Graphene has been extensively studied for possible future technical applications due to its unique electronic, transport, and mechanical properties. For practical applications, graphene often needs to be placed in a medium or on a substrate. The interfacial interaction between graphene and other materials can greatly affect the performance of graphene-based devices, but has not been well explored. My thesis research focused on developing a better understanding of the interface of pristine and chemically/mechanically modified graphene sheets with ionic liquids (ILs) as well as amorphous silica (a-SiO₂) surfaces using first principles-based atomistic modeling which combines density functional theory, classical molecular dynamics, and Metropolis Monte Carlo. The major focus of my thesis research was on investigating the interfacial structure and capacitance between graphene and ILs; graphene-based materials and ILs have been regarded as viable candidates for supercapacitor electrodes and electrolytes, respectively. Particular emphasis was placed on elucidating the relative contributions of the electric double layer (EDL) capacitance at the graphene/IL interface and the quantum capacitance of graphene-like electrodes. More specifically, we first determined the microstructure (such as orientation, packing density, cation-anion segregation) of chosen ILs near planar graphene electrodes with various surface charge densities. Based on the calculated IL microstructure for each system, the EDL capacitance was then evaluated with particular attention to the effect of cation-anion size difference. We also examined the influence of the chemical and mechanical modifications of graphene-like electrodes on the supercapacitor performance. Especially, mechanisms underlying chemical doping-induced enhancement of the total interfacial capacitance were addressed through analysis of electrode quantum capacitance changes resulting from electronic structure modifications. A part of my effort was also devoted to examining the binding interaction of graphene with a-SiO₂ (which is not yet clearly understood despite its scientific and technological importance). In particular, we attempted to evaluate quantitatively the adsorption strength of graphene on the a-SiO₂ surface, which has been under debate mainly due to the difficulty of direct measurement. / text

Despite the widespread interest in graphene electronics over the last decade, high-performance graphene field-effect transistors (GFETs) on flexible substrates have been rarely achieved, even though this atomic sheet is widely understood to have greater prospects for flexible electronic systems. In this work, we investigate the realization of high-performance graphene field effect transistors implemented on flexible plastic substrates. The optimum device structure for high-mobility and high-bendability is suggested with experimental comparison among diverse structures including top-gate GFETs (TG-GFETs), single/multi-finger embedded-gate GFETs with high-k dielectrics (EG-highk/GFETs), and embedded-gate GFETs with hexagonal boron nitride (h-BN) dielectrics. Flexible graphene transistors with high-k dielectric afforded intrinsic gain, maximum carrier mobility of 8,000 cm²/V·s, and importantly 32 GHz cut-off frequency. Mechanical studies reveal robust transistor performance under repeated bending down to 0.7 mm bending radius whose tensile strain corresponds to 8.6%. Passivation techniques, with robust mechanical and chemical protection in order to operate under harsh environments, for embedded-gate structures are also covered. The integration of functional coatings such as highly hydrophobic fluoropolymers combined with the self-passivation properties of the polyimide substrate provides water-resistant protection without compromising flexibility, which is an important advancement for the realization of future robust flexible systems based on graphene. / text

Optical properties of graphene/GaN hybrid structure were investigated by using a variety of optical spectroscopy techniques including low-temperature photoluminescence (PL) spectroscopy, time-resolved PL (TRPL) spectroscopy, confocal scanning micro-Raman spectroscopy. Single-layer graphene grown by chemical vapor deposition was transferred to GaN epilayer surface, which is verified by the Raman spectrum with a sharp characteristic peak at ~2690 cm-1and a homogeneous Raman image. Three main band-edge emissions including the free exciton A transition (denoted as FXA), the donor bound exciton transition (denoted as DX) and the third peak (denoted as Ix) were well resolved in the PL spectra of the hybrid structure as well as the as-grown GaN epilayer at low temperatures. Interestingly, the FXA transition and Ix line of the GaN epilayer were found to be dramatically altered by the top graphene layer while the DX is almost unaffected. The intensity of Ix line substantially drops after the transfer of graphene layer on GaN, indicating surface defect nature of the Ix line. More interestingly, an unpredictable dip structure develops in the FXA peak when the temperature is beyond 50 K. Similar spectral structure change also occurred in the emission of free exciton B (referred as FXB)with higher transition energy .A free exciton dissociation and electron transfer model was proposed to explain the “dip effect”. More supporting evidence to the model was found in the time-resolved PL spectra of the hybrid structure and the control sample. The results showed the significant influence of graphene monolayer on the fundamental optical properties of GaN. / published_or_final_version / Physics / Master / Master of Philosophy

