Defects In Transition Metal Dichalcogenides Research Articles

Role of chalcogen vacancies and hydrogen in the optical and electrical properties of bulk transition-metal dichalcogenides

Like in any other semiconductor, point defects in transition-metal dichalcogenides (TMDs) are expected to strongly impact their electronic and optical properties. However, identifying defects in these layered two-dimensional materials has been quite challenging with controversial conclusions despite the extensive literature in the past decade. Using first-principles calculations, we revisit the role of chalcogen vacancies and hydrogen impurity in bulk TMDs, reporting formation energies and thermodynamic and optical transition levels. We show that the S vacancy can explain recently observed cathodoluminescence spectra of MoS2 flakes and predict similar optical levels in the other TMDs. In the case of the H impurity, we find it more stable sitting on an interstitial site in the Mo plane, acting as a shallow donor, and possibly explaining the often observed n-type conductivity in some TMDs. We also predict the frequencies of the local vibration modes for the H impurity, aiding its identification through Raman or infrared spectroscopy.

Defect Engineering in Transition Metal Dichalcogenide-Based Gas Sensors

Since the discovery of innovative two-dimensional (2D) materials, significant efforts have been dedicated to exploring their intriguing properties and emerging applications. Among all candidates, transition metal dichalcogenides (TMDs) have proven to be exceptional for gas sensing, while defects engineering has been introduced to modify the pristine TMDs for better gas sensing performances. In this review, we systematically summarize types of defects, advanced characterization techniques, and state-of-the-art controllable synthetic methods. Various types of defects in TMDs can induce diverse changes in chemical and electron structures, which are closely correlated with gas sensing ability. Therefore, connections between defects and gas sensing mechanisms and performances have been addressed based on both defect categories and electron affinity of gases. This review will be a guide for researchers in defective materials and open up the field of precisely synthesis chemistry and deepen the understanding of the underlying effects of defects in other 2D materials.

CVD growth of large-area, continuous, and defect-free MoS2 multilayer films from solution-cast seed nanoflakes

Defects in transition metal dichalcogenides are among the primary source of their chemical reactivity, including their response to water. The CVD growth of defect-free continuous molybdenum disulfide (MoS2) films has been particularly challenging. In this work, we demonstrate the growth of large-area continuous films of MoS2 using solution-cast MoS2 seed nanoflakes, both with and without the assistance of alkali halide. The MoS2 seed nanoflakes serve as nucleation sites for the in-plane growth of few-layer MoS2, which is accompanied by localized vertical growth of MoS2 particles. For films grown with the combination of seed and NaCl, very low oxygen content was observed together with the stoichiometric ratio of Mo: S being close to the ideal value of 1: 2. These films show humidity-insensitive electrical response over a wide interval (5%–95% RH). This is attributed to very low concentrations of edge and basal-plane defects, leading to low water dissociation and restricted proton transport.

Topological superconductivity of line defects in transition metal dichalcogenides

Topological superconductivity of line defects in transition metal dichalcogenides

Enhancement of the E12g and A1g Raman modes and layer identification of 2H‐WS2 nanosheets with metal coatings

AbstractRaman spectroscopy in transition metal dichalcogenides (TMDCs) helps determine their structural information and layer dependency. Because it is non‐destructive and fast, it is an archetypal spectroscopic technique to investigate the structure and defects in TMDCs. In our earlier study, we used a metal‐dielectric coating to enhance Raman signal of WS2 because the Raman Spectra measured from WS2 coated on the standard Si/SiO2 was significantly lower. This metal‐dielectric coating allowed access to the otherwise unavailable E12g and A1g modes of WS2. In this study, we compare the Raman spectra of WS2 on a Si/SiO2 to that of metal layers (Au [200 nm] and Al [200 nm]). A significant enhancement in the Raman signal of 2‐3L WS2 is observed for both the Au and Al coatings. Although 200 nm Au coating enhances the Raman Signal better than the 10 nm Au coating, it does not resolve the other signature vibrations other than E12g and A1g. Whereas, coating based on Al (200 nm) and Al (10 nm) both enhance the Raman signal of WS2. In the case of Al coating, we report well‐resolved vibrational modes such as the ones below <300 cm−1in a few layered WS2. This enhancement allows us to quantify the number of layers and also investigate the presence of defects. Photoluminescence measurements on 2–3L and 15L WS2 on Al (200 nm) confirm this.

