3d Electronic States Research Articles

Novel electronic structures from anomalous stackings in NbS2 and MoS2

We show that in some transition metal dichalcogenides, minority regions of the cleaved sample surfaces show—unexpectedly and anomalously—a finite number of 2D electronic states instead of the expected 3D valence bands. In the case of NbS2, in addition to the typical spectrum associated with bulk 2Ha stacking, we also find minority regions with electronic structures consistent with few layers of 3R stacking. In MoS2, we find areas of both bulk 2Hc and 3R stackings, and regions exhibiting finite-layer quantization of both types. We further find evidence for a more exotic 4Ha stacking of MoS2, in which the valence band maximum is quasi-2D. The results highlight how variation of the interlayer stacking of van der Waals materials beyond the commonly reported bulk polytypes can yield novel electronic structures. Published by the American Physical Society 2024

New Insights into the ORR Catalysis on Pt Alloy Nanoparticles from an Element Specific d-Band Analysis.

Bolstered by their unique atomic structures and tailored compositions, nanoalloys exhibit extraordinary properties making them ideal materials to solve challenges in energy storage and conversion catalysis. However, a quantitative description of the structure-property relationships using an accurate descriptor-based model for nanoalloys, ranging from bimetallic to multimetallic compositions, is needed to drive efficient material design toward high-performance catalysis. In this work, we highlight the electronic property and catalytic activity relationship from an element specific d-band analysis of Pt-based alloy catalysts using X-ray absorption near-edge spectroscopy (XANES). Using a series of L10-MPt/Pt (M = Fe, Co, Ni) core/shell alloy catalysts with well-defined atomic structures, we quantified subtle differences in the Pt d-electron states and correlated the Pt d-band structure to their superior catalytic activity toward the oxygen reduction reaction (ORR). Our analysis used the upper d-band edge position as a predictive descriptor for the mass activity toward the ORR instead of the commonly used d-band center position. Together with density functional theory calculations and Nørskov d-band theory, the upper d-band edge position for the Pt states, derived from experimental measurements, elucidates new physical insights into the ORR performance of the L10-MPt/Pt core/shell catalysts. An element specific Pt d-band analysis using XANES overcomes challenges in traditional X-ray photoelectron spectroscopy-based valence d-band analysis, which cannot distinguish signals from independent elements in nanoalloys. Thus, the insights from the element specific d-band analysis presented in this work are a promising approach to determine structure-property relationships in a variety of transition metal nanoalloys and will be useful in the design of future high-performance catalysts.

Coordination Structure Modulation in Group-VIB Metal Doped Ag3PO4 Augments Active Site Density for Electrocatalytic Conversion of N2 to NH3.

Doping is considered a promising material engineering strategy in electrochemical nitrogen reduction reaction (NRR), provided the role of the active site is rightly identified. This work concerns the doping of group VIB metal in Ag3PO4 to enhance the active site density, accompanied by d-p orbital mixing at the active site/N2 interface. Doping induces compressive strain in the Ag3PO4 lattice and inherently accompanies vacancy generation, the latter is quantified with positron annihilation lifetime studies (PALS). This eventually alters the metal d-electronic states relative to Fermi level and manipulate the active sites for NRR resulting into side-on N2 adsorption at the interface. The charge density deployment reveals Mo as the most efficient dopant, attaining a minimum NRR overpotential, as confirmed by the detailed kinetic study with the rotating ring disk electrode (RRDE) technique. In fact, the Pt ring of RRDE fails to detect N2H4, which is formed as a stable intermediate on the electrode surface, as identified from in-situ attenuated total reflectance-infrared (ATR-IR) spectroscopy. This advocates the complete conversion of N2 to NH3 on Mo/Ag3PO4-10 and the so-formed oxygen vacancies formed during doping act as proton scavengers suppressing hydrogen evolution reaction resulting into a Faradaic efficiency of 54.8% for NRR.

Optimizing the adsorption of intermediates through modulating Mn d-band centers by the Mn-Ce heterojunction for high-performance lithium-oxygen batteries

