Symposium Organizers
Hua Zhou, Argonne National Laboratory
Panchapakesan Ganesh, Oak Ridge National Laboratory
Anna Kimmel, University College London
Dong Su, Brookhaven National Laboratory
Symposium Support
Argonne National Laboratory, Advanced Photon Source
TC07.01: Functional Defects by Design for Energy Storage and Harvest I
Session Chairs
Panchapakesan Ganesh
Anna Kimmel
Monday PM, November 27, 2017
Hynes, Level 2, Room 207
8:00 AM - *TC07.01.01
Like People, Solids are Imperfect—Probing Defects in Battery Materials and Perovskite Solar Cells
Saiful Islam 1
1 , University of Bath, Bath United Kingdom
Show AbstractBreakthroughs in lithium batteries and perovskite solar cells require advances in new materials and underpinning science. Indeed, a greater understanding of these energy materials requires characterization of their underlying defect chemistry and their transport behaviour. In this context, combined modelling-experimental studies are now a powerful approach for investigating such properties. This presentation will describe studies [1,2] in two principal areas: (i) structural and electrochemical insights into Li-rich oxide electrodes for lithium-ion batteries; (ii) defect chemistry and ion transport in hybrid perovskite solar cell materials (based on methylammonium lead iodide), which have shown rapidly rising power-conversion efficiencies.
[1] J. Billaud et al., Adv. Energy Mater., 1601043 (2017); Y. Yuan et al., Nature Comm., 7, 13374 (2016).
[2] C. Eames et al., Nature Commun., 6, 7497 (2015); N. Aristidou et al., Nature Comm., 8, (2017).
8:30 AM - *TC07.01.02
Functional Defects in Battery Electrodes
Chris Van de Walle 1
1 , University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractMaterials for battery electrodes need to engineered for high storage performance as well as adequate electronic and ionic conductivity. Defects and impurities significantly impact these properties, and we address their role with a rigorous formalism for treating energetics and electronic properties, combined with first-principles calculations using density functional theory with a hybrid functional. I will discuss two illustrative examples.
Layered NaMnO2 has promising applications as a cathode material for sodium-ion batteries. We find that both Na and Mn vacancies act as acceptors with the induced holes trapped as small polarons, while O vacancies are deep defecs. Cation antisites, especially Mn on a Na site, have low formation energies and can be detrimental to performance. Both electronic conduction, via polaron hopping, and ionic conduction, through sodium-vacancy migration, are significantly affected by the presence of point defects. I will discuss strategies to improve the electrical conduction and storage performance of NaMnO2, including how to optimize the conditions of synthesis and the impact of impurity doping.
As a second example, the proton-conductive perovskite-type LaFeO3 is a promising negative-electrode material for Ni/metal-hydride (Ni-MH) batteries. Here we find that La vacancies and Sr substitutional on La sites are shallow acceptors with the induced holes trapped as small polarons, while O and Fe vacancies are deep defect centers. We find that hole-polaron hopping is the major electronic conduction mechanism, and that this is impacted by defects and impurities. At the same time, protons can be trapped by cation vacancies to form defect complexes, leading to both capacity fade and slow kinetics. We show that acceptor doping (for instance with Sr impurities) is beneficial to enhance both polaron and proton conductivity.
Our overall goal is not only to elucidate fundamental charge-transport mechanism in these electrode materials, but also to provide guidance about optimum growth conditions and doping strategies for experimentalists to further improve electrode performance.
Work performed in collaboration with Zhen Zhu and Hartwin Peelaers, and supported by DOE.
9:00 AM - TC07.01.03
Mechanistic Studies of FeS|P Electrocatalysts Using Synchrotron X-Ray Spectroscopy and Scattering
Zhenxing Feng 1 , Maoyu Wang 1 , Binghong Han 2 , Zishan Wu 3 , Hailiang Wang 3
1 School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, Oregon, United States, 2 Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, Illinois, United States, 3 Department of Chemistry, Yale University, West Haven, Connecticut, United States
Show AbstractTransition metal phosphosulfides represent an emerging category of earth-abundant electrocatalyst materials that show even better catalytic activity than corresponding sulfides and phosphides. To systematically study how catalytic properties are linked to their intrinsic atomic and electronic structures, we have performed electrochemical measurements for the hydrogen evolution reaction (HER) on FeS|P nanomaterials with different ratio of phosphor and sulfur doping. With advanced characterizations from synchrotron based X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD) and pair distribution function (PDF) on the same sets of catalysts, we found Fe-P bond plays an important role in promoting catalytic activities of FeS|P nanomaterials. The electronic structure and Fe-P hybridization information obtained from sulfur and phosphor K-edge X-ray absorption near edge structure (XANES) further support our atomic structure findings, and also provide insights on how these intrinsic parameters influence materials’ macroscopic properties. Through these comprehensive electrochemical and physical characterizations, we have summarized strategies for the design of transition metal phosphosulfides as electrocatalysts for HER and possible other electrochemical reactions.
9:15 AM - TC07.01.04
Conceptual Design of the Functional Defects and Dopants in Carbon-Based Nanomaterials
Rodion Belosludov 1 , Victor Nemykin 2
1 , Tohoku University, Sendai Japan, 2 Department of Chemistry , University of Manitoba, Winnipeg, Manitoba, Canada
Show AbstractThe carbon-based (carbonaceous) materials recently take a lot of scientific interest as the state-of-the-art of carbon-based adsorbent that may apply for effective, clean and low energy requirement storage/separation process [1]. Among various strategies that improve the adsorption properties of carbonaceous materials, N-doping, surface functionalization and extra-framework ions can be selected as the most common. In the case of nitrogen doping the concentration as well as nitrogen effect on the long-term stability of microporous carbons, need to be controlled. Therefore, the realization of the uniform nanomaterials based on thermally and chemically robust structures containing nitrogen and extra-framework ions is one of the possible solutions for controlling the doping concentration and its thermal stability. Porphyrins may consider as interesting building blocks. These thermally and chemically robust molecules found a variety of applications ranging from traditional dyes and pigments to more contemporary cancer therapies, environmental and biochemical sensors.
We have presented a conceptual design for functional 3D porphyrin-based nanostructures, which would bridge the gap between the well-known fullerenes and nanotubes and a new class of the functional nanomaterials. We have explored three major motifs for functional nanostructures which vary by three- or four-fold topology, porosity, degree of conjugation, and electronic structures [2]. The stability of proposed nanocages, nanobarrels and nanotubes generated by conversion from nanobarrels was revealed on the basis of density functional (DFT) and molecular dynamics (MD) calculations, whereas their optical properties were assessed using a time-dependent density functional (TDDFT) approach.
In comparison with pure nanocarbon structures the walls in the proposed structures contain ordered holes (defect) owing to the specific geometry of the corresponding building blocks. This peculiarity opens the possibility of adsorbing small molecules not only inside and outside nanostructures but also in the intermediate spaces. Thus, the ability to store large quantities of methane (106–216 cm3(STP)/cm3) was observed in all cases with several compounds being close to or exceeding the DOE target of 180 cm3(STP)/cm3. The electronic structures, redox and optical properties of new conceptual nanostructures could be easily tuned via their size, topology, and the presence of bridging sp3 carbon atoms. TDDFT calculations demonstrate that by varying the degree of conjugation we can easily adjust the low-energy transitions between 417 and 751 nm, thus covering the majority of the solar spectrum. These additional features may potentially be used as analogues of quantum dots for solar cells and imaging/ bioimaging applications.
REFERENCES
[1] Y. Zhao et al. RSC Adv. 5 (2015) 30310.
[2] R. V. Belosludov et al. Phys. Chem. Chem. Phys. 18 (2016) 13503.
9:30 AM - TC07.01.05
Density Functional Study of Impurity States in Mg2Si Doped with Li, Na and Ag
Takafumi Ogawa 1 , Ayako Konishi 1 , Akihide Kuwabara 1 2
1 , Japan Fine Ceramics Center, Nagoya Japan, 2 , National Institute for Materials Science, Tsukuba Japan
Show AbstractThermoelectric energy conversion enables the energy in waste heat to be put to useful work, e.g., to improve the efficiency of photovoltaic devices. Magnesium silicide-based solid solutions Mg2X (X = Si, Ge, Sn) are promising thermoelectric materials for use at moderate temperatures (between 500 K and 800 K). While the thermoelectric figure of merit, ZT, over 1.0 of the n-type materials is often according to experiment and theoretical calculations, the ZT of p-type materials is at most 0.8 according to theoretical calculations and even lower according to experiment. The ZT of p-type Mg2X compounds thus needs to be improved if practical thermoelectric modules are to be made of this material. Calculations of ZT require the carrier density as an input parameter, but to accurately determine carrier densities the atomistic and electronic states of acceptor dopants in the p-type material need to be known. In this study, we focused on Mg2Si as representative of this series of compounds and we calculated defect formation energies of impurities, i.e., acceptor dopants, and native point defects in p-type Mg2Si doped with Li, Na, and Ag using density functional theory with a hybrid functional. We also calculated volume densities of point defects and carriers in doped Mg2Si under conditions of charge neutrality.
For all three dopants, the calculated defect formation energies as a function of the Fermi level indicate that, in addition to substitutional impurities on Mg sites, interstitial impurities are also energetically stable compared to intrinsic point defects. Interstitial impurity defects have a charge state of +1, raising or suppressing densities of electrons and holes, respectively. This is not surprising because interstitial Mg with +2 charge state is the most stable defect in non-doped Mg2Si [1]. From the calculated densities under charge neutrality conditions, we find that, while Li is considerably more soluble than Na or Ag, hole densities in Li-doped Mg2Si are lower than those in Na- or Ag-doped Mg2Si. These results highlight the importance of charge compensation between substitutional and interstitial impurities when selecting dopants for designing p-type Mg2Si based materials that exhibit good thermoelectric properties.
[1] X. Liu, et al., Adv. Elec. Mater. 2, 1500284 (2016).
10:15 AM - *TC07.01.06
Towards a Framework for Modelling Ionic Diffusivity in Disordered Alloys—Application to Mixed Conducting Perovskites
Namhoon Kim 1 , Bin Ouyang 1 , Nicola Perry 2 , Elif Ertekin 1
1 , University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 , International Institute for Carbon Neutral Energy Research , Kyushu Japan
Show AbstractMany high-temperature electrochemical device components, such as fuel and electrolysis cell electrodes, rely on materials that simultaneously exhibit ionic and electronic conductivity. Rational design of such materials via computational modeling and simulation has long been a challenge for several reasons. For instance, multiscale considerations are important since features such as crystalline quality, grain size, and orientation affect ionic diffusivity. The complexity of the materials themselves is also challenging, as they tend to be disordered alloys whose composition and stoichiometry are very sensitive to the thermodynamic environment. We present here our effort to build a design framework to predict the ionic conductivity of disordered alloy perovskite oxides. The framework has been applied to compositions within the mixed conducting (Sr,La)(Ti1-x,Fex)O3-y perovskite to better understand the roles of point defect chemistry and electronic structure on the resulting properties and diffusivity. Our approach consists of three steps. First, first-principles electronic structure methods are used to establish defect equilibria and enable prediction of stoichiometry and oxygen vacancy content under different thermodynamic environments. We show that the composition space can be well-described as a solid solution between the band insulator SrTiO3 and the ordered oxygen vacancy compound Sr2Fe2O5. Our simulations are compared to experimental measurements of optical absorption to assess composition effects on electronic structure. Second, we use a cluster expansion to predict representative alloy configurations under different temperatures and gas environments. An expression for the free energy of the alloy system, including contributions from mixing enthalpy and both vibrational and configurational entropy, is derived and phase stability of the alloys are determined. Third, to determine diffusion transport coefficients we apply a generalization of the concept of connected diffusion networks (well-defined for crystalline systems) to disordered materials. The diffusivity tensor is obtained from site and transition state energies as steady-state solutions to the diffusion master equation. This represents a scalable approach to capturing properties such as diffusivity in complex disordered alloy materials.
10:45 AM - TC07.01.08
Defect-Mediated Mechanics in Non-Stoichiometric Oxide Films—Simulation and Experiment
Jessica Swallow 1 , Mostafa Youssef 1 , Krystyn Van Vliet 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractChemomechanical coupling is a hallmark of the functional oxides widely used for energy conversion and storage applications including solid oxide fuel cells (SOFCs). The oxides used for electrodes and electrolytes in such devices rely on the presence of oxygen vacancies to enable important properties including ionic conductivity and gas exchange reactivity. However, such defects can also cause chemical expansion, or coupling between material volume and point defect content, such that the electrode materials “breathe” with changes in effective oxygen partial pressure.1 Such chemomechanical coupling is particularly relevant with the recent interest in micro-SOFCs, which have the potential to decrease operating temperatures and enable portable applications by using thin film components. In turn, thin films provide opportunities for strain engineering to enhance functional properties, as well as challenges due to interfacial stress that can lead to cracking and delamination in operando. Furthermore, thin films present a particular challenge for design and control of functional defects in such oxides, as experimental results have shown that defect chemistry can differ significantly from bulk counterparts under the same experimental conditions and thus require novel in situ characterization methods.
Here we explore the influence of point defects, including oxygen vacancies and cation dopants, on the expansion and elastic properties of the functional oxide PrxCe1-xO2-δ (PCO) bulk and thin films, using a combination of density functional theory (DFT + U) simulations and in situ mechanical characterization experiments. PCO is a model mixed ionic-electronic conducting oxide with the fluorite structure and a well-established defect chemistry model. Computationally, we show that the biaxial elastic stiffness of PCO decreases with increased oxygen vacancy content in both bulk and slab forms. We consider the relative influences of oxygen vacancies and cation dopants on this trend and highlight local details of structural changes in the presence of such defects. Experimentally, we show that these PCO films exhibit a decrease in Young’s elastic modulus E during this chemical expansion, but that this decrease in E can be larger than predicted based on bulk defect models. By linking the results of simulation and experiment, we evaluate the relative importance of increased oxygen vacancy content, anisotropy, and extended defect structures in affecting the mechanical properties of oxides subject to chemical expansion under operando conditions. This work informs the design of micro-SOFCs, emphasizing the need to characterize thin films separately from bulk counterparts. The results also demonstrates how functional defect chemistry can directly influence stress and strain development in devices through both changes in material volume due to chemical expansion and changes in elastic properties.
[1] J.G. Swallow et al., Nat. Mater., DOI: 10.1038/nmat4898 (2017).
11:00 AM - TC07.01.09
Considering Fast Electron/Cation Coupled Transport within Inorganic Ionic Matrices
Paul Smith 1 , Kenneth Takeuchi 1 , Esther Takeuchi 1 2 , Amy Marschilok 1
1 , Stony Brook University, Stony Brook, New York, United States, 2 , Brookhaven National Laboratory, Upton, New York, United States
Show AbstractA chemistry-focused conceptual framework with representative case studies towards batteries capable of fast charge conduction involving both facile ion transport and rapid electron transfer will be presented. Examples based on lithium based batteries will be presented, however, the general concepts provided apply to a wide array of battery systems. On a bulk level, resistance in a battery manifests as polarization, or the difference between the standard (theoretical) potential and usable output. High polarization results in reduced work and increased heat output.
Electrochemical energy storage mechanisms demand the concomitant movement of electrons and positive ions. In cases where electron mobility is rate limiting, a conductive matrix can support higher current with less polarization due to reduced electronic resistance. However, for most cases, the rate limiting process is ion mobility. Thus, facilitating ion transport would reduce sources of polarization in the battery and increase efficiency. The quantitative factors which govern speed of ion diffusion will be highlighted.
11:15 AM - TC07.01.10
Defect Genome in Cubic Perovskites—A Case Study for Proton Conducting Fuel Cell Application
Janakiraman Balachandran 1 , Panchapakesan Ganesh 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractDiscovering new solid materials that exhibit fast ion transport properties are indispensable for the development of next generation electrochemical systems. The ion transport in a material is governed by the energetics of formation, interaction and migration of point defects. Further, these defect energies exhibit multivariate, non-linear relationships with chemical and structural identities of the material.
Creating large defect datasets (aka defect genome) across large material search space can provide qualitative insights and quantitative correlations between these relationships that are critical to accelerate the search and in turn the discovery new fast materials. In this work, we create such a defect genome in over eighty cubic perovskites to study various types of defect formation and interactions (vacancy, interstitials and substitutional dopants), that are relevant for proton conducting fuel cells (PCFC) applications.
This large defect dataset, whose computational complexity is equivalent to performing atleast 13,000 unit cell relaxations is generated by employing a material modeling framework. The framework combines (i)ab initio material models, (ii) automated high-throughput workflows, and (iii) data analytics tools. The resultant defect energies when correlated with structural/chemical identities of materials, defect induced local distortions enables us to identify (i) important descriptors of defect energies and (ii) mechanisms that influence defect formation and interaction. Further, by performing comparitive analysis of the resultant data, we are also able to identify a few promising new compounds that can exhibit fast proton transport. This approach can be generalized to not only create such defect datasets to study not only ionic transport in other families of materials, but they can also be created to analyze the influence of such point defects for other energy applications such as hydrogen storage, CO2 capture, solid state lighting, thermoelectrics and solid-state batteries.
Acknowledgement
This research was sponsored by the Laboratory Directed Research and Development Program (LDRD) of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Offi�ce of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.
References
J.Balachandran, L. Lin, J.A. Anchell, C.A. Bridges, P. Ganesh, \Defect Genome of Cubic Perovskites for fuel cell applications" (in review)
TC07.02: Understanding Defects in Energy Materials by Versatile In Situ Probes
Session Chairs
Monday PM, November 27, 2017
Hynes, Level 2, Room 207
1:30 PM - *TC07.02.01
X-Ray Studies of Oxygen Vacancy Behavior in Complex Oxide Heterostructures
Dillon Fong 1 , Huajun Liu 2
1 Materials Science Division, Argonne National Laboratory, Lemont, Illinois, United States, 2 , Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore Singapore
Show AbstractDespite the difficulty of tracking oxygen vacancies by X-ray methods, this is one of the few techniques able to monitor their behavior in situ as they form during synthesis, during chemical reaction, and in electric fields. In this presentation, I will discuss recent experiments focused on understanding vacancy behavior at surfaces and interfaces, under applied electric fields, and in electrochemically active environments, often in tandem with electrical transport characterization. I will also describe methods for understanding vacancy-induced phase transitions using coherent X-ray scattering.
2:00 PM - TC07.02.02
Control of Oxygen Vacancy Ordering of Strontium Cobaltites via Molecular Beam Epitaxy
Tassie Andersen 1 , Seyoung Cook 1 , Gang Wan 2 , Hawoong Hong 2 , Laurence Marks 1 , Dillon Fong 2
1 , Northwestern University, Evanston, Illinois, United States, 2 , Argonne National Laboratory, Lemont, Illinois, United States
Show AbstractDue to the enormous range of electronic, magnetic, and optical properties that perovskite-derived metal oxides offer they have been a keen focus of synthesis, characterization, and device incorporation efforts in energy storage and conversion, information processing, and sensing devices. The strontium cobalt oxide system, Sr-Co-O, is suited to many of these applications as a functional oxide due to the flexible valence state of the Cobalt transition metal acting as a B-site cation. The series of stable oxygen-vacancy ordered perovskite-derived SrCoOx phases (SCOx) can support properties that can be tuned from an antiferromagnetic insulator (x = 2.5) to a ferromagnetic metal (x = 3.0). Other members of the Sr-Co-O system can adopt more complex perovskite-derived structures such as the Ruddlesden-Popper, Srn+1ConO3n+1 (RP-SCO). In these structures, too, it is the oxygen vacancy ordering which gives rise to tunable properties.
This work focuses on growth and control of vacancy content and Sr-Co-O phase via molecular beam epitaxy (MBE). Growth conditions for the Brownmillerite structure, SrCoO2.5, and an oxygen deficient member of the Ruddlesden-Popper series, Sr3Co2O6, have been identified. Growth conditions for these two phases show close agreement with thermodynamic calculations, indicating that thermodynamics are often an important consideration even in growth methods normally considered to be kinetic in nature.
Strontium cobalt oxide films were synthesized using an oxide MBE equipped with in-situ X-ray diffractometer at Sector 33-IDE of the Advanced Photon Source. Films were grown epitaxially on SrTiO3 (001) via shuttered deposition of SrO/CoO1-2 unit-cells with different deposition schemes under varying growth conditions. During and after growth scattered intensity in the out-of-plane direction (00L) was measured with 8 keV X-rays. X-ray Absorption Near-Edge Spectroscopy spectra at the Co K-edge (7.71 keV) were measured. Together these two methods were used to understand the atomic-scale evolution of the structure of all films grown.
Direct growth of two distinct oxygen-vacancy ordered Sr-Co-O phases in close agreement with thermodynamic predictions indicates these principles can be applied to other complex oxide systems to stabilize desired phases via oxide MBE growth methods.
2:15 PM - TC07.02.03
Exsolution of Highly Active Catalysts—Uncovering the Relationship between Parent Oxide Chemistry and Catalyst Performance
Jiayue Wang 1 , Bilge Yildiz 1 , Alexander Opitz 1 2
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Institute of Chemical Technologies and Analytics, Vienna University of Technology, Vienna Austria
Show AbstractMuch effort has been made during the past decade to develop environmentally friendly technologies to change our current energy supply towards sustainable resources. Much of the targeted technologies, such as solid oxide fuel cells (SOFC) and electrolyser cells (SOEC), rely on catalytically active nanoparticles that are supported on oxides. These nanostructured catalysts exhibit enhanced activity towards desired reactions due to their high specific area and thus provide a cost-effective way to increase the performance of the respective system. A recent advance in catalyst design is to exsolve metal nanoparticles at the surface of a supporting oxide. In the exsolution process, catalytically active transition metals are first dissolved in the parent oxide under oxidizing conditions, and the nanostructured particles can then be released from the host matrix to the surface upon reduction. Compared to traditional deposition techniques, the nanoparticle catalysts from exsolution have two additional appealing characteristics. First of all, the exsolved particles are more resilient to particle sintering due to strong metal-oxide interaction. Secondly, the exsolved metal could be re-integrated into the host matrix upon re-oxidation, which opens up the possibility of oxidative regeneration.
While being an exciting and promising pathway for high-performance oxide-metal composite catalysts, the approach towards exsolution has thus far been a mainly empirically engineered one, without a clear understanding and control of the underlying mechanisms to optimize catalyst composition and dispersion. Studies in the literature indicate that the exsolution products are highly dependent on both surface chemistry (e.g., surface stoichiometry and process environment) and bulk chemistry (e.g., composition and extended defects) of the parent oxide. However, so far it remains unclear how these parameters affect the exsolution process, which is partly related to the fact that to date mainly powder samples were studied.
In this work, we aim to uncover the factors that control the exsolution process on well-defined perovskite-type thin-film samples. In particular, we quantify the effect of crystallographic orientation, bulk composition, extended defects, and process conditions on the obtained exsolved particles. Ambient pressure X-ray photoelectron spectroscopy (APXPS) combined with the ability to control temperature, gas composition and electrochemical polarization on the studied materials, is the necessary platform to conduct this mechanistic study. In situ quantification of the amount of exsolved particles, surface oxidation states, and surface composition is a powerful tool to disentangle and reveal the dominant factors affecting the kinetics of exsolution. Moreover, the catalytic properties of the prepared oxide supported metal particles will be addressed for electrolysis of H2O and CO2, and thus, allow for correlating catalyst preparation conditions and performance.
