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We have performed density functional theory calculations to explore the possibility to overcome the linear scaling relations in the oxygen reduction reaction (ORR) using local inhomogeneities on Pt...

Janus MoSSe monolayers were recently synthesised by replacing S by Se on one side of MoS$_2$ (or vice versa for MoSe$_2$). Due to the different electronegativity of S and Se these structures carry a finite out-of-plane dipole moment. As we show here by means of density functional theory (DFT) calculations, this intrinsic dipole leads to the formation of built-in electric fields when the monolayers are stacked to form $N$-layer structures. For sufficiently thin structures ($N4$. Based on band structure calculations and the Mott-Wannier exciton model, we compute the energies of intra- and interlayer excitons as a function of film thickness suggesting that the Janus multilayer films are ideally suited for achieving ultrafast charge separation over atomic length scales without chemical doping or applied electric fields. Finally, we explore a number of other potentially synthesisable 2D Janus structures with different band gaps and internal dipole moments. Our results open new opportunities for ultrathin opto-electronic components such as tunnel diodes, photo-detectors, or solar cells.

Using computational screening we identify materials with potential use as light absorbers in photovoltaic or photoelectrochemical devices. The screening focuses on compounds of up to three different chemical elements which are abundant and nontoxic. A prescreening is carried out based on information from the Inorganic Crystal Structure Database and Open Quantum Materials Database. The light absorption, carrier mobility, defect tolerance, and stability of the materials are assessed by a set of simple computational descriptors. The identified 74 materials include a variety of pnictogenides, chalcogenides, and halides. Several recently investigated light absorbers, such as CsSnI3, CsSnBr3, and BaZrS3, appear on the list.

The electrochemical oxidation of methane to methanol at remote oil fields where methane is flared is the ultimate solution to harness this valuable energy resource. In this study we identify a fundamental surface catalytic limitation of this process in terms of a compromise between selectivity and activity, as oxygen evolution is a competing reaction. By investigating two classes of materials, rutile oxides and two-dimensional transition metal nitrides and carbides (MXenes), we find a linear relationship between the energy needed to activate methane, i.e. to break the first C-H bond, and oxygen binding energies on the surface. Based on a simple kinetic model we can conclude that in order to obtain sufficient activity oxygen has to bind weakly to the surface but there is an upper limit to retain selectivity. Few potentially interesting candidates are found but this relatively simple description enables future large scale screening studies for more optimal candidates.

Recent experiments on II–IV–V2 type nitrides and phosphides have shown that these materials are potential candidates as photovoltaic absorbers. The materials space for such light absorbers can be expanded by elemental substitution of the different species, and thanks to the small energy difference of the polymorphs, the space can be extended by tuning the atomic structure as well. Using electronic structure calculations, we explore chalcopyrite, kesterite, and wurtzite polymorphs of II–IV–V2 and III–III–V2 materials for light absorption especially in the visible range. Based on the thermodynamic stability, band gap, and charge carrier effective masses, we discuss the posssibility for the materials containing nontoxic elements to act as PV absorbers. Additionally, the systematic mapping of the materials space provides trends in thermodynamic and electronic properties which can be exploited further to tune these properties via elemental substitution and/or alloying.

We use density functional theory calculations to explore different polymorphs of a new class of 2D materials commonly known as MXenes,which are primarily carbides and nitrides of transition metals. The stability of the M2X, M3X2, and M4X3 polymorphs in their bare and functionalized forms is assessed via the calculated standard heat of formation. We find that most of the MXenes are metallic, and we investigate their performance as electrocatalysts for the hydrogen evolution reaction(HER) using the free energy of hydrogen adsorption at equilibrium coverage as an activity descriptor. For a given type of metal, we find that the hydrogen adsorption energy can vary by up to 0.5 eV depending on the number of metal layers in the structure, suggesting that the catalytic activity of MXenes can be tuned by controlling the layer thickness. On the basis of a combined stability and activity analysis of 72 different MXenes, we identify several new promising nonprecious HER electrocatalysts.

The sulfide perovskite LaYS3 has been recently identified as a promising wide band gap photoabsorber material by computational screening techniques. In this study, we combine experiment and theory to comprehensively characterize LaYS3 thin films produced by sulfurization of sputter-deposited precursors. An attractive feature of LaYS3 is its optimal band gap (2.0 eV) for application as a wide band gap photoabsorber in tandem solar energy conversion devices. Promisingly, the LaYS3 films are photoconductive, with a grain size in excess of 1 μm and comparable recombination time scales to state-of-the-art hybrid halide perovskite absorbers. Although the fabrication of solar cells based on LaYS3 absorbers is complicated by the high temperature necessary to grow the compound, complete solar cells could be produced in this work by growing LaYS3 on refractory metal back contacts. These are the first reported solar cells based on a sulfide perovskite absorber. A major reason for their poor performance could be the highly localized trap states observed directly by photoluminescence imaging of LaYS3, which may also explain the surprisingly long carrier lifetimes and the low carrier mobility found in this material

