Electronic and Molecular Adsorption Properties of Pt-Doped BC6N: An Ab-Initio Investigation.

In the last two decades, significant efforts have been particularly invested in two-dimensional (2D) hexagonal boron carbon nitride h-BxCyNz because of its unique physical and chemical characteristics. The presence of the carbon atoms lowers the large gap of its cousin structure, boron nitride (BN), making it more suitable for various applications. Here, we use density functional theory to study the structural, electronic, and magnetic properties of Pt-doped BC6N (Pt-BC6N, as well as its adsorption potential of small molecular gases (NO, NO2, CO2, NH3). We consider all distinct locations of the Pt atom in the supercell (B, N, and two C sites). Different adsorption locations are also considered for the pristine and Pt-doped systems. The formation energies of all Pt-doped structures are close to those of the pristine system, reflecting their stability. The pristine BC6N is semiconducting, so doping with Pt at the B and N sites gives a diluted magnetic semiconductor while doping at the C1 and C2 sites results in a smaller gap semiconductor. We find that all doped structures exhibit direct band gaps. The studied molecules are very weakly physisorbed on the pristine structure. Pt doping leads to much stronger interactions, where NO, NO2, and NH3 chemisorb on the doped systems, and CO2 physiorb, illustrating the doped systems' potential for gas purification applications. We also find that the adsorption changes the electronic and magnetic properties of the doped systems, inviting their consideration for spintronics and gas sensing.

Hybrid G/BN@2H-MoS2 Nanomaterial Composites: Structural, Electronic and Molecular Adsorption Properties.

Hybrid structures often possess superior properties to those of their component materials. This arises from changes in the structural or physical properties of the new materials. Here, we investigate the structural, electronic, and gas-adsorption properties of hybrid structures made from graphene/hexagonal boron nitride and 2H-molybdenum disulfide (G/BN@MoS2) monolayers. We consider hybrid systems in which the G/BN patch is at the Mo plane (model I) and the S plane (model II). We find that the implanted hexagon of G or BN in MoS2 alters its electronic properties: G@MoS2 (I,II) are metallic, while BN@MoS2 (I) is an n-type conducting and BN@MoS2 (II) is semiconducting. We study the molecular adsorption of some diatomic gases (H2, OH, N2, NO, CO), triatomic gases (CO2, NO2, H2S, SO2), and polyatomic gases (COOH, CH4, and NH3) on our hybrid structures while considering multiple initial adsorption sites. Our results suggest that the hybrid systems may be suitable materials for some applications: G@MOS2 (I) for oxygen reduction reactions, BN@MoS2 (I,II) for NH3-based hydrogen production, and G@MoS2 (I) and BN@MoS2 (I,II) for filtration of No, Co, SO2, H2S, and NO2.

Hybrid MXene-Graphene/Hexagonal Boron Nitride Structures: Electronic and Molecular Adsorption Properties.

Recent advances in experimental techniques allow for the fabrication of hybrid structures. Here, we study the electronic and molecular adsorption properties of the graphene (G)/hexagonal boron nitride (h-BN)-MXenes (Mo2C) hybrid nanosheets. We use first-principles calculations to explore the structure and electronic properties of the hybrid structures of G-2H-Mo2C and h-BN-2H-Mo2C with two different oxygen terminations of the Mo2C surface. The embedding of G or h-BN patches creates structural defects at the patch-Mo2C border and adds new states in the vicinity of the Fermi energy. Since this can be utilized for molecular adsorption and/or sensing, we investigate the ability of the G-M-O1 and BN-M-O1 hybrid structures to adsorb twelve molecules. Generally, the adsorption on the hybrid systems is significantly higher than on the pristine systems, except for N2 and H2, which are weakly adsorbed on all systems. We find that OH, NO, NO2, and SO2 are chemisorbed on the hybrid systems. COOH may be chemisorbed, or it may dissociate depending on its location at the edge between the G/h-BN and the MXene. NH3 is chemisorbed/physisorbed on the BN/G-M-O1 systems. CO, H2S, CO2, and CH4 are physisorbed on the hybrid systems. Our results indicate that the studied hybrid systems can be used for molecular filtration/sensing and catalysis.