Admitting that rare-earth (RE)-based transition metal oxides (TMO) are emerging as a promising catalyst for lithium-oxygen (Li-O2) batteries, the understanding of their electrocatalytic mechanism and active sites remains ambiguous. In this study, CeO2 nanoparticles modified MnOOH nanowires (CeO2@MnOOH NWs) is prepared by a simple solvothermal method as highly effective cathode catalysts for Li-O2 batteries. The incorporation of CeO2 nanoparticles into MnOOH facilitates a robust interaction between the 3d electronic state of MnOOH and the 4f electronic state of CeO2 at the Fermi level (Ef). The strong electron interaction at the heterogeneous interface between MnOOH and CeO2 promotes internal electron transfer. Compared to pristine MnOOH, the prepared CeO2@MnOOH NWs exhibits enhanced electrocatalytic activity and improved electrocatalytic stability. According to experimental analysis and density functional theory (DFT) results, it is demonstrated that there is a slight negative shift in the d-band center of the Mn site, resulting in faster electron transfer rates and higher conductivity. This acceleration of charge transfer optimizes the adsorption strength of the oxygen-containing intermediate (LiO2), thereby promoting the kinetics of the oxygen reduction/evolution reactions (ORR/OER) and reducting the reaction over-potential. As a result, the electrochemical performances of Li-O2 batteries are greatly enhanced.

Improvement in hot carrier dynamics of all-inorganic halide perovskite CsPbI3 on doping Cu

Hot carrier extraction is crucial for efficient solar energy harvesting, and lead halide perovskite nanocrystals (NCs) are potential candidates for photovoltaic and light-emitting applications. Therefore, swift extraction of hot carriers is an immediate requirement to improve the energy conversion efficiency, which need longer thermalization time. To address this issue, we synthesized nominally Cu-doped CsPbI3 NCs with enhanced structural and optical characteristics compared to undoped CsPbI3 NCs. We investigated the hot carrier dynamics in both the NCs at different fluences using ultrafast transient absorption spectroscopy. Interestingly, we observed very fast thermalization at higher fluences that indicated breaking of the phonon bottleneck. On the contrary, doped NCs preserved the effects and decayed over a longer period of time possibly due to increase in size and introduction of shallow trap states of Cu 3d and Cu 4s electrons in the conduction band, as computed using density functional theory. Notably, as the carrier–carrier interaction increased, we observed a dominating bandgap renormalization in the doped system compared to the undoped system. Overall, our studies improve the understanding of Cu doping in enhancing the hot carrier dynamics in perovskites and open possibilities for further investigation in the quantum phenomenon of these materials.

Charge doping and electric field tunable ferromagnetism and Curie temperature of the MnS2 monolayer.

Two-dimensional ferromagnets with a long-range ferromagnetic ordering at finite temperature present a bright prospect for their potential applications in nanoscale spintronic devices. The tuning of their intrinsic ferromagnetism and Curie temperature is essential for the development of next-generation data storage and spintronic devices. In this work, the electronic structures, ferromagnetism and Curie temperature of two-dimensional MnS2 monolayer are controlled by charge doping and electric field using first principles calculations. The results show that the dynamic and thermal stability of monolayer MnS2 for all of the cases can be still maintained. Moreover, there is no existence of phase transition and all MnS2 monolayers at any charge doping concentrations and electric field intensities favor ferromagnetic coupling. For the manipulation of electron doping, the calculated total magnetic moment Mtot of the MnS2 monolayer exhibits an increase from 3.112 to 3.491μB per unit cell. Further analysis indicates that a transition from half-metal to metal occurs by introducing the charge doping and vertical electric field, and the Mn 3d electronic states are the major determinants of ferromagnetism. Additionally, the charge doping enables the magnetic anisotropy energy to transform from an in-plane easy axis to the magnetization direction out of the plane. The Curie temperature Tc of the MnS2 monolayer can be moderately enhanced above room temperature by hole doping and application of a vertical electric field. Remarkably, Tc reaches its peak at 767 K at a hole doping concentration of -0.8e. This work enriches the microscopic understanding of the tuning mechanism of ferromagnetism and supplies a sound theoretical basis for subsequent experimental studies.

(Invited) Unravelling Structural Implications on Charge Transport Mechanism in Nanostructured Electrode Materials for Energy Storage Devices

