This thesis presents theoretical studies of the optical and electronic properties of defects in diamond and of the mechanisms of doping graphene. The birefringence of the four petalled defect commonly observed in CVD diamond is explained by four linear arrays of dislocations along ⟨110⟩ directions with ⟨110⟩ Burgers vectors. Such an arrangement of dislocations reproduces the extension and the features of the birefringence patterns observed experimentally. Density functional theory via the AIMPRO code was used to study the electronic and optical properties of different nitrogen-related point defects in diamond. It was found that the zero-phonon luminescence line of the NV− defects can split in the presence of a surface or other NV− defects. Since VNH and VN2 are expected to have similar optical properties, the optical transi- tions for VN2 were used to correct the transitions for VNH calculated by local density approximation. The absorption band at 2.38 eV (520 nm) observed in CVD diamond is then attributed to an internal transition of VNH. The weak zero-phonon line and broad vibronic sidebands for VN− and VN−2 and its absence for VNH− is explained by the large structural change when the defect is excited. Finally, different mechanisms for doping graphene were considered. The calculations predict the electropositive metals, such as Ti and Cr, act as donors, while molecules with strong electron affinity, such as F4-TCNQ, act as acceptors in graphene. An unexpected mechanism of doping graphene was shown by Au which dopes bilayer graphene but not single layer. In the presence of water, electrochemical reactions on the graphene can also lead to p or n-type doping.

Graphene is a two-dimensional sp2 hybridized carbon lattice that is also the fundamental building block of graphite. Graphene has attracted significant interest recently due to its distinctive electrical, optical and mechanical properties. These properties have spurred research directed at modifying graphene for use in a variety of electronic, optoelectronic, and sensor technologies. However, before graphene can be used in products, it is necessary to find methods to tune, modify, grow and integrate graphene features while substantially boosting device performance and maintaining current processing compatibility and ease of integration with existing manufacturing infrastructure. This dissertation focuses on developing techniques for controllably doping the graphene layer through scalable, industry friendly and simple chemical doping; using self-assembled monolayer compounds, photo-acid and photo-base generators, polymers and metal-organic species. We have, in fact, demonstrated simple p-n junctions fabricated in this manner. Characteristic I-V curves indicate the superposition of two separate Dirac points from the p and n regions, confirming an energy separation of neutrality points within the complementary regions; Raman studies of these methods have shown that these processes result in extremely low defect levels in the graphene. Our simple methods for producing patterned doping profiles in graphene films and devices open up a variety of new possibilities for forming complex doping profiles in a simple manner in graphene. This work can enable rapid testing, such as controlled work function tuning, complex doping profiles and simple post-fabrication tuning, of concepts for graphene that may be useful in both interconnect and transparent conductor applications. In addition to graphene doping, we also investigated approaches to the synthesis of few-layer graphene flakes, since current techniques still produce inferior materials. Exfoliation of Graphene Sheets by an Electron Donor Surfactant was demonstrated to generate few-layers graphene flakes that rival the electrical quality of reduce graphene-oxide (rGO) flakes. Last but not least, Diels-Alder adducts on silica were explored as a controllable carbon precursor for pristine graphene; these allow for a rational direct-growth-of-graphene-on-surface reaction mediated by copper catalyst, without the use of flammable precursors, such as methane, that are used in current methods of chemical vapor deposition synthesis of graphene.

