Two-dimensional Electronic Structure Research Articles

Weakening O-Intermediates Adsorption Strength Over the Pd Metallene via Lewis-Acidic Site Modulation for Enhanced Oxygen Reduction.

The reasonable design and modulation of the electronic properties of Pd metallene are acknowledged as a promising avenue for enhancing the oxygen reduction reaction (ORR) in anion exchange membrane fuel cells (AEMFCs), yet they remain a formidable challenge. Herein, a thin-sheet structure of Zr-doped Pd metallene (PdZr metallene) with abundant defects is proposed using a facile wet-chemical approach for efficient and highly durable ORR electrocatalysis. Multiple microstructural analyses uncover that orchestrated electronic and oxophilic regulation of PdZr metallene via Lewis-acidic Zr site modulation could concurrently optimize the electronic configuration of Pd, downshift the d-band center of Pd, and, thus, promote the intrinsic activity. Benefiting from the unique two-dimensional morphology and electronic structure optimization facilitated by the Zr coupling effect, the resultant PdZr metallene demonstrates significantly enhanced ORR electrocatalytic performance in basic solutions, with a high half-wave potential (E1/2) of 0.87 V and commendable stability for 30 000 s, surpassing those of Pd metallene and various advanced Pd-based catalysts reported in the literature. Encouragingly, the PdZr metallene-based AEMFC achieves an increased maximum power density (90.4 mW cm-2) and impressive robustness over 12 h in an alkaline environment, manifesting the practical application of PdZr metallene in AEMFCs. This study showcases the applicability of PdZr metallene via Lewis acid site regulation for fabricating highly active electrocatalysts for high-performance AEMFCs.

Giant thermoelectric effect governed by unique two-dimensional electronic structure and strong anharmonicity in layered nitrides

Giant thermoelectric effect governed by unique two-dimensional electronic structure and strong anharmonicity in layered nitrides

Ideal two-dimensional electronic structure in bulk lead titanate for superior thermoelectrics

An ideal two-dimensional (2D) electronic structure is revealed in n-type tetragonal perovskite PbTiO3 through first-principle calculations. This opens exciting prospects for its applications in thermal-to-electrical energy conversion. The electronic structure is regarded as ideally 2D when it is highly dispersive only along two axes, while along the remaining axis, it is nearly non-dispersive with a bandwidth smaller than 1 meV – insignificant when compared to thermal energy above room temperature (i.e., kBT ≥ 25.9 meV). As a basis for comparison, the electronic structure of the cubic perovskite SrTiO3 is calculated. It is nonideal 2D with the bandwidth along the less dispersive axis larger than the thermal energy. The ideal 2D electronic structure in PbTiO3 leads to a 2D-like density of states, resembling a step function around the conduction minimum while for SrTiO3, the density of states remains 3D-like with a square-root energy dependence. Consequently, n-type PbTiO3 exhibits superior effective band degeneracy within the Fermi window compared to SrTiO3, even though PbTiO3 is nondegenerate at the conduction band minimum, while cubic SrTiO3 is three-fold degenerate. This demonstrates the important role of the ideality in the 2D electronic structure. The ab-initio results demonstrate the potential of PbTiO3 as a promising thermoelectric material.

Ferroelectric Organic Semiconductor: [1]Benzothieno[3,2-b][1]benzothiophene-Bearing Hydrogen-Bonding -CONHC14H29 Chain.

