Interparticle Distance Decreases Research Articles

Slow Hot-Exciton Cooling and Enhanced Interparticle Excitonic Coupling in HgTe Quantum Dots.

Rapid hot-carrier/exciton cooling constitutes a major loss channel for photovoltaic efficiency. How to decelerate the hot-carrier/exciton relaxation remains a crux for achieving high-performance photovoltaic devices. Here, we demonstrate slow hot-exciton cooling that can be extended to hundreds of picoseconds in colloidal HgTe quantum dots (QDs). The energy loss rate is 1 order of magnitude smaller than bulk inorganic semiconductors, mediated by phonon bottleneck and interband biexciton Auger recombination (BAR) effects, which are both augmented at reduced QD sizes. The two effects are competitive with the emergence of multiple exciton generation. Intriguingly, BAR dominates even under low excitation fluences with a decrease in interparticle distance. Both experimental evidence and numerical evidence reveal that such efficient BAR derives from the tunneling-mediated interparticle excitonic coupling induced by wave function overlap between neighboring HgTe QDs in films. Thus, our study unveils the potential for realizing efficient hot-carrier/exciton solar cells based on HgTe QDs. Fundamentally, we reveal that the delocalized nature of quantum-confined wave function intensifies BAR. The interparticle excitonic coupling may cast light on the development of next-generation photoelectronic materials, which can retain the size-tunable confinement of colloidal semiconductor QDs while simultaneously maintaining high mobilities and conductivities typical for bulk semiconductor materials.

Transformations in crystals of DNA-functionalized nanoparticles by electrolytes.

Colloidal crystals have applications in water treatments, including water purification and desalination technologies. It is, therefore, important to understand the interactions between colloids as a function of electrolyte concentration. We study the assembly of DNA-grafted gold nanoparticles immersed in concentrated electrolyte solutions. Increasing the concentration of divalent Ca2+ ions leads to the condensation of nanoparticles into face-centered-cubic (FCC) crystals at low electrolyte concentrations. As the electrolyte concentration increases, the system undergoes a phase change to body-centered-cubic (BCC) crystals. This phase change occurs as the interparticle distance decreases. Molecular dynamics analysis suggests that the interparticle interactions change from strongly repulsive to short-range attractive as the divalent-electrolyte concentration increases. A thermodynamic analysis suggests that increasing the salt concentration leads to significant dehydration of the nanoparticle environment. We conjecture that the intercolloid attractive interactions and dehydrated states favour the BCC structure. Our results gain insight into salting out of colloids such as proteins as the concentration of salt increases in the solution.

Concentration dependence of yield stress of bentonite suspension and corresponding particle interactions

The prediction of the concentration dependence of the yield stress of bentonite suspension is important for both practical use and fundamental study. Whilst a great amount of research work has been undertaken concerning the theoretical prediction based on the Face-Face repulsive interactions alone, the effect of inclination angle α and separation distance D between clay particles has seemingly received little attention in the research literature. This article presents the prediction model of yield stress developed using the strength of interparticle interactions between two non-parallel platy particles of finite dimensions, aimed at providing insights into the evolution of interparticle interaction with increasing the mass fraction. The repulsive force between the particles for a range of values of geometrical parameters computed using the Gouy-Chapman theory of electrical double layer, along with the calculation presented on van der Waals force, provide a more quantitative means of evaluating the interparticle force in bentonite suspension. It was found that as α and D between particles increase, the magnitude of interparticle force including repulsive and attractive force decreases quickly. The ultimate comprehensive investigation indicated that as the mass fraction of bentonite suspension increases, interparticle interaction propagates from attractive interaction to repulsive attraction, and finally shifts to attractive interaction. In this process, inter-particle separation distance decreases, accompanied by a transition process from a larger inclination angle to a nearly parallel orientation between the particles. The prediction model of yield stress considering the influence of concentration variation on α and D yields a good prediction of experimental data. The results of the present study should help interpret test results in cases where it may not be entirely possible to maintain a parallel particle orientation.