2:30 PM - TC07.02.04
Probing the Surface Chemistry of Oxide Electrodes at Controlled Oxygen Activity in a UHV-Based XPS Analyser
Andreas Nenning 1 2 3 , Jurgen Fleig 2
1 , Massachusetts Institute of Technology, Cambridge, MA, Austria, 2 Department of Chemical Technologies and Analytics, TU Vienna, Vienna Austria, 3 Department of Materials, ETH Zürich, Zurich Switzerland
Show AbstractMixed conducting oxides gain increasing interest as anodes in solid oxide fuel cells, due to their large electrochemically active surface area and excellent redox stability, compared to state-of-the-art Ni-YSZ cermets. However, to understand and optimize these materials better, information of the surface chemistry and reaction mechanisms is important, but scarce. Here we present a method to employ such investigations at operation temperature in a standard UHV-based XPS analyzer. With the presented method, also the oxygen partial pressure in the investigated electrode (but not in the UHV chamber) can be precisely controlled from roughly 10-4 mbar to the reducing decomposition of the electrode, by application of a voltage versus an oxygen buffering reference electrode. The absence of a gas phase prohibits the identification of atmospheric adsorbates, but still the effect of different defect energetics at the surface, electrode polarization, and the thermo-chemical stability window are experimentally accessible. In combination with ambient pressure XPS measurements, the UHV characterization can deliver essential reference data by differentiating between surface species originating from atmospheric adsorbates and those originating from the different defect energetics of electrode bulk and surface. This new technique was applied to investigate the near surface oxidation states of transition the transition metals found on La0.6Sr0.4FeO3-δ, SrTi0.3Fe0.7O3-δ and La0.7Sr0.2Cr0.9Ni0.1O3 at different oxygen partial pressures. The results relate excellently to those of previous ambient pressure XPS measurements[1, 2]. When the oxygen partial pressure in the working electrode goes below the NiO/Ni and FeO/Fe redox pairs, the exsolution of metallic Ni or Fe nanoparticles was monitored in-situ. Understanding and control of the exsolution process will lead to morphologically stable and coking-resistant catalysts [3] for usage of hydrocarbon fuels and CO2 electrolysis.
References
[1] A. Nenning, A. Opitz, C. Rameshan, R. Rameshan, R. Blume, M. Hävecker, A. Knop-Gericke, G. Rupprechter, B. Klötzer, J. Fleig, The Journal of Physical Chemistry C (2015).
[2] A.K. Opitz, A. Nenning, C. Rameshan, R. Rameshan, R. Blume, M. Hävecker, A. Knop-Gericke, G. Rupprechter, J. Fleig, B. Klötzer, Angewandte Chemie International Edition (2014).
[3] D. Neagu, G. Tsekouras, D.N. Miller, H. Ménard, J.T.S. Irvine, Nat Chem 5 (2013) (11) 916.
2:45 PM - TC07.02.05
Detection of the Onset of Plasticity in Micro-Crystals—In Situ Deformation of InSb Micro-Pillars under Synchrotron Coherent X-Ray Nanobeam
Ludovic Thilly 1 , Vincent Jacques 2 , Christoph Kirchlechner 3
1 , Institut Pprime, Futuroscope France, 2 , LPS, Orsay France, 3 , Max-Planck-Inst, Düsseldorf Germany
Show AbstractCoherent x-ray micro-diffraction was used to detect and count phase defects (stacking faults, SFs, left in the crystal after the glide of partial dislocations) preliminarily introduced by deformation of InSb single-crystalline micro-pillars. Diffraction patterns were recorded by scanning the coherent nanobeam along the pillars axis: peak splitting is observed in the diffraction pattern associated to the top region, in agreement with the presence of a few SFs located in the upper part of the deformed pillars. Simulations of coherent diffraction patterns were also performed considering SFs randomly distributed in the illuminated volume: they show that not only the number of defects but also the size of the defected volume influences the maximum intensity of the pattern, allowing for a precise counting of defects [Physical Review Letters, 111 (2013), 065503].
Recently, diffraction measurements were performed in-situ, during compression, to detect the first lattice defects, i.e. the first events of the plastic deformation appearing in InSb micro-pillars.
3:45 PM - TC07.02.06
In Situ Spectroscopic Characterization of Oxygen-Induced Defects at the Graphene Oxide/Perovskite Interfaces
Muge Acik 1 , In Kee Park 2 , Geunsik Lee 2 , Richard Rosenberg 3
1 Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois, United States, 2 Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan Korea (the Republic of), 3 Advanced Photon Source, X-Ray Science Division, Argonne National Laboratory, Lemont, Illinois, United States
Show AbstractGraphene incorporated perovskite-based solar cells have been demonstrated for low-cost and scalable production as an alternative to metal oxide and polymer -based perovskite photovoltaics. The varying PCE at ~0.62-18% needs to be investigated to unravel challenges for solar device improvement. Graphene oxide (GO) and its reduced form (RGO) recently emerge as an electron (ETL) or hole (HTL) transport layers in these devices. Organic-inorganic methylammonium lead halides, MAPbT3 (T=I, Br, Cl and MA=CH3NH3) are deposited on graphene films as light harvesting layers because of their exciting optoelectronic properties: tunable bandgap, long electron-hole diffusion lengths and high electron/hole mobility. Nevertheless, halide-based perovskites require in situ characterization to understand perovskite growth mechanisms and explain interfacial chemical changes at GO/MAPbT3 film interfaces. Understanding the origin of perovskite degradation mechanisms is lacking for the evolution of thermal reduction of GO at the ETL/perovskite/HTL interfaces. In particular, incomplete lead precursor conversion, inconsistent crystallite formation, weak cation-anion-solvent coordination, uncontrolled stoichiometry, high lead content, and high surface roughness are detrimental effects for device stability and reliability.
To address scalability and stability issues, we examine degradation, nucleation and growth mechanisms during annealing in RGO during halide-based perovskite growth. First, GO thin films (3-5 layers) are deposited by vacuum filtration, followed by spin coating perovskites, and then annealed on GO. Chemical interactions are interpreted at perovskite/RGO interfaces for the grain size, orientation, boundaries, and surface/bulk effects using variable-temperature (≤400°C, Ar(g)) in situ infrared absorption and raman spectroscopy, as well as XPS, SEM, XRD, and AFM. MAPbI3 growth on GO indicates poor perovskite conversion, forming Pb(NO3)2 at GO/MAPbI3 interfaces after annealing that are modified by unsaturated Pb states. Oxygen-induced chemical reactions yield defects in RGO that occur during MAPbI3 growth. In contrast, MAPbBr3 growth on GO results in improved chemical stability with heat because of crystal reorientation on RGO after perovskite nucleation without forming Pb(NO3)2 and having metallic lead. These findings suggest novel solar device designs. 1Acik et al. (2017) in prep. 2Acik et al. (2017), Ad. En. Mat. 3Acik et al. J. Mater. Chem. A 4 (2016) 6185. 4Acik et al. Nature Mater. 9 (2010) 840.
This work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Office of Science User Facility, and supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. XPS performed at the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under contract No. DE-AC02-06CH11357.
4:00 PM - TC07.02.07
Correlation of Defect Property and Electric Transport Properties of Metal Oxide Electrodes and Solid Electrolytes
Artur Braun 1
1 , Empa-Swiss Federal Laboratories for Materials Science and Technology, Duebendorf Switzerland
Show AbstractThe term "defect" comes with a very negative connotation, although defects account for many of the benign functionalities which make designed materials so valuable in modern technology. I will show detailed studies where x-ray spectroscopy and neutron scattering elucidate various kinds of benign defects, such as permanent 1 or transient electron holes 2, oxygen vacancies, excitons and polarons 3 which lend electrode and electrolyte materials the necessary high electronic and ionic conductivity for deployment in electrochemical energy conversion and storage components such as batteries, fuel cells and photoelectrochemical cells. It is particularly rewarding to carry out the electroanalytical methods in parallel with the scattering and spectroscopy experiments, which in fortunate cases to test theoretical models with virtually mathematical precision 4.
1. Braun A, et al. Pre-edges in oxygen (1s) x-ray absorption spectra: A spectral indicator for electron hole depletion and transport blocking in iron perovskites. Applied Physics Letters 94, (2009).
2. Braun A, et al. Direct Observation of Two Electron Holes in a Hematite Photoanode during Photoelectrochemical Water Splitting. Journal of Physical Chemistry C 116, 16870-16875 (2012).
3. Braun A, Chen Q. Experimental neutron scattering evidence for proton polaron in hydrated metal oxide proton conductors. Nature Communications 8, 15830 (2017).
4. Braun A. X-ray Studies on Electrochemical Systems - Synchrotron Methods for Energy Materials. Walter De Gruyter GmbH (2017).
4:15 PM - TC07.02.08
In Situ Characterization of Polycrystalline Solar Cell Absorbers—Performance at the Nanoscale under Operating Conditions
Michael Stuckelberger 1 , Bradley West 1 , Tara Nietzold 1 , Barry Lai 2 , Jorg Maser 2 , Mariana Bertoni 1
1 , Arizona State University, Tempe, Arizona, United States, 2 , Argonne National Laboratory, Argonne, Illinois, United States
Show AbstractUnderstanding the mechanism of charge collection and its relation to elemental composition and structural variations at the nanoscale is of critical importance for developing efficient solar cell materials. Although this is true for silicon solar cells, it is much more critical for polycrystalline thin-film absorber layers such as II/VI, III/V materials deposited by methods faster than epitaxial growth, or perovskite solar cells (PSCs). A tool capable of correlating composition and properties on a nano- pixel-to-pixel basis could help point to the origins of performance losses as well as degradation pathways and kinetics.
Correlative X-ray microscopy with a resolution down to the nanoscale offers a method to fill this gap. In this contribution we will present results from a modified in-situ stage used for studying the electrical performance (current and voltage) and elemental distribution in CIGS, CdTe and PSC solar cells with high sensitivity, voxel sizes <40nm and under various operating conditions (light, bias, temperature). With this tool we can map cells under operation giving access to the charge collection efficiency and changes in recombination pathways with nanoscale lateral resolution.
Further upgrades to the tool allow to collect information under growth and/or processing conditions enabling access to kinetics and transport information, which is crucial to understand degradation mechanisms and even more important to engineer the next generation absorbers.
4:30 PM - TC07.02.09
Superfast Transient Absorption Imaging of Grain Boundary and Nano-Defects in Graphene
Kai-Chih Huang 1 2 , Jeremy McCall 1 , Pu Wang 1 2 , Chien-Sheng Liao 1 2 , Gregory Eakins 1 , Chen Yang 1 2 , Ji-Xin Cheng 1 2
1 , Purdue University, West Lafayette , Indiana, United States, 2 , Boston University, Boston , Massachusetts, United States
Show AbstractGraphene, as one of the 2-D single atomic nano-material, is becoming attractive for several applications including future electronics based on its outstanding electrical, mechanical, and chemical properties. Large-area graphene films are manufactured by chemical vapour deposition (CVD) technique and roll-to-roll process on an industrial scale. During the manufacturing process, nano-scale defect, which may appear during growth or processing, strongly impacts the electrical transport properties. This nano-scale defect, however, cannot be detected by a traditional optical microscope. Raman spectroscopy can directly characterize the defect density in graphene based on the molecular vibrational spectrum, but suffer from long acquisition time on second scale. Recently developed time-resolved transient absorption (TA) microscopy provides a non-destructive characterization method by measuring the carrier relaxation dynamics in graphene. Nevertheless, the acquisition time of a time-resolved TA image still takes minutes, which is far too slow compared with roll-to-roll process.
Here, we present a superfast TA microscope as a real-time graphene characterization tool. We improved spatial acquisition speed by line-illuminated scanning scheme and parallel detection with our lab-designed tuned amplifier (TAMP) and photodiode array. The acquisition speed of a time-resolved TA signal was improved to 92 µs by resonant delay tuning with the angle-to-delay convertor. We successfully quantified the surface coverage, degree of wrinkles, defect density and layer numbers of graphene with high contrast and high speed. Compared with the 2.3% contrast difference in a traditional optical microscope, we have reached 35:1 contrast ratio of single layer graphene. The imaging speed was able to catch the roll-to-roll process (0.3 m/min), which is 100 times faster than the state-of-art wide-field Raman or pump-probe microscopy under the same resolution and field of view. The image speed can be further improved to 1,000 frame per second at a specific temporal delay. Future work will include the study of other 2-D nano-materials and thin films.
4:45 PM - TC07.02.10
Studying Break-In Phenomena in Lithium-Ion Batteries through In Operando Acoustic and Impedance Measurements
Thomas Hodson 1 2 , Kevin Knehr 1 2 , Daniel Steingart 1 2
1 MAE, Princeton University, Princeton, New Jersey, United States, 2 ACEE, Princeton University, Princeton, New Jersey, United States
Show AbstractLithium-Ion Batteries (LiBs) have become ubiquitous in modern technologies, such as in the burgeoning electric vehicle and consumer electronics markets. LiBs undergo complex structural and electrochemical changes as the batteries age. It is difficult, however, to determine the nature of these complex processes due to the lack of proper in situ and in operando characterization techniques. Acoustic measurements of Li-ion batteries have recently shown promise as a technique for structural characterization over the cycle life of the battery due to the ability to operate this technique in operando. [1] In this study, we use electrochemical acoustic and impedance spectroscopy measurements to provide in operando analysis of break-in effects of LiBs. We further analyze how break-in conditions, such as varying the rate of current, affect the overall performance of the battery.
[1] Hsieh, A. G., et al. Energy & environmental science 8.5 (2015): 1569-1577.
Symposium Organizers
Hua Zhou, Argonne National Laboratory
Panchapakesan Ganesh, Oak Ridge National Laboratory
Anna Kimmel, University College London
Dong Su, Brookhaven National Laboratory
Symposium Support
Argonne National Laboratory, Advanced Photon Source
TC07.03: Functional Defects by Design for Energy Storage and Harvest II
Session Chairs
Panchapakesan Ganesh
Anna Kimmel
Tuesday AM, November 28, 2017
Hynes, Level 2, Room 207
8:00 AM - *TC07.03.01
Energy Materials by Computational Design—Defects, Disorder and Heterostructural Alloys
Stephan Lany 1
1 , National Renewable Energy Laboratory, Lakewood, Colorado, United States
Show AbstractThe increasing compositional and structural complexity of energy materials provides both opportunities and challenges for computational materials design. Defect equilibria from first principles calculated defect formation energies yield a “defect phase diagram”, expressing, e.g., the doping and defect densities level as a function of the composition and growth parameters in photovoltaic and wide gap materials. In ternary or multinary systems, the formation of cation exchange defects can lead to disorder. Monte-Carlo simulations show that disorder can have profound effects on both the electronic structure and the defect properties [1]. Isovalent substitution creates an alloy where the materials properties are usually tuned by variation of the chemical composition. In heterostructural alloys, however, the properties respond also to the change of the atomic structure, opening new avenues for tailored properties. Recent work has shown that under certain conditions, heterostructural alloys can open up a wide space of metastable single phase compositions above the solubility limit [2], thereby providing access to this novel phase space for materials design.
[1] P.P. Zawadzki, A. Zakutayev, S. Lany, Phys. Rev. Appl. 3, 034007 (2015); Phys. Rev. B 92, 201204(R) (2015).
[2] A. Holder et al., Sci. Adv. 3, e1700270 (2017).
8:30 AM - TC07.03.02
Thermodynamics of Defects and Impurities in ZnSnN2
Jie Pan 1 , Elisabetta Arca 1 , John Perkins 1 , Andriy Zakutayev 1 , Stephan Lany 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractZnSnN2 (ZTN) is a promising candidate of the next-generation earth-abundant absorber material for photovoltaic (PV) applications. Although ZTN has many desirable properties for PV, such as a suitable direct band gap, high absorption coefficient, and low effective masses of the carriers, the high degeneracy in ZTN films makes it cumbersome for device fabrication. As the high carrier concentration comes from ionized defects, a detailed thermodynamics study of defects/defect pairs in ZTN becomes necessary to guide experiments by reducing the degeneracy. In this contribution, we present a density functional theory (DFT) based thermodynamic simulation of defects in ZTN. The formation energies of ionized defects, e.g., ZnSn, VN, ON, and possible defect pairs, e.g., ZnSn+VN, ZnSn+ON, ZnSn+SnZn, are determined from DFT+U calculations with GW band gap corrections. The energetics of defects/defect pairs is then transferred into a thermodynamic simulation to calculate defect phase diagram as a function of accessible elemental chemical potentials. At this point, non-equilibrium nitrogen chemical potentials are considered within the accessible range of sputtering deposition with activated N sources.
8:45 AM - TC07.03.03
Toward Quantitative Metrics to Screen for Defect Tolerance in Novel Semiconducting Materials
Rachel Kurchin 1 , Prashun Gorai 2 3 , Anuj Goyal 2 3 , Tonio Buonassisi 1 , Vladan Stevanovic 3 2
1 , MIT, Cambridge, Massachusetts, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractDefect tolerance (resilience of electronic transport properties to defects) has emerged as a key screening parameter to accelerate the discovery of novel semiconductor materials. This interest is inspired by hybrid lead halide perovskites, which achieve the outstanding photovoltaic (PV) performance despite high defect concentrations introduced by solution-based methods. In this work, we advance computationally inexpensive, quantitative screening metrics for defect tolerance and demonstrate their correlation with DFT defect calculations as well as experimental measurements of minority-carrier lifetime in nearly a dozen emergent halide materials. These quantitative screening metrics incorporate elements of chemistry and crystal structure, and are predicated upon simple tight-binding type insights that allow intuitive consideration of the origins and behaviors of defect states.
To validate our models, we theoretically and experimentally compare differences in defect properties of binary iodides comprising indium, lead, and bismuth cations. We predict that anion vacancy formation energies increase from indium to bismuth in binary iodides. We correlate this observation with the cation-cation electronic orbital overlap in the presence of an anion vacancy, which is determined by both quantitative structural considerations (bond angles and distances) and energetic considerations (mismatch in valence s-orbital energies of isolated cation and anion). With these quantitative metrics in mind, we demonstrate experimentally that the first growths of these binary iodides using similar methods yield approximately an order of magnitude longer lifetime for indium iodide than bismuth iodide, consistent with our predictions. We apply these quantitative screening criteria to nearly thousands of semiconductors in online databases such as the ICSD and materialsproject.org, ranking classes of compounds in accordance to their predicted defect tolerance.
9:00 AM - *TC07.03.04
Oxygen Off-Stoichiometry and Defect Entropies in Solar Thermochemical Water Splitting Materials
Christopher Wolverton 1
1 Department and Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States
Show AbstractA large solid-state entropy of reduction increases the thermodynamic efficiency of metal oxides for two-step thermochemical water splitting (TWS). The configurational entropy arising from oxygen off-stoichiometry in the oxide has been the focus of most previous work on the entropy of TWS. We present a high-throughput density functional theory (HT-DFT) study of more than 11,000 cubic and distorted perovskite ABO3 compounds to screen for thermodynamically favorable two-step TWS materials. We screen compounds based on the following: (a) thermodynamic stability and (b) oxygen vacancy formation energy that allow favorable TWS. From our screening process, we identify 139 materials as potential new candidates for TWS application. In addition, in an effort to understand the favorable thermodynamics of CeO2 for TWS, we examine a different source of entropy, the on-site electronic configurational entropy arising from excitations of electrons from a lower to higher weakly splitted states. We find that this electronic entropy is sizable in all lanthanides, and reaches a maximum value of ~4.7 kB per oxygen vacancy for Ce+4=Ce+3 reduction. The unique and large positive electronic entropy in ceria contributes to its excellent water-splitting performance as well as its superior properties for other high-temperature catalytic redox reactions. *Work was performed in collaboration with Tony Emery, Shahab Naghavi, Heine Hansen, James Saal, Vinay Hegde, Scott Kirklin, Fei Zhao, and Vidvuds Ozolins.
9:30 AM - TC07.03.05
Defect Screening of Complex Oxides for high ZT Thermoelectrics
Alex Ganose 1 2 , W. Winnie Leung 1 , Adam Jackson 1 , R. Palgrave 1 , David Scanlon 1 2
1 , University College London, London United Kingdom, 2 , Diamond Light Source, Didcot United Kingdom
Show AbstractThermoelectric materials, used to convert thermal into electrical energy, present a promising route for renewable energy generation. The range of applications for thermoelectrics is broad, with industries from manufacturing to the automotive likely to benefit from efficient recycling of waste thermal energy.1 The dimensionless figure of merit for thermoelectrics, ‘ZT’, depends on both electronic and thermal transport properties, with a material considered promising if its ZT exceeds ~1.5. Unfortunately, despite over 50 years’ development, the champion thermoelectric materials, such as Bi2Te3, show lack-lustre performance and are costly to produce due to their reliance on tellurium.2 Significant research effort has been spent attempting to produce oxide based thermoelectrics due to their earth-abundance, chemical stability and dramatically reduced costs. However, all attempts to produce high performance n-type oxide thermoelectrics have failed, often due to their high lattice conductivity which limits obtainable ZT.3
Standard packages now exist for calculating ZT from an electronic band structure, with the results being dependent on two major approximations: a fixed lattice thermal conductivity and electronic chemical potential (Fermi level). Typically, the Fermi level is assumed without knowledge of the true response of the material to defects and doping, which can lead to incorrect predictions of high ZT capability.
In this work, we have used rational chemical design to pinpoint a series of layered oxides that should exhibit degenerate n-type conductivity, whilst still possessing very low lattice thermal conductivity.4 We employ state of the art methods to calculate the lattice thermal conductivity, using many-body perturbation theory to capture phonon-phonon scattering processes. We also use rigorous defect chemistry analysis, performed using hybrid density functional theory, to explicitly consider the intrinsic and extrinsic defect behaviour and obtain a physical and realistic doping density and Fermi level. Combining these methods, we have predicted the largest ZT of any oxide thermoelectric material previously reported and provide guidance on the growth conditions to enhance thermoelectric power conversion.
References
[1] L. E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 321, 1457–1461 (2008)
[2] M. W. Gaultois, T. D. Sparks, C. K. H. Borg, R. Seshadri, W. D. Bonificio, and D. R. Clarke, Data-driven review of thermoelectric materials: performance and resource considerations, Chemistry of Materials 25, 2911–2920 (2013)
[3] G. Tan, L-D. Zhao, and M. G. Kanatzidis, Rationally designing high-performance bulk thermoelectric materials, Chemical Reviews 116, 12123–12149 (2016)
[4] Alex M. Ganose, W. W. Leung, Adam J. Jackson, R. G. Palgrave, and David O. Scanlon, Submitted (2017)
TC07.04: Defects in Semiconductors for Light Harvest
Session Chairs
Monica Burriel
Annamaria Petrozza
Tuesday PM, November 28, 2017
Hynes, Level 2, Room 207
10:15 AM - *TC07.04.01
Mitigating Defect-Induced Band Tailing in Kesterites
Talia Gershon 1 , Yun Seog Lee 1 , Priscilla Antunez 1 , Douglas Bishop 1 , Oki Gunawan 1 , Tayfun Gokmen 1 , Richard Haight 1
1 , IBM T.J. Watson Research Center, Yorktown Heights, New York, United States
Show AbstractThe photovoltaic absorber Cu2ZnSn(SxSe1-x)4 (CZTSSe) has attracted interest due to the earth-abundance of its constituents and the realization of high performance (12.6% efficiency). However, efficiency improvements in CZTSSe devices have been limited by the inability to increase the open-circuit voltage (Voc) beyond present values. This shortfall in Voc is correlated with severe band tailing in the absorber, which itself is correlated with an exceptionally high density of Cu/Zn antisites. Spatial inhomogeneity in these defects, leading to electrostatic fluctuations in the bands, is a leading explanation for the band tailing and consequent Voc loss in Kesterite photovoltaics.
