Defect-Induced Modification of Low-Lying Excitons and Valley Selectivity in Monolayer Transition Metal Dichalcogenides

Excitons in metal-halide perovskites from first-principles many-body perturbation theory

Metal-halide perovskites are a structurally, chemically, and electronically diverse class of semiconductors with applications ranging from photovoltaics to radiation detectors and sensors. Understanding neutral electron-hole excitations (excitons) is key for predicting and improving the efficiency of energy-conversion processes in these materials. First-principles calculations have played an important role in this context, allowing for a detailed insight into the formation of excitons in many different types of perovskites. Such calculations have demonstrated that excitons in some perovskites significantly deviate from canonical models due to the chemical and structural heterogeneity of these materials. In this Perspective, I provide an overview of calculations of excitons in metal-halide perovskites using Green's function-based many-body perturbation theory in the GW + Bethe-Salpeter equation approach, the prevalent method for calculating excitons in extended solids. This approach readily considers anisotropic electronic structures and dielectric screening present in many perovskites and important effects, such as spin-orbit coupling. I will show that despite this progress, the complex and diverse electronic structure of these materials and its intricate coupling to pronounced and anharmonic structural dynamics pose challenges that are currently not fully addressed within the GW + Bethe-Salpeter equation approach. I hope that this Perspective serves as an inspiration for further exploring the rich landscape of excitons in metal-halide perovskites and other complex semiconductors and for method development addressing unresolved challenges in the field.

Electrical tuning of quantum light emitters in hBN for free space and telecom optical bands

Quantum light emitters (also known as single photon emitters) are known to be the heart of quantum information technologies. Irrespective of possessing ideal single photon emitter properties, quantum emitters in 2-D hBN defect structures, exhibit constrained quantum light emission within the 300-700 nm range. However, this emission range cannot fully satisfy the needs of an efficient quantum communication applications such as quantum key distribution (QKD), which demands the quantum light emission in fiber optic telecom wavelength bands (from 1260 to 1625 nm) and the free space optical (FSO) (UV-C-solar blind band-100 to 280 nm) wavelength ranges. Hence, there is a necessity to tune the quantum light emission into these two bands. However, the most promising technique to tune the quantum light emitters in hBN here, is still a matter of debate and till date there is no experimental and theoretical assurances. Hence, this work will focus on one of the most promising simple techniques known as Stark electrical tuning of the quantum light emission of hBN defect structures (NBVN, VB, CB, CBVN, CBCN, CBCNCBCN complex, and VBO2). These hBN defects are designed and sandwiched as metal/graphene/hBN defect structure/graphene/metal heterostructure and electrically tuned towards FSO and fiber optic bands (tuning range from UV-C to O-band IR region) region, using constrained DFT computations. The external electric field predicted to yield an atomic bond angle tilt associated with this point defect structure creates out-of-plane dipole moments, enabling the tuning of quantum emission. This electrical tuning technique leads to a simple passive photonic component which enables easier compatibility with quantum circuits and it is found to be one of the perfect alternative solutions, which does not require much external hardware setup to implement as compared to earlier published strain induced tuning experiments.

Photovoltage and Photocurrent Absorption Spectra of Sulfur Vacancies Locally Patterned in Monolayer MoS2

We report on the optical absorption characteristics of selectively positioned sulfur vacancies in monolayer MoS2, as observed by photovoltage and photocurrent experiments in an atomistic vertical tunneling circuit at cryogenic and room temperature. Charge carriers are resonantly photoexcited within the defect states before they tunnel through an hBN tunneling barrier to a graphene-based drain contact. Both photovoltage and photocurrent characteristics confirm the optical absorption spectrum as derived from ab initio GW and Bethe-Salpeter equation approximations. Our results reveal the potential of single-vacancy tunneling devices as atomic-scale photodiodes.

Accelerating GW Calculations of Point Defects with the Defect-Patched Screening Approximation

The GW approximation has been widely accepted as an ab initio tool for calculating defect levels with the many-electron effect included. However, the GW simulation cost increases dramatically with the system size, and unfortunately, large supercells are often required to model low-density defects that are experimentally relevant. In this work, we propose to accelerate GW calculations of point defects by reducing the simulation cost of many-electron screening, which is the primary computational bottleneck. The random-phase approximation of many-electron screening is divided into two parts: one is the intrinsic screening, calculated using a unit cell of pristine structures, and the other is the defect-induced screening, calculated using the supercell within a small energy window. Depending on specific defects, one may only need to consider the intrinsic screening or include the defect contribution. This approach avoids the summation of many conduction states of supercells and significantly reduces the simulation cost. We have applied it to calculate various point defects, including neutral and charged defects in two-dimensional and bulk systems with small or large bandgaps. The results are consistent with those from the direct GW simulations. This defect-patched screening approach not only clarifies the roles of defects in many-electron screening but also paves the way to fast screen defect structures/materials for novel applications, including single-photon sources, quantum qubits, and quantum sensors.

Tunable Interlayer Delocalization of Excitons in Layered Organic-Inorganic Halide Perovskites

Layered organic-inorganic halide perovskites exhibit remarkable structural and chemical diversity and hold great promise for optoelectronic devices. In these materials, excitons are thought to be strongly confined within the inorganic metal halide layers with interlayer coupling generally suppressed by the organic cations. Here, we present an in-depth study of the energy and spatial distribution of the lowest-energy excitons in layered organic-inorganic halide perovskites from first-principles many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation. We find that the quasiparticle band structures, linear absorption spectra, and exciton binding energies depend strongly on the distance and the alignment of adjacent metal halide perovskite layers. Furthermore, we show that exciton delocalization can be modulated by tuning the interlayer distance and alignment, both parameters determined by the chemical composition and size of the organic cations. Our calculations establish the general intuition needed to engineer excitonic properties in novel halide perovskite nanostructures.

