Browse Articles

Explicit core-hole single-particle methods for <i>L</i>- and <i>M</i>-edge x-ray absorption and electron energy-loss spectra

Single-particle methods based on Kohn–Sham unoccupied states to describe near-edge X-ray absorption (XAS) spectra are routinely applied for the description of K-edge spectra, as there is no complication due to spin–orbit (SO) coupling. L- and M-edge spectra are often addressed via variants of time-dependent density functional theory (TDDFT) based on SO calculations. Here, we present a computationally efficient implementation based on single-particle calculations with core holes within the frozen-core approximation. Combined with a semiempirical energy shift and a fixed SO splitting for each core level, this allows for a computationally cheap, while overall accurate, prediction of experimental spectra on the absolute energy scale. The spectra are compared to about 40 times slower linear-response TDDFT calculations for molecules and show similar or even better matches with experiment. An exception is multiplet effects that we analyze in detail and show that they cannot be covered by a single-particle approximation. A similar picture emerges for solids, where good qualitative and sometimes even quantitative agreement to experimental XAS and electron energy-loss spectra is achieved.

Arguments for blind peer review also need to be recognized

Constrained nuclear–electronic orbital method for periodic density functional theory: Application to H2 chemisorption on Si(001) surfaces

The nuclear–electronic orbital (NEO) method provides a powerful computational framework for incorporating nuclear quantum effects (NQE) in electronic structure calculations beyond the Born–Oppenheimer approximation. By incorporating additional constraints to the position operator on quantum particles like protons, the NEO method enables calculation of effective potential that accounts for NQE. In this work, we present a new constrained NEO (cNEO) formulation for density functional theory (cNEO-DFT) calculations in the context of extended periodic systems. Using the nudged elastic band method, we discuss an application of the cNEO-DFT approach to studying the adsorption of a hydrogen molecule on the Si(001) surfaces. The calculation shows how NQE impacts the reaction energetics. The proton density changes are computed along the reaction pathways. This work demonstrates the capability of the new cNEO-DFT method to study a wide range of chemical processes, such as surface reactions where the quantum nature of light atoms like protons is non-negligible.

Autism is on the rise: what’s really behind the increase?

A density functional theory study of thermally activated water splitting on the CuWO4 (010) surface

CuWO4 shows promise as a suitable material for solar-driven water splitting to aid progress towards sustainable and carbon-free energy generation. In this work, we report a computational study of catalytic water splitting and hydrogen generation at CuWO4 surfaces, employing calculations based on the density functional theory with on-site Coulomb and long-range dispersion corrections (DFT+U-D3). We have analyzed three potential thermodynamic and kinetic reaction profiles at the pristine CuWO4 surface and one potential profile at the reduced CuWO4 surface, examining the structural and electronic properties of the intermediates, as well as the transition states along the different pathways. Our findings reveal that along the pathway on the reduced surface, oxygen vacancies introduced in the surface led to under-coordinated Cu and W atoms and localized excess electrons, which significantly enhance the hydrogen evolution reaction.

Shell-thickness dependent Fano resonance in molecular catalyst functionalized CdSe/ZnS core/shell QDs

Hybrid photocatalysts consisting of molecular catalyst functionalized semiconductors have attracted intense recent interest in solar fuel applications. Charge transfer interactions between the molecular catalyst and semiconductor have long been recognized to affect catalyst properties by controlling photoinduced charge separation across the semiconductor/molecule interface. In this paper, we investigate how such an interaction can also affect Fano resonance between the catalyst vibration and the intraband absorption of semiconductors. Using [Re(3,3′-disulfide-2,2′-bipyridine)(CO)3Cl] (ReS2) functionalized CdSe/ZnS core/shell quantum dots (QDs) as a model system, we show that the CO stretching mode of the catalyst can interact with the broad intraband absorption of conduction band (CB) electrons. Detailed analysis shows that the Fano resonance asymmetry factor q decreases at larger ZnS shell thicknesses. This experimental finding is consistent with a theoretical model that assumes the vibronic interaction leading to the observed Fano resonances is mediated by effective charge transfer interactions between the QD conduction band electron and the adsorbed catalyst. Because of the type I band alignment in the CdSe/ZnS QDs, an increasing shell thickness leads to a decreasing CB electron density at the ZnS shell surface, reducing electronic coupling and the charge transfer interaction with the adsorbed catalysts.

