Showing papers on "Electronic band structure published in 2017"

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

Abstract: Encapsulated few-layer InSe exhibits a remarkably high electronic quality, which is promising for the development of ultrathin-body high-mobility nanoelectronics. A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound1,2. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes3,4. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103 cm2 V−1 s−1 and 104 cm2 V−1 s−1 at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5 eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides5,6,7 and black phosphorus8,9,10,11.

Understanding band gaps of solids in generalized Kohn–Sham theory

Abstract: The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. However, the gap in the band structure of the exact multiplicative Kohn–Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density-functional theory. Here, we give a simple proof of a theorem: In generalized KS theory (GKS), the band gap of an extended system equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from metageneralized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential. The theorem also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules, solid aluminum arsenide, and solid argon provide numerical illustrations.

Lattice relaxation and energy band modulation in twisted bilayer graphene

Abstract: We theoretically study the lattice relaxation in the twisted bilayer graphene (TBG) and its effect on the electronic band structure. We develop an effective continuum theory to describe the lattice relaxation in general TBGs and obtain the optimized structure to minimize the total energy. At small rotation angles $l{2}^{\ensuremath{\circ}}$, in particular, we find that the relaxed lattice drastically reduces the area of the AA stacking region and forms a triangular domain structure with alternating AB and BA stacking regions. We then investigate the effect of the domain formation on the electronic band structure. The most notable change from the nonrelaxed model is that an energy gap of up to 20 meV opens at the superlattice subband edges on the electron and hole sides. We also find that the lattice relaxation significantly enhances the Fermi velocity, which was strongly suppressed in the nonrelaxed model.

Determination of band offsets, hybridization, and exciton binding in 2D semiconductor heterostructures.

Abstract: Combining monolayers of different two-dimensional semiconductors into heterostructures creates new phenomena and device possibilities. Understanding and exploiting these phenomena hinge on knowing the electronic structure and the properties of interlayer excitations. We determine the key unknown parameters in MoSe2/WSe2 heterobilayers by using rational device design and submicrometer angle-resolved photoemission spectroscopy (μ-ARPES) in combination with photoluminescence. We find that the bands in the K-point valleys are weakly hybridized, with a valence band offset of 300 meV, implying type II band alignment. We deduce that the binding energy of interlayer excitons is more than 200 meV, an order of magnitude higher than that in analogous GaAs structures. Hybridization strongly modifies the bands at Γ, but the valence band edge remains at the K points. We also find that the spectrum of a rotationally aligned heterobilayer reflects a mixture of commensurate and incommensurate domains. These results directly answer many outstanding questions about the electronic nature of MoSe2/WSe2 heterobilayers and demonstrate a practical approach for high spectral resolution in ARPES of device-scale structures.

Experimental realization and characterization of an electronic Lieb lattice

Abstract: Geometry, whether on the atomic or nanoscale, is a key factor for the electronic band structure of materials. Some specific geometries give rise to novel and potentially useful electronic bands. For example, a honeycomb lattice leads to Dirac-type bands where the charge carriers behave as massless particles [1]. Theoretical predictions are triggering the exploration of novel 2D geometries [2-10], such as graphynes, Kagome and the Lieb lattice. The latter is the 2D analogue of the 3D lattice exhibited by perovskites [2]; it is a square-depleted lattice, which is characterised by a band structure featuring Dirac cones intersected by a flat band. Whereas photonic and cold-atom Lieb lattices have been demonstrated [11-17], an electronic equivalent in 2D is difficult to realize in an existing material. Here, we report an electronic Lieb lattice formed by the surface state electrons of Cu(111) confined by an array of CO molecules positioned with a scanning tunneling microscope (STM). Using scanning tunneling microscopy, spectroscopy and wave-function mapping, we confirm the predicted characteristic electronic structure of the Lieb lattice. The experimental findings are corroborated by muffin-tin and tight-binding calculations. At higher energies, second-order electronic patterns are observed, which are equivalent to a super-Lieb lattice.

