Substitution Of Atoms Research Articles

Atomic-scale electronic inhomogeneity in single-layer iron chalcogenide alloys revealed by machine learning of STM/S data

Chemical pressure from the isovalent substitution of Se by a larger Te atom in the epitaxial film of iron chalcogenide FeSe can effectively tune its superconducting, topological, and magnetic properties. However, such substitution during epitaxial growth inherently leads to defects and structural inhomogeneity, making the determination of alloy composition and atomic sites for the substitutional Te atoms challenging. Here, we utilize machine learning to distinguish between Se and Te atoms in scanning tunneling microscopy images of single-layer FeSe1−xTex on SrTiO3(001) substrates. Defect locations are first identified by analyzing spatial-dependent dI/dV tunneling spectra using the K-means clustering method. After excluding the defect regions, the remaining dI/dV spectra are further analyzed using the singular value decomposition method to determine the Se/Te ratio. Our findings demonstrate an effective and reliable approach for determining alloy composition and atomic-scale electronic inhomogeneity in superconducting single-layer iron chalcogenide films.

Control of Ag acceptor concentration and pn-junction depth in single crystalline Mg2Si photodiodes

We have investigated the relationship between the Ag concentration and the pn-junction depth in the Mg2Si pn-junction photodiodes fabricated by thermal diffusion of the Ag acceptor. The Ag concentration profiles and pn-junction depths in the samples annealed between 400 and 550 °C were studied by secondary ion mass spectroscopy and electron beam-induced current (EBIC) images. We observed two kinds of lattice diffusions of substitutional and interstitial Ag atoms with two different diffusion coefficients, of which activation energies were ∼0.97 and 0.75 eV, respectively. The depth of pn-junction observed by EBIC images increased with annealing temperature and annealing time. On the other hand, the average Ag concentration did not depend on the annealing time but depended on the annealing temperature. These results indicate that the average Ag concentration and pn-junction depth in Mg2Si photodiodes can be controlled by annealing temperature and annealing time, respectively. This study would contribute to the development of Mg2Si pn-junction photodiodes.

Analysis of heat conductivity mechanisms in PbSnTe solid solutions

In the paper, a theoretical calculation of the coefficient of thermal conductivity of solid solutions of PbSnTe was carried out. The contribution of phonon scattering on substitution atoms to the effect of reducing thermal conductivity has been established. The composition of the PbSnTe solid solution, characterized by the lowest values of the lattice component of the thermal conductivity coefficient klat, was determined. The concentration of intrinsic charge carriers in solid solutions is calculated and their influence on the thermoelectric parameters of the material is shown.

Topological phase transition in the antiferromagnetic topological insulator MnBi_2Te_4 from the point of view of axion-like state realization

This work aims to study the conditions of topological phase transition (TPT) between the topological and trivial states in the antiferromagnetic topological insulator (AFM TI) MnBi_2Te_4 and propose some theory about the relationship of this TPT with the possibility of axion-like state realization in this material. Using the density functional approach we have analyzed the changes in the electronic and spin structure of topological surface states (TSSs) and the nearest conduction and valence bands (CB and VB) including the changes in the bulk band gap as well as the Dirac point (DP) gap in TSSs under variation of the spin-orbit coupling strength in the region of the TPT for infinite crystal and slab with a surface both. We have shown that in both cases the TPT occurs with inversion of the contributions of the Bi-p_z and Te-p_z states of different parity at the gap edges related to change in the gap sign. In the case of surface calculations, the Bi-p_z and Te-p_z states at the edges of the bulk band gap and their inversion at the TPT point are transformed into the TSSs with an energy gap at the DP. In this case the TPT takes place without closing the band gap, i.e. with a “jump” through zero and the formation of the nonzero gap during such a transition. Our calculations show that the TPT point is also characterized by an inversion of the out-of-plane spin polarization s_z at the Gamma point for lower and upper parts of the Dirac cone and a significant spatial redistribution of the TSSs between the surface and the bulk. We suppose that the nonzero Dirac gap can have some relationship with the formation of the axion-like state, which presumably couples nonmagnetic spin-orbit and magnetic contributions at the boundary between the topological and trivial phases for a system with parameters close to the TPT conditions. A practically realized system is proposed - the AFM TI with a stoichiometry close to that of MnBi_2Te_2Se_2 with partial (about 50%) substitution of Te atoms for Se atoms in MnBi_2Te_4 which can be an experimental platform for the implementation and experimental analysis of the TPT and the corresponding possibility of the axion-like state realization in Condensed Matter. Besides, such system could serve as a good platform for studying the dynamic axion effect, where the axion field fluctuations are maximised when a small external field is applied to the system which state is close to the TPT.

