Spatially Selective, High-Density Placement of Polyfluorene-Sorted Semiconducting Carbon Nanotubes in Organic Solvents

A simple simulation-derived descriptor for the deposition of polymer-wrapped carbon nanotubes on functionalized substrates

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.

Chemical and topographical patterns combined with solution shear for selective-area deposition of highly-aligned semiconducting carbon nanotubes

Selective deposition of semiconducting carbon nanotubes (s-CNTs) into densely packed, aligned arrays of individualized s-CNTs is necessary to realize their potential in semiconductor electronics. We report the combination of chemical contrast patterns, topography, and pre-alignment of s-CNTs via shear to achieve selective-area deposition of aligned arrays of s-CNTs. Alternate stripes of surfaces favorable and unfavorable to s-CNT adsorption were patterned with widths varying from 2000 nm down to 100 nm. Addition of topography to the chemical contrast patterns combined with shear enabled the selective-area deposition of arrays of quasi-aligned s-CNTs (∼14°) even in patterns that are wider than the length of individual nanotubes (>500 nm). When the width of the chemical and topographical contrast patterns is less than the length of individual nanotubes (<500 nm), confinement effects become dominant enabling the selective-area deposition of much more tightly aligned s-CNTs (∼7°). At a trench width of 100 nm, we demonstrate the lowest standard deviation in alignment degree of 7.6 ± 0.3° at a deposition shear rate of 4600 s^-1, while maintaining an individualized s-CNT density greater than 30 CNTs μm^-1. Chemical contrast alone enables selective-area deposition, but chemical contrast in addition to topography enables more effective selective-area deposition and stronger confinement effects, with the advantage of removal of nanotubes deposited in spurious areas via selective lift-off of the topographical features. These findings provide a methodology that is inherently scalable, and a means to deposit spatially selective, aligned s-CNT arrays for next-generation semiconducting devices.

Dense Assembly of Finely Patterned Semiconducting Single-Walled Carbon Nanotubes via a Selective Transfer Method of Nanotube-Attracting Layers

Development of technology for assembled single-walled carbon nanotube (SWCNT) film with the fine resolution is an essential technique for penetrating practical electronic applications. A promising approach is the assembly method by adding a chemical-functionalizing substrate to enhance affinity between the SWCNTs and the substrate. However, the various introduced approaches for solution-based assembly have suffered from low SWCNT deposition selectivity or low SWCNT deposition density. Herein, we demonstrated a facile method for selectively assembling semiconducting SWCNT network on the substrate. The substrate was prepared via a transfer printing of a poly-l-lysine (PLL)-coated poly(dimethylsiloxane) (PDMS) stamp. The thermal-assisted transfer method enabled an ultrafine PLL pattern (≤4 μm) and a high transfer yield (96.5%) by only one-time stamping without a change of the SWCNT-attracting nature. So, semiconducting SWCNTs were deposited on the patterned regions selectively and precisely. The benefit of the patterned semiconducting SWCNTs was lowering leakage current and turn-on voltage in the transfer characteristics by suppressing attachment of unnecessary SWCNT network. They showed excellent electrical performance, a log₁₀(I_on/I_off) ratio of 4.76, and an average value of linear field-effect mobility of 7.56 cm²/(V s). This research provides a simple but high-quality assembling technique of semiconducting SWCNTs, thereby improving the feasibility of solution-processed SWCNT-TFTs.

Facile Fabrication of Semiconducting Single-Walled Carbon Nanotubes Patterns on Flexible Substrate Based on a Photoimmobilization Technique

Single-walled carbon nanotubes (SWCNTs) have attracted significant attention due to their outstanding properties. For their wide applications in electronics and optoelectronics, pure semiconducting SWCNTs (s-SWCNTs) and their precise placement are preconditions. Recent advances have focused on developing effective strategies to separate s-SWCNTs from raw SWCNTs, a mixture of metallic and semiconducting nanotubes, and deposit s-SWCNTs on target substrates. Herein, a polyfluorene-based alternative copolymer (PFBP) containing the benzophenone group was employed. PFBP achieved higher yield for s-SWCNTs than the well-studied poly(9,9-dioctylfluorene) through solution process. Subsequently, the dispersed s-SWCNTs were immobilized on a flexible polyethylene terephthalate in a facile manner by the photoreactive benzophenone group upon exposure to UV irradiation, and chemically robust patterns were fabricated from micro to macro scales through photomasks. Our method accomplished by utilizing photoimmobilization is a simple cleaning procedure and an important step forward in pitch scaling for further applications of conjugated polymer wrapped s-SWCNTs.

Nanoscale Patterning of Carbon Nanotubes: Techniques, Applications, and Future

Carbon nanotube (CNT) devices and electronics are achieving maturity and directly competing or surpassing devices that use conventional materials. CNTs have demonstrated ballistic conduction, minimal scaling effects, high current capacity, low power requirements, and excellent optical/photonic properties; making them the ideal candidate for a new material to replace conventional materials in next-generation electronic and photonic systems. CNTs also demonstrate high stability and flexibility, allowing them to be used in flexible, printable, and/or biocompatible electronics. However, a major challenge to fully commercialize these devices is the scalable placement of CNTs into desired micro/nanopatterns and architectures to translate the superior properties of CNTs into macroscale devices. Precise and high throughput patterning becomes increasingly difficult at nanoscale resolution, but it is essential to fully realize the benefits of CNTs. The relatively long, high aspect ratio structures of CNTs must be preserved to maintain their functionalities, consequently making them more difficult to pattern than conventional materials like metals and polymers. This review comprehensively explores the recent development of innovative CNT patterning techniques with nanoscale lateral resolution. Each technique is critically analyzed and applications for the nanoscale-resolution approaches are demonstrated. Promising techniques and the challenges ahead for future devices and applications are discussed.

Solvent-Mediated Affinity of Polymer-Wrapped Single-Walled Carbon Nanotubes for Chemically Modified Surfaces

Semiconducting single-walled carbon nanotube (s-CNT) arrays are being explored for next-generation semiconductor electronics. Even with the multitude of alignment and spatially localized s-CNT deposition methods designed to control s-CNT deposition, fundamental understanding of the driving forces for s-CNT deposition is still lacking. The individual roles of the dispersant, solvent, target substrate composition, and the s-CNT itself are not completely understood because it is difficult to decouple deposition parameters. Here, we study poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6'-[2,2'-{bipyridine}])] (PFO-BPy)-wrapped s-CNT deposition from solution onto a chemically modified substrate. We fabricate various self-assembled monolayers (SAMs) to gain a greater understanding of substrate effects on PFO-BPy-wrapped s-CNT deposition. We observe that s-CNT deposition is dependent on both the target substrate and s-CNT dispersion solvent. To complement the experiments, molecular dynamics simulations of PFO-BPy-wrapped s-CNT deposition on two different SAMs are performed to obtain mechanistic insights into the effect of the substrate and solvent on s-CNT deposition. We find that the global free-energy minimum associated with favorable s-CNT adsorption occurs for a configuration in which the minimum of the solvent density around the s-CNT coincides with the minimum of the solvent density above a SAM-grafted surface, indicating that solvent structure near a SAM-grafted surface determines the adsorption free-energy landscape driving s-CNT deposition. Our results will help guide informative substrate design for s-CNT array fabrication in semiconductor devices.