Session S12: Computational Materials Design - Novel Oxides and Chalcogenides

High-throughput screening for novel p-type transparent semiconducting oxides using hydrogen descriptor

The ultimate transparent electronic devices require complementary and symmetrical pairs of n-type and p-type transparent semiconducting oxides (TSOs). While several n-type TSOs like InGaZnO are available and being used in consumer electronics, there are practically no p-type oxides that are comparable to the n-type counterpart in spite of tremendous efforts to discover them. Recently, high-throughput screening with the density-functional-theory calculations attempted to identify candidate p-type TSOs, but none of suggested materials was verified experimentally, implying need for a better theoretical predictor. Here we propose a highly reliable and computationally efficient descriptor for p-type dopability - the hydrogen impurity energy. We show that the hydrogen descriptor can distinguish well known p-type and n-type oxides. Using the hydrogen descriptor, we screen most binary oxides and a selected pool of ternary compounds that covers Sn²⁺- and Cu¹⁺-bearing oxides as well as oxychalcogenides. As a result, we suggest La₂O₂Te and CuLiO as promising p-type oxides. In addition, we identify simple descriptors that correlate to the hydrogen impurity energy. Using these simplified descriptors, we screen 17,000+ oxides in Aflow database, and identify a new class of p-type candidates.   

Computational Discovery of Novel Ternary Oxyfluorides

Anion engineering of crystalline materials has seen a resurgence in interest. In particular, oxyfluoride compounds present an opportunity to create materials that possess the advantageous properties of both oxides and fluorides. We present here a high-throughput ab initio density functional study that (a) critically assesses the thermodynamic stability and energetics of currently reported oxyfluoride compounds, (b) using phase stability analysis within the framework of the Open Quantum Materials Database (OQMD), discover new, hiterto-unreported, ternary oxyfluoride compounds. Our study thus opens up a new class of materials with a wide variety of properties for further experimental exploration.   

Considerations in the Stability of Ternary Oxide Alloys

Computational high-throughput screening has generated much interest in materials science because of the possibility to guide the development of functional materials along with the ability to enhance the understanding of fundamental properties. Identification of stable crystalline materials for a mixture of two (three) components requires examination of the lowest thermodynamic energy of approximately 2^N (3^N) configurations, where N is the number of atoms in the unit cell. Cluster expansion-based energy functions offer a numerically efficient approach for estimating the stability of new potential alloys. Combining this approach with the nested sampling algorithm, which is a Bayesian Markov chain Monte Carlo method, allows for a one-shot calculation of the phase diagram as a function of composition and temperature. Our results for stable ternary and quaternary mixtures in various crystalline symmetries of group-III oxides with the formula (In_xGa_yAl_z)₂O₃ will be presented. A new stable InGaO₃ compound is reported as a potential transparent conductor material. In addition, the key aspects that determine the stability of these materials will be highlighted.   

Mining Unexplored Chemistries for Phosphors for High-Color-Quality White-Light-Emitting Diodes

There is a critical need for new earth-abundant phosphors to enable next-generation, highly efficient solid-state lighting. We report the discovery of Sr₂LiAlO₄, the first known Sr-Li-Al-O quaternary crystal, via a carefully-targeted data-driven structure prediction and screening effort using density functional theory calculations. Sr₂LiAlO₄ is predicted and experimentally confirmed to be a thermodynamically and thermally stable phosphor host that can be excited with near-UV/blue sources. The Eu²⁺ and Ce³⁺ activated Sr₂LiAlO₄ phosphors exhibit broad green-yellow/blue emissions, respectively, with excellent thermal quenching resistance of > 88% intensity at 150°C. A prototype phosphor-converted white LED utilizing Sr₂LiAlO₄-based phosphors yields an excellent color rendering index exceeding 90. Sr₂LiAlO₄ therefore exhibits great potential for industrial applications in low-cost, high-color-quality WLEDs.   

