Publications by authors named "Andriy Zakutayev"

36 Publications

Instrument for spatially resolved, temperature-dependent electrochemical impedance spectroscopy of thin films under locally controlled atmosphere.

Rev Sci Instrum 2021 Jun;92(6):065105

National Renewable Energy Laboratory, Materials Science Center, Golden, Colorado 80401, USA.

We demonstrate an instrument for spatially resolved measurements (mapping) of electrochemical impedance under various temperatures and gas environments. Automated measurements are controlled by a custom LabVIEW program, which manages probe motion, sample motion, temperature ramps, and potentiostat functions. Sample and probe positioning is provided by stepper motors. Dry or hydrated atmospheres (air or nitrogen) are available. The configurable heater reaches temperatures up to 500 °C, although the temperature at the sample surface is moderated by the gas flow rate. The local gas environment is controlled by directing flow toward the sample via a glass enclosure that surrounds the gold wire probe. Software and hardware selection and design are discussed. Reproducibility and accuracy are quantified on a Ba(Zr,Y)O proton-conducting electrolyte thin film synthesized by pulsed laser deposition. The mapping feature of the instrument is demonstrated on a compositionally graded array of electrocatalytically active Ba(Co,Fe,Zr,Y)O thin film microelectrodes. The resulting data indicate that this method proficiently maps property trends in these materials, thus demonstrating the reliability and usefulness of this method for investigating electrochemically active thin films.
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http://dx.doi.org/10.1063/5.0024875DOI Listing
June 2021

Synthesis of ZnNbNternary nitride semiconductor with wurtzite-derived crystal structure.

Authors:
Andriy Zakutayev

J Phys Condens Matter 2021 Jul 8;33(35). Epub 2021 Jul 8.

National Renewable Energy Laboratory, Golden CO 80401 United States of America.

Binary III-N nitride semiconductors with wurtzite crystal structure such as GaN and AlN have been long used in many practical applications ranging from optoelectronics to telecommunication. The structurally related ZnGeNor ZnSnNderived from the parent binary compounds by cation mutation (elemental substitution) have recently attracted attention, but such ternary nitride materials are mostly limited to II-IV-Ncompositions. This paper demonstrates synthesis and characterization of zinc niobium nitride (ZnNbN)-a previously unreported II-V-Nternary nitride semiconductor. The ZnNbNthin films are synthesized using a one-step adsorption-controlled growth that locks in the targeted stoichiometry, and a two-step deposition/annealing method that suppresses the loss of Zn and N. Measurements indicate that this sputtered ZnNbNcrystalizes in cation-disordered wurtzite-derived structure, in contrast to chemically related rocksalt-derived MgNbNcompound, also synthesized here for comparison using the two-step method. The estimated wurtzite lattice parameter ratio of ZnNbNis 1.55, and the optical absorption onset is at 2.1 eV. Both of these values are lower compared to published ZnNbNcomputational values of/= 1.62 and= 3.5-3.6 eV. Additional theoretical calculations indicate that this difference is due to cation disorder in experimental samples, suggesting a way to tune the structural parameters and the resulting properties of heterovalent ternary nitride materials. Overall, this work expands the wurtzite family of nitride semiconductors to include ZnNbN, and suggests that related II-V-Nand other ternary nitrides should be possible to synthesize.
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http://dx.doi.org/10.1088/1361-648X/abfab3DOI Listing
July 2021

Metal chalcogenides for neuromorphic computing: emerging materials and mechanisms.

Nanotechnology 2021 Jun 22;32(37). Epub 2021 Jun 22.

Materials Science Center, National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, CO 80401, United States of America.

The approaching end of Moore's law scaling has significantly accelerated multiple fields of research including neuromorphic-, quantum-, and photonic computing, each of which possesses unique benefits unobtained through conventional binary computers. One of the most compelling arguments for neuromorphic computing systems is power consumption, noting that computations made in the human brain are approximately 10times more efficient than conventional CMOS logic. This review article focuses on the materials science and physical mechanisms found in metal chalcogenides that are currently being explored for use in neuromorphic applications. We begin by reviewing the key biological signal generation and transduction mechanisms within neuronal components of mammalian brains and subsequently compare with observed experimental measurements in chalcogenides. With robustness and energy efficiency in mind, we will focus on short-range mechanisms such as structural phase changes and correlated electron systems that can be driven by low-energy stimuli, such as temperature or electric field. We aim to highlight fundamental materials research and existing gaps that need to be overcome to enable further integration or advancement of metal chalcogenides for neuromorphic systems.
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http://dx.doi.org/10.1088/1361-6528/abfa51DOI Listing
June 2021

Triple ionic-electronic conducting oxides for next-generation electrochemical devices.

Nat Mater 2021 Mar 21;20(3):301-313. Epub 2020 Dec 21.

Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO, USA.