Plasmon-induced excitation energy transfer in silver nanoparticle dimers: A real-time TDDFTB investigation.

Using real-time quantum dynamics calculations, we perform theoretical investigations of light-induced interactions and electronic excitation transfer in a silver nanoparticle dimer. Real-time time-dependent density functional tight-binding (RT-TDDFTB) calculations provide details of the quantum dynamical processes at an electronic/atomistic level with attosecond resolution. The computational efficiency of RT-TDDFTB allows us to examine electronic dynamics up to picosecond time scales. With time scales varying over six orders of magnitude, we provide insight into interactions between the nanoparticle and laser and between nanoparticles. Our results show that the coupling between nanoparticle monomers is dependent on the separation distance between the nanoparticles in the dimer. As the interparticle distance is varied, the dipole-dipole interactions and electronic excitation transfer mechanisms are markedly different. At large distances (from 50 to 20 Å), the energy transfer from NP1 to NP2 becomes more efficient as the interparticle distance decreases. The total dipole moment of the Ag14 nanoparticle dimer increases linearly at an interparticle distance of 20 Å and reaches its maximum after 1.2ps. The electronic excitation transfer is also the most efficient at 20 Å. At short distances, back-transfer effects reduce the ability of the dimer and NP1 to accept energy from the incident electric field. We attribute the distance-dependent features of the nanoparticle dimer to the beating between the laser acting on NP1 and the back transfer from NP2 to NP1.

TD-DFTB study of optical properties of silver nanoparticle homodimers and heterodimers.

The absorption spectra for face-centered cubic nanoparticle dimers at various interparticle distances are investigated using time-dependent density functional tight binding. Both homodimers and heterodimers are investigated in this work. By studying nanoparticles at various interparticle distances and analyzing their vertical excitations, we found that as the interparticle distance decreases, a red shift arises from contributions of the transition dipole moment that are aligned along the z-axis with nondegenerate features; blue shifts occur for peaks that originate from transition dipole moment components in the x and y directions with double degeneracy. When the nanoparticles are similar in size, the features in the absorption spectra become more sensitive to the interparticle distances. The best-fit curves from vertical excitation energy in the form of AR-b for ΔEredshift/ΔEblueshift vs R are determined. In this way, we determined trends for absorption peak shifts and how these depend on the interparticle distance.

Preparation of PBT/POE‐g‐GMA/PP ternary blends with good rigidity–toughness balance by core–shell particles

ABSTRACTCompared with poly(butylene terephthalate)/glycidyl methacrylate grafted poly(ethylene–octene) (PBT/POE‐g‐GMA) binary blends, supertough PBT‐based ternary blends with little rigidity loss were successfully obtained by adding rigid polypropylene (PP) into PBT/POE‐g‐GMA blends to construct core–shell particles during melt blending. The effects of PP content and type on the phase morphology and mechanical properties of the blends were systematically investigated. Theoretical predictions and scanning electron microscopy observation showed that a core–shell structure was formed in PBT matrix with PP as the core and POE‐g‐GMA as the shell. The mechanical property tests showed that POE‐g‐GMA and PP had significant synergistic toughening effect. When PP with high melt flow index (H‐PP) was used, PBT/POE‐g‐GMA/H‐PP (70/15/15) blends possessed the highest Izod notched impact strength, which was 1.9‐fold compared with PBT/POE‐g‐GMA (70/30) binary blends, while the tensile performance loss was little. The essential work of fracture tests was performed to evaluate the fracture resistance of different samples. The results demonstrated that PBT/POE‐g‐GMA/PP ternary blends possessed much better resistance to crack propagation than PBT/POE‐g‐GMA binary blends. The decrease of interparticle distance and the fibrillation of core–shell particles activated intense matrix shear yielding, which was the reason for the high crack resistance of ternary blends. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48872.

