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Data availability: Data will be made available on request. Appendix A. Supplementary data: Supplementary data to this article can be found online at: https://doi.org/10.1016/j.ultsonch.2022.106158. Copyright © 2022 The Authors. The volume of fluid (VOF) and continuous surface force (CSF) methods were used to develop a bubble dynamics model for the simulation of bubble oscillation and implosion dynamics under ultrasound. The model was calibrated and validated by the X-ray image data acquired by ultrafast synchrotron X-ray. Coupled bubble interactions with bulk graphite and freely moving particles were also simulated based on the validated model. Simulation and experiments quantified the surface instability developed along the bubble surface under the influence of ultrasound pressure fields. Once the surface instability exceeds a certain amplitude, bubble implosion occurs, creating shock waves and highly deformed, irregular gas-liquid boundaries and smaller bubble fragments. Bubble implosion can produce cyclic impulsive stresses sufficient enough to cause µs fatigue exfoliation of graphite layers. Bubble-particle interaction simulations reveal the underlying mechanisms for efficient particle dispersion or particle wrapping which are all strongly related to the oscillation dynamics of the bubbles and the particle surface properties. UK Engineering and Physical Sciences Research Council (Grant Nos. EP/R031819/1; EP/R031665/1; EP/R031401/1; EP/R031975/1); Royal Society.

The surface energy based solubility parameters theory was applied\ud to model the degree of polystyrene-functionalisation of MWCNTs\ud in six different organic solvents. The experimental characterization of\ud the polymer-functionalized MWCNTs is consistent with the predictions of this model providing a breakthrough towards the rational design of functionalized MWCNTs based on thermodynamic parameters.

The endohedral fullerene 15N@C60 has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

By addressing the challenge of controlling molecular motion, mechanically interlocked molecular machines are primed for a variety of applications in the field of nanotechnology. Specifically, the designed manipulation of communication pathways between electron donor and acceptor moieties that are strategically integrated into dynamic photoactive rotaxanes and catenanes may lead to efficient artificial photosynthetic devices. In this pursuit, a novel [3]rotaxane molecular shuttle consisting of a four-station bis-naphthalene diimide (NDI) and central C60 fullerene bis-triazolium axle component and two mechanically bonded ferrocenyl-functionalized isophthalamide anion binding site-containing macrocycles is constructed using an anion template synthetic methodology. Dynamic coconformational anion recognition-mediated shuttling, which alters the relative positions of the electron donor and acceptor motifs of the [3]rotaxane’s macrocycle and axle components, is demonstrated initially by 1H NMR spectroscopy. Detailed steady-state and time-resolved UV–vis–IR absorption and emission spectroscopies as well as electrochemical studies are employed to further probe the anion-dependent positional macrocycle–axle station state of the molecular shuttle, revealing a striking on/off switchable emission response induced by anion binding. Specifically, the [3]rotaxane chloride coconformation, where the ferrocenyl-functionalized macrocycles reside at the center of the axle component, precludes electron transfer to NDI, resulting in the switching-on of emission from the NDI fluorophore and concomitant formation of a C60 fullerene-based charge-separated state. By stark contrast, in the absence of chloride as the hexafluorophosphate salt, the ferrocenyl-functionalized macrocycles shuttle to the peripheral NDI axle stations, quenching the NDI emission via formation of a NDI-containing charge-separated state. Such anion-mediated control of the photophysical behavior of a rotaxane through molecular motion is unprecedented.

We measure the electron spin resonance spectrum of the endohedral fullerene molecule $^{15}\mathrm{N@C}_{60}$ at pressures ranging from atmospheric pressure to 0.25 GPa, and find that the hyperfine coupling increases linearly with pressure. We present a model based on van der Waals interactions, which accounts for this increase via compression of the fullerene cage and consequent admixture of orbitals with a larger hyperfine coupling. Combining this model with theoretical estimates of the bulk modulus, we predict the pressure shift and compare it to our experimental results, finding fair agreement given the spread in estimates of the bulk modulus. The spin resonance linewidth is also found to depend on pressure. This is explained by considering the pressure-dependent viscosity of the solvent, which modifies the effect of dipolar coupling between spins within fullerene clusters., 5 pages, 2 figures

Fullerene dyads bridged with perfluorinated linking groups have been synthesized through a modified arc-discharge procedure. The addition of Teflon inside an arc-discharge reactor leads to the formation of dyads, consisting of two C60 fullerenes bridged by -CF2- groups. The bridging groups consisting of electronegative atoms, lead to different energy levels and to new features in the photoluminescence spectrum. A suppression of the singlet oxygen photosensitization, indicated that the radiative decay from singlet to singlet state is favoured against the intersystem crossing singlet to triplet transition.

The endohedral fullerene 15N@C60 has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock’s projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

The endohedral fullerene 15N@C60 has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock’s projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

Electron spin qubits in molecular systems offer high reproducibility and the ability to self-assemble into larger architectures. However, interactions between neighboring qubits are "always on," and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both effectively turn on or off interqubit coupling mediated by dipolar interactions and benefit from the long nuclear spin decoherence times (T2n). We transfer qubit states between the electron and N15 nuclear spin in N15@C60 with a two-way process fidelity of 88%, using a series of tuned microwave and radio frequency pulses and measure a nuclear spin coherence lifetime of over 100 ms. © 2011 American Physical Society.