Revealing broken valley symmetry of quantum emitters in WSe2 with chiral nanocavities

Single photon emission of quantum emitters (QEs) carrying internal degrees of freedom such as spin and angular momentum plays an important role in quantum optics. Recently, QEs in two-dimensional semiconductors have attracted great interest as promising quantum light sources. However, whether those QEs are characterized by the same valley physics as delocalized valley excitons is still under debate. Moreover, the potential applications of such QEs still need to be explored. Here we show experimental evidence of valley symmetry breaking for neutral QEs in WSe₂ monolayer by interacting with chiral plasmonic nanocavities. The anomalous magneto-optical behaviour of the coupled QEs suggests that the polarization state of emitted photon is modulated by the chiral nanocavity instead of the valley-dependent optical selection rules. Calculations of cavity quantum electrodynamics further show the absence of intrinsic valley polarization. The cavity-dependent circularly polarized single-photon output also offers a strategy for future applications in chiral quantum optics.

Room-Temperature Photoluminescence Mediated by Sulfur Vacancies in 2D Molybdenum Disulfide

Atomic defects in monolayer transition metal dichalcogenides (TMDs) such as chalcogen vacancies significantly affect their properties. In this work, we provide a reproducible and facile strategy to rationally induce chalcogen vacancies in monolayer MoS₂ by annealing at 600 °C in an argon/hydrogen (95%/5%) atmosphere. Synchrotron X-ray photoelectron spectroscopy shows that a Mo 3d_5/2 core peak at 230.1 eV emerges in the annealed MoS₂ associated with nonstoichiometric MoS_x (0 < x < 2), and Raman spectroscopy shows an enhancement of the ∼380 cm^-1 peak that is attributed to sulfur vacancies. At sulfur vacancy densities of ∼1.8 × 10¹⁴ cm^-2, we observe a defect peak at ∼1.72 eV (referred to as LX_D) at room temperature in the photoluminescence (PL) spectrum. The LX_D peak is attributed to excitons trapped at defect-induced in-gap states and is typically observed only at low temperatures (≤77 K). Time-resolved PL measurements reveal that the lifetime of defect-mediated LX_D emission is longer than that of band edge excitons, both at room and low temperatures (∼2.44 ns at 8 K). The LX_D peak can be suppressed by annealing the defective MoS₂ in sulfur vapor, which indicates that it is possible to passivate the vacancies. Our results provide insights into how excitonic and defect-mediated PL emissions in MoS₂ are influenced by sulfur vacancies at room and low temperatures.

Exchange-Driven Intermixing of Bulk and Topological Surface States by Chiral Excitons in Bi_{2}Se_{3}

Topological surface states (TSS) in the prototypical topological insulator (TI) Bi_{2}Se_{3} are frequently characterized using optical probes, but electron-hole interactions and their effect on surface localization and optical response of the TSS remain unexplored. Here, we use ab initio calculations to understand excitonic effects in the bulk and surface of Bi_{2}Se_{3}. We identify multiple series of chiral excitons that exhibit both bulk and TSS character, due to exchange-driven mixing. Our results address fundamental questions about the degree to which electron-hole interactions can relax the topological protection of surface states and dipole selection rules for circularly polarized light in TIs by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light.

Diffusion of Excitons in a Two-Dimensional Fermi Sea of Free Charges

Propagation of light-emitting quasiparticles is of central importance across the fields of condensed matter physics and nanomaterials science. We experimentally demonstrate diffusion of excitons in the presence of a continuously tunable Fermi sea of free charge carriers in a monolayer semiconductor. Light emission from tightly bound exciton states in electrically gated WSe₂ monolayer is detected using spatially and temporally resolved microscopy. The measurements reveal a nonmonotonic dependence of the exciton diffusion coefficient on the charge carrier density in both electron and hole doped regimes. Supported by analytical theory describing exciton-carrier interactions in a dissipative system, we identify distinct regimes of elastic scattering and quasiparticle formation determining exciton diffusion. The crossover region exhibits a highly unusual behavior of an increasing diffusion coefficient with increasing carrier densities. Temperature-dependent diffusion measurements further reveal characteristic signatures of freely propagating excitonic complexes dressed by free charges with effective mobilities up to 3 × 10³ cm²/(V s).

Abnormal behavior of preferred formation of the cationic vacancies from the interior in a γ-GeSe monolayer with the stereo-chemical antibonding lone-pair state

Two-dimensional (2D) materials tend to have the preferable formation of vacancies at the outer surface. Here, contrary to the normal notion, we reveal a type of vacancy that thermodynamically initiates from the interior part of the 2D backbone of germanium selenide (γ-GeSe). Interestingly, the Ge-vacancy (V_Ge) in the interior part of γ-GeSe possesses the lowest formation energy amongst the various types of defects considered. We also find a low diffusion barrier (1.04 eV) of V_Ge, which is half of those of sulfur vacancies in MoS₂. The facile formation of mobile V_Ge is rooted in the antibonding coupling of the lone-pair Ge 4s and Se 4p states near the valence band maximum, which also exists in other gamma-phase MX (M = Sn, Ge; X = S, Te). The V_Ge is accompanied by a shallow acceptor level in the band gap and induces strong infrared light absorption and p-type conductivity. The V_Ge located in the middle cationic Ge sublattice is well protected by the surface Se layers - a feature that is absent in other atomically thin materials. Our work suggests that the unique well-buried inner V_Ge, with the potential of forming structurally protected ultrathin conducting filaments, may render the GeSe layer an ideal platform for quantum emitting, memristive, and neuromorphic applications.

