited a decreasing trend with the increase in magnetic nanoparticle concentration and the decrease in magnetic field strength. Specifically, as the magnetic nanoparticle concentration increased from 5 % to 20 %, the SLP diminished from 457.3 mW/g (at 5 % concentration and 5 kA/m) to 1.24 mW/g (at 20 % concentration and 1 kA/m). This decrease in SLP can be attributed to enhanced dipole–dipole interactions between particles at smaller interparticle distances as concentration increases, as well as reduced magnetic responsiveness to weaker magnetic fields. Even though Brownian relaxation was likely hindered in the gel structure due to the immobilization of magnetic nanoparticles, the observed magnetic heating effect remains significant. Given the limited existing data on rotating magnetic field effects on tissue mimicking ferrogels, this study offers novel insights, potentially aiding in the optimization of magnetic hyperthermia treatments while highlighting the benefits of rotating over alternating magnetic fields.

Effect of Hydrophobic Moieties on the Assembly of Silica Particles into Colloidal Crystals

To boost the implementation of colloidal crystals (CCs) in separation science, the effects of the most common chromatographic reversed phases, that is, butyl and octadecyl, on the assembly of silica particles into CCs and on the optical properties of CCs are investigated. Interestingly, particle surface modification can cause phase separation during sedimentation because the assembly is highly sensitive to minute changes in surface characteristics. Solvent-induced surface charge generation through acid-base interactions of acidic residual silanol groups with the solvent is enough to promote colloidal crystallization of modified silica particles. In addition, solvation forces at small interparticle distances are also involved in colloidal assembly. The characterization of CCs formed during sedimentation or via evaporative assembly revealed that C4 particles can form CCs more easily than C18 particles because of their low hydrophobicity; the latter can only form CCs in tetrahydrofuran when C18 chains with a high bonding density have extra hydroxyl side groups. These groups can only be hydrolyzed from trifunctional octadecyl silane but not from a monofunctional one. Moreover, after evaporative assembly, CCs formed from particles with different surface moieties exhibit different lattice spacings because their surface hydrophobicity and chemical heterogeneity can modulate interparticle interactions during the two main stages of assembly: the wet stage of crystal growth and the late stage of nano dewetting (evaporation of interparticle solvent bridges). Finally, short, alkyl-modified CCs were effectively assembled inside silica capillaries with a 100 μm inner diameter, laying the foundation for future chromatographic separation using capillary columns.

The Effect of the Core on the Absorption in a Hybrid Semiconductor Quantum Dot—Metal Nanoshell System

We examine the optical absorption in a hybrid structure composed of a metal nanoshell and a semiconductor quantum dot, while interacting with a linearly polarized probe electromagnetic field. First, we derive the equations of motion, in the rotating wave approximation. Then we procced to the derivation of analytical expressions for the linear susceptibility of the metal nanoshell and the semiconductor quantum dot. The imaginary part of the susceptibility expresses the absorption coefficient. We find that by properly engineering the thickness of the metal nanoshell, the material of the dielectric core and the interparticle distance, we may achieve an optimum response. We identify the emergence of two distinct types of hybrid exciton states. One of them emerges in the strong exciton–plasmon coupling regime for low values of the dielectric constant and the radius of the dielectric core. This type of hybrid exciton exhibits an amplified gain without population inversion and a quenched absorption resonance accompanied by a suppressed exciton lifetime. The second type of hybrid exciton emerges in the weak exciton–plasmon coupling regime and presents the opposite spectral characteristics. Here, the exciton lifetime presents a substantial increase, especially for small interparticle distances, in which case the semiconductor quantum dot and the metal nanoshell are strongly coupled with one another.

Electroactive substrates for surface-enhanced Raman spectroscopy based on overgrown gold-nanoparticle arrays by electrodeposition on indium tin oxide

Highly homogeneous and robust SERS conductive surfaces with large gold coverages and small inter-particle distances for a combined electrochemical-SERS investigation to monitor switchable redox molecules and electroactivated interfacial reactions.

Silver Nanoparticle Chains for Ultra-Long-Range Plasmonic Waveguides for Nd3+ Fluorescence.

