Density Functional Calculations Research Articles

Enhanced Photocatalytic Activity of Surface‐Modified TiO2 with Bimetallic AuPd Nanoalloys for Hydrogen Generation

Herein, commercial titania (TiO2‐P25) is modified with mono‐ and bi‐metallic (Au, Pd, and AuPd) nanoparticles synthesized by chemical reduction method using NaBH4 as a strong reducing agent at room temperature. Bimetallic AuPd nanoalloys homogeneous in size and well dispersed on the TiO2 surface are obtained. The charge‐carrier dynamics, which is a key factor in photocatalysis, is studied by time‐resolved microwave conductivity. The results reveal that surface modification plays a crucial role in charge‐carrier separation, increasing the activity under UV–vis light irradiation. The bimetallic AuPd nanoalloys formation is confirmed by high‐angle annular dark field scanning transmission electron microscopy and corroborated by semiempirical molecular dynamics simulations (Gupta‐LAMMPS). The surface‐modified TiO2 with bimetallic AuPd nanoalloys exhibits higher photocatalytic activity compared to TiO2 modified with their monometallic counterparts. The experimental results are also supported by density functional theory and density functional tight binding calculations, which show that alloying AuPd with low Pd content presents significant synergetic effects for hydrogen generation under UV–vis light from aqueous triethanolamine solutions. Additionally, the AuPd/TiO2 photocatalysts are stable with cycling.

On-Surface Synthesis of Organolanthanide Sandwich Complexes.

The synthesis of lanthanide-based organometallic sandwich compounds is very appealing regarding their potential for single-molecule magnetism. Here, it is exploited by on-surface synthesis to design unprecedented lanthanide-directed organometallic sandwich complexes on Au(111). The reported compounds consist of Dy or Er atoms sandwiched between partially deprotonated hexahydroxybenzene molecules, thus introducing a distinct family of homoleptic organometallic sandwiches based on six-membered ring ligands. Their structural, electronic, and magnetic properties are investigated by scanning tunneling microscopy and spectroscopy, X-ray absorption spectroscopy, X-ray linear and circular magnetic dichroism, and X-ray photoelectron spectroscopy, complemented by density functional theory-based calculations. Both lanthanide complexes self-assemble in close-packed islands featuring a hexagonal lattice. It is unveiled that, despite exhibiting analogous self-assembly, the erbium-based species is magnetically isotropic, whereas the dysprosium-based compound features an in-plane magnetization.

High-κ Dielectric (HfO2)/2D Semiconductor (HfSe2) Gate Stack for Low-Power Steep-Switching Computing Devices.

Herein, a high-quality gate stack (native HfO2 formed on 2D HfSe2) fabricated via plasma oxidation is reported, realizing an atomically sharp interface with a suppressed interface trap density (Dit ≈ 5 × 1010 cm-2 eV-1). The chemically converted HfO2 exhibits dielectric constant, κ ≈ 23, resulting in low gate leakage current (≈10-3 A cm-2) at equivalent oxide thickness ≈0.5 nm. Density functional calculations indicate that the atomistic mechanism for achieving a high-quality interface is the possibility of O atoms replacing the Se atoms of the interfacial HfSe2 layer without a substitution energy barrier, allowing layer-by-layer oxidation to proceed. The field-effect-transistor-fabricated HfO2/HfSe2 gate stack demonstrates an almost ideal subthreshold slope (SS) of ≈61 mV dec-1 (over four orders of IDS) at room temperature (300 K), along with a high Ion/Ioff ratio of ≈108 and a small hysteresis of ≈10 mV. Furthermore, by utilizing a device architecture with separately controlled HfO2/HfSe2 gate stack and channel structures, an impact ionization field-effect transistor is fabricated that exhibits n-type steep-switching characteristics with a SS value of 3.43 mV dec-1 at room temperature, overcoming the Boltzmann limit. These results provide a significant step toward the realization of post-Si semiconducting devices for future energy-efficient data-centric computing electronics.

