Klie R.F.,University of Illinois at Chicago |
Qiao Q.,University of Illinois at Chicago |
Paulauskas T.,University of Illinois at Chicago |
Ramasse Q.,SuperSTEM |
And 4 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012
Our ability to directly characterize the atomic and electronic structures is crucial to developing a fundamental understanding of structure-property relationships in complex-oxide materials. Here, we examine one specific example, the misfit-layered thermoelectric material Ca 3Co 4O 9, which exhibits a high Seebeck coefficient governed by spin-entropy transport as well as hopping-mediated electron transport. However, the role of oxygen and its bonding with cobalt in thermoelectric transport remains unclear. We use atomic-resolution annular bright-field imaging to directly image the oxygen sublattice and to combine our experimental data with multislice image calculations to find that the oxygen atoms in the CoO 2 subsystem are highly ordered, while the oxygen-atomic columns are displaced in the Ca 2CoO 3 subsystem. Atomic-column-resolved electron energy-loss spectroscopy and spectrum image calculations are used to quantify the bonding in the different subsystems of incommensurate Ca 3Co 4O 9. We find that the holes in the CoO 2 subsystem are delocalized, which could be responsible for the p-type conductivity found in the CoO 2 subsystem. © 2012 American Physical Society. Source
Pennycook S.J.,Oak Ridge National Laboratory |
Pennycook S.J.,University of Tennessee at Knoxville |
Zhou H.,North Carolina State University |
Chisholm M.F.,Oak Ridge National Laboratory |
And 5 more authors.
Acta Materialia | Year: 2013
Complex oxides are of intense interest due to their diverse properties, such as colossal magnetoresistance and superconductivity. Their complexity arises not only from the number of constituent elements, but also from their tolerance of non-stoichiometry and the structural complexity of these perovskite-based materials, e.g. the distortions and rotations of the oxygen octahedra surrounding the B-site cation. For these reasons, misfit accommodation in these materials is far more complex than in simpler materials, and can involve several different mechanisms simultaneously. In some cases, interfaces can be free from any misfit dislocations, lattice mismatch being accommodated via incorporation of oxygen vacancies, which take an ordered periodic arrangement. Interfaces may also present a perturbation to the octahedral rotations that can dramatically affect properties, not just close to the interface but through the entire film. In oxygen ion conducting materials, the oxygen sublattice may even melt in some situations. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source
The interfaces between the two oxides (represented in this idealized, atomically abrupt model by the yellow and purple bands) create an electric field. The field separates electrons (silver) excited by sunlight (gold), which could be used to catalyze hydrogen fuel production. Nobody wants a laptop computer that stops working when a cloud passes by. Storing sunlight as fuel that can be later used to drive fuel cells requires new materials. Scientists demonstrated such a material. They combined two oxides on the atomic scale. The interface between the oxide materials, one containing strontium and titanium (SrTiO ) and one containing lanthanum and chromium (LaCrO ), absorbs visible light, producing electrons and holes that might be useful for catalyzing reactions, such as producing hydrogen fuel. However, if there is nothing to pull those electrons and holes apart, they will quickly annihilate one another without doing anything useful. By carefully synthesizing this material as a series of alternating layers, the international team created a built-in electric field that could help separate the excited electrons and holes and improve the material's performance as a catalyst. This material opens up new scientific frontiers to solve a persistent energy challenge: storing solar energy for later use. Fuel cells capable of running on hydrogen fuel created by solar energy could allow people to heat their homes and run their computers on solar energy even in the dark of night. By depositing thin layers of SrTiO and LaCrO on a crystalline substrate, the investigators at the Pacific Northwest National Laboratory, Argonne National Laboratory, SuperSTEM, and the University of Oxford controlled how the ions at the interfaces bond to each other. This allowed them to engineer an electric field in the material that can be used to separate electrons and holes. Using X-ray spectroscopy techniques, they confirmed that the electric field was present and that the ions in the material shift positions as a result. Ultra-high resolution measurements using an electron microscope helped confirm this ionic shift. All of these experimental results were backed up by computational modeling of the materials, which predicted behavior in excellent agreement with what was observed. The electric field in these materials is present because the researchers were able to precisely control the growth process so that each interface has either a positive or negative charge. Alternating positively and negatively charged interfaces in the material produce nanoscale electric fields that can interact with the electrons and holes that are excited by solar energy. Electrons may then be driven to the surface, where they could interact with water molecules to break their bonds and produce hydrogen fuel. Researchers are continuing to explore the properties of these superlattices using cutting-edge X-ray measurements at synchrotrons around the world and using other advanced microscopy techniques to look at the chemical makeup of the interfaces. Explore further: New method to reduce the optical band gap of strontium titantate thin films More information: Ryan B. Comes et al. Interface-Induced Polarization in SrTiO -LaCrO Superlattices , Advanced Materials Interfaces (2016). DOI: 10.1002/admi.201500779
