No description available.

Reduced graphene oxide (RGO) sheet, a chemically functionalized atomically thin carbon sheet, provides a convenient pathway for producing large quantities of graphene via solution processing. The easy processibility of RGO sheet and its composites offer interesting electronic, chemical and mechanical properties that are currently being explored for advanced electronics and energy based materials. However, a clear understanding of electron transport properties of RGO sheet is lacking which is of great significance for determining its potential application. In this dissertation, I demonstrate fabrication of high-yield solution based graphene field effects transistor (FET) using AC dielectrophoresis (DEP) and investigate the detailed electronic transport properties of the fabricated devices. The majority of the devices show ambipolar FET properties at room temperature. However, the mobility values are found to be lower than pristine graphene due to a large amount of residual defects in RGO sheets. I calculate the density of these defects by analyzing the low temperature (295 to 77K) charge transport data using space charge limited conduction (SCLC) with exponential trap distribution. At very low temperature (down to 4.2 K), I observe Coulomb blockade (CB) and Efros-Shklovskii variable range hopping (ES VRH) conduction in RGO implying that RGO can be considered as a graphene quantum dots (GQD) array, where graphene domains act like QDs while oxidized domains behave like tunnel barriers between QDs. This was further confirmed by studying RGO sheets of varying carbon sp 2 fraction from 55 – 80 % and found that both the localization length and CB can be tuned. From the localization length and using confinement effect, we estimate tunable band gap of RGO sheets with varying carbon sp 2 fraction. I then studied one dimensional RGO nanoribbon iv (RGONR) and found ES VRH and CB models are also applicable to the RGONR. However, in contrast to linear behavior of decrease in threshold voltage (Vt) with increasing temperature (T) in the RGO, sub linear dependence of Vt on T was observed in RGONR due to reduced transport pathways. Finally, I demonstrate synthesis and transport studies of RGO/nanoparticles (CdS and CeO2) composite and show that the properties of RGO can be further tuned by attaching the nanoparticles.

Novel two dimensional nanoscale materials like graphene and metal dichalcogenides (MX2) have attracted the attention of the scientific community, due to their rich physics and wide range of potential applications. It has been shown that novel graphene based transparent conductors and radiofrequency transistors are competitive with the existing technologies. Graphene’s properties are influenced sensitively by adsorbates and substrates. As such not surprisingly, physical properties of graphene are found to have a large variability, which cannot be controlled at the synthesis level, reducing the utility of graphene. As a part of my doctorate dissertation, I have developed atomic hydrogen as a novel technique to count the scatterers responsible for limiting the carrier mobility of graphene field effect transistors on silicon oxide (SiO2) and identified that charged impurities to be the most dominant scatterer. This result enables systematic reduction of the detrimental variability in device performance of graphene. Such sensitivity to substrates also gives an opportunity for engineering device properties of graphene using substrate interaction and atomic scale vacancies. Stacking graphene on hexagonal boron-nitride (h-BN) gives rise to nanoscale periodic potential, which influences its electronic graphene. Using state-of-the-art atomic-resolution scanning probe microscope, I correlated the observed transport properties to the substrate induced extrinsic potentials. Finally in efforts to exploit graphene’s sensitivity to discover new sensor technologies, I have explored noncovalent functionalization of graphene using peptides. Molybdenum disulfide (MoS2) exhibits thickness dependent bandgap. Transistors fabricated from single layer MoS2 have shown a high on/off ratio. It is expected that ad-atom engineering can be used to induce on demand a metal-semiconductor transition in MoS2. In this direction, I have iii explored controlled/reversible fluorination and hydrogenation of monolayer MoS2 to potentially derive a full range of integrated circuit technology. The in-depth characterization of the samples is carried out by Raman/photoluminescence spectroscopy and scanning tunneling microscopy

No description available.

Solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used to anal- yse numerous graphene-sheet based materials in an attempt to study their effects on the performance of polymer electrolyte membrane fuel cell (PEM-FC) mate- rials. It has been noted in the literature that fuel cells which incorporated these materials (e.g. functionalized graphene, doped carbon nanotubes (CNTs), etc.) displayed increased performance over a wider range of environmental conditions, chiefly temperature and relative humidity. The inter-material interactions behind this phenomenon are poorly described at best. Due to its extreme site speci ficity and sensitivity to minute differences in nuclear electromagnetic environments, ss- NMR is an ideal tool for investigating the complicated interactions at work in these systems. While the electronically conductive, amorphous, non-stoichiometric, and low spin-density nature of these materials presented challenges to the collection of NMR spectra, the results presented here display the remarkable utility of this method in the study of analogues and derivatives of graphene. Graphene Oxide (GO), a derivative of graphene, has intrinsic proton conduc- tivity which is similar to Na fon, the most popular proton exchange membrane material currently used in fuel cells. Research into acid-functionalized graphene oxides and determining the role of acidic groups in increasing proton conductivity will help to improve polymer electrolyte membrane performance in fuel cell sys- tems. Multinuclear solid-state NMR (ssNMR) spectroscopy was used to analyse the structure and dynamics of GO and a number of sulfonic acid derivatives of GO, both novel and previously reported. 13C spectra showed the disappearance of surface-based oxygen groups upon GO functionalization, and can be used to identify linker group carbon sites in previously synthesized and novel functional- ized GO samples with high speci city. Dehydration of these samples allows the collection of 1H spectra with resolved acidic proton and water peaks. The effect of dehydration on the proton spectrum is partially reversible through rehydration. Deuteration of the acidic groups in high temperature and acidic conditions was virtually unsuccessful, indicating that only the surface and not the intercalated functional groups play a role in the enhanced proton conductivity of ionomer / functionalized GO composites. Increased surface area and increased delamination of functionalized GO is suggested to be important to improved PEM-FC perfor- mance. This synthesis and method of analysis proves the utility of ssNMR in the study of structure and dynamics in industrially relevant amorphous carbon ma- terials, despite the obvious di culties caused by naturally broad signals and low sensitivity. Graphene and carbon nanotubes (CNTs) have been investigated closely in re- cent years due to their apparent positive effect on the electrochemical performance of new fuel cell and battery systems as catalyst stabilizers, supports, or as metal- free catalysts. This is particularly true for doped graphene and CNTs, where only a small amount of doping with nitrogen and/or phosphorus can have a re- markable effect on materials performance. A direct link between structure and function in these materials is, as of yet, unclear. Doped graphene and CNTs were synthesized using varied chemical vapour deposition (CVD)-based methods, and ssNMR was used to unambiguously identify dopant atom sites, revealing that these particular synthesis methods result in highly homogeneous populations of installed phosphorus and nitrogen atoms. We present the first experimental 15N spectrum for graphitic nitrogen in N-doped graphene. 15N-labeled nitrogen doped graphene synthesized as reported here produces mainly graphitic nitrogen sites located on the edges of sheets and around defect sites. 1H-1H and 1H-15N corre- lations were also used to probe dopant nitrogen sites in labelled and unlabelled N-doped graphene. A nearly homogeneous population of phosphorus in P-doped graphene is found, with an overwhelming majority of graphitic phosphorus and a small amount of phosphate oligomer. Similar findings are noted for the phos- phorus sites in phosphorus and nitrogen co-doped CNTs with a minor change in chemical shift, as would be expected from two chemically similar phosphorus sites in carbon allotropes (CNTs versus graphene sheets) with signifi cantly different electronic structures. Ionomeric sulfonated polyether ether ketone (SPEEK) membranes were doped with functionalized graphenes, and the proton conductivities of these composite membranes was measured at fuel cell operational temperatures and percent relative humidities (%RH). The differences in proton conductivity between pure SPEEK membranes and composites with different dopants and doping levels at varied conditions were investigated through high-fi eld 1H ssNMR. It was found that high- speed MAS was able to dehydrate membranes under water-saturated conditions, and so lower %RH conditions were better able to produce reliable ssNMR results. The addition of graphitic dopants appeared to have an overall detrimental effect on the bulk proton conductivity of membranes, while concurrently these doped membranes had a broadened operational temperature window. In an attempt to explore the positive influence of nitrogen doping on the effec- tive lifetime of carbon-supported platinum catalysts used in automotive hydrogen fuel cell systems, solid-state NMR was employed to probe the difference (if any) between doped catalyst supports made from different carbon and nitrogen sources. 1H spectroscopy showed a variety of sites present in the doped samples; some likely from residual starting material but others from novel sites within the doped cat- alyst supports. Double-quantum and 2D 1H experiments were used to examine the structure of these catalysts, while 13C CPMG experiments (see Chapter 2) revealed subtle differences in the nuclear relaxation rates of these materials, poten- tially related to their electronic conductivity. The results of the ssNMR analysis were insuffcient to provide an unambiguous picture of the dopant sites within these carbon black samples; this was due in equal parts to the lack of isotopically labelled dopants, the effects of electronic induction and ring current shifts on data acquisition and analysis, and the broad array of different 13C chemical shift en- vironments present in the carbon black itself. While the data is still interesting spectroscopically, suggestions are made at the end of this chapter to expand upon the lessons learned through this study to produce more useful results from similar samples in the future. / Thesis / Doctor of Philosophy (PhD) / Solid-state nuclear magnetic resonance (ssNMR) spectroscopy was used to anal- yse numerous graphene-sheet based materials in an attempt to study their effects on the performance of polymer electrolyte membrane fuel cell (PEM-FC) materials. It has been noted in the literature that fuel cells which incorporated these materials (e.g. functionalized graphene / graphite, doped carbon nanotubes (CNTs), etc.) displayed increased performance over a wider range of environmental conditions, chiefly temperature and relative humidity. The inter-material interactions behind this phenomenon are poorly described at best. Due to its extreme site specifi city and sensitivity to minute differences in nuclear electromagnetic environments, ss- NMR is an ideal tool for investigating the complicated interactions at work in these systems. While the electronically conductive, amorphous, non-stoichiometric, and low spin-density nature of these materials presented challenges to the collection of NMR spectra, the results presented here display the remarkable utility of this method in the study of analogues and derivatives of graphene. Covalently functionalized graphene / graphite was synthesized, and the struc- tures of several derivatives were recorded with remarkable resolution, such that functional group carbons were resolvable. The proton dynamics of this material were remarkably slow, and so improvements in composite PEM ion conductiv- ity were proposed to be caused by surface interactions between dopant and poly- mer. The proton dynamics of ionomer graphene composites were also investigated through ssNMR. A number of graphene and CNT samples doped with phosphorus and 15N-labelled nitrogen were also analysed, and the synthesis methods employed were found to produce chemically homogeneous dopant sites with few by-products. Absent isotopic labelling, nitrogen dopant sites in carbon black samples were found to affect the relaxation properties of protons within nitrogen doped carbon black.