First-principles study of the native defects with charge states in ZrSe[formula omitted

Study of the native defects in transition metal dichalcogenides (TMDCs) is of fundamental interest and potential technological importance. The atomic and electronic structures of native defects with charge states in ZrSe2 are systematically investigated for the first time based on first-principles calculations with the optB86R-vdW and HSE06 functionals. We identify the configurations of relatively stable defects including Zr interstitial (Zrint), Se vacancy (VSe) and Zr antisite (ZrSe). The positive charged Zr interstitial defect (Zrint2+) has the lowest formation energy and thus is most commonly seen in ZrSe2, which is in good agreement with experimental observation. The ZrSe defect is found to form a DX-like configuration. Moreover, the non-interacting Zr Frenkel pairs are the most favorable form of intrinsic defects. The electronic structure analyses of these defects reveal that the extra Zr atoms adjacent to the defects act as donors, which are responsible for the native n-type conductivity of as-grown ZrSe2. Our study can provide an insight into understanding the properties of native defects in ZrSe2.

Experimental and theoretical studies of native deep-level defects in transition metal dichalcogenides

Transition metal dichalcogenides (TMDs), especially in two-dimensional (2D) form, exhibit many properties desirable for device applications. However, device performance can be hindered by the presence of defects. Here, we combine state of the art experimental and computational approaches to determine formation energies and charge transition levels of defects in bulk and 2D MX2 (M = Mo or W; X = S, Se, or Te). We perform deep level transient spectroscopy (DLTS) measurements of bulk TMDs. Simultaneously, we calculate formation energies and defect levels of all native point defects, which enable identification of levels observed in DLTS and extend our calculations to vacancies in 2D TMDs, for which DLTS is challenging. We find that reduction of dimensionality of TMDs to 2D has a significant impact on defect properties. This finding may explain differences in optical properties of 2D TMDs synthesized with different methods and lays foundation for future developments of more efficient TMD-based devices.

Sulfur Line Vacancies in MoS2 for Catalytic Hydrogen Evolution Reaction

Defects in transition metal dichalcogenides play important roles in the field of the catalytic hydrogen evolution reaction (HER). However, the use of defective MoS2 as HER catalysts remains controversial because the types of defects are various, including zero-dimensional point defects, one-dimensional linear defects, and two-dimensional plane defects. Recently, novel structures of linear defects have drawn more and more attention, and it is necessary to explore their unique properties. This review focuses on the formation mechanism, fabrication method, accurate atomic structure, and catalytic hydrogen evolution mechanism of sulfur line vacancies in MoS2 as electrocatalysts. The structure–activity relationship between line defects and catalytic performance is discussed in detail. This will provide a route for the design of excellent catalysts by engineering line defects.

Electrocatalytic Reduction of N2 to NH3 Over Defective 1T'-WX2 (X=S, Se, Te) Monolayers.

Defects in transition metal dichalcogenides (TMDs) can serve as active sites in catalytic reactions. In this work, by means of first-principles calculations, the catalytic activities of WX2 (X=S, Se, Te) monolayers in the 1T' phase with both vacancy defects (missing chalcogen atoms, X Vd ) and antisite defects (replacing chalcogen atoms with W atoms, X Ad ) were evaluated for the nitrogen reduction reaction (NRR). Results showed that all these defective catalysts had great potential toward electrocatalytic ammonia synthesis by exhibiting low limiting potentials (UL ). Over 1T'-WTe2 @Te Vd , 1T'-WS2 @S Ad , 1T'-WSe2 @Se Ad , and 1T'-WTe2 @Te Ad , the corresponding UL values were -0.49, -0.21, -0.19, and -0.15 V, much smaller than that of the benchmark catalyst, the Ru (0001) surface (UL =-0.98 V). Furthermore, the hydrogen evolution reaction (HER) was inhibited. 1T'-WX2 monolayers with the antisite defects showed better NRR activity than those with the vacancy defects because of the smaller steric hindrance at the former. Results suggest that the steric effect at the active surface sites should be utilized to develop better catalysts.