Heterostructure of Ta5+-substituted Na3V2(PO4)3–TaN promoting the activation of V4+/V5+ redox couple for high performance sodium-ion batteries

Currently, low capacity with poor kinetics seriously hinders the development of Na3V2(PO4)3 (NVP). Herein, a simultaneous optimized scheme of Ta5+ substitution and carbon nanotubes (CNTs) enwrapping is proposed. The substitution of hypervalent Ta5+ at V3+ site introduces a favorable n-type doping effect to generate extra electrons, increasing electron concentration to accelerate electronic conductivity. Ta5+ behaves as a pillar ion to expand Na + migration channel and stabilize the skeleton. Moreover, a new phase of TaN is produced, which embeds into the Na3V2(PO4)3 phase to generate grain boundary. Benefiting from this heterojunction, Na+ diffusion rate could be effectively promoted. The addition of CNTs further restricts agglomeration growth of particles, making the grains smaller and more uniform. Benefiting from the synergetic improved kinetics, a higher voltage platform at 3.9 V is activated. Due to the investigations of ex-situ X-ray photoelectron spectroscopy, the extra plateau is derived from the V4+/V5+ redox couple. Moreover, Density Functional Theoretical calculation suggests the favorable Ta5+ substitution optimizes the electronic structure of NVP to reduce the band gap. Accordingly, the modified NVPT/C@CNTs0.15 submits 125.5 mA h g−1 at 0.1C, and delivers a high value of 77.5 mA h g−1 at 100 C with retention rate of 81.03% after 12000 cycles.

Phase comparison and equation of state for Ta2O5

Tantalum pentoxide (Ta2O5) is among the most technologically useful heavy transition metal oxides. Unfortunately, its crystal structure is the subject of long-standing and unresolved disagreement. Among other consequences, this uncertainty has made it impossible to formulate a robust high pressure equation of state for Ta2O5. Here, we report the results of high pressure x-ray diffraction experiments indexed against a comprehensive list of proposed Ta2O5 phases. Five of the proposed phases produce good matches to experimental observations, and the compressibility parameters for these phases are all consistent within uncertainty. This means that regardless of the particular phase chosen, the Ta2O5 equation of state can be described with bulk modulus K0=138±3.68  GPa and pressure derivative K0′=1.82±0.45 . Combining these experimental results with new density-functional theory calculations allows us to identify the λ phase as the best structural model of Ta2O5 at ambient conditions.

Achieving ultra-strong and ductile CoNi-based FCC multi-principal element alloys via alloying with refractory Mo and W

FCC single-phase multi-principal element alloys (MPEAs) often suffer from insufficient strength, which limits their wide applications. Here a CoNi-based FCC MPEA (Co44Ni44Mo9W3, at. %) was designed by alloying with refractory Mo and W. The Co44Ni44Mo9W3 alloy possesses an ultrahigh yield strength of ∼1050 MPa and good ductility of ∼30 % at ambient temperature. The excellent mechanical properties were correlated to its superior lattice friction stress and grain boundary strengthening compared to most reported FCC MPEAs and conventional alloys. By density-functional theory calculations, such extraordinary mechanical properties were further attributed to its high lattice distortion, low intrinsic stacking fault energy and high unstable stacking fault energy due to the addition of Mo and W. Transmission electron microscopy investigations revealed that the high strength-ductility combination arises from the synergistic effect of planar slip bands, stacking faults and deformation nano-twins.

Influence of caffeine on the crystallinity and thermal stability of MAPbI3: Experiment and density-functional theory calculations