Composition design, optical gap and stability investigations of lead-free halide double perovskite Cs2AgInCl6

Abstract: The discovery of lead-free double perovskites provides a feasible way of searching for air-stable and environmentally benign solar cell absorbers. Herein we report the design and hydrothermal crystal growth of double perovskite Cs2AgInCl6. The crystal structure, morphology related to the crystal growth habit, band structure, optical properties, and stability are investigated in detail. This perovskite crystallized in a cubic unit cell with the space group Fm3m and is composed of [AgCl6] and [InCl6] octahedra alternating in a ordered rock-salt structure, and the as-obtained crystal size is dependent on the hydrothermal reaction time. Cs2AgInCl6 is a direct gap semiconductor with a wide band gap of 3.23 eV obtained experimentally and 3.33 eV obtained by DFT calculation. This theoretically predicted and experimentally confirmed optical gap is a prototype of the band gaps that are direct and optically allowed except at the single high-symmetry k-point, which didn't raise interest before but have potential applications in future technologies. Cs2AgInCl6 material with excellent moisture, light and heat stability shows great potential for photovoltaic and other optoelectronic applications via further band gap engineering.

Signature of Metallic Behavior in the Metal–Organic Frameworks M3(hexaiminobenzene)2 (M = Ni, Cu)

Abstract: The two-dimensionally connected metal-organic frameworks (MOFs) Ni3(HIB)2 and Cu3(HIB)2 (HIB = hexaiminobenzene) are bulk electrical conductors and exhibit ultraviolet-photoelectron spectroscopy (UPS) signatures expected of metallic solids. Electronic band structure calculations confirm that in both materials the Fermi energy lies in a partially filled delocalized band. Together with additional structural characterization and microscopy data, these results represent the first report of metallic behavior and permanent porosity coexisting within a metal-organic framework.

Impact of the Electronic Band Structure in High-Harmonic Generation Spectra of Solids.

Abstract: Advanced first-principles calculations reveal that the generation of high harmonic spectra is enhanced by the inhomogeneity of the electron\penalty1000\-\allowhyphens{}-nuclei potential, which is obtained from the band structure.

Spin–orbit-coupled fermions in an optical lattice clock

Abstract: Engineered spin-orbit coupling (SOC) in cold-atom systems can enable the study of new synthetic materials and complex condensed matter phenomena. However, spontaneous emission in alkali-atom spin-orbit-coupled systems is hindered by heating, limiting the observation of many-body effects and motivating research into potential alternatives. Here we demonstrate that spin-orbit-coupled fermions can be engineered to occur naturally in a one-dimensional optical lattice clock. In contrast to previous SOC experiments, here the SOC is both generated and probed using a direct ultra-narrow optical clock transition between two electronic orbital states in 87Sr atoms. We use clock spectroscopy to prepare lattice band populations, internal electronic states and quasi-momenta, and to produce spin-orbit-coupled dynamics. The exceptionally long lifetime of the excited clock state (160 seconds) eliminates decoherence and atom loss from spontaneous emission at all relevant experimental timescales, allowing subsequent momentum- and spin-resolved in situ probing of the SOC band structure and eigenstates. We use these capabilities to study Bloch oscillations, spin-momentum locking and Van Hove singularities in the transition density of states. Our results lay the groundwork for using fermionic optical lattice clocks to probe new phases of matter.

Observation of topological valley modes in an elastic hexagonal lattice

Abstract: We report on the experimental observation of topologically protected edge waves in a two-dimensional elastic hexagonal lattice The lattice is designed to feature $K$-point Dirac cones that are well separated from the other numerous elastic wave modes characterizing this continuous structure We exploit the arrangement of localized masses at the nodes to break mirror symmetry at the unit-cell level, which opens a frequency band gap This produces a nontrivial band structure that supports topologically protected edge states along the interface between two realizations of the lattice obtained through mirror symmetry Detailed numerical models support the investigations of the occurrence of the edge states, while their existence is verified through full-field experimental measurements The test results show the confinement of the topologically protected edge states along predefined interfaces and illustrate the lack of significant backscattering at sharp corners Experiments conducted on a trivial waveguide in an otherwise uniformly periodic lattice reveal the inability of a perturbation to propagate and its sensitivity to backscattering, which suggests the superior waveguiding performance of the class of nontrivial interfaces investigated herein