We examine the temperature dependence of the electron spin relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero-field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S> or =1 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find an Arrhenius temperature dependence for N@C60 , indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170 K.

Photoluminescence spectroscopic measurements have allowed the acquisition of high resolution spectra at low temperature for the endohedral metallofullerenes, Er2 @ C82 (isomer I) and Er2 C2 @ C82 (isomer I). The characteristic emission in the 1.5-1.6 μm region corresponds to the 4 I13 / 2 (m) → 4 I15 / 2 (n) transitions of the Er3 + ion for both molecules. The emission arising from Er2 C2 @ C82 (I) appears acuminated (narrow lines that taper to a point) when compared with that of Er2 @ C82 (I). The Er2 C2 @ C82 (I) emission linewidths are comparable to those found in crystals, making this molecule of interest for applications where accessible, well-defined quantum states are required. © 2009 Elsevier B.V. All rights reserved.

Motivated by the possibility of observing photoluminescence and electron paramagnetic resonance from the same species located within a fullerene molecule, we initiated an EPR study of Er3+ in ErSc2N@C80. Two orientations of the ErSc2N rotor within the C80 fullerene are observed in EPR, consistent with earlier studies using photoluminescence excitation (PLE) spectroscopy. For some crystal field orientations, electron spin relaxation is driven by an Orbach process via the first excited electronic state of the 4I15/2 multiplet. We observe a change in the relative populations of the two ErSc2N configurations upon the application of 532 nm illumination, and are thus able to switch the majority cage symmetry. This photoisomerization, observable by both EPR and PLE, is metastable, lasting many hours at 20 K. © 2008 The American Physical Society.

We review recent progress towards implementing quantum information processing protocols using electron and nuclear magnetic resonance in the molecule N-14@C-60. Using conventional spectrometers, we measure the errors inherent in quantum manipulations of spins. By applying composite pulses, we correct the dominant error to a high degree. Identifying a two qubit computational basis out of the electron and oF nuclear spin levels of N-14@C-60, we implement an ultrafast phase gate on the nuclear qubit by exciting an electron spin transition. (c) 2007 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

The endohedral fullerene molecule, N@C60, is a candidate for molecular spin qubits (quantum bits) and spin probes owing to its exceptional electron spin properties. Advancements in the processing of N@C60 are key to obtaining samples of high purity on a reasonable timescale. We investigate enrichment by high throughput processing (flow rate of 18 L h(-1) and operating pressure of 1.5-2 MPa) using high performance liquid chromatography (HPLC) as a means of scaling N@C60 production. We use detection by electron paramagnetic resonance (EPR) spectroscopy to map N@C60 during processing, and through the reconstruction of the peak position in the chromatogram, we are able to determine the retention time and relative purity of N@C60 without the need for its isolation. Based on this, we establish a procedure for time-efficient, high throughput processing to isolate N@C60 in high purity.

A method has been developed for the attachment of a dithiolane group to endohedral metallofullerenes via a 1,3-dipolar cycloaddition reaction. This sulfur-containing functional group serves as an anchor, enabling efficient immobilisation of endohedral fullerenes on Au(111) surfaces at room temperature, directly from the solution phase. The functionalised fullerenes form disordered monolayers that exhibit no long-range ordering, which is attributed to both the strong bonding of the dithiolane anchor to the surface and to the conformational flexibility of the functional group. Endohedral fullerenes Er(3)N@C(80) and Sc(3)N@C(80) have been used as models for functionalisation and subsequent surface deposition. Their chemical reactivity towards dithiolane functionalisation and their surface behaviour have been compared to that of C(60). The endohedral fullerenes appear to be significantly less reactive towards the functionalisation than C(60), however they bind in a similar manner to a gold surface as their dithiolane terminated C(60) counterparts. The optical activity of Er(3)N@C(80) molecules is preserved after attachment of the functional group. We report a splitting of the endohedral Er(3+) emission lines due to the reduction in symmetry of the functionalised fullerene cage, as compared to the highly symmetrical icosahedral C(80) cage of pristine Er(3)N@C(80).

Electron spin qubits in molecular systems offer high reproducibility and the ability to self assemble into larger architectures. However, interactions between neighbouring qubits are 'always-on' and although the electron spin coherence times can be several hundred microseconds, these are still much shorter than typical times for nuclear spins. Here we implement an electron-nuclear hybrid scheme which uses coherent transfer between electron and nuclear spin degrees of freedom in order to both controllably turn on/off dipolar interactions between neighbouring spins and benefit from the long nuclear spin decoherence times (T2n). We transfer qubit states between the electron and 15N nuclear spin in 15N@C60 with a two-way process fidelity of 88%, using a series of tuned microwave and radiofrequency pulses and measure a nuclear spin coherence lifetime of over 100 ms., Comment: 5 pages, 3 figures with supplementary material (8 pages)

Endohedral fullerenes encapsulating single nitrogen atoms (N@C60 and N@C70) are spin active, due to the presence of unpaired nitrogen electron spins. High reactivity of nitrogen leads to a poor stability of these molecules at elevated temperatures, drastically restricting possibilities of their chemical treatment. In this study, N@C60 and N@C70 in toluene solutions were subjected to high pressure (HP) up to 0.8 GPa and annealed for 1, 2, 10 or 24 h, at either room or elevated temperatures. In each case the number of surviving molecules was evaluated based on the relative spin counts of ex situ EPR spectra obtained before and after the treatment. The enhanced thermal stability of N@C60 and N@C70 under HP conditions is attributed to inhibited formation of azabridges, which under normal pressure facilitate the escape of nitrogen from the fullerene cage. © 2009 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.

