Vacancy Formation Energy Research Articles

Cu-Content La1−XSrxNi1−YCuyO3−Δ Perovskites As Cobalt-Free Cathode Materials for High-Performance It-SOFCs

Solid Oxide Fuel Cells (SOFCs) are one of the most attractive energy conversion and storage devices, which can be applied for decentralized energy applications. However, the high operating temperature making SOFCs still unmarketable, which indicates that lowering the working temperature of SOFCs while still maintaining a high-power output is very crucial. For Intermediate-Temperature Solid Oxide Fuel Cells (IT-SOFCs), new cathode materials with high electrocatalytic activity and stability are necessary to provide a stable and high performance of cells. Perovskite-structured oxide (ABO3−δ) is one group of the most interesting material candidates for IT-SOFCs, presenting great potential in chemical composition modifications, yielding the design and gain of desired physicochemical and electrochemical properties [1]. The simple perovskite LaCuO3 is one of the well-studied Cu-content oxides with a high conductivity (106 S·cm−1), but it can be hardly obtained and suffers with stability issues in air [1, 2]. To stabilize the perovskite structure and to improve electrochemical properties of Cu-content oxides A-site and B-site modification strategy can be applied. It has been noted that Cu- and Ni-containing LaNi0.5Cu0.5O3−δ simple perovskite possesses low cathodic polarization of 0.056 Ω·cm2 at 800 °C, and a high power output of 870 mW·cm−2 at 900 °C [1, 3]. The generation of oxygen vacancies by strontium doping contributes to the increase of ionic conductivity component, and the Sr dopant can reduce the oxygen vacancy formation energy and increase the ionic conductivity of La1−xSrxMO3−δ (M = Fe, Mn) perovskites [1,4]. Therefore, in this work, Cu-content La1−xSrxNi1−yCuyO3−δ oxides with strontium doping at the A-site were evaluated as very promising Co-free cathode material candidates for IT-SOFCs.In this work, all La1−xSrxNi1−yCuyO3−δ compounds have been successfully obtained using soft chemistry. The room-temperature crystal structure of all oxides was assigned to the R-3c trigonal system. The substitution of La with Sr lowers the phase transition temperature from the R-3c space group to Pm-3m simple perovskite while increases the oxygen non-stoichiometry and the thermal expansion coefficient of studied materials. All investigated materials are chemically compatible with GDC-10, while unstable in contact with LSGM and 8YSZ. The manufactured anode-supported IT-SOFC with the La0.95Sr0.05Ni0.5Cu0.5O3−δ cathode presents an excellent power density of 445 mW·cm−2 at 650 °C in humidified H2. The results confirmed that La1−xSrxNi1−yCuyO3−δ perovskites can be potentially qualified as promising cathode candidates for IT-SOFCs, yielding promising electrochemical performance in the intermediate-temperature range [1].

Oxygen Non-Stoichiometry and Point Defect Equilibria in (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ

Compositionally complex oxides (CCOs), also referred to as high entropy oxides, have several different cations (typically 5 or more) occupying a single site. CCOs have shown unique dielectric, ionic conductivity, and thermal conductivity properties, in addition to promising electrochemical properties for solid oxide fuel cells (SOFCs) [1,2]. In this work, the A-site of the common SOFC cathode La0.8Sr0.2MnO3-δ (LSM) is modified with additional lanthanide and alkali elements, resulting in the composition (La1/6Pr1/6Nd1/6Gd1/6Ba1/6Sr1/6)MnO3- δ. Typically, the average lattice parameter is correlated with bond length and strength, and thus energetics of oxygen vacancy formation energy. In the compositionally complex system, there is a wide range of cation radii on the A-site and so bond strengths are expected to vary significantly, resulting in a distribution of oxygen vacancy formation energies. Initial results have shown that the amount of oxygen released during reduction is significantly greater for the studied CCO in comparison with La2/3Sr1/3MnO3-δ, the latter having the same amount of total acceptor dopant and with a similar lattice parameter as the CCO. The significant enhancement in oxygen loss is an indicator of a lower reduction enthalpy and suggests a significant impact from atomic structure that is not captured in long-range structure parameters (i.e., average lattice parameter). The oxygen non-stoichiometry measured by thermogravimetric analysis in ~10-5 – 1 atm O2 at 1200 oC – 1450 oC will be presented and the point defect equilibria analysis will be discussed. Comparisons with existing high temperature experimental and modeling studies on LSM will also be discussed.SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525References[1] Xu, Y., et al., J. Adv. Ceram., 11 (2022) 794, DOI:10.1007/s40145-022-0573-7[2] J. Dabrowa, et al., J. Mater. Chem. A, 8 (2020) 24455; DOI:10.1007/s40145-022-0573-7