An alkylamide-substituted [1]benzothieno[3,2-b][1]benzothiophene (BTBT) derivative of BTBT-CONHC14H29 (1) and C8H17-BTBT-CONHC14H29 (2) were prepared to design the multifunctional organic materials, which can show both ferroelectric and semiconducting properties. Single-crystal X-ray structural analyses of short-chain (-CONHC3H7) derivatives revealed the coexistence of two-dimensional (2D) electronic band structures brought from a herringbone arrangement of the BTBT π core and the one-dimensional (1D) hydrogen-bonding chains of -CONHC3H7 chains. The thin films of 1 and 2 fabricated on the Si/SiO2 substrate surface have monolayer and bilayer structures, respectively, resulting in conducting layers parallel to the substrate surface, which is suitable for a channel layer of organic field-effect transistors (OFETs). The thin film of 1 indicated a hole mobility μFET = 2.4 × 10-5 cm2 V-1 s-1 and threshold voltage VTh = - 29 V, whereas that of 2 showed a μFET = 2.1 × 10-2 cm2 V-1 s-1 and threshold voltage VTh = -9.7 V. Both 1 and 2 formed the smectic E (SmE) phase above 410 and 369 K, respectively, where the existence of a hole transport pathway was confirmed in the SmE phase. The ferroelectric hysteresis behavior was observed in bulk 1 and 2 in the polarization-electric field (P-E) curves at the SmE phase. 1 showed the remanent polarization Pr = 2.3 μC cm-2 and coercive electric field Ec = 5.2 V μm-1, whereas the Pr and Ec of 2 were 3.4 μC cm-2 and 7.0 V μm-1 at the conditions of 453 K and 1 Hz. Introduction of alkylamide units into the BTBT π core has the potential to develop the external stimulus-responsive organic semiconductors brought from both ferroelectricity and semiconducting properties.

Revealing the two-dimensional electronic structure and anisotropic superconductivity in a natural van der Waals superlattice (PbSe) 1.14NbSe2

Van der Waals superlattices are important for tailoring the electronic structures and properties of layered materials. Here we report the superconducting properties and electronic structure of a natural van der Waals superlattice (${\mathrm{PbSe})}_{1.14}{\mathrm{NbSe}}_{2}$. Anisotropic superconductivity with a transition temperature ${T}_{c}$ = 5.6 $\ifmmode\pm\else\textpm\fi{}$ 0.1 K, which is higher than monolayer ${\mathrm{NbSe}}_{2}$, is revealed by transport measurements on high-quality samples. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the two-dimensional electronic structure and a charge transfer of 0.43 electrons per ${\mathrm{NbSe}}_{2}$ unit cell from the blocking PbSe layer. In addition, polarization-dependent ARPES measurements reveal a significant circular dichroism with opposite contrast at K and ${\mathrm{K}}^{\ensuremath{'}}$ valleys, suggesting a significant spin-orbital coupling and distinct orbital angular momentum. Our work suggests natural van der Waals superlattice as an effective pathway for achieving intriguing properties distinct from both the bulk and monolayer samples.

Understanding the electronic structure of Y2Ti2O5S2 for green hydrogen production: a hybrid-DFT and GW study.

Utilising photocatalytic water splitting to produce green hydrogen is the key to reducing the carbon footprint of this crucial chemical feedstock. In this study, density functional theory (DFT) is employed to gain insights into the photocatalytic performance of an up-and-coming photocatalyst Y2Ti2O5S2 from first principles. Eleven non-polar clean surfaces are evaluated at the generalised gradient approximation level to obtain a plate-like Wulff shape that agrees well with the experimental data. The (001), (101) and (211) surfaces are considered further at hybrid-DFT level to determine their band alignments with respect to vacuum. The large band offset between the basal (001) and side (101) and (211) surfaces confirms experimentally observed spatial separation of hydrogen and oxygen evolution facets. Furthermore, relevant optoelectronic bulk properties were established using a combination of hybrid-DFT and many-body perturbation theory. The optical absorption of Y2Ti2O5S2 weakly onsets due to dipole-forbidden transitions, and hybrid Wannier-Mott/Frenkel excitonic behaviour is predicted to occur due to the two-dimensional electronic structure, with an exciton binding energy of 0.4 eV.

Ambipolar Nickel Dithiolene Complex Semiconductors: From One- to Two-Dimensional Electronic Structures Based upon Alkoxy Chain Lengths.