By replacing Cu in CZTSSe with Ag, whose covalent radius is ~15% larger than that of Cu and Zn, the density of I-II antisite defects drops, consistent with theoretical predictions. I will discuss the fundamental properties of the mixed Ag-Cu kesterite compound as a function of Ag/(Ag+Cu) ratio. The extent of band tailing is shown to decrease with increasing Ag. Additionally, the pinning of the Fermi level in CZTSSe, attributed to heavy defect compensation and band tailing is not observed in the pure-Ag compound, offering further evidence of improved electronic structure. I will also present out current progress and understanding of devices based on the pure-Ag Kesterite as well as what is known about loss mechanisms in these devices.
10:45 AM - *TC07.04.02
Functional and Dysfunctional Defects and Doping: The Good, the Bad and the Ugly
Alex Zunger 1
1 Renewable and Sustainable Energy Institute, University of Colorado at Boulder, Boulder, Colorado, United States
Show AbstractA number of effects discovered in Quantum Materials and attributed to exciting “new physics” often turn out —if modeled carefully-- to reflect good-old defect physics. Examples include the non-exotic but valid explanation of conductivity at the interface of polar insulators (such as STO-LAO) once thought to reflect polar catastrophe, or the conductivity of undoped half- Heusler compounds. Even pristine, as-grown compounds without any attempt at impurity/chemical doping exhibit systematic off-stoichiometry and consequently have systematic trends in the free carriers they exhibit. DFT calculations clarify that such natural non- Daltonian off stoichiometry reflects the balance between different competing phases that would make the target compound. For example, in ABC Half- Heusler compounds, one observes spontaneous deviations from 1:1:1 stoichiometry which depends largely on the identity of the B atom: (i) Bulk ABC compounds containing 3d elements in the B position (ZrNiSn and ZrCoSb) are predicted to be naturally 3d rich with the B=3d interstitials being the prevailing shallow donors, resulting in an overall uncompensated n-type character. In contrast, (ii) bulk ABC compounds containing 5d elements in the B position (ZrPtSn, ZrIrSb, and TaIrGe) are predicted to be naturally C rich and A poor, promoting hole-producing C-on-A antisite defects and resulting in p-type character. The Modern Theory of Defects and Doping reminds us how to model defect formation and compensation as a feedback effect.
This was supported by the Office of Science, Basic Energy Science, MSE Division under Grant No. DE-FG02- 13ER46959
11:15 AM - TC07.04.03
Cation Off-Stoichiometry in Wurtzite-Based Ternary Nitrides
Elisabetta Arca 1 , Stephan Lany 1 , John Perkins 1 , Aaron Holder 1 , Wenhao Sun 3 2 , John Mangum 4 , Gerbrand Ceder 3 2 , Brian Gorman 4 , Glenn Teeter 1 , Adele Tamboli 1 4 , Andriy Zakutayev 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , University of California, Berkeley, Berkeley, Colorado, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 , Colorado School of Mines, Golden, Colorado, United States
Show AbstractSemiconducting nitrides are a class of technological relevant compounds that are crucial components of many optoelectronic devices. To date, most of the research activity was dedicated to traditional III-Vs (GaN, AlN, InN etc.), or other technologically relevant compound (TiN, Si3N4 etc.). Beside traditional binary III-V nitrides, a new class of ternary nitrides is emerging, those based on wurtzite-derived structure, for example ZnSnN2 and ZnGeN2 in the II-IV-V group of materials. This class of materials is characterized by the capability to tolerate large cation off-stoichiometry (10s of %) without compromising the crystal symmetry. Such flexible compositions may provide an additional degree of freedom to tune materials properties, such as electron concentration in Zn1+xSn1-xN2. Optimization of these materials alongside discovery of new compounds with wide miscibility region, might help overcoming the limitation of the traditional III-V nitrides.
In this contribution we will discuss wurtzite-based ternary nitrides ZnSnN2 and Zn3MoN4 that can tolerate large ranges of off-stoichiometry on their cation sub-lattice. This occurs by accommodating defects that do not disrupt the long-range order, thus preserving their crystallographic symmetry. In particular, we will report on a theoretically predicted and experimentally synthesized new material, Zn3MoN4. By using our combinatorial physical vapor deposition and spatially-resolved characterization approach, coupled with Density Functional Theory (DFT) calculations, we show that this compound retains its crystallographic properties despite introducing substantial off-stoichiometry and disorder in the cation sub-lattice. X-ray Photoelectron Spectroscopy (XPS) is used to confirm the oxidation state of the three elements, whereas Rutherford Back Scattering (RBS) is used to confirm the N to metal ratio. Changes in the optical and electrical properties were determined and will be reported as a function of Zn-Mo ratio.
11:30 AM - TC07.04.04
Towards P-Type Doping of Metal-Oxide Semiconductors
Fernando Sabino 1 , Anderson Janotti 1
1 , University of Delaware, Newark, Delaware, United States
Show AbstractOne of the great challenges in using oxides as active layer in electronic devices is to control their conductivity. Most of the oxide semiconductors show unintentional n-type conductivity, the cause of which has been extensively discussed in the literature. This can be explaining by the typically high electron affinity, i.e., the position of the conduction band is relatively low with respect to the vacuum level, and most of the impurities that give up electrons are shallow donors. As for p-type doping, the valence band, that is derived from oxygen p-orbitals, is typically too low with respect to the vacuum level, so that most impurities that have one less valence electron than the host atoms are deep acceptors. In addition, holes in the valence band of these materials tend to become localized in the form of small polarons, characterized by a local lattice distortion. Using density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional (HSE), we investigate a series of possible acceptor impurities in titanates, such as SrTiO3 and TiO2, paying special attention to those impurities that cause only small distortions in the lattice. We discuss the differences between thermodynamic and optical ionization energies and possible ways to avoid passivation by native donor defects such as oxygen vacancies.
11:45 AM - TC07.04.05
Au Hyperdoped Si—High Sub-Band Gap Absorption Aided by Au-Defect Complexes
Wenjie Yang 1 , Naheed Ferdous 2 , Quentin Hudspeth 3 , Philippe Chow 3 , Jeffrey Warrender 3 , Elif Ertekin 2 , Jim Williams 1
1 Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory, Australia, 2 Department of Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, Champaign, Illinois, United States, 3 Benet Laboratories, US Army ARDEC, Watervliet, New York, United States
Show AbstractUsing ion implantation followed by pulsed laser melting, it is possible to incorporate Au into single crystalline Si at highly non-equilibrium concentrations. Such hyperdoped Si exhibits enhanced sub-band gap optical absorption, allowing for Si-based infrared light detection [1]. While we have shown that the enhanced absorption increases with increasing Au concentration, the lateral distribution of Au becomes inhomogeneous at high Au concentrations [2]. Vertical filaments containing up to 1.4 at. % Au in disorder structures are observed in the hyperdoped layer. Despite these disorder structures, however, a significant fraction of the Au still occupies substitutional lattice positions and introduces surprisingly small uniaxial strain. Furthermore, such structures continue to enhance sub-band gap absorption. In this work, we explore the defective structure of the Au-rich filaments by examining the local lattice environment around the Au impurities using a combination of techniques, including Rutherford Backscattering Spectrometry and Channelling (RBS/C), High-Resolution Transmission Electron Microscopy (HRTEM), and High-Resolution X-ray Diffraction (HR-XRD). In addition, the possibility of vacancy/Au-vacancy complexes in the hyperdoped layer is investigated with the aid of Density Functional Theory (DFT) calculations to elucidate the origin of enhanced sub-band gap absorption in these materials.
[1] J. P. Mailoa et al., Nature communications 5, 3011 (2014).
[2] W. Yang, A. J. Akey, L. A. Smillie, J. P. Mailoa, B. C. Johnson, J. C. McCallum, D. Macdonald, T. Buonassisi, M. J. Aziz, and J. S. Williams, Under review, 2017.
TC07.05: Observing and Crafting Functional Defects in Low Dimensional Materials
Session Chairs
Tuesday PM, November 28, 2017
Hynes, Level 2, Room 207
1:30 PM - *TC07.05.01
Atom by Atom Fabrication of Novel Phases and Devices via Electron Beams
Sergei Kalinin 1 , Ondrej Dyck 1 , Albina Borisevich 1 , Artem Maksov 1 , Eva Zarkadoula 1 , Stephen Jesse 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractAtom by atom design and manufacturing of materials, structures, and devices remains the ultimate, and yet not achieved, goal of nanotechnology. The reigning paradigms are scanning probe microscopy (SPM) and synthesis. SPM assembly dates back to seminal experiments by Don Eigler, who demonstrated single atom manipulation and writing. However, stability and throughput remain issues, and only in the last decade synergy of STM and surface chemistry was used to make several-qubit devices. The molecular machines approach harnesses the power of modeling and synthetic chemistry to build individual functional blocks, yet strategies for structural assembly remain uncertain. In this presentation, I discuss the research activity coordinated by the Institute for Functional Imaging of Materials (IFIM) towards the third paradigm—the use of atomically focussed beam of scanning transmission electron microscope to control and direct matter on atomic scales. Traditionally, STEMs are perceived only as imaging tools, and any beam induced modifications are undesirable beam damage. In the last five years, our team and several groups worldwide demonstrated that beam induced modifications can be more precise. We have demonstrated ordering of oxygen vacancies, single defect formation in 2D materials, and beam induced migration of single interstitials in diamond like lattices. What is remarkable is that these changes often involve one atom or small group of atoms, and can be monitored real time with atomic resolution. This fulfills two out of three requirements for atomic fabrication. I will introduce several examples of beam-induced emergence of novel non-equilibrium phases which are created and detected via atomically resolved imaging. This approach is further extended towards fabrication on atomic level, and I will demonstrate how beam control, rapid image analytics, and image- and ptychography based feedback allows for controlling matter on atomic level.
This research is supported by and performed at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, BES DOE.
1. S.V. Kalinin, A. Borisevich, and S. Jesse, Fire up the Atom Forge, Nature 539, 485 (2016).
2. S. Jesse, A. Belianinov, J. Fowlkes, A. Lupini, P. Rack, R. Unocic, B. Sumpter, S.V. Kalinin, and O. Ovchinnikova, Directing Matter: Towards Atomic Scale 3D Nanofabrication, ACS Nano 10, 5600 (2016).
3. X. Sang, A. Lupini, J. Ding, S.V. Kalinin, S. Jesse, and R. Unocic, Precision controlled atomic resolution scanning transmission electron microscopy using spiral scan pathways, Sci Rep., in print
4. S. Jesse, Q. He, A.R. Lupini, D.N. Leonard, M.P. Oxley, O. Ovchinnikov, R. Unocic, A. Tselev, M. Fuentes-Cabrera, B.G. Sumpter, S.J. Pennycook, S.V. Kalinin, and A.Y. Borisevich, Atomic-level sculpting of crystalline oxides: towards bulk nanofabrication with single atomic plane precision, Small 11, 5895 (2015).
2:00 PM - *TC07.05.02
Atomic Electron Tomography—Probing 3D Structures and Material Properties at the Single-Atom Level
Jianwei Miao 1
1 Department of Physics and Astronomy and California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California, United States
Show AbstractTo understand material properties and functionality at the most fundamental level, one must know the 3D positions of atoms with high precision. For crystalline materials, crystallography has provided this information since the pioneering work of Max von Laue, William Henry Bragg, and William Lawrence Bragg over a century ago. However, perfect crystals are rare in nature. Real materials often contain crystal defects, surface reconstructions, nanoscale heterogeneities, and disorders, which strongly influence material properties and performance. Here, I will present atomic electron tomography (AET) for 3D structure determination of crystal defects and disordered materials at the single-atom level (1). Using a Fourier based iterative algorithm, we first demonstrated electron tomography at 2.4-Å resolution without assuming crystallinity in 2012 (2). We then applied AET to image the 3D structure of grain boundaries and stacking faults and the 3D core structure of edge and screw dislocations at atomic resolution (3). Furthermore, in combination of AET and atom tracing algorithms, we localized the coordinates of individual atoms and point defects in materials with a 3D precision of ~19 pm, allowing direct measurements of 3D atomic displacements and the full strain tensor (4). More recently, we determined the 3D coordinates of 6,569 Fe and 16,627 Pt atoms in an FePt nanoparticle, and correlated chemical order/disorder and crystal defects with material properties at the individual atomic level (5). We identified rich structural variety with unprecedented 3D detail including atomic composition, grain boundaries, anti-phase boundaries, anti-site point defects and swap defects. We showed that the experimentally measured coordinates and chemical species with 22 pm precision can be used as direct input for density functional theory calculations of material properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy (5). Looking forward, AET will not only advance our ability in 3D atomic structure determination of crystal defects and disordered materials, but also transform our understanding of materials properties and functionality at the fundamental level.
References
1. J. Miao, P. Ercius and S. J. L. Billinge, Science 353, aaf2157 (2016).
2. M. C. Scott, C. C. Chen, M. Mecklenburg, C. Zhu, R. Xu, P. Ercius, U. Dahmen, B. C. Regan and J. Miao, Nature 483, 444–447 (2012).
3. C. C. Chen, C. Zhu, E. R. White, C.-Y. Chiu, M. C. Scott, B. C. Regan, L. D. Marks, Y. Huang and J. Miao, Nature 496, 74–77 (2013).
4. R. Xu, C.-C. Chen, L. Wu, M. C. Scott, W. Theis, C. Ophus, M. Bartels, Y. Yang, H. Ramezani-Dakhel, M. R. Sawaya, H. Heinz, L. D. Marks, P. Ercius and J. Miao, Nature Mater. 14, 1099-1103 (2015).
5. Y. Yang, C.-C. Chen, M. C. Scott, C. Ophus, R. Xu, A. Pryor Jr, L. Wu, F. Sun, W. Theis, J. Zhou, M. Eisenbach, P. R. C. Kent, R. F. Sabirianov, H. Zeng, P. Ercius and J. Miao, Nature 542, 75-79 (2017).
2:30 PM - TC07.05.03
Tuning the Band Structure of Graphene Nanoribbons through Defect Interaction Driven Edge Patterning
Lin Du 1 , Andre Muniz 2 , Dimitrios Maroudas 1
1 Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States, 2 Department of Chemical Engineering, Federal University of Rio Grande do Sul, Porto Alegre Brazil
Show AbstractGraphene nanoribbons (GNRs) with widths narrower than 10 nm have outstanding electronic, thermal, and mechanical properties and are considered for both front-end and back-end technologies in future generations of high-performance and low-power-consumption electronics, as well as for photovoltaic device technologies. The electronic band structure of the GNRs can be modified and their bandgap can be tuned by controlling the GNR width and the orientation of its edges. Here, we report a defect engineering strategy for tuning the band structure of GNRs based on patterning of the GNR edges. The defects of interest are nanopores, or vacancy clusters, which can be introduced into GNRs in regular arrangements by ion etching or electron irradiation. We have conducted a systematic analysis of pore-edge interactions in GNRs using atomic-scale computations of interaction energies, which have revealed strongly attractive interactions for nanopores in the vicinity of GNR edges. These attractive interactions provide the thermodynamic driving force for nanopore migration toward the GNR edge, leading to its coalescence with the GNR edge through a sequence of carbon ring reconstructions. We have studied in detail nanopore dynamics near GNR edges at high temperature based on molecular-dynamics (MD) simulations according to a reliable interatomic potential and constructed the optimal kinetic pathways of the mechanisms mediating the coalescence of the nanopore and the GNR edge using climbing-image nudged elastic band calculations. We have found that the post-coalescence morphological evolution of an armchair GNR edge leads to the formation of a V-shaped edge pattern consisting of zigzag linear segments (facets). First-principles density functional theory (DFT) calculations of the electronic structure of such patterned GNRs show that the zigzag segments formed at the armchair edges can be used to tune the GNR band structure. We find a linear dependence of the bandgap of the patterned GNRs on the linear density of the zigzag edge atoms, which is controlled by the size and concentration of the pores introduced in the defect-engineered GNR.
2:45 PM - TC07.05.04
Interaction of an Energetic Ar Molecular Cluster Beam with Graphene
Songkil Kim 1 , Anton Ievlev 1 , Jacek Jakowski 1 , Ivan Vlassiouk 1 , Matthew Burch 1 , Chance Brown 1 , Alex Belianinov 1 , Bobby Sumpter 1 , Stephen Jesse 1 , Olga Ovchinnikova 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractAbstract
Manipulation of low dimensional nanomaterials provides intriguing opportunities to design new functional materials as well as to develop next-generation device applications. To manipulate properties of low dimensional nanomaterials, extensive study has been conducted so far for interaction of energetic particles with low dimensional nanomaterials. However, most of the research has been focused on utilizing electron or light/heavy ion beams to study irradiation effects on alternation of structural, mechanical and electrical properties of nanomaterials. In this study, we investigated the effect of Argon molecular cluster beam irradiation on both defect formation and removal of organic contaminants on graphene. An Argon cluster beam was generated using the Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) combined with Atomic Force Microscopy (AFM). The ToF-SIMS allows for conducting in-situ monitoring of defect formation as well as organic contaminants removal. This leads to accomplishments of a high degree of controls over modification of graphene. A systematic study has been conducted to provide in-depth understanding about defect formation of graphene by synergistic theoretical and experimental approaches. Raman spectra clearly indicate that suspended graphene is more susceptible to Ar cluster beam irradiation than supported graphene on a SiO2/Si substrate under the same irradiation conditions. The underlying mechanisms for the experimentally observed phenomena are demonstrated by theoretical analysis using the first-principles molecular dynamics calculations.
Acknowledgement
This work was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy (DOE) Office of Science User Facility.
3:30 PM - TC07.05.05
Thermal Conductivity of Graphene with Irradiation-Induced Defects
Asanka Weerasinghe 1 , Ashwin Ramasubramaniam 2 , Dimitrios Maroudas 3
1 Department of Physics, University of Massachusetts Amherst, Amherst, Massachusetts, United States, 2 Department of Mechanical and Industrial Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States, 3 Department of Chemical Engineering, University of Massachusetts Amherst, Amherst, Massachusetts, United States
Show AbstractDefect engineering through irradiation and chemical functionalization of graphene are promising routes for fabrication of carbon nanostructures and two-dimensional metamaterials with unique properties and function. In recent computational studies, we found that, for an inserted vacancy concentration of 5-10%, a vacancy-induced crystalline-to-amorphous transition occurs in graphene accompanied by a brittle-to-ductile transition in its mechanical response as well as introduction of localized electronic states near the Fermi level. Here, we report results of a systematic analysis of thermal transport in these electron-irradiated, including irradiation-induced amorphous, graphene sheets with emphasis on the dependence of their thermal conductivity on the inserted defect density. Furthermore, we use hydrogenation as a means of studying the effects of chemical functionalization and defect passivation on the thermal transport properties of irradiated graphene. Using molecular-dynamics (MD) simulations according to a reliable bond-order potential, we prepare and equilibrate the irradiated configurations, pure and hydrogenated, and subsequently conduct non-equilibrium MD simulations at constant heat flux. While the thermal conductivity of irradiated graphene decreases precipitously from that of perfect graphene upon introducing a low vacancy concentration (less than 1%) in the graphene lattice, further decrease of the thermal conductivity with increasing vacancy concentration exhibits a weaker dependence on vacancy concentration until the amorphization threshold. Beyond the onset of amorphization, the dependence of thermal conductivity on vacancy concentration becomes significantly weaker, and the thermal conductivity of irradiated graphene practically reaches a plateau value. We find that hydrogenation does not affect the thermal conductivity of the irradiated graphene sheets if the hydrogenated C atoms remain sp2-hybridized. However, upon inducing sp3 hybridization of these C atoms with additional hydrogenation, the thermal conductivity of the irradiated sheets is reduced further as long as the irradiated structure remains crystalline. Beyond the amorphization threshold, defect passivation by hydrogenation does not have any detectable effect on the thermal conductivity of irradiated graphene. Our results indicate that the combination of defect engineering and functionalization could be used very effectively to tailor the thermomechanical properties of graphene for a range of structural and electronic applications.
3:45 PM - TC07.05.06
Atomic Manipulation via Electron Beam—Modifying Graphene
Artem Maksov 1 2 , Ondrej Dyck 2 , Stephen Jesse 2 , Bobby Sumpter 2 , Sergei Kalinin 2
1 Bredesen Center, University of Tennessee, Knoxville, Knoxville, Tennessee, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe discovery of the new compounds and materials have long remained one of the preponderant directions in materials science and condensed matter physics alike, with the two primary pathways towards this goal being experimental synthesis and theoretical prediction. Here, we explore the potential of controlled modification of matter by electron and ion beam irradiation to create the new nanoscale structures, which can then be identified via atomically-resolved imaging in-situ.
Using the model graphene layer, we explore the evolution of bonding and structural configuration for different damage mechanisms, ranging from subtractive knock-on processes to removal and redeposition of individual atoms. The structural evolution is explored via clustering and dynamic analysis of atomic configuration, establishing statistically significant transformation pathways and enabling local phase identification. We use molecular dynamics (MD) with reactive force field (ReaxFF) to generate large datasets of simulations representing potential irradiation induced damage mechanisms resulting in defects and clusters of defects, and use data mining and machine learning techniques to gain better understanding of their structural effects such as nanoscale strain fluctuations and defect diffusion. We use selected optimized areas as input to density functional theory (DFT) simulation to calculate magnetic and electronic properties. We further correlate discovered structural features with computed properties. Finally, we demonstrate correspondence to atomic resolution scanning transmission electron microscopy (STEM) experiments demonstrating patterning and assembling matter atom by atom via controlled motion of atomically defined beams and defect creation.
This research was conducted at and partially supported BS, SVK at the Center for Nanophase Materials Sciences, which is a US DOE Office of Science User Facility. Research for OD, SJ was sponsored by Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U.S. Department of Energy. AM acknowledges fellowship support from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education.
4:00 PM - *TC07.05.07
Point Defects and Anelasticity in Pure and Doped Ceria
Anatoly Frenkel 2 , Olga Kraynis 1 , Ellen Wachtel 1 , Igor Lubomirsky 1
2 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States, 1 , Weizmann Inst of Science, Rehovot Israel
Show AbstractRare-doped ceria, in general, and Gd-doped ceria (GDC), in particular, is one of the most important and well-studied oxygen ion conductors having broad technological application. During the past decade, it has been the subject of a large number of experimental and theoretical investigations. In addition to the practical need to further increase its already high ionic conductivity at moderately elevated temperatures, doped ceria also exhibits a number of unusual mechanical and electromechanical effects: dependence of the ion diffusion coefficient on mechanical strain, as well as pronounced anelasticity, room temperature creep, and non-classical electrostriction under ambient conditions. One objective of these studies has been to characterize the local lattice distortions introduced by oxygen vacancies and/or strain and their role in modulating both ion conductivity and mechanical properties, especially anelasticity and electrostriction. To this end, cation-cation and cation-anion bond lengths have been measured as a function of dopant concentration, strain and applied electric field. Comparison of the structural data (XAS and XRD) with the data on elastic and anelastic properties are the focus of the current presentation.
X-ray absorption spectroscopy (EXAFS) is the method of choice for determining bond lengths, which differ from those based on the average X-ray diffraction (XRD) structure. For GDC, the XRD/EXAFS discrepancy indicates that the local structure deviates from fluorite. Increase in Gd-content causes expansion of the lattice but it is accompanied by contraction of the Ce-O bond, pointing to increase in local distortion with Gd-content. The modulated excitation EXAFS data collected under external electric field from 10mol% GDC demonstrated the presence of Ce-O bonds that are at least 4.6% shorter than the average. An applied electric field eliminates these bonds, thereby increasing local symmetry. Recent 3D-reconstruction of the local distortions obtained with high-energy resolution fluorescence detection, indicated that the short bonds are randomly oriented and their length can vary widely (0-10%). These results have led us to suggest that the strong elastic dipoles (E > 1 eV) formed by the local distortions, can nevertheless reorient and, most unusually, change their strength. Although such behavior has not been observed previously and certainly requires further study, it can nevertheless provide a satisfactory explanation for a range of anelastic effects in GDC. However, a complete explanation for the electrostrictive behavior is still elusive.
