Multishell Fullerenes Research Articles

Origin of metallic-like behavior in disordered carbon nano-onions

Carbon nano-onions are a class of nanomaterials that can exhibit long electron spin relaxation times at room temperature and thus hold promise as potential building blocks for spintronics and quantum information processing devices. Despite first being synthesized 30 years ago, there exists a gap in understanding electronic and magnetic properties of these nanostructures. Here we investigate the origin of the metallic-like behavior that has been observed experimentally in disordered nano-onions. Employing a density functional tight-binding approach and starting from ordered multi-shell fullerenes, we develop realistic models of highly disordered nano-onions comprised of nanometer-scale graphitic flakes. We find that multiple parameters such as flake size, edge structure and substituent groups can give rise to in-gap metallic states, effectively closing the HOMO-LUMO gap.

Simulation of multi-shell fullerenes using Machine-Learning Gaussian Approximation Potential

Multi-shell fullerenes "buckyonions" were simulated, starting from initially random configurations, using a density-functional-theory (DFT)-trained machine-learning carbon potential within the Gaussian Approximation Potential (ML-GAP) Framework [Volker L. Deringer and Gabor Csanyi, Phys. Rev. B 95, 094203 (2017)]. A large set of such fullerenes were obtained with sizes ranging from 60 ~ 3774 atoms. The buckyonions are formed by clustering and layering starts from the outermost shell and proceed inward. Inter-shell cohesion is partly due to interaction between delocalized $\pi$ electrons into the gallery. The energies of the models were validated ex post facto using density functional codes, VASP and SIESTA, revealing an energy difference within the range of 0.02 - 0.08 eV/atom after conjuagte gradient energy convergence of the models were achieved with both methods.

Fundamental Study of the Dispersion of Carbon Nano-Onions

Since the discovery of the fullerene C60 in 1985 by Curl, Kroto and Smalley, carbon nanomaterials have been the focus of interdisciplinary chemical research. Multi-shell fullerenes, known as carbon nano-onions (CNOs) discovered by Ugarte in 1992, are structured by concentric shells of carbon atoms. Different methods for their synthesis have been developed and their properties have been widely studied. In addition, the chemical functionalization of CNOs has been investigated and several synthetic pathways were found to be applicable for the introduction of a variety of functional groups. The purpose of this project is to make the oxidized CNOs bear a relative centrifugal force at different time durations to analyzed how well the material disperses. This in turn, will provide a fundamental concept for the synthesis efficiency. According to preliminary results, there is a tendency on which the concentrated CNOs difference in weight gets narrow, as duration of the centrifugal force increases. Chemically modified CNOs were probed in different fields of application and have revealed to be a promising nanomaterial that attracts a growing interest among researchers and opens new avenues for investigation. Its development starts using nano-diamonds; a carbon nanomaterial of strong structure which undergoes through a temperature of approximately 1650°C inside a furnace to obtain the CNOs structure. The CNOs are then oxidize through synthesis that requires a constant temperature and stirring to attain oxygen based functional groups. Another way to obtain the oxygen based functional groups is through heating the sample at 400°C for two hours; these procedures are then analyzed to determine the efficiency of functionalization. The preliminary elemental detection of oxygen atoms was gathered by Energy-Dispersive Spectroscopy. To detect states in the CNO’s molecular system, Raman Spectroscopy was utilized to confirm the carbon structure of CNOs by the identification of the peaks D and G characteristic of this carbon material. An in-depth characterization analysis was done using X-Ray Diffraction, providing detailed information about chemical composition, and crystalline structure. The morphology and microstructure of CNOs were studied using a Transmission Electron Microscope and Scanning Electron Microscopy. Through Cyclic Voltammetry, CNO’s ink was prepared to characterize electrochemically comparing both functionalization processes. To study its catalytic properties, Oxygen Reduction Reaction (ORR) experiments was conducted utilizing Rotating Ring-Disk Electrode (RRDE). The ORR experiments were carried out by varying the rotation rates of the RRDE at 100, 400, 900, 1600 and 2500 rpm.

Carbon nano-onions as fluorescent on/off modulated nanoprobes for diagnostics.

