Chapter 1 describes work towards the synthesis of graphene nanoribbons with varying widths and edge structures. Interest in graphene comes from the high electron mobility at room temperature, exceptional thermal conductivity, and superior mechanical properties.¹ These properties enable graphene’s use in numerous applications such as transparent conducting electrodes, gas detection, transistors, energy storage devices, and polymer composites.¹ Density functional theory has predicted that the electronic properties of GNRs differ with changes in length, width, and differences in edge structure.⁵ First polyacetylene ladder polymers were developed as intermediates for nanoribbons with zig-zag edge structures. Experiments have shown evidence for polyacetylene structures within the material although conversion is too low to be used as a precursor for graphene nanoribbons. Next tetraethynylethene monomers were synthesized to study their use as monomers for Bergman polymerization in hopes of producing armchair edged nanoribbons. Polymers were made with both alkyl and carboxylic acid functionality. ortho-Acylphenols are useful reagents in the synthesis of many natural products, pharmaceuticals, agrichemicals, flavors, and fragrances²⁷,²⁸. For this reason, ketone directed hydroxylation of arenes catalyzed by Pd was developed by Dong and coworkers. During this work it was discovered that 1-acetylnaphthalene would polymerize under the reaction conditions. Chapter 2 describes the author’s efforts to understand the polymerization mechanism through the synthesis of a variety of substituted acetylnaphthalene derivatives and their polymerization. / text

I measured the transport properties of graphene nanoribbons as a function of the charged impurity density in ultra-high vacuum to directly probe the effect of dimensional confinement. Coulomb impurities create charge puddles in graphene sheets, which dominate transport properties. My results shed light on recent predictions about the fundamental mechanisms behind gaps in conductance that appear in transport measurements.

The graphitic systems have attracted intensive attention recently due to the discovery of graphene, a single layer of graphite. The low-energy band structure of graphene exhibits an unusual linear dispersion relation which hosts massless Dirac fermions and leads to intriguing electronic and optical properties. In particular, due to the high mobility and tunability, graphene and graphitic materials have been recognized as promising candidates for future nanoelectronics and optoelectronics. Electron-phonon coupling (EPC) plays a significant role in electronic and optoelectronic devices. Therefore, it is crucial to understand EPC in graphitic materials and then manipulate it to achieve better device performance. In the first part of this thesis, we explore EPC between Dirac-like fermions and infrared active phonons in graphite via infrared magneto-spectroscopy. We demonstrate that the EPC can be tuned by varying the magnetic field. The second part of this thesis deals with magnetoplasmons in quasineutral graphene nanoribbons. Multilayer epitaxial graphene grown on the carbon terminated silicon carbide surface behaves like single layer graphene. Plasmons are excited in the nanoribbons of undoped multilayer epitaxial graphene. In a magnetic field, the cyclotron resonance can couple with the plasmon resonance forming the so-called upperhybrid mode. This mode exhibits a distinct dispersion relation, radically different from that expected for conventional two dimensional systems.

Graphene nanostructures directly grown on SiC are appealing for their potential application to nano-scale electronic devices. In particular, epitaxial sidewall graphene nanoribbons have been a promising candidate in ballistic transport and band gap engineering. In this thesis, we study graphene nanoribbons by utilizing both nano-lithography and natural step bunching to control the step morphology of the SiC(0001) surface in order to guide the growth of graphene which initiates at step edges, and study their respective characteristics. With scanning tunneling microscopy and spectroscopy (STM/STS), we explore the local atomic and electronic structures of the graphene nanoribbons down to atomic scale. It is found that nanoribbon formation depends critically on nanofacet orientation, nanofacet density, and growth conditions. Under some conditions, nanoribbons grow predominantly on the nanofacet. Significant electronic density-of-states features, resolved by STS, are found to depend strongly on proximity to strained graphene near the step edge. Experimental results are compared to Molecular Dynamics simulations to better understand the origin of the discrete electronic states.

Phonon properties of AB (Bernal) stacked bilayer graphene (BLG) with various types of defects have been investigated theoretically. Forced Vibrational (FV) method has been employed to compute the phonon modes of disordered BLG. A downward linear shift of E2g mode frequencies has been observed with an increasing amount of defect concentration. Moreover, two identical E2g peaks have been observed in PDOS of the bilayer system where the individual layer contains 12C and 13C atoms respectively. From computed typical mode patterns of in-plane K-point optical mode phonons, it has been noticed that phonons become strongly localized around a few nanometers area at the presence of defects and localized modes increase with the increasing amount of defect concentration. The edge effect on the localized phonon modes has also been discussed for bilayer armchair graphene nanoribbons (BiAGNRs). The impact of defects on the phonon conduction properties has also been studied for BiAGNRs. My investigated results can be convenient to study the thermal conductivity and electron-phonon interaction of bilayer graphene-based nanodevices and to interpret the Raman and infrared spectra of disordered system.

