Determination of formation and ionization energies of charged defects in two-dimensional materials

Vacancy Defects in 2D Ferroelectric In2Se3 and the Conductivity Modulation by Polarization-Defect Coupling

α-In2Se3 is a promising two-dimensional (2D) ferroelectric semiconductor with unique phase transition behaviors and intrinsic n-type conductivity. However, the origin of this conductivity and the impact of defects on the phase transition remain unclear. In this study, we employed the WLZ method to calculate vacancies' formation energy and ionization energy in monolayer α-In2Se3 and identified the defect-bound band edge states. Our results reveal a strong polarization-defect coupling effect, where the bottom-layer selenium vacancy drives intrinsic n-type conductivity in the sample with upward polarization while reversing the polarization-induced deep p-type defect. Furthermore, we demonstrate that a vacancy stabilizes the ferroelectric phase and reduces the phase transition rate to the paraelectric phase. Finally, we propose a defect-engineered ferroelectric field-effect transistor model that controls the resistance by leveraging the polarization-defect coupling effect. This work highlights the significant roles of vacancy defects in 2D α-In2Se3, offering strategies to design In2Se3 electronic devices at the nanoscale.

Tunable magnetic and electronic properties of CrS2/VS2 lateral superlattices

Two-dimensional (2D) lateral heterostructures and superlattices, especially those based on transition metal dichalcogenides, boast exceptional properties for electronics, optoelectronics, and photovoltaics. Our study delves into the intricate superlattice architecture of CrS2/VS2, as well as its magnetic and electronic attributes, utilizing the framework of density functional theory. The CrS2/VS2 superlattice, crafted by seamlessly stitching together CrS2 and VS2 monolayers along their armchair interfaces, demonstrates remarkable stability and magnetism. Notably, the magnetic phase transitions exhibited by this superlattice are intricately linked to its overall size and sublattice width. Furthermore, the electronic structures of these CrS2/VS2 superlattices exhibit a strong dependence on the widths of the CrS2 and VS2 ribbons. In smaller superlattices, spin-down electrons establish semiconductor-semiconductor contacts with a distinct type II band alignment. Conversely, spin-up electrons forge metal-metal contacts, facilitating spin-dependent 2D electron segregation. However, in larger superlattices, the electronic states are more constrained, leading to metal-semiconductor contacts that exhibit ohmic conductivity within a single spin channel. This versatility in integrating various magnetic and contact modes fosters multiple structural configurations, ushering in an exciting new paradigm characterized by significant tunability. This advancement holds immense promise for the development and application of multifunctional spintronic devices, offering a wide range of possibilities for future technological innovations.

Exploring Intrinsic and Extrinsic p-Type Dopability of Atomically Thin β-TeO2 from First Principles

Two-dimensional (2D) β-TeO2 has gained attention as a promising material for optoelectronic and power device applications, thanks to its transparency and high hole mobility. However, the mechanisms driving its p-type conductivity and dopability remain elusive. In this study, we investigate the intrinsic and extrinsic point defects in monolayer and bilayer β-TeO2, the latter of which has been experimentally synthesized, using the Heyd-Scuseria-Ernzerhof (HSE) + D3 hybrid functional. Our results reveal that most intrinsic defects are unlikely to contribute to p-type doping in 2D β-TeO2. Moreover, Si and H contamination could further impair p-type conductivity. Since the point defects do not contribute to p-type conductivity, we suggest two possible mechanisms for hole conduction: hopping conduction via localized impurity states, and substrate effects. We also explored substitutional p-type doping in 2D β-TeO2 with 10 trivalent elements. Among these, the Bi dopant is found to exhibit a relatively shallow acceptor transition level. However, all the dopants introduce deep localized states, where hole polarons are trapped by the lone pairs of Te atoms. Interestingly, monolayer β-TeO2 shows potential advantages over bilayers due to reduced self-compensation effects for p-type dopants. These findings provide valuable insights into defect engineering strategies for future electronic applications involving 2D β-TeO2.

Synergy Between Dynamic Behavior of Hydrogen Defects and Non-Radiative Recombination in Metal-Halide Perovskites

In organic-inorganic hybrid perovskite solar cells (PSCs), hydrogen defects introduce deep-level trap states, significantly influencing non-radiative recombination processes. Those defects are primarily observed in MA-PSCs rather than FA-PSCs. As a result, MA-PSCs demonstrated a lower efficiency of 23.6% compared to 26.1% of FA-PSCs. In this work, both hydrogen vacancy (VH -) and hydrogen interstitial (Hi -) defects in MAPbI3 bulk and on surfaces, respectively are investigated. i) Bulk VH - defects have dramatic impact on non-radiative recombination, with lifetime varying from 67 to 8 ns, depending on whether deprotonated MA0 are ion-bonded or not. ii) Surface H-defects exhibited an inherent self-healing mechanism through a chemical bond between MA0 and Pb2+, indicating a self-passivation effect. iii) Both VH - and Hi - defects can be mitigated by alkali cation passivation; while large cations are preferable for VH - passivation, given strong binding energy of cation/perovskite, as well as, weak band edge non-adiabatic couplings; and small cations are suited for Hi - passivation, considering the steric hindrance effect. The dual passivation strategy addressed diverse experimental outcomes, particularly in enhancing performance associated with cation selections. The dynamic connection between hydrogen defects and non-radiative recombination is elucidated, providing insights into hydrogen defect passivation essential for high-performance PSCs fabrication.