The magnetic properties of ErSc2 N @ C80, Er2 ScN @ C80 and Er3 N @ C80 metallofullerenes are characterized in the temperature range 2 to 300 K up to a magnetic field of 7 T. The magnetic susceptibility of these fullerenes follows the Curie-Weiss law. The fitting parameters to the Curie-Weiss law provide an effective magnetic moment μeff of Er3 + ion in each of the fullerenes. The magnetic moment decreases with the increase in number of Er3 + ions inside the cage. This is related to crystal-field effects, intramolecular interactions and the local quenching of the angular momentum of the ion within the fullerene cage. © 2008 Elsevier B.V. All rights reserved.

We examine environmental effects of surrounding nuclear spins on the electron spin relaxation of the N@C60 molecule (which consists of a nitrogen atom at the centre of a fullerene cage). Using dilute solutions of N@C60 in regular and deuterated toluene, we observe and model the effect of translational diffusion of nuclear spins of the solvent molecules on the N@C60 electron spin relaxation times. We also study spin relaxation in frozen solutions of N@C60 in CS2, to which small quantities of a glassing agent, S2Cl2 are added. At low temperatures, spin relaxation is caused by spectral diffusion of surrounding nuclear 35Cl and 37Cl spins in the S2Cl2, but nevertheless, at 20 K, T2 times as long as 0.23 ms are observed., Comment: 7 pages, 6 figures

We examine the temperature dependence of the relaxation times of the molecules N@C60 and N@C70 (which comprise atomic nitrogen trapped within a carbon cage) in liquid CS2 solution. The results are inconsistent with the fluctuating zero field splitting (ZFS) mechanism, which is commonly invoked to explain electron spin relaxation for S > 1/2 spins in liquid solution, and is the mechanism postulated in the literature for these systems. Instead, we find a clear Arrhenius temperature dependence for N@C60, indicating the spin relaxation is driven primarily by an Orbach process. For the asymmetric N@C70 molecule, which has a permanent non-zero ZFS, we resolve an additional relaxation mechanism caused by the rapid reorientation of its ZFS. We also report the longest coherence time (T2) ever observed for a molecular electron spin, being 0.25 ms at 170K., 6 pages, 6 figures V2: Updated to published version

The long spin lifetimes exhibited by endohedral fullerenes imply that they can store quantum information and hence have the potential to be used as qubits. Production of asymmetrical endohedral fullerene dimers (e.g. iNC(60)-iNC(70)) could lead to ordered arrays of ABABAB sequences. This kind of molecular self-assembly offers potential for creating large qubit arrays, a precondition for any scalable quantum computer. We present our progress towards the production and characterisation of endohedral fullerenes, fullerene dimers and array structures and their potential use in quantum computation.

Graphenoids are nanometer-sized flakes that share the same honeycomb framework of graphene. Novel chemistry has allowed the bottom-up synthesis of molecular graphenoids with atomic-like control of their structure. The electronic properties of these finite-sized graphene nanoislands actively depend on the topology of their edges and their pi-electron network. Rational design of the topology in terms of size, conjugation, and presence of defects, has highlighted exotic magnetic properties that were predicted for quantum confined graphene structures. The presence of free spins, and the similarity to the graphene lattice would render them promising materials for all carbon-based spintronic and quantum devices. While a large number of structures are being synthetised by chemists everywhere, the quantum properties of these molecules remain widely unexplored. In this work, we study the quantum properties of three categories of graphenoids: units with pentagonal defects, segments with zigzag edges, and ring-shaped molecules. Defects have been observed in graphene and are expected to play a key role in its optical, electronic, and magnetic properties. However, because most of the studies focused on the structural characterization, the implications of topological defects on the physicochemical properties of graphene remain poorly understood. Zigzag segments of carbon edges are a fundamental ingredient for most proposals of graphene nanostructures: they allow nontrivial topologies, where spin states can be used for quantum information processing and new communication pathways. Ring structures are intriguing for their aromatic and antiaromatic properties, and because they could show compelling quantum interference patterns. In this thesis, we use EPR to study the quantum properties of ensemble of isolated molecules. We measure coherence times at different temperatures and in different hosting matrices in order to evaluate structural and environmental effects on the properties. The molecular graphenoids obtain large coherence times up to room temperature that we compare to metal-based molecular systems and inorganic systems. We use advanced decoupling sequences to increment the coherence of the systems. Finally, we suggest synthetic strategies to remove the main channels of decoherence. These results shine new light on the fundamental quantum properties of topological defects and edge states in nanographene structures.

