Crystals

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17 pages, 868 KiB

Open AccessArticle

Impact of Vacancies on Diffusive and Pseudodiffusive Electronic Transport in Graphene

by Alessandro Cresti, Thibaud Louvet, Frank Ortmann, Dinh Van Tuan, Paweł Lenarczyk, Georg Huhs and Stephan Roche

Crystals 2013, 3(2), 289-305; https://doi.org/10.3390/cryst3020289 - 8 Apr 2013

Cited by 6 | Viewed by 7253

Abstract

We present a survey of the effect of vacancies on quantum transport in graphene, exploring conduction regimes ranging from tunnelling to intrinsic transport phenomena. Vacancies, with density up to 2%, are distributed at random either in a balanced manner between the two sublattices [...] Read more.

We present a survey of the effect of vacancies on quantum transport in graphene, exploring conduction regimes ranging from tunnelling to intrinsic transport phenomena. Vacancies, with density up to 2%, are distributed at random either in a balanced manner between the two sublattices or in a totally unbalanced configuration where only atoms sitting on a given sublattice are randomly removed. Quantum transmission shows a variety of different behaviours, which depend on the specific system geometry and disorder distribution. The investigation of the scaling laws of the most significant quantities allows a deep physical insight and the accurate prediction of their trend over a large energy region around the Dirac point. Full article

(This article belongs to the Special Issue Graphenes)

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21 pages, 737 KiB

Open AccessArticle

The Origin of Raman D Band: Bonding and Antibonding Orbitals in Graphene

by Ken-ichi Sasaki, Yasuhiro Tokura and Tetsuomi Sogawa

Crystals 2013, 3(1), 120-140; https://doi.org/10.3390/cryst3010120 - 22 Feb 2013

Cited by 56 | Viewed by 12931

Abstract

In Raman spectroscopy of graphite and graphene, the D band at ∼ 1355 cm⁻¹ is used as the indication of the dirtiness of a sample. However, our analysis suggests that the physics behind the D band is closely related to a very [...] Read more.

In Raman spectroscopy of graphite and graphene, the D band at ∼ 1355 cm⁻¹ is used as the indication of the dirtiness of a sample. However, our analysis suggests that the physics behind the D band is closely related to a very clear idea for describing a molecule, namely bonding and antibonding orbitals in graphene. In this paper, we review our recent work on the mechanism for activating the D band at a graphene edge. Full article

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11 pages, 519 KiB

Open AccessArticle

Effects of Strain on Notched Zigzag Graphene Nanoribbons

by Jack Baldwin and Y. Hancock

Crystals 2013, 3(1), 38-48; https://doi.org/10.3390/cryst3010038 - 23 Jan 2013

Cited by 6 | Viewed by 7365

Abstract

The combined effects of an asymmetric (square or V-shaped) notch and uniaxial strain are studied in a zigzag graphene nanoribbon (ZGNR) device using a generalized tight-binding model. The spin-polarization and conductance-gap properties, calculated within the Landauer–B¨uttiker formalism, were found to be tunable for [...] Read more.

The combined effects of an asymmetric (square or V-shaped) notch and uniaxial strain are studied in a zigzag graphene nanoribbon (ZGNR) device using a generalized tight-binding model. The spin-polarization and conductance-gap properties, calculated within the Landauer–B¨uttiker formalism, were found to be tunable for uniaxial strain along the ribbon-length and ribbon-width for an ideal ZGNR and square (V-shaped) notched ZGNR systems. Uniaxial strain along the ribbon-width for strains 10% initiated significant notch-dependent reductions to the conduction-gap. For the V-shaped notch, such strains also induced spin-dependent changes that result, at 20% strain, in a semi-conductive state and metallic state for each respective spin-type, thus demonstrating possible quantum mechanisms for spin-filtration. Full article

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10 pages, 435 KiB

Open AccessArticle

Thermostability, Photoluminescence, and Electrical Properties of Reduced Graphene Oxide–Carbon Nanotube Hybrid Materials

by Fuchi Liu, Yong Cao, Mingdong Yi, Linghai Xie, Wei Huang, Nujiang Tang, Wei Zhong and Youwei Du

Crystals 2013, 3(1), 28-37; https://doi.org/10.3390/cryst3010028 - 21 Jan 2013

Cited by 32 | Viewed by 11731

Abstract

Reduced graphene oxide–carbon nanotube (RGO–CNT) hybrid materials were prepared by a simple catalyst-free route. The thermostability, photoluminescence (PL) and electrical properties of RGO–CNTs were investigated systematically. The results revealed that compared to RGO, RGO–CNTs showed multicolor PL, and higher thermostability and conductivity. The [...] Read more.