Designing Organic Spin-Gapless Semiconductors via Molecular Adsorption on C4N3 Monolayer

Spin-gapless semiconductor (SGS), a class of zero-gap materials with fully spin-polarized electrons and holes, offers significant potential for high-speed, low-energy consumption applications in spintronics, electronics, and optoelectronics. Our first-principles calculations revealed that the Pca21 C4N3 monolayer exhibits a ferromagnetic ground state. Its band structure displays SGS-like characteristics, with the energy gap between the valence and conduction bands near the Fermi level in the spin-down channel much smaller than the one in the other spin channel. To enhance its SGS properties, we introduced electrons into the Pca21 C4N3 monolayer by adsorbing the CO gas molecule on its surface. Stable gas adsorption (CO@C4N3) effectively narrowed the band gap in the spin-down channel without changing the band gap in the spin-up channel obviously. Moreover, injecting holes into the CO@C4N3 system could increase the net magnetic moments and induce an SGS-to-metallic phase transition, while injecting electrons into the CO@C4N3 system is able to lower the net magnetic moments and cause an SGS-to-half-metallic phase transition. Our findings not only underscore a new promising material for practical metal-free spintronics applications but also illustrate a viable pathway for designing SGSs.

The Case for a Defect Genome Initiative

The Materials Genome Initiative (MGI) has streamlined the materials discovery effort by leveraging generic traits of materials, with focus largely on perfect solids. Defects such as impurities and perturbations, however, drive many attractive functional properties of materials. The rich tapestry of charge, spin, and bonding states hosted by defects are not accessible to elements and perfect crystals, and defects can thus be viewed as another class of "elements" that lie beyond the periodic table. Accordingly, a Defect Genome Initiative (DGI) to accelerate functional defect discovery for energy, quantum information, and other applications is proposed. First, major advances made under the MGI are highlighted, followed by a delineation of pathways for accelerating the discovery and design of functional defects under the DGI. Near-term goals for the DGI are suggested. The construction of open defect platforms and design of data-driven functional defects, along with approaches for fabrication and characterization of defects, are discussed. The associated challenges and opportunities are considered and recent advances towards controlled introduction of functional defects at the atomic scale are reviewed. It is hoped this perspective will spur a community-wide interest in undertaking a DGI effort in recognition of the importance of defects in enabling unique functionalities in materials.

First-principles theory of the nitrogen interstitial in hBN: a plausible model for the blue emitter

Color centers in hexagonal boron nitride (hBN) have attracted considerable attention due to their remarkable optical properties enabling robust room temperature photonics and quantum optics applications in the visible spectral range. On the other hand, identification of the microscopic origin of color centers in hBN has turned out to be a great challenge that hinders the in-depth theoretical characterization, on-demand fabrication, and development of integrated photonic devices. This is also true for the blue emitter, which is a result of irradiation damage in hBN, emitting at 436 nm wavelength with desirable properties. Here, we propose the negatively charged nitrogen split interstitial defect in hBN as a plausible microscopic model for the blue emitter. To this end, we carried out a comprehensive first-principles theoretical study of the nitrogen interstitial. We carefully analyzed the accuracy of first-principles methods and showed that the commonly used HSE hybrid exchange-correlation functional fails to describe the electronic structure of this defect. Using the generalized Koopman's theorem, we fine-tuned the functional and obtained a zero-phonon photoluminescence (ZPL) energy in the blue spectral range. We showed that the defect exhibits a high emission rate in the ZPL line and features a characteristic phonon side band that resembles the blue emitter's spectrum. Furthermore, we studied the electric field dependence of the ZPL and numerically showed that the defect exhibits a quadratic Stark shift that is perpendicular to plane electric fields, making the emitter insensitive to electric field fluctuations in the first order. Our work emphasizes the need for assessing the accuracy of common first-principles methods in hBN and exemplifies a workaround methodology. Furthermore, our work is a step towards understanding the structure of the blue emitter and utilizing it in photonics applications.

