The highlights here document the progress and impact of scientific research within the Center for Next Generation of Materials Design. Most of these highlights are associated with published journal articles by our principal investigators.
Methylammonium lead iodide (MAPbI3) exhibits exceptional photovoltaic performance. But a significant controversy remains over the existence and impact of ferroelectricity on the photovoltaic response. We confirm ferroelectricity in MAPbI3 single crystals and demonstrate mediation of the electronic response by ferroelectric domain engineering.
For some heterostructural alloys, a metastable low-density polymorph can be the preferred structure for non-phase-separated intermediate compositions. For the MnSe-MnTe system, where this effect was predicted theoretically, the low-density metastable wurtzite phase was successfully synthesized using non-equilibrium thin-film growth.
Physical Descriptor for the Gibbs Energy of Inorganic Crystalline Solids and Temperature-Dependent Materials Chemistry December 2018
Developed a machine-learned descriptor enabling the chemically accurate high-throughput prediction of temperature-dependent thermodynamics (<40 meV/at MAE).
Demonstrated predictive synthesis of MnO2 polymorphs based on principle of remnant metastability—that synthesizable metastable phases nucleate under thermodynamic conditions where they were once the lowest free-energy phase, and then grow into conditions where they are metastable. Using in-situ X-ray diffraction measurements, we verify that this framework can predict which phases are more likely to form as well as the progression order of phases.
New, stable and metastable Zn-Mo-N alloys with wurtzite-derived crystal structure were theoretically predicted and experimentally synthesized. A broad range of properties—from insulating and transparent Zn3MoN4 to conductive and absorptive ZnMoN2—is realized by tuning the composition.
Amorphous Precursors: A Route to Polymorph Synthesis (January 2018)
Using theory-guided synthesis, we have synthesized TiO2 thin films with greater than 80% brookite fraction without substrate templating or the presence of helper ions via solid-phase crystallization of amorphous precursors.
Thermodynamic Routes to Novel Metastable Nitrogen-Rich Nitrides (December 2017)
We formulate how reactive nitrogen precursors can stabilize metastable nitrogen-rich nitrides during synthesis. With this synthesis strategy, we use DFT to predict 22 new nitrogen-rich binary nitrides stabilizable under ΔµN2 = +1 eV/N.
Design of Metastable Tin Titanium Nitride Semiconductor Alloys (November 2017)
We predicted and synthesized new mixed-metal nitride alloys with improved optoelectronic properties. Specifically, we synthesized metastable (Sn,Ti)3N4 spinels using non-equilibrium synthesis. These (Sn,Ti)3N4 alloys have lower hole effectives masses and better transport than Sn3N4.
Metastable (SnCa)Se with improved thermoelectric functionality was realized by combining theory-guided alloying with non-equilibrium growth.
A Framework for Automating Point-Defect Calculations (September 2017)
We completed and rigorously validated an open-source Python framework to automate first-principles point-defect calculations.
Developed computational framework for using neural networks to learn analytical potentials, non-linear density functionals, and structure-property relationships based on high-accuracy ab initio data. Demonstrated use of neural networks to create easily evaluated local DFT charge-density functionals for range of properties for model gas-phase system, NH3.
We rationalized the synthesis of all common MnO2 polymorphs by forming off-stoichiometric intermediates during aqueous synthesis. We also derived general rules governing off-stoichiometric polymorph stabilization by alkali ions and hydration applicable to transition metal oxide chemistries.
We discovered wide metastable regions in the phase diagram of heterostructural alloys. The materials lie above the phase-separated free-energy minimum, but are stable against spinodal composition fluctuations.
ALD Oxides for Higher Performance Power Transistors (February 2017)
Successfully grew single-crystalline (Mg,Ca)O films epitaxially on InAlN transistors with unprecedentedly low density of defects and electron traps at the oxide/semiconductor interface.
Successfully determined the synthesis space (substrate temperature, oxygen partial pressure, substrate selection) for the targeted growth of β-Ga2O3 thin films through theory-guided experiments. Grew high-quality, oriented β-Ga2O3 films.
Revealed the thermodynamic landscape of inorganic crystalline metastability. Proposed principle of Remnant Metastability: "Observable metastable phases are remnants of thermodynamic conditions where they were once the lowest free-energy phase."
Characterized the interfacial electronic band alignment using new high-throughput measurements by coupling spatially resolved photoelectron spectroscopy (PES) mapping with combinatorially deposited crossed-gradient thin-film samples.
Local Amorphous Structure Controls Polymorph Formation (October 2016)
Used grazing incidence X-ray scattering measurements to identify local structural differences in amorphous thin-film precursors that subsequently determine the polymorph formed upon crystallization.
Epitaxial Polymorph Stabilization through a Computational Approach to Substrate Selection (May 2016)
Developed a computational framework combining calculations of formation energy, elastic strain energy, and topological lattice matching to guide substrate selection for epitaxial materials growth.
We provide a new perspective on the promising properties, unexplored chemistry, and metastable character of nitride semiconductors for solar energy conversion.
Through a collaboration with the Center for Computational Design of Functional Layered Materials EFRC, we have established that the new, non-empirical, SCAN (Strongly Constrained and Appropriately Normed Semilocal Density Functional) exchange-correlation functional for density functional theory provides a uniquely accurate first-principles model of polymorph energetics and properties.
Bismuth Triiodide (BiI3) – A Candidate Photovoltaic Absorber (December 2015)
We identified BiI3 as a candidate photovoltaic absorber using computational design criteria based on the methyl ammonium lead iodide perovskites. Initial experiments demonstrate room-temperature photoluminescence with application-relevant lifetimes.
Transition Metal Oxide Semiconductors (Sept 2015)
Comprehensive assessment of semiconducting transition metal oxides using unbiased electronic structure calculations with integrated treatment of s, p, and d electrons.
We developed a theoretical approach to search for new and realizable metastable polymorphs in ionic systems.
The key role that band-edge orbital character has on defect tolerance (gained from MAPbX3 perovskites) underlies a new joint data-mining and theory approach to screen materials for long minority-carrier lifetimes, which is a critical photovoltaic absorber property.
Thin films of the metastable spinel γ-Sn3N4 were synthesized by sputtering and characterized for semiconducting properties, such as absorption spectra, electrical transport, ionization potential, and minority-carrier diffusion length.
The mission of the Center for Next Generation of Materials Design is to dramatically transform the discovery of functional energy materials through multiple-property search, incorporation of metastable materials into predictive design, and the development of theory to guide materials synthesis.
The Center for Next Generation of Materials Design is creating a high-throughput computational tool based on first-principles theory to predict the formation energy of polymorphs, including new unknown structures.
The Center for Next Generation of Materials Design is designing a novel semiconducting transition metal oxide alloy with absorption in the visible and with favorable electron and hole transport properties.