In this work, the effect of caffeine concentration (2 %, 4 %, 6 % and 8 %) on crystallinity and thermal stability of MAPbI3 was successfully studied. MAPbI3 showed stability at lower temperatures ranges between 0 and 90 °C, however, after adding caffeine the thermal stability showed an improvement towards higher temperatures [196–242 °C]. All the samples exhibited intense diffraction peaks at ∼2θ = 19.66° and 39.81°, respectively assigned to the (112) and (141) planes of tetragonal perovskite structure which indicates that the desired perovskite film was formed. MAPbI3@Caffeine (8 %) and MAPbI3@Caffeine (6 %) exhibited high PL intensity quenching which indicates their high charge carrier extraction efficacy which can be attributed to the higher crystallinity and fewer defect states of the film compared to MAPbI3@Caffeine (2 %) and MAPbI3@Caffeine (4 %) samples. MAPbI3@Caffeine (8 %) absorption spectra had the shifts to low wavelength in the range 491–532 nm which indicated that caffeine may allow sufficient photon into MAPbI3 due to the estimated bandgap. The density functional theory calculations suggest increased interactions at high caffeine concentration with corresponding switch from metallic to semiconducting properties. The stability of MAPbI3 can be attributed to increased cationic interaction between I 5p orbital states and C 2p, N 2p orbital states as indicated in the PDOS and electron density difference analysis.

Cryogenic Thermal Shock Effects on Optical Properties of Quantum Emitters in Hexagonal Boron Nitride.

Solid-state quantum emitters are vital building blocks for quantum information science and quantum technology. Among various types of solid-state emitters discovered to date, color centers in hexagonal boron nitride have garnered tremendous traction in recent years, thanks to their environmental robustness, high brightness, and room-temperature operation. Most recently, these quantum emitters have been employed for satellite-based quantum key distribution. One of the most important requirements to qualify these emitters for space-based applications is their optical stability against cryogenic thermal shock. Such an understanding has, however, remained elusive to date. Here, we report on the effects caused by such thermal shock that induces random, irreversible changes in the spectral characteristics of the quantum emitters. By employing a combination of structural characterizations and density functional calculations, we attribute the observed changes to lattice strain caused by cryogenic temperature shock. Our study sheds light on the stability of the quantum emitters under extreme conditions─similar to those countered in outer space.

Deciphering multistep transformation mechanism of cobalt-vanadium bimetallic sulfides anode in sodium storage

Transition metal sulfides as a high specific capacity sodium-ion batteries (SIBs) anode which have been suffering from successive phase transitions bringing about stress changes, poor electronic conductivity and ineffective ion transport kinetics. Combining self-sacrificial templates and ion-exchange transition reaction mechanisms, herein, cobalt, vanadium bimetallic sulfides hollow nano-cubic boxes with nitrogen-doped nanocarbon coating layer (CoV2S4@NC) are designed to solve these symbiosis problems. With a combination of in-situ Raman, ex-situ XPS/TEM, and density-functional theory calculations, the synergistically coordination mechanism between cobalt and vanadium binary metal during multi-step conversion processes is responsible for the faster charge transfer kinetics and boosted Na+ pseudocapacitance storage capability. Furthermore, the hollow cubic structure combined with the carbon shell layer can release the internal stress during phase transitions and avoid the recurrent growth of SEI film by volume change during the electrochemical process, thus realizing a superior electrode/electrolyte interface. Thus, when used as anode materials for SIBs, the prepared anode materials display excellent electrochemical performance, delivering capacity of 492 mA h g−1 at a current density of 10 A g−1 and high capacity stability with 484 mAh g−1 after 1000 cycles at 5 A g−1. More importantly, the assembled full cell (Na3V2(PO4)3//CoV2S4@NC) also shows high capacity of 574 mAh g−1 with 94.7% capacity retention after 85 cycles, suggesting the potential application of bimetallic sulfides in SIBs.

The Constructing of the Oxide Phase Diagram for Fluoride Adsorption on La-Fe-Al: A Collaborative Study of Density Functional Calculation and Experimentation.

