Analysis of the reactivity and selectivity of fullerene dimerization reactions at the atomic level

Single molecular spectroscopy: identification of individual fullerene molecules

We report the molecule-by-molecule spectroscopy of individual fullerenes by means of electron spectroscopy based on scanning transmission electron microscopy. Electron energy-loss fine structure analysis of carbon 1s absorption spectra is used to discriminate carbon allotropes with known symmetries. C(60) and C(70) molecules randomly stored inside carbon nanotubes are successfully identified at a single-molecular basis. We show that a single molecule impurity is detectable, allowing the recognition of an unexpected contaminant molecule with a different symmetry. Molecules inside carbon nanotubes thus preserve their intact molecular symmetry. In contrast, molecules anchored at or sandwiched between atomic BN layers show spectral modifications possibly due to a largely degraded structural symmetry. Moreover, by comparing the spectrum from a single C(60) molecule and its molecular crystal, we find hints of the influence of solid-state effects on its electronic structure.

Multiple reaction pathways of metallofullerenes investigated by transmission electron microscopy

Recent advances in molecule-by-molecule transmission electron microscopy (TEM) have provided time-series structural information of individual molecules supported by nano-carbon materials, enabling researchers to trace their motions and reactions. In this paper, the chemical reactions of fullerenes and metallofullerene derivatives, focusing on their deformation process, are reviewed and discussed based on the single-molecule-resolved TEM analysis.

Evaluating the effects of carbon nanoreactor diameter and internal structure on the pathways of the catalytic hydrosilylation reaction

Three different types of carbon nanoreactors, double-walled nanotubes (DWNT), multi-walled nanotubes (MWNT) and graphitised carbon nanofibers (GNF) have been appraised for the first time as containers for the reactions of phenylacetylene hydrosilylation catalysed by a confined molecular catalyst [Rh₄(CO)₁₂]. Interactions of [Rh₄(CO)₁₂] with carbon nanoreactors determining the ratio of β-addition products are unchanged for all nanoreactors and are virtually unaffected by the confinement of [Rh₄(CO)₁₂] inside carbon nanostructures. Conversely, the relative concentrations of reactants affecting the ratio of addition and dehydrogenative silylation products is very sensitive to nanoscale confinement, with all nanoreactors demonstrating significant effects on the distribution of reaction products as compared to control experiments with the catalyst in bulk solution or adsorbed on the outer surface of nanoreactors. Surprisingly, the widest nanoreactors (GNF) change the reaction pathway most significantly, which is attributed to the graphitic step-edges inside GNF providing effective anchoring points for the catalyst and creating local environments with greatly altered concentrations of reactants as compared to bulk solution. Possessing diameters significantly wider than molecules, GNF impose no restrictions on the transfer of reactants while providing the strongest confinement effects for the reaction. Furthermore, GNF facilitate the effective recyclability of the catalyst and thus represents a superior nanoreactor system to carbon nanotubes.

Growth of carbon nanotubes via twisted graphene nanoribbons

Carbon nanotubes have long been described as rolled-up graphene sheets. It is only fairly recently observed that longitudinal cleavage of carbon nanotubes, using chemical, catalytical and electrical approaches, unzips them into thin graphene strips of various widths, the so-called graphene nanoribbons. In contrast, rolling up these flimsy ribbons into tubes in a real experiment has not been possible. Theoretical studies conducted by Kit et al. recently demonstrated the tube formation through twisting of graphene nanoribbon, an idea very different from the rolling-up postulation. Here we report the first experimental evidence of a thermally induced self-intertwining of graphene nanoribbons for the preferential synthesis of (7, 2) and (8, 1) tubes within parent-tube templates. Through the tailoring of ribbon’s width and edge, the present finding adds a radically new aspect to the understanding of carbon nanotube formation, shedding much light on not only the future chirality tuning, but also contemporary nanomaterials engineering.

Carbon nanotubes can be considered as rolled-up small sheets of graphene. Here Lim and colleagues demonstrate this process, by fabricating carbon nanotubes through a thermally induced process of self-intertwining of graphene nanoribbons.