CO adsorption on MgO thin-films: formation and interaction of surface charged defects

Two-dimensional (2D) materials formed by thin-films of metal oxides that grow on metal supports are commonly used in heterogeneous catalysis and multilayer electronic devices. Despite extensive research on these systems, the effects of charged defects at supported oxides on surface processes are still not clear. In this work, we perform spin-polarized density-functional theory (DFT) calculations to investigate formation and interaction of charged magnesium and oxygen vacancies, and Al dopants on MgO(001)/Ag(001) surface. The results show a sizable interface compressive effect that decreases the metal work function as electrons are added on the MgO surface with a magnesium vacancy. This surface displays a larger formation energy in a water environment (O-rich condition) even with additional Al-doping. Under these conditions, we found that a polar molecule such as CO is more strongly adsorbed on the low-coordination oxygen sites due to a larger contribution of the channeled electronic transport with the silver interface regardless of the surface charge. Therefore, these findings elucidate how surface intrinsic vacancies can influence or contribute to charge transfer, which allows one to explore more specific reactions at different surface topologies for more efficient catalysts for CO2 conversion.

Strong interlayer coupling and unusual antisite defect-mediated p-type conductivity in GePx (x = 1, 2)

As an emerging candidate for anisotropic two-dimensional materials, the group IV-V family (e.g. GeP, GeP₂) has appealing applications in photoelectronics. However, their intrinsic point defect properties, which largely determine the device performance and optimization, are still poorly explored. In our study, through density functional theory (DFT) calculations, antisite defects were affirmed to be dominant with the lowest formation energies in 2D GeP_x semiconductors because of the similar atomic size and electronegativity of elemental components, which is in contrast to previous calculations and experimental speculation. These antisite defects could introduce relatively shallow states within the bandgap in bulk cases. The transition energy levels and electronic structures of defects reveal that Ge_P and P_Ge antisites act as dominant acceptors and donors, respectively. Strong interlayer coupling between anions results in a significant upshift of the valence band maximum (VBM) and shallower acceptor behaviors of GeP_x. Together with the dominant Ge_P antisite defect, the large upshift of the VBM in GeP leads to a remarkable transition of conductivity from intrinsic in the monolayer to p-type in the bulk. Such a synergistic effect in GeP₂ is rather weak due to the strong inherent intralayer coupling of anions. Our research provides deep insights into the strong anion coupling effects on the electronic structures and defect properties of GeP and GeP₂, which sheds light on defect engineering and electronic applications of GeP_x based semiconductors.

Are formation and adsorption energies enough to evaluate the stability of surface-passivated tin-based halide perovskites?

Surface passivation is one of the effective and widely-used strategies to enhance the stability of halide perovskites with reduced surface defects and suppressed hysteresis. Among all existing reports, the formation and adsorption energies are popularly used as the decisive descriptors for screening passivators. Here, we propose that the often-ignored local surface structure should be another critically important factor governing the stability of tin-based perovskites after surface passivation, but has no detrimental effect on the stability of lead-based perovskites. It is verified that poor surface structure stability and deformation of the chemical bonding framework of Sn-I caused by surface passivation are ascribed to the weakened Sn-I bond strength and facilitated formation of surface iodine vacancy (V_I). Therefore, the surface structure stability represented by the formation energy of V_I and Sn-I bond strength should be used to accurately screen preferred surface passivators of tin-based perovskites.

Electronic and magnetic properties of charged point defects in monolayer CrI3

The two-dimensional magnetic material CrI₃ has gained considerable attention owing to its promising applications in photoelectric and spin-related devices. Recently, various structural defects in CrI₃ have been identified; however, the charge states of these defects have been mainly ignored. Here, we report on an investigation of the charged defects in monolayer CrI₃, focused on the electronic and magnetic properties of the five most stable point defects using first-principles calculations. For positively charged I vacancies and negatively charged Cr vacancies, a blue- and red-shift of defect states near the Fermi level can be observed because of the atom relaxation. Our results also indicate that, among the five defects, the Cr interstitial defect has the smallest ionization energy of 0.34 eV, which makes its ionization easiest. Furthermore, a 0.2 μ_B enhancement of the magnetic moment on the nearest Cr atom can be found for the I vacancy and Cr interstitial defect. The investigation contributes to the atomic-scale comparison and understanding of the charged defects of monolayer CrI₃.

Electronic state evolution of oxygen-doped monolayer WSe2 assisted by femtosecond laser irradiation

Electronic states are significantly correlated with chemical compositions, and the information related to these factors is especially crucial for the manipulation of the properties of matter. However, this key information is usually verified by after-validation methods, which could not be obtained during material processing, for example, in the field of femtosecond laser direct writing inside materials. Here, critical evolution stages of electronic states for monolayer tungsten diselenide (WSe₂) around the modification threshold (at a Mott density of ∼10¹³ cm^-2) are observed by broadband femtosecond transient absorption spectroscopy, which is associated with the intense femtosecond-laser-assisted oxygen-doping mechanism. First-principles calculations and control experiments on graphene-covered monolayer WSe₂ further confirm this modification mechanism. Our findings reveal a photochemical reaction for monolayer WSe₂ under the Mott density condition and provide an electronic state criterion to in situ monitor the degrees of modification in monolayer transition metal dichalcogenides during the femtosecond laser modification.