Using complementary microscopy and spectroscopy techniques, researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) say they have solved the structure of lithium- and manganese-rich transition metal oxides, a potentially game-changing battery material. Up to now, scientists have been divided into three schools of thought on the material's structure, but after nearly four years of study a team led by Alpesh Khushalchand Shukla and Colin Ophus have concluded that the least popular theory is in fact the correct one. They recently published their findings in Nature Communications. Other co-authors were Berkeley Lab scientists Guoying Chen and Hugues Duncan, and Quentin Ramasse and Fredrik Hage at SuperSTEM in Daresbury, UK. This material is important because it could potentially lead to batteries with double the energy capacity of today’s most commonly-used lithium-ion batteries, due to the extra lithium in its structure. "However, it doesn't come without problems, such as voltage fade, capacity fade and DC resistance rise," said Shukla. "It is immensely important that we clearly understand the bulk and surface structure of the pristine material. We can't solve the problem unless we know the problem." A viable battery with a marked increase in storage capacity would not only shake up the cell phone and laptop markets, it would also transform the market for electric vehicles (EVs). "The problem with the current lithium-ion batteries found in laptops and EVs now is that they have been pushed almost as far as they can go," said Ophus. "If we're going to ever double capacity, we need new chemistries." Using state-of-the-art electron microscopy techniques at the National Center for Electron Microscopy (NCEM) at Berkeley Lab's Molecular Foundry and at SuperSTEM, the researchers were able to image the material at atomic resolution. Because previous studies have been ambiguous about the structure, the researchers minimized ambiguity by looking at the material from different directions, or zone axes. "Misinterpretations from electron microscopy data are possible because individual two-dimensional projections do not give you the three-dimensional information needed to solve a structure," Shukla said. "So you need to look at the sample in as many directions as you can." Scientists have been divided on whether the material structure is single trigonal phase, double phase or defected single monoclinic phase: the ‘phase’ of a material refers to the arrangement of the atoms with respect to each other. Ophus, a project scientist at the Molecular Foundry, explains how easy it is for researchers to reach different conclusions: "The two-phase and one-phase model are very closely related. It's not like comparing an apple to an orange – it's more like comparing an orange and a grapefruit from very far away. It's hard to tell the difference between the two." In addition to viewing the material at atomic resolutions along multiple zone axes, the researchers made another important decision: to view entire particles rather than just a subsection. "Imaging with very high fields of view was also critical in solving the structure," Shukla said. "If you just look at one small part you can't say that the whole particle has that structure." Putting the evidence together, Shukla and Ophus are fairly convinced that the material is indeed defected single phase. "Our paper gives very strong support for the defected single-phase monoclinic model and rules out the two-phase model, at least in the range of compositions used in our study," said Ophus, whose expertise is in understanding structure using a combination of computational methods and experimental results. "We need to know what goes on at the atomic scale in order to understand the macroscopic behavior of new emerging materials, and the advanced electron microscopes available at national facilities such as SuperSTEM or NCEM are essential in making sure their potential is fully realized," added Ramasse, director of SuperSTEM. In addition to solving the structure of the bulk material, which has been studied by other research groups, the team also solved the surface structure. This is different to the bulk structure, consisting of just a few layers of atoms on select crystallographic facets. "The intercalation of lithium starts at the surface, so understanding the surface of the pristine material is very important," Shukla said. On top of the STEM (scanning transmission electron microscopy) imaging used for the bulk, they had to use additional techniques to solve the surface structure, including EELS (electron energy loss spectroscopy) and XEDS (X-ray energy dispersive spectroscopy). "We show for the first time which surface structure occurs, how thick it is, how it's oriented in relation to the bulk, and in particular on what facets the surface phase does and doesn't exist," Ophus said. An important part of the study was the quantity and quality of the samples studied. The scientists started with lab-made samples, prepared by Duncan, a chemistry postdoc in Chen’s lab whose research focuses on lithium-ion batteries. Duncan used a molten-salt method that produces high-quality discrete primary particles that are impurity-free, making them ideal for fundamental characterization. Taking a conservative approach, the researchers also decided to procure and analyze two commercial samples from two different companies. "We could have finished the paper a year earlier, but because there was so much controversy we wanted to make sure we didn't leave any stone unturned," said Shukla. Although a scientist with Berkeley Lab's Energy Storage and Distributed Resources Division at the time he did this work, Shukla has since become a consulting scientist at Envia Systems while continuing to be affiliated with Berkeley Lab as a user of the Molecular Foundry. In the end, it took nearly four years to complete the research. Ophus calls it a "tour de force of microscopy" because of its thoroughness. This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.
