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.

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.

Thermoelectricity Enhanced Electrocatalysis.

We show that thermoelectric materials can function as electrocatalysts and use thermoelectric voltage generated to initiate and boost electrocatalytic reactions. The electrocatalytic activity is promoted by the use of nanostructured thermoelectric materials in a hydrogen evolution reaction (HER) by the thermoelectricity generated from induced temperature gradients. This phenomenon is demonstrated using two-dimensional layered thermoelectric materials Sb2Te3 and Bi0.5Sb1.5Te3 where a current density approaching ∼50 mA/cm2 is produced at zero potential for Bi0.5Sb1.5Te3 in the presence of a temperature gradient of 90 °C. In addition, the turnover frequency reaches to 2.7 s-1 at 100 mV under this condition which was zero in the absence of temperature gradient. This result adds a new dimension to the properties of thermoelectric materials which has not been explored before and can be applied in the field of electrocatalysis and energy generation.

Impact of the intermixed phase and the channel network on the carrier mobility of nanostructured solar cells.

We analyzed the impact of the complex channel network of donor and acceptor domains in nanostructured solar cells on the mobility of the charge carriers moving by thermally activated hopping. Particular attention was given to the so called intermixed phase, or interface roughness, that has recently been shown to promote an increase in the cell efficiency. The domains were obtained from a Monte Carlo simulation of a two-species lattice gas. We generated domain morphologies with controllable channel size and interface roughness. The field and density dependence of the carrier hopping mobility in different morphologies was obtained by solving a master equation. Our results show that the mobility decreases with roughness and increases with typical channel sizes. The deleterious effect of the roughness on the mobility is quite dramatic at low carrier densities and high fields. The complex channel network is shown to be directly responsible for two potentially harmful effects to the cell performance: a remarkable decrease of the mobility with increasing field and the accumulation of charge at the domains interface, which leads to recombination losses.

Two-dimensional cobalt telluride as a piezo-tribogenerator.

Two-dimensional (2D) materials have been shown to be efficient in energy harvesting. Here, we report the use of waste heat to generate electricity via the combined piezoelectric and triboelectric properties of 2D cobalt telluride (CoTe2). The piezo-triboelectric nanogenerator (PTNG) produced an open-circuit voltage of ∼5 V under 1 N force and the effect of temperature in the range of 305-363 K shows a four-fold energy conversion efficiency improvement. The 2D piezo-tribogenerator shows excellent characteristics with a maximum voltage of ∼10 V, fast response time, and high responsivity. Density functional theory was used to gain further insights and validation of the experimental results. Our results could lead to energy harvesting approaches using 2D materials from various thermal sources and dissipating waste heat from electronic devices.

Reformulated space-charge-limited current model and its application to disordered organic systems.

We have reformulated a traditional model used to describe the current-voltage dependence of low mobility materials sandwiched between planar electrodes by using the quasi-electrochemical potential as the fundamental variable instead of the local electric field or the local charge carrier density. This allows the material density-of-states to enter explicitly in the equations and dispenses with the need to assume a particular type of contact. The diffusion current is included and as a consequence the current-voltage dependence obtained covers, with increasing bias, the diffusion limited current, the space-charge limited current, and the injection limited current regimes. The generalized Einstein relation and the field and density dependent mobility are naturally incorporated into the formalism; these two points being of particular relevance for disordered organic semiconductors. The reformulated model can be applied to any material where the carrier density and the mobility may be written as a function of the quasi-electrochemical potential. We applied it to the textbook example of a nondegenerate, constant mobility material and showed how a single dimensionless parameter determines the form of the I(V) curve. We obtained integral expressions for the carrier density and for the mobility as a function of the quasi-electrochemical potential for a Gaussianly disordered organic material and found the general form of the I(V) curve for such materials over the full range of bias, showing how the energetic disorder alone can give rise, in the space-charge limited current regime, to an I∝V(n) dependence with an exponent n larger than 2.

The theoretical current-voltage dependence of a non-degenerate disordered organic material obtained with conductive atomic force microscopy.

We develop a simple continuum model for the current voltage characteristics of a material as measured by the conducting atomic force microscopy, including space charge effects. We address the effect of the point contact on the magnitude of the current and on the transition voltages between the different current regimes by comparing these with the corresponding expressions obtained with planar electrodes.

Extraction of Two-Dimensional Aluminum Alloys from Decagonal Quasicrystals.

Atomically thin metallic alloys are receiving increased attention due to their prospective applications as interconnects/contacts in two-dimensional (2D) circuits, sensors, and catalysts, among others. In this work, we demonstrate an easily scalable technique for the synthesis of 2D metallic alloys from their 3D quasicrystalline precursors. We have used aluminum (Al)-based single-phase decagonal quasicrystal Al66Co17Cu17 alloy to extract the corresponding 2D alloy structure. The 2D layered Al alloy possesses 2-fold decagonal quasicrystalline symmetry and consists of two- or three-layer-thick sheets with a lateral dimension of microns. These 2D metallic layers were combined with the atomic layers of tungsten disulfide to form the stacked heterostructures, which is demonstrated to be a stable and efficient catalyst for hydrogen evolution reaction.

