Kinetic Monte Carlo Calculations Research Articles

Unraveling the Dynamics of Solid-Electrolyte Interphase (SEI) Formation on Lithium Metal: Insights from Multiscale Modeling

We need batteries to get the best out of renewable energies and other low-carbon energy sources, and the most promising path forward is to invest in high-energy-density battery technologies. In this regard, lithium metal is the best candidate for the anode electrode as it is the lightest metal on earth and has the lowest electrochemical potential (-3.04 V vs. SHE). However, the challenge to overcome for mass commercializing lithium-metal batteries (LMB) is stabilizing the solid electrolyte interphase that naturally grows at the electrode-electrolyte interface to avoid uncontrolled reactions leading to lithium depletion. Since the successful introduction of lithium-ion batteries (LIB), it has been known that tuning of the SEI composition and morphology is fundamental to prolong the battery life span; a well-engineered SEI layer simultaneously serves as an electron barrier and favors Li+ ion conduction across it, ensuring proper battery performance. However, the engineering of the SEI layer in LMB is more complex than its counterpart LIB batteries, given that lithium metal is overwhelmingly more reactive than graphite and the overall Li+ ion current across it is orders of magnitude higher. The up-to-date knowledge on the SEI morphology indicates that inorganic phases produced upon the decomposition of lithium salts present in the electrolyte tends to grow dominantly buried within the SEI layer, acting as an intermediate between the surviving lithium metal and the outermost organic SEI phase grown after decomposition of solvent molecules. The effect of electrolyte additives and diluents is to tweak the SEI composition. The narrative built after years of experimentation tells that proper battery functioning depends heavily on the ability of the incoming Li+ ions to desolve near the SEI outermost layer without triggering further side reactions, penetrate the inner SEI, and deposit in the anode as reduced lithium. However, the intricacies of the elementary reactions and the stage where the reduction takes place still fall beyond detailed comprehension, even though the development of in situ and operando analytical techniques has brought us a level of understanding on the subject that was unimaginable a few decades ago.In this talk, we discuss the instrumental impact that multiscale modeling techniques have played in clarifying the elementary steps of the SEI formation on pristine lithium metal, providing a comprehensive understanding of the precursors formed in the early stages of Solid-Electrolyte Interphase (SEI) growth. For instance, density functional theory (DFT) calculations have revealed that electrolyte diluents, such as TTE, previously considered inert to lithium metal, actively modify the liquid electrolyte solvation structure by weakly interacting with Li+ ions. Additionally, these diluents decompose against lithium metal, releasing fluoride ions and inundating the SEI layer with LiF. Similarly, DFT-based molecular dynamics (MD) techniques, including ab initio (AIMD) and reactive classical molecular dynamics methods (ReaxFF MD, among others), have facilitated a picoseconds (ps) scale observation of the temporal evolution of these SEI precursors, propelling a mechanistic understanding of the dynamics of the SEI formation; it is now recognized that there is an initial and rapid electrolyte decomposition occurring within the first tens of picoseconds after contact with lithium metal, preceding a longer-scale mass transfer segregation process leading to the formation of a myriad of organic and inorganic microphases within the SEI structure. However, the accessible time and space windows to MD methods fall within few hundred ps, leaving out the study of the morphological aspects of the SEI over extended cycling. In this sense, we discuss through kinetic Monte Carlo (kMC) calculations the evolution of the SEI formation on lithium metal and confirm the critical role that dislocations and grain boundaries play in ensuring proper lithium plating and mobility within the SEI layer over cycling. We confirm that bulk lithium diffusion within the inorganic SEI microphases, such as LiF and Li2O, is significantly lower than lithium mobility across LiF/Li2O interfaces. We also discuss the impact of electrolyte tunning on SEI morphology and provide insights into developing an SEI formation strategy based on the electrolyte molecules electrochemical stability and their relative contents of heteroatom species, such as F and O, to seed in the SEI composition and morphology, which we believe could have a significant impact on the development of stable LMB batteries.

A Method Probing High-Temperature Oxidation Behavior of Crystalline Materials.

