No description available.

No description available.

The interactions between fullerene cages and Si {100) surfaces are studied using density functional theory. This has previously been studied and the molecules are found to bind in four different locations upon the dimerised Si (100) surface. These are above the dimer row bound to one or two dimers (rl and r2 respectively) and within the dimer trench bound to two or four dimers (t2 and t4 respectively). Here we focus on the r2 and t4 configurations as these were found to have stronger binding energies due to the four bonds forming between cage and surface rather than two. The rl and t2 sites are actually metastable and minor displacements of the cages result in them falling in to one of the energy minima of the t4 and r2 sites. All of the configurations discovered by Godwin et al. via PLATO, are verified as are three of the additional configurations from the study by Hobbs et al. which used the SIESTA software package. A more complete basis set is employed here to ascertain the effect it has upon the basis set superposition error. It is found to reduce it to negligible levels.

In this thesis, the design and synthesis of novel C60 fullerene compounds with different functional groups is reported. Chapter 1 introduces the background of the project with a general introduction into the covalent and non-covalent types of bonding interactions, and the chemistry and reactions of C60 fullerenes. Chapter 2 focuses on co-crystallisation of pristine C60 with selected aromatic compounds. Single crystals have been obtained and characterised by X-ray diffraction. Chapter 3 is concerned with the synthesis and characterisation of novel fullerene compounds by using Prato cycloaddition reactions, such novel compounds are further characterised in the solid state by single crystal X-ray diffraction. Hirshfeld surfaces are used to investigate the non-covalent interaction of the obtained crystal structures. Chapter 4 describes the synthesis and characterisation of novel fullerene compounds by using Bingel cycloaddition reactions. Crystallisation experiments have been attempted and theoretical studies using the semi-empirical method, parameterized model number 3, have been employed to investigate the charge transfer between the ligand and the C60 molecule. The novel molecules in this work have been characterised by infrared and NMR spectroscopies, and mass spectrometry. Single crystal X-ray diffraction was used when appropriate.

No description available.

The studies in this thesis give deep insights on the large scale preparation of graphene and the fabrication and properties of novel monolayered quantum dots (QDs). Graphene has received remarkable attention due to its interesting physical and chemical properties. Among various preparations for graphene, the solvothermal deoxidation of graphene oxide (GO) is highly attractive as it potentially offers a relatively economical and scalable manufacturing route for use in industrial applications. Unfortunately, the deep deoxidation of GO and highly dispersable reduced GO (rGO) are difficult to achieve using this approach, although the reasons for this deoxidation remain unclear. This thesis shows that the agglomeration/self-assembly of partially reduced GO (p-rGO) sheets in the solvothermal deoxidation reaction suppresses the deep deoxidation of GO and led to low dispersibility/electrical conductivity of the product. By tuning the surface energy of the solvent to minimize the surface enthalpy of the dispersion, these technical problems can be ameliorated and full deoxidation of GO with high dispersibility and electrical conductivity achieved. In this thesis, an alternative novel and effective route to fabricating graphene QDs (GQDs, lateral size ~ 20 nm) is also described. This technique of delaminating layered structures has also been developed to produce monolayered QDs of boron nitride (BN, lateral size of ~ 10 nm), tungsten disulfide (WS2, lateral size ~ 8-15 nm) and molybdenum disulfide (MoS2, lateral size of ~ 8-20 nm). This has opened up many opportunities in studying these interesting materials with reduced dimensions, with new behaviours and properties emerging from the various QDs. The zigzag edges of GQDs led to the appearance of new band gaps and give strong blue-green luminescence centred at 420 nm wavelength (quantum yield of ~7.6%). In monolayered BN QDs, carbene-replaced zigzag edges, carbon-replaced N vacancy point and BOx- (x = 1 and 2) species added new luminescence at around 425 nm wavelength (quantum yield of ~2.5%). Strong luminescence was created by the reduced dimensions of WS2 and MoS2 monolayered QDs causing them to became direct semiconductors. The reduced lateral dimensions also caused marked quantum confinement effects to arise, such as large blue shifts in absorption features of BN, WS2 and MoS2 monolayered QDs. The formation of monolayered WS2 and MoS2 QDs also led to their valance bands being split by giant spin-orbit coupling effects to a far greater degree than is observed form monolayered sheets. The studies suggest strongly that these features are likely to be tunable with lateral dimensions, which makes the QDs potentially very interesting for applications. Although these uses may include spintronics, optoelectronics and even quantum computing, their application in biology is demonstrated by all the monolayered QDs being used as non-toxic fluorescent labels in confocal microscopy of biological cells.