Exciton dispersion and exciton-phonon interaction in solids by time-dependent density functional theory

Understanding, predicting, and ultimately controlling exciton band structure and exciton dynamics are central to diverse chemical and materials problems. Here, we have developed a first-principles method to determine exciton dispersion and exciton-phonon interaction in semiconducting and insulating solids based on time-dependent density functional theory. The first-principles method is formulated in planewave bases and pseudopotentials and can be used to compute exciton band structures, exciton charge density, ionic forces, the non-adiabatic coupling matrix between excitonic states, and the exciton-phonon coupling matrix. Based on the spinor formulation, the method enables self-consistent noncollinear calculations to capture spin-orbital coupling. Hybrid exchange-correlation functionals are incorporated to deal with long-range electron-hole interactions in solids. A sub-Hilbert space approximation is introduced to reduce the computational cost without loss of accuracy. For validations, we have applied the method to compute the exciton band structure and exciton-phonon coupling strength in transition metal dichalcogenide monolayers; both agree very well with the previous GW-Bethe-Salpeter equation and experimental results. This development paves the way for accurate determinations of exciton dynamics in a wide range of solid-state materials.

Solvent-Type Passivation Strategy Controls Solid-State Self-Quenching-Resistant Behavior in Sulfur Dots

Treatment of sulfur dots with polyethylene glycol (PEG) has been an efficient way to achieve a high luminescence quantum yield, and such a PEG-related quantum dot (QD)-synthesis strategy has been well documented. However, the polymeric insulating capping layer acting as the "thick shell" will significantly slow down the electron-transfer efficiency and severely hamper its practical application in an optoelectric field. Especially, the employment of synthetic polymers with long alkyl chains or large molecular weights may lead to structural complexity or even unexpected changes of physical characteristics for QDs. Therefore, in sulfur dot preparation, it is a breakthrough to use short-chain molecular species to replace PEG for better control and reproducibility. In this article, a solvent-type passivation (STP) strategy has been reported, and no PEG or any other capping agent is required. The main role of the solvent, ethanol, is to directly react with NaOH, and the generated sodium ethoxide passivates the surface defects. The afforded STP-enhanced emission sulfur dots (STPEE-SDs) possess not only the self-quenching-resistant feature in the solid state but also the extension of fluorescence band toward the wavelength as long as 645 nm. The realization of sulfur dot emission in the deep-red region with a decent yield (8.7%) has never been reported. Moreover, a super large Stokes shift (300 nm, λ_ex = 345 nm, λ_em = 645 nm) and a much longer decay lifetime (109 μs) have been found, and such values can facilitate to suppress the negative influence from background signals. Density functional theory demonstrates that the surface passivation via sodium ethoxide is dynamically favorable, and the spectroscopic insights into emission behavior could be derived from the passivation effect of the sulfur vacancy as well as the charge-transfer process dominated by the highly electronegative ethoxide layer.

Strain tunable quantum emission from atomic defects in hexagonal boron nitride for telecom-bands

This study presents extending the tunability of 2D hBN Quantum emitters towards telecom (C-band - 1530 to 1560 nm) and UV-C (solar blind - 100 to 280 nm) optical bands using external strain inducements, for long- and short-range quantum communication (Quantum key distribution (QKD)) applications, respectively. Quantum emitters are the basic building blocks of this QKD (quantum communication or information) technologies, which need to emit single photons over room temperature and capable of tuning the emission wavelength to the above necessary range. Recent literature revealed that quantum emitters in 2D hBN only has the ability to withstand at elevated temperatures and aggressive annealing treatments, but density functional theory (DFT) predictions stated that hBN can only emit the single photons from around 290 to 900 nm (UV to near-IR regions) range. So, there is a need to engineer and further tune the emission wavelength of hBN quantum emitters to the above said bands (necessary for efficient QKD implementation). One of the solutions to tune the emission wavelength is by inducing external strain. In this work, we examine the tunability of quantum emission in hBN with point defects by inducing three different normal strains using DFT computations. We obtained the tunability range up to 255 nm and 1589.5 nm, for the point defects viz boron mono vacancies (V_B) and boron mono vacancies with oxygen atoms (V_BO₂) respectively, which can enhance the successful implementation of the efficient QKD. We also examine the tunability of the other defects viz. nitrogen mono vacancies, nitrogen mono vacancy with self-interstitials, nitrogen mono vacancy with carbon interstitials, carbon dimers and boron dangling bonds, which revealed the tunable quantum emission in the visible, other UV and IR spectrum ranges and such customized quantum emission can enhance the birth of other quantum photonic devices.

Ultra-Sensitive Extinction Measurements of Optically Active Defects in Monolayer MoS2

We utilize cavity-enhanced extinction spectroscopy to directly quantify the optical absorption of defects in MoS₂ generated by helium ion bombardment. We achieve hyperspectral imaging of specific defect patterns with a detection limit below 0.01% extinction, corresponding to a detectable defect density below 1 × 10¹¹ cm^-2. The corresponding spectra reveal a broad subgap absorption, being consistent with theoretical predictions related to sulfur vacancy-bound excitons in MoS₂. Our results highlight cavity-enhanced extinction spectroscopy as efficient means for the detection of optical transitions in nanoscale thin films with weak absorption, applicable to a broad range of materials.

Tuning colour centres at a twisted hexagonal boron nitride interface

The colour centre platform holds promise for quantum technologies, and hexagonal boron nitride has attracted attention due to the high brightness and stability, optically addressable spin states and wide wavelength coverage discovered in its emitters. However, its application is hindered by the typically random defect distribution and complex mesoscopic environment. Here, employing cathodoluminescence, we demonstrate on-demand activation and control of colour centre emission at the twisted interface of two hexagonal boron nitride flakes. Further, we show that colour centre emission brightness can be enhanced by two orders of magnitude by tuning the twist angle. Additionally, by applying an external voltage, nearly 100% brightness modulation is achieved. Our ab initio GW and GW plus Bethe-Salpeter equation calculations suggest that the emission is correlated to nitrogen vacancies and that a twist-induced moiré potential facilitates electron-hole recombination. This mechanism is further exploited to draw nanoscale colour centre patterns using electron beams.

