Stability of multishell fullerenes

Metal Contact Induced Unconventional Field Effect in Metallic Carbon Nanotubes

One-dimensional carbon nanotubes (CNTs) are promising for future nanoelectronics and optoelectronics, and an understanding of electrical contacts is essential for developing these technologies. Although significant efforts have been made in this direction, the quantitative behavior of electrical contacts remains poorly understood. Here, we investigate the effect of metal deformations on the gate voltage dependence of the conductance of metallic armchair and zigzag CNT field effect transistors (FETs). We employ density functional theory calculations of deformed CNTs under metal contacts to demonstrate that the current-voltage characteristics of the FET devices are qualitatively different from those expected for metallic CNT. We predict that, in the case of armchair CNT, the gate-voltage dependence of the conductance shows an ON/OFF ratio of about a factor of two, nearly independent of temperature. We attribute the simulated behavior to modification of the band structure under the metals caused by deformation. Our comprehensive model predicts a distinct feature of conductance modulation in armchair CNTFETs induced by the deformation of the CNT band structure. At the same time, the deformation in zigzag metallic CNTs leads to a band crossing but not to a bandgap opening.

Self-folding and self-scrolling mechanisms of edge-deformed graphene sheets: a molecular dynamics study

Graphene-based nanofolds (GNFs) are edge-connected 2D stacked monolayers that originate from single-layer graphene. Graphene-based nanoscrolls (GNSs) are nanomaterials with geometry resembling graphene layers rolled up into a spiral (papyrus-like) form. Both GNS and GNF structures induce significant changes in the mechanical and optoelectronic properties of single-layer graphene, aggregating new functionalities in carbon-based applications. Here, we carried out fully atomistic reactive (ReaxFF) molecular dynamics simulations to study the self-folding and self-scrolling mechanisms of edge-deformed graphene sheets. We adopted initial armchair edge-scrolled graphene (AESG(φ, θ)) structures with similar (or different) twist angles (φ, θ) in each edge, mimicking the initial configuration that was experimentally developed to form biscrolled sheets. The results showed that AESG(0, 2π) and AESG(2π, 2π) evolved to single-folded and two-folded fully stacked morphologies, respectively. As a general trend, for twist angles higher than 2π, the self-deformation process of AESG morphologies yields GNSs. Edge twist angles lower than π are not enough for triggering the self-deformation processes. In the AESG(0, 3π) and AESG(3π, 3π) cases, after a relaxation period, their morphology transition towards GNSs occurred rapidly. In the AESG(3π, 3π) dynamics, a metastable biscroll was formed by the interplay between the left- and right-sided partial scrolling while forming a unique GNS. At high-temperature perturbations, the edge folding and scrolling transitions to GNFs and GNSs occurred within an ultrafast time-period. Remarkably, the AESG(2π, 3π) evolved to a dual state that combines folded and scrolled structures in a temperature-independent process.

Band Structure and Contact Resistance of Carbon Nanotubes Deformed by a Metal Contact

Capillary and van der Waals forces cause nanotubes to deform or even collapse under metal contacts. Using ab initio band structure calculations, we find that these deformations reduce the band gap by as much as 30%, while fully collapsed nanotubes become metallic. Moreover, degeneracy lifting due to the broken axial symmetry, and wave functions mismatch between the fully collapsed and the round portions of a CNT, lead to a 3 times higher contact resistance. The latter we demonstrate by contact resistance calculations within the tight-binding approach.

