We design a facile approach to investigate the role benzene derivatives play in the capacitance enhancement of graphene-based supercapacitors. The main reason is attributed to the pseudocapacitance of the aromatic molecules rather than the former one. Meanwhile, we find that the para and ortho substituted benzene derivatives contribute much more than the meta substituted ones. In addition, we fabricate an all-solid-state flexible MSC based on metal-reduced GO. The as-fabricated MSC shows high areal capacitance and excellent reliability, which makes it a promising energy storage candidate for wearable electronics. Based on the work of MSC, we achieve a flexible ac line-filter that is not only competitive against commercial product but also suitable for mass production. Meanwhile, we produce a three-dimensional graphene/polydimethylsiloxane composite that gives a thermal resistance as small as 14 mm2K/W, which is comparable to commercial products. What’s more, a convenient transient program that saves much time is developed to measure the thermal resistance.

M. Tech. Polymer Technology. / The gas permeability of composite epoxy resins formulated with graphene platelets and glass flakes was investigated. The purpose of researching the gas permeability of the composite resins was to develop a possible coating system that could prevent or limit the release of radioactive gases like tritium from irradiated graphite waste which may accumulate in underground repositories. Helium was used as a substitute gas to simulate the diffusive properties of tritium gas.

This thesis describes electronic transport experiments in graphene from the hopping to the ballistic regime. The first experiment studies dual-gated bilayer graphene devices. By applying an electric field with these dual gates, we can open a band gap in bilayer graphene and observe an increase in resistance of over six orders of magnitude as well as a strongly non-linear behavior in the transport characteristics. A temperature-dependence study of resistance at large electric field at the charge neutrality point shows the change in the transport mechanism from a hopping dominated regime at low temperature to a diffusive regime at high temperature. / Physics

Plasmonic waves in solid-state are caused by collective oscillation of mobile charges inside or at the surface of conductors. In particular, surface plasmonic waves propagating at the skin of metals have recently attracted interest, as they reduce the wavelength of electromagnetic waves coupled to them by up to ~10 times, allowing one to create miniaturized wave devices at optical frequencies. In contrast, plasmonic waves on two-dimensional (2D) conductors appear at much lower infrared and THz-GHz frequencies, near or in the electronics regime, and can achieve far stronger wavelength reduction factor reaching well above 100. In this thesis, we study the unique machinery of 2D plasmonic waves behind this ultra-subwavelength confinement and explore how it can be used to create various interesting devices. To this end, we first develop a physically intuitive theoretical formulation of 2D plasmonic waves, whose two main components---the Coulomb restoration force and inertia of the collectively oscillating charges---are combined into a transmission-line-like model. We then use this formulation to create various ultra-subwavelength 2D plasmonic devices. For the 2D conductor, we first choose GaAs/AlGaAs heterostructure---a 2D electron gas consisting of massive (m*>0) electrons---demonstrating plasmonic bandgap crystals, interferometers, and negatively refracting metamaterials. We then examine a 2D plasmonic device based on graphene, a 2D electron gas consisting of effectively massless (m*=0) electrons. We theoretically show and experimentally demonstrate that the massless electrons in graphene can surprisingly exhibit a collective mass when subjected to a collective excitation, providing the inertia that is essential for the propagation of 2D plasmonic waves. Lastly, we theoretically investigate the thermal current fluctuation behaviors in massive and massless electron gases. While seemingly unrelated on first sight, we show that the thermal current fluctuation is actually intimately linked to the collective mass of the massive or massless electron gas. Thus, we show that the thermal current fluctuation behaviors can also be described by the same theoretical framework introduced earlier, suggesting a possibility to design new concept devices and experiments based on this linkage. / Engineering and Applied Sciences

