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Crystals, Volume 3, Issue 1 (March 2013), Pages 1-274

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Research

Jump to: Review

Open AccessArticle Is the Registry Between Adjacent Graphene Layers Grown on C-Face SiC Different Compared to That on Si-Face SiC
Crystals 2013, 3(1), 1-13; doi:10.3390/cryst3010001
Received: 16 November 2012 / Revised: 17 December 2012 / Accepted: 9 January 2013 / Published: 15 January 2013
Cited by 5 | PDF Full-text (1475 KB) | HTML Full-text | XML Full-text
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 [...] 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
(This article belongs to the Special Issue Graphenes)
Open AccessArticle Bound States and Supercriticality in Graphene-Based Topological Insulators
Crystals 2013, 3(1), 14-27; doi:10.3390/cryst3010014
Received: 16 November 2012 / Revised: 17 December 2012 / Accepted: 9 January 2013 / Published: 21 January 2013
Cited by 3 | PDF Full-text (330 KB) | HTML Full-text | XML Full-text
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 [...] 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
(This article belongs to the Special Issue Graphenes)
Open AccessArticle Thermostability, Photoluminescence, and Electrical Properties of Reduced Graphene Oxide–Carbon Nanotube Hybrid Materials
Crystals 2013, 3(1), 28-37; doi:10.3390/cryst3010028
Received: 8 November 2012 / Revised: 27 December 2012 / Accepted: 9 January 2013 / Published: 21 January 2013
Cited by 10 | PDF Full-text (435 KB) | HTML Full-text | XML Full-text
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. [...] 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
(This article belongs to the Special Issue Graphenes)
Open AccessArticle Effects of Strain on Notched Zigzag Graphene Nanoribbons
Crystals 2013, 3(1), 38-48; doi:10.3390/cryst3010038
Received: 19 November 2012 / Revised: 28 December 2012 / Accepted: 9 January 2013 / Published: 23 January 2013
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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 [...] 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
(This article belongs to the Special Issue Graphenes)
Open AccessCommunication Solvent-Included Ternary Charge-Transfer Salt (BEDT-TTF)2GaCl4(C6H5Cl)0.5, (BEDT-TTF = Bis(ethyelenedithio)-tetrathiafulvalene)
Crystals 2013, 3(1), 112-119; doi:10.3390/cryst3010112
Received: 27 November 2012 / Revised: 10 January 2013 / Accepted: 25 January 2013 / Published: 18 February 2013
Cited by 2 | PDF Full-text (352 KB) | HTML Full-text | XML Full-text
Abstract
The synthesis, crystal structure, and conductivity of a solvent-included ternary charge-transfer salt (BEDT-TTF)2GaCl4(C6H5Cl)0.5 (1) is described and interpreted. Electrochemical oxidation of neutral bis(ethyelenedithio)-tetrathiafulvalene (BEDT-TTF) in the presence of (Me4N)Ga(C [...] Read more.
The synthesis, crystal structure, and conductivity of a solvent-included ternary charge-transfer salt (BEDT-TTF)2GaCl4(C6H5Cl)0.5 (1) is described and interpreted. Electrochemical oxidation of neutral bis(ethyelenedithio)-tetrathiafulvalene (BEDT-TTF) in the presence of (Me4N)Ga(C2O4)Cl2 in a mixture of C6H5Cl and C2H5OH yields crystals of 1. Compound 1 crystallized as a monoclinic C 2/c space group with cell parameters: a = 47.0615(4) Å, b = 6.7895(1) Å, c = 31.6297(4) Å, β = 132.064(1)°, V = 7503.0(2) Å3, Z = 8 at 293 K and a = 46.4767(5) Å, b = 6.7398(1) Å, c = 31.0778(4) Å, β = 131.630(1)°, V = 7267.4(2) Å3, Z = 8 at 120 K. The formal charge of the donor molecule was assigned as +0.5 from bond lengths in the TTF core. The donor molecules stack with C―···S contacts along the c direction and side-to-side SS contacts along the b direction to form a two-dimensional donor layer on the bc plane. In the anion sheet, C6H5Cl chain is sandwiched by two GaCl4 chains with ClCl contacts. Compound 1 shows semiconductive behavior with Ea = 124 meV between room-temperature and 150 K and σ300K = 1 S·cm−1. Full article
(This article belongs to the Special Issue Molecular Conductors)
Open AccessArticle The Origin of Raman D Band: Bonding and Antibonding Orbitals in Graphene
Crystals 2013, 3(1), 120-140; doi:10.3390/cryst3010120
Received: 19 November 2012 / Revised: 7 January 2013 / Accepted: 1 February 2013 / Published: 22 February 2013
Cited by 8 | PDF Full-text (737 KB) | HTML Full-text | XML Full-text
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 [...] 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
(This article belongs to the Special Issue Graphenes)
Open AccessArticle Syntheses and Crystal Structures of Ferrocenoindenes
Crystals 2013, 3(1), 141-148; doi:10.3390/cryst3010141
Received: 18 January 2013 / Revised: 15 February 2013 / Accepted: 18 February 2013 / Published: 27 February 2013
