Search

First-principles calculations were employed to study the molecular and dissociative adsorption of CO 2 , H 2 O, O 2 and N 2 on a LaB 6 (100) surface. The adsorption energy calculation results indicate that these gas molecules can form thermodynamically stable adsorption structures. Dissociative adsorption is always accompanied by more electron transfer compared to molecular adsorption, which is one of the necessary conditions for dissociation to occur. Bader charge analysis indicates that the adsorption of gas molecules on the LaB 6 (100) surface is always accompanied by electron transfer from the surface to the adsorbed molecules. This electron transfer forms a dipole moment from the adsorbed molecule towards the surface, leading to an increase in the work function of LaB 6 (100). This mechanism is a crucial aspect of the LaB 6 (100) poisoning effect. Additionally, LaB 6 (100) sometimes exhibits a certain degree of resistance to poisoning. In such cases, after the adsorption of gas molecules, local regions of the LaB 6 (100) surface become negatively charged, which counteracts the poisoning effect to some extent. Using electronic density of states and electron localization function analyses, we examined the nature of the chemical bonds. It was found that La tends to form non-covalent bonds with the adsorbates, whereas B tends to form covalent bonds with them. The 2p orbitals of B play a significant role in the formation of these covalent bonds in most cases.

Structural symmetry breaking plays a pivotal role in fine-tuning the properties of nano-layered materials. Here, based on the first-principles approaches we propose a Janus monolayer of metallic CoSb by breaking the out-of-plane structural symmetry. Specifically, within the CoSb monolayer by replacing the top-layer 'Sb' with 'As' atoms entirely, the Janus Co 2 AsSb monolayer can be formed, whose structure is confirmed via structural optimization and ab initio molecular dynamics simulations. Notably, the Janus Co 2 AsSb monolayer demonstrates stability at an elevated temperature of 1200 K, surpassing the stability of the CoSb monolayer, which remains stable only up to 900 K. We propose that both the CoSb and Janus Co 2 AsSb monolayers could serve as capable anode materials for power-driven metal-ion batteries, owing to their substantial theoretical capacity and robust binding strength. The theoretical specific capacities for Li/Na reach up to 1038.28/1186.60 mA h g -1 for CoSb, while Janus Co 2 AsSb demonstrates a marked improvement in electrochemical storage capacity of 3578.69/2215.38 mA h g -1 for Li/Na, representing a significant leap forward in this domain. The symmetry-breaking effect upgrades the CoSb monolayer, as a more viable contender for power-driven metal-ion batteries. Furthermore, electronic structure calculations indicate a notable charge transfer that augments the metallic nature, which would boost electrical conductivity. These simulations demonstrate that the CoSb and Janus Co 2 AsSb monolayers have immense potential for application in the design of metal-ion battery technologies.

Tetradymite compounds, such as Bi 2 Te 3 , crystallizing in rhombohedral structures have triggered tremendous research interest from the scientific community because of their intriguing properties. Herein, using the state-of-the-art first-principles calculations, we identify that La 2 X 2 Y (X = I, Br, Cl; Y = Ge, Te) nanosheets exhibit a ternary tetradymite-type structure with extraordinary electrical and electrochemical properties. It is first demonstrated that the layered La 2 X 2 Y compounds exhibit weak interlayer coupling with cleavage energies in the range of ∼0.28-0.38 J m -2 , allowing the ready separation of monolayers that can be synthesized by mechanical exfoliation from their bulk counterparts. Next, we predict that La 2 X 2 Ge nanosheets exhibit a semiconducting nature, and upon physical realistic strain, a Dirac cone can be realized. These findings can be exploited in the transport properties. Furthermore, we comprehensively investigated the electrochemical properties of the predicted systems to evaluate their potential use in metal-ion (Li/Na) batteries. Our detailed analyses reveal that the Li (Na) adatoms are sufficiently mobile on the surface of the studied systems. For instance, the binding energy for the Li (Na) adatom on La 2 I 2 Ge is -2.24(-1.79) eV with a diffusion barrier of as small as ∼0.31(0.20) eV. Subsequently, the maximum theoretical specific capacity for Li (Na) reaches as high as 887(1064) mA h g -1 , which can be attributed to a much higher storage capacity compared to previously identified 2D anode materials. These findings substantiate that the predicted nanosheets could be synthesized to explore their potential applications in future metal-ion batteries.