Influence of Structural Disorder on the Magnetic Order in FeRhCr Alloys

Magnetic phase transitions in alloys are highly influenced by the sample preparation techniques. In the present research, electronic and magnetic properties of Fe48Cr3Rh49 alloys with varying cooling rates were studied, both experimentally and theoretically. The degree of crystalline ordering was found to depend on the cooling rate employed after annealing the alloy. Modeling of alloy structures with different degrees of crystalline ordering was carried out via strategic selection of substitution positions and distances between chromium atoms. Theoretical calculations revealed significant changes in magnetic and electronic properties of the alloy with different substitutions. A comprehensive analysis of the calculated and experimental data established correlations between structural characteristics and parameters governing the magnetic phase transition. In this study, we also developed a method for evaluating the magnetic properties of the alloys obtained under different heat treatments. The proposed approach integrates atom substitution and heat treatment parameters, offering precise control over alloy manufacturing to effectively tune their essential magnetic properties.

Regulating fluorescent properties and ESIPT behavior of novel flavone-based fluorophore by replacing oxygen atom in proton donor/acceptor with sulfur atom: A TD-DFT study

Recently, the fluorescent phenomena of a new excited state intramolecular proton transfer (ESIPT) based fluorophore 3-hydroxy-2-(naphthalen-2-yl)-4H-chromen-4-one (HFN) has been reported experimentally. However, the first-hand dynamic information of its ESIPT process is still lacking theoretically. Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the luminescent features and ESIPT processes of HFN and its two derivatives were studied in detail. Two HFN derivatives, 3-mercapto-2-(naphthalen-2-yl)-4H-chromen-4-one (TFN1) and 3-hydroxy-2-(naphthalen-2-yl)-4H-chromene-4-thione (TFN2) were obtained by replacing the oxygen atom of proton donor and proton acceptor in HFN with sulfur atom, respectively. The changes and effects of fluorescent properties and ESIPT process generated by different proton donors and proton acceptors are demonstrated. The simulated maximum absorption and emission peaks of HFN are well consistent with the experimental data. After replacing the oxygen atoms in proton donor and proton acceptor with sulfur atoms, the absorption wavelength of HFN slightly blue-shifts and red-shifts, respectively. And the fluorescence wavelengths of HFN obviously red-shift. The S0→S1 transitions of HFN and TFN1 are dominated by ππ* charge transfer transition, whereas the S0→S1/S0→S2 transition of TFN2 is a mixed charge transfer transition of nπ* and ππ*. The substitution of oxygen atom by sulfur atom in the proton donor or proton acceptor of HFN obviously decreases or increases the ESIPT barrier, due to its stronger proton-donating ability of S–H group or weaker proton-accepting ability of sulfur atom, respectively.

Stability and magnetic properties of Nd[formula omitted]Fe[formula omitted]X (X = Al, Si, Cu, Zn, and Ga)

The stability and magnetic properties of the RE6Fe13X compounds, as well as their preferential localization in Nd-Fe-B magnets, have been investigated by first-principles calculations. We find that the RE6Fe13X (X = Al, Si, Zn, and Ga) have negative formation energies and are more likely to form in the triple junctions, while Nd6Fe13Cu with the formation energy of 0.73 eV tends to localize in the thin intergranular phase. These compounds contribute to the exchange decoupling of Nd2Fe14B grains and thus enhance the coercivity of Nd-Fe-B magnets. Such decoupling can be more pronounced in the cases of Nd6Fe13Cu and Nd6Fe13Zn, which exhibit antiferromagnetic ordering in the Fe sublattice according to our calculations. We propose that the partial substitution of Nd atoms with Pr can gradually reduce the formation energy of these phases and maintain their original antiferromagnetic states.