To date, the oxidation behavior of crystal materials is not fully understood; additional research is needed to understand the oxidation of materials. Herein, density functional theory (DFT) calculations and a 3D kinetic Monte Carlo (KMC) model are used to investigate the infiltration and diffusion behaviors of oxygen atoms within the crystal. Oxygen molecules readily adsorbes on crystal surfaces of the material and rapidly dissociates, verified by both first-principles calculations and energy-dispersive spectrometer (EDS) results. The infiltration ability of oxygen atoms into the inner crystal layers is affected by the surrounding oxygen atom, lattice compactness, and other factors. Energy-barrier calculations show that crystal thin/dense layers have significant effects on the crystal oxidation process, so high-pressure technology is used to investigate this correlation experimentally. KMC calculations and thermogravimetric analyses (TGA) show the infiltration behavior of oxygen atoms in the main crystal plane (211) toward the inner layers has the highest proportion to the actual high-temperature oxidation behavior of the title material. The results of both the KMC calculations and thermal experiments show the material peeled off upon further oxidation, which accelerates oxidation. At the same time, high-pressure treatment increases the oxidation resistance of materials at lower temperatures (<600°C).

(Invited) Approaches for Mechanistic Understanding of Electrodeposition Processes for Fabricating Micro/Nano Structures and Devices

Electrochemical deposition processes have precise controllability, selectivity and uniformity, and have been widely applied for fabricating various functional micro/nano structures and devices. In order to achieve further higher controllability for developing reliable process for manufacturing, molecular-level understanding of the deposition processes is required. Furthermore, this can be be also utilized to understand various applications such as the electrode processes of secondary batteries using metal anodes and so on. For this, we have attempted to apply theoretical approaches such as density functional theory (DFT) and kinetic Monte Carlo (KMC) calculations[1.2], as well as experimental methods such as surface enhanced Raman microscopy equipped with plasmonic sensors[3-5]. In this paper, these approaches with some of the resent results will be described. The analysis using DFT calculations provided the molecular-level insights for the catalytic activity of the electroless deposition reaction process at the surface, with the effects of the additive species and solvation. Also, in combination with the KMC calculation, multi-scale simulation for the nucleation and growth process at the Zn anode surface for the secondary battery was attempted. While these approaches enable quantitative discussion for the mechanistic understanding of the processes, we also attempted to obtain "real" data from experimental approaches. For this, we applied surface enhanced Raman scattering (SERS) and developed "plasmonic sensors" with controlled nanostructures to obtain high signal enhancement at electrode-electrolyte interface. We have developed two types of the plasmonic sensors which could achieve extremely high sensitivity. Multi-confocal type SERS microscopy was also developed for mapping and imaging analyses of the electrode surface as well as local pH distributions. By using these techniques, various processes at the electrode surfaces and interfaces have been investigated and modelling of the processes has been carried out.This work was financially supported in part by “Development of Systems and Technology for Advanced Measurement and Analysis” program from Japan Science and Technology Agency, and Grant-in-Aid for Scientific Research, MEXT, Japan.[1] Y. Onabuta, M. Kunimoto, H. Nakai, T. Homma, Electrochim. Acta, 307, 536 (2019).[2] Y. Onabuta, M. Kunimoto, S. Wang, Y. Fukunaka, H. Nakai, T. Homma, J. Electrochem. Soc., 169, 092504 (2022).[3] M. Yanagisawa, M. Saito, M. Kunimoto, T. Homma, Appl. Phys. Express, 9 , 122002 (2016).[4] M. Kunimoto, F. Yamaguchi, M. Yanagisawa, T. Homma, J. Electrochem. Soc., 166, D212 (2019).[5] T. Wang, M. Kunimoto, M. Yanagisawa, M. Morita, T. Abe, T. Homma, Energy Env. Mat., in press (2022).

Investigation of the Reaction Mechanisms in La1-XSrxCo1-YFeyO3 Electrode by DFT Calculations

