Multiple Vacancies Research Articles

Two-Layer Silicene on the SiC Substrate: Lithiation Investigation in the Molecular Dynamics Experiment.

The functioning of the lithium-ion battery anode composed of silicene/SiC composite is studied by molecular dynamics. In this composite, silicene has multiple vacancy defects. Approximately the same degree of lithium filling in such an anode is considered for both horizontal and vertical intercalations. During the horizontal intercalation lithium atoms not only fill the channel and deposit on its walls, but also penetrate into the substrate. In both cases, the self-diffusion coefficients of lithium atoms have similar values. However, the process of filling the system with lithium occurs with a smoother total energy change when the intercalation is performed vertically. A detailed study of the lithium atoms packing via the construction of Voronoi polyhedra for each of the systems under consideration shows the better uniformity of the Li atoms distribution over the volume of the system during the vertical intercalation.

Effect of Multiple Oxygen Vacancies on the Optical and Thermodynamic Properties of La0.75Sr0.25Co0.25Fe0.75O3−δ Perovskite

The oxygen vacancy concentration of perovskites changes with the work condition in applications such as oxygen carriers and oxygen sensors. To explore the effect of multiple oxygen vacancies on the optical and thermodynamic properties, we perform first-principles calculations to investigate the formation of multiple oxygen vacancies in the paramagnetic La0.75Sr0.25Co0.25Fe0.75O3−δ. Our results show that the higher VO concentration in La0.75Sr0.25Co0.25Fe0.75O3−δ occurs at the conditions of higher temperatures and lower oxygen partial pressures. The electrons liberated upon oxygen vacancy formation are partially localized on the nearby Fe/Co sites and partially delocalized through the lattice, with the result that the unoccupied Fe/Co states near the Fermi level decrease and the Fermi level shows an upward shift. From the results of optical absorption, the peaks from 250 to 600 nm are weakened and blue-shifted with an increase of oxygen vacancy concentration due to the decrease of empty Fe states and an upward shift of the Fermi level. The obvious change of optical absorption with oxygen vacancy concentration indicates that La0.75Sr0.25Co0.25Fe0.75O3−δ could be a good candidate for optical oxygen sensors in the visible spectrum.

Improved optical, dielectric, impedance, and magnetic properties of (BiFeO3)0.6(CaTiO3)0.4 for multifunctional utilities

The investigation of the multifaceted properties of (BiFeO3)0.6(CaTiO3)0.4, synthesized following a solid solution reaction mechanism is reported. The temperature-dependent dielectric and tanδ loss analysis sheds light on the temperature-dependent phase transitions. The structural deformation from the rhombohedral structure for pure BFO to the orthorhombic phase is evident from the Rietveld refinement. The optical energy gap investigation carried out through UV–Visible spectroscopy points out the potential useful photovoltaic applicability of the ceramic. The XPS analysis reveals the existence of multiple valency states of Fe, Ti, and oxygen defects which in turn might explain the underlying phenomena of hopping mechanisms and magnetic exchange interactions responsible for enhancing the ferroelectric, dielectric, and magnetic properties in the sample. The impedance and modulus analysis reveal the semiconducting and capacitive properties of the ceramic useful for device applications. Enhanced dielectric properties, weak ferromagnetic ordering due to suppression of G-type antiferromagnetism and double exchange paths, and reduced value of optical band gap as compared to the previous report are the novel findings in this communication.

The impact of apartment vacancies on nearby housing rents over multiple time periods: application of smart meter data

PurposeThis study aims to explore the spatial externalities of apartment vacancy rates on housing rent by considering multiple vacancy durations.Design/methodology/approachThis research uses smart meter data to measure unobservable vacant houses. This study made a significant contribution by applying building-level smart meter data to housing market analysis. It examined whether vacancy duration significantly affected apartment rent and whether the relationship between apartment rent and vacancy rate differed depending on the level of housing rent.FindingsThe primary finding indicates that there is a significant negative correlation between apartment rent and vacancy duration. Considering the spatial externalities of apartment vacancy rates, the apartment vacancy rates of surrounding buildings did not show any statistical significance. Moreover, quantile regression results indicate that although the bottom 10% of apartment rent levels showed a negative correlation with all vacancy durations, the top 10% showed no statistical significance related to vacancies.Practical implicationsThis study measures the extent of spatial externalities that can differentiate taxation based on housing vacancies.Originality/valueThe findings indicate that landlords have asymmetric information about their buildings compared with the surrounding buildings, and the extent to which price adjusts for long-term vacancies differs depending on the level of apartment rent.

