Chalcogen Vacancies Research Articles

Role of Chalcogen Defect Introducing Metal-Induced Gap States and Its Implications for Metal-TMDs' Interface Chemistry.

The contact resistance of the transition metal dichalcogenide (TMD) devices is not comparable to that of their silicon counterparts, probably due to a lack of clarity in their interface chemistry. Looking beyond the conventional Schottky-Mott rule, the metal chalcogen orbital overlaps, tunnel barrier, and metal-induced gap states (MIGSs) are crucial factors determining different metals' contact properties with TMDs. Exploring their properties helps TMDs' contact resistance engineering, driven mainly by their orbital overlaps and perturbing parameters. This work presents the interface chemistry of TMDs (MoS2, MoSe2, WS2, and WSe2) with different metals (Au, Cr, Ni, and Pd) in detail using density functional theory computations. Additionally, the work discusses the role of the chalcogen vacancy and interstitial defects in the metal-TMD interactions and corresponding MIGS features. The investigations reveal that Au does not show any significant MIGS due to its weak interactions with all the TMDs. However, other investigated metals have a strong affinity with TMDs, making significant MIGS contributions. All the metals offer n-type doping characteristics to TMDs due to valence charge transfer from the metals toward TMDs. The chalcogen vacancy boosts the orbital overlaps of the TMDs with all the metals. The vacancy reduces metal-TMD interfacial distance, which can be a promising technique to reduce the tunnel barrier and contact resistance. The MIGS and defect-induced gap states (DIGSs) reflect the possibility of Fermi-level pinning in the TMDs' contacts with Cr, Ni, and Pd. Besides, the work discloses that the chalcogen vacancy converts an n-type Pd-TMD interface into p-type due to reverse charge transfer after the vacancy. Chalcogen interstitial impurity also helps with contact resistance engineering for some metal-TMD systems by reducing the bond distance of the metal TMDs. Our study highlights the possibility of defect-assisted and MIGS-based contact engineering at the metal-TMD interfaces.

Charge quenching at defect states in transition metal dichalcogenide–graphene van der Waals heterobilayers

The electronic and spin properties of van der Waals materials are greatly influenced by the atomic features of the interface. Combining $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ methods, model Hamiltonian, and quantum master equation, the authors calculate the electronic transition times and the dynamics of electron population and depopulation of chalcogen vacancies at the interface between graphene and transition metal dichalcogenides. Their findings give insight into the semiclassical dynamics, reveal the interplay between lattice symmetry and spin through the spin-orbit interaction, and examine its impact on the charge quenching.

Tunable magneto-optical properties in MoS2 via defect-induced exciton transitions

The presence of chalcogen vacancies in monolayer transition metal dichalcogenides (TMDs) leads to excitons with mixed localized-delocalized character and to reduced valley selectivity. Recent experimental advances in defect design in TMDs allow for a close examination of such mixed exciton states as a function of their degree of circular polarization under external magnetic fields, revealing strongly varying defect-induced magnetic properties. A theoretical understanding of these observations and their physical origins demands a predictive, structure-sensitive theory. In this work, we study the effect of chalcogen vacancies on the exciton magnetic properties in monolayer ${\mathrm{MoS}}_{2}$. Using many-body perturbation theory, we show how the complex excitonic picture associated with the presence of defects---with reduced valley and spin selectivity due to hybridized electron-hole transitions---leads to a structurally controllable exciton magnetic response. We find a variety of $g$-factors with changing magnitudes and sign depending on the exciton energy and character. Our findings suggest a pathway to tune the nature of the excitons---and by that their magneto-optical properties---through defect architecture.

Optically Active Chalcogen Vacancies in Monolayer Semiconductors

AbstractDefect engineering of atomically thin semiconducting crystals is an attractive route to developing single‐photon sources and valleytronic devices. For these applications, defects with well‐defined optical characteristics need to be generated in a precisely controlled manner. However, defect‐induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS2, WS2, MoSe2, and WSe2 via proton‐beam irradiation. Defect‐induced emissions are found to occur ≈100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high‐excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe‐Salpeter‐equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.

Carrier Multiplication in Transition Metal Dichalcogenides Beyond Threshold Limit.