Puckered Penta-like PdPX (X = O, S, Te) Semiconducting Nanosheets: First-Principles Study of the Mechanical, Electro-Optical, and Photocatalytic Properties.

The atomic, electronic, optical, and mechanical properties of penta-like two-dimensional PdPX (X = O, S, Te) nanosheets have been systematically investigated using density functional theory calculations. All three PdPX nanosheets exhibit dynamic and mechanical stability on the basis of an analysis of phonon dispersions and the Born criteria, respectively. The PdPX monolayers are found to be brittle structures. Our calculations demonstrate that the PdPX nanosheets exhibit semiconducting characteristics with indirect band gaps of 0.93 (1.99), 1.34 (2.11), and 0.74 (1.51) eV for X = O, S, Te, respectively, using the PBE (HSE06) functional, where PdPTe is the best material for visible-light photocatalytic water splitting. Our findings give important basic characteristics of penta-like two-dimensional PdPX materials and should motivate further theoretical and experimental investigations of these interesting materials.

Substitutional transition metal doping in MoSi2N4 monolayer: structural, electronic and magnetic properties.

Monolayer MoSi2N4 (MoSiN) was successfully synthesized last year [Hong et al., Science369, 670 (2020)]. The MoSiN monolayer exhibited semiconducting characteristics and exceptional ambient stability, calling for more studies of its properties. Here, we conduct first-principle calculations to examine the structural, magnetic, and electronic properties of substitutional doping of MoSiN monolayers with transition metals (TM) at the Mo site (TM-MoSiN). We find that the Sc-, Y-, Ti-, and Zr-MoSiN are metallic systems, while Mn-, Tc-, and Ru-MoSiN are n-type conducting. The Fe-MoSiN is a dilute magnetic semiconductor, and the Ni-MoSiN is a metal (or half-metal). The inclusion of spin-orbit coupling turns them into a half-metal and a semimetal, respectively. We also find that the work function of TM-MoSiN and the bond lengths between the TM and neighbor atoms increase as the atomic radius and electronegativity of the TM atom increase, respectively. The Fe-, Co-, and Ni-MoSiN may be used in spintronic devices, while Mn-, Rh- and Pd-MoSiN could be utilized for spin filter applications.

Metal dichalcogenide nanomeshes: structural, electronic and magnetic properties.

Motivated by the successful preparation of two-dimensional transition metal dichalcogenide (2D-TMD) nanomeshes in the last three years, we use density functional theory (DFT) to study the structural stability, mechanical, magnetic, and electronic properties of porous 2H-MoX2 (X = S, Se and Te) without and with pore passivation. We consider structures with multiple, systematically created pores. The molecular dynamics simulations and cohesive energy calculations showed the stability of the 2D-TMD nanomeshes, with larger stability for those with smaller pores. The lattice undergoes some deformations to accommodate the pore energetically, and as the pore size increases Young's modulus decreases. In most cases, the missing metal atoms disrupt the spin states' even population, resulting in some nanomeshes becoming magnetic. The electronic gaps of the MoX2 nanomesh systems are diminished because of the emergence of pore-edge localized mid-gap metal 4d states in the spin-polarized spectrum, making some systems half-metallic. The oxygen passivation of the pore edges of 2D-TMD nanomeshes restores the even population of spin states, and makes those systems metallic. Our results can be used in different applications such as spintronics, ion chelation, and molecular sensing applications.

Two-dimensional buckled tetragonal cadmium chalcogenides including CdS, CdSe, and CdTe monolayers as photo-catalysts for water splitting.

