94 research outputs found
A kinetic Monte Carlo simulation method of van der Waals epitaxy for atomistic nucleation-growth processes of transition metal dichalcogenides
No full textAbstractControlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.</jats:p
Kinetic Stability of Bulk LiNiO₂ and Surface Degradation by Oxygen Evolution in LiNiO₂-Based Cathode Materials
No full textDue to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is limited to current UTD affiliates (use the provided Link to Article).Capacity degradation by phase changes and oxygen evolution has been the largest obstacle for the ultimate commercialization of high-capacity LiNiO₂-based cathode materials. The ultimate thermodynamic and kinetic reasons of these limitations are not yet systematically studied, and the fundamental mechanisms are still poorly understood. In this work, both phenomena are studied by density functional theory simulations and validation experiments. It is found that during delithiation of LiNiO₂, decreased oxygen reduction induces a strong thermodynamic driving force for oxygen evolution in bulk. However, oxygen evolution is kinetically prohibited in the bulk phase due to a large oxygen migration kinetic barrier (2.4 eV). In contrast, surface regions provide a larger space for oxygen migration leading to facile oxygen evolution. These theoretical results are validated by experimental studies, and the kinetic stability of bulk LiNiO₂ is clearly confirmed. Based on these findings, a rational design strategy for protective surface coating is proposed.International Energy Joint R&D Program (No. 20168510011350) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Knowledge Economy, Korean government. This work was also supported by the L&F Co.'s World Class 300 Project of the Korea Institute of Advancement of Technology (KIAT) funded by the Ministry of Trade, Industry and Energy (No. S2483103).Erik Jonsson School of Engineering and Computer ScienceSchool of Natural Sciences and Mathematic
Effects of alloying elements on the solubility, diffusivity, and permeability of hydrogen in TiC from first-principles calculation
No full textFirst-principles calculations were utilized to explore the effects of Zr, Nb, Ta, and W on the solubility, diffusivity, and permeability of hydrogen in TiC. It is found that the trigonal site of H in TiC, which is encircled by three Ti atoms lying a {111} plane, has been identified as thermodynamically more stable. The addition of alloying elements will destroy the equilibrium distribution of the Ti-H interaction and reduce the structural stability of interstitial H. Furthermore, the solubility, diffusivity, and permeability of H in Ti32C32 and Ti31MC32 (M = Zr, Nb, Ta, and W) at finite temperatures are derived, and the results are discussed and compared with similar experiments in the literature. Drawing upon the influence of elemental alloying on H retention, we deeply understand experimental observations and provide experimentalists with a potentially valuable recommendation for controlling hydrogen permeability
Graph Encryption for Shortest Path Queries with k Unsorted Nodes
No full textShortest distance queries over large-scale graphs bring great benefits to various applications, i.e., save planning time and travelling expenses. To protect the sensitive nodes and edges in the graph, a user outsources an encrypted graph to an untrusted server without losing the query ability. However, no prior work has considered the user requirement of the shortest path with k unsorted nodes. In particular, we are concerned with how to securely find the shortest path by passing k nodes that do not have a fixed traverse order. To solve the problems, we propose Gespun (stands for Graph encryption for shortest path queries with k unordered nodes). It includes an oracle encryption scheme that is provably secure against the semi-honest server. Specifically, we compute the shortest paths and distances for all nodes locally to obtain path-distance oracles. We transform the shortest paths to a sequence of secure codes by using a pseudo-random permutation to protect the structure privacy. We encrypt the shortest distance by using additively homomorphic encryption. Second, we pack the oracles in link-list nodes and store them in an array-based dictionary after another permutation. Next, we construct a search graph to compute the shortest path while guaranteeing that the path passes the required k nodes. We formally prove that Gespun is adaptively semantically-secure in the random oracle. We implement a prototype of Gespun and evaluate its performance. Experiments results demonstrate that Gespun is efficient, e.g., a query over 6301 nodes, 20777 edges, and 5 unsorted nodes only needs 483 ms to get queried results. We believe that our research problem span new research that soon promotes a new line of graph encryption schemes.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Cyber Securit
Optimal rate list decoding of folded algebraic-geometric codes over constant-sized alphabets