As silicon-based transistor technology continues to scale ever downward, anticipation of the fundamental limitations of ultimately-scaled devices has driven research into alternative device technologies as well as new materials for interconnects and packaging. Additionally, as power dissipation becomes an increasingly important challenge in highly miniaturized devices, both the implementation and verification of high mobility, high thermal conductivity materials, such as low dimensional carbon nanomaterials, and the experimental investigation of heat transfer in the nanoscale regime are requisite to continued progress. This work furthers the current understanding of structure-property relationships in low dimensional carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene, through use of combined thermal conductance and transmission electron microscopy (TEM) measurements on the same individual nanomaterials suspended between two micro-resistance thermometers. Through the development of a method to measure thermal contact resistance, the intrinsic thermal conductivity, [kappa], of multi-walled (MW) CNTs is found to correlate with TEM observed defect density, linking phonon-defect scattering to the low [kappa] in these chemical vapor deposition (CVD) synthesized nanomaterials. For single- (S) and double- (D) walled (W) CNTs, the [kappa] is found to be limited by thermal contact resistance for the as-grown samples but still four times higher than that for bulk Si. Additionally, through the use of a combined thermal transport-TEM study, the [kappa] of bi-layer graphene is correlated with both crystal structure and surface conditions. Theoretical modeling of the [kappa] temperature dependence allows for the determination that phonon scattering mechanisms in suspended bi-layer graphene with a thin polymeric coating are similar to those for the case of graphene supported on SiO₂. Furthermore, a method is developed to investigate heat transfer through a nanoscale point contact formed between a sharp silicon tip and a silicon substrate in an ultra high vacuum (UHV) atomic force microscope (AFM). A contact mechanics model of the interface, combined with a heat transport model considering solid-solid conduction and near-field thermal radiation leads to the conclusion that the thermal resistance of the nanoscale point contact is dominated by solid-solid conduction. / text

