Initio Density Functional Theory Simulations Research Articles

Characterizing ABS–copper chemistry-dependent adhesion: From the atomic to macro level

The electroplating of copper layers on acrylonitrile–butadiene–styrene (ABS) polymer surfaces is a common need in industrial applications. The development of new eco-friendly techniques to promote ABS–copper adhesion is crucial due to the harsh and expensive chemicals used in the existing processes. The initial path involves establishing a better understanding on chemistry-dependent ABS–copper adhesion mechanisms. By formulating the material with more functional groups, the adhesion between ABS and the coated copper layer was characterized through a multiscale approach. At the atomic scale, we performed ab initio density functional theory simulations to obtain the variations in the adsorption energy. At the nano and micro scales, we adopted atomic force microscopy to measure the adhesion forces on the bulk polymer surfaces. At the macro scale, we compared these adhesion results with both 90-degree peeling tests and finite element model analyses. This study presents the first comprehensive approach for bridging information gaps across the scales (from nano to macro) with computational and experimental data, facilitating the design of formulated ABS additives.

MoS2 Field-Effect Transistor Performance Enhancement by Contact Doping and Defect Passivation via Fluorine Ions and Its Cyclic Field-Assisted Activation.

MoS2-based field-effect transistors (FETs) and, in general, transition metal dichalcogenide channels are fundamentally limited by high contact resistance (RC) and intrinsic defects, which results in low drive current and lower carrier mobilities, respectively. This work addresses these issues using a technique based on CF4 plasma treatment in the contacts and further cyclic field-assisted drift and activation of the fluorine ions (F-), which get introduced into the contact region during the CF4 plasma treatment. The F- ions are activated using cyclic pulses applied across the source-drain (S/D) contacts, which leads to their migration to the contact edges via the channel. Further, using ab initio molecular dynamics and density functional theory simulations, these F- ions are found to bond at sulfur (S) vacancies, resulting in their passivation and n-type doping in the channel and near the S/D contacts. An increase in doping results in the narrowing of the Schottky barrier width and a reduction in RC by ∼90%. Additionally, the passivation of S vacancies in the channel enhances the mobility of the FET by ∼150%. The CF4 plasma treatment in contacts and further cyclic field-assisted activation of F- ions resulted in an ON-current (ION) improvement by ∼90% and ∼480% for exfoliated and CVD-grown MoS2, respectively. Moreover, this improvement in ION has been achieved without any deterioration in the ION/IOFF, which was found to be >7-8 orders.

Interfacial phenomena and surface protection of N80-carbon steel in acidic environments using thiazolidinediones: An experimental and computational analysis

In petrochemical engineering, a pressing challenge is the corrosion of N80 carbon steel (N80-CS) in strong acidic conditions like 15 wt% HCl, impacting both mechanical integrity and economic viability. Herein, the corrosive behavior of N80-CS in a 15 wt% HCl solution was evaluated with two thiazolidinediones, (Z)− 5-benzylidenethiazolidine-2,4-dione (BT) and 5-(4-fluorobenzylidene) thiazolidine-2,4-dione (FBT). The surface and interface phenomena were evaluated using electrochemical testing, Scanning electron microscope (SEM), atomic force microscopy (AFM), and computational modelling. Electrochemical impedance spectroscopy (EIS) revealed that, at optimum concentrations, the polarization resistance (Rp) manifested a notable increase from an initial 5.51 Ω cm2 to 80.46 and 39.64 Ω cm2 when BT and FBT were introduced, respectively. This corresponded to inhibition efficiencies of 93% for BT and 86% for FBT. This results in a substantial reduction in effective double layer capacitance due to the adsorption of inhibitor molecules. The Rp values of BT compound peaked at 158.02 Ω cm2 after a 24-hour immersion period, later declining to 82 Ω cm2 after 42 h, thus evidencing the enhanced stability of the inhibitor’s protective layer on the steel surface. The potentiodynamic polarization (PDP) technique validated that both compounds function as mixed-type corrosion inhibitors. Their adsorption followed the Langmuir adsorption isotherm model, indicating a mixed mechanism of protection via both physical and chemical interactions between the inhibitor molecules and the steel surface. SEM and AFM analyses further revealed significant alterations in the surface morphology and roughness of N80-CS, affirming the performance of these inhibitors. Ab initio Density Functional Theory (DFT) simulations further supported the experimental results, particularly explaining the superior performance of BT and its exceptional adsorption characteristics. This study confirms the anti-corrosive potential of thiazolidinediones and sets the groundwork for future research and improved inhibitor design.

