First-principles Theory Research Articles

Band gap reduction in highly-strained silicon beams predicted by first-principles theory and validated using photoluminescence spectroscopy

A theoretical study of the band gap reduction under tensile stress is performed and validated through experimental measurements. First-principles calculations based on density functional theory (DFT) are performed for uniaxial stress applied in the [001], [110] and [111] directions. The calculated band gap reductions are equal to 126, 240 and 100 meV at 2% strain, respectively. Photoluminescence spectroscopy experiments are performed by deformation applied in the [110] direction. Microfabricated specimens have been deformed using an on-chip tensile technique up to ∼1% as confirmed by back-scattering Raman spectroscopy. A fitting correction based on the band gap fluctuation model has been used to eliminate the specimen interference signal and retrieve reliable values. Very good agreement is observed between first-principles theory and experimental results with a band gap reduction of, respectively, 93 and 91 meV when the silicon beam is deformed by 0.95% along the [110] direction.

Physics-constrained Data-Driven Variational method for discrepancy modeling

A data-driven discrepancy modeling method is presented that variationally embeds measured data in the modeling and analysis framework. The proposed method exploits the variationally derived loss function that is comprised of the residual between the first-principles theory and sensor-based measurements to augment the physics-based model. The method was first developed in the context of linear elasticity (Masud and Goraya, J. Appl. Mech. 89 (11) 2022) wherein the relation between the discrepancy model and the loss terms was derived to show that the data embedding terms behave like residual-based least-squares regression functions. An interpretation of the stabilization tensor as a kernel function was formally established and its role in assimilating a-priori knowledge of the problem in the modeling method was highlighted. The present paper employs linear elastodynamics as a model problem where Data-Driven Variational (DDV) method incorporates high-fidelity data into forward calculations. This results in driving the problem with not only the boundary and initial conditions, but also with the sensor data that is available at only a small subset of the total domain. The effect of the loss function on the time-dependent response of the system is investigated under a variety of loading conditions and model discrepancies. The energy and Morlet wavelet analyses reveal that the problem with embedded data recovers the energy and the fundamental frequency band of the target system. Time histories of strain energy and kinetic energy of a cantilever beam undergoing damped oscillations are recovered by including known data in an undamped model. This highlights the discrepancy modeling attributes of the method under combined effects of parameter and model discrepancy.

Efficient photocatalytic dehydrogenation and synergistic selective oxidation of benzyl alcohol to benzaldehyde for Zn0.5Cd0.5S co-modified with MoS2 nanoflowers and g-C3N4 nanosheets

Zn0.5Cd0.5S solid solution co-modified by MoS2 nanoflowers and g-C3N4 nanosheets (C3N4/MoS2/Zn0.5Cd0.5S) was synthesized with highly efficient photocatalytic dehydrogenation and synergistic selective oxidation of benzyl alcohol (BA) to benzaldehyde (BAD). The photocatalytic dehydrogenation rate and BAD yield are 1.7 mmol·g−1·h−1 and 35.8% for C3N4/MoS2/Zn0.5Cd0.5S, and selective oxidation to BAD is almost 100%. Reaction selectivity was explored by detecting all the substances of the solution after reaction for Zn0.5Cd0.5S and C3N4/Zn0.5Cd0.5S using gas chromatography-mass spectrometry (GCMS), and analysis results indicated that BA was converted to other substances because of the insufficient hole oxidation capacity of the samples. The mechanism for highly efficient photocatalytic dehydrogenation and synergistic selective oxidation of BA to BAD was systematically investigated by electron paramagnetic resonance (EPR) and first-principle density-functional theory calculations. The EPR spectra of the phenyl-N-tert-butyl-nitrone (PBN)-carbon centered radical indicated that ·C induced by photo-generated holes are the key intermediate of the photocatalytic dehydrogenation of BA. The loading of flower-like MoS2 accelerates charge transport and enhances the hole oxidation ability of Zn0.5Cd0.5S, and the introduction of g-C3N4 has a negative valence band potential that can collect photogenerated holes of Zn0.5Cd0.5S, thereby inhibiting photocorrosion. Therefore, the photocatalytic dehydrogenation and synergistic selection of BA to BAD for the C3N4/MoS2/Zn0.5Cd0.5S ternary heterostructure were greatly improved. The research work would provide a feasible, sustainable, economical, and potential technique for the highly efficient dehydrogenation and synergistic selective oxidation of BA based on photocatalysis that excludes the usage of noble metals and sacrificial reagents.