The synthesis of hybrid nanostructures comprised by CNTs and nanoparticles is a promising field of research due to their potential applications in catalysis, energy storage, energy generation and biomedical technologies. However, the field lacks of synthetic guidelines for the rational design of hybrid nanostructures. This Thesis aims to stablish synthetic guidelines that consider the forces involved during the synthesis of iron oxide nanoparticle-CNT hybrids where polymers are used to mediate the interactions between the nanomaterials. The functionalisation of MWCNTs with polystyrene is studied as the first level of hierarchy representing the polymer-MWCNT interactions. Non-covalent methods were found the most effective way to create polystyrene functionalised MWCNTs. It was found that the solvent used during the functionalisation was the parameter that had a higher effect in the amount of polystyrene found in the MWCNTs. The surface energy solubility parameter theories were proposed as a predictive synthetic model for the design of polymer functionalised MWCNTs. The predictions of the model were compared with the amount of polystyrene found in the functionalised MWCNTs synthesised in different solvents, finding good agreement between the predictions and the experimental evidence. The next level of nanostructural hierarchy was addressed by studying the synthesis of polystyrene coated iron oxide nanoparticle-CNT hybrids. The solvent was found to play a key factor in the amount of polystyrene coated iron oxide nanoparticles found in the hybrid structures. The surface energy based solubility parameter theory was used to understand the amount of iron oxide nanoparticles found in the hybrids synthesised in different solvents, showing a better agreement with the experimental results than other parameters previously highlighted in the literature. The results and ideas proposed in this Thesis will contribute to the future development of the rational synthesis of nanoparticle-CNT hybrid structures.

The discovery of graphene has attracted significant attention and research on the material as well as providing avenues of research into other two-dimensional (2D) materials. Expanding the properties of graphene requires integration with other systems, such as molecules, or other low dimensional materials, to provide new functionalities and applications. In particular, the strong sp2 bond between carbon atoms in graphene gives a unique opportunity for adatom adsorption. Decorating the surface of graphene with adatoms and nanoclusters is one approach that can alter the band structure of graphene and introduce dopants that can modify the p-type and n-type behavior. Moreover, the integration of other 2D materials with graphene can significantly alter their properties and also lead to observation of new structure or phenomenon. The first chapter is based on synthesizing, extracting and separating endohedral metallo-fullerenes (EMFs) and their integration with 2D graphene systems. Optimal set of process parameters for the arc discharge apparatus for the production of EMFs was identified along with the best solvent extraction and separation methodologies. The EMFs studied were La@C82, Y@C82, Sc3N@C80 and Gd3N@C80. EPR was used to confirm purity and characteristics of the EMFs. Gd based metallofullerene (Gd3N@C80) molecules was then used to integrate with the graphene system and used to create single adatoms and nanoclusters on a graphene surface. An in-situ heating holder within an aberration corrected scanning transmission electron microscope was used to track the adhesion of endohedral metallofullerenes to the surface of graphene, followed by Gd metal ejection and diffusion across the surface. Hydrogen was shown to be used to reduce the temperature of EMF fragmentation and metal ejection, enabling Gd nanocluster formation on graphene surfaces at temperatures as low as 300oC. The second part of the project was to study the other 2D materials in their heterostructures with graphene. Lead Iodide (PbI2) is a large band gap 2D layered material that has potential for 2D semiconductor applications. However, atomic level imaging of PbI2 monolayer crystal structure and its fundamental defects has been limited due to challenges in obtaining suitable quality thin crystals. In this work, liquid-exfoliation was used to produce monodisperse 2D monolayer PbI2 nanodisks (30-40nm in diameter and >99% monolayer purity) and deposit them onto suspended graphene supports that have high electron beam transparency to enable atomic resolution annular dark field scanning transmission electron microscopy (ADF-STEM) of PbI2. Strong epitaxial alignment of PbI2 monolayers with the underlying graphene lattice occurred, with PbI2 zig-zag edge commensurate with the graphene arm-chair edge direction, leading to a phase shift from the 1T to 1H structure to increase the level of commensuration in the two lattice spacings (PbI2 and graphene). Then the fundamental point vacancy structures in PbI2 monolayers were imaged directly, showing rapid vacancy migration to the edges and self-healing. Zig-zag edges were also observed to dominate the PbI2 nanodisks. Nanopores were produced by electron beam irradiation of the PbI2 nanodisks and they underwent migration as an intact entity throughout the lattice. These results provided a detailed insight into the atomic structure and defects in monolayer PbI2, and the impact of the strong van der Waals interaction with graphene, which has importance for future applications in opto-electronics. The third chapter studies the role of graphene in another newly emerging 2D material, Palladium Diselenide (PdSe2). Technologically challenging, controllable transformation between the semiconducting and metallic phases of transition metal chalcogenides (TMDs) is of particular importance. Here, controlled laser irradiation could be used to ablate PdSe2 thin films using high power, or trigger the local transformation of PdSe2 into a metallic phase PdSe2-x using lower laser power. PdSe2 material demonstrated strong sensitivity to laser exposure where high laser power resulted in local material degradation and formation of Pd nanoparticles (NPs), while lower laser power could be used to controllably modify the PdSe2 phase, making it Se-deficient and resulting in a PdSe2-x phase. Four regions within the film were observed after laser exposure (1) hole region where the PdSe2 film and graphene were fully damaged, (2) PdSe2 film was modified forming Pd NPs; (3) the phase change of the material was observed where PdSe2 transforms into PdSe2-x and (4) unmodified area of PdSe2. The presence and absence of graphene considerably changed the hole formation area and the phase transformation.

