Shanling Wang

Recent observations of superconductivity in low-dimensional systems composed of twisted, untwisted, or rhombohedral graphene have attracted significant attention. One-dimensional moiré superlattices and flat bands have interestingly been identified in collapsed chiral carbon nanotubes (CNTs), opening up new avenues for the tunability of the electronic properties in these systems. The nucleation of hexagonal moiré superlattices and other types of stacking faults has also been demonstrated in partially collapsed and uncollapsed carbon nano-onions (CNOs). Here, we report a novel investigation on the dynamics of stacking fault nucleation within the multilayered lattices of micrometer-scale vertically oriented films of multiwall CNTs (MWCNTs), resulting from the pyrolysis of molecular precursors consisting of ferrocene or dimethyl ferrocene, at low vapor flow rates of ~5–20 mL/min. Interestingly, local nucleation of moiré-like superlattices (as stacking faults) was found when employing dimethyl ferrocene as the pyrolysis precursor. The morphological and structural properties of these systems were investigated with the aid of scanning and transmission electron microscopies, namely SEM, TEM, and HRTEM, as well as X-ray diffraction (XRD) and Raman point/mapping spectroscopy. Deconvolution analyses of the Raman spectra also demonstrated a local surface oxidation, possibly occurring on defect-rich interfaces, frequently identified within or in proximity of bamboo-like graphitic caps. By employing high-temperature Raman spectroscopy, we demonstrate a post-growth re-graphitization, which may also be visualized as an alternative way of depleting the oxygen content within the MWCNTs’ interfaces through recrystallization.

Occurrence of magnetic-correlation-phenomena in multi-layered carbon-materials has recently attracted an important attention for applications in magnetic devices and spintronics. In this study, exfoliated highly-oriented-pyrolytic-graphite (HOPG) lamellae exhibiting hexagonal-Moiré-superlattices, with periodicity of ∼13 nm (1st category, θ _rot ∼ 1.09° ) and ∼36 nm (2nd category, θ _rot ∼ 0.39°) were investigated. Raman-spectroscopy evidenced weak D, D’ and intense G bands. In 1st category, magnetization versus field, ZFC- FC magnetic-curves from 2 K to 300 K and T-ESR revealed presence of uncorrelated and correlated ferromagnetic clusters at T* ∼ 150 K together with a critical transition at T_c ∼ 50 K, compatible with percolative-ferromagnetic-correlation. Comparative measurements on the 2nd category, revealed an analogue trend, with at T* ∼ 50–60 K together with an irreversibility at T_c ∼ 40 K, indicative of competing ferromagnetic/antiferromagnetic-correlations.

Carbon nano-onions are approximately spherical nanoscale graphitic shells. When filled with ferromagnetic Fe₃C/α-Fe nanocrystals, these structures have several important applications, such as point electron-sources, magnetic data recording, energy storage, and others, that exploit the interaction of either or both the shells and the magnetic moments in the filling. Despite these applications receiving much recent attention, little is known about the structural relationship between the carbon shells and the internal nanocrystal. In this work, the graphitic c-axis lattice-spacing in Fe₃C/α-Fe-filled multi-shell structures was determined by XRD in the temperature range from 130 K to 298 K. A significant reduction in the c-axis lattice-spacing was observed in the multi-shell structures. A defect-induced magnetic transition was probed and ascribed to the formation of randomly oriented ferromagnetic clusters in the recrystallized disclination-rich regions of the CNOs-shells, in agreement with the percolative theory of ferromagnetism.

We show a novel chemical vapour deposition (CVD) approach, in which the large-scale fabrication of ferromagnetically-filled cm-scale buckypapers is achieved through the deposition of a mesoporous supported catalyst (SBA-16) on a silicon substrate. We demonstrate that SBA-16 has the crucial role of promoting the growth of carbon nanotubes (CNTs) on a horizontal plane with random orientation rather than in a vertical direction, therefore allowing a facile fabrication of cm-scale CNTs buckypapers free from the onion-crust by-product observed on the buckypaper-surface in previous reports. The morphology and composition of the obtained CNTs-buckypapers are analyzed in detail by scanning electron microscopy (SEM), Energy Dispersive X-ray (EDX), transmission electron microscopy (TEM), high resolution TEM (HRTEM), and thermogravimetric analysis (TGA), while structural analysis is performed by Rietveld Refinement of XRD data. The room temperature magnetic properties of the produced buckypapers are also investigated and reveal the presence of a high coercivity of 650 Oe. Additionally, the electrochemical performances of these buckypapers are demonstrated and reveal a behavior that is compatible with that of a pseudo-capacitor (resistive-capacitor) with better performances than those presented in other previously studied layered-buckypapers of Fe-filled CNTs, obtained by pyrolysis of dichlorobenzene-ferrocene mixtures. These measurements indicate that these materials show promise for applications in energy storage systems as flexible electrodes.

One of the major challenges in the field of carbon nanomaterials consists of understanding their growth mechanism dynamics. Recent reports have shown preliminary steady state chemical vapour deposition (CVD) experiments for the encapsulation of FePd-based alloys inside carbon-based nanostructures. However, very little is known about their growth mechanism dynamics. Here we investigate the possible presence of local changes of “carbon-to-metal ratio” in the growth mechanism of carbon nanostructures grown from FePd-based and Fe₃C catalyst-particles by steady state CVD and viscous-boundary-layer chemical vapour synthesis (CVS) experiments involving the pyrolysis of ferrocene/dichlorocyclooctadiene-palladium mixtures and of ferrocene/dichlorobenzene mixtures. In the first case, we demonstrate the observation of an unusual growth mechanism in which a direct transition from a spherically elongated carbon-onion-like (CNOs-like) morphology (low carbon-to-metal ratio condition) to a carbon fiber-like morphology (high carbon-to-metal ratio condition) is present within the same carbon structure. We attribute such transitions to the variation of the Cl-radicals concentration during the pyrolysis process, which implies a variation in the local “carbon to metal ratio” parameter. The reported mechanism is then compared in detail with that of Fe₃C filled CNTs obtained by CVD of ferrocene/dichlorobenzene mixtures and with those reported in the literature for other transition metal catalyst systems. In contrast, when a viscous boundary layer is created between the pyrolysed ferrocene/dichlorocyclooctadiene-palladium precursors and a rough surface, radial CNT structures filled with large quantities of both FePd₃ and Fe₃C crystals are found as major reaction products. The presence of filled-CNOs within the radial structures-core (resulting from the homogeneous nucleation of particles in the viscous boundary layer) suggests that fluctuations in the local “carbon-to-metal ratio” are also present in this type of mechanism. The morphological, cross-sectional and structural properties of the obtained carbon structures are analyzed in detail by SEM, TEM, STEM, HRTEM, ED and X-ray diffraction.

FePd₃-type alloys have attracted strong interest due to their unusual pressure-induced Invar behaviour characterized by anomalously low thermal expansion properties. However, little is known about the factors controlling their magnetization properties. Here we present a chemical vapour deposition approach which allows the encapsulation of FePd₃ alloys into a spherical type of carbon nanomaterial consisting of concentrically arranged distorted-carbon-layers. A dependence of the magnetic properties of this soft ferromagnetic phase on the crystal-grain-size is found by comparing the results in the present study with those reported in literature. The fabricated samples are characterized in detail by electron microscopy, x-ray and electron diffraction and magnetometry.