Blocking and remanence properties of weakly and highly interactive cobalt ferrite based nanoparticles

We compare both magnetic blocking properties and remanence curves for dilute ferrofluid and powder samples of ferrite magnetic nanoparticles. Low field DC magnetization, AC susceptibility, isothermal remanent magnetization and DC demagnetization techniques are employed to investigate the role of interparticle magnetic interactions on the superparamagnetic relaxation, the magnetic anisotropy and on the super-spin-glass state in closely packed particles. The samples used herein are 3 nm sized spinel-type nanocrystals made of a cobalt ferrite core covered by a layer of maghemite on its outermost surface and can be obtained as aqueous colloidal dispersions thanks to this core-shell strategy. They show large anisotropy attributed to an enhanced surface contribution and the blocking temperature is shifted towards higher values as interparticle distance decreases. For all investigated diluted liquids and powder samples the frequency dependency of the peak temperature is well accounted by a Vogel–Fulcher law, with the insertion of a phenomenological temperature associated to the magnitude of interparticle dipolar interactions. The fractional change of the peak temperature per decade of frequency enlights the presence of interactions between particles in dilute liquids and of a spin-glass-like state in powder samples. The remanence curves always show global demagnetizing behavior, attributed to the combination of both spin surface disorder and interparticle dipolar interactions, the former being predominant in isolated nanoparticles and the latter in powder samples. However, in the most compacted powder, exchange interaction between surface ions of different particles becomes more pronounced and promotes an additive magnetizing effect.

Ion-Induced Formation of Nanocrystalline Cellulose Colloidal Glasses Containing Nematic Domains.

Controlling the assembly of colloids in dispersion is a fundamental approach toward the production of functional materials. Nanocrystalline cellulose (NCC) is a charged nanoparticle whose colloidal interactions can be modulated from repulsive to attractive by increasing ionic strength. Here, we combine polarized optical microscopy, rheology, and small-angle scattering techniques to investigate (i) the concentration-driven transition from isotropic dispersion to cholesteric liquid crystals and (ii) salt-induced NCC phase transitions. In particular, we report on the formation of NCC attractive glasses containing nematic domains. At increasing NCC concentration, a structure peak was observed in small-angle X-ray scattering (SAXS) patterns. The evolution of the structure peak demonstrates the decrease in NCC interparticle distance, favoring orientational order during the isotropic-cholesteric phase transition. Small amounts of salt reduce the cholesteric volume fraction and pitch by a decrease in excluded volume. Beyond a critical salt concentration, NCC forms attractive glasses due to particle caging and reduced motility. This results in a sharp increase in viscosity and formation of viscoelastic glasses. The presence of nematic domains is suggested by the appearance of interference colors and the Cox-Merz rule failure and was confirmed by an anisotropic SAXS scattering pattern at q ranges associated with the presence of nematic domains. Thus, salt addition allows the formation of NCC attractive glasses with mechanical properties similar to those of gels while remaining optically active owed to entrapped nematic domains.

Influence of ion shadowing effect on average inter-particle distance in dusty plasma crystals

Average distance between particles of dust in dusty plasma crystals is sensitive to the temperature of neutral atoms in a glow discharge plasma. In some experiments a significant decrease of average inter-particle distance after cooling to low temperatures is observed: the distance at 100 K is up to 4 times less than at 300 K. In this work dusty plasma system is studied by the method of numerical solving of Newton equations which is similar to the method of molecular dynamics. It is shown that the model of dusty plasma interaction including only the Debye potential and electrostatic parabolic trap may be not enough to explain this effect. Adding ion shadowing potential to this model increases the difference between the values of inter-particle distance at 100 and 300 K by 2–3 times. Comparison of theoretical results with experiments of other groups shows that some of the experimental points may be well approximated by this model.