Inspection of Line Defects in Transition Metal Dichalcogenides Using a Microscopic Hyperspectral Imaging Technique

The line defects of two-dimensional (2D) transition metal dichalcogenides (TMDs) play a vital role in determining their device performance. In this work, a microscopic hyperspectral imaging technique based on differential reflectance was introduced for the online inspection of line defects in TMDs. Upon comparison of the measurement results of imaging and spectra, the relationship between optical contrast and differential reflectance spectra was established. A light selection method was proposed to optimize the optical contrast of line defects. Via application of an image processing algorithm, an automatic detection of the line defects with a classification accuracy of 95% was achieved for WS2, MoS2, and MoSe2. This work not only provides a microscopic hyperspectral imaging technique for detecting 2D material defects but also introduces a versatile design strategy for developing an advanced machine vision spectroscopic system.

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.

Exciton-dominant photoluminescence of MoS2 by a functionalized substrate.

Transition metal dichalcogenides (TMDs) have been considered as promising candidates for transparent and flexible optoelectronic devices owing to their large exciton binding energy and strong light-matter interaction. However, monolayer (1L) TMDs exhibited different intensities and spectra of photoluminescence (PL), and the characteristics of their electronic devices also differed in each study. This has been explained in terms of various defects in TMDs, such as vacancies and grain boundaries, and their surroundings, such as dielectric screening and charged impurities, which lead to non-radiative recombination of trions, low quantum yield (QY), and unexpected doping. However, it should be noted that the surface conditions of the substrate are also a critical factor in determining the properties of TMDs located on the substrate. Here, we demonstrate that the optical and electrical properties of 1L MoS2 are strongly influenced by the functionalized substrate. The PL of 1L MoS2 placed on the oxygen plasma-treated SiO2 substrate was highly p-doped owing to the functional groups of -OH on SiO2, resulting in a strong enhancement of PL by approximately 20 times. The PL QY of 1L MoS2 on plasma-treated SiO2 substrate increased by one order of magnitude. Surprisingly, the observed PL spectra show the suppression of non-radiative recombination by trions, thus the exciton-dominant PL led to a prolonged lifetime of MoS2 on the plasma-treated substrate. The MoS2 field-effect transistors fabricated on plasma-treated SiO2 also exhibited a large hysteresis in the transfer curve owing to charge trapping of the functional groups. Our study demonstrates that the functional groups on the substrate strongly affect the characteristics of 1L MoS2, which provides clues as to why MoS2 exfoliated on various substrates always exhibited different properties in previous studies.

Point and complex defects in monolayer PdSe2 : Evolution of electronic structure and emergence of magnetism

Layered transition-metal dichalcogenides (TMDs) constitute an emerging class of materials that provide researchers a platform to realize fundamental studies and to design promising optoelectronic devices. While defects are an almost unavoidable part of TMDs, they bring additional interesting properties absent in defect-free layers. Moreover, the controlled introduction of defects in TMDs makes it possible to tailor the electromagnetic properties of the materials. Here we report defect-induced properties of single-layer $\mathrm{Pd}{\mathrm{Se}}_{2}$ and demonstrate the emergence of magnetism at the nanoscale. Our first-principle calculations indicate that Se vacancies create in-gap defect states, which can be responsible for trapping of carriers. The complex square ${V}_{\mathrm{Pd}+4\mathrm{Se}}$ vacancy induces spin-polarized states with a total local magnetic moment of $2\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per defect, making it possible to introduce magnetization into $\mathrm{Pd}{\mathrm{Se}}_{2}$ and therefore expand the family of two-dimensional (2D) magnets. The defect formation energies are much lower compared to many other TMD materials that can explain the presence of a large number of Se defects after mechanical exfoliation of $\mathrm{Pd}{\mathrm{Se}}_{2}$ layers, while the central location of the Pd atoms preserves them from exfoliation-induced defect formation. The negatively charged vacancies are prone to form and in many cases demonstrate spin-polarized states. The small diffusion barrier of ${\mathrm{V}}_{\mathrm{Se}}$ in 2D $\mathrm{Pd}{\mathrm{Se}}_{2}$ leads to an easy room-temperature migration, while ${\mathrm{V}}_{\mathrm{Pd}}$ demonstrates a high diffusion barrier preventing its spontaneous migration. As a guide for experimentalists, we simulate and characterize scanning tunneling microscope images in valence and conduction states and estimate the electron-beam energy for a controllable production of various defect patterns. These intriguing findings make $\mathrm{Pd}{\mathrm{Se}}_{2}$ an ideal platform to study defect-induced phenomena.