Plasmonic waveguides have been shown to be a promising approach to confine and transport electromagnetic energy beyond the diffraction limit. However, ohmic losses generally prevent their integration at micrometric or millimetric scales. Here, we present a gain-compensated plasmonic waveguide based on the integration of linear chains of Ag nanoparticles on an optically active Nd3+-doped solid-state gain medium. By means of dual confocal fluorescence microscopy, we demonstrate long-range optical energy propagation due to the near-field coupling between the plasmonic nanostructures and the Nd3+ ions. The subwavelength fluorescence guiding is monitored at distances of around 100 µm from the excitation source for two different emission ranges centered at around 900 nm and 1080 nm. In both cases, the guided fluorescence exhibits a strong polarization dependence, consistent with the polarization behavior of the plasmon resonance supported by the chain. The experimental results are interpreted through numerical simulations in quasi-infinite long chains, which corroborate the propagation features of the Ag nanoparticle chains at both excitation (λexc = 590 nm) and emission wavelengths. The obtained results exceed by an order of magnitude that of previous reports on electromagnetic energy transport using linear plasmonic chains. The work points out the potential of combining Ag nanoparticle chains with a small interparticle distance (~2 nm) with rare-earth-based optical gain media as ultra-long-range waveguides with extreme light confinement. The results offer new perspectives for the design of integrated hybrid plasmonic-photonic circuits based on rare-earth-activated solid-state platforms.

Synthesis and characterization of silver-indium and antimony selenide: role in photocatalytic degradation of dyes

These days, water contamination poses a severe threat to the ecosystem and demands immediate care. This study examined the need to reduce water pollution using clean, renewable energy (solar light irradiations) for the degradation of Congo red by Silver-indium and antimony selenide with chemical composition AgInSbSe3. The sample was fabricated through a hydrothermal technique. The synthesized sample was characterized through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM) and Ultraviolet-Visible spectroscopy. The X-ray diffraction confirms crystalline structure of the synthesized sample. The SEM analysis reveals irregular grains and exhibits a very small inter-particle distance. SEM provides the morphology of the synthesized sample, the grain size of the synthesized sample was 0.58 μm. FTIR results revealed specific absorption bands in the range of 400–4000 cm−1; optical properties are studied through UV-Vis-spectroscopy. The synthesized sample has 1.97 eV bandgap which is suitable for degradation of organic pollutants. The photocatalytic activity of the material is checked by degrading the Congo red dye under direct sunlight irradiation and for the 75 min illumination 77.8% degradation efficiency is attained.

Micromagnetic Simulations of Clusters of Nanoparticles with Internal Structure: Application to Magnetic Hyperthermia

Micromagnetic simulation results on dynamic hysteresis loops of clusters of iron oxide nanoparticles (NPs) with internal structure composed of nanorods are compared with the widely used macrospin approximation. Such calculations allowing for nanorod-composed NPs is facilitated by a previously developed coarse-graining method based on the renormalization group approach. With a focus on applications to magnetic hyperthermia, we show that magnetostatic interactions improve the heating performance of NPs in chains and triangles, and reduce heating performance in fcc arrangements. Hysteresis loops of triangular and fcc systems of complex NPs are not recovered within the macrospin approximation, especially at smaller interparticle distances. For triangular arrangements, the macrospin approximation predicts that magnetostatic interactions reduce loop area, in contrast to the complex NP case. An investigation of the local hysteresis loops of individual NPs and macrospins in clusters reveals the impact of the geometry of their neighbors on individual versus collective magnetic response, inhomogeneous heating within clusters, and further differences between simulating NPs with internal structure and the use of the macrospin approximation. Capturing the internal physical and magnetic structure of NPs is thus important for some applications.

Efficient Biexciton State Preparation in a Semiconductor Quantum Dot Coupled to a Metal Nanoparticle with Linearly Chirped Gaussian Pulses.

We consider a hybrid nanostructure composed of a semiconductor quantum dot placed near a spherical metallic nanoparticle, and study the effect of the nanoparticle on the population transferral from the ground to the biexciton state of the quantum dot, when using linearly chirped Gaussian pulses. For various values of the system parameters (biexciton energy shift, pulse area and chirp, interparticle distance), we calculate the final population of the biexciton state by performing numerical simulations of the non-linear density matrix equations which describe the coupled system, as well as its interaction with the applied electromagnetic field. We find that for relatively large values of the biexciton energy shift and not very small interparticle distances, the presence of the nanoparticle improves the biexciton state preparation, since it effectively increases the area of the applied pulse. For smaller biexciton energy shifts and smaller distances between the quantum dot and the nanoparticle, the performance is, in general, degraded. However, even in these cases we can still find ranges of parameter values where the population transfer to the biexciton state is accomplished with high fidelity, when using linearly chirped Gaussian pulses. We anticipate that our results may be exploited for the implementation of novel nanoscale photonic devices or future quantum technologies.

