Static charging of graphene and graphite slabs

The effect of external static charging of graphene and its flakes are investigated by using first-principles calculations. While the Fermi level of negatively charged graphene rises and then is quickly pinned by the parabolic, nearly free electron like bands, it moves down readily by removal of electrons from graphene. Excess charges accumulate mainly at both surfaces of graphite slab. Even more remarkable is that Coulomb repulsion exfoliates the graphene layers from both surfaces of positively charged graphite slab. The energy level structure, binding energy and and spin-polarization of specific adatoms adsorbed to a graphene flake can be monitored by charging.Comment: accepted for publication in APL (after 226 days

Elastic and plastic deformation of graphene, silicene, and boron nitride honeycomb nanoribbons under uniaxial tension: A first-principles density-functional theory study

This study of elastic and plastic deformation of graphene, silicene, and boron nitride (BN) honeycomb nanoribbons under uniaxial tension determines their elastic constants and reveals interesting features. In the course of stretching in the elastic range, the electronic and magnetic properties can be strongly modified. In particular, it is shown that the band gap of a specific armchair nanoribbon is closed under strain and highest valance and lowest conduction bands are linearized. This way, the massless Dirac fermion behavior can be attained even in a semiconducting nanoribbon. Under plastic deformation, the honeycomb structure changes irreversibly and offers a number of new structures and functionalities. Cagelike structures, even suspended atomic chains can be derived between two honeycomb flakes. Present work elaborates on the recent experiments [C. Jin, H. Lan, L. Peng, K. Suenaga, and S. Iijima, Phys. Rev. Lett. 102, 205501 (2009)] deriving carbon chains from graphene. Furthermore, the similar formations of atomic chains from BN and Si nanoribbons are predicted.Comment: http://prb.aps.org/abstract/PRB/v81/i2/e02410

Structures of Fluorinated Graphenes and Their Signatures

Recent synthesis of fluorinated graphene introduced interesting stable derivatives of graphene. In particular, fluorographene (CF), namely fully fluorinated chair conformation, is found to display crucial features, such as high mechanical strength, charged surfaces, local magnetic moments due to vacancy defects and a wide band gap rapidly reducing with uniform strain. These properties, as well as structural parameters and electronic densities of states are found to scale with fluorine coverage. However, most of the experimental data reported to date neither for CF, nor for other CnF structures complies with the results obtained from first-principles calculations. In this study, we attempt to clarify the sources of disagreements.Comment: Phys. Rev. B 83, 115432 (2011

Two and one-dimensional honeycomb structures of silicon and germanium

Based on first-principles calculations of structure optimization, phonon modes and finite temperature molecular dynamics, we predict that silicon and germanium have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, they are ambipolar and their charge carriers can behave like a massless Dirac fermions due to their pi- and pi*-bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.Comment: 4 pages, 3 figures and 1 table. (published in Physical Review Letters

Two and one-dimensional honeycomb structures of silicon and germanium

Based on first-principles calculations of structure optimization, phonon modes and finite temperature molecular dynamics, we predict that silicon and germanium have stable, two-dimensional, low-buckled, honeycomb structures. Similar to graphene, they are ambipolar and their charge carriers can behave like a massless Dirac fermions due to their pi- and pi*-bands which are crossed linearly at the Fermi level. In addition to these fundamental properties, bare and hydrogen passivated nanoribbons of Si and Ge show remarkable electronic and magnetic properties, which are size and orientation dependent. These properties offer interesting alternatives for the engineering of diverse nanodevices.Comment: 4 pages, 3 figures and 1 table. (published in Physical Review Letters

Size modulation and defects in graphene based ribbons : magnetism and charge confinement

Ankara : The Program of Material Science and Nanotechnology and the Institute of Engineering and Science of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 56-59.In this thesis, we investigated the effects of vacancy and heterojunction formation on electronic and magnetic properties of graphene nanoribbons (GNRs) by using first principles pseudopotential plane wave method within Density Functional Theory. Graphene based materials are expected to be very important in future technology. Through understanding of all the factors which influence their physical properties is essential. We have shown that electronic and magnetic properties of graphene nanoribbons can be affected by defect-induced itinerant states. The band gaps of armchair nanoribbons can be modified by hydrogen saturated holes. Defects due to periodically repeating vacancies or divacancies induce metallization, as well as magnetization in non-magnetic semiconducting nanoribbons due to the spin-polarization of local defect states. Antiferromagnetic ground state of semiconducting zigzag ribbons can change to ferrimagnetic state upon creation of vacancy defects, which reconstruct and interact with edge states. Even more remarkable is that all these effects of vacancy defects are found to depend on their geometry and position relative to edges. We also predicted that periodically repeated junctions of graphene ribbons of different widths form multiple quantum well structures having confined states. These quantum structures are unique, since both constituents of heterostructures are of the same material. The width as well as the band gap, for specific superlattices are modulated in direct space. Orientation of constituent nanoribbons, their widths, lengths and the symmetry of the junction are some of the crucial structural parameters to engineer electronic properties of these systems. Our further studies on nanoribbons and nanoribbon superlattices showed the strong dependence of band gaps and magnetic moments on applied uniaxial stress. This thesis presents an extensive study of size modulation and defect formation on graphene nanoribbons.Topsakal, MehmetM.S