Graphene has been one of the most interesting and widely investigated materials in the past decade. Because of its high mobility, high current density, inherent strength, high temperature stability and other properties, scientists consider it a promising material candidate for the future all-carbon electronics. However, graphene still exhibits a number of problems such as an unknown interface structure and no sizable band gap. Therefore, the purpose of this thesis is to probe and solve these problems to make graphene suitable for electronics. The work focuses on high-quality C-face epitaxial graphene, which is grown on the (000-1) face (C-face) of hexagonal silicon carbide using the confinement-controlled sublimation method. C-face epitaxial graphene has much higher mobility compared to Si-face graphene, resulting from its special stacking order and interface structure, the latter of which is not fully understood. Thus, the first part of the work consists of a project, which is to investigate and modify the interface and the surface of C-face graphene by silicon deposition and annealing. Results of this project show that silicon can intercalate into the graphene-SiC interface and form SiC by bonding carbon atoms on the graphene surface. Another crucial problem of graphene is the absence of a band gap, which prevents graphene from becoming an ideal candidate for traditional digital logic devices. Therefore, the second project of this work is devoted to introducing a wide band gap into the graphene electronic structure by growing from a nitrogen-seeded SiC. After successful opening of a band gap, a pre-patterning method is applied to improve graphene thickness variations, orientational epitaxy, and the gapped electronic structure.

Over the past decade there has been a great deal of interest in graphene, a 2-dimensional allotrope of carbon with exceptional mechanical and electrical properties. Its outstanding mobility, minimal size, and mechanical stability make it an appealing material for use in next generation electronic devices. Epitaxial graphene growth on silicon carbide is a reliable, scalable method for the production of high quality graphene films. Recent work has shown that the SiC can be patterned prior to graphitization, in order to selectively grow graphene nanostructures. Graphene nanoribbons grown using this technique do not suffer from the rough edges caused by lithographic patterning, and recent measurements have revealed extraordinary transport properties. In this thesis the magnetic properties of these nanoribbons are investigated through spin polarized current injection. The sensitivity of these nanoribbons to spin polarized current is interesting from a fundamental physics standpoint, and may find applications in future spintronic devices.

As devise sizes approach the nanoscale, novel device geometries and materials are considered, and new types of essential physics becomes important and new physical switching mechanism are considered, and as our intuitive understanding of device behavior is stretched accordingly, increasing first-principles simulation is required to understand and predict device behavior. To this end, initially I worked to capture the richness of the confinement and transport physics in quantum-wire devices. I developed an efficient fully three dimensional atomistic quantum transport simulator within a nearest-neighbor atomistic tight-binding framework. However, I soon adapted this work to the study of transport in graphene mono-layer and bilayer nano-ribbons. Motivated by proposals for use of nano-ribbons to create band gaps in otherwise gapless graphene monolayers, I studied the effects of edge disorder in such graphene nano-ribbon FETs. I found that ribbon widths sufficiently narrow to produce useful bandgaps, would also lead to an extreme sensitivity to ribbon-edge roughness and associated performance degradation and device-to-device variability. Going beyond conventional switching but staying with the graphene material system, to model electron-hole condensation in two graphene monolayers separated by a tunnel dielectric potentially beyond room temperature, I developed a self-consistent atomistic tight-binding treatment of the required interlayer exchange interaction within non-local Hartree-Fock mean-field theory. Such condensation, associated many-body enhanced interlayer current flow, and gate-control thereof is the basis for the beyond-CMOS Bilayer-pseudoSpin Field Effect Transistor (BiSFET) proposed by colleagues. I studied the effect of various system parameters and on interlayer charge imbalance on the strength of the condensate state. I also modeled the critical current, the maximum interlayer current that can be supported by the condensate, its detailed dependence on the nature and strength of the required interlayer bare tunneling and on charge imbalance. The results presented here are expected to be used to refine devices models of the BiSFET, and may serve as guides to experiments to observe such a condensate state. / text

This thesis describes the fabrication and characterization of an inorganic catalyst based glucose Biofuel cell using nanoporous (mesoporous) silica thin-film as a functional membrane. The desired use of nanoporous silica based biofuel cell is for a blood vessel implantable device. Blood vessel implantable Biofuel Cells (iBFCs) are subjected to higher glucose concentrations and blood flow rates. However, reduction in the implant thickness is critical for the intra-vascular implantable Biofuel cells. Platinum thin-film (thickness: 25 nm) deposited on Silicon substrate (500 [mu]m) served as the anode while Graphene pressed on Stainless steel mesh (175 [mu]m) was used as the cathode. Control experiments involved the use of surfactant-coated polypropylene membrane (50 [mu]m) and Activated Carbon (198 [mu]m) electrodes. Preliminary results show that nanoporous silica thin film (270 nm) is capable of replacing the conventional polymer based membranes with an increased power density output of as high as 10 [mu]W/cm2 under physiological conditions. in-vitro (5 [mu]W/cm2) and in-vivo (10 [mu]W/cm2) experiments demonstrate the potential of ultra-thin iBFCs towards powering future medical implants. / text