Triple ionic-electronic conductors (TIECs) are materials that can simultaneously transport electronic species alongside two ionic species. The recent emergence of TIECs provides intriguing opportunities to maximize performance in a variety of electrochemical devices, including fuel cells, membrane reactors and electrolysis cells. However, the potential application of these nascent materials is limited by lack of fundamental knowledge of their transport properties and electrocatalytic activity. The goal of this Review is to summarize and analyse the current understanding of TIEC transport and electrochemistry in single-phase materials, including defect formation and conduction mechanisms. We particularly focus on the discovery criteria (for example, crystal structure and ion electronegativity), design principles (for example, cation and anion substitution chemistry) and operating conditions (for example, atmosphere) of materials that enable deliberate tuning of the conductivity of each charge carrier. Lastly, we identify important areas for further advances, including higher chemical stability, lower operating temperatures and discovery of n-type TIEC materials.
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http://dx.doi.org/10.1038/s41563-020-00854-8DOI Listing
March 2021

Three-Dimensional Mapping of Resistivity and Microstructure of Composite Electrodes for Lithium-Ion Batteries.

Nano Lett 2020 Nov 30;20(11):8081-8088. Epub 2020 Oct 30.

Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, United States.

Nanoparticle silicon-graphite composite electrodes are a viable way to advance the cycle life and energy density of lithium-ion batteries. However, characterization of composite electrode architectures is complicated by the heterogeneous mixture of electrode components and nanoscale diameter of particles, which falls beneath the lateral and depth resolution of most laboratory-based instruments. In this work, we report an original laboratory-based scanning probe microscopy approach to investigate composite electrode microstructures with nanometer-scale resolution via contrast in the electronic properties of electrode components. Applying this technique to silicon-based composite anodes demonstrates that graphite, SiO nanoparticles, carbon black, and LiPAA binder are all readily distinguished by their intrinsic electronic properties, with measured electronic resistivity closely matching their known material properties. Resolution is demonstrated by identification of individual nanoparticles as small as ∼20 nm. This technique presents future utility in multiscale characterization to better understand particle dispersion, localized lithiation, and degradation processes in composite electrodes for lithium-ion batteries.
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http://dx.doi.org/10.1021/acs.nanolett.0c03074DOI Listing
November 2020

Synthesis of Tunable SnS-TaS Nanoscale Superlattices.

Nano Lett 2020 Oct 29;20(10):7059-7067. Epub 2020 Sep 29.

Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States.

Nanoscale superlattices represent a compelling platform for designed materials as the specific identity and spatial arrangement of constituent layers can lead to tunable properties. A number of kinetically stabilized, nonepitaxial superlattices with almost limitless structural tunability have been reported in telluride and selenide chemistries but have not yet been extended to sulfides. Here, we present SnS-TaS nanoscale superlattices with tunable layer architecture. Layered amorphous precursors are prepared as thin films programmed to mimic the targeted superlattice; subsequent low temperature annealing activates self-assembly into crystalline nanocomposites. We investigate structure and composition of superlattices comprised of monolayers of TaS and 3-7 monolayers of SnS per repeating unit. Furthermore, a graded precursor preparation approach is introduced, allowing stabilization of superlattices with multiple stacking sequences in a single preparation. Controlled synthesis of the architecture of nanoscale superlattices is a critical path toward tuning their exotic properties and enabling integration with electronic, optical, or quantum devices.
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http://dx.doi.org/10.1021/acs.nanolett.0c02115DOI Listing
October 2020

Transient Evolution of the Built-in Field at Junctions of GaAs.

ACS Appl Mater Interfaces 2020 Sep 28;12(36):40339-40346. Epub 2020 Aug 28.

Materials and Chemical Science and Technology Directorate, National Renewable Energy Laboratory, Golden, Colorado 80401, United States.

Built-in electric fields at semiconductor junctions are vital for optoelectronic and photocatalytic applications since they govern the movement of photogenerated charge carriers near critical surfaces and interfaces. Here, we exploit transient photoreflectance (TPR) spectroscopy to probe the dynamical evolution of the built-in field for n-GaAs photoelectrodes upon photoexcitation. The transient fields are modeled in order to quantitatively describe the surface carrier dynamics that influence those fields. The photoinduced surface field at different types of junctions between n-GaAs and n-TiO, Pt, electrolyte and p-NiO are examined, and the results reveal that surface Fermi-level pinning, ubiquitous for many GaAs surfaces, can have beneficial consequences that impact photoelectrochemical applications. That is, Fermi-level pinning results in the primary surface carrier dynamics being invariant to the contacting layer and promotes beneficial carrier separation. For example, when p-NiO is deposited there is no Fermi-level equilibration that modifies the surface field, but photogenerated holes are promoted to the n-GaAs/p-NiO interface and can transfer into defect midgap states within the p-NiO resulting in an elongated charge separation time and those transferred holes can participate in chemical reactions. In contrast, when the Fermi-level is unpinned via molecular surface functionalization on p-GaAs, the carriers undergo surface recombination faster due to a smaller built-in field, thus potentially degrading their photochemical performance.
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http://dx.doi.org/10.1021/acsami.0c11474DOI Listing
September 2020

Investigating the Effects of Lithium Phosphorous Oxynitride Coating on Blended Solid Polymer Electrolytes.