The incursion of the physical sciences into the medical sphere has led to myriad investigations in the use of nanomaterials for diagnostic and therapeutic purposes. In particular, the fullerenes show great promise in a number of areas because of their unique molecular properties. The chemistry of these carbon building blocks can be harnessed through covalent functionalisation and incorporation into supramolecular architectures such as extended self-assembled structures and molecular machines based on mechanically interlocked molecules. This thesis describes the synthesis and characterisation of such supramolecular structures incorporating fullerenes, whose potential utility in nanomedicine is demonstrated. Water-solubilisation of fullerene C60 is achieved in the first part of this thesis by covalent functionalisation with triethylene glycol monoethyl ether. The resulting complex molecular structure is elucidated and a new method developed for interpreting elemental and thermal gravimetric analyses. It is highly soluble in water (37 mg/mL) and exhibits concentration-dependent photoluminescent behaviour suitable for biomedical applications. Ultraviolet-Visible absorption data points to the formation of self-assembled structures. Over time, these self-assembled structures grow considerably into hydrogels. Optical and scanning electron microscopies show the hydrogels have a tricontinuous hierarchical structure of great promise for use in drug delivery. The same functionalisation protocol is applied to higher fullerenes C70, C84 and C90–92, and extensive hierarchical structures are formed for the latter two. It is proposed that the presence of corannulene-like hydrophobic regions in the functionalised fullerenes affects the formation of hierarchical structures. Atomic force microscopy nanoindentation shows that the strongest of the formed hydrogels (C90–92 derivative, E = 432 ± 286 kPa) fares remarkably well against other supramolecular hydrogel competitors. In the second part of this thesis, fullerene C60 is functionalised for incorporation into a rotaxane-based molecular machine. Two new iodo-alkyne functionalised fullerene derivatives are isolated in this process. C60 is incorporated into a four-station [3]rotaxane with two peripheral naphthalene diimide stations and two ferrocene-functionalised macrocycles via a copper-catalysed click reaction. The anion-induced motion of the macrocycles leads to a tuneable photoluminescent response due to photoinduced electron transfer. C60 plays a crucial electron-accepting role in this setup. In this way, this fullerene-containing mechanically interlocked architecture demonstrates a unique platform for the sensing of chloride ions. The utility of this [3]rotaxane is expanded into a theranostic tool by the demonstration of tuneable singlet oxygen generation by the central fullerene moiety concomitant with the tuneable photoluminescence.

Fullerenes have a number of unique properties that make them interesting for use in molecular electronics. They are readily functionalized to produce a wide range of physical and chemical properties as well as creating a number of methods by which they can be combined with an electrode for electron transport. Fullerenes are also able to encapsulate atoms or clusters of atoms to further expand their properties, leading to a number of materials which show huge potential for applications in the field of quantum information. In particular the endohedral fullerene N@C60 shows one of the longest coherence lifetimes of any molecular system, making it an ideal candidate for a qubit. This thesis presents one method of functionalising fullerenes for combining their many attractive electronic properties with those of graphene. These functionalized fullerenes display a number of interesting properties in their own right. In particular, they show the possibility to selectively quench or sensitise the formation of singlet oxygen, which has dramatic implications for the use of fullerenes in medical applications. Electron transport measurements of the fullerene show excited vibrational states which confirm the presence of fullerene molecules. Finally, the presence of long-lived vibrational states makes fullerenes appealing for use in thermoelectrics, which is studied in detail.

Transition metal dichalcogenides (TMDs) are semiconducting two-dimensional (2D) materials with direct bandgaps for the monolayers. Recently, much attention has been attracted to its synthesis, properties and applications in nanoelectronics and optoelectronics. The DPhil project focused on growing 2D materials developing chemical vapour deposition (CVD) approaches to grow 2D materials, including molybdenum disulphide (MoS2), tungsten disulphide (WS2), and hexagonal boron nitride (hBN), based on which vertical layered heterostructures (VLHs) were fabricated via sequential transfer. Furthermore, a range of characterisation techniques were employed to investigate the structural, vibrational, optical and thermal properties of both these monolayers and multilayer stacks. An atmospheric pressure CVD (APCVD) method was first developed to grow monolayer MoS2 crystals on silicon substrates with an amorphous oxide layer (SiO2/Si). A gradient of morphologies ranging from strictly triangular to highly dendritic shapes were attained with the growth dynamics controlled by tailoring the local concentrations of the precursors. In addition, the growth procedure was programmed and the conditions were optimized for large domain size. The monolayer MoS2 dendrites show good electrocatalytic performance toward hydrogen evolution reactions (HER). Subsequently, this manner was applied to synthesise monolayer MoS2 on crystalline strontium titanate (SrTiO3) substrate. The crystal shape was dependent on the surface terminations of the substrate, explained by the greatest interfacial van der Waals (vdW) bonding between MoS2 monolayers and SrTiO3 at the maximum commensuration of their lattices. The as-grown MoS2 was annealed either under vacuum or in sulphur environment, leading to either degradation or defect annihilation, respectively, as indicated by Raman and photoluminescence (PL) spectroscopy and X-ray photoelectron spectroscopy (XPS). The focus of the following studies were on the interlayer interactions between TMD monolayers and the optical properties of the as-constituent WS2:hBN:MoS2 and WS2:hBN:WS2 VLHs. The involvement of hBN separators restricts the interlayer charge transfer and instead enables Förster energy transfer, switching the observation of PL quenching to enhancement. The level of PL enhancement could be determined by the layer number of hBN, the excitation power, the lattice strain, and the temperature, which were revealed by PL spectroscopy and transient absorption spectroscopy, as well as by theoretical modelling. The layer number of hBN adjusts the separation distance and thereby the interlayer coupling between the TMD monolayers; at higher excitation power, the possibility of interlayer energy transfer increases due to the higher exciton density, which results in a larger quantum yield (QY) of the heterosystem; the presence of strain induces shifts in the bandgap of TMDs and alters the relative offsets in the band structure generating the type II heterojunction where the interlayer interactions take place; the nonradiative decay channels are suppressed at cryogenic temperatures, and the efficiency of PL emission can be improved. These VLHs hold potential for advanced devices with desirable optical performance.