Origin of the Size-Dependent Stokes Shift in CsPbBr3 Perovskite Nanocrystals

Abstract: The origin of the size-dependent Stokes shift in CsPbBr3 nanocrystals (NCs) is explained for the first time. Stokes shifts range from 82 to 20 meV for NCs with effective edge lengths varying from ∼4 to 13 nm. We show that the Stokes shift is intrinsic to the NC electronic structure and does not arise from extrinsic effects such as residual ensemble size distributions, impurities, or solvent-related effects. The origin of the Stokes shift is elucidated via first-principles calculations. Corresponding theoretical modeling of the CsPbBr3 NC density of states and band structure reveals the existence of an intrinsic confined hole state 260 to 70 meV above the valence band edge state for NCs with edge lengths from ∼2 to 5 nm. A size-dependent Stokes shift is therefore predicted and is in quantitative agreement with the experimental data. Comparison between bulk and NC calculations shows that the confined hole state is exclusive to NCs. At a broader level, the distinction between absorbing and emitting states in...

Gaussian-Based Coupled-Cluster Theory for the Ground-State and Band Structure of Solids.

Abstract: We present the results of Gaussian-based ground-state and excited-state equation-of-motion coupled-cluster theory with single and double excitations for three-dimensional solids. We focus on diamond and silicon, which are paradigmatic covalent semiconductors. In addition to ground-state properties (the lattice constant, bulk modulus, and cohesive energy), we compute the quasiparticle band structure and band gap. We sample the Brillouin zone with up to 64 k-points using norm-conserving pseudopotentials and polarized double- and triple-ζ basis sets, leading to canonical coupled-cluster calculations with as many as 256 electrons in 2176 orbitals.

Epitaxial Growth and Band Structure of Te Film on Graphene

Abstract: Tellurium (Te) films with monolayer and few-layer thickness are obtained by molecular beam epitaxy on a graphene/6H-SiC(0001) substrate and investigated by in situ scanning tunneling microscopy and spectroscopy (STM/STS). We reveal that the Te films are composed of parallel-arranged helical Te chains flat-lying on the graphene surface, exposing the (1 × 1) facet of (1010) of the bulk crystal. The band gap of Te films increases monotonically with decreasing thickness, reaching the near-infrared band for the monolayer Te. An explicit band bending at the edge between the monolayer Te and graphene substrate is visualized. With the thickness controlled in the atomic scale, Te films show potential applications of electronics and optoelectronics.

Femtosecond electron-phonon lock-in by photoemission and x-ray free-electron laser.

Abstract: The interactions that lead to the emergence of superconductivity in iron-based materials remain a subject of debate. It has been suggested that electron-electron correlations enhance electron-phonon coupling in iron selenide (FeSe) and related pnictides, but direct experimental verification has been lacking. Here we show that the electron-phonon coupling strength in FeSe can be quantified by combining two time-domain experiments into a “coherent lock-in” measurement in the terahertz regime. X-ray diffraction tracks the light-induced femtosecond coherent lattice motion at a single phonon frequency, and photoemission monitors the subsequent coherent changes in the electronic band structure. Comparison with theory reveals a strong enhancement of the coupling strength in FeSe owing to correlation effects. Given that the electron-phonon coupling affects superconductivity exponentially, this enhancement highlights the importance of the cooperative interplay between electron-electron and electron-phonon interactions.

HfSe 2 and ZrSe 2 : Two-dimensional semiconductors with native high-κ oxides.