Theoretical Study on the Raman Effect Due to Magnons in Two-Dimensional Magnets.

Raman spectroscopy is one of the most useful experimental tools for studying elementary excitations in two-dimensional (2D) materials. The Raman scattering due to phonons was widely employed for detecting structural evolutions, especially those caused by magnetic phase transitions in 2D magnets. A first-principles theory of the Raman scattering effect caused by magnons is still lacking. We theoretically study the magnon Raman effect in 2D magnet CrI3. We propose a first-principles method and have calculated the intensity of circularly polarized Raman signals due to different magnon modes in the CrI3 monolayer and bilayers. The calculated Raman intensities due to magnons in the CrI3 monolayer and the rhombohedral bilayer are consistent with the selection rule deduced from the magnon pseudoangular moment and the parity of magnon modes. We also find that the selection rule is violated in the symmetry-broken monoclinic bilayer due to interlayer coupling.

Adsorption and Sensing of SF6 Decomposition Products by a Pd-Doped MoTe2 Monolayer: A First-Principles Study.

The detection of sulfur hexafluoride (SF6) decomposition components has become one of the best ways to diagnose early latent insulation faults in gas-insulated equipment, which can effectively prevent sudden accidents by identifying such faults. In this paper, we by first-principles theory investigated the adsorption and sensing behaviors of four typical SF6 decomposition components (H2S, SO2, SOF2, and SO2F2) on the pristine Pd-doped MoTe2 monolayer. The adsorption energy, work function, recovery time, charge density difference, density of state, and band structure of the adsorption structures are obtained as well as analyzed. The results indicate that the Pd dopant prefers to be trapped at the TMo site, with a binding energy of -2.25 eV. The Pd-MoTe2 chemisorbs the remaining gases except SO2xF2, with the adsorption capacity ranking as SOF2 > SO2 > H2S. The adsorption of gas molecules reduces the bandgap of Pd-MoTe2, thereby increasing conductivity. On the other hand, the recovery time of the Pd-MoTe2 monolayer material at a temperature of 398 K demonstrates its excellent gas desorption performance toward four decomposition gases. The research results provide a theoretical basis for Pd-MoTe2 to detect SF6 decomposition components, thus, promoting the stable operation of the power system.

DFT computation of rare earth element doped TiO2 anatase: Tunable absorption spectra for water splitting application

The electronic band structure, density of states and optical properties of pure TiO2 and TiO2 doped with different elements (La, Ce, Hf and Pr) have been calculated using the first-principles density general function theory. The calculated results show that the La and Ce doped materials show a negative shift in the conduction band and an insignificant valence band shift. the band energy band gap values of La (Eg = 1.904 eV), Ce (Eg = 1.756 eV), Hf (Eg = 2.005 eV) and Pr (Eg = 1.970 eV) doped TiO2 are smaller than those of undoped TiO2 (Eg = 2.035 eV), among which, Ce doped anatase TiO2 has a narrower band width, smaller band gap value, high redshift and higher photocatalytic activity in the visible region.

Prediction of mechanical, electronic and optical properties of monolayer 1T Si-dichalcogenides via first-principles theory

This study utilizes first-principles calculations to predict the mechanical, electronic, and optical properties of 1 T Si-dichalcogenides (SiX2; X = S, Se, and Te). The results of dynamic and static stability calculations demonstrate the high stability of all three structure. Due to their mechanical flexibility, 1 T- SiX2 structures can undergo strain quite well compared to other 2D structures. Furthermore, we employ a hybrid functional method to investigate the materials’ energy band structures and electronic configuration. Our findings indicate that while 1 T-SiS2 and 1 T-SiSe2 structures are semiconductors and their electronic properties can be controlled through mechanical strain, while 1 T-SiTe2 expresses as a metallic material at all states. The high absorption light coefficient of 1 T-SiX2 indicates polarization in the ultraviolet and sensitivity under strain, revealing the potential for application in nanoelectronics and optical devices. Overall, the study enhances our understanding of the properties of 1 T-SnX2 structures and highlights their potential for future technological applications.