A Systematical Ab Initio Study of Diffusivity and Ionic Conductivity of Ln2NiO4+ δ (Ln=La, Nd, Pr) Reversible Solid Oxide Cells (r-SOCs)

The Ln2NiO4+δ (Ln=La, Nd, Pr) reversible solid oxide cells (r-SOCs) exhibit a relatively high oxygen diffusivity and surface reaction activity, compatible thermal expansion coefficient with solid electrolyte, and oxygen over-stoichiometry enabling the transportation of oxygen ions. The atomistic simulation technique provides detailed information concerning the diffusion mechanisms of the materials. In the present study, we systematically investigated the vacancy formation energy, thermal expansion, diffusivity, and ionic conductivity of the Ln2NiO4 + δ (δ=0.125) compound through the ab initio approach. Especially considering the change of the oxygen transport properties caused by the thermal expansion effect, a combination of quasi-harmonic approximation (QHA) and a linear regression model is used to study and reproduce the temperature-dependent properties of the Ln2NiO4 + δ.

Building Negative-Thermal-Expansion Protective Layers on the Grain Boundary of Ni-rich Cathodes Enables Safe and Durable High Voltage Lithium-Ion Batteries.

Ni-rich layered oxide cathode materials demonstrate high energy densities for Li-ion batteries, but the electrochemically driven thermal runaway and mechanical degradation remain their long-standing challenges in practical applications. Herein, it presents a novel ZrV2 O7 (ZVO) coating with negative thermal expansion properties along the secondary particles and primary particle grain boundaries (GBs), to simultaneously enhance the structural and thermal stability of LiNi0.8 Co0.1 Mn0.1 O2 (NCM811). It unveils that, such an architecture can significantly enhance the electronic conductivity, suppress the microcracks of GBs, alleviate the layered to spinel/rock-salt phase transformation, and meanwhile relieve the lattice oxygen loss by increasing the oxygen vacancy formation energy increased (1.43 vs 1.90eV). Consequently, the ZVO-coatedNCM811 material demonstrates a remarkable cyclability with 86.5% capacity retention after 100 cycles, and an outstanding rate performance of 30C under a high-voltage of 4.6V, outperforming the state-of-the-art literature. More importantly, the Li+ transportation can be readily blocked at 120°C by the negative-thermal-expansion ZVO coating, thus avoiding the high-temperature thermal runaway.

A novel methodology to estimate hydrogen diffusivity and its applications in revealing hydrogen effects in CoCrNi medium-entropy alloy versus 316L stainless steel

Hydrogen diffusivity is one of the important factors affecting the susceptibility to hydrogen embrittlement of metals and alloys. Here, we proposed a novel approach to estimate hydrogen diffusivity in FCC alloys by performing nanoindentation experiments with continuous stiffness measurement. In consideration of the plastic zone size around the indentation, the relationship between hydrogen-induced hardness change and the depth from the sample surface was established for two typical FCC alloys—316L stainless steel (SS) and CoCrNi medium-entropy alloy (MEA)—subjected to hydrogen-charging and subsequent room-temperature aging. By combining with the estimation of the through-thickness distribution of hydrogen concentration, hydrogen diffusivity values were successfully determined, which agreed well with the literature data. It was revealed that CoCrNi MEA exhibited a hydrogen diffusivity ∼0.68 times higher than that in SS316L, and that hydrogen-induced softening upon long-term aging (at room temperature) was observed in CoCrNi but not in SS316L. The underlying mechanisms for the distinct hydrogen-related phenomena in CoCrNi MEA were elucidated based on the density functional theory calculations of hydrogen solution energy and vacancy formation energy.