Endohedral nitrogen fullerenes (ENFs) have been proposed as building blocks for quantum information processing due to very long relaxation time for their incarcerated electron spins. However, fabricating quantum devices based on this exotic material is still limited by the low yield of ENFs synthesis and various difficulties in subsequent molecular engineering, including the chemical sensitivity of the molecules, assembling approaches of the molecular architecture, and control of spin-spin coupling between qubits. My contributions towards removing the aforementioned limitations by studying the synthesis and functionalisation of ENFs are presented herein. My aim has been to pave the way towards quantum devices based on ENFs. Firstly, I enhanced the ENFs production yield by a factor of five. I accomplished this by optimizing the ion implantation apparatus and parameters during the synthesis of raw ENFs, in addition to adjusting the column and eluent during the purification of ENFs by high performance liquid chromatography (HPLC). Secondly, I established (for the first time) a spin-compatible protocol for performing Bingel reactions on ENFs. Utilizing the developed method, I also demonstrated the feasibility of chemically modifying ENFs for different molecular requirements via synthesizing a series of ENF derivatives with rigid configuration, long molecular aspect ratio, and amphiphilic properties. Subsequently, I covalently assembled ENFs at microscopic levels and axially aligned ENFs at macroscopic levels, respectively. At the microscopic scale, I synthesized ENF- containing dyads and dimers, and developed a method of coaxially dimerizing ENFs with rigid bridge molecules, which provides material foundation for multi-qubit manipulations with spin couplings. At the macroscopic scale, I achieved the best orientational alignment of ENFs reported to date by embedding elongated ENF derivatives within a liquid crystal, which is critical for ensemble qubit with anisotropic spin properties. Benefiting from the good alignment, a controllability of the ensemble spin anisotropy is demonstrated with zero-field splitting of ENF derivatives. Finally, I studied the electron spin dipolar coupling in ENFs by measuring and comparing of the coupling strength at different conditions. I discovered that the electron spin dipolar coupling in N@C60-CuPc dyads can be chemically tuned by altering the lengths of the spacing groups between the two spin centres, and be physically adjusted by changing the sample concentrations to aggregate the sample and suppress the Cu electron spin. In summary, aiming for ENF-based quantum devices, I made progress in both the material production and the molecular engineering of ENFs.

Endohedral fullerenes are remarkable molecules with unique electronic, magnetic, photovoltaic and quantum properties. They have found a number of applications in various fields. Among all the potential applications, one particularly interesting application is to employ them as building blocks for a quantum computer. However, the scientific and commercial exploitation of these materials is held back by their low productivity. In this project, the arc discharge synthesis of endoedral fullerenes which remains the most promising candidate for producing bulk quantities of Endohedral Metallofullerenes (EMFs) was divided into 4 steps, and in this thesis, I discuss the attempts that had been made to optimise each of them. In this project, I focus on Y@C82, which is a spin active endohedral fullerene with attractive quantum properties. I obtained sights into the dynamics of the Soxhlet extraction of fullerenes, and developed an optimised Soxhlet scheme which is able to extract 95% of Y@C82 from the soot within 60 hours (Chapter 4). Further progress on the optimization of Soxhlet extraction was made when a two-stage Soxhlet extraction/purification was independently developed (chapter 9). This method employs two solvents (toluene and DMF) to extract fullerenes with different dipolar moment. It was not only able to extract fullerenes effectively from the soot, but also to purify EMFs/Trimetallic nitride template(TNT) into high purity (97%). Efforts were made to search for the best conditions for generating Y@C82 with a patented pilot arc discharge system. After analysing the data of the yield of fullerenes in various conditions with a "22 factorial design of experiment", I believe the yields of Y@C82 can be increased by using high He pressure (Chapter 5). The scaling up the production of EMFs was also tackled from a more fundamental and theoretical aspect. Although fullerenes have been efficiently synthesized by several methods to date, the formation mechanism of these materials remains a mystery. The study of the fullerene formation mechanism in arc discharge is particularly rare due to intrinsic technical difficulties. In chapter 6, I propose a new "bottom up" formation scenario in the arc discharge synthesis of fullerenes that adopts the so called "closed network growth". Attempts that were made to improve the efficiency and safety of the current system were introduced in Chapter 7. Concepts to develop a more efficient safe arc discharge system were suggested and discussed in the same chapter. The Lewis acid separation method was reported to be an efficient approach to remove the empty cages from the EMFs1, however, this method is only suitable for a lab equipped with specialized facilities and cannot be characterized as generic (Chapter 8). Finally, I have applied a functionalization scheme to C60 which may be a promising scheme to functionalize spin-active metallofullerenes to produce a two-qubit quantum information system (Chapter 10).