Η ανακάλυψη των νανοσωλήνων άνθρακα το 1991 από τον Iijima και οι εκπληκτικές ιδιότητες που βρέθηκε ότι παρουσιάζουν προκάλεσε τεράστιο ενδιαφέρον στην επιστημονική κοινότητα και πολλές ερευνητικές ομάδες ανά τον κόσμο ξεκίνησαν την προσπάθεια να εκμεταλλευτούν τις ιδιότητες αυτές στην ανάπτυξη νέων συνθέτων υλικών και εφαρμογών. Από τις σημαντικότερες εφαρμογές που εξαρχής οι νανοσωλήνες δοκιμάστηκαν, ήταν για την ενίσχυση των μηχανικών, ηλεκτρικών και θερμικών ιδιοτήτων των πολυμερών. Πολύ γρήγορα ωστόσο διαπιστώθηκε ότι η ανάμιξη νανοσωλήνων με πολυμερή είναι εξαιρετικά δύσκολη, με αποτέλεσμα τα νανοσύνθετα υλικά που παράγονταν είτε να έχουν πολύ μικρή ενίσχυση των ιδιοτήτων τους, είτε να παρουσιάζουν ιδιότητες υποδεέστερες από την πολυμερική μήτρα. Το πρόβλημα πηγάζει από την σχεδόν μονοδιάστατη δομή των νανοσωλήνων, με αποτέλεσμα να έχουν την τάση να συσσωματώνονται και έτσι να μην διασπείρονται ομογενώς σε μεγάλες συγκεντρώσεις μέσα στην πολυμερική μήτρα με τις υπάρχουσες τεχνικές μορφοποίησης των πολυμερών. Τα παραγόμενα νανοσύνθετα υλικά είναι χαμηλής περιεκτικότητας σε νανοσωλήνες άνθρακα και παρουσιάζουν φτωχές ιδιότητες καθώς, εντέλει, τα συσσωματώματα δρουν ως ατέλειες, και όχι ως ενισχυτικά μέσα. Η παρούσα εργασία φιλοδοξεί να δώσει μια κατεύθυνση προς τη λύση του παραπάνω προβλήματος, της ανομοιογενούς δηλαδή διασποράς νανοσωλήνων σε πολυμερικές μήτρες και να παράγει ένα νανοσύνθετο υλικό με υψηλή περιεκτικότητα σε νανοσωλήνες και βελτιωμένες ιδιότητες σε σχέση με την πολυμερική μήτρα. Η βασική ιδέα είναι η παρασκευή ξηρών υμενίων νανοσωλήνων άνθρακα (τα λεγόμενα buckypapers), αποτελούμενων δηλαδή από δίκτυα νανοσωλήνων άνθρακα περιπλεγμένων μεταξύ τους και κατόπιν η διαβροχή τους από το πολυμερές. Η μέθοδος αυτή έχει το μεγάλο πλεονέκτημα ότι τα υμένια παρουσιάζουν πολύ υψηλό βαθμό ομοιογένειας, η οποία και διατηρείται και μετά την εισροή του πολυμερούς. Το αποτέλεσμα είναι τα παραγόμενα νανοσύνθετα να είναι ομογενή και υψηλής περιεκτικότητας σε νανοσωλήνες. Πιο συγκεκριμένα, στην παρούσα εργασία παρασκευάστηκαν νανοσύνθετα υλικά εποξειδικής μήτρας με πολυφλοιικούς νανοσωλήνες άνθρακα ως ενισχυτική φάση σε μορφή υμενίων. Η παρασκευή τους έγινε σε τέσσερα στάδια. Αρχικά οι νανοσωλήνες τροποποιήθηκαν επιφανειακά με δύο διαφορετικές χημικές μεθόδους (οξείδωση-εποξείδωση), ώστε να αποφευχθεί η συσσωμάτωσή τους και για να επιτευχθεί καλύτερη χημική συνάφεια με την εποξειδική ρητίνη. Το δεύτερο στάδιο περιελάμβανε την παρασκευή των υμενίων νανοσωλήνων άνθρακα με ή χωρίς οξείδια του γραφενίου, δηλαδή σχηματίστηκαν μακροδομές νανοσωλήνων, οι οποίες και αποτέλεσαν την ενισχυτική φάση για τα νανοσύνθετα υλικά. Στο τρίτο στάδιο παρασκευάστηκαν προεμπτισμένα με ρητίνη υμένια νανοσωλήνων άνθρακα (prepregs), με την εμβάπτιση των υμενίων σε εποξειδική ρητίνη και την κατάψυξή τους, ενώ κατά το τέταρτο στάδιο πραγματοποιήθηκε η παραγωγή των νανοσυνθέτων από τα προεμποτισμένα υμένια με χρήση αυτόκλειστου φούρνου. Στη συνέχεια τα υμένια και τα νανοσύνθετα που παρασκευάστηκαν χαρακτηρίσθηκαν ώστε να μελετηθεί η δομή και οι ιδιότητες των υλικών αυτών. Οι νανοσωλήνες χαρακτηρίσθηκαν θερμικά με θερμοσταθμική ανάλυση και δομικά με φωτοηλεκτρονιακή φασματοσκοπία ακτίνων Χ και ηλεκτρονική μικροσκοπία. Η δομή των υμενίων μελετήθηκε με ποροσιμετρία υδραργύρου και ηλεκτρονική μικροσκοπία. Μηχανικά, τα υμένια δοκιμάστηκαν με εφελκυσμό και μελετήθηκε η μικρομηχανική τους με φασματοσκοπία Raman. Τα νανοσύνθετα που παρασκευάστηκαν μελετήθηκαν δομικά με ηλεκτρονική μικροσκοπία σάρωσης και προσδιορίστηκαν οι μηχανικές, ηλεκτρικές και θερμικές τους ιδιότητες. Η μελέτη των πειραματικών αποτελεσμάτων έδειξε ότι η χημική τροποποίηση της επιφάνειας των νανοσωλήνων επηρεάζει τις ιδιότητες των υμενίων όπως το πορώδες, το οποίο με τη σειρά του επηρεάζει τις ιδιότητες των παραγόμενων νανοσυνθέτων. Υπάρχει δηλαδή άμεση συσχέτιση δομής-ιδιοτήτων μεταξύ των νανοσωλήνων, των υμενίων και των νανοσυνθέτων. Τα παραγόμενα νανοσύνθετα υλικά εμφανίζουν ενισχυμένες μηχανικές, ηλεκτρικές και θερμικές ιδιότητες οι οποίες μπορούν να φανούν χρήσιμες για την ανάπτυξη νέων εφαρμογών. / The discovery of carbon nanotubes in 1991 by Iijima and their explicit properties has attracted for many years the scientific interest and many research groups around the world have started an effort to take advantage of these properties for the development of new materials and applications. Carbon nanotubes have been used mostly for the reinforcement of the mechanical, electrical and thermal properties of polymers, but very soon it was found that the dispersion of these nanomaterials into polymers is very difficult and the produced nanocomposites presented negligible reinforcement of their properties, or even inferior than the properties of polymeric matrices. The problem arises from the quasi-one dimensional structure of the nanotubes, leading them to form bundles and preventing them to be dispersed homogenous and in large amounts by the current polymers processing. The aim of the current thesis is to solve the problem of the inhomogeneous dispersion of nanotubes into polymers and to fabricate a high volume fraction nanocomposite material of carbon nanotubes with improved properties compared to the matrix. The basic idea is the formation of thin sheets of randomly entangled nanotubes, the so-called buckypapers, and the impregnation of them by the matrix. The main advantage of this method is that buckypapers are homogenous materials and this homogeneity remains after the infusion of the polymer. More specifically, at this work polymeric nanocomposite materials with carbon nanotubes as reinforcement at the form of buckypapers were produced. The production involved four steps. Firstly, the surface of the nanotubes was modified by two different chemical processed (oxidation-epoxidation) to prevent the formation of bundles and to achieve a better chemical compatibility with the matrix. At a second stage buckypapers were prepared to consist the reinforcement material for nanocomposites. The third step was the soaking of buckypapers into an epoxy resin and the refrigeration to form prepregs and on the final step prepregs were place on an autoclave oven for the curing and the production of the nanocomposites. The nanotubes and the produced buckypapers and nanocomposites characterized to study their structure and properties. Nanotubes characterized by thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Buckypapers structure was studied by porosity measurements and SEM, and their mechanical properties by tensile experiments and Raman spectroscopy. Nanocomposites were studied by SEM and their mechanical, electrical and thermal properties were determined. The study of the experimental results showed that the chemical modification of the nanotubes affects the properties of the buckypapers such as the porosity, which also influences the properties of the subsequent nanocomposites. So there is a strong structure-properties correlation between nanotubes, buckypapers and nanocomposites. The produced nanocomposites presented improved mechanical, electrical and thermal properties that may be useful for the development of new applications.