Air-stable single-component ambipolar organic semiconductors that conduct both holes and electrons are highly desired but have been rarely realized. Neutral nickel bis(dithiolene) complexes are promising candidates that fulfill the stringent electronic requirements of shallow HOMO levels and deep LUMO levels, which can reduce the carrier injection barrier to overcome the work function of gold electrodes and ensure air stability. However, most nickel bis(dithiolene) analogs that have been characterized as ambipolar semiconductors have twisted molecular structures that hinder the effective intermolecular interactions required for carrier conduction. To address this issue, we synthesized planar alkoxy-substituted nickel bis(dithiolene) analogs that facilitate dense packing with effective intermolecular interactions. Remarkably, changing the methoxy substituents to ethoxy or propoxy groups led to a dramatic change in the packing mode, from one-dimensional to herringbone-like, while maintaining effective intermolecular interactions. These materials overcome the usual trade-off between crystallinity and solubility; they are highly crystalline, even in their film forms, and are highly soluble in organic solvents. They are therefore readily solution-processable to form semiconducting layers with well-defined and well-ordered structures in field-effect transistors. Devices based on these compounds exhibited efficient ambipolar characteristics, even after several months of exposure to air, achieving high carrier mobilities of up to 10-2 cm2 V-1 s-1 and large on/off ratios of up to 105, which are the top-class performances achieved for a single-component ambipolar semiconductor material driven in air.

Design, Synthesis, and Optoelectronic Properties of the High-Purity Phase in Layered AETMN2 (AE = Sr, Ba; TM = Ti, Zr, Hf) Semiconductors.

We report the synthesis and optoelectronic properties of high phase-purity (>94 mol %) bulk polycrystals of KCoO2-type layered nitrides AETMN2 (AE = Sr, Ba; and TM = Ti, Zr, Hf), which are expected to exhibit unique electron transport properties originating from their natural two-dimensional (2D) electronic structure, but high-purity intrinsic samples have yet been reported. The bulks were synthesized using a solid-state reaction between AENH and TMN precursors with NaN3 to achieve high N chemical potential during the reaction. The AETMN2 bulks are n-type semiconductors with optical band gaps of 1.63 eV for SrTiN2, 1.97 eV for BaZrN2, and 2.17 eV for BaHfN2. SrTiN2 and BaZrN2 bulks show degenerated electron conduction due to the natural high-density electron doping and paramagnetic behavior in all of the temperature ranges examined, while such unintentional carrier generation is largely suppressed in BaHfN2, which exhibits nondegenerated electron conduction. The BaHfN2 sample also exhibits weak ferromagnetic behavior at temperatures lower than 35 K. Density functional theory calculations suggest that the high-density electron carriers in SrTiN2 come from oxygen impurity substitution at the N site (ON) acting as a shallow donor even if the high-N chemical potential synthesis conditions are employed. On the other hand, the formation energy of ON becomes larger in BaHfN2 because of the stronger TM-N chemical bonds. Present results demonstrate that the easiness of impurity incorporation is designed by density functional calculations to produce a more intrinsic semiconductor in wider chemical conditions, opening a way to cultivating novel functional materials that are sensitive to atmospheric impurities and defects.

Two-dimensional electronic structure for high thermoelectric performance in halide perovskite Cs2Au(I)Au(III)I6.

Pb- or Sn-based halide perovskites usually exhibit poor thermoelectric performance, arising from their low electrical conductivity or oxidation state instability. It is highly desired to search for new halide perovskites with good thermoelectric properties. In this work, the thermally stable mixed-valence halide perovskite Cs2Au(I)Au(III)I6 is revealed to be a highly promising thermoelectric material with high in-plane power factor and ultralow lattice thermal conductivity using first-principles calculations. The high in-plane power factor is achieved due to the novel two-dimensional electronic structure near the Fermi level driven by the weak interaction between AuI-5d and I-p orbitals. In addition, the small group velocities and short phonon lifetimes give rise to ultralow lattice thermal conductivity in Cs2Au(I)Au(III)I6. These excellent electronic and thermal properties lead to a high ZT value, which is close to 1 at 300 K and ∼4 at 800 K. Our results suggest that the 2D electronic structure from the weak interaction between d and p crystal orbitals is a promising route to design high-efficiency halide double perovskite thermoelectric materials.