Abstract: The success of silicon as a dominant semiconductor technology has been enabled by its moderate band gap (1.1 eV), permitting low-voltage operation at reduced leakage current, and the existence of SiO2 as a high-quality "native" insulator. In contrast, other mainstream semiconductors lack stable oxides and must rely on deposited insulators, presenting numerous compatibility challenges. We demonstrate that layered two-dimensional (2D) semiconductors HfSe2 and ZrSe2 have band gaps of 0.9 to 1.2 eV (bulk to monolayer) and technologically desirable "high-κ" native dielectrics HfO2 and ZrO2, respectively. We use spectroscopic and computational studies to elucidate their electronic band structure and then fabricate air-stable transistors down to three-layer thickness with careful processing and dielectric encapsulation. Electronic measurements reveal promising performance (on/off ratio > 106; on current, ~30 μA/μm), with native oxides reducing the effects of interfacial traps. These are the first 2D materials to demonstrate technologically relevant properties of silicon, in addition to unique compatibility with high-κ dielectrics, and scaling benefits from their atomically thin nature.

Interlayer Excitons and Band Alignment in MoS2/hBN/WSe2 van der Waals Heterostructures

Abstract: van der Waals heterostructures (vdWH) are ideal systems for exploring light–matter interactions at the atomic scale. In particular, structures with a type-II band alignment can yield detailed insight into carrier-photon conversion processes, which are central to, for example, solar cells and light-emitting diodes. An important first step in describing such processes is to obtain the energies of the interlayer exciton states existing at the interface. Here we present a general first-principles method to compute the electronic quasi-particle (QP) band structure and excitonic binding energies of incommensurate vdWHs. The method combines our quantum electrostatic heterostructure (QEH) model for obtaining the dielectric function with the many-body GW approximation and a generalized 2D Mott–Wannier exciton model. We calculate the level alignment together with intra- and interlayer exciton binding energies of bilayer MoS2/WSe2 with and without intercalated hBN layers, finding excellent agreement with experimenta...

An Empirical, yet Practical Way To Predict the Band Gap in Solids by Using Density Functional Band Structure Calculations

Abstract: Band structure calculations based on density functional theory (DFT) with local or gradient-corrected exchange-correlation potentials are known to severely underestimate the band gap of semiconducting and insulating materials. Alternative approaches have been proposed: from semiempirical setups, such as the so-called DFT+U, to hybrid density functionals using a fraction of nonlocal Fock exchange, to modifications of semilocal density functionals. However, the resulting methods appear to be material dependent and lack theoretical rigor. The rigorous many-body perturbation theory based on GW methods provides accurate results but at a very high computational cost. Hereby, we show that a linear correlation between the electronic band gaps obtained from standard DFT and GW approaches exists for most materials and argue that (1) this is a strong indication that the problem of predicting band gaps from standard DFT calculation arises from the assignment of a physical meaning to the Kohn–Sham energy levels rather...

Boosting the Thermoelectric Performance of (Na,K)-Codoped Polycrystalline SnSe by Synergistic Tailoring of the Band Structure and Atomic-Scale Defect Phonon Scattering

Abstract: We report the high thermoelectric performance of p-type polycrystalline SnSe obtained by the synergistic tailoring of band structures and atomic-scale defect phonon scattering through (Na,K)-codoping. The energy offsets of multiple valence bands in SnSe are decreased after Na doping and further reduced by (Na,K)-codoping, resulting in an enhancement in the Seebeck coefficient and an increase in the power factor to 492 μW m–1 K–2. The lattice thermal conductivity of polycrystalline SnSe is decreased by the introduction of effective phonon scattering centers, such as point defects and antiphase boundaries. The lattice thermal conductivity of the material is reduced to values as low as 0.29 W m–1 K–1 at 773 K, whereas ZT is increased from 0.3 for 1% Na-doped SnSe to 1.2 for 1% (Na,K)-codoped SnSe.