Recent Progress in Double-Layer Honeycomb Structure: A New Type of Two-Dimensional Material

Two-dimensional (2D) materials are no doubt the most widely studied nanomaterials in the past decade. Most recently, a new type of 2D material named the double-layer honeycomb (DLHC) structure opened a door to achieving a series of 2D materials from traditional semiconductors. However, as a newly developed material, there still lacks a timely understanding of its structure, property, applications, and underlying mechanisms. In this review, we discuss the structural stability and experimental validation of this 2D material, and systematically summarize the properties and applications including the electronic structures, topological properties, optical properties, defect engineering, and heterojunctions. It was concluded that the DLHC can be a universal configuration applying to III-V, II-VI, and I-VII semiconductors. Moreover, these DLHC materials indeed have exotic properties such as being excitonic/topological insulators. The successful fabrication of DLHC materials further demonstrates it is a promising topic. Finally, we summarize several issues to be addressed in the future, including further experimental validation, defect engineering, heterojunction engineering, and strain engineering. We hope this review can help the community to better understand the DLHC materials timely and inspire their applications in the future.

Wafer-scale high aspect-ratio sapphire periodic nanostructures fabricated by self-modulated femtosecond laser hybrid technology

Sapphire nanostructures with a high aspect-ratio have broad applications in photoelectronic devices, which are difficult to be fabricated due to the properties of high transparency and hardness, remarkable thermal and chemical stability. Although the phenomenon of laser-induced periodic surface structures (LIPSS) provides an extraordinary idea for surface nanotexturing, it suffers from the limitation of the small depth of the nanostructures. Here, a high-efficiency self-modulated femtosecond laser hybrid technology was proposed to fabricate nanostructures with high aspect-ratios on the sapphire surface, which was combined backside laser modification and subsequent wet etching. Due to the refractive index mismatch, the focal length of the laser could be elongated when focused inside sapphire. Thus, periodic nanostructures with high-quality aspect ratios of more than 55 were prepared on the sapphire surface by using this hybrid fabrication method. As a proof-of-concept, wafer-scale (∼2 inches) periodic nanostripes with a high aspect-ratio were realized on a sapphire surface, which possesses unique diffractive properties compared to typical shallow gratings. The results indicate that the self-modulated femtosecond laser hybrid technology is an efficient and versatile technique for producing high aspect-ratio nanostructures on hard and transparent materials, which would propel the potential applications in optics and surface engineering, sensing, etc.

Modulation doping and charge density wave transition in layered PbSe-VSe2 ferecrystal heterostructures

Controlling charge carrier concentrations remains a major challenge in the application of quasi-two-dimensional materials. A promising approach is the modulation doping of transport channels via charge transfer from neighboring layers in stacked heterostructures. Ferecrystals, which are metastable layered structures created from artificial elemental precursors, are a perfect model system to investigate modulation doping, as they offer unparalleled freedom in the combination of different constituents and variable layering sequences. In this work, differently stacked combinations of rock-salt structured PbSe and VSe₂ were investigated using X-ray photoelectron spectroscopy. The PbSe layers act as electron donors in all heterostructures, with about 0.1 to 0.3 donated electrons per VSe₂ unit cell. While they initially retain their inherent semiconducting behavior, they themselves become metallic when combined with a larger number of VSe₂ layers, as evidenced by a change of the XPS core level lineshape. Additional analysis of the valence band structure was performed for selected stacking orders at different sample temperatures to investigate a predicted charge density wave (CDW) transition. While there appear to be hints of a gap opening, the data so far is inconclusive and the application of spatially resolved techniques such as scanning tunneling microscopy is encouraged for further studies.

Antisite defect qubits in monolayer transition metal dichalcogenides

Being atomically thin and amenable to external controls, two-dimensional (2D) materials offer a new paradigm for the realization of patterned qubit fabrication and operation at room temperature for quantum information sciences applications. Here we show that the antisite defect in 2D transition metal dichalcogenides (TMDs) can provide a controllable solid-state spin qubit system. Using high-throughput atomistic simulations, we identify several neutral antisite defects in TMDs that lie deep in the bulk band gap and host a paramagnetic triplet ground state. Our in-depth analysis reveals the presence of optical transitions and triplet-singlet intersystem crossing processes for fingerprinting these defect qubits. As an illustrative example, we discuss the initialization and readout principles of an antisite qubit in WS2, which is expected to be stable against interlayer interactions in a multilayer structure for qubit isolation and protection in future qubit-based devices. Our study opens a new pathway for creating scalable, room-temperature spin qubits in 2D TMDs.

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.

