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Defect physics as key to understanding complex materials

FEATURED RESEARCH - POSTED ON AUG 08, 2016

Dr. Khang Hoang's research in the Center for Computationally Assisted Science and Technology (CCAST) focuses on modeling, theory, and design of materials for energy-related applications. Using methods such as first-principles calculations, Dr. Hoang investigates complex phenomena and processes in materials at the electronic and atomic level. In such calculations, certain processes, including those that are not easily accessed by experimental means, can be isolated and studied. More importantly, they can provide physical insights into the mechanism that are governing the most relevant processes. Such an understanding is crucial for explaining, predicting, and optimizing materials' properties, and ultimately for rational design of materials with better performance. His research has resulted in multiple publications in high-ranked scientific journals, including Physical Review Letters, Angewandte Chemie International Edition, and Chemistry of Materials.

One major theme in Dr. Hoang's current research is developing a detailed understanding of the thermodynamics and kinetics of and the growth–structure–property relationship in structurally and chemically complex functional materials via comprehensive defect studies. Specific materials under investigation include complex oxides, chalcogenides, and nitrides for batteries, solid-oxide fuel cells, thermoelectrics, photovoltaics, and optoelectronics. His research is currently supported by the U.S. Department of Energy and by CCAST. Samples from Dr. Hoang’s work are given below:

Figure 1: Mechanisms for lithium extraction in lithium-ion battery cathode materials: on the left, involving oxidation at the transition-metal site, e.g., in layered LiMnO2 and, on the right, at the oxygen site, e.g., in lithium-rich layered Li2MnO3. Large gray spheres are Li, medium blue are Mn, and small red are O.

Defect physics in complex transition-metal oxides: Materials for battery electrodes are often complex oxides in which defects can be essential or detrimental to the functional properties. Dr. Hoang has developed a comprehensive approach based first-principles defect calculations that connects defect physics with electrochemistry [Phys. Rev. Appl. 3, 024013 (2015)]. The approach is unique in its ability to effectively investigate defect landscapes, lithium extraction and insertion mechanisms, ionic and electronic conduction, and effects of doping. A major accomplishment in this area is the discovery of different defect landscapes in cathode material LiFePO4 (LFP) and, most importantly, specific conditions under which LFP samples free or with only a minimal amount of the unwanted iron antisite defects can be synthesized [Chem. Mater23, 3003 (2011)]. Another major discovery is about the intrinsic mechanism for lithium extraction in Li2MnO3: the delithiation process is found to involve oxidation at the oxygen site, instead of the transition-metal site like in other electrode materials, and the formation of bound oxygen-hole polarons, i.e., localized electron holes on oxygen [Phys. Rev. Appl. 3, 024013 (2015); see also Figure 1]. The finding explains the unconventional electrochemical properties of Li2MnO3 and related lithium-rich layered oxide materials, opening the door to utilizing the oxygen-oxidation mechanism–in combination with the conventional mechanism involving the transition metal–in design of high-capacity battery materials.

Figure 2: Mechanisms for the decomposition of lithium amide (LiNH2): on the left, involving the formation of intrinsic defects in the interior of the material and, on the right, at the surface (symbolized by the vertical black dash-dotted line). Large gray spheres are Li, medium blue are N, and small pink are H.

Hydrogen desorption kinetics in complex hydrides: Hydrogen storage in the form of complex hydrides has attracted attention because of the high volumetric and gravimetric densities. However, like most hydrogen storage materials, these hydrides have unfavorable thermodynamics and slow hydrogen uptake and release. In order to address the latter issue, Dr. Hoang and collaborators have developed a computational approach based on first-principles defect calculations to understanding the hydrogen desorption kinetics of complex hydrides. Based on this approach, they have uncovered the underlying atomistic, defect-mediated mechanisms for decomposition and dehydrogenation in metal alanates, borohydrides, and amides, and identified processes that act as rate-limiting steps in the reaction kinetics [see, e.g., Phys. Chem. Chem. Phys16, 25314 (2014)]. Their results provide explanations for the experimental observations, e.g., the particle-size dependence of the dehydrogenation kinetics, and specific solutions for improving the storage performance of the materials. Dr. Hoang's work also offers a general framework for understanding the reaction kinetics in bulk versus nanoscale systems [Angew. Chem. Int. Ed50, 10170 (2011); see also Figure 2].