Multishell fullerenes, known as carbon nano-onions (CNOs), have emerged as a platform for bioimaging because of their cell-penetration properties and minimal systemic toxicity. Here, we describe the covalent functionalization of CNOs with a π-extended distyryl-substituted boron dipyrromethene (BODIPY) dye with on/off modulated fluorescence emission activated by an acidic environment. The switching properties are linked to the photoinduced electron transfer (PET) characteristics of the dimethylamino functionalities attached to the BODIPY core. The on/off emission of the fluorescent CNOs is fast and reversible both in solution and in vitro, making this nanomaterial suitable as pH-dependent probes for diagnostic applications.

Processing of Onion-like Carbon for Electrochemical Capacitors

Multi-shell fullerenes known as onion-like carbon (OLC) are especially attractive in applications relative to energy storage, such as electrochemical capacitors, due to a near-spherical shape of particles, their nanoscale diameters and high conductivity leading to fast rate performance. Because of this, onion-like carbon can be fabricated into electrodes, used as a conductive additive, and may have potential in composites and additive manufacturing. However due to agglomeration of OLC particles, creating a stable, aqueous dispersion for ink production or formulating composites proves challenging. We explore how attrition milling, acid treatment, and probe sonication can be employed to decrease agglomeration and provide colloidal stability in aqueous media. We also investigate how the electrochemical performance changes with each processing step as well as the treatments in succession. When tested in electrochemical capacitors, the processing increases the capacitance by a factor of three, due to an added pseudocapacitive contribution which is not present in untreated OLC. As a result, the processing of OLC proves to be advantageous for the production of stable, aqueous solutions, which also exhibit improved electrochemical properties suitable for functional inks, conductive additives, and fabrication of composite electrodes.

Photoionization of multishell fullerenes studied by ab initio and model approaches*

Photoionization of two buckyonions, C60@C240 and C20@C60, is investigated by means of time-dependent density-functional theory (TDDFT). The TDDFT-based photoabsorption spectrum of C60@C240, calculated in a broad photon energy range, resembles the sum of spectra of the two isolated fullerenes, thus illustrating the absence of strong plasmonic coupling between the fullerenes which was proposed earlier. The calculated spectrum of the smaller buckyonion, C20@C60, differs significantly from the sum of the cross sections of the individual fullerenes because of strong geometrical distortion of the system. The contribution of collective electron excitations arising in individual fullerenes is evaluated by means of plasmon resonance approximation (PRA). An extension of the PRA formalism is presented, which allows for the study of collective electron excitations in multishell fullerenes under photon impact. An advanced analysis of photoionization of buckyonions, performed using modern computational and analytical approaches, provides valuable information on the response of complex molecular systems to the external electromagnetic field.

Surface analysis of zinc-porphyrin functionalized carbon nano-onions.

Multishell fullerenes, known as carbon nano-onions (CNOs), are an interesting class of carbon-based nanomaterials. They display several unique properties, such as a large surface area to volume ratio, a low density, and a graphitic multilayer morphology, which have made them appealing for several applications in many fields, including biology. Chemical functionalization of CNOs dramatically enhances their solubility and attenuates their inflammatory properties, thereby increasing their applicability especially in the fields of biology and medicine. CNOs functionalized with fluorescent probes can be used for cellular imaging. In this article, detailed surface characterization of CNOs functionalized with a zinc porphyrin (ZnTPP) as the fluorescent probe is presented. In particular, time-of-flight secondary ion mass spectrometry and x-ray photoelectron spectroscopy provide a detailed surface characterization of the organic functionalities introduced via "click chemistry" and clearly demonstrate the success of the CNOs functionalization process. XPS data reveal the presence of Zn and N, whilst ToF-SIMS is able to identify specific fragments related to the presence of the ZnTPP, such as the quasimolecular mass peak [C32N4H20Zn](+) and the molecular mass peak [C44N4H28Zn](+). Moreover, ions fragments deriving from the triazole ring formed by the click chemistry reaction have been identified by ToF-SIMS analysis, proving unambiguously the covalent binding of the fluorescent molecules to the CNOs surfaces.

High temperature stability of onion-like carbon vs highly oriented pyrolytic graphite.