Carrier multiplication (CM), multiexciton generation by absorbing a single photon, enables disruptive improvements in photovoltaic conversion efficiency. However, energy conservation constrains the threshold energy to at least twice bandgap (2 ). Here, a below threshold limit CM in monolayer transition metal dichalcogenides (TMDCs) is reported. Surprisingly, CM is observed with excitation energy of only 1.75 due to lattice vibrations. Electron-phonon coupling (EPC) results in significant changes in electronic structures, which favors CM. Indeed, the strongest EPC in monolayer MoS2 leads to the most efficient CM among the studied TMDCs. For practical applications, chalcogen vacancies can further lower the threshold by introducing defect states within bandgap. In particular, for monolayer WS2 , CM occurs with excitation energy as low as 1.51 . The results identify TMDCs as attractive candidate materials for efficient optoelectronic devices with the advantages of high photoconductivity and efficientCM.

Vacancy localization effects on MX2 transition-metal dichalcogenides: A systematic ab initio study

Two-dimensional transition-metal dichalcogenide (${\mathrm{MX}}_{2}$) vacancy formation energetics are extensively investigated. Within an ab initio approach, we study the ${\mathrm{MX}}_{2}$ systems, with M = Mo, W, Ni, Pd, and Pt, and X = S, Se, and Te. Here we classify that chalcogen vacancies are always energetically favorable over the transition-metal ones. However, for late transition metals Pd $4d$, and Pt $5d$ the metal vacancies are experimentally achievable, bringing up localized magnetic moments within the semiconducting matrix. By quantifying the localization of the chalcogen vacancy states we show that it rules the intra- and intervacancy interactions that establish both the number of vacancy states neatly lying within the semiconducting gap, as well as its electronic dispersion and spin-orbit coupling splitting. Combining different vacancies and phase variability $1T$ and $1H$ of the explored systems allows us to construct a guiding picture of the vacancy state localization.

Enhanced Emission from Defect Levels in Multilayer MoS2

AbstractRealizing stimulated emission from defects in 2D‐layered semiconductors has the potential to enhance the sensitivity of characterizing their defects. However, stimulated emission from defects in layered materials presents a different set of challenges in carrier lifetime and energy level design and is not achieved so far. Here, photoluminescence (PL) spectroscopy, Raman spectroscopy, and first‐principles theory are combined to reveal an anomalous PL intensity–temperature relation and strong polarization effects at a defect emission peak in annealed multilayer MoS2, suggesting defect‐based stimulated emission. The emergence of stimulated emission behavior is also controllable (by temperature) and reversible. The observed stimulated emission behavior is supported by a three‐level system involving two defect levels from chalcogen vacancies and a pump level from the conduction band edge. First‐principles calculations show that the special indirect gap that enables stimulated emission is not native to pristine bulk MoS2 and only emerges under thermal strain.

Autonomous scanning probe microscopy investigations over WS2 and Au{111}

Individual atomic defects in 2D materials impact their macroscopic functionality. Correlating the interplay is challenging, however, intelligent hyperspectral scanning tunneling spectroscopy (STS) mapping provides a feasible solution to this technically difficult and time consuming problem. Here, dense spectroscopic volume is collected autonomously via Gaussian process regression, where convolutional neural networks are used in tandem for spectral identification. Acquired data enable defect segmentation, and a workflow is provided for machine-driven decision making during experimentation with capability for user customization. We provide a means towards autonomous experimentation for the benefit of both enhanced reproducibility and user-accessibility. Hyperspectral investigations on WS2 sulfur vacancy sites are explored, which is combined with local density of states confirmation on the Au{111} herringbone reconstruction. Chalcogen vacancies, pristine WS2, Au face-centered cubic, and Au hexagonal close-packed regions are examined and detected by machine learning methods to demonstrate the potential of artificial intelligence for hyperspectral STS mapping.

Defects, band bending and ionization rings in MoS2

Chalcogen vacancies in transition metal dichalcogenides are widely acknowledged as both donor dopants and as a source of disorder. The electronic structure of sulphur vacancies in MoS2 however is still controversial, with discrepancies in the literature pertaining to the origin of the in-gap features observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies. Here we use a combination of scanning tunnelling microscopy and STS to study embedded sulphur vacancies in bulk MoS2 crystals. We observe spectroscopic features dispersing in real space and in energy, which we interpret as tip position- and bias-dependent ionization of the sulphur vacancy donor due to tip induced band bending. The observations indicate that care must be taken in interpreting defect spectra as reflecting in-gap density of states, and may explain discrepancies in the literature.

