Jong Hyun Jung

A Rigorous Method of Calculating Exfoliation Energies from First Principles

The exfoliation energy, the energy required to peel off an atomic layer from the surface of a bulk material, is of fundamental importance in the science and engineering of two-dimensional materials. Traditionally, the exfoliation energy of a material has been obtained from first-principles by calculating the difference in the ground-state energy between (i) a slab of N atomic layers ( N ≫ 1) and (ii) a slab of N - 1 atomic layers plus an atomic layer separated from the slab. In this paper, we prove that the exfoliation energy can be obtained exactly as the difference in the ground-state energy between a bulk material (per atomic layer) and a single isolated layer. The proposed method is (i) tremendously lower in computational cost than the traditional approach because it does not require calculations on thick slabs, (ii) still valid even if there is a surface reconstruction of any kind, (iii) capable of taking into account the relaxation of the single exfoliated layer (both in-plane lattice parameters and atomic positions), and (iv) easily combined with all kinds of many-body computational methods. As a proof of principles, we calculated exfoliation energies of graphene, hexagonal boron nitride, MoS2, and phosphorene using density-functional theory. In addition, we found that the in-plane relaxation of an exfoliated layer accounts for 5% of one-layer exfoliation energy of phosphorene while it is negligible (<0.4%) in the other cases.

Universality of periodicity as revealed from interlayer-mediated cracks

A crack and its propagation is a challenging multiscale materials phenomenon of broad interest, from nanoscience to exogeology. Particularly in fracture mechanics, periodicities are of high scientific interest. However, a full understanding of this phenomenon across various physical scales is lacking. Here, we demonstrate periodic interlayer-mediated thin film crack propagation and discuss the governing conditions resulting in their periodicity as being universal. We show strong confinement of thin film cracks and arbitrary steering of their propagation by inserting a predefined thin interlayer, composed of either a polymer, metal, or even atomically thin graphene, between the substrate and the brittle thin film. The thin interlayer-mediated controllability arises from local modification of the effective mechanical properties of the crack medium. Numerical calculations incorporating basic fracture mechanics principles well model our experimental results. We believe that previous studies of periodic cracks in SiN films, self-de-bonding sol-gel films, and even drying colloidal films, along with this study, share the same physical origins but with differing physical boundary conditions. This finding provides a simple analogy for various periodic crack systems that exist in nature, not only for thin film cracks but also for cracks ranging in scale.

A computational study on hydrogen storage in potential wells using K-intercalated graphite oxide

Using ab initio electronic structure calculations and grand canonical Monte Carlo simulations, we investigate the storage capacity of hydrogen molecules in a potential well created inside potassium-intercalated graphite oxide layers. We show that the binding energy of hydrogen located between layers of graphite oxide mainly originates from the dispersion interaction, and it is further increased slightly by induced dipole interactions. Its strength is fairly insensitive to the precise positions of the hydrogen molecules on the graphene plane, so the system may be described as a quasi-two-dimensional potential well. In this situation, the storage capacity is enhanced by the corresponding Boltzmann factor based on equilibrium thermodynamics. The trend of storage capacity with different geometries and chemical compositions of the scaffold materials is explained. With the present model, the density functional theory calculations and grand canonical Monte Carlo simulations predict a 2.5 wt% hydrogen storage capacity at room temperature at 10 MPa. For a model with increased potential depth, the storage capacity is predicted to increase up to 5.5 wt%.

Isosteric Heat of Potential Confinement in the Hydrogen Storage Material

In the hydrogen storage problem, if an attractive potential well is formed inside the void space of porous materials, the storage gas density is expected to increase significantly compared to the H2 gas density outside the material. Actually, the overall H2 density inside the material is enhanced basically by a Boltzmann factor of exp[−U/kT] where U (<0) is some averaged potential energy. Corresponding to this negative potential energy, latent heat is released in the H2 gas confinement process. We theoretically investigate the energetics involved during the H2 storage in the potential well and, from the equilibrium thermodynamic principles, we derive a formalism for the isosteric heat of potential confinement of the H2 gas. Since the gas density inside the potential well increases tremendously, the van der Waals equation is adopted to describe the nonideal gas behavior of H2. We compare our results to the well-known expression for the isosteric heat of adsorption where, unlike our case, the molecules are bound to specific adsorption sites in the material.