The thermodynamic stability of onion-like carbon (OLC) nanostructures with respect to highly oriented pyrolytic graphite (HOPG) was determined in the interval 765–1030 K by the electromotive force (emf) measurements of solid electrolyte galvanic cell: (Low) Pt|Cr3C2,CrF2,OLC|CaF2s.c.|Cr3C2,CrF2,HOPG|Pt (High). The free energy change of transformation HOPG = OLC was found positive below 920.6 K crossing the zero value at this temperature. Its trend with temperature was well described by a 3rd degree polynomial. The unexpected too high values of jointly to the HR-TEM, STEM and EELS evidences that showed OLC completely embedded in rigid cages made of a Cr3C2/CrF2 matrix, suggested that carbon in the electrodes experienced different internal pressures. This was confirmed by the evaluation under constant volume of by the ratio for OLC (0.5 MPa K−1) and HOPG (8 Pa K−1) where and are the isobaric thermal expansion and isothermal compressibility coefficients, respectively. The temperature dependency of the pressure was derived and utilized to calculate the enthalpy and entropy changes as function of temperature and pressure. The highest value of the internal pressure experienced by OLC was calculated to be about 7 GPa at the highest temperature. At 920.6 K, and values are 95.8 kJ mol−1 and 104.1 JK−1 mol−1, respectively. The surface contributions to the energetic of the system were evaluated and they were found negligible compared with the bulk terms. As a consequence of the high internal pressure, the values of the enthalpy and entropy changes were mainly attributed to the formation of carbon defects in OLC considered as multishell fullerenes. The change of the carbon defect fraction is reported as a function of temperature.

Energy shift of collective electron excitations in highly corrugated graphitic nanostructures: Experimental and theoretical investigation

Effect of corrugation of hexagonal carbon network on the collective electron excitations has been studied using optical absorption and X-ray photoelectron spectroscopy in conjunction with density functional theory calculations. Onion-like carbon (OLC) was taken as a material, where graphitic mantle enveloping agglomerates of multi-shell fullerenes is strongly curved. Experiments showed that positions of π and π + σ plasmon modes as well as π → π* absorption peak are substantially redshifted for OLC as compared with those of highly ordered pyrolytic graphite and thermally exfoliated graphite consisted of planar sheets. This effect was reproduced in behavior of dielectric functions of rippled graphite models calculated within the random phase approximation. We conclude that the energy of electron excitations in graphitic materials could be precisely tuned by a simple bending of hexagonal network without change of topology. Moreover, our investigation suggests that in such materials optical exciton can transfer energy to plasmon non-radiatively.

Density functional theory based molecular dynamics simulation study on the bulk modulus of multi-shell fullerenes

We investigate the bulk modulus of single- and double-shell fullerenes as a function of diameter and shell distance at various temperatures using the DFT-based molecular dynamics simulation method. We found that total energies depend on the diameter and shell distance. The obtained results indicate that, with increasing diameter of single-shell fullerenes, the bulk modulus increases up to C180 fullerene and then decreases for larger diameter fullerenes. The bulk modulus of double-shell fullerenes is higher than both its internal and external constructing fullerenes and decreases as the shell distance increases. The bulk moduli of the considered systems are very weakly affected by the temperature.

Diffuse Interstellar Bands in (Proto-) Fullerene-Rich Environments

AbstractThe recent infrared detection of fullerenes (C60 and C70) in Planetary Nebulae (PNe) and R Coronae Borealis (RCB) stars offers a beautiful opportunity for studying the diffuse interstellar bands (DIBs) in sources where fullerenes are abundant. Here we present for the first time a detailed inspection of the optical spectra of the hot RCB star DY Cen and two fullerene PNe (Tc 1 and M 1-20), which permits us to directly explore the fullerenes - DIB connection. The DIB spectrum of DY Cen (García-Hernández et al. 2012a) is remarkably different from that in fullerene PNe (García-Hernández & Díaz-Luis 2013). In particular, Tc 1 displays unusually strong 4428 Å and 6309 Å DIBs, which are normal (or not seen) in DY Cen. On the other hand, DY Cen displays an unusually strong 6284 Å DIB that is found to be normal in fullerene PNe. We also report the detection of new broad and unidentified features centered at 4000 Å and 6525 Å in DY Cen and Tc 1, respectively. We suggest that the new 4000 Å band seen in DY Cen may be related to the circumstellar proto-fullerenes seen at infrared wavelengths (García-Hernández et al. 2012a). However, the intense 4428 Å DIB (probably also the 6309 Å DIB and the new 6525 Å band) may be related to the presence of larger fullerenes (e.g., C80, C240, C320, and C540) and buckyonions (multishell fullerenes such as C60@C240 and C60@C240@C540) in the circumstellar envelope of Tc 1 (García-Hernández & Díaz-Luis 2013).

