Graphene became a popular material as it has been prepared in University of Manchester since 2004. In our job, we heat SiC in order to decompose silicon on the Si-surface of SiC. We used LEED and AUGER technique in order to check the quality of graphene sample. First we observed the (6root3 x 6root3)R30 reconstruction on the surface of our sample and then we found Carbon peak in AUGER spectrum. After that, we measure R-T and Hall effect by PPMS to do the further research of the electronic transport in graphene/SiC system, in this part of work we tried to analyse the R-T result by hopping theory and calculate Hall mobility and carrier concentration from Hall effect result. In the conclusion we found the active energy is totally different while the process of preparation is changed.

This thesis discusses correlation effects in graphene and bilayer graphene. The discovery of graphene was awarded 2010 Nobel Prize in Physics. Graphene is one of the most intriguing topics around the world. Its flexibilities make it a very promising material in device physics. From theoretical point of view, graphene connects condensed matter physics to quantum field theory, and is an excellent candidate for model studies. Furthermore, it stimulates researches in low-dimensional electron systems. Bilayer graphene is an interesting variant of graphene, and is one of the new directions in developing low-dimensional systems. Due to honeycomb lattice symmetry, the low-energy effective Hamiltonian of a graphene is described by gapless Dirac equation a(_σ^→).p round K(K^1) point. In this thesis, symmetry of Dirac equation is reviewed. In graphene, there are four copies of gapless Dirac equations. In addition, spin-orbit couplings are also discussed by using point-group techniques. We calculate screening and collective modes by using lattice Green's functions within random phase approximations. Some typical models on honeycomb lattice are reviewed, including Haldane model and Kane-Mele model. Interaction effects are further discussed within the Hubbard and extended models. It is reported there are some interesting phases both in doped and undoped cases. Graphene ribbons are also discussed in this thesis: zigzag ribbons and armchair ones. We investigate the attractive-U Kane-Mele-Hubbard model by using a mean-field theory, and find strong superconducting correlations along the edge, analogous to edge magnetism in positive U case. We investigate mesoscopic spin Hall effect on the surface of a three-dimensional topological insulator using McMillan Green's function techniques, and discuss the robustness of edge states and stabilities against interactions in topological insulator. Bilayer graphene is also investigated. Our study follows the recent experiments and theoretical proposals. As suggested by previous works, quantum spin Hall state and layer antiferromagnetic state are two most possible candidates of the ground state. We propose by tiny doping, a half-metallic state can be realized based on layer antiferromagnetic state. The responses to in-plane and perpendicular magnetic fields are also reviewed. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

The strategic importance of microelectronics is reflected in its ubiquity in the global production network and in our daily lives. Above all, the microelectronics revolution has been enabled and driven by the scalability of the silicon transistor and the computational efficiency of its CMOS architecture. While the semiconductor industry has been incredibly adept at pushing the boundaries of scaling in the last few decades, many factors suggest that silicon technology is running into scientific and practical limitations to further scaling. Thus, the push for a beyond-silicon computing platform is imperative. Akin to the transition from bipolar to MOSFET technology or from NMOS to CMOS architecture, the industry is once again looking for the next disruptive technology to continue the exponential growth of computing power. In 2004, two research groups, one from the University of Manchester and the other from Georgia Tech, reported on the electrical properties of ultrathin graphite. Their findings demonstrated the stability of graphene, an atomic layer of graphite, as well as its superb carrier mobility, spurring the semiconductor industry to invest in the material as a candidate for a beyond-silicon computing platform. Within this framework, this thesis explores the promise of graphene as a material and technological platform for electronic devices. The objectives of the thesis are (i) to elucidate opportunities and challenges in the design and fabrication of graphene field-effect devices, and (ii) to advance a new device platform based on graphene p-n junctions.

No description available.

Efficient Transfer of Graphene –Physical and Electrical Performance Perspective Graphene has become one of the most widely used atomic crystal structure materials since its first experimental proof by Geim-Novoselov in 2004 [1]. This is attributed to its reported incredible carrier mobility, mechanical strength and thermal conductivity [2] [3] [4]. These properties suggest interesting applications of Graphene ranging from electronics to energy storage and conversion [5]. In 2008, Chen et al reported a 40,000 cm2V-1s-1 mobility for a Single Layer Graphene (SLG) on SiO2 compared to 285 cm2V-1s-1 for silicon channel devices [6]. Chemical vapor deposition (CVD) is a common method for growing graphene on a metal surface as a catalyst for graphene nucleation. This adds a necessary transfer step to the target substrate ultimately desired for graphene devices fabrication. Interfacing with graphene is a critical challenge in preserving its promising high mobility. This initiated the motivation for studying the effect of intermediate interfaces imposed by transfer processes. In this work, few layers graphene (FLG) was grown on copper foils inside a high temperature furnace. Then Raman spectroscopy was performed on grown graphene sample to confirm few (in between 3-10) layers. Afterwards the sample was cut into three pieces and transferred to 300 nm SiO2 on Si substrates using three techniques, namely: (i) pickup transfer with top side of Graphene brought in contact with SiO2 [7], (ii) Ploy (methyl methacrylate) (PMMA) transfer with Graphene and a PMMA support layer on top scooped from bottom side [8], and (iii) a modified direct transfer which is similar to PMMA transfer without the support layer [9]. Comparisons were done using Raman spectroscopy to determine the relative defectivity, Scanning Electron Microscopy (SEM) to observe discontinuities and Atomic Force Microscopy (AFM) to measure surface roughness. Then we conclude with electrical data based on the contact resistivity measured for layers transferred using these different techniques. Contacts were deposited using e-beam thermal evaporation and contact resistivity was calculated using Transmission Line Method (TLM) [10]. To date no comparative analysis for the aforementioned transfer methods has been done to determine which is the most efficient [5]. Our contributions are: (i) determining the most efficient method, (ii) reporting a lift-off process for Graphene, (ii) and reporting lower specific contact resistivity for no-post transfer processing Graphene.