Investigation of the electronic, optical, elastic, mechanical and vibrational properties of CuI using HSE03

Ab Initio density functional theory (DFT) simulations have been employed to systematically explore the electronic, optical, elastic, mechanical and vibrational properties. In this study, we revealed that [Formula: see text]-CuI has a wide direct bandgap energy of 3.21 eV, is pure covalent and brittle. We also found that the core level is made up of I s electron orbitals, the valence band is constructed with I p orbitals, and the Cu s orbital states mainly contribute to the conduction band minimum (CBM). The reflectivity of CuI is reported to be low (35.9% for the light reflected), showing high material absorption. A high absorption coefficient of [Formula: see text] is also reported. The elastic and mechanical properties can further confirm the mechanical stability of the CuI system, derived from DFT-calculated elastic constants and phonon dispersion from density functional perturbation theory (DFPT) calculations.

Influence of Capping Ligands, Solvent, and Thermal Effects on CdSe Quantum Dot Optical Properties by DFT Calculations.

Cadmium selenide nanomaterials are very important materials in photonics, catalysis, and biomedical applications due to their optical properties that can be tuned through size, shape, and surface passivation. In this report, static and ab initio molecular dynamics density functional theory (DFT) simulations are used to characterize the effect of ligand adsorption on the electronic properties of the (110) surface of zinc blende and wurtzite CdSe and a (CdSe)33 nanoparticle. Adsorption energies depend on ligand surface coverage and result from a balance between chemical affinity and ligand-surface and ligand-ligand dispersive interactions. In addition, while little structural reorganization occurs upon slab formation, Cd···Cd distances become shorter and the Se-Cd-Se angles become smaller in the bare nanoparticle model. This originates mid-gap states that strongly influence the absorption optical spectra of nonpassivated (CdSe)33. Ligand passivation on both zinc blende and wurtzite surfaces does not induce a surface reorganization, and thus, the band gap remains nonaffected with respect to bare surfaces. In contrast, structural reconstruction is more apparent for the nanoparticle, which significantly increases its highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap upon passivation. Solvent effects decrease the band gap difference between the passivated and nonpassivated nanoparticles, the maximum of the absorption spectra being blue-shifted around 20 nm by the effect of the ligands. Overall, calculations show that flexible surface cadmium sites are responsible for the appearance of mid-gap states that are partially localized on the most reconstructed regions of the nanoparticle that can be controlled through appropriate ligand adsorption.

Defect pairing in Fe-doped SnS van der Waals crystals: a photoemission and scanning tunneling microscopy study.

We investigate the effect of low concentrations of iron on the physical properties of SnS van der Waals crystals grown from the melt. By means of scanning tunneling microscopy (STM) and photoemission spectroscopy we study Fe-induced defects and observe an electron doping effect in the band structure of the native p-type SnS semiconductor. Atomically resolved and bias dependent STM data of characteristic defects are compared to ab initio density functional theory simulations of vacancy (VS and VSn), Fe substitutional (FeSn), and Fe interstitial (Feint) defects. While native SnS is dominated by acceptor-like VSn vacancies, our results show that Fe preferentially occupies donor-like interstitial Feint sites in close proximity to VSn defects along the high-symmetry c-axis of SnS. The formation of such well-defined coupled (VSn, Feint) defect pairs leads to local compensation of the acceptor-like character of VSn, which is in line with a reduction of p-type carrier concentrations observed in our Hall transport measurements.

