Electronic Structure Research Articles

Ferromagnetic stability optimization via oxygen-vacancy control in single-atom Co/TiO 2 nanostructures

Oxygen vacancies and their correlation with the nanomagnetism and electronic structure are crucial for applications in dilute magnetic semiconductors design applications. Here, we report on cobalt single atom-incorporated titanium dioxide (TiO 2 ) monodispersed nanoparticles synthesized using a thermodynamic redistribution strategy. Using advanced synchrotron-based X-ray techniques and simulations, we find trivalent titanium is absent, indicating trivalent cations do not influence ferromagnetic (FM) stability. Density functional theory calculations show that the FM stability between Co 2+ ions is very weak. However, electron doping from additional oxygen vacancies can significantly enhance this FM stability, which explains the observed room-temperature ferromagnetism. Moreover, our calculations illustrate enhanced FM interactions between Co Ti + V O complexes with additional oxygen vacancies. This study explores the electronic structure and room-temperature ferromagnetism using monodispersed nanocrystallites with single-atom-incorporated TiO 2 nanostructures. The strategies described herein offer promise in revealing magnetism in other single-atom-incorporated nanostructures.

Band structure database of layered intercalation compounds with various intercalant atoms and layered hosts

AbstractHere we provide a database comprising electronic band structures of 9,004 layered intercalation compounds, where atoms are intercalated into a host layered compound with different intercalant atoms, along with 468 structures related to the layered host compounds. Additionally, we provide properties derived from the electronic states such as band gap as well as stability-related properties like formation energies. Direct comparison of the band structures before and after intercalation is generally challenging due to changes in their space group and k-path. However, in this study, we developed new k-paths consistent with the host materials, allowing for the direct comparison of band structures before and after intercalation. This enables direct and quantitative discussion of the band structure changes induced by the intercalations and provides a valuable database for intercalant-driven band engineering. Layered intercalation compounds are widely used in many fields, including superconductivity and energy applications, and understanding of electronic structures is necessary. The feature of our database holds promises for the development of layered compounds with enhanced functionalities through database utilization.

A brief review on Ce and Zr-based phase-separated metallic glasses

AbstractPhase-separated metallic glasses (MGs) have attracted a lot of interest recently because they offer a unique opportunity to design composites or alloys with hierarchical microstructure at various length scales. Phase-separated MGs differ from other MGs in terms of their structure and physical properties. Though a lot of theoretical work has been done, there is still a lack of understanding regarding the mechanism underlying phase separation in MGs. In general, phase separation in many MG systems is explained on the basis of nucleation and growth or spinodal decomposition mechanisms. On the other hand, the phase separation in Ce-based MGs is examined based on changes in the electronic structure of Ce atoms. This opens up a new direction of research for delineating issues pertaining to phase separation in amorphous systems. The present brief review aims to provide a comprehensive overview of the phase separation phenomenon in Ce- and Zr-based MG systems. It is broadly divided into two sections: the first section gives a brief introduction into the phase separation in MG systems, mechanisms of phase separation, micro-structural and thermal characteristics, and advantages of phase separation. The second section discusses some of the recent work on Ce- and Zr-based phase-separated MGs with respect to their design and properties. Graphical Abstract

Development of an electrochemically approximated simulation model and a hardware substitution cell approach for thermal management battery system tests

AbstractBattery tests require a high degree of safety-related preparation and constant monitoring of the operating parameters to guarantee the smooth running of tests without incidents or exceptional events such as a thermal runaway. As the handling before, during, and especially after the tests is relatively complex and sometimes just as costly, it is important to reduce these risks and costs as well as to find an alternative to conventional battery tests. Such an approach is being developed by the T-cell project of the Institute of Thermodynamics and Sustainable Propulsion Systems at Graz University of Technology, founded by the Austrian Research Promotion Agency (FFG). A thermal substitution cell, which resembles a battery cell on the outside, is to reflect the surface temperature distribution of a chemical cell without there being any cell chemistry inside the substitution cell. Rather, the interior should not be part of the observation and the thermal requirements should be provided by an internal heating option. Such a measurement approach requires, among other things, a control and regulation unit, without which it would not be possible to transfer the thermal behavior of a battery cell to the substitution cell. Together with the electronic structure of the substitution cell and a circuit environment including a battery simulation model established at a later date, this control/regulation module forms an overall package that considerably facilitates investigations at cell, module, and pack level by substitution of a certain amount of cells using system symmetry advantages. A suitable simulation model was constructed and parameterized for this purpose from an electrochemical and a thermal network. As a first step, the data, which were partly determined empirically but also derived from real cell measurements in the literature, were adapted to the requirements of the simulation environment, so that the real cell used could be simulated thermally in principle. The simulation observations represent the state of the art of the model and are continuously improved by measurements carried out at the institute, thus building up the overall system in more detail.

