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The assembly of a two-dimensional (2D) nematic liquid crystal at an interface between two liquids can be exploited to assemble densely packed and highly aligned arrays of rod-like nanoparticles. This method is especially relevant to creating arrays of semiconducting carbon nanotubes (CNTs) for high-performance electronics. When a dense solvent containing CNTs flows over a less dense water subphase in a confined channel, the locally aligned arrays of nanoparticles align globally with the flow direction and can be transferred to the substrate. For large substrates and long channels, the dense solvent tends to slow and create a pool, which then drops through the interface and disturbs the delicate deposition process. Understanding this phenomenon is critical to improving and scaling up similar manufacturing processes. Here, data are collected, and an empirical model is developed to understand and predict the pooling behavior of a suspended fluid flowing over a less dense subphase. The model is demonstrated with two different solvents and proves to be accurate within +/− 15%. With a better understanding of the physics governing the system, the model is then used to suggest methods for minimizing pooling behavior.

 

Directed self-assembly of block copolymers (BCPs) enables nanofabrication at sub-10 nm dimensions, beyond the resolution of conventional lithography. However, directing the position, orientation, and long-range lateral order of BCP domains to produce technologically-useful patterns is a challenge. Here, we present a promising approach to direct assembly using spatial boundaries between planar, low-resolution regions on a surface with different composition. Pairs of boundaries are formed at the edges of isolated stripes on a background substrate. Vertical lamellae nucleate at and are pinned by chemical contrast at each stripe/substrate boundary, align parallel to boundaries, selectively propagate from boundaries into stripe interiors (whereas horizontal lamellae form on the background), and register to wide stripes to multiply the feature density. Ordered BCP line arrays with half-pitch of 6.4 nm are demonstrated on stripes >80 nm wide. Boundary-directed epitaxy provides an attractive path towards assembling, creating, and lithographically defining materials on sub-10 nm scales.

 

To exploit their charge transport properties in transistors, semiconducting carbon nanotubes must be assembled into aligned arrays comprised of individualized nanotubes at optimal packing densities. However, achieving this control on the wafer‐scale is challenging. Here, solution‐based shear in substrate‐wide, confined channels is investigated to deposit continuous films of well‐aligned, individualized, semiconducting nanotubes. Polymer‐wrapped nanotubes in organic ink are forced through sub‐mm tall channels, generating shear up to 10 000 s⁻¹uniformly aligning nanotubes across substrates. The ink volume and concentration, channel height, and shear rate dependencies are elucidated. Optimized conditions enable alignment within a ±32° window, at 50 nanotubes µm⁻¹, on 10 × 10 cm²substrates. Transistors (channel length of 1–5 µm) are fabricated parallel and perpendicular to the alignment. The parallel transistors perform with 7× faster charge carrier mobility (101 and 49 cm²V⁻¹s⁻¹assuming array and parallel‐plate capacitances, respectively) with high on/off ratio of 10⁵. The spatial uniformity varies ±10% in density, ±2° in alignment, and ±7% in mobility. Deposition occurs within seconds per wafer, and further substrate scaling is viable. Compared to random networks, aligned nanotube films promise to be a superior platform for applications including sensors, flexible/stretchable electronics, and light emitting and harvesting devices.

 

Tangential flow interfacial self-assembly (TaFISA) is a promising scalable technique enabling uniformly aligned carbon nanotubes for high-performance semiconductor electronics. In this process, flow is utilized to induce global alignment in two-dimensional nematic carbon nanotube assemblies trapped at a liquid/liquid interface, and these assemblies are subsequently deposited on target substrates. Here, we present an observational study of experimental parameters that affect the interfacial assembly and subsequent aligned nanotube deposition. We specifically study the water contact angle (WCA) of the substrate, nanotube ink composition, and water subphase and examine their effects on liquid crystal defects, overall and local alignment, and nanotube bunching or crowding. By varying the substrate chemical functionalization, we determine that highly aligned, densely packed, individualized nanotubes deposit only at relatively small WCA between 35 and 65°. At WCA (< 10°), high nanotube bunching or crowding occurs, and the film is nonuniform, while aligned deposition ceases to occur at higher WCA (>65°). We find that the best alignment, with minimal liquid crystal defects, occurs when the polymer-wrapped nanotubes are dispersed in chloroform at a low (0.6:1) wrapper polymer to nanotube ratio. We also demonstrate that modifying the water subphase through the addition of glycerol not only improves overall alignment and reduces liquid crystal defects but also increases local nanotube bunching. These observations provide important guidance for the implementation of TaFISA and its use toward creating technologies based on aligned semiconducting carbon nanotubes. 

