Novel phase diagram behavior and materials design in heterostructural semiconductor alloys

Structure and composition control the behavior of materials. Isostructural alloying is historically an extremely successful approach for tuning materials properties, but it is often limited by binodal and spinodal decomposition, which correspond to the thermodynamic solubility limit and the stability against composition fluctuations, respectively. We show that heterostructural alloys can exhibit a markedly increased range of metastable alloy compositions between the binodal and spinodal lines, thereby opening up a vast phase space for novel homogeneous single-phase alloys. We distinguish two types of heterostructural alloys, that is, those between commensurate and incommensurate phases. Because of the structural transition around the critical composition, the properties change in a highly nonlinear or even discontinuous fashion, providing a mechanism for materials design that does not exist in conventional isostructural alloys. The novel phase diagram behavior follows from standard alloy models using mixing enthalpies from first-principles calculations. Thin-film deposition demonstrates the viability of the synthesis of these metastable single-phase domains and validates the computationally predicted phase separation mechanism above the upper temperature bound of the nonequilibrium single-phase region

High-Throughput Experimental Study of Wurtzite Mn1–xZnxO Alloys for Water Splitting Applications

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn 1-x Zn x O wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn 1-x Zn x O thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with x varying from 0 to 1. The solubility limit of ZnO in MnO was determined using the disappearing phase method from X-ray diffraction and X-ray fluorescence data and found to increase with decreasing substrate temperature due to kinetic limitations of the thin-film growth at relatively low temperature. Optical measurements indicate the strong reduction of the optical band gap down to 2.1 eV at x = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn 1-x Zn x O alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn 1-x Zn x O compositions above x = 0.4. The wurtzite Mn 1-x Zn x O samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 μA cm -2 for 673 nm-thick films. These Mn 1-x Zn x O films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn 1-x Zn x O materials with Ga dramatically increases the electrical conductivity of Mn 1-x Zn x O up to ∼1.9 S/cm for x = 0.48, but these doped samples are not active in water splitting. Mott-Schottky and UPS/XPS measurements show that the presence of dopant atoms reduces the space charge region and increases the number of mid-gap surface states. Overall, this study demonstrates that Mn 1-x Zn x O alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties

Design of Semiconducting Tetrahedral Mn_{1−x}Zn_{x}O Alloys and Their Application to Solar Water Splitting

Transition metal oxides play important roles as contact and electrode materials, but their use as active layers in solar energy conversion requires achieving semiconducting properties akin to those of conventional semiconductors like Si or GaAs. In particular, efficient bipolar carrier transport is a challenge in these materials. Based on the prediction that a tetrahedral polymorph of MnO should have such desirable semiconducting properties, and the possibility to overcome thermodynamic solubility limits by nonequilibrium thin-film growth, we exploit both structure-property and composition-structure relationships to design and realize novel wurtzite-structure Mn_{1−x}Zn_{x}O alloys. At Zn compositions above x≈0.3, thin films of these alloys assume the tetrahedral wurtzite structure instead of the octahedral rocksalt structure of MnO, thereby enabling semiconductor properties that are unique among transition metal oxides, i.e., a band gap within the visible spectrum, a band-transport mechanism for both electron and hole carriers, electron doping, and a band lineup suitable for solar hydrogen generation. A proof of principle is provided by initial photo-electrocatalytic device measurements, corroborating, in particular, the predicted favorable hole-transport properties of these alloys

Theory-Guided Synthesis of a Metastable Lead-Free Piezoelectric Polymorph.

Many technologically critical materials are metastable under ambient conditions, yet the understanding of how to rationally design and guide the synthesis of these materials is limited. This work presents an integrated approach that targets a metastable lead-free piezoelectric polymorph of SrHfO3 . First-principles calculations predict that the previous experimentally unrealized, metastable P4mm phase of SrHfO3 should exhibit a direct piezoelectric response (d33 ) of 36.9 pC N-1 (compared to d33 = 0 for the ground state). Combining computationally optimized substrate selection and synthesis conditions lead to the epitaxial stabilization of the polar P4mm phase of SrHfO3 on SrTiO3 . The films are structurally consistent with the theory predictions. A ferroelectric-induced large signal effective converse piezoelectric response of 5.2 pm V-1 for a 35 nm film is observed, indicating the ability to predict and target multifunctionality. This illustrates a coupled theory-experimental approach to the discovery and realization of new multifunctional polymorphs

Theory-Guided Synthesis of a Metastable Lead-Free Piezoelectric Polymorph.