Stimulated Raman adiabatic passage in a quantum system near a plasmonic nanoparticle

We investigate theoretically the population transfer process in a Λ-type three-level quantum system (QS) near a metallic nanosphere using the stimulated Raman adiabatic passage (STIRAP) technique. We combine density matrix quantum dynamical calculations with first-principle electromagnetic calculations, which quantify the influence of the plasmonic nanoparticle on the electric field of the pump and Stokes pulses in STIRAP as well as on the spontaneous emission rates within the Λ-type system. We study the population transfer process by varying the free-space spontaneous emission rate, the distance of the QS from the nanosphere, the polarization direction with respect to the nanoparticle surface and the relative strength of the pump and Stokes pulses used in STIRAP. We find that when the pump and Stokes fields have tangential and radial polarizations with respect to the nanosphere surface, the transfer efficiency is improved due to the increase of the decay rate of the excited state to the target state relatively to the decay to the initial state. The optimal population transfer is achieved for small interparticle distances, moderate free space spontaneous decay rate, large values of the pump Rabi frequency and small values of the Stokes Rabi frequency. When we exchange the polarization directions of the pump and Stokes fields we can still find a range of parameters where the population transfer remains efficient, but larger Stokes Rabi frequencies are necessary to overcome the increased decay rate from the excited state back to the initial state.

Particle radial velocity and concentration kernels estimation in isotropic grid turbulence experiments of inertialess particles at small separation distances

We report experimental measurements of kinematic and dynamic particle concentration kernels conditioned by the separation distances of solid inertialess particles in isotropic turbulence by three-dimensional particle tracking velocimetry with particle diameters smaller than the Kolmogorov length scale. Particle radial relative velocity statistics are measured from the dissipation to the integral length-scale range. The radial scaling of particle and fluid relative velocity variance $\langle w_r(r)^{2} \rangle \sim r^{2/3}$ in the inertial subrange, consistent with Kolmogorov's theory, is reported, while a new scaling is found for small distances due to finite-size effects between particles. The measured concentration kernels at small separation distances therefore deviate from those in the theory of Saffman & Turner (J. Fluid Mech., vol. 1, 1956, pp. 16–30) at small inter-particle distances due to hydrodynamic interactions. A real kernel taking into account the history of the particle tracks and excluding multiple events is also calculated, while the normalised particle concentration kernels are found to be essentially insensitive to the flow Reynolds number.

Efficient exciton generation in a semiconductor quantum-dot–metal-nanoparticle composite structure using conventional chirped pulses

We consider a nanostructure consisting of a semiconductor quantum dot coupled to a metal nanoparticle, and show with numerical simulations that the exciton state of the quantum dot can be robustly generated from the ground state even for small interparticle distances, using conventional chirped pulses with Gaussian and hyperbolic secant envelopes. The asymmetry observed in the final exciton population with respect to the chirp sign of the applied pulses is explained using the nonlinear density matrix equations describing the system, and is attributed to the real part of the parameter emerging from the interaction between excitons in the quantum dot and plasmons in the metal nanoparticle. The simplicity of the conventional chirped pulses, which can also be easily implemented in the laboratory, make the proposed robust quantum control scheme potentially useful for the implementation of ultrafast nanoswitches and quantum information processing tasks with semiconductor quantum dots.