Tuning the Plasma Frequency in Correlated Transition Metal Oxides

Transparent conductors, materials that bring together electrical conductivity with optical transparency, are usually designed starting with a wide transparent insulator, which is then doped to introduce charge carriers. However, this approach is often limited in the maximum conductivity that can be obtained because of both defect scattering and doping bottlenecks. An alternative approach is to design a metal that has weak interband absorption and a plasma frequency that is close to but below the lower end of the visible spectrum. In this talk, we present a systematic first principles study of the effect of biaxial strain, octahedral rotations, and layering on the transparent conducting properties of d1 perovskites. We employ Density Functional Theory in conjunction with Dynamical Mean Field Theory (DFT+DMFT) to predict the correlation induced suppression of the plasma frequency and show that it can be significant even in the 4d transition metal oxides. We show that factors such as polyhedral connectivity induce changes much more significant than strain or octahedral rotations in weakly correlated oxides.   

Exploring Polymorphism in High-valent Tantalum Trioxides

Tantalum oxides (TaO, TaO₂, and Ta₂O₅) are key oxide materials for modern electronic devices, such as dynamic random-access memory and sensors, possessing the usual +2, +4, and +5 valence charge state of Ta. Interestingly, new forms of tantalum oxides have been proposed with an unusual stoichiometry of TaO₃ and are predicted to have a high-valent charge state of Ta⁶⁺. However, not much is known about these highvalent TaO₃ nor is the possibility of structural polymorphism (as found in other more well-known heavy metal oxides like WO₃ and MoO₃) explored. In this work, we have collected structural motifs from previous studies about TaO₃, especially synthesis process. Using first-principles density-functional theory calculations, we study the thermodynamics and lattice dynamics of polymorphs of TaO₃ (namely, the cubic ReO₃, new stacked and sheet structures) and explore the electronic and optical properties of these new TaO₃ polymorphs for potential device applications.   

Exploring the RNiO3 and RVO3 Phase Diagrams with Data Analytics

The ABO₃ perovskites exhibit complex electron-lattice interactions that complicate the identification of primary pathways to tune physical properties. Representation theory provides a framework us to decompose low-symmetry structures into orthonormal symmetry-breaking lattice modes of the parent aristotype. Statistical analyses may then be used to study structure-property relationships based on these symmetry-modes and macroscopic observables [1]. As test cases, we explore the metal-insulator transitions and complex spin orders of the rare-earth nickelates and spin and orbital orders of vanadate perovskites. Analyses of experimental vs. ab initio data reveal significant differences in predicted trends depending on how the crystal structures were obtained. We show how physical priors can help explain this discrepancy and illustrate why context matters in conducting statistical studies.
[1] P.V. Balachandran et al., Phys. Rev. B. 88, 054101 (2013).   

Microscopic origins of the large piezoelectricity of lead-free (Ba,Ca)(Zr,Ti)O3

In light of directives around the world to eliminate toxic materials in various technologies, finding lead-free materials with high piezoelectric responses is an important current scientific quest. The recent discovery of a large electromechanical conversion near room temperature in (1 − x)Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)TiO₃ compounds has attracted a lot of attention. Strikingly, the origin of such a strongly desired response is not conclusively understood.

Here, we report the development of a large-scale atomistic scheme providing a microscopic insight into this technologically-promising material. The use of such a scheme reveals that high piezoelectricity in this material originates from the existence of large fluctuations of the polarization in the orthorhombic state arising from the combination of flat free-energy surfaces, a fragmented local structure, and the narrow temperature window around room temperature at which this orthorhombic phase is the equilibrium state.

Nature Communications 8, 15944 (2017)   

Band structure, optical properties, and band alignment of Cu-based delafossites

Recently, CuGaO₂ has been shown to hold great potential as inorganic hole transport layers (HTLs). In this work, we study the electronic band structure, optical properties and band alignment of all the Cu-based delafossites using the state-of-the-art GW-BSE approach. All the delafossites are shown to be indirect large band gap semiconductors with large exciton binding energies, varying from 0.24 to 0.44 eV, in consistent with experimental findings. Taking into consideration the exciton binding energy, the calculated band gap agrees quite well with experiments. Based on the GW band structure, we calculated the band alignment of the delafossites using the Anderson’s rule. By comparing the band alignment between the delafossites and MAPbI₃, we predict that, beside CuGaO₂, CuScO₂, CuYO₂, and CuCrO₂ are also suitable as HTLs in MAPbI₃ based solar cells. In addition, the suitability of CuCrO₂ as HTL has also been verified by our experimental collaborators.   