IL and AIF acknowledge the NSF-BSF program grant 2015679. AIF acknowledges support by NSF Grant number DMR-1606840.
4:30 PM - TC07.05.08
Defect Engineering and Chemical Characterization of h-BN Using Functionalized AFM Cantilevers for Applications in Heterogeneous Catalytic Reactions
Yi Ding 1 , Laurene Tetard 1
1 , University of Central Florida, Orlando, Florida, United States
Show AbstractCO2 capture and conversion requires catalytic activation for which metal-free systems are highly sought-after. Heterogeneous catalysis is of special interest due to advantages in recyclability and because it is thought to be more environmentally friendly. Recent studies reveal that several 2D materials can be used as catalysts due to defects. In layered hexagonal boron nitride (h-BN), defects such as vacancies and edges are thought to act as the active sites.
Here we introduce defects by heat treatment of h-BN. We show that defects induce changes in structural and electrical properties of the h-BN layers. In addition, we study the nature of the interaction between the defect and selected molecules of interest in the CO2 reaction using force spectroscopy with a chemically functionalized atomic force microscopy (AFM) cantilever tip. We discuss how the binding properties change with different molecule-defect systems. Our results highlight how functional AFM can be exploited to determine catalytic activity of defected materials as with defect laden h-BN for CO2 capture and production of higher alcohols.
4:45 PM - TC07.05.09
ZnO-Au Nanowire Based Chemical Sensors under Mixing Oxidative Gas Stream—A Mechanistic Study of Gas Detection Based on Multiple Sensing Modes
Bo Zhang 1 , Pu-Xian Gao 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractThe adsorption of oxidative gas NO2 and O2 on metal oxide semiconductor surface has been extensively studied and tremendous sensing device towards trace amount gas detection has also been developed. The dissociative adsorption is energetically preferred at the surface oxygen vacancies while one of the resulting oxygen atom filling the oxygen vacancies.1 Different from single gas detection and calibration, the reaction of mixing gas (NO2 and O2) and ZnO-Au hybrids scenario is studied in this work. Using solution phase deposition methods, heterostructured hybrid ZnO-Au nanowires have been successfully synthesized. The adhesion of Au and the resulted formation of structural defects greatly enhances the efficiency of gas dissociative chemisorption.2 The reversible nitrate specie formation is proved by the XPS spectra while the amount of detected nitrate is clearly dependent on the Au content. In high temperature, the formation of nitrite could also be found at the trace of migrated Au nanoparticles through the High Resolution TEM characterization. In addition, the response is further enhanced when exposed to mixing gas. The obtained sensitivity may be amplified towards two- component gases in resistance mode. Utilizing the impedance mode, selective detection based on the integration of phase angle, impedance frequency spectra could also be achieved on a single-device platform.
Symposium Organizers
Hua Zhou, Argonne National Laboratory
Panchapakesan Ganesh, Oak Ridge National Laboratory
Anna Kimmel, University College London
Dong Su, Brookhaven National Laboratory
Symposium Support
Argonne National Laboratory, Advanced Photon Source
TC07.06: Interrogating Evolving Defects in Energy Materials by Multimodality Imaging
Session Chairs
Wednesday AM, November 29, 2017
Hynes, Level 2, Room 207
8:00 AM - *TC07.06.01
Atomic Structures, Chemistry and Vacancies of Grain Boundaries and Surfaces in Functional Energy Materials
Yuichi Ikuhara 1 2 3
1 , University of Tokyo, Tokyo Japan, 2 , Japan Fine Ceramics Center, Nagoya Japan, 3 WPI, Tohoku University, Sendai Japan
Show AbstractGrain boundary (GB) and surface atomic structures are influenced by the segregated dopants and vacancies, and it is thus needed to investigate the sites of vacancies and dopants segregated at GBs and surfaces, which are related to the material’s properties[1]. In this study, GBs and surfaces of functional oxide energy materials such as Yttria-stabilized ZrO2 (YSZ) and LixFePO4 olivine were systematically invetigated by by combining aberration-corrected STEM, ABF, EELS, EDS and first princples calculations. STEM observations were mainly performed using ARM-200F (200kV, JEOL) equipped with CEOS Cs-corrector and Grand ARM-300F(300kV, JEOL). For theoretical approach, static lattice and density functional theory (DFT) calculations were used to understand the GB and surface reconstruction behavior.
GBs of YSZ were characterized to understand ytrrium and vacancy segregation behavior. The ionic conductivity of YSZ are limited by the presence of GBs, which is at least two orders of magnitude lower than that of bulk. So far Y segregation in the GBs has been intensively studied, but the local oxygen vacancy and Y distributions across GBs are still unclear. In this study, the oxygen, vacancy and Y distributions were characterized for several types of YSZ GBs fabricated by the bicrystal method [2]. Considering the observation results and the dynamical scattering simulation, it was found that the oxygen concentrations increase in all the GBs. These results indicate local point defect distribution in GBs is mainly governed by long-range electric interactions. It was also found that the orderd segregation structures are formed for several CSL(Coincidence Site Lattice) GBs.
The properties of lithium ion battery (LIB) cathodes strongly depend on the diffusion of lithium ions during charge/discharge process. Since this behavior determines the stability, lifetime and reliability, direct visualization of lithium site is required to understand the mechanism of the diffusion of lithium ions. In this study, ABF STEM were applied to directly observe the (010) surface of the olivine LixFePO4 and delithiated olivine (FePO4) [3], and the mechanism of the lithiation/delithiation was discussed based on the observation results. It was found that P and Fe atom columns undergo comparatively large displacements near the surface, which was consistent with the results predicted from first-principles calculations. The magnitudes of the P and Fe displacements were also found to depend on the location of the outmost Li sites.
Acknowledgements
A part of this work was supported by the Research & Development Initiative for Scientific Innovation of New Generation Batteries II (RISING II) from NEDO.
References
[1] Y. Ikuhara, . J.Elect. Microsc., 60, S173-S188 (2011) (Review)
[2] B.Feng, T.Yokoi, A.Kumamoto, M.Yoshiya, Y.Ikuhara and N.Shibata, Nat. Commun., 7, 11079 (2016)
[3] S. Kobayashi, C.A. J. Fisher, T.Kato, Y.Ukyo, T.Hirayama and Y.Ikuhara, Nano Lett. 16 (2016) 5409
8:30 AM - TC07.06.02
Multi-Scale Characterization of Mechanical Properties of Thermoelectric Alloys
Matthieu Aumand 1 2 , Guillaume Amiard 2 , Ran He 1 , Zhifeng Ren 1 , Ludovic Thilly 2 , Ken White 1
1 , University of Houston, Houston, Texas, United States, 2 , University of Poitiers, Poitiers France
Show AbstractIncreasing the figure of merit, ZT, of thermoelectric alloys is a challenge that is currently attempted through various metallurgy methods, including nanostructuring and dislocation engineering. However, such techniques raise questions about the mechanical reliability of these new alloys, and the deformation mechanisms are yet to be known. Focusing on the p-type Half-Heusler Hf0.44Zr0.44Ti0.12CoSb0.8Sn0.2, we present here a series of experiments completing each other and conducted at two scales in order to assess the deformation mechanisms of this alloy.
1) Macroscopic scale: the alloy is tested in compression with the Paterson machine, making use of a gaseous confining pressure of 400 MPa, preventing crack propagation during compression testing. The failure mechanisms are studied trough SEM, unveiling specific properties of such alloy. In order to further understand these mechanisms, small scale testing becomes necessary through local and controlled deformation.
2) Microscopic scale: fracture mechanics is studied with a controlled crack insertion via nanoindentation. The crack tip regions are extracted by FIB to perform TEM observations for plastic zone investigation, and the volume containing a crack is analyzed by 3D EBSD, a tomographic reconstruction, analyzing crack path with respect to the crystallographic environment. After preliminary EBSD maps, micropillars (single- or polycrystalline) are carved by FIB and compressed via nanoindentation. The single crystalline pillars are compressed in order to enforce plasticity by dislocation activity, and the polycrystalline pillars aim at understanding the grain cohesion forces.
8:45 AM - TC07.06.03
Elucidating the Grain Boundary Conductivity Distribution by Correlating Local Composition and Character
William Bowman 2 1 , Amith Darbal 3 , Peter Crozier 2
2 Materials Science and Engineering, Arizona State University, Tempe, Arizona, United States, 1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 , AppFive LLC, Tempe, Arizona, United States
Show AbstractHigh ionic conductivity is desired to optimize electrolyte performance, though it is significantly degraded by grain boundaries (GBs), which act as blocking layers in polycrystalline electrolytes. Given the rich diversity in GB types, and the complex interplay between structure, composition, and chemistry at the atomic and nanoscale [1,2], there is considerable opportunity to elucidate fundamental science and performance optimization of GBs. Hence, studies should rely on GB datasets correlated across many length scales, with the aim of generalizing high spatial resolution observations to an entire GB population. This should facilitate bottom-up design of GBs with optimized properties, which remains a considerable challenge. By combining suitable modeling approaches with experimental measurements interrogating materials over different length scales, it becomes possible to estimate the electrical properties of individual GBs.
Here a novel correlated approach is employed combining precession-enhanced nanodiffraction (PEND) orientation imaging and electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning TEM to elucidate the GB transport properties in a previously reported (Gd,Pr)xCe1-xO2-δ solid solution [1]. GB transport properties are interpreted based on nanoscale EELS characterization of solute segregation, which are generalized to the entire GB population based on GB character determined using PEND orientation imaging. Experimental data were combined with a thermodynamic modeling framework developed for concentrated ceria solutions [3]. The parameters in the model were adjusted to match the experimentally-determined cation distribution, and the model then allowed the oxygen vacancy distribution of GB population to be determined. By combining the oxygen vacancy distribution with a solute composition-dependent migration enthalpy, it was possible to estimate the conductivity variation within the GB population.
Acknowledgements
W.J.B. acknowledges the NSF’s Graduate Research Fellowship (DGE-1211230) for financial support. P.A.C. and W.J.B. acknowledge support of NSF grant DMR-1308085. The authors acknowledge access to ASU’s John M. Cowley Center for High Resolution Electron Microscopy.
References
[1] W.J. Bowman, J. Zhu, R. Sharma, P.A. Crozier. Solid State Ion. 272 (2015) 9-17.
[2] W.J. Bowman, M.N. Kelly, G.S. Rohrer, C.A. Hernandez, P.A. Crozier. Nanoscale. (In review).
[3] D.S. Mebane, R.A. De Souza, Energy Environ. Sci. 8 (2015) 2935-2940.
9:00 AM - TC07.06.04
Identification of Intrinsic Defects of BiI3 for Photoelectronic Applications
Sung Beom Cho 1 2 , Yoon Myung 1 3 , Jaume Gazquez 4 , Parag Banerjee 1 2 , Rohan Mishra 1 2
1 Mechanical Engineering and Materials Science, Washington University in St. Louis, Saint Louis, Missouri, United States, 2 Institute of Materials Science and Engineering, Washington University in St. Louis, St. Louis, Missouri, United States, 3 Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul Korea (the Republic of), 4 , Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Catalonia Spain
Show AbstractBiI3 is a layered-structure semiconductor with a moderate bandgap of 1.7 eV and has high electron-hole pair generation efficiency.1,2 Historically, it has been intensively investigated for X-ray and gamma-ray detector. More recently, it has gained attention as a promising photovoltaic material. However, all the above applications require a thorough understanding of the atomic-scale structure of the material, including defects, and their effect on the electronic properties, which has been lacking. We have used a combination of density functional theory (DFT) calculations and aberration-corrected scanning transmission electron microscopy (STEM) to identify stable point defects and stacking faults in BiI3 films and their effect on its functionalities. Based on DFT calculations, we find intrinsic defects, such as both Bi and I interstitials and vacancies have sufficiently low formation energy to be present under ambient conditions. While these dominant intrinsic point defects show deep thermodynamic transition levels, they do not lead to mid-gap states that can act as optical recombination centers. As a result, we find intrinsic defects in BiI3 act as centers to trap thermally excited carriers. We have also calculated the effect of intrinsic doping using the calculated formation energies. Though the deep-level defects do not affect the optical properties of BiI3, they act as compensating defects that pin the Fermi-level.
From STEM Z-contrast imaging, we observe plenty of stacking faults in the BiI3 thin films. We have identified the atomic configuration of these stacking faults using a combination of STEM images and the total energy of promising models calculated using DFT. We find the stacking faults are triggered by Bi bulk-like line defects that can be formed by the alignment of Bi interstitials and Bi antisites. These line defects have a formation energy of 0.24 eV/nm and are observed to span a length between 3-8 nm. This Bi bulk-like line defect separates one I layer, then triggers stacking faults. Also, interestingly, this line defect show metallic character according to DFT calculation. We will discuss the implications of these defects on the optical properties including the excitonic states. Our combined study intrinsic defects on BiI3 provides guidance to control the defect structure for improved photodetector and photovoltaic applications by changing the chemical potential.
1. N.J. Podraza, W. Qiu, B.B. Hinojosa, M.A. Motyka, S.R. Philpot, J.E. Baciak, S. Trolier-Mckinstry, and J.C. Nino: Journal of Applied Physics 114, 033110 (2013).
2. A. Beer, R.K. Willardson, and E. Weber, Semiconductors for Room Temperature Nuclear Detector Applications, Academic Press (1995)
9:15 AM - TC07.06.05
Oxygen Vacancy Motions at a Fluorite-Bixbyite Interface
Xiang Gao 1 , Dongkyu Lee 1 , Ho Nyung Lee 1 , Matthew Chisholm 1
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractIn the pursuit of fast ion conductors, rapidly growing attention is being directed to an emerging facet of heterostructures, which enable faster ion conduction than either of the parent counterparts. However, understanding the origin of enhanced ion conduction in heterostructures is still in debate. Various scenarios have been proposed, including epitaxial strain, structural ordering, space-charge effects, and interface reconstruction. In particular, due to the lack of direct information, whether and how the heterointerface can contribute to enhanced ion conduction in heterostructures are still far from being fully understood. Here we report the direct evidence of remarkably enhanced interfacial oxygen ion conduction in a novel fluorite-bixbyite (CeO2-Y2O3) superlattice nanobrush, studied by state-of-the-art scanning transmission electron microscopy (STEM). Our results show the formation of thin interfacial layers in the superlattices that are subject to considerable oxygen loss and lattice expansion. Such an interface layer is found to serve as an excellent ionic conduction pathway. STEM imaging found also that the oxygen atoms neighboring the interfacial layer were able to migrate rapidly through the interface. Furthermore, we will discuss the reason for the formation of oxygen vacancies in the nanobrushes, and, show the ability to manipulate the oxygen vacancy movements in this material.
*This work was supported by the U.S. Department of Energy, Office of science, Basic Energy Sciences, Materials Sciences and Engineering Division.
10:00 AM - *TC07.06.06
Nanoscale Functional Imaging of Materials for Energy Applications
Marina Leite 1
1 , University of Maryland, College Park, College Park, Maryland, United States
Show AbstractThe further development of energy harvesting and storage systems requires mapping the role of defects in device performance. In particular, the understanding of the physical and chemical processes that take place at the mesoscale constructs composing solar cells is critical for the advancing the state-of-knowledge of non epitaxial materials that can be implemented as high-efficiency and low-cost photovoltaics. We realize and apply a functional imaging platform to resolve, with nanoscale spatial resolution, the local electrical response (performance) of perovskite and polycrystalline solar cells. To temporally resolve the local electrical response of perovskites we implement a 4D microscopy based on illuminated Kelvin probe force microscopy (KPFM). We measure nanoscale variations in open-circuit voltage (Voc) > 300 mV, not revealed by conventional microscopy tools. Using fast-KPFM (16 seconds/scan) while maintaining high spatial sensitivity, we map the real-time dynamics of the Voc with nanoscale spatial resolution. We find a post-illumination ‘residual Voc’, resulting from ion migration. Our results show, for the first time, the real-time and nanoscale transient behavior of the Voc in perovskites. Concerning polycrystalline materials, we map spatial variations in Voc >20% in CIGS solar cells. We further implement a correlative microscopy to establish the relationship between the material electrical, structural and chemical properties. Our functional imaging can be extended to probe the stability of other energy harvesting systems, ranging from lead-free perovskites to photoelectrochemical cells.
10:30 AM - TC07.06.07
Probing Polar Domains across Phase Transitions in Ferroelectric-Like Metals Using Nonlinear Optical Imaging
Haricharan Padmanabhan 1 , Shiming Lei 1 , Zhiqiang Mao 3 , Jak Chakhalian 2 , Venkatraman Gopalan 1
1 , The Pennsylvania State University, State College, Pennsylvania, United States, 3 , Tulane University, New Orleans, Louisiana, United States, 2 , University of Arkansas–Fayetteville, Fayetteville, Arkansas, United States
Show AbstractPolar metals are a rare class of materials that exhibit polar order, despite the presence of free charges that typically screen long-range electrostatic forces. These materials, by virtue of their broken inversion symmetry, exhibit a finite second harmonic response to incident electromagnetic radiation. This gives nonlinear optical imaging techniques such as second harmonic generation (SHG) microscopy the unique ability to probe polar mesoscale structures and defects in these materials, which may be inaccessible using traditional techniques such as piezoforce microscopy, due to their metallic nature.
In this work, a low-temperature scanning confocal SHG microscopy tool developed in-house is used to probe the mesoscale structure of the ferroelectric-like metals Ca3Ru2O7 and LiOsO3. It is observed that Ca3Ru2O7 exhibits a rich ferroelectric-like domain structure; a feature that until now, has not been observed in a metal. Temperature-dependent imaging across the metal-insulator transition1 of Ca3Ru2O7 at 48 K and the polar to non-polar transition2 of LiOsO3 at 140 K will be used to study the evolution of the domain structure at these critical points, allowing an insight into the physics underlying the polar-metallicity of these materials. Preliminary temperature-dependent SHG measurements clearly identify the nature of the aforementioned phase transitions as first-order and second-order respectively. Finally, polarization-dependent SHG measurements reveal that both these materials exhibit a large anisotropic second-order optical susceptibility an order of magnitude greater than comparable classical ferroelectrics. This, in combination with their electrically conducting nature, is an enticing prospect for future optoelectronic applications.
1. Cao, G., McCall, S., Crow, J. & Guertin, R. Observation of a Metallic Antiferromagnetic Phase and Metal to Nonmetal Transition in Ca3Ru2O7. Phys. Rev. Lett. 78, 1751–1754 (1997).
2. Shi, Y. et al. A ferroelectric-like structural transition in a metal. Nat. Mater. 12, 1024–1027 (2013).
10:45 AM - TC07.06.08
Probing Localized Strain Induced by Extended and Point Defects in Solution-Derived YBa2Cu3O7-δ Nanocomposite Films
Roger Guzman 1 , Jaume Gazquez 1 , Bernat Mundet 1 , Laia Soler 1 , Júlia Jareño 1 , Juri Banchewski 1 , Mariona Coll 1 , Susagna Ricart 1 , Xavier Obradors 1 , Teresa Puig 1
1 , ICMAB-CSIC, Barcelona Spain
Show AbstractThe investigation of the atomic structure of individual defects is critical to the understanding and precise controlling of the physical properties of materials. And although defects are sometimes detrimental to functionality, in high temperature superconductors (HTS) are necessary for providing pinning of magnetic flux and allowing high currents to be carried. Moreover, a strong enhancement on the vortex pinning in HTS YBa2Cu3O7-δ (YBCO) films is also found to be controlled by nanostrain [1], which is attributed to elastic distortions of the crystal lattice at the nanoscale level.
Using aberration-corrected scanning transmission electron microscopy (STEM), we explore the atomic structure of individual defects of solution-derived YBa2Cu3O7 (YBCO) nanocomposites, where the inclusion of incoherent secondary phase nanoparticles within the YBCO matrix dramatically increases the density of Y1Ba2Cu4O7 (Y124) intergrowths, the commonest defect in YBCO thin films. Combining High Angle Annular Dark Field (HADDF) with Low Angle Annular Dark Field (LAADF) imaging and local strain analyses we are able to map and quantify the lattice deformations associated to the defects. The formation of the Y124 is found to trigger a concatenation of strain-derived interactions with other (extended) defects and the concomitant nucleation of intrinsic (point) defects, which weave a web of randomly distributed nanostrained regions that profoundly transform the vortex-pinning landscape of the YBCO nanocomposite thin films [2].
Finally, we will compare the microstructure of conventional high-quality solution-derived trifluoroacetate-YBCO nanocomposites with new fluorine-free films based on a novel transient-liquid assisted growth method (TLAG). Due to the particular growth characteristics of TLAG, this new route provides ultra-high growth rates having a strong influence on the type and density of the defects observed in the films, and thus on the resulting pinning landscape. Accordingly, TLAG envisages an enormous potential for low-cost and high-performance coated conductors.
Authors acknowledge financial support from Spanish Ministry of Economy and Competitiveness through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496), EU-FP7 NMP-LA-2012-280432 EUROTAPES, ERC-AdG-2014-669504 ULTRASUPERTAPE and MINECO MAT2014-51778-C2-1-R.
References
[1] Llordés et al., Nanoscale strain-induced pair suppression as a vortex-pinning mechanism in high-temperature superconductors. Nature Materials, 11, 329 (2012).
[2] Guzman et al., Probing localized strain in solution-derived YBa2Cu3O7-δ nanocomposite thin films. Physical Review Materials 1, 024801 (2017).
11:00 AM - TC07.06.09
Atomic-Scale STEM Study of Lattice Arrangements and Structural Disorder in LiVO2
Qiang Zheng 1 , Miaofang Chi 2 , Wei Tian 3 , Jiaqiang Yan 1 , Brian Sales 1
1 , Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 , Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractDue to interesting structural and magnetic behaviors [1, 2] and high-capacity as electrodes [3], LiVO2 has attracted much attention for several decades. LiVO2 crystallizes in an ordered rocksalt structure with space group R-3m, and it undergoes a first-order magnetic transition at Tt ~500 K, changing from a high-temperature (HT) paramagnetic phase with Curie–Weiss susceptibility to a low-temperature (LT) nonmagnetic phase without long-range magnetic order. Although several competing explanations have been proposed for this transition, exact crystal structure and arrangements of Li, V and O triangular lattices below and above Tt are quite lacking, hindering better understanding of this transition.
In this work, we use aberration-corrected scanning transmission electron microscopy (STEM) to study small structure change and evolution of V triangular lattices in LiVO2. HAADF-STEM imaging along several important crystallographic directions shows V clustering in each layer. Complex stacking behavior between V layers is reflected by the appearance of pronounced diffuse rods of the superstructure reflections in FFT patterns. We then combine neutron diffuse scattering and HAADF imaging to study the evolution of V clustering and structural disorder below and above Tt. ABF-STEM and EELS are also used to reveal lattice arrangements of two light elements Li and O, and V L2,3 edge and O K edge to show the local electron behaviors around V, respectively. Finally, we will discuss the influence of lattice arrangements and structural disorder on the magnetic-to-nonmagnetic transition of LiVO2.
[1] J.B. Goodenough, Phys. Rev. 120, 67 (1960).
[2] H.F. Pen, et al., Phys. Rev. Lett. 78 ,1323 (1997).
[3] M. Sathiya, et al., Nat. Mater. 14, 230 (2015).
