Subscriber access provided by Caltech Library Article Local Structure and Bonding of Carbon Nanothreads Probed by High-Resolution Transmission Electron Microscopy Stephen J. Juhl, Tao Wang, Brian Vermilyea, Xiang Li, Vincent H. Crespi, John V. Badding, and Nasim Alem J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13405 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 8, 2019 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Page 1 of 13 Journal of the American Chemical Society 1 2 3 4 5 6 7 Local Structure and Bonding of Carbon Nanothreads Probed by 8 High-Resolution Transmission Electron Microscopy 9 10 Stephen J. Juhl,1 Tao Wang,2 Brian Vermilyea,2 Xiang Li,1,† Vincent H. Crespi,1,2,3,4 John V. 11 1,2,3,4 3,4,* 12 Badding, and Nasim Alem 13 1 14 Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, USA 15 2Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, USA 16 3Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 17 16802, USA 18 19 4Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA 20 ABSTRACT: Carbon nanothreads are a new one-dimensional sp3-bonded nanomaterial of CH stoichiometry synthesized 21 from benzene at high pressure and room temperature by slow solid-state polymerization. The resulting threads assume 22 crystalline packing hundreds of microns across. We show high-resolution electron microscopy (HREM) images of 23 hexagonal arrays of well-aligned thread columns that traverse the 80–100 nm thickness of the prepared sample. Diffuse 24 25 scattering in electron diffraction reveals that nanothreads are packed with axial and/or azimuthal disregistry between 26 them. Layer lines in diffraction from annealed nanothreads provide the first evidence of translational order along their 27 length, indicating that this solid-state reaction proceeds with some regularity. HREM also reveals bends and defects in 28 nanothread crystals that can contribute to the broadening of their diffraction spots, and electron energy-loss spectroscopy 29 confirms them to be primarily sp3 hybridized, with less than 27% sp2 carbon, most likely associated with partially saturated 30 “degree-4” threads. 31 32 33 34 12 35 INTRODUCTION and hydrocarbon molecules. Their properties have been 36 extensively investigated computationally. For example, 1 37 Carbon nanothreads, predicted independently in 2001, they may uniquely combine extreme strength with 2 3 3 38 2011, and 2014, are sp -bonded one-dimensional (1D) flexibility and resilience,13-15 have higher load transference 39 nanomaterials synthesized by slow polymerization of solid to polymer matrices than carbon nanotubes due to more 4 40 benzene at high pressure. They assemble into close- irregular surface topography,16-17 and may support 41 packed near-hexagonal crystals ordered across hundreds torsional deformations ~3 times those of sp2 nanotubes 5 42 of microns (even when the benzene precursor is for which flattening degrades mechanical performance.17 43 polycrystalline) apparently guided by a uniaxial Modeling also suggests certain nanothreads may behave 44 component to the applied pressure and packing as resonators with higher quality factors than carbon 45 constraints of rod-like objects under pressure. Many fully- nanotubes,18 could exhibit electronic properties tunable 46 6 saturated “degree-6” and partially-unsaturated “degree- by tensile strain,19 and may show superplastic behavior.20 47 7 4” nanothread structures have been enumerated by Their thermal conductivity is predicted to be tunable 48 group theoretical analysis. Nanothreads are chemically depending on axial structural order.21-22 The ability of 49 8-9 versatile: they can be formed from heteroaromatic narrow, stiff,13 straight nanothreads to spatially organize 50 molecules such as pyridine10 and their exterior hydrogens 51 both interior heteroatoms and exterior groups relative to can be substituted without disrupting the carbon each other may be broadly useful in application areas such 52 backbone,11 in contrast to sp2 carbon nanotubes whose 53 as the life sciences, photovoltaics, battery materials, and sidewall functionalization requires a change in carbon 54 catalysis.12,23 As hydrocarbons, nanothreads can likely be hybridization. Nanothreads are thus both nanomaterials 55 made more robust against degradation (e.g. 56 1 57 58 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 13 1 2 3 photodegradation and environmental chemical attack) time layer lines in electron diffraction on samples aligned 4 than can other stiff 1D molecules that can organize at a non-zero angle to the thread axis: i.e. nanothreads 5 attached chemical functions (e.g. DNA, cellulose, etc.).4,24- have axial translational order over at least a few 6 25 Similar to hydrogen-terminated diamond surfaces, nanometers. 