Dimensionality-Inhibited Chemical Doping in Two-Dimensional Semiconductors: The Phosphorene and MoS2 from Charge-Correction Method

Despite the great appeal of two-dimensional semiconductors for electronics and optoelectronics, to achieve the required charge carrier concentrations by means of chemical doping remains a challenge due to large defect ionization energies (IEs). Here, by decomposing the defect IEs into three parts based on ionization process, we propose a conceptual picture that the large defect IEs are caused by two effects of reduced dimensionality. While the quantum confinement effect makes the neutral single-electron point defect levels deep, the reduced screening effect leads to high energy cost for the electronic relaxation. The first-principles calculations for black phosphorus and MoS₂ do demonstrate the general trend. Using BP monolayer either embedded into dielectric continuum or encapsulated between two hBN layers, we demonstrate the feasibility of increasing the screening to reduce the defect IEs. Our analysis is expected to help achieve effective carrier doping and open ways toward more extensive applications of 2D semiconductors.

Point Defects in Monolayer h-AlN as Candidates for Single-Photon Emission

A single-photon emission (SPE) system based on a solid state is one of the fundamental branches in quantum information and communication technologies. The traditional bulk semiconductors suffered limitations of difficult photon extraction and long radiative lifetime. Two-dimensional (2D) semiconductors with an entire open structure and low dielectric screening can overcome these shortcomings. In this work, we focus on monolayer h-AlN due to its wide band gap and the successful achievement of SPE compared to its bulk counterpart. We systematically investigate the properties of point defects, including vacancies, antisites, and impurities, in monolayer h-AlN by employing hybrid density functional theory calculations. The -1 charged Al vacancy (V_Al^-) and +1 charged nitrogen antisite (N_Al⁺) are predicted to achieve SPE with the zero-phonon lines of 0.77 and 1.40 eV, respectively. Moreover, the charged point-defect complex C_AlV_N⁺, which is composed of vacancies and carbon substitutions, also can be used for SPE. Our results extend the avenue for realizing SPE in 2D semiconductors.

Modulation Doping: A Strategy for 2D Materials Electronics

It remains a remarkable challenge to develop practical techniques for controllable and nondestructive doping in two-dimensional (2D) materials for their use in electronics and optoelectronics. Here, we propose a modulation doping strategy, wherein the perfect n-/p-type channel layer is achieved by accepting/donating electrons from/to the defects inside an adjacent encapsulation layer. We demonstrate this strategy in the heterostructures of BN/graphene, BN/MoS₂, where the previously believed useless deep defects, such as the nitrogen vacancy in BN, can provide free carriers to the graphene and MoS₂. The carrier density is further modulated by engineering the surroundings of the encapsulation layer. Moreover, the defects and carriers are naturally separated in space, eliminating the effects of Coulomb impurity scattering and thus allowing high mobility in the 2D limit. This doping strategy provides a highly viable route to tune 2D channel materials without inducing any structural damage, paving the way for high-performance 2D nanoelectronic devices.

Type-II lateral SnSe/GeTe heterostructures for solar photovoltaic applications with high efficiency

Recently, lateral heterostructures based on two-dimensional (2D) materials have provided new opportunities for the development of photovoltaic nanodevices. In this work, we propose a novel lateral SnSe/GeTe heterostructure (LHS) with high photovoltaic performance and systematically investigate the structural, electronic and optical properties of the lateral heterostructure by using first-principles calculations. Our results show that this type of heterostructure processes excellent stability due to the small lattice mismatch and formation energy and also covalent bonding at the interface, which is greatly beneficial for the epitaxial growth of heterostructures. These heterostructures are semiconductors with type-II band alignment and their electronic properties can be effectively tuned by the size and composition ratio of the heterostructures. More importantly, it is found that these heterostructures possess high absorption over a wide range of visible light and high power conversion efficiency (up to 22.3%). These extraordinary properties make the SnSe/GeTe lateral heterostructures ideal candidates for photovoltaic applications.

Fully Light-Controlled Memory and Neuromorphic Computation in Layered Black Phosphorus

Imprinting vision as memory is a core attribute of human cognitive learning. Fundamental to artificial intelligence systems are bioinspired neuromorphic vision components for the visible and invisible segments of the electromagnetic spectrum. Realization of a single imaging unit with a combination of in-built memory and signal processing capability is imperative to deploy efficient brain-like vision systems. However, the lack of a platform that can be fully controlled by light without the need to apply alternating polarity electric signals has hampered this technological advance. Here, a neuromorphic imaging element based on a fully light-modulated 2D semiconductor in a simple reconfigurable phototransistor structure is presented. This standalone device exhibits inherent characteristics that enable neuromorphic image pre-processing and recognition. Fundamentally, the unique photoresponse induced by oxidation-related defects in 2D black phosphorus (BP) is exploited to achieve visual memory, wavelength-selective multibit programming, and erasing functions, which allow in-pixel image pre-processing. Furthermore, all-optically driven neuromorphic computation is demonstrated by machine learning to classify numbers and recognize images with an accuracy of over 90%. The devices provide a promising approach toward neurorobotics, human-machine interaction technologies, and scalable bionic systems with visual data storage/buffering and processing.

