Two-dimensional covalent organic frameworks (2D COFs) are a class of modular polymeric crystals with high porosities and large surface areas, which position them as ideal candidates for applications in gas storage and separation technologies. In this work, we study the influence of pore geometry on the anisotropic heat transfer mechanisms in 2D COFs through systematic atomistic simulations. More specifically, by studying COFs with varying pore sizes and gas densities, we demonstrate that the cross-plane thermal conductivity along the direction of the laminar pores can either be decreased due to solid–gas scattering (for COFs with relatively smaller pores that are ≲2 nm) or increased due to additional heat transfer pathways introduced by the gas adsorbates (for COFs with relatively larger pores). Our simulations on COF/methane systems reveal the intricate relationship among gas diffusivities, pore geometries, and solid–gas interactions dictating the modular thermal conductivities in these materials. Along with the understanding of the fundamental nature of gas diffusion and heat conduction in the porous framework crystals, our results can also help guide the design of efficient 2D polymeric crystals for applications with improved gas storage, catalysis, and separation capabilities.

The prospect of combining two-dimensional materials in vertical stacks has created a new paradigm for materials scientists and engineers. Herein, we show that stacks of two-dimensional covalent organic frameworks are endowed with a host of unique physical properties that combine low densities, high thermal conductivities, and highly negative Poisson’s ratios. Our systematic atomistic simulations demonstrate that the tunable mechanical and thermal properties arise from their singular layered architecture comprising strongly bonded light atoms and periodic laminar pores. For example, the negative Poisson’s ratio arises from the weak van der Waals interactions between the two-dimensional layers along with the strong covalent bonds that act as hinges along the layers, which facilitate the twisting and swiveling motion of the phenyl rings relative to the tensile plane. The mechanical and thermal properties of two-dimensional covalent organic frameworks can be tailored through structural modularities such as control over the pore size and/or interlayer separation. We reveal that these materials mark a regime of materials design that combines low densities with high thermal conductivities arising from their nanoporous yet covalently interconnected structure.

Hydrogen-bonded organic frameworks (HOFs) are a class of nanoporous crystalline materials formed by the assembly of organic building blocks that are held together by a network of hydrogen-bonding interactions. Herein, we show that the dynamic and responsive nature of these hydrogen-bonding interactions endows HOFs with a host of unique physical properties that combine ultraflexibility, high thermal conductivities, and the ability to “self-heal”. Our systematic atomistic simulations reveal that their unique mechanical properties arise from the ability of the hydrogen-bond arrays to absorb and dissipate energy during deformation. Moreover, we also show that these materials demonstrate relatively high thermal conductivities for porous crystals with low mass densities due to their extended periodic framework structure that is comprised of light atoms. Our results reveal that HOFs mark a new regime of material design combining multifunctional properties that make them ideal candidates for gas storage and separation, flexible electronics, and thermal switching applications.

The design of innovative porous crystals with high porosities and large surface areas has garnered a great deal of attention over the past few decades due to their remarkable potential for a variety of applications. However, heat dissipation is key to realizing their potential. We use systematic atomistic simulations to reveal that interpenetrated porous crystals formed from two-dimensional (2D) frameworks possess remarkable thermal conductivities at high porosities in comparison to their three-dimensional (3D) single framework and interpenetrated 3D framework counterparts. In contrast to conventional understanding, higher thermal conductivities are associated with lower atomic densities and higher porosities for porous crystals formed from interpenetrating 2D frameworks. We attribute this to lower phonon–phonon scattering and vibrational hardening from the supramolecular interactions that restrict atomic vibrational amplitudes, facilitating heat conduction. This marks a new regime of materials design combining ultralow mass densities and ultrahigh thermal conductivities in 2D interpenetrated porous crystals.

The complete understanding of the mechanical and thermal responses to strain in hybrid organic–inorganic perovskites holds great potential for their proper functionalities in a range of applications, such as in photovoltaics, thermoelectrics, and flexible electronics. In this work, we conduct systematic atomistic simulations on methyl ammonium lead iodide, which is the prototypical hybrid inorganic–organic perovskite, to investigate the changes in their mechanical and thermal transport responses under uniaxial strain. We find that the mechanical response and the deformation mechanisms are highly dependent on the direction of the applied uniaxial strain with a characteristic ductile- or brittle-like failure accompanying uniaxial tension. Moreover, while most materials shrink in the two lateral directions when stretched, we find that the ductile behavior in hybrid perovskites can lead to a very unique mechanical response where negligible strain occurs along one lateral direction while the length contraction occurs in the other direction due to uniaxial tension. This anisotropy in the mechanical response is also shown to manifest in an anisotropic thermal response of the hybrid perovskite where the anisotropy in thermal conductivity increases by up to 30% compared to the unstrained case before plastic deformation occurs at higher strain levels. Along with the anisotropic responses of these physical properties, we find that uniaxial tension leads to ultralow thermal conductivities that are well below the value predicted with a minimum thermal conductivity model, which highlights the potential of strain engineering to tune the physical properties of hybrid organic–inorganic perovskites.