Interactions and chemical transformations of coronene inside and outside carbon nanotubes

By exposing flat and curved carbon surfaces to coronene, a variety of van der Waals hybrid heterostructures are prepared, including coronene encapsulated in carbon nanotubes, and coronene and dicoronylene adsorbed on nanotubes or graphite via π-π interactions. The structure of the final product is determined by the temperature of the experiment and the curvature of the carbon surface. While at temperatures below and close to the sublimation point of coronene, nanotubes with suitable diameters are filled with single coronene molecules, at higher temperatures additional dimerization and oligomerization of coronene occurs on the surface of carbon nanotubes. The fact that dicoronylene and possible higher oligomers are formed at lower temperatures than expected for vapor-phase polymerization indicates the active role of the carbon surface used primarily as template. Removal of adsorbed species from the nanotube surface is of utmost importance for reliable characterization of encapsulated molecules: it is demonstrated that the green fluorescence attributed previously to encapsulated coronene is instead caused by dicoronylene adsorbed on the surface which can be solubilized and removed using surfactants. After removing most of the adsorbed layer, a combination of Raman spectroscopy and transmission electron microscopy was employed to follow the transformation dynamics of coronene molecules inside nanotubes.

Optically enhanced charge transfer between C60 and single-wall carbon nanotubes in hybrid electronic devices

In this article we probe the nature of electronic interactions between the components of hybrid C60-carbon nanotube structures. Utilizing an aromatic mediator we selectively attach C60 molecules to carbon nanotube field-effect transistor devices. Structural characterization via atomic force and transmission electron microscopy confirm the selectivity of this attachment. Charge transfer from the carbon nanotube to the C60 molecules is evidenced by a blue shift of the Raman G(+) peak position and increased threshold voltage of the transistor transfer characteristics. We estimate this charge transfer to increase the device density of holes per unit length by up to 0.85 nm(-1) and demonstrate further optically enhanced charge transfer which increases the hole density by an additional 0.16 nm(-1).

Bottom-up molecular-assembly of Ru(ii)polypyridyl complex-based hybrid nanostructures decorated with silver nanoparticles: effect of Ag nitrate concentration

Facile and templateless one-pot synthesis of Ru(ii)polypyridyl complex-based hybrid nanostructures decorated with silver nanoparticles (Ag NPs) with variable morphologies.

Conformational analysis of single perfluoroalkyl chains by single-molecule real-time transmission electron microscopic imaging

Whereas a statistical average of molecular ensembles has been the conventional source of information on molecular structures, atomic resolution movies of single organic molecules obtained by single-molecule real-time transmission electron microscopy have recently emerged as a new tool to study the time evolution of the structures of individual molecules. The present work describes a proof-of-principle study of the determination of the conformation of each C-C bond in single perfluoroalkyl fullerene molecules encapsulated in a single-walled carbon nanotube (CNT) as well as those attached to the outer surface of a carbon nanohorn (CNH). Analysis of 82 individual molecules in CNTs under a 120 kV electron beam indicated that 6% of the CF2-CF2 bonds and about 20% of the CH2-CH2 bonds in the corresponding hydrocarbon analogue are in the gauche conformation. This comparison qualitatively matches the known conformational data based on time- and molecular-average as determined for ensembles. The transmission electron microscopy images also showed that the molecules entered the CNTs predominantly in one orientation. The molecules attached on a CNH surface moved more freely and exhibited more diverse conformation than those in a CNT, suggesting the potential applicability of this method for the determination of the dynamic shape of flexible molecules and of detailed conformations. We observed little sign of any decomposition of the specimen molecules, at least up to 10(7) e·nm(-2) (electrons/nm(2)) at 120 kV acceleration voltage. Decomposition of CNHs under irradiation with a 300 kV electron beam was suppressed by cooling to 77 K, suggesting that the decomposition is a chemical process. Several lines of evidence suggest that the graphitic substrate and the attached molecules are very cold.

Regioselective control of aromatic halogenation reactions in carbon nanotube nanoreactors

The use of single-walled carbon nanotubes as effective nanoreactors for preparative chemical reactions has been demonstrated for the first time. Extreme spatial confinement of reactant molecules inside nanotubes has been shown to drastically affect both the regioselectivity and kinetics of aromatic halogenation reactions.