Environmental Screening Effects in 2D Materials: Renormalization of the Bandgap, Electronic Structure, and Optical Spectra of Few-Layer Black Phosphorus

Abstract: Few-layer black phosphorus has recently emerged as a promising 2D semiconductor, notable for its widely tunable bandgap, highly anisotropic properties, and theoretically predicted large exciton binding energies. To avoid degradation, it has become common practice to encapsulate black phosphorus devices. It is generally assumed that this encapsulation does not qualitatively affect their optical properties. Here, we show that the contrary is true. We have performed ab initio GW and GW plus Bethe–Salpeter equation (GW-BSE) calculations to determine the quasiparticle (QP) band structure and optical spectrum of one-layer (1L) through four-layer (4L) black phosphorus, with and without encapsulation between hexagonal boron nitride and sapphire. We show that black phosphorus is exceptionally sensitive to environmental screening. Encapsulation reduces the exciton binding energy in 1L by as much as 70% and completely eliminates the presence of a bound exciton in the 4L structure. The reduction in the exciton bindin...

Effective mass and Fermi surface complexity factor from ab initio band structure calculations

Abstract: The effective mass is a convenient descriptor of the electronic band structure used to characterize the density of states and electron transport based on a free electron model. While effective mass is an excellent first-order descriptor in real systems, the exact value can have several definitions, each of which describe a different aspect of electron transport. Here we use Boltzmann transport calculations applied to ab initio band structures to extract a density-of-states effective mass from the Seebeck Coefficient and an inertial mass from the electrical conductivity to characterize the band structure irrespective of the exact scattering mechanism. We identify a Fermi Surface Complexity Factor: $${N}_{{\rm{v}}}^{\ast }{K}^{\ast }$$ from the ratio of these two masses, which in simple cases depends on the number of Fermi surface pockets $$({N}_{{\rm{v}}}^{\ast })$$ and their anisotropy K *, both of which are beneficial to high thermoelectric performance as exemplified by the high values found in PbTe. The Fermi Surface Complexity factor can be used in high-throughput search of promising thermoelectric materials. A simple method for determining a material’s thermoelectric properties is developed by researchers in the United States and Belgium. Jeffrey Snyder from Northwestern University and his co-workers’ model could simplify the search for materials that efficiently generate electricity from waste heat. Even though the environment of an electron in a solid is very complex, the way an electron moves through a solid’s lattice of atoms can be treated as if it is moving in free space. However, because of the influence of its environment an effective mass, not its true mass, is used to model the movement of electrons and that material’s properties. But this effective-mass can be defined in several ways depending on which material property is being modeled. Snyder et al. determine that the ratio of two different effective masses, as computed from different electronic properties, could be a good method to identify novel thermoelectric materials and can be associated with the “complexity” of the electronic structure.

Dynamic band structure tuning of graphene moir\'e superlattices with pressure

Abstract: Heterostructures of atomically-thin materials have attracted significant interest owing to their ability to host novel electronic properties fundamentally distinct from their constituent layers. In the case of graphene on boron nitride, the closely-matched lattices yield a moir\'e superlattice that modifies the graphene electron dispersion and opens gaps both at the primary Dirac point (DP) and the moir\'e-induced secondary Dirac point (SDP) in the valence band. While significant effort has focused on controlling the superlattice period via the rotational stacking order, the role played by the magnitude of the interlayer coupling has received comparatively little attention. Here, we modify the interaction between graphene and boron nitride by tuning their separation with hydrostatic pressure. We observe a dramatic enhancement of the DP gap with increasing pressure, but little change in the SDP gap. Our surprising results identify the critical role played by atomic-scale structural deformations of the graphene lattice and reveal new opportunities for band structure engineering in van der Waals heterostructures.

Integrating band structure engineering with all-scale hierarchical structuring for high thermoelectric performance in PbTe system