The first-principles calculations of photocatalytic water splitting and photoelectric properties of two-dimensional MoxW1-xS2 and MoS2xSe2(1-x) alloys

The realization of ternary alloys by atomic substitution of two-dimensional (2D) transition metal dichalcogenides (TMDs) is an effective solution to modulate the optoelectronic properties of 2D materials. The MoxW1-xS2 and MoS2xSe2(1-x) (0 ≤ x ≤ 1) alloys with high stability are established and their electronic, optical and photocatalytic performances are systematically investigated based on the first-principles calculations. Our results indicate that the MoxW1-xS2 and MoS2xSe2(1-x) alloys remain direct bandgaps and optical adsorption has excellent ability to capture ultraviolet and visible light. Suitable bandgap at HSE06 level (>1.23 eV) and band edges satisfy the conditions of photocatalytic water splitting under acidic or neutral solutions. Biaxial strain can enhance the alloys direct bandgap ranges. The introductions of W and S in MoS2 and MoSe2 monolayers cause their intrinsic electronic properties to change. The bandgap values, effective masses and work functions as well as optical adsorption properties of the MoxW1-xS2 and MoS2xSe2(1-x) show different behaviors due to the d-d, p-p and d-p orbitals hybridization between Mo, W, S and Se atoms. The absorption coefficients of Mo0·5W0·5S2 and MoS1·5Se0.5 are significantly higher theirs pure binary TMDs. The work provides a valuable theoretical guidance of TMDs alloys application in 2D flexible photoelectric devices and photocatalytic water splitting.

Electronic and Surface Properties of Aluminum (111) Surface Modified by Interstitial and Substitutional Titanium Incorporation

This study investigates the influence of interstitial and substitutional titanium atoms on the electronic properties of aluminum surfaces using density functional theory (DFT). The study focuses on three variables: the presence and arrangement of Ti interstitials on the aluminum surface, the behavior of Ti substitutional and interstitial impurities, and the energetic stability and structural properties of these systems. Multiple DFT methods are employed to derive conclusions regarding the impact of these variables on the surface properties of aluminum. The study provides valuable insights into how different states of interstitial and substitutional Ti can alter the physical characteristics and performance behaviors of the aluminum surface. The understanding of these effects could enable engineers to design more efficient materials with enhanced properties suitable for various industries.

Study on the modulating effect of halogen atom substitution on the detection range of water content detection probes in organic solvents

Fluorescence probes based on the variations of aggregation state (Aggregation-Induced Emission (AIE) and Aggregation-Caused Quenching (ACQ)) have received widespread attention due to their simplicity, efficiency and intuitiveness. However, typical probes are highly sensitive to changes in polarity and slight variations in the external environment can cause a complete change in the aggregation state. With the aim of expanding the detection range of the molecular probe, this work adopts a different design strategy from adjusting the molecular backbone but regulates the fluorescence behavior of the Schiff base molecular backbone by introducing different halogen atoms. Systematic studies show that when chlorine serves as substitutional atoms (3,5-Cl Salen), the probe can achieve full-range detection of water content (0–100 vol%) in ethanol and DMF. To our knowledge, the 3,5-Cl Salen represents the best water content probe in organic molecules. Experimental and theoretical studies have shown that the adjustment of halogen atoms can linearly change the charge distribution on the benzene ring and precisely control the strength of intermolecular interactions. At the same time, we developed a fluorescent filter paper based on 3,5-Cl Salen and used smartphones for rapid, sensitive and precise on-site measurement of water content in organic solvents.

Iodination of propargylpyrazole in the presence of cadmium (II) acetate

This paper describes an efficient synthetic method of iodine addition to the triple bond and substitution of the CH-acid hydrogen atom of propargylpyrazole in the presence of cadmium (II) acetate at room temperature, the role of solvents and the ratio of reagents during iodination are optimized.