Solid Oxide Cells (SOCs) are electrochemical devices working at high temperatures for a clean hydrogen and electricity production in electrolysis and fuel cell modes, respectively. This technology has attracted a growing attention thanks to its high efficiency without the use of expensive catalysts. Although SOCs could play a key role in the energy transition, its commercialization at large scale is still limited due to performance and degradation issues. The state of the art oxygen electrode, La1-xSrxCo1-yFeyO3 (LSCF) exhibits a high electrochemical activity combined with a good mixed ionic and electronic conductivity. However, LSCF is prone to decompose under operation with a Strontium segregation at the surface reducing the cell performance over time [1].To date, the driving force of Sr segregation in LSCF and its impact on the cell performance is not precisely understood. However, to be able to discern these phenomena, it is still needed to better understand at atomic scale the mechanisms occurring at the electrode in the absence of Sr segregation. Therefore, we carried out a first study on the bulk properties of the parent material LSF [3] and LSCF using Density Functional Theory (DFT) calculations. Now we turn our attention to the surface properties of LSCF. In particular, the oxygen reduction reaction is decomposed in fuel cell mode into a sequence of elementary reactions. Thereby, a special focus lies on the identification of the rate determining steps limiting the overall reaction.To shade light into this, we modeled the A-site terminated (100) surface of LSCF using a symmetric slab. Our calculations revealed the existence of peroxide species at the surface arising from the reductive adsorption of a dioxygen molecule into a surface oxygen vacancy. Incorporation and dissociation of the peroxide requires a surface oxygen vacancy or sub-surface oxygen vacancy. To make this reaction happen, the peroxide and the vacancy have to encounter each other. Thereby it is still unclear weather the oxygen vacancy diffuses towards the peroxide, or vice-versa. Due to the particular orientation of the peroxide, which does not stand perpendicularly out of the (100) plane, but leans towards the direction between two A-site surface ions (cf. Fig. 1), four possible orientations of the peroxide exist. Thus, on-site rotations of the peroxide and its hopping to nearest neighbor sites are considered, as well as the diffusion of oxygen vacancies on surface and sub-surface. The Nudged Elastic Band (NEB) method is used to calculate the activation barriers. Diffusion coefficients are then determined using Transition State Theory (TST) combined with Kinetic Monte Carlo (KMC) calculations. These diffusion coefficients will be used as inputs to improve the existing elementary kinetic models [3] in order to investigate more precisely the performance and durability of SOCs under working conditions.[1] Chen, K., Jiang, S.P. Surface Segregation in Solid Oxide Cell Oxygen Electrodes: Phenomena, Mitigation Strategies and Electrochemical Properties. Electrochem. Energ. Rev. 3, 730–765 (2020). https://doi.org/10.1007/s41918-020-00078-z[2] Hartmann, C., Geneste, G., Laurencin, J., Hole polarons in LaFeO3 and La1 − x Sr x FeO3 − δ : Stability, trapping, mobility, effect of Sr concentration, and oxygen vacancies. Accepted in Phys. Rev. B. [3] E. Effori, J. Laurencin, E. D. R. Silva, M. Hubert, T. David, M. Petitjean, G. Geneste, L. Dessemond, and E. Siebert, J.Electrochem. Soc. 168, 044520 (2021). Figure 1

Influence of carbon on the evolution of irradiation defects in tungsten

We have systematically investigated the influence of carbon (C) impurity on the aggregation, migration, and annihilation of neutron irradiation defects as well as the mechanical properties of tungsten (W), based on the object kinetic Monte Carlo (OKMC) calculations in combination with the theoretical models. Interestingly, although C has a strong attraction with both vacancy-type and interstitial-type irradiation defects, the C addition promotes the aggregation of former but inhibits latter in W. On the one hand, due to the weak repulsion of di-vacancy, it is difficult to form vacancy clusters spontaneously. However, the presence of C changes the interaction between vacancies from repulsion to attraction, thus promoting their clustering in W and leading to the increase of average size of vacancies cluster. On the other hand, because of the strong attraction, self-interstitial atoms (SIAs) tend to form clusters spontaneously. However, the C addition will slow down the diffusivity of SIAs due to the strong attraction between them, thereby blocking the aggregation of interstitial-type defects. Therefore, the number density of SIA clusters will increase with the increasing of C concentration, but the average size decreases significantly. Besides, the influence of C on the annihilation ratio of neutron irradiation defects is dependent on the temperature, which is mainly because the stability of C-SIA complexes and the migration of vacancies are strongly temperature dependent. Further, it is found that the increment of Vickers hardness in irradiated W at low temperature increases with the increasing of C concentration, which should be attributed to the enhanced effect of C on the retention of irradiation defects. When the C concentration is 2.5–5 appm, the model predictions are consistent with the experimental results. Consequently, although the concentration of impurity is extremely low in nuclear materials, it has significant effect on the defect evolution and mechanical properties under irradiation.

Aggregation of amphiphilic nanocubes in equilibrium and under shear.