Surface wettability effect on heat transfer across solid-water interfaces

• Mechanisms of surface wettability effect on interfacial heat transfer are unraveled. • Hydrophilic surface promotes heat transfer through enhanced hydrogen-bonded network. • Molecular roughness with high biphasic contact area is beneficial to heat transfer. • Multiple and shallow vacancies are better than the deep but fewer ones on substrate. Although it has been found that the heat transfer across solid–liquid interface can be affected by the surface wettability, the underlying mechanism remains ambiguous. Herein non-equilibrium molecular dynamics simulations are conducted to unravel the microscopic mechanisms of heat transfer across solid-water interfaces. Specifically, two aspects of the solid surface wettability are investigated including modifying surface chemistry and introducing molecular roughness. We find that enlarging the solid-water interaction strength facilitates the formation of hydrogen-bonded network of water, hence enhancing the interfacial heat transfer. In addition, introducing molecular roughness can promote heat transfer by increasing the solid-water contact area. When the contact area remains the same, the substrates with multiple and shallow vacancies are more beneficial to heat transfer than those with deep but fewer vacancies. This work sheds insights into the mechanistic understanding of the heat transfer process and provides theoretical guidance for the development of efficient heat transfer devices.

Unidentified major p-type source in SnSe: Multivacancies

Tin selenide (SnSe) is considered a robust candidate for thermoelectric applications due to its very high thermoelectric figure of merit, ZT, with values of 2.6 in p-type and 2.8 in n-type single crystals. Sn has been replaced with various lower group dopants to achieve successful p-type doping in SnSe with high ZT values. A known, facile, and powerful alternative way to introduce a hole carrier is to use a natural single Sn vacancy, VSn. Through transport and scanning tunneling microscopy studies, we discovered that VSn are dominant in high-quality (slow cooling rate) SnSe single crystals, while multiple vacancies, Vmulti, are dominant in low-quality (high cooling rate) single crystals. Surprisingly, both VSn and Vmulti help to increase the power factors of SnSe, whereas samples with dominant VSn have superior thermoelectric properties in SnSe single crystals. Additionally, the observation that Vmulti are good p-type sources observed in relatively low-quality single crystals is useful in thermoelectric applications because polycrystalline SnSe can be used due to its mechanical strength; this substance is usually fabricated at very high cooling speeds.

Photon Emission from Hollow Ions Near Surfaces

Ions with multiple inner-shell vacancies frequently arise due to their interaction with different targets, such as (intense) light pulses, atoms, clusters or bulk material. They are formed, in addition, if highly charged ions approach surfaces and capture electrons at rather large distances. To explore the interaction of such hollow ions and their subsequent relaxation, photon spectra in different frequency regions have been measured and compared to calculations. To support these and related measurements, we here show within the framework of the Jena Atomic Calculator (Jac) how (additional) electrons in outer shells modify photon emission and lead to characteristic shifts in the observed spectra. Further, for highly charged Ar ions in KLm(m=1…8) configurations, we analyze the mean relaxation time for their stabilization into the different ground configurations. These examples demonstrate how a powerful and flexible toolbox such as Jac will be useful (and necessary) in order to model the photon and electron emission of ions as they occur not only near surfaces but also in astro-, atomic and plasma physics.

Strategies for solving the excited-state self-consistent-field problem for highly excited and multiply ionized states

The dynamics of molecules exposed to intense x-ray radiation involve a large number of multiply ionized and highly excited electronic configurations. To model these dynamics a reliable and efficient electronic structure model is imperative. Employing the Hartree-Fock-Slater electronic structure model in combination with the maximum overlap method, we quantify the associated convergence failures when calculating electronic states of carbon monoxide with multiple vacancies in the core and valence levels. We characterize these cases and describe strategies to overcome the convergence problems. The described techniques not only eliminate all convergence issues for CO but also result in a significant reduction of convergence failures for simulations of the x-ray-induced multiple ionization dynamics of the phenol molecule.