11:30 AM - TC07.06.11
Reading out Molecular States—Machine Learning Approach for Analysis of Molecular Assemblies on Surfaces
Artem Maksov 1 2 , Maxim Ziatdinov 2 , Sergei Kalinin 2
1 Bredesen Center, University of Tennessee, Knoxville, Knoxville, Tennessee, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractRecent advances in scanning probe techniques such as scanning tunneling microscopy (STM) have allowed us to study structural and functional parameters of materials with a picometer precision. While the long-range symmetry properties discovered through scattering techniques can give us information about averaged compositions, order parameters, and thermal and phonon properties, the knowledge of local structure and disorder can lead to discovery of novel functionalities not predicted by averaged models. Therefore, it is crucial to be able to accurately identify individual structural units such as atoms or molecules on the scale of hundreds or thousands, as well as more complex patterns they form. This necessitates creation of reliable and reproducible methods for automatic data processing, structure – property relationship mining, and physics extraction. We employ machine learning algorithms to analysis of ultra-high vacuum STM images of self-assembled monolayer of p-bowl sumanene molecules on gold. In particular, we combine pattern recognition, Markov random field (MRF) model, and convolutional neural networks (cNN) in a framework that allows to identify individual molecules within the STM image, and to classify their structural and rotational states based on information inferred from density functional theory simulations. We represent the recognized self-assembled structure as a graph with nodes corresponding to molecules and edges corresponding to nearest neighbor connectivity. We use observed intensity and neighborhood information to infer structural states using MRF, and we use machine vision through cNN combined with MRF refinement to infer rotational states. Obtained full decoding allows us to construct pair density functions to analyze disorder-property relationships. We demonstrate effectiveness of the framework on synthetic images, and further apply it to experiment to study the spatial mapping of correlation between identified order parameters, and to achieve better understanding the reaction pathway of conformational changes.
This research was conducted at and partially supported SVK, MZ at the Center for Nanophase Materials Sciences, which is a US DOE Office of Science User Facility. A.M. acknowledges fellowship support from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education.
11:45 AM - TC07.06.12
Assessing Defects and Failure Mechanisms in Highly Doped Epitaxially Encapsulated Micro-Scale Sensors Using Micro and Nano X-Ray Computer Tomography
Lizmarie Comenencia Ortiz 1 , David Heinz 1 , Ian Flader 1 , Yunhan Chen 1 , Thomas Kenny 1
1 Mechanical Engineering, Stanford University, Stanford, California, United States
Show AbstractMicro Electro Mechanical Systems (MEMS) are micro-scale structures with mechanical and electrical components manufactured with similar materials and processes as semiconductor circuits for applications as high performance inertial sensors, timing references, and others. Epitaxial encapsulation is a unique packaging technology for MEMS, where pure silicon structures are packaged free of contamination that is crucial for high performance and stability. The devices are manufactured at the Stanford Nanofabrication Facility with features from tens of nanometers to hundreds of microns, and are monolithically encapsulated with silicon and silicon dioxide at the wafer level. This process imposes a susceptibility of the surfaces to fatal adhesion (stiction), and difficulties in examining surfaces after encapsulation.
The surface features of epi-sealed encapsulated MEMS remain a mystery after the devices are sealed. The low transmission of highly doped silicon to IR radiation precludes the use of typical imaging and characterization tools. In this paper, we discuss the use of Micro and Nano X-ray Computed Tomography (XCT) to study the precise device topology and assess the defects in the fabrication ultra-clean epi-sealed sensors without destructive intrusion into the cavity.
Former XCT studies have been limited to devices with features above the millimeter scale and non-encapsulated MEMS. A large gap remains in studying the topology of highly doped encapsulated micro-scale sensors. Assessing defects and failure modes is vital to understanding their behavior for continued development.
Using a Zeiss X-Radia Micro XCT instrument and a Nano XCT at the Stanford Linear Accelerator Center National Laboratory, we studied devices from variations of our fabrication process, and devices before and after stiction tests to assess defects in the structures and failure. We analyzed dozens of structures including accelerometers, gyroscopes, and other sensors with resolution from 30nm to 0.7um. We reconstructed three-dimensional images, and used a systematic method to analyze each device layer to study and find defects to predict failure modes. The results showed that Micro XCT was effective to analyze the basic structure of the device, and assess stiction in the larger structures. The Nano XCT results showed the detailed topology of structural sections and revealed small defects in the features including surface roughness, silicon migration, and overetch. Our findings allowed us to map failures and defects to specific steps in the fabrication process, such as an accelerometer mass adhered to the substrate that was traced to a step prior to the cavity seal.
A combination of Micro and Nano XCT studies of ultra-clean encapsulated MEMS sensors provides a unique insight into the as-fabricated device features and into material transfer and roughening/smoothing processes that effect stiction, in addition to other characteristics that determine device performance.
TC07.07: Engineering and Control of Functional Defects in Energy Materials
Session Chairs
Zhenxing Feng
Talia Gershon
Wednesday PM, November 29, 2017
Hynes, Level 2, Room 207
1:30 PM - *TC07.07.01
Design of Chemically Stable Proton Conducting Ceramic Materials for Solid Oxide Cells
Enrico Traversa 1
1 , School of Energy Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu, Sichuan China
Show AbstractHigh temperature proton conductor (HTPC) oxides are promising electrolytes to reduce the solid oxide fuel cell (SOFC) working temperature at 600°C or below, due to their lower activation energy for proton conduction (0.3-0.6 eV), with respect to oxygen-ion conducting electrolytes. Y-doped barium zirconate (BZY) owns thermodynamic stability against CO2 and H2O reaction and high bulk conductivity, but refractory nature and low conductivity values for sintered pellets due to the presence of blocking grain boundaries. We have recently made significant progresses in the development of chemically-stable HTPC electrolytes for SOFCs by improving the BZY sinterability, obtaining a chemically stable, sinterable electrolyte that showed a conductivity of 0.01 S/cm at 600°C. Moreover, we developed tailored cathode materials with low overpotential, following a rational approach considering concomitant electron, proton and oxygen-ion conductivities, given the different species involved in the cathode reactions. Nonetheless, the best-obtained power output was deceivingly low. In this presentation, we report the results obtained in using impregnation as a strategy to tailor the microstructure of cathode materials at the nanometer level, to increase the triple phase boundary and thus the SOFC performance. The recent systematic investigation of the defect design through varying the dopant in barium zirconate conductivity and microstructure will be also reported.
2:00 PM - TC07.07.02
The Impact of Strain State on the Oxygen Defect Chemistry and Catalytic Activity for Ruddlesden-Popper Phases Oxides
Fei Li 1 , Huijun Chen 1 , Yapeng Zhang 1 , Jiao Li 1 , Chenghao Yang 1 , Jiang Liu 1 , Meilin Liu 2 , Yan Chen 1
1 , South China University of Technology, Guangzhou China, 2 , Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractPerovskite-based oxides have been widely studied as the electrode materials for Solid Oxide Fuel Cell,Solid Oxide Electrolysis Cell and other low temperature electrochemical energy devices such as alkaline membrane fuel cells and metal-air batteries. Ionic defect, particularly oxygen defect, was known to play an essential role in determining the electronic structure, ionic conductivity and surface catalytic activity of these oxides. Furthermore, the change of oxygen defect states can drive A site or B site cations toward surface, leading to surface cation segregation. Manipulating oxygen defect chemistry of perovskite-based oxides, therefore, put forward a feasible way to achieve high performance electrode materials. Ruddlesden-Popper (RP) phases oxides have attracted great attention due their capability to accommodate both oxygen vacancy and oxygen interstitial as the dominate defects, and the anisotropic ionic diffusion and surface exchange characteristics. In this work, we explored the possibility of tuning the defect chemistry and catalytic activity of Ruddlesden-Popper phase oxides by controlling the lattice strain.
Nickel based RP phases oxides, K2NiO4 (K=La, Nd, Pr), thin films on single crystal yttrium-doped zirconia (YSZ) substrates grown by pulsed laser deposition were used as the model systems. The strain states of thin films were tuned by using YSZ substrate with different orientation, including (110), (001) and (111). The oxygen non-stoichiometry of the thin films with different strain states were characterized using high resolution X-ray diffraction and X-ray photoelectron spectroscopy. The defects state, surface composition and electrochemical activity as the anode of Solid oxide Fuel Cell were found to closely depend on the strain state of thin films. Our results can be helpful for the rational design of better electrode materials for various applications.
2:15 PM - TC07.07.03
In Situ Control of Oxygen Defect Type and Concentration in Layered Cuprate-Based Thin Films by Electrical Bias
Chang Sub Kim 1 , Harry Tuller 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe types and concentrations of defects present in oxide materials can have a significant impact on their properties as well as interfacial reaction kinetics such as oxygen exchange with the atmosphere. Two most commonly used methods to achieve a desired oxygen defect concentration, or equivalently oxygen nonstoichiometry, are through 1) doping with donors and/or acceptors, and 2) controlling the oxygen partial pressure and temperature in which they are equilibrated or annealed. These approaches, however, are limited by solubility limits of dopants, the range of oxygen partial pressures readily experimentally achievable, and requires knowledge of the applicable defect chemical model. In this study, we control oxygen defect types and concentrations in promising rare earth cuprate (RE2CuO4: RE = rare earth) SOFC cathode materials by applied electrical potential across an yttria-stabilized zirconia supporting electrolyte. These layered perovskites can incorporate both oxygen interstitials and vacancies thereby broadening the range of investigations. Oxygen nonstoichiometry values are determined by in-situ measurement of chemical capacitance and are correlated with surface kinetics and electronic properties.
3:30 PM - TC07.07.04
Enhanced Tungsten Oxide Photoelectrochemical Charge Transfer and Stability by Hierarchically Structure and Oxygen Deficience
Peng Chen 1 , Matthew Baldwin 1 , Prab Bandaru 1
1 , University of California, San Diego, La Jolla, California, United States
Show AbstractThe role of non-stoichiometry in a hierarchically structured WO3-x electrode, constituted from nanoscale fuzziness as well as microscale wire morphology, on the photoelectrochemical response is investigated. The particular role of the metal oxidation state and related non-stoichiometry is significantly enhancing the photoelectrochemical (PEC) response in the oxidation of water through the oxygen evolution reaction (OER). Here, we consider tungsten oxide as an example. we also consider the role of a unique fuzzy surface coupled with micro-wire morphology specifically induced through helium plasma bombardment of metallic tungsten substrates, in promoting such superior photoresponse. We studied the role of non-stoichiometry in WO3-x by varying x through diverse thermal oxidation. It was concluded, based on detailed structural and electrochemical characterization, that there is an optimal value of x for enhanced photocurrents. Through such optimization, we observed a large photocurrent density that was relatively stable over more than 110 hours – and is among the highest reported values to date for tungsten oxides.
Furthermore, we interpret such results through equivalent circuit models considering the charge transfer resistance that influences the current density as well as capacitance that monitors the extent of charge storage and the relative roles of oxygen vacancies and hole carriers responsible for the photoelectrochemical character. It has been well recognized that enhancing OER efficiency could usher in a new paradigm in the harness of solar radiation and energy utilization. Our experiments and related analyses offer several ideas and principles to overcome the challenges to increasing such efficiency. Our findings, in addition to marking new scientific territory related to defect engineering would enable crucial insights into electrode design for electrochemical systems and solar energy harness.
Through x-ray photoelectron spectroscopy (XPS) studies, the relative amounts of the various oxidation states of the constituent W are probed with respect to the observed response. It is concluded that an intermediate/optimal number of vacancies, yielding a W6+/ (W5+ + W4+) ratio of around 2, would be beneficial for increasing the photocurrent. It is posited that defect engineering combined with optimized band structure modulation could be used for enhanced photocurrent density as well as electrode stability. The work would help considerably elucidate the role of defects as well as charge carriers for oxygen evolution reaction (OER) efficiency increase.
3:45 PM - TC07.07.05
Construction of Atomic Scale Defect-Rich Semiconductors for Photocatalytic Reduction of N2 or CO2 under Ambient Condition
Songmei Sun 1
1 International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka Japan
Show Abstract
Photocatalytic solar fuels production represents one of the most potential strategy for replacing the dying-up fossil feedstocks and dealing with the environmental problems caused by the combustion of fossil fuels. However, this technology has not been widely applied because of its low solar energy conversion efficiency which is usually limited by the microstructure and the electronic structure of the semiconductor photocatalysts. After more than 10 years studies on this area, we found well designed defect-rich atomic scale materials have peculiar advantageous as high performance photocatalysts for N2 or CO2 reduction under ambient conditions. Here we systematically present a discussion in detail on the relationship between the crystal microstructure, the electronic structure and the photocatalytic performance which is apparently affected by the light response, charge carrier separation/migration, and reactants activation etc. The material is focused on atomic scale bismuth, tungsten, and molybdenum based semiconductor material. Their facile synthesis process will also be introduced.
4:00 PM - *TC07.07.06
Oxygen Ion Conduction and Surface Exchange Tuned by Rare Earth Dopant Cations in Ceria Epitaxial Thin Films
Carmela Aruta 1
1 , National Research Council, Rome Italy
Show AbstractFunctional properties caused by mobile ions, such as ionic conductivity and surface reaction, are of great importance for a wide range of applications, such as catalysts, gas sensors, memristors, electrochemical energy storage/conversion systems, and microsolid oxide fuel cells. New possibilities in thin film fabrication allow growth of oxide thin films with a more precise control of the structure and chemical stoichiometry, thus unveiling new perspectives in the study of technologically important properties of oxide materials. While this approach is quite established in the field of nanoelectronics, has been more recently adopted also to study ion conducting materials, raising the question of whether by using epitaxial thin films the functionalities based on mobile oxygen ions can be properly tuned. In this context, doped ceria is widely investigated for the intricate interrelationship between microstructure and chemical substitution defects affecting the transport and catalytic properties. We will discuss the results obtained on the epitaxial doped ceria films by complementary state-of-art techniques, in particular by x-ray photoemission and absorption measurements by synchrotron radiation on in-situ UHV transferred samples. We show how the amount of doping and the different ion radius size of the rare-earth dopants affect the ion conductivity and the activation of the oxygen exchange surface reaction in a non-trivial manner.[1,2]
References
[1] Nan Yang et al. ACS Appl. Mater. Interfaces 8, 14613 (2016)
[2] Nan Yang et al. J. Phys. Chem. C 121, 8841 (2017)
4:30 PM - TC07.07.07
Elastic Properties of Thin Films of Eu-Doped Ceria via Raman Scattering, 3D Optical Profilometry and X-Ray Diffraction
Olga Kraynis 1 , Evgeniy Makagon 1 , Igor Lubomirsky 1 , Tsachi Livneh 2
1 Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel, 2 Physics, Nuclear Research Center Negev, Beer Sheva Israel
Show AbstractDoped ceria, an extensively studied ionic conductor, displays elastic anomalies which make its incorporation into practical thin-film based devices (MEMS, SOFC, oxygen sensors) technically challenging. In such devices, the films can experience large tensile or compressive strain, either retained from deposition processes or developed during heterogeneous layer stacking. Deviation from linear elasticity, combined with this anisotropic strain, produces variability in film mechanical behavior with respect to temporal response and strain magnitude. Understanding these effects is a precondition for practical application of doped ceria films.
To this end, we designed a strain-relaxation process in which a 10 mol% Eu-doped ceria (10EDC) film, prepared by RF sputtering with residual in-plane compressive strain <1%, is moved to the “relaxed” state by substrate removal. The resulting membrane undergoes buckling, indicating strain relaxation. To evaluate the relevant Poisson’s ratio, we determined the degree of strain relaxation by performing high resolution, 3D mapping of the membrane profile and calculating its surface area with an in-house program based on a triangulation algorithm. In parallel, the lattice constants for the strained film state were determined by X-ray diffraction (XRD). Together, these allowed for the derivation of the Poisson’s ratio. Then we measured the position of the Raman active F2g symmetric vibrational mode in both the strained and relaxed states. Combining these measurements with the XRD- determined lattice constants and the Poisson’s ratio, we derived the F2g mode- Grüneisen parameter.
The values of the Poisson’s ratio and the F2g-mode Grüneisen parameter found here are significantly lower than literature values for pure and doped ceria ceramics. Our group earlier reported that the Poisson’s ratio of Gd-doped ceria thin films is dependent on the degree of strain(1), while time-dependent elasticity- anelasticity- was detected in the Raman spectra of such films (2). These elastic anomalies have been attributed to defect-induced local lattice distortions, which are able to release strain through relaxation of elastic dipolar strain fields. Our current findings therefore give support to two caveats: a) strain engineering for 10EDC films must include the possibility of both time-dependent and strain-dependent deviations from linear elasticity; and b) the use of Raman spectroscopy for the evaluation of absolute strain in thin films of doped ceria, requires consideration of their mechanical behavior, as well as of the thermal and temporal history. Assuming that the elastic properties of doped ceria ceramics are also characteristic of thin films appears not to be justified.
1. N. Goykhman et al., On the Poisson ratio of thin films of Ce0.8Gd0.2O1.9 II: Strain-dependence. J Electroceram 33,180 (2014).
2. O. Kraynis et al., Inelastic relaxation in Gd-doped ceria films: Micro-Raman spectroscopy. Scripta Materialia, (2017).
4:45 PM - TC07.07.08
Tuning Transport Properties of Nickel-Doped Zinc Oxide for Thermoelectric Applications by Microstructure Modulations
Ido Koresh 1 , Yaron Amouyal 1
1 Materials Science and Engineering, Technion, Haifa Israel
Show AbstractOxide thermoelectric (TE) materials offer good oxidation resistance and chemical stability at high temperatures, and are usually non-toxic and inexpensive, which make them commercially viable. As a part of our ongoing study of oxide TE materials [1-4], We investigate the effects of microstructure evolution on transport properties of nickel-doped zinc oxide (ZnO) for TE waste heat recovery at high temperatures [5]. A 3 at. % supersaturated Ni-alloyed ZnO solid solution was prepared by sintering at 1400 oC followed by controlled nucleation and growth of sub-micrometer size NiO-precipitates by aging at 750, 800, and 900 oC for different durations. Minimum thermal conductivity as low as 8.0 W m-1K-1 at 700 °C is obtained for samples aged at 750 °C for 8 h due to precipitates with high number density of 1.3 1020 m-3, which initiate phonon scattering. In turn, as-quenched samples exhibit the highest electrical conductivity, ca. 17.9 S cm-1 at 700 °C. Further nucleation and growth of precipitates taking place for longer annealing durations reduce electrical conductivity and increase Seebeck coefficients, which is associated with dilution of the ZnO-matrix from Ni-atoms. This study provides us with guidelines for optimization of TE Ni-doped ZnO.
References
1. A. Graff, Y. Amouyal, Reduced thermal conductivity in niobium-doped calcium-manganate compounds for thermoelectric applications. Appl. Phys. Lett. 105, 181906 (2014)
2. A. Graff, Y. Amouyal, Effects of Lattice Defects and Niobium Doping on Thermoelectric Properties of Calcium Manganate Compounds for Energy Harvesting Applications. J. Electron. Mater. 45, 1508–1516 (2016)
3. A. Baranovskiy, Y. Amouyal, Dependence of electrical transport properties of CaO(CaMnO3)m (m = 1, 2, 3, ∞) thermoelectric oxides on lattice periodicity. J. Appl. Phys. 065103 121 (2017)
4. A. Baranovskiy, Y. Amouyal, Structural stability of calcium-manganate based CaO(CaMnO3)m (m = 1, 2, 3, ∞) compounds for thermoelectric applications. J. Alloys Compd. 687, 562–569 (2016)
5. I. Koresh, Y. Amouyal, Effects of microstructure evolution on transport properties of thermoelectric nickel-doped zinc oxide. J. Europ. Ceram. Soc. 37 (11) 3541-3550 (2017).
TC07.08: Poster Session: Design, Control and Advanced Characterization of Functional Defects in Materials
Session Chairs
Thursday AM, November 30, 2017
Hynes, Level 1, Hall B
8:00 PM - TC07.08.01
BaMoO4:Tm3+ Phosphors—Design of Persistent Luminescent Materials
Ana Paula Marques 1 , Roseli Künzel 1 , Nancy Kuniko Umisedo 2 , Renato Latini 1 , Elisabeth Yoshimura 2 , Emico Okuno 2 , L.F. Ceridório 1
1 , Universidade Federal de São Paulo, Diadema Brazil, 2 , Universidade de São Paulo, São Paulo Brazil
Show AbstractMolybdates with scheelite-type tetragonal structure, are interesting for phosphor applications, optical fibers, scintillators, magnets, sensors and catalysts. [1] Molybdates how BaMoO4 can be doped with rare earth ions to modify the color and properties of emission optical. Thulium ions are an excellent blue activator and play an important role in the design of persistent luminescent materials. Persistent phosphors are materials that have the ability to capture charge carriers at structural defects or impurity sites (called traps) and to release them gradually. [2] This work reports the investigation on the structural and luminescent properties of Barium Thulium Molybdate (Ba1-xTmx)MoO4 microcrystals (with x = 1 or 3), synthesized by a co-precipitation method and grown in a microwave-assisted hydrothermal system. XRD and Raman studies showed the formation of a single scheelite phase. FTIR spectra presents the characteristic active vibrational modes, corresponding to the antisymmetric stretches [F2(ν3) vibrations] and bending modes [F2(ν4) vibrations] of the Mo-O bonds. UV-Vis absorption spectra exhibit bands attributed to electronic transitions within the MoO4-2 complex and the band gap was estimated for the samples, with a range of 3.98-3.55 eV. Under ultraviolet (359 nm) excitation, photoluminescence (PL) spectra present the characteristic emission bands at 453 nm and 545 nm, which are due to the 1D2-3F4 and 1D2-3H4,5 transitions, respectively, from Tm+3 ions. Thermoluminescence (TL) and optically stimulated luminescence (OSL) measurements were performed with powdered samples previously irradiated with beta radiation. OSL signal from the sample, stimulated by the blue light, is very strong, and, for long stimulation times (16 hours or more) remains practically constant. The trap depth, associated with trap levels located inside the band-gap, were determined from TL data using different methods of glow curve analysis. A long term OSL signal, measured in the range 270 - 380 nm under CW stimulation in the blue (470 nm), takes the form of a stable emission with the stimulation time, which can be assigned to a persistent luminescence in the ultraviolet energy range.
Keywords: persistent luminescence, barium molybdate, rare-earth, thermoluminescence, OSL
Acknowledgements
This work was supported by CNPq and FAPESP (#2016/20578-5, 2013/07437-5, 2010/16437-0, 2010/06816-4).
References
[1] A. P. A. Marques, F. C. Picon, D. M. A. Melo, P. S. Pizani, E.R. Leite, J.A. Varela, E. Longo, J. Fluoresc 18 (2008) 51-59.
[2] K Van den Eeckhout, D. Poelman, P. F. Smet, Materials 6(2013), 2789-2818.
8:00 PM - TC07.08.02
NIR-Photocatalytic Peptide Formation on Chiral Tungsten Oxide Nanoparticles
Mahshid Chekini 1 , Shuang Jiang 1 2 , Marisa Martinelli 3 , Nicholas Kotov 1
1 Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 School of Chemical Engineering and Technology, Tianjin University, Tianjin China, 3 College of Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractPrimary biomolecules formed on earth in the presence of inorganic materials. Amino acids are one of these biomolecules with a considerable amount (10-3 to 10-7) in primitive oceans. Besides amino acids importance in living metabolism, they are the basic components of peptides, proteins and living organisms. Peptide formation at the presence of bulk inorganic materials such as clay, pyrite and metal oxides at harsh condition has been reported. Among different minerals, ceramics, especially metal oxides are a good candidate since amino acid polymerization is widely reported for different metal oxides, even in the presence of water and low temperatures.
We explored the role of different type of nano-sized inorganic tungsten oxide nanoparticles in amino acid condensation and formation of peptides. Formation of esters in the presence of nanoparticles was evidenced by IR and Raman spectroscopy. Analysis of surface separated species by mass-spectrometry also showed more amino acid esterification in the presence of tungsten oxide nanoparticles in ethanol.