Approaches to modelling irradiation-induced processes in transmission electron microscopy

The recent progress in high-resolution transmission electron microscopy (HRTEM) has given rise to the possibility of in situ observations of nanostructure transformations and chemical reactions induced by electron irradiation. In this article we briefly summarise experimental observations and discuss in detail atomistic modelling of irradiation-induced processes in HRTEM, as well as mechanisms of such processes recognised due to modelling. Accurate molecular dynamics (MD) techniques based on first principles or tight-binding models are employed in the analysis of single irradiation-induced events, and classical MD simulations are combined with a kinetic Monte Carlo algorithm to simulate continuous irradiation of nanomaterials. It has been shown that sulphur-terminated graphene nanoribbons are formed inside carbon nanotubes as a result of an irradiation-selective chemical reaction. The process of fullerene formation in HRTEM during continuous electron irradiation of a small graphene flake has been simulated, and mechanisms driving this transformation analysed.

Movies of molecular motions and reactions: the single-molecule, real-time transmission electron microscope imaging technique

"The truth is, the Science of Nature has been already too long made only a work of the Brain and the Fancy: It is now high time that it should return to the plainness and soundness of Observations on material and obvious things," proudly declared Robert Hooke in his highly successful picture book of microscopic and telescopic images, "Micrographia" in 1665. Hooke's statement has remained true in chemistry, where a considerable work of the brain and the fancy is still necessary. Single-molecule, real-time transmission electron microscope (SMRT-TEM) imaging at an atomic resolution now allows us to learn about molecules simply by watching movies of them. Like any dream come true, the new analytical technique challenged the old common sense of the communities, and offers new research opportunities that are unavailable by conventional methods. With its capacity to visualize the motions and the reactions of individual molecules and molecular clusters, the SMRT-TEM technique will become an indispensable tool in molecular science and the engineering of natural and synthetic substances, as well as in science education.

Endohedral and exohedral hybrids involving fullerenes and carbon nanotubes

Since fullerenes and carbon nanotubes (CNTs) were discovered, these materials have attracted a great deal of attention in the scientific community due to their unique structures and properties. The properties of both carbon allotropes can be modulated by chemical functionalization, and merging fullerenes and CNTs combines the electronic and optical properties of CNTs with the excellent electron acceptor characteristic of fullerenes; moreover, a synergistic effect of these hybrids can be found, as the properties of both the nanotube and the fullerene are affected by the presence of the other. In these hybrids, the fullerene can be located inside (endohedral) or outside (exohedral) the CNT and both types of hybrid have specific features. CNT-fullerene hybrids have been studied for various applications, including photovoltaics, optical limiting and flame retardancy amongst others. This review outlines the progress in research on CNT-fullerene hybrids, including endohedral and exohedral combinations, their properties, functionalization, applications and outlook.

Electrons for single molecule diffraction and imaging

We demonstrate the potential of electrons for single molecule diffraction and imaging using C₆₀ molecules confined inside single-walled carbon nanotubes (C₆₀s@SWCNT or peapod) as a model system. A 25 nm diameter electron beam from a field emission gun source is used to record diffraction patterns from individual peapods using imaging plates. The electron beam illuminates about 25 C₆₀ molecules. Experimentally, we found that the molecules diffract inside ∼15% of the host nanotubes. With the help of simulations, we explore the limits of electron molecular diffraction and its sensitivity to the molecular configurations. We show that the combination of electron diffraction and electron direct imaging provides the best approach to single molecule imaging.

Intrinsic catalytic structure of gold nanoparticles supported on TiO2

Despite the fragility of TiO(2) under electron irradiation, the intrinsic structure of Au/TiO(2) catalysts can be observed by environmental transmission electron microscopy. Under reaction conditions (CO/air 100 Pa), the major {111} and {100} facets of the gold nanoparticles are exposed and the particles display a polygonal interface with the TiO(2) support bounded by sharp edges parallel to the 〈110〉 directions.

Single molecule electronics and devices

The manufacture of integrated circuits with single-molecule building blocks is a goal of molecular electronics. While research in the past has been limited to bulk experiments on self-assembled monolayers, advances in technology have now enabled us to fabricate single-molecule junctions. This has led to significant progress in understanding electron transport in molecular systems at the single-molecule level and the concomitant emergence of new device concepts. Here, we review recent developments in this field. We summarize the methods currently used to form metal-molecule-metal structures and some single-molecule techniques essential for characterizing molecular junctions such as inelastic electron tunnelling spectroscopy. We then highlight several important achievements, including demonstration of single-molecule diodes, transistors, and switches that make use of electrical, photo, and mechanical stimulation to control the electron transport. We also discuss intriguing issues to be addressed further in the future such as heat and thermoelectric transport in an individual molecule.