Reduced graphene oxide–carbon nanotube (RGO–CNT) hybrid materials were prepared by a simple catalyst-free route. The thermostability, photoluminescence (PL) and electrical properties of RGO–CNTs were investigated systematically. The results revealed that compared to RGO, RGO–CNTs showed multicolor PL, and higher thermostability and conductivity. The RGO–CNTs therefore have important potential applications in the fields of photonic and electrical devices. Full article

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14 pages, 330 KiB

Open AccessArticle

Bound States and Supercriticality in Graphene-Based Topological Insulators

by Denis Klöpfer, Alessandro De Martino and Reinhold Egger

Crystals 2013, 3(1), 14-27; https://doi.org/10.3390/cryst3010014 - 21 Jan 2013

Cited by 9 | Viewed by 6223

Abstract

We study the bound state spectrum and the conditions for entering a supercritical regime in graphene with strong intrinsic and Rashba spin-orbit interactions within the topological insulator phase. Explicit results are provided for a disk-shaped potential well and for the Coulomb center problem. [...] Read more.

We study the bound state spectrum and the conditions for entering a supercritical regime in graphene with strong intrinsic and Rashba spin-orbit interactions within the topological insulator phase. Explicit results are provided for a disk-shaped potential well and for the Coulomb center problem. Full article

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13 pages, 1475 KiB

Open AccessArticle

Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC

by Leif I. Johansson, Chao Xia, Jawad Ul Hassan, Tihomir Iakimov, Alexei A. Zakharov, Somsakul Watcharinyanon, Rositza Yakimova, Erik Janzén and Chariya Virojanadara

Crystals 2013, 3(1), 1-13; https://doi.org/10.3390/cryst3010001 - 15 Jan 2013

Cited by 8 | Viewed by 7829

Abstract

Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron [...] Read more.

Graphene grown on C-face SiC substrates using two procedures, high and low growth temperature and different ambients, was investigated using Low Energy Electron Microscopy (LEEM), X-ray Photo Electron Electron Microscopy (XPEEM), selected area Low Energy Electron Diffraction (μ-LEED) and selected area Photo Electron Spectroscopy (μ-PES). Both types of samples showed formation of μm-sized grains of graphene. The sharp (1 × 1) μ-LEED pattern and six Dirac cones observed in constant energy photoelectron angular distribution patterns from a grain showed that adjacent layers are not rotated relative to each other, but that adjacent grains in general have different azimuthal orientations. Diffraction spots from the SiC substrate appeared in μ-LEED patterns collected at higher energies, showing that the rotation angle between graphene and SiC varied. C 1s spectra collected did not show any hint of a carbon interface layer. A hydrogen treatment applied was found to have a detrimental effect on the graphene quality for both types of samples, since the graphene domain/grain size was drastically reduced. From hydrogen treated samples, μ-LEED showed at first a clear (1 × 1) pattern, but within minutes, a pattern containing strong superstructure spots, indicating the presence of twisted graphene layers. The LEED electron beam was found to induce local desorption of hydrogen. Heating a hydrogenated C-face graphene sample did not restore the quality of the original as-grown sample. Full article

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Review

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18 pages, 295 KiB

Open AccessReview

A Study on Graphene—Metal Contact

by Wenjun Liu, Jun Wei, Xiaowei Sun and Hongyu Yu

Crystals 2013, 3(1), 257-274; https://doi.org/10.3390/cryst3010257 - 18 Mar 2013

Cited by 65 | Viewed by 20217

Abstract

The contact resistance between graphene and metal electrodes is crucial for the achievement of high-performance graphene devices. In this study, we review our recent study on the graphene–metal contact characteristics from the following viewpoints: (1) metal preparation method; (2) asymmetric conductance; (3) annealing [...] Read more.

The contact resistance between graphene and metal electrodes is crucial for the achievement of high-performance graphene devices. In this study, we review our recent study on the graphene–metal contact characteristics from the following viewpoints: (1) metal preparation method; (2) asymmetric conductance; (3) annealing effect; (4) interfaces impact. Full article

(This article belongs to the Special Issue Graphenes)

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43 pages, 2024 KiB

Open AccessReview

Epitaxial Graphene and Graphene–Based Devices Studied by Electrical Scanning Probe Microscopy

by Olga Kazakova, Vishal Panchal and Tim L. Burnett

Crystals 2013, 3(1), 191-233; https://doi.org/10.3390/cryst3010191 - 13 Mar 2013

Cited by 68 | Viewed by 17663

Abstract

We present local electrical characterization of epitaxial graphene grown on both Si- and C-faces of 4H-SiC using Electrostatic Force Microscopy and Kelvin Probe Force Microscopy in ambient conditions and at elevated temperatures. These techniques provide a straightforward identification of graphene domains with [...] Read more.

