Asymmetric van der Waals Forces Drive Orientation of Compositionally Anisotropic Nanocylinders within Smectic Arrays: Experiment and Simulation

Abstract

Understanding how micro- and nanoparticles interact is important for achieving bottom-up assembly of desired structures. Here, we examine the self-assembly of two-component, compositionally asymmetric nanocylinders that sediment from solution onto a solid surface. These particles spontaneously formed smectic arrays. Within the rows of an array, nanocylinders tended to assemble such that neighboring particles had the same orientation of their segments. As a probe of interparticle interactions, we classified nanocylinder alignments by measuring the segment orientations of many sets of neighboring particles. Monte Carlo simulations incorporating an exact expression for the van der Waals (vdW) energy indicate that differences in the vdW interactions, even when small, are the key factor in producing observed segment alignment. These results point to asymmetrical vdW interactions as a potentially powerful means of controlling orientation in multicomponent cylinder arrays, and suggest that designing for these interactions could yield new ways to control self-assembly.

Limiting the valence: advancements and new perspectives on patchy colloids, soft functionalized nanoparticles and biomolecules

“…162 When focusing on one-component systems of anisotropic particles decorated with anisotropic bonding patterns, most of the results accumulated so far in the literature deal with Janus-like nonspherical entities -mainly elongated shapes carrying one or at most two patches -assembling into a vast variety of fiber-like structures with diverse applications. Janus nano-cylinders that form vertical, horizontal or even smectic arrays, 163,164 ellipsoids with one patch in a Janus-like or ''kayak'' fashion that form ordered assemblies 165 or even field-sensitive colloidal fibers, 166 ''Mickey Mouse''-shaped colloidal molecules that form tubular aggregates, 167 and silica rods coated with gold tips that self-assemble into different multipods 168 are just a few examples. The susceptibility of Janus-like anisotropic units to external fields can also be used to drive the assembly into string-like structures.…”

Self‐Assembled Binary Mixtures of Partially Etched Nanowires

“…Pt, Rh, and Au are all metallic, with large Hamaker constants (20–40 × 10 −20 J in vacuum) . Although in some cases, relatively small differences in Hamaker values can influence assembly outcomes, we do not see evidence of clustering or demixing in our co‐assemblies.…”

“…The lower standing percentage and ordered rows are a consequence of VDW, specifically from the interactions with the 1.5Au‐2.5E‐1.5Au wires present in the second layer. VDW preferentially forms multilayered rows of PENs by aligning the metal cores . Therefore, order created in the top layer templated the rows observed in the lower layer.…”

“…We have previously introduced partially etched nanowires (PENs) as a model system to understand the interparticle forces that drive assembly of shape‐ and composition‐anisotropic particles . PENs are hybrid nanotube/nanowire particles, generally ≈300 nm diameter, and ranging from 2 to 12 μm length, consisting of a silica shell and a partial metal core . Like solid nanowires of similar dimensions, PENs sediment from aqueous suspension under the action of gravity, leading to high surface concentrations on an underlying substrate.…”

“…There, electrostatic repulsions from the silica shell and VDW attractions from the metal core lead to assemblies in which particle long axes are either parallel or perpendicular to the surface depending on PEN core patterns. In horizontal assemblies, PEN core patterns lead to variations in VDW attractions along the particle length, which influences their relative orientations . PENs with metal cores at one end and large terminal etched segments adopt vertical orientations (metal core down) that maximize VDW attractions between adjacent particles …”