Cited by 1 | PDF Full-text (271 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Ferrocenoindenes display planar chirality and thus represent valuable ligands for asymmetric catalysis. Here, we report on the synthesis of novel 3-(1,1-dibromomethylene)ferroceno[1,2-a]indene, (Z)-3-(1-bromomethylene)-6-iodoferroceno[1,2-a]indene, and benzo[5,6-f]ferroceno[2,3,a]inden-1-one. Any application-oriented design of chiral catalysts requires fundamental knowledge about the ligands involved, not only in [...] Read more.
Ferrocenoindenes display planar chirality and thus represent valuable ligands for asymmetric catalysis. Here, we report on the synthesis of novel 3-(1,1-dibromomethylene)ferroceno[1,2-a]indene, (Z)-3-(1-bromomethylene)-6-iodoferroceno[1,2-a]indene, and benzo[5,6-f]ferroceno[2,3,a]inden-1-one. Any application-oriented design of chiral catalysts requires fundamental knowledge about the ligands involved, not only in terms of atom-connectivity, but also in terms of their three-dimensional structure and steric demand. Therefore, the crystal structures of 2-ferrocenylbenzoic acid, ferroceno[1,2-a]indene, and (Z)-3-(1-bromomethylene)-6-iodoferroceno[1,2-a]indene have been determined. The bond-lengths that can be retrieved therefrom also allow for an estimation of the reactivity of the aryl-iodo, bromo-methylidene and dibromomethylidene moieties. Full article
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Open AccessArticle Generation of Light Scattering States in Cholesteric Liquid Crystals by Optically Controlled Boundary Conditions
Crystals 2013, 3(1), 234-247; doi:10.3390/cryst3010234
Received: 18 February 2013 / Revised: 4 March 2013 / Accepted: 11 March 2013 / Published: 18 March 2013
PDF Full-text (1131 KB) | HTML Full-text | XML Full-text | Supplementary Files
Abstract
Circularly polarized light was previously employed to stimulate the reversible and reconfigurable writing of scattering states in cholesteric liquid crystal (CLC) cells constructed with a photosensitive layer. Such dynamic photodriven responses have utility in remotely triggering changes in optical constructs responsive to [...] Read more.
Circularly polarized light was previously employed to stimulate the reversible and reconfigurable writing of scattering states in cholesteric liquid crystal (CLC) cells constructed with a photosensitive layer. Such dynamic photodriven responses have utility in remotely triggering changes in optical constructs responsive to optical stimulus and applications where complex spatial patterning is required. Writing of scattering regions required the handedness of incoming radiation to match the handedness of the CLC and the reflection bandwidth of the CLC to envelop the wavelength of the incoming radiation. In this paper, the mechanism of transforming the CLC into a light scattering state via the influence of light on the photosensitive alignment layer is detailed. Specifically, the effects of: (i) the polarization state of light on the photosensitive alignment layer; (ii) the exposure time; and (iii) the incidence angle of radiation on domain formation are reported. The photogenerated light-scattering domains are shown to be similar in appearance between crossed polarizers to a defect structure that occurs at a CLC/air interface (i.e., a free CLC surface). This observation provides strong indication that exposure of the photosensitive alignment layer to the circularly polarized light of appropriate wavelength and handedness generates an out-of-plane orientation leading to a periodic distortion of the original planar structure. Full article
(This article belongs to the Special Issue Advances in Liquid Crystals)
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Open AccessArticle Iminium Salts by Meerwein Alkylation of Ehrlich’s Aldehyde
Crystals 2013, 3(1), 248-256; doi:10.3390/cryst3010248
Received: 25 January 2013 / Revised: 6 March 2013 / Accepted: 11 March 2013 / Published: 18 March 2013
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Abstract
4-(Dimethylamino)benzaldehyde is alkylated at the N atom by dialkyl sulfates, MeI, or Me3O BF4. In contrast, ethylation by Et3O BF4 occurs selectively at the O atom yielding a quinoid iminium ion. 4-(Diethylamino)benzaldehyde is alkylated only [...] Read more.
4-(Dimethylamino)benzaldehyde is alkylated at the N atom by dialkyl sulfates, MeI, or Me3O BF4. In contrast, ethylation by Et3O BF4 occurs selectively at the O atom yielding a quinoid iminium ion. 4-(Diethylamino)benzaldehyde is alkylated only at O by either Et or Me oxonium reagent. The iminium salts are prone to hydrolysis giving the corresponding hydrotetrafluoroborates. Five crystal structures were determined. Full article