Atom substitution of the solid-state electrolyte Li10GeP2S12 for stabilized all-solid-state lithium metal batteries

Solid-state electrolyte Li10GeP2S12 (LGPS) has a high lithium ion conductivity of 12 mS cm−1 at room temperature, but its inferior chemical stability against lithium metal anode impedes its practical application. Among all solutions, Ge atom substitution of the solid-state electrolyte LGPS stands out as the most promising solution to this interface problem. A systematic screening framework for Ge atom substitution including ionic conductivity, thermodynamic stability, electronic and mechanical properties is utilized to solve it. For fast screening, an enhanced model DopNetFC using chemical formulas for the dataset is adopted to predict ionic conductivity. Finally, Li10SrP2S12 (LSrPS) is screened out, which has high lithium ion conductivity (12.58 mS cm−1). In addition, an enhanced migration of lithium ion across the LSrPS/Li interface is found. Meanwhile, compared to the LGPS/Li interface, LSrPS/Li interface exhibits a larger Schottky barrier (0.134 eV), smaller electron transfer region (3.103 Å), and enhanced ability to block additional electrons, all of which contribute to the stabilized interface. The applied theoretical atom substitution screening framework with the aid of machine learning can be extended to rapid determination of modified specific material schemes.

Comparative Study of Magnetic Properties of (Mn1−xAxIV)Bi2Te4 AIV = Ge, Pb, Sn

We investigated the magnetic properties of the antiferromagnetic (AFM) topological insulator MnBi2Te4 with a partial substitution of Mn atoms by non-magnetic elements (AIV = Ge, Pb, Sn). Samples with various element concentrations (10–80%) were studied using SQUID magnetometry. The results demonstrate that, for all substitutes the type of magnetic ordering remains AFM, while the Néel temperature (TN) and spin-flop transition field (HSF) decrease with an increasing AIV = Ge, Pb, Sn concentration. The rate of decrease varies among the elements, being highest for Pb, followed by Sn and Ge. This behavior is attributed to the combined effects of the magnetic dilution and lattice parameter increase on magnetic properties, most prominent in (Mn1−xPbx)Bi2Te4. Besides this, the linear approximation of the experimental data of TN and HSF suggests higher magnetic parameters for pure MnBi2Te4 than observed experimentally, indicating the possibility of their non-monotonic variation at low concentrations and the potential for enhancing magnetic properties through doping MnBi2Te4 with small amounts of nonmagnetic impurities. Notably, the (Mn1−xPbx)Bi2Te4 sample with 10% Pb substitution indeed exhibits increased magnetic parameters, which is also validated by local-probe analyses using ARPES. Our findings shed light on tailoring the magnetic behavior of MnBi2Te4-based materials, offering insights into the potential applications in device technologies.

BN-bicyclohexyl material for enhanced reversible dehydrogenation reaction for hydrogen storage: Density functional theory approach

Hydrogen is considered as an environmentally friendly energy resource to replace fossil fuels. This study aims to enhance the dehydrogenation efficiency of liquid organic hydrogen carrier (LOHC) materials by investigating the dehydrogenation and hydrogenation mechanisms through hetero atom (B and N) substitution of bicyclohexyl, as well as by applying the high-efficiency catalysts, palladium and ruthenium. To achieve this, BN-bicyclohexyl, a novel material based on bicyclohexyl, was proposed using density functional theory (DFT) calculations, and the synthesis of the new material was evaluated through enthalpy and free energy calculations. Mulliken charge analysis showed that the B and N atoms substituted in bicyclohexyl can induce dihydrogen bonding, resulting in efficient dehydrogenation at the substitution site. The dehydrogenation efficiency of BN-bicyclohexyl (1.53 eV in activation energy) surpasses that of bicyclohexyl (2.77 eV) on the Pd(111) surface. However, on the Ru(001) surface, BN-bicyclohexyl exhibits a slightly lower hydrogenation efficiency than bicyclohexyl, with an increase of 0.39 eV in activation energy. This trend highlights BN as a promising candidate for enhancing the capability of LOHCs. Notably, the dehydrogenation efficiency of BN-bicyclohexyl is significantly higher than that of bicyclohexyl, which go beyond slightly lower hydrogenation efficiency than bicyclohexyl.

An approach to investigate the crystallographic unit cell of human tooth enamel.