Ab initio calculations and empirical potential assessments of the energy and structure of symmetric tilt grain boundaries in tungsten

Molecular statics/dynamics (MS/MD) based on empirical interatomic potentials (IAPs) is an effective means for conducting grain boundary (GB) related studies. The accuracy of these MS/MD simulations, however, can be greatly affected by the IAP used. Unfortunately, the large discrepancies between the ab-initio values make it difficult to evaluate, improve and further develop IAPs. Motivated by this issue, in the present work we perform a systematic investigation of 〈100〉 and 〈110〉 symmetric tilt GBs in tungsten (W) via atomistic simulations, including the stable structure, GB energy, work of separation, excess volume, and vacancy formation energy. We here use a two-step relaxation process, initially obtaining the candidate structures from MS and then performing DFT validation of these structures to determine the ground state structure, where six empirical IAPs and four exchange–correlation (xc) functionals are used. And the step-like feature is revealed that can be used to reduce the cost of time-consuming optimization processes. Taking the results yielded by PBEsol xc-functional as a benchmark, it is shown that for the same xc-functional, the relative errors of GB energy and work of separation are almost identical, independent of the GB character. The performance of six typical empirical IAPs considered for W is also evaluated by comparing it with the DFT data. The quantities chosen for the comparison include the structure and energetic properties of GBs, as mentioned before. We demonstrate that these empirical IAPs tend to better describe only part of the properties of GBs; for example, some IAPs can predict the energetic properties well, while others can better describe the structure. This suggests that a fully reliable description of GBs remains an elusive objective, requiring further efforts in the development and optimization of the IAP.

Enhanced radical scavenging performance of Pt@CeO2 via strong metal-support interaction effect for improving durability of proton exchange membrane fuel cell

Since scavenging free radical is regarded as an efficient strategy to enhance the durability of proton exchange membrane fuel cells (PEMFCs), strong metal-support interaction (SMSI) effect between metal and reducible oxide supports is employed as a strategy to moderate the number of accessible active sites for reduction-oxidation reaction. Enriched Pt–CeO2 interface to enhance SMSI effect is achieved as well via constructing multicore-shell structure Pt@CeO2. Theoretical calculation confirms that SMSI effect at Pt–CeO2 interface endows electrons flowing from Pt to CeO2 to reduce the formation energy of oxygen vacancies, while Pt@CeO2 with higher magnitude of lattice distortion plan provides enriched Pt–CeO2 interface that made more concentration of Ce3+ ions feasible. Results show that better free radical scavenging performance is achieved with higher concentration of Ce3+ ions in Pt@CeO2. Based on the open circuit voltage (OCV) test, the Pt@CeO2-modified membrane in this work decays at only 70% of the commercial membrane, while 65% of the maximum power is still maintained after the test. This superior performance to the commercial membrane indicates this designed structure with SMSI effect could be considered as a potential route to solve the free radical problem in PEM for enhanced durability of PEMFCs.

