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.

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.

Clusters of transition metals, W, Re, and Os, upon encapsulation within a single-walled carbon nanotube (SWNT) exhibit marked differences in their affinity and reactivity with the SWNT, as revealed by low-voltage aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM). Activated by an 80 keV electron beam, W reacts only weakly with the SWNT, Re creates localized defects on the sidewall, and Os reacts readily causing extensive defect formation and constriction of the SWNT sidewall followed by total rupture of the tubular structure. AC-HRTEM imaging at the atomic level of structural transformations caused by metal−carbon bonding of π-and σ-character demonstrates what a crucial role these types of bonds have in governing the interactions between the transition metal clusters and the SWNT. The observed order of reactivity W < Re < Os is independent of the metal cluster size, shape, or orientation, and is related to the metal to nanotube bonding energy and the amount of electronic density transferred between metal and SWNT, both of which increase along the triad W, Re, Os, as predicted by firstprinciples density functional theory calculations. By selecting the appropriate energy of the electron beam, the metal−nanotube interactions can be controlled (activated or precluded). At an electron energy as low as 20 keV, no visible transformations in the nanotube in the vicinity of Os-clusters are observed. ■ INTRODUCTIONTransition metals (d-elements) form the largest block of the Periodic Table and offer the widest variety of magnetic, optical, catalytic, and other functional properties. The rich chemistry of transition metals when combined with the mechanical, electric, thermal, and chemical properties of carbon nanostructures, such as single-walled carbon nanotubes (SWNTs), may lead to the generation of new families of functional materials which harness the synergy of the resultant metal−nanocarbon interactions. Recent investigations of metal−SWNT heterostructures have opened new highly promising avenues for applications in catalysis, 1 hydrogen storage, 2 and electronic devices. 3 Therefore, the quest for complete understanding of the nature of bonding between carbon nanotubes and transition metals is becoming increasingly important as illustrated by a recent flurry of theoretical studies on interactions between transition metals and SWNTs. 4 However, experimental measurements are significantly impeded because of the typical polydispersity of nanotubes (i.e., SWNTs of different lengths, diameters and helicities are present in the same sample), the lack of their intrinsic solubility, and by ubiquitous impurities in SWNT samples (e.g., amorphous carbon, graphitic particles, residual metal catalyst). While conventional spectroscopic methods that integrate over larger volumes (e.g., XPS, Raman, etc.) can be applied for characterizing the bulk physicochemical properties, high-resolution transmission electron microscopy (HRTEM) is now rapidly becoming an excellent local-probe tool for study...

The main objective of this Account is to assess the challenges of transmission electron microscopy (TEM) of molecules, based on over 15 years of our work in this field, and to outline the opportunities in studying chemical reactions under the electron beam (e-beam). During TEM imaging of an individual molecule adsorbed on an atomically thin substrate, such as graphene or a carbon nanotube, the e-beam transfers kinetic energy to atoms of the molecule, displacing them from equilibrium positions. Impact of the e-beam triggers bond dissociation and various chemical reactions which can be imaged concurrently with their activation by the e-beam and can be presented as stop-frame movies. This experimental approach, which we term ChemTEM, harnesses energy transferred from the e-beam to the molecule via direct interactions with the atomic nuclei, enabling accurate predictions of bond dissociation events and control of the type and rate of chemical reactions. Elemental composition and structure of the reactant molecules as well as the operating conditions of TEM (particularly the energy of the e-beam) determine the product formed in ChemTEM processes, while the e-beam dose rate controls the reaction rate. Because the e-beam of TEM acts simultaneously as a source of energy for the reaction and as an imaging tool monitoring the same reaction, ChemTEM reveals atomic-level chemical information, such as pathways of reactions imaged for individual molecules, step-by-step and in real time; structures of illusive reaction intermediates; and direct comparison of catalytic activity of different transition metals filmed with atomic resolution. Chemical transformations in ChemTEM often lead to previously unforeseen products, demonstrating the potential of this method to become not only an analytical tool for studying reactions, but also a powerful instrument for discovery of materials that can be synthesized on preparative scale.

Graphitized carbon nanofibers (GNFs) act as efficient templates for the growth of gold nanoparticles (AuNPs) adsorbed on the interior (and exterior) of the tubular nanostructures. Encapsulated AuNPs are stabilized by interactions with the step-edges of the individual graphitic nanocones, of which GNFs are composed, and their size is limited to approximately 6 nm, while AuNPs adsorbed on the atomically flat graphitic surfaces of the GNF exterior continue their growth to 13 nm and beyond under the same heat treatment conditions. The corrugated structure of the GNF interior imposes a significant barrier for the migration of AuNPs, so that their growth mechanism is restricted to Ostwald ripening. Conversely, nanoparticles adsorbed on smooth GNF exterior surfaces are more likely to migrate and coalesce into larger nanoparticles, as revealed by in situ transmission electron microscopy imaging. The presence of alkyl thiol surfactant within the GNF channels changes the dynamics of the AuNP transformations, as surfactant molecules adsorbed on the surface of the AuNPs diminished the stabilization effect of the step-edges, thus allowing nanoparticles to grow until their diameters reach the internal diameter of the host nanofiber. Nanoparticles thermally evolved within the GNF channel exhibit alignment, perpendicular to the GNF axis due to interactions with the step-edges and parallel to the axis because of graphitic facets of the nanocones. Despite their small size, AuNPs in GNF possess high stability and remain unchanged at temperatures up to 300 °C in ambient atmosphere. Nanoparticles immobilized at the step-edges within GNF are shown to act as effective catalysts promoting the transformation of dimethylphenylsilane to bis(dimethylphenyl)disiloxane with a greater than 10-fold enhancement of selectivity as compared to free-standing or surface-adsorbed nanoparticles.

Self-optimising chemical systems have experienced a growing momentum in recent years. Herein, we review algorithms used for the self-optimisation of chemical reactions in an accessible way for the general chemist.

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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.