Many-particle induced band renormalization processes in few- and mono-layer MoS2

Band renormalization effects play a significant role for two-dimensional (2D) materials in designing a device structure and customizing their optoelectronic performance. However, the intrinsic physical mechanism about the influence of these effects cannot be revealed by general steady-state studies. Here, band renormalization effects in organic superacid treated monolayer MoS₂, untreated monolayer MoS₂and few-layer MoS₂are quantitatively analyzed by using broadband femtosecond transient absorption spectroscopy. In comparison with the untreated monolayer, organic superacid treated monolayer MoS₂maintains a direct bandgap structure with two thirds of carriers populated at K valley, even when the initial exciton density is as high as 2.05 × 10¹⁴cm^-2(under 400 nm excitations). While for untreated monolayer and few-layer MoS₂, many-particle induced band renormalizations lead to a stronger imbalance for the carrier population between K and Q valleys inkspace, and the former experiences a direct-to-indirect bandgap transition when the initial exciton density exceeds 5.0 × 10¹³cm^-2(under 400 nm excitations). Those many-particle induced band renormalization processes further suggest a band-structure-controlling method in practical 2D devices.

Iron and oxygen vacancies at the hematite surface: pristine case and with a chlorine adatom

Defect complexes play critical roles in the dynamics of water molecules in photoelectrochemical cell devices. For the specific case of hematite (α-Fe₂O₃), iron and oxygen vacancies are said to mediate the water splitting process through the localization of optically-derived charges. Using first-principles methods based on density-functional theory we show that both iron and oxygen vacancies can be observed at the surface. For an oxygen-rich environment, usually under wet conditions, the charged iron vacancies should be more frequent. As sea water would be an ideal electrolyte for this kind of device, we have also analyzed the effect of additional chlorine adsorption on this surface. While the chlorine adatom kills the charged oxygen vacancies, entering the void sites, it will not react with the iron vacancies, keeping them active during water splitting processes.

Machine Learning-Enabled Design of Point Defects in 2D Materials for Quantum and Neuromorphic Information Processing

Engineered point defects in two-dimensional (2D) materials offer an attractive platform for solid-state devices that exploit tailored optoelectronic, quantum emission, and resistive properties. Naturally occurring defects are also unavoidably important contributors to material properties and performance. The immense variety and complexity of possible defects make it challenging to experimentally control, probe, or understand atomic-scale defect-property relationships. Here, we develop an approach based on deep transfer learning, machine learning, and first-principles calculations to rapidly predict key properties of point defects in 2D materials. We use physics-informed featurization to generate a minimal description of defect structures and present a general picture of defects across materials systems. We identify over one hundred promising, unexplored dopant defect structures in layered metal chalcogenides, hexagonal nitrides, and metal halides. These defects are prime candidates for quantum emission, resistive switching, and neuromorphic computing.

Fermi level dependence of gas-solid oxygen defect exchange mechanism on TiO2 (110) by first-principles calculations

In the same way that gases interact with oxide semiconductor surfaces from above, point defects interact from below. Previous experiments have described defect-surface reactions for TiO₂(110), but an atomistic picture of the mechanism remains unknown. The present work employs computations by density functional theory of the thermodynamic stabilities of metastable states to elucidate possible reaction pathways for oxygen interstitial atoms at TiO₂(110). The simulations uncover unexpected metastable states including dumbbell and split configurations in the surface plane that resemble analogous interstitial species in the deep bulk. Comparison of the energy landscapes involving neutral (unionized) and charged intermediates shows that the Fermi energy E_F exerts a strong influence on the identity of the most likely pathway. The largest elementary-step thermodynamic barrier for interstitial injection trends mostly downward by 2.1 eV as E_F increases between the valence and conduction band edges, while that for annihilation trends upward by 2.1 eV. Several charged intermediates become stabilized for most values of E_F upon receiving conduction band electrons from TiO₂, and the behavior of these species governs much of the overall energy landscape.

Carrier Dynamics and Transfer across the CdS/MoS2 Interface upon Optical Excitation

Carrier dynamics across the interface of heterostructures have important technological, photovoltaic, and catalytic implications. Using first-principles time-dependent density functional theory, we have systematically investigated the charge transfer of excited carriers from CdS to MoS₂ and found that two interdependent mechanisms are responsible for the transfer, one slow and one fast. While the slower process may be attributed to typical electron-phonon coupling, the interfacial dipole resulting from this transfer enables a fast secondary process involving a level crossing of the excited carrier state in CdS with receiving states in MoS₂. An analysis based on the interfacial binding energy reveals that the Cd-terminated (001) interface is by far the most energetically favorable, which in addition to its calculated fast resonant electron transfer suggests it is a good candidate to explain the experimentally observed charge transfer between CdS and MoS₂.