Effect of point defects on electronic and excitonic properties in Janus-MoSSe monolayer

Defects in transition-metal dichalcogenides (TMDs) monolayers are ubiquitous and have great effects on the electronic and optical properties. As a consequence, an in-depth understanding of how defects influence the performance of materials is crucial for the design and manipulation of electronic and optoelectronic devices. In this work, we investigate the effect of four common point defects, i.e., ${V}_{\text{S}}$, ${V}_{\text{Se}}$, ${V}_{{\text{MoS}}_{3}}$, and ${V}_{{\text{MoSe}}_{3}}$ on the electronic and excitonic properties in the Janus-MoSSe monolayer, by employing the first-principles method combined with the $GW$-Bethe-Salpeter equation. Results show that the presence of defects indeed alters the electronic and optical properties of the Janus-MoSSe monolayer, but in different ways. For monochalcogen vacancies, the first bright exciton (${X}_{1}$) is more localized and has a smaller exciton radius as compared with that in the pristine Janus-MoSSe monolayer. In addition, the exciton binding energies become much larger, with the value up to 1.58 eV. As a result, excitons are easy to combine with defects, becoming centers of luminescence. While for the systems with ${\mathrm{MoS}}_{3}$ and ${\mathrm{MoSe}}_{3}$ vacancy clusters, the distribution of ${X}_{1}$ has changed significantly even though the exciton binding energies are comparable with that of the pristine system. Further analysis indicates that this can be ascribed to the competition mechanism between the existence of defects and Coulomb screening effect. Based on these findings, we obtained a thorough understanding of how point defects influence the nature of the Janus-TMD monolayer, which provides useful theoretical guidance for further experimental research.

Defect activated optical Raman modes in single layer MoSe2

In the last decade, transition metal dichalcogenides (TMDs) have been intensively synthesized/studied thus linking their morphological aspect to their physical properties, and consequently leading to the understanding of the possible benefits of defects in such materials. Nevertheless, for future applications, quantifying and identifying defects in TMDs is still a milestone to reach in order to better employ these materials in optoelectronic devices. Raman Spectroscopy has been successfully employed in graphene to quantify punctual or line defects. In this paper, we bombarded monolayer MoSe2 with He ions and found out the existence of three defect activated Raman bands around 250–300 cm−1. Density functional theory calculations were employed to obtain the electronic and phonon dispersion bands, making it possible to infer that these bands arise from inter-valley Raman double resonance processes. Interestingly, the same punctual defect model, that allows one to predict the defect concentration at which graphene starts to become amorphous, also works for TMDs. Hence, this work opens the door to the macroscopic quantification of defects in TMDs, which is essential for technological applications.

(Invited) Ab Initio Exciton and Phonon Dynamics in Transition Metal Dichalcogenides