In recent years, fluoride pollution in water is a problem that has attracted much attention from researchers. The removal of fluoride-containing wastewater by adsorption with metal oxide as an adsorbent is the most common treatment method. Based on this, the effect of the doping ratio of La2O3, Fe2O3, and Al2O3 on the fluoride-removal performance was discussed by constructing a phase diagram. In this study, the adsorption mechanism of nanocrystalline lanthanum oxide terpolymer was investigated by density functional theory calculation and experiment. The optimal pH condition selected in the experiment was three, and the adsorption kinetics of fluoride ions were more consistent with the quasi-second-order kinetic model. The adsorption thermodynamics was more consistent with the Langmuir model. When the La-Fe-Al ternary composite oxides achieved the optimal adsorption efficiency for fluoride ions, the mass synthesis ratio was Al2O3:(Fe2O3:La2O3 = 1:2) = 1:100, resulting in a fluoride ion removal rate of up to 99.78%. Density functional calculations revealed that the La-Fe-Al ternary composite oxides had three important adsorption sites for La, Fe, and Al. Among them, the adsorption capacity for HF was Fe2O3 > La2O3 > Al2O3, and for F- was La2O3 > Al2O3 > Fe2O3. This provided good guidance for designing adsorbents to remove fluoride.

Porous carbon-coated silicon composites for high performance lithium-ion batterie anode

With the rapid development of silicon-based lithium-ion battery anode, the commercialization process highlights the importance of low-cost and short-flowproduction processes. The porous carbon/silicon composites (C/Si) are prepared by one-step calcination using zinc citrate and nano-silicon as the primary raw materials at a temperature of 950 °C. In this composite, silicon nanoparticles are coated with a porous carbon shell with an average pore diameter of 4.49nm, which improves the structural stability and electrochemical performance of the anode. Density-functional theory calculations show that the porous carbon enhances the electrical conductivity, facilitates the interlayer migration of Li-ion, and strengthens the gain effect of stress on conductivity. Additionally, stress further reduces the diffusion barriers of Li-ion. The porous C/Si anode demonstrates outstanding specific capacity and rate capability, maintaining a stable specific capacity of 1170 mAh/g at a current density of 0.2 A g−1 after 100 cycles and retaining a reversible specific capacity of 944 mAh/g even at a charging current density of 2 A g−1. The carbon/silicon anode materials prepared using this strategy present a streamlined manufacturing process and exceptional performance, as well as avoiding the post-washing step, providing a pathway toward the scalable application of silicon-based anodes.

Origin of discrete donor–acceptor pair transitions in 2D Ruddlesden–Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light–matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (≃120μeV) and long decay times (5–8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden–Popper perovskites, (PEA)2(MA)n−1PbnI3n+1, particularly in phases with n≥2, has been identified as a mechanism of donor–acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of ≃10 nm. The analysis of the spectral and electronic characteristics of these single donor–acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions.

A Nickel-doped, lanthanum gallium germanate-based phosphor as an ultra-broadband short-wavelength infrared region emitter for optical imaging

Short-wavelength infrared region light source (SWIR, λ = 1000–2000 nm) is useful for optical imaging of biological tissues for lack of auto-fluorescence, low intrinsic absorption, and low scattering. Here, we report the synthesis, characterization, and mechanistic studies of a Ni2+-doped, lanthanum gallium germanate-based phosphor, La3Ga5GeO14:Ni2+, that emits SWIR with an ultra-broad bandwidth emission of 1000–2000 nm, a full-width at half maximum (FWHM) of 340 nm, which covers near infrared-II and III regions. The analysis of crystal structure suggests that Ga3+ ions in [GaO6] octahedron lattices are replaced by Ni2+ ions, possibly due to the similar radius shared by both ions. The observed ultra-broadband SWIR luminescence with large Stokes shift is most likely originated from the influence of a strong crystal field on the extent of the cleavage of energy levels of Ni2+ ions, which is supported by the charge density data of [GaO6] octahedra in La3Ga5GeO14 derived from state density functional calculation. A SWIR pc-LED was constructed by packaging the La3Ga5GeO14:Ni2+ phosphor with a near-ultraviolet LED chip, and its potential application in optical imaging of human bodies was explored.

