Topological transitions in carbon nanotube networks via nanoscale confinement

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.

High-Rate Assembly of Nanomaterials on Insulating Surfaces Using Electro-Fluidic Directed Assembly

Conductive or semiconducting nanomaterials-based applications such as electronics and sensors often require direct placement of such nanomaterials on insulating surfaces. Most fluidic-based directed assembly techniques on insulating surfaces utilize capillary force and evaporation but are diffusion limited and slow. Electrophoretic-based assembly, on the other hand, is fast but can only be utilized for assembly on a conductive surface. Here, we present a directed assembly technique that enables rapid assembly of nanomaterials on insulating surfaces. The approach leverages and combines fluidic and electrophoretic assembly by applying the electric field through an insulating surface via a conductive film underneath. The approach (called electro-fluidic) yields an assembly process that is 2 orders of magnitude faster compared to fluidic assembly. By understanding the forces on the assembly process, we have demonstrated the controlled assembly of various types of nanomaterials that are conducting, semiconducting, and insulating including nanoparticles and single-walled carbon nanotubes on insulating rigid and flexible substrates. The presented approach shows great promise for making practical devices in miniaturized sensors and flexible electronics.

Inter-allotropic transformations in the heterogeneous carbon nanotube networks

The allotropic transformations of carbon provide an immense technological interest for tailoring the desired molecular structures in the scalable nanoelectronic devices. Herein, we explore the effects of morphology and geometric alignment of the nanotubes for the re-engineering of carbon bonds in the heterogeneous carbon nanotube (CNT) networks. By applying alternating voltage pulses and electrical forces, the single-walled CNTs in networks were predominantly transformed into other predetermined sp² carbon structures (multi-walled CNTs and multi-layered graphitic nanoribbons), showing a larger intensity in a coalescence-induced mode of Raman spectra with the increasing channel width. Moreover, the transformed networks have a newly discovered sp²-sp³ hybrid nanostructures in accordance with the alignment. The sp³ carbon structures at the small channel are controlled, such that they contain up to about 29.4% networks. This study provides a controllable method for specific types of inter-allotropic transformations/hybridizations, which opens up the further possibility for the engineering of nanocarbon allotropes in the robust large-scale network-based devices.

High-rate nanoscale offset printing process using directed assembly and transfer of nanomaterials

High-rate nanoscale offset printing using a newly developed reusable template enables the assembly of nanomaterials into nanostructures followed by their transfer onto a flexible substrate in a few minutes. The developed template can potentially be reused more than 100 times in the offset printing process without any additional functionalization. This approach provides a new way for the printing of flexible devices with nanoscale patterns.

Shaping van der Waals nanoribbons via torsional constraints: scrolls, folds and supercoils

Interplay between structure and function in atomically thin crystalline nanoribbons is sensitive to their conformations yet the ability to prescribe them is a formidable challenge. Here, we report a novel paradigm for controlled nucleation and growth of scrolled and folded shapes in finite-length nanoribbons. All-atom computations on graphene nanoribbons (GNRs) and experiments on macroscale magnetic thin films reveal that decreasing the end distance of torsionally constrained ribbons below their contour length leads to formation of these shapes. The energy partitioning between twisted and bent shapes is modified in favor of these densely packed soft conformations due to the non-local van der Waals interactions in these 2D crystals; they subvert the formation of supercoils that are seen in their natural counterparts such as DNA and filamentous proteins. The conformational phase diagram is in excellent agreement with theoretical predictions. The facile route can be readily extended for tailoring the soft conformations of crystalline nanoscale ribbons, and more general self-interacting filaments.

Sculpting carbon bonds for allotropic transformation through solid-state re-engineering of -sp2 carbon

Carbon forms one of nature's strongest chemical bonds; its allotropes having provided some of the most exciting scientific discoveries in recent times. The possibility of inter-allotropic transformations/hybridization of carbon is hence a topic of immense fundamental and technological interest. Such modifications usually require extreme conditions (high temperature, pressure and/or high-energy irradiations), and are usually not well controlled. Here we demonstrate inter-allotropic transformations/hybridizations of specific types that appear uniformly across large-area carbon networks, using moderate alternating voltage pulses. By controlling the pulse magnitude, small-diameter single-walled carbon nanotubes can be transformed predominantly into larger-diameter single-walled carbon nanotubes, multi-walled carbon nanotubes of different morphologies, multi-layered graphene nanoribbons or structures with sp(3) bonds. This re-engineering of carbon bonds evolves via a coalescence-induced reconfiguration of sp(2) hybridization, terminates with negligible introduction of defects and demonstrates remarkable reproducibility. This reflects a potential step forward for large-scale engineering of nanocarbon allotropes and their junctions.

Bundling dynamics regulates the active mechanics and transport in carbon nanotube networks and their nanocomposites

High-density carbon nanotube networks (CNNs) continue to attract interest as active elements in nanoelectronic devices, nanoelectromechanical systems (NEMS) and multifunctional nanocomposites. The interplay between the network nanostructure and its properties is crucial, yet current understanding remains limited to the passive response. Here, we employ a novel superstructure consisting of millimeter-long vertically aligned single walled carbon nanotubes (SWCNTs) sandwiched between polydimethylsiloxane (PDMS) layers to quantify the effect of two classes of mechanical stimuli, film densification and stretching, on the electronic and thermal transport across the network. The network deforms easily with an increase in the electrical and thermal conductivities, suggestive of a floppy yet highly reconfigurable network. Insight from atomistically informed coarse-grained simulations uncover an interplay between the extent of lateral assembly of the bundles, modulated by surface zipping/unzipping, and the elastic energy associated with the bent conformations of the nanotubes/bundles. During densification, the network becomes highly interconnected yet we observe a modest increase in bundling primarily due to the reduced spacing between the SWCNTs. The stretching, on the other hand, is characterized by an initial debundling regime as the strain accommodation occurs via unzipping of the branched interconnects, followed by rapid rebundling as the strain transfers to the increasingly aligned bundles. In both cases, the increase in the electrical and thermal conductivity is primarily due to the increase in bundle size; the changes in network connectivity have a minor effect on the transport. Our results have broad implications for filamentous networks of inorganic nanoassemblies composed of interacting tubes, wires and ribbons/belts.