Delamination Mechanics of Carbon Nanotube Micropillars.

The adhesion of carbon nanotube (CNT) forests to their growth substrate is a critical concern for many applications. Here, we measured the delamination force of CNT forest micropillars using in situ scanning electron microscopy (SEM) tensile testing. A flat tip with epoxy adhesive first established contact with the top surface of freestanding CNT pillars and then pulled the pillars in displacement-controlled tension until delamination was observed. An average delamination stress of 6.1 MPa was measured, based on the full pillar cross-sectional area, and detachment was observed to occur between catalyst particles and the growth substrate. Finite element simulations of CNT forest delamination show that force and strain are heterogeneously distributed among CNTs during tensile loading and that CNTs progressively lose adhesion with increased displacement. Based on combined experiments and simulations, an adhesion strength of approximately 350 MPa was estimated between each CNT and the substrate. These findings provide important insight into CNT applications such as thermal interfaces, mechanical sensors, and structural composites while also suggesting a potential upper limit of tensile forces allowed during CNT forest synthesis.

Isolating the Roles of Hydrogen Exposure and Trace Carbon Contamination on the Formation of Active Catalyst Populations for Carbon Nanotube Growth.

Limited understanding of the factors influencing the yield of carbon nanotubes (CNTs) relative to the number of catalyst particles remains an important barrier to their large-scale production with high quality, and to tailoring CNT properties for applications. This lack of understanding is evident in the frequent use of Edisonian approaches to give high-yield CNT growth, and in the sometimes-confusing influence of trace residues on the reactor walls. In order to create conditions wherein CNT yield is reproducible and to enable large-scale and reliable CNT synthesis, it is imperative to understand-fundamentally-how these common practices impact catalytic activity and thus CNT number density. Herein, we use ambient pressure-X-ray photoelectron spectroscopy (AP-XPS) to reveal the influence of carbon and hydrogen on the coupling between catalyst reduction and CNT nucleation, from an iron catalyst film. We observe a positive correlation between the degree of catalyst reduction and the density of vertically aligned CNTs (forests), verifying that effective catalyst reduction is critical to CNT nucleation and to the resulting CNT growth yield. We demonstrate that the extent of catalyst reduction is the reason for low CNT number density and for lack of self-organization, lift-off, and growth of CNT forests. We also show that hydrocarbon byproducts from consecutive growths can facilitate catalyst reduction and increase CNT number density significantly. These findings suggest that common practices used in the field-such as reactor preconditioning-aid in the reduction of the catalyst population, thus improving CNT number density and enabling the growth of dense forests. Our results also motivate future work using AP-XPS and complementary metrology tools to optimize CNT growth conditions according to the catalyst chemical state.

A Scalable Route to Nanoporous Large-Area Atomically Thin Graphene Membranes by Roll-to-Roll Chemical Vapor Deposition and Polymer Support Casting.

Scalable, cost-effective synthesis and integration of graphene is imperative to realize large-area applications such as nanoporous atomically thin membranes (NATMs). Here, we report a scalable route to the production of NATMs via high-speed, continuous synthesis of large-area graphene by roll-to-roll chemical vapor deposition (CVD), combined with casting of a hierarchically porous polymer support. To begin, we designed and built a two zone roll-to-roll graphene CVD reactor, which sequentially exposes the moving foil substrate to annealing and growth atmospheres, with a sharp, isothermal transition between the zones. The configurational flexibility of the reactor design allows for a detailed evaluation of key parameters affecting graphene quality and trade-offs to be considered for high-rate roll-to-roll graphene manufacturing. With this system, we achieve synthesis of uniform high-quality monolayer graphene ( ID/ IG < 0.065) at speeds ≥5 cm/min. NATMs fabricated from the optimized graphene, via polymer casting and postprocessing, show size-selective molecular transport with performance comparable to that of membranes made from conventionally synthesized graphene. Therefore, this work establishes the feasibility of a scalable manufacturing process of NATMs, for applications including protein desalting and small-molecule separations.

Direct fabrication of graphene on SiO2 enabled by thin film stress engineering.

We demonstrate direct production of graphene on SiO2 by CVD growth of graphene at the interface between a Ni film and the SiO2 substrate, followed by dry mechanical delamination of the Ni using adhesive tape. This result is enabled by understanding of the competition between stress evolution and microstructure development upon annealing of the Ni prior to the graphene growth step. When the Ni film remains adherent after graphene growth, the balance between residual stress and adhesion governs the ability to mechanically remove the Ni after the CVD process. In this study the graphene on SiO2 comprises micron-scale domains, ranging from monolayer to multilayer. The graphene has >90% coverage across centimeter-scale dimensions, limited by the size of our CVD chamber. Further engineering of the Ni film microstructure and stress state could enable manufacturing of highly uniform interfacial graphene followed by clean mechanical delamination over practically indefinite dimensions. Moreover, our findings suggest that preferential adhesion can enable production of 2-D materials directly on application-relevant substrates. This is attractive compared to transfer methods, which can cause mechanical damage and leave residues behind.