Quantum information processing (QIP), which inherently utilizes quantum mechanical phenomena to perform information processing, may outperform its classical counterpart at certain tasks. As one of the physical implementations of QIP, the electron-spin based architecture has recently attracted great interests. Endohedral fullerenes with unpaired electrons, such as N@C60, are promising candidates to embody the qubits because of their long spin decoherence time. This thesis addresses several fundamental aspects of the strategy of engineering the N@C60 molecules for applications in QIP.Chemical functionalization of N@C60 is investigated and several different derivatives of N@C60 are synthesized. These N@C60 derivatives exhibit different stability when they are exposed to ambient light in a degassed solution. The cyclopropane derivative of N@C60 shows comparable stability to pristine N@C60, whereas the pyrrolidine derivatives demonstrate much lower stability. To elucidate the effect of the functional groups on the stability, an escape mechanism of the encapsulated nitrogen atom is proposed based on DFT calculations. The escape of nitrogen is facilitated by a 6-membered ring formed in the decomposition of the pyrrolidine derivatives of N@C60. In contrast, the 4-membered ring formed in the cyclopropane derivative of N@C60 prohibits such an escape through the addends.Two N@C60-porphyrin dyads are synthesized. The dyad with free base porphyrin exhibits typical zero-field splitting (ZFS) features due to functionalization in the solid-state electron spin resonance (ESR) spectrum. However, the nitrogen ESR signal in the second dyad of N@C60 and copper porphyrin is completely suppressed at a wide range of sample concentrations. The dipolar coupling between the copper spin and the nitrogen spins is calculated to be 27.0 MHz. To prove the presence of the encapsulated nitrogen atom in the second dyad, demetallation of the copper porphyrin moiety is carried out. The recovery of approximately 82% of the signal intensity confirms that the dipolar coupling suppresses the ESR signal of N@C60.To prepare ordered structure of N@C60, the nematic matrix MBBA is employed to align the pyrrolidine derivatives of N@C60. Orientations of these derivatives are investigated through simulation of their ESR spectra. The derivatives with a –CH3 or phenyl group derived straightforward from the N-substituent of the pyrrolidine ring are preferentially oriented based on their powder-like ESR spectra in the MBBA matrix. An angle of about is also found between the directors of fullerene derivatives and MBBA. In contrast, the derivatives with a –CH₂ group inserted between the phenyl group and the pyrrolidine ring are nearly randomly distributed in MBBA. These results illustrate the applicability of liquid crystal as a matrix to align N@C60 derivatives for QIP applications.

Nanomaterials have attracted great interest due to the unique physical properties and great potential in the applications of nanoscale devices. Two dimensional atomic crystals, which are atomic thickness, especially graphene, have triggered the gold rush recently due to the fascinating high mobility at room temperature for future electronics. The crystal structure of nanomaterials will have great influence on their physical properties. Thus, this thesis is focused on developing the methods to control the crystal structure of nanomaterials, namely quantum dots as semiconductor, boron nitride (BN) as insulator, graphene as semimetal, with low cost for their applications in photonics, structural support and electronics. In this thesis, firstly, Mn doped ZnSe quantum dots have been synthesized using colloidal synthesis. The shape control of Mn doped ZnSe quantum dots has been achieved from branched to spherical by switching the injection temperature from kinetics to thermodynamics region. Injection rates have been found to have effect on controlling the crystal phase from zinc blende to wurtzite. The structural-property relationship has been investigated. It is found that the spherical wurtzite Mn doped ZnSe quantum dots have the highest quantum yield comparing with other shape or crystal phase of the dots. Then, the Mn doped ZnSe quantum dots were deposited onto the BN sheets, which were micron-sized and fabricated by chemical exfoliation, for high resolution imaging. It is the first demonstration of utilizing ultrathin carbon free 2D atomic crystal as support for high resolution imaging. Phase contrast images reveal moiré interference patterns between nanocrystals and BN substrate that are used to determine the relative orientation of the nanocrystals with respect to the BN sheets and interference lattice planes using a newly developed equation method. Double diffraction is observed and has been analyzed using a vector method. As only a few microns sized 2D atomic crystal, like BN, can be fabricated by the chemical exfoliation. Chemical vapour deposition (CVD) is as used as an alternative to fabricate large area graphene. The mechanism and growth dynamics of graphene domains have been investigated using Cu catalyzed atmospheric pressure CVD. Rectangular few layer graphene domains were synthesized for the first time. It only grows on the Cu grains with (111) orientation due to the interplay between atomic structure of Cu lattice and graphene domains. Hexagonal graphene domains can form on nearly all non-(111) Cu surfaces. The few layer hexagonal single crystal graphene domains were aligned in their crystallographic orientation over millimetre scale. In order to improve the alignment and reduce the layer of graphene domains, a novel method is invented to perform the CVD reaction above the melting point of copper (1090 ºC) and using molybdenum or tungsten to prevent the balling of the copper from dewetting. By controlling the amount of hydrogen during the growth, individual single crystal domains of monolayer over 200 µm are produced determined by electron diffraction mapping. Raman mapping shows the monolayer nature of graphene grown by this method. This graphene exhibits a linear dispersion relationship and no sign of doping. The large scale alignment of monolayer hexagonal graphene domains with epitaxial relationship on Cu is the key to get wafer-sized single crystal monolayer graphene films. This paves the way for industry scale production of 2D single crystal graphene.