Η παρούσα διπλωματική εργασία πραγματοποιήθηκε στο εργαστήριο παρασκευής και μηχανικών ιδιοτήτων συνθέτων υλικών στο ΙΤΕ/ΙΕΧΜΗ στη πάτρα. Το κύριο μέρος της παρούσης μεταπτυχιακής εργασίας είναι η μεταφορά μονού στρώματος γραφενίου σε υποστρώματα PDMS και ο χαρακτηρισμός του μέσω της φασματοσκοπίας Raman καθώς επίσης και η μελέτη της αλληλεπίδρασης του γραφενίου με το υπόστρωμα. Και τέλος, εξίσου σημαντικό μέρος αποτελεί η θερμομηχανική μελέτη του μονοστρωματικού γραφενίου, το οποίο έχει αναπτυχθεί στην επιφάνεια του χαλκού. Πιο αναλυτικά, το πρώτο κεφάλαιο της διπλωματικής εργασίας αποτελεί μια πρώτη εισαγωγή στο γραφένιο, όπου απαντά σε διάφορα ερωτήματα όπως, πως ανακαλύφθηκε, τι είναι, τι ιδιότητες έχει, πως παρασκευάζεται, ποία είναι η δομή του (κρυσταλλική, ηλεκτρονική) και σε πολλά άλλα. Στη συνέχεια, στο δεύτερο κεφάλαιο περιγράφονται οι πειραματικές τεχνικές, οι φασματοσκοπικές μέθοδοι καθώς και οι πειραματικές διατάξεις που χρησιμοποιήθηκαν κατά την εκπόνηση της διατριβής. Για τον χαρακτηρισμό και την μελέτη του γραφενίου πάνω σε διάφορα υποστρώματα (PDMS, PMMA, Si/SiO2, χαλκού) χρησιμοποιούνται, κυρίως, η φασματοσκοπία Raman, αλλά και η οπτική μικροσκοπία, η ηλεκτρονική μικροσκοπία σάρωσης (SEM) και η διάταξη θέρμανσης-ψύξης (THMS600) της εταιρίας Linkam. Στο τρίτο κεφάλαιο περιγράφονται οι διαδικασίες παρασκευής των προς μελέτη δοκιμίων. Αρχικά αναλύουμε τον τρόπο με τον οποίο παρασκευάσαμε δοκίμια PDMS (πoλυδιμέθυλο σιλοξάνιο). Η πρώτη μέθοδος περιγράφει τον τρόπο με τον οποίο το γραφένιο, το οποίο έχει αναπτυχθεί σε υπόστρωμα χαλκού, μεταφέρεται σε άλλο υπόστρωμα, ενώ η δεύτερη μέθοδος (scotch tape method) περιγράφει πως μεταφέρουμε το γραφένιο από τον γραφίτη με χρήση κολλητικής ταινίας. Τέλος, αναλύουμε τον τρόπο με τον οποίο παρασκευάσαμε δοκίμια (πολυμεθακριλικός-μεθυλεστέρας) PMMA/Gr/Cu τα οποία χρησιμοποιήσαμε για την έρευνά μας. Το τέταρτο κεφάλαιο αποτελεί μια φασματοσκοπική μελέτη του γραφενίου. Περιγράφει το πρώτο μέρος των πειραματικών δεδομένων που αναφέρονται στην μελέτη του γραφενίου το οποίο τοποθετήσαμε σε υπόστρωμα PDMS. Για την μελέτη αυτή διαθέτουμε δυο δείγματα. Και στα δυο αυτά δείγματα έχουμε τοποθετήσει φύλλα γραφενίου πάνω σε PDMS. Στο πρώτο δείγμα έχουμε ακολουθήσει την μέθοδο της μικρομηχανικής αποφλοίωσης για να μεταφέρουμε το γραφένιο στην επιφάνεια του PDMS, ενώ στο δεύτερο έχουμε μεταφέρει γραφένιο, αφού πρώτα έχει αναπτυχθεί σε χαλκό με την μέθοδο CVD. Στο πρώτο μας δείγμα έχουμε μεταφέρει εκτός από μονά στρώματα γραφενίου, διπλά και τριπλά στρώματα αυτού. Επίσης, στο δεύτερο δείγμα, με κατάλληλη επεξεργασία έχουμε αναπτύξει ένα επαναλαμβανόμενο μοτίβο οπών. Σκοπός μας, λοιπόν, είναι η μελέτη και η ανάλυση της συμπεριφοράς του γραφενίου στην επιφάνεια του PDMS καθώς και η σύγκριση αυτής με άλλα υποστρώματα. Επίσης μελετάμε τις διαφορές, όσον αφορά των διαφορετικών μεθόδων μεταφοράς του γραφενίου που χρησιμοποιήσαμε, μεταξύ των δυο δειγμάτων που διαθέτουμε. Επιχειρούμε, δηλαδή, με την χαρτογράφηση των δειγμάτων μας να κατανοήσουμε τον τρόπο με τον οποίο τοποθετήθηκε (κάθισε) το γραφένιο πάνω στην επιφάνεια του υποστρώματος αλλά και τον τρόπο που αλληλεπιδρά με αυτό. Επιπρόσθετα παραθέτουμε και αναλύουμε φάσματα Raman για ένα, δύο και τρία στρώματα γραφενίου. Και τέλος, ερευνούμε τη διαφορετική συμπεριφορά του γραφενίου σε υπόστρωμα (supported graphene) και στον αέρα (suspended graphene). Στο πέμπτο κεφάλαιο ασχολούμαστε με την συμπεριφορά του γραφενίου, το οποίο αναπτύχθηκε στην επιφάνεια του χαλκού με τη μέθοδο της χημικής εναπόθεσης ατμών. Πρωταρχικός μας στόχος είναι η μελέτη της αλληλεπίδρασης του γραφενίου με το υπόστρωμα του χαλκού και κατ’ επέκταση η θερμομηχανική συμπεριφορά της μονής στρώσης γραφενίου, η οποίας εκτιμάται με τη θερμοκρασιακή εξάρτηση από την φασματοσκοπία Raman στην περιοχή θερμοκρασιών