Size, structure, and helical twist of graphene nanoribbons controlled by confinement in carbon nanotubes

Carbon nanotubes (CNTs) act as efficient nanoreactors, templating the assembly of sulfur-terminated graphene nanoribbons (S-GNRs) with different sizes, structures, and conformations. Spontaneous formation of nanoribbons from small sulfur-containing molecules is efficiently triggered by heat treatment or by an 80 keV electron beam. S-GNRs form readily in CNTs with internal diameters between 1 and 2 nm. Outside of this optimum range, nanotubes narrower than 1 nm do not have sufficient space to accommodate the 2D structure of S-GNRs, while nanotubes wider than 2 nm do not provide efficient confinement for unidirectional S-GNR growth, thus neither can support nanoribbon formation. Theoretical calculations show that the thermodynamic stability of nanoribbons is dependent on the S-GNR edge structure and, to a lesser extent, the width of the nanoribbon. For nanoribbons of similar widths, the polythiaperipolycene-type edges of zigzag S-GNRs are more stable than the polythiophene-type edges of armchair S-GNRs. Both the edge structure and the width define the electronic properties of S-GNRs which can vary widely from metallic to semiconductor to insulator. The encapsulated S-GNRs exhibit diverse dynamic behavior, including rotation, translation, and helical twisting inside the nanotube, which offers a mechanism for control of the electronic properties of the graphene nanoribbon via confinement at the nanoscale.

Unravelling the Role of the Compressed Gas on Melting Point of Liquid Confined in Nanospace

Phase behaviors of the liquids in nanospaces are of particular interest to understand the thermodynamics of the liquid on the nanoscale and for their applications that involve the confined systems. However, in many cases, the inconsistent observations of melting point variation for confined liquids are often revealed by different groups. Ionic liquids are a special kind of liquid. Here, by using the merits of the nonvolatile nature of ionic liquids, we realized the encapsulation of ionic liquids inside of mesopores silica oxide nanoparticles with a complete removal of compressed gas under high-vacuum condition; the completely confined ionic liquid formed a crystalline-like phase. It was found that compressed gas plays an important role in changing the melting point of the confined ionic liquid.

Using the graphene Moiré pattern for the trapping of C60 and homoepitaxy of graphene

The graphene Moiré superstructure offers a complex landscape of humps and valleys to molecules adsorbing and diffusing on it. Using C(60) molecules as the classic hard sphere analogue, we examine its assembly and layered growth on this corrugated landscape. At the monolayer level, the cohesive interactions of C(60) molecules adsorbing on the Moiré lattice freeze the molecular rotation of C(60) trapped in the valley sites, resulting in molecular alignment of all similarly trapped C(60) molecules at room temperature. The hierarchy of adsorption potential well on the Moiré lattice causes diffusion-limited dendritic growth of C(60) films, as opposed to isotropic growth observed on a smooth surface like graphite. Due to the strong binding energy of the C(60) film, part of the dentritic C(60) films polymerize at 850 K and act as solid carbon sources for graphene homoepitaxy. Our findings point to the possibility of using periodically corrugated graphene in molecular spintronics due to its ability to trap and align organic molecules at room temperature.

Multiple intra-tube junctions in the inner tube of peapod-derived double walled carbon nanotubes: theoretical study and experimental evidence

The coalescence process of fullerenes in the hollow core of single walled carbon nanotubes is systematically explored by the kinetic Monte Carlo method. Two elongation (or growth) modes via the coalescence (i) between an inner tube and fullerenes and (ii) between neighboring inner tubes are identified. It is found that the coalescence of two inner tubes mostly creates a very stable intra-tube junction which is composed of multiple pentagon-heptagon pairs. As a consequence, the study predicts that the inner tube of peapod derived double walled carbon nanotubes (DWNTs) must contain many intra-tube junctions. Careful high resolution transmission electron microscopy observation on peapod-grown DWNT sample provides experimental evidence of the presence of the junctions.

