Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2015"

Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system

Abstract: One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression in which the intercept gives the band-gap and the fitting exponent identifies the electronic transition as direct or indirect (see Tauc et al., Phys. Status Solidi 15, 627 (1966); these are often called “Tauc” plots). While the technique is powerful and simple, the accuracy of the fitted band-gap result is seldom stated or known. We tackle this question by refitting a large number of Tauc plots from the literature and look for trends. Nominally pure zinc oxide (ZnO) was chosen as a material with limited intrinsic deviation from stoichiometry and which has been widely studied. Our examination of the band gap values and their distribution leads to a discussion of some experimental factors that can bias the data and lead to either smaller or larger apparent values than would be expected. Finally, an easily evaluated figure-of-merit is defined that may help guide more accurate Tauc fitting. For samples with relatively sharper Tauc plot shapes, the population yields Eg(ZnO) as 3.276 ± 0.033 eV, in good agreement with data for single crystalline material.

Brillouin zone and band structure of β‐Ga2O3

Abstract: Gallium oxide is increasingly used in a variety of applications, but confusion reigns over the Brillouin zone and the band structure of monoclinic β-Ga2O3. We present a detailed study of the shape of the Brillouin zone and the location of high-symmetry points. Combined with a study of electronic structure based on hybrid density functional theory, this allows us to derive an accurate band structure. We discuss the nature of the band gap and the location of the band extrema.

Synthesis of carbon nanotubes by the laser ablation method: Effect of laser wavelength

Abstract: The effect of laser wavelength on single-wall carbon nanotubes synthesis yield and their properties was studied. A double-pulse Nd:YAG laser, working at a wavelength of 355 or 1064 nm, was used for carbon nanotubes production. The synthesized carbon nanotubes (CNTs) were investigated using the SEM/STEM microscopy and Raman spectroscopy. The results show that the useful range of UV laser radiation fluence is narrower and the properties of synthesized CNTs depend much more on the laser fluence than in the case of infrared laser radiation.

First‐principles theory of acceptors in nitride semiconductors

Abstract: Acceptor defects and impurities play a critical role in the performance of GaN-based devices. Mg is the only acceptor impurity that gives rise to p-type conductivity, while other acceptors (such as CN impurities and VGa defects) act as sources of compensation and trapping. From the point of view of theory, understanding the physics of acceptor species in GaN has long been a challenge. In the past, limitations of computational techniques made it difficult to quantitatively predict crucial quantities such as thermodynamic and optical transition levels. However, advances in first-principles calculations, including the use of hybrid functionals in density functional theory, have led to a resurgence in efforts to understand properties of acceptors in nitrides. After briefly discussing advances in theoretical techniques, we review recent computational work on acceptor impurities in GaN and compare theoretical results with the available experimental data. We also present new hybrid density functional calculations on the transition levels of VGa and its complexes with O and H impurities. The results indicate that donor impurities significantly lower VGa transition levels, and that VGa3H and VGaON2H complexes give rise to yellow luminescence. We also discuss the properties of acceptor impurities in AlN and InN.

Controllable thermal expansion of large magnitude in chiral negative Poisson's ratio lattices

Abstract: Lattices of controlled thermal expansion are presented based on planar chiral lattice structure with Poisson’s ratio approaching -1. Thermal expansion values can be arbitrarily large positive or negative. A lattice was fabricated from bimetallic strips and the properties analyzed and studied experimentally. The eective thermal expansion coecient of the lattice is about = 3:5 10 4 K 1 . This is much larger in magnitude than that of constituent metals. Nodes were observed to rotate as temperature was changed corresponding to a Cosserat thermoelastic solid.

Spin and valley dynamics of excitons in transition metal dichalcogenide monolayers

Abstract: Monolayers of transition metal dichalcogenides, namely, molybdenum and tungsten disulfides and diselenides demonstrate unusual optical properties related to the spin–valley locking effect. Particularly, excitation of monolayers by circularly polarized light selectively creates electron–hole pairs or excitons in non‐equivalent valleys in momentum space, depending on the light helicity. This allows studying the inter‐valley dynamics of charge carriers and Coulomb complexes by means of optical spectroscopy. Here, we present a concise review of the neutral exciton fine structure and its spin and valley dynamics in monolayers of transition metal dichalcogenides. It is demonstrated that the long‐range exchange interaction between an electron and a hole in the exciton is an efficient mechanism for rapid mixing between bright excitons made of electron–hole pairs in different valleys. We discuss the physical origin of the long‐range exchange interaction and outline its derivation in both the electrodynamical and urn:x-wiley:15213951:media:pssb201552211:pssb201552211-math-0001 approaches. We further present a model of bright exciton spin dynamics driven by an interplay between the long‐range exchange interaction and scattering. Finally, we discuss the application of the model to describe recent experimental data obtained by time‐resolved photoluminescence and Kerr rotation techniques.