Machine Learning Diffusion Monte Carlo Forces

Diffusion Monte Carlo (DMC) is one of the most accurate techniques available for calculating the electronic properties of molecules and materials, yet it often remains a challenge to economically compute forces using this technique. As a result, ab initio molecular dynamics simulations and geometry optimizations that employ Diffusion Monte Carlo forces are often out of reach. One potential approach for accelerating the computation of "DMC forces" is to machine learn these forces from DMC energy calculations. In this work, we employ Behler-Parrinello Neural Networks to learn DMC forces from DMC energy calculations for geometry optimization and molecular dynamics simulations of small molecules. We illustrate the unique challenges that stem from learning forces without explicit force data and from noisy energy data by making rigorous comparisons of potential energy surface, dynamics, and optimization predictions among ab initio density functional theory (DFT) simulations and machine-learning models trained on DFT energies with forces, DFT energies without forces, and DMC energies without forces. We show for three small molecules─C2, H2O, and CH3Cl─that machine-learned DMC dynamics can reproduce average bond lengths and angles within a few percent of known experimental results at one hundredth of the typical cost. Our work describes a much-needed means of performing dynamics simulations on high-accuracy, DMC PESs and for generating DMC-quality molecular geometries given current algorithmic constraints.

Understanding External Pressure Effects and Interlayer Orbital Exchange Pathways in the Two-Dimensional Magnet─Chromium Triiodide

Recent experiments have shown that exfoliated few-layer CrI3, a prototypical van der Waals magnet, undergoes a phase transition from the high-temperature monoclinic structure to the low-temperature rhombohedral structure under pressure. To understand how the magnetism of CrI3 responds to these structural changes, we perform ab initio density functional theory simulations on bilayer CrI3. We simulate the interlayer lateral shift-dependent potential energy surface of bilayer CrI3 to examine the stability and magnetism as a function of external pressure. Using the hybrid PBE0 functional, we are able to give qualitatively correct exchange coupling energies, without using an on-site Coulomb interaction correction. Thus, we avoid using tunable parameters. The results show that under external pressure, the monoclinic crystal structure is destabilized in comparison with the rhombohedral structure, in agreement with the observed phase transition in few-layer CrI3 devices under pressure. We also look into the microscopic origins of the interlayer exchange coupling. We identify the competing orbital pathways that favor ferromagnetic and antiferromagnetic kinetic exchange, respectively, which are consistent with previous reports. This study opens a new direction of using hybrid functionals with Gaussian orbitals and a cluster-based approach for obtaining Heisenberg J values to accurately simulate the magnetic properties of 2D materials.

Towards the Growth of Hexagonal Boron Nitride on Ge(001)/Si Substrates by Chemical Vapor Deposition.

The growth of hexagonal boron nitride (hBN) on epitaxial Ge(001)/Si substrates via high-vacuum chemical vapor deposition from borazine is investigated for the first time in a systematic manner. The influences of the process pressure and growth temperature in the range of 10−7–10−3 mbar and 900–980 °C, respectively, are evaluated with respect to morphology, growth rate, and crystalline quality of the hBN films. At 900 °C, nanocrystalline hBN films with a lateral crystallite size of ~2–3 nm are obtained and confirmed by high-resolution transmission electron microscopy images. X-ray photoelectron spectroscopy confirms an atomic N:B ratio of 1 ± 0.1. A three-dimensional growth mode is observed by atomic force microscopy. Increasing the process pressure in the reactor mainly affects the growth rate, with only slight effects on crystalline quality and none on the principle growth mode. Growth of hBN at 980 °C increases the average crystallite size and leads to the formation of 3–10 well-oriented, vertically stacked layers of hBN on the Ge surface. Exploratory ab initio density functional theory simulations indicate that hBN edges are saturated by hydrogen, and it is proposed that partial de-saturation by H radicals produced on hot parts of the set-up is responsible for the growth.