Effects of charging on two-dimensional honeycomb nanostructures

Ankara : The Department of Materials Science and Nanotechnology and the Graduate School of Engineering and Science of Bilkent Univ., 2012.Thesis (Ph. D.) -- Bilkent University, 2012.Includes bibliographical references leaves 82-88.In this thesis we employ state-of-the-art first-principles calculations based on density functional theory (DFT) to investigate the effects of static charging on two-dimensional (2D) honeycomb nanostructures. Free standing, single-layer graphene and other similar single-layer honeycomb structures such as boron nitride (BN), molybenum disulfide (MoS2), graphane (CH) and fluorographene (CF) have been recently synthesized with their unusual structural, electronic and magnetic properties. Through understanding of the effects of charging on these nanostructures is essential from our points of view in order to better understand their fundamental physics and developing useful applications. We show that the bond lengths and hence 2D lattice constants increase as a result of electron removal from the single layer. Consequently, phonons soften and the frequencies of Raman active modes are lowered. Three-layer, wide band gap BN and MoS2 sheets are metalized while these slabs are wide band semiconductors, and excess positive charge is accumulated mainly at the outermost atomic layers. Excess charges accumulated on the surfaces of slabs induce repulsive force between outermost layers. With increasing positive charging the spacing between these layers increases, which eventually ends up with exfoliation when exceeded the weak van der Waals (vdW) attractions between layers. This result may be exploited to develop a method for intact exfoliation of graphene, BN and MoS2 multilayers. In addition we also show that the binding energy and local magnetic moments of specific adatoms can be tuned by charging. We have addressed the deficiencies that can occur as an artifact of using plane-wave basis sets in periodic boundary conditions and proposed advantages of using atomicorbital based methods to overcome these deficiencies. Using the methods and computation elucidated in this thesis, the effects of charging on periodic as well as finite systems and the related properties can now be treated with reasonable accuracy. The adsorption of oxygen atoms on graphene have been investigated extensively before dealing with the charging of graphene oxide (GOX). The energetics and the patterns of oxygen coverage trends are shown to be directly related with the amount of bond charge at the bridge sites of graphene structure. We finally showed that the diffusion barriers for an oxygen atom to migrate on graphene surface is significantly modified with charging. While the present results comply with the trends observed in the experimental studies under charging, we believe that there are other factors affecting the reversible oxidation-reduction processes.Topsakal, MehmetPh.D

Ecotourism research: a bibliometric review

Makale (Review)WOS:000956643100003The paper aims to examine the trends and evolution of ecotourism research in the last 20 years, highlighting co-citations, collaborations, and emerging research themes. A total of 1387 ecotourism-related research from the Web of Science (WoS) database from 1991 to 2021 were analyzed in some bibliometric analyses using CiteSpace V software. The WoS database was scanned using the terms “ecotourism” and “eco-tourism.” While scanning, the “Hospitality, Leisure, Sport, and Tourism” field was selected for filtering, and articles published by September 12, 2021, were accessed. The most frequently studied cluster of topics related to ecotourism includes the creation of local benefits, ecotourism intentions, and community empowerment. The three most frequently used keywords related to ecotourism are “ecotourism,” “tourism,” and “conservation.” The outcome of this research constitutes a unique contribution to the area of ecotourism. The study provides an in-depth bibliometric analysis of publications and identification of important research trends. It also identifies possible avenues for future research in the knowledge domain of ecotourism.O presente trabalho tem como objetivo examinar as tendências e a evolução da investigação em ecoturismo nos últimos 20 anos, destacando as co-citações, as colaborações e os temas emergentes da investigação. Um total de 1387 artigos relacionados com ecoturismo da base de dados Web of Science (WoS) de 1991 a 2021 foram estudados através de análises bibliométricas usando o software CiteSpace V. Foi feita uma busca usando os termos “ecotourism” e “eco-tourism”. Durante a busca, o campo “Hospitality, Leisure, Sport, and Tourism” foi selecionado para filtragem e os artigos publicados até 12 de setembro de 2021 foram acessados. Os temas mais frequentemente estudados incluem a criação de benefícios locais, as intenções de ecoturismo e o empoderamento comunitário. As três palavras-chave mais utilizadas são "ecoturismo", "turismo" e "conservação". O resultado desta pesquisa constitui uma contribuição única para o campo do ecoturismo. O estudo fornece uma análise bibliométrica aprofundada de publicações e identificação de tendências de pesquisa importantes. Também identifica possíveis pistas para futuras investigações no domínio do conhecimento do ecoturismo