Dynamic response of sandwich panels with auxetic cores

Abstract: Effective properties and dynamic response of a sandwich panel made of two face sheets and auxetic core are analyzed in this study by computer simulations. The inner composite layer is made of a cellular auxetic structure immersed in a filler material of a given Poisson's ratio (filler material fills the voids in structure). Each cell is composed of an auxetic structure (re-entrant honeycomb or rotating square), i.e., exhibiting negative Poisson's ratio without any filler. Influence of filler material on the effective properties of the sandwich panel is investigated. The proposed structure shows interesting structural characteristics and dynamic properties. Our results clearly show that it is possible to create auxetic sandwich panels made of two solid materials with positive Poisson's ratio. This is even possible if the filler material is nearly incompressible, but can move in out-of-plane direction. Moreover, effective Young's modulus of such sandwich panels becomes very large if the Poisson's ratio of the filler material tends to −1.

Light induced TiO2 phase transformation: Correlation with luminescent surface defects

Abstract: The light induced structural phase transition of TiO2 nanoparticles from anatase to rutile structure is reported with different distribution of defect related surface states. Pristine, defective, and surface passivated samples were irradiated in vacuum condition by intragap visible wavelength to achieve the phase transformation. The surface states were studied by means of intragap excited steady state and time-resolved photoluminescence spectroscopy. Two bands were clearly observed, the first component centered at about 470 nm with time decay in the ns range and the second one peaked at 600 nm, with a lifetime in the order of µs. The bands are assigned to 5Ti3+ species and to 6Ti3+-OH species located at the surface of the anatase TiO2 nanoparticles. The transition mechanism to the rutile phase is interpreted in the framework of oxygen adsorption and desorption phenomena with the involvement of the surface defects.

Sixteen years GaN on Si

Abstract: GaN, the basis of high brightness LEDs and high power, high frequency FET's has developed to the second important semiconductor after Si. Although epitaxial growth of thin layers on sapphire has been developed already in the late 1980s the growth on Si, which offers lower cost and new application fields, was hampered mostly by thermal mismatch and chemical incompatibility leading to destructive melt-back etching. In the last 16 years, GaN on Si has developed from a niche academic work to part of GaN mainstream technology. This article gives an overview on the breakthrough developments and different methods to solve the problems associated with GaN growth on Si as well as an overview on the different application fields.

Cellular plates with auxetic rectangular perforations

Abstract: The work describes a cellular structure configuration with a rectangular perforation topology exhibiting auxetic (negative Poisson's ratio) in-plane behaviour. The rectangular voids used in this structure produce a rigid rotating squares effect. Changes in the sizing parameters of aspect ratio, intercell spacing, and number of unit cells are made through non-dimensional modelling. The effects on the key properties of in-plane Poisson's ratio, Young's modulus and shear modulus are investigated. Through numerical modelling and experimental testing the auxetic behaviour is confirmed, with increased negative Poisson's ratio values in comparison to rhomboidal patterns of perforations available in open literature.

Low-kinetic energy impact response of auxetic and conventional open-cell polyurethane foams

Abstract: This paper reports quasi-static and low-kinetic energy impact testing of auxetic and conventional open-cell polyurethane foams. The auxetic foams were fabricated using the established thermo-mechanical process originally developed by Lakes. Converted foams were subject to compression along each dimension to 85% and 70% of the unconverted dimension during the conversion process, corresponding to linear compression ratios of 0.85 and 0.7, respectively. The 0.7 linear compression ratio foams were confirmed to have a re-entrant foam cell structure and to be auxetic. Impact tests were performed for kinetic energies up to 4 J using an instrumented drop rig and high speed video. A flat dropper was employed on isolated foams, and a hemisperical shaped dropper on foams covered with a rigid polypropylene outer shell layer. The flat dropper tests provide data on the rate-dependency of the Poisson’s ratio in these foam test specimens. The foam Poisson’s ratios were found to be unaffected by the strain rate for the impact energies considered here. Acceleration-time data are reported along with deformation images from the video footage. The auxetic samples displayed a 6 times reduction in peak acceleration, showing potential in impact protector devices such as shin or thigh protectors in sports equipment applications. Keywords : auxetic, foam, impact, negative Poisson’s ratio