High activity and stability of cobalt-free SmBa0.5Sr0.5Fe2O5+δ perovskite oxide as cathode material for solid oxide fuel cells

It is very important to develop cathode material with high stability and strong catalytic activity to realize the commercial application of solid oxide fuel cells (SOFCs). Here, we prepared a cobalt-free cathode material which partial substitution of Sr for Ba in SmBa0.5Sr0.5Fe2O5+δ (SBSF) sample and systematically investigated its electrochemical performance. Density functional theory (DFT) results showed that the oxygen vacancy formation energy of SBSF was lower than that of SmBaFe2O5+δ (SBF). The local density of state (LDOS) results showed that bandwidth center energy of SBSF between O-2p and Fe-3d obviously overlapped compared with parent SBF. The XRD spectra show that the SBSF formed a perovskite structure and had good chemical compatibility with La0.9Sr0.1Ga0.83Mg0.17O3-δ (LSGM) and CeO2-based electrolyte, respectively. At 800 °C, the polarization resistance and maximum power density of SBSF cathode on LSGM electrolyte were 0.023 Ω cm2 and 874 mWcm−2, respectively. In addition, the polarization resistance of SBSF electrode was stable during the 100 h test. The preliminary results show that SBSF oxide is a promising cathode material for SOFCs.

Accelerated Surface Reconstruction through Regulating the Solid-Liquid Interface by Oxyanions in Perovskite Electrocatalysts for Enhanced Oxygen Evolution.

A comprehensive understanding of surface reconstruction was critical to developing high performance lattice oxygen oxidation mechanism (LOM) based perovskite electrocatalysts. Traditionally, the primary determining factor of the surface reconstruction process was believed to be the oxygen vacancy formation energy. Hence, most previous studies focused on optimizing composition to reduce the oxygen vacancies formation energy, which in turn facilitated the surface reconstruction process. Here, for the first time, we found that adding oxyanions (SO42-, CO32-, NO3-) into the electrolyte could effectively regulate the solid-liquid interface, significantly accelerating the surface reconstruction process and enhancingoxygen evolution reaction (OER) activities. Further studies indicated that the added oxyanions would adsorb onto the solid-liquid interface layer, disrupting the dynamic equilibrium between the adsorbed OH- ions and the OH- ions generated during surface reconstruction process. As such, the OH- ions generated during surface reconstruction process could be more readily released into the electrolyte, thereby leading to an acceleration of the surface reconstruction. Thus, it was expected that our finding would provide a new layer of understanding to the surface reconstruction process in LOM-based perovskite electrocatalysts.

Synergistic passivation of high stability CsPb(Br/I)3 perovskite quantum dots with double ligands for WLEDs

The all-inorganic perovskite CsPbI3 quantum dots are difficult to achieve pure red stable luminescence due to their poor lattice thermodynamic stability and non-radiative recombination loss caused by surface halide vacancies. Therefore, the pure red perovskites are usually achieved by mixing the composition of halides (Br and I). In this work, CsPb(Br/I)3 quantum dots (QDs) were synthesized by the hot-injection method under the synergistic effect of homophthalic acid (HA), 2-bromoethanesulphonic acid sodium salt (SBES), and oleylamine (OAm). By adjusting the molar ratio of passivates, the synergistic effect of multiple ligands can not only passivate the unliganded Pb2+-related defects on the surface of NCs, but also increase the formation energy of halide vacancies. Meanwhile, due to the bidentate structure of HA, smaller size of perovskites nanocrystals were obtained. Eventually, synergistic boost in device performance is achieved for color-by-blue white light-emitting diodes (WLEDs) with CIE coordinates of (0.3319, 0.3297) and have an external quantum efficiency (EQE) of 10.13%. This strategy paves an efficient way for improving the efficiency and stability of CsPb(Br/I)3 NCs.

Accelerated Surface Reconstruction through Regulating the Solid‐Liquid Interface by Oxyanions in Perovskite Electrocatalysts for Enhanced Oxygen Evolution