A Catalog of Diffuse Interstellar Bands in Fullerene-Containing Planetary Nebulae

AbstractLarge fullerenes and fullerene-based molecules have been proposed as carriers of diffuse interstellar bands (DIBs). The recent detection of the most common fullerenes (C60 and C70) around some Planetary Nebulae (PNe) now enable us to study the DIBs in fullerene-rich space environments. We have studied the presence of DIBs in the optical spectra (~3300-9400 Å) of two fullerene-containing PNe (Tc 1 and M 1-20). Special attention is given to DIBs which are found to be unusually intense in fullerene-containing PNe; several of these DIBS have not previously been reported. Fullerenes larger than C60 (and C70) and multishell fullerenes may be possible candidate carriers for the unusual DIBs seen in fullerene-rich environments.

Diffuse interstellar bands in fullerene planetary nebulae: the fullerenes – diffuse interstellar bands connection

We present high-resolution (R ~ 15 000) VLT/UVES optical spectra of two planetary nebulae (PNe; Tc 1 and M 1-20) where C60 (and C70) fullerenes have already been found. These spectra are of high quality (S/N > 300) for PN Tc 1, which permits us to search for the expected electronic transitions of neutral C60 and diffuse interstellar bands (DIBs). Surprisingly, we report the non-detection of the most intense optical bands of C60 in Tc 1, although this could be explained by the low C60 column density estimated from the C60 infrared bands if the C60 emission peaks far away from the central star. The strongest and most common DIBs in both fullerene PNe are normal for their reddening. Interestingly, the very broad 4428 Å DIB and the weaker 6309 Å DIB are found to be unusually intense in Tc 1. We also report the detection of a new broad (FWHM ~ 5 Å) unidentified band at ~6525 Å. We propose that the 4428 Å DIB (probably also the 6309 Å DIB and the new 6525 Å band) may be related to the presence of larger fullerenes (e.g., C80, C240, C320, and C540) and buckyonions (multishell fullerenes such as C60@C240 and C60@C240@C540) in the circumstellar envelope of Tc 1.

Investigation of high- and low-frequency electrostatic oscillations in multishell fullerenes

A theoretical analysis of the high- and low-frequency electrostatic oscillations in multishell fullerenes is presented within the framework of the fluid theory in conjunction with Poisson's equation. The electron and ion components of each shell of fullerenes are regarded as the two-species plasma system, in which the perturbed electron number density is deduced by means of the quantum hydrodynamic model, while the ion density perturbation follows the classical expression. Analytical expressions of the dispersion relations are obtained for the plasmon and quantum ion-acoustic wave oscillations in the system.

NANODIAMONDS ENVELOPED IN GLASSY CARBON SHELLS AND THE ORIGIN OF THE 2175 Å OPTICAL EXTINCTION FEATURE

It is shown that small nanodiamond particles encapsulated into cages of glassy carbon may contribute to the 2175 A (5.67 eV) interstellar extinction feature, in the normalized interstellar extinction curve, for a ratio between the size of the diamond core to its overall exterior radius in the range 0.6-0.9. Results for optical absorption by diamond nanoclusters encased in multishell fullerenes are also presented. These are also possible candidates to explain the feature.

Dipole polarizability of onion-like carbons and electromagnetic properties of theircomposites

Onion-like carbons (OLC) obtained by thermal transformation of nanodiamonds areagglomerates of multi-shell fullerenes, often covered by an external graphitic mantle. Forthe present work, elemental OLC units were constructed on the computer by coalescence ofseveral two-layer fullerenes, in a structure similar to carbon peapods with a corrugatedexternal wall. The electrical polarizability of such pod-of-peas fullerenes has beencomputed by a classical monopole–dipole atomistic theory. The description ofpod-of-peas fullerenes was further simplified by representing them as linear arrays ofpoint-like objects, whose polarizability matches that of the starting molecules.Calculations demonstrated that the static polarizability of spherically shapedassemblies of these arrays, modeling real OLC materials, is weakly dependent on thegeometry of its constituent molecules and is chiefly proportional to the volume ofthe whole cluster. It increases with increasing filling fraction of the pod-of-peasfullerenes in the OLC aggregate. The polarizability so obtained can be used inMaxwell–Garnett theory to predict the permittivity of OLC-based composites, atleast for static excitations. Experimental results obtained at GHz frequenciesreveal a weak attenuation for OLC- and nanodiamond-based polydimethylsiloxanecomposites. In these silicone composites, we did not find long chains of coupledOLCs. Quite separated clusters were found instead, which contribute little to thepolarizability and to the dielectric properties, in good agreement with our theoreticalpredictions.

