Centrifuge-Free Separation of Solution-Exfoliated 2D Nanosheets via Cross-Flow Filtration.

Solution-processed graphene is a promising material for numerous high-volume applications including structural composites, batteries, sensors, and printed electronics. However, the polydisperse nature of graphene dispersions following liquid-phase exfoliation poses major manufacturing challenges, as incompletely exfoliated graphite flakes must be removed to achieve optimal properties and downstream performance. Incumbent separation schemes rely on centrifugation, which is highly energy-intensive and limits scalable manufacturing. Here, cross-flow filtration (CFF) is introduced as a centrifuge-free processing method that improves the throughput of graphene separation by two orders of magnitude. By tuning membrane pore sizes between microfiltration and ultrafiltration length scales, CFF can also be used for efficient recovery of solvents and stabilizing polymers. In this manner, life cycle assessment and techno-economic analysis reveal that CFF reduces greenhouse gas emissions, fossil energy usage, water consumption, and specific production costs of graphene manufacturing by 57%, 56%, 63%, and 72%, respectively. To confirm that CFF produces electronic-grade graphene, CFF-processed graphene nanosheets are formulated into printable inks, leading to state-of-the-art thin-film conductivities exceeding 104 S m-1 . This CFF methodology can likely be generalized to other van der Waals layered solids, thus enabling sustainable manufacturing of the diverse set of applications currently being pursued for 2D materials.

Elucidating and Mitigating High-Voltage Degradation Cascades in Cobalt-Free LiNiO2 Lithium-Ion Battery Cathodes.

LiNiO2 (LNO) is a promising cathode material for next-generation Li-ion batteries due to its exceptionally high capacity and cobalt-free composition that enables more sustainable and ethical large-scale manufacturing. However, its poor cycle life at high operating voltages over 4.1 V impedes its practical use, thus motivating efforts to elucidate and mitigate LiNiO2 degradation mechanisms at high states of charge. Here, a multiscale exploration of high-voltage degradation cascades associated with oxygen stacking chemistry in cobalt-free LiNiO2 , is presented. Lattice oxygen loss is found to play a critical role in the local O3-O1 stacking transition at high states of charge, which subsequently leads to Ni-ion migration and irreversible stacking faults during cycling. This undesirable atomic-scale structural evolution accelerates microscale electrochemical creep, cracking, and even bending of layers, ultimately resulting in macroscopic mechanical degradation of LNO particles. By employing a graphene-based hermetic surface coating, oxygen loss is attenuated in LNO at high states of charge, which suppresses the initiation of the degradation cascade and thus substantially improves the high-voltage capacity retention of LNO. Overall, this study provides mechanistic insight into the high-voltage degradation of LNO, which will inform ongoing efforts to employ cobalt-free cathodes in Li-ion battery technology.

Graphene-enabled and directed nanomaterial placement from solution for large-scale device integration.

Directed placement of solution-based nanomaterials at predefined locations with nanoscale precision limits bottom-up integration in semiconductor process technology. We report a method for electric-field-assisted placement of nanomaterials from solution by means of large-scale graphene layers featuring nanoscale deposition sites. The structured graphene layers are prepared via either transfer or synthesis on standard substrates, and then are removed once nanomaterial deposition is completed, yielding material assemblies with nanoscale resolution that cover surface areas >1 mm2. In order to demonstrate the broad applicability, we have assembled representative zero-dimensional, one-dimensional, and two-dimensional semiconductors at predefined substrate locations and integrated them into nanoelectronic devices. Ultimately, this method opens a route to bottom-up integration of nanomaterials for industry-scale applications.

Comprehensive Enhancement of Nanostructured Lithium-Ion Battery Cathode Materials via Conformal Graphene Dispersion.

