Graham A. Rance

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 nanoribbons 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.

van der Waals Interactions between Nanotubes and Nanoparticles for Controlled Assembly of Composite Nanostructures

We have demonstrated that ubiquitous van der Waals forces are significant in controlling the interactions between nanoparticles and nanotubes. The adsorption of gold nanoparticles (AuNPs) on nanotubes (MWNTs) obeys a simple quadratic dependence on the nanotube surface area, regardless of the source of AuNPs and MWNTs. Changes in the geometric parameters of the components have pronounced effects on the affinity of nanoparticles for nanotubes, with larger, more polarizable nanostructures exhibiting stronger attractive interactions, the impact of which changes in the following order MWNT diameter > AuNP diameter > MWNT length.

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.

Assembly, Growth, and Catalytic Activity of Gold Nanoparticles in Hollow Carbon Nanofibers

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.

Palladium nanoparticles on carbon nanotubes as catalysts of cross-coupling reactions

The macroscopic properties of composite nanotube–nanoparticle superstructures are determined by a complex interplay of structural parameters at the nanoscale. The catalytic performance of different carbon nanotube–palladium nanoparticle catalysts, where nanoparticles were formed either directly onto nanotubes or preformed prior to deposition on nanotubes using different types of surfactants, were tested in cross-coupling reactions. The decoration of multi-walled carbon nanotubes with preformed thiolate-stabilised palladium nanoparticles yielded the optimum catalyst, exhibiting high activity and stability towards carbon–carbon bond formation and excellent recyclability, retaining high activity from cycle to cycle. The type of carbon nanotube support has pronounced effects on the density of deposited nanoparticles, with more polarisable MWNT able to uptake the highest number of nanoparticles per unit surface area as compared to other carbon nanostructures (MWNT > DWNT > SWNT ∼ GNF). Microscopic investigation of the nanoscale morphology found that nanoparticles increase in size during catalysis. The extent of growth is dependent on the type of nanocarbon support, with wider MWNT possessing lower curvature and thus retarding the growth and coalescence of nanoparticles to a greater extent than other carbon nanostructures (SWNT ≫ DWNT > MWNT ∼ GNF). The type of halogen X in the C–X bond activated by palladium appears to influence the evolution of nanoparticles during catalysis, with X = Br having the greatest effect as compared to X = Cl or I. Overall, preformed thiolate-stabilised palladium nanoparticles deposited on MWNT from solution was found to possess the most functional catalytic properties, with optimum activity, stability and recyclability in a range of cross-coupling reactions.

Chemistry at the Nanoscale: Synthesis of an N@C60–N@C60 Endohedral Fullerene Dimer

Endohedral fullerenes—carbon cages with one or more heteroatoms incarcerated within them—are one of the most exotic classes of molecules. They possess a wealth of fascinating functional properties, including magnetism and photoactivity. While the properties of individual endohedral fullerene molecules are remarkable, the full functional potential of many of these species will likely be realized in systems where two or more of these molecules are connected. One prominent example is the endohedral fullerene radical species N@C60. The highly symmetric central location of the nitrogen atom within the C60 cage, [2] with minimal mixing between the nitrogen atom and fullerene electron wavefunctions, imparts a degree of isolation to the nitrogen radical that is usually only attainable using an ion trap or in an atomic gas. This isolation allows the nitrogen center to retain a S3/2 electron spin ground state and to have an extraordinarily long electron spin coherence time (T2e of 250 ms). [4] This remarkable spin coherence time has lead many research groups to study the potential and feasibility of an N@C60based quantum computer over the past decade. Owing to a number of synthetic challenges associated with N@C60, experimental electron spin resonance studies have so far been restricted to probing the interaction of ensembles of molecules each containing a single N@C60 spin center. These challenges center on the low-yielding production methods available to synthesize N@C60, thus producing at best a 500 ppm N@C60/C60 mixture, [2, 6] and the time consuming processing techniques currently required to enrich the ratio of N@C60 to C60. [7] In addition, compared to C60, chemically functionalized N@C60 derivatives have significantly lower thermal and photo stability. 9] To further explore the potential of N@C60 as a quantum computing element, including the creation of controlled entanglement between electron spins, an array of at least two N@C60 centers with fixed separation is required to understand the interaction of neighboring spin centers with one another and investigate methods of selective manipulation of one of the spin centers. Herein we describe a one-pot method, using a double 1,3dipolar cycloaddition, for the synthesis of a two-center N@C60 molecule with fixed spatial separation (Scheme 1), thus providing a platform for future experiments to probe the nature of the electron interaction between two N@C60 molecules to assess the potential capabilities of a 2 qubit N@C60 quantum computer. To the best of our knowledge, this is the first endohedral fullerene dimer comprising two chemically linked N@C60 spin centers. A Prato 1,3-dipolar cycloaddition reaction was chosen to link the two N@C60 molecules owing to its proven compatibility with the stability of N@C60 and the wide range of amino acid and aldehyde derivatives that it has been reported to be compatible with. A dibenzaldehyde-terminated oligo(pphenylene polyethylene) (OPE) molecule (Ald-3Bz) was synthesized and used as the spacing unit because of its rigidity and modular construction. The incorporation of long alkyl chains on the central phenyl group enhances the inherent poor solubility of the rigid, linear molecule, thus allowing the reaction to be performed at high concentration and making the product fullerene dimer (dimer-3Bz) soluble in common solvents such as chloroform. Previous work has shown that the structure of the amino acid derivative used makes a significant difference to the rate and yield of the 1,3-dipolar cycloaddition reaction. Work within our group has found that the N-(4-(hexyloxy)benzyl)glycine amino acid derivative greatly enhances the dimer formation reaction rate compared to more commonly used amino acid derivatives such as N(ethyl)glycine (see the Supporting Information). In addition, the amino group produces a fullerene dimer that can be purified using standard single-pass HPLC methods. We synthesized three grams of a N@C60/C60 mixture using an ion implantation method, thus yielding an average purity of 50 ppm of N@C60 in C60. Extensive purification of this sample followed. After 19 recycling HPLC runs, a high purity N@C60/C60 peak was isolated. The integrated area of the N@C60/C60 peak indicated the total mass of N@C60/C60 to be 10 mg. This sample then underwent quantitative ESR analysis of the total number of N@C60 spin centers present, thereby giving a lower bound for the mass of N@C60 to be 4.6 mg. Optimized reaction conditions and an efficient purification method for the one-pot dimer synthesis were identified and scaled down to work using only 20 mg of the C60 starting [*] B. J. Farrington, Prof. G. A. D. Briggs, Dr. K. Porfyrakis Department of Materials, University of Oxford Oxford OX1 3PH (UK) E-mail: kyriakos.porfyrakis@materials.ox.ac.uk