Interest in the properties of transition metal dichalcogenides (TMDs) has increased due to the discovery of the coupling between spin and valley degrees of freedom, which can be seen experimentally using a circularly polarised laser. After excitation the newly formed carrier populations must move towards the other valley until balance is reached. However, this relaxation process is not entirely understood in the literature, where the relative importance of the electron-electron (e-e) or electron-phonon (e-p) interactions is still a subject of debate. Previous works on WSe2 [A. Molina- Sánchez, et al - Nano letters, 2017] have shown that the e-p interaction is a good candidate to describe the relaxation process. Using a fully ab-initio framework based on the Baym-Kadanoff equations [P. M. M. C. de Melo and A. Marini, Phys. Rev. B 93, 155102 (2016)] we study the influence of the e-p interaction on MoSe2 after its excitation by a laser field. We show how phonons allow carrier relaxation and how the Kerr signal and total magnetisation are affected at different temperatures, with the latter exhibiting a non-monotonic behaviour as the temperature increases [M Ersfeld et al - Nano Letters 2019 19 (6), 4083-4090]. An important conclusion is that long lived spin states probably reside within defects. We calculate the spectral signatures of point defects in TMDs, finding two main classes based on the presence of in-gap states, and estimating the experimental resolution needed to provide quantification of the defect concentration [P de Melo et al. https://arxiv.org/abs/2010.10222]. The localization of excitonic states around the defect provides a benchmark for scanning probe characterization (figure shows a S vacancy exciton wave function, the green ball is the hole position). Figure 1

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.

Field-effect-transistor-based ion sensors: ultrasensitive mercury(II) detection via healing MoS2 defects.

The contamination of water with heavy metal ions represents a harsh environmental problem resulting from societal development. Among various hazardous compounds, mercury ions (Hg2+) surely belong to the most poisonous ones. Their accumulation in the human body results in health deterioration, affecting vital organs and eventually leading to chronic diseases, and, in the worst-case scenario, early death. High selectivity and sensitivity for the analyte of choice can be achieved in chemical sensing using suitable active materials capable of interacting at the supramolecular level with the chosen species. Among them, 2D transition metal dichalcogenides (TMDCs) have attracted great attention as sensory materials because of their unique physical and chemical properties, which are highly susceptible to environmental changes. In this work, we have fabricated MoS2-based field-effect transistors (FETs) and exploited them as platforms for Hg2+ sensing, relying on the affinity of heavy metal ions for both point defects in TMDCs and sulphur atoms in the MoS2 lattice. X-ray photoelectron spectroscopy characterization showed both a significant reduction of the defectiveness of MoS2 when exposed to Hg2+ with increasing concentration and a shift in the binding energy of 0.2 eV suggesting p-type doping of the 2D semiconductor. The efficient defect healing has been confirmed also by low-temperature photoluminescence measurements by monitoring the attenuation of defect-related bands after Hg2+ exposure. Transfer characteristics in MoS2 FETs provided further evidence that Hg2+ acts as a p-dopant of MoS2. Interestingly, we observed a strict correlation of doping with the concentration of Hg2+, following a semi-log trend. Hg2+ concentrations as low as 1 pM can be detected, being way below the limits imposed by health regulations. Electrical characterization also revealed that our sensor can be efficiently washed and used multiple times. Moreover, the developed devices displayed a markedly high selectivity for Hg2+ against other metal ions as ruled by soft/soft interaction among chemical systems with appropriate redox potentials, being a generally applicable approach to develop chemical sensing devices combining high sensitivity, selectivity and reversibility, to meet technological needs.

Unraveling Structural and Optical Properties of Two-Dimensional MoxW1–xS2 Alloys

Ordered defects in two-dimensional (2D) transition-metal dichalcogenides (TMDs) could function as single-photon emitters and quantum-dot qubits in quantum information technologies. Alloying is a commonly used method to introduce such ordered defects in TMDs. However, the structural, vibrational, optical, and excitonic properties of the ordered defects in TMDs remain poorly understood. In this work, we provide a comprehensive study of 2D MoxW1–xS2 alloys from first principles. We found that defect stripes can be formed in MoxW1–xS2 alloys with the lowest energy configuration at x ∼ 0.5. We identify a unique Raman fingerprint for the striped defect configurations. Both defect quantum dots and striped defects in MoxW1–xS2 are shown to trap excitons. The optical gap of the MoxW1–xS2 alloys redshifts as the Mo content increases, but the excitons remain strongly bound. We predict that excitons at the lateral heterostructure of WS2 and MoS2 are delocalized with a strong charge-transfer character.

Locally Engineering and Interrogating the Photoelectrochemical Behavior of Defects in Transition Metal Dichalcogenides

Transition metal dichalcogenides (TMDs) are attractive materials for a variety of applications in solar energy conversion and electrocatalysis, due to their favorable optical and electrical propert...