AbstractA comprehensive understanding of surface reconstruction was critical to developing high performance lattice oxygen oxidation mechanism (LOM) based perovskite electrocatalysts. Traditionally, the primary determining factor of the surface reconstruction process was believed to be the oxygen vacancy formation energy. Hence, most previous studies focused on optimizing composition to reduce the oxygen vacancy formation energy, which in turn facilitated the surface reconstruction process. Here, for the first time, we found that adding oxyanions (SO42−, CO32−, NO3−) into the electrolyte could effectively regulate the solid–liquid interface, significantly accelerating the surface reconstruction process and enhancing oxygen evolution reaction (OER) activities. Further studies indicated that the added oxyanions would adsorb onto the solid–liquid interface layer, disrupting the dynamic equilibrium between the adsorbed OH− ions and the OH− ions generated during surface reconstruction process. As such, the OH− ions generated during surface reconstruction process could be more readily released into the electrolyte, thereby leading to an acceleration of the surface reconstruction. Thus, it was expected that our finding would provide a new layer of understanding to the surface reconstruction process in LOM‐based perovskite electrocatalysts.

The impact of vacancy formation energies on the nucleation and growth of helium (He) bubbles in low-activation multicomponent vanadium-based alloys

Generally, the complexity of concentrated solid solution alloys (CSAs) can effectively enhance their radiation tolerance. We studied the effect of helium (He) ion irradiation on body centered cubic (BCC) V, VCr, VCrFe and VCrFeMn alloys with the increase of complexity. The comparison shows the void swelling in terms of the size and density of bubbles in V, VCrFe and VCrFeMn decreases with the increase of the complexity after the same He ion irradiation. However, binary VCr alloy has the smallest void swelling although it has only two types of elements, that is less complexity, which is significantly different from VCrFe and VCrFeMn subjected to the same He ion irradiation. Meanwhile, the irradiated VCr lattice exhibits almost no shrinkage. First-principles calculations reveal that the high vacancy formation energy (VFE) of VCr could prevent He-vacancy clusters from absorbing vacancies to grow. The results suggest that the VFE could play a more important role in enhancing the radiation tolerance under He ion irradiation in addition to the compositional complexity.

Oxygen vacancy engineered electron structure of porous CexCo3−xO4 nanorods for wide spectrum photocatalytic hydrogen evolution

Development of high-performance semiconductors for photocatalytic hydrogen evolution is a promising solution to solve the current energy crisis. In this work, a porous and oxygen vacancy-enriched CexCo3−xO4-Vo nanorods (CexCo3−xO4-Vo NRs) with adjust electron structure were fabricated by Ce-doping for wide spectrum photocatalytic hydrogen evolution. Because Ce doping regulates the Co3+/Co2+ atomic ratio of Co3O4 nanorods (Co3O4 NRs), resulting in a reduction of oxygen vacancy formation energy and confirmed by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy measurements (XPS) studies. The formation of oxygen vacancy leads to a modification in the positions of conduction band and valence band positions of CexCo3−xO4-Vo NRs. The ultraviolet photoelectron spectroscopy (UPS) spectra results indicate that the generation of oxygen vacancies leads to a more negative Fermi level in CexCo3−xO4-Vo NRs. This result directly results in an enhanced capacity for H+ reduction of CexCo3−xO4-Vo NRs. Density functional theory (DFT) calculations demonstrate that Ce0.15Co2.85O4-Vo NRs have faster electron transfer rates. In addition, the porous rod-like structure endows it abundant electron diffusion pathways, facilitates electrolyte infiltration, and establishes an ideal electrolyte/catalyst contact interface. Among them, Ce0.15Co2.85O4-Vo NRs have the best photocatalytic hydrogen evolution activity (3912.8 μmol g−1), which were 2.2 times more than that of Co3O4 NRs, and an excellent stability also obtained. This work provides a new idea for elements doping Co3O4 NRs to in situ-induced oxygen vacancies for enhanced photocatalytic hydrogen evolution.