μεταξύ -1500C και 2400C. Για να επιτευχθεί η θερμοκρασιακή εξάρτηση του γραφενίου, τα φάσματα Raman λήφθηκαν και κατά την ψύξη και την θέρμανση των δειγμάτων με χρήση της διάταξής THM600 της εταιρίας Linkam. Στη συνέχεια σε αντίστοιχο δείγμα γραφενίου πάνω σε χαλκό εναποθέτουμε PMMA με τρείς διαφορετικούς ρυθμούς στροφών (4000rpm, 5000rpm, 6000rpm), με σκοπό να διαπιστώσουμε εάν και πώς με την εναπόθεση του πολυμερούς το φύλλο του γραφενίου επηρεάστηκε σε σχέση με πριν. / This thesis took place in the composite materials laboratory at ITE/IEXMH in Patras. The main part of this thesis is the transfer of a single-layer of graphene to a PDMS substrate and its characterization via Raman spectroscopy as well as the study of the interaction of graphene with the substrate. Last but not least the thermomechanical study of a single-layer of graphene which was grown on a copper surface. More specifically the first chapter of this thesis is a first introduction to graphene, answering various questions such as, how it was discovered, what it is, what properties it has, how it is produced, what its structure is (crystalline, electronic) and many others. The second chapter discusses the experimental techniques, the spectroscopic methods and also the experimental configurations that were used in the preparation of this thesis. In order to, characterize and study graphene placed on various substrates (PDMS, PMMA, Si/〖SiO〗_2, Cu) Raman spectroscopy is mainly used. We also use other methods, such as optical microscopy, scanning electron microscopy (SEM) and the Linkam’s company heating-cooling stage (THMS 600). The third chapter describes the preparation processes of the samples under consideration. We begin by analyzing the way in which we produce the PDMS samples. We then continue by examining the two main methods of transferring graphene. The first method describes how the graphene, grown on a copper substrate, is transferred on another, while the second one that is the micromechanical exfoliation method (also known as the scotch tape method), describes the way in which we transfer graphene from graphite to a substrate using a scotch tape. Finally, we analyze the way in which we produced PMMA/Gr/Cu samples which were used for our research. The fourth chapter constitutes a spectroscopic study of graphene. It describes the first part of our experimental data which pertain to the study of graphene placed on PDMS substrate. Our purpose is to study and analyze the graphene behavior on the PDMS surface as well as to compare it to other substrates. We also point out the differences of the various transfer methods of the graphene that was used to produce our samples. By mapping our samples we attempt to comprehend the way in which the graphene was placed over the substrate surface and the way it interacts with it. In addition, we present and analyze Raman spectra for one, two and three graphene layers. Finally, we investigate the difference in the behavior of graphene placed on a substrate (supported graphene) and free standing graphene (suspended graphene). The fifth chapter discusses the behavior of graphene which was developed on a copper surface via the chemical deposition method. Our initial goal is to study the interaction of graphene with the copper substrate and furthermore its temperature dependence in the temperature range between 150 (_ ^o)C and 240 (_ ^o)C. In order to achieve the temperature dependence of graphene, Raman spectra were obtained using the Linkam’s company stage THM 600 both during cooling and heating of the samples. Moreover, we place PMMA on a corresponding sample of graphene on copper with three different rates of speed (4000 rpm, 5000 rpm, 6000 rpm) in order to see whether and how the graphene sheet was affected by the deposition of the polymer.