No description available.

Thesis (M.S.)--Oregon State University, 2011. / Printout. Includes bibliographical references (leaves 95-102). Also available on the World Wide Web.

In this work we have investigated supercapacitor electrodes prepared from pure and 1-pyrenecarboxylic acid (PCA)-functionalized graphene flakes obtained from liquid phase chemical exfoliation method. The performances of the supercapacitor devices fabricated using the graphene electrodes were tested using cyclic voltammetry, constant current charging-discharging and by electrochemical impedance spectroscopy (EIS) The specific capacitances obtained (using 6M KOH aqueous solution as an electrolyte) were found to be ~ 30 F/g and ~ 200 F/g for pure graphene and PCA functionalized graphene electrodes respectively. A comprehensive understanding of the effect of surface fictionalization on the electrochemical double layer capacitance was obtained in the light of equivalent circuit modeling and EIS data analysis. Information obtained from the EIS spectrum analysis revealed the possibility of occurrence of pseudocapacitance due to the presence of surface functional groups on the graphene flakes. Further, the wettability by KOH significantly increases upon functionalizing the graphene surfaces. These results shows PCA functionalized graphene membrane electrodes have the potential for high performance as supercapacitor electrode material.

Graphene is a promising material for chemical sensing applications and many studies have focused on incorporating graphene into \sgfet s sensors. The purpose of this work is to get a deeper understanding of the physics governing the surface interaction of graphene in \sgfet s with ions and charged molecules. With a clearer understanding of how these interactions register in the conductivity of graphene, it then may be possible to design the ultrasensitive sensors that are often predicted to be possible when using graphene. Epitaxial graphene (EG) and graphene produced by chemical vapor deposition (CVD) were used to fabricate \sgfet s that were tested under different ionic strength conditions and concentrations of charged proteins. To get a clearer picture of the electrostatic gating effect in ionic solutions, we analyzed our data combining two models: the electrical double layer model, which accounts for the distribution of ions inside the solution, and a ionization model that accounts for ionizable groups on the graphene surface. This gave us an insight into the influence of charged groups fixed to the surface on the gating effect which is fundamental to the performance of \sgfet s as sensors. Using our experimental data we were also able to estimate the density of charged impurities in two carrier density regimes. For high densities, we found a correlation between our estimated impurities and the surface charge that suggests that the ionizable groups act as impurities. For small carrier densities, we modeled the carriers using a self-consistent approximation (SCA). The impurities estimated from the SCA model do not seem to be related to the ionizable groups and so the origin of the conductivity for small density seems to be originated by the permanently charged impurities only. Our estimation of the charged impurities for our charged-protein adsorption experiments showed a relation between their values and the protein concentration. This shows that the proteins interact with the graphene as charged impurities. Overall, our experiments allowed us to gain a deeper understanding of the interaction of charged particles with graphene. The analysis performed in this work gives a guide for the development of graphene \sgfet s sensors by engineering the impurities at the surface to optimize the sensitivity. The design of receptors for specific sensing that do not require charged targets is possible with engineering the charge that the receptor presents to graphene when the analyte concentration changes.

The experimental discovery of graphene in 2004 has opened up a new research field in the direction of atomically thin two-dimensional layered materials for exploration of many fundamental research problems and technological applications. The charge carriers in graphene are massless Dirac fermions due to which they exhibit absence of localization, thereby giving rise to huge intrinsic mobilities and ballistic transport even at room temperatures. But it was observed that the extrinsic disorder and intrinsic structural disorder can significantly influence the transport in graphene films. This thesis focuses on three different aspects of graphene -disorder, magnetism and proximity-induced superconductivity. We have reported conductance fluctuations-based transport studies to investigate these aspects as they provide more detailed information than what can be obtained from the standard transport measurements. Even though these conductivity fluctuations pose a serious bottleneck for various applications, they can also yield useful insights into the various scattering mechanisms and the symmetry properties of graphene. In the first half of the thesis, we describe the measurement of low frequency 1/f noise in large area polycrystalline graphene films to understand the role of grain boundaries in charge carrier transmission in graphene. TEM studies on the low and high angled GBs formed in these graphene samples showed that they form distinct disordered regions of varying widths depending on the tilt angle of the GBs. At low temperatures, the 1/f noise measurements indicated spontaneous breaking of time reversal symmetry across graphene grain boundaries which suggests the magnetic nature of these grain boundaries. In the second half of the thesis, we will concentrate on the universal conductance fluctuations (UCF) in graphene which is the manifestation of quantum interference phenomena at low temperatures. We find that the absolute magnitude of the UCF is directly related to various symmetry-breaking disorder present in graphene. We also discuss how the UCF can be used to study the nature of proximity-induced superconducting correlations in graphene. In the end, we have proposed new device schemes for the integration of ferromagnetic and superconducting materials with graphene.