Home > Press > Thin-film solar cells: How defects appear and disappear in CIGSe cells: Concentration of copper plays a crucial role Abstract: Copper-indium-gallium-selenide (CIGSe) solar cells have the highest efficiency of polycrystalline thin-film solar cells. The four elements comprising CIGSe are vapour-deposited onto a substrate together to form a very thin layer of tiny chalcopyrite crystals. It is an exceedingly complex process controlled by many variables. This is why CIGSe modules in standard industrial formats have not yet attained the record efficiency already demonstrated at laboratory scale. One possible cause: defects that reduce the efficiency level can form during the course of fabrication. A collaboration of German, Israeli, and British teams has now conducted detailed studies of how different fabrication techniques influence the microstructure. They were able for the first time to observe the defects as these formed during deposition and under what conditions they self-healed by using in-situ X-ray diffraction and fluorescence analysis capabilities at the BESSY II X-ray source. Additional copper helps defects heal Vapour deposition of thin CIGSe films is a complex process. Indium, gallium, and selenium are first deposited on the substrate. The deposition of the copper and selenium atoms takes place in a second step. These atoms migrate into the In-Ga-Se layer. Tiny CIGSe crystals of chalcopyrite form there. The concentration of copper only reaches the correct value over the course of this second step. The prior copper-poor phase is characterised by numerous defects within the crystal. The defects increasingly disappear with the addition of copper and selenium. If more copper and selenium atoms are added after reaching the "right" ratio, then these two elements no longer fit into the existing crystal matrix and deposit themselves as copper and selenium grains in and on the polycrystalline CIGSe layer. This is actually problematic, since the grains must be removed afterwards. Nevertheless, they apparently have an important function in reducing the defects to near zero, as the current work shows. Analysing growing structures of elements in real time Dr. Roland Mainz and his colleagues at HZB were able to observe the changes to the film structure during deposition using X-ray diffraction at the EDDI beamline of BESSY II - in real time. At the same time, they were able to use X-ray fluorescence to analyse the elemental composition of the thin-film layer as it grew. Simultaneous observation with two methods enabled them to obtain a new insight: "The annihilation of the defects takes place very rapidly - just prior to the excess copper-selenium grains being deposited on the surface of the CIGSe film and the film entering the copper-rich phase. So far, we had only understood the copper-rich phase as being important for the growth of the grains. Now we know that it also plays an important role in the elimination of the defects", explains Mainz. Improving vapour deposition processes for high-quality CIGSe films Helena Stange, co-author of the study, simulated the influence of the various types of defects on the diffraction signal. The in-situ observations fit extremely well with the simulations and with the results derived from different imaging processes used to study the samples in various stages of deposition by teams at the Max Planck Institute for Solid State Research in Stuttgart, the SuperSTEM Lab in Daresbury, England, and at the Racah Institute, Jerusalem. An additional important result is that the temperature during deposition represents a relatively uncritical parameter for defect elimination. As soon as the layer reaches the copper-rich state, it makes little difference whether the process takes place at 400 degrees Celsius or 530 degrees Celsius. This insight is also of assistance in improving the procedure for depositing onto large surface areas. Instead of trying to maintain as homogenous a temperature as possible over the entire surface, other parameters could be optimised. The collaboration is part of the Helmholtz Virtual Institute of "Microstructure control for thin-film solar cells" that has been funded from 2012 to 2018. For more information, please click If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.