Session M5: Focus Session: Computational Discovery and Design of New Materials: Semiconductors, Molecular Systems and Interfaces

ReaxFF-based molecular dynamics studies on reactions at complex material surfaces

The ReaxFF method provides a highly transferable simulation method for atomistic scale simulations on chemical reactions at the nanosecond and nanometer scale. It combines concepts of bond-order based potentials with a polarizable charge distribution. Since it initial development for hydrocarbons in 2001, we have found this concept to be highly transferable, leading to applications to elements all across the periodic table, including all first row elements, metals, ceramics and ionic materials. In this presentation we will provide an overview of recent developments of the ReaxFF method for reactions at the complex material interfaces, in particular TiO$_2$/water, silica/water and graphite/oxygen interfaces. We will describe the ReaxFF parameter development process and show how, by employing parallel molecular dynamics methods, ReaxFF can assist in bridging the gap between atomistic-scale simulations and experiment. We will also discuss new developments in metadynamics and Monte Carlo based implementations of ReaxFF, which enable us to extend molecular dynamics simulation times to beyond hundreds of nanoseconds.   

Reliable Modeling of Complex Organic/Metal Interfaces

The understanding of electronic properties of complex organic/metal interfaces requires a reliable method for the prediction of their structure and stability. The bonding at complex interfaces arises from delicate balance between covalent bonds, van der Waals (vdW) forces, charge transfer, and Pauli repulsion. We developed a method based on density-functional theory with vdW interactions (PBE+vdW$^{\rm surf}$ [1]) to accurately model adsorbates on surfaces, by a synergetic linkage of the PBE+vdW [2] for intermolecular interactions with the Lifshitz-Zaremba-Kohn theory [3] for the dielectric screening within the substrate surface. This method is demonstrated to reliably model a multitude of molecules on metal surfaces [1,4], leading to an accuracy of 0.1 {\AA} in adsorption heights and 0.1 eV in binding energies wrt experiments. To demonstrate the predictive power of the PBE+vdW$^{\rm surf}$, we design a novel type of single-molecule push button switch, by carefully controlling the stability and activation barrier between a chemically bound state and a physically bound state for benzene derivatives adsorbed on metal surfaces.\\[4pt] [1] Ruiz, \textit{et al.}, PRL (2012).\\[0pt] [2] Tkatchenko and Scheffler, PRL (2009).\\[0pt] [3] Zaremba and Kohn, PRB (1976).\\[0pt] [4] Wagner, \textit{et al.}, PRL (2012).   

A High-Throughput Computational Search for New Transparent Conducting Oxides

Transparent conducting oxides (TCOs) are critical to many technologies from solar cells to electronics. However, finding materials that combine the two antagonistic properties of large conductivity and transparency to the visible light can be extremely challenging. In this talk, we will present a high-throughput screening approach aimed at discovering new high-performance TCOs. Combining different \emph{ab initio} techniques from density functional theory to GW, we evaluated thousands of oxides in terms of essential TCO properties (e.g., band gap and carrier transport). From these results, we will present new interesting compounds as well as discuss the chemistries likely to form high performance TCOs.   

Generation and analysis of the largest ab initio database for metal borides

Boron-based materials have been observed in a remarkable variety of crystal structures with outstanding superconducting, mechanical, and refractory properties. Aiming to provide a systematic description of known compounds and to identify new synthesizable candidate materials, we have generated an extensive ab initio database spanning over 40 binary and ternary metal boride systems at ambient and gigapascal pressures. The considered crystal structures include known prototypes listed in the ICSD as well as brand-new prototypes found with an evolutionary search implemented in MAISE [1]. Having examined over 15,000 entries of calculated formation enthalpies, we find a number of surprising disagreements between theory and experiment regarding the ground state crystal structures and identify over a dozen systems in which novel compounds are expected to form under high pressures. Data mining of the ab initio information has revealed trends in the electronic, magnetic, vibrational, and elastic properties which can help fine-tune the metal boride materials for specific applications. [1] Module for Ab Initio Structure Evolution, \underline {http://maise-guide.org}   

Mechanistic Design of New Materials and Processes through Multifunctional Atomic-Scale Simulations

Multifunctional systems that contain heterogeneous interfaces are ubiquitous in numerous applications, including catalysis, electronic devices, friction, and coatings. Traditionally, computational studies of these complex interfacial systems have relied on methods such as first-principles density functional theory (DFT), because of the difficulty in describing the changes in bonding environment with empirical approaches. Here, empirical, charge optimized many-body (COMB) potentials are used in classical, atomic-scale simulations to examine several model systems that involve heterogeneous material interfaces or surface reactions at size scales that are much larger than are currently tractable with traditional DFT methods. . The COMB potentials allow for dynamic charge transfer between atoms and across interfaces, and are demonstrated to describe metallic, covalent, and ionic bonding across interfaces and at surfaces. The simulations yield mechanistic insights that allow for the design of materials and optimization of process conditions for several applications, including catalysis, thin-film growth, and supported two-dimensional materials with well-defined interfacial interactions.   