ACS Appl Mater Interfaces 2020 Sep 27;12(36):40749-40758. Epub 2020 Aug 27.

National Renewable Energy Laboratory, Materials Science Center, Golden, Colorado 80401, United States.

Solid-state electrolytes are very promising to enhance the safety of lithium-ion batteries. Two classes of solid electrolytes, polymer and ceramic, can be combined to yield a hybrid electrolyte that can synergistically combine the properties of both materials. Chemical stability, thermal stability, and high mechanical modulus of ceramic electrolytes against dendrite penetration can be combined with the flexibility and ease of processing of polymer electrolytes. By coating a polymer electrolyte with a ceramic electrolyte, the stability of the solid electrolyte is expected to improve against lithium metal, and the ionic conductivity could remain close to the value of the original polymer electrolyte, as long as an appropriate thickness of the ceramic electrolyte is applied. Here, we report a bilayered lithium-ion conducting hybrid solid electrolyte consisting of a blended polymer electrolyte (BPE) coated with a thin layer of the inorganic solid electrolyte lithium phosphorous oxynitride (LiPON). The hybrid system was thoroughly studied. First, we investigated the influence of the polymer chain length and lithium salt ratio on the ionic conductivity of the BPE based on poly(ethylene oxide) (PEO) and poly(propylene carbonate) (PPC) with the salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The optimized BPE consisted of 100 k molecular weight PEO, 50 k molecular weight PPC, and 25(w/w)% LiTFSI, (denoted as PEO100PPC50LiTFSI25), which exhibited an ionic conductivity of 2.11 × 10 S/cm, and the ionic conductivity showed no thermal memory effects as the PEO crystallites were well disrupted by PPC and LiTFSI. Second, the effects of LiPON coating on the BPE were evaluated as a function of thickness down to 20 nm. The resulting bilayer structure showed an increase in the voltage window from 5.2 to 5.5 V (vs Li/Li+) and thermal activation energies that approached the activation energy of the BPE when thinner LiPON layers were used, resulting in similar ionic conductivities for 30 nm LiPON coatings on PEO100PPC50LiTFSI25. Coating BPEs with a thin layer of LiPON is shown to be an effective strategy to improve the long-term stability against lithium.
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http://dx.doi.org/10.1021/acsami.0c09113DOI Listing
September 2020

Combinatorial Synthesis of Magnesium Tin Nitride Semiconductors.

J Am Chem Soc 2020 May 22;142(18):8421-8430. Epub 2020 Apr 22.

Materials and Chemistry Science and Technology Directorate, National Renewable Energy Laboratory, Golden, Colorado 80401, United States.

Nitride materials feature strong chemical bonding character that leads to unique crystal structures, but many ternary nitride chemical spaces remain experimentally unexplored. The search for previously undiscovered ternary nitrides is also an opportunity to explore unique materials properties, such as transitions between cation-ordered and -disordered structures, as well as to identify candidate materials for optoelectronic applications. Here, we present a comprehensive experimental study of MgSnN, an emerging II-IV-N compound, for the first time mapping phase composition and crystal structure, and examining its optoelectronic properties computationally and experimentally. We demonstrate combinatorial cosputtering of cation-disordered, wurtzite-type MgSnN across a range of cation compositions and temperatures, as well as the unexpected formation of a secondary, rocksalt-type phase of MgSnN at Mg-rich compositions and low temperatures. A computational structure search shows that the rocksalt-type phase is substantially metastable (>70 meV/atom) compared to the wurtzite-type ground state. Spectroscopic ellipsometry reveals optical absorption onsets around 2 eV, consistent with band gap tuning via cation disorder. Finally, we demonstrate epitaxial growth of a mixed wurtzite-rocksalt MgSnN on GaN, highlighting an opportunity for polymorphic control via epitaxy. Collectively, these findings lay the groundwork for further exploration of MgSnN as a model ternary nitride, with controlled polymorphism, and for device applications, enabled by control of optoelectronic properties via cation ordering.
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http://dx.doi.org/10.1021/jacs.0c02092DOI Listing
May 2020

Wide Band Gap Chalcogenide Semiconductors.

Chem Rev 2020 May 6;120(9):4007-4055. Epub 2020 Apr 6.

Materials Science Center, National Renewable Energy Laboratory Golden, Colorado 80401, United States.