Many technologically critical materials are metastable under ambient conditions, yet the understanding of how to rationally design and guide the synthesis of these materials is limited. This work presents an integrated approach that targets a metastable lead-free piezoelectric polymorph of SrHfO3 . First-principles calculations predict that the previous experimentally unrealized, metastable P4mm phase of SrHfO3 should exhibit a direct piezoelectric response (d33 ) of 36.9 pC N-1 (compared to d33 = 0 for the ground state). Combining computationally optimized substrate selection and synthesis conditions lead to the epitaxial stabilization of the polar P4mm phase of SrHfO3 on SrTiO3 . The films are structurally consistent with the theory predictions. A ferroelectric-induced large signal effective converse piezoelectric response of 5.2 pm V-1 for a 35 nm film is observed, indicating the ability to predict and target multifunctionality. This illustrates a coupled theory-experimental approach to the discovery and realization of new multifunctional polymorphs

High-Throughput Experimental Study of Wurtzite Mn1–xZnxO Alloys for Water Splitting Applications

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn1-xZnxO wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn1-xZnxO thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with x varying from 0 to 1. The solubility limit of Mn1-xZnxO was determined using the disappearing phase method from X-ray diffraction and X-ray fluorescence data and found to increase with decreasing substrate temperature due to kinetic limitations of the thin-film growth at relatively low temperature. Optical measurements indicate the strong reduction of the optical band gap down to 2.1 eV at x = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn1-xZnxO alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn1-xZnxO compositions above x = 0.4. The wurtzite Mn1-xZnxO samples with the 0.4 < x < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 mu A cm(-2) for 673 nm-thick films. These Mn1-xZnxO films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn1-xZnxO materials with Ga dramatically increases the electrical conductivity of Mn1-xZnxO up to similar to 1.9 S/cm for x = 0.48, but these doped samples are not active in water splitting. Mott-Schottky and UPS/ XPS measurements show that the presence of dopant atoms reduces the space charge region and increases the number of midgap surface states. Overall, this study demonstrates that Mn1-xZnxO alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.U.S. Department of Energy (DOE) [DE-AC36-08GO28308]; Office of Science; Office of Basic Energy Sciences (BES), Energy Frontier Research Center "Center for Next Generation of Materials Design: Incorporating Metastability"; Indo-US Science and Technology Forum [BASE 2014/F-8]Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Integer Charge Transfer and Hybridization at an Organic Semiconductor/Conductive Oxide Interface

We investigate the prototypical hybrid interface formed between PTCDA and conductive <i>n</i>-doped ZnO films by means of complementary optical and electronic spectroscopic techniques. We demonstrate that shallow donors in the vicinity of the ZnO surface cause an <i>integer</i> charge transfer to PTCDA, which is clearly restricted to the first monolayer. By means of DFT calculations, we show that the experimental signatures of the anionic PTCDA species can be understood in terms of strong hybridization with localized states (the shallow donors) in the substrate and charge back-donation, resulting in an effectively integer charge transfer across the interface. Charge transfer is thus not merely a question of locating the Fermi level above the PTCDA electron-transport level but requires rather an atomistic understanding of the interfacial interactions. The study reveals that defect sites and dopants can have a significant influence on the specifics of interfacial coupling and thus on carrier injection or extraction

Self-Doping and Electrical Conductivity in Spinel Oxides: Experimental Validation of Doping Rules

Self-doping of cations on the tetrahedral and octahedral sites in spinel oxides creates “anti-site” defects, which results in functional optical, electronic, magnetic, and other materials properties. Previously, we divded the III–II spinel family into four doping types (DTs) based on first-principle calculations in order to understand their electrical behavior. Here, we present experimental evidence on two prototype spinels for each major doping type (DT1 and DT4) that test the first principles calculations. For the DT-1 Ga₂ZnO₄ spinel, we show that the anti-site defects in a stoichiometric film are equal in concentration and compenstate each other, whereas, for nonstoichiometric Cr₂MnO₄, a representative DT-4 spinel, excess Mn on the tetrahedral sites becomes electrically inactive as the Mn species switch from (III) to (II). The agreement between experiment and theory validates the Doping Rules distilled from the theoretical framework and significantly enhances our understanding of the defect chemistry of spinel oxides