Efficient energy storage systems based on lithium-ion batteries represent a critical technology across many sectors including consumer electronics, electrified transportation, and a smart grid accommodating intermittent renewable energy sources. Nanostructured electrode materials present compelling opportunities for high-performance lithium-ion batteries, but inherent problems related to the high surface area to volume ratios at the nanometer-scale have impeded their adoption for commercial applications. Here, we demonstrate a materials and processing platform that realizes high-performance nanostructured lithium manganese oxide (nano-LMO) spinel cathodes with conformal graphene coatings as a conductive additive. The resulting nanostructured composite cathodes concurrently resolve multiple problems that have plagued nanoparticle-based lithium-ion battery electrodes including low packing density, high additive content, and poor cycling stability. Moreover, this strategy enhances the intrinsic advantages of nano-LMO, resulting in extraordinary rate capability and low temperature performance. With 75% capacity retention at a 20C cycling rate at room temperature and nearly full capacity retention at -20 °C, this work advances lithium-ion battery technology into unprecedented regimes of operation.

Solution-Processed Dielectrics Based on Thickness-Sorted Two-Dimensional Hexagonal Boron Nitride Nanosheets.

Gate dielectrics directly affect the mobility, hysteresis, power consumption, and other critical device metrics in high-performance nanoelectronics. With atomically flat and dangling bond-free surfaces, hexagonal boron nitride (h-BN) has emerged as an ideal dielectric for graphene and related two-dimensional semiconductors. While high-quality, atomically thin h-BN has been realized via micromechanical cleavage and chemical vapor deposition, existing liquid exfoliation methods lack sufficient control over h-BN thickness and large-area film quality, thus limiting its use in solution-processed electronics. Here, we employ isopycnic density gradient ultracentrifugation for the preparation of monodisperse, thickness-sorted h-BN inks, which are subsequently layer-by-layer assembled into ultrathin dielectrics with low leakage currents of 3 × 10(-9) A/cm(2) at 2 MV/cm and high capacitances of 245 nF/cm(2). The resulting solution-processed h-BN dielectric films enable the fabrication of graphene field-effect transistors with negligible hysteresis and high mobilities up to 7100 cm(2) V(-1) s(-1) at room temperature. These h-BN inks can also be used as coatings on conventional dielectrics to minimize the effects of underlying traps, resulting in improvements in overall device performance. Overall, this approach for producing and assembling h-BN dielectric inks holds significant promise for translating the superlative performance of two-dimensional heterostructure devices to large-area, solution-processed nanoelectronics.

Hot spot dynamics in carbon nanotube array devices.

We report on the dynamics of spatial temperature distributions in aligned semiconducting carbon nanotube array devices with submicrometer channel lengths. By using high-resolution optical microscopy in combination with electrical transport measurements, we observe under steady state bias conditions the emergence of time-variable, local temperature maxima with dimensions below 300 nm, and temperatures above 400 K. On the basis of time domain cross-correlation analysis, we investigate how the intensity fluctuations of the thermal radiation patterns are correlated with the overall device current. The analysis reveals the interdependence of electrical current fluctuations and time-variable hot spot formation that limits the overall device performance and, ultimately, may cause device degradation. The findings have implications for the future development of carbon nanotube-based technologies.

Thickness sorting of two-dimensional transition metal dichalcogenides via copolymer-assisted density gradient ultracentrifugation.

Two-dimensional transition metal dichalcogenides have emerged as leading successors to graphene due to their diverse properties, which depend sensitively on sample thickness. Although solution-based exfoliation methods hold promise for scalable production of these materials, existing techniques introduce irreversible structural defects and/or lack sufficient control over the sample thickness. In contrast, previous work on carbon nanotubes and graphene has shown that isopycnic density gradient ultracentrifugation can produce structurally and electronically monodisperse nanomaterial populations. However, this approach cannot be directly applied to transition metal dichalcogenides due to their high intrinsic buoyant densities when encapsulated with ionic small molecule surfactants. Here, we overcome this limitation and thus demonstrate thickness sorting of pristine molybdenum disulfide (MoS2) by employing a block copolymer dispersant composed of a central hydrophobic unit flanked by hydrophilic chains that effectively reduces the overall buoyant density in aqueous solution. The resulting solution-processed monolayer MoS2 samples exhibit strong photoluminescence without further chemical treatment.

Subnanowatt carbon nanotube complementary logic enabled by threshold voltage control.

