Stephen J. Juhl

Compressed glassy carbon: An ultrastrong and elastic interpenetrating graphene network

The compression of glassy carbon forms a series of lightweight, ultrastrong, hard, elastic, and conductive carbons. Carbon’s unique ability to have both sp2 and sp3 bonding states gives rise to a range of physical attributes, including excellent mechanical and electrical properties. We show that a series of lightweight, ultrastrong, hard, elastic, and conductive carbons are recovered after compressing sp2-hybridized glassy carbon at various temperatures. Compression induces the local buckling of graphene sheets through sp3 nodes to form interpenetrating graphene networks with long-range disorder and short-range order on the nanometer scale. The compressed glassy carbons have extraordinary specific compressive strengths—more than two times that of commonly used ceramics—and simultaneously exhibit robust elastic recovery in response to local deformations. This type of carbon is an optimal ultralight, ultrastrong material for a wide range of multifunctional applications, and the synthesis methodology demonstrates potential to access entirely new metastable materials with exceptional properties.

Carbon Nitride Nanothread Crystals Derived from Pyridine

Carbon nanothreads are a new one-dimensional sp3 carbon nanomaterial. They assemble into hexagonal crystals in a room temperature, nontopochemical solid-state reaction induced by slow compression of benzene to 23 GPa. Here we show that pyridine also reacts under compression to form a well-ordered sp3 product: C5NH5 carbon nitride nanothreads. Solid pyridine has a different crystal structure from solid benzene, so the nontopochemical formation of low-dimensional crystalline solids by slow compression of small aromatics may be a general phenomenon that enables chemical design of properties. The nitrogen in the carbon nitride nanothreads may improve processability, alters photoluminescence, and is predicted to reduce the bandgap.

Tetracyanomethane under Pressure: Extended CN Polymers from Precursors with Built-in sp3 Centers

Tetracyanomethane, C(CN)4, is a tetrahedral molecule containing a central sp3 carbon that is coordinated by reactive nitrile groups that could potentially transform to an extended CN network with a significant fraction of sp3 carbon. High-purity C(CN)4 was synthesized, and its physiochemical behavior was studied using in situ synchrotron angle-dispersive powder X-ray diffraction (PXRD) and Raman and infrared (IR) spectroscopies in a diamond anvil cell (DAC) up to 21 GPa. The pressure dependence of the fundamental vibrational modes associated with the molecular solid was determined, and some low-frequency Raman modes are reported for the first time. Crystalline molecular C(CN)4 starts to polymerize above ∼7 GPa and transforms into an interconnected disordered network, which is recoverable to ambient conditions. The results demonstrate feasibility for the pressure-induced polymerization of molecules with premeditated functionality.

Low Dose Characterization of Diamondoid Carbon Nanothreads by Transmission Electron Microscopy

Soft materials present a difficult case for characterization in the transmission electron microscope (TEM). Due to the use of high energy electrons, samples degrade rapidly from knock-on damage and radiolysis. Still, the atomic-scale structural and chemical information that can be gained from the TEM are attractive for newly discovered materials with nanoscale crystallinity. Diamondoid carbon nanothreads are one such material, which has been produced by compressing benzene to extremely high pressures in a diamond anvil cell.[1] X-ray diffraction data shows the nanothreads consist of cylindrically-symmetric columns of charge that are packed in a two-dimensional hexagonal lattice.[1] However, pair-distribution function analysis shows that the axial order extends to only 15 nm. Therefore, it is ideal to probe the atomic and chemical structure of diamondoid carbon nanothreads through a battery of low-dose TEM techniques.

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

Carbon nanothreads are the first in a new class of one-dimensional sp 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.

Synthesis of Bulk BC8 Silicon Allotrope by Direct Transformation and Reduced-Pressure Chemical Pathways.

Phase-pure samples of a metastable allotrope of silicon, Si-III or BC8, were synthesized by direct elemental transformation at 14 GPa and ∼900 K and also at significantly reduced pressure in the Na-Si system at 9.5 GPa by quenching from high temperatures ∼1000 K. Pure sintered polycrystalline ingots with dimensions ranging from 0.5 to 2 mm can be easily recovered at ambient conditions. The chemical route also allowed us to decrease the synthetic pressures to as low as 7 GPa, while pressures required for direct phase transition in elemental silicon are significantly higher. In situ control of the synthetic protocol, using synchrotron radiation, allowed us to observe the underlying mechanism of chemical interactions and phase transformations in the Na-Si system. Detailed characterization of Si-III using X-ray diffraction, Raman spectroscopy, (29)Si NMR spectroscopy, and transmission electron microscopy are discussed. These large-volume syntheses at significantly reduced pressures extend the range of possible future bulk characterization methods and applications.