Structure, dynamics and ionic conductivities of ternary ionic liquid/lithium salt/DMSO mixtures

In this paper we report classical Molecular Dynamics and ab initio Density Functional Theory simulations of the structure and single particle dynamics of ternary mixtures of a protic (ethylammonium nitrate, EAN) and an aprotic (1-Ethyl-3-methylimidazolium tetrafluoroborate, EMIMBF4) ionic liquid, lithium salts with common anion and a molecular cosolvent, dimethyl sulfoxide (DMSO). The coordination numbers and electrical conductivities of the different mixtures have been calculated throughout the whole concentration range, and the differences between mixtures with the protic and aprotic ionic liquids were analyzed. In both cases, the evolution of the electrical conductivity is seen to correlate well with the formation of mixed tetrahedral solvation complexes of lithium cations with ionic liquid anions and cosolvent molecules. The differences in the solvation and charge transport mechanisms in hydrogen bonded mixtures and those based in the aprotic ionic liquid are analyzed. Our conclusions indicate that the major feature behind the electrical conductivity of the ternary mixtures is the composition of the solvation shell of the metal cations in the mixtures and the rate at which anions in it are replaced by DMSO molecules.

Enantioseparation of novel anti-inflammatory chiral sulfoxides with two cellulose dichlorophenylcarbamate-based chiral stationary phases and polar-organic mobile phase(s)

Chiral sulfoxides represent a class of substances of great importance in many fields, including medicinal chemistry dealing with the synthesis of novel cyclooxygenase-2 (COX-2) inhibiting/NO donors. In the present study, two nitrooxyethyl sulfoxides along with their metabolites, hydroxyethyl derivatives have been successfully enantioresolved with two cellulose tris(3,5-dichlorophenylcarbamate)-based chiral stationary phases (CSPs), one with a coated (CSP 1) and the other (CSP 2) with an immobilized chiral selector. The immobilized selector in CSP 2 produced comparable-to-better performances than the coated one in CSP 1 (with α and RS values up to 1.94 and 6.32, respectively) running the analysis with a polar-organic phase made up with an ethanol/2-propanol (80:20, v/v) mobile phase. Electronic circular dichroism studies coupled to ab initio time-dependent density functional theory simulations allowed us to determine the elution order of three out of four compounds. For the two hydroxyethyl derivatives the same enantiomeric elution order [(S)<(R)] was obtained with both CSPs, while the compounds containing the −ONO2 group experienced a different elution order depending on the coated or immobilized nature of the chiral selector [(S)<(R) with CSP 1 and (R)<(S) with CSP 2]. A molecular modeling study based on docking simulations was performed to gain a deeper insight into the enantioseparation mechanism of the hydroxyethyl derivatives on both CSPs.

Water adsorption behaviour on (001) pyrophyllite surface from ab initio Density Functional Theory simulations

The present study focuses on the interaction between water and a single layer of the (001) pyrophyllite surface. Pyrophyllite [Al2Si4O10(OH)2] is an important phyllosilicate characterized by strong in-plane covalent bonds and weak van der Waals interactions holding the structural layers together. The investigation was carried out by ab initio Density Functional Theory simulations, performed both at 0 K and room conditions (300 K, 1 bar) using static and molecular dynamics approaches, respectively. The adsorption of one to three water molecules, investigated by static method, showed that the hydration is completely driven by van der Waals interactions and, as the water content increases, so does the tendency of the solvent to self-assemble. In fact, the binding energies per H2O molecule were calculated as −17.2 kJ mol−1, −11.8 kJ mol−1, and − 11.6 kJ mol−1 for the adsorption of one, two, and three water molecules, respectively. The results showed that water forms structures that resemble those typically found in the gas phase, i.e. water dimer and triangular cluster. Regarding the latter, the ab initio molecular dynamics simulations proved that this self-assembly is stable over the considered time for the run. These findings are in good agreement with the few previous data reported in the literature and could be of great use for both experimental and theoretical applications in both mineralogy and petrography and for the development of bidimensional van der Waals material from natural sources.

