Fabrication of Ellipsoidal Mesostructures in Block Copolymers via a Step-Shear Deformation

Ellipsoids have attracted abiding attention because of their shape-dependent, anisotropic properties. In some applications, e.g., photonic crystals, both positional and orientational order of the ellipsoidal packing are required. We propose a versatile, facile, and efficient strategy to fabricate positionally and orientationally ordered crystals of soft ellipsoids in block copolymers via a step-shear deformation. Starting from the thermodynamically stable, equilibrium spherical mesophase of the copolymer material, the step-shear deformation provides an instantaneous, anisotropic stimulus to deform the spherical domains into ellipsoids and simultaneously stretches the macromolecular conformations. Subsequently, at fixed strain, the molecular stress relaxes to an equilibrium where the shape and orientation of the obtained ellipsoids are dictated by the packing frustration. Since the residual molecular stress is minuscule, the lattice relaxation via slippage in the absence of external stress is protracted, i.e., the crystal of soft ellipsoids with positional and orientational order is pseudometastable. Our strategy also allows for low volume fractions of ellipsoids (compared to colloidal systems). Both single-chain-in-mean-field (SCMF) simulations and self-consistent field theory (SCFT) calculations are employed to demonstrate the pseudometastability of the obtained ellipsoids. Varying the magnitude of the step-shear strain and the composition of the block copolymer, we can control the asphericity and orientation of the ellipsoidal domains independently. Our study provides a new concept for fabricating soft, positionally and orientationally ordered crystals of ellipsoids with potential applications in engineering functional materials

Uniform Distance Scaling Behavior of Planet–Satellite Nanostructures Made by Star Polymers

Planet–satellite nanostructures from RAFT star polymers and larger (planet) as well as smaller (satellite) gold nanoparticles are analyzed in experiments and computer simulations regarding the influence of arm number of star polymers. A uniform scaling behavior of planet–satellite distances as a function of arm length was found both in the dried state (via transmission electron microscopy) after casting the nanostructures on surfaces and in the colloidally dispersed state (via simulations and small-angle X-ray scattering) when 2-, 3-, and 6-arm star polymers were employed. This indicates that the planet–satellite distances are mainly determined by the arm length of star polymers. The observed discrepancy between TEM and simulated distances can be attributed to the difference of polymer configurations in dried and dispersed state. Our results also show that these distances are controlled by the density of star polymers end groups, and the number of grabbed satellite particles is determined by the magnitude of the corresponding density. These findings demonstrate the feasibility to precisely control the planet–satellite structures at the nanoscale

Free Energy of Defects in Ordered Assemblies of Block Copolymer Domains

We investigate commonly occurring defects in block copolymer thin films assembled on chemically nanopatterned substrates and predict their probability of occurrence by computing their free energies. A theoretically informed 3D coarse grain model is used to describe the system. These defects become increasingly unstable as the strength of interactions between the copolymer and the patterned substrate increases and when partial defects occur close to the top surface of the film. The results presented here reveal an extraordinarily large thermodynamic driving force for the elimination of defects. When the characteristics of the substrate are commensurate with the morphology of the block copolymer, the probability of creating a defect is extremely small and well below the specifications of the semiconductor industry for fabrication of features having characteristic dimensions on the scale of tens of nanometers. We also investigate how the occurrence of defect changes when imperfections arise in the underlying patterns and find that, while defects continue to be remarkably unstable, stretched patterns are more permissive than compressed patterns