Engineering novel surface electronic states via complex supramolecular tessellations

Interfacial complex supramolecular tessellations were employed to derive novel two-dimensional electronic structures from Shockley surface-state, giving rise to Kagome or Dirac bands as well as hybrid band structures through quantum dots coupling.

Interfacial eg orbital reconstruction: Modulation of Metal–Insulator transitions of ultrathin NdNiO3 films by two-dimensional electronic gas

Tunable metal–insulator transitions (MIT) of ultrathin NdNiO3 (NNO) film grown on the LaAlO3/SrTiO3 two-dimensional electronic gas structures have been investigated by interfacial eg orbital reconstruction. Spectroscopy studies suggest that the polar catastrophe of the two-dimensional electronic gas results in Ni2+ to Ni3+ charge transfer of the NNO owing to the band-alignment condition. The orbital-hybridization of Ni 3d and O 2p can directly increase in-plane eg orbital occupancy, and the physical mechanism and modulation of the MIT can be clearly elucidated. This study not only provides new insight into the relationship between the electronic properties and orbital occupation, but it will also promote future application of epitaxial NNO thin films.

Interexciton nonradiative relaxation pathways in the peridinin-chlorophyll protein

Excitation energy transfer can be unusually efficient and structurally robust when mediated by molecular excitons, which arise in a photosynthetic light-harvesting protein when delocalized excitations of the electronic chromophores are created upon absorption of light. Here we report results from two-dimensional electronic spectroscopy and electronic structure calculations, revealing that excitation energy is transferred between the peridinin and chlorophyll excitons in the peridinin-chlorophyll protein from marine dinoflagellates over a delocalized, two-step pathway. Upon absorption of light by the peridinins in the strong, mid-visible absorption band, excitation energy is trapped by the chlorophylls in less than 50 fs at room temperature. The overall process is slowed only by a few fs upon replacement of chlorophyll a with chlorophyll b. The key step in the pathway transfers excitation from higher-energy peridinin excitons with more extensive delocalization to the lowest-energy peridinin exciton, which is delocalized only over peridinin 614 and the neighboring chlorophyll. A two-step pathway transfers energy from peridinin excitons to chlorophylls Excitation energy is trapped by chlorophyll acceptors in less than 50 fs Delocalization collapses as excitation progresses irreversibly down the pathway Transfer rates are robust when chlorophyll a is replaced with chlorophyll b Tilluck et al. reveal how excitation energy transfer in the peridinin-chlorophyll protein from marine dinoflagellates involves quantum coherent mixing of the excited states of the clustered chromophores in each protein domain. The results suggest that solar photons might be captured efficiently in photocatalytic devices and solar cells by delocalized nanomaterials.

Electron Spectrum and Interband Magneto-Absorption of Light by Two-Dimensional Systems with Antidots

The degeneracy of Landau levels in perforated two-dimensional electronic structures is lifted in a perpendicular magnetic field. The cylindrical symmetry of the problem with a circular antidot holds and the magnetic quantum number m = 0, ±1, ±2, … is a good quantum number. The splitting of Landau levels for the antidot smaller than the magnetic length, as well as the frequencies and intensities of interband transition lines for normal semiconductors and transition metal dichalcogenide monolayers, has been analytically found.

Thermoelectric Properties of InA Nanowires from Full-Band Atomistic Simulations.

In this work we theoretically explore the effect of dimensionality on the thermoelectric power factor of indium arsenide (InA) nanowires by coupling atomistic tight-binding calculations to the Linearized Boltzmann transport formalism. We consider nanowires with diameters from 40 nm (bulk-like) down to 3 nm close to one-dimensional (1D), which allows for the proper exploration of the power factor within a unified large-scale atomistic description across a large diameter range. We find that as the diameter of the nanowires is reduced below d < 10 nm, the Seebeck coefficient increases substantially, as a consequence of strong subband quantization. Under phonon-limited scattering conditions, a considerable improvement of ~6× in the power factor is observed around d = 10 nm. The introduction of surface roughness scattering in the calculation reduces this power factor improvement to ~2×. As the diameter is decreased to d = 3 nm, the power factor is diminished. Our results show that, although low effective mass materials such as InAs can reach low-dimensional behavior at larger diameters and demonstrate significant thermoelectric power factor improvements, surface roughness is also stronger at larger diameters, which takes most of the anticipated power factor advantages away. However, the power factor improvement that can be observed around d = 10 nm could prove to be beneficial as both the Lorenz number and the phonon thermal conductivity are reduced at that diameter. Thus, this work, by using large-scale full-band simulations that span the corresponding length scales, clarifies properly the reasons behind power factor improvements (or degradations) in low-dimensional materials. The elaborate computational method presented can serve as a platform to develop similar schemes for two-dimensional (2D) and three-dimensional (3D) material electronic structures.

