First-principles calculation of alloy phase diagrams: The renormalized-interaction approach

Two-dimensional III-nitride alloys: electronic and chemical properties of monolayer Ga(1-x)AlxN

Potential applications of III-nitrides have led to their monolayer allotropes, i.e., two-dimensional (2D) III-nitrides, having attracted much attention. Recently, alloying has been demonstrated as an effective method to control the properties of 2D materials. In this study, the stability, and the electronic and chemical properties of monolayer Ga(1-x)AlxN alloys were investigated employing density functional theory (DFT) calculations and the cluster expansion (CE) method. The results show that 2D Ga(1-x)AlxN alloys are thermodynamically stable and complete miscibility in the alloys can be achieved at ambient temperature (>85 K). By analyzing CE results, the atomic arrangement of 2D Ga(1-x)AlxN was revealed, showing that Ga/Al atoms tend to mix with the Al/Ga atoms in their next nearest site. The band gaps of Ga(1-x)AlxN random alloys can be tuned by varying the chemical composition, and the corresponding bowing parameter was calculated as -0.17 eV. Biaxial tensile strain was also found to change the band gap values of Ga(1-x)AlxN random alloys ascribed to its modifications to the CBM positions. The chemical properties of Ga(1-x)AlxN can also be significantly altered by strain, making them good candidates as photocatalysts for water splitting. The present study can play a crucial role in designing and optimizing 2D III-nitrides for next-generation electronics and photocatalysis.

In2Se3, In2Te3, and In2(Se,Te)3 Alloys as Photovoltaic Materials

In its lowest-energy three-dimensional (3D) hexagonal crystal structure (γ phase), In₂Se₃ has a direct band gap of ∼1.8 eV and displays high absorption coefficient, making it a promising semiconductor material for optoelectronics. Incorporation of Te allows for tuning the band gap, adding flexibility to device design and extending the application range. Here we report results of hybrid density functional theory calculations to assess the electronic and optical properties of γ-In₂Se₃, γ-In₂Te₃, and γ-In₂(Se_1-xTe_x)₃ alloys, and initial experiments on the growth and characterization of γ-In₂Se₃ thin films. The predicted band gap of 1.84 eV for γ-In₂Se₃ is in good agreement with the absorption onset derived from transmission and reflection spectra of thin films. We show that incorporation of Te gives γ-In₂(Se_1-xTe_x)₃ alloys with a band gap ranging from 1.84 eV down to 1.23 eV, thus covering the optimal band gap range for single-junction solar cells. In addition, the γ-In₂Se₃/γ-In₂(Se_1-xTe_x)₃ bilayer could be employed in tandem solar-cell architectures absorbing at E_g ≈ 1.8 eV and at E_g ≤ 1.4 eV, toward overcoming the ∼33% efficiency set by the Shockley-Queisser limit for single junction solar cells. We also discuss band gap bowing and mixing enthalpies, aiming at adding γ-In₂Se₃, γ-In₂Te₃, and γ-In₂(Se_1-xTe_x)₃ alloys to the available toolbox of materials for solar cells and other optoelectronic applications.

Equilibrium phase diagrams of isostructural and heterostructural two-dimensional alloys from first principles

Alloying is a successful strategy for tuning the phases and properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). To accelerate the synthesis of TMDC alloys, we present a method for generating temperature-composition equilibrium phase diagrams by combining first-principles total-energy calculations with thermodynamic solution models. This method is applied to three representative 2D TMDC alloys: an isostructural alloy, MoS_2(1-x)Te_2x , and two heterostructural alloys, Mo_1-x W _x Te₂ and WS_2(1-x)Te_2x . Using density-functional theory and special quasi-random structures, we show that the mixing enthalpy of these binary alloys can be reliably represented using a sub-regular solution model fitted to the total energies of relatively few compositions. The cubic sub-regular solution model captures 3-body effects that are important in TMDC alloys. By comparing phase diagrams generated with this method to those calculated with previous methods, we demonstrate that this method can be used to rapidly design phase diagrams of TMDC alloys and related 2D materials.

