The Poisson's ratio is a fundamental measure of the elastic-deformation behaviour of materials. Although negative Poisson's ratios are theoretically possible, they were believed to be rare in nature. In particular, while some studies have focused on finding or producing materials with a negative Poisson's ratio in bulk form, there has been no such study for nanoscale materials. Here we provide numerical and theoretical evidence that negative Poisson's ratios are found in several nanoscale metal plates under finite strains. Furthermore, under the same conditions of crystal orientation and loading direction, materials with a positive Poisson's ratio in bulk form can display a negative Poisson's ratio when the material's thickness approaches the nanometer scale. We show that this behaviour originates from a unique surface effect that induces a finite compressive stress inside the nanoplates, and from a phase transformation that causes the Poisson's ratio to depend strongly on the amount of stretch.

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.

We conducted molecular statics simulations to investigate the negative Poisson's ratio (auxetic behavior) of periodic porous graphene structures based on the rotating rigid unit mechanism. To obtain a negative Poisson's ratio, simple voids were periodically introduced into graphene. We showed that the Poisson's ratio of the designed graphene structure is strongly dependent on the aspect ratio of the voids, and it can approach the theoretical limit of À1.0. More importantly, the graphene periodic structure maintains its auxetic behavior even under large strains (e $ 0.20). Hence, it can be employed in a wide range of applications requiring structures that can endure large deformation. In addition, we found that the key factor in the auxeticity of the investigated structures is the deformation occurring at the void tips.

MD) is a simple process that utilizes heat to drive vapor to pass through a porous hydrophobic membrane and obtains clean and fresh water in the permeate, while liquid water is blocked by surface hydrophobicity. [2] The potential of using lowgrade heat has made MD a promising low-cost and sustainable water desalination technology. [3] MD is also a key technology in many zero-discharge sustainable processes because the "last-mile" of such processes often involves a high-salinity water solution that is failed to be treated by other desalination technologies, while MD can in principle harvest 100% water from either polluted or high salinity waters. [4] The reported MD membranes are typically prepared from hydrophobic polymers such as polypropylene (PP), poly(vinylidene fluoride) (PVDF), and poly(tetrafluoroethylene) (PTFE). [5] However, these membranes have suffered from low flux. Structural optimization using, for example, sandwiched, hierarchical, or Janus structures, has been studied extensively to improve the flux. [6] Nevertheless, the best-reported membrane flux thus far is less than 80 L m −2 h −1 (LMH) in the direct contact membrane distillation (DCMD) mode even under a high temperature gradient of 90/20 °C. [2a,6a,7] As a result, applications of these membranes in sustainable processes such as solar-driven MD systems have shown far below the practically required water capacity.In our previous work, we found a graphitic carbon nanowire membrane showing a superior water flux of 400 LMH at 90/20 °C in vacuum membrane distillation process (VMD) mode because of the fast water transport on the graphitic surface and the short transport pathway. [8] However, the 1D forest-like carbon nanowires also caused profound concentration polarization. In contrast, the atomically thin 2D graphene, another graphitic carbon material, with a similar surface masstransfer property to the 1D carbon nanowires/tubes, could ameliorate these challenges, while holds potentially even shorter transport distance thus higher MD performance. [9] However, because of its impermeability and insufficient strength over large areas, a functional graphene membrane will require postsynthesis pore generation and transfer to porous support. Both have been proved to be of great challenge in the current studies. [9c,10] Membrane distillation has attracted great attention in the development of sustainable desalination and zero-discharge processes because of its possibility of recovering 100% water and the potential for integration with low-grade heat, such as solar energy. However, the conventional membrane structures and materials afford limited flux thus obstructing its practical application. Here, ultrathin nanoporous graphene membranes are reported by selectively forming thin graphene layers on the top edges of a highly porous anodic alumina oxide support, which creates short and fast transport pathways for water vapor but not liquid. The process avoids the challenging pore-generation and substrate-transfer processes required to prepare regu...

This report employed molecular statics simulation and densityfunctional-theory calculation to study the Poisson's ratios of face-centered-cubic materials. We provide numerical and theoretical evidences to show that cubic materials can exhibit auxetic behavior in a principal direction under proper loading conditions. When a stress perpendicular to the loading direction is applied, cubic materials can exhibit a negative Poisson's ratio at finite strain. The negative Poisson's ratio behavior, including its direction and value, is highly dependent on the direction and magnitude of the transversely applied stresses. As a result, we show that it is possible to tune the direction and magnitude of the negative Poisson's ratio behavior of cubic materials by controlling the transverse loadings.

We utilized molecular statics (MS) simulations to investigate the auxeticity of single layer black phosphorus (SLBP). Our simulation results show that the SLBP has a negative in‐plane Poisson's ratio in the zigzag direction when the applied strain along the armchair direction exceeds 0.018. We show that the interplay between bond stretching and bond rotating modes determines the in‐plane Poisson's ratio behavior. While the bond stretching mode always tends to increase the in‐plane auxeticity, the bond rotating mode might increase or decrease the in‐plane auxeticity. Furthermore, we show that graphite also exhibits an in‐plane negative Poisson's ratio at finite strains due to a similar mechanism.

The coefficient of thermal expansion, which measures the change in length, area, or volume of a material upon heating, is a fundamental parameter with great relevance for many applications. Although there are various routes to design materials with targeted coefficient of thermal expansion at the macroscale, no approaches exist to achieve a wide range of values in graphene-based structures. Here, we use molecular dynamics simulations to show that graphene origami structures obtained through pattern-based surface functionalization provide tunable coefficients of thermal expansion from large negative to large positive. We show that the mechanisms giving rise to this property are exclusive to graphene origami structures, emerging from a combination of surface functionalization, large out-of-plane thermal fluctuations, and the three-dimensional geometry of origami structures.

When materials are under stretching, occurrence of lateral contraction of materials is commonly observed. This is because Poisson’s ratio, the quantity describes the relationship between a lateral strain and applied strain, is positive for nearly all materials. There are some reported structures and materials having negative Poisson’s ratio. However, most of them are at macroscale, and reentrant structures and rigid rotating units are the main mechanisms for their negative Poisson’s ratio behavior. Here, with numerical and theoretical evidence, we show that metal [100] nanowires with asymmetric cross-sections such as rectangle or ellipse can exhibit negative Poisson’s ratio behavior. Furthermore, the negative Poisson’s ratio behavior can be further improved by introducing a hole inside the asymmetric nanowires. We show that the surface effect inducing the asymmetric stresses inside the nanowires is a main origin of the superior property.