Abstract: PbTe1−xSex-2%Na-y%SrTe system is investigated and a high maximum ZT of 2.3 at 923 K for PbTe0.85Se0.15-2%Na-4%SrTe is reported. This is achieved by performing electronic band structures modifications as well as all-scale hierarchical structuring and combining the two effects. It is found that high ZTs in PbTe0.85Se0.15-2%Na-4%SrTe are possible at all temperature from 300 to 873 K with an average ZTave of 1.23. The high performance in PbTe1−xSex-2%Na-y%SrTe can be achieved by either choosing PbTe-2Na-4SrTe or PbTe0.85Se0.15-2Na as a matrix. At room temperature the carrier mobility shows negligible variations as SrTe fraction is increased, however the lattice thermal conductivity is significantly reduced from ≈1.1 to ≈0.82 W m−1 K−1 when 5.0% SrTe is added, correspondingly, the lattice thermal conductivity at 923 K decreases from ≈0.59 to ≈0.43 W m−1 K−1. The power factor maxima of PbTe1−xSex-2Na-4SrTe shift systematically to higher temperature with rising Se fractions due to bands divergence. The maximum power factors reach ≈27, ≈30, ≈31 μW cm−1 K−2 for the x = 0, 0.05, and 0.15 samples peak at 473, 573, and 623 K, respectively. The results indicate that ZT can be increased by synergistic integration of band structure engineering and all-scale hierarchical architectures.

Ultrathin β-tellurium layers grown on highly oriented pyrolytic graphite by molecular-beam epitaxy

Abstract: Monolayer tellurium (Te) or tellurene has been suggested by a recent theory as a new two-dimensional (2D) system with great electronic and optoelectronic promises. Here we present an experimental study of epitaxial Te deposited on highly oriented pyrolytic graphite (HOPG) by molecular-beam epitaxy. Scanning tunneling microscopy of ultrathin layers of Te reveals rectangular surface cells with the cell size consistent with the theoretically predicted β-tellurene, whereas for thicker films, the cell size is more consistent with that of the [100] surface of the bulk Te crystal. Scanning tunneling spectroscopy measurements show that the films are semiconductors with the energy band gaps decreasing with increasing film thickness, and the gap narrowing occurs predominantly at the valence-band maximum (VBM). The latter is understood by strong coupling of states at the VBM but a weak coupling at conduction band minimum (CBM) as revealed by density functional theory calculations.

Band structure evolution during the ultrafast ferromagnetic-paramagnetic phase transition in cobalt.

Abstract: The evolution of the electronic band structure of the simple ferromagnets Fe, Co, and Ni during their well-known ferromagnetic-paramagnetic phase transition has been under debate for decades, with no clear and even contradicting experimental observations so far. Using time- and spin-resolved photoelectron spectroscopy, we can make a movie on how the electronic properties change in real time after excitation with an ultrashort laser pulse. This allows us to monitor large transient changes in the spin-resolved electronic band structure of cobalt for the first time. We show that the loss of magnetization is not only found around the Fermi level, where the states are affected by the laser excitation, but also reaches much deeper into the electronic bands. We find that the ferromagnetic-paramagnetic phase transition cannot be explained by a loss of the exchange splitting of the spin-polarized bands but instead shows rapid band mirroring after the excitation, which is a clear signature of extremely efficient ultrafast magnon generation. Our result helps to understand band structure formation in these seemingly simple ferromagnetic systems and gives first clear evidence of the transient processes relevant to femtosecond demagnetization.

Mapping the electron band structure by intraband high-harmonic generation in solids

Abstract: High-harmonic generation (HHG) has recently been extended to solids, enabling all-optical reconstruction of electron band structure. However, material absorption of above-the-bandgap interband harmonics used for this purpose in earlier work limits the applicability of this promising technique. Here, sub-100-fs mid-infrared pulses tunable within the range of wavelengths from 5.0 to 6.7 μm are used to study HHG in ZnSe. Below-the-bandgap high-order harmonics generated by such driver pulses fall within the transparency range of a solid material, thus removing absorption-related limitations on the depth of HHG. Such harmonics are shown to be ideally suited to probe the nonlinearities of electron bands, enabling an all-optical mapping of the electron band structure in bulk solids.