Transition-metal Impurities in PtSe2

PtSe₂ is a recently synthesized, semiconducting two-dimensional (2D) transition metal dichalcogenide whose ground state structure is the 1T rather than the more typical 1H phase. For applications of 2D materials in areas such as spintronics, there is a need for stable, semiconducting magnetic materials. As in the case of three-dimensional, bulk semiconductors, creation of dilute magnetic semiconductors through doping with magnetic elements provides an appealing route. In the present work, we have investigated the inclusion of transition-metal impurities as cation dopants in PtSe₂ through the use of density functional theory calculations. We find that both hole and electron magnetism can be stabilized in PtSe₂ with minimal impact on the relative stability between structural phases and explore the nature of the magnetic moment within the context of the local electronic structure. Furthermore, we examine the magnetic coupling between multiple impurities in order to determine which have the more robust ferromagnetic coupling.   

Electric and Optical Behaviors of SiC(GeC)/MoS2 Heterostructures: A First Principles Study†

Hybrid structures have attracted a great deal of attention because of their excellent properties,
which can open up a way we could not foresee in materials science and device physics. Here,
we investigate the electric and optical behaviors of SiC(GeC)/ MoS2 heterostructures, using first
principle calculations based on density functional theory. The non􀀀covalent bond exists between
the junctions due to the weak orbital coupling. Both junctions have optically active band gaps,
smaller than SiC or GeC and MoS2 layers, which result in the enhanced optical adsorption under
visible􀀀light irradiation. A small number of electron transfer from SiC/GeC to MoS2 causing its
n􀀀doped. Furthermore, the charge density states of the valence band maximum and conduction
band minimum are localized at the different sides, thus the electron􀀀holes pairs are spatially
separated. Our results provide a potential scheme for the photovoltaic materials.   

Effects of Adsorption of CO and NO Molecules on MoS2/Graphene Heterostructures

Gases play a pivotal-role in controlling chemical and physical processes in environment. The adsorption of carbon-monoxide and nitric oxide on MoS₂/graphene heterostructures have been investigated using spin-polarized density functional theory - with generalized gradient approximation, incorporating van der Waals interaction. In this study, the first-principles calculations are performed to investigate - geometries, energetics of MoS₂/graphene heterostructures followed by the gaseous adsorption at different possible adsorption sites. Electronic band structures and density of states calculations suggest that the states at the Fermi level are contributed by graphene, which may increase the conductivity of the composite system. In addition, Mo-d states are observed around the Fermi level to increase total density of states in all systems. Our findings suggest that the heterostructures can physisorb these molecules and may act as useful supportive materials for gas sensing. The mechanism for these two gaseous adsorption are different with varying binding energies due to different chemical reactivity of constituent elements. In summary, our results demonstrate the possibility of using 2D van der Waals heterostructures as sensors for the detection of toxic gases.   

Piezomagnetic response in CrPS4 monolayer

We performed first-principles calculations to investigate the magnetic, mechanical and electronic properties of the tetrachalcogenide CrPS₄. Although bulk CrPS₄ has been shown to exhibit a low-dimensional antiferromagnetic (AFM) ground state where ferromagnetic (FM) Cr-chains are coupled antiferromagnetically, our calculations indicated that the monolayer can be transformed to an FM material by applying a uniaxial tensile strain of ≥4% along the FM Cr-chain direction. The AFM-to-FM transition is explained to be driven by an increase of the exchange interaction induced by a decrease in the distance between the FM Cr-chains. A huge nonlinear piezomagnetism was predicted at the strain-induced magnetic phase boundary. Our study provides insight about rational design of single-layer magnetic materials for a wide range of spintronic devices and energy applications.