Manipulating magnons via ultrafast magnetization modulation

We demonstrate how fundamental properties of magnons may be manipulated using femtosecond laser pulses by performing ab initio real-time time-dependent density functional theory simulations. To illustrate this, we show how the spin-wave dynamics in ${\mathrm{Fe}}_{50}{\mathrm{Ni}}_{50}$ can be manipulated in three different ways by tailoring the applied laser pulse to excite optically induced intersublattice transfer (OISTR): (1) element-selective destruction of magnon modes depending on the laser intensity, (2) delay-dependent freezing of the magnon mode into a transient noncollinear state (where the delay is in the pulse peak with respect to the start of simulations), and (3) OISTR-driven renormalization of the optical magnon frequency. Harnessing such processes would significantly speed up magnonic devices.

Identifying alternative ferroelectric materials beyond Hf(Zr)O2

A database-driven approach combined with ab initio density functional theory (DFT) simulations is used to identify and simulate alternative ferroelectric materials beyond Hf(Zr)O2. The database-driven screening method identifies a class of wurtzite ferroelectric materials. DFT simulations of wurtzite magnesium chalcogenides, including MgS, MgSe, and MgTe, show their potential to achieve improved ferroelectric (FE) stability, simple atomistic unit cell structure, and large FE polarization. Strain engineering can effectively modulate the FE switching barrier height for facilitating FE switching. The effect of the piezoelectric property on the FE switching barrier heights is also examined.

Imaging and manipulation of adsorbed Pb adatom structures on the Si(100) surface by noncontact atomic force microscopy

We performed imaging and manipulation of epitaxially grown Pb adatom structures on the Si(100) surface by noncontact atomic force microscopy (NC-AFM) in ultra-high-vacuum (UHV) and at cryogenic temperatures. We observe several distinct contrast modes during imaging, which we assign to termination of the scanning probe tip by either a single Si, or single Pb, atom, via quantitative comparison of atomic resolution force spectroscopy experiments with ab initio density functional theory (DFT) simulations. We show that the Pb adatom structures can be controllably manipulated via mechanochemical means and identify a novel semideterministic manipulation strategy that arises from the combination of low temperature operation and the asymmetric diffusion barriers present on the Si(100) surface.

Multiscale analysis of dislocation loops and voids in tungsten

We performed ab initio density functional theory simulations of $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}$ interstitial dislocation loops, closed and open vacancy loops, $\ensuremath{\langle}100\ensuremath{\rangle}$ interstitial loops, and voids in tungsten, using simulation cells involving from 2000 to 2700 atoms. The size of the loops transcends the microscopic scale and reaches the mesoscopic scale where asymptotic elasticity treatment applies. Comparing the formation energies of dislocation vacancy loops and voids, we conclude that a void remains the most energetically favorable vacancy defect over the entire range of sizes investigated here. A closed $\frac{1}{2}\ensuremath{\langle}111\ensuremath{\rangle}$ vacancy loop is more stable than an open loop if the number of vacancies in the loop is greater than $\ensuremath{\sim}45$, corresponding to the diameter of a loop of approximately 1.8 nm. We have also computed elastic dipole tensors and relaxation volumes of loops and voids, representing the source terms in continuum models for radiation induced stresses and strains in the material. A detailed analysis of metastable configurations of closed and open vacancy loops performed using molecular statics simulations shows that vacancy loop configurations are not unique, and significant fluctuations of defect structures may occur in the course of microstructural evolution under irradiation.

Simulated infrared and Raman spectroscopy, complex dielectric function and refractive index dataset of monoclinic C2/m stoichiometric clinochlore Mg6Si4O10(OH)8 as obtained from Density Functional Theory.