Core-shell nanoparticles (NPs) are active research areas for their unique properties and wide applications. By changing the elemental composition in the core and shell, a series of core-shell NPs with specific functions can be obtained, where the sizes of the core and shell also influence the properties. X-ray photoelectron spectroscopy (XPS) is useful in this context as a means of quantitatively analyzing such NPs. The empirical formula proposed by Shard [ J. Phys. Chem. C , 2012, 116 (31), 16806-16813] for calculating the shell thickness of the spherical core-shell NPs has been verified by Powell et al. [ J. Phys. Chem. C , 2016, 120 (39), 22730-22738] through a simulation of XPS with Simulation of Electron Spectra for Surface Analysis (SESSA) software. However, real core-shell NPs are not necessarily ideal spheres; such NPs can have rich shapes and uneven thicknesses. This work aims to extend the Shard formula to non-ideal core-shell NPs. We have used a Monte Carlo simulation method to study the XPS signal variation with the shell thickness for several modeled non-spherical shapes of core-shell NPs including some complex geometric structures which are numerically constructed with finite-element triangular meshes. Five types of non-spherical shapes, i.e. egg, ellipsoid, rod, rough-surface, and star shapes, are considered, while the size parameters are varied over a wide range. The equivalent radius and equivalent thickness are defined to characterize the average size of the nanoparticles for the use of the Shard formula. We have thus derived an extended Shard formula for the specific core-shell NPs, with which the relative error between the predicted shell thickness and the real thickness can be reduced to less than 10%.

Knowledge of absolute secondary electron yield ( δ ) is important for various applications of electron emission materials. Besides, it is also crucial to know the dependence of δ on primary electron energy E p and material properties like atomic number Z. The available experimental database of δ reveals a large discrepancy among the measurement data, while the oversimplified semi-empirical theories of secondary electron emission can only present the general shape of the yield curve but not the absolute yield value. This limits not only the validation of a Monte Carlo model for theoretical simulations but also presents large uncertainties in the applications of different materials for various purposes. In applications, it is highly desirable to have the knowledge of the absolute yield of a material. Therefore, it is highly desirable to establish the relationship of the absolute yield with material and electron energy based on the available experimental data. Recently, machine learning (ML) methods have been increasingly used for the prediction of material properties mainly based on the atomistic calculations with the first-principles theory. We propose here the application of ML models to a material property study, starting with experimental observations and unfolding the relationship of δ with basic material properties and primary electron energy. Our ML models are able to predict δ ( E p )-curve covering a wide energy range of 10 eV-30 keV for unknown elements within the uncertainty range of the experimental data and can suggest more reliable data among the scattered experimental data.

An efficient N-centered radical intramolecular cyclization reaction of alkenyl amides induced by visible light was described. In this process, an alkenyl amide underwent 5-exo / 6-endo cyclization to selectively yield two critical alkaloid structures, namely isoindolinones and isoquinolinones.

We present the combined experimental and theoretical investigations of the optical properties of amorphous carbon. The reflection electron energy loss spectra (REELS) spectra of carbon were measured using a cylindrical mirror analyzer under ultrahigh vacuum conditions at primary electron energies of 750, 1000 and 1300 eV. The energy loss function and thereby the refractive index n and the extinction coefficient k were determined from these REELS spectra in a wide loss energy range of 2-200 eV by applying our reverse Monte Carlo method. The high accuracy of the obtained optical constants is justified with the ps- and f-sum rules. We found that our present optical constants of amorphous carbon fulfill the sum rules with the highest accuracy compared with the previously published data. Therefore, we highly recommend to replace the previous data with the present ones for practical applications. Moreover, we present the atomic scattering factors of amorphous carbon obtained from the dielectric function to predict its optical constants at a given density.