We investigate the self-assembly of amphiphilic nanocubes into finite-sized aggregates in equilibrium and under shear, using molecular dynamics (MD) simulations and kinetic Monte Carlo (KMC) calculations. These patchy nanoparticles combine both interaction and shape anisotropy, making them valuable models for studying folded proteins and DNA-functionalized nanoparticles. The nanocubes can self-assemble into various finite-sized aggregates ranging from rods to self-avoiding random walks, depending on the number and placement of the hydrophobic faces. Our study focuses on suspensions containing multi- and one-patch cubes, with their ratio systematically varied. When the binding energy is comparable to the thermal energy, the aggregates consist of only few cubes that spontaneously associate/dissociate. However, highly stable aggregates emerge when the binding energy exceeds the thermal energy. Generally, the mean aggregation number of the self-assembled clusters increases with the number of hydrophobic faces and decreases with increasing fraction of one-patch cubes. In sheared suspensions, the more frequent collisions between nanocube clusters lead to faster aggregation dynamics but also to smaller terminal steady-state mean cluster sizes. The results from the MD and KMC simulations are in excellent agreement for all investigated two-patch cases, whereas the three-patch cubes form systematically smaller clusters in the MD simulations compared to the KMC calculations due to finite-size effects and slow aggregation kinetics. By analyzing the rate kernels, we are able to identify the primary mechanisms responsible for (shear-induced) cluster growth and breakup. This understanding allows us to tune nanoparticle and process parameters to achieve desired cluster sizes and shapes.

Mechanism insight into MnO for CHx(x = 1 to 3) hydrogenation and C1-C1 coupling processes on Co(0001) surface: A DFT and kMC study

The mechanism of CHx(x = 1–3) hydrogenation and C1-C1 coupling processes is essential for both Fischer-Tropsch synthesis to ethylene and C5+ products. Here we illustrate the effect mechanism of MnO on Co catalysts in CHx(x = 1–3) hydrogenation and C1-C1 coupling processes through density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. Specifically, compared to Co(0001), all hydrogenation reactions barriers of CHx(x = 0–3) on MnO/Co(0001) get enhanced, and MnO raises CH* coverage by 9 times hence promoting C-chain propagation and reducing methane formation. Studies on C1-C1 coupling reactions between CHx(x = 0–3) suggest that the C1-C1 coupling on MnO/Co(0001) has lower energy barriers. The main coupling mode on Co(0001) is CH* + CH* → CHCH* + *. While on MnO/Co(0001), it is mainly CH* + CH* → CHCH* + * and CH* + CH2* → CHCH2* + *. Through the study of C2Hx(x = 0–5) hydrogenation, the formation paths of ethylene and ethane on both surfaces were obtained. MnO lowers the desorption energy barrier and increases the deep hydrogenation energy barrier of ethylene, therefore improving the olefin/paraffin (O/P) ratio of C2 species. At 503–533 K, the O/P ratio of C2 species on MnO/Co(0001) is 11–15 times higher than that on Co(0001), and increasing temperature is beneficial to O/P ratio. The absence of C2+ species may limit the accuracy, and a more complete model containing C5 species will be conducted in future.

DIFUSIÓN DE HIDRÓGENO EN HCP Zr

We study the behavior of hydrogen (H) in theαandβphases of Zr and in theβphase of Nb, using the Siesta andVasp first principles codes based on the density functional theory (DFT). We calculate the hydrogen solution energy, the binding energy of the H-H and H-Nb complexes, as well as, the mixing energy of the Zr-Nb system. Furthermore, we have proposed a simple model for the study of the hydrogen diffusion (DH) mediated by interstitial mechanism in hcp Zr. We find that interstitial hydrogen diffuses isotropically according to DH=1.1×10−7exp(−42 KJ.mol−1/RT)(m2/s). Our results are in good agreement with experimental data and other Kinetic Monte Carlo (KMC) calculations. Finally, the formation energies of the three most stable hydrides in the hcp Zr phase were also calculated.

Modelling of breakaway swelling in intermetallic fuels during low-temperature irradiation

A new interpretation of the breakaway swelling observed in high-density dispersion fuels (U6Fe, U3Si, etc.) under irradiation in research reactors is proposed on the base of Mansur's pore coalescence mechanism for randomly distributed immobile pores owing to their growth and pair impingement. This mechanism was further developed in the previous papers of the authors by considering triple and multiple collisions and was modified in the current paper in application to equilibrium gas-filled pores (bubbles). New analytical solutions in the mean-field approximation for the case of pair and triple collisions of equilibrium pores at the early stage of irradiation (with the swelling less than 50–60%) are presented. For higher swellings it is necessary to use a more accurate statistical approach, based on kinetic Monte Carlo calculations and considering multiple collisions of growing pores. It is shown that at high burnups, a sharp increase in the growth rate of the fuel swelling begins at a relatively high value of the fuel swelling (about 60%), in a qualitative agreement with observations for various intermetallic compounds.