Wide band gap semiconductors are essential for today's electronic devices and energy applications because of their high optical transparency, controllable carrier concentration, and tunable electrical conductivity. The most intensively investigated wide band gap semiconductors are transparent conductive oxides (TCOs), such as tin-doped indium oxide (ITO) and amorphous In-Ga-Zn-O (IGZO), used in displays and solar cells, carbides (e.g., SiC) and nitrides (e.g., GaN) used in power electronics, and emerging halides (e.g., γ-CuI) and 2D electronic materials (e.g., graphene) used in various optoelectronic devices. Compared to these prominent materials families, chalcogen-based (Ch = S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out because of their propensity for p-type doping, high mobilities, high valence band positions (i.e., low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent semiconductors, as well as the theoretical and experimental underpinnings of the corresponding research methods. We proceed to summarize progress in wide band gap ( > 2 eV) chalcogenide materials-namely, II-VI MCh binaries, CuMCh chalcopyrites, CuMCh sulvanites, mixed-anion layered CuMCh(O,F), and 2D materials-and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications-for example, photovoltaic and photoelectrochemical solar cells, transistors, and light emitting diodes-that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this Review aims to inspire continued research on this emerging class of transparent semiconductors and thereby enable future innovations for optoelectronic devices.
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http://dx.doi.org/10.1021/acs.chemrev.9b00600DOI Listing
May 2020

Combined Spatially Resolved Characterization of Antireflection and Antisoiling Coatings for PV Module Glass.

ACS Comb Sci 2020 04 11;22(4):197-203. Epub 2020 Mar 11.

SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States.

Characterization of photovoltaic (PV) module materials throughout different stages of service life is crucial to understanding and improving the durability of these materials. Currently the large-scale of PV modules (>1 m) is imbalanced with the small-scale of most materials characterization tools (≤1 cm). Furthermore, understanding degradation mechanisms often requires a combination of multiple characterization techniques. Here, we present adaptations of three standard materials characterization techniques to enable mapping characterization over moderate sample areas (≥25 cm). Contact angle, ellipsometry, and UV-vis spectroscopy are each adapted and demonstrated on two representative samples: a commercial multifunctional coating for PV glass and an oxide combinatorial sample library. Best practices are discussed for adapting characterization techniques for large-area mapping and combining mapping information from multiple techniques.
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http://dx.doi.org/10.1021/acscombsci.9b00213DOI Listing
April 2020

Probing the Evolution of Surface Chemistry at the Silicon-Electrolyte Interphase via In Situ Surface-Enhanced Raman Spectroscopy.

J Phys Chem Lett 2020 Jan 20;11(1):286-291. Epub 2019 Dec 20.

Materials and Chemical Science and Technology Directorate , National Renewable Energy Laboratory , 15013 Denver West Parkway , Golden , Colorado 80401 , United States.

We present a novel spectroscopic technique for in situ Raman microscopy studies of battery electrodes. By creating nanostructures on a copper mesh current collector, we were able to utilize surface-enhanced Raman spectroscopy (SERS) to monitor the evolution of the silicon anode-electrolyte interphase. The spectra show reversible Si peak intensity changes upon lithiation and delithiation. Moreover, an alkyl carboxylate species, lithium propionate, was detected as a significant SiEI component. Our experimental setup showed reproducible and stable performance over multiple cycles in terms of both electrochemistry and spectroscopy.
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http://dx.doi.org/10.1021/acs.jpclett.9b03284DOI Listing
January 2020

Intrinsic Properties of Individual Inorganic Silicon-Electrolyte Interphase Constituents.

ACS Appl Mater Interfaces 2019 Dec 3;11(50):46993-47002. Epub 2019 Dec 3.

Materials and Chemical Science and Technology Directorate , National Renewable Energy Laboratory , 15013 Denver West Parkway , Golden , Colorado 80401 , United States.

Because of the complexity, high reactivity, and continuous evolution of the silicon-electrolyte interphase (SiEI), "individual" constituents of the SiEI were investigated to understand their physical, electrochemical, and mechanical properties. For the analysis of these intrinsic properties, known SiEI components (i.e., SiO, LiSiO, LiSiO, LiSiO, LiO, and LiF) were selected and prepared as amorphous thin films. The chemical composition, purity, morphology, roughness, and thickness of prepared samples were characterized using a variety of analytical techniques. On the basis of subsequent analysis, LiF shows the lowest ionic conductivity and relatively weak, brittle mechanical properties, while lithium silicates demonstrate higher ionic conductivities and greater mechanical hardness. This research establishes a framework for identifying components critical for stabilization of the SiEI, thus enabling rational design of new electrolyte additives and functional binders for the development of next-generation advanced Li-ion batteries utilizing Si anodes.
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http://dx.doi.org/10.1021/acsami.9b18252DOI Listing
December 2019

High-Throughput Experimental Study of Wurtzite Mn Zn O Alloys for Water Splitting Applications.

ACS Omega 2019 Apr 24;4(4):7436-7447. Epub 2019 Apr 24.

Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States.