One of the key challenges in photoelectrochemical water splitting is to identify efficient semiconductors with band gaps of the order of ∼2 eV to operate as the large-band-gap component in water splitting tandem devices. Here, we address this challenge by extensive computational screening of ternary sulfides followed by synthesis and confirmation of the properties of one of the most promising materials. The screening focusses on materials with ABS3 composition taking both perovskite and non-perovskite structures into consideration, and the material selection is based on descriptors for thermodynamic stability, light absorption, charge mobility, and defect tolerance. One of the most promising candidates identified is LaYS3. This material was synthesized directly in thin-film form demonstrating its stability, crystal structure, light absorption, and strong photoluminescence. These data confirms its potential applicability in tandem photoelectrochemical devices for hydrogen production.

We explore the possibility of modulating the electronic band edges of the transition metal dichalcogenides (TMDs) via alloying of different semiconductors within the same group (intragroup alloying). The stability of the ordered alloys is assessed from the calculated mixing enthalpy, which is found to be negative for several alloys and below 20 meV/atom for all of the alloys. We explore to what extent the electronic properties like the band gap and band edge positions of the alloy can be evaluated by taking the weighted average of the corresponding properties of the pristine systems. In general, this approach works well with the only exception being Cr-containing compounds. Because the calculated properties of the alloys are very similar to the weighted averages, we expect that the trends observed for the ordered alloys will also hold for more realistic disordered alloys.

We present a density functional theory study of stacked MXenes with the aim of assessing the accuracy of the most widely used exchange–correlation (xc) functionals for calculating properties relevant for battery electrode materials, namely, mechanical stability, open-circuit voltages, and lithium diffusion barriers. The open-circuit voltage results are nearly independent of the choice of xc-functional, while the lithium diffusion barriers are shown to be heavily dependent on the selected xc-functional, with diffusivities spanning 5 orders of magnitude. This is shown to be due to differences in the computed interlayer distances between the MXene monolayers, which are found to be linearly related to the diffusion barriers. This underlines the importance of starting from reliable crystal structures when modeling MXenes as battery electrode materials. Using an experimental reference materials database, we show a general trend in lattice parameter accuracy for 2D stacked materials with the chosen xc-functionals. Our results reveal that the optB88-vdW functional provides the best average accuracy for predicting the out-of-plane lattice parameter and hence also the best estimate for the transition-state barriers, while PBE with D3 damping performs only slightly worse on average.

In his comment Maździarz 2019 (2D Mater. 6 048001) raises doubts concerning the reliability of our test for dynamical (in particular elastic) stability of monolayer materials, which neglects the shear components of the stiffness tensor and only considers the sign of the planar stiffness coefficients. We agree that our analysis has not been complete, but find that it suffices in practice except for very few cases (less than 1% of the materials). Nevertheless, for completeness we are currently calculating the shear components of the elastic tensor for all the materials in the C2DB.

In the present study, using density functional calculations we have investigated a possible mechanism for the structural phase transition of the semiconducting bilayer 2H-MoS2 via lithiation. The results indicate that the addition of lithium to the bilayer 2H-MoS2 transforms the bilayer to a heterostructure of the 2H and 1T structures instead of a complete conversion to the 1T bilayer structure. Therefore, we propose that the desired synthesis of the 1T-MoS2 from the bulk 2H-MoS2 takes place through the hybrid 2H–1T structure. Our finding gives physical insight into the experimentally described microscopic mechanism of the phase transition in MoS2 and enriches the atomic scale understanding of the interaction of MoS2 with the alkali ions and other transition-metal dichalcogenides manifesting a similar phase transition.

In the present work, we have investigated the electrocatalytic activity of the oxygen reduction reaction (ORR), O2 + 4H+ + 4e− → 2H2O, for (Pt)n clusters (n = 1, 2, 3, 5, 7, 10 and 12) adsorbed on semiconducting (2H) and metallic (1T) MoS2 monolayers using first principles density functional theory. We have considered four elementary reactions involved in ORR within a unified electrochemical thermodynamic framework and the corresponding Gibbs adsorption free energies of the key intermediates (*OOH, *O, *OH) associated with each step have been calculated. The results indicate that the reduction of adsorbed hydroxyl (*OH) to water (*OH + H+ + e− → H2O) is the bottleneck step in the ORR process. The adsorption free energy of *OH (ΔG*OH) is found to be the thermodynamic descriptor for the present systems. Eventually, the ORR activity has been described as a function of ΔG*OH and a volcano plot predicting (Pt)7/2H-MoS2 as the best ORR catalyst amongst the (Pt)n/MoS2 heterosystems with an overpotential value of 0.33 V has been established. Our finding proposes a new promising electrocatalyst towards better activity for ORR with very small amount of Pt loading.