First-Principles Calculations of the Diffusivity of Interstitial Helium in Alpha-U Considering Anisotropy, Isotope Effects, and Quantum Effects

The interstitial migration of helium in alpha-U, which is planned to be used as a tritium storage material for nuclear fusion reactors and as a metallic fuel for advanced nuclear reactors, is studied by first-principles calculations. First, all migration paths are identified using the nudged elastic band method, and the migration barrier is determined for each path considering the thermal expansion of the lattice. In addition, the jump attempt frequencies are determined by applying harmonic transition state theory coupled with vibration analysis. Subsequently, the diffusion coefficient is evaluated numerically by kinetic Monte Carlo calculations using the determined migration barriers and jump attempt frequencies for all migration paths. Diffusion in the [010]-direction is found to be the most unlikely until sufficiently high temperature, while [001]-diffusion is the most dominant diffusion direction through the whole temperature range. The isotope effect, which is important because the beta decay of tritium produces helium-3, is not large and approaches the classical limit as the temperature increases. The quantum tunneling crossover temperature is computed to be approximately 100 K for helium-3 and 86 K for helium-4, which ensures the validity of the present calculation results over a wide temperature range. The diffusion coefficients are obtained as D=(9.67×10−4)×exp(−0.202eVkT)cm2/s for helium-3 and D=(8.48×10−4)×exp(−0.201eVkT)cm2/s for helium-4 over a temperature range from 200 to 900 K.

Mechanism insight into MnO for CO activation and O removal processes on Co(0001) surface: A DFT and kMC study

Direct production of olefins from syngas is of great energy and economic significance. Recently, many researches have reported that Mn modified Co based catalysts are of great application prospect. However, the effect mechanism of Mn is still not clear. This work illustrates how the addition of Mn promotes the CO dissociation and O removal processes by comparing Co(0001) surfaces with MnO/Co(0001) surfaces through density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. Specifically, CO tends to dissociate through the H-assisted path via CHO intermediate on both surfaces while direct dissociation may also occur on MnO/Co(0001). The addition of MnO can stabilize the transition states and reduce the reaction barrier by its bonding with the transition states in the direct dissociation path and CHO intermediate path, thus improving the dissociation efficiency significantly. Besides, instead of promoting the CO dissociation on Co(0001), the dissociation path via COH intermediate consumes the already dissociated C by its inverse reactions. On the contrary, MnO/Co(0001) can inhibit the reverse reactions of the COH intermediate pathway. The absorbed O tends to be removed as H2O through OH disproportionation path on both surfaces. The effect of temperature on the path of water generation is negligible. The low CO2 selectivity advantage of MnO/Co(0001) over Co(0001) becomes more and more apparent with the raise of temperature. The addition of Mn doesn’t change the benefit of low water gas shift activity on Co based catalysts.