Van der Waals (vdW) heterostructure-based electrodes have invoked tremendous research interest due to their intriguing properties and their capability to break the limitations of the restricted properties of single-material systems. Herein, based on first-principles approaches, we propose that the black phosphorene/blue phosphorene (BLK-P/BLE-P) vdW heterostructure can be a capable anode material for power-driving lithium-ion batteries (LIBs), as it exhibits a large theoretical capacity, together with a relatively strong binding strength compared with the individual BLK-P and BLE-P monolayers. Our calculation results show that the Li adatom prefers to intercalate into the interlayer of the BLK-P/BLE-P vdW heterostructure due to the synergistic interfacial effect, resulting in a high binding strength and a diffusivity comparable to the BLK-P and BLE-P monolayers. Subsequently, the theoretical specific capacity is found to be as high as 552.8 mA h g -1 , which can be attributed to the much higher storage capacity of Li adatoms in the BLK-P/BLE-P vdW heterostructure. Furthermore, electronic structure calculations reveal that a large amount of charge transfer assists in semiconductor to metallic transition upon lithiation, which would ensure good electrical conductivity. These simulations prove that the BLK-P/BLE-P heterostructure has great potential in LIBs and is essential for future battery design.

Electron stopping power (SP) is of great importance in theoretical and applied research areas specifically for Monte Carlo simulation studies in many microanalysis and surface analysis techniques, radiation dosimetry, and the design of particle detectors. However, experimental data are available for a dozen elemental materials only. On the other hand, the Bethe analytical expression of the SP is applicable at high energies only whereas no generally accepted formula exists at lower energies. We employed ensemble machine learning (ML) methods with the available experimental database for the prediction of SPs of electrons with energies from 100 keV down to 1 eV, in elements over the entire periodic table. With a small training database for electron SPs, we applied various algorithms individually as well as their ensembles, which have the credibility to enhance the prediction accuracy in the case of a small training database. Based on the model's performance evaluation tests, we concluded that the stacked generalization is more accurate than the individual algorithms. Using this method, we were able to predict the electron SPs for 54 elements (in total) including 12 elements that were present in the training database as well as for 42 elements beyond the training database over a wide energy range (1 eV to 100 keV). Compared to other theoretical approaches, the ML predicted SPs show very good agreement with the available experimental data at all energies. Moreover, unlike other theoretical approaches, the ML model does not need dielectric function data and other physical parameters which involve complex calculations. Using our ML model, we have predicted SPs for a further 14 elements for which no theoretical SPs are available because of the lack of good dielectric function data.

Since anthrax spores are highly lethal to human beings and animals, and also potential biological warfare agents, it is highly desirable to develop simple, robust and easy-device sensors that are rapid and sensitive, and can selectively detect the Bacillus anthrax biomarker dipicolinic acid (DPA) quantitatively. Herein, we report a ratiometric luminescent "turn-on" sensor via mixing Eu 3+ ions, sodium polyacrylate and internal luminescence reference in water, which can identify the Bacillus anthrax biomarker dipicolinic acid (DPA) quantitatively with outstanding selectivity over aromatic ligands and amino acids, even including all of the 5 DPA isomers.