Chiral topologies for composite morphing structures – Part I: Development of a chiral rib for deformable airfoils

Abstract: The paper moves from a technological process developed in previous works to produce chiral honeycombs made of thin composite laminates. Such approach is applied in this work to manufacture the morphing ribs for a variable camber wing-box. The specifications for such components are obtained by developing a finite element model of a demonstrator, which is designed taking into account aeroelastic performances, structural, and technological issues. In the first part of the paper, the design of such a demonstrator is presented and the role of composite chiral ribs with auxetic behavior is outlined. Production, testing, and numerical studies of manufacturing trials are performed to assess the technological process applied to small-sized chiral units made of different materials, to investigate their mechanical properties, and to validate a numerical approach for design and analysis. A complete chiral composite rib is then produced and tests are carried out to verify the overall structural response and to validate the numerical approach.

Self-organized van der Waals epitaxy of layered chalcogenide structures

Abstract: Highly oriented Sb2Te3 films were successfully deposited by RF-magnetron sputtering on both crystalline and amorphous substrates. A novel deposition mechanism and method are proposed based on van der Waals epitaxy. Due to the selective reactivity of the top surface atoms of the substrate with sputtered atoms, a Te monolayer is the first layer formed on the substrate, resulting in the subsequent layer-by-layer growth of the Sb2Te3 film independent of the crystallinity of the substrates. We believe that this method can be applied to the mass production of a wide range of various van der Waals solids, such as transition metal dichalcogenides and topological insulators for future electronics devices.

Vacancy-type defects and their annealing behaviors in Mg-implanted GaN studied by a monoenergetic positron beam

Abstract: Vacancy-type defects in Mg-implanted GaN were probed using a monoenergetic positron beam. Mg ions of multiple energies (15–180 keV) were implanted to provide a 200-nm-deep box profile with Mg concentration of 4 × 1019 cm−3. The major defect species of vacancies introduced by Mg-implantation was a complex between Ga-vacancy (VGa) and nitrogen vacancies (VNs). After annealing above 1000 °C, these defects started to agglomerate, and the major defect species became (VGa)2 coupled with VNs. The defect reaction occurred between not only the defects introduced by the implantation but also the defects introduced by an excess Mg-doping. The depth distribution of vacancy-type defects agreed well with that of implanted Mg, and no large change in the distribution was observed up to 1300°C annealing. Relationships between photoluminescence bands and vacancy-type defects introduced by Mg-implantation are also discussed.

Enhancement of the in-plane stiffness of the hexagonal re-entrant auxetic honeycomb cores

Abstract: In the present work, two new designs of honeycomb structures were presented based on the basic re-entrant structure namely, splined-reentrant and stiffened-reentrant honeycombs. The new structures were designed in order to enhance the in-plane stiffness and maintaining the auxetic behavior of the structure. In this study, finite elements modeling and experimental work were carried out to evaluate the in-plane properties of the new designs. The effect of the geometrical parameters such as rib length and rib thickness of the unit cell of the new designs on the in-plane properties was investigated. Finite elements results showed that the in-plane stiffness of the new designs was improved significantly compared to the basic re-entrant structure. Also, the stiffened-reentrant structure showed better enhancement of the stiffness than the splined-reentrant structure. For example the modulus of elasticity of the stiffened-reentrant structure exceeds 16× that of the basic re-entrant structure in x-direction and was over two times in the y-direction with lower values of the negative Poisson's ratio. Compression tests were carried on honeycomb samples made of steel using laser cutting technique with different geometrical parameters. Test results for the three designs were compared with the finite element results and they were in a good agreement.

Annealing temperature dependence of photovoltaic properties of solar cells containing Cu2SnS3 thin films produced by co-evaporation

Abstract: The ternary compound Cu2SnS3 (CTS) is composed of elements that are low in cost, non-toxic, and abundant in the Earth's crust. In addition, CTS is a p-type semiconductor with a high reported absorption coefficient of more than 104 cm−1 and a band gap energy of 0.92–1.77 eV. It is, therefore, considered to be a suitable candidate for the absorber layer in thin film solar cells. In the present study, CTS thin films were produced by first depositing precursor films by co-evaporation of Cu, Sn, and S, and then annealing them. Solar cells were then fabricated using the CTS films as absorber layers, and the dependence of their photovoltaic properties on the annealing temperature was investigated. The solar cell using the CTS thin film annealed at 570 °C exhibited an open-circuit voltage of 248 mV, a short-circuit current density of 33.5 mA/cm2, a fill factor of 0.439, and a conversion efficiency of 3.66%.