Probing the Spin-Polarized Electronic Band Structure in Monolayer Transition Metal Dichalcogenides by Optical Spectroscopy

Abstract: We study the electronic band structure in the K/K′ valleys of the Brillouin zone of monolayer WSe2 and MoSe2 by optical reflection and photoluminescence spectroscopy on dual-gated field-effect devices. Our experiment reveals the distinct spin polarization in the conduction bands of these compounds by a systematic study of the doping dependence of the A and B excitonic resonances. Electrons in the highest-energy valence band and the lowest-energy conduction band have antiparallel spins in monolayer WSe2 and parallel spins in monolayer MoSe2. The spin splitting is determined to be hundreds of meV for the valence bands and tens of meV for the conduction bands, which are in good agreement with first-principles calculations. These values also suggest that both n- and p-type WSe2 and MoSe2 can be relevant for spin- and valley-based applications.

Giant tunable Rashba spin splitting in a two-dimensional BiSb monolayer and in BiSb/AlN heterostructures

Abstract: The search for novel two-dimensional giant Rashba semiconductors is a crucial step in the development of the forthcoming nanospintronic technology. Using first-principles calculations, we study a stable two-dimensional crystal phase of BiSb having buckled honeycomb lattice geometry, which is yet unexplored. The phonon, room temperature molecular dynamics, and elastic constant calculations verify the dynamical and mechanical stability of the monolayer at 0 K and at room temperature. The calculated electronic band structure reveals the direct band gap semiconducting nature of a BiSb monolayer with the presence of a highly mobile two-dimensional electron gas (2DEG) near the Fermi level. Inclusion of spin-orbit coupling yields the giant Rashba spin-splitting of a 2DEG near the Fermi level. The calculated Rashba energy and Rashba splitting constant are 13 meV and 2.3 eV\AA{}, respectively, which are amongst the largest yet known Rashba spin splitting parameters in 2D materials. We demonstrate that the strength of the Rashba spin splitting can be significantly tuned by applying in-plane biaxial strain on the BiSb monolayer. The presence of the giant Rashba spin splitting together with the large electronic band gap (1.6 eV) makes this system of peculiar interest for optoelectronics applications. Furthermore, we study the electronic properties of BiSb/AlN heterostructures having a lattice mismatch of 1.3% at the interface. Our results suggest that a BiSb monolayer and BiSb/AlN heterostructure systems could be potentially used to develop highly efficient spin field-effect transistors, optoelectronics, and nanospintronic devices. Thus, this comprehensive study of two-dimensional BiSb systems can expand the range of possible applications in future spintronic technology.

Excitonic and Polaronic Properties of 2D Hybrid Organic–Inorganic Perovskites

Abstract: We theoretically characterize the unusual white-light emission properties of two-dimensional (2D) hybrid organic–inorganic perovskites with an APbX4 structure (where A is a bidentate organic cation and X = Cl, Br). In addition to band structure calculations including corrections due to spin–orbit couplings and electron–hole interactions, a computationally intensive molecular cluster approach is exploited to describe the excitonic and polaronic properties of these 2D perovskites at the atomistic level. Upon adding or removing an electron from the neutral systems, we find that strongly localized small polarons form in the 2D clusters. The polaron charge density is distributed over just ∼1.5 lattice sites, which is consistent with the calculated large polaron binding energies, on the order of ∼0.4–1.2 eV.

Dark excitons in transition metal dichalcogenides

Abstract: Monolayer transition metal dichalcogenides (TMDs) exhibit a remarkably strong Coulomb interaction that manifests in tightly bound excitons. Due to the complex electronic band structure exhibiting several spin-split valleys in the conduction and valence band, dark excitonic states can be formed. They are inaccessibly by light due to the required spin-flip and/or momentum transfer. The relative position of these dark states with respect to the optically accessible bright excitons has a crucial impact on the emission efficiency of these materials and thus on their technological potential. Based on the solution of the Wannier equation, we present the excitonic landscape of the most studied TMD materials including the spectral position of momentum- and spin-forbidden excitonic states. We show that the knowledge of the electronic dispersion does not allow to conclude about the nature of the material's band gap, since excitonic effects can give rise to significant changes. Furthermore, we reveal that an exponentially reduced photoluminescence yield does not necessarily reflect a transition from a direct to a non-direct gap material, but can be ascribed in most cases to a change of the relative spectral distance between bright and dark excitonic states.