Fixing the gap between "nano-scaled" pieces and "product-scale" materials, devices or machines is an ineluctable challenge that people have to tackle. Herein, we show that combining self-assembly and electrospinning processes results in the fabrication of anisotropic fluorescent nanofibers (PDI@PVDF) in which the well-defined rod-like perylene bisimide derivative assemblies are embedded in a highly oriented way along the axis of the poly(vinylidene fluoride) (PVDF) fiber. Compared to fragile individual PDI assemblies, the electrospinning anisotropic fluorescent PDI@PVDF nanofibers not only maintain high sensitivity for aniline vapour but also exhibit an unexpected short response time for both quenching and recovering. The results demonstrate that electrospinning assistance is a versatile and effective strategy to maintain the anisotropy of fluorescent nanomaterials, building a bridge between self-assembled nano-rods and practical materials., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The physical and chemical properties of monolayers can be tuned by selective combinations so as to be useful for device applications. Here we present a density functional theory study on the structural, electronic and optical properties of three transition metal dichalcogenide (TMD) heterobilayers, ZrS2/HfS2, ZrSe2/HfSe2 and SnS2/SnSe2. These heterobilayers are predicted to be energetically and dynamically stable structures. The band structure calculation result shows that ZrS2/HfS2, ZrSe2/HfSe2 and SnS2/SnSe2 heterobilayers are semiconductors with indirect band gaps. The efficient charge carrier separation in ZrS2/HfS2 and ZrSe2/HfSe2 heterobilayers indicates that they can be employed in energy harvesting devices. Contrary to the previous report on the ZrS2/HfS2 heterobilayer, we found it to have an intrinsic type-II band alignment which is required in p-n junction diodes and tunnel field effect transistors, and the same behavior was observed in ZrSe2/HfSe2 and SnS2/SnSe2 for the first time. The ZrS2/HfS2 and ZrSe2/HfSe2 heterobilayers reveal enhanced optical absorption both in the ultraviolet and visible regions as compared to their respective monolayers, whereas the parallel and perpendicular part of the optical absorption of the SnS2/SnSe2 heterobilayer revealed an anisotropic behavior; the perpendicular part is largely improved in the higher energy region, and the parallel part of the optical absorption is improved in the ultraviolet region.

The geometric, electronic and magnetic properties of a nitric oxide (NO) adsorbed Fe3O4(100) surface have been investigated using density functional theory (DFT) calculations. NO molecules preferentially bond with surface Fe(B) atoms via their N atoms. The generalized gradient approximation (GGA) is not recommended to be used in such a strongly correlated system since it provides not only an overestimation of the adsorption energy and an underestimation of the Fe(B)-N bond length, but also magnetic quenching of the adsorbate and the bonded Fe(B) atoms. In contrast, a tilted geometry and magnetization of the adsorbate and the bonded Fe(B) atom are obtained after including the strong on-site Coulomb interactions through a Hubbard term (GGA+U). The spin-down 2π* states of the NO molecule are filled and broadened due to the adsorbate-substrate interaction and the molecule-molecule interaction. The surface spin polarization close to the Fermi level is expected to be greatly enhanced by the NO adsorption which has significance for interface design in spintronic devices.

A new family of biphen[n]arenes (n = 4-8) with 2,2'-disubstituents (2,2'-BPns) has been designed and synthesized. The host-guest properties of the 2,2'-BPns have been examined; they exhibit strong binding affinities towards selected organic cationic guests.

In this paper, a novel quantum-trajectory Monte Carlo simulation method is developed to study electron beam interaction with a crystalline solid for application to electron microscopy and spectroscopy. The method combines the Bohmian quantum trajectory method, which treats electron elastic scattering and diffraction in a crystal, with a Monte Carlo sampling of electron inelastic scattering events along quantum trajectory paths. We study in this work the electron scattering and secondary electron generation process in crystals for a focused incident electron beam, leading to understanding of the imaging mechanism behind the atomic resolution secondary electron image that has been recently achieved in experiment with a scanning transmission electron microscope. According to this method, the Bohmian quantum trajectories have been calculated at first through a wave function obtained via a numerical solution of the time-dependent Schrödinger equation with a multislice method. The impact parameter-dependent inner-shell excitation cross section then enables the Monte Carlo sampling of ionization events produced by incident electron trajectories travelling along atom columns for excitation of high energy knock-on secondary electrons. Following cascade production, transportation and emission processes of true secondary electrons of very low energies are traced by a conventional Monte Carlo simulation method to present image signals. Comparison of the simulated image for a Si(110) crystal with the experimental image indicates that the dominant mechanism of atomic resolution of secondary electron image is the inner-shell ionization events generated by a high-energy electron beam.