<i>Ab initio</i> study of electromigration in liquid GeAsSe alloys for selector devices

Selenide amorphous alloys are of interest for applications in selector devices that exploit a sort of reversible dielectric breakdown called ovonic threshold switching. In the on-state of the device, the system is typically brought into the supercooled liquid phase above the glass transition temperature, where the atomic mobility is sufficiently high to cause demixing driven by the electric field. The electromigration force F responsible for ionic migration is proportional to the electric field E via the effective charge Z* (F = |e|Z*E, where e is the electron charge), which is thus of great relevance for the electrothermal modeling of the devices. In this work, we computed Z* for a prototypical GeAsSe selector alloy by leveraging a non-equilibrium Green’s function method based on density functional theory. The effective charges in the metallic liquid were obtained by calculating the atomic forces, including the wind force, in the presence of both an electric field and an electronic current.

Maximizing nanoparticle light absorption: Size, geometry, and a prospect for metal alloys

In this work, we show how to maximize absorption of plasmonic nanoparticles in terms of size, geometry, and material. We determined that the main effect dictating the optimal amount of optical losses is radiation damping, and how it depends on nanoparticle size and geometry. Based on this, we find that for many combinations of sizes and geometries, losses in pure metals are far from optimal. To overcome the aforementioned issue, alloying is presented as a straightforward and flexible way of modulating the optical losses. Furthermore, strategies for tuning the optical losses to values above, between, and even below those in pure plasmonic metals are developed in terms of selecting the right alloy composition. In some cases, alloys showed a multifold increase in absorption when compared to pure plasmonic metals. The physical reasons governing such changes are elucidated based on the electronic structure changes during alloying of different metals, which enables generalization of the results to other systems. In addition to increasing absorption, electronic structure changes can also be utilized for channeling the absorbed energy to suit different purposes, such as hot carrier generation for photocatalysis or solar energy harvesting. Overall, these results establish alloying as a powerful tool for designing nanostructures for applications that utilize light absorption.

Climate models need more frequent releases of input data — here’s how to do it

Predicting energetic and entropic driving forces with coarse-grained models

Low resolution coarse-grained (CG) models provide exceptional computational efficiency for simulating soft materials. Consequently, many studies employ CG models to determine free energy surfaces along order parameters or reaction coordinates of interest. However, because CG models average over atomic details, it is challenging to determine the energetic and entropic contributions to the resulting free energy surfaces. In this work, we present a rigorous and predictive CG framework for computing these energetic and entropic driving forces based upon simulations at a single temperature. This dual approach employs distinct variational principles to independently approximate the exact CG interaction potential, W(R), and its energetic component, EW(R). This dual approach determines the free energy surface, aφ(x), along an order parameter, φ(x), via simulations with W(R). The dual approach then determines the energetic driving force, ūφ(x), by evaluating EW(R) for the sampled configurations. The entropic driving force, s̄φ(x), is indirectly inferred, s̄φ(x)=ūφ(x)−aφ(x)/T. Importantly, this entropic contribution reflects both the CG configuration distribution and the atomic details that have been eliminated from the CG model. We demonstrate that the dual approach reasonably describes the energetic and entropic driving forces between a pair of nonpolar solutes in a polar solvent. In contrast, naïvely estimating energetics with the CG interaction potential provides a qualitatively incorrect description for these driving forces.

