Schwarzites and Triply Periodic Minimal Surfaces: From Pure Topology Mathematics to Macroscale Applications.

Schwarzites are porous (spongy-like) carbon allotropes with negative Gaussian curvatures. They are proposed by Mackay and Terrones inspired by the works of the German mathematician Hermann Schwarz on Triply-Periodic Minimal Surfaces (TPMS). This review presents and discusses the history of schwarzites and their place among curved carbon nanomaterials. The main works on schwarzites are summarized and are available in the literature. Their unique structural, electronic, thermal, and mechanical properties are discussed. Although the synthesis of carbon-based schwarzites remains elusive, recent advances in the synthesis of zeolite-templates nanomaterials have brought them closer to reality. Atomic-based models of schwarzites are translated into macroscale ones that are 3D-printed. These 3D-printed models are exploited in many real-world applications, including water remediation and biomedical ones.

Reversible actuation of α-borophene nanoscrolls.

In this work, we proposed and investigated the structural and electronic properties of boron-based nanoscrolls (armchair and zigzag) using the DFTB+ method. We also investigated the electroactuation process (injecting and removing charges). A giant electroactuation was observed, but the results show relevant differences between the borophene and carbon nanoscrolls. The molecular dynamics simulations showed that the scrolls are thermally and structurally stable for a large range of temperatures (up to 600 K), and the electroactuation process can be easily tuned and can be entirely reversible for some configurations.

Tetra-penta-deca-hexagonal-graphene (TPDH-graphene) hydrogenation patterns: dynamics and electronic structure.

The advent of graphene has renewed the interest in other 2D carbon-based materials. In particular, new structures have been proposed by combining hexagonal and other carbon rings in different ways. Recently, Bhattacharya and Jana have proposed a new carbon allotrope, composed of different polygonal carbon rings containing 4, 5, 6, and 10 atoms, named tetra-penta-deca-hexagonal-graphene (TPDH-graphene). This unusual topology results in interesting mechanical, electronic, and optical properties with several potential applications, including UV protection. Like other 2D carbon structures, chemical functionalizations can be used to tune TPDH-graphene's physical/chemical properties. In this work, we investigate the hydrogenation dynamics of TPDH-graphene and its effects on its electronic structure, combining DFT and fully atomistic reactive molecular dynamics simulations. Our results show that H atoms are mainly incorporated on tetragonal ring sites (up to 80% at 300 K), leading to the appearance of well-delimited pentagonal carbon stripes. The electronic structure of the hydrogenated structures shows the formation of narrow bandgaps with the presence of Dirac cone-like structures, indicative of anisotropic transport properties.

Scalable Synthesis and Characterization of Multilayer γ-Graphyne, New Carbon Crystals with a Small Direct Band Gap.

γ-Graphyne is the most symmetric sp2/sp1 allotrope of carbon, which can be viewed as graphene uniformly expanded through the insertion of two-carbon acetylenic units between all the aromatic rings. To date, synthesis of bulk γ-graphyne has remained a challenge. We here report the synthesis of multilayer γ-graphyne through crystallization-assisted irreversible cross-coupling polymerization. A comprehensive characterization of this new carbon phase is described, including synchrotron powder X-ray diffraction, electron diffraction, lateral force microscopy, Raman spectroscopy, infrared spectroscopy, and cyclic voltammetry. Experiments indicate that γ-graphyne is a 0.48 eV band gap semiconductor, with a hexagonal a-axis spacing of 6.88 Å and an interlayer spacing of 3.48 Å, which is consistent with theoretical predictions. The observed crystal structure has an aperiodic sheet stacking. The material is thermally stable up to 240 °C but undergoes transformation at higher temperatures. While conventional 2D polymerization and reticular chemistry rely on error correction through reversibility, we demonstrate that a periodic covalent lattice can be synthesized under purely kinetic control. The reported methodology is scalable and inspires extension to other allotropes of the graphyne family.

Synthesis and Characterization of Biotene: A New 2D Natural Oxide From Biotite.

In this work, the synthesis and characterization of ultrathin metal oxide, called biotene, using liquid-phase exfoliation from naturally abundant biotite are demonstrated. The atomically thin biotene is used for energy harvesting using its flexoelectric response under multiple bending. The effective flexoelectric response increases due to the presence of surface charges, and the voltage increases up to ≈8 V, with a high mechano-sensitivity of 0.79 V N-1 for normal force. This flexoelectric response is further validated by density functional theory (DFT) simulations. The atomically thin biotene shows an increased response in the magnetic field and thermal heating. The synthesis of two-dimensional (2D) metal-oxide biotene suggests a wealth of future 2D-oxide material for energy generation and energy harvesting applications.

