Diffusion Monte Carlo (DMC) calculations have been performed to study the adsorption of a single Pt atom on pristine graphene. We obtain the adsorption energy curves of a single Pt...
Quantum Monte Carlo (QMC) and density-functional theory (DFT) calculations were carried out to study cohesion energetics of two-dimensional (2D) sheets of boron atoms called borophenes. Our QMC calculations confirmed the polymorphism among free-standing borophenes that was reported in previous DFT studies. Although Perdew–Burke–Ernzerhof (PBE) calculations significantly overestimate the cohesive energies of 2D boron sheets, DFT-PBE relative energetics with respect to each other among various free-standing borophenes are found to be in quantitative agreement with the corresponding QMC results. This suggests that one can make reliable predictions for relative stability of different boron sheets through DFT-PBE calculations. Our analysis of PBE formation energies of borophenes on metal surfaces shows that the polymorphic range is extended for borophenes on the Ag(111) and the Au(111) surfaces beyond that of free-standing borophenes, reflecting recent experimental synthesis of β12 and χ3 boron sheets on the Ag(111) surface. We have also found that a hexagonal borophene can be stabilized through charge transfer from a metal surface and is energetically favored on the Al(111) surface over other borophene structures. Finally, it is found that the bilayer formation could be energetically favored over its monolayer form for the borophene–Au system, especially for borophenes with hexagonal hole densities η lower than 1/9. This leads to our prediction that in addition to its monolayer form, a bilayer η = 1/12 borophene can be synthesized on the Au(111) surface, opening a new possibility for borophene-based electronic devices.
We have carried out diffusion Monte Carlo calculations for an A1B–1-stacked bilayer blue phosphorene to find that it undergoes a semiconductor–metal transition as the interlayer distance decreases. While the most stable bilayer structure is a semiconducting one with two monolayers coupled through a weak van der Waals interaction, the metallic bilayer at a shorter interlayer distance is found to be only metastable. This is in contrast to a recent theoretical prediction based on a random phase approximation that the metallic phase would be the most stable bilayer configuration of blue phosphorene. Our analysis of charge density distributions reveals that the metastable metallic phase is induced by interlayer chemical bonding and intralayer charge redistributions. This study enriches our understanding of interlayer binding of a blue phosphorene and contributes to the establishment of correct energetic order between its different phases, which will be essential in devising an experimental pathway for a metallic phosphorene.
Path-integral Monte Carlo calculations have been performed to study the 4 He adsorption on γ-graphyne, a planar network of benzene rings connected by acetylene bonds. Assuming the 4 Hesubstrate interaction described by a pairwise sum of empirical 4 He-carbon interatomic potentials, we find that unlike α-graphyne, a single sheet of γ-graphyne is not permeable to 4 He atoms in spite of its large surface area. One-dimensional density distributions computed as a function of the distance from the graphyne surface reveal a layer-by-layer growth of 4 He atoms. A partially-filled 4 He monolayer is found to exhibit different commensurate solid structures depending on the helium coverage; it shows a C 2/3 commensurate structure at an areal density of 0.0491Å −2 , a C 3/3 structure at 0.0736Å −2 , and a C 4/3 structure at 0.0982Å −2 . While the promotion to the second layer starts beyond the C 4/3 helium coverage, the first 4 He layer is found to form an incommensurate triangular solid when compressed with the development of the second layer. PACS numbers: 67.25.bd, 67.25.bh, For the past few decades, a system of 4 He atoms adsorbed on a substrate has been intensively studied to investigate physical properties of low-dimensional quantum fluids. Carbon allotropes have often been chosen as substrates for this purpose because they provide strong enough interactions for 4 He adsorbates to show multiple distinct layered structures [1]. As a result of the interplay between 4 He-4 He and 4 He-substrate interaction, these helium adlayers are known to exhibit rich phase diagrams including various commensurate and incommensurate solids. On the surface of graphite, a monolayer of 4 He atoms is crystallized to a C 1/3 commensurate solid at an areal density of 0.0636Å −2 and goes through various domain structures before freezing into an incommensurate triangular solid as the helium coverage increases [2,3] . Similar quantum phase transitions were predicted for the 4 He monolayer on a single graphene sheet [4][5][6]. While no superfluidity has been observed in the first 4 He layer, the second layer on graphite does show finite superfluid response at intermediate helium coverages as first revealed by torsional oscillator measurements of Crowell and Reppy [3]. Whether this second-layer superfluid phenomenon is related to two-dimensional supersolidity is still an ongoing issue pursued heavily by some experimentalists.The 4 He adsorption on the surface of a carbon allotrope other than graphite or graphene has recently been investigated. While 4 He atoms adsorbed on the interstitials or the groves of carbon nanotube bundles showed characteristics of one-dimensional quantum fluid [7,8], a series of theoretical calculations predicted well-distinct layered structures for 4 He atoms adsorbed on the outer surfaces of fullerene molecules with each near-spherical helium layer exhibiting various quantum states depending on the number of 4 He adatoms [9][10][11][12]. More recently, graphynes, sp-sp 2 hybridized two-dimensional networks of carbon atoms...
We performed fixed-node diffusion Monte Carlo (DMC) calculations to investigate structural and energetic properties of graphenylene (GPNL), a two-dimensional network of sp2-bonded carbon atoms with large near-circular pores, and its H2 separation performance for gas mixtures. It is found that energetic stability of a GPNL monolayer is comparable to that of γ-graphyne, as evidenced by its large cohesive energy of 6.755(3) eV/atom. Diffusion barriers of several gas molecules, including hydrogen, through a GPNL membrane, were determined from the analysis of their adsorption energies depending on the adsorption distance, which led to our estimation for hydrogen selectivity with respect to other target molecules. DMC hydrogen selectivity of a GPNL monolayer was found to be exceptionally high at 300 K, as high as 1010 to 1011 against CO and N2 gases. This, along with high hydrogen permeance due to its generic pore structure, leads us to conclude that GPNL is a promising membrane to be used as a high-performance hydrogen separator from gas mixtures. We find that when compared to our DMC results, DFT calculations tend to overestimate H2 selectivity, which is understood to be mostly due to their inaccurate description of short-range repulsive interactions.
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