Thermal and structural dependence of auxetic properties of composite materials

Abstract: Negative Poisson's ratio has already been discovered for many geometrical structures. In most cases, however, the metamaterials built upon such geometries are foams or cellular solids. In this paper, a unidirectional fibrous composite built of two constituent materials of different thermomechanical properties has been studied. The resultant composite is a solid material in its volume. In order to obtain a material of the required properties, both the geometry of fibers and the influence of temperature on both materials have been investigated.

Band gap bowing and optical polarization switching in Al1−xGaxN alloys

Abstract: We present a detailed theoretical study of the band gap bowing of wurtzite AlGaN alloys over the full composition range. Our theoretical framework is based on an atomistic tight-binding model, including local strain and built-in potential variations due to random alloy fluctuations. We extract a bowing parameter for the band gap of eV, which is in good agreement with experimental data. Our analysis shows that the bowing of the band gap mainly arises from bowing of the conduction band edge; for the composition dependence of the valence band edge energy we find a close to linear behavior. Finally, we investigate the wave function character of the valence band edge as a function of GaN content x. Our analysis reveals an optical polarization switching around , which is in the range of reported experimental data.

On the third‐generation Calphad databases: An updated description of Mn

Abstract: Aiming for better extrapolations and predictabilities of thermodynamic properties of materials, new thermodynamic models are implemented in the third-generation Calphad databases. In these models, ...

Inducing out‐of‐plane auxetic behavior in needle‐punched nonwovens

Abstract: We describe here a heat-compression protocol used on needle-punched commercial nonwoven fabrics which induces an out-of-plane auxetic response in these treated fabrics. The as-received samples, however, are not auxetic and show a decrease in thickness when stretched uniaxially. Using micro-CT imaging it was found that the vertical fiber bundles/columns, produced in the needle punching step of fabric manufacture, are tilted and buckled as a result of the heat-compression treatment. It is suggested that it is likely the reorientation of these columns during subsequent uniaxial strain that drives thickness increase (auxetic response). Variation of instantaneous Poisson's ratio with axial strain for as-received and treated nonwovens.

Growth of high-quality AlN layers on sapphire substrates at relatively low temperatures by metalorganic chemical vapor deposition

Abstract: We report a three-step method to grow high-quality AlN heteroepitaxial layers on sapphire substrates at relatively low temperatures by metalorganic chemical vapor deposition (MOCVD) without the use of epitaxial lateral overgrowth (ELO) or pulse atomic layer epitaxy (PALE) method. The three-layer AlN structure comprises a 15-nm thick buffer layer, a 50-nm thick intermediate layer, and a 3.4-µm thick template layer grown at 930, 1130, and 1100 °C sequentially on the c-plane sapphire substrate. The resulting AlN layer had smooth surface with well-defined terraces and low root-mean square (RMS) roughnesses of 0.50 and 0.07 nm for 20 × 20 and 1 × 1 µm2 atomic force microscopy (AFM) scans. Band-edge emission was observed at 208 nm by room temperature (RT) photoluminescence (PL) measurements. The total threading dislocation density was 2.5 × 109/cm2 as determined by transmission electron microscopy (TEM), which is comparable to those of some AlN layers recently grown at significantly higher temperatures. Growth evolution was studied and correlated to the TEM results. The residual impurity concentrations were comparable to those of AlN layers grown at higher temperatures, i.e., 1200–1600 °C. This study demonstrates the high quality AlN layers on sapphire substrates can be grown at achievable temperatures for most of the modern MOCVD systems.