Two-dimensional coalescence dynamics of encapsulated metallofullerenes in carbon nanotubes

We report on the coalescence of a two-dimensional (2D) chain of La@C(82) metallofullerene molecules encapsulated inside a single-wall carbon nanotube (SWNT). 2D packing of metallofullerenes is known to adopt a zigzag arrangement and cause elliptical distortion to the cross-section of the SWNT host. We show that after coalescence of the metallofullerenes into an inner nanotube the carbon nanotube host returns to its original circular cross-section. This is due to a relaxation of the strain caused by the packing of the encapsulated La@C(82) molecules into the nanotube. We identify the formation of some novel but transient fullerene-based structures formed during the intermediate stages of coalescence of the La@C(82) into an inner nanotube. These results highlight the flexible nature of SWNTs and their ability to adapt their cross-sectional profile depending upon forces induced by material encapsulated within.

Carbon nanotubes: from nano test tube to nano-reactor

Confinement of molecules and atoms inside carbon nanotubes provides a powerful strategy for studying structures and chemical properties of individual molecules at the nanoscale. In this issue of ACS Nano, Allen et al. explore the nanotube as a template leading to the formation of unusual supramolecular and covalent structures. The potential of carbon nanotubes as reactors for synthesis on the nano- and macroscales is discussed in light of recent studies.

Transformations of carbon adsorbates on graphene substrates under extreme heat

We describe new phenomena of structural reorganization of carbon adsorbates as revealed by in situ atomic-resolution transmission electron microscopy (TEM) performed on specimens at extreme temperatures. In our investigations, a graphene sheet serves as both a quasi-transparent substrate for TEM and as an in situ heater. The melting of gold nanoislands deposited on the substrate surface is used to evaluate the local temperature profile. At annealing temperatures around 1000 K, we observe the transformation of physisorbed hydrocarbon adsorbates into amorphous carbon monolayers and the initiation of crystallization. At temperatures exceeding 2000 K the transformation terminates in the formation of a completely polycrystalline graphene state. The resulting layers are bounded by free edges primarily in the armchair configuration.

Electron microscopic imaging of a single Group 8 metal atom catalyzing C-C bond reorganization of fullerenes

Heating a bulk sample of [60]fullerene complexes, (η(5)-C(5)H(5))MC(60)R(5) (M = Fe, Ru, R = Me, Ph), produces small hydrocarbons because of coupling of R and C(5)H(5) via C-C and C-H bond activation. Upon observation by transmission electron microscopy, these complexes, encapsulated in single-walled carbon nanotubes, underwent C-C bond reorganization reactions to form new C-C bond networks, including a structure reminiscent of [70]fullerene. Quantitative comparison of the electron dose required to effect the C-C bond reorganization of fullerenes and organofullerenes in the presence of a single atom of Ru, Fe, or Ln and in the the absence of metal atoms indicated high catalytic activity of Ru and Fe atoms, as opposed to no catalytic activity of Ln. Organic molecules such as hydrocarbons and amides as well as pristine [60]fullerene maintain their structural integrity upon irradiation by ca. 100 times higher electron dose compared to the Ru and Fe organometallics. The results not only represent a rare example of direct observation of a single-metal catalysis but also have implications for the use of single metal atom catalysis in Group 8 metal heterogeneous catalysis.

Reactions of the inner surface of carbon nanotubes and nanoprotrusion processes imaged at the atomic scale

Although the outer surface of single-walled carbon nanotubes (atomically thin cylinders of carbon) can be involved in a wide range of chemical reactions, it is generally thought that the interior surface of nanotubes is unreactive. In this study, we show that in the presence of catalytically active atoms of rhenium inserted into nanotubes, the nanotube sidewall can be engaged in chemical reactions from the inside. Aberration-corrected high-resolution transmission electron microscopy operated at 80 keV allows visualization of the formation of nanometre-sized hollow protrusions on the nanotube sidewall at the atomic level in real time at ambient temperature. Our direct observations and theoretical modelling demonstrate that the nanoprotrusions are formed in three stages: (i) metal-assisted deformation and rupture of the nanotube sidewall, (ii) the fast formation of a metastable asymmetric nanoprotrusion with an open edge and (iii) a slow symmetrization process that leads to a stable closed nanoprotrusion.