Pure hydrogen production via water splitting is an ideal strategy for producing clean and sustainable energy. Two-dimensional (2D) cadmium chalcogenide single-layers with a tetragonal crystal structure, namely Tetra-CdX (X = S, Se, and Te) monolayers, are theoretically predicted by means of density functional theory (DFT). Their structural stability and electronic and optical properties are investigated. We find that Tetra-CdX single-layers are thermodynamically stable. Their stability decreases as we go down the 6A group in the periodic table, i.e., from X = S to Se, and Te which also means that the electronegativity decreases. All considered novel monolayers are indirect band gap semiconductors. Using the HSE06 functional the electronic band gaps of CdS, CdSe, and CdTe monolayers are predicted to be 3.10 eV, 2.97 eV, and 2.90 eV, respectively. The impact of mechanical strain on the physical properties was studied, which indicates that compressive strain increases the band gap and tensile strain decreases the band gap. The optical properties of the Tetra-CdX monolayers show the ability of these monolayers to absorb visible light. Due to the suitable band gaps and band edge positions of Tetra-CdX, these newly discovered 2D materials are promising for photocatalytic water splitting.

Semiconducting Chalcogenide Alloys Based on the (Ge, Sn, Pb) (S, Se, Te) Formula with Outstanding Properties: A First-Principles Calculation Study.

Very recently, a new class of the multicationic and -anionic entropy-stabilized chalcogenide alloys based on the (Ge, Sn, Pb) (S, Se, Te) formula has been successfully fabricated and characterized experimentally [Zihao Deng et al., Chem. Mater. 32, 6070 (2020)]. Motivated by the recent experiment, herein, we perform density functional theory-based first-principles calculations in order to investigate the structural, mechanical, electronic, optical, and thermoelectric properties. The calculations of the cohesive energy and elasticity parameters indicate that the alloy is stable. Also, the mechanical study shows that the alloy has a brittle nature. The GeSnPbSSeTe alloy is a semiconductor with a direct band gap of 0.4 eV (0.3 eV using spin-orbit coupling effect). The optical analysis illustrates that the first peak of Im(ε) for the GeSnPbSSeTe alloy along all polarization directions is located in the visible range of the spectrum which renders it a promising material for applications in optical and electronic devices. Interestingly, we find an optically anisotropic character of this system which is highly desirable for the design of polarization-sensitive photodetectors. We have accurately predicted the thermoelectric coefficients and have calculated a large power factor value of 3.7 × 1011 W m-1 K-2 s-1 for p-type. The high p-type power factor is originated from the multiple valleys near the valence band maxima. The anisotropic results of the optical and transport properties are related to the specific tetragonal alloy unit cell.

Effect of pore-size disorder on the electronic properties of semiconducting graphene nanomeshes.

Graphene nanomeshes (GNMs) are novel materials that recently raised a lot of interest. They are fabricated by forming a lattice of pores in graphene. Depending on the pore size and pore lattice constant, GNMs can be either semimetallic or semiconducting with a gap large enough (∼ 0.5 eV) to be considered for transistor applications. The fabrication process is bound to produce some structural disorder due to variations in pore size. Recent electronic transport measurements in GNM devices (ACS Appl. Mater. Interfaces 10, 10 362, 2018) show a degradation of their bandgap in devices having pore-size disorder. It is therefore important to understand the effect of such variability on the electronic properties of semiconducting GNMs. In this work we use the density functional-based tight binding formalism to calculate the electronic properties of GNM structures with different pore sizes, pore densities, and with hydrogen and oxygen pore edge passivations. We find that structural disorder reduces the electronic gap and the carrier group velocity, which may interpret recent transport measurements in GNM devices. Furthermore, the trend of the bandgap with structural disorder is not significantly affected by the change in pore edge passivation. Our results show that even with structural disorder, GNMs are still attractive from a transistor device perspective.

Dopant site in indium-doped SrTiO3 photocatalysts.