Plasmon coupling and aging effect in CsCl–Ag thin films

Cesium chloride-silver thin films were prepared on glass substrate by thermal evaporation technique. Optical, structural and morphological properties of as deposited of various thicknesses and aged films were studied. Multiple plasmon bands have been observed in the UV-visible spectra of all the nano-composite films due to formation of near-spherical silver nano-particles. Position and intensity of the peak were found to be a function of particle size and inter-particle distance. The prominent plasmon peak of as deposited films showed a red shift, due to the increase in particle size, with increasing film thickness at the rate of 0.06 nm shift per 1 nm film thickness and peak at higher wavelength showed 0.75 nm red shift per 1 nm change in film thickness due to the decrease in inter-particle distance. Also, in aged sample the film revealed 18 nm blue shift with 216 h of aging due to an increase in inter-particle separation between silver nanoparticles. The manuscript investigates and reports the cause for the changes observed.

In situ X-ray scattering observation of two-dimensional interfacial colloidal crystallization

Charged colloids at interfaces hold such a simple configuration that their interactions are supposed to be fully elucidated in the framework of classical electrostatics, yet the mysterious existence of attractive forces between these like-charged particles has puzzled the scientific community for decades. Here, we perform the in situ grazing-incidence small-angle X-ray scattering study of the dynamic self-assembling process of two-dimensional interfacial colloids. This approach allows simultaneous monitoring of the in-plane structure and ordering and the out-of-plane immersion depth variation. Upon compression, the system undergoes multiple metastable intermediate states before the stable hexagonal close-packed monolayer forms under van der Waals attraction. Remarkably, the immersion depth of colloidal particles is found to increase as the interparticle distance decreases. Numerical simulations demonstrate the interface around a colloid is deformed by the electrostatic force from its neighboring particles, which induces the long-range capillary attraction.

Interaction of silver nanoparticles with Lysozyme: Functional and structural investigations

Biological implications of protein adsorption on nanoparticles’ surfaces make interaction studies of paramount importance. Herein, mutual effects of lysozyme and silver nanoparticles on each counterpart have been studied using spectroscopic techniques. Surface Plasmon Resonance (SPR) studies showed decrease of interparticle distance upon interaction with biomolecule, and Dynamic Light Scattering (DLS) confirmed formation of nanoparticles aggregates. In consistency with SPR and DLS analyses, Scanning Electron Microscopy (SEM) showed that protein concentration plays a critical role either in guiding formation of stable complexes or dictating irreversible aggregations of nanoparticles. Analysis of Circular Dichroism (CD) spectroscopy data showed that protein retained most of its helical structure upon interaction with silver nanoparticles. Kinetic investigations also revealed that lysozyme almost maintained its catalytic activity, together with a slight decrease in the enzyme-substrate binding affinity. Results of this effort encourage combined utility of spectroscopic techniques to gain further insight into challenges at the interface for both bio and nano counterparts, prior to exploit variety of theranostic applications.

Plasmon coupling between silver nanoparticles: Transition from the classical to the quantum regime

We explore plasmon coupling between silver nanoparticles (AgNPs) as two AgNPs approach each other within a subnanometer distance. We prepare AgNP dimers with two 21-nm AgNPs separated by alkanedithiol linkers in high yield. Changing the length of the alkanedithiol linkers enables us to control the interparticle distance down to the subnanometer level on the molecular scale. We observe that the longitudinal plasmon coupling band, which is sensitive to the interaction between AgNPs, gradually redshifts as the interparticle distance decreases. This observation is fully consistent with the classical electromagnetic model. The redshift of the plasmon coupling, however, undergoes a drastic change when the interparticle distance reaches ∼1nm. The longitudinal plasmon coupling band vanishes and a new intense band appears at a shorter wavelength. This band redshifts as the nanogap further narrows, but crosses over to a blueshift at ∼0.7nm. A comparison of our observation with finite-difference time-domain simulations reveals that this band arises from quantum effects. Controlled assembly of AgNP dimers in combination with simulations allows us to observe the transition of the plasmon coupling from the classical to the quantum regime at the ensemble level.