This article reports a simulated dataset of the vibrational (infrared and Raman) and optical properties (complex dielectric function and refractive index) of clinochlore, an important mineral belonging to the phyllosilicate family [1]. The data here reported were calculated from ab initio Density Functional Theory (DFT) simulations at B3LYP level, including a correction for the dispersive forces (B3LYP-D* approach) and all-electron Gaussian-type orbitals basis sets. This dataset was calculated between 0 cm–1 and 4000 cm–1 and comprises infrared, reflectance and Raman spectra, frequency-dependent complex dielectric function and complex refractive index of clinochlore. The data was validated against available experimental spectroscopic results reported in literature and can be of help in several application fields, for instance fundamental georesource exploration and exploitation, in applied mineralogy, geology, material science, and as a reference to assess the quality of other theoretical approaches.

Interface Matters: Enhanced Photoluminescence and Long-Term Stability of Zero-Dimensional Cesium Lead Bromide Nanocrystals via Gas-Phase Aluminum Oxide Encapsulation.

Cesium lead halide perovskite nanocrystals (PNCs), while possessing facile chemical synthesis routes and high photoluminescence (PL) properties, are still challenged by issues of instability and degradation. Although atomic layer deposition (ALD) of metal oxides has been one of the common encapsulation approaches for longer term stability, its application inevitably resulted in severe loss of emission efficiency and at times partial loss of structural integrity of perovskites, creating a bottleneck in its practical viability. We demonstrate a nondestructive modified gas-phase technique with codeposition of both precursors trimethylaluminum and water to dramatically enhance the PL emission in zero-dimensional (0D) Cs4PbBr6 PNCs via alumina encapsulation. X-ray photoelectron spectroscopy analysis of Cs4PbBr6 films reveals the alumina deposition to be accompanied by elemental composition changes, particularly by the reduction of the excessive cesium content. Ab initio density functional theory simulations further unfold that the presence of excess Cs on the surface of PNCs leads to decomposition of structural [PbBr6]4- octahedra in the 0D perovskite lattice, which can be prevented in the presence of added hydroxyl groups. Our study thus unveils the pivotal role of the PNC surface composition and treatment in the process of its interaction with metal oxide precursors to control the PL properties as well as the stability of PNCs, providing an unprecedented way to use the conventional ALD technique for their successful integration into optoelectronic and photonic devices with improved properties.

The influence of alloying on Zn liquid metal embrittlement in steels

In this study we investigate Zn liquid metal embrittlement at Fe grain boundaries (GBs) by employing ab initio density functional theory (DFT) simulations. We consider three different GBs and two common alloying elements in steels (Si and Al) and evaluate how the interplay of the elements affects Zn enrichment and embrittlement of the GBs. Our simulations show that Zn has stronger tendency to enrich at both GBs and surfaces than Al and Si. By increasing the amount of Al and Si in the bulk, Zn can be reduced at both GBs and surfaces. This is quantified for a large temperature range by the newly introduced replacement potency, which shows that compared to Al, Si has a larger potency to decrease the amount of Zn at GBs and surfaces. Regarding GB embrittlement, our results demonstrate that Si a small positive, Al a small negative, and Zn a strong negative effect on GB cohesion. In combination with Zn both Al and Si are able to somewhat reduce the detrimental effect of Zn.

The atomic structure of a bulk metallic glass resolved by scanning tunneling microscopy and ab-initio molecular dynamics simulation

Scanning tunneling microscopy (STM) technique is widely used to study the electronic and structural properties of nano structures. Here we study the atomic structure of a Ni–Nb BMG by means of ultra high vacuum (UHV) STM in local areas of nanometer scale size, and report the direct atomic structure observation supported by ab initio molecular dynamics (MD) and density functional theory (DFT) simulations used to understand the observed structural and electronic features. It was possible to resolve the atomic surface structure of a bulk metallic glassy sample while MD simulation explained relatively large interatomic distances observed using STM.