Penta-graphene: A new carbon allotrope

A 2D metastable carbon allotrope, penta-graphene, composed entirely of carbon pentagons and resembling the Cairo pentagonal tiling, is proposed. State-of-the-art theoretical calculations confirm that the new carbon polymorph is not only dynamically and mechanically stable, but also can withstand temperatures as high as 1000 K. Due to its unique atomic configuration, penta-graphene has an unusual negative Poisson's ratio and ultrahigh ideal strength that can even outperform graphene. Furthermore, unlike graphene that needs to be functionalized for opening a band gap, penta-graphene possesses an intrinsic quasi-direct band gap as large as 3.25 eV, close to that of ZnO and GaN. Equally important, penta-graphene can be exfoliated from T12-carbon. When rolled up, it can form pentagon-based nanotubes which are semiconducting, regardless of their chirality. When stacked in different patterns, stable 3D twin structures of T12-carbon are generated with band gaps even larger than that of T12-carbon. The versatility of penta-graphene and its derivatives are expected to have broad applications in nanoelectronics and nanomechanics.

Auxetics and Other Systems of Anomalous Characteristics

Tetrahex-carbon is a recently predicted two dimensional (2D) carbon allotrope which is composed of tetragonal and hexagonal rings. Unlike flat graphene, this new 2D carbon structure is buckled, possesses a direct band gap ~ 2.6 eV and high carrier mobility with anisotropic feature. In this work, we employ first-principles density-functional theory calculations to explore mechanical properties of tetrahex-C under uniaxial tensile strain. We find that tetrahex-C demonstrates ultrahigh ideal strength, outperforming both graphene and penta-graphene. It shows superior ductility and sustains uniaxial tensile strain up to 20% (16%) till phonon instability occurs, and the corresponding maximal strength is 38.3 N/m (37.8 N/m) in the zigzag (armchair) direction. It shows intrinsic negative Poisson's ratio. This exotic in-plane Poisson's ratio takes place when axial strain reaches a threshold value of 7% (5%) in the zigzag (armchair) direction. We also find that tetrahex-C holds a direct band gap of 2.64 eV at the center of Brillouin zone. This direct-band-gap feature maintains intact upon strain application with no direction-indirect gap transition. The ultrahigh ideal strength, negative Poisson's ratio and integrity of direct-gap under strain in tetrahex-C suggest it may have potential applications in nanomechanics and nanoelectronics.

Preface: phys. stat. sol. (b) 245/11

Silicon dioxide or silica, normally existing in various bulk crystalline and amorphous forms, was recently found to possess a two-dimensional structure. In this work, we use ab initio calculation and evolutionary algorithm to unveil three new two-dimensional (2D) silica structures whose thermal, dynamical, and mechanical stabilities are compared with many typical bulk silica. In particular, we find that all three of these 2D silica structures have large in-plane negative Poisson's ratios with the largest one being double of penta graphene and three times of borophenes. The negative Poisson's ratio originates from the interplay of lattice symmetry and Si-O tetrahedron symmetry. Slab silica is also an insulating 2D material with the highest electronic band gap (>7 eV) among reported 2D structures. These exotic 2D silica with in-plane negative Poisson's ratios and widest band gaps are expected to have great potential applications in nanomechanics and nanoelectronics.

In isotropic materials, it is known that values of Poisson's ratio larger than one half are thermodynamically inadmissible, for such values would lead to negative strain energy under certain loads. Although a negative Poisson's ratio is not forbidden by thermodynamics, it is rare in crystalline solids. With the development of modem fiber reinforced composite materials, the effective Poisson's ratio of laminated fiber reinforced composites shows a peculiar behavior as it becomes larger than one half or less than zero. In this article, a study of negative in-plane Poisson's ratio for a general class of randomly-oriented composite laminates is presented. A simple random statistical analysis has been presented. It is demonstrated that composite laminates with negative in-plane Poisson's ratio could be achieved by using the specific values of independent elastic constants E1, E2, G12, and v12 in each lamina. Also, the influence of the lamina material properties to the negative in-plane Poisson's ratio of the composite laminates is presented. The results from this statistical analysis provide a set of general guidelines for designing composite laminates with a special character-the negative in-plane Poisson's ratio.