Screening of Excitons by Organic Cations in Quasi-Two-Dimensional Organic-Inorganic Lead-Halide Perovskites

Interlayer organic cations in quasi-two-dimensional halide perovskites are a versatile tuning vehicle for the optoelectronic properties of these complex systems, but chemical intuition for this design route is yet to be established. Here, we use density functional theory, the GW approximation, and the Bethe-Salpeter equation approach to understand the contribution of the organic cation to the quasiparticle band gap and exciton binding energy of layered perovskites. We show that organic cations in quasi-two-dimensional perovskites contribute significantly to the dielectric screening in these systems, countering quantum confinement effects on the quasiparticle band gap and the exciton binding energy. Using a simple electrostatics model inspired by parallel-plate capacitors, we decouple the organic cation and inorganic layer contributions to the effective dielectric constants and show that dielectric properties of layered perovskites are broadly tunable via the interlayer cation, providing a direct means of tuning photophysical properties for a variety of applications.

Photoluminescence Induced by Substitutional Nitrogen in Single-Layer Tungsten Disulfide

The electronic and optical properties of two-dimensional materials can be strongly influenced by defects, some of which can find significant implementations, such as controllable doping, prolonged valley lifetime, and single-photon emissions. In this work, we demonstrate that defects created by remote N₂ plasma exposure in single-layer WS₂ can induce a distinct low-energy photoluminescence (PL) peak at 1.59 eV, which is in sharp contrast to that caused by remote Ar plasma. This PL peak has a critical requirement on the N₂ plasma exposure dose, which is strongest for WS₂ with about 2.0% sulfur deficiencies (including substitutions and vacancies) and vanishes at 5.6% or higher sulfur deficiencies. Both experiments and first-principles calculations suggest that this 1.59 eV PL peak is caused by defects related to the sulfur substitutions by nitrogen, even though low-temperature PL measurements also reveal that not all the sulfur vacancies are remedied by the substitutional nitrogen. The distinct low-energy PL peak suggests that the substitutional nitrogen defect in single-layer WS₂ can potentially serve as an isolated artificial atom for creating single-photon emitters, and its intensity can also be used to monitor the doping concentrations of substitutional nitrogen.

Mitigation of Edge and Surface States Effects in Two-Dimensional WS2 for Photocatalytic H2 Generation

Large scale development of the 2D transition metal di-chalcogenides (TMDC) relies on landmark improvement in performance, which could emerge from nanostructuration. Using p-WS₂ nanoflakes with different degrees of exfoliation and fracturing, perspectives were provided to develop high-surface-area 2D p-WS₂ films for the photocatalytic hydrogen generation. The critical role of inter-nanoflakes contacts within high-surface-area 2D films was demonstrated, highlighting the benefit of plane/plane versus edge/plane contacts. Evidence of the high density of surface states displayed by these 2D films was provided through electrochemical measurements. In addition to operating as recombination centers, the surface states were shown to give rise to deleterious Fermi-level pinning (FLP), which dramatically decreased the efficiency of charge carrier separation. Lastly, promising strategies yielding FLP suppression via surface states modification were proposed. In particular, use of a multifunctional ultrathin film displaying healing, catalytic, and n-type semiconduction properties was shown to greatly enhance charge carrier separation and transport to the photo-electrode/electrolyte interface. When the 2D photoelectrodes were fabricated with the above prerequisites (i. e., a high proportion of plane/plane contacts and a successful surface states chemical modification), a photocurrent up to 4.5 mA cm^-2 was achieved for the first time on 2D p-WS₂ photocathodes for hydrogen generation.

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2

The density-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron-hole pairs within a WSe₂ homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron-hole plasma above 7 × 10¹² cm^-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.

Carbon defect qubit in two-dimensional WS2

Identifying and fabricating defect qubits in two-dimensional semiconductors are of great interest in exploring candidates for quantum information and sensing applications. A milestone has been recently achieved by demonstrating that single defect, a carbon atom substituting sulphur atom in single layer tungsten disulphide, can be engineered on demand at atomic size level precision, which holds a promise for a scalable and addressable unit. It is an immediate quest to reveal its potential as a qubit. To this end, we determine its electronic structure and optical properties from first principles. We identify the fingerprint of the neutral charge state of the defect in the scanning tunnelling spectrum. In the neutral defect, the giant spin-orbit coupling mixes the singlet and triplet excited states with resulting in phosphorescence at the telecom band that can be used to read out the spin state, and coherent driving with microwave excitation is also viable. Our results establish a scalable qubit in a two-dimensional material with spin-photon interface at the telecom wavelength region.

Recent work has demonstrated controlled fabrication of single carbon defect spins in the two-dimensional material WS₂. Here, the authors use ab initio methods to determine the electronic and optical properties of this defect, establishing it as a viable qubit candidate operating close to the telecom band.

Tunable band gaps and optical absorption properties of bent MoS2 nanoribbons

The large tunability of band gaps and optical absorptions of armchair MoS₂ nanoribbons of different widths under bending is studied using density functional theory and many-body perturbation GW and Bethe-Salpeter equation approaches. We find that there are three critical bending curvatures, and the non-edge and edge band gaps generally show a non-monotonic trend with bending. The non-degenerate edge gap splits show an oscillating feature with ribbon width n, with a period [Formula: see text], due to quantum confinement effects. The complex strain patterns on the bent nanoribbons control the varying features of band structures and band gaps that result in varying exciton formations and optical properties. The binding energy and the spin singlet-triplet split of the exciton forming the lowest absorption peak generally decrease with bending curvatures. The large tunability of optical properties of bent MoS₂ nanoribbons is promising and will find applications in tunable optoelectronic nanodevices.

Study of photogenerated exciton dissociation in transition metal dichalcogenide van der Waals heterojunction A2-MWS4: a first-principles study

In order to explore the photocatalytic hydrogen production efficiency of the MoS₂/WSe₂ heterostructure (A2-MWS₄) as a photocatalyst, it is highly desirable to study the photogenerated exciton dissociation related to photocatalysis. The electronic properties, optical absorption, and lattice dynamic properties of A2-MWS₄ were investigated using a first-principles approach. The results show that the type II energy band alignment of A2-MWS₄ facilitates the dissociation of photogenerated excitons (electrons and holes). The highly localized d-state electrons of A2-MWS₄ induce the formation of internal potentials that promote the dissociation of photogenerated excitons. The hot carrier diffuses its extra energy into the lattice by scattering with phonons and forms a hot spot in the lattice while releasing phonons, which are dragged away from the hot spot by Ridley decay to promote exciton dissociation. These findings could provide insights for research studies on photochemical reactions and photovoltaic devices.