Data‐driven Design of Enhanced In‐based Catalyst for CO2 to Methanol Reaction

AbstractThe environmental impact of unsustainable CO2 emissions calls for immediate action. One of the main methods for large‐scale reduction of CO2 emissions is conversion of carbon dioxide to valuable feedstocks like energy carriers or chemicals. Realization of this goal requires catalysts showing high‐performance characteristics under the relevant industrial conditions. In recent years, In2O3‐based catalysts have been discussed as target materials for conversion of CO2 to methanol in the presence of hydrogen. Optimization of this catalytic system via conventional routes requires massive resources in terms of extensive testing, synthesis, and characterization. In our study, we take an alternative approach and exploit high‐throughput computation to create a database for oxygen vacancy formation energy and explore a large number of candidates, which motivates the selection of a subset of materials for the synthesis of bulk and supported catalysts, to be tested in hydrogenation of CO2 to methanol. The method shows the impact, as results show improved performance for selected candidates, compared to the reference In2O3‐system. This confirms the potential of the selected descriptors as criteria for a new way of targeted design of alternative catalysts.

The reconciliation and validation of a combined interatomic potential for the description of Xe in γU-Mo

A U-Mo alloy has been selected as the fuel design for the conversion of high-performance research reactors in the United States. Efforts are ongoing to describe the fuel evolution as a function of time, for a variety of different reactor conditions. The accurate prediction of fuel evolution under irradiation requires the implementation of correct thermodynamic properties into mesoscale and continuum-level fuel performance modeling codes. Molecular dynamics has proven to be a valuable tool to parameterize or inform these higher-length scale models. However, there are currently inaccuracies in the only available U-Mo-Xe potential, which limits the predictive capabilities of molecular dynamics to inform critical phenomena in these fuel systems such as fission gas swelling. This work provides an updated U-Mo-Xe ternary interatomic potential which combines existing potentials in a reconciled format. The validation of the interatomic potential is performed by analyzing the phase stability and vacancy formation energies. Subsequently, Xe solution energies and an equation of state to describe Xe bubbles in U-Mo are calculated, providing 1) evidence of the significant differences between the prior ternary potential and the currently presented potential, and 2) updated data/tools for implementation into mesoscale simulation methodologies to study fission gas bubble evolution.

Performance enhancement of HfO2-based resistive random-access memory devices using ZnO nanoparticles

In this study, resistive random-access memory (ReRAM) devices with ZnO nanoparticles (NPs) are suggested to enhance performance and reduce variation in device switching parameters. The ZnO NPs are formed by annealing ZnO prepared via atomic layer deposition on HfO2, which is verified using transmission electron microscopy, x-ray diffraction pattern, and atomic force microscopy. The depth profile analysis of x-ray photoelectron spectroscopy shows that oxygen diffuses from HfO2 to ZnO NPs during annealing. This can be explained by the calculation results using density functional theory (DFT) where the formation energy of oxygen vacancies is reduced at the interface of ZnO NPs and HfO2 compared to single HfO2. The fabricated ZnO NPs ReRAM demonstrates reduced forming voltage, stable resistive switching behavior, and improved cycle-to-cycle uniformity in a high-resistance state.

Artificially controlled nanoscale chemical reduction in VO2 through electron beam illumination

Chemical reduction in oxides plays a crucial role in engineering the material properties through structural transformation and electron filling. Controlling the reduction at nanoscale forms a promising pathway to harvest functionalities, which however is of great challenge for conventional methods (e.g., thermal treatment and chemical reaction). Here, we demonstrate a convenient pathway to achieve nanoscale chemical reduction for vanadium dioxide through the electron-beam illumination. The electron beam induces both surface oxygen desorption through radiolytic process and positively charged background through secondary electrons, which contribute cooperatively to facilitate the vacancy migration from the surface toward the sample bulk. Consequently, the VO2 transforms into a reduced V2O3 phase, which is associated with a distinct insulator to metal transition at room temperature. Furthermore, this process shows an interesting facet-dependence with the pronounced transformation observed for the c-facet VO2 as compared with the a-facet, which is attributed to the intrinsically different oxygen vacancy formation energy between these facets. Remarkably, we readily achieve a lateral resolution of tens nanometer for the controlled structural transformation with a commercial scanning electron microscope. This work provides a feasible strategy to manipulate the nanoscale chemical reduction in complex oxides for exploiting functionalities.