Adsorption of Water Molecules on Pristine and Defective NiPX3 (X: S, Se) Monolayers

AbstractLayered transition metal trichalcogenides MPX3 (M: transition metal; X: S, Se) demonstrate a wide spectrum of properties and are widely proposed as effective materials for the water splitting reactions. Among these materials, NiPX3 are the most promising ones because their electronic structures, band gaps and positions of the valence and conduction bands edges fit to the potentials characteristic for the oxygen and hydrogen evolution reactions. Here, first steps of a detailed theoretical description on the adsorption of water molecules on pristine and defected (chalcogen vacancies) surfaces of NiPX3 are presented and it is shown that in all cases a physisorption takes the place with adsorption energies do not exceeding −650 meV and water dissociative adsorption is unfavorable. This work provides a general description for water molecules interaction with MPX3 and can serve as a basis for further studies on more complicated water/MPX3 reactions.

Density functional theory based HSE06 calculations to probe the effects of defect on electronic properties of monolayer TMDCs

Defects usually play an important role in modifying the properties of the materials, therefore in this investigation an attempt has been made to understand the effect of chalcogen vacancy defect in various monolayer transition metal dichalcogenides (TMDCs). En route to probe the effect of defect, the density functional theory (DFT) calculations have been carried out. The results obtained from DFT calculations reveal that: (i) the nature of both chalcogen and metal significantly influences the dissociation energy, (ii) the work function of defective TMDC alters only marginally for both single or double vacancies, (iii) chalcogen vacancy converts direct gap (KV → KC) semiconductors into indirect gap semiconductors (ΓV → KC), and (iv) electronic bands clearly indicate that the d-orbital of the transition metal plays a vital role in the formation of impurity band.

NO Electroreduction by Transition Metal Dichalcogenides with Chalcogen Vacancies

AbstractNitric oxide electroreduction reaction (NOER) is one of the most attractive routes for ammonia synthesis and NOx‐related pollutant treatment. However, current research on NOER catalysts mainly focus on noble metals and single atom catalysts, while low‐cost transition metal dichalcogenides (TMDs) are rarely considered, let alone their product selectivity. Herein, using first‐principles calculations, we systematically investigate the performance of a series of single‐layer TMDs (MoS2, MoSe2, MoTe2, TaTe2 and WTe2) in the 1T’ phase as electrocatalysts for NOER. Our results reveal that defective 1T’‐MoS2 and 1T’‐MoSe2 monolayers (with the most common sulphur and selenium vacancies, respectively) exhibit excellent activity for NOER as well as high selectivity for ammonia, which can be correspondingly yielded at 0 and −0.06 V potentials, comparable to the best Pt‐based and single atom catalysts. Furthermore, these two catalysts efficiently suppress the competing hydrogen evolution reaction (HER). Thus, single‐layer TMDs synthesized with chalcogen vacancies may serve as efficient catalysts for electrochemical ammonia synthesis from pollutants and electrocatalytic denitrification.

Single-Atom Vacancy Doping in Two-Dimensional Transition Metal Dichalcogenides

ConspectusFaced with the growing quests of higher-performance chips, developing new channel semiconductors immune to short channel effects has become a realistic option for continuing Moore’s Law. With outstanding gate electrostatic capacitance, stable chemical properties, and suitable bandgap, two-dimensional (2D) transition metal dichalcogenides (TMDCs) are considered as potential candidates for next-generation channel materials. However, the practical applications of 2D TMDCs are severely limited by stable, precise, and controllable doping technologies, due to their ultrathin body and dangling bond-free surface. Compared to three-dimensional semiconductors, donors in 2D semiconductors need larger ionization energy which can be attributed to the reduced screening of Coulomb interaction and the larger bandgap induced by quantum confinement. Limited by the ultrathin body of 2D TMDCs and the strong film–substrate charge transfer, typical silicon-based substitutional doping technology encounters some headache difficulties in 2D TMDCs and hardly achieves high-concentration doping. The other two doping technologies also cannot take on this task either; local gate electrostatic doping cannot leave the aid of the external electric field. And surface charge transfer doping of molecule adsorbents behaves unstably (e.g., thermal desorption) or ineffectively modifies the original electronic structure. Fortunately, single-atom vacancies can effectively and precisely adjust the carrier concentration of 2D TMDCs and significantly enhance their conductivity. Therefore, clarifying the work rules and function mechanism of single-atom vacancy doping in 2D TMDCs is beneficial in creating a brand-new optimization strategy of electrical properties and overcoming the technical obstacles of the “lab-to-fab” transition for their practical applications in high-performance electronics and optoelectronics.In this Account, we summarize the state-of-the-art progress in single-atom vacancy doping in 2D TMDCs and highlight the applications in optoelectronic and electronic devices. First, the common defects and the density-largest-defect type in 2D TMDCs are demonstrated through experimental characterizations. Second, the healing and manufacturing strategies of chalcogen vacancies in 2D TMDCs are systematically summarized. Third, we clarify the doping mechanism of single-atom vacancies in 2D TMDCs and its regulation of the electrical properties including carrier concentration and carrier mobility. Fourth, the correlations between chalcogen vacancies in 2D TMDCs and the optical signals from Raman and photoluminescence spectroscopies are established, which will help to quickly and nondestructively evaluate the chalcogen vacancy concentration. Fifth, the current applications of single-atom vacancy doping of 2D TMDCs materials are reviewed, including complementary metal–oxide semiconductor (CMOS) logic inverters, homojunctions, Schottky diodes, and photovoltaic devices. Finally, the potential challenges and future development trends of single-atom vacancy doping for next-generation electronic and optoelectronic devices are pointed out. Overall, this Account guides on controllable and precise doping technologies for researchers in these fields from materials, electronics, and optoelectronics to promote the practical applications of 2D TMDCs.