The surface defects can change the interaction energy barriers of adsorbents and facilitate peptide bond formation, also the potential role of surface defects at tungsten oxide nanoparticles should be considered. Under-coordinated metal ions at defect sites can form strong bonds with carboxyl groups and increase the amino acids retention time at the surface. Higher retention time increases the chance of amino acid encounter, interaction and possibly peptide bond formation. The formation of Asp-Asp ethanol ester with the mass of 276 a.u. is observed for nanoparticles stabilized using L-aspartic dried at 70°C. While we obtained dimer of aspartic proline dipeptide with a mass of 230 a.u. for nanoparticles that are carrying both amino acids at the same time.
Since heating and dehydration is an essential step in the formation of peptides and tungsten oxide nanoparticles show strong absorption in NIR region, we examined the samples by shining under heat lamp. By increasing the amount of amino acid molecules on the nanoparticles surface, and shining NIR for 1 hour, we observed the appearance of peptide of aspartic acid. The effect of different illumination duration and different NIR wavelength intervals and also different type of amino acids are studied for optimizing the effect.
Since the nanoparticles, the reactants and the products are chiral, further studies are required to explore the use of circularly polarized NIR radiation to understand its effect on the reaction. These experiments might increase the selectivity and control over the formation of desired synthetic peptides.
8:00 PM - TC07.08.03
The Preparation and Ferromagnetism of Single Crystal ε-Fe3N(111) Film on SrTiO3(100) Substrate
Yaping Qi 1
1 , The University of Hong Kong, Hong Kong Hong Kong
Show AbstractWe report the growth of single crystal hexagonal ε-Fe3N(111) film on SrTiO3(100) substrate by ablating high-purity iron target in activated nitrogen. The measurements of reflection high-energy diffraction, x-ray diffraction, atomic force microscopy and vibrating sample magnetometer reveal that the ε-Fe3N(111) films have well crystallization, smooth surface, and strong ferromagnetism. The root mean square roughness of the ε-Fe3N film surface is 0.25 nm. The saturation magnetization values are ~900 and ~1050 emu/cm3 at 300 and 10 K, respectively. The in-plane and out-of-plane coercivity forces are 550 and 675 Oe, respectively. Meanwhile, the film has obvious magnetic anisotropy.
8:00 PM - TC07.08.04
Study of the Structural, Morphological, Photoluminescent and Photocatalytic Properties of the Ag3Mox (PO4)1-X System
Aline Trench 1 , Letícia Guerreiro da Trindade 1 , Marcelo Assis 1 , Thales Machado 1 , Clayane Carvalho Dos Santos 1 , Elson Longo 1
1 , Federal University of Sao Carlos, Sao Carlos Brazil
Show AbstractSilver phosphate (Ag3PO4) has received considerable attention from the scientific community because of its phototocatalytic and photoluminescent properties. Structural modification by doping is an effectual and simple strategy to improve the electronic properties of materials. In this sense, we evaluated the semiconductor Ag3Mx(PO4)1-x (in which M are ions Mo6+) obtained by method of aqueous co-precipitation and characterized by X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) absorption spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), spectroscopy X-ray excited photoelectron (EDX) and x-ray fluorescence (XRF). Photoluminescence (PL) measurements was perform and we observed that the presence of dopant caused deep defects in the material, evidenced by the displacement of the emission band to larger wavelengths. These defects was relate to the presence of vacancies, interstitial atoms, displacements and stacking failures. The photocatalytic activity of the Ag3PO4 pure and doped was evaluate on the degradation of Rhodamine B under irradiation of visible light. The results showed that the photocatalytic activity of the samples was influence directly by the percentage amount of Mo, which affected the surface area available for photoreactions. The doped sample with lower concentration of Mo6+ providing a pollutant degradation of 91.86% in 5 minutes, while the pure silver phosphate sample yielded only 53.53% in the same time interval. This represent improve in efficiency of 71%. The increase of light absorption and of the adsorption capacity of pollutants are the main factors to increase the activity of these photocatalysts. In our case, these conditions were reach by carried out the doping of silver phosphate with ions of Mo6+. In order to propose a photocatalytic mechanism, tests of photocatalytic activity were perform by the addition of appropriate reactive species scavengers. The result showed that the photodegradation is mainly due to the presence of the superoxide radical (O2-).
8:00 PM - TC07.08.05
Localized Levels of Low-Temperature-Grown InxGa1-xAs on InP Substrates
Shunsuke Tsurisaki 1 , Yoriko Tominaga 1 , Momoko Deura 2 , Yutaka Kadoya 1
1 , Hiroshima University, Hiroshima Japan, 2 , The University of Tokyo, Tokyo Japan
Show AbstractLow-temperature-grown (LTG)-InxGa1-xAs samples were grown on InP(001) substrates using molecular beam epitaxy (MBE) at the growth temperatures of 200–220 °C in this study. The carrier densities as a function of temperature for the LTG InxGa1-xAs were measured by using the Hall-effect measurements to clarify the defect (localized) levels at the inside of its forbidden band. Simulation fitting based on the charge neutrality of carriers and impurities to measured carrier densities revealed that deep localized levels tend to be formed below the conduction band minimum (CBM) of LTG InxGa1-xAs grown at 220 °C in comparison with the case of the samples grown at 200 °C.
LTG-GaAs-based III-V compound semiconductors have been studied actively because they are candidate materials to realize photoconductive antennas (PCAs) activated by femtosecond fiber lasers with wavelengths of 1.5 μm for terahertz (THz) wave emission and detection. These lasers are less expensive and more compact than conventional optical sources, and therefore, THz time-domain spectroscopy systems with these PCAs will become more practical. Although these materials are required to have high carrier mobility, an ultrashort carrier lifetime, and high resistivity, it is still difficult to obtain these characteristics in LTG-GaAs-based compounds. In LTG GaAs, the high density of defects such as excess As, Ga vacancy and the formation of As precipitates which is induced by thermal annealing generate the high resistivity and short carrier lifetime. Therefore, it is essential to reveal the key defects and defect levels to achieve efficient THz-wave emission and detection in LTG-GaAs-based compounds. In this study, we investigated the defect levels of the LTG InxGa1-xAs on InP substrates by the analysis of the results of Hall effect measurements.
LTG-InxGa1-xAs samples (x = 0.42–0.48), with thicknesses of 2.0 μm, were grown on (100)InP substrates by MBE at temperatures of 200 and 220 °C. Be was doped into InxGa1-xAs layers with a concentration in the range of 3.00×1017–18 cm-3. After the growth, the samples were annealed at 550 °C for 1 h in an H2 atmosphere. We performed X-ray diffraction (XRD) and Hall effect measurements at a temperature in the range of 120–300 K for these samples.
The (400) reflection XRD showed that the sample grown at 220 °C exhibited clear XRD peak while the samples grown at 200 °C did broad spectra with low intensities, suggesting that crystalline quality deteriorates drastically at a growth temperature between 200 and 220 °C. The measured carrier densities using Hall effect measurements were fitted by simulation based on the charge neutrality of carriers and impurities. These simulation fitting showed that the localized level lay 100 meV below the CBM and its density was 3.40×1018 cm-3 for LTG In0.45Ga0.55As grown at 220 °C. As for LTG In0.44Ga0.56As grown at 200 °C, it was also shown that it lay 10-18 meV below the CBM and its density was 3.26-3.35×1018 cm-3.
8:00 PM - TC07.08.06
Structural, Optical and Electrical Properties of Zn3N2 Films Deposited by dc or rf Magnetron Sputtering
Junjun Jia 1 , Hironori Kamijo 1 , Shin-ichi Nakamura 1 , Yuzo Shigesato 1
1 , Aoyama Gakuin University, Kanagawa Japan
Show AbstractIII-nitrides semiconductors, including GaN, InN and AlN, have been utilized for the electronic devices such as light-emitting diodes and have been studied for power-electronic devices. On the metal nitride semiconductors other than III-nitrides, very few have been reported. In this study, we have focused on Zinc nitride (Zn3N2) as one of the new nitride semiconductors. Zn3N2 is a group-II nitride semiconductor with a cubic anti-bixbyite crystal structure. This material shows high transparency from the visible to near infrared regions because it has a wide band gap and a large refractive index[1][2]. Therefore, Zn3N2 can be expected to be utilized as a transparent electrode of the solar cells or GaN-LEDs. In this study, the Zn3N2 films were tried to deposited by reactive dc or rf magnetron sputtering and there structure, electrical and optical properties were investigated.
In the case of the dc sputtering, Zn3N2 films were deposited on synthetic quartz substrates at 473 K by reactive sputtering with 50kH pulsing using a Zn-Al alloy target (Al: 1.5wt%, 130mm×400mm in size) with He, Ne or Ar as the sputtering gases. X-ray diffraction patterns showed that the films deposited using Ar and N2 were metallic films, whereas the films deposited using Ne or He were confirmed to be polycrystalline Zn3N2 by XRD. Transmission electron microscope (TEM, JEOL JEM-4010, 400 kV) revealed that the crystal growth behaviors of these films were very different. The film deposited using Ne showed porous “boulder-like” structure. On the other hand, the film deposited using He showed dense and typical columnar structure.
On the other hand, the Zn3N2 films deposited on synthetic quartz substrates at 473 K by the reactive RF magnetron sputtering using a Zn target. In the case of RF sputtering, we succeeded to deposit Zn3N2 thin films under wide range of the deposition conditions. TEM images show that all Zn3N2 films were dense and columnar structure. The minimum resistivity of 2.9×10-3Ωcm was obtained at N2 flow ratio of 40%.
References:
1) N. Yamada, et al., J. J. Apple. Phys., 53, 05FX01(2014)
2) K. Kuriyama, et al., Physical Review. 48. 4 (1993)
8:00 PM - TC07.08.07
Low Frequency Noise Characteristics for MoTe2 Thin Film Transistors Adopted with Hydrophobic Polymer Encapsulation
Jong Hun Hong 1 , Jae Hyun Ryu 1 , Geun Woo Baek 1 , Sung Hun Jin 1
1 , Incheon National University, Incheon Korea (the Republic of)
Show AbstractTransition metal dichalchogenides (TMDCs) have been actively researched toward various electronic applications due to their novel electrical, optical, and chemical properties along with ideal two dimensional structure. Among various TMDCs, many research activities have been predominantly focused on MoS2 TFTs because of its abundance, nontoxicity, easy preparation of MoS2 films through exfoliation or/and CVD. However, MoTe2, as one of promising candidates, has just begun to be explored for the application of logic circuits and optical sensing application, particularly due to its smaller semiconducting energy band gap (Eg~0.8 or 1.24 eV in a single layer form), suitable for near infrared wavelength and ambipolar electronic behaviors. However, most of research activities for MoTe2 field effect transistors have been focused on high performance device implementation, understanding on gas (or chemical) adsorption effects, along with transport behaviors in ultra-low temperature.
Interestingly there have been few reports on investigation of device instability of MoTe2 FETs before and after novel hydrophobic polymer encapsulation. In this study, we implemented reliable MoTe2 FETs with the proposed passivation technology and their low frequency characterization have been investigated as a diagnostic tool for understanding on intrinsic and extrinsic process parameters to determine device instability. Furthermore, systematic analysis on the change of low frequency characteristics, followed by bias temperature stresses, has been tried to trace the origins of instability of devices associated with functional defects located in nanoscale interface. In parallel, device instability model has been proposed to decouple the intrinsic origins of device instabilities with the extrinsic factors, followed by experimental validation with the proposed model.
8:00 PM - TC07.08.09
Electronic Structure of Polyethylene—Role of Chemical, Morphological and Interfacial Complexity
Lihua Chen 1 , Huan Tran 1 , Rampi Ramprasad 1
1 , University of Connecticut, Storrs, Connecticut, United States
Show AbstractThe electronic structure of an insulator encodes essential signatures of its short-term electrical performance and long-term reliability. A critical long-standing challenge though is that key features of the electronic structure of an insulator (and its evolution) under realistic conditions have not been entirely accessible, either via experimental or computational approaches, due to the inherent complexities involved. In this comprehensive study, we reveal the role of chemical and morphological imperfections that inevitably exist within the technologically important prototypical and pervasive insulator, polyethylene (PE), and at electrode/PE interfaces. Large-scale density functional theory computations and long-time molecular dynamics simulations were employed to accurately recover, explain and unravel a wide variety of experimental data obtained during the electrical degradation of PE. This scheme has allowed us to directly and realistically address the role of chemical, morphological and interfacial complexity in determining electronic structure. These efforts take us a step closer to understanding and potentially controlling dielectric degradation and breakdown.
8:00 PM - TC07.08.10
Quantum Chemical Modeling and Calculation of Trap States for Electrical Breakdown Strength Enhancement of Oil Impregnated Paper
Guanghao Qu 1 , Yuanwei Zhu 1 , Daomin Min 1 , Shengtao Li 1
1 State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an China
Show AbstractOil impregnated paper (OIP) is composed of insulating oil and cellulose with excellent electrical and mechanical performance; it has been widely used in power equipment such as converter transformers, cables, and capacitors. The electrical breakdown strength of OIP is about three times or higher than that of its single components. The object of this work is to determine whether the breakdown field of OIP can be tuned with oil by introducing trap states. In this work, the trap parameters of OIP were calculated by using the quantum chemical theory, and the volume resistivity and breakdown field were measured. In order to describe the electronic structure of cellulose well, a minimum degree of polymerization should be established so that even when additional β-D-glucose (C6H12O5) monomers are introduced, the HOMO and LUMO levels of the chain model remain unchanged. To this end, the double numerical plus polarization (DNP) basis and Grimme DFT-D dispersion correction method were used to fully relaxed molecular configurations. Then dodecylbenzene (DB, C6H5-C12H25), benzyltoluene (BT, C6H5-CH2-C6H5-CH3) and phenylethlphenylethane (PEPE, C6H5-CH(CH3)-C6H5-C2H5) were separately placed near the single chain of cellulose to model three kinds of OIPs. The calculation results show that the depth of traps of cellulose with DB, BT and PEPE are 0.93 eV, 1.12 eV and 1.23 eV, respectively, corresponding to the dc breakdown field 233.5 kV/mm, 240.7 kV/mm and 254.1 kV/mm, respectively, in the experiment. The density of traps induced by BT or PEPE with two benzene rings is a factor of 2 higher than that of DB with single benzene ring, which plays a greater role in suppressing the accumulation of space charges and enhancing electric breakdown strength. It was found that electrical breakdown strength of OIP is proportional to the depth and density of traps from experimental and computational results. Our results indicated that the variation of molecular structure of the insulating oil using aromatic and aliphatic segments allowed for tuning the electrical breakdown strength through introduction of additional trap states, and better control of resistivity.
8:00 PM - TC07.08.11
Effect of Point Defects on the Phase Transformation of NiTi Shape Memory Alloys
Aria Mansouri Tehrani 1 , Jakoah Brgoch 1
1 Chemistry, University of Houston, Houston, Texas, United States
Show AbstractShape memory alloys are a fascinating class of smart materials with applications ranging from stents to actuators. Among the known materials displaying this phenomenon, NiTi alloys have attracted the most attention due to their efficient shape recovery, mechanical robustness, and biocompatibility. The phase transformation between the cubic B2 and monoclinic B19’ crystal structures are responsible for the shape memory and pseudoplasticity behavior within these alloys. Although the composition-transformation relationship has been extensively studied, the influence of point defects that are undoubtedly present in NiTi are not well understood. Here, Density Functional Theory (DFT) is used to study the energetics and the influence of point defects (vacancies, anti-sites, and substitutional) on the electronic structure of NiTi alloys while their equilibrium concentration is determined using a grand-canonical formalism. Further, we have employed Molecular Dynamic (MD) simulations to determine the origin of changes on both thermal and stress induced phase transformations in NiTi upon point defect incorporation. Our results suggest that increasing the defect concentration hinders the transformation through a pinning mechanism, leading to a predicted 80 K change in the transformation temperature with 3% of vacancies. Moreover, anti-site defects have a greater impact than vacancies causing a most substantial shift in the transformation temperatures. The combination of computational methods allow to study this significant and complex phenomena at different lengths and time scales providing new insight into the involved mechanism. The results presented here also allows for a more fine-tune control over the transformation temperature and elastic recovery.
8:00 PM - TC07.08.13
Persistent Photoconductivity Due to Hole-Hole Correlation in CdS with Applications to Solar Cells and Neuromorphic Computing
Han Yin 1 , Rafael Jaramillo 2
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractLarge and persistent photoconductivity (PPC) in semiconductors is due to the trapping of photo-generated minority carriers at crystal defects. PPC in cadmium sulfide (CdS) has long been reported, and is relevant to the operation of thin-film solar cells that use CdS as an n-type layer. Theory has suggested that anion vacancies in II-VI semiconductors are responsible for PPC due to negative-U behavior, whereby two minority carriers become kinetically trapped by lattice relaxation following photo-excitation (Lany and Zunger, Phys. Rev. B 72, 035215 (2005)). By performing a detailed analysis of PPC decay at long times in CdS, we provide experimental support for this negative-U model of PPC. We also show that PPC is correlated with sulfur deficiency. We use this understanding to vary the photoconductivity of CdS films over nine orders of magnitude, and vary the PPC characteristic decay time from seconds to 104 seconds, by controlling the activities of Cd2+ and S2- ions during chemical bath deposition. We suggest a screening method to identify other materials with long-lived, non-equilibrium, photo-excited states based on the results of ground-state calculations of atomic rearrangements following defect redox reactions, with a conceptual connection to organic molecular photo-response.
Understanding and controlling PPC in II-VI materials informs their application in thin film solar cells, and may enable new applications such as electro-optic elements in neuromorphic imaging systems. We will discuss the role of PPC in enabling artificial synaptic behavior, and demonstrate results using resonant optical transitions to excite and inhibit synaptic weights.
8:00 PM - TC07.08.14
Local Structures and Phase Stability of Anatase TiO2 Films Prepared by Reactive Sputtering with Nb and/or N Doping
Toshihiro Okajima 1 , Junjun Jia 2 , Hiroyoshi Nishiyama 2 , Yuzo Shigesato 2
1 , Kyushu Synchrotron Light Research Center, Tosu, Saga, Japan, 2 Graduate School of Science and Engineering, Aoyama Gakuin University, Sagamihara, Kanagawa, Japan
Show AbstractRutile and anatase TiO2 films are widely used in various industrial applications [1,2]. Rutile TiO2 film is utilized as the optical coating material due to its high refractive index, and anatase TiO2 film is applied as photocatalysts and transparent electrodes. There are different applications according to the crystal phases in thin films. Therefore, controlling the crystal phases and conversion between rutile and anatase TiO2 thin films is important for these applications. Our latest report revealed the Nb and/or N doping effected the selective anatase TiO2 films growth by reactive sputter depositions [3]. In order to reveal the local structures and the phase stability in the anatase TiO2 films, we investigated XAFS measurements and first principled DFT calculations for Nb and/or N doped TiO2 films prepared by reactive sputtering process in this study.
We first confirmed the enhancements of the anatase TiO2 phase growth by the Nb and/or N doping by using the XRD profiles. The XAFS measurements were also supported the results. The obtained atomic distances from the EXAFS analysis between Nb and O in the Nb doped TiO2 were determined to be about 0.201 nm, which approach the value in NbO2 with oxygen six-fold coordination. The Nb K-edge XANES spectra also revealed the Nb ions in the Nb doped TiO2 exist as Nb5+ ions. These results indicate the Nb ions are substituted at Ti4+ site in the Nb doped TiO2 films forming Nb5+. On the other hand, the intensities of radial distribution functions obtained from EXAFS spectra of N doped and Nb and N co-doped anatase TiO2 films clearly became weak comparing with those of non-doped and Nb doped films, indicating the oxygen vacancies were induced by N doping. We also investigated the formation energies of Nb doped TiO2 by using first principles DFT calculations to examine the phase stabilities of the TiO2 films. The origins of selective growth of anatase phase with Nb and/or N doping will be discussed.
[1] J. M. Bennett, et. al., Applied Optics 28, 3303 (1989), [2] Y. Furubayashi, et. al., Appl. Phys. Lett. 86, 252101 (2005), [3] H. Nishiyama, et. al., A3-P20-013, 26th Annual Meeting of MRS-J (2016).
8:00 PM - TC07.08.16
First-Principles Investigations of Grain Boundaries in InAs
Altynbek Murat 1 , Masahiko Matsubara 1 , Enrico Bellotti 1
1 , Boston University, Boston, Massachusetts, United States
Show AbstractPolycrystalline InAs based materials are potential candidates for low cost wafer-scale detector applications due to their excellent optical properties. However, grain boundaries (GBs) in polycrystalline InAs are very challenging to eliminate and are expected to have a profound effect on the electronic properties of the material and the device efficiency. Thus, an in-depth understanding of the electronic structure of the GBs in InAs and of the role played by the passivation mechanisms is required. We employ first-principles density functional approach to investigate the electronic, structural, and transport properties of GBs in InAs. In particular, we study the energetics and passivation mechanisms of low-Σ GBs to calculate their relative stability and experimental feasibility. We find that the symmetric tilt twin boundaries are the most stable GBs in InAs, in excellent agreement with the stable GBs established for CdTe. We also discuss the effect of different passivation and doping mechanisms on the electronic properties. Understanding the exact nature of the GB electronic structure as well as their passivation mechanism is a key step for the development of wafer-scale InAs based devices.
8:00 PM - TC07.08.18
Bulk Properties of Hong-Type NASICON as Solid Electrolyte—Vacancy Formation and Ion Migration
Su Hwan Kim 1 , Dae Yeon Hwang 1 , Sang Kyu Kwak 1 , Youngsik Kim 1
1 School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan Korea (the Republic of)
Show AbstractHong-type sodium super ionic conductor (NASICON) compounds have been considered as strong candidates for solid electrolyte materials in sea water battery. However, there have been few theoretical studies on the bulk properties of Hong-type NASICON. Therefore, theoretical analysis on Hong-type NASICON electrolyte was performed to investigate the structural stability and migration path of Na ion by density functional theory (DFT) calculation. Four types of NASCION structure were constructed according to the ratio of Si and P (i.e., Si3, Si2P, SiP2, and P3) by following doping schema; Starting from Na4Zr2Si3O12, P was doped on tetrahedral Si site, and one Na was eliminated to maintain charge neutrality. Then, we calculated the vacancy formation energies for all elements of NASICON and migration barrier of Na ion. The result showed that the formation energy of neutral Si/P vacancy was calculated to be ~2 eV, while that of Zr and O was higher than 5 eV. In electronic structures, O and P vacancy induced vacancy state between valence band and conduction band, which could trap charge carriers. Especially, P vacancy induced the dumbbell shape of oxygen configuration, which could block the path of Na migration. Additionally, Na migration at the path 2->3 or 3->2 was calculated to be the dominant migration path with the lowest energy barrier in all types of NASICON.
Symposium Organizers
Hua Zhou, Argonne National Laboratory
Panchapakesan Ganesh, Oak Ridge National Laboratory
Anna Kimmel, University College London
Dong Su, Brookhaven National Laboratory
Symposium Support
Argonne National Laboratory, Advanced Photon Source
TC07.09: Design and Engineering Functional Defects for Electronics and Computing
Session Chairs
Ho Nyung Lee
Petro Maksymovych
Thursday AM, November 30, 2017
Hynes, Level 2, Room 207
8:00 AM - *TC07.09.01
Quantized Dislocations
Mingda Li 1 , Gang Chen 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractCrystal dislocation, as one common type of materials line defect, plays an important role in materials mechanical properties, which has been extensively studied. However, the quantification of how dislocations affect functional properties – such as electrical, thermal and superconducting properties – has been challenging. This talk will introduce our recent theoretical work in constructing a quantized theory of dislocation, which aims at treating dislocations at a fundamental quantum field theoretical level, without empirical parameters. The resulting quasiparticles associated with the dislocation are named as “dislons” [1]. We apply the dislon concept to treat electron-dislocation scattering and phonon-dislocation scattering [2,3]. Building on the dislon concept, we also evaluate how dislocations can change the superconducting transition temperature [4].
