Frenkel excitons in heat-stressed supramolecular nanocomposites enabled by tunable cage-like scaffolding.

Delocalized Frenkel excitons-coherently shared excitations among chromophores-are responsible for the remarkable efficiency of supramolecular light-harvesting assemblies within photosynthetic organisms. The translation of nature's design principles to applications in optoelectronic devices has been limited by the fragility of the supramolecular structures used and the delicate nature of Frenkel excitons, particularly under mildly changing solvent conditions and elevated temperatures and upon deposition onto solid substrates. Here, we overcome those functionalization barriers through composition of stable supramolecular light-harvesting nanotubes enabled by tunable (~4.3-4.9 nm), uniform (±0.3 nm) cage-like scaffolds. High-resolution cryogenic electron microscopy, combined with scanning electron microscopy, broadband femtosecond transient absorption spectroscopy and near-field scanning optical microscopy revealed that excitons within the cage-like scaffolds are robust, even under extreme heat stress, and control over nanocomposite dimensions is maintained on solid substrates. Our bio-inspired nanocomposites provide a general framework for the development of next-generation organic devices made from stable supramolecular materials.

Self-assembly of magnetic colloids with radially shifted dipoles.

Anisotropic potentials in Janus colloids provide additional freedom to control particle aggregation into structures of different sizes and morphologies. In this work, we perform Brownian dynamics simulations of a dilute suspension of magnetic spherical Janus colloids with their magnetic dipole moments shifted radially towards the surface of the particle in order to gain valuable microstructural insight. Properties such as the mean cluster size, orientational ordering, and nucleation and growth are examined dynamically. Differences in the structure of clusters and in the aggregation process are observed depending on the dipolar shift (s)-the ratio between the displacement of the dipole and the particle radius-and the dipolar coupling constant (λ)-the ratio between the magnetic dipole-dipole and Brownian forces. Using these two dimensionless quantities, a structural "phase" diagram is constructed. Each phase corresponds to unique nucleation and growth behavior and orientational ordering of dipoles inside clusters. At small λ, the particles aggregate and disaggregate resulting in short-lived clusters at small s, while at high s the particles aggregate in permanent triplets (long-lived clusters). At high λ, the critical nuclei formed during the nucleation process are triplets and quadruplets with unique orientational ordering. These small clusters then serve as building blocks to form larger structures, such as single-chain, loop-like, island-like, worm-like, and antiparallel-double-chain clusters. This study shows that dipolar shifts in colloids can serve as a control parameter in applications where unique size, morphology, and aggregation kinetics of clusters are required.

Broadband chiral hybrid plasmon modes on nanofingernail substrates.

There is significant interest in the utility of asymmetric nanoaperture arrays as substrates for the surface-enhanced detection, fluorescence, and imaging of individual molecules. This work introduces obliquely-cut, out-of-plane, coaxial layered structures on an aperture edge. We refer to these structures as nanofingernails, which emphasizes their curved, oblique, and out-of-plane features. Broadband coupling into chiral hybrid plasmon modes and helicity-dependent near-field scattering without circular dichroism are demonstrated. The unusually-broadband, multipolar modes of nanofingernail micropore structures exhibit phase retardation effects that may be useful for achieving spatial overlap at different frequencies. The nanofingernail geometry shows new potential for simultaneous polarization-enhanced hyperspectral imaging on apertured, plasmonic surfaces.

Floor- or Ceiling-Sliding for Chemically Active, Gyrotactic, Sedimenting Janus Particles.