We used high-throughput experimental screening methods to unveil the physical and chemical properties of Mn Zn O wurtzite alloys and identify their appropriate composition for effective water splitting application. The Mn Zn O thin films were synthesized using combinatorial pulsed laser deposition, permitting for characterization of a wide range of compositions with 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 = 0.5 associated with the rock salt-to-wurtzite structural transition in Mn Zn O alloys. Transmission electron microscopy results show evidence of a homogeneous wurtzite alloy system for a broad range of Mn Zn O compositions above = 0.4. The wurtzite Mn ZnO samples with the 0.4 < < 0.6 range were studied as anodes for photoelectrochemical water splitting, with a maximum current density of 340 μA cm for 673 nm-thick films. These Mn Zn O films were stable in pH = 10, showing no evidence of photocorrosion or degradation after 24 h under water oxidation conditions. Doping Mn Zn O materials with Ga dramatically increases the electrical conductivity of Mn Zn O up to ∼1.9 S/cm for = 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 Zn O alloys hold promise for photoelectrochemical water splitting, which could be enhanced with further tailoring of their electronic properties.
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http://dx.doi.org/10.1021/acsomega.8b03347DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6648451PMC
April 2019

Ternary nitride semiconductors in the rocksalt crystal structure.

Proc Natl Acad Sci U S A 2019 Jul 3;116(30):14829-14834. Epub 2019 Jul 3.

Materials Science Center, National Renewable Energy Laboratory, Golden, CO 80401;

Inorganic nitrides with wurtzite crystal structures are well-known semiconductors used in optical and electronic devices. In contrast, rocksalt-structured nitrides are known for their superconducting and refractory properties. Breaking this dichotomy, here we report ternary nitride semiconductors with rocksalt crystal structures, remarkable electronic properties, and the general chemical formula Mg N ( = Ti, Zr, Hf, Nb). Our experiments show that these materials form over a broad metal composition range, and that Mg-rich compositions are nondegenerate semiconductors with visible-range optical absorption onsets (1.8 to 2.1 eV) and up to 100 cm V⋅s electron mobility for MgZrN grown on MgO substrates. Complementary ab initio calculations reveal that these materials have disorder-tunable optical absorption, large dielectric constants, and electronic bandgaps that are relatively insensitive to disorder. These ternary Mg N semiconductors are also structurally compatible both with binary N superconductors and main-group nitride semiconductors along certain crystallographic orientations. Overall, these results highlight Mg N as a class of materials combining the semiconducting properties of main-group wurtzite nitrides and rocksalt structure of superconducting transition-metal nitrides.
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http://dx.doi.org/10.1073/pnas.1904926116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6660719PMC
July 2019

A map of the inorganic ternary metal nitrides.

Nat Mater 2019 07 17;18(7):732-739. Epub 2019 Jun 17.

Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.

Exploratory synthesis in new chemical spaces is the essence of solid-state chemistry. However, uncharted chemical spaces can be difficult to navigate, especially when materials synthesis is challenging. Nitrides represent one such space, where stringent synthesis constraints have limited the exploration of this important class of functional materials. Here, we employ a suite of computational materials discovery and informatics tools to construct a large stability map of the inorganic ternary metal nitrides. Our map clusters the ternary nitrides into chemical families with distinct stability and metastability, and highlights hundreds of promising new ternary nitride spaces for experimental investigation-from which we experimentally realized seven new Zn- and Mg-based ternary nitrides. By extracting the mixed metallicity, ionicity and covalency of solid-state bonding from the density functional theory (DFT)-computed electron density, we reveal the complex interplay between chemistry, composition and electronic structure in governing large-scale stability trends in ternary nitride materials.
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http://dx.doi.org/10.1038/s41563-019-0396-2DOI Listing
July 2019

COMBIgor: Data-Analysis Package for Combinatorial Materials Science.

ACS Comb Sci 2019 07 7;21(7):537-547. Epub 2019 Jun 7.

National Renewable Energy Laboratory , 15013 Denver West Parkway , Golden , Colorado 80401 , United States.

Combinatorial experiments involve synthesis of sample libraries with lateral composition gradients requiring spatially resolved characterization of structure and properties. Because of the maturation of combinatorial methods and their successful application in many fields, the modern combinatorial laboratory produces diverse and complex data sets requiring advanced analysis and visualization techniques. In order to utilize these large data sets to uncover new knowledge, the combinatorial scientist must engage in data science. For data science tasks, most laboratories adopt common-purpose data management and visualization software. However, processing and cross-correlating data from various measurement tools is no small task for such generic programs. Here we describe COMBIgor, a purpose-built open-source software package written in the commercial Igor Pro environment and designed to offer a systematic approach to loading, storing, processing, and visualizing combinatorial data. It includes (1) methods for loading and storing data sets from combinatorial libraries, (2) routines for streamlined data processing, and (3) data-analysis and -visualization features to construct figures. Most importantly, COMBIgor is designed to be easily customized by a laboratory, group, or individual in order to integrate additional instruments and data-processing algorithms. Utilizing the capabilities of COMBIgor can significantly reduce the burden of data management on the combinatorial scientist.
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http://dx.doi.org/10.1021/acscombsci.9b00077DOI Listing
July 2019

An Inter-Laboratory Study of Zn-Sn-Ti-O Thin Films using High-Throughput Experimental Methods.

ACS Comb Sci 2019 05 22;21(5):350-361. Epub 2019 Apr 22.

National Institute of Standards and Technology (NIST) , Gaithersburg , Maryland 20899-3460 , United States.