Computational design of quantum defects in two-dimensional materials

Missing atoms or atom substitutions (point defects) in crystal lattices in two-dimensional (2D) materials are potential hosts for emerging quantum technologies, such as single-photon emitters and spin quantum bits (qubits). First-principles-guided design of quantum defects in 2D materials is paving the way for rational spin qubit discovery. Here we discuss the frontier of first-principles theory development and the challenges in predicting the critical physical properties of point defects in 2D materials for quantum information technology, in particular for optoelectronic and spin-optotronic properties. Strong many-body interactions at reduced dimensionality require advanced electronic structure methods beyond mean-field theory. The great challenges for developing theoretical methods that are appropriate for strongly correlated defect states, as well as general approaches for predicting spin relaxation and the decoherence time of spin defects, are yet to be addressed.

Neutralizing Defect States in MoS2 Monolayers

We report a method to neutralize the mid-gap defect states in MoS₂ monolayers using laser soaking of an organic/transition metal oxide (TMO) blend thin film. The treated MoS₂ monolayer shows negligible emission from defect states as compared to the as-exfoliated MoS₂, accompanied by a photoluminescence quantum yield improvement from 0.018 to 4.5% at excitation power densities of 10 W/cm². The effectiveness of the method toward defect neutralization is governed by the polaron pair generated at the organic/TMO interface, the diffusion of free electrons, and the subsequent formation of TMO radicals at the MoS₂ monolayer. The treated monolayers are stable in air, vacuum, and acetone environments, potentially enabling the fabrication of defect-free optoelectronic devices based on 2D materials and 2D/organic heterojunctions.

Phonon Screening of Excitons in Semiconductors: Halide Perovskites and Beyond

The ab initio Bethe-Salpeter equation (BSE) approach, an established method for the study of excitons in materials, is typically solved in a limit where only static screening from electrons is captured. Here, we generalize this framework to include dynamical screening from phonons at lowest order in the electron-phonon interaction. We apply this generalized BSE approach to a series of inorganic lead halide perovskites, CsPbX_{3}, with X=Cl, Br, and I. We find that inclusion of screening from phonons significantly reduces the computed exciton binding energies of these systems. By deriving a simple expression for phonon screening effects, we reveal general trends for their importance in semiconductors and insulators, based on a hydrogenic exciton model. We demonstrate that the magnitude of the phonon screening correction in isotropic materials can be reliably predicted using four material specific parameters: the reduced effective mass, static and optical dielectric constants, and frequency of the most strongly coupled longitudinal-optical phonon mode. This framework helps to elucidate the importance of phonon screening and its relation to excitonic properties in a broad class of semiconductors.

Moiré excitons in defective van der Waals heterostructures

Defects are inevitable in van der Waals (vdW) materials, but their role on moiré excitons remains unknown. While both moiré and defect potentials can trap excitons in twisted vdW heterostructures, their interplay has not been explored thus far. In this work, we perform first-principles simulations to elucidate the interplay of the defect and moiré potentials in tailoring the excitonic properties in twisted vdW heterostructures. Our work provides an in-depth study of defect-trapped moiré excitons in vdW heterostructures and establishes defect engineering as a promising strategy to tailor optoelectronic responses on demand.

Excitons can be trapped by moiré potentials in van der Waals (vdW) heterostructures, forming ordered arrays of quantum dots. Excitons can also be trapped by defect potentials as single photon emitters. While the moiré and defect potentials in vdW heterostructures have been studied separately, their interplay remains largely unexplored. Here, we perform first-principles calculations to elucidate the interplay of the two potentials in determining the optoelectronic properties of twisted MoS₂/WS₂ heterobilayers. The binding energy, charge density, localization, and hybridization of the moiré excitons can be modulated by the competition and cooperation of the two potentials. Their interplay can also be tuned by vertical electric fields, which can either de-trap the excitons or strongly localize them. One can further tailor the interplay of the two potentials via defect engineering to create one-dimensional exciton lattices with tunable orientations. Our work establishes defect engineering as a promising strategy to realize on-demand optoelectronic responses.

The role of chalcogen vacancies for atomic defect emission in MoS2

For two-dimensional (2D) layered semiconductors, control over atomic defects and understanding of their electronic and optical functionality represent major challenges towards developing a mature semiconductor technology using such materials. Here, we correlate generation, optical spectroscopy, atomic resolution imaging, and ab initio theory of chalcogen vacancies in monolayer MoS2. Chalcogen vacancies are selectively generated by in-vacuo annealing, but also focused ion beam exposure. The defect generation rate, atomic imaging and the optical signatures support this claim. We discriminate the narrow linewidth photoluminescence signatures of vacancies, resulting predominantly from localized defect orbitals, from broad luminescence features in the same spectral range, resulting from adsorbates. Vacancies can be patterned with a precision below 10 nm by ion beams, show single photon emission, and open the possibility for advanced defect engineering of 2D semiconductors at the ultimate scale.

Theory and Ab Initio Calculation of Optically Excited States-Recent Advances in 2D Materials

Recent studies of the optical properties of 2D materials have reported unique phenomena and features that are absent in conventional bulk semiconductors. Many of these interesting properties, such as enhanced light-matter coupling, gate-tunable photoluminescence, and unusual excitonic optical selection rules arise from the nature of the two- and multi-particle excited states such as strongly bound Wannier excitons and charged excitons. The theory, modeling, and ab initio calculations of these optically excited states in 2D materials are reviewed. Several analytical and ab initio approaches are introduced. These methods are compared with each other, revealing their relative strength and limitations. Recent works that apply these methods to a variety of 2D materials and material-defect systems are then highlighted. Understanding of the optically excited states in these systems is relevant not only for fundamental scientific research of electronic excitations and correlations, but also plays an important role in the future development of quantum information science and nano-photonics.