Hydrogen sensing by plasmon decoupling effect in nanostructured Pd/Au films

Plasmon coupling effect occurs in plasmonic nanostructures when interparticle distances are in the order of particle size leading to spectral shifts in the plasmonic band. This effect has been recently highlighted for measurement of fluctuations in the interparticle distance at nanoscale level. In this study, nanostructured thin Au films were deposited on quartz substrates by pulsed laser deposition (PLD) for sensing of hydrogen gas. A blue shift from 730 to 560 nm in LSPR of Au films was observed when substrate temperatures rises from 25 to 600 °C due to variation in morphology of films from a continuous surface composed of tiny agglomerates to granular surface composed of bigger particles with increased interparticle spacing. For plasmon coupling sensing of hydrogen, a thin Pd film was deposited on top of nanostructured Au films. Upon hydrogen exposure, up to12 nm blue shift within few seconds was observed depending on hydrogen concentration. Based on field emission scanning electron microscope (FESEM) images and finite-difference time-domain (FDTD) simulations, this plasmon sensing is explained by hydrogen-induced decoupling due to the formation of surface stresses in Pd, which can affect the LSPR via an increase in interparticle spacing of Au nanoislands. • Nanostructured Au/quartz films were deposited prepared by pulsed laser deposition at different substrate temperatures. • The interparticle distance of Au films increases with temperature, leading to blue shift in the gold plasmonic absorption. • A thin Pd layer as hydrogen catalyst was deposited on Au/quartz samples by DC magnetron sputtering. • Hydrogen interaction led to a further blue shift, which was greater for films with smaller interparticle distance. • Using finite difference time domain (FDTD) simulation, this effect was attributed to a plasmon decoupling effect.

Effect of size and interparticle distance of nanoparticles on the formation of bubbles induced by nanosecond laser

When a laser beam is irradiated on nanoparticles (optical absorbers) in a liquid, massive heat is generated, which causes vaporization of a nearby liquid, followed by the formation of bubbles. Despite the potential application of these bubbles for cancer diagnosis and therapy, solar energy harvesting, and local chemical reaction enhancement, little is known on how the size and interparticle distance of optical absorbers affect laser-induced bubble formation. Additionally, observing the bubble dynamics near nanomaterials is challenging due to their fast growth and immediate collapse. In this study, we trapped bubbles using hydrogels and investigated how their diameter changed with the size and interparticle distance of nanoparticles or their agglomerates and the energy level of laser irradiation. We found that the size of trapped bubbles increased with softer hydrogels, higher laser energy level, larger agglomerate size, and smaller interparticle distance among agglomerates. Based on these observations, we suggest that nanobubbles initially formed on nanoparticles merged to become microbubbles.

The short-range correlation of asymmetric nucleonic matter at finite temperature: The LOCV approach

The short-range correlation of the asymmetric nucleonic matter (ASM) is perused at finite temperature in the lowest order constrained variational (LOCV) method. It is observed that, by increasing the temperature, the LOCV ASM static non-central distribution functions become weak. For small inter-particle distances, a universal behavior is found for the LOCV ASM state-dependent density distributions. Using the expectation value of the one-pion exchange part of the AV18 interaction, the scaling factor of the pion absorption cross section (the number of quasideuterons), for the different proton to neutron ratios R, is also estimated in the LOCV formalism. It is shown that, by increasing (decreasing) the ratio R (the temperature), the number of quasideuterons grows. Considering the scaling factor, the Bethe-Levinger (BL) factor for the ASM is also investigated. It is demonstrated that, at the saturation density and temperature 20 MeV, the BL factor for the ASM with R=1(0.5) becomes 4.3 (4.5).

Efficacy of the radial pair potential approximation for molecular dynamics simulations of dense plasmas

Macroscopic simulations of dense plasmas rely on detailed microscopic information that can be computationally expensive and is difficult to verify experimentally. In this work, we delineate the accuracy boundary between microscale simulation methods by comparing Kohn–Sham density functional theory molecular dynamics (KS-MD) and radial pair potential molecular dynamics (RPP-MD) for a range of elements, temperature, and density. By extracting the optimal RPP from KS-MD data using force matching, we constrain its functional form and dismiss classes of potentials that assume a constant power law for small interparticle distances. Our results show excellent agreement between RPP-MD and KS-MD for multiple metrics of accuracy at temperatures of only a few electron volts. The use of RPPs offers orders of magnitude decrease in computational cost and indicates that three-body potentials are not required beyond temperatures of a few eV. Due to its efficiency, the validated RPP-MD provides an avenue for reducing errors due to finite-size effects that can be on the order of ∼20%.

