Search

The role of interlayer bonds in the two-dimensional (2D) materials "beyond graphene" and so-called van der Waals heterostructures is vital, and understanding the nature of these bonds in terms of strength and type is essential due to a wide range of their prospective technological applications. However, this issue has not yet been properly addressed in the previous investigations devoted to 2D materials. In our work, by using first-principles calculations we perform a systematic study of the interlayer bonds and charge redistribution of several representative 2D materials that are traditionally referred as van der Waals systems. Our results demonstrate that one can distinguish three main types of inter-layer couplings in the considered 2D structures: one atom thick membranes bonded by London dispersion forces (graphene, hBN), systems with leading electrostatic interaction between layers (diselenides, InSe and bilayer silica) and materials with so-called dative or coordination chemical bonds between layers (ditelurides). We also propose a protocol for recognising the leading type of interlayer bonds in a system that includes comparison of interlayer distances, binding energies and redistribution of the charge densities in interlayer space. Such an approach is computationally cheap and can be used to further prediction of chemical and physical properties such as charge density waves (CDW), work function and chemical stability at ambient conditions., Comment: 35 pages, 6 figures, 3 tables

The discrepancy between the bright theoretical projections for two-dimensional (2D) Janus structures and the lack of experimental realisation of these structures motivated us to study the effect of structural disorder on the stability of MoSSe, SnSSe, PtSSe, In2SSe and GaInSe2. The calculation results demonstrate that the difference between metal-sulfur and metal-selenium bonds makes Janus structures frustrated and less energetically favourable than less ordered allotropes of the same compounds. This result explains the difficulties encountered in experimental fabrication of these materials. In the bulk, there is an additional contribution to the total energy from dipole-dipole interactions between layers with a Janus structure that can overcome the energetic cost of structural frustration in layers for compounds with sufficiently large dipole moments. However, the entropic contribution to the free energy decreases the favourability of the ordered Janus structure. The calculation results are used to make recommendations to enable the discovery and synthesis of 2D materials with Janus structures., Comment: 11 pages, 3 figures, 3 tables, to appear in PCCP as Communication

In this work we report results of step-by-step modeling of the oxidation of free-standing boron monolayers of different types. Results of the calculations demonstrate that the process of the oxidation is always exothermic and lead toward the formation of foam-like boron oxide films with incorporated non-oxidized small boron clusters. Some of these boron-oxide films demonstrate the presence of chemically stable magnetic centers. Evaluation of the physical properties of oxidized boprophene sheets (OBS) demonstrate it possible application in solar energy, as sensors and coating against leakage of hydrogen., Comment: 14 pages, 5 figures, accepted in Physica E journal

First principles calculations of magnetic and electronic properties of VSe2 describing the transition between two structural phases(H,T) were performed. Results of the calculations evidence rather low energy barrier ( 0.60 eV for monolayer) for transition betweenthe phases. The energy required for the deviation of Se atom or whole layer of selenium atoms on a small angle up to 10 degrees from initial positions is also rather low, 0.32 and 0.19 eV/Se, respectively. The changes in band structure of VSe2 caused by these motions of Se atoms should be taken into account for analysis of the experimental data. Simulations of the strain effects suggest that the experimentally observed T phase of VSe2 monolayer is the ground state due a substrate-induced strain. Calculations of the difference in total energies of ferromagnetic and antiferromagnetic configurations evidence that the ferromagnetic configuration is the ground state of the system for all stable and intermediate atomic structures. Calculated phonon dispersions suggest visible influence of magnetic configurations on vibrational properties.

Single- and few-layered InSe flakes are produced by the liquid-phase exfoliation of beta-InSe single crystals in 2-propanol, obtaining stable dispersions with a concentration as high as 0.11 g/L. Ultracentrifugation is used to tune the morphology, i.e., the lateral size and thickness of the as-produced InSe flakes. We demonstrate that the obtained InSe flakes have maximum lateral sizes ranging from 30 nm to a few um, and thicknesses ranging from 1 to 20 nm, with a max population centred at ~ 5 nm, corresponding to 4 Se-In-In-Se quaternary layers. We also show that no formation of further InSe-based compounds (such as In2Se3) or oxides occurs during the exfoliation process. The potential of these exfoliated-InSe few-layer flakes as a catalyst for hydrogen evolution reaction (HER) is tested in hybrid single-walled carbon nanotubes/InSe heterostructures. We highlight the dependence of the InSe flakes morphologies, i.e., surface area and thickness, on the HER performances achieving best efficiencies with small flakes offering predominant edge effects. Our theoretical model unveils the origin of the catalytic efficiency of InSe flakes, and correlates the catalytic activity to the Se vacancies at the edge of the flakes., Comment: 39 pages