Extrapolated Defect Transition Level in Two-Dimensional Materials: The Case of Charged Native Point Defects in Monolayer Hexagonal Boron Nitride

Defect formation energy as well as the charge transition level (CTL) plays a vital role in understanding the underlying mechanism of the effect of defects on material properties. However, the accurate calculation of charged defects, especially for two-dimensional materials, is still a challenging topic. In this paper, we proposed a simplified scheme to rescale the CTLs from the semilocal to the hybrid functional level, which is time-saving during the charged defect calculations. Based on this method, we systematically calculated the formation energy of four kinds of intrinsic point defects in two-dimensional hexagonal boron nitride (2D h-BN) by uniformly scaling the supercells by which we found a time-saving method to obtain the "special vacuum size" (Komsa, H.-P.; Berseneva, N.; Krasheninnikov, A. V.; Nieminen, R. M. Phys. Rev. X, 2014, 4, 031044). Native defects including nitrogen vacancy (V_N), boron vacancy (V_B), nitrogen atom anti-sited on boron position (N_B), and boron atom anti-sited on nitrogen position (B_N) were calculated. The reliability of our scheme was verified by taking V_N as a probe to conduct the hybrid functional calculation, and the rescaled CTL is within the acceptable error range with the pure HSE results. Based on the results of CTLs, all the native point defects in the h-BN monolayer act as hole or electron trap centers under certain conditions and would suppress the p- or n-type electrical conduction of h-BN-based devices. Our rescale method is also suitable for other materials for defect charge transition level calculations.

Single-photon emitters in hexagonal boron nitride: a review of progress

This report summarizes progress made in understanding properties such as zero-phonon-line energies, emission and absorption polarizations, electron-phonon couplings, strain tuning and hyperfine coupling of single photon emitters in hexagonal boron nitride. The primary aims of this research are to discover the chemical nature of the emitting centres and to facilitate deployment in device applications. Critical analyses of the experimental literature and data interpretation, as well as theoretical approaches used to predict properties, are made. In particular, computational and theoretical limitations and challenges are discussed, with a range of suggestions made to overcome these limitations, striving to achieve realistic predictions concerning the nature of emitting centers. A symbiotic relationship is required in which calculations focus on properties that can easily be measured, whilst experiments deliver results in a form facilitating mass-produced calculations.

Flat Boron: A New Cousin of Graphene

The mechanical exfoliation of graphene from graphite provides the cornerstone for the synthesis of other 2D materials with layered bulk structures, such as hexagonal boron nitride, transition metal dichalcogenides, black phosphorus, and so on. However, the experimental production of 2D flat boron is challenging because bulk boron has very complex spatial structures and a rich variety of chemical properties. Therefore, the realization of 2D flat boron marks a milestone for the synthesis of 2D materials without layered bulk structures. The historical efforts in this field, particularly the most recent experimental progress, such as the growth of 2D flat boron on a metal substrate by chemical vapor deposition and molecular beam epitaxy, or liquid exfoliation from bulk boron, are described.

Two-Level Quantum Systems in Two-Dimensional Materials for Single Photon Emission

Single photon emission (SPE) by a solid-state source requires presence of a distinct two-level quantum system, usually provided by point defects. Here we note that a number of qualities offered by novel, two-dimensional materials, their all-surface openness and optical transparence, tighter quantum confinement, and reduced charge screening-are advantageous for achieving an ideal SPE. On the basis of first-principles calculations and point-group symmetry analysis, a strategy is proposed to design paramagnetic defect complex with reduced symmetry, meeting all the requirements for SPE: its electronic states are well isolated from the host material bands, belong to a majority spin eigenstate, and can be controllably excited by polarized light. The defect complex is thermodynamically stable and appears feasible for experimental realization to serve as an SPE-source, essential for quantum computing, with Re_MoV_S in MoS₂ as one of the most practical candidates.

Non-phase-separated 2D B-C-N alloys via molecule-like carbon doping in 2D BN: atomic structures and optoelectronic properties

Two-dimensional (2D) B-C-N alloys have recently attracted much attention but unfortunately, Chemical Vapor Deposition (CVD) B-C-N alloys typically phase separate. In spite of that, our analysis of the B-C-N alloy fabricated by electron-beam irradiation suggests that non-phase-separated B-C-N may in fact exist with a carbon concentration up to 14 at%. While this analysis points to a new way to overcome the phase-separation in 2D B-C-N, by first-principles calculations, we show that these B-C-N alloys are made of motifs with even numbers of carbon atoms, in particular, dimers or six-fold rings (in a molecule-like form), embedded in a 2D BN network. Moreover, by tuning the carbon concentration, the band gap of the B-C-N alloys can be reduced by 35% from that of BN. Due to a strong overlap of the wavefunctions at the conduction band and valance band edges, the non-phase-separated B-C-N alloys maintain the strong optical absorption of BN.