Review

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Open AccessReview Indirect Exchange and Ruderman–Kittel–Kasuya–Yosida (RKKY) Interactions in Magnetically-Doped Graphene
Crystals 2013, 3(1), 49-78; doi:10.3390/cryst3010049
Received: 16 November 2012 / Revised: 13 December 2012 / Accepted: 9 January 2013 / Published: 30 January 2013
Cited by 32 | PDF Full-text (473 KB) | HTML Full-text | XML Full-text
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 [...] 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)
Open AccessReview Metals on Graphene: Interactions, Growth Morphology, and Thermal Stability
Crystals 2013, 3(1), 79-111; doi:10.3390/cryst3010079
Received: 30 November 2012 / Revised: 14 January 2013 / Accepted: 24 January 2013 / Published: 31 January 2013
Cited by 32 | PDF Full-text (3399 KB) | HTML Full-text | XML Full-text
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 [...] 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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Open AccessReview Photoaligning and Photopatterning — A New Challenge in Liquid Crystal Photonics
Crystals 2013, 3(1), 149-162; doi:10.3390/cryst3010149
Received: 22 January 2013 / Revised: 17 February 2013 / Accepted: 20 February 2013 / Published: 1 March 2013
Cited by 9 | PDF Full-text (1226 KB) | HTML Full-text | XML Full-text
Abstract Photoalignment possesses obvious advantages in comparison with the usually “rubbing” treatment of the substrates of liquid crystal display (LCD) cells. The application of the photoalignment and photopatterning nanotechnology for the new generation of photonic and display devices will be reviewed. Full article
(This article belongs to the Special Issue Advances in Liquid Crystals)
Open AccessReview Prevention of Graphene Restacking for Performance Boost of Supercapacitors—A Review
Crystals 2013, 3(1), 163-190; doi:10.3390/cryst3010163
Received: 6 December 2012 / Revised: 6 February 2013 / Accepted: 16 February 2013 / Published: 6 March 2013
Cited by 17 | PDF Full-text (21263 KB) | HTML Full-text | XML Full-text
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 [...] 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)
Open AccessReview Epitaxial Graphene and Graphene–Based Devices Studied by Electrical Scanning Probe Microscopy
Crystals 2013, 3(1), 191-233; doi:10.3390/cryst3010191
Received: 10 December 2012 / Revised: 17 February 2013 / Accepted: 26 February 2013 / Published: 13 March 2013
Cited by 22 | PDF Full-text (2024 KB) | HTML Full-text | XML Full-text
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 [...] 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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Open AccessReview A Study on Graphene—Metal Contact
Crystals 2013, 3(1), 257-274; doi:10.3390/cryst3010257
Received: 9 January 2013 / Revised: 19 February 2013 / Accepted: 27 February 2013 / Published: 18 March 2013
Cited by 26 | PDF Full-text (295 KB) | HTML Full-text | XML Full-text
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) [...] 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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