Mn-doped SnTe monolayer as toxic gas scavenger or sensor based on first-principles study

Using first-principles theory, we investigated the stability and feasibility on Mn-doped SnTe monolayer and the interactions of Mn-doped SnTe monolayer (Mn-SnTe) for the adsorption of toxic gases H2S, SO2, NO, NO2 and Cl2. The results show that the Mn-SnTe monolayer has a weak interaction for H2S and strong adsorption energies of −1.840, −4.123, −2.954 and −3.578 eV for SO2, NO, NO2 and Cl2, respectively. In addition, due to the high sensitivity of Mn-SnTe monolayer to adsorb SO2 and Cl2, the bandgaps of the developed systems are reduced by about 79.96% and 100%, respectively. The results indicate that the Mn-SnTe monolayer has a high sensitivity for the detection of SO2 and Cl2 gases. Our calculations provide a theoretical basis for the development of Mn-SnTe monolayer for potential applications as sensors or scavengers for toxic gases.

Thermal conductivity of glasses: first-principles theory and applications

Predicting the thermal conductivity of glasses from first principles has hitherto been a very complex problem. The established Allen-Feldman and Green-Kubo approaches employ approximations with limited validity—the former neglects anharmonicity, the latter misses the quantum Bose-Einstein statistics of vibrations—and require atomistic models that are very challenging for first-principles methods. Here, we present a protocol to determine from first principles the thermal conductivity κ(T) of glasses above the plateau (i.e., above the temperature-independent region appearing almost without exceptions in the κ(T) of all glasses at cryogenic temperatures). The protocol combines the Wigner formulation of thermal transport with convergence-acceleration techniques, and accounts comprehensively for the effects of structural disorder, anharmonicity, and Bose-Einstein statistics. We validate this approach in vitreous silica, showing that models containing less than 200 atoms can already reproduce κ(T) in the macroscopic limit. We discuss the effects of anharmonicity and the mechanisms determining the trend of κ(T) at high temperature, reproducing experiments at temperatures where radiative effects remain negligible.

Enhancing adsorption and sensing performances of pristine and Ni cluster doped HfSe2 monolayers to SF6 decomposition gases: A DFT study

In this work, we used first-principles theory to examine the adsorption performance of three typical sulfur hexafluoride (SF6) decomposed gases (SO2, SOF2, and SO2F2) on a pristine HfSe2 and cluster Nin (n = 1–3) doped HfSe2 monolayers. The potential applications of Nin-HfSe2 as the SF6 decomposition gas sensors are studied using band structures and adsorption energy. The density of states, charge transfer, and recovery time of the SF6 gas products adsorbed on the pristine HfSe2 and Nin-HfSe2 monolayers. The Nin clusters behaved as electron acceptors transferring the electrons from the HfSe2 monolayer. The findings demonstrate that Ni1-HfSe2 has weak adsorption for SO2F2 but excellent adsorption performance for SO2 and SOF2. The Ni3-HfSe2 monolayer is the desired adsorbent for SO2, SOF2, and SO2F2 storage or removal. The results are advantageous to improve the gas sensing capabilities of HfSe2 materials and deposit a foundation for SF6 decomposed gas detection list on HfSe2 sensors.

A first-principles study into Pt-embedded NiS2 monolayer as an outstanding gas sensor upon CO and HCHO dry-type reactors

This work purposes Pt-embedded NiS2 (Pt-NiS2) monolayer as a promising gas sensor upon CO and HCHO in dry-type reactors on basis of first-principles theory. The Pt-NiS2 monolayer is modelled through the replacement of a S atom by a Pt atom, and the formation energy is −0.57 eV, implying the exothermicity for Pt-emending on the NiS2 surface. The CO and HCHO systems are respectively clarified as chemisorption and physisorption, given their adsorption energy of −1.98 and −0.77 eV. The analyses of band structure in line with density of state show the deformation of electronic properties in Pt-NiS2 monolayer in gas adsorptions, and the sensing responses in CO and HCHO systems are obtained as −99.6% and −99.2%, respectively. Moreover, the work function analysis exhibits the stronger sensitivity in the HCHO system. Our theoretical simulations put forward the strong potentials of Pt-NiS2 monolayer to be a gas sensing material for CO or HCHO detection, thus realizing the evaluation of the operation conditions of the dry-type reactors, and we hope further experimental investigations can further verity our hypothesis in the near future.

