The initial stages of melting of graphene between 4000 K and 6000 K.

Graphene and its analogues have some of the highest predicted melting points of any materials. Previous work estimated the melting temperature for freestanding graphene to be a remarkable 4510 K. However, this work relied on theoretical methods that do not accurately account for the role of bond breaking or complex bonding configurations in the melting process. Furthermore, experiments to verify these high melting points have been challenging. Practical applications of graphene and carbon nanotubes at high temperatures will require a detailed understanding of the behavior of these materials under these conditions. Therefore, we have used reliable ab initio molecular dynamics calculations to study the initial stages of melting of freestanding graphene monolayers between 4000 and 6000 K. To accommodate large defects, and for improved accuracy, we used a large 10 × 10 periodic unit cell. We find that the system can be heated up to 4500 K for 18 ps without melting, and 3-rings and short lived broken bonds (10-rings) are observed. At 4500 K, the system appears to be in a quasi-2D liquid state. At 5000 K, the system is starting to melt. During the 20 ps simulation, diffusion events are observed, leading to the creation of a 5775 defect. We calculate accurate excitation energies for these configurations, and the pair correlation function is presented. The modified Lindemann criterion was calculated. Graphene and nanotubes together with other proposed high melting point materials would be interesting candidates for experimental tests of melting in the weightless environment of space.

Computational Study of Quasi-2D Liquid State in Free Standing Platinum, Silver, Gold, and Copper Monolayers.

Recently, freestanding atomically thick Fe metal patches up to 10 atoms wide have been fabricated experimentally in tiny pores in graphene. This concept can be extended conceptually to extended freestanding monolayers. We have therefore performed ab initio molecular dynamics simulations to evaluate the early melting stages of platinum, silver, gold, and copper freestanding metal monolayers. Our calculations show that all four freestanding monolayers will form quasi-2D liquid layers with significant out-of-plane motion and diffusion in the plane. Remarkably, we observe a 4% reduction in the Pt most likely bond length as the system enters the liquid state at 2400 K (and a lower effective spring constant), compared to the system at 1200 and 1800 K. We attribute this to the reduced average number of bonds per atom in the Pt liquid state. We used the highly accurate and reliable Density Functional Theory (DFT-D) method that includes dispersion corrections. These liquid states are found at temperatures of 2400 K, 1050 K, 1600 K, and 1400 K for platinum, silver, gold, and copper respectively. The pair correlation function drops in the liquid state, while the bond orientation order parameter is reduced to a lesser degree. Movies of the simulations can be viewed online (see Supplementary Material).

Glitter in a 2D monolayer.

We predict a highly stable and robust atomically thin gold monolayer with a hexagonal close packed lattice stabilized by metallic bonding with contributions from strong relativistic effects and aurophilic interactions. We have shown that the framework of the Au monolayer can survive 10 ps MD annealing simulations up to 1400 K. The framework is also able to survive large motions out of the plane. Due to the smaller number of bonds per atom in the 2D layer compared to the 3D bulk we observe significantly enhanced energy per bond (0.94 vs. 0.52 eV per bond). This is similar to the increase in bond strength going from 3D diamond to 2D graphene. It is a non-magnetic metal, and was found to be the global minima in the 2D space. Phonon dispersion calculations demonstrate high kinetic stability with no negative modes. This 2D gold monolayer corresponds to the top monolayer of the bulk Au(111) face-centered cubic lattice. The close-packed lattice maximizes the aurophilic interactions. We find that the electrons are completely delocalized in the plane and behave as 2D nearly free electron gas. We hope that the present work can inspire the experimental fabrication of novel free standing 2D metal systems.

Ten new predicted covalent organic frameworks with strong optical response in the visible and near infrared.

We use density functional theory to predict and evaluate 10 novel covalent organic frameworks (COFs), labeled (X4Y)(BDC)3, (X = C/Si; Y = C, Si, Ge, Sn, and Pb), with topology based on metal organic framework isoreticular metal-organic framework (IRMOF-1), but with new elements substituted for the corner atoms. We show that these new materials are stable structures using frequency calculations. For two structures, (C4C and Si4C) molecular dynamics simulations were performed to demonstrate stability of the systems up to 600 K for 10 ps. This demonstrates the remarkable stability of these systems, some of which may be experimentally accessible. For the C4C material, we also explored the stability of isolated corners and linkers and vacuum and started to build the structure from these pieces. We discuss the equilibrium lattice parameters, formation enthalpies, electronic structures, chemical bonding, and mechanical and optical properties. The predicted bulk moduli of these COFs range from 18.9 to 23.9 GPa, larger than that of IRMOF-1 (ca. 15.4 GPa), and larger than many existing 3D COF materials. The band gaps range from 1.5 to 2.1 eV, corresponding to 600-830 nm wavelength (orange through near infrared). The negative values of the formation enthalpy suggest that they are stable and should be experimentally accessible under suitable conditions. Seven materials distort the crystal structure to a lower space group symmetry Fm-3, while three materials maintain the original Fm-3m space group symmetry. All of the new materials are highly luminescent. We hope that this work will inspire efforts for experimental synthesis of these new materials.

Calculation of hydrogen storage capacity of metal-organic and covalent-organic frameworks by spillover.

We have used accurate ab initio quantum chemistry calculations together with a simple model to study the hydrogen storage capacity of metal-organic and covalent-organic frameworks by spillover. Recent experiments by Tsao et al. [J. Am. Chem. Soc. 131, 1404 (2009)] {based on an earlier work by Li and Yang [J. Am. Chem. Soc. 128, 8136 (2006)]} have found that IRMOF-8 with bridged Pt catalysts can reversibly store up to 4.7 wt % of hydrogen at room temperature and 100 bar. We have calculated the binding energy for multiple H atoms on model molecules. By counting active storage sites, we predict a saturation excess storage density at room temperature of 5.0 wt % for IRMOF-8. We also predict storage densities of 4.5 wt % for IRMOF-1, 5.4 wt % for MOF-177, 4.5 wt % for COF-1, and 5.7 wt % for IRMOF-15 and IRMOF-16. This suggests that the current experimental H storage results for IRMOF-8 are well optimized. However, for other materials such as MOF-177 and COF-1, the experimental results are not yet optimized, and significantly more H can be stored on these materials. We also find that significant strain will result from shrinkage of the linker molecules as H atoms are loaded onto the crystals.