Graphene – a two-dimensional honeycomb lattice of sp2-bonded carbon atoms – has been attracting a great deal of research interest since its first experimental realization in 2004, due to its various novel properties and its potential for applications in futuristic nanodevices. Being the fundamental building block for carbon allotropes of other dimensionality, it can be stacked to form 3d graphite or rolled into 1d nanotube. Graphene is the thinnest known material in the universe, and one of the strongest materials ever measured in terms of its in-plane Young modulus and elastic stiffness. The charge carriers in graphene exhibit giant mobility as high as 20 m2/Vs, have almost zero effective mass, and can travel for micrometers without scattering even at ambient conditions. Graphene can sustain current densities six orders of magnitude higher than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and renders easy accessibility to optical probes. Electron transport in graphene is described by a Dirac-type equation, which allows the investigation of “relativistic” quantum phenomena in a benchtop experiment. This results in the observation of a number of very peculiar electronic properties from an anomalous quantum Hall effect to Kien paradox and the absence of localization. All these enticing features make this material an excellent candidate for application in various electronic, photonic and optoelectronic devices. For instance, its ballistic ambipolar transport and high carrier mobility are the most useful traits for making ultrafast and low-power electronic devices. Its high surface area shouldmake it handy in manufacturing tough composite materials. The extreme thinness of graphene could also lead to more efficient field emitters that release electrons in the presence of strong electric fields. Its robustness and light weight are useful for micromechnical resonators. The tunability of its properties could make it possible to build so-called spin-valve transistors, as well as ultra-sensitive chemical detectors. Many of such applications of graphene require tuning of its properties, which can be achieved by varying the number of layers or/and by doping. There are several ways to dope graphene: (i)electrochemically gated doping, (ii)molecular charge-transfer doping, and (iii) substitutional doping by atoms like Boron or Nitrogen.Moreover, for graphene, a zero band gap semiconductor in its pristine form, to become a versatile electronic device material it is mandatory to find means to open up a band gap and tune the size of the band gap. Several strategies have been adopted to engineer such a band gap in graphene in a controlled way. Some of these are based on the ability to control the geometry of graphene layers, some use graphene-substrate interactions, while others are based on chemical reactions of atoms or molecules with the graphene layer. Motivated by these considerations, in this thesis we present a systematic and thorough study of the structural, electronic and vibrational properties of graphene and their dependence on the number of layers, and on doping achieved electrochemically, molecularly and substitutionally, using first principles density functional theory (DFT). In Chapter 1, we give an introduction to the hitherto beguiling world of graphene. Here, we briefly discuss the structure, novel properties and potential applications of graphene, and the motivation for this thesis. In Chapter 2, an overview of the DFT formalism adopted here is given. We clearly state the theorems of the formalism and the approximations used when performing calculations. We succinctly explain how the various quantities like total energies, forces, stresses etcetera are calculated within this formalism. We also discuss how phonon frequencies, eigenvectors, electron-phonon couplings are obtained by using density functional perturbation theory (DFPT), which calculates the full dynamical matrices through the linear response of electrons to static perturbations induced by ionic displacements. Calculations are done first using a fully ab-initio approach within the standard Born-Oppenheimer approximation, and then time-dependent perturbation theory is used to explore the effects of dynamic response. In Chapter 3, using such first-principles density-functional theory calculations, we determine the vibrational properties of ultra-thin n(1,2,...,7)-layer graphene films and present a detailed analysis of their zone-center phonons. We present the results (including structural relaxations, phonons, mode symmetries, optical activities) for bulk Graphite, single-layer graphene and ultrathin n-layer graphene films. and discuss the underlying physics of our main results together with a pictorial representation of the phonon modes. We demonstrate that a low-frequency (∼ 112 cm−1 ) optical phonon with out-of-plane displacements exhibits a particularly large sensitivity to the number of layers, although no discernible change in the interlayer spacing is found as n varies. Frequency shifts of the optical phonons in bilayer graphene are also calculated as a function of its interlayer separation and interpreted in terms of the inter-planar interaction. The surface vibrational properties of n-layer graphene films are presented in Chapter 4, which renders a detailed and thorough analysis of all the surface phonon modes by determining, classifying and identifying them accurately. The response of surface modes to the presence of adsorbed hydrogen molecules is determined. As an illustrative adsorbate, hydrogen is chosen here mainly because of its huge importance in fuel cell technology and as a molecular sensor. We demonstrate that a doubly degenerate surface phonon mode with low-frequency (~ 35cm−1)exhibits a particularly large sensitivity to the adsorption of hydrogen molecules, as compared to other surface modes. Futhermore, we show that a low-frequency (108.8 cm−1)bulk-like phonon with out-of-plane displacements is also very sensitive and gets upshifted by as much as 21 cm−1 due to this adsorption. In Chapter 5, we determine the adiabatic frequency shift of the and phonons in a monolayer graphene as a function of both electron and hole doping. The doping is simulated here to correspond to electrochemically gated graphene. Compared to the results for the E2g -Γ phonon (Raman G band), the results for the phonon are dramatically different, while those for the phonon are not so different. Furthermore, we calculate the frequency shifts, as a function of the charge doping, of the (K + ΔK) phonons responsible for the Raman 2D band –a key finger print of graphene, where [ΔK] is determined by the double resonance Raman process. Doping graphene with electron donating or accepting molecules is an interesting approach to introduce carriers into it, analogous to electrochemical doping accomplished in graphene when used in a field-effect transistor. In Chapter 6, we use first-principles density-functional theory to determine changes in the electronic structure and vibrational properties of graphene that arise from the adsorption of aromatic molecules such as aniline and nitrobenzene. Identifying the roles of various mechanisms of chemical interaction between graphene and the adsorbed molecules, we bring out the contrast between electrochemical and molecular doping of graphene. Our estimates of various contributions to shifts in the Raman active modes of graphene with molecular doping are fundamental to the possible use of Raman spectroscopy in (a)characterization of the nature and concentration of carriers in graphene arising from molecular doping, and (b) graphene-based chemical sensors. Graphene doped electrochemically or through charge-transfer with electron-donor and acceptor molecules, shows marked changes in electronic structure, with characteristic signatures in the Raman spectra. Substitutional doping, universally used in tuning properties of semiconductors, could also be a powerful tool to control the electronic properties of graphene. In Chapter 7, we present the structure and properties of boron and nitrogen doped graphenes, again using first-principles density functional theory. We demonstrate systematic changes in the carrier-concentration and electronic structure of graphenes with B/N-doping, accompanied by a stiffening of the G-band and change of the defect related D-band in the Raman spectra. Such n/p -type graphenes obtained without external fields or chemical agents should find device applications.

