Enhanced Tunnelling in a Hybrid of Single-Walled Carbon Nanotubes and Graphene

Transparent and conductive films (TCFs) are of great technological importance. The high transmittance, electrical conductivity and mechanical strength make single-walled carbon nanotubes (SWCNTs) a good candidate for their raw material. Despite the ballistic transport in individual SWCNTs, however, the electrical conductivity of their networks is limited by low efficiency of charge tunneling between the tube elements. Here, we demonstrate that the nanotube network sheet resistance at high optical transmittance is decreased by more than 50% when fabricated on graphene and thus provides a comparable improvement as widely adopted gold chloride ($\mathrm{AuCl_3}$) doping. However, while Raman spectroscopy reveals substantial changes in spectral features of doped nanotubes, no similar effect is observed in presence of graphene. Instead, temperature dependent transport measurements indicate that graphene substrate reduces the tunneling barrier heights while its parallel conductivity contribution is almost negligible. Finally, we show that combining the graphene substrate and $\mathrm{AuCl_3}$ doping, the SWCNT thin films can exhibit sheet resistance as low as 36 $\Omega$/sq. at 90% transmittance.Comment: 21 pages, 6 figure

Creation of single vacancies in hBN with electron irradiation

Understanding electron irradiation effects is vital not only for reliable transmission electron microscopy characterization, but increasingly also for the controlled manipulation of two-dimensional materials. The displacement cross sections of monolayer hBN are measured using aberration-corrected scanning transmission electron microscopy in near ultra-high vacuum at primary beam energies between 50 and 90 keV. Damage rates below 80 keV are up to three orders of magnitude lower than previously measured at edges under poorer residual vacuum conditions where chemical etching appears to have been dominant. Notably, is possible to create single vacancies in hBN using electron irradiation, with boron almost twice as likely as nitrogen to be ejected below 80 keV. Moreover, any damage at such low energies cannot be explained by elastic knock-on, even when accounting for vibrations of the atoms. A theoretical description is developed to account for lowering of the displacement threshold due to valence ionization resulting from inelastic scattering of probe electrons, modelled using charge-constrained density functional theory molecular dynamics. Although significant reductions are found depending on the constrained charge, quantitative predictions for realistic ionization states are currently not possible. Nonetheless, there is potential for defect-engineering of hBN at the level of single vacancies using electron irradiation.Comment: 38 pages, 15 figure

The morphology of doubly-clamped graphene nanoribbons

Understanding the response of micro/nano-patterned graphene to mechanical forces is instrumental for applications such as advanced graphene origami and kirigami. Here, we analyze free-standing nanoribbons milled into single-layer graphene by focused ion beam processing. Using transmission electron microscopy, we show that the length L of the structures determines their morphology. Nanoribbons with L below 300 nm remain mainly flat, whereas longer ribbons exhibit uni-axial crumpling or spontaneous scrolling, a trend that is well reproduced by molecular dynamics simulations. We measure the strain of the ribbons as well as their crystallinity by recording nanometer-resolved convergent beam electron diffraction maps, and show that the beam tails of the focused ion beam cause significant amorphization of the structures adjacent to the cuts. The expansive or compressive strain in the structures remains below 4%. Our measurements provide experimental constraints for the stability of free-standing graphene structures with respect to their geometry, providing guidelines for future applications of patterned graphene

Buckyball sandwiches

Two-dimensional (2D) materials have considerably expanded the field of materials science in the past decade. Even more recently, various 2D materials have been assembled into vertical van der Waals heterostacks, and it has been proposed to combine them with other low-dimensional structures to create new materials with hybridized properties. We demonstrate the first direct images of a suspended 0D/2D heterostructure that incorporates C60 molecules between two graphene layers in a buckyball sandwich structure. We find clean and ordered C60 islands with thicknesses down to one molecule, shielded by the graphene layers from the microscope vacuum and partially protected from radiation damage during scanning transmission electron microscopy imaging. The sandwich structure serves as a 2D nanoscale reaction chamber, allowing the analysis of the structure of the molecules and their dynamics at atomic resolution.© The Author

Atomic-Scale Deformations at the Interface of a Mixed-Dimensional van der Waals Heterostructure

Molecular self-assembly due to chemical interactions is the basis of bottom-up nanofabrication, whereas weaker intermolecular forces dominate on the scale of macromolecules. Recent advances in synthesis and characterization have brought increasing attention to two- and mixed-dimensional heterostructures, and it has been recognized that van der Waals (vdW) forces within the structure may have a significant impact on their morphology. Here, we suspend single-walled carbon nanotubes (SWCNTs) on graphene to create a model system for the study of a 1D–2D molecular interface through atomic-resolution scanning transmission electron microscopy observations. When brought into contact, the radial deformation of SWCNTs and the emergence of long-range linear grooves in graphene revealed by the three-dimensional reconstruction of the heterostructure are observed. These topographic features are strain-correlated but show no sensitivity to carbon nanotube helicity, electronic structure, or stacking order. Finally, despite the random deposition of the nanotubes, we show that the competition between strain and vdW forces results in aligned carbon–carbon interfaces spanning hundreds of nanometers.Copyright © 2018 American Chemical Societ