Development of epitaxial AlxSc1−xN for artificially structured metal/semiconductor superlattice metamaterials

Abstract: Epitaxial nitride rocksalt metal/semiconductor superlattices are emerging as a novel class of artificially structured materials that have generated significant interest in recent years for their potential application in plasmonic and thermoelectric devices Though most nitride metals are rocksalt, nitride semiconductors in general have hexagonal crystal structure We report rocksalt aluminum scandium nitride (Al,Sc)N alloys as the semiconducting component in epitaxial rocksalt metal/semiconductor superlattices The AlxSc1−xN alloys when deposited directly on MgO substrates are stabilized in a homogeneous rocksalt (single) phase when x < 051 Employing 20 nm TiN as a seed layer on MgO substrates, the homogeneity range for stabilizing the rocksalt phase has been extended to x < 082 for a 120 nm film The rocksalt AlxSc1−xN alloys show moderate direct bandgap bowing with a bowing parameter, B = 141 ± 019 eV The direct bandgap of metastable rocksalt-AlN is extrapolated to be 470 ± 020 eV The tunable lattice parameter, bandgap, dielectric permittivity, and electronic properties of rocksalt AlxSc1−xN alloys enable high quality epitaxial rocksalt metal/AlxSc1−xN superlattices with a wide range of accessible metamaterials properties

The emergence of auxetic material as a result of optimal isotropic design

Abstract: Using both mathematical and numerical methods, the optimal distributions of material characterized by the Young modulus and Poisson ratio (as well as other moduli of isotropy) maximizing the overall stiffness of an inhomogeneous isotropic elastic 3D body transmitting a given surface loading to a given support are constructed. The overall stiffness of the body is defined as the inverse of the work of external forces on displacements, called here the compliance of the structure. The isoperimetric condition bounds the integral of the trace of the Hooke tensor. It is proved that isotropic composite materials forming the bodies of extremely high stiffness exhibit negative Poisson ratio in large subdomains, which points at the significance of the auxetic material in modern structural design. The obtained results show that the whole range of possible variation of the Poisson ratio is used, from −1 to 1/2, which proves usefulness of the auxetic materials.

Dependence of the Raman spectrum characteristics on the number of layers and stacking orientation in few-layer graphene

Abstract: Few-layer graphene (FLG) samples prepared by two methods (chemical vapor deposition (CVD) followed by transfer onto SiO2/Si substrate and mechanical exfoliation) are characterized by combined optical contrast and micro-Raman mapping experiments. We examine the behavior of the integrated intensity ratio of the 2D and G bands (A2D/AG) and of the 2D band width (Γ2D) as a function of the number of layers (N). For our mechanically exfoliated FLG, A2D/AG decreases and Γ2D increases with N as expected for commensurately stacked FLG. For CVD FLG, both similar and opposite behaviors are observed and are ascribed to different stacking orders. For small (respectively, large) relative rotation angle between consecutive layers (θ), the values of the A2D/AG ratio is smaller (respectively, larger) and the 2D band is broader (respectively, narrower) than for single-layer graphene. Moreover, the A2D/AG ratio decreases (respectively, increases) and, conversely, Γ2D increases (respectively, decreases) as a function of N for small (respectively, large) θ. An intermediate behavior has also been found and is interpreted as the presence of both small and large θ within the studied area. These results confirm that neither A2D/AG nor Γ2D are definitive criteria to identify single-layer graphene, or to count N in FLG.

Auxeticity of face‐centered cubic metal (001) nanoplates

Abstract: We present the results of an atomistic study on the Poisson's ratios of face-centered cubic metal (001) nanoplates under tensile loading. Here, we find that the behavior of the Poisson's ratios of metal nanoplates is strongly dependent on the characteristics of a phase transformation that takes place in their bulk counterparts as well as on the amount of compressive stress induced in the nanoplates. In addition, we discuss the effects of the nanoplate thickness and temperature on the mechanical behavior of the nanoplates. As the thickness decreases, the amount of compressive stress increases. As a result, the metal nanoplates become more auxetic. Higher temperatures cause the phase transformation to occur sooner. Thus, strongly auxetic nanoplates can be obtained by raising the temperature. In addition to investigating the effects of the thickness and temperature, we compare the behaviors of the Poisson's ratios of (001) nanoplates of six different metals. Interestingly, the behaviors of the Poisson's ratios of the metal nanoplates differ, even though their corresponding bulks have similar and positive Poisson's ratios. This is because the six metals exhibit large differences in their surface stresses as well as in the critical strains for the phase transformation.

