Noa Lachman

A high performance hybrid asymmetric supercapacitor via nano-scale morphology control of graphene, conducting polymer, and carbon nanotube electrodes

An asymmetric supercapacitor, exploiting ultra-high density aligned activated graphene flakes as the positive electrode and nm-scale conformal coating of a conducting polymer poly(ethylenedioxythiophene) on aligned carbon nanotubes as the negative electrode, is introduced. By complementary tailoring of the asymmetric electrodes, the layered device exhibits a wide 4 V electrochemical window, with the highest power density (149 kW L−1) and energy density (113 Wh L−1) reported thus far for carbon-based supercapacitors.

The Evolution of Carbon Nanotube Network Structure in Unidirectional Nanocomposites Resolved by Quantitative Electron Tomography

Carbon nanotube (CNT) reinforced polymers are next-generation, high-performance, multifunctional materials with a wide array of promising applications. The successful introduction of such materials is hampered by the lack of a quantitative understanding of process-structure-property relationships. These relationships can be developed only through the detailed characterization of the nanoscale reinforcement morphology within the embedding medium. Here, we reveal the three-dimensional (3D) nanoscale morphology of high volume fraction (V(f)) aligned CNT/epoxy-matrix nanocomposites using energy-filtered electron tomography. We present an automated phase-identification method for fast, accurate, representative rendering of the CNT spatial arrangement in these low-contrast bimaterial systems. The resulting nanometer-scale visualizations provide quantitative information on the evolution of CNT morphology and dispersion state with increasing V(f), including network structure, CNT alignment, bundling and waviness. The CNTs are observed to exhibit a nonlinear increase in bundling and alignment and a decrease in waviness as a function of increasing V(f). Our findings explain previously observed discrepancies between the modeled and measured trends in bulk mechanical, electrical and thermal properties. The techniques we have developed for morphological quantitation are applicable to many low-contrast material systems.

Impact of carbon nanotube length on electron transport in aligned carbon nanotube networks

Here, we quantify the electron transport properties of aligned carbon nanotube (CNT) networks as a function of the CNT length, where the electrical conductivities may be tuned by up to 10× with anisotropies exceeding 40%. Testing at elevated temperatures demonstrates that the aligned CNT networks have a negative temperature coefficient of resistance, and application of the fluctuation induced tunneling model leads to an activation energy of ≈14 meV for electron tunneling at the CNT-CNT junctions. Since the tunneling activation energy is shown to be independent of both CNT length and orientation, the variation in electron transport is attributed to the number of CNT-CNT junctions an electron must tunnel through during its percolated path, which is proportional to the morphology of the aligned CNT network.

Exohedral physisorption of ambient moisture scales non-monotonically with fiber proximity in aligned carbon nanotube arrays.

Here we present a study on the presence of physisorbed water on the surface of aligned carbon nanotubes (CNTs) in ambient conditions, where the wet CNT array mass can be more than 200% larger than that of dry CNTs, and modeling indicates that a water layer >5 nm thick can be present on the outer CNT surface. The experimentally observed nonlinear and non-monotonic dependence of the mass of adsorbed water on the CNT packing (volume fraction) originates from two competing modes. Physisorbed water cannot be neglected in the design and fabrication of materials and devices using nanowires/nanofibers, especially CNTs, and further experimental and ab initio studies on the influence of defects on the surface energies of CNTs, and nanowires/nanofibers in general, are necessary to understand the underlying physics and chemistry that govern this system.

Sensitivity of Carbon Nanotubes to the Storage of Stress in Polymers

Residual stress in polymers arises from the freezing of unstable molecular conformations. Residual stress is critical because its relaxation can cause shrinkage, defects, and fractures of polymer materials. The storage of stress is purposely enhanced to develop shape memory materials. Unfortunately, the storage of mechanical stress is still poorly controlled and understood. An approach to sense the storage of stress based on the spectroscopic response of carbon nanotubes is explored. The Raman response of nanotubes exhibits a variable sensitivity to strain when embedded in polymers that have experienced different thermal and mechanical treatments. This unique feature opens up new possibilities for the use of carbon nanotubes as mechanical nanosensors.

Synthesis of polymer bead nano-necklaces on aligned carbon nanotube scaffolds

Here, we report the fabrication of aligned carbon nanotube (A-CNT)/conducting polymer (CP) heterostructures with both uniform conformal and periodic beaded polymer morphologies via oxidative chemical vapor deposition of poly(ethylenedioxythiophene). Periodic beaded CP morphologies are realized utilizing the Plateau-Rayleigh instability to transform the original uniform conformal film, yielding a beaded CP morphology with a >50% enhancement in specific surface area (SSA). Modeling indicates that this SSA increase originates from the internal volume of the A-CNTs becoming available for adsorption, and that these internal A-CNT surfaces, if they could be made accessible to electrolyte ions, could lead to >30% enhancement of specific gravimetric and volumetric capacitances of current state-of-the-art A-CNT/CP heterostructures.

Electromagnetic scattering from multiple Carbon Nanotubes with experimentally determined shapes and distributions

Electromagnetic scattering from Carbon Nanotubes (CNT) has received wide interest in the past decade. Many different CNT configurations have been computationally investigated such as single CNTs, infinite planar arrays of CNTs, finite arrays with simple distributions, and bundles of CNTs. In all of the previously reported configurations, the CNTs were perfectly straight and they were arranged in a uniform distribution. However, in commercial CNT composites the CNTs typically exhibit highly complex shapes and distributions. The goal of this work is to simulate and characterize the electromagnetic scattering from multiple CNTs with realistic shapes and distributions that resemble those found in commercial composites.