27 research outputs found
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Wannier–Koopmans method calculations for transition metal oxide band gaps
No full textThe widely used density functional theory (DFT) has a major drawback of underestimating the band gaps of materials. Wannier–Koopmans method (WKM) was recently developed for band gap calculations with accuracy on a par with more complicated methods. WKM has been tested for main group covalent semiconductors, alkali halides, 2D materials, and organic crystals. Here we apply the WKM to another interesting type of material system: the transition metal (TM) oxides. TM oxides can be classified as either with d0 or d10 closed shell occupancy or partially occupied open shell configuration, and the latter is known to be strongly correlated Mott insulators. We found that, while WKM provides adequate band gaps for the d0 and d10 TM oxides, it fails to provide correct band gaps for the group with partially occupied d states. This issue is also found in other mean-field approaches like the GW calculations. We believe that the problem comes from a strong interaction between the occupied and unoccupied d-state Wannier functions in a partially occupied d-state system. We also found that, for pseudopotential calculations including deep core levels, it is necessary to remove the electron densities of these deep core levels in the Hartree and exchange–correlation energy functional when calculating the WKM correction parameters for the d-state Wannier functions
Wannier Koopman method calculations of the band gaps of alkali halides
No full textCorrecting the band structure within the density functional theory (DFT) formalism is a long term goal for its development. Recently, we have proposed a Wannier Koopman method (WKM) to correct the DFT bandgap using the Kohn-Sham equation. Previous tests show that WKM works well for common semiconductors. Here, we test its accuracy in terms of predicting the bandgap of extreme ionic crystals: alkali halides. We found that the WKM can accurately reproduce the alkali halide bandgaps with accuracy in par with the GW method. On the other hand, the hybrid functional with common parameters, which work well for common semiconductors, significantly under-estimate the alkali halides. Published by AIP Publishing.National Materials Genome Project of China [2016YFB0700600]; National Natural Science Foundation of China [21603007, 51672012]; Shenzhen Science and Technology Research Grant [JCYJ20150729111733470, JCYJ20151015162256516]; Office of Science (SC), Basic Energy Science (BES), Materials Science and Engineering Division (MSED), of U.S. Department of Energy (DOE) [DE-AC02-05CH11231, KC2301]SCI(E)ARTICLE511
Defect Engineering in Titanium-Based Oxides for Electrochemical Energy Storage Devices
No full textDefect engineering involves the manipulation of the type, concentration, mobility or spatial distribution of defects within crystalline structures and can play a pivotal role in transition metal oxides in terms of optimizing electronic structure, conductivity, surface properties and mass ion transport behaviors. And of the various transition metal oxides, titanium-based oxides have been keenly investigated due to their extensive application in electrochemical storage devices in which the atomic-scale modification of titanium-based oxides involving defect engineering has become increasingly sophisticated in recent years through the manipulation of the type, concentration, spatial distribution and mobility of defects. As a result, this review will present recent advancements in defect-engineered titanium-based oxides, including defect formation mechanisms, fabrication strategies, characterization techniques, density functional theory calculations and applications in energy conversion and storage devices. In addition, this review will highlight trends and challenges to guide the future research into more efficient electrochemical storage devices.No Full Tex
Few-Layer Fe-3(PO4)(2).8H(2)O: Novel H-Bonded 2D Material and Its Abnormal Electronic Properties
No full textUsing first-principles calculations, we study the structural and electronic properties of a new layered hydrogen-bonded 2D material Fe-3(PO4)(2).8H(2)O. Interestingly, unlike other common 2D materials, such as layered van der Waals 2D materials, the band gap of 2D Fe-3(PO4)(2).8H(2)O-(010)-(1 x 1) is smaller than bulk Fe-3(PO4)(2).8H(2)O, which does not obey the normal quantum confinement effect and can be attributed to the edge states and the hydrogen bonds between the layers. We also find that the band-gap variation with the reduced layers depends on the length of the interlayer hydrogen bond and the stronger interlayer hydrogen bond leads to the larger band gap.National Materials Genome Project [2016YFB0700600]; Guangdong Innovation Team Project [2013N080]; Shenzhen Science and Technology Research [ZDSY20130331145131323, JCYJ20140903101633318, JCYJ20140903101617271]SCI(E)[email protected]; [email protected]
First-Principles Study of Cu<sub>9</sub>S<sub>5</sub>: A Novel p-Type Conductive Semiconductor
No full textThe role of M@Ni6 superstructure units in honeycomb-ordered layered oxides for Li/Na ion batteries
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Rapid Mining of Fast Ion Conductors via Subgraph Isomorphism Matching
No full textThe rapidly evolving field of inorganic solid-state electrolytes (ISSEs) has been driven in recent years by advances in data-mining techniques, which facilitates the high-throughput computational screening for candidate materials in the databases. The key to the mining process is the selection of critical features that underline the similarity of a material to an existing ISSE. Unfortunately, this selection is generally subjective and frequently under debate. Here we propose a subgraph isomorphism matching method that allows an objective evaluation of the similarity between two compounds according to the topology of the local atomic environment. The matching algorithm has been applied to discover four structure types that are highly analogous to the LiTi2(PO4)(3) NASICON prototype. We demonstrate that the local atomic environments similar to LiTi2(PO4)(3) endow these four structures with favorable Li diffusion tunnels and ionic conductivity on par with those of the prototype. By further taking into account the electronic structure and electrochemical stability window, 13 compounds are identified to be potential ISSEs. Our findings not only offer a promising approach toward rapid mining of fast ion conductors without limitation in the compositional range but also reveal insights into the design of ISSEs according to the topology of their framework structures.THEO
Wannier-Koopmans method calculations of organic molecule crystal band gaps
No full textIt is important to accurately predict the band gaps of crystals, including organic crystals, with low computational cost. Despite the significant underestimation of the crystal band gap by the density functional theory (DFT), a recently proposed Wannier-Koopmans method (WKM) based on DFT calculations seems to yield accurate band gaps for a wide class of materials including common semiconductors, alkali halides and 2D materials. It is nevertheless important to test the limit of WKM, in particular in systems with unique characteristics. In this work, we apply the WKM to 10 organic small molecule crystals and find that the WKM calculated band gaps agree well with GW results. We also introduce a new way to calculate the Wannier functions in the WKM calculations