First-principles calculation of Cu2SnS3 and related compounds

Abstract: We evaluate the electronic structures of Cu2SnS3 (CTS) and related compounds, Cu2GeS3 (CGS), Cu2SiS3 (CSS), Cu2SnSe3 (CTSe), Cu2GeSe3 (CGSe), and Cu2SiSe3 (CSSe), using a first-principles calculation. To determine band-gap energy, Eg, without use of the reported crystal structure, we used the Heyd–Scuseria–Ernzerhof (HSE) screened hybrid density functional after geometry optimization by a DFT-D2 approach in consideration of the van der Waals (vdW) forces. The Eg values of the monoclinic CTS and related compounds with a space group of Cc were calculated using the HSE functional with α = 0.32. The determined Eg values were 0.88 eV for CTS, 1.54 eV for CGS, 2.87 eV for CSS, 0.45 eV for CTSe, 0.72 eV for CGSe, and 1.72 eV for CSSe. Additionally, we discuss the reason for the difference in Eg between CTS and related compounds from density of states (DOS).

High-energy-product MnBi films with controllable anisotropy

Abstract: High-anisotropy NiAs-type MnBi films are produced by in situ annealing of Bi/Mn/Bi trilayers, [Bi/Mn/Bi]n multilayers, and subsequent magnetic field annealing. Phase components, crystallographic anisotropy, and magnetic properties of the MnxBi100−x thin films exhibits strong dependence on Mn concentration. High-purity MnBi thin films with perfect c-axis orientation are obtained by carefully controlling the Mn/Bi ratio. An energy product of 16.3 MGOe, which is the highest value reported so far, is achieved for the x = 50 film of thickness t = 100 nm. The MnBi thick film (t = 2 μm) changes from isotropic to anisotropic after magnetic field annealing. Depending on the direction of the applied field during magnetic field annealing, the MnBi thick film may have out-of-plane or in-plane anisotropy. This control of anisotropy direction enables applications of MnBi films in permanent-magnet, spintronic devices, or magnetic micro-electro-mechanical systems. In addition, the room-temperature magnetocrystalline anisotropy constant and saturation polarization of the hard magnetic MnBi phase are determined to be K = K1 + K2 = 15.0 Mergs cm−3 and Js = 8.2 kG, respectively.

Mechanical metamaterials with anisotropic and negative effective mass-density tensor made from one constituent material

Abstract: If one replaces the bulk masses in a three-dimensional (3D) mass-and-spring model of a solid by composite masses made of different springs connected to a heavy internal mass, one obtains a paradigm for a metamaterial with an effectively frequency-dependent and anisotropic mass-density tensor. Besides the different internal eigenfrequencies, components of this tensor can become negative. Today, such structures can be fabricated by using 3D printing – usually, however, only out of one constituent material and with constraints regarding the maximum ratio of unit cell size to minimum feature size. Here, we use numerical phonon band structure calculations to study what performance can be expected with current fabrication technology.

Scaling of Lennard-Jones liquid elastic moduli, viscoelasticity and other properties along fluid-solid coexistence

Abstract: Static and dynamical properties of the Lennard–Jones (LJ) fluid along the fluid–solid coexistence line are determined by molecular dynamics simulation. A number of properties, such as the radial distribution function, Einstein frequency, mean force, root mean square force, and normalised time correlation functions are shown to be essentially invariant or structurally isomorphic along this line when scaled by so-called macroscopic variables (MRU). Other quantities subject to MRU such as the potential energy, pressure and infinite frequency compressional modulus are not constant along this line of states but can be reproduced using simple formulae of the form for Roskilde fluids. The elastic moduli fall within the domain of isomorphism theory. A generalised Cauchy relationship in which the infinite frequency longitudinal modulus is proportional to the longitudinal modulus of the fluid was found to be obeyed very well for the LJ fluid phase along this coexistence line.

From flat graphene to bulk carbon nanostructures

Abstract: Studies on thin sheets and related materials are of high importance nowadays because of their great potential in various applications. The latest success in the production of graphene opens many new opportunities for the construction of novel three-dimensional carbon nanostructures that exhibit good mechanical and electronic properties together with high specific surface area. Such novel nanostructures based on graphene sheets are very promising for energy storage devices, supercapacitors and stretchable electronics, to name a few. In this work, the mechanical responses of new bulk carbon nanostructures under hydrostatic pressure or shear strain are investigated, respectively, via molecular dynamics simulations. The size effect of the structural units on the strength of crumpled graphene is analyzed. The studied bulk carbon nanostructures are found to be extremely stable against diamondization. It is shown that the structures and mechanical properties of bulk carbon nanomaterials can be altered by severe plastic shear deformation. Shear strain leads to the formation of stable structures, even at relatively small strain.