Surface Plasmon Properties of Silver Nanostructures Fabricated Using Extremely Non-Equilibrium, Hot and Dense Plasma

We present fabrication of silver nanostructures on glass substrates using highly energetic and high fluence material ions generated by one shot of hot, dense and extremely non-equilibrium plasma such as found in modified dense plasma focus (DPF) device. The substrates were first placed at 4.0 cm and 6.0 cm from the top of anode. Nanodots and nanocapsules are observed in the scanning electron microscopy (SEM) images of silver ions deposited with DPF shot on glass. The interparticle distance is found to decrease whereas mean size of nanodots is found to increase slightly when the distance of glass substrate is increased from 4.0 cm to 6.0 cm. The X-ray diffraction (XRD) pattern of the fabricated nanostructures has peaks at 2θ equals 37.90o, 44.24o and 64.20o which correspond to (111), (200) and (220) planes of silver having face-centered cubic structure. The nanostructures obtained on glass placed at 4.0 cm show surface plasmon resonance (SPR) peak at 420 nm whereas nanostructures obtained on glass placed at 6.0 cm has a SPR peak at 428 nm. Redshift of the SPR peak is attributed to increased interaction as a result of decrease in interparticle distance of the nanostructures as well as increase in mean size of nanodots.

Toughening with little rigidity loss and mechanism for modified polypropylene by polymer particles with core–shell structure

Toughening with little rigidity loss was achieved by adding high density polyethylene (HDPE) into polypropylene/ethylene-propylene random copolymer (PP/EPR) blends to fabricate a series of PP/EPR/HDPE ternary blends with core–shell dispersed particles. Morphology observations revealed that the addition of HDPE leads to the appearance of core–shell dispersed particles in PP matrix, i.e., HDPE core encapsulated in EPR shell. Dynamic mechanical analysis results showed that the introduction of HDPE could increase glass transition temperature (Tg) and loss factor peak area of EPR in PP/EPR/HDPE blends, which is similar to the effect of adding EPR in PP/EPR. The shrinkage behavior results suggested that the increase of glass transition temperature of EPR was induced by the mismatch of thermal expansion coefficients of components and the larger peak area was ascribed to the stronger relaxation friction of EPR. According to percolation of stress volumes, the interparticle distance was proposed to be a key factor of toughening effect of rubber particles in thermoplastic and thereby a toughening mechanism about the equivalent rubber content was established to explain balanced toughness-strength improvement of PP/EPR/HDPE blends with core–shell particles. The rubber particles with core–shell structure lead to the increase of particle size and the decrease of interparticle distance on the premise of not increasing actual rubber content, resulting in a notable improvement of toughness, while the ‘hard’ core made from HDPE component provides a satisfied rigidity.

A numerical study of the effect of particle properties on the radial distribution of suspensions in pipe flow

We employ a Lagrangian–Lagrangian (LL) numerical formalism to study two- and three-dimensional (2D, 3D) pipe flow of dilute suspensions of macroscopic neutrally buoyant rigid bodies at flow regimes with Reynolds numbers (Re) between 0.1 and 1400. A validation study of particle migration over a wide spectrum of Re and average volumetric concentrations demonstrates the good predictive attributes of the LL approach adopted herein. Using a scalable parallel implementation of the approach, 3D direct numerical simulation is used to show that (1) rigid body rotation affects the behavior of a particle laden flow; (2) an increase in neutrally buoyant particle size decreases radial migration; (3) a decrease in inter-particle distance slows down the migration and shifts the stable position further away from the channel axis; (4) rigid body shape influences the stable radial distribution of particles; (5) particle migration is influenced, both quantitatively and qualitatively, by the Reynolds number; and (6) the stable radial particle concentration distribution is affected by the initial concentration. The parallel LL simulation framework developed herein does not impose restrictions on the shape or size of the rigid bodies and was used to simulate 3D flows of dense, colloidal suspensions of up to 23,000 neutrally buoyant ellipsoids.