Strontium titanate, SrTiO3, with the perovskite ABO3 structure is known as one of the most efficient photocatalyst materials for the overall water splitting reaction. Doping with appropriate metal cations at the A site or at the B site substantially increases the quantum yield to split water into H2 and O2. The site occupied by the guest dopant in the SrTiO3 host thus plays a key role in dictating the water splitting activity. However, little is known about the detailed structure of the dopant site in the host lattice. In this study, the local structure of In3+ cations, which were shown to improve the water splitting activity of SrTiO3, is investigated with X-ray absorption fine structure spectroscopy and density functional theory (DFT) calculations. The In3+ cations exclusively substitute for Ti4+ cations at the B site to form InO6 octahedra. Further optical experiments using UV-Vis diffuse reflectance spectroscopy and DFT calculations of the density of states indicate that the substitution of In3+ for Ti4+ does not alter the band structure and bandgap energy (remaining at 3.2 eV). The mechanism underlying the increased water splitting activity is discussed in relation to occupation of the B site by In3+ cations.

Graphene and graphene nanomesh supported nickel clusters: electronic, magnetic, and hydrogen storage properties.

Small-sized nanoparticles are widely used in applications such as catalysis, nanoelectronics, and hydrogen storage. However, the small size causes a common problem: agglomeration on the support template. One solution is to use templates that limit the mobility of the nanoparticles. Graphene nanomeshes (GNMs) are two dimensional porous structures with controllably passivated pores. In this work, we employ first principles calculations to investigate the potential for using GNMs as support templates for Ni clusters and, at the same time, study their magnetic and hydrogen storage properties. We consider two Ni clusters (Ni6 and Ni13) and two GNMs (O-terminated and N-terminated), comparing our results to those of isolated Ni clusters and those of Ni clusters on graphene. High stability of the Ni clusters is found on the N-GNM in contrast to the O-GNM. We quantify the hydrogen storage capacity by calculating the adsorption energy for multiple H2 molecules. The values on Ni x /N-GNM are significantly reduced as compared to the corresponding isolated Ni x clusters, but a high hydrogen storage capacity is maintained. The fact that Ni x /N-GNM hosts spin polarization is interesting for spintronic applications.

Unravelling the interplay of geometrical, magnetic and electronic properties of metal-doped graphene nanomeshes.

Graphene nanomeshes (GNMs), formed by creating a superlattice of pores in graphene, possess rich physical and chemical properties. Many of these properties are determined by the pore geometry. In this work, we use first principles calculations to study the magnetic and electronic properties of metal-doped nitrogen-passivated GNMs. We find that the magnetic behaviour is dependent on the pore shape (trigonal versus hexagonal) as dictated by the number of covalent bonds formed between the 3d metal and the passivating N atoms. We also find that Cr and V doped trigonal-pore GNMs, and Ti doped GNMs are the most favourable for spintronic applications. The calculated magnetic properties of Fe-doped GNMs compare well with recent experimental observations. The studied systems are useful as spin filters and chemical sensors.

Defect engineering of the electronic transport through cuprous oxide interlayers.

The electronic transport through Au-(Cu2O)n-Au junctions is investigated using first-principles calculations and the nonequilibrium Green's function method. The effect of varying the thickness (i.e., n) is studied as well as that of point defects and anion substitution. For all Cu2O thicknesses the conductance is more enhanced by bulk-like (in contrast to near-interface) defects, with the exception of O vacancies and Cl substitutional defects. A similar transmission behavior results from Cu deficiency and N substitution, as well as from Cl substitution and N interstitials for thick Cu2O junctions. In agreement with recent experimental observations, it is found that N and Cl doping enhances the conductance. A Frenkel defect, i.e., a superposition of an O interstitial and O substitutional defect, leads to a remarkably high conductance. From the analysis of the defect formation energies, Cu vacancies are found to be particularly stable, in agreement with earlier experimental and theoretical work.