Designing high-efficiency organic semi-conductors for organic photodetectors assisted by machine learning and property prediction

Quantum chemical calculations have been widely utilized to design and explore organic semi-conductors for various photovoltaics applications. Photovoltaic devices such as photodetectors work on the principle of photo-electric-effect and widely used for optical communications. In recent years, machine learning has become famous in many research fields. In present study, we have predicted reorganization energy of organic semi-conductors with the aid of machine learning. Statistical analysis is used to select best molecular descriptors (features). About 10 machine learning models are tested for selecting the best one. Among various tested models, gradient forest regressor and random forest regressor are the top models. By using our new designing approach, easy to synthesize building blocks are connected to design new organic semiconductors. Their reorganization energies are predicted using fast machine learning method. For this purpose, already trained and hyper parameter optimized gradient boosting regressor is used. It is generally accepted that to obtain higher charge mobility molecules should have lower reorganization energy. Ten organic semi-conductors with lower reorganization energy are selected for density functional calculations and their energy levels are calculated. The chemical similarity analysis using clustering and heatmap is also done for selected organic semi-conductors. Synthetic accessibility score is calculated and majority of semi-conductors are easy to synthesize. These molecules are potential candidates for device fabrication. The proposed python-based pipeline has ability to select the molecules for organic photodetectors in fast and cheaper way.

Boosting hydroxyl radical generation for highly efficient electrooxidation of sulfamethazine via facile doping strategy

The one-electron water oxidation reaction (WOR) to generates hydroxyl radicals (•OH) plays a pivotal role in electrochemical advanced oxidation processes (EAOPs). In this study, we present a novel and facile strategy involving Ho-doping to regulate the electronic configuration of the conventionally nonactive PbO2 electrode. Experimental findings demonstrate that PbO2 doped with 2.4% Ho achieves the highest degradation performance of sulfamethazine (SMZ) (∼100%; kobs = 0.093 min−1) and highest •OH generation efficiency with a relatively concentration of •OH = 1.9 × 107. The doped electrodes exhibit a notable enhancement compare to the undoped electrode, which elevating SMZ degradation efficiency and •OH generation by factors of 1.45 and 3.8, respectively. Electrochemical characterization and degradation results elucidate that the modulation of electron transfer rate and oxygen evolution potential are crucial factors governing the performance of the Ho-doped PbO2 anode. Specifically, maintaining a suitably high oxygen evolution potential (OEP) enables the thermodynamically feasible production of •OH, while a relatively fast electron transfer rate ensures the generation of H2O+ (the oxidation precursor of •OH) with kinetic feasibility. Furthermore, a degradation pathway for SMZ is proposed based on the identification of transformation mechanism using density functional calculation (DFT) and the detection of intermediates via LC-MS/MS. This study provides an universal strategy for simultaneously enhancing the OEP and optimizing the electron transfer rate of an EAOP anode through the modulation of anodic electronic configuration, thereby advancing the field of efficient water treatment applications.

Raman Spectroscopy of Na3Co2SbO6

Raman spectroscopy together with density functional calculations were used to study lattice dynamics in a layered honeycomb cobaltite Na3Co2SbO6, which can host a field-induced phase related with the Kitaev physics. We show that there develops an additional mode well above Neel temperature (at ≈200 K) at 525 cm–1, which origin can be related to electronic excitation to one of  j3/2 doublets. Moreover, our theoretical calculations demonstrate that the highest frequency intensive mode related to the oxygen vibrations is very sensitive to type of the magnetic order. Thus, we propose to use the softening of this mode as a hallmark of the transition to a fully polarized regime, which is stabilized in Kitaev materials in strong magnetic fields.

Synergistic regulation of chloride anion recognition using a triple-functional sites receptor with two different cationic effectors.