In this dissertation we discuss the electronic properties, structural configurations, and reaction mechanisms of chemically functionalized graphene and charged atomic metals. In general, we analyze fundamental atomic scale and nanoscale systems with density functional theory in order to investigate chemical reaction energetics for peroxide synthesis as well as methanol production without carbon emission. These systems were found to be tunable via the addition of cationic and anionic charges, change in transition metal type, and modification through chemical functionalization. Furthermore, transition state theory was used to predict an optimal configuration for chemically functionalized graphene, efficient use of anionic atomic gold and palladium for synthesis of water to peroxide, and clean conversion of methane to methanol without carbon dioxide emission utilizing anionic gold.

Graphene is widely considered to be a promising candidate for a new generation of electronics, but there are many outstanding fundamental issues that need to be addressed before this promise can be realized. This thesis focuses on the production and properties of graphene grown epitaxially on the carbon terminated face (C-face) of hexagonal silicon carbide leading to the construction of a novel graphene transistor structure. C-face epitaxial graphene multilayers are unique due to their rotational stacking that causes the individual layers to be electronically decoupled from each other. Well-formed C-face epitaxial graphene single layers have exceptionally high mobilities (exceeding 10,000 cm^2/Vs), which are significantly greater than those of Si-face graphene monolayers. This thesis investigates the growth and properties of C-face single layer graphene. A field effect transistor based on single layer graphene was fabricated and characterized for the first time. Aluminum oxide or boron nitride was used for the gate dielectric. Additionally, an all graphene/SiC Schottky barrier transistor on the C-face of SiC composed of 2DEG in SiC/Si2O3 interface and multilayer graphene contacts was demonstrated. A multiple growth scheme was adopted to achieve this unique structure.