Structural and Thermal Stability of Graphyne and Graphdiyne Nanoscroll Structures.

Graphynes and graphdiynes are generic names for families of two-dimensional carbon allotropes, where acetylenic groups connect benzenoid-like hexagonal rings, with the coexistence of sp and sp2 hybridized carbon atoms. The main differences between graphynes and graphdiynes are the number of acetylenic groups (one and two for graphynes and graphdiynes, respectively). Similarly to graphene nanoscrolls, graphyne and graphdiynes nanoscrolls are nanosized membranes rolled into papyrus-like structures. In this work we studied through molecular dynamics simulations, using reactive potentials, the structural and thermal (up to 1000 K) stability of α,ß,γ-graphyne and α,ß,γ-graphdiyne scrolls. Our results demonstrate that stable nanoscrolls can be created for all the structures studied here, although they are less stable than corresponding graphene scrolls. This can be elucidated as a result of the higher graphyne/graphdiyne structural porosity in relation to graphene, and as a consequence, the π-π stacking interactions decrease.

Hybrid MoS2/h-BN Nanofillers As Synergic Heat Dissipation and Reinforcement Additives in Epoxy Nanocomposites.

Two-dimensional (2D) nanomaterials as molybdenum disulfide (MoS2), hexagonal boron nitride (h-BN), and their hybrid (MoS2/h-BN) were employed as fillers to improve the physical properties of epoxy composites. Nanocomposites were produced in different concentrations and studied in their microstructure, mechanical and thermal properties. The hybrid 2D mixture imparted efficient reinforcement to the epoxy leading to increases of up to 95% in tensile strength, 60% in ultimate strain, and 58% in Young's modulus. Moreover, an enhancement of 203% in thermal conductivity was achieved for the hybrid composite as compared to the pure polymer. The incorporation of MoS2/h-BN mixture nanofillers in epoxy resulted in nanocomposites with multifunctional characteristics for applications that require high mechanical and thermal performance.

Multiscale Geometric Design Principles Applied to 3D Printed Schwarzites.

Schwartzites are 3D porous solids with periodic minimal surfaces having negative Gaussian curvatures and can possess unusual mechanical and electronic properties. The mechanical behavior of primitive and gyroid schwartzite structures across different length scales is investigated after these geometries are 3D printed at centimeter length scales based on molecular models. Molecular dynamics and finite elements simulations are used to gain further understanding on responses of these complex solids under compressive loads and kinetic impact experiments. The results show that these structures hold great promise as high load bearing and impact-resistant materials due to a unique layered deformation mechanism that emerges in these architectures during loading. Easily scalable techniques such as 3D printing can be used for exploring mechanical behavior of various predicted complex geometrical shapes to build innovative engineered materials with tunable properties.

Exfoliation of a non-van der Waals material from iron ore hematite.

With the advent of graphene, the most studied of all two-dimensional materials, many inorganic analogues have been synthesized and are being exploited for novel applications. Several approaches have been used to obtain large-grain, high-quality materials. Naturally occurring ores, for example, are the best precursors for obtaining highly ordered and large-grain atomic layers by exfoliation. Here, we demonstrate a new two-dimensional material 'hematene' obtained from natural iron ore hematite (α-Fe2O3), which is isolated by means of liquid exfoliation. The two-dimensional morphology of hematene is confirmed by transmission electron microscopy. Magnetic measurements together with density functional theory calculations confirm the ferromagnetic order in hematene while its parent form exhibits antiferromagnetic order. When loaded on titania nanotube arrays, hematene exhibits enhanced visible light photocatalytic activity. Our study indicates that photogenerated electrons can be transferred from hematene to titania despite a band alignment unfavourable for charge transfer.

Lightweight Hexagonal Boron Nitride Foam for CO2 Absorption.

Weak van der Waals forces between inert hexagonal boron nitride (h-BN) nanosheets make it easy for them to slide over each other, resulting in an unstable structure in macroscopic dimensions. Creating interconnections between these inert nanosheets can remarkably enhance their mechanical properties. However, controlled design of such interconnections remains a fundamental problem for many applications of h-BN foams. In this work, a scalable in situ freeze-drying synthesis of low-density, lightweight 3D macroscopic structures made of h-BN nanosheets chemically connected by poly(vinyl alcohol) (PVA) molecules via chemical cross-link is demonstrated. Unlike pristine h-BN foam which disintegrates upon handling after freeze-drying, h-BN/PVA foams exhibit stable mechanical integrity in addition to high porosity and large surface area. Fully atomistic simulations are used to understand the interactions between h-BN nanosheets and PVA molecules. In addition, the h-BN/PVA foam is investigated as a possible CO2 absorption and as laser irradiation protection material.