The synergistic regulation of the multi-functional sites on one receptor molecule with different cationic effectors for anion recognition is scarce to be well understood from the experiment and theory. In this work, a new anion receptor with three functional zones including ether hole, biurea and double bipyridine groups (EUPR) is designed expecting to enhance the chloride anion recognition together with a rational synthesis path being proposed based on four simple and mature organic reaction steps. The conformational structures of the designed receptor EUPR and the binding behaviors for three kinds of ions (Cl-, Na+, and Ag+) are deeply investigated by using density functional theoretical calculations. It is found that Cl- binding via the hydrogen bond interaction can be significantly enhanced and synergistically regulated by the two kinds of cations and the corresponding conformational changes of receptor EUPR. Especially, the conformational pre-organization of receptor caused by the encapsulation of sodium ion into ether hole is benefit to the binding for Cl- in both thermodynamics and kinetics. Na+ binding, in turn, can ever be enhanced by chloride anion, whereas it seems that Ag+ binding cannot always be enhanced by chloride anion, reflecting an electrical complementary matching and mutual enhancement effect for different counter ions. Moreover, solvent effect calculations indicate that EUPR may be an ideal candidate structure for Cl- recognition by strategy of counter ion enhancement in water. Additionally, a visual study of intermolecular noncovalent interaction (NCI) and molecular electrostatic potential (ESP) are used for the analysis on the nature of interactions between receptor and bound ions.

Photoluminescence of Three- and Four-Coordinate Platinum(0) Phosphine Complexes

Abstract Six platinum complexes bearing monodentate phosphine ligands were prepared and their structures and photophysical properties examined. The complexes were photoluminescent in the solid state, with a maximum quantum yield of 72%. The tris(phosphine) platinum complexes displayed more red-shifted emissions than the tetrakis complexes. The nature of the emissive excited states is discussed based on the results of density functional theory and time-dependent density-functional theory calculations.

Boron and carbon in two-dimensional nitrogene structures: Heterodoping, doping, and chemical functionalization

We perform density functional calculations to determine the effects of simultaneous doping with carbon (C) and boron (B) on the electronic properties of nitrogene. Our procedure substitutionally incorporates the two dopants into the nitrogen monolayer. We analyze the relative position between dopants by considering different configurations. Band structure, density of states, band gaps, magnetic stability energies (energy difference between states with total magnetization with spin S=1 and S=0), and energetic stability were examined. The codoped structures exhibit a spin−dependent band gap with a strength of 1.04–2.68 eV, which is reduced compared to that in nitrogene (3.80 eV). The B-C codoped nitrogene incorporated ferromagnetism with well-localized flat bands around the Fermi level. A spin−polarized ground state was exhibited by the systems with a total magnetization of S=1, in which magnetic moments were localized mainly at the C atom and its first neighbors. The systems displayed an antiferromagnetic ordering when the total spin was fixed to S=0. The valence band maximum (VBM) was dominated by spin−up, while the conduction band minimum (CBM) was dominated by spin−down. Dopants placed as second neighbors in the nitrogene structures exhibited the most prominent band gap and magnetic stability.

Resonant electron capture by polycyclic aromatic hydrocarbon molecules: Effects of aza-substitution.

Resonant electron capture by aza and diaza derivatives of phenanthrene (7,8-benzoquinoline and 1,10-phenanthroline) and anthracene (acridine and phenazine) at incident free electron energies (Ee) in the range of 0-15eV was studied. All compounds except 7,8-benzoquinoline form long-lived molecular ions (M-) at thermal electron energies (Ee ∼ 0eV). Acridine and phenazine also form such ions at epithermal electron energies up to Ee = 1.5-2.5eV. The lifetimes (τa) of M- with respect to electron autodetachment are proportional to the extent of aza-substitution and increase on going from molecules with bent geometry of the fused rings (azaphenanthrenes) to linear isomers (azaanthracenes). These regularities are due to an increase in the adiabatic electron affinities (EAa) of the molecules. The EAa values of the molecules under study were comprehensively assessed based on a comparative analysis of the measured τa values using the Rice-Ramsperger-Kassel-Marcus theory, the electronic structure analysis using the molecular orbital approach, as well as the density functional calculations of the total energy differences between the molecules and anions. The only fragmentation channel of M- ions from the compounds studied is abstraction of hydrogen atoms. When studying [M-H]- ions, electron autodetachment processes were observed, the τa values were measured, and the appearance energies were determined. A comparative analysis of the gas-phase acidity of the molecules and the EAa values of the [M-H]· radicals revealed their proportionality to the EAa values of the parent molecules.