Anisotropic Transport and Quantum Oscillations in the Quasi-One-Dimensional TaNiTe5: Evidence for the Nontrivial Band Topology.

The past decade has witnessed the burgeoning discovery of a variety of topological states of matter with distinct nontrivial band topologies. Thus far, most materials that have been studied possess two-dimensional or three-dimensional electronic structures, with only a few exceptions that host quasi-one-dimensional (quasi-1D) topological electronic properties. Here we present clear-cut evidence for Dirac Fermions in the quasi-1D telluride TaNiTe5. We show that its transport behaviors are highly anisotropic, and we observe nontrivial Berry phases via quantum oscillation measurements. The nontrivial band topology is further corroborated by first-principles calculations. Our results may help to guide the future quest for topological states in this new family of quasi-1D ternary chalcogenides.

Quantized Reaction Pathways for Solution Synthesis of Colloidal ZnSe Nanostructures: A Connection between Clusters, Nanowires, and Two-Dimensional Nanoplatelets.

The morphology of nanocrystals serves as a powerful handle to modulate their functional properties. For semiconducting nanostructures, the shape is no less important than the size and composition, in terms of determining the electronic structure. For example, in the case of nanoplatelets (NPLs), their two-dimensional (2D) electronic structure and atomic precision along the axis of quantum confinement makes them well-suited as pure color emitters and optical gain media. In this study, we describe synthetic efforts to develop ZnSe NPLs emitting in the ultraviolet part of the spectrum. We focus on two populations of NPLs, the first having a sharp absorption onset at 345 nm and a previously unreported species with an absorption onset at 380 nm. Interestingly, we observe that the nanoplatelets are one step in a quantized reaction pathway that starts with (zero-dimensional (0D)) magic-sized clusters, then proceeds through the formation of (one-dimensional (1D)) nanowires toward the (2D) "345 nm" species of NPLs, which finally interconvert into the "380 nm" NPL species. We seek to rationalize this evolution of the morphology, in terms of a general free-energy landscape, which, under reaction control, allows for the isolation of well-defined structures, while thermodynamic control leads to the formation of three-dimensional (3D) nanocrystals.

Designing Two-Dimensional Properties in Three-Dimensional Halide Perovskites via Orbital Engineering.

Manipulating the orbital hybridization between the metal cation and the halide anion to achieve novel properties is highly desired. Here, we present an orbital engineering strategy to construct two-dimensional (2D) electronic structures in three-dimensional (3D) halide perovskites by rationally controlling the hybridization between the d orbitals of the metal cations and the halide p orbitals. Taking Cs2Au(I)Au(III)I6 as an example, we demonstrate that the flat conduction band and valence band at the band edges can be achieved simultaneously by combining two metal cations with different d orbital configurations using first-principles calculations. The band structure and predicted carrier mobilities show huge anisotropy along in-plane and out-of-plane directions, confirming the 2D electronic properties. In addition, the strong anisotropic optical and mechanical properties (e.g., 2D-like properties) are also presented. Our work provides orbital engineering guidance for achieving low-dimensional properties with strong anisotropy in 3D halide perovskites for novel electronic and photonic applications.