Chemically active particles achieve motility without external forces and torques ("self-propulsion") due to catalytic chemical reactions at their surfaces, which change the chemical composition of the surrounding solution (called "chemical field") and induce hydrodynamic flow of the solution. By coupling the distortions of these fields back to its motion, a chemically active particle experiences an effective interaction with confining surfaces. This coupling can lead to a rich behavior, such as the occurrence of wall-bound steady states of "sliding". Most active particles are density mismatched with the solution and, thus, tend to sediment. Moreover, the often employed Janus spheres, which consist of an inert core material decorated with a cap-like, thin layer of a catalyst, are gyrotactic (i.e., "bottom-heavy"). Whether or not they may exhibit sliding states at horizontal walls depends on the interplay between the active motion and the gravity-driven sedimentation and alignment, such as the gyrotactic tendency to align the axis along the gravity direction being overcome by a competing, activity-driven alignment with a different orientation. It is therefore important to understand and quantify the influence of these gravity-induced effects on the behavior of model chemically active particles moving in the vicinity of walls. For model gyrotactic, self-phoretic Janus particles, here we study theoretically the occurrence of sliding states at horizontal planar walls that are either below ("floor") or above ("ceiling") the particle. We construct "state diagrams" characterizing the occurrence of such states as a function of the sedimentation velocity and of the gyrotactic response of the particle, as well as of the phoretic mobility of the particle. We show that in certain cases sliding states may emerge simultaneously at both the ceiling and the floor, while the larger part of the experimentally relevant parameter space corresponds to particles that would exhibit sliding states only either at the floor or at the ceiling-or there are no sliding states at all. These predictions are critically compared with the results of previous experimental studies, as well as with our dedicated experiments carried out with Pt-coated, polystyrene-core, or silica-core Janus spheres immersed in aqueous hydrogen peroxide solutions.

Surface tension anomaly observed for chemically-modified Janus particles at the air/water interface.

The behavior of Janus particles fabricated from core silica particles decorated with gold nanoparticles on one hemisphere is studied at the air/water interface. An unexpected reduction in the effective surface tension is observed in the presence of these chemically-modified Janus particles. Experiments on the interfacial behavior of a variety of control particles, including the physically-modified Janus particles made from the same core silica particles coated with a thin gold layer, do not exhibit significant surface tension effects. We hypothesize that the chemical modification of particles in form of a Janus structure is needed to alter the surface tension and attribute the surfactant-like behavior of these particles to the presence of immersion forces.

Effect of Orientation and Wetting Properties on the Behavior of Janus Particles at the Air-Water Interface.

The adhesion force and contact angle of gold-capped silica Janus particles and plain silica particles at an air-water interface are studied via colloidal atomic force microscopy. Particles are attached to cantilevers at various orientations, and wetting properties of the gold surface are varied through modification with dodecanethiol. Thiol modification increases the hydrophobicity of the gold surface, thereby increasing the difference between the contact angles of the gold hemisphere and the silica hemisphere and, thus, increasing the degree of amphiphilicity of the Janus particle. Subsequently, the colloidal probe is pushed into a stationary bubble from the water phase followed by retraction back into the water phase. Adhesion force is found to be higher for Janus particles than isotropic silica particles, regardless of orientation of the anisotropic hemisphere. Particles with their polar half oriented toward the water and apolar half facing the air show an increase in adhesion force and contact angle as the degree of amphiphilicity of the particles increases. For particles of the reverse orientation, no significant difference is observed as wetting properties change. Both adhesion force and contact angle display an inverse relationship with a cap angle for particles with a higher degree of amphiphilicity. These results are of importance for using Janus particles to stabilize interfaces as well as for understanding the equilibrium height of Janus particles at the interface, which will impact capillary interactions and thus self-assembly.

Impact of Surface Amphiphilicity on the Interfacial Behavior of Janus Particle Layers under Compression.

Langmuir monolayers of silica/gold Janus particles with two different degrees of amphiphilicity have been examined to study the significance of particle surface amphiphilicity on the structure and mechanical properties of the interfacial layers. The response of the layers to the applied compression provides insight into the nature and strength of the interparticle interactions. Different collapse modes observed for the interfacial layers are linked to the amphiphilicity of Janus particles and their configuration at the interface. Molecular dynamics simulations on nanoparticles with similar contact angles provide insight on the arrangement of particles at the interface and support our conclusion that the interfacial configuration and collapse of anisotropic particles at the air/water interface are controlled by particle amphiphilicity.