Efficient conversion of solar energy into electricity or fuels requires the identification of new semiconductors with optimal optical and electronic properties. We discuss the current and future role that computational screening is expected to play in this challenge. We discuss the identification of new computable descriptors characterising optimal materials performance, and we outline different search strategies in the materials screening. Finally, we describe some of the screening results obtained for perovskites, 2D materials, and for materials extracted from crystallographic databases.

The concentration of Eu3+ ion/dopant in the LaVO4 monazite nanocrystal phase cannot be increased by the conventional synthetic procedures. We demonstrate a unique three-step methodology to increase the doping concentration of Eu3+ in the LaVO4 monazite nanocrystals. In the first step, Eu3+ is doped (10%) in the zircon LaVO4 nanocrystal phase, which does not have a limitation in terms of Eu3+ ion loading. In the second step, high pressure (∼5 GPa) is utilized to transform the zircon crystal phase to the monazite phase. In the third step, the pressure is brought back to the atmospheric level, wherein it is observed that the monazite crystal phase is retained in its metastable phase with the 10% Eu3+ ion doping concentration. The phase transitions have been characterized via electrical resistivity data, XRD, Raman spectroscopy, fluorescence spectroscopy, TEM, and density functional simulations.

We analyze an approach toward stabilizing high-pressure rocksalt (RS) CdSe phases at atmospheric pressures using a ZnS core–CdSe shell1–ZnS shell2 architecture. The approach exploits the lattice mismatch between ZnS core and CdSe shell1, which generates bulk stress in the CdSe shell1 in wurtzite structure, thus driving the transition of wurtzite shell1 to RS shell1 to release the stress from the system, leading to the stabilization of the CdSe RS phase. Density functional computations suggest that a coherent interface between ZnS/CdSe and the sufficiently large unstrained ZnS core is an important ingredient for this synthetic strategy.

Organic−inorganic halide perovskites have proven highly successful for photovoltaics but suffer from low stability, which deteriorates their performance over time. Recent experiments have demonstrated that low dimensional phases of the hybrid perovskites may exhibit improved stability. Here we report first-principles calculations for isolated monolayers of the organometallic halide perovskites (C4H9NH3)2MX2Y2, where M = Pb, Ge, Sn and X,Y = Cl, Br, I. The band gaps computed using the GLLB-SC functional are found to be in excellent agreement with experimental photoluminescence data for the already synthesized perovskites. Finally, we study the effect of different defects on the band structure. We find that the most common defects only introduce shallow or no states in the band gap, indicating that these atomically thin 2D perovskites are likely to be defect tolerant.

Localized electronic states formed inside the band gap of a semiconductor due to crystal defects can be detrimental to the material's optoelectronic properties. Semiconductors with a lower tendency to form defect induced deep gap states are termed defect-tolerant. Here we provide a systematic first-principles investigation of defect tolerance in 29 monolayer transition metal dichalcogenides (TMDs) of interest for nanoscale optoelectronics. We find that the TMDs based on group VI and X metals form deep gap states upon creation of a chalcogen (S, Se, Te) vacancy, while the TMDs based on group IV metals form only shallow defect levels and are thus predicted to be defect-tolerant. Interestingly, all the defect sensitive TMDs have valence and conduction bands with a very similar orbital composition. This indicates a bonding/antibonding nature of the gap, which in turn suggests that dangling bonds will fall inside the gap. These ideas are made quantitative by introducing a descriptor that measures the degree of similarity of the conduction and valence band manifolds. Finally, the study is generalized to nonpolar nanoribbons of the TMDs where we find that only the defect sensitive materials form edge states within the band gap.

We explore the possibilities of hydrogen evolution by basal planes of 2D metal dichalcogenides and oxides in the 2H and 1T class of structures using the hydrogen binding energy as a computational activity descriptor. For some groups of systems like the Ti, Zr, and Hf dichalcogenides the hydrogen bonding to the 2H structure is stronger than that to the 1T structure, while for the Cr, Mo, and W dichalcogenides the behavior is opposite. This is rationalized by investigating shifts in the chalcogenide p levels comparing the two structures. We find that usually for a given material only at most one of the two phases will be active for the hydrogen evolution reaction; however, in most cases the two phases are very close in formation energy, opening up the possibility for stabilizing the active phase. The study points to many new possible 2D HER materials beyond the few that are already known.