Adsorption of water on the pristine and defective semiconducting 2D CrPX 3 monolayers (X: S, Se)

The effect of vacancy and water adsorption on the electronic structure of semiconducting 2D trichalcogenide material CrPX 3 (X: S, Se) is studied using state-of-the-art density functional theory (DFT) approach. It is found that chalcogen vacancies play a minor role on the electronic structure of CrPX 3 in the vicinity of the Fermi level leading to the slightly reduced band gap for these materials, however, inducing strongly localised defect states which are placed in the energy gap formed by the valence band states. Our DFT calculations show that the interaction of water molecules with CrPX 3, pristine and defective, can be described as physisorption and the adsorption energy for H2O is insensitive to the difference between pristine and chalcogen-defective surface of trichalcogenide material. These results are the first steps for the theoretical description of the ambient molecules interaction with 2D semiconducting CrPX 3 material, that is important for its future experimental studies and possible applications.

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.

Phase-controllable laser thinning in MoTe2

Laser thinning of two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) has been considered a promising method to tune the bandgaps of TMDs via precise control of their thickness. However, the laser irradiation generates numerous chalcogen vacancies, which are known to cause a phase transition in polymorphic TMDs such as MoTe2. Therefore, the delicate control of the thickness and the phase during laser thinning is highly demanded to study the intrinsic properties of few-layered TMDs. Here, we report power-dependent laser thinning and phase control of semiconducting hexagonal MoTe2 (2H-MoTe2). High-resolution X-ray nano diffraction with synchrotron radiation showed that laser-thinned 2H-MoTe2 with low laser power (<2 mW) retained its hexagonal diffraction patterns with a single crystal orientation. In contrast, a phase transition to monoclinic (1T′) MoTe2 occurred during laser thinning at a high laser power level. Confocal Raman spectroscopy and atomic force microscopy (AFM) revealed that the low-power laser thinning of 2H-MoTe2 retained the crystal structure whereas high-power laser thinning created considerable amount of chalcogen vacancies and a phase transition. Power-dependent laser thinning thus provides a promising way to control the thickness and the phase of polymorphic 2D TMDs for next-generation optoelectronic devices.

Nearly hyperuniform, nonhyperuniform, and antihyperuniform density fluctuations in two-dimensional transition metal dichalcogenides with defects

Hyperuniform many-body systems in $d$-dimensional Euclidean space are characterized by completely suppressed (normalized) infinite-wavelength density fluctuations, and appear to be endowed with novel exotic physical properties. In this work, we analyze the effects of localized defects on the density fluctuations across length scales and on the hyperuniformity property of experimental samples of two-dimensional transition metal dichalcogenides. In particular, we extract atomic coordinates from time series annular dark field-scanning transmission electron microscopy imaging data of 2D tungsten chalcogenides with the 2H structure showing continuous development and evolution of electron-beam induced defects, and construct the corresponding chemical-bonding informed coordination networks between the atoms. We then compute a variety of pair statistics and bond-orientational statistics to characterize the samples. At low defect concentrations, the corresponding materials are nearly hyperuniform. As more defects are introduced, the (approximate) hyperuniformity of the materials is gradually destroyed, and the system becomes non-hyperuniform even when the material still contains a significant amount of crystalline regions. At high defect concentrations, the structures become antihyperuniform with diverging (normalized) large-scale density fluctuations. Overall, the defected materials possess varying degrees of orientation order, and there is apparently no intermediate hexatic phase emerging. We also construct a minimalist structural model and demonstrate that the experimental samples can be essentially viewed as perturbed honeycomb crystals with small correlated displacements and double chalcogen vacancies.