The development of new and novel electrode materials for energy storage devices has become the intensive research by the materials science community because of its importance in the portable electronic devices, hybrid electric vehicles and many other applications. Since the discovery by Goodenough and his co-workers on the electrochemical behavior of olivine LiFePO4 (LFP), different theoretical and experimental investigations have been undertaken to optimize it as cathode material in lithium-ion batteries and trying to understand the charge/discharge mechanism. Despite having many fascinating electrochemical properties, the main drawback of LFP lies with its low gravimetric density and poor electrical conductivity (both electronic and ionic) which limits the lithium intercalation/deintercalation rates, and hence the practical specific capacity. Therefore, it becomes necessary to gain the fundamental understanding of electronic structure of the LFP system by adopting appropriate experimental technique. We exploit the combined Mössbauer and X-ray absorption spectroscopy to unravel the electronic structure and local site symmetry of Fe in olivine structured LFP with different crystallite sizes (CS). The lattice parameters are found to contract with decrease in CS monotonously, whereas the electronic structural parameters exhibit two different regions with threshold anomaly around ≈30 nm CS. The 57Fe Mössbauer studies reveal the coexistence of Fe2+ and Fe3+sites and their relative concentration are mainly determined by the CS, which provides the comprehensive insight into the electronic structure of LFP at mesoscopic level. The soft X-ray absorption unravels unequivocally the valance states of Fe 3d electrons in the proximity of Fermi level, which are prone to the local lattice distortion. An obtained spectra fingerprint the effect of CS supplying rich information on valence state of iron, lithium-ion vacancy concentration, covalency and crystal field. The unique structural and electronic properties of the LFP are closely interlinked with changes in the bonding character, which shows the strong dependency on the CS. The evolution of the 3d states is in overall agreement with the local lattice distortion and provides the origin of the size effects on the electronic structure olivine phosphate and other transition metal ion containing materials. We observe polaronic conductivity enhancement of approximately two orders of magnitude at the nanoscale level as compared with its bulk counterpart. The volumetric changes with respect to crystallite size are related to the compressive strain resulting into the improvement in the electronic diffusivity. The nano-crystalline LFP with better kinetics will open the new avenue for its usage as cathode material in rechargeable batteries.

D-electron regulation of Ru clusters via conductive V4C3Tx MXene and cation vacancy to boost efficient oxygen evolution

The design of economical and efficient noble-metal-based catalysts is paramount for sustainable energy production. However, modulating the electronic structures of noble metals so that they serve as efficient active sites remains challenging. In this study, we develop a multi-interfacial Ru/NiFe-LDH@V4C3Tx composite by modulating its d-electronic states using electron donors and cation vacancies. The fabricated Ru/NiFe-LDH@V4C3Tx catalyst exhibits a high mass activity of 7741 mA mgRu−1 at an overpotential of 270 mV and a low overpotential of 231 mV at 10 mA cm−2. Our experimental and density functional theory calculation investigations confirm that the synergistic effect of the V4C3Tx MXene and cation vacancies on the electronic structure lowers the d-band center of Ru and weakens the adsorption of the *OOH intermediate at the active Ru atom site. Thus, the catalytic performance of Ru/NiFe-LDH@V4C3Tx is significantly improved owing to its reduced energy barrier of the rate-determining step (RDS).

Key role of eg* band broadening in nickel-based oxyhydroxides on coupled oxygen evolution mechanism

A coupled oxygen evolution mechanism (COM) during oxygen evolution reaction (OER) has been reported in nickel oxyhydroxides (NiOOH)-based materials by realizing eg* band (3d electron states with eg symmetry) broadening and light irradiation. However, the link between the eg* band broadening extent and COM-based OER activities remains unclear. Here, Ni1-xFexOOH (x = 0, 0.05, 0,2) are prepared to investigate the underlying mechanism governing COM-based activities. It is revealed that in low potential region, realizing stronger eg* band broadening could facilitate the *OH deprotonation. Meanwhile, in high potential region where the photon utilization is the rate-determining step, a stronger eg* band broadening would widen the non-overlapping region between dz2 and a1g* orbitals, thereby enhancing photon utilization efficiency. Consequently, a stronger eg* band broadening could effectuate more efficient OER activities. Moreover, we demonstrate the universality of this concept by extending it to reconstruction-derived X-NiOOH (X = NiS2, NiSe2, Ni4P5) with varying extent of eg* band broadening. Such an understanding of the COM would provide valuable guidance for the future development of highly efficient OER electrocatalysts.

Capturing the 2D and 3D electronic states of novel and exotic materials

New instrument visualizes the Fermi surfaces of novel materials.

High-energy photoemission final states beyond the free-electron approximation

Three-dimensional (3D) electronic band structure is fundamental for understanding a vast diversity of physical phenomena in solid-state systems, including topological phases, interlayer interactions in van der Waals materials, dimensionality-driven phase transitions, etc. Interpretation of ARPES data in terms of 3D electron dispersions is commonly based on the free-electron approximation for the photoemission final states. Our soft-X-ray ARPES data on Ag metal reveals, however, that even at high excitation energies the final states can be a way more complex, incorporating several Bloch waves with different out-of-plane momenta. Such multiband final states manifest themselves as a complex structure and added broadening of the spectral peaks from 3D electron states. We analyse the origins of this phenomenon, and trace it to other materials such as Si and GaN. Our findings are essential for accurate determination of the 3D band structure over a wide range of materials and excitation energies in the ARPES experiment.