Rational Passivation of Sulfur Vacancy Defects in Two-Dimensional Transition Metal Dichalcogenides

Structural defects vary the optoelectronic properties of monolayer transition metal dichalcogenides, leading to concerted efforts to control defect type and density via materials growth or postgrowth passivation. Here, we explore a simple chemical treatment that allows on-off switching of low-lying, defect-localized exciton states, leading to tunable emission properties. Using steady-state and ultrafast optical spectroscopy, supported by ab initio calculations, we show that passivation of sulfur vacancy defects, which act as exciton traps in monolayer MoS₂ and WS₂, allows for controllable and improved mobilities and an increase in photoluminescence up to 275-fold, more than twice the value achieved by other chemical treatments. Our findings suggest a route for simple and rational defect engineering strategies for tunable and switchable electronic and excitonic properties through passivation.

Flat-Band-Enabled Triplet Excitonic Insulator in a Diatomic Kagome Lattice

The excitonic insulator (EI) state is a strongly correlated many-body ground state, arising from an instability in the band structure toward exciton formation. We show that the flat valence and conduction bands of a semiconducting diatomic Kagome lattice, as exemplified in a superatomic graphene lattice, can possibly conspire to enable an interesting triplet EI state, based on density-functional theory calculations combined with many-body GW and Bethe-Salpeter equation. Our results indicate that massive carriers in flat bands with highly localized electron and hole wave functions significantly reduce the screening and enhance the exchange interaction, leading to an unusually large triplet exciton binding energy (∼1.1  eV) exceeding the GW band gap by ∼0.2  eV and a large singlet-triplet splitting of ∼0.4  eV. Our findings enrich once again the intriguing physics of flat bands and extend the scope of EI materials.

Identification of Point Defects in Atomically Thin Transition-Metal Dichalcogenide Semiconductors as Active Dopants

Selective doping in semiconductors is essential not only for monolithic integrated circuity fabrications but also for tailoring their properties including electronic, optical, and catalytic activities. Such active dopants are essentially point defects in the host lattice. In atomically thin two-dimensional (2D) transition-metal dichalcogenides (TMDCs), the roles of such point defects are particularly critical in addition to their large surface-to-volume ratio, because their bond dissociation energy is relatively weaker, compared to elemental semiconductors. In this Mini Review, we review recent advances in the identifications of diverse point defects in 2D TMDC semiconductors, as active dopants, toward the tunable doping processes, along with the doping methods and mechanisms in literature. In particular, we discuss key issues in identifying such dopants both at the atomic scales and the device scales with selective examples. Fundamental understanding of these point defects can hold promise for tunability doping of atomically thin 2D semiconductor platforms.

Defect-related dynamics of photoexcited carriers in 2D transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDs) exhibit enormous potential in the field of optoelectronics. The high performance of TMD materials and optoelectronic devices significantly depends on processes involved in photoelectric conversion, including photo-excitation, relaxation, transportation, and recombination. Remarkably, inevitable defects in materials prolong or shorten the characteristic time of these processes and even bring about new photoelectric conversion channels, namely, the defect-related relaxation pathways of photoexcited carriers tailor the performance of photoelectric applications. In recent years, there have been numerous investigations in exploring the variant transient signals caused by defects in TMDs utilizing ultrafast spectroscopies. They have the capability in providing an accurate and overall representation of ultrafast processes owing to the subtle temporal resolution. The defect-related mechanisms occurring in different time scales (from femtosecond (fs) to microsecond (μs)) play influential roles throughout the relaxation process of photoexcited species. Herein, we review the defect-related relaxation mechanisms of photoexcited species in TMDs according to the time scale utilizing ultrafast spectroscopy techniques. By interpreting and summarizing the defect-related transient signals, we furnish the direction in material design and performance optimization.

Impact of S-Vacancies on the Charge Injection Barrier at the Electrical Contact with the MoS2 Monolayer

Making electrical contacts to semiconducting transition metal dichalcogenides (TMDCs) represents a major bottleneck for high device performance, often manifesting as strong Fermi level pinning and high contact resistance. Despite intense ongoing research, the mechanism by which lattice defects in TMDCs impact the transport properties across the contact-TMDC interface remains unsettled. Here we study the impact of S-vacancies on the electronic properties at a MoS₂ monolayer interfaced with graphite by photoemission spectroscopy, where the defect density is selectively controlled by Ar sputtering. A clear reduction of the MoS₂ core level and valence band binding energies is observed as the defect density increases. The experimental results are explained in terms of (i) gap states' energy distribution and (ii) S-vacancies' electrostatic disorder effect. Our model indicates that the Fermi level pinning at deep S-vacancy gap states is the origin of the commonly reported large electron injection barrier (∼0.5 eV) at the MoS₂ ML interface with low work function metals. At the contact with high work function electrodes, S-vacancies do not significantly affect the hole injection barrier, which is intrinsically favored by Fermi level pinning at shallow occupied gap states. Our results clarify the importance of S-vacancies and electrostatic disorder in TMDC-based electronic devices, which could lead to strategies for optimizing device performance and production.

Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites' photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

Valley manipulation in monolayer transition metal dichalcogenides and their hybrid systems: status and challenges

Recently, the emerging conceptual valley-related devices have attracted much attention due to the progress on generating, controlling, and detecting the valley degree of freedom in the transition metal dichalcogenide (TMD) monolayers. In general, it is known that achieving valley degree of freedom with long valley lifetime is crucial in the implementation of valleytronic devices. Here, we provide a brief introduction of the basic understandings of valley degree of freedom. We as well review the recent experimental advancement in the modulation of valley degree of freedom. The strategies include optical/magnetic/electric field tuning, moiré patterns, plasmonic metasurface, defects and strain engineering. In addition, we summarize the corresponding mechanisms, which can help to obtain large degree of polarization and long valley lifetimes in monolayer TMDs. Based on these methods, two-dimensional valley-optoelectronic systems based on TMD heterostructures can be constructed, providing opportunities for such as the new paradigm in data processing and transmission. Challenges and perspectives on the development of valleytronics are highlighted as well.