Guided fractures in graphene mechanical diode-like structures.

The concept of a diode is usually applied to electronic and thermal devices but very rarely for mechanical ones. A recently proposed fracture rectification effect in polymer-based structures with triangular void defects has motivated us to test these ideas at the nanoscale using graphene membranes. Using fully-atomistic reactive molecular dynamics simulations we showed that robust rectification-like effects exist. The fracture can be 'guided' to more easily propagate along one specific direction than its opposite. We also observed that there is an optimal value for the spacing between each void for the rectification effect.

Controlling Movement at Nanoscale: Curvature Driven Mechanotaxis.

Locating and manipulating nano-sized objects to drive motion is a time and effort consuming task. Recent advances show that it is possible to generate motion without direct intervention, by embedding the source of motion in the system configuration. In this work, an alternative manner to controllably displace nano-objects without external manipulation is demonstrated, by employing spiral-shaped carbon nanotube (CNT) and graphene nanoribbon structures (GNR). The spiral shape contains smooth gradients of curvature, which lead to smooth gradients of bending energy. It is shown that these gradients as well as surface energy gradients can drive nano-oscillators. An energy analysis is also carried out by approximating the carbon nanotube to a thin rod and how torsional gradients can be used to drive motion is discussed. For the nanoribbons, the role of layer orientation is also analyzed. The results show that motion is not sustainable for commensurate orientations, in which AB stacking occurs. For incommensurate orientations, friction almost vanishes, and in this instance, the motion can continue even if the driving forces are not very high. This suggests that mild curvature gradients, which can already be found in existing nanostructures, could provide mechanical stimuli to direct motion.

On the adsorption mechanism of caffeine on MAPbI3 perovskite surfaces: a combined UMC-DFT study.

Recently, it was experimentally shown that the performance and thermal stability of the perovskite MAPbI3 were improved upon the adsorption of a molecular layer of caffeine. In this work, we used a hybrid methodology that combines uncoupled monte carlo (UMC) and density functional theory (DFT) simulations to carry out a detailed and comprehensive study of the adsorption mechanism of a caffeine molecule on the surface of MAPbI3. Our results showed that the adsorption distance and energy of a caffeine molecule on the MAPbI3 surface are 2.0 Å and -0.3 eV, respectively. The caffeine/MAPbI3 complex presents a direct bandgap of 2.38 eV with two flat intragap bands distanced 1.15 and 2.18 eV from the top of valence bands. Although the energy band levels are not significantly shifted by the presence of caffeine, the interaction MAPbI3/perovskite is enough to affect the bands' dispersion, particularly the conduction bands.

Few-Wall Carbon Nanotube Coils.

While various electronic components based on carbon nanotubes (CNTs) have already been demonstrated, the realization of miniature electromagnetic coils based on CNTs remains a challenge. Coils made of single-wall CNTs with accessible ends for contacting have been recently demonstrated but were found unsuitable to act as electromagnetic coils because of electrical shorting between their turns. Coils made of a few-wall CNT could in principle allow an insulated flow of current and thus be potential candidates for realizing CNT-based electromagnetic coils. However, no such CNT structure has been produced so far. Here, we demonstrate the formation of few-wall CNT coils and characterize their structural, optical, vibrational, and electrical properties using experimental and computational tools. The coils are made of CNTs with 2, 3, or 4 walls. They have accessible ends for electrical contacts and low defect densities. The coil diameters are on the order of one micron, like those of single-wall CNT coils, despite the higher rigidity of few-wall CNTs. Coils with as many as 163 turns were found, with their turns organized in a rippled raft configuration. These coils are promising candidates for a variety of miniature devices based on electromagnetic coils, such as electromagnets, inductors, transformers, and motors. Being chirally and enantiomerically pure few-wall CNT bundles, they are also ideal for fundamental studies of interwall coupling and superconductivity in CNTs.

Temperature Effects on the Fracture Dynamics and Elastic Properties of Popgraphene Membranes.

Popgraphene (PopG) is a new 2D planar carbon allotrope which is composed of 5-8-5 carbon rings. PopG is intrinsically metallic and possesses excellent thermal and mechanical stability. In this work, we report a detailed study of the thermal effects on the mechanical properties of PopG membranes using fully-atomistic reactive (ReaxFF) molecular dynamics simulations. Our results showed that PopG presents very distinct fracture mechanisms depending on the temperature and direction of the applied stretching. The main fracture dynamics trends are temperature independent and exhibit an abrupt rupture followed by fast crack propagation. The reason for this anisotropy is due to the fact that y-direction stretching leads to a deformation in the shape of the rings that cause the breaking of bonds in the pentagon-octagon and pentagon-pentagon ring connections, which is not observed for the x-direction. PopG is less stiff than graphene membranes, but the Young's modulus value is only 15 % smaller.