High-throughput experimental (HTE) techniques are an increasingly important way to accelerate the rate of materials research and development for many technological applications. However, there are very few publications on the reproducibility of the HTE results obtained across different laboratories for the same materials system, and on the associated sample and data exchange standards. Here, we report a comparative study of Zn-Sn-Ti-O thin films materials using high-throughput experimental methods at National Institute of Standards and Technology (NIST) and National Renewable Energy Laboratory (NREL). The thin film sample libraries were synthesized by combinatorial physical vapor deposition (cosputtering and pulsed laser deposition) and characterized by spatially resolved techniques for composition, structure, thickness, optical, and electrical properties. The results of this study indicate that all these measurement techniques performed at two different laboratories show excellent qualitative agreement. The quantitative similarities and differences vary by measurement type, with 95% confidence interval of 0.1-0.2 eV for the band gap, 24-29 nm for film thickness, and 0.08 to 0.37 orders of magnitude for sheet resistance. Overall, this work serves as a case study for the feasibility of a High-Throughput Experimental Materials Collaboratory (HTE-MC) by demonstrating the exchange of high-throughput sample libraries, workflows, and data.
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http://dx.doi.org/10.1021/acscombsci.8b00158DOI Listing
May 2019

Interplay between Composition, Electronic Structure, Disorder, and Doping due to Dual Sublattice Mixing in Nonequilibrium Synthesis of ZnSnN :O.

Adv Mater 2019 Mar 22;31(11):e1807406. Epub 2019 Jan 22.

Materials Science Center, National Renewable Energy Laboratory, Golden, CO, 80401, USA.

The opportunity for enhanced functional properties in semiconductor solid solutions has attracted vast scientific interest for a variety of novel applications. However, the functional versatility originating from the additional degrees of freedom due to atomic composition and ordering comes along with new challenges in characterization and modeling. Developing predictive synthesis-structure-property relationships is prerequisite for effective materials design strategies. Here, a first-principles based model for property prediction in such complex semiconductor materials is presented. This framework incorporates nonequilibrium synthesis, dopants and defects, and the change of the electronic structure with composition and short range order. This approach is applied to ZnSnN (ZTN) which has attracted recent interest for photovoltaics. The unintentional oxygen incorporation and its correlation with the cation stoichiometry leads to the formation of a solid solution with dual sublattice mixing. A nonmonotonic doping behavior as a function of the composition is uncovered. The degenerate doping of near-stoichiometric ZTN, which is detrimental for potential applications, can be lowered into the 10 cm range in highly off-stoichiometric material, in quantitative agreement with experiments.
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http://dx.doi.org/10.1002/adma.201807406DOI Listing
March 2019

Mechanical Properties and Chemical Reactivity of Li SiO Thin Films.

ACS Appl Mater Interfaces 2018 Nov 26;10(44):38558-38564. Epub 2018 Oct 26.

Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States.

Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO on Si is often inevitable. However, it is not clear if this layer has a positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties of the SiO lithiation products and by the convolution of chemical and electrochemical effects during the anode lithiation process. In this study, Li SiO thin films as model materials for lithiated SiO were deposited by magnetron sputtering at ambient temperature, with the goal of (1) decoupling chemical reactivity from electrochemical reactivity and (2) evaluating the physical and electrochemical properties of Li SiO . X-ray photoemission spectroscopy analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO can be achieved through sputtering. Our density functional theory calculations also confirm that the possible phases formed by lithiating SiO are very close to the measured film compositions. Scanning probe microscopy measurements show that the mechanical properties of the film are strongly dependent on lithium concentration, with a ductile behavior at a higher Li content and a brittle behavior at a lower Li content. The chemical reactivity of the thin films was investigated by measuring the AC impedance evolution, suggesting that Li SiO continuously reacts with the electrolyte, in part because of the high electronic conductivity of the film determined from solid-state impedance measurements. The electrochemical cycling data of the sputter-deposited Li SiO /Si films also suggest that Li SiO is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties.
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http://dx.doi.org/10.1021/acsami.8b10895DOI Listing
November 2018

Mechanical Properties and Chemical Reactivity of LixSiOy Thin Films.

ACS Appl Mater Interfaces 2018 11 15;10(44):38558-38564. Epub 2018 Oct 15.

Silicon (Si) is a commonly studied candidate material for next-generation anodes in Li-ion batteries. A native oxide SiO2 on Si is often inevitable. However, it is not clear if this layer has positive or negative effect on the battery performance. This understanding is complicated by the lack of knowledge about the physical properties, and by convolution of chemical and electrochemical effects during the anode lithiation process. In this study, LixSiOy thin films as model materials for lithiated SiO2 were deposited by magnetron sputtering at ambient temperature, with the goal of 1) decoupling chemical reactivity from electrochemical reactivity, and 2) evaluating the physical and electrochemical properties of LixSiOy. XPS analysis of the deposited thin films demonstrate that a composition close to previous experimental reports of lithiated native SiO2, can be achieved through sputtering. Our density functional theory calculations also confirm that possible phases formed by lithiating SiO2 are very close to the measured film compositions. Scanning probe microscopy measurements show the mechanical properties of the film are strongly dependent on lithium concentration, with ductile behavior and higher Li content and brittle behavior at lower Li content. Chemical reactivity of the thin films was investigated by measuring AC impedance evolution, suggesting that LixSiOy continuously reacts with electrolyte, in part due to high electronic conductivity of the film determined from solid state impedance measurements. Electrochemical cycling data of sputter deposited LixSiOy/Si films also suggest that LixSiOy is not beneficial in stabilizing the Si anode surface during battery operation, despite its favorable mechanical properties.
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http://dx.doi.org/10.1021/acsami.8b10895DOI Listing
November 2018