We present local electrical characterization of epitaxial graphene grown on both Si- and C-faces of 4H-SiC using Electrostatic Force Microscopy and Kelvin Probe Force Microscopy in ambient conditions and at elevated temperatures. These techniques provide a straightforward identification of graphene domains with various thicknesses on the substrate where topographical determination is hindered by adsorbates and SiC terraces. We also use Electrostatic Force Spectroscopy which allows quantitative surface potential measurements with high spatial resolution. Using these techniques, we study evolution of a layer of atmospheric water as a function of temperature, which is accompanied by a significant change of the absolute surface potential difference. We show that the nanoscale wettability of the material is strongly dependent on the number of graphene layers, where hydrophobicity increases with graphene thickness. We also use micron-sized graphene Hall bars with gold electrodes to calibrate work function of the electrically conductive probe and precisely and quantitatively define the work functions for single- and double-layer graphene. Full article

(This article belongs to the Special Issue Graphenes)

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Graphical abstract

28 pages, 21263 KiB

Open AccessReview

Prevention of Graphene Restacking for Performance Boost of Supercapacitors—A Review

by Jiantong Li and Mikael Östling

Crystals 2013, 3(1), 163-190; https://doi.org/10.3390/cryst3010163 - 6 Mar 2013

Cited by 107 | Viewed by 21600

Abstract

Graphene is a promising electrode material for supercapacitors mainly because of its large specific surface area and high conductivity. In practice, however, several fabrication issues need refinement. The restacking of graphene flakes upon being packed into supercapacitor electrodes has become a critical challenge [...] Read more.

Graphene is a promising electrode material for supercapacitors mainly because of its large specific surface area and high conductivity. In practice, however, several fabrication issues need refinement. The restacking of graphene flakes upon being packed into supercapacitor electrodes has become a critical challenge in the full utilization of graphene’s large specific surface area to further improve the device performance. In this review, a variety of recent techniques and strategies are overviewed for the prevention of graphene restacking. They have been classified into several categories to improve and facilitate the discussion on the underlying ideas. Based on the overview of the existing techniques, we discuss the trends of future research in the fields. Full article

(This article belongs to the Special Issue Graphenes)

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33 pages, 3399 KiB

Open AccessReview

Metals on Graphene: Interactions, Growth Morphology, and Thermal Stability

by Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho and Michael C. Tringides

Crystals 2013, 3(1), 79-111; https://doi.org/10.3390/cryst3010079 - 31 Jan 2013

Cited by 144 | Viewed by 16273

Abstract

Graphene, a single atomic layer of graphite, has been a material of recent intensive studies due to its novel electronic and structural properties and its potential applications in the emerging area of carbon-based electronic devices. Metal on graphene growth is one of the [...] Read more.

Graphene, a single atomic layer of graphite, has been a material of recent intensive studies due to its novel electronic and structural properties and its potential applications in the emerging area of carbon-based electronic devices. Metal on graphene growth is one of the current research interests, aiming at improving and manipulating the electronic and magnetic properties of graphene through metal atom adsorption or doping to meet various requirements in device applications. In this paper, we will give an overview of recent experimental and computational investigation of interaction, growth morphology, and thermal stability of various metals on graphene grown on 6H-SiC(0001) substrate. Full article

(This article belongs to the Special Issue Graphenes)

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30 pages, 473 KiB

Open AccessReview

Indirect Exchange and Ruderman–Kittel–Kasuya–Yosida (RKKY) Interactions in Magnetically-Doped Graphene

by Stephen R. Power and Mauro S. Ferreira

Crystals 2013, 3(1), 49-78; https://doi.org/10.3390/cryst3010049 - 30 Jan 2013

Cited by 89 | Viewed by 14147

Abstract

Magnetically-doped graphene systems are potential candidates for application in future spintronic devices. A key step is to understand the pairwise interactions between magnetic impurities embedded in graphene that are mediated by the graphene conduction electrons. A large number of studies have been undertaken [...] Read more.

Magnetically-doped graphene systems are potential candidates for application in future spintronic devices. A key step is to understand the pairwise interactions between magnetic impurities embedded in graphene that are mediated by the graphene conduction electrons. A large number of studies have been undertaken to investigate the indirect exchange, or RKKY (Ruderman-Kittel-Kasuya-Yosida), interactions in graphene. Many of these studies report a decay rate faster than expected for a two-dimensional material and the absence of the usual distance dependent oscillations. In this review we summarize the techniques used to calculate the interaction and present the key results obtained to date. The effects of more detailed parameterisations of the magnetic impurities and graphene host are considered, as are results obtained from ab initio calculations. Since the fast decay of the interaction presents an obstacle to spintronic applications, we focus in particular on the possibility of augmenting the interaction range by a number of methods including doping, spin precession and the application of strain. Full article

(This article belongs to the Special Issue Graphenes)

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