A Global-Optimization Study of the Phase Diagram of Free-Standing Hydrogenated Two-Dimensional Silicon

Abstract

We perform a comprehensive study of the phase space of two-dimensional hydrogenated silicon. To scan the composition space, we employ an efficient constrained global structural prediction method, capable of exploring all possible bonding patterns on an equal footing. We find an interesting landscape of crystalline geometries, among which hydrogenated versions of mono- and bilayer silicene and haeckelites. The lowest energy phases, however, are buckled, with geometrical features similar to those of surface reconstructions. We observe that many structures can be described as connected nanowires exhibiting shared silicon pentagons. All studied systems are semiconducting with sizable band gaps. Considering the similarity of some of these structures with recently synthesized 2D silicon phases and their low formation energies, we expect that these 2D materials can be produced experimentally.

Evidence for “Unusual” Exchange–Correlation on Si(111) 7 × 7: Limitations of Density Functional Calculations for Charge Transfer Interactions on Semiconductor Surfaces

“…The silicon‐rich adlayers also have the lowest surface energy, significantly lower than an equivalent DAS like adlayer array. [ 135,136 ]…”

“…The silicon-rich adlayers also have the lowest surface energy, significantly lower than an equivalent DAS like adlayer array. [135,136] SiC(0001) p 3 Â p 3 is one of the first of these systems theoretically explored. Experimentally, this surface is found to possesses narrow filled and empty surface states but when calculated within DFT using an AA structure, it possesses a single half-filled band.…”

“…[135] In energy-optimized DFA calculations, AOA arrays are found to have strong twisting distortions of the triad of atoms below the AA as well as the lateral displacements of the lower atoms of the hexagonal structure. [135] New comprehensive search procedures appear to be needed to find these unusual states within DFT, [135,136] instead of the nominal Helmann-Feynman force minimization procedures currently used. [137] Whether such DFT-based searches will find a corresponding higher density (DFA) phase of the 7 Â 7 (or 5 Â 5) remains to be resolved.…”

“…SiC exhibits a range of polytypes whose energies are nearly degenerate but have a surprising range of surface properties and structures. [133][134][135][136] These numerous polytypes range from having a zinc-blende structure with pure cubic stacking of the Si-C double layers in the [111] direction and wurtzite structure with pure hexagonal stacking in the [0001] direction. Some polytypes consist of hexagonal or rhombohedral combinations with these aforementioned stacking sequences.…”

Prediction and Characterization of Graphitic Structures at Diamond Grain Boundaries

“…In this approach, the adiabatic potential energy surface is explored by iteratively performing consecutive short molecular dynamics steps to escape from local minima, followed by local geometry relaxations that take into account both atomic and cell variables. The predictive power of this approach was already demonstrated in a wide variety of systems, ranging from bulk crystals to low-density structures, crystals with defects, ,, quasi two-dimensional materials, − and more. In particular, combining the MHM with adequate constraints, we have studied recently the geometrical reconstruction of internal interfaces in silicon and revealed the relation between recurrent bonding patterns and electronic properties at grain boundaries …”