Towards Reliable Predictions of Molecular Materials

While dispersion interactions are known to be essential to the stability and accurate prediction of molecular-crystal structures, the vast majority of computational methods use simple pairwise approximations to model these interactions, ignoring the non-additive, many-body nature of long-range electron correlation. Here we use the recently developed many-body dispersion (MBD) method (PRL 108, 236402; PNAS 109, 14791) together with a representative database of molecular crystals, to illustrate how important electrodynamic screening and many-body contributions are to crystal stability. Crucially, these MBD contributions allow DFT calculations to reach the highly coveted ``chemical accuracy'' with respect to high-level calculations and experiments in both the crystalline and gaseous phases. This ability to treat molecular solids and their components on such an accurate and equal footing is essential for controled and informed design of complex materials.   

Property optimization in isovalent and aliovalent semiconductor alloys based on MnO

Materials for solar energy conversion need to fulfill specific targets in regard of the band-structure, optical properties, carrier transport, and doping. In order to design or discover novel materials that satisfy multiple requirements, we employ design principles to select a range of material compositions where those properties are likely to occur, and then evaluate them computationally. Here we are addressing the design of semiconductor alloys based on the d$^{\mathrm{5}}$ oxide MnO, which was recently identified as an interesting base material for semiconducting transition metal oxides [PRB 85, 201202(R)]. To calculate the properties for different alloy compositions with the many-body GW method, we modeled the alloy systems by searching for special quasi-random structures (SQS). In isovalent alloys, the SQS was chosen such that the correlation functions were as close as possible to the ideal random alloy. For aliovalent alloys where strong short-range-ordering is expected, the target correlation functions for the SQS search were determined by a Monte-Carlo simulation based on a cluster expansion of the total energy for the alloy. The optical properties determined from GW calculations for such SQS alloy structures are compared with available experimental data.   

Abundant defects and defect clusters in kesterite Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$

Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$ are drawing intensive attention as the light-absorber materials in thin-film solar cells. A large variety of intrinsic defects can be formed in these quaternary semiconductors, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. We will present our first-principles calculation study on a series of intrinsic defects and defect clusters in Cu$_2$ZnSnS$_4$ and Cu$_2$ZnSnSe$_4$, and discuss: (i) strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the dominant Cu$_{\mathrm{Zn}}$ antisites and Cu vacancies which determine the intrinsic p-type conductivity, and their dependence on the elemental ratios; (iii) the high population of charge-compensated defect clusters (like V$_{\mathrm{Cu}}+$Zn$_{\mathrm{Cu}}$ and 2Cu$_{\mathrm{Zn}}+$Sn$_{\mathrm{Zn}})$ and their contribution to non-stoichiometry ; (iv) the deep-level defects which act as recombination centers. Based on the calculation, we will explain the experimental observation that Cu poor and Zn rich conditions give the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu$_2$ZnSn(S,Se)$_4$ cells with high S composition.   

Strain Induced Photoabsorption of CuGa$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$O$_2$

Delafossite oxides are a family of materials that hold promise for photocatalytic, thermoelectric, and other cutting edge applications. These materials are of interest because they exhibit a disparity between their optical and electronic band gaps due to inversion symmetry according to the Laporte selection rule. Though they appear transparent, their electronic structure suggests that they should absorb visible light, aside from conduction and valence band parity. We use B-site substitution to break inversion symmetry and allow the absorption of visible light. Here we present computational and experimental electronic and optical results of B-site substitution of the delafossite CuGaO$_{2}$ with Fe which supports the inversion symmetry theory of the band gap disparity. Included are experimental and computational absorption spectra for CuGa$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$O$_{2}$. We find and explain an interesting increase optical absorption in the visible range at the 5{\%} Fe substitution level. To the best of our knowledge computational results to this degree of percentage accurate substitution or alloying have not been performed on this or similarly complicated systems.   

Massive computational search for n-type organic semiconductors

In a search for n-type organic materials, which are rare compared to p-type, we calculate the optimized geometries and electronic structures for millions of conjugated oligomers. A good n-type material (electron conductor) must have a low-lying LUMO level (high electron affinity) in order to avoid chemical reactions that create electron traps. For high conductivity, it must have a low barrier to electron hopping, indicated by an internal reorganization energy in the meV range. The calculations use the adapted Su-Schrieffer-Heeger tight-binding Hamiltonian and include both neutral and singly charged structures. The group of structures with a combination of low-lying LUMO levels and small internal reorganization energies is presented as candidates for n-type organic semiconductor materials. The data are also used to directly compute the optical band gaps and exciton binding energies, while the HOMO and LUMO levels are used to estimate cyclic voltammetry oxidation and reduction potentials. Application of the methodology to other organic materials searches is discussed.   

Theoretical study of LaOXS \{X=Cu, Ag\} layered oxide sulphides

The ternary oxides, owing to the mismatch between the energy levels of the transition metal {\em d}-orbitals and the deep oxygen {\em p}-orbitals, typically show a limited dispersivity of the valence band maxima (VBM) and relatively heavy masses that make them not favorable in applications as p-type transparent conducting oxides (TCOs). In a hope to increase the {\em p}-{\em d} hybridization and preserve large band gaps in oxides with the addition of sulphur atoms, we studied the reported layered quarternary oxysulphides (LaCuOS, LaAgOS) using density functional theory with G0W0 self energy corrections. We confirmed that the VBM is mainly contributed by the antibonding state of Cu/Ag-{\em d} and S-{\em p} and the hole effective mass increases upon Cu substitution by Ag, which has a deeper {\em d} level than the Cu {\em d} one.