Buckling-driven delamination of carbon nanotube forests

We report buckling-driven delamination of carbon nanotube (CNT) forests from their growth substrates when subjected to compression. Macroscale compression experiments reveal local delamination at the CNT forest-substrate interface. Results of microscale flat punch indentations indicate that enhanced CNT interlocking at the top surface of the forest accomplished by application of a metal coating causes delamination of the forest from the growth substrate, a phenomenon not observed in indentation of as-grown CNT forests. We postulate that the post-buckling tensile stresses that develop at the base of the CNT forests serve as the driving force for delamination

Effects of morphology on the micro-compression response of carbon nanotube forests

This study reports the mechanical response of distinct carbon nanotube (CNT) morphologies as revealed by flat punch in situ nanoindentation in a scanning electron microscope. We find that the location of incipient deformation varies significantly by changing the CNT growth parameters. The initial buckles formed close to the growth substrate in 70 and 190 µm tall CNT forests grown with low pressure chemical vapor deposition (LPCVD) and moved to ~100 µm above the growth substrate when the height increased to 280 µm. Change of the recipe from LPCVD to CVD at pressures near atmospheric changed the location of the initial buckling event from the bottom half to the top half of the CNT forest. Plasma pretreatment of the catalyst also resulted in a unique CNT forest morphology in which deformation started by bending and buckling of the CNT tips. We find that the vertical gradients in CNT morphology dictate the location of incipient buckling. These new insights are critical in the design of CNT forests for a variety of applications where mechanical contact is important

Compressive response of vertically aligned carbon nanotube films gleaned from in situ flat-punch indentations

We report the mechanical behavior of vertically aligned carbon nanotube films, grown on Si substrates using atmospheric pressure chemical vapor deposition, subjected to in situ large displacement (up to 70 μm) flat-punch indentations. We observed three distinct regimes in their indentation stress–strain curves: (i) a short elastic regime, followed by (ii) a sudden instability, which resulted in a substantial rapid displacement burst manifested by an instantaneous vertical shearing of the material directly underneath the indenter tip by as much as 30 μm, and (iii) a positively sloped plateau for displacements between 10 and 70 μm. In situ nanomechanical indentation experiments revealed that the shear strain was accommodated by an array of coiled carbon nanotube “microrollers,” providing a low-friction path for the vertical displacement. Mechanical response and concurrent deformation morphologies are discussed in the foam-like deformation framework with a particular emphasis on boundary conditions

Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An in-Situ Study

We report mechanical behavior and strain rate dependence of recoverability and energy dissipation in vertically aligned carbon nanotube (VACNT) bundles subjected to quasi-static uniaxial compression. We observe three distinct regimes in their stress–strain curves for all explored strain rates from 4 × 10^(–2) down to 4 × 10^(–4) /sec: (1) a short initial elastic section followed by (2) a sloped plateau with characteristic wavy features corresponding to buckle formation and (3) densification characterized by rapid stress increase. Load–unload cycles reveal a stiffer response and virtually 100% recoverability at faster strain rates of 0.04/sec, while the response is more compliant at slower rates, characterized by permanent localized buckling and significantly reduced recoverability. We propose that it is the kinetics of attractive adhesive interactions between the individual carbon nanotubes within the VACNT matrix that governs morphology evolution and ensuing recoverability. In addition, we report a 6-fold increase in elastic modulus and gradual decrease in recoverability (down to 50%) when VACNT bundles are unloaded from postdensification stage as compared with predensification. Finally, we demonstrate energy dissipation capability, as revealed by hysteresis in load–unload cycles. These findings, together with high thermal and electrical conductivities, position VACNTs in the “unattained-as-of-to-date-space” in the material property landscape

Conductive 3D nano-biohybrid systems based on densified carbon nanotube forests and living cells

Conductive biohybrid cell-material systems have applications in bioelectronics and biorobotics. To date, conductive scaffolds are limited to those with low electrical conductivity or 2D sheets. Here, 3D biohybrid conductive systems are developed using fibroblasts or cardiomyocytes integrated with carbon nanotube (CNT) forests that are densified due to interactions with a gelatin coating. CNT forest scaffolds with a height range of 120–240 µm and an average electrical conductivity of 0.6 S/cm are developed and shown to be cytocompatible as evidenced from greater than 89% viability measured by live-dead assay on both cells on day 1. The cells spread on top and along the height of the CNT forest scaffolds. Finally, the scaffolds have no adverse effects on the expression of genes related to cardiomyocyte maturation and functionality, or fibroblast migration, adhesion, and spreading. The results show that the scaffold could be used in applications ranging from organ-on-a-chip systems to muscle actuators. Graphical abstract: [Figure not available: see fulltext.