Measuring, Modeling, and Predicting the Magnetic Assembly Rate of 2D-Staggered Janus Particle Chains.

The assembly of magnetic Janus particles in a quasi-two-dimensional environment with a dipole moment shifted from the center and oriented perpendicular to the Janus cap height is studied with optical microscopy and found to adhere to a general model accounting for the particle dipole strength, the particle Brownian dynamics, the initial concentration, and, most importantly, the magnetic dipole shift. The particle aggregates are treated as diffusing spherocylinders with length and width dependent on the magnetic dipole shift. Aggregation occurs irreversibly once particle aggregates enter within a distance at which Brownian and dipole forces are equal, defined as the capture distance. The capture distance model is expressed as a general Smoluchowski coagulation rate kernel for chains of an arbitrary length, dipole strength, and dipole shift, allowing for aggregation rate predictions for related systems.

Self-assembly of magnetic colloids with shifted dipoles.

The self-assembly of colloidal magnetic Janus particles with a laterally displaced (or shifted), permanent dipole in a quasi-two-dimensional system is studied using Brownian dynamics simulations. The rate of formation of clusters and their structures are quantified for several values of dipolar shift from the particle center, which is nondimensionalized using the particle's radius so that it takes values ranging from 0 to 1, and examined under different magnetic interaction strengths relative to Brownian motion. For dipolar shifts close to 0, chain-like structures are formed, which grow at long times following a power law, while particles of shift higher than 0.2 generally aggregate in ring-like clusters that experience limited growth. In the case of shifts between 0.4 and 0.5, the particles tend to aggregate in clusters of 3 to 6, while for all shifts higher than 0.6 clusters rarely contain more than 3 particles due to the antiparallel dipole orientations that are most stable at those shifts. The strength of the magnetic interactions hastens the rate at which clusters are formed; however, the effect it has on cluster size is lessened by increases in the shift of the dipoles. These results contribute to better understand the dynamics of magnetic Janus particles and can help the synthesis of functionalized materials for specific applications such as drug delivery.

Experimental Study of the Motion of Patchy Particle Swimmers Near a Wall.

In this work, we demonstrate our ability to precisely tailor the surface activity of self-propelled active colloids by varying the size of the active area. The quasi two-dimensional autonomous motion of spherical patchy particle swimmers is studied in a chemical environment in the vicinity of a solid boundary. Oxidative decomposition of hydrogen peroxide into oxygen and water occurs only on a well-defined Pt-coated section of the polystyrene particle surface. The asymmetric distribution of product molecules interacting with the particle leads to the autonomous motion, which is characterized as the patch size varies from 11 to 25 to 50% of the particle surface area. The phoretic motion of patchy particle swimmers is analytically predicted by a model developed by Popescu et al. and shows good agreement with the experimentally observed velocities when the influence of the wall on the preferential rotational motion of the particles near the solid boundary is considered. The study illustrates the potential to precisely engineer the motion of particles by controlling their properties rather than depending on changes in the environment.

Investigation on electrical surface modification of waste to energy ash for possible use as an electrode material in microbial fuel cells.

With the world population expected to reach 8.5 billion by 2030, demand for access to electricity and clean water will grow at unprecedented rates. Municipal solid waste combusted at waste to energy (WtE) facilities decreases waste volume and recovers energy, but yields ash as a byproduct, the beneficial uses of which are actively being investigated. Ash is intrinsically hydrophobic, highly oxidized, and exhibits high melting points and low conductivities. The research presented here explores the potential of ash to be used as an electrode material for a microbial fuel cell (MFC). This application requires increased conductivity and hydrophilicity, and a lowered melting point. Three ash samples were investigated. By applying an electric potential in the range 50-125 V across the ash in the presence of water, several key property changes were observed: lower melting point, a color change within the ash, evidence of changes in surface morphologies of ash particles, and completely wetting water-ash contact angles. We analyzed this system using a variety of analytical techniques including sector field inductively coupled plasma mass spectrometry, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, and tensiometry. Ability to make such surface modifications and significant property changes could allow ash to become useful in an application such as an electrode material for a MFC.