Shockwavelike Behavior across Social Media.

Online communities featuring "anti-X" hate and extremism, somehow thrive online despite moderator pressure. We present a first-principles theory of their dynamics, which accounts for the fact that the online population comprises diverse individuals and evolves in time. The resulting equation represents a novel generalization of nonlinear fluid physics and explains the observed behavior across scales. Its shockwavelike solutions explain how, why, and when such activity rises from "out-of-nowhere," and show how it can be delayed, reshaped, and even prevented by adjusting the online collective chemistry. This theory and findings should also be applicable to anti-X activity in next-generation ecosystems featuring blockchain platforms and Metaverses.

Probing temperature effects on the stacking fault energy of GH3536 superalloy using first-principles theory

Probing temperature effects on the stacking fault energy of GH3536 superalloy using first-principles theory

The role of electron extinction in the breakdown strength of nanocomposite capacitors

A first-principles theory of electrical breakdown in nanocomposite capacitors, which considers the trapping and scattering (extinction) of electrons originating from the presence of nanoinclusions in the polymer matrix, is developed. The breakdown strength relative to its value for a neat polymer is expressed in terms of two parameters, one of which is determined by the volume density of the nanoinclusion polarizability and the other one is proportional to the electron trap surface density around an inclusion, while the effect of electron scattering is shown to be insignificant. A comparison of the theoretical predictions with diverse experimental data demonstrates an excellent agreement and suggests an effective tool for the design of nanocomposite capacitors.

Modulation of the Discharge and Corrosion Properties of Aqueous Mg-Air Batteries by Alloying from First-Principles Theory

Mg-air batteries have been rapidly developed with the proposal of several corrosion inhibition strategies for magnesium anodes. However, researchers have been puzzled by the limited improvement in discharge voltage caused by alloying and the theoretical mechanism of alloying on corrosion. Here, a detailed thermodynamic study was carried out combined with the stepwise hydroxide-assisted mechanism for metal dissolution in alkaline media and the Mg7B (B is the alloying elements) alloy model using first-principles theory. The effects of alloying on the discharge voltage and corrosion of magnesium anodes were simulated for alloying elements intensively investigated. The results indicate that within the range of electrochemical polarization, the discharging of Mg-air batteries at high current densities has a large overpotential while the impact of alloying on the voltage of Mg-air batteries is less than 0.15 V at high current densities. Consequently, without considering other polarization reductions, alloying cannot profoundly change the discharge voltage of Mg-air batteries; contrarily, an increase in corrosion resistance will result in a decrease in voltage for elements with an electrode potential higher than magnesium.

First-principles calculation of Fe-Mo-Cr alloys based on special quasi-random structure

Mo element has an important effect on ferritic stainless steel. It is of great significance to study the role of the Mo element in the composition design of stainless steel alloys. In this paper, the formation energy, magnetic moment, electronic structure, mechanical and high-temperature mechanical properties of Fe-Mo binary alloys and Fe-Mo-Cr ternary alloys were obtained by density functional theory of first principles. The results showed that the formation energy increases gradually with the increase of Mo content. The alloys have a stable system when the Mo concentration is less than 3.125%. The addition of Cr increases the formation energy, but when Mo content is 3.125%, the formation energy is lower than that of Fe-Mo binary alloys. Fe atoms are ferromagnetic state and Mo atoms are anti-ferromagnetic state, there is an opposite magnetic effect between them. With the increase of Mo concentration, Mo atoms change from anti-ferromagnetic to paramagnetic. Mo atoms become paramagnetic when Mo concentration reaches 100%. Mo element has little effect on the shear modulus of the alloys. The hardness and compressive strength decrease with increasing Mo concentration. The addition of Cr atom also reduces the hardness. Fe-Mo alloys have better thermal stability when Mo concentration is 12.5%. The Cr atom significantly changes the thermodynamic properties of the alloys. The volume thermal expansion coefficient of Fe-Mo-Cr ternary alloys decreases with the increase of Mo concentration, and the highest temperature appears near the Mo concentration of 12.5∼18.75%.