Emotional AI is here — let’s shape it, not shun it

Extended line list for the B2Σ+–X2Σ+ band system of CaF for use in laser-induced plasma studies

Calcium monofluoride (CaF) is an important species for the sensitive detection of fluorine with laser-induced breakdown spectroscopy (LIBS). A line list for the B2Σ+–X2Σ+ transition system of CaF was calculated and extended from v ≤ 20 to v = 35 using empirical potential functions obtained through a direct-potential fit approach with observed transition frequencies. The line strengths were predicted using the scaled transition dipole moments calculated ab initio. The simulated intensities and transition positions with the present line list agree well with the spectra of high-resolution molecular spectroscopy from the literature and of LIBS from this work. Based on the simulated spectra of the CaF B–X system, the derived temperatures with the LIBS spectra vary across different wavelength regions. For example, with the same spectrum of delay time t = 13 μs, the obtained temperatures with λ ≈ 514–529 nm and λ ≈ 514–570 nm are 4000 and 5500 K, respectively. However, this discrepancy decreases with increasing delay time. For t = 23 μs, the obtained temperatures (3700 ± 200–4000 ± 100 K) are already in agreement within the estimated uncertainties. Self-absorption in the spectrum also influences the simulated temperature. Excluding the influences from self-absorption, the derived temperature (6000 ± 200 K) with the CaF molecular spectrum of λ ≈ 529–542 nm of t = 13 μs is close to the ionization temperature (6600 K) obtained with the Saha–Boltzmann fit from the observed atomic/ionic lines of Ca in the plasma. The present line list can be adopted in plasma diagnostics and in sensitive detections of fluorine with the CaF spectrum in hot environments.

I trawl coastlines to study the impacts of microplastics on marine life

Sum frequency dual null angle approach and its use in surface hyperpolarizability ratio measurements

Null angle measurements are a recognized method for accurately determining the ratio of optical constants in linear and nonlinear optical spectroscopy. Here, we extend the established null angle scheme in vibrational sum-frequency generation where the sum-frequency beam is linearly polarized at ±45° to include a second scheme where the IR beam is polarized at ±45°. We illustrate that measurement of the null angles obtained in both schemes may be used together to calibrate the SFG response between three polarization combinations. We then demonstrate that these two null angles provide the required phase information to determine the surface hyperpolarizability ratio, even at buried interfaces where calibration is typically more difficult, and without requiring a heterodyne scheme. This makes extracting the electronic structure information directly from the SFG spectra more accurate and truly independent of the molecular orientation distribution.

Assessing high-risk human papillomavirus-based cervical precancer screening recommendations and implications among women aged 60/65 years and older in Ghana

My moonshot to preserve endangered species

Computational screening of single-atom catalysts supported on triazine-based graphite carbon nitride for 1,2-dichloroethane dechlorination

In this study, we systematically investigated the performance of eight transition metal atom-loaded triazine-based graphitic carbon nitride (TM@TGCN) for the catalysis of 1,2-dichloroethane (1,2-DCE) dechlorination reaction (DCEDR) by density functional theory calculations. Through the five-step screening method, the suitable catalysts, respectively, applicable to the generation of vinyl chloride (CH2CHCl), ethylene (CH2CH2), and ethane (CH3CH3) were finally determined. The limiting potential of Fe@TGCN for reducing 1,2-DCE to CH3CH3 is lower, at −0.47 V (gauche-C2H4Cl2) and −0.50 V (trans-C2H4Cl2), respectively. The activity mechanism indicates that Fe@TGCN is at the vertex of the volcano plot, confirming that the intensity of its interaction with the reactants is in optimal equilibrium. In addition, we further examined the influence of hydroxyl modification on the selectivity of DCEDR. The results show that hydroxyl modification significantly weakens the adsorption strength of intermediates (such as *CH2CH2Cl) through a steric hindrance effect and electron delocalization, as well as reduces the desorption energy of CH2CH2 and enhances its selectivity. This study provides theoretical guidance for the rational design of DCEDR electrocatalysts and reveals the key role of ligand modification strategies in regulating the reaction pathway.

A fused weighted federated learning-based adaptive approach for early-stage drug prediction

How a self-taught biologist transformed nature writing — and inspired Darwin

Microbiota-driven antitumour immunity mediated by dendritic cell migration