Effect of atomic configuration and spin-orbit coupling on thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 solid solutions

Through a combination of density functional theory calculations and cluster-expansion formalism, the effect of the configuration of the transition metal atoms and spin-orbit coupling on the thermodynamic stability and electronic bandgap of monolayer 2H-Mo1-xWxS2 is investigated. Our investigation reveals that, in spite of exhibiting a weak ordering tendency of Mo and W atoms at 0 K, monolayer 2H-Mo1-xWxS2 is thermodynamically stable as a single-phase random solid solution across the entire composition range at temperatures higher than 45 K. The spin-orbit coupling effect, induced mainly by W atoms, is found to have a minimal impact on the mixing thermodynamics of Mo and W atoms in monolayer 2H-Mo1-xWxS2; however, it significantly induces change in the electronic bandgap of the monolayer solid solution. We find that the band-gap energies of ordered and disordered solid solutions of monolayer 2H-Mo1-xWxS2 do not follow Vegard's law. In addition, the degree of the SOC-induced change in band-gap energy of monolayer 2H-Mo1-xWxS2 solid solutions not only depends on the Mo and W contents, but for a given alloy composition it is also affected by the configuration of the Mo and W atoms. This poses a challenge of fine-tuning the bandgap of monolayer 2H-Mo1-xWxS2 in practice just by varying the contents of Mo and W.

Electronic-structure methods for materials design

The accuracy and efficiency of electronic-structure methods to understand, predict and design the properties of materials has driven a new paradigm in research. Simulations can greatly accelerate the identification, characterization and optimization of materials, with this acceleration driven by continuous progress in theory, algorithms and hardware, and by adaptation of concepts and tools from computer science. Nevertheless, the capability to identify and characterize materials relies on the predictive accuracy of the underlying physical descriptions, and on the ability to capture the complexity of realistic systems. We provide here an overview of electronic-structure methods, of their application to the prediction of materials properties, and of the different strategies employed towards the broader goals of materials design and discovery.

Cluster expansion based configurational averaging approach to bandgaps of semiconductor alloys

Configurationally disordered semiconducting materials including semiconductor alloys [e.g., (GaN)_1-x(ZnO)_x] and stoichiometric materials with fractional occupation (e.g., LaTiO₂N) have attracted a lot of interest recently in search for efficient visible light photo-catalysts. First-principles modeling of such materials poses great challenges due to the difficulty in treating the configurational disorder efficiently. In this work, a configurational averaging approach based on the cluster expansion technique has been exploited to describe bandgaps of ordered, partially disordered (with short-range order), and fully disordered phases of semiconductor alloys on the same footing. We take three semiconductor alloys [Cd_1-xZn_xS, ZnO_1-xS_x, and (GaN)_1-x(ZnO)_x] as model systems and clearly demonstrate that semiconductor alloys can have a system-dependent short-range order that has significant effects on their electronic properties.

Stability, Electronic Structure, and Phase Diagrams of Novel Inter- Semiconductor Compounds

High-technology electronic devices are based on highly specialized core materials that make their operation pos sible : semiconductors. Unfortunately, the range of mate rial properties that make high-technology devices work is extremely narrow, since the sheer number of useful semiconductors is small. Hence, a major challenge has been to predict and develop new, potentially useful semiconductors. We describe here how the use of state- of-the-art techniques in both quantum and statistical mechanics can lead to predictions of new, stable, and ordered semiconductor alloys. A number of laboratories have already grown experimentally these new materials; efforts to characterize their useful material properties are ongoing in the United States, Japan, and Europe. This work describes the theoretical methodologies of our ap proach, and shows how supercomputers make possible the quantum-mechanical architectural analysis of new materials.