(Invited) Automated Multiscale Design Flow for Organic LED Design

Material scientists can synthesize many organic semiconductors and dopants. While this presents a unique opportunity to device designers, discovering carrier transport parameters becomes a challenge for simulators. To overcome this, a seamless flow that links quantum chemistry calculations to TCAD driven from a single front end GUI is presented here. The developed multiscale simulation workflow links the microscopic to device scale, thereby connecting fundamental chemistry with device design in a parameter-free simulation of a 5-layer red oLED stack with an α-NPD:Ir(MDQ)2(acac) emission layer.In materials development, quantum chemistry approaches are applied to compute fundamental molecular properties such as orbital energies (HOMO/LUMO) or excitation spectra. Continuum models such as drift diffusion on the other hand are efficient tools for device designers to simulate characteristics such I-V or quantum efficiency of multilayer OLED stacks, based on parametric models.There are shortcomings that hinder the full exploitation of virtual design for organic electronics. Quantum chemistry methods used to compute properties of single molecules in vacuum neglects the interaction between molecules in organic thin films. Examples include material specific shifts in transport levels (ionization potential, IP, and electron affinity, EA) due to polarization effects, or energy disorder. Mesoscopic material properties such as charge carrier mobility or accurate transport levels (IP/EA) cannot be computed with these methods. These material-specific quantities, however, are essential input parameters for drift diffusion models. Device level simulations of multilayer OLEDs therefore rely on time-consuming and costly measurements.Consequently, the translation of microscopic properties to the device level is not possible. Further, established methods do not allow developers to analyse chemical structure impact on device performance.Digital twins of organic layers, constructed by mimicking physical vapour deposition, the properties of molecules embedded in thin films are calculated by taking into account environmental effects on a full quantum-mechanical level.1–5 Besides ab-initio access to transport energies, these molecular quantities are used to compute rates for all microscopic processes that determine device performance, including charge hops between molecules, exciton formation or quenching events. Based on these rates, kinetic Monte-Carlo (KMC) simulations are used to compute material-specific macroscopic properties such as charge carrier mobilities, which are automatically fed to drift diffusion models.6–9Drift diffusion calculations were performed using Silvaco Atlas.10,11 Quantum mechanical and KMC calculations provided the affinity, LUMO and HOMO values, material disorder and carrier mobility for the OLED stack.This seamless workflow allows full virtual design that features: Linking fundamental chemistry to device design for accurate design rules.Insight into particle distributions and processes that cannot be measured experimentally.Discovery of the best material and device performance within the design space.No parametrization of drift diffusion models via experiment is required. All inputs computed from first principles.Steering the development of physics based SPICE compact models. Funding was received from the European Union Horizon 2020 research and innovation programme. Grant agreement No. 646176 (EXTMOS) Neumann, T., Danilov, D., Lennartz, C. & Wenzel, W. Modeling disordered morphologies in organic semiconductors. Journal of Computational Chemistry vol. 34 2716–2725 (2013).Friederich, P., Rodin, V., von Wrochem, F. & Wenzel, W. Built-In Potentials Induced by Molecular Order in Amorphous Organic Thin Films. ACS Appl. Mater. Interfaces 10, 1881–1887 (2018).Friederich, P., Symalla, F., Meded, V., Neumann, T. & Wenzel, W. Ab Initio Treatment of Disorder Effects in Amorphous Organic Materials: Toward Parameter Free Materials Simulation. Journal of Chemical Theory and Computation vol. 10 3720–3725 (2014).Friederich, P., Meded, V., Symalla, F., Elstner, M. & Wenzel, W. QM/QM Approach to Model Energy Disorder in Amorphous Organic Semiconductors. Journal of Chemical Theory and Computation vol. 11 560–567 (2015).Friederich, P. et al. Molecular Origin of the Charge Carrier Mobility in Small Molecule Organic Semiconductors. Advanced Functional Materials vol. 26 5757–5763 (2016).Symalla, F. et al. Multiscale Simulation of Photoluminescence Quenching in Phosphorescent OLED Materials. Advanced Theory and Simulations vol. 3 1900222 (2020).Symalla, F. et al. 19‐4: Boosting OLED Performance with Ab‐initio Modeling of Roll‐off and Quenching Processes. SID Symposium Digest of Technical Papers vol. 50 259–262 (2019).Symalla, F. et al. 26-4: Computer-Aided Optimization of Multilayer OLED Devices. SID Symposium Digest of Technical Papers vol. 49 340–342 (2018).Symalla, F. et al. Charge Transport by Superexchange in Molecular Host-Guest Systems. Phys. Rev. Lett. 117, 276803 (2016).Introduction to Silvaco© ATLAS TCAD Software. Modeling and Electrothermal Simulation of SiC Power Devices 19–50 (2019) doi:10.1142/9789813237834_0003.SILVACO, ATLAS User’s Manual-DEVICE SIMULATION SOFTWARE, USA, Santa Clara, 2020 Figure 1

Computational Study of Solute Effects in Tungsten under Irradiation

Tungsten (W) is suitable for solid targets of spallation neutron source due to its high neutron yield. The prediction of radiation effects of W is, therefore, of importance; especially, the influence of solute elements are complex and are not clearly known to date. We discuss here the solute effects using the first principles and kinetic Monte Carlo (KMC) calculations and show that Re and Os, which are nuclear transmutation products of W, can largely change the stability and mobility of radiation defects. Such influences of the solute elements seem to explain the unsolved mechanism of the microstructural evolution of W-based materials under irradiation.