Hysteresis loss and field dependence of magnetic entropy change of Zn-doped Mn5Ge3 system

In this work, a series of Mn5Ge3-xZnx samples were prepared by the arc-melting method. The Rietveld structure refinement of the powder XRD spectrum was performed using the FullProf program. The lattice parameters and cell volume increase with the doping of Zn atoms. From the magnetization curve at 5 K, the compounds show a decreased magnetic moment of the Mn atom with increasing Zn content, resulting from mixing the 3d electronic state of Mn atoms and the 3d electronic state of Zn atoms. The small hysteresis loss and soft ferromagnetism of Mn5Ge3-xZnx indicate that it is suitable for magnetic refrigeration. Thermomagnetic M(T) curves show an increase in thermal hysteresis concurrent with increasing Zn concentration, which is verified by the DSC curve. It is found that the magnetic entropy change and effective magnetic moment decrease with the increase of Zn content, which can be attributable to the doping of Zn atoms resulting in the weakening of the exchange interaction between Mn-Mn atoms and the decrease of magnetic entropy change. In addition, the specific power-law relationships between the magnetic entropy change, the maximum magnetic entropy change, the relative cooling capacity, and the magnetic field of this system were determined, which provides a tool for evaluating the performance of the magnetic refrigerant in the entire magnetic field range. Finally, the specific heat was calculated by the magnetic entropy change obtained by the experiment, and the specific heat curve proves that the sample undergoes a second-order phase transition, consistent with the results of Arrott plots. The TC obtained by the specific heat -ΔCP−T curve is in good agreement with the TC obtained by the magnetic measurement, revealing that TC obtained by the specific heat is also an effective method.

Flat Bands for Electrons in Rhombohedral Graphene Multilayers with a Twin Boundary

AbstractTopologically protected flat surface bands make thin films of rhombohedral graphite an appealing platform for searching for strongly correlated states of 2D electrons. In this work, rhombohedral graphite is studied with a twin boundary stacking fault and the semimetallic and topological properties of low‐energy bands, localized at the surfaces and at the twinned interface, are analyzed. An effective 4‐band low energy model is derived, where the full set of Slonczewski–Weiss–McClure (SWMcC) parameters is implemented, and the conditions for the bands are found to be localized at the twin boundary, protected from the environment‐induced disorder. This protection together with a high density of states at the charge neutrality point, in some cases—due to a Lifshitz transition, makes this system a promising candidate for hosting strongly‐correlated effects.

Optimization of oxygen evolution activity by tuning e*g band broadening in nickel oxyhydroxide

In Ni(OH)2, a greater extent of band (3d electron states with egsymmetry) broadening can facilitate electron transfer from the electrocatalyst to the external circuit, leading to higher OER catalytic performance.

Comparative theoretical study of x-ray magnetic circularly polarized emission from ferromagnetic Fe, Co, and Ni

X-ray magnetic circularly polarized emission (XMCPE) is a novel magneto-optical phenomenon in which characteristic x-rays emitted from a magnetized material are circularly polarized. In this letter, we report a comparative theoretical study of XMCPE spectra of the K α emission from ferromagnetic Fe, Co, and Ni. Calculated XMCPE spectra have characteristic tail structures on the low-energy side of the K α emission peaks. Comparison of the spectra shows that Fe, Co, and Ni have broad, intermediate, and narrow tail structures, respectively. Since these tail structures originate from electron excitations in the 3d conduction bands, this difference in tail structure width can be explained by the difference in the spin-polarized 3d electron states among the three metals.

α−As2Te3 as a platform for the exploration of the electronic band structure of single layer β−tellurene

Arsenic telluride, ${\mathrm{As}}_{2}{\mathrm{Te}}_{3}$, is a layered van der Waals (vdW) semiconducting material usually known for its thermoelectric properties. It is composed of layers stacked together via weak vdW interactions, which can consequently be exfoliated into thin two-dimensional layers. Here, we studied the electronic properties of the $\ensuremath{\alpha}$ phase of ${\mathrm{As}}_{2}{\mathrm{Te}}_{3}$ by using angle-resolved photoemission spectroscopy (ARPES) and density-functional theory (DFT). In addition to the spectroscopic signature of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Te}}_{3}$, we were able to isolate anisotropic 2D electronic states, decoupled from the $\ensuremath{\alpha}\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Te}}_{3}$ electronic structure, that we propose to ascribe to single layer (SL) $\ensuremath{\beta}\ensuremath{-}\mathrm{tellurene}$. Our findings are supported by theoretical investigations using DFT, which reproduce the main ARPES experimental features. Our work thereby proposes $\ensuremath{\alpha}\ensuremath{-}{\mathrm{As}}_{2}{\mathrm{Te}}_{3}$ (100) surface as an interesting platform for the experimental exploration of the electronic band structure of SL $\ensuremath{\beta}\ensuremath{-}\mathrm{tellurene}$, which has been difficult to experimentally access otherwise.

