This work is dedicated to electron transport in atomically thin crystals. We explore hydrodynamic effects in the electron liquid of graphene and perform a comprehensive study of electronic and optical properties of a novel 2D semiconductor - indium selenide(InSe). Graphene hosts a high quality electron system with weak phonon coupling such that electron-electron scattering can be the dominant process responsible for the establishment of local equilibrium of the electronic system above liquid nitrogen temperatures. Under these conditions, charge carriers are expected to behave as a viscous fluid with a hydrodynamic behaviour similar to classical gases or liquids. In this thesis, we aimed to reveal this hydrodynamic behaviour of the electron fluid by studying transport properties of high-quality graphene devices. To amplify the hydrodynamic effects, we used a special measurement geometry in which the current was injected into the graphene channel and the voltage was measured at the contact nearest to the injector. In this geometry we detected a negative signal which is developed as a result of the viscous drag between adjacent fluid layers, accompanied by the formation of current vortices. The magnitude of the signal allowed us to perform the first measurement of electron viscosity. In order to understand how an electron liquid enters the hydrodynamic regime we studied electron transport in graphene point contacts. We observed a drop in the point contact resistance upon increasing temperature. This drop was attributed to the interaction-induced lubrication of the point contact boundaries that was found to be strong enough to prevent momentum relaxation of charge carriers. The viscosity of the electron fluid was measured over a wide range of temperatures and at different carrier densities. Experimental data was found to be in good agreement with many-body calculations. In this work we also studied transport properties of two-dimensional InSe. We observed high electron mobility transport, quantum oscillations and a fully developed quantum Hall effect. In optical studies, we revealed that due to the crystal symmetry a monolayer InSe features suppressed recombination of electron-hole pairs.

The aim of this thesis is to investigate different functionalization strategy of graphene nanomaterials for graphene-based different biotechnological applications such as graphene-directed stem cell growth and differentiation and graphene-based biosensors. Chemical functionalization of graphene is required in many biological applications; in this thesis we have focused on exploiting the carboxylic groups available on GO molecules and non-covalent functionalization of graphene. GO has been a promising material for stem cell culture due to high specific surface area, ease of functionalization, its ability to support cell proliferation and to not cause cytotoxicity when stem cells are cultured on its substrate. The impact of biochemical functionalization on stem cell differentiation was not widely researched, and many research groups worldwide have been focusing on GO and rGO surfaces only. The approach of this thesis is to fabricate and characterize different graphene-based substrates to investigate the impact of biochemical functionalization of GO in directing adipose stem cell differentiation and to influence the gene expression pathways of Schwann-like differentiated adipose stem cells. The fabrication of graphene based biosensors is still challenging as biological molecules need to be attached to graphene-based sensors to increase both the specificity and the selectivity of the biosensors. In this thesis, two different chemical functionalization approaches were considered. Firstly, the covalent immobilization of membrane proteins embedded on a lipid nanodisc structure on GO was achieved. Secondly, the feasibility of using dip-pen nanolithography as a tool to locally functionalize graphene arrays with phospholipids was demonstrated. Phospholipid interface layer can act as bioactive layer which can be used for the protein insertion of tail-anchoring recombinant proteins as a new route for a non-covalent biological functionalization of graphene array.

