Monochromated Low-Dose Aberration-Corrected Transmission Electron Microscopy of Diamondoid Carbon Nanothreads

Abstract

Carbon nanothreads are the first in a new class of one-dimensional sp 3 carbon nanomaterials that was recently discovered through compression-induced polymerization of benzene in a Paris-Edinburgh cell [1]. Their diamond-like structure is predicted to have interesting mechanical properties that are on par with or greater than carbon nanotubes [2]. In addition, the solid-state synthesis is entirely driven by kinetic control and devoid of waste, which may open up new routes to fabricate efficient carbon nanomaterials [3,4]. Therefore, it is important to probe the atomic and chemical structure of nanothreads to elucidate the reaction mechanisms that result in their synthesis and determine their macroscale physical and chemical properties.X-ray diffraction (XRD) studies and density functional theory (DFT) calculations strongly suggest that carbon nanothreads form a well-aligned, hexagonally-packed structure with an inter-thread spacing of ~6.5 angstroms [1]. Likewise, electron diffraction patterns reveal higher order reflections that agree with these experimentally and theoretically-derived models. Direct observation of the atomic structure of nanothreads is complicated by the low threshold of carbon nanomaterials for beam damage in the electron microscope [5]. Similarly, carbon nanothreads' one-dimensional tetrahedral structure is akin to polymers, which makes them susceptible to electron beam damage. In this study, the structural degradation of carbon nanothreads is examined through selected-area electron diffraction, and the critical dose is determined. An electron gun monochromator is then employed to reduce the electron dose and minimize beam irradiation and damage [6] while imaging the nanothreads on a high-resolution aberration-corrected transmission electron microscope. High-resolution images and their Fourier transforms in Figure 1 demonstrate the effect of electron dose on the sample. Figure 1a shows the structural degradation of the nanothreads when exposed to a high electron dose, which is indicated by the amorphous morphology. Figure 1b presents a low-dose image of the highly-oriented nanothread structure, which displays 6.4-angstrom inter-thread spacing and large-scale crystallinity in the sample. These direct observations agree with the predicted 6.5-angstrom hexagonally-packed lattice shown in Figure 1c. This study further employs low-dose imaging techniques to explore a variety of sample preparation techniques, which are intended to separate individual nanothreads in order to determine their atomic and chemical structure.

The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR

“…X-ray and electron diffraction experiments reveal symmetric two-dimensional spot patterns that provide compelling evidence for hexagonal single crystal packings hundreds of microns across of carbon (and carbon nitride) nanothreads that are spaced ∼6.5 Å apart, in good agreement with the spacing predicted by modeling of degree-6 nanothreads . The threads and their hexagonal packing are evident in transmission electron microscopy (TEM) imaging as well. , However, although NMR spectroscopy has revealed that nanothreads consist primarily (75–80%) of sp 3 -bonded carbon, the details of the chemical structure along their length are not yet understood. Fifty different degree-6 nanothread structures with 6 sp 3 -carbons per benzene formula unit (Figure ) have been enumerated by theory .…”

“… , Many of the pure degree-6 and pure degree-4 nanothread structures have approximately the same diameter and thus cannot be easily distinguished from each other or from mixtures of both degrees merely by geometric packing considerations, although detectable differences in lattice constants are expected in certain cases . Uniform “beading” patterns parallel to the nanothread axes and ∼15 nm in length (i.e., ∼100 carbon bond lengths) are observed in TEM images, suggesting that local order can be present over at least this distance …”

“…2 The threads and their hexagonal packing are evident in transmission electron microscopy (TEM) imaging as well. 1,17 However, although NMR spectroscopy has revealed that nanothreads consist primarily (75−80%) of sp 3 -bonded carbon, 1 the details of the chemical structure along their length are not yet understood. Fifty different degree-6 nanothread structures with 6 sp 3 -carbons per benzene formula unit (Figure 1) have been enumerated by theory.…”

“…2 Uniform "beading" patterns parallel to the nanothread axes and ∼15 nm in length (i.e., ∼100 carbon bond lengths) are observed in TEM images, suggesting that local order can be present over at least this distance. 17 Knowledge of the atomic structure of nanothreads is important to understanding both their structure−property relations for applications 2,11,13 and the nontopochemical synthesis reaction, and also for improving synthesis protocols to enable the production of large quantities of pure nanothreads of desired structure, hybridization, and composition. Solid-state nuclear magnetic resonance (NMR) spectroscopy is an excellent method for comprehensive structural and quantitative 18 compositional analysis of organic materials 19−27 and has played a seminal role in elucidating structure−property relations as new carbon nanomaterials such as sp 2 -bonded nanotubes 28 and sp 3 -bonded nanodiamond 29 have emerged.…”

Spatially Resolved Functional Group Analysis of OLED Materials Using EELS and ToF-SIMS

“…Further, the alignment and optimization of the STEM for EELS measurements were performed in areas away from the regions of interest to prevent preirradiation damage. Additionally, the electron beam was blanked before and after the EELS measurements to further protect the sample. − …”