Collaborative Motion of Helium and Self-Interstitial Atoms Enhanced Self-Healing Efficiency of Irradiation-Induced Defects in Tungsten

The self-healing of nuclear materials is dependent on the motion pattern of irradiation-induced defects. Helium (He) is a typical impurity element in nuclear materials and plays a crucial role on the microstructure evolution under irradiation. Here, we systematically investigate the interactions of He with self-interstitial atoms (SIAs) in tungsten (W) as well as their influences on the kinetic behaviors of SIAs and defects annihilation, using first-principles method and object kinetic Monte Carlo calculations. It is found that there are attractive interactions between He and SIAs, which become stronger with the increasing of SIA numbers. Therefore, He has significant effect on the migration behaviors of SIAs in W. Specifically, the He-SIA1 and He-SIA2 complexes adopt a three-dimensional (3D) migration pattern with the energy barrier of 0.38 and 0.61 eV, respectively, which is completely different from the fast 1D migration of SIAs in W (≤ 0.033 eV) without He. Such unexpected collaborative 3D motion of He-SIA complexes increases the probability of Frenkel pairs recombination and reduces the number of surviving defects. Consequently, our calculations reveal the enhancing effect of He on the self-healing efficiency in W, governed by the concerted 3D motion of He-SIA complexes. These will improve our fundamental understanding of the influence of impurities on the evolution of irradiation defects in nuclear materials.

An effective method to calculate atomic movements in 3D objects with tuneable stochasticity (3DO-SKMF)

We present an effective computer simulation method, called 3D object stochastic kinetic modelling framework (3DO-SKMF), to calculate atomic movements in 3D objects including surface segregation and Gibbs–Thomson effect (surface curvature). Objects with any kind of shapes can easily be considered thanks to the flexibility and versatility of the model and code. Accordingly, the model and the computer code can be used in a wide variety of applications: nanoparticles, nanorods, nanotubes, nanopillars, etc. To increase the versatility of the model, it includes stochastic noise in a tuneable manner. This means that if the noise level is zero, the model is completely deterministic (mean-field), whereas by increasing the noise level the result gets closer and closer to that obtained by a kinetic Monte Carlo calculation. This allows us to calculate processes with activation barriers. Besides demonstrating the capabilities of the model, we also reproduce an experimental result showing decomposition of Ag–Cu nanoparticles. Program summaryProgram title: 3DO-SKMFCPC Library link to program files:http://dx.doi.org/10.17632/776b7txmhv.1Licensing provisions: CC BY NC 3.0Programming language: ISO CNature of problem: Atomistic simulation of nano-objects with free surfaces by calculating occupation probabilities. The model includes surface segregation and Gibbs–Thomson effect (surface curvature).Solution method: Numerical solution of a system of differential equations with taking into account the conservation of matter. The number of equations is equal to the number of lattice sites in a 3D lattice.Additional comments including restrictions and unusual features: The model contains parameters that can be determined from macroscopic measurements, still in the background it follows an atomistic approach. Note that the provided code is written for nanospheres (hollow and solid), nanowires and nanotubes, however objects with any kind of shapes can easily be considered. Due to the flexibility of the code, with minor change of initialization, any 3D object can be simulated. The published code uses the face centred cubic lattice (fcc), its modification for other lattices is, however, straightforward.

Different time scales in dynamic systems with multiple outcomes.

Stochastic biochemical and transport processes have various final outcomes, and they can be viewed as dynamic systems with multiple exits. Many current theoretical studies, however, typically consider only a single time scale for each specific outcome, effectively corresponding to a single-exit process and assuming the independence of each exit process. However, the presence of other exits influences the statistical properties and dynamics measured at any specific exit. Here, we present theoretical arguments to explicitly show the existence of different time scales, such as mean exit times and inverse exit fluxes, for dynamic processes with multiple exits. This implies that the statistics of any specific exit dynamics cannot be considered without taking into account the presence of other exits. Several illustrative examples are described in detail using analytical calculations, mean-field estimates, and kinetic Monte Carlo computer simulations. The underlying microscopic mechanisms for the existence of different time scales are discussed. The results are relevant for understanding the mechanisms of various biological, chemical, and industrial processes, including transport through channels and pores.