Structural transformation of vapor grown carbon nanofibers studied by HRTEM

Vapor grown carbon nanofibers have been extensively manufactured and investigated in recent years. In this study commercially available vapor grown carbon nanofibers subjected to different processing and post processing conditions were studied employing high resolution TEM images. The analysis showed that the fibers consist primarily of conical nanofibers, but can contain a significant amount of bamboo nanofibers. Most conical nanofibers were found to consist of an ordered inner layer and a disordered outer layer, with the cone angle distribution of the inner layers indicating that these cannot have a stacked cone structure but are compatible with a cone-helix structure. Fibers that have been heat treated to temperatures above 1,500 °C undergo a structural transformation with the ordered inner layers changing from a cone-helix structure to a highly ordered multiwall stacked cone structure. The bamboo nanofibers were found to have a tapered multiwall nanotube structure for the wall and a multishell fullerene structure for the cap of each segment, surrounded by a disordered outer layer. When these fibers are heat treated the disordered outer layers transform to an ordered multiwall nanotube structure and merge with the wall of each segment. The end caps of each segment transform from a smooth multiwall fullerene structure to one consisting of disjointed graphene planes. A reaction-diffusion mechanism is proposed to explain the growth and structure of the bamboo nanofibers.

Noncovalent Interactions between Graphene Sheets and in Multishell (Hyper)Fullerenes

The intershell and interlayer interaction (complexation) energies of C60 inside C240 (C60@C240) and of graphene sheets are investigated by all-electron density functional theory (DFT) using generalized gradient approximation (GGA) functionals and a previously developed empirical correction for dispersion (van der Waals) effects (DFT−D method). Large Gaussian basis sets of polarized triple-ζ quality that provide very small basis set superposition errors (<10% of ΔE) are employed. The theoretical approach is first applied to graphene sheet model dimers of increasing size (up to (C216H36)2). The interaction energies are extrapolated to infinite lateral size of the sheets. The value of −66 meV/atom obtained for the interaction energy of two sheets supports the most recent experimental estimate for the exfoliation energy of graphite (−52 ± 5 meV/atom). The interlayer equilibrium distance (334 ± 3 pm) is also obtained accurately. The binding energy of C60 inside C240 is calculated to be −184 kcal mol-1 which is about 89% of the corresponding value of a similarly sized graphene sheet model dimer. Geometric relaxation of the monomers upon complexation and nonadditivity (multilayer) effects are found to be negligible. The various contributions to the binding (Pauli exchange repulsion, electrostatic and induction, dispersion) are comparatively analyzed for the sheets and for C60@C240. The binding in both systems is that of typical van der Waals complexes; that is, the dispersion contributions play a major role as also indicated by the fact that conventional GGA functionals yield purely repulsive interactions. The plots of the electrostatic potential of the fragments often used as tools for analysis lead here to qualitatively wrong conclusions. The relatively large binding energy of C60@C240 can be explained by favorable dispersion, induction, and charge-transfer interaction contributions but reveals no special role of the π orbitals. According to population analyses, about 0.67 electrons are transferred from the inner to the outer cage in C60@C240 upon complex formation.

Nanoscale modeling of an embedded multi-shell fullerene and its application to vibrational analysis

In this paper, nanoscale modeling of a multi-shell fullerene embedded in an elastic medium and its application to vibrational analysis is investigated. The spherical layers of the multi-shell fullerene are concentrically nested, with carbon–carbon van der Waals interactions between them. Also, the whole multi-shell fullerene is influenced by polymer–carbon van der Waals forces from the surrounding elastic medium. The elasticity generated by the carbon–carbon bonds is assumed to be distributed isotropically over the fullerene surfaces. Following derivation of explicit equations for the motion of the multi-shell fullerene, vibrational behavior of a double-shell fullerene is analyzed and resonant frequencies and the associated mode shapes are determined.

Graphitic onions as reaction cells on the nanoscale

Multishell fullerene clusters (carbon onions) encapsulating iron crystals of a few nanometers in size are subjected to intense electron irradiation at high temperature in an electron microscope. A reaction between graphite and iron, leading to the formation of iron carbide (cementite) in the core of the clusters, is observed in situ. The reaction in these spherical nanoencapsulates is explained by the occurrence of high pressure, mobile carbon interstitial atoms, and the irradiation-induced lowering of the kinetic barrier. Thus, carbon onions can be used as high-pressure cells for inducing a chemical reaction in nanometer-sized particles.