Orbital Delocalization and Enhancement of Magnetic Interactions in Perovskite Oxyhydrides

Recent experiments showed that some perovskite oxyhydrides have surprisingly high magnetic-transition temperature. In order to unveil the origin of this interesting phenomenon, we investigate the magnetism in SrCrO₂H and SrVO₂H on the basis of first-principles calculations and Monte Carlo simulations. Our work indicates that the Cr-O-Cr superexchange interaction in SrCrO₂H is unexpectedly strong. Different from the previous explanation in terms of the H⁻ ion substitution induced increase of the Cr-O-Cr bond angle, we reveal instead that this is mainly because the 3d orbitals in perovskite oxyhydrides becomes more delocalized since H⁻ ions have weaker electronegativity and less electrons than O²⁻ ions. The delocalized 3d orbitals result in stronger Cr-O interactions and enhance the magnetic-transition temperature. This novel mechanism is also applicable to the case of SrVO₂H. Furthermore, we predict that SrFeO₂H will have unprecedented high Neel temperature because of the extraordinarily strong Fe-H-Fe σ-type interactions. Our work suggests the anion substitution can be used to effectively manipulate the magnetic properties of perovskite compounds.

First-principles calculations of structural, electronic, and thermodynamic properties of monolayer Si1−xGexC sheet

The bowing coefficient of structural parameters is calculated. A band gap transition is also observed. The T–x phase diagram is calculated and shows a critical temperature of 187.4 K.

Stability and spinodal decomposition of the solid-solution phase in the ruthenium-cerium-oxide electro-catalyst

The phase diagram of Ru-Ce-O was calculated by a combination of ab initio density functional theory and thermodynamic calculations. The phase diagram indicates that the solubility between ruthenium oxide and cerium oxide is very low at temperatures below 1100 K. Solid solution phases, if existing under normal experimental conditions, are metastable and subject to a quasi-spinodal decomposition to form a mixture of a Ru-rich rutile oxide phase and a Ce-rich fluorite oxide phase. To study the spinodal decomposition of Ru-Ce-O, Ru0.6Ce0.4O2 samples were prepared at 280 °C and 450 °C. XRD and in situ TEM characterization provide proof of the quasi-spinodal decomposition of Ru0.6Ce0.4O2. The present study provides a fundamental reference for the phase design of the Ru-Ce-O electro-catalyst.

Adaptive cluster expansion approach for predicting the structure evolution of graphene oxide

An adaptive cluster expansion (CE) method is used to explore surface adsorption and growth processes. Unlike the traditional CE method, suitable effective cluster interaction (ECI) parameters are determined, and then the selected fixed number of ECIs is continually optimized to predict the stable configurations with gradual increase of adatom coverage. Comparing with traditional CE method, the efficiency of the adaptive CE method could be greatly enhanced. As an application, the adsorption and growth of oxygen atoms on one side of pristine graphene was carefully investigated using this method in combination with first-principles calculations. The calculated results successfully uncover the structural evolution of graphene oxide for the different numbers of oxygen adatoms on graphene. The aggregation behavior of the stable configurations for different oxygen adatom coverages is revealed for increasing coverages of oxygen atoms. As a targeted method, adaptive CE can also be applied to understand the evolution of other surface adsorption and growth processes.

Anomalous Alloy Properties in Mixed Halide Perovskites

Engineering halide perovskite through mixing halogen elements, such as CH3NH3PbI3-xClx and CH3NH3PbI3-xBrx, is a viable way to tune its electronic and optical properties. Despite many emerging experiments on mixed halide perovskites, the basic electronic and structural properties of the alloys have not been understood and some crucial questions remain, for example, how much Cl can be incorporated into CH3NH3PbI3 is still unclear. In this Letter, we chose CsPbX3 (X = I, Br, Cl) as an example and use a first-principle calculation together with cluster-expansion methods to systematically study the structural, electronic, and optical properties of mixed halide perovskites and find that unlike conventional semiconductor alloys, they exhibit many anomalous alloy properties such as small or even negative formation energies at some concentrations and negligible or even negative band gap bowing parameters at high temperature. We further show that mixed-(I,Cl) perovskite is hard to form at temperature below 625 K, whereas forming mixed-(Br,Cl) and (I,Br) alloys are easy at room temperature.