No description available.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第22709号 / エネ博第406号 / 新制||エネ||78(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 坂口 浩司, 教授 松田 一成, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DFAM

2004年石墨烯的發現是二維（2D）材料發展的關鍵性時刻。近年來，由於從2D材料出現的新性質和應用，許多非石墨烯層狀材料也成為重要的研究課題。 在本論文中，我們利用密度泛函理論（DFT）進行了對二硫化鉬奈米帶與其加上各種原子鏈的混合結構做了各種研究如結構，電子性質，能帶隙，局部電子態密度（LDOS）和磁化的性質。從我們的研究發現，二硫化鉬與不同的原子鏈混合時會改變原本的半導體性質,而有半金屬和導體的性質出現. / A new area of two-dimensional (2D) materials started in 2004, when graphene was successfully isolated from graphite. In recent years, there has been lots of research topic focusing on other(non-graphene) layered materials due to the new properties and applications that were found in 2D confinement. Within this thesis, an ab-initio study of MoS2 nanoribbon with a wide variety of atomic chains deposited on it is performed by utilizing the framework of density functional theory(DFT). Properties like the structural, band gaps, electronic properties, local electronic density of states (LDOS) and magnetization are determined. We have found that depositing atomic chains,the band gap of MoS2 nanoribbons can be engineered, changing the initially semiconductor ribbon into half metallic and conductors.

No description available.

Graphene can be used to create circuits that are almost superconducting, potentially speeding electronic components by as much as 1000 times [1]. Such blazing speed might also help produce ever-tinier computing devices with more power than your clunky laptop [2]. Graphite is a polymorph of the element carbon [3]. Graphite is made up of tiny sheets of graphene. Graphene sheets stack to form graphite with an interplanar spacing of 0.335 nm, which means that a stack of 3 million sheets would be only one millimeter thick. [1] This nano scale 2 dimensional sheet is graphene. Novoselov and Geim's discovery is now the stuff of scientific legend, with the two men being awarded the Nobel Prize in 2010 [4]. In 2004, two Russian-born scientists at the University of Manchester stuck Scotch tape to a chunk of graphite, then repeatedly peeled it back until they had the tiniest layer possible [2]. Graphene has exploded on the scene over the past couple of years. "Six years ago, it didn't exist at all, and next year we know that Samsung is planning to release their first mobile-phone screens made of graphene." - Dr Kostya Novoselov [4]. It is a lattice of hexagons, each vertex tipped with a carbon atom. At the molecular level, it looks like chicken wire [4]. There are two common lattice formations of graphene, armchair and zigzag. The most studied edges, zigzag and armchair, have drastically different electronic properties. Zigzag edges can sustain edge surface states and resonances that are not present in the armchair case Rycerz et al., 2007 [5]. This research focused on the armchair graphene nanoribbon formation (acGNR). Graphene has several notable properties that make it worthy of research. The first of which is its remarkable strength. Graphene has a record breaking strength of 200 times greater than steel, with a tensile strength of 130GPa [1]. Graphene has a Young's modulus of 1000, compared to just that of 150 for silicon [1]. To put it into perspective, if you had a sheet of graphene as thick as a piece of cellophane, it would support the weight of a car. [2] If paper were as stiff as graphene, you could hold a 100-yard-long sheet of it at one end without its breaking or bending. [2] Another one of graphene's attractive properties is its electronic band gap, or rather, its lack thereof. Graphene is a Zero Gap Semiconductor. So it has high electron mobility at room temperature. It's a Superconductor. Electron transfer is 100 times faster than Silicon [1]. With zero a band gap, in the massless Dirac Fermion structure, the graphene ribbon is virtually lossless, making it a perfect semiconductor. Even in the massive Dirac Fermion structure, the band gap is 64meV [6]. This research began, as discussed in Chapter 2, with an armchair graphene nanoribbon unit cell of N=8. There were 16 electron approximation locations (ψ) provided per unit cell that spanned varying Fermi energy levels. Due to the atomic scales of the nanoribbon, the carbon atoms are separated by 1.42Å. The unit vector is given as, ~a = dbx, where d = 3αcc and αcc = 1.42°A is the carbon bond length [5]. Because of the close proximity of the carbon atoms, the 16 electron approximations could be combined or summed with their opposing lattice neighbors. Using single line approximation allowed us to reduce the 16 points down to 8. These approximations were then converted into charge densities (ρ). Poisson's equation, discussed in Chapter 3, was expanded into the 3 dimensional space, allowing us to convert ρ into voltage potentials (φ). Even though graphene is 2 dimensional; it can be used nicely in 3 dimensional computations without the presence of a substrate, due to the electric field lines and voltage potential characteristics produced being 3 dimensional. Subsequently it was found that small graphene sheets do not need to rest on substrates but can be freely suspended from a scaffolding; furthermore, bilayer and multilayer sheets can be prepared and characterized.