Yes / We report on the predicted structural disruption of an adenosine-binding DNA aptamer adsorbed via noncovalent interactions on aqueous graphene. The use of surface-adsorbed biorecognition elements on device substrates is needed for integration in nanofluidic sensing platforms. Upon analyte binding, the conformational change in the adsorbed aptamer may perturb the surface properties, which is essential for the signal generation mechanism in the sensor. However, at present, these graphene-adsorbed aptamer structure(s) are unknown, and are challenging to experimentally elucidate. Here we use molecular dynamics simulations to investigate the structure and analyte-binding properties of this aptamer, in the presence and absence of adenosine, both free in solution and adsorbed at the aqueous graphene interface. We predict this aptamer to support a variety of stable binding modes, with direct base−graphene contact arising from regions located in the terminal bases, the centrally located binding pockets, and the distal loop region. Considerable retention of the in-solution aptamer structure in the adsorbed state indicates that strong intra-aptamer interactions compete with the graphene−aptamer interactions. However, in some adsorbed configurations the analyte adenosines detach from the binding pockets, facilitated by strong adenosine−graphene interactions.

Yes / We combine single molecule force spectroscopy measurements with all-atom metadynamics simulations to investigate the cross-materials binding strength trends of DNA fragments adsorbed at the aqueous graphite C(0001) and Au(111) interfaces. Our simulations predict this adsorption at the level of the nucleobase, nucleoside, and nucleotide. We find that despite challenges in making clear, careful connections between the experimental and simulation data, reasonable consistency between the binding trends between the two approaches and two substrates was evident. On C(0001), our simulations predict a binding trend of dG > dA ≈ dT > dC, which broadly aligns with the experimental trend. On Au(111), the simulation-based binding strength trends reveal stronger adsorption for the purines relative to the pyrimadines, with dG ≈ dA > dT ≈ dC. Moreover, our simulations provide structural insights into the origins of the similarities and differences in adsorption of the nucleic acid fragments at the two interfaces. In particular, our simulation data offer an explanation for the differences observed in the relative binding trend between adenosine and guanine on the two substrates.