Two-Step Growth of Uniform Monolayer MoS2 Nanosheets by Metal-Organic Chemical Vapor Deposition.

To achieve large area growth of transition metal dichalcogenides of uniform monolayer thickness, we demonstrate metal–organic chemical vapor deposition (MOCVD) growth under low pressure followed by a high-temperature sulfurization process under atmospheric pressure (AP). Following sulfurization, the MOCVD-grown continuous MoS2 film transforms into compact triangular crystals of uniform monolayer thickness as confirmed from the sharp distinct photoluminescence peak at 1.8 eV. Raman and X-ray photoelectron spectroscopies confirm that the structural disorders and chalcogen vacancies inherent to the as-grown MOCVD film are substantially healed and carbon/oxygen contaminations are heavily suppressed. The as-grown MOCVD film has a Mo/S ratio of 1:1.6 and an average defect length of ∼1.56 nm, which improve to 1:1.97 and ∼21 nm, respectively, upon sulfurization. The effect of temperature and duration of the sulfurization process on the morphology and stoichiometry of the grown film is investigated in detail. Compared to the APCVD growth, this two-step growth process shows more homogenous distribution of the triangular monolayer MoS2 domains across the entire substrate, while demonstrating comparable electrical performance.

Reinforcement learning in discrete action space applied to inverse defect design

Reinforcement learning (RL) algorithms that include Monte Carlo Tree Search (MCTS) have found tremendous success in computer games such as Go, Shiga and Chess. Such learning algorithms have demonstrated super-human capabilities in navigating through an exhaustive discrete action search space. Motivated by their success in computer games, we demonstrate that RL can be applied to inverse materials design problems. We deploy RL for a representative case of the optimal atomic scale inverse design of extended defects via rearrangement of chalcogen (e.g. S) vacancies in 2D transition metal dichalcogenides (e.g. MoS2). These defect rearrangements and their dynamics are important from the perspective of tunable phase transition in 2D materials i.e. 2H (semi-conducting) to 1T (metallic) in MoS2. We demonstrate the ability of MCTS interfaced with a reactive molecular dynamics simulator to efficiently sample the defect phase space and perform inverse design—starting from randomly distributed S vacancies, the optimal defect rearrangement of defects corresponds a line defect of S vacancies. We compare MCTS performance with evolutionary optimization i.e. genetic algorithms and show that MCTS converges to a better optimal solution (lower objective) and in fewer evaluations compared to GA. We also comprehensively evaluate and discuss the effect of MCTS hyperparameters on the convergence to solution. Overall, our study demonstrates the effectives of using RL approaches that operate in discrete action space for inverse defect design problems.

Vacancy-triggered and dopant-assisted NO electrocatalytic reduction over MoS2.

Nitric oxide electroreduction reaction (NOER) is an efficient method for NH3 synthesis and NOx-related pollutant treatment. However, current research on NOER catalysts mainly focuses on noble metals and single atom catalysts, while low-cost transition metal dichalcogenides (TMDCs) are rarely considered. Herein, by applying density functional theory (DFT) calculations, we study the catalytic performance of NOER over 2H-MoS2 monolayers with the most common S vacancies and some Mo atoms substituted by transition metal atoms (denoted as TM-MoS2@VS). Our results show that an S vacancy and a heteroatom substitution tend to form a first nearest neighbour (1NN) pair, which greatly improves the NOER catalytic performance of 2H-MoS2. The S vacancy site can trigger NOER by strongly adsorbing a NO molecule and elongating the NO bond, while the heteroatom dopant can assist NOER by tuning the electron donating capability of 2H-MoS2 which breaks the linear scaling relations among key reaction intermediates. At low NO coverage, NH3 can be correspondingly yielded at -0.06 and -0.38 V onset potentials over the Pt- and Au-doped MoS2 catalysts with S vacancies (Pt-MoS2@VS and Au-MoS2@VS). At high NO coverage, N2O/N2 is thermodynamically favored. Meanwhile, the competing hydrogen evolution reaction (HER) is suppressed. Thus, the Pt-MoS2@VS catalysts are promising candidates for NOER. In addition, coupling the substitutional doping of Mo atoms to S vacancies presents great potential in improving the catalytic activity and selectivity of MoS2 for other reactions. In general, the strategy of coupling hetero-metal doping and chalcogen vacancy can be extended to enhance the catalytic activity of other TMDCs.