Στην παρούσα εργασία υπολογίστηκε η ενέργεια διαμορφώσεων γραφενίου στις οποίες τα άτομα άνθρακα αποκλίνουν από το επίπεδο εξαγωνικό πλέγμα. Η πρώτη περίπτωση που εξετάσθηκε ήταν η απόκλιση από το επίπεδο, των ατόμων του ενός εκ των δύο υποπλεγμάτων του γραφενίου, θεωρώντας μία άπειρη περιοδική δομή. Χρησιμοποιήσαμε τη θεωρία γραμμικού συνδυασμού ατομικών τροχιακών (Linear Combination of Atomic Orbitals-LCAO) προκειμένου να κατασκευάσουμε τις ενεργειακές ζώνες για τα s, px, py και pz τροχιακά των ατόμων άνθρακα στο γραφένιο και μελετήσαμε τη μορφή τους συναρτήσει της κατακόρυφης απόκλισης z των ατόμων του ενός υποπλέγματος από το επίπεδο. Στη συνέχεια, με προσαρμογή ab initio δεδομένων για την ενέργεια παραμόρφωσης του δεσμού άνθρακα-άνθρακα στο επίπεδο γραφένιο, εκτιμήσαμε τη συνάρτηση απωστικής ενέργειας μεταξύ των ατόμων άνθρακα. Έτσι, για την περίπτωση του άπειρου περιοδικού πλέγματος, υπολογίσαμε την ολική ενέργεια ανά άτομο συναρτήσει της κατακόρυφης απόστασης των δύο ατόμων της μοναδιαίας κυψελίδας. Στη συνέχεια, προκειμένου να μελετήσουμε ημιτονοειδείς κυματώσεις διαφορετικού μήκους κύματος και πλάτους, κατασκευάσαμε πλέγματα γραφενίου διαφορετικών μεγεθών και υπολογίσαμε την ολική ενέργεια για διάφορες διαμορφώσεις των πλεγμάτων αυτών. Προέκυψαν έτσι ορισμένες διαμορφώσεις πλέγματος ενεργειακά προτιμητέες σε σχέση με την επίπεδη διαμόρφωση του γραφενίου. Τέλος, μελετήθηκε η αλληλεπίδραση τύπου Van der Waals μεταξύ των ατόμων επίπεδου υποστρώματος SiO2 και των ατόμων του γραφενίου καθώς και η μεταβολή στην ολική ενέργεια των κυματώσεων του γραφενίου που προκύπτει από την αλληλεπίδραση αυτή. / In the present thesis, we studied cases of carbon atom deviations from planarity in graphene’s hexagonal lattice, with respect to graphene’s total energy. The first case studied, is the deviation of the atoms of the one crystal sublattice of graphene, in an infinite crystal lattice. Linear Combination of Atomic Orbitals (LCAO) was used in order to obtain the energy bands for the s, px, py and pz atomic orbitals in graphene. Graphene’s band structure was studied with respect to the deviation z ofthe atoms of the one crystal sublattice of graphene. In order to obtain an empirical formula for repulsive energy between carbon atoms in graphene, we fitted ab initio results for graphene bond stretching potential, in graphene’s plane. Subsequently, we calculated the total energy per carbon atom with respect to the distance z, for an infinite graphene lattice. In order to study ripples of sinusoidal form in graphene’s structure, we created graphene lattices of different sizes and then we calculated the electronic, the repulsive and the total energy for different ripple configurations. Configurations which are energetically more favourable with respect to flat graphene were found, providing thus a ground state with ripples at very low temperatures. Lastly, Van der Waals interaction between a flat SiO2 substrate and graphene lattices on top of it was studied, with respect to changes of graphene’s total energy that result from the graphene-substrate interaction.

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