Characterization of Elastic Modulus Across the (AlSc)N System Using DFT and Substrate-Effect-Corrected Nanoindentation.

IEEE Trans Ultrason Ferroelectr Freq Control 2018 11 16;65(11):2167-2175. Epub 2018 Aug 16.

Knowledge of accurate values of elastic modulus of (AlSc)N is required for design of piezoelectric resonators and related devices. Thin films of (AlSc)N across the entire composition space are deposited and characterized. Accuracy of modulus measurements is improved and quantified by removing the influence of substrate effects and by direct comparison of experimental results with density functional theory calculations. The 5%-30% Sc compositional range is of particular interest for piezoelectric applications and is covered at higher compositional resolution here than in previous work. The reduced elastic modulus is found to decrease by as much as 40% with increasing Sc concentration in the wurtzite phase according to both experimental and computational techniques, whereas Sc-rich rocksalt-structured films exhibit little variation in modulus with composition.
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http://dx.doi.org/10.1109/TUFFC.2018.2862240DOI Listing
November 2018

Combinatorial Nitrogen Gradients in Sputtered Thin Films.

ACS Comb Sci 2018 07 7;20(7):436-442. Epub 2018 Jun 7.

Materials Science Center , National Renewable Energy Laboratory , Golden , Colorado 80401 , United States.

High-throughput synthesis and characterization methods can significantly accelerate the rate of experimental research. For physical vapor deposition (PVD), these methods include combinatorial sputtering with intentional gradients of metal/metalloid composition, temperature, and thickness across the substrate. However, many other synthesis parameters still remain out of reach for combinatorial methods. Here, we extend combinatorial sputtering parameters to include gradients of gaseous elements in thin films. Specifically, a nitrogen gradient was generated in a thin film sample library by placing two MnTe sputtering sources with different gas flows (Ar and Ar/N) opposite of one another during the synthesis. The nitrogen content gradient was measured along the sample surface, correlating with the distance from the nitrogen source. The phase, composition, and optoelectronic properties of the resulting thin films change as a function of the nitrogen content. This work shows that gradients of gaseous elements can be generated in thin films synthesized by sputtering, expanding the boundaries of combinatorial science.
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http://dx.doi.org/10.1021/acscombsci.8b00035DOI Listing
July 2018

Negative-pressure polymorphs made by heterostructural alloying.

Sci Adv 2018 04 20;4(4):eaaq1442. Epub 2018 Apr 20.

National Renewable Energy Laboratory, Golden, CO 80401, USA.

The ability of a material to adopt multiple structures, known as polymorphism, is a fascinating natural phenomenon. Various polymorphs with unusual properties are routinely synthesized by compression under positive pressure. However, changing a material's structure by applying tension under negative pressure is much more difficult. We show how negative-pressure polymorphs can be synthesized by mixing materials with different crystal structures-a general approach that should be applicable to many materials. Theoretical calculations suggest that it costs less energy to mix low-density structures than high-density structures, due to less competition for space between the atoms. Proof-of-concept experiments confirm that mixing two different high-density forms of MnSe and MnTe stabilizes a Mn(Se,Te) alloy with a low-density wurtzite structure. This Mn(Se,Te) negative-pressure polymorph has 2× to 4× lower electron effective mass compared to MnSe and MnTe parent compounds and has a piezoelectric response that none of the parent compounds have. This example shows how heterostructural alloying can lead to negative-pressure polymorphs with useful properties-materials that are otherwise nearly impossible to make.
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http://dx.doi.org/10.1126/sciadv.aaq1442DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5930396PMC
April 2018

An open experimental database for exploring inorganic materials.

Sci Data 2018 04 3;5:180053. Epub 2018 Apr 3.

Computational Sciences Center, National Renewable Energy Laboratory, Golden, CO 80401, USA.

The use of advanced machine learning algorithms in experimental materials science is limited by the lack of sufficiently large and diverse datasets amenable to data mining. If publicly open, such data resources would also enable materials research by scientists without access to expensive experimental equipment. Here, we report on our progress towards a publicly open High Throughput Experimental Materials (HTEM) Database (htem.nrel.gov). This database currently contains 140,000 sample entries, characterized by structural (100,000), synthetic (80,000), chemical (70,000), and optoelectronic (50,000) properties of inorganic thin film materials, grouped in >4,000 sample entries across >100 materials systems; more than a half of these data are publicly available. This article shows how the HTEM database may enable scientists to explore materials by browsing web-based user interface and an application programming interface. This paper also describes a HTE approach to generating materials data, and discusses the laboratory information management system (LIMS), that underpin HTEM database. Finally, this manuscript illustrates how advanced machine learning algorithms can be adopted to materials science problems using this open data resource.
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http://dx.doi.org/10.1038/sdata.2018.53DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5881410PMC
April 2018

Redox-Mediated Stabilization in Zinc Molybdenum Nitrides.