The development of graphene-based electronics may produce a new generation of electronic devices with enhanced performance over traditional materials. However, quality graphene electronics will require large-area, continuous graphene film produced by chemical vapor deposition (CVD), which is typically not stable when free-standing. Instead, CVD graphene may be coupled with polymer thin films to produce polymer/graphene laminates (PGLs), which show improved mechanical stability and good electrical conductivity over large areas. For example, single-layer graphene (SLG) has been coupled with polyetherimide (PEI) to produce thin film audio speakers with significantly enhanced energy efficiency compared to traditional speaker designs. However, the poor design and fabrication of PGLs may degrade the interfacial interactions of polymers with graphene and make the interface more susceptible to deformation. Interfacial weakening then limits their long-term reliability and performance in micro- and nano-electromechanical systems (MEMS/NEMS), in which materials are constantly subjected to dynamic loads. In this dissertation, we construct a framework for developing PGL thin films with controlled interfacial properties based on the rational design of polymer substrates and careful consideration of fabrication-induced defects. We evaluate this framework from several angles. First, the design of PEI/SLG film thickness is explored for controlling mechanical mixing and composite Young's modulus. Then, the role of thermal annealing during PEI/SLG fabrication is examined and its effect of mechanical mixing studied. Finally, we investigate mechanical fatigue in PEI/SLG thin films under dynamic loading with different pre-tensions. The design of mechanical properties in PGLs is crucial for ensuring long-term stability and operational performance. The Young's modulus is an important mechanical parameter governed by mechanical mixing in PGLs, and better mechanical mixing is facilitated by improved stress transfer efficiency at the polymer/graphene interface. Thus, the control of mechanical mixing through PGL design is an important research topic for developing PGL thin films with good mechanical performance. To this end, we evaluated the design of PEI/SLG thin films with precisely controlled thicknesses to afford control over mechanical mixing and the composite Young's modulus. PEI concentrations of 3–6 wt% were used to spin-coat PEI/SLG films at precisely controlled thicknesses of ~200–1000 nm, which enabled the design of PEI/SLG thin films based on thickness control. The design of PEI/SLG film thickness afforded control over the volume fraction of SLG, which is related to the composite Young's modulus of the films via the well-known mixing rule. To validate this approach, we measured the composite Young's modulus of PEI/SLG films and modelled the relationship between modulus and film thickness (i.e., volume fraction) using the mixing rule. However, we found that linear regression analysis yielded an unexpectedly high effective Young's modulus of 1.12 ± 0.05 TPa for SLG. Further investigation revealed the larger-than-expected value was due to the gradual deterioration of mechanical mixing between SLG and PEI at film thicknesses > 250 nm. Overall, our results demonstrate that the control of mechanical mixing in PGL thin films is achievable through the physical design of film thickness, which fits well within the proposed framework for PGL development. The fabrication of PGL materials is another important topic for controlling PGL properties. For example, the transfer process required for removing PGL thin films from a CVD substrate is known to introduce mechanical defects and electronic doping if improper processing conditions are used. However, little is known about the impact of thermal annealing on the properties of PGL materials. Normally, thermal annealing is thought to aid in the removal solvent and stress after polymer deposition on SLG, but we show that thermal annealing instead induces substantial structural damage and reduced film properties when poor conditions are used. Specifically, thermally annealing PEI/SLG in air past the Tg of PEI (~217 °C) caused widespread structural damage due to oxidation of the underlying Cu substrate. This conclusion was supported through an in-depth mechanistic investigation of annealing-induced deformation. First, changing the annealing atmosphere to high-purity nitrogen prevented widespread structural deformation from occurring during annealing, regardless of temperature. Second, the onset of film deformation during annealing in air was strongly associated with temperatures above the glass transition temperature of PEI. Finally, Arrhenius analysis yielded an activation energy of 159 kJ/mol for the deformation process, almost twice that associated with Cu oxidation and instead closer in scale to that of glass transitions in amorphous polymers. A physical failure mechanism was proposed based on the in-plane shrinkage of PEI during the glass-to-rubber transition caused by the release of internal stress induced by spin-coating. In-plane contraction of PEI would then cause compression of SLG as internal stress was released, which we confirmed from a significant blue-shift in the 2D band of SLG by ~30 cm-1 after annealing both with and without visible deformation. Importantly, we demonstrate that common secondary processing steps, like thermal annealing, will have critical effects on film properties if conditions are not properly controlled. The mechanical fatigue and cycling stability of PGL thin films is another topic of interest for developing reliable PGLs with long operational lifetimes. Mechanical failure analysis might be used to evaluate failure mechanisms governing interfacial fatigue in PGL thin films. However, little work has been done understanding mechanical fatigue in PGLs. To this end, we explore the use of mechanical bulge testing for the evaluation of cycling fatigue in PGLs and construct a custom instrument, known as a bulge test apparatus (BTA), to perform fatigue measurements. In general, our BTA test platform provides a means of analyzing different fabrication/design conditions. To test its use, we prepared PEI/SLG thin films with different tensioning weights between 0 and 30 g. Then, the well-established methods for bulge testing were applied to the dynamic loading of PEI/SLG using the BTA. Specifically, the evolution of bulge-test-derived pre-stress in PEI/SLG was monitored as a function of cycle number and tensioning weight, which revealed large fluctuations in pre-stress with mechanical cycling up to ~200k cycles, which was unexpected for these films. To investigate the underlying mechanism, we further employed Raman spectroscopy, optical microscopy, and scanning electron microscopy (SEM) to determine the strain state of SLG before and after cycling and probe for structural changes. Raman measurements revealed that mechanical cycling induced a large redshift in the 2D and G peak positions of SLG by ~25.4 and ~10.1 cm-1, respectively, for 30 g-tensioned PEI/SLG, with similar shifts observed for 20 g-tensioned films. Optical and SEM images showed possible changes in the surface structure of SLG after cycling accompanying the shift in Raman characteristics. We discuss the possibility of interfacial failures involving in-plane slippage and out-of-plane buckling of SLG based on our results. If validated, the use of BTA may provide future insight regarding mechanical fatigue and failure at the PGL interface during dynamic loading. The practical relevance of methods for determining the influence of design and fabrication characteristics in PGLs will no doubt be invaluable for further development. Additional work in this area will be necessary to connect our analytical approach by BTA to underlying failure mechanisms in PGLs. / Doctor of Philosophy / Due to its record-breaking properties, graphene has tremendous promise for use in next-generation consumer electronics, like ultra-thin audio speakers and flexible displays. However, on its own, single-layer graphene (SLG) is not stable enough for practical uses. Therefore, in this dissertation, I construct a framework for developing polymer substrates to stabilize and enhance graphene based on the design and fabrication of polymer/graphene laminates (PGLs). To explore this framework, I design polyetherimide (PEI)/SLG thin films with controlled mechanical properties, evaluate fabrication-induced defects, and develop an analytical approach for measuring mechanical fatigue in these films. The ability to precisely control thin film mechanics has important implications for the application of PGLs. However, it is challenging in PGL systems to enhance mechanical properties due to the limited amount of graphene that is used. Therefore, I show that the design of PEI/SLG film thickness can effectively control mechanical reinforcement, even at very low loadings of graphene. I further investigate the design of a controlled pre-tensioning in PEI/SLG and show that tensioning can be used to modify the mechanical response. However, using a custom mechanical testing platform, I also determine that mechanical fatigue may increase with pre-tensioning as well. The impact of fabrication procedures on the properties of PGL thin films is another important aspect of PGL development that is often overlooked. Specifically, thermal annealing may introduce residual contamination and/or structural defects that reduce material properties. Therefore, I present a detailed study of thermal annealing-induced structural failures in PEI/SLG films, which shows that annealing at high temperature induces substrate oxidation and structure damage in the films. The results of this annealing study demonstrate the potential negative effects of fabrication on PGL development. Finally, this dissertation ends by discussing future directions in PGL design and fabrication that will need to be addressed in the coming years for further development of PGL thin film electronics.

