Abstract
We introduce a simple criterion to identify two-dimensional (2D) materials based on the comparison between experimental lattice constants and lattice constants mainly obtained from Materials-Project (MP) density functional theory (DFT) calculation repository. Specifically, if the relative difference between the two lattice constants for a specific material is greater than or equal to 5%, we predict them to be good candidates for 2D materials. We have predicted at least 1356 such 2D materials. For all the systems satisfying our criterion, we manually create single layer systems and calculate their energetics, structural, electronic, and elastic properties for both the bulk and the single layer cases. Currently the database consists of 1012 bulk and 430 single layer materials, of which 371 systems are common to bulk and single layer. The rest of calculations are underway. To validate our criterion, we calculated the exfoliation energy of the suggested layered materials, and we found that in 88.9% of the cases the currently accepted criterion for exfoliation was satisfied. Also, using molybdenum telluride as a test case, we performed X-ray diffraction and Raman scattering experiments to benchmark our calculations and understand their applicability and limitations. The data is publicly available at the website http://www.ctcms.nist.gov/~knc6/JVASP.html.
Highlights
We chose δ greater than or equal to 5% as screening criterion to identify possible 2D materials, where δ is the relative difference between Inorganic Crystal Structure Database (ICSD) and materials project (MP)-PBE (DFT database computed using the PBE functional) lattice constants (a, b and c lattice constants chosen as in the ICSD and MP databases) for bulk materials in non-cubic crystal systems: δ = lPBE−lICSD where l ∈ a, b, c lICSD
Please note that we didn’t perform any of the density functional theory (DFT) calculations needed to compute δ, we obtained the necessary quantities from the Materials Project database through REST API41
As we found a lot of compounds with relative error in lattice constants greater than 5% (Fig. 1a), we noticed that PBE functional may not be the ideal functional to investigate 2D materials, and tested if the optB88 exchange-correlation functional[38, 39] could be a better choice
Summary
Two-dimensional (2D) materials[1, 2] have great potential in sub-micron level electronics[3], flexible and tunable electronics[4], superconductivity[5], photo-voltaic[6], water purification[7], sensors[8], thermal management[9], ethanol distillation and energy storage[10], medicine[11], quantum dots[12, 13] and composites[14,15,16]. These databases are quite homogenous among themselves and encompasses many of the ICSD materials data While they provide a large amount of electronic structure, surface, interface and mechanical properties data, the choice of the PBE functional, fixed plane-wave energy cutoff and fixed K-point selection may not be well suitable for characterizing 2D materials[36]. As evident from the above discussion, while there are many DFT-based databases available nowadays, there is still a significant need for a systematic evaluation of 2D material properties in monolayer, bilayer, and multi-layer forms, and for a comparison to their 3D bulk counterpart Such properties should be calculated for all experimentally observed structures, not just conventional 2H and 1T, using DFT-functionals more suitable to the 2D case, such as optB88 functional[38, 39]. We repeat the DFT procedure for all these materials with better vdW functionals, such as optB88 to compute energetics and all other properties mentioned above
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