Large tunability in the mechanical and thermal properties of carbon nanotube-fullerene hierarchical monoliths

Reversible and high-contrast thermal conductivity switching in a flexible covalent organic framework possessing negative Poisson's ratio

The ability to dynamically and reversibly control thermal transport in solid-state systems can redefine and propel a plethora of technologies including thermal switches, diodes, and rectifiers. Current material systems, however, do not possess the swift and large changes in thermal conductivity required for such practical applications. For instance, stimuli responsive materials, that can reversibly switch between a high thermal conductivity state and a low thermal conductivity state, are mostly limited to thermal switching ratios in the range of 1.5 to 4. Here, we demonstrate reversible thermal conductivity switching with an unprecedented 18× change in thermal transport in a highly flexible covalent organic framework with revolving imine bonds. The pedal motion of the imine bonds is capable of reversible transformations of the framework from an expanded (low thermal conductivity) to a contracted (high thermal conductivity) phase, which can be triggered through external stimuli such as exposure to guest adsorption and desorption or mechanical strain. We also show that the dynamic imine linkages endow the material with a negative Poisson's ratio, thus marking a regime of materials design that combines low densities with exceptional thermal and mechanical properties.

Supramolecular Interactions Lead to Remarkably High Thermal Conductivities in Interpenetrated Two-Dimensional Porous Crystals

The design of innovative porous crystals with high porosities and large surface areas has garnered a great deal of attention over the past few decades due to their remarkable potential for a variety of applications. However, heat dissipation is key to realizing their potential. We use systematic atomistic simulations to reveal that interpenetrated porous crystals formed from two-dimensional (2D) frameworks possess remarkable thermal conductivities at high porosities in comparison to their three-dimensional (3D) single framework and interpenetrated 3D framework counterparts. In contrast to conventional understanding, higher thermal conductivities are associated with lower atomic densities and higher porosities for porous crystals formed from interpenetrating 2D frameworks. We attribute this to lower phonon-phonon scattering and vibrational hardening from the supramolecular interactions that restrict atomic vibrational amplitudes, facilitating heat conduction. This marks a new regime of materials design combining ultralow mass densities and ultrahigh thermal conductivities in 2D interpenetrated porous crystals.

Highly Negative Poisson's Ratio in Thermally Conductive Covalent Organic Frameworks

The prospect of combining two-dimensional materials in vertical stacks has created a new paradigm for materials scientists and engineers. Herein, we show that stacks of two-dimensional covalent organic frameworks are endowed with a host of unique physical properties that combine low densities, high thermal conductivities, and highly negative Poisson's ratios. Our systematic atomistic simulations demonstrate that the tunable mechanical and thermal properties arise from their singular layered architecture comprising strongly bonded light atoms and periodic laminar pores. For example, the negative Poisson's ratio arises from the weak van der Waals interactions between the two-dimensional layers along with the strong covalent bonds that act as hinges along the layers, which facilitate the twisting and swiveling motion of the phenyl rings relative to the tensile plane. The mechanical and thermal properties of two-dimensional covalent organic frameworks can be tailored through structural modularities such as control over the pore size and/or interlayer separation. We reveal that these materials mark a regime of materials design that combines low densities with high thermal conductivities arising from their nanoporous yet covalently interconnected structure.

Thermally conductive ultra-low-k dielectric layers based on two-dimensional covalent organic frameworks

As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m^-1 K^-1) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties.

A modified "gel-blowing" strategy toward the one-step mass production of a 3D N-doped carbon nanosheet@carbon nanotube hybrid network for supercapacitors

In this work, we have realized the synchronous and large-scale synthesis of one-dimensional (1D) carbon nanotubes (CNTs) on two-dimensional (2D) N-doped carbon nanosheets (NCNS) by a one-step annealing of a Ni-containing gel precursor. Upon heating, the gel is "blown" into large-sized 2D NCNS with uniformly embedded Ni nanoparticles that can catalyze the in situ CNT growth, forming a three-dimensional (3D) N-doped carbon nanosheet@carbon nanotube (NCNS@CNT) hybrid. Different from our previous "gel-blowing" strategy for 2D nanosheets, the modified "gel-blowing" strategy is capable of producing 3D architecture by employing a new complexing agent and introducing ethanol as a carbon source. Importantly, this method can be easily scaled up by annealing more gel precursors with an increased amount of ethanol. The introduction of CNTs endows NCNS@CNTs with higher quality and larger specific surface area (SSA) than pure NCNS. Consequently, the electrochemical performance of 3D NCNS@CNTs is much superior to that of 2D NCNS and found to be related with the annealing temperature. The optimized NCNS@CNTs can deliver a specific capacitance of 124 F g-1 at 1 A g-1 and maintain 88% of their initial value after 10 000 cycles at 1 A g-1. Furthermore, NiO nanosheets are deposited on the NCNS@CNT framework to study its function as a conductive host. The as-fabricated hybrid electrode exhibits a high specific capacitance of 660 F g-1 at 1 A g-1 and 532 F g-1 at 20 A g-1, which is also better than its counterpart using NCNS as substrates. This method provides a simplified and low-cost way towards the mass production of NCNS@CNTs for energy application and beyond.

Advanced biomedical applications of carbon nanotube

With advances in nanotechnology, the applications of nanomaterial are developing widely and greatly. The characteristic properties of carbon nanotubes (CNTs) make them the most selective candidate for various multi-functional applications. The greater surface area of the CNTs in addition to the capability to manipulate the surfaces and dimensions has provided greater potential for this nanomaterial. The CNTs possess greater potential for applications in biomedicine due to their vital electrical, chemical, thermal, and mechanical properties. The unique properties of CNT are exploited for numerous applications in the biomedical field. They are useful in both therapeutic and diagnostic applications. They form novel carrier systems which are also capable of site-specific delivery of therapeutic agents. In addition, CNTs are of potential application in biosensing. Many recently reported advanced systems of CNT could be exploited for their immense potential in biomedicine in the future.