No / The performance of cement-based materials can be controlled and tailored by adjusting the characteristics of reinforced nano inclusions. Therefore, the lateral size effect of graphene on the nano/micro-structures of ultra-high performance concrete (UHPC) was explored, and then the mechanical properties were investigated to analyze the structure–property correlation of composites in this paper. The test results show that due to nucleation site effect and the formation of core–shell elements, incorporating graphene with lateral size of > 50 µm improves the polymerization degree and mean molecule chain length of C-S-H gel by 242.6% and 56.3%, respectively. Meanwhile, the porosity and average pore volume of composites is reduced by 41.4% and 43.4%. Furthermore, graphene can effectively inhibit the initiation and propagation of cracks by crack-bridging, crack-deflection, pinning and being pulled-out effect, and the wrinkling characteristic is conductive to the enhancement of pinning effect. These improvements on nano- and micro- structures result in that the compressive strength, compressive toughness and three-point bending modulus of UHPC are increased by 43.5%, 95.7% and 39.1%, respectively, when graphene with lateral size of > 50 µm and dosage of 0.5% is added. Compared to graphene with lateral size of > 50 µm, graphene with average lateral size of 10 µm has less folds and larger effective size, then reducing the distance between core–shell elements. Hence, the addition of graphene with average lateral size of 10 µm leads to 21.1% reduction for Ca(OH)2 crystal orientation index, as well as 30.0% increase for three-point bending strength. It can be, therefore, concluded that the lateral size of graphene obviously influences the nano/micro-structures of UHPC, thus leading to the significantly different reinforcing effects of graphene on mechanical behaviors of UHPC.

The topic of this thesis is the study of nitrogen functionalities in nitrogen-doped reduced graphene oxide using Raman spectroscopy. Specifically, the project set out to investigate if the Raman active nitrogen-related vibrational modes of graphene can be identified via isotope labelling. Previous studies have used Raman spectroscopy to characterise nitrogen doped graphene, but none has employed the method of isotope labelling to do so. The study was conducted by producing undoped, nitrogen-doped and nitrogen-15-doped reduced graphene oxide and comparing the differences in the first-order Raman spectrum of the samples. Results of this study are inconclusive. However, some indications linking the I band to nitrogen functionalities are found. Also, a hypothetical Raman band denoted I* possibly related to \spt{3} hybridised carbon is introduced in the same spectral area as I. This indication of a separation of the I band into two bands, each dependent on one of these factors could bring clarity to this poorly understood spectral area. As the results of this study are highly speculative, further research is needed to confirm them and the work presented here serves as a preliminary investigation.