Information processors that harness quantum mechanics may be able to outperform their classical counterparts at certain tasks. Quantum information processing (QIP) can utilize the quantum mechanical phenomenon of entanglement to implement quantum algorithms. Endohedral fullerenes, where atoms, ions or clusters are trapped in a carbon cage, are a class of nanomaterials that show great promise as the basis for a solid-state QIP architecture. Some endohedral fullerenes are spin–active, and offer the potential to encode information in their spin-states. This thesis addresses the challenges of how to engineer the components of a scalable QIP architecture based on endohedral fullerenes. It focuses on the synthesis and characterization of molecules which may, in the future, permit the demonstration of entanglement; the optical read-out of quantum states; and the creation of quasi-one-dimensional molecular arrays. Due to its long spin decoherence time, N@C60 is the selected as the basic molecular unit for ‘coupled’ fullerene pairs, molecular systems for which it may be possible to demonstrate entanglement. To this end, isolated fullerene pairs, in the form of spin-bearing fullerene dimers, are created. This begins with the processing of N@C60 at the macroscale and leads towards the synthesis of 15N@C60-15N@C60 dimers at the microscale. High throughput processing is introduced as the most efficient technique to obtain high purity N@C60 on a reasonable timescale. A scheme to produce symmetric and asymmetric fullerene dimers is also demonstrated. EPR spectroscopy of the dimers in the solid-state confirms derivatization, whilst permitting the modelling of spin–spin interactions for 'coupled' fullerene pairs. This suggests that the optimum inter–spin separation for which to observe spin–spin coupling in powders is circa 3 nm. Motivated by the properties of the trivalent erbium ion for the optical detection of quantum states, optically–active erbium–doped fullerenes are also investigated. These erbium metallofullerenes are synthesized and isolated as individual isomers. They are characterized by low temperature photoluminescence spectroscopy, emitting in the infra- red at a wavelength of 1.5 μm. The luminescence is markedly different where a C2 cluster is trapped alongside the erbium ions in the fullerene cage. Er2C2@C82 (isomer I) exhibits emission linewidths that are comparable to those observed for Er3+ in crystals. Finally, the discovery of a novel praseodymium-doped fullerene is reported. The balance of evidence favours the structure being assigned as Pr2@C72. This novel endohedral fullerene forms quasi-one-dimensional arrays in carbon nanotubes, which is a useful proof-of-principle of how a scaled fullerene-based architecture may be achieved.

Quantum information processing (QIP) has the potential to reduce the complexity of many classically ‘hard’ computational problems. To implement quantum information algorithms, a suitable physical quantum computer architecture must be identified. One approach is to store quantum information in the electron spins of an array of paramagnetic N@C60 endohedral fullerene molecules, using the electron-electron dipolar interaction to permit the formation of the entangled quantum states needed to implement QIP. This thesis explores two different chemical methods to create two-spin centre arrays that contain N@C60. The first method uses a double 2,3 dipolar cycloaddition reaction to a dibenzaldehyde-terminated oligo-p-phenylene polyethynylene (OPE) unit , to create an (S3/2, S3/2) N@C60-N@C60 dimer with a fixed spin centre separation of 2.7 nm. The second approach is via a self-assembly scheme in which a Lewis base functionalised N@C60 molecule coordinates to an antiferromagnetic metallic ring magnet to form a (S3/2, S3/2) two-spin centre N@C60-Cr7Ni system with an inter-spin separation of 1.4 nm. In both systems, a significant perturbation of the electron spin transition energies is observed using CW ESR, this perturbation is shown to be well accounted for by the inclusion of an electron-electron dipolar coupling term in the electron spin Hamiltonians. To create entanglement in an ensemble of two-spin centre molecules, the dipolar coupling interaction must lie within a narrow distribution. To achieve this not only the separation but also the orientation of the inter-spin axis with respect to the applied magnetic field must be controlled for which a method of macroscopic alignment is required. The potential of using a uniaxially drawn liquid crystal elastomer to exert uniaxial order on fullerene dimers is tested, finding that the degree of alignment is insufficient, possibly a result of the propensity for the fullerene molecules to phase separate from the elastomer. This phase separation is shown to restrict N@C60 phase coherence lifetime to 1.4 µs at 40 K due to instantaneous spin diffusion. The electron spin environment of both N@C60 and an N@C60-C60 dimer in a polymer matrix is examined using polystyrene as the host matrix. By deuteration of the polystyrene matrix, a maximum phase coherence lifetimes of 48 µs and 21 µs are measured for the N@C60 and N@C60-C60 dimer, respectively. The concept of reading out the electron spin state of N@C60 molecules by coupling it to a spin system that can be probed using optically detected magnetic resonance (ODMR) such as an NV- centre has been previously suggested. To this end, the photostability of N@C60 under 637 nm laser illumination has been examined in solution. The effect of the presence of an atmospheric concentration of oxygen is striking, affording a 57-fold retardation in the photodecomposition of N@C60 compared to a degassed solution. When ambient oxygen is present, the average number of excitations that are required to cause decomposition is ≈60000. Finally, for future UV photophysics experiments involving N@C60, the best solvent to use was found to be decalin, finding that it significantly slowed decomposition of N@C60 in both ambient and degassed solutions. The conclusions of this work make a significant contribution to the field of QIP with N@C60, showing that there is a bright future for N@C60.