We examine if the bundling of semiconducting carbon nanotubes (CNTs) can increase the transconductance and on-state current density of field effect transistors (FETs) made from arrays of aligned, polymer-wrapped CNTs. Arrays with packing density ranging from 20 to 50 bundles  μm −1 are created via tangential flow interfacial self-assembly, and the transconductance and saturated on-state current density of FETs with either (i) strong ionic gel gates or (ii) weak 15 nm SiO 2 back gates are measured vs the degree of bundling. Both transconductance and on-state current significantly increase as median bundle height increases from 2 to 4 nm, but only when the strongly coupled ionic gel gate is used. Such devices tested at −0.6 V drain voltage achieve transconductance as high as 50 μS per bundle and 2 mS  μm −1 and on-state current as high as 1.7 mA  μm −1 . At low drain voltages, the off-current also increases with bundling, but on/off ratios of ∼10 5 are still possible if the largest (95th percentile) bundles in an array are limited to ∼5 nm in size. Radio frequency devices with strong, wraparound dielectric gates may benefit from increased device performance by using moderately bundled as opposed to individualized CNTs in arrays. 

Controlling the deposition of polymer-wrapped single-walled carbon nanotubes (s-CNTs) onto functionalized substrates can enable the fabrication of s-CNT arrays for semiconductor devices. In this work, we utilize classical atomistic molecular dynamics (MD) simulations to show that a simple descriptor of solvent structure near silica substrates functionalized by a wide variety of self-assembled monolayers (SAMs) can predict trends in the deposition of s-CNTs from toluene. Free energy calculations and experiments indicate that those SAMs that lead to maximum disruption of solvent structure promote deposition to the greatest extent. These findings are consistent with deposition being driven by solvent-mediated interactions that arise from SAM-solvent interactions, rather than direct s-CNT-SAM interactions, and will permit the rapid computational exploration of potential substrate designs for controlling s-CNT deposition and alignment. 

Semiconducting carbon nanotubes promise faster performance and lower power consumption than Si in field-effect transistors (FETs) if they can be aligned in dense arrays. Here, we demonstrate that nanotubes collected at a liquid/liquid interface self-organize to form two-dimensional (2D) nematic liquid crystals that globally align with flow. The 2D liquid crystals are transferred onto substrates in a continuous process generating dense arrays of nanotubes aligned within ±6°, ideal for electronics. Nanotube ordering improves with increasing concentration and decreasing temperature due to the underlying liquid crystal phenomena. The excellent alignment and uniformity of the transferred assemblies enable FETs with exceptional on-state current density averaging 520 μA μm −1 at only −0.6 V, and variation of only 19%. FETs with ion gel top gates demonstrate subthreshold swing as low as 60 mV decade −1 . Deposition across a 10-cm substrate is achieved, evidencing the promise of 2D nanotube liquid crystals for commercial semiconductor electronics. 

Long-lived photon-stimulated conductance changes in solid-state materials can enable optical memory and brain-inspired neuromorphic information processing. It remains challenging to realize optical switching with low-energy consumption, and new mechanisms and design principles giving rise to persistent photoconductivity (PPC) can help overcome an important technological hurdle. Here, we demonstrate versatile heterojunctions between metal-halide perovskite nanocrystals and semiconducting single-walled carbon nanotubes that enable room-temperature, long-lived (thousands of seconds), writable, and erasable PPC. Optical switching and basic neuromorphic functions can be stimulated at low operating voltages with femto- to pico-joule energies per spiking event, and detailed analysis demonstrates that PPC in this nanoscale interface arises from field-assisted control of ion migration within the nanocrystal array. Contactless optical measurements also suggest these systems as potential candidates for photonic synapses that are stimulated and read in the optical domain. The tunability of PPC shown here holds promise for neuromorphic computing and other technologies that use optical memory.