In this work fabrication and studies of transistor structures based on an atomic sheet of graphite, graphene, are described. Since graphene technology is in its early stages, the development and optimisation of the fabrication process are very important. In this work the impact of various fabrication conditions on the quality of graphene devices is investigated, in particular the effects on the carrier mobility of the details of the mechanical exfoliation procedure, such as environmental conditions and humidity, source of graphite and wafer cleaning procedure. In addition, a comparison is made between the conventional e-beam lithorgaphy and lithography-free fabrication of samples. It was also demonstrated that water and other environmental species play an important role in graphene-to-substrate adhesion and can also contribute to the carrier scattering in graphene. A technique for creating suspended metal gates was developed for the fabrication of graphene p-n-p structures, and charge transport has been studied in such top-gated graphene devices. Depending on the relation between the carrier mean free path and the length of the top-gate we have realized three distinct transport regimes through the p-n-p structure: a) diffusive across the structure; b) ballistic in the regions of p-n junctions but diffusive in the n-region; c) ballistic across the whole p-n-p structure. The second regime has revealed the chiral nature of carriers in graphene. This was demonstrated by comparing the experimental resistance of a single p-n junction with results of electrostatic modeling in the diffusive model. In the third regime we have observed oscillations of the device resistance as a function of carrier concentration in the n-region, which are also dependent on magnetic field. These oscillations have been demonstrated to be a direct consequence of a Fabri-Perot-like interference effect in the graphene p-n-p structures.

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.

Graphene is a new two-dimensional (2D) material with unique electrical transport, optical and mechanical properties. However, monolayer graphene (MLG) is a gapless semiconductor, which limits its relevance for transistor applications where a large on/off ratio of the current is required. In this work the investigation of transport properties of few-layer graphene (FLG) is presented. These 2D electronic systems offer a novel solution to the problem concerned the absence of an energy gap in single layer graphene, since they exhibit an electric field and stacking-dependent band gap in the energy dispersion. Thus far, a clear observation of a band-gap in multilayer graphene (e.g. Bernal-stacked bilayers) in transport measurements was hindered by the presence of disorder. Here we develop a reliable and effective method of fabrication of high-quality suspended double-gated graphene devices, which are of crucial importance for probing the low energy dispersion of few-layer graphene. The current annealing technique, described in details, improves transport characteristics like carrier mobility, which is typically higher than ∼ 104 cm2/Vs for our multilayer devices. Electrical transport experiments on suspended dual-gated ABC-stacked trilayer are performed. We report the direct evidence of the opening of a tunable band-gap with an external perpendicular electric field, ranging from 0 meV up to 5.2 meV for an electric field of 117 mV/nm. Thermally activated transport is observed in these samples over the temperature range 0.5 - 80 K. The values of energy gap extracted from both temperature dependence of minimum conductivity measurements and non-linear I –V characteristics correlate well. Our experimental results are in a good agreement with theoretical approximation, based on self-consistent tight-binding calculations. The high quality of our ABC trilayer samples is also demonstrated by a particularly high on/off ratio of the current (250 at applied electrical displacement as low as 80 mV/nm), which makes these devices promising for future semiconductor electronics. FLG samples with reduced disorder allow us to observe quantum Hall effect (QHE) at magnetic field as low as 500 mT. We present the first study of electric field- induced new QH states in ABC trilayer graphene (TLG). The transitions between spin-polarized and valley polarized phases of the sample at the charge neutrality point are investigated. Resolved novel broken symmetry states along with observed Lifshitz transition in rhombohedral TLG display exciting phenomena attributed to rich physics in these interactive electronic systems.