Genesis and evolution of extended defects: The role of evolving interface instabilities in cubic SiC

Emerging wide bandgap semiconductor devices such as the ones built with SiC have the potential to revolutionize the power electronics industry through faster switching speeds, lower losses, and higher blocking voltages, which are superior to standard silicon-based devices. The current epitaxial technology enables more controllable and less defective large area substrate growth for the hexagonal polymorph of SiC (4H-SiC) with respect to the cubic counterpart (3C-SiC). However, the cubic polymorph exhibits superior physical properties in comparison to its hexagonal counterpart, such as a narrower bandgap (2.3 eV), possibility to be grown on a silicon substrate, a reduced density of states at the SiC/SiO2 interface, and a higher channel mobility, characteristics that are ideal for its incorporation in metal oxide semiconductor field effect transistors. The most critical issue that hinders the use of 3C-SiC for electronic devices is the high number of defects in bulk and epilayers, respectively. Their origin and evolution are not understood in the literature to date. In this manuscript, we combine ab initio calibrated Kinetic Monte Carlo calculations with transmission electron microscopy characterization to evaluate the evolution of extended defects in 3C-SiC. Our study pinpoints the atomistic mechanisms responsible for extended defect generation and evolution, and establishes that the antiphase boundary is the critical source of other extended defects such as single stacking faults with different symmetries and sequences. This paper showcases that the eventual reduction of these antiphase boundaries is particularly important to achieve good quality crystals, which can then be incorporated in electronic devices.

Experimentally Validated Hopping-Transport Model for Energetically Disordered Organic Semiconductors

Charge transport in disordered organic semiconductors occurs by hopping of charge carriers between localized sites that are randomly distributed in a strongly energy dependent density of states. Extracting disorder and hopping parameters from experimental data like temperature dependent current-voltage characteristics typically relies on parametrized mobility functionals that are integrated in a drift-diffusion solver. Surprisingly, the functional based on the extended Gaussian disorder model (eGDM) has been extremely successful at this, despite it being based on the assumption of nearest neighbor hopping (nnH) on a regular lattice. We here propose a variable range hopping (VRH) model that has been integrated in a freeware drift-diffusion solver. The mobility model has been calibrated using kinetic Monte Carlo calculations and shows good agreement with the Monte Carlo calculations over the experimentally relevant part of the parameter space. The model is applied to temperature-dependent space charge limited current (SCLC) measurements of different systems. In contrast to the eGDM, the VRH model provides a consistent description of both p-type and n-type devices. We find a critical ratio of aNN/$\alpha$ (mean inter-site distance / localization radius) of ~3 below which hopping to non-nearest neighbors becomes important around room temperature and the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder values in the range 45-120 meV are found, without any clear correlation with photovoltaic performance when the same active layer is used in an organic solar cell.

Correlation of structural and optical properties using virtual materials analysis.

Thin film growth of ${\textrm{TiO}}_2$TiO2 by physical vapor deposition processes is simulated in the Virtual Coater framework resulting in virtual thin films. The simulations are carried out for artificial, simplified deposition conditions as well as for conditions representing a real coating process. The study focuses on porous films which exhibit a significant anisotropy regarding the atomistic structure and consequently, to the index of refraction. A method how to determine the effective anisotropic index of refraction of virtual thin films by the effective medium theory is developed. The simulation applies both, classical molecular dynamics as well as kinetic Monte Carlo calculations, and finally the properties of the virtual films are compared to experimentally grown films especially analyzing the birefringence in the evaluation.

Theoretical Study of Moisture-Pretreated Lithium as Potential Material for Natural Gas Upgrading

For efficient natural gas upgrading at near ambient temperatures, moisture-pretreated lithium is proposed as a reactive adsorbent, and its separation performance was examined for selected gas mixtures (involving combination of CH4, CO2, O2, N2, and/or H2) by theoretical studies. For evaluating the proposed adsorbent, we used density functional theory calculations as well as temperature-programmed kinetic Monte Carlo calculations. Contributions of different mechanisms for the gas capture were examined, including diffusion, adsorption, and reaction. Among these mechanisms, reaction was found to be the dominant factor during the gas-capture process. In contrast to the conventional reactive adsorbents where gases directly react with adsorbents and thus are captured, the gas-capture mechanism herein follows the reaction between gases and species generated upon pre-exposure of lithium to moisture. The proposed separation process is thermodynamically favorable and would progress spontaneously. It is shown that f...