The results of X-ray photoemission (XPS) and valence bands spectroscopy, optically stimulated electron emission (OSEE) measurements and density functional theory based modeling of graphene oxide (GO) placed on Cu via an electrophoretic deposition (EPD) are reported. The comparison of XPS spectra of EPD prepared GO/Cu composites with those of as prepared GO, strongly reduced GO, pure and oxidized copper demonstrate the partial (until C/O ratio about two) removal of oxygen-containing functional groups from GO simultaneously with the formation of copper oxide-like layers over the metallic substrate. OSEE measurements evidence the presence of copper oxide phase in the systems simultaneously with the absence of contributions from GO with corresponding energy gap. All measurements demonstrate the similarity of the results for different thickness of GO cover of the copper surface. Theoretical modeling demonstrates favorability of migration of oxygen-containing functional groups from GO to the copper substrate only for the case of C/O ratio below two and formation of Cu-O-C bonds between substrate and GO simultaneously with the vanishing of the energy gap in GO layer. Basing on results of experimental measurements and theoretical calculations we suggest the model of atomic structure for Cu/GO interface as Cu/CuO/GO with C/O ratio in gapless GO about two., Comment: 22 pages, 14 figures, accepted to Thin Solid Films journal

By means of vibrational spectroscopy and density functional theory (DFT), we investigate CO adsorption on phosphorene-based systems. We find stable CO adsorption at room temperature on both phosphorene and bulk black phosphorus. The adsorption energy and vibrational spectrum have been calculated for several possible configurations of the CO overlayer. We find that the vibrational spectrum is characterized by two different C-O stretching energies. The experimental data are in good agreement with the prediction of the DFT model and unveil the unusual C-O vibrational band at 165-180 meV, activated by the lateral interactions in the CO overlayer.

Based on the results of first-principles calculations we demonstrate that significant distortion of graphene sheets caused by adsorption of fluorine atoms leads to the formation of metastable patterns for which the next step of fluorination is considerably less energetically favorable. Existence of these stable patterns oriented along the armchair direction makes possible the synthesis of various CFx structures. The combination of strong distortion of the nonfluorinated graphene sheet with the doping caused by the polar nature of C-F bonds reduces the energy cost of migration and the energy of migration barriers, making possible the migration of fluorine atoms on the graphene surface as well as transformation of the shapes of fluorinated areas. The decreasing energy cost of migration with increasing fluorine content also leads to increasing numbers of single fluorine adatoms, which could be the source of magnetic moments., Comment: 16 pages, 6 figures (one figure added), accepted in PCCP

First principle modeling of nitrogen- and boron-doped phosphorene demonstrates the tendency toward formation of highly ordered structures. Nitrogen doping leads to the formation of -N-P-P-P-N- lines. Further transformation to -P-N-P-N- lines across the chains of phosphorene occurs with increasing band gap and increasing nitrogen concentration, which coincides with the decreasing chemical activity of N-doped phosphorene. In contrast to the case of nitrogen, boron atoms prefer to form -B-B- pairs with the further formation of -P-P-B-B-P-P- patterns along the phosphorene chains. The low concentration of boron dopants converts the phosphorene from a semiconductor into a semimetal with the simultaneous enhancement of its chemical activity. Co-doping of phosphorene by both boron and nitrogen starts from the formation of -B-N- pairs, which provide flat bands and the further transformation of these pairs to hexagonal BN lines and ribbons across the phosphorene chains., Comment: 21 pages, 8 figures, 2 tables, to appear at PCCP

Despite the highly porous nature with significantly large surface area, metal organic frameworks (MOFs) can be hardly used in electronic, and optoelectronic devices due to their extremely poor electrical conductivity. Therefore, the study of MOF thin films that require electron transport or conductivity in combination with the everlasting porosity is highly desirable. In the present work, thin films of Co3(NDC)3DMF4 MOFs with improved electronic conductivity are synthesized using layer-by-layer and doctor blade coating techniques followed by iodine doping. The as-prepared and doped films are characterized using FE-SEM, EDX, UV/Visible spectroscopy, XPS, current-voltage measurement, photoluminescence spectroscopy, cyclic voltammetry, and incident photon to current efficiency measurements. In addition, the electronic and semiconductor property of the MOF films are characterized using Hall Effect measurement, which reveals that in contrast to the insulator behavior of the as-prepared MOFs, the iodine doped MOFs behave as a p-type semiconductor. This is caused by charge transfer induced hole doping into the frameworks. The observed charge transfer induced hole doping phenomenon is also confirmed by calculating the densities of states of the as-prepared and iodine doped MOFs based on density functional theory. Photoluminescence spectroscopy demonstrate an efficient interfacial charge transfer between TiO2 and iodine doped MOFs, which can be applied to harvest solar radiations., Comment: Main paper (19 pages, 6 figures) and supplementary information (15 pages, 10 figures), accepted in ACS Appl. Materials & Interfaces