The secretary problem with multiple job vacancies and batch candidate arrivals

We extend the secretary problem with multiple vacancies to allow batch arrival of candidates. We establish structural properties of the optimal policies. We show that the optimal reward is convex and submodular in the values of candidates, which means that there is benefit for having a candidate pool with more variable or less interdependent values. Similar properties continue to hold when there are multiple classes of vacancies. Our model is applicable to recruitment, dynamic auctions and sequential investment.

Organizational personnel career development

The aim of the study is to analyse opportunities for career development of organizational personnel. The object of the study includes processes providing organizational career development. The subject of the research is interdependence between personal professional development of employees and organizational performance. The research methods are description and analysis.The processes providing career progression can be divided into direct, actively encouraging employees to improve professional skills, and indirect, creating favourable work environment and commitment to achieving organizational relevant goals. Under the modern conditions of societal transformations, it is highly important to pay closer attention to interrelation between personal development and organizational aims since a customary vision of career as vertical climbing a corporate ladder is receding into the past. Opportunities for continuous training, entrepreneurial activity, and multiple vacancies are changing a vision of career as a finite process within an organization. An essential role in the system of personnel management should be played by employee career development planning which establishes connections between financial and nonfinancial incentives, on the one hand, and employee performance, on the other hand, – with respect to their personal interests, social characteristics and motivation level.The author concludes that two opposite tendencies influence personnel management system functioning: change and conservation. The necessity for change is caused by transforming market and social conditions, employee personal growth, and organizational life cycle. The tendency for status quo conservation is dictated by the desire of any system for stability and sustainability.

Superior electrochemical water oxidation in vacancy defect-rich 1.5 nm ultrathin trimetal-organic framework nanosheets

Engineering the 2D metal-organic framework (MOF) material can enrich the metal-unsaturated edges as active sites for catalysis, however, introducing multiple heterointerfaces and vacancy defects into 2D MOF and exploring their effects on electrocatalytic oxygen evolution reaction (OER) remain a major challenge. Here we construct ultrathin and highly curved 2D FeCoNi trimetal-organic framework nanosheets (FeCoNi-MOFs) with only approximately 1.5 nm thickness and abundant oxygen vacancies. The formed defect-rich FeCoNi-MOFs display outstanding OER performance with a much smaller overpotential of 254 mV at 10 mA cm−2 and remarkable stability for over 100 h in alkaline solutions. This is the highest OER activity level attained for direct-MOF catalysts. Theoretical analysis of FeCoNi MOFs with rich oxygen vacancies further suggests that the increased Fermi level with multiple heterointerfaces and the addition of oxygen vacancies co-facilitate the pre-oxidation of low-valence metals and the reconstruction/deprotonation of intermediate metal−OOH, thus enhancing electron transport efficiency.

Tailoring the cationic and anionic sites of LaFeO3-based perovskite generates multiple vacancies for efficient water oxidation

A simple strategy involving Cr and Mo tailoring and post-reduction process to generate multivacancies in LaFeO3 is reported, which substantially accelerates the OER electroactivity of LaFeO3 with overpotential of 263 mV at 10 mA cm−2.

Multiple Vacancies on (111) Facets of Single-Crystal NiFe2 O4 Spinel Boost Electrocatalytic Oxygen Evolution Reaction.

Oxygen evolution reaction (OER) as the rate-determining reaction of water splitting has been attracting enormous attention. At present, only some noble-metal oxide materials (IrO2 and RuO2 ) have been reported as efficient OER electrocatalysts for OER. However, the high cost and scarcity of these noble-metal oxide materials greatly hamper their large-scale practical application. Herein, we synthesize 100% (111) faceted NiFe2 O4 single crystals with multiple vacancies (cation vacancies and O vacancies). The (111) facets can supply enough platform to break chemical bonds and enhance electrocatalytic activity, due to its high density of atomic steps and kink atoms. Compared to NiFe2 O4 (without vacancies), the as-synthesized NiFe2 O4 -Ar (with vacancies) exhibits a dramatically improved OER activity. The NiFe2 O4 -Ar-30 shows the lowest onset potential (1.45 V vs RHE) and the best electrocatalytic OER activity with the lowest overpotential of 234 mV at 50 mA cm-2 . Furthermore, based on the theoretical calculations that the introduction of multiple vacancies can effectively modulate the electronic structure of active centers to accelerate charge transfer and reaction intermediates adsorption,which can reduce the reaction energy barrier and enhance the activity of electrochemical OER.