Effect of interfacial defects on the electronic properties of MoS2 based lateral T–H heterophase junctions

Our systematic study shows significant improvement in transport properties of MoS2-based lateral T–H heterophase junctions when interfacial defects are present.

Synergistic vacancy defects and mechanical strain for the modulation of the mechanical, electronic and optical properties of monolayer tungsten disulfide

Monolayer transition metal dichalcogenides (TMDs) are the potential candidate materials in nanoelectronic and optoelectronic applications due to their unique physical and chemical properties. Although both defect and strain greatly alter the structural, physical and chemical properties of monolayer TMDs, the defective monolayer TMDs under applied strain have not been adequately studied. In this paper, the synergistic effects of sulfur vacancy defects and mechanical strain on the mechanical, electronic and optical properties of monolayer tungsten disulfide (WS2) have been systematically studied using first principles density functional theory. The results indicate that the sulfur vacancy formation energy increases linearly with increasing sulfur vacancy concentration under different strains. The strain energy and stress of monolayer WS2 with different sulfur vacancy concentrations increase with increasing applied strain in the strain range of -10% to 10%. The band gap of monolayer WS2 decreases with increasing sulfur vacancy concentration under different strains. Moreover, compared with unstrained conditions, 5% compressive strain increases the band gap at a larger vacancy concentration and the case is just opposite at a smaller vacancy concentration, while 5% tensile strain decreases the band gap. The band gap of monolayer WS2 with different sulfur vacancy concentrations firstly increases and then shrinks with increasing applied strain under compressive strain, whereas it decreases monotonically under tensile strain in the strain range of -10% to 10%. In the visible-light wavelength region, the out-of-plane absorption coefficient under different strains increases with increasing sulfur vacancy concentration. Furthermore, 5% compressive strain enhances the absorption coefficient and 5% tensile strain decreases the absorption coefficient. Hence, the synergistic effects of sulfur vacancy defects and mechanical strain in monolayer TMDs can open new avenues for their applications in nanoelectronic and optoelectronic devices.

Quantum Information and Algorithms for Correlated Quantum Matter

Discoveries in quantum materials, which are characterized by the strongly quantum-mechanical nature of electrons and atoms, have revealed exotic properties that arise from correlations. It is the promise of quantum materials for quantum information science superimposed with the potential of new computational quantum algorithms to discover new quantum materials that inspires this Review. We anticipate that quantum materials to be discovered and developed in the next years will transform the areas of quantum information processing including communication, storage, and computing. Simultaneously, efforts toward developing new quantum algorithmic approaches for quantum simulation and advanced calculation methods for many-body quantum systems enable major advances toward functional quantum materials and their deployment. The advent of quantum computing brings new possibilities for eliminating the exponential complexity that has stymied simulation of correlated quantum systems on high-performance classical computers. Here, we review new algorithms and computational approaches to predict and understand the behavior of correlated quantum matter. The strongly interdisciplinary nature of the topics covered necessitates a common language to integrate ideas from these fields. We aim to provide this common language while weaving together fields across electronic structure theory, quantum electrodynamics, algorithm design, and open quantum systems. Our Review is timely in presenting the state-of-the-art in the field toward algorithms with nonexponential complexity for correlated quantum matter with applications in grand-challenge problems. Looking to the future, at the intersection of quantum information science and algorithms for correlated quantum matter, we envision seminal advances in predicting many-body quantum states and describing excitonic quantum matter and large-scale entangled states, a better understanding of high-temperature superconductivity, and quantifying open quantum system dynamics.

Valley-polarized local excitons in WSe2/WS2 vertical heterostructures

Two-dimensional van der Waals heterostructures (vdWHs) are drawing growing interest in the investigation of their valley polarization properties of localized excitons. However, most of the reported vdWHs were made by micro-mechanical peeling, limiting their large-scale production and practical applications. Furthermore, the circular polarization characters of localized excitons in WSe₂/WS₂ heterostructures remain elusive. Here, a bidirectional-flow physical vapor deposition technique was employed for the synthesis of the WSe₂/WS₂ type-II vertical heterostructures. The interfaces of such heterojunctions are sharp and clean, making the neutral excitons of the constituent layers quenched, which significantly highlights the luminescence of the local excitons. The circular polarization of localized excitons in this WSe₂/WS₂ heterostructure was demonstrated by circularly-polarized PL spectroscopy. The degree of the circular polarization of the localized excitons was determined as 7.17% for σ^- detection and 4.78% for σ⁺ detection. Such local excitons play a critical role in a quantum emitter with enhanced spontaneous emission rate that could lead to the evolution of LEDs. Our observations provide valuable information for the exploration of intriguing excitonic physics and the applications of innovative local exciton devices.

Assessing the role of plasma-engineered acceptor-like intra- and inter-grain boundaries of heterogeneous WS2-WO3 nanosheets for photocurrent characteristics

High-temperature annealing in tungsten disulfide resulted in heterogeneous WS₂-WO₃ in which intra- (within WS₂ and WO₃) and inter- (between WS₂ and WO₃) grain boundaries were observed, which were highly critical for charge transport and recombination. The heterogeneous WS₂-WO₃ phase was evidenced by observing the coexistence of d-spacing values of 0.26 nm (WS₂) and 0.37 nm (WO₃) in transmission electron microscopic (TEM) studies. Further systematic high-resolution TEM studies elucidated that intra-grain boundaries separated crystallites within WS₂ and WO₃, while inter-grain boundaries separated WS₂ from WO₃. As WS₂ and WO₃ are both n-type, these defects are acceptor-like in the grain boundaries and they actively participate in the capture (trapping) process, which impedes charge transport characteristics in the heterogeneous WS₂-WO₃ films. Plasma treatment in the heterogeneous WS₂-WO₃ film, for 60 minutes using argon, energetically modulated the defects in the intra/inter-grain boundaries, as evidenced from detailed comparative photocurrent characteristics obtained individually in (i) pristine WS₂, (ii) heterogeneous WS₂-WO₃ and (iii) Ar plasma-treated heterogeneous WS₂-WO₃ films under blue and green lasers, along with AM1.5 (1 sun) illumination. Detrimental roles (trapping/de-trapping and scattering) of grain boundary states on photoelectrons were seen to be significantly suppressed under the influence of plasma.