Nature inspired solid-liquid phase amphibious adhesive.

Here we report a new class of bio-inspired solid-liquid adhesive, obtained by simple mechanical dispersion of PVDF (polyvinylidene fluoride) (solid spheres) into PDMS (polydimethylsiloxane) (liquid). The adhesive behavior arises from strong solid-liquid interactions. This is a chemical reaction free adhesive (no curing time) that can be repeatedly used and is capable of instantaneously joining a large number of diverse materials (metals, ceramic, and polymer) in air and underwater. The current work is a significant advance in the development of amphibious multifunctional adhesives and presents potential applications in a range of sealing applications, including medical ones.

On the structural stability and optical properties of germanium-based schwarzites: a density functional theory investigation.

Since graphene was synthesized the interest in building new 2D and 3D structures based on carbon allotropes has been growing every day. One of these 3D structures is know as carbon schwarzites. Schwarzites consist of carbon nanostructures possessing the shape of Triply-Periodic Minimal Surfaces (TPMS), which is characterized by a negative Gaussian curvature introduced by the presence of carbon rings with more than six atoms. Some examples of schwarzite families include: primitive (P), gyroid (G) and diamond (D). Previous studies considering different element species of schwarzites have investigated the mechanical, electrical and thermal properties. In this work, we investigated the stability of germanium (Ge) schwarzites using density functional theory with the GGA exchange-correlation functional. We chose one structure of each family (P8bal), (G688) and (D688). It was observed that regions usually flat in carbon schwarzites acquire buckled configurations as previously observed on silicene and germanene monolayers. The investigated structures presented a semiconducting bandgap ranging from 0.13 to 0.27 eV. We also performed calculations of optical properties within the linear regime, where it was shown that Ge schwarzite structures absorb light from infrared to ultra-violet frequencies. Therefore, our results open new perspectives of materials that can be used in optoelectronics device applications.

3D Printed Tubulanes as Lightweight Hypervelocity Impact Resistant Structures.

Lightweight materials with high ballistic impact resistance and load-bearing capabilities are regarded as a holy grail in materials design. Nature builds these complementary properties into materials using soft organic materials with optimized, complex geometries. Here, the compressive deformation and ballistic impact properties of three different 3D printed polymer structures, named tubulanes, are reported, which are the architectural analogues of cross-linked carbon nanotubes. The results show that macroscopic tubulanes are remarkable high load-bearing, hypervelocity impact-resistant lightweight structures. They exhibit a lamellar deformation mechanism, arising from the tubulane ordered pore structure, manifested across multiple length scales from nano to macro dimensions. This approach of using complex geometries inspired by atomic and nanoscale models to generate macroscale printed structures allows innovative morphological engineering of materials with tunable mechanical responses.

The structure of graphene on graphene/C60/Cu interfaces: a molecular dynamics study.

Two experimental studies reported the spontaneous formation of amorphous and crystalline structures of C60 molecules intercalated between graphene and a surface. The findings observed included interesting phenomena ranging from reaction between fullerene C60s ('C60s' stands for plural of C60) under graphene to graphene sheets sagging between C60s and control of strain in these sheets. Motivated by this work, we performed fully atomistic reactive molecular dynamics simulations to investigate the formation and thermal stability of graphene sheet wrinkles as well as graphene attachment to and detachment from a surface when the sheet is laid over a previously distributed array of C60 molecules on a copper surface at different temperatures. As graphene compresses the C60s against the surface, and graphene attachment to the surface in between C60s depends on the height of the wrinkles in the graphene sheet, configurations with both frozen and non-frozen fullerenes were investigated in the simulations in order to examine the experimental result of stable, sagged graphene sheets when the distance between C60s is about 4 nm and the height of the wrinkles in the sheet is about 0.8 nm. Below a distance of 4 nm between fullerenes, the graphene is predicted to become locally suspended and less strained. The simulations predict that this happens when the fullerenes can deform under the compressive action of the graphene sheet. If the fullerenes are kept frozen, spontaneous 'blanketing' of graphene is predicted only when the distance between neighbouring C60s is equal to or great than about 7 nm. These predictions agree with a mechanical model relating the rigidity of a graphene sheet to the energy of graphene-surface adhesion. This work further reveals the structure of intercalated molecules and the role of stability and sheet wrinkling on the preferred configuration of graphene. This study thus might assist in the development of two-dimensional confined nanoreactors for chemical reactions.