Self-assembly of a sulphur-terminated graphene nanoribbon within a single-walled carbon nanotube

The ability to tune the properties of graphene nanoribbons (GNRs) through modification of the nanoribbon's width and edge structure widens the potential applications of graphene in electronic devices. Although assembly of GNRs has been recently possible, current methods suffer from limited control of their atomic structure, or require the careful organization of precursors on atomically flat surfaces under ultra-high vacuum conditions. Here we demonstrate that a GNR can self-assemble from a random mixture of molecular precursors within a single-walled carbon nanotube, which ensures propagation of the nanoribbon in one dimension and determines its width. The sulphur-terminated dangling bonds of the GNR make these otherwise unstable nanoribbons thermodynamically viable over other forms of carbon. Electron microscopy reveals elliptical distortion of the nanotube, as well as helical twist and screw-like motion of the nanoribbon. These effects suggest novel ways of controlling the properties of these nanomaterials, such as the electronic band gap and the concentration of charge carriers.

Electron beam stimulated molecular motions

Electron microscopy with advances in aberration correction has the power to resolve atoms in single molecules. However, its application is limited by electron irradiation induced molecular motions. A better understanding of damage mechanisms is required to achieve the full potential of electron imaging. Here, we report a direct observation of molecular motions stimulated by an electron beam, which allows us to study the breakdown and formation of molecular bonds using C(60)'s encapsulated inside single-walled carbon nanotubes as a model system. An activation energy of 100 s meV is estimated for the observed molecular motions based on van der Waals interactions. We demonstrate that the molecular confinement can significantly increase the electron energy threshold for breaking the vdW bonds.

Effect of Peierls transition in armchair carbon nanotube on dynamical behaviour of encapsulated fullerene

The changes of dynamical behaviour of a single fullerene molecule inside an armchair carbon nanotube caused by the structural Peierls transition in the nanotube are considered. The structures of the smallest C20 and Fe@C20 fullerenes are computed using the spin-polarized density functional theory. Significant changes of the barriers for motion along the nanotube axis and rotation of these fullerenes inside the (8,8) nanotube are found at the Peierls transition. It is shown that the coefficients of translational and rotational diffusions of these fullerenes inside the nanotube change by several orders of magnitude. The possibility of inverse orientational melting, i.e. with a decrease of temperature, for the systems under consideration is predicted.

Atomic scale growth dynamics of nanocrystals within carbon nanotubes

The confined interior region of carbon nanotubes has proved to be an effective "nano-test-tube" to conduct chemical reactions in a restricted volume. It also benefits from being thin and relatively transparent to electrons, enabling structural characterization using high-resolution transmission electron microscopy. This permits real-time monitoring of chemical reactions with atomic resolution. Here, we have studied the dynamics of single Pr atoms released from Pr(2)@C(72) metallofullerenes. We show that the Pr atoms form small nanoclusters that subsequently coalesce to ordered, stable nanocrystals within the confines of a carbon nanotube. This process has been tracked in situ with atomic-resolution using low-voltage aberration-corrected high-resolution transmission electron microscopy. We reveal that nanocrystal formation within a nanotube does not generally occur by the addition of single atoms to one pre-existing cluster but rather through aggregation of several smaller clusters. These results provide some of the deepest insights into the dynamics of single-atom behavior in the solid state.

Chemically reactive species remain alive inside carbon nanotubes: a density functional theory study

The behavior of alkyl guest radicals inside carbon nanotube hosts with different diameters is analyzed using density functional theory (DFT) calculations. Here the inner alkyl radicals are assumed to be formed by decomposition of their precursors, which had been incorporated into the tubes. DFT calculations show that inner alkyl radicals prefer to exist separately from the nanotube wall (separate form) rather than forming an inner covalent bond with the wall (bound form). Keeping a radical apart from the inner wall is more likely for a more bulky radical inside a smaller diameter tube. A key to the preference for the separate forms over the bound forms is that the bound forms gain a weak attraction due to the formation of a bond with the inner wall. The weak attraction, ascribed to the inertness of the inner surface, is counteracted by destabilization due to deformations of a tube and radical induced by guest-host coupling. The energy balance argument illuminates that the inertness of the inner wall makes an alkyl radical species remain alive inside a tube and retain its reactivity. These findings can help us to understand experimental results where chemical reactions inside a tube proceed after guests are activated.