Mechanical behavior of carbon nanotube forests under compressive loading

Carbon nanotube (CNT) forests are an important class of nanomaterials with many potential applications due to their unique properties such as mechanical compliance, thermal and electrical conductance, etc. Their deformation and failure in compression loading is critical in any application involving contact because the deformation changes the nature of the contact and thus impacts the transfer of load, heat, and charge carriers across the interface. The micro- and nano-structure of the CNT forest can vary along their height and from sample to sample due to different growth parameters. The morphology of CNTs and their interaction contribute to their mechanical behavior with change of load distribution in the CNT forest. However, the relationship is complicated due to involvement of many factors such as density, orientation, and entanglement of CNTs. None of these effects, however, are well understood. This dissertation aims to advance the knowledge of the structure-property relation in CNT forests and find methodologies for tuning their mechanical behavior. The mechanical behavior of CNT forests grown with different methodologies is studied. Furthermore, the effects of coating and wetting of CNT forests are investigated as methods to tailor the degree of interaction between CNTs. In situ micro-indentation of uncoated CNT forests with distinct growth-induced structures are performed to elucidate the effects of change of morphology along the height of CNT forests on their deformation mechanism. CNT aerial density and tortuosity are found to dictate the location of incipient deformation along height of CNT forests. Macro-compression testing of uncoated CNT forests reveals mechanical failure of CNT forests by delamination at the CNT-growth substrate. Tensile loading of CNT roots due to post-buckling bending of CNTs is proposed to be the cause of this failure and simple bending theory is shown to estimate the failure load to be on the same order of magnitude as experimental measurements. Furthermore, delamination is observed to occur in the in situ micro-indentation of CNT forests coated with aluminum on the top surface, which demonstrates the role of the mechanical constraints within the CNT forest in the occurrence of delamination at the CNT-substrate interface. In addition, this dissertation explores the mechanical behavior of CNT forests coated conformally (from top to bottom) with alumina by atomic layer deposition. In situ micro-indentation testing demonstrates that the deformation mechanism of CNT forests does not change with a thin coating (2 nm) but does change with a sufficiently thick coating (10 nm) that causes fracturing of the hybrid nanotubes. Ex situ flat punch and Berkovich indentations reveal an increase in stiffness of the CNT forests that are in range with those predicted by compression and bending theories. An increase in the recoverability of the CNTs is also detected. Finally, solvent infiltration is proposed as a method of decreasing stiffness of CNT forests and changing the deformation mechanism from local to global deformations (i.e., buckling in the entire height). Presence of solvents between CNTs decreases the van der Waals forces between them and produces CNT forests with lower stiffness. The results demonstrate the effect of interaction between CNTs on the mechanical behavior. This dissertation reveals important information on the mechanical behavior of CNT forests as it relates to CNT morphology and tube-to-tube interactions. In addition, it provides a framework for future systematic experimental and theoretical investigations of the structure-property relationship in CNT forests, as well as a framework for tuning the properties of CNT forests for diverse applications.PhDCommittee Co-Chair: Cola, Baratunde; Committee Member: Garmestani, Hamid; Committee Member: Graham, Samuel; Committee Member: Kumar, Satish; Committee Member: Zhu, Tin

PH-Responsive DNA Nanolinker Conjugated Hybrid Materials for Electrochemical Microactuator and Biosensor Applications

Copyright © 2018 American Chemical Society. Carbon nanotube (CNT)-based composite or hybrid materials have been broadly used for various biomedical applications such as microactuators, sensors, capacitors, and flexible electronic textiles because of their appealing physical and electrical properties and energy-storage functions. However, to enable application-based specific functionalities (e.g., sensing, responding, and deformation) it is essential that smart stimulus-responsive elements be incorporated into the CNT-based materials. A pioneering approach in integrating stimulus-responsive molecules or linkers is to utilize multistranded DNA structures, such as i-motif DNA with a four-folded structure, which shows reversible conformational changes upon pH alteration. Herein, a pH-responsive CNT-based hybrid material is developed by conjugating i-motif DNA as a pH-responsive nanosized cross-linker. To fabricate microfibers, we spun the i-motif DNA nanolinker-conjugated CNT-based hybrid material in a proton-rich coagulation bath. The attained hybrid microfibers are composed of partially aligned nanowires with â50 nm diameters that are formed in the protonation process by self-assembly of the i-motif DNA nanolinker-conjugated CNT-based hybrid material. The hybrid microfibers showed high electrical conductivity (â27 S/cm), excellent capacitance in a biological medium (â59.9 F/g at pH 5 and â47.8 F/g at pH 8), and stable microactuation without creep behavior. Furthermore, the conjugated i-motif DNA in the hybrid microfibers undergoes conformational changes from a four-folded structure (pH 5) to a random coil structure (pH 8), thus enabling unique dual-pH reversibility in the microfibers, namely switchable microporosity, electrochemical redox activity, and hydrogen peroxide sensing activity. Consequently, the designed stimulus-responsive hybrid microfiber can be used for microactuation and biosensing applications.