Electric field analyses on monolayer semiconductors: the example of InSe

Properties of an InSe monolayer under external vertical electric fields.

Native defects and substitutional impurities in two-dimensional monolayer InSe

As a burgeoning two-dimensional (2D) semiconductor, InSe holds unique electronic properties and great promise for novel 2D electronic devices. To advance the development of 2D InSe devices, the exploration of n-type and p-type conductivities of InSe is indispensable. With first-principles calculations, we investigate the properties of native defects and substitutional impurities in monolayer InSe, including formation energies and ionization energies. As the traditional jellium scheme encounters an energy divergence for charged defects in 2D materials, an extrapolation approach is adopted here to obtain convergent energies. We find that In vacancy is a deep acceptor and Se vacancy is an electrically neutral defect. All the studied substitutional dopants at In or Se sites have high ionization energies in the range of 0.41-0.84 eV. However, electrons may transport through the defect-bound band edge states in X_Se (X = Cl, Br, and I) as a potential source of n-type conductivity.

Atomistic Corrective Scheme for Supercell Density Functional Theory Calculations of Charged Defects

A new method to correct formation energies of charged defects obtained by supercell density-functional calculations is presented and applied to bulk, surface, and low-dimensional systems. The method relies on atomistic models and a polarizable force field to describe a material system and its dielectric properties. The polarizable force field is based on a minimal set of fitting parameters, it accounts for the dielectric screening arising from ions and electrons separately, and it can be easily implemented in any software for atomistic molecular dynamics simulations. This work illustrates both technical aspects and applications of the new corrective scheme. The method is tested on systems in vacuo to validate the energy scheme. It is applied to charged defects in the bulk and at the surface of realistic materials to achieve comparison with published results obtained by using available corrective schemes based on continuum electrostatics treatments. Moreover, to demonstrate its generality, the method is used to correct the formation energy obtained by DFT of a singly negatively charged S vacancy in monolayer, bilayer, trilayer and bulk MoS₂.

Slow cooling and efficient extraction of C-exciton hot carriers in MoS2 monolayer

In emerging optoelectronic applications, such as water photolysis, exciton fission and novel photovoltaics involving low-dimensional nanomaterials, hot-carrier relaxation and extraction mechanisms play an indispensable and intriguing role in their photo-electron conversion processes. Two-dimensional transition metal dichalcogenides have attracted much attention in above fields recently; however, insight into the relaxation mechanism of hot electron-hole pairs in the band nesting region denoted as C-excitons, remains elusive. Using MoS₂ monolayers as a model two-dimensional transition metal dichalcogenide system, here we report a slower hot-carrier cooling for C-excitons, in comparison with band-edge excitons. We deduce that this effect arises from the favourable band alignment and transient excited-state Coulomb environment, rather than solely on quantum confinement in two-dimension systems. We identify the screening-sensitive bandgap renormalization for MoS₂ monolayer/graphene heterostructures, and confirm the initial hot-carrier extraction for the C-exciton state with an unprecedented efficiency of 80%, accompanied by a twofold reduction in the exciton binding energy.

Light-matter interaction in atomically thin transition metal dichalcogenides is dominated by excitonic effects and hot-carrier relaxation/extraction mechanisms. Here, the authors report that the C exciton in two-dimensional MoS₂ exhibits a slower hot-carrier cooling than band-edge excitons.

Surface Fe vacancy defects on haematite and their role in light-induced water splitting in artificial photosynthesis

A pathway for water dissociation near a surface Fe vacancy site on a hematite surface with photogenerated holes.

Alloy Engineering of Defect Properties in Semiconductors: Suppression of Deep Levels in Transition-Metal Dichalcogenides

Developing practical approaches to effectively reduce the amount of deep defect levels in semiconductors is critical for their use in electronic and optoelectronic devices, but this still remains a very challenging task. In this Letter, we propose that specific alloying can provide an effective means to suppress the deep defect levels in semiconductors while maintaining their basic electronic properties. Specifically, we demonstrate that for transition-metal dichalcogenides, such as MoSe_{2} and WSe_{2}, where anion vacancies are the most abundant defects that can induce deep levels, the deep levels can be effectively suppressed in Mo_{1-x}W_{x}Se_{2} alloys at low W concentrations. This surprising phenomenon is associated with the fact that the band edge energies can be substantially tuned by the global alloy concentration, whereas the defect level is controlled locally by the preferred locations of Se vacancies around W atoms. Our findings illustrate a concept of alloy engineering and provide a promising approach to control the defect properties of semiconductors.