Atomic Infusion Induced Reconstruction Enhances Multifunctional Thermally Conductive Films for Robust Low‐Frequency Electromagnetic Absorption

AbstractDeterministic fabrication of highly thermally conductive composite film with satisfying low‐frequency electromagnetic (EM) absorption performance exhibits great potential in advancing the application of 5G smart electric devices but persists challenge. Herein, a multifunctional flexible film combined with hetero‐structured Fe6W6C‐FeWO4@C (FWC−O@C) as the absorber and aramid nanofibers (ANFs) as the matrix was prepared. Driven by an atomic gradient infusion reduction strategy, the carbon atoms of absorbers can be precisely relocated from carbon shell to core oxometallate lattice, triggering in situ carbothermic reduction for customization of unique oxometallate‐carbide heterojunctions and surface geometrical structure. Such an atoms reconstruction process effectively regulates interface electronic structure and magnetic configuration, resulting in enhanced polarization loss from abundant heterointerfaces and crystal defects and magnetic loss from hierarchical structure endowed magnetic coupling interaction, which jointly contributes to the efficient low‐frequency EM absorption performance. Eventually, optimized FWC−O@C microplate exhibits a broad absorption bandwidth surpassed the entire C band, and the assembled FWC−O@C/ANFs composite film also performs a high thermal conductivity over 2500 % higher than that of the pure ANFs. These findings provide a new insight into the atomic reconstruction affected EM properties and a generalized methodological guidance for preparing multifunctional thermally conductive composite films.

Complex charge density waves in simple electronic systems of two-dimensional III2–VI3 materials

AbstractCharge density wave (CDW) is the phenomenon of a material that undergoes a spontaneous lattice distortion and modulation of the electron density. Typically, the formation of CDW is attributed to Fermi surface nesting or electron-phonon coupling, where the CDW vector (QCDW) corresponds to localized extreme points of electronic susceptibility or imaginary phonon frequencies. Here, we propose a new family of multiple CDW orders, including chiral Star-of-David configuration in nine 2D III2–VI3 van der Waals materials, backed by first-principles calculations. The distinct feature of this system is the presence of large and flat imaginary frequencies in the optical phonon branch across the Brillouin zone, which facilitates the formation of the diverse CDW phases. The electronic structures of 2D III2–VI3 materials are relatively simple, with only III-s,p and VI-p orbitals contributing to the formation of the CDW order. Despite that, the CDW transitions involve both metal-to-insulator and insulator-to-insulator transitions, accompanied by a significant increase in the bandgap caused by an enhanced electronic localization. Our study not only reveals a new dimension in the family of 2D CDWs, but is also expected to offer deeper insights into the origins of the CDWs.

Mn‐Doped CoFeP Nanosheets as Effective Electrocatalysts for Superior Overall Water Splitting

For green hydrogen, the exploitation of advanced electrocatalysts is crucial, and transition metal phosphides have great potential in water splitting due to their abundant reserves and optimized electronic structure. Herein, a synergistic approach involving Mn doping and phosphorization is applied to CoFe‐layered double hydroxide nanoflowers to produce a series of bifunctional catalysts Mn‐doped CoFeP (Mn‐CoFeP). Electrochemical evaluations demonstrate that the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) performance of Mn(10%)‐CoFeP both have notable improvement in 1 M KOH, with the overpotentials of only 160 and 239 mV to achieve current densities of 10 and 100 mA cm−2, respectively. Comparative electrocatalytic analysis indicates that moderate Mn doping mainly contributes to improve the OER performance, while the phosphorization significantly enhances the HER activity, resulting in effective bifunctional catalysis. For overall water decomposition, a total hydrolysis electrolysis cell equipped with the Mn(10%)‐CoFeP bifunctional catalyst requires only 1.64 V to reach a current density of 10 mA cm−2. Furthermore, it performs stable operation for 10 h at a current density of 10 mA cm−2 with a current maintenance rate of 83.6%. This work offers new insights into preparing effective bifunctional electrocatalysts, advancing clean energy development.