Figure 3: Rare-earth, e.g., Er3+, excitation scheme following a band-to-band transition. The Er-related defect level D acts as a carrier trap; an electron captured here can subsequently recombine nonradiatively with a free hole from the valence band (VB) or a hole at some acceptor level and transfer energy to the Er 4shell.

Luminescent rare-earth centers in wide-gap semiconductors: Rare-earth (RE) dopants have received great attention as they lead to sharp intra-f shell optical transitions which are desirable for spintronic and optoelectronic applications. The Er center in, e.g., GaN, can be excited by a direct absorption of energy into the 4f-electron core (resonant excitation) or indirectly by energy transfer from the host (band-to-band excitation; see Figure 3). The latter excitation mechanism is believed to be mediated by Er-related defects in which the presence of defect levels in the host band gap, acting as carrier traps, may have a crucial role. Using first-principles calculations, Dr. Hoang studies the interaction between the Er dopant and the host GaN, including intrinsic defects and other impurities that may be present in the host material, and identify possible optically active Er centers [Phys. Status Solidi RRL 9, 722 (2015)]. He finds that multiple Er centers with different Stark splittings, luminescence decay dynamics, and excitation cross sections are possible in Er-doped GaN samples.

Selected Recent Publications (see Google Scholar for complete publications):
• K. Hoang and M. Johannes, "Defect physics and chemistry in layered mixed transition-metal oxide cathode materials: (Ni,Co,Mn) vs (Ni,Co,Al)," Chem. Mater. 28, 1325 (2016).
• K. Hoang, "Hybrid density functional study of optically active Er3+ centers in GaN," Phys. Status Solidi RRL 9, 722 (2015).
• K. Hoang, "Defect physics, delithiation mechanism, and electronic and ionic conduction in layered lithium manganese oxide cathode materials," Phys. Rev. Appl. 3, 024013 (2015).
• K. Hoang, A. Janotti, and C. G. Van de Walle, "The role of native defects in the transport of charge and mass and the decomposition of Li4BN3H10," Phys. Chem. Chem. Phys. 16, 25314 (2014).
• K. Hoang, "Understanding the electronic and ionic conduction and lithium over-stoichiometry in LiMn2O4 spinel," J. Mater. Chem. A 2, 18271 (2014).
• K. Hoang and M. D. Johannes, "Defect chemistry in layered transition-metal oxides from screened hybrid density functional calculations," J. Mater. Chem. A 2, 5224 (2014); featured as "Hot Article".
• K. Hoang, A. Janotti, and C. G. Van de Walle, "Mechanisms for the decomposition and dehydrogenation of Li amide/imide," Phys. Rev. B85, 064115 (2012).
• K. Hoang and M. D. Johannes, "First-principles studies of the effects of impurities on the ionic and electronic conduction in LiFePO4," J. Power Sources 206, 274 (2012).
• K. Hoang and C. G. Van de Walle, "Mechanism for the decomposition of lithium borohydride," Int. J. Hydrogen Energy 37, 5825 (2012).
• K. Hoang, A. Janotti, and C. G. Van de Walle, "Decomposition mechanism and the effects of metal additives in the kinetics of lithium alanate," Phys. Chem. Chem. Phys14, 2840 (2012).
• K. Hoang and M. D. Johannes, "Tailoring native defects in LiFePO4: Insights from first-principles calculations," Chem. Mater. 23, 3003 (2011).
• K. Hoang, A. Janotti, and C. G. Van de Walle, "The particle-size dependence of the activation energy for decomposition of lithium amide," Angew. Chem. Int. Ed. 50, 10170 (2011).


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