This thesis focuses on developing support materials for direct methanol fuel cell (DMFC) catalysts. The approach involves using graphene based materials including reduced graphene oxide (rGO), reduced graphene oxide-activated carbon (rGO-AC) hybrid and reduced graphene oxide-silicon carbide (rGO-SiC) hybrid as a support for Pt and Pt-Ru nanoparticles. Pt/rGO and Pt-Ru/rGO catalysts were synthesized by three chemical reduction methods: (1) modified polyol, (2) ethylene glycol (EG) reduction and (3) mixed reducing agents (EG + NaBH4) methods. The synthesized catalysts were characterized by physical and electrochemical techniques. The results demonstrated that Pt/rGO-3 and Pt-Ru/rGO-3 catalyst synthesized with Method-3 exhibit higher electrochemical active surface area (ECSA) than the other rGO supported and Vulcan supported commercial electrocatalysts. In addition, Pt/rGO-3 and Pt-Ru/rGO-3 catalysts showed better oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) activities, respectively. The DMFC tests under different cell temperature (30, 50 and 70°C) and methanol concentration (1, 2 and 4 M) conditions further demonstrated the higher catalytic activity of the catalysts. The peak power density obtained with Pt/rGO-3 cathode and Pt-Ru/rGO-3 anode catalysts at 70°C with 1 M methanol was 63.3 mW/cm2 which is about 59 % higher than that of commercial Pt/C and Pt-Ru/C catalysts. The enhanced performance was attributed to the highly accessible and uniformly dispersed nanoparticles on rGO support with large surface area and high conductivity. Pt/rGO-AC (reduced graphene oxide-activated carbon) and Pt-Ru/rGO-AC catalysts were synthesized with various rGO:AC support ratios by using biomass derived AC. The results showed that the catalysts with content of 20 wt. % AC support (Pt/rGO-AC20 and Pt-Ru/rGO-AC20) exhibited higher ECSA, better catalytic activity and stability among all the tested catalysts. With 1 M methanol and 70°C cell temperature, the MEA with Pt/rGO-AC20 cathode and Pt-Ru/rGO-AC anode catalysts gave 19.3 % higher peak power density (75.5 mW/cm2), than that of Pt/rGO-3 and Pt-Ru/rGO-3 catalysts. The better DMFC performance was due to the incorporation of AC particles into rGO structure which builds electron-conductive paths between rGO sheets, facilitates the transport of reactant and products and provides higher specific surface area for the uniform distribution of nanoparticles. Pt/rGO-SiC catalysts were synthesized with variable silicon carbide (SiC) content in the hybrid support. Pt/rGO-SiC10 (10 wt. % of SiC support) catalyst showed higher ECSA and better catalytic activity compared to the Pt/SiC, Pt/rGO-3 and Pt/rGO-SiC20 catalysts. In addition, the Pt/rGO-SiC10 gave 14.2 % higher DMFC performance than the Pt/rGO-3 catalyst in terms of power density. The high performance can be attributed to the insertion of the SiC nanoparticles into rGO structure that improves the conductivity and stability of the catalyst by playing a spacer role between rGO layers. In summary, the overall results showed that the catalytic performance of the catalysts followed the trend in terms of support material: rGO-AC20 > rGO-SiC10 > rGO > Vulcan. The study demonstrated that the novel rGO-AC and rGO-SiC hybrids are promising catalyst supports for direct methanol fuel cell applications.

Advances in experimental techniques such as nanofabrication, characterization and synthesis have resulted in the development of many novel and interesting materials and devices. Surfaces and interfaces play an indispensible role for nanoelectronics development. ZnO and graphene have drawn tremendous research interests in recent years, due to their exceptional merits in electrical, optical and magnetic applications. This thesis attempts to ferret out the current experimental research progress, particularly, the frontiers of ZnO and graphene based surfaces and interfaces, and employs first principles to explore their electronic structures, to acquire mechanistic understanding of experimental findings, and to shed light on rational design of functional devices. / Finally, the magnetic properties of graphene by organic molecule modification are investigated by first principles method. For the first time, we demonstrate that methoxyphenyl group can introduce a delocalized p-type ferromagnetism into graphene sheet, with the Curie temperature (T c) above room temperature. Each aryl group can totally induce 1 muB into molecule/graphene system. Moreover, an around 1.1 eV direct band gap is introduced into both majority and minority spin bands of graphene by methoxyphenyl group modification. Zigzag graphene nanoribbon (GNR) shows strong site-specific magnetism by aryl group adsorption near the edge. At specific site of GNR, each molecule could totally induce 3∼4 mu B into molecule/GNR hybrid system. / First, we study the controllable modulation of the electronic structures of ZnO(10 1¯ 10) surface functionalized by various types of carboxylic acids. The calculated structural results are consistent with the experimental ones attained by the Fourier transform infrared attenuated total reflectance (FT-IR-ATR). Mercapto-acetic acid molecules are found to contribute an abundance of band gap states into ZnO. Mercapto-acetic monolayer functionalized ZnO (10 1¯ 10) is on the verge of metal-to-insulator transition, which is consistent with the experimental finding of an conductivity increase by 6 orders of magnitude. Mercapto-acetic acid functionalized ZnO (10 1¯ 10) surface shows a strong configuration-dependence for both electronic structure and adsorption energy. Moreover, mercapto-acetic acid molecule functionalized ZnO also shows facet-dependent characteristic in which the monolayer functionalized ZnO (2 1¯ 1¯ 0) does not show metal-to-insulator transition. Acetic acid does not contribute to the band gap states of ZnO (10 1¯ 10), whereas benzoic acid and 9-anthracenecarboxylic acid do contribute an abundance of band gap states to ZnO(10 1¯ 10). / Second, we study the band gap opening of graphene bilayer by F4-TCNQ doping and externally applied electric filed effects. With F4-TCNQ concentration of 8.0x1013 molecules/cm2, the electrostatic charge transfer between each F4-TCNQ molecule and graphene is 0.45 e, and the built-in electric field Ebi between the graphene layers could achieve 0.070 V/A. The charge transfer and band gap opening of the F4-TCNQ doped bilayer graphene can be further modulated by externally applied electric field (Eext ). At 0.077 eV/A, the gap opening at the Dirac point ( K) DeltaEK = 306 meV and the band gap Eg 253 meV are around 71% and 49% larger than those of the pristine bilayer under the same Eext. By combining F4-TCNQ molecular doping and Eext, the p-type semiconductor bilayer graphene are attained, with the band gap and hole concentration varied in a wide range. / These four theoretical sub-topics stem from the experimental advances in ZnO and graphene based surfaces and interfaces. They form the mechanistic understanding of the respective surfaces and interfaces down to the molecular level. / Third, the self-assembly mechanism of PTCDA ultrathin films on graphene with the coverage in a range of 0.3∼3 monolayers (MLs) are interrogated by first principles method. For alpha modification mode, with critical thickness of 1 ML, the growth of PTCDA on graphene follows the Stranski-Krastanov (SK) growth mode. In contrast, for beta modification mode, the PTCDA can form two complete MLs on graphene substrate. From the thermodynamical viewpoint, alpha modification mode is more stable than beta modification mode. At 1 ML, the PTCDA follows a continuous and planar˙ packing arrangement on graphene, which is almost unperturbed by typical defects in graphene substrate. This is in consistentcy with the experimental findings. For alpha modification mode with 2 and 3 ML coverage, the bulk-like phases appear. At the same time, the total charge transfer between PTCDA and graphene per 5✓3x5 super cell at 2 MLs saturates with 0.42e, which is larger than those of 1 or 3 ML coverage. / Tian, Xiaoqing. / Adviser: Jianbin Xu. / Source: Dissertation Abstracts International, Volume: 73-03, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