Synthesis of Iron(IV) Alkynylide Complexes and Their Reactivity to Form 1,3‐diynes

The isolation of thermally unstable and highly reactive organoiron(IV) complexes is a challenge for synthetic chemists. In particular, the number of examples where the C‐based ligand is not part of the chelating ligand remains scarce. These compounds are of interest because they could pave the way to designing catalytic cycles of bond forming reactions proceeding via organoiron(IV) intermediates. Herein, we report the synthesis and characterization, including single‐crystal X‐ray diffraction, of a family of alkynylferrates(III) and Fe(IV) alkynylide complexes. The alkynylferrates(III) are formed by transmetalation of the Fe(III) precursor [(N3N’)FeIII] (N3N’ is tris(N‐tert‐butyldimethylsilyl‐2‐amidoethyl)amine)) with lithium alkynylides, and their further one‐electron oxidation enables the synthesis of the corresponding Fe(IV) alkynylides. The electronic structure of this family of organometallic Fe(III) and Fe(IV) complexes has been thoroughly investigated by spectroscopic methods (EPR, NMR, 57Fe Mössbauer, X‐Ray absorption (XAS) and emission (XES) spectroscopies) and theoretical calculations. While alkynylferrates(III) are sluggish to engage into C–C bond forming processes, the Fe(IV) alkynylides react to afford 1,3‐diynes at room temperature. A bimolecular reductive elimination from a bimetallic iron(IV) intermediate to form the 1,3‐diynes is proposed based on the mechanistic investigations performed.

Synthesis of Iron(IV) Alkynylide Complexes and Their Reactivity to Form 1,3‐diynes

The isolation of thermally unstable and highly reactive organoiron(IV) complexes is a challenge for synthetic chemists. In particular, the number of examples where the C‐based ligand is not part of the chelating ligand remains scarce. These compounds are of interest because they could pave the way to designing catalytic cycles of bond forming reactions proceeding via organoiron(IV) intermediates. Herein, we report the synthesis and characterization, including single‐crystal X‐ray diffraction, of a family of alkynylferrates(III) and Fe(IV) alkynylide complexes. The alkynylferrates(III) are formed by transmetalation of the Fe(III) precursor [(N3N’)FeIII] (N3N’ is tris(N‐tert‐butyldimethylsilyl‐2‐amidoethyl)amine)) with lithium alkynylides, and their further one‐electron oxidation enables the synthesis of the corresponding Fe(IV) alkynylides. The electronic structure of this family of organometallic Fe(III) and Fe(IV) complexes has been thoroughly investigated by spectroscopic methods (EPR, NMR, 57Fe Mössbauer, X‐Ray absorption (XAS) and emission (XES) spectroscopies) and theoretical calculations. While alkynylferrates(III) are sluggish to engage into C–C bond forming processes, the Fe(IV) alkynylides react to afford 1,3‐diynes at room temperature. A bimolecular reductive elimination from a bimetallic iron(IV) intermediate to form the 1,3‐diynes is proposed based on the mechanistic investigations performed.

On the Nature of the Out-Of-Plane Distortions in Subporphyrins.

The field of subporphyrins has garnered great interest in recent years owing to its unique structure and associated properties. They exhibit spectroscopic features similar to porphyrins and find applications in various optoelectronic devices, photodynamic therapy etc. Most of the synthesized subporphyrins have boron coordination with an axial ligand and exhibits a bowl-shaped geometry. The first isolation of a stable free-base subporphyrin is achieved recently with mesityl groups at two of the meso positions and anthracene at the other. X-ray studies reveal a markedly non-planar structure different from the bowl shape and is attributed to the steric hindrance of the inner N-H bonds. Herein, we report a systematic quantum chemical investigation assisted by symmetry principles on molecular models to characterize the out-of-plane (OOP) distortions observed so far in subporphyrins and unveil the electronic reasons. Correlation of the frontier molecular orbital (FMO) landscape between the D4h porphyrin and D3h subporphyrin gives insight into their electronic structure relative to one another and the nature of OOP distortions. Further the effect of a π-donor cum σ-acceptor substituent at the meso/beta positions of the subporphyrin ring as well as the impact of boron incorporation in the central cavity on the OOP distortions are also discussed.