An optimal band gap that allows for a high solar-to-fuel energy conversion efficiency is one of the key factors to achieve sustainability. We investigate computationally the band gaps and optical spectra of functional perovskites composed of layers of the two cubic perovskite semiconductors ${\mathrm{BaSnO}}_{3}$ and ${\mathrm{BaTaO}}_{2}\mathrm{N}$. Starting from an indirect gap of around $3.3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ for ${\mathrm{BaSnO}}_{3}$ and a direct gap of $1.8\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ for ${\mathrm{BaTaO}}_{2}\mathrm{N}$, different layerings can be used to design a direct gap of the functional perovskite between $2.3$ and $1.2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. The variations of the band gap can be understood in terms of quantum confinement and tunneling. We also calculate the light absorption of the different heterostructures and demonstrate a large sensitivity to the detailed layering.

The need for prediction of accurate electronic binding energies has led to the development of different schemes for combining density functional calculations, typically at the level of the generalized gradient approximation (GGA), with experimental information. We analyze one such scheme by Stevanovic´ et al. [Phys. Rev. B85, 115104 (2012)PRBMDO1098-012110.1103/PhysRevB.85.115104] for predictions of compound enthalpies of formation using fitted elemental-phase reference energies. We show that different versions of GGA with or without +U and a meta-GGA (TPSS) lead to comparable accuracy after fitting the reference energies. Our results also show that the recently developed mBEEF, a Bayesian error estimation functional, gives comparable accuracy with the other functionals even without the fitting. The mBEEF functional furthermore supplies an ensemble estimate of the prediction errors in reasonable agreement with the actual errors. We also show that using the fitting scheme on the mBEEF ensemble leads to improved accuracy including realistic error estimation.

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA. Electronic bandgap calculations are presented for 2400 experimentally known materials from the Materials Project database and the bandgaps, obtained with different types of functionals within density functional theory and (partial) self-consistent GW approximation, are compared for 20 randomly chosen compounds forming an unconventional set of ternary and quaternary materials. It is shown that the computationally cheap GLLB-SC potential gives results in good agreement (around 15%) with the more advanced and demanding eigenvalue-self-consistent GW. This allows for a high-throughput screening of materials for different applications where the bandgaps are used as descriptors for the efficiency of a photoelectrochemical device. Here, new light harvesting materials are proposed to be used in a one-photon photoelectrochemical device for water splitting by combining the estimation of the bandgaps with the stability analysis using Pourbaix diagrams and with the evaluation of the position of the band edges. Using this methodology, 25 candidate materials are obtained and 5 of them appear to have a realistic possibility of being used as photocatalyst in a one-photon water splitting device.

We present a density functional study of the structural behavior of zinc oxide nanostructures in basic growth condition which consequently leads to the formation of few layers of hydroxylated rocksalt structure over the wurtzite ZnO structure. We demonstrate the greater stability of the few layers of hydroxylated zinc oxide polar surface in rocksalt structure as compared to wurtzite structure. This coerces the near-surface layers of the nanostructure to acquire rocksalt structure giving rise to a trilayer structure consisting of a layer of hydroxyls on ZnO surface, rocksalt near-surface layers, and wurtzite bulk(or wurtzite sub-surface). The formation of coherent interface between rocksalt and wurtzite structure forces the hydroxylated trilayer structure to have lattice constant in between that of a rocksalt and wurtzite structure. Further, the hydroxylated rocksalt structure in the trilayer configuration is stable up to a critical size of the trilayer above which the increasing strain due to lattice mismatch between rocksalt and wurtzite structure overcomes the stabilizing effect of the hydroxylated rocksalt structure.

The stabilization and growth of nanocrystals in “non-native” structures is explored via density functional calculations. Non-native and “native” bulk structures differ in their discrete translational symmetry. Computations suggest that the lower surface energy of the non-native structures always facilitates their stabilization in the early stages of crystal growth. In the compound semiconductors considered here, the transition pathways between non-native and native structures involve planar or near-planar depolarized layers and the growth conditions have significant effects on the stabilization and growth of non-native structures. The findings of this study help in identifying heuristics for the synthesis of non-native nanocrystals.

We introduce the Computational 2D Materials Database (C2DB), which organises a variety of structural, thermodynamic, elastic, electronic, magnetic, and optical properties of around 1500 two-dimensional materials distributed over more than 30 different crystal structures. Material properties are systematically calculated by state-of-the art density functional theory and many-body perturbation theory (G$_0\!$W$\!_0$ and the Bethe-Salpeter Equation for $\sim$200 materials) following a semi-automated workflow for maximal consistency and transparency. The C2DB is fully open and can be browsed online or downloaded in its entirety. In this paper, we describe the workflow behind the database, present an overview of the properties and materials currently available, and explore trends and correlations in the data. Moreover, we identify a large number of new potentially synthesisable 2D materials with interesting properties targeting applications within spintronics, (opto-)electronics, and plasmonics. The C2DB offers a comprehensive and easily accessible overview of the rapidly expanding family of 2D materials and forms an ideal platform for computational modeling and design of new 2D materials and van der Waals heterostructures., Comment: Add journal reference and DOI; Minor updates to figures and wording