Metal-semiconductor-metal transition in zigzag carbon nanoscrolls

Similar to rolling up paper, carbon nanoscrolls (CNSs) can be rolled from graphene nanoribbons (GNRs) using physical approaches. Owing to their peculiar one-dimensional nanostructures, CNSs have attracted great attention over the past few years. In this study, we have investigated the effects of bending deformation on the electronic properties of zigzag CNSs (ZCNSs) during the rolling process from zigzag GNRs (ZGNRs) by means of first-principles calculations. It is found that a metal-semiconductor-metal transition is observed. By analyzing charge density and density of states, the origin of this electronic property transition is discussed. Furthermore, we find that the metal-semiconductor-metal transition in ZCNSs is independent of ribbon width as well as spin-orbit interaction. Our results of the metal-semiconductor-metal transition in the ZCNSs are robust and may open potential applications in nano-electromechanical devices based on the ZCNSs.

Synthesis of single layer graphene on Cu(111) by C60 supersonic molecular beam epitaxy

High kinetic energy impacts between inorganic surfaces and molecular beams seeded by organics represent a fundamental tool in materials science, particularly when they activate chemical–physical processes leading to nanocrystals' growth.

Graphenylene Nanotubes

A new type of carbon nanotube, based on the graphenylene motif, is investigated using density functional and tight-binding methods. Analogous to conventional graphene-based nanotubes, a two-dimensional graphenylene sheet can be "rolled" into a seamless cylinder in armchair, zigzag, or chiral orientations. The resulting nanotube can be described using the familiar (n,m) nomenclature and possesses 4-, 6-, and 12-membered rings, with three distinct bond lengths, indicating a nonuniform distribution of the electron density. The dodecagonal rings form pores, 3.3 Å in diameter in graphenylene, which become saddle-shaped paraboloids in smaller-diameter nanotubes. Density functional theory predicts zigzag nanotubes to be small-band gap semiconductors, with a generally decreasing band gap as the diameter increases. Interestingly, the calculations predict metallic characteristics for armchair nanotubes with small diameters (<2 nm), and small-band gap semiconducting characteristics for larger-diameter ones. Graphenylene nanotubes with indices mod(n-m,3) = 0 exhibit a band gap approximately equal to that of armchair graphenylene nanotubes with comparable diameter.

Symmetric nested complexes of fullerenes

Large fullerenes such as C180 and C116 can be used as hosts for other molecules of the same family. Based on this idea, two complexes were designed, one in which C180 accepts a C60 molecule as a guest and the other in which C20 was placed inside C116. The behavior of these new assemblies was closer to that of a large endohedral complex rather than onion-like. There were marked differences between the systems. In the first system, there was minor interaction between the two cages but the association resulted in a more stable thermodynamic state. In the second system, there was strong electronic interchange between the cages, and the thermodynamic results suggest that such a combination might be useful for forming stable C20.

Wetting transition for carbon nanotube arrays under metal contacts

Structural arrays with nanoscale spacing arise in many device concepts. Carbon nanotube transistors are an extreme example, where a practical technology will require arrays of parallel nanotubes with spacing of order 10 nm or less. We show that with decreasing pitch there is a first-order transition, from a robust structure in which the metal wets the substrate between tubes, to a poorly wetting structure in which the metal rides atop the nanotube array without touching the substrate. The latter is analogous to the superhydrophobic "lotus leaf effect." There is a sharp minimum in the delamination energy of metal contacts at the transition pitch. We discuss implications for contact resistance and possible mitigation strategies.

High Stability of Faceted Nanotubes and Fullerenes of Multiphase Layered Phosphorus: A Computational Study

We present a paradigm in constructing very stable, faceted nanotube and fullerene structures by laterally joining nanoribbons or patches of different planar phosphorene phases. Our ab initio density functional calculations indicate that these phases may form very stable, nonplanar joints. Unlike fullerenes and nanotubes obtained by deforming a single-phase planar monolayer at substantial energy penalty, we find faceted fullerenes and nanotubes to be nearly as stable as the planar single-phase monolayers. The resulting rich variety of polymorphs allows us to tune the electronic properties of phosphorene nanotubes and fullerenes not only by the chiral index but also by the combination of different phosphorene phases. In selected phosphorene nanotubes, a metal-insulator transition may be induced by strain or by changing the number of walls.