In this work, we develop a new flexible formalism to calculate anharmonic interatomic interactions from first principles at arbitrary order. Using the recently-developed slave-mode basis, we Taylor-expand the potential with a minimal number of independent coefficients. The anharmonic dynamical tensor, a higher-order generalization of the dynamical matrix in strain+reciprocal space, is calculated via a generalized frozen phonon methodology. We perform high-throughput calculations, emphasizing efficiency with multidimensional finite differences and Hellman-Feynman forces. Applying the methodology to graphene, we show convergence through fifth order terms. Our calculated force constants produce stress-strain curves, bond-length relaxations, and phonon spectra that agree well with those expected within DFT. We show that to fully capture anharmonic effects, long-range interactions must be included.

The acoustoelectric effect in graphene and graphene nanoribbons (GNRs) on lithium niobate surface acoustic wave (SAW) devices was studied experimentally. Monolayer graphene produced by chemical vapour deposition was transferred to the SAW devices. The photoresponse of the acoustoelectric current (Iae) was characterised as a function of SAW frequency and intensity, and illumination wavelength (using 450 nm and 735 nm LEDs) and intensity. Under illumination, the measured Iae increased by more than the measured decrease in conductivity, while retaining a linear dependence on SAW intensity. The latter is consistent with the piezoelectric interaction between the graphene charge carriers and the SAWs being described by a relatively simple classical relaxation model. A larger increase in Iae under an illumination wavelength of 450 nm, compared to 735 nm at the same intensity, is consistent with the generation of a hot carrier distribution. The same classical relaxation model was found to describe Iae generated in arrays of 500 nm-wide GNRs. The measured acoustoelectric current decreases as the nanoribbon width increases, as studied for GNRs with widths in the range 200 – 600 nm. This reflects an increase in charge carrier mobility due to increased doping, arising from damage induced at the nanoribbon edges during fabrication. 2 Lastly, the acoustoelectric photoresponse was studied as a function of graphene nanoribbon width (350 – 600 nm) under an illumination wavelength of 450 nm. Under illumination, the nanoribbon conductivity decreased, with the largest percentage decrease seen in the widest GNRs. Iae also decreased under illumination, in contrast to the acoustoelectric photoresponse of continuous graphene. A possible explanation is that hot carrier effects under illumination lead to a greater decrease in charge carrier mobility than the increase in acoustoelectric attenuation coefficient. This causes the measured decrease in Iae.

No description available.