In this Letter, we demonstrate thin-film single-walled carbon nanotube (SWCNT) complementary metal-oxide-semiconductor (CMOS) logic devices with subnanowatt static power consumption and full rail-to-rail voltage transfer characteristics as is required for logic gate cascading. These results are enabled by a local metal gate structure that achieves enhancement-mode p-type and n-type SWCNT thin-film transistors (TFTs) with widely separated and symmetric threshold voltages. These complementary SWCNT TFTs are integrated to demonstrate CMOS inverter, NAND, and NOR logic gates at supply voltages as low as 0.8 V with ideal rail-to-rail operation, subnanowatt static power consumption, high gain, and excellent noise immunity. This work provides a direct pathway for solution processable, large area, power efficient SWCNT advanced logic circuits and systems.

Aerosol jet printed, low voltage, electrolyte gated carbon nanotube ring oscillators with sub-5 µs stage delays.

A central challenge for printed electronics is to achieve high operating frequencies (short transistor switching times) at low supply biases compatible with thin film batteries. In this report, we demonstrate partially printed five-stage ring oscillators with >20 kHz operating frequencies and stage delays <5 µs at supply voltages below 3 V. The fastest ring oscillator achieved 1.2 µs delay time at 2 V supply. The inverter stages in these ring oscillators were based on ambipolar thin film transistors (TFTs) employing semiconducting, single-walled carbon nanotube (CNT) networks and a high capacitance (∼1 µF/cm(2)) ion gel electrolyte as the gate dielectric. All materials except the source and drain electrodes were aerosol jet printed. The TFTs exhibited high electron and hole mobilities (∼20 cm(2)/(V s)) and ON/OFF current ratios (up to 10(5)). Inverter switching times t were systematically characterized as a function of transistor channel length and ionic conductivity of the gel dielectric, demonstrating that both the semiconductor and the ion gel play a role in switching speed. Quantitative scaling analysis suggests that with suitable optimization low voltage, printed ion gel gated CNT inverters could operate at frequencies on the order of 1 MHz.

Large-area, electronically monodisperse, aligned single-walled carbon nanotube thin films fabricated by evaporation-driven self-assembly.

By varying the evaporation conditions and the nanotube and surfactant concentrations, large-area, aligned single-walled carbon nanotube (SWCNT) thin films are fabricated from electronically monodisperse SWCNT solutions by evaporation-driven self-assembly with precise control over the thin film growth geometry. Tunability is possible from 0.5 µm stripes to continuous thin films. The resulting SWCNT thin films possess highly anisotropic electrical and optical properties that are well suited for transparent conductor applications.

Flexible gigahertz transistors derived from solution-based single-layer graphene.

Flexible electronics mostly relies on organic semiconductors but the limited carrier velocity in polymers and molecular films prevents their use at frequencies above a few megahertz. Conversely, the high potential of graphene for high-frequency electronics on rigid substrates was recently demonstrated. We conducted the first study of solution-based graphene transistors at gigahertz frequencies, and we show that solution-based single-layer graphene ideally combines the required properties to achieve high speed flexible electronics on plastic substrates. Our graphene flexible transistors have current gain cutoff frequencies of 2.2 GHz and power gain cutoff frequencies of 550 MHz. Radio frequency measurements directly performed on bent samples show remarkable mechanical stability of these devices and demonstrate the advantages of solution-based graphene field-effect transistors over other types of flexible transistors based on organic materials.

Sorting single-walled carbon nanotubes by electronic type using nonionic, biocompatible block copolymers.

As-synthesized single-walled carbon nanotubes (SWNTs) typically possess a range of diameters and electronic properties. This polydispersity has hindered the development of many SWNT-based technologies and encouraged the development of postsynthetic methods for sorting SWNTs by their physical and electronic structure. Herein, we demonstrate that nonionic, biocompatible block copolymers can be used to isolate semiconducting and metallic SWNTs using density gradient ultracentrifugation. Separations conducted with different Pluronic block copolymers reveal that Pluronics with shorter hydrophobic chain lengths lead to higher purity semiconducting SWNTs, resulting in semiconducting purity levels in excess of 99% obtained for Pluronic F68. In contrast, X-shaped Tetronic block copolymers display an affinity for metallic SWNTs, yielding metallic purity levels of 74% for Tetronic 1107. These results suggest that high fidelity and high yield density gradient separations can be achieved using nonionic block copolymers with rationally designed homopolymer segments, thus generating biocompatible monodisperse SWNTs for a range of applications.