Effect of Doped Carbon Atoms on Electronic Structure and Optical Properties of Monolayer 1T-ZrS2

In this paper, a method based on density functional theory is used to replace varying numbers of sulfur atoms with carbon atoms in the original monolayer ZrS2 system, resulting in a new doped system. The theory is based on the First Principle. The photovoltaic properties of the new carbon-atom doping systems have been calculated and investigated. The pristine system and the carbon-atom doped systems were structurally optimized using the automatic optimization method. It was found that the stability of the structure decreases as the number of carbon atoms increases. Pristine monolayer 1T-ZrS2 is an indirect bandgap material. The results show that after doping carbon atoms in monolayer 1T-ZrS2, the p-type conductivity of the system increases and exhibits metallicity. The density of states analysis shows that the conduction band consists mainly of S-3p, Zr-4d, Zr-4p, Zr-5s and C-2p orbitals, while the valence band consists mainly of S-3p, S-3s, Zr-4d, C-2p and C-2s orbitals. It is concluded that strong hybridization between Zr-d and S-p orbitals is exhibited by both the pristine and doped systems. The analysis of the optical properties shows that the peak absorption coefficient and reflectivity peaks are blue-shifted in the doped system, and these peaks are lower than in the pristine system. The peaks of the real and imaginary parts of the dielectric function are also blue shifted. With the increase of doping concentration, the system’s energy loss decreases, indicating that proper doping can effectively reduce the system’s energy loss. The above studies provide theoretical support for applying ZrS2 in nano-optoelectronics.

Substrate-induced strain upshot on the optical and optoelectronic properties of trilayer MoS2.

The characteristics of 2D layered MoS2 film are highly dependent on the substrate it is grown on which leaves us privileged to achieve unique and tunable properties. In this study, trilayer MoS2 films have been grown on fused quartz, crystalline quartz (z-cut), sapphire (0001), and silicon (100) substrates. MoS2 film grows as freestanding on amorphous fused quartz, while it experiences an in-plane tensile strain on the sapphire and silicon. Unprecedentedly we show that due to a large mismatch in the lattice parameter as well as in the thermal expansion coefficient, MoS2 grows with a significant compressive strain both along both in-plane on the crystalline quartz. The developed strain causes an alteration in its electronic structure, causing a 30 meV blue shift in the photoluminescence peak and an increased band gap in addition to fewer sulphur vacancies. Comparatively, the film on sapphire having tensile strain along the in-plane exhibits more sulphur vacancies increasing the electron density. The photoresponse time, photosensitivity, and charge separation distinctly vary for the MoS2 films depending on the substrates. This study underscores the influence of substrate on MoS2 film opening further research scopes on tunable properties owing to 2D layer-substrate interactions.

Engineering ZnIn2S4 Nanosheets with Zinc Vacancies: Unleashing Enhanced Photocatalytic Degradation of Tetracycline.

Defect engineering is a highly effective strategy for accelerating charge transfer and enhancing the performance of photocatalysts. In this study, ZnIn2S4 nanosheets were designed and prepared with controlled Zn vacancies to optimize the electronic band structure and localized charge density of ZnIn2S4. EPR results confirmed the formation of Zn vacancies. This modification enabled efficient capture of photoexcited charges in defect centers, thereby prolonging the carrier's lifetime. Theoretical calculations demonstrated that these vacancies induced the formation of new defect states and highly efficient surface reaction sites. As anticipated, under visible light irradiation, the photocatalytic tetracycline removal rate of the ZnIn2S4 nanosheets with Zn vacancies reached 82.8% within 60 min, significantly higher than that observed for the pristine ZnIn2S4 sample. These findings offer valuable insights into the deliberate construction of metal-vacancy-containing photocatalytic nanomaterials for the enhanced degradation of micropollutants.