Intrinsic and Extrinsic Defect-Related Excitons in TMDCs

We investigate the excitonic peak associated with defects and disorder in low-temperature photoluminescence of monolayer transition metal dichalcogenides (TMDCs). To uncover the intrinsic origin of defect-related (D) excitons, we study their dependence on gate voltage, excitation power, and temperature in a prototypical TMDC monolayer MoS₂. Our results suggest that D excitons are neutral excitons bound to ionized donor levels, likely related to sulfur vacancies, with a density of 7 × 10¹¹ cm^-2. To study the extrinsic contribution to D excitons, we controllably deposit oxygen molecules in situ onto the surface of MoS₂ kept at cryogenic temperature. We find that, in addition to trivial p-doping of 3 × 10¹² cm^-2, oxygen affects the D excitons, likely by functionalizing the defect sites. Combined, our results uncover the origin of D excitons, suggest an approach to track the functionalization of TMDCs, to benchmark device quality, and pave the way toward exciton engineering in hybrid organic-inorganic TMDC devices.

Spin-Valley Locking Effect in Defect States of Monolayer MoS2

Valley pseudospin in two-dimensional (2D) transition-metal dichalcogenides (TMDs) allows optical control of spin-valley polarization and intervalley quantum coherence. Defect states in TMDs give rise to new exciton features and theoretically exhibit spin-valley polarization; however, experimental achievement of this phenomenon remains challenges. Here, we report unambiguous valley pseudospin of defect-bound localized excitons in CVD-grown monolayer MoS₂; enhanced valley Zeeman splitting with an effective g-factor of -6.2 is observed. Our results reveal that all five d-orbitals and the increased effective electron mass contribute to the band shift of defect states, demonstrating a new physics of the magnetic responses of defect-bound localized excitons, strikingly different from that of A excitons. Our work paves the way for the manipulation of the spin-valley degrees of freedom through defects toward valleytronic devices.

Quasi-degenerate states and their dynamics in oxygen deficient reducible metal oxides

The physical and chemical properties of oxides are defined by the presence of oxygen vacancies. Experimentally, non-defective structures are almost impossible to achieve due to synthetic constraints. Therefore, it is crucial to account for vacancies when evaluating the characteristics of these materials. The electronic structure of oxygen-depleted oxides deeply differs from that of the native forms, in particular, of reducible metal oxides, where excess electrons can localize in various distinct positions. In this perspective, we present recent developments from our group describing the complexity of these defective materials that highlight the need for an accurate description of (i) intrinsic vacancies in polar terminations, (ii) multiple geometries and complex electronic structures with several states attainable at typical working conditions, and (iii) the associated dynamics for both vacancy diffusion and the coexistence of more than one electronic structure. All these aspects widen our current understanding of defects in oxides and need to be adequately introduced in emerging high-throughput screening methodologies.

Defect creation in WSe2 with a microsecond photoluminescence lifetime by focused ion beam irradiation

Defect engineering is important for tailoring the electronic and optical properties of two-dimensional materials, and the capability of generating defects of certain types at specific locations is meaningful for potential applications such as optoelectronics and quantum photonics. In this work, atomic defects are created in single-layer WSe2 using focused ion beam (FIB) irradiation, with defect densities spanning many orders of magnitude. The influences of defects are systematically characterized. Raman spectroscopy can only discern defects in WSe2 for a FIB dose higher than 1 × 1013 cm-2, which causes blue shifts of both A'1 and E' modes. Photoluminescence (PL) of WSe2 is more sensitive to defects. At cryogenic temperature, the low-energy PL induced by defects can be revealed, which shows redshifts and broadenings with increased FIB doses. Similar Raman shifts and PL spectrum changes are observed for the WSe2 film grown by chemical vapor deposition (CVD). A four microsecond-long lifetime is observed in the PL dynamics and is three orders of magnitude longer than the often observed delocalized exciton lifetime and becomes more dominant for WSe2 with increasing FIB doses. The ultra-long lifetime of PL in single-layer WSe2 is consistent with first-principles calculation results considering the creation of both chalcogen and metal vacancies by FIB, and can be valuable for photo-catalytic reactions, valleytronics and quantum light emissions owing to the longer carrier separation/manipulation time.

Nonlinear Dark-Field Imaging of One-Dimensional Defects in Monolayer Dichalcogenides

One-dimensional defects in two-dimensional (2D) materials can be particularly damaging because they directly impede the transport of charge, spin, or heat and can introduce a metallic character into otherwise semiconducting systems. Current characterization techniques suffer from low throughput and a destructive nature or limitations in their unambiguous sensitivity at the nanoscale. Here we demonstrate that dark-field second harmonic generation (SHG) microscopy can rapidly, efficiently, and nondestructively probe grain boundaries and edges in monolayer dichalcogenides (i.e., MoSe₂, MoS₂, and WS₂). Dark-field SHG efficiently separates the spatial components of the emitted light and exploits interference effects from crystal domains of different orientations to localize grain boundaries and edges as very bright 1D patterns through a Čerenkov-type SHG emission. The frequency dependence of this emission in MoSe₂ monolayers is explained in terms of plasmon-enhanced SHG related to the defect's metallic character. This new technique for nanometer-scale imaging of the grain structure, domain orientation and localized 1D plasmons in 2D different semiconductors, thus enables more rapid progress toward both applications and fundamental materials discoveries.