Tunable Electronic Properties of Two-Dimensional Transition Metal Dichalcogenide Alloys: A First-Principles Prediction

We investigated the composition-dependent electronic properties of two-dimensional transition-metal dichalcogenide alloys (WxMo1-xS2) based on first-principles calculations by applying the supercell method and effective band structure approximation. It was found that hole effective mass decreases linearly with increasing W composition, and electron effective mass of alloys is always larger than that of their binary constituents. The different behaviors of electrons and holes in alloys are attributed to the fact that metal d-orbitals have different contributions to conduction bands of MoS2 and WS2 but almost identical contributions to valence bands. We examined the conduction polarity of WxMo1-xS2 monolayer alloys with four metal electrode materials (Au, Ag, Cu, and Pd). It suggests the main carrier type for transport in transistors could change from electrons to holes as W composition increases if high work function metal contacts were used. The tunable electronic properties of two-dimensional transition-metal dichalcogenide alloys make them attractive for electronic and optoelectronic applications.

Overcoming the phase inhomogeneity in chemically functionalized graphene: the case of graphene oxides

The inhomogeneous phase, which usually exists in graphene oxides (GOs), is a long-standing problem that has severely restricted the use of GOs in various applications. By using first-principles based cluster expansion, we find that the existence of phase separation in conventional GOs is due to the extremely strong attractive interactions of oxygen atoms at different graphene sides. Our Monte Carlo simulations show that this kind of phase separation is not avoidable under the current experimental growth temperature. In this Letter, the idea of oxidizing graphene on a single side is proposed to eliminate the strong double-side oxygen attractions, and our calculations show that well-ordered GOs could be obtained at low oxygen concentrations. These ordered GOs behave as quasi-one-dimensional narrow-gap semiconductors with quite small electron effective masses, which can be useful in high-speed electronics. Our concept could be widely applied to overcome the phase inhomogeneity in various chemically functionalized two-dimensional systems.

Strong Dzyaloshinskii-Moriya interaction and origin of ferroelectricity in Cu2OSeO3

By performing density functional calculations, we investigate the origin of the Skyrmion state and ferroelectricity in Cu2OSeO3. We find that the Dzyaloshinskii-Moriya interactions between the two different kinds of Cu ions are extremely strong and induce the helical ground state and the Skyrmion state in the absence and presence of a magnetic field, respectively. On the basis of the general model for the spin-order induced polarization, we propose that the ferroelectric polarization of Cu2OSeO3 in the collinear ferrimagnetic state arises from an unusual mechanism, i.e., the single-spin-site contribution due to the spin-orbit coupling.

High Indium Content Ingan Films and Quantum Wells

InGaN films and InGaN/GaN quantum wells with high indium content have been grown by MOVPE and characterised to evaluate the growth process and the indium incorporation efficiency. The characterisation techniques include photoluminescence, DC X-ray and TEM. The closed spaced vertical rotating disk reactor configuration results in a very high Indium incorporation for InGaN material, compared to other configurations. InGaN layers with an indium composition up to 56 % have been deposited which still exhibit very good optical properties (intense PL emission). The influence of various growth conditions on the InGaN composition and quality have been investigated to optimize the layer quality. TEM diffraction patterns have shown that the ternary InGaN layer can be chemically ordered. The In and Ga atoms occupy respectively the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P6 3mc symmetry group of the wurtzite GaN.

"Narrow" graphene nanoribbons made easier by partial hydrogenation

It is highly desirable to produce narrow-width graphene nanoribbons (GNRs) with smooth edges in large scale. In an attempt to solve this difficult problem, we examined the hydrogenation of GNRs on the basis of first principles density functional calculations. Our study shows that narrow GNRs can be readily obtained from wide GNRs by partial hydrogenation. The hydrogenation of GNRs starts from the edges of GNRs and proceeds gradually toward the middle of the GNRs so as to maximize the number of carbon-carbon pi-pi bonds, hence effectively leading to narrower GNRs. Furthermore, the partially hydrogenated wide GNRs have similar electronic and magnetic properties as those of the narrow GNRs representing their graphene parts. Therefore, partial hydrogenation of wide GNRs should be a practical and reliable method to produce narrow GNRs in large scale.