Human tooth enamel (HTE) is the hardest tissue in the human body and its structural organization shows a hierarchical composite material. At the nanometric level, HTE is composed of approximately 97% hydroxyapatite [HAP, Ca10(PO4)6(OH)2] as inorganic phase, and of 3% as organic phase and water. However, it is still controversial whether the hexagonal HAP phase crystallizes in P63/m or another space group. The observance in HTE of Ca2+, Mg2+ and Na+ ions using X-ray characteristic energy-dispersive spectroscopy in the scanning electron microscope has been explained by substitutions in the HAP unit cell. Thus, Ca2+ can be replaced by Na+ and Mg2+ ions; the PO43- group can be replaced by CO32- ions; and the OH- ions can also be replaced by CO32-. A unit-cell model of the hexagonal structure of HTE is not fully defined yet. In this work, density functional theory calculations are performed to study the hexagonal HAP unit cell when substitution by OH-, CO32-, Mg2+ and Na+ ions are carried out. An approach is presented to study the crystallographic unit cell of HTE by examining the changes resulting from the inclusion of these different ions in the unit cell of HAP. Enthalpies of formation and crystallographic characteristics of the electron diffraction patterns are analysed in each case. The results show an enhancement in structural stability of HAP with OH defects, atomic substitution of Mg2+, carbonate and interstitial Na+. Simulated electron diffraction patterns of the generated structures show similar characteristics to those of human tooth enamel. Hence, the results explain the indiscernible structural changes shown in experimental X-ray diffractograms and electron diffraction patterns.

First principles investigation of CO and CO2 adsorption on graphene nanoribbon modified by ZrOx

Carbon dioxide (CO2) is normally emitted from anthropogenic and natural sources, and it is a greenhouse gas that is directly linked to climate change. CO is a main sink of OH molecules in the troposphere to produce CO2. Precise evaluation of the concentrations of the two gases is essential for controlling their emission. The influence of Armchair-graphene nanoribbon (GNR) modification by ZrOx (where x=0,1,or2) on its adsorption of CO and CO2 is inspected in this investigation. First principles computations that employ density functional theory (DFT) are utilized to assess gas adsorption by evaluation of the adsorption energy (Ega) and length (D), exchange of charge between the gas and the structure (∆QT), density of states (DOS), along with the band structure. The modification of GNR is established by atomic substitution (doping) or deposition on GNR structure (decoration). The results indicate outstanding enhancement of CO and CO2 adsorption on the modified GNR structures. However, doping is more efficient than decoration for adsorption of both gases. In particular, the Zr doped GNR has the highest capacity for both gases' adsorption, where the adsorption energy for CO and CO2 increases 18.4 and 16.5 times, respectively, reference to the pristine GNR. The outcomes of this investigation promote the utilization of ZrOx doping of GNR as an approach for the fabrication of highly sensitive and selective environmental CO and CO2 sensors.

Design of New Inorganic Crystals with the Desired Composition Using Deep Learning.

New solid-state materials have been discovered using various approaches from atom substitution in density functional theory (DFT) to generative models in machine learning. Recently, generative models have shown promising performance in finding new materials. Crystal generation with deep learning has been applied in various methods to discover new crystals. However, most generative models can only be applied to materials with specific elements or generate structures with random compositions. In this work, we developed a model that can generate crystals with desired compositions based on a crystal diffusion variational autoencoder. We generated crystal structures for 14 compositions of three types of materials in different applications. The generated structures were further stabilized using DFT calculations. We found the most stable structures in the existing database for all but one composition, even though eight compositions among them were not in the data set trained in a crystal diffusion variational autoencoder. This substantiates the prospect of the generation of an extensive range of compositions. Finally, 205 unique new crystal materials with energy above hull <100 meV/atom were generated. Moreover, we compared the average formation energy of the crystals generated from five compositions, two of which were hypothetical, with that of traditional methods like atom substitution and a generative model. The generated structures had lower formation energy than those of other models, except for one composition. These results demonstrate that our approach can be applied stably in various fields to design stable inorganic materials based on machine learning.

Ab initio anharmonic analysis of complex vibrational spectra of phenylacetylene and fluorophenylacetylenes in the acetylenic and aromatic C-H stretching region.