This doctoral thesis is focused on the preparation of graphene layers by Chemical Vapor Deposition (CVD) and their utilization for fabrication and characterization of field effect transistors. The theoretical part of the thesis deals with different methods of graphene production and measurement of its transport properties. In the first part of the experimental section the growth of polycrystalline graphene and individual graphene crystals with sizes up to 300 m is investigated. Further, graphene layer was also grown on an atomically flat copper foils, which were fabricated in order to achieve the growth of graphene of higher quality. Subsequently, the transport properties of field effect transistors produced from the grown layers were measured. The last two chapters deal with a doping of graphene layer by gallium atoms and X-ray radiation. Whereas the deposition of gallium atoms on the graphene surface causes chemical doping of graphene layer by charge transfer, X-ray irradiation of graphene field effect transistors induces the ionization of positively charged defects in dielectrics, which electrostatically dope a graphene layer.

Electronic systems are an indivisible part of modern life. Every day, new materials, devices, passive components, antennas for wireless communication are needed to be designed and developed. In particular, flexible and biocompatible wearable devices are urgent required for medical and industrial applications. The great hope lies in the materials with high crystalline quality and flexibility such as graphene and other 2D semiconductors and insulators. Doping is a conventional tool for tailoring of the electronic properties of the functional materials. Here we examine application of the widely used the electron donor species to the graphene and hexagonal boron nitride monolayer (h-BN). For each we determine surface-interface properties and the full electronic band structure using the combination of the surface science methods such as angle-integrated and angle resolved photoemission (XPS, ARPES), electron diffraction (LEED) and photo absorption (XAS). As the result we provided insight into mechanisms underlying the doping gating of the graphene h-BN monolayer by the alkali metals. We fully characterized their surface and interface structure. Finally we studied the interplay between electrons and phonons in the doped graphene and we demonstrated that Ca-doped graphene is the promising candidate for realizing superconductivity in graphene.

Als Graphen bezeichnet man eine einzelne freistehende Lage des Schichtkristalls Graphit. Im Gegensatz zur mechanischen Isolation von Graphit bietet die Züchtung auf Siliziumkarbid eine Methode zur großflächigen Herstellung von Graphen. Aufgrund der besonderen physikalischen Eigenschaften werden für Graphen viele verschieden Einsatzmöglichkeiten in diversen Bereichen prognostiziert. Mit seiner hohen Ladungsträgerbeweglichkeit ist Graphen besonders als Kanalmaterial für Feldeffekttransistoren (FET) interessant. Allerdings muss hierfür unter anderem ein geeignetes FET-Isolatormaterial gefunden werden. In dieser Arbeit wird eine detaillierte, theoretische Beschreibung der Graphen-FETs vorgestellt, die es erlaubt die steuerspannungsabhängige Hall-Konstante zu berechnen. Mit der dadurch möglichen Analyse können wichtige Kenngrößen, wie z. B. die Grenzflächenzustandsdichte des Materialsystems bestimmt werden. Außerdem wurden zwei Methoden zur Isolatorabscheidung auf Graphen untersucht. Siliziumnitrid, welches mittels plasmaangeregter Gasphasenabscheidung aufgetragen wurde, zeichnet sich durch seine n-dotierende Eigenschaft aus. Damit ist es vor allem für quasi-freistehendes Graphen auf Siliziumkarbid interessant. Bei der zweiten Methode handelt es sich um einen atomaren Schichtabscheidungsprozess, der ohne eine Saatschicht auskommt. An beiden Graphen- Isolator-Kombinationen wurde die neue Charakterisierung mittels der Hall-Datenanalyse angewandt.:1 Einleitung 2 Graphen 3 Methoden 4 Die Hall-Konstante von Graphen 5 Siliziumnitrid als Dielektrikum für Graphentransistoren 6 Aluminiumoxid auf epitaktischem Graphen 7 Zusammenfassung A Anhang