Electrical and Thermal Transport Properties of Ge1–xPbxCuySbyTeSe2y

AbstractBalancing the contradictory relationship between thermoelectric parameters, such as effective mass and carrier mobility, is a challenge to optimize thermoelectric performance. Herein, the exceptional thermoelectric performance is realized in GeTe through collaboratively optimizing the carrier and phonon transport via stepwise alloying Pb and CuSbSe2. The formation energy of Ge vacancy is efficiently bolstered by alloying Pb, which reduces carrier density and carrier scattering to maintain superior carrier mobility in GeTe. Additionally, CuSbSe2, acting as an n‐type dopant, further modulates carrier density and validly equilibrates carrier mobility and effective mass. Accordingly, the promising power factor of 45 µW cm−1 K−2 is achieved at 723 K. Meanwhile, point defects are found to significantly suppress phonons transport to descend lattice thermal conductivity by Pb and CuSbSe2 alloying, which barely impacts the carrier mobility. A combination with superior carrier mobility and lower lattice thermal conductivity, a maximum ZT of 2.2 is attained in Ge0.925Pb0.075Cu0.005Sb0.005TeSe0.01, which corresponds to a 100% promotion compared with that of intrinsic GeTe. This study provides a new indicator for optimizing carrier and phonon transport properties by balancing interrelated thermoelectric parameters.

First-principle study on physical properties and vacancy formation of face-centered cubic Co–Ni–Cu–Mo–W high entropy alloys

This paper investigated the physical properties and vacancy formation of face-centered cubic (FCC) Co–Ni–Cu–Mo–W high-entropy alloys (HEAs) using first-principle calculation. Firstly, we analyzed the effect of W and Cu content on the physical properties of alloys. The calculation results show that increasing W content increased the thermodynamic stability, dislocation-energy factor and dislocation width of Co–Ni–Cu–Mo–W HEAs. The fracture energy increased with increasing W atoms at the fracture interface. However, the coefficient of thermal expansion of HEAs decreased with increasing W content. The effect of Cu on the physical properties of Co–Ni–Cu–Mo–W HEAs was contrary to that of the W element. Secondly, we investigated the effect of neighboring atoms on vacancy formation energy (Evf) of Co–Ni–Cu–Mo–W alloys. The results show that the Evf increased with more first-neighboring W and Mo atoms, and the Evf decreased with more first-neighboring Cu atoms. Lastly, we investigated the partial electronic density of state (PDOS) of the first-neighboring atoms before and after forming vacancy. The PDOS in sites with higher Evf changed more obviously after forming vacancy compared with the sites with lower Evf, which indicate that the atoms at higher Evf site have strong bonding effect with neighboring atoms.

Microscale spectroscopic mapping of defect evolution and filling in large-area growth of monolayer MoS2

Defects in monolayer transition metal dichalcogenides are known to be problematic in that they usually deteriorate optical and electronic properties, thus acting as a constraint in nanodevice applications. In this regard, understanding defect-related strain and charge distributions is intriguing to identify the inhomogeneous energy landscape of two-dimensional materials. Herein, simultaneous Raman and photoluminescence results unveil how strain and charge carriers evolve over the large-area growth of monolayer MoS2. Unexpectedly, spatially-resolved correlation analysis between the in-plane and out-of-plane phonon frequencies of MoS2 reveals that both compressive strain and charge carriers increase as the morphology evolves from a triangular flake to a partially coalesced film and, finally, to a fully covered film, indicating that the number of sulfur vacancies increases with surface coverage. Theoretically calculated formation energies of sulfur vacancies support the evolution scenarios with respect to morphological variations. Photoluminescence mappings of excitons and trions provide further quantitative information on electron concentrations that are consistent with Raman correlation data. Notably, chemical treatment of defective MoS2 with a p-type dopant leads to a relaxation of compressive strain and a decrease in excess electrons, where in particular, the p-doping effects are more prominent in the fully covered film region than in the partially coalesced region.