Strain Rate Dependent Shear Plasticity in Graphite Oxide.

Graphene oxide film is made of stacked graphene layers with chemical functionalities, and we report that plasticity in the film can be engineered by strain rate tuning. The deformation behavior and plasticity of such functionalized layered systems is dominated by shear slip between individual layers and interaction between functional groups. Stress-strain behavior and theoretical models suggest that the deformation is strongly strain rate dependent and undergoes brittle to ductile transition with decreasing strain rate.

Defect-Free Carbon Nanotube Coils.

Carbon nanotubes are promising building blocks for various nanoelectronic components. A highly desirable geometry for such applications is a coil. However, coiled nanotube structures reported so far were inherently defective or had no free ends accessible for contacting. Here we demonstrate the spontaneous self-coiling of single-wall carbon nanotubes into defect-free coils of up to more than 70 turns with identical diameter and chirality, and free ends. We characterize the structure, formation mechanism, and electrical properties of these coils by different microscopies, molecular dynamics simulations, Raman spectroscopy, and electrical and magnetic measurements. The coils are highly conductive, as expected for defect-free carbon nanotubes, but adjacent nanotube segments in the coil are more highly coupled than in regular bundles of single-wall carbon nanotubes, owing to their perfect crystal momentum matching, which enables tunneling between the turns. Although this behavior does not yet enable the performance of these nanotube coils as inductive devices, it does point a clear path for their realization. Hence, this study represents a major step toward the production of many different nanotube coil devices, including inductors, electromagnets, transformers, and dynamos.

The structural and dynamical aspects of boron nitride nanotubes under high velocity impacts.

This communication report is a study on the structural and dynamical aspects of boron nitride nanotubes (BNNTs) shot at high velocities (∼5 km s(-1)) against solid targets. The experimental results show unzipping of BNNTs and the formation of hBN nanoribbons. Fully atomistic reactive molecular dynamics simulations were also carried out to gain insights into the BNNT fracture patterns and deformation mechanisms. Our results show that longitudinal and axial tube fractures occur, but the formation of BN nanoribbons from fractured tubes was only observed for some impact angles. Although some structural and dynamical features of the impacts are similar to the ones reported for CNTs, because BNNTs are more brittle than CNTs this results in a larger number of fractured tubes but with fewer formed nanoribbons.

Solid-Vapor Reaction Growth of Transition-Metal Dichalcogenide Monolayers.

Two-dimensional (2D) layered semiconducting transition-metal dichalcogenides (TMDCs) are promising candidates for next-generation ultrathin, flexible, and transparent electronics. Chemical vapor deposition (CVD) is a promising method for their controllable, scalable synthesis but the growth mechanism is poorly understood. Herein, we present systematic studies to understand the CVD growth mechanism of monolayer MoSe2 , showing reaction pathways for growth from solid and vapor precursors. Examination of metastable nanoparticles deposited on the substrate during growth shows intermediate growth stages and conversion of non-stoichiometric nanoparticles into stoichiometric 2D MoSe2 monolayers. The growth steps involve the evaporation and reduction of MoO3 solid precursors to sub-oxides and stepwise reactions with Se vapor to finally form MoSe2 . The experimental results and proposed model were corroborated by ab initio Car-Parrinello molecular dynamics studies.

Unzipping carbon nanotubes at high impact.

The way nanostructures behave and mechanically respond to high impact collision is a topic of intrigue. For anisotropic nanostructures, such as carbon nanotubes, this response will be complicated based on the impact geometry. Here we report the result of hypervelocity impact of nanotubes against solid targets and show that impact produces a large number of defects in the nanotubes, as well as rapid atom evaporation, leading to their unzipping along the nanotube axis. Fully atomistic reactive molecular dynamics simulations are used to gain further insights of the pathways and deformation and fracture mechanisms of nanotubes under high energy mechanical impact. Carbon nanotubes have been unzipped into graphene nanoribbons before using chemical treatments but here the instability of nanotubes against defect formation, fracture, and unzipping is revealed purely through mechanical impact.

Novel nanoscroll structures from carbon nitride layers.

Nanoscrolls (papyrus-like nanostructures) are very attractive structures for a variety of applications, owing to their tunable diameter and large accessible surface area. They have been successfully synthesized from different materials. In this work, we investigate, through fully atomistic molecular dynamics simulations, the dynamics of scroll formation for a series of graphene-like carbon nitride (CN) two-dimensional systems: g-CN, triazine-based g-C3 N4 , and heptazine-based g-C3 N4 . Our results show that stable nanoscrolls can be formed for each of these structures. Possible synthetic routes to produce these nanostructures are also addressed.