This work is supported by S3TEC, an Energy Frontier Research Center funded by U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES) under Award No. DESC0001299/DE-FG02-09ER46577 and DARPA MATRIX Program Contract HR0011-16-2-0041.
[1] Mingda Li, Yoichiro Tsurimaki, Qingping Meng, Yimei Zhu, Gerald D. Mahan, Gang Chen, “Theory of Electron-Phonon-Dislon Interacting System - Toward a Quantized Theory of Dislocations”, arXiv:1708.07143.
[2] Mingda Li, Wenping Cui, Mildred S. Dresselhaus, and Gang Chen, “Electron energy can oscillate near a crystal dislocation,” New Journal of Physics, 19, 013033 (2017).
[3] Mingda Li, Zhiwei Ding, Qingping Meng, Jiawei Zhou, Yimei Zhu, Hong Liu, Mildred S. Dresselhaus, and Gang Chen, “Nonperturbative quantum nature of the dislocation-phonon interaction,” Nano Letters, 17, 1587-1594 (2017).
[4] Mingda Li, Qichen Song, Te-Huan Liu, Laureen Meroueh, Gerald D. Mahan, Mildred S. Dresselhaus, and Gang Chen, “Tailoring Superconductivity with Quantum Dislocations, Nano Letters, 17, 4604–4610 (2017).
8:30 AM - TC07.09.02
Defect-Induced Electronic Correlations in Oxide Materials
Frank Lechermann 1
1 , University of Hamburg, Hamburg Germany
Show AbstractThe first-principles investigation of point defects, such as dopants,
vacancies or anti-structure atoms, is a longstanding important research
field in materials science.
In principle, point defects are also prominent in various hallmark problems
of correlated materials, e.g. the metal-insulator transition in Cr-doped
V2O3. The rising field of oxide heterostructures additionally stimulates
various questions concerning the interplay of defects and demanding
electronic states.
Advancements in the combination of density functional theory (DFT) and
dynamical mean-field theory (DMFT) render it possible to address defect
problems with a performance that resembles state-of-the-art materials
science treatments. Relevant many-body effects beyond the capabilities of
static-correlation calculational schemes become reachable therein.
Using such elaborate DFT+DMFT techniques, the importance of oxygen
vacancies in introducing and/or modifying electron correlation effects in oxide
materials will be discussed in the present talk.
8:45 AM - TC07.09.03
Coupling of Strain and Defect Energetics in VO2 and Its Influence on Metal-Insulator Transition Characteristics
Janakiraman Balachandran 1 , Yogesh Sharma 1 , Ilkka Kylanpaa 1 , Jaron Krogel 1 , Ho Nyung Lee 1 , Paul Kent 1 , Panchapakesan Ganesh 1 , Olle Heinonen 2
1 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 , Argonne National Laboratory, Lemont, Illinois, United States
Show AbstractVO2 is a canonical example of a strongly correlated system that exhibits a wide range of polymorphs such as monoclinic (M1, M2) and rutile (R) which each exhibiting unique electronic and optical properties. VO2 undergoes a metal-insulator transition (MIT) around 340K accompanied by a first order phase transition from high temperature metallic rutile phase to low temperature insulating monoclinic phase. Understanding and controlling this MIT by varying chemical composition, and applying external stimuli, such as strain, temperature, and electric field is necessary to realize VO2 based devices. Recent experimental advances have enabled unprecedented control of modifying the stoichiometry of VO2 at localized, microscopic length scales, in strained thin film geometries. However, gaining mechanistic insights on how these chemical and mechanical modifications influences the structural and electronic properties requires development of ab initio models. Mean-field based density functional theory (DFT) methods are notorious for their poor predictive capabilities in VO2.
In this talk, we will explore the coupling between experimentally realizable strain and oxygen vacancy (VOq) formation as well as diffusion energies in VO2 using beyond-DFT methods (e.g. DFT+U, meta- and hybrid DFT). These results will be further benchmarked against very accurate many-body Quantum Monte Carlo (QMC) calculations. This comparison enables us to quantify the errors, understand the source of errors and come up with strategies to minimize these errors.
Finally, these ab initio results are quantitatively compared against experimental measurements of electrical properties on VO2 epitaxial films. This comparison provides insights on how external stimulus such as electrical field and mechanical pressure when applied experimentally through scanning probe (SPM) tips modify VOq defect energies, which influences the electronic/structural properties and in turn the metal-insulator-transition.
Acknowledgement
This work was supported by the Center for Predictive Simulation of Functional Materials, through U.S. Department of Energy, Office of Science, Basic Energy Sciences, Computational Materials Sciences Program. This research used resources of the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.
9:00 AM - TC07.09.04
Local Electrical and Mechanical Control of Metal-Insulator Transition in Epitaxial Vanadium Dioxides
Yogesh Sharma 2 , Janakiraman Balachandran 2 , Changhee Sohn 2 , Liam Collins 2 , Nina Balke 2 , Panchapakesan Ganesh 2 , Olle Heinonen 1 , Ho Nyung Lee 2
2 , Oak Ridge National Laboratory, Knoxville, Tennessee, United States, 1 Materials Science Division, Argonne National Laboratory, Lemont, Illinois, United States
Show AbstractStrongly correlated vanadium oxide (VO2) is one of the most promising materials exhibiting temperature-driven metal-insulator transition (MIT) in the vicinity of room temperature. It has been reported that MIT behavior in VO2 is very sensitive to the oxygen stoichiometry, which can alter its phase-transition temperature. As the oxygen stoichiometry in VO2 can be modified readily by electrochemical and mechanical means, such as electric field and strain, it is important to understand their responses to these external stimuli towards the mechanism of phase transition. In this work, we explore the control of MIT in epitaxial VO2/TiO2(001) thin films by electric fields and mechanical pressure using scanning probe microscopy approaches. We identified that the applied electric voltage and mechanical force to the SPM-tip could locally modulate surface chemical potential and conductivity of an insulating VO2 film and thus the MIT temperature, which were also seen by temperature dependence of surface chemical potential variation throughout the MIT. Based on our results, we discuss about the energetics of oxygen vacancy formation and associated phenomena induced by the applied electrical field and mechanical force, providing a deeper understanding on the MIT behavior in VO2.
9:15 AM - TC07.09.05
Defects-Modulated Elastic Strain Engineering in Nanoscale Semiconductor/VO2 Hybrids
Yiping Wang 1 , Xin Sun 1 , Toh-Ming Lu 1 , Jian Shi 1
1 , Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractSignificant epitaxial strain (~1%) is conventionally believed to exist only below the critical thickness limit at tens of nanometer scale, which greatly hinders the possible applications of strain engineering. The development of nanocomposite materials recently attempts to overcome the barrier by the inclusion of instantaneous phase separation between two materials during growth that ends up with a larger surface area, thus promoting large-scale strain. Here as a step forward, we report the growth of vertically aligned M2-phase VO2 hybrid nanowire array that circumvents the pairing requirements for common nanocomposite and at the same time introduces the dynamic bulk strain by triggering the VO2 phase transformation. By controlling the growth temperature and oxygen vapor pressure, a dense array of VO2 nanowire arrays with hole doping can be obtained by a template growth from V2O5 droplets. Revealed by Raman spectroscopy, the as-grown free-standing VO2 nanoforrest stabilizes itself in the M2 phase as a result of the oxygen rich template of V2O5, different from the commonly observed M1 phase. The M2-R phase transition renders a larger lattice contraction compared to the conventional M1-R trend and thus is more promising for more effective strain enginneering. Our findings also shed light on the third component besides temperature and strain which can modulate the phase of VO2 in a more facile way. As a second step for the demonstration of VO2 hybrid material, various optically active and strain sensitive semiconductors including CdS, ZnO and halide perovskite have been deposited on the VO2 nanowire, all of which show non-linear temperature depedent optical properties across the MIT temperature of VO2. The utilization of phase transition from strongly correlated oxides thus provides a very promising approach for the realization of strain at bulk.
9:30 AM - TC07.09.06
Dopant Cation Diffusion in Barium Titanate Using Metadynamics and Umbrella Sampling
Robyn Ward 1 , Colin Freeman 1 , Derek Sinclair 1 , John Harding 1
1 , Univ of Sheffield, Sheffield United Kingdom
Show AbstractDopant cations such as yttrium, gadolinium and dysprosium are regularly used in barium titanate based ceramic capacitors to increase the electrical stability of the device [1], thus increasing the lifetime of the component. It is thought that they do this by trapping intrinsic oxygen vacancies and so prevent charged species from building up at the electrodes. Our understanding of the system suggests that Gd, Y and Dy can all trap oxygen to a similar degree. This suggests that the difference in ability between these ions to increase the lifetime of the component may be connected to their distribution in the lattice – the more even the distribution of the ions, the more effective they will be at trapping oxygen.
In this work we use advanced sampling techniques – metadynamics and umbrella sampling [2] – coupled with molecular dynamics simulations - to explore the diffusion pathways for self-diffusion in barium titanate, oxygen diffusion around rare earths, and rare earth diffusion. The metadynamics simulations use the Plumed plug-in (version 1.3 [3]) patched to the DL_POLY Classic (version 1.9 [4]) molecular dynamics code to explore the diffusion pathways possible for each ion type. The potentials used were that of Freeman et al [5] with rare-earth potentials from Lewis and Catlow [6]. The shell model was used for oxygen and barium ions. Simulations were run with an NVT ensemble with a box size of 2560 atoms. Umbrella sampling was then used (still employing the Plumed 1.3 plugin) to calculate an accurate activation energy for each of the diffusion pathways identified by the metadynamics simulations. We also show results on changes in pathway length, variations in pathway and barriers to diffusion for different ions, comparing our results where possible with the available experimental data [7].
[1] J. Itoh, D.C. Park, N. Ohashi, I. Sakaguchi, I.Yashima, H. Haneda, J. Ceram. Soc. Japan, 110, 495–500, (2002).
[2] C. Abrams and G. Bussi, Entropy, 16, 163–199, (2014).
[3] M. Bonomi, D. Branduardi, G. Bussi, C. Camilloni, D. Provasi, P. Raiteri, D. Donadio, F. Marinelli, F. Pietrucci, R.A. Broglia and M. Parrinello, Comp. Phys. Comm. 180, 1961-1972 (2009).
[4] W.Smith, T.R.Forester, J. Molec. Graphics, 14, 136-141, (1996).
[5] C. L. Freeman, J. A. Dawson, H.R. Chen, J. H. Harding, L.B. Ben, and D. C. Sinclair, J. Mater. Chem., 21, 4861-4868 (2011).
[6] G. V. Lewis and C. R. A. Catlow, J. Phys. C. Solid State Phys., 18, 1149-1161 (1985).
[7] M. Kessel, R. A. De Souza, and M. Martin, Phys. Chem. Chem. Phys., 17, 12587-12597, (2015).
9:45 AM - TC07.09.07
Direct Observation of the Fe Impurity Levels in BaTiO3 Single Crystal by Electrical Measurement
Issei Suzuki 1 , Andreas Klein 1
1 , Technical University of Darmstadt, Darmstadt Germany
Show AbstractEven nominally undoped BaTiO3 inevitably has the natural impurity inside; especially Fe is the most common impurity of BaTiO3 because of its earth-abundance. Fe impurity would make defect levels within the band gap of BaTiO3 [1]. It is important to study the energy of the defect levels possibly caused by Fe in order to design the interface involving BaTiO3 layer. Fe impurity is reported to be usually Fe3+ and/or Fe4+ in the BaTiO3 and makes Fe3+/Fe4+ defect level at 0.5-0.8 eV above the valence band maximum [1]. Also, Fe2+/Fe3+ level has been predicted to exist in reduced BaTiO3 at 0.5-0.7 eV below the conduction band minimum by the calculations [1,2] and observed by optical measurement [3]; however, this defect level is yet to be directly observed by electrical measurement.
In this work, activation energy of the BaTiO3 single crystals were measured with the various oxidation states. Starting with the highly reduced conductive BaTiO3 single crystal obtained by having been annealed in either ultra-high vacuum or H2-N2 atmosphere, the electrical conductivity and the activation energies were monitored during stepwise slow re-oxidation of the sample. The latter was controlled by heating in the controlled atmosphere of 1%O2-99%N2 at a moderate temperature for a given time. The evolution of activation energies upon re-oxidation combining the defect calculation realized experimental identification of the Fe impurity levels of BaTiO3 for the first time. The results imply that Fe2+ exists in the reduced BaTiO3 and there is Fe2+/Fe3+ level at approximately 0.7 eV below the conduction band minimum which agrees well with the calculated prediction [1,2].
[1] B.A. Wechsler and M. B. Klein, J. Opt. Soc. Am. B, 5, 1711, 1988
[2] F. M. Michel-Calendini et al., Jpn. J. Appl. Phys., 24, 656, 1985
[3] A. Mazur et al., Radiation Effect & Defects in Solids, 150, 281, 1999
10:30 AM - TC07.09.08
Electronic Transport and Ferroelectric Switching in Ion-Bombarded, Defect-Engineered BiFeO3 Thin Films
Sahar Saremi 1 , Ruijuan Xu 1 , Liv Dedon 1 , Ran Gao 1 , Lane Martin 1 2
1 , University of California, Berkeley, Berkeley, California, United States, 2 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractDespite continued interest in the multiferroic BiFeO3 for a diverse range of applications, use of this material has been limited by its poor electrical resistivity. This is often related to the difficulties in deterministic control of this material due to the complex nature of its chemistry and penchant for defects (i.e., the presence of multiple cation and anion species and their corresponding point defects, complexes, and clusters). In order to reduce the electronic leakage in BiFeO3, chemical doping/alloying (i.e., introduction of extrinsic defects/dopants) has traditionally been applied. The extent to which the resistivity can be enhanced using chemical alloying is, however, limited by the solid solubility, as well as by simultaneous changes in the ferroelectric properties (e.g., reduction in polarization). In other systems such as group IV and III-V semiconductors, it has been shown that, in addition to extrinsic dopants, controlled introduction of intrinsic defects (i.e., vacancies, interstitials, and antisites related to the constituent elements) via ion bombardment can be an alternative and effective route to enhance resistivity. In this work, we address the leakage problem in BiFeO3 thin films via defect engineering achieved via high-energy ion bombardment. The ability of this ion-bombardment technique to control the concentration of intrinsic defects beyond the thermodynamic limits enables tuning of the resistivity by orders of magnitude, here demonstrated in some of the most resistive BiFeO3 thin films reported to date.
High leakage in as-grown BiFeO3 thin films is shown to be due to the presence of moderately shallow isolated trap states, which form during growth and dope the lattice with charge. Ion bombardment is shown to be an effective way to reduce this free carrier transport (by up to ~4 orders of magnitude) by trapping the charge carriers deep in the band gap as a result of the formation of bombardment-induced deep-lying defect complexes and clusters. The ion bombardment, however, is also found to give rise to an increased resistance to switching as a result of an increase in defect concentration. Using a combination of piezoresponse force microscopy, macroscale capacitor-based switching kinetics, and first-order reversal curve analysis it is shown that the increased defect concentrations, while having no evident impact on the domain structure or the switching mechanism, do give rise to a systematic increase in the coercivity, an extension of the defect-related creep regime, an increase in the pinning activation energy, a decrease in the switching speed, and a broadening of the field distribution of switching. Ultimately, such defect engineering routes to enhance ferroelectric device performance require finding an optimum range of ion dosage to achieve maximum enhancement in resistivity with minimum impact on the ferroelectric switching.
10:45 AM - TC07.09.09
Cation-Eutectic Transition via Sublattice Melting in CuInP2S6/In4/3P2S6 van der Waals Layered Crystals
Michael Susner 1 , Marius Chyasnavichyus 2 , Alexander Puretzky 2 , Qian He 2 , Benjamin Conner 2 , Yang Ren 3 , David Cullen 2 , Panchapakesan Ganesh 2 , Petro Maksymovych 2 , Michael McGuire 2
1 , Air Force Research Laboratory, WPAFB, Ohio, United States, 2 , Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 , Argonne National Laboratory, Argonne, Illinois, United States
Show Abstract
Single crystals of the van-der-Waals layered ferrielectric material CuInP2S6 spontaneously phase separate when synthesized with Cu deficiency. Here we identify a route to form and tune intralayer heterostructures between the corresponding ferrielectric (CuInP2S6) and paraelectric (In4/3P2S6 ) phases through control of chemical phase-separation. We conclusively demonstrate that Cu-deficient Cu1-xIn1+x/3P2S6 forms a single phase at high temperature. We also identify the mechanism by which the phase separation proceeds upon cooling. Above 500 K both Cu+ and In3+ become mobile, while P2S64- anions maintain their structure. We therefore propose that this transition can be understood as eutectic melting on the cation sublattice. Such a model suggests that the transition temperature for the melting process is relatively low because it requires only a partial reorganization of the crystal lattice. As a result, varying the cooling rate through the phase transition controls the lateral extent of chemical domains over several decades in size. At the fastest cooling rate, the dimensional confinement of the ferrielectric CuInP2S6 phase to nanoscale dimensions suppresses ferrielectric ordering due to intrinsic ferroelectric size-effect. Intralayer heterostructures can be formed, destroyed, and reformed by thermal cycling, thus enabling the possibility of finely tuned ferroic structures which can potentially be optimized for specific device architectures.
11:00 AM - TC07.09.10
Extended Defects in Thin Films and Heterostructures of Complex Oxides
Peter Sushko 1 , Iffat Nayyar 1 , Steven Spurgeon 1 , Arun Devaraj 1 , Scott Chambers 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractInterface-defined materials exhibit novel phenomena and tunable functions that can be enriched or suppressed by defects and deviations from ideality that emerge during their synthesis. To take full advantage of this class of materials, it is essential to characterize individual defects and extended defect structures, understand their origin and the nature of interaction with internal interfaces, and to develop protocols for their control. Epitaxial thin films and heterostructures of complex oxides are convenient materials systems to introduce and study such defect structures.
Here we will consider examples of thin oxide films synthesized using molecular beam epitaxy where intentional and unintentional deviations from the ordered structures dramatically affect their properties. First, we consider the case of epitaxial chromium ferrite spinel (Fe3–xCrxO4), where replacing Fe atoms with Cr converts it from a metal (x=0) to a semiconductor (x=1) to an insulator (x=2). Ab initio simulations suggest that Cr (Cr3+) preferentially substitutes Fe (Fe2+ and Fe3+) at the octahedral lattice sites. This trend creates a possibility to manipulate the electron distribution in the octahedral sub-lattice by judiciously selecting the concentration and spatial distribution of the Cr species. For example, formation of Cr-rich regions would displace the electron charge towards Fe-rich regions, leading to charge disproportionation between them. This effect can be used constructively to engineer materials with low-energy optical absorption, promote electron-hole separation, and open new channels for the electron transport.
In contrast, unintentional extended defects in La2MnNiO6 (LMNO) - a ferromagnetic semiconductor with a theoretical magnetic moment of 5 mB per formula unit - can lower this moment by a factor of four. Analysis of the initial stages of LMNO growth on SrTiO3 (001) indicates no detectable defects in the 3-5 nm region near the interface, while NiO inclusions begin to appear at larger distances. Ab initio simulations of prototype point defects as a function of LMNO thickness suggest that ordered double perovskite LMNO films, grown with molecular beam epitaxy, have a built-in electric field which favors the formation of oxygen vacancies for thicker films and, in turn, promotes the formation of Mn-Ni anti-site defects and NiO precipitates.
S.A. Chambers, T.C. Droubay, T.C. Kaspar, I.H. Nayyar, M.E. McBriarty, S.M. Heald, D.J. Keavney, M.E. Bowden, P.V. Sushko, Electronic and optical properties of a semiconducting spinel (Fe2CrO4), Adv. Funct. Mater. 27, 1605040 (2017)
S.R. Spurgeon, Y. Du, T. Droubay, A. Devaraj, X. Sang, P. Longo, P. Yan, P.G. Kotula, V. Shutthanandan, J.M. LeBeau, Ch. Wang, P.V. Sushko, S.A. Chambers, Multidimensional analysis of nanoscale phase separation in complex materials systems, Chem. Mater. 28, 3814 (2016)
S.R. Spurgeon et al., Cation disorder and oxygen deficiency-mediated phase separation in double perovskite oxides, (under review)
11:15 AM - TC07.09.11
Tunable Metal-to-Insulator Transition in La1-xNdxNiO3 Thin Films
Ryan Desautels 1 , Amanda Huon 3 2 , John Nichols 3 , Robert Green 4 5 , Steven May 2 , George Sawatzky 4 5 , Ho Nyung Lee 3 , Michael Fitzsimmons 1
1 Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Materials Science, Drexel University, Philadelphia, Pennsylvania, United States, 4 Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada, 5 Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia, Canada
Show AbstractTo elucidate the role of the NiO6 octahedral rotation on transport and magnetic properties, we have grown a series of coherently strained, 60 unit cell thick, La1-xNdxNiO3 single layer alloys on (001) SrTiO3. Growth on (001) SrTiO3 by pulsed laser epitaxy imparts a nominal tensile strain ranging from 1.9-2.4% depending on x. A Nd-doped LaNiO3 alloy will see a slight increase in ε as pure LaNiO3 has a tensile strain of ε(bulk) ≈ 1.9%. Transport measurements using the Van der Pauw configuration indicated three distinct regions of resistivity: i) an insulating state with no metal-to-insulator transition (x = 1), ii) a measurable metal-to-insulator transition (x = 4/8-7/8), and iii) metallic state with no metal-to-insulator transition (x < 4/8). Pure NdNiO3 is an insulator at all temperatures, contrary to previous experiments of NdNiO3 under tensile strain[1]. With the introduction of Nd (likely via site substitution), we observed a metal-to-insulator transition that increased with increasing x above x = 3/8. In addition, a transition that appears to be first order with the width of the temperature hysteresis increasing with increasing Nd content was also observed. Below x = 4/8, the thin films appear to be metallic down to the lowest temperatures measured, 2 K. Resonant elastic x-ray scattering experiments indicate that the films have the typical q=(1/4,1/4,1/4) antiferromagnetic ordering with Néel temperatures similar to the metal-to-insulator transition temperature, in agreement with previous experiments[2]. In this work, we have shown that with addition of Nd, an atom with a slightly smaller ionic radius (compared to La), results in the gradual transition from the metallic phase to the insulating phase; the result of a systematic reduction in the tolerance factor leading to a change in the NiO6 octahedra rotation.
S. Middey, J. Chakhalian, P. Mahadevan, J. W. Freeland, A. J. Millis, and D. D. Sarma, Physics of Ultrathin Films and Heterostructures of Rare-Earth Nickelates, Annu. Rev. Mater. Res., 46, (2016) 305-334 DOI: 10.1146/annurev-matsci-070115-032057
J. B. Torrance, P. Lacorre, A. I. Nazzal, E. J. Ansaldo, and Ch. Niedermayer, Systematic study of insulator-metal transitions in perovskites RNiO3 (R = Pr, Nd, Sm, Eu) due to closing of charge-transfer gap, Phys. Rev. B, 45, (1992) 8209-8212 DOI: 10.1103/PhysRevB.45.8209
11:30 AM - TC07.09.12
Study of Extended Defects in NiO Microcrystals Fabricated by a Vapor-Solid Method
María Taeño 1 , David Maestre 1 , Ana Cremades 1 , Javier Piqueras 1
1 , Univ Complutense de Madrid, Madrid Spain
Show AbstractNickel oxide (NiO) is a wide band gap p-type semiconductor with good electrical, optical and magnetic properties, as well as excellent chemical and thermal stability. This material has recently demonstrated potential applicability in electrochemical capacitors, alkaline batteries, smart windows and gas sensing, among others, exceeding in some cases the performance of conventional n-type semiconductor oxides. Achieving a high control of the shape and size of NiO can lead to optimize most of these applications, however NiO is usually synthesized in form of nanoparticles, ceramic or thin films by sol-gel, electrospinning or hydrothermal methods, and less has been done in the fabrication of micro- and nanostructures with variable morphology, so far.