Work presented here has been centered around the growth of epitaxial graphene via the thermal decomposition of 4H silicon carbide wafers. Improvements to ultra high vacuum growth procedures used within the research group have been made via the optimization of annealing times and temperatures. The optimization involved the use of surface science techniques such as low energy electron diffraction, atomic force microscopy, low energy electron microscopy and Raman spectroscopy amongst others to monitor changes in surface reconstructions, lateral grain sizes of graphene domains and graphene coverage on the surface as the growth parameters were varied. Improvements observed via the surface science techniques such as increasing the lateral domain grain sizes from 10s nm to 100s nm and increasing the graphene film coverage were linked to the betterment of the electronic properties of the graphene films (electronic measurements carried out by Graham Creeth), this linking lead to published work. The mechanical properties of these films were also measured via the use of Raman spectroscopy to probe the formation of strains within the graphene and compare growth carried out on the silicon carbide (0001) face to literature work carried out on the (0001) face to show evidence of graphene-substrate decoupling within the films grown here, this work also lead to a publication. Alternate growth procedures have also been investigated. This involved carrying out annealing processes in inert argon gas atmospheres. Atomically terraced substrates were produced via annealing in argon gas atmospheres at temperatures of ~1500°C. These terraced substrates where then subsequently graphitised by increasing the annealing temperature to ~1600°C allowing for a single stage substrate preparation and graphitisation process. A result not published elsewhere. A Nanoprobe system has been used to manipulate the graphene films grown under argon atmosphere and make 4-probe electrical transport measurements allowing sheet resistance measurements to be made.

No description available.

In this thesis the nonlinear and nonequilibrium properties of graphene are experimentally investigated using degenerate four--wave mixing and time--resolved pump--probe spectroscopy. High quality exfoliated natural graphite and large area epitaxial graphene on silicon carbide are investigated with femtosecond and picosecond ultrafast pulses in the near--infrared. A bespoke technique for suspending exfoliated graphene is also presented. In Chapter 3, the third--order nonlinear susceptibility of graphene is measured for the first time and shows a remarkably large response. Degenerate four--wave mixing at near--infrared wavelengths demonstrates an almost dispersionless emission over a broad spectral range. Quantum kinetic theory is employed to estimate the magnitude of the response and is in good agreement with the experimental data. The large susceptibility enables high contrast imaging, with a monolayer flake contrast of the order 10^{7} times higher than for standard reflection imaging. The degenerate four--wave mixing technique is utilised in Chapter 4 to measure the interface carbon signal of epitaxially grown graphene on silicon carbide. Comparable third--order signal from the silicon carbide bulk prevents true interface imaging. Excluding the third--order emission from detection by elongating the emission to outside a band--pass filter range allows for pure interfacial luminescence imaging. Features within the two growth faces are investigated with Raman spectroscopy. Nonlinear measurements are an increasingly popular tool for investigating fundamental properties of graphene. Chapter 5 investigates the influence of ultrafast pulses on the nonlinear response of graphene. High instantaneous intensities at the sample are shown to reduce the nonlinear emission by a factor or two. Comparing the Raman peak positions, widths and intensities before and after irradiation points to a huge doping of the samples, of the order 500 meV. In Chapter 6 the relaxation of photoexcited carriers is measured via time--resolved pump--probe spectroscopy, where a layer dependence of hot phonon decay is observed. Single layer flakes are observed to relax faster than bilayers and trilayers, with an asymptote reached at approximately four layers. Removing the substrate and measuring fully suspended samples reveals the same trend, suggesting that substrate interactions are not the cause of the enhanced decay. The decay mechanism is therefore intrinsic to graphene, perhaps due to coupling to out--of--plane, flexural phonons. The thickness dependence of epitaxial graphene on silicon carbide is compared to that of exfoliated flakes where the layer dependence is not observed. Phonon relaxation times, however, are in good agreement. Predictions for future investigations into this novel material based on the works here are suggested in Chapter 7. Preliminary pump--probe measurements at high carrier concentrations are an example of such progress, which will offer an insight into further decay mechanisms in graphene.