No full textWe construct a new list-decodable family of asymptotically good algebraic-geometric (AG) codes over fixed alphabets. The function fields underlying these codes are constructed using class field theory, specifically Drinfeld modules of rank 1, and designed to have an automorphism of large order that is used to “fold” the AG code. This generalizes earlier work by the first author on folded AG codes based on cyclotomic function fields. The recent linear-algebraic approach to list decoding can be applied to our new codes, and crucially, we use the Chebotarev density theorem to establish a polynomial upper bound on the list-size for list decoding up to an error fraction approaching 1 – R where R is the rate. The list decoding can be performed in polynomial time given polynomial amount of pre-processed information about the function field.Our construction yields algebraic codes over constant-sized alphabets that can be list decoded up to the Singleton bound — specifically, for any desired rate R ∊ (0, 1) and constant ∊ > 0, we get codes over an alphabet size that can be list decoded up to error fraction 1 – R – ∊ confining close-by messages to a subspace with elements. Previous results for list decoding up to error-fraction 1 – R – ∊ over constant-sized alphabets were either based on concatenation or involved taking a carefully chosen subcode of algebraic-geometric codes. In contrast, our result shows that these folded algebraic-geometric codes themselves have the claimed list decoding property. Further, our methods to get function fields with the properties needed for constructing and decoding the code might be of independent algebraic interest.Published versio
Alloying effects of Zr, Nb, Ta, and W on thermodynamic and mechanical properties of TiC based on first-principles calculation
No full textFirst-principles calculation has been used to study the temperature-dependent thermodynamic and mechanical properties of TiC with additions of transition metal elements through the combination of quasi-harmonic Debye model and thermal electronic excitation. It is found that the substitution behaviors of Zr, Nb, Ta, and W doped into TiC are not only structurally stable, but also would increase its melting temperature. To further determine the alloying effects, the mechanical parameters of doped TiC at finite temperatures have been established by means of ductility, Vickers hardness, fracture toughness, and machinability damage tolerance, from which we reveal that it is feasible to substitute Ti by W in TiC. The computed results are in good agreements with experimental observations in the literature, and are discussed in terms of electronic structures and bond characteristics
Beating the probabilistic lower bound on -perfect hashing
No full textFor an integer , a perfect -hash code is a block code over
of length in which every subset
of elements is
separated, i.e., there exists such that
,
where denotes the th position of
. Finding the maximum size of perfect -hash codes of
length , for given and , is a fundamental problem in combinatorics,
information theory, and computer science. In this paper, we are interested in
asymptotic behavior of this problem. Precisely speaking, we will focus on the
quantity .
A well-known probabilistic argument shows an existence lower bound on ,
namely \cite{FK,K86}.
This is still the best-known lower bound till now except for the case
\cite{KM}. The improved lower bound of was discovered in 1988 and there
has been no progress on the lower bound of for more than years. In
this paper we show that this probabilistic lower bound can be improved for
from to and all odd integers between and , and \emph{all
sufficiently large} .Comment: arXiv admin note: text overlap with arXiv:1010.5764 by other author
Nucleation and growth of WSe <sub>2</sub> : enabling large grain transition metal dichalcogenides
No full textThe limited grain size (< 200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe₂ is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe₂ thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).NSF Award No. 1407765; National Research Foundation of Korea no. 2013K1A4A3055679Erik Jonsson School of Engineering and Computer Scienc
The Research on Exogenous Problems of Farmers’ Piritual and Cultural Education in China
No full textThe author studied and analyzed the exogenous problems of the farmers’ spiritual and cultural education, and found out: In today’s China, the exogenous problems of the farmers’ spiritual and cultural education mainly reflected in the separation of spiritual and cultural education is from social environment, political system, economic development, and cultural concepts etc. Then the author put forward to the countermeasures and suggestions aimed at optimizing the allocation of famers’ spiritual and cultural educations resources, environment and evaluation system construction and so on
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