This thesis is devoted to the calculation of thermal contact resistance in various molecular systems based on carbon nanotubes (CNTs) and few layer graphene (FLG). This work has been performed through equilibrium molecular dynamics (EMD) simulations. We adopted the temperature difference fluctuations method in our EMD calculations. This method only needs the input of the temperatures of the subsystems whereas the heat flux, which is involved in all the other approaches, remains more difficult to compute in terms of simulation time and algorithm. Firstly, three cases were studied to validate this method, namely: (i) Si/Ge superlattices; (ii) diameter modulated SiC nanowires; and (iii) few-layer graphenes. The validity of the temperature difference fluctuations method is proved by equilibrium and non-equilibrium MD simulations. Then, by using this method, we show that an azide-functionalized polymer (HLK5) has a lower contact resistance with CNT than the one between CNT and PEMA, because HLK5 could form covalent bonds (C-N bonds) with CNT through its tail group azide, while only weak Van der Waals interactions exist in the case of CNT-PEMA contact. The data from our EMD simulations match with the results from experiments in a reasonable range. We then report the thermal contact resistance between FLG and a SiO2 substrate, which could be tuned with the layer number. Taking advantage of the resistive interface, we show that a SiO2 /FLG superlattices have a thermal conductivity as low as 0.30 W/mK, exhibiting a promising prospect in nano-scale thermal insulation. In the last part, we investigated the layer number dependence of the cross-plane thermal resistances of suspended and supported FLGs. We show that the existence of a silicon dioxide substrate can significantly decrease the cross-plane resistances of FLGs with low layer numbers, and the effective thermal conductivities were increased accordingly. The Frenkel-Kontorova model was introduced to explain the substrate-induced band gaps in FLG dispersion relations and the corresponding thermal energy transfer. The enhanced thermal conduction in the cross-plane direction is ascribed to the phonon radiation that occurs at the FLG-substrate interface, which re-distributes the FLG in-plane propagating energy to the cross-plane direction and to the substrate.

This thesis reports results from the investigation of optically-induced carrier dynamics in graphite and graphitic carbon nanostructures. In this first set of experiments, the dynamics of photo-excited carriers in exfoliated graphene and thin graphitic films are studied by optical pump-probe spectroscopy. Samples ranging in thickness from 1 to 260 carbon layers are deposited onto an oxidized silicon substrate. Time-resolved reflectivity and transmissivity are measured at 1300 nm, following excitation by 150 fs, 800 nm pump pulses at room temperature. Two time scales are identified over which the extracted transient dielectric function returns to its quiescent value. A fast decay time of ~200 fs in graphene is associated with hot phonon emission and increases to ~300 fs for thicknesses greater than only a few carbon layers. The slow decay time, associated with hot phonon interaction and/or carrier recombination, increases more gradually, from ~2.5 to 5 ps over ~30 layers. A simple model suggests the thickness dependence of the slow decay time is likely a result of thermal diffusion into the substrate. In the second set of experiments, coherently-controlled two-colour injection photocurrents are generated via quantum interference of single- and two-photon absorption in bulk graphite and a variety of single-walled carbon nanotube samples, such as a CVD-grown aligned forest of nanotubes (tube diameter dt = 2.5 ± 1.5 nm), and both arc discharge (dt = 1.44 ± 0.15 nm) and HiPco (dt = 0.96 ± 0.14 nm) nanotube films separated by electronic type (metallic vs. semiconducting). At pump wavelengths of 1500 and 750 nm, the emitted terahertz radiation is used to estimate a peak current density of ~12 kA/cm² in graphite and a peak current of ~8 nA per nanotube. From the dependence of the injected current on pump polarization, the relative values of the current injection tensor elements are measured, and information is gained on the alignment and birefringence of the nanotube samples. The dependence of the injected current on pump wavelength implies that the currents are likely based on band-band electronic transitions and not on excitonic effects, which govern most linear optical processes.