Οι εξαιρετικά καλές μηχανικές ιδιότητες που εμφανίζουν οι νανοσωλήνες άνθρακα τους καθιστούν ιδανικό ενισχυτικό μέσο για πολυμερικές μήτρες με σκοπό την παραγωγή σύνθετων πολυμερικών υλικών υψηλής αντοχής. Για την παρασκευή όμως τέτοιων υλικών είναι αναγκαία η τροποποίηση της επιφάνειας των νανοσωλήνων με σκοπό την δημιουργία «ενεργούς» διεπιφάνειας αλληλεπίδρασης μεταξύ αυτών και της πολυμερικής μήτρας. Σε αυτό το πλαίσιο, στο πρώτο μέρος της παρούσας εργασίας, έγινε συστηματική μελέτη της χημικής και ηλεκτροχημικής τροποποίησης της επιφάνειας νανοσωλήνων άνθρακα, έτσι ώστε να βρεθεί το καλύτερο οξειδωτικό μέσο και οι καταλληλότερες συνθήκες οξείδωσης για να επιτευχθεί η ενσωμάτωσή τους σε πολυμερική μήτρα. Για την μελέτη αυτή χρησιμοποιήθηκαν η Φασματοσκοπία Φωτοηλεκτρονίων από ακτίνες-Χ και Υπεριώδες (XPS, UPS), η Φασματοσκοπία Raman και η θερμοσταθμική ανάλυση TGA. Αρχικά τροποποιήθηκαν νανοσωλήνες άνθρακα πολλαπλού τοιχίου σε μορφή υμενίου (φιλμ) με την τεχνική της ηλεκτροχημικής οξείδωσης χρησιμοποιώντας ως ηλεκτρολύτη διάλυμα χλωριούχου νατρίου (ΝaCl). Οι νανοσωλήνες είχαν τοποθετηθεί στη θέση της ανόδου σε σταθερή βάση στήριξης. Τα αποτελέσματα από την φασματοσκοπία XPS έδειξαν ότι αν και επιτυγχάνεται οξείδωση, υπάρχει δυσαναλογία ως προς τα ποσοστά των λειτουργικών ομάδων που επάγωνται στην επιφάνεια του υμενίου σε σχέση με την ένταση της οξειδωτικής διαδικασίας. Στη συνέχεια, μελετώντας υμένια που οξειδώθηκαν σε σταθερή βάση χρησιμοποιώντας HNO3 ως ηλεκτρολύτη, παρατηρήθηκε ότι δεν επιτυγχάνεται ομοιόμορφη ηλεκτροχημική οξείδωση και στις δύο πλευρές των υμενίων. Έτσι σχεδιάστηκε περιστροφικό σύστημα για την άνοδο (περιστροφική βάση), η οποία και χρησιμοποιήθηκε περαιτέρω. Έχοντας κάνει αυτή την αλλαγή στο ηλεκτροχημικό κελί έγιναν μελέτες όπου χρησιμοποιήθηκαν ως ηλεκτρολύτες διαλύματα νιτρικού οξέος (ΗΝΟ3), υδροχλωρικού οξέος (ΗCl) και καυστικής ποτάσας (ΚΟΗ). Βρέθηκε ότι το νιτρικό οξύ οδηγεί (υπό κατάλληλες συνθήκες) σε ικανοποιητική αλλά και ελεγχόμενη τροποποίηση της επιφάνειας. Το υδροχλωρικό οξύ, όπως αναμενόταν, λειτούργησε ως ένα αποτελεσματικό οξειδωτικό μέσο για τον καθαρισμό των νανοσωλήνων από επικαθίσεις άμορφου άνθρακα, ενώ η καυστική ποτάσα έδειξε ότι είναι ισχυρό οξειδωτικό μέσο δημιουργώντας λειτουργικές ομάδες στην επιφάνεια του φιλμ νανοσωλήνων άνθρακα σε λιγότερο χρόνο οξείδωσης. Στην συνέχεια μελετήθηκε η απλή χημική οξείδωση σκόνης νανοσωλήνων άνθρακα χρησιμοποιώντας ως οξειδωτικά μέσα μείγμα θεϊκού οξέος (Η2SO4) και υπεροξειδίου του υδρογόνου (Η2Ο2) γνωστό και ως piranha, διάλυμα υδροχλωρικού οξέος και νιτρικού οξέος αλλά και μείγμα υδροξείδιου του αμμωνίου με υπεροξείδιο του υδρογόνου. Βρέθηκε ότι η οξείδωση με piranha δημιουργούσε σχετικά χαμηλό αριθμό δομικών ατελειών στην επιφάνεια των νανοσωλήνων ενώ ταυτόχρονα οδηγούσε σε ικανοποιητικού βαθμού τροποποίηση επάγοντας επιφανειακές λειτουργικές ομάδες. Έτσι η διαδικασία τροποποίησης με piranha ήταν αυτή που επιλέχθηκε για τη προετοιμασία των υμενίων νανοσωλήνων που θα αποτελούσαν το ενισχυτικό υλικό στο σύνθετο πολυμερικό υλικό. Η μελέτη των δειγμάτων σύνθετων πολυμερικών υλικών σε διάφορα ποσοστά εποξειδικής ρητίνης έγινε με XPS. Στα φάσματα που προήλθαν από τα σύνθετα υλικά εντοπίστηκαν χαρακτηριστικά που αποτελούν ενδείξεις ότι η χημική συγγένεια μεταξύ του υμενίου και της πολυμερικής μήτρας είχε επιτευχθεί. Αξιοσημείωτη είναι όμως και η επιφανειακή ανομοιομορφία που παρατηρήθηκε στα συγκεκριμένα υλικά. Στο δεύτερο μέρος της εργασίας μελετήθηκαν δύο υβριδικά μόρια που προήλθαν από προσάρτηση πολυμερικών αλυσίδων στην επιφάνεια νανοσωλήνων άνθρακα μονού τοιχίου. Οι πολυμερικές αλυσίδες προήλθαν από το μονομερές του μεθακρυλικού μεθυλεστέρα (ΜΜΑ) και του ακρυλικού οξέως (ΑΑ). Τα αποτελέσματα τον μετρήσεων με XPS έδειξαν ότι η πολυμερική αλυσίδα και στις δύο περιπτώσεις περιβάλλει την επιφάνεια του νανοσωλήνα στον οποίο έχει προσαρτηθεί. Επιπλέον εντοπίστηκαν φασματοσκοπικά χαρακτηριστικά που αποτελούν ενδείξεις ότι αλυσίδες έχουν προσαρτηθεί χημικά στην επιφάνεια των νανοσωλήνων. Αλλά και η φασματοσκοπία Raman