For graphene to be a viable platform for nanoscale devices, high quality growth and structures are necessary. This means structuring the SiC surface to prevent graphene from having to be patterned using standard microelectronic processes. Presented in this thesis are new processes aimed at improving the graphene as well as devices based on high quality graphene nanoribbons. Amorphous carbon (aC) corrals deposited prior to graphene growth are demonstrated to control SiC step-flow. SiC steps are shown to be aligned by the presence of the corrals and can increase SiC terrace widths. aC contacts deposited and crystallized during graphene growth are shown as a way to contact graphene without metal lift-off. Observation of the Quantum Hall Effect demonstrates the high quality of the graphene grown alongside the nanocrystalline graphite contacts. Continuing the ballistic transport measurements on sidewall graphene nanoribbons, the invasive probe effect is observed using an atomic force microscope (AFM) based technique that spatially maps the invasive probe effect. Cleaning experiments demonstrate the role of scattering due to resist residues and environmental adsorbates on graphene nanoribbons. Finally, switches based on junctions formed in the graphene nanoribbons are shown as a route toward graphene based devices.

Thesis (MSc)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The goal of this thesis is to investigate the effects of distorting a graphene lattice and connect- ing this distorted graphene sheet to a superconductor. At low energies the possible excitation states in graphene are restricted to two distinct regions in momentum space called valleys. Many electronic applications are possible if one can design a graphene system where excitations can be forced to occupy a single valley in a controllable way. Investigating the spectrum of the distorted graphene sheet reveals that, if the chemical potential is chosen to coincide with a bulk Landau level, the normal-superconductor interface always supports propagating modes in both directions. Excitations from opposite valleys travel in opposite directions along the interface. The spectrum of a distorted graphene sheet terminated by an armchair edge, in contrast, is dis- persionless. We verify this insulating nature of the armchair edge for finite samples by numerical means. Furthermore, we verify previous analytical results pertaining to a graphene sheet with NS interface and an applied perpendicular real magnetic field numerically. In the process, it is shown that considering graphene sheets of perfect width is not necessary, as long as the width a few magnetic lengths away from the interface is well-defined. By then considering a finite graphene sheet, terminated by armchair edges, that is distorted and connected to a superconductor, we find bound states near the NS interface that can be changed by distorting the graphene lattice further. / AFRIKAANSE OPSOMMING: Die doel van hierdie tesis is om die uitwerking van die vervorming van ’n grafeenrooster te ondersoek wanneer die met ’n supergeleier verbind word. By lae energieë word die moontlike opwekkings in grafeen beperk tot twee aparte gebiede van momentumruimte — die sogenaamde valleie. Verskeie elektroniese toepassings is moontlik indien ’n grafeenstelsel ontwerp kan word waar opwekkings slegs ’n enkele vallei beset en die besetting beheer kan word. Deur die spektrum van die vervormde grafeenrooster te ondersoek word daar gevind dat, indien die chemiese potensiaal gekies word om saam te val met ’n Landauvlak, die NS-tussenvlak geleiding in beide rigtings ondersteun. Opwekkings van verskillende valleie beweeg in teenoorgestelde rigtings langs die tussenvlak. Daarteenoor is die spektrum van ’n vervormde grafeenroster met ’n leunstoelrand dispersieloos. Ons bevestig hierdie insulerende gedrag van ’n leunstoelrand vir eindige grafeen- roosters deur middel van ’n numeriese berekening. Verder word vorige analitiese resultate wat verband hou met ’n grafeenrooster met normaal-supergeleiertussenvlakstelsel en loodregte mag- neetveld op die vlak bevestig deur middel van numeriese berekeninge. In die proses word dit ook aangedui dat die grafeenrooster nie ’n perfekte wydte hoef te hˆe nie, solank die wydte goed gedefinieer is vir ’n paar magnetiese lengtes in die omgewing van die tussenvlak. Deur dan die eindige grafeenrooster met leunstoelrande te koppel aan ’n supergeleier word gebonde toe- stande naby aan die NS tussenvalk gevind. Hierdie toestande kan gemanipuleer word deur die grafeenrooster verder te vervorm.