Dynamics and Structure of Monolayer Polymer Crystallites on Graphene

Graphene-based nanostructured systems and van der Waals heterostructures comprise a material class of growing technological and scientific importance. Joining materials with vastly different properties, polymer/graphene heterosystems promise diverse applications in surface and nanotechnology, including photovoltaics or nanotribology. Fundamentally, molecular adsorbates are prototypical systems to study confinement-induced phase transitions exhibiting intricate dynamics, which require a comprehensive understanding of the dynamical and static properties on molecular time and length scales. Here, we investigate the dynamics and the structure of a single polyethylene chain on free-standing graphene by means of molecular dynamics simulations. In equilibrium, the adsorbed polymer is orientationally linked to the graphene as two-dimensional folded-chain crystallite or at elevated temperatures as a floating solid. The associated superstructure can be reversibly melted on a picosecond time scale upon quasi-instantaneous substrate heating, involving ultrafast heterogeneous melting via a transient floating phase. Our findings elucidate time-resolved molecular-scale ordering and disordering phenomena in individual polymers interacting with solids, yielding complementary information to collective friction and viscosity, and linking to recent experimental observables from ultrafast electron diffraction. We anticipate that the approach will help in resolving nonequilibrium phenomena of hybrid polymeric systems over a broad range of time and length scales

Photoluminescence Quantum Yield and Matrix-Induced Luminescence Enhancement of Colloidal Quantum Dots Embedded in Ionic Crystals

The incorporation of colloidal quantum dots (QDs) into solid matrices, especially ionic salts, holds several advantages for industrial applications. Here, we demonstrated via absolute measurements of photoluminescence quantum yields (PL-QY) that the photoluminescence of aqueous CdTe QDs can be considerably increased upon incorporation into a salt matrix with a simple crystallization procedure. Enhancement factors of up to 2.8 and a PL-QY of 50 to 80%, both in NaCl crystals and incorporated in silicone matrices, were reached. The fact that the achievable PL enhancement factors depend strongly on PL-QY of the parent QDs can be described by the change of the dielectric surrounding and the passivation of the QD surface, modifying radiative and nonradiative rate constants. Time-resolved PL measurements revealed noncorrelating PL lifetimes and PL-QY, suggesting that weakly emissive QDs of the ensemble are more affected by the enhancement mechanism, thereby influencing PL-QY and PL lifetime in a different manner

Colloidal Nanocrystals Embedded in Macrocrystals: Robustness, Photostability, and Color Purity

The incorporation of colloidal quantum dots (QDs) into ionic crystals of various salts (NaCl, KCl, KBr, etc.) is demonstrated. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Moreover, the ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment and as a result providing them with extraordinary high photo- and chemical stability. A prototype of a white light-emitting diode (WLED) with a color conversion layer consisting of this kind of mixed crystals is demonstrated. These materials may also find applications in nonlinear optics and as luminescence standards

Kinetics of Nanoscale Self-Assembly Measured on Liquid Drops by Macroscopic Optical Tensiometry: From Mercury to Water and Fluorocarbons

Various molecules are known to form self-assembled monolayers (SAMs) on the surface of liquids. We present a simple method of investigating the kinetics of such SAM formation on sessile drops of various liquids such as mercury, water and fluorocarbon. To measure the surface tension of the drops we used an optical tensiometer that calculates the surface tension from the axisymmetric drop shape and the Young–Laplace relation. In addition, we estimated the SAM surface coverage fraction from the surface tension measured by other techniques. With this methodology we were able to optically detect concentrations as low as tenths of ppb increments of SAM molecules in solution and to compare the kinetics of SAM formation measured as a function of molecule concentration or chain length. The analysis is performed in detail for the case of alkanethiols on mercury and then shown to be more general by investigating the case of SAM formation of stearic acid on a water droplet in hexadecane and of perfluorooctanol on a Fluorinert FC-40 droplet in ethanol

Simulation of Defect Reduction in Block Copolymer Thin Films by Solvent Annealing

Solvent annealing provides an effective means to control the self-assembly of block copolymer (BCP) thin films. Multiple effects, including swelling, shrinkage, and morphological transitions, act in concert to yield ordered or disordered structures. The current understanding of these processes is limited; by relying on a theoretically informed coarse-grained model of block copolymers, a conceptual framework is presented that permits prediction and rationalization of experimentally observed behaviors. Through proper selection of several process conditions, it is shown that a narrow window of solvent pressures exists over which one can direct a BCP material to form well-ordered, defect-free structures