Unveiling the strong interaction among hadrons at the LHC

One of the key challenges for nuclear physics today is to understand from first principles the effective interaction between hadrons with different quark content. First successes have been achieved using techniques that solve the dynamics of quarks and gluons on discrete space-time lattices1,2. Experimentally, the dynamics of the strong interaction have been studied by scattering hadrons off each other. Such scattering experiments are difficult or impossible for unstable hadrons3–6 and so high-quality measurements exist only for hadrons containing up and down quarks7. Here we demonstrate that measuring correlations in the momentum space between hadron pairs8–12 produced in ultrarelativistic proton–proton collisions at the CERN Large Hadron Collider (LHC) provides a precise method with which to obtain the missing information on the interaction dynamics between any pair of unstable hadrons. Specifically, we discuss the case of the interaction of baryons containing strange quarks (hyperons). We demonstrate how, using precision measurements of proton–omega baryon correlations, the effect of the strong interaction for this hadron–hadron pair can be studied with precision similar to, and compared with, predictions from lattice calculations13,14. The large number of hyperons identified in proton–proton collisions at the LHC, together with accurate modelling15 of the small (approximately one femtometre) inter-particle distance and exact predictions for the correlation functions, enables a detailed determination of the short-range part of the nucleon-hyperon interaction.

Rheology of high-aspect-ratio nanocarbons dispersed in a low-viscosity fluid

Printing inks typically consist of a functional component dispersed within a low-viscosity resin/solvent system where interparticle interactions would be expected to play a significant role in dispersion, especially for the high-aspect-ratio nanocarbons such as the graphite nanoplatelets (GNPs). Rheology has been suggested as a method for assessing the dispersion of carbon nanomaterials in a fluid. The effects of phase volume of ammonia plasma-functionalized GNPs on a near-Newtonian low-viscosity thermoplastic polyurethane (TPU) resin system have been studied using shear and quiescent oscillatory rheology. At low concentrations, the GNPs were well dispersed with a similar shear profile and viscoelastic behavior to the unfilled TPU resin, as viscous behavior prevailed indicating the absence of any long-range order within the fluid. Particle interactions increased rapidly as the phase volume tended toward maximum packing fraction, producing rapid increases in the relative viscosity, increased low shear rate shear thinning, and the elastic response becoming increasingly frequency independent. The nanoscale dimensions and high-aspect-ratio GNPs occupied a large volume within the flow, while small interparticle distances caused rapid increases in the particle–particle interactions to form flocculates that pack less effectively. Established rheological models were fitted to the experimental data to model the effect of high-aspect-ratio nanocarbon on the viscosity of a low-viscosity system. Using the intrinsic viscosity and the maximum packing fraction as fitting parameters, the Krieger–Dougherty (K–D) model provided the best fit with values. There was good agreement between the estimates of aspect ratio from the SEM images and the predictions of the aspect ratio from the rheological models. The fitting of the K–D model to measured viscosities at various phase volumes could be an effective method in characterizing the shape and dispersion of high-aspect-ratio nanocarbons.

Modeling memory and learning with molecular dynamics

The present study transfers molecular dynamics (MD) in condensed matter physics to simulate human memory. In our MD model, a spatial representation of memory of an object is identified where constituent elements of the object are depicted in terms of physical particles. Learning is implemented as attractive forces between particles, which drive particles to gather gradually together and form closely packed clusters eventually. With this, the robustness of the memory is elucidated by the total distance between particles within the cluster. Commonly, a well-established memory of an object corresponds to a cluster with small inter-particle distances. As a demonstration, the present MD model captures a set of interference effects, including retroactive interference, proactive interference and interference between concurrently learned materials that are similar to each other. Different from the existing computational models, our MD model, relying essentially on only one parameter of the force between particles, may be useful to simulate and understand diverse phenomena associated with memory and learning.

How gravitational softening affects galaxy stability – I. Linear mode analysis of disc galaxies

Linear perturbation is used to investigate the effect of gravitational softening on the retrieved two-armed spiral eigenmodes of razor-thin stellar discs. We explore four softening kernels with different degrees of gravity bias, and with/without compact support (compact in the sense that they yield exactly Newtonian forces outside the softening kernel). These kernels are applied to two disc galaxy models with well-known unsoftened unstable modes. We illustrate quantitatively the importance of a vanishing linear gravity bias to yield accurate frequency estimates of the unstable modes. As such, Plummer softening, while very popular amongst simulators, performs poorly in our tests. The best results, with excellent agreement between the softened and unsoftened mode properties, are obtained with softening kernels that have a reduced gravity bias, obtained by compensating for the sub-Newtonian forces at small interparticle distances with slightly super-Newtonian forces at radii near the softening length. We present examples of such kernels that, moreover, are analytically simple and computationally cheap. Finally, these results light the way to the construction of softening methods with even smaller gravity bias, although at the price of increasingly complex kernels.