Sc@B28-, Ti@B28, V@B28+, and V@B292-: Spherically Aromatic Endohedral Seashell-like Metallo-Borospherenes.

Transition-metal-doped boron nanoclusters exhibit unique structures and bonding in chemistry. Using the experimentally observed seashell-like borospherenes C2 B28-/0 and Cs B29- as ligands and based on extensive first-principles theory calculations, we predict herein a series of novel transition-metal-centered endohedral seashell-like metallo-borospherenes C2 Sc@B28- (1), C2 Ti@B28 (2), C2 V@B28+ (3), and Cs V@B292- (4) which, as the global minima of the complex systems, turn out to be the boron analogues of dibenzenechromium D6h Cr(C6H6)2 with two B12 ligands on the top and bottom interconnected by four or five corner boron atoms on the waist and one transition-metal "pearl" sandwiched at the center in between. Detailed molecular orbital, adaptive natural density partitioning (AdNDP), and iso-chemical shielding surface (ICSS) analyses indicate that, similar to Cr(C6H6)2, these endohedral seashell-like complexes follow the 18-electron rule in bonding patterns (1S21P61D10), rendering spherical aromaticity and extra stability to the systems.

Perfect Core-Shell Octahedral B@B38 + , Be@B38 , and Zn@B38 with an Octa-Coordinate Center as Superatoms Following the Octet Rule.

The front cover artwork is provided by Prof. Si-Dian Li's group at Shanxi University, China. The image shows the smallest perfect core-shell octahedral borospherene Oh B@B38 + and its endohedral metallo-borospherene analogs Oh Be@B38 and Oh Zn@B38 obtained at first-principles theory. Read the full text of the Research Article at 10.1002/cphc.202200947.

Spin-orbit-coupling-induced band splitting in two-dimensional hybrid organic-inorganic perovskites: Importance of organic cations

We investigate how the spin-orbit-coupling-induced lifting of the spin degeneracy depends indirectly on constituting organic motifs in two-dimensional hybrid organic-inorganic perovskites. We examine experimentally synthesized structures using first-principles electronic structure theory. In particular, the effect of the organic spacer cation packing and of the methyl ammonium orientation within the inorganic layer are elucidated. Both the organic spacer packing and subtle changes in methyl ammonium orientations can cause enough distortions of lead iodide octahedral cages to induce the spin splitting. Interestingly, the momentum-independent persistent spin texture splitting rather than the traditional Rashba/Dresselhaus splitting is observed for a structure in which organic spacer molecules are modified with fluorine substitution. Our calculation on these experimentally reported structures highlights the important role of the organic constituents in these hybrid organic inorganic perovskites for the spin splitting.

Using Surface-Enhanced Raman Spectroscopy to Unravel the Wingtip-Dependent Orientation of N-Heterocyclic Carbenes on Gold Nanoparticles

N-Heterocyclic carbenes (NHCs) are an attractive alternative to thiol ligands when forming self-assembled monolayers on noble-metal surfaces; however, relative to the well-studied thiol monolayers, comparatively little is known about the binding, orientation, and packing of NHC monolayers. Herein, we combine surface-enhanced Raman spectroscopy (SERS) and first-principles theory to investigate how the alkyl "wingtip" groups, i.e., those attached to the nitrogens of N-heterocyclic carbenes, affect the NHC orientation on gold nanoparticles. Consistent with previous literature, smaller wingtip groups lead to stable flat configurations; surprisingly, bulkier wingtips also have stable flat configurations likely due to the presence of an adatom. Comparison of experimental SERS results with the theoretically calculated spectra for flat and vertical configurations shows that we are simultaneously detecting both NHC configurations. In addition to providing information on the adsorbate geometry, this study highlights the extreme SERS enhancement of vibrational modes perpendicular to the surface.