Functionalized Fullerenes in Self-Assembled Monolayers

Anisotropy of intermolecular and molecule-substrate interactions holds the key to controlling the arrangement of fullerenes into 2D self-assembled monolayers (SAMs). The chemical reactivity of fullerenes allows functionalization of the carbon cages with sulfur-containing groups, thiols and thioethers, which facilitates the reliable adsorption of these molecules on gold substrates. A series of structurally related molecules, eight of which are new fullerene compounds, allows systematic investigation of the structural and functional parameters defining the geometry of fullerene SAMs. Scanning tunnelling microscopy (STM) measurements reveal that the chemical nature of the anchoring group appears to be crucial for the long-range order in fullerenes: the assembly of thiol-functionalized fullerenes is governed by strong molecule-surface interactions, which prohibit formation of ordered molecular arrays, while thioether-functionalized fullerenes, which have a weaker interaction with the surface than the thiols, form a variety of ordered 2D molecular arrays owing to noncovalent intermolecular interactions. A linear row of fullerene molecules is a recurring structural feature of the ordered SAMs, but the relative alignment and the spacing between the fullerene rows is strongly dependent on the size and shape of the spacer group linking the fullerene cage and the anchoring group. Careful control of the chemical functionality on the carbon cages enables positioning of fullerenes into at least four different packing arrangements, none of which have been observed before. Our new strategy for the controlled arrangement of fullerenes on surfaces at the molecular level will advance the development of practical applications for these nanomaterials.

Transport and encapsulation of gold nanoparticles in carbon nanotubes.

Nanoparticles confined in small volumes exhibit functional properties different from that of the bulk material. Furthermore, the smaller the volume available then the greater the effects of confinement are observed to be. Metallic nanoparticles encapsulated within carbon nanotubes have been proposed for many applications ranging from catalysis to quantum storage devices. In this study we examine encapsulation of discrete gold nanoparticles (AuNP) within multi-wall carbon nanotubes (MWNT), with internal diameter less than 10 nm. During the encapsulation process AuNP undergo Ostwald ripening allowing them to reach a diameter that precisely matches the internal diameter of MWNT (snug fit). The use of supercritical CO2 as a processing medium enables efficient transport and irreversible encapsulation of AuNP into narrow nanotubes. Once inside MWNT, the nanoparticles are unable to grow further and retain their spheroidal shape. This dynamic behaviour observed for AuNP differs significantly from the behaviour of molecular guest-species under similar conditions.

Controlling the Regioselectivity of the Hydrosilylation Reaction in Carbon Nanoreactors

Hollow graphitized carbon nanofibres (GNF) are employed as nanoscale reaction vessels for the hydrosilylation of alkynes. The effects of confinement in GNF on the regioselectivity of addition to triple carbon-carbon bonds are explored. A systematic comparison of the catalytic activities of Rh and RhPt nanoparticles embedded in a nanoreactor with free-standing and surface-adsorbed nanoparticles reveals key mechanisms governing the regioselectivity. Directions of reactions inside GNF are largely controlled by the non-covalent interactions between reactant molecules and the nanofibre channel. The specific π-π interactions increase the local concentration of the aromatic reactant and thus promote the formation of the E isomer of the β-addition product. In contrast, the presence of aromatic groups on both reactants (silane and alkyne) reverses the effect of confinement and favours the formation of the Z isomer due to enhanced interactions between aromatic groups in the cis-orientation with the internal graphitic step-edges of GNF. The importance of π-π interactions is confirmed by studying transformations of aliphatic reactants that show no measurable changes in regioselectivity upon confinement in carbon nanoreactors.

Assembly, structure and electrical conductance of carbon nanotube–gold nanoparticle 2D heterostructures

Thiol-functionalised multiwalled carbon nanotubes (MWNTs, average outside diameter ∼10 nm) assemble into thin films on a liquid–liquid interface. Combined with citrate-stabilised gold nanoparticles (NPs, average diameter ∼10 nm) MWNTs form uniform, extended quasi-2D nanotube-nanoparticle heterostructures which, being on average only a few tens of nm thick, are self-supporting on the microscale and can span macroscopic surface areas up to 10 cm2. MWNTs in these heterostructures are interwoven and interlinked with nanoparticles. The nanotube : nanoparticle ratio in the film can be conveniently controlled by the ratio of components in the liquid phase. The electrical resistance of MWNT-NP composites varies only slightly with the percentage of nanoparticles incorporated in the film, indicating that the electrical properties of these structures are mostly defined by nanotubes. The effect of the presence of nanoparticles on the resistance of MWNT-NP films is highly dependent on the scale of the measurements (electrode geometry) and is qualitatively different for the sub-50 μm regime as compared to the macroscopic regime.