Variation in surface plasmonic response due to the reorganization of Au nanoparticles in Langmuir-Blodgett film

Layer-by-layer structures of dodecanethiol-encapsulated Au nanoparticles have been formed on Si(001) and quartz substrates at different surface pressures by Langmuir-Blodgett method. Optical absorption spectra and out-of-plane structural information have been obtained from UV-Vis spectroscopy and X-ray reflectivity measurements, respectively. With time the thickness of the film decreases keeping the layered structure unchanged but finally monolayer like structure forms. Localized surface plasmon resonance peaks obtained from the UV-Vis spectra show that coupling between Au nanoparticles occurs at the initial stage of the reorganization process as the interparticle distance decreases and as a result, a redshift in the plasmon peak wavelength takes place. Maximum redshift occurs for the monolayer and the peak shift linearly decreases for the multilayer structures. After prolonged reorganization when all layered structures transform into monolayer like structure again redshift occurs but in this process the redshift is reverse with respect to the previous one. In the later process, redshift is minimum for the monolayer structure and increases nearly linearly for the multilayer structures. Two different mechanisms responsible for these two processes are proposed.

Spatially Selective Au Nanoparticle Deposition and Raman Analysis of Ion-Irradiated Single-Wall Carbon Nanotubes

Electroless deposition of gold nanoparticles (Au-NPs) onto ion-irradiated single-wall carbon nanotubes (SWCNTs) represents a selective method to spatially profile defects by scanning electron microscopy (SEM). High purity SWCNT papers were irradiated with 150 keV 11B+ and subsequently exposed to 0.01 M KAuBr4(aq) to nucleate Au-NPs. Based on statistical image analysis, a 30 s KAuBr4(aq) exposure is sufficient to indicate the presence of defects by Au-NP nucleation, resulting in particle radii of ∼14 nm and minimal coalescence. A decrease in interparticle distance along individual SWCNT bundles and an increase in Au-NP areal density from 3 to 58 Au-NPs/μm2 are measured over the fluence range of 1 × 1013–1 × 1015/cm2, respectively. Raman analysis shows that the G′-band peak position progressively downshifts for the SWCNTs due to ion irradiation, whereas purified SWCNTs exhibit an upshift from charge transfer with KAuBr4(aq). A strong correlation between the Au-NP areal density from SEM and the relative ratios for the Raman D and G′ peaks confirms the method is directly monitoring ion irradiation-induced structural damage. Overall, the procedure provides a rapid assessment of SWCNT defect density based on microscopy analysis and shows that SWCNTs can be used as a fluence-based dosimeter or adequate support for controlled NP deposition.

Strongly modified four-wave mixing in a coupled semiconductor quantum dot-metal nanoparticle system

We study the four-wave mixing effect in a coupled semiconductor quantum dot-spherical metal nanoparticle structure. Depending on the values of the pump field intensity and frequency, we find that there is a critical distance that changes the form of the spectrum. Above this distance, the four-wave mixing spectrum shows an ordinary three-peaked form and the effect of controlling its magnitude by changing the interparticle distance can be obtained. Below this critical distance, the four-wave mixing spectrum becomes single-peaked; and as the interparticle distance decreases, the spectrum is strongly suppressed. The behavior of the system is explained using the effective Rabi frequency that creates plasmonic metaresonances in the hybrid structure. In addition, the behavior of the effective Rabi frequency is explained via an analytical solution of the density matrix equations.

Interaction of two dielectric macroparticles

The electrostatic interaction of two charged dielectric spherical particles with a nonuniform freecharge distribution over their surfaces in an external homogeneous electric field is considered. An exact solution for the electric field potential is obtained, and an analytical expression for the interaction force between these two particles is found. The case of a uniform free-charge distribution is considered in detail, and the region of parameters in the plane “the ratio of the radii vs. the ratio of the charges,” where repulsion between two like-charged particles turns into attraction as the interparticle distance decreases is established.