Vibrational spectra in the acetylenic and aromatic C-H stretching regions of phenylacetylene and fluorophenylacetylenes, viz., 2-fluorophenylacetylene, 3-fluorophenylacetylene, and 4-fluorophenylacetylene, were measured using the IR-UV double resonance spectroscopic method. The spectra, in both acetylenic and aromatic C-H stretching regions, were complex exhibiting multiple bands. Ab-initio anharmonic calculations with quartic potential using B97D3/6-311++G(d,p) and vibrational configuration interaction were able to capture all important spectral features in both the regions of the experimentally observed spectra for all four molecules considered in the present work. Interestingly, for phenylacetylene, the spectrum in the acetylenic C-H stretching region emerges due to anharmonic coupling of modes localized on the acetylenic moiety along with the other ring modes, which also involve displacements on the acetylenic group, which is in contrast to what has been proposed and propagated in the literature. In general, this coupling scheme is invariant to the fluorine atom substitution. For the aromatic C-H stretching region, the observed spectrum emerges due to the coupling of the C-H stretching with C-C stretching and C-H in-plane bending modes.

Ultraviolet electronic spectroscopy of heavily substituted naphthalene derivatives

Context. The so-called bump spectral signature observable on interstellar extinction curves, peaking at 217.5 nm, is commonly assigned to π* ← π transitions from carbonaceous carriers, but the exact nature of the carbonaceous carriers remains debated. Aims. To constrain the chemical structures associated with the bump carriers, we record and compare the UV spectra of a large variety of carbonaceous molecules to this interstellar feature. Methods. Large carbonaceous molecules, such as polycyclic aromatic hydrocarbons (PAHs), were produced by a combustion process stabilized at low pressure under rich flame conditions. Species were extracted and probed through resonance enhanced multiphoton ionization spectroscopy coupled to a time-of-flight mass spectrometer. Masses and absorption profiles of the carbonaceous molecules were measured, and their spectra were compared to the bump feature. Results. Species showing a specific mass progression starting at mass 128 u visible up to mass 394 u with a characteristic progression of +14 u present a common electronic absorption band profile peaking asymptotically around 220 nm. The first masses were assigned to a naphthalene C10H8 molecule and two of its derivatives: C10H7CH3 and C10H7C2H5. The mass progression of +14 u is explained by successive H atom substitutions by CH3 functional groups. This mass distribution was thus assigned to naphthalene derivatives with large aliphatic carbon substitution. This derivative family shows an electronic band assigned to S3 ← S0 transitions involving electron promotion within the π aromatic orbitals of the naphathlene chromophore. More importantly, after a few substitutions, the position of the band converges asymptotically to a value close to the interstellar bump signature, independent of the molecule size. Conclusions. Based on the asymptotic behavior of the larger members in the species distribution, a similar band position is expected from double aromatic ring substructures within hydrogenated amorphous carbons (HACs). Similar to the conclusions of previous works, we find substituted naphthalene units as substructures of interstellar HACs to be good candidates as carriers of the bump feature.

Facile scalable multilevel structure engineering makes Ti0.667Nb1.333O4 a new promising lithium-ion battery anode

Titanium/Niobium oxide-based lithium-ion battery anodes are characterized by their excellent safety features, superior rate capability and high theoretical capacity. However, the intrinsic poor electron conductivity of the metal oxides significantly limits anode performance. In this work, Ti0.667Nb1.333O4 (TNO4) has been synthesized and utilized as the lithium-ion battery anode for the first time. A facile scalable multilevel structure engineering in terms of electronic, morphology, and surface structures is applied to TNO4, leading to an effective performance improvement. The electronic structure, in regard to the band gap and lithium-ion diffusion energy barrier, is well tuned by a stoichiometric atomic substitution of titanium by niobium. The reduced band gap (0.38 eV vs. 3.10 eV) and increased energy barrier along the c-axis (0.13 eV vs. 0.04 eV) accelerate electrochemical kinetics and increase actual capacities. In combination with particle size reduction and surface carbon composition, the electrochemical performance is further improved. The lithiation/delithiation mechanism of the TNO4 anode is investigated by in situ X-ray diffraction (XRD) and density functional theory (DFT) computations. In situ transmission electron microscopy (TEM) is applied to reveal the structure change upon lithiation and delithiation, where a limited volume change is observed. Differential electrochemical mass spectrometry (DEMS) shows a slight gassing process of TNO4/carbon.