Asymmetric Charge Distribution in Atomically Precise Metal Nanoclusters for Boosted CO2 Reduction Catalysis.

Recently, atomically precise metal nanoclusters (NCs) have been widely applied in CO2 reduction reaction (CO2RR), achieving exciting activity and selectivity and revealing structure-performance correlation. However, at present, the efficiency of CO2RR is still unsatisfactory and cannot meet the requirements of practical applications. One of the main reasons is the difficulty in CO2 activation due to the chemical inertness of CO2. Constructing symmetry-breaking active sites is regarded as an effective strategy to promote CO2 activation by modulating electronic and geometric structure of CO2 molecule. In addition, in the subsequent CO2RR process, asymmetric charge distributed sites can break the charge balance in adjacent adsorbed C1 intermediates and suppress electrostatic repulsion between dipoles, benefiting for C-C coupling to generate C2+ products. Although compared to single atoms, metal nanoparticles, and inorganic materials the research on the construction of asymmetric catalytic sites in metal NCs is in a newly-developing stage, the precision, adjustability and diversity of metal NCs structure provide many possibilities to build asymmetric sites. This review summarizes several strategies of construction asymmetric charge distribution in metal NCs for boosting CO2RR, concludes the mechanism investigation paradigm of NCs-based catalysts, and proposes the challenges and opportunities of NCs catalysis.

All-optical manipulation of bandgap dynamics via coherent phonons

The ability to actively and dynamically control electronic states at ultrafast timescales opens up a wide range of potential applications across optoelectronics, quantum computing and sensing, energy conversion and storage, etc. Yet, achieving dynamic electronic manipulation via coherent phonons has posed a considerable challenge. Here, employing time-resolved high-harmonic generation (tr-HHG) spectroscopy, we demonstrate the manipulation of bandgap dynamics in a BaF2 crystal by coherent phonons. The tr-HHG spectrum perturbed by a triply degenerate phonon mode T2g exhibits simultaneously a remarkable two-dimensional (2D) sensitivity, i.e., in both intensity and energy domains. The dynamic compression and enhancement of the harmonics in the intensity domain showed a π/2 phase shift compared to the manifestation of shifts of the harmonics in the energy domain, an astounding example of a physical phenomenon being observed simultaneously in two different perspectives. We employed a quantum model incorporating the electron–phonon coupling to complement our experimental observations, successfully reproducing the results. In addition, we demonstrated complete control over the strength and initial phase of the coherent phonon oscillations by varying the incident electric field polarizations across different crystal orientations. Our findings lay a foundation for engineering the electronic structure through coherent phonons within the terahertz frequency and picosecond to nanosecond time regimes.

Crystalline/Amorphous Interface Engineering and d-sp Orbital Hybridization Synergistically Boosting the Electrocatalytic Performance of PdCu Bimetallene toward Formic Acid-Assisted Overall Water Splitting.

Advanced electrocatalysts capable of bifunctional catalysis for formic acid oxidation (FAOR) and hydrogen evolution reaction (HER) have garnered significant attention due to their exceptional energy efficiency. In this research, we have meticulously designed a PdCu bimetallene characterized by numerous crystalline/amorphous (c/a) interfaces and robust d-sp orbital hybridization, achieved by integrating the p-block metalloid boron within the PdCu matrix (B-PdCu-c/a). The B-PdCu-c/a bimetallene revealed a multitude of surface atoms and unsaturated defect sites, offering abundant catalytic active sites and an optimized electronic structure. The B2-PdCu-c/a exhibited the best performance in FAOR and HER, achieving a mass activity of 1106 mA mgcat-1 and an overpotential of 52 mV, respectively. Significantly, the two-electrode configuration of B2-PdCu-c/a∥B2-PdCu-c/a attained a low cell voltage of 0.19 V at 10 mA cm-2 during formic acid-assisted overall water splitting. Density functional theory (DFT) calculations indicated that c/a interface engineering and d-sp orbital hybridization synergistically optimized the electronic configuration of pristine PdCu bimetallene. This led to an elevation of the d-band center and an accumulation of charge at the c/a interface, which enhanced the adsorption of intermediates, facilitated C-H bond cleavage, and balanced the adsorption-desorption of hydrogen, thereby improving electrocatalytic activities for FAOR and HER, respectively. This study not only presents a viable strategy for effectively tuning the electronic configuration of bimetallene but also offers valuable insights into the development of electrocatalysts.