Directed Motion of Metallodielectric Particles by Contact Charge Electrophoresis.

We investigate the dynamics of metallodielectric Janus particles moving via contact charge electrophoresis (CCEP) between two parallel electrodes. CCEP uses a constant voltage to repeatedly charge and actuate conductive particles within a dielectric fluid, resulting in rapid oscillatory motion between the electrodes. In addition to particle oscillations, we find that micrometer-scale Janus particles move perpendicular to the field at high speeds (up to 600 µm/s) and over large distances. We characterize particle motions and propose a mechanism based on the rotation-induced translation of the particle following charge transfer at the electrode surface. The propulsion mechanism is supported both by experiments with fluorescent particles that reveal their rotational motions and by simulations of CCEP dynamics that capture the relevant electrostatics and hydrodynamics. We also show that interactions among multiple particles can lead to repulsion, attraction, and/or cooperative motions depending on the position and phase of the respective particle oscillators. Our results demonstrate how particle asymmetries can be used to direct the motions of active colloids powered by CCEP.

Control of photo-induced voltages in plasmonic crystals via spin-orbit interactions.

There is wide interest in understanding and leveraging the nonlinear plasmon-induced potentials of nanostructured materials. We investigate the electrical response produced by spin-polarized light across a large-area bottom-up assembled 2D plasmonic crystal. Numerical approximations of the Lorentz forces provide quantitative agreement with our experimentally-measured DC voltages. We show that the underlying mechanism of the spin-polarized voltages is a gradient force that arises from asymmetric, time-averaged hotspots, whose locations shift with the chirality of light. Finally, we formalize the role of spin-orbit interactions in the shifted intensity patterns and significantly advance our understanding of the physical phenomena, often related to the spin Hall effect of light.

Mechanical Stability of Polystyrene and Janus Particle Monolayers at the Air/Water Interface.

The compressional instability of particle-laden air/water interfaces is investigated with plain and surface-anisotropic (Janus) particles. We hypothesize that the amphiphilic nature of Janus particles leads to both anisotropic particle-particle and particle-interface interactions that can yield particle films with unique collapse mechanisms. Analysis of Langmuir isotherms and microstructural characterization of the homogeneous polystyrene particle films during compression reveal an interfacial buckling instability followed by folding, which is in good agreement with predictions from classical elasticity theory. In contrast, Janus particle films exhibit a different behavior during compression, where the collapse mode occurs through the subduction of the Janus particle film. Our results suggest that particle-laden films comprised of surface-anisotropic particles can be engineered to evolve new material properties.

Collapse of Particle-Laden Interfaces under Compression: Buckling vs Particle Expulsion.

Colloidal particles can bind to fluid interfaces with a capillary energy that is thousands of times the thermal energy. This phenomenon offers an effective route to emulsion and foam stabilization where the stability is influenced by the phase behavior of the particle-laden interface under deformation. Despite the vast interest in particle-laden interfaces, the key factors that determine the collapse of such an interface under compression have remained relatively unexplored. In this study, we illustrate the significance of the particle surface wettability and presence of electrolyte in the subphase on interparticle interactions at the interface and the resulting collapse mode. Various collapse mechanisms including buckling, particle expulsion, and multilayer formation are reported and interpreted in terms of particle-particle and particle-interface interactions.

Molecular dynamics simulations: insight into molecular phenomena at interfaces.