In the first part of this work, we describe a theory of the ground states and charge gaps in the fractional quantum Hall states of graphene. The theory relies on knowledge of these properties for filling fractions smaller than one. Then, by the application of two mapping rules, one is able to obtain these properties for fractional quantum Hall states at arbitrary fillings, by conceiving the quantum Hall ferromagnets as vacua on which correlated electrons or correlated holes are added. The predicted charge gaps and phase transitions between different fractional quantum Hall states are in good agreement with recent experiments. In the second part, we investigate the low energy theory for the neutral Landau level of bilayer graphene. We closely analyze the way different terms in the Hamiltonian transform under the action of particle-hole conjugation symmetries, and identify several terms that are relevant in explaining the lack of such symmetry in experiments. Combining an accurate parametrization of the electronic structure of bilayer graphene with a systematic account of the impact of screening we are able to explain the absence of particle-hole symmetry reported in recent experiments. We also study the energetics of fractional quantum Hall states with coherence between n=0 and n=1 cyclotron quantum numbers, and obtain a general formula to map the two-point correlation function from their well-known counterparts made from only n=0 quantum numbers. Bilayer graphene has the potential for realizing these states which have no analogue in other two-dimensional electron systems such as Gallium Arsenide. We apply this formula to describe the properties of the n=0/n=1 coherent Laughlin state which displays nematic correlations. / text

Graphene has recently become the focus of enormous attention for experimentalists and theorists alike mainly due to its unique electronic properties. However, the limited way in which one can control these properties is a major obstacle for device applications. The unifying theme of this thesis is to propose and thoroughly justify ways to control the electronic properties of graphene and carbon nanotubes by light or static electric and magnetic fields and to harness these properties for optoelectronic applications. A linearly polarized excitation is shown to create a strongly anisotropic distribution of photoexcited carriers in graphene, where the momenta of photoexcited carriers are aligned preferentially normal to the polarization plane. This effect offers an experimental tool to generate highly directional photoexcited carriers which could assist in the investigation of "direction-dependent phenomena" in graphene-based nanostructures. The depolarization of hot photoluminescence is used to study relaxation processes in graphene, both free standing and grown on silicon carbide. This analysis is extended to include the effect of a magnetic field, thereby allowing one to obtain the momentum relaxation times of hot electrons. The analysis of momentum alignment in the high frequency regime shows that a linearly polarized excitation allows the spatial separation of carriers belonging to different valleys. Quasi-metallic carbon nanotubes are considered for terahertz applications. They are shown to emit terahertz radiation when a potential difference is applied across their ends and their spontaneous emission spectra have a universal frequency and bias voltage dependence. It is shown that the same intrinsic curvature which opens the gap in the quasi-metallic carbon nanotube energy spectrum also allows optical transitions in the terahertz range. The exciton binding energy in narrow-gap carbon nanotubes is calculated and found to scale with the band gap and vanishes as the gap decreases, even in the case of strong electron-hole attraction. Therefore, excitonic effects should not dominate in narrow-gap nanotubes. Contrary to widespread belief, it is shown that full confinement is possible for zero-energy states in pristine graphene. The exact analytical solutions for the zero-energy modes confined within a smooth one-dimensional potential V = α/ cosh (βx) are presented. This potential provides a good fit for the potential profiles of top-gated graphene structures. It is shown that there is a threshold value of the characteristic potential strength α/β for which the first mode appears, in striking contrast to the non-relativistic case. A relationship between the characteristic strength and the number of modes within the potential is found. An experimental setup is proposed for the observation of these modes. The proposed geometry could be utilized in future graphene-based devices with high on/off current ratios.

The solid acid catalyst of the Keggin-type 12-tungstophosphoric acid (H3PW12O40, HPW) is supported on partially reduced graphene oxide (PRGO) nanosheets for acid-catalyzed reactions. HPW is a new class of catalyst with a good thermal stability and high Bronsted acidity in order to replace common mineral acids. However, it has low specific surface area (1-5 m2/g). Therefore, the possibility of PRGO as a catalytic support for HPW is investigated due to its high surface area (2630 m2/g) and good thermal stability. The synthesis of HPW-GO catalyst is prepared using microwave and laser irradiation without using any chemical reducing agents. The HPW-GO catalysts are characterized by Ultraviolet-visible spectroscopy (UV-Vis), Fourier Transform Infrared Spectroscopy (FT-IR), Raman Spectroscopy, X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD) techniques, and Transmission Electron Microscopy (TEM). Also, the surface acidity is measured by a non-aqueous titration of n-butyl amine. Furthermore, the application for catalysts is tested by three acid-catalyzed reactions: Esterification, Friedel-Crafts acylation, and Pechmann condensation. The greatest acidity for the microwave irradiation method is with the loading of 85 wt% HPW-GO and 60wt% HPW-GO for laser irradiation. The results observed provide an excellent opportunity for PRGO as a catalytic support for HPW for acid-catalyzed reactions.