πιστοποίησε ότι η προσάρτηση των πολυμερικών αλυσίδων έχει επιτευχθεί και στις δύο περιπτώσεις. Μέσω της θερμοσταθμικής ανάλυσης TGA φαίνεται μάλιστα το υβριδικό μόριο με προσαρτημένες αλυσίδες PAA να είναι μεγαλύτερου μοριακού βάρους από το αντίστοιχο υβριδικό μόριο με προσαρτημένες αλυσίδες PMMA. / The excellent mechanical properties of carbon nanotubes render them as ideal reinforcing materials for the development of polymer-based composite materials of high mechanical strength. However, for the production of such composites, the surface modification of carbon nanotubes appears to be a necessary step in order to form an “active” interface with the polymeric matrix. In this context, within the first part of this work, a systematic study of the chemical and electrochemical surface modification of carbon nanotubes took place. The scope was to identify the proper oxidation conditions that lead to the maximum number of surface functional groups while, at the same time, keeping the graphitic lattice intact by preventing defect formation. Two surface sensitive techniques, X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoelectron Spectroscopy (UPS) as well as Raman Spectroscopy and Thermogravimetric Analysis (TGA) were employed for this study. Multi-wall carbon nanotubes in the form of buckypapers were initially modified by electrochemical oxidation, using NaCl. In the electrochemical cell, the buckypapers were placed at the anode position on an immovable holder. XPS showed that the surface functionalisation was achieved. Nevertheless, quantitative analysis demonstrated that the quantity of the functional groups was not related to the intensity of the oxidation conditions. Studying both sides of buckypapers, oxidised in the same way by HNO3 clearly proved that the side of the film that “faces” the procedure is more affected. For this reason a new electrochemical cell was designed, where the film holder (anode) was able to rotate during the oxidation procedure. Using this new cell, buckpaper electrochemical oxidation was studied in ΗΝΟ3, ΗCl and ΚΟΗ solutions. It was found that HNO3 (under proper conditions) can lead to successful surface modification in a controllable manner. As expected HCl had a very mild effect, mainly cleaning the film surface from amorphous carbon contamination. Finally, KOH was found to act as a fast oxidising surface agent but in a less controllable way. Multi-wall carbon nanotubes in the form of powder were modified by a wet oxidation method using Η2SO4-Η2Ο2 (piranha), HCl, HNO3 and HNO4-Η2Ο2 solutions. The treatment with piranha solution was found to lead to the formation of a relatively low number of structural defects while, at the same time, it was inducing a moderate number of surface functional groups. Thus, piranha solution was selected to be used for the preparation of the reinforcing material (in the form of a buckypaper) for the composite polymer that was the final product of these efforts. The XPS study of the produced composite polymeric materials was performed on samples prepared with different quantities of epoxy resin (polymeric matrix). The spectra originating from the composite samples had both the spectroscopic features of the nanotubes and the polymer. Although surface inhomogeneity was observed, there was evidence of an active interface between the nanotubes and the matrix. The second and shorter part of this work was dedicated to the study of two hybrid molecules prepared by grafting polymeric chains on the surface of single wall carbon nanotubes. The polymer chains originated from the methacrylic methylester (MMA) and acrylic oxide (AA) monomers. XPS measurements showed that in both cases the polymer chains wrap around the nanotubes. Furthermore, there was spectroscopic evidence that the chains have been chemically attached to the nanotubes. This was also supported by Raman spectroscopy measurements. Thermogravimetric analysis showed that the hybrid molecule with the PAA chains attached was of higher molecular weight compared to that with PMMA chains.