Molecular dynamics simulations, when aptly devised, can enhance our fundamental understanding of a system, set up a platform for testing theoretical predictions, and provide insight and a framework for further experimental studies. This feature article highlights the importance of molecular dynamics simulations in understanding interfacial phenomena using three case studies involving liquid-liquid and solid-liquid interfaces. After briefly reviewing molecular dynamics methods, we discuss velocity slip at a liquid-liquid interface, the coalescence of liquid drops in suspension and in free space, and the behavior of colloidal nanoparticles at a liquid-liquid interface. We emphasize the utility of simple intermolecular potentials and generic liquids. The case studies exemplify the significant insight gained through the molecular modeling approach regarding the interfacial phenomena studied. We conclude the highlight with a brief discussion illustrating potential shortcomings and pitfalls of molecular dynamics simulations.

Nanoparticles at liquid interfaces: rotational dynamics and angular locking.

Nanoparticles with different surface morphologies that straddle the interface between two immiscible liquids are studied via molecular dynamics simulations. The methodology employed allows us to compute the interfacial free energy at different angular orientations of the nanoparticle. Due to their atomistic nature, the studied nanoparticles present both microscale and macroscale geometrical features and cannot be accurately modeled as a perfectly smooth body (e.g., spheres and cylinders). Under certain physical conditions, microscale features can produce free energy barriers that are much larger than the thermal energy of the surrounding media. The presence of these energy barriers can effectively "lock" the particle at specific angular orientations with respect to the liquid-liquid interface. This work provides new insights on the rotational dynamics of Brownian particles at liquid interfaces and suggests possible strategies to exploit the effects of microscale features with given geometric characteristics.

Template-assisted GLAD: approach to single and multipatch patchy particles with controlled patch shape.

Template-assisted glancing angle deposition (GLAD) is explored for the fabrication of single and multipatch patchy particles with one or more patches of controlled but asymmetric shape. The template is used to ensure the formation of uniform patchy particles, whereas rotation of the template gives access to a large number of asymmetric patch shapes caused by the shadowing effect of the templating groove and/or the neighboring particle. Careful analysis with a straightforward geometric model reveals the effect of the angle of incidence, rotational angle, groove size, and particle size on the patch shape. Initial magnetic field assembly results are presented to illustrate the removal of patchy particles from their template and accessibility to a large number of patchy particles. Two-patch particles with overlapping patches are also accessible by means of secondary metal vapor deposition. The connectivity of these two patches on each particle and the predictable size of the overlapping section provide access to functional patchy particles. The combination of the template-assisted GLAD method with rotation of the template and secondary evaporation is demonstrated to be a good method for fabricating patchy particles with a variety of asymmetric patch shapes, sizes, and multipatches where every particle of a batch carries exactly the same patch pattern and thereby provides valuable input on experimentally accessible patch shapes for future experimental and computational studies of patchy particles.

Viscosity-dependent Janus particle chain dynamics.

Iron oxide (Fe3O4) Janus particles assemble into staggered chains parallel to the field lines in an ac electric field. Subsequent application of an external magnetic field leads to contraction of the staggered chains into double chains. The relation between the viscosity of the surrounding solution and the contraction rate of the iron oxide Janus particle chains is studied. Further, the influence of particle size and chain length (i.e., number of particles in chain) on the contraction rate is investigated. The base material for the Janus structure is silica (SiO2) with particle sizes of 1, 2, and 4 µm, and the cap material is Fe3O4. Addition of increasing amounts of glycerol to the aqueous system reveals that the contraction dynamics strongly correlate with the viscosity of the solution. The average chain contraction rate for each particle size can be fitted in the low viscosity range from 1 to 30 mPa·s with a power function of the form A/µ(0.9) - B/µ, in which the coefficients A and B are particle size, electric field, and magnetic-field-dependent constants. Using this function, the viscosity of an unknown solution can be determined, thereby pointing to the potential application of these Janus particle chain assemblies as in situ microviscometers.