Crashworthiness analysis of cylindrical tubes filled with conventional and negative Poisson's ratio foams

Elastic band structures of two-dimensional solid phononic crystal with negative Poisson's ratios

The 21st Winterschool on Electronic Properties of Novel Materials (IWEPNM 2007) took place from 10 March to 17 March 2007. It continued the tradition of winterschools of this type in Kirchberg/Tirol in its well-known atmosphere of collegiality and emphasis on scientific exchange between young and experienced scientists. The sessions at the IWEPNM 2007 focused on quantum information, carbon nanostructure synthesis, purification and separation, the characterization and properties of carbon nanostructures, fullerenes, endohedrals, and fullerides, excitons in carbon nanotubes, the chemical treatment of carbon nanotubes, nanotube filling and double-walled nanotubes, graphite and graphene, non-carbonaceous nanostructures, electron and heat transport in carbon nanostructures, composites with carbon nanotubes, and on theory and applications of these materials. This proceedings volume is divided into according sections. The program of the Winterschool was composed of invited oral presentations, poster presentations and mini-workshops. The oral presentations were scheduled in morning and evening sessions, the mini-workshops took place during the afternoons, and the posters were presented and discussed in two late-evening sessions. In 2007 the Winterschool celebrated an anniversary: It had been of taking place for the 20th time at the Hotel Sonnenalp, a hotel ideally suited for the close interactions of international participants from many countries. We are grateful to the manager of Hotel Sonnenalp, Frau Edith Mayer, and her staff for the local arrangements as well as for her patience with many special requests during the meeting. We owe special thanks to the hotel staff for their careful organization of the celebration. The event benefited substantially from support of the Universitat Wien and the Verein zur Forderung der Winterschulen in Kirchberg, as well as from numerous industrial sponsors. We greatly appreciate their financial contribution without which the meeting would have hardly become possible. Finally, special acknowledgments go to all contributors of the IWEPNM 2007 and to the authors of this volume, the proceedings of the Winterschool. Viera Skakalova was instrumental in compiling and editing the proceedings. In the name of all participants who have enjoyed the Winterschools since 1985 we express special thanks to Hans Kuzmany for his outstanding engagement in organizing the IWEP for so many years. His experience and advice will be needed for future Winterschools. The next Winterschool in this series will take place from 1 March to 8 March 2008, again in Kirchberg/Tirol, and will be organized by Christian Thomsen from Berlin. Wien, Darmstadt, Stuttgart, Berlin, 2007

Preface: phys. stat. sol. (b) 245/3

Elastic properties of a material composed of alternating layers of negative and positive Poisson's ratio

to mechanical metamaterials and related systems with particular emphasis on auxetic systems. Such materials show counter-intuitive, negative behaviours. For example, auxetic materials exhibit negative Poisson’s ratios which mean that these systems get fatter rather than thinner when stretched. Due to the superior properties exhibited by these systems, their study and characterisation has experienced a signifi cant increase, particularly in the last decade, something which may be perceived from the always increasing number of publications and patents in these areas. In fact, this issue is the seventh in a series of special issues published in physica status solidi (b) dedicated to auxetic materials and related systems (see [1–6]).

Spatial tuning of the positive and negative Poisson's ratio of metamaterials through gradient design

Two-dimensional (2D) materials with negative Poisson's ratio (NPR) attract considerable attention because of their exotic mechanical properties. We propose a new 2D material, monolayer GaPS4, which shows NPR for both in-plane (-0.033) and out-of-plane (-0.62) directions. Such coexistence of NPR in two distinctdirections could be explained by its corner- and edge-shared tetrahedra pucker structure. GaPS4 has an ultralow cleavage energy of 0.23 J m-2 according to our calculation, such thatexfoliation of the bulk material is feasible for the preparation of mono- and few-layer GaPS4. Direct wide band gap of 3.55 eV and moderate electron mobility have been revealed in monolayer GaPS4, while the direct gap feature is robust within a strain range of -6% to 6%. These findings render 2D GaPS4 a promising candidate for applications in nanoelectronics and low-dimensional electromechanical devices.

In this work, continuous fiber reinforced thermoplastic negative Poisson's ratio structures (CFNPRSs) with rotating squares are fabricated by 3D printing based on a symmetrical orthogonal and one‐stoke path planning method. The influences of the printing path on the Poisson's ratio, elastic modulus and energy absorption of the structures under compression are systematically researched. The distribution of continuous fiber at the hinge has a great influence on the compression behavior of the structures. As the number of cross laps of continuous fibers at the hinge increases, the negative Poisson's ratio effect decreases while the elastic modulus and energy absorption increase. The printed CFNPRSs with no cross lap have the most obvious negative Poisson's ratio effect, with an average Poisson's ratio of −0.61. A comparative study on Poisson's ratio of the 3D printed polylactic acid negative Poisson's ratio structures (PANPRSs) is carried out, and the results show that the PANPRSs cannot achieve the negative Poisson's ratio effect because the insufficient stiffness of the printed rotation units violates the rigid assumption in theoretical model. Furthermore, the printed CFNPRSs with a lower relative density of 0.18 has an even better negative Poisson's ratio effect than the existing fiber‐reinforced auxetic structures fabricated by 3D printing. This work can provide a significant reference for the preparation of lightweight functional structures using 3D printing.Highlights The CFNPRSs with rotating squares are prepared by 3D printing technique. The path planning of the fiber at the hinge affects the properties of CFNPRSs. The negative Poisson's ratio of CFNPRSs is more obvious than that of PANPRSs. The relative density and Poisson's ratio of CFNPRSs have obvious advantages.