Thermodynamic theory of epitaxial alloys: first-principles mixed-basis cluster expansion of (In, Ga)N alloy film

Epitaxial growth of semiconductor alloys onto a fixed substrate has become the method of choice to make high quality crystals. In the coherent epitaxial growth, the lattice mismatch between the alloy film and the substrate induces a particular form of strain, adding a strain energy term into the free energy of the alloy system. Such epitaxial strain energy can alter the thermodynamics of the alloy, leading to a different phase diagram and different atomic microstructures. In this paper, we present a general-purpose mixed-basis cluster expansion method to describe the thermodynamics of an epitaxial alloy, where the formation energy of a structure is expressed in terms of pair and many-body interactions. With a finite number of first-principles calculation inputs, our method can predict the energies of various atomic structures with an accuracy comparable to that of first-principles calculations themselves. Epitaxial (In, Ga)N zinc-blende alloy grown on GaN(001) substrate is taken as an example to demonstrate the details of the method. Two (210) superlattice structures, (InN)(2)/(GaN)(2) (at x = 0.50) and (InN)(4)/(GaN)(1) (at x = 0.80), are identified as the ground state structures, in contrast to the phase-separation behavior of the bulk alloy.

First principle calculations of structural, electronic, thermodynamic and optical properties of Pb(1-x)Ca(x)S,Pb(1-x)Ca(x)Se and Pb(1-x)Ca(x)Te ternary alloys

Using first principles total energy calculations within the full potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic, thermodynamic and optical properties of Pb(1-x)Ca(x)S, Pb(1-x)Ca(x)Se and Pb(1-x)Ca(x)Te ternary alloys. The effect of composition on lattice parameter, bulk modulus, band gap, refractive index and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence were observed for the three alloys. Using the approach of Zunger and co-workers, the microscopic origins of band gap bowing have been detailed and explained. The disorder parameter (gap bowing) was found to be mainly caused by the chemical charge transfer effect. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing, ΔH(m), as well as the phase diagram. It was shown that all of these alloys are stable at low temperature. The calculated refractive indices and optical dielectric constants were found to vary nonlinearly with Ca composition.

Cluster expansions in multicomponent systems: precise expansions from noisy databases

We have performed a systematic analysis of the numerical errors contained in the databases used in cluster expansions of multicomponent alloys. Our results underscore the importance of numerical noise in the determination of the effective cluster interactions and in the expansion determination. The relevance of the size of and information contained in the input database is highlighted. It is shown that cross-validatory approaches by themselves can produce unphysical expansions characterized by non-negligible, long-ranged coefficients. A selection criterion that combines both forecasting ability and a physical limiting behavior for the expansion is proposed. Expansions performed under this criterion exhibit the remarkable property of noise filtering. A discussion of the impact of this unforeseen characteristic of the cluster expansion method on the modeling of multicomponent alloy systems is presented.

Origins of nonstoichiometry and vacancy ordering in Sc1-x squarexS

Whereas nearly all compounds A(n)B(m) obey Dalton's rule of integer stoichiometry (n: m, both integer), there is a class of systems, exemplified by the rocksalt structure Sc1-x squarexS, that exhibits large deviations from stoichiometry via vacancies, even at low temperatures. By combining first-principles total energy calculations with lattice statistical mechanics, we scan an astronomical number of possible structures, identifying the stable ground states. Surprisingly, all have the same motifs: (111) planes with (112) vacancy rows arranged in (110) columns. Electronic structure calculations of the ground states (identified out of approximately 3x10(6) structures) reveal the remarkable origins of nonstoichiometry.