7 nanothreads are expected to have a negative electron 8 affinity, which may make them attractive for field emission EXPERIMENTAL 9 applications.4 All of these properties are sensitive to the Carbon nanothreads were imaged using an FEI Titan3 G2 10 details of the nanothread structure at length scales from high-resolution TEM equipped with a spherical 11 atomic to micron, motivating multidisciplinary structural aberration-corrected lens and an electron 12 investigations. monochromator (see Supporting Information). 13 Nanothreads have been studied by x-ray diffraction,4-5 Nanothread samples were prepared for TEM imaging and 14 spectroscopy by crushing in an agate mortar with 15 which reveals near-six-fold diffraction patterns characteristic of pseudohexagonal crystals with an a-axis cyclohexene or liquid nitrogen (both gave similar results) 16 and drop-casting onto lacey carbon grids. As 17 lattice parameter of 6.47 Å. The reduced real-space pair hydrocarbons, nanothreads are susceptible to electron 18 distribution function indicates a spatial correlation length beam damage, although less so than conventional 19 of about 15 nm for translational order in the a-b plane, i.e. polymers such as polyethylene30-32 apparently due to the 20 perpendicular to the nanothread axes. The fraction of sp3- 21 bonded carbon is 75% to 80% according to nuclear robustness of their carbon backbone. The 2D 22 magnetic resonance (NMR) spectroscopy.4,26 NMR also pseudohexagonal packing of nanothreads was observed 2 23 reveals that nanothread crystals have fully saturated to expand for doses above 30 electrons/Å . Thus, we used 24 degree-6 regions at least 2.5 nm long. Degree-6 threads a low electron dose from a Nelsonian illumination scheme 25 comprise approximately a third of that sample, with in combination with the monochromator aperture to 33-35 26 partially saturated degree-4 threads containing sp2 carbon minimize beam damage. 27 comprising another third. NMR also provides intriguing RESULTS AND DISCUSSION 28 suggestions of a 4+2-based reaction pathway.26-27 The carbon K-edge of the EELS spectrum at 284 eV 29 Although there is evidence of defect sites such as ortho-, 30 acquired from the edge of a nanothread particle can be fit meta-, or para-coordinated phenyl rings, there is no 31 by three Gaussian peaks associated with the 1s-to-π*(sp2), evidence for well-organized non-thread minority phases. 32 1s-to-σ*(sp3), and 1s-to-σ*(sp2) transitions (Figure 1).36 The details of the spatial arrangement of degree-4 and 33 The sp2 and sp3 contents estimated by the two-window degree-6 threads relative to each other is not yet known. 34 method with a glassy carbon sp2 standard37 (Figure S1) are Preliminary transmission electron microscopy (TEM) 35 27% and 73%, respectively, consistent with the range imaging of nanothreads shows parallel striations with 6.4- 36 determined by solid-state NMR.4 This level of agreement Å spacing, larger than the 5.6-Å spacing expected for the 37 between a highly local probe (TEM) and a global probe hexagonal {1010} planes of nanothreads with a 6.47-Å a- 38 (ss-NMR) suggests reasonable sample uniformity. NMR 39 axis lattice constant, with no information about axial order reveals that the sp2 carbon is associated with degree-4 40 or interthread registry.4 nanothreads and a small fraction of benzene linkers. The 41 Electron microscopy plays a seminal role in thickness of several samples was estimated to be 80–100 42 characterizing the structure and bonding of new carbon nm using the log-ratio method in EELS (see Supporting 43 nanomaterials28-29 by directly imaging unique modes of 44 Information).