J Am Chem Soc 2018 03 14;140(12):4293-4301. Epub 2018 Mar 14.

National Renewable Energy Laboratory , Golden , Colorado 80401 , United States.

We report on the theoretical prediction and experimental realization of new ternary zinc molybdenum nitride compounds. We used theory to identify previously unknown ternary compounds in the Zn-Mo-N systems, ZnMoN and ZnMoN, and to analyze their bonding environment. Experiments show that Zn-Mo-N alloys can form in broad composition range from ZnMoN to ZnMoN in the wurtzite-derived structure, accommodating very large off-stoichiometry. Interestingly, the measured wurtzite-derived structure of the alloys is metastable for the ZnMoN stoichiometry, in contrast to the ZnMoN stoichiometry, where ordered wurtzite is predicted to be the ground state. The formation of ZnMoN-ZnMoN alloy with wurtzite-derived crystal structure is enabled by the concomitant ability of Mo to change oxidation state from +VI in ZnMoN to +IV in ZnMoN, and the capability of Zn to contribute to the bonding states of both compounds, an effect that we define as "redox-mediated stabilization". The stabilization of Mo in both the +VI and +IV oxidation states is due to the intermediate electronegativity of Zn, which enables significant polar covalent bonding in both ZnMoN and ZnMoN compounds. The smooth change in the Mo oxidation state between ZnMoN and ZnMoN stoichiometries leads to a continuous change in optoelectronic properties-from resistive and semitransparent ZnMoN to conductive and absorptive ZnMoN. The reported redox-mediated stabilization in zinc molybdenum nitrides suggests there might be many undiscovered ternary compounds with one metal having an intermediate electronegativity, enabling significant covalent bonding, and another metal capable of accommodating multiple oxidation states, enabling stoichiometric flexibility.
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http://dx.doi.org/10.1021/jacs.7b12861DOI Listing
March 2018

Core Levels, Band Alignments, and Valence-Band States in CuSbS for Solar Cell Applications.

ACS Appl Mater Interfaces 2017 Dec 21;9(48):41916-41926. Epub 2017 Nov 21.

Stephenson Institute for Renewable Energy and Department of Physics, University of Liverpool , Liverpool L69 7ZF, U.K.

The earth-abundant material CuSbS (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInGaSe (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies.
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http://dx.doi.org/10.1021/acsami.7b14208DOI Listing
December 2017

Novel phase diagram behavior and materials design in heterostructural semiconductor alloys.

Sci Adv 2017 06 7;3(6):e1700270. Epub 2017 Jun 7.

National Renewable Energy Laboratory, Golden, CO 80401, USA.

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.
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http://dx.doi.org/10.1126/sciadv.1700270DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5462504PMC
June 2017

Understanding and Control of Bipolar Self-Doping in Copper Nitride.

J Appl Phys 2016 05 5;119(18). Epub 2016 May 5.

National Renewable Energy Laboratory, Golden, Colorado 80401 USA.

Semiconductor materials that can be doped both n-type and p-type are desirable for diode-based applications and transistor technology. Copper nitride (CuN) is a metastable semiconductor with a solar-relevant bandgap that has been reported to exhibit bipolar doping behavior. However, deeper understanding and better control of the mechanism behind this behavior in CuN is currently lacking in the literature. In this work, we use combinatorial growth with a temperature gradient to demonstrate both conduction types of phase-pure, sputter-deposited CuN thin films. Room temperature Hall effect and Seebeck effect measurements show n-type CuN with an electron density of 10 cm for low growth temperature (≈ 35 °C) and p-type with a hole density between 10 cm and 10 cm for elevated growth temperatures (50 °C to 120 °C). Mobility for both types of CuN was ≈ 0.1 cm/Vs to 1 cm/V. Additionally, temperature-dependent Hall effect measurements indicate that ionized defects are an important scattering mechanism in p-type films. By combining X-ray absorption spectroscopy and first-principles defect theory, we determined that V defects form preferentially in p-type CuN while Cu defects form preferentially in n-type CuN; suggesting that CuN is a compensated semiconductor with conductivity type resulting from a balance between donor and acceptor defects. Based on these theoretical and experimental results, we propose a kinetic defect formation mechanism for bipolar doping in CuN, that is also supported by positron annihilation experiments. Overall, the results of this work highlight the importance of kinetic processes in the defect physics of metastable materials, and provide a framework that can be applied when considering the properties of such materials in general.
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http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5061149PMC
http://dx.doi.org/10.1063/1.4948244DOI Listing
May 2016
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