A Defect Study and Classification of Brown Diamonds with Deformation-Related Color

For this study, the properties of a large sample of various types of brown diamonds with a deformation-related (referred to as “DR” in this work) color were studied to properly characterize and classify such diamonds, and to compare them to pink to purple to red diamonds. The acquisition of low temperature NIR spectra for a large range of brown diamonds and photoexcitation studies combined with various treatment experiments have opened new windows into certain defect characteristics of brown diamonds, such as the amber centers and naturally occurring H1b and H1c centers. It was determined that the amber centers (referred to as “AC” in this work) exhibit rather variable behaviors to annealing and photoexcitation; the annealing temperature of these defects were determined to range from 1150 to >1850 °C and it was found that the 4063 cm−1 AC was the precursor defect of many other ACs. It is suggested that the amber centers in diamonds that contain at least some C centers are essentially identical to the ones seen in diamonds without C centers, but that they likely have a negative charge. The study of the naturally occurring H1b and H1c link them to the amber centers, specifically to the one at 4063 cm−1. Annealing experiments have shown that the H1b and H1c defects and the 4063 cm−1 AC were in line with each other. The obvious links between these defects points towards our suggestion that the H1b and H1c defects are standalone defects that consist of multiple vacancies and nitrogen and that they are—in the case of brown diamonds—a side product of the AC formation. A new classification of DR brown diamonds was elaborated that separates the diamonds in six different classes, depending on type and AC. This classification had been completed recently with the classification of brown diamonds with a non-deformation-related color (referred to as “NDR”), giving a total of 11 classes of brown diamonds.

Itinerant ferromagnetism in van der Waals Fe5−xGeTe2 crystals above room temperature

Two-dimensional (2D) van der Waals (vdW) magnets have recently attracted increasing attention, as they provide a novel system for exploring 2D magnetism. However, intrinsic ferromagnetism in 2D systems has almost exclusively been observed at low temperatures, limiting their technological relevance. ${\mathrm{Fe}}_{\mathrm{N}}{\mathrm{Ge}\mathrm{Te}}_{2}$ ($N=3$, 4, and 5) systems are currently becoming the most attractive 2D vdW materials due to their relatively high Curie temperatures and large saturation magnetization. However, the nature of their complex yet intriguing magnetic behaviors is still unclear, in part due to the multiple inequivalent iron sites and iron vacancies. Here, we show evolution of magnetic ordering transitions in ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ with high Curie temperature and a strong saturation magnetization using photoemission electron microscopy and transport measurements. At 275 K, the ferromagnet transitions to a ferrimagnet, and below 110 K transitions to a state with glassy clusters. These are evidenced from temperature-dependent magnetic stripe domain evolution and anisotropic magnetoresistance measurements. Our findings show a clear magnetic ground state of ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ at room temperature which signals that ${\mathrm{Fe}}_{5\ensuremath{-}x}{\mathrm{Ge}\mathrm{Te}}_{2}$ system is a very promising candidate for spintronic devices and provides a material design pathway to further increase the Curie temperature and saturation moments in vdW ferromagnets.

Electronic transport properties of graphene with Stone-Wales defects and multiple vacancy chains: a theoretical study

Recent advances in controlled synthesis of graphene nanodevices urge the understanding of various defects’ effect on the electronic transport properties, such as Stone-Wales defects, single vacancy, double vacancies and multiple vacancy chains. In this work, we systematically investigated these defects in single-layer graphene, by using first principle calculations combined with the non-equilibrium Green’s function method. The calculated current-voltage curves reveal that these defects can lead to current decrease compared with pristine graphene. Besides, corresponding transmission spectra and device density of states indicate that some defect induced electron states can strongly enhance the transport of electron between electrodes at certain energy levels, while others are only localized around the defect sites. Moreover, the distinct results of graphene with multiple vacancy chains demonstrate that both the number and arrangement of vacancy defects could affect the electronic transport properties of graphene nanodevices. We also verified that these vacancy defects could be easily identified by using a small source-drain voltage and sweeping the gate voltage applied on the graphene field effect transistors. These results are helpful to further understand vacancy defects’ impact on the transport properties of graphene nanodevices, and inspiring to tune the electronic behaviors of two-dimensional nanodevices through controlled defect engineering modifications.