In this work, a catalyst free evaporation-deposition method, using metallic nickel as precursor, has been employed to fabricate NiO micro and nanostructures. Thermal treatments were carried out at temperatures ranging from 800 to 1200 oC and durations of 10 – 15 h under a controlled Argon flow. Treatments at temperatures of 800 and 900 oC lead to the growth of rods some microns length with a low aspect ratio. By increasing the temperature of the treatment up to 1000 oC, large microcrystals are grown on the surface of the treated pellet, most of them exhibiting surfaces with big amounts of ordered hollow cavities with square sections of hundreds of nm, resembling inverted square pyramids. Besides, some other crystals surfaces exhibit a layered structure formed by piled terraces tens of nm high. Thermal treatments performed at higher temperature, up to 1200 oC, also result in microcrystals with holey or terraced surfaces, although the former appear in a lower concentration. The presence of these ordered cavities, which confers a high surface to volume ratio to the samples, could be associated with stress and formation of dislocations during the growth process, which finally result in the appearance of pinholes in energetically favourable surfaces. Catalytic and gas sensing devices could take advantage of this ordered holey appearance.
The micro- and nanostructures have been characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy and cathodoluminescence in a SEM. Raman and XRD confirm that the samples consists of NiO with cubic rock-salt structure, and no other nickel oxides or rest from the metallic Ni precursors are observed. The cathodoluminescence of the samples is formed by an emission centered at 1.7 eV and a wide emission between 2.2 and 2.5 eV, which origin is still under discussion, although should be associated with the extended defects formed during the growth.
11:45 AM - TC07.09.13
Observation and Tuning of Oxygen Anion Mobility in Gd-Doped Ceria Memristive Devices
Andreas Nenning 1 2 , Rafael Schmitt 2 , Roman Korobko 2 , Jennifer Rupp 1 2
1 , Massachusetts Institute of Technology, Cambridge, MA, Austria, 2 Department of Materials, ETH Zürich, Zurich, Zurich, Switzerland
Show AbstractResistive switching is a recently extensively studied phenomenon for use in non-volatile memories as promising building blocks of future electronics. In such devices transition metal oxides are operated at high electric field strengths to initiate non-linear and hysteretic current-voltage relations. Despite the known importance of oxygen vacancy defects in metal-oxide memristive devices, systematic strategies on tuning their concentration and mobility are rare.
In this work, we utilized the fluorite-type ceria-gadolinia system Ce1-xGdxO2-x/2 (GDC) with x = 0.03-0.3 to tune the oxygen vacancy concentration in order to correlate structure and defect chemistry to the memristive properties. The mixed conduction of GDC is well investigated at high temperature, which helps in understanding and interpreting the observed switching characteristics.
For this purpose Pt-GDC-Pt sandwich structures with a very thin (4 nm) top electrode were fabricated as model switching units to allow Raman spectroscopic investigation during electroforming and resistive switching. The strong voltage-induced changes in the Raman spectra were used to track the generation of defects and defect clusters during electroforming and resistive switching.
These experimental results directly prove that the electronic charge carriers generated by the formation and redistribution of oxygen vacancies at high electric fields are the origin of memristance in doped ceria films, and that the switching happens in a rather homogeneous manner. The importance of oxygen vacancy migration in our model is further verified by the doping study. Doping levels of 10-20 mol% Gd with the highest ionic conductivity2 resulted in good switching properties, whereas for low (<10 mol%) and high (>20 mol%) concentrations no resistive switching is observed. In both cases the number of mobile oxygen vacancies is not sufficient, either because of the low extrinsic doping level, or because of the formation of bixbyite-like domains with ordered or clustered "immobile" oxygen vacancies. This clustering is observed by the appearance of new vibrational modes in Raman spectroscopy at higher doping levels. Excitingly, "free" (randomly distributed) at low doping and "trapped" clustered vacancies at high doping levels could be observed and their impact on the resistive switching was clearly demonstrated3. In conclusion, we observed a clear correlation of oxygen vacancy mobility and switching characteristics. We suggest that a high concentration of mobile oxyen vacancies is of general importance when designing oxide materials for future computing applications.
References
[1] Schweiger, S., Kubicek, M., Messerschmitt, F., Murer, C., J.LM. Rupp (2014). ACS nano, 8(5), 5032-5048.
[2] T. Zhang, J. Ma, H. Cheng, S. Chan, Mater. Res. Bull. 41 (2006) (3) 563.
[3] R. Schmitt, R. Korobko, J. Spring, J.L.M. Rupp, “Design of Oxygen Vacancy Configuration for Memristive Systems”, accepted at ACS Nano
TC07.10: Design and Engineering Functional Defects for Photonics and Optoelectronics
Session Chairs
Thursday PM, November 30, 2017
Hynes, Level 2, Room 207
1:30 PM - *TC07.10.01
Understanding Defect Physics in Metal-Halide Perovskites for Optimizing Optoelectronic Devices
Annamaria Petrozza 1
1 , Fondazione Istituto Italiano di Tecnologia, Milano Italy
Show AbstractSemiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects.
Here I will review our understanding in the identification of key parameters which must be taken into consideration in order to evaluate the suscettibility of the perovkite crystals (2D and 3D) to the formation of defects, allowing one to proceed through a predictive synthetic procedure. I will discuss the role of defect physics in determing the open circuit voltage of metal halide perovskite solar cells and present technological strategies for the optimization of devices which include: 1) the engineering of the charge extracting layer (CEL), which accounts not only for the energy level alignment between the CELs and the perovskite, but also for the quality of the microstructure of the perovskite bulk film that is driven by the substrate surface; and 2) the use of inks based on colloidal suspensions of nanoparticles which lead to a high level of control over the material quality and device reliability, and offer more versatile processing routes by decoupling crystal growth from film formation.
2:00 PM - TC07.10.02
Design of Novel N-Type Transparent Conducting Oxides by Transition Metal Doping in In2O3
Jian Xu 1 , Jianbo Liu 1 , Shunning Li 1 , Baixin Liu 1 , Su-Huai Wei 2 , Bing Huang 2
1 School of Materials Science and Engineering, Tsinghua University, Beijing China, 2 , Beijing Computational Science Research Center, Beijing China
Show AbstractWith the emerging era of optoelectronic technologies, design of novel n-type transparent conducting oxide (TCO) materials that can outperform the conventional Sn-doped In2O3 (ITO), has stimulated extensive interest in the past decade. Transition-metal (TM), e.g., Mo, has been recognized as alternative dopants to achieve higher carrier densities in In2O3 than Sn, but there is a long-standing debate on the role of Mo in In2O3, partially due to the puzzling experimental observations. In this work, we have unveiled that Mo could be a good n-type dopant in In2O3 only if the growth temperature is sufficiently high, agreeing with the experimental observations well. By using state-of-the-art hybrid functional calculations, we have found a general rule for the doping behaviors of TM in In2O3. The present work not only has clarified a long-standing debate that whether Mo is a good dopant for n-type In2O3, but also provides a general route for the selection of suitable TM dopants to achieve higher carrier densities as compared to ITO, meanwhile maintaining the high charge carrier mobility and optical transparency of In2O3. Our study can provide valuable guidelines for future design of novel TCO materials in the field of optoelectronics.
2:15 PM - TC07.10.03
Ab Initio Study of the Effect of Hydrogen Centers on the Conductivity of In2O3
Amit Samanta 1 , Joel Varley 1 , Vincenzo Lordi 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractTransparent conductive electrode materials, like In2O3, are used in a variety of opto-electronic applications like advanced displays, smart windows, optical sensors, and solar cells. In spite of their widespread use, the effect of external impurities like water or hydrogen, that can be incorporated during film deposition or aging, remains unclear. We have systematically studied the stability of hydrogen centers and their propensity to form defect complexes with different charge states in In2O3 using ab initio density functional theory calculations with on-site Coulomb interactions. The formation free energies of the defects and Gibbs free energy changes associated with different defect reactions were calculated including the vibrational free energy contributions and the effects of oxygen, hydrogen, and water partial pressures. By simultaneously solving the laws of mass action and the charge balance condition, we obtained the defect concentrations at experimentally relevant conditions of temperature and partial pressures. The equilibrium defect concentrations were then used to obtain the defect stability diagrams as functions of temperature and partial pressures of oxygen, hydrogen, and water in the surroundings, from which changes in film conductivity associated with shifts in Fermi level could be predicted.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
2:30 PM - TC07.10.04
From Narrow Gap to Ultra Wide Band Gap Systems—The End of Band Gap Problem Excuses for Defect Calculations
Peter Schultz 1 , Arthur Edwards 2 , Renee Van Ginhoven 2 , Andrew Pineda 2
1 , Sandia National Labs, Albuquerque, New Mexico, United States, 2 , Air Force Research Laboratory (AFRL), Kirtland AFB, New Mexico, United States
Show AbstractModeling is an essential aspect of characterizing defect physics and enabling defect engineering in functional materials. The band gap problem—the inability of density functional theory (DFT) to directly predict the fundamental band gap—has long served as a ready excuse for the ongoing inability of DFT to contribute quantitatively to understanding, predicting, and controlling defect behavior in semiconductors and insulators. A shibboleth of modern defect physics asserts one cannot hope to get defect charge levels correct if one cannot get the band gap correct—but is this belief justified? Using local functionals with (occasionally severe) band gap problems, we describe the results of comprehensive defect calculations in over a dozen materials, ranging from narrow band gap systems to ultra-wide band gap systems, including group IV, III-V, II-VI, and I-VII materials. We find not only that the band gap problem does not prevent quantitative defect level calculations, but that the ‘defect band gap’—the range of defect levels accessible to local charge transitions—has excellent correlation with the fundamental experimental band gap. This is true for alkali halides with the widest gaps, and for InAs where the DFT gap nominally is zero; it captures the difference in band gaps between cubic and wurtzite structures in GaN and SiC; and demonstrates quantitative accuracy for defect level positions where unambiguous experimental data is available. Careful supercell size tests, up to 1000 atoms, confirm convergence to asymptotically dilute defect level limits in each material, and explicitly discriminate localized defect levels in the gap from shallow levels and band edges. The presence of a “band gap problem” cannot be used as a valid excuse for the failure to describe defects. Careful control of boundary conditions and explicit verification of convergence of computational approximations prove more crucial in obtaining reliable results. With these new developments, DFT can used with greater confidence in materials where experiment is less definitive in identifying and characterizing defects. — Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.
2:45 PM - TC07.10.05
Thermodynamic Routes to Novel Stable and Metastable Nitrogen-Rich Nitrides
Wenhao Sun 1 2 , Aaron Holder 3 , Bernardo Orvañanos 1 , Elisabetta Arca 3 , Andriy Zakutayev 3 , Stephan Lany 3 , Gerbrand Ceder 2
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractCompared to oxides, the nitrides are relatively unexplored, making them a promising chemical space for novel materials discovery. Of particular interest are nitrogen-rich nitrides, which often possess useful semiconducting properties for electronic and optoelectronic applications. However, such nitrogen-rich compounds are generally metastable, and the lack of a guiding theory for their synthesis has limited their exploration. Here, we examine the thermodynamics of how reactive nitrogen precursors can stabilize metastable nitrogen-rich compositions during materials synthesis. We map these thermodynamic strategies onto a predictive computational search; training a data-mined ionic substitution algorithm specifically for nitride discovery, which we combine with grand-canonical DFT phase stability calculations to compute stabilizing nitrogen chemical potentials. We identify 117 new binary and ternary metal nitride chemical spaces with nitrogen-rich nitrides. By formulating rational thermodynamic routes to metastable compounds, we expand the search space for functional technological materials beyond equilibrium phases and compositions.
3:30 PM - TC07.10.06
Residual Stress Depth Gradient Analysis in GaN Heterostructures
Michael Reisinger 2 , Manuel Tomberger 3 , Johannes Zechner 4 , Jozef Keckes 1 2
2 , Department of Materials Physics, Leoben Austria, 3 , Infineon Technologies Austria AG, Villach Austria, 4 , KAI Kompetenzzentrum Automobil- u. Industrieelektronik GmbH, Villach Austria, 1 , Erich Schmid Institut, Leoben Austria
Show AbstractGaN and related III – nitride based wide bandgap semiconductors are promising materials for the application in microelectronic and optoelectronic devices. Since there is a lack of native substrates, the control of residual stress gradients is an important aspect for the fine-tuning of electrical and optical properties of high quality heteroepitaxial structures. Consequently, the thermal and epitaxial mismatches have to be counterbalanced (i) by adjusting the deposition process parameters and (ii) by selecting a sophisticated multilayer design.
In this contribution, we have characterized the residual stress depth gradient of an AlxGa1-xN based multilayer structure by using in – situ and ex – situ stress analysis.
In industry the wafer curvature is routinely monitored during the epitaxial growth of semiconductor heterostructures. Hence the deposition parameters are adjusted on basis of in-situ curvature data in order to obtain wafers with a negligible overall residual stress to avoid large wafer bow after deposition. However, the residual stress depth gradient of epitaxial heterostructures can be determined using additional ex-situ XRD, TEM or FIB based characterization techniques. In our study we prove that the curvature evolution during layer growth can be used for the reliable determination of the epitaxial strain as a function of the layer thickness. Therefore, we evaluated the in-situ curvature data on basis of the Stoney equation, in order to obtain an in-situ live stress profile. Complementary, an ex-situ focused ion-beam (FIB) based Ion beam layer Removal (ILR) analysis was performed in order to reveal the complex stress gradient across the deposited heterostructure with a spatial resolution of 100 nm. The comparison of both stress profiles show, that the ex-situ and in-situ study display the same trend with a discrepancy between the absolute stress values. Subsequently, we show that the mismatch is caused by relaxation processes and thermal strain generation during cooling down from deposition to room temperature. With this approach we have the opportunity to separate thermal and epitaxial stress, which is crucial for the understanding and development of semiconductor deposition processes.
3:45 PM - TC07.10.07
Predicting Vacancy Formation Energies from Statistical Learning of Bulk Properties
Joel Varley 1 , Amit Samanta 1 , Vincenzo Lordi 1
1 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractPoint defects play an important role in determining a wide variety of materials properties and detailed analyses of their role often necessitates the evaluation of their formation energies and transition levels. Generally, formation of point defects involves displacing few atoms from their ideal positions meaning that defect formation is very localized process that involves very few degrees of freedom of a system. In this talk, we will illustrate how machine learning techniques can be used to predict cation vacancy formation energies and defect levels in II-VI and III-V materials with a quantifiable uncertainty using a set of simple descriptors. These descriptors are based on bulk properties and can be evaluated easily from ab initio simulations, or in some case from experimental data. We apply our model to predict defect levels for vacancies within the Cd-Zn-Se-Te alloy space and find good agreement with explicit calculations of different alloy compositions. Our analysis opens avenues for predicting the stability of defects in complex materials and alloys by training a coarse-grained model which can significantly simplify experimental design and material synthesis.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
4:00 PM - TC07.10.08
New Insights into Si Diffusion in III-V Semiconductor Alloys
Mardochee Reveil 1 , Hsien-Lien Huang 1 , Huang-Ta Chen 1 , Jason Liu 1 , Paulette Clancy 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractSilicon is a common dopant in III-V semiconductor materials such as InAs, InGaAs and GaAs. Those compound semiconductors, composed of elements from groups III and V in the periodic table, have desirable properties such as high electron mobility and small band gaps that make them excellent candidates for a wide range of electronic and photonic applications from high-performance transistors to lasers and optical detectors. Doping III-Vs with Si at high concentrations produces the counter-intuitive result that it not only saturates the level of dopant activation, as given by the total carrier concentration, but also considerably improves Silicon’s diffusivity. The underlying mechanism for this phenomenon was not clearly understood. In this set of computational studies, we investigate the atomistic origin of such experimentally observed diffusion and activation behavior. Our findings from ab initio calculations using Density Functional Theory techniques suggest that certain Si-defect complexes, such as split interstitials, might play an important role in this concentration-dependent behavior. Using InAs, InGaAs and GaAs as model systems, we explore the different diffusional pathways of silicon dopants and intrinsic defects such as vacancies, anti-sites and interstitials. We show that As is the most mobile species in all three compounds, whereas In is hardly mobile. We confirm that Si prefers to remain on the In/Ga sublattice, leading to n-type doping, but can also occupy As sites. We show that Si can also occupy interstitial sites, which are kinetically favorable for diffusion. We use our findings coupled with previously reported formation energy data from our research group to propose new diffusion models that explain the interconnection between atomistic jumps of all four species (Si, As, In and Ga) and how they affect dopant activation and modify trends in Si diffusion. These results significantly advance our understanding of diffusion and activation of group IV dopants such as Si in III-V compound materials.
4:15 PM - TC07.10.09
Examining the Valence of Thermally Diffused Cerium in Lithium Silicate Glass and the Related Radiation-Induced Optical Coloration
Michael Moore 1 , Steven Zinkle 1 , Jason Hayward 1
1 , University of Tennessee, Knoxville, Tennessee, United States
Show AbstractNeutron imaging often requires nonlinear detection systems that can accurately represent the spatial features of an irradiated object. While thin film and microchannel plate detectors have been heavily researched for this application, little effort has been made to create selective scintillating regions within structured silicate glass detectors. This is likely because most commercially available glass scintillators are drawn from a melt that uniformly contains an activator, with cerium-activated lithium silicates being one of the most common glass scintillators used. This paper presents the continued research of diffusing trivalent cerium in lithium loaded glass. The properties of cerium diffusion have been assayed with Rutherford Backscattering Spectrometry. Radiation-induced color centers formed on the diffusion samples as an effect of using the ion beam for these measurements. The valence state of the diffused cerium was then assayed as a function of depth with X-ray Photoelectron Spectroscopy, and the signal from cerium for the color centers was compared to the unirradiated sample surface.
4:30 PM - TC07.10.10
Exploiting Defect Processes in Chalcogenide Glass Films for Gradient Index Optics Applications
Myungkoo Kang 1 , Andrew Swisher 2 , Alexej Pogrebnyakov 2 , Liu Liu 2 , Andrew Kirk 3 , Stephen Aiken 3 , Clara Rivero-Baleine 3 , Charmayne Smith 1 , Carlo Pantano 2 , Theresa Mayer 2 , Kathleen Richardson 1
1 , University of Central Florida, Orlando, Florida, United States, 2 , The Pennsylvania State University, University Park, Pennsylvania, United States, 3 , Lockheed Martin Corporation, Orlando, Florida, United States
Show AbstractMicrolens arrays have become increasingly important for a wide range of applications, such as compact imaging, optical sensing, fiber coupling, and thermo-photovoltaic energy conversion. As a result, there has been growing demand for the development of low-cost, scalable fabrication techniques capable of manufacturing such high performance components. Although significant progress has been made using conventional spherical lenses, chromatic aberration and small operational bandwidth remains a lingering problem for infrared imaging. These challenges are difficult to overcome due to the lack of materials capable of broadband transmission as well as the feasibility for manufacturing multi-element/multi-material systems with micro-scale dimensions. Novel gradient refractive index (GRIN) materials can be engineered to provide dispersive properties which lie far outside those found in nature, providing new degrees of freedom for optical design as well as the potential for use in new applications. This work reports a novel photo-thermal process to spatially modulate high-index nanocrystals within a meta-stable thin film chalcogenide glass, composed of Ge-As-Pb-Se (GAP-Se) constituents, thereby achieving ultra-low dispersion over an unprecedented bandwidth of 1 to 12 µm wavelength while enabling control of an arbitrary index gradient required for GRIN optics.1 Spatially tailorable refractive index change is induced using a two-step fabrication approach, capable of fabricating achromatic optical components. Sub-bandgap laser exposure is used to create non-bonded, atomic defects in the metastable, homogeneous thin film GAP-Se glass which lead to the formation of Pb-rich amorphous secondary phases within the film. These phases are then subsequently crystallized into a high-index crystal phase by thermal treatment. The nanocrystal density is modulated by the laser dose, providing a spatially tailorable index change up to ~ 0.1. The measured chromatic properties after the index change were found to be superior to conventional homogeneous infrared media throughout the entire extent of the bandwidth.
1. Kang, M.; Swisher, A. M.; Pogrebnyakov, A. V.; Liu, L.; Kirk, A.; Aiken, S.; Rivero-Baleine, C.; Smith, C.; Pantano, C. G.; Mayer, T. S.; Richardson, K. Ultra-Low Dispersion Multicomponent Thin Film Chalcogenide Glass for Broadband Gradient Index Optics. (under review)
4:45 PM - TC07.10.11
Structural Characterization of Functional ZnO-InN Alloy Films
Junjun Jia 1 , Tomohiko Hara 1 , Yuzo Shigesato 1
1 , Aoyama Gakuin University, Kanagawa Japan
Show AbstractZnO film has been widely applied as transparent electrodes in various electronic devices, such as flat display panel and solar cell, due to high visible transmittance and good conductivity by doping other Group III elements (Al or Ga). ZnO is also expected to find applications in the optical electronic devices that allows light emission or absorption over a spectrum from the UV to the visible region, and the combination of ZnO with CdO or MgO creates opportunities for band gap engineering. However, because the crystal structure of ZnO is different from one of CdO and MgO, the phase separation occurs and the range of band gap engineering is limited1). Recently, indium nitride (InN) was reported to be easy alloy with ZnO because InN has the same crystal structure to ZnO2), and the band gap of ZnO can be adjusted from 1.6 to 2.7 eV by changing the composition x of (ZnO)x(InN)1−x.
In this study, we reported the fabrication method of (ZnO)x(InN)1−x film and clarified its physical properties with different x. (ZnO)x(InN)1−x films with the thickness of 200 nm were deposited on synthetic quartz glass by dc or rf magnetron sputtering. Ar was used as the sputtering gas, and N2 or N2O gas were used as the reactive gas. The anion contents (O and N) were adjusted by changing the gas flow ratio.
X-ray diffraction pattern showed a clear (002) diffraction peak of the wurtzite type from (ZnO)x(InN)1−x films. This peak shifted from the ZnO (002) plane to the InN (002) plane with increasing the indium content. The chemical composition ratios (Zn/(Zn+In)) in (ZnO)x(InN)1−x films varied from 14 to 91 at.%, which was evaluated by Electron Probe Micro Analyzer (EPMA). Moreover, we observed the local structure of metal-oxygen bonds for (ZnO)x(InN)1−x films using extended X-ray absorption fine structure (EXAFS) measurements, and compared them with those from Raman spectroscopy. Optical bandgaps shifted to lower photon energies with the increase in indium. Such tendency could be attributed to the formation of InN, which was confirmed by the chemical shifts in X-ray photoelectron spectroscopy (XPS). Subsequently, it was examined whether or not a proportional relationship (Vegard’s rule) exists between the lattice constant and the concentration of the composition element. In the region of the Zn/(Zn+In) ＝14~70 at.%, it confirmed that the lattice constant deviates slightly from Vegard's law, but almost agrees. These experimental results open up a new route to realize the bandgap engineering of ZnO thin films.
[1] K Sakurai et al., J. Cryst. Growth 237, 514 (2002).
[2] N Itagaki et al., Materials Research Express 1, 036405 (2014).