Breakdown of bulk-projected isotropy in surface electronic states of topological Kondo insulator SmB6(001)

The topology and spin-orbital polarization of two-dimensional (2D) surface electronic states have been extensively studied in this decade. One major interest in them is their close relationship with the parities of the bulk (3D) electronic states. In this context, the surface is often regarded as a simple truncation of the bulk crystal. Here we show breakdown of the bulk-related in-plane rotation symmetry in the topological surface states (TSSs) of the Kondo insulator SmB6. Angle-resolved photoelectron spectroscopy (ARPES) performed on the vicinal SmB6(001)-p(2 × 2) surface showed that TSSs are anisotropic and that the Fermi contour lacks the fourfold rotation symmetry maintained in the bulk. This result emphasizes the important role of the surface atomic structure even in TSSs. Moreover, it suggests that the engineering of surface atomic structure could provide a new pathway to tailor various properties among TSSs, such as anisotropic surface conductivity, nesting of surface Fermi contours, or the number and position of van Hove singularities in 2D reciprocal space.

On the Theory of Optical Diode on Iron Ions in FeZnMo3O8

The parameters of the interaction of 3d electron states with an electromagnetic wave, as well as the probabilities of magnetic and electric dipole transitions between states of the ground term of an iron ion, which is split by the crystal field, exchange interaction, and spin–orbit coupling, have been calculated. The dependences of the absorption lines on the magnitude and direction of the applied magnetic field have been determined. It has been shown that the optical diode effect discovered in [Sh. Yu, B. Gao, J. W. Kim, S.-W. Cheong, M. K. L. Man, J. Madeo, K. M. Dani, and D. Talbayev, Phys. Rev. Lett. 120, 037601 (2018)] can be explained by the interference of magnetic and electric dipole transitions.

Correlated States of 2D Electrons near the Landau Level Filling ν=1/7.

The ground state of two-dimensional electron systems (2DESs) at low Landau level filling factors (ν≲1/6) has long been a topic of interest and controversy in condensed matter. Following the recent breakthrough in the quality of ultrahigh-mobility GaAs 2DESs, we revisit this problem experimentally and investigate the impact of reduced disorder. In a GaAs 2DES sample with density n=6.1×10^{10}/cm^{2} and mobility μ=25×10^{6} cm^{2}/V s, we find a deep minimum in the longitudinal magnetoresistance (R_{xx}) at ν=1/7 when T≃104 mK. There is also a clear sign of a developing minimum in R_{xx} at ν=2/13. While insulating phases are still predominant when ν≲1/6, these minima strongly suggest the existence of fractional quantum Hall states at filling factors that comply with the Jain sequence ν=p/(2mp±1) even in the very low Landau level filling limit. The magnetic-field-dependent activation energies deduced from the relation R_{xx}∝e^{E_{A}/2kT} corroborate this view and imply the presence of pinned Wigner solid states when ν≠p/(2mp±1). Similar results are seen in another sample with a lower density, further generalizing our observations.

The ligand field in low-crystallinity metal-organic frameworks investigated by soft X-ray core-level absorption spectroscopy.

The ligand field (LF) of transition metal ions is a crucial factor in realizing the mechanism of novel physical and chemical properties. However, the low-crystallinity state, including the amorphous state, precludes the clarification of the electronic structural relationship of transition metal ions using crystallographic techniques, ultraviolet and infrared optical methods, and magnetometry. Here, we demonstrate that soft X-ray 2p → 3d core-level absorption spectroscopy (L2,3-edge XAS) systematically revealed the local 3d electronic states, including in the LF, of nitrogen-coordinated transition-metal ions for low-crystallinity cyanide-bridged metal-organic frameworks (MOFs) M[Ni(CN)4] (MNi; M = Mn, Fe, Co, Ni) and Ni[Pd(CN)4] (NiPd). In NiNi and NiPd, N-coordinated Ni ions with square-planar symmetry exhibit strong orbital hybridization and ligand-to-metal charge transfer effects. In MnNi, FeNi, and CoNi, the correlation between the crystalline electric field splitting in the LF and the transition metal-nitrogen bonding length is revealed using the multiplet LF theory. Regardless of the different local symmetries, our results indicate that L2,3-edge XAS is a powerful tool for gaining element-specific knowledge about the transition-metal ion characterizing the functionality of low-crystallinity MOFs and will be the foundation for an attractive platform, such as adsorption/desorption materials.