In this thesis, the design, fabrication and characterisation of graphene electromechanical resonators have been presented. Graphene features ultrahigh Young’s modulus and large surface to volume ratio that make it ideal for radio frequency (RF) components, sensors and other micro/nano-electromechanical systems (MEMS/NEMS). A novel batch fabrication process for graphene electromechanical resonators has been developed by using poly-Si film as sacrificial layer. Previously reported fabrication processes of graphene resonators use SiO2 as sacrificial layer only because graphene is visible on 300nm SiO2/Si substrate. However, the wet etching of SiO2 involves HF, which is not compatible with metal connections or SiO2 serving as dielectric or passivation layer in graphene NEMS devices. Moreover, the liquid surface tension during drying after wet etching could damage graphene bridges even critical point drying is used. Therefore, in this work, poly-Si is adopted as the sacrificial material. To facilitate the fabrication of graphene resonators, a poly-Si/SiO2/Si substrate has been designed and optimised to make graphene visible under optical microscope for the first time to the author’s knowledge. Chemical vapour deposition (CVD)-grown monolayer graphene sheet has been transferred onto the optimised poly-Si/SiO2/Si substrate and patterned into strips. Metal electrodes have been deposited by lift-off process to make electrical connections, which is prerequisite for integrating graphene resonator into practical devices. The graphene bridges have been released by etching the poly-Si layer with XeF2 in vapour phase, which completely avoids the capillary force induced damage to the graphene bridges. De-fluorination process has been performed by hydrazine reduction to recover graphene’s conductivity. This fabrication process is scalable for massive production of graphene electromechanical resonators, thus furthering their practical application. One-source current mixing characterisation setup has been constructed to test the graphene resonators. Besides the fundamental peak, the activation and enhancement of the second mode of doubly clamped resonator by electrostatic actuation have been observed for the first time. The second mode amplitude reaches 95% of the fundamental mode, whereas only odd higher modes of small intensity have been reported before in other MEMS/NEMS resonators actuated by electrostatic force or magnetomotive force. The findings in this thesis could lead to substantial increase of the sensitivity of sensors based on the graphene resonators. Modal analysis based on Euler-Bernoulli equation has been performed to understand the mechanism behind the activation and enhancement of the second mode. Finite element analysis agrees very well with experimental results and complies with the theoretical model. Finally, a set of novel alignment marks has been designed, which can be incorporated to process mechanically exfoliated 2D material flakes of micron size and irregular shape with conventional photolithography tools, as have been demonstrated by the successful fabrication of a graphene transistor. This optical alignment technique provides an alternative for prototype device development besides electron beam lithography to prevent electron-induced damage to fragile 2D materials.

Modified graphene and low dimensional carbon nano-electronic devices have the potential to supersede current technologies in many respects although manufacturing and understanding these materials poses a significant challenge which requires an incremental approach. Doping of graphene, a prerequisite for modifying the electronic properties, is still poorly understood.Band-modulation is therefore difficult to control. Resonant tunneling induced through the incorporation of impurity clusters has not yet been addressed. On the other hand electronspin correlations in modified graphenes have hardly been studied. In this work we address these issues through a tandem approach of theoretical and experimental studies. This work begins with an ab-initio study of the electronic properties of bilayer graphene and the modifications induced through the substitutional incorporation of isolated nitrogen impurities.Nitrogen modification results in a change from a zero-gap semiconductor to a metal as a result of nitrogen incorporation while charge density calculations show the localization of charge in the vicinity of the impurity. This work on isolated impurities was then extended to impurity clusters. The quantum transport properties of impurity clusters distributed within a high bandgap material were then studied. Different geometrical configurations of the impurity clusters were studied to tune quantum interference to control the carrier lifetime. The effects of randomly distributed clusters were also studied to interpret the effects of disorder. These studies provide insight into the transport properties of naturally grown quantum dot systems such as reduced graphene oxide which consists of low defect density graphene nano-islands randomly distributed in oxygen and free radical functionalized graphene which was studied experimentally. Resistance was recorded as a function of temperature for graphene oxide and reduced graphene oxide two terminal devices. Evidence of mesoscopic resistance fluctuations, charge carrier activation and enhanced elastic scattering was found while the magnetic properties of reduced graphene oxide showed a phase transition from ferromagnetism at low temperatures to diamagnetism at higher temperatures. Finally, the Kondo effect was demonstrated in reduced graphene oxide through transport and magnetoresistance measurements which were interpreted within the Fermi liquid description of the Kondo effect. These effects were explained through the microstructure of reduced graphene oxide and illustrate the significance of spin in reduced graphene oxide. These studies will inform the design of functionalized graphene spin-polarized devices and spin valves.