Li diffusion in Si and LiSi: Nuclear quantum effects and anharmonicity.

The diffusion of Li in bulk Si and crystalline LiSi is investigated over a wide range of temperatures employing first-principles calculations based on density functional theory, transition state theory, and the kinetic Monte Carlo method. Nuclear quantum effects are incorporated by computing the vibrational spectrum and its effect on the effective energy barrier. The Li diffusion coefficient in bulk Si calculated with such quantum effects is ∼33% lower than the classical limit near room temperature due to higher effective energy barrier and tends to the classical limit at a high temperature (>1000 K). The presence of anharmonicity, estimated by the quasiharmonic approximation and the cBΩ model, increases the diffusion coefficient by ∼60%. For Li diffusion in LiSi with multiple vacancy jumps, we obtain an effective diffusion barrier of 0.27 eV ± 0.01 eV. In the Li-Si system, the quantum mechanical effects are only marginally significant at room temperature.

Identifying the Origin of Ti3+ Activity toward Enhanced Electrocatalytic N2 Reduction over TiO2 Nanoparticles Modulated by Mixed-Valent Copper.

The ambient electrocatalytic N2 reduction reaction (NRR) enabled by TiO2 has attracted extensive recent attention. Previous studies suggest the formation of Ti3+ in TiO2 can significantly improve the NRR activity, but it still remains unclear what kinds of Ti3+ are effective. Herein, it is demonstrated that mixed-valent Cu acts as an effective dopant to modulate the oxygen vacancy (VO ) concentration and Ti3+ formation, which markedly improves the electrocatalytic NRR performance. In 0.5 m LiClO4 , this electrocatalyst attains a high Faradic efficiency of 21.99% and a large NH3 yield of 21.31 µg h-1 mgcat. -1 at -0.55 V vs reversible hydrogen electrode, which even surpasses most reported Ti-based NRR electrocatalysts. Using density function theory calculations, it is evidenced that mixed-valent Cu ions modulate the TiO2 (101) surface with multiple oxygen vacancies, which is beneficial for generating different Ti3+ 3d1 defect states localized below the Fermi energy. N2 activation and adsorption are effectively strengthened when Ti3+ 3d1 defect states present the splitting of eg and t2g orbitals, which can be modulated by its coordination structure. The synergistic roles of the three ion pairs formed by the VO defect, including Cu1+ -Ti4+ , Ti3+ -Ti4+ and Ti3+ -Ti3+ , are together responsible for the enhanced NRR performance.

Stability and energetics of HenVm complexes in Fe–Cr alloys: Ab initio study

Density functional theory (DFT) calculations have been implemented to investigate the helium trapping effect in multi-vacancies, and the stability and energetics of HenVm complexes (n = 0–6, m = 0–4). The existence of Cr inhibits the capture of helium atoms in multiple vacancies to some extent. A stronger helium self-trapping aggregation than that in pure iron was found in Fe–Cr alloys. In the early stage of nucleation, the HenVm complexes with higher density in Fe–Cr alloys provide more annihilation sites for the irradiation induced recombination of Frankel pairs, and have higher irradiation swelling resistance than pure iron. A new mechanism has been found that the possibility of the formation of helium-vacancy complexes depends on the helium atoms (n) or vacancies (m). Our simulation results corroborate that when the n/m ratio tends to increase, there is spontaneous emission of helium atoms or self-interstitial atoms (SIAs), and it is difficult to grow helium-vacancy-type complexes in Fe–Cr alloys, which provides necessary information for explaining complex experimental phenomena.

Shuangyin Wang.

"The secret of being a successful scientist is to do your own and labeled research … In my first experiment I unintentionally functionalized carbon nanotubes, on which Pt nanoparticles were deposited uniformly. Luck is part of research …" Find out more about Shuangyin Wang in his Author Profile.