Spintronics reaches beyond typical charge-based information storage technologies by utilizing an addressable degree of freedom for electron manipulation, the electron spin polarization. With mounting experimental data and improved theoretical understanding of spin manipulation, spintronics has become a potential alternative to charge-based technologies. However, for a long time, spintronics was not thought to be feasible without the ability to electrostatically control spin conductance at room temperature. Only recently, graphene, a 2D honeycomb crystalline allotrope of carbon only one atom thick, was identified because of its predicted, long spin coherence length and experimentally realized electrostatic gate tunability. However, there exist several challenges with graphene spintronics implementation including weak spin-orbit coupling that provides excellent spin transfer yet prevents charge to spin current conversion, and a conductivity mismatch due to the large difference in carrier density between graphene and a ferromagnet (FM) that must be mitigated by use of a tunnel barrier contact. Additionally, the usage of graphene produced via CVD methods amenable to semiconductor industry in conjunction with graphene spin valve fabrication must be explored in order to promote implementation of graphene-based spintronics. Despite advances in the area of graphene-based spintronics, there is a lack of understanding regarding the coupling of industry-amenable techniques for both graphene synthesis and lateral spin valve fabrication. In order to make any impact on the application of graphene spintronics in industry, it is critical to demonstrate wafer-scale graphene spin devices enabled by wafer-scale graphene synthesis, which utilizes thin film, wafer-supported CVD growth methods. In this work, high-quality graphene was synthesized using a vertical cold-wall furnace and catalyst confinement on both SiO2/Si and C-plane sapphire wafers and the implementation of the as-grown graphene for fabrication of graphene-based non-local spin valves was examined. Optimized CVD graphene was demonstrated to have ID/G ≈ 0.04 and I2D/G ≈ 2.3 across a 2" diameter graphene film with excellent continuity and uniformity. Since high-quality, large-area, and continuous CVD graphene was grown, it enabled the fabrication of large device arrays with 40 individually addressable non-local spin valves exhibiting 83% yield. Using these arrays, the effects of channel width and length, ferromagnetic-tunnel barrier width, tunnel barrier thickness, and level of oxidation for Ti-based tunnel barrier contacts were elucidated. Non-local, in-plane magnetic sweeps resulted in high signal-to-noise ratios with measured ΔRNL across the as-fabricated arrays as high as 12 Ω with channel lengths up to 2 µm. In addition to in-plane magnetic field spin signal values, vertical magnetic field precession Hanle effect measurements were conducted. From this, spin transport properties were extracted including: spin polarization efficiency, coherence lifetime, and coherence distance. The evaluation of industry-amenable production methods of both high-quality graphene and lateral graphene non-local spin valves are the first steps toward promoting the feasibility of graphene as a lateral spin transport interconnect material in future spintronics applications. By addressing issues using a holistic approach, from graphene synthesis to spin transport implementation, it is possible to begin assessment of the challenges involved for graphene spintronics.

The work presented in this dissertation attempts to form an understanding of the importance of polymer connectivity and nanoparticle shape and curvature on the formation of non-covalent interactions between polymer and nanoparticles by monitoring the dispersion of nanoparticles in copolymers containing functionalities that can form non-covalent interactions with carbon nanoparticles. The first portion of this study is to gain a fundamental understanding of the role of electron donating/withdrawing moieties on the dispersion of the fullerenes in copolymers. UV- Vis spectroscopy and x-ray diffraction were used to quantify the miscibility limit of C60 fullerene with the incorporation of electron donor-acceptor interactions (EDA) between the polymer and fullerene. The miscibility and dispersion of the nanoparticles in a polymer matrix are interpreted to indicate the extent of intermolecular interactions, in this case non-covalent EDA interactions. Experimental data indicate that the presence of a minority of interacting functional groups within the polymer chains leads to an optimum interaction between polymer and fullerene. This is further affirmed by density functional theory (DFT) calculations that specify the binding energy between interacting monomers and fullerenes. The second portion focuses on the impact of sample preparation on the dispersion of graphene nanocomposites. Visualization and transparency are used to quantify the dispersion of graphene in the polymer matrix. In addition, differential scanning Calorimetry (DSC) also provides insight into the efficiency of the preparation process in forming a homogeneous sample, where rapid precipitation and solvent evaporation are studied. Examining the change in glass transition temperature, Tg, with nanoparticle addition also provides insight into the level of interaction and dispersion in the graphene nanocomposites. The approach of utilizing non-covalent interactions to enhance the dispersion of polymer nanocomposites is realized by varying the functional group in the copolymer chains, while the impact of nanoparticle shape is also examined. The optimum enhancement of dispersion is interpreted in terms of the improvement of interaction between polymer and nanocomposites. This interpretation leads to the conclusion that chain connectivity and the ability of the polymer to conform to the nanoparticle shape are two important factors that govern the formation of non-covalent interactions in polymer nanocomposites.

With experimentation on graphene (an atomic layer of graphite) becoming more and more common it is imperative that we have the capability to shape the material beyond the random manner in which it is deposited by mechanical exfoliation. This capability would be invaluable not only for the interesting electronic and optical properties that can be obtained, but also potentially for characterizing the mechanical resonators that we have been able to fabricate here at Pomona College by suspending few-layer graphene sheets over trenches in SiO2. We propose novel methods for etching graphene that should allow us to shape the material when used in conjunction with our e-beam lithography capabilities.