Spatial Transformations of Atoms

Based on the experimentally established existence of atoms with spatially one- and two-dimensional electronic structures that can be paired with conventional three-dimensional atoms, a formal theoretical substantiation is proposed in favor of the possibility of simultaneous presence of all three types of atoms with spatial dimension DI = D1, D2, D3 . Despite the fundamental difference in the computational procedure, the results of the present study seem to be quite adequate being a significant addition to the results of previous V. V. Skobelev’s works. The conclusions of the work can be verified in experiments, though the principal feasibility of such experiments is not obvious. This last problem needs further substantiation and is not discussed in this work.

Electrical Properties of Two-Dimensional Materials Used in Gas Sensors.

In the search for gas sensing materials, two-dimensional materials offer the possibility of designing sensors capable of tuning the electronic band structure by controlling their thickness, quantity of dopants, alloying between different materials, vertical stacking, and the presence of gases. Through materials engineering it is feasible to study the electrical properties of two-dimensional materials which are directly related to their crystalline structure, first Brillouin zone, and dispersion energy, the latter estimated through the tight-binding model. A review of the electrical properties directly related to the crystalline structure of these materials is made in this article for the two-dimensional materials used in the design of gas sensors. It was found that most 2D sensing materials have a hexagonal crystalline structure, although some materials have monoclinic, orthorhombic and triclinic structures. Through the simulation of the mathematical models of the dispersion energy, two-dimensional and three-dimensional electronic band structures were predicted for graphene, hexagonal boron nitride (h-BN) and silicene, which must be known before designing a gas sensor.

First-principles study of electronic structure, magnetic and optical properties of laminated molybdenum oxides

According to the pseudopotential plane-wave method of first-principles calculation based on the spin density functional theory, the electronic structure, magnetic and optical properties of laminated molybdenum oxides (orthonormal and monoclinic MoO&lt;sub&gt;3&lt;/sub&gt;) are studied theoretically. The interlaminar dissociation energy, band-structure, spin polarization, dielectric function, and the optical absorption/reflectivity in a charged state are systematically calculated to explore the potential technology applications of laminated MoO&lt;sub&gt;3&lt;/sub&gt; as electrochromic or electromagnetic materials in optoelectronic devices. The semilocal GGA-PW91 and nonlocal HSE06 exchange-correlation functional are employed to obtain the more accurate crystal structure and band gap respectively. The cleavage energy results indicate that the single layers can easily flake off from the bulk material of these molybdenum oxides. The band structure and atomic-projected density of states prove that the conduction band minimum and valence band maximum are mainly derived from the atom-orbitals bonding oriented in layer-plane, representing characteristic two-dimensional electronic structure. The spin polarized calculations imply that the evident magnetic-moment will engender in MoO&lt;sub&gt;6&lt;/sub&gt; octahedron layers of the perfect MoO&lt;sub&gt;3&lt;/sub&gt; due to the substantial spin polarization of Mo and vertex O atoms which are ferromagnetic-coupling to produce significant net magnetic moments, essentially accounting for the magnetic source of bulk MoO&lt;sub&gt;3&lt;/sub&gt;. The Mo vacancy reduces the electronic density of states derived from the spin polarized d-orbitals, leading the net magnetic moment to decrease, while the O&lt;sub&gt;I&lt;/sub&gt; vacancy can reduce the density of spin-down states in the MoO&lt;sub&gt;3&lt;/sub&gt;, resulting in the significant improvement of net magnetic moment. The existence of O&lt;sub&gt;II&lt;/sub&gt; vacancy leads to the energetic spin-splitting of O-2p and Mo-4d orbital states, and thus increasing net magnetic moment by raising the electronic density of polarized spin-up states. The electron spin polarization of Mo-4d orbital component dominantly contributes to the bulk magnetism. The laminated MoO&lt;sub&gt;3&lt;/sub&gt; presents a significant optical response in the visible region with obvious anisotropy of optical absorption spectra, which will represent a considerable blue shift or new low-frequency absorption peaks for visible light when loading charges. The calculation results demonstrate that the laminated molybdenum oxides have evident electrochromic property with controllable magnetic moment, which provides theoretical basis and technical data for developing novel functional materials with high performance to be used in electromagnetic or optoelectronic devices.