Local Density Approximation for Excited States

The ground state of an homogeneous electron gas is a paradigmatic state that has been used to model and predict the electronic structure of matter at equilibrium for nearly a century. For half a century, it has been successfully used to predict ground states of quantum systems via the local density approximation (LDA) of density functional theory (DFT), and systematic improvements in the form of generalized gradient approximations and evolution thereon. Here, we introduce the LDA for excited states by considering a particular class of nonthermal ensemble states of the homogeneous electron gas. These states find sound foundation and application in ensemble DFT—a generalization of DFT that can deal with ground and excited states on equal footing. The ensemble LDA is shown to successfully predict difficult low-lying excitations in atoms and molecules for which approximations based on local spin density approximation and time-dependent LDA fail. Published by the American Physical Society 2024

Gas-phase preparation of silylacetylene (SiH3CCH) through a counterintuitive ethynyl radical (C2H) insertion.

Elementary reaction mechanisms constitute a fundamental infrastructure for chemical processes as a whole. However, while these mechanisms are well understood for second-period elements, involving those of the third period and beyond can introduce unorthodox reactivity. Combining crossed molecular beam experiments with electronic structure calculations and molecular dynamics simulations, we provide compelling evidence on an exotic insertion of an unsaturated sigma doublet radical into a silicon-hydrogen bond as observed in the barrierless gas-phase reaction of the D1-ethynyl radical (C2D) with silane (SiH4). This pathway, which leads to the D1-silylacetylene (SiH3CCD) product via atomic hydrogen loss, challenges the prerequisite and fundamental concept that two reactive electrons and an empty orbital are required for the open shell, unsaturated radical reactant to insert into a single bond.

Intramolecular Polarization Contributions to the pKa's of Carboxylic Acids Through the Chain Length Dependence of Vibrational Tag-Shifts in Cryogenically Cooled Pyridinium-(CH2)n-COOH (n = 1-7) Cations.

The pKa's of acids are known to depend on the ionic strength of an electrolyte solution, an effect that qualitatively results from electrostatic interactions of the acid and conjugate base with proximal ions. Here, we explore an intramolecular variation on this theme in which a carboxylic acid group is tethered to a pyridinium cationic charge center with increasingly long alkyl linkages in the series Py+-(CH2)n-COOH, with n = 1-7. The effective acidities of the carboxylic acid group in the isolated cations are determined by recording the red-shifts in the frequencies of the acid OH stretches upon attachment to D2 and N2 molecules, measured by using cryogenic ion spectroscopy. The short chains indeed lead to substantial increases in acidity, with the effect falling off in the range of n = 4-5. The response of the CO stretch on the acid head group is surprisingly strong and, in contrast to the expected red shift of the OH, displays a blue shift as the chain length is reduced. Electronic structure calculations recover these trends and indicate that the proximal cationic charge center acts to draw electron density away from the acid C═O bond. This acts to blue-shift the CO stretch while weakening the C-C bond that attaches the head group to the chain.

Nanomaterials for Energy Conversion

Nanomaterials have emerged as a cornerstone in advancing energy conversion technologies, offering unparalleled potential for improving efficiency and sustainability. Their unique properties—such as high surface area, tunable electronic structure, and quantum effects—enable superior performance in applications like solar cells, fuel cells, thermoelectric devices, and batteries. By manipulating nanoscale architectures, researchers can enhance energy harvesting, storage, and conversion processes. This study explores the role of nanomaterials in energy conversion, focusing on their design, synthesis, and application in cutting-edge technologies. The integration of nanomaterials with advanced energy systems has shown significant potential to address challenges such as limited energy resources and environmental concerns. Special emphasis is placed on nanostructured semiconductors, quantum dots, and graphene-based materials for their contributions to photovoltaic and catalytic processes. Despite their advantages, challenges remain, including scalability, stability, and cost-efficiency. This research aims to provide a comprehensive overview of recent advancements, evaluate existing challenges, and highlight future directions for nanomaterials in energy conversion, ultimately contributing to a sustainable energy future.