The topology of fullerenes

Fullerenes are carbon molecules that form polyhedral cages. Their bond structures are exactly the planar cubic graphs that have only pentagon and hexagon faces. Strikingly, a number of chemical properties of a fullerene can be derived from its graph structure. A rich mathematics of cubic planar graphs and fullerene graphs has grown since they were studied by Goldberg, Coxeter, and others in the early 20th century, and many mathematical properties of fullerenes have found simple and beautiful solutions. Yet many interesting chemical and mathematical problems in the field remain open. In this paper, we present a general overview of recent topological and graph theoretical developments in fullerene research over the past two decades, describing both solved and open problems. WIREs Comput Mol Sci 2015, 5:96-145. doi: 10.1002/wcms.1207 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

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 3^rd 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.

Carbon nanotube deformation and collapse under metal contacts

In carbon nanotube transistors, typically part of the nanotube is covered by a metal contact. This covered region plays an important role due to the significant electron transfer length. Here we predict that capillary and van der Waals forces cause the nanotube to deform or even collapse under the metal. Nanotubes are known to collapse when their diameters are above some critical value around 4 nm. Under the metal, we find that spontaneous collapse occurs for diameters down to ∼ 1.5-1.6 nm, close to the range used in high-performance transistors. Even at smaller diameters, we find surprisingly large deformations that could significantly affect the electronic structure.

Spectroscopic signatures of the carbon buckyonions C60@C180 and C60@C240: a dispersion-corrected DFT study

We have investigated, using dispersion corrected DFT methods, the structure and the spectroscopic properties of carbon buckyonions C60@C180 and C60@C240. C60, C180 and C240 showed a noticeable variation of their geometries in C60@C180 and C60@C240, upon encapsulation. Inclusion of the dispersion correction term in the calculations has a significant effect on the geometry. C60@C180 has a large positive interaction energy, while for C60@C240 a negative value is found indicating that only C240 can easily accommodate C60. In both cases dispersion interactions strongly contribute to the stabilization of the complexes. Vibrational frequencies, electronic transitions and NMR properties have been computed. The results show that encapsulation leads to appreciable variation in the characteristic resonances thus offering a useful tool for a spectroscopic identification of these species.

Extensive energy landscape sampling of nanotube end-caps reveals no chiral-angle bias for their nucleation

In the formation of a carbon nanotube (CNT) nucleus, a hemispherical fullerene end-cap, a specific pattern of six pentagons encodes what unique (n,m) chirality a nascent CNT would inherit, with many possible pentagon patterns corresponding to a single chirality. This configurational variety and its potential role in the initial stages of CNT catalytic growth remain essentially unexplored. Here we present large-scale calculations designed to evaluate the intrinsic energies of all possible CNT caps for selected chiralities corresponding to tube diameters d ≲ 1 nm. Our quantitative analysis reveals that for all chiral angles χ the energy scale variability associated with the CNT caps is small, compared to that of the CNT/catalyst interface. Such a flat energy landscape cannot therefore be a dominant factor for chiral distribution and lends further credibility to interface-controlled scenarios for selective growth of single-walled CNT of desired chirality.

Thin single-walled carbon nanotubes with narrow chirality distribution: constructive interplay of plasma and Gibbs-Thomson effects

Multiscale, multiphase numerical modeling is used to explain the mechanisms of effective control of chirality distributions of single-walled carbon nanotubes in direct plasma growth and suggest effective approaches to further improvement. The model includes an unprecedented combination of the plasma sheath, ion/radical transport, species creation/loss, plasma-surface interaction, heat transfer, surface/bulk diffusion, graphene layer nucleation, and bending/lift-off modules. It is shown that the constructive interplay between the plasma and the Gibbs-Thomson effect can lead to the effective nucleation and lift-off of small graphene layers on small metal catalyst nanoparticles. As a result, much thinner nanotubes with narrower chirality distributions can nucleate at much lower process temperatures and pressures compared to thermal CVD. This approach is validated by a host of experimental results, substantially reduces the amounts of energy and atomic matter required for the nanotube growth, and can be extended to other nanoscale structures and materials systems, thereby nearing the ultimate goal of energy- and matter-efficient nanotechnology.

Optical excitations in carbon nanoscrolls

The optical properties of carbon nanoscrolls in the presence of uniform electric fields are investigated by using gradient approximation. Absorption spectra exhibit rich prominent peaks structures, which is caused by one-dimensional sub-bands. The numbers, spectral intensities, and energies of the absorption peaks are strongly dependent on the geometry and the electric field strength. There exists an optical selection rule originating from the two equivalent sublattices in graphene. The two-fold degeneracy of the absorption peaks can be lifted by the inter-wall interactions or the electric field. The variations of the absorption peak energies with the geometry and field strength are also explored. These theoretical predictions can be validated by optical absorption measurements.

Snap-Through Instability of Graphene on Substrates

We determine the graphene morphology regulated by substrates with herringbone and checkerboard surface corrugations. As the graphene-substrate interfacial bonding energy and the substrate surface roughness vary, the graphene morphology snaps between two distinct states: (1) closely conforming to the substrate and (2) remaining nearly flat on the substrate. Since the graphene morphology is strongly tied to the electronic properties of graphene, such a snap-through instability of graphene morphology can lead to desirable graphene electronic properties that could potentially enable graphene-based functional electronic components (e.g. nano-switches).

Toward a Reversible Isolation of a C20 Fullerene Inside a Tetraureacalix[4]arene Dimer. A Theoretical Study

The potential stabilization of normally unstable C20, the smallest fullerene, via its encapsulation inside a tetraureacalix[4]arene dimer has been analyzed using molecular mechanics calculations with different force fields, the self-consistent-charge density-functional tight-binding with dispersion correction (SCC-DFTB-D) model, and standard density-functional-theory (DFT) calculations. The interaction energies obtained for the C20 complex have been compared with analogous values calculated for numerous complexes of the tetraureacalix[4]arene dimer with other guests. Results of the calculations with all force fields and SCC-DFTB-D predict that the binding of C20 occurs with the highest selectivity. On the other hand, standard DFT calculations fail to correctly describe the stabilization of the complexes under study as standard DFT generally does not treat dispersion interactions properly. Predicted relative stabilities of the complexes are discussed in conjunction with available experimental data. Molecular dynamics simulations reveal the instability of the guest-free capsular dimer, which decomposes on a 1-ns time scale, while dimeric complexes with guests remained intact during the 5-ns simulation time, indicating the guest-driven formation of the molecular capsule.

A theoretical model of the photoabsorption spectra of carbon buckyonions

A theoretical model has been developed to predict the photoabsorption spectra of spherical carbon buckyonions in the region dominated by the pi-plasmon feature. This model makes use of the microscopic electronic structure of the system, which is provided by an effective Huckel one-electron model. The important screening effects are treated within the random phase approximation, whose form is an extension to the dynamic case of the one derived in a previous work for the static polarizabilities of these species. A systematic analysis as a function of the buckyonion size is performed. We compare the spectra obtained in this way with those derived from a different representation of the electron motion, namely a two-dimensional spherical electron gas, and from a classical dielectric model.

Zipper mechanism of nanotube fusion: theory and experiment

We propose a new microscopic mechanism to explain the unusually fast fusion process of carbon nanotubes. We identify the detailed pathway for two adjacent (5,5) nanotubes to gradually merge into a (10,10) tube, and characterize the transition states. The propagation of the fused region is energetically favorable and proceeds in a morphology reminiscent of a Y junction via a